Conductivity Mapping in Graphene through Scattering-Type Scanning Near-Field Optical Microscopy in the Mid-Infrared and Terahertz Spectral Region with 25nm Spatial Resolution

Abstract

Terahertz (THz) imaging and spectroscopy has emerged as a powerful tool for analysis of biological specimens or for studying and controlling low-energy excitations in solid-state systems [1]. One major limitation of THz spectroscopy is, however, the spatial resolution that is limited by diffraction to about 100-1000 µm, making it impossible to extract intrinsic, local material characteristics of nanoscale Graphene or other 2D-material structures or devices. Scattering-type scanning near-field optical microscopy (s-SNOM) bypasses the diffraction limit, enabling optical measurements with extreme sub-wavelength spatial resolution of below 20 nm [2]. THz time-domain s-SNOM spectroscopy of a semiconductor static random access memory (SRAM) sample enables quantitative analysis of free carrier concentration and scattering rates of nanoscale device structures at unprecedented surface sensitivity. Similarly, THz s-SNOM measurements at single frequencies enabled highlighting highly conductive nanostructures in transistor devices [3]. Using a similar experimental concept to generate a local photo-current by THz radiation allows for studying nanoscale conductivity in a biased graphene device [4] or to characterize carrier scattering in a graphene sheet that has been encapsulated in hexagonal boron nitride (h-BN) and placed on a split metallic film [5]. Using an SRAM sample as a method to calibrate the amplitude contrast of the s-SNOM THz measurement with known free carrier concentrations the concept is extended to graphene. Single to few layer graphene on h-BN is imaged with a broadband THz pulse and reveals changing amplitude contrast independently of layer number (Fig. (a), (b), zoom in (d)). In a corresponding s-SNOM measurement carried out at 10 µm an inverted contrast can be observed (Fig. (c)). This contrast inversion is a direct consequence of the scattering frequency dependent free carrier response in graphene [6]. Extending the already established concepts of mid-IR s-SNOM imaging and spectroscopy of 2D materials [7] to the THz frequency range a plethora of fundamental and applied new insights into graphene and other 2D device structures can be expected from THz near-field nanoscopy measurements. [1] M. Tonouchi, Nat. Photonics 2007, 1, 97; [2] F. Keilmann, R. Hillenbrand, Phil. Trans. R. Soc. Lond. A 2004, 362, 787; [3] A. J. Huber et al. Nano Lett. 2008, 8, 3766; [4] P. Alonso-Gonzalez, et al., Nat. Nanotechnol. 2016, 12, 31; [5] B. Lundeberg, et al., Science 2017, 357, 187; [6] J. Horng et al., Phys. Rev. B 2011, 83, 165113; [7] D. N. Basov et al., Science 2016, 354, 1992. Figure 1