Luminescence Properties of Silicon Clusters: Chain, Ladder and Cubic Structures
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Yoshihiko KANEMITSU Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
ABSTRACT We have studied the luminescence properties of chemically synthesized Si 8 clusters and Si-based polymers with different backbone structures. The optical properties of Si 8 clusters depend strongly on the shape of clusters. The luminescence properties of the cubic Si 8 cluster are different from those of the chain and ladder Si 8 clusters. Moreover, the ladder and network Si polymers whose backbones are constructed by the organosilicon units with three Si-Si bonds exhibit broad photoluminescence spectra in the visible region. These luminescence properties are entirely different from those of the chain structure constructed by the organosilicon units having two Si-Si bonds. The unique optical properties of silicon clusters and polymers are discussed.
INTRODUCTION Very recently, silicon low-dimensional nanostructures such as porous Si and Si nanocrystallites have attracted interest from viewpoints of both fundamental physics and optical-device applications, because they exhibit strong visible luminescence even at room temperature [1]. However, the mechanism of the strong luminescence from silicon nanostructures is not clear. Natural analogues of silicon nanostructures such as quantum dots, wires, and sheets are chemically synthesized Si clusters and Si-based polymers [2-5]. Modem organic synthesis and purification techniques can produce new silicon materials with controlled molecular weight. These techniques have some advantages over the widely-used laserevaporation methods for clusters [6,7] or the electrochemical methods for nanocrystallites [1]: For example, the size and shape of Si clusters can be exactly controlled. The study of the chemically-synthesized Si materials helps to understand the electronic properties of vaporphase Si clusters [6,7] and visible light-emitting silicon materials [1]. In this paper, we report the luminescence properties of chemically synthesized Si clusters and Si-based polymers for the understanding of the luminescence mechanism in silicon nanostructures.
LUMINESCENCE FROM SMALL SILICON CLUSTERS The synthesis procedure of Si 8 clusters is illustrated in Fig.1. Synthetic, purification, and characterization methods were described in Ref. [8,9]. The dangling bonds are terminated by the methyl and phenyl groups in the chain structure [Fig. 1(a)], by the isopropyl groups in the ladder structure [Fig. 1(b)], and by 1,1,2-trimethylpropyl groups in the cubic structure [Fig. 81 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
1(c)]. In each structures, the optical properties are not sensitive to the kind of the organic terminations. The optical absorption and luminescence properties of these clusters reflect the electronic structures of the central Si skeleton structures. Saturatedly bonded Si8 clusters terminated by an organic substituent have no dangling bonds and become a new model material for small silicon clusters.
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