Researchers have used MS Modeling's CASTEP to study the effect of nitrogen substitutional impurities on the electronic properties of single-wall carbon nanotubes.
Fine Tuning Electronic Properties of Carbon Nanotubes
Such an understanding will enable the electronic properties of carbon nanotubes to be fine tuned. This should lead to the design of better electronic devices, leading to the use of carbon nanotubes in many nanotechnologies and molecular electronics. Carbon nanotubes are long, thin cylinders of bound carbon atoms, about 10 000 times thinner than a human hair, and can be single- or multi-walled. They have remarkable electronic and mechanical properties that depend on atomic structure and more precisely on the manner in which the graphene sheet is wrapped to form a nanotube (chirality). They can vary from being metallic to semiconducting.
Potential Applications of Carbon Nanotubes
Carbon nanotubes are a hot research area, fuelled by experimental breakthroughs that have led to realistic possibilities of using them in a host of commercial applications: field emission-based flat panel displays, novel semiconducting devices in microelectronics, hydrogen storage devices, chemical sensors, and most recently in ultra-sensitive electromechanical sensors. As a result they represent a real-life application of nanotechnology.
Challenges to the Full Commercialization of Carbon Nanotubes
However, two major challenges remain an obstacle to the full commercialization of nanotube-based nanotechnologies and molecular electronic devices:
• The manipulation of individual tubes is difficult owing to their size, and
• The ability to manipulate nanotube properties to suit the application has to be achieved.
Modelling the Effect of Nitrogen Impurities on Semiconducting Properties of Carbon Nanotubes
Reporting in Physical Review Letters (2003, 91(10), 105502), Professor Michael Payne and team at the Cavendish Laboratory, University of Cambridge, UK, used MS Modeling's CASTEP to study the effect of introducing nitrogen impurities in semiconducting zigzag and metallic armchair single-walled nanotubes.
Doping Carbon Nanotubes
In semiconducting nanotubes, introducing impurities, a process known as doping, is the main method of tuning properties to make electronic devices. Doping is also a way of creating chemically active impurity sites.
Optimizing Dopant Concentrations
Using CASTEP, the researchers found that, at low concentrations of nitrogen impurity (less than 1 atom%), the impurity site becomes chemically and electronically active. In addition, the team found that an inter-tube covalent bond can form between neighboring nanotubes with impurity sites facing each other.
Figure 1. The effect of nitrogen doping in two zigzag nanotubes. The left image shows the charge density, the right image shows the density of the HOMO orbital (red the highest density, blue the lowest). The chemical bond is formed between the two carbon atoms that have the maximum spin density (red).
Nanotube Manipulation and Doping
These findings open the door to the possibility of nanotube manipulation via the formation of tunnel junctions between suitably doped nanotubes. Nanotube properties could also be controlled by selective functionalization through ligand docking at the impurity sites.
Advantages of Using the CASTEP Software
Professor Michael Payne says, "CASTEP enabled us to treat a system of several hundred atoms, necessary in order to study the intertube covalent bond and the isolated impurity, whose electronic state decays very slowly."
"Treating the system at the ab initio level also allowed us to predict experimental observables which will help in synthesizing this structure," added Professor Payne. "In the future, we hope to study applications of the doped nanotubes, such as the tunnel junction or an enhanced gas sensor. This will require computing non-equilibrium electronic structures, which is at the cutting edge of current quantum mechanical modeling."