UCI CFSE - Core Initiatives

The School of Physical Sciences' Center for Solar Energy supports multiple research initiatives:

Core Initiative: Metal-Semiconductor Hybrid Nanowires Utilizing Plasmonics for Concentrating Solar Radiation.

Principle Investigator: Dean John Hemminger

New nanofabrication capabilities have opened the door to studying the fundamentals of solar energy conversion with new nanoarchitectures. Atomic steps on the surface of highly ordered pyrolytic graphite (HOPG) can be used to nucleate silver or gold nanoparticles during the deposition by hot-filament evaporation. During this deposition process, metal adatoms incident on the HOPG surface diffuse laterally until encountering a step edge and aggolomerating with other metal atoms to form a metal nucleus.

Figure 1. SEM images of silver nanoparticle arrays on a HOPG substrate grown at 400 ºC deposition temperature with 5.5 nm mass thickness of silver. (a), (b), and (c) are images with different scales.
One specific concept that we are exploring is composite nanowires in which noble metal particles (Ag or Au) are connected by segments of a direct-gap semiconductor (e.g., CdSe, CdTe) as shown in Figure 2 below. With techniques that study the surface of the materials, we will study how the metal’s surface plasmons affect the properties of the semiconductor. Plasmons can localize incident light and enhance solar light absorption, therefore having the ability to increase the efficiency of ultra-thin solar cells.

Figure 2. Silver or gold nanoparticles for 1-D nanoaparticle ensembles on a HOPG substrate. Then, a semiconductor can be electrodeposited on these 1-D ensembles resulting in the formation of hybrid semiconductor/metal nanowires.
Relevant Publications

W. Luo, W. van der Veer, P. Chu, D.L. Mills, R.M. Penner, J.C. Hemminger, Polarization-Dependent Surface Enhanced Raman Scattering from Silver 1D Nanoparticle Arrays, J. Phys. Chem. C 112 (2008) 11609. pdf.

M.J. Krisch, R. DŐAuria, M.A. Brown, D.J. Tobias, J.C. Hemminger, The Effect of an Organic Surfactant on the Liquid-Vapor Interface of an Electrolyte Solution, J. Phys. Chem. C 111 (2007) 13497. pdf.

Q. Li, M.A. Brown, J.C. Hemminger, R.M. Penner, Luminescent Polycrystalline Cadmium Selenide Nanowires Synthesized by Cyclic Electrodeposition/Stripping Coupled with Step Edge Decoration, Chem. Mater. 18 (2006) 3432. pdf.

Core Initiative: Quantum Dot Solar Cells

Principle Investigator: Professor Matt Law

Solar cells based on neat films of electronically-coupled PbSe nanocrystals sandwiched between two electrodes are excellent model systems for studying junction formation, multi-exciton dynamics and charge transport in quantum dot solids. Moreover, these devices are promising for efficient, low-cost solar energy conversion because they can be processed in solution and may produce photocurrent that is enhanced by multiple exciton generation (MEG).

Figure 1. (a) A cartoon of the PbSe NC Schottky solar cell. Light is incident through the glass. (b) The device stack in cross section, showing the thickness and schematic roughness of each layer.
Schottky-type PbSe nanocrystal cells were recently shown to yield large short-circuit current densities (> 20 mA/cm²) and power conversion efficiencies above 2%. Our efforts focus on improving the efficiency and stability of this new class of devices and to demonstrate MEG-enhanced device performance.

Relevant Publications

M. Law, M.C. Beard, S. Choi, J.M. Luther, M.C. Hanna, A.J. Nozik, Determining the Internal Quantum Efficiency of PbSe Nanocrystal Solar Cells with the Aid of an Optical Model, Nano Lett. ASAP pdf.

J.M. Luther, M. Law, M.C. Beard, Q. Song, M.O. Reese, R.J. Ellingson, A.J. Nozik, Schottky Solar Cells Based on Colloidal Nanocrystal Films, Nano Lett. 8 (2008) 3488. pdf.

M. Law, J.M. Luther, Q. Song, B.K. Hughes, C.L. Perkins, A.J. Nozik, Structural, Optical, and Electrical Properties of PbSe Nanocrystal Solids Treated Thermally or with Simple Amines, J. Am. Chem. Soc. 130 (2008) 5974. pdf.

M. Law, J.M. Luther, Q. Song, C. L. Perkins, M.C. Beard, A.J. Nozik, Structural, Optical, and Electrical Properties of Self-Assembled Films of PbSe Nanocrystals Treated with 1,2-Ethanedithiol, ACS Nano 2 (2008) 271. pdf.

Core Initiative: Molecular Machines for Solar-Powered Photochemistry

Principle Investigator: Professor Alan F. Heyduk

Natural photosynthesis, carried out in green plants and purple photosynthetic bacteria, powers the planet by harnessing solar energy to drive endothermic chemical reactions. These endothermic reactions produce oxygen and sugars, which are used as energy carriers in every facet of modern society.

Figure 1. Schematic of energy transfer in natural photosynthesis.
An attractive alternative to further consumption of fossil fuel resources and deterioration of our environment is the development of a new artificial photosynthesis, which stores 58 kcal/mol of solar energy by splitting water into oxygen gas and hydrogen fuel. Despite the overwhelming importance of developing such a strategy, little progress has been made towards a workable system for artificial photosynthesis, owing to the slow rates of inter-conversion for the fundamental multi-electron reactions of water splitting.

Figure 2. Schematic of water splitting using metal catalysts.
We believe that the development of new catalysts to promote water-splitting reactions will provide the fundamental understanding necessary to overcome slow multi-electron reaction rates in strategies for artificial photosynthesis. As such we are developing a new class of molecular architectures that work to capture solar energy and channel it into the formation of hydrogen and oxygen gas.

Relevant Publications

R.A. Zarkesh, J.W. Ziller, A.F. Heyduk, Four-Electron Oxidative Formation of Aryl Diazenes Using a Tantalum Redox-Active Ligand Complex, Angew. Chem. Int. Ed. 47 (2008) 4715. pdf.

S.M. Carter, A. Sia, M.J. Shaw, A.F. Heyduk, Isolation and Characterization of a Neutral Imino-semiquinone Radical, J. Am. Chem. Soc. 130 (2008) 5838. pdf.

A.F. Heyduk, D.G. Nocera, Hydrogen Produced from Hydrohalic Acid Solutions by a Two-Electron Mixed-Valence Photocatalyst, Science 293 (2001) 1639. pdf.

Core Initiative: Direct Solar Thermal-to-Electrical Energy Conversion using Thermoelectric Nanowires.

Principle Investigator: Professor Reginald M. Penner

The heat produced by infrared photons emitted by the sun represents lost energy. How might this energy be recovered? Thermoelectric materials provide a way to directly convert heat to electrical power, but this conversion process is inefficient. The figure-of-merit for state-of-the-art thermoelectric materials, ZT, has been stuck at 1.0 - 1.2 since the discovery of Bi₂Te₃ fifty years ago. According to theory, nanowires may exhibit ZT values considerably larger than 1.0. So far, however, few experimental measurements of ZT for nanowires have been reported. One experimental problem is that nanowires must be extremely long in addition to having nanoscopic width, as shown in Figure 1 below.

Figure 1. a) Schematic of a thermoelectric device using nanowires. The wires have to be long so the device can easily be electrically contacted. b) Graph based on theory showing increased power output with decreased width of thermoelectric material.
Stoichiometric and single-phase PbTe nanowire arrays have been prepared using a new method, photoresist-bottomed lithographically patterned nanowire electrodeposition (PB-LPNE). PB-LPNE provides for control over wire width and thickness, photolithographic patterning, and the suspension of nanowires across 25-micrometer air gaps separating photoresist supports.

Figure 2. Synthesis of air-suspended PbTe nanowires. a,b) SEM images of suspended PbTe nanowire arrays at 2 micron pitch density. The air gap between the PbTe nanowires and the glass surface is 2 microns in this case.
The resulting nanowire-substrate system provides new opportunities for the investigation of optical, electrical, and thermoelectric properties.

Relevant Publications

Y. Yang, S.C. Kung, D.K. Taggart, C. Xiang, F. Yang, M.A. Brown, A.G. Gźell, T.J. Kruse, J.C. Hemminger, R.M. Penner, Synthesis of PbTe Nanowire Arrays using Lithographically Patterned Nanowire Electrodeposition, Nano Lett. 8 (2008) 2447. pdf.

E.J. Menke, M.A. Brown, Q. Li, J.C. Hemminger, and R.M. Penner, Bismuth Telluride (Bi₂Te₃) Nanowires: Synthesis by Cyclic Electrodeposition/Stripping, Thinning by Electrooxidation, and Electrical Power Generation, Langmuir 22 (2006) 10564. pdf.,

E.J. Menke, Q. Li, and R.M. Penner, Bismuth Telluride (Bi₂Te₃) Nanowires Synthesized by Cyclic Electrodeposition/Stripping Coupled with Step Edge Decoration, Nano Lett. 4 (2004) 2009. pdf.

Core Initiative: Developing Sustainable Materials for Solar Energy Conversion

Principle Investigator: Professor Matt Law

Fundamental to solving our energy problems is the discover of new materials that can effectively harness the sun's energy but are based on abundant and non-toxic elements. The current research is focused on the synthesis of quantum dots, nanorods, and nanowires made from sustainable materials using mulitple synthetic techniques.

Figure 1. SEM image of a ZnO nanorod array
Relevant Publications

M. Law, L.E. Greene, A. Radenovic, T. Kuykendall, J. Liphardt, P. Yang, ZnO-Al₂O₃ and ZnO-TiO₂ Core-Shell Nanowire Dye-Sensitized Solar Cells, J. Phys. Chem. B 110 (2006) 22652. pdf.

M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P. Yang, Nanowire dye-sensitized solar cells, Nat. Mater. 4 (2005) 455. pdf.

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