We then searched for a suitable model of the IF growth based on a rational analysis of each reaction product by high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) techniques. Gradually a consistent growth model of the IF nanoparticles has emerged. The following years were spent working to demonstrate that the IF growth mechanism could be rationalized, and to find the optimal conditions and construct the reactors for IF synthesis. Furthermore, a detailed study of their structure and properties was undertaken.

My main area of interest is the elucidation of the growth mechanism of the fullerene-like nanoparticles and nanotubes in layered compounds, and, in particular, to address one of the most intriguing questions in this field, i.e., what are the driving forces that lead to folding of the nanoparticles of layered compounds into the fullerene-like structures?

Until the 1980s when carbon fullerenes were discovered, it was believed that only compounds with an asymmetric layered structure in which the unit cell of one layer differs from that of the second layer (for example, asbestos) may form geometrically closed structures, such as cylinders. Also, in 1930 Pauling had pointed out that the bending required should not be expected for symmetrical layered compounds like graphite and MoS₂. In fact, we can establish that the causes of the bending that occur in asbestos and in nanoparticles of graphite or any inorganic layered compound are very different. A small fragment of a graphite layer is not stable against folding, due to the large number of dangling bonds on the fragment edges: under certain conditions it leads to the formation of carbon fullerenes and nanotubes. This phenomenon is not unique to carbon, and the same effect is also observed in molybdenum or tungsten disulfides.

In the case of IF–WS₂ or IF-MoS₂ formation using the chemical reaction between the oxide and H₂S, the main driving force for the bending of the sulfide layers is somewhat different. The IF material is obtained from oxide particles smaller than ca. 0.3 µm, while larger oxide particles give rise to sulfide platelets. The key step in the reaction mechanism is the formation of the first closed sulfide layer on the surface of the oxide nanoparticle, which serves as a template. The fast formation of the first sulfide layer on top of the oxide nanoparticles isolates them from each other and prevents their coalescence into larger particles. It should be emphasized that only the synergy of an autocatalytic mechanism of oxide reduction and oxygen-sulfur exchange process can provide the necessary conditions for the fast growth of the first sulfide layer surrounding an oxide nanoparticle.

Subsequently, sulfide layers grow in a quasi-spiral mechanism from the surface of the oxide nanoparticles inward towards its center. The curvature of the sulfide layers increases as their diameter decreases, which has a dramatic effect on their electronic and optical properties. Perfect, defect-free curved sulfide layers are usually observed in WS₂ (MoS₂) nanotubes (Fig. 1b), which fold in one direction only and hence suffer from a smaller strain as compared to the fullerene-like nanoparticles. The highest curvature was observed in single-wall MoS₂ nanotubes with a diameter less than one nm, as reported by Remskar et al. (Science, 292: 479, 2001).

My most-cited paper overall is the 1995 Science paper, "High-rate, gas-phase growth of MoS₂ nested inorganic fullerenes and nanotubes," (Feldman Y, et al., 267[5195]: 222-5, 13 January 1995), in which we first demonstrated a reproducible synthesis of uniform IF-MoS₂ nanoparticles and nanotubes by the gas-phase reaction of H₂S and vaporized MoO₃. The following paper, "Bulk synthesis of inorganic fullerene-like MS₂ (M=Mo, W) from the respective trioxides and the reaction mechanism," (Feldman Y, et al., Journal of the American Chemical Society 118[23]: 5362-7, 12 June 1996), described the growth mechanism of IF-WS₂ nanoparticles. This work also reported a synthetic strategy for the production of macroscopic quantities of a pure IF-WS₂ phase with a very high yield. On the basis of this pioneering research a scaled-up reactor for the production of IF-WS₂ was built, which enabled us to proceed to a systematic study of the structural and physical properties of the IF nanostructures.

In particular, the 1997 Nature paper has presented results of tribological tests of IF-WS₂ nanoparticles carried-out in the laboratory of Prof. Lev Rapoport from the Holon Institute of Technology. The IF nanoparticles demonstrated tribological properties that were superior to that of the regular (bulk) WS₂ (MoS₂) platelets, which are well-known solid lubricants. It is known that the low friction of WS₂ or other metal dichalcogenides as well as graphite is usually attributed to the interplanar mechanical weakness, intrinsic to their layered crystal structures. Under the action of a shear force, inter-crystalline slip occurs in the weak interplanar regions of the layered platelets. This mechanism is responsible for the formation of smooth transfer films by wear: the new surfaces, created by separating the weakly bonded sandwiches, are quite inert. However, there is a major obstacle to lubrication by these platelets: the presence of unsaturated or dangling bonds at their lateral edges. The synthesized IF nanospheres overcome this problem, since the curved, nested nanostructures have no exposed reactive edges. Follow-up research confirmed these results and, moreover, showed some new advantages of the IF nanoparticles and nanotubes, with numerous potential applications.

First of all, a startup company named "NanoMaterials Ltd." ("ApNano Materials, Inc.") was founded in 2002. The company was granted an exclusive license to manufacture, commercialize, and sell the unique nanotechnology products based on IF nanostructures by the Weizmann Institute of Science.

Currently, the most compelling application for the IF materials is in the field of tribology, where already MoS₂ and WS₂ platelets have found substantial number of applications. IF nanoparticles reduce friction and wear significantly, even better than the conventional lubricants especially under high loads, prolonging device service life, saving energy, and lowering maintenance costs and downtimes. They can be used in products ranging from power tools and machines to airplanes and satellites as well as medical devices. IF is effective as an additive for enhancing the performance of oils and greases, as a dry powder, as a thin film coating of various sorts, and for impregnating self-lubricating parts.

The tests of inorganic nanotubes’ properties show that they have a favorable potential to be used in various technologies: as nanoprobes for microelectronic components imaging, high-resolution flat-panel displays, tunable semiconductors, and molecular-sized transistors. Numerous other anticipated applications of IF materials include nanocomposites with enhanced strength and endurance, special inks, catalysts, energy storage, solar cells, and other products.

As an expert in different characterization techniques such as X-ray diffraction, transmission electron microscopy, and atomic force microscopy, I am engaged now in various materials research projects. Besides being in collaboration with Prof. Tenne and his students, there are other interesting projects in which I am involved. For example, a new phenomenon is observed in amorphous BaTiO₃ thin films that are supported by silicon. In collaboration with Dr. Igor Lubomirsky’s group we found that after certain heating treatment they showed relatively high pyro- and piezoeffects, in spite of remaining amorphous, and therefore were coined quasi-amorphous ceramic films.

Dr. Yishay (Isai) Feldman
Weizmann Institute of Science
Rehovot, Israel