Evolution of the electronic band structure of twisted bilayer graphene upon doping

Shengqiang Huang,Kanokporn Chattrakun,Matthew Yankowitz,Brian J. LeRoy,Arvinder Sandhu

doi:10.1038/s41598-017-07580-3

Shengqiang Huang, Kanokporn Chattrakun + Show 3 more

Open Access

https://doi.org/10.1038/s41598-017-07580-3

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Abstract

The electronic band structure of twisted bilayer graphene develops van Hove singularities whose energy depends on the twist angle between the two layers. Using Raman spectroscopy, we monitor the evolution of the electronic band structure upon doping using the G peak area which is enhanced when the laser photon energy is resonant with the energy separation of the van Hove singularities. Upon charge doping, the Raman G peak area initially increases for twist angles larger than a critical angle and decreases for smaller angles. To explain this behavior with twist angle, the energy separation of the van Hove singularities must decrease with increasing charge density demonstrating the ability to modify the electronic and optical properties of twisted bilayer graphene with doping.

Highlights

Twisted bilayer graphene has drawn a lot of theoretical attention because of its unique electronic band structure, which depends on the twist angle between the two graphene monolayers[1,2,3,4]
The band structure of Twisted bilayer graphene (tBLG) is predicted to depend on the potential difference between the two layers[1, 33,34,35], which can be generated by charge doping or electrostatic gating
The variation of the intensity of the Raman G peak of tBLG as a function of charge density demonstrates that G peak quenching is not a universal behavior but rather a consequence of the electronic band structure depending on density

Summary

Experimental setup

A polymer electrolyte top gate was employed to induce charge carriers in the sample[38] (see details in the Method section). Debye layers are formed near the graphene and the side gate electrode where ions accumulate. The gate voltage was constant during the acquisition of the Raman spectra and was changed slowly at a rate of 1 mV/s between measurements to ensure the polymer electrolyte remained stable. The slightly darker areas (e.g. blue circle in the inset of Fig. 1(b)) correspond to approximately a 12 degree twist angle which absorbs more visible light owing to the enhanced DoS in this energy range[42, 43]

Twist angle determination by STM and Raman measurements

Charge doping induced by a polymer top gate

Raman G peak area dependence on charge doping

Methods

Author Contributions

Findings

Additional Information