Mid-Infrared Mapping of Four-Layer Graphene Polytypes Using Near-Field Microscopy

Abstract

The mid-infrared (MIR) spectral region attracts attention for accurate chemical analysis using photonic devices. Few-layer graphene (FLG) polytypes are promising platforms, due to their broad absorption in this range and gate-tunable optical properties. Among these polytypes, the noncentrosymmetric ABCB/ACAB structure is particularly interesting, due to its intrinsic bandgap (8.8 meV) and internal polarization. In this study, we utilize scattering-scanning near-field microscopy to measure the optical response of all three tetralayer graphene polytypes in the 8.5-11.5 μm range. We employ a finite dipole model to compare these results to the calculated optical conductivity for each polytype obtained from a tight-binding model. Our findings reveal a significant discrepancy in the MIR optical conductivity response of graphene between the different polytypes than what the tight-binding model suggests. This observation implies an increased potential for utilizing the distinct tetralayer polytypes in photonic devices operating within the MIR range for chemical sensing and infrared imaging.

Keywords: Bernal; Few-layer graphene; Mid-IR nano-imaging; Optical conductivity; Raman spectroscopy; Rhombohedral; Scanning near-field optical microscopy; Stacking order.

Figure 1

4LG far-field Raman identification and near-field setup. (A) Different possible polytypes of 4LG, which are characterized by shifted carbon pairs in each graphene layer. The ABCB/ACAB polytypes represent two possible stackings of the 4LG, which are identical when flipped. (B) Schematic of the s-NSOM system utilized in this work. The abbreviations BS, PM, WM, RM, and QCL correspond to the beam splitter, parabolic mirror, wedge mirror, reference mirror, and quantum cascade laser, respectively. (C) Raman peak measurements are obtained using a 532-nm laser source for the 2D and G peaks of each observed 4LG polytype. Subtle differences in peak shape can be observed, particularly for the ABAB and ABCA stackings.

Figure 2

Far-field Raman and SHG measurements and 2 μm near-field optical scans on a tetralayer graphene (4LG) sample. (A) Optical microscopy image of the studied flake. The contrast of each section of the flake is related to the number of graphene layers in it. The flake has a 4-layer section between two 5-layer sections. (B) A G peak Raman scan of the flake was conducted, with two dashed red lines indicating the 4-layer section. The shaded area in Figure 1B indicates the filter used to generate the image. The scan shows two distinct peak shape sections, corresponding to ABCA and ABAB, and a third section not corresponding to either. (C) An SHG scan displays a small triangular zone with an SHG signal in the 4-layer section. (D) AFM topography scan of the 4-layer section of the flake, with boundaries marked by dashed red lines. (E, F) s-SNOM amplitude (panel (E)) and phase signals (panel (F)). The combination of amplitude and phase images clearly shows all three possible 4LG polytypes.

Figure 3

MIR 8.5–10.95 μm s-SNOM scan data for 4LG and a schematic of the analytical model used. (A) Amplitude (top) and optical phase (bottom) s-SNOM scan images of the 4LG sample. All images represent the third-harmonic-order deconvolution, S₃, of the s-SNOM signal normalized by the substrate’s signal, which helps to reduce far-field noise and compensate for changes in laser power between scans. Horizontal lines visible in near-field results are due to previously performed SHG measurements of the sample. Each image color scale was set individually to maximize the contrast between the different polytypes in that image. All scale bars in the image are for 4 μm (B). The measured spectrum of the s-SNOM optical third harmonic amplitude and phase signal in the 8.5–11 μm range for each 4LG polytype. The spectrum shows a wavelength-dependent increase in the amplitude and phase differences between the different polytypes. (C) Schematic showing the FDM and its components. The incident beam (E₀) generates a dipole P in the AFM probe (gold triangle). This dipole generates a mirror dipole P* through the interaction of the dipole charge Q₀ with the surface. The interaction between the s-SNOM probe and the surface changes the scattered light (E_S) going to the detector.

Figure 4

Point spectroscopy s-NSOM experimental measurements for 4LG graphene and the theoretical FDM model. The experimental results are compared with the predicted FDM model results for the optical conductivity calculated by the tight binding model. The FDM model can predict the lower part of the spectral scan range yet slightly deviates from the experimental measurements from 9.5 μm. Furthermore, experimental results show a larger amplitude and phase difference between the Bernal and rhombohedral polytypes than the theoretical in-plane optical conductivity predicted. (See the discussion in the text.)