Image of the Month

Image of the Month
posted July 1, 2016
july-2016

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.

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Image of the Month
posted 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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Image of the Month
posted May 1, 2016
may-iom

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; TSTM=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.

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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.

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