Mini reviewOptical properties of polysilanes with various silicon skeletons
In this account, I present a brief overview of the optical properties of polysilanes with various silicon skeletons reviewing previous studies and supplying new experimental results.
Introduction
Polysilane (polysilylene) consisting of a Si–Si main chain and organic side chains has received considerable attention due to the emission in the UV-visible region, which is a feature different from inorganic silicons [1], [2]. The linear Si–Si chain of a polysilane can be considered to be a pseudo-one-dimensional silicon chain when compared with a three-dimensional crystalline silicon (c-Si) [2]. One of the expectations for the study on polysilane is that we can bridge the interdisciplinary field between one-dimensional and three-dimensional silicons with polysilanes having various Si-skeletons. The advantage of organic silicons is the flexibility in the molecular design and synthesis. The emission properties of polysilanes with various Si-skeletons provide important information on the relationship between the Si dimensionality and physical properties. Polysilanes with various kinds of silicon skeletons have been reported in the last decade, for example, branched polysilane [poly(silylene-co-silyne)] [3], [4], network polysilane (polysilyne) [5], [6], [7], [8], [9], [10], ladder polysilane [11], organosilicon nanocluster (OSI) [12], [13], [14], polysilane dendrimer [15], [16], [17], [31], [32], [33], [34], etc. The network polysilane is synthesized by the reaction of trichloroorganosilane, and the branched polysilane prepared by the copolymerization of the dichloro- and trichloro-organosilanes has an intermediate structure between the linear and network polysilanes. The ratio of the branched Si–Si chain to the linear Si–Si chain can be varied by the feed composition of the monomers [4]. As a three-dimensionally branched polysilane, an organosilicon monocluster (OSI) was prepared from tetrachlorosilane. The OSI has a hyperbranched structure involving a four-coordinate silicon atom [12]. Fig. 1 shows the classification of polysilanes based on the Si-dimensionalities, where only Si chains are illustrated schematically. In this paper, I present a brief overview of the optical properties of polysilanes with various dimensionalities reviewing previous studies and supplying new experimental results. The information of the optical properties of the organic Si chain may contribute to understanding the visible emission properties of inorganic silicon such as porous silicon. The emission properties of silicon with a low-dimensional structure are a topic of great interest. In the field of inorganic silicon, the visible emission from porous silicon has been an exciting research area for the past decade [18], [19], [20], [21]. The properties of a visible light emission from porous silicon are significantly different from those of crystalline silicon (c-Si) and amorphous silicon (a-Si), which show no emission at room temperature and very weak emission at low temperature in the near-IR region, respectively [22]. Silicon is the main semiconductor in the electronics industry, but one drawback of silicon is its nonemissive property. The visible emission from porous silicon was explained by the quantum confinement effect of the column-like structure with a nanometer size [18]. Later, the oxidized structure on the silicon surface, which is called siloxene, was suggested as another reason for the visible emission [23], [24]. However, the origin of the visible emission has not yet been clarified, because the oxidized surface structure of porous silicon is not homogeneous and cannot be adequately characterized.
Section snippets
Synthesis and characterization of polysilanes with various silicon skeletons
The most common procedure to prepare a linear polysilane is the condensation of dichlorodiorganosilane by the Kipping reaction using a sodium metal in toluene at 110 °C with rapid stirring. The molecular weight distribution of linear poly(methylphenylsilylene) becomes narrow by the addition of crown ether [25]. Polysilyne having a silicon network structure was prepared by similar procedures using trichloroorganosilane as a monomer. Branched polysilanes were prepared by copolymerization of
Absorption and emission spectra of branched polysilanes
The electronic state of the linear polysilane (polysilylene) is characterized by the σ-conjugation which is caused by overlapping of the Si 3sp3 orbitals along the Si–Si chain. The absorption and emission spectra of polysilane changed dramatically with branching structure. In Fig. 5, Fig. 6, the absorption and emission spectra of linear, branched, and network polysilanes are compared [4]. The branched polysilanes were synthesized by the copolymerization of dichloromethylphenylsilane and
Excited states of branched polysilanes
The visible broad emission from polysilanes has been related to the branching point of the Si–Si chain [33]. However, the assignment of the excited structure around the branching point is difficult because these polysilanes do not have a regular structure. The inhomogeneity of the Si–Si chain causes the complicated kinetics of the emission processes and analysis of the polydispersed emission kinetics has many difficulties and obscurities. In contrast, a polysilane dendrimer has regular
Size effects in the optical properties
In the above sections, the effects of the Si branching structure on the optical properties of polysilanes were discussed. Recently, the visible emission from porous silicon is one of the exciting problems and the origin of the visible emission has been discussed in relation to the quantum size effect of the nanostructure [18], [19], [20], [21]. The sizes of the polysilanes are just nanometer, and the quantum size effect should also be considered in the optical properties. In the field of
Materials beyond the border between organic and inorganic silicons
The OSI is a chemically synthesized three-dimensional silicon with a nanometer size and it can be handled in air and solvents because of the organic side chains around the Si nanocluster. The solubility of the OSI decreases with increasing the size, therefore, there is a limitation of the size synthesized chemically in solution. One of the purposes of this study is the material design and synthesis beyond the border between organic and inorganic materials. Polysilanes and inorganic nanosilicon
Materials
The OSI was obtained by the reaction of silicon tetrachloride using Mg metal in THF at 10 °C under ultrasonic field and the replacement of the remaining Si–Cl group with alkyl group (n-propyl or tert-butyl group). After the reaction was completed, the mixture was poured into methanol and the OSI was obtained as a precipitate. To remove the oxidized products, the precipitates were purified by column chromatography using silica gel. The details of the procedures are described elsewhere [13], [14],
Conclusion
The advantage of organic silicons is the flexibility in the molecular design and synthesis. Polysilanes with various Si skeletons provide a model to study the effect of Si dimensionality on the optical properties, where linear polysilane, network polysilane, and OSI can be considered as pseudo one-, two-, and three-dimensional silicons, respectively. The emissions of polysilanes with branching Si chain are characterized by the dual emission in the UV and visible regions and the large Stokes
Acknowledgements
This work was supported by Grants-in-Aid for Scientific Research (Nos. 1155900 and 13650942) from the Ministry of Education, Culture, Sports, Science, and Technology.
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