Two-dimensional Nanoparticle Arrays Research Articles

Time-resolved measurements of the ultra-low dissolution rates of two-dimensional arrays of platinum nanoparticles

Methods for the quantitative evaluation of platinum (Pt) dissolution are required to allow catalyst lifetimes in polymer electrolyte fuel cells (PEFCs) to be predicted. However, it is challenging to detect very slow Pt dissolution rates occurring at low potential fluctuations, such as during load cycles, as may occur in automotive fuel cells. The dissolution behavior of Pt nanoparticles (NPs) is also greatly affected by the catalyst layer thickness, particle size and inter-particle distance, and it is difficult to assess these effects separately. To address these problems, the present work prepared two-dimensionally distributed Pt NPs using coaxial arc plasma deposition. An online ICP-MS system was subsequently used to evaluate the dissolution rate of these NPs. The results showed that Pt dissolved primarily during cathodic scans in association with potential cycling with a high upper potential limit, such as would occur in a PEFC start-stop cycle. This dissolution was accompanied by the formation of Pt oxide. There was little difference in dissolution behavior between Pt NPs and bulk Pt. An extremely slow time-resolved dissolution rate was successfully measured in the low potential region below 1.0 V vs. a standard hydrogen electrode, indicating that anodic dissolution was the predominant Pt dissolution mechanism. In this case, a clear difference in dissolution behavior between the NPs and bulk metal was observed. The onset potential for NPs dissolution was lower than that for bulk Pt and smaller NPs dissolved faster than larger ones.

High-Quality Infrared Metalenses Based on Germanium Dimers

Modern methods of nanophotonics allow creating miniature devices that change the direction of light propagation, modulate the phase front, and control the outcoming state of the polarization. One of the promising areas of research is the development of flat optics elements based on planar analogues of metamaterials—dielectric metasurfaces, which are two-dimensional arrays of subwavelength nanoparticles with a high refractive index and low absorption coefficient. However, the resonances of such nanoscatterers have usually a low quality factor. Symmetry breaking of particle can lead to the excitation of a high-Q quasi-bound states in the continuum. In this work, we numerically study infrared metasurfaces that support such resonances and are formed by dimers of germanium nanocuboids. The possibility of focusing radiation to a point and line by 300‑nm-thick spherical and cylindrical metalenses is shown.

Photoisomerization-controlled wavelength-tunable plasmonic lasers.

We demonstrate photoisomerization-controlled wavelength-tunable plasmonic lasers by integrating spiropyran derivative-doped PMMA films with two-dimensional Ag nanoparticle arrays. The controllable transformation between spiropyran derivatives and its isomers with different refractive indices by photoexcitation allows for a dynamical and continuous change of the refractive index in the host PMMA film, which is able to tune the lattice plasmon resonance, and hence the lasing wavelength. This result opens up a new avenue for engineering wavelength tunable plasmonic lasers toward practical photonic integration.

Anisotropic effect of dipolar interaction in ordered ensembles of nanoparticles

We implement extensive computer simulations to investigate the hysteresis characteristics in the ordered arrays (lx × ly) of magnetic nanoparticles as a function of aspect ratio Ar = ly/lx, dipolar interaction strength hd, and external magnetic field directions. We have considered the aligned anisotropy case, α is the orientational angle. It provides an elegant en route to unearth the explicit role of anisotropy and dipolar interaction on the hysteresis response in such a versatile system. The superparamagnetic character is dominant with weak dipolar interaction (hd ≤ 0.2), resulting in the minimal hysteresis loop area. Remarkably, the double-loop hysteresis emerges even with moderate interaction strength (hd ≈ 0.4), reminiscent of antiferromagnetic coupling. These features are strongly dependent on α and Ar. Interestingly, the hysteresis loop area increases with hd, provided Ar is enormous, and the external magnetic field is along the y-direction. The coercive field μoHc, remanent magnetization Mr, and the heat dissipation EH also depend strongly on these parameters. Irrespective of the external field direction and weak dipolar interaction (hd ≤ 0.4), there is an increase in μoHc with hd for a fixed α and Ar ≤ 4.0. The dipolar interaction also elevates Mr as long as Ar is huge and the field is along the y-direction. EH is minimal for negligible and weak dipolar interaction, irrespective of Ar, α, and the field directions. Notably, the magnetic interaction enhances EH if Ar is enormous and the magnetic field is along the long axis of the system. These results are beneficial in various applications of interest such as digital data storage and spintronics. (a) Schematic of the two-dimensional array of nanoparticles. The variation of heat dissipation EH as a function of dipolar interaction strength and anisotropy axis orientation is shown in (b)-(d). The magnetic field is applied along the x-direction in (b). In contrast, the magnetic field is applied along y-direction in (c) and (d).

Near-Field Electromagnetic Coupling Effects in Optical Spectra of One-Dimensional and Two-Dimensional Arrays of Metal Nanoparticles

Near-Field Electromagnetic Coupling Effects in Optical Spectra of One-Dimensional and Two-Dimensional Arrays of Metal Nanoparticles

Light scattering and absorption by two-dimensional arrays of nano and micrometer monodisperse spherical silver particles

The problem of light scattering and absorption is considered for a free-standing two-dimensional array (monolayer) of nano- and micrometer spherical silver particles. To solve this problem for short- and imperfect long-range ordered arrays the statistical theory of multiple scattering of waves is used. The key points of the developed semi-analytical method are described. The method is based on the quasicrystalline approximation and the multipole expansion of electromagnetic fields and the dyadic Green's function in terms of vector spherical wave functions. Results for the angular distribution of scattered light, the coefficients of coherent transmission and reflection, incoherent scattering, and absorption of a monolayer are obtained for a spectral range corresponding to the nanoparticle plasmonic resonance. Some features of light propagation through two-dimensional arrays of particles with different spatial order are investigated and discussed. The results are in close agreement with the existing experimental data for a partially ordered array and the calculations for a regular array obtained under the extended low-energy electron diffraction theory.

Universal Transfer Printing of Micelle-Templated Nanoparticles Using Plasma-Functionalized Graphene.

Nanostructure incorporation into devices plays a key role in improving performance, yet processes for preparing two-dimensional (2D) arrays of colloidal nanoparticles tend not to be universally applicable, particularly for soft and oxygen-sensitive substrates for organic and perovskite-based electronics. Here, we show a method of transferring reverse micelle-deposited (RMD) nanoparticles (perovskite and metal oxide) on top of an organic layer, using a functionalized graphene carrier layer for transfer printing. As the technique can be applied universally to RMD nanoparticles, we used magnetic (γ-Fe2O3) and luminescent (methylammonium lead bromide (MAPbBr3)) nanoparticles to validate the transfer-printing methodology. The strong photoluminescence from the MAPbBr3 under UV illumination and high intrinsic field of the γ-Fe2O3 as measured by magnetic force microscopy (MFM), coupled with Raman measurements of the graphene layer, confirm that all components survive the transfer-printing process with little loss of properties. Such an approach to introducing uniform 2D arrays of nanoparticles onto sensitive substrates opens up new avenues to tune the device interfacial properties.

Deterministic aperiodic photonic crystal with a 2D array of metallic nanoparticles as polarization-sensitive dichroic filter

We demonstrate the possibility of using a two-dimensional array of spheroidal metallic nanoparticles embedded in a one-dimensional photonic crystal to obtain a narrow-bandpass, polarization-sensitive dichroic filter operating in the near-UV and visible domains. The optical anisotropy of the array of identically oriented nanoparticles results in two spectrally distinct plasmon resonances independently excited for two mutually orthogonal linear polarization states of light, which ensures polarization and spectral selectivity of the composite structure. The narrow transmission bands of the filter are defect modes due to a layer located at the center of the structure and hosting the nanoparticle array. In order to suppress these transmission windows, it is essential that the defect modes closely coincide with the plasmon resonances excited in the array. We show that the use of deterministic aperiodic distributed reflectors surrounding the defect layer makes it possible to adjust the spectral positions of two defect modes in two separate bandgaps in order to achieve such a coincidence. Among the various parameters governing the precise position of transmittivity windows of the filter, we establish the strong influence of the thickness of the defect layer. We also show that a strong localization of the optical field in the plane of the nanoparticle array is essential to enhance the efficiency of plasmonic excitation and obtain the desired control of the defect modes. Our study opens up possibilities for the further development of polarization-controlled nanophotonic devices.

Hot Carrier Generation in Two-Dimensional Silver Nanoparticle Arrays at Different Excitation Wavelengths under On-Resonant Conditions

We evaluated the hot carrier generation in two-dimensional (2D) silver nanoparticle (AgNP) arrays under light illumination at different wavelengths, 458, 532, 671, and 785 nm. The 2D AgNP arrays we...

Plasmonic Metasurface for Spatially Resolved Optical Sensing in Three Dimensions.

The highly localized sensitivity of metallic nanoparticles sustaining localized surface plasmon resonance (LSPR) enables detection of minute events occurring close to the particle surface and forms the basis for nanoplasmonic sensing. To date, nanoplasmonic sensors typically consist of two-dimensional (2D) nanoparticle arrays and can therefore only probe processes that occur within the array plane, leaving unaddressed the potential of sensing in three dimensions (3D). Here, we present a plasmonic metasurface comprising arrays of stacked Ag nanodisks separated by a thick SiO2 dielectric layer, which, through rational design, exhibit two distinct and spectrally separated LSPR sensing peaks and corresponding spatially separated sensing locations in the axial direction. This arrangement thus enables real-time plasmonic sensing in 3D. As a proof-of-principle, we successfully determine in a single experiment the layer-specific glass transition temperatures of a bilayer polymer thin film of poly(methyl methacrylate), PMMA, and poly(methyl methacrylate)/poly(methacrylic acid), P(MMA-MAA). Our work thus demonstrates a strategy for nanoplasmonic sensor design and utilization to simultaneously probe local chemical or physical processes at spatially different locations. In a wider perspective, it stimulates further development of sensors that employ multiple detection elements to generate distinct and spectrally individually addressable LSPR modes.

Electrotunable Nanoplasmonics for Amplified Surface Enhanced Raman Spectroscopy.

Tuning the properties of optical metamaterials in real time is one of the grand challenges of photonics. Being able to do so will enable a class of adaptive photonic materials for use in applications such as surface enhanced Raman spectroscopy and reflectors/absorbers. One strategy to achieving this goal is based on the electrovariable self-assembly and disassembly of two-dimensional nanoparticle arrays at a metal |liquid interface. As expected, the structure results in plasmonic coupling between NPs in the array but perhaps as importantly between the array and the metal surface. In such a system, the density of the nanoparticle array can be reversiblycontrolled by the variation of electrode potential. Theory suggests that due to a collective plasmon-coupling effect less than 1 V variation of electrode potential can give rise to a dramatic simultaneous change in optical reflectivity from ∼93% to ∼1% and the amplification of the SERS signal by up to 5 orders of magnitude. Thisis experimentallydemonstrated using a platform based on the voltage-controlled assembly of 40 nm Au-nanoparticle arrays at a TiN/Ag electrode in contact with an aqueous electrolyte. We show that all the physics underpinning the behavior of this platform works precisely as suggested by the proposed theory, setting the electrochemical nanoplasmonics as a promising direction in photonics research.

Preservation of entanglement and quantum correlations next to periodic plasmonic nanostructures

We study quantum correlations dynamics of two identical V-type quantum systems initially prepared in an extended Werner-like state, where each one independently interacts with a plasmonic nanostructure. Each V-type system can be decomposed as a two-level system with an additional third external level acting as an “umbrella level.” As the plasmonic nanostructure, we use a two-dimensional array of metal-coated dielectric nanoparticles. For the calculations, we combine quantum dynamics calculations using the density matrix equations and classical electromagnetic calculations. In order to describe the entanglement, we use the measure of entanglement of formation, while we use quantum discord to describe the total quantum correlations of our composite system. We find that the presence of the plasmonic nanostructure leads to high suppression of spontaneous emission rates along with a high degree of quantum interference. These phenomena affect the evolution of both entanglement and quantum discord, while they significantly prolong their dynamics.

Ion Irradiation Shaping of Dense Two-dimensional Arrays of Au Nanoparticles Embedded in Silica Studied via TEM

Ion Irradiation Shaping of Dense Two-dimensional Arrays of Au Nanoparticles Embedded in Silica Studied via TEM

Analytical model of resonant electromagnetic dipole-quadrupole coupling in nanoparticle arrays

An analytical model for investigations of multipole coupling effects in the finite and infinite nanoparticle arrays supporting electromagnetic resonances is presented and discussed. This model considers the contributions of both electric and magnetic modes excited in the nanoparticles, including electric and magnetic dipoles and electric and magnetic quadrupoles. The magnetic quadrupole propagator (Green's tensor) that describes the electromagnetic field generated by a point magnetic quadrupole source in all wave zones is derived. As an example, we apply the developed model to study infinite two-dimensional rectangular periodic arrays of spherical silicon nanoparticles supporting the dipole and quadrupole resonant responses. The correctness and accuracy of the analytical model are confirmed by the agreement of its results with the results of full-wave numerical simulations. Using the developed model, we show the electromagnetic coupling between electric dipole and magnetic quadrupole moments as well as between magnetic dipole and electric quadrupole moments even for the case of an infinite rectangular periodic array of spherical nanoparticles. The strong suppression of the dipole or quadrupole moment due to the coupling effects is demonstrated and discussed for spherical nanoparticle arrays. The analytical expressions for the reflection and transmission coefficients written with the effective dipole and quadrupole polarizabilities are derived for normal light incidences and zero-order diffraction. The derived expressions are applied to explain the lattice anapole (invisibility) states when the incident light is transmitted unperturbed through the silicon nanoparticle array. The important role of dipole and quadrupole excitations in scattering compensation resulting in the lattice anapole effect is explicitly demonstrated. The presented approach can be used for designing metasurfaces and further utilizing them in developing ultrathin functional optical elements.

Nonreciprocal Asymmetric Polarization Encryption by Layered Plasmonic Metasurfaces.

As flexible optical devices that can manipulate the phase and amplitude of light, metasurfaces would clearly benefit from directional optical properties. However, single layer metasurface systems consisting of two-dimensional nanoparticle arrays exhibit only a weak spatial asymmetry perpendicular to the surface and therefore have mostly symmetric transmission features. Here, we present a metasurface design principle for nonreciprocal polarization encryption of holographic images. Our approach is based on a two-layer plasmonic metasurface design that introduces a local asymmetry and generates a bidirectional functionality with full phase and amplitude control of the transmitted light. The encoded hologram is designed to appear in a particular linear cross-polarization channel, while it is disappearing in the reverse propagation direction. Hence, layered metasurface systems can feature asymmetric transmission with full phase and amplitude control and therefore expand the design freedom in nanoscale optical devices toward asymmetric information processing and security features for anticounterfeiting applications.

Monolayer Arrays of Nanoparticles on Block Copolymer Brush Films.

Two-dimensional arrays of nanoparticles (NPs) have widespread applications in optical coatings, plasmonic sensors, and nanocomposites. Current bottom-up approaches that use homogeneous NP templates, such as silane self-assembled monolayers or homopolymers, are typically plagued by NP aggregation, whereas patterned block copolymer (BCP) films require specific compositions for specific NP distributions. Here, we show, using polystyrene- b-poly(4-vinylpyridine) (PS- b-P4VP) and gold NPs (AuNPs) of various sizes, that a nanothin PS- b-P4VP brushlike coating (comprised of a P4VP wetting layer and a PS overlayer), which is adsorbed onto flat substrates during their immersion in very dilute PS- b-P4VP tetrahydrofuran solutions, provides an excellent template for obtaining dense and well-dispersed AuNPs with little aggregation. These non-close-packed arrays have similar characteristics regardless of immersion time in solution (about 10-120 s studied), solution concentration below a critical value (0.1 and 0.05 mg/mL studied), and AuNP diameter (10-90 nm studied). Very dilute BCP solutions are necessary to avoid deposition, during substrate withdrawal, of additional material onto the adsorbed BCP layer, which typically leads to patterned surfaces. The PS brush coverage depends on immersion time (adsorption kinetics), but full coverage does not inhibit AuNP adsorption, which is attributed to PS molecular rearrangement during exposure to the aqueous AuNP colloidal solution. The simplicity, versatility and robustness of the method will enable applications in materials science requiring dense, unaggregated NP arrays.

Tunable Lattice Plasmon Resonances in 1D Nanogratings

Lattice plasmon resonances or surface lattice resonances (SLRs) supported in two-dimensional (2D) metal nanoparticle arrays have extremely narrow line widths and highly localized electric field enhancements, which are key properties for realizing plasmon lasers and hybrid solid-state lighting devices. This paper reports lattice plasmons in one-dimensional (1D) metal nanogratings with broadband tunability (over 400 nm) far beyond their 2D counterparts at visible wavelengths. The large wavelength tunabilities of 1D or line-SLRs are from the lower symmetry of the structures compared to 2D arrays based on nanoparticles. We demonstrate that line-SLRs exhibit a Fano-like character based on coupling between an out-of-plane plasmon excitation and 1D Bragg diffraction modes. We show how the height and periodicity of the grating determine the spectral properties of the line-SLRs. By adjusting the line height, we achieved high-quality lattice resonances, even in index-mismatched environments.

Tunable coloration of diamond films by encapsulation of plasmonic Ag nanoparticles

Chemical vapor deposition (CVD) grown diamond films are widely used as coating materials. While diamond thin films are typically clear or slightly gray in color, we demonstrate that diamond films of different optical properties and exhibiting different colors can be prepared by embedding Ag nanoparticles into the diamond film. Microwave plasma heating of Ag thin films deposited onto H-terminated diamond films grown on transparent fused quartz substrates leads to de-wetting and formation of a high density, two-dimensional array of size-controlled Ag nanoparticles. Surprisingly, scanning electron microscopy and transmission electron microscopy show that starting with Ag films <30 nm diameter, dewetting and subsequent growth of a second diamond layer encapsulates the Ag nanoparticles, forming dense 2-dimensional arrays of diamond-encapsulated Ag nanoparticles. Under conditions used here, initial Ag films >30 nm thickness lead to more polydisperse nanoparticles and the formation of Ag/diamond core/shell structures. By changing the thickness of the deposited Ag layer, the Ag nanoparticle size distribution can be changed, leading to changes in optical properties. Three-dimensional electromagnetic field modeling was performed for SiO2-diamond/Ag/diamond-air structures produced by dewetting of 20 nm thick Ag films show good qualitative agreement with experimental spectra.

Compositional effect of two-dimensional monodisperse AuPd bimetallic nanoparticle arrays fabricated by block copolymer nanopatterning on catalytic activity of CO oxidation.

We present a straightforward approach for fabricating AuPd nanocatalysts based on self-assembled block copolymer nanopatterning. CO oxidation was carried out on AuPd alloy nanoparticles and the activity increased with increasing Pd content; this synergistic activity can be precisely tuned by varying the chemical composition. Theoretical calculations suggest that Pd clusters are required to bind and dissociate O2 for oxidation of coadsorbed CO molecules. Our results suggest that the geometric configuration of the Pd atoms on the surface are a key factor in the catalytic activity.

Tailoring two-dimensional nanoparticle arrays into various patterns

A simple and effective technique has been developed to fabricate patterns of nanoparticle arrays. Lithographically fabricated structures in resists serve as scissors to tailor two-dimensional nanoparticle arrays on a flat poly(dimethylsiloxane) (PDMS) stamp. The desired patterns of nanoparticle arrays remaining on the PDMS stamp after tailoring can be printed onto solid substrates. Various regular nanoparticle patterns, such as squares, triangles, disks, and pentagons, can be easily prepared using this technique. Arbitrary nanoparticle patterns as complex as Chinese characters have been successfully demonstrated. Moreover, nanoparticle stripes with width ranging from micrometers to quasi single nanoparticle diameter have also been achieved. Nanoparticle stripes have been integrated into electronic devices for transport measurements.