Research

My interests include synthetic inorganic and materials chemistry, optical properties of semiconductor quantum wires, metallic nanoparticles, nanowire and nanotube growth mechanisms, electrical transport in nanowires, and nanocrystalline and nanocomposite structural materials.

Semiconductor Quantum Wires and the Influence of Geometric Dimensionality on Quantum Confinement: Quantum-confinement effects are the dramatic changes in electronic and optical properties occurring in small semiconductor crystallites as a result of the geometric confinement of electrons and holes. When an electron-hole pair in an excited nanocrystal is squeezed into a dimension approaching the bulk exciton Bohr radius (~2-60 nm), the effective band gap of the semiconductor increases with decreas innanocrystal size. Thus, the magnitude of quantum confinement depends upon nanocrystal size and composition. But how about the nanocrystal shape? One may reasonably wonder which nanocrystal shape- the quantum well (layer), quantum wire, quantum rod (short wire), orquantum dot - should exhibit the inherently stronger quantum-confinement effects.The answer is known theoretically: 3D confinement is stronger than 2D confinement, which in turn is stronger than 1D confinement. Thus, the magnitude of quantum confinement should increase in the order wells < wires < rods < dots. My group is now providing quantitative experimental ver-ification of these predictions. We grow soluble, diameter-controlled quantum wires by solution chemistry using monodisperse metallic-nanoparticle catalysts. Spectroscopic characterization of the wires, from which their band gaps and other optical properties are determined, is conducted in collaboration with Prof. Loomis. The size dependences of the quantum-wire band gaps and other properties are compared to those of the corresponding dots, rods, and wells, and to the results of high-level theoretical calculations provided by the group of Dr. Lin-Wang Wang (Lawrence Berkeley National Lab.). Our work affirms that bodybuilders, distance runners, architects, and quantum mechanics all agree: in function, performance, and behavior - shape matters.

Electrical Transport in Boron-based Nanowires. As integrated electronics continue to shrink toward the nanometer scale, much effort is focused on identifying nanoscale components to serve as the active devices and interconnects in nanoelectronic circuitry. Boron and metal-boride nanowires should have the ideal strengths, stabilities, and conductivities for such applications. We are growing such nanowires by catalyzed CVD, and studying their electrical properties in collaboration with Prof. Jia G. Lu at UC-Irvine.

Selected Publications

• H. Yu, J. Li, R.A. Loomis, L.-W. Wang, and W.E. Buhro,* "Two- versus three-dimensional quantum confinement in indium phosphide wires and dots," Nature Mater., 2, 517 (2003).
• W.E. Buhro,* H. Yu, J. Li, R.A. Loomis, P.C. Gibbons, and L.-W. Wang, "Cadmium selenide quantum wires and the transition from 3D to 2D confinement," J. Am. Chem. Soc., 125, 16168 (2003).
• C.J. Otten, O.R. Lourie, M.-F. Yu, J.M. Cowley, M.J. Dyer, R.S. Ruoff,* and W.E. Buhro,* "Crystalline boron nanowires," J. Am. Chem. Soc., 124, 4564 (2002).