Volume 6, Issue 47 October 2009
From Nanosolar to Spintronics
Michael Crommie, a Berkeley professor of physics (left) with graduate student Matt Comstock. The monitor depicts one of Crommie's experiments stimulating molecules with laser light. Image credit: courtesy Michael Crommie
Atoms and molecules may be tiny, but they hold big promise to Michael Crommie. A Berkeley professor of physics, Crommie envisions a day when molecular machines, quantum computers, and molecule-sized solar cells are commonplace. To turn these dreams into reality, Crommie is uncovering the principles that govern the behavior of nanoscale structures.
"I'm interested in how we can create very small structures and control them, and to understand the mechanisms that make them behave the way they do," Crommie says.
His quest isn't going to be easy. The world operates very differently at the nanoscale, where objects are on the order of one billionth of a meter in size. Here, the often mind-bending rules of quantum physics govern the properties of individual molecules.
That's particularly true for nanomagnets. Magnets the size of a single atom are attracted to iron filings in the same way as a standard horseshoe magnet. But that's where the resemblance ends. "Unlike the magnet you stick on your refrigerator door, which has a magnetic field that can point in any direction, the magnetic field of nanomagnets can only point in discrete directions. In other words, they are quantized," Crommie says.
A copper surface with triangular cobalt islands interspersed. Green specks on the islands are individual iron atoms. The map was obtained with a spin-polarized scanning tunneling microscope, which measured the spin of each iron atom—a feat Crommie's laboratory was the first to accomplish. The size of the image is 36nm x 36nm. Image credit: courtesy Michael Crommie
Configuring nanomagnets in rows or other two-dimensional arrays can alter their electromagnetic properties in unexpected ways, says Crommie. One configuration might align the electron spins of each nanomagnet all in the same direction, making the array a "ferromagnet," while another arrangement might cause the spins to align antiparallel, forming an "anti-ferromagnet."
Crommie is exploring how to link the magnetic spins of nanomagnets by combining them with other materials. Recently, he and his collaborators took two vanadium atoms and linked their spins together through a single organic molecule called tetracyanoethylene. "We were able to detect the coupling between the spins; one influenced the other through the molecule," Crommie says. "This is an important step forward in the creation of larger quantum spin structures and networks."
Being able to predict and control these properties could lead to a whole new paradigm for electronic devices. Where today's electronic circuits use electric fields to create regions that encourage or discourage the flow of electrons, new "spintronic" circuits might control these pathways via the alignment of nanomagnets. Spintronic circuits could potentially be smaller, generate less heat, and be better adapted for many applications.
One of those is quantum computing. "In a classical computer, the bits represent either zero or one, on or off. But in a quantum computer, they could be zero or one at the same time. So you can encode much more information in the bits. That is potentially vastly more efficient than classical computers for problems such as data encryption."
A chain of nanophotovoltaic molecules on a gold surface as imaged by a scanning tunneling microscope. Seven molecules can be seen in the chain, each of which is designed to behave as a solar cell. The image is a result of a research collaboration between the Crommie, Segalman, and Tilley research groups at UC Berkeley. Image credit: courtesy Michael Crommie
The electrical properties of atoms are equally central to another of Crommie's projects: creating solar cells the size of a single molecule. This work is part of the Lawrence Berkeley Laboratory Helios Project, which seeks to develop the means to perform artificial photosynthesis. "The idea is to find new ways to convert sunlight to fuel," Crommie says.
The solar cells people currently use work well, but the panels themselves are expensive to manufacture and difficult to mass produce. Crommie and his coworkers want to replace them with nanostructures such as photovoltaic organic molecules. These nanosolar cells might be simply sprayed onto surfaces, and mass produced cheaply.
Others have created molecular aggregates capable of converting sunlight into electricity, but these are extremely inefficient. Crommie suspects much of the problem stems from the large number of disordered interfaces an electron must navigate in these nanophotovoltaic structures. He is currently exploring photovoltaic processes down at the single molecule level, from the absorption of photons to the transfer of electrons from one molecule to another, and he hopes to eventually optimize this process.
Nanoscale research demands special equipment and laboratories insulated from vibrations, electrical fields, temperature shifts, and even acoustical noise. In search of better experimental conditions, he says, "the Physics Department has been upgrading the labs that we have, but it's not enough. We need new labs."
To that end, Crommie and others have secured an $11 million grant from the Department of Commerce to build a state-of-the-art laboratory for nanoscale research on campus. The laboratory will encompass 10,000 square feet in the basement of a new Campbell Hall, constructed when the current, seismically vulnerable building is torn down. Crommie, for one, can't wait.