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Arthur J. NOZIK
Office Phone: 303 735 5703, 303 384 6603
E-mail: arthur_nozik@nrel.gov
FAX: 303 384 6655
Lab: National Renewable Energy Laboratory
Lab Phone: 303
Group Website:
Ph.D.: Yale University, 1967
Electron Transfer Dynamics Across Semiconductor-Molecule Interfaces and Size Quantization Effects in Semiconductors
Professor Nozik and his research group at the National Renewable Energy Laboratory (NREL) in Golden are engaged in two areas of research that deal with phenomena at semiconductor-molecule interfaces: (1) the dynamics of electron relaxation and transfer across these interfaces and (2) size quantization effects in ultra small (2 - 25 nm) semiconductor nanocrystals and structures (called quantum dots, quantum rods, and quantum wells).
Electron Relaxation Dynamics
Photoelectrochemical energy conversion of light to electricity or fuels (e.g., hydrogen, alcohols, or hydrocarbons) and heterogeneous photocatalysis depend upon the efficient spatial separation and subsequent interfacial charge transfer of electrons and positively charged holes (the two charges carriers) that are created upon the absorption of photons in the semiconductor or by the molecules at semiconductor-molecule interfaces. The separated electrons and holes can produce electrical power or drive electrochemical oxidation-reduction reactions with redox molecules at the semiconductor surfaces.
A critically important fundamental issue is the dynamics of relaxation of the photogenerated charge carriers. The relaxation processes include carrier or molecule cooling, radiative and non-radiative recombination, and electron and hole transfer across the semiconductor-molecule interface. Systems of interest include electron transfer from photoexcited dye molecules adsorbed on the semiconductor surface, as well as charge transfer from illuminated semiconductors to molecules at the interface.
These dynamics are studied theoretically and experimentally. The experimental studies utilize ultrafast time-resolved transient laser spectroscopy in the fs to ns time regime. This includes fs visible to mid-IR transient absorption spectroscopy, fs luminescence up-conversion spectroscopy,fs terahertz spectroscopy, and time-correlated (ps to ns) single photon counting measurements. Steady-state electrochemical impedance spectroscopy, photocurrent spectroscopy, photomodulation spectroscopy, and quartz crystal micro/nanobalance measurements are also used in these studies.
Semiconductor Quantum Dots and Nanostructures
When electrons and holes in semiconductors are confined to ultra-small regions of space (typically 1-25 nm), the semiconductor structure enters the regime of size quantization, wherein the electronic energy levels of the system become discrete rather than quasi-continuous, and the optical and electronic properties of the semiconductor become strongly size-dependent. Such structures are called quantum dots or nanocrystals, quantum rods, or quatnum wells depending upon their shape and dimensionality of the quantum confinement. We produce and study these quantization effects in colloidal semiconductor nanocrystals produced via chemical synthesis, as well as in quantized semiconductor structures produced via epitaxial growth in a Metallorganic Organic Chemical Vapor Deposition (MOCVD) reactor; III-V semiconductors are typically the materials we study. Quantum dots and nanostructures are of great scientific interest and also have many important potential applications in quantum dot lasers, as photocatalysts, and in solar energy conversion. They show remarkable properties such as absorption and emission spectra that can shift by several eV as a function of quantum dot size, photoluminescence blinking, long-range energy transfer, enhanced non-linear optical effects, enhanced photoredox properties, and enhanced utilization of hot electrons via impact ionization (an inverse Auger process) or hot electron transport, interfacial transfer, and conversion.
Selected Publications
"III-V Quantum Dots and Quantum Dot Arrays: Synthesis, Optical Properties, Photogenerated Carrier Dynamics, and Applications to Photon Conversion," in Semiconductor and Metal Nanocrystals, V. Klimov, ed., Marcel Dekker, Inc., (2004) (with O.I. Micic).
"Growth of InP Nanostructures via Reaction of Indium Droplets with Phosphide Ions: Synthesis of InP Quantum Rods and InP-TiO2 Composites", JACS, in press (2004) (with J.M. Nedeljkovic, O.I. Micic, S.P. Ahrenkiel, A. Miedaner)
"Electron Relaxation in Colloidal InP Quantum Dots with Photogenerated Excitons or Chemically Injected Electrons," J. Phys. Chem. 107, 102 (2003) (with J.L. Blackburn, R.J. Ellingson, and O.I. Micic).
"Theoretical and Experimental Investigation of Electronic Structure and Relaxation in Colloidal Nanocrystalline Indium Phosphide Quantum Dots", Phys. Rev. B. 67, 075308 (2003) (with R.J. Ellingson, J.L. Blackburn, J. Nedeljkovic, G. Rumbles, M. Jones, and H. Fu).
"Electron Transfer Dynamics in Quantum Dot/Titanium Dioxide Composites Formed by in situ Chemical Bath Deposition, J. Phys. Chem. B 107, 14154 (2003) (with J.L. Blackburn and D.C. Selmarten).
"Electronic Coupling in InP Nanoparticle Array's", Nano Letters, 3, 1695 (2003) (with M.C. Beard, G.M. Turner, J.E. Murphy, O.I. Micic, M.C. Hanna, and C. Schmuttenmaer).
"Photo-enhancement of Luminescence in Colloidal CdSe Quantum Dot Solutions", J. Phys. Chem. B, 107, 11346 (2003) (with M. Jones, J. Nedeljkovic, R. Ellingson, and G. Rumbles).
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