  We report on temperature dependent photocurrent measurements of high-quality
dual-gated monolayer graphene (MLG) p-n junction devices. A photothermoelectric
(PTE) effect governs the photocurrent response in our devices, allowing us to
track the hot electron temperature and probe hot electron cooling channels over
a wide temperature range (4 K to 300 K). At high temperatures ($T > T^*$), we
found that both the peak photocurrent and the hot spot size decreased with
temperature, while at low temperatures ($T < T^*$), we found the opposite,
namely that the peak photocurrent and the hot spot size increased with
temperature. This non-monotonic temperature dependence can be understood as
resulting from the competition between two hot electron cooling pathways: (a)
(intrinsic) momentum-conserving normal collisions (NC) that dominates at low
temperatures and (b) (extrinsic) disorder-assisted supercollisions (SC) that
dominates at high temperatures. Gate control in our high quality samples allows
us to resolve the two processes in the same device for the first time. The peak
temperature $T^*$ depends on carrier density and disorder concentration, thus
allowing for an unprecedented way of controlling graphene's photoresponse.
