# Fabrication and Luminescent Properties of Zn-Cu-In-S/ZnS Quantum Dot Films under UV Excitation
## 1 Introduction  
Quantum dots (QDs) are inorganic semiconductor nanocrystals with sizes in the nanoscale range [1]. The size, shape, and consequently the number of electrons they contain can be accurately controlled by chemical processes [2]. During fluorescence stimulation, quantum dots are excited by radiation of specific energy and emit in different wavelengths, depending on their size [3; 4; 5]. Theirexcellent chemical and physical properties, such as the high energy conversion efficiency, the high extinction coefficient, tunable emitting wavelength, the broad excitation spectrum, and the resistance to photobleaching have attracted the interest of the scientific community providing quite impressive results [6; 7; 8; 9; 10; 11; 12]. QDs are 20 times brighter and 100 times more stable than traditional fluorescent indicators [13]. In the area of biology, because of their microscopic size and high penetrating ability, they can enter tissue and cells with ease. QDs make the long-term real-time monitoring and tracking of molecules and cells more feasible. During the last years, QDs have been employed in many areas such as solar cells, transistors, light-emitting diodes (LEDs), medicine and biology applications, as well as in quantum computing [14; 15; 16; 17; 18; 19; 20; 21; 22]. QDs that contain cadmium or mercury have been used in the above areas; however, for biomedical applications, QDs must meet additional criteria since toxic compounds are not allowed. High toxicity in the biological environment can destroy cells and tissues. European Union Directive 2011/65/EU \"Restriction of the use of certain hazardous substances\" (RoHS) [23] has set the limits of hazardous toxic compounds, such as cadmium or mercury, to protect humans as well as the environment. Under the prism of nontoxicity, cadmium-free ZnCuInS/ZnS QDs might be an interesting candidate for medical applications. Another important factor which dictated the use of ZnCuInS/ZnS QDs was the fact that they emit light in the green region of the optical spectrum, being compatible with the most common digital optical sensors that are utilized in biomedical instrumentation [24; 25; 26; 27; 28]. Multiple methods have been reported for the preparation of QD films [29; 30; 31; 32]. Previous work of our group introduced a simple method for the fabrication of composite films using a mixture of powder scintillators and PMMA. PMMA is a well-known thermoplastic which is widely used in biomedical applications. [28; 33; 34; 35]. The in vitro and in vivo biocompatibility of PMMA has been studied extensively, and its resins are broadly used in medicine [36]. In ophthalmology, intraocular lenses are made of PMMA, while in orthopedic surgery, PMMA is used as bone cement in joint replacements, screw fixation in bone, and filler for bone cavities and skull defects [36; 37]. Furthermore, in dentistry PMMA is used in dental prostheses [38; 39]. In this study, the luminescence efficiency of four ZnCuInS/ZnS/PMMA samples, prepared with the previously mentioned method, was evaluated for possible use in medical applications. Volume homogeneity as a measure of QD dispersion of the composite films was assessed using medical X-ray images.