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1. Uncovering the Effect of Lattice Strain and Oxygen Deficiency on Electrocatalytic Activity of Perovskite Cobaltite Thin Films

   https://doi.org/10.1002/advs.201801898  

   Liu, Xi ; Zhang, Lei ; Zheng, Yun ; Guo, Zheng ; Zhu, Yunmin ; Chen, Huijun ; Li, Fei ; Liu, Peipei ; Yu, Bo ; Wang, Xinwei ; et al ( January 2019 , Advanced Science)

   Developing cost effective electrocatalysts with high oxygen evolution reaction (OER) activity is essential for large‐scale application of many electrochemical energy systems. Although the impacts of either lattice strain or oxygen defects on the OER performance of oxide catalysts have been extensively investigated, the effects of both factors are normally treated separately. In this work, the coupled effects of both strain and oxygen deficiency on the electrocatalytic activity of La_0.7Sr_0.3CoO_3−δ(LSC) thin films grown on single crystal substrates (LaAlO3 (LAO) and SrTiO3 (STO)) are investigated. Electrochemical tests show that the OER activities of LSC films are higher under compression than under tension, and are diminished as oxygen vacancies are introduced by vacuum annealing. Both experimental and computational results indicate that the LSC films under tension (e.g., LSC/STO) have larger oxygen deficiency than the films under compression (e.g., LSC/LAO), which attribute to smaller oxygen vacancy formation energy. Such strain‐induced excessive oxygen vacancies in the LSC/STO increases thee_gstate occupancy and enlarges the energy gap between the O 2p and Co 3d band, resulting in lower OER activity. Understanding the critical role of strain–defect coupling is important for achieving the rational design of highly active and durable catalysts for energy devices.

    

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2. Field-induced resistance peak in a superconducting niobium thin film proximity coupled to a surface reconstructed SrTiO3

   https://doi.org/10.1038/s41535-020-0242-4  

   Singh, Akhilesh Kr. ; Kar, Uddipta ; Redell, Matthew D. ; Wu, Tsung-Chi ; Peng, Wei-Hsiang ; Das, Bipul ; Kumar, Satish ; Lee, Wei-Cheng ; Lee, Wei-Li ( July 2020 , npj Quantum Materials)

   Oxygen vacancy is known to play an important role for the physical properties in SrTiO₃(STO)-based systems. On the surface, rich structural reconstructions had been reported owing to the oxygen vacancies, giving rise to metallic surface states and unusual surface phonon modes. More recently, an intriguing phenomenon of a huge superconducting transition temperature enhancement was discovered in a monolayer FeSe on STO substrate, where the surface reconstructed STO (SR-STO) may play a role. In this work, SR-STO substrates were prepared via thermal annealing in ultra-high vacuum followed by low energy electron diffraction analyses on surface structures. Thin Nb films with different thicknesses (d) were then deposited on the SR-STO. The detailed studies of the magnetotransport and superconducting property in the Al(1 nm)/Nb(d)/SR-STO samples revealed a large positive magnetoresistance and a pronounced resistance peak near the onset of the resistive superconducting transition in the presence of an in-plane field. Remarkably, the amplitude of the resistance peak increases with increasing fields, reaching a value of nearly 57% of the normal state resistance at 9 T. Such resistance peaks were absent in the control samples of Al(1 nm)/Nb(d)/STO and Al(1 nm)/Nb(d)/SiO₂. Combining with DFT calculations for SR-STO, we attribute the resistance peak to the interface resistance from the proximity coupling of the superconducting niobium to the field-enhanced long-range magnetic order in SR-STO that arises from the spin-polarized in-gap states due to oxygen vacancies.

    

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3. The Role of Oxygen Transfer in Oxide Heterostructures on Functional Properties

   https://doi.org/10.1002/admi.202101867  

   Corey, Zachary ; Han, Henry H. ; Kang, Kyeong Tae ; Wang, Xuejing ; Lalk, Rebecca A. ; Paudel, Binod ; Roy, Pinku ; Sharma, Yogesh ; Yoo, Jinkyoung ; Jia, Quanxi ; et al ( January 2022 , Advanced Materials Interfaces)

   A variety of mechanisms are reported to play critical roles in contributing to the high carrier/electron mobility in oxide/SrTiO₃(STO) heterostructures. By using La_0.95Sr_0.05TiO₃(LSTO) epitaxially grown on different single crystal substrates (such as STO, GdScO₃, LaAlO₃, (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7, and CeO₂buffered STO) as the model systems, the formation of a conducting substrate surface layer (CSSL) on STO substrate is shown at relatively low growth temperature and high oxygen pressure (725 °C, 5 × 10^–4 Torr), which contributes to the enhanced conductivity of the LSTO/STO heterostructures. Different from the conventional oxygen vacancy model, this work reveals that the formation of the CSSL occurs when growing an oxide layer (LSTO in this case) on STO, while neither annealing nor the growth of an Au layer alone at the exact same growth condition generates the CSSL in STO. It demonstrates that the oxide layer actively pulls oxygen from STO substrate at given growth conditions, leading to the formation of the CSSL. The observations emphasize the oxygen transfer across film/substrate interface during the synthesis of oxide heterostructures playing a critical role in functional properties.

    

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4. Memristive Behavior Enabled by Amorphous–Crystalline 2D Oxide Heterostructure

   https://doi.org/10.1002/adma.202000801  

   Yin, Xin ; Wang, Yizhan ; Chang, Tzu‐hsuan ; Zhang, Pei ; Li, Jun ; Xue, Panpan ; Long, Yin ; Shohet, J. Leon ; Voyles, Paul M. ; Ma, Zhenqiang ; et al ( April 2020 , Advanced Materials)

   The emergence of memristive behavior in amorphous–crystalline 2D oxide heterostructures, which are synthesized by atomic layer deposition (ALD) of a few‐nanometer amorphous Al₂O₃layers onto atomically thin single‐crystalline ZnO nanosheets, is demonstrated. The conduction mechanism is identified based on classic oxygen vacancy conductive channels. ZnO nanosheets provide a 2D host for oxygen vacancies, while the amorphous Al₂O₃facilitates the generation and stabilization of the oxygen vacancies. The conduction mechanism in the high‐resistance state follows Poole–Frenkel emission, and in the the low‐resistance state is fitted by the Mott–Gurney law. From the slope of the fitting curve, the mobility in the low‐resistance state is estimated to be ≈2400 cm²V⁻¹s⁻¹, which is the highest value reported in semiconductor oxides. When annealed at high temperature to eliminate oxygen vacancies, Al is doped into the ZnO nanosheet, and the memristive behavior disappears, further confirming the oxygen vacancies as being responsible for the memristive behavior. The 2D heterointerface offers opportunities for new design of high‐performance memristor devices.

    

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5. In Situ Scattering Studies of Superconducting Vacancy‐Ordered Monoclinic TiO Thin Films

   https://doi.org/10.1002/apxr.202300086  

   Baksi, Merve ; Hong, Hawoong ; Kumah, Divine P. ( November 2023 , Advanced Physics Research)

   The structural and transport properties of vacancy‐ordered monoclinic superconducting titanium oxide (TiO) thin films grown by molecular beam epitaxy are investigated. The evolution of the crystal structure during growth is monitored by in situ synchrotron X‐ray diffraction. Long‐range ordering of Ti and O vacancies in the disordered cubic phase stabilizes the vacancy‐ordered monoclinic TiO phase. The reduced structural disorder arising from vacancy‐ordering is correlated with a superconductor‐metal transition (SMT) in contrast to the superconductor‐insulator transition (SIT) observed in cubic TiO, orthorhombicTi₂O₃, and the Magneli γ −Ti₃O₅and γ −Ti₄O₇phase. Magnetoresistance measurements for the SIT phases indicate superconducting fluctuations persisting in the normal phase. These results confirm the role of disorder related to Ti and O vacancies and structural inhomogeneity in determining the electronic properties of the normal state of titanium oxide‐based superconductors.

    

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