## 1 Introduction  
Nanocrystalline transition metal oxide semiconductors have attracted enormous interest in recent years, both from a scientific and industrial point of view. Within the oxides of transition metals, copper oxides (CuO and Cu\({}_{2}\)O), which are inexpensive, \(p\)-type semiconductors, have received substantial consideration due to their exceptional physical and chemical properties [1]. Their reported optical bandgaps range between 2.1 and 2.6 eV for cuprous oxide (Cu\({}_{2}\)O) and between 1.2 and 2.1 eV for cupric oxide (CuO), respectively. A metastable copper oxide (i.e., paramelaconite, Cu\({}_{4}\)O\({}_{3}\)), an intermediate compound between the previous two, has also been reported. Due to their non-toxicity and abundant availability, they represent advantageous and promising materials for device applications. Since copper is multivalent, it forms several stoichiometric and/or non-stoichiometric oxides. Recent publications on copper oxides highlight the current level of knowledge concerning electrical and optical features, as well as potential, advanced applications. CuO is considered a \(p\)-type semiconductor owing to the existence of acceptor levels attributed to copper vacancies, the charge compensation of which can either be explained by the formation of an equivalent concentration of Cu\({}^{3+}\) or by the presence of impurities. Thus, electron correlation effects and spin dynamics are among their many potentially useful physical properties. CuO films are extensively used not only in numerous devices such as high-temperature superconductors, since the superconductivity of these materials is related to Cu-O bonds, but also in thin film transistors, smart windows, optical limiters, photoelectric and photovoltaic applications, sensors for infrared (IR) radiation and other high-tech modules. Moreover, CuO is also used in various chemical applications, including as a catalyst material due to its high chemical activity and selectivity in redox (reduction and oxidation) reactions [2] and is a catalyst system of importance for industrial applications such as the synthesis of methanol and in the production of rayon. It is used in various mechanical applications, as a pigment in ceramics and even as a dietary supplement. It is an excellent photocathode for photoelectrochemical reactions [3; 4; 5; 6; 7; 8], including water separation [9; 10], the removal of arsenic and organic pollutants from wastewater [11] and in practical energy storage/conversion/anodes in Li-ion batteries and other dry/wet cell applications [12]. In nanoscale form, CuO has also been extensively studied in the literature, with potential use as a nanofluid material for heat transfer applications. Additionally, in nanoparticulate form, it has been reported to exhibit antimicrobial activity, as it can be synthesized with uncommon crystal morphology and/or exceptionally high surface area; for example, particles ranging between 20 and 95 nm exhibit a mean surface area of \(\sim\)16 m\({}^{2}\)/g. Additionally, as quantum dots (QD), these particles exhibit higher sensitivity in electrochemical applications than their micro-counterparts do. Due to its specific chemical and physical features, CuO is used as a standard material in understanding various chemical and/or physical nanomaterial properties that are generated by certain transformations and morphology, i.e., mainly due to size reduction and/or doping, it is one of the few metal oxide \(p\)-type semiconductors that have been reported to have an indirect bandgap and a direct bandgap, depending on process parameters [11]. Thin CuO films can be produced by annealing Cu films under controlled vacuum or in air: the annealing environments yield mixed phases of copper oxide exhibiting variable physical properties. Reports in the literature confirm that annealing improves optical transmittance by 55% on the visible spectrum, as well as electrical mobility to 47 cm\({}^{2}\)/Vs.