# A Review of Double-Walled and Triple-Walled Carbon Nanotube Synthesis and Applications
## 6 Conclusions and Future Perspectives  
The development of powerful methods for growing CNTs has reached the state-of-the-art for SWNT growth, and the expectation is that the growth techniques will eventually reach such a state for the growth of few-walled carbon nanotubes (such as DWNTs and TWNTs), as well. The main parameters in the catalytic growth of DWNTs and TWNTs are: (1) the size and shape of metal particles or the thickness of metal films; (2) the use of promoters (Mo and S); and (3) reaction conditions (e.g., temperature, pressure and gas flow rate). Basically, the dimensions of the catalyst particles determine the number of walls in the carbon nanotube structure resultant from the growth process. It is worth mentioning that the metal catalyst acts like a reservoir for carbon atoms. The concentration of Mo or S in the catalyst particles/films plays an important role in determining both the size of catalyst particles and the deposition rate of carbon atoms during the carbon nanotube growth time. Indeed, the decomposition rate of hydrocarbons on the catalysts surface, as well as the carbon diffusion (segregation) into (out of) the catalyst particles/films both change the number of walls in the process of synthesizing the carbon nanotube structures. For example, the increase in the number of walls constituting the CNTs in the growth process was observed with increasing the concentration of Mo in the catalyst medium. The growth of TWNTs is considerably harder when compared to the growth of DWNTs. In fact, the reaction window for the selective growth of TWNTs is much narrower than that for DWNTs, and any small variation in the growth conditions is already enough to jeopardize the synthesis of TWNTs. As discussed throughout the text, the growth techniques are already able to achieve high selectivity (>80%) for DWNTs and TWNTs species relative to other carbon structures produced simultaneously in the growth procedure. The control of DWNT and TWNT diameters is also in an advanced stage, although there is still a way to go as it comes to the chirality-specific growth of such systems. However, as mentioned above, the synthesis of DWNTs and TWNTs did reach a stage in which more sophisticated science can now be carried out, not only in terms of applications, but also in terms of fundamental science related to electrons, phonons, electron-electron interactions, electron-phonon interactions and phonon-phonon interactions (also known as many-body interactions), which are well understood in monolayer graphene and its multi-layer counterparts [199; 200; 201; 202; 203]. Both DWNTs and TWNTs, which are concentric SWNTs, have interesting phenomena related to their IL interactions [204; 205; 202; 203]. These IL interactions are mediated by van der Waals forces, and they change as the chirality of the tubes composing the DWNTs and TWNTs changes, which therefore modifies how strongly the electronic and vibrational structuresof each SWNT constituent will mix together in DWNT and TWNT systems. In fact, these IL interactions when well understood can be used in device applications, which work in the infrared regime [202; 203; 204; 205]. The chirality and metallicity of the DWNTs and TWNTs constituents are certainly important to understand in depth the electronic and thermal properties, as well, and a good understanding of such properties is still elusive for few-walled carbon nanotubes. Regarding the use of DWNTs and TWNTs in biological applications, there still remains much to understand about their toxicity in living organisms and also their role as potential drug-delivery agents [182]. The mechanical robustness and the variety of metallicities observed in DWNTs and TWNTs could also be explored, for example to enhance the quality and resolution of scanning probe microscopy (SPM) probes [206; 207]. Moreover, as we reported in Section 5, even though there are some applications that have already been tested using DWNTs and TWNTs, there are also various predictions still awaiting confirmation.