Image of the Month

Image of the Month
posted August 1, 2016
iotm-august-thumb

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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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.

Microscope:
RHK PanScan Microscope

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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.

Microscope:
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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.

Microscope:
RHK PanScan Freedom Microscope

Control System:
RHK R9 Control System

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