|
چکیده
|
Nanostructured materials especially II-VI semiconductors have become a subject of intensive research for their extraordinary properties compared to their bulk counterparts [1]. Blue shift of the optical absorption spectrum, size dependent luminescence, enhanced oscillator strength, and nonlinear optical effect are some examples of the interesting properties exhibited by these nanoparticles. All these properties are various manifestations of the so called quantum size effect which arises due to the increasing quantum confinement of the electrons and holes with diminishing size of the crystallites and the consequent charges in the electronic structures. CdS Nanoparticles were synthesized from solution containing Cd(ClO4)2.6H2O (0.02 molL-1), urea (0.25 molL-1), and thioacetamide (0.02 molL-1) to obtain 100 ml solution. Finally, the pH of solution was adjusted to 4.0 by drop wise addition of 2 molL-1 HCl solution. The solution was maintained for 5 h at room temperature (25 ºC). The solution turned light yellow after 30 minutes, visually manifesting the initial formation of CdS nanoparticles and remained optically clear.The CdS nanoparticles solution shows a well-resolved absorption maximum of the first electronic transition indicating a sufficiently narrow size distribution of the CdS nanoparticles, which shifts to the shorter wavelengths with decreasing size of the nanoparticles as a consequence of the quantum confinement. The value of the energy band gap, Eg, was calculated from the UV-Vis spectrum by plotting (αhυ)2 versus hυ, and was 3.2 eV. α, h, and υ signify absorption coefficient, Planck constant, and frequency, respectively. Our band gap value of the CdS nanoparticles is somewhat larger than the typical value of the bulk CdS (ca. 2.42 eV); probably due to the quantum size effect as expected from the nanosized nature of the CdS nanoparticles [2]. The crystallinity of the CdS nanoparticles was nvestigated by X-ray diffraction (XRD. In this work, CdS nanoparticles can be indexed as cubic (sphalerite) in structure, showing (111), (220) and (311) broadened peaks [3]. This behaviour is perhaps due to the presence of urea molecules on the CdS nanoparticles surface. The Average nanoparticle size estimated from the Debey-Scherrer formula is about 28 nm. TEM image of the CdS nanoparticles supports the nanocrystallite size estimated from the XRD data. The characteristic CdS nanoparticles are observed with a relatively narrow particle size distribution (12-50 nm range).
|