Scientists create first artificial graphite intercalation compound made by layer-by-layer stacking of graphene
28 Apr 2013. ONDL scientists report today in Nature Nanotechnology a major breakthrough in the development of a robust transfer methodology that can transfer monolayers of graphene with high integrity from their growth substrate to any applications substrate, including those having soft or fragile layers (Song et al, “A general graphene transfer method to soft surfaces”, doi:10.1038/nnano.2013.63). This enables the development of new applications of graphene and the preparation of new artificial layered compounds which may have interesting new properties.
A graphene sheet comprises of one monolayer of carbon atoms bonded into a honeycomb structure. This material has attracted a lot of attention in recent years because of its unique electronic, optical and mechanical properties. Although it was first available as tiny slivers stripped from small single crystals of the mineral graphite or of artificially-produced highly-oriented pyrolytic graphite, recent advances in chemical vapor deposition (CVD) have made large sheets of graphenes grown on copper foils available almost at industrial scale. However in order to be most useful, these graphene monolayers need to be transferred out of their growth substrate onto the applications substrate. This is where a substantial stumbling block has existed for many years up till now. The problem is that these graphene sheets are fragile (because they are made of many mosaics joined together with defects) and are further susceptible to contamination. Previous methods to transfer the graphene suffer from sheet cracking, sheet contamination and the inability to place the sheet accurately at the desired site.
Now the team of ONDL scientists from the Departments of Chemistry and Physics has overcome these challenges by inserting a special layer, called the self-release layer, between an elastomeric (i.e., rubber) stamp and the graphene sheet. This layer provides a low work of adhesion on the stamp that facilities the delamination of the graphene and its subsequent accurate placement on the destination substrate. “It is a remarkably simple idea that works, very much akin to the use of an anti-stick coating that you can then subsequently removed under very mild conditions,” says Prof Lay-Lay Chua, the Principal Investigator who led the team.
Using the new method, these scientists demonstrated that ultra-thin polymer capacitors and low-operation-voltage field-effect transistors can be fabricated by transferring the graphene sheet to give the capacitor electrode or gate electrode on a thin stack of organic polymer materials. This opens new applications of graphenes as ultrathin electrodes in devices which were previously not possible. Furthermore, unlike traditional evaporated metal films, which damage the underlying organic film due to metal atom penetration that cause early dielectric breakdown, the transferred graphene sheets do not compromise the integrity of the underlying layers. “This leads to superior dielectric breakdown strengths even in very thin films of the insulator,” says Ms Jie Song, a PhD student on the project.
In addition, the scientists grew the first “artificial” graphite intercalation compound by stacking alternate monolayers of graphene and a pi-electron acceptor, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). F4TCNQ has been known for some years to be able to p-dope graphene monolayers, but this is the first time it has been assembled with graphene into an artificial compound. On the other hand, the traditional graphite intercalation compounds can be formed only under very harsh conditions to intercalate molecules or ions into the graphite interlayer spacing. Now because of the transfer method that has been developed, it has possible to stack graphene and F4TCNQ layer-by-layer. The compound exhibits good electrical conductivity and an unprecedented stability to heating in the ambient. “This work may stimulate new research directions in materials science to stack two-dimensional materials and molecular materials together in different ways to achieve new properties,” enthuses Prof Chua.
This project is collaboration with Dr Geok-Kieng Lim, principal scientist at DSO National Laboratories and adjunct professor at Department of Physics, NUS; and with the group of Prof Peter Ho, from Department of Physics, NUS.