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: 13.59).
10/2009; 9(12):4172-6. DOI: 10.1021/nl9023935
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.
Available from: Yangfan hu
- "Uncovering the nature of its intrinsic ripples   is one of the most challenging and vital problems concerning suspended graphene . On one hand, its academic interest derives from the PeierlseLandaueMermin argument, raised almost 80 years ago, about the non-existence of low-dimensional crystalline state [3e5]. "
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ABSTRACT: The intrinsic ripples of suspended graphene have attracted intensive attention due to their influence on the electronic transport and other properties. Negative thermal expansion (NTE), another unconventional phenomenon found in graphene, can be utilized to control the intrinsic ripples in a reversible way, thus opening new perspective for application. In this case, understanding the mutual relation and physical origin of the intrinsic ripples and NTE is crucial, especially since they are both widely observed in other 2D materials. Here we clarify through lattice dynamical analysis that at low temperature the two phenomena are both intrinsic for any 2D crystals with a honeycomb structure (or any monatomic 2D crystals). We find that the intrinsic ripples, generally believed to be caused by thermal fluctuation, have another origin that is the appearance of soft ZA modes near long wavelength limit when the lattice constant is shortened. Moreover, the soft ZA modes and NTE have the same physical origin at low temperature. At finite temperature, NTE is dominantly caused by the “vibrational elongation” effect owing to large out-of-plane fluctuation according to our calculation based on self-consistent phonon theory.
Carbon 12/2015; 95:239. DOI:10.1016/j.carbon.2015.08.022 · 6.20 Impact Factor
Available from: Somayeh Fardindoost
- "In addition, the integrated intensity of the 2D mode is enhanced for the suspended part compared to the supported region. On the supported portion of the sample, the data are more widely scattered, which is in agreement with the previous reports of measurements on several different suspended graphene flakes    . With regard to D-mode, which reflects the defect and disorder density, however, no disorder is observed for either the suspended or the supported parts. "
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ABSTRACT: In this paper we present flexible strain sensors made of graphene flakes fabricated,
characterized, and analyzed for the electrical actuation and readout of their mechanical
vibratory response in strain-sensing applications. For a typical suspended graphene membrane
fabricated with an approximate length of 10 μm, a mechanical resonance frequency around
136 MHz with a quality factor (Q) of ~60 in air under ambient conditions was observed. The
applied strain can shift the resonance frequency substantially, which is found to be related to
the alteration of physical dimension and the built-in strain in the graphene flake. Strain sensing
was performed using both planar and nonplanar surfaces (bending with different radii of
curvature) as well as by stretching with different elongations.
Journal of Micromechanics and Microengineering 04/2015; 25(7). DOI:10.1088/0960-1317/25/7/075016 · 1.73 Impact Factor
Available from: Changgu Lee
- "We conclude that the microwave annealing process can replace previously proposed thermal annealing techniques such as strain engineering     , thermal oxidation , hydrogenation and dehydrogenation , removing residues and wrinkles  , intentional ripple formation   that aim to tailor the electronic band structure of graphene on silicon and induce doping. In particular, we attempted to apply microwave dielectric heating to the induction of a strain in graphene similar to that of a previous report, which demonstrated ripple formation in suspended graphene sheets . "
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ABSTRACT: We propose microwave-induced annealing as a rapid, simple, and effective method of controlling surface doping and strain in graphene. Raman spectroscopy was used to confirm that heavy and uniform p-type (1.2 × 1013 cm−2) doping can be achieved within only 5 min without unintended defects by placing graphene onto a substrate with a sufficiently high dielectric constant and exposing graphene and its substrate to microwave irradiation. Further, we showed that ripples are formed in suspended graphene when it is exposed to microwave irradiation. Silicon has a sufficiently high dielectric constant (11.9) and graphene is commonly deposited on silicon-based substrates, so our proposed microwave-induced annealing technique can be used for the rapid manipulation of the properties of graphene at low cost.
Carbon 02/2014; 67:673-679. DOI:10.1016/j.carbon.2013.10.056 · 6.20 Impact Factor
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