Synthesis of Highly Photoluminescent CdTe Nanocrystals and Their Incorporation into Glass Matrices
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Synthesis of Highly Photoluminescent CdTe Nanocrystals and Their Incorporation into Glass Matrices Norio Murase,* and Chunliang Li Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan ABSTRACT Photoluminescence (PL) efficiency of thioglycolic-acid (TGA)-stabilized CdTe nanocrystals in water depends strongly on the amount of TGA upon synthesis. The less the amount, the higher the PL efficiency obtained if the colloid is not precipitated. When the molar ratio of TGA against Cd2+ upon preparation is ca. 1.3, PL efficiency of as-prepared solution becomes the maximum and exceeds 60% in red-emitting region. In this case, the spectral widths of the PL become the minimum. We have encapsulated these highly luminescent nanocrystals into glass matrices by using a sol-gel method. Silane coupling agent having an amino-group is effective to preserve the surface of the nanocrystals. The glass thus prepared is stable and shows PL efficiency as high as 40%. INTRODUCTION Colloidal syntheses of semiconductor nanocrystals have attracted much attention because of their size-dependent excitonic emissions. Since aqueous syntheses have an advantage of their simplicity when compared with that of organometalic routes [1], we have selected TGA-stabilized CdTe. [2, 3] The amount of surfactant gives a significant effect on PL efficiency in the case of colloidal synthesis of ZnSe.[4] Since the ratio of TGA against Cd2+ upon preparation has been traditionally fixed to ca. 2.4, the effect of amount of TGA on the PL efficiency of CdTe nanocrystals becomes a matter of our interest. In this report, a highly photoluminescent CdTe nanocrystals were prepared by optimizing an amount of surfactant (TGA). After that, an incorporation of them into glass matrices is demonstrated by considering the surface condition of the nanocrystals. A kind of silane coupling agent was found to be effective for the preservation of the surface. The prepared glass was very transparent and fragile like a cover glass. Their PL efficiency was maintained at least 8 months. It is therefore fair to expect that it can easily be used for developing devices such as solid state lightings.
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EXPERIMENTAL CdTe nanocrystals in water were prepared based on a previously reported method.[5] Typically, 1.095 g (2.61 mmol) of Cd(ClO4)2•6H2O was dissolved in 200 mL of pure water (Milli-Q, Synthetic grade), and TGA was added at 6 molar ratios between 1.00 and 2.43 against Cd2+. Then, the pH value of the solution was adjusted to 11.4 by dropwise addition of aqueous NaOH solution. After the solution was saturated with Ar, H2Te gas generated from the reaction between Al2Te3 and H2SO4 was passed through the solution. The molar ratio of Te2- to Cd2+ was fixed at 0.47 in all the experiments. Clusters were formed quickly after the introduction of H2Te. CdTe nanocrystals grew gradually by refluxing the solution at ca. 100 °C. When the solution was turbid after some period of reflux, it was
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