Tailorable, visible light emission from silicon nanocrystals

Abstract

Crystalline, size-selected Si nanocrystals in the size range 1.8-10 nm grown in inverse micellar cages exhibit highly structured optical absorption and photoluminescence (PL) across the visible range of the spectrum. The most intense PL for the smallest nanocrystals produced (-2 nm) was in the blue (-365 nm) with a radiative lifetime of -1 ns and is attributed to direct recombination at zone center. There is presently a large research effort aimed at exploring physical and chemical means to induce a useful level of visible photoluminescence (PL) from silicon (Si). The approaches includel porous silicon and quantum wires and dots. These two approaches, which may be mechanistically related via quantum confinement, have considerable potential but are poorly understood. Visible PL has been observedl from Si nanocrystals, or quantum dots, produced by a variety of techniques including aerosols,2 colloids,3 and ion implantation.4 However, all of these techniques produce large distributions of cluster sizes resulting in broad, unstructured optical absorption and PL features which limit usefulness and make definitive interpretation of results difficult. DISCLAIMERWe have used inverse micelles as reaction vessels5 to produce useful quantities of Si nanocrystals which have been remarkable in their size monodispersity, the sharpness and richness of their spectral features and tailorability of their properties.In what follows we describe the synthesis and some of the properties. Details of the work will be published elsewhere.6 An anhydrous ionic salt (e.g., SiX4, where X = Cl, Br, or I) is dissolved in the hydrophilic interior of a solution of micelles in octane with complete absence of water. The surfactants used, nonionic aliphatic pol yethers or quatemary ammonium cationic surfactants, are dissolved in anhydrous tetrahydro furan (THF) and dried over Na metal. We next reduce Si(IV)to Si(0) using an anhydrous metal hydride, (usually 1M LiAl~in THF). The reduction is rapid with vigorous bubbling of Hz gas. One can determine the progress of the reaction by following the disappearance of the Si(IV) charge transfer absorption from the precursor solution.Nanocrystals with diameters between 1.8 and -10 nm were produced. Spectroscopy and high pressure liquid chromatography (HPLC) with on-line absorbance, conductivity and refractive index detectors were used to demonstrate 100% reduction of the Si(IV) to the final Si (0) nanocrystal form. The nanocrystals were characterized by high-resolution transmission electron microscopy.The optical absorption spectra of our nanocrystals are much richer in spectral features than earlier nanocrystal spectra making it possible to assess the role of quantum confinement inSi. An example is shown in Fig. 1 for a sample of 2 nm nanocrystals. The figure also shows the spectrum of bulk Si where the spectral features reflect the details of the band structure shown in the inset in Fig. 2. Specifically, the long absorption tail between 1. The close resemblance in the shape of the two spectra in F...

The Microemulsion Synthesis of Hydrophobic and Hydrophilic Silicon Nanocrystals

“…The solution-phase synthesis of silicon nanocrystals has been previously reported by Kauzlarich and co-workers using a variety of reducing agents, [16,17] by Korgel and co-workers at high temperatures [18,19] and pressures, and by Wilcoxon et al in micelles. [20,21] In this study, we have also synthesized silicon nanoparticles in micelles focusing on achieving smaller particle size distributions than previously reported by using powerful hydride reducing agents. The synthesis of highly monodisperse particles is significant because the electronic and thus the optical properties are highly dependent on the particle size.…”

Surface brightens up Si quantum dots: direct bandgap-like size-tunable emission

“…In view of past reports on fast emission in Si-QDs, it should be noted that this situation is very different from the frequently invoked bulk-like C-C recombination from hot carrier states in oxidized Si-QDs 26 and for alkyl-capped Si-QDs. 30 In addition to the increased radiative rates, our calculations predict also enhancement of the band-edge absorption cross-section, when compared to H-and O-terminated Si-QDs (Supplementary Fig. S6).…”

ZnS/Si composite nano-structured thin films and their photoluminescence

“…In recent years, low-dimensional semiconductors, especially composites, have attracted much interest because of their valuable photoluminescent (PL) properties [1][2][3], especially visible PL at room temperature of nanometer-sized Si [4][5][6][7] and II-VI materials such as ZnS [8][9][10][11][12][13][14][15][16][17][18]. Much research was focused on lightemitting materials, modification of their optical properties [17], and their potential applications in devices in electronics, optoelectronics and sensors.…”