# A Review of Double-Walled and Triple-Walled Carbon Nanotube Synthesis and Applications
## 5 Applications Involving DWNTs and TWNTs
These double-walled systems can also be used to fabricate nano-p-n junction devices wherein one SWNT exhibits a p-type semiconducting behavior and the other SWNT exhibits an n-type behavior [163]. These cylindrical nano-p-n junctions are designed so that electrode contacts are placed individually in each SWNT constituting the DWNT, as shown in Figure 10b. With this design, the IL interactions give rise to a measurable current flow [163]. [PERSON] _et al._[163] observed that the barrier heights at the nano-p-n junction in a device as the one shown in Figure 10b are different for electrons and holes, which is confirmed by the asymmetric (and unusual) ambipolar behavior of the devices, as observed in Figure 10c,d. In their work, they show that the ambipolar behavior that was observed could be addressed only by considering the transport between the constituent SWNTs, which is mediated by the IL interactions. The nano-p-n junctions are more likely to exist at the location where the carbon atoms in hexagonal lattices have the highest alignment at the interlayer space. In other words, their work suggests that the DWNTs need to be commensurate [163]. To the best knowledge of the authors, devices like the one described above have not been fabricated using TWNTs. It is worth mentioning that TWNTs would be natural candidates to build nano-p-n-p or nano-n-p-n junctions, which would allow the fabrication of cylindrical nano-transistors. Nano-supercapacitors can also be fabricated with DWNTs [164, 165, 166]. Namely, [PERSON] _et al._[166] used DNA-dispersed DWNTs to fabricate electrodes for supercapacitors. Their work demonstrates that the DNA-dispersed electrode films work much better than those made of DWNTs in bundles, and these electrodes double the capacitance obtained in films fabricated with pristine DWNTs. In the  
Figure 10: (**a**) Resistance as a function of temperature at various magnetic fields B. There is a steep drop in the resistance for temperatures below the critical temperature of 6.8 K. Reproduced with permission from [28], Copyright Nature Publishing Group, 2012. (**b**) The left panel shows the transmission electron microscopy (TEM) image of a DWNT. The inset shows the DWNT between the source (S) and drain (D) electrodes. The right panel shows the design of the nano-p-n junction device. Reproduced with permission from [163], Copyright AIP Publishing LLC, 2009. (**c**,**d**) The source-drain current plotted against the source-drain voltage and the back-gate voltage, respectively. The asymmetric ambipolar behavior is observed in both plots. Reproduced with permission from [163], Copyright AIP Publishing LLC, 2009.  
experiment, they measured a supercapacitance of about 67 F/g for the DNA-dispersed DWNTs films [166]. [PERSON] _et al._[164], on the other hand, used bromine-doped DWNTs to demonstrate the existence of a radial charge distribution within the positive carbon electrode of a cylindrical DWNT molecular capacitor. In these molecular capacitors, bromine acts as a radial gate, which mediates very different band filling effects on each SWNT constituting the DWNT system. They demonstrate that in such capacitors, the nanoscale electrostatics dominates over the quantum confinement inherent for these one-dimensional structures [164]. Multi-walled carbon nanotubes have been suggested as good platforms for fabricating oscillator devices [167; 168; 169; 170; 171; 172]. [PERSON]'s group [167] proposed in their calculations that DWNTs and TWNTs are good candidates for functional nano-oscillators, where the number of walls, the metallicity and chiralities of the nanotubes composing the few-walled systems are important parameters for determining the characteristics and performances of the resulting electronic oscillator device. [PERSON] and [PERSON] [168] also performed molecular dynamics calculations suggesting TWNTs as interesting platforms for fabricating nano-oscillators. They simulated the characteristic frequencies of coupled TWNTs oscillators, in which the IL interactions play an important role.