The
band-gap energy of most bulk semiconductors tends to increase
as the temperature decreases. However, non-monotonic temperature dependence
of the emission energy has been observed in semiconductor quantum
dots (QDs) at cryogenic temperatures. Here, using stable and highly
efficient CdSe/CdS/ZnS QDs as the model system, we quantitatively
reveal the origins of the anomalous emission red-shift (∼8
meV) below 40 K by correlating ensemble and single QD spectroscopy
measurements. About one-quarter of the anomalous red-shift (∼2.2
meV) is caused by the temperature-dependent population of the band-edge
exciton fine levels. The enhancement of electron-optical phonon coupling
caused by the increasing population of dark excitons with temperature
decreases contributes an ∼3.4 meV red-shift. The remaining
∼2.4 meV red-shift is attributed to temperature-dependent electron-acoustic
phonon coupling.