Alivisatos Research Articles

University of California appoints Paul Alivisatos interim director of Lawrence Berkeley National Laboratory

University of California appoints Paul Alivisatos interim director of Lawrence Berkeley National Laboratory

Photodeposition of Pt on Colloidal CdS and CdSe/CdS Semiconductor Nanostructures

Photodeposition of Pt on Colloidal CdS and CdSe/CdS Semiconductor Nanostructures Gordana Dukovic, Maxwell G. Merkle, James H. Nelson, Steven M. Hughes, and A. Paul Alivisatos* Department of Chemistry, University of California, Berkeley and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Semiconductor photocatalysis has been identified as a promising avenue for the conversion of solar energy into environmentally friendly fuels, most notably by the production of hydrogen from water.[1–5] Nanometer-scale materials in particular have attracted considerable scientific attention as the building blocks for light-harvesting applications.[6,7] Their desirable attributes include tunability of the optical properties with size, amenability to relatively inexpensive low-temperature processing, and a high degree of synthetic sophistication leading to increasingly complex and multi-functional architectures. For photocatalysis in particular, the high surface- to- volume ratios in nanoscale materials should lead to an increased availability of carriers for redox reactions on the nanoparticle surface. Recombination of photoexcited carriers directly competes with photocatalytic activity.[3] Charge separation is often achieved with multi-component heterostructures. An early example is the case of TiO2 powders functionalized with Pt and RuO2 particles, where photoexcited electrons are transferred to Pt (the reduction site) and holes to RuO2 (the oxidation site).[8] More recently, many colloidally synthesized nanometer-scale metal–semiconductor heterostructures have been reported.[7,9,10] A majority of these structures are made by thermal methods.[7,10] We have chosen to study photochemical formation of metal–semiconductor heterostructures. The detailed understanding of the mechanisms involved in photodeposition of metals on nanometer-scale semiconductors is necessary to enable a high degree of synthetic control. At the same time, because the results of metal deposition can be directly observed by electron microscopy, it can be used to understand how factors such as nanocrystal composition, shape, carrier dynamics, and surface chemistry influence the photochemical properties of semiconductor nanocrystals. In this communication, we report on the photodeposition of Pt on colloidal CdS and CdSe/CdS core/shell nanocrystals. Among the II–VI semiconductors, CdS is of particular interest because it has the correct band alignment for water photolysis[2] and has been demonstrated to be photocatalytically active.[11–16] We have found that the photoexcitation of CdS and CdSe/CdS in the presence of an organometallic Pt precursor leads to deposition of Pt nanoparticles on the semiconductor surface. Stark differences are observed in the Pt nanoparticle location on the two substrates, and the photodeposition can be completely inhibited by the modification of the semiconductor surface. Our results suggest that tuning of the semiconductor band structure, spatial organization and surface chemistry should be crucial in the design of photocatalytic nanostructures.

HYBRID SOLAR CELL FABRICATED

A NEW SOLAR CELL USES A semiconducting film of cadmium selenide nanorods dispersed in an organic polymer. The hybrid cells could find use in intermediate applications that require more solar power conversion efficiency than conventional solar panels but less than the highly efficient, expensive cells made for satellites, according to the group at the University of California, Lawrence Berkeley National Laboratory, that developed the device. Chemistry professor A. Paul Alivisatos, graduate student Wendy U. Huynh, and postdoctoral researcher Janke J. Dittmer fabricated the photovoltaic device by spin-casting a solution of CdSe nanocrystals and the conjugated polymer poly-3(hexylthiophene) onto an indium tin oxide glass substrate coated with a transparent conducting polymer electrode [ Science , 295, 2425 (2002)]. An aluminum electrode was then deposited on top of the semiconducting film There is a great deal of interest in a new generation of solar cells that can be 'painted on', as opposed to deposited by...

Surveying physics

What will be the hottest areas of physics in the next decade? Quantum technologies, complex systems, biophysics, new materials, cosmology, and high-energy physics are the six areas identified as priorities in Physics in a New Era: An Overview. The report, out this month, is the culmination of a survey by the National Research Council, which over the past five or so years has published a series of field-specific reports. Physics in a New Era: An Overview looks not just at physics research in the US, but also at the conditions under which it is carried out and at its impact on society.The overview makes nine key policy recommendations. They include increasing funding for basic physics research, developing mechanisms for the US to participate in international research collaborations, revamping undergraduate physics education, reestablishing basic research as a high priority in the Department of Energy’s defense labs, and fostering collaborations among government, academia, and industry. Physics in a New Era: An Overview may be ordered from National Academies Press by e-mail at [email protected] or from their Web site at http://www.nap.edu. Survival of the curriculum. The Kansas State Board of Education has restored Darwinian evolution, the Big Bang, and the age of Earth to the K–12 science curriculum, ending a controversy that began in 1999 when creationists on the board stripped such “controversial” science from the state’s education standards. (See Physics Today, November 1999, page 59, and October 2000, page 75.)A contentious election last November that centered on the evolution issue resulted in the defeat of three conservative board members. On 14 February, the new board, by a 7–3 vote, approved new science standards that had been developed by a 27-member committee of science teachers and other experts. The new standards will be used in developing tests that will be administered statewide this spring.Referring to the elections that put creationists in the majority on the board of education, then removed them, John Staver, director of Kansas State University’s Center for Science Education, noted, “I said when this began a year and a half ago that democracy got us into this, and democracy would get us out, and it did.” Staver was the cochair of the committee that wrote the new education standards. Nano journal. Physical, chemical, and biological structures, and processes and applications at the nanoscale are the topics spanned by Nano Letters, a new peer-reviewed journal published by the American Chemical Society. Articles are posted online regularly and collected in print monthly. The online multimedia version is free through June at http://www.pubs.acs.org/journals/nalefd/. Paul Alivisatos, a professor of chemistry and materials science at the University of California, Berkeley, is the journal’s founding editor. Scanning for alien lasers. A team led by Harvard physicist Paul Horowitz hopes to spot laser pulses from extraterrestrials with a new optical telescope. The 1.8-meter telescope will scour the northern sky, seeking flashes from fixed objects or aliens on the move.It’s easy to make beamed laser pulses brighter than stars, says Horowitz. The telescope will view any one spot for only about a minute, he adds, “but if they are sending flashes frequently, we’ll see it.”This latest tool joining the search for extraterrestrial intelligence (SETI) is scheduled to go online early next year 65 kilometers west of Boston, in Harvard, Massachusetts. The Planetary Society is paying for the telescope, with half the $350 000 tab donated by entrepreneur David Brown of Santa Barbara, California.© 2001 American Institute of Physics.

Semiconductor Nanocrystals

The following is an edited transcript of the presentation given by A. Paul Alivisatos, recipient of the Outstanding Young Investigator Award, at the 1995 MRS Spring Meeting in San Francisco.The work I will describe on semiconductor nanocrystals started with the realization that it is possible to precipitate a semiconductor out of an organic liquid. We can precipitate out a semiconductor as a colloid—a very small-sized semiconductor with reduced dimensionality—that will show large, quantum size effects. A dream at that time was to make an electronic material by such a process in a liquid beaker, by starting with an organic fluid and somehow injecting something into the fluid to make very small particles, which we could use in electronics. The materials we use in electronics today have perfect crystalline order. We are able to put in dopants very specifically, or control precisely their arrangements in space in enormously complicated ways. The level of purity of electronic materials is so high that making an electronic material in a wet chemistry approach seems almost impossible. If, in addition, we specify that the size must be controlled precisely, we recognize the project is a problem for basic research, yet not one ready for applications. Many fundamental problems arise if we try to make semiconductor particles, in a liquid, of such high quality that they can be used as electronic materials.