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Hydrogen spillover is a catalytic process that occurs by surface reaction and subsequent diffusion to reversibly provide a massive amount of hydrogen dopants in correlated oxides, but the mechanism at the surface of correlated oxides with metal catalyst are not well understood. Here we show that a significant amount of oxygen is released from the surface of correlated VO₂ films during hydrogen spillover, contrary to the well-established observation of the formation of hydrogen interstitials in the bulk part of VO₂ films. By using ambient-pressure X-ray photoelectron spectroscopy, we prove that the formation of surface oxygen vacancies is a consequence ofmore » a favorable reaction for the generation of weakly adsorbed H₂O from surface O atoms that have low coordination and weak binding strength. Our results reveal the importance of in situ characterization to prove the dynamic change during redox reaction and present an opportunity to control intrinsic defects at the surface.« less

Unprecedented hydrogen detection at room temperature by Au nanoclusters supported on a self-activated graphene microchannel is demonstrated.

Here, determining the origin of the insulating gap in the monoclinic VO₂(M1) is a long-standing issue. The difficulty of this study arises from the simultaneous occurrence of structural and electronic transitions upon thermal cycling. Here, we compare the electronic structure of the M1 phase with that of single crystalline insulating VO₂(A) and VO₂(B) thin films to better understand the insulating phase of VO₂. As these A and B phases do not undergo a structural transition upon thermal cycling, we comparatively study the origin of the gap opening in the insulating VO₂ phases. By x-ray absorption and optical spectroscopy, we findmore » that the shift of unoccupied t_2g orbitals away from the Fermi level is a common feature, which plays an important role for the insulating behavior in VO₂ polymorphs. The distinct splitting of the half-filled t_2g orbital is observed only in the M1 phase, widening the bandgap up to ~0.6 eV. Our approach of comparing all three insulating VO₂ phases provides insight into a better understanding of the electronic structure and the origin of the insulating gap in VO₂.« less

The magnetic stability of E' centers and the peroxy radical on the surface of α-quartz is investigated with first-principles calculations to understand their role in magnetic flux noise in superconducting qubits (SQs) and superconducting quantum interference devices (SQUIDs) fabricated on amorphous silica substrates. Paramagnetic E' centers are common in both stoichiometric and oxygen deficient silica and quartz, and we calculate that they are more common on the surface than the bulk. However, we find the surface defects are magnetically stable in their paramagnetic ground state and thus will not contribute to 1/f noise through fluctuation at millikelvin temperatures.