Small colloidal particles (10-35 A) of crystalline McS2 and WS2 exhibiting size quantization effects in their optical absorption spectra are believed to form by simple dissolution of crystalline powder in acetonitrile. This conclusion is supported by electron microscopy, electron diffraction, X-ray diffraction, EDAX, and chemical analyses. We speculate that the MoS2 and WS2 clusters are formed by cleavage of the van der Waals layers by penetrating solvent molecules, a process related to the well-known formation of colloids from layered clay structures. Preliminary experiments with layered WSe2 suggest that this material may also form colloids by a similar mechanism.Recently, several papers have reported quantization effects in very small colloidal particles consisting of layered semiconductor materials, such as Pb12, Bi13, Hg12, Bi2S3, and Sb2S3.14 A major feature of the results for layered iodide compounds is the presence of well-resolved peaks in the absorption spectra of the colloidal sol~tions,~-~ in addition to the large blue shift of the fundamental absorption shift that is characteristic of quantized semiconductor particles. The spectral peaks have been attributed either to preferred particle sizes,' termed "magic numbers", or/and to multiple excited-state transitions for a single particle size.2 In the previous work, the colloids were prepared by reaction of the appropriate metal salts with iodide or sulfide ions. The identification of the spectra of these colloidal sols with quantized particles has recently been q~e s t i o n e d .~.~In this Letter, we present very strong evidence that the layered semiconductors MoS2 and WS2 (direct band gaps of 1.7 and 1.8 eV, respectively) form stable colloids of quantized particles upon simple dissolution of the solid crystalline material in acetonitrile; the concentration of the colloids formed in this manner is low, typically about 1 X M, and dependent upon temperature. We speculate that colloid formation occurs because the bonding between adjacent MoS, and WS, layers is weak (van der Waals type), and the organic solvent molecules can penetrate between the van der Waals bonded layers (a well-known process that can lead to formation of intercalation compounds with appropriate organic molecules) and shatter the crystal into small clusters.611 This process is apparently related to that which occurs when clays, also having layered structures, are contacted with liquids to produce colloidal clay dispersions.12The colloids were prepared under an argon atmosphere to prevent formation of oxidized species that could interfere with the spectra of colloidal particle^;^ also, no added stabilizing agents were used. Two procedures were followed. In the first, the purest crystalline powders commercially available (Aldrich MoS2, 99+%;Atomergic WS2, 99.95%) were first "washed" in acetonitrile at 65 "C for 24 h to remove soluble and/or leachable impurities. The washed solid was then isolated and reheated in pure acetonitrile (Burdick-Johnson UV spectral grade) at 60 "C for 3 days.Mic...
