  Optically levitated nanomechanical resonators in vacuum perform ultrahigh
sensitivity for mechanical quantities by overcoming the limitations of clamped
resonators. However, the generally levitated silica nanoparticles (NPs) with
low absorption and high transparence still face difficulties surviving in high
vacuum with unclear reason. By monitoring the physicochemical properties like
scattering, mass and density of amorphous silica NPs during pumping process. we
propose that the loss of NPs may arises from the motional instability induced
by laser heating lead releasing at low pressure. In this work, two types of NPs
are heat treated from 100 to 1200 degree Celsius to release impurities before
being loaded into an optical trap. The high vacuum levitation ratio for both
NPs increase obviously after heat treatment. In particular, for NPs heated to
600 degree Celsius, the ratio strikingly improves from ~30% to 100% and ~0 to
85% for two types of NPs. The loss mechanism is further confirmed by their
relatively stable physicochemical parameters during pumping process. This work
paves a way for wide application of levitated nano-resonators and indicates
that levitated vacuum optomechanical systems could be a promising tool for
dynamics and in-situ studying of small particles like aerosols and dusts.
