Raman spectroscopy of ripple formation in suspended graphene.

Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089, USA.
Nano Letters (Impact Factor: 13.03). 10/2009; 9(12):4172-6. DOI: 10.1021/nl9023935
Source: PubMed

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.

  • [Show abstract] [Hide abstract]
    ABSTRACT: Graphene is nature's thinnest elastic membrane, and its morphology has important impacts on its electrical, mechanical, and electromechanical properties. Here we report manipulation of the morphology of suspended graphene via electrostatic and thermal control. By measuring the out-of-plane deflection as a function of applied gate voltage and number of layers, we show that graphene adopts a parabolic profile at large gate voltages with inhomogeneous distribution of charge density and strain. Unclamped graphene sheets slide into the trench under tension; for doubly clamped devices, the results are well-accounted for by membrane deflection with effective Young's modulus E = 1.1 TPa. Upon cooling to 100 K, we observe buckling-induced ripples in the central portion and large upward buckling of the free edges, which arises from graphene's large negative thermal expansion coefficient.
    Nano Letters 10/2012; · 13.03 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Graphene grown by a coronene (C-graphene) source is transferred to an SiO2 surface, and its Raman spectra are investigated in annealing environments of O2, Ar, and N2. An irreversible doping effect is observed in all the annealing environments, which is attributed to the enhancement of substrate doping. Compared with the mechanically exfoliated graphene on SiO2, stronger remnant stress remains in the transferred C-graphene, and wrinkles prevail on the surface. It is found that the defect density increases only after O2 annealing, and the full width half maximum (FWHM) of the G and 2D bands in the Raman spectrum increases in all the annealing atmospheres. We suggest that the increase of FWHM is caused by the crystalline disorders.
    Spectroscopy Letters 06/2014; 47(6). · 0.67 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Theoretical research has predicted that ripples of graphene generates effective gauge field on its low energy electronic structure and could lead to zero-energy flat bands, which are the analog of Landau levels in real magnetic fields. Here we demonstrate, using a combination of scanning tunneling microscopy and tight-binding approximation, that the zero-energy Landau levels with vanishing Fermi velocities will form when the effective pseudomagnetic flux per ripple is larger than the flux quantum. Our analysis indicates that the effective gauge field of the ripples results in zero-energy flat bands in one direction but not in another. The Fermi velocities in the perpendicular direction of the ripples are not renormalized at all. The condition to generate the ripples is also discussed according to classical thin-film elasticity theory.
    Physical review. B, Condensed matter 12/2012; 87(20). · 3.77 Impact Factor