Single-bowtie-antenna-based Terahertz Time-domain Spectroscopy of Semiconducting van der Waals Materials: Chemical Treatment and Limitations

초록

We investigate changes in terahertz (THz) transmission characteristics by placing micrometer-sized van der Waals materials at the gap of metal bowtie antennas resonant in the THz frequency range. Specifically, the electrical conductivity of MoS2 with a thickness of tens of nanometers is tuned by chemical doping using N-methyl-2-pyrrolidone (NMP), and DC current-voltage characteristics are measured using the bowtie antenna electrodes. Corresponding changes in THz transmission are measured to analyze the impact of conductivity modulation on the THz responses. First, MoS2 flakes with electrical conductivities of 8.8 x 10(-3) S/cm and 5.7 x 10(-3) S/cm showed no change in antenna transmission, indicating semiconducting off-state behavior. After NMP treatment, the electrical conductivities increased by up to an order of magnitude, reaching 8.7 x 10(-2) S/cm and 3.7 x 10(-2) S/cm, respectively, yet no significant change in THz transmittance was observed. This result is consistent with previous theoretical predictions that transmission changes become observable when the conductivity of the material at the bowtie gap reaches approximately 10(2) S/cm or higher. These findings enable estimation of the lower conductivity threshold required for effective THz modulation and nanomaterial detection and advance the understanding of doping effects in van der Waals materials for THz applications.

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