Abstract
An environmentally friendlier solution processing has been introduced to fabricate zirconium oxide (ZrO2) films on quartz substrates, using spin coating of simple water-based solution. The films cured with UV-A = 330 nm for different times (40, 80, 120 min) were investigated for structural and optical properties and compared with thermally annealed film (at 350 °C). XRD and Raman spectroscopy showed amorphous structure in all the samples with no significant phase transformation with UV-A exposure. AFM microscopy showed smooth and crack free films with surface roughness ≤2 nm that reduced with UV-A exposure. Ultraviolet-visible (UV–Vis) spectroscopy demonstrated optical transmittance ≥88% and energy band gap variations as 4.52–4.70 eV. Optical constants were found from spectroscopic ellipsometry (SE). The refractive index (n) values, measured at 470 nm increased from 1.73 to 2.74 as the UV-A exposure prolonged indicating densification and decreasing porosity of the films. The extinction coefficient k decreased from 0.32 to 0.19 indicating reduced optical losses in the films under the UV-A exposure. The photoluminescence (PL) spectra exhibited more pronounced UV emissions which grew intense with UV-A exposure thereby improving the film quality. It is concluded that UV-A irradiation can significantly enhance the optical properties of ZrO2 films with minimal changes induced in the structure as compared to thermally treated film. Moreover, the present work indicates that water-based solution processing has the potential to produce high-quality ZrO2 films for low cost and environmental friendlier technologies. The work also highlights the use of UV-A radiations as an alternate to high temperature thermal annealing for improved quality.
Highlights
Zirconia (ZrO2) has emerged as an excellent material due to its unique properties such as wide band gap (3–7.8 eV) [1,2], high level stability, tuneable conductivity, and high refractive index, making it suitable for high performance bio medical [3], sensing [4,5], catalyst [6] and many more applications
We do not observe any crystallization peak in the XRD pattern of ZrO2 film whether it is treated thermally (Z-TA) or with UV-A, appearance of a broad hump/band approves that all the films are amorphous
That the hump is centered at 2θ~25°, which corresponds to monoclinic -ZrO2 reflections (011) and (110) [25,26] and demonstrates that the short range order may be emerging towards the monoclinic [27] phase as indicated by the rise in the hump’s intensity with prolonged UV-A exposure, which on the other hand allows the ZrO2 films to maintain the amorphous structure
Summary
Zirconia (ZrO2) has emerged as an excellent material due to its unique properties such as wide band gap (3–7.8 eV) [1,2], high level stability (chemical, thermal, and electrical), tuneable conductivity, and high refractive index, making it suitable for high performance bio medical [3], sensing [4,5], catalyst [6] and many more applications. A different approach that can decrease the curing temperature of solution-processed oxide films and improve the properties, has been developed and has gained increasing interest because the modern era of technology is heading towards transparent and flexible applications which require low temperature processing. The paradigm of heat treatment has changed in due course of its development to low temperature annealing [15] and even better the ultraviolet (UV)/photo annealing [10,11,16,17] The latter approach has emerged as a promising alternative to high temperature treatments because the film surface can be directly processed, without heating, using appropriate energy of light which can enhance the ionic diffusion in the oxide films as well [18]. UV radiation exposure of films is done to improve the bonding of films and it is assumed that no oxygen will be lost during the exposure
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