MS Modeling’s quantum mechanical tools CASTEP and DMol3 have been used to study the properties (structural, mechanical, vibrational, and electronic) of carbon and boron-nitride nanotubes.
Carbon Nanotubes and Their Properties
If nanotube technology is to reach its full commercial potential, the ability to control and fine-tune properties such as these will be vital to manufacture of tailored devices. 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 be either metallic or semiconducting.
Potential Applications of Carbon Nanotubes
Carbon nanotubes are a hot research area owing to their novel properties, fuelled by experimental breakthroughs that have led to realistic possibilities of using them in a host of commercial nanoelectronic 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.
In addition, their high strength extends their potential application sphere to include composite reinforced materials.
Boron Nitride Nanotubes
Boron-nitride nanotubes also show potential for similar applications, and may even improve on the performance of carbon nanotubes, as they can tolerate heat, have a constant band-gap that is independent of tube-diameter and chirality. It has also been shown that boron-nitride coated carbon nanotubes show better field emission than non-coated ones.
Nanotube Studies Carried out at Wrights-Patterson and Rice University
Researchers at the Airforce Base Research Laboratory (Wrights-Patterson) and the Rice University, Houston, TX, used MS Modeling’s density functional theory (DFT) codes CASTEP and DMol3 to study and compare the properties (structural, mechanical, vibrational, and electronic) of single-walled carbon and boron-nitride nanotubes, looking at the effects (if any) of inter-nanotube coupling.
The studies concluded:
• While Resonant Raman spectroscopy has become a key experimental technique for studying the optical and electronic properties in nanotubes, theory and models are important for predictive purposes as well as detailed analysis of observations. This work demonstrates various ways in which DFT methods can impact on this, including (a) testing and validation of simpler model relationship between nanotube structure and RBM, (b) quantifying the effect of tube interactions, and thereby the difference between single and multiple tube materials, (c) prediction of RBMs beyond the case of carbon nanotubes, here including boron-nitride nanotubes. For example, the study reveals that a model proposed by Bachilo et al. for predicting RBMs of isolated semiconducting tubes does not hold for large diameter tubes
• DFT methods give a detailed picture of variation in the structural, mechanical, and electonic properties of both C and BN nanotubes as a function of their radius, chirality, and interactions. It reveals features with potentially significant impact for applications. The location of the van Hove singularity, which for example impacts optical transitions, was studied, revealing that tube interactions do not always lead to an outward expansion with respect to the Fermi energy, but to an inward shift for tubes of smaller radius.