First-order hyperpolarizability of ZnS nanocrystal quantum dots studied by hyper-Rayleigh scattering

Hyper-Rayleigh scattering technique was used to measure for the first time the first-order hyperpolarizability (β) of ZnS nanocrystals with 2.5 nm mean diameter. Results show that the ‘per ZnS particle’ β value is 2.34×10⁻²⁷ esu and the ‘per ZnS formula unit’ β value is 1.63×10⁻²⁸ esu. An increase by at least two orders of magnitude in the β value per ZnS formula unit is found when compared with bulk ZnS. Two possible contributions originating from nanocrystal surface electric field and solvent field were experimentally excluded. Other two contributions, bulk-like contribution and surface contribution, are considered. Especially, the latter is emphasized due to the special surface structure of nanocrystals.