Characterizing intrinsic charges in top gated bilayer graphene device by Raman spectroscopy

doi:10.1016/j.carbon.2012.03.006

Carbon

Volume 50, Issue 10, August 2012, Pages 3435-3439

https://doi.org/10.1016/j.carbon.2012.03.006 Get rights and content

Abstract

In this work we study the behavior of the optical phonon modes in bilayer graphene devices by applying top gate voltage, using Raman scattering. We observe the splitting of the Raman G band as we tune the Fermi level of the sample, which is explained in terms of mixing of the Raman (E_g) and infrared (E_u) phonon modes, due to different doping in the two layers. We theoretically analyze our data in terms of the bilayer graphene phonon self-energy which includes non-homogeneous charge carrier doping between the graphene layers. We show that the comparison between the experiment and theoretical model not only gives information about the total charge concentration in the bilayer graphene device, but also allows to separately quantify the amount of unintentional charge coming from the top and the bottom of the system, and therefore to characterize the intrinsic charges of bilayer graphene with its surrounding environment.

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: $E = (n_{top} + n_{bot}) | e | / (2 ϵ_{0})$ where n_top and n_bot are the charge carriers coming from the top and the bottom of bilayer graphene, respectively, $ϵ_{0}$ 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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¹: D.L. Mafra and P. Gava contributed equally for the paper.

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Characterizing intrinsic charges in top gated bilayer graphene device by Raman spectroscopy

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusion

Acknowledgements

Phys Rep

CARBON

Asymmetry gap in the electronic band structure of bilayer graphene

Phys Rev B

The electronic properties of graphene

Rev Mod Phys

Controlling the electronic structure of bilayer graphene

Science

Biased bilayer graphene- semiconductor with a gap tunable by the electric field effect

Phys Rev Lett

Gate-induced insulating state in bilayer graphene devices

Nat Mater

Direct observation of a widely tunable bandgap in bilayer graphene

Nature

Observation of an electric field induced band gap in bilayer graphene by infrared spectroscopy

Phys Rev Lett

Infrared spectroscopy of electronic bands in bilayer graphene

Phys Rev B

Probing the electrostatic environment of bilayer graphene using Raman spectra

Phys Rev B

Raman spectrum of graphene and graphene layers

Phys Rev Lett

Breakdown of the adiabatic Born–Oppenheimer approximation in graphene

Nat Mater