Crystal Reconstruction of PdO Film Caused by Oxygen Vacancy Generation Through Vacuum Annealing

The oxides of platinum group metals are of significant interest for sustainable energy, electronic, and spintronic applications.^1-3Among them, palladium oxide (PdO) has been widely studied as a catalyst in powder form and a Dirac semimetal in thin film form.^4,5 Epitaxial growth provides a systematic apprach to modify their structure, phase, and strain state. However, synthesizing phase pure PdO thin films by molecular beam epitaxy (MBE) remains challenging due to its low vapor pressure and oxidation potential.⁶ In this study, we show that tetragonal structured PdO thin films grown on MgO (001) substrates using oxygen plasma-assistant MBE often result in mixed (100)- and (001)-orientation. Our ab initio calculations reveal interfacial structures and stabilities of the PdO/MgO interfaces for both (100)- and (001)-orientations of the PdO films. We demonstrate that crystal structure reconstruction in PdO films occurs through the accumulation of oxygen vacancies indcued by post-annealing in vacuum resulting in a single crstalline film in the metastable configuration stablized by the interfacial interactions. This study suggests that vacuum annealing is a promising post-processing strategy for synthesizing single crystalline thin films and enhances our understanding of the role of oxygen vacacies in structural reconstruction.<br/><br/>Reference<br/>1. F. Y. Yu, Z. L. Lang, L. Y. Yin, K. Feng, Y. J. Xia, H. Q. Tan, H. T. Zhu, J. Zhong, Z. H. Kang and Y. G. Li, Nat. Commun. <b>11,</b> 490 (2020).<br/>2. J. Du, J. Quinson, D. M. Zhang, F. Bizzotto, A. Zana and M. Arenz, ACS Catal. <b>11,</b> 820-828 (2021).<br/>3. B. J. Kim, H. Ohsumi, T. Komesu, S. Sakai, T. Morita, H. Takagi and T. Arima, Science <b>323,</b> 1329-1332 (2009).<br/>4. F. Tang, H. C. Po, A. Vishwanath and X. G. Wan, Nature <b>566,</b> 486 (2019).<br/>5. C. X. Wang, F. Yang, W. Yang, L. Ren, Y. H. Zhang, X. L. Jia, L. Q. Zhang and Y. F. Li, Rsc Adv <b>5,</b> 27526-27532 (2015).<br/>6. S. Nair, Z. F. Yang, D. Y. Lee, S. L. Guo, J. T. Sadowski, S. Johnson, A. Saboor, Y. Li, H. Zhou, R. B. Comes, W. C. Jin, K. A. Mkhoyan, A. Janotti and B. Jalan, Nat Nanotechnol <b>18,</b> 1005 (2023).