The School of Physical Sciences Center for Solar Energy supports three core research initiatives:
Core Initiative: Metal-Semiconductor Hybrid Nanowires Utilizing Plasmonics for Concentrating Solar Radiation.
Principle Investigator: Dean John Hemminger
New nanofabrication capabilities open the door to new nanoarchitectures that may enable more efficient photovoltaics. One specific concept that we are exploring are 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 the Figure below:
Figure 1. Atomic steps on a graphite surface (left) can be used to nucleate silver or gold nanoparticles during the deposition by hot-filmament evaporation. The result (middle) are 1-D nanoparticle ensembles. Then, a semiconductor can be electrodeposited on these 1D ensembles resulting in the formation of hybrid semiconductor/metal nanowires.
In principle, these composite nanowires will be able to efficiently collect and concentrate light while simultaneously effecting its conversion to electrical energy. The SEM images in the Figure below show 1D ensembles of 2-5 nm diameter silver nanoparticles prepared by the evaporation of silver onto hot (>300 oC) HOPG surfaces. During this deposition process, silver adatoms incident on the HOPG surface diffuse laterally until encountering a step edge and aggolomerating with other silver atoms to form a silver nucleus.
Hybrid semiconductor/metal nanowires will be obtained by exploiting these 1D metal nanoparticle to nucleate an electrodeposited semiconductor, such as CdSe, data for which is shown below.
Figure 3. Hybrid semiconductor/metal nanowires will be obtained by exploiting these 1D metal nanoparticle to nucleate an electrodeposited semiconductor, such as CdSe, data for which is shown below.
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.
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.
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 (in fact, just one) have been reported. One experimental problem is that nanowires must be extremely long in addition to having nanoscopic width, as shown in the Figure below:
We have developed methods for synthesizing nanowires that are more than 1 mm in total length using electrodeposition. Typical examples of Bi2Te3 nanowires are shown below:
Now, using this same nanowire synthesis strategy, we are developing a versatile and modular "test bench" that makes possible efficient electrochemical synthesis of nanowires composed of Bi2Te3 and PbTe, and the in-place evaluation of key thermoelectric metrics, including ZT, s (the electrical conductivity), S, and k (the thermal conductivity). This test bench (Figure) will be based upon a new nanofabrication technology that we have just developed, called Lithographically Patterned Nanowire Electrodeposition or LPNE.
Figure. Test bench for nanowire synthesis and thermal testing: LEFT: Nanowires of Bi2Te3 or PbTe, are electrodeposited on a polystyrene-coated glass surface using the LPNE method (see below). Nickel ohmic contacts are evaporated through a contact mask. MIDDLE: A photoresist etch mask is applied, exposed and developed. RIGHT: Polystyrene spacer is stripped by exposure to acetone or by oxygen plasma etch. The segments of nanowires between the inner electrical contacts are now suspended in air (k = 0.01 W m-1 K-1). The suspended length, l, of nanowires is variable, as indicated.
