Session Q20: Focus Session: Graphene: Local Probes

Imaging Dynamics of Carbon Atoms in a Graphene Sheet

Using a transmission electron aberration--corrected microscope capable of simultaneous atomic spatial resolution and 1-second temporal resolution, we record the dynamics of carbon atoms surrounding a hole in a suspended, graphene sheet. As the hole grows, bonds rearrange and atoms are ejected by the electron beam. We study the mechanism of edge reconstruction and demonstrate the stability of the ``zigzag'' edge configuration both theoretically and by image analysis of the data. ~We discuss other phenomena from the movie such as pentaheptite edge reconstruction and defect-induced strain.   

Spatially resolved tunneling spectroscopy of graphene

We have performed spatially resolved scanning tunneling spectroscopy measurements on single layer graphene. The graphene was prepared on SiO$_2$ by the mechanical exfoliation technique. Electrodes were deposited using a shadow mask technique to eliminate the use of PMMA. This fabrication procedure produced significantly cleaner devices. Atomic resolution topography images were obtained without any cleaning procedure unlike previous STM studies of graphene devices on SiO$_2$ which required high temperature cleaning. Low temperature tunneling spectroscopy measurements showed strong variations in the local density of states as a function of position and energy. These measurements indicated localized regions in the graphene with varying charge density. Possible origins of this charge localization will be discussed.   

Scanning tunneling spectroscopy of Landau levels near a zigzag edge of graphene

The quantum Hall plateaus observed in the transverse conductance of a two dimensional electron system in the presence of a magnetic field are believed to be a direct consequence of edge states. However, these states have not been characterized directly. For example, an outstanding question is how the Landau levels evolve from the bulk towards a sample edge. Using a low temperature high magnetic field scanning tunneling microscope, we studied the spatial dependence of electronic states in graphene in the presence of a magnetic field. We will report on the evolution of well resolved bulk Landau levels into edge states upon approaching a zigzag edge. The results, obtained by monitoring the scanning tunneling spectroscopy peaks corresponding to the first few Landau levels, will be compared to theoretical predictions.   

Graphene: Two-dimensional carbon at atomic resolution

Graphene is a crystalline single layer of carbon atoms that can be viewed as an individual atomic plane extracted from graphite. Freely suspended mono-layer graphene is the thinnest possible membrane that is conceivable with currently known materials. Yet, it is remarkably stable under electron irradiation and thus opens unprecedented opportunities for electron microscopic studies. First, the graphene membrane structure and its defects are of outstanding interest for science and applications of this promising new material. Static deformations, topological defects, various vacancy configurations, substitutions, adatoms, and the two-dimensional equivalent of dislocations are detected by transmission electron microscopy (TEM). Further, graphene membranes can serve as a perfect sample support for transmission electron microscopy. Its contribution to the TEM image signal can be filtered out completely and adsorbed atoms and molecules on the graphene sheet can be imaged as if they were suspended in free space. Finally, graphene membranes constitute an ideal test object for microscopic developments due to the precisely defined structure and high stability in an electron beam at energies below the knock-on threshold. Numerous results from aberration-corrected transmission electron microscopy, showing precise atomic configurations in graphene, are presented, and new insights into various aspects of defects in graphene are discussed.   

Scanning tunneling spectroscopy of charged impurities in graphene

The electronic properties of graphene were investigated by scanning tunneling microscopy. Mono- and bilayer flakes were prepared by exfoliation on 300 nm SiO$_2$ substrates. The samples were measured in ultra high vacuum by scanning tunneling spectroscopy at 5 K. In these experiments, we have investigated the effect of two types of charged impurities, either random impurities from the oxide substrate or controlled doping of the graphene with potassium ions. For the potassium ions, the density is controlled by varying the dosage time of potassium ions. Initial results indicate that charged impurities act to create local puddles in the graphene film which act as electron and hole doped regions.   

The Effect of a Lattice Defect on Graphene Landau Levels: A Scanning Tunneling Spectroscopy Study

We present tunneling differential conductance (dI/dV) spectra and 2D conductance maps acquired over both N- and P-type defects in magnetic fields up to 8 T. The measurements were performed on multilayer epitaxial graphene using scanning tunneling microscopy and spectroscopy at 4 K under ultra high vacuum conditions. Landau levels are found to follow the local potential (determined independently at near-zero magnetic field) until an instability is reached close to the defect. Spectral shifts at high magnetic field are modeled using the low-field-derived potential maps. The source of the tunnel instability will be discussed.   

Scanning tunneling microscope study of striated carbon ridges in few-layer epitaxial graphene formed on 4H-SiC(000-1)

Scanning tunneling microscopy (STM) is used to study carbon ridge defects found in few-layer graphene formed on the C-face of 4H-SiC(000-1) at growth temperatures between 1475\r{ }C and 1550\r{ }C. STM images reveal that ridges are characterized by a striated exterior surface formed from out-of-plane distortions in the hexagonal graphene lattice. While ridge formation is likely explained by compressive in-plane stresses and small values of the bending modulus for few-layer graphene, the striated structure along the ridges argues for a localized unidirectional stress in the material directed along the ridge length.   

Probing impurity states on graphene/SiC with Scanning Tunneling Microscope

Graphene has attracted a lot of attention for its unique electronic structure, high electron mobility and hence the unusual transport and spectroscopic properties. Impurities, like adatoms, vaccancies, and on-lattice doping, on graphene sheets also generate interesting phenomena and play very important roles in graphene's unusual properties. For example, conducting electrons scattered off by the impurity atoms can have resonance states. These resonance states around the impurity can be probed by Scanning Tunneling Spectroscopy(STS), through imaging the electronic states in real space at different energy levels.   

Connecting the Microscopic and Macroscopic Transport Properties of Graphene

Graphene with its extraordinary physical properties has attracted a lot of attention in the scientific community as a model system for two-dimensional (2D) condensed-matter physics and as a prospective material for nanoscale electronic device engineering. In contrast to conventional 2D electron systems, in which the 2D gas is buried inside of the device, the unique structure of graphene enables direct exploration of its both macroscopic and microscopic properties using electron transport and scanning probe microscopy (SPM) techniques. This allows one to connect the graphene macroscopic properties with their microscopic origins. We are fabricating graphene devices for both electrical transport and SPM measurements to investigate the physics in graphene ranging from the quantum Hall effect to superconducting proximity effects, in particular the role of disorder and metal-graphene interface effects. In this presentation we will report on the results of our graphene device fabrication and initial electrical and SPM measurements.   

Magnetic Quantization of Exfoliated Graphene Probed with Scanning Tunneling Spectroscopy

Recent scanning tunneling spectroscopy studies have shown the excellent magnetic quantization of Dirac fermions in epitaxial graphene on SiC [1]. Landau level lifetimes of $\approx $ 0.4 ps were observed, indicating the high mobility of these graphene samples. In this talk, we compare the magnetic quantization properties of exfoliated graphene to what was previously measured for epitaxial graphene [1]. The exfoliated graphene sample was made by removing graphene layers from natural graphite, then placing them onto a SiO$_{2}$/Si substrate. A gold electrode, used for the tunneling bias, was deposited using a shadow mask technique. Magnetic quantization of the graphene is then probed with both the application of an external magnetic field and with an external gate voltage applied to the Si substrate below the graphene. Through the electric field effect, the external gate voltage will change the carrier density in the graphene, giving us new information about the screening and carrier dependence of the Landau level lifetimes. \\[4pt] [1] D. L. Miller et al., \textit{Science} \textbf{324}, 924 (2009).   

Atomic structure of progressively reduced graphene oxide

Using molecular dynamics simulations, we have studied the evolution of epoxy and hydroxyl functional groups on graphene oxide (GO) during high temperature thermal reduction. We find that the reduced GO sheets are characterized by a large number of stable hole-like defects formed by breaking of C-C bonds in the basal plane. These defects are always decorated by the carbonyl (C=O) groups and are formed due to the strain in the basal plane created by epoxy and hydroxyl functional groups that are located close to each other. With very few exceptions, the carbonyl groups that are observed in Raman spectroscopy and other experimental studies are generally attributed to the C=O terminations of the edges. However, our study using first principles calculations and a reactive force field approach clearly shows that the formation of carbonyl groups within the graphene basal plane is energetically favorable compared to other well-known functional groups such as epoxies and ethers. We have identified the specific reaction mechanisms that lead to the formation of these holes starting from particular initial configurations of epoxy and hydroxyl functional groups. These configurations can be readily found on GO sheets with random distribution of epoxy and hydroxyl groups and do not require an ordered arrangement of any particular functional groups.   

Electronic inhomogeneities of epitaxial graphene on Ru(0001) probed by dynamic STM and STS measurements

Epitaxial growth of graphene on Ru(0001) surfaces is a powerful route to obtain wafer-scale graphene layers. Nevertheless the graphene-Ru(0001) interaction is expected to play an important role in electronic and chemical properties of the grown graphene layer. We have performed dynamic scanning tunneling microscopy (dyn-STM) and scanning tunneling spectroscopy (STS) at temperatures down to 300 mK on graphene epitaxialy grown on Ru(0001). We have found that both the local tunneling barrier height (LBH) obtained from the dyn-STM measurements and the local density of electronic states (LDOS) deduced from the STS measurements show a Moir\'{e}-like distribution. This inhomogeneity on the electronic properties of graphene on Ru(0001) is induced by local variations of the carbon -- ruthenium interaction due to the lattice mismatch between the graphene and the Ru(0001) lattices.   

Thermal Imaging of Graphene Devices

We have performed scanning thermal microscopy on voltage-biased single-layer graphene devices by employing a microfabricated tip with an integrated thermocouple sensor at the apex. The lattice-temperature profile of the graphene channel as well as the temperatures of the substrates and the electrodes are measured with high spatial resolution as a function of both bias and gate voltages. The results allow us to understand various heat dissipation mechanisms in these technologically-important devices.