Image of the Month

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 Li_0.9Mo₆O₁₇.

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.

Microscope:
RHK PanScan Microscope

Control System:
RHK R9 Control System

Figure 1: Atomic positions obtained by density functional calculations overlaid on an image created by a scanning tunneling microscope of D2O-covered step edges on Pt(553). STM image of D2O-covered step edges on Pt(553) [Tip type: cut and pull Pt/Ir; 4.5×4.5nm²; T_STM=25K; V=−1V , I= −9pA (Phys. Rev. Lett. 116, 136101)

The interaction of platinum with water plays a key role in (electro)catalysis. Herein, we describe a combined theoretical and experimental study that resolves the preferred adsorption structure of water wetting the Pt(111)-step type with adjacent (111) terraces. Double stranded lines wet the step edge forming water tetragons with dissimilar hydrogen bonds within and between the lines. Our results qualitatively explain experimental observations of water desorption and impact our thinking of solvation at the Pt electrochemical interface.

Credits:
Manuel J. Kolb¹, Rachael G. Farber², Jonathan Derouin², Cansin Badan¹, Federico Calle-Vallejo¹, Ludo B. F. Juurlink¹, Daniel R. Killelea², and Marc T. M. Koper¹ (Phys. Rev. Lett. 116, 136101)

1 Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA, Leiden, The Netherlands

2 Department of Chemistry and Biochemistry, Loyola University Chicago, Chicago, Illinois 60660, USA

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

Microscope:
RHK PanScan Freedom Microscope

Control System:
RHK R9 Control System

Figure 1: Adsorption of DDQT molecules on Au(111) in the intermediate coverage regime. STM image [set point 100 mV, 5 pA] of a large areafeaturing a finite-sized 2D crystal of DDQT with individual DDQT dimers in the vicinity (J. Phys. Chem. C 2015, 119, 26959−26967)

Charge transport in electronic applications involving molecular semiconductor materials strongly depends on the electronic properties of molecular-scale layers interfacing with external electrodes. In particular, local variations in molecular environments can have a significant impact on the interfacial electronic properties. In this study, we use scanning tunneling microscopy and spectroscopy to investigate the self-assembly regimes and resulting electronic structures of alkyl-substituted quaterthiophenes adsorbed on the Au(111) surface. We find that at dilute molecular concentrations, dimerized cis conformers were formed, while at higher concentrations corresponding to small fractions of a submonolayer, the molecular conformation converted to trans, with the molecules self-assembled into ordered islands. At approximately half-monolayer concentrations, the structure of the self-assembled islands transformed again showing a different type of the trans conformation and qualitatively different registry with the Au(111) lattice structure. Molecular distributions are observed to vary significantly due to variations in local molecular environments, as well as due to variations in the Au(111) surface reactivity. While the observed conformational diversity suggests the existence of local variations in the molecular electronic structure, significant electronic differences are found even with molecules of identical apparent adsorption configurations. Our results show that a significant degree of electronic disorder may be expected even in a relatively simple system composed of conformationally flexible molecules adsorbed on a metal surface, even in structurally well-defined self-assembled molecular layers.

Credits:
Dmitry A. Kislitsyn, Benjamen N. Taber, Christian F. Gervasi, Stefan C. B. Mannsfeld, Lei Zhang,
Alejandro L. Briseno, and George V. Nazin (J. Phys. Chem. C 2015, 119, 26959−26967)
Images and data graciously provided by George Nazin, University of Oregon, Eugene, Oregon.

Microscope:
RHK PanScan Freedom Microscope

Control System:
RHK Technology Control System

Non-contact AFM images showing metal-organic coordination chains of platinum(II)dipyridinyl-tetrazine on the reconstructed Au(100) surface.  The model in the lower right panel is based on the molecular resolution image shown in the lower left.

Credits:
Daniel Skomski, Christopher D. Tempas, Kevin A. Smith, and Steven L. Tait*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States

Microscope:
RHK Technology AFM/STM UHV 7500

Control System:
RHK Technology SPM 1000 Control System

Reference:
D. Skomski, C. D. Tempas, K. A. Smith, and S. L. Tait

“Redox-Active On-Surface Assembly of Metal-Organic Chains with Single-Site Pt(II),”
Journal of the American Chemical Society, 136, 9862-9865 (2014).
DOI: 10.1021/ja504850f

Abstract:
The formation and stabilization of well-defined transition-metal single sites at surfaces may open new routes to achieve higher selectivity in heterogeneous catalysts. Organic ligand coordination to produce a well-defined oxidation state in weakly reducing metal sites at surfaces, desirable for selective catalysis, has not been achieved. Here, we address this using metallic platinum interacting with a dipyridyl tetrazine ligand on a single crystal gold surface. X-ray photoelectron spectroscopy measurements demonstrate the metal−ligand redox activity and are paired with molecular-resolution scanning probe microscopy to elucidate the structure of the metal−organic network. Comparison to the redox-inactive diphenyl tetrazine ligand as a control experiment illustrates that the redox activity and molecular-level ordering at the surface rely on two key elements of the metal complexes: (i) bidentate binding sites providing a suitable square-planar coordination geometry when paired around each Pt, and (ii) redox-active functional groups to enable charge transfer to a well-defined Pt(II) oxidation state. Ligand-mediated control over the oxidation state and structure of single-site metal centers that are in contact with a metal surface may enable advances in higher selectivity for next generation heterogeneous catalysts.

Figure 1: Clean H terminated Si 100 surface 100nm*100nm
Scanning parameters:
Sample bias: -3V;   Current:-0.45nA;  Scanning angle:45 deg; Integral gain: 200 m/As; Pixel: 1024*1024; Scanning speed: 476 nm/s

Preparing atomically clean samples are a primary requirement for developing patterned dopant devices based on hydrogen resist lithography.  The samples from this study are used to develop moderately low temperature silicon overgrowth to ensure epitaxial Si on Si growth.  The overall process involves, hydrogen lithography, phosphine dosing and subsequent dopant activation, and then silicon overgrowth to encapsulate the patterned devices.  Quality Si epitaxial overgrowth is essential to good local electronic properties for the encased atomic-scale phosphor-based donor devices.  The silicon (100) samples ( 4 mm x 10 mm ) were first chemically cleaned using  an RCA Piranha recipe and HF dipped to passivate the surface. The samples were then load locked into a UHV system for thermal processing. Thermal processing includes a couple of 1200 °C flashes (maintaining low -10 Torr pressure all the time) and a 1050 °C anneal for a few hours. The samples were then hydrogen terminated in situ with an atomic hydrogen source and moved to the Pan Scan STM chamber for RT imaging using the recent new design RHK all metal tip holder.  W polycrystalline tips were used.

Credits:
Images and data graciously provided by Richard Silver, NIST.

Microscope:
RHK PanScan Microscope

Control System:
RHK Technology Control System

The whole family of the observed B4PB molecular STs. a,b, Models of the STs(a) and the corresponding STM images of B4PB-ST-n (n =0, 1, 2, 3or4)(b). The number in each image in b indicates the number (Tn) of the B4PB molecules that participate in the corresponding molecular ST. The STM image for n =4 (b) is a cropped section of an imperfect B4PB-ST-4 structure (the largest one achieved in experiments to date). To illustrate the ST-4 structure clearly, the missing bottom-right corner, separated by the dashed line, was artificially added to the structure and is composed of modeled molecules. The length given below each STM image in b indicates the horizontal size of the corresponding STM image. Imaging conditions: constant height, Vbias =20 mV, I =1 nA (n = 0, 1 and 2); constant current, Vbias =50 mV, I =10 pA (n = 3); constant current, Vbias = −100 mV, I =15 pA (n =4).

Credits: Images and data graciously provided by Professor Kai Wu. Jian Shang, Yongfeng Wang, Min Chen, Jingxin Dai1, Xiong Zhou, Julian Kuttner, Gerhard Hilt, Xiang Shao, J. Michael Gottfried and Kai Wu.

SKLSCUSS, BNLMS, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. 2Key Laboratory for the Physics andChemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, China. 3Fachbereich Chemie, Philipps-Universität Marburg,Hans-Meerwein-Strasse, Marburg 35032, Germany. 4Department of Chemical Physics, School of Chemistry and Materials Science, University of Scienceand Technology of China, Hefei 230026, China. 5SPURc, 1 CREATE Way, #15-01, CREATE Tower, Singapore 138602, Singapore.

Microscope: Unisoku low-temperature STM (USM-1200S)

Control System: RHK Technology R9 Control System

Abstract:
Fractals, being “exactly the same at every scale or nearly the same at different scales” as defined by Benoit B. Mandelbrot, are complicated yet fascinating patterns that are important in aesthetics, mathematics, science and engineering. Extended molecular fractals formed by the self-assembly of small-molecule components have long been pursued but, to the best of our knowledge, not achieved. To tackle this challenge we designed and made two aromatic bromo compounds (4,4″-dibromo-1,1′:3′,1″-terphenyl and 4,4‴-dibromo-1,1′:3′,1″:4″,1‴-quaterphenyl) to serve as building blocks. The formation of synergistic halogen and hydrogen bonds between these molecules is the driving force to assemble successfully a whole series of defect-free molecular fractals, specifically Sierpiński triangles, on a Ag(111) surface below 80 K. Several critical points that govern the preparation of the molecular Sierpiński triangles were scrutinized experimentally and revealed explicitly. This new strategy may be applied to prepare and explore various planar molecular fractals at surfaces.

3C-AFM current maps of Array 1 at various applied magnetic fields between ±434 Oe. Insets show position on a resistance versus field hysteresis loop measured for one particle. Saturated antiparallel (low current) and parallel (high current) states are marked with antiparallel or parallel arrows.

Arrays of nanoscale magnetic tunnel junctions (MTJs) are being pursued for bit patterned media (BPM) and magnetoresistive random access memory (MRAM). Device performance depends on coercive field and critical bias switching distributions. Many well-established techniques allow for the characterization of ensemble average behavior, but conventional device-by-device characterization requires cumbersome processing steps. In addition to costly and complicated fabrication steps, the devices are often obscured for imaging.

Credits: Stephan K. Piotrowski, Samuel D. Oberdick, and Sara A. Majetich
Physics Department, Carnegie Mellon University

Microscope: RHK Technology AFM/STM UHV 350

Control System: RHK Technology R9 Control System

Abstract:
Arrays of nanoscale magnetic tunnel junctions (MTJs) are being pursued for bit patterned media (BPM) and magnetoresistive random access memory (MRAM). Device performance depends on coercive field and critical bias switching distributions. Many well-established techniques allow for the characterization of ensemble average behavior, but conventional device-by-device characterization requires cumbersome processing steps. In addition to costly and complicated fabrication steps, the devices are often obscured for imaging.

This image, recorded with molecular resolution at 150 °C, provides direct evidence for the stability of the metal-organic chains at high temperature on the Au(100) surface. Elevated-temperature STM image collected with the sample held at 425 K showing platinum(II) dipyridyltetrazine 1D chains on the reconstructed Au(100) surface.

The formation and stabilization of well-defined transition-metal single sites at surfaces may open new routes to achieve higher selectivity in heterogeneous catalysts. Organic ligand coordination to produce a well-defined oxidation state in weakly reducing metal sites at surfaces, desirable for selective catalysis, has not been achieved. Here, we address this using metallic platinum interacting with a dipyridyl tetrazine ligand on a single crystal gold surface. X-ray photoelectron spectroscopy measurements demonstrate the metal−ligand redox activity and are paired with molecular-resolution scanning probe microscopy to elucidate the structure of the metal−organic network. Comparison to the redox-inactive diphenyl tetrazine ligand as a control experiment illustrates that the redox activity and molecular-level ordering at the surface rely on two key elements of the metal complexes: (i) bidentate binding sites providing a suitable square-planar coordination geometry when paired around each Pt, and (ii) redox-active functional groups to enable charge transfer to a well-defined Pt(II) oxidation state. Ligand-mediated control over the oxidation state and structure of single-site metal centers that are in contact with a metal surface may enable advances in higher selectivity for next generation heterogeneous catalysts.

Credits: Daniel Skomski, Christopher D. Tempas, Kevin A. Smith, and Steven L. Tait*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States

Microscope: RHK Technology AFM/STM UHV 7500

Control System: RHK Technology SPM 1000 Control System

Mid infrared plasmon mode observed in doped semiconductor SrTiO₃ fabricated by Argon ion etching. Color represents near field optical signal, which is overlaid on top of the topography profile in a 24µm square area.

The image is obtained by a RHK broadband variable temperature scanning near field optical microscope.

Credits: Professor Cheng Cen, Department of Physics and Astronomy, West Virgina University, Morgantown, WV

Microscope: RHK broadband variable temperature scanning near field optical microscope.

Control System: RHK R9-STM and PMC100

These figures show the arrangement of 5 and 6-membered rings of D₂O on the Pt(111) surface, imaged at 80 K.

Credits: Daniel Killelea, Rachael Farber and Jon Derouin at Loyola University,Chicago and Ludo Juurlink at Leiden University, The Netherlands

Microscope: RHK PanScan Flow-LT STM

Control System: RHK R9-STM and PMC100