Site-Specific Growth of Width-Tailored Graphene Nanoribbons on

Aug 28, 2012 - width-tailored GNR directly onto an insulating substrate. Predeposition of .... which are denoted by the red circle in Figure 3e, are i...
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Site-Specific Growth of Width-Tailored Graphene Nanoribbons on Insulating Substrates Wooseok Song,† Soo Youn Kim,† Yooseok Kim,† Sung Hwan Kim,† Su Il Lee,† Inkyung Song,† Cheolho Jeon, † and Chong-Yun Park*,†,‡ †

BK21 Physics Research Division, Sungkyunkwan University, Suwon 440-746, Republic of Korea Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea

‡

ABSTRACT: The band-gap opening in graphene is a key factor in developing graphene-based field-effect transistors. Although graphene is a gapless semimetal, a band gap opens when graphene is formed into a graphene nanoribbon (GNR). Moreover, the band-gap energy can be manipulated by the width of the GNR. In this study, we propose a site-specific synthesis of a width-tailored GNR directly onto an insulating substrate. Predeposition of a diamond-like carbon nanotemplate onto a SiO2/Si wafer via focused ionbeam-assisted chemical vapor deposition is first utilized for growth of the GNR. These results may present a feasible route for growing a width-tailored GNR onto a specific region of an insulating substrate.

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INTRODUCTION Graphene has recently received a great deal of attention for potential use in graphene-based nanoelectronics due to its remarkable electrical properties, including bipolar conductance, ballistic transport over an ∼0.4 μm length, half-integer quantum Hall effect, and extremely high electron mobility at room temperature.1−3 However, the gapless semimetallic nature of graphenebased nanoelectronics is a major hurdle for the advancement of graphene-based field-effect transistors (FETs). Quasi onedimensional structures called graphene nanoribbons (GNRs) have been proposed to overcome this obstacle because a band gap can be generated by reducing the graphene width. In a previous study, GNRs with a width of less than 10 nm exhibited a high on/off ratio up to 107, which could be useful for room-temperature transistor operation with excellent switching speed and high carrier mobility.4 Thus far, GNRs have been produced by various methods, including electron-beam lithographic patterning,5 chemical exfoliation,4 longitudinal unzipping of carbon nanotubes,6−8 inorganic nanowire templates,9,10 chemical vapor deposition (CVD) growth,11 bottom-up fabrication,12 nanosphere lithography,13 and dip-pen nanolithography.14 Unfortunately, the width distribution of GNRs seems to be quite broad, and substantial damage to the edge and basal plane inevitably occurs in the production process. Thus, to meet the demands of the target applications, a reliable method for the direct production of width-tailored GNRs is needed. Furthermore, site-specific positioning of GNRs on insulating substrates still remains a great challenge. Hence, this paper presents width-tailored GNRs that were synthesized directly on a SiO2 substrate at a desired position using a diamond-like carbon nanotemplate (DLCNT) predeposited by focused ion-beam-assisted CVD (FIB-CVD).

Figure 1a. The predeposition process of the DLCNT was implemented as follows: a phenanthrene (C14H10) vapor was introduced into a vacuum chamber while the designated area was irradiated with a Ga+ ion beam. The Ga+ ion beam operating at 30 keV was focused to a spot with a size of ∼20 nm at a 91.3 pA beam current. Accordingly, the phenanthrene precursor in the designated area was decomposed into volatile and nonvolatile components by the Ga+ ion beam irradiation, and a DLCNT was eventually formed on the SiO2/Si substrate. Note that this method, FIB-CVD, facilitates direct patterning of size-controlled carbon nanostructures on arbitrary substrates. In previous reports, size-, shape-, and position-controlled carbon nanostructures were fabricated by FIB-CVD, which is useful for applications in nanomechanics.15 GNRs were synthesized on the DLCNT using a conventional thermal CVD (TCVD) system. The DLCNT on the SiO2/Si substrate was heated to 900− 1000 °C inside a TCVD reactor with a flow of H2 (10 sccm). A flow of CH4 (20 sccm) was subsequently introduced as a carbon feedstock with H2 gas to synthesize the GNR on the DLCNT for 10−300 min, as depicted in Figure 1b,c. The feedstock was then turned off, after which the TCVD reactor was cooled to room temperature (∼30 °C/min) with flowing H2 gas.

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RESULTS AND DISCUSSION Figure 1d−g shows a typical optical microscope image and Raman intensity maps of the Si peak (∼520 cm−1), G band (∼1550 cm−1), and 2D band (∼2700 cm−1) of the DLCNT deposited by FIBCVD, which were obtained using a confocal Raman spectrometer (WiTec, CRM200) with an excitation wavelength of 532 nm. It is noteworthy that the Raman spectra of the DLCNT in Figure 1h

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EXPERIMENTAL SECTION First, a width-tailored DLCNT was predeposited onto a SiO2 (300 nm)/Si(001) substrate using FIB-CVD, as depicted in © 2012 American Chemical Society

Received: April 10, 2012 Revised: August 10, 2012 Published: August 28, 2012 20023

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Figure 1. Schematic representation of the GNR growth process: (a) Size-tailored DLCNT was predeposited onto the desired area of a SiO2/Si substrate. (b) CH4 feedstock was introduced into the TCVD reactor at the growth temperature. (c) The width-tailored GNR was synthesized on the DLCNT. (d) Optical microscope images (scale bar = 10 μm) and normalized intensity maps of the (e) Si peak (∼520 cm−1), (f) G band (∼1550 cm−1), (g) 2D band (∼2700 cm−1), (h) Raman spectrum, (i) SEM image (scale bar = 5 μm), and (j) EDS carbon mapping image of the DLCNT.

Figure 2. (a) Optical microscope image (scale bar = 10 μm) and intensity maps of the (b) G band (∼1590 cm−1) and (c) 2D band (∼2700 cm−1) of the GNRs on 1, 3, and 5 nm thick DLCNTs synthesized at 1000 °C for 60 min using TCVD. Raman spectra of the GNRs on (d) the 1 nm thick DLCNT, (e) the 3 nm thick DLCNT, and (f) the 5 nm thick DLCNT. Spectra were recorded at an excitation wavelength of 532 nm. 20024

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exhibits a broad peak around 1550 cm−1, which is identical to that of previously reported DLC.15,16 Because DLC is a good electrical insulator, it is likely that transfer-free fabrication of GNRbased FETs can be accomplished using our approach, particularly since the practical realization of a high-frequency graphene transistor on a DLC substrate has been previously demonstrated.17 Finally, Figure 1j presents the chemical identification of the DLCNT, which was performed by energy-dispersive X-ray spectroscopy (EDS) analysis. The effect of DLCNT thickness on GNR growth was investigated by Raman analysis. The GNRs were synthesized using a mixture of CH4 (20 sccm) and H2 (10 sccm) at 1000 °C for

60 min. Figure 2a−c displays an optical microscope image and the intensity maps of the G and 2D bands of the GNRs on the DLCNTs with various thicknesses. These maps reveal a distinct and intense pattern with increasing DLCNT thickness. The broad peak of the DLCNT (Figure 1h) splits into the sharp D and G bands after GNR synthesis, as shown in Figure 2d−f. By increasing the DLCNT thickness, the full width at half-maximum (fwhm) of the G band decreases and the 2D band appears clearly. Also, the significant red shift of the G band likely indicates the structural transition from a nanocrystalline structure to graphite, namely, the enlargement of the crystal size.18,19 On the basis of this result, the 5 nm thick DLCNT was especially effective for obtaining a GNR showing typical Raman fingerprints: the D, G, and 2D bands, as shown in Figure 2f. In addition, the Raman spectra did not exhibit any noticeable changes when a GNR was synthesized on 10−30 nm thick DLCNTs (not shown here). Figure 3 shows optical microscope images and Raman spectra of the GNRs synthesized on a 5 nm thick DLCNT at (a, b) 900, (c, d) 950, and (e, f) 1000 °C for 60 min. A clear splitting of the G and D bands and the presence of the 2D band are observed for the GNR synthesized at 1000 °C. At 1000 °C, black particles, which are denoted by the red circle in Figure 3e, are identifiable as Ga particles induced by heat-driven precipitation since Ga+ ions were used for the fabrication of the DLCNT via FIB-CVD. However, the Ga particles were almost eliminated by increasing the growth time (Figure 4).20 Additionally, a heat-induced structural disruption of the SiO2 substrate is observed for the GNR synthesized at 1050 °C. Although the optimized synthesis temperature may influence the dielectric breakdown of the SiO2 layer, the DLCNT is an insulating layer, which results in the fabrication of GNR-based FETs that lack any leakage current. Nevertheless, the full details of the GNR growth mechanism are uncertain. Nicholas et al. presented graphene flakes that were grown over a pre-existing graphene sheet via “template growth”, in which the nucleation rate of additive flakes on defect sites and the edge of the first graphene layer was higher than that on the basal

Figure 3. Optical microscope images (scale bar = 10 μm) and Raman spectra of the GNRs synthesized on the 5 nm thick DLCNT at (a, b) 900, (c, d) 950, and (e, f) 1000 °C for 60 min, respectively.

Figure 4. (a−e) Optical microscope images (scale bar = 10 μm) and normalized intensity maps of the (f−j) G band (∼1590 cm−1), (k−o) D band (∼1350 cm−1), and (p−t) 2D band (∼2700 cm−1) of the DLCNT, the annealed DLCNT, and the GNRs grown on DLCNTs at 1000 °C for 60, 180, and 300 min, respectively. 20025

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Figure 5. Raman spectra of the (a) DLCNT, (b) annealed DLCNT, and of GNRs synthesized on the DLCNT at 1000 °C for (c) 10, (d) 60, (e) 180, and (f) 300 min, respectively. (g) ID/IG and crystallite size, (h) G-band position and 2D-band fwhm, and (i) I2D/IG of the GNRs (red triangles) as a function of growth time and CVD graphenes (blue dotted lines) transferred on the as-annealed DLCNT.

plane.21 In principle, the formation of additive flakes should take a longer time compared to the first layer. However, this difficulty might be compensated by many defect sites at the surface of the first layer. Here, we employed the DLCNT consisting of trigonal (sp2) and tetrahedral (sp3) bonded carbon and a C−H bond, which acts as the first layer. Hence, the density of nucleation sites on the DLCNT for the adsorption of carbon atoms seems to be higher than that on the graphene, thereby the GNR could be easily formed on top of the DLCNT. The structural evolution of GNRs for various growth times was studied by Raman mapping, as shown in Figure 4. The asannealed DLCNT was prepared at 1000 °C for 1 min without introducing CH4 feedstock (Figure 4b,g,l,q). Optical microscope images reveal that no significant change is observed before or after the annealing. The GNR seems to be formed on the DLCNT surface with increasing growth time, as shown in Figure 4a−e. The intensity of the G band related to sp2-bonded carbon significantly increases after GNR synthesis (Figure 4h), and marginally increases with increasing synthesis time (Figure 4i,j). The intensity of the D band also significantly increases, due to the splitting of the G and D bands after GNR synthesis (Figure 4m). However, a growth time dependency is not observed, as shown in Figure 4n,o. After GNR synthesis, the 2D band abruptly appears,

and its intensity increases with increasing growth time, as shown in Figure 4q−t. Figure 5 exhibits the Raman spectra of the DLCNT, annealed DLCNT, and GNRs synthesized for 10−300 min. All the spectra were normalized to a G band and deconvoluted with Gaussian and Lorentzian functions. The spectrum of the DLCNT was decomposed into two Gaussian components of the D band (1375 cm−1) and G band (1551 cm−1), as seen in Figure 5a. After annealing, the position and fwhm of the D and G bands change significantly, which is presumably a result of local graphitization, as shown in Figure 5b.22 The graphene fingerprints, Lorentzian line-shaped G, D, D′, and 2D bands, clearly appear after GNR growth, as seen in Figure 5c−f. With increasing growth time, the intensity of peaks related to the DLCNT gradually decreases, while that of the GNR substantially increases. The ID/IG and crystallite size of the GNRs are summarized in Figure 5g. Note that the crystallite size (La) of graphene was determined according to the equation, La (nm) = 560/El4 (ID/IG)−1,23 where El is the excitation laser energy used in the Raman measurement in electronvolt units. The ID/IG decreases significantly with increasing growth time, implying that the crystallite size of the GNRs increases. It is notable that the ID/IG is much smaller than those in previous reports, including the noncatalytic CVD growth of graphene on insulating substrates.24,25 An abrupt red 20026

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Figure 6. AFM images of (a) the DLCNTs with 100, 200, 300, and 500 nm widths and (b) after GNR synthesis (temperature, 1000 °C; time, 300 min). (c) Height profiles of the DLCNTs and the GNRs corresponding to (a) and (b). (e) SEM image of the DLCNTs (scale bar = 6 μm). Normalized intensity maps (scale bar = 5 μm) of the (d) G band (∼1590 cm−1) and (f) 2D band (∼2700 cm−1) of the 300 nm wide GNRs (dotted circle) synthesized on DLCNTs.

this result, we approximately estimate that one-layer GNRs are synthesized for 10 and 60 min growth times and two-layer GNRs are synthesized up to 180 min. Figure 6a,e shows atomic force microscope (AFM, Seiko SPM400) and SEM images of the DLCNTs formed on a SiO2 (300 nm)/Si substrate via FIB-CVD, indicating that widthtunable (100, 200, 300, and 500 nm) fabrication of GNRs is achieved. The 20 nm thick DLCNTs are adopted for the AFM and SEM measurements. The AFM image of the GNRs synthesized on the DLCNTs at 1000 °C for 300 min is shown in Figure 6b. After GNR synthesis, the height of the patterns increases slightly (∼7.5%), as seen in Figure 6c. Figure 6d,f presents the intensity maps of the G (∼1590 cm−1) and 2D bands (∼2700 cm−1) of 300 nm wide GNRs (dotted circle) synthesized on DLCNTs at 1000 °C for 300 min. Consequently, we have successfully demonstrated the width-tunable synthesis of GNRs on an insulating substrate utilizing the predeposition of DLCNT via FIB-CVD.

shift of the G band and decrease in the 2D-band fwhm with increasing growth time are manifestations of the formation of graphene and the enlargement of the crystallite size, as shown in Figure 5h. Therefore, the 5 nm thick DLCNT, 1000 °C (growth temperature), and 300 min (growth time) are especially effective for growing high-quality GNRs. In addition, the thickness of synthesized GNRs was investigated by comparing the intensity ratio of 2D to G bands (I2D/IG) between the GNRs and CVD graphenes (one-layer, two-layer, and three-layer) transferred onto the as-annealed DLCNT film. One-layer graphene was synthesized by TCVD (temperature, 1000 °C; time, 30 min; gases, H2 (20 sccm) and CH4 (40 sccm)) using a 25 μm thick Cu foil (99.8%, Alfa Aesar) and transferred onto the as-annealed DLCNT film, and two-layer and three-layer graphenes were prepared by the layer-by-layer stacking technique. Figure 5i shows the plot of I2D/IG as a function of growth time. The blue lines correspond to the I2D/IG of one-layer, two-layer, and threelayer graphenes on the as-annealed DLCNT film. On the basis of 20027

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Figure 7. (a) Schematic diagram of electrochemically gate tuned GNR FETs. (b) Optical microscope images (scale bar = 100 μm) of GNR FETs with widths of 100, 200, 300, and 500 nm. Transfer characteristics (IDS−VG) at VDS = 0.1 V of (c) 500, (d) 300, (e) 200, and (f) 100 nm wide GNR FETs. (g) Output characteristics (IDS−VDS) of GNR FETs. (h) Plot of Eg ∼ kBT·ln(Ion/Ioff) for GNRs of various widths.

Notes

In addition, electrical transport measurements of width-tailored GNRs were conducted. We fabricated electrochemically gated GNR FETs with widths of 500, 300, 200, and 100 nm, as depicted in Figure 7a.26 Platinum and 1-butyl-3-methylimidazolium (BmimPF6) were employed as source and drain electrodes and an ionic liquid. All devices have a fixed channel length of approximately 1 mm, as shown in Figure 7b. Transfer curves (IDS−VG) at VDS = 0.1 V of GNR FETs show the Dirac point at negative VG and slightly asymmetric hole and electron conduction regardless of the width of GNRs, as shown in Figure 7c−f. This result is presumably influenced by unintentional n-type doping of the GNRs from residual Ga particles due to a workfunction difference between Ga (4.2 eV)27 and graphene (4.5−4.8 eV).28 Output characteristics (IDS−VDS) of GNR FETs reveal that the electrical conductivity gradually decreases with decreasing the width of GNRs, as shown in Figure 7g. The band gap of width-tailored GNRs was estimated by Ion/Ioff ∼ exp(Eg/kBT) (where kB is the Boltzmann’s constant and T is temperature). Figure 7h shows a plot of Eg ∼ kBT·ln(Ion/Ioff) versus GNR width, in which our results seem to follow the same trend as the previous report, including chemically derived GNR FETs.4 We believe that this approach provides a feasible and reliable route for obtaining band-gap-controlled GNRs if the patterning limitation (≥100 nm) of the FIB-CVD system can be overcome.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the WCU program (World Class University, R31-2008-000-10029-0) and the Basic Science Research Program (2011-0004421) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST).

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(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Griorieva, I. V.; Firsov, A. A. Science 2004, 306, 666− 669. (2) Zhang, Y.; Tan, Y.-W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201−204. (3) Novoselov, K. S.; Jiang, Z.; Zhang, Y.; Morozov, S. V.; Stormer, H. L.; Zeitler, U.; Maan, J. C.; Boebinger, G. S.; Kim, P.; Geim, A. K. Science 2007, 315, 1379. (4) Li, X.; Wang, X.; Zhang, L.; Lee, S.; Dai, H. Science 2008, 319, 1229−1231. (5) Han, M. Y.; Ozyilmaz, B.; Zhang, Y.; Kim, P. Phys. Rev. Lett. 2007, 98, 206805. (6) Jiao, L.; Wang, X.; Diankov, G.; Wang, H.; Dai, H. Nat. Nanotechnol. 2010, 5, 321−325. (7) Kosynkin, D. V.; Higginbotham, A. L.; Sinitskii, A.; Lomeda, J. R.; Dimiev, A.; Price, B. K.; Tour, J. M. Nature 2009, 458, 872−876. (8) Jiao, L.; Zhang, L.; Wang, X.; Diankov, G.; Dai, H. Nature 2009, 458, 877−880. (9) Yu, W. J.; Chae, S. H.; Perello, D.; Lee, S. Y.; Han, G. H.; Yun, M.; Lee, Y. H. ACS Nano 2010, 4, 5480−5486. (10) Wang, R.; Hao, Y.; Wang, Z.; Gong, H.; Thong, J. T. Nano Lett. 2010, 10, 4844−4850. (11) Delgado, J. C.; Herrera, J. M. R.; Jia, X.; Cullen, D. A.; Muramatsu, H.; Kim, Y. A.; Hayashi, T.; Ren, Z.; Smith, D. J.; Okuno, Y.; Ohba, T.; Kanoh, H.; Dresselhaus, M. S.; Terrones, M. Nano Lett. 2008, 8, 2773− 2778. (12) Cai, J.; Ruffieux, P.; Jaafar, R.; Bieri, M.; Braun, T.; Blankenburg, S.; Muoth, M.; Seitsonen, A. P.; Saleh, M.; Feng, X.; Mullen, K.; Fasel, R. Nature 2010, 466, 470−473.

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CONCLUSIONS We demonstrated a facile route for direct growth of widthtailored GNRs on insulating substrates. Optimum conditions for growing high-quality GNRs were robustly established: the 5 nm thick DLCNT, 1000 °C (growth temperature), and 300 min (growth time) were especially effective. This method provides a reliable and reproducible production of width-tailored GNRs on a specified region of an insulating substrate, which is highly applicable for future electronics.

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REFERENCES

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(13) Liu, L.; Zhang, Y.; Wang, W.; Gu, C.; Bai, X.; Wang, E. Adv. Mater. 2011, 23, 1246−1251. (14) Shin, Y.-S.; Son, J. Y.; Jo, M. H.; Shin, Y.-H.; Jang, M. H. J. Am. Chem. Soc. 2011, 133, 5623−5625. (15) Matsui, S.; Kaito, T.; Fujita, J.-I.; Komuro, M.; Kanda, K.; Haruyama, Y. J. Vac. Sci. Technol., B 2000, 18, 3181−3184. (16) Kalish, R.; Lifshitz, Y.; Nugent, K.; Prawer, S. Appl. Phys. Lett. 1999, 74, 2936−2938. (17) Wu, Y.; Lin, Y.-M.; Boi, A. A.; Jenkins, K. A.; Xia, F.; Farmer, D. B.; Zhu, Y.; Avouris, P. Nature 2011, 472, 74−78. (18) Ferrari, A. C.; Robertson, J. Phys. Rev. B 2000, 61, 14095−14107. (19) Ferrari, A. C. Diamond Relat. Mater. 2002, 11, 1053−1061. (20) Kometani, R.; Haruyama, Y.; Kanda, K.; Kaito, T.; Matsui, S. Jpn. J. Appl. Phys. 2007, 46, 7987−7990. (21) Nicholas, N. W.; Connors, L. M.; Ding, F.; Yakobson, B. I.; Schmidt, H. K.; Hauge, R. H. Nanotechnology 2009, 20, 245607. (22) Kanda, K.; Yamada, N.; Okada, M.; Igaki, J.-Y.; Kometani, R.; Matsui, S. Diamond Relat. Mater. 2009, 18, 490−492. (23) Cancado, L. G.; Takai, K.; Enoki, T.; Endo, M.; Kim, Y. A.; Mizusaki, H.; Jorio, A.; Coelho, L. N.; Paniago, R. M.; Pimenta, M. A. Appl. Phys. Lett. 2006, 88, 163106. (24) Sun, J.; Cole, M. T.; Lindvall, N.; Teo, K. B. K.; Yurgens, A. Appl. Phys. Lett. 2012, 100, 022102. (25) Rummeli, M. H.; Bachmatiuk, A.; Scott, A.; Borrnert, F.; Warner, J. H.; Hoffman, V.; Lin, J.-H.; Cuniberti, G.; Buchner, B. ACS Nano 2010, 4, 4206−4210. (26) Chen, F.; Qing, Q.; Xia, J.; Li, J.; Tao, N. J. Am. Chem. Soc. 2009, 131, 9908−9909. (27) Michaelson, H. B. J. Appl. Phys. 1977, 48, 4729−4733. (28) Yu, Y.-J.; Zhao, Y.; Ryu, S.; Brus, L. E.; Kim, K. S.; Kim, P. Nano Lett. 2009, 9, 3430−3434.

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