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Atomic concertina

A project funded by FAPESP helps unveil the behavior of nanostructures

fernando sato/unicampMalleable as a concertina, the nanotube, below, stretches without breakingfernando sato/unicamp

Interested in discovering how nature functions at its most intimate level – the scale of atoms, the block that form matter – Argentine physicist Daniel Ugarte had to do more than plan his experiments. He had to learn how to assemble and adjust the super powerful microscopes he uses and even design the buildings of the National Synchrotron Light Laboratory/LNLS that house these devices, which are sensitive to the subtle vibrations on the ground caused by passing cars on the street. During his fifteen years in Brazil, Ugarte has published a number of discoveries which are fundamental for understanding how matter behaves at this extremely reduced scale and for the development of the science of electronics in the future. The latest findings, described in the January issue of Nature Nanotechnology, is the identification of a totally unexpected atomic structure:  a square-shaped tube with a thickness of less than 0.5 nanometers (a millionth of a millimeter) – this is the smallest three-dimensional structure formed by silver, the same material that has been used for thousand of years to make jewelry and coins.

The discovery that silver takes on this format naturally is one more success case in the most comprehensive electronic microscope laboratory in Brazil, which Ugarte began to assemble in 1998 at the LNLS. Today, the Electronic Microscope Lab/LME – the construction of which was funded by R$ 6 million provide by the Financiadora de Estudos e Projetos/Finep funding agency  – is part of the Centro de Nanociência e Nanotecnologia Cesar Lattes nanoscience and nanotechnology center.  The LME facility has six high performance rooms that take up 600 square meters, and five powerful microscopes purchased at a cost of R$ 8 million financed by FAPESP.

The LME facility is equipped with three electronic transmission microscopes, used to analyze the atomic arrangement of materials at different levels of resolution, and two electronic scanning microscopes that produce three-dimensional images.  Of these, the most powerful is the analytical transmission microscope, purchased in 2005, and at the final stage of installation.  This microscope is able to identify the chemical components that comprise the material being studied. In the ten years since it was founded – celebrated this month – the laboratory has hosted approximately 400 researchers from different institutions, who were trained at the lab to use these microscopes and conduct the measurements they needed.  This work has generated hundreds of scientific papers.

Nanotubos_BandoneonPaul Krok/Wikimedia CommonsThe experiment that revealed the new nanotube was conducted by Ugarte and Peruvian physicist Maureen Lagos, a doctorate student studying under Ugarte at the State University of Campinas/Unicamp.  The researchers placed a very thin silver blade, dozens of atoms thick and thousands of atoms wide, under a high resolution transmission microscope. This microscope produces images that are amplified millions of times, enough to distinguish the atoms.  The physicists then bombarded the silver blade with electron beams (negatively-charged electric particles), expelling thousands of atoms and giving it the appearance of Swiss cheese. By making holes in the blade, they shaped ultra-microscopic rods eight atoms thick, which made them lengthen spontaneously until they broke apart, in the manner of a piece of chewing gum stretched at both ends.

The full stretching occurs in a few seconds – and the atoms do not remain still, even though they have been cooled down to a temperature of 150 degree negative.  Ugarte and Lagos, with the help of physicists Jefferson Bettini and Varlei Rodrigues, filmed the transformation of the rods to analyze the transformation step by step. As the metal stretched and thinned out in the middle, the atoms reorganized themselves until they formed a string one atom thick and then the string broke. Between the rod format and the string of pearls, the physicists from Campinas witnessed the appearance of the tiniest three-dimensional hollow structure that silver can turn into:  a tube, the base of which is formed by four atoms.

Tension and rotations
It was not easy to identify this structure.  The two-dimensional images only showed the profile of the tube:  sequences of two atoms, with three other atoms inserted between them that piled up like marbles. As the physicists knew that silver atoms tend to organize themselves into a three-dimensional structure, they soon realized that where they saw two atoms there were actually four atoms. “Two of the atoms were hidden”, Ugarte explains.

The problem was knowing how many atoms existed in the layers where the physicists saw three atoms.  Initially, Ugarte and Lagos thought that these layers also hid another two atoms – and that this layer had a total of five atoms, as other models suggested. If this actually occurred, the center of the tube should have been solid rather than hollow.  When Ugarte and Lagos re-evaluated the distances between the atoms, they concluded that there was some mistake. In fact, the three-atom layers actually contained four atoms – one atom more than had been previously believed. These layers seemed as if they had three atoms because they had rotated at 45 degrees around the central axis, covering one of the atoms. “We knew what had happened, but we didn’t understand the reason”, says the Argentine physicist.

On the basis of this information, physicists Douglas Galvão and Fernando Sato, both from Unicamp, created a computer program that simulates the movement of the atoms and the forces that keep them united.  They concluded that the hollow nanotubes appeared when the silver rod was submitted to high tension and the distance between the atoms in one row narrowed slightly while the atoms in the next row rotated at 45 degrees, lengthening the structure, in the manner of – as Ugarte describes it – a bandoneon, the instrument played by tango musicians. When fully stretched, the hollow nanotube was almost twice as long as the length of the rod in the beginning. “We hope that these structures will also form in copper wires, which will probably be used as the electric conductors of nanocircuits in the future”, says. If this materializes, the copper nanowires will gain more elasticity and resistance. “Knowing how these structures become deformed”, says Ugarte, “is crucial to understanding two little-known basic properties for the manipulation of nanomaterials:  attrition and adhesion.”

The projects
1. High resolution electronic microscope center (nº 96/04241-5); Modality Infraestrutura 3 Program; Coordinator Daniel Ugarte – Unicamp and LNLS; Investment R$ 2.621.484,09 (FAPESP)
2. Analytical transmission electron microscope for spectroscopic nanocharacterization of materials (nº 02/04151-9); Modality Regular Research Awards; Coordinator Daniel Ugarte – Unicamp and LNLS;
Investment R$ 5.241.219,61 (FAPESP)
3. Equipment maintenance and repair; Modality Equipment Repair; Coordinator Daniel Ugarte – Unicamp and LNLS; Investment R$ 189.298,06 (FAPESP)
4. Grants for master and doctorate studies; Modality Grant for master and doctorate studies; Coordinator Daniel Ugarte – Unicamp and LNLS; Investment R$ 602.737,95 (FAPESP)

Scientific Article
LAGOS, M.J. et al. Observation of the smallest metal nanotube with a square cross-section. Nature Nanotechnology. v. 4. p. 149-152. 25 jan. 2009.