Chemical vapor deposition of graphene at very low pressures

Abstract

The chemical vapor deposition (CVD) of hydrocarbons appears as the most suitable graphene production method for large area applications such as flexible displays and photovoltaics. However, the CVD mechanisms have not been fully understood and therefore a complete control over the morphology of the produced sheets has yet to be achieved. In this thesis we report a systematic investigation of graphene CVD at very low pressures performed on a cold wall reactor using copper substrates. A combination of scanning electron microscopy images and Raman spectroscopy measurements had demonstrated that the initial stages of graphene growth is strongly dependent of the growth temperature (T) and the nature of the carbon precursor. By using a high molecular weight liquid carbon precursor, growth of large graphene crystals (~up to 300 m) at very high rates (up to 3 m2.s-1) was achieved. For high temperature growth (T>900ºC), the shape and symmetry of the grains were found to depend on the underlying symmetry of the Cu crystal, whereas for lower temperatures (<900ºC), mostly rounded grains are observed. The temperature dependence of graphene nucleation density was determined, showing two thermally activated regimes, with activation energy values of (6±1) eV for 900 ºC < T < 960 ºC and (9±1) eV for T > 960 ºC. The comparison of such dependence with the temperature dependence of Cu surface self-diffusion suggests that graphene growth at high temperatures and low pressures is strongly influenced by copper surface rearrangement. A growth model was propose that incorporates Cu surface self-diffusion as an essential process to explain the orientation correlation between graphene and Cu crystals, and which can clarify the difference generally observed between graphene domain shapes in atmospheric-pressure and low-pressure chemical vapor deposition. In addition, we show that strain is induced over the graphene films during the coalescence of graphene individual domains.