Ag Nanoparticle Arrays Research Articles

LSPR anisotropy minimization by sequential growth of Ag nanoparticles on nanoripple patterned Si surface for SERS Application

The present work describes the formation of low-aspect-ratio Ag nanoparticles (NPs) arrays by sequential deposition of Ag from different growth directions on a nanoripple-patterned Si substrate, produced by low-energy ion beam irradiation. It is observed that the growth of Ag-NPs on ripple surfaces is direction-dependent due to the asymmetric ridge of the ripple pattern. This asymmetric ridge can be utilized to tune the shape of NP from elongated to spherical and in turn, assists in minimizing the LSPR anisotropy. The Finite-Difference-Time-Domain (FDTD) simulations demonstrate that the interparticle gap, major, and minor axis combinations of a NP are crucial in minimizing anisotropic near-field interaction. NPs grown perpendicular to the ripple from either direction lead to an elongated chain of nanoparticles; however, if a proper combination of sequential growth times is chosen then the aspect ratio of NPs can be tuned from elongation to spherical ones. Interestingly, a LSPR shift of 209 nm was observed when NPs are grown for 60 min on ripple patterns from one direction only. On the other hand, it reduces to 54 nm when NPs are sequentially grown for 30 min from each of directions, perpendicular to the ripples. SERS measurements also evidence a minimized anisotropic nature in sequentially grown NP arrays; hence, the growth direction as well as optimized growth times together can be used to control the LSPR anisotropy of NP arrays which can be used to fabricate such isotropic SERS substrates.

Electrodynamic Interference and Induced Polarization in Nanoparticle-Based Optical Matter Arrays.

Optical matter (OM) arrays are self-organizing, ordered arrangements of nanometer- to micrometer-size particles, where interparticle forces are mediated by incident and scattered coherent light. The structures that form and their dynamics depend on the properties (e.g., material, size) of the constituent particles, as well as the incident and scattered light. While significant progress has been made toward understanding how the OM arrays are affected by the phase, polarization, and intensity profile of the incident light, the polarization induced in the particles and the light scattered by OM arrays have received less attention. In this paper, we establish the roles of electrodynamic interference, many-body coupling, and induced-polarization concomitant with the coherent light scattered by OM arrays. Experiments and simulations together demonstrate that the spatial profile and directionality of coherent light scattered by OM arrays in the far field are primarily influenced by interference, while electrodynamic coupling (interactions) and the associated polarization induced in the nanoparticle constituents have a quantitative wavelength-dependent effect on the total amount of light scattered by the arrays. Furthermore, the electrodynamic coupling in silver nanoparticle OM arrays is significantly enhanced by constructive interference and increases superextensively with the number of particles in the array. Particle size, and hence polarizability, also has a significant effect on the strength of the coupling. Finally, we simulate larger hexagonal OM arrays of Ag nanoparticles to demonstrate that the electrodynamic coupling and scattering enhancement observed in small OM arrays develop into surface lattice resonances observed in the infinite array limit. Our work provides insights for designing OM arrays to tune many-body forces and the coherent light that they scatter.

Ag-Assisted Dry Exfoliation of Large-Scale and Continuous 2D Monolayers.

Two-dimensional (2D) semiconductors have generated considerable attention for high-performance electronics and optoelectronics. However, to date, it is still challenging to mechanically exfoliate large-area and continuous monolayers while retaining their intrinsic properties. Here, we report a simple dry exfoliation approach to produce large-scale and continuous 2D monolayers by using a Ag film as the peeling tape. Importantly, the conducting Ag layer could be converted into AgOx nanoparticles at low annealing temperature, directly decoupling the conducting Ag with the underlayer 2D monolayers without involving any solution or etching process. Electrical characterization of the monolayer MoS2 transistor shows a decent carrier mobility of 42 cm2 V-1 s-1 and on-state current of 142 μA/μm. Finally, a plasmonic enhancement photodetector could be simultaneously realized due to the direct formation of Ag nanoparticles arrays on MoS2 monolayers, without complex approaches for nanoparticle synthesis and integration processes, demonstrating photoresponsivity and detectivity of 6.3 × 105 A/W and 2.3 × 1013 Jones, respectively.

Annealing temperature effects on monolayer WS2-veiled Ag nanoparticle array for surface catalytic reaction

Plasmonic-WS2 hybrids have attracted widespread interest for plasmon driven catalytic reactions. In this work, a Ag nanoparticles (NPs)/WS2 hybrid was fabricated by utilizing a one-step anodized Al template-assisted vacuum thermal evaporation technique and wet transfer method. To optimize the catalytic performance, the morphological evolution and corresponding changes in the catalytic properties of the Ag NPs/WS2 hybrid at different thermal annealing temperatures were investigated. It was found that the surface-enhanced Raman scattering (SERS) and catalytic activity of the hybrid were optimized by tuning the annealing temperature, with the optimal SERS and catalytic properties observed at 290 °C. These results may open new avenues for improving the efficiency and expanding the research field of plasmon-driven reactions.

Sol-Gel Synthesis of Uniform Arrays of Ag and Au Nanoparticles

The obtaining of uniform arrays of silver and gold nanoparticles with a surface density up to 3.3∙109 cm–2 on the zinc oxide buffer layers by sol-gel method is described. The variations of the solution composition and synthesis mode, layers coating and subsequent heat treatment were carried out. The absorption spectra of the obtained samples had a peak near 400–570 nm corresponding to the plasmon resonance in the Ag and Au nanoparticles. Wavelength and shape of Ag and Au nanoparticles plasmon peak varied depending on the synthesis mode: the use of ZnO buffer layers leads to an increase in the intensity of the nanoparticles plasmon peak, the annealing leads to a gradual decrease and broadening of the absorption peak of Ag and mixed Ag and Au nanoparticles arrays, but does not affect the peak of Au nanoparticles.

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.

Au/Ag SERS active substrate for broader wavelength excitation

Self-organized Ag nanoparticles arrays capped with Au layer are produced by depositing Ag nanoparticles on ion beam produced ripple patterned Si substrate followed by Au deposition. Au/Ag combination of nanoparticles shows intermediate plasmonic properties of Ag and Au nanoparticles by overcoming the associated drawbacks. Such combination redshifts the Localized Surface Plasmon Resonance (LSPR) compared to pure Ag and makes them suitable to excite the surface plasmon by both 532 nm and 785 nm laser for a wider range SERS application. The optimized Au deposition decreases the interparticle gap and elongates the nanoparticles along the ripple direction and makes a larger redshift in the LSPR. FDTD simulation also revealed that the electric field enhancement is higher along the ripple direction for both exciting lasers and shows that the LSPR of the combined Au/Ag system is shifted towards 785 nm. The limit of detection of Crystal Violet using the optimized Au/Ag substrate is found to be 10−10 M for 532 nm laser and 10−8 M for 785 nm laser, respectively. The substrate shows good spatial uniformity with a relative standard deviation of less than 10% with SERS enhancement factors in the order of 104-105 for both lasers. These studies suggest that the as-prepared substrate is a promising candidate for detecting molecules sensitive to different wavelengths of laser excitation.

On-Demand Electrochemical Fabrication of Ordered Nanoparticle Arrays using Scanning Electrochemical Cell Microscopy.

Well-ordered nanoparticle arrays are attractive platforms for a variety of analytical applications, but the fabrication of such arrays is generally challenging. Here, it is demonstrated that scanning electrochemical cell microscopy (SECCM) can be used as a powerful, instantly reconfigurable tool for the fabrication of ordered nanoparticle arrays. Using SECCM, Ag nanoparticle arrays were straightforwardly fabricated via electrodeposition at the interface between a substrate electrode and an electrolyte-filled pipet. By dynamically monitoring the currents flowing in an SECCM cell, individual nucleation and growth events could be detected and controlled to yield individual nanoparticles of controlled size. Characterization of the resulting arrays demonstrate that this SECCM-based approach enables spatial control of nanoparticle location comparable with the terminal diameter of the pipet employed and straightforward control over the volume of material deposited at each site within an array. These results provide further evidence for the utility of probe-based electrochemical techniques such as SECCM as tools for surface modification in addition to analysis.

Arrays of Ag-nanoparticles decorated TiO2 nanotubes as reusable three-dimensional surface-enhanced Raman scattering substrates for molecule detection.

Three-dimensional surface-enhanced Raman scattering (SERS) substrates usually provide more hot spots in the excitation light beam and higher sensitivity when compared with the two-dimensional counterpart. Here a simple approach is presented for the fabrication of arrays of Ag-nanoparticles decorated TiO2 nanotubes. Arrays of ZnO nanorods were fabricated in advance by a hydrothermal method. Then TiO2 nanotube arrays were achieved by immersing the arrays of ZnO nanorods in an aqueous solution of (NH4)2TiF6 for 1.5 h. Vertically aligned TiO2 nanotube arrays were modified with dense Ag nanoparticles by Ag mirror reaction. High density of Ag nanoparticles decorated on the fabricated TiO2 nanotubes provide plenty of hotspots for Raman enhancement. In addition, the fabricated array of Ag nanoparticles modified TiO2 nanotubes can serve as a reusable SERS substrate because of the photocatalytic activity of the TiO2 nanotubes. The SERS substrate adsorbed with analyte molecules can realize self-cleaning in deionized water after UV irradiation for 2.5 h. The sensitivity of the fabricated SERS substrate was investigated by the detection of organic dye molecules. The detectable concentration limits of rhodamine 6G (R6G), malachite green (MG) and methylene blue (MB) were found to be 10−12 M, 10−9 M and 10−8 M, respectively. The enhancement factor (EF) of the three-dimensional SERS substrate was estimated to be as high as ∼1.4×108. Therefore, the prepared Ag nanoparticles modified TiO2 nanotube arrays have promising potentials to be applied to rapid and trace SERS detection of organic chemicals.

Plasmon resonance of gold and silver nanoparticle arrays in the Kretschmann (attenuated total reflectance) vs. direct incidence configuration

While the behaviour of plasmonic solid thin films in the Kretschmann (also known as Attenuated Total Reflection, ATR) configuration is well-understood, the use of discrete nanoparticle arrays in this optical configuration is not thoroughly explored. It is important to do so, since close packed plasmonic nanoparticle arrays exhibit exceptionally strong light-matter interactions by plasmonic coupling. The present work elucidates the optical properties of plasmonic Au and Ag nanoparticle arrays in both the direct normal incidence and Kretschmann configuration by numerical models, that are validated experimentally. First, hexagonal close packed Au and Ag nanoparticle films/arrays are obtained by air–liquid interfacial assembly. The numerical models for the rigorous solution of the Maxwell’s equations are validated using experimental optical spectra of these films before systematically investigating various parameters. The individual far-field/near-field optical properties, as well as the plasmon relaxation mechanism of the nanoparticles, vary strongly as the packing density of the array increases. In the Kretschmann configuration, the evanescent fields arising from p- and s-polarized (or TM and TE polarized) incidence have different directional components. The local evanescent field intensity and direction depends on the polarization, angle of incidence and the wavelength of incidence. These factors in the Kretschmann configuration give rise to interesting far-field as well as near-field optical properties. Overall, it is shown that plasmonic nanoparticle arrays in the Kretschmann configuration facilitate strong broadband absorptance without transmission losses, and strong near-field enhancement. The results reported herein elucidate the optical properties of self-assembled nanoparticle films, pinpointing the ideal conditions under which the normal and the Kretschmann configuration can be exploited in multiple light-driven applications.

Cu/Ag Nanoparticle-Based Surface-Enhanced Raman Scattering Substrates for Label-Free Bacterial Detection

Surface-enhanced Raman spectroscopy (SERS) is a promising analytical technique for fast and accurate disease detection due to its attractive features. However, realizing label-free direct detection is still challenging as most probes have extremely low Raman cross sections and little affinity to SERS substrates. Disappointingly, SERS spectra of most bacteria and other biological samples look similar as the differences in their molecular compositions are subtle and not detectable. Hence, the fabrication of highly enhancing plasmonic nanoparticles with excellent uniformity is demanded. Moreover, as SERS substrates are not reusable, cost-effective and simplistic fabrication methods are critical. Here, we report a facile approach to synthesizing Ag nanoparticle array on Cu-foil in less than 3 min, using only Cu-foil, silver nitrate, and hydroquinone. We employed the idea of galvanic replacement in combination with a seed-mediated particle-growth approach. The label-free bacterial detection has shown that our Cu/Ag nanoparticle substrate is superior to highly acclaimed Ag nanocubes. Creating strong second and third-generation SERS hot-spots through cooperative interaction of homo- (Ag–Ag) and heterogeneous (Ag–Cu) surfaces contributes mainly to the observed excellent enhancement. Interestingly, direct liquid bacteria sample analysis showed a 6-fold higher detection sensitivity than completely dried samples. We believe that our approach will offer remarkable advantages and change how SERS substrates are prepared and conducted in SERS-based bacterial detection.

Effects of annealing temperature on morphological and optical properties of Ag nanoparticle array and its application in SERS

Ordered metal nanoparticle (NP) arrays have attracted tremendous attention for potential plasmonic applications due to their unique surface plasmon resonance. Herein, we fabricated highly ordered Ag NP arrays employing anodized aluminum (Al) templates and the vacuum thermal evaporation technique. The morphological evolution and corresponding optical property changes of the Ag NP arrays during annealing treatment has been investigated. It is found that there is a transition point approximately at annealing temperature of 290°C, where the shape of Ag NPs is most spherical and the peak position of quadrupole mode has a largest wavelength about 500 nm. Then, the surface-enhanced Raman spectra of Rhodamine 6G on Ag NP arrays annealed at different temperatures has been demonstrated at an excitation wavelength of 532 nm. The maximum SERS enhancement is also observed at 290°C. Our findings provide a way to further improve the performance of the plasmonic structures.

Second-harmonic generation of embedded plasmonic nanoparticle arrays via interparticle coupling

Efficient nonlinear frequency conversion, such as second-harmonic generation in ultracompact structures, is essential for the development of modern nanophotonic devices. Here, we demonstrate intense second-harmonic emission in scalable embedded Ag nanoparticle arrays fabricated by ion implantation into BK7 glass. The interparticle coupling effect significantly enhances the local field at the nanogap (gap size ∼ 1 nm) of two neighboring Ag nanoparticles and finally amplifies second-harmonic emission generated at the surface of plasmonic nanoparticles. Notably, the intensity of second-harmonic emission in embedded Ag nanoparticle arrays is comparable to that of two-dimensional transition metal dichalcogenides under the excitation of a fundamental wave at 1064 nm and independent of the incident polarization angles. Our work offers a promising strategy on the rapid fabrication of low-cost nonlinear optical nanostructures with great environmental stability.

Fabry-Perot interference pattern scattered by a sub-monolayer array of nanoparticles

Understanding scattering of visible and infrared photons from nanomaterials and nanostructured materials is increasingly important for imaging, thermal management, and detection, and has implications for other parts of the electromagnetic spectrum (e.g., x-ray scattering and radar). New, interesting reports of photon scattering as a diagnostic probe, from inelastic x-ray scattering and interference to ‘nano-FTIR’ microscopy using infrared photons, have been published and are under active investigation in laboratories around the world. Here, we report, for the first time to our best knowledge, the experimental discovery of a Fabry–Perot interference pattern that is scattered by the sub-monolayer array of plasmonic Ag nanoparticles, and confirm it analytically and with rigorous numerical FDTD simulations.

Coupling of plasmonic nanoparticles on a semiconductor substrate via a modified discrete dipole approximation method.

Understanding the plasmonic coupling between a set of metallic nanoparticles (NPs) in a 2D array, and how a substrate affects such coupling, is fundamental for the development of optimized optoelectronic structures. Here, a simple semi-analytical procedure based on discrete dipole approximation (DDA) is reported to simulate the far-field and near-field properties of arrays of NPs, considering the coupling between particles, and the effect of the presence of a semiconductor substrate based on the image dipole approach. The method is validated for Ag NP dimers and single Ag NPs on a gallium nitride (GaN) substrate, a semiconductor widely used in optical devices, by comparison with the results obtained by the finite element method (FEM), indicating a good agreement in the weak coupling regime. Next, the method is applied to square and random arrays of Ag NPs on a GaN substrate. The increase in the surface density of NPs on a GaN substrate mainly results in a redshift of the dipolar resonance frequency and an increase in the near-field enhancement. This model, based on a single dipole approach, grants very low computational times, representing an advantage to predict the optical properties of large NP arrays on a semiconductor substrate for different applications.

In situ SERS monitoring of plasmon-driven catalytic reaction on gap-controlled Ag nanoparticle arrays under 785 nm irradiation

Plasmon-enhanced photocatalysis has attracted considerable attention due to its low energy consumption and high energy throughput. Surface-enhanced Raman scattering (SERS) is a highly sensitive and label-free nondestructive tool to investigate plasmon-driven photocatalytic reactions. Herein, we present a facile method to fabricate gap-controlled Ag nanoparticle (NP) arrays with uniform and high-density distribution of hot spots, which can be employed as both efficient plasmonic photocatalysts and stable SERS platforms. The plasmon-driven catalytic reaction of 4-nitrobenzenethiol (4NBT), which transforms it into p, p′-dimercaptoazobenzene (DMAB), is detected by using an in situ SERS technique at the excited wavelength of 785 nm. According to the temperature and laser power density dependent photocatalytic reaction rates observed on the Ag NP arrays, we quantitatively determined that the reductive coupling of 4NBT is more likely to occur as the gap decreases. The finite-difference time-domain (FDTD) simulation results demonstrate that the plasmonic hot spots are significantly enhanced with a decrease in gap, which in turn reduces activation energy. The gap-controlled Ag NP arrays are efficient for both promotion and detection of plasmon-driven catalytic reactions, and may pave a pathway for implementing efficient plasmonic photocatalytic platforms.

Super-resolution imaging of plasmonic nanostructures by microsphere-assisted microscopy.

We fabricate both triangularly and circularly shaped Au, Ag, and Cr nanoparticle arrays and observe the imaging properties of these plasmonic nanostructures by BaTiO3 glass (BTG) microsphere-assisted microscopy. We experimentally find that the resolution of triangularly shaped Ag nanoparticle arrays is higher than that of Au and Cr ones, and a gap resolution of ∼λ/7.7 is demonstrated for the circularly shaped Au, Ag, and Cr nanostructures. Numerical simulations show that when a fully immersed BTG microsphere is dispersed on the surface of a plasmonic nanostructure sample, an enhanced electric field is generated in the vicinity of the sample, especially at the gap of the microsphere and the sample, due to the focusing effect of the microsphere and the excitation of localized surface plasmon resonance in the plasmonic nanostructure. The enhanced electric field in Ag nanostructures is significantly stronger than that in Au and Cr ones. Besides, the microsphere collects, amplifies, and propagates the enhanced near-field information to the far field, resulting in the improvement of imaging resolution.

Development of self-cleaning SERS-active nanostructures based on ZnO nanorods and Ag nanoparticles

This work demonstrates the possibility of using a combination of zinc oxide nanorods with silver nanoparticles as a self-cleaning SERS substrate. In addition to Raman scattering enhancement such structures demonstrate the self-cleaning effect during UV treatment. The ZnO nanorods (NRs) array was synthesized on a ZnO seed layer by the hydrothermal method. The Ag nanoparticles (NPs) array was formed by vacuum thermal evaporation over the ZnO NRs. Rhodamine-B 230 μM solution has been detected using the formed SERS-substrates without additional mathematical processing of the Raman spectra. Subsequent UV radiation treatment showed a 3-fold decrease in the intensity of the spectral peaks of the analyte.

Acoustofluidic multimodal diagnostic system for Alzheimer's disease

Alzheimer's disease (AD) is a progressive and irreversible neurodegenerative brain disorder that affects tens of millions of older adults worldwide and has significant economic and societal impacts. Despite its prevalence and severity, early diagnosis of AD remains a considerable challenge. Here we report an integrated acoustofluidics-based diagnostic system (ADx), which combines triple functions of acoustics, microfluidics, and orthogonal biosensors for clinically accurate, sensitive, and rapid detection of AD biomarkers from human plasma. We design and fabricate a surface acoustic wave-based acoustofluidic separation device to isolate and purify AD biomarkers to increase the signal-to-noise ratio. Multimodal biosensors within the integrated ADx are fabricated by in-situ patterning of the ZnO nanorod array and deposition of Ag nanoparticles onto the ZnO nanorods for surface-enhanced Raman scattering (SERS) and electrochemical immunosensors. We obtain the label-free detections of SERS and electrochemical immunoassay of clinical plasma samples from AD patients and healthy controls with high sensitivity and specificity. We believe that this efficient integration provides promising solutions for the early diagnosis of AD.

Silver nanoparticle array on weakly interacting epitaxial graphene substrate as catalyst for hydrogen evolution reaction under neutral conditions

The paucity of research on hydrogen evolution reaction (HER) under neutral conditions, which is a more sustainable way to produce H2 compared to acidic and alkaline HER, encourages the development of efficient catalytic materials and devices and deeper investigation of the mechanisms behind neutral HER. We present an electrode concept for facilitating HER under neutral conditions. The concept entails the use of annealing-reshaped silver (Ag) nanoparticle array on monolayer epitaxial graphene (MEG) on 4H-SiC. Measurements of HER performance show more positive onset potential of the cathodic HER for Ag-decorated MEG compared to that for pristine MEG, indicating improved water dissociation at Ag/MEG electrodes. Complementary morphological characterization, absorption measurements, and Raman mapping analysis enable us to ascribe the enhanced catalytic performance of electrodes decorated with 2 nm thick annealed Ag on the synergetic effect originating from simultaneous water reduction on circular Ag nanoparticles of 31 nm in diameter and on compressively strained Ag-free graphene regions. The overall results pave the way toward development of stable van der Waals heterostructure electrodes with a tunable metal–carbon interaction for fast HER under neutral conditions.