Nanometric bends and twists

About the tiny and complex world of inorganic nanotubes and the good news they bring to our world

Dr. Yifat Kaplan-Tashiri, who recently completed her research work for her third degree in Prof. Rashef Tana's laboratory in this department, chose to investigate the mechanical properties of inorganic nanotubes. These structures are similar in manner
In principle to the organic nanotubes, which are made of carbon (the more familiar ones), but they do not contain carbon. Inorganic nanotubes made of tungsten disulfide were first discovered and produced in Prof. Tana's laboratory about 15 years ago. Since then it has been discovered that they can be created from a variety of other inorganic materials. Each of the materials has unique properties that can offer desirable benefits - depending on the intended use. However, because the inorganic nanotubes are quite difficult to manufacture (compared to their carbon cousins), only a few research groups in the world are studying them, and their properties have not yet been studied in depth and extensively.
Carbon nanotubes, on the other hand, have been thoroughly studied, and special methods for testing their mechanical properties were previously developed in Prof. Wagner's laboratory. Applying these methods to multi-layered nanotubes made of tungsten disulfide, which Dr. Rita Rosenzweig produced in Prof. Tana's laboratory, allowed Dr. Kaplan-Ishari to investigate their properties
the mechanics. These inorganic nanotubes were subjected to "abbreviated training", which included a series of stretches, bends and squeezes, with their behavior examined using a scanning electron microscope. Based on these observations, she calculated their "fitness degree", that is, the point at which they break, tear or wear out under the effort. Comparing the experimental values ​​she received to theoretical calculations based on quantum mechanics, made in his laboratory
of Prof. Gotthard Seifert (G. Seifert) at the University of Dresden, indicated an almost perfect match. In other words, the nanotubes were found to be exactly as strong as theoretically predicted, and hence, virtually defect-free. In order to confirm the unusual findings, Dr. Kaplan-Tashii conducted four additional series of experiments, using different methods - and reached the same conclusion in all of them. The results of this study, in which Dr. Sidney Cohen and Dr. Constantine Gertzman from the department also participated
for chemical research infrastructures, were recently published in the scientific journal "Records of the National Academy of Sciences of the USA" (PNAS).
The absence of defects means that inorganic nanotubes are extremely strong structures - more durable than any other known material. These materials could be used in the future as a basis for a new generation of advanced nanometer components. Similar types of inorganic nanotubes were registered as a patent by the company "Yade" - which promotes industrial technological applications based on the inventions of Weizmann Institute of Science scientists - and already today they are produced commercially in Israel and around the world for various uses, such as the lubricants produced by the company "Nanomaterials" ".

the missing link

Following the research of Dr. Kaplan-Tashiri, who was able to prove that inorganic nanotubes are strong and without defects, the question arose, how this unique perfection is possible. To answer the question, the properties of these nanomaterials must be deeply understood, and above all, their structure must be characterized in detail and precisely. At this point, Dr. Maya Bar-Sedan entered the picture, also a former research student in Prof. Tana's laboratory. Her research focused on the structural properties of the nanotubes. As a post-doctoral researcher in the group of Dr. Luther Houben (L. Houben), at the center
For microscopy and electron spectroscopy at the Yulich Research Institute in Germany, Dr. Bar-Sedan uses advanced microscopic methods, and combines imaging methods with image processing technologies based on developments made at the research center of Prof. Knut Urban (K. Urban.) All of these allow her to determine the structure of multilayered inorganic nanotubes, atom by atom.
The nanotubes can be described as a surface of atoms rolled into a cylindrical shape. There are three types of such rolls, depending on the axis of rolling: tubes formed by rolling along the horizontal axis of the surface, "zigzag" tubes formed by rolling along the vertical axis, and chiral tubes obtained when rolling the surface diagonally. Multilayered nanotubes are a kind of "babushkas" consisting of many cylinders placed inside each other. The structure and shape of the nanotubes, as well as other properties such as the arrangement of the atoms, their diameter and chiral angle, determine their mechanical properties, such as elasticity, strength, electrical conductivity and heat conduction.
Dr. Bar-Sedan discovered that the two or three outermost layers of inorganic nanotubes are always identical to each other, and these are always tubes formed by vertical or horizontal rolling. A chiral layer will then be found, and the inner layers will again be of one of the other types. These findings, recently published in the scientific journal "Records of the National Academy of Sciences of the USA" (PNAS), explain the exact match between the strength data obtained in the theoretical calculations and the experimental results: the theoretical calculations were made on long and wide tubes,
And in the experiments, the outer layers of the tubes were tested - which indeed only have these shapes. In addition, these new insights proved to be of great significance when it comes to deciphering the structure of inorganic nanotubes, and will help to improve their production process. In this way, it will be possible to produce even better and stronger nanotubes, which will enable the development of additional uses for these materials in the future.

A plot twist

Prof. Ernesto Yoslevitz and post-doctoral researcher Dr. Kabori Sathomedvan Nagfriya, also from the Department of Materials and Surfaces at the Weizmann Institute, are studying how carbon nanotubes react - mechanically and electrically - when they are twisted. Following this, Prof. Tana contacted them, to check if it is possible to investigate inorganic nanotubes made of tungsten disulfide in the same way, and to check their rigidity - that is, how difficult it is to wind them.

When the team, which included Dr. Nagfaria, Dr. Kaplan-Eshari and research student Ohad Goldbert, approached to wind the nanotubes, they encountered an unexpected phenomenon: the tubes began to creak - like rusty hinges on an old door.

Creaks caused by such frictions, in a mechanism called "sticking-sliding", are familiar to physicists who deal with large-scale phenomena, such as earthquakes or playing the violin. But helical stick-slip on an atomic scale has never been observed, as far as is known.
What causes squeaks? Preliminary experiments showed that when the inorganic nanotubes twist, they get stuck - in contrast to the carbon tubes, in which the outer layers slide uniformly around the inner layers. The researchers noticed that in the first stage, all the layers "stick" to each other and twist together, but after twisting beyond a certain angle, the outer layer slips and twists around the inner layers. then begins
Another cyclical series of adhesions and slips. What is the explanation for this strange behavior? What happens at the critical angle, which causes adhesion to alternate with sliding? And why have torsional squeaks not been observed until now at the atomic level?
To try and explain the new phenomenon, Prof. Yoslevitz and Dr. Nagfaria recruited Prof. Seifert for their help, and together they proposed a simple theoretical model that explains the effect of friction on nanotube structures. The study was recently published in the scientific journal Physical Review Letters.

It turns out that the key to the mystery of the squeaks lies in the surface of the inorganic nanotubes: the atoms protrude from the rolled surface and create a wavy and rough structure. Since the structure of the outer layers is always the same - as Dr. Bar-Sedan found - the layers are "locked" on top of each other, like a stack of corrugated tin plates. The scientists calculated that this phenomenon forces the layers to remain stuck together even when they twist. At a certain point, the force that twists the pipe becomes stronger than the force that "locks" the layers together. This is the critical angle where the layers start to slide on top of each other. The wavy surface also explains the series of cyclic slippages that follow - when the layers sliding against each other rub against each other. Carbon nanotubes, on the other hand, have smooth surfaces. Therefore, the friction is smaller, and the sliding movement is uniform and without squeaks.

Melting Pot

Research student Ronen Kreizman, from Prof. Tana's laboratory, together with Dr. Anna Elvo Yaron from the Department of Materials and Surfaces and Dr. Ronit Popovitz-Biro from the Department of Chemical Research Infrastructures, and with Prof. Malcolm Green and Ben Davis and the student Song Yu Hong from the University of Oxford, discovered Another and surprising way to physically penetrate into the heart of nanotubes made of tungsten disulfide. They did this by melting an inorganic material with a low melting temperature near the tubes. Kreizman discovered that capillary forces pull the molten liquid into the cavity of the tube, where it hardens and forms a thin nanotube by itself. The discovery, which shows for the first time an inorganic nanotube formed inside another inorganic nanotube, was recently published in the scientific journal Angewandte Chemie International Edition.
These findings may have important applications, since it is possible that with this method it will be possible to produce nanotubes even from "reluctant" inorganic materials. Previous attempts to make nanotubes from these materials did not go well, because they are unstable in this structure. Using the method developed by Kraizman, nanotubes made of tungsten disulfide can be used as a "casting mold" and as protective armor, and will allow less stable materials (in this case, lead iodide) to form a tubular structure inside their core. The researchers hope that this research will mark the starting point for production Inorganic nanotubes from a variety of materials, inside existing nanotubes and outside them, and thus it will be possible to considerably expand the variety of existing nanostructures.
Following this, the way will be paved for the development of a wide range of components and applications with desirable and unique properties. In Kreizman's next research, together with Dr. Bar-Sedan, he plans to analyze the structure of the inner nanotubes. Such an analysis, combined with chemical and physical characterization, will make it possible to verify that the potential offered by his previous research can indeed be realized - a "melting pot" in the form of a nanotube, which is used to produce other nanotubes.

Shortcut:

The question: How can nanotubes be produced from inorganic materials that reveal instability in a tubular structure?

The findings: It turns out that it is possible to melt the inorganic material near another, stable inorganic nanotube. The molten material is drawn into the stable tube cavity, condenses inside it and forms a thin nanotube.

personal

Maya Bar-Sedan was born in Tel Aviv in 1975. She became interested in science when her first baby tooth fell out - then she received as a gift a book about building airplanes and the principles of aviation. She completed her undergraduate studies in chemical engineering at the Technion, and went on to graduate studies in the laboratory of Prof. Shimon Reich, at the Weizmann Institute of Science, where she dealt with superconductors. In her PhD research work, under the guidance of Prof. Rashef Tana, she synthesized different types of inorganic fullerenes. Among other things, she succeeded in producing a new type of inorganic fullerenes: nano-octahedrons with a hexagonal and rhombic cross section. Together with colleagues in Germany, she studied the properties of fullerenes using microscopy
Electron penetrates, and discovered that their unique structure dictates a state of a metal-like conductor, in contrast to their starting material. Dr. Bar-Sedan is the mother of a daughter, about eight years old, and twins, a boy and a girl, about six years old. She spends her free time traveling and reading.

personal

Yifat Kaplan-Tashiri was born in Holon in 1975. She completed her master's and third degree studies in the laboratory of Prof. Rashef Tana at the Weizmann Institute, and won many awards - the most recent of which is the 2007 Outstanding Doctoral Dissertation award she received from the Israel Chemical Society. Currently, as a postdoctoral researcher in Dr. Catherine Willets' group at the University of Texas at Austin, she plans to combine advanced spectroscopic and microscopic methods to study single molecules. Dr. Kaplan-Tashiri is married to Elad and mother to Rona, about a year old. Besides nanotubes, she is also interested in playing the piano, pottery and reading.