Negative thermal quenching of photoluminescence emissions in multilayers of hexagonal boron nitride

Hexagonal boron nitride (<i>h</i>-BN) exhibits an atomically flat surface and a large bandgap over 6 eV, capable of hosting stable, isolated, optically active defect centers, thus offering remarkable opportunities as nanoelectronics and photonics. The optoelectronic physics behind is of fundamental importance, yet underexplored. Here, we report the temperature-dependent photoluminescence characteristics from 93 K to 293 K in visible and infrared bands of<i>h</i>-BN multilayers, prepared using chemical exfoliation method. In contrast to the usually observed thermal quenching effect in the negative thermal quenching (NTQ) behavior has been obtained at emissions of 2.25 eV and 1.46 eV throughout the entire temperature region. After hydrogen plasma irradiation, these<i>h</i>-BN multilayers exhibit higher levels of point vacancies of<i>V</i>_Band<i>V</i>_N, and more pronounced NTQ behavior, especially in the higher temperature region. The activation energies associated with the thermal quenching process were extracted by fitting the experimental results using an analytical theory, and become significantly larger for the hydrogen irradiated<i>h</i>-BN. A possible mechanism for this NTQ behavior is proposed, suggesting multiple channels for electron trapping under photo-excitation and de-trapping at elevated temperatures. The voltammetric test on<i>h</i>-BN multilayers further confirms a higher density of non-radiative channels induced by hydrogen plasma treatment, manifesting their significant role in the abnormal thermal quenching phenomenon.