Novel exciton interactions observed in compositionally-defined bundles of carbon nanotubes

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Laboratory researchers in collaboration with the National Institute of Standards and Technology (NIST) have discovered an optical signature of novel exciton interactions in small bundles of carbon nanotubes (CNTs). The quantum tunneling action could impact energy distribution in carbon nanotube networks and enable potential use in light-harvesting films and other applications. The journal Nature Communications has published their findings.

Carbon nanotubes are cylinders of graphene with their atoms arranged in hexagons. CNTs have potential as near-infrared light emitters and nanoscale semiconductor materials for electronics and optoelectronics applications. Excitons effectively carry energy in CNTs as tightly bound and highly correlated pairs of negative and positive charge (electrons and holes) and are created when light is absorbed by the material. This discovery provides clear evidence that in small bundles of CNTs, in which each nanotube has an identical structure, typical excitons that are bound to a single nanotube (intratube excitons) are now accompanied by excitons that instead tunnel across closely interacting nanotubes (intertube excitons).

Interactions between individual elements of nanomaterials can give rise to new emergent behaviors. Intertube excitons are a one-dimensional analogue to a broader class of interlayer excitons found in two-dimensional materials like transition metal dichalcogenides that are also of growing interest for their optical and electronic properties. Exotic behaviors such as exciton condensation may result. Intertube excitons of CNTs could add to the range of observed exciton behaviors and potentially impact how energy moves around in CNT networks of interest as light-emitting films or for light harvesting applications. While intertube excitons had previously been suggested based on observations of light absorption and emission in CNT bundles, limitations of these optical probes have restricted further study.

The Los Alamos/NIST team showed that Raman spectroscopy (a form of light scattering) can provide more extensive characterization of intertube excitons. The team used chemical separations to isolate a sample of a single type of CNT structure. The nanotubes in these samples were then bundled to force interactions between individual nanotubes. The researchers measured the intensity of Raman scattered light as the wavelength of light was varied to provide a profile or map of the CNT exciton energies. The team found a previously unobserved sharp feature in the Raman profile of the bundled CNTs (red-circled region in the Figure). The investigators did not find this unexpected feature for non-interacting individual CNTs.

Theoretical analysis showed that the unique packing geometry produced in bundles comprised of a single CNT structure results in chains of closely interacting carbon atoms that promote the formation of intertube excitons (Figure inset, upper right). Further analysis showed that the intertube excitons by themselves cannot interact with light in a way that generates the sharp feature. Instead, an interaction (depicted in lower left inset, Figure) between the intertube excitons and intratube excitons leads to an exciton scattering process that is accompanied by a quantum interference. The Raman spectral signature of such an interference, known as a Fano resonance, is exactly the sharp asymmetric feature observed experimentally.

The Fano interference has been well known in atomic and condensed matter physics. It is becoming increasingly important as a means for creating new optical behaviors in nanoscale composite materials such as plasmonic assemblies and metamaterials, as routes to manipulate light. The team’s findings reveal this behavior as a new class of exciton response in CNT assemblies, suggesting such Fano resonances may be found in a broader class of 2D quantum composite materials, for which expanded optical functionality of interest for optoelectronic and photonic applications may result. 

Reference: “Resonance Raman Signature of Intertube Excitons in Compositionally-Defined Carbon Nanotube Bundles,” Nature Communications 9, 637 (2018); doi: 10.1038/s41467-018-03057-7). https://www.nature.com/articles/s41467-018-03057-7 Authors: Stephen Doorn, Erik Haroz, and Hagen Telg (Center for Integrated Nanotechnologies, MPA-CINT); Andrei Piryatinski (Physics of Condensed Matter and Complex Systems, T-4); Oleksiy Roslyak (Fordham University, CINT user); Jared Crochet and Juan Duque (Physical Chemistry and Applied Spectroscopy, C-PCS); Jeffrey Simpson (Towson University and NIST); and Angela Hight Walker (NIST).

This work was supported in part by the Laboratory Directed Research and Development (LDRD) program and was performed in part at both the Center for Integrated Nanotechnologies (a DOE Office of Science user facility jointly operated by Sandia and Los Alamos National Laboratories) and the National Institute of Science and Technology in Gaithersburg, MD.

The work supports the Laboratory’s Energy Security mission area and its Materials for the Future and Science of Signatures science pillars by developing new methods to define, probe, and model novel materials interactions with the aim of developing technologically useful quantum materials. Such understanding aids efforts to develop materials with predictable performance and controlled functionality, such as for photovoltaic and optoelectronic devices. Technical contacts: Stephen Doorn and Andrei Piryatinski

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Image 1:

Caption 1:

Photo. Steve Doorn works on an instrument used for spectroscopic characterization of carbon nanotubes.

 

Image 2:

Caption 2:

Figure. Plot of exciton energy resonances determined by resonance Raman spectroscopy for a carbon nanotube bundle limited solely to (6,5) nanotubes. The red-circled feature highlights the new resonance behavior introduced by intertube interactions in bundles, depicted in the upper right inset. The lower left inset depicts the interaction between cross-polarized intertube (blue) and intratube (red) excitons.