The effect of PbS core size on the temperature-dependent photoluminescence (PL) of PbS/CdS quantum dots (QDs) in the temperature range of 100-300 K was thoroughly investigated and compared with shell-free PbS QDs. The core/shell QDs show significantly smaller PL intensity variation with temperature at a smaller PbS size, while a larger activation energy when the PbS domain size is relatively large, suggesting both different density and different distribution of defects/traps in the PbS and PbS/CdS QDs. The most remarkable difference consists in the PbS size dependence of the energy gap temperature coefficient (dE/dT). The PbS/CdS QDs show unusual non-monotonic dE/dT variation, resulting in the reversal of the dE/dT difference between the PbS and PbS/CdS QDs at a larger PbS size. In combination with theoretical calculations, we find that, although lattice dilation and carrier-phonon coupling are generally considered as dominant terms, the unique negative contribution to dE/dT from the core/shell interfacial strain becomes most important in the relatively larger-core PbS@CdS QDs.
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