Carbon Nanostructures

Scanning tunneling microscopy (STM) enables us to explore the atomic-scale properties of carbon nanostructures, such as graphene and graphene nanoribbons (GNRs). Graphene is a two dimensional material formed by carbon atoms sp2 bonded together in a honeycomb lattice, whereas a GNR is a narrow strip of graphene whose electronic behavior is dominated by quantum confinement and its edge geometry.

Graphene has a linear relativistic-like band structure consisting of two cones that meet at the so-called “Dirac point.” The energy of the Dirac point with respect to the Fermi level can be shifted through electrostatic gating and doping via surface adsorbates.

Charged Impurities and Atomic Collapse on Graphene

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.

In nonrelativistic quantum mechanics, a hydrogen-like atom is stable regardless of the value of the nuclear charge Z. This is no longer true in relativistic quantum mechanics, where “atomic collapse” states emerge at Z>170. These atomic collapse states represent electrons falling to the center, accompanied with spontaneous positron emission. Although Z>170 is far beyond any currently accessible nuclei, we can explore similar phenomena using graphene as a platform for conducting ultra-relativistic experiments. Using the STM tip to perform atomic manipulation, we build “artificial nuclei” on graphene to visualize the atomic collapse regime.

Strain in Graphene

Shown below is a topographic image 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.”

Graphene Nanoribbons

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. We 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.

Bottom-up fabrication enables us to fine-tune the atomic edge structures and widths of GNRs, and hence controllably tailor their electronic properties. Collaborating with chemists, different molecules are synthesized and deposited onto surfaces. Through chemical reactions called polymerization and cyclization, we can grow GNRs with atomically precise edges and different widths on surfaces.

Tuning the bandgaps of GNRs

Using this bottom-up approach, we are able to tune the bandgaps of the GNRs (as shown above), make heterojunctions at the molecular level (as shown below) and characterize their unique electronic properties.

Bottom-up fabrication of GNR heterojunctions

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.