Inorganic fullerenes of MX2 (M=W,Mo;X=S,Se)
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Mat, Res. Soc. Symp. Proc. Vol. 359 01995 Materials Research Society
Because carbon fullerenes can be considered as a metastable form of carbon, it was suggested'l that the ideal growth conditions for CF could be provided through a gas phase reaction. In this case, each nanocluster is isolated in the reaction chamber and the only means for it to release its extra energy is through collisions with the noble carrier gas (He in most experiments). Development of a similar strategy for the IF would be desirable, but requires careful consideration and detailed knowledge of the phase equilibria in the reaction chamber. However, the choice of source materials for a gas phase reaction is fairly extensive. For example, preparation of MoS2 powders and films by the gas phase reaction between MoCIx (X=3-5) and H2S1 2 or MoF6 and H2S 13 has been demonstrated. Alternatively, pulsed laser evaporation 14 , metal organic chemical vapor deposition 15 , and so forth, can be used to form IF from the gas phase. In order to take advantage of the sublimation of M003-x at relatively low temperatures (>6501C), a reactor was built that allowed for a gas phase reaction
between a stream of gaseous molybdenum suboxide and H2S. This reactor was used to prepare, reproducibly, a few milligrams of an almost pure IF phase in each run 16. The production of copious amounts of IFs allowed, a systematic study of the properties of the IF, the initial results of which are also reported here. Furthermore, the analysis of the growth conditions provided some clues for the pathway to IF production. Minor changes in the reactor design resulted in the production of substantial amount of nanotubes of up to 5flm in length and diameters of 10 to 20 nm. In order to form MoS2 from H2S and MoO3 in a reducing atmosphere at elevated temperatures, the Mo-S-O ternary phase diagram should be considered 17 . The following series of reactions are relevant for the growth of MoS2: MoO3(s) + XH2(g) --> MoO3-x(g) + XH20(g)
(1)
MoO3-x(g) +(1-X)H2(g) +2H2S(g) --> MoS2(s) + (3-X)H20(g)
(2)
MoO3(s) + 3H2(g) --> Mo(s)+3H20(g)
(3)
MoO3-x(g) + (1-X)H2(g) +XH2S(g) --> MoO2-xSx(s)+H2O(g)
(4)
MoO3-x(g) +3H2S(g) --> MoS3(s) + (3-X)H20(g) +XH2(g)
(5)
MoS3(s) --> MoS2(s) + S(g)
(6)
Nonstoichiometry commonly occurs in metaloxides, including the molybdenurr compounds. One may also take advantage of the fact that some of thE substoichiometric oxides are very volatile and sublime at temperatures as low a,6500C (reaction 1). However, if the reducing atmosphere is too strong, metalli( molybdenum, which has an exceedingly low vapor pressure at these temperature, (reaction 3), occurs. On the other hand if the reducing atmosphere in the reactor is no sufficiently strong, oxi-sulfides of molybdenum with an orthorhombic structure ar( collected on the substrate (for example, reaction 4). Careful control of the reducin(
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atmosphere in the reaction chamber is a prerequisite for the successful production of the elusive IF phase through reaction 2. The dominant phase in the Mo-S p
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