Session L12: Focus Session: Graphene: Growth, Mechanical Exfoliation, and Properties - Growth on Single Crystals

Graphene growth on coinage-metal substrates

The low solubility of carbon in Cu and Au gives these coinage metals advantages as substrates for graphene growth. Namely, growth occurs exclusively by surface processes, avoiding the complications of C segregating from the bulk of the metal substrate. However, the relatively weak interactions of Cu and Au with graphene can lead to mosaic films having large ranges of in-plane orientations. This talk will emphasize understanding the relationship between the microstructure of graphene sheets and the mechanisms of island nucleation and growth. We use low-energy electron microscopy (LEEM) to observe growth. We find that bunches of substrate steps on Cu(111) can generate misorientation boundaries in a graphene sheet as it overgrows the steps [1]. Thus, growth on rough Cu(111) leads to large rotational disorder. Optimized growth on smooth Cu(111) and Au(111), however, produces islands all in close registry to a single in-plane orientation. On Cu(100), the most abundant grain orientation of commercial Cu foils, graphene islands align around two equivalent in-plane Cu directions [2]. This inherent source of disorder from symmetry mismatch is further compounded by large spreads of orientation around the equivalent directions. The substrate choice also affects the microscopic growth mechanism. The rate that C diffuses to the graphene islands limits growth on Cu(111) [and likely on Au(111)]. The sheet edges are then morphologically unstable, with dendritic islands at low temperature and six-fold loped islands at higher temperature. In contrast, growth on Cu(100) is limited by the rate of C attaching to the graphene edge. This mechanism, combined with the symmetry mismatch, produces two-fold islands. Finally, the coinage metals will be compared to other transition metal substrates. \\[4pt] [1] Phys. Rev. B 84, p. 155425 (2011). \\[0pt] [2] Nano Lett. 10, p. 4890 (2010).   

Growth and Characterization of Graphene on Single Crystal Cu Substrates

One of the key issues for the use of CVD graphene in device applications is the influence of defects on the transport properties of the graphene. Therefore, it is important to understand the influence of the substrate on the orientation of the graphene. Growth of graphene films on Cu(111) has the potential for producing films with a low defect density because of the hexagonal symmetry of the substrate and relatively small lattice mismatch, whereas growth on Cu(100) is expected to result in multi-domain growth because of its square symmetry. In this study, graphene films were grown on Cu single crystal substrates, and characterized with LEEM, LEED, SEM, AFM, and Raman spectroscopy. The clean Cu substrates were prepared by sputtering and annealing in UHV. For the initial growth studies, the samples were transferred to a tube furnace for graphene growth using a technique optimized for Cu foils. The UHV system has recently been modified with a button heater compatible with the conditions needed for graphene growth to enable in-situ growth and characterization.   

Suppression of Grain Boundaries in Graphene Growth on Superstructured Mn-Cu(111) Surface

A standing obstacle in epitaxial graphene growth on metal substrates is the prevalence of undesirable grain boundaries (GB) that severely degrade the electronic, transport and mechanical properties of graphene. Employing density functional theory calculations, we demonstrate that the inherent multi-orientational degeneracy of the graphene islands on Cu(111) is the underlying reason for the prevalence of GB. We propose a possible solution, by invoking a functionalized Cu(111) surface to lift the orientational degeneracy of graphene islands and consequently suppress the creation of GB. We have identified the candidate substrate---a superstructured Mn-Cu(111) alloyed surface, which is experimentally achievable and ensures a single orientation for the graphene islands. The proposed approach promises to drastically improve the quality of epitaxial graphene without compromising on efficiency and yield.   

Rotational homogeneity in graphene grown on Au(111)

The set of properties offered by the (111) surface of gold makes it intriguing as a platform on which to study the fundamental processes that underpin graphene growth on metals. Among these are the low carbon solubility and an interaction strength with graphene that is predicted to be smaller than most transition metals. We have investigated this synthesis process using low-energy electron microscopy and diffraction to monitor the sample surface in real time, and found that the resulting graphene film possesses a remarkable degree of rotational homogeneity. The dominant orientation of the graphene is aligned with the Au lattice, with a small minority rotated by 30 degrees. The origins of this in-plane structuring are puzzling because angularly resolved photo-emission spectroscopy and scanning tunneling microscopy experiments both suggest only a relatively small interaction between the two materials. Finally, the implications of these findings for the growth of high structural-quality graphene films are discussed.   

Fundamental growth mechanisms of graphene on nickel surfaces

CVD growth of graphene over transition metal surfaces has become a main approach for synthesizing large area graphene wafers. The low carbon solubility in copper makes a good material for monolayer graphene synthesis, while carbon dissolution and re-segregation from the bulk of other transition metals, namely nickel, make these materials more demanding for controlling graphene growth. However, lower growth temperatures and defined graphene orientation relative to the substrate are some benefits for graphene synthesis on nickel compared to copper. Here we thoroughly characterize the fundamental growth processes of graphene on nickel substrate with the aim to find growth procedures that enable controlled graphene synthesis. We also identify defect structures in graphene that are formed as a consequence of the nickel substrate.   

DFT studies of Graphene on Ni(111) and Surface Nickel Carbide Ni$_{2}$C

Graphene with its unique transport properties supported on ferromagnetic materials is a promising candidate for the fabrication of spin-filtering devices. In order to study the growth of graphene on metal surfaces, graphene on Ni(111) is a perfect system from a structural point of view. The CVD growth of graphene on Ni(111) was studied with STM. The experiments showed not only perfect aligned (1x1) structures, but also several moir\'{e} patterns are observed. They are due to grain rotations of graphene on both Ni(111) and also surface nickel carbide Ni$_{2}$C. During CVD growth carbon atoms segregate into the Ni bulk. With an increasing carbon concentration first the surface carbide and then a graphene layer on top is built. We studied the systems using the DFT program package VASP with the DFT-D2 method of Grimme to include van der Waals interactions. In our theoretical analysis we tried to understand the formation of the graphene and Ni$_{2}$C on Ni(111). We calculated the stability of the surface carbide phase and the binding energies of rotated and unrotated graphene on Ni(111) as well as on Ni$_{2}$C in order to understand the experimental findings. Graphene and Ni$_{2}$C have no epitaxial relationship due to their incommensurate lattices, leading to a spread in grain rotations.   

Graphene nucleation and growth on the transition metal surfaces: the role of pentagon, metal step and magic carbon clusters

The nucleation behavior of graphene on transition metal surfaces, either on a terrace or near a step edge, is systematically explored using density functional theory calculations. The supported carbon clusters, CN~(N=1$\sim $24), on the Ni(111) surface are carefully optimized [1,2]. A structural transformation from a C chain to a sp$^{2}$~C network at~C$_{12}$ and the most stable structures of sp$^{2}$ graphene islands contain one to three pentagons. In agreement with experimental observations, our calculations show that graphene nucleation near a metal step edge is superior to that on a terrace. Besides, ground state structures of supported CN (N = 16$\sim $26), clusters on four selected transition metal surfaces: (Rh(111), Ru(0001), Ni(111) and Cu(111)) are explored [3]. A core-shell structured of C$_{21}$ stands out as a magic cluster, which is one of the dominating carbon precursors in graphene CVD growth and has been observed in experimental STM images. The energy barrier of two C$_{21}$ clusters' coalescence is computed to illustrate their influence on the kinetics of graphene CVD growth at different temperatures. \\[4pt] [1] J. Gao, et al,. J. Am. Chem. Soc. 133, 5009 (2011). \\[0pt] [2] J. Gao, et al., J. Phys. Chem. C 115, 17695 (2011). \\[0pt] [3] Q. Yuan, et al., J. Am. Chem. Soc. (accepted).   

Optimizing Epitaxial Cu and Ni Films on Al$_2$O$_3$(0001) for Uniform Graphene Growth

Copper and nickel are the most commonly used substrates for the growth of graphene by chemical vapor deposition. While cold-rolled polycrystalline foils are most often selected for their commercial availability and ability to withstand the high temperatures required for graphene growth, (111) crystal faces have been shown to offer better growth characteristics on both materials. We deposited Cu and Ni films onto single crystal Al$_2$O$_3$(0001) using magnetron sputtering at temperatures between $250 ^{\circ}$C and $700 ^{\circ}$C. This gave films with pure (111) texture but with two epitaxial in-plane orientations as measured by x-ray diffraction and electron backscatter diffraction. Upon heating to graphene CVD temperatures ($900 ^{\circ}$C to $1000 ^{\circ}$C), the grain boundaries widen and deepen into trenches that prevent the growth of uniform graphene over large areas. Reactive sputtering of a thin layer of Al$_2$O$_3$ before depositing the metal results in a single in-plane orientation over $>90\%$ of the film for Ni. In addition, gradually increasing the temperature during metal deposition suppresses the formation of deep trenches under graphene CVD conditions. We compare CVD graphene grown on the optimized films with that grown on commercial foils.   

Kinetic Pathways towards Fabrication of Narrow Graphene Nanoribbons on Stepped Metal Surfaces

Graphene nanoribbons (GNRs) are predicted to exhibit intriguing electronic transport properties that strongly depend on their widths. To this end, one standing challenge is controlled fabrication of narrow GNRs with sizeable band gaps. In this study, we use first-principles approaches to explore the possibility of growing narrow GNRs along the step edges of catalytic metal surfaces. By minimizing the lattice mismatches of the growing graphene islands, optimizing their adsorption geometries, and exploiting the diffusion anisotropy of selective precursor monomers on the stepped surfaces, we identify the growth conditions under which the precursor monomers can be nucleated at the steps and then grow along the steps. Our studies point to kinetic pathways towards controlled fabrication of some of the narrowest GNRs with zigzag or armchair edges, and are expected to stimulate experimental efforts to realize these predictions.   

Molecular Beam Epitaxy of graphene nanocrystals

The ability to produce large-area graphene films on commonly used dielectric substrates can lead to many technological applications. We demonstrate the fabrication of large area conducting graphene nanocrystalline films on arbitrary dielectric substrates by MBE (molecular beam epitaxy) using a solid carbon source that can offer the integration of capable graphene production with high flexibility and variety in ultra-high vacuum environment together with state of the art thin film technology. Synchrotron x-ray and Raman spectroscopies show that the films consist on graphene nanocrystals oriented parallel to the sample surface. The growth rate is a key parameter that determines the bonding environment. Careful control of the growth conditions results in the production of predominantly sp$^{2}$-bonded carbon thin films on arbitrary substrates, with the potential of growing large graphene grains on epitaxial substrates.