Session C16: Scanning Tunneling Microscopy/Spectroscopy of Graphene

Imaging nonlocal transport in graphene using scanning gate microscopy

Nonlocal transport measurements are designed to detect when charge injected by a current probe induces voltages far from the classical current path. While a range of exotic forces can induce nonlocal transport of Dirac fermions in graphene such as bandstructure topology [1], Zeeman spin Hall [2], and many-body interactions [3], it is important to understand the role of density fluctuations around the Dirac point where nonlocality can be most pronounced. We use scanning gate microscopy to image current flow and nonlocal signals directly in high-mobility graphene encapsulated by hexagonal boron nitride. Despite being located several mean-free paths from the current injector, Hall voltage probes parallel with current path display an order of magnitude larger nonlocal signal than expected around the Dirac point. SGM images captured at different carrier density are consistent with current spreading due to percolation. Such long range charge transport should be considered when designing devices and calculating the relaxation length of nonlocal currents. [1] R. V. Gorbachev, \textit{et al.}, Science, 346, 6208, 448-451 (2014) [2] D. A. Abanin, \textit{et al.}, Science, 332, 6027, 328-330 (2011) [3] D. A. Bandurin, arXiv:1509.04165 (2015)   

New Approaches to Edge-Doping Graphene Nanoribbons

Graphene nanoribbons (GNRs) are narrow semiconducting strips of graphene that exhibit novel electronic and magnetic properties. New bottom-up fabrication techniques enable atomic-scale precision in GNR synthesis. The use of these techniques to reliably tune the position and size of GNR band gaps is an important challenge that also has relevance for the question of whether GNRs are viable for future nanotechnologies. We have used scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) to investigate how the geometry of heteroatom incorporation alters the electronic structure of bottom-up fabricated chevron-type GNRs. We find that the addition of nitrogen into the GNR edge via a five-membered ring yields a reduced band gap compared to the behavior of pristine, undoped chevron GNRs.   

Scanning tunneling microscopy of atomically precise graphene nanoribbons exfoliated onto H:Si(100)

Atomically precise graphene nanoribbons (GNRs) are promising materials for next generation transistors due to their well-controlled bandgaps and the high thermal conductivity of graphene. The solution synthesis of graphene nanoribbons offers a pathway towards scalable manufacturing. While scanning tunneling microscopy (STM) can access size scales required for characterization, solvent residue increases experimental difficulty and precludes band-gap determination via scanning tunneling spectroscopy (STS). Our work addresses this challenge through a dry contact transfer method that cleanly transfers solution-synthesized GNRs onto H:Si(100) under UHV using a fiberglass applicator. The semiconducting silicon surface avoids problems with image charge screening enabling intrinsic bandgap measurements. We characterize the nanoribbons using STM and STS. For chevron GNRs, we find a 1.6 eV bandgap, in agreement with computational modeling, and map the electronic structure spatially with detailed spectra lines and current imaging tunneling spectroscopy. Mapping the electronic structure of graphene nanoribbons is an important step towards taking advantage of the ability to form atomically precise nanoribbons and finely tune their properties.   

LT-STM/STS studies of clean armchair edge

It was predicted and observed that the passivated zigzag edges of graphene host highly localized edge state [1]. This edge state is predicted to be spin-polarized, which is appealing for spintronic applications. In contrast, no edge state was expected at passivated armchair graphene edge. Here we report low temperature scanning tunneling microscopy and spectroscopy (STM/STS) studies of electronic properties of clean monoatomic step edges on cleaved surface of HOPG. Most of step edges are armchair edges, in agreement with previous STM results. We observed only $(\sqrt{3}\times\sqrt{3})R30^{\circ}$ superstructure near armchair edges, which has been reported in previous STM studies [2, 3, 4]. On the other hand, no honeycomb superstructure was observed in our STM data. In addition, our STM results reveal an intriguing localized electronic state at clean armchair edges. Spectroscopic and spatial evolution of this edge state will be presented. [1] Fujita et al, JSPJ, 65, 1920, (1996). [2] Niimi et al, PRB, 73, 085421 (2006). [3] Giunta and Kelty, J. Chem. Phys., 114, 1807 (2001). [4] Sakai, et al, PRB, 81, 235417(2010).   

Exotic Charge Polarization near Dirac Cone Merging Transition in Graphene-based Systems

Extreme strain in graphene yields a fascinating charge distribution around a Coulomb impurity. Graphene's band structure is characterized by gapless Dirac cones but can be made gapped by application of intense strain. The cones become increasingly elliptic continually merging, until the spectrum has an exotic, highly anisotropic, semi-Dirac, nature. This situation can also occur in various artificially engineered lattices. The unusual spectrum leads to an unconventional charge distribution around a Coulomb impurity. Crucially, unlike isotropic graphene, the polarization charge density exhibits long-range oscillatory tails far from the impurity. Such exotic behavior is due to the anisotropy of the polarization, and occurs even at zero chemical potential (i.e. unrelated to Friedel-type physics). The angular and radial functions are intrinsically coupled, hence a litany of distinct angular distributions is observed through multiple distance regimes. The density can approach infinity at angles where it was once close to zero, be negative all around the impurity, or have its polarity fluctuate along different directions. Thus our results could have important implications for STM experiments probing polarization charge around impurities in highly anisotropic Dirac systems.   

Experimental Evidence of Giant Non-reciprocity of WGM Modes in Graphene

Klein scattering in a circular graphene pn junction can lead to Whispering Gallery Mode(WGM) type resonances[1]. Utilizing the electrostatic potential induced by the probe of a scanning tunneling microscope, we create graphene electron resonators defined by circular pn junctions. These quasi-confined WGM states can be probed by tunneling spectroscopy measurements. With small applied magnetic fields, we observe a large energy splitting of the WGM states, displaying a manifestation of non-reciprocity due to Klein scattering of massless Dirac fermions in graphene~[2]. \begin{enumerate} \item Y. Zhao, et. al., \textit{Science }348(6235), 672-675 \item J. F. Rodriguez-Nieva, L. S. Levitov,\textit{ arXiv: 150806609} \end{enumerate}   

The synthesis of planar sp2-bonded system from molecular building blocks

Biphenyl and pyrene molecules were deposited onto atomically flat Cu (100) surface as building blocks for the synthesis of planar, conjugated, sp2-bonded system. In situ STM observation confirmed the formation of highly-ordered lattice structure after annealing under UHV condition, as a result of the substrate-assisted dehydrogenation. The electronic properties of the system were examined by STS and will be presented.   

Tunneling spectroscopy in metal-hexagonal boron nitride-graphene structures

Tunneling spectroscopy provides a tool to probe the density of states of various electronic materials. Here, we report vertical tunneling transport in van der Waals heterostructures with hexagonal boron nitride (hBN) as a tunnel barrier between graphene (or graphite) and a metal (Cr/Au) electrode. We observe a strong suppression of tunneling at low biases, with a gap of about 130 meV in the dI/dV spectrum for metal-hBN-graphene (or graphite) structures. In the graphene devices, the finite zero-bias tunnel resistance was found to depend on the electron density of the graphene layer, while the gap remained unaffected. We also tested graphite-hBN-graphite junctions, in which the strong suppression of tunneling at low energies was found to be absent. We interpret the signatures in the context of phonon-mediated processes in such vertical heterostructures.   

Modification of Electronic Surface States by Graphene Islands on Cu(111)

The interaction of graphene with copper is of interest for graphene applications due to the frequent use of copper in the chemical vapor deposition (CVD) growth of graphene. We grew pristine graphene islands on Cu(111) by dissociating ethylene gas in an ultra high vacuum environment. In situ low-temperature scanning tunneling microscopy (STM) was used to measure the physical and electronic structure of the surface with atomic resolution, enabling us to compare the graphene-covered regions to bare Cu(111). We observed a shift of the Rydberg-like series of images potential states (IPS) to lower energies and a decrease in linewidth in graphene-covered regions, indicating a decrease in local work function and reduced coupling to the copper bulk states. In some cases, the first of these states were split, which may correspond to the dual Rydberg series which has been predicted for graphene. By measuring the dispersion of the Shockley surface state, we found that the band edge and effective mass are influenced by the graphene layer. We will extend these findings [SM Hollen, et al. Phys. Rev B 91, 195425 (2015)] to our study of in situ graphene devices and present preliminary STM and transport data with respect to gate voltage.   

Screening of a Coulomb Charge by Dirac Electrons in Graphene

Single-atom vacancies in graphene exhibit a rich variety of electronic phenomena ranging from mid-gap states to Kondo screening. Here we report on a new phenomenon showing that vacancies can host a positive charge which can be built up gradually by applying voltage pulses with the tip of a scanning tunneling microscope. The response of the conduction electrons to this charge, which is monitored with scanning tunneling and Landau level spectroscopy, and compared to numerical simulations, exhibits an unusual electron-hole asymmetry. On the p-doped side screening is weak. In this regime, as the charge is increased its interaction with the conduction electrons undergoes a transition into a regime where itinerant electrons are trapped in quasi-bound states (QBS) resembling an artificial atom. We observe the equivalent of the atomic 1S and 2S states as well as the emergence of a new satellite of the 1S state resulting from the broken sublattice symmetry at the vacancy site. In contrast, on the n-doped side screening is very efficient: as soon as the n-doped regime is entered the charge is screened and the QBS disappear. We show that the QBS are gate tunable and that the trapping mechanism can be turned on and off, providing a new mechanism to control electrons in graphene.   

Computer simulation of STM images of vertical heterostructures of graphene/hexagonal boron nitride with intercalated atoms

Using density functional theory, we did computational simulations of scanning tunneling microscopy of vertical graphene/hexagonal boron nitride heterostructures with an intercalated atom (Li, K, Cr, Mn, Co or Cu). A plane-wave basis set was employed with a kinetic energy of 400 eV. The form of the Perdew-Burke-Ernzerhof type was utilized for the exchange-correlation energy functional. To obtain the more accurate result, the van der Waals interaction was also considered. In the computer-simulated scanning tunneling microscopy (STM) images in the Tersoff-Hamann scheme, we demonstrated that the single impurity atom between Gr and hBN sheets is detectable. We observed three different STM patterns on the graphene side. These can be classified by group 1 (Li, Co, and Cu), group 2 (Cr and Mn), and group 3 (K), which have hexagonal, circular, and wide bright spot patterns around the impurity atom, respectively. Although Co and Cu are both in group 1, the Co atom shows stronger d orbital character than the Cu atom. Interestingly, in the case of the Co atom, the simulated STM images are quite different at bias voltages of -0.1 V and +0.1 V. While C $p_z$-Co $d_{yz}$ hybridization occurs at the bias voltage of -0.1 V, C $p_z$-Co $d_{xz}$ hybridization occurs at the bias voltage of +0.1 V.   

Growth and analysis of polymorphic graphene with STM and LEEM-IV for applications in molecular self-assembly and organic electronics

Graphene has aroused tremendous interest due to its remarkable electronic and mechanical properties, and is of interest for use in organic electronic devices such as organic photovoltaic cells. We present an analysis of a novel graphene system grown on Ru(0001) in the presence of atomic hydrogen and carbon vapor using STM and LEEM-IV. Structural studies completed with STM show a wide array of moire superlattice sizes ranging from 0.9 to 3.0 nm. Preliminary LEEM and LEEM-IV results confirm the presence of ordered graphene atop the Ru(0001) surface. Investigation using LEEM-IV provides information about the carbon layer thickness; also, micro-LEED-IV determines the precise atomic reconstruction of the interface region. In this regard, we believe the hydrogen present in the system to be interstitial at the carbon-ruthenium interface thus passivating the ruthenium surface, decoupling, and lifting the carbon layer from the substrate. The structural polymorphism displayed by this system is of interest for the study of directed self-assembly. Control over moire size can aid in future work using graphene as a template for self-assembled growth of organic electronics.   

Scanning Tunneling Microscopy Studies of Crystalline Hydrogenation of Graphene Grown on Cu(111)

Because of the sensitivity of 2D material surfaces, chemical functionalization can be exploited to tune the electronic structure of these materials. For example, hydrogen bonding to carbon atoms in graphene tunes the material from a semi-metal to a wide-gap insulator. We developed a method for a reproducible epitaxial growth of graphene on Cu(111) in the ultra-high vacuum chamber of a scanning tunneling microscope (STM). We find that hydrogen atoms can be bonded to the graphene in a nanoscale region using a novel field-emission process, whereby physisorbed H$_{2}$ is cracked in situ using the STM tip. This method produced crystalline surfaces of hydrogen-terminated graphene with 4.2{\AA} lattice, which has proven difficult to produce using conventional atomic beam methods which typically produced disordered hydrogenation. Additionally, this hydrogenation process is reversible and we are able to recover the pristine graphene by H desorption during STM imaging at a high bias. STM images after the dehydrogenation process showed the same atomic lattice and Moir\'{e} pattern as the pristine graphene, with the exception of additional point defects. STM spectra show the suppression of the Cu surface state on the hydrogenated graphene, but the opening of a wide-gap was not observed.   

Experimental Investigation of the Electronic Properties of Twisted Bilayer Graphene by STM and STS

The electronic properties of graphene multilayers depend sensitively on their stacking order. A twisted angle is treated as a unique degree of freedom to tune the electronic properties of graphene system. Here we study electronic structures of the twisted bilayers by scanning tunneling microscopy (STM) and spectroscopy (STS). We demonstrate that the interlayer coupling strength affects both the Van Hove singularities and the Fermi velocity of twisted bilayers dramatically. This removes the discrepancy about the Fermi velocity renormalization in the twisted bilayers and provides a consistent interpretation of all current data. Moreover, we report the experimental evidence for non-Abelian gauge potentials in twisted graphene bilayers by STM and STS. At a magic twisted angle, about 1.11°, a pronounced sharp peak is observed in the tunnelling spectra due to the action of the non-Abelian gauge fields. Because of the effective non-Abelian gauge fields, the rotation angle could transfer the charge carriers in the twisted bilayers from massless Dirac fermions into well localized electrons, or vice versa, efficiently. This provides a new route to tune the electronic properties of graphene systems, which will be essential in future graphene nanoelectronics.   

Ab initio study of friction of graphene flake on graphene/graphite or SiC surface

Recently, the rich dynamics of graphene flake on graphite or SiC surfaces are revealed from atomic force microcopy experiments. The studies toward to the understanding of microscopic origin of friction are getting a lot of attention. Despite the several studies of these systems using molecular dynamics methods, density functional theory based investigations are limited because of the huge system sizes. In this study, we investigated the frictional force on graphene flake on graphite or SiC surfaces from pseudopotential planewave calculations based on density functional theory. In both cases, graphene flake (24 C) on graphite or SiC surface, bilayer flake is introduced by freezing the top layer as well as the bottom layer of the surface slab. After fixing the load with these frozen layers, we checked the relative motion of the flake over the surface. A minimum energy is reached when the flake is moved on graphene to attain AB stacking. We also conclude that edge reconstruction because of the finite size of the flake is very critical for frictional properties of the flake; therefore the saturation of dangling bonds with hydrogen is also addressed. Not only the symmetric configurations remaining parameter space is extensively studied. Supported by TUBITAK Project No: 114F162.