Carbon Nanotube Spheres Fabricated with Light

Michael A. Greenwood

As more potential photonic applications are being explored for carbon nanotubes, which are valued for properties such as ultrafast recovery times and optical nonlinearity, the construction and handling of nanotubes remain an issue.

Nanotubes are prone to becoming entangled with other nanotubes, and it is difficult to separate them. Once separated, depositing them precisely in the correct spot can be laborious. Some of the construction methods, meanwhile, require steps such as chemical functionalization, which add time and cost.

Investigators at the University of Tokyo and at Alnair Laboratories Corp., also in Tokyo, report that they have constructed sphere-shaped carbon nanotube superstructures using an optical deposition technique that relies on changes in light intensity and not on chemistry.

Researcher Ken Kashiwagi, from the University of Tokyo, described the process and setup as “simple” and said that it could lead to the efficient use of carbon nanotubes in a variety of photonic applications.

The group constructed the spheres by taking light at a wavelength of 1560 nm from a laser diode and amplifying it with a high-power erbium-doped fiber amplifier. The light, which emitted from the end of a cleaved fiber, was directed into a droplet of carbon nanotube-dispersed dimethylformamide solution.

By controlling the light’s power, they deposited a drop of solution directly onto the fiber’s tip. The amount of power required to do this depended on the size of the nanotube bundle and on the extent of entanglement. Smaller bundles required higher optical power and larger bundles, lower power. The team deposited the droplet with 21.5 dBm of power.

The droplet was heated by the light, causing a microbubble to form at the end of the fiber and to grow as it was heated. Carbon nanotubes that had been deposited through adsorption formed the exterior of this sphere-shaped bubble while, throughout the process, the dimethylformamide solution evaporated.

Researchers used a light-injection technique to construct carbon nanotube spheres, which were coated onto the surface of a microbubble that was formed by the evaporation of dimethylformamide solution. Reprinted with permission of Optics Express.

The scientists found that spheres formed from smaller bubbles generally were unstable, whereas spheres with a diameter in excess of 15 μm were stable and could be manipulated. They created spheres with diameters as large as 100 μm, easily manipulating the size by changing the optical intensity and the duration of the injected light.

After creating the spheres, they reported that they could manipulate or handle them with relative ease. They demonstrated this by moving a sphere from one fiber to another and by precisely adjusting its location. After verifying the presence of carbon nanotubes in the sphere by using it as a saturable absorber, they constructed a passively mode-locked fiber laser, which yielded a broad lasing spectrum.

Optics Express, Feb. 8, 2008, pp. 2528-2532.