# Formation of a Colloidal CdSe and ZnSe Quantum Dots via a Gamma Radiolytic Technique
## 1 Introduction  
Over the past two decades, semiconductor nanoparticles or quantum dots with sizes between 1 and 20 nm, have received much attention because of the quantum confinement effect of their surface electrons. In this dimension, the particles are in the transition between the bulk and molecular regimes and contain hundreds to a few thousand atoms only. The electronic, optical, and magnetic properties of quantum dots can be tailored by changing the material composition and varying the physical size [1; 2; 3]. In particular, group II-VI compounds such as narrow band gap CdSe quantum dots and wider band gap ZnSe quantum dots, have become intriguing materials prospects for the development of electro-optical, optical, and biomedical devices, such as light-emitting diodes [4], photo detectors [5], lasers [6], solar cells [7], and biomedical imaging applications [8; 9]. The method of synthesis and, to some extent, the local environment can influence the particle size and physical properties of quantum dots. An organic polymer containing hydrophilic functional groups, such as poly(vinyl pyrrolidone) (PVP) or poly(vinyl alcohol) (PVA), is often used as a capping agent to prevent uncontrolled growth and aggregation of the particles [10]. The particles are dispersed and suspended homogeneously in the colloidal solution [11]. Some of the techniques associated with the synthesis of colloidal CdSe and ZnSe quantum dots are chemical synthesis [12; 13], photochemicalsynthesis [14], micro emulsion [15], low temperature methods [16], and sol-gel methods [17], among others. Most of these methods have successfully produced particles with desired sizes or shapes and electronic or optical properties. Colloidal CdSe nanoparticles synthesized using the micro emulsion method have shown narrow particle sizes between 2.10 and 2.78 nm with band gaps of 2.8 and 1.8 eV, respectively [18]. Particles in the size range of 2.8-3.0 nm with a band gap range of 2.76-2.95 eV have been prepared by the chemical method [19]. The microwave radiation method can produce larger particles in the size range of 10 to 19 nm with a narrow band gap of approximately 1.79 eV [20]. CdSe nanoparticles with sizes of 43 nm and a narrow band gap of 1.75 eV were prepared by the hydrothermal method [21]. However, the narrow size ZnSe nanoparticles of approximately 4 nm with a band gap of 3.3 eV were fabricated using the microwave method [20]. ZnSe nanoparticles of 5.14 nm with a band gap of 3.2 eV have been prepared using the micro emulsion method [22]. Larger sized particles at 11.79 nm can be prepared by chemical methods with a band gap of 3.3 eV [23]. ZnSe nanoparticles with a size between 14.3 and 17.9 nm, and with a wider band gap up to 4.48 eV, have been synthesized using hydrothermal methods [24]. The gamma radiolytic method is another technique that can be used to produce colloidal nanomaterials [25; 26; 27; 28; 29; 30; 31; 32]. This technique was originally used for the synthesis of colloidal silver nanoparticles [25; 26], colloidal platinum nanoparticles [27], and colloidal nickel-aluminum and copper-aluminum alloy nanoparticles [28; 29], where the particle size of metal nanoparticles was found to decrease with increasing gamma radiation dose. This observation was explained by the competition between the nucleation and aggregation processes, where higher-dose nucleation dominates over aggregation, which allows for particle growth in which smaller particles are likely to be produce [30]. Recently, colloidal cadmium sulfide (CdS) and zinc sulfide (ZnS) quantum dots in aqueous PVA solution were fabricated using the same method [31; 32]. The particle size of colloidal CdS nanoparticles was found to increase from 13.31 nm at 10 kGy to 21.07 nm at 40 kGy with band gaps of 3.15 and 3.03 eV respectively [31].The particle size of the colloidal ZnS nanoparticles was also found to increase from 53 to 59 nm and the band gap decreased from 3.74 to 3.68 eV upon increasing the dose from 10 to 50 kGy [32].