Publications

Surface modification of tungsten disulfide (WS2) nanoparticles (NPs) by a photo-initiator (PI) for radical curing of acrylate nano-structured systems has been studied with the purpose of acquiring covalent bonding of the acrylate matrix to the NPs surface. This surface functionalization was expected to photosensitize both the PI and the NPs thereby enhance the photocuring of the acrylate as well as improving its mechanical properties. The studied PI was the commonly used Bis(acyl) phosphane oxides (BAPOs). For surface modification of the WS2 NPs, the BAPO was functionalized first with Tri(methoxy) silane (TMESI) to yield TMESI-BAPO moieties. Subsequently, the functionalized BAPO was chemisorbed to the WS2 NPs resulting in WS2 NPs surface modified by TMESI-BAPO pendant moieties. The functionalization and surface modification were confirmed using spectroscopic techniques as well as electron microscopy, and thermal measurements. Thermal gravimetric analysis (TGA) displayed 40 wt% weight loss for TMESI-BAPO modified WS2 upon heating to 400 °C. The modified WS2 NPs were incorporated into acrylate resin to study the curing kinetics and resulting mechanical properties compared to neat WS2. Nanostructured acrylate with TEMSI-BAPO modified WS2 NPs exhibited an increase of 83% in storage modulus and toughness compared to neat acrylate.

The radiation curing paradigm of opaque WS2 nanoparticle (NP)-based epoxy/acrylate nanocomposites was studied and found to exhibit both a reduction in viscosity and an enhanced degree of curing when incorporating WS2 NPs. The objective of this study was to investigate the mechanical, thermal, and physical properties of a radiation-induced and cured epoxy/acrylate blend containing 0.3 to 1.0 wt.% WS2 NPs. Experimental results indicate that the tensile toughness increased by 22% upon optimizing the NP content compared to that of WS2-free formulations. Tensile fractured surfaces with different WS2 NP contents were analyzed with a scanning electron microscope and an atomic force microscope and showed distinctive morphology depending on the WS2 NP content, supporting the results of the tensile test. The energy required to break shear adhesion specimens demonstrated an increase of up to 60% compared to that of the neat resin. The glass transition temperature determined by dynamic mechanical analysis presented similar or higher values upon WS2 NP incorporation. Furthermore, up to 80% improvement in impact strength was demonstrated when WS2 NPs were dispersed in the epoxy/acrylate blend. It was concluded that the surface chemistry and dispersion level of the WS2 NPs are the major variables affecting the macro properties of cationically radiation-cured resins and their adhesion properties. This study is the first to demonstrate the possibility for radiation-induced curing of opaque NPs based on WS2 that serve as both a reinforcement nanoparticle at low concentrations and an enhancement of the degree of curing.

Intelligent materials with adaptive response to external stimulation lay foundation to integrate functional systems at the material level. Here, with experimental observation and numerical simulation, we report a delicate nano-electro-mechanical-opto-system naturally embedded in individual multiwall tungsten disulfide nanotubes, which generates a distinct form of in-plane van der Waals sliding ferroelectricity from the unique combination of superlubricity and piezoelectricity. The sliding ferroelectricity enables programmable photovoltaic effect using the multiwall tungsten disulfide nanotube as photovoltaic random-access memory. A complete “four-in-one” artificial vision system that synchronously achieves full functions of detecting, processing, memorizing, and powering is integrated into the nanotube devices. Both labeled supervised learning and unlabeled reinforcement learning algorithms are executable in the artificial vision system to achieve self-driven image recognition. This work provides a distinct strategy to create ferroelectricity in van der Waals materials, and demonstrates how intelligent materials can push electronic system integration at the material level.

One-dimensional (1D) analogues of two-dimensional (2D) layered materials, especially nanotubes exhibit unique properties, which are distinct from the 2D flakes. The nanotubes and fullerene-like nanoparticles from layered transition metal dichalcogenides (TMDs) are one of the prime foci of this field in the last 30 years. In this concise review, we present the advancement made in the TMDs nanotubes and fullerene-like nanoparticles over the last few years. The synthesis and structure of TMDs nanotubes such as WS2/MoS2 are briefly described. The mechanical properties of single WS2 nanotubes were examined by in-situ electron microscopy techniques and are briefly discussed. Their reinforcement effects in polymer composites are also presented, as well as their superior tribological behavior. The unique optoelectronic properties of WS2 (MoS2) nanotubes are presented. Thus, the bulk photovoltaic effect and superconductivity exhibited by WS2 nanotubes, which are a manifestation of their 1D structure and low symmetry, are revisited briefly. The strong light-matter interaction of nanotubes resulting in polariton quasiparticles and their evolution as a function of nanotube diameter are explained. Last but not least, a new family of misfit layered nanotubes and their exceptional physical properties are briefly touched upon.

Exploring the behavior of nanocrystals with varying shapes and sizes under high pressure is crucial to understanding the relationship between the morphology and properties of nanomaterials. In this study, we investigated the compression behaviors of WS2 nanotubes (NT-WS2) and fullerene-like nanoparticles (IF-WS2) by in situ high-pressure X-ray diffraction (XRD) and Raman spectroscopy. It was found that the bulk modulus of NT-WS2 is 81.7 GPa, which is approximately twice as large as that of IF-WS2 (46.3 GPa). This might be attributed to the fact that IF-WS2 with larger d-spacing along the c-axis and higher defect density are more compressible under isotropic pressure than NT-WS2. Thus, the slender NT-WS2 possess a more stable crystal structure than the IF-WS2. Our findings reveal that the effects of morphology and size play crucial roles in determining the high-pressure properties of WS2 nanoparticles, and provide significant insight into the relationship between structure and properties.

Misfit layered compounds (MLCs) MX-TX2, where M, T = metal atoms and X = S, Se, or Te, and their nanotubes are of significant interest due to their rich chemistry and unique quasi-1D structure. In particular, LnX-TX2 (Ln = rare-earth atom) constitute a relatively large family of MLCs, from which nanotubes have been synthesized. The properties of MLCs can be tuned by the chemical and structural interplay between LnX and TX2 sublayers and alloying of each of the Ln, T, and X elements. In order to engineer them to gain desirable performance, a detailed understanding of their complex structure is indispensable. MLC nanotubes are a relative newcomer and offer new opportunities. In particular, like WS2 nanotubes before, the confinement of the free carriers in these quasi-1D nanostructures and their chiral nature offer intriguing physical behavior. High-resolution transmission electron microscopy in conjunction with a focused ion beam are engaged to study SmS-TaS2 nanotubes and their cross-sections at the atomic scale. The atomic resolution images distinctly reveal that Ta is in trigonal prismatic coordination with S atoms in a hexagonal structure. Furthermore, the position of the sulfur atoms in both the SmS and the TaS2 sublattices is revealed. X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and X-ray absorption spectroscopy are carried out. These analyses conclude that charge transfer from the Sm to the Ta atoms leads to filling of the Ta 5d z 2 level, which is confirmed by density functional theory (DFT) calculations. Transport measurements show that the nanotubes are semimetallic with resistivities in the range of 10–4 Ω·cm at room temperature, and magnetic susceptibility measurements show a superconducting transition at 4 K.

Several nanotubular structures from chalcogenide-based misfit layer compounds (MLC) were reported in recent years. MLCs consist of a stacking of two alternating and dissimilar (2D) atomic layers, e. g. one with rocksalt structure (MX) and the other- TX2 – with hexagonal layer structure. The layers are held together by weak van der Waals forces, i. e. they can be exfoliated with scotch-tape. Furthermore, in analogy to intercalation compounds, partial charge transfer between the layers with dissimilar work function results also in polar forces between the MX and TX2 layers. The mismatch between the alternating (asymmetric) layers and the seaming of the dangling bonds at the edges drives them to form tubular (and also scroll-like) structures. New structural characterization whereby the nanotubes were bisected into lamella via focused ion beam and examined by TEM, are reported.

This conference contribution is focused on decoration of WS2 nanotubes (NT-WS2) with gold and silver nanoparticles via facile routes implying direct reaction of tungsten disulfide with water-soluble AuIII and AgI species at 100°C. The underlying mechanism of these interactions will be discussed in details based on extensive studies of reaction mixtures and resulting metal–NT-WS2 nanocomposites, including thorough X-ray photoelectron spectroscopy (XPS) analysis. Surprising features in optical spectra of the designed nanocomposites would be reported, including suppression of plasmon resonance in tiny noble metal nanoparticles (

Tungsten disulfide polycrystalline microfibers were successfully synthesized by a process involving electrospinning, calcination, and sulfidation steps. We used an aqueous solution of silicotungstic acid (H4SiW12O40) and polyvinyl alcohol as precursors for the synthesis of composite fibers by the needle-less electrospinning technique. The obtained green composite fibers (av. diam. 460 nm) were converted by calcination in air to tungsten oxide WO3 fibers with traces of SiO2 and a smaller diameter (av. diam. 335 nm). The heat treatment of the WO3 fibers under flowing H2/H2S/N2 stream led to conversion to tungsten disulfide WS2 with retention of the fibrous morphology (av. diam. 196 nm). Characterization of the intermediate and final fibers was performed by the XRD, SEM, TEM, HAADF STEM EDS, elemental analyses ICP-OES, and IR spectroscopy methods.

We present density functional theory calculations of the vibrational and electronic properties of the misfit-layer compound (MLC) LaS–CrS2 and its isolated sublayers to identify the vibrational modes of the compound. From the comparison of two model systems, (i) the fully relaxed misfit-layer compound supercell and (ii) its sublayers as isolated systems, we extract the charges, which are transferred between the two sublayers and the concomitant changes of the structural parameters upon formation of the supercell. The deformation within each sublayer indicates a strong influence of the charge transfer, confirming the important role of the interlayer interaction between the two sublayers. By comparing the vibrational properties of the two model systems, we assign several frequency regimes within the vibrations of the supercell to phonon branches of the sublayers. From polarized Raman spectra, we find that some of the Raman modes are directly inherited from the Γ-point phonons of the hypothetical isolated 1T-CrS2 layer; other Raman modes are backfolded from the A1g-associated phonon branch of the 1T-CrS2 onto the misfit-layer compound MLC Γ-point, which is a direct consequence of the supercell formation in LaS–CrS2.

Misfit layered compounds (MLCs) have been studied in the literature for the last 40 years. They are generally made of an alternating sequence of two monolayers, a distorted rocksalt structure, and a hexagonal layered compound. In a typical MLC, the c-axis is common to the two monolayers and so is one of the axes in the layer plan. However, the two compounds are non-commensurate along at least one axis, and the ratio between the two axes is an irrational number making the MLC a non-stoichiometric compound. The two main families of MLC are those based on metal dichalcogenides and CoO2 as the hexagonal layered compound. Traditionally, ternary MLCs were prepared and studied, but some quaternary and multinary MLC minerals have been known for many years. Over the last few years, interest in MLCs with four and even larger number of atoms has grown. Doping or alloying of a ternary MLC permits precise control of the charge carrier density and hence the electrical, thermoelectric, catalytic, and optical properties of such compounds. In this short review, some of these developments will be discussed with the main emphasis put on quaternary MLC nanotubes belonging to the chalcogenide series. The synthesis, structural characterization, and some of their properties are considered. Some recent developments in quaternary cobaltite MLCs and recent studies on exfoliated MLCs are discussed as well.

WS2 inorganic nanotubes (WS2-NT) have been incorporated into Polylactic Acid (PLA) by melt mixing to create a bio-degradable, mechanically reinforced nanocomposite filament. The filament was then processed by Fused Filament Fabrication (FFF) 3D-printer, and the morphology and characteristics before and after printing were compared. We found that addition of WS2-NT to PLA by extrusion mixing increases the elastic modulus, yield strength and strain-at-failure by 20%, 23% and 35%, respectively. Moreover, we found that the printing process itself improves the dispersion of WS2-NT within the PLA filament, and does not require changing of the printing parameters compared to pure PLA. The results demonstrate the advantage of WS2-NT as reinforcement specifically in 3D-printable polymers, over more traditional nano-reinforcements such as graphene and carbon nanotubes. WS2-NT based 3D-printable nanocomposites can be used for variety of applications from custom-made biodegradable scaffold of soft implants such as cartilage-based organs and biodegradable soft stents to the more general easy-to-apply nano-reinforced polymers.

Proton exchange membranes with high through-plane proton conductivity are a critical component of high-performance fuel cells, electrolyzers, and batteries. However, isotropically distributed proton-conducting channel structures of current membranes present a limitation. Herein, a proton exchange membrane with straight proton-conducting channels aligned in the thickness direction is fabricated, achieved by magnetic field-induced alignment of proton-conductive, paramagnetic, and one-dimensional (1D) tungsten disulfide nanotubes (pms-WS2) distributed in a perfluorinated sulfonic acid (Nafion) membrane. The pms-WS2 nanotubes feature straight WS2 nanotubes as a core, a polystyrenesulfonate (PSS) skin layer, and surface-decorated Fe3O4 nanoparticles. A molecular dynamics simulation suggests that straight proton-conducting channels are constructed at the interface of Nafion/pms-WS2 due to densely populated sulfonic acids. Spectroscopic investigation and magnetization measurements verify the through-plane alignment of pms-WS2 under a weak through-plane magnetic field (0.035 T) during the removal of solvent from the membrane cast. Compared with a recast Nafion membrane with the same thickness, the through-plane aligned composite membrane exhibits 69% higher proton conductivity and 51% higher power performance in a proton exchange membrane fuel cell, demonstrating its efficacy. The through-plane alignment of a proton-conductive inorganic 1D material promises improved power performance of advanced electrochemical devices.

The global impact of carbonaceous emissions from the internal combustion engine and coal-fired power plants has stimulated efforts to mitigate global warming and deterioration of the habitable biosphere. These efforts resulted in the maturation of new technologies to harvest and store the natural energies available-wind, waves, geothermal, sun and outer space radiation - for conservation and security. Inorganic layered compounds (2D materials), like MoS2, TiS2 and CoO2 played a major role in the upbringing of novel energy technologies, which can one day replace fossil fuel in the transport industry as well as in other energy-consuming sectors. In this perspective, the history of various concepts explored in energy-related research using bulk layered compounds (2D materials), is briefly reviewed, first. The recent addition of 2D nanostructures, in energy conversion and storage, has added significant momentum to this research field. Particularly, this perspective places a great emphasis on the study of inorganic fullerene-like nanoparticles and nanotubes from 2D materials, and to some extent also on the atomically thin single layer solids, that apparently surpasses the performance of other respective allotropes in the pursuit of their use in energy storage/conversion, catalysis, and sensing.

Core-shell nanoparticles provide a unique morphology to exploit electronic interactions between dissimilar materials, conferring upon them new or improved functionalities. MoS2 is a layered transition-metal disulfide that has been studied extensively for the hydrogen evolution reaction (HER) but still suffers from low electrocatalytic activity due to its poor electronic conductivity. To understand the fundamental aspects of the MoS2-Au hybrids with regard to their electrocatalytic activity, a single to a few layers of MoS2 were deposited over Au nanoparticles via a versatile procedure that allows for complete encapsulation of Au nanoparticles of arbitrary geometries. High-resolution transmission electron microscopy of the Au@MoS2 nanoparticles provides direct evidence for the core-shell morphology and also reveals the presence of morphological defects and irregularities in the MoS2 shell that are known to be more active for HER than the pristine MoS2 basal plane. Electrochemical measurements show a significant improvement in the HER activity of AugMoS(2) nanoparticles relative to freestanding MoS2 or Au-decorated MoS2. The best electrochemical performance was demonstrated by the Au nanostars-the largest Au core employed here-encapsulated in a MoS2 shell. Density-functional theory calculations show that charge transfer occurs from the Au to the MoS2 layers, producing a more conductive catalyst layer and a better electrode for electrochemical HER The strategies to further improve the catalytic properties of such hybrid nanoparticles are discussed.

Misfit-layered compounds (MLCs) are formed by the combination of different lattices and exhibit intriguing structural and morphological characteristics. MLC SrxLa1-xS-TaS2 nanotubes with varying Sr composition (10, 20, 40, and 60 Sr atom %, corresponding to x = 0.1, 0.2, 0.4 and 0.6, respectively) were prepared in the present study and systematically investigated using a combination of high-resolution electron microscopy and spectroscopy. These studies enable detailed insight into the structural aspects of these phases to be gained at the atomic scale. The addition of Sr had a significant impact on the formation of the nanotubes with higher Sr content, leading to a decrease in the yield of the nanotubes. This trend can be attributed to the reduced charge transfer between the rare earth/S unit (LaxSr1-xS) and the TaS2 layer in the MLC which destabilizes the MLC lattice. The influence of varying the Sr content in the nanotubes was systematically studied using Raman spectroscopy. Density functional theory calculations were carried out to support the experimental observations.

Composites of poly(l-lactic acid) (PLLA) reinforced by adding inorganic nanotubes of tungsten disulfide (INT-WS2) were prepared by solvent casting. In addition to the pristine nanotubes, PLLA nanocomposites containing surface modified nanotubes were studied as well. Several surface-active agents, including polyethylene imine (PEI), were studied in this context. In addition, other biocompatible polymers, like poly d,l-lactic acid (PDLLA) and others were considered in combination with the INT-WS2. The nanotubes were added to the polymer in different proportions up to 3 wt %. The dispersion of the nanotubes in the nanocomposites were analyzed by several techniques, including X-ray tomography microscopy (Micro-XCT). Moreover, high-temperature rheological measurements of the molten polymer were conducted. In contrast to other nanoparticles, which lead to a considerable increase of the viscosity of the molten polymer, the WS2 nanotubes did not affect the viscosity significantly. They did not affect the complex viscosity of the molten PLLA phase, either. The mechanical and tribological properties of the nanocomposites were found to improve considerably by adding the nanotubes. A direct correlation was observed between the dispersion of the nanotubes in the polymer matrix and its mechanical properties.

Self-lubricating films are of immense importance in various tribological applications. Nanoparticles are often used as the lubricating component in such films. Inorganic fullerene-like (IF) nanoparticles of WS2 have been used in the past for variety of tribological applications including for self-lubricating polymer and metallic films. IF nanoparticles from MoS2 with superior tribological behavior were reported in the past. However, such nanoparticles, which are available in minute amounts, were not applied for self-lubricating coatings in the past. In the current work, inorganic fullerene-like nanoparticles of MoS2 (IF-MoS2) were co-evaporated with titanium (cobalt) on metal substrates. The films were characterized by different techniques and were shown to have good adhesion to the underlying substrate. Furthermore, tribological tests indicated that such coatings exhibit improved friction coefficient (roughly 0.1) and very small wear under relatively high load.

Herein, the use of highly concentrated sunlight for materials science research is reviewed. Specific research directions include: (1) the generation of inorganic nanostructures, some of which had eluded experimental realization with conventional synthetic processes, and (2) elucidating the processes governing the degradation of organic and perovskite-based photovoltaic materials and devices, along with accelerated assessment of their stability. Both approaches employ solar concentrators capable of producing flux densities exceeding those of terrestrial solar radiation by up to three orders of magnitude, and are geared toward either creating extensive ultrahot reactor conditions conducive to the rapid, safe synthesis of unusual nanomaterials or judiciously interrogating photovoltaic devices.

The synthesis and characterization of nanotubes from misfit layered compounds (MLCs) of the type (LnS)(1+y)TaS2 (denoted here as LnS-TaS2; Ln=Pr, Sm, Gd, and Yb), not reported before, are described (the bulk compound YbS-LaS2 was not previously documented). Transmission electron microscopy and selected area electron diffraction showed that the interlayer spacing along the c axis decreased with an increase in the atomic number of the lanthanide atom, which suggested tighter interaction between the LnS layer and TaS2 for the late lanthanides. The Raman spectra of the tubules were studied and compared to those of the bulk MLC compounds. Similar to the bulk MLCs, the Raman spectra could be divided into the low-frequency modes (110-150cm(-1)) of the LnS lattice and the high-frequency modes (250-400cm(-1)) of the TaS2 lattice. The Raman spectra indicated that the vibrational lattice modes of the strained layers in the tubes were stiffer than those in the bulk compounds. Furthermore, the modes of the late lanthanides were higher in energy than those of the earlier lanthanides, which suggested larger charge transfer between the LnS and TaS2 layers for the late lanthanides. Polarized Raman measurements showed the expected binodal intensity profile (antenna effect). The intensity ratio of the Raman signal showed that the E-2g mode of TaS2 was more sensitive to the light-polarization effect than its A(1g) mode. These nanotubes are expected to reveal interesting low-temperature quasi-1D transport behavior.

First, the mechanisms leading to the formation of nanotubes from layered (2-D) materials are briefly discussed. Two main mechanisms are evoked: (1) The asymmetry of the layer along the c-axis, which leads to spontaneous folding, as revealed first by Pauling in 1930; (2) The seaming of the layer due to the abundance of dangling bonds in the rim atoms of the 2-D nanoclusters. This mechanism was discussed first in connection with carbon fullerenes and carbon nanotubes, some 30 years ago and was further extended to inorganic 2-D materials in 1992. In the second part of this work, the formation mechanism of nanotubes from misfit layered compounds (MLC) is deliberated. Here, the two forces are shown to work in synergy leading to facile formation of nanotubes from ternary misfit compounds. This synergy is demonstrated through the versatile chemistry, which has been employed to synthesize MLC nanotubes. Furthering in complexity, few recent examples of nanotubes from quaternary chalcogenide-based MLC are briefly discussed.

Screening effect in finite-length carbon nanotubes (CNT) and their agglomerates hinders significantly the electromagnetic interaction in composite materials. Screening effect is strong in the microwave range, and it decreases with increasing frequency resulting in a strong frequency dependence of the effective conductivity of the composite. Since screening effect is rather small in the terahertz range, the effective conductivity in this range is determined directly by the intrinsic conductivity of the inclusions. The ratio of the microwave to terahertz effective conductivities was proposed as a parameter to estimate how effectively carbon nanotube inclusions contribute to the electromagnetic performance of composite materials in the microwave range. CNT film was considered as a material where maximal possible interaction of the CNTs with EM field occurs. Single-walled CNT films and CNT-based composite materials, as well as hybrid film comprising mixtures of WS
2 nanotubes and CNTs were fabricated and measured in the microwave and terahertz ranges. The electromagnetic field interaction with the inclusions has been estimated for all the samples fabricated.

Inorganic fullerene-like closed-cage nanoparticles of MoS2 and WS2 (IF-MoS2; IF-WS2), are synthesized in substantial amounts and their properties are widely studied. Their superior tribological properties led to large scale commercial applications as solid lubricants in numerous products and technologies. Doping of these nanoparticles can be used to tune their physical properties. In the current work, niobium (Nb) doping of the nanoparticles is accomplished to an unprecedented low level (

We report here a new methodology for the formation of freestanding nanotubes composed of individual gold nanoparticles (NPs) cross-linked by coordination complexes or porphyrin molecules using WS
2 nanotubes (INT-WS
2) as a template. Our method consists of three steps: (i) coverage of these robust inorganic materials with monodispersed and dense monolayers of gold NPs, (ii) formation of a molecular AuNP network by exposing these decorated tubes to solutions containing a ruthenium polypyridyl complex or meso-tetra(4-pyridyl)porphyrin, and (iii) removal of the INT-WS
2 template with a hydrogen peroxide solution. Nanoindentation of the template-free AuNP tubes with atomic force microscopy indicates a radial elastic modulus of 4 GPa. The template-free molecular AuNP tubes are characterized using scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, and micro-Raman spectroscopy. The methodology provides a convenient and scalable strategy for the realization of molecular AuNP tubes with a defined length and diameter, depending on the dimensions of the template.

The review concerns the development and the state-of-the-art in studies on the surface modification methods aimed at fabricating promising nanocomposites based on multilayer inorganic tubular and onion (fullerene-like) MoS2 and WS2 nanostructures. The synthetic details and structural features of these materials are considered. Considerable attention is paid to targeted functionalization of molybdenum and tungsten disulfide nanostructures and to fundamental principles that underlie their ability to chemical interactions. The functional properties and applications of the obtained materials are described.

Keywords: Chemistry, Multidisciplinary

Recently, polyhedral; octahedral fullerene-like structures and cylindrical nanotubes of gallium sulfide have been produced in far from equilibrium conditions using a high power solar synthesis and Pb as growth promoter. Their exact atomistic structure and their formation path remain unknown. Here, the models of fullerenes and nanotubes are designed for both stable - and metastable -polymorphs of GaS. Their stability and electronic properties were investigated as a function of both the morphology and size using density-functional tight-binding method. The results manifest that, construction principles for GaS fullerenes are different from those for classical dichalcogenide fullerenes. In contrast to the bulk, the kinetic stability of fullerene-like structures of the -GaS phase is found to be larger, than that for -GaS which is ascribed to a sterical specificity of intersected -GaS layers. We predict that, the -GaS phase should arise at least as an intermediate state during the synthesis of polyhedral GaS nanoparticles. Notwithstanding the polymorphic type, the size and the morphology of GaS nanostructures, they are all semiconductors. Finally, the possible composition and electronic properties of Pb||GaS interface within the Pb@GaS nanoparticles are discussed.

The study of inorganic nanometer-scale materials with hollow closed-cage structures, such as inorganic fullerene-like (IF) nanostructures and inorganic nanotubes (INTs), [GRAPHIC] is a rapidly growing field. Numerous kinds of IF nanostructures and INTs were synthesized for a variety of applications, particularly for lubrication, functional coatings, and reinforcement of polymer matrices. To date, such nanostructures have been synthesized mostly by heating a transition metal or oxide thereof in the presence of precursor gases, which are however toxic and hazardous. In this context, one frontier of research in this field is the development of new avenues for the green synthesis of IF structures and INTs, directly from the bulk of layered compounds. In the present work, we demonstrate a simple room-temperature and environmentally friendly approach for the synthesis of IF nanostructures and INTs via ultrashort-pulse laser ablation of a mixture of transition-metal dichalcogenides in bulk form mixed with Pb/PbO, in ambient air. The method can be considered as a synergy of photothermally and photochemically induced chemical transformations. The ultrafast-laser-induced excitation of the material, complemented with the formation of extended hot annealing regions in the presence of the metal catalyst, facilitates the formation of different nanostructures. Being fast, easy, and material-independent, our method offers new opportunities for the synthesis of IF nanostructures and INTs from different bulk metal chalcogenide compounds. On the basis of the capabilities of laser technology as well, this method could advantageously be further developed into a versatile tool for the simultaneous growth and patterning of such nanostructures in preselected positions for a variety of applications.

The dielectric and electrical characteristics of the semiconductive WS2 nanotubes/epoxy composites were studied as a function of the nanotubes concentration and the pressure applied during their molding. In addition, the ability of WS2 nanotubes to serve as stress sensors in epoxy based nanocomposites, for health-monitoring applications, was studied. The nanocomposite elements were loaded in three-point bending configuration. The direct current was monitored simultaneously with stress-strain measurements. It was found that, in nanocomposites, above the percolation concentrations of the nanotubes, the electrical conductivity increases considerably with the applied load and hence WS2 nanotubes can be potentially used as sensors for health monitoring of structural components.

New types of core-shell nanoparticles are reported: Pb@GaS fullerene-like and nanotubular structures, achieved via the continuously high reactor temperatures and ultra-hot strong-gradient annealing environments created by highly concentrated sunlight. Structural and chemical characterizations suggest a formation mechanism where vaporized Pb condenses into nanoparticles that are stabilized as they become covered by molten GaS, the ensuing crystallization of which creates the outer layers. Hollow-core GaS fullerene-like nanoparticles and nanotubes were also observed among the products, demonstrating that a single solar procedure can generate a variety of core-shell and hollow nanostructures. The proposed formation mechanisms can account for their relative abundance and the characterization data.

The simple process of a liquid wetting a solid surface is controlled by a plethora of factors-surface texture, liquid droplet size and shape, energetics of both liquid and solid surfaces, as well as their interface. Studying these events at the nanoscale provides insights into the molecular basis of wetting. Nanotube wetting studies are particularly challenging due to their unique shape and small size. Nonetheless, the success of nanotubes, particularly inorganic ones, as fillers in composite materials makes it essential to understand how common liquids wet them. Here, we present a comprehensive wetting study of individual tungsten disulfide nanotubes by water. We reveal the nature of interaction at the inert outer wall and show that remarkably high wetting forces are attained on small, open-ended nanotubes due to capillary aspiration into the hollow core. This study provides a theoretical and experimental paradigm for this intricate problem.

We present Raman spectra of misfit layer (PbS)(1.14)NbS2 nanotubes and lead intercalated NbS2 nanotubes. They represent interesting model systems to investigate the nature of interlayer interaction in layered materials. A direct correlation to the Raman modes of the parent 2H-NbS2 compound exists, but some modes are seen drastically upshifted in frequency in the misfit layer and intercalated compound while others remain almost unchanged. On the basis of the Raman spectroscopic investigations and with the help of supporting calculations we examine different interlayer bonding mechanisms and contribute to the discussion as to why these frequency shifts occur.

Polyvinyl alcohol (PVA) is a biocompatible, semi-crystalline and water soluble polymer with moderate tensile properties. To improve the thermal and mechanical properties of PVA, as well as to reduce water uptake, structural modification by glutaric acid (GA) and nanoparticle reinforcement by tungsten disulphide nanotubes (WSNTs) were used to prepare PVA based composites. We observed a significant drop in the water uptake of GA crosslinked PVA, an indication of the formation of a network. Fourier transform infrared spectroscopy was applied to confirm the presence of covalent bonds formed during the crosslinking. Crosslinked PVA and composites are found to have higher thermal stability and mechanical properties compared to their un-crosslinked counterparts. Tensile tests show that the presence of WSNTs increases the strength (up to 25%), modulus (up to 120%) and toughness (up to 80%) of the pristine as well as the crosslinked PVA. (C) 2016 Elsevier Ltd. All rights reserved.

The process engineering of Mg alloys is nevertheless compromised due to their relatively poor mechanical properties. To address some of these difficulties, metal-matrix composites (MMCs) of Mg-alloys have been investigated for sometime. In the present work, small amounts (up to 1 wt%) of WS2 nanotubes are mixed with Mg-alloy (AZ31) using melt-stirring process above 700 degrees C. The new MMC nano-composites exhibit much superior mechanical properties vis a vis the pristine alloy. Metallographic investigation demonstrates that the average grain size has been reduced in inverse proportion to the added amounts of nanotubes up to 1 wt%. Physical considerations suggest that the main mechanism responsible for the reinforcement effect lies in the mismatch between the thermal expansion coefficients of the metal and the nanotubes. This mismatch induced large density of dislocations in the grain boundaries in the vicinity of the nanotube-matrix interface, which obstruct the crack propagation. (C) 2015 Elsevier B.V. All rights reserved.

Palladium nanoparticles were deposited on multiwall WS2 nanotubes. The composite nanoparticles were characterized by a variety of techniques. The Pd nanoparticles were deposited uniformly on the surface of WS2 nanotubes. An epitaxial relationship between the (111) plane of Pd and the (013) plane of WS2 was mostly observed. The composite nanoparticles were found to perform efficiently as catalysts for cross-coupling (Heck and Suzuki) reactions. The role of the nanotubes' support in the catalytic process is briefly discussed.

Thermoset adhesives such as epoxy exhibit high stiffness, strength and adhesion properties, but relatively poor toughness, elongation and conductivity. Nanofillers such as carbon (CNTs) and tungsten disulfide (WS2) nanotubes (INTs) and fullerene-like (IF) nanoparticles (NPs) possess excellent mechanical properties and also high electrical (thermal) conductivity. The quality of dispersion of the nanofillers in the thermoset adhesives has a major effect on the final properties of the nanocomposite adhesive. Dispersion techniques such as solution mixing, ultrasonication, three-roll milling, etc. have been used. Significant enhancements in stiffness, tensile strength, shear and peel strengths, fracture toughness, glass transition temperature, CLTE (coefficient of linear thermal expansion) among others, can be achieved at low CNTs concentrations (less than 1 wt.%), while at higher content of the CNTs, agglomeration reduces the toughness. Also for the WS2 nanoparticles, the lower concentrations (up to 1 wt.%) are preferable in order to improve the mechanical performance of the epoxy-based composites. Enhanced dispersion and interfacial interactions, resulting in improved mechanical performance of the nanocomposites containing CNTs, could be achieved by covalent and noncovalent modifications. The dispersion of WS2 nanostructures in the polymer medium was obtained using simple techniques like shear mixing and sonication without surface modifications. Moreover, it was found that the IF-NPs could covalently interact with the epoxy, without any surface modifications. The toughening mechanism of polymer matrices by the tubular nanofillers is mainly attributed to pull-out and crack bridging of the nanotubes. At the same time, inorganic nanocomposite adhesives exhibit promising results. IF structures of WS2 were found to exhibit crack deflection and bowing mechanisms. Electrically and thermally conductive adhesives have been obtained by adding CNTs, whereas conductive nanocomp

Inorganic, fullerene-like (IF) nanoparticles of tungsten disulfide (WS2) and molybdenum disulfide (MoS2) have been synthesised in the past and their useful tribological properties have been studied quite extensively. Rhenium doping of such nanoparticles was also reported and studied to some extent. Herein, further studies of the physico-chemical properties are reported in connection to lubrication under mild loads. Due to their self-repelling character, the doped nanoparticles form tessellated monolayers on certain substrates. When mixed in small amounts, the IF nanoparticles lead to a reduction in the viscosity of water-based gels. Furthermore, the added nanoparticles lead to a precipitous reduction in traction force (friction) under mild loads. The lubrication mechanism of such nanoparticles is briefly discussed.

Impaired salivary gland (SG) function leading to oral diseases is relatively common with no adequate solution. Previously, tissue engineering of SG had been proposed to overcome this morbidity, however, not yet clinically available. Multiwall inorganic (tungsten disulfide [WS2]) nanotubes (INT-WS2) and fullerene-like nanoparticles (IF-WS2) have many potential medical applications. A yet unexplored venue application is their interaction with SG, and therefore, our aim was to test the biocompatibility of INT/IF-WS2 with the A5 and rat submandibular cells (RSC) SG cells. The cells were cultured and subjected after 1 day to different concentrations of INT-WS2 and were compared to control groups. Growth curves, trypan blue viability test, and carboxyfluorescein succinimidyl ester (CFSE) proliferation assay were obtained. Furthermore, cells morphology and interaction with the nanoparticles were observed by light microscopy, scanning electron microscopy and transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy. The results showed no significant differences in growth curves, proliferation kinetics, and viability between the groups compared. Moreover, no alterations were observed in the cell morphology. Interestingly, TEM images indicated that the nanoparticles are uptaken by the cells and accumulate in cytoplasmic vesicles. These results suggest promising future medical applications for these nanoparticles.

The synthesis of nanotubes from layered compounds has generated substantial scientific interest. "Misfit" layered compounds (MLCs) of the general formula [(MX)(1+x)](m)[TX2](n), where M can include Pb, Sb, rare earths; T = Cr, Nb, and X = S, Se can form layered structures, even though each sub-system alone is not necessarily a layered or a stable compound. A simple chemical method is used to synthesize these complex nanotubes from lanthanide-based misfit compounds. Quaternary nanotubular structures formed by partial substitution of the lanthanide atom in nanotubes by other elements are also confirmed. The driving force and mechanism of formation of these nanotubes is investigated by systematic temperature and time-dependent studies. A stress-inducement mechanism is proposed to explain the formation of the nanotubes. The resulting materials may find applications in fields that include thermoelectrics, light emitters, and catalysis and address fundamental physical issues in low dimensions.

This minireview outlines the main scientific directions in the research of inorganic nanotubes (INT) and fullerene-like (IF) nanoparticles from layered compounds, in recent years. In particular, this review describes to some detail the progress in the synthesis of new nanotubes, including those from misfit compounds; core-shell and the successful efforts to scale-up the synthesis of WS2 multiwall nanotubes. The high-temperature catalytic growth of nanotubes, via solar ablation is discussed as well. Furthermore, the doping of the IF-MoS2 nanoparticles and its influence on the physiochemical properties of the nanoparticles, including their interesting tribological properties are briefly discussed. Finally, the numerous applications of these nanoparticles as superior solid lubricants and for reinforcing variety of polymers are discussed in brief.

A new two-step procedure for the synthesis of MoS2 nanotubes using lead as a growth promoter is reported. In the first step, molybdenum suboxide nanowhiskers containing a small amount of lead atoms were created by exposing a pressed MoS2+Pb mixture to highly compressed shock-heated argon gas, with estimated temperatures exceeding 9900 K. In the second step, these molybdenum suboxide nanowhiskers served as templates for the sulfidization of the oxide into MoS2 nanotubes (by using H2S gas in a reducing atmosphere at 820 degrees C). Unlike the case of WS2 nanotubes, the synthesis of a pure phase of MoS2 nanotubes from molybdenum oxide has proven challenging, due mostly to the volatile nature of the latter at the high requisite reaction temperatures (>800 degrees C). In contrast, the nature and apparent reaction mechanism of the method reported herein are amenable to future scale-up. The high-temperature shockwave system should also facilitate the synthesis of new nanostructures from other layered materials.

Carbon fullerenes and nanotubes revolutionized understanding of the reactivity of nanoscale compounds. Subsequently, our group and others discovered analogous inorganic compounds with hollow, dosed nanostructures. Such inorganic nanostructures offer many applications, particularly in the energy and electronics industries. One way to create inorganic nanostructures is via misfit layered compounds (MLC), which are stacks of alternating two-dimensional molecular slabs, typically held together via weak van der Waals forces. They contain "misfits" in their a-b plane structures that can make them unstable, leading to collapse of the slabs into tubular nanostructures. For example, metal chalcogenide MLCs of the general formula (MX)(1+y)/TX2 (M = Sn, Pb, Bi, Sb, and other rare earths; T= Sn, Ti, V, Cr, Nb,Ta, etc; X = S or Se) consist of a superstructure of alternating layers where the MX unit belongs to a (distorted NaCl) orthorhombic symmetry group (O), the TX2 layer possesses trigonal (T) or octahedral symmetry, and the two layers are held together via both van der Waals and polar forces. A misfit in the a axis or both a and b axes of the two sublattices may lead to the formation of nanostructures as the lattices relax via scrolling. Previous research has also shown that the abundance of atoms with dangling bonds in the rims makes nanoparticles of compounds with layered structure unstable in the planar form, and they tend to fold into hollow closed structures such as nanotubes. This Account shows that combining these two triggers, misfits and dangling bond annihilation in the slab rims, leads to new kinds of nanotubes from MLCs. In particular, we report the structure of two new types of nanotubes from misfits, namely, the SnS/SnS2 and PbS/NbS2 series. To decipher the complex structures of these nanotubes, we use a range of methods: high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), selected area electron diffraction (

Theoretical analysis of experimental data on unzipping multilayered WS2 nanotubes by consequent intercalation of lithium atoms and 1-octanethiol molecules [C. Nethravathi, et al., ACS Nano, 2013, 7, 7311] is presented. The radial expansion of the tube was described using continuum thin-walled cylinder approximation with parameters evaluated from ab initio calculations. Assuming that the attractive driving force of the 1-octanethiol molecule is its reaction with the intercalated Li ions ab initio calculations of a 1-octanethiol molecule bonding with Li+ were carried out. In addition, the non-chemical interactions of the 1-octanethiol dipole with an array of positive point charges representing Li+ were taken into account. Comparing between the energy gain from these interactions and the elastic strain energy of the nanotube allows us to evaluate a value for the tube wall deformation after the implantation of 1-octanethiol molecules. The ab initio molecular dynamics simulation confirmed our estimates and demonstrated that a strained WS2 nanotube, with a decent concentration of 1-octanethiol molecules, should indeed be unzipped into the WS2 nanoribbon.

Next-generation catalysts for water splitting are crucial towards a renewable hydrogen economy. MoS2 and WS2 represent earth-abundant, noble metal cathode alternatives with high catalytic activity at edge sites. One challenge in their development is to nanostructure these materials in order to achieve increased performance through the creation of additional edge sites. In this work, we demonstrate a simple route to form nanostructured-WS2 using sonochemical exfoliation to break interlayer and intralayer bonds in WS2 nanotubes. The resulting few-layer nanoflakes are similar to 100 nm wide with a high density of edge sites. WS2 nanoflakes are utilized as cathodes for the hydrogen evolution reaction (HER) and exhibit superior performance to WS2 nanotubes and bulk particles, with a lower onset potential, shallower Tafel slope and increased current density. Future work may employ ultra-small nanoflakes, dopant atoms, or graphene hybrids to further improve electrocatalytic activity.

Al2O3/IF-WS2 nanocomposite coating was prepared in a two-step process. The Al2O3 nano-fibers are the matrix of the composite and the nanoparticles of WS2 with fullerene-like structure (IF-WS2) are the dispersed phase material, which can improve the mechanical and tribological properties of the nanocomposite. Detailed structural analysis of the nanocomposites was performed using scanning electron microscopy (SEM) and X-ray diffraction. These analyses show that the IF nanoparticles are entrapped in the microporous surface-film of the alumina matrix. The structure and mechanical properties of the nanosized Al2O3/IF-WS2 nanocomposite film was analyzed. This analysis demonstrates that the hardness and composition of the film exhibits graded functional material characteristics, suggesting possible tribological applications. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Polymer-inorganic nanoparticle composites are an area of great research interest. Inorganic fullerene-like (IF) structures and inorganic nanotubes (INT) are used for producing composite materials with specific goals of reinforcement of the polymer and ameliorating its thermal stability. Here, a composite material containing INT and conducting polymer (polyaniline (PANI)) is synthesized by an in situ oxidative polymerization of the monomer in the presence of WS2 nanotubes. The structure and electrical behavior of PANI/INT composite is studied and compared with the results of pristine PANI (synthesized under the same conditions without INT). Most remarkably, INT-WS2 are found to play the role of an active doping agent. The highest conductivity is obtained for 0.85 wt% (ca. 0.06 at%) nanotubes content, two orders of magnitude higher than that of pristine PANI. This work suggests a new approach to control the host-guest interaction in the polymer nanocomposite via (Lewis) acid-base equilibrium.

We report a reasonably high yield (similar to 50%) synthesis of silicon carbide (SiC) nanowires from silicon oxides and carbon in vacuum, by novel solar and lamp photothermal ablation methods that obviate the need for catalysis, and allow relatively short reaction times (similar to 10 min) in a nominally one-step process that does not involve toxic reagents. The one-dimensional core/shell beta-SiC/SiOx nanostructures-characterized by SEM, TEM, HRTEM, SAED, XRD and EDS-are typically several microns long, with core and outer diameters of about 10 and 30 nm, respectively. HRTEM revealed additional distinctive nanoscale structures that also shed light on the formation pathways.

This study describes a new method for fabrication of thin composite films using physical vapor deposition (PVD). Titanium (Ti) and hybrid films of titanium containing tungsten disulphide nanoparticles with inorganic fullerene-like structure (Ti/IF-WS2) were fabricated with a modified PVD machine. The evaporation process includes the pulsed deposition of IF-WS2 by a sprayer head. This process results in IF-WS2 nanoparticles embedded in a Ti matrix. The layers were characterized by various techniques, which confirm the composition and structure of the hybrid film. The Ti/IF-WS2 shows better wear resistance and a lower friction coefficient when compared to the Ti layer or Ti substrate. The Ti/IF films show very good antireflective properties in the visible and near-IR region. Such films may find numerous applications, for example, in the aerospace and medical technology.

We investigated the optical properties of rhenium-doped MoS2 nanoparticles and compared our findings with the pristine and bulk analogues. Our measurements reveal that confinement softens the exciton positions and reduces spin-orbit coupling, whereas doping has the opposite effect. We model the carrier-induced exciton blue shift in terms of the Burstein-Moss effect. These findings are important for understanding doping and finite length scale effects in low-dimensional nanoscale materials.

Insertion of endoscopes and other medical devices into the human body are ubiquitous, especially among aged males. The applied force for the insertion/extraction of the device from the urethra must overcome endoscope-surface-human-tissue interactions. In daily practice a gel is applied on the endoscope surface, in order to facilitate its entry into the urethra, providing also for local anesthesia. In the present work, a new solid-state lubricant has been added to the gel, in order to reduce the metal-urethra interaction and alleviate the potential damage to the epithelial tissue. For that purpose, a urethra model was designed and fabricated, which allowed a quantitative assessment of the applied force for extraction of the endoscope from a soft polymer-based ring. It is shown that the addition of MoS2 nanoparticles with fullerene-like structure (IF-MoS2) and in particular rhenium-doped nanoparticles (Re:IF-MoS2) to Esracain gel applied on the metal-lead reduced the friction substantially. The Re:IF-MoS2 showed better results than the undoped fullerene-like nanoparticles and both performed better than the gel alone. The mechanism of friction reduction is attributed to fullerenes' ability to roll and act as a separator between the active parts of the model.

The synthesis of rhenium-doped fullerene-like MoS2 nanoparticles (Re:IF-MoS2) is described. Careful inductively coupled plasma mass spectrometry analysis reveals that the concentration of the rhenium atoms in the IF-MoS2 lattice is more than 10-fold smaller than the weighted amount of this atom in the precursor powder. High resolution scanning transmission electron microscopy in high angle annular dark field mode is used to decipher the isolated rhenium atoms in the MoS2 lattice and confirm that these atoms substitute for molybdenum atoms. Scanning electron microscopy analysis shows that, in contrast to the undoped nanoparticles which tend to agglomerate, the Re:IF-MoS2 nanoparticles self-assemble on substrate surfaces forming an ordered tessellated monolayer. Theoretical quantum-chemical calculations show that the Re level is some 180 meV below the conduction band. Furthermore, the rhenium electrons are fairly localized in the MoS2 lattice. At a higher concentration the rhenium atoms form a mini-band below the conduction band which coincides with the Fermi level of the lattice.

The electrical properties of WS2 nanotubes (NTs) were studied through measuring 59 devices. Important electrical parameters, such as the carrier concentration, mobility, and effective barrier height at the contacts, were obtained through fitting experimental non-linear I-V curves using a metal-semiconductor-metal model. The carrier mobility was found to be several orders of magnitude higher than that have been reported previously for WS2 NTs. Water absorption was found to decrease the conductivity and carrier mobility of the NTs, and could be removed when the sample was dried. Oxygen absorption also slightly decreased the conductivity of WS2 NTs. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4752440]

WS2 nanostructures hold structural characteristics which suggest they will be suitable for heterogeneous catalysis in the hydrodesulfurization (HDS) process. In this work, WS2 nanotubes (INT-WS2) were coated with cobalt nanoparticles using electroless plating method. Prior to cobalt deposition, the nanotubes surface was activated using palladium seeding process. The deposited cobalt nanoparticles had hcp crystal structure and formed non-uniform layer on the nanotubes surface. The catalytic reactivity of the produced cobalt coated nanotubes toward thiophene decomposition was characterized by an atmospheric flow reactor. The coated nanotubes revealed good catalytic reactivity toward thiophene mineralization. Further, the adsorption kinetics of thiophene on coated INT-WS2 was studied by thermal desorption spectroscopy (TDS). The cobalt coated system was found to be more catalytically active than the pristine INT-WS2 system. This result is promising since further optimization of the nanofabrication process of the catalyst should increase the conversion rates even further. (C) 2012 Published by Elsevier Ltd.

A synthetic route for preparation of inorganic WS2 nanotube (INT)-colloidal semiconductor quantum dot (QD) hybrid structures is developed, and transient carrier dynamics on these hybrids are studied via transient photoluminescence spectroscopy utilizing several different types of QDs. Measurements reveal efficient resonant energy transfer from the QDs to the INT upon photoexcitation, provided that the QD emission is at a higher energy than the INT direct gap. Charge transfer in the hybrid system, characterized using QDs with band gaps below the INT direct gap, is found to be absent. This is attributed to the presence of an organic barrier layer due to the relatively long-chain organic ligands of the QDs under study. This system, analogous to carbon nanotube-QD hybrids, holds potential for a variety of applications, including photovoltaics, luminescence tagging and optoelectronics.

Metallic films impregnated with fullerene-like-WS2 (MoS2) nanoparticles were fabricated on stainless steel and Ti-Ni substrates using galvanic and electroless deposition. The coatings were obtained from aqueous suspensions containing the metallic salts as well as the dispersed nanoparticles. Tribological tests showed that the films have low friction and wear. Such coatings could be useful for numerous civilian and defense-related applications.

Inorganic nanoparticles of layered [two-dimensional (2D)] compounds with hollow polyhedral structure, known as fullerene-like nanoparticles (IF), were found to have excellent lubricating properties. This behavior can be explained by superposition of three main mechanisms: rolling, sliding, and exfoliation-material transfer (third body). In order to elucidate the tribological mechanism of individual nanoparticles in different regimes, in situ axial nanocompression and shearing forces were applied to individual nanoparticles using a high resolution scanning electron microscope. Gold nanoparticles deposited onto the IF nanoparticles surface served as markers, delineating the motion of individual IF nanoparticle. It can be concluded from these experiments that rolling is an important lubrication mechanism for IF-WS(2) in the relatively low range of normal stress (0.96 +/- 0.38 GPa). Sliding is shown to be relevant under slightly higher normal stress, where the spacing between the two mating surfaces does not permit free rolling of the nanoparticles. Exfoliation of the IF nanoparticles becomes the dominant mechanism at the high end of normal stress; above 1.2 GPa and (slow) shear; i.e., boundary lubrication conditions. It is argued that the modus operandi of the nanoparticles depends on their degree of crystallinity (defects); sizes; shape, and their mechanical characteristics. This study suggests that the rolling mechanism, which leads to low friction and wear, could be attained by improving the sphericity of the IF nanoparticle, the dispersion (deagglomeration) of the nanoparticles, and the smoothness of the mating surfaces.

In this work chromium-rich coatings impregnated with fullerene-like (IF)-WS2 nanoparticles were deposited on stainless steel substrates. The coatings were obtained from a trivalent chromium bath at pH 2 by galvanostatic electrodeposition. Zinc and cobalt salts were added to the aqueous solution in small amounts serving as cationic growth promoters. Photodeposition of tin-palladium nanoparticles was used as seeding enhancer for the co-deposition of the fullerene-like nanoparticles. The coatings were characterized by a number of techniques and were found to show a decreasing gradient of the IF nanoparticles towards the film-substrate interface. Tribological tests showed that in contrast to the substrate and the pure metal coating, the IF-containing films exhibit low friction and wear.

We report on the synthesis of inorganic fullerene-like molybdenum disulfide (MoS(2)) nanoparticles by pulsed laser ablation (PLA) in water. The final products were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and resonance Raman spectroscopy, etc. Cell viability studies show that the as-prepared MoS2 nanoparticles have good solubility and biocompatibility, which may show a great potential in various biomedical applications. It is shown that the technique of PLA in water also provides a green and convenient method to synthesize novel nanomaterials, especially for biocompatible nanomaterials.

The growth mechanism of WS2 nanotubes is briefly discussed. Two distinct growth mechanisms can be delineated, leading to somewhat different products: 1) thick (50-150 nm) and very long (20-50 microns and above) nanotubes consisting of many (> 20) layers, and 2) slender (20-25 nm) nanotubes with 5-10 layers. The synthesis of large amounts of pure WS2 nanotubes belonging to the first category in the large-scale fluidized-bed reactor is described. Characterization of the nanotubes, which grow catalyst-free by a number of analytical techniques, is reported. The nanotubes reveal highly crystalline order, suggesting very good mechanical behavior and numerous applications, especially in the field of nanocomposites.

A track record of generating novel fullerene like and nanotubular inorganic nanostructures from Cs2O, SiO2x WS2 and MoS2 by photothermal ablation with highly concentrated sunlight and ultra bright lamp light is re viewed, and augmented with new results for exfoliated MoS2 and carbon as well as carbon nanotubes

Various inorganic salts can be encapsulated inside the comparatively narrow (0.8-2 nm) hollow core of single-walled carbon nanotubes (SWNTs) by molten phase capillary wetting. A new synthetic strategy is presented allowing the formation of one dimensional (1D) inorganic crystals or core-shell nanotubular structures by using multiwall WS2 nanotubes as host templates. Molten phase wetting with CsI results in the formation of 1D crystal structures inside WS2 nanocapillaries with a Moire pattern. The relatively large diameter of the WS2 nanotube (with inner and outer diameters of ca. 10 and 20 nm, respectively), allows a conformal folding of the guest PbI2 layers (PbI2@WS2 core-shell nanotubes) on the interior wall of the WS2 nanotube-template, thus leading to relatively defect-free core-shell inorganic nanotubular structures, which have not been previously observed within carbon nanotubes (CNTs). Core-shell WS2@MoS2 nanotubes can be obtained by the gas-phase reaction of MoCl5 with sulfur in the presence of WS2 nanotubes. The mechanism of imbibition/solidification of the molten salt into the hollow cores of MoS2 nanotubes has been studied by molecular dynamics simulations, showing major differences between layered compounds and those with quasi-isotropic structure. Theoretical considerations also show the conditions for the stability of such core-shell 1D nanostructures. These new strategies can open up many possibilities for the synthesis of new nanotubular structures.

Nanoparticles of materials with layered structure are able to spontaneously form closed-cage nanostructures such as nested fullerene-like nanoparticles and nanotubes. This propensity has been demonstrated in a large number of compounds such as WS2, NiCl2, and others. Layered metal oxides possess a higher ionic character and consequently are stiffer and cannot be evenly folded. Vanadium pentoxide (V2O5), a layered metal oxide, has received much attention due to its attractive qualities in numerous applications such as catalysis and electronic and optical devices and as an electrode material for lithium rechargeable batteries. The synthesis by pulsed laser ablation (PLA) of V2O5 hollow nanoparticles, which are closely (nearly) associated with inorganic "fullerene-like" (NIF-V2O5) nanoparticles, but not quite as perfect, is reported in the present work. The relation between the PLA conditions and the NIF-V2O5 morphology is elucidated. A new mechanism leading to hollow nanostructure via crystallization of lower density amorphous nanoparticles is proposed. Transmission electron microscopy (TEM) is used extensively in conjunction with structural modeling of the NIF-V2O5 in order to study the complex 3-D structure of the NIF-V2O5 nanoparticles. This structure was shown to be composed of facets with their low-energy surfaces pointing outward and seamed by defective domains. These understandings are used to formulate a formation mechanism and may improve the function of V2O5 in its many uses through additional morphological control. Furthermore, this study outlines which properties are required from layered compounds to fold into perfectly closed-cage IF nanoparticles.

New materials and techniques pertaining to the synthesis of inorganic nanotubes have been ever increasing since the initiation of the field in 1992. Recently, WS(2) nanotubes, which are produced now in large amounts, were filled with molten lead iodide salt by a capillary wetting process, resulting in PbI(2)@WS(2) core-shell nanotubes. This work features progress in the synthesis of new core-shell nanotubes, including BiI(3)@WS(2) nanotubes produced in a similar same manner. In addition, two new techniques for obtaining core shell nanotubes are presented. The first is via electron-beam irradiation, i.e., in situ synthesis within a transmission electron microscope. This synthesis results in SbI(3) nanotubes, observed either in a hollow core of WS(2) ones (SbI(3)@WS(2) nanotubes), or atop of them (WS(2)@SbI(3) nanotubes). The second technique involves a gaseous phase reaction, where the layered product employs WS(2) nanotubes as nucleation sites. In this case, the MoS(2) layers most often cover the WS(2) nanotube, resulting in WS(2)@MoS(2) core shell nanotubes. Notably, superstructures of the form MoS(2)@WS(2)@MoS(2) are occasionally obtained. Using a semi-empirical model, it is shown that the PbI(2) nanotubes become stable within the core of MoS(2) nanotubes only above a critical core diameter of the host (>12 nm); below this diameter the PbI(2) crystallizes as nanowires. These model calculations are in agreement with the current experimental observations, providing further support to the growth mechanism of such core-shell nanotubes.

WS2 nanotubes have been filled and intercalated by molten phase caesium iodide. The presence of caesium iodide inside the WS2 nanotubes has been determined using high-resolution transmission electron microscopy (HRTEM) coupled with electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDS). Noticeably, a Moire pattern was observed due to the interference between encapsulated CsI and WS2 layers. The intercalation of CsI into the host concentric WS2 lattices resulted in an increase in the interplanar spacing.

WS2 inorganic nanotubes (INT) and inorganic fullerene-like nanoparticles (IF) are well-known for their high mechanical strength and as superior solid lubricants. The outermost WS2 layer is considered to be fully bonded; thus, it was suggested that the interactions of these WS2 nanostructures with their surroundings are governed by purely van der Waals (vdW) interactions. However, in the case of IF-WS2 nanoparticles the faceted surface may contain sites with nonsaturated coordination, which in turn, react with the surrounding media. Gold nanoparticles (GNP) were used as probes for the IF-WS2 surface defects mapped by both scanning and transmission electron microscopy. The interaction between the GNP and the reactive surface was investigated using INT-WS2 as model and was characterized by atomic force microscopy (AFM).

Using a new quartz-made reactor, large amounts of fullerene-like (IF) MoS2 nanoparticles were synthesized by reacting MoO3 vapor with H2S in a reducing atmosphere. The nanoparticles were found to be of high crystalline order; with an average size of 70 nm and consist of more than 30 closed shells. Extensive tribological testing of the nanoparticles in two types of synthetic oils- poly-alpha olefins (PAO)- was carried out and compared to that of bulk (2H platelets) MoS2 and IF-WS2. These tests indicated that under high pressure and relatively low humidity, the IF-MoS2 exhibited a friction coefficient as low as 0.03 and the smallest wear rate of the measured systems. However, its performance was found to be lower in comparison to IF-WS2 after 2500 cycles, due probably to its inferior chemical stability. This study indicates that the tribological performance of the IF nanoparticles depends strongly on their crystalline order and size.

The characterization of nanostructures to the atomic dimensions becomes more important, as devices based on a single particle are being produced. In particular, inorganic nanotubes were shown to host interesting properties making them excellent candidates for various devices. The WS2 nanotubes outperform the bulk in their mechanical properties offering numerous applications especially as part of high strength nanocomposites. In contrast, their electrical properties are less remarkable. The structure-function relationship can be investigated by aberration-corrected high-resolution transmission electron microscopy (HRTEM), which enables the insight into their atomic structure as well as performing spectroscopic measurements down to the atomic scale. In the present work, the deciphering of atomic structure and the chiral angle of the different shells in a multiwall WS2 nanotube is demonstrated. In certain cases, the helicity of the structure can also be deduced. Finally, first electron energy loss spectra (EELS) of a single tube are presented, acquired by a new acquisition technique that allows for high spatial resolution (denoted StripeSTEM). The measured band gap values correspond with the values found in literature for thin films, obtained by spectroscopic techniques, and are higher than the values resulting from STM measurements.

The growth mechanism of WS2 nanotubes in the large-scale fluidized-bed reactor is studied in greater detail. This study and careful parameterization of the conditions within the reactor lead to the synthesis of large amounts (50-100 g/batch) of pure nanotubes, which appear as a fluffy powder, and (400-500 g/batch) of nanotubes/nanoplatelets mixture (50:50), where nanotubes usually coming in bundles. The two products are obtained simultaneously in the same reaction but are collected in different zones of the reactor, in a reproducible fashion. The characterization of the nanotubes, which grow catalyst-free, by a number of analytical techniques is reported. The majority of the nanotubes range from 10 to 50 micron in length and 20-180 nm in diameter. The nanotubes reveal highly crystalline order, suggesting very good mechanical behavior with numerous applications.

In this article a comparison between inorganic nanoparticles with hollow closed structure and the carbon fullerenes and nanotubes is undertaken. First, the structural evolution of inorganic fullerene-like (IF) nanoparticles of MoS(2) as a function of their size is examined in some detail and compared to that of carbon and BN fullerenes. It is shown that hollow closed structures of MoS(2) are stable above 3 nm (app 10(3) atoms). In the range of 3-8 nm (10(3)-10(5)) nanooctahedra with metallic character are the most stable form of MoS(2) Semiconducting nanotubes and quasispherical IF nanoparticles become the stable-most form beyond that size and the bulk (platelets) are stable above about 0.2 mu m. The stability of inorganic nanotubes is also discussed. The scaling-up of the synthesis of IF-WS(2) and the very recent successful synthesis of large, amounts of pure WS(2) nanotubes are briefly described. The stability of IF and INT of MoS(2) (WS(2)) under pressure and that of carbon is also discussed. Applications of the IF-WS(2) as superior solid lubricants, which lead to their recent commercialization, is demonstrated.

The synthesis of WS2 inorganic nanotubes (INT) and inorganic fullerene-like (IF) structures in 1992 signified the opening of a fertile and challenging field of scientific endeavor. These structures were the first of a long and ever-expanding series of INT and IF structures. Although initially much of the effort concentrated on the synthesis of INT and IF from compounds with layered structures, recently there his been a surge of efforts to synthesize crystalline and polycrystalline nanotubular structures from compounds with quasi-isotropic structures, like spinels, BaTiO3, SiO2, TiO2, and many others. The present review summarizes some of the progress in this field in recent years. Much of the progress in this field was achieved through strong interaction between theoretical and experimental work. This article has four themes: (a) new synthetic approaches leading to new kinds of Wand INT; (b) study of the molecular structure Of Such nanoparticles with new tools, such as aberration-corrected transmission electron microscopy (TEM) and high-angle annular dark field (HAADF); (c) recent progress in the investigation of the properties of such nanostructures; and (d) examples of applications for which clear progress has been accomplished, in particular in solid lubrication and high-strength nanocomposites.

A new type of composite metal-nanoparticle coating that significantly reduces the friction force of various surfaces, particularly archwires in orthodontic applications, is demonstrated. The coating is based on electrodeposited Ni film impregnated with inorganic fullerene-like nanospheres of tungsten disulphide. The first encouraging tests have shown reduction of up to 60% of the friction force between coated rectangular archwires and self-ligating brackets in comparison with uncoated archwires. The coating not only significantly reduces friction of commercial archwires but also maintains this low value of friction for the duration of the tests in comparison to archwires coated with nickel film without the nanoparticles. The coated surfaces of the wires were examined by scanning electron microscopy equipped with energy dispersive analyzer and by x-ray powder diffraction methods before and after the friction tests. Using these analyses, it was possible to qualitatively estimate the state of the Ni+IF-WS(2) coating before and after the friction test compared to Ni coated wires without IF-WS(2).

The adsorption kinetics of thiophene on WS(2) nanoparticles with fullerene-like (onion-like) structure has been studied at ultra-high vacuum conditions by sample temperature ramping techniques. At low temperatures, thiophene adsorbs molecularly. The formation of H(2)S and alkanes is evident at greater temperatures on fully sulfided as well as reduced and oxidized WS(2) nanoparticles.

The recent progress in intercalation of nanotubes and fullerene-like nanoparticles with various metals was reviewed. Nanotubes from layered materials and also those from non layered compounds were discussed. While nanotubes from layered compounds are generically crystalline those of non layered compounds have in most cases a polycrystalline structure. Different intercalation routes were presented and their relative merits and pitfalls were discussed. Furthermore, the changes in the structural and physical properties of the nanoparticles which accompany the intercalation reaction were described. Special attention was paid to the implementation of the nanotubes as a potential electrode material in lithium based batteries.

The time dependent chemical changes occuring at the surface of inorganic fullerene-like (IF) nanoparticles of WS2 were investigated using X-ray photoelectron spectroscopy (XPS) and compared to those of bulk powder, 2H-WS2. It was possible to follow the long term surface oxidation and carbonization occuring at defects on the outermost surface (0001) molecular layers of the inorganic fullerene-like nanoparticles. Vacuum annealing was shown to remove most of these contaminants and bring the surface close to its pristine stoichoimetric composition. In accordance with previous measurements, further evidence was obtained for the existence of water molecules, which were entrapped in the hollow core and interstitial defects of the fullerene-like nanoparticles during the synthesis. These water molecules were also shown to be removable by the vacuum annealing process. Chemically resolved electrical measurements (CREM) in the XPS showed that the IF samples had become less p-type after the vacuum annealing. Finally, tribological measurements showed that the vacuum annealed IF samples performed better as an oil additive than the non-annealed IF samples and the 2H-WS2 powder. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

WS2 inorganic fullerene-like (IF) nanoparticles were subjected to intercalation with potassium, sodium, and rubidium atoms in heated sealed ampules. The product of the intercalation process was not pure and was composed of both intercalated and nonintercalated phases. X-ray diffraction measurements under inert conditions of the intercalated powders showed that the interlayer expansion was correlated with the alkali metal radius. Small increase of the a-axis was observed as well and was explained on the grounds of the WS2 band structure. The XPS analysis of the rubidium intercalated material showed a rise in the Fermi energy as a result of the intercalation, endowing the originally p-type nanoparticles an n-type character.

Inorganic fullerene-like (IF) nanoparticles and nanotubes of WS(2) were synthesized by a gas phase reaction starting from WCl(n) (n = 4, 5, 6) and H(2)S. The effect of the various metal chloride precursors on the formation of the products was investigated during the course of the study. Various parameters have been studied to understand the growth and formation of the IF-WS(2) nanoparticles and nanotubes. The parameters that have been studied include flow rates of the various carrier gases, heating of the precursor metal chlorides and the temperature at which the reactions were carried out. The best set of conditions wherein maximum yields of the high quality pure-phase IF-WS(2) nanoparticles and nanotubes are obtained have been identified. A detailed growth mechanism has been outlined to understand the course of formation of the various products of WS(2).

Solar ablation creates the sharp radiative and temperature gradients, as well as the high-temperature annealing environment, that favor nanomaterial syntheses. Using highly concentrated sunlight, we generated fullerene-like MoS2, ranging from single-walled nanotubes and closed-cage structures to their larger multi-walled counterparts. TEM, HRTEM and EDS unambiguously established the nanostructures, some achieving fundamentally minimum sizes predicted by molecular structural theory. Irradiation of MoS2 and powdered mixtures of MoS2 + SiO2 in evacuated quartz ampoules also generated nanofibers and nanospheres of amorphous SiO2: the first production of SiO2 nanostructures purely from quartz. Also, solar ablation of MoS2 + SiO mixtures produced nanowires and nanospheres of crystalline Si.

Back in 1992 it was proposed that nanoparticles of layered compounds will be unstable against folding and will close up into fullerene-like structures (IF) and nanotubes. In the years that followed nanotubes and fullerene-like structures were synthesized from numerous compounds with layered structure. More recently, crystalline and noncrystalline nanotubes of compounds with a 3D, i.e., quasi-isotropic lattice have been intensively investigated. In view of their eminent applications potential, much effort and substantial progress has been achieved in the scaling-up of the synthesis of inorganic nanotubes and fullerene-like nanoparticles of WS(2) and MoS(2) and also other compounds. Early on it was suggested that hollow nano-octahedra consisting of a few hundred MoS(2) moieties make the true analogs of C(60), etc. This notion has been advanced considerably in recent years through a combined experimental-theoretical effort. Substantial progress has been accomplished in the use of such nanoparticles for tribological applications and lately for impact resilient nanocomposites. These tests indicated that IF-MoS(2) and IF-WS(2) are heading for large-scale applications in the automotive, machining, aerospace, electronics, defense, medical and numerous other kinds of industries. A few products based on these nanoparticles have been recently commercialized by "ApNano Materials, Inc" ("NanoMaterials, Ltd.", see also www.apnano.com). Most recently, a manufacturing facility for the commercialization of these nanomaterials has been erected and sales of the product started. Novel applications of inorganic nanotubes and fullerene-like nanoparticles in the fields of catalysis; microelectronics; Li rechargeable batteries; medical and optoelectronics will be discussed.

Fullerene-like WS2 (MoS2) nanoparticles (IF) have been studied in the past as solid lubricants. Using the tribological ball-on-flat experiments, it was shown that the size of the aggregates and their distribution determine the penetration and entrapping of the IF nanoparticles at the interface. It is expected that the wedge clearance at the inlet of the contact, i.e., the oblique-angle entrance to the contact zone between the two mating tribological surfaces, as well as the average surface roughness, can limit the supply of the lubricant into the interface in, e.g., the block-on-ring experiment. In the present series of experiments, the Stribeck curve was designed first using a linear loading scheme and pure oil. It was concluded that a wedge clearance (oblique-angle) in the inlet of the contact zone leads to entrapment of the IF nanoparticles and their compaction, which hamper the supply of the fluid lubricant into the interface. A ball-on-flat and flat block-on-ring friction devices with wedge clearance in the inlet of the contact can distort the efficacy of IF. Procedures for improving the supply of the IF nanoparticles to the contact zone and improving thereby their efficacy are considered.

It is already well established today that numerous materials form closed-cage structures, of which carbon fullerenes and nanotubes are a special case [1]. Inorganic fullerene-like nanoparticles (designated IF) and inorganic nanotubes (INT) have been produced by different routes and experimental techniques, achieving persistent growth of a variety of materials and structural wealth within them. The research in this area has focused on synthesizing new IF and INT materials and understanding their different properties as well as scaling up the synthetic process in order to make it suitable for industrial applications. In this review, the main synthetic procedures to obtain inorganic fullerene-like nanoparticles and nanotubes will be discussed alongside with the different mechanisms that affect the morphology of the final product. The main differences between the morphologies will be presented. Some general considerations relating the properties of the parent compound with the morphology of the product will be mentioned.

To investigate phonon confinement in nanoscale metal dichalcogenides, we measured the low-temperature specific heat of layered and nanoparticle WS2. Below 9 K, the specific heat of the nanoparticles deviates from that of the bulk counterpart. Further, it deviates from the usual T (3) dependence below 4 K due to finite size effects that eliminate long wavelength acoustic phonons and interparticle-motion entropy. This separation of nanoscale effects from T-3 dependence can be modeled by assuming that the phonon density of states is flexible, changing with size and shape. We invoke relationships between the low-temperature T (3) phonon term, Young's modulus, and friction coefficient to assess the difference in the tribological properties. On the basis of this analysis, we conclude that the improved lubrication properties of the nanoparticles are extrinsic.

Composite coatings of Co + fullerene-like WS2 nanoparticles on stainless steel substrate were obtained through electroless deposition, using DMAB ( dimethyl borane complex, 97%) as the reducing agent, and by electroplating in acidic solution. Phase analysis results show that the coatings consist of Co and the fullerene-like WS2 nanoparticles alone. Tribological measurements show reduced wear and friction of the composite coatings as compared with the pure cobalt film or the stainless steel substrate.

MoS2 nanooctahedra are believed to be the smallest stable closed-cage structures of MoS2, i.e., the genuine inorganic fullerenes. Here a combination of experiments and density functional tight binding calculations with molecular dynamics annealing are used to elucidate the structures and electronic properties of octahedral MoS2 fullerenes. Through the use of these calculations MoS2 octahedra were found to be stable beyond n(Mo) > 100 but with the loss of 12 sulfur atoms in the six corners. In contrast to bulk and nanotubular MoS2, which are semiconductors, the Fermi level of the nanooctahedra is situated within the band, thus making them metallic-like. A model is used for extending the calculations to much larger sizes. These model calculations show that, in agreement with experiment, the multiwall nanooctahedra are stable over a limited size range of 10(4)-10(5) atoms, whereupon they are converted into multiwall MoS2 nanoparticles with a quasi-spherical shape. On the experimental side, targets of MoS2 and MoSe2 were laser-ablated and analyzed mostly through transmission electron microscopy. This analysis shows that, in qualitative agreement with the theoretical analysis, multilayer nanooctahedra of MoS2 with 1000-25 000 atoms (Mo + S) are stable. Furthermore, this and previous work show that beyond similar to 10(5) atoms fullerene-like structures with quasi-spherical forms and 30-100 layers become stable. Laser-ablated WS2 samples yielded much less faceted and sometimes spherically symmetric nanocages.

Although graphite, with its anisotropic two-dimensional lattice, is the stable form of carbon under ambient conditions, on nanometre length scales it forms zero- and one-dimensional structures, namely fullerenes and nanotubes, respectively. This virtue is not limited to carbon and, in recent years, fullerene-like structures and nanotubes have been made from numerous compounds with layered two-dimensional structures. Furthermore, crystalline and polycrystalline nanotubes of pure elements and compounds with quasi-isotropic (three-dimensional) unit cells have also been synthesized, usually by making use of solid templates. These findings open up vast opportunities for the synthesis and study of new kinds of nanostructures with properties that may differ significantly from the corresponding bulk materials. Various potential applications have been proposed for the inorganic nanotubes and the fullerene-like phases. Fullerene-like nanoparticles have been shown to exhibit excellent solid lubrication behaviour, suggesting many applications in, for example, the automotive and aerospace industries, home appliances, and recently for medical technology. Various other potential applications, in catalysis, rechargeable batteries, drug delivery, solar cells and electronics have also been proposed.

Electrical resistivity and Hall measurements, of pellets compacted from IF-WS2 nanoparticles and 2H-WS, powder were done. Electrical transport measurements were carried out on pellets by the van der Pauw method in a wide temperature range. Arrhenius plots for conductivities of the WS2 samples (2H, IF and IF+treatment) exhibit marked variations in (partial derivative In sigma T-1)/partial derivative T-1 with temperature. Resistivity of IF-WS2 pellets is higher than that of 2H-WS2 pellets. It was found that the.electrical properties of IF-WS2 powder vary with aposteriory heat treatment under vacuum. The H-1 NMR measurements show that the prepared product contains water (and possibly some hydrogen) molecules that occupy the voids in the central part of the fullerene-like nanoparticles and the nanopores between the adhering IF-WS2 particles. Defects in the IF-WS, structure, arising due to the strain release during the folding of the layers, may result in additional sites for the absorbed water. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Inorganic fullerene-like nanoparticles of WS2 (IF-WS2), are synthesized by a reaction of tungsten oxide with molecular hydrogen and hydrogen sulfide. The synthesized nanoparticles appear as large agglomerates (> 40 microns), each one counting thousands of IF nanoparticles. H-1 nuclear magnetic resonance study of these nanoparticles is reported. The measurements show that the prepared product contains water (and possibly some hydrogen) molecules that occupy the voids in the central part of the fullerene-like nanoparticles and the nanopores between the adhering IF-WS2 particles. Defects in the IF-WS2 structure, arising due to the strain release during the folding of the layers, may result in additional sites for the absorbed water. Vacuum annealing of the powder leads to substantial reduction in the amount of absorbed water molecules.

Current-voltage characteristics measured using STM on fullerene-like WS2 nanoparticles show zero-bias current and contain segments in which the tunneling current flows opposite to the applied bias voltage. In addition, negative differential conductance peaks emerge in these reversed current segments, and the characteristics are hysteretic with respect to the change in the voltage sweep direction. Such unusual features resemble those appearing in cyclic voltammograms, but are uniquely observed here in tunneling spectra measured in vacuum, as well as in ambient and dry atmosphere conditions. This behavior is attributed to tunneling-driven electrochemical processes.

Uneven malaligned teeth are a problem afflicting large numbers of people, having significant economic and societal repercussions. Sliding a tooth along an archwire during orthodontic treatment involves a frictional type of force which resists this movement, causing a number of adverse effects. First, using excessive orthodontic force, leads to unwanted movements of the anchor teeth and increasing the risk of damage to the roots of the teeth. Furthermore, the frictional force is distributed unevenly between the archwire and the brackets interface, leading to strong adhesion between the wire and the bracket's corner. This force-asymmetry causes lengthening of treatment and frequent visits for fine-tuning of the orthodontic appliances. Despite numerous efforts to lower the friction, no satisfactory solution to this issue has been obtained. In the present work a self-lubricating metal coating containing fullerene-like WS2 (IF) nanoparticles is demonstrated. Such coatings significantly reduce archwire friction, and may alleviate the adverse complications. Moreover, a number of other medical applications of the self-lubricating coatings are foreseen.

We measured the infrared vibrational properties of bulk and nanoparticle WS2 in order to investigate the structure-property relations in these materials. In addition to the symmetry-breaking effects of local strain, nanoparticle curvature modifies the local charging environment of the bulk material. Performing a charge analysis on the xy-polarized E-1u vibrational mode, we find an approximate 1.5:1 intralayer charge difference between the layered 2H material and inorganic fullerene-like (IF) nanoparticles. This effective charge difference may impact the solid-state lubrication properties of nanoscale metal dichalcogenides.

We obtained a Raman signal from an individual WS2 nanotube mounted on an atomic force microscopy cantilever tip. We discuss the implications for simultaneous investigations of the mechanical properties of WS2 nanotubes by combining different experimental methods. From the orientation dependence of this nanotube's resonant Raman intensity, we estimate the ratio of the perpendicular to parallel polarizabilities alpha(XX)/alpha(ZZ)approximate to 0.16. We compare the WS2 nanotube with single-walled carbon nanotubes and expect a similarly strong depolarization effect for multiwalled carbon nanotubes.

Motivated by the discovery of the C-60 molecule (buckminsterfullerene), the search for inorganic counterparts of this closed-cage nanostructure started in 1992 with the discovery of nested fullerene-like nanoparticles of WS2. Inorganic fullerene-like (IF) materials have since been found in numerous two-dimensional compounds and are available in a variety of shapes that offer major applications such as in lubricants and nanocomposites. Various synthetic methodologies have been employed to achieve the right conditions for the constricted or templated growth needed for the occurrence of this new phase. In this study, IF-TaS2 is produced from a volatile chloride precursor in the gas phase and in small yield by room temperature laser ablation both in argon and in liquid CS2. For the gas-phase reaction, a high yield of IF nanoparticles was obtained between 400 and 600 degrees C with a low concentration of the precursor gas. The average size and the Yield of the IF-TaS2 nanoparticles decrease with temperature. Above 600 degrees C, IF nanoparticles were found in low yields and at sizes below 20 nm. The stability of the IF nanoparticles produced by the gas-phase reaction is discussed in the light of two existing theoretical models. Laser ablation in argon leads to IF nanoparticles filled with clusters of TaS2. Agglomeration of the nanoparticles can be avoided by laser ablation in liquid CS2.

We present the results from an experimental study of the magnetotransport of superconducting wires of amorphous indium-oxide having widths in the range 40-120 nm. We find that, below the superconducting transition temperature, the wires exhibit clear, reproducible, oscillations in their resistance as a function of magnetic field. The oscillations are reminiscent of those that underlie the operation of a superconducting quantum interference device.

Fullerene-like WS2 (MoS2) nanoparticles (IF) have been studied in the past. It was shown that the IF nanopaticles appear to form a protective film allowing increased load capacity of the rubbed pairs under mixed lubrication. The main objective of the present work is to evaluate the behavior of the IF nanoparticles under severe contact conditions-the transition to seizure. The effect of the IF nanoparticles burnished to porous alumina matrix on friction and wear of alumina-Si3N4 pair under high contact pressure is also considered. It was shown that when the Gap between the contact surfaces is smaller than the size of the IF nanoparticles, there is no effect of the nanopaprticles on the friction force. With load increasing, the IF nanoparticles penetrate into the interface, protecting the rubbed surfaces from a direct contact and thus increase the critical point of transition to seizure. Burnishing of the porous alumina surfaces by the IF solid lubricant nanoparticles provides very low friction coefficient and wear loss under high contact pressure. The IF-WS2 nanoparticles are found to be preserved in the rough summits, fill the valleys, and the pores of the sintered alumina and thus limit the straight contact between the ceramic surfaces. Although the external sheets of the outermost layers of the IF-WS2 nanoparticles were peeled-off, the pristine IF nanoparticles remain in the valleys and pores of the alumina supplying the solid lubricant sheets to the contact area during a long-term test. (c) 2005 Elsevier B.V. All rights reserved.

Recently, the behavior of inorganic fullerene-like (IF) WS2 nanoparticles in the interface of steel-on-steel pair has been analyzed. It was shown that originally when the gap between the contact surfaces is smaller than the size of the IF nanoparticles, there is no effect of the nanoparticles on the friction force. During the test stiff IF nanoparticles can plough the surface of hard steel samples and penetrate into the interface under friction. Molecular sheets of WS2 from the delaminated IF nanoparticles, which reside in the valleys of the rough surfaces cover the contact spots and thus decrease the number of adhered spots at the transition to seizure. The goal of the present work was to study the behavior of IF nanoparticles in the interface of ceramic surfaces. The friction tests were performed using a ball-on-flat device. A silicon nitride ball was slid against an alumina flat with maximum contact pressure close to 2 GPa. SEM, TEM and AFM techniques have been used in order to assess the behavior of IF nanoparticles in the interface. The behavior of IF nanoparticles in the much harder ceramic interfaces was found to be appreciably different from the steel pair. The pristine IF nanoparticles are damaged in the inlet of the contact during the first few cycles and thin shells of broken nanoparticles gradually cover the middle range of the contact surface. Different modes of deformation and destruction of the IF nanoparticles are exhibited when going from the middle to edge area of the contact. While aggregates of the pristine nanoparticles are formed at the edge of the contact, thin shells of broken IF nanoparticles are observed in the middle area where contact pressure is maximum. Mechanical stability and damage of IF nanoparticles in the ceramic interface are discussed.

Individual WS2 multiwalled nanotubes, 2-3 micron in length, and with 15 - 25 nm diameter were mounted on AFM cantilevers tips. The nanotube orientation along the cantilever cone axis was confirmed by scanning electron microscopy (SEM). Micro-Raman spectra of these individual nanotubes showed similar vibrational frequencies as in the bulk material. The highly anisotropic shape of the nanotubes, however, leads to a strong antenna effect as it is known from single-walled carbon nanotubes. A qualitative assessment of the radiation field screening for polarization perpendicular to the nanombe axis is given.

Following the discovery of carbon fullerenes and carbon nanotubes, it was hypothesized that nanoparticles of inorganic compounds with layered (two-dimensional) structure, such as MoS2, will not be stable against folding and form nanotubes and fullerene-like structures: IF. The synthesis of numerous other inorganic nanotubes has been reported in recent years. Various techniques for the synthesis of inorganic nanotubes, including high-temperature reactions and strategies based on 'chemie douce' (soft chemistry, i.e. low-temperature) processes, are described. First-principle, density functional theory based calculations are able to provide substantial information on the structure and properties of such nanotubes. Various properties of inorganic nanotubes, including mechanical, electronic and optical properties, are described in brief. Some potential applications of the nanotubes in tribology, protection against impact, (photo)catalysis, batteries, etc., are discussed.

Cesium oxides are materials of great interest to the photodetection industry because of their relatively low work function (similar to1 eV). Used mainly as coating films for photoemissive devices, they provide high wavelength thresholds and high photocurrents. However, they are unstable, air-sensitive, and hygroscopic, rendering them short-lived and limiting their applications. Although the technology of these devices is highly developed, their characterization on the micro- and nanoscale suffers from their poor chemical stability and poor crystallinity. In the present study, cesium oxides were synthesized from the elements and were characterized using a combination of chemical and structural analysis techniques. Because the reaction products were extremely sensitive to humidity, sample analysis without atmospheric exposure was essential, and techniques were developed for the transfer of the samples to the measurements systems. Extensive data obtained from X-ray energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), X-ray diffraction (XRD), and Raman microscopy were obtained. Raman spectra with bands at 103, 742, and 1134 cm(-1) strongly confirmed the presence of the oxide, peroxide, and superoxide ions, respectively, as well as the absence of carbonate as an impurity. The A(1g) mode Of Cs2O was detected as an anti-Stokes band at 103 cm(-1). This study provides further insight into the reactivity of the various cesium oxides.

Inorganic fullerene-like structures (IF) of the layered hafnium sulfide, Hf2S, have been synthesized by the laser ablation of HfS3 in tert-butyl disulfide medium. Apart from the Hf2S IFs exhibiting quasi-spherical as well as faceted nested-shell geometries, quasi-spherical nanoparticles of HfS were observed by this means. Whereas Hf2S has anti-NbS2 structure with S layers sandwiched between two Hf layers, HfS has the nonlayered WC-type structure. The nanoparticles of HfS show excess sulfur in the core, and they do not possess closed-shell geometry. The mechanism of formation of these nanoparticles has also been discussed.

Metal dichalcogenide nanotubes and in particular those Of WS2 were shown to exhibit some unique physical and chemical properties, which offer numerous applications for this kind of nanophase material. Using a fluidized bed reactor (FBR), WS2 nanotubes were obtained in substantial amounts recently, rendering a systematic study of their properties possible. The FBR synthesized nanotubes are multiwalled (5-7 layers); open-ended; long (

Recently, it has been established that WS2 and MoS2 nanoparticles (inorganic fullerene-like, IF) mixed with oil, and impregnated into porous matrix of powdered materials appear to enhance the tribological properties of mating surfaces in definite loading range in comparison to typical metal dichalcogenide solid lubricants. The main results have been obtained under relatively low pressures. It is important to evaluate the tribological properties of IF when the concentrated contact is obtained. The effect of the IF in oil was studied using pin-on-disk tester in the regime of mixed lubrication. The interaction between the full film and the asperity contact fractions has been considered and the time evolution of the friction force was evaluated. The states of the mating surfaces and the nanoparticles before and after the friction test were studied by transmission electron microscopy (TEM), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). It was established that the IF nanoparticles mixed with oil allow to decrease the fraction of straight asperity contact under mixed lubrication regime and thus improve their tribological properties. TEM analysis showed that the shape of the IF nanoparticles is preserved under low loads. It was found that some of the IF nanoparticles were damaged after the friction at the maximal load of 420 N. The IF nanopaticles appear to form a protective film allowing increased load capacity of the rubbed pairs. The role of the IF solid lubricants as a part of a third body is discussed in this work. The mechanism of friction and wear of the IF nanoparticles are discussed. (C) 2003 Elsevier Science B.V. All rights reserved.

Tin disulfide pellets were laser ablated in an inert gas atmosphere, and closed cage fullerene-like (IF) nanoparticles were produced. The nanoparticles had various polyhedra and short tubular structures. Some of these forms contained a periodic pattern of fringes resulting in a superstructure. These patterns could be assigned to a superlattice created by periodic stacking of layered SnS2 and SnS. Such superlattices are reminiscent of misfit layer compounds, which are known to form tubular morphologies. This mechanism adds up to the established mechanism for IF formation, namely, the annihilation of reactive dangling bonds at the periphery of the nanoparticles. Additionally, it suggests that one of the driving forces to form tubules in misfit compounds is the annihilation of dangling bonds at the rim of the layered structure.

WS2 nanoparticle with closed-cage structure (fullerene-like IF) are already being synthesized in macroscopic amounts from the respective oxide nanoparticles. They have been studied as superior solid lubricants under harsh conditions in recent years. Under severe contact conditions both fluids and greases are squeezed out from the contact area and consequently do not provide adequate lubrication conditions. Addition of even a small amount of IF nanoparticles to the oil was found to reduce the friction coefficient and wear rate, and increase the load-bearing capacity of the friction pairs. Furthermore, IF nanoparticles were impregnated into polymer and metal coatings and into self-lubricating porous metal parts, and were found to alleviate both friction and wear remarkably well. In another set of experiments, IF nanoparticles were shown to provide excellent tribological behavior for the contact between a ceramic alumina block and silicon nitride ball. The mechanism for the improved tribological behavior of the IF nanoparticles is being investigated. In addition to the rolling friction, gradual exfoliation of the IF onions and transfer of monomolecular WS2 sheets onto the metal surface (third body transfer) is shown to play a major role in alleviating friction and wear. This work suggests numerous applications for this new solid-state nanolubricant.

Non-carbon inorganic fullerene-like (IF) nanoscale materials have recently attracted intense interest due to their nested hollow and nanotube structures. In this letter, IF-WS2 nanoparticles prepared by solid-gas reaction were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results show that the IF-WS2 nanoparticles have a nested hollow closed spherical structure with diameter of 100-150 nm.

Recently, it has been established that WS(2) and MoS(2) nanoparticles (IF) mixed in oil, and impregnated into porous matrix of powdered materials appear to enhance the tribological properties in definite loading range in comparison to typical metal dichalcogenides particles. Modification of the contact surface by IF-WS(2) nanoparticles from oil, grease and from pores of densified powdered samples is considered in this work. The role of the IF solid lubricants as a part of a third body is discussed also in this work. The effect of the IF in oil was studied using a pin-on-disk tester. The study of IF in greases and impregnated into the pores of densified matrices was performed using a ring-block tester. The state of the mating surfaces and the nanoparticles before and after the friction test has been studied by TEM, SEM and XPS. It was established that the IF nanoparticles form a protective film allowing an increase in the load capacity of rubbed pairs. The tribological behavior of lithium grease with IF was found to be appreciably better than that of a commercially available grease incorporating MoS(2) platelets (2H). It is found that the IF nanoparticles lead to improved tribological behavior of greases with the IF nanoparticles under very high loads. Impregnation of the IF-WS(2) into the pores of densified powdered materials provides a reduced friction coefficient and wear rate over a wide load range in comparison to 2H solid lubricant. The main advantages of the IF nanoparticles is ascribed to the release and furnishing of the nanoparticles from the open pores onto the metal surface and their confinement at the interface, which is not possible with dense flat surfaces. It is expected that the spherical shape of IF nanoparticles facilitate their sliding/rolling between the rubbed surfaces. (C) 2002 Elsevier Science B.V. All rights reserved.

Inorganic Fullerene-like (IF)-MoS2 nanoparticles were tested under boundary lubrication and ultra-high vacuum (UHV) and were found to give an ultra-low friction coefficient in both cases compared to hexagonal (h)-MoS2 material. Previous works made by Rapoport et al. with IF-WS2 revealed that the benefit effect of the inorganic fullerene-like materials decreases at high loads and sliding velocities. Nevertheless, under the conditions used in our experiments using high contact pressure (maximum pressure above 1.1 GPa in oil and 400 MPa in high vacuum) and slow sliding velocities (1.7 mm/s in oil test and 1 mm/s in hi-h vacuum), friction always decreases and stabilizes at about 0.04 for 800 cycles in both cases. Therefore, IF-MoS2 material appears to be a good candidate for use in various environments in regard to other MoS2 crystal structures. Wear mechanisms were investigated using both High Resolution TEM and surface analyses (XPS) on the wear tracks. Wear particles collected from the flat wear scar show several morphologies, suggesting at least two lubricating mechanisms. As spherical particles are found in the wear debris, rolling may be a possible event. However, flattened and unwrapped IF-MoS2 particles are often observed after friction. In this case, low friction is thought to be due either to sliding between IF-MoS2 external flattened planes or to slip between individual unwrapped MoS2 sheets. (C) 2002 Elsevier Science B.V. All rights reserved.

Active laser ablation has been used for the synthesis of NiCl2 nanotubes and fullerene-like nanoparticles (see Figure). Spectroscopic measurements on the structures showed that the NiCl2 layers in the nanotubes were quite perfectly crystalline. The growth is thought to occur via a vapor-liquid-solid mechanism through which many different, surprising shapes have been obtained.

One of the main advantages of powder metallurgy technology is controlled porosity for self-lubrication, To decrease friction and wear of contact pairs, solid lubricants such as WS2 and MoS2 are used, Recently, inorganic fullerene-like supromolecules of WS2 and MoS2 with structures closely related to carbon fullerenes and nonotubes have been synthesized, Experiments showed that the supromolecules possess lubricating properties superior to those of layered materials, in which each unit cell contains two layers with a hexagonal arrangement. It is expected that the fabrication of densified powders containing solid lubricant nonoporticles will lead to the development of a new class of composites with improved mechanical properties. The main goal of the present work was to study material and process parameters for the formation of a porous matrix and the impregnation of nonoporticles, Self-lubricating sintered bronze, iron and iron-nickel base composites were produced. friction and wear results for the powder materials impregnated with layered materials and supromolecules as solid lubricants are presented.

An overview about the characteristic structure of metal chalcogenide nanotubes is given in this paper. On the basis of atomistic calculations, parameters for a model are derived, describing the stability of tubular structures. Especially the interplay between the strain energy in the tubes and the energy due to unsaturated (dangling) bonds in layer stripes and the role of the van der Waals interaction in multilayer stripes and tubes are discussed. In agreement with experimental observations it is shown that nanotubes with a diameter larger than about 6 nm are more stable than stripes of a corresponding width. The van der Waals interaction stabilizes multilayered stripes, leads to multiwalled nanotubes, and can explain the nonexistence of single-wall metal chalcogenide nanotubes.

The impregnation of inorganic fullerene-like nanoparticles of WS2 (IF) allows to improve effectively the tribological properties of powdered materials in comparison to the impregnation of oil or commercially available layered WS2 (211) particles. The main goal of this work was to determine the dominant lubrication regimes under friction of the bronze, iron, and iron-nickel porous matrixes impregnated with 2H and IF solid lubricants. The tribological tests were performed at laboratory atmosphere (humidity similar to50%) using a ring-on-block tester at the sliding speed of V = 1 m/s, and the loads of 150-1000 N. The wear of the metal bodies was measured by an eddy current probe system and by weighting of the samples before and after the test. In order to evaluate the radial clearance, the profiles of wear blocks were analyzed by profile projector. Than these data were used in the calculation of the Sommerfeld reciprocal values. Friction and wear results were presented as the Stribeck curves. The critical Sommerfeld reciprocals were evaluated from these curves. The Stribeck curves were compared with the Morgan's curves for different ranges of non-dimensional permeability, psi. These results are used then in calculation of the permeability, Phi. Three lubrication regimes as: quasi-hydrodynamic, boundary or mixed, and dry friction were revealed under friction of porous samples in the load-range studied. It was found that the impregnation of IF nanoparticles provides the regime of quasi-hydrodynamic lubrication in the widest range of loads in comparison to the reference sample and the sample impregnated with 2H-WS2. Fe-Ni samples exhibited the highest wear resistance and provided the widest range of quasi-hydrodynamic lubrication in comparison to bronze and iron powdered composites. The effect of IF on the regimes of lubrication is explained on the base of the third-body model. It is expected that the sliding/rolling of the IF nanoparticles in the boundary of the first

Recent experimental results demonstrate that it is possible to grow a variety of different multiphase, nested nanotube structures. This paper predicts the structure and energetics of such multiphase nanotubes. There are several distinct contributions to the energetics: the internal and external surface energies, the energy of the interface between the different phases, the long-range (van der Waals) interactions between interfaces, and the elastic bending energy. We perform energy minimizations to compare the energies of two- and three-layer films and nanotubes. We present physical guidelines, quantitative theory, and structure maps that show how materials and geometric parameters influence the stability of competing structures.

Heterojunction thin-film solar cells are usually fabricated from n-type window material (large bandgap) in its front and p-type absorber material (small bandgap) in the back. In this work, an inverted cell with p-type window material in front and a rear n-type absorber is presented. A diamond/CdTe photovoltaic cell has been fabricated to demonstrate the concept of an inverted p-n heterojunction solar cell. A systematic study of the influence of various experimental parameters on the behavior of the cell has been undertaken and is briefly discussed. It is shown that the solar cell open-circuit voltage V-OC is 230 mV with an internal quantum efficiency of 70% at 500 nm. Steps to further improve the photoresponse and the photovoltaic conversion efficiency of the cell are discussed. (C) 2001 The Electrochemical Society.

In the past few years, inorganic fullerene-like (e.g. IF) supramolecules of metal dichalcogenide MX2 (M=Mo,W, etc.; X=S, Se), materials with structures closely related to (nested) carbon fullerenes and nanotubes have been synthesized. Recent experiments showed that IF possess lubricating properties superior to those of commercially available layered solid lubricant (2H-WS2 polytype) in a wide range of operating conditions. It was shown before that the impregnation of a small amount of such nanoparticles into porous bronze matrix largely improve the tribological properties of bronze-steel contact pairs. In the present work, the effect of the PV (pressure-velocity) parameter on the transition to seizure for powdered bronze-graphite, iron-graphite and iron-nickel-graphite composites impregnated 2H and IF lubricant has been studied. The tribological tests were performed using a ring-block tester at loads of 150-1200 N and sliding speeds from 0.5 to 1.7 m/s. It was found that impregnation of IF into the pores improves the tribological properties of the powdered composites in comparison to 2H-WS2 solid lubricant. Furthermore, it was established that the impregnation of oil together with the IF nanoparticles allows to provide very high load bearing capacity of the powdered materials. It is suggested that the transition to seizure occurs when the wear debrises accumulate in the pores on the surface. This process practically blocks the surface pores and limits the supply of the solid lubricant particles to the contact surface. The main advantage of the IF nanoparticles is attributed to: (a) slow release and supply of nanoparticles from the open pores to the surface; (b) sliding/rolling of the IF between the rubbing surfaces; and (c) prevention of the accumulation of the agglomerated wear particles in the pores. The model of third body was used in order to explain the effect of the wear particles, oil and solid lubricant particles on the friction and wear behavior of powde

In the past few years, inorganic fullerene-like (IF) supramolecules of metal dichalcogenide WS2 and MoS2 with structures closely related to (nested) carbon fullerenes and nanotubes have been synthesized. Recent experiments showed that IF added to oil and impregnated into the porous matrixes possess lubricating properties superior to those of layered WS2 and MoS2 (2H platelets). The main goal of this work was to analyze the mechanism of friction of fullerene-like nanoparticles, Friction and wear behavior of IF in different contact conditions is studied. Third body model is considered. Sliding/rolling of the IF nanoparticles in the boundary of the first bodies and in between the wear particles (third body) is supposed to facilitate the shear of the lubrication film, Broken and oxidized 2H-WS2 small pieces adhered to wear debris do not provide high tribological properties especially under high loads.

Recently the concept of the inverted photovoltaic cell has become more attainable as a practical cell. This thin film cell consists of a p-n heterojunction in which the window layer is p-type and the absorber layer is n-type. The feasibility of a new inverted p-n heterojunction p-diamond/n-CdTe solar cell has been demonstrated. The non-optimized solar cell structure grown on semi-transparent p-diamond yielded an open circuit voltage of 0.23 V and a short circuit current of 1.54 mA/cm(2) when illuminated. (C) 2001 Elsevier Science B.V. All rights reserved.

We report on the investigation of surface-plasmon excitation of anisotropic VS,, hollow nanoparticles in a near-field geometry by means of a scanning transmission electron microscope. The shell thickness influence on the electron-energy-loss-spectroscopy spectra is experimentally observed and is analyzed within a classical dielectric formalism. As for the isotropic case, we evidence one symmetric (tangential) and one antisymmetric (radial) mode. We point out the intriguing fact that, for the anisotropic case, one can relate these modes to the interband transition of the in-plane component of the dielectric tensor and to the bulk-plasmon energy of the out-of-plane component.

In the past few years, WS2 and MoS2 nanoparticles (IF) with structures closely related to (nested) carbon fullerenes and nanotubes have been synthesized. It was shown before that addition of a small amounts of such nanoparticles to lubricating fluid largely improve the tribological properties of contact surfaces. In the present work, impregnation of the nanoparticles into a porous powder (bronze-graphite) matrix is demonstrated. The critical point to seizure of the powder matrix impregnated with mineral oil, 2H-WS2 platelets and IF nanoparticles was measured and is 300, 570 and 950 N, respectively. The friction coefficient of the powder matrix lubricated by oil and oil + IF-WS2 was measured. It was found that the addition of the IF nanoparticles in the oil decreased the friction coefficient by 30-50% as compared to the surfaces lubricated by the oil alone. The main effect of TF nanoparticles in comparison to 2H-WS2 platelets appears to be the storage of the nanoparticles for long periods of times within the pores and the gradual release and furnishing to the contact surface. Once released from the pores. IF nanoparticles protect better the rubbed surfaces against straight contact. (C) 2001 Elsevier Science B.V. All rights reserved.

Two methods for the synthesis of CdTe nanoparticles in zirconia sol-gel films are demonstrated. The nanoparticles were obtained by chemical reduction of Te(IV) using reducing agent (hydrazine) or tin chloride. Particle sizes ranging from 6 to 20 nm in diameter could be prepared by varying the experimetal parameters. The size and crystalline structure of the particles were characterized by optical absorption, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. The film morphology was characterized by scanning force microscopy. The film obtained by SnCl2 method is smooth and homogeneous. The dense structure of CdTe nanoparticles of a few nm in diameter is revealed. The films prepared with hydrazine are porous as a result of evolution of the decomposition gaseous products during the reduction. Advantages and disadvantages of the two methods are discussed.

The inorganic analogs of carbon fullerenes and nanotubes, like MoS(2) and BN, are reviewed. It is argued that nanoparticles of 2D layered compounds are inherently unstable in the planar configuration and prefer to form closed cage structures. The progress in the synthesis of these nanomaterials. and, in particular. the large-scale synthesis of BN, WS(2) and V(2)O(5) nanotubes, are described. Some of the electronic, optical and mechanical properties of these nanostructures are reviewed. The red-shift of the energy gap with shrinking nanotube diameter is discussed as well as the suggestion that zigzag nanotubes exhibit a direct gap rather than an indirect gap, as is prevalent in many of the bulk 2D materials. Some potential applications of these nanomaterials are presented as well, most importantly the superior tribological properties of WS(2) and MoS(2) nested fullerene-like structures (onions).

Inorganic fullerene-like (hollow onionlike) nanoparticles (IF) and nanotubes have attracted considerable interest in recent years, due to their unusual crystallographic morphology and their interesting physical properties. IF-MoS2 and nanotubes were first synthesized by a gas-phase reaction from MoO3 powder, This process consists of three steps: (1) evaporation of the MoO3 powder as molecular clusters; (2) condensation of the oxide clusters to give MoO3-x nanosize particles; (3) sulfidization of the suboxide nanoparticles to generate IF nanoparticles. The evaporation of MoO3 (step I) and the IF particle formation from the oxide nanoparticles (step 3) have been investigated already, while the mechanism for the suboxide nanoparticles formation (step 2) has not been studied before and is reported here. According to the present model, a partial reduction of the trioxide molecular clusters (3-5 molecules) leads to the formation of MoO3-x nanoparticles (5-300-nm particles size)-the precursor for IF-MoS2. A mathematical model, which takes into account the diffusion of the reactants into the reaction zone, the chemical reactions, and the boundary conditions obtained from the experiments, is established and solved. Based on the comprehensive understanding of the IF-MoS2 growth mechanism from MoO3 powder and the solution of the diffusion equations, a gas-phase reactor, which allowed reproducible preparation of a pure IF-MoS2 powder (50 mg per batch) with controllable sizes, is demonstrated.

Multiple beam interferometry (MBI) can be used in the surface forces apparatus for in situ topographical imaging in real-time of the contact between two shearing surfaces at ultrahigh resolution in the normal direction at the same time as friction forces are measured. Simultaneous measurements were made of the friction forces between two shearing mica surfaces separated by WS2 (inorganic) fullerene, and non-fullerene WS2 nanoparticle additives in tetradecane. The results were correlated with the very different transfer layers formed in each case, as visualized by in situ MBI and ex situ atomic force microscopy. (C) 2000 Elsevier Science S.A. All rights reserved.

Recently, a method to produce bulk quantities of pure multiwall WS2 nanotubes, which could reach several microns in length, has been developed. A detailed study of the growth mechanism of these WS2 nanotubes has been undertaken, which is reported hereby. A series of experiments were conducted to define the key parameters, which determine the shape of the WS2 nanotubes. An alternative approach for the synthesis of WS2 nanotubes, starting from long WO3-x nanowhiskers, which can be extended for the synthesis of other nanotubes, is described as well.

The synthesis of a pure phase of very long and hollow WS, nanotubes from short but asymmetric oxide nanoparticles is described. In this process, the oxide nanoparticles grow along their longest axis; and subsequently their outermost layer is being sulfidized, while the growing oxide tip remains uncoated as long as the nanowhisker continues to grow. Thereafter, the sulfide cup is closed and a slow diffusion controlled sulfidization of the oxide core is completed within 60-120 min.

Communication: Inorganic fullerene-like nested nanostructures (IFs) such as the WS2 Ifs shown in the Figure deposited on an a-SiOx-coated TEM grid are confirmed to confer tribological advantages when added to a lubricant fluid (tetradecane) between two shearing mica surfaces. It is shown that the lower friction is associated with friction-induced material transfer of WS2 from the Ifs to the mica surfaces.

Raman and resonance Raman spectra of MoS2 nanoparticles, in the form of inorganic fullerenelike nanoparticles with diameters ranging from 200 to 2000 Angstrom in size, and platelets ranging from 50 to 5000 Angstrom in size, are presented. Off resonance, the first-order Raman bands are broadened, but not significantly shifted, and no additional bands are observed, indicating that the atomic structure is preserved, at least locally, in the nanoparticles. The broadening effect is assigned to phonon confinement by facet boundaries. In the resonance Raman spectra of the nanoparticles, several additional first-order peaks are observed. The electron-phonon coupling responsible for the strong-resonance conditions is identified through dynamic band calculations. Using temperature-dependent resonance Raman measurements, we assign these peaks to zone-boundary phonons activated by disorder and finite-size effects. By analyzing the position of the dispersive peak at 429 (-1) under resonance conditions, it was possible to probe the softening of modes propagating in the c-axis cm direction. [S0163-1829(99)08827-X].

Complex tungsten oxides, consisting of nonstoichiometric oxides of the form WO3-x and stoichiometric lamellar oxides of the form {001}(R) WnO3n-1 (n = 3 to 6) have been observed incorporated within 2H-WX2 (X = S or Se) inorganic fullerene-like (IF) structures by HRTEM, These encapsulates were formed from a gas-solid reaction between H2X and disordered WO3-x precursors exhibiting a range of particle sizes and morphologies. The microstructures of most of the encapsulated oxides could be described in terms of {hkl}(R) crystallographic shear (CS) structures formed relative to an ReO3-type (R) substructure. Smaller spheroidal WO3-x encapsulates were frequently found to exhibit random {103}(R) CS defects of the Wadsley type, while larger, needle encapsulates were found to form exclusively {001}(R) WnO3n-1 type lamellar structures that were predominantely ordered. Spheriodal encapsulates with randomly spaced {001}(R) CS planes were also observed encapsulated inside 2H-WSe2 IF structures. The growth and morphologies of the encapsulating 2H-WX2 shells were profoundly influenced by those of the precursor oxides used in their formation. Ordering mechanisms were proposed with respect to the formation of the ordered encapsulated oxides from the disordered precursors. (C) 1999 Academic Press.

The preparation of WC encapsulated within 2H-WS2 inorganic fullerene-like (LF) cages by sulfidisation is described; the encapsulates were prepared from both high surface area (high S-g) and low surface area (low S-g) WC precursors; for low S-g WC, partial conversion of the carbide to the sulfide only was achieved with 2-20 IF layers obtained around carbide particles, resulting in WC/2H-WS2 composites; the amount of conversion obtained in the case of the high S-g WC depended on the method of preparation of the precursor; total conversion to 2H-WS2 was obtained from a precursor obtained by carburisation of small WO3 particles, while only partial conversion was obtained in the instance of high S-g WC prepared from ball-milled low S-g WC.

The tribological properties of textured WS2, MoS2 and WSe2 films, which were prepared using an ultra-thin interlayer of Ni/Cr (van der Waals rheotaxy technique) on quartz substrate, were determined in ambient conditions. Using scanning force microscope adapted for tribological measurements, very low (0.04 and below) friction coefficients and little wear were measured on flat areas of the films, Macroscopic (engineering) tribological measurements, using the reciprocating ball on flat tribometer, exhibit somewhat higher friction coefficients. Compactization of the films under the load and little wear were observed for the films even after a few hundred cycles. X-ray photoelectron spectroscopy of the WS, film after the wear experiment confirmed that some oxidation took place within the wear track, but the overall integrity of the film was preserved. These measurements indicate that highly textured films of this kind are promising candidates for tribological coatings, where oil-free lubrication is required. (C) 1998 Elsevier Science S.A. All rights reserved.

Optical-absorption spectroscopy of inorganic fullerenelike MoS2 and WS2 (IF-MoS2 and IF-WS2) is reported in the range 400-800 nm, at temperatures between 4-300 K, and compared to the corresponding bulk (2H) material. A systematic study of the effect of IF size and number of atomic layers on the optical properties shows that the semiconductivity of the layered material is preserved in the IF structures. Nevertheless, all TF with number of layers (n)>6 exhibit a decrease in the A and B exciton energies. This redshift becomes larger as additional inner layers are formed, until a saturation value is reached (n>10). We assign this redshift to the deformations, curvature, and discommensuration between adjacent atomic layers the structure must accommodate in order to form an IF structure. An increase in the exciton energies is observed in IF consisting of a few sulfide layers (n7% along the c axis leads to a convergence of the levels K-1 and K-4, which is displayed in the absorption spectra of IF with 1 or 2 layers.

The deposition of copper on boron-doped diamond thin-film electrodes was investigated, Three kinds of boron-doped diamond films were studied: a film deposited on a silicon wafer, a second grown on a tungsten substrate. and a third, similar to the first, but surface-amorphized (bombarded) by ion beam irradiation. The films were investigated by a number of techniques. Linear potential sweep experiments in 0.1 M H2SO4 + 0.001 M CuSO4 solution showed that, besides the peak due to bulk copper deposition, a "prewave" peak is present in the voltammogram of the first two electrodes and not in the third one. An electrochemical activation at negative potential was imperative for the appearance of the "prewave" peak. The amount of copper involved was less than 2% of a monolayer. The comparison between spectroscopic and electrochemical investigations leads to the interpretation that the "prewave" peak corresponds to an underpotential deposition of Cu on graphitic inclusions in the diamond electrode, or along the grain boundaries, probably by intercalation.

Lately MoS2 and WS2 fullerene-like nanoparticles, generically termed inorganic fullerene-like (IF) materials, were reproducibly obtained by gas phase(4,6) and solid state(6) reactions and the growth mechanism of such structures was elucidated(6). The present work deals with the synthesis of IF-WSe2 from tungsten oxide and selenium vapors. Thermodynamic calculations of the sulfidization and selenization processes are provided, and indicate salient differences between the two processes. Notwithstanding, the growth model of IF-WSe2 is similar to that of sulfide fullerene-like material. The IF-WSe2 was characterized by X-ray powder diffraction, transmission electron microscopy and high resolution transmission electron microscopy.

Structures with negative curvature were predicted and experimentally observed in carbon nanoparticles belonging to an expanded class of fullerenes (named Schwartzites). To obtain negative curvature in such structures, it is necessary to dispose heptagons or octagons in the fullerene network which consists of hexagons and pentagons. This phenomenon allows one to construct complicated structures, of which only a few of were experimentally realized. In the present work, structures with negative curvature are shown to exist also in the recently discovered inorganic fullerene-like (IF) materials. A number of MoS2 nanoparticles with complicated topologies having negative curvature, are described.

STM experiments were performed to study modifications on the surface of crystalline and amorphous transition metal chalcogenides. Pulses with different parameters were applied to the STM tip, at predefined locations, while scanning the surface of WSe2 and MoS3. Different types of structures were formed on the crystalline WSe2 surface: (a) holes, (b) mounds, (c) ringlike structures. The three types were formed at different ranges of voltage. In some cases local cleavage pf the surface was observed. An explanation is offered for the creation of such a variety of structures. On the amorphous MoS3 surface the pulses created small, round mounds. The size of the structures grows with the voltage of the applied pulse. I-V characteristics taken on these structures show semiconducting behavior with a gap of 1.1 eV, similar to the gap of bulk MoS2. TEM images show that nested polyhedral structures are formed in the STM experiments, with sizes that agree with the observed structures. Similar round crystalline structures are formed on the surface of MoS2 grown under different conditions. This is the first time that such crystallization is observed by pulsing the STM tip.

The different methods for encapsulating crystalline materials inside fullerene related structures are reviewed. The relationships between the mode of encapsulation and the crystallisation behaviour obtained in each case are described. In particular, the mechanisms of morphological and orientational control of crystallite growth inside carbon nanotubes and the comparative encapsulation behaviours of materials encapsulated by physical and catalytic methods are described and discussed. The encapsulation of defect tungsten oxide structures within inorganic fullerene-like structures are also described.

Highly textured films of 2H-WS2 can be obtained by sulfurization of up to 4 mu m thick WO3 layers in the presence of hydrogen using amorphous (quartz glass, glassy carbon) or crystalline (sapphire, muscovite, highly oriented pyrolytic graphite) substrates. Best conditions have been found employing (00.1) oriented sapphire substrates with a 5 nm thick nickel layer interposed between substrate and oxide film (0.5 to 4 mu m thickness) and a reaction temperature ranging from 973 to 1173 K. Depending on time the crystallites, oriented with their hexagonal basal planes parallel to the substrate, exhibited a lateral extension of up to 20 mu m and a thickness of less than or equal to 300 nm. Conductivity measurements of the films showed a p-type conductivity in the range from 0.1 to 3 Omega(-1) cm(-1) and a lateral mobility as high as 105 cm(2) V-1 s(-1) at room temperature. The conductivity type has been confirmed by ultraviolet photoelectron and x-ray photoelectron spectroscopy which were compared with n-type single crystals. A freestanding film pealed off from a Pt coated quartz substrate and mounted on a brass holder was investigated photoelectrochemically. Using a 0.2 M Fe2+/Fe3+ redox electrolyte in 0.5 M H2SO4 an open-circuit voltage of approximate to 100 mV and a short-circuit current of 5 mA/cm(2) has been detected for the first time.

Amorphous precursors, like MoS3 (WS3), were shown before to be an ideal precursor for the growth of inorganic fullerene-like material in a rather slow crystallization process which lasts anything from 1 h at 800-900 degrees C-1,C-2 to a few years at ambient conditions.(3,4) Using a few microsecond short electrical pulses from the tip of a scanning tunneling microscope, crystallization of amorphous MoS3 (a-MoS3) nanoparticles, which were electrodeposited on a Au substrate into MoS2 nanocrystallites with a fullerene-like structure (IF-MoS2), is demonstrated. The (outer) shell of each nanocrystallite is complete, which suggests that the reaction extinguishes itself upon completion of the crystallization of the MoS2 layers. A completely different mode of crystallization is observed in the case of continuous a-MoS3 films. Here tiny (2-3 nm thick) 2H-MoS2 platelets are observed after the electrical pulse, suggesting a very rapid dissipation of the thermal energy through the gold substrate, in the continuous domain. Since the reaction mechanism in both cases is believed to be the same, it is likely that the main stimulus for the chemical reaction/crystallization of the IF material results from the slow heat dissipation from the nanoparticle. The exothermicity of the chemical reaction may further promote the rate of the process.

Recently, milligram quantities of MoS2 fullerene-like nanotubes and negative curvature polyhedra (generically called inorganic fullerene-like material, IF), were reproducibly obtained by a gas phase reaction from an oxide precursor (Feldman, Y.; Wasserman, E.; Srolovitz, D.J.; Tenne, R. Science 1995, 267, 222. Srolovitz, D. J.; Safran, S.A.; Homyonfer, M.; Tenne R. Phys. Rev. Lett. 1995, 74, 1778). The present work focuses on the mechanism of the synthesis of IF-MS(2) (M = W, Mo). The IF material is obtained from oxide particles smaller than ca. 0.2 mu m, while larger oxide particles result in 2H-MS(2) platelets. The key step in the reaction mechanism is the formation of a closed layer of MS(2), which isolates the nanoparticle from its surroundings and prevents its fusion into larger particles. Subsequently, the oxide core of the nanoparticle si progressively converted into a sulfide nanoparticle with an empty core (IF). Taking advantage of this process, we report here a routine for the fabrication of macroscopic quantities of a pure IF-WS2 phase with a very high yield. As anticipated, the size distribution of the IF material is determined by the size distribution of the oxide precursor. The present synthesis paves the way for a systematic study of these materials which are promising candidates for, e.g., solid lubrication.

The electrochemical behavior of boron-doped diamond films has been investigated in a number of neutral and alkaline solutions with and without nitrate ions. Two kinds of diamond electrode were studied: self-supported film (100 mu m) (sample A) and diamond film (10 mu m) supported on a Si substrate (sample B). It was found that water oxidation and reduction appear at much larger polarizations for diamond electrodes, as compared with platinum and platinized platinum electrodes. In particular, the higher (cathodic) overpotential for hydrogen reduction permits efficient nitrate reduction to ammonia. The underlying Si substrate is shown to take part in the electrochemistry of the diamond electrodes. In the case of the Si-supported electrode (sample B) the reaction with the Si substrate was imminent. For the free-standing diamond electrode (sample A) various impurities in the grain boundaries and at the back of the electrode, including the back metallic contact, intervened with the electrochemistry of the diamond electrode, but to a much lesser extent than with sample B. Meticulous cleaning and careful working practices permitted this interference to be excluded altogether in sample A.

WSe2 films (1-4 mu m thick) prepared over a molten NixSey substrate (by the so-called van der Waals rheotaxy process) at elevated temperatures, where characterized through a number of techniques. They were shown to possess very good optical and electronic properties, in addition to having (almost) perfect type-II texture, i.e. c axis perpendicular to the substrate. Ramifications of this work to photovoltaics and solid lubrication, is briefly discussed.

The dark-field diffraction contrast of helical nanotubes (NTs) is shown to be asymmetric when an NT is tilted at appropriate angle with respect to the incident electron beam. This phenomenon was used for the chirality determination of multi-shell NTs observed in MoS2 layered compound. Both kinds of NT - helical and non-helical were found. In the case of helical NTs only right-hand chirality was observed.

Anisotropic materials with layered structure, like MoS2 and WSe2, play an important role in a number of technologies. Some of these applications (lubrication, photovoltaics) require polycrystalline films oriented with their c axis perpendicular to the substrate surface (type-II texture), which is the thermodynamically favorable texture. However, films with the substrate parallel to c (type-I texture) are usually obtained. We report that an ultrathin (

We report here the effects of systematic cation and dissolved salt variation on n-WSe2/aqueous polyiodide photoelectrochemistry. It is shown that Ag+, highly soluble in concentrated iodide electrolytes, enhances photopotential and photoelectrochemical cell power. The variation of four highly soluble iodide salts containing Na+, K+, Li+, or Zn2+ cations are discussed in conjunction with activity and conductivity Zinc iodide permits the nominal dissolution of up to 27 m I- into the photoelectrolyte. Lithium polyiodide electrolytes enhance observed photocurrent and appear to be the preferred cation at conventional illumination. The effects of other cations, Mg2+, NH4+, H+, Cd2+, and Ca2+ on illuminated WSe2 in polyiodide are also reported, and the passivation by certain CaI2 electrolytes and the decomposition of certain CdI2 electrolytes discussed.