Jan 13, 2009
Carbon nanotubes know where to grow
Carbon nanotubes (CNTs) are ideal conductors of heat and electricity, extremely strong, chemically inert and non-toxic. Many scientists believe that they will not be able to develop better materials than CNTs. CNTs are particularly promising for future-generation electronic and optical devices when the dimensions of these devices shrink to molecular scales. However, the length and position of both ends of a CNT must be controlled so that it can become part of an electronic device or circuit. Furthermore, the CNT must be mounted on electrodes so that it can be electrically wired to the circuit. Currently, chemical vapor deposition (CVD) provides the only viable route for controlled growth of CNTs. However, the length and growth sites of each CNT are randomly determined. Therefore, device fabrication using CNTs has not been easy.
Researchers at the University of Nebraska, Lincoln, US, have developed a new method involving the application of optical near-field effects in a laser-assisted chemical vapor deposition process, which can precisely control the lengths and positions in CNT growth at a relatively low substrate temperature.
In this method, the team first designs and fabricates metallic electrodes with sharp apexes on silicon substrates covered with a layer of insulating silicon dioxide. A carbon dioxide laser is then used to irradiate a large area on the substrate surface, which is placed in a vacuum chamber filled with the gas precursors necessary for CNT growth. Optical near-field effects occur due to the interactions between the electrodes and the incident laser beam.
Numerical simulations revealed that significant enhancement of local heating takes place at the electrode tips under laser irradiation, which is about one order of magnitude higher than the rest of the electrodes. The nanoscale localized heating at the electrode tips determines the locations where the CNTs start to grow.
With the help of an electrical field by applying a bias voltage between the electrodes, CNTs could be grown in a self-aligned manner to form a bridge between two opposite electrodes. This method has two obvious advantages: precise position/length control and low substrate temperature.
The new approach provides a viable and cost-effective way to fabricate CNT devices and structures, which have potential applications in electronics, photonics, nanoscale electromechanical devices, sensors and probes.
The researchers presented their work in Nanotechnology.
About the author
The work was performed at the University of Nebraska, Lincoln (UNL) and supported by the National Science Foundation (NSF), US. W Xiong and M Mahjouri-Samani are PhD students in the Laser-Assisted Nano-Engineering (LANE) Laboratory. Dr Yunshen Zhou is a postdoctoral research fellow who led the experimental efforts. Prof. Yongfeng Lu, who directs the LANE laboratory, is the Lott Professor of Engineering at UNL. Prof. Lu proposed the concept of using optical near-fields to control CNT growth three years ago in a research proposal submitted to NSF.