Wednesday, 4 June 2014

Graphene At Your Fingertips


Over the past few years, researchers have developed numerous methods for synthesizing graphene. The synthesis of high-quality graphene is usually prepared by a complex and costly process – epitaxial growth on transition metal surfaces via chemical vapor deposition (CVD) using high-purity hydrocarbons as precursors . This kind of synthesis is in conflict with one of the desired applications – transistor arrays – since a metal support short cuts the graphene layer.The presence of a metal surface is a basic requirement for the CVD synthesis of graphene because its catalytic effect is necessary for the process.The one way to circumvent the problem is to firstly grow graphene on a thin metal film laying on an insulating surface. Secondly, the metal film is removed by chemical etching. That way, the graphene layer gets in direct contact to the insulator. Etching, however, introduces a new challenge, since it can induce defects in the graphene layer and can spoil its unique electronic properties.

                          The challenge to be overcome can be reduced by use of metals with very small chemical interaction with the graphene layer, on some of this metals – such as silver – the performance of carbon uptake during a CVD process decreases with decreasing interaction. Therefore, we have used liquid precursor deposition (LPD) synthesis, which is an adequate alternative to CVD.

                    In LPD, the precursor molecules are provided via the liquid phase, e.g. by rinsing the metal surface with an organic solvent. In this process, weakly interacting metals take enough carbon from the precursor molecules to ensure graphene growth.In a new study, published online in Langmuir ("Graphene from Fingerprints: Exhausting the Performance of Liquid Precursor Deposition"), Müller, who is first author of the paper, and his fellow researchers demonstrate the reliability of graphene growth by LPD using an unconventional liquid – a human fingerprint.Müller concedes that, on first sight, graphene formation by a fingerprint may seem like a useless experiment. "Nobody would seriously consider the application of this synthesis route in a technological production step. However, our experiments prove the reliability of LPD synthesis because graphene growth from fingerprints, although starting with a highly uncontrolled way to deposit carbon, achieves the same results as the LPD synthesis route using pure synthetic precursors."LPD synthesis by using synthetic precursors and by using fingerprints provided the same results in terms of graphene monolayer formation. The precursors just affect the initial amount of material that is provided after deposition.

                                    Epitaxial graphene is expected to be the only way to obtain large-area sheets of this two-dimensional material for applications on an industrial scale. So far, there are different recipes for epitaxial growth of graphene, using either intrinsic carbon, such as the selective desorption of silicon from a SiC surface, or extrinsic carbon, as via the chemical vapor deposition (CVD) of simple hydrocarbons on transition metal surfaces. In addition, even liquid precursor deposition (LPD) provides well-ordered graphene monolayers. It will be shown that graphene formation on transition metal surfaces by LPD synthesis is a very robust mechanism that even works if carbon is provided in a quite undefined way, namely by using a human fingerprint as a liquid precursor. Graphene growth from fingerprints provides well-ordered monolayers with the same quality as LPD grown graphene using ultrapure synthetic single precursors. The reliability of the self-assembly process of graphene growth on transition metals by LPD therefore offers a simple and extremely robust synthesis route for epitaxial graphene and may give access to production pathways for substrates for which the CVD method fails.
                Since the fingerprint contains a broad mixture of different chemicals, its excellent performance for graphene formation is an interesting result in terms of suitable or unsuitable precursor.Experiments show that graphene does not only grow from organic 'waste' but also self assembles directly out of the 'waste', showing that the synthesis of monolayer graphene on transition metals is an extremely robust process.LPD may give access to a wider range of substrate materials for graphene growth. The results from this study may contribute to a possible future synthesis route for graphene on insulators, thereby disposing all the problems in handling the current transfer techniques.

source:Langmuir ,nanowerk