A new solution phase synthetic method for preparing silicon nanoparticles has been developed by the reduction of silicon tetrachloride using sodium metal at relatively low temperature (164 °C) and ambient pressure. Small (average size 1 nm) and large (average size 4 nm) silicon nanoparticles coated with n-butyl, methoxy, and hydroxyl groups were produced using this method. HRTEM show that the average size of these large silicon nanoparticles is about 4 nm. AFM measurement shows the average size of small hydroxyl-coated silicon nanoparticles is about 1 nm. All particles luminesce in the blue region of the visible spectrum and no red emissions were observed. The small particles exhibit blue photoluminescence (PL) with average emission lifetimes ranging from 3.5 to 5.0 ns. The photoluminescence (PL) of these silicon nanoparticles are attributed to both the quantum size effect and surface states. The silanone groups are not the origin of the PL in the silicon nanoparticles.

Introduction

Nanoparticles are proving to be of great interest to the scientific community due to their unique physical and chemical properties, which result mainly from their size (1, 2, 3). Semiconductor nanoparticles have potential applications in optoelectronic devices such as display devices, optoelectronic sensors, lasers etc.. Most research work has focused on the II-VI and III-V binary semiconductors because these semiconductors can easily be made to luminesce due to their direct band gap structure (22, 23). For decades it was thought that silicon had poor photoluminescence emissions, thus compound semiconductor materials such as GaAs and InP were used widely in display devices and lasers. However, these compounds are toxic to the environment and are costly to make. Besides, these semiconductor materials are proving difficult to use for one of the most important potential application in optoelectronics, to provide a direct link between electronic data processing and optical telecommunications. Silicon nanoparticles are much easier to integrate into silicon integrated circuits because they have a common lattice. Research on silicon nanoparticles started in 1990 after Canham discovered that porous silicon (PSi) can give intense photoluminescence (PL) (1). Canham and Cullis also provided supporting evidence that the PL of porous silicon is due to the quantum size effect, because the emission intensified and shifted to higher energy (i.e. blue shift) when the crystallite silicon nanoparticles became smaller. Brus and coworkers synthesized 3-8 nm surface-oxidized silicon nanoparticles by a homogeneous gas-phase nucleation following pyrolysis of dilute disilane in He. They proved that the red PL of PSi was due to quantum size effect (4), because the wavelength of maximum emission shifted toward the blue when the sizes of the silicon nanoparticles became smaller.
The basic theory behind the quantum size effect states that there are a finite number of quantum states available to the valence electrons in the nanoparticles. When the particle size gets smaller, the energy gap between HOMO and LUMO becomes larger. The electrons in the HOMO level need greater energy to be excited to the LUMO level, then the excited electrons relax to the lowest energy level of the first excited state and finally back to their ground state with emission of light, i.e. photoluminescence. The wavelength of the emitted light is determined by the energy gap of the nanoparticles.
The discovery of PL in the red region of the visible spectrum from PSi made possible the use of PSi for optical applications. However, highly porous silicon is fragile and very chemically reactive. Thus, synthetic silicon nanoparticles were suggested as a possible alternative to PSi. The synthesis of silicon nanoparticles has been reported using gas-phase pyrolysis of silanes (3, 4, 5, 6); ultrasonic dispersion of PSi in organic solvents (7); evaporation and laser ablation of silicon wafers in an inert atmosphere (8, 9); or through high pressure, high temperature solution phase methods (10). Previously solution-route strategies for silicon nanoparticle synthesis required either highly reactive materials (11, 12), or high temperature and pressure (10). None of these synthetic methods were able to make large quantities of silicon particles in an economic way. Bley, et al., reacted the Zintl salt KSi with silicon tetrachloride under argon to produce silicon nanoparticles with an average particle size of 2.5 nm. But KSi is synthesized by reacting excess K with silicon at 650 ºC for 3 days and subliming away the excess K at 275 ºC under vacuum for several hours. In addition, KSi is very air and water sensitive. The synthesis of silicon nanoparticles by Heath (10) was carried out at relatively high temperature (385 ºC) and pressure (>100 atm) with rapid stirring for 3 to 7 days. He used sodium metal to reduce SiCl4 and RSiCl3 in a non-polar organic solvent. Size control of the nanoparticles was obtained by using octyl as R to cap the surface of the particles.
The silicon nanoparticles prepared from these methods luminescence in both the red and blue regions of the spectrum and have wide size distributions. The PL mechanism for silicon nanoparticles has yet to be determined. However, theoretical models primarily fall into the following categories: the quantum size effect, surface states, and/or defects. The red luminescence in silicon nanoparticles has largely been attributed to the quantum confinement effect (6), but the origin of the blue luminescence is still unknown. The surface state mechanism claims that surface localized states such as a Si-oxide interface are likely to serve as radiative recombination centers. Gole, et al., proposed a model which suggested that the PL of silicon nanoparticles was due to a silanone (Si=O) group on the particle surface (13). No single model can explain the PL observed for all of various silicon nanoparticles prepared by different methods. Wolkin, et al., proposed that the quantum confinement effect and surface states simultaneously contribute to the Si PL. In his experiments, calculations from a self-consistent tight-binding method (33) were used to explain this phenomenon (14). However, both Gole and Wolkin used PSi to test their models. Brus, et al., (4) synthesized surface-oxidized silicon nanoparticles to investigate the origin of PL for silicon nanoparticles. Their silicon nanoparticles had hydroxyl and oxide groups on the surface as indicated by the IR spectroscopy, but they simply attribute PL to the quantum size effect. Yang et al., prepared silicon nanoparticles terminated with organic alkyl groups such as methyl, ethyl, n-butyl, and n-octyl (15, 16). Their results suggested that the observed UV-blue PL emission is consistent with optical transitions in quantum-confined silicon nanoparticles. But the way they prepared larger nanoparticles was to anneal the smaller particles at different temperatures (16) and this annealing process also changed the surface of the silicon nanoparticles as evidenced by the FT-IR spectra. Korgel, et al., reported a new synthetic method to produce organic-monolayer passivated silicon nanocrystals in a supercritical fluid (21). Silicon nanocrystals ranging from 1.5 to 4.0 nm in diameter could be prepared in "significant quantities". The nature of their method is to thermally degrade the silicon precursor, diphenylsilane. But the reaction has to be run at 500 °C and 345 bar. Korgel claimed that the PL of the silicon nanocrystals was due to the quantum size effect, but their PL spectra showed the opposite trend.
We aim to develop a solution phase route to synthesize silicon nanoparticles at relatively low temperature and ambient pressure. Another goal of our research is to investigate the origin of the photoluminescence observed in the silicon nanoparticles. To do this, silicon nanoparticles with different size distributions and coated with three different groups such as hydroxyl, methoxy and n-butyl needed to be prepared. The PL of silicon nanoparticles which have various sizes, and the same coating group versus those with the same size distribution, and various surface groups are compared to elucidate the origin of PL for the silicon nanoparticles.

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