  We study the photoresponse of single-layer MoS2 field-effect transistors by
scanning photocurrent microscopy. We find that, unlike in many other
semiconductors, the photocurrent generation in single-layer MoS2 is dominated
by the photo-thermoelectric effect and not by the separation of photoexcited
electron-hole pairs across the Schottky barriers at the MoS2/electrode
interfaces. We observe a large value for the Seebeck coefficient for
single-layer MoS2 that, by an external electric field, can be tuned between
-4x10^2 uV/K and -1x10^5 uV/K. This large and tunable Seebeck coefficient of
the single-layer MoS2 paves the way to new applications of this material such
as on-chip thermopower generation and waste thermal energy harvesting.
