Session W11: Focus Session: Graphene Structure, Stacking, Interactions: Local Probes and Microscopy

Direct Imaging of a Two-Dimensional Silica Glass on Graphene

Large-area graphene substrates [1] are a promising lab bench for synthesizing and characterizing novel low-dimensional materials such as two-dimensional (2D) glasses. Unlike 2D crystals such as graphene, 2D glasses are almost entirely unexplored--yet they have enormous applicability for understanding amorphous structures, which are difficult to probe in 3D. We report direct observations of the structure of an amorphous 2D silica supported on graphene. To our knowledge, these results represent the first discovery of an extended 2D glass. The 2D glass enables aberration-corrected scanning transmission electron microscopy and spectroscopy, producing the first atomically-resolved experimental images of a glass. The images strikingly resemble Zachariasen's seminal 1932 cartoons of a 2D continuous random network glass [2] and allow direct structural analyses not possible in 3D glassy materials. DFT calculations indicate that van der Waals interactions with graphene energetically favor the 2D structure over bulk SiO$_{2}$, suggesting that graphene can be instrumental in stabilizing new 2D materials. [1] J. C. Meyer et al., Nature \textbf{454}, 319--322 (2008). [2] W. H. Zachariasen, J. Am. Chem. Soc. \textbf{54}, 3841--3851 (1932).   

Imaging defects on epitaxial graphene/SiC(0001) using non-contact AFM with a Q-plus sensor

STM has been commonly used to study the atomic structures of resonant scatters such as vacancies and adsorbates on graphene, which are leading factors limiting its mobility. However, since STM probes primarily the local density of states, complex patterns are often observed when imaging defects on graphene, making it challenging to determine their atomic structures using STM alone. In this work, we carried out an integrated study of defects on epitaxial graphene/SiC(0001) using non-contact AFM with Q-plus sensors in addition to STM. With atomic resolution AFM imaging, straight forward identifications of single- and di-vacancy defects can be made. These results and their implications for understanding electron scattering in epitaxial graphene on SiC(0001) will be presented at the meeting.   

Direct Imaging of Charged Impurities in Substrates used for Graphene Devices

The use of hexagonal boron nitride (h-BN) as a substrate for graphene led to approximately an order of magnitude improvement in electron mobility compared to graphene on SiO$_{2}$. One hypothesis for the improvement is a reduction in trapped charge density on the surface of h-BN compared to SiO$_{2}$. We address this directly by mapping local potential fluctuations above the bare substrates h-BN and SiO$_{2}$ using Kelvin probe microscopy in ultra-high vacuum. We compare the results to a model of randomly distributed charges in a 2D plane at the surface of an insulating substrate. For SiO$_{2}$, the results are well modeled by a 2D charge density of $\sim$ 2.5x10$^{11}$ cm$^{-2}$. Previous measurements of charged impurity scattering in graphene indicates that this density of substrate charges would limit graphene mobility to 20,000 cm$^{2}$/Vs, in good agreement with the maximum values reported for graphene on SiO$_{2}$. h-BN displays potential fluctuations that are approximately an order of magnitude lower than SiO$_{2}$, consistent with an order of magnitude improvement in mobility in graphene/h-BN devices. This work was supported by the US ONR MURI program, and the U. of MD NSF-MRSEC under Grant No. DMR 05-20471.   

Self-Organized Graphene Nanoribbons on SiC(0001) Studied with Scanning Tunneling Microscopy

Graphene nanoribbons grown directly on nanofacets of SiC$(0001)$ offer an attractive union of top-down and bottom-up fabrication techniques. Nanoribbons have been shown to form on the $<1\bar{1}0n>$ facets of templated silicon carbide substrates,\footnote{Sprinkle et al., \emph{Nat. Nanotech.} \textbf{5}, 727 (2010).} but also appear spontaneously along step-bunches on vicinal SiC(0001) miscut slightly towards $<1\bar{1}00>$. These self-organized graphene nanoribbons were characterized with low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) in ultra-high vacuum. Our measurements indicate that the graphene forms a continuous ``buffer layer'' across the SiC(0001) terraces during nanoribbon formation, with the zigzag edge of the buffer layer aligned parallel to the step-bunched nanofacets. Scanning tunneling microscopy/spectroscopy (STM/STS) was used to characterize the topography and electrical characteristics of the graphene nanoribbons. These measurements indicate that the graphene nanoribbons are highly-crystalline with predominantly zigzag edges.   

Imaging epitaxial graphene on SiC(0001) using STM with functionalized W tips

Epitaxial graphene on SiC(0001) is studied using scanning tunneling microscopy with W tips functionalized by transition-metal (Cr, Fe) coatings, enabling the imaging of states within a few meV of the Fermi level that are not accessible with conventional W tips. First-principles modeling of these tips as pyramidal structures on W(110) indicates that an apex atom is stable for the Cr/W(110) tip but not for the Fe/W(110) or W/W(110) tips. Further calculations of the tunneling current show that the Cr- and Fe-coated tips can get significantly closer to the substrate than a bare W tip at a given current, and that the Cr (Fe) tip states contributing to the tunneling at low bias are spatially more localized than the W tip states. These characteristics lead to increased resolution, making possible the selective imaging of the complex electronic properties of the epitaxial graphene on SiC(0001)1,2.   

Scanned probe studies of dielectric screening and charge puddles in epitaxial graphene on SiC(0001)

Epitaxial growth of graphene on SiC(0001) produces wafer-scale monolayer films suitable for large scale device applications. However, the presence of the buffer layer beneath the graphene produces n-doping of 10$^{12}$-10$^{13}$ cm$^{-2}$ and limits mobility to $\sim $10$^{3}$ cm$^{2}$/Vs. Recently H intercalation has produced p-doped samples with similar carrier density and improved mobility. Transport measurements on processed devices show evidence of charge impurity scattering, but these measurements cannot show whether the transport behavior is due to top gate dielectrics or intrinsic to the as-grown graphene. Here we use scanning microwave microscopy to look at graphene samples and bare SiC substrates to extract information about the screening role of the buffer layer. This data is complemented by earlier results suggesting charge puddles due to random impurities to not exist in epitaxial samples.   

Scanning Tunneling Microscopy Study of Mechanically-Stacked Double Layer Graphene

Bilayer graphene has drawn considerable attention due to deviation from Dirac Fermion picture such as anomalous quantum hall effect and a tunable band gap in their spectrum. While a pristine Bernal (AB) stacked bilayer graphene can be synthesized by mechanical exfoliation, growth on a SiC single crystal and epitaxial growth on metal substrates, separate control of the top and the bottom layers has seldom been performed. In this study, artificially modified 2D layers were demonstrated with individually stacked double layer graphene. Large-area graphene was grown on a Cu foil by chemical vapor deposition (CVD). CVD-grown graphene layers were transferred successively onto several insulating substrates with minimum chemical process for realizing bilayer graphene. Mechanically-stacked double layer graphene was mainly investigated using scanning tunneling microscopy and spectroscopy. The artificial bilayer graphene showed Moire patterns as determined by misalignment angle. In spatially resolved spectrums of local density of states, dependence on separation distance between two graphene layers and their corrugation was measured. In addition, we confirmed less charging effects of graphene on BN thin film than on SiO2 or SiN.   

Scanning Tunneling Spectroscopy of Potassium on Graphene

We investigate the effect of charged impurities on the electronic properties of large single crystal CVD grown graphene using scanning tunneling microscopy. Mono- and multilayer crystals were prepared by transferring graphene from copper onto exfoliated boron nitride flakes on 300 nm SiO$_2$ substrates. The boron nitride provides an ultra flat surface for the graphene. Potassium atoms are controllably deposited on the graphene at low temperature by heating a nearby getter source. Scanning tunneling spectroscopy and transport measurements were performed in ultra high vacuum at 4.5 K. Transport measurements demonstrate the shifting of the Dirac point as the samples are doped, while STM measurements demonstrate the size, arrangement and local electronic influence of the potassium atoms.   

Nanoscale electronic and optical investigations of functionalized graphene

A rigorous understanding of light-matter interactions at the nanometer scale is pivotal in the development of nanoscale device applications. Graphene and its functionalized derivatives, due to their unique properties, promise unexpected capabilities as a platform for such devices, which has led to significant interest in graphene-based nano-optical, optoelectronic, and photovoltaic applications. Here, we will describe our efforts to resolve and understand the structural, electronic and optical properties of these systems. We will present a UHV STM study of the structural and electronic properties of C60 molecules deposited on graphene that was grown epitaxially on SiC(0001), which serves as a model system for the study of molecule-surface interactions. Our results indicate reduced coupling of the molecules to the graphene and underlying substrate, compared to those on metallic substrates, suggesting a path for developing molecular-scale electronic and optically active ``devices'' that are not dominated by the substrate. We will also discuss our efforts to correlate these STM studies with the optical properties of the system using a UHV STM that incorporates confocal optical microscopy and spectroscopy at the tip-sample junction with integrated high-numerical aperture optics.   

Three Different Translations Can Each Convert the Top Plane of Graphite to Graphene

The discovery of graphene, a unique two-dimensional electron system with extraordinary physical properties, has ignited tremendous research activity in both science and technology. Graphene can be obtained from graphite by moving its top layer until it becomes locally decoupled from the bulk. However, a detailed microscopic understanding of this process has yet to be completed. Here we present STM images of the top plane of graphite, which has been transformed into graphene. In addition, we also present STM images which reveal several intermediate stages in between pure graphene and pure graphite. Density functional theory was used to simulate STM images from a six-layer slab of graphite. We also moved the top layer of graphite incrementally in three different directions. Vertical movement of the top layer by about 0.1 nm created graphene. A continuous transition from pure graphite to pure graphene was observed with the simulations. Horizontal movement of the top layer can also create graphene in two ways. In one configuration, the carbon atoms perfectly align with the layer below, while in the second the carbon atoms have no vertical alignment with the layer below. Other significant details found between graphite and graphene will be discussed.   

Graphene on Ru(0001): The four hills in the 25 on 23 structure

A single layer of sp2 hybridized carbon on Ru(0001) accommodates in a 23 on 25 superstructure with 625 carbon honeycombs as found by surface x-ray diffraction (SXRD) [1]. In a significant computational effort 25x25 unit cells of graphene were relaxed on 23x23 Ru(0001) unit cells with up to 6 substrate layers. The density functional theory calculations that take van der Waals interactions into account predict 4 protrusions, quantum dots [2] or ``hills'' in the unit cell with two kinds of hills: 3 $\Omega$-type hills with a honeycomb around the summits, and one T-type hill with a single carbon atom on the summit. This prediction is confirmed by state of the art low temperature scanning tunneling microscopy and is in line with the SXRD data in Ref.[1].\\[4pt] [1] Martoccia et al. Phys. Rev. Lett. 101 (2008) 126102.\\[0pt] [2] Zhang et al. J. Phys.: Condens. Matter 22 (2010) 302001.   

Thickness Determination of Multilayer Graphene Using Transmission Electron Microscopy

With dark field transmission electron microscopy and select area electron diffraction (SAED) crystallographic grain boundaries in graphene can be easily imaged. We present a complete, quantitative theoretical model of the SAED pattern that allows determination of the number of layers. Grain boundary maps of single and multilayer graphene grown by chemical vapor deposition will be shown.   

High resolution transmission electron microscopy of lattice dynamics of graphene

Lattice dynamics of carbon atoms in graphene was investigated by aberration corrected ultra-high resolution transmission electron microscopy near the holes and the grain boundaries. We studied in-situ formation of various unusual defect structures in graphene under various conditions. In this presentation we will show the stability and dynamics of the atoms at the holes, grain boundaries and the defects and discuss their formation mechanism. This work will also elaborate on the electronic properties of the defects in light of recent experimental and theoretical progress.   

High resolution transmission electron microscope Imaging and first-principles simulations of atomic-scale features in graphene membrane

Ultra-thin membranes such as graphene[1] are of great importance for basic science and technology applications. Graphene sets the ultimate limit of thinness, demonstrating that a free-standing single atomic layer not only exists but can be extremely stable and strong [2--4]. However, both theory [5, 6] and experiments [3, 7] suggest that the existence of graphene relies on intrinsic ripples that suppress the long-wavelength thermal fluctuations which otherwise spontaneously destroy long range order in a two dimensional system. Here we show direct imaging of the atomic features in graphene including the ripples resolved using monochromatic aberration-corrected transmission electron microscopy (TEM). We compare the images observed in TEM with simulated images based on an accurate first-principles total potential. We show that these atomic scale features can be mapped through accurate first-principles simulations into high resolution TEM contrast. [1] Geim, A. K. {\&} Novoselov, K. S. \textit{Nat. Mater. }\textbf{6}, 183-191, (2007). [2] Novoselov, K. S.\textit{et al. Science }\textbf{306}, 666-669, (2004). [3] Meyer, J. C. \textit{et al. Nature }\textbf{446}, 60-63, (2007). [4] Lee, C., Wei, X. D., Kysar, J. W. {\&} Hone, J. \textit{Science }\textbf{321}, 385-388, (2008). [5] Nelson, D. R. {\&} Peliti, L. \textit{J Phys-Paris }\textbf{48}, 1085-1092, (1987). [6] Fasolino, A., Los, J. H. {\&} Katsnelson, M. I. \textit{Nat. Mater. }\textbf{6}, 858-861, (2007). [7] Meyer, J. C. \textit{et al. Solid State Commun. }\textbf{143}, 101-109, (2007).   

Interference effects in electronic transport in mesoscopic graphene

Graphene consists of a monolayer of carbon atoms arranged in a honeycomb lattice and has been intensively studied due to its fascinating physical properties. We study transport in mesoscopic graphene systems, in particular, conductance oscillations due to interference of the Dirac electrons in phase-coherent transport. Green's functions are calculated in the tight-binding model via $T$-matrix formalism, and the conductance is then obtained using the Landauer-B\"{u}ttiker formalism. We consider a measurement set-up consisting of two STM tips, as well as transport from a contact and a STM tip. Regular Bloch oscillations are obtained, as well as richer structures when different types of isolated impurities are considered.