Molybdenum oxide 2-D flakes: role of thickness and annealing treatment on the optoelectronic properties of the material

Molybdenum oxide (MoO_x) thin films were grown by using a simple, economic and scalable DC Cressington sputtering unit. For thickness measurement with the quartz balance, during the deposition, MoO_x density was set to an average value of 5.58 g/cm³ which is the mean density of the two main oxidation states, MoO₂ (6.47 g/cm ³) and MoO₃, (4.69 g/cm ³), respectively. Afterward, as-deposited samples were annealed in air at 400 °C for 60 min. Then onwards, X-ray diffraction (XRD), field emission scanning microscopy, atomic force microscopy (AFM), UV–visible (UV–Vis) spectroscopy and photoconductivity studies were carried out to investigate the differences appearing in the structural, morphological and optoelectronic properties of the as-deposited and annealed samples. Using AFM and SEM, it was observed a decrease in the average roughness as well in the grain size for thicker samples (after annealing), whereas contrary behavior was found for thinner samples. Presence of α-MoO₃ and sub-oxide phases such as β-MoO₂ and γ-Mo₄O₁₁ with predominance of MoO₂ was observed before annealing. XRD and XPS analysis revealed a better crystalline structure for annealed samples in which the MoO₃ phase was observed in majority. Depending on the thickness and oxidation states, the electrical behavior of the samples was found to be varied from semimetal to semiconductor. After annealing, all samples shown a semiconductor behavior, with an increase in photoconductivity response for the films with a lower band gap. The aim of the present work is to provide useful information over the control of the morphology and functionality of the thin films with a view to their use for future prospective application especially for the fabrication of hole transport layer in the new generation of solar cells.