Second-order optical nonlinearity of surface-capped CdS nanoparticles and effect of surface modification Original Research Article

Surface-capped CdS nanoparticles (5 nm mean diameter) with hexametaphosphate and CTAB (hexadecyltrimethyl-ammonium bromide) in chloroform were prepared by colloidal chemistry and phase transfer method. The second-order nonlinear optical response was measured by incoherent hyper-Rayleigh scattering (HRS) technique. Results show that the surface-capped CdS nanoparticles have a large first-order hyperpolarizability β value of 1.30×10−26 esu per particle. Surface capping molecule modification was performed by adding pyridine to a chloroform sol of the surface-capped CdS nanoparticles where pyridine can be bound to the CdS nanoparticle surface through Cd–N chemical bonds. A reduction in HRS signal intensity was observed when increasing pyridine concentration. This is explained in terms of a two-level model approximation derived from molecular chromophores, when considering the influence of the formation of Cd–N coordinating bonds on the charge-transfer transition energy of surface molecule-like scatterers of Cd–S polar bonds.