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Vanadium dioxide (VO2) is a popular candidate for electronic and optical switching applications due to its well-known semiconductor-metal transition. Its study is notoriously challenging due to the interplay of long and short range elastic distortions, as well as the symmetry change, and the electronic structure changes. The inherent coupling of lattice and electronic degrees of freedom opens the avenue towards mechanical actuation of single domains. In this work, we show that we can manipulate and monitor the reversible semiconductor-to-metal transition of VO2 while applying a controlled amount of mechanical pressure by a nanosized metallic probe using an atomic force microscope. At a critical pressure, we can reversibly actuate the phase transition with a large modulation of the conductivity. Direct tunneling through the VO2-metal contact is observed as the main charge carrier injection mechanism before and after the phase transition of VO2. The tunneling barrier is formed by a very thin but persistently insulating surfacelayer of the VO2. The necessary pressure to induce the transition decreases with temperature. In addition, we measured the phase coexistence line in a hitherto unexplored regime. Our study provides valuable information on pressure-induced electronic modifications of the VO2 properties, as well as on nanoscale metal-oxide contacts, which can help in the future design of oxide electronics., Y.A.B. and K.S. contributed equally. 30 pages, 4 figures, Supplemental Material (28 pages, 16 figures)

Correlated metals with high carrier density and strongly correlated electron effects provide an alternative route to achieve transparent conducting materials, different from the conventional degenerately doped wide-bandgap transparent conducting oxides (TCO). The extremely low electrical resistivity and high optical transparency in the ultraviolet-visible spectral range shown in 4d correlated metals present an advantage over conventional TCOs. However, most of the 4d correlated metals are grown epitaxially on single crystal substrates. Here, it has been shown that Ca2Nb3O10 nanosheets with different buffer layers promote the growth of high-quality 4d2 SrMoO3 films on fused silica substrates, overcoming the use of expensive and size-limited single-crystal substrates. The room temperature electrical resistivity of SrMoO3 is as low as 61 µΩ cm, the lowest reported value on amorphous transparent substrates to date, without compromising its high optical transmittance. 4d1 correlated metal SrNbO3 on Ca2Nb3O10 nanosheets also exhibits similarly high optical transmittance but a higher room temperature resistivity of 174 µΩ cm. These findings facilitate the use of highly conducting and transparent 4d correlated metals not only as TCOs on technologically relevant substrates for the applications in the ultraviolet-visible spectral range but also as electrodes for other oxide-based thin film technologies.

High entropy materials are a new pathway in the development of high-activity (electro )catalysts because of the inherent tunability and coexistence of multiple potential active sites, which may lead to earth-abundant catalyst materials for energy-efficient electrochemical energy storage. In this report, we identify how the multi-cation composition in high entropy perovskite oxides (HEO) contributes to high catalytic activity for the oxygen evolution reaction (OER), i.e. the key kinetically limiting half-reactions in several electrochemical energy conversion technologies, including green hydrogen generation. We compare the activity of the (001) facet of LaCr0.2Mn0.2Fe0.2Co0.2Ni0.2O3-δ with the parent compounds (single B-site in the ABO3 perovskite). While the single B-site perovskites roughly follow the expected volcano-type activity trends, the HEO clearly outperforms all of its parent compounds with 2.8 to 100 times higher currents at fixed overpotential. As all samples were grown as an epitaxial layer, our results indicate an intrinsic composition–function relationships, avoiding the effects of complex geometries or unknown surface composition. In-depth X-ray photoemission studies reveal a synergistic effect of simultaneous oxidation and reduction of different transition metal cations during adsorption of reaction intermediates. The surprisingly high OER activity demonstrates that HEOs are a highly attractive, earth-abundant new material class for high-activity OER electrocatalysts, possibly allowing fine-tuning the activity beyond the scaling limits of mono- or bimetallic oxides.