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Teng et al. Microstructures 2023;3:2023019 https://dx.doi.org/10.20517/microstructures.2023.07 Page 23 of 29

In the present manuscript, we concentrated on an overview of the preparation, morphological and
electronic structure characterization, and applications of filled CNTs heterostructures. With the aid of the
advanced HR-TEM and AC-TEM techniques, the structures of the internal substances filling in the
heterostructures are thoroughly examined in situ. Due to the spatial confinement effect of CNT, for some
substances, the structures inside the CNT are significantly different from the bulk state and sensitive to the
diameter of the CNT, which can be manipulated to design its electronic behavior. Various unstable nano-
materials have been achieved through nano chemical reactions inside the nanotubes, suggesting that the
confined hollow nano space of CNTs serves as a platform for novel nanomaterials. Furthermore,
spectroscopic techniques like OAS, Raman, and XAS provide researchers with simple approaches to
investigate the modified electronic properties of nanotubes that occur as a result of the encapsulation of
various compounds inside their channels. For example, both P and N doping of the CNT were observed
when the electron donor and electron acceptor were filled. Therefore, the established methods and
knowledge for modifying the electronic properties of CNTs provide the fundamentals for functioning CNTs
devices.

Although various filled CNTs heterostructures have been synthesized, many challenges remain for further
large-scale applications. First of all, many external factors affect the filling process, leading to low
reproducibility of internal filling quality and filling yield. Therefore, it is necessary to explore and
summarize more experienced filling mechanisms to guide the synthesis of high-quality samples. The yield
of CNT filling may vary due to various factors, such as the size of CNTs, the properties of filling materials,
the filling method, and the conditions of filling. In some cases, the yield may be high, even close to 100%,
while in other cases, the yield may be relatively low or even unable to fill. Regarding the practical aspects of
CNT filling, it is important to consider the expected applications and required characteristics of the filled
CNTs. For example, if the filled CNTs are intended to be used as catalyst supports, the filling material
should be selected based on its catalytic activity and stability, and the filling process should be optimized to
ensure uniform distribution of the filling material throughout the CNTs. The filling method also has
practical significance. For example, if a solvent-based filling method is used, it may be difficult to completely
remove the solvent from the filled CNTs, which can affect the final performance of the material. Second, the
nature of heterostructures has not been investigated in depth. Based on obtaining high-quality
heterostructure samples, understanding the structural, electrical, and spectral properties of the sample from
the perspective of a single tube is the next essential step in further applications. Third, it is difficult to
separate high-purity semiconducting filled CNTs heterostructures for field-effect transistor applications.
When the SWCNT is filled with substance, the weight and surface charge distribution of the CNT are both
altered, resulting in mature separation conditions that are no longer applicable. Finally, in scenarios where
filled carbon nanotubes are used in vacuum or liquid media, there is a potential for the filling material to
diffuse out, reducing the stability of the structure. One potential method to minimize diffusion is to
functionalize the CNT walls with appropriate chemical groups, which can help immobilize the guest
material inside the CNT. Another approach is to use encapsulation techniques, such as coating the CNT
with protective layers or combining the filled CNT with larger composite materials.

In conclusion, despite many challenges, the filled CNTs heterostructures would provide abundant
opportunities for future interdisciplinary fundamental research and emerging applications in
nanoelectronic devices, energy, storage, catalysis, and other fields.

DECLARATIONS
Authors’ contributions
Conducted the literature review and drafted the original version: Teng Y, Li J