  Atomically thin transition metal dichalcogenides have emerged as promising
candidates for sensitive photodetection. Here, we report a photoconductivity
study of biased mono- and bilayer molybdenum disulfide field-effect
transistors. We identify photovoltaic and photoconductive effects, which both
show strong photogain. The photovoltaic effect is described as a shift in
transistor threshold voltage due to charge transfer from the channel to nearby
molecules, including SiO2 surface-bound water. The photoconductive effect is
attributed to the trapping of carriers in band tail states in the molybdenum
disulfide itself. A simple model is presented that reproduces our experimental
observations, such as the dependence on incident optical power and gate
voltage. Our findings offer design and engineering strategies for atomically
thin molybdenum disulfide photodetectors, and we anticipate that the results
are generalizable to other transition metal dichalcogenides as well.
