Raman spectroscopy of ripple formation in suspended graphene.
ABSTRACT Using Raman spectroscopy, we measure the optical phonon energies of suspended graphene before, during, and after thermal cycling between 300 and 700 K. After cycling, we observe large upshifts ( approximately 25 cm(-1)) of the G band frequency in the graphene on the substrate region due to compression induced by the thermal contraction of the underlying substrate, while the G band in the suspended region remains unchanged. From these large upshifts, we estimate the compression in the substrate region to be approximately 0.4%. The large mismatch in compression between the substrate and suspended regions causes a rippling of the suspended graphene, which compensates for the change in lattice constant due to the compression. The amplitude (A) and wavelength (lambda) of the ripples, as measured by atomic force microscopy, correspond to an effective change in length Deltal/l that is consistent with the compression values determined from the Raman data.
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ABSTRACT: We study the mechanism of wrinkling of suspended graphene, by means of atomistic simulations. We argue that the structural instability under edge compression is the essential physical reason for the formation of periodic ripples in graphene. The ripple wavelength and out-of-plane amplitude are found to obey 1/4-power scaling laws with respect to edge compression. Our results also show that parallel displacement of the clamped boundaries can induce periodic ripples, with oscillation amplitude roughly proportional to the 1/4 power of edge displacement. The results are fundamental to graphene's applications in electronics.Physical review. B, Condensed matter 04/2011; 83. · 3.77 Impact Factor
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ABSTRACT: Despite the recent progress in understanding the geometric structures of defects and edges in a graphene device (GD), how such defects and edges affect the transport properties of the device have not been clearly defined. In this study, the surface geometric structure of a GD was observed with an atomic force microscope (AFM) and the spatial variation of the transport current by the gating tip was measured with scanning gate microscopy (SGM). It was found that geometric corrugations, defects and edges directly influence the transport current. This observation is linked directly with a proposed scattering model based on macroscopic transport measurements.International Journal of High Speed Electronics and Systems 03/2011; 20(1):205.
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ABSTRACT: Localized energy exchange and mechanical coupling across a few nm gap at corrugated graphene-substrate interface remain a great challenge to study. In this work, an infrared laser is used to excite an unconstrained epitaxial graphene/SiC interface to induce a local thermal non-equilibrium. The interface behavior is uncovered using a second laser beam for Raman excitation. Using Raman peaks for dual thermal probing, the temperature difference across a gap of just a few nm is determined precisely. The interfacial thermal conductance is found extremely low: 410±7 Wm-2K-1, indicating poor phonon transport across the interface. By decoupling of the graphene's mechanical and thermal behavior from the Raman wavenumber, the stress in the graphene is found extremely low, uncovering its flexible mechanical behavior. Based on interface-enhanced Raman, it is found the increment of interface separation between graphene and SiC can be as large as 2.9 nm when the local thermal equilibrium is destroyed.Nanoscale 05/2014; · 6.73 Impact Factor