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Teng et al. Microstructures 2023;3:2023019 https://dx.doi.org/10.20517/microstructures.2023.07 Page 15 of 29

Figure 7. Raman characterization of carbon-nanotube nanopeapods: (A) Raman of MnX @SWCNT heterostructures at 1.96 eV laser
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energy (Reproduced with permission [123] . Copyright 2012, IOP Science). (B) Raman of MnX @SWCNTs heterostructures at 1.58 eV
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laser energy (Reproduced with permission [123] . Copyright 2012, IOP Science). (C) Raman of SWCNTs and NiX @SWCNTs at 2.41 eV
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laser energy (Reproduced with permission [124] . Copyright 2012, Wiley). (D) Raman of SWCNTs and NiX @SWCNTs at 1.58 eV laser
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energy (Reproduced with permission [124] . Copyright 2012, Wiley).
XAS spectra of NiX @SWCNTs (X = Cl, Br) heterostructures measured by Kharlamova et al. are shown in
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[124]
Figure 8A . Partial information on nickel halide nanotubes can be obtained from the absorption peaks of
C 1s. C 1s can be simply understood as the minimum energy required to excite an electron in a 1s orbital.
An additional spectral feature A* appears below the p formant A. This property can be attributed to the
interaction between the wall and the filling material. Compared with the spectra of AgX@SWCNTs and the
original nanotubes [Figure 8B], SWCNTs interact with the AgX crystal of the plugged layer . This
[102]
additional spectrum characteristic can be attributed to the energy level that is reduced by electron transition
to the reduction of the transfer of the plug-in charge.

FUNCTIONAL APPLICATIONS OF FILLED CARBON-NANOTUBE HETEROSTRUCTURES
Carbon nanotubes have excellent conductivity , optical , and thermal properties , mechanical
[133]
[132]
[5]
properties , and flexibility , which can be applied to them in nano-electronics , photovoltaic ,
[135]
[134]
[4]
[2]
[136]
[137]
thermoelectric power generation , energy storage , catalytic , and other important areas. SWCNTs
[138]
[139]
can be a metal or semiconductor, depending on their atomic structure . The method of filling can not
only control its electronic structure but also retain the above many excellent properties, which greatly
[12]
accelerates the industrialization application of carbon nanotubes.
Nanoelectronics
Charge transfer between the filled materials and CNT is common, which alters the electrical transport
properties in a CNT. Li et al. prepared field-effect transistors (FET) with C and C @SWCNT peapods as
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channel materials and measured the transfer characteristics at room temperature [Figure 9A] . The I -V
[26]
g
DS
curves of the C and C peapods FET show P-type hole-dominate transport characteristics, which are
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similar to those of pristine SWCNTs. When C is replaced with azafullerene (C N and C N), the FETs
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exhibit typical N-type electron-dominate transport characteristics [Figure 9B] . Furthermore, a
[26]