AVS 60th International Symposium and Exhibition
    Graphene and Other 2D Materials Focus Topic Thursday Sessions
       Session GR-ThP

Paper GR-ThP6
Characterisation of Mo Oxidation States in MoOx and MoSx Layer Materials Using X-ray Photoelectron Spectroscopy

Thursday, October 31, 2013, 6:00 pm, Room Hall B

Session: Graphene and Other 2D Materials Poster Session
Presenter: A.J. Roberts, Kratos Analytical Limited, UK
Authors: A.J. Roberts, Kratos Analytical Limited, UK
N.A. Fairley, CasaXPS, UK
B. Mendoza-Sánchez, Trinity College Dublin, Ireland
C. Moffitt, Kratos Analytical Limited, UK
Correspondent: Click to Email
The characterisation of layered materials has been dominated by graphene in recent years. There are however other examples of layered materials which retain stability down to a single monolayer and whose properties are complementary to graphene. Of these materials both MoS2 and α-phase MoO3 are of interest due to electronic and optical properties which may lead to applications as diverse as optoelectronics and light emitting devices for the former material or energy storage and catalysis for the latter. The chemical state and therefore electronic structure of these materials is readily characterised using x-ray photoelectron spectroscopy. In the case of MoO3, which may be reduced quite easily, there are a number of stable phases where the Mo becomes progressively reduced until reaching the stable dioxide with the formal oxidation state Mo(IV). Here we present a study of electrochemically reduced MoO3 where a mixture of lower valence oxides have been generated with concentrations varying as Mo(V) > Mo (IV) > Mo (VI) with MoO2 as the main component created below 0.185 V (vs Ag/AgCl). The stability of α-MoO3 under x-ray irradiation has also been studied with particular focus on the possibility of x-ray / in-vacuo reduction of Mo(VI) causing misinterpretation of the valance of the samples generated electrochemically. To complement the work on the oxide material we also present a study of the oxidation state of MoS2 where the Mo(IV) disulphide has been reduced in ultra-high vacuum by preferential sputtering of the sulfur. This relatively aggressive form of reduction leads to the creation of Mo(0) metallic-like molybdenum and demonstrates the extreme care required for in-vacuum sputter cleaning of layered materials.