Temperature-Dependent Broadband Terahertz Behavior of Metal-Free Multiwalled Carbon Nanotubes

This study focuses on the temperature-dependent terahertz (THz) response of a metal-catalyst-free multiwalled carbon nanotubes (MWCNTs) film. The presence of metal catalyst particles challenges the understanding of pure response of the MWCNTs; hence, a distinct method is adopted for the development of pure MWCNTs excluding metal catalyst particles. Utilizing the MWCNTs obtained by this method, a film of ∼40 μm thickness is drop-casted on a high-resistance Si substrate. With the help of terahertz time domain spectroscopy (THz-TDS), the MWCNTs films are characterized for the broadband frequency range (0.2–1 THz) with temperature variation from 24 to 123 °C. Our experiments reveal that an increase in the sample temperature leads to a decrease in THz transmissions due to enhanced THz conductivity. Further, decreasing temperature brings back its response in the reverse manner; however, the two paths are slightly deviated from each other, inducing a temperature-induced hysteresis effect. We attribute this to the temperature-dependent THz response of MWCNTs to π-electron transitions and the existence of defect states. Moreover, the establishment of scattering junctions at high temperature is dedicated to the observed hysteresis effect. Our study also reveals the applicability of these MWCNTs films as THz broadband absorbers, low pass filters, and modulators. Hence, this study can be very useful in incorporating low-dimensional materials in order to realize THz quantum devices.