Image of the Month

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Image of the Month
Posted Date: April 1, 2013
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Left Image – STM images of (a, b) the buffer layer and (d) QFMLG. Panel (a) shows the long-range periodicity imposed on the buffer layer by the substrate. The solid and dashed diamond designates the (6 x 6). Images in panel (a) were taken with a sample bias of +1.7 V. Under optimal tunneling conditions (main image in panel a) as opposed to earlier stages (inset in (a)) the atomic lattice superimposed on the (6 · 6) periodicity is revealed. Panels (b, d) are zoomed-in images of the buffer layer and QFMLG imaged with a sample bias of -0.223 V and +0.103 V, respectively. The upper insets in (b, d) present the structural models of the buffer layer and QFMLG, respectively. The lower insets in panels (b) and (d) are zoomed in 2D Fast Fourier Transforms (2DFFT) of one of the (1 · 1) spots of the graphene lattice with the quasi-(6 · 6) satellite spots visible only on the buffer layer. Scale bar 0.58 nm-1. Panel (c) shows atomically resolved STM images taken on the buffer layer and QFMLG and the corresponding line profiles along the graphene periodicity. The STM images in panel (c) have been filtered to remove noise. All measurements were taken in constant-current mode with the current set to 0.3 nA.

Right Image– (a) Current vs. voltage (I–V) curves and (b) differential conductance spectra acquired on the buffer layer (red line) and on QFMLG (blue line). The I–V curves in (a) are an average of multiple curves. The spectrum of the buffer layer reveals a low density of states ranging from around -0.5 V to +0.5 V, whereas hydrogen intercalation restores the semimetallic behavior of QFMLG expected for pristine graphene. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Credits:
Sarah Golera,b, Camilla Colettia,c, Vincenzo Piazzaa, Pasqualantonio Pingueb, Francesco Colangelob, Vittorio Pellegrinib, Konstantin V. Emtsevc, Stiven Fortic, Ulrich Starkec, Fabio Beltrama,b, Stefan Heunb
aCenter for Nanotechnology Innovation @ NEST, Istituto Italiano di Tecnologia, Piazza San Silvestro 12, 56127 Pisa, Italy
bNEST, Istituto Nanoscienze – CNR and Scuola Normale Superiore, Piazza San Silvestro 12, 56127 Pisa, Italy
cMax-Planck-Institut fuer Festkoerperforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany

Microscope:
RHK Technology UHV 7000

Control System:
RHK Technology SPM 1000 Control System

Reference :
CARBON 51 (2013) 249–254

Abstract:
On the SiC(0001) surface (the silicon face of SiC), epitaxial graphene is obtained by subli- mation of Si from the substrate. The graphene film is separated from the bulk by a car- bon-rich interface layer (hereafter called the buffer layer) which in part covalently binds to the substrate. Its structural and electronic properties are currently under debate. In the present work we report scanning tunneling microscopy (STM) studies of the buffer layer and of quasi-free-standing monolayer graphene (QFMLG) that is obtained by decou- pling the buffer layer from the SiC(0001) substrate by means of hydrogen intercalation. Atomic resolution STM images of the buffer layer reveal that, within the periodic structural corrugation of this interfacial layer, the arrangement of atoms is topologically identical to that of graphene. After hydrogen intercalation, we show that the resulting QFMLG is relieved from the periodic corrugation and presents no detectable defect sites.

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Image of the Month
Posted Date: March 1, 2013
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Figure 1. (b) dI/dV measured at an applied load of 72 nN on fluorinated graphene and pristine graphene. A band gap of 2.9 eV was measured after fluorination.
Figure 2. 500 × 500 nm2 images of (a) topography and (b) friction measured on the fluorinated graphene using contact mode AFM (applied load = 71 nN). (c) Plot of friction force versus applied load measured on pristine and on fluorinated graphene.

Credits:
Sangku Kwon,†,⊥ Jae-Hyeon Ko,‡,⊥ Ki-Joon Jeon,§ Yong-Hyun Kim,*,‡,∥ and Jeong Young Park*,†,∥
Graduate School of EEWS (WCU), KAIST, Daejeon 305-701, Republic of Korea
Graduate School of Nanoscience and Technology (WCU), KAIST, Daejeon 305-701, Korea §School of Electrical Engineering, University of Ulsan, Ulsan 680-749, Republic of Korea
KAIST Institute for the NanoCentury, KAIST, Daejeon 305-701, Republic of Korea

Microscope:
RHK Technology UHV 7500

Control System:
RHK Technology SPM 1000 Control System

Reference:
Nano Lett. 2012, 12, 6043−6048

Abstract:
Atomically thin graphene is an ideal model system for studying nanoscale friction due to its intrinsic two-dimensional (2D) anisotropy. Furthermore, modulating its tribological properties could be an important milestone for graphene-based micro- and nanomechanical devices. Here, we report unexpectedly enhanced nanoscale friction on chemically modified graphene and a relevant theoretical analysis associated with flexural phonons. Ultrahigh vacuum friction force microscopy measurements show that nanoscale friction on the graphene surface increases by a factor of 6 after fluorination of the surface, while the adhesion force is slightly reduced. Density functional theory calculations show that the out-of-plane bending stiffness of graphene increases up to 4-fold after fluorination. Thus, the less compliant F-graphene exhibits more friction. This indicates that the mechanics of tip-to- graphene nanoscale friction would be characteristically different from that of conventional solid-on-solid contact and would be dominated by the out-of-plane bending stiffness of the chemically modified graphene. We propose that damping via flexural phonons could be a main source for frictional energy dissipation in 2D systems such as graphene.

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Image of the Month
Posted Date: February 1, 2013
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STM images of the Cu(100) surface with TPA and NaCl, after annealing to 160 °C. (a) Part of a large island made up of the Na−TPA α phase. (b) Molecular and atomic resolution of the island in (a). (c) Zoom in of (b), with a schematic representation of the orientation of the Na−TPA α phase on the copper surface.

Credits:
Daniel Skomski, Sabine Abb,, and Steven L. Tait*,
†Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States
University of Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany

Microscope:
RHK Technology AFM/STM UHV 7500

Control System:
RHK Technology SPM 1000 Control System

Reference:
J. Am. Chem. Soc. 2012, 134, 14165−14171

Abstract:
Ionic bonding in supramolecular surface networks is a promising strategy to self-assemble nanostructures from organic building blocks with atomic precision. However, sufficient thermal stability of such systems has not been achieved at metal surfaces, likely due to partial screening of the ionic interactions. We demonstrate excellent stability of a self-assembled ionic network on a metal surface at elevated temperatures. The structure is characterized directly by atomic resolution scanning tunneling microscopy (STM) experiments conducted at 165 °C showing intact domains. This robust nanometer-scale structure is achieved by the on-surface reaction of a simple and inexpensive compound, sodium chloride, with a model system for carboxylate interactions, terephthalic acid (TPA). Rather than distinct layers of TPA and NaCl, angle resolved X-ray photoelectron spectroscopy experiments indicate a replacement reaction on the Cu(100) surface to form Na−carboxylate ionic bonds. Chemical shifts in core level electron states confirm a direct interaction and a +1 charge state of the Na. High-temperature STM imaging shows virtually no fluctuation of Na−TPA island boundaries, revealing a level of thermal stability that has not been previously achieved in noncovalent organic-based nanostructures at surfaces. Comparable strength of intermolecular ionic bonds and intramolecular covalent bonds has been achieved in this surface system. The formation of these highly ordered structures and their excellent thermal stability is dependent on the interplay of adsorbate−substrate and ionic interactions and opens new possibilities for ionic self-assemblies at surfaces with specific chemical function. Robust ionic surface structures have potential uses in technologies requiring high thermal stability and precise ordering through self-assembly.

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Image of the Month
Posted Date: January 1, 2013
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Fig. 1. (A) An STM topography scan of Ti0.87O2 nanosheets on Pt(200 nm)/Si of an area of 0.83 um × 0.83 um. Only a faint contrast is observed due to the presence of the nanosheets. (B) di/dz map of the same area from (A). A large contrast in the image reflects a larger difference in di/dz on the nanosheet and Pt/Si surface. Here, tunneling current is 0.5 nA and sample bias voltage is +2.5 V.

Microscope:
RHK Technology UHV 7000

Control System:
RHK Technology SPM 1000 Control System

Credits:
Avijit Kumara, Suresh Kumar C. Palanisamyb, Jelmer M. Botera, Chris Hellenthala, Johan E. ten Elshofb, Harold J.W. Zandvlieta
aPhysics of Interfaces and Nanomaterials, MESA+Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
bInorganic Materials Science, MESA+Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

Reference:
Applied Surface Science 265 (2013) 201– 204

Abstract:
We report scanning tunneling microscopy (STM) and spectroscopy (STS) measurements on single layer Ti0.87O2 nanosheets deposited on Pt(200 nm)/Si substrates. These nanosheets have a band gap of 3.8 eV and are virtually invisible in conventional STM images. However, an impressive contrast is observed in the di/dz map, which allows for a clear identification of the nanosheets. The di/dz signal is obtained by adding a high frequency, small amplitude modulation to the z-piezo using a lock-in amplifier.

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