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.
In conventional thin-film heteroepitaxy, the formation of misfit dislocations (and their effect on electronic and optical properties) for highly lattice-mismatched systems greatly limits the materials that can be integrated together across a broad spectrum of areas. Several well-known ‘critical thickness’ models of great historical significance -- such as the Matthews Blakesless model -- are used to estimate when these defects will form. We are interested in extending these critical thickness models to the newly realized 2D superlattices. Due to lattice mismatch, we consider the formation of topological defects that are the equivalent of misfit dislocations at the interface of two-dimensional graphene/boron nitride superlattices, which have been synthesized for the first time in the laboratory last year. In analogy to thin-film superlattices, a ‘critical superlattice pitch’ exists beyond which the formation of these dislocations is favorable. In contrast to thin-film epitaxy however, for two-dimensional superlattices we also find that critical thicknesses are greatly enhanced relative to their thin-film analogue, thanks to strain-relief mechanisms that are unique to 2D (buckling and out of plane deformation).
Green energy from wasted heat sounds like a free lunch, but thermoelectric materials do provide a promising vehicle. Dictated by the second thermodynamic law, a great amount of energy input is being (has to be) wasted in the form of heat. As global economic activities explode, thermal waste also spikes due to the sharp rise of energy expenses. Among all the others, thermoelectric materials convert heat directly and cleanly into electricity, without any moving parts and very high specific power (i.e., remarkable advantage in maintenance and scaling). However, due to tightly interrelated but competing requirements in material properties, the pursuit of high thermoelectric performance has proven strenuous for nearly 200 years. Nonetheless, the figure of merit experienced a sudden jump thanks to improvements in nanoscale fabrication and synthesis. We are currently studying phonon transport on two-dimensional superlattices using molecular dynamics, lattice dynamics, as well as density functional perturbation theory. Our primary concerns are two-fold: i) the anomalous transport physics, such as the dependence of thermal properties on structural dimensions; and ii) possible high-throughput designs for practical applications.