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ABSTRACT: The Young's modulus of graphene is estimated by measuring the strain applied by a pressure difference across graphene membranes using Raman spectroscopy. The strain induced on pressurized graphene balloons can be estimated directly from the peak shift of the Raman G band. By comparing the measured strain with numerical simulation, we obtained the Young's modulus of graphene. The estimated Young's modulus values of single- and bilayer graphene are 2.4 ± 0.4 and 2.0 ± 0.5 TPa, respectively.
Nano Letters 08/2012; 12(9):4444-8. · 13.20 Impact Factor
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ABSTRACT: The dependence of the photocurrent generated in a Pd/graphene/Ti junction
device on the incident photon polarization is studied. Spatially resolved
photocurrent images were obtained as the incident photon polarization is
varied. The photocurrent is maximum when the polarization direction is
perpendicular to the graphene channel direction and minimum when the two
directions are parallel. This polarization dependence can be explained as being
due to the anisotropic electron-photon interaction of Dirac electrons in
graphene.
07/2012;
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Ik-Su Byun, Duhee Yoon,
Jin Sik Choi,
Inrok Hwang,
Duk Hyun Lee,
Mi Jung Lee,
Tomoji Kawai,
Young-Woo Son,
Quanxi Jia,
Hyeonsik Cheong,
Bae Ho Park
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ABSTRACT: Monolayer graphene is one of the most interesting materials applicable to next-generation electronic devices due to its transport properties. However, realization of graphene devices requires suitable nanoscale lithography as well as a method to open a band gap in monolayer graphene. Nanoscale hydrogenation and oxidation are promising methods to open an energy band gap by modification of surface structures and to fabricate nanostructures such as graphene nanoribbons (GNRs). Until now it has been difficult to fabricate nanoscale devices consisting of both hydrogenated and oxidized graphene because the hydrogenation of graphene requires a complicated process composed of large-scale chemical modification, nanoscale patterning, and etching. We report on nanoscale hydrogenation and oxidation of graphene under normal atmospheric conditions and at room temperature without etching, wet process, or even any gas treatment by controlling just an external bias through atomic force microscope lithography. Both the lithographically defined nanoscale hydrogenation and oxidation have been confirmed by micro-Raman spectroscopy measurements. Patterned hydrogenated and oxidized graphene show insulating behaviors, and their friction values are several times larger than those of graphene. These differences can be used for fabricating electronic or electromechanical devices based on graphene.
ACS Nano 08/2011; 5(8):6417-24. · 10.77 Impact Factor
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ABSTRACT: The thermal expansion coefficient (TEC) of single-layer graphene is estimated with temperature-dependent Raman spectroscopy in the temperature range between 200 and 400 K. It is found to be strongly dependent on temperature but remains negative in the whole temperature range with a room temperature value of (-8.0 ± 0.7) × 10(-6) K(-1). The strain caused by the TEC mismatch between graphene and the substrate plays a crucial role in determining the physical properties of graphene, and hence its effect must be accounted for in the interpretation of experimental data taken at cryogenic or elevated temperatures.
Nano Letters 08/2011; 11(8):3227-31. · 13.20 Impact Factor
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Jin Sik Choi,
Jin-Soo Kim,
Ik-Su Byun,
Duk Hyun Lee,
Mi Jung Lee,
Bae Ho Park,
Changgu Lee, Duhee Yoon,
Hyeonsik Cheong,
Ki Ho Lee,
Young-Woo Son,
Jeong Young Park,
Miquel Salmeron
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ABSTRACT: Graphene produced by exfoliation has not been able to provide an ideal graphene with performance comparable to that predicted by theory, and structural and/or electronic defects have been proposed as one cause of reduced performance. We report the observation of domains on exfoliated monolayer graphene that differ by their friction characteristics, as measured by friction force microscopy. Angle-dependent scanning revealed friction anisotropy with a periodicity of 180° on each friction domain. The friction anisotropy decreased as the applied load increased. We propose that the domains arise from ripple distortions that give rise to anisotropic friction in each domain as a result of the anisotropic puckering of the graphene.
Science 06/2011; 333(6042):607-10. · 31.20 Impact Factor
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ABSTRACT: Under homogeneous uniaxial strains, the Raman 2D band of graphene involving two-phonon double-resonance scattering processes splits into two peaks and they altogether redshift strongly depending on the direction and magnitude of the strain. Through polarized micro-Raman measurements and first-principles calculations, the effects are shown to originate from significant changes in resonant conditions owing to both the distorted Dirac cones and anisotropic modifications of phonon dispersion under uniaxial strains. Quantitative agreements between the calculation and experiment enable us to determine the dominant double-resonance Raman scattering path, thereby answering a fundamental question concerning this key experimental analyzing tool for graphitic systems.
Physical Review Letters 04/2011; 106(15):155502. · 7.37 Impact Factor
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ABSTRACT: The thermal conductivity of suspended single-layer graphene was measured as a
function of temperature using Raman scattering spectroscopy on clean samples
prepared directly on a prepatterned substrate by mechanical exfoliation without
chemical treatments. The temperature at the laser spot was monitored by the
frequency of the Raman 2$D$ band of the Raman scattering spectrum, and the
thermal conductivity was deduced by analyzing heat diffusion equations assuming
that the substrate is a heat sink at ambient temperature. The obtained thermal
conductivity values range from $\sim$1800 Wm$^{-1}$K$^{-1}$ near 325 K to
$\sim$710 Wm$^{-1}$K$^{-1}$ at 500 K.
03/2011;
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ABSTRACT: We studied variations in the Raman spectrum as a function of the number of graphene layers by using micro-Raman spectroscopy and atomic force microscopy (AFM). The sample preparation was done by micromechanical cleaving of natural graphite on an ∼300 nm SiO2 layer. The Raman G band (∼1580 cm −1), G * band (∼2450 cm −1) and 2D band (∼2700 cm −1) varied as functions of the number of graphene layers. The Raman 2D band was especially sensitive to the number of graphene layers. These features are related to the electronic band structure of graphene. Moreover, areas of different numbers of graphene layers were clearly identified using spatially-resolved micro-Raman imaging spectroscopy.
Journal- Korean Physical Society 10/2009; 5530. · 0.45 Impact Factor
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ABSTRACT: The intensity ratio between two major Raman bands in graphene is one of the most important pieces of information for physics of graphene and has been believed to represent various intrinsic properties of graphene without critical assessment of extrinsic effects. We report a micro-Raman spectroscopy study on the Raman intensity ratio of the 2D band to the G Raman band of graphene varying the thickness of dielectric layers (SiO2) underneath it. The ratio is shown to change by almost 370% when the thickness is varied by 60%. The large variation in the ratio is well explained by theoretical calculations considering multiple Raman scattering events at the interfaces. Our analysis shows that the interference effect is critical in extracting the intrinsic 2D to G intensity ratio and therefore must be taken into account in extracting various physical properties of graphene from Raman measurements.
Phys. Rev. B. 09/2009; 80(12).
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ABSTRACT: The intensity ratio between two major Raman bands in graphene is one of the most important information for physics of graphene and has been believed to represent various intrinsic properties of graphene without critical assessment of extrinsic effects. We report a micro Raman spectroscopy study on the Raman intensity ratio of the 2D band to the G Raman band of graphene varying the thickness of dielectric layers (SiO_2) underneath it. The ratio is shown to change by almost 370% when the thickness is varied by 60%. The large variation in the ratio is well explained by theoretical calculations considering multiple Raman scattering events at the interfaces. Our analysis shows that the interference effect is critical in extracting the intrinsic 2D to G intensity ratio and therefore must be taken into account in extracting various physical properties of graphene from Raman measurements. Comment: 21 pages, 5 figures. Phys. Rev. B; in press
08/2009;
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ABSTRACT: We used micro‐Raman spectroscopy and atomic force microscopy to study variations of the Raman spectrum as a function of the number of graphene layers. Samples were prepared by micromechanical cleaving of natural graphite on a ∼ 300‐nm SiO2 layer. The variations of Raman G band ( ∼ 1,580 cm−1), G∗ band ( ∼ 2,450 cm−1), and 2D band ( ∼ 2,700 cm−1) were observed as a function of the number of graphene layers. Raman 2D band is especially sensitive to the number of graphene layers. These features are related to the electronic band structure of graphene. Moreover, the areas of different number of graphene layers were clearly identified using spatially resolved micro‐Raman imaging spectroscopy. Polarized micro‐Raman spectroscopy on single‐layer graphene shows strong polarization dependences of double‐resonance Raman intensities. The Raman intensity of the double‐resonant 2D band is maximum when the excitation and detection polarizations are parallel and minimum when they are orthogonal, whereas that of the G band is isotropic. A calculation shows that this strong polarization dependence is a direct consequence of inhomogeneous optical absorption and emission mediated by electron‐phonon interactions involved in the second‐order Stokes‐Stokes Raman scattering process.
AIP Conference Proceedings. 04/2009; 1119(1):232-232.
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ABSTRACT: Spatially resolved and polarized micro-Raman spectroscopy on microcrystalline graphene shows strong polarization dependences of double-resonance Raman intensities. The Raman intensity of the double-resonant 2D band is maximum when the excitation and detection polarizations are parallel and minimum when they are orthogonal, whereas that of the G band is isotropic. A calculation shows that this strong polarization dependence is a direct consequence of inhomogeneous optical absorption and emission mediated by electron-phonon interactions involved in the second-order Stokes-Stokes Raman scattering process.
Nano Letters 01/2009; 8(12):4270-4. · 13.20 Impact Factor
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ABSTRACT: Spatially resolved and polarized micro-Raman spectroscopy on microcrystalline graphene shows strong polarization dependences of double-resonance Raman intensities. The Raman intensity of the double-resonant 2 D band is maximum when the excitation and detection polarizations are parallel and minimum when they are orthogonal, whereas that of the G band is isotropic. A calculation shows that this strong polarization dependence is a direct consequence of inhomogeneous optical absorption and emission mediated by electron-phonon interactions involved in the second-order Stokes-Stokes Raman scattering process.
Nano Letters 11/2008; · 13.20 Impact Factor
