Hyper-Rayleigh scattering of CdS nanoparticles with different surfaces in solution

Large third-order susceptibilities, chi((3)) or gamma, have been observed for II-VI semiconductor nanoparticles. However, there are only few studies on the second-order susceptibilities because it is usually believed that the centrosymmetry or near-centrosymmetry of the particles eliminate the beta (or chi((2))) to zero or very small value. Here the Hyper-Rayleigh scattering (HRS) technique is used to measure the second-order NLO responses of nanoscale CdS Colloids with different surfaces in solution, which are denoted by CdS/Cd2+, CdS/S2-, CdS/SC(NH2)(2), CdS/AOT(-) (AOT(-) is a kind of anionic surfactant) and CdS/Py. The result shows that the "per particle" beta values for CdS nanoparticles (or the beta values per CdS unit) are very large. And the beta values are different for CdS nanoparticles with different surfaces. Time dependent experiment show that the HRS signal decreases remarkably as time goes by. Further studies reveal that it has multi-photon fluorescence (MPF) emission under the radiation of 1064 nm for newly made sample, but for aged stable sample the MPF is rather weak. All these experiments show that the HRS and MPF signals are very sensitive to the changes of the nanoparticle surface or the nanoparticle/solution interface. They also give the evidences proving that the surface termination of the crystalline lattice that creates a condition of non-centrosymmetry is contributing to the large beta values for CdS nanoparticles.