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Double-Walled Carbon Nanotubes Overview

                                      
Carbon nanotubes are materials that possess remarkable properties and offer extraordinary possibilities. This article gives a brief overview of the physio-chemical nature and characterization of double-walled carbon nanotubes (DWNTs).
Double-walled carbon nanotubes are a synthetic blend of both single-walled and multi-walled nanotubes, which show properties that are intermediate between both types. Double-walled carbon nanotubes may adopt four different permutations coming from the electronic type, which may be metallic or semiconducting, of their inner and outer walls. Such polydispersity limits the utility of double-walled carbon nanotubes in applications like thin film electronics. The density gradient ultracentrifugation can be used to address this source of heterogeneity by the means of producing double-walled carbon nanotubes with well-defined outer-wall electronic types.
Optical absorption value of sorted double-walled carbon nanotubes reveals the outer-wall’s 96% and 98% of purity for sorted semiconducting and metallic units. Some experimental studies found that even though both walls are semiconducting, double-walled carbon nanotubes might behave as a metal. Electrical characterization of semiconducting and metallic outer-wall double-walled carbon nanotubes directly confirms the efficacy of these separations in thin transistors, with semiconducting double-walled carbon nanotubes devices that yield on and off ratios 2 orders of magnitude that is higher than a comparable metallic double-walled carbon nanotubes devices. This complication of their overall electrical behavior has limited the utility of double-walled carbon nanotubes to applications such as thin film electronics.
Double-walled carbon nanotubes are comprised of exactly two concentric nanotubes separated by 0.35 – 0.40 nm, with sufficient band gaps for use in field-effect transistors. The inner and outer walls of double-walled carbon nanotubes have optical and Raman scattering characteristics of each wall. Double-walled carbon nanotubes also exhibit several beneficial properties observed from multiple-walled carbon nanotubes, such as improved lifetimes and current densities for field emission and high stability under aggressive chemical, mechanical, and thermal treatments along with the flexibility observed with single-walled carbon nanotubes.

Double-walled carbon nanotubes applications:
Double-walled carbon nanotubes are used in biological systems as imaging and therapeutic agents.
Double-walled carbon nanotubes are used in gas sensors.
Double-walled carbon nanotubes are used in field emission displays.
Double-walled carbon nanotubes are used in photovoltaics
Double-walled carbon nanotubes are used in solar cell fabrication processes.

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