Gas-Phase Production of Molybdenum Carbide, Nitride, and Sulfide Clusters and Nanocrystallites

Molybdenum carbide, nitride, and sulfide clusters were created via laser ablation in the presence of dilute and neat reactive carrier gases. Distributions of the neutral products were characterized by time-of-flight mass spectrometry after photoionization with 193 nm radiation. The carbide clusters show an increase in ion intensity up to Mo₈C₁₂ at which point there is a sharp drop in intensity. The latter suggests that the Mo ₈C₁₂ neutral or ion is particularly stable, which we attribute to a Met-Car-like structure analogous to that observed for other early transition-metal carbides. Carbide clusters containing 10-23 Mo atoms exhibit a Mo_xC_x+3 stoichiometry, while those containing >23 Mo atoms are closer to Mo_xC_x+2, indicative of near cubic nanocrystallite structures. At low mass (Mo_x, x ≤ 6), cluster ions produced in expansions of ammonia gas contained up to three nitrogen atoms; however, heavier species (Mo_x, x ≤ 40) appear to be pure molybdenum metal clusters. The mass distributions for the sulfide clusters indicate a "magic number" structure at Mo₆S ₄⁺ which is attributed to a stable structure previously observed for the [Cu₆S₄]^- anion. Also, the dependence of cluster distributions on the fluence of the ionizing laser was investigated to gain insight on the observed cluster ion distributions using a simple, qualitative kinetic model.