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Estimating Young’s modulus of graphene with Raman scattering enhanced by micrometer tip
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 255703 (http://iopscience.iop.org/0957-4484/25/25/255703) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 207.162.240.147 This content was downloaded on 11/05/2017 at 07:36 Please note that terms and conditions apply.
You may also be interested in: Atomic force and shear force based tip-enhanced Raman spectroscopy and imaging S S Kharintsev, G G Hoffmann, P S Dorozhkin et al. Possibilities of functionalized probes in optical near-field microscopy Eugene G Bortchagovsky, Ulrich C Fischer and Thomas Schmid Elastic and nonlinear response of nanomechanical graphene devices M Annamalai, S Mathew, M Jamali et al. Electronic properties of graphene: a perspective from scanning tunneling microscopy and magnetotransport Eva Y Andrei, Guohong Li and Xu Du Plasmonic optical antenna design for performing tip-enhanced Raman spectroscopy and microscopy S S Kharintsev, G G Hoffmann, A I Fishman et al. Near-field photonics: tip-enhanced microscopy and spectroscopy on the nanoscale Neil Anderson, Alexandre Bouhelier and Lukas Novotny Fabrication of Near-Field Plasmonic Tip by Photoreduction for Strong Enhancement in Tip-Enhanced Raman Spectroscopy Takayuki Umakoshi, Taka-aki Yano, Yuika Saito et al. Theoretical study of carbon-based tips for scanning tunnelling microscopy C González, E Abad, Y J Dappe et al. Tip-enhanced Raman scattering microscopy: Recent advance in tip production Yasuhiko Fujita, Peter Walke, Steven De Feyter et al.
Nanotechnology Nanotechnology 25 (2014) 255703 (5pp)
doi:10.1088/0957-4484/25/25/255703
Estimating Young’s modulus of graphene with Raman scattering enhanced by micrometer tip Shao-Wei Weng1,2, Wei-Hsiang Lin1, Wei-Bin Su1, En-Te Hwu1, Peilin Chen3, Tsong-Ru Tsai2 and Chia-Seng Chang1 1
Institute of Physics, Academia Sinica, Nankang, Taipei 11529, Taiwan Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan 3 Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan 2
E-mail: [email protected] Received 11 February 2014, revised 16 April 2014 Accepted for publication 30 April 2014 Published 4 June 2014 Abstract
We demonstrate that the Raman intensities of G and 2D bands of a suspended graphene can be enhanced using a gold tip with an apex size of 2.3 μm. The enhancement decays with the tipgraphene distance exponentially and remains detectable at a distance of 1.5 μm. Raman mappings show that the enhanced area is comparable to the apex size. Application of a bias voltage to the tip can attract the graphene so that Raman signals are intensified. The exponential enhancement-distance relationship enables the measurement of the graphene deformation, and the Young’s modulus of graphene is estimated to be 1.48 TPa. Keywords: tip-enhanced Raman scattering, graphene, Young’s modulus (Some figures may appear in colour only in the online journal) 1. Introduction
Because the tip used in TERS studies is typically nanometer-sized, investigations on the response of Raman scattering using a micrometer-size tip are rather sparse. One advantage of employing a micrometer-sized tip is that the enhanced region generated in Raman mapping can immediately be optically visualized. In practice, this requires that optical microscopy combined with Raman spectroscopy be operated under the transmission mode, and that the sample is on a transparent substrate, such as glass. However, Yang et al have demonstrated that using a dielectric substrate degrades the enhancement effect [26]. Thus, a more ideal situation involves suspending the observed object without a supporting substrate. Due to the fact that graphene is a transparent material that can be suspended [27–31], it is an adequate material for studying TERS at the micrometer scale. In this study, we show that the enhancement of Raman scattering can be generated by a gold tip with an apex of 2.3 μm on the suspended graphene, and the enhanced area is comparable to the apex size. Moreover, the enhancement exponentially decays with the increment of the tip-graphene distance and remains detectable even at a distance as large as 1.5 μm.
.Tip-enhanced Raman scattering (TERS) spectroscopy [1–3] combined with scanning probe microscopy is capable of identifying the chemical properties of surface structures with nanometer spatial resolution [4]. Consequently, TERS has attracted considerable attention in the past decade regarding improving its enhancement factor and spatial resolution, as well as probing the properties of nanostructures using a gold or silver tip of a nanometer apex [5–13]. Zhang et al recently demonstrated that the spatial resolution of TERS can reach 1 nm, and the chemical properties of a single molecule can be detected [14]. Graphene [15], the thinnest material in nature, possesses unique electronic, thermal, and mechanical properties, such as near ballistic transport at room temperature [16], high mobility of charge carrier [17], superior thermal conductivity [18, 19], and extremely high Young’s modulus [20]. Therefore, this material has great potential applications in transistors with frequencies approaching the GHz range [21], as well as in supercapacitors [22], ultrafast photodetectors [23], transparent conductive films [24], and solar cells [25]. 0957-4484/14/255703+05$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
S-W Weng et al
Nanotechnology 25 (2014) 255703
The Young’s modulus of graphene has been measured using the nano-indentation technique in atomic force microscopy (AFM) [19] and the graphene balloon combined with Raman spectroscopy [32]. Here, we demonstrate an alternative method for estimating the Young’s modulus of graphene, with which the suspended graphene at approximately 60 μm in size was deformed by applying a bias voltage to the nearby tip. The graphene deformation is measured with the exponential enhancement-distance relationship, and its Young’s modulus is estimated with the model of a circular membrane subjected to a point load.
2. Experimental methods Commercial graphene grown on a Cu foil was used to fabricate our sample. The Cu foil was first etched by a Fe(NO3)3 solution to leave a single-layer graphene floating on the liquid surface. The Fe(NO3)3 solution was subsequently diluted using DI water mixed with isopropyl alcohol until the solution became colorless. The solution was gradually expelled from the container, in which we placed a Ni mesh with hexagonal holes at the bottom. The floating graphene was in contact with the mesh after the solution was expelled completely. No polymer material was used in the entire process. The gold tip was produced by electrochemical etching, and its apex size was derived from the scanning electron microscopic (SEM) image. The sample and tip were mounted on a module (figure 1) where the tip position can be adjusted with two steppers in both x and z directions, and the sample was made mobile in the y direction with a fine screw. We used this module to control the gold tip to approach the suspended graphene. The contact of the tip and the graphene was monitored by the measurement of the resistance between them. The stepper movement in the z-direction was measured by an interferometer. We utilized the contact of the tip and the graphene as a reference point, and thus the distance between the tip and the graphene could be determined precisely. This module was combined with a micro-Raman microscope, thereby allowing us to observe the tip location and TERS. For acquiring the Raman spectra, we employed a 532 nm laser of 0.1 mW power without polarization and a 100× object lens with a numerical aperture of 0.95. The laser spot size is about 1 μm.
Figure 1. Schematic diagram of a micro-Raman microscope
combined with a module in which a gold tip can be moved by two steppers in the x and z directions; and the position of the sample can be adjusted in the y direction using a fine screw.
contact with the graphene, which was verified by monitoring the resistance between the tip and the graphene. Since the focal point was at the graphene, the apex image in figure 2(d) is clearer than those in figures 2(b) and (c). The spot size in figure 2(d) is approximately 3 μm, which is comparable to the tip size of 2.3 μm measured by SEM in the figure 2(e) inset. Figure 2(e) displays the Raman spectra of suspended graphene acquired at the region where the gold tip apex appears. The intensities of 2D and G bands of the graphene change with the tip-graphene distance. Intensities of both bands are the weakest when no tip is present, and they are evidently enhanced when the tip appears at a distance of 1.5 μm from the graphene. The band intensities are the highest as the tip touches the graphene. Obviously, the enhanced band intensities are caused by the appearance of the gold tip, which is a typical TERS behavior. These results demonstrate that the TERS phenomenon does not exclude the size of the tip apex and the tip-sample distance beyond the nanometer scale. Figure 2(f) shows the Raman mapping of the 2D band of suspended graphene without the presence of a tip. When the tip is at a distance of 1.5 μm from the graphene, as exhibited in figure 2(g), the region marked by a dashed circle is illuminated. Figures 2(i) and (j) show that the same region grows brighter as the tip-graphene distance is reduced to 0.5 μm and 0 (contact), respectively, which is consistent with the spectra illustrated in figure 2(e). The size of the enhanced region is approximately 3 μm, implying that the tip size determines the enhanced area.
3. Results and discussion Figure 2(a) shows an optical image of the graphene suspended above a hexagonal hole. The hole is surrounded by the step structure (marked by an arrow), which is another hexagon with a side length of 34.6 μm and is higher than the hole edge by 4 μm. Therefore, the graphene is suspended from the step structure instead of the edge of the hole. The bright spot marked by an arrow in figure 2(b) is the tip apex located near the graphene at a distance of 1.5 μm. The distance between the apex and the graphene was reduced to 0.5 μm, as shown in figure 2(c). Figure 2(d) shows the apex in 2
S-W Weng et al
Nanotechnology 25 (2014) 255703
Figure 3. (a) Raman spectra of the graphene at various tip-graphene distances. Inset: an optical image of a gold tip appearing below the graphene. The apex size of the tip is 7 μm. (b) and (c) are the enhancement factors of the 2D and G bands versus the tip-graphene distance for apex sizes of 2.3 and 7 μm, respectively.
Figure 2. (a)–(c), and (d) are optical images of the same area. The
scale bar is 10 μm. (a) Graphene suspended above a hexagonal hole. (b) The bright spot marked by an arrow is the apex of a gold tip that is near the graphene at a distance of 1.5 μm. (c) The distance between the apex and graphene is 0.5 μm. (d) The apex is in contact with graphene. (e) Raman spectra of the graphene acquired at the region where the apex of the gold tip appears for different tipgraphene distances. Inset: scanning electron microscope image of the gold tip with a 2.3 μm apex size. (f)–(h), and (i) are mappings of the Raman intensity of the 2D band, corresponding to the situations shown in (a)–(c), and (d), respectively. The scale bar is 3 μm.
distance as follows: F = A exp ( − d λ ) R + 1,
(1)
where A and λ are constants, d is the tip-graphene distance, and R is the apex size. The solid curves in figures 3(c) and (d) are derived from equation (1) with optimal As and λs. Each curve fits the data well, indicating that equation (1) is valid, and that the enhancement factor indeed decays with tipgraphene distance exponentially. The TERS induced by a micrometer-size tip manifests the essential characteristics of nanoscale TERS: the enhanced area comparable to the apex size, the exponential relationship between enhancement and distance [33], and the weakening of enhancement with the apex size. This implies that outstanding TERS behaviors at the nanometer scale could be induced as well by the micrometer-size tip. As an example, the enhanced area that determines the spatial resolution of nanoscale TERS is known to vary with the polarization and angle of the incident light [26]. Similar influence by these factors on the enhanced area could also be realized at the micrometer regime. It is known that the D band can reflect the existence of defects in the graphene [34]. In figure 3(a), the intensity of the D band increases with decreasing the tip-graphene distance, indicating that the region where the 7 μm tip is located has defects. On the contrary, the region where the 2.3 μm tip is located has no defect; therefore, in figure 2(e), there is no signal of D band in 1.5−, 1.0−, and 0.5 μm spectra. However,
The inset in figure 3(a) reveals an optical image of a gold tip with a 7 μm apex just below the suspended graphene. The appearance of the tip enhances the intensities of both the 2D and G bands, and again their intensities increase as the tipgraphene distance decreases, as shown in figure 3(a). The enhancement factor is defined as the ratio between the band intensities with and without the tip appearance. Figures 3(b) and (c) depict the enhancement factors of the 2D and G bands versus the tip-graphene distance for two apex sizes of 2.3 and 7 μm, respectively. The enhancement factor of the 2.3 μm apex is always larger than that of the 7 μm apex at the same distance. This indicates that the apex size has an essential influence on the enhancement factor. The enhancement factors for individual 2D and G bands are nearly the same for the two apex sizes, and the decay of the enhancement factor as a function of the distance is close to the exponential form. Moreover, when the tip is in contact with graphene, the ratio of the 2.3 μm enhancement factor to the 7 μm enhancement factor is approximately 3 for both bands, which is roughly equal to the ratio of the apex size. Therefore, we define the relationship between the enhancement factor (F) and the 3
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Nanotechnology 25 (2014) 255703
a previous study demonstrated that the D band can be induced using a gold tip to contact the graphene [35]. Hence, we can observe the appearance of the D band in the spectrum where the 2.3 μm tip is in contact with the graphene in figure 2(e). Furthermore, in figure 3(a), the intensity of the D band in the spectrum of the 7 μm tip in contact with the graphene is more pronounced, which is attributed to the fact that the observed D band is the superposition of the contact-induced D band and the tip-enhanced D band. Previous studies have demonstrated that suspended graphene can be deformed when a bias voltage is applied to a nearby electrode [27, 36]. Recently, Chan et al observed that the electric field can result in the expansion of Pb films [37]. In this work, we demonstrate that the deformation of graphene can be measured with the exponential enhancementdistance relationship shown in figure 3(c). We used the same tip shown in figure 2 to approach the graphene until the enhancement factor of the 2D band reached 3.3. Using the exponential fitting curve shown in figure 3(c), we determined that it corresponded to a tip-graphene distance of 0.86 μm. We subsequently applied a bias voltage between the tip and the graphene and observed that the intensities of the 2D and G bands were enhanced, as exhibited in figure 4(a), in which the spectra at 0 V and 10 V are shown. The two spectra at 0 V were the spectrum before the bias voltage was applied and after the bias voltage was reduced to 0 V. Three Raman mappings of the 2D band in figure 4(a) inset also reveal that the region above the tip (marked by a dashed circle) at 5 V (middle) is brighter than the regions at 0 V (left and right). The enhancement indicates that the suspended graphene is attracted to the tip due to the electrostatic force. Moreover, figure 4(a) also shows that after the bias voltage was reduced to 0 V, the original intensities of the 2D and G bands were recovered. This indicates that the flexible graphene returned to its original position after the voltage was turned off. Figure 4(b) shows that the enhancement factor of the 2D band changed from 3.3 to 6.3 as voltage increased gradually from 0 to 12 V. Referring to the fit curve in figure 3(c), we infer that at 12 V, the tip-graphene distance d is 0.11 μm, and the displacement (deformation) w of graphene attracted by the tip is 0.75 μm. Since the apex size is 2.3 μm, a similar size of graphene would be pulled toward the tip, and the suspended graphene (from the side view) would thus be deformed into a curve, as illustrated in the inset of figure 4(b). The Young’s modulus of the graphene can be estimated using d and w at 12 V. The tip-graphene distance of 0.11 μm is much smaller than the size of the tip apex. Therefore, we first approximate the apex and the graphene region near the apex using two parallel discs with a diameter D of 2.3 μm. The electrostatic force P between the discs follows: P = ε0 V2π D 2 8d2 ,
Figure 4. (a) Raman spectra of the graphene acquired at the region
where the apex of the gold tip appears for 0 V and 10 V. Inset: mappings of the Raman intensity of the 2D band reveal that the region of the tip location (marked by a dashed circle) at 5 V (middle) is brighter than the ones at 0 V (left and right). (b) The enhancement factor of the 2D band is changed from 3.3 to 6.3 as the voltage is increased gradually from 0 to 12 V. Inset: schematic illustration showing that from the side view, the graphene is deformed into a curve when a bias voltage is applied to the tip.
freestanding circular graphene subjected to a point force through nano-indentation in AFM [20]. They described the force-displacement behavior of the graphene by formulating an equation in which the applied force is equal to the summation of a force, proportional to the cube of displacement, and a force caused by the pretension σ in the graphene, which is σπw. Lee et al reported that the average pretension is approximately 0.7 N m−1 for graphene membranes of 1 and 1.5 μm in diameter. Using this value to calculate the force resulting from the pretension at 12 V, the value 1.65 μN is obtained, which is much larger than the applied electrostatic force of 0.22 μN at 12 V. This implies that a displacement of 0.75 μm is not possible when a pretension of 0.7 N m−1 exists in a hexagonal suspended graphene with a size of approximately 60 μm. Thus, the pretension that occurs in this study is much smaller than 0.7 N m−1. Since the relevant size in this study is much larger than 1.5 μm, we assume that the force caused by the pretension can be neglected, and that the
(2)
where V is the applied bias voltage, and ε0 is the permittivity of free space. Using equation (2), the force at 12 V is 0.22 μN. Because the tip apex is much smaller than the suspended graphene, we consider the electrostatic force as a point force. Lee et al investigated the deformation of a 4
S-W Weng et al
Nanotechnology 25 (2014) 255703
[4] Notingher I and Elfick A 2005 J. Phys. Chem. B 109 15699 [5] Anderson N, Hartschuh A, Cronin S and Novotny L 2005 J. Am. Chem. Soc. 127 2533 [6] Pettinger B, Ren B, Picardi G, Schuster R and Ertl G 2004 Phys. Rev. Lett. 92 096101 [7] Kalkbrenner T, Håkanson U and Sandoghdar V 2004 Nano Lett. 4 2309 [8] Zhang W, Cui X, Yeo B-S, Schmid T, Hafner C and Zenobi R 2007 Nano Lett. 7 1401 [9] Steidtner J and Pettinger B 2008 Phys. Rev. Lett. 100 236101 [10] Stadler J, Schmid T and Zenobi R 2010 Nano Lett. 10 4514 [11] Reparaz J S et al 2013 Nanotechnology 24 185704 [12] Chan T-S, Dvoynenko M M, Liu C-Y, Wang J-K and Wang Y-L 2009 Opt. Lett. 34 2248 [13] Ichimura T, Fujii S, Verma P, Yano T, Inouye Y and Kawata S 2009 Phys. Rev. Lett. 102 186101 [14] Zhang R et al 2013 Nature 498 82 [15] Novoselov K S, Geim A K, Morozov S V, Jiang D, Zhang Y, Dubonos S V, Grigorieva I V and Firsov A A 2004 Science 306 666 [16] Du X, Skachko I, Barker A and Andrei E Y 2008 Nat. Nanotechnology 3 491 [17] Morozov S V, Novoselov K S, Katsnelson M I, Schedin F, Elias D C, Jaszczak J A and Geim A K 2008 Phys. Rev. Lett. 100 016602 [18] Balandin A A, Ghosh S, Bao W, Calizo I, Teweldebrhan D, Miao F and Lau C N 2008 Nano Lett. 8 902 [19] Chen S S, Li Q Y, Zhang Q M, Qu Y, Ji H X, Ruoff R S and Cai W W 2012 Nanotechnology 23 365701 [20] Lee C, Wei X, Kysar J W and Hone J 2008 Science 321 385 [21] Lin Y-M, Jenkins K A, Valdes-Garcia A, Small J P, Farmer D B and Avouris P 2008 Nano Lett. 9 422 [22] Yoo J J et al 2011 Nano Lett. 11 1423 [23] Xia F, Mueller T, Lin Y-M, Valdes-Garcia A and Avouris P 2009 Nat. Nanotechnology 4 839 [24] Becerril H A, Mao J, Liu Z, Stoltenberg R M, Bao Z and Chen Y 2008 ACS Nano 2 463 [25] Wang X, Zhi L and Müllen K 2008 Nano Lett. 8 323 [26] Yang Z, Aizpurua J and Xu H 2009 J. Raman Spectrosc. 40 1343 [27] Bunch J S, van der Zande A M, Verbridge S S, Frank I W, Tanenbaum D M, Parpia J M, Craighead H G and McEuen P L 2007 Science 315 490 [28] Bao W, Miao F, Chen Z, Zhang H, Jang W, Dames C and Lau C N 2009 Nat. Nanotechnology 4 562 [29] Koenig S P, Boddeti N G, Dunn M L and Bunch J S 2011 Nat. Nanotechnology 6 543 [30] Min K and Aluru N R 2011 Appl. Phys. Lett. 98 013113 [31] Huang C-W, Lin B-J, Lin H-Y, Huang C-H, Shih F-Y, Wang W-H, Liu C-Y and Chui H-C 2012 Nanoscale Res. Lett. 7 533 [32] Lee J-U, Yoon D and Cheong H 2012 Nano Lett. 12 4444 [33] Yano T, Ichimura T, Taguchi A, Hayazawa N, Verma P, Inouye Y and Kawata S 2007 Appl. Phys. Lett. 91 121101 [34] Ni Z H et al 2010 Nano Lett. 10 3868 [35] Wang P J, Zhang D, Li L L, Li Z P, Zhang L S and Fang Y 2012 Plasmonics 7 555 [36] AbdelGhany M, Ledwosinska E and Szkopek T 2012 Appl. Phys. Lett. 101 153102 [37] Chan W Y, Huang H S, Su W B, Lin W H, Jeng H-T, Wu M K and Chang C S 2012 Phys. Rev. Lett. 108 146102 [38] Komaragiri U and Begley M R 2005 J. Appl. Mech. 72 203
applied electrostatic force is proportional to the displacement cube. Furthermore, the hexagonal shape of the suspended graphene is approximated by a circle of radius of 30 μm. According to the derivation of Komaragiri et al [38], the relationship between displacement and a point force exerted on a circular membrane of radius r without pretension obeys: 1 3
w r = f (v) ( P Erh) ,
(3)
where E is Young’s modulus, h is the graphene thickness (0.335 nm), and f(ν) = 1.0491 − 0.1462ν − 0.15827ν2. ν is Poisson’s ratio and is equal to 0.165 for graphite. By substituting equation (2) into equation (3), the Young’s modulus of the graphene is determined to be E = 1.48 TPa. This value is comparable to the 1.1 TPa acquired using nano-indentation in AFM [20]. Lee et al demonstrated that the positions of the G and 2D bands would change when the graphene is subjected to an external stress of 101 300 N m−2 [33]. However, we did not observe obvious changes in the positions of the G and 2D bands with applied voltage. This is due to the stress that is only about 78 N m−2 at 12 V. The changes of positions of the G and 2D bands are unable to be detected under such a minute stress.
4. Conclusions In summary, using the suspended graphene, we demonstrate that the essential features of nanoscale TERS are still preserved in the micro-scale TERS. In particular, the enhancement remains detectable at a distance of 1.5 μm, which is observed for the first time and is unexpected for the theoretical prediction. By combining TERS with the application of a bias voltage to deform suspended graphene, we develop an alternative method for estimating the Young’s modulus of graphene.
Acknowledgments The authors are grateful to C S Chen, T H Liao, C C Cheng, and C H Hsieh for their technical support. This work is supported by the National Science Council (Grant No.: NSC 100-2112-M-001-018-MY3) and Academia Sinica of Taiwan.
References [1] Stöckle R, Suh Y D, Deckert V and Zenobi R 2000 Chem. Phys. Lett. 318 131 [2] Anderson M S 2000 Appl. Phys. Lett. 76 3130 [3] Hayazawa N, Inouye Y, Sekhat Z and Kawata S 2000 Opt. Commun. 183 333
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Estimating Young’s modulus of graphene with Raman scattering enhanced by micrometer tip
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 255703 (http://iopscience.iop.org/0957-4484/25/25/255703) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 207.162.240.147 This content was downloaded on 11/05/2017 at 07:36 Please note that terms and conditions apply.
You may also be interested in: Atomic force and shear force based tip-enhanced Raman spectroscopy and imaging S S Kharintsev, G G Hoffmann, P S Dorozhkin et al. Possibilities of functionalized probes in optical near-field microscopy Eugene G Bortchagovsky, Ulrich C Fischer and Thomas Schmid Elastic and nonlinear response of nanomechanical graphene devices M Annamalai, S Mathew, M Jamali et al. Electronic properties of graphene: a perspective from scanning tunneling microscopy and magnetotransport Eva Y Andrei, Guohong Li and Xu Du Plasmonic optical antenna design for performing tip-enhanced Raman spectroscopy and microscopy S S Kharintsev, G G Hoffmann, A I Fishman et al. Near-field photonics: tip-enhanced microscopy and spectroscopy on the nanoscale Neil Anderson, Alexandre Bouhelier and Lukas Novotny Fabrication of Near-Field Plasmonic Tip by Photoreduction for Strong Enhancement in Tip-Enhanced Raman Spectroscopy Takayuki Umakoshi, Taka-aki Yano, Yuika Saito et al. Theoretical study of carbon-based tips for scanning tunnelling microscopy C González, E Abad, Y J Dappe et al. Tip-enhanced Raman scattering microscopy: Recent advance in tip production Yasuhiko Fujita, Peter Walke, Steven De Feyter et al.
Nanotechnology Nanotechnology 25 (2014) 255703 (5pp)
doi:10.1088/0957-4484/25/25/255703
Estimating Young’s modulus of graphene with Raman scattering enhanced by micrometer tip Shao-Wei Weng1,2, Wei-Hsiang Lin1, Wei-Bin Su1, En-Te Hwu1, Peilin Chen3, Tsong-Ru Tsai2 and Chia-Seng Chang1 1
Institute of Physics, Academia Sinica, Nankang, Taipei 11529, Taiwan Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan 3 Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan 2
E-mail: [email protected] Received 11 February 2014, revised 16 April 2014 Accepted for publication 30 April 2014 Published 4 June 2014 Abstract
We demonstrate that the Raman intensities of G and 2D bands of a suspended graphene can be enhanced using a gold tip with an apex size of 2.3 μm. The enhancement decays with the tipgraphene distance exponentially and remains detectable at a distance of 1.5 μm. Raman mappings show that the enhanced area is comparable to the apex size. Application of a bias voltage to the tip can attract the graphene so that Raman signals are intensified. The exponential enhancement-distance relationship enables the measurement of the graphene deformation, and the Young’s modulus of graphene is estimated to be 1.48 TPa. Keywords: tip-enhanced Raman scattering, graphene, Young’s modulus (Some figures may appear in colour only in the online journal) 1. Introduction
Because the tip used in TERS studies is typically nanometer-sized, investigations on the response of Raman scattering using a micrometer-size tip are rather sparse. One advantage of employing a micrometer-sized tip is that the enhanced region generated in Raman mapping can immediately be optically visualized. In practice, this requires that optical microscopy combined with Raman spectroscopy be operated under the transmission mode, and that the sample is on a transparent substrate, such as glass. However, Yang et al have demonstrated that using a dielectric substrate degrades the enhancement effect [26]. Thus, a more ideal situation involves suspending the observed object without a supporting substrate. Due to the fact that graphene is a transparent material that can be suspended [27–31], it is an adequate material for studying TERS at the micrometer scale. In this study, we show that the enhancement of Raman scattering can be generated by a gold tip with an apex of 2.3 μm on the suspended graphene, and the enhanced area is comparable to the apex size. Moreover, the enhancement exponentially decays with the increment of the tip-graphene distance and remains detectable even at a distance as large as 1.5 μm.
.Tip-enhanced Raman scattering (TERS) spectroscopy [1–3] combined with scanning probe microscopy is capable of identifying the chemical properties of surface structures with nanometer spatial resolution [4]. Consequently, TERS has attracted considerable attention in the past decade regarding improving its enhancement factor and spatial resolution, as well as probing the properties of nanostructures using a gold or silver tip of a nanometer apex [5–13]. Zhang et al recently demonstrated that the spatial resolution of TERS can reach 1 nm, and the chemical properties of a single molecule can be detected [14]. Graphene [15], the thinnest material in nature, possesses unique electronic, thermal, and mechanical properties, such as near ballistic transport at room temperature [16], high mobility of charge carrier [17], superior thermal conductivity [18, 19], and extremely high Young’s modulus [20]. Therefore, this material has great potential applications in transistors with frequencies approaching the GHz range [21], as well as in supercapacitors [22], ultrafast photodetectors [23], transparent conductive films [24], and solar cells [25]. 0957-4484/14/255703+05$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
S-W Weng et al
Nanotechnology 25 (2014) 255703
The Young’s modulus of graphene has been measured using the nano-indentation technique in atomic force microscopy (AFM) [19] and the graphene balloon combined with Raman spectroscopy [32]. Here, we demonstrate an alternative method for estimating the Young’s modulus of graphene, with which the suspended graphene at approximately 60 μm in size was deformed by applying a bias voltage to the nearby tip. The graphene deformation is measured with the exponential enhancement-distance relationship, and its Young’s modulus is estimated with the model of a circular membrane subjected to a point load.
2. Experimental methods Commercial graphene grown on a Cu foil was used to fabricate our sample. The Cu foil was first etched by a Fe(NO3)3 solution to leave a single-layer graphene floating on the liquid surface. The Fe(NO3)3 solution was subsequently diluted using DI water mixed with isopropyl alcohol until the solution became colorless. The solution was gradually expelled from the container, in which we placed a Ni mesh with hexagonal holes at the bottom. The floating graphene was in contact with the mesh after the solution was expelled completely. No polymer material was used in the entire process. The gold tip was produced by electrochemical etching, and its apex size was derived from the scanning electron microscopic (SEM) image. The sample and tip were mounted on a module (figure 1) where the tip position can be adjusted with two steppers in both x and z directions, and the sample was made mobile in the y direction with a fine screw. We used this module to control the gold tip to approach the suspended graphene. The contact of the tip and the graphene was monitored by the measurement of the resistance between them. The stepper movement in the z-direction was measured by an interferometer. We utilized the contact of the tip and the graphene as a reference point, and thus the distance between the tip and the graphene could be determined precisely. This module was combined with a micro-Raman microscope, thereby allowing us to observe the tip location and TERS. For acquiring the Raman spectra, we employed a 532 nm laser of 0.1 mW power without polarization and a 100× object lens with a numerical aperture of 0.95. The laser spot size is about 1 μm.
Figure 1. Schematic diagram of a micro-Raman microscope
combined with a module in which a gold tip can be moved by two steppers in the x and z directions; and the position of the sample can be adjusted in the y direction using a fine screw.
contact with the graphene, which was verified by monitoring the resistance between the tip and the graphene. Since the focal point was at the graphene, the apex image in figure 2(d) is clearer than those in figures 2(b) and (c). The spot size in figure 2(d) is approximately 3 μm, which is comparable to the tip size of 2.3 μm measured by SEM in the figure 2(e) inset. Figure 2(e) displays the Raman spectra of suspended graphene acquired at the region where the gold tip apex appears. The intensities of 2D and G bands of the graphene change with the tip-graphene distance. Intensities of both bands are the weakest when no tip is present, and they are evidently enhanced when the tip appears at a distance of 1.5 μm from the graphene. The band intensities are the highest as the tip touches the graphene. Obviously, the enhanced band intensities are caused by the appearance of the gold tip, which is a typical TERS behavior. These results demonstrate that the TERS phenomenon does not exclude the size of the tip apex and the tip-sample distance beyond the nanometer scale. Figure 2(f) shows the Raman mapping of the 2D band of suspended graphene without the presence of a tip. When the tip is at a distance of 1.5 μm from the graphene, as exhibited in figure 2(g), the region marked by a dashed circle is illuminated. Figures 2(i) and (j) show that the same region grows brighter as the tip-graphene distance is reduced to 0.5 μm and 0 (contact), respectively, which is consistent with the spectra illustrated in figure 2(e). The size of the enhanced region is approximately 3 μm, implying that the tip size determines the enhanced area.
3. Results and discussion Figure 2(a) shows an optical image of the graphene suspended above a hexagonal hole. The hole is surrounded by the step structure (marked by an arrow), which is another hexagon with a side length of 34.6 μm and is higher than the hole edge by 4 μm. Therefore, the graphene is suspended from the step structure instead of the edge of the hole. The bright spot marked by an arrow in figure 2(b) is the tip apex located near the graphene at a distance of 1.5 μm. The distance between the apex and the graphene was reduced to 0.5 μm, as shown in figure 2(c). Figure 2(d) shows the apex in 2
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Figure 3. (a) Raman spectra of the graphene at various tip-graphene distances. Inset: an optical image of a gold tip appearing below the graphene. The apex size of the tip is 7 μm. (b) and (c) are the enhancement factors of the 2D and G bands versus the tip-graphene distance for apex sizes of 2.3 and 7 μm, respectively.
Figure 2. (a)–(c), and (d) are optical images of the same area. The
scale bar is 10 μm. (a) Graphene suspended above a hexagonal hole. (b) The bright spot marked by an arrow is the apex of a gold tip that is near the graphene at a distance of 1.5 μm. (c) The distance between the apex and graphene is 0.5 μm. (d) The apex is in contact with graphene. (e) Raman spectra of the graphene acquired at the region where the apex of the gold tip appears for different tipgraphene distances. Inset: scanning electron microscope image of the gold tip with a 2.3 μm apex size. (f)–(h), and (i) are mappings of the Raman intensity of the 2D band, corresponding to the situations shown in (a)–(c), and (d), respectively. The scale bar is 3 μm.
distance as follows: F = A exp ( − d λ ) R + 1,
(1)
where A and λ are constants, d is the tip-graphene distance, and R is the apex size. The solid curves in figures 3(c) and (d) are derived from equation (1) with optimal As and λs. Each curve fits the data well, indicating that equation (1) is valid, and that the enhancement factor indeed decays with tipgraphene distance exponentially. The TERS induced by a micrometer-size tip manifests the essential characteristics of nanoscale TERS: the enhanced area comparable to the apex size, the exponential relationship between enhancement and distance [33], and the weakening of enhancement with the apex size. This implies that outstanding TERS behaviors at the nanometer scale could be induced as well by the micrometer-size tip. As an example, the enhanced area that determines the spatial resolution of nanoscale TERS is known to vary with the polarization and angle of the incident light [26]. Similar influence by these factors on the enhanced area could also be realized at the micrometer regime. It is known that the D band can reflect the existence of defects in the graphene [34]. In figure 3(a), the intensity of the D band increases with decreasing the tip-graphene distance, indicating that the region where the 7 μm tip is located has defects. On the contrary, the region where the 2.3 μm tip is located has no defect; therefore, in figure 2(e), there is no signal of D band in 1.5−, 1.0−, and 0.5 μm spectra. However,
The inset in figure 3(a) reveals an optical image of a gold tip with a 7 μm apex just below the suspended graphene. The appearance of the tip enhances the intensities of both the 2D and G bands, and again their intensities increase as the tipgraphene distance decreases, as shown in figure 3(a). The enhancement factor is defined as the ratio between the band intensities with and without the tip appearance. Figures 3(b) and (c) depict the enhancement factors of the 2D and G bands versus the tip-graphene distance for two apex sizes of 2.3 and 7 μm, respectively. The enhancement factor of the 2.3 μm apex is always larger than that of the 7 μm apex at the same distance. This indicates that the apex size has an essential influence on the enhancement factor. The enhancement factors for individual 2D and G bands are nearly the same for the two apex sizes, and the decay of the enhancement factor as a function of the distance is close to the exponential form. Moreover, when the tip is in contact with graphene, the ratio of the 2.3 μm enhancement factor to the 7 μm enhancement factor is approximately 3 for both bands, which is roughly equal to the ratio of the apex size. Therefore, we define the relationship between the enhancement factor (F) and the 3
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a previous study demonstrated that the D band can be induced using a gold tip to contact the graphene [35]. Hence, we can observe the appearance of the D band in the spectrum where the 2.3 μm tip is in contact with the graphene in figure 2(e). Furthermore, in figure 3(a), the intensity of the D band in the spectrum of the 7 μm tip in contact with the graphene is more pronounced, which is attributed to the fact that the observed D band is the superposition of the contact-induced D band and the tip-enhanced D band. Previous studies have demonstrated that suspended graphene can be deformed when a bias voltage is applied to a nearby electrode [27, 36]. Recently, Chan et al observed that the electric field can result in the expansion of Pb films [37]. In this work, we demonstrate that the deformation of graphene can be measured with the exponential enhancementdistance relationship shown in figure 3(c). We used the same tip shown in figure 2 to approach the graphene until the enhancement factor of the 2D band reached 3.3. Using the exponential fitting curve shown in figure 3(c), we determined that it corresponded to a tip-graphene distance of 0.86 μm. We subsequently applied a bias voltage between the tip and the graphene and observed that the intensities of the 2D and G bands were enhanced, as exhibited in figure 4(a), in which the spectra at 0 V and 10 V are shown. The two spectra at 0 V were the spectrum before the bias voltage was applied and after the bias voltage was reduced to 0 V. Three Raman mappings of the 2D band in figure 4(a) inset also reveal that the region above the tip (marked by a dashed circle) at 5 V (middle) is brighter than the regions at 0 V (left and right). The enhancement indicates that the suspended graphene is attracted to the tip due to the electrostatic force. Moreover, figure 4(a) also shows that after the bias voltage was reduced to 0 V, the original intensities of the 2D and G bands were recovered. This indicates that the flexible graphene returned to its original position after the voltage was turned off. Figure 4(b) shows that the enhancement factor of the 2D band changed from 3.3 to 6.3 as voltage increased gradually from 0 to 12 V. Referring to the fit curve in figure 3(c), we infer that at 12 V, the tip-graphene distance d is 0.11 μm, and the displacement (deformation) w of graphene attracted by the tip is 0.75 μm. Since the apex size is 2.3 μm, a similar size of graphene would be pulled toward the tip, and the suspended graphene (from the side view) would thus be deformed into a curve, as illustrated in the inset of figure 4(b). The Young’s modulus of the graphene can be estimated using d and w at 12 V. The tip-graphene distance of 0.11 μm is much smaller than the size of the tip apex. Therefore, we first approximate the apex and the graphene region near the apex using two parallel discs with a diameter D of 2.3 μm. The electrostatic force P between the discs follows: P = ε0 V2π D 2 8d2 ,
Figure 4. (a) Raman spectra of the graphene acquired at the region
where the apex of the gold tip appears for 0 V and 10 V. Inset: mappings of the Raman intensity of the 2D band reveal that the region of the tip location (marked by a dashed circle) at 5 V (middle) is brighter than the ones at 0 V (left and right). (b) The enhancement factor of the 2D band is changed from 3.3 to 6.3 as the voltage is increased gradually from 0 to 12 V. Inset: schematic illustration showing that from the side view, the graphene is deformed into a curve when a bias voltage is applied to the tip.
freestanding circular graphene subjected to a point force through nano-indentation in AFM [20]. They described the force-displacement behavior of the graphene by formulating an equation in which the applied force is equal to the summation of a force, proportional to the cube of displacement, and a force caused by the pretension σ in the graphene, which is σπw. Lee et al reported that the average pretension is approximately 0.7 N m−1 for graphene membranes of 1 and 1.5 μm in diameter. Using this value to calculate the force resulting from the pretension at 12 V, the value 1.65 μN is obtained, which is much larger than the applied electrostatic force of 0.22 μN at 12 V. This implies that a displacement of 0.75 μm is not possible when a pretension of 0.7 N m−1 exists in a hexagonal suspended graphene with a size of approximately 60 μm. Thus, the pretension that occurs in this study is much smaller than 0.7 N m−1. Since the relevant size in this study is much larger than 1.5 μm, we assume that the force caused by the pretension can be neglected, and that the
(2)
where V is the applied bias voltage, and ε0 is the permittivity of free space. Using equation (2), the force at 12 V is 0.22 μN. Because the tip apex is much smaller than the suspended graphene, we consider the electrostatic force as a point force. Lee et al investigated the deformation of a 4
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[4] Notingher I and Elfick A 2005 J. Phys. Chem. B 109 15699 [5] Anderson N, Hartschuh A, Cronin S and Novotny L 2005 J. Am. Chem. Soc. 127 2533 [6] Pettinger B, Ren B, Picardi G, Schuster R and Ertl G 2004 Phys. Rev. Lett. 92 096101 [7] Kalkbrenner T, Håkanson U and Sandoghdar V 2004 Nano Lett. 4 2309 [8] Zhang W, Cui X, Yeo B-S, Schmid T, Hafner C and Zenobi R 2007 Nano Lett. 7 1401 [9] Steidtner J and Pettinger B 2008 Phys. Rev. Lett. 100 236101 [10] Stadler J, Schmid T and Zenobi R 2010 Nano Lett. 10 4514 [11] Reparaz J S et al 2013 Nanotechnology 24 185704 [12] Chan T-S, Dvoynenko M M, Liu C-Y, Wang J-K and Wang Y-L 2009 Opt. Lett. 34 2248 [13] Ichimura T, Fujii S, Verma P, Yano T, Inouye Y and Kawata S 2009 Phys. Rev. Lett. 102 186101 [14] Zhang R et al 2013 Nature 498 82 [15] Novoselov K S, Geim A K, Morozov S V, Jiang D, Zhang Y, Dubonos S V, Grigorieva I V and Firsov A A 2004 Science 306 666 [16] Du X, Skachko I, Barker A and Andrei E Y 2008 Nat. Nanotechnology 3 491 [17] Morozov S V, Novoselov K S, Katsnelson M I, Schedin F, Elias D C, Jaszczak J A and Geim A K 2008 Phys. Rev. Lett. 100 016602 [18] Balandin A A, Ghosh S, Bao W, Calizo I, Teweldebrhan D, Miao F and Lau C N 2008 Nano Lett. 8 902 [19] Chen S S, Li Q Y, Zhang Q M, Qu Y, Ji H X, Ruoff R S and Cai W W 2012 Nanotechnology 23 365701 [20] Lee C, Wei X, Kysar J W and Hone J 2008 Science 321 385 [21] Lin Y-M, Jenkins K A, Valdes-Garcia A, Small J P, Farmer D B and Avouris P 2008 Nano Lett. 9 422 [22] Yoo J J et al 2011 Nano Lett. 11 1423 [23] Xia F, Mueller T, Lin Y-M, Valdes-Garcia A and Avouris P 2009 Nat. Nanotechnology 4 839 [24] Becerril H A, Mao J, Liu Z, Stoltenberg R M, Bao Z and Chen Y 2008 ACS Nano 2 463 [25] Wang X, Zhi L and Müllen K 2008 Nano Lett. 8 323 [26] Yang Z, Aizpurua J and Xu H 2009 J. Raman Spectrosc. 40 1343 [27] Bunch J S, van der Zande A M, Verbridge S S, Frank I W, Tanenbaum D M, Parpia J M, Craighead H G and McEuen P L 2007 Science 315 490 [28] Bao W, Miao F, Chen Z, Zhang H, Jang W, Dames C and Lau C N 2009 Nat. Nanotechnology 4 562 [29] Koenig S P, Boddeti N G, Dunn M L and Bunch J S 2011 Nat. Nanotechnology 6 543 [30] Min K and Aluru N R 2011 Appl. Phys. Lett. 98 013113 [31] Huang C-W, Lin B-J, Lin H-Y, Huang C-H, Shih F-Y, Wang W-H, Liu C-Y and Chui H-C 2012 Nanoscale Res. Lett. 7 533 [32] Lee J-U, Yoon D and Cheong H 2012 Nano Lett. 12 4444 [33] Yano T, Ichimura T, Taguchi A, Hayazawa N, Verma P, Inouye Y and Kawata S 2007 Appl. Phys. Lett. 91 121101 [34] Ni Z H et al 2010 Nano Lett. 10 3868 [35] Wang P J, Zhang D, Li L L, Li Z P, Zhang L S and Fang Y 2012 Plasmonics 7 555 [36] AbdelGhany M, Ledwosinska E and Szkopek T 2012 Appl. Phys. Lett. 101 153102 [37] Chan W Y, Huang H S, Su W B, Lin W H, Jeng H-T, Wu M K and Chang C S 2012 Phys. Rev. Lett. 108 146102 [38] Komaragiri U and Begley M R 2005 J. Appl. Mech. 72 203
applied electrostatic force is proportional to the displacement cube. Furthermore, the hexagonal shape of the suspended graphene is approximated by a circle of radius of 30 μm. According to the derivation of Komaragiri et al [38], the relationship between displacement and a point force exerted on a circular membrane of radius r without pretension obeys: 1 3
w r = f (v) ( P Erh) ,
(3)
where E is Young’s modulus, h is the graphene thickness (0.335 nm), and f(ν) = 1.0491 − 0.1462ν − 0.15827ν2. ν is Poisson’s ratio and is equal to 0.165 for graphite. By substituting equation (2) into equation (3), the Young’s modulus of the graphene is determined to be E = 1.48 TPa. This value is comparable to the 1.1 TPa acquired using nano-indentation in AFM [20]. Lee et al demonstrated that the positions of the G and 2D bands would change when the graphene is subjected to an external stress of 101 300 N m−2 [33]. However, we did not observe obvious changes in the positions of the G and 2D bands with applied voltage. This is due to the stress that is only about 78 N m−2 at 12 V. The changes of positions of the G and 2D bands are unable to be detected under such a minute stress.
4. Conclusions In summary, using the suspended graphene, we demonstrate that the essential features of nanoscale TERS are still preserved in the micro-scale TERS. In particular, the enhancement remains detectable at a distance of 1.5 μm, which is observed for the first time and is unexpected for the theoretical prediction. By combining TERS with the application of a bias voltage to deform suspended graphene, we develop an alternative method for estimating the Young’s modulus of graphene.
Acknowledgments The authors are grateful to C S Chen, T H Liao, C C Cheng, and C H Hsieh for their technical support. This work is supported by the National Science Council (Grant No.: NSC 100-2112-M-001-018-MY3) and Academia Sinica of Taiwan.
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