  We describe the application of nanoelectromechanical systems (NEMS) to
ultrasensitive mass detection. In these experiments, a modulated flux of atoms
was adsorbed upon the surface of a 32.8 MHz NEMS resonator within an ultrahigh
vacuum environment. The mass-induced resonance frequency shifts by these
adsorbates were then measured to ascertain a mass sensitivity of 2.53x10^-18 g.
In these initial measurements, this sensitivity is limited by the noise in the
NEMS displacement transducer; the ultimate, limits of the technique are set by
fundamental phase noise processes. Our results and analysis indicate that mass
sensing of individual molecules will be realizable with optimized NEMS devices.
