Converting silicon hydride (-SiH) to oxygen backed silicon hydride (-OSiH) on porous silicon leads to a shift in the wavelength of photoluminescence (PL) maximum from 670 to 605 nm, corresponding to an increase of 0.2 eV on emission energy. The results implied that silicon hydride, which links to the surfaces of silicon nano-crystals (SiNCs) via oxygen atoms, is directly responsible for the wavelength change of the PL peak.Silicon nano-crystals (SiNCs) are one of the most attractive nano-functional materials due to their electronic and photonic properties, their compatibility in biological environments, as well as their popularity in the semiconductor microelectronics industry.1-4 Understanding the origin of the photoluminescence (PL) of SiNCs is fundamentally important, considering the possibility of their widespread applications. The surface of SiNCs containing silicon hydride groups could readily react with 1-ene compounds to form relatively stable Si-C bonds, 5,6 introducing organic functional groups on the surface, facilitating the covalent attachment of organic and biological moieties that open the potential of such inorganic nano-materials to many biological applications. 1,4,7,8 Since the discovery of the PL of SiNCs at room temperature in 1990, 9 the mechanism of the PL generation has been the centre of many studies. Various theories have been proposed in order to reveal the physical and chemical grounds. To date, however, research data have seldom led to the complete understanding of such emission. Quantum connement (QC) theory 10 proposed in earlier years has been challenged by many experimental observations, which showed that PL was inu-enced by not only geometric dimensions of SiNCs, but also surface states of chemical groups, such as silicon oxide species.11-14 Crystal defects and dangling bonds have been regarded as the causes of the PL emission, but the effect of the oxidation of surface silicon hydride was not fully accounted for.15,16 It becomes well accepted that the PL of SiNCs is a complicated process and QC theory cannot be adequately applied for full explanation. Besides surface states of chemical groups, other factors, such as particle sizes, also play important roles in the mechanism of the PL. 7,11,[17][18][19][20] Time resolved PL spectra revealed that there are two types of PL emissions from SiNCs, namely, fast decaying "F band" blue emissions and slow decaying "S band" red emissions.
21Although short life-time F band emissions were believed to be originated from the core of SiNCs, 20 both F and S bands were found to be inuenced by oxygen on SiNCs.22,23 Recently a long lived blue band and a UV band were also found to be associated with oxidized silicon.24 Although the oxidation of silicon in ambient air has been known to cause wavelength shis of PL maximum ever since the discovery of the PL of porous silicon for S band emission decades ago, the detailed mechanism was unclear. There are too many unknown factors for both S and F band emissions when oxygen is involved. Therefore, rev...