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Page 2 of 29 Teng et al. Microstructures 2023;3:2023019 https://dx.doi.org/10.20517/microstructures.2023.07

INTRODUCTION
CNTs have received widespread attention since they were confirmed to exist in 1991 by Japanese scientist
[1]
[3]
Iijima due to their one-dimensional (1D) nanostructure with remarkable electronic , optical ,
[2]
mechanical , and thermal properties . Because of these characteristics, CNTs are critical for the
[5]
[4]
advancement of electronics and nanoelectronics . For single-walled carbon nanotubes (SWCNTs), the
[6]
nanotube band structure is determined by the nanotube radius and chirality, which can be either metallic or
semiconductor . Using a SWCNT as the gate and molybdenum disulfide (MoS ) as channel materials, a
[7]
2
one-nanometer physical gate length transistor with a subthreshold swing of ~65 mV dev at 298 K was
-1
achieved, breaking the five-nanometer-limitation of Si technology . Furthermore, as separation methods
[8]
[9]
for high-purity semiconductor single-walled carbon nanotube (s-SWCNT, > 99.99%) have matured, high-
performance high-density arrays s-SWCNT field effect transistors have been developed , positioning s-
[10]
SWCNTs as the candidate elements for the manufacture of next-generation electronic devices.
Another intriguing feature of CNT is its unusual tubular structure with a nanometer diameter, which makes
it an ideal nanoscale vessel for restricted chemical reactions as well as a powerful method to control the
[11]
electronic structure of CNTs by filling it with specific substances . Since the feasibility of filling carbon
[12]
nanotubes with guest substances was predicted in 1992, and the Pb@MWCNT was first synthesized in
[13]
1993 , attempts to fill the CNTs with gas, liquid, and solid substances have been extensively explored.
According to the aforementioned pioneering works, numerous new nanoclusters , nanowires , and
[15]
[14]
[17]
nanoribbons guests were observed inside CNTs, which were different from that in their bulk states due
[16]
to the confined space of 1D nanotube channels. To exhibit the influence of the diameter of the CNTs, SnTe
nanowires, for example, were transformed from monatomic chains to curvilinear chains, hyperbolic chains,
and then to 2 × 2 rock salt . CNT template-assisted growth can help produce 1D materials with high aspect
[18]
ratios. However, nanoscale materials are thermodynamically unstable and susceptible to degradation by air
and water. Materials encapsulated within CNTs can be protected from reactions with the surrounding
medium, particularly oxidation, when exposed to air. Additionally, nanoscale guest materials within CNTs
can be stabilized by the strong carbon walls acting as a barrier. In a highly confined space, X@CNTs provide
a new combination to strengthen and stabilize chemical elements, enabling chemically stable new crystal
structures . As a result, the filled carbon-nanotube heterostructures pave the way for research into
[19]
confinement-stabilized nonequilibrium materials and related emergent physical phenomena.
On the other hand, the carbon nanotube’s electronic band structure would be modified by the interaction
between the carbon wall and the guest substance [12,20] . Synthesized SWCNT samples consist of a mixture of
metallic and semiconducting nanotubes, resulting in uneven properties. Customizing the electronic
properties of SWCNTs is crucial for their advanced applications. Therefore, filling CNTs to control their
electronic structure is necessary. For strongly interacting heterostructure systems, allowing for electron
exchange between the encapsulated electron donor or electron acceptor materials and the CNTs, which
controls the electronic properties of CNTs. For instance, the electronic acceptor 1,1′-didodecyl-4,4′-
bipyridinium dihexafluorophosphate (Viol) was filled into the metallic SWCNT to transit it into the
semiconducting state . For weakly interacting heterostructure systems, although the electron transfer is
[21]
hindered, the radial vibrations of the carbon nanotubes are suppressed, causing them to deform . The
[22]
[23]
single CNT in discrete-filled heterostructure systems would split into a series of quantum dots . Therefore,
CNTs can be designed with the help of filling for particular applications, such as sensors , p-n
[24]
[25]
junctions , transistors and so forth.
[26]
The purpose of this manuscript is to give an overview of the state-of-the-art research on filled CNTs
heterostructures, from synthesis to application. The filling methods and mechanism are covered in detail in