My main research interests lie in exploring the local electronic, magnetic, and mechanical properties of atomic and molecular structures at surfaces. I am interested in studying how local interactions between atomic-scale structures affect their microscopic behavior, and how quantum mechanical effects might influence nanodevice behavior in very small structures. My main experimental tool is scanned probe microscopy, which can be used in combination with other experimental tools 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 several instruments capable of performing atomic and molecular manipulation, and we are currently exploring the physical parameters necessary to controllably position atoms and molecules into new and useful nanoscale configurations.
Carbon Nanostructures: Recent advances in synthesis have made it possible to fabricate whole new classes of carbon nanostructures such as buckyballs, endohedral fullerenes, nanotubes, and single sheets of graphene. These materials are extremely flexible and hold great promise for new generations of nanodevices. My group is using scanned probe techniques to explore and modify the properties of these materials at the single atom / single molecule level. Some of our successes include the first energy-resolved mapping of a single buckyball, single-atom electronic doping of an individual C60 molecule, mapping electron-phonon coupling strength across the surface of a single endohedral fullerene, and the first atomically-resolved spectroscopy of a single layer of graphene. Future projects will include exploring atomic-scale modification of carbon nanostructures and investigation of their device potential.
Spin-based Nanostructures: When magnetic structures are shrunk down to single-atom or single-molecule sizes, then quantum spin effects dominate their behavior. This is a particularly fascinating regime in condensed matter, where individual spins can interact with the excitations of the condensed matter environment. Much of the interest in this area derives from its great promise for revolutionary applications in spintronics and quantum information. My group is using scanned probe techniques to explore individual magnetic atoms, small spin clusters, and magnetic molecules. Some of our successes here include the first spectroscopic characterization of a single Kondo impurity, exploration of coupled systems of two and three atomic spins at a surface, and the first spin-polarized measurement of a single magnetic adatom. We are currently exploring spin-polarized atomic-scale structures and coupled magnetic molecule systems with an eye toward quantum information applications.
Molecular Machines: Molecular machines are well-known to exist in the biological realm, but it has proved difficult to control the mechanical behavior of synthetically fabricated machines at truly molecular scales. Nevertheless, this is a very promising direction with many exciting NEMS applications (Nano-Electro-Mechanical Systems). In my group we are interested in creating molecular machines from the ground up by assembling and actuating molecular-mechanical building blocks. One of our successes includes the first observation of reversible optically-actuated mechanical switching of a single molecule (azobenzene) at a surface. We are currently exploring the physics of new molecular systems whose electro-mechanical state can be remotely switched, and our goal is to assemble these into functional molecular machines at small lengthscales.
Nano-photovoltaics: Current state-of-the-art solar cells require carefully processed semiconducting interfaces that extend over macroscopic distances (i.e., extended n-p junctions). An alternative method for creating solar cells, however, is to engineer nanoscale elements that can be combined to create microscopic n-p junctions. These can then be combined in large quantities to create composite photovoltaics with spatially distributed n-p interfaces. A benefit of this approach is that such nano-photovoltaic building blocks can be mass-produced cheaply and so this technique is potentially more scalable than current state-of-the-art semiconductor photovoltaics. The down side is that nano-photovoltaics currently have very low efficiencies. My group is currently exploring the microscopic physics of nano-photovoltaic interfaces, with the goal of understanding and optimizing the processes that allow us to convert sunlight into usable electrical energy in molecular-scale structures.
"Observing Spin Polarization of Individual Magnetic Adatoms", by Y. Yayon, V. W. Brar, L. Senapati, S. C. Erwin, M. F. Crommie, Phys. Rev. Lett., in press (cond-mat/0703741v1) (2007).
"Scanning tunneling spectroscopy of inhomogeneous electronic structure in monolayer and bilayer graphene on SiC", by V. W. Brar, Y. Zhang, Y. Yayon, A. Bostwick, T. Ohta, J. L. McChesney, K. Horn, Eli Rotenberg, M. F. Crommie (cond-mat/arXiv:0706.3764v1) (2007).
"Reversible photomechanical switching of individual engineered molecules at a surface" by M. J. Comstock, N. Levy, A. Kirakosian, J. W. Cho, F. Lauterwasser, J. H. Harvey, D. A. Strubbe, J. M. J. Fréchet, D. Trauner, S. G. Louie and M. F. Crommie, Phys. Rev. Lett., in press (cond-mat/0612201) (2006).
"Direct Visualization of the Molecular Jahn-Teller Effect in an Insulating K4C60 Monolayer" by A. Wachowiak, R. Yamachika, K. H. Khoo, Y. Wang, M. Grobis, D-H Lee, Steven G. Louie, and M. F. Crommie, Science310, 468 (2005).
"Spatially Dependent Inelastic Tunneling in a Single Metallofullerene" by M. Grobis, K. H. Khoo, R. Yamachika, Xinghua Lu, K. Nagaoka, Steven.G. Louie, M. F. Crommie, H. Kato, and H. Shinohara, Phys. Rev. Lett. 94, 136802 (2005).
"Controlled atomic doping of a single C60 molecule" by R. Yamachika, M. Grobis, A. Wachowiak, and M. F. Crommie, Science 304, 281 (2004).
"Spatially Mapping the Spectral Density of a Single C60 Molecule" by Xinghua Lu, M. Grobis, K. H. Khoo, S. G. Louie, and M. F. Crommie, Phys. Rev. Lett. 90, 096802 (2003).
"Noise Spectroscopy of a Single Spin with Spin-Polarized Scanning Tunneling Microscopy" by Z. Nussinov, M. F. Crommie, and A. V. Balatsky, Phys. Rev. B.68, 085402 (2003).
"Kondo Response of a Single Antiferromagnetic Trimer" by T. Jamneala, V. Madhavan, and M. F. Crommie, Phys. Rev. Lett. 87, 256804 (2001).
"Observing electronic scattering in atomic-scale structures on metals" by M. F. Crommie, Journal of Electron Spectroscopy and Related Phenomena 109, 1 (2000).
“Tunneling into a single magnetic atom: Spectroscopic evidence of the Kondo resonance" by V. Madhavan, W. Chen, T. Jamneala, M. F. Crommie, and N. S. Wingreen, Science 280, 567 (1998).
“Confinement of electrons to quantum corrals on a metal surface" by M. F. Crommie, C. P. Lutz, and D. M. Eigler, Science 262, 218 (1993).