Small silicon clusters are among the most promising objects for the development of nanoelectronics and nanooptics, as they possess photoluminescence, which is absent in bulk material and also for their compatibility with currently dominating silicon-based technology of microelectronics. In this regard, the problem of production nanoclusters of high uniformity with predetermined features is crucial.
Using the evolutionary algorithm we investigate the uniformity problem via exploring the structure of the clusters, for the structure determines all of the equilibrium properties of the clusters.
We modeled the clusters of Si10H2m formula where m=0...11. The case of Si10H22 corresponds to maximal passivation degree. It's been found that only Si10H16 has the structure that resembles the bulk diamond lattice of the bulk Si, while for other m's the structure is very sensitive to the composition. Despite the diversity of the structures some regularities are discussed.
For small passivation degrees (m6) the coordination number of silicon atoms differs from 4 and is very sensitive to the distance threshold. Besides the most stable structures, the low energy metastable isomers were also found using topology structure classification method.
The data obtained were used to perform the uniformity study. The problem formulation was as follows. Given the ratio of average bound hydrogen molecules per clusters (N(H2)), find the equilibrium distribution of Si10H2m clusters at arbitrary temperature. We considered two cases: the zero-temperature case and the case of finite temperatures (T=500 K). We found that at T=0 K only the most stable structures of several clusters (Si10, Si10H14, Si10H16, Si10H20 and Si10H22) appear in the ensemble, which is either uniform (at N(H2)=0,7,8,10, or 11 respectively) or represents the binary mixture in intermediate cases. As the temperature rises, Si10H12 and Si10H18 clusters emerge in the ensemble. So the ensemble can co! ntain only the bare silicon cluster or the clusters which have only 4-coordinated Si atoms. The second effect of elevated temperature is the appearance of low lying isomers. In some cases the isomer share can be prevailing. As for uniformity, it disappears regardless of the H2 gas concentration. We developed the analytical explaination for this effect.