Nanotechnology specialists have developed a new method for fine-tuning the optoelectronic properties of single-walled carbon nanotubes (SWCNTs), opening up the possibilities for using them in the manufacture of a wide range of optical electronic devices, such as photocells, solar cells, laser diodes, optical couplers, and LEDs.

The new approach was devised by researchers from the Skoltech Center for Photonics and Quantum Materials (CPQM) in Moscow, who made the breakthrough by applying an aerosolized dopant solution on the surface of the SWCNTs.

The recent introduction to the mobile phone market of devices with bendable and foldable screens has created a surge in demand for the development of materials that are both electronically conductive, as well as lightweight, strong, and crucially, flexible. Carbon nanotubes, with their unique combination of all of these properties, have therefore become an ideal candidate for research as a cutting-edge raw material for use in the manufacture of top-end electronic devices.  

As the scientific journal, explains, “Transparent conductive films (TCFs) made using an advanced solution of SWCNT, are seen as the key element of flexible and transparent electronics. As opposed to the customary n-type transparent rigid conductors, such as tin-doped indium oxide or aluminum-doped zinc oxide, the flexible and stretchable SWCNT films have p-type (hole-type) conductivity.”

However, using SWCNTs to make bendable, electronically conductive films still has a significant drawback. Primarily that manufacturers have poor control over the SWCNT's electronic properties. If TCFs are to have a more widespread use, then producers of electronics would require more control over the resistance levels of SWCNTs, as well as their Fermi levels, particularly when applying CNTs for optoelectronic applications.

Carbon nanotubes are commonly treated with a doping agent, and it is the application of these doping agents that laid out both the problem and the solution to researchers. As Alexey Tsapenko, a Ph.D. student and the study’s lead author, outlines, “SWCNT conductivity is enhanced using one of the three most common doping methods: drop-casting, spin-coating or dip-coating, which can significantly decrease the resistance of pristine SWCNT films (up to 15 times) but fail to ensure spatial uniformity and have poor scalability. This leads to non-uniform evaporation of the liquid solvent, resulting in a coffee-ring effect. Moreover, none of these techniques enable precise control over the Fermi level in the SWCNT films.”

To solve this problem required a new doping method, one that could ensure a controllable, uniform, and easily reproducible doping of SWCNT.

This the researchers have now achieved, publishing their results in the ACS’s Journal of Physical Chemistry Letters, where the team claims the discovery of, “… a novel, scalable, and simple method for aerosol doping of single-walled carbon nanotube (SWCNT) films. This method is based on aerosolization of a dopant solution (HAuCl4 in ethanol) and time-controlled deposition of uniform aerosol particles on the nanotube film surface.”

The results are impressive, and show great flexibility in controlling conductivity, as, “The approach developed allows fine-tuning of the SWCNT work function in the range of 4.45 (for pristine nanotubes) to 5.46 eV, controllably varying the sheet resistance of the films from 79 to 3.2 Ω/□ for the SWCNT films with 50% transmittance (at 550 nm).”

“Our method allows easy tuning of SWCNT film parameters thanks to time-controlled deposition of doping aerosol particles,” says Tsapenko.

Additionally, the CNT fine-tuning approach can also be used on the electronic structures of other low-dimensional materials.

However, using the process on SWCNTs for the development of highly conductive transparent films will be the focus of further research, as the team hope to provide an alternative to existing indium-tin oxide electrodes, as well as pushing the limits of flexible optoelectronics.

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