Phase-dependent structural and electrochemical properties of single crystalline MnS thin films deposited by DC reactive sputtering

Abstract

Single crystalline α- and γ-MnS thin films have been deposited on Si and ITO substrates by reactive DC sputtering (Ar:H2S 2:1) of a manganese target for electrochemical energy storage application. We found that working pressure was one of the major parameters while optimizing the crystallinity of thin films, whereas the phase tuning (γ to α) was primarily controlled by temperature variations. The temperature was varied from RT to 450 °C, keeping the gas pressure constant at 10 mTorr optimized value, resulting in a transition between two different polymorphs of MnS as confirmed by XRD results. AFM and contact angle measurements were also performed to study the surface roughness, wetting properties, and surface energy calculations of prepared thin films. α-MnS films prepared at 400 °C were found to have a maximum contact angle of 118° and a minimum free surface energy (γSV) of 8.38 mN/m. Moreover, we have also studied the phase dependent electrochemical properties and found that γ-MnS thin films prepared at ambient substrate temperature displayed the highest specific capacitance of 178.3 F/g at a scan rate of 5 mV/s with superior charge-discharge rates in neutral electrolytes. As the substrate temperature was increased to 300 °C, we observed a continuous decrease in the respective specific capacitance values, and α-MnS electrodes were found to have a minimum specific capacitance of 120 F/g. The enhanced electrochemical performance of γ-MnS thin films can be attributed to the superior water interacting properties (θw = 90.4°) and its wurtzite structure, which enables easy penetration of electrolytes into the active materials.