An LCD Built with Graphene
Ultrathin flakes of carbon may offer advantages over indium tin oxide.
According to a team of researchers from the UK and Russia, thin sheets of carbon could improve LCDs. The group recently demonstrated that graphene — a two-dimensional layer of carbon — can be used to make the conducting transparent thin films required as electrodes in LCDs and other photonic devices.
The insets show two stages of an LCD built using graphene instead of the commonly used indium tin oxide for one of its electrodes. The graphs show the normalized transmission — as a percentage of the maximum — versus the voltage applied to the electrode. As with other LCDs, the transmitted light falls as voltage is increased, eventually dropping to less than 1 percent. Courtesy of Kostya S. Novoselov, University of Manchester.
Unlike the metal oxides now used for such electrodes, graphene is chemically stable. Furthermore, metal oxides suffer from nonuniform absorption across the visible spectrum, and the new material does not.
“A single layer of graphene absorbs only 2.3 percent of light, independent of the light’s wavelength,” said research team member Kostya S. Novoselov of the University of Manchester in the UK.
Other members of the group were from the Institute for Microelectronics Technology in Cherno?golovka, Russia, and from Graphene Industries Ltd., also in Manchester. Novoselov said that the research was done, in part, because graphene’s light absorption is determined solely by fundamental physical constants.
At its heart, an LCD consists of top and bottom transparent electrodes sandwiching uniformly aligned liquid-crystal material. Polarized light can pass through the stack or be blocked, depending upon the voltage of one electrode relative to the other. It is this ability to selectively turn pixels on and off that makes the device work.
Indium tin oxide is the most commonly used electrode material, but it can inject oxygen and indium ions into the active media of a device. Indium tin oxide films also can be highly resistive when optical transmittance is above 95 percent.
In building demonstration LCDs, the investigators used graphene flakes for one electrode and In2O3 for the other. They created the flakes on a glass slide via micromechanical cleavage. They located the flakes using a Nikon microscope and confirmed that they were monolayers by using a Raman microscope from Renishaw plc of Wotton-under-Edge, UK.
The researchers deposited 55-nm chromium/gold contacts around the monolayer flakes, causing the graphene to cover holes in the otherwise opaque metallization. They applied a 40-nm-thick polyvinyl alcohol alignment layer on top of the graphene and topped that with liquid-crystal material. They also built a control, which was the same as their device except that it did not contain graphene.
When they applied a voltage across the test device, the transmission of both white and monochromatic 505-nm light through the device changed, but the control was unaffected. The contrast ratio — the difference between maximum transmission and the level of opaqueness when 100 VAC was applied — was better than 100:1 for the demonstration device. The electrical resistance and the light transmission of the flakes also were better than what could be obtained with In2O3.
However, chipping off graphene flakes is not suitable for mass manufacturing. Various schemes have been proposed to overcome this problem, but the researchers devised their own method. They made a graphene suspension directly from graphite by using dimethylformamide for chemical exfoliation. They spray-deposited the suspension onto a preheated glass slide, producing approximately 1.5-nm-thick films over centimeter-size areas. The film consisted of overlapping flakes, with a maximum thickness of five layers.
Novoselov said that developing the technique did not require a lot of trial and error. “Once you know the principle — that you need to find a chemical which would wet graphene — the rest is pretty simple.”
The optical transmission of the film was better than 90 percent, and the sheet resistance was about 5000 Ω, making it suitable for some applications. Performance could be improved through optimization of the processing, according to the group.
Nano Letters, ASAP Edition, April 30, 2008, doi: 10.1021/nl080649i.
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