Black phosphorus quantum dots (BPQDs) are promising candidates
for semiconductor photoluminescence (PL) nanomaterials with the desired
blue-violet emission and size-tuned optical response. Here, three
prominent PL peaks were observed in the BPQDs with uniform nanosize,
which were fabricated by pulsed-laser ablation of black phosphorus
crystals. A sharp increase in the peak intensity was observed in the
narrow temperature range of 225–250 K. This anomalous temperature
dependence of the PL intensity is derived from negative thermal quenching,
in which more electrons that are restricted in the intermediate states
are thermally activated and excited to the lowest unoccupied molecular
orbital (LUMO) with increasing temperature. The intermediate states
were formed due to the defects induced by laser ablation. Consequently,
the radiative transitions from LUMO to the σinner, σouter, and surface states dominate. By contrast,
in the temperature ranges of 200–225 and 250–320 K,
the PL intensity decreases because of normal thermal quenching, which
usually occurs in conventional PL materials. The fitting result shows
a nonmonotonicity similar to that in the negative thermal quenching
semiconductors, reflecting the competition between normal and negative
thermal quenching with changing temperature. Furthermore, the PL peak
positions are blue-shifted with increasing temperature, which is associated
with the stronger electron–phonon coupling and the lattice
thermal expansion caused by transverse optical vibrations. The peak
full width at half-maximum values broadened with temperature, which
is attributed to the exciton–phonon interaction. This work
is of great significance for not only a comprehensive understanding
of the PL mechanism of the BPQDs but also the design of BPQDs-based
optoelectrical nanodevices.