In this work, stable blue-green luminescent colloidal silicon nanocrystals (SiNCs) are fabricated by nanosecond pulsed laser ablation of a silicon target in dimethyl sulfoxide (DMSO). Transmission electron microscopy and X-ray diffraction analysis have shown the formation of spherical silicon nanocrystals in the colloid with size range of 2-5 nm. Our results show that the DMSO stabilizes the silicon nanocrystals via oxide formations on the nanocrytals surfaces by a simple route of laser ablation and a schematic representation of the process is suggested. The colloid exhibits strong blue luminescent emissions in the spectral range of 455-465 nm when excited at wavelengths near the direct band gap of the silicon nanocrystal. The luminescent emission band shifts to longer wavelengths (green light) if the excitation wavelength increases toward the indirect band gap of the SiNCs. The oxidized SiNCs with quantum confinement effects are shown to be responsible for visible photoluminescence of the colloid. The observed blue-green emission of the colloid makes it a good candidate for display, solid-state lighting and biological luminescent based devices.
This work was conducted experimentally to investigate the material removal rate and its mechanisms during the single-pulse and double-pulse nanosecond laser ablation of a silicon wafer in distilled water. The laser ablation processes were performed under the same experimental conditions with the same total pulse energy (E single pulse = E double pulse ). The amount of ablated material was estimated for all of the processes based on measuring the dimensions (depths and widths) and volumes of the laser-induced craters on the silicon wafer.The results indicate that double-pulsed laser processing can result in a higher material removal rate compared to the more common single-pulse process, when the inter-pulse delay time is less than the pulse duration. The higher ablation yield in the double-pulse process can be due to the higher coupling efficiency of the second laser pulse with the melted target induced by the laser pre-pulse, leading to the more efficient laser energy absorption and deposition within the irradiated region. The double-pulse nanosecond laser processing with delay time of ~5 ns not only results in a higher material removal rate, but also leads to preparation of silicon nanoparticles with a greater mean particle size compared to that of the more common singlepulse laser ablation process.
In this paper we introduce a comparative approach for studying the emission properties of silicon nanocrystal (Si-nc) colloids prepared by single pulse and double pulse laser ablation processes of a silicon wafer in distilled water. Experiments were conducted to investigate the luminescence properties of the colloids considering the size distributions and surface characteristics of the synthesized Si-ncs. The results indicated that single pulse and double pulse laser ablation processes under similar experimental conditions can lead to the preparation of Si-nc colloids with almost the same size distributions and different surface chemistry. The results show that double pulse laser processing with an inter-pulse delay time of ~5 ns can produce Si-nc colloids with a much greater emission intensity (about fivefold to tenfold) in the orange-red region (550-700 nm) of the visible spectrum. Based on the detailed analysis of the Si-nc size distribution and surface characteristics, the observed prominent orange-red emission is possibly due to a different type of Si-OH surface termination that protects the nanocrystal core upon inward oxidation.
Silicon nanocrystals with various size distributions and surface states may yield a wide range of light emission, which makes them a good candidate for color-tuning devices. In this study, emission properties of 2–6 nm sized colloidal silicon nanocrystals (SiNCs) in water are investigated at different excitation wavelengths. Nanosecond pulsed laser ablation of a silicon wafer in water has been carried out to fabricate surface passivated silicon nanocrystals with various size distributions in a simple one-step route. We observed green–red emissions with several peaks in the range of 505–675 nm at different applied excitation wavelengths ranging from 375 to 450 nm. The observed experimental peak emissions are found to be in good agreement with the model of the quantum confinement size effect and radiative centers associated with the Si = O and Si–O–H surface states. This work significantly contributed to understanding the effect of surface states for efficient emission of silicon nanocrystals.
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