A cleaner, more efficient way to produce single-photon emitters

Introduction of organic color centers into airborne nanotubes using a vapor phase reaction. a A schematic of a functionalized SWCNT suspended through a trench on a Si substrate. b A scanning electron micrograph of a tube after functionalization and series of PL measurements. The particles on top are patterned catalysts for the growth of SWCNTs and the nanotube is indicated by an arrow. c Representative PL spectra of an identical airborne SWCNT (10.5) before and after functionalization taken with a laser power of 10 μW and an excitation energy of 1.59 eV. PL intensity maps of (d)E₁₁(e)E⁻₁₁and (f)E^−∗₁₁ emission from a tube (9.7) where the intensity is integrated in a window of 37.4, 32.5 and 28.5 meV centered on each emission peak, respectively. The color scales are normalized to the maximum intensities in the respective maps. The dark features to the right of the tube are caused by the reflection of the excitation laser from the bottom of the trench. The white broken lines indicate the edges of the trench. g A reflection image in the same area, where the lighter and darker regions correspond respectively to the surface of the substrate and the bottom of the trench. Scale bars in panels (b, d–g) are 1.0 μm. Credit: Nature Communication (2022). DOI: 10.1038/s41467-022-30508-z

RIKEN researchers have created an efficient source of single photons for emerging quantum technologies by adding molecules to carbon nanotubes using a reaction that occurs in the vapor phase.

Quantum technologies are poised to revolutionize computing and communications, promising benefits such as secure communication, ultra-sensitive sensing, and parallel computing. Many of these applications require light sources capable of generating single photons, the smallest possible light packets, on demand.

A promising source of single photons in the infrared wavelength range used in telecommunications are carbon nanotubes – cylinders of graphene sheets with a diameter of about one nanometer – which have been given new functions, or functionalized, by adding an organic molecule.

The cleanest way to do this would be to use carbon nanotubes suspended through an air gap, but unfortunately this is not compatible with the usual approach of functionalizing carbon nanotubes, which takes place in solutions. “Solution-functionalized carbon nanotubes tend to be very short and have defects everywhere,” notes Yuichiro Kato of the RIKEN Center for Advanced Photonics (RAP).

Now Kato and Daichi Kozawa, also from RAP, and their colleagues have developed a method of functionalizing carbon nanotubes that can be performed in the vapor phase, and thus on nanotubes suspended through a trench in a silicon substrate.

“We grew nanotubes quite long and functionalized them in the vapor phase, so that they have no contact with solutions, which contain a lot of impurities,” Kato explains. “This method allowed us to introduce organic molecules without also incorporating unwanted defects.”

The study was a collaboration born out of a pre-pandemic interaction at an international conference. Kato and Kozawa’s team at RAP produced the nanotubes in suspension, then sent them to chemists at the University of Maryland in the United States for functionalization, who then sent them back for analysis. “YuHuang Wang from the University of Maryland is a great chemist, and he was the one who became interested in the possibility of doing these reactions in the vapor phase,” Kato explains. “It took us a few turns, but we could see good emission from the organic molecules on the nanotubes.”

The team verified the optical performance of their carbon nanotubes by performing spectroscopic measurements on more than 2,000 of them. They found that the number of organic molecules introduced per nanotube increased with smaller diameter nanotubes, and they were able to model this effect in terms of greater reactivity of narrower nanotubes.

The study is published in Nature Communicationand the team now intends to optimize the functionalization process so that only one organic molecule is introduced per nanotube.