# A Review of Double-Walled and Triple-Walled Carbon Nanotube Synthesis and Applications
## 5 Applications Involving DWNTs and TWNTs  
As we mentioned above, CNTs have been discussed as powerful materials for producing the next generation of electronic devices. Indeed, several applications were already tested, but not explored enough due to the limitations of synthesizing high quality few-walled carbon nanotubes,although this limitation no longer exists for SWNT synthesis, which has reached the state-of-the-art in chirality-specific production [154; 155; 156]. The use of DWNTs and TWNTs as interconnects, which usually represent 70% of the impedance of today's integrated circuits, is considered to have the potential to play a key role in improving the performance of future integrated circuits. The successful use of CNTs relies on the number of CNT walls of the CNTs and also on the density of tubes making the interconnections. [PERSON] and co-authors in 2010 [157] approached this problem by synthesizing DWNTs and TWNTs directly on doped silicon utilizing a method called \"integration in vias\", as shown in Figure 9a. Again, CNT growth has been identified as a major drawback in helping to control the species being grown, as well as the density and specific number of walls at the resulting carbon nanotube [157]. Double-walled carbon nanotubes have also been identified as materials with interesting electronic transport properties (such as chirality dependence and inner/outer tube selectivity) and superconductivity [158; 159; 160; 161; 162]. [PERSON] _et al._[158], as shown in Figure 9b, investigated the transport properties in films containing pea pod-derived DWNTs and CVD DWNTs. They found that the synthesis technique matters to determine the performance of the films. Indeed, the lowest resistivity was found for films obtained from CVD-DWNTs. The resistivity was lower in CVD-DWNTs than in both SWNT and pea pod-derived DWNT films. The result was correlated with the total content of metallic tubes that the CVD-DWNT sample contains. According to their DWNT chirality analysis, the metallic content of the inner and outer tubes for pea pod-derived DWNTs was found to be 15% and 25%, whereas the corresponding values for the inner and the outer tubes of the CVD-DWNTs corresponded to 35% and 20%. The low metallic content in the inner tubes of the pea pod-DWNTs, as explained above, is related to their special growth conditions, such as their high temperature thermal treatment. Moreover, no significant difference in the electrical resistivities of SWNT films and the pea pod-DWNT films is observed, which indicates the predominant role of the outer tubes in the transport. The results demonstrated that conductive films of DWNTs can have their transport properties tailored by the method used to grow the DWNT systems [158]. Devices utilizing bundles of MWNTs, including DWNTs and TWNTs, also showed superconductivity behavior at low temperatures [28; 61; 62; 63; 64]. [PERSON] _et al._[28] measured devices containing bundles of DWNTs and observed clear signatures of supercurrents for temperatures below the average critical temperature of 6.8 K, as shown in Figure 10a, while [PERSON] _et al._[63] observed superconductive behavior in their bundled DWNT samples for temperatures below the critical temperature of 15 K.  
Figure 9: **(a)** The selective growth of DWNTs and TWNTs is achieved on doped silicon via TiN structures in specific regions over the surface. **(b)** Relative resistance as a function of temperature for the SWNT, pea pod-derived and CVD-DWNT films before and after vacuum ultraviolet treatment. It is observed that the ultraviolet (UV) treatment largely increases the relative resistance of the films. Reproduced with permission from [158], Copyright American Chemical Society, 2011. These systems, DWNTs, can be used to manufacture self-organized nano-Schottky junctions wherein one layer exhibits a semiconducting behavior and the other exhibits a metallic behavior [163].