According to Rice University researchers, armchair nanotubes get their exclusive bright colors due to excitons or hydrogen-like objects. Their findings can be seen in their online edition of the Journal of the American Chemical Society.
Armchair carbon nanotubes get their name due to the "U"-shaped arrangement of the atoms at their open tips, these metals are single-dimensional and do not have any band gap. Since the absence of band gap causes the electrons to flow between ends with minimal resistivity, there is a possibility of obtaining armchair quantum wires.
The Rice researchers have proved that absorption of light by armchair nanotubes is similar to semiconductors. According to Junichiro Kono, Rice physicist, absorption of photons promotes an electron from an immobile state to a conducting state by absorption of photons and leaving a remnant of positively charged "holes". This electron-hole pair results in an exciton with a neutral charge.
Researchers have proved that it is possible to control silver and gold nanoparticles in such a way as to reflect bright hues. Due to their size, the absorption and emission of specific color light was possible by plasma resonance. Scientists have noticed that semiconducting nanoparticles or quantum dots show colors with respect to their band gaps, which are size-dependent. Since armchair nanotubes do not have any band gaps, plasma resonance occurs at wavelengths external to the visible spectrum in metallic carbon nanotubes. The researchers found that the actual color source in batches of pure armchair nanotubes in solution is due to excitons.
It was also noted that armchair nanotubes do not have band gaps but they have an exclusive electronic structure that favors specific wavelengths for light absorption.
The researchers strived to create pure single-walled carbon nanotube batches using ultracentrifugation. Nanotubes were spun in a mixture of solutions having a range of densities, which are up to 250,000 times the gravitational force. The tubes moved naturally towards separated solutions based on their own densities to form a colorful "nano parfait." The researchers were able to produce pure armchair nanotubes from a range of synthesis techniques.
Source: http://www.rice.edu