4C07
Optical and Dynamic Properties of Water-soluble CdTe Quantum Dots
School of Science & Technology, Kwansei Gakuin University○A. Mandal, J. Nakayama, N. Tamai
Introduction: The study of semiconductor nanocrystals has received wide interest in past two
decades. Nanometer sized semiconductor particles and their size dependent physicochemical
properties are currently a vital field in various branches of chemistry, physics and biology. In
particular semiconductor quantum dots (QDs) are very attractive as biological labels, because oftheir small size, emission tunability, superior photostability and longer photoluminescence (PL)decay times in comparison with dyes. One of the major challenges is to obtain water soluble
QDs with a high PL quantum yields. The most interesting subject is to understand the size andshape dependence on optical properties of semiconductor nanomaterials. The single
chromophore optical microscopy and spectroscopic techniques have allowed us to study theoptical properties without the blurring effects of size distribution. These techniques have lead tothe discovery of new phenomena in optical properties of semiconductor nanoparticles, such as
blinking and spectral diffusion, that are not at all observed on the ensemble level. One of thetechnological challenges of dealing with quantum-size materials, both in synthesis andapplication, is to limit the size of growing clusters and to prevent them from interacting witheach other. If clusters do interact, they condense and collapse into the bulk solid. This
interaction must be prevented for the clusters to exhibit quantum confinement. In our case forsingle molecule measurements the quantum confinement has been done by around 1%
Trehalose or PVA polymer.
Experimental: All chemicals used were of the highest purity commercially available. Firstly,
highly luminescent CdTe QDs were synthesized in aqueous media at different pH. In a typicalsynthesis 2.35 mmol of Cd(ClO4)2.6H20 is dissolved in about 125 ml water and 5.7 mmol of thethioglycolic acid was added followed by adjusting the desired pH by addition of 1M NaOH
solution. Under this condition, H2Te gas is passed through the resulting solution with a slow N2flow for ~20 minutes. CdTe precursors are formed at this stage, which is accompanied byorange color. The size of the QDs growing is controlled by the duration of reflux time and caneasily be monitored by absorption and emission spectra. UV-Vis absorption andphotoluminescence spectra were measured using HITACHI U-3210 and Fluoro Max-2
spectrophotometer respectively. The fluorescence lifetimes were measured with a Picosecond
Ti:Sa cavity dumped laser (λex=410 nm, 2 MHz repetition rate) by TCSPC method. For single
particle measurements CdTe solution was diluted by 1% PVA. Spin coating solution wasprepared by 3000 rpm on a clean cover glass slip. Confocal microscopy is used to study
fluorescence intermittency of individual CdTe QDs excited at 524 nm.

Results and discussion: All samples show a well-resolved absorption maximum of the firstelectronic transition indicating a sufficiently narrow size distribution of the CdTe QDs, whichshifts to the longer wavelengths with increasing size of the nanocrystals. Emission maxima ofCdTe QDs increases with increase in particle size. Similarly average lifetime is increased byincrease in particle size.

Fig.1: Emission spectra (a) and decay profiles (b) of CdTe QDs in presence of acid and base.
The photoluminescence of TGA capped CdTe QDs were investigated before and after
addition of inorganic acid and base. By addition of dilute acid the PL intensity as well as
quantum yields of the CdTe is enhanced (Fig.1a). Intensity reaches maximum and thendecreases with further decrease of pH. By using time resolved study we observed that bylowering the pH of the solution average lifetime increases upto a certain pH (Fig.1b). Thechanges of fluorescence intensity were reversible when the pH of the solution is increased byusing NaOH solution. We observed that spectra are simply enhanced during the pH decrease but
the peak position of the emission remains fixed. This indicates that the changes of pH mainly
influence the surface and not the size of the QDs. Confocal microscopy is used to study
fluorescence intermittency of individual CdTe QDs. Under continuous excitation theluminescence emission switches `On` and `Off` by sudden stochastic jumps. The intermittency
was detected only in the single nanocrystal fluorescence experiments. In the ensemble, thisphenomenon is averaged out. The analysis of fluorescence intermittency has lead to theconclusion that such “blinking” events occurs due to discrete electron transfer events thatsequentially ionize and neutralize the QDs turning it “on” and “off”. Effect of environments on
luminescence properties of CdTe single QDs will be discussed on the basis of these results.