# A Review of Double-Walled and Triple-Walled Carbon Nanotube Synthesis and Applications
## 2 Synthesis and Characterization of DWNTs
### The Catalytic Chemical Vapor Deposition Method
#### 2.2.3 Annealing of Confined Molecules inside SWNTs  
The annealing of confined molecules inside a SWNT provides a different concept for producing DWNTs. Since a CNT is a tubular carbon material, there is a hollow empty space inside the CNT. This empty space can be usefully filled with some small molecules or atoms, and then, the effect of the encapsulation of various molecules and atoms by SWNTs can be studied. The \"pea pod\" was first reported and named for the hybridized structure of SWNTs encapsulating fullerenes (such as C\({}_{60}\)C [130], C\({}_{70}\)C [131], _etc._) (Figure 7a). When external stimuli, such as heat [PERSON] _et al_. (2012), an electron beam [133] and UV light [134], were applied to the pea pod (Figure 7b), the C\({}_{60}\) molecules inside the SWNTs partially coalesced with neighboring C\({}_{60}\) molecules to eventually be transformed thereby forming the inner tube of a DWNT (Figure 7c). Besides fullerenes, other molecules also worked as precursors for forming the inner tube of DWNTs. Successful DWNT formation by thermal annealing or electron beam irradiation of SWNTs with encapsulated molecules, such as coronene (C\({}_{24}\)H\({}_{12}\)) [135], anthracene (C\({}_{14}\)H\({}_{10}\)) [136], picene (C\({}_{22}\)H\({}_{14}\)) [137], pentacene (C\({}_{22}\)H\({}_{14}\)) [137], Perylenetetracarboxylic diamondride (PTDCA) (C\({}_{24}\)H\({}_{8}\)O\({}_{6}\)) [137; 138], ferrocene (FeCp\({}_{2}\)) [139; 140; 141], tris(cyclopentadienyl) gadolinium (GdCp\({}_{3}\)) [139; 140] and organometallic fullerene complexes [142], was reported, as well. [PERSON] _et al_. [137] found that there are two different intermediate states of the inner tube depending on the encapsulated molecular species. In the case of PTCDA, graphene nanoribbon-like intermediates were formed during annealing (Figure 7d), whereas fullerenes, picene and pentacene formed deformed fullerene-like intermediates prior to DWNT formation (Figure 7e). The graphene nanoribbon-like intermediates are possible because the oxygen-terminated edges of PTCDA molecules release their oxygen, to generate CO or CO\({}_{2}\) molecules upon annealing, and the remaining perylene tetraradicals are merged into the graphene nanoribbon structure. The helically-twisted single or double graphene nanoribbons are further merged into an inner tube upon continuous annealing. Ferrocene (FeCp\({}_{2}\)), which is commonly used for CNT growth as a metal catalyst precursor for the catalytic chemical vapor deposition (CCVD) floating catalyst method, also works as a precursor for the inner tube growth [139; 140; 141]. In the case of ferrocene, the inner tube formed at a low temperature of 600 \({}^{\circ}\)C [139]. It was proposed that the formation mechanism of the inner tube is different from the fullerene coalescence and graphene nanoribbon merging methods, since these coalescence-derived inner tube formation methods do not occur at this low temperature. The encapsulated ferrocene (Figure 7f) would first decompose into iron carbide (Fe\({}_{3}\)C, Figure 7g), which could be responsible for the catalytic growth of these inner tubes. The chirality distribution of the encapsulated molecules-derived inner tube of the DWNT is dependent on the choice of the precursor molecules [137]. Even when fullerenes with a similar structure, such as C\({}_{60}\) and C\({}_{70}\), are used as the precursor, these molecules produce inner tubes with a different chirality distribution. The C\({}_{60}\)-derived inner tubes tend to be \((6,4)\) and \((6,5)\) tubes, whereas  
Figure 7: Various molecular encapsulated SWNT for DWNT preparation. TEM image of (**a**) pea pod (C\({}_{60}\)@SWNT), (**b**) intermediate structure and (**c**) completed transformation into a DWNT upon annealing. Reproduced with permission from [132], Copyright Elsevier, 2001. (**d**) Perylenetetracarboxylic diamondride (PTCDA)-derived graphene nanoribbon-like intermediate structure. Reproduced with permission from [137], Copyright Nature Publishing Group, 2013.