Temperature-Dependent\nBroadband Terahertz Behavior\nof Metal-Free Multiwalled Carbon Nanotubes

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