Microwave shielding of transparent and conducting single-walled carbon nanotube films

Abstract

The authors measured the transport properties of single-walled carbon nanotube ͑SWCNT͒ films in the microwave frequency range from 10 MHz to 30 GHz by using the Corbino reflection technique from temperatures of 20-400 K. Based on the real and imaginary parts of the microwave conductivity, they calculated the shielding effectiveness for various film thicknesses. Shielding effectiveness of 43 dB at 10 MHz and 28 dB at 10 GHz are found for films with 90% optical transmittance, which suggests that SWCNT films are promising as a type of transparent microwave shielding material. By combining their data with those from the literature, the conductivity of SWCNT films was established in a broad frequency range from dc to visible.Single-walled carbon nanotubes ͑SWCNTs͒ are emerging as building blocks of electronics for a variety of applications. In particular, films of nanotubes have found potential applications for electronics and optoelectronics. 1 However, an understanding of the transport mechanism that governs the response to dc and ac electric fields is required for application. It has been established that the overall resistance of the films is determined by the junction resistance between different tubes. 2,3 To fully explore the potential applications of NT films as a type of electrical and photonic material, it is important to map out the conductivity in a wide frequency range. Ac conductance measurements of a percolating NT network of up to 1 MHz have shown a universal power law in frequency, which is commonly observed in systems with randomly distributed barriers. 4 In the terahertz range, the effective Maxwell-Garnet model has been introduced where both the metallic and semiconducting NTs were embedded in a dielectric host. 5 Optical conductance in the far infrared and visible range has been obtained by measuring the reflectance of NT films and a Kramers-Kronig transformation. [6][7][8] Study of conductance in the microwave frequency range is important for high speed NT thin film field effect transistors. Microwave conductivity of individual SWCNT and operation as transistor at 2.6 GHz have been measured by Burke and co-workers. 9 They also gave a rf circuit model for carbon nanotubes. 10 However, there is a paucity of conductivity measurements on SWCNT films in this frequency range. So far, a few groups have measured with a cavity setup, which can only lead to conductivity values at a few discrete frequencies. 11,12 Here we use the Corbino reflectance setup 13 which allows the measurement of conductivity in a continuous frequency range from the rf to the microwave.Electromagnetic radiation at radio and microwave frequencies, such as those emanated from cell phones, tend to interfere with electronic devices. The electromagnetic interference ͑EMI͒ leakage from radio frequency to microwave is still a serious problem for our society. The primary mecha-nism of EMI shielding is usually reflection. Thin metal foil and metal grids are commonly used for this purpose. Recently light-weight, flexible, and highly effecti...

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