Highly monodisperse silicon nanoparticles (1.57 +/- 0.21 nm) are synthesized with a covalently attached alkyl monolayer on a gram scale. Infrared spectroscopy shows that these silicon nanoparticles contain only a few oxygen atoms per nanoparticle. XPS spectra clearly show the presence of unoxidized Si and attached alkyl chains. Owing to the relatively efficient synthesis (yields approximately 100-fold higher than of those previously reported) the molar extinction coefficient epsilon can be measured: epsilon(max) = 1.7 x 10(-4) M(-1)cm(-1), only a factor of 4 lower than that of CdS and CdSe nanoparticles of that size. The quantum yield of emission ranges from 0.12 (C(10)H(21)-capping) to 0.23 (C(16)H(33)-capping). UV/Vis absorption and emission spectroscopy show clear vibrational progressions (974 +/- 14 cm(-1); up to five vibrational bands visible at room temperature), resembling bulk SiC phonons, which support the monodispersity observed by TEM. This was also confirmed by time-resolved fluorescence anisotropy measurements, which display a strictly monoexponential decay that can only be indicative of monodisperse, ball-shaped nanoparticles.

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Blue-emitting amine-terminated Si nanoparticles (NPs; size, 1.57 ( 0.24 nm) are functionalized with a Ru(bpy) 2 (spb) 2+ [bpy ) 2,2 ′ -bipyridine; spb ) 4-(p-N-succinimidylcarboxyphenyl)-2,2′-bipyridine] complex. The distance between the dye and the Si core is controlled by different alkyl chain lengths (-C 3 H 6 , -C 6 H 12 , and -C 11 H 22 ), and the thus formed Si NPs are two-chromophore systems that exhibit dual-emission in two separate regions: blue (∼450 nm, from Si core) and red (∼630 nm, from Ru dye). By measuring the Si/Ru ratio, the extinction coefficient of amino-terminated Si NPs was experimentally determined for the first time (2.6 × 10 5 M -1 cm -1 ). Energy transfer from Si NPs to acceptor molecules [Ru(bpy) 2 (spb) 2+ ] is observed by steady-state and time-resolved fluorescence, and its distance-dependent efficiency is shown to be up to 55% in the case of a short alkyl spacer (-C 3 H 6 ). Energy transfer rates are for all examined cases in the (0.2-2.2) × 10 9 s -1 range.

The photophysical properties are described of silicon nanoparticles protected with a shell of n-butyl chains, and with an asymmetric size distribution with a peak between 2 and 3 nm, and a tail extending up to 7 nm. The excited nanoparticles decay via multiexponential luminescence on a time scale of a few nanoseconds. Longer-lived nonluminescent dark states were observed by nanosecond transient absorption spectroscopy. The average lifetime of the luminescent excited states increased somewhat with increasing wavelengths of excitation and emission. The nanosecond transient absorption spectra shifted to longer wavelengths with time. Probably, these observations are related to the size distribution of the particles: larger particles are excited at longer wavelength, emit at longer wavelength, and have longer-lived and red-shifted nanosecond transient absorption spectra. Two-photon excited luminescence showed broader emission spectra than one-photon excited luminescence at the same excitation energies. The two-photon cross-sections were found to be surprisingly small.