The behavior of Device B at 300 K is generally similar to Device A discussed above. The Fermi level alignment results in a formation of upward Schottky-like potential barrier, photogenerated electrons are being accumulated in the QD thin film, and photogenerated holes are being transferred to graphene (Figure ). At 250 K, in contrast, Fermi levels of the QDs and graphene may equalize and, therefore, the gate voltage induced Fermi level shift of graphene may become decisive for the formation of an electron or hole blocking potential barrier (Figure ). Further cooling of Device B would change the averaged surface dipole strength back to a similar level as at room temperature. The origin of such non-monotonic behavior is likely related to the complex interplay between several temperature-dependent factors such as QD-surface/ligand interatomic distance, ligand vibrations and rotations, and PbS bandgap narrowing (Figure ), which altogether may impact on the surface dipoles in a sophisticated way. This is in stark contrast to Devices A (based on partially oxidized QDs), where trap states may entirely dominate the responsivity dependence on temperature. The responsivity and phase measurements of Devices B1-B5 are highly similar in their appearance and only vary in responsivity magnitude as the film thickness (92-171 nm) decreases, but differ strongly from those of Devices A. To exclude any dependence of the ligand exchange duration on the device performance, Devices B were briefly exposed to 1,2ethandithiol and characterized, followed by a longer ligand exposure time and second characterization with only minor changes in the responsivity behavior (Figure ). The high dependence of responsivity measurements on the temperature and the gate voltage could potentially be used as a sensing method for QD surface treatments, and vice versa, surface treatments could be used for hybrid photodetector optimization with a response at a desired temperature and gate voltage. It is probable that some hybrid detectors with their tested surface treatments are non-responsive at room temperature, but may function at lower temperatures and optimized gate voltages. Therefore, temperature-dependent measurements might be essential for the discovery and optimization of functional QD-ligand-graphene hybrid detectors. The best performance was achieved for Device B1 at the temperature 80 K and the gate voltage of 2 V where the wavelength, frequency and irradiance dependence were additionally investigated. The spectral dependence of the photoresponse shows a quantum confinement-related excitonic peak at 1320 nm (Figure ) with a slight bathochromic shift in comparison to the room temperature responsivity spectrum (Figure ). The frequency dependence of responsivity, normalized to the drain current, is shown together with the drain-current-normalized noise-current in Figure . Dividing the responsivity with the fitted noise and normalizing it with the detector area results in the specific detectivity, which is plotted as the inset. The specific detectivity increases up to 27 Hz and decreases at higher frequencies. At 27 Hz a tradeoff between reduced 1/f noise and decreased responsivity is found, which is in the range of the fast rise times (below the measurement resolution for the recorded time traces of 61 ms) as observed for the detector (Figure ). Up to this point, the devices were characterized at high irradiance to obtain a clear signal, which results in lower responsivity and specific detectivity values. Higher responsivity can be achieved with increased drain bias, which also leads to higher noise-current-both parameters are linearly dependent on the drain bias within an ohmic operational range-leaving the specific detectivity unaffected. However, reducing the irradiance leads to higher responsivities and thus higher specific detectivities, as shown in Figure . When the irradiance is reduced until the signal-to-noise ratio approaches unity, responsivity values as high as 7180 AW -1 at a drain current of 2.5 μA were measured. Together with the corresponding drain-currentnormalized noise-current (5.4×10 -4 Hz -0.5 ) a specific detectivity of 8.4×10 Jones is calculated using the following equation: