Presence Of Silicon Research Articles

Why does the presence of silicon atoms improve the emission properties of biphenyl derivatives? - Verification of various hypotheses by experiment and theory.

In the course of studying silicon modifications to improve emission properties of commonly used organic compounds, biphenyl with dimethylsilylvinyl groups in the para position (3-Si) was investigated. A comparative study was performed on the exact C-analogue (3-C) and expanded to biphenyl and dimethylbiphenyl to emphasize the general trend observed. Compound 3-Si displayed emission properties clearly different than all of the investigated hydrocarbon compounds, i.e. twice stronger fluorescence (Φf = 0.6) and a 3-times larger radiative rate constant as compared to 3-C in acetonitrile. Searching for the source of the unique emission of 3-Si, singlet and triplet processes were investigated for all of the compounds using steady-state and time-resolved methods, and their principal photophysical parameters are reported. Experimental work was supported by the theoretical predictions obtained using the EOM-CCSD method. The results led to the conclusion that the strong emission of 3-Si must be due to silicon's presence that enhanced intensity borrowing from the strongly allowed S0 → S2 transition and the larger S1 → S0 transition moment.

Optical and microstructural characterization of porous silicon using photoluminescence,SEM and positron annihilation spectroscopy

We have studied the dependence of porous silicon morphology and porosityon fabrication conditions. N-type (100) silicon wafers with resistivity of2–5 Ω cm were electrochemically etched at various current densities and anodization times. Surfacemorphology and the thickness of the samples were examined by scanning electronmicroscopy (SEM). Detailed information of the porous silicon layer morphology withvariation of preparation conditions was obtained by positron annihilation spectroscopy(PAS): the depth-defect profile and open pore interconnectivity on the sample surface hasbeen studied using a slow positron beam. Coincidence Doppler broadening spectroscopy(CDBS) was used to study the chemical environment of the samples. The presence ofsilicon micropores with diameter varying from 1.37 to 1.51 nm was determined by positronlifetime spectroscopy (PALS). Visible luminescence from the samples was observed, whichis considered to be a combination effect of quantum confinement and the effect ofSi = O double bondformation near the SiO2/Si interface according to the results from photoluminescence (PL) and positron annihilationspectroscopy measurements. The work shows that the study of the positronium formedwhen a positron is implanted into the porous surface provides valuable information on thepore distribution and open pore interconnectivity, which suggests that positron annihilationspectroscopy is a useful tool in the porous silicon micropores’ characterization.

Nanocrystal formation in annealeda-SiO0.17N0.07:H films

Silicon nanocrystals have been fabricated by annealing amorphous hydrogenatedsilicon-rich oxynitride (SRON) films in vacuum for 4 h over the temperature range850–1150 °C. X-ray photoelectron spectroscopy confirmed the composition of the film to beSiO0.17N0.07. Glancing angle x-ray diffraction results revealed consistent silicon crystallite sizes of nm for films annealed at temperatures , increasing to nm for films annealed at 1150 °C. The room temperature photoluminescence spectra of the samples annealed at850 and 950 °C comprised luminescent peaks from silicon nanocrystals and luminescence from the defectsin Si–O system. However, only peaks from defects in Si–O system were present inthe luminescence spectra from samples annealed at temperatures greater than950 °C. For the samplesannealed at 850 and 950 °C, the presence of strong Si–N bonds prevented the coalescence of smallersilicon crystallites into larger crystallites. Larger, non-luminescent siliconcrystallites were only formed in films annealed at temperatures greater than950 °C, where the energetics of coalescing particles overcame the strong Si–N bonding in SRONfilms. High-resolution transmission electron microscopy analysis confirmed the presence ofsilicon nanocrystallites. A proposed growth mechanism of silicon nanocrystals is discussed.