Publications

1D multiwall tungsten disulfide nanotubes were discovered more than three decades ago, and their large-scale fabrication has been worked out systematically over the years. Building on this knowledge, reports on the more complex coreshell nanotubular structures have emerged in recent years. In the present work, coreshell tungsten disulfide and tungsten diselenide nanotubes (WS₂@WSe₂NTs) have been materialized through high-temperature selenidation of WS₂nanotubes and fine WO₃powder, yielding large quantities of epitaxially aligned heterostructures. Advanced high-resolution electron microscopy revealed the formation of complex heterointerfaces with well-defined atomic arrangements between the constituent phases. Optical characterization shows distinctive electronic properties arising from the heterostructure architecture, indicating strong interfacial coupling between the WS₂and WSe₂constituents. The unique electronic and structural characteristics of these nanotubes position them as promising candidates for enhanced photocatalytic applications and quantum device integration, where precise control over electronic states and charge transfer dynamics is critical.

Tungsten disulfide is an important compound with layered structure (2D-material). Chemically it is less stable than tungsten trioxide and hence undergoes oxidation upon exposure to the ambient atmosphere. Although nanoparticles of tungsten disulfide are metastable, hollow-closed nanostructures, i.e. fullerene-like WS₂ (IF-WS₂) and nanotubes thereof are kinetically stabilized because they present their basal (001) surface to the outer world. In this work, the oxidation of IF-WS₂ and nanotubes stored under different conditions for 1726 years have been studied. It was found that the degree of oxidation depends on the storage conditions. In fact, nanoparticles which were stored for long time and protected from the ambient atmosphere oxidized only marginally, while others kept under less favorable conditions suffered severe oxidation. Furthermore, the degradation of the IF nanoparticles is autocatalytic with clear acceleration over time. This work has also practical significance, since the shelf-life of IF-WS₂ nanoparticles, which are used as solid-state lubricants commercially have not been studied before.

Inorganic layered compounds (2D-materials), particularly transition metal dichalcogenide (TMDC), are the focus of intensive research in recent years. Shortly after the discovery of carbon nanotubes (CNTs) in 1991, it was hypothesized that nanostructures of 2D-materials can also fold and seam forming, thereby nanotubes (NTs). Indeed, nanotubes (and fullerene-like nanoparticles) of WS₂ and subsequently from MoS₂ were reported shortly after CNT. However, TMDC nanotubes received much less attention than CNT until recently, likely because they cannot be easily produced as single wall nanotubes with well-defined chiral angles. Nonetheless, NTs from inorganic layered compounds have become a fertile field of research in recent years. Much progress has been achieved in the high-temperature synthesis of TMDC nanotubes of different kinds, as well as their characterization and the study of their properties and potential applications. Their multiwall structure is found to be a blessing rather than a curse, leading to intriguing observations. This concise minireview is dedicated to the recent progress in the research of TMDC nanotubes. After reviewing the progress in their synthesis and structural characterization, their contributions to the research fields of energy conversion and storage, polymer nanocomposites, andunique optoelectronic devices are being reviewed. These studies suggest numerous potential applications for TMDC nanotubes in various technologies, which are briefly discussed.

Nanocomposite materials, integrating nanoscale additives into a polymer matrix, hold immense promise for their exceptional property amalgamation. This study delves into the fabrication and characterization of polyetherimide (PEI) nanocomposite strings fortified with multiwall WS₂ nanotubes. The manufacturing process capitalizes on the preferential alignment of WS₂ nanotubes along the string axis, corroborated by scanning electron microscopy (SEM). Mechanical measurements unveil a remarkable acceleration of strain hardening in the nanocomposite strings, chiefly attributed to the WS₂ nanotubes. Structural analyses via X-ray diffraction (XRD) and wide-angle X-ray scattering (WAXS) reveal intriguing structural alterations during tensile deformation. Notably a semi-crystalline framework ∼100 nm in diameter surrounding the WS₂ nanotubes emerges, which is stabilized by the π-π interactions between the PEI chains. The amorphous majority phase (97% by volume) undergoes also major structural changes upon strain becoming more compact and closing-up of the distance beweeetn the PEI chains. Dynamic mechanical analysis (DMA) demonstrates improved thermal stability of the evolved semi-crystalline π-π oriented PEI molecules, characterized by delayed thermal \u201cstructural melting\u201d, underscoring the pivotal role of the WS₂ nanotubes in reinforcing the nanocomposite. The insight gained in this study of WS₂ nanotube-reinforced PEI nanocomposite strings, could offer diverse applications for such tailor-made polymeric materials.

Uranium is a high-value energy element, yet also poses an appreciable environmental burden. The demand for a straightforward, low energy, and environmentally friendly method for encapsulating uranium species can be beneficial for long-term storage of spent uranium fuel and a host of other applications. Leveraging on the low melting point (60 °C) of uranyl nitrate hexahydrate and nanocapillary effect, a uranium compound is entrapped in the hollow core of WS₂ nanotubes. Followingly, the product is reduced at elevated temperatures in a hydrogen atmosphere. Nanocrystalline UO₂ nanoparticles anchor within the WS₂ nanotube lumen are obtained through this procedure. Such methodology can find utilization in the processing of spent nuclear fuel or other highly active radionuclides as well as a fuel for deep space missions. Moreover, the low melting temperatures of different heavy metal-nitrate hydrates, pave the way for their encapsulation within the hollow core of the WS₂ nanotubes, as demonstrated herein.

The inclusion of tungsten disulphide nanotubes (WS₂ NTs) in chitosan, plasticized with glycerol, facilitates the formation of a polyelectrolyte complex. The glycerol interrupts the intramolecular hydrogen bonding between chitosan chains allowing positively charged protonated amines of chitosan to form a complex with negatively charged oxygen ions chemisorbed to the tungsten atoms in defects. These interactions, with the unique mechanical and chemical properties of WS₂ NTs, result in a chitosan film with superior properties relative to unfilled chitosan. Even at low WS₂ NT loadings (≤1 wt%), the Young's modulus (E) increases by 59%, tensile strength (σ) by 40% and tensile toughness by 74%, compared to neat chitosan, without sacrificing ductility. Addition of highly dispersed WS₂ NTs significantly improves the gas barrier properties of chitosan, with a 50% reduction in oxygen permeability, while the addition of both glycerol and WS₂ NTs to chitosan effectively reduces the carbon dioxide permeability by 80% and the water vapor transmission rate by 90%. The intrinsic antimicrobial efficacy of chitosan against both Gram-positive and Gram-negative bacteria is enhanced on inclusion of WS₂ NTs. Polyelectrolyte complexation of WS₂ NTs and glycerol-plasticized chitosan provides a cost-effective, sustainable route to biodegradable films with desirable mechanical, gas barrier properties, and antimicrobial efficacy suitable for food packaging applications.

WS₂ nanotubes present many new technologies under development, including reinforced biocompatible polymers, membranes, photovoltaic-based memories, ferroelectric devices, etc. These technologies depend on the aspect ratio (length/diameter) of the nanotubes, which was limited to 100 or so. A new synthetic technique is presented, resulting in WS₂ nanotubes a few hundred micrometers long and diameters below 50 nm (aspect ratios of 2000-5000) in high yields. Preliminary investigation into the mechanistic aspects of the two-step synthesis reveals that W₅O₁₄ nanowhisker intermediates are formed in the first step of the reaction instead of the ubiquitous W₁₈O₄₉ nanowhiskers used in the previous syntheses. The electrical and photoluminescence properties of the long nanotubes were studied. WS₂ nanotube-based paper-like material was prepared via a wet-laying process, which could not be realized with the 10 μm long WS₂ nanotubes. Ultrafiltration of gold nanoparticles using the nanotube-paper membrane was demonstrated.

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.

Functionalisation of nanofillers is required for the promotion of strong interfacial interactions with polymers and is essential as a route for the preparation of (nano)composites with superior mechanical properties. Tungsten disulphide nanotubes (WS₂ NTs) were functionalized using (3-aminopropyl) triethoxysilane (APTES) for preparation of composites with poly(lactic acid) (PLA). The WS₂ NTs : APTES ratios used were 1 : 1, 1 : 2 and 1 : 4 WS₂ NTs : APTES. The APTES formed siloxane networks bound to the NTs via surface oxygen and carbon moieties adsorbed on the WS₂ NTs surface, detected by X-ray photoelectron spectroscopy (XPS) studies and chemical mapping using energy dispersive X-ray spectroscopy in the scanning transmission electron microscope (STEM-EDS). The successful silane modification of the WS2 NTs was clearly evident with both significant peak shifting by as much as 60 cm(-1) for Si-O-Si vibrations (FTIR) and peak broadening of the A(1g) band in the Raman spectra of the WS2 NTs. The evolution of new bands was also observed and are associated with Si-CH₂-CH₂ and, symmetric and assymetric -NH³⁺ deformation modes (FTIR). Further evidence for functionalization was obtained from zeta potential measurements as there was a change in surface charge from negative for pure WS₂ NTs to positive for APTES modified WS₂ NTs. Additionally, the thermal stability of APTES was shifted to much higher temperatures as it was bound to the WS₂ NTs. The APTES modified WS2 NTs were organophilic and readily dispersed in PLA, while presence of the pendant amine and hydroxyl groups resulted in strong interfacial interactions with the polymer matrix. The inclusion of as little as 0.5 wt% WS2 NTs modified with 2.0 wt% APTES resulted in an increase of 600% in both the elongation at break (a measure of ductility) and the tensile toughness relative to neat PLA, without impacting the stiffness or strength of the polymer.

The synthesis of fundamentally small MoS₂ nanotubes and nanocones(horns) that have proven elusive in prior studies has been achieved via ablation of a precursor mixture of crystallites of MoS₂ + MoO₃ by highly concentrated solar radiation. The special far-from-equilibrium conditions achieved in the solar furnace prove conducive to the generation of these singular nanostructures. Extensive electron microscopy and characterization results (transmission electron microscopy (TEM), electron diffraction (ED), X-ray diffraction (XRD), scanning TEM (STEM), and high angle annular dark field (HAADF)) reveal a range of nanoparticle shapes and sizes based on which reaction mechanisms are proposed. Molecular dynamics simulations indicate that the sizable thermal fluctuations intrinsically produced in the high-temperature solar reactor soften the MoS₂ nanostructures, yielding corrugated layers that favor nanostructures with only a few layers, in agreement with the experimental observations.

Cationic photo-initiated and polymerized epoxies are characterized by good adhesion, high modulus, zero volatiles, low shrinkage and living polymerization characteristics. Radiationcured acrylate resins are characterized by rapid initial curing with increased initial strength. The combination of radiation-cured acrylates and epoxies may present advantageous attributes. Thus, the system investigated is a hybrid epoxy/methyl acrylate and three different initiators for cationic polymerization of epoxies, the radical reaction of acrylates and the thermal initiator. When incorporating additives like opaque WS₂ nanoparticles (NPs), absorption of the photo radiation takes place, which may lead to low photo activity. Curing kinetics measurements revealed that the absorbing/masking effect of WS₂ was insignificant, and surprisingly, the level of curing was enhanced when the WS₂ NPs were incorporated. FTIR results demonstrated that covalent bonds were formed between the inorganic fullerenes (IF-WS₂) and the crosslinked matrix. Viscosity measurements showed a surprising reduction of five to ten times in the low-shear viscosity upon NPs incorporation compared to neat resins. It was concluded that the decrease of viscosity by the inorganic NPs, in addition to the enhanced level of conversion, has profound advantages for structural adhesives and 3D printing resins. To the best of our knowledge, this investigation is the first to report on a radiation-induced curing system containing opaque WS₂ NPs that leads to an enhanced degree of curing and reduced shear viscosity.

Even though WS₂nanotubes (NTs-WS₂) have great potential as anode materials for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) thanks to their unusual layered structure, their conductivity and cycling stability are far from satisfactory. To tackle these issues, carbon-coated WS₂(NTs-WS₂@C) nanocomposites were prepared through a facile synthesis method that involved precipitating a carbon precursor (20% sucrose) on WS₂nanotubes, followed by annealing treatment under an argon environment. Thanks to the presence of highly conductive and mechanically robust carbon on the outer surface, NTs-WS₂@C nanocomposites show improved electrochemical performance compared with bare NTs-WS₂. After 60 cycles at 80 mA g^-1current density, the cells display high capacities of 305 mAh g^-1in LIBs and 152 mAh g^-1in SIBs, respectively. As the current density increases to 600 mA g^-1, it provides specific capacities of 209 and 115 mAh g^-1, correspondingly. The enhanced electrochemical performance in LIBs and SIBs is primarily attributed to the synergistic effects of the tubular architecture of WS₂, carbon network and stable nanocomposite structure, which can effectively constrain volume variation during the metal ions intercalation/deintercalation processes.

Nanotubes of transition metal dichalcogenides such as WS2 and MoS2 offer unique quasi-1D properties and numerous potential applications. Replacing sulfur by selenium would yield ternary WS2(1x)Se2x (0 ≤ x ≤ 1; WSSe) nanotubes, which are expected to reveal strong modulation in their absorption edge as a function of selenium content, x Se. Solid WO2.72 oxide nanowhiskers were employed as a sacrificial template to gain a high yield of the nanotubes with a rather uniform size distribution. Though sulfur and selenium belong to the same period, their chemical reactivity with oxide nanowhiskers differed appreciably. Here, the closed ampoule technique was utilized to achieve the completion of the solidvapor reaction in short time scales instead of the conventional flow reactor method. The structure and chemical composition of the nanotubes were analyzed in detail. X-ray and electron diffractions indicated a systematic modulation of the WSSe lattice upon increasing the selenium content. Detailed chemical mapping showed that the sulfur and selenium atoms are distributed in random positions on the anion lattice site of the nanotubes. The optical excitonic features and absorption edges of the WSSe nanotubes do not vary linearly with the composition x Se, which was further confirmed by density functional theory calculations. The WSSe nanotubes were shown to exhibit strong lightmatter interactions forming excitonpolariton quasiparticles, which was corroborated by finite-difference time-domain simulations. Transient absorption analysis permitted following the excited state dynamics and elucidating the mechanism of the strong coupling. Thus, nanotubes of the ternary WSSe alloys offer strong band gap tunability, which would be useful for multispectral vision devices and other optoelectronic applications.

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.

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.

Asymmetric two-dimensional (2D) structures (often named Janus), like SeMoS and their nanotubes, have tremendous scope in material chemistry, nanophotonics, and nanoelectronics due to a lack of inversion symmetry and time-reversal symmetry. The synthesis of these structures is fundamentally difficult owing to the entropy-driven randomized distribution of chalcogens. Indeed, no Janus nanotubes were experimentally prepared, so far. Serendipitously, a family of asymmetric misfit layer superstructures (tubes and flakes), including LaX-TaX2 (where X = S/Se), were synthesized by high-temperature chemical vapor transport reaction in which the Se binds exclusively to the Ta atoms and La binds to S atoms rather than the anticipated random distribution.With increasing Se concentration, the LaS-TaX2 misfit structure gradually transformed into a new LaS-TaSe2-TaSe2 superstructure. No misfit structures were found for xSe = 1. These counterintuitive results shed light on the chemical selectivity and stability of misfit compounds and 2D alloys, in general. The lack of inversion symmetry in these asymmetric compounds induces very large local electrical dipoles. The loss of inversion and time-reversal symmetries in the chiral nanotubes offers intriguing physical observations and applications.

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 metalNT-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 (

Analyzing naturally occurring layered alumo(magnesium)silicates (asbestos) minerals, Pauling proposed in 1930 that the strain in compounds with asymmetric structure, like kaolinite (hallyosite), lends itself for folding of the layers. Once transmission electron microscopy gained sufficient resolution in 1950, chrysotile and hallyosites nanotubes were discovered fully confirming this early hypothesis. Following the discovery of carbon fullerene (C₆₀) by Kroto and Smalley and carbon nanotubes by Iijima, Tenne proposed a new mechanism for the formation of nanotubes from inorganic compounds with layered structure via seaming of their chemically reactive rims. Early on, nanotubes of WS₂ and subsequently MoS₂, GaSe and BN were found experimentally and in silico and many others followed over the years. Here, different mechanisms for the formation of nanotubes from inorganic compounds with layered structure are analyzed with a few examples. Few potential applications of such nanotubes are briefly discussed, as well. Finally, several frontiers and scientific challenges in this field in the years ahead are presented

The synthesis of high-quality WS2 and more so of MoS2 (multiwall) nanotubes in substantial amounts from oxide precursors is a very challenging and important undertaking. While progress has been offered by a recent report, the present work presents another step forward in the synthesis of MoS2 nanotubes with a narrow size distribution and better crystallinity than before. WilliamsonHall analysis of the X-ray diffraction data is used to analyze the crystallinity and strain in the nanotubes. This analysis shows that the crystallinity and average diameter of the WS2 and MoS2 nanotubes reported here (type II) are better than those obtained according to the previous methods (type I). Size selection by centrifugation reported by others is used here to prepare several fractions of WS2 and MoS2 nanotubes according to their average diameter. The high refractive index of WS2 and more so MoS2 enables the nanotubes to trap light by total internal reflection, turning them into nanocylindrical resonators and thereby supporting cavity-mode resonances. The extinction, net absorption, and transient absorption of suspensions of MoS2 and WS2 nanotubes of different (average) diameters were investigated. A strong coupling effect between optical cavity modes and the A and B excitons was observed for the WS2 and MoS2 nanotubes with diameters above 80 and 60 nm, respectively. These conclusions are also supported by transient absorption measurements. Finite-difference time-domain (FDTD) calculations support the experimental findings, confirming the strong coupling effect in the WS2 and MoS2 nanotubes. These results are important not only for their own sake but also because they may bear on the new photocatalytic applications of such nanotubes.

The non-stoichiometric misfit layered compounds (MLC) of the general formula ((MX)(1+y))(m)(TX2)(n) (abbreviated herein as MX-TX2) have been investigated quite extensively over the last 30 years. Here MX is a atomic slab of a material with distorted rocksalt structure and TX2 is a layered compound with hexagonal (octahedral) coordination between the metal T atom and the chalcogen X atom. Recognizing the mismatch between the two (MX and TX2) sublattices, nanotubes from the MLC of different compositions were described in the past. In particular, semimetallic nanotubes belonging to the family LnX-TaX2 with Ln = rare earth atom and X = S, Se, Te have been studied in the past. While some of them, like LaS-TaS2 were obtained with moderately high yields, others like YbS-TaS2 were scarce. In the present study, a new strategy for promoting the yield of such MLC nanotubes by alloying the LaS sublattice with another Ln atom is proposed. Detailed transmission electron microscopy investigation of the (mixed) Ln(x)La((1-x))S-TaS2 (Ln = Pr, Sm, Ho, Yb) nanotubes show clearly that the substituting Ln atom resides in the rocksalt LaS sublattice of the nanotubes. Raman measurements show distinct differences between mixed tubes with open-shell (Pr, Sm, Ho) and close-shell (La, Yb) rare-earth atoms. Density functional calculations show that the interplay between two important factors determine the enhanced stability of the mixed nanotubes- the size and electronic structure of the substituting rare-earth atom. The smaller is the substituting rare-earth atom (larger Z number), the more dissimilar it is to the original La atom. This dissimilarity enhances the incommensurability between the Ln(x)La((1-x))5 and the TS2 subunits, promoting thereby the stability of the mixed MLC. However, the electronic structure of the Ln atom was found to play a more significant role. The MLC lattice of the LaS-TaS2 is electron-rich and consequently the 4d(z)(2) level of Ta is full. The unoccupied 4f levels of the substituent open-shell atoms (Pr, Sm, Ho), which are positioned below the Fermi level, serve as electron acceptors. Consequently, the Ln substitution is found to enhance the stability of the mixed lattice and nanotubes thereof. This strategy can be employed for enhancing the yield of these and other misfit nanotubes using different substituents of the right size and energy profile.

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.

We present the analysis of a family of nanotubes (NTs) based on the quaternary misfit layered compound (MLC) YxLa1-xS-TaS2. The NTs were successfully synthesized within the whole range of possible compositions via the chemical vapor transport technique. In-depth analysis of the NTs using electron microscopy and spectroscopy proves the in-phase (partial) substitution of La by Y in the (La,Y)S subsystem and reveals structural changes compared to the previously reported LaS-TaS2 MLC-NTs. The observed structure can be linked to the slightly different lattice parameters of LaS and YS. Raman spectroscopy and infrared transmission measurements reveal the tunability of the plasmonic and vibrational properties. Density-functional theory calculations showed that the YxLa1-xS-TaS2 MLCs are stable in all compositions. Moreover, the calculations indicated that substitution of La by Sc atoms is electronically not favorable, which explains our failed attempt to synthesize these MLC and NTs thereof.

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.

Multiwall WS2 nanotubes (and fullerene-like nanoparticles thereof) are currently synthesized in large amounts, reproducibly. Other than showing interesting mechanical and tribological properties, which offer them a myriad of applications, they are recently shown to exhibit remarkable optical and electrical properties, including quasi-1D superconductivity, electroluminescence, and a strong bulk photovoltaic effect. Here, it is shown that, using a simple dispersion-fractionation technique, one can control the diameter of the nanotubes and move from pure excitonic to polaritonic features. While nanotubes of an average diameter >80 nm can support cavity modes and scatter light effectively via a strong coupling mechanism, the extinction of nanotubes with smaller diameter consists of pure absorption. The experimental work is complemented by finite-difference time-domain simulations, which shed new light on the cavity mode-exciton interaction in 2D materials. Furthermore, transient absorption experiments of the size-fractionated nanotubes fully confirm the steady-state observations. Moreover, it is shown that the tools developed here are useful for size control of the nanotubes, e.g., in manufacturing environment. The tunability of the light-matter interaction of such nanotubes offers them intriguing applications such as polaritonic devices, in photocatalysis, and for multispectral sensors.

Strong coupling of electric transition dipoles with optical or plasmonic resonators modifies their light-matter interaction and, therefore, their optical spectra. Semiconducting WS2 nanotubes intrinsically provide the dipoles through their excitonic resonances, and the optical cavity via their cylindrical shape. We investigate the nonequilibrium light-matter interaction in WS2 nanotubes in the time domain using femtosecond transient extinction spectroscopy. We develop a phenomenological coupled oscillator model with time-dependent parameters to describe the transient extinction spectra, allowing us to extract the underlying nonequilibrium electron dynamics. We find that the exciton and trion resonances shift due to many-body effects of the photogenerated charge carriers and their population dynamics on the femto- A nd picosecond timescale. Our results show that the time-dependent phenomenological model quantitatively reproduces the nonequilibrium optical response of strongly coupled systems.

The photovoltaic effect in traditional pn junctionswhere a p-type material (with an excess of holes) abuts an n-type material (with an excess of electrons)involves the light-induced creation of electronhole pairs and their subsequent separation, generating a current. This photovoltaic effect is particularly important for environmentally benign energy harvesting, and its efficiency has been increased dramatically, almost reaching the theoretical limit1. Further progress is anticipated by making use of the bulk photovoltaic effect (BPVE)2, which does not require a junction and occurs only in crystals with broken inversion symmetry3. However, the practical implementation of the BPVE is hampered by its low efficiency in existing materials410. Semiconductors with reduced dimensionality2 or a smaller bandgap4,5 have been suggested to be more efficient. Transition-metal dichalcogenides (TMDs) are exemplary small-bandgap, two-dimensional semiconductors11,12 in which various effects have been observed by breaking the inversion symmetry inherent in their bulk crystals1315, but the BPVE has not been investigated. Here we report the discovery of the BPVE in devices based on tungsten disulfide, a member of the TMD family. We find that systematically reducing the crystal symmetry beyond mere broken inversion symmetrymoving from a two-dimensional monolayer to a nanotube with polar propertiesgreatly enhances the BPVE. The photocurrent density thus generated is orders of magnitude larger than that of other BPVE materials. Our findings highlight not only the potential of TMD-based nanomaterials, but also more generally the importance of crystal symmetry reduction in enhancing the efficiency of converting solar to electric power.

Advanced nanomaterials play a prominent role in nanoscience and nanotechnology developments, opening new frontiers in these areas. Among these nanomaterials, due to their unique characteristics and enhanced chemical and physical properties, inorganic nanotubes have been considered one of the most interesting nanostructures. In recent years, important progress has been achieved in the production and study of these nanomaterials, including boron nitride, transition metal dichalcogenide nanotubular structures, misfit-based nanotubes and other hybrid/doped nanotubular objects. This review is devoted to the in-depth analysis of recent studies on the synthesis, atomic structures, properties and applications of inorganic nanotubes and related nanostructures. Particular attention is paid to the growth mechanism of these nanomaterials. This is a crucial point for the challenges ahead related to the mass production of high-quality defect-free nanotubes for a variety of applications.

Orthodontic teeth movement that is performed by a sliding archwire-bracket system is always affected by an inevitable friction (FR) force created at the wire-bracket interface. Overcoming the frictional impediment requires application of increased orthodontic load. Excessive force can cause undesirable teeth movement, increase root resorption depending on individual response, as well as inefficient treatment. To date, attempts have mostly been directed at reducing FR by altering the bracket design. These attempts have been successful to some extent, although no satisfactory solution for FR reduction of both stainless steel and nickel titanium wires was achieved. Therefore, the development of wires with low FR coefficients is of great importance. This has been successfully achieved by coating the orthodontic wires with inorganic fullerene-like tungsten disulfide nanoparticles (IF-WS₂ NP). The IF-WS₂ NP was first described by Prof. Reshef Tenne in 1992. It was shown that under certain reducing and sulfidizing conditions at elevated temperatures, tungsten oxide (WO₃) nanoparticles could form nested IF-WS₂ nanostructures creating layers that resemble an onion or a nanotube. The layers are weakly connected through van der Waals forces only. The IF-WS₂ NPs provide excellent lubricity and allow remarkable improvement of FR and wear properties under different contact conditions. Coating the wires by using an adhesion matrix (nickel or cobalt) into which IF-WS₂ NP were impregnated, brought about a significant reduction of the orthodontic force required to move the coated wire along the bracket. The toxicity of the IF-WS₂ NP was tested at various modes of exposure in animals and in vitro in human cell cultures. The results obtained showed no toxic effect of IF-WS₂ NP in all the performed tests. Obviously, biocompatibility approval from the relevant regulatory agencies is mandatory for future clinical use of IF-WS₂ NP-coated orthodontic appliances.

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.

Transition metal dichalcogenide materials have recently been shown to exhibit a variety of intriguing optical and electronic phenomena. Focusing on the optical properties of semiconducting WS2 nanotubes, we show here that these nanostructures exhibit strong light-matter interaction and form exciton-polaritons. Namely, these nanotubes act as quasi 1-D polaritonic nano-systems and sustain both excitonic features and cavity modes in the visible-near infrared range. This ability to confine light to subwavelength dimensions under ambient conditions is induced by the high refractive index of tungsten disulfide. Using "finite-difference time-domain'' (FDTD) simulations we investigate the interactions between the excitons and the cavity mode and their effect on the extinction spectrum of these nanostructures. The results of FDTD simulations agree well with the experimental findings as well as with a phenomenological coupled oscillator model which suggests a high Rabi splitting of similar to 280 meV. These findings open up possibilities for developing new concepts in nanotube-based photonic devices.

In this paper, we demonstrate the formation of hybrid nanostructures consisting of two distinctive components mainly in a one-to-one ratio. Thermolysis of inorganic nanotubes (INT) and closed-cage, inorganic fullerene-like (IF) nanoparticles decorated with a dense coating of metallic nanoparticles (M = Au, Ag, Pd) results in migration of relatively small NPs or surface-enhanced diffusion of atoms or clusters, generating larger particles (ripening). AuNP growth on the surface of INTs has been captured in real time using in situ electron microscopy measurements. Reaction of the AuNP-decorated INTs with an alkylthiol results in a chemically induced NP fusion process at room temperature. The NPs do not dissociate from the surfaces of the INTs and IFs, but for proximate IFs we observed fusion between AuNPs originating from different IFs.

Due to their favourable and rich electronic and optical properties, group-VI-B transition-metal dichalcogenides (TMDs) have attracted considerable interest. They have earned their position in the materials portfolio of the spintronics and valleytronics communities. The electrical performance of TMDs is enhanced by rolling up the two-dimensional (2D) sheets to form quasi-one-dimensional (1D) tubular structures. The fabrication of p-n junctions out of these tubular TMDs would boost their potential for optoelectronic devices as such junctions represent a fundamental building block. Here, we report the realization of a p-n junction out of a single, isolated WS₂-nanotube (WS₂-NT). Light-emitting diode operation and photovoltaic behaviour were observed based on such p-n junctions. The emitted light as well as the photovoltaic effect exhibit strong linear polarization characteristics due to the quasi-1D nature. The external quantum efficiency for the photovoltaic effect reaches a value as high as 4.8%, exceeding by far that of 2D TMDs and even approaching the internal quantum efficiency of the 2D TMDs. This efficiency improvement indicates that TMD nanotubes are superior candidates over 2D TMDs for optoelectronic applications.

We report here a unique and efficient methodology for the surface functionalization of closed-cage inorganic fullerene-like (IF) nanoparticles and inorganic nanotubes (INTs) composed of two-dimensional nanomaterials of transition-metal chalcogenides (MS
₂; M = W or Mo). The first step is the physical coverage of these robust inorganic materials with monodispersed and dense monolayers of gold, silver, and palladium nanoparticles. The structural continuity at the interface between the IF/INT and the metallic nanoparticles is investigated. Lattice matching between these nanocrystalline materials and strong chemical affinity lead to efficient binding of the metallic nanoparticles onto the outer sulfide layer of the MS
₂-based structures. It is shown that this functionalization results in narrowing of the IF/INT optical band gap, increased work function, and improved surface-enhanced Raman scattering. In the second step, functionalization of the surface-bound nanoparticles is carried out by a ligand-exchange reaction. This ligand exchange involving the tetraoctylammonium bromide capping layer and an alkyl thiol enhances the solubility (∼10×) of the otherwise nearly insoluble materials in organic solvents. The scope of this method is further demonstrated by introducing a ruthenium(II) polypyridyl complex on the surface of the surface-bound AuNPs to generate fluorescent multicomponent materials.

This study conducted tensile and hardness tests to compare the mechanical properties of Mg alloys and metal matrix composites with and without WS₂ or multi-walled carbon nanotubes. We also examined the microscopic structures to investigate their failure characteristics. Our results indicate that AZ61 presented the best mechanical properties following the addition of WS₂ and solution heat treatment, and increasing the content of WS₂ improved the mechanical properties. This performance of this combination even outperformed that of metal matrix composites containing multi-walled carbon nanotubes. This study developed an image processing method, in which X-ray images of the metal matrix composite specimens undergo filtering, binarization, edge enhancement, and morphological calculations to determine the area of internal defects in Mg metal matrix composites. The results indicated reduced percentage of internal defect area values in AZ31-T4 and AZ61-T4 following the addition of 0.2% WS₂ inorganic nanotube (INT). X-ray diffraction results revealed that the solution heat treatment process causes the WS₂ to dissolve into the Mg base, thereby shifting the peak to the right. This improves solid solution strengthening and enhances the mechanical properties. Further observation of the microscopic structures indicated that the AZ61-T4 possesses finer grains than does the AZ31-T4, which is consistent with the existing literature.

Tins work decribes the enhancement of the electrical figures of merit of an Electrolyte Gated Thin-Film Transistor (EG-TFT) comprising a nanocomposite of n-type tungsten disulfide (WS2) nanotubes (NTs) dispersed in a regio-regular p-type poly(3-hexylthiophene-2,5-diyl) (P3HT) polymeric matrix. P3HT/WS2 nanocomposites loaded with different concentrations of NTs, serving as EG-TFTs electronic channel materials have been studied and the formulation has been optimized. The resulting EG-TFTs figures of merit (field-effect mobility, threshold voltage and on off ratio) are compared with those of the device comprising a bare P3HT semiconducting layer The optimized P3HT/WS2 nanocomposite, comprising a 60% by weight of NTs; results in an improvement of all the elicited figures of merit with a striking ten-fold increase in the field-effect mobility and the on/off ratio along with a sizable enhancement of the in-water operational stability of the device,

The nature of the interface between a reinforcement and the matrix is crucial in determining the degree of dispersion and mechanical properties. Controlling the interface is particularly important when 2D materials are used as reinforcements, as their flat nature eliminates bonding mechanisms such as mechanical lock-in. For example, we have previously shown that unfunctionalised graphene in an epoxy matrix has just a tenth of the interfacial strength of a carbon fibre. Herein, we have developed chemical functionalization of two-dimensional (2D) materials as a route to control their interface with thermosets, thermoplastics and elastomers. One well-established method for functionalizing graphene is the use of aryl diazonium chemistry. This functional step is typically applied on as-made powder, however we have found that graphene can be simultaneously functionalized and exfoliated from graphite by electroreduction. This one-step process prevents aggregation, increases thin-flake yield and leads to high quality dispersions. However, similarly structured inorganic 2D materials such as transition metal dichalcogenides (TMDs) have generally been considered to be chemically inert, with few studies into the covalent modification and functionalization of them. Previously, it was believed that only the 1T phase of TMDs could be functionalised by aryl diazonium due to the difference in electronic structure. However, in this work we demonstrate that the more stable and naturally occurring 2H-phase can be successfully functionalised via aryl diazonium chemistry. We have measured the interfacial strength of these functionalized materials within different matrices using Raman mapping of in-situ mechanical tests of single flakes and applying a simple shear lag model. Bulk composites were then produced with flakes processing different aspect ratios to prove that these micromechanics approaches can be translated to the macroscale.

Keywords: Chemistry, Multidisciplinary

The study of inorganic nanometer-scale materials with hollow closed-cage structures, such as inorganic fullerene-like (IF) nanostructures and inorganic nanotubes (INTs), 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 roomerature 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.

Owing to their outstanding variety of properties, nanoparticles attracted considerable interest for developing a new age of novel polymer nanocomposites and adhesives. Among them, inorganic nanotubes (INTs) of tungsten disulfide (WS₂) and carbon nanotubes (CNTs) have been identified as unique candidates for many industrial applications by virtue of their superior mechanical, thermal and tribological properties. In this work, a comparative study between INT-WS₂ and multi-walled carbon nanotubes (MWCNTs) regarding the properties of structural polyurethane (PU) adhesives is presented. Specifically, we evaluate the thermomechanical properties, chemical and micro-phase structure, bonded joints performance in lap shear and peel modes, and fracture mechanisms, with aiming to highlight some of the differences between these nanotubes. The added content spanned over the range of 0.3, 0.6, 0.9 and 1.2 wt% of INT-WS₂, while CNTs are incorporated at the same volumetric fractions (e.g., 0.1, 0.2, 0.3 and 0.4 wt%). According to the results of the tests, in all measured aspects, INT-WS₂ outperformed the carbon counterparts. With regards to the thermomechanical attributes, the hard segment Tg of the PU increased by 14 °C at 0.6 wt% INTs, whereas CNTs recorded only modest improvement of 8 °C at the nanocomposite loaded with 0.4 wt%. The structural bonding performance indicates an improvement of 117% and 38% in the shear and peel strengths at 0.9 and 1.2 wt% INTs, respectively, while CNTs show moderate changes. Interestingly, the changes are also reflected by the interplay between the hard and soft segments of the PU, and different fracture mechanisms operating within the nanocomposites. At any rate, the combined attributes of >20 MPa in the shear strength and approximately 2 N/mm in (average) peel strength, targets the PU/INT nanoadhesives as promising alternative for conventional epoxy systems which has similar shear strength with inferior peel strength.

Chirality of materials are known to affect optical, magnetic and electric properties, causing a variety of nontrivial phenomena such as circular dichiroism for chiral molecules, magnetic Skyrmions in chiral magnets and nonreciprocal carrier transport in chiral conductors. On the other hand, effect of chirality on superconducting transport has not been known. Here we report the nonreciprocity of superconductivity-unambiguous evidence of superconductivity reflecting chiral structure in which the forward and backward supercurrent flows are not equivalent because of inversion symmetry breaking. Such superconductivity is realized via ionic gating in individual chiral nanotubes of tungsten disulfide. The nonreciprocal signal is significantly enhanced in the superconducting state, being associated with unprecedented quantum Little-Parks oscillations originating from the interference of supercurrent along the circumference of the nanotube. The present results indicate that the nonreciprocity is a viable approach toward the superconductors with chiral or noncentrosymmetric structures.

We study for the first time the resonant torsional behaviors of inorganic nanotubes, specifically tungsten disulfide (WS₂) and boron nitride (BN) nanotubes, and compare them to that of carbon nanotubes. We have found WS₂ nanotubes to have the highest quality factor (Q) and torsional resonance frequency, followed by BN nanotubes and carbon nanotubes. Dynamic and static torsional spring constants of the various nanotubes were found to be different, especially in the case of WS₂, possibly due to a velocity-dependent intershell friction. These results indicate that inorganic nanotubes are promising building blocks for high-Q nanoelectromechanical systems (NEMS).

In this report, a systematic study of the effects of p- and n-type doping of the inorganic fullerene (IF) MoS₂ on the efficiency of the hydrogen evolution reaction (HER) is described. Active edge site enriched IF-MoS₂, promoted by strategically introducing Nb (p-type) and Re (n-type) dopants (below 500 ppm), enables facile HER over a range of pH values. Experimental results suggest that although Nb-doping on IF-MoS₂ leads to better electrocatalytic HER activity in an alkaline medium with an onset potential difference of 80 mV, Re-doping gives excellent activity in an acidic medium. The present work presents a systematic study of HER activity by finely tuning the activity in different electrolyte media with varied pH values through deliberate doping of the parent catalyst with p- and n- type materials. The doped IF-MoS₂ catalysts exhibit excellent catalytic activity even with sea water as an electrolyte.

The relationship between structure and properties has been followed for different nanoscale forms of tungsten disulfide (2H-WS₂) namely exfoliated monolayer and few-layer nanoplatelets, and nanotubes. The similarities and differences between these nanostructured materials have been examined using a combination of optical microscopy, scanning and high-resolution transmission electron microscopy and atomic force microscopy. Photoluminescence and Raman spectroscopy have also been used to distinguish between monolayer and few-layer material. Strain induced phonon shifts have been followed from the changes in the positions of the A_1g and E_2g¹ Raman bands during uniaxial deformation. This has been modelled for monolayer using density functional theory with excellent agreement between the measured and predicted behaviour. It has been found that as the number of WS₂ layers increases for few-layer crystals or nanotubes, the A_1g mode hardens whereas the E_2g¹ mode softens. This is believed to be due to the A_1g mode, which involves out of plane atomic movements, being constrained by the increasing number of WS₂ layers whereas easy sliding reduces stress transfer to the individual layers for the E_2g¹ mode, involving only in-plane vibrations. This finding has enabled the anomalous phonon shift behaviour in earlier pressure measurements on WS₂ to be resolved, as well as similar effects in other transition metal dichalcogenides, such as molybdenum disulfide, to be explained.

Various nanotubes from ternary misfit compounds have been reported in recent years. In the present work, the detailed atomic structure and chemical configuration of misfit-layered nanotubes based on the TbS-CrS₂ are reported. These analyses have been developed via different transmission electron microscopy techniques, including high-resolution scanning transmission electron microscopy, electron diffraction, and electron energy loss spectroscopy. These structural analyses show that two different kinds of nanotubes can be produced: a "regular" nanotube and a "wavy" one. Both kinds of nanotubes show the alternating arrangements of the TbS and CrS₂ subsystems; however, the wavy ones present a nearly periodically deficiency in terbium. In addition to the structural investigation, the chemical analyses have proved that the outer layer of both kinds of nanotubes is composed of the elements Cr and S. All these findings helped to understand the growth mechanism during the sulfurization reaction taking place in the synthesis process.

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.

DC and AC electrical properties of hybrid films, consisting of carbon nanotubes and tungsten disulfide nanotubes (and fullerene like nanoparticles) were studied within the 2 - 300 K temperature range and over the 20 Hz - 1 MHz frequency range. The temperature dependences of the resistance R(T) exhibit behavior typical for the fluctuation-induced tunneling model in the intermediate temperature range. Analysis of the dependences of real and imaginary components of the impedance on the frequency (Z(f ) and Z(f )) demonstrates the rising role of the contact barriers between carbon nanotubes inside hybrid films, consisting of the carbon nanotubes and inorganic tungsten disulfide nanotubes as the temperature was decreased. The active component of the impedance was found to prevail in the AC electrical properties of the hybrid films, consisting of multiwall carbon nanotubes and WS₂ nanoparticles over the entire available temperature range.

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 °C. The new MMC nanocomposites 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.

Multilayer films of WS₂ nanotubes decorated with gold nanoparticles are prepared for the first time using nanocomposite assemblage on the water-heptane interface and film transition onto optically transparent or semiconducting surfaces. The film morphology resembles a mosaic structure of 10-25 square micron areas with in-plane textured nanotubes. Optical properties demonstrate several features around 490, 545, and 675 nm connected either with excitonic transitions or scattering guided by high anisotropy of the nanotubes and their texturing peculiarities in the films.

The incorporation of nanostructures into nanoelectronic and nanoelectromechanical systems is a long sought-after goal. In the present article, we report the first torsional electromechanical measurements of pure inorganic nanotubes. The WS₂ nanotubes exhibited a complex and reproducible electrical response to mechanical deformation. We combined these measurements with density-functional-tight-binding calculations to understand the interplay between mechanical deformation, specifically torsion and tension, and electrical properties of WS₂ nanotubes. This yielded the understanding that the electrical response to mechanical deformation may span several orders of magnitude on one hand and detect several modes of mechanical deformation simultaneously on the other. These results demonstrate that inorganic nanotubes could thus be attractive building blocks for nanoelectromechanical systems such as highly sensitive nanometric motion sensors.

We report the synthesis and supporting density-functional-theory computations for a closed-cage, misfit layered-compound superstructure from PbS-SnS₂, generated by highly concentrated sunlight from a precursor mixture of Pb, SnS₂, and graphite. The unique reactor conditions created in our solar furnace are found to be particularly conducive to the formation of these nanomaterials. Detailed structural and chemical characterization revealed a spontaneous inside-out formation mechanism, with a broad range of nonhollow fullerene-like structures starting at a diameter of ∼20 nm and a wall thickness of ∼5 layers. The computations also reveal a counterintuitive charge transfer pathway from the SnS₂ layers to the PbS layers, which indicates that, in contrast to binary-layered compounds where it is principally van der Waals forces that hold the layers together, polar forces appear to be as important in stabilizing superstructures of misfit layered compounds. (Figure Presented).

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. When two systems meet: Interactions between two systems comprising different in-plane periodicities can result in misfit layered compounds even if each subsystem alone is not a layered or stable compound. A combination of two independent stimuli, namely the incommensurability of the misfit lattice (often leading to folding and scrolling) and the reactivity of the layer rim atoms, provides a new strategy to synthesize misfit nanotubes.

Theoretical analysis of experimental data on unzipping multilayered WS ₂ 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.

Tubular structures of the MS-TaS₂ with (M = Pb, Sn, Sb, Bi) misfit layered compounds are reported. The lattice mismatch between the alternating MS and TaS₂ layers leads to a variety of chiral tubular structures. Such tubular structures are studied via scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). For the PbS-TaS₂ and SnS-TaS ₂ tubules, relative in-plane orientations as well as folding vectors of the two subsystems can be determined. However, almost ring-like SAED patterns are obtained for SbS-TaS₂ nanotubes precluding exact determination of the relative in plane orientation. Also, very complex diffraction patterns were obtained for BiS-TaS₂ nanotubes.

Ureteral stents and urethral catheters are commonly used medical devices for maintaining urinary flow. However, long-term placement (>30 days) of these devices in the urinary tracts is limited by the development of encrustation, a phenomenon that holds a prevalence of 50% within this patient population, resulting in a great deal of morbidity to the patients. Here we report the influence of surface coating of an all-silicone catheter with rhenium-doped fullerene-like molybdenum disulfide (Re:IF-MoS₂) nanoparticles on the growth and attachment of in vitro encrustation stones. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRD) analyses indicated a remarkable attenuation in encrustation occupation on the Re:IF-MoS₂-coated catheter surfaces compared to neat catheters. The doped nanoparticles displayed a unique tendency to self-assemble into mosaic-like arrangements, modifying the surface to be encrustation-repellent. The mechanism of encrustation retardation on the surface coated catheters is discussed in some detail. The ramification of these results for the clogging of other body indwelling devices is briefly discussed. This journal is

The synthesis of inorganic nanotubes (INT) from layered compounds of a small size (

Carbon fullerenes and nanotubes revolutionized understandingof the reactivity of nanoscale compounds. Subsequently, our group and others discovered analogous inorganic compounds with hollow, closed nanostructures. Such inorganic nanostructures offer many applications, particularly in the energy and electronics industries.One way to create inorganic nanostructures is via misfit layer-ed 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 (SAED) analyses, scanning electron microscopy (SEM), and Cs-corrected scanning transmission electron microscopy (STEM) in the high-angle annular dark-field mode (HAADF). In both new types, the lattice mismatch between the two alternating sublayers dictates the relative layer-stacking order and leads to a variety of chiral tubular structures. In particular, the incommensuration (a type of misfit) of the SnS₂/SnS system in both the (in plane) a and b directions leads to a variety of relative in-plane orientation and stacking orders along the common c-axis. Thus the SnS/SnS₂ nanotubes form superstructures with the sequence O-T and O-T-T, and mixtures thereof. We also report nanotubes of the misfit layered compound (PbS)_1.14NbS₂, and of NbS ₂ intercalated with Pb atoms, with the chemical formula PbNbS ₂. Thus, the possibility to use two kinds of folding mechanisms jointly offers a new apparatus for the synthesis of unique 1-D nanostructures of great complexity and a potentially large diversity of physicochemical properties.

Closed-cage (fullerene-like) nanoparticles (NPs) of WS₂ are currently produced in large amounts and were investigated as additives to thermoplastics and thermosetting polymers. The nanoinduced morphology and the resulting enhanced fracture toughness of epoxy/WS₂ nanocomposites were studied. The morphology of the epoxy nanocomposites was induced by controlled WS₂ surface chemistry. The WS₂ NPs used were either untreated or chemically treated with acryloxy, which is compatible, and alkyl silane, which is incompatible, respectively, with the epoxy matrix. In the case where the acryloxy silane was used to treat the WS₂ particles, good dispersion and compatibility were obtained in the epoxy resin_. Moreover, a distinct nodular morphology was induced on fracture as a result of nucleation by the compatible NPs. In the case where the alkyl silane treatment was used cavitation morphology was induced, following mechanical loading, which is the result of incompatibility with the epoxy resin. The fracture toughness results showed an increase of 70% for nanocomposites contains alkyl-treated WS₂ compared with the neat epoxy. Modeling of the nodular morphology enabled the determination of optimal concentration of the WS₂ in epoxy (0.3% by weight). Two main fracture mechanisms were observed, crack bowing around the nodular boundaries in the case of compatibility between the nanoparticle and the epoxy and particle-induced cavitation in the case of incompatibility, respectively. These results are of significant importance both for epoxy-based adhesives and fiber composites. POLYM. ENG. SCI., 53:2624-2632, 2013. © 2013 Society of Plastics Engineers

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.

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-MoS ₂ 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.

WS₂ nanoribbons have been synthesized by chemical unzipping of WS₂ nanotubes. Lithium atoms are intercalated in WS₂ nanotubes by a solvothermal reaction with n-butyllithium in hexane. The lithiated WS₂ nanotubes are then reacted with various solvents - water, ethanol, and long chain thiols. While the tubes break into pieces when treated with water and ethanol, they unzip through longitudinal cutting along the axes to yield nanoribbons when treated with long chain thiols, 1-octanethiol and 1-dodecanethiol. The slow diffusion of the long chain thiols reduces the aggression of the reaction, leading to controlled opening of the tubes.

We report the first transistor based on inorganic nanotubes exhibiting mobility values of up to 50 cm² V^-1 s^-1 for an individual WS₂ nanotube. The current-carrying capacity of these nanotubes was surprisingly high with respect to other low-dimensional materials, with current density at least 2.4 × 10⁸ A cm^-2. These results demonstrate that inorganic nanotubes are promising building blocks for high-performance electronic applications.

Recent progress in the synthesis and the applications of inorganic nanotubes and fullerene-like nanoparticles of layered compounds is reviewed in short brief. New synthetic pathways have been developed for this purpose. Thus, the use of heavy metal catalysts, like lead and bismuth, allowed the synthesis of nanotubes from the "misfit" SnS/SnS₂ superstructure; WSe₂ and other compounds. Scaling up of the synthesis of multiwall WS₂ nanotubes is also briefly discussed. The use of such nanotubes to reinforce polymer matrices, and for humidity sensors, is described.

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.

Mo₂C nanoparticles encapsulated within MoS₂ inorganic fullerene-like nanoparticles and nanotubes were produced by carbothermal reaction at 1200-1300 °C inside a vertical induction furnace. The particles were analyzed using various electron microscopy techniques and complementary methods.

Inorganic fullerene-like (IF) nanoparticles (NP) and inorganic nanotubes (INT) of layered compounds, such as WS₂, have been of particular interest due to their unique structural characteristics. Recently, the catalytic decomposition of thiophene using INT of WS₂ decorated with Co NP was demonstrated. This finding also suggests that these materials could be also suitable for the photocatalytic treatment of pollutants in wastewaters. In the present work, the photocatalytic decomposition of methyl orange (MO) in aqueous solution using Co-coated INT-WS₂ as well as other NP was investigated. The photocatalytic reactivity under visible light illumination of this photocatalyst was measured and compared with that of various IF and INT and TiO₂ (P25). The Co NP-coated INT-WS₂ exhibited the highest photodegradation of MO among the studied NP. The significant enhancement in the photoactivity of the hybrid nanostructure can be attributed to the combination of the metallic Co NP and the semiconducting WS₂ nanotubes. The hybrid nanostructure enables the efficient light absorption by the INT and the subsequent charge separation of the hybrid semiconductormetal NP under visible light illumination. In addition, Raman spectroscopy technique was used to verify that the MO was decomposed by Co-coated nanotubes and not adsorbed in large amounts on the hybrid NP surface.

A new procedure for the synthesis of MoS ₂ nanotubes is reported, and additionally demonstrated for MoSe ₂, WS ₂, and WSe ₂. Highly concentrated sunlight creates continuous high temperatures, strong temperature gradients, and extended hot annealing regions, which, together with a metallic (Pb) catalyst, are conducive to the formation of different inorganic nanotubes. Structural characterization (including atomic resolution images) reveals a three-step reaction mechanism. In the first step, MoS ₂ platelets react with water-air residues, decompose by intense solar irradiation, and are converted to molybdenum oxide. Subsequently, the hot annealing environment leads to the growth of Pb-stabilized MoO _3-x nanowhiskers. Shortly afterward, the surface of the MoO _3-x starts to react with the sulfur vapor supplied by the decomposition of nearby MoS ₂ platelets and becomes enveloped by MoS ₂ layers. Finally, the molybdenum oxide core is gradually transformed into MoS ₂ nanotubes. These findings augur well for similar syntheses of as yet unattained nanotubes from other metal chalcogenides.

Tubular structures of the SnS ₂/SnS misfit compound, which are currently prepared in large amounts, are reported. The lattice mismatch between the two alternating sublayers of SnS ₂ and SnS leads to a variety of chiral tubular structures. Such tubular structures are studied via high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). The diversity of the structures manifests itself through different stacking orders of SnS ₂ and SnS layers along their common c-axis and their relative in-plane orientation. Folding vectors and chiral angles of both subsystems can be determined.

We propose that a photodetector based on nanotubes formed from layered structure may have a faster response than nanowires or nanobelts. The layered compound tungsten disulfide (WS ₂) can absorb visible and near-infrared lights. We fabricated photodetectors based on individual WS ₂ nanotubes. The photodetectors exhibited a remarkable response to excitation with 633 and 785 nm light. The nanotube-based photodetectors exhibited short rise and decay times of a few hundred μs, high on/off ratio, and high spectral responsivity and external quantum efficiency. Our results imply that WS ₂ nanotubes are prospective candidates for high-performance nanoscale optoelectronic devices.

The electrostatic effects in tribological systems have been studied in the past, especially with regards to data storage media. Nanoparticles (NP) of WS2 and MoS2 with fullerene-like structure (IF) have been studied in the past and showed very good tribological behavior. Being semiconductors, their electrical properties can be controlled by, e.g., substituting the lattice Mo (W) atoms with Re (n-type conductivity) and Nb (p-type conductivity) atoms. In this study doping of IF-MoS2, and to a lesser degree IF-WS ₂, NP with small amounts (

Lubricating nanoparticles: The effect of doping semiconductor hollow closed-fullerene-like nanoparticles of MoS ₂ and WS ₂ has been overlooked to date. Rhenium doping of these nanoparticles leads to a marked increase in the nanoparticle's conductivity, reduced agglomeration, and a great reduction in friction and wear (see picture) that approaches superlubricity.

The phenomenon of a partial 2H→1T phase transition within multiwalled WS ₂ nanotubes under substitutional rhenium doping is discovered by means of high-resolution transmission electron microscopy. Using density-functional tight-binding calculations for the related MoS ₂ compound, we consider a possible origin of this phase transition, which was known formerly only for WS ₂ and MoS ₂ intercalated by alkali metals. An interplay between the stability of layered or nanotubular forms of 2H and 1T allotropes is found to be intimately related with their electronic structures and electro-donating ability of an impurity.

It is now possible to synthesize inorganic nanotubes (INT) of WS2 in a pure phase and substantial amounts. Due to their crystalline perfection, they are characterized by excellent mechanical behavior. This study is dedicated to the investigation of the effect of INT-WS2 on the mechanical, thermal, adhesion and tribological properties of epoxy based nanocomposites. Various concentrations up to 1.0 wt% of the INT were added to the epoxy matrix. First a method was developed to mix the nanotubes in the epoxy resin matrix. A combination of both magnetic stirring and ultrasonic mixing was used. The adhesion, fracture toughness and strain energy release rate were studied. The INT-WS2 were found to significantly improve all these properties. The wear of the nanotubes-reinforced epoxy was eight-times lower than that of pure epoxy. These results suggest numerous applications.

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 (MoS₂) 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.

Inorganic layered materials can form hollow multilayered polyhedral nanoparticles. The size of these multi-wall quasi-spherical structures varies from 4 to 300 nm. These materials exhibit excellent tribological and wear-resisting properties. Measuring and evaluating the stiffness of individual nanoparticle is a non-trivial problem. The current paper presents an in situ technique for stiffness measurements of individual WS₂ nanoparticles which are 80 nm or larger using a high resolution scanning electron microscope (HRSEM). Conducting the experiments in the HRSEM allows elucidation of the compression failure strength and the elastic behavior of such nanoparticles under uniaxial compression.

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 Moiré 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, thusleading to relatively efect-free core-shell inorganic nanotubular structures, which have not been previously observed within carbon nanotubes (CNTs). Core-shell WS ₂@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.

New materials and techniques pertaining to the synthesis of inorganic nanotubes have been ever increasing since the initiation of the field in 1992. Recently, WS2 nanotubes, which are produced now in large amounts, were filled with molten lead iodide salt by a capillary wetting process, resulting in PbI₂@WS₂ core-shell nanotubes. This work features progress in the synthesis of new core-shell nanotubes, including Bil₃@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 SbI₃ nanotubes, observed either in a hollow core Of WS₂ ones (Sbl₃@WS ₂ nanotubes), or atop of them (WS₂@Sbl₃ nanotubes). The second technique involves a gaseous phase reaction, where the layered product employs WS₂ nanotubes as nucleation sites. In this case, the MoS₂ layers most often cover the WS₂ nanotube, resulting in WS₂@MoS₂ core-shell nanotubes. Notably, superstructures of the form MoS₂@WS₂ are occasionally obtained. Using a semi-empirical model, it is shown that the Pbl₂ nanotubes become stable within the core of MoS2 nanotubes only above a critical core diameter of the host (>12 nm); below this diameter the PbI₂ 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.

Nanoparticles of inorganic compounds with layered (2D) structures, like graphite and MoS₂, were shown to be unstable in the planar from and fold on themselves forming seamless hollow structures like multiwall nanotubes and fullerene-like nanoparticles. The present concise tutorial review reports on the salient developments in this field over the last several years. Numerous applications for such nanophases have been proposed, like solid lubricants, ultra-strong nanocomposites, catalysts, etc.

WS₂ belongs to a family of layered metal dichalcogenide compounds that are known to form cylindrical (inorganic nanotubes-INT) and polyhedral nanostructures - onion or nested fullerene-like (IF) particles. The outermost layers of these IF nanoparticles can be peeled under shear stress, thus IF nanoparticles have been studied for their use as solid lubricants. However, the IF nanoparticles tend to agglomerate, presumably because of surface structural defects induced by elastic strain and curvature, a fact that has a deleterious effect on their tribological properties. In the present work, chemical modification of the IF-WS₂ surface with alkyl-silane molecules is reported. The surface-modified IF nanoparticles display improved dispersion in oil-based suspensions. The alkyl-silane coating reduces the IF-WS₂ nanoparticles' tendency to agglomerate and consequently improves the long-term tribological behavior of oil formulated with the IF additive.

Using a new quartz-made reactor, large amounts of fullerene-like (IF) MoS ₂ nanoparticles were synthesized by reacting MoO ₃ vapor with H ₂S 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) MoS ₂ and IF-WS ₂. These tests indicated that under high pressure and relatively low humidity, the IF-MoS ₂ 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-WS ₂ 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.

Inorganic fullerene-like WS₂ and MoS₂ nanoparticles have been synthesized using exclusively solid precursors, by reaction of the corresponding metal oxide nanopowder, sulfur and a hydrogen-releasing agent (NaBH₄ or LiAlH₄), achieved either by conventional furnace heating up to a 900 °C or by photothermal ablation at far higher temperatures driven by highly concentrated white light. In contrast to the established syntheses that require toxic and hazardous gases, working solely with solid precursors permits relatively safer reactor conditions conducive to industrial scale-up.

We report on the first nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) investigation of inorganic fullerene-like MoS ₂ nanoparticles. Spectra of bulk 2H-MoS₂ samples have also been measured for comparison. The similarity between the measured quadrupole coupling constants and chemical shielding anisotropy parameters for bulk and fullerene-like MoS₂ reflects the nearly identical local crystalline environments of the Mo atoms in these two materials. EPR measurements show that fullerene-like MoS₂ exhibits a larger density of dangling bonds carrying unpaired electrons, indicative of them having a more defective structure than the bulk sample. The latter observation explains the increase in the spin-lattice relaxation rate observed in the NMR measurements for this sample in comparison with the bulk 2H- MoS₂ ones.

The growth mechanism of WS 2 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 MoS₂ 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 MoS₂ are stable above 3 nm (app 103 atoms). In the range of 3-8 nm (103-105) nanooctahedra with metallic character are the most stable form of MoS₂ Semiconducting nanotubes and quasispherical IF nano-particles become the stable-most form beyond that size and the bulk (platelets) are stable above about 0.2 μm. 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 WS₂ nanotubes are briefly described. The stability of IF and INT of MoS₂ (WS₂) under pressure and that of carbon is also discussed. Applications of the IF-WS₂ as superior solid lubricants, which lead to their recent commercialization, is demonstrated.

Further understanding of the growth mechanism and the detailed structure of fullerene-like MoS₂ (IF-MoS₂) nanoparticles was achieved by using a new kind of reactor. The annealed nanoparticles consist of >30 closed layers and their average diameter is 50-80 nm although a small (

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-WS₂ coating before and after the friction test compared to Ni coated wires without IF-WS₂.

The adsorption kinetics of thiophene on WS₂ 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 H₂S and alkanes is evident at greater temperatures on fully sulfided as well as reduced and oxidized WS₂ nanoparticles.

The time dependent chemical changes occurring 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 occurring 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 stoichiometric 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.

The temporal chemical changes occurring at the surface of fullerene-like (IF) nanoparticles of WS₂ were investigated using X-ray photo-electron spectroscopy (XPS) and compared to those of bulk powder (2H) of the same material. It is possible to follow the long term (surface oxidation and carbonization) occurring at defects on the outermost surface (0001) layer of the fullerene-like nanoparticles. Similar but perhaps more distinctive changes are observed on the prismatic (hk0) surfaces of the 2H powder. Vacuum annealing is shown to remove most of these changes and bring the surface close to its stoichiometric composition. In accordance with previous measurements, further evidence is obtained for the existence of water molecules which are entrapped in the hollow core and interstitial defects of the fullerene-like nanoparticles during the synthesis. They are also shown to be removed by the vacuum annealing process. Chemically resolved electrical measurements (CREM) in the XPS show that the vacuum annealed IF samples become more intrinsic. Finally, tribological measurements show that the vacuum annealed IF samples perform better as an additive to oil than the non-annealed IF samples and the bulk (2H) platelets powder.

The mechanical properties of individual WS2 nanotubes were investigated and directly related to their atomic structure details by in situ transmission electron microscope measurements. A brittle mode deformation was observed in bending tests of short (ca. 1 mu m in length) multilayer nanotubes. This mode can be related to the atomic structure of their shells. In addition, longer nanotubes (6-7 mu m in length) were deformed in situ scanning electron microscope, but no plastic deformation was detected. A "sword-in-sheath" fracture mechanism was revealed in tensile loading of a nanotube, and the sliding of inner shells inside the outermost shell was imaged "on-line". Furthermore, bending modulus of 217 GPa was obtained from measurements of the electric-field-induced resonance of these nanotubes.

We present the advancement of electron tomography for three-dimensional structure reconstruction of fullerene-like particles toward atomic-scale resolution. The three-dimensional reconstruction of nested molybdenum disulfide nanooctahedra is achieved by the combination of low voltage operation of the electron microscope with aberration-corrected phase contrast imaging. The method enables the study of defects and irregularities in the three-dimensional structure of individual fullerene-like particles on the scale of 2-3 Å. Control over shape, size, and atomic architecture is a key issue in synthesis and design of functional nanoparticles. Transmission electron microscopy (TEM) is the primary technique to characterize materials down to the atomic level, albeit the images are two-dimensional projections of the studied objects. Recent advancements in aberration-corrected TEM have demonstrated single atom sensitivity for light elements at subångström resolution. Yet, the resolution of tomographic schemes for three-dimensional structure reconstruction has not surpassed 1 nm³, preventing it from becoming a powerful tool for characterization in the physical sciences on the atomic scale. Here we demonstrate that negative spherical aberration imaging at low acceleration voltage enables tomography down to the atomic scale at reduced radiation damage. First experimental data on the three-dimensional reconstruction of nested molybdenum disulfide nanooctahedra is presented. The method is applicable to the analysis of the atomic architecture of a wide range of nanostructures where strong electron channeling is absent, in particular to carbon fullerenes and inorganic fullerenes.

The temporal chemical changes occurring at the surface of fullerence-like (IF) nanoparticles of WS2 were investigated using X-ray photoelectron spectroscopy (XPS) and compared to those of bulk powder (2H) of the same material. It is possible to follow the long term (surface oxidation and carbonization) occurring at defects on the outermost surface (0001) layer of the fullerene-like nanoparticles. Similar but perhaps more distinctive changes are observed oil the prismatic (hk0) surfaces of the 2H powder. Vacuum annealing is shown to remove most of these changes and bring the surface close to its stoichiometric composition. In accordance with previous measurements, further evidence is obtained for the existence of water molecules which are entrapped in the hollow core and interstitial defects of the fullerene-like nanoparticles during the synthesis. They are also shown to be removed by the vacuum annealing process. Chemically resolved electrical measurements (CREM) in the XPS show that the vacuum annealed IF samples become more intrinsic. Finally, tribological measurements show that the vacuum annealed IF samples perform better as an additive to oil than the non-annealed IF samples and the bulk (2H) platelets powder.

In this paper, we report on the synthesis and applications of semiconducting nanostructures. Nanostructures of interest were zinc oxide (ZnO) nanowires and tungsten disulfide WS₂ nanotubes where transistors/phototransistors and photovoltaic (PV) energy conversion cells have been fabricated. ZnO nanowires were grown with both high- and low-temperature approaches, depending on the application. Individual ZnO nanowire side-gated transistors revealed excellent performance with a field-effect mobility of 928 cm² V·s. ZnO networks were proposed for large-area macroelectronic devices as a less lithographically intense alternative to individual nanowire transistors where mobility values in excess of 20 cm² /V·s have been achieved. Flexible PV devices utilizing ZnO nanowires as electron acceptors and for photoinduced charge separation and transport have been presented. Phototransistors were fabricated using individual WS₂ nanotubes, where clear sensitivity to visible light has been observed. The results presented here simply reveal the potential use of inorganic nanowires/tubes for various optoelectronic devices.

Reactors driven by highly concentrated sunlight can create conditions well suited to the synthesis of inorganic nanomaterials. We report the experimental realization of a broad range of closed-cage (fullerene-like) nanostructures, nanotubes and/or nanowires for MoS₂, SiO₂ and Si, achieved via solar ablation. The solar technique generates the strong temperature and radiative gradients - in addition to the extensive high-temperature annealing environment - conducive to producing such nanostructures. The identity of the nanostructures was established with TEM, HRTEM and EDS. The fullerene-like and nanotube MoS₂ configurations achieved fundamentally minimum sizes predicted by molecular structural theory. Furthermore, our experiments represent the first time SiO₂ nanofibers and nanospheres have been produced purely from quartz. The solar route is far less energy intensive than laser ablation and other high-temperature chemical reactors, simpler and less costly.

Inorganic fullerene-like (IF) nanoparticles of WS2 and MoS2 with extremely useful tribological characteristics have been realized. One of the disadvantages in application of IF is their aggregation. Two aspects of the problem will be considered: the effect of the aggregates on friction and wear and the effect of the loading scheme on the efficacy of IF.It was found out that the reproducibility of the friction results for the pairs lubricated with oil +IF is determined by the size and the size- distribution of the aggregates. Decreasing the size of the ggregates increases the probability for the IF penetration into the interface and decrease remarkably the friction coefficient. It was reveiled that wedge clearance in the inlet of contact affect considerably on penetration of IF into interface. The application of friction device with wedge clearance in the inlet of the contact limits the efficacy of the IF. The analysis of results allows to conclude that one of the ways to improve the efficacy of IF nanoparticles is decreasing their size down to a value lesser than 50 nm. Another important way allowing to limit the agglomeration of the IF nanoparticles and thus to improve their efficacy is by the application of surfactants.

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.

IF-MO_1-xNb_xS₂ nanoparticles have been synthesized by a vapor-phase reaction involving the respective metal halides with H₂S. The IF-MO_1-xNb_xS₂ nanoparticles, containing up to 25% Nb, were characterized by a variety of experimental techniques. Analysis of the powder X-ray powder diffraction, X-ray photoelectron spectroscopy, and different electron microscopy techniques shows that the majority of the Nb atoms are organized as nanosheets of NbS₂ within the MoS₂ host lattice. Most of the remaining Nb atoms (3%) are interspersed individually and randomly in the MoS₂ host lattice. Very few Nb atoms, if any, are intercalated between the MoS₂ layers. A sub-nanometer film of niobium oxide seems to encoat the majority of the nanoparticles. X-ray photoelectron spectroscopy in the chemically resolved electrical measurement mode (CREM) and scanning probe microscopy measurements of individual nanoparticles show that the mixed IF nanoparticles are metallic independent of the substitution pattern of the Nb atoms in the lattice of MoS₂ (whereas unsubstituted IF-MoS₂ nanoparticles are semiconducting). Furthermore the IF-MO_1-xNb_xS₂ nanoparticles are found to exhibit interesting single electron tunneling effects at low temperatures.

To investigate phonon confinement in nanoscale metal dichalcogenides, we measured the low-temperature specific heat of layered and nanoparticle WS ₂. Below 9 K, the specific heat of the nanoparticles deviates from that of the bulk counterpart. Further, it deviates from the usual T³ 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 T³ 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 T³ 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 10⁴-10⁵ 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 ∼10⁵ 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.

We have proposed in 1992 that nanoparticles of layered compounds will be unstable against folding and close into fullerene-like structures and nanotubes (IF). Nanotubes and fullerene-like structures were prepared from numerous compounds with layered and recently also non-layered structure by various groups. Much progress has been achieved in the synthesis of inorganic nanotubes and fullerene-like nanoparticles of WS₂ and MoS₂ and many other metal dichalcogenides over the last few years. 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-MoS₂ and IF-WS₂ 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". Novel applications of inorganic nanotubes and fullerene-like nanoparticles in the fields of catalysis; microelectronics; Li rechargeable batteries; medical and opto-electronics will be discussed.

Electrical resistivity and Hall measurements, of pellets compacted from IF-WS₂ nanoparticles and 2H-WS₂ 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 (∂ ln ∂T^-1)/∂T^-1 with temperature. Resistivity of IF-WS₂ pellets is higher than that of 2H-WS ₂ pellets. It was found that the electrical properties of IF-WS ₂ powder vary with aposteriory heat treatment under vacuum. The ¹H 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-WS₂ structure, arising due to the strain release during the folding of the layers, may result in additional sites for the absorbed water.

Inorganic fullerene-like ( IF) nanoparticles of MoS2 were synthesized using gas-phase reaction starting from MoCl5 and H2S. The IF-MoS2 nanoparticles are spherical and in some cases faceted with diameters in general ranging between 20 and 80 nm. The IF-MoS2 nanoparticles have large hollow cores, filled in some cases with amorphous material. Various parameters have been investigated to understand the growth and formation of the IF-MoS2 nanoparticles. The parameters that have been studied include flow rates of the various carrier gases, temperature at which the reaction was carried out, time of the reaction and heating of the precursor material. The best set of conditions wherein maximum yields of the IF-MoS2 nanoparticles are obtained have been identified. Additionally, annealing the as-obtained samples or heating them in a mixture of H-2 along with H2S improves the crystallinity and reduces the amorphous material filling in the core. Apart from the fullerene-like nanoparticles under certain experimental conditions nanotubes of MoS2 have also been obtained nonetheless in small yields.

Electrical resistivity and Hall effect measurements of pellets compacted from fullerene-like WS₂ nanoparticles (IF-WS₂) and bulk 2H-WS₂ powder were carried out using the van der Pauw method over a wide temperature range. In addition IF-WS₂ pellets were annealed at elevated temperatures under vacuum in a specially designed system. Arrhenius plots for the conductivities of the WS₂ samples (2H, IF and IF+annealing) exhibit marked uprise of ∂ In (σT^-1)/ ∂T^-1 with temperature. The resistivity of the non-annealed IF-WS₂ pellets is higher by 2-8 orders of magnitude than that of 2H-WS₂ pellets, whereas the resistivity of the annealed IF pellets is higher than that of the non-annealed ones. Hall Effect measurements at 300 K show p-type conductivity and similar carrier concentration for both types of materials. The carrier mobility of 2H-WS₂ platelets is found to be in the range of the reported values. However, IF-WS₂ pellets have shown an unusually low mobility for a semiconducting material. The experimental data was found to be in a good agreement with a model used for analyzing the conductivity of polycrystalline semiconductors, which takes into consideration fluctuations of the barrier heights among the different nanoparticles as well as within a single nanoparticle boundary.

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.

Fullerene-like WS₂ (MoS₂) nanoparticles (IF) have been studied in the past. Their efficacy as additives for lubrication fluids has been demonstrated. It was shown that the IF nanoparticles are usually delaminated in the inlet of the smooth contact. Thin sheets of broken IF nanoparticles can be entrapped between the rubbed surfaces and thus favorably affect the friction and the wear. Friction pairs at real mechanical macrosystems are often subjected to friction-induced vibrations. It was shown that the mechanical excitations can improve the supplying and preservation of fluid lubricant film in the interface. It can be hypothesized that under vibrations in a definite range of frequencies and amplitudes, the probability for small IF aggregates to be entrapped into the interface is increased. The main goal of this work was to study the effect of artificial mechanical excitations on the friction and wear of contact pairs rubbed with nanoparticles. In order to avoid friction-induced excitations, a new ball-on-flat friction device was developed. The frequency and the amplitude of the ball were varied using a motion of miniature micromotor attached to the ball holder. The behavior of IF nanoparticles in friction tests with and without external mechanical excitations was compared with the tribological behavior of the contact pair lubricated with pure paraffin oil. It was found that the external mechanical excitation of the mechanical parts rubbed with nanoparticles allows a penetration of these nanoparticles into interface. This effect leads to a remarkable shortening of the run-in period and improves the tribological properties of contact pairs. From the present results it may be anticipated that the accidental friction-induced vibrations, which are determined by the stiffness and damping force of the device, lead to preferential penetration of the IF nanoparticles into the contact area, affecting thereby the tribological behavior of the interface.

The mechanical properties of materials and particularly the strength are greatly affected by the presence of defects; therefore, the theoretical strength (≈10% of the Young's modulus) is not generally achievable for macroscopic objects. On the contrary, nanotubes, which are almost defect-free, should achieve the theoretical strength that would be reflected in superior mechanical properties. In this study, both tensile tests and buckling experiments of individual WS₂ nanotubes were carried out in a high-resolution scanning electron microscope. Tensile tests of MoS₂ nanotubes were simulated by means of a density-functional tight-binding-based molecular dynamics scheme as well. The combination of these studies provides a microscopic picture of the nature of the fracture process, giving insight to the strength and flexibility of the WS₂ nanotubes (tensile strength of ≈16 GPa). Fracture analysis with recently proposed models indicates that the strength of such nanotubes is governed by a small number of defects. A fraction of the nanotubes attained the theoretical strength indicating absence of defects.

A new class of inorganic nanostructures with a closed cage structure has been discovered. These inorganic fullerenes are multi-walled (i.e. onion-like) spherical particles which can also be wrapped to form nanotubes. They are extremely effective as solid lubricants, particularly under extreme conditions. The development and initial applications of these materials are reviewed.

Motivated by the discovery of the C₆₀ 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 CS ₂. For the gas-phase reaction, a high yield of IF nanoparticles was obtained between 400 and 600°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°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.

The excellent shock-absorbing performance of WS₂ and MoS ₂ nanoparticles with inorganic fullerene-like structures (IFs) under very high shock wave pressures of 25 GPa is described. The combined techniques of X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, thermal analysis, and transmission electron microscopy have been used to evaluate the diverse, intriguing features of shock recovered IFs, of interest for their tribological applications, thereby allowing improved understanding of their antishock behavior and structure-property relationships. Two possible failure mechanisms are proposed and discussed. The supershock-absorbing ability of the IF-WS₂ enables them to survive pressures up to 25 GPa accompanied with concurrent temperatures of up to 1000 °C without any significant structural degradation or phase change making them probably the strongest cage molecules now known.

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.

(Figure Presented) Fullerene-like Cs₂O nanoparticles were prepared by laser ablation of 3R-Cs₂O powder in evacuated quartz ampoules. The Cs₂O closed cages, such as the faceted nanoparticle shown in the picture, are remarkably stable as compared with the corresponding extremely unstable but technically important bulk compound, which makes them potentially useful in applications involving cesium oxide coatings, for example, photoemissive devices and catalytic converters.

The dynamic pressure-resitance capacity of inorganic fullerene (IF) cages in uniaxial shockwave pressures up to 30 GPa, was studied. It was observed that to a limit of 25 GPa, except those IFs mixed with nanotubes, others suffered light damages. It was also observed that a perfectly hollow and small IF cage consists an ability to withstand much higher shock wave pressure. These inorganic fullerenes can be used as lubricant at severe loading conditions, or as a coating or composite phase in cases of anti-shock or high-pressure environment.

Nanoparticles of WS₂ and MoS₂ with a closed cage structure (fullerene-like) that are termed IF phases are synthesized in large amounts in a pure form. These nanoparticles were shown to play a favorable role as solid lubricants under severe conditions where fluids are unable to support the heavy load and are squeezed away from the contact area. Various tribological scenarios are presented for these superior solid lubricants, demonstrating the large scale potential for applications of these materials. The mechanism of action of these solid lubricants is briefly discussed. Various other potential applications of IF phases for nanocomposites with high impact resistance; in rechargeable batteries and in optical devices are discussed in short.

Fullerene-like WS₂ (MoS₂) nanoparticles (IF) have been studied in the past. Their efficacy as additives for lubrication fluids has been demonstrated. Recently, IF-WS₂ nanoparticles were confined inside a porous and densified bronze-graphite matrix, prepared by powder metallurgy (PM) technique. Substantial reduction in both friction and wear, and an increase in the critical load were observed. Novel applications of IF nanoparticles as development of polymer nanocomposites, burnishing, and friction of ceramic materials under severe contact conditions were presented. Polymer nanococomposites with embedded IF nanoparticles exhibited excellent tribological properties. The main friction mechanism was the transfer of the IF nanoparticles from the surface of the polymer composite to the surface of the mating steel disk leading to a reduction in both the friction coefficient and wear rate of the contact surfaces. This is an abstract presented at the World Tribology Congress III (Washington, DC 9/12-16/2005).

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.

When dispersed in a synthetic polyalphaolefin (PAO) base oil, inorganic fullerene-like (IF-MS₂) nanoparticles of metal dichalcogenides (IF-MoS₂, IF-WS₂, IF-NbS₂) lead to a significant reduction of both friction and wear under boundary lubrication. The effect of the contact pressure on the tribological properties of IF nanoparticles is particularly interesting. Results show that the higher is the pressure, the lower is the friction coefficient. The effect of the concentration shows that, even used at a low concentration (0.1%wt), IF-MS₂ is able to decrease friction (0.05) compared to base oil only (0.08). A steady state friction coefficient of 0.04 was reached with IF-WS₂ at 1%wt in PAO. Friction-induced transformation of the IF-MS₂ nanoparticles into H-MS₂ single sheets was evidenced by High Resolution Transmission Electron Microscopy (HRTEM). Some of these superimposed sheets are found in incommensurate positions, thus possibly explaining the very low friction coefficient of 0.04 obtained with IF-WS₂. In-situ Raman spectroscopy was performed during a friction test to follow this structural modification. The lubrication mechanism of IF-MS₂ is very similar to a "drug delivery system". A very low concentration of additives is sufficient and the activation is obtained by the opening of the nested structure, like in certain micellar structures. Furthermore, no chemical reaction is required to obtain interesting properties. Thus, fullerene-like nanoparticles are active at the very beginning of the test and even at ambient and low temperature.

Adding guest atoms to inorganic nanotubes, known as 'doping', influences their room-temperature magnetic properties - properties that could be exploited in 'spintronic' devices and computer memory.

A study of the tribological behavior of nested inorganic fullerene-like (IF) nanoparticles of WS₂, as a potential additive to base oils is presented. Friction measurement results obtained from three different test rigs over a wide range of normal loads and sliding velocities are shown. Stribeck curves are used to reveal the lubrication regimes where the IF are most effective. It is found that the addition of IF-WS₂ nanoparticles to the base oils results in up to 50% reduction in friction coefficient in the mixed lubrication regime. The mechanism of improved friction and wear behavior with the IF additive is discussed.

Inorganic fullerene-like materials have been identified as being of potentially utmost importance for many industrial applications. MoS₂ and WS₂ hollow nanoparticles have been identified as strong candidates for tribological applications such as solid lubricants. The main goal of this work was to evaluate the mechanical properties of solid lubricant particles in ensemble under hydrostatic pressure. The behavior of nanopowders under compression has been described on the basis of constitutive models of continuum mechanics. The model will be applied to an isotropic compaction of copper (well-studied medium), fullerene-like (IF-WS₂) nanoparticles and a natural powder of 2H-WS₂ platelets. The morphology of individual nanoparticles and nanoparticle ensembles will be examined and discussed. Another aspect of this work was to study the applicability and limitations of the proposed constitutive model for the understanding of the tribological behavior of solid lubricant nanoparticles. Compression with the maximal pressure (500 MPa) showed that the shape of the IF nanoparticles is preserved. The dominant mechanism of damage was found to be the delamination or peeling-off of the external sheets of hollow nanoparticles. Strong destruction of 2H-WS₂ platelets was observed under compression.

The Young's modulus of WS₂ nanotubes is an important property for various applications. Measurements of the mechanical properties of individual nanotubes are challenged by their small size. In the current work, atomic force microscopy was used to determine the Young's modulus of an individual multiwall WS₂ nanotube, which was mounted on a silicon cantilever. The buckling force was measured by pushing the nanotube against a mica surface. The average Young's modulus of an individual WS₂ nanotube, which was calculated by using Euler's equation, was found to be 171 GPa. First-principle calculations of the Young's modulus of MoS₂ single-wall nanotubes using density-functional-based tight-binding method resulted in a value (230 GPa) that is close to that of the bulk material. Furthermore, the diameter dependence of the Young's modulus in both zigzag and armchair configuration was studied and was found to approach the bulk value for nanotubes with few-nanometer diameters. Similar behavior is expected for WS ₂ nanotubes. The mechanical behavior of the WS₂ nanotubes as atomic force microscope imaging tips gave further support for the measured Young's modulus.

Pronounced surface diffusion is observed during x-ray photoelectron spectroscopy measurements of 2H platelets and inorganic fullerene-like (IF) MS2 (M=WMo) powders, intercalated with alkaline (A=K,Na) elements. Using controlled surface charging the intercalants migrate towards the surface, where they oxidize. This dry deintercalation is controllable via external charging parameters, yet showing that internal chemical and structural parameters play an important role in the process. Diffusion rates out of 214 matrixes are generally higher than in corresponding IF samples. Clear differences are also found between Mo and W-based systems. Application of this approach into surface modification and processing is, proposed. (C) 2003 American Vacuum Society.

The development of inorganic and fullerene-like nanoparticles from layered compounds is discussed. It is found that most of the inorganic layered materials comprise more than one atomic layer with strong chemical bonds interconnecting between the atoms of the different layers. The Young's modulus of WS₂ nanotubes is also elaborated.

It was shown previously that nanoparticles of layered comppunds form closed-cage and nanotubular structures. These nanostructures received the generic name of inorganic fullerene-like (IF) materials. In particular nanoparticles of metal dihalides, like NiCl₂ and CdCl₂, were shown to form such closed-cage structures in the past. In the present report faceted IF-CdI₂ nanoparticles encapsulating a Cd core are reported. These nanoparticles were obtained using electron beam irradiation of the source powder of CdI₂. The faceted nanoparticles exhibit one of two morphologies: hexagonal or elongated rectangular characters. The nanoparticles can be considered as core (Cd)-shell (CdI₂) nanostructures. Consistent with previous observations, this study shows that the seamless structure of the IF materials can stabilize phases, which are otherwise unstable Under the electron-beam irradiation. The growth mechanism of these nanostructures is briefly discussed.

Laser ablation has been extensively used for the synthesis of nanoparticles of various sorts, and in particular single wall carbon nanotubes and C₆₀ molecules. NiCl₂ nanotubes were recently also produced using this technique. While fullerene-like NiCl₂ structures can be obtained through regular ablation, vapor phase enriched with CCl₄ gas (reactive ablation) is necessary for the synthesis of the nanotubes. The experimental results indicate that the synthesis of such nanotubes is much more difficult than the synthesis of say MoS₂ or WS₂ nanotubes. Moreover, the NiCl₂ nanotubes are of larger diameter and consist on the average of more layers than their MoS₂ predecessors. First principle calculations show that single layer NiCl₂ nanotubes of diameter smaller than 54 nm are unstable and lose their outer chlorine atoms. In contrast, MoS₂ nanotubes with diameter of 2 nm and larger are found to be stable using the same kind of calculations. To gain better understanding of the differences between the materials, a review of the mechanical properties of layered metal dihalide and metal dichalcogenide compounds is undertaken. First principle calculations show that the Young's and bending moduli of NiCl₂ are almost twice larger than those of MoS₂. The large ionicity of NiCl₂ entails much larger shear and stacking fault energies for this compound as compared to MoS₂, which explains its smaller propensity to bend and fold. These observations are supported by analysis of the corresponding Raman modes. Furthermore, metal dihalide compounds are very hygroscopic making their handling, and especially their analysis more difficult. This analysis explains the greater difficulties to grow NiCl₂ nanotubes or fullerene-like nanoparticles, as compared to their MoS₂ analogues.

Multiwall WS₂ nanotubes of 40-50 nm diameter were functionalized with n-octadecyl phosphonic acid by sonication in toluene and blended with mixtures of polystyrene (PS) and polymethylmethacrylate (PMMA) to form new nanocomposite (NC) materials. The surface and domain structures were studied by atomic force microscopy (AFM), scanning transmission X-ray microscopy (STXM) and transmission electron microscopy (TEM) for various levels of loading of nanotubes up to 20 wt%. Phase-separated domain size and surface roughness of the nanocomposite films were found to be dramatically reduced relative to the pure homopolymer blend and good dispersal of the nanotubes in the blend matrix was attained.

The shock-wave resistance of WS₂ nanotubes has been studied and compared to that of carbon nanotubes. Detailed structural features of post-shock samples were investigated using HRTEM, XRD, and Raman spectroscopy. WS₂ nanotubes are capable of withstanding shear stress caused by shock waves of up to 21 GPa, although some nanotube tips and nanoparticles containing multiple structural defects in the bending regions are destroyed. Small WS₂ species, consisting of only a few layers, are extruded from the nanotubes. Well-crystallized tube bodies were found to exhibit significant stability under shock, indicating high tensile strength. XRD and Raman analyses have confirmed this structural stability. Under similar shock conditions, WS₂ tubes are more stable than carbon nanotubes, the latter being transformed into a diamond phase. WS₂ nanotubes containing small concentrations of defects possess significantly higher mechanical strength, and, as a consequence, hollow WS₂ nanoparticles are expected to act as excellent lubricants under much higher loading than was previously thought.

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.

Following the discovery of fullerenes and carbon nanotubes, it was shown that nanoparticles of inorganic layered compounds, like MoS₂, are unstable in the planar form and they form closed cage structures with polyhedral or nanotubular shapes. Various issues on the structure, synthesis, and properties of such inorganic fullerene-like structures are reviewed, together with some possible applications.

It was proposed that nanoparticles of layered compounds are not stable in the planar form and they spontaneously fold into fullerene-like structures and nanotubes (IF phases). Considering the spherical topology of the IF nanoparticles, it was proposed that they could ideally suit tribological applications. In this paper the authors demonstrate the preparation of a metal matrix IF-nanoparticles composite coating by electroless code-position. It was found that the IF-nanoparticle was able to significantly improve the tribological characteristics of the electroless composite coatings. Moreover, the spherical IF nanoparticles can promote easier sliding between the two mating metal surface.

The Raman spectrum of Cs₂O₂ excited at 632.8 nm shows a single O₂^-2 fundamental band at wavenumber of 742.5 cm^-1 and second and third overtones at 1468.5 and 2176.8 cm^-1, respectively. The harmonic wavenumber ω_e = 759.5 ± 0.7 cm^-1 and anharmonicity constant ω_eχ_e = 8.4 ± 0.3 cm^-1 were calculated directly from the measurements. These results are compared with ω_e = 1151.0 ± 1.5 cm-^-1 and ω_eχ_e = 9.6 ± 0.5 cm^-1 recently found for O₂^- ion in CsO₂ by others. The lower ω_eχ_e of O₂^-2 relative to that of O₂^- indicates a broader (smaller curvature) anharmonic oscillator potential well.