Everything we see around us is the surface of a material. Surfaces and interfaces are important because they represent the boundary between two materials, or a material and its environment. Surface and interface phenomena are at the core of the performance of all electronic and optical devices, determining for example band alignment at a heterojunction or the catalytic properties for surface reactions. Understanding surface and interface properties at the atomic scale is challenging. Inside of the material (bulk), properties are uniform because each atom is surrounded by other similar atoms. On the other hand, surface atoms may have dissimilar atoms above them, or may not have any atoms when the environment is vacuum.
Copper Indium Diselenide is a high-efficiency thin-film photovoltaic material. It's high efficiency arises from a favorable defect chemistry both in the bulk and at surfaces and interfaces in the solar cell. In collaboration with Prof. Angus Rockett's group, we are assessing the stable surface reconstruction mechanisms at the surface of CuInSe and the cousin material AgInSe. Although Cu and Ag are isovalent, scanning tunneling microscopy images show that both material facet to the polar (112) surface, but the surface reconstruction mechanisms are different. It is known that there are missing Cu atoms (vacancy) at the surface. Through the first-principles analysis, we can show that compensating fields generated by Cu vacancies at the surface stabilizes the entire material. We are now trying to reveal the regions in chemical potential phase space where vacancy and/or antisite -mediated reconstructions are energetically stable for the two materials.
Hydrogen fuel provides an excellent means of storing and transporting environmentally clean energy because it has a high energy-to-weight ratio and does not produce carbon emissions. Current methods of production of hydrogen fuel involve electrolysis using electricity that was generated in ways that contribute to greenhouse gas emissions. However, water splitting catalyzed on the surface of a photovoltaic material has the potential to directly convert sunlight into a transportable, zero-emission fuel source. A few material surfaces, such as titanium dioxide, some perovskites, and several others have been identified as candidates for efficient water splitting. These materials often perform best when doped with foreign atoms at or near the surface. In collaboration with several experimental group at Kyushu University, we are investigating the viability of doped nanosheets (calcium niobate and lepidocrocite TiO2) for water splitting. To this end, we perform density functional theory calculations to determine the electronic structure of the materials as well as barriers and possible reaction pathways. Comparison of these should allow us to pick the best dopants and dopant sites to catalyze the reaction.
In collaboration with Prof. Ed Seebauer's group, we are exploring surface reconstructions and mechanisms of oxygen incorporation from exposed zinc oxide surfaces into the near-surface bulk. Our work here is focused on understanding how various zinc oxide surfaces can adsorb molecular oxygen and then incorporate into the interior. Defect engineering from the surface can give rise to controllable profiles in the interior, with the long-term goal of controlling surface band bending profiles and potentially the surface chemistry.