Laser production and deposition of light-emitting silicon nanoparticles

Silicon clusters and nanoparticles are produced by CO2-laser-induced decomposition of silane in a flow reactor. In contrast to conventional techniques, the particles are expanded, directly after production, through a conical nozzle into a high vacuum chamber and then transferred into a molecular beam apparatus where they an analyzed in situ by time-of-flight mass spectrometry (TOF-MS). The analysis reveals that the flow reactor emits, besides small clusters, also high-purity silicon crystallites with diameters between 2 and 20 nm. It is found that the particles' velocity strongly correlates with their mass. This feature and the fact that the particles are produced in the pulsed mode enable us, by introducing a chopper into the cluster beam, to considerably reduce the dispersion of their size distribution and to perform size-selected low-energy cluster deposition on various substrates. High-resolution electron transmission micrographs demonstrate the capabilities of the new apparatus and reveal interesting details of the crystalline structure of silicon nanoparticles as a function of their size. The monodispersed silicon films have been further characterized by studying their luminescence and Raman scattering behavior. As predicted by theoretical models, the peak of the luminescence curve shifts with decreasing particle size to smaller wavelengths (higher energies). Structured thin films are obtained by shaping the duster beam with a mask and depositing the nanocrystals at low energy on a sapphire substrate. Upon illumination with ultraviolet radiation, the structured film exhibits strong photoluminescence (PL) in the red. (C) 2000 Elsevier Science B.V. All rights reserved.