Volume 1, Issue 1
Crystallizing Nanoscience
Paul Alivisatos has created nanocrystals of various shapes and sizes, including spheres, rods, and tetrapods. (courtesy LBL)
UC Berkeley chemist Paul Alivisatos, director of the Molecular Foundry under construction at Lawrence Berkeley National Laboratory (LBL), likens the facility to a kitchen. The comparison is surprisingly apt. When it's completed in 2006, the Department of Energy-funded, $85 million research center will provide scientists with the latest appliances, ingredients, and recipes to cook up state-of-the-art nanoscale materials, atom by atom. From quantum dots to biomolecular nanomotors, the kinds of novel structures that will emerge promise nothing short of a revolution in science and industry.
"Right now, scientists are inhibited in what they can do by the fact that they can't always get access to the materials they'd like to work with," Alivisatos says. "The Foundry will keep track of what nanofabrication techniques work well and make those available so all scientists can become experts. That will certainly accelerate the pace of innovation."
For a hint of what lies ahead for the Molecular Foundry, one only needs to look inside Alivisatos's own laboratory. Alivisatos's research focuses on nanocrystals, chemically-pure clusters of anywhere from 100 to 100,000 atoms. (A nanometer is one-billionth of a meter.) During the last decade, Alivisatos and his students pioneered the growth of high-quality nanocrystals by injecting semiconductors like cadmium selenide or cobal into pure, hot, soap-like films called surfactants. The beauty of the nanocrystals is that their physical and chemical properties can be tuned by controlling their size and surface.
An assortment of cobalt nanocrystals manufactured by the Alivisatos research group. (courtesy LBL)
In 1999, Alivisatos and LBL colleague Shimon Weiss reported a groundbreaking biomedical application for quantum dots, nanocrystals that emit different colors of light when a laser shines on them. The researchers discovered that the quantum dots' luminescent properties make them ideal "bar codes" to tag biological materials like DNA and proteins for detection. Now, in collaboration with Carolyn Larabell, a cell biologist and microscopist at LBL, Alivisatos is hoping to use similar nanocrystals to help visualize the interior of individual living cells. Inserted into a cell, the nanocrystals will act as contrast agents when bombarded with beams from the new X-Ray Microscope at LBL's Advanced Light Source.
"Ultimately, this could impact cancer research," Alivisatos says. 'There are a lot of things going on inside cells that we don't understand and cancer causes it all to go haywire."
Along with his life science efforts, Alivisatos is harnessing the unique properties of nanocrystals for renewable energy applications. Two years ago, he and his colleagues fashioned flexible solar cells from rod-shaped nanocrystals. More recently though, Alivisatos and fellow UC Berkeley chemistry professor Gabor Somorjai have shown that a nanocrystal they developed has promise as a nanoscale reactor for future fuel cells. The researchers' microporous nanocrystal features a hollow interior loaded with catalyst particles. Fuel cells, Alivisatos explains, depend on such catalytic systems to facilitate the chemical reaction that produces electrical power.
The Alivisatos research group fabricated this panel of eight plastic solar cells from inorganic nanorods and semiconducting polymers. (courtesy the researchers)
To synthesize the hollow structure, the researchers took advantage of an unusual atomic mechanism similar to the Kirkendall Effect, in which the difference in diffusion rates between two metallic materials causes pores to form. Here, oxidation of a cobalt nanocrystal resulted in the metal diffusing outward, leaving a hollow chamber inside. Encapsulating catalyst materials inside the self-contained but porous crystal results in an entirely different geometry for catalytic systems, Alivisatos says.
While Alivisatos continues to create new kinds of nanocrystals, he's also exploring ways to use the structures as building blocks for complex structures. To that end, he and LBL scientist Alex Liddle have developed a method to place nanoparticles in precise locations on a silicon chip to form transistors.
The technique is based on electron beam lithography, a method of fabricating nanoscale features on silicon substrates. The researchers' first pattern an array of wires on a silicon wafer with gaps where the nanocrystals will be placed. Then, they coat the wafer with a plastic layer that's subsequently riddled with holes, just 50 nanometers in size, above the gaps in the wires. Finally, a liquid solution containing 2 nm nanocrystals is flowed over the wafer. As it evaporates, the nanocrystals fall through the holes to land between the gaps in the wires where they complete the transistors. Future research is aimed at improving the reliability of the contacts between the nanocrystals and the metal leads.
"We'd really like to make devices where you use nanoparticles on a one-by-one basis," Alivisatos says. "We're far from there, but our research is certainly shifting in that direction."
And undoubtedly, those research results will become dog-eared pages in the Molecular Foundry's recipe book.