The impact of morphological structure and flexo-chemical strains on the electric transport mechanisms in molybdenum-disulfide-oxide nanoflakes

Electric conduction mechanisms in pressed powder samples consisting of molybdenum-disulfide-oxide (MoS<i>_x</i>O<i>_y</i>) nanoflakes depending on their content and structure have been investigated. The MoS<i>_x</i>O<i>_y</i>nanoflakes were synthesized in the temperature range of 130 °C-180 °C by reaction of (NH₄)₆Mo₇O₂₄with thiourea in aqueous solution followed by aerial oxidation. The chemical composition and structure of the powders have been determined by means of XPS, EDS and Raman spectroscopy. The obtained nanoflakes are 10-20 nm thick and self-assembled into 'nanoflower'-shaped agglomerates forming powder particles. The agglomerates in powders synthesized at different temperatures are shown to consist of MoS₂and molybdenum oxides/sulfoxides whose content ratios differ from each other in powders depending on their synthesis temperature. The current versus voltage (<i>I-V</i>) dependences of the pressed powder films manifest a hysteresis-like behavior with substantial dependence on this ratio. For the samples with the highest content of the Mo oxide/sulfoxide nanoflake forms (⩾50%) one observes the negative differential conductivity (NDC) in the<i>I-V</i>characteristics and a very large difference between the forward and backward<i>I-V</i>branches at small DC biases. However, the samples with low content of these forms have slightly non-linear<i>I-V</i>characteristics, narrower hysteresis loops and the absence of NDC. All samples manifest a long-lasting (tens of seconds) transient charge/discharge process after switching 'on/off' the voltage across the sample and ability of large charge accumulation, with the specific capacitance equal to achieving 12 ÷ 75 F g^-1depending on the powder synthesis temperature. These phenomena provide evidence of the important role of interface charges in the MoS<i>_x</i>O<i>_y</i>powder electric conduction mechanisms. To describe theoretically the observed<i>I-V</i>curves, polar and electric-transport properties of the pressed MoS<i>_x</i>O<i>_y</i>nanoflake films, the Landau-Cahn-Hilliard approach considering flexo-chemical field has been used. The revealed features of electric conduction and charge accumulation look interesting for possible applications in nanoelectronics and charge storage devices.