Suklyun Hong

Phase stability and elastic moduli of Cr2Nb by first-principles calculations

The phase stability and elastic moduli of Cr2Nb are investigated by first-principles calculations. Heats of formation are calculated and compared for the three Laves phases (C15, C14, and C36). It is found that the C15 phase is the ground-state structure with the lowest energy and the C36 phase is an intermediate state between C15 and C14. These three phases, however, are very close in energy, indicating low stacking fault energies in this system. For the ground-state C15 phase, we calculate three elastic constants from which the shear and Young’s moduli are obtained. It is found that these calculated moduli are smaller than the experimental values obtained from polycrystals.

Ab initio study of the effect of water adsorption on the carbon nanotube field-effect transistor

We perform density-functional calculations to investigate the effect of adsorbed water molecules on carbon nanotubes (CNTs). Noting that the H2O molecule has much wider energy gap than the CNT, we find that the charge transfer between them is negligible. We discuss that several recent publications, which claimed a substantial electron transfer from the water molecule to the CNT, have been based on incautious interpretations of the Mulliken population analysis. We suggest that the effect of humidity on nanotube devices may be attributed to various indirect effects enhanced by water vapors, rather than the carrier generations by the physisorbed H2O molecules.

Modification of Electrical Properties of Graphene by Substrate-Induced Nanomodulation

A periodically modulated graphene (PMG) generated by nanopatterned surfaces is reported to profoundly modify the intrinsic electronic properties of graphene. The temperature dependence of the sheet resistivity and gate response measurements clearly show a semiconductor-like behavior. Raman spectroscopy reveals significant shifts of the G and the 2D modes induced by the interaction with the underlying grid-like nanostructure. The influence of the periodic, alternating contact with the substrate surface was studied in terms of strain caused by bending of graphene and doping through chemical interactions with underlying substrate atoms. Electronic structure calculations performed on a model of PMG reveals that it is possible to tune a band gap within 0.14-0.19 eV by considering both the periodic mechanical bending and the surface coordination chemistry. Therefore, the PMG can be regarded as a further step toward band gap engineering of graphene devices.

Crystal Shape of a Nickel Particle Related to Carbon Nanotube Growth

In order to investigate the relevance of the equilibrium crystal shape of a nickel particle during carbon nanotube growth, we have performed self-consistent pseudopotential density-functional calculations. The nickel particles equilibrium shape is obtained from the Wulff construction using the calculated surface energies. To understand the role of facets of the nickel particle, we investigate the adsorption and diffusion of the carbon atom on the nickel surfaces. The desorption energy of the carbon atom and the activation energy for carbon diffusion are found to be very different on different low-index facets, thus diffusion behaviors will also differ. It is found that the {111} and {110} facets are likely to be more reactive compared to the {100} facet. Therefore, the facets of the nickel particle will play an important role in carbon nanotube growth.

THEORETICAL STUDY OF HYDROGEN-COVERED DIAMOND (100) SURFACES : A CHEMICAL-POTENTIAL ANALYSIS

The bare and hydrogen-covered diamond (100) surfaces were investigated through pseudopotential density-functional calculations within the local-density approximation. Different hydrogen coverages, ranging from one to two, were considered. These corresponded to different structures (1\ifmmode\times\else\texttimes\fi{}1, 2\ifmmode\times\else\texttimes\fi{}1, and 3\ifmmode\times\else\texttimes\fi{}1) and different hydrogen-carbon arrangements (monohydride, dihydride, and configurations in between). Assuming the system was in equilibrium with a hydrogen reservoir, the formation energy of each phase was expressed as a function of hydrogen chemical potential. As the chemical potential increased, the stable phase successively changed from bare 2\ifmmode\times\else\texttimes\fi{}1 to (2\ifmmode\times\else\texttimes\fi{}1):H, to (3\ifmmode\times\else\texttimes\fi{}1):1.33H, and finally to the canted (1\ifmmode\times\else\texttimes\fi{}1):2H. Setting the chemical potential at the energy per hydrogen in

The presence of CH3NH2 neutral species in organometal halide perovskite films

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Elastic properties and stacking fault energies of Cr2Ta

and in a free atom gave the (3\ifmmode\times\else\texttimes\fi{}1):1.33H and the canted (1\ifmmode\times\else\texttimes\fi{}1):2H phase as the most stable one, respectively. However, after comparing with the formation energy of

Systematic study of electronic structure and band alignment of monolayer transition metal dichalcogenides in Van der Waals heterostructures

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Indirect-direct band gap transition through electric tuning in bilayer MoS2

, only the (2\ifmmode\times\else\texttimes\fi{}1):H and (3\ifmmode\times\else\texttimes\fi{}1):1.33H phases were stable against spontaneous formation of

Realistic adsorption geometries and binding affinities of metal nanoparticles onto the surface of carbon nanotubes

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