Preparation and Optical Properties Assessment of CdSe Quantum Dots ()
1. Introduction
QDs (Quantum dots), also known as colloidal semiconductor nanocrytals, are generally composed of II-VI and III-V groups of table of elements. Their optical and electrical properties are strongly size dependent [1-4]. High quality semiconductor nanocrystals have many applications such as thin film light-emitting devices, non-linear optical devices, solar cells and life science [5-8]. On the other hand, most of chemical materials which used for their production are toxic, expensive, and even explosive.
Cadmium selenide (CdSe) can have both of solid hexagonal or cubic crystal structures with dark red appearance. It is an n-type semiconductor material with a band gap of 1.74 eV at 300˚K. The molecular weight of CdSe is 191.37g/mol where Cd is 58.74% and Se is 41.26% [1]. Bulk form of CdSe is not very interesting but CdSe nanoparticles are one of the most interesting semiconductors which many current researches have focused on their characteristics and applications. Researchers are concentrating on developing controlled synthesis of CdSe nanoparticles. It has useful properties for optoelectronic devices, laser diodes, nanosensing, biomedical imaging and high efficiency solar cells [1-4].
In this study, CdSe nanoparticles were prepared and the crystal structure, particles size distribution and optical properties of them were investigated. The optical properties of the CdSe nanoparticles were also assessed at different temperatures.
2. Experimental
2.1. Materials and Methods
CdSe nanoparticles were synthesized by chemical precipitation method [9]. For this purpose, three solutions of Cadmium Chloride (CdCl2·4H2O), Mercaptoethanol (ME) and Sodium Selenide (Na2SeO3·5H2O) were prepared in the distilled deionized water, under vigorous stirring (all from Merck Company). At first, CdCl2 solution was poured into a three spout balloon container and at the meanwhile, ME solution was added to the same balloon. Finally, Sodium Selenide solution was added to the balloon by the same way and under atmosphere control condition (N2). The resulting solution was washed by deionized water and then was centrifuged in order to remove any impurity aggregate. Then, the precipitated sample was dried at room temperature. All processes were done at room temperature [1].
2.2. Properties Assessment
The crystal structure and optical properties of CdSe QDs were characterized by XRD (X-ray diffraction, Bruker D8 ADVANCE λ = 0.154 nm Cu Kα radiation) [10] and UV-Vis spectrophotometer (ultra violet-visible, UV- 2600 Shimadzu, Japan). STM (scanning tunneling microscope, NATSICO Iran) were used for investigation of particles size distribution. The optical properties of the CdSe nanoparticles were also investigated at different temperatures from 10˚C to 70˚C.
3. Results and Discussion
3.1. Structure Analysis
The structure of the CdSe QDs was investigated by XRD. Figure 1 demonstrates the XRD pattern of the CdSe QDs. It can be seen that, the sample has single phase and also has the cubic crystal structure.
According to the standard JCPDS (Joint Committee on Powder Diffraction Standards) card No. 19-0191, the diffraction peaks correspond to the (111), (220) and (311) are the crystal plane. The mean size of the particles was determined by Debye-Scherer formula. It was calculated 2.4 nm for CdSe QDs [11].
Figure 2 demonstrates the STM photograph of the CdSe sample. The size was determined around 3 nm from STM photograph.