Volume 2, Issue 9 November 2002

2002
2001

The science-fiction fantasy of nanotechnology — building novel structures, devices, and materials at the atomic or molecular scale — is becoming a reality. For the great potential of nanoscience and nanotechnology to be fully realized, however, research efforts must cross many disciplines, from electrical engineering, mechanical engineering, materials science, and computer science to bioengineering, chemistry, and physics. Nowhere is this cross-disciplinary approach fostered more than at UC Berkeley. Each month, Lab Notes is proud to present the work of nanotechnology researchers from the College of Engineering and our collaborators across the campus. Nanocrystals, Quantum Dots, and Nature's Own Assembly Line by David Pescovitz Paul Alivisatos, Janke Dittmer, and Wendy Huynh with one of their nanorod-based solar cells in the upper-right foreground. Courtesy LBNL Chemist Paul Alivisatos's pioneering research into tiny nanocrystals and nanorods is paying off in big ways. Chemically-pure clusters of anywhere from 100 to 100,000 atoms, Alivisatos's nanocrystals and nanorods have myriad applications that impact the macroworld — from tagging biological samples for genetic analysis and drug discovery to the creation of plastic solar cells that can be painted onto any surface. The beauty of these nanomaterials, Alivisatos explains, is that their unusual properties — predicted by quantum mechanics — can be tuned by controlling the crystal's size and surface. For example, the materials' ability to emit or absorb different colors of light or conduct electricity can be altered. Alivisatos's latest small tech innovation nanotechnology is a potentially giant leap for solar energy. Several months ago, the group reported a technique to make flexible solar cells that could someday provide power for next-generation mobile phones, handheld computers, and wearable electronics. The first prototypes boast efficiencies of 1.7 percent. This means that they can only convert 1.7 percent of the energy they receive from the sun into electricity, far less than the 10 percent efficiency of today's commercial photovoltaics. "Our efficiency is not good enough yet by a factor of 10, but this technology has the potential to do a lot better," says Alivisatos, who is also part of the Materials Science Division of Lawrence Berkeley National Laboratory. "There is a pretty clear path for us to take to make this perform much better." A panel of eight plastic solar cells based on inorganic nanorods and semiconducting polymers. The shiny ovals are the aluminum back electrodes of the individual solar cells. UC Berkeley photo The solar cells were born from a previous breakthrough by the Alivisatos Research Group — the growth of two-dimensional rod-shapped nanocrystals made from cadmium selenide, a semiconducting material. Previously, all nanocrystals were dot-like spherical structures. The solar cells consist of nanorods with (relatively speaking) large surface areas dispersed in an organic polymer and sandwiched between electrodes. Unlike traditional solar cells which are manufactured in expensive clean rooms, plastic solar cells are created in beakers. Alivisatos and his colleagues hope that the efficiency of the solar cells can be increased by developing methods to align them perpendicular to the electrodes — rather than haphazardly mixing them in the polymer — so the loss of electrical current can be minimized. They also hope to tune the nanorods to absorb varying colors of sunlight to further increase the efficiency. "This opens up all sorts of new applications, like putting solar cells on clothing to power LEDs, radios, or small computer processors," says post-doctoral fellow Janke Dittmer, who with Alivisatos and graduate student Wendy Huynh co-authored a paper reporting their development in Science magazine. In this cross-section of mouse cells labeled with two different sizes of semiconductor nanocrystals, or quantum dots, nuclei show up as green and actin fibers show up as red under the same illumination. Courtesy Paul Alivisatos In October, Nanosys Incorporated — a Palo Alto-based firm co-founded by Berkeley scientists — signed an exclusive licensing agreement in October for a broad set of Alivisatos's nanotechnology patents. One potential application might be to grow nanorods into large plates of Light-Emitting Diodes (LEDs) for lightweight computer displays. The solar cells are only the most recent in a long line of breakthroughs from the Alivisatos group. In 1997, Alivisatos and UC Berkeley physicist Paul McEuen built a single-electron nanocrystal transistor. And in 1999, the work of Alivisatos and LBNL colleague Shimon Weiss led to the development of quantum dots, nanocrystals that emit different colors of light when a laser shines on them. The use of these nanocrystals as barcode-like tags to detect and trace biological materials — proteins or DNA, for instance — led to Alivisatos, Weiss, and several others founding a company, Quantum Dot, in 1998. Will nanomaterials yield big results for solar energy? We want to hear from you... Alivisatos's latest merging of nano and bio is to employ nature's own assembly lines to arrange nanocrystals into more complex structures. One technique, Alivisatos explains, is to piggyback nanocrystals on single strands of DNA. When single strands of DNA recognize complementary sequences on other strands, they pair off to form a familiar double helix. The DNA, he explains, acts a template for creating desired nanocrystal molecules. Alivisatos believes that this directed DNA assembly technique could result in nanoscale devices as complex as today's semiconductor circuits, potentially enabling the creation of ultra-powerful and tiny nanocomputers. "We've shown how organic chemistry can direct the assembly of inorganic nanocrystals," Alivisatos says. "That's the first step in bringing DNA from the biological world to the material world." Alivisatos Group Homepage UC Berkeley Press Release Nanosys Quantum Dot Corporation Lab Notes is published online by the Public Affairs Office of the UC Berkeley College of Engineering. The Lab Notes mission is to illuminate groundbreaking research underway today at the College of Engineering that will dramatically change our lives tomorrow. Editor, Director of Public Affairs: Teresa Moore Writer, Researcher: David Pescovitz Designer: Robyn Altman Subscribe or send comments to the Engineering Public Affairs Office: lab-notes@coe.berkeley.edu. © 2002 UC Regents. Updated 11/1/02.