Optical and electronic properties of Si nanoclusters synthesized in inverse micelles

Abstract

Highly crystalline, size-selected silicon (Si) nanocrystals in the size range 2--10 nm were grown in inverse micelles and their optical absorption and photoluminescence (PL) properties were studied. High resolution TEM and electron diffraction results show that these nanocrystals retain their cubic diamond structures down to sizes \ensuremath{\sim}4 nm in diameter, and optical absorption data suggest that this structure and bulklike properties are retained down to the smallest sizes produced (\ensuremath{\sim}1.8 nm diameter containing about 150 Si atoms). High pressure liquid chromatography techniques with on-line optical and electrical diagnostics were developed to purify and separate the clusters into pure, monodisperse populations. The optical absorption revealed features associated with both the indirect and direct band-gap transitions, and these transitions exhibited different quantum confinement effects. The indirect band-gap shifts from 1.1 eV in the bulk to \ensuremath{\sim}2.1 eV for nanocrystals \ensuremath{\sim}2 nm in diameter and the direct transition at $\ensuremath{\Gamma}({\ensuremath{\Gamma}}_{25}\ensuremath{-}{\ensuremath{\Gamma}}_{15})$ blueshifts by 0.4 eV from its 3.4 eV bulk value over the same size range. Tailorable, visible, room temperature PL in the range 700--350 nm (1.8--3.5 eV) was observed from these nanocrystals. The most intense PL was in the violet region of the spectrum (\ensuremath{\sim}365 nm) and is attributed to direct electron-hole recombination. Other less intense PL peaks are attributed to surface state and to indirect band-gap recombination. The results are compared to earlier work on Si clusters grown by other techniques and to the predictions of various model calculations. Currently, the wide variations in the theoretical predictions of the various models along with considerable uncertainties in experimental size determination for clusters less than 3--4 nm, make it difficult to select among competing models.