Nanocrystalline CuO Thin Films for H<sub>2</sub>S Monitoring: Microstructural and Optoelectronic Characterization

Abstract

Nanocrystalline copper oxide (CuO) thin films were deposited onto glass substrates by a spin coating technique using an aqueous solution of copper acetate. These films were characterized for their structural, morphological, optoelectronic properties by means of X-ray diffraction (XRD) scanning electron microscopy (SEM), UVspectroscopy and four probe method. The CuO films are oriented along (1 1 1) plane with the monoclinic crystal structure. These films were utilized in H 2 S sensors. The dependence of the H 2 S response on the operating temperature, H 2 S concentration of CuO film (annealed at 700˚C) was investigated. The CuO film showed selectivity for H 2 S. The maximum H 2 S response of 25.2% for the CuO film at gas concentration of 100 ppm at operating temperature 200˚C was achieved.

Physical Properties of Copper Oxide Thin Films Prepared by Sol–Gel Spin–Coating Method

“…Our results corroborate those found in the literature [18,41]. Dattarya et al [30] ascribed this shift in energy band gap, e.g., to the increase in crystallites size and the reduction of the amorphous phase for the annealing CuO films. In this case, the mean crystallite size increases from 15.8 nm to 32.2 nm after annealing from 350°C -550°C.…”

“…These results are in accordance with the results of previous works. [30]. This improvement can be attributed to the coalescences of grains at higher annealing temperature which is confirmed by the decreasing of the full width at half maximum of peaks in the XRD patterns.…”

“…Table 1 lists the band gap values for all samples, which show a decrease of the band gap from 3.5 to 2.44 eV with the annealing temperature, which is related to the quality of the film due to the annealing out of the structural defects [30] and to the increase crystallites size. Our results corroborate those found in the literature [18,41].…”

Ultrafast Charge-Carrier Dynamics of Copper Oxide Nanocrystals

“…This measured bandgap is within the range seen in the literature for CuO. [12][13][14][15][16][17] There is also a notable Urbach tail, extending out to lower energies, which is characteristic of optical absorption via defect states in disordered semiconductor media. 24 The authors note that the bandgap for CuO nanocrystals will, in general, depend on the nanocrystal size.…”

“…A major challenge to the understanding of CuO is the nanocrystalline morphology that the material typically forms. The differing nanocrystal sizes and phases of the nanocrystalline morphology have led to disparities for the optical absorption characteristics of the CuO nanocrystals, with reports of an indirect bandgap of approximately 1.2 eV , and/or a direct bandgap ranging from 1.3 to 3.0 eV. ,− The nanocrystalline morphology also leads to complexities in the evolution of the ultrafast charge-carrier dynamics within the CuO nanocrystals, due largely to midgap trap states within the bandgap created by the nanocrystals’ large surface area. Therefore, an accurate understanding of the fundamental charge-carrier trapping and relaxation dynamics in CuO nanocrystals is essential to future applications of CuO nanocrystals.…”