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In this work, we study the Raman spectra of twisted bilayer graphene samples as a function of their twist-angles (θ), ranging from 0.03º to 3.40º, where local θ are determined by analysis of their associated moiré superlattices, as imaged by scanning microwave impedance microscopy. Three standard excitation laser lines are used (457, 532, and 633 nm wavelengths), and the main Raman active graphene bands (G and 2D) are considered. Our results reveal that electron-phonon interaction influences the G band's linewidth close to the magic angle regardless of laser excitation wavelength. Also, the 2D band lineshape in the θ < 1º regime is dictated by crystal lattice and depends on both the Bernal (AB and BA) stacking bilayer graphene and strain soliton regions (SP) [1]. We propose a geometrical model to explain the 2D lineshape variations, and from it, we estimate the SP width when moving towards the magic angle.

We have fabricated graphene devices on lightly doped Si substrates and show that pronounced changes in resistance versus gate voltage, R(Vg), characteristics of these devices at 77 K are induced by the variation in the charge distribution in substrate with both gate voltage and illumination. The R(Vg) of the graphene devices in the dark shows remarkable changes as the carriers in the underlying substrate go through accumulation, depletion, and inversion regimes. We demonstrate the possibility of using a graphene device as an optical-latch.

Coherence length (L c) of the Raman scattering process in graphene as a function of Fermi energy is obtained with spatially coherent tip-enhanced Raman spectroscopy. L c decreases when the Fermi energy is moved into the neutrality point, consistent with the concept of the Kohn anomaly within a ballistic transport regime. Since the Raman scattering involves electrons and phonons, the observed results can be rationalized either as due to unusually large variation of the longitudinal optical phonon group velocity v g, reaching twice the value for the longitudinal acoustic phonon, or due to changes in the electron energy uncertainty, both properties being important for optical and transport phenomena that might not be observable by any other technique.

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