  Contrary to most materials, graphene exhibits a negative thermal expansion
coefficient (TEC), i.e it contracts when heated. This contraction is due to the
thermal excitation of low energy out-of-plane vibration modes. These flexural
modes have been reported to govern the electronic transport and the mechanical
response of suspended graphene. In this work, we systematically investigate the
influence of defects in the TEC of suspended graphene membranes. Controlled
introduction of low densities of mono-vacancies reduces the graphene TEC, up to
one order of magnitude for a defect density of $5\times10^{12}$~cm$^{-2}$ . Our
molecular dynamics simulations reproduce the observed trend and show that TEC
reduction is due to the suppression of out-of-plane fluctuations caused by the
strain fields created by mono-vacancies in their surrounding areas. These
results highlight the key role of defects in the properties of "real-life"
graphene, and pave the way for future proposals of electronic and mechanical
defect engineering.
