Delivered by Publishing Technology to: jacob busch IP: 129.15.118.74 On: Thu, 30 Jul 2015 17:14:25 Copyright: American Scientific Publishers Copyright © 2015 American Scientific Publishers All rights reserved Printed in the United States of America Article Journal of Nanoscience and Nanotechnology Vol. 15, 1203–1208, 2015 www.aspbs.com/jnn Ripple-Free Graphene Sheets on Alkanethiol Self-Assembled Monolayers Provided from Unzipped Multi-Walled Carbon Nanotubes Hyunmo Koo 1 , Nam-Suk Lee 2 , Hoon-Kyu Shin 2 , Jaegeun Noh 3 , Yutaka Majima 4 , and Gyoujin Cho 1 ∗ 1 Department of Printed Electronics of World Class University Program, Sunchon National University, Jeonnam 540-742, Korea 2 National Center for Nanomaterials Technology, Pohang University of Science and Technology, Pohang 790-784, Korea 3 Department of Chemistry, Hanyang University, Seoul 133-791, Korea 4 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan Octanethiol (C8S, CH3(CH2)7SH) self-assembled monolayers/Au(111) were utilized as an inert sur- face to provide ripple-free graphene oxide layers provided from chemically unzipped multi-walled carbon nanotubes (MWCNTs). The resulting graphene oxide monolayers were character ized with atomic resolution by UHV-STM. The honeycomb structure for the graphene monolayer and “three- for-six” triangular pattern for the multi-layer graphene sheets on C8S SAMs were clearly observed without ripples by the high-resolution UHV-STM. These results provide new insight into the prepa- ration of ripple-free graphene monolayers. Keywords: Graphene Oxide Monolayer, Self-Assembled Monolayer, Scanning Tunneling Microscopy. 1. INTRODUCTION Graphene is a very promising material for the next generation in microelectronics, especially for flexible electronics, 1–3 because it has high mechanical flexibil- ity and excellent conductivity and mobility properties. However, those great potentials of graphene in elec- tronic devices have been attenuated due to the influ- ence of the ambient environment, primarily the substrate. For example, the carrier mobility of graphene fabricated on SiO 2 of Si will be deteriorated by the charge trap- ping in oxide. Furthermore, due to the influence from the substrate, such as surface induced ripples, also gen- erate electron–hole puddles, 4 5 the suppression of weak localization, 6 decreased carrier mobility 7 and enhanced chemical reactivity. 8–10 Therefore, in order to eliminate the substrate induced perturbation and ripples on the graphenes, suspended graphene 11 and atomically flat graphene 12 on mica have been reported. However, those methods still somewhat ∗ Author to whom correspondence should be addressed. apart from practically fabricating ripple-free graphene monolayer, called “ultra-flat graphene monolayer,” on sub- strates with minimizing any possible interaction induced from the substrates. Here, we have developed self- assembled monolayers (SAMs) 13 of normal alkyl thiol on gold surface as a general substrate to provide ripple-free graphene while any interaction with the substrates can be avoided due to inert alkyl group of SAMs. By using SAMs, ripple-free and environmentally stable graphene monolayer should be prepared on the any gold evaporated substrate with high reliability using a simple and low cost process. The fabrication method for the ripple-free graphene monolayer with minimizing the interaction from substrates should be simple and costless while the reliability always should be guaranteed so that the method may be used as a general one to fabricate the ultra-flat graphene mono- layer on any substrates. As a consequence of developing the general method, we are employing unzipping chemical reaction of multi-walled carbon nanotubes (MWCNTs) to produce slightly oxidized graphene sheets 14 15 and SAMs J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 2 1533-4880/2015/15/1203/006 doi:10.1166/jnn.2015.9333 1203 Delivered by Publishing Technology to: jacob busch IP: 129.15.118.74 On: Thu, 30 Jul 2015 17:14:25 Copyright: American Scientific Publishers Ripple-Free Graphene Sheets on Alkanethiol Self-Assembled Monolayers Provided from Unzipped MWCNTs Kooetal. as an inert substrate. By unzipping the MWCNTs, the slightly oxidized graphene sheets (SOG) can be mass pro- duced, and the dispersed solution of the SOG can be sim- ply fabricated onto the SAMs on gold evaporated substrate. This method will be used in the near future as a general costless method to directly fabricate graphene monolayers on any gold evaporated substrate. Furthermore, by exploit- ing SAMs, not only the direct interaction of the substrate to the graphene oxide monolayer can be minimized but also variations in the surface structure of SAMs will be use tailor the electrical properties of the graphene monolayers for using in various nanoscale applications. 13 16 17 In this work, we would like to demonstrate a gen- eral method for fabricating SOG monolayers without any ripples or deformations using octanethiol (C8S, CH 3 (CH 2 ) 7 SH) SAMs on substrates. We prepared mono- and multi-layer of SOG on C8S SAMs by simple drop-casting of the SOG dispersed solution prepared by chemically unzipped MWCNTs. The resulting SOG layers were investigated using ultrahigh vacuum scanning tunnel- ing microscopy (UHV-STM) to elucidate the ultra-flat and ripple-free SOG monolayer with the honeycomb structure and the “three-for-six” triangular structure for the multi- layers without any ripples. 2. EXPERIMENTAL DETAILS Unzipping reaction of MWCNTs was carried out using exactly same procedures in the report from Tour’s group 15 except using MWCNTs provided from Hanhwa Nanotech Co. (CM-95, Korea) with 10–15 nm of mean diameter. The dispersed ethanol solution of SOG sheets was prepared with 20 mL of ethanol and 8 mg of SOG sheets through sonication for 30 min. The resulting solution was filtered using 0.2 m filter before drop casting on the SAMs on Au(111) substrate using a micro-syringe. The Au(111) substrates were fabricated by vacuum deposition of gold onto freshly cleaved mica sheets pre- baked at 300  C with a base pressure of 10 −7 to 10 −8 Torr. After deposition, the substrates were annealed at 350  C in the same vacuum chamber for 2 h. The substrate was briefly flame-annealed and quenched in ethanol to form an atomically flat Au(111) surface. The Au(111) substrate was immersed ina1mMsolu- tion of octanethiol (C8S) (Sigma Aldrich, USA) in ethanol at 50  C for 12 h and the samples were rinsed with ethanol two times and dried in a pure N 2 flow. Au(111) substrate is not only widely used for the bottom electrode in elec- tronic devices, but it is also used for the STM character- ization. The resulting partially oxidized graphene sheets casted substrate was dried at room temperature for 10 min in a desiccator. Then, the sample was introduced into the UHV-STM (Omicron, Germany). Scanning tunneling microscopy (STM) image obser- vations were carried out using a Micro-STM (Omicron, Germany) with the tungsten tip prepared by the electrochemical etching method. All STM images were obtained in ultrahigh vacuum (39 ×10 −11 torr) using a constant current mode at room temperature. Bias voltages ranging from −500 to 500 mV were used to get images while the tunneling currents between the tip and the sam- ple were kept in the range from 20 to 500 pA. 3. RESULTS AND DISCUSSION Figure 1(a) shows TEM images of as-prepared few- layer SOG from the unzipping reaction of MWCNTs. Figure 1(b) is the high-resolution TEM image of the red box in Figure 1(a). Hexagonal structures of the graphene are confirmed. Fourier transform of image (Fig. 1(c)) shows the typical hexagonally arranged lattice of carbon in graphene. To further characterize unzipped SOG, X-ray photoe- mission spectroscopy (XPS; Fig. 1(d)) was used to confirm the degree of oxidation on unzipped SOG by checking the intensity of C–O–C, C O and COOH signals in the broad ranges of 286 eV to 289 eV. Based on XPS results, the resulting graphene sheets would be less densely oxidized because the intensity of C–O–C, C O and COOH peaks (Fig. 1(d)) is a lot weaker than fully oxidized graphene sheets where ratio of C–C peak is almost same to those of C–O–C (including C O) peaks. 15 Figure 2 shows the STM images of the C8S SAMs, which were used for the buffer layer between the SOG monolayer and the Au(111) substrate. STM images of C8S SAMs in Figure 2(a) show the typical features of a gold surface covered by alkanethiol SAMs such as etch pits and domain boundaries. The depth of the pits was 0.25 nm, which is close to the monatomic step height of the Au(111) surface, and therefore, these pits were assigned to the vacancy islands (VIs) on the gold sur- face. The molecularly resolved STM image in Figure 2(b) presents the well-known ( √ 3 × √ 3)R30  structure and c(4 × 2) super lattice showing a difference in molecu- lar brightness in the unit cell and clear domain bound- aries. The high resolution STM image (7 nm ×7 nm) of the Au(111) surface showed the ( √ 3× √ 3)R30  structure, which is known to be a typical structure of C8S SAMs, as shown in Figure 2(c). The ( √ 3 × √ 3)R30  unit cell is indicated by the rhombus with black lines. Each side of the rhombus was determined to be 2.88 Å× √ 3 =499 Å. The molecularly resolved STM image in Figure 2(c) clearly shows well-ordered C8S SAMs with a densely packed structure and an atomically uniform surface. Figure 3 show STM images of the unzipped SOG on C8S SAMs. In Figure 3(a), point A and B indicate the STM images (scan size: 100 nm ×100 nm) of the mono- and the bi-layer of unzipped SOG. The measured height difference between the area A and B is ≈0.35 nm which is well matched with the value of the graphene mono- layer but not from graphene oxide monolayer. 19 Further- more, the SOG from the other spots also show the same 1204 J. Nanosci. Nanotechnol. 15, 1203–1208, 2015 Delivered by Publishing Technology to: jacob busch IP: 129.15.118.74 On: Thu, 30 Jul 2015 17:14:25 Copyright: American Scientific Publishers Kooetal. Ripple-Free Graphene Sheets on Alkanethiol Self-Assembled Monolayers Provided from Unzipped MWCNTs (a) (b) (c) 295 290 285 280 0.0 0.2 0.4 0.6 0.8 1.0 Intensity (a.u.) Binding energy (eV) (d) C-C C-O C=O COOH Figure 1. (a) TEM image of few-layer SOG sheets on the carbon coated copper grid (800 ×800 nm 2 ). (b) High-resolution TEM image of the SOG sheet. (c) Fourier transform image of the hexagonal type which is indicating the graphene lattice. (d) XPS analysis of unzipped SOG sheets. thickness of 0.35 nm. Those results could be proved from XPS results on unzipped SOG which may be par- tially oxidized only at the edges. Even though exfoliated graphenes on a solid substrate are known to have ripples due to the Van der Waals interaction with the substrate, 20–22 the unzipped SOG on C8S SAMs/Au(111) are charac- terized as the mono- and bi-layers SOG without ripples Figure 2. STM image (V b = 05V,I = 500 pA) of the C8S SAMs. The STM image in (a) shows the typical features of a gold surface covered with an alkanethiol SAM such as etch pits and domain boundaries. Both depths of the pits are around 0.25 nm, which is close to the monatomic step height of the Au(111) surface. High-resolved STM image in (b) presents the well-known c(4 ×2) superlattice with the difference in molecular brightness in the unit cell and clear domain boundaries. The high resolution STM image (7 nm×7 nm) of the Au(111) surface shows the ( √ 3× √ 3)R30  structure, as shown in (c), which is known to be another typical structure of C8S SAMs. The ( √ 3 × √ 3)R30  unit cell is indicated by the rhombus with black lines. Each side of the rhombus measures 2.88 Å × √ 3 = 499 Å. over the STM scan areas which ranged over 100 nm by 100 nm. Figures 3(b) and (c) are high-resolution STM images of a mono- and bi-layer SOG shown in Figure 3(a). Figure 3(b) shows a constant current STM image of the monolayer SOG on the area A in Figure 3(a). As shown in Figure 3(b), the honeycomb has the inter-carbon length J. Nanosci. Nanotechnol. 15, 1203–1208, 2015 1205 Delivered by Publishing Technology to: jacob busch IP: 129.15.118.74 On: Thu, 30 Jul 2015 17:14:25 Copyright: American Scientific Publishers Ripple-Free Graphene Sheets on Alkanethiol Self-Assembled Monolayers Provided from Unzipped MWCNTs Kooetal. A B (a) 100 nm (b) (c) 7 nm 7 nm a b (d) Graphene sheet Probability Height (nm) OT SAMs 2.5 nm 3 nm 0.000 0.002 0.004 0.006 0.008 –0.08 –0.04 0.00 0.04 0.08 Figure 3. Images for STM topographs; (a) SOG bilayer (100 nm ×100 nm) on C8S SAMs/Au(111) measured with values of V b = 05 V and I = 40 pA. The black line shows the cross-sectional profile in inset. The measured height of SOG monolayer is ≈0.35 nm, consistent with the measured height of the SOG bilayers in (a). (b) Constant current STM image (V b =−05V,I = 60 pA) of the SOG monolayer measured at the area A in (a). As shown in (b), the honeycomb has the side-length of 0.14 nm, corresponding to the size of one hexagonal carbon ring. (c) STM image (V b =−05V,I = 60 pA) of the bilayer SOG measured at the area B in (a). Illustrated hexagon with three circles is superimposed to highlight the graphene honeycomb lattice. The insets in each figure show the 7 nm ×7 nm STM images of the (b) and (c), respectively. (d) Histogram of the height values acquired from the STM images of graphene and C8S SAMs (red and blue squares represent the SOG monolayer and the C8S SAMs, respectively). The histograms are well-described by Gaussian distributions with standard deviations of 0.037 and 0.035 nm for SOG monolayer and C8S SAMs, respectively. of 0.14 nm, corresponding to the size of one hexago- nal carbon ring. Figure 3(c) shows a STM image of the bilayer SOG measured on the area B in Figure 3(a). The illustrated hexagon with three circles is superimposed to highlight the graphene honeycomb lattice and the three carbon atoms imaged as protrusions. The insets show the 7nm×7 nm STM images of the Figures 3(b) and (c), respectively. Our experimental results in the range of 7 ×7nm 2 area show that the SOG has no ripples on the C8S SAMs which is used as the buffer layer between the SOG monolayer and Au(111) substrate. The height variation of 0.05 nm, which is a very small value compared with the previously- reported values of 0.19 nm, 21 0.5 nm, 20 and 0.78 nm, 23 was confirmed by the section analysis as presented on the STM images in Figure 3. The height variation is also comparable with the histograms based on STM images of 7 nm×7nm of a SOG monolayer including 512 ×512 measurement points each. The apparent height histogram of SOG mono- layer and C8S SAMs was shown in Figure 3(d). The histogram of SOG monolayer (red) is well-described by Gaussian distribution with a full width at half maximum (FWHM) of 0.037 nm, while C8S SAMs (blue) exhibits a FWHM of only 0.035 nm. The Gaussian fits of the SOG and the C8S SAMs coincide with each other in the error range of ±0.002 nm. This means that the C8S SAMs are well-formed with the densely packed structure and uni- form at the atomic level. Consequently, SOG monolayers are deposited onto the substrate without ripples. Previ- ous reports show that the height variation of the graphene monolayer on a SiO 2 substrate is 0.19 to 0.78 nm. 20 21 23 On the other hand, our experimental results show that the height variation of the SOG monolayer on the C8S SAMs decreased to 0.037 nm. By the effect of the C8S SAMs, ripple-free ultra-flat SOG monolayers are formed in the range of 7×7nm 2 area. 1206 J. Nanosci. Nanotechnol. 15, 1203–1208, 2015 Delivered by Publishing Technology to: jacob busch IP: 129.15.118.74 On: Thu, 30 Jul 2015 17:14:25 Copyright: American Scientific Publishers Kooetal. Ripple-Free Graphene Sheets on Alkanethiol Self-Assembled Monolayers Provided from Unzipped MWCNTs Figure 4. STM images (V b =−05V,I = 20 pA) of the SOG monolayer without ripples on the C8S SAMs and the edge structure. (a) The STM image of the SOG monolayer shows the honeycomb structure in which the section indicated by an arrowhead mark shows a slightly overlapped section at the edge in the SOG monolayer. (b) The enlarged image of the white dot square presented in (a). (c) The STM image at the boundary between the SOG monolayer and the SOG bilayer. The image enlarged at the black dot square is presented in (d). As illustrated in the STM image, it can be verified that the edge structure of the SOG bilayer is a zigzag pattern. The edge structures of unzipped SOG monolayer have also been studied. Figure 4 shows the STM images of the SOG monolayer and their edge structure without rip- ples on the C8S SAMs. Figure 4(a) represents the hon- eycomb structure of the SOG monolayer at the upper and lower area. Here, the section indicated by an arrow- head mark shows a slightly overlapped image of the edge of the SOG monolayer. In this study, we deter- mined SOG monolayer using the cross-sectional analysis (below 0.1 nm height). Figure 4(b) shows the magnified STM image of the edge structure of the SOG mono- layer of the white dot square in Figure 4(a). As in the pre-reported studies, 13 the SOG unzipped with the longi- tudinal direction of the CNTs shows a zigzag structure at the edge. Here, the edge structure shows an arm- chair as shown in Figure 4(b). Figure 4(c) shows the STM image of the honeycomb structure of the mono- layer and “three-for-six” triangular pattern of the bilayer SOG at the center and left lower area, respectively. The enlarged image of the black dot square is presented in Figure 4(d). As illustrated in the STM image, it can be ver- ified that the edge structure of the SOG bilayer is a zigzag pattern. 4. CONCLUSION We demonstrated that ripple-free SOG monolayers were prepared from chemically unzipped SOG on C8S SAMs and characterized by using TEM and STM. We observed the honeycomb and “three-for-six” triangular patterns at a mono- and bi-layer of SOG on C8S SAMs using high resolution STM (scan size: 7 nm × 7 nm). The mea- sured height variation of SOG monolayer on C8S SAMs is 0.037 nm, which is much smaller than those previ- ously reported. Because of the C8S SAMs, as the buffer layer between the SOG monolayer and Au(111) substrate, the additional deformation would be minimized. Conse- quently, for the development of future graphene-based nano-electronic devices, those attained results would pro- vide valuable information to understand the variations in J. Nanosci. Nanotechnol. 15, 1203–1208, 2015 1207 Delivered by Publishing Technology to: jacob busch IP: 129.15.118.74 On: Thu, 30 Jul 2015 17:14:25 Copyright: American Scientific Publishers Ripple-Free Graphene Sheets on Alkanethiol Self-Assembled Monolayers Provided from Unzipped MWCNTs Kooetal. electrical properties originated from the ripple formation in the near future. Acknowledgment: This work was supported by the Ministry Of Trade, Industry and Energy (MOTIE) through the project of GTFAM Regional Innovation Center (RIC). This work is also supported by a Grant-in-Aid for Sci- entific Research on Innovative Areas (No. 20108011, pi-space) from MEXT, Japan; the Global Centers of Excel- lence (COE) Program of “Photonics Integration-Core Elec- tronics,” MEXT; and Collaborative Research Project of Materials and Structures Laboratory, Tokyo Institute of Technology. References and Notes 1. Y. Liu, L. Huang, G. L. Guo, L. C. Ji, T. Wang, Y. Q. Xie, F. Liu, and A. Y. Liu, J. Nanosci. Nanotechnol. 12, 6480 (2012). 2. L. Wang, C. Li, D. Wang, Z. Dong, F. X. Zhang, and J. Jin, J. Nanosci. Nanotechnol. 13, 5461 (2013). 3. G. Eda, G. Fanchini, and M. Chhowalla, Nature Nanotech. 3, 270 (2008). 4. J. Martin, N. Akerman, G. Ulbricht, T. Lohmann, J. H. Smet, K. Von Klitzing, and A. Yacoby, Nature Phys. 4, 144 (2008). 5. A. Deshpande, W. Bao, F. Miao, C. N. Lau, and B. J. Leroy, Phys. Rev. B 79, 205411 (2009). 6. S. V. Morozov, K. S. Novoselov, M. I. Katsnelson, F. Schedin, L. A. Ponomarenko, D. Jiang, and A. K. 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Nanotechnol. 15, 1203–1208, 2015 . 2015 www.aspbs.com/jnn Ripple-Free Graphene Sheets on Alkanethiol Self-Assembled Monolayers Provided from Unzipped Multi-Walled Carbon Nanotubes Hyunmo Koo 1 , Nam-Suk Lee 2 , Hoon-Kyu Shin 2 , Jaegeun. Ripple-Free Graphene Sheets on Alkanethiol Self-Assembled Monolayers Provided from Unzipped MWCNTs Figure 4. STM images (V b =−05V,I = 20 pA) of the SOG monolayer without ripples on the C8S SAMs. Publishers Ripple-Free Graphene Sheets on Alkanethiol Self-Assembled Monolayers Provided from Unzipped MWCNTs Kooetal. electrical properties originated from the ripple formation in the near future. Acknowledgment: