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Dirac Charge Dynamcs in Graphene by Infrared Spectroscopy Print
Wednesday, 29 October 2008 00:00

 

Graphene—a single layer of carbon atoms arranged in a honeycomb lattice—has very high conductivity that can be tuned by applying a gate voltage. The charge carriers in graphene can travel ballistically over great distances (~1 micron) without scattering. These unusual electronic properties make graphene a promising candidate for future nanoelectronics. Using infrared spectromicroscopy at ALS Beamline 1.4, a group of researchers from the University of California at San Diego, Columbia University, and the ALS has succeeded in probing the dynamical properties of the charge carriers in graphene with an accuracy never before achieved. Their results have uncovered signatures of many-body interactions in graphene and have demonstrated the potential of graphene for novel applications in optoelectronics.

Infrared View of Graphene

Graphene's potential for superseding silicon in next-generation electronic circuits has made it a hot topic of research—infrared hot! Graphene's unusual electronic properties arise from the fact that the carbon atom has four electrons, three of which are tied up in bonding with its neighbors. But the unbound fourth electrons are in orbitals extending vertically above and below the plane, and the hybridization of these spreads across the whole graphene sheet. The hybridized electrons interact with the periodic field of the hexagonal crystal lattice and form Dirac fermions, described by cone-like energy bands. One interesting consequence of this unique band structure is that the electrons in graphene are "sort of free." Unlike electrons in other materials, the electrons in graphene move ballistically—without collisions—over great distances, even at room temperature. As a result, the ability of the electrons in graphene to conduct electrical current is 10 to 100 times greater than those in a normal semiconductor like silicon at room temperature. This makes graphene a very promising candidate for future electronic applications. Li et al. employed infrared synchrotron radiation to probe the charge carriers in graphene. This infrared view uncovers several new aspects of Dirac fermions in graphene.

Graphene is a two-dimensional crystal consisting of a single layer of carbon atoms arranged hexagonally.

Graphene displays many intriguing electronic phenomena, such as massless Dirac quasiparticles with linear energy–momentum dispersion due to the interactions between electrons and the honeycomb lattice. This unique Dirac nature, along with other features, makes graphene an promising material for replacing silicon in future generations of electronic devices. However, the experimental study of graphene is still in its infancy. Little is known about the dynamical properties of the quasiparticles in graphene, such as quasiparticle lifetime and the effects of many-body interactions, because it is extremely difficult to perform spectroscopic measurements on a single monolayer of graphene.

Infrared measurements can probe the dynamical properties of quasiparticles over a wide energy range, and therefore can provide some of the most interesting information about the electronic properties of a material. In this work, the researchers employed infrared synchrotron radiation to probe the quasiparticle dynamics of graphene. The high brightness and tight focus of the synchrotron beam (less than 10 micrometers) enabled infrared measurements of one atomic layer of carbon.

Left: A schematic of the graphene sample integrated in a gate-tunable device. Infrared transmission and reflectance were measured with applied gate voltages. Right: the band structure of graphene, with the Fermi energy (EF) and the absorption threshold at twice the Fermi energy (dotted line).

The sample consisted of exfoliated graphene integrated in a gate-tunable device. An applied gate voltage changes the carrier density in the sample, and thus modifies the transmission and reflectance of the sample. From these changes, the optical conductivity of graphene could be extracted from the absorption spectrum of the infrared light. The researchers found that the optical conductivity of graphene is dominated by a threshold feature at twice the Fermi energy (the highest energy occupied by charge carriers).

The transmission ratio of the graphene device as a function of gate voltage. Voltage-tunable transmission and reflectance of graphene demonstrate its potential for novel applications in optoelectronics.

The optical conductivity of graphene at different gate voltages, which is the absorption spectrum of infrared light. The threshold feature is due to the absorption onset at twice the Fermi energy.

The Fermi energy is a manifestation of the intrinsic properties of the quasiparticles in a material. In a normal two-dimensional material, the Fermi energy changes linearly with the density of the carriers. But in a system of two-dimensional Dirac quasiparticles, the Fermi energy varies as the square root of the carrier density. Indeed, the researchers observed the latter intriguing behavior, which confirms the Dirac nature of the charge carriers in graphene.

Most interestingly, the scientists found several signatures of many-body effects in graphene. First, theories predict that, in ideal graphene, the absorption is negligible at low energy below twice the Fermi energy. Instead, considerable absorption of infrared light was observed in the low-energy region. This absorption may arise from disorder effects and/or many-body interactions—interactions between electrons and the honeycomb lattice, or mutual interactions among electrons. Another finding involved the speed of the charge carriers. Without mutual interactions, charge carriers in graphene will travel at a constant speed at any energy. Interestingly, the researchers found that the speed of the charge carriers increases systematically as their energy is lowered. This behavior may stem from electron–electron interactions according to theoretical predictions. The team hopes to study much cleaner samples to acquire a complete understanding of these observations.

Besides the broad implications for the fundamental properties of graphene, this study has also demonstrated voltage-tunable transmission and reflectance in graphene, which is very promising for future device applications such as tunable optical switches and modulators.

 


 

Research conducted by Z.Q. Li and D.N. Basov (University of California, San Diego); E.A. Henriksen, Z. Jiang, P. Kim, H.L. Stormer (Columbia University); and Z. Hao and M.C. Martin (ALS).

Research funding: Work at UCSD is supported by DOE (No. DE-FG02-00ER45799). Research at Columbia University is supported by the DOE (No. DE-AIO2-04ER46133 and No. DE-FG02-05ER46215), NSF (No. DMR-03-52738 and No. CHE-0117752), NYSTAR, and the Keck Foundation. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this research: Z.Q. Li, E.A. Henriksen, Z. Jiang, Z. Hao, M.C. Martin, P. Kim, H.L. Stormer, and D.N. Basov, "Dirac charge dynamics in graphene by infrared spectroscopy," Nature Physics 4, 532 (2008).