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Open AccessArticle

X-Ray Spectroscopy of Ultra-Thin Oxide/Oxide Heteroepitaxial Films: A Case Study of Single-Nanometer VO2/TiO2

1
Department of Physics, Applied Physics and Astronomy, Binghamton University, Binghamton, NY 13902, USA
2
Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA
3
Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
4
National Synchrotron Light Source-II, Brookhaven National Laboratory, Upton, NY 11973, USA
5
Kavli Institute at Cornell for Nanoscale Science, Ithaca, NY 14853, USA
*
Author to whom correspondence should be addressed.
Academic Editor: Jan Ingo Flege
Materials 2015, 8(8), 5452-5466; https://doi.org/10.3390/ma8085255
Received: 30 July 2015 / Revised: 12 August 2015 / Accepted: 14 August 2015 / Published: 21 August 2015
(This article belongs to the Special Issue Epitaxial Materials 2015)
Epitaxial ultra-thin oxide films can support large percent level strains well beyond their bulk counterparts, thereby enabling strain-engineering in oxides that can tailor various phenomena. At these reduced dimensions (typically < 10 nm), contributions from the substrate can dwarf the signal from the epilayer, making it difficult to distinguish the properties of the epilayer from the bulk. This is especially true for oxide on oxide systems. Here, we have employed a combination of hard X-ray photoelectron spectroscopy (HAXPES) and angular soft X-ray absorption spectroscopy (XAS) to study epitaxial VO2/TiO2 (100) films ranging from 7.5 to 1 nm. We observe a low-temperature (300 K) insulating phase with evidence of vanadium-vanadium (V-V) dimers and a high-temperature (400 K) metallic phase absent of V-V dimers irrespective of film thickness. Our results confirm that the metal insulator transition can exist at atomic dimensions and that biaxial strain can still be used to control the temperature of its transition when the interfaces are atomically sharp. More generally, our case study highlights the benefits of using non-destructive XAS and HAXPES to extract out information regarding the interfacial quality of the epilayers and spectroscopic signatures associated with exotic phenomena at these dimensions. View Full-Text
Keywords: heteroepitaxial systems and interfaces; rational design of nanoscale materials; structure-function relationship heteroepitaxial systems and interfaces; rational design of nanoscale materials; structure-function relationship
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MDPI and ACS Style

Quackenbush, N.F.; Paik, H.; Woicik, J.C.; Arena, D.A.; Schlom, D.G.; Piper, L.F.J. X-Ray Spectroscopy of Ultra-Thin Oxide/Oxide Heteroepitaxial Films: A Case Study of Single-Nanometer VO2/TiO2. Materials 2015, 8, 5452-5466.

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