Prof. Xihong Peng's Computational Group

I. Semiconduting nanostructrues

We studied various semiconducting nanostructures, including Si quantum dots, Si/Ge homogeneouse and core-shell hetero nanowires, carbon nanotubes, graphene nanoribbons, group III-V AlN/GaAs/InAs nanowires. We investigated structural, mechanical, and electronical properties of these nanostructures, such as band gap, band structure, effective masses of charge carriers, workfunction, and strain depedence of those properties.

II. Hydrogen storage

The goal of this project is to investigate atomic processes involved in hydrogenation and dehydrogenation of magnesium hydride by employing first principles electronic structrue calculations. A fundamental understanding of the mechanism of hydrogenation and dehydrogenation of magnesium hydride is essential for tailoring the material to a fast kinetics and a low hydrogen decomposition temperature. We systematically investigate the effects of strain, catalyst, and defect on the hydrogenation and dehydrogenation of magnesium hydride.

III. CO2 interacts with brookite TiO2

We studied the interactions of CO2 with the (210) surface of brookite TiO2 using first-principle calculations on cluster and periodic slab systems. Charge and spin density analyses were implemented to determine if charge transfer to the CO2 molecule occurred and whether this charge transfer was comparable to that seen with the anatase TiO2 (101) surface. The project is collaborated with Prof. Andino's research group.

IV. Tin-doped chalcopyrite materials

We performed density functional theory calculations to understand the effect of Tin doping in chalcopyrite material AgInS2 (AIS). The dopant formatin energy for Sn occuping different sites in AIS with different doping rate were calcualted to determine the energetically most favorable site. The band structures and density of states of the doped AIS were analyzed to better understanding experimental data from photoelectrochemical evaluation of nano-textured Sn-doped AIS films. The project is collaborated with Prof. Chan's group.

V. Proton-exchange-membrane fuel cells

Fuel cells, more specifically Proton Exchange Membrane Fuel Cells (PEMFCs) are projected to be the major electrochemical energy conversion device in hydrogen economy. However, the slow kinetics of the cathodic oxygen reduction reaction (ORR), and high cost of the Pt-based noble metal electrocatalysts still pose an obstacle for their commercial viability. This research effort is to identify and design alternative electrocatalysts (noble metal alloys) with high ORR performance and durability using low Pt content on carbon support, along with fluid dynamic simulation of electrode structure and experimental testing conducted in Prof. MadaKannan's fuel cell lab.

VI. 2D Phosphorene

Recently fabricated two dimensional (2D) puckered honeycomb phosphorene crystal structures demonstrated great potential in applications of electronics. Mechanical strain was demonstrated to be able to significantly modify the electronic properties of phosphorene and few-layer black phosphorus. Compared to other 2D materials, such as graphene, phosphorene demonstrates superior flexibility with an order of magnitude smaller Young's modulus. This is especially useful in practical large-magnitude-strain engineering. It can sustain tensile strain up to 27% and 30% in the zigzag and armchair directions, respectively. In addition, it was found that the band gap of phosphorene experiences a direct-indirect-direct transition when axial strain is applied. A moderate −2% compression in the zigzag direction can trigger this gap transition. Effective masses of carriers in the armchair direction are an order of magnitude smaller than that of the zigzag axis, indicating that the armchair direction is favored for carrier transport.

VII. Clathrates as Anodes for Li-ion batteries

Types I and II Si and Ge clathrate materials recently been studied for their electrochemical properties as anodes for lithium-ion batteries due to their unique cage structures and ability to incorporate extrinsic guest atoms. First-principles density functional theory (DFT) calculations were performed to investigate the type I Si and Ge clathrate compounds with and without the guest Ba atoms to understand the optimal structural configurations of small degrees of lithiation and Li diffusion paths inside the clathrates. The results showed that Li insertion into framework or Ba vacancies could stabilize the clathrate structures and it is energetically feasible for multiple guest atoms to be placed in the Si24 cages. For Ba-doped Ge clathrates, it was found that Li insertion into the three framework vacancies in Ba8Ge43 is energetically favorable, with a calculated lithiation voltage of 0.77 V versus Li/Li+ [2]. However, the high energy barrier (1.6 eV) for Li diffusion between vacancies and around Ba guest atoms suggests that framework vacancies are unlikely to significantly contribute to lithiation processes unless the Ba guest atoms are absent. The results from this study can elucidate the preferred structural configurations for Li in type I, Ba-doped Si and Ge clathrates and also be informative for efforts related to understanding the structures obtained after electrochemical insertion of Li into the clathrates.