Characterizing intrinsic charges in top gated bilayer graphene device by Raman spectroscopy
Introduction
Bilayer graphene has attracted a lot of attention recently because of its special low energy electronic dispersion, in which a tunable band gap can be opened by application of a transverse electric field [1], [2], [3], [4], [5], [6], [7], [8]. Such device is desirable for low energy photo-emitters and detectors possessing a high tunability by the control of charge concentrations on the graphene layers. Recent experimental demonstration of this tunable band gap in bilayer graphene was based on the absorption measurements in the infrared region [6], [7], [8] or by electric transport measurements [4], [5]. However the tunable band gap bilayer graphene device operation can be greatly influenced by the surrounding environment.
Typically, unintentional doping charges coming from the top and the bottom of the system can accumulate on bilayer graphene, giving rise to an unintentional electric field which determines a non-homogeneous doping between the layers and the opening of a band gap in the band structure, without any applied electric field [9]. In this work we use Raman spectroscopy to monitor the unintentional charge coming from the top and the bottom of the system, which gives information on the electrostatic environment of the sample and which helps to characterize the bilayer devices for further applications.
The band gap opening and tunability in bilayer graphene is based on the application of an electric field E perpendicular to the layers, given by:where ntop and nbot are the charge carriers coming from the top and the bottom of bilayer graphene, respectively, is the vacuum permittivity, and e is the electronic charge. Raman spectroscopy has already shown to be a fast and non-destructive tool to characterize graphene samples [10], [11] and doping effects [12], [13], [14], [15], [16], however no carefully analysis has been done to demonstrate the effect of non-homogeneous doping in bilayer graphene devices. Recent theoretical calculations made by Gava et al. [9] suggest that from the analysis of the Raman spectra of gated bilayer graphene it is possible to quantitatively identify the amount of non-intentional charges coming from the atmosphere and from the substrate and to characterize the electrostatic environment of few-layers graphene. In this work we study the dependence of the G band of bilayer graphene on the gate voltage. From the direct comparison between the experimental and the theoretically simulated Raman spectra, and from the analysis of the positions, full width at half maximum (FWHM) and relative intensities of the two Raman peaks as a function of the electron concentration, we were able to estimate the charge unintentionally accumulated on the device from the environment.
Section snippets
Experimental details
Fig. 1(a) shows the bilayer-graphene field-effect transistor (FET) used in the experiment. Graphene samples were produced by micro-mechanical cleavage of graphite and deposited on Si covered with 300 nm of SiO2. Top gating was achieved by using a polymer electrolyte consisting of polyethylene glycol (PEG) and NaClO4 with ratio concentration of 1:0.25, and the gate voltage was applied between a gold electrode in contact with the graphene layer and a platinum wire electrode inserted in the
Results and discussion
The interface between graphene and polymer electrolyte has been shown experimentally to behave like a double layer capacitor of thickness in the order of nanometers [17]. Therefore, the geometric capacitance of the electrolyte is very high compared to bottom gate devices where the thickness of the dielectric material is much larger (typically 300 nm).
Capacitance measurements of the polymeric electrolyte used in the experiment were performed by Impedance Spectroscopy with frequency analyzer
Conclusion
In summary, a detailed analysis of the G band of top gated bilayer graphene is presented. We observed that, unlike in the unbiased case where the G Raman band is composed by only one peak, the gate voltage breaks the inversion symmetry and the G band splits in two modes, that are combinations of the symmetric and anti-symmetric modes of the unbiased bilayer graphene. We analyze the dependence of the frequency and the relative intensities of the peaks with higher and lower frequency as a
Acknowledgements
We would like to acknowledge Nacional de Grafite (Brazil) for providing us the graphite samples. D.L.M. and L.M.M. acknowledges the support from the Brazilian Agency CNPq. Calculations were performed at the IDRIS supercomputing center (Projects No. 081202 and No. 081387).
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D.L. Mafra and P. Gava contributed equally for the paper.