We dispersed electrochemical etched silicon into a colloid of ultrasmall ultrabright Si nanoparticles. Direct imaging using transmission electron microscopy shows particles of ∼1 nm in diameter, and infrared and electron photospectroscopy show that they are passivated with hydrogen. Under 350 nm excitation, the luminescence is dominated by an extremely strong blue band at 390 nm. We replace hydrogen by a high-quality ultrathin surface oxide cap by self-limiting oxidation in H2O2. Upon capping, the excitation efficiency drops, but only by a factor of 2, to an efficiency still two-fold larger than that of fluorescein. Although of slightly lower brightness, capped Si particles have superior biocompatability, an important property for biosensing applications.

Silicon has been anodized such that the porous layer is passivated with a homogeneous stretching phase by incorporating H2O2 in the anodization mixture. Fourier transform infrared spectroscopy measurements show that the Si–H stretching mode oriented perpendicular to the surface at ∼2100 cm−1 dominates the spectrum with negligible contribution from the bending modes in the 600–900 cm−1 region. Material analysis using Auger electron spectroscopy shows that the samples have very little impurities, and that the luminescent layer is very thin (5–10 nm). Scanning electron microscopy shows that the surface is smoother with features smaller than those of conventional samples.

Intrinsic localized radiative surface states belonging to Si-Si dimers on the surface of silicon nanocrystallites have been recently predicted. We examine the various photoexcitation pathways involved in populating these molecular states. We include both direct excitation from the ground state and indirect excitation from the photoexcited delocalized excitonic states via quantum tunneling and thermal activation. We determine the absorption and excitation spectra and the quantum efficiency of the photoluminescence as a function of the crystallite size. Our calculation gives a dramatic enhancement in the efficiency for sizes below a critical size of about 1.4 nm.