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

Silicon clusters and nanocrystals containing up to several thousand atoms p er cluster have been generated by pulsed CO2-laser-induced decomposition of SIH, in a flow reactor. By introducing a conical nozzle into the reaction zone, the nascent clusters are extracted into a molecular beam apparatus wh ere they are analyzed with a time-of-flight mass spectrometer. Compared to the well-established laser vaporization method, this technique is capable o f producing considerably larger silicon clusters with diameters in the nano meter size regime. A time-of-flight study of the neutral silicon clusters h as 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 wit h quasi-size-selected neutral clusters. An investigation of the fragmentati on behavior of the ionized silicon clusters as a function of the fluence of the ionizing ArF excimer laser reveals that intermediate-size Si-n cluster s (n=22-100) fragment by fission, yielding Si-6(+) -Si-11(+) while nanometr ic silicon clusters evaporate single Si+ and Si-2(+) ions if the fluence of the ionizing laser is large enough. At the same time, multiply charged nan oclusters are observed. The probability of multiple ionization scales with the size of the nanoclusters. [S0163-1829(99)15203-2].