The identification of handedness for single carbon nanotubes becomes possible by scientists from Peking University in China

In the past three decades, various optical spectroscopic techniques have been demonstrated to identify the chiral indices (n, m) of carbon nanotubes unequivocally even at the single-tube level. However, the accurate characterization of the handedness via optical spectroscopy has been unrealizable for single nanotubes. Here, we developed the Rayleigh scattering circular dichroism to fulfil the dream of complete structure identification of both chiral indices and handedness for single nanotubes.

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The last thirty years have witnessed enormous progress in carbon nanotubes, driven by their unique physical properties and remarkable applications. To explore the full potential of nanotubes, high-throughput and non-invasive characterization on their accurate structure is a prerequisite. Through intensive research, various optical spectroscopic techniques, including Raman scattering, photoluminescence, Rayleigh scattering, and polarization spectroscopy, have been successfully demonstrated to identify the chiral indices (n, m) and probe the physical properties of carbon nanotubes unequivocally even at the single-tube level.

However, the very basic structural parameter of a single carbon nanotube, i.e., the inherent handedness (left-handed or right-handed helicity, Fig. 1), can’t be determined by optical spectroscopy yet. Conventional circular dichroism techniques, which are widely used to determine the handedness of macroscopic chiral structures, have failed for single carbon nanotubes due to the extremely weak chiroptical signals compared to the strong excitation light in a single nanotube. As a consequence, their development of sophisticated handedness-related devices remains stagnant.

Fig. 1 Schematic geometric structure of carbon nanotubes with different helicities.

Here, in the work just published in Nature Nanotechnology, we demonstrated the complete structure identification of individual carbon nanotubes for the first time by using a high-sensitivity Rayleigh scattering circular dichroism (Ray-CD) spectroscopic technique. In our setup, Rayleigh scattering of an individual suspended carbon nanotube is collected by an oblique objective (Fig. 2a). Thus, compared to absorption measurements with a large background of excitation light, Rayleigh scattering in our configuration is free from the interference of the transmitted light. Therefore, the dichroic signal measured in Rayleigh scattering can be three to four orders of magnitude larger than that measured using an absorption circular dichroism method, enabling the extremely weak chiroptical signal from a single nanotube detectable (Fig. 2b-c). Moreover, we successfully realized the Rayleigh circular dichroism detection of 30 individual chirality-defined carbon nanotubes in a broad optical spectral range and explored their handedness distributions statistically. Furthermore, we revealed their structure-dependent chiral properties by combining with the tight-binding calculations.

Fig. 2 (a) Schematic of the Ray-CD experimental setup. LLTF, laser line tuneable optical filter; PEM, photoelastic modulator; SWNT, single-walled carbon nanotube; PMT, photomultiplier tube. (b-c) Typical Ray-CD results for two representative nanotubes. Optical transitions were marked above each peak, based on which chiral indices of these nanotubes were determined. Furtherly, their handedness were identified by the signs of Ray-CD at peaks and their type category.

This work fulfils the final dream of identifying the complete structure of single nanotubes efficiently and rapidly, and represents one of the cornerstones for nanotube characterization by optical spectroscopy. Moreover, this new methodology can be universal for characterizing the chiral properties of other single nanomaterials, therefore, offering a facile platform for the exploration of low-dimensional chiral physics and chiral devices.

For more information, please read our recent publication in Nature Nanotechnology (https://www.nature.com/articles/s41565-021-00953-w)

Corresponding author: Kaihui Liu, Professor of Physics, Peking University. Kaihui Liu's group focuses on the study of growth mechanism and spectral physics of low-dimensional materials. They designed the meter-scale single-crystal manufacturing technique and equipment, built the single-nanomaterial-level in-situ optical characterization system and developed the low-dimensional material composite fibers for all-fiber integration devices. These works have been published in Nature (2020, 2019), Nature Nanotechnology (2020, 2016, 2013, 2012), Nature Photonics (2021, 2019), Nature Physics (2014), Nature Chemistry (2019) and Nature Communications (2018, 2014, 2013).

Fengrui Yao

Phd, Peking Unversity