Recently, milligram quantities of MoS2 fullerene-like nanotubes and negative curvature polyhedra (generically called inorganic fullerene-like material, IF), were reproducibly obtained by a gas phase reaction from an oxide precursor (Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R. Science 1995, 267, 222. Srolovitz, D. J.; Safran, S. A.; Homyonfer, M.; Tenne, R. Phys. Rev. Lett. 1995, 74, 1778). The present work focuses on the mechanism of the synthesis of IF-MS2 (M = W, Mo). The IF material is obtained from oxide particles smaller than ca. 0.2 μm, while larger oxide particles result in 2H-MS2 platelets. The key step in the reaction mechanism is the formation of a closed layer of MS2, which isolates the nanoparticle from its surroundings and prevents its fusion into larger particles. Subsequently, the oxide core of the nanoparticle is progressively converted into a sulfide nanoparticle with an empty core (IF). Taking advantage of this process, we report here a routine for the fabrication of macroscopic quantities of a pure IF-WS2 phase with a very high yield. As anticipated, the size distribution of the IF material is determined by the size distribution of the oxide precursor. The present synthesis paves the way for a systematic study of these materials which are promising candidates for, e.g., solid lubrication.

Layered metal disulfides-MS(2) (M = Mo, W) in the form of fullerene-like nanoparticles and in the form of platelets (crystallites of the 2H polytype) have been intercalated by exposure to alkali metal (potassium and sodium) vapor using a two-zone transport method. The composition of the intercalated systems was established using X-ray energy dispersive spectrometer and X-ray photoelectron spectroscopy (XPS). The alkali metal concentration in the host lattice was found to depend on the kind of sample and the experimental conditions. Furthermore, an inhomogeneity of the intercalated samples was observed. The product consisted of both nonintercalated and intercalated phases. X-ray diffraction analysis and transmission electron microscopy of the samples, which were not exposed to the ambient atmosphere, showed that they suffered little change in their lattice parameters. On the other hand, after exposure to ambient atmosphere, substantial increase in the interplanar spacing (3-5 A) was observed for the intercalated phases. Insertion of one to two water molecules per intercalated metal atom was suggested as a possible explanation for this large expansion along the c-axis. Deintercalation of the hydrated alkali atoms and restacking of the MS(2) layers was observed in all the samples after prolonged exposure to the atmosphere. Electric field induced deintercalation of the alkali metal atoms from the host lattice was also observed by means of the XPS technique. Magnetic moment measurements for all the samples indicate a diamagnetic to paramagnetic transition after intercalation. Measurements of the transport properties reveal a semiconductor to metal transition for the heavily K intercalated 2H-MoS(2). Other samples show several orders of magnitude decrease in resistivity and two- to five-fold decrease in activation energies upon intercalation. These modifications are believed to occur via charge transfer from the alkali metal to the conduction band of the host lattice. Recovery of the pristine compound properties (diamagnetism and semiconductivity) was observed as a result of deintercalation.

The diffusion and electromigration of Ag in crystals of CdxHgl_xTe is studied, as a function of original doping level and of the concentration of mercury. In materials with x = 0.55-0.8, Ag dopes p-type, when diffusing in at <125~ This should be contrasted to what is found in n-CdTe, where in-diffusion of Ag at 200~ increases the net donor density, leaving the material n-type. Our results show that the higher is the mercury content or the hole concentration in CdxHgl_~Te (x = 0.55-0.8), the faster Ag will diffuse in these materials. We explain our results, building on earlier suggestions made for Hg-rich materials, by assuming that silver diffuses by way of a substitutional-interstitial mechanism; i.e., it is present as two species with opposite charge, one of which dominates and is practically immobile, while the minority species diffuses rapidly. These forms equilibrate, at room temperature, within a few seconds, something that can be understood by postulating silver-mercury complex formation. If both forms of silver are bound to mercury, then this hypothesis explains the strong influence of mercury content on the diffusion behavior.

Bulk Synthesis of Inorganic Fullerene-Like MS2 (M: Mo, W) from the Respective Trioxides and the Reaction Mechanism.-A model for the growth mechanism of inorganic fullerene-like (IF) MS2 ( M: Mo, W) materials from oxide nanoparticles is presented. Reaction of the top surface of the latter with H2S gas leads to formation of a closed layer of MS2 within the first seconds of reaction. This layer prevents agglomeration to larger particles. Subsequently the oxide core, which is completely reduced into MoO2 or W18O49 by fast H2 diffusion into the nanoparticles, is progressively converted into the corresponding sulfide nanoparticle with an empty core (IF) by slow diffusion-controlled reaction. The size of the IF particle is controlled by the size of the oxide nanoparticle serving as precursor. -(FELDMAN, Y.; FREY, G. L.; HOMYONFER, M.; LYAKHOVITSKAYA, V.; MARGULIS, L.; COHEN, H.; HODES, G.; HUTCHISON, J. L.; TENNE, R.; J. Am. Chem.