Photoluminescence silicon compounds

Crystalline silicon is the most widely used semiconductor material today, with a maiket share of above 90%. Because of its indirect electronic band structure, however, the material is not able to emit light effectively and therefore carmot be used for key applications like light-emitting diodes or lasers. Selected one- or two-dimensional silicon compounds like linear or branched polysilylenes [1] or layered structures like siloxene [2], however, possess a direct band gap and therefore exhibit intense visible photoluminescence. Siloxene, a solid-state polymer with a sheet-like layered structure and an empirical formula Si H (OH) , in particular, is considered as an alternative material for Si-based liuninescent devices. Detailed studies of stmctural and photophysical properties of the material, however, are strraigly impeded by its insolubility in organic solvents. 

As for silicon, secondary ion mass spectrometry (SIMS) is the most widely used profiling analysis technique for deuterium diffusion studies in III-V compounds. Deuterium advantageously replaces hydrogen for lowering the detection limit. The investigations of donor and acceptor neutralization effects have been usually performed through electrical measurements, low temperature photoluminescence, photothermal ionization spectroscopy (PTIS) and infrared absorption spectroscopy. These spectroscopic investigations will be treated in a separated part of this chapter. 

Germanenko et al. [153] suggested an explosive detector based on the photolumines-cence of silicon nanostructures. Silicon nanocrystals are first prepared by laser vaporization (LVCC). After suspension in methanol, the silicon nanocrstyals are excited by a laser at 355 nm, resulting in photoluminescence. They found that nitrotoluenes quench the photoluminescence from the silicon nanocrystals. Quenching rate constants for a number of nitro-compounds were presented. 

Siloxene (Si6H603) , a fascinating compound ever since its discovery by Wohler [1], is prepared by reacting CaSi2 with aqueous HCl. As the photoluminescence shown by porous silicon is attributed to the presence of siloxene layers, numerous investigations have been performed both on Wohler (JVS) and Kautsky siloxene (KS). Nevertheless, the structure of siloxene has not yet been cleared up and therefore structure models including planes, chains or cycles are discussed [2]. 

The photoluminescence emission obtained from n-electron systems such as stilbene or benzene is red-shifted into the blue part of the visible spectrum if a silicon atom is attached to the conjugated moiety. Additional substitutions on the silicon atom affect the spectral features of the molecule as demonstrated with the derivatives of TCA 2 such as the diphenylsilacyclobutene compound. Diol derivatives 5 of this molecule that were also investigated include the spirocyclics 1 and the cyclosiloxanes. 

Several different types of phenyl groups containing silicon-based compounds show an intense blue photoluminescence (PL) upon excitation with UV light [1]. For silacyclobutenes 1, the exocyclic substituents R at silicon affect both the strength and wavelength of the PL (Fig. 1). 

Mouri et al. reported compound 36, which can be considered as a ladder-type structure comprising stilbenes linked via sulfur and silicon atoms [94], The compound was prepared from the diacetylene 35 by the addition of ferr-butyllithium, followed by elemental sulfur (68 % yield). Oxidation of 36 with mCPBA in dichloromethane gave the bis(5,iS-dioxide) 37 in 45 % yield. Both absorption and emission maxima are significantly red shifted for the 5,S-dioxide and the photoluminescence quantum efficiency is improved (43 % and 50 % for 36 and 37, respectively). 

Nitride compounds can also be categorized as binary, ternary, quartemary, and multinary based on the number of elements included. Binary covalent nitrides, such as GaN and AIN, are not easily considered as host lattices for phosphors due to the fact that they lack suitable crystal sites for activators. Ternary, quartemary, and multinary covalent nitride compounds, typically silicon-based nitrides, have distinctive and rigid crystal stmctures. They have suitable crystal sites for activators and have a versatile stmcture, which allows the doped RE ions to exhibit useful photoluminescence [33]. 

Figure 1. Photoluminescence emission spectra at room temperature of differently silicon-substituted tolane cycloadduct (TCA) compounds with general fomula 1,1-R2-2,3-diphenyl-4-neopentyl-l-silacyclo-2-butene where R = ethyl (trace 1), R = methyl (trace 2), R = hydroxyl (trace 3), and R = phenylethynyl or C UsC=C (trace 4). Excitation wavelength 320 nm, spectral bandwidth 1 nm.

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