Gold Raspberry Shell Grown onto Nonspherical Lithium Niobate Nanoparticles for Second Harmonic Generation and Photothermal Applications

Abstract

Nanoparticles (NPs) containing a lithium niobate (LiNbO3) core and a gold shell are prepared using a combination of seeded-growth and layer-by-layer approaches. The method includes first the surface charge reversal of lithium niobate with branched polyethyleimine, second, the electrostatic binding of gold seeds, and third, successive reduction steps of gold chloride onto the gold-seeded LiNbO3 NPs for the progressive and surface-directed growth of the gold shell. The influence of three synthesis parameters, namely, pH, initial density of gold seeds covering the lithium niobate core, and gold chloride concentration, on the NPs’ characteristics (structural properties, plasmon band, surface charge, hydrodynamic diameter) is studied. The progress of the gold shell growth is investigated by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. In addition, it is shown that LN@Au core–shell NPs can emit a second harmonic generation signal when excited by a femtosecond laser. Finally, photothermal properties are studied, showing an increase of temperature of 8.6 °C upon infrared excitation, with an estimated light-to-heat conversion efficiency of 40% and a specific absorption rate of 8000 W g−1.