Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T6: Focus Session: Graphene - Heterostructures, Overlayers |
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Sponsoring Units: DMP Chair: Debdeep Jena, University of Notre Dame Room: 302 |
Thursday, March 21, 2013 8:00AM - 8:12AM |
T6.00001: Direct Evidence for van der Waals Hetero-epitaxy of Graphene on Hexagonal Boron Nitride Haomin Wang, Shujie Tang, Ang Li, Xiaoming Xie, Mianheng Jiang We report on direct evidence for van der Waals (vdW) hetero-epitaxy of graphene grown on hexagonal boron nitride (hBN). Rotational misalignment of graphene on hBN produces a moir\'e pattern detectable by scanning probe microscopy (SPM) as a small modulation of the probe/surface friction. With the help of moir\'e interferometry and atomic resolution imaging, we obtained a fundamental insight into the growth behavior of single-crystalline graphene grown on h-BN substrates. It is found that the graphene grown by chemical vapor deposition mainly locks into one crystallographic orientation with respect to the h-BN substrate, while the graphene edges are parallel to armchair direction. The Moir\'e pattern on graphene/h-BN confirms that the rotational misalignment of graphene is definitely less than 0.05 $^{\circ}$ with respect to h-BN. It is also noticed that the vdW interaction plays a critical role in releasing the interfacial stress in the epitaxial graphene on h-BN. Our work shines light on creating artificial moir\'e interferometry in nanometer scale, which provides an invaluable scientific tool of atomic analyses on graphene based hetero-junction. [Preview Abstract] |
Thursday, March 21, 2013 8:12AM - 8:24AM |
T6.00002: DUV-Vis-NIR Structural and Compositional Imaging of Two-Dimensional Heterostructures Robin Havener, Cheol-Joo Kim, Lujie Huang, Adam Tsen, Mark Levendorf, Jiwoong Park Recent advances have allowed precise stacking and lateral stitching of various two-dimensional materials in complex geometries, but characterizing these structures remains a challenge. Here, we use a DUV-Vis-NIR (\textless\ 200-1000 nm) hyperspectral microscope to image composition and structural features in graphene and hexagonal boron nitride (h-BN) heterostructures with micron-scale resolution. We provide high-contrast images of h-BN at its absorption peak (6.1 eV), and map the quantitative full optical functions of single-layer graphene and h-BN in a device geometry. Stacking these materials provides an additional rotational degree of freedom which can produce unique optical signatures, allowing all-optical structural imaging. We characterize the optical response of twisted bilayer graphene, which exhibits an absorption peak whose energy varies with relative rotation angle from the infrared to the DUV ($\sim$ 4.0 eV), by combining hyperspectral imaging with dark-field transmission electron microscopy. By establishing such structure-property relationships, we enable controlled device fabrication on silicon substrates. [Preview Abstract] |
Thursday, March 21, 2013 8:24AM - 8:36AM |
T6.00003: Fabrication and characterization of graphene/MoS$_{2}$ heterojunctions Amelia Barreiro, Chul-ho Lee, Inanc Meric, Lei Wang, James Hone, Ken Shephard, Philip Kim We have fabricated graphene/MoS$_{2}$ /graphene co-laminated heterojunctions using a micromechanical manipulation technique. In order not to mask the transport properties of the heterojunctions, ohmic contact resistances need to be established. With the purpose of avoiding the formation of a Schottky barrier between the metal electrode and the MoS$_{2}$, different metals with work functions lower than the MoS$_{2}$ are tested. Once having obtained ohmic contacts, we are able to access the intrinsic transport properties of the heterojunctions and form Schottky diodes at the interface of the two layered materials. We will discuss the implications of the stacked heterojunction geometry to build novel transistors. [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 8:48AM |
T6.00004: Atomically-Smooth MgO films grown on Epitaxial Graphene by Pulsed Laser Deposition Sean Stuart, Andreas Sandin, Jack Rowe, Dan Dougherty, Marc Ulrich The growth of high quality insulating films on graphene is a crucial materials science task for graphene electronic and spintronic applications. It has been demonstrated that direct spin injection from a magnetic electrode to graphene is possible using MgO tunnel barriers of sufficient quality. We have used pulsed laser deposition (PLD) to grow thin magnesium oxide films directly on epitaxial graphene on SiC(0001). We observe very smooth film morphologies (typical rms roughness of $\sim$ 0.4 nm) that are nearly independent of film thickness and conform to the substrate surface which had $\sim$ 0.2 nm rms roughness. Surface roughness of 0.04 nm have been recorded for $\sim$ 1nm films with no pinholes seen by AFM. XPS and XRD data show non crystalline, hydroxylated MgO films with uniform coverage. This work shows that PLD is a good technique to produce graphene-oxide interfaces without pre-deposition of an adhesion layer or graphene functionalization. The details and kinetics of the deposition process will be described with comparisons being made to other dielectric-on-graphene deposition approaches. [Preview Abstract] |
Thursday, March 21, 2013 8:48AM - 9:00AM |
T6.00005: Initial stages of growth of pentacene on graphene Gvido Bratina, Manisha Chikkara, Egon Pavlica, Aleksandar Matkovic, Angela Beltao\v{s}, Djordje Jovanovic, Danka Stojanovic, Rado\v{s} Gajic We have examined by scanning probe microscope submonolayer coverages of pentacene on graphene fabricated by chemical vapor deposition (CVD) and exfoliated graphene. Inherent to CVD-graphene, even upon transferring onto SiO$_{2}$ substrates is the presence of varying surface density of folds-grafolds. By means of Kelvin force microscopy we observe about 0.3 eV higher workfunction on multiply-folded grafolds, but within our resolution, observe no change in workfunction for singly folded grafolds. By atomic force microscopy we observe that grafolds act as nucleation centers for pentacene, inducing three-dimensional (3D) morphology of pentacene layers in the nucleation phase of growth. Moreover, the resulting elongated islands exhibit a preferential orientation perpendicular to the dominant direction of a grafold. We associate this behavior in terms of elastic strain and enhanced chemical reactivity of the grafolds. This type of morphology is at strong variance with the morphology of pentacene layers that we observe on exfoliated graphene. There we observe two-dimensional (2D) islands whose height of 1.5 nm corresponds to a thin-film phase of pentacene. We observe the onset of 3D island nucleation on the surface of the 2D islands that have attained a critical size. We interpret this behavior in terms of surface energy of pentacene that depends on the underlying substrate. [Preview Abstract] |
Thursday, March 21, 2013 9:00AM - 9:12AM |
T6.00006: Intrinsic Electron-Hole Puddles in Graphene on Hexagonal Boron-Nitride Menno Bokdam, Taher Amlaki, Geert Brocks, Paul J. Kelly When graphene is placed on top of hexagonal boron nitride (h-BN), the 1.7{\%} lattice mismatch between the honeycomb lattices of graphene and h-BN leads to the formation of superstructures that are observed as moir\'{e} patterns in scanning tunneling microscopy images [1,2]. Using first-principles calculations and ignoring the incommensurability, we observed the formation of a dipole layer at the graphene\textbar h-BN interface [3]. The strength and direction of this dipole layer depends sensitively on the local bonding of the carbon atoms to the substrate i.e. on the details of how the graphene layer is positioned on top of h-BN. The dipole layer is accompanied by a step in the electrostatic potential, which ranges from $+$120 to $-$30 meV depending on the configuration. Because the lattice mismatch is so small, the local bonding configuration varies slowly in a graphene\textbar h-BN superstructure. We predict that the Dirac cone will follow the slowly varying potential created by the interface dipole layer even when screening effects are included. This then leads to the formation of regions of electron- and hole-doped graphene: intrinsic electron-hole puddles that will limit the mobility in this system. We make a comparison with graphene on molybdenum disulphide (MoS$_{\mathrm{2}})$ where a dipole layer is also formed but where we do not expect intrinsic electron-hole puddles to be formed. [1] R. Decker et al., Nano Lett. 11, 2291-2295 (2011) [2] J.M. Xue et al., Nature Mat. 10, 282-285 (2011) [3] M. Bokdam et al., Nano Lett. 11, 4631-4635 (2011) [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:24AM |
T6.00007: Accurate effective model Hamiltonian for non-commensurate graphene on hexagonal boron nitride substrate Jeil Jung, Zhenhua Qiao, Allan MacDonald High quality hexagonal boron nitride (h-BN) crystals have emerged as a promising substrate and barrier-material for graphene nanoelectronic devices. The influence of the h-BN substrate on graphene's electronic properties is sometimes observable, but often extremely weak. We develop a theory of the h-BN graphene interaction that is based on first-principles electron tunneling amplitudes calculated as a function of horizontal displacement between commensurate honeycomb lattices. The effective Hamiltonian we derive is valid for arbitrary rotation angles between adjacent graphene and h-BN sheets. [Preview Abstract] |
Thursday, March 21, 2013 9:24AM - 9:36AM |
T6.00008: Electronic structure of graphene-topological insulator heterostructures Christopher Triola, Junhua Zhang, Enrico Rossi We have studied the electronic structure of heterostructures consisting of graphene in close proximity to a strong three dimensional topological insulator (3DTI). We find that in the presence of a momentum dependent tunneling the low-energy band structure of graphene is qualitatively modified due to the hybridization of the two-dimensional bands of the 3DTI surface with the bands of graphene. One of the effects of the hybridization is to effectively shift the two spin-degenerate Dirac cones of pristine graphene in opposite directions in momentum space. We also show how, by tuning separately the doping in graphene and the 3DTI surface, some of the qualitative features of the hybridized bands can be controlled. [Preview Abstract] |
Thursday, March 21, 2013 9:36AM - 9:48AM |
T6.00009: Spin textures in graphene-topological insulator heterostructures Junhua Zhang, Christopher Triola, Enrico Rossi We study the spin texture of the bands of heterostructures formed by graphene and strong three dimensional topological insulators (3DTIs). We find that in these systems, via the proximity effect, graphene can acquire nontrivial spin textures and we identify the conditions for their realization. The presence of spin textures in the graphene layer opens the possibility to realize ideal 2D spin-selective systems with the unique properties of graphene, such as the extremely high room-temperature mobility. In addition, we find that in graphene-3DTI heterostructures some of the spin structures are characterized by the locking of the spin and valley degrees of freedom and should allow the realization of novel valley-spintronics effects. [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:00AM |
T6.00010: Chiral superfluid states in hybrid graphene heterostructures Enrico Rossi, Junhua Zhang We study the hybrid heterostructure formed by one sheet of single layer graphene (SLG) and one sheet of bilayer graphene (BLG) separated by a thin film of dielectric material. In general it is expected that interlayer interactions can drive the system to a spontaneously broken symmetry state characterized by interlayer phase coherence. The peculiarity of the SLG-BLG heterostructure is that the electrons in the layers (SLG and BLG) have different chiralities. We find that the difference of chirality between electrons in the two layers causes the spontaneously broken symmetry state to be N-fold degenerate. Moreover, we find that some of the degenerate states are chiral superfluid states, topologically distinct from the usual layer-ferromagnetism. The chiral nature of the ground state opens the possibility to realize topologically protected midgap states. \medskip\\ Work supported in part by the Jeffress Memorial Trust, Grant No. J-1033 [Preview Abstract] |
Thursday, March 21, 2013 10:00AM - 10:12AM |
T6.00011: Ab-initio investigation of one-dimensional graphene-silicene superlattices Lars Matthes, Karsten Hannewald, Friedhelm Bechstedt Since the two-dimensional (2D) crystal graphene was rediscovered in 2004 by Geim et al. there has been a strong interest in tailoring its properties in order to achieve a broad usability in manifold applications. Furthermore, due to massless electrons appearing in graphene it is also a playground for theoretical physicists for testing basic physical theories of high energy physics in a solid state system. Recently, also a silicon based 2D honeycomb crystal, called silicene, was discovered. Due to the similar crystal structure, silicene shares many properties with graphene, e.g., massless fermions. Here we present first-principles studies of electronic and structural properties of graphene-silicene superlattices. Our investigations provide insights to the physics of heterostructures consisting of materials where both may contain massless fermions and a vanishing electronic gap around the Fermi-energy. Finally, we also discuss the importance of the 1D interface between those 2D crystals [2].\\[4pt] [1] P. Vogt et al., PRL 108, 155501 (2012)\\[0pt] [2] L. Matthes et. al, PRB 86, 205409 (2012) [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T6.00012: Design of Ordered Graphene Oxides by First-Principles based Cluster Expansion Approach Bing Huang, Hongjun Xiang, Suhuai Wei The inhomogeneous phase, which usually exists in graphene oxides (GOs), is a long-standing problem that has severely restricted the use of GOs in various applications. By using first-principles based cluster expansion, we find that the existence of phase separation in conventional GOs is due to the extremely strong attractive interactions of oxygen atoms at different graphene sides. Our Monte Carlo simulations show that this kind of phase separation is not avoidable under the current experimental growth temperature. In this Letter, the idea of oxidizing graphene on single-side is proposed to eliminate the strong double-side oxygen attractions, and our calculations show that well-ordered GOs could be obtained at low oxygen concentrations. These ordered GOs behave as quasi-one-dimensional narrow-gap semiconductors with quite small electron effective masses, which can be useful in high-speed electronics. Our concept could be widely applied to overcome the inhomogeneous phases in various chemically functionalized two-dimensional systems. [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T6.00013: Electronic and transport properties in graphene oxide frameworks Pan Zhu, Vincent Meunier We report a detailed theoretical study of the electronic and transport properties of a series of graphene oxide frameworks (GOFs) using first-principles calculations based on density functional theory. The pillar molecular structure of GOFs determine that with various linear boronic acid pillars and different pillar concentrations, GOF structures can be fine tuned and exhibit various electronic properties. Based on ideal GOF structures, we predict that GOFs' electronic properties, such as band gap, can be modified controllobly by an appropriate choice of pillaring units and pillar concentration. The quantum transport properties of several systems with various linear boronic acid pillars are also evaluated. The variation of conductance arising from different pillar composition is shown to be potentially useful for practical applications. [Preview Abstract] |
Thursday, March 21, 2013 10:36AM - 10:48AM |
T6.00014: Dispersions of non-covalently functionalized graphene with minimal stabilizer Dorsa Parviz, Sriya Das, Fahmida Irin, Micah Green Pyrene derivatives are promising substitutes of surfactants and polymers for stabilization of graphene in aqueous dispersions. We demonstrate that pyrene derivatives stabilize single- to few-layer graphene sheets, yielding exceptionally higher graphene/stabilizer ratio in comparison with conventional stabilizers. Parameters such as stabilizer concentration, initial graphite concentration, type and number of functional groups, counterions, the pH and the polarity of dispersion media were shown to affect the adsorption process and final graphene concentration. The effectiveness of pyrene derivatives is determined by the type, number and electronegativity of functional groups and counterion. It also depends on the distance between functional group and pyrene basal plan, the pH of the dispersion (as shown by zeta potential measurements) and the relative polarity between stabilizer and solvent. Stability of the dispersions against centrifugation, pH and temperature changes and lyophilization was investigated. These dispersions also show promise for applications to polymer nanocomposites, organic solar cells, conductive films, and inkjet-printed electronic devices. [Preview Abstract] |
Thursday, March 21, 2013 10:48AM - 11:00AM |
T6.00015: Formation of transferable transparent pristine graphene films at water/heptane interface A.J. Oyer, J-M.Y. Carrillo, S.J. Woltornist, D.H. Adamson, A.V. Dobrynin We present a method of forming one to four layer thick pristine graphene films on glass substrates. These transparent and electrically conductive films are formed from natural graphite without the use of chemical treatment. The films are initially formed at a water/heptane interface and then transferred to a glass slide. Computer simulations of the graphene sheets at water/heptane interface show that the films are metastable, kinetically trapped assemblies. To evaluate stability of the film we used the Weighted Histogram Analysis Method to calculate the potential of the mean force and the height of the local potential barrier for single sheet and double sheet assembly of the graphene at water/heptane interface. The film structure on a glass slides was analyzed by Raman spectroscopy, optical microscopy, and transmission electron microscopy. These measurements show that the films are composed of overlapping graphene sheets one to four layers thick covering approximately 80{\%} of the substrate. These low cost films are expected to find applications in the economical replacement of current inorganic transparent conductive films. [Preview Abstract] |
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