Carbon nanotubes – tubes formed from a repeating arrangement of carbon atoms with diameter of the order of a billionth of a meter – have remarkable mechanical, electronic and optical properties. Their potential applications range from ultra-strong ropes to ultra-small transistors, as well as field-emission displays, biosensors and optical switches. Unfortunately it is not yet possible to produce carbon nanotubes on a large scale with controlled properties (such as diameter and chirality – the degree of spiral in the arrangement of the carbon atoms). One important method for producing tubes is to use small particles of a metal such as nickel, which at high temperatures catalyse the decomposition of a carbon-containing gas forming carbon nanotubes which ‘grow’ on each metal particle. This process has not yet been fully understood, but recent work at the University of Surrey sheds new light on the interaction between the catalysts and the carbon atoms involved in the growth.
“There is still a hot debate about whether carbon nanotubes grow from catalysts as a result of carbon diffusing through or on the surface of the catalyst”, said Dr Vlad Stolojan, who led the research team. “This is mainly because the result of the growth process can only be observed at room temperature, after the process is completed. Through analysing the physics behind the controlled growth reversal that we observed, we concluded that the steady-state part of the growth process is surface-driven and demonstrated that the carbon nearest to the catalyst’s surface is highly mobile”.
A carbon nanotube, with its Ni catalyst at the top, shrinks in a controlled manner under electron beam irradiation. After ~7 minutes of irradiation at 75A/cm2, ~60nm length of the carbon nanotube has been consumed and the holey carbon film supporting the tube can be seen (arrow). The measurement of the reversal rate and the high-resolution analysis of the crystalline structure reveal that the growth process of carbon nanotubes from catalysts is a surface-driven process.
Stolojan and his co-workers studied the reversal process with high spatial resolution, in a transmission electron microscope, and have shown that the catalyst remains attached to the nanotube throughout the irradiation sequence, whilst an equivalent of 1 carbon atom is consumed per every nickel atom in the catalyst. By considering the effects of heating and irradiation, they have discovered that the carbon atoms at the catalyst surface are very easily removed (also confirmed by theoretical simulations), followed by a rapid rearrangement of the nanotube’s atoms around the catalyst. They have also discovered that changes in the nanotube’s growth direction are linked to a sudden rotation of the catalyst.
The observed controlled growth reversal under the high-energy electron irradiation will allow for controlling the height of individual nanotubes within patterned arrays, thus offering three-dimensional control of nanotube arrays for field-emission applications.
“The ability to observe the behaviour of the catalyst during the growth-reversal of the nanotube is exciting, as it allows the reverse-engineering of the steady-state growth process. Ultimately, this can help establish the relationship between the catalyst’s crystalline structure and the chirality of the resulting nanotube; the control of the chirality being the true ‘holy grail’ of carbon nanotube growers.” said Prof Ravi Silva, the Director of the Advanced Technology Institute, University of Surrey from the UK.
Stuart Miller | alfa
Climate cycles may explain how running water carved Mars' surface features
02.12.2016 | Penn State
What do Netflix, Google and planetary systems have in common?
02.12.2016 | University of Toronto
A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.
Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...
In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.
“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...
The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.
The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...
Broadband rotational spectroscopy unravels structural reshaping of isolated molecules in the gas phase to accommodate water
In two recent publications in the Journal of Chemical Physics and in the Journal of Physical Chemistry Letters, researchers around Melanie Schnell from the Max...
The efficiency of power electronic systems is not solely dependent on electrical efficiency but also on weight, for example, in mobile systems. When the weight of relevant components and devices in airplanes, for instance, is reduced, fuel savings can be achieved and correspondingly greenhouse gas emissions decreased. New materials and components based on gallium nitride (GaN) can help to reduce weight and increase the efficiency. With these new materials, power electronic switches can be operated at higher switching frequency, resulting in higher power density and lower material costs.
Researchers at the Fraunhofer Institute for Solar Energy Systems ISE together with partners have investigated how these materials can be used to make power...
16.11.2016 | Event News
01.11.2016 | Event News
14.10.2016 | Event News
02.12.2016 | Medical Engineering
02.12.2016 | Agricultural and Forestry Science
02.12.2016 | Physics and Astronomy