Gas-phase characterization of silicon nanoclusters produced by laser pyrolysis of silane

Silicon clusters and nanocrystals containing up to several thousand atoms per cluster have been generated by pulsed CO₂-laser-induced decomposition of SiH₄ in a flow reactor. By introducing a conical nozzle into the reaction zone, the nascent clusters are extracted into a molecular beam apparatus where they are analyzed with a time-of-flight mass spectrometer. Compared to the well-established laser vaporization method, this technique is capable of producing considerably larger silicon clusters with diameters in the nanometer size regime. A time-of-flight study of the neutral silicon clusters has shown that the velocity of the particles strongly depends on their size. This feature enables one, by introducing a chopper into the cluster beam, to considerably reduce the size distribution and to perform experiments with quasi-size-selected neutral clusters. An investigation of the fragmentation behavior of the ionized silicon clusters as a function of the fluence of the ionizing ArF excimer laser reveals that intermediate-size Siₙ clusters (n=22-100) fragment by fission, yielding Si₆⁺-Si₁₁⁺, while nanometric silicon clusters evaporate single Si⁺ and Si₂⁺ ions if the fluence of the ionizing laser is large enough. At the same time, multiply charged nanoclusters are observed. The probability of multiple ionization scales with the size of the nanoclusters.