Synergistic Effects on Second Harmonic Generation of Hybrid CdSe-Au Nanoparticles

Hybrid semiconductor-metal nanoparticles exhibit a combination of properties from the disparate components or even, more interestingly, synergetic properties which arise from the coupling between the two materials. In this work, we study the second harmonic generation (SHG) in CdSe-Au hybrid nanoparticles in comparison with their components, using the Hyper-Rayleigh scattering (HRS) method. Possible contribution of symmetry effects was studied by comparing symmetric two-sided gold-tipped CdSe nanoclumbbells with asymmetric one-sided quantum dot-Au (QD-Au) hybrids. A simplistic view of a symmetry effect is disproved in this case by the experimental data, which shows an unexpected reduction in the SHG response in both cases, compared to the respective mixtures of Au and CdSe nanoparticles. For CdSe-Au hybrids with a long semiconductor rod segment, we find that the SHG response corresponds to a sum of the contributions from the semiconductor and the metal components. However, for QD-Au and smaller dumbbells, the SHG response is smaller than expected from a simple sum of the contribution from both components. This reduction is assigned to the effects related to the CdSe-Au interfacial region within these hybrids. A first plausible contribution to the reduction is the dephasing induced by the gold, leading to diminished SHG from the CdSe component. This reduced response of the semiconductor component is accompanied by reduced SHG from the gold component which is assigned to a partial change of the surface of the gold once an interface with CdSe is formed. These observations regarding the SHG response manifest the unique properties that arise from the combination of a semiconductor and a metal within one hybrid nanoparticle.