Fig. 1

(a) Typical absorption spectrum (red curve) and HPL spectrum (blue curve) of QDs samples with its core size of \(\sim 3.3\) nm and a shell thickness of about 1.4 nm. Gray area denotes the range of excitation energies that could possibly generate net energy up-conversion in PL spectra. (b) Schematic of PL processes with sub-band absorption, where \(\varepsilon _{\text {ex}}\) is less than the \(\bar{\varepsilon }_{\text {em}}\) of the HPL spectrum.