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Image of the Month
Posted Date: February 1, 2013
institution

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
institution

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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Image of the Month
Posted Date: December 1, 2012
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Figure 6. (a) Scanning electron microscope image of a 200 nm wide Pt line; (b) 3D topographic of a 2.5 μm x 2.5 μm region where the 200 nm wide Pt line lies on top of a 1 μm wide Pt line; (c) thermal image of the same region shown in panel b. The temperature rise of the 200 nm line is seen to be lower in the region where it intersects the 1 μm wide line because the 1 μm wide Pt line acts as a fin.

Microscope: RHK Technology, Customized UHV7500 STM/AFM

Control System: RHK Technology SPM1000

Credits – Kyeongtae Kim, Wonho Jeong, Woochul Lee, and Pramod Reddy – University of Michigan, Ann Arbor

Reference – ACS Nano, 2012, 6 (5), pp 4248–4257

Abstract – Understanding energy dissipation at the nanoscale requires the ability to probe temperature fields with nanometer resolution. Here, we describe an ultra-high vacuum (UHV)-based scanning thermal microscope (SThM) technique that is capable of quantitatively mapping temperature fields with ∼15 mK temperature resolution and ∼10 nm spatial resolution. In this technique, a custom fabricated atomic force microscope (AFM) cantilever, with a nanoscale AuCr thermocouple integrated into the tip of the probe, is used to measure temperature fields of surfaces. Operation in an UHV environment eliminates parasitic heat transport between the tip and the sample enabling quantitative measurement of temperature fields on metal and dielectric surfaces with nanoscale resolution. We demonstrate the capabilities of this technique by directly imaging thermal fields in the vicinity of a 200 nm wide, self-heated, Pt line. Our measurements are in excellent agreement with computational results; unambiguously demonstrating the quantitative capabilities of the technique. UHV-SThM techniques will play an important role in the study of energy dissipation in nanometer-sized electronic and photonic devices and the study of phonon and electron transport at the nanoscale.
Keywords: scanning thermal microscopy . ultrahigh vacuum . quantitative temperature profiling . nanoscale thermal contact . thermocouple probe

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Image of the Month
Posted Date: October 1, 2012
iotm-october-2012

Description:
Microscope: RHK Technology UHV 300 STM
Control System: RHK Technology SPM1000 Control System

Credits:
Jose A. Hinojosa Jr., Jason F. Weaver – Department of Chemical Engineering, University of Florida, Gainesville

Reference:
Surface Science 605 (2011) 1797–1806

Abstract:
We used scanning tunneling microscopy (STM) to characterize PdO(101) thin films grown on Pd(111), and the structural changes that occur during isothermal decomposition.We find that the PdO(101) thin films have high-quality surface structures that are characterized by large, crystalline terraces with low concentrations of point defects. Small domains of single-layer oxide are also present on the top layer of relatively thick PdO(101) films grown at 500 K. The thinner PdO(101) films exhibit negligible quantities of such domains, apparently because new domains agglomerate rapidly as the film thickness decreases. We find that the isothermal decomposition rate of a PdO(101) film at 720 K exhibits an autocatalytic regime in which the rate of oxygen desorption increases as the oxide decomposes. Our STM results demonstrate that reduced sites created during oxide decomposition catalyze further PdO decomposition, and reveal strong kinetic anisotropies in the decomposition. The kinetic anisotropies produce one-dimensional reaction fronts that propagate preferentially along the atomic rows of the PdO(101) surface, resulting in the formation of long chains of reduced sites.We also find that reduced sites promote oxygen recombination in neighboring rows of the Pd(101) structure, causing loops and larger aggregates of reduced sites to form. The promotion of decomposition across the atomic rows can qualitatively explain the autocatalytic desorption kinetics. Finally, the STM images provide evidence that underlying PdO(101) layers transfer oxygen to reduced surface domains, thus producing large domains of PdO(101) islands that coexist with reduced domains as well as the larger PdO(101) terraces of the initial surface. Re-oxidation of the surface acts to sustain the autocatalytic decomposition kinetics, and provides a mechanism for oxygen atoms to ultimately evolve from the subsurface of the PdO(101) film.

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