Researchers simulate formation of carbon nanotube serpentines

A Brazilian study will allow scientists to control the properties of materials used to produce electronic nanocircuits. The study was on the cover of Physical Review Letters.

By Elton Alisson

Agência FAPESP – A group of researchers from Universidade Estadual de Campinas’s Gleb Wataghin Physics Institute (Unicamp- IFGW), in collaboration with colleagues from the federal universities of Roraima and Minas as well as Israel’s Weizmann Institute, has managed to simulate and develop a model for the formation of carbon nanotubes.

The results of the study, which are part of doctoral studies undertaken through a FAPESP fellowship, were among the highlights on the cover of the March 2013 edition of Physical Review Letters.

The formation of serpentines will allow scientists to use carbon nanotubes in the production of electronic nanocircuits. Although carbon nanotubes were discovered decades ago, sparking interest because of their unique mechanical, electronic, optical and thermal properties, they still present challenges for use in areas such as nanoelectronics.

This difficulty arises because it is still impossible to control the formation of large quantities of these cylindrical structures – which are essentially light, hollow straws that act as conductors and are ten times more resistant than steel – on a nanometer scale so that they have a consistent diameter, length and electrical properties.

One alternative explored by some research groups around the globe is the development of a single very long nanotube in the form of a serpentine several microns in length and with parallel segments having the same electrical properties throughout. Nevertheless, until such a nanotube was developed, no one knew how such materials would be formed.

“There was only a vague idea of the how carbon nanotube serpentines formed, but the group led by Professor Ernesto Joselevich, of the Weizmann Institute, was a pioneer in the development of this material, seeking a physics model to better understand this system in order to control it,” said Leonardo Dantas Machado, the first author of the study.

“In collaboration with the Joselevich group and other experimental physicists, we began to reproduce the process by which these structures form to see how they emerge,” said Machado.

Utilizing high performance graphics boards, the researchers used computers to simulate how the synthesis of carbon nanotube serpentines occurs in processes in which catalyst nanoparticles are placed on uneven quartz substrates – with steps similar to those of a staircase – that are inserted into quartz crystalline tubes and placed in a furnace with automatic temperature control and argon, ethylene and hydrogen gas flows. The nanotubes are grown and self-assemble into serpentines during this procedure, and this cannot be observed directly. For this reason, it was important to simulate the process.

The scientists showed through simulations that by placing long carbon nanotubes (approximately 1 micron in length) on the surface of these stepped quartz substrates and applying force – somewhat like a light push over a short interval of time at the top of the “staircase” – the nanotubes fall along the steps in oscillatory movements like falling spaghetti sliding on the surface of a colander. While the part of the nanotube that is in contact with the stepped substrate forms structures like a serpentine, the suspended part exhibits random movements, like the head of a serpent.

“We managed to see, through simulations that involved millions of atoms, how nanotube carbon serpentines form, as experimental physicists had predicted,” said Professor Douglas Soares Galvão of the Group of Solid Organics and New Materials at IFGW’s Department of Applied Physics.

“Although the details are not exactly the same as those the Israeli group imagined, we can verify in the simulation that the hypothesis they had of the ‘falling spaghetti model’ was correct,” said Galvão, Machado’s research adviser for his doctorate.

Other results from the simulation showed that the gas flow and the placement of nanoparticles on the extremities of the substrate are important in the formation of the serpentine because they help reduce oscillations, thus creating more uniform serpentines.

The researchers also observed that for the formation of serpentines, the presence of the steps is much more important than the type of material that comprises the substrate.

“We did a test with graphite, for example, and found that, even with a very smooth substrate, as long as there are steps it can be used to form carbon nanotube serpentines,” explained Machado.

Applications

According to the researchers, one immediate application for carbon nanotube serpentines is the production of electronic nanocircuits with greater precision and more predictable behavior.

These nanotube serpentines can be easily transferred from one substrate to another because they have enough mechanical stability and are single nanotubes, with all parallel segments having the same properties throughout their extension. Because of these properties, carbon nanotubes can be used to build nanocircuits.

“With the serpentines we can conduct a circuit in which all the parallel segments have the same electronic properties because the segments are made from the same tube,” explained Machado.

Currently, the Brazilian group is simulating carbon structures in other types of substrates and with different serpentine geometries. “Whenever there is a new system of ordered carbon nanotubes in a substrate with gas flow, we can attempt to better understand how the formation of this system occurred and better control its properties,” affirmed Machado.

The Dynamics of the formation of carbon nanotube serpentines (doi: 10.1103/PhysRevLett.110.105502), by Machado and others, can be read in Physical Review Letters at prl.aps.org/pdf/PRL/v110/i10/e105502.