During the last three years, we have modified the set-up of a conventional scanning electron microscope in order to enable the observation of catalyst surface dynamics under controlled atmosphere and temperature. Using this instrument, we investigate chemical vapor deposition (CVD) growth of graphene on different metal catalysts. Since the experiments are performed in the chamber of a microscope, it is possible to observe a complete CVD process from substrate annealing through graphene nucleation and growth and, finally, substrate cooling in real time at nanometer-scale resolution without the need of sample transfer. The nucleation and growth of single layer graphene can be investigated at temperatures of up to 1000°C, while at the same time, surface dynamics of the active metal catalyst can be imaged and directly related to the catalytic activity (Figure 1).[1] Due to the high sensitivity of the secondary electron signal to changes in the work function and charge transfer at the surface, we are able to visualize different degrees of graphene-substrate coupling [2] as well as the stacking sequence of few layer graphene. The in situ SEM image in Figure 2a and the plot in Figure 2b illustrate the step-wise variation of the contrast that allows identification of up to 9 individual graphene layers on platinum substrates, starting with the brightest first layer in contact with the substrate. In addition, the in situ SEM images of edge misalignment between mutual layers and individual sheets provide real-time information on the evolution of the rotation angle between growing layers and formation of the stacking order. The growth behavior of graphene on nickel, copper and platinum substrates shows characteristic differences that are related to the catalytic activity and carbon solubility of the respective catalysts (Figure 3).

In the case of Cu and Pt substrates, we observe grain orientation dependent growth dynamics. Real-time imaging during growth thus allows us to directly visualize and study the catalytic activity of differently oriented surfaces.            
ESEM observations during graphene growth highlight the dynamic nature of catalysts and reveal the sensitive response of the surface to changes in the chemical potential of the gas phase. In situ scanning electron microscopy furthermore covers the spatial resolution of complementary in situ techniques that provide spectroscopic information, such as ambient pressure X-ray and Raman spectroscopy. It completes the spectroscopic data with visual information and spatially resolved chemical dynamics.

References:

[1] Zhu-Jun Wang et al., ACS Nano, 2015, 9 (2), 1506-1519

[2] Piran R. Kidambi et al., Nano Lett., 2013, 13 (10), 4769-4778

Figures:

Figure 1: a) shows colorized snapshots taken during low-pressure CVD growth of graphene on copper at 1000°C. The growth and nucleation behaviour can directly be abstracted from the recorded images as shown in b).

Figure 2: The ESEM image (a) and the corresponding plot (b) show that up to 9 graphene layers can be distinguished within the contrast range.

Figure 3: Graphene growth shows characteristic differences for different metal catalysts: a) Ni, b) Cu and d) Pt

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EMC Abstracts - https://emc-proceedings.com/abstract/direct-observation-of-cvd-graphene-growth-and-related-surface-dynamics-of-active-metal-catalysts-by-in-situ-scanning-electron-microscopy/