The effects of electronic coupling between capping molecules and quantum dots on the light absorption and emission of octyl, styryl, and 4-ethynylstyryl terminated silicon quantum dots.

Optical properties of silicon quantum dots (Si QDs) are greatly influenced by their size and surface chemistry. We report the micro-emulsion synthesis of hydrogen terminated Si QDs, with the modification of quenching the remaining reducing agent LiAlH4 with CuSO4. Subsequent functionalization was carried out with different capping molecules, including 1-octene, phenylacetylene, and 1,4-diethynylbenzene, to give octyl, styryl, and 4-ethynylstyryl terminated silicon quantum dots, respectively. The optical properties of the three kinds of Si QD synthesized, with the extended conjugation length, were examined. The effects of surface chemistry on the optical properties of the Si QD, obtained using ultraviolet-visible absorption spectroscopy and photoluminescence spectroscopy, were compared to the extension of electron and hole wavefunctions into the capping molecules, which were estimated from modified particles in a box calculation. The observed quantum yield increased from 2% to 2.5% and 9.0% and the average lifetime decreased with increasing conjugation length of the octyl Si QD, the styryl Si QD, and the 4-ethynylstyryl Si QD, which were ascribed to the effect of electronic coupling between the capping molecules and the Si QD. A tentative model in which the strong electronic interaction through covalent bonding induced a more direct band gap character for light emission was proposed by tuning the ground state wavefunctions of the electron and hole in wave vector space.