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Page 8 of 29 Teng et al. Microstructures 2023;3:2023019 https://dx.doi.org/10.20517/microstructures.2023.07

[77]
characteristics of Te nanowire crystal rather than the interaction between Te and CNT [Figure 3C] .
However, the situation became more complicated for element S of the same group as Te after being filled. It
[78]
was observed that the sulfur chains confined within the carbon nanotubes lacked a definite structure and
[46]
were markedly different from the previously proposed linear or zigzag structures . Instead, they tended to
assume conformations resembling the cyclo-S allotrope. Density functional theory (DFT) calculations
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showed that a more disordered sulfur chain structure was more stable than linear and zigzag conformations
when the sulfur chain was confined within the SWCNT. Various nanowires, such as Eu [Figure 3D] and
[48]
[15]
Sn [Figure 3E] , are filled in carbon nanotubes using this simple process.
For element materials with high melting point temperatures or strong chemical active and unstable,
multistep sequential chemical transformations within the pre-filled nanotube is an effective method. Novel
[84]
[83]
[79]
metal nanowires (Ag , Pt , etc.), 3d metal (Fe , Co , Ni etc.), and carbon nanowires filled inside
[82]
[80]
[81]
CNT were synthesized with this method. For example, the nanowire Fe, Co, Ni, and their alloys@CNTs can
be synthesized by the pyrolysis of related pre-filled metallocene@CNT. The Fe nanowire filled inside CNT
has a diameter of 20-40 nm with a filling of 60% and exhibits strong ferromagnetic behavior at room
[85]
temperature . Similarly, the ferromagnetic nanowires filled CNTs with different reaction precursors were
also reported in several other pieces of literature . As a true 1D nanocarbon, the sp hybridized carbon
[86]
1
chain is predicted to have novel chemical and physics properties, but because of its strong chemical activity
and extreme instability, it is difficult to synthesize in common conditions . In 2016, the sp hybridized
[87]
1
carbon chain inside DWCNTs (LLCC@DWCNT) was first synthesized by Shi et al., which provides a
practical strategy for the preparation of the true 1D carbon chain [Figure 3F] .
[84]
Organic and metal-organic molecule cluster
The vast majority of organic molecules are nanosized but less stable in the atmospheric environment .
[88]
Filling the molecule inside the cavity of CNT can improve molecular stability while also serving as a
precursor for the preparation of other complex materials, such as graphene nanoribbons , polymers
[16]
compounds , and inorganic metal compounds .
[14]
[89]
[91]
[90]
C was the first molecule studied inside CNTs . Following that, many other larger fullerenes and
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endohedral metallofullerenes were successfully filled and were used to study the interaction,
[92]
intermolecular spacing, molecular orientation, molecular motion, and reaction behavior in the confined
space. According to TEM measurements of C @CNTs, the intermolecular spacing in the nanotube is
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shorter than in bulk crystals, which is coincident with the van der Waals forces that induce compression of
fullerenes inside the nanotube [Figure 4A] . The stacking mode of C in CNTs is sensitive to nanotubes’
[61]
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internal diameters, demonstrating the confinement effect inside nanotubes. C @SWCNTs is relatively
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-6
stable below 800 °C in the vacuum state of less than 1 × 10 Torr, whereas fullerenes in nanotubes gradually
polymerize above 1,000 ℃ or under the electron beam. A series of intermediate structures, such as a dimer,
trimer, and chain, gradually transform to form complete nanotubes. Endohedral metallofullerenes have
electric dipole moments caused by metal atoms within the carbon cage, which can affect their behavior
inside nanotubes. Using Ce@C as a model molecule, the influence of the dipolar interactions on molecular
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orientations in CNTs was investigated by using HR-TEM [Figure 4B]. The molecules have a tendency to
[93]
align their dipolar moments along the nanotube axis, enhancing electrostatic interactions between nearby
molecules, which does not occur in Ce@C crystals or solutions.
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Because of the exact geometrical match between fullerene and the interior of a nanotube, some globular
non-fullerene molecules, such as o-carborane , octasiloxane [Figure 4C] , metal-organic cluster ,
[95]
[94]
[14]
[89]
polyoxometalates (POM) [Figure 4D] , were also tried to fill inside the CNTs. Because it has a smaller