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OPEN

received: 18 November 2014
accepted: 22 June 2015
Published: 24 July 2015

Method for Controlling Electrical
Properties of Single-Layer
Graphene Nanoribbons via
Adsorbed Planar MolecularNanoparticles
Hirofumi Tanaka1,2, Ryo Arima1, Minoru Fukumori1, Daisuke Tanaka1,†, Ryota Negishi3,
Yoshihiro Kobayashi3, Seiya Kasai4, Toyo Kazu Yamada5 & Takuji Ogawa1
A simple method for fabricating single-layer graphene nanoribbons (sGNRs) from double-walledcarbon nanotubes (DWNTs) was developed. A sonication treatment was employed to unzip the
DWNTs by inducing defects in them through annealing at 500 °C. The unzipped DWNTs yielded
double-layered GNRs (dGNRs). Further sonication allowed each dGNR to be unpeeled into twosGNRs. Purification performed using a high-speed centrifuge ensured that more than 99% of theformed GNRs were sGNRs. The changes induced in the electrical properties of the obtained sGNR by
the absorption of nanoparticles of planar molecule, naphthalenediimide (NDI), were investigated. The
shape of the I-V curve of the sGNRs varied with the number of NDI nanoparticles adsorbed. This was
suggestive of the existence of a band gap at the narrow-necked part near the NDI-adsorbing area ofthe sGNRs.

Ever since remarkable two-dimensional electron system of graphene was discovered1, it has come to
be used in a variety of applications. Because graphene’s carrier mobility is extremely high (2 ×  105 cm2V−1 s−1)2,3, it can be used as a silicon substitute in transistors that have to operate at high frequencies.
Large-area graphene sheets have been used to successfully fabricate circuits on SiC/Si wafers4. On theother hand, as scaled-down graphene-based devices and wiring are developed, it has become important to investigate the electrical properties of graphene nanoribbons (GNRs), particularly single-layer
GNRs (sGNRs). Several methods of fabricating GNRs have been reported, including lithographic5–7 andchemical8 processes. A popular method of synthesizing GNRs in large amounts is to unzip carbon nanotubes (CNTs) directly in solution9–13. The synthesis of sGNR by this technique involves first obtaining
multilayered GNRs (mGNRs) by unzipping multiwalled carbon NTs (MWNTs, number of layers ≥  3)in solution14 and then unpeeling one of the layers of the mGNRs. Although we have also been able tosuccessfully obtain sGNRs directly by unzipping SWNTs in solution, we intend to report these results
elsewhere. This is because, in the present study, we focused on the changes induced in the electrical properties of sGNRs by the adsorption of organic nanoparticles onto them, given that the sGNRs exhibited1

Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan. 2Graduate
School of Life Science and Systems Engineering, Kyushu Institute of Technology (Kyutech), 2-1 Hibikino,
Wakamatsu, Kitakyushu 808-0196, Japan. 3Graduate School of Engineering, Osaka University, 2-1 Yamadaoka,
Suita, Osaka 565-0871, Japan. 4Research Center for Integrated Quantum Electronics, Hokkaido University, North
13, West 8, Sapporo 060-8628, Japan. 5Graduate School of Advanced Integration Science, Chiba University, 133, Yayoi, Inage, Chiba 263-8522, Japan. †Current address: Kwansei Gakuin University, 669-1337 Hyogo, Japan.
Correspondence and requests for materials should be addressed to H.T. (email: tanaka@brain.kyutech.ac.jp)
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semiconductor-like characteristics because their width was less than 10 nm. In this study, we were able
to unzip double-walled carbon nanotubes (DWNTs) instead of MWNTs to obtain semimetallic sGNRs
with a width of 15–50 nm.
In this study, our aim was to devise a method for obtaining sGNRs in high yields. DWNTs were usedas the starting material instead of MWNTs, in contrast to what has been done in previous studies. We
found that unzipping DWNTs yielded double-layered GNRs (dGNRs), which were easier to unpeel andseparate into two individual sGNRs by further sonication.
The second aim of the study was to investigate how the electrical properties of sGNRs are affected
when nanoparticles of planar molecules are adsorbed onto them. Controlling the electrical properties ofsGNRs, including their band gap, is also important for the development of graphene-based electronics.
Only a few methods for controlling the band structure of graphene have been reported. In one method,
the band structure of graphene is controlled by forming a nanomesh by fabricating periodic holes (called“necks”) of varying sizes at different distances15. Although this method is a breakthrough in the development of graphene-based electronics, the structures of the devices based on such nanomeshes would still
be very complex, and the devices time consuming to fabricate. Developing easier methods of controllingthe electrical properties of graphene is therefore essential.
We attempted to achieve this goal by modulating the adsorption of nanoparticles of organic molecules
onto graphene. We have already reported the effect of adsorbed organic nanoparticles on the electricalproperties of individual SWNTs. When nanoparticles of 5,15-bispentylporphyrinato zinc(II) (porphyrin)
is adsorbed on an SWNT, conduction through the nanoparticles is followed by Fowler-Nordheim tunnelling16. On the other hand, if N,N’-bis(n-alkyl)tetracarbonatenaphthalenediimide (Cx-NDI, where x is
the number of methylene units in the alkyl side chains) is adsorbed, conduction is followed by Schottky
emission17. These differences are attributable to the differences in the highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the porphyrin (donor) and Cx-NDI(acceptor) molecules. Further, we also determined the I-V characteristics of the graphene samples, since
we were also interested in elucidating the changes induced in the electrical properties of graphene bythese particles. It was found that the electrical properties of graphene could be controlled merely bycontrolling the number of nanoparticles adsorbed.

Experimental

We obtained the dGNRs as well as the sGNRs via sonication treatments performed using a previously reported procedure11. DWNTs having diameters of 3–15 nm and synthesised by catalyst-based
chemical vapour deposition (CVD) were obtained commercially (Tokyo Ohka Kogyo Co., Ltd.). These
were used as the starting material instead of MWNTs. The purchased DWNTs were annealed in air
at 500 °C to induce defects in them. They were then dispersed in an organic solution of 7.5 mg ofpoly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) (PmPV) in 20 ml of 1,2-dichloroethane.
Under sonication performed at 37 kHz and 600 W, the annealed DWNTs could be unzipped into
high-quality dGNRs. Further, by increasing the sonication time from 0.5 h to 16 h while maintaining
the power at the same level, the dGNRs could be unpeeled to form sGNRs. The solution was then ultracentrifuged at 45620 G (20000 rpm; TOMY Suprema21 High Speed Centrifuge) for 2 h to remove any
nanotubes that remained unzipped, as well as the amorphous carbon-like impurities present in the starting material. The supernatant was then cast onto a cleaved mica or SiO2 substrate, and the substrate wasannealed at 350 °C for 1 h in air to remove all of the PmPV, yielding dispersed sGNRs. Raman spectroscopy was performed on the sGNRs using a RENISHAW inVia Reflex/Stream Line system and a Horiba
HR800 system with 633 and 785 nm excitation lasers. The electrical properties of the sGNRs were measured using point-contact current-imaging atomic force microscopy (PCI-AFM)18,19 and a three-electrodefield-effect transistor (FET) structure. The unzipping process described above is illustrated in Fig. 1.
Next, a toluene solution of the organic molecule C15-NDI was cast on the substrate precast with thesGNRs as per a previously reported procedure19. During the evaporation of the solvent, self-assembled
nanoparticles of C15-NDI were formed on the sGNRs through π -π stacking. The electrical characteristics of the complex were also measured by PCI-AFM and a three-electrode FET structure with a backgate using a four-probe system (Lakeshore, ProbeStation TTP). Electrodes with a Ni(5 nm)/Au(30 nm)
structure were fabricated through ultraviolet (UV) lithography. The measurements were controlled with
an Advantest R6245 two-channel I-V source monitor, which was interfaced with a computer through a
GPIB-SCSI board as per the NI-488.2 protocol. The data were collected using a self-devised procedureand the Igor Pro 4.0 (WaveMetrics) software.

Results and Discussion

The AFM images of the DWNTs used as the starting material showed that they had diameters of 3–15 nm
and lengths greater than 1000 nm (Fig. 2(a)). Annealing at 500 °C and the subsequent sonication allowed
the DWNTs to be unzipped, resulting in dGNRs (Fig. 2(b)). No SWNTs resulting from the unzipping of
only the outer walls of the DWNTs were observed. This suggested that all the unzipped DWNTs formed
dGNRs. The dGNRs had a height of 1.6 nm, a value similar to that reported previously13. The observed
thickness of the dGNRs was slightly higher than the theoretical value, owing to the roughness of the
SiO2 substrate and the defects induced in the dGNRs by sp3 bonding. Figure 2(c) shows an AFM imageof the Y-shaped point at which a dGNR splits or unpeels into two sGNRs; the GNRs corresponding to
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Figure 1. Procedure for obtaining single-layered graphene nanoribbons (sGNRs) from double-walledcarbon nanotubes (DWNTs). (a) A defect is created in a DWNT by annealing. (b) Sonicating the DWNT
allows it to be unzipped, starting at the defect. (c) The DWNT is unzipped completely and (d) a flat, doublelayered GNR (dGNR) is obtained. (e) Further sonication splits the dGNR into two individual sGNRs, and(f) the sGNRs can be separated by centrifugation. The sGNR obtained from the outer layer of the DWNT(red) is slightly wider than that obtained from the inner layer (blue), owing to the geometry of the DWNT.

this stage of the unpeeling process are called yGNRs, in order to distinguish them from the dGNRs andsGNRs. The height and width of the yGNRs along the AA′ line (double-layered area) were 1.64 nm and33.8 nm, respectively (see Fig. 2(b)). Similarly, those along the BB’ line (single-layered area) were 0.87 nm
and 31.2 nm, respectively, those for the sGNR on the left and 0.62 nm and 34.2 nm, respectively, for thesGNR on the right (see Fig. 2(c,e,f)). These values were determined from the AFM profiles of the GNRs.
Although the observed widths were greater than the actual values, owing to the curvature of the AFM
tip, the difference in the widths could be compared in the same image. The difference in the width owing
to blurriness caused by the roundness of the cantilever tip can be expected to be less than 0.5 nm in thisrange, if the same cantilever is used20. Figure 2(c) shows which sGNR was formed from the inner wall of
the DWNT and which one was obtained from the outer wall. The sGNR on the left is 3 nm narrower thanthe one on the right. Since the ribbons formed from the inner walls of the DWNTs would be narrower, it
can be concluded that the sGNR on the left in the figure is formed from an inner wall and the one on theright is obtained from an outer wall. Further, the reason the sGNR on the left looks thicker than the oneon the right is that the former was formed from the upper layer of the dGNR and was not touching thesurface of the substrate. A difference in the widths of approximately 3 nm corresponds to a difference of
1 nm in the outer and inner diameters of the DWNTs. Finally, the Raman spectra confirmed that sGNRs
were obtained in high yields using the unzipping process with no existence of DWNT residue because
no peaks attributable to the radial breathing mode were observed (Fig. S3).
Thus, it was confirmed that DWNTs are a better starting material than MWNTs for obtaining
semi-metallic sGNRs in high yields. The spring constant of DWNTs is considered higher than that of
MWNTs. This is because the smaller radius of DWNTs results in the bond angle between neighbouring
carbon atoms being higher. This suggests that a greater amount of energy is stored in the case of DWNTs.
This energy can be used for the unzipping of the CNTs along their long axis. Although we found that
SWNTs are also suitable for producing narrower sGNRs, we will report these results elsewhere, because
we wished to focus on opening the band gap of the semimetallic GNRs (wider than 10 nm) in the present
work. The I-V characteristics of the sGNRs and dGNRs obtained from DWNTs were determined (Fig.
S4). Both types of GNRs exhibited semimetallic properties.
Next, the solution of C15-NDI was drop cast onto the Si substrate. As a result, the C15-NDI molecules, which are planar, were adsorbed as nanoparticles on the sGNRs. The I-V characteristics of thesGNRs were measured again. Figure 3(a–d) show AFM images of sGNRs with C15-NDI nanoparticles
adsorbed on their surfaces by the drop casting of solutions of various concentrations. By increasing theconcentration of the C15-NDI solution, the number of nanoparticles adsorbed on the substrate surface
as well as on the sGNRs increased. The I-V curves of the GNR/C15-NDI complex (Fig.3 (f)) were determined by PCI-AFM (Fig. 3(e)) for different concentrations (Fig. 3(g)). The part of the I-V curve corresponding to dI/dV =  0 represents the plateau width (PW) of the complex. As the concentration of the
C15-NDI solution was increased, the current decreased gradually while the PW increased. Figure 3(h)shows the PW values plotted against the number of nanoparticles adsorbed per unit length of an sGNR.
The value of PW varied in proportion to the number of adsorbed nanoparticles. It was also found thatthe number of adsorbed nanoparticles reached a maximum when the concentration of the solution was
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Figure 2. AFM images of (a) a DWNT used as the starting material, (b) a dGNR obtained by the unzippingof the DWNT, (c) the region at which the dGNR splits into two sGNRs, (d) a resulting sGNR. Each scalebar is 50 nm. (e) Line profile of the dGNR in (c), obtained along the line A-A′. The height and width of thedGNR are 1.64 nm and 33.8 nm, respectively. (e) Line profile of the dGNR in (c), obtained along the line
B-B′ . The height and width of the sGNR on the left are 0.87 nm and 31.2 nm, respectively, and those of thesGNR on the right are 0.62 nm and 34.2 nm, respectively. The insets in (a,b,d) show the height profile along
the white line in each case. The images (a–d) correspond to different samples.

approximately 1.0 g L−1. The collected data included information about the band structure of the sGNRs,
because the sGNRs transformed into a p-type semiconductor material; this was determined from the
FET-based measurements (Fig. 4(a)), even though the PW values are not directly indicative of the band
gap itself. The reason the PW value changed with the number of adsorbed nanoparticles is the following: the positions where the nanoparticles are adsorbed become localised electron-trapping sites. As the
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Figure 3. (a–d) AFM images of sGNRs after the adsorption of molecular nanoparticles of NDI onto
them by the drop casting of C15-NDI solutions of different concentrations. The concentrations of thesolutions were (a) 0.05 g L−1, (b) 0.1 g L−1, (c) 0.5 g L−1, and (d) 1.0 g L−1. (e) Schematic of the setup used
to determine the I-V characteristics of the NDI-adsorbing sGNRs using PCI-AFM. (f) Molecular structureof C15-NDI. (g) I-V curves of the C15-NDI/GNR complex for different concentrations of the C15-NDI
solution. As the concentration of the solution was increased, (i) the current decreased gradually and (ii) theplateau width (PW) increased. (h) The values of the PW plotted as a function of the number of C15-NDI
nanoparticles adsorbed on the sGNRs per unit length of the sGNRs. Owing to steric hindrance, the upper
limit for the number of nanoparticles adsorbed per unit length was approximately 0.1.

number of molecules adsorbed on a GNR is approximately 10 times larger than the number of conduction electrons, the electrons trapped by the nanoparticles no longer contribute to conduction, and hole
conduction mainly occurs in the other areas of the sGNR. This phenomenon is similar to conduction
through the “necks” in a graphene nanomesh15. The conduction path must become narrower as the sizeof the adsorbed nanoparticles increases. It has also been reported that, if the effective width of sGNRs
is low, their band gap is similar to that of a semiconductor21. This is probably the primary reason why
sGNRs transformed into a semiconductor material from a semimetallic one, when the C15-NDI nanoparticles were adsorbed onto their surfaces. To determine whether this was indeed the case, we analysed
the I-V curves obtained using a back-gated FET structure (inset of Fig. 4(a)) and a four-probe system.
Figure 4(b) shows the I-V curves obtained using FET structures fabricated from the GNR/C15-NDI
complexes synthesised using C15-NDI solutions with concentrations of 0.1 g L−1 and 1.0 g L−1. The differential conductance (dI/dV-V) curves derived from these I-V curves are shown in Fig. 4(c). The curve
corresponding to the 0.1 g L−1 solution exhibits three peaks, while that corresponding to the 1.0 g L−1
solution contains nine peaks. The peaks are indicated by arrows and are evidence of the sudden changes
in the current (called kinks) during the measurements.
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Figure 4. Results obtained using the FET structure. (a) Gate property of the FET structure based on the
C15-NDI/GNR complex. The sGNRs exhibited p-type semiconductor behavior after the adsorption of the
NDI nanoparticles. The inset shows the back-gated FET structure employed for the measurements. (b) I-V
curves of the C15-NDI/GNR complex at Vg =  0, obtained for C15-NDI solution concentrations of 0.1 g L−1
and 1.0 g L−1. (c) Differential conductance values obtained from (b). Three peaks are observed in the curve
corresponding to the 0.1 g L−1 solution and nine peaks in one corresponding to the 1.0 g L−1 solution. Thesepeaks indicate the points at which Zener-like tunnelling occurred, strongly implying the existence of a band
gap in the NDI-adsorbing sGNR. (d) Schematic showing the electron path in an NDI-adsorbing sGNR. The
NDI nanoparticle becomes an electron-trapping site, and current flows through the necks of the sGNR.

It was assumed that the appearance of these kinks proved that the band gap of the sGNRs opened up
around the neck structures, owing to the adsorption of the C15-NDI nanoparticles. This was because of
the following reason: there are a number of high-resistance points (the previously mentioned necks) in
the NDI-adsorbing sGNRs, because the holes are screened in very short distance of the neck along thelong axis of the sGNRs. In such cases, when a bias is applied to the entire GNR, only a partial voltage isapplied to the high-resistance regions of the necks. Every high-resistance neck forms a high-electron-fielddomain, giving rise to Zener-like tunnelling. When this type of tunnelling occurs, the current flowing
through the sGNR increases suddenly, giving rise to the kinks noticed in the I-V curves, because theband gap is small enough to break down. Thus, the breakdown of current introduces the peaks observedin the dI/dV-V curves. To put it simply, the number of kinks in the I-V curve of a system equals thenumber of times tunnelling occurs in the system. Further, the number of times tunnelling occurs is equal
to the number of semiconducting necks in an NDI-adsorbing sGNR. This proves that the high-resistanceregions of an NDI-adsorbing sGNR break down at high voltages, even though this does not directly
prove the existence of a band gap. In other words, the occurrence of tunnelling strongly implies thanan NDI-adsorbing sGNR has a band gap. Thus, we can conclude that the band gap of an sGNR can becontrolled by controlling the number of C15-NDI nanoparticles that are adsorbed onto it. This, in turn,can be varied by controlling the number of necks. This hypothesis is strongly supported by the FET data
shown in Fig. 4(a), which is indicative of p-type semiconductor behavior.
The technique used in the present work to control the electrical properties, especially the opening
band gap, of an sGNR by controlling the number of C15-NDI nanoparticles adsorbed onto it is a promising one and should aid in the fabrication of graphene-based nanodevices.
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Conclusion

It was shown that DWNTs are a much better starting material than MWNTs for unzipping and forming sGNRs. The unzipping of DWNTs yielded double-layered GNRs, which could be peeled to form
single-layered GNRs. The electrical properties of the sGNRs could be controlled by varying the numberof C15-NDI nanoparticles adsorbed onto them, making the technique a promising approach for band-gap
engineering. The band gap of the sGNRs was attributable to the necking structures formed around theadsorbed C15-NDI nanoparticles. This technique should also be useful for controlling the functionality of conventional semiconductor devices. The plateau widths of the I-V curves of the NDI-adsorbingsGNRs varied in proportion to the number of adsorbed C15-NDI particles. Further, the electrical properties of the sGNRs also changed, with the sGNRs going from being semimetallic in nature to a p-typesemiconductor, after the adsorption of the molecular C15-NDI nanoparticles.

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Acknowledgments

This work was partially supported by a Grant-in-Aid for Scientific Research (Nos. 24510150, 15H03531)
and a Grant-in-Aid for Scientific Research on Innovative Areas (Nos. 20111012 and 25110002) from the
Ministry of Education, Culture, Sports, Science and Technology, Japan and by the Inamori Foundation.
One of the authors (HT) thanks the International Training Program of the Japan Society for the Promotionof Science for support for a stay at Rice University. HT also thanks Prof. S. Qiao of Fudan University,
China, and Prof. K. F. Kelly of Rice University, USA, for fruitful discussions and Profs. K. Maeda and Y.
Horibe of Kyutech, Japan, for support with the experiments.

Author Contributions

H.T. and T.O. led the research study. R.A., M.F. and D.T. fabricated and characterised the GNR devices.
R.N. and Y.K. measured the Raman spectra and analysed the spectra. S.K. and T.K.Y. developed themodel to explain conduction in GNRs. All the authors contributed to writing this paper and discussed
the results.

Additional Information

Supplementary information accompanies this paper at http://www.nature.com/srep
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Tanaka, H. et al. Method for Controlling Electrical Properties of Single-Layer
Graphene Nanoribbons via Adsorbed Planar Molecular Nanoparticles. Sci. Rep. 5, 12341; doi: 10.1038/
srep12341 (2015).

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