The first-order metal-insulator phase transition in VO(2) is characterized by an ultrafast several-orders-of-magnitude change in electrical conductivity and optical transmittance, which makes this material an attractive candidate for the fabrication of optical limiting elements, thermochromic coatings, and Mott field-effect transistors. Here, we demonstrate that the phase-transition temperature and hysteresis can be tuned by scaling VO(2) to nanoscale dimensions. A simple hydrothermal protocol yields anisotropic free-standing single-crystalline VO(2) nanostructures with a phase-transition temperature depressed to as low as 32 degrees C from 67 degrees C in the bulk. The observations here point to the importance of carefully controlling the stoichiometry and dimensions of VO(2) nanostructures to tune the phase transition in this system.

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Soft X-ray ͑200 to 1000 eV͒ absorption spectroscopy at the O K-edge and the metal L II,III -edges, in both the fluorescence yield ͑FY͒ and the partial electron yield ͑PEY͒ mode, has been used to probe the electronic structure of electrochemically deintercalated Li 1Ϫx Ni 0.5 Mn 0.5 O 2 . FY and PEY spectra of the transition metal L II,III -edges, indicated that Mn ions remain mostly unchanged in the Mn 4ϩ state at all levels of charge. However, the Ni FY L-edge spectra show a continuous shift to higher energy during charge, but remain mostly unchanged in the PEY data. The results of the FY data show that the Ni ions in the bulk are oxidized form Ni 2ϩ to Ni 4ϩ during charge. The difference between the surface-sensitive PEY data and the bulk-sensitive FY data indicates that the surface of Li 1Ϫx Ni 0.5 Mn 0.5 O 2 has a different electronic structure than the bulk. The shift in the O K-edge to lower energies and the development of a shoulder on the low energy side of the first pre-edge peak indicates that the holes compensating the lithium ion deintercalation are located in O 2p states as well as Ni 3d states. These results show that soft X-ray absorption is a powerful technique for studying the electronic structure of new battery materials and it provides unique complementary information to that obtained from hard X-ray ͑above 1000 eV͒ absorption studies at the transition metal K-edges.

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