A Clear Future for Graphitic Electrodes
Of the many scientific topics in the current limelight, none seem to hold as much promise as graphene. Graphene is an atom-thick sheet of the mineral graphite, found in pencils. This atom-thick sheet of carbon is one of—if not the most—electrically conductive, heat conductive, and strongest substances known. In this modern era of solar energy conversion, computing with light, and similar technologies, there has been a considerable thrust to develop transparent electrodes. The properties of graphene have led to broad speculation of the integration of this material into next-generation electronics such as tablet computers and solar cells. The problem, however, is that the individual layers of graphene can be difficult to isolate, and, despite being among the strongest materials, they are particularly fragile.
Writing in Nature Communications, Wenzhong Bao, Jiayu Wan, and their co-workers at the Nanostructures for Electrical Energy Storage Center (NEES) have reported a new method to conduct electrical and optical measurements of multilayer graphene. They also report chemically modifying it to increase its transparency and conductivity.
By stacking the layers of graphene, the material becomes stronger and they form what is known as ultrathin graphite (UTG). Unfortunately, as the graphene layers stack up, the ultrahigh performance of the individual graphene layers begins to decrease. Most notable are the decreases in how well it conducts electricity as well as its transparency. Many studies have been done trying to increase the conductivity of graphitic materials by "doping" the material with non-carbon atoms to modify the chemical structure of the layers and allow for better electron mobility throughout the material. These doping studies have led to new routes to novel graphene-based materials with new electromagnetic properties.
Method of Measuring. The studies start with custom-built devices that seal the UTG with contact electrodes and a lithium source as the counter electrode in thin transparent glass. The lithium electrode allows for controlled release of lithium that intercalates into the UTG on the other electrode when a voltage is applied. Because the UTG is sealed in a transparent cell, optical properties can be measured concurrent with lithium intercalation giving real-time data. The researchers conducted a number of intercalation studies with a different number of layers of graphene. The thickness of the UTG material was measured with atomic force microscopy that resolves the thickness difference of individual layers.
Increasing Transparency and Conductivity. Using an optical microscope, clear increases in transparency can be observed with an optimum ratio of one lithium atom to six carbon (graphene) atoms. If the charge of the device is reversed, the lithium can be removed from the UTG, which then reverts to its initial opacity. The researchers conducted the optical transmission studies with a white light source and observed wavelength-dependent transmission. Depending on the amount of lithium intercalated, the transparency for specific wavelengths of light varied with red/infrared light favored for LiC12 and blue/green favored for LiC6.
Electrical measurements of the intercalated UTG show a significantly reduced electrical resistance versus the unmodified UTG, consistent with other doping studies. Although the measured conductivity of the doped material is roughly one-third of the theoretical limit of these types of materials, owing to certain effects the lithium has on the structure, the conductivity is quite impressive.
To directly compare the lithium-intercalated UTG with other high-performance transparent electrodes, the team developed a conductivity/transparency-defined figure of merit. By this comparative measure, lithium-UTG exceeds all other carbon-based electrodes and appears to surpass any other uniform material including indium-tin-oxide, the industry standard.
To verify the feasibility of using lithium-intercalated UTG, the team generated millimeter-scale electrodes. Although these larger devices show reduced conductivity and transparency compared to the test scale, they still outperformed indium-tin-oxide in the conductivity/transparency figure of merit. The team's device preparation and analytical method can be extended to other doped graphene systems.
Bao W, J Wan, X Han, X Cai, H Zhu, D Kim, D Ma, Y Xu, JN Munday, HD Drew, MS Fuhrer, and L Hu. 2014. "Approaching the Limits of Transparency and Conductivity in Graphitic Materials through Lithium Intercalation." Nature Communications 5:1-9. DOI: 10.1038/ncomms5224
JW, WB, and LH were funded by Nanostructures for Electrical Energy Storage Center, an Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. MF was funded by Office of Naval Research Multidisciplinary University Research Initiative and Australian Research Council Laureate Fellowship. XH was funded by a National Science Foundation Civil, Mechanical and Manufacturing Innovation Award. The team acknowledges the support of the Maryland Nanocenter.