We are studying carbon nanostructures with scanning tunneling microscopy (STM). In particular, we are exploring monolayer graphene grown by a chemical vapor deposition (CVD) process and graphene nanoribbons (GNRs) created by unzipping nanotubes or by molecular precursors. Graphene is a two dimensional material formed by carbon atoms sp2 bonded together in a honeycomb lattice.
As a consequence of symmetry, graphene has a linear relativistic-like band structure consisting of two cones that meet at the so-called “Dirac Point.” When graphene is neutrally doped, the Fermi Level coincides with the Dirac Point energy. However, by application of a back gate voltage (Vg) or by the deposition of charged impurities, we can dope the graphene and shift the Dirac Point relative to the Fermi Level.
At Crommie Research Group, we are particularly interested in how atomic impurities and defects affect the electronic structure of graphene. Below is a scanning tunneling microscopy (STM) differential conductance map that shows an “ionization ring” around a cobalt atom on graphene. The STM tip acts as a top gate that can discharge or charge a cobalt atom by pushing a cobalt ionization state above or below the Fermi Level. Inside the ring, the cobalt atom is ionized by the tip.
Below is topography of a “graphene nanobubble” on epitaxial graphene grown on Pt(111). The deformation of the graphene lattice induces Landau Levels that are analogous to that of a 300 Tesla magnetic field. The ability to create pseudo-magnetic fields in graphene opens up the possibility of controlling its electronic properties via “strain engineering.”
A graphene nanoribbon (GNR) is a strip of graphene of width on the nanometer scale. The existence of the edges of GNRs makes them very different from bulk graphene. GNRs are predicted to have tunable energy gaps and magnetic properties, determined by the width and the edge geometry of GNRs.
Researchers at Crommie Research Group are interested in studying the physical properties of GNRs locally on the atomic scale. Scanning tunneling microscopy (STM) and spectroscopy (STS) are used to show the existence of the edges states and possible magnetism at the edges. They have also applied hydrogen plasma to control the edge terminals; In collaboration with theorists, the thermodynamics of the edge terminals are studied. Below is shown an STM constant current image of the edge of a GNR on a gold substrate.
The study of graphene will deepen our understanding of related carbon systems and hold great promise for new generations of devices. In the future, we hope to explore more unusual behaviors in graphene. We will learn more about charge impurities in graphene, nanobubbles, and nanoribbons.
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