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One of the biggest practical issues surrounding graphene is the commonplace methods that require the transfer of graphene onto an intended substrate. Transfer-free graphene is one key area currently being developed to not only help improve the yield of graphene, but to improve the efficiency of the applications that it features in. A team of researchers from Japan and Malaysia have recently synthesised transfer-free graphene onto a silicon dioxide substrate, by a solid-liquid-solid reaction. They also achieved reaction temperatures lower than previous efforts have yielded.
Graphene is most commonly grown by chemical vapour deposition (CVD) methods. The indirect growth of graphene on to foil, normally copper, followed by transferring onto a substrate, is a major factor for loss of yield when synthesising graphene. Most reactions involving the synthesis of graphene require high temperatures, with recent efforts from other research groups only yielding a reaction temperature of 450-700 °C. Whilst this is fine for small scale production, to really thrust graphene production to new commercial heights, an industry-friendly method is required that lowers the temperature of the graphene synthetic process.
Graphene transfer methods not only play a major role in the yield of graphene, but also in the effectiveness of certain applications. One notable example is that of graphene-based pressure sensors. Many of these sensors either employ a graphene sheet suspended above another membrane (commonly silicon-based), or a membrane with holes etched into the molecular sheet- which provides an area for graphene sheets to be grafted onto. In the former, the transfer process plays a major role in providing effective coverage across the whole membrane.
For the latter, many of the holes require the graphene at different tensions to measure the pressure. If the transfer method is not effective, it does not only affect the yield but also the tension of the graphene sheet, affecting the overall efficiency of the sensor- a problem which could be done without. This effect is multiplied when more than one graphene sheets are transferred, which is often the case for many applications, not just in sensors. This is just one example of the many applications that rely on inconsistent graphene transfer methods. As such, the transfer processes of graphene play a much greater role for graphene-based applications than may people realise.
The Transfer-Free Process
Having noted that a variety of metals can be used to grow graphene on insulating substrates through solid-liquid-solid reactions, the researchers tested out these methods to produce a transfer-free method that uses low temperatures.
The researchers heated layers of a non-CVD-compatible tin catalyst, with carbon on a silicon dioxide substrate in a vacuum. Amorphous carbon is commercially available, so unlike CVD, the method negated the decomposition process of carbon-containing molecules. The deposition of a thin layer was carried out using a pulsed laser deposition (PLD) method, controlling both the catalyst and carbon deposition rates. The researchers performed annealing at 250 °C and 5 x 10-5 Pa of pressure.
The graphene layer was sandwiched between catalytic layers and not freestanding on the surface, so a simple chemical etching technique was required to expose the graphene sheet. The researchers characterised the graphene sheet using an optical microscope (VHX-500 digital microscope) and atomic force microscopy (AFM) (JSPM-5200) to measure the domain size; with Raman Spectroscopy (NRS 3300 laser Raman spectrometer, 532.08 nm) and Auger electron spectroscopy (AES) (JAMP-7800) being used to measure the removal of the catalyst.
The highest temperature exhibited throughout the reaction was 250 °C, which is significantly lower than any previous efforts have so far produced and is down to the choice of catalyst. The size of the graphene sheet is currently limited with a maximum size of 5 µm obtained to date. However, research is now being undertaken to increase the size and yield. The combination of a transfer free process at low-temperatures provides an exciting platform for commercially-viable processes in the future and could potentially explore new dimensions of graphene properties and applications.
Vishwakarama R., Rosmi M. S., Takahashi K., Wakamatsu Y., Yaakob Y., Araby M. I., Kalita G., Kitazawa M., Tanemura M., Transfer free graphene growth on SiO2 substrate at 250 °C, Scientific Reports, 2017, 7, 43756