- Nano Express
- Open Access
Observation of strain effect on the suspended graphene by polarized Raman spectroscopy
Nanoscale Research Letters volume 7, Article number: 533 (2012)
We report the strain effect of suspended graphene prepared by micromechanical method. Under a fixed measurement orientation of scattered light, the position of the 2D peaks changes with incident polarization directions. This phenomenon is explained by a proposed mode in which the peak is effectively contributed by an unstrained and two uniaxial-strained sub-areas. The two axes are tensile strain. Compared to the unstrained sub-mode frequency of 2,672 cm−1, the tension causes a red shift. The 2D peak variation originates in that the three effective sub-modes correlate with the light polarization through different relations. We develop a method to quantitatively analyze the positions, intensities, and polarization dependences of the three sub-peaks. The analysis reflects the local strain, which changes with detected area of the graphene film. The measurement can be extended to detect the strain distribution of the film and, thus, is a promising technology on graphene characterization.
Raman and surface-enhanced Raman spectroscopy have been widely used to investigate vibration properties of materials[1–6]. Recently, they have been used as powerful technologies to characterize the phonons of graphene[7–14]. With its one to several atomic layers, graphene is the thinnest sp2 allotrope of carbon, and therefore holds many unique electrical and optical properties, interesting scientists and technologists[15–19]. The unique properties change with the number of atomic layers, defects, and dopants. These factors also affect graphene's phonon modes, and therefore, Raman spectroscopy is a useful method to reflect the variation of the properties[20–22]. In addition, Raman spectroscopy can be employed to determine strain, which modifies the characteristic of materials, such as band structure, and thus influences the performance of corresponding devices[23–25]. Recently, several groups study strain of graphene by artificially bending or stretching the film and then measuring the corresponding Raman spectra[26–30].
Suspended graphene are fabricated by mechanical exfoliation of graphene flakes onto the oxidized silicon wafer. First, ordered squares with areas of 6 μm2 are defined by photolithography on an oxidized silicon wafer with oxide thickness of 300 nm. Reactive ion etching is then used to etch the squares to a depth of 150 nm. Micromechanical cleavage of HOPG with scotch tape is then used to deposit the suspended graphene flakes over the indents, as shown in the schematic of Figure1a. Optical micrograph and atomic force microscopy (AFM) image, as shown in Figure1b,c, were used to characterize the suspended graphene. The surface of suspended graphene was bulging as indicated by AFM cross-section. The strain and defects of graphene are usually measured by Raman spectroscopy. To understand the strain of the suspended graphene, a micro-Raman microscope was used to perform Raman polarization-dependence measurements. A 532-nm frequency-doubling Nd-YAG laser serves as the excitation light source. The polarization and power of the incident light were adjusted by a half-wave plate and a polarizer. The laser power was monitored by a power meter and maintained through these measurements. The excitation laser power measured by the power meter is 8 mW. The laser power is measured on the delivered path between the polarizer and spectroscopy. After the delivery of laser light, the power of laser on the graphene surface is finally about 0.45 mW. The laser beam was focused by a ×50 objective lens (NA = 0.75) to the sample with a focal spot size of approximately 0.5 μm, representing the spatial resolution of the Raman system. The scattered radiation was collected backward with the same objective lens and polarization-selected by a polarization analyzer. Finally, the radiation was sent to a 55-cm spectrometer plus a liquid-nitrogen-cooled charge-coupled device for spectral recording. For the polarization dependent measurement, the polarization of incident (scattered) laser is controlled by the polarizer (analyzer). In the exploration, the polarization direction of the incident and scattered light are variable and fixed, respectively. The variability of polarization direction of the scattered light is 20°.
Results and discussion
Polarized Raman spectra of 2D modes under incident lights with different polarization angle (Φ) are shown in Figure2. The Φ is defined as the included angle between the incident polarization direction and the analyzer. The clear anti-symmetric spectra with different polarization angle can be observed in the spectra. Fitting of the 2D peaks is done using a double-Lorentzian function for Φ of (a) 0° and (b) 90°.
The 2D peak originates from four-step Stokes-Stokes double-resonance Raman scattering. According to the previous review, the 2D band split when strain is applied. The two angles relating to the two maximum peak positions are exhibited in Figure3, respectively. The two Lorentzian peaks for Figure3a are at 2,646 (2D−) and 2,660 (2D+) cm−1, while that for Figure3b are at 2,647 (2D−) and 2,662 (2D+) cm−1. To systematically analyze the sub-peak of the 2D modes, the spectra of 2D modes with different polarization angle can be fitted by double-Lorentz function, and the 2,647 and 2,660 cm−1 by average of all the peak positions of 2D+ and 2D− modes with different polarization angles. The 2D0 showed the peak position at 2,672 cm−1 as an original 2D peak in the unstrained graphene. The peak positions of 2D+ and 2D− modes compared with an original 2D peak both red shifts. The results showed that the 2D+ and 2D− modes are both tensile strains on suspended graphene. To understand the strain effect of the suspended graphene, fitting the 2D peak of graphene by a triple-Lorentzian function whose three peaks correspond to the sub-modes of 2D+, 2D−, and 2D0 with Φ = 40°, which is just an example of all the spectra, is shown in Figure3c.
To systematically analyze the intensities of the fitting sub-peaks, the plots of I(2D+), I(2D−), and I(2D0) as functions of Φ is shown in Figure4. The analysis of the suspended graphene was shown in Figure4a. The 2D+ and 2D− sub-bands which have the peak positions of 2,647 and 2,660 cm−1, respectively, showed a prominent sinusoidal intensity modulation with a period of 180°. Both the modulation of the 2D+ and 2D− bands can be fitted by a function of cos2(θA − θP), where θA and θP are the polarization angles of the analyzer and polarizer, respectively. Both the intensities of 2D+ and 2D− modes are the maximum when Φ = 0° and minimum when Φ = 90°. This result of suspended graphene is very different from those of the supported graphene by previous research. Compared with supported graphene, the same direction of tensile strain on the suspended graphene by analyzing 2D+ and 2D− modes can be obtained.
The strains of the 2D+ and 2D− can be calculated by employing the Grüneisen parameter (γ) for the 2D mode of graphene. The corresponding equation is written as, where ω0 is the 2D peak position at zero strain and Δω is the shift caused by the strain of ε. For the uniaxial strain,is the uniaxial strain and is the relative strain in the perpendicular direction due to Poissons’ ratio of graphene. In addition, the γ has been measured as 1.24 from the experiment on CNTs. Hence the stains of for the 2D+ and 2D− are estimated as 0.44% and 0.93%, respectively. Based on these results, the distribution of strain on the suspended graphene can be obtained through our analysis.
Another interesting phenomenon measured from our sample can be observed in Figure4b. The analysis of the supported graphene which was used the same method in Figure4a was shown in Figure4b. The 2D+ and 2D− sub-bands having the peak positions of 2,651 and 2,661 cm−1, respectively, showed a prominent sinusoidal intensity modulation with a period of 180°. Both the modulation of the 2D+ and 2D− bands can be fitted by a function of cos2(θA − θP), where θA and θP are the polarization angles of the analyzer and polarizer, respectively. Both the intensities of 2D+ and 2D− modes are the maximum when Φ = 1° and minimum when Φ = 91°. Using the same calculation, the stains of for the 2D+ and 2D− are estimated as 0.44% and 0.93%, respectively. The result in Figure4b is similar with Figure4a. Based on the results, we believed the strain will be relaxed to a new condition during the fabricated process of substrate.
We have explored the suspended graphene by polarized Raman spectroscopy. In the exploration, the polarization direction of the incident and scattered light are variable and fixed, respectively. The position and intensity of the graphene's 2D peak is modified by the incident polarization, and the modification is explained by a proposed biaxial-strained model. In this model, the 2D peak is contributed by three effective areas related to unstrained and two tensile-strained graphene, respectively. The two strains are uniaxial and in the same directions. The strength of the strains is quantified through our analysis. This analytical method can be used to probe strain and help us understand the situation of suspended graphene. Hence, this method provides great application potential on graphene-based electrical and optical devices, whose performance usually relies on strain.
CWH received his BS degree in Electronic Engineering from the National University of Kaohsiung, Kaohsiung, Taiwan, in 2008. He studied his MS degree in 2008 and PhD degree immediately in 2009. Currently, he is a PhD candidate in the Department of Photonics, National Cheng Kung University, Tainan, Taiwan. He focuses on the property of graphene and surface plasmon resonance of nanoparticles.
BJL received his BS degree in Physics from the National Chung-Hsing University, Taichung, Taiwan, in 2010. He received his MS degree in Photonics from the National Cheng Kung University, Tainan, Taiwan, in 2012. His research interests mainly include Raman measurement of graphene. He is in compulsory military service now.
Suëtaka W: Surface Infrared and Raman Spectroscopy: Methods and Applications. New York: Plenum; 1995.
Wang JK, Tsai CS, Lin CE, Lin JC: Vibrational dephasing dynamics at hydrogenated and deuterated semiconductor surfaces: symmetry analysis. J Chem Phys 2000, 113: 5041–5052. 10.1063/1.1289928
Kneipp K, Moskovits M, Kneipp H: Surface-Enhanced Raman Scattering: Physics and Applications. Berlin: Springer; 2006.
Wang HH, Liu CY, Wu SB, Liu NW, Peng CY, Chan TH, Hsu CF, Wang JK, Wang YL: Highly Raman-enhancing substrates based on silver nanoparticle arrays with tunable sub-10 nm gaps. Adv Mater 2006, 18: 491–495. 10.1002/adma.200501875
Liu CY, Dvoynenko MM, Lai MY, Chan TH, Lee YR, Wang JK, Wang YL: Anomalously enhanced Raman scattering from longitudinal optical phonons on Ag-nanoparticle-covered GaN and ZnO. Appl Phys Lett 2010, 96: 033109. 10.1063/1.3291041
Huang CH, Lin HY, Chen ST, Liu CY, Chui HC, Tzeng YH: Electrochemically fabricated self-aligned 2-D silver/alumina arrays as reliable SERS sensors. Opt Express 2011, 19: 11441–11450. 10.1364/OE.19.011441
Ferrari AC, Meyer JC, Scardaci V, Casiraghi C, Lazzeri M, Mauri F, Piscanec S, Jiang D, Novoselov KS, Roth S, Geim AK: Raman spectrum of graphene and graphene layers. Phys Rev Lett 2006, 97: 187401.
Malard LM, Pimenta MA, Dresselhaus G, Dresselhaus MS: Raman spectroscopy in graphene. Phys Rep 2009, 473: 51–87. 10.1016/j.physrep.2009.02.003
Gao LB, Ren WC, Liu BL, Saito R, Wu ZS, Li SS, Jiang CB, Li F, Cheng HM: Surface and interference coenhanced Raman scattering of graphene. ACS Nano 2009, 3: 933–939. 10.1021/nn8008799
Schedin F, Lidorikis E, Lombardo A, Kravets VG, Geim AK, Grigorenko AN, Novoselov KS, Ferrari AC: Surface-enhanced Raman spectroscopy of graphene. ACS Nano 2010, 4: 5617–5626. 10.1021/nn1010842
Wu D, Zhang F, Liu P, Feng X: Two-dimensional nanocomposites based on chemically modified graphene. Chemistry 2011, 17: 10804–10812. 10.1002/chem.201101333
Calizo I, Bao WZ, Miao F, Lau CN, Balandin AA: The effect of substrates on the Raman spectrum of graphene: graphene-on-sapphire and graphene-on-glass. Appl Phys Lett 2007, 91: 201904. 10.1063/1.2805024
Calizo I, Miao F, Bao W, Lau CN, Balandin AA: Variable temperature Raman microscopy as a nanometrology tool for graphene layers and graphene-based devices. Appl Phys Lett 2007, 91: 071913. 10.1063/1.2771379
Calizo I, Bejenari I, Rahman M, Liu G, Balandin AA: Ultraviolet Raman microscopy of single and multilayer graphene. J Appl Phys 2009, 106: 043509. 10.1063/1.3197065
Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, Grigorieva IV, Firsov AA: Electric field effect in atomically thin carbon films. Science 2004, 306: 666–669. 10.1126/science.1102896
Geim AK, Novoselov KS: The rise of graphene. Nat Mater 2007, 6: 183–191. 10.1038/nmat1849
Geim AK: Graphene: status and prospects. Science 2009, 324: 1530–1534. 10.1126/science.1158877
Balandin AA: Thermal properties of graphene and nanostructured carbon materials. Nat Mater 2011, 10: 569–581. 10.1038/nmat3064
Ghosh S, Nika DL, Pokatilov EP, Balandin AA: Heat conduction in graphene: experimental study and theoretical interpretation. New J Phys 2009, 11: 095012. 10.1088/1367-2630/11/9/095012
Compagnini G, Forte G, Giannazzo F, Raineri V, La Magna A, Deretzis I: Ion beam induced defects in graphene: Raman spectroscopy and DFT calculations. J Mol Struct 2011, 993: 506–509. 10.1016/j.molstruc.2010.12.065
Sahoo S, Palai R, Katiyar RS: Polarized Raman scattering in monolayer, bilayer, and suspended bilayer graphene. J Appl Phys 2011, 110: 044320. 10.1063/1.3627154
Cancado LG, Jorio A, Ferreira EHM, Stavale F, Achete CA, Capaz RB, Moutinho MVO, Lombardo A, Kulmala TS, Ferrari AC: Quantifying defects in graphene via Raman spectroscopy at different excitation energies. Nano Lett 2011, 11: 3190–3196. 10.1021/nl201432g
Castro Neto AH, Guinea F, Peres NMR, Novoselov KS, Geim AK: The electronic properties of graphene. Rev Mod Phys 2009, 81: 109–162. 10.1103/RevModPhys.81.109
Huang MY, Yan HG, Heinz TF, Hone J: Probing strain-induced electronic structure change in graphene by Raman spectroscopy. Nano Lett 2010, 10: 4074–4079. 10.1021/nl102123c
Pezzotti G: Spatially resolved Raman and cathodoluminescence probes in electronic materials: basics and applications. Phys Status Solidi A-Appl Mat 2011, 208: 976–999. 10.1002/pssa.201000785
Ni ZH, Yu T, Lu YH, Wang YY, Feng YP, Shen ZX: Uniaxial strain on graphene: Raman spectroscopy study and band-gap opening. ACS Nano 2008, 2: 2301–2305. 10.1021/nn800459e
Mohiuddin TMG, Lombardo A, Nair RR, Bonetti A, Savini G, Jalil R, Bonini N, Basko DM, Galiotis C, Marzari N, Novoselove KS, Geim AK, Ferrari AC: Uniaxial strain in graphene by Raman spectroscopy: G peak splitting, Gruneisen parameters, and sample orientation. Phys Rev B 2009, 79: 205433.
Huang MY, Yan HG, Chen CY, Song DH, Heinz TF, Hone J: Phonon softening and crystallographic orientation of strained graphene studied by Raman spectroscopy. Proc Natl Acad Sci USA 2009, 106: 7304–7308. 10.1073/pnas.0811754106
Sasaki K, Wakabayashi K, Enoki T: Polarization dependence of Raman spectra in strained graphene. Phys Rev B 2010, 82: 205407.
Yoon D, Son YW, Cheong H: Strain-dependent splitting of the double-resonance Raman scattering band in graphene. Phys Rev Lett 2011, 106: 155502.
Yoon D, Moon H, Son YW, Samsonidze G, Park BH, Kim JB, Lee Y, Cheong H: Strong polarization dependence of double-resonant Raman intensities in graphene. Nano Lett 2008, 8: 4270–4274. 10.1021/nl8017498
Yoon D, Son YW, Cheong H: Strain-dependent splitting of the double-resonance Raman scattering band in graphene. Phys Rev Lett 2011., 106:
Liu F, Ming PM, Li J: Ab initio calculation of ideal strength and phonon instability of graphene under tension. Phys Rev B 2007, 76: 064120.
Reich S, Jantoljak H, Thomsen C: Shear strain in carbon nanotubes under hydrostatic pressure. Phys Rev B 2000, 61: 13389–13392. 10.1103/PhysRevB.61.R13389
We wish to acknowledge the support of this work by the National Science Council, Taiwan under contact no. NSC 98-2112-M-006-004-MY3.
The authors declare that they have no competing interests.
CWH and BJL carried the experimental parts: the acquisition, analysis, and interpretation of data. CWH also had been involved in drafting the manuscript. HYL and CHH performed the analysis and interpretation of data. They also had been involved in revising the manuscript. FYS and WHW (Institute of Atomic and Molecular Sciences, Academia Sinica) prepared the samples, suspended the graphene using micromechanical method, and captured the OM and AFM images. CYL has made substantial contributions to the conception and design of the study, and the critical revision of the manuscript for important intellectual content. HCC, the corresponding author, had made substantial contributions to the conception and design of the study, and had been involved in drafting the manuscript and revising it critically for important intellectual content. All authors read and approved the final manuscript.
About this article
Cite this article
Huang, CW., Lin, BJ., Lin, HY. et al. Observation of strain effect on the suspended graphene by polarized Raman spectroscopy. Nanoscale Res Lett 7, 533 (2012). https://doi.org/10.1186/1556-276X-7-533
- Graphene strain
- Polarization Raman spectroscopy
- suspended graphene
- 78.67.Wj (optical properties of graphene)
- 74.25.nd (Raman and optical spectroscopy)
- 63.22.Rc (phonons in graphene)