Observation of strain effect on the suspended graphene by polarized Raman spectroscopy
© Huang et al.; licensee Springer. 2012
Received: 11 July 2012
Accepted: 9 September 2012
Published: 26 September 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.
KeywordsGraphene 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)
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].
Results and discussion
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.
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.
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