of the Dirac point and modulation of conductivity. Further
studies are underway to elucidate the detailed charge transport
mechanisms of spiropyran on pristine graphene FETs and will
be reported in due course. The versatile nature of LbL
assembly integrated with the smart photoresponsive molecules
as a means of controlling the device performance can be of
potential interest in the design of new graphene based optical
electronic devices and sensors.
This research is supported by WCU Program through the
Korea Science and Engineering Foundation funded by the
MEST (R31-2008-000-20012-0), the NRF (2010-0003219 and
2009-0083540), the Principal Research Program in the KIMS,
and a grant (Code No. 2011-0031639) from the Center
for Advanced Soft Electronics under the Global Frontier
Research Program of the MEST, Republic of Korea.
Fig. 3 (a) Dirac voltage shift (ꢀDVDirac) and minimum conductance
ratio (GUV/Gwhite) between UV and white light irradiation as a
function of the number of bilayers. (b) Time-course measurements
of drain current of (top) SP-functionalized and (bottom) unmodified
2-bilayer graphene multilayer FETs (VD = ꢀ0.1 V and VG = ꢀ5 V).
graphene multilayers containing many defect sites increased owing
to ring opening of spiropyran to fully conjugated merocyanine,
thus increasing the density of charge carriers in the graphene
channel.27 In addition, the increased dipole moment of the
merocyanine form leads to dielectric screening for an enhanced
carrier mobility in graphene FETs as similarly observed by Tao
and co-workers.29
Notes and references
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By taking advantage of the LbL assembly, we also observed
that the degree of Dirac voltage shift (DVDirac) under UV
illumination could be precisely tunable by the number of
bilayers in graphene multilayers. As shown in Fig. 3a, inter-
estingly, the shift in Dirac voltage was most pronounced in the
2-bilayer film, which then gradually decreased with an increased
number of bilayers. For example, the Dirac voltage shifts of 11.3
and 4.1 V were observed for 2- and 5-bilayer graphene FETs
upon UV radiation for 5 min, respectively. This result stems from
the reduced influence of SP molecules on the interfacial graphene
channel as the number of bilayers increases (as the film becomes
thicker), which eventually becomes saturated after 6-bilayer of
graphene multilayers. In accord with the Dirac voltage shift, the
conductance change ratio of UV vs. white light illumination
(GUV/Gwhite) diminishes as the film continues to become thicker
(Fig. 3a). Taken together, this observation supports the argument
that the observed photoresponsive effect originates from the
interaction between SP molecules and graphene surfaces. More-
over, it should be noted that the number of bilayers of graphene
films is the key factor in tuning both the degree of doping and
electrical conductance of graphene films. These interesting
features highlight the advantages of LbL assembly as a nanoscale
bottom-up assembly that would have otherwise been hard to
achieve with other techniques. Finally, in order to investigate the
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graphene multilayer FETs, ID was monitored as a function of
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In conclusion, we present a facile method for achieving
optical switching of the Dirac point in reduced graphene oxide
multilayer FETs that are non-covalently functionalized with a
photoresponsive spiropyran derivative. LbL assembly afforded a
facile solution-based protocol for controlled assembly of graphene
nanosheets. We found that the photoresponsive transition from
spiropyran to merocyanine induced the observed optical switching
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c
10980 Chem. Commun., 2012, 48, 10978–10980
This journal is The Royal Society of Chemistry 2012