Dr. Junwen Li
Dr. Junwen Li
The Power of Density Functional Theory Simulations
From Model Hamiltonian to Energy Materials
In 1929 Dirac wrote: “The general theory of quantum mechanics is now almost complete, … the difficulty is only that the exact application of these laws leads to equations much too complicated to be soluble. It therefore becomes desirable that approximate practical methods of applying quantum mechanics should be developed ….” The density functional theory (DFT) born in 1964 is just such an approximate practical method which replaces many-electron wave functions with electron density. Owing to the rapid development of massively parallel computing facilities, computational modeling using DFT plays an increasingly important role in both fundamental and practical materials studies.
Here I will present my research projects as examples to illustrate how DFT is used to establish model Hamiltonian and to investigate energy materials. I will start with building tight-binding model for graphene nanoribbons. The nearest-neighbor tight-binding model has been extensively used in the study of physical properties of carbon nanostructures. However, our DFT calculations reveal that third-nearest-neighbor interactions are needed in the tight-binding description of armchair graphene nanoribbons. We further find that this longer range interaction had been hidden in the carbon nanotube tight-binding model for more than 10 years. Then, I will talk about energy materials related to hydrogen and photovoltaics, two promising alternative and clean energy solutions. Hydrogen evolution from water can be easily achieved by electrolysis at large overpotentials that can be lowered with expensive Pt-based catalysts. Replacement of Pt with cost-effective and earth-abundant electrocatalysts would be significantly beneficial for large scale hydrogen generation. The chemically exfoliated WS2 nanosheets can exhibit enhanced electrocatalytic activity. We find that different from previously recognized active edges in MoS2, the surface of WS2 is taking part in the catalytic reaction. The tensile strain in as-exfoliated nanosheets is responsible for the observed low overpotentials. For the photovoltaic materials, I will discuss the spintronics in the emerging organic-inorganic halide perovskite CH3NH3PbI3 solar cell absorbers. Most perovskite solar cells are polycrystalline, permeated by grain boundaries which are typically recombination centers. One of the mysteries of perovskite materials is the remarkable conversion efficiency which persists in spite of these grain boundaries. Similar questions apply to other successful polycrystalline photovoltaic materials, such as CdTe and CuInxGa1-xSe2. We show that the strong Rashba-type spin-orbit coupling in perovskite materials provides a great opportunity of using spin excitations to study the charge current distribution and therefore, the role of grain boundaries in polycrystalline solar cell materials. Finally, I will conclude with my future research plan including nonlinear optics, secondary phases in Cu2ZnSnSxSe4-x solarcell absorbers, catalysts for photocatalytic water splitting and large scale DFT simulation package development