Semiconductor Nanocrystals, Gold Nanostructures, Toxic Metals in the Environment

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On the computational side, my research program seeks to model and thereby understand the properties of (i) semiconductor nanocrystals, (ii) gold nanostructures, and (iii) toxic metals in the environment. On the theoretical side, my research program seeks to (i) examine the quantum potential as a measure of reactivity and bonding and (ii) obtain an accurate expression for the kinetic energy that is a direct functional of the electron density. For ground state calculations we primarily use the Kohn-Sham formulation of density functional theory (DFT) with both gradient-corrected and hybrid functionals. For excited state calculations we primarily use time-dependent density functional theory (TD-DFT) with long-range corrected functionals. For finite temperature calculations we primarily use Born-Oppenheimer molecular dynamics (BOMD). ******Ligand-passivated semiconductor nanocrystals (NCs), also called quantum dots, have applications in many areas including photovoltaic devices. Upon adsorption of a photon with energy greater than the band gap, an electron-hole pair is formed and, if it can be separated, an electric current can be generated. Due to quantum confinement, a smaller NC has a larger band gap which can make better use of higher energy photons. Our current focus is on CdSe nanocrystals and related structures in which Se is replaced by Te or Cd is replaced by Cu2. Gold nanostructures have properties which are very different from those of the bulk metal. We established the stability of helical gold nanorods (both bare and with adsorbed molecules) and compared them to alternative compact and cage structures. Our current focus is on the optoelectronic properties of helical gold nanorods and their chiral applications as catalysts and biosensors. For toxic metals in the environment, our current focus is on arsenic. Arsenic exists in both inorganic and organic forms that can be interconverted by biogeochemical processes. Less is known about the organic forms and the ways in which they bind to inorganic soil particles. We are examining the properties of organic arsenic and iron-(oxyhydr) oxide complexes, and organic arsenic on goethite surfaces.******Reactivity and bonding are at the heart of chemistry and, although measures such as the Laplacian of the electron density and the electron localization function have been established, other measures could provide important new insights. In this regard, we plan to examine the quantum potential, which is a direct functional of the electron density, ?. In the Kohn-Sham formulation of DFT, the kinetic energy is obtained accurately (via N one-electron orbitals) as an indirect functional of ? but the scaling is nonlinear. We plan to construct an accurate direct (orbital-free) kinetic energy functional.**