Raman imaging of doping domains in graphene on SiO2

Abstract

We present spatially resolved Raman images of the G and 2D lines of single-layer graphene flakes. The spatial fluctuations of G and 2D lines are correlated and are thus shown to be affiliated with local doping domains. We investigate the position of the 2D line-the most significant Raman peak to identify single-layer graphene-as a function of charging up to ͉n͉Ϸ4 ϫ 10 12 cm −2 . Contrary to the G line which exhibits a strong and symmetric stiffening with respect to electron and hole doping, the 2D line shows a weak and slightly asymmetric stiffening for low doping. Additionally, the linewidth of the 2D line is, in contrast to the G line, doping independent making this quantity a reliable measure for identifying single-layer graphene. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2816262͔Graphene has attracted increasing attention over the last few years. 1,2 Its unique electronic properties, 3,4 mainly due to the linear energy versus momentum dispersion and the electron hole symmetry near the charge neutrality point, make it an interesting nanomaterial for high mobility electronics. 5,6 Raman spectroscopy has proven to be a powerful tool to distinguish single-layer graphene from few-layer graphene and graphite. [7][8][9][10][11] The particular electronic structure of graphite and graphene leads to Kohn anomalies in the phonon dispersion at the ⌫ and K points. 12-14 Additionally, the BornOppenheimer approximation, which is usually employed for the calculation of phonon frequencies, is no longer valid because the vibration period is much smaller than the electronmomentum relaxation time. Furthermore, a pronounced stiffening of the Raman G line upon positive or negative charging ͑p or n doping͒ of the graphene sheet is observed. [15][16][17] In this paper, we investigate how spatially resolved Raman spectroscopy can be used to probe doping domains and local charge fluctuations. While electron-hole puddles ͑i.e., local charge fluctuations͒ have been predicted to be responsible for the finite conductance at vanishing ͑av-erage͒ charge carrier density 18 and have recently been observed using a scanning single electron transistor, 19 the identification of different doping domains might be desirable to investigate graphene devices. Here, we report on Raman measurements on back gate controlled charged graphene and on Raman imaging of doping fluctuations of isolated graphene flakes. We focus on the correlation between the shifts of the G line and 2D line ͑or D * line͒. The latter one is the most significant Raman peak in single-layer graphene. 7,9 Within the low charging regime ͑up to ±4 ϫ 10 12 cm −2 ͒ obtained in our experiments, the 2D line stiffens for both electron and hole charging while its linewidth ͑in contrast to the G line͒ is not affected by charging. A good correlation between the shift of G and 2D lines is observed. However, the spectral resolution and lateral resolution are not sufficient to resolve electron-hole puddles, as shown in Ref. 19. Therefore, we refer to charging ͑i.e., doping͒ domains ra...

Probing the Intrinsic Properties of Exfoliated Graphene: Raman Spectroscopy of Free-Standing Monolayers

“…This observation is surprising, since no significant change in the 2D linewidth has been reported as a function of doping level for electrostatically gated samples. 13,[15][16][17] In addition to the reduced linewidth for free-standing graphene compared with supported graphene, we also find a marked difference in the line shape. The Raman spectra in Fig.…”

Raman spectroscopy and imaging of graphene

“…On the other hand, the 2D band is redshifted for electron doping and blueshifted for hole doping, due to the charge transfer induced modifi cation of the equilibrium lattice parameter [25]. This makes Raman spectroscopy an effective technique to determine the doping type and dopant concentration in graphene [26,27]. The effects of temperature on the Raman spectra of graphene have also been studied and the lattice inharmonicity of graphene investigated [28].…”

Plasma-enhanced chemical vapor deposition of graphene on copper substrates

“…In the case of single-layer graphene, the 2D band is fitted by a single narrow Lorentzian function. A FWHM of 33 cm −1 (measured with 532 nm) for the 2D band is typically assigned to SLG, 37 while for few-layer samples the 2D feature is becoming significantly broader and asymmetric. The bilayer has a much broader and up-shifted 2D band with respect to single-layer graphene due to its special electronic structure, consisting of two conduction bands and two valence bands.…”