The Nanotechnology Research Institute (NRI) of the National Institute of Advanced Industrial Science and Technology (AIST), an independent administrative institution, has developed thins film of oriented single wall carbon nanotubes that emit polarized light.

Preparing high quality thin films is an important prerequisite for many industrial applications of single-wall carbon nanotubes (SWNTs). For conventional methods of making SWNT films, however, it has been very difficult to prevent tubes from aggregating to form bundles. If bundled, strong electronic interactions among tubes would hinder them from exhibiting intrinsic semiconductor properties of SWNT, especially light emission and photoelectric conversion.

The NRI-AIST has succeeded in preparing thin films of uniformly dispersed individual tubes by using gelatin, a biopolymer, as a dispersion medium, and further in orienting separate tubes in a fixed direction by mechanical stretching. (See Photo.) When the film is illuminated with (non- polarized) visible light, an emission of near-infrared rays strongly polarized in the direction of tube alignment is observed (Fig. 1). This is the first implementation of light emitting SWNT film, and the additional feature of polarization is expected to provide a significant momentum for exploiting the optoelectronic functions of SWNTs, as well as for enhancing the understanding of their electronic properties.

The result of the present study appeared in the February 14 issue of the Applied Physics Letters, published by the American Institute of Physics: "Highly polarized adsorption and photoluminescence of stretch-aligned single-wall carbon nanotubes dispersed in gelatin films", Y. Kim, N. Minami and S. Kazaoui, Appl. Phys. Lett. 86, 073103 (2005).

Photo. A stretched gelatin film dispersed with single-wall nanotubes

Fig. 1. Polarized photoluminescence spectra of an aligned SWNT film When an aligned SWNT film is illuminated with non-polarized visible light, the film emits polarized light in the near-infrared region; the emission intensity differs drastically depending on the direction of the polarization, parallel (//) or perpendicular (⊥) to the direction of the tube alignment.

Single wall carbon nanotube (SWNT) is characterized by a tubular structure as if a graphite sheet is rolled up, and has a unique property of becoming either metallic or semiconducting, depending on in which direction the sheet is rolled. Other unique features include energy gap scaling with inverse of the tube diameter despite its single-element (carbon) constitution, marked anisotropy presenting different physical properties in axial and radial directions, and placing all the constituent elements on the surface. Owing to these extraordinary properties, SWNT is expected to work as an unprecedented type of semiconductor. While a number of studies have been made on the use of semiconducting SWNTs for field effect transistors (FET), little progress has been achieved for the use of another important feature of semiconductors, namely, optoelectronics such as photoelectric conversion and electroluminescence. Earlier works showed that surfactant-aided dispersion of SWNTs in water emit light, but thin films prepared from such a system lose the light-emitting capability because of SWNT aggregation. This makes it difficult to investigate the optoelectronic properties of SWNT-based films.

The NRI-AIST has been intensively working on SWNT films over couples of years, in the belief that realization of light-emitting SWNT films must open the way to the practical application of unique optoelectronic features of SWNT.

The NRI-AIST had already succeeded in preparing homogeneous thin films of aligned SWNT by the Langmuir-Blodgett (LB) technique. [AIST Today, October 2002; Japanese Journal of Applied Physics, Part 1, December 2003; and Patent filed July 2004.] While the LB process has an advantage of layer-by-layer deposition, strong aggregation of tubes inevitably occurs during the chemical synthesis of solubilized SWNTs, hindering the intrinsic optoelectronic features of semiconducting SWNTs from being exhibited.

For exploiting the optoelectronic features of the SWNT system, it is essential to isolate and align individual tubes; this is what has been achieved in the present study taking advantage of gelatin's favorable properties, dispersing ability and stretchability.

In the present study, a quite simple method is used to prepare thin films; dispersing raw SWNTs in aqueous gelatin solution, from which thin film is prepared retaining the isolated state of individual, homogeneously dispersed SWNTs (Fig. 2). The gelatin film has been used as an excellent dispersion medium for photographic receptor since 100 or more years ago, and the present success is attributable to this historical heritage. In particular, the prevention of SWNT's aggregation owes much to the gelation property of gelatin solution. When a warm aqueous solution of gelatin is left to cool, it turns from the fluid state of sol to non-fluid gel at around 40 °C, just like in the cooking process for jelly. This freezes the move of dispersed SWNTs to prevent tubes from aggregating during the subsequent drying.

The newly prepared SWNT film is optically homogeneous and demonstrates SWNT-specific light emission in the near infrared region, which is attributed to the interband optical transitions in semiconducting SWNTs. While the emission feature is lost for aggregated tubes due to the inter-tube interaction, the present method realized light-emitting SWNT films because tubes are isolated by gelatin's dispersing ability. Moreover, we have succeeded in aligning SWNTs by mechanically stretching the film. The tube alignment is corroborated by the measurement of optical anisotropy, such as polarized absorption, polarized photoluminescence and birefringence. (Figs. 1, 3 and 4.) If SWNTs are not aligned, that is, randomly oriented, such optical anisotropy cannot be observed.

Casting an aqueous dispersion of SWNT/gelatin on a substrate	Desiccation	Peeling off	Swelling in alcohol	Mechanical stretching
Fig. 2. Preparation and stretching of an SWNT/gelatin thin film

Fig. 4. Optical anisotropy of carbon nanotube Because of the large structural anisotropy, the optical properties of SWNT change drastically depending on the direction of polarization (direction of light wave oscillation).