B. Kohn

Improved one-phonon confinement model for an accurate size determination of silicon nanocrystals

In this article, we show how the well-known one-phonon confinement model can be improved to determine the diameter of silicon nanocrystalline spheres from the optical phonon wave-number shift, even using a physical-meaning weighting function. We show that the fundamental parameter is the knowledge of the phonon dispersion. The accuracy of our approach is supported by experimental data obtained by selective UV Raman scattering on nanocrystalline silicon thin films produced by size-selected silicon cluster beam deposition.

Structured films of light-emitting silicon nanoparticles produced by cluster beam deposition

Crystalline silicon nanoparticles (quantum dots) with diameters around 4 nm are produced via CO2-laser-induced decomposition of SiH4 in a flow reactor and subsequently transferred into a freely propagating cluster beam. Thin structured films are then obtained by shaping the beam with a mask and depositing the nanoclusters at low energy on a sapphire substrate. Upon illumination with ultraviolet radiation, the nanoparticles exhibit strong photoluminescence in the red.

Reactions of iron clusters with oxygen and ethylene: Observation of particularly stable species

Iron clusters have been produced by CO2-laser-induced decomposition of iron pentacarbonyl in a flow reactor. The absorption of CO2 laser photons was achieved by using SF6 as a sensitizer. By adding an oxidizing gas, N2O, or a hydrocarbon, C2H4, molecules which are also dissociated in the laser field, the iron clusters may react with several radicals. The as-synthesized species are extracted from the reaction zone by a conical nozzle and expanded into the source chamber of a cluster beam apparatus where they are analyzed with a time-of-flight mass spectrometer. In the experiment with N2O, we observe a magic peak at m=856 amu which can be readily assigned to the particularly stable Fe13O8 cluster. If C2H4 is added to the reactant gas, the mass spectrum reveals a magic peak at mass m=884 amu. Using deuterated ethylene, the magic peak shifts by 12 amu to larger masses, indicating that the magic cluster contains 12 hydrogen atoms. With the given restrictions, we readily derive the molecular formula Fe13C12H12....

Synthesis and characterization of iron clusters coated with hydrocarbons

Iron clusters were produced by CO2-laser-induced decomposition of iron pentacarbonyl in a flow reactor using SF6 as a sensitizer. By adding hydrocarbon molecules (e.g. C2H4), which were also dissociated in the laser field, the iron clusters were allowed to react with several radicals. The as-synthesized species were extracted from the reaction zone by a conical nozzle and expanded into the source chamber of a cluster beam apparatus where they were analyzed with a time-of-flight mass spectrometer. At sufficiently high C2H4 concentration, we observed the appearance of a magic peak in the mass spectrum at mass m equals 884 amu. Using C2D4 instead of C2H4, the magic peak shifted by 12 amu to larger masses, indicating that the magic cluster must contain 12 hydrogen atoms. With the given restrictions, we readily derive the sum formula Fe13C12H12. Chemical stability and symmetry considerations suggest that the detailed chemical formula of the magic cluster is Fe13(C2H26 or Fe13(CequalsCH2)6 and that its structure corresponds to a Fe13 icosahedron with six HCequalsCH or CequalsCH2 groups bound to six pairs of the 12 iron surface atoms.

Molecular Beams of Silicon Clusters and Nanoparticles Produced by Laser Pyrolysis of Gas Phase Reactants

In recent years, there has been increasing interest in the synthesis and characterization of nanosized particles [1]. Due to their finite size, they often exhibit physical properties which may significantly differ from those of their bulk counterparts. Research in this direction is strongly motivated by the possibility of designing nanostructured materials that possess novel electronic, optical, magnetic, chemical, or mechanical properties. Since silicon is of great importance for the microelectronics industry a detailed knowledge of the physical and chemical properties of silicon in its mesoscopic state between the atom and bulk matter is particularly desirable.