A chain is only as strong as its weakest link. This saying is true for materials as well; it is the defects that often ultimately determine a material's strength.
Nanotubes may buck this trend, as their defects are limited. Because of their infinitesimal size, however, it's hard to prove this experimentally.
The first synthesis of inorganic nanotubes, composed of tungsten disulfide, took place in the lab of Prof. Reshef Tenne of the Materials and Interfaces Department more than 15 years ago. When Dr. Ifat Kaplan-Ashiri, recently a student in Tenne's group in the Institute's Faculty of Chemistry, decided to investigate the mechanical properties of these nanotubes – made of compounds other than carbon – she turned to Prof. Daniel Wagner of the same department.
Wagner researches carbon nanotubes, and he has developed special techniques to probe their mechanical properties. Applying Wagner's techniques to multiwalled tungsten disulfide nanotubes synthesized in Tenne's lab by Dr. Rita Rosentsveig, Kaplan-Ashiri put them through a series of stretching, bending and compression "exercises" while observing their behavior under a scanning electron microscope. When her results were compared with theoretical values obtained by the group of Prof. Gotthard Seifert of Dresden University of Technology, Germany, the values matched almost exactly. In other words, the nanotubes were as strong as the theory predicted – virtually defect-free. Also participating in the project were Drs. Sidney R. Cohen and Konstantin Gartsman of the Chemical Research Support Department.
The Missing Link
Dr. Maya Bar Sadan, also a former Tenne student, researches the structural properties of nanotubes. Now a postdoctoral fellow at the Julich Research Centre, Germany, Bar Sadan uses state-of-the-art electron microscopy techniques to determine the structure of multiwalled, inorganic nanotubes, atom by atom.
A nanotube basically consists of a sheet of atoms that has been rolled up into a seamless cylinder. But that sheet can roll in different ways: "armchair-wise" on the horizontal plane, "zigzag" on the vertical plane or "chiral" diagonally. Multiwalled nanotubes are, like Russian dolls, composed of multiple cylinders nested inside one another.
Bar Sadan, working with Dr. Lothar Houben, found that the first two or three outer layers of multiwalled, inorganic nanotubes are always rolled identically, either armchair or zigzag. Inside is a chiral layer, with the innermost layers reverting back to either the armchair or zigzag conformation. Knowing the tubes' structure can aid in refining their growth mechanism, helping to create more perfect nanotubes with even greater strength.
Tenne also approached Prof. Ernesto Joselevich and his post-doctoral student Nagapriya Kavoori also of the Materials and Interfaces Department, who had developed a method to test nanotube mechanics by twisting them.
Kavoori, together with Ohad Goldbart and Kaplan-Ashiri of Tenne's group, had a surprise: When twisted, the inorganic nanotubes started creaking like the hinges of an old door! This creaking – a type of friction known to physicists as "stick-slip" behavior – takes place in everything from earthquakes to violins, but it has never before been observed in twisting on the atomic scale.
What was causing this creaking? Preliminary observations showed that at the onset of twisting, the multiple walls "stick" and twist as one; but beyond a certain angle, the outer layer "slips" and twists around the inner walls.
Joselevich, Kavoori and Seifert came up with a simple theoretical model. Unlike smooth-walled carbon nanotubes, inorganic nanotubes have a bumpy, corrugated surface. As Bar Sadan's research had shown, the outer walls are rolled identically, so they stack up like sheets of corrugated tin. The scientists calculated that this stacking would cause the layers to stick initially; but when the force of the twisting became stronger than the "locking" force between the corrugated walls, it caused them to repeatedly stick and slip.
A Melting Pot
Another student in Tenne's group, Ronen Kreizman, working together with Oxford University student Sung You Hong under the supervision of Profs. Malcolm Green and Ben Davis, has discovered yet another way to get to the core of nanotubes – literally: By melting an inorganic material with a lower melting point than tungsten disulfide, Kreizman found that the liquid is drawn into a nanotube's straw-like hollow cavity, where it then solidifies into a new nanotube.
This is the first ever report of a perfectly crystalline inorganic nanotube being produced within a nanotube. Certain inorganic materials tend to be unstable in tubular form, resisting synthesis into nanotubes. Now, however, thanks to Kreizman, this feat seems to be possible.
Dr. Ronit Popovitz-Biro of Chemical Research Support and Dr. Ana Albu-Yaron of the Materials and Interfaces Department also participated in this research.
Prof. Ernesto Joselevich's research is supported by the Helen and Martin Kimmel Center for Nanoscale Science; the Gerhardt Schmidt Minerva Center on Supramolecular Architectures.
Prof. Reshef Tenne's research is supported by the Helen and Martin Kimmel Center for Nanoscale Science; and the Phyllis and Joseph Gurwin Fund for Scientific Advancement. Prof. Tenne is the incumbent of the Drake Family Professorial Chair in Nanotechnology.
Prof. Daniel Hanoch Wagner is the incumbent of the Livio Norzi Professorial Chair.
Ifat and Maya
Dr. Ifat Kaplan-Ashiri pursued her M.Sc. and Ph.D. degrees in the lab of Prof. Reshef Tenne, garnering many prizes and honors, the most recent being the Outstanding Ph.D. Student Award of 2007, bestowed by the Israeli Chemical Society. Kaplan-Ashiri recently started her postdoctoral studies in the group of Dr. Katherine Willets at the University of Texas at Austin where she intends to combine atomic force microscopy and Raman spectroscopy to study single molecules.
Israeli-born Kaplan-Ashiri is married to Elad; they have one daughter. Apart from nanotubes, she likes to research the "pleasurable properties" of playing the piano, pottery and reading.
Dr. Maya Bar Sadan received a B.Sc. in chemical engineering from the Technion and an M.Sc. from the Weizmann Institute, studying superconductors. Her Ph.D. research was carried out under Tenne. Investigating inorganic nanotube properties together with German colleagues, she found they can change from semiconductors to a metal-like state. Bar Sadan is a recent recipient of a National Postdoctoral Award for Advancing Women in Science.
The mother of an 8-year-old daughter and 6-year-old twins, Bar Sadan likes to hike and read in her spare time.