Surface Modification And Photoluminescence Of Silicon Nanocrystals

Silicon nanocrystals (Si NCs) exhibit interesting electronic and optical properties, which may find applications in many fields such as optoelectronic, photovoltaics, display and bioimaging. In this work, we focus on the dependence of the optical properties of Si NCs on their size and surface modification.Freestanding Si NCs are synthesized in a plasma system. The NC size ranges from 2 to10 nm. The surface of Si NCs are modified by means of hydrosilylation. Size-selective precipitation is then used to prepare a series of samples with different Si-NC sizes. Photoluminescence (PL) from Si NCs is measured. The quantum yield and lifetime of the PL are employed to work out the radiative/nonradiative recombination rates of Si NCs. It is found that the relationship between the PL energy and NC size for Si NCs can be very well described by the theory of effective mass approximation. The PL quantum yield non-monotonically changes with the decrease of the NC size from 10 to 2 nm. The optimum PL quantum yield is obtained when the NC size is-2.8 nm, which corresponds to the PL wavelength of 744 nm. We show that the nonradiative rate of Si NCs exponentially increases with the decrease of the NC size from 10 to 2 nm. This correlates to the dependence of the area density of dangling bonds at the NC surface on the NC size, as evidenced by electron paramagnetic resonance (EPR) measurements. The radiative rate of Si NCs exponentially increases with the decrease of the NC size from 10 to-2.8 nm, consistent with the well-known quantum confinement effect. However, the radiative rate of Si NCs slightly decreases with the further decrease of the NC size from-2.8 to 2 nm. We assume that the tunneling of electrons and holes from the NC core to the NC surface for a Si NC may become significant when the size of the Si NC is rather small. The enhanced tunneling effect should be responsible for the decrease of the radiative rate of Si NCs when the NC size changes from～2.8 to 2 nm.We have compared the methods of heating and ultraviolet (UV) radiation for the hydrosilylation of Si NCs.1-octadecene,1-dodecene,1-octene,1-pentene and styrene are used to initiate the hydrosilylation reaction. The PL from the resulting hydrosilylated Si NCs is systematically characterized. It is found that UV radiation is beneficial to the reduction of cross-linking during hydrosilylation. For hydrosilylation with 1-octadecene,1-dodecene and 1-octene heating and UV radiation lead to the similar PL properties of hydrosilylated Si NCs. When 1-pentene and styrene are used for hydrosilylation, UV radiation gives rise to the higher PL quantum yields of hydrosilylated Si NCs than heating. EPR measurements have been carried out to study the dangling bonds at the surface of Si NCs hydrosilylated with both heating and UV radiation. It is observed that a smaller area density of dangling bonds corresponds to a higher PL quantum yield for a hydrosilylated Si-NC sample..