Metal Nanoparticle Arrays Research Articles

Improvement of directivity in plasmonic nanoantennas based on structured cubic gold nanoparticles

An array of metallic nanoparticles can diffract or concentrate the incident electromagnetic wave and behave as an antenna. In this paper, the effects of the inner sub-wavelength structure of nanoparticles are studied on the directivity of the plasmonic nanoantenna, which is coated on the output of a waveguide. Three 5*5 element configurations are analyzed: nanocubes, nanoshells, and nanoframes array. Numerical results are obtained using the 3D FDTD technique. The results show that structured nanoantennas can improve the antenna's directivity due to the plasmonic properties and hybridization mechanism. Between the three configurations investigated in the 250–800 nm wavelength range, the nanoshell array exhibits maximum and minimum amounts of its directivity at 321.5 nm and 591 nm, respectively. At 558 nm, nanoframes and nanoshells' arrays show the same amount of directivity, and from the wavelength greater than 558 nm, the nanoframe array has the best performance. The results may help design and fabricate directive optical fiber endcaps.

Charge transfer plasmons in nanoparticle arrays on graphene: Theoretical development

The properties of charge transfer plasmons (CTPs) in periodic metallic nanoparticle arrays (PMNPAs) on the single-layer graphene surface are studied within a computationally efficient original hybrid quantum-classical model. The model is based on the proven assumption that the carrier charge density in doped graphene remains unchanged under plasmon oscillations. Calculated CTP frequencies for two PMNPA geometries are shown to lie within the THz range and to be factorized, i.e., presented as a product of two independent factors determined by the graphene charge density and the PMNPA geometry. Equations are derived for describing the CTP frequencies and eigenvectors, i.e., oscillating nanoparticle charge values. It is shown that the CTP plasmons having a band structure containing a wave vector and a band number, like to phonons in periodic media, can be divided into an acoustic mode and optical CTP modes. For the acoustic modes, the CTP group velocity tends to zero at k→0, but reaches a value of ∼VFermi in graphene inside the Brillouin zone, while for the optical modes, the group velocity dispersion is extremely weak, although their energy is higher than the acoustic plasmon energies. It is shown that the calculated dependence of CTP frequencies on the carrier concentration in graphene is in good agreement with experimental data. We believe that the proposed model can help in designing various graphene-based terahertz nanoplasmonic devices of complex geometry due to very high computational efficiency.

Long-range molecular energy transfer mediated by strong coupling to plasmonic topological edge states

Abstract Strong coupling between light and molecular matter is currently attracting interest both in chemistry and physics, in the fast-growing field of molecular polaritonics. The large near-field enhancement of the electric field of plasmonic surfaces and their high tunability make arrays of metallic nanoparticles an interesting platform to achieve and control strong coupling. Two dimensional plasmonic arrays with several nanoparticles per unit cell and crystalline symmetries can host topological edge and corner states. Here we explore the coupling of molecular materials to these edge states using a coupled-dipole framework including long-range interactions. We study both the weak and strong coupling regimes and demonstrate that coupling to topological edge states can be employed to enhance highly-directional long-range energy transfer between molecules.

Wet Chemical Engineering of Nanostructured GRIN Lenses

AbstractGradient‐index (GRIN) lenses have long been recognized for their importance in optics as a result of their ability to manipulate light. However, traditional GRIN lenses are limited on a scale of tens of microns, impeding their integration into nanoscale optical devices. This study presents a groundbreaking self‐assembled method that overcomes this limitation, allowing for constructing GRIN lenses at an extremely small dimension. The self‐assembly process offers several advantages, including creating highly precise, scalable, cost‐effective, and complex structures that eliminate the need for intricate and time‐consuming manual assembly. By engineering densely packed arrays of metallic nanoparticles, exceptional control over the local refractive index has been achieved. This is accomplished by layer‐by‐layer assembly of gold nanoparticles of different sizes over silica beads. A GRIN lens light‐sink is built where light is preferentially directed toward the center, which is corroborated by measuring the fluorescence of Rhodamine B (RhB) in the inside. Unlike traditional bulky macroscopic GRIN lenses, light‐sinks boast a size under 2.5 µm. Notably, the self‐focusing effects of this design allowed us to track the growth of single‐nanoparticle layers using SERS (Surface‐Enhanced Raman Spectroscopy). These results pave the way for designing and developing lens‐like devices at the nanoscale, allowing unprecedented light manipulation.

Interplay between gain and loss in arrays of nonlinear plasmonic nanoparticles: toward parametric downconversion and amplification.

With the help of a theoretical model and finite-difference time-domain (FDTD) simulations based on the hydrodynamic-Maxwell model, we examine the effect of difference-frequency generation (DFG) in an array of L-shaped metal nanoparticles (MNPs) characterized by intrinsic plasmonic nonlinearity. The outcomes of the calculations reveal the spectral interplay between gain and loss in the vicinity of the fundamental frequency of the localized surface plasmon resonances. Subsequently, we identify different array thicknesses and pumping regimes facilitating parametric amplification and spontaneous parametric downconversion. Our results suggest that the parametric amplification regime becomes feasible on a scale of hundreds of nanometers and spontaneous parametric downconversion on the scale of tens of nanometers, opening up new exciting opportunities for developing building blocks of photonic metasurfaces.

Ag@Au yolk shells decorated onto silica spheres for enhanced plasmonic applications

Silica surfaces covered with a homogenous layer of metal nanoparticles exhibit superior plasmonic properties when compared with pristine colloidal counterparts, as they have higher scattering probability with enhanced potential utility. Precise galvanic replacement reactions performed on such sacrificial templates are of great interest. Herein, we demonstrate the robust formation of Ag@Au yolk shells decorated onto the dielectric silica spheres via a direct one-step galvanic replacement procedure, evidently initiating the deposition of Au onto low surface energy Ag facets (prefixed onto the silica spheres), resulting in a disjointed framework of compact dealloyed metal lobes, wherein their facets remain smooth enough with a high degree of crystallinity and exhibit maximal porosity. Our combined spectroscopic data analyses corroborate the plausible mechanism governing solely the formation of Ag@Au core-shell onto the silica spheres rather than their hollow counterparts. Furthermore, we made the respective monolayer films using the well-known air-water interface self-assembly technique, which allowed us to easily decorate different substrates with a uniform array of these surface-coupled metal nanoparticles. Enhanced refractive index (RI) sensing applications based on the sensitivity and FOM (figure of merit) calculations demonstrate the versatile nature of our synthetic protocol for direct synthesis of optical metamaterials along with their intriguing light-matter interactions, in general. Also, the plasmonic vector field of 2-D assembled nanoparticle monolayer displays intrigued SERS enhancement of crystal violet dye when compared with their respective drop-casted films, thus establishing a novel method for producing diverse meta-structures based efficient SERS substrates with promising applications in ultrasensitive chemical and biological sensing.

Tuning and total resonant suppression of reflection in the photonic bandgap range of Bragg reflector by two-dimensional nanoparticle array

The influence of a dielectric layer with an embedded 2D array of metal nanoparticles on the spectral characteristics of a distributed Bragg reflector is theoretically studied and numerically validated. A significant dependence of the reflectivity of the hybrid structure on the location of the nanoparticle array relative to the maxima and minima of the optical field in the surface dielectric layer is demonstrated. It is found that the application of a dilute ensemble of nanoparticles (the interparticle distance is from 2 to 5 times larger than the nanoparticle size) in the region of high optical field localization makes it possible to obtain a total suppression of reflection in the photonic bandgap range of distributed Bragg reflector. Contrariwise, if the optical field is almost zero at the nanoparticle array location, its impact on the scattered light is negligible, that is, the resonant nanoparticles are masked by a highly reflective photonic structure. The target wavelength can be tuned inside the photonic bandgap range by adjusting the shape of nanoparticles and interparticle distance in the array.

Conduction Mechanism Switching from Coulomb Blockade to Classical Critical Percolation Behavior in Disordered Nanoparticle Array

AbstractLarge, open‐gate transistors made from metal nanoparticle arrays offer possibilities to build new electronic devices, such as sensors. A nanoparticle necklace network (N3) of Au particles from 300 K to cryogenic temperatures exhibit a nonohmic I–Vd behavior, I ≈ (Vd–VT)ζ, where VT is a conduction gap and ζ is a constant critical exponent. The conduction gap in N3, made from disordered networks of 1D chains of 10 nm diameter Au particles exhibits room temperature (RT) gating. Although the I–Vd behavior at RT is identical to Coulomb blockade, the conduction is modulated by field‐assisted tunneling exhibiting classical critical behavior. In this study, based on three results, invariance of VT on gating, invariance of VT on temperature, and zero–bias conductance, a sharp transition temperature at ≈140 K is discovered where the conduction mechanism switches from Coulomb blockade to classical critical percolation behavior. The N3 architecture allows the reconciliation of the Coulomb blockade versus activation process as a sharp thermal transition to serve as a model system to study the exotic behavior in nanogranular‐metallic materials. The novel global critical behavior to local Coulomb blockade governed transition in these N3 architectures may potentially lead to novel sensors and biosensors.

SERS active substrates for neonicotinoids studies

Different basic substrates, – Si wafers, (001) SiO2, printer paper, Al2O3, micro-processed (001) SiO2 or diamond abrasive films, have been used to create active Ag and Au nanostructures. In this lecture, we report the use of pulsed-laser deposition and thermal deposition both followed by pulsed-laser annealing; the results are compared. Advanced substrates of Au and Ag on Si were produced in view of surface-enhanced Raman spectroscopy (SERS) detection of the imidacloprid (Nuprid 200 SP) neonicotinoid insecticide in amounts much smaller than those ordinarily applied in agricultural medicine. The SERS peaks intensity rose by at least one order of magnitude after the pulsed-laser annealing of the metal films and nanoparticles arrays formation. The enhancement factor (EF) was estimated to be >5×104, the limit of detection (LOD) reached being < 0.5 nM. The properties of the advanced substrates were compared and discussed. The importance of SERS as a relatively inexpensive and simple method is emphasized in regulating, monitoring and controlling the level of such chemicals as environmental pollutants, thus precluding harming the human and, especially, honey bees’ health.

Effects of "defective" plasmonic metal nanoparticle arrays on the opto-electronic performance of thin-film solar cells: computational study.

This computational study investigates the effects of common defects that occur while fabricating arrays of plasmonic metal nanoparticles (NPs) on the absorbing layer of the solar cells for enhancing their opto-electronic performance. Several "defects" in an array of plasmonic NP arrays on solar cells were studied. The results demonstrated no major changes in the performance of solar cells in the presence of "defective" arrays when compared to a "perfect" array with defect-free NPs. The results indicate that relatively inexpensive techniques may be used to fabricate "defective" plasmonic NP arrays on solar cells and still obtain a significant enhancement in opto-electronic performance.

Basics of the LSPR Sensors for Soft Matter at Interfaces

An important class of localized surface plasmon resonance (LSPR)–based sensors implies the fabrication of an array of plasmonic metal nanoparticles on the support in combination with a thin protective dielectric layer. If needed, this layer can be covered, e.g., by a suitable thin biological layer, e.g., a lipid bilayer with receptors. The attachment of analyte (e.g., protein molecules or vesicles) to such interfaces is tracked via its indirect optical effect on the LSPR-related peak extinction wavelength. Such sensors have been commercialized and are now used to study biological soft matter. The length scale of the local field able in probing analyte around plasmonic nanoparticles is in this case on the order of 20 nm. Conceptually, these LSPR sensors are similar to the SPR sensors which were developed much earlier. Herein, the similarities and differences in the formalisms used to interpret SPR and LSPR measurements are discussed in detail. In particular, the exponential and power-law attenuation functions employed in these formalisms to describe the drop of the field are compared from various perspectives. The applicability of the power-law attenuation function in the context of LSPR is illustrated by using a generic model describing spherically shaped plasmonic metal nanoparticles. This model is also employed to illustrate the sensitivity of LSPR sensors with respect to various quantities. Among more specific results, the available expressions for the signal reduction factor for analyte nanoparticles of various shapes are collected and complemented by new ones. In addition, the equation describing the LSPR signal related to analyte attachment to a rough surface is presented.

Surface lattice resonance effect of double-ring array of metallic nano-particles

Surface lattice resonances due to regular periodic array of metallic nanoparticles can be attributed to the mutual coupling between the localized surface plasmon resonances of different nanoparticles. A comparison of resonant effect between the single particle and the array shows that the resonance line width can be significantly reduced. In this paper, we extend the coupled dipole approximation to solving the electromagnetic characteristics of the particle ring structures with rotational symmetry, and propose an analytical model for the double ring array of metallic nano-particles. Furthermore, we derive the general resonant condition of the double ring array and investigate some concrete cases in detail. It shows that the full resonance of the whole array depends crucially on the structural parameters, whose enhancement factor can be extremely high. But a slight change in the structural parameter willlead the enhancement factor to decrease sharply. We also find that the radiation field of the full resonance effect will be independent of the external field, which provides us a simple approach to producing a localized optical field with complex space distribution. This proposed structure can possess potential applications in various fields such as metasurface, optoelectronics, optical manipulation, communication, and biosensing.

Fabrication of Centimeter-Scale Plasmonic Nanoparticle Arrays with Ultranarrow Surface Lattice Resonances.

Plasmonic surface lattice resonances (SLRs) supported by metallic nanoparticle (NP) arrays show diverse applications including nanolasers, sensors, photocatalysis, and nonlinear optics. However, to rationally fabricate high-quality plasmonic NP arrays with ultranarrow SLR line widths over large areas remains challenging. This article describes a general approach for the efficient fabrication of centimeter-scale inorganic NP arrays with precisely controlled NP size, composition, position, and lattice geometry. This method combines the processes of solvent-assisted soft lithography and in situ site-specific NP growth to reproducibly create many replicates of NP arrays without utilizing cleanroom and specialized equipment. For demonstration, we show that Au NP arrays exhibit ultranarrow SLRs with a line width of 4 nm and a quality factor of 218 toward the theoretical limit.

Lattice Resonances for Thermoplasmonics

Thanks to their ability to support localized surface plasmons, metallic nanostructures have emerged as ideal tools to transduce light into heat at the nanoscale, giving birth to the field of thermoplasmonics. When arranged in a periodic array, the localized plasmons of metallic nanostructures can interact coherently to generate a collective mode known as a lattice resonance. This collective mode, whose wavelength is controlled by the periodicity of the array, produces a stronger and more spectrally narrow optical response than that of the localized plasmons supported by the individual nanostructures. Motivated by the exceptional properties of the lattice resonances of periodic arrays of metallic nanoparticles, here, we investigate their use for applications in thermoplasmonics. Through a comprehensive analysis based on a coupled dipole model, we show that arrays supporting a lattice resonance absorb more energy per nanoparticle, and thus achieve a much larger increase in temperature under pulsed illumination conditions, than those that do not support such a mode. On the contrary, for continuous wave illumination conditions, we find that the temperature increase is mostly independent of the array period for the systems under consideration. Furthermore, by analyzing arrays with two nanoparticles per unit cell, we show that it is possible to engineer their lattice resonances to selectively absorb light in one of the nanoparticles without exciting the other. The results of this work pave the way for the development of thermoplasmonics applications exploiting the exceptional optical response and tunability provided by lattice resonances.

Formation and Characterization of 2D Closely Packed Arrays of Bare Gold Nanoparticles without Aggregation.

Uniform 2D arrays of metal nanoparticles (NPs) have received significant attention in the field of molecular sensing using localized surface plasmon resonance. Generally, metal NPs bear organic surface-modifying molecules to prevent aggregation and form 2D metal NP arrays. However, surface-modifying molecules negatively affect molecular sensing. Previously, we developed a technique for forming a 2D bare metal NP array, denoted the sandwich (SW) technique. However, the formation mechanism of these 2D metal NP arrays remains unknown and therefore the experimental conditions of the SW technique are not optimized. Here, we observed the formation of a 2D Au NP (d: 60 nm) array using the SW technique with an optical microscope. Moderate drying conditions of the colloidal droplets sandwiched between two parallel substrates were necessary for forming 2D Au NP arrays. We then optimized the drying conditions and obtained a 2D Au NP array. This array was uniform, and the Au NPs were arranged at distances of 4.5 nm with hexagonal periodicity, without aggregation. Further, the 2D Au NP arrays exhibited excellent spot-to-spot reproducibility in surface-enhanced Raman scattering.

Perfect absorption and phase singularities induced by surface lattice resonances for plasmonic nanoparticle array on a metallic film.

The formation of pairs of perfect absorption associated with phase singularities in the parameter space using the hybridized structure constructed with a metallic nanoparticle array and a metallic film is promising to enhance light-mater interactions. However, the localized plasmon resonances of the array possess strong radiative losses, which is an obstacle to improve the performances for many applications. On the contrary with the subwavelength array hybridized structure, this study shows that by enlarging the lattice spacing, the oscillator strength of the nanoparticles can be enhanced with the formation of surface lattice resonance, thereby leading to similar but much narrower pairs of perfect absorption due to the interactions with the Fabry-Pérot cavity modes. Furthermore, when the surface plasmon polariton mode shift to the same spectral range associated with the enlarged lattice spacing, the coupling and mode hybridization with the surface lattice resonance result in an anticrossing in the spectra. Although the resonance coupling does not enter the strong coupling regime, the quality factors (∼ 134) and near-field enhancements (∼ 44) are strongly enhanced for the hybridized resonance modes due to the effectively suppressed radiative losses compared with that of the localized plasmon resonances, which make the hybridized structure useful for the design of functional nanophotonic device such as biosensing, multi-model nanolasing, and high-quality imaging.

Plasmonic Surface Lattice Resonances in Suspended Symmetric Double-Layer Gratings

Surface lattice resonances (SLRs) with high-quality factors supported by metal nanoparticle arrays are useful for plasmonic nanolasers, biochemical sensors, and surface-enhanced Raman spectroscopy. Most nanoparticle arrays are fabricated on a substrate, and the refractive index mismatch between the substrate and superstrate suppresses the performance of SLRs. In this work, we propose unique SLRs excited in suspended, self-aligned symmetric double-layer gratings with index-matched environment. The self-aligned double-layer gratings are fabricated using a single-step electron beam lithography and exhibit a Fano-like spectra resulting from interference between out-of-plane plasmonic resonances and diffraction modes. By changing the incident angle and refractive index of the surrounding medium, the SLRs can be tuned from visible to near-infrared regions with a high-quality factor of 120.

Growth of Highly-Ordered Metal Nanoparticle Arrays in the Dimpled Pores of an Anodic Aluminum Oxide Template.

A reliable, scalable, and inexpensive technology for the fabrication of ordered arrays of metal nanoparticles with large areal coverage on various substrates is presented. The nanoparticle arrays were formed on aluminum substrates using a two-step anodization process. By varying the anodization potential, the pore diameter, inter-pore spacing, and pore ordering in the anodic aluminum oxide (AAO) template were tuned. Following a chemical etch, the height of the pores in the AAO membrane were reduced to create a dimpled membrane surface. Periodic arrays of metal nanoparticles were subsequently created by evaporating metal on to the dimpled surface, allowing for individual nanoparticles to form within the dimples by a solid state de-wetting process induced by annealing. The ordered nanoparticle array could then be transferred to a substrate of choice using a polymer lift-off method. Following optimization of the experimental parameters, it was possible to obtain cm2 coverage of metal nanoparticles, like gold and indium, on silicon, quartz and sapphire substrates, with average sizes in the range of 50-90 nm. The de-wetting process was investigated for a specific geometry of the dimpled surface and the results explained for two different film thicknesses. Using a simple model, the experimental results were interpreted and supported by numerical estimations.

Evolutionary optimization of the short-circuit current enhancement in organic solar cells by nanostructured electrodes

Using a particle swarm optimization algorithm (a population-based stochastic optimization technique) combined with 3D finite-difference time-domain simulations, we inverse design periodic arrays of metallic nanoparticles on indium-tin-oxide electrodes and nanoholes in metallic thin films working as electrodes in P3HT (Poly(3-hexylthiophene-2,5-diyl)):PCBM ([6,6]-Phenyl C61 butyric acid methyl ester) organic solar cells to achieve the maximum short-circuit currents (Jsc). Nanohole-array electrodes have large optical losses, leading to a net reduction of Jsc compared to a reference solar cell. On the other hand, nanoparticle arrays can lead to a significant enhancement of Jsc of up to 20%. Detailed simulations show that this enhancement is caused by the grating coupling of the incident light to surface plasmon polaritons at the interface of the metal electrode and the hole transport layer, leading to the enhancement of the electromagnetic field in the organic blend.

Simplified numerical modeling for Fano interference-induced asymmetric light reflectance effect using equivalent medium theory.

Localized surface plasmons exhibit promising capabilities in optoelectronic devices. In most cases, the metal nanoparticle arrays are located on interfaces or inside optical cavities. Fano interferences have been observed and explained via the interference between the waves generated by the localized surface plasmon and dielectric interfaces. Conventionally, these Fano interferences are modeled using the modified Fresnel equation. However, certain issues persist in the fundamental physics or in the numerical calculation process. Here, we adopt the equivalent medium theory (Maxwell-Garnett theory, MGT) to calculate and elucidate Fano interferences in different structures, in the region comprising nanoparticle arrays and dielectrics equivalent to a homogeneous layer of media via the mean field theory. Using this method, the Fano interference can be modeled by mixing different materials, i.e., metals and dielectrics in these cases. Furthermore, a multiple-layered equivalent medium theory is proposed to significantly improve the scalability of this simplified numerical method. In other words, this method can be easily extended to nanoparticles with different shapes, sizes, and materials; in addition, it exhibits robust practicability. Compared with the modified Fresnel equation and finite-difference time-domain methods, this MGT-based method can effectively minimize the calculation process, which is beneficial to the prospective application of plasmon photonics.