  The thermal conductivities of pristine and defective tungsten diselenide
(WSe2) are investigated by using equilibrium molecular dynamics method. The
thermal conductivity of WSe2 increases dramatically with size below a
characteristic with of ~ 5 nm and levels off for broader samples and reaches a
constant value of ~2 W/mK. By introducing atomic vacancies, we discovered that
the thermal conductivity of WSe2 is significantly reduced. In particular, the W
vacancy has a greater impact on thermal conductivity reduction than Se
vacancies: the thermal conductivity of pristine WSe2 reduced by ~60% and ~70%
with the adding of ~1% of Se and W vacancies, respectively. The reduction of
thermal conductivity is found to be related with the decrease of mean free path
(MFP) of phonons in the defective WSe2. The MFP of WSe2 decreases from ~4.2 nm
for prefect WSe2 to ~2.2 nm with the adding of 0.9% Se vacancies. More
sophisticated types of point defects, such as vacancy clusters and anti-site
defects, are explored in addition to single vacancies, and are found to
dramatically renormalize the phonons. The reconstruction of the bonds leads to
localized phonons in the forbidden gap in the phonon density of states which
leads to the drop of thermal conduction. This work demonstrates the influence
of different defects on thermal conductivity of single-layer WSe2, providing
insight into the process of defect-induced phonon transport as well as ways to
improve heat dissipation in WSe2-based electronic devices.
