Abnormal Negative Thermal Quenching of Photoluminescence (PL) in Laser-Induced Exfoliated Black Phosphorus Quantum Dots for Applications as a Semiconductor PL Nanomaterial

Black phosphorus quantum dots (BPQDs) are promising candidates for semi-conductor 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 sigma(inner), sigma(outer), 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.