Group IV crystalline solids act as a diverse class of materials for basic research, aesthetic interest, as well as a plethora of technological uses. In particular, Si and Ge have been of great importance as elemental semiconductors in a broad range of structures ranging from the first transistor to the extremely fast processors used today. In general, my research interests focus on multifunctional semiconducting nanostructures relevant to biomaterials and nanoscale electronics (and the two are not necessarily mutually exclusive!).
Nanoscale Silicon-Based Biomaterials. For this class of materials, we seek to successfully construct a rapidly-adaptive platform based on electrically-responsive, mechanically-robust tunable artificial nanostructures that are not only biocompatible, but furthermore bioactive, and whose activity can be altered not only by physical dimension and chemical composition but external stimuli as well. Recent focus has entailed studies of the bottom up synthesis of elemental silicon dots and wires, top down fabrication of spongy porous Si structures, and fundamental studies of surface modification and diffusion from these matrices. Incorporation of the proper inorganic component to the nanostructures brings mechanical strength and semiconductive character; Porosity allows for the release of therapeutic release of useful substances from the material, as well as proper vasculature & neural in-growth to the scaffold; in some cases, composite formulation with biopolymers brings tunability to the structure in terms of biodegradability.
‘Doped’ Silicon and Germanium Nanocrystals and Nanowires. Two existing fundamental strategies designed to overcome silicon’s intrinsic indirect bandgap and the accompanying absence of efficient light emission are: (1) the formation of visibly-luminescent, quantum-confined nanophase Si and (2) rare earth incorporation into single crystal Si. In a synergistic combination of these approaches, we are expending extensive effort into the incorporation of optically-active rare earth dopants into discrete Si nanoparticles as well as Si and Ge nanowires, and systematically investigating these nanostructures as a function of size & dimension. We have recently succeeded in the first preparation of discrete erbium-doped silicon nanoparticles prepared by the co-pyrolysis of disilane and a volatile Erbium b-diketonate complex. The characteristic Er3+ near IR PL signature at 1540 nm is observed in these nanoparticles, mechanistically via a carrier-mediated process.
The long-term goal of this work is to produce a more systematic understanding of how the nature of rare earth - semiconductor charge carrier interactions evolve as the particle size of the Si or Ge host changes. It is hoped that the information gleaned from these studies will be of extensive value in the design of new Si or Ge-based opto-electronic systems as well as new materials demonstrating useful properties.
Some Recent Selected Publications
“Porous Silicon-Based Scaffolds for Tissue Engineering and Other Biomedical Applications,” Coffer, J.; Whitehead, M.A.; Nagesha, D.; Mukherjee, P.; Akkaraju, G.;Totolici, M.;Saffie, R.; Canham, L.. Phys. Stat. Sol (a) , 2005 , 202 , 1451.
“Biorelevant Calcification and Non-Cytotoxic Behavior in Silicon Nanowires ” Nagesha, D.; Whitehead, M.A.; Coffer, J.L., Adv. Mater . 2005 , 17 , 924,
“Fabrication and Optical Properties of Erbium-Doped Germanium Nanowires”, Wu, J.; Coffer, J.L.; Punchaipetch, P.; Wallace, R.M. Adv. Mater ., 2004 ,16, 1444.