NANO LETTERS
Growth of Semiconducting Graphene on Palladium
2009 Vol. 9, No. 12 3985-3990
Soon-Yong Kwon,†,‡ Cristian V. Ciobanu,*,§ Vania Petrova,| Vivek B. Shenoy,⊥ Javier Baren˜o,|,# Vincent Gambin,¶ Ivan Petrov,| and Suneel Kodambaka*,† Department of Materials Science and Engineering, UniVersity of California Los Angeles, Los Angeles, California 90095, DiVision of Engineering, Colorado School of Mines, Golden, Colorado 80401, Frederick Seitz Materials Research Laboratory, UniVersity of Illinois, Urbana, Illinois 61801, DiVision of Engineering, Brown UniVersity, ProVidence, Rhode Island 02912, and Northrop Grumman Space and Technology, Redondo Beach, California 90278 Received July 5, 2009; Revised Manuscript Received August 31, 2009
ABSTRACT We report in situ scanning tunneling microscopy studies of graphene growth on Pd(111) during ethylene deposition at temperatures between 723 and 1023 K. We observe the formation of monolayer graphene islands, 200-2000 Å in size, bounded by Pd surface steps. Surprisingly, the topographic image contrast from graphene islands reverses with tunneling bias, suggesting a semiconducting behavior. Scanning tunneling spectroscopy measurements confirm that the graphene islands are semiconducting, with a band gap of 0.3 ( 0.1 eV. On the basis of density functional theory calculations, we suggest that the opening of a band gap is due to the strong interaction between graphene and the Pd substrate. Our findings point to the possibility of preparing semiconducting graphene layers for future carbon-based nanoelectronic devices via direct deposition onto strongly interacting substrates.
Graphene1,2sa two-dimensional crystalline sheet of carbon atoms arranged in a honeycomb latticesgenerated enormous interest in the research community owing to its ultrathin geometry and properties such as high carrier mobility,3 excellent thermal conductivity,4 and high mechanical strength.5 One of the attractive features of free-standing graphene, a semimetal, is its semiconducting behavior6,7 at length scales below 500 Å with a size-dependent band gap.8-10 Previous reports11-13 have shown that a band gap can also be opened in graphene grown on insulating SiC(0001) and BN(0001) via interactions with the substrate. Here, we report the formation of semiconducting graphene layers with a band gap of 0.3 ( 0.1 eV on Pd(111), a metallic substrate. Using in situ scanning tunneling microscopy (STM) and spectroscopy (STS), we determine the electronic structure of graphene islands grown in situ via chemical * Towhomcorrespondencemaybeaddressed.E-mail:
[email protected],
[email protected]. † Department of Materials Science and Engineering, University of California Los Angeles. ‡ Present address: School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Korea. § Division of Engineering, Colorado School of Mines. | Frederick Seitz Materials Research Laboratory, University of Illinois. ⊥ Division of Engineering, Brown University. # Present address: Department of Physics, Chemistry and Biology (IFM) Linko¨ping University, SE-581 83 Linko¨ping, Sweden. ¶ Northrop Grumman Space and Technology. 10.1021/nl902140j CCC: $40.75 Published on Web 09/28/2009
2009 American Chemical Society
vapor deposition on Pd(111). In contrast to recent reports on nanoribbons, where the band gap originates from size/ edge effects,8-10 the band gap in epitaxial graphene on palladium is caused by a strong interaction with the Pd substrate and the ensuing breaking of translational symmetry between the two hexagonal close-packed sublattices of graphene. Our experiments illustrate the control over the electronic properties of graphene through interactions with substrates in well-defined epitaxial configurations. This approach opens up the possibility of preparing metal-semiconducting graphene structures and metal-doped graphenebased devices with potentially new applications. Using STM, we followed the formation and growth of graphene on Pd(111) during ethylene deposition over a range of pressures, substrate temperatures, and times. Panels A and B of Figure 1 are representative STM images of graphene islands acquired from a Pd(111) surface in situ during ethylene deposition at 968 K. In our experiments, island sizes vary between 200 and 2000 Å and are commonly observed at or near the Pd step edges as in Figure 1A, or spanning across multiple terraces as in Figure 1B. During ethylene deposition, we observe graphene islands on the surface, which likely form via precipitation of the carbon atoms dissolved in the substrate. This is plausible because C dissolves readily in Pd with a temperature-dependent solubility14 that increases from 0.2 atom % at 723 K to 1.4 atom %
Figure 1. STM images of graphene on Pd(111) acquired in situ during growth. (A and B) Derivative filled-state STM images (VT ) -1.3 V) acquired from a Pd(111) sample at 968 K during exposure to ethylene. Images show graphene islands near or spanning across substrate steps. IT ) 0.13 nA in (A) and 0.22 nA in (B). Fourier transforms (insets) of the images in (A) and (B) show 6-fold symmetry. (C) High-resolution filled-state STM image (VT ) -1.3 V, IT ) 0.13 nA) of a graphene island at 968 K. (D) Surface height profile along the white line shown in panel C. Graphene forms a hexagonal Moire´ pattern with a spatial periodicity of 21 ( 1 Å. (E) Atomic model showing the [21j1j0] orientation of graphene (honeycomb lattice) aligned with the Pd [11j0] atoms (large spheres). This specific arrangement of the two lattices is consistent with the most frequently observed Moire´ periodicity of 21 Å.
at 1023 K. The decrease of the substrate temperature following the high-temperature deposition and/or extended deposition times lead to an increase of the C supersaturation in the bulk, which eventually results in precipitation of crystalline carbon at the surface.15 A similar process has been previously reported for graphene growth on other metals.16-19 In all our growth experiments, graphene islands appear instantaneously, presumably due to faster nucleation and growth rates compared to the STM scan rates. After graphene islands are formed, their shapes and sizes do not change significantly with time and appear to be independent of C2H4 pressure at a given temperature. More interestingly, we observe considerable rearrangement of the substrate surface (refer to Figure S1 in Supporting Information for more details). Similar morphological changes of the substrate have been recently reported for graphene growth on ruthenium and attributed to the displacement of substrate atoms by carbon.20 We now focus on the structure of graphene islands. Fourier transforms (FT) of the STM images (shown as insets in panels A and B of Figure 1) indicate that the islands exhibit an ordered superstructure with a 6-fold symmetry. Figure 1C is a higher resolution image of the ordered superstructure within a graphene island. These superstructures are Moire´ patterns formed by the superposition of the honeycomb lattice of graphene and the hexagonal lattice of Pd(111) and have also been observed on other metal surfaces.17-20 From the FTs, we measure spatial periodicities of ∼19 and ∼20 Å for the islands in panels A and B of Figure 1, respectively. 3986
Figure 2. Surface morphologies of graphene islands on Pd substrate. (A and B) Higher magnification STM images showing portions of the graphene island in Figure 1B. (C and D) Surface height profiles obtained along the lines highlighted in panels A and B, respectively. The observed step height difference between the substrate and the graphene island is
