Here we report graphene systems' nano-Raman hyperspectral imaging based on tip-enhanced Raman scattering (TERS). The vibrational and electronic structures are modulated within the graphene-related materials, leading to nano-scale changes in the behavior of electrons and phonons that can be used for spectral imaging. Furthermore, we utilize a He-focused ion beam to do nanolithography on graphene. We then show that the tiny features on graphene made by the He-focused ion beam can only be visualized under nanometer-scaled spectroscopy imaging. We have also imaged low-angle reconstructed twisted bilayer graphene, and our observations highlight the relevance of solitons and topological points for the structures' vibrational and electronic properties, relevant in the context of twistronics.
The correlated Stokes-anti-Stokes Raman scattering mediated by phonons was introduced in 1977 by Klyshko. In the last two decades, it has been broadly studied experimentally, including results on diamond, graphene and transparent liquid. The theoretical description of non-resonant Stokes-anti-Stokes pair production was shown to be formally similar to the BCS theory of superconductivity, raising attention to the study of pair production as a function of the Raman shift. Intriguingly, the pair production efficiency is not symmetric with respect to the positive versus negative detuning from phonon resonance, a result that was shown for a 180fs pulsed laser, and that remains without a theoretical explanation. Here we show the asymmetry is persistent in diamond measured with lasers of different pulse widths (180 fs and 5 ps) and different wavelengths (633 nm versus 785 nm).
In the emerging field of twistronics, new electronic devices based on bilayer graphene have shown distinct electronic properties that depend on the rotational misalignment of one crystalline layer with respect to another. Given present methods of preparing these bilayers, there is always some uncertainty in the actual versus targeted twist angle of a specific bilayer that can only be resolved by measuring the moiré patterns that are unique to a specific twist angle. Traditional methods enabling such a measurement, Transmission Electron Microscopy and Scanning Tunneling Microscopy, impose serious restrictions on the types of substrates supporting the bilayers, which, in turn, constrains the subsequent fabrication of any devices. We report here a new, non-destructive method to measure moiré patterns of bilayer graphene deposited on any smooth substrate, using the scanning probe technique known as scanning microwave impedance microscopy (sMIM) which enables the simultaneous generation of localized topography, capacitance and conductance images with nanometer 1 scale resolution . Moiré patterns were observed in samples prepared on various substrates with twist angles ranging from 0.02 to 6.7 degrees, beyond which the moiré patterns are too small to be resolved by the sMIM probes. We present some possible reasons for the various contrast mechanisms. Addressing the problem of variations across a bilayer surface due to localized moiré distortions that result from the tensile and shear forces involved in transferring a twisted bilayer to a substrate, we demonstrate how sMIM can precisely map the twist angle distribution across the film, and enable direct device and circuit routing.
The implications of spatial coherence in tip-enhanced Raman spectroscopy of two-dimensional systems will be discussed. The prototype material is graphene. Based on massive data with improved resolution based on a plasmon-tuned tip pyramid, we show that in the tip-enhanced strong field regime, interference affects the spectral outcome utilized, for example, to quantify defects. The Raman figure of merit, i.e. the relative intensity of the defect-activated Raman band, depends on the TERS enhancement. Graphene sitting on different substrates is also analyzed. Super-resolution is shown to be due to the field configuration resulting from the coupled tip-sample-substrate system, exhibiting a non-trivial spatial surface distribution. The field distribution and the symmetry selection rules are different for non-gap versus gap mode configurations. This influences the overall enhancement which depends on the Raman mode symmetry and substrate structure.
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