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A Method for Fabrication of Graphene Oxide Nanoribbons from Graphene Oxide Wrinkles Xiaozhu Zhou, Gang Lu, Xiaoying Qi, Shixin Wu, Hai Li, Freddy Boey, and Hua Zhang* School of Materials Science and Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore 639798, Singapore ReceiVed: August 17, 2009; ReVised Manuscript ReceiVed: September 21, 2009
We report a novel approach for direct fabrication of graphene oxide nanoribbons (GONRs) on the 3-aminopropyltriethoxysilane (APTES)-modified SiOx surface with varied widths and lengths by plasma etching of graphene oxide (GO) wrinkles (GOWs), in which top layers of GOWs are used as sacrificial layers. AFM images show that single- and double-layer GONRs are easily achieved, and also confirm that these GONRs are obtained from GOWs, which normally form during absorption of large GO sheets with a size of at least several micrometers on APTES-modified SiOx substrates. Raman mapping and scanning electron microscopy (SEM) were performed to further confirm the attainment of GONRs by this method. A GONR with a width as narrow as 15 nm was obtained. Reduced graphene oxide nanoribbons (rGONRs) can be readily obtained by chemical reduction of GONRs. Introduction Graphene, a single layer of carbon bonded in a honeycomb lattice structure, has stimulated a myriad of research because of its exceptional electrical, mechanical, and thermal properties.1–4 However, the graphene, made by a mechanical cleavage method1 or grown epitaxially from surfaces5 or by CVD method,6,7 has a zero-bandgap without semiconducting properties, hindering its application in nanoelectronics. Therefore, it is of great necessity to open the bandgap in graphene. One typical approach is to make graphene nanoribbons (GNRs), which have been theoretically predicted to be semiconducting with the width reduced down to sub-10 nm.8,9 Thus resulting semiconducting characteristics arise from the quantum confinement effect and edge effect from the small width.8–10 Recent experimental progress has revealed such properties with a great promise for real applications, such as in p-type and n-type graphene field effect transistors.11–13 The all-semiconducting properties offered by GNRs might rival or even replace CNTs because the extreme chirality is required for CNTs to be metals or semiconductors.14,15 Reliable production of such nanoribbons is also needed for various investigations.16 Thus, how to make GNRs is key to their applications. A few methods for making GNRs have been reporteds recently.11,12,17–20 Among them, both chemically sonicating expandable graphite11 and physically and chemically unzipping CNTs12,18 show great promise. GNRs with a width down to 5 nm have been achieved and show outstanding electrical performance.11 However, new, simple methods used to produce GNRs are still required to investigate the potential applications of this material, such as in biology, electronics, magnetism, and catalysis.16 Herein, we report a simple method for fabrication of graphene oxide (GO) nanoribbons (GONRs) from GO wrinkles (GOWs)21,22 by plasma etching. GOWs have been widely observed, and their formation mechanism have been proposed theoretically. Although GOWs were recently created by manipulating GO with AFM,22 in our experiment, GOWs automatically formed when * To whom correspondence should be addressed. Phone: +65-67905175. Fax: +65-67909081. E-mail:
[email protected]. Website: http://www. ntu.edu.sg/home/hzhang/.
the large GO sheets with a size of at least several micrometers adsorbed on the 3-aminopropyltriethoxysilane (APTES)-modified SiOx substrates. Importantly, since reduced graphene oxide nanoribbons (rGONRs) can be obtained by chemical reduction of GONRs,23 our method also directly offers a route to fabricate rGONRs. Results and Discussion On the basis of our previous report,23 after the synthesized GO was adsorbed onto APTES-modified SiOx substrates, the GOWs frequently formed if the GO sheets are large enough (greater than several micrometers). Note that the density of GO sheets adsorbed on substrates can be controlled by the incubation time and concentration of GO solution.23 Figure 1A shows two typical GOWs, which contain three overlapped layers of GO and are responsible for the production of GONRs, as discussed below. The GOWs, presumably originating from the oxidation process,22,24 are difficult to avoid. Inspired by the recent GNR fabrication with plasma etching,19 in which silicon nanowires were used as etch masks, we tried to use the top layers (first or second layer, indicated in Scheme 1A) of the GOWs as sacrificial layers to make double- or even single-layer GONRs by plasma etching. As expected, the results are quite promising. The obtained single- and double-layer GONRs are characterized by AFM, Raman, and SEM. Here, it should be clarified that the obtained ribbons are actually graphene oxide nanoribbons (GONRs). However, upon hydrazine reduction23 or heat treatment,25 the GONRs can be reduced to rGONRs, which could be used for fabrication of rGONR-based nanodevices. The process for fabrication of GONRs is schematically represented in Scheme 1. Plasma etching with O2 or Ar will result in the formation of GONRs with double layers (Scheme 1, part B) or single layer (Scheme 1, part C) from the GOWs (Scheme 1, part A) after the GOWs are exposed to plasma for various times. Typically, in our experiment, an etching time of 20 s gave double-layer GONRs, whereas an etching time of 30 s will give single-layer GONRs when the pressure of O2 is 140 mTorr and the power used is 6.8 W. The obtained double- and single-layer GONRs are monitored during the plasma etching process (Figure 1). A typical GOW
10.1021/jp9079298 CCC: $40.75 2009 American Chemical Society Published on Web 10/09/2009
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Figure 1. AFM topographic images used to monitor the plasma etching process. (A) GOWs form in a single GO sheet with an average height of 3.3 nm. (B) After 20 s of etching, double-layer GONRs are obtained with an average height of 2.3 nm. (C) After an additional 10 s of etching of B, single-layer GONRs are obtained with an average height of 1.2 nm. All scale bars ) 500 nm. Height scale ) 20 nm.
SCHEME 1: Process for Fabrication of GONRsa
a (A) GONR fabricated from GOWs by plasma etching. (B) Doubleor (C) single-layer GONRs are obtained by varying etching time (t2 > t1). Drawing is not to scale. The underlying SiOx substrate is not shown.
with a height of ∼3.3 nm as measured by AFM (Figure 1A) indicates three layers of GO sheets overlapped, in accordance with the proposed formation manner of GOWs (Scheme 1, part A), since the height of a single-layer GO is ∼1 nm.26 After 20 s of plasma etching, the single-layer GO sheet surrounding the GOWs is etched away, and the GONR with a height of ∼2.3 nm is obtained (Figure 1B), which means that the doublelayer GONR was obtained. With continuous plasma etching for another 10 s, the single-layer GONR with a height of ∼1.2 nm is achieved (Figure 1C). These results clearly show that our experiment achieved the atomic layer control through the plasma etching, and the controlled layer-by-layer etching of the GOWs was obtained by using the top or second layer of GOWs as the sacrificial layer. It is worth noting that during the fabrication of GO sheets in our experiment,23 the mild sonication will yield large pieces of GO sheets with a size of several to 10 µm, and the wrinkles formed after adsorption of GO sheets on the APTES-modified Si/SiOx substrates. During the strong sonication, only small GO sheets with a size of a few hundred nanometers are obtained, leading to no formation of wrinkles, because the wrinkle lines may be difficult to form in a small GO sheet. Thus, it is reasonably believed that longer GONRs could be fabricated when GO sheets are large enough. GONRs with different length and width are also achieved using this method. Figure 2A-C shows GONRs with a height of ∼2 nm, corresponding to the double-layer GO. This can be achieved by selectively etching the top layer of the GOWs and the other surrounding single-layer GO. To simplify the experiment and make it easily controllable, we changed only the etching time with a fixed etching power and gas pressure (vide
Figure 2. AFM topographic images of double- and single-layer GONRs. (A-C) Double-layer GONRs with heights and widths of (A) 2.3 and 30 nm, (B) 2.0 and 50 nm, and (C) 2.3 and 90 nm, respectively. (D-G) single-layer GONRs with heights and widths of (D) 0.9 and 15 nm, (E) 1.1 and 25 nm, (F)1.3 and 40 nm, and (G)1.4 and 100 nm, respectively. The measurement was conducted in the middle of the ribbons. All scale bars ) 100 nm.
supra). The width of such GONRs varies, depending on the width of the GOWs. Double-layer GONRs with average widths of 30, 50, and 90 nm, respectively, are achieved (Figure 2A-C). The length of the GONRs typically ranges from a few hundred nanometers to a few micrometers. Figure 2B shows a ribbon with a length of 2 µm. If an etching time of 30 s is carried out, single-layer GONRs can be obtained directly from the GOWs (Figure 2D-G), in which the height of GONRs is ∼1 nm, in agreement with the height of single-layer GO.26 Meanwhile, different widths of GONRs with different lengths were obtained, which are dependent on those of the GOWs. Until now, the smallest width of GONR we obtained is 15 nm (Figure 2D). After GONR was reduced by hydrazine23 or heat treatment,25 reduced graphene oxide nanoribbons (rGONRs) can be obtained. GNR with a width of around this value is interesting because the narrower GNRs will give a high Ion/Ioff value, which is needed in transistor applications.12 As expected, the plasma condition plays a critical role in the attainment of single- or double-layer GONRs. This is evidenced by the etching process, which is monitored by AFM, as shown in Figure 1. In addition, GONRs obtained from overetch is
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Figure 4. SEM image of Ag NPs grown on a GONR. The conditions for growth of Ag NPs on GONR: heat the GONR substrate in 0.1 M AgNO3 at 75 °C for 30 min under N2 protection. Inset: AFM topographic image and a height profile of Ag NPs grown on a GONR. All scale bars ) 200 nm.
Figure 3. (A) AFM, (B) SEM, and (C) Raman mapping of the D-band (1315-1400 cm-1) further confirm that the GONRs are fabricated. (D) The corresponding spectra of positions at 1, 2, and 3 in C, respectively. Clearly, outside GONRs, no signal of D and G bands were detected; e.g., in position 3. All scale bars ) 1 µm.
shown in Figure S1 of the Supporting Information, further evidencing the criticality of etching time. The longer etching time used in Figure S1 gives the discontinuity in the remaining ribbons and GO sheets, indicating that the overetch occurred. Therefore, the etching time should be accurately controlled on the basis of the designed layers of GONRs. After plasma etching, the single layer GO sheet surrounding the GOWs was etched away, and GONRs were formed, which was proved by AFM (Figure 1). From the Raman mapping (Figure 3), it is further confirmed that GONRs were fabricated because the area surrounding GONRs gives no signal of GO, which means that the surrounding GO was completely etched away by plasma etching. Note that the GONRs will give Raman spectra similar to those of GO. Positions 1 and 2 show the signal of D and G bands; position 3 shows no detectable signal of D and G bands, which is the same as that in the other empty area without GO adsorption. This verifies the successful fabrication of GONRs by our method. After GONRs were reduced in a hydrazine vapor environment,23 the obtained rGONR was further characterized by TEM. Figure S2 shows a typical rGONR with a width of ∼50 nm. Note that the width of the rGONR should be larger if its roll-up behavior is taken into account. It is worth mentioning that GONRs are frequently observed to be surrounded by some residual GO sheets (Figure S3). This can be avoided if there is no overlapping of GO sheets after they are adsorbed on the APTES-modified SiOx. In our experiment, less overlapping is observed when a lower concentration of GO solution is used. However, the number of GONRs produced is also decreased. How to optimize experimental conditions to realize isolated but dense GONRs is currently ongoing in our lab. Last but not least, thus-fabricated GONRs are subject to decoration with Ag NPs, following our reported method.23 The composites of graphene NPs have also
been reported elsewhere.27–29 Figure 4 shows the SEM and AFM images of a GONR on which Ag NPs were grown after the substrate was immersed in a 0.1 M AgNO3 aqueous solution at 75 °C for 30 min. The size of the Ag NPs obtained here (∼6 nm) is similar to that reported in our previous paper.23 The obtained Ag NP-GONR composite might have applications in nanoelectronics and optics. Conclusion In summary, with an accurate control of plasma etching conditions, single- or double-layer GONRs are successfully obtained by plasma etching of GOWs, in which the top layers of GOWs act as sacrificial layers. GONRs as narrow as 15 nm and as long as 2 µm were obtained. The GONRs are wellcharacterized by AFM, Raman, SEM and TEM, and were used as templates to in situ reduce AgNO3 without any reducing agent to obtain Ag NP-GONR composites. Our results show that even the single atomic GO sheet can still be used as a sacrificial layer in the plasma etching process for successful fabrication of GONRs with controlled layers. Experimental Section Fabrication of Graphene Oxide Nanoribbons (GONRs). The method to synthesize the graphene oxide (GO) and adsorption of GO on the 3-aminopropyltriethoxysilane (APTES)modified SiOx surface was reported elsewhere.23 The plasma etching was carried out in a Harrick Plasma PDC-32G-2 (Harrick Plasma, Ithaca, New York) which has both O2 and Ar gas inlets. The SiOx substrates adsorbed with GO were put on a quartz tray and then placed in the plasma chamber. Plasma etching conditions (i.e. plasma gas chosen, gas pressure, plasma power, and etching time) were optimized for fabrication of GONRs. Oxygen with a pressure of 140 mTorr was used unless otherwise indicated. The plasma etching was operated with a power of 6.8 W. Under our experimental conditions, 20- and 30-s etching will generate double- and single-layer GONRs, respectively. The experimental conditions for growth of Ag NPs on GONRs are the same as those in our previous report.23 Briefly, the GONR substrates were immersed in a bottle
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containing 5 mL of aqueous solution of 0.1 M AgNO3, which was purged with N2 for 15 min and under N2 protection during the experiment. The system was heated to 75 °C and maintained for 30 min. After the reaction was complete, the synthesized Ag nanoparticles (Ag NPs) were protected from oxidation by injection of 50 µL of 2 mM 16-mercaptohexadecanoic acid (MHA) ethanolic solution into the reaction solution and incubation for 1-2 min. The obtained MHA-passivated Ag NPs on GONRs were rinsed with water and dried with N2. Characterization. All AFM images were obtained by using Dimension 3100 (Veeco, Santa Barbara, CA) in tapping mode with a Si tip (Veeco; resonant frequency, 320 kHz; spring constant, 42 N m-1) under ambient conditions with a scanning rate of 1 Hz and scanning line of 512. Raman spectra were recorded with a WITec CRM200 confocal Raman microscopy system with an excitation line of 488 nm and an air cooling charge coupled device (CCD) as the detector (WITec Instruments Corp, Ulm, Germany). The Raman band of a silicon wafer at 520 cm-1 was used as a reference to calibrate the spectrometer. A D-band Raman mapping image is shown in Figure 3. Scanning electron microscopy (SEM) was performed using a JEOL JSM-6700 field-emission scanning electron microanalyzer (JEOL Ltd., Tokyo, Japan) at an accelerating voltage of 12 kV. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2010 transmission electron microscopy (JEOL Ltd., Tokyo, Japan) with an accelerating voltage of 200 kV. Reduced graphene oxide nanoribbons (rGONRs) on a lacey carbon TEM grid were obtained by first depositing GO on the TEM grid, undergoing plasma etching, and then being subjected to hydrazine reduction before TEM analysis. Acknowledgment. We thank Mr. Yanping Xu for Raman measurement. This work was supported by a Start-Up Grant from NTU, AcRF Tier 1 (RG 20/07) from MOE and an A*STAR SERC Grant (no. 092 101 0064) from A*STAR in Singapore. Supporting Information Available: Additional AFM and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666–669. (2) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183–191.
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