Changes in major charge transport by molecular spatial orientation in graphene channel field effect transistors

Article abstract of DOI:10.1039/c3cc42591f

Changes in major charge transport of graphene channel transistors in terms of the spatial orientation of adsorbed functional molecules were demonstrated. In contrast to the horizontally (physically) bound molecules, the vertically (chemically) bound molecules did not change major charge carriers of graphene channels, revealing the molecular orientation-dependent doping effects.

Full text of DOI:10.1039/c3cc42591f

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Changes in major charge transport by molecular
spatial orientation in graphene channel field effect
transistors†
Cite this: Chem. Commun., 2013,
49, 6289
Received 9th April 2013,
Accepted 22nd May 2013
Misook Min, Sohyeon Seo, Junghyun Lee, Sae Mi Lee, Eunhee Hwang and
Hyoyoung Lee*
DOI: 10.1039/c3cc42591f
www.rsc.org/chemcomm
Changes in major charge transport of graphene channel transistors in the charge transport of the chemically attached molecules is urgently
terms of the spatial orientation of adsorbed functional molecules required, however, only fragmentary information on the molecular
were demonstrated. In contrast to the horizontally (physically) bound doping effects via chemisorption of the functional groups on the
molecules, the vertically (chemically) bound molecules did not change graphene channels has been reported. For example, benzene diazo-
major charge carriers of graphene channels, revealing the molecular nium salts, which can form carbon–carbon bonding with sp2 carbon
orientation-dependent doping effects.
atoms of graphene, with para-positioned electron acceptors were
chemically attached to the p-type graphene channel, producing a
Graphene is a two-dimensional (2D) honeycomb lattice with excel- p-type doped graphene FET.10,11 Nonetheless, the charge transport
lent electronic properties such as high charge carrier mobility.1 mechanism after molecular chemisorption is still ambiguous because
Many research groups have reported that 2D-graphene can be most previous papers have reported the p-type graphene with the
effectively used for various applications due to its high surface molecules having only an electron withdrawing group. As a semi-
area.2 A semiconducting graphene channel in a field effect conducting channel, there are two different graphene backbone
transistor (FET) has shown excellent performance in chemical frameworks such as hole-dominated chemical vapour deposition
and biomolecular sensing.3,4 One of the issues regarding graphene (CVD)-grown graphene (p-type p-CVD-G) and electron-dominated,
FET applications is to control the transport properties of graphene nitrogen-doped reduced graphene oxide (n-type n-RGO).2,12 Thus,
channels by electron distribution of the functional molecules physi- it is required to consider electronic characteristics of graphene back-
cally or chemically adsorbed.5–7 In particular, to modify major charge bones themselves as well as molecular functionalization.
transport by functional molecules chemically attached on a graphene
The chemisorbed aromatic molecules should be in the upright
channel FET has become an important issue because the chemically (vertical) position via carbon–carbon bonding to the graphene
modified graphene channel should provide stronger molecular adhe- channel, while the physisorbed molecules should be oriented with
sion on the graphene surface than the physically modified one for horizontal molecular stacking via p–p electron interaction, parallel to
realizing the device stabilization. When the graphene channel FET is the graphene channel (Scheme 1a and b). Various molecular
to be used as a biomolecular sensor, it is usually required to functionalities (e.g., electron donating or withdrawing functions)
chemically immobilize an antibody on the graphene channel for could facilitate to modify the charge transport behavior of p- or
sensing specific antigens as well as enhancing the sensitivity and n-type graphene channels (Scheme 1c).
selectivity of various kinds of antigens.8 Thus, it is important to clearly
understand changes in the major charge transport of the graphene
channel FET immobilized with an antibody before applying as a real
biosensor.9 It is desired not to change the major charge transport of
the chemically modified graphene channel FET with antibodies, but
to change the major charge transport with antigens for use as
molecular biosensors. To date, even though an understanding of
National Creative Research Initiative, Center for Smart Molecular Memory,
Department of Chemistry, Sungkyunkwan University, 300 Cheoncheon-dong,
Jangan-gu, Suwon, Gyeonggi-do, Republic of Korea. E-mail: hyoyoung@skku.edu;
Scheme 1 Illustration of molecular spatial orientations via (a) physisorption
versus (b) chemisorption for graphene channel FETs. (c) Schematic band structures
of graphene channels regarding the nature of graphene and a functional group
(R: electron donating or electron withdrawing group).
Fax: +82-031-290-5934; Tel: +82-031-299-4566
† Electronic supplementary information (ESI) available: Experimental methods,
AFM images, XPS data, TEM data, and a table displaying Raman spectra analysis
data. See DOI: 10.1039/c3cc42591f
c
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Chem. Commun., 2013, 49, 6289--6291 6289

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Herein, we carefully designed two different graphene channel graphene channel, whereas the electron donating groups produced
FETs and four physisorbed and five chemisorbed molecules (experi- the electron-doped graphene channel. However, the G-bands of all
mental methods, ESI†). Two different graphenes, p-CVD-G and chemisorbed samples with various functional molecules including
n-RGO, were deposited on a SiO2/Si substrate pre-patterned with the benzene molecule were down-shifted from that of pristine
Au source–drain electrodes. Functionalized benzene molecules (e.g., n-RGO (Fig. 1d), indicating that all n-RGO channels with chemi-
nitrobenzene (–NO2), carboxylbenzene (–COOH), bromobenzene sorbed molecules produced only the electron-doped n-RGO channel
(–Br), and aminobenzene (–NH2)) and their diazonium salts including (i.e., n-type). Plots in Fig. 1c show that the 2D- and G-bands for
benzene (–H) diazonium salts were prepared for physisorption and n-doping shifted to the reverse direction while the 2D- and G-bands
chemisorption, respectively.13 X-ray photoelectron spectroscopy (XPS) for p-doping shifted to the same direction (Fig. S4, ESI†).5 On the
and transmission electron microscopy (TEM) confirmed the physis- other hand, plots in Fig. 1d show that the 2D- and G-bands
orbed and chemisorbed molecules on each graphene (Fig. S1 and S2, for n-doping shifted to the reverse direction regardless of the
ESI†).14,15 The atomic force microscopy (AFM) image of an n-RGO presence of electron donating or electron withdrawing groups.
channel identified a well-made FET (Fig. S3a, ESI†). Two different Such unexpected behavior in Raman shifts for the chemisorption
molecular spatial orientations (Scheme 1a and b) due to is assumed due to the new C–C bonding leading to vertical
adsorption processes were clearly identified in AFM images locations of functional groups. Raman spectra results indicate
showing a difference in height corresponding to the molecular that the main transport property of the molecularly chemisorbed
length (Fig. S3b and c, ESI†), indicating that the molecular graphene FET is not influenced by the chemisorbed molecules
orientation to a graphene plane can be well controlled.
due to less interaction between the vertically attached functional
Fig. 1a and b show Raman spectra recorded to characterize molecules and the graphene channel.
changes in the sp2 carbon structure of the n-RGO framework
Finally, the electronic transport behavior of each molecularly-
regarding the adsorption process. Via molecular chemisorption, modified n-RGO FET was confirmed from the transfer curves of the
the sp2 bonding of the n-RGO framework was converted into the source–drain current (Ids) versus the gate voltage (Vg) (Fig. 2). The
sp3 hybrid bonding between an electron-deficient phenyl group of Dirac point (DP)17,18 near 0 Vg of the pristine n-RGO FETs shifted
molecules and electron-rich sp2 carbons of n-RGO, resulting in with regard to the doped carriers on n-RGO by the molecular
the structural defects of the sp2 carbon network of the n-RGO adsorption. Hole-doped n-RGO (p-type) or electron-doped n-RGO
framework and an increase in the I(D)/I(G) values (e.g., 0.24–0.42) (n-type) channel effects were observed in the n-RGO FETs physically
(Table S1, ESI†).16 On the other hand, the structural change in the modified by phenyl-electron withdrawing groups (e.g., –NO2) and
sp2 carbon network for the molecular physisorption did not occur, phenyl-electron donating groups (e.g., –NH2), respectively (Fig. 2a).
resulting in no change in the I(D)/I(G) values (e.g., +0.01/À0.04). Thus, DPs of the phenyl-NO2, –COOH, and –Br/n-RGO FETs
Furthermore, the molecular functionality induced the G-band slightly shifted to the positive voltage direction while the DP of the
shifts; the physically adsorbed benzene molecules with the
electron withdrawing groups induced a slight up-shift compared
to that of the pristine n-RGO, while the benzene molecule with
the electron donating group induced a down-shift (Fig. 1c).
Thus, the electron withdrawing groups produced the hole-doped
Fig. 2 Normalized transfer curves of the source–drain current (Ids) versus the
gate voltage (Vg) for the n-RGO channel FETs modified with the molecular (a)
physisorption and (b) and (c) chemisorption. The black and red curves represent
the responses of the n-RGO FETs before and after the molecular adsorption,
respectively. The source–drain voltage was 0.5 V. (d) Plots of the average DP
positions for physisorption and chemisorption with the error bars of standard
deviation, which were obtained by each 5–10 places with 2–3 samples. The back
arrow indicates the shift of the DP position.
Fig. 1 (a) and (b) Raman spectra of (a) physically- and (b) chemically-functionalized
n-RGO. (c) and (d) Plots of the peak positions for the G-band and the 2D-band with
regard to (c) physically- and (d) chemically-functionalized n-RGO, respectively. Peak
position plots were average values with the error bars denoting the standard
deviations, which were obtained by each 5–10 places with 2–3 samples.
c
6290 Chem. Commun., 2013, 49, 6289--6291
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physisorbed benzene molecules, with the electron withdrawing
groups (e.g., –Br) and electron donating groups (e.g., –NH2),
respectively (Fig. 3a). In order to examine the true molecular
functionality effects, the pristine p-CVD-G FETs with similar DP
positions (e.g., 30–40 Vg) were used. For the vertically oriented
molecules, the molecularly chemisorbed phenyl-Br (an electron
withdrawing group) and phenyl-NH2 (an electron donating
group) on the p-type prone p-CVD-G resulted in significant
shifts of DPs (from near 0 Vg of the pristine p-CVD-G FETs) to
the positive direction and only hole-doped channel FET devices
(p-type CVD-G FETs). Regardless of the molecular functionality
whether it produces electron-doping or hole-doping in p-CVD-G,
molecular doping effects via vertical chemisorption do not seem
to originate from the functional groups apart from the p-CVD-G
surface (Fig. 3b).
In conclusion, the physisorbed molecules horizontally aligned
to the graphene channel changed the major charge carriers of the
graphene FET. However, the chemisorbed molecules vertically
aligned to the graphene channel could not change the major
charge transport. Our novel finding promises a new molecular
doping strategy of graphene channels for various applications,
especially for molecular and biomolecular sensors.
Fig. 3 Normalized transfer curves of the source–drain current (Ids) versus the
gate voltage (Vg) of p-CVD-G channels for the molecular (a) physisorption and (b)
chemisorption of phenyl-Br and phenyl-NH2. The black and red curves represent
the responses of the p-CVD-G FETs before and after the molecular adsorption,
respectively. The source–drain voltage was 0.5 V. The back arrow indicates the
shift of the DP position.
phenyl-NH2/n-RGO FET slightly shifted to the negative voltage
This work was supported by the Creative Research Initiatives
direction. However, the chemically modified n-RGO-phenyl deriva- research fund (project title: Smart molecular memory) of MEST/
tives including RGO-phenyl-H led to a DP shift to a negative voltage NRF. M. Min is grateful to Dr G. S. Bang for helpful advice.
direction, revealing only an electron-doped n-RGO channel effect,
Notes and references
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This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 6289--6291 6291