A significant part of my research focuses on computational characterization of structural, electronic, and optical properties of materials via atomistic scale simulation techniques from the first principles such as density-functional theory, many-body perturbation theory, density-functional perturbation theory, etc.
Essentially, first-principles simulation techniques such as density-functional theory makes approximations directly on top of the Schrödinger equation, which is the fundamental equation governing the quantum mechanical world, and study materials properties from the atomistic level. My research uses these approaches to understand properties of materials including:
Strutural properties such as hardness, bulk modulus, lattice constants.
Electronic energies to understand phase stability, surface stability, defects, etc.
Electronic transport to study mobility, electrical conductivity, superconductivity, etc.
Optical absorption to understand how materials response upon excitation via photons.
These characterization techniques are essential in materials discovery for many useful applications such as photovoltaics, high-power electronics, LEDs, and transistors. They are also very useful in explaining experimental findings. We have collaborated actively with experimental groups to understand energy landscapes of complicated teneary compounds, their electronic structures, and predict applications.
An example of the simulated surface structure of wurtzite ZnO and the corresponding electronic structure and density of states of the surface.
Theoretical models all have their limitations. It is important to understand what is the limitation of the models, and where we can improve them so that they describes the physical processes more accurately. More specifically I have worked with developing and implementing models to understand complicated physical processes related to optical excitation via perturbation theory into simulation tools such as the Vienna Ab initio Simulation Package (VASP) and Quantum Espresso. Some examples are:
Dynamical screening in the Bethe-Salpeter equation approach to understand optical excitations in materials with large exciton-binding energies.
Phonon-assisted optical processes to understand the absorption process in indirect semiconductors and doped semiconductors/metals.
Phonon-assisted two-photon process to understand absorption of high-intensity photons in indirect semiconductors.
An example of the atomic structure of an organic crystal Naphthalene who exhibit very strong exciton-binding, and requires the consideration of frequency-dependent dielectric screening models to accurately predict optical absorption (right panel, black: standard technique, colored: dynamical screening we developed. )
Computational techniques are the most useful and powerful when it can be connected to practical applications. I have worked with both developing tools in the atomistic level to understand materials properties in practical conditions, such as at finite temperature and under doping, as well as using mesoscale simulation tools to solve for continuum level equations to study structures using the atomistic scale simulations as foundations to provide the necessary inputs. Some examples that I've worked with before and currently developing are:
Implementing a framework to consider free-carrier absorption, treating various mechanisms on the same footing to understand optical loss in doped semiconductors and metals
Understanding electronic transport properties at finite temepratures considering electron-phonon interactions and electron-ionized-impurity interactions for doped semiconductors
Mesoscale simulation of optical profiles in layered systems solving the Maxwell's equations using Multiphysics tools such as COMSOL, Nextnano.
Silicon, the most popular solar cell material (adopted from [here]) and an example of the simulated absorption coefficient considering free-carrier absorption for n-type doped silicon (middle panel) and the short-circuit current density of silicon-based solar cell of different thickness and carrier density achieved using the absorption coefficient as an input.