Razvan A. Nistor

A generalization of the charge equilibration method for nonmetallic materials.

Assigning effective atomic charges that properly reproduce the electrostatic fields of molecules is a crucial step in the construction of accurate interatomic potentials. We propose a new approach to calculate these charges, which as previous approaches are, is based on the idea of charge equilibration. However, we only allow charge to flow between covalently bonded neighbors by using the concept of so-called split charges. The semiempirical fit parameters in our approach do not only reflect atomic properties (electronegativity and atomic hardness) but also bond-dependent properties. The new method contains two popular but hitherto disjunct approaches as limiting cases. We apply our methodology to a set of molecules containing the elements silicon, carbon, oxygen, and hydrogen. Effective charges derived from electrostatic potential surfaces can be predicted more than twice as accurately as with previous works, at the expense of one additional fit parameter per bond type controlling the polarizability between two bonded atoms. Additional bond-type parameters can be introduced, but barely improve the results. An increase in accuracy of only 30% over existing techniques is achieved when predicting Mulliken charges. However, this could be improved with additional bond-type parameters.

The Role of Chemistry in Graphene Doping for Carbon-Based Electronics

Graphene forms an important two-dimensional (2D) material class that displays both a high electronic conductivity and optical transparency when doped. Yet, the microscopic origin of the doping mechanism in single sheet or bulk intercalated systems remains unclear. Using large-scale ab initio simulations, we show the graphene surface acts as a catalytic reducing/oxidizing agent, driving the chemical disproportionation of adsorbed dopant layers into charge-transfer complexes which inject majority carriers into the 2D carbon lattice. As pertinent examples, we focus on the molecular SbCl(5) and HNO(3) intercalates, and the solid compound AlCl(3). Identifying the microscopic mechanism for the catalytic action of graphene is important, given the availability of large area graphene sheets, to spur research into new redox reactions for use in science and technology.

Evidence for electronic gap-driven metal-semiconductor transition in phase-change materials

Phase-change materials are functionally important materials that can be thermally interconverted between metallic (crystalline) and semiconducting (amorphous) phases on a very short time scale. Although the interconversion appears to involve a change in local atomic coordination numbers, the electronic basis for this process is still unclear. Here, we demonstrate that in a nearly vacancy-free binary GeSb system where we can drive the phase change both thermally and, as we discover, by pressure, the transformation into the amorphous phase is electronic in origin. Correlations between conductivity, total system energy, and local atomic coordination revealed by experiments and long time ab initio simulations show that the structural reorganization into the amorphous state is driven by opening of an energy gap in the electronic density of states. The electronic driving force behind the phase change has the potential to change the interconversion paradigm in this material class.