  Two-dimensional hexagonal boron nitride (hBN) has attracted large attentions
as platforms for realizations for integrated nanophotonics and collective
effort has been focused on the spin defect centers. Here, the temperature
dependence of the resonance spectrum in the range of 5-600 K is investigated.
The zero-field splitting (ZFS) parameter D is found to decrease monotonicly
with increasing temperature and can be described by Varshni empirical equation
perfectly, while E almost does not change. We systematically study the
differences among different hBN nanopowders and provide an evidence of edge
effects on ODMR of VB- defects. Considering the proportional relation between D
and reciprocal of lattice volume, the thermal expansion might be the dominant
reason for energy-level shifts. We also demonstrate that the VB- defects still
exist stably at least at 600 K. Moreover, we propose a scheme for detecting
laser intensity using the VB- defects in hBN nanopowders, which is based on the
obvious dependence of its D value on laser intensity. Our results are helpful
to gain insight into the spin properties of VB- and for the realizations of
miniaturized, integrated thermal sensor.
