The synthesis of large, defect-free two-dimensional materials is a major challenge toward industrial applications (photovoltaics, semiconductors, etc.). Chemical vapor deposition (CVD) is to date the most promising method to produce large, high quality graphene sheets. However, polycrystalline copper generates defects in the graphene layer, and after cooling, the layer forms domains and wrinkles that significantly degrade its quality. The researchers are working to develop a new process on liquid metal catalysts under CVD growth conditions.

Liquid metal catalysts, e.g., molten copper, have been employed for the fast growth of uniform graphene film of the highest quality, while using temperature, pressure and flow conditions that are comparable to those used with solid catalysts. However, the lack of in situ techniques led to empirical growth recipes. Thus, an in situ multiscale monitoring coupled with a real time control of the growth parameters is necessary for efficient synthesis. This allows to control operando the morphology, at large scale from the atom to the millimeter, to organize the interactions of the graphene crystals and to optimize the obtained film. Until now, there were significant hurdles against the realization of in situ monitoring techniques including heat and evaporation of the liquid metal, curve and dynamic of the liquid surface, and the presence of reactive CVD gas at close to atmospheric pressure.

IRIG researchers [collaboration] have succeeded in monitoring the growth of graphene on liquid copper via four complementary methods (Figure 1) applied in situ and in real time: synchrotron X-ray diffraction and X-ray reflectivity, Raman spectroscopy and radiation mode optical microscopy (Figure 2). Synchrotron X-rays confirm the superior crystallinity of the produced, monolayer-thick graphene.

Figure 1: Configuration of four complementary in situ, operando and real time methods: synchrotron X-ray diffraction and reflectivity, Raman spectroscopy, and radiation-mode optical microscopy, applied to a graphene layer grown on liquid copper.
© Francesco La Porta /CEA


Figure 2: Radiation-mode optical microscopy of self-organised hexagonal graphene flakes on liquid copper (Image scale: ~1 mm).
© MEM

Real-time monitoring allows to control the size, shape and purity of the crystals and to optimize the growth rate. Finally, the experimental observations associated with a modelization allowed to understand the growth mechanisms.

This process proves to be reliable and opens the way to the rapid production (Figure 3) of defect-free single-crystal graphene on surfaces of several square centimeters, suitable for various electronic applications. This real-time monitoring and control methodology is also useful for the scientific study of other 2D materials.


Figure 3: Squetch of a new scaled-up reactor capable of growing large graphene flakes on liquid metal catalysts.
© CEA