Enormous Hyper-Rayleigh Scattering from Nanocrystalline Gold Particle Suspensions

Abstract

The recent emergence of advanced technological applications for colloidal gold suspensions and related particle assemblies and interfaces has created a demand for new chemical and physical techniques with which to characterize them. For macroscopic samples/interfaces, coherent second harmonic generation (SHG) has proven itself a useful characterization tool due, at least in part, to metal-based plasmon enhancement. In an effort to defeat or bypass the size restrictions inherent to SHG, we have utilized a related incoherent methodology, hyper-Rayleigh scattering (HRS), to interrogate aqueous colloidal suspensions of 13 nm diameter gold particles. The nanoscale particles have proven to be remarkably efficient scatterers; when evaluated in terms of the first hyperpolarizability ( ), HRS signals from the gold particles substantially surpass those observable from the best available molecular chromophores. Moreover, the present experiments indicate that is highly sensitive to colloid aggregation and imply that HRS is an effective tool for the characterization of symmetry-reducing perturbations of nanoscale interfaces.The intentional miniaturization of metal/solution interfaces to the point where colloidal stabilization of suspensions of solvated metal particles occurs has been possible, at least at a rudimentary level, for more than a century. Arguably, however, only over the past decade has the concept generated widespread excitement among chemists and materials scientists. Fueling the contemporary interest are demonstrated and emerging applications involving medical screening, 1 chemical sensing, 2 singleelectron conduction (Coulomb blockade behavior), 3 optical frequency tripling, 4 and new materials-assembly schemes. [5][6][7] Accompanying the new applications chemistry is a need for interface characterization. Among the more attractive methodologies for macroscopic metal/solution interface characterization is optical second harmonic generation (SHG). 8 For otherwise centrosymmetric metal structures, interface formation creates an asymmetry providing for interface localized signal generation. SHG has been utilized to interrogate surface symmetry, 9,10 surface charge, 11 adsorbate coverage, 12 and/or adsorbate orientation 13 on gold surfaces as well as to interrogate gold particles at the liquid/air interface. 14 However, the coherent nature of conventional SHG and the necessity of utilizing a finite wavelength of radiation place a practical lower limit on sample size. 15 On the other hand, implementation of incoherent second harmonic generation or hyper-Rayleigh scattering (HRS) methods 16 should circumvent the interface size limitation (albeit at the expense of much smaller signal intensity). 17,18 On the basis of a successful application of HRS to nanoscale silica/water interfaces (surface acidity characterization) 19 and on the basis of a brief report of HRS from silver colloid samples (molecular adsorption effects), 20 we reasoned that the methodology might be applicable to nanoscale gold/water interfaces...

Hyper-Rayleigh scattering studies of silver, copper, and platinum nanoparticle suspensions

“…This work follows several recent reports of substantial HRS from nanocrystalline gold particle suspensions [7][8][9][10] and reports of SHG from highly concentrated gold nanoparticle samples [11,12]. HRS signals from gold nanoparticles were observed to exceed by factors of up to 10 5 those observed from suspensions of similarly sized wide bandgap semiconductor particles [13] and silicondioxide insulator particles [14].…”

“…HRS measurements were performed by using a previously described setup [7]. Briefly, the output from a mode-locked Ti:sapphire laser (82 MHz, 80 fs pulses, 1.0 W average power at 820 nm) was focused into a solution of nanoparticles.…”

“…In contrast, similarly sized tetrahedral platinum particles yielded no detectable HRS. Recalling that b 0 gold is similarly large ð$ 2800 Â 10 À30 esu for incident light at 820 nm [7]), it seems clear that the key difference between platinum and the coinage metals is the absence of a visible-region plasmon band. (The values reported in [7] are incorrectly labeled b/atom; they are, in fact, b 0 values.…”

“…Recalling that b 0 gold is similarly large ð$ 2800 Â 10 À30 esu for incident light at 820 nm [7]), it seems clear that the key difference between platinum and the coinage metals is the absence of a visible-region plasmon band. (The values reported in [7] are incorrectly labeled b/atom; they are, in fact, b 0 values. The seemingly much smaller values reported in [10], on the other hand, are values of b/atom.…”

Nonlinear optics on nano/micro-hierarchical structures on metals: focus on symmetric and plasmonic effects

“…To account for the hyper-Rayleigh scattering (HRS) [37,38], the reflected SHG light was collected through a focusing lens with a large numerical aperture. Here, HRS is incoherent second-harmonic (SH) scattering from small feature structures based on Mie theory; up to now, HRS signals from spherical and non-spherical lacking centrosymmetric structures have been observed [39,40]. Hence, the consideration of HRS is important, and the experimental setup has been referred to in some papers [38,41].…”