Optically active defects improve carbon nanotubes

The properties of carbon-based nanomaterials can be altered and engineered through the deliberate introduction of certain structural “flaws” or defects. However, the challenge is to control the number and type of these defects.

In the case of carbon nanotubes – microscopically small tubular compounds that pro-dr. The chemists and physicists at Heidelberg University, led by Jan Zhaumseel, emit light, have now demonstrated a new reaction pathway to enable such defect control.

This typically results in strongly active defects — so-called sp3 defects — that are more luminescent and can emit single photons, namely particles of light. Efficient emission of near infrared light is important for applications in telecommunications and biological imaging.

Defects are generally considered to be something “bad” that negatively affects the properties of a material, making it less perfect. However, in some nanomaterials such as carbon nanotubes, these “imperfections” may result in something “good” and enable new functionality. Here, the exact type of defect is important.

Carbon nanotubes have rolled sheets of hexagonal lattice of sp2 carbon atoms, as they are also in benzene. These hollow tubes are approximately one nanometer in diameter and up to several micrometers long.

Through some chemical reactions, some sp2 carbon atoms of the lattice can be converted into sp3 carbon, which is also found in methane or diamond. This alters the local electronic structure of carbon nanotubes and results in an alternatively active defect.

These sp3 defects also emit light in the near-infrared and are overall more luminescent than nanotubes that have not been functional. Due to the geometry of the carbon nanotube, the exact position of the introduced sp3 carbon atoms determines the optical properties of the defects.

“Unfortunately, so far there is little control over what constitutes the defect,” Jan Zumsel, a professor at the Institute of Physical Chemistry and a member of the Center for Advanced Materials at the University of Heidelberg.

Heidelberg Scientific and his team have recently demonstrated a new chemical reaction pathway that enables defect control and selective fabrication of only a specific type of sp3 defect. These alternatively active defects are “better” than “already initiated” flaws.

Not only are they more luminescent, they also show single-photon emission at room temperature, Proc. Zumseel explains. In this process, only one photon is emitted at a time, which is a prerequisite for quantum cryptography and highly secure telecommunications.

Pro. According to Simon Settle, a doctoral student in Zumseil’s research group and the first author of the paper, this new method of reporting these results is very simple nucleophilic addition and does not require any special equipment.

“We are only just beginning to explore potential applications. Many chemical and photophysical aspects are still unknown. However, the goal is to create even better defects.”

The research is part of “Tryons and SP3-Defects in Single-Walled Carbon Nanotubes for Optoelectronics” (Trifacts), led by Proc.

Funded by an ERC Consolidated Grant from Zumseel and the European Research Council (ERC). Its goal is to understand and engineer the electronic and optical properties of defects in carbon nanotubes.

“The chemical differences between these defects are subtle and the desired bond configuration is usually formed only in a minority of nanotubes.

Being able to produce large numbers of nanotubes with a specific defect and controlled defect density, paves the way for optoelectronic devices as well as electrically pumped single-photon sources, which are the future in quantum cryptography. Applications are essential, ”says Prof. Zumseel.

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