Graphene makes for better optical displays

31 Mar 2008

The cost and quality of LCDs could be improved by replacing conventional indium tin oxide electrodes with graphene.

A new technique that boosts graphene production could offer an alternative to using expensive indium tin oxide (ITO) electrodes in LCDs. The approach could lead to lower cost and better quality LCDs, according to research by a team from the University of Manchester in the UK and the Institute for Microelectronics Technology in Chernogolovka in Russia.

LCDs typically consist of a thin layer of liquid crystal sandwiched between a pair of polarizers. The setup either blocks or allows light from travelling through the display, depending upon an electric field applied across the polarizers. Conventional LCD displays use ITO electrodes to apply the electric field, but indium is rare and expensive and ITO can release both indium and oxygen ions. Such stray ions can reduce the effectiveness of the alignment layer, causing undesirable "image sticking".

Graphene offers much greater stability, and so avoids the problem of stray ions preventing the liquid crystal from aligning correctly. Perhaps more importantly, however, the team found that graphene had an optical transmission of about 98%, significantly higher than the 82–85% of standard ITO.

What is doubly impressive about graphene is that it can achieve high optical transmission with a corresponding sheet resistance (a 2D measure of resistivity) of just 6 kΩ. With an added alignment layer of polyvinyl alcohol, which has the side effect of reducing resistance, this figure drops to 400 Ω. Further chemical doping can reduce the sheet resistance to 50 Ω. ITO, on the other hand, has to trade resistance for transparency. Indeed, if an ITO electrode is made thin enough to rival the transparency of graphene, its sheet resistance skyrockets.

Faster production
Until now, manufacturing sizable quantities of graphene has been difficult, since the usual technique of micromechanical cleavage can only produce a few flakes at a time. However, the group has developed a new production technique that can reap larger quantities.

They begin by placing crystals of graphite in a bath of dimethylformamide (DMF) and then sonicate it with ultrasound for over three hours. Graphite is hydrophobic, which means that it tends to clump together in water, but in DMF the sonication allows it to "dissolve" into flakes. Next, the researchers centrifuge the mixture for 10 minutes to remove thick flakes from the monolayer flakes of graphene, which they subsequently spray onto a glass slide. Finally, they anneal the slides for two hours at 250 °C amid hydrogen and argon gas. Although the thickness is not consistent over the slide — it varies between one and four layers of graphene — the optical properties match those of graphene produced by micromechanical cleavage.

Although the group has no plans to commercialize, interest has been received from the LCD industry. The team will shortly be publishing more fundamental results on graphene's optical properties.