Optical studies of single metal nanoparticles

The optical properties of metal nanoparticles are dominated by the surface plasmon resonance (SPR), which is a collective oscillation of the conduction electrons of the particle. The SPR is responsible for the brilliant colors of solutions of Ag and Au nanoparticles, and also gives rise to field enhancements at the surface of the particles, which are the basis of surface enhanced spectroscopies such as surface enhanced Raman spectroscopy (SERS). The frequency of the SPR, and the size of the field enhancements, depend sensitively on the size and shape of the particles, and the presence of nearby particles. Because samples of metal nanoparticles are inherently heterogeneous, the question of how size, shape and couplings between particles affect their optical properties is best studied by single particle experiments.

This themed issue in PCCP brings together a number of papers on optical studies of single metal nanoparticles. One of the major themes is understanding how couplings between particles affect their spectra and the field enhancements. The Perspective by Tong et al. (DOI: 10.1039/C3CP44361B) reviews recent progress in understanding electromagnetic couplings in dimers and aggregates, with an emphasis on effects such as Fano resonances, and new quantum phenomena in systems with very small gaps. The effect of gap size is also the focus of the Perspective by Misawa and co-workers (DOI: 10.1039/C2CP43681G), who review recent experimental work on the optical properties of fabricated Au nanoparticle dimers. In particular, they discuss how the SPR spectrum and two-photon luminescence depend on gap size. The article by Shimada et al. (DOI: 10.1039/C3CP43128A) describes near field optical microscopy studies that allow the optical fields in one-dimensional arrays of Au nanospheres to be visualized. Comparison to finite difference time-domain calculations reveals the role of localized and propagating plasmon modes in the response. Knappenberger and co-workers (DOI: 10.1039/C3CP43271D) used a different technique – second harmonic generation – to examine the fields between dimers of Au nanoparticles. Polarization resolved measurements reveal that the fields are chiral for certain dimers, due to non-zero magnetic dipolar contributions to the nonlinear optical response. Mulvaney and co-workers discuss more conventional Rayleigh scattering measurements of end-to-end linked Au nanorod dimers and trimers, which are sensitive to the coupling (DOI: 10.1039/C3CP44657C). The article by Cheng et al. (DOI: 10.1039/C3CP43270J) examines the electromagnetic fields at the surface of arrays of metal nanoparticles using scattering near-field optical microscopy. The measurements show high field enhancements in the gaps between the nanoparticles, and that there is a significant difference in the phase of the field for the gaps compared to the particles. These results are significant for understanding hot-spots in SERS substrates. These papers show interesting effects due to interfaces. In contrast, the paper by Ahn et al. (DOI: 10.1039/C3CP43365F) shows that the longitudinal plasmon resonance of single AgAuAg nanorods is insensitive to composition. This implies that the interfaces between metals within a single nanoparticle do not affect the plasmon oscillation.

One of the main consequences of the high fields created by metal nanoparticle junctions is an increase in the Raman scattering cross-section. Suh and co-workers used correlated atomic force microscopy and optical measurements (Rayleigh scattering and SERS) to study dimers and trimers of Au–Ag core–shell nanoparticles (DOI: 10.1039/C3CP43817A). These experiments provide an understanding of how the structure of the dimer or trimer determines the wavelengths and polarization that give the best SERS response. SERS is also the focus of the article by Jeong and co-workers (DOI: 10.1039/C2CP43252H), where it was shown that junctions between nanoparticles and nanowires result in 60× enhancement of Raman scattering. The enhancement is strongly polarization dependent, indicating significant antenna effects from the longitudinal SPR modes of the nanowire. The paper by Murakoshi and co-workers (DOI: 10.1039/C3CP43728K) also involves the SERS effect, however, here the authors use SERS from carbon nanotubes to examine heating in metal nanostructures from laser illumination. Creating new substrates for SERS is also an important topic. Ren and co-workers (DOI: 10.1039/C3CP43857K) discuss the synthesis of Au nanoparticles with controllable size and surface roughness. These materials provide a reproducible substrate for SERS studies, which may have advantages compared to aggregates. Developing new nanoparticle systems is also the focus of the article by Wongravee et al., (DOI: 10.1039/C2CP42758C) who present optical and TEM measurements of shape evolution (nanospheres to nanoplates) of Ag nanoparticles under reactive conditions.

Theory development is also important for this field. The article by Ishihara and co-workers (DOI: 10.1039/C2CP43442C) describes self-consistent calculations of the optical response of a plasmonic nanoparticle coupled to a semiconductor nanoparticle, or a molecular system. The authors show how the optical response of the system depends on structure, and also investigate nanogaps. These calculations are complex, and in many cases, it is valuable to quickly compare measurements to calculated spectra. The article by Le Ru and co-workers (DOI: 10.1039/C3CP44124E) presents simple closed form expressions for the absorption and scattering of metal nanoparticles of arbitrary diameter, that go beyond the well known quasi-static limit Mie theory expression. The results are compared to the full solution of Maxwell's equations, and show improvement compared to previous work in this area.

Single nanoparticles are also being used in a variety of applications. The Perspective by Van Duyne and co-workers (DOI: 10.1039/C3CP44574G) gives an overview of recent progress in using single metal nanoparticles as sensors. Both far-field and near-field experiments are discussed, along with the advantages and disadvantages of the different techniques. The article by Fairbairn et al. (DOI: 10.1039/C2CP43162A) presents hyperspectral imaging results for single hollow Au nanoparticles. This technique allows light scattered from the nanoparticle to be differentiated from the background, increasing speed and selectivity. These advantages make hyperspectral imaging a promising technique for revealing interactions between NPs and cells, for example. The ability to collect data quickly is at the core of correlation spectroscopies, which are a valuable tool for understanding microenvironments in biological systems. Metal nanoparticles can be used in correlation spectroscopies in several ways. Correlation measurements are typically done by luminescence. The paper by Loumaigne et al. (DOI: 10.1039/C2CP43294C) presents power dependent measurements of luminescence from single Au nanoparticles in solution. The results show that the Au NPs do not blink, making them potentially superior to quantum dots or molecules. The luminescence intensity was observed to increase nonlinearly with intensity, which was attributed to a heating effect of the nanoparticle. The article by Cichos and co-workers (DOI: 10.1039/C3CP44092C) discusses photothermal correlation spectroscopy of metal nanoparticles in solution. The authors present an analysis of the signal that allows the extraction of sample concentrations and size distributions, as well as parameters related to the focus of the laser beams in the sample. This quantification is important for applications of this technique. Link and co-workers (DOI: 10.1039/C2CP43966B) discuss a different application: using Au nanorods incorporated into a liquid crystal to control the polarization of light. This work combines simulations and single particle measurements to understand the important factors in these devices.

This themed issue also has several articles that deal with ultrafast measurements of single nanoparticles, which is a topic near and dear to the editor's hearts. Arbouet and co-workers (DOI: 10.1039/C2CP43273K) present transient absorption and two-photon luminescence studies of single Au nanoplates. The two-phonon luminescence images reveal the near-field structure of the plasmon modes of the particles, and the transient absorption measurements give information about energy dissipation of the acoustic modes. The data and analysis indicates a weak mechanical coupling of the nanoparticles to the substrate for this system. The article by Major et al. (DOI: 10.1039/C2CP43330C) is concerned with transient absorption studies of single, suspended Au nanowires. By recording traces in air and in a liquid environment, the effect of the liquid on the damping of the acoustic vibrational modes of the nanowire can be determined. The results are in good agreement with a continuum mechanics analysis of the damping. Finally, Borri and co-workers (DOI: 10.1039/C2CP43451B) present an ultrafast study of single Au nanoparticles using a phase sensitive four-wave-mixing technique. Polarization dependent studies show that the signal has a contribution from the coherent response of the surface plasmon resonance, which is not observed in conventional transient absorption measurements. This new technique provides detailed information about the ultrafast response of these materials. The article by Blythe et al. (DOI: 10.1039/C2CP43152A) also describes new measurements: ground state depletion microscopy studies of single fluorophores attached to the surface of gold nanowires. The results show the emitting molecules couple to the plasmon modes of the nanowire, which may affect the perceived location of the fluorophore.

Overall, the collection of articles in this themed issue shows that single particle studies of metal nanoparticles is a very active, international field. These studies are extremely important for advancing applications, such as SERS and molecular sensing, as well as for improving our fundamental understanding of how metal nanoparticle interact with light as well as with their environment.

Plate1 A collection of the graphical abstracts from this themed collection

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