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Page 16 of 29 Teng et al. Microstructures 2023;3:2023019 https://dx.doi.org/10.20517/microstructures.2023.07

Figure 8. XAS characterization of carbon-nanotube nanopeapods: (A) XAS of SWCNTs and NiX @SWCNTs (X = Cl, Br) (Reproduced
2
with permission [124] . Copyright 2012, Wiley). (B) XAS of SWCNTs and AgX@SWCNTs (X = Cl, Br, I) (Reproduced with permission [102] .
Copyright 2010, Elsevier).

photoswitching behavior was also observed in C N peapods FET devices, indicating the charge transfer
59
[140]
from azafullerene to SWCNT [Figure 9C] . Shimada et al. investigated the transport characteristics of
M@C (M = Gd, Dy) metallofullerenes nanopeapods FETs with ambipolar behavior. However, in the case
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of C -peapods, all devices exhibited metallic properties [Figure 9D] .
[63]
90
Yang et al. further studied the temperature-dependent charge transport characteristics of Dy@C peapods
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FET. A transition from p-type to n-type conduction has been observed as the temperature decreases from
room temperature to 265 K, indicating that charge transfers from the Dy@C to the conductance band of
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carbon nanotubes at low temperatures [Figure 9E] . At a temperature lower than 215 K, metallic behavior
[23]
occurred, suggesting that additional electrons are continuously injected into the conductance band, shifting
the Fermi level into the conduction band. Under 75 K, the device became a single-electron transistor with
irregular coulomb blockade oscillation, meaning that the inside Dy@C splits the tube into discrete
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quantum dots. The transport properties of other fullerene nanopeapods FET are also investigated
[Figure 9F].

It is also possible to produce p-n junctions within individual CNTs by partially filling the acceptor or donor.
The examples were demonstrated in the heterostructures of partially filled CsI, CsC 60 [25] , and Fe
[141]
nanoparticles inside the SWCNTs where ultimate heterostructures of electron donor and acceptor were
realized within the cavity of a SWCNT, yielding the air-stable rectifying performance.

Additional means can be used to tune the properties of the heterostructure-based electronic device if the
target filler substance has a unique property, such as a spin-crossover (SCO) molecule or magnetic
[96]
cluster . Giménez-López Mdel et al. encapsulated Mn Ac, a single-molecule magnet (SMM), into
[14]
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MWCNTs, resulting in a new type of heterostructure that combines the magnetic properties of the SMM