The interaction between these atomically-thin layers and the substrate on which they are commonly deposited has a dramatic (and often detrimental) influence on the electron transport and optical properties. Fabrication of suspended samples makes it possible to obtain quasi-intrinsinc graphene samples, in which residual charged impurities can be neglected. We are investigating the fundamental properties of graphene and few layer graphene under such ideal conditions. To this end, we develop “lithography-free” processes to fabricate pristine freestanding graphene devices in the StNano clean room (link) and carry out combined optical (micro-Raman spectroscopy) and electron transport measurements. Our main focus is on the coupling between vibrational (optical phonons) and electronic (electron-hole pairs) excitations in graphene field effect transistors. Owing to the greatly reduced residual charge inhomogeneity in freestanding samples, we hope to probe electronic and vibrational excitations in the immediate vicinity of the charge neutrality point (known as the Dirac Point in monolayer graphene).
Figure 1. Our home built experimental setup for combined optical and electrical measurements.
Some recent results:
We have been investigating the Raman response of freestanding graphene flakes, with a particular emphasis on the popular (two-phonon intervalley) Raman 2D mode. This mode is routinely used in the graphene community to distinguish a graphene monolayer from a (Bernal or Rhomohedral) stack of N-graphene layers. We have performed a comprehensive study of the Raman 2D mode in graphene monolayers, motivated by recent reports of asymmetric 2D-mode line shapes in freestanding graphene. For photon energies in the range 1.53–2.71 eV, the 2D-mode Raman response of freestanding samples appears as bimodal, in stark contrast with the Lorentzian approximation that is commonly used for supported monolayers. The transition between the freestanding and supported cases is mimicked by electrostatically doping freestanding graphene at carrier densities above 2 × 1011 cm–2. This result quantitatively demonstrates that low levels of charging can obscure the intrinsically bimodal 2D-mode line shape of monolayer graphene. In pristine freestanding graphene, we observe a broadening of the 2D-mode feature with decreasing photon energy that cannot be rationalized using a simple one-dimensional model based on resonant inner and outer processes. This indicates that phonon wavevectors away from the high-symmetry lines of the Brillouin zone must contribute to the 2D-mode, so that a full two-dimensional calculation is required to properly describe multiphonon-resonant Raman processes.
Figure 2 (left) Optical image showing a graphene sample with a freestanding region bridging a micro-trench. Raman 2D mode spectra (center) and linewidth (right) at various photon energies.
Intrinsic Line Shape of the Raman-2D mode in Freestanding Graphene Monolayers
S. Berciaud, X. Li, H. Htoon, L.E. Brus, S.K. Doorn and T.F. Heinz
Nano Letters 13, 3517 (2013) DOI: 10.1021/nl400917e
Probing the Intrinsic Properties of Exfoliated Graphene : Raman Spectroscopy of Free-Standing Monolayers
S. Berciaud, S. Ryu, L. E. Brus & T. F. Heinz
Nano Letters 9, 346 (2009) DOI: 10.1021/nl8031444
We have performed a detailed spatially resolved Raman study on a set of freestanding graphene monolayers. While we find, as previously reported, that freestanding graphene is quasi-undoped, our study also reveals that non-negligible built-in strain occurs in these samples. The level of built-in strain varies significantly from one sample to another and can be estimated with accuracy from the correlation of the frequencies of the G and 2D Raman modes. Sample-dependent compressive and tensile strains as high as 0.1% are reported.
These results pave the way for mechanical studies on freestanding graphene using Raman spectroscopy as a sensitive probe. For this purpose, we are currently investigating pressurized graphene blisters.
Probing Built-in Strain in Graphene Monolayers by Raman SpectroscopyD. Metten, F. Federspiel, M. Romeo and S. Berciaud
Physica Status Solidi b 250, 2681 (2013) DOI: 10.1002/pssb.201300220
Figure 3 Spatially averaged values of the G and 2D peak frequencies for eight different freestanding samples The red solid line is a linear fit to the data revealing a slope of (2.2 ± 0.1), that is characteristic of pure strain.