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.

• Silicon and Germanium Nanocrystals and Nanowires. Two existing fundamental strategies designed to overcome the intrinsic indirect bandgap of Si (or Ge) 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 focused our energies alng two main lines, with the goal of producing constructs relevant to two different optoelectronic components. The first, a light emissive source, is formed by introducing erbium ions into Si nanocrystals as well as into Si or Ge nanowires whose near infrared emitted light is initially generated by energy transfer from the host semiconductor.  The second is to fabricate structures capable of modulating or guiding this emitted light along well-defined one dimensional geometries of the associated oxides (e.g. SiO2 or GeO2).

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.

• Medicinal Surface Modification of Silicon Nanowires: Impact on Calcification and Stromal Cell Proliferation, Ke Jiang, Dongmei Fan, Yamina Belabassi, Giridhar Akkaraju, Jean-Luc Montchamp and Jeffery L. Coffer, ACS Appl. Mater. Interfaces , 2009 , 1 (2), 266–269.
• High-porosity poly(caprolactone)/mesoporous silicon scaffolds : calcium phosphate induction and biological response to fibroblasts and bone precursor cells, M. A. Whitehead, P. Mukherjee , G. Akkaraju, L. T. Canham, and J. L Coffer, Tissue Engineering A , 2008 , 14(1): 195-206 .   
• Oxidized Germanium as a Broad-Band Sensitizer for Er-Doped SnO2 Nanofibers, Ji Wu, Jeffery L. Coffer, Yuejian Wang, and Roland Schulze, J. Phys. Chem. C , 2009 , 113 (1), 12–16.