My main research interest is in exploring the local electronic properties of atomic-scale structures at surfaces. I am also interested in studying how local interactions between atomic-scale structures affect the morphology and dynamics of mesoscopic systems. My main experimental tool is the ultra-high vacuum cryogenic scanning tunnelling microscope, which can be used to both fabricate atomic-scale structures and probe them spectroscopically.
Atomic manipulation: The tip of a scanning tunneling microscope (STM) can be used to position individual atoms and molecules on a conducting surface, allowing the fabrication of precise atomic-scale structures. We have built a low temperature STM that is capable of performing atomic manipulation, and we are currently exploring the physical parameters necessary to controllably position atoms and molecules on different substrates. The ability to manipulate an atom depends on the relative strength of its coupling to the surface and to the STM tip. By characterizing these interactions we extend our ability to fabricate structures at the atomic-scale, and further our understanding of how adsorbates bind to surfaces.
Visualizing low dimensional electronic behavior: Electrons at some metal surfaces occupy a 2-dimensional surface state, known as a Shockley state. These electrons can be observed with an STM, allowing direct visualization of the quantum mechanical state density of 2-dimensional electronic wavefunctions. We are currently studying such systems in an effort to better understand how 2-d electrons interact with adsorbates, artificially fabricated atomic-scale structures, and surface reconstructions.
Magnetic nanostructures: One of the smallest magnetic structures imaginable is a single magnetic atom in a non-magnetic host. If the host is a metal, then spin-flip scattering between conduction electrons and the magnetic impurity leads to a many-body groundstate where an extended cloud of electrons screens the local impurity moment. This is known as the Kondo effect, and the influence of the "Kondo screening cloud" on macroscopic material properties has been well studied. We are currently using a low temperature STM to directly investigate the local electronic properties of individual Kondo impurities at a clean metal surface. We are using the atomic manipulation capabilities of our STM to probe magnetic interaction effects between individual Kondo impurities and also to study artificially fabricated clusters of magnetic atoms. These studies help us to understand how the properties of microscopic magnetic structures evolve with the overall size of a system.
Nanostructures on semiconductors: Microfabricated circuit elements on semiconducting substrates form the core of current high technology. We are using a low temperature STM to probe the limits of atomic manipulation on semiconducting surfaces. Single atom manipulation on semiconductors holds the possibility of providing us with the ability to fabricate 1-dimensional wires only a single atom wide. These structures are predicted to exhibit novel electronic behavior, such as the Luttinger liquid groundstate. Such studies bring us closer to the goal of exploring electrical circuitry at the ultimate level of miniaturization.
Carbon nanostructures: sp2 bonded carbon lattices have been shown to yield a fabulous diversity of molecular structures with striking electronic and structural properties. One exciting aspect of these "fullerines" is that their properties can be tuned through doping and structural modification, opening possibilities for new device applications. We are interested in characterizing the electronic properties of "bare" fullerine molecules, as well as atomically-fabricated and molecularly functionalized fullerine structures. Current projects involve C60 and carbon nanotubes deposited onto metal and semiconducting surfaces.
In the News
UC Berkeley physicists have used graphene to build lightweight ultrasonic loudspeakers and microphones, enabling people to mimic bats or dolphins’ ability to use sound to communicate and gauge the distance and speed of objects around them.
Researchers at Lawrence Berkeley National Laboratory and the University of California, Berkeley, have developed a new precision approach for synthesizing graphene nanoribbons from pre-designed molecular building blocks. Using this process the researchers have built nanoribbons that have enhanced properties—such as position-dependent, tunable bandgaps—that are potentially very useful for next-generation electronic circuitry.
The first experimental observation of a quantum mechanical phenomenon that was predicted nearly 70 years ago holds important implications for the future of graphene-based electronic devices.
Direct Imaging by Berkeley Lab Researchers Confirms the Importance of Electron-Electron Interactions in Graphene
Perhaps no other material is generating as much excitement in the electronics world as graphene. For the vast potential of graphene to be fully realized, however, scientists must first learn more about what makes graphene so super. The latest step in this direction has been taken by researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley.