Chapter 3 CVD growth of highly-oriented monolayer graphene on Cu/Ni(111) foil
3.3 CVD growth and characterization of graphene on Cu/Ni(111) foils
3.3.1 Preparation and characterization of Cu/Ni(111) foils
30
3.3 CVD growth and characterization of graphene on Cu/Ni(111) foils
31
XPS depth-profile measurements showed that the bulk composition was uniform, further confirming that Cu and Ni were well mixed in the alloy (Figure 3.6). The top atomic layer(s) have a different composition and have an ordered structure of Cu and Ni atoms rather than randomly substituted Ni (Cu6Ni1; see discussion of LEED results below). Figure 3.3b-e show scanning electron microscopy (SEM) images of the graphene grown at 1075 °C on Cu/Ni(111) alloy substrates (growth conditions given in Figure 3.1). It can be seen that the graphene first nucleates and then grows into islands that (almost) all have the same orientation across the imaged area (∼800 μm; Figure 3.3b-d) and, that upon extended growth time, eventually coalesce into a continuous film. All the graphene islands are hexagonal with sizes in the range 50–100 µm after 1 min exposure to methane/hydrogen gas mixture. Like the islands, the continuous graphene film was identically oriented in almost all regions. We have found between 1 to 2% mis-oriented graphene islands in different samples (the OM image of a typical sample is given in Figure 3.7).
Figure 3.5. Characterization of Cu(111) and Cu/Ni(111) foils. (a) XRD patterns of pure Cu(111), Ni- plated Cu(111) and Cu/Ni(111) alloy foils. (b, c) EBSD of Cu(111) and Cu/Ni(111) alloy foils, respectively. (d) AFM image of Cu(111) foil (surface roughness is 0.82 nm after polishing). (e) AFM image of Cu/Ni(111) alloy foil (surface roughness is 1.25 nm after polishing).
32
Figure 3.6. XPS depth profile measurements. (a) Cu/Ni(111) alloy foil. (b) Monolayer graphene on Cu/Ni(111) alloy foil.
In addition to transferred samples, we have also analyzed several SEM images of the as-grown graphene islands on the Cu/Ni(111) substrate and found that the misorientation angle is essentially random (ranging from greater than 0 to less than 30°) with respect to the aligned islands. This broad range of misorientation angles in misoriented graphene islands has been previously observed for graphene grown on Cu(111).94 Thus, a large majority of the islands are epitaxial with the substrate and closely aligned with each other (marked with yellow dashed lines in Figure 3.7). Full coverage of the highly-oriented monolayer graphene film can be achieved in 5 min, which is much shorter than the growth time reported in previous studies.91, 94, 112-113
Figure 3.7. Optical image of a transferred graphene island sample on SiO2-on-Si substrate. The yellow dashed lines indicate the direction of oriented graphene islands. The red dashed line shows one mis-oriented island.
33
Figure 3.8a shows a photograph of the Cu/Ni(111) alloy foil and we note its mirror-like surface; Figure 3.8b shows a SEM image of the same foil. The uniform color of the EBSD maps (Figure 3.8c–d) for the out-of-plane (z) and in-plane (rolling direction) (y) maps indicates that the Cu/Ni(111) alloy foil is a single crystal over a large area (about 2 cm × 3.5 cm). Combined data from XRD and EBSD analysis prove that the whole Cu/Ni alloy foil is a single crystal. The LEED pattern of the Cu/Ni(111) alloy foil obtained at 75 eV (the spot size is approximately 1 mm) has two sets of hexagonal patterns (Figure 3.8e). These two sets of satellite LEED spots (see detailed analysis in Figure 3.9) indicate the formation of (√7×√7)R19.1º and (√7×√7)R-19.1º superstructures (they have the same periodicity but different chiral angles). The models of the superstructures in Figure 3.9c-d show that the Ni atoms are periodically arranged on the surface; the atomic ratio of Cu and Ni is 6:1 from the unit cell shown in Figure 3.9e, corresponding to a Ni percentage of 14.3 at.% at the surface.
This result indicates that the surface is Ni-rich relative to the bulk, and we note other studies where, for Cu-rich Cu/Ni alloys, Ni was reported to segregate to the alloy surface.114-116 More interestingly, we find that the same superstructure was detected by LEED with or without graphene grown on the surface, for the bulk composition of 5.9 at.% Ni (for 1.3, 2.4, 3.9, 6.2, and 7.8 (bulk) at.% Ni, an identical superstructure was found for the graphene-coated samples; we did not study if it was present prior to graphene growth for these compositions).
Figure 3.8. Characterization of Cu/Ni(111) alloy foils. (a) Optical image of a Cu/Ni(111) alloy foil (5.9 at.% Ni). (b) SEM image of the Cu/Ni(111) alloy foil. (c) Inverse pole figure orientation coloring scheme of the EBSD maps. (d) EBSD maps taken over four 100×100 μm2 areas across a 2 cm × 3.5 cm Cu/Ni foil. Out-of-plane (z) data represent the surface orientation, while the in-plane (y) measurements indicate the azimuthal angle. (e) LEED measurement of the Cu/Ni(111) alloy foil.
34
To the best of our knowledge, such single crystal Cu/Ni(111) foils, and also the superstructure of Ni in the Cu lattice in the top atomic layer(s) in a Cu/Ni(111) crystal (surface is Ni- rich) has never been reported. The exceptionally fast growth of epitaxial graphene on a metal foil surface motivated us (see below) to study the kinetics of growth of the graphene islands.
Figure 3.9. LEED investigations of the Cu/Ni(111) alloy. (a) LEED patterns of Cu/Ni(111) alloy (5.9 at.% Ni) at different incident energies. (b) Superstructure analysis from the LEED pattern taken at 75 eV. (c, d) Corresponding optimized models of (b). (e) The unit cell is illustrated with solid lines.