Two‐Terminal Graphene Oxide Devices for Electrical Modulation of Broadband Terahertz Waves

Abstract

THz waves transmission amplitude at room temperature. Notably, we found that the amount of the trapped charge carriers within the fully randomized, complex molecular defects (i.e., localized impurity states (LIS)) of GO can be precisely adjusted by the DC biasing with versatile and simple device architecture (i.e., a two-terminal, interdigitated electrode device); the excitation of the electrically trapped charge carrier in turn leads to a large change in broadband THz transmission (e.g., ≈ 30% at 1.0 THz). This dramatic change in broadband THz transmission (i.e., ≈0.3–2 THz) was achieved with a deepsubwavelength (< λ /10,000) thick GO paper (≈ 500 nm) without any assistance of the foreign structural motifs (e.g., metamaterials, plasmonic structures, and Fabry–Perot architectures). Systematic THz spectroscopy of the chemically reduced GO further revealed the critical role of GO’s complex molecular defects in the active control of broadband THz transmission. Also, we observed the trapped charge carriers within LIS of GO can additionally provide interesting hysteretic behavior in the control of broadband THz transmission, which could be applied to the THz-based photonic memory devices. Figure 1 a–c presents schematic and macro/microscopic images of the two-terminal GO devices used in the present work. The GO device mainly consists of two functional parts: (i) GO paper (i.e., multistacked GO fl akes) and (ii) two-terminal, interdigitated gold (Au) electrodes (Au microwire array) patterned on the polyimide (PI) fl exible substrate (2.6 μm in thickness). High-quality monolayer GO fl akes, prepared by acidic chemical exfoliation of graphite, [ 23 ] were dispersed in deionized water; then, GO aqueous solutions were vacuumfi ltrated in order to obtain mechanically robust, freestanding paper platform (≈500 nm in averaged thickness). The transmission electron microscope image and UV–vis spectroscopy of the obtained GO fl akes are summarized in Figures S1 and S2 in the Supporting Information. In particular, UV–vis spectroscopy and its Tauc plot analysis verify the presence of both energy bandgap (3.4−4.0 eV) and LIS, which makes GO contrast to graphene with zero bandgap. The mechanical robustness of the paper-type fi lm can facilitate the transfer of GO onto any desired substrate; herein, by using solvent-assisted transfer printing (see Figure S3 in the Supporting Information), GO paper was directly integrated onto the surface of the interdigitated, two-terminal electrodes (100 nm thick Au micro wire array with 10 nm Ti adhesion layer), as shown in Figure 1 b–d. Scanning electron microscope image in Figure 1 d shows the large-area GO paper (1 cm × 2 cm), conformably printed onto the predeveloped electrodes. The average roughness of GO (i.e., root-mean-square) was measured about 180 nm, which is still far below the wavelength of interest (Figure S4 in the Two-Terminal Graphene Oxide Devices for Electrical Modulation of Broadband Terahertz Waves