  We report the electrical detection of captured gases through measurement of
the tunneling characteristics of gas-mediated molecular junctions formed across
nanogaps. The gas sensing nanogap device consists of a pair of vertically
stacked gold electrodes separated by an insulating spacer of (~1.5 nm of
sputtered amorphous Si and ~4.5 nm ALD SiO2) which is notched ~10 nm along the
edges of the top electrode. The exposed gold surface is functionalized with a
self-assembled monolayer (SAM) of conjugated thiol linker molecules. When the
device is exposed to a target gas (1, 5-diaminopentane), the SAM layer
electrostatically captures the target gas molecules forming a molecular bridge
across the nanogap. The gas capture lowers the barrier potential for electron
tunneling from ~5 eV to ~0.9 eV across the notched edge regions and establishes
additional conducting paths for charge transport between the gold electrodes,
leading to a substantial decrease in junction resistance. We demonstrated an
output resistance change of greater than 100000000 times on exposure to 80 ppm
of diamine target gas as well as ultra-low standby power consumption of smaller
than 15 pW, confirming electron tunneling through molecular bridges for
ultra-low power gas sensing.
