Chemistry and physics share tremendous potential at the nanoscale. This is where chemistry excels and where physics predicts that many properties can be tuned. For example, quantum states, charging, spin, phonons, and plasmons show large effects at the nm scale, which can be investigated by using colloidal synthesis to chemically precipitate nanostructures. The research in the group is driven by physical concepts and enabled by synthesis.
Quantum Confined Semiconductors
Nanocrystals of semiconductor materials show very strong quantum confinement effects, so controlling the size leads to exquisite tuning of energy levels. The group is studying the effect of small amounts of additional charges, i.e. quantum dot ions, on the optical, magnetic, and electronic properties of the dots. We synthesize semiconductor nanocrystals, controlling their sizes and surfaces, and use microscopy and nonlinear spectroscopy to study basic aspects of electron dynamics and interaction in such strongly confined structures. We currently focus on the doping, the unusual infrared response, and the electrochromic effects of these nanocrystals, as well as the potentially novel electrical transport properties in films made of these artificial atoms.
The above image demonstrates the conductivity through thin, quantum dot films. As the potential is more reducing (-), electrons are added to the CdSe quantum dots. The conductance of a film of dots (red line) first increases linearly with the occupation of the 1Se orbital (dashed blue line), up to ½ filling, at which point it decreases. The conductance then increases again as the 1Pe shell (dashed green line) occupation increases. Shell occupation is controlled by the electrochemical potential and measured by the optical absorption of the sample. Science 300, 1277 (2003)
Plasmonic Metal Nanoparticles
A metal object of dimensions much smaller than the wavelength of light exhibit a very strong optical response due to a collective excitation of all its valence electrons, called a “plasmon.” The frequency at which this resonance occurs is tightly determined by the composition and shape of the object, and its linewidth is dictated by the losses caused by the scattering of the electrons. Thus, gold and silver are the best, and most chemically stable, materials for such particles. Since the resonance is a shape, rather than size, effect, optimizing colloidal syntheses to yield specific shapes is an important goal. Needle shape nanostructures are particularly advantageous, as they focus external electric fields to very high values at very specific points. Our research aims to observe the maximum effect of these large local fields on the photoresponse of individual nanoparticles and assemblies.
The above image demonstrates how solutions of gold and gold/silver core/shell colloids can exhibit very different colors, effectively covering the visible spectrum. This is not a quantum effect, but is caused by controlling the locations of the plasmon resonances of the particles. J. Phys. Chem. B, 108, 5882 (2004)