Long-range interaction in regular metallic nanostructure arrays can provide the possibility to manipulate their optical properties, governed by the excitation of localized surface plasmon (LSP) resonances. When assembling the nanoparticles in an array, interactions between nanoparticles can result in a strong electromagnetic coupling for specific grating constants. Such a grating effect leads to narrow LSP peaks due to the emergence of new radiative orders in the plane of the substrate, and thus, an important improvement of the intensity of the local electric field. In this work, we report on the optical study of LSP modes supported by square arrays of gold nanodiscs deposited on an indium tin oxyde (ITO) coated glass substrate, and its impact on the surface enhanced Raman scattering (SERS) of a molecular adsorbate, the mercapto benzoic acid (4-MBA). We estimated the Raman gain of these molecules, by varying the grating constant and the refractive index of the surrounding medium of the superstrate, from an asymmetric medium (air) to a symmetric one (oil). We show that the Raman gain can be improved with one order of magnitude in a symmetric medium compared to SERS experiments in air, by considering the appropriate grating constant. Our experimental results are supported by FDTD calculations, and confirm the importance of the grating effect in the design of SERS substrates.

Nanometric gaps in plasmonic structures can lead to huge optical near fields and, related, to strongly enhanced interaction of molecules and light. Nanocavities formed by sphere-on-film or cube-on-film systems were recently established as promising systems, eventually reaching the strong coupling regime. However, such structures are limited with respect to being bound to a surface and by having no means of adjusting the resonance wavelength to the requirements of arbitrary analytes, independent of the gap width. We suggest and investigate in this paper silver-gold compound nanocuboid dimers in colloidal solution as potential structures to mitigate both limitations. We analyze the dimers' plasmonic properties by a combination of optical spectroscopy, electron energy loss spectroscopy, and numerical simulations, focusing on the longitudinal dimer geometry and the dominant light-coupled plasmon mode. We then calculate optical field enhancements under light and electron excitation to assess the cuboid dimer's potential in sensing and spectroscopy applications.

Site-selective surface functionalization of anisotropic gold nanoparticles represents a major breakthrough for fully exploiting nanoparticle anisotropy. In this paper, we explore an original strategy for the regioselective functionalization of lithographically designed gold nanorods (AuNRs), based a combination of photo-induced plasmon excitation and aryl diazonium salt chemistry.

The controlled assembly of anisotropic plasmonic nanoparticles (NPs) into highly SERS-active substrates remains particularly challenging for the production of long-term stable NP assemblies in suspension. In this work, we report a simple and efficient strategy to assemble gold nanorods (AuNRs) into dimers. The pH-dependent assembly was triggered using the bifunctional molecular linker BPE (1,2-bis(4-pyridyl)ethylene) and quenched with silver nitrate. The resulting AuNR dimers were encapsulated in mesoporous silica shell and proved to be stable in water for at least 5 months. Taking advantage of the large Raman scattering cross-section of the linker BPE, we conducted a detailed study of the enhancement ability of these NR dimers using solution-based surface enhanced Raman scattering (SERS). Both experimental (SERS) and theoretical (discrete dipole approximation) studies of the near-field characteristics revealed a two-orders of magnitude increase of the SERS enhancement factor for the dimers as compared to isolated AuNRs. Besides thermal and colloidal stability, mesoporous silica coating of AuNRs imparts other notable advantages due to its porosity and biocompatibility, which make these core-shell plasmonic platforms promising for future bio-applications.

Controlling the surface grafting of species at the nanoscale remains a major challenge, likely to generate many opportunities in materials science. In this work, we propose an original strategy for chemical surface functionalization at the nanoscale, taking advantage of localized surface plasmon (LSP) excitation. The surface functionalization is demonstrated through aryl film grafting (derived from a diazonium salt), covalently bonded at the surface of gold lithographic nanostripes. The aryl film is specifically grafted in areas of maximum near field enhancement, as confirmed by numerical calculation based on the discrete dipole approximation method. The energy of the incident light and the LSP wavelength are shown to be crucial parameters to monitor the aryl film thickness of up to ∼30 nm. This robust and versatile strategy opens up exciting prospects for the nanoscale confinement of functional layers on surfaces, which should be particularly interesting for molecular sensing or nanooptics.

In this article, we aim to investigate the sensitivity of regular arrays of hybrid plasmonic nanostructures to variations in properties of the local environment (temperature, refractive index, polymer thickness), in the context of sensing and active plasmonic applications. A proper description and characterization of such hybrid systems is indeed essential in order to provide designed criteria for efficient stimuli-responsive devices. As an ideal model, we introduce a novel kind of hybrid plasmonic core-shell system made of lithographic gold nanorods (GNRs), coated by a thermosensitive polymer shell based on poly(N-isopropylacrylamide) (pNIPAM). The grafting of the polymer on the GNRs results from a multistep but simple approach in order to confine the pNIPAM brushes on the GNRs and to control the thickness of the polymer coating. We show that the optical response of the plasmonic hybrid structures (GNR@pNIPAM) is strongly modified upon a variation of the external temperature, due to a physical change of the conformation of the polymer coating. These thermo-induced changes of the optical properties can be optimized by changing the aspect ratio of the GNRs and the polymer thickness to obtain very efficient optical reporters of the polymer state in a controlled and reversible manner. This work could provide an important step toward the use of GNR@pNIPAM structures for applications spanning from opto-mechanical modulators to nanoscale adhesion and molecular sensing.

We investigated composite films of gold nanoparticles (NPs)/liquid crystal (LC) defects as a model system to understand the key parameters, which allow for an accurate control of NP anisotropic self-assemblies using soft templates. We combined spectrophotometry, Raman spectroscopy, and grazing incidence small-angle X-ray scattering with calculations of dipole coupling models and soft sphere interactions. We demonstrate that dense arrays of elementary edge dislocations can strongly localize small NPs along the defect cores, resulting in formation of parallel chains of NPs. Furthermore, we show that within the dislocation cores the inter-NP distances can be tuned. This phenomenon appears to be driven by the competition between "soft (nano)sphere" attraction and LC-induced repulsion. We evidence two extreme regimes controlled by the solvent evaporation: (i) when the solvent evaporates abruptly, the spacing between neighboring NPs in the chains is dominated by van der Waals interactions between interdigitated capping ligands, leading to chains of close-packed NPs; (ii) when the solvent evaporates slowly, strong interdigitation between the is avoided, leading to a dominating LC-induced repulsion between NPs associated with the replacement of disordered cores by NPs. The templating of NPs by topological defects, beyond the technological inquiries, may enable creation, investigation, and manipulation of unique collective features for a wide range of nanomaterials.

We describe a simple and effective strategy to couple gold nanorods (GNRs) into end to end dimers and freeze the assembly in water. The assembly is initiated using cysteine and driven by hydrogen bonding between two cysteine. We show that the aging of GNRs samples impacts both the assembly kinetics and the final yield of GNRs dimers. The addition of an appropriate amount of silver nitrate induces the immediate termination of GNRs dimerization and stabilizes the small aggregates in solution for at least 24 h.

In the presence of oriented smectic liquid crystal defects, hybrid systems of nanoparticles/liquid crystals form straight chains of nanoparticles of length longer than tens of micrometers and width equal to one single nanoparticle. The interparticle distance in a chain can be varied between a few micrometers and 1.5 nm, highlighting the control of optical absorption by light polarization monitored by gold nanoparticle concentration.

We introduce a novel experimental and analytical method for discerning rare surface-enhanced Raman scattering (SERS) events observable at the nanoscale. We show that the kinetics of the Raman activity recorded on an isolated nanostructure is punctuated by intense and rare events of large amplitude and spectral variations. The fluctuations of thousands of SERS spectra were analyzed statistically in terms of power density functions, and the occurrence of the rare events was quantified by a wavenumber statistics. Our analysis enables one to extract valuable and unique spectroscopic signature of Raman variations usually hidden in time-average or space-average measurements. We illustrate our approach using molecular surface dynamics of gold adatoms on nanoparticles.

The grafting of stimuli-responsive polymer brushes on plasmonic structures provides a perfectly controlled two-dimensional active device with optical properties that can be modified through external stimuli. Herein, we demonstrate thermally induced modifications of the plasmonic response of lithographic gold nanoparticles functionalized by thermosensitive polymer brushes of (poly(N-isopropylacrylamide), PNIPAM). Optical modifications result from refractive local index changes due to a phase transition from a hydrophilic state (swollen regime) to a hydrophobic state (collapsed regime) of the polymer chains occurring in a very small range of temperatures. The refractive index of the polymer in aqueous solution is estimated in both states, deduced from the discrete dipole approximation (DDA) method. The combination of lithographic gold NPs and thermoresponsive polymer chains leads to a new generation of perfectly calibrated and dynamically controlled hybrid gold/polymer system for real-time nanosensors.

In the present work, the combination of chemical immobilization with electron beam lithography enables the production of sensitive and reproducible SERS-active areas composed of stochastic arrangements of gold nanoparticles. The number of nanoparticles was varied from 2 to 500. Thereby a systematic analysis of these SERS-active areas allows us to study SERS efficiency as a function of the number of nanoparticles. We found that the experimental parameters are critical, in particular the size of the SERS-active area must be comparable to the effective area of excitation to obtained reproducible SERS measurements. The sensitivity has also been studied by deducing the number of NPs that generate the enhancement. With this approach we demonstrates that the maximum enhancement, the best sensitivity, is obtained with the smallest number of nanoparticles that is resonant at a given excitation wavelength.

Surface-enhanced Raman spectroscopy (SERS) is now a well-established technique for the detection, under appropriate conditions, of single molecules (SM) adsorbed on metallic nanostructures. However, because of the large variations of the SERS enhancement factor on the surface, only molecules located at the positions of highest enhancement, so-called hot-spots, can be detected at the single-molecule level. As a result, in all SM-SERS studies so far only a small fraction, typically less than 1%, of molecules are actually observed. This complicates the analysis of such experiments and means that trace detection via SERS can in principle still be vastly improved. Here we propose a simple scheme, based on selective adsorption of the target analyte at the SERS hot-spots only, that allows in principle detection of every single target molecule in solution. We moreover provide a general experimental methodology, based on the comparison between average and maximum (single molecule) SERS enhancement factors, to verify the efficiency of our approach. The concepts and tools introduced in this work can readily be applied to other SERS systems aiming for detection of every single target molecule.

The surface plasmon fields of gold nanorods with a diameter of 100 nm and lengths of 1–5 \(\mu\)m are imaged by using far-field Raman scattering of methylene blue adsorbed on the rods. When optically exciting the nanorods under total internal reflection with wave vector and electric field vector orientations along the rod axis, the plasmon field intensity along this axis is observed to be periodically modulated. This modulation is attributable to a beating of the exciting light wave and the nanorod plasmon mode. The plasmon wavelength deduced from the beat frequency is 379 nm, which is considerably smaller than the exciting laser wavelength of 647 nm. In general, Raman imaging is shown to be a powerful technique to probe local plasmon fields using far-field spectroscopy.

Trimesitylbodipy (TMB), a new 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene derivative with bulky side groups, was synthesized in order to develop nanostructured fluorescent devices. Fluorescence properties of TMB were studied in dichloromethane, in amorphous films prepared by rapid evaporation of a solution and in micro- and nanocrystals. It was compared to the spectroscopy of Mesitylbodipy (MB) to see if the mesityl groups of TMB modify fluorescence in the solid state. A set-up including a space and time correlated photon-counting photo-multiplier was used to record fluorescence intensity and lifetime images of solids. For both dyes, the formation of fluorescent excimers was observed. There is evidence to suggest energy transfer from monomer to both fluorescent and non-fluorescent excimers. MB fluorescence is still quenched in the crystalline state but in TMB monocrystals the fluorescence comes from only one excimer with a lifetime of 9.5ns. TMB nanocrystals with a monoexponential fluorescence decay were also prepared.

We analyze the optical properties of mu m-sized rings of gold nanoparticles by the combination of extinction spectroscopy, surface enhanced Raman scattering (SERS) spectroscopy and microscopic dark field imaging in two variants. The imaging process relies on either SERS from methylene blue dye molecules adsorbed on the nanoparticles or elastically scattered light. Whereas elastically scattered light images are governed by the coherence and intensity of the light scattered from the particles, SERS images reflect the optical near field of the particles averaged incoherently over a surface area corresponding to the point spread function of the microscope. From the analysis of the extinction spectra, scattered light and SERS images, we find that near field interaction of the single gold nanoparticles in the rings plays a minor role. Both scattered light and SERS images are well reproduced by simple model calculations. Due to the different signal generation and coherence properties, the combination of both imaging methods is a useful means in the characterization of optical properties of nanostructured metal surfaces.

Polymethylthiophene polybutylthiophene and polyoctylthiophene films have been synthesized on electrochemically roughened gold electrodes by anodic oxidation of the corresponding monomers. In situ and ex situ surface-enhanced Raman scattering (SERS) studies of these films lead to high quality spectra the intensity of which is more than one hundred times greater than that obtained, under identical conditions, on platinum electrodes. Marked modifications of the positions, widths and relative intensities of the Raman bands observed during the polymer doping–dedoping process are attributed to structural transformations. During the redox process, the oxidized to reduced intensity ratio of the symmetric ν(CC) mode of polyalkylthiophene ring obeys a Nernst equation when the applied potential to the working electrode is varied. In addition, the solubility of polyalkylthiophenes in organic solvents allows analysis of their SERS spectra in colloidal silver solutions. The very low concentration (10−7–10−10M) and the mild experimental conditions used in these experiments indicate that a huge amplification of the Raman signal takes place.

The vibrational dephasing rate is calculated for H2-rare gas mixtures between 85 and 300 K. The semiclassical calculation which only considers unbounded trajectories is based on binary interaction of particles through a Lennard—Jones potential. A comparison is made with three-dimensional and with collinear results obtained when a purely repulsive potential is considered. It is shown that the temperature dependence of the vibrational dephasing rate increases monotonically as T rises. Experimental investigation of Q1 (1) Raman lines for n-H2 perturbed by neon, argon and krypton between 100 and 300 K has been realized. The comparison between calculated and measured linewidths suggests that bounded states have to be taken into account at the lowest temperatures considered here.

Rotation and rotation–vibration Raman spectra of HD compressed by argon were recorded at room temperature (72 to 695 amagat) and 175 K (95 to 364 amagat). At low densities, an important motional narowing in the Q branch conceals the vibrational broadening, while this last mechanism becomes the main contribution to the bandwidth when pressure is sufficiently increased. Discussion of the results in the impact limit is given for the low density range where a linear density dependence of the rotational linewidths is observed. The validity of second order calculations in a semiclassical approach is discussed. The case of the HD–Ar system is compared to that of pure HD for which new data complete previous experimental studies.

A detailed comparison of two extensions of the Debye model for molecular reorientation in liquids, Gordon’s extended J-diffusion model and the rotational Fokker–Planck–Langevin model, is presented. It is shown that the two models, although they represent very different physical pictures of rotational dynamics in fluids, lead to remarkably similar reorientational correlation functions, memory functions, correlation times and spectral densities. Only for values of the angular momentum correlation time appropriate to low density fluids do the two models give significantly different results, but the validity of the FPL model is questionable in this region. The results of an infrared study of the ν3 band of N2O in N2O/O2 along the coexistence curve from 105–153 K, and in N2O/N2 along the 80 bar isobaric line from 100–300 K are presented. The reorientational correlation functions from these measurements, and from earlier infrared studies of CO/N2 and Raman studies of CF4 and N2O liquids are compared with correlation functions computed with the EDJ and FPL models. Neither the EDJ nor the FPL model gave a substantially better description of the spectral band shape data.