Salvy P. Russo

Enhanced Charge Carrier Mobility in Two-Dimensional High Dielectric Molybdenum Oxide

We demonstrate that the energy bandgap of layered, high-dielectric α-MoO(3) can be reduced to values viable for the fabrication of 2D electronic devices. This is achieved through embedding Coulomb charges within the high dielectric media, advantageously limiting charge scattering. As a result, devices with α-MoO(3) of ∼11 nm thickness and carrier mobilities larger than 1100 cm(2) V(-1) s(-1) are obtained.

Physisorption-Based Charge Transfer in Two-Dimensional SnS2 for Selective and Reversible NO2 Gas Sensing

Nitrogen dioxide (NO2) is a gas species that plays an important role in certain industrial, farming, and healthcare sectors. However, there are still significant challenges for NO2 sensing at low detection limits, especially in the presence of other interfering gases. The NO2 selectivity of current gas-sensing technologies is significantly traded-off with their sensitivity and reversibility as well as fabrication and operating costs. In this work, we present an important progress for selective and reversible NO2 sensing by demonstrating an economical sensing platform based on the charge transfer between physisorbed NO2 gas molecules and two-dimensional (2D) tin disulfide (SnS2) flakes at low operating temperatures. The device shows high sensitivity and superior selectivity to NO2 at operating temperatures of less than 160 °C, which are well below those of chemisorptive and ion conductive NO2 sensors with much poorer selectivity. At the same time, excellent reversibility of the sensor is demonstrated, which has rarely been observed in other 2D material counterparts. Such impressive features originate from the planar morphology of 2D SnS2 as well as unique physical affinity and favorable electronic band positions of this material that facilitate the NO2 physisorption and charge transfer at parts per billion levels. The 2D SnS2-based sensor provides a real solution for low-cost and selective NO2 gas sensing.

On fitting a gold embedded atom method potential using the force matching method

We fit a new gold embedded atom method (EAM) potential using an improved force matching methodology which included fitting to high-temperature solid lattice constants and liquid densities. The new potential shows a good overall improvement in agreement to the experimental lattice constants, elastic constants, stacking fault energy, radial distribution function, and fcc/hcp/bcc lattice energy differences over previous potentials by Foiles, Baskes, and Daw (FBD) [Phys. Rev. B 33, 7983 (1986)] Johnson [Phys. Rev. B 37, 3924 (1988)], and the glue model potential by Ercolessi et al. [Philos. Mag. A 50, 213 (1988)]. Surface energy was improved slightly as compared to potentials by FBD and Johnson but as a result vacancy formation energy is slightly inferior as compared to the same potentials. The results obtained here for gold suggest for other metal species that further overall improvements in potentials may still be possible within the EAM framework with an improved fitting methodology. On the other hand, we also explore the limitations of the EAM framework by attempting a brute force fit to all properties exactly which was found to be unsuccessful. The main conflict in such a brute force fit was between the surface energy and the liquid lattice constant where both could not be fitted identically. By intentionally using a very large number of spline sections for the pair potential, electron-density function, and embedding energy function, we eliminated a lack of functional freedom as a possible cause of this conflict and hence can conclude that it must result from a fundamental limitation in the EAM framework.

Structural Relaxation and Relative Stability of Nanodiamond Morphologies

Presented here are the results of ab initio Density Functional Theory (DFT) relaxations performed on nanocrystalline diamond structures of cubic {100}, octahedral {111} and cuboctahedral morphologies, up to approximately 1 nm in diameter. Results show that in this size range, the crystal morphology plays an important role in the structural stability of the crystals, in the absence of external fields. While the surfaces of the cubic crystals exhibited reconstruction and relaxations comparable to that of bulk diamond, the surfaces of the octahedral and cuboctahedral crystals showed the transition from sp3 to sp2 bonding. Our results demonstrate the inward transition of nanodiamond clusters into carbon onion-like structures, with preferential exfoliation of the (111) surfaces, in agreement with recent experimental observations. The results of this study will provide a better understanding of the effects of nanodiamond morphology on the stability of diamondoid nanostructures and nanodevices.

Size dependent phase stability of carbon nanoparticles: Nanodiamond versus fullerenes

Over the past 15 years, a number of studies have reported findings comparing the relative stability of diamond and graphite, at the nanoscale. In light of more recent experimental and theoretical results concerning the transformation of nanodiamonds into carbon-onions, it is considered important to extend this body of work to included fullerenes. Presented here is a study of the phase stability of carbon nanoparticles, with particular attention given to the relative stability of nanodiamonds and fullerenes. The structural energies have been calculated using density functional theory within the generalized gradient approximation using the Vienna ab initio simulation package, and used to determine the standard heat of formation for respective carbon phases as a function of the number of carbon atoms. Our results show that in contrast to previously reported studies, nanodiamond is not necessarily the stable phase a the nanoscale, but instead occupies a “window” of stability between ∼1.9 and ∼5.2 nm.

Safe, stable and effective nanotechnology: phase mapping of ZnS nanoparticles

Zinc sulfide (ZnS) nanoparticles are of interest for their luminescent and catalytic properties which are useful in bio-medical, electronic and photovoltaic devices. However, ZnS nanoparticles undergo reversible and irreversible phase transformations under ambient conditions, which affect the material properties and their biological and environmental significance. In this paper the current knowledge, drawn from experimental, computational and theoretical studies, is reviewed and gaps in this knowledge-base are identified. Based on this assessment, it is suggested that the development of a nanoscale phase diagram for ZnS may provide a way of rapidly assessing the complicated parameter-space occupied by this problem, and serve to highlight the size, temperature, pressure and chemical regimes that may provide the most stable and appropriate size, shape and phase for specific applications.

Ab initio modelling of the stability of nanocrystalline diamond morphologies

Ab initio (density functional theory) relaxation of nanocrystalline diamond structures of cubic, octahedral and cuboctahedral morphologies, up to about 1.3 nm in diameter are presented. Results show that the crystal morphology plays an important role in the structural stability, in the absence of external fields. Our results illustrate the transition of nanocrystalline diamonds (nanodiamonds) into onion-like structures showing preferential exfoliation of the (111) surfaces, in agreement with experimental observations. The cohesive energy for the relaxed nanodiamond is also examined and compared with bulk diamond. We have found that the cohesive energy of nanodiamond differs from that of bulk diamond by approximately 0.34 eV.

Nonvolatile-Memory Characteristics of

The nonvolatile-memory (NVM) characteristics of AlO^- -implanted Al₂O₃ structures are reported and shown to exhibit promising behaviors, including fast program/erase speeds and high-temperature data retention. Photoconductivity spectra show the existence of two dominant trap levels, located at around 2 and 4 eV below the conduction band minimum of Al₂O₃, and our calculations show that these levels are likely attributed to the defects in the Al₂O₃, such as the Al-O divacancy. The relative concentrations of these defects vary with the implant fluence and are shown to explain the NVM characteristics of the samples irradiated to different fluences.

\hbox{AlO}^{-}

The formation of single nanocrystals of the non-magnetic mineral iron pyrite (FeS2) depends on the concentration of sulfur present during synthesis. In nature pyrite nanocrystals are often observed in sulfide-rich sediments, or during intracellular biomineralization in multicellular magnetotactic bacteria. However, characterizing these nanocrystals and understanding the formation processes in either of these sulfidic environments is challenging, as anisotropic crystal growth, alteration and dissolution are linked to the crystallographic orientation of surface facets, and in the latter case to the presence of water and microbial activity. In the present study we use a multi-scale thermodynamic model capable of describing the stability (formation) of nanocrystals as a function of size, shape, temperature and chemical environment, and use it to examine the morphological stability of pyrite nanocrystals formed in sulfidic environments common to the different formation routes. Physical parameters such as the supersaturation of sulfur and temperature are investigated, based on parameters obtained from first principles calculations.

-Implanted

We have used the density functional theory (DFT) method with a plane wave-pseudopotential basis to compute the structure and properties of a model xanthate molecule (HOCS2−) and its adsorption characteristics on the pyrite FeS2(110) and (111) surfaces. Molecular calculations revealed that HOCS2− and CH3CH2OCS2− have similar head group electronic structure and, therefore, the use of the model xanthate is a justifiable approximation in simulations of xanthate head group attachment to FeS2 surfaces. Results from DFT calculations suggest that HOCS2− readily undergoes dissociation at the fourfold coordinated surface Fe on the (110) surface, and the bridging S of the (111) surface. These results suggest that xanthate may undergo chemisorption at defect sites on real FeS2 surfaces, which contain low-coordinate Fe sites and sites in proximity to cleaved S–S bonds.