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Twenty-two GFETs with different channel
geometries were processed in line perpendicular to the channel length using 280
fs pulse laser as shown in Fig. 1a. We chose the pulse energy of 2 nJ and repetition
rate of 500 kHz and varied the photon flux by changing the number of pulses per
?m (see Table S1). The laser beam was focused to a spot diameter of 2 ?m. As it
is clearly seen from resistance over time plot (see Fig. 1b), measured during
the treatment, the changes of graphene properties during laser processing are remarkable.
The in-situ resistance change during
laser processing provides information about linearity of graphene modification
and local probing of graphene electrical properties. Resistance growth during
GFET treatment gradually depends on photon flux. The micro-Raman map (Fig. 1c), measured for
another structure with the same treatment parameters, confirmed the local
modification of graphene lattice. Increase of the D band intensity, the blue
shift of the G and 2D bands, appearance of the D band after fs laser
irradiation (Fig. 1d, Table S2) prove the two-photon local oxidation of
graphene. Moreover, the ID/ID’
ratio of 13 shows that defects in the processed area are mostly sp type
(epoxy and hydroxyl groups) according to others works.The process of fs laser pulses interaction with
graphene lattice is rather complicated. It includes mechanical, chemical and
structural modification. Notably, we observed the mechanical attachment of
graphene to the substrate (Fig. 2a) that leads to a closer contact between
substrate and graphene and can enhance charge trapping from graphene to the substrate.
The strong change in graphene properties was also observed on a rather
localized level, through STM spectroscopy measurements (Fig. S2), showing
non-Ohmic behavior of I-V curve in the area of fs laser treatment. R-V curve of
a GFET after irradiation is drastically altered (Fig. 2b) and even dual Dirac points
were occasionally observed. They are related to two regions with different
doping levels, showing the formation
of p-p+ heterojunctions due to laser-induced oxidation. The strong
blue shift of the G band (Fig. 2c) in processed area indicates an increase in
hole concentration caused by fs laser local oxidation. For pristine graphene, the
charge carrier mobilities were 1400 and 1000 cm V s for holes and electrons, respectively. After the local functionalization through
fs laser irradiation of the GFET at the point of maximum transconductance we
observed that the mobilities are almost on order of magnitude lower (170 and 90
cm V s for holes and electrons,
respectively) and the relation between the hole and electron mobility is
increased (Table S3, Fig. S3). The increase of the accumulated energy fluence
raises the probability of partial thermal ablation of defects in graphene, such
as wrinkles, bilayer islands, or atomic vacancies that lead to non-monotonic
growth of resistance.

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