Tunable s-SNOM for Nanoscale Infrared Optical Measurement of Electronic Properties of Bilayer Graphene

Abstract

Here we directly probe the electronic properties of bilayer graphene using s-SNOM measurements with a broadly tunable laser source over the energy range from 0.3 to 0.54 eV. We tune an OPO/OPA system around the interband resonance of Bernal stacked bilayer graphene (BLG) and extract amplitude and phase of the scattered light. This enables us to retrieve and reconstruct the complex optical conductivity resonance in BLG around 0.39 eV with nanoscale resolution. Our technique opens the door toward nanoscopic noncontact measurements of the electronic properties in complex hybrid 2D and van der Waals material systems, where scanning tunneling spectroscopy cannot access the decisive layers.

Figure 1

(a) Sketch of a far-field IR measurement at few-layer graphene (FLG) used to obtain the spectra of monolayer and bilayer graphene in (b). The plot is adapted from Mak et al. Insets show the bandstructure in single and BLG, which gives rise to the conductivity resonance at around 0.37 eV. (c) Schematic of the s-SNOM measurement with a tunable OPO laser used in this work. The tip oscillates at a frequency Ω. It is illuminated by the tunable laser and scatters the light. (d) Estimated s-SNOM near-field amplitude (S₃/S₃(SiO₂)) and phase (φ₃ – φ₃ (SiO₂)) contrasts for BLG on SiO₂/Si are depicted in the plot.

Figure 2

(a) Light microscope image of the few-layer graphene flake and Raman map around the 2D-peak (details in SI). (b–d) Infrared s-SNOM amplitude s(ω) (top row) and phase φ(ω) (bottom row): images taken at three different photon energies of (b) 0.282, (c) 0.390, and (d) 0.5 eV. The images are referenced to the signal on the SiO₂ substrate on the left and were recorded with a different tip than the reference spectra in Figure 3. Note that the color bars of the phase are asymmetric. The scale bars correspond to 4 μm each.

Figure 3

Spatially averaged third demodulation order normalized amplitude and phase contrast of BLG/SiO₂ in (a) and SLG/SiO₂ in (b). The respective contrast is normalized with respect to SiO₂. An inversion of the phase contrast between SLG and BLG happens around 0.31 eV. The data sets were recorded with a different tip than the images in Figure 2. The positon of the recorded images, from which the data points were extracted can be found in the SI (Figure S4).