Strongly Coupled Plasmonic Modes on Macroscopic Areas via Template-Assisted Colloidal Self-Assembly (original) (raw)
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The Journal of Physical Chemistry Letters, 2013
We investigated the near-and far-field response of 1D chains of Au nanoparticles (NPs) fabricated with high structural control through template guided self-assembly. We demonstrate that the density of poly(ethylene glycol) ligands grafted onto the NP surface, in combination with the buffer conditions, facilitate a systematic variation of the average gap width (g) at short separations of g < 1.1 nm. The overall size (n) of the individual clusters was controlled through the template. The ability to independently vary n and g allowed for a rational tuning of the spectral response in individual NP clusters over a broad spectral range. We used this structural control for a systematic investigation of the electromagnetic coupling underlying the superradiant cluster mode. Independent of the chain length, plasmon coupling is dominated by direct neighbor interactions. A decrease in coupling strength at separations ≲0.5 nm indicates the presence of nonlocal or quantum-mechanical coupling mechanisms. Figure 4. (a) HRTEM pictures of 1D clusters formed from NP NC in T40 buffer; scale bar represents 50 nm. (b) Cumulative probability plots for the interparticle gap distributions obtained under three different experimental conditions as defined in the legend.
Langmuir : the ACS journal of surfaces and colloids, 2012
The plasmonic properties of self-assembled layers of rod- and branched-shaped gold nanoparticles were investigated using optical techniques. Nanoparticles were synthesized by a surfactant-guided, seed-mediated growth method. The layers were obtained by gradual assembly of nanoparticles at the interface between a polar and a nonpolar solvent and were transferred to a glass slide. Polarization and angle-dependent extinction measurements showed that the layers made of gold nanorods were governed by an effective medium response. The response of the layers made by branched gold particles was characterized by random light scattering. Microscopic mapping of the spatial mode structure demonstrates a uniform optical response of the nanoparticle layers down to a submicrometer length scale.
Self-orienting nanocubes for the assembly of plasmonic nanojunctions
Nature Nanotechnology, 2012
Plasmonic hot spots are formed when metal surfaces with high curvature are separated by nanoscale gaps and an electromagnetic field is localized within the gaps. These hot spots are responsible for phenomena such as subwavelength focusing 1,2 , surface-enhanced Raman spectroscopy 3 and electromagnetic transparency 4 , and depend on the geometry of the nanojunctions between the metal surfaces 5 . Direct-write techniques such as electron-beam lithography can create complex nanostructures with impressive spatial control 6 but struggle to fabricate gaps on the order of a few nanometres or manufacture arrays of nanojunctions in a scalable manner. Self-assembly methods, in contrast, can be carried out on a massively parallel scale using metal nanoparticle building blocks of specific shape 7,8 . Here, we show that polymer-grafted metal nanocubes can be self-assembled into arrays of one-dimensional strings that have well-defined interparticle orientations and tunable electromagnetic properties. The nanocubes are assembled within a polymer thin film and we observe unique superstructures derived from edge-edge or face-face interactions between the nanocubes. The assembly process is strongly dependent on parameters such as polymer chain length, rigidity or grafting density, and can be predicted by free energy calculations. A significant challenge in the self-assembly of shaped nanoparticles is the formation of non-close-packed nanoparticle groupings that adopt specific interparticle orientations, which precludes the use of assembly methods that involve colloidal crystallization or jamming. Several strategies have been developed for achieving programmable assembly through anisotropic chemical modification of the nanoparticle surface, including the use of DNA linkers 9,10 , block co-polymers 11,12 or patchy particles 13-15 . However, these approaches are solution-based, require extensive chemical modification of nanoparticle surfaces, and ultimately necessitate post-processing to transfer the nanoparticle assemblies into a dielectric material or onto a solid support.
2019
Chains of metallic nanoparticles sustain strongly confined surface plasmons with relatively low dielectric losses. To exploit these properties in applications, such as waveguides, the fabrication of long chains of low disorder and a thorough understanding of the plasmon-mode properties, such as dispersion relations, are indispensable. Here, we use a wrinkled template for directed self-assembly to assemble chains of gold nanoparticles. With this up-scalable method, chain lengths from two particles (140 nm) to 20 particles (1500 nm) and beyond can be fabricated. Electron energy-loss spectroscopy supported by boundary element simulations, finite-difference time-domain, and a simplified dipole coupling model reveal the evolution of a band of plasmonic waveguide modes from degenerated single-particle modes in detail. In striking difference from plasmonic rod-like structures, the plasmon band is confined in excitation energy, which allows light manipulations below the diffraction limit. T...
Journal of the Optical Society of America B, 2012
We investigate nanostructured surfaces consisting of a hexagonal lattice of polymeric pillars embedded in a gold matrix. These systems are prepared by a new fabrication technique based on plasma assisted deposition and colloidal lithography. A complete characterization of such surfaces is performed by angle resolved reflectance and transmittance measurements. Both delocalized and localized plasmonic modes can be identified: their reciprocal interplay allows to observe spectral features and to detect refractive index changes related to one of the sample interfaces by measurements performed with a light beam incident from the opposite side. This intriguing behaviour, together with ease of use and low cost of the deposition procedure, make this kind of nanostructures particularly interesting in biosensing applications.
Plasmonic Shaping in Gold Nanoparticle Three-Dimensional Assemblies
The Journal of Physical Chemistry C, 2013
When a large number of similar gold particles are organized into complex architectures, the dipolar plasmon spectrum of the individual plasmonic entities gives rise to a broader, red-shifted feature centered around 750 nm. In this work, we show that superstructures fabricated using the convective assisted capillary force assembly method (CA-CFA) and excited at that wavelength display a subwavelength patterning of their optical field intensity that results from the self-consistent coupling between the colloidal nanoparticles. First, we demonstrate the fabrication of shape-controlled threedimensional assemblies of metallic nanocrystals using the CA-CFA method. In a second step, the absorption band resulting from the mutual coupling between the metallic building blocks is exploited to excite a coupled plasmon mode and map the twophoton luminescence (TPL) by scanning a tightly focused light beam. Highly resolved TPL images show that the morphology of the plasmonic particle assemblies has a strong impact on their optical response. A model based on a rigorous optical Gaussian beam implementation inside a generalized propagator derived from a three-dimensional Green dyadic function accurately reproduces the TPL maps revealing the influence of interparticle separation and thus coupling between the individual particles. Finally, we show that the spatial distribution of the electric field intensity can be controlled by tuning the linear polarization of the optical excitation.
Touching Gold Nanoparticle Chain Based Plasmonic Antenna Arrays and Optical Metamaterials
The control of light−material interactions at the nanoscale requires optical elements with sizes much smaller than the wavelength of light. Plasmonic nanostructures and optical metamaterials enable drastic control and manipulation of light at such small scales. However it is quite challenging to further reduce the size of resonant elements using conventional plasmonic nanostructures. In this paper, we propose novel optical resonators that rely on the conducting plasmon mode of touching nanoparticle chains that enable significant size reduction when compared with widely used nanostripe antennas and U-shaped split-ring resonators. We employ full-field electromagnetic simulations to study the resonance mechanisms of nanoparticle chain arrays. In comparison with the nanobar plasmonic antennas, a nanoparticle chain based antenna with similar physical sizes operates at larger wavelengths, opening routes for deep subwavelength plasmonic resonators. Moreover, using nanoshell chain arrays, we demonstrate an optical resonator that is 10 times smaller than the resonance wavelength (λ/10). Similarly, nanoparticle-based split-ring resonators provide significant size reduction that could be used for smaller metamaterial and metasurface building blocks. Designing nanoparticle-based resonant elements is a promising route for achieving optical metamaterials with smoother resonance dispersion and lower optical losses. M etal nanostructures support localized surface plasmon resonances at optical frequencies that can be controlled by the size, shape, and dielectric permittivity of the environment. 1−6 The field of plasmonics is dedicated to the study of optical properties of various metal nanostructures including chemically synthesized metal nanoparticles (bottom-up), 7 as well as top-down fabricated 8 metallic gratings, antennas, and nanostructure arrays, enabling strong light−material interactions such as enhanced scattering and absorption and highly localized electric fields. Similarly, optical metamaterials are composed of metal nanostructures with unique geometries, facilitating localized electric and/or magnetic resonances. 9−15 Initially, the fields of plasmonics and metamaterials progressed as two separate disciplines with very minimal overlap. However, after realization of optical metamaterials using plasmonic nanostructures, the interactions and exchange between the fields of plasmonics and metamaterials are increased drastically. Here, we propose novel metamaterial building blocks composed of metal nanoparticle chains that strongly benefit between the synergy of plasmonics and metamaterials. Although localized surface plasmon resonances (LSPRs) of metallic nanoparticles are very effective in increasing the interactions between photons and materials, one can further enhance these interactions using coupled plasmonic nano-particles, 5 such as dimers, 16−18 trimers, 19 and an even larger number of nanoparticles resulting in nanoclusters. 20−24 The distance between plasmonic nanoparticles turns out to be an extremely important parameter governing the coupling between the nanoparticles. Recently, numerous studies have revealed exciting quantum plasmonic effects for metal nanoparticles that are separated by a distance of 0.5 nm or below. 25−28 Although much attention has been devoted to dimers with ultrasmall gap distances, it is worth investigating what happens when particles are brought into contact. It has been shown that touching nanoparticles behave like a single nanoparticle that supports the conducting plasmon mode but not the hybridized resonance mode. Effectively, one can model the resulting nanostructure as a single nanoparticle rather than two individual nanoparticles. Touching nanoparticles and nanodisks have been also described in detail using coordinate transformation. 29,30 In this article, we propose ultrasmall optically resonant elements based on touching nanoparticle chains with unique advantages over conventional top-down-fabricated plasmonic antennas and metamaterials. Electromagnetic wave simulations predict that the resonance wavelength of nanoparticle chain based metamaterial arrays are much larger than their counterparts that are formed from stripe and split-ring resonator arrays with the same physical size. Moreover, metal nanoparticle chain based metamaterials provide stronger field localization around the contact points, paving the way toward highly sensitive, ultracompact biosensors. We also extend our studies to
Self-Assembled Plasmonic Nanoparticle Clusters
Science, 2010
Optical Nanoengineering Optics and electronics operate at very different length scales. Surface plasmons are light-induced electronic excitations that are being pursued as a route to bridge the length scales and bring the processing speed offered by optical communication down to the size scales of electronic chip circuitry. Now, Fan et al. (p. 1135 ) describe the self-assembly of nanoscale dielectric particles coated with gold. Functionalization of the gold surface with polymer ligands allowed controlled production of clusters of nanoparticles. The optical properties of the self-assembled nanostructures depended on the number of components within the cluster and each structure could be selected for its unique optical properties. Such a bottom-up approach should help in fabricating designed optical circuits on the nanoscale.