Purer carbon nanotubes potentially enable brighter light sources and more effective solar cells.

Ultraclean carbon nanotubes hold promise for advances in optical fiber communications, solar cells and LEDs

Carbon atoms can assemble in numerous structural forms called allotropes, e.g. a diamond, graphite, or graphene. These forms can result in distinct properties for materials that consist of the same element. One such allotrope, a cylindrically structured molecule known as a carbon nanotube, has been the subject of much scientific research for the past twenty years because of its extraordinary tensile strength, unique electrical properties, and efficient heat conduction. It has well-established applications in nanoelectronics and more recently has attracted tremendous interest as a nanomaterial for next-generation optoelectronics (electronic devices that source, detect and control light for optical fiber communications, solar cells and LEDs) and quantum photonic devices that have the potential to revolutionize information processing, telecommunications, sensing and measurement.

Despite the promise of this innovative material, its light emission has generally been dimmer than theorists had expected. The majority of experiments on carbon nanotubes to date reveal low quantum efficiencies as well as dependence on the environment and chemical processing. This is detrimental to their usefulness in devices and other applications. According to Dr. Stefan Strauf, Professor in the Department of Physics and Engineering Physics and Director of the NanoPhotonics Laboratory at Stevens Institute of Technology, “Understanding the intrinsic photophysical properties of carbon nanotubes is very interesting scientifically and also essential to realizing efficient devices.”

To address these inefficiencies, Dr. Strauf and collaborators James Hone and Chee Wei Wong from Columbia University have devised an improved fabrication process for carbon nanotubes, potentially leading to brighter light sources and more effective solar cells based on the material. They were able to increase the spontaneous light emission from an individual carbon nanotube by two orders of magnitude compared to previously reported experiments. They were also able to achieve a fourfold prolonged coherence time of the light emission.  The results of their work, titled “Prolonged Spontaneous Emission and Dephasing of Quantum Dot Excitons in Air-Bridged Carbon Nanotubes,” were published in the July 11 edition of Nature Communications (Issue 4, Article Number 2152, doi:10.1038/ncomms3152).

“Dr. Strauf’s groundbreaking advances with carbon nanotubes represent a significant scientific breakthrough that could herald technological innovation in numerous important industries such as quantum computing and solar energy,” says Dr. Michael Bruno, Dean of the Charles V. Schaefer, Jr. School of Engineering and Science.

Previous experiments have reported carbon nanotubes with spontaneous light emission times on the picosecond scale, while theorists had predicted intrinsic optical lifetimes of several nanoseconds. Dr. Strauf and his collaborators surmised that this disparity was due to masking caused by impurities in the material, which are the result of contamination from the substrate (material upon which the experimental processes take place) and surfactant (a chemical that works like a detergent to separate and disperse nanotubes in order to prevent clumping). The researchers therefore used sophisticated techniques to grow and arrange carbon nanotubes in order to mitigate unintentional impurities and reveal the true extent of the material’s optical capabilities. They prepared about 1,000 pairs of pillar posts, each 3 micrometers apart, in a silicon wafer and topped them with a metal catalyst. They then deposited the carbon in the form of an ambient chemical vapor and intensely heated the preparation, creating many carbon nanotubes that bridged the pillars. The growth suspended in air prevents the substrate and surfactant from blending into the nanotubes and diminishing their effectiveness. They also heated the nanotubes for shorter periods (2-10 minutes) than previous experiments. The shorter heating times meant less residual amorphous carbon, resulting in ultraclean nanotubes that emit a much brighter.

carbon nanotubes

Carbon nanotubes have attracted great interest for optoelectronics because of the unique ability of the material to maintain the stability of electron states called excitons even at room temperature, as opposed to the extreme cold usually required. An exciton comes about when a (negatively charged) electron in a carbon nanotube is excited (raised to a higher energy level) but remains bound to a positively charged “hole” in the lower energy level. The exciton thus carries energy but not a net electric charge. Photons are absorbed when the electron enters the exciton state and light is emitted when the electron recombines with the hole. The emission can be used to create devices like LEDs, lasers, and quantum light sources, while the absorption can be used to create solar cells or photodetectors.

While the prolonged radiative emission is promising for device applications, the researchers also were able to maintain a longer coherence time of the emitted light from the exciton recombination in these individual carbon nanotubes, finding four-fold prolonged values compared to previous ensemble measurements. This discovery could spark new discussions about the nature of the underlying mechanism which causes the dephasing that makes it difficult to sustain quantum effects long enough to allow for practical quantum information processing. A breakthrough in preserving coherence could lead to quantum computers with unprecedented power, allowing researchers to approach unwieldy problems and rendering most cryptography obsolete.

According to Dr. Rainer Martini, Director of the Department of Physics and Engineering Physics, “This work constitutes a major advance in carbon-nanotube based photonics and will generate even more interdisciplinary inquiry in this field.”

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