Abstract
Structural changes of MoO3 thin films deposited on thick copper substrates upon annealing at different temperatures were investigated via ex situ X-Ray Absorption Spectroscopy (XAS). From the analysis of the X-ray Absorption Near-Edge Structure (XANES) pre-edge and Extended X-ray Absorption Fine Structure (EXAFS), we show the dynamics of the structural order and of the valence state. As-deposited films were mainly disordered, and ordering phenomena did not occur for annealing temperatures up to 300 °C. At ~350 °C, a dominant α-MoO3 crystalline phase started to emerge, and XAS spectra ruled out the formation of a molybdenum dioxide phase. A further increase of the annealing temperature to ~500 °C resulted in a complex phase transformation with a concurrent reduction of Mo6+ ions to Mo4+. These original results suggest the possibility of using MoO3 as a hard, protective, transparent, and conductive material in different technologies, such as accelerating copper-based devices, to reduce damage at high gradients.
Highlights
Molybdenum-based oxides are amongst the most adaptable and functional oxides due to their unique characteristics and tunable properties [1,2,3]
From the analysis of the X-ray Absorption Near-Edge Structure (XANES) pre-edge and Extended X-ray Absorption Fine Structure (EXAFS), we show the dynamics of the structural order and of the valence state
To establish the optimal annealing temperature for an ordered MoO3 film on a copper substrate, we investigated the structural evolution of annealed MoO3 films by X-ray Absorption Spectroscopy (XAF) in X-ray Absorption Near-Edge Structure (XANES) and in the Extended X-ray Absorption Fine Structure (EXAFS) regions [17,18,19,20]
Summary
Molybdenum-based oxides are amongst the most adaptable and functional oxides due to their unique characteristics and tunable properties [1,2,3]. MoO3 coating can significantly enhance the electronic and mechanical properties of copper-based devices via its high WF and relatively higher hardness, compared with copper [5,13,14,15], without affecting the surface conductivity. Because the thermal evaporation deposition produces disordered and poorly adhesive MoO3 coatings [5], with the aim to obtain ordered MoO3 coatings, we tried to optimize the annealing procedure. This method minimized the formation of MoO2, which must be avoided, since the slightly off-axis position of Mo atoms in the MoO2 phase causes the lowering of the WF (~4.6 eV) [3,16]. To establish the optimal annealing temperature for an ordered MoO3 film on a copper substrate, we investigated the structural evolution of annealed MoO3 films by X-ray Absorption Spectroscopy (XAF) in X-ray Absorption Near-Edge Structure (XANES) and in the Extended X-ray Absorption Fine Structure (EXAFS) regions [17,18,19,20]
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