  Gas sensors built using two-dimensional (2D) MoS2 have conventionally relied
on a change in field-effect-transistor (FET) channel resistance or a change in
Schottky contact/pn homojunction barrier. This report demonstrates, for the
first time, an NO2 gas sensor that leverages a gate tunable type II WSe2
(p)/MoS2 (n) heterojunction to realize a 4x enhancement in sensitivity, 8x
lower limit of detection and improved dynamic response when compared to an MoS2
FET sensor on the same flake. Comprehensive sensing measurements over a range
of analyte concentrations, gate biases and MoS2 flake thicknesses indicate a
novel two-fold electrical response to NO2 exposure underlying the enhanced
sensitivity of the heterojunction- (i) a series resistance change that leads to
an exponential change in thermionic current at high bias, and, (ii) a carrier
concentration change that leads to a linear change in interlayer recombination
current near zero bias. The heterojunction diode also exhibits fast and tunable
recovery under negative gate biasing. All-electrical (gate controlled) sensing
and recovery operation at room temperature makes this a simple, low-overhead
sensor. The ability to sense tri-nitro toluene (TNT) molecules down to a
concentration of 80PPB highlights its potential as a comprehensive chemical
sensing platform.
