Integrated Calendar

The two natural allotropes of carbon, diamond and graphite, are extended networks of sp3- and sp2-
hybridized carbon atoms, respectively. By mixing different hybridizations and geometries of carbon, one
could conceptually construct countless synthetic allotropes. In this talk, I will introduce graphullerene, a
new two-dimensional superatomic allotrope of carbon combining three- and four-coordinate carbon
atoms. The constituent subunits of graphullerene are C60 fullerenes that are covalently interconnected
within a molecular layer, forming graphene-like hexagonal sheets. The most remarkable thing about the
synthesis of graphullerene is that the solid-state reaction produces large polyhedral crystals (hundreds of
micrometers in lateral dimensions), rather than an amorphous or microcrystalline powder as one would
typically expect from polymerization chemistry.
Similar to graphite, the crystals can be mechanically exfoliated to produce molecularly thin flakes with
clean interfaces—a critical requirement for the creation of heterostructures and optoelectronic devices.
We find that polymerizing the fullerenes leads to a large change in the electronic structure of C60 and the
vibrational scattering mechanisms affecting thermal transport. Furthermore, imaging few-layer
graphullerene flakes using transmission electron microscopy and near-field nano-photoluminescence
spectroscopy reveals the existence of moiré-like superlattices.
The discovery of a superatomic cousin of graphene demonstrates that there is an entire family of
higher and lower dimensional forms of carbon that may be chemically prepared from molecular
precursors.

The need for affordable large scale energy storage has risen dramatically with the increase in usage of renewable energy sources. In recent years, beyond Li batteries such as Na ion batteries (SIB), gained much interest due to limited Lithium resources. However, SIBs are still far from meeting the demands in terms of electrochemical performance, rendering research on SIBs very important.
During battery cycling, chemical and electrochemical processes result in the formation of an interphase between the anode and electrolyte called the solid electrolyte interphase (SEI). The effect of the SEI on electrochemical performance cannot be overstated, as its composition and structure dictate interfacial ionic transport in the battery cell. Since the SEI is very thin (10-50 nm) and is composed of disordered, organic, and inorganic phases it is extremely difficult to characterize at the atomic-molecular level.
In this seminar I will present methodology developed for probing the native SEI formed in SIBs by using nuclear magnetic resonance (NMR) and signal enhanced NMR by exogenous and endogenous dynamic nuclear polarization (DNP). Employing these techniques enabled us to gain information on the chemical composition of the SEI together with important insights into the SEI’s structural gradient formed with different Na electrolytes. Correlating the compositional and structural information acquired with the SEI’s function can assist in designing SIBs with improved performance and longer lifetime.

Quasicrystals are exotic materials that have symmetries that once thought to be impossible for matter. The first known examples were synthesized in the laboratory 30 years ago, but could Nature have beaten us to the punch? This talk will describe the decades-long search to answer this question, resulting in one of the strangest scientific stories you are ever likely to hear.

No two events are alike. But still, we learn, which means that we implicitly decide what events are similar enough that experience with one can inform us about what to do in another. We have suggested that this relies on parsing of incoming information into “clusters” according to inferred hidden (latent) causes. In this talk, I will present a computational model of this latent-cause inference process, and show supporting data from a variety of behavioral experiments in humans and rodents spanning from simple conditioning to memory to social decision making. I will also briefly discuss the relevance of this theory to mental health treatments.

Inorganic semiconductor nanomaterials have been under extensive research for the last few decades for their fascinating optical and electronic properties. Our group demonstrated the guided growth approach for planar semiconductor nanowires (NWs), by taking advantage of the epitaxial relations between the substrate and the inorganic overlayer. Epitaxy enables one to control crystallographic orientation, growth directions, and properties of the nanostructures. Among the inorganic semiconductors, the family of chiral inorganic semiconductor nanomaterials has recently become a focal point of many studies owing to their unique behavior in condensed matter physics and their potential in spintronics and circularly polarized optoelectronics for information technology. Despite the extensive studies on the enantioselective growth of chiral crystals induced by chiral molecules, “chiral epitaxy”, namely the enantioselective growth of chiral crystals by epitaxy on chiral crystal surfaces has not yet been demonstrated. Here, we explore the interaction between intrinsically chiral inorganic semiconductor nanomaterials and various chiral and asymmetric surfaces.
In one chapter, we demonstrate the enantioselective guided growth of Te NWs on a chiral plane of ReSe2. In order to determine the handedness of the NWs and the substrate, we made special modifications to a known handedness-determination STEM method, which allowed us to analyze both Te and ReSe2 structures. To the best of our knowledge, this is not just the first guided growth of Te on ReSe2, but it is also the first demonstration of enantioselective crystallization of a chiral crystal induced by its epitaxial relations with a chiral surface of a different crystal. Namely, this is the first demonstration of chiral epitaxy.
In the second chapter, we study the guided growth of the chiral wide-bandgap semiconductor α-TeO2 along the asymmetric nanogrooves of annealed M-plane sapphire (α-Al2O3). The NWs show both straight and helical morphologies depending on their crystallographic orientation. This is the first demonstration of the guided growth of TeO2 NWs. In addition, this system demonstrates the formation of helical nanostructures with coherent handedness, controlled by interaction with an asymmetric substrate. All the NWs were characterized with SEM, AFM, TEM, EDS, and Raman spectroscopy.
Overall, this work presents a new approach for enantioselective growth of chiral nanocrystals based on epitaxy. This gas-phase process should be suitable for a wide range of inorganic nanomaterials, and compatible with fabrication processes for integration into functional devices.