AVS2011 Session SS-FrM: Surface Science on Graphene

MoS2 is a semiconducting material consisting of sulfur-molybenum-sulfur tripledecker layers loose bound by van der Waals interactions. MoS2 has been used technologically for a long time, for instance as lubricant, where similar to graphite its layered character was employed. Recently, its electronic characteristics have attained increased attention with the finding that it transitions from an indirect bandgap semiconductor at 1.6eV gap to a direct bandgap one at 1.9eV gap at the transition from multilayers to a single layer. A transistor has been constructed from a MoS2 and shown appreciable properties. The increased bandgap and high fluorescence yield may also suggest applications of the material for photonic or photocatalytic applications.

MoS2 can be exfoliated mechanically similar to graphene. While this method is simple, it is hard to control and not amendable to mass production of thin films. Solution-based processes have been proposed and may provide a scalable source of a mixture of single and multilayer material. Here we show an alternative avenue for the fabrication of MoS2 monolayers: growth of MoS2 on a sulfur-preloaded copper surface. In contrast to all other methods, this route has the potential of providing exclusively monolayer material, as the sulfur source is only available until the substrate is covered. Practically, this approach is related to the growth of graphene monolayers on copper or ruthenium films, where segregation of carbon to the surface is employed in aggregating a carbonaceous layer that transforms into graphene under the correct conditions.

Small MoS2 triangles of a few nanometers in size have been grown previously on gold in a dilute H2S athmosphere. Here we show significantly larger patches, tens of nanometers in size. In contrast to gold, copper forms a multitude of sulfur surface coverages and also readily absorbs sulfur into the bulk. Thus, we can preload the substrate with a specific amount sulfur using an easy to handle liquid precursor, benzenethiol. In previous work we have shown that heating to below 400K removes the phenyl group of benzenethiol reliably from copper leaving sulfur coverages behind.

The graphite surface consists of π conjugated system. When the π conjugated system is broken, the non-bonding π electronic states are known to form on the surface. Recently, the non-bonding π electronic states at the Fermi level of the graphite-related materials are expected to the active sites for the specific chemical reaction such as oxygen-reduction reaction in the fuel cell [1]. It is thus important to understand the interaction between an oxygen molecule and the graphite surface for the efficient usage of the graphite-related materials. We have reported previously that the defects induced by Ar⁺ ion bombardment on the graphite surface significantly affects the gas-graphite interaction based on the measurements of the angular intensity distributions of He and Ar beam scattered from the pristine and the defect induced graphite surfaces [2]. The difference in the gas-surface interaction has been ascribed to the local breaking of the π conjugated system of graphite by defect formation. To further investigate the effect of the modification of the graphite electronic states on the gas-surface interaction, especially for the oxygen adsorption, we have measured angular intensity distributions of O₂ from electronically modified graphite surfaces, namely potassium intercalated graphite, nitrogen-doped graphite (graphite bombarded by N₂⁺ ion) and defective graphite (graphite bombarded by Ar⁺ ion). The detail of scattering features as well as the effects of the electronic modification of graphite on the oxygen adsorption will be discussed in detail with our recent STM and STS results.

*E-mail: nakamura@ims.tsukuba.ac.jp [mailto:nakamura@ims.tsukuba.ac.jp]

[1] S.F. Huang, K. Terakura, T. Ozaki, T. Ikeda, M. Boero, M. Oshima, J. Ozaki and S. Miyata, Phys. Rev. B, 80, 235410 (2009).

[2] J. Oh, T. Kondo, D. Hatake, Y. Honma, K. Arakawa, T. Machida, J. Nakamura, J. Phys.: Condens. Matter, 22, 304008 (2010).

Graphene devices are based on finite size flakes (e.g. nanoribbons) in contact with dielectrics or other materials, and therefore require control of edges and depend on the control of processing methods (often involving vapor or wet chemistry). Graphene oxide (GO) represents an interesting system from which much can be learned about oxygen interaction with graphene. Furthermore, studying the reduction of GO provides a powerful way to understand the stability of oxygen species and the role of trapped molecules. We have studied both the thermal and chemical reduction of single- and multi-layer GO using in situ infrared (IR) absorption spectroscopy under a variety of conditions. For the commonly used as-synthesized GO, we find that water molecules play an important role in both defect formation (evident from CO₂ evolution)¹ and carbonyl-termination of defect edges at intermediate annealing temperatures (150-250 C).² We also find that a very stable edge configuration appears after high temperature anneals (> 850C), involving edge-ether termination of atomically straight zigzag edges and characterized by an anomalously strong IR absorption.³ The situation is dramatically different when water is replaced by alcohols or more complex molecules (e.g. ionic liquids). In general, defect formation is greatly suppressed (no CO₂ evolution) with less carbonyl formation and a reduced density of atomically straight, edge-ether terminated edges. This talk will summarize the current understanding of the mechanisms involved in thermal reduction and suggest pathways for developing stable graphene nanostructures with reasonable electrical properties.

1. Acik et al., Generation and capture of CO₂ and CO in graphite oxide stacks during thermal reduction Mater. Res. Soc. Symp. Proc., 1205E, 1205 (2010).

2. Acik et al., The Role of Intercalated Water in Multilayered Graphene Oxide. ACS Nano 4, 5861 (2010).

Graphene on transition metal substrates often forms superlattices that are manifested as Moiré patterns in scanning tunneling microscopy (STM) images. Such graphene superlattices can serve as templates for the formation of periodic arrays of metal nanoclusters with a uniform size distribution, a situation that is ideal for model catalyst studies. We have used an ultra high vacuum (UHV) STM to investigate graphene growth on Pt(111) from precursor hydrocarbon species. Different periodicities in the Moiré patterns are observed corresponding to different orientations of the graphene layer with respect to the Pt(111) lattice. Various graphene orientations are possible because of a relatively weak graphene-Pt interaction. Following Pt deposition onto the graphene-covered areas of the surface, small Pt nanoclusters were observed. While graphene on Pt(111) only weakly interacts with the substrate, which leads to a weak corrugation in the superlattice compared to other transition metals, such as Ru, our results show that even this weak corrugation is sufficient to serve as a template for the formation of mono-dispersed Pt nanoclusters. These Pt nanoclusters are relatively stable and only undergo agglomeration for annealing temperatures above 600 K.

Graphene has aroused tremendous interest due to its remarkable electronic and mechanical properties. The lack of a band-gap, however, causes a serious challenge for implementing graphene as a material for electrical switches and therefore creative ways of inducing this band-gap are needed. We will present a STM/LEED/DFT study of the monolayer graphene on Ru(0001) system in the presence of hydrogen. STM reveals a diverse array of Moiré superlattice sizes ranging from 0.9 to 2.4 nm in the presence of hydrogen adlayer structures, as confirmed by LEED. Density functional theory calculations show a correlation between the Moiré superstructure sizes and the hydrogen coverage and the opening of a band-gap in the graphene/H/Ru(0001) system for some of the Moiré/hydrogen adlayer coverages.

Graphene growth on metal surfaces (Ni, Pt, Ir, Rh and Cu) has been studied extensively [1]. Ni(111) is special among these metals because it is closely lattice matched with graphene (a_graphene= 0.246 nm vs. a_Ni(111)= 0.249 nm) allowing the growth of graphene with a single domain and in registry with the substrate [2]. However, compared to most other metal substrates the interaction between Ni and graphene is rather large, resulting in a small metal-carbon distance and a large shift of the graphene π-band compared to freestanding graphene. In order to de-couple graphene from the Ni-substrate other weaker interacting metals such as Cu and Au have been successfully intercalated between the graphene and Ni-substrate [3]. These metals have, however, a different lattice parameter and consequently the registry between the substrate and graphene is lost. Here we demonstrate a new approach that weakens the metal-graphene interaction without destroying the lattice registry. By intercalating Sn-atoms an ordered √3 × √3 R30° Sn-Ni alloy is formed at the interface. The Sn intercalation process is characterized by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). In this alloy Sn substitutes for surface Ni atoms without changing the lattice parameter of the substrate and consequently the registry between the metal substrate and graphene is maintained. DFT simulations indicate that Sn alloying with Ni weakens the interaction of graphene with the metal substrate and consequently increasing the graphene-substrate distance and restoring the graphene π-band close to the position of free-standing graphene. Atomic-resolution scanning tunneling microscopy (STM) imaging reveals that the alloy periodicity is reproduced in the graphene layer, i.e. a √3 × √3 R30° superstructure is imposed on the graphene by the alloy substrate. This indicates a variation of the local density of states for C-atoms located on top of Sn-substrate sites compared to Ni-sites. Further experimental and theoretical characterization of the influence of the substrate on the electronic and structural properties of graphene is ongoing.

[1] J. Wintterlin and M.-L. Bocquet, Surf. Sci. 603, 1841-1852 (2009)

[2] J. Lahiri et al., Nano Lett. 11, 518-522 (2010)

[3] A. Varykhalov et al., Phys. Rev. Lett., 101, 157601 (2008)

We have just synthesized in Marseille silicene sheets [1], i.e., atom-thin two-dimensional graphene-like silicon layers with an in-plane Si-Si interatomic distance of 0.23 nm [2], upon in-situ epitaxial growth on silver (111) surfaces. The honeycomb atomic structure is revealed in Scanning Tunnelling Microscopy, while the long-range epitaxial order is confirmed by sharp 4x4 Low Energy Electron Diffraction patterns. Dirac cones at the K and K’ points of the silicene Brillouin zone, evidenced in High-Resolution Synchrotron Radiation Angle-Resolved PhotoElectron Spectroscopy measurements, point to massless relativistic fermions with a Fermi velocity of 1.3E6 m/s, as theoretically predicted [3], quite the same as graphene, and four times higher than previously obtained on a one-dimensional grating of silicene nano-ribbons [4]. Density Functional Theory calculations including the Ag(111) substrate confirm the stability of the epitaxial arrangement. The demonstration that silicon can form sheets of silicene, a two dimensional honeycomb structure, which does not exist in Nature, is tantalizing for new Physics. Silicon being the workhorse of electronics industry, this synthesis could have a major impact for novel devices because of the compatibility with existing Si technologies.

[1] P. Vogt , P. De Padova, C. Quaresima, J. Avila, E. Frantzeskakis, M.C. Asensio and G. Le Lay, submitted

[2] G. G. Guzman-Verri and L.C. Lew Yan Voon, Phys. Rev. B 76, 75132 (2007).

[3] M. Houssa, G. Pourtois, M. Heyns, V.V. Afanas'ev, and A. Stesmans,

[4] P. De Padova et al., Appl. Phys. Lett., 96, 261905 (2010)