Fabrication and wetting characteristics of copper thin film: An active layer for SPR-based sensor applications

Abstract

In this work, a simple and two-step process was demonstrated to develop multifunctional Cu-based thin films that would be suitable for thin film photoactive devices. Cu thin films on quartz glass substrates were prepared using a sputtering technique followed by a thermal treatment. The samples were annealed at high temperatures such as 200, 400, and 600 °C for 2 h in a tubular furnace. Surface morphology and elemental composition were investigated using a high-resolution scanning electron microscope (FESEM) and SEM-aided energy dispersion spectroscopy (EDS), respectively. At high temperatures, the thin films were found to have clusters and voids. Detailed studies on optical properties such as ultraviolet–visible (UV–vis) absorptions, energy band gaps and Urbach energies have been carried out. A red shift in absorption edges (from 464 to 616 nm), a decrease in energy band gaps (from 2.38 to 1.54 eV) and an increase in Urbach energies (from 193 to 272 meV) were observed for those samples annealed at higher temperatures. Sessile drop tests were carried out to find the wetting contact angle (WCA) and to demonstrate the hydrophobicity of the thin film of pristine Cu and those treated at high temperatures. An approximate WCA of 71.9° was determined for the Cu thin film. Following treatment at 200 °C and 400 °C, respectively, the samples' surfaces increased in hydrophobicity by 92.4° and 85.2°. Nevertheless, the same thin film's WCA was decreased and its hydrophilicity increased during additional annealing. Cu-based thin films have been suggested as the active layer in an SPR sensor model, and the spectrum and angular resolved reflectance properties have been thoroughly investigated. At spectral wavelengths of 600, 700, and 800 nm, the optimum thickness of Cu thin film was determined to be 40 nm at SPR angles of 44.7°, 42.7°, and 42.15°.