Synthesis Of CdSe@ZnS Quantum Dots Via Non-TOPO Hydrothermal Techniques G.Ramalingam†, J.Madhavan†*, R.Jerald Vijay†, M.Vimalan¶, P.Sagayaraj† †
Department of Physics, Loyola College, Chennai- 600 034, India. Department of Physics, S.T.Hindu College, Nagercoil - 629 002 India. *Corresponding author:E-mail:
[email protected],
[email protected]. ¶
Abstract. We describe the synthesis of CdSe@ZnS semiconductor QDs by simple hydrothermal technique without TOPO solvent. Transmission electron microscopy (TEM) consistent with nanocrystals contains a core of nearly monodispersive CdSe@ZnS of 4-7 nm diameters. Optical absorption and photoluminescence (PL) measurement as well as EDX demonstrated good quality of obtained nanocrystallites, and the QDs exhibit strong luminescence at room temperature. Keyword: Semiconductor compounds, Quantum dots, TEM, Photoluminescence. PACS: 71.20.Nr, 73.63.Kv, 68.37.Lp, S78.55.-m
The synthesis procedure of CdSe was already reported [1]. Similarly ZnS nanoparticles was synthesized using the same procedure by replacing Cd(NO3)2 instead of Zn(NO3)2 as source materials, and further coupling with CdSe and ZnS by adding, L-Cysteine, Hydrazine hydrate(N2H4) was added. Water used as solvent thought the experiment. Finally, the CdSe/ZnS colloidal nanoparticles were synthesized by hydrothermal techniques at 180 °C.
INTRODUCTION Colloidal semiconductor nanocrystals or quantum dots (QDs) have been the focus of much research effort in the past decade. The development of these colloidal QDs have allowed the concept of quantum confinement and dimensional control of electronic and optical properties to find entirely new areas of application, for instance in fluorescents labeling of biological specimens. At the single-particle level, colloidal QDs exhibit surprising behavior in their photoluminescence (PL) characteristics [1-4]. To obtain high-quality luminescent properties such as emission, color, color purity and stability etc; the surface structure of semiconductor nanocrystals must be strictly controlled. A Varity of methods have been employed to synthesize semiconductor QDs. These methods include the hot coordination solvent method using tri-n-octylphosphine oxide (TOPO) and tri octyl phosphine (TOP) which is identified as a prominent method to synthesize semiconductor QDs. However, this method requires high temperature greater than 250 °C and expensive materials. We have attempted to synthesize CdSe/ZnS QDs without using of TOP/TOPO as solvent. The QDs are characterized by TEM, EDAX, UV and PL studies.
TEM ANALYSIS TEM photograph of CdSe/ZnS Qds show that they are monodispersive QDs. The spacing between nanocrystals indicates that an organic (L-Cystine) matrial is likely to be capping of ZnS. A typical size distribution, estimated from the TEM images, and is shown in Figure 1. The average nanoparticles diameters is d=5 nm. The L-Cystine and thioactimanie is used for good capping ligands al well as starting materials of ZnS respectively. To be sure that our nanocrystals have distinct core-shell structure, we separated and purified nanoctystals before and after growing of the product. To achieve good reproducibility in the synthesis of core-shell nanoctystals, we paid special attentions to optimization of each preparation stage. A narrow size distribution of core-shell particles is strongly desirable to eliminate size selection procedures. The growth of ZnS shell on CdSe core is accompanied by
SYNTHESIS OF QDS
Solid State Physics, Proceedings of the 55th DAE Solid State Physics Symposium 2010 AIP Conf. Proc. 1349, 379-380 (2011); doi: 10.1063/1.3605893 © 2011 American Institute of Physics 978-0-7354-0905-7/$30.00
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significant broadening of particles size distribution. We found the parameter (L-Cysteine and N2H4, thioactimanie) affecting size distribution of colloidal nanoparticles play a vital role. To grow the shell, the molecular precursors (further referred to as “monomers/capping ligands”) are slowly added into a colloidal solution of core nanocrystals.
distribution and the controllable optical properties as a function of their size. The exciton peak has broadened the absorption edges of QDs is 515 nm (in Figure 4). And the lower emission at 550-650 nm with the absorption extinction is assigned to the metal-to-ligand charge transfer (MLCT) in the triplet manifold. For comparison, spectral assignment of CdSe@ZnS is straightforward, in which the lowest lying absorption, the absorption of TOPO capped CdSe@ZnS QDs is also depicted [5]; it exhibited and emission peak at ~580 nm [4]. Fluorescence spectra
FIGURE.1. TEM image of CdSe@ZnS QDs
The kinetics of shell growth can be limited either by diffusion of monomers towards the nanocrystal surface or by the rate of decomposition of monomers on the nanocrystal surface. Theoretical consideration predicts that diffusion-limited growth should always result in narrowing of the particle size [5]. The ring pattern- selected area electron diffraction (SAED) is shown in Figure 2.which confirms the QDs.
FIGURE.4. Absorption and luminescence spectra of CdSe@ZnS.
were obtained at room temperature using a JY Fluorolog-3-11Spectrofluorimeter, an excitation wavelength of 350 nm was used. The PL spectrum (figure. 4) shows the distinct narrow emission peak of the CdSe/ZnS QDs at 560 nm. This peak can be attributed to the PL can be separate “bare” ZnS nanocrystals, which are created in ZnS shell growth, their formation is promoted by the large lattice mismatch parameter which also induces the presence of surface defects at the core/shell interface[5].
FIGURE.2.SAED pattern of QDs
EDAX-Analysis
CONCLUSION
Chemical purity and stoichiometry of the samples were investigated by EDAX. Figure.3 indicates energy representative spectrum of the CdSe/ZnS nanoparticles. The strong peaks related to Cd, Se, Zn and S are found in the spectrum. The elemental atomic percentage of CdSe/ZnS is Cd (30.7 %), Se (15.7 %), Zn (30.6 %) and S (23.10 %).
We have demonstrated that it is possible to synthesize CdSe@ZnS QDs without using TOP/TOPO solvent. The results of TEM, EDAX and UV, PL studies reveal the size, composition and optical properties of CdSe@ZnS QDs.
ACKNOWLEDGMENTS The authors thank Prof. B.S.Murty, Department of Metallurgy and Materials Science, Indian Institute of Technology, Madras for TEM facility and for useful suggestions.
REFERENCES FIGURE.3. EDAX spectrum of CdSe/ZnS
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Ultraviolet –visible (UV-Vis) adsorption spectra were measured at room temperature on Cary 5E Uvvis spectrometer. Chemically synthesized CdSe@ZnS QDs exhibit nearly monodispersive size
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