Structural, Optical, and Electronic Properties of Ni_xCd_1–xS Quantum Dots: Implications for Photodetection Applications

X-ray diffraction (XRD) and transmission electron microscopy (TEM) have been used to study the structural and morphological characteristics of pure and Ni-doped CdS (CdS:Ni x ) ( x = 0–6 atomic %) QDs synthesized via the hydrothermal method. Inductively coupled plasma mass spectrometry (ICP-MS) and EDS measurements have been carried out for quantifying the elemental composition. Due to Ni-doping, the short-range structural disorder causes a significant variation in the intensity of the longitudinal optical (LO) modes of Raman spectra. Optical absorption has been broadened with Ni doping, resulting in a reduction of the band gap from 2.42 eV (for CdS) to 2.36 eV (for CdS:Ni6). Photoluminescence (PL) spectra show various peaks associated with surface defects, near band emission (NBE), sulfur vacancies, photoinduced charge carrier separation, and recombination processes. To investigate the chemical states and valence band spectra of Ni-doped CdS QDs, X-ray photoemission spectroscopy (XPS) has been utilized. To realize their applicability in electronic devices, bilayer heterostructure photodetectors (PDs) have been fabricated on the glass substrate by using CdS:Ni x QDs and sol–gel-derived SnO 2 (as a charge transport layer), viz., Glass/SnO 2 /CdS:Ni x QD PDs. The performance of the device has been improved with a Ni-doping concentration in CdS QDs. The optimized device has been achieved with high photocurrent (28.2 mA/cm 2 ), high figure-of-merit performance having a responsivity of 2.23 A/W, and detectivity of 2.1 × 10 13 Jones at a wavelength of approximately 450 nm under 5 V external bias for CdS:Ni6 QD heterostructure PD. Furthermore, this PD shows good response kinetics and are quite stable over time indicating its operational stability.