Origin of Luminescence from Silicon Nanocrystals: a Near Edge X-ray Absorption Fine Structure (NEXAFS) and X-ray Excited Optical Luminescence (XEOL) Study of Oxide-Embedded and Free-Standing Systems

We report an X-ray-based investigation of the formation and luminescent properties of oxide-embedded silicon nanocrystals (Si-NCs) formed by the thermolysis of hydrogen silsesquioxane (HSQ). Near edge X-ray absorption fine structure (NEXAFS) spectroscopy at the Si K-edge was used to monitor the evolving electronic characteristics of silicon during the thermal disproportionation of HSQ, from noncrystalline elemental silicon clusters at 500 °C to well-defined Si-NCs in a SiO2-like matrix at 1100 °C. X-ray excited optical luminescence (XEOL) was employed to examine the luminescent properties of the oxide-embedded Si-NCs and identified two emission bands centered at 780 and 540 nm in the optical spectrum. Energy selective XEOL and X-ray photoelectron spectroscopy (XPS) determined that the 540 nm emission band is a result of a well-defined interfacial species bridging the Si-NCs with the surrounding oxide matrix. Energy selective XEOL spectroscopy of oxide-free, freestanding hydride-terminated Si-NCs conclusively established that the emission from sub-5nm Si-NCs is a result of quantum confinement effects.