Our main research area concerns understanding the fundamental properties of materials with a size in between individual molecules and macroscopic objects. These nanometer scale materials have physical characteristics that are strong functions of their size and shape, with properties that can be easily manipulated to address a given application. Our investigations in this area are currently focused on fundamental photophysical studies of carbon nanotubes and semiconductor nanocrystal quantum dots, and the integration of these materials into both novel devices for solar energy conversion and biological sensors. These studies are highly interdisciplinary, and lie at the interface between chemistry, optics, physics, applied physics, and materials science.

Carbon nanotubes consist of a hexagonal network of carbon atoms rolled up into a cylinder. Nanotubes are typically microns long, while their diameter is usually only around 1 nanometer. The quantum confinement of electrons around the nanotube results in unexpected properties. For example, carbon nanotubes can be either metallic or semiconducting, depending on the diameter and helicity of the carbon lattice. We are concerned with determining fundamental electronic and optical characteristics of single carbon nanotubes (Figure 1). These investigations are carried out using atomic force microscopy, as well as single molecule and ultrafast optical spectroscopy.

Figure 1. Fluorescence spectrum and (inset) a fluorescence image from an individual single walled carbon nanotube.

Figure 2. Fluorescence from different sized CdSe quantum dots.

Inorganic semiconductor particles containing a few thousand atoms, known as semiconductor quantum dots (Figure 2) also have unique electronic and optical properties. Using state-of-the-art experimental techniques (electrostatic force microscopy and single molecule optical spectroscopy), we are investigating the optical emission characteristics of individual quantum dots and the effect of permanent charges on this emission.

A significant remaining challenge for materials chemistry is to connect nanometer-sized materials to the macroscopic world. To that end, we are also developing simple synthetic methods to make nanocrystals and carbon nanotubes, and are exploring chemical modification of their surfaces. Our eventual goal is to build both integrated solar cell devices and biological sensors using nanomaterials as building blocks. Also, in collaboration with Professor Bren from the University of Rochester, we have began experiments to understand the folding of proteins (cytochromes c) on the single molecule level.