9232 | Phys. Chem. Chem. Phys., 2017, 19, 9232--9245 This journal is © the Owner Societies 2017 Cite this: Phys. Chem. Chem. Phys., 2017, 19, 9232 Electronic properties of reduced molybdenum oxides K. Inzani, a M. Nematollahi, b F. Vullum-Bruer, a T. Grande, a T. W. Reenaas b and S. M. Selbach* a The electronic properties of MoO 3 and reduced molybdenum oxide phases are studied by density functional theory (DFT) alongside characterization of mixed phase MoO x films. Molybdenum oxide is utilized in compositions ranging from MoO 3 to MoO 2 with several intermediary phases. With increasing degree of reduction, the lattice collapses and the layered MoO 3 structure is lost. This affects the electronic and optical properties, which range from the wide band gap semiconductor MoO 3 to metallic MoO 2 . DFT is used to determine the stability of the most relevant molybdenum oxide phases, in comparison to oxygen vacancies in the layered MoO 3 lattice. The non-layered phases are more stable than the layered MoO 3 structure for all oxygen stoichiometries of MoO x studied where 2 r x o 3. Reduction and lattice collapse leads to strong changes in the electronic density of states, especially the filling of the Mo 4d states. The DFT predictions are compared to experimental studies of molybdenum oxide films within the same range of oxygen stoichiometries. We find that whilst MoO 2 is easily distinguished from MoO 3 , intermediate phases and phase mixtures have similar electronic structures. The effect of the different band structures is seen in the electrical conductivity and optical transmittance of the films. Insight into the oxide phase stability ranges and mixtures is not only important for under- standing molybdenum oxide films for optoelectronic applications, but is also relevant to other transition metal oxides, such as WO 3 , which exist in analogous forms. 1 Introduction In order to utilize molybdenum oxide films, it is important to understand how their optical and electrical properties change with stoichiometry and crystal structure. Molybdenum oxide has applications in numerous optical and electronic devices, including organic light emitting diodes, photodetectors, gas sensors, photovoltaics, batteries and multi-chromic coatings. 1–9 The layered structure of MoO 3 has proved to be useful for creating many highly oriented nanostructures, such as two- dimensional flakes and belts, as well as nanorods, nanowires and nanoparticles. 3,10–18 These architectures are often seen in the morphology of thin films, where they can be exploited for their novel properties. 14,16–24 For such wide-ranging applica- tions, there exist many combinations of stoichiometry, crystal- linity and morphology. 6,25–29 In addition, phase mixtures of MoO x compositions with 2 o x o 3 can be present in films and devices. 30–34 Many reported devices rely on sub-stoichiometric MoO 3 , with mid-gap states necessary for device operation. 1,26,27,31,33,35–38 However, it is unlikely that oxygen vacancies remain as point defects, as ordering of oxygen vacancies has been observed at very low concentrations. 39 The crystal structures of molybdenum oxide phases can be split into three groups: those based on the layered MoO 3 structure, those based on the distorted ReO 3 structure, and MoO 2 which exists as a distorted rutile structure. The stable, orthorhombic polymorph of MoO 3 is built up of distorted MoO 6 octahedra which are edge and corner sharing in two directions, forming bilayers which are held together by dispersed interac- tions. The separation of the layers is known as the van der Waals gap. There are oxygen ions on three symmetrically inequivalent positions – O1, singly coordinated and pointing towards the van der Waals gap, O2, corner-sharing and O3, edge-sharing. Metastable structures based on crystallographic shear of MoO 3 have been observed in the reduction of MoO 3 . 40–43 These fall into the series Mo n O 3nm+1 , in which only Mo 18 O 52 has been produced in a pure form. 44 Upon further reduction, the lattice collapses into ReO 3 type structures with crystallographic shear planes. These are described by the Magne ´li series Mo n O 3n1 , for which Mo 9 O 26 and Mo 8 O 23 are known to exist. 45,46 Early calcula- tions based on a cluster model found that oxygen vacancy for- mation is more favourable when accompanied by crystallographic a Department of Materials Science and Engineering, NTNU Norwegian University of Science and Technology, N-7491 Trondheim, Norway. E-mail: selbach@ntnu.no; Tel: +47-73-59-40-99 b Department of Physics, NTNU Norwegian University of Science and Technology, N-7491 Trondheim, Norway Received 29th January 2017, Accepted 13th March 2017 DOI: 10.1039/c7cp00644f rsc.li/pccp PCCP PAPER Published on 13 March 2017. Downloaded by Universiteit Twente on 30/04/2017 21:40:28. View Article Online View Journal | View Issue