Chapter 6 Single-crystal graphene grown on twinned Cu substrates
6.2 Experimental methods and results
6.2.1 Preparation of twinned Cu foils
In this work, three different processing methods for Cu foils before annealing are designed. For the first one, the commercial Cu foil was kept in a flat shape and supported by a quartz slab in the furnace, as displayed in Figure 6.1(a). Then the Cu foil was annealed at 1010 °C for 1 hour in a H2
environment. After that, we oxidized the annealed foil to check its surface structure, and the optical image in Figure 6.1(b) shows the surface orientation is very uniform without obvious surface orientation difference. The EBSD map in Figure 6.1(c) shows a strong {001}<100> cubic texture with an average grain size of ~ 300 μm for the annealed Cu foil, which is consistent with previous reports.352,353 It is worth noting that a very small fraction of twined grains (the blue area) can be observed. For the second process, the commercial Cu foil was bended by the furnace tube wall (see Figure 6.1(d)) and then annealed. Both optical image (Figure 6.1(e)) and EBSD map (Figure 6.1(f)) of the annealed bended Cu foil show various surface orientations but the sizes of grains are much larger than that in the annealed flat Cu foil, indicating that extra stress can significantly promote the migration and growth of grain boundaries during annealing.
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Figure 6.1 (a) Photo of a flat Cu foil. (b) Optical image of the annealed flat Cu foil after oxidization.
(c) EBSD map of the annealed flat Cu foil. (d) Photo of a bended Cu foil. (e) Optical image of the annealed bended Cu foil after oxidization. (f) EBSD map of the annealed bended Cu foil.
For the third processing method, the polycrystalline Cu foil was not only bended but also manipulated by a microhardness indenter in the central area to induce a preferred nucleation site and further a preferred growth of Cu grains at this site, as shown in Figure 6.2(a). The tailored Cu foil was annealed for one hour in a H2 environment at 1010 °C. After annealing, the Cu foil was then oxidized, and its optical image is shown in Figure 6.2(b). Obviously, there are two contrasts in the optical image, suggesting that there are two types of Cu surfaces on the annealed Cu foil. To figure out the surface orientation of the annealed Cu foil, EBSD was performed and the maps of three representative areas are displayed in Figure 6.2(c). It is revealed that all dark areas are single crystalline with (116) crystalline surface and light areas show (111) crystalline surface, and there is no in-plane rotation within the single crystalline areas and grain boundaries only exist in the transition areas between the two single crystalline areas. The pole and inverse pole figure in Figure 6.2(d) proved that there are no other crystalline orientations in the annealed Cu foil, further supporting the EBSD results. In Figure 6.2(e), High resolution transmission electron microscope (HRTEM) image of the grain boundary shows that the two grains with (116) and (111) surfaces are mirror symmetric with respect to their grain boundary, indicating the twin characteristic of the annealed Cu foil. Combining with the corresponding selected area electron diffraction (SAED) pattern (insert of Figure 6.2(e)), the two grains were coaxial along one
<111> direction and had a 60° misorientation around their <111> coaxis, which is consistent with the most popular grain boundary in FCC TMs due to its lowest formation energy, called <111>/60°twin boundary.354,355
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Figure 6.2 (a) Photo of a bended Cu foil with microhardness indent (shown in insert) in center area. (b) Optical image of an oxidized twinned Cu foil. (c) EBSD maps of three representative areas of the twinned Cu foil marked in (b), with scale bar of 500 m; (d) (111) Pole figure and inverse pole figure of the twinned Cu foil. (e) HRTEM and SAED images of the grain boundary in the twinned Cu foil.
6.2.2 Synthesis of unidirectional graphene islands on twinned Cu foils
Using ambient pressure chemical vapor deposition (APCVD) method, graphene was synthesized on such a twined Cu foil and the typical growth process versus the growth time is shown in Figure 6.3(a). At nucleation stage, the coverage rate kept quite low for a certain time period, implying the nucleation incubation process. During growth, the coverage rate increased significantly, which was followed by a final slow growth period, indicating a self-limited growth of graphene film. After ~15 minutes growth, graphene islands merged and a continuous graphene film was formed. Three representative optical microscopy images of graphene islands grown on the twined Cu foil were shown.
Despite that graphene islands on the two sides of a Cu twin grain boundary showed different shapes, which were perfect hexagons on the Cu(111) surface (light part) and elongated hexagons on the Cu(116) surface (dark part), their hexagonal shapes and the 120o inner angle of the vertices proved their single crystalline nature. It is worth noting that all graphene islands showed the same orientation with one pair of their ZZ edges paralleled with the Cu grain boundary, which was in sharp contrast with previous reports of graphene grown on polycrystalline Cu foils, where graphene islands show random orientations. Hydrogen etching at 1000ºC for 10 min was also performed to verify the crystallinity of the obtained graphene film,356,357 and the results are shown in Figure 6.3(b). It can be seen that all etched holes were hexagonal and unidirectionally aligned, confirming that the synthesized graphene film was single crystalline with only point defects. Furthermore, the crystallinity of graphene grown on a 5×5
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mm2 area Cu foil with abundant twin grain boundaries was further characterized by LEED. Figure 6.3(d) show the diffraction patterns of the synthesized graphene at different but uniformly distributed locations of the Cu foil in Figure 6.3(c), it turns out that the graphene film over the whole area was single crystalline.
Figure 6.3 (a) Growth process of graphene on the twin Cu foil. (b) Optical images of graphene films on the twin Cu foil after hydrogen etching. (c) Photo and EBSD map of the twin Cu foil. (d) LEED patterns of 16 different areas of graphene on the twin Cu foil.