Optical properties of CdS nanocrystalline thin films in the abrupt phase transition from zinc blende to wurtzite
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Optical properties of CdS nanocrystalline thin films in the abrupt phase transition from zinc blende to wurtzite C. G. Torres‑Castanedo1 · J. Márquez‑Marín2 · R. Castanedo‑Pérez2 · G. Torres‑Delgado2 · M. A. Aguilar‑Frutis3 · S. Arias‑Cerón4 · O. Zelaya‑Ángel5 Received: 30 March 2020 / Accepted: 8 August 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract A set of cadmium sulfide (CdS) thin films was grown on glass substrates by the chemical bath deposition technique at different bath temperatures (Tb). A microwave oven was used to heat the precursor aqueous solution employed to prepare the films in the 60–97 °C interval. The average crystallite size of the CdS films lies in the 7–20 nm range, calculated from X-ray diffraction data. The diffraction patterns reveal that the crystalline structure of CdS nanoparticles is cubic zinc blende (ZB) for 60 ≤ Tb ≤ 93 °C, hexagonal wurtzite (WZ) for 95 ≤ Tb ≤ 97 °C, and ZB-WZ mix of phases for the critical temperature Tb ≅ 94 °C (Tbc). The mixture of both phases is supported by Transmission Electron Microscopy. The CdS films show preferred orientation along (111) and (002) directions for ZB and WZ, respectively. The optical properties reveal significant changes at Tbc, namely, the energy band gap, photoluminescence emission, and refractive index. The photoluminescence results show an additional band at the critical phase transition temperature due to the presence of a high-density Cd interstitial/vacancies, produced by the mix of phases. Furthermore, high-energy transitions above the conduction band also exhibit splitting due to the phase transition.
1 Introduction The II–VI compound semiconductors are widely studied due to their promising applications in nano-dimensional structures. Cadmium sulfide (CdS) is a semiconductor compound that was initially investigated as a fluorescent material for image reproduction [1, 2]. Nowadays, CdS is widely employed in solar cells, optoelectronic devices, lasers, gas sensors, biological, and pharmaceutic products [3–7]. * O. Zelaya‑Ángel [email protected] 1
Depto. de Física, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Mexico 07360, México
2
Unidad Querétaro, Centro de Investigación y de Estudios Avanzados del I.P.N., 76230 Querétaro, QRO, Mexico
3
Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada del I.P.N., 11500 Mexico, Mexico
4
Depto. de Ingeniería Eléctrica, SEES, Centro de Investigación y de Estudios Avanzados del I.P.N., 07360 Mexico, Mexico
5
Facultad de Ingeniería, Universidad Autónoma de Querétaro, Centro Universitario, 76010 Querétaro, QRO, Mexico
Recently, Li et al. [8] published a review on the nanostructured cadmium sulfide as a promising semiconductor for profitable optoelectronic devices. According to this report, CdS nanocrystals can be utilized to prepare functional lasers, waveguides, photovoltaics, among others. There has been an increasing interest in nanostructured CdS due to a wide variety of industri
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