Chemical Vapour Deposition as a possible route for large-scale production of graphene

Graphene is a 2D layer of carbon atoms arranged in a honeycomb lattice. Unlike usual semiconductors, charge carriers in graphene behave as massless Dirac particles, characterized by Fermi velocity v_F ~ 10^6 m/s, and exhibit a perfect electron/hole symmetry, making this material a gapless semiconductor. These astonishing features explain the most intriguing properties of graphene: charge carriers mobilities at room temperature up to 200 000 cm^2/(Vs), anomalous Quantum Hall Effect, weak localization providing minimal conductivity also for vanishing density of states, perfect tunneling through rectangular strong potential barriers, superior thermal conductivity (5000 W/(mK) for suspended monolayer graphene), almost perfect transparency over a wide range of the electromagnetic spectrum. These peculiarities, combined with its inherent excellent mechanical properties (Young’s modulus of 1 TPa) and the demonstrated possibility of gating graphene to fabricate Field Electron Transistors, make this material the most promising candidate for many applications, ranging from Nanoelectronics to Optoelectronics, Photonics, Spintronics and even Metrology. However, a scalable, cost-effective and efficient method to grow monolayer graphene over large scale areas, as required by electronic industries, is still lacking. The “groundbreaking experiments” performed by the Nobel Prizes K. S. Novoselov and A. K. Geim in 2004 led for the first time to grow graphene on an insulating substrate, allowing to investigate its properties: nonetheless, the technique they have developed (micromechanical exfoliation of Highly Oriented Pyrolytic Graphite by means of adhesive tape), although quite straightforward and cost-effective, is messy (monolayer graphene flakes must be searched after deposition) and not up-scalable: therefore, though this method provides high quality and quite large graphene crystals, ideal for basic research, it is not suitable for industrial purposes. Various attempts have been carried out during the years to conceive more efficient graphene growth techniques and nowadays one of the most powerful methods to achieve the goal is the Chemical Vapour Deposition (CVD) through decomposition, activated by a catalytic substrate, of a carbon precursor. The interest for this technique stems from its suitability to match the requirements of device fabrication technologies and from the quite low temperatures (T ~ 900 °C) required for graphene growth. However, it has some drawbacks related to the polycrystalline nature of the catalytic substrate limiting the growth of large graphene crystals and to the difficulties (mainly related to dewetting of the substrate) arising while processing thin film catalysts (especially Cu) at the temperatures needed. In this thesis the results concerning the research about CVD processes performed at I.N.Ri.M. in the last three years will be presented. Emphasis will be devoted to the presentation of results concerning the deposition of graphene on Cu films, since in this field an in situ technique allowing for a real time control of dewetting occurring on Cu surface has been developed. Moreover, results concerning a laser induced etching technique to reduce the number of layers on top of Cu, discovered while characterizing the samples with Raman analysis, will be also reported. Both the techniques represent good achievements towards a better control of CVD processes and therefore towards the full exploitation of this technique as a promising route to grow high quality monolayer graphene, over large scale areas.