Image of the Month

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Image of the Month
Posted Date: April 1, 2017
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(c) STM image of a branch of a fractal. The sample bias is 0.5 V and the tunnel current is 0.2 nA. (d) Differential conductivity recorded at the environment of the fractal, red curve (region (i) of (c)), and on the fractal, blue curve (region (i) of (c)), and on the fractal, blue curve (region (ii) of (c)). The sample bias for both curves is 0.5 V and the tunnel current is 0.2 nA.

Abstract
The basic science responsible for the fascinating  shapes of ice crystals and snowflakes is still not understood. Insufficient knowledge of the interaction potentials and the lack of relevant experimental access to the growth process are to blame for this failure. Here, we study the growth of fractal nanostructures in a two-dimensional  (2D) system, intercalated between mica and graphene. Based on our scanning tunneling spectroscopy data, we provide compelling evidence that these fractals are 2D ice. They grow while they are in material contact with the atmosphere at 20 ºC and without significant thermal contact to the ambient. The growth is studied in situ, in real time and space at the nanoscale. We find that the growing 2D ice nanocrystals assume a fractal shape, which is conventionally attributed to Diffusion Limited Aggregation (DLA). However, DLA requires a low mass density mother phase, in contrast to the actual currently present high mass density mother phase. Latent heat effects and consequent transport of heat and molecules are found to be the key ingredients for understanding the evolution of the snow (ice) flakes. We conclude that not the local availability of water molecules (DLA), but rather them having the locally required orientation is the key factor for incorporation into the 2D ice nanocrystal. In combination with the transport of latent heat, we attribute the evolution of fractal 2D ice nanocrystals to local temperature dependent rotation limited aggregation. The ice growth occurs under extreme supersaturation, i.e., the conditions closely resemble the natural ones for the growth of complex 2D snow (ice) flakes and we consider our findings crucial for solving the “perennial” snow (ice) flake enigma. © 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4926467]

Reference:
Journal of Chemical Physics 143, 034702 (2015)

Credits:
Pantelis Bampoulis,1,2,a) Martin H. Siekman,1  E. Stefan Kooij,1 Detlef Lohse,2 Harold J. W. Zandvliet,1  and Bene Poelsema1

1Physics of Interfaces and Nanomaterials, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands
2Physics of Fluids and J. M. Burgers Centre for Fluid Mechanics, MESA+ Institute of Nanotechnology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands

Microscope:
RHK UHV3500 STM/AFM with Inverted Viewport

Control System:
RHK SPM1000

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Image of the Month
Posted Date: March 7, 2017
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Figure 2 | Imaging silicon atoms with terahertz-driven scanning tunnelling microscopy (TD-STM). a. Topographic images of the Si(111)-(7 x 7) surface measured in constant-current mode with standard STM at Vd.c. = -0.5 V, ETHz,pk = 0 V cm-1, Id.c. = -20 pA b. and terahertz-driven STM (TD-STM) of the same region with Vd.c. = 0 V, ETHz,pk = -200 V cm-1ITHz,avg = -20 pA

Abstract
Ultrafast control of current on the atomic scale is essential for future innovations in nanoelectronics. Extremely localized transient electric fields on the nanoscale can be achieved by coupling picosecond duration terahertz pulses to metallic nanostructures. Here, we demonstrate terahertz scanning tunnelling microscopy (THz-STM) in ultrahigh vacuum as a new platform for exploring ultrafast non-equilibrium tunnelling dynamics with atomic precision. Extreme terahertz-pulse-driven tunnel currents up to 107 times larger than steady-state currents in conventional STM are used to image individual atoms on a silicon surface with 0.3 nm spacial resolution. At terahertz frequencies, the metallic-like Si(111)-(7 x 7) surface is unable to screen the electric field from the bulk, resulting in a terahertz tunnel conductance that is fundamentally different than that of the steady state. Ultrafast terahertz-induced band bending and non-equilibrium charging of surface states open new conduction pathways to the bulk, enabling extreme transient tunnel currents to flow between the tip and sample.

Public Reference:
NATURE PHYSICS: DOI: 10.1038/NPHYS4047

Credits:
Vedran Jelic1*, Krzysztof Iwaszczuk2, Peter H. Nguyen1, Christopher Rathje3, Graham J. Hornig1, Haille M. Sharum1, James R. Hoffman1, Mark R. Freeman1 and Frank A. Hegmann1*

1Department of Physics, University of Alberta, Edmonton, Alberta T6G 2E1, Canada. 2DTU Fotonik – Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark. 34th Physical Institute, University of Gottingen, 37077 Gottingen, Germany.
*e-mail: [email protected];, [email protected]

Microscope:
RHK UHV3500 STM/AFM with Inverted Viewport

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Image of the Month
Posted Date: February 8, 2017
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Figure 2 | Charge density wave (CDW) transition in MTBs. (a) STM images of a single MTB at low temperatures (120 K) exhibit three times the periodicity than the atomic corrugation imaged at room temperature. In (b) a larger scale low-T STM image and the corresponding cross-section along the indicated MTB is shown that measured the periodicity of the CDW as B1.0 nm. The schematic in (c) illustrates the relationship between CDW period and nesting vector q = 2kF. Also the opening of a band gap at kF is illustrated. Temperature dependent resistance measurements, shown in (d), indicate two CDW transitions. The transitions at 235 and 205 K correspond to incommensurate and commensurate CDW transitions, respectively. Depending on the applied bias voltage we also observe a drop in resistance below the CDW transition temperatures, which is attributed to CDW-sliding. The inset shows the control measurement on a bare MoS2 substrate and shows no transitions.

Material line defects are one-dimensional structures but the search and proof of electron behaviour consistent with the reduced dimension of such defects has been so far unsuccessful. Here we show using angle resolved photoemission spectroscopy that twin-grain boundaries in the layered semiconductor MoSe2 exhibit parabolic metallic bands. The one-dimensional nature is evident from a charge density wave transition, whose periodicity is given by kF/π, consistent with scanning tunnelling microscopy and angle resolved photoemission measurements. Most importantly, we provide evidence for spin- and charge-separation, the hallmark of one-dimensional quantum liquids. Our studies show that the spectral line splits into distinctive spinon and holon excitations whose dispersions exactly follow the energy-momentum dependence calculated by a Hubbard model with suitable finite-range interactions. Our results also imply that quantum wires and junctions can be isolated in line defects of other transition metal dichalcogenides, which may enable quantum transport measurements and devices.

Credits:
Yujing Ma1, Horacio Coy Diaz1, José Avila2,3, Chaoyu Chen2,3, Vijaysankar Kalappattil1, Raja Das1, Manh-Huong Phan1, Čadež4,5, Josè M.P. Carmelo4,5,6, Maria C. Asensio2,3 & Matthias Batzill1

1 Department of Physics, University of South Florida, Tampa, Florida 33620, USA. 2 Synchrotron SOLEIL, L’Orme des Merisiers, Saint Aubin-BP 48, Gif sur Yvette Cedex 91192, France. 3 Universite ́ Paris-Saclay, L’Orme des Merisiers, Saint Aubin-BP 48, Gif sur Yvette Cedex 91192, France. 4 Beijing Computational Science Research Center, Beijing 100193, China. 5 Center of Physics of University of Minho and University of Porto, Oporto P-4169-007, Portugal. 6 Department of Physics, University of Minho, Campus Gualtar, Braga P-4710-057, Portugal.

Microscope:
PanScan Freedom UHV

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RHK R9 Control System

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Image of the Month
Posted Date: January 1, 2017
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FIG. 1. (a) At left, the crystal structure for TbTe3 with a black rectangle outlining the unit cell. The dotted line indicates the a-c cleave plane between the double Te layers. At right, the square lattice of the Te layer which is exposed by cleaving as well as the locations of the closest Tb ions in the rare-earth block layer directly below. The unit cell is again shown in the structures at right for reference. The crystal structures were constructed using Vesta software. (b) Topographic image taken over a 90 A ̊ square region at I = 65 pA, VSample = −350 mV. The Te square lattice of the exposed surface can be clearly seen as well as superimposed “stripes” associated with a unidirectional CDW state along the a1 crystal axis.

We studied TbTe3 using scanning tunneling microscopy (STM) in the temperature range of 298–355 K. Our measurements detect a unidirectional charge density wave (CDW) state in the surface Te layer with a wave vector consistent with that of the bulk qCDW = 0.30 ± 0.01c∗. However, unlike previous STM measurements, and differing from measurements probing the bulk, we detect two perpendicular orientations for the unidirectional CDW with no directional preference for the in-plane crystal axes (a or c axis) and no noticeable difference in wave vector magnitude. In addition, we find regions in which the bidirectional CDW states coexist. We propose that observation of two unidirectional CDW states indicates a decoupling of the surface Te layer from the rare-earth block layer below, and that strain variations in the Te surface layer drive the local CDW direction to the specific unidirectional or, in rare occurrences, bidirectional CDW orders observed. This indicates that similar driving mechanisms for CDW formation in the bulk, where anisotropic lattice strain energy is important, are at play at the surface. Furthermore, the wave vectors for the bidirectional order we observe differ from those theoretically predicted for checkerboard order competing with stripe order in a Fermi-surface nesting scenario, suggesting that factors beyond Fermi-surface nesting drive CDW order in TbTe3. Finally, our temperature-dependent measurements provide evidence for localized CDW formation above the bulk transition temperature TCDW.

Credits:
Ling Fu,1 Aaron M. Kraft,1 Bishnu Sharma,1 Manoj Singh,1 Philip Walmsley,2,3 Ian R. Fisher,2,3 and Michael C. Boyer1,*
1Department of Physics, Clark University, Worcester, Massachusetts 01610, USA
2Geballe Laboratory for Advanced Materials and Department of Applied Physics, Stanford University, Stanford, California 94305-4045, USA
3Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA

Microscope:
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Image of the Month
Posted Date: November 1, 2016
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Figure 1: Phase-separated Mott/pseudogap electronic structure at 5.5% doping. a, Different spectra in the phase-separated region: Mott like spectrum (blue), with the chemical potential pinned to the UHB, and mixed Mott/pseudogap spectrum (red). The spectra are the average of 180 spectra inside the white circles in c. b, Phenomenological fit function to simultaneously extract both the Mott and pseudogap size. It consists of a density of states (dot-dashed) multiplied by the Mott gap (dashed) plus states inside the Mott gap with a V-shaped pseudogap (dotted). c, The Mott parameter as defined in the text identifies pseudogap puddles (red) and pure Mott regions (blue). Green circles indicate La dopant locations. The triangle on the colour bar indicates the value of the black contour. Inset, definition of the Mott parameter: the integrated DOS inside Mott gap (red) normalized by the one outside the gap (blue). d, Local density of states spectra along the white line in c (each corresponding to a single measurement). The separation is sharp in the sense that a Mott spectrum becomes a pseudogap spectrum within roughly a nanometre. e, ΔPG map extracted from the fitting procedure. The square indicates the region displayed in Fig. 4. Inset, the correlation between ΔMott and ΔPG.

It is widely believed that high-temperature superconductivity in the cuprates emerges from doped Mott insulators. When extra carriers are inserted into the parent state, the electrons become mobile but the strong correlations from the Mott state are thought to survive—inhomogeneous electronic order, a mysterious pseudogap and, eventually, superconductivity appear. How the insertion of dopant atoms drives this evolution is not known, nor is whether these phenomena are mere distractions specific to hole-doped cuprates or represent genuine physics of doped Mott insulators. Here we visualize the evolution of the electronic states of (Sr1-x Lax)2IrO4, which is an effective spin-1/2 Mott insulator like the cuprates, but is chemically radically di_erent2,3. Using spectroscopic-imaging scanning tunneling microscopy (SI-STM), we find that for a doping concentration of x ≈5%, an inhomogeneous, phase separated state emerges, with the nucleation of pseudogap puddles around clusters of dopant atoms. Within these puddles, we observe the same iconic electronic order that is seen in underdoped cuprates.  We investigate the genesis of this state and find evidence at low doping for deeply trapped carriers, leading to fully gapped spectra, which abruptly collapse at a threshold of x≈4%.Our results clarify the melting of the Mott state, and establish phase separation and electronic order as generic features of doped Mott insulators. (Nature Physics Letters. 19 SEPTEMBER 2016 | DOI: 10.1038/NPHYS3894 )

Credits:
I. Battisti1, K. M. Bastiaans1, V. Fedoseev1, A. de la Torre2,3, N. Iliopoulos1, A. Tamai2, E. C. Hunter4, R. S. Perry5, J. Zaanen1, F. Baumberger2,6 and M. P. Allan1*

1Leiden Institute of Physics, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands.

2Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland.

3Department of Physics, California Institute of Technology, Pasadena, California 91125, USA.

4School of Physics and Astronomy, The University of Edinburgh, James Clerk Maxwell Building, Mayfield Road, Edinburgh EH9 2TT, UK. 5London Centre for Nanotechnology and UCL Centre for Materials Discovery, University College London, LondonWC1E 6BT, UK.

6Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland.

Images and data graciously provided by Milan Allan, Leiden Institute of Physics, Leiden University, Leiden, The Netherlands.

Microscope:
Custom Modified Unisoku STM Microscope

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RHK R9 Control System

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Image of the Month
Posted Date: October 1, 2016
PanScan Freedom Helium Free

Figure 1: STM images of Ag(111) after exposure to AO (atomic oxygen) at Tdep = 490 K. Exposure duration is labeled in the upper left corner of each image, and the scale bar is in the lower right corner. Panels (A) and (B) show that the p(4 X 5√3) domain was predominant after brief exposures, as were areas of clean Ag(111) with isolated O adatoms that were observed as black depressions, as shown in panel (B). Panels (C)−(F) show that, with increasing AO exposure, several domains coexisted until the surface became uniformly covered in the striped pattern after 300 and 600 s exposures. Imaging conditions for each image were as follows: (A) i = 280 pA, V = 1.0 V; (B) i = 300 pA, V = 800 mV; (C) I = 260 pA, V = 0.400 mV; (D) i = 200 pA, V = 800 mV; (E) i = 300 pA, V = 900 mV; and (F) i = 260 pA, V = 0.970 mV. (ACS Catal. 2016, 6, 4640−4646)

A long-standing challenge in the study of heterogeneously catalyzed reactions on silver surfaces has been the determination of what oxygen species are of greatest chemical importance. This is due to the coexistence of several different surface reconstructions on oxidized silver surfaces. A further complication is subsurface oxygen (Osub ). Osub  are O atoms absorbed into the near surface region of a metal, and are expected to alter the surface in terms of chemistry and structure; however, these effects have yet to be well characterized. We studied oxidized Ag(111) surfaces after exposure to gas-phase O atoms to determine how Osub  is formed and how its presence alters the surface structure. Using a combination of surface science techniques to quantify Osub  formation and the resultant surface structure, we observed that once 0.1 ML of Osub  formed, the surface was dramatically, and uniformly, reconstructed to striped structures at the expense of all other surface structures. Furthermore, Osub  formation was hindered at temperatures above 500 K. The thermal dependence for Osub  formation suggests that, under the industrial catalytic conditions of 475− 500 K for the epoxidation of ethylene to ethylene oxide, Osub  would be present and is a factor in the subsequent reactivity of the catalysts. These findings point to the need for the incorporation of Osub  into catalytic models, as well as further theoretical investigation of the resultant structure observed in the presence of Osub. (ACS Catal. 2016, 6, 4640−4646)

Credits:
Jonathan Derouin,† Rachael G. Farber,† Marie E. Turano,† Erin V. Iski,‡ and Daniel R. Killelea*,†

(ACS Catal. 2016, 6, 4640−4646)

†Department of Chemistry & Biochemistry, Loyola University Chicago, 1068 W. Sheridan Rd., Chicago, Illinois 60660, United States

‡Department of Chemistry and Biochemistry, The University of Tulsa, 800 S. Tucker Dr., Tulsa, Oklahoma 74104, United States

Images and data graciously provided by Dan Killelea, Loyola University Chicago, Chicago, Illinois.

Microscope:
RHK PanScan Freedom Microscope

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RHK R9 Control System

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Image of the Month
Posted Date: September 1, 2016
SEP16IOM-Preview

Figure 1:  The sample is graphene grown on SiC by Joshua Robinson’s group at Penn State. The image is a large scale (50nm) high resolution (2048px) simultaneously collected dI/dV and topo image. Taken at 200mV and 0.1nA. dI/dV setting is 10mV excitation at 1kHz. The Moire pattern is clear in the main image, while the zoom shows atomic resolution.

Graphene grown on SiC is one of the most promising routes for producing large-scale graphene devices and heterostructures. Here we studied topography and variations in local density of states of graphene grown on SiC by Joshua Robinson’s group at Penn State.  This characterization of the base graphene growth is in preparation for developing work that aims to understand the local electronic states across graphene heterostructures. The figure shows a large scale image (50nm) taken at a high resolution (2048px) and a simultaneously collected dI/dV map (at 200mV and 0.1nA) from Shawna Hollen’s group at University of New Hampshire. The large scale image shows a superposition of a Moire pattern and electronic variations, while the zoom shows details down to atomic resolution.

Shruti Subramanian in Joshua Robinson’s group at Penn State grew the graphene on SiC samples. Jake Riffle in Shawna Hollen’s group at University of New Hampshire took the STM/STS data.

Credits:
Jake Riffle2, Shruti Subramanian1, Joshua Robinson1 and Shawna Hollen2

Department of Materials Science and Engineering and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

University of New Hampshire, Department of Physics, 9 Library Way, Durham, NH 03824

Images and data graciously provided by Professor Shawna Hollen, University of New Hampshire, Durham, New Hampshire.

Microscope:
RHK PanScan Freedom STM/AFM

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RHK R9 Control System

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Image of the Month
Posted Date: August 1, 2016
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Figure 1:  STM images of atomic level oxidation at two sequential exposure times, t = 3 min and t = 4 min. Circles indicate areas of change, e.g., a bright site converting to dark after additional SMB−O2 exposure, and an area where a single bright site changed into a pair of adjacent bright sites. Images were taken at 2 V and 230 pA.. (DOI: 10.1021/acs.jpcc.6b01360   J. Phys. Chem. C 2016, 120, 8191−8197)

The site-specific locations of molecular oxygen reactivity on Si(111)-(7 Å~  7) surfaces were examined using kinetic energy selected supersonic molecular beams in conjunction with in situ  scanning tunneling microscopy. We herein present a detailed visualization of the surface as it reacts in real-time and real-space when exposed to molecular oxygen with translational energy Ei  = 0.37 eV. Atomically resolved images reveal two channels for oxidation leading to the formation of dark and bright reaction sites. The darks sites dominate the reaction throughout the range of exposures sampled and exhibit almost no preference for occurrence at the corner or inner adatom sites of the reconstructed (7 Å~  7) unit cell. The bright sites show a small preference for corner vs. inner site reactivity on the reconstructed (7 Å~  7) unit cell. The bright site corner preference seen here at elevated kinetic energies and with selected incident kinematics is smaller than that typically observed for more conventional thermal (background dosed) oxidation processing. These observations suggest that two adsorption pathways, trapping-mediated chemisorption and direct chemisorption, occur simultaneously when using energetic molecular oxygen but with modified relative probability as compared with thermal dosing. These results demonstrate the efficacy of using angle- and energy-selected supersonic molecular beams to gain a topographical diagram of the accessible reactive potential surface energy and precise control of semiconductor oxidation, a process that is of growing importance as we seek to create high-quality and precisely defined oxides having atomic dimensions.

Credits:
Bryan Wiggins, L. Gaby Avila-Bront, Ross Edel, and S. J. Sibener*

The James Franck Institute and Department of Chemistry, The University of Chicago 929 East 57th Street, Chicago, Illinois 60637, United States

Images and data graciously provided by Steve Sibener, University of Chicago, Chicago, Illinois.

Microscope:
RHK Custom PanScan STM/AFM

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RHK R9 Control System

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Image of the Month
Posted Date: July 1, 2016
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Figure 1:  Empty states STM images of 0.4 ML Si deposition on bare Si (100) surface at 250 C (deposition rate is 0.4 ML/min) and H terminated at 140 ◦C. (Applied Surface Science 378 (2016) 301–307)

Low temperature Si epitaxy has become increasingly important due to its critical role in the encapsulation and performance of buried nanoscale dopant devices. We demonstrate epitaxial growth up to nominally 25 nm, at 250C, with analysis at successive growth steps using STM and cross section TEM to reveal the nature and quality of the epitaxial growth. STM images indicate that growth morphology of both Si on Si and Si on H-terminated Si (H: Si) is epitaxial in nature at temperatures as low as 250C. For Si on Si growth at 250C, we show that the Si epitaxial growth front maintains a constant morphology after reaching a specific thickness threshold. Although the in-plane mobility of silicon is affected on the H: Si surface due to the presence of H atoms during initial sub-monolayer growth, STM images reveal long range order and demonstrate that growth proceeds by epitaxial island growth albeit with noticeable surface roughening.

Credits:
Xiao Deng1,2, Pradeep Namboodiri2,, Kai Li2, Xiqiao Wang2,3, Gheorghe Stan2, Alline F. Myers2, Xinbin Cheng1, Tongbao Li1, Richard M. Silver2

1 School of Physics Science and Engineering, Tongji University, Shanghai 200092, People’s Republic of Chinab

2 National Institute of Standards and Technology, Gaithersburg, MD 20899, United States

3 University of Maryland, College Park, MD 20740, United States

Images and data graciously provided by Pradeep Namboodiri, NIST, Gaithersburg, Maryland.

Microscope:
RHK PanScan Microscope

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RHK R9 Control System

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Image of the Month
Posted Date: June 1, 2016
RHK-June-IOM-2016-Fu-PRB_93_045430

Figure 1: Zoomed 5 nm image. Using Fourier filtering of the larger raw image, the superlattice is removed, and only the crystal lattice remains. Specifically, in filtering, we selected all of the crystallographic Bragg peaks connected by reciprocal lattice vectors appearing in the FFT. (PHYSICAL REVIEW B 93, 045430 (2016))

We use scanning tunneling microscopy to study the lithium molybdenum purple bronze (Li0.9Mo6O17) at room temperature. Our measurements allow us to identify the single-crystal cleave plane and show that it is possible to obtain clean cleaved surfaces reflecting the crystal structure without the complications of nanoscale surface disorder. In addition to the crystal lattice, we observe a coexisting discommensurate superlattice with wave vectors q = 0.5a ± 0.25b. We propose that the origin of the superstructure is a surface reconstruction that is driven by cleaving along a crystal plane that contains in-plane MoO4 tetrahedra connected to out-of-plane MoO6 octahedra through corner-sharing oxygens. When combined with spectroscopic measurements, our studies show a promising avenue through which to study the complex physics within Li0.9Mo6O17.

Credits:
Ling Fu,1  Aaron M. Kraft,1  Martha Greenblatt,2  and Michael C. Boyer1,*  (Phys. Rev. B 93, 045430 (2016))

1 Department of Physics, Clark University, Worcester, Massachusetts 01610, USA

2 Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey 08854, USA

Images and data graciously provided by Michael Boyer, Clark University, Worcester, Massachusetts.

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RHK PanScan Microscope

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RHK R9 Control System

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