ja048222a_si_001.mpg (1.32 MB)
Luminescence Temperature Antiquenching of Water-Soluble CdTe Quantum Dots: Role of the Solvent
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posted on 2004-08-25, 00:00 authored by Sander F. Wuister, Celso de Mello Donegá, Andries MeijerinkLuminescence temperature antiquenching (LTAQ) is observed for water-soluble CdTe quantum
dots (QDs) capped with aminoethanethiol (AET). The efficient exciton emission (quantum efficiency of
∼40% at 300 K) is quenched almost completely as the QD solutions are cooled to below 230 K and is fully
recovered around 270 K upon warming up to room temperature (LTAQ). Temperature-dependent lifetime
measurements show that the quenching rate is high, resulting in an on/off behavior. No LTAQ is observed
for CdTe QDs capped with aminoundecanethiol (AUT). The LTAQ is explained by the influence of solvent
freezing on the surface of the QD core. Freezing of the solvation water molecules surrounding the QD will
induce strain in the capping shell, due to the interaction between water and the charged heads of the
capping molecules. Short carbon chains (AET) will propagate the strain to the QD surface, creating surface
quenching states, whereas long and flexible chains (AUT) will dissipate the strain, thus avoiding surface
distortion. Freezing-point depression by the addition of methanol results in a lowering of the transition
temperature. Additional support is provided by the size dependence of the LTAQ: smaller particles, with
higher local ionic strength due to a higher density of charged NH3+ surface groups, experience a lower
transition temperature due to stronger local freezing-point depression.