Creating large graphene coatings
Graphene films that can be deposited over large areas of a substrate may be possible using a technique developed by US researchers. The material’s perfect 2D crystallographic nature and electrical properties will enable high-speed and flexible circuitry in electronic devices.
Scientists at the New Jersey Institute of Technology (NJIT) in Newark, USA, used modified chemical mechanical polishing, which is typically employed to even out the surface of a semiconductor wafer. This enables coverage of a substrate hundreds of microns wide that is made from silicon, anodised aluminium oxide or a copper screen.
‘Chemical mechanical polishing is a well-known technique,’ explains Professor Haim Grebel. ‘The stumbling block to overcome was reversing the polishing procedure – here we polish the raw graphite onto the wafer rather than polishing off the end-user wafer itself.’
A highly orientated pyrolitic graphite ingot was lowered over the substrate. Using 2D computer aided translation, the team controlled the graphite to ‘write’ desired features onto the substrate.
After annealing the material for three hours at 800-900ºC, a monolayer of carbon atoms was created on the surface. ‘We determined that the graphene is highly crystalline throughout the film,’ says Grebel. ‘[Although] more is to be done to confirm the size of the domains (if any).’
The technique can cover large areas of solid or perforated substrates. ‘Existing techniques are either too restrictive and accommodate only particular substrates, too obscure to be used with modern large-scale integration methods, or cannot be scaled up to achieve large coverage,’ says Grebel. The team has so far deposited the film over an area that is one inch-square.
Grebel explains that the tool also overcomes contamination caused by patterning graphene using lithography, as it removes many of the contact steps.
However, a co-discoverer of graphene, Professor Andre Geim of The University of Manchester, UK, is ‘not impressed’ with the New Jersey team’s work. ‘There have been dozens of papers offering a [technique for] mass production of graphene, and this one is mediocre at best,’ says Geim. ‘Quality and quantity are important. [The NJIT team] show a few small graphene flakes. There is nothing there except for an idea.’
Grebel admits that maintaining a single crystal throughout the film is a difficulty to overcome as the process is scaled up. ‘Of course we have some improvements to make, but that does not take away from the fact we have demonstrated graphene coverage which extends much more than what anyone has achieved before.’