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University of Africa Journal of Science University of Africa Journal of Science (U.A.J.S., Vol. 1, No.1, 90 -95) Physicochemical Stability of ZnS Quantum Dots Stabilized by Gum Arabic Hatim Mohamed El-Khair Dirar* Abstract ZnS quantum dots synthesized as soft matter utilizing Gum Arabic as stabilizer had exhibited quantum size effect, which is evidently observed from the significant blue shift in absorption and emission peak. Strong physicochemical stability has been attained and can be attributed to the perfect surface passivation done by Gum Arabic. Key words: ZnS Quantum Dots. Introduction The study of semiconductor quantum dots (QDs) has been advancing at rapid pace. Much of interest in II-VI compound materials such as CdS [Eychuler et al., 1991], CdSe [Huang et al., 1999] and ZnS [Hatim et al., 2002] stems from the fact that their optical properties can be systematically tuned by variation of their size. However optical properties of these QDs have been found to be very sensitive to the physical and chemical properties of their surfaces [Alivisatos,1996]. For example photoluminescence (PL) properties of bare structure QDs showed a detectable surface emission, which is attributed to the recombination of the carriers at the surface trap sites with different energies. Surface defect states such as dangling bonds and vacancies are the main sources of these trap states. These trap states can induce thermaland photo-instability when the QDs are subjected to thermal- and photooxidation, respectively. Upon subjecting CdS QDs to the thermal annealing at different temperature, rapid decrease in the intensity of PL was observed as the temperature increases [Eychuler et al., 1991]. *Department of Physics, School of Physics and Applied Physics, Faculty of Science and Technology, Al-Nelain University, Khartoum, Sudan. * Department of Physics, Faculty of Pure and Applied Science, International University of Africa, Khartoum, Sudan. University of Africa Journal of Science The increase of temperature enhances the nonradiative and radiative recombination at band edge and trap states, respectively. Semiconductor QDs coated with inorganic shells such as CdSe/CdS [Huang et al., 2000] and CdSe/ZnS [Peng et al., 1997] had exhibited enhanced band edge emission with highest quantum yields (>50%) [Huang et al., 2000]. QDs can also be passivated with long chain organic surfactant such as trioctylphosphine oxide (TOPO) [Browen Katari, 1994]. These capped QDs have room temperature PL quantum yields as high as 10% with very long fluorescence lifetime and often have some non-band-edge luminescence [Norris et al., 1994]. Slight changes in temperature can cause a large perturbation in optical characteristics of the QDs system and thus limit their applications. Our objective is to seek for a better way to improve the optical and thermal characteristics of ZnS QDs, by means of stabilization techniques. In this work we reported, for the first time, the enhancement of the band edge emission from ZnS QDs stabilized by Gum Arabic (GA). Such capping leads to highly stable robust structure ZnS QDs. As being depicted by the structural characterization. Materials and Methods Synthesis of ZnS QDs: The stabilized media are composed of an aqueous solution, well degassed and adjusted to a proper pH. The precursors fro Zn+2 and S-2 ionic species were chosen to be ZnNO3 and Na2S, respectively. The absorption and emission spectra for the given samples were measured by UV-Vis. 1200 and USB 2000 emission spectrophotometers, respectively. Physicochemical stability test is done by subjecting the direct illumination as well as oxidation for different time exposure and hence the stability has been ensured by the optical characterization techniques. Results and discussion The absorption edge of ZnS QDs depicted in figure 1, had exhibited strong blue shift band, with respect to the bulk ZnS, centered at nm. This shift is due to the quantum size effect (Alivisatos, 1996), which is detectable at the entirely measured size. The absence of a multiple transitions indicates the University of Africa Journal of Science change of band gap from indirect to direct. Such changes can prevent formation of assisted phonon and hence improving the quantum yield of ZnS QDs [Alivisatos, 1996]. Absorption spectra of ZnS QDs capped by GA Abs S1 S2 S3 S4 250 300 350 400 450 500 Wavelength (nm) Figure 1 shows the absorption spectra of ZnS QDs stabilized by Gum Arabic The emission spectra depicted in figure 2 had exhibited very narrow blueshifted emission spectra, which are ranging from 330 to 380 nm. Such shifts are due to the quantum confinement phenomena [Eychuler et al., 1991]. The single peak obtained is attributed to the near band edge emission. The absence of any red shifted emissions which are obtained in previous works and which might be due to the surface defect states is a good indicator for the removal of these defects[Hatim Mohamed et al., 2002]. Therefore, GA has proven to be a most perfect stabilizer compared to others [Huang et al., 2000]. University of Africa Journal of Science Figure 2 shows the emission spectra of ZnS QDs stabilized by Gum Arabic. Emission spectra from ZnS QDs shown in figure 3, represent the optical characterization for the set of samples before and after being exposed in an open and illuminated by sun light for 0, 15, 30 and 45 minutes exposure time. The features of the spectra have shown that, the emission peaks were blueshifted, with respect to the bulk ZnS, with steady state intensities for a chosen exposure time. The above spectral features indicate good photo-chemical stability; such stability can be attributed to the complete surface passiviation, and hence removal of defects caused by S vacancies act as radiative trap states with energy closed to the bandedge [William and Allen, 1983]. University of Africa Journal of Science Photo-chemical Stability for ZnS Capped GA Emission (a.u.) S3 S3 S3 S3 300 350 400 0 min 15 min 30 min 45 min 450 500 550 Wavelength (nm) Figure 3 indicates the Photo-chemical stability of ZnS QDs stabilized by Gum Arabic Conclusion ZnS quantum dots stabilized by GA had exhibited quantum size effect and highest photo-chemical stability. Such improved physical properties can be attributed to the perfect surface passiviation attained by Gum Arabic. References Alivisatos, A. P. (1996). J. Phys. Chem., 100 13226. Browen Katari J. E., Colvin V. L. and Alivisattos A. P. (1994). J. Phys. Chem. 98 : 4109-4117. Eychuler A. Hasselberth A, Kastikas L. and Weller H. (1991). J. of Luminescence, 48 & 49: 745. Hatim Mohamed, Kunji Chen, Ling Xu, Minghai Li and Xinfan Huang (2002). University of Africa Journal of Science Chinese Physics Letter, Vol. 19, No. 7: 967 Huang H.B., Xu L. Chen H.M., Huang X.F., Chen K.J. and Feng D. (1999). Acta Physica Sinica, 8 : 40. Huang X.F., Xu L., Zhu J., Chen H.M., and Chen K.J. (2000). J. of Material Science, 35 : 1375. Norris D. J., Sacara A., Murray C. B., Bawendi M. G. (1994). Phys. Rev. Lett. 72 : 2612. Peng X.G., Schlamp M.C., Kadavanich A.V. and Alivisatos, (1997). J. Am.Chem. Soc., 119 : 7019. William G. Becker and Allen, (1983). J. Bard, J. Phys. Chem., 87, 4888-4893.