The Crommie group's experiments used exfoliated graphene, individual flakes made by mechanically cleaving a sheet of atoms from a larger chunk of carbon. The group attached electrodes to both the graphene flake and an underlying substrate consisting of a conducting layer of silicon, which was separated from the flake by an insulating layer of silicon dioxide. The experimental setup was thus able to uniquely incorporate two distinct voltage differences, that between the tip of the STM and the surface (the "bias" voltage) and that between the graphene flake and the underlying substrate (the "gate" voltage).
"The purpose of controlling the gate voltage is to vary the density of the charge carriers in the graphene," Crommie says. "The purpose of varying the STM bias voltage is to perform spectroscopy, so we can look at the graphene's local density of states at different energies. We want to know where are the electrons? How are they behaving?"
These questions are of particular interest because of graphene's odd electronic properties. The carbon atoms in graphene are arranged at the corners of hexagons, as in chicken wire, with three of each atom's four electrons involved in molecular bonds with its neighbors; these are sigma orbitals that lie in the plane of the material. The remaining electrons are in pi orbitals extending above and below the plane. The hybridization of the pi orbitals spreads across the graphene sheet, and the unconfined electrons are free to move as high-speed "relativistic quasiparticles," so-called Dirac fermions which act as if they have no mass.
The plot of energy states for Dirac fermions in graphene looks quite different from that of a conventional 3-D semiconductor, which typically consists of two opposing parabolic curves, a lower-energy valence band and a higher-energy conduction band, with a band gap between them that no charge carriers can occupy.
Graphene's unusual electronic properties
By contrast, the Dirac fermion energy states of graphene can be represented as two cones with their vertices meeting at a point of minimum electronic density, called the Dirac point. Thus one might expect the spectrum of the density of states resulting from electrons tunneling into graphene to be linear, following the smooth edge of the touching cones.
"When we plotted the LDOS spectra of our gated graphene flakes, however, we found a gap-like feature that was centered on the Fermi energy — no matter how we changed the density of charge carriers in the graphene with the gate voltage and no matter where we looked on the flake," Crommie says.
The Fermi energy is the energy of the highest occupied electronic state in a graphene flake, and is the reference energy for this kind of measurement. "Almost no electrons tunneling from the STM tip could enter the graphene at low energies within this gap region, but at slightly higher energies there was an abrupt, giant enhancement in tunneling, like a floodgate opening up for electrons." And this was not the only odd feature in the graphene spectrum.
"There was another feature in the spectrum, a local minimum of states, which moved in a very regular way as we changed the gate voltage and thus the density of charge carriers in the material," Crommie says. The research team was able to unambiguously identify this feature as the mark of electrons tunneling from the STM tip to the Dirac point itself, the minimum in graphene density of states.
And what of the mystery gap itself? "We realized that this is not a true energy gap; it is not a feature of the electronic band structure of graphene," Crommie says. "Rather it marks the interaction of the tunneling electrons with phonons, the quantized vibrations of the graphene lattice."
Naturally occurring vibrations in the graphene sample are minimal for the Crommie group's STM setup, since it is kept very cold (just four degrees above absolute zero). However, when the bias voltage between the tip and graphene sample increased above a special threshold of 63 millivolts, "then each tunneling electron is able to create a phonon vibration in the graphene sheet, which allows the electron to get into the graphene much easier," Crommie says.
Indeed, this "phonon-assistance" causes the electron tunneling conductance to suddenly increase by more than 10 times, as phonons essentially open a new channel for electrons to flow through. Says Crommie, "We call it a phonon floodgate."
An underlying cause for this new channel arises from the carbon sigma orbitals, which normally don't conduct electrons (as the pi orbitals do), but which are brought into play when the graphene sheet vibrates. "When a phonon is created, the sigma orbital kind of rubs up against the pi orbital and acts like grease to help insert a tunneling electron into graphene," Crommie says.
"We started this research by simply asking, what do you see when you measure a graphene device with STM?" Crommie says. "In the process, we discovered a completely unexpected phonon floodgate. This gives us new insight into how electrons and phonons behave in graphene and creates new opportunities for future graphene-based nanodevice applications." ###
"Giant phonon-induced conductance in scanning tunneling spectroscopy of gate-tunable graphene," by Yuanbo Zhang, Victor W. Brar, Feng Wang, Caglar Girit, Yossi Yayon, Melissa Panlasigui, Alex Zettl, and Michael F. Crommie, appears in the advance online publication of Nature Physics at dx.doi.org/10.1038/nphys1022.
This research was sponsored by the U.S. Department of Energy's Office of Science.
Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California. Visit our website at www.lbl.gov.
Contact: Paul Preuss paul_preuss@lbl.gov 510-486-6249 DOE/Lawrence Berkeley National Laboratory
Tags: Nano or Nanotechnology and Nanotech
No comments:
Post a Comment