  Porous, atomically thin graphene membranes have interesting properties for
filtration and sieving applications because they can accommodate small pore
sizes, while maintaining high permeability. These membranes are therefore
receiving much attention for novel gas and water purification applications.
Here we show that the atomic thickness and high resonance frequency of porous
graphene membranes enables an effusion based gas sensing method that
distinguishes gases based on their molecular mass. Graphene membranes are used
to pump gases through nanopores using optothermal forces. By monitoring the
time delay between the actuation force and the membrane mechanical motion, the
permeation time-constants of various gases are shown to be significantly
different. The measured linear relation between the effusion time constant and
the square root of the molecular mass provides a method for sensing gases based
on their molecular mass. The presented microscopic effusion based gas sensor
can provide a small, low-power alternative for large, high-power,
mass-spectrometry and optical spectrometry based gas sensing methods.
