Image of the Month

Image of the Month
posted April 1, 2016
iotm-apr2016

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

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Image of the Month
posted January 1, 2016
iotm-jan2016

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 Society136, 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.

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Image of the Month
posted December 1, 2015
iotm-dec2015

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

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Image of the Month
posted May 1, 2015
iotm-may-2015

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.

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