I know they are transparent when formed into a thin film, but is there a method to alter the structure that will make them more transparent in their bulk form?
Is this make sense? Optically transparent means it's wide band gap semiconductor. Currently nanotube products are mixture of many types of the nanotubes. If you could produce only wide gap types, it may be transparent. But the band gaps of carbon nanotubes are up to 2eV (according to Wikipedia). If so, it's transparent only for red right.
Carbon nanotubes can never be transparent in visible light while maintaining the usual properties that are well known, i.e., high electrical and thermal conductivity as well as high mecanical strength due to the aromatic structure and the related electronic structure. For changing so much the optical properties, you have to change their electronic structure. So, either this is not carbon anymore, or the structure is so different that these are not tubes anymore ... If you need transparent nanotubes, go for totally different composition.
Alain
I do not agree with that, Alain, CNTs have wide varieties of electrical property. Some is metallic but some is ssemiconductor. Though I'm not a specialist, you can pick it from Wikipedia. I agree with that most of CNTs are almost zero band gap as graphite. But there are exceptions. In my understanding, thin CNTs tend to be wide band gap due to the distortion from graphite structure.
Whether they are metallic or semi-conducting, the electrical conductivity is not compatible with transparency, unless the material is extremely thin. Even like this, it would not be really transparent, stricto sensu, but just absorbing or reflecting a small part of the light. Therefore, "transparent" devices can be obtained with CNTs but CNTs themselves are not transparent by themselves.
The question open a scientific argument .Comments by Yoshiki and Alain are very clear. Thank you so much Yoshiki and Alain for worthy comments
I appreciate everybody to paying attention to our discussion.
CNTs with wide band gap do not have high electric conductivity; it is also exceptional cases of CNTs. In addition, electric conductivity does not means it is not transparent. Most of metals are transparent for UV range because the plasma frequency is below the frequancy of the light. So, if the wide band gap CNTs were successfully doped to p-type or n-type, they can be conductive.
If only proper spices of CNTs were sorely produced. The property must be very different with known CNTs. It is very interesting. So far, I do not know such a way to produce CNTs.
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