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.
Prof. Xihong Peng's Computational Group
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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.