Photoluminescence Properties and Zeta Potential of Water-Dispersible CdTe Nanocrystals
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Photoluminescence Properties and Zeta Potential of Water-Dispersible CdTe Nanocrystals Masanori Ando, Chunliang Li and Norio Murase Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Kansai Center, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, JAPAN ABSTRACT The photoluminescent (PL) properties and zeta potential of green-emitting CdTe nanocrystals (diameter: 3 nm) capped with a stabilizing surfactant, thioglycolic acid (TGA), have been investigated as a function of pH of the aqueous solution. The green PL intensity reached the maximum at pH5.1 and was somewhat lower in the pH range of 6-10, which was similar to the previously reported result. However, when the pH was at and below 4, the green PL intensity decreased drastically. The relative ratio of the dissociation form of the carboxyl group of TGA showed a large diminution at and below pH5 accompanied by a significant decrease of the absolute value of zeta potential. Since the absolute value of zeta potential reflects the stability of nanocrystals, the results obtained shows that the TGA-capped CdTe nanocrystals are stable only in basic to neutral regions and that the agglomeration of the nanocrystals in acidic range reflects the transition from the dissociated (charged) form to the non-dissociated (non-charged) form of a carboxyl group in TGA. Encapsulation of nanocrystals in glass is a promising way to further improve the long-term photostability of nanocrystals. Therefore, we chose an alkoxide having an amino group for a matrix for the encapsulation. The amino group has a good affinity to TGA as well as promotes the sol-gel reaction. As the result, the CdTe nanocrystals have been dispersed finely in the glass matrix without a deterioration of PL intensity.
INTRODUCTION Over the years, semiconductor nanocrystals (quantum dots (QDs)) attract much attention due to their strong, size-dependent photoluminescence (PL) [1-3]. Many possible applications are expected ranging from optical, optoelectronic to biological fields [4-6]. To date, two solution based syntheses of QDs have been reported. The first method is a non-aqueous solution based one which utilizes a thermal decomposition of organometallic compound, and trioctyl phosphine is used as a hot coordinating solvent [1,7]. Hydrophobic QDs are obtained by this method. The second method is an aqueous solution based one in which hydrophilic QDs are prepared by using organic capping surfactants such as thioglycolic acid (TGA) [2,8]. The QDs synthesized by the second method usually retain their PL for longer time in ambient than the QDs prepared by the first method. Since the PL intensity depends on the pH of the solution [2], we have measured zeta potential as an indicator of the stability of particles in solution. As the result, we have found that the transition between the dissociated form and nondissociated form of carboxyl group of the stabilizing surfactant plays an important role in the diminution of PL intensity and stability of QDs in acidic region. In
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