Size dependence of second harmonic generation in CdSe nanocrystal quantum dots

Second harmonic generation in CdSe nanocrystal quantum dots is observed by Hyper-Rayleigh scattering. The size dependence of the first hyperpolarizibility beta was investigated for the nanocrystals in solution. The value of beta per nanocrystal decreases with size down to approximately 13 Angstrom in radius. An increase is observed with further size reduction. The unit cell normalized second harmonic coefficient, beta, shows a substantial systematic enhancement in small diameters. The observed size dependence of the second harmonic generation is explained assuming two contributions. The first is a bulklike contribution, from the noncentrosymmetric nanocrystal core, and the second, a contribution from the nanocrystal surface. The latter contribution is most significant in small nanocrystals with a substantial proportion of surface atoms. This result suggests that the second harmonic generation technique may be used as a probe of nanocrystal surfaces, in analogy to its common use as a probe of bulk surfaces.