AVS2010 Session GR+TF-TuA: Graphene and 2D Nanostructures

The incredible material properties of graphene have spurred intense interest among chemists, physicists and engineers towards potentially exciting electronic applications. Much like nanotubes, graphene electrons have high mobilities due to the sharp curvature of their bands at the Gamma point that reduces their effective masses, as well as long scattering lengths due to symmetry selection rules among their pseudospin separated bands. However, a potential problem with graphene is its metallicity, which makes its ON-OFF ratio unacceptable for digital logic. Effort is under way to mitigate this by opening bandgaps through various chemical and electrostatic means. I will argue that any such band-gap opening leads to an inevitable reduction in mobility even if we manage to do so without affecting its scattering length. The trade-off arises from a fundamental asymptotic constraint on all graphitic materials (epitaxial graphene, strained graphene, nanoribbons, nanotubes, and bilayer graphene) that pins the high energy electrons away from the Gamma point to an ultimately linear dispersion. However, opening a bandgap by width confinement, e.g. in a nanoribbons, can provide distinct electrostatic if not material advantages. The presence of diffuse boundary conditions at the edges, along with strain and edge roughness, systematically erases any signs of chirality and metallicity in GNRs, making their widths the single arbiter of metallicity. This allows us to envisage wide-narrow-wide (WNW) nanoribbons monolithically patterned out of a single template into both switches and interconnects. The 2-D electrostatics of the source-drain contact edge capacitances improves the gate control, allowing the current to show a highly desirable saturation characteristic. Furthermore, the presence of C-C bonds at the channel-contact interface makes metal induced gap states relatively ineffective in pinning the bands, promoting Ohmic behavior. I will quantify the advantages and disadvantages of WNW devices, and compare with alternate GNR switches, such as utilizing electron focusing in p-n junctions.

Graphene has been grown by chemical vapor deposition of C₂H₄ on a monolayer of h-BN(0001) formed by atomic layer deposition (BCl₃, NH₃) on Ru(0001). AES, STM and LEED confirm a graphene-like overlayer, with near-zero DOS near the Fermi level, in registry with a BN R30(√3x√3) substrate. Raman spectra reveal graphene “G” and “2D” features with relative intensities indicative of single layer graphene. A large (350 cm^-1) redshift in the 2D feature relative to HOPG indicates significant BN-to-graphene charge transfer. The charge transfer is confirmed by photoemission/angle-resolved inverse photoemission spectroscopies (PES/ARIPES), that demonstrate filling of the lowest unoccupied graphene state ( π*) near the Brillouin zone center. These results are in direct contrast to PES/ARIPES results for graphene/ Cu, and reported results for graphene/SiC(0001), that show empty graphene π* states. The data show that the BN layer acts as an n-type dopant for graphene. For the graphene/BN heterojunctions, the ARIPES-determined dispersion of the unoccupied graphene σ*(Γ₁+) state yields an effective mass of 0.05 m_e , in excellent agreement with reported transport measurements on graphene sheets, and indicating that BN doping does not fundamentally alter the graphene electronic structure. The direct growth of graphene on dielectric substrates, and the controlled exploitation of graphene/substrate heterojunction properties, are critical issues for practical device fabrication. The implications of direct CVD of undoped graphene and graphene/BN heterojunctions on high dielectric constant substrates for device applications will be discussed in light of recent results in our laboratories for graphene thermal and free radical-assisted CVD on OH-terminated MgO(111).

Acknowledgements: Work at UNT was supported by the Global Research Consortium of the Semiconductor Research Corporation through Task ID 1770.001, and through ONR under award no. N00014-08-1-1107 through a subcontract with Texas State University at San Marcos. Work at UNL was supported by the Defense Threat Reduction Agency (Grant No. HDTRA1-07-1-0008), and the NSF "QSPINS" MRSEC (DRM-0820521) at UNL. The authors also thank Luigi Colombo and Adam Pirkle for acquisition of the Raman spectra.

Ultrathin carbon nanomembranes have recently attracted enormous interest. We report a molecular route to the fabrication of a monolayer or few layers of free-standing graphenoid and graphene nanomembranes based on molecular self-assembly, electron processing and pyrolysis. Aromatic biphenyl self-assembled monolayers (SAMs) are cross-linked by electron irradiation. The cross-linking results in mechanically stable graphenoid sheets with the thickness of a single molecule (~1 nm) and arbitrary sized. The graphenoid sheets can be lifted from their surface and transferred to another solid substrate or holey structures, where they become free-standing nanomembranes. Upon annealing (pyrolysis) up to 1300 K the molecular sheets transform into nanocrystalline graphene phase. This transformation is accompanied by a drop of the sheet resistivity from ~10⁸ to ~10 kΩ/sq and a 2D insulator to metal transition. We characterize the insulator to metal transition by electrical transport measurements as well as by complementary spectroscopic and microscopic techniques. A plethora of applications of the suggested molecular route to free-standing ultrathin carbon materials is feasible that take advantage from the fact that the large scale fabrication, control over the thickness and nanostructuring are easily controlled.

[1] A. Turchanin, A. Beyer, C. T. Nottbohm, X. H. Zhang, R. Stosch, A. Sologubenko, J. Mayer, P. Hinze, T. Weimann, and A. Gölzhäuser: One Nanometer Thin Carbon Nanosheets with Tunable Conductivity and Stiffness, Adv. Mater. 21, 1233-1237 (2009)

[2] A. Turchanin, D. Käfer, M. El-Desawy, C. Wöll, G. Witte, and A. Gölzhäuser: Molecular Mechanisms of Electron-Induced Cross-Linking in Aromatic SAMs, Langmuir 25, 7342-7352 (2009).

Graphene is a single monolayer thick carbon sheet with remarkably high electron mobility. Its unique structural and electronic properties make it an interesting material for nanoscale electronic and sensing devices. The addition of functionalities increases its reactivity toward certain materials and thus broadens its applications. One significant impediment to realizing the potential of graphene is the development of an industrially viable approach to producing precisely engineered functionalities over large areas. In this respect, plasmas are an ideal candidate but problems associated with the large fluxes of high-energy ions are a significant concern. Electron beam generated plasmas, characterized by low incident ion energies (< 5 eV), have been used to functionalize graphene without any damage [1]. We discuss the use of this system to controllably introducing oxygen, hydrogen, fluorine, nitrogen or ammine containing groups at different concentrations. The reversibility of the functionalization via low-temperature annealing will also be discussed. This work was supported by the Office of Naval Research. M.B. appreciates the support of the National Research Council.

Chemically functionalized semiconductor surfaces have been widely explored due to their potential for enabling molecular electronic and sensing devices that are compatible with conventional microelectronic technology [1]. Thus far, the vast majority of work in this field has focused on established semiconductors including silicon, germanium, and gallium arsenide. Meanwhile, the condensed matter physics community has diverted substantial experimental and theoretical effort to graphene, an emerging electronic material with superlative carrier mobility and exotic charge transport phenomena such as the quantum Hall effect.

In an attempt to unify these two fields, we have been exploring strategies for forming and interrogating organic self-assembled monolayers on graphene surfaces. In particular, we have recently demonstrated that self-assembled monolayers of perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA) can be formed on graphene surfaces via gas-phase deposition in ultra-high vacuum (UHV) environments at room temperature [2]. Molecular-scale resolution scanning tunneling microscopy (STM) images reveal long-range order in the PTCDA monolayers, while scanning tunneling spectroscopy (STS) measurements yield distinct electronic features associated with the PTCDA that are not observed on pristine graphene.

In addition to UHV STM characterization, this talk will summarize our most recent efforts to nanopattern self-assembled monolayers on graphene at the sub-10 nm scale. Nanopatterning chemically functionalized graphene presents opportunities for tailoring the electronic and chemical properties of graphene nanoribbons in addition to providing a molecular-scale resolution template for subsequent materials growth on graphene surfaces.

[1] M. A. Walsh and M. C. Hersam, “Atomic-scale templates patterned by ultrahigh vacuum scanning tunneling microscopy on silicon,” Annual Review of Physical Chemistry, 60, 193 (2009).

[2] Q. H. Wang and M. C. Hersam, “Room-temperature molecular-resolution characterization of self-assembled organic monolayers on epitaxial graphene,” Nature Chemistry, 1, 206 (2009).