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Background information

Silicon nanoparticles have attracted much attention due to their potential application in the optoelectronic devices. Since the discovery of room-temperature luminescence from anodized porous silicon, this phenomenon from nanosized silicon is now widely investigated.  Various techniques can be employed in producing nanosized silicon crystals such as colloidal systems using organic liquids, gas evaporation techniques, pulsed laser ablation, self-assembly techniques, silane plasma formation techniques, plasma-enhanced chemical vapour deposition and photoelectrochemical techniques.

Colloidal systems using organic liquids to synthesise the nanosized silicon crystals is desired as a simple dilution will eliminate the interparticle interaction of the nanocrystals. Thus, ensure the narrow distribution of particle sizes. Colloidal systems also eliminate the possibilities of unintended oxidation by capping the nanocrystals with a layer of solvent molecules. Selection of inactive solvents such as hexane, acetone and alcohols is found to be able to control the interparticle interaction of active metals such as calcium, zinc and magnesium. 

Luminescence of infra-red or red region was due to quantum size effect, while a surface effect was presented for blue or green luminescence as a possible mechanism. If the photoluminescence (PL) originates from quantum confinement, then the PL will vary according to the change in the average size of the nanoparticles.  As the nanoparticles aggregate, the size increases thus shifting the PL to the lower energy region. It is favourable to use colloid for surface modification of Si nanoparticles. We envisage that the emission peak position and its intensity will be affected by the selection of capping solvent molecules.

Objectives

The objectives of this research project are:

·     To investigate the feasibility of using micro-emulsion techniques in preparing unique silicon nanocrystals in room temperature conditions

·     To elucidate the factors influencing the growth of nanocrystals via micro-emulsion techniques

·     To investigate the photoluminescence properties of silicon nanocrystals in relation to their particle sizes

·     To investigate, if any, structural defects or interfaces of silicon nanocrystals using high resolution electron microscopy

Funding for this project

This project has successfully secured a MOSTI e-Science funding effectively from January 2007 onwards for a duration of two years. An ISAT New Zealand travel grant was obtained which enabled part of the TEM analyses required for this project. The TEM analyses were carried out over two separate trips to the MacDiarmid Institute for Advanced Materials and Nanotechnology of Victoria University of Wellington in New Zealand. Geok Bee would like to thank MTSF for the initial funding which has made the preliminary development of the project a success.

Methodology

1 μL of SiCl₄ (99.8+%, Acros Organics) was added to 100mL of toluene (J. T. Baker) followed by 1.5 g of tetraoctylammonium bromide (TOAB) (98.0% Merck). The solution was left stirring for 1 hour. Then, 1.5 mL of 0.1 M NaBH₄ (R & M Chemicals) in 0.4 M NaOH was added dropwise to the solution. The reduction was rapid resulting in vigorous bubbling of H₂ gas. The solution was then left to react for 3 hours. The strong reducing agent was then quenched with 20 mL of methanol (Fischer Scientifics), upon which the solution became transparent. At this stage of the reaction, the silicon nanocrystals were terminated by hydrogen and encapsulated in the inverse micelles. Half of the resultant solution is kept.      

The remaining half of the resultant solution was then added with 1 μL of 0.1mM chloroplatinic acid hexahydrate (CAH) (40% Pt, Merck) in THF followed by 2 mL of 1-heptene (98%, Merck). CAH served as a catalyst for the capping process of 1-heptene onto the surface of silicon nanocrystals. The final product was silicon nanocrystals capped with 1-heptene.

Samples were then purified which involved the removal of excess solvents via rotary evaporation and the sample was re-dispersed in hexane solution. The hexane phase was purified by liquid-liquid extraction with n-methyl formamide (99+%, Acros Organics) and followed by de-ionised water to remove reducing agent by-products and surfactant. The final samples were purified hydrogen terminated sample and purified 1-heptene capped sample.

Fourier Transform Infra-red spectroscopy (FTIR) was used to identify and characterise the terminating species on the particle surfaces. The spectra were collected on a Perkin Elmer FTIR spectrometer, obtained at the room temperature by smearing the sample solution onto a KBr plate by allowing the solvent to evaporate. The PL spectra were obtained from a Perkin Elmer PE-LS 55 spectrophotometer. A slit width of 5.0 nm was used for both the excitation and emission monochromators throughout the analysis process. Both samples were examined at different excitation wavelength varied from 310 nm ? 335 nm, at an interval of 5 nm. Si nanoparticles were characterised by ex-situ observations with a 200 keV transmission electron microscope. HRTEM micrographs were captured employing a Philips Tecnai 20 high resolution transmission electron microscope (HRTEM) operated with an accelerating voltage of 200 keV with an LaB₆ gun source coupled with STEM, EDS and CCD.

Outcome of research  

Photoluminescent silicon nanocrystals have been produced via the microemulsion systems. CTAB and TOAB are found to be useful to create stable inverse micelles to control the particle sizes of the silicon crystals.  The use of LiAlH₄ is found to be less favourable as compared to NaBH₄ due to the Al contamination. The results of the project have been presented in international and national conferences. Furthermore, a manuscript has been sent for publication at international journals and is currently under the reviewing process.