Article pubs.acs.org/Langmuir
Minimizing Unintentional Strain and Doping of Single-Layer Graphene on SiO2 in Aqueous Environments by Acid Treatments Katsuya Masuda and Masahito Sano* Department of Polymer Science and Engineering, Yamagata University, 4-3-16 Jyonan, Yonezawa, Yamagata 992-8513, Japan S Supporting Information *
ABSTRACT: The effects of treating SiO2/Si with either acidic or alkaline solutions on single-layer graphene were investigated using Raman microscopy. It is well-known that in air graphene on SiO2 is unintentionally strained and hole-doped to different degrees, varying widely by sample. It is also known that various amine compounds act as electron donors to graphitic materials. In this study, a SiO2/Si substrate was simply dipped in either a concentrated HCl solution or pH 9.0 NaOH solution and then rinsed, prior to transferring graphene on it. The G and 2D peaks were followed at a fixed position on a single-layer graphene flake in water and various concentrations of pH 7.4 tris(hydroxymethyl)aminomethane (Tris) buffer. The results demonstrate that these treatments reduce the sample variation, improve the stability against Tris, and even bring some graphene samples close to a freestanding state. The Raman analysis reveals that the main effect of dipping is to relieve strain. The undoping effect on some samples is explained by the HCl solution becoming trapped between the graphene and SiO2 surface.
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INTRODUCTION
Tris is a necessary condition for using graphene as a platform for biochemical studies. In addition to doping, graphene on a SiO2/Si substrate is often unintentionally strained. This result occurs during the handling processes and is also caused by adhesion to a SiO2/Si surface that has nanometer scale roughness. This behavior results in either compression or tensile strain on graphene, modulating the electronic properties in various degrees.19,20 Presently, it appears that straining on graphene is inevitable on a SiO2/Si surface due to its intrinsic physical roughness and chemical inhomogeneity. Thus, a technique to relax the strain needs to be developed for stable and reproducible operation of graphene electronic devices. To minimize the unintentional strain and doping, selfassembled monolayers have been introduced on the SiO2 surface as a buffer layer.21−24 We are interested in other methods that do not involve organic films because they may interfere with the device performance. A recent theoretical study indicates that one of the stable oxygen radicals in a SiO2 network strongly hole-doped graphene.25 This behavior suggests the possibility that acid or alkali treatments may neutralize active dopants. Raman spectroscopy is a powerful tool to evaluate electronic and vibrational properties of graphene.26 The main peaks consist of D-band (∼1350 cm−1), G-band (∼1580 cm−1), and 2D band (∼2675 cm−1). The D peak is usually associated with graphene defects and organic adsorbates. The G and 2D peaks
Graphene is a zero band gap semiconductor with a linear band dispersion at the Dirac point.1 This means that graphene can be electron- or hole-doped in a similar manner by shifting Fermi energy across the Dirac point. Currently, this outcome can be controllably achieved by either electrical2 or molecular doping.3 Graphene also possesses a chemically inert, flat planar surface with a macroscopic area. These properties make graphene an attractive nanomaterial for applications in various electronic devices, including field-effect transistors,4 electrochemical electrodes,5 and biosensors.6 However, the same linear dispersion causes the electronic properties to be susceptible to doping when placed on a solid substrate that supports graphene.7 For instance, simply placing graphene on a SiO2/Si substrate, which is the most popular solid support for transistors and sensors, is reported to induce hole-doping.8−11 The doping level varies from sample to sample and even depends on the position within a single graphene flake.12−14 Although atmospheric oxygen and water are known to enhance the effect,15,16 the mechanism of unintentional doping is not well understood. Substrates are not the only materials that affect the electronic properties. In electrochemical or bioapplications, graphene is submerged in liquid. It is well-known that amines act as electron donors to single-walled carbon nanotubes (SWCNTs).17 In particular, tris(hydroxymethyl)aminomethane (Tris), a common buffer agent used in biochemical studies, was observed to reduce proteins by electron transfer through SWCNTs.18 Although there has been no report suggesting that graphene reacts in the same way as a SWCNT, stability against © XXXX American Chemical Society
Received: January 27, 2015 Revised: April 15, 2015
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Figure 1. Raman spectra from the fixed position on a single-layer graphene with various Tris concentrations. The contribution from Tris-HCl buffer has been subtracted.
Figure 2. Center frequency of the G peak in air, water, and various Tris concentrations for (a) the untreated samples and (b) the acid- and alkalitreated samples with rinsing (filled and half-filled marks) and without rinsing (empty marks). The black and red dashed line correspond to the values of graphite and freestanding graphene, respectively. and adhesive tape on these SiO2/Si substrates. The adhesive residues were removed by heating to 350 °C in air for 1 h. For each concentration of Tris, the equimolar HCl solution was added to adjust the pH to 7.4 with D2O as water. Confocal Raman microscopy (Almega XR, Nicolet) was operated with a 532 nm laser with a beam spot of approximately 0.7 μm. The graphene sample was placed at the bottom of a closed cell that could hold liquid inside and exchange the liquid through the side openings. The top face of the cell was made with cover glass for Raman measurements. For a given graphene sample, a single-layer graphene flake, whose typical size was several micrometers long, was selected by measuring a Raman spectrum in air. With the laser beam fixed at the same position, the cell was filled with pure water. This procedure caused the focal point to move slightly due to the different refractive indexes of air and water. Although refocusing might move the laser spot along the graphene plane, the distance moved was undetectable under microscopic observation with respect to a reference marker placed on the substrate beforehand. After obtaining a spectrum in water, the water was replaced by the lowest concentration Tris buffer, and the spectrum was acquired without changing the beam position. Subsequently, the lower concentration buffer was replaced with the higher one, and the measurement was repeated.
originate from the Brillouin zone centered E2g phonon and double-resonance boundary phonons, respectively. In particular, the 2D peak is sensitive to the number of stacked graphene layers.27−29 A symmetric 2D line shape with the large intensity ratio of 2D to G is a characteristic of single-layer graphene. The center frequencies and line widths of G and 2D peaks of a single-layer graphene shift by either doping (change in charge density) or mechanical strain. Here, we follow a method developed by Lee et al.30 to decompose the observed peak shifts to doping and strain contributions. In this study, single-layer graphene on SiO2, immersed in Tris-HCl buffer, is investigated using Raman spectroscopy. The laser beam was fixed at the same position, focused on a particular graphene, while changing the Tris concentration. We demonstrate that simply dipping the SiO2 substrate in concentrated HCl before transferring graphene frees unintentional strain and doping, making the graphene behave like freestanding graphene.
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EXPERIMENTAL SECTION
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A Si substrate with 300 nm thick SiO2 was sonicated (100 W bath sonicator) for 5 min in acetone and washed with hot water. For the acid treatment, the cleaned SiO2/Si substrate was immersed in concentrated HCl (37%) for 1 h and then dried immediately by blowing N2 over the surface. Some samples were further rinsed thoroughly with water and dried. For the alkali treatment, the cleaned SiO2/Si substrate was immersed in a NaOH solution at pH 9.0 for 1 h, followed by thorough rising with water and drying. Graphene samples were made by the micromechanical exfoliation method using HOPG
RESULTS AND DISCUSSION With no liquid inside the cell, we determined whether the graphene was single-layer in air. The graphene was classified to be defect-free and single-layer by the absence of the D peak, the 2D peak having a symmetric line shape, and a 2D to G peak ratio greater than 2. Figure 1 shows the D, G, and 2D peaks at various Tris concentrations. The peak at D band increases with B
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Figure 3. G line width in air, water, and various Tris concentrations for (a) the untreated samples and (b) the acid- and alkali-treated samples with rinsing (filled and half-filled marks) and without rinsing (empty marks). The black and red dashed line correspond to the values of graphite and freestanding graphene, respectively.
Figure 4. Center frequency of the 2D peak in air, water, and various Tris concentrations for (a) the untreated samples and (b) the acid- and alkalitreated samples with rinsing (filled and half-filled marks) and without rinsing (empty marks). The black and red dashed line correspond to the values of graphite and freestanding graphene, respectively.
freestanding graphene and remain completely insensitive to Tris. The line widths of the G peaks are shown in Figure 3. The G line width is known to increase when either electron- or holedoped graphene is undoped. In air, the widths of nearly all untreated samples stay close to the graphite value and then increase as Tris is added (SD = 2.4 cm−1). On the other hand, the widths of most treated samples are below the graphite values in air. Placement in water causes large shifts among some samples. Once in water, however, the widths of all samples stay close to those of graphite and freestanding even after Tris is introduced (SD = 1.7 cm−1). These behaviors are consistent with the lowering of the G center frequency, suggesting either that strain is relieved or that hole-doping is weakened by bringing them in aqueous environments. Figure 4 reveals the response of the 2D center frequency to Tris. Whereas the G center frequency is independent of the excitation laser energy, the 2D center frequency increases with the laser energy.32 The literature values of freestanding graphene have been measured with a 514.5 nm laser.30,31 Here, we have estimated the value at 532 nm to be slightly smaller than the literature values, with the understanding that the value may vary by several cm−1. The 2D center frequency is known to increase (decrease) upon compression (tensile) strain or hole (electron) doping. In air, most samples show upshift, indicating that the samples are compressed or holedoped in accordance with the G center frequency and line width. Then, with the exception of a sample having a large jump, the untreated samples show a slight decrease, whereas
the Tris concentration, suggesting that it is caused by Tris molecules being adsorbed to the graphene. Whereas a clear downshift of the G peak is observed, no changes in the line shape are recognizable. To follow the subtle change, each peak was fitted with a Lorentzian curve (Supporting Information). Figure 2 presents the center frequency of the G peak in air, water, and various concentrations of Tris for the untreated and treated samples. The values of graphite and freestanding graphene30,31 are also indicated as references. In Figures 2−6, a line connecting data points follows the response of a particular graphene sample. The same sample is denoted using the unique colored and shaped symbol. Because NaOH might have etched the SiO2 surface, we used only the rinsed samples. As Figures 2−6 show, the rinsed alkali-treated samples have yielded the same results as the rinsed acid-treated samples. In all cases, the G center frequency in air is upshifted, indicating that all graphene samples are either hole-doped or strained. This result is in good agreement with other studies.8−16 For the untreated samples, additions of Tris tend to lower the G center frequency. It is evident that the relative response to Tris varies widely from sample to sample (the standard deviation among sample variation SD = 1.9 cm−1). In contrast, the sample variations are small among the rinsed treated samples (SD = 1.1 cm−1). For the unrinsed acid-treated samples, the upshifted frequencies in air are lowered when placed in water. After being in water, the frequencies remain nearly constant even when Tris is introduced. Most amazingly, some unrinsed acid-treated samples recover the value of C
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Figure 5. 2D line width in air, water, and various Tris concentrations for (a) the untreated samples and (b) the acid- and alkali-treated samples with rinsing (filled and half-filled marks) and without rinsing (empty marks). The red dashed line corresponds to the value of freestanding graphene.
Figure 6. Intensity ratio of 2D to G in air, water, and various Tris concentrations for (a) the untreated samples and (b) the acid- and alkali-treated samples with rinsing (filled and half-filled marks) and without rinsing (empty marks).
Figure 7. 2D-G space for (a) the untreated samples and (b) the acid- and alkali-treated samples with rinsing (darker color) and without rinsing (lighter color). The values of freestanding graphene define the green colored origin. The black lines with the slopes of 2.5 and 0.7 are the strain and hole-doping axes, respectively.
have the large sample dependence (SD = 5.0 cm−1) and values greater than that of freestanding graphene, while the treated samples have hardly any sample dependence (SD = 1.5 cm−1) that remains close to the freestanding value and are insensitive to the Tris concentration. The 2D:G intensity ratio is often used as a measure of graphene “quality”. However, it is difficult to obtain a consistent response to Tris as given in Figure 6. The data points are also widely scattered when the ratio is plotted against the G center frequency (not shown). We have also investigated the 2D/G peak area instead of peak height and obtained a similar result. This result indicates that we must be cautious about using the 2D/G ratio for a measure of quality. All data consistently indicate that each treatment minimizes the sample variability and improves the stability against Tris
the treated samples remain constant against Tris once in water. The sample shows that the large 2D jump is not accompanied by a large change in the G peak. Presently, we do not understand why this phenomenon occurs. Similar to the G band, 2D frequency measurements of the untreated samples have the largest sample dependence (SD = 4.5 cm−1), while the rinsed treated samples vary little (SD = 2.5 cm−1). The unrinsed acid-treated samples that give the G values of freestanding graphene stay close to the corresponding freestanding 2D frequency. The 2D line width is presented in Figure 5. Because of double resonance processes, effects of strain or doping on the 2D line width are complicated. Many researchers have reported that the 2D line width hardly changes when the doping level is electrically modulated.2,8,33 It is clear that the untreated samples D
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the SiO2 surface. Because the untreated SiO2 surface is slightly hydrophobic, it is unlikely that water flows into the vacant space when the graphene sample is brought into water. The similar acid and alkali treatments applied here are known to increase roughness and improve water wettability on glass.36 We expect that the present treatments have allowed water to spread more easily into the vacant space. In this case, both front and back faces of the stained part of graphene are surrounded by water, creating the situation resembling a freely dispersed graphene in water. Then, the surface tensions acting on both faces relax the compressional stress. If nanometer-sized HCl droplets had remained on the SiO2 surface, they must be strongly adhered and are likely hidden inside the roughed valleys. In this case, the HCl droplets could not make contact with graphene in air. Once in aqueous environments, however, the water that has spread into the vacant space dissolve HCl, and the hydronium ions could diffuse to the graphene surface for transferring charges. Since the volume that water has spread is on the nanometer scale, the pH must be much lower than our pH measurement with a 2 μL droplet and is sufficient to undope the hole-doped graphene. The present acid treatment is technically simpler to apply than the previously studied self-assembled monolayers. It has an advantage of not including organic films that are not stable at high temperatures and lack reliability for long-time use. The present treatment is, however, only effective in aqueous environments whereas self-assembled monolayers is applicable in air.
within the similarly treated batch. Furthermore, the unrinsed acid treatment brings the spectral characteristics toward those of freestanding graphene. It is therefore important to analyze the factors causing these results. The effects of mechanical strain and charge doping on the G and 2D center frequency are relatively well understood. Recently, Lee et al. have demonstrated a graphical method to decompose the observed shifts into each contribution.30 The 2D center frequency is plotted against the G center frequency. The values of a strainfree, uncharged graphene define the origin of 2D-G space. As both frequencies are known to change linearly with strain, a line passing through the origin with a definite slope defines “the strain axis”. Changes induced by hole doping can also be approximated as linear provided that the doping level is not very large. Hole doping only moves the G center frequency upward; thus, another line starting from the origin to the right with a definite slope defines “the hole-doping axis”. Then, any point in the nonorthogonal 2D-G space can be decomposed vectorially into strain and hole-doping components. In Figure 7, the value of freestanding graphene is used as an origin to construct a 2D-G space (the identical plots using the symbols used in Figures 2−5 are given in the Supporting Information). The black lines passing through the origin define the strain and hole-doping axes, respectively. The slopes of 2.5 and 0.7 are the averages of the values listed in Lee et al. For the untreated samples, most of the points scatter over the first quadrant, indicating that the samples are both compressionally strained and hole-doped. The points at low G and high 2D center frequencies imply that this particular sample is strongly strained but not doped. This scattering over the first quadrant explains why the untreated sample has the large sample variability. The acid and alkali treatments gather most points closer to the doping axis. Contrary to our initial expectation that these treatments would neutralize dopant species on a SiO2 substrate, they instead relieve strain. For the untreated samples, the red points distribute closer to the origin than the blue points, implying that electrons transferred from Tris neutralize hole doping to some degree. For the rinsed acid- and alkali-treated samples, the effect of Tris is limited. These observations indicate that electron transfer from Tris is not sufficient to undope hole-doped graphene. Some unrinsed acid-treated samples, however, have reached the origin. Previous electrical measurements have shown that lowering solution pH electron-dopes graphene.34,35 Although no residue was observable by the naked eye when the silicon substrate was taken out of the HCl solution and the entire substrate was heated to 350 °C, very small HCl droplets might have been trapped in the surface defects. To verify this possibility, we measured the pH of a 2 μL pure water droplet placed on a surface of the unrinsed acid-treated sample. Over ten measurements gave pH = 5, only slightly more acidic than the pure water, implying that the trapped HCl droplets, even if existed, must be very small amounts (see Supporting Information for experimental details). Relieving strain and neutralizing hole doping were achieved by treating the SiO2 substrate prior to transferring graphene on it. We must conclude that the treatment had modified the SiO2 surface, not graphene. Interestingly, the results of this SiO2 modification appear on graphene not in air but in aqueous environments. Because a graphene flake is compressionally strained, some parts of the flake must be free from adhesion to the SiO2 surface. This implies that there are some nanometer scale vacant spaces between the strained area of graphene and
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CONCLUSIONS The spectral response of the untreated samples to Tris depends heavily on a particular sample. The acid and alkali treatments not only reduce this sample dependence but also make graphene insensitive to a change of the Tris concentration. The Raman analysis shows that the role played by the acid and alkali treatments is to relieve strain in graphene on SiO2. This study demonstrates that the acid treatment without rinse is capable of making graphene on SiO2 behave like freestanding graphene. In the present study, this behavior occurred in half of the graphene flakes randomly selected over many flakes on a SiO2 surface. Finding additional conditions to increase this occurrence would lead to more stable and reliable graphene that is suitable for applications in electrochemical devices and biosensors.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
A full Raman spectrum and Lorentzian fitting. This material is available free of charge via the Internet at http://pubs.acs.org.
Corresponding Author
*E-mail:
[email protected] (M.S.). Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Castro Neto, A. H.; Guinea, F.; Peres, N. M. R. The Electronic Properties of Graphene. Rev. Mod. Phys. 2009, 81, 109−162. (2) Yan, J.; Zhang, Y.; Kim, P.; Pinczuk, A. Electric Field Effect Tuning of Electron-Phonon Coupling in Graphene. Phys. Rev. Lett. 2007, 98, 166802. (3) Voggu, R.; Das, B.; Rout, C. S.; Rao, C. N. R. Effects of Charge Transfer Interaction of Graphene with Electron Donor and Acceptor
E
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(24) Lee, W. H.; Park, J.; Kim, Y.; Kim, K. S.; Hong, B. H.; Cho, K. Control of Graphene Field-Effect Transistors by Interfacial Hydrophobic Self-Assembled Monolayers. Adv. Mater. 2011, 23, 3460−3464. (25) Nistor, R.; Kuroda, M. A.; Maarouf, A. A.; Martyna, G. J. Doping of Adsorbed Graphene from Defects and Impurities in SiO2 Substrates. Phys. Rev. B 2012, 86, 041409. (26) Malard, L. M.; Pimenta, M. A.; Dresslhaus, G.; Dresselhaus, M. S. Raman Spectroscopy in Graphene. Phys. Rep. 2009, 473, 51−87. (27) Gupta, A.; Chen, G.; Joshi, P.; Tadigadapa, S.; Eklund, P. C. Nano Lett. 2006, 6, 2667−2673. (28) Graf, D.; Molitor, F.; Ensslin, K.; Stampfer, C.; Jungen, A.; Hierold, C.; Wirtz, L. Spatially Resolved Raman Spectroscopy of Single- and Few-Layer Graphene. Nano Lett. 2007, 7, 238−242. (29) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. (30) Lee, J. E.; Ahn, G.; Shim, J.; Lee, Y. S.; Ryu, S. Optical Separation of Mechanical Strain from Charge Doping in Graphene. Nat. Commun. 2012, 3, 1024. (31) Berciaud, S.; Ryu, S.; Brus, L. E.; Heinz, T. F. Probing the Intrinsic Properties of Exfoliated Graphene: Raman Spectroscopy of Free-Standing Monolayers. Nano Lett. 2009, 9, 346−352. (32) Malard, L. M.; Mafra, D. L.; Doorn, S. K.; Pimenta, M. A. Resonance Raman Scattering in Graphene: Probing Phonons and Electrons. Solid State Commun. 2009, 149, 1136−1139. (33) Stampfer, C.; Wirtz, L.; Jungen, A.; Graf, D.; Molitor, F.; Hierold, C.; Ensslin, K. Raman Imaging of Doping Domains in Graphene on SiO2. Appl. Phys. Lett. 2007, 91, 241907. (34) Ang, P. K.; Chen, W.; Wee, A. T. S.; Loh, K. P. Solution-Gated Epitaxial Graphene as pH Sensor. J. Am. Chem. Soc. 2008, 130, 14392− 14393. (35) Cheng, Z.; Li, Q.; Li, Z.; Zhou, Q.; Fang, Y. Suspended Graphene Sensors with Improved Signal and Reduced Noise. Nano Lett. 2010, 10, 1864−1868. (36) Eske, L. D.; Galipeau, D. W. Characterization of SiO2 Surface treatments Using AFM, Contact Angles and a Novel Dewpoint Technique. Colloids Surf., A 1999, 154, 33−51.
Molecules Examined Using Raman Spectroscopy and Cognate Techniques. J. Phys.: Condens. Matter 2008, 20, 472204. (4) Schwierz, F. Graphene Transistors. Nat. Nanotechnol. 2010, 5, 487−496. (5) Wu, S.; He, Q.; Tan, C.; Wang, Y.; Zhang, H. Graphene-Based Electrochemical Sensors. Small 2013, 9, 1160−1172. (6) Kuila, T.; Bose, S.; Khanra, P.; Mishra, A. K.; Kim, N. H.; Lee, J. H. Recent Advances in Graphene-Based Biosensors. Biosens. Bioelectron. 2011, 26, 4637−4648. (7) Wang, Y.; Ni, Z.; Yu, T.; Shen, Z.; Wang, H.; Wu, Y.; Chen, W.; Wee, A. Raman Studies of Monolayer Graphene: The Substrate Effect. J. Phys. Chem. C 2008, 112, 10637−10640. (8) Casiraghi, C.; Pisana, S.; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C. Raman Fingerprint of Charged Impurities in Graphene. Appl. Phys. Lett. 2007, 91, 233108. (9) Shi, Y.; Dong, X.; Chen, P.; Wang, J.; Li, L. Effective Doping of Single-Layer Graphene from Underlying SiO2 Substrates. Phys. Rev. B 2009, 79, 115402. (10) Romero, H. E.; Shen, N.; Joshi, P.; Gutierrez, H. R.; Tadigadapa, S. A.; Sofo, J. O.; Eklund, P. C. n-Type Behavior of Graphene Supported on Si/SiO2 Substrates. ACS Nano 2008, 2, 2037−2044. (11) Rudenko, A. N.; Keil, F. J.; Katsnelson, M. I.; Lichtenstein, A. I. Interfacial Interactions between Local Defects in Amorphous SiO2 and Supported Graphene. Phys. Rev. B 2011, 84, 085438. (12) Ziegler, D.; Gava, P.; Güttinger, J.; Molitor, F.; Wirtz, L.; Lazzeri, M.; Saitta, A. M.; Stemmer, A.; Mauri, F.; Stampfer, C. Variations in the Work Function of Doped Single- and Few-Layer Graphenes Assessed by Kelvin Probe Force Microscopy and Density Functional Theory. Phys. Rev. B 2011, 83, 235434. (13) Castellanos-Gomez, A.; Smit, R.; Agraït, N.; Rubio-Bollinger, G. Spatially Resolved Electronic Inhomogeneities of Graphene Due to Subsurface Charges. Carbon 2012, 50, 932−938. (14) Burson, K. M.; Cullen, W. G.; Adam, S.; Dean, C. R.; Watanabe, K.; Taniguchi, T.; Kim, P.; Fuhrer, M. S. Direct Imaging of Charged Impurity Density in Common Graphene Substrates. Nano Lett. 2013, 13, 3576−3580. (15) Ryu, S.; Liu, L.; Berciaud, S.; Yu, Y.; Liu, H.; Kim, P.; Flynn, G. W.; Brus, L. E. Atmospheric Oxygen Binding and Hole Doping in Deformed Graphene and a SiO2 Substrate. Nano Lett. 2010, 10, 4944− 4951. (16) Lee, Y. G.; Kang, C. G.; Cho, C.; Kim, Y.; Hwang, H. J.; Lee, B. H. Quantitative Analysis of Hysteretic Reactions at the Interface of Graphene and SiO2 Using the Short Pulse I-V Method. Carbon 2013, 60, 453−460. (17) Kong, J.; Dai, H. Full and Modulated Chemical Gating of Individual Carbon Nanotubes by Organic Amine Compounds. J. Phys. Chem. B 2001, 105, 2890−2893. (18) Nakashima, T.; Sano, M. Single-Walled Carbon Nanotubes as 1D Array of Reacting Sites: Reaction Kinetics of Reduction of Cytochrome c in Tris Buffer. J. Phys. Chem. C 2011, 115, 20931− 20936. (19) Teague, M. L.; Lai, A. P.; Velasco, J.; Hughes, C. R.; Beyer, A. D.; Bockrath, M. W.; Lau, C. N.; Yeh, N. Evidence for Strain-Induced Local Conductance Modulations in Single-Layer Graphene on SiO2. Nano Lett. 2009, 9, 2542−2546. (20) Deshpandle, A.; Bao, W.; Miao, F.; Lau, C. N.; LeRoy, B. J. Spatially Resolved Spectroscopy of Monolayer Graphene on SiO2. Phys. Rev. B 2009, 79, 205411. (21) Lafkioti, M.; Krauss, B.; Lohmann, T.; Zschieschang, U.; Klauk, H.; Klitzing, K.; Smet, J. H. Graphene on a Hydrophobic Substrate: Doping Reduction and Hysteresis Suppression under Ambient Conditions. Nano Lett. 2010, 10, 1149−1153. (22) Yan, Z.; Sun, Z.; Lu, W.; Yao, J.; Zhu, Y.; Tour, J. M. Controlled Modulcation of Electronic Properties of Graphene by Self-Assembled Monolayers on SiO2 Substrates. ACS Nano 2011, 5, 1535−1540. (23) Wang, X.; Xu, J.; Wang, C.; Du, J.; Xie, W. High-Performance Graphene Devices on SiO2/Si Substrate Modified by Highly Ordered Self-Assembled Monolayers. Adv. Mater. 2011, 23, 2464−2468. F
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