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Electronic Structure of an Ultrathin Molybdenum Oxide Film P. A. Dementeva, E. V. Ivanovaa, M. N. Lapushkina, *, D. A. Smirnovb, and S. N. Timoshnevc a Ioffe

b

Institute, St. Petersburg, 194021 Russia Institut für FestKörper und Materialphysik, Technische Universität Dresden, 01062 Dresden, Germany c Alferov University, Russian Academy of Sciences, St. Petersburg, 194021 Russia *e-mail: [email protected] Received June 4, 2020; revised June 4, 2020; accepted June 4, 2020

Abstract—The electronic structure of an ultra-thin molybdenum oxide film obtained by oxidation of molybdenum at an oxygen pressure of 1 Torr and the effect of adsorption of sodium atoms on its electronic structure are studied by photoelectron spectroscopy. Photoemission spectra from the valence band and core levels of O 2s, Mo 3d, Mo 3p, and Na 1p are studied upon synchrotron excitation in the photon energy range 80– 600 eV. It is shown that in the formed oxide film, molybdenum is in two states: Mo6+ and Mo4+. On the surface of the oxide, oxygen is induced both in the composition of the oxides and in hydroxyl. It was shown that MoO3 is formed on the surface, and MoO2 at a distance from the surface. The deposition of Na atoms leads to intercalation of the molybdenum oxide layer. Keywords: molybdenum oxides, photoemission, sodium, intercalation DOI: 10.1134/S1063783420100030

1. INTRODUCTION Transition metal oxides and, in particular, molybdenum oxides are not only subjects of scientific studies but also are widely used for designing various sensors, transducers, thin-film transistors, solar cells, “smart” windows, energy storage systems, biosensors, catalysts, photo- and electrochrome materials, emitters for surface ionization in mass-spectroscopy, and etc. An advanced direction of using molybdenum oxides is designing of the energy storage systems; however, the case in point is not only the development of lithiumion batteries and supercapacitors, but also the use of other alkali metals instead of lithium (sodium or potassium), which can sharply decrease the cost of the devices. The development of sodium-ion batteries on the base of MoO3 and MoO2 has the highest perspectives [1–7]. It should be noted, as well, that the studies are mainly devoted to the morphological and electrochemical characteristics of the sodium–molybdenum oxide systems, and the change in the electronic structures of the oxides at its intercolation with sodium is scantily known. There are many molybdenum oxides with different stoichiometries and structures. Oxide MoO3 is most interesting both from the point of view of the studies and its practical application. Several MoO3 phases are known: the thermodynamically stable orthorhombic α phase, the metastable monoclinic β phase, and the metastable hexagonal h phase. The α-MoO3 phase has the layered structure consisted of two-layer MoO6

octahedron layer bounded between the layers by the Van der Waals forces [8] with the following lattice parameters: a = 0.396 nm, b = 1.386 nm, and c = 0.376 nm. It is shown that this st