Influence of passivation effects on the fractionation and optical properties of alkyl-passivated mechanochemically synthesized silicon nanocrystals

Visible emission from the indirect bandgap semiconductor silicon offers the prospect of merging conventional very large scale integration (VLSI) fabrication techniques with the luminescent properties of direct bandgap optoelectronic devices. In addition, the inherently lower cytotoxity of solubilized silicon nanoparticles, compared to traditional III-V compounds, allows for more effective incorporation into biological systems as fluorescent labeling.;Blue-green luminescent (2.48--3.10 eV) alkyl-passivated silicon nanoparticles (Si NPs) were synthesized utilizing a mechanochemical procedure by high energy ball milling single crystal silicon (1 0 0) wafer in the presence of reactive alkyne species as first reported in Heintz et al. ADVANCED MATERIALS 2007, Vol. 19, pages 3984--3988. An affinity based fractionation protocol utilizing normal phase liquid chromatography (NPLC) was developed to further isolate nanoparticle samples possessing comparable optical properties. The subsequent fractionation process allows for the efficient recovery of excess alkyne, purification, and separation of 1-hexyne terminated silicon nanoparticles. This preparative column procedure isolates silicon nanoparticles with predominantly nonpolar surfaces as confirmed by FTIR. Samples exhibit photoluminescence emission energies of 2.46 eV for the retained fractions and 2.95 eV for eluted fractions, illustrating a dependence on the polarity of surface passivations.;The influence of passivation on the optical properties of silicon nanoparticles functionalized with increasing alkyne chainlength was investigated by employing this standardized NPLC fractionation protocol. Samples produced with 1-hexyne, 1-octyne, 1-decyne, and 1-dodecyne passivations were subjected to equivalent NPLC configurations and collection retention volumes. All samples display a broad luminescence spanning from 350 nm (3.53 eV) to 570 nm (2.18 eV) and display peak intensity at an emission of 420 nm (2.95 eV) when excited with 360 nm (3.44 eV). In addition, all nanoparticles elute at comparable retention volumes and possess consistent emission energies indicating similar sample polarities and negligible optical dependence on passivant length. Alkyl passivated nanoparticles milled in increasing chainlength possess an equivalent quantum yield of 3.0 % post NPLC fractionation, and a multi-exponential decay radiative lifetime on the order of 1--2 nanoseconds.;The developed fractionation process utilizing normal phase liquid chromatography has shown to be a successful route for the isolation of passivated silicon nanoparticles. An additional advantage to the inherent simplicity is the significantly lower cost compared to conventional entropic based separation media of almost an order of magnitude less. The presented method offers a direct approach for the recovery and isolation of passivation of silicon nanoparticles, without the need for hazardous chemicals or large amounts of waste. The combination of intense blue-green emission and short lifetimes make them appealing candidates for potential optoelectronic applications.