To hot carriers cooling is about 50 ps and

To investigate the
optoelectronic response in fabricated structures we irradiated the samples with
the same fs laser at lower photon fluxes. Experiments with fs laser reveal the
photocurrent generation in both pristine and modified structures. The
two-step mechanism of photocurrent dynamics after laser pump was observed. The
first step is related to intrinsic charges relaxation after hot carriers generation
in graphene. The average time of hot carriers cooling is about 50 ps and in our
case we observed only consequences of this process (the minimal resolution of
the multimeter is 1 ms, the total time of irradiation was ~ 5 ms). The second part of the decay we associate with charge
carriers trapping at the graphene-SiO interface. The photocurrent, generated in the channel during the
laser treatment, is strongly related to the graphene width. Narrow
graphene ribbons provide higher response to laser irradiation. The narrow
graphene channel has higher body effect in response to light irradiation
compare to wide ribbons where contact effect dominates. Additional data on
photocurrent measurements with fs laser can be found.

Optoelectronic measurements with
continuous wave 532 nm laser were performed to investigate the origin of the
generated photocurrent. The laser was focused to a ~ 0.7 m spot and the laser
power was controlled by neutral density filter in the range of 0.1 W – 0.5 mW.
We compared the laser power dependence of generated photocurrent in pristine
and functionalized areas. Figure  shows the typical power dependence of the
photocurrent for pristine and functionalized with two
different photon fluxes GFETs. The photocurrent for pristine graphene is found
to saturate at 2×10 W cm and the saturation limit for
functionalized graphene goes beyond 1×10 W cm, which
is rather high. The non-unity exponent may be as a result of complex
processes of electron-hole generation, trapping, and recombination, implying carrier transport and electron trapping in
the processed graphene photodetector. The calculated photoresponsivity for GFETs varies from 0.3 to 100 mA W-1 and highly depends on power
density and applied voltages V and V. These values
are comparable or exceed the results from similar works for single-layer
graphene photodetectors.

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Additionally, photocurrent
signals were measured while laser light at 100 W scanned the channel.
A photocurrent as large as 7 nA was measured directly at and close to the functionalized
area. As expected, no significant photocurrent was observed for pristine
graphene areas. The photocurrent generation area is wider than the spatial region
of the junctions, which indicates that the excited carriers stayed hot across
the GFET channel and contributed to the photocurrent.

The shift of the G
band, as discussed previously, and shown in , can be quantitatively
translated into a charge density. We found that the fs
laser treatment of single layer graphene increases the charge density of up to
1×10 cm. The significant change in hole
concentration at the edges of irradiated region defines sharp p-p heterojunctions.