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Page 12 of 29 Teng et al. Microstructures 2023;3:2023019 https://dx.doi.org/10.20517/microstructures.2023.07

Another type of novel material is composed of 1D chains that are weakly held together by van der Waals
interactions. Transition metal tri chalcogenides MX (TMTs, M = transition metal, X= S, Se, Te) are
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representative examples that allow unusual ground states and collective mode electronic transport in bulk. It
is fascinating to isolate and manipulate quasi-1D bulk materials in the few-chain limit because new physics
can be induced by the new degree of low dimensionality. In 2018, Pham et al. synthesized the single- or few-
[51]
chain limit of NbSe encapsulated in protective MWCNT cavities [Figure 5D] . Static and dynamic charge-
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induced structural torsional waves observed by TEM are not found in bulk NbSe . Following that, the few-
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to-single chain limits of HfTe 3 [108] in the MWCNT cavity were achieved. They discovered that the typically
parallel chains in the HfTe @MWCNT system spiral around each other once three chains are reached, while
3
at the same time, a short-wavelength trigonal antiprismatic rocking distortion takes place, opening a
prominent energy gap. Later, they concentrate on MX family members that are not stable in bulk and have
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been synthesized in the few to a single-chain limit of MTe (M = Nb, V, Ti) within the nanoconfined cavity
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of MWCNTs [Figure 5E] .
[50]
Electronic structure of filled CNTs heterostructures
Spectroscopy can energetically analyze the interaction between fillers and carbon nanotubes, allowing for a
preliminary understanding of the filling material on the electronic structure . Common types of the
[112]
spectrum include optical absorption spectroscopy (OAS), Raman spectroscopy, and X-ray absorption
spectroscopy (XAS).

Optical absorption spectroscopy (OAS)
Optical absorption spectroscopy (OAS) is an experimental technique that measures the ability of a sample
to absorb light at different wavelengths. Since energy states are continuous, a substance can only absorb
photons with a specific energy in a continuous spectrum. By measuring the amount of light absorbed by the
sample, information about the material’s electronic and molecular structure can be determined, resulting in
spectral information . OAS is a clear and accurate method to investigate the electronic structure of the
[113]
filled CNTs heterostructures. The OAS technique also has some limitations. One of the main drawbacks is
that it can only provide information on the surface or near-surface of the sample, and it is difficult to obtain
information about deeper structures. OAS has been used to characterize CoBr 2 [114] , FeX 2 [115] , AgX ,
[102]
[120]
[103]
[49]
[119]
[49]
CdX 2 [116] , ZnX , CuX (X = Cl, Br, I), PrCl 3 [117] , TbCl 3 [118] , GaSe , GaTe , Bi Se , SnTe , and Bi Te .
[52]
3
2
2
2
3
The following takes the four heterostructures systems selected in Figure 6A-D as examples to briefly explain
the influence of material filling on SWCNTs. The curve of unfilled SWCNTs has several obvious absorption
S
peaks, and the first two peaks, S1 (E ) and S2 (E ) are connected with the bandgap transition between
S
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Van-Hove singularities in semiconductor SWCNTs . The appearance of the M1 (E ) peak at about
[102]
M
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1.8 eV is because of the inter-band transition of the first Van-Hove singularity in the metal tube. The last
peak around 2.4 eV corresponds to a shift in E semiconductor SWCNTs.
S
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Compared with the original data, the OAS of filled heterostructures changed significantly. The change in
the spectrum is caused by the local interaction between the carbon atom and the filling atom, which further
proves that the change in the electronic properties of SWCNTs is caused by the filling of the inner channel
[103]
of the carbon tube . The most significant and common phenomenon after being filled with foreign
materials is that the optical transition at E is inhibited or even completely quenched because of a charge
S
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transfer between the packed substances and the carbon wall . The direction and path of the charge
[116]
transfer hinge on the properties of the filled materials. Metal halides usually act as electron acceptors,
[115]
leading to the depletion of electrons in CNTs . Another obvious feature is that all the peaks move towards
the low energy region, which can be explained by the shrinking of the energy gap between Van-Hove