Optical and electronic properties of Si nanoclusters synthesized in inverse micelles

Highly crystalline, size-selected silicon (Si) nanocrystals in the size ran ge 2-10 nm were,grown in inverse micelles and their optical absorption and phoeoluminescence (PL) properties were studied. High resolution TEM and ele ctron diffraction results show that these nanocrystals retain their cubic d iamond structures down to sizes similar to 4 nm in diameter, and optical ab sorption data suggest that this structure and bulklike properties an retain ed down to the smallest sizes produced (similar to 1.8 nm diameter containi ng about 150 Si atoms). High pressure liquid chromatography techniques with on-line optical and electrical diagnostics were developed to purify and se parate the clusters into pure, monodisperse populations. The optical absorp tion revealed features associated with both the indirect and direct band-ga p transitions, and these transitions exhibited different quantum confinemen t effects. The indirect band-gap shifts from 1.1 eV in the bulk to similar to 2.1 eV for nanocrystals similar to 2 nm in diameter and the direct trans ition at Gamma(Gamma(25)-Gamma(15)) blueshifts by 0.4 eV from its 3.4 eV bu lk 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 (similar to 3 65 nm) and is attributed to direct electron-hole recombination. Other less intense PL peaks are attributed to surface state and to indirect band-gap r ecombination. 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 variou s models along with considerable uncertainties in experimental size determi nation for clusters less than 3-4 nm, make it difficult to select among com peting models. [S0163-1829(99)02328-0].