Mechanochemical Synthesis of Blue Luminescent Alkyl/Alkenyl-Passivated Silicon Nanoparticles

Silicon is a key material in the microelectronics industry. Recently, there has been great interest in nanocrystalline phases of silicon due to their size dependent electronic and optical properties. Nanoparticles with physical dimensions less than the bulk Bohr exciton radius of silicon (4 nm) typically display intense photoluminescence due to quantum size effects and have potential use both in optoelectronic devices and as fluorescent biomarkers. In the case of crystalline silicon nanoparticles, the photoluminescence (PL) will shift to higher energies as the average size of the nanoparticles is reduced. Particle sizes of the order of ∼ 1 nm have been termed “ultrabright” and show an intense emission in the blue region of the visible spectrum. In addition to a dependence on the particle size, the photoluminesence of silicon nanoparticles is also influenced by the nature of the passivating layer on the particle surface. Passivation of the surface provides chemical stability to air-oxidation and Ostwald ripening and also ties down defect states at the surface caused by dangling bonds. Successful passiviation of the silicon surface has been recently achieved by chemically inert alkyl groups. A surface passivated with strong Si–C bonds is chemically stable and the effective band gap of the silicon nanoparticle is relatively unperturbed by the alkyl passivation. Alkyl passivated nanoparticles have been previously produced by chemical and electrochemical etching of bulk silicon, the reduction of halosilanes, the oxidation of metal silicides, and the thermal decomposition of silanes in the gas phase and supercritical fluids. Each of these approaches requires the use of highly reactive or corrosive chemicals and often requires the modification of unstable hydrogen or halogen terminated surfaces. Direct approaches, commonly involving the mechanical scribing of silicon in the presence of reactive organic reagents, have found success in the patterning of silicon surfaces though reaction of a freshly exposed surface with the organic reagent. However, these techniques are limited to large and regular surfaces, and are not practical for use with nanoparticles. Here we describe a novel top-down procedure for the synthesis of stable alkyl/alkenyl passivated silicon nanoparticles using high energy ball milling (HEBM). The main advantage of this mechanochemical approach is the simultaneous production of silicon nanoparticles and the chemical passivation of the particle surface by alkyl/alkenyl groups covalently linked through strong Si–C bonds. The overall procedure for production of alkyl/alkenyl passivated silicon nanoparticles is illustrated in Figure 1. A milling vial is loaded under inert atmosphere with non-spherical millimeter-sized pieces of semiconductor-grade silicon and either an alkene or alkyne. Stainless steel milling balls are added to the vial, which is then sealed and placed in the high energy ball mill. The ongoing impacts and collisions of the milling balls during HEBM impart a significant amount of mechanical energy to the system which causes the silicon pieces to fracture, thus reducing particle size and creating fresh silicon surface. The newly-created surface is highly reactive, and provides sites for direct reaction between the silicon and the alkene or alkyne. The alkene or alkyne reacts with the silicon surface resulting in the formation of a covalent Si–C bond. In the case of mechanical scribing, it has been proposed that such a reaction occurs through surface radicals and reactive Si=Si bond sites. These reactive sites are well characterized on reconstructed surfaces of silicon under ultrahigh vacuum conditions. As HEBM proceeds, particle sizes are reduced into the nano-domain, and the direct reaction continues with the large amount of resulting fresh surface. After HEBM, the vial is allowed to sit undisturbed allowing any larger particles to settle, leaving the functionalized nanoparticles in solution. This procedure has shown to be effective for both alkenes and alkynes. However, a higher reactivity of alkynes relative to alkenes over comparable milling times has shown them to provide a higher yield of solubilized nanoparticles, and thus will be the focus of this work. Figure 2 is a series of transmission electron microscope (TEM) images of suspended silicon nanoparticles produced by milling for 24 hours in 1-octyne. Figure 2A shows a large number of nanoparticles of various sizes. In addition to the single large nanoparticle in the center of the image, there are a few nanoparticles, such as those shown in by high-resolution TEM in Figure 2B and C, within the 5–10 nm diameter range. It is clear from Figure 2A, however, that the majority of the particles found are significantly smaller than 4 nm. These nanoparticles, denoted by the arrows in the figure, are the nanoparticles small enough to be subject to quantum confineC O M M U N IC A TI O N