Several functional groups and graphene and charge traps occupation

Several physical mechanisms
can give rise to a photoresponse, but only two can play a significant role in
graphene-based photodetectors. These are the photovoltaic and the photo-thermoelectric
effects.36 In the case of photovoltaic effect, incident photons
generate a density of carriers that are separated, and induce current at the electrodes
when an electric field is applied. In our experiments the photocurrent in
processed structures has linear dependence on bias voltage without any photovoltaic
effect (Fig. 4a). The photo-thermoelectric effect, on the other hand,
originates from a difference in Seebeck coefficients from areas with different
density of states. The different local temperatures at the junction result in
the diffusion of carriers after irradiation. Hot-carrier dynamics are generally
recognized to dominate photocurrent generation in supported graphene devices
because of inefficient cooling of electrons with the lattice.41 Bolometric effect is negligible due to electrodes
passivation with photostructurable polyimide.

The gate-modulated current
gives clear evidence of Dirac point shift under continuous laser irradiation. The
shift can reach up to 30 V in backgate measurements (Fig. 4b) converting the
graphene channel from heavy p-doped to almost intrinsic. Such decrease of the
hole concentration and increase of the off-current can be related to the raise
of electron concentration through reducing the energy barrier between attached
functional groups and graphene and charge traps occupation at the graphene-SiO2
interface.42 In pristine graphene we did not observe prominent
shift under laser irradiation (Fig. S6).

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By tuning the gate
voltage Vgs, the majority carriers in the processed graphene channel
change from holes to electrons. The photocurrent is minimal near the Dirac
point (Fig. 4c). It rises with increasing of the |Vgs-VDirac|
difference for both types of carriers. When we consider the energy levels far
from Fermi level, all carriers are involved in charge transport and the
quantity of charges generated upon laser irradiation is minimal, hence the
photocurrent tends to zero. For these observations the photo-thermoelectric
effect dominates as was shown previously by Xu et al.43 The physical picture
of photocurrent generation due to the photo-thermoelectric effect can be
described as following: after the electrons are excited from the valence band
to the conduction band, they relax back to the Fermi level on the time scale of
femtoseconds by phonon emission and form a hot Fermion distribution.44, 45 The hot free carriers
tend to diffuse from the pristine part of the channel into the functionalized
one due to the temperature gradient across the channel, which leads to a
positive current for hole doped graphene. In this case the photocurrent can be
formulated as