DWCNTs consist of two individual carbon nanotubes, with one embedded inside the other. The differences in diameters and the chirality of the two different nanotubes lead to a varying degree of interaction between the two, while at the same time the properties of the individual nanotubes being separate from each other. It is this wide variety of possibilities that have made DWCNTs a focus of interest for carbon nanotube research. Varying chirality allows a range of inner-wall outer-wall interactions to occur, because the chirality determines whether the nanotube will be semiconducting or metallic. It is possible to achieve metallic-metallic, semiconducting-metallic, metallic-semiconducting or semiconducting-semiconducting interactions. In addition to this, the metallic and semiconducting properties can vary depending upon the exact lattice parameters, which enables a wide range of possible property combinations.代理Ossila 双壁碳纳米管 Ossila碳纳米管M2016L1
 

DWCNTs also have a large advantage over single-walled carbon nanotubes, as it is possible to modify the outer nanotube without changing the properties of the inner nanotube. This modification could be either through functionalisation (to add solubilising groups), or the doping of the structure (to alter the properties). This allows the double-walled system to maintain functionality of a single-walled nanotube whilst simultaneously having the solubility of functionalised nanotubes. This combination makes double-walled systems attractive for use as additives in composite materials as it allows high doping concentrations without affecting the properties of the nanotube overall.代理Ossila 双壁碳纳米管 Ossila碳纳米管M2016L1
 

The biggest barriers for DWCNTs - with regards to further research and commercialisation - are their synthesis and purification. The yields produced by various synthesis techniques can vary from around 50% to 90% for arc discharge. Similarly, for catalytic chemical vapour deposition the yields can vary from 70% to 85%. The remainder of the nanotubes synthesised using these techniques are a mixture of single-walled and multi-walled nanotubes which then need to be purified to obtain individual double-walled nanotubes. The process of purification is much more difficult. Methods such as high-temperature oxidation result in preferential oxidation of single-walled nanotubes over double-walled. However, the process can damage the remaining nanotubes and will leave residual multi-walled contaminants behind. Other processes, such as ultra-centrifugation, can be used to obtain high-purity DWCNT samples and sort double-walled samples by outer diameter. However this process is labour and time intensive making commercialisation and large scale production of high purity DWCNTs difficult.
 

Just like with single-walled carbon nanotubes, there are many different areas in which DWCNT's can be applied due to their impressive mechanical and electrical properties. In addition double-walled nanotubes show an increase in the mechanical strength, thermal stability, and also chemical stability over that of single-walled nanotubes. However, the ability to combine different nanotube types have the potential to result in interesting optical, electronic and mechanical properties that are not possible with single-walled nanotubes, and could result in the most interesting research in the coming years.