These materials exhibit quantum confinement effects when their sizes are comparable with the diameter of the bulk exciton, leading to split the continuum of electronic energy levels into discrete states with an effective band gap blue shifted from that of the bulk [7][8][9].There are two different synthetic methods for producing CdTe nanostructures, the organometallic route and synthesis in an aqueous solution [10][11][12][13]. The organometallic method usually requires hard conditions such as high temperature, toxic reagents and solvents, and a rigorous process for protecting synthesized nanostructures from oxidation, while the aqueous synthesis method requires mild conditions and is a simpler one [14,15]. Compared with organometallic route, aqueous synthesis has many advantages such as biocompatibility, water solubility, environmental friendliness, and low cost [16]. The aqueous synthetic approach allows the resulting nanostructures to be tuned in terms of their size, shape, and structure on the nanometer scale [17]. The capping agent is one of the most important factors in the synthesis of nanostructures. Some of the commonly used methods for surface passivation include protection by self-assembled monolayers, the most popular being thiol-functionalized organics; encapsulation in the water pools of reverse microemulsions; and dispersion in polymer matrixes [18]. The thiol-functionalized organic compounds are highly reactive chemically and pose potential environmental and biological risks. It is well known that capping ligands attached on the nanostructure surface have strong effects on the size, morphology, and compatibility of nanostructures with different chemical mediums [19]. Therefore, it is of great importance to have a proper selection of appropriate capping ligands, which would lead to the desired particle structures. Sodium citrate is one of the most common agents used in the synthesis of metallic and semiconductor nanostructures because Abstract The CdTe nanocrystals were successfully synthesized using a facile room temperature method with non-toxic and environmentally friendly capping agent. The obtained CdTe nanocrystals were characterized by UV-Vis absorption spectroscopy, transmission electron microscopy, and z-scan technique. A photoluminescence peak with full width at half maximum of around 15 nm could be obtained, which indicates a homogeneous and narrow size distribution for the CdTe nanocrystals. The magnitude and sign of the third-order nonlinear refractive index (n 2 ) and nonlinear absorption coefficient (β) of the CdTe nanocrystals were determined by using of the closed-and open-aperture z-scan at different intensities, respectively. The negative sign of n 2 indicated that there is a self-defocusing effect in the sample. The nonlinear refractive index, n 2 , and the nonlinear absorption coefficient, β, were in order of 10 −7 cm 2 W −1 and 10 −2 -10 −3 cmW −1, respectively.
