Silicon clusters transferability

One of the great issues in the field of silicon clusters is to understand their photoluminescence (PL) and finally to tune the PL emission by controlling the synthetic parameters. The last two chapters deal with this problem. In experiments described by F. Huisken et al. in Chapter 22, thin films of size-separated Si nanoparticles were produced by SiLL pyrolysis in a gas-flow reactor and molecular beam apparatus. The PL varies with the size of the crystalline core, in perfect agreement with the quantum confinement model. In order to observe an intense PL, the nanocrystals must be perfectly passivated. In experiments described by S. Veprek and D. Azinovic in Chapter 23, nanocrystalline silicon was prepared by CVD of SiH4 diluted by H2 and post-oxidized for surface passivation. The mechanism of the PL of such samples includes energy transfer to hole centers within the passivated surface. Impurities within the nanocrystalline material are often responsible for erroneous interpretation of PL phenomena. 

The clustering reactions of SiD + (n = 0-3) and Si2D + (n = 0-6) cations with deuterated disilane, Si2D6, have been measured in a FTMS study110. The dominant pathway for these reactions was found to correspond to silylene transfer and SiD4 elimination. The overall reactivity of disilane compared to monosilane was found to be higher, and this was explained by the fact that the silicon-silicon bond in disilane is considerably weaker (76 kcalmol-1) than the Si—H bond of SiFLt (88 kcalmol-1)111. Thus the insertion of Si+ into the Si—Si bond was calculated to be 17 kcalmol-1 more favorable than Si+ insertion into the Si—H bond of SiFFt106,112. 

A number of other groups have investigated the clustering reactions of small cation silicon species with silanes and other small molecules. The ion-molecule reactions occurring between SiHv+ (x = 0-3) cations and neutral ammonia, as well as the reactions between NHt+ cations and SiH4 were studied by FIMS113,114. The main channel for the reaction between SiHv+ ions and NH3 was found to correspond to the elimination-addition reaction, well-known for silanes (equation 20), which formally corresponds to the transfer of a nitrene-unit (NH)113. 

The obtained results on the reaction mechanism can be summarized as follows The metal silicides form cluster structures which represent electron buffer systems. They can be oxidized or reduced easily by surface reactions. The adsorption of SiCl4 molecules at the cluster surface is immediately followed by an electron transfer from the cluster to the silicon atom of SiCl4, the cluster is oxidized. As a result of such a process a silylene species is formed at the surface of the catalyst. Chloride ions act as counter ions to the positive cluster, supporting the redox step (Eq. 4). 

Photoluminescence excitation (PLE) spectroscopy was carried out at 77K on oxidized porous silicon containing iron/erbium oxide clusters. The novel PLE spectrum of the 1535 nm Er PL band comprises a broad band extending from 350 to 570 nm and very week bands located at 640, 840, and 895 nm. The excitation at wavelengths of 400 - 560 nm was shown to be the most effective. No resonant PLE peaks related to the direct optical excitation of Er by absorption of pump photons were observed. The lack of the direct optical excitation indicates conclusively that Er is in the bound state and may be excited by the energy transfer within the clusters. 

Optical properties of dielectrics can be modified by incorporating nanosize clusters of foreign materials. Recently Si nanoclusters were shown to excite rare-earth element Er in the silica glass host [1,2]. The favorable effect of Si nanoclusters on the photoluminescence of Er in oxidized porous silicon (OPS) was also demonstrated [3], In silica hosts doped with Si nanoclusters it was shown that the excitation energy can be transferred from nanoclusters to Er ions located in a silicalike environment near the clusters. Nowadays, there is a principal interest to incorporate Er ions inside clusters due to influence on the excitation process. 

The replacement of the all electron potential with a pseudopotential has a number of advantages. Core states are not considered, and only the properties of the chemically active valence states are reproduced. Moreover, the ion-core pseudopotentials are highly transferable. The same potentials can be used in clusters, molecules, solids or the liquid state. In the case of silica, the 1 s, 2s, 2p > states in silicon are treated as chemically inert... 

Gal and coworkers observed the formation of ionic clnsters with np to five silicon and nitrogen atoms upon ionization of silane/ammonia mixtnres. Again, the silylene and nitrene transfers via addition-elimination reactions were fonnd to be the most important pathways leading to the formation of larger clusters (equation 21). However, it was concluded in these studies that chain propagation leading to the formation of large-sized clusters was not possible in these systems. 

Because this reaction involves the transfer of a charged proton, it is essential that the cluster is neutral. This has been achieved by saturating the terminal bonds of the silicon by hydrogen. 

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