Gold-seeded Lithium Niobate Nanoparticles: Influence of Gold Surface Coverage on Second Harmonic Properties

Abstract

Hybrid nanoparticles composed of an efficient nonlinear optical core and a gold shell can enhance and tune the nonlinear optical emission thanks to the plasmonic effect. However the influence of an incomplete gold shell, i.e., isolated gold nano-islands, is still not well studied. Here LiNbO3 (LN) core nanoparticles of 45 nm were coated with various densities of gold nano-seeds (AuSeeds). As both LN and AuSeeds bear negative surface charge, a positively-charged polymer was first coated onto LN. The number of polymer chains per LN was evaluated at 1210 by XPS and confirmed by fluorescence titration. Then, the surface coverage percentage of AuSeeds onto LN was estimated to a maximum of 30% using ICP-AES. The addition of AuSeeds was also accompanied with surface charge reversal, the negative charge increasing with the higher amount of AuSeeds. Finally, the first hyperpolarizability decreased with the increase of AuSeeds density while depolarization values for Au-seeded LN were close to the one of bare LN, showing a predominance of the second harmonic volumic contribution.

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

“…For a seeded‐growth mechanism, the ratio between the gold seeds and the precursor of Au species to be reduced is a factor influencing the homogeneous or inhomogeneous growth of Au NPs. [ 32 ] In a previous study, we found from inductive coupled plasma atomic emission spectroscopy (ICP‐AES) analysis that the maximum surface coverage of LN by AuSeeds at ≈30% is already attained for both AuSeeds:LN ratios (1000 and 3000) [ 24 ] whereas X‐ray photoelectron spectroscopy (XPS) data revealed different Au:Nb atomic ratios at 0.91 and 1.28, respectively (Table S1, Supporting Information). Effect of these two AuSeeds densities on the final LN@Au structures and properties is thus worth to be studied.…”

“…Interestingly, if free Au NPs are observed in LN@Au samples prepared from LN@BPEI@AuSeeds3000, previous TEM investigations revealed the absence of unattached AuSeeds for every AuSeeds:LN ratio studied after their attachment to LN@BPEI. [ 24 ] Upon addition of NH 2 OH, HCl to catalyze the reduction of Au 3+ to Au 0 , lack of space area between the closely packed Au NPs at the surface of the LN@BPEI@AuSeeds3000 might explain the release of weakly bonded AuSeeds from the LN surface and their subsequent growth to produce unbound Au NPs. Due to the lower surface density coverage before reduction, this latter scenario is less prone to occur for the LN@BPEI@AuSeeds1000 samples that finally result in a more homogeneous population of LN@Au NPs.…”

“…Preparation of Au-Decorated LN NPs (LN@BPEI@AuSeeds NPs): The preparation of LN@BPEI@AuSeeds NPs was achieved in a two-step process as detailed previously. [24] First, AuSeeds were synthesized using the protocol reported by Duff et al [48] In brief, 540 µL of 0.1 m NaOH was added to 45.5 µL of water followed by 2.14 µL of a 4.2 mm solution of THPC. While stirring, 360 µL of 25 mm HAuCl 4 •3H 2 O solution was added to the mixture and the stirring was continued for 10 min, with the mixture undergoing a color change from colorless to brown-red within less than 30 s. The AuSeeds dispersion was stored at 4 °C and used for further procedures up to two months postsynthesis.…”

“…The preparation of LN@BPEI@AuSeeds NPs was achieved in a two‐step process as detailed previously. [ 24 ] First, AuSeeds were synthesized using the protocol reported by Duff et al. [ 48 ] In brief, 540 µL of 0.1 m NaOH was added to 45.5 µL of water followed by 2.14 µL of a 4.2 m m solution of THPC.…”

“…Nanoparticles (LN@BPEI@AuSeeds NPs) Synthesis and characterizations of LN@BPEI@AuSeeds NPs were previously described. [24] Briefly, LN core NPs exhibit an average diameter of 34 ± 12 nm as determined by transmission electron microscopy (TEM) and a hydrodynamic diameter of 135 ± 15.7 nm. [25] As shown in Figure S1 (Supporting Information), a pseudospherical shape was observed, with an average degree of sphericity of 0.8.…”