Polarized Optomechanical Response of Silver Nanodisc Monolayers on an Elastic Substrate Induced by Stretching

A monolayer assembly of silver nanodisks (AgNDs) was fabricated on the surface of a polydimethylsiloxane (PDMS) polymer substrate using the Langmuir–Blodgett technique. Upon stretching the PDMS substrate, the localized surface plasmon resonance (LSPR) spectrum of the AgND monolayer is blue-shifted when the incident light excitation is polarized parallel to the stretching direction. Conversely, a red shift in the LSPR spectrum of the AgND monolayer is observed in the case of light polarization orthogonal to the stretching direction. The magnitude of the shift in the LSPR spectrum is proportional to the degree of stretching of the PDMS substrate. Stretching PDMS in one direction causes its shrinking in the orthogonal direction. Consequently, the interparticle distance between individual AgNDs on the PDMS surface increases in the same direction as the mechanical stretching and simultaneously decreases in the orthogonal direction. The different optical responses of the AgND assembly on the surface of stretched PDMS when excited with different polarization directions is due to the changing strength of the plasmon field coupling, which is inversely proportional to the separation gap between the AgNDs. The experimentally measured LSPR spectra upon stretching the PDMS substrate to different lengths and varying the incident light polarization were confirmed using the discrete dipole approximation calculation technique. The same optical response was obtained for an AgND monolayer sandwiched between two PDMS substrates. Covering the surface of the AgND monolayer on the PDMS substrate with another PDMS layer on top eliminates their deformation after multiple stretching–shrinking cycles and increases its chemical stability

Tunable Plasmonic Neutral Density Filters and Chromatic Polarizers: Highly Packed 2D Arrays of Plasmonic Nanoparticle on Elastomer Substrate

Highly packed gold nanocube (AuNC) 2D arrays sandwiched between two layers of polydimethylsiloxane (PDMS) substrates act as an optical neutral density filter (NDFs) and a chromatic polarizer. Upon mechanical stretching, the intensity of the absorption spectrum of the AuNC 2D arrays-PDMS is found to decrease evenly in the UV, visible, and NIR regions of the electromagnetic spectrum. The color of the polarized light transmitted through the filter is dependent on its angle of polarization. The localized surface plasmon resonance (LSPR) extinction spectrum of the AuNC arrays arises mainly from scattering rather than absorption, unlike standard NDFs where their function is based on light absorption. Absorption of light causes heat generation that has a negative impact on the function of the NDFs. The ordering of the AuNCs inside the array after stretching was examined by dark field imaging, polarization-dependent optical measurements, and surface-enhanced Raman scattering spectroscopy

Plasmon Resonance Hybridization of Gold Nanospheres and Palladium Nanoshells Combined in a Rattle Structure

Gold and palladium nanoparticles are characterized by their localized surface plasmon resonance (LSPR). In contrast with the sharp LSPR spectrum of gold nanoparticles, palladium nanoparticles had a broad LSPR spectrum. Palladium–gold nanorattles (PdAuNRT) are an ideal system with optical properties that are a hybrid of gold and palladium nanoparticles. The PdAuNRTs consisted of small gold nanospheres (AuNSs) located inside hollow palladium nanospheres (PdHNSs) of larger sizes without touching each other. PdAuNRTs of various sizes were synthesized by systematic variation of the experimental parameters. Interestingly, for the PdAuNRTs, where PdHNSs and AuNSs are separated by a distance, it was found that the broad plasmon resonance band of the PdHNSs hybridizes with the sharp plasmon resonance of the AuNSs located in its center. This was further confirmed experimentally by optical absorption measurements and theoretically using discrete dipole approximation technique. The plasmon resonance hybridization resulted in broadening of the LSPR spectrum of the PdAuNRTs and the appearance of a dip due to a Fano resonance

Surface-Enhanced Raman Spectroscopy of Double-Shell Hollow Nanoparticles: Electromagnetic and Chemical Enhancements

Enhancements of the Raman signal by the newly prepared gold–palladium and gold–platinum double-shell hollow nanoparticles were examined and compared with those using gold nanocages (AuNCs). The surface-enhanced Raman spectra (SERS) of thiophenol adsorbed on the surface of AuNCs assembled into a Langmuir–Blodgett monolayer were 10-fold stronger than AuNCs with an inner Pt or Pd shell. The chemical and electromagnetic enhancement mechanisms for these hollow nanoparticles were further proved by comparing the Raman enhancement of nitrothiophenol and nitrotoulene. Nitrothiophenol binds to the surface of the nanoparticles by covalent interaction, and Raman enhancement by both the two mechanisms is possible, while nitrotoulene does not form any chemical bond with the surface of the nanoparticles and hence no chemical enhancement is expected. Based on discrete dipole approximation (DDA) calculations and the experimental SERS results, AuNCs introduced a high electromagnetic enhancement, while the nanocages with inner Pt or Pd shell have a strong chemical enhancement. The optical measurements of the localized surface plasmon resonance (LSPR) of the nanocages with an outer Au shell and an inner Pt or Pd shell were found, experimentally and theoretically, to be broad compared with AuNCs. The possible reason could be due to the decrease of the coherence time of Au oscillated free electrons and fast damping of the plasmon energy. This agreed well with the fact that a Pt or Pd inner nanoshell decreases the electromagnetic field of the outer Au nanoshell while increasing the SERS chemical enhancement

Effective Optoelectrical Switching by Using Pseudo-Single Crystal of Monolayer Array of 2D Polymer–Plasmonic Nanoparticles System

The Langmuir–Blodgett (LB) technique is used to assemble molecular compounds and colloidal nanoparticles into a monolayer on the surface of a substrate. Although the LB technique was used successfully in the assembly of the nanoparticles and molecules into monolayer, ordering of the nanoparticles or the molecules inside the LB film was not highly controlled. Functionalization of long chain polymers, which are able to semicrystallize into a monolayer, with the surface of colloidal nanoparticles can lead to highly ordered 2D arrays upon LB assembly. Poly­(ethylene glycol) (PEG) with an average molecular weight of 30 000 g/mol is able to organize both isotropic 40 nm gold nanocubes (AuNCs) and anisotropic 40 nm gold nanorods into highly ordered 2D arrays. The separation distances between the nanoparticles in such LB assemblies are comparable, and the distances decreased upon increasing LB surface pressure. A strong sharp localized surface plasmon resonance (LSPR) spectrum of 2D AuNC arrays of full width at half-maximum (fwhm) of 46 nm is obtained when they are organized into highly ordered, well-separated 2D arrays on the surface of a substrate. The high efficiency of the plasmonic AuNC 2D arrays is exploited for optoelectrical switching applications. The LSPR peak of the AuNC 2D arrays coated with a thin film of poly­(3-hexyl­thiophene), an electrochromic polymer that changes its optical properties upon electrochemical oxidation, reversibly blue-shifts by 9 nm upon the electrochemical oxidation of the polymer and shifts back after electrochemical reduction. The narrow LSPR spectrum of the fabricated 2D arrays decreases the value of the figure of merit and thus improves their sensing efficiency

Optical Properties of Gold Nanorattles: Evidences for Free Movement of the Inside Solid Nanosphere

Gold nanorattles (AuNRTs), hollow gold nanospheres with internal small solid gold nanospheres (AuNSs), were prepared with different sizes. The presence of AuNS inside the hollow gold nanospheres in the nanorattle shape was found to improve their sensing efficiency. The sensitivity factor of the nanorattles is in the range of 450 nm/RIU, while the individual hollow nanosphere’s efficiency is ∼300 nm/RIU. This improvement is due to the strong plasmon field on the cavity and around the inner gold nanosphere as shown by using the discrete dipole approximation (DDA) calculations. Interestingly, this nanoparticle produces a strong enhancement for the interaction of light at 850 nm due to the excitation of both the inner sphere and outer nanoshell, despite being the fact that NIR radiation (850 nm) has very low energy to excite the inner gold nanosphere when present alone. Comparing the experimental and simulated scattering spectrum for a single colloidal nanorattle suggests that the interior gold nanosphere moves freely inside the gold nanoshell. When the rattle is dried, the nanosphere adheres to the inner surface as shown from the experimental and theoretical results. Unlike nanospheres and nanoshells, the nanorattles have three plasmon peaks in addition to a shoulder. This allows the AuNRTs to be useful in applications in the visible and near IR spectral regions

Dynamic Template for Assembling Nanoparticles into Highly Ordered Two-Dimensional Arrays of Different Structures

The proper assembly of nanoparticles can enhance their properties and improve their applicability. Likewise, imprudent assembly can damage the unique properties of the nanomaterials. Accordingly, finding robust techniques for making ordered assemblies of nanoparticles is a hot topic in materials science research. In this work, the Langmuir–Blodgett (LB) technique was used to assemble polyethylene glycol (PEG)-functionalized gold nanocubes (AuNCs) into highly packed two-dimensional (2D) arrays with different structures. This technique is based on creating polymeric micelles within the AuNC monolayer, which drives the nanocubes to assemble into a highly packed structure even at low LB surface pressures. Interestingly, the micelles could be made more diffuse by changing the LB trough surface pressure, which allowed for tuning the width and the structure of the AuNC 2D arrays. The areas occupied by the micelles appeared as voids that separated the AuNC arrays and prevented the formation of a uniform monolayer of AuNCs. The polymer micelles were therefore able to act as dynamic soft templates, and the separation distances between individual nanocubes as well as the 2D array structure were controlled by changing the chain length of the PEG functionalization on the surface of the nanocubes. Theoretical calculations of the attractive and repulsive forces and the balance between them presented a good prediction for the optimum separation distance between the AuNCs inside the 2D arrays

Overgrowth of Silver Nanodisks on a Substrate into Vertically Aligned Nanopillars for Chromatic Light Polarization

Vertically aligned and well-separated 1D silver nanopillars (AgNPLs) are prepared on a large-area quartz surface using a robust colloidal chemical technique. Silver nanodisk (AgND) monolayers were first deposited on quartz using the Langmuir–Blodgett technique, and the presence of the substrate induced asymmetric chemical overgrowth of the AgNDs into AgNPLs. The height and diameter of the prepared AgNPLs were controlled by changing the rate of the overgrowth reaction. Chloride ions were used during overgrowth to etch the silver atoms that formed sharp features on the sides of the AgNDs and to limit growth in the lateral direction. The grown AgNPLs displayed two surface plasmon resonance modes corresponding to the transverse and longitudinal electron oscillations. The intensity of the longitudinal mode increased by a factor of 9 while the intensity of the transverse mode decreased by a factor of 2.5 upon increasing the angle of incidence of the exciting light from 0° to 60°. This interesting property makes these AgNPL arrays on quartz useful as chromatic light polarizers

Simultaneous Reduction of Metal Ions by Multiple Reducing Agents Initiates the Asymmetric Growth of Metallic Nanocrystals

Thermodynamically unfavorable metallic nanocrystals can be prepared only by the growth of the nanocrystals under kinetically controlled experimental conditions. The common technique to drive the growth of metallic nanocrystals under kinetic control is to adjust the rate of the generation of metal atoms to be slower than the rate of deposition of such atoms onto the surface of nanocrystal nuclei, which form in the first step of the nanoparticle synthesis. The kinetically controlled growth leads to the formation of seeds with crystal defects, which are needed for the growth of anisotropic nanocrystals such as silver nanodisks (AgNDs). The simultaneous multiple asymmetric reduction technique (SMART) is introduced here to successfully prepare AgNDs of controllable sizes and on a large scale within a few seconds. The SMART is simply based on the simultaneous reduction of silver ions with a strong reducing agent such as borohydride (redox potential of 1.24 V) and a weak reducing agent such as l-ascorbic acid (redox potential of 0.35 V) in the presence of a polyvinylpyrrolidone capping agent. The random formation and deposition of silver atoms by the two different reducing agents generated stacking faults in the growing nanocrystal. The hexagonal close-packed {111} layers of silver atoms were then deposited on the surface of the growing nanocrystal containing stacked faults along the [111] plane. This initiated asymmetric growth necessary for the formation of platelike seeds with planar twin defects, which is required for the formation of anisotropic AgNDs

Silver Nanodisk Monolayers with Surface Coverage Gradients for Use as Optical Rulers and Protractors

Colloidal silver nanodisks (AgNDs) are assembled into a monolayer with a coverage density gradient (CDG) on the surface of flat and cylindrical substrates using the Langmuir–Blodgett (LB) technique. Compressing the LB monolayers during transfer to the substrates causes the CDG assembly of the AgNDs. By functionalizing the AgNDs with poly­(ethylene glycol), it is possible to control their order inside the LB monolayer assembly by changing the deposition surface pressure. Well-separated AgNDs, 2D aggregates with different numbers of particles, and highly packed 2D arrays are formed as the deposition surface pressure is increased. Localized surface plasmon resonance (LSPR) spectra collected at different separation distances from the highest coverage spot (HCS) of the CDG AgND arrays on a flat substrate are blue-shifted, and the shift increases systematically upon increasing the distance. The relationship among the LSPR peak position, the peak intensity at a fixed wavelength, and the corresponding separation distance from the HCS is fitted exponentially. A similar systematic blue shift in the LSPR spectrum of the CDG AgND monolayer on a cylindrical substrate is obtained when the substrate is rotated at different angles relative to the HCS. The fabricated CDG AgND monolayers can potentially be used for optically measuring distances and angles