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
Sharpening the X-ray view of the nanocosm
23.03.2018 | Changchun Institute of Optics, Fine Mechanics and Physics
Drug or duplicate?
23.03.2018 | Fraunhofer-Institut für Angewandte Festkörperphysik IAF
Satellites in near-Earth orbit are at risk due to the steady increase in space debris. But their mission in the areas of telecommunications, navigation or weather forecasts is essential for society. Fraunhofer FHR therefore develops radar-based systems which allow the detection, tracking and cataloging of even the smallest particles of debris. Satellite operators who have access to our data are in a better position to plan evasive maneuvers and prevent destructive collisions. From April, 25-29 2018, Fraunhofer FHR and its partners will exhibit the complementary radar systems TIRA and GESTRA as well as the latest radar techniques for space observation across three stands at the ILA Berlin.
The "traffic situation" in space is very tense: the Earth is currently being orbited not only by countless satellites but also by a large volume of space...
An international team of researchers has discovered a new anti-cancer protein. The protein, called LHPP, prevents the uncontrolled proliferation of cancer cells in the liver. The researchers led by Prof. Michael N. Hall from the Biozentrum, University of Basel, report in “Nature” that LHPP can also serve as a biomarker for the diagnosis and prognosis of liver cancer.
The incidence of liver cancer, also known as hepatocellular carcinoma, is steadily increasing. In the last twenty years, the number of cases has almost doubled...
In just a few weeks from now, the Chinese space station Tiangong-1 will re-enter the Earth's atmosphere where it will to a large extent burn up. It is possible that some debris will reach the Earth's surface. Tiangong-1 is orbiting the Earth uncontrolled at a speed of approx. 29,000 km/h.Currently the prognosis relating to the time of impact currently lies within a window of several days. The scientists at Fraunhofer FHR have already been monitoring Tiangong-1 for a number of weeks with their TIRA system, one of the most powerful space observation radars in the world, with a view to supporting the German Space Situational Awareness Center and the ESA with their re-entry forecasts.
Following the loss of radio contact with Tiangong-1 in 2016 and due to the low orbital height, it is now inevitable that the Chinese space station will...
Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, provider of research and development services for OLED lighting solutions, announces the founding of the “OLED Licht Forum” and presents latest OLED design and lighting solutions during light+building, from March 18th – 23rd, 2018 in Frankfurt a.M./Germany, at booth no. F91 in Hall 4.0.
They are united in their passion for OLED (organic light emitting diodes) lighting with all of its unique facets and application possibilities. Thus experts in...
A new scenario seeking to explain how Mars' putative oceans came and went over the last 4 billion years implies that the oceans formed several hundred million...
23.03.2018 | Event News
19.03.2018 | Event News
16.03.2018 | Event News
23.03.2018 | Materials Sciences
23.03.2018 | Agricultural and Forestry Science
23.03.2018 | Physics and Astronomy