Silver Thin Films Research Articles

Spatially Controlled Fabrication of Surface-Enhanced Raman Scattering Hot Spots through Photoinduced Dewetting of Silver Thin Films.

A well-designed narrow gap between noble metal nanostructures plays a prominent role in surface-enhanced Raman scattering (SERS) to concentrate electromagnetic fields at the local point, called a "hot spot". However, SERS-active substrate fabrication remains a substantial hurdle due to the high process cost and the difficulty of engineering efficient plasmonic hot spots at the target area. In this study, we demonstrate a simple photolithographic method for generating ultrasensitive SERS hot spots at desired positions. The solid-state dewetting of a Ag thin film (thickness of ∼10 nm) using a continuous-wave laser (∼1 MW/cm2) generates a closely packed assembly of hemispherical Ag nanoislands. Some of these nanoislands provide substantial plasmonic-field enhancement that is sufficient for single-molecule detection and plasmon-catalyzed chemical reaction. Such hot spot structures can be patterned on the substrate with a spatial resolution of better than 1 μm. In integrated analytical devices, the patterned SERS hot spots can be used as position-specific chemical-sensing elements.

Near Infrared Metal-insulator-metal Surface Plasmon Resonances for Refractive Index Sensors

This work reports the optical properties of surface plasmon resonance (SPR) based on the metal-insulator-metal (MIM) structure towards a refractive index sensor. The MIM-SPR structure operating near infrared region consists of lateral periodicity of subwavelength gold patterns placed on a stack of thin silica spacer and silver film (acting as a reflector) on a silicon substrate. The reflection spectra and the electric field distributions of MIM-SPR structures can be tuned by modifying the geometrical properties and have been numerically investigated by using Lumerical’s finite-difference time-domain (FDTD) solutions. The square lattice configuration of 1200 nm to 1400 nm pitch of gold micro-disks of thickness from 80 nm to 120 nm have been conducted. The size of these considered gold patterns, i.e., the diameter of the micro-disks is in the range of 900 nm to 1000 nm. The proposed MIM-SPR structure possessing sensitivity of 370 nm per refractive index unit (RIU), can be applicable for a wide variety of plasmonic sensing, in particular for refractometric biosensors.

Silver oxide coatings deposited on leathers to prevent diabetic foot infections

Diabetic foot is one of the most serious complications of diabetes, the appearance of foot ulcers is highly susceptible to the prevalence of infections. In this sense, thin films of silver (Ag) and silver oxide (AgOx) were deposited by the dc reactive magnetron sputtering technique on different leather substrates for application in insoles and/or footwear inside in order to functionalize them, thus promoting antibacterial properties. A detailed characterization is described to the uncoated and coated leathers. Cytotoxicity and antimicrobial activity were evaluated and, a silver antimicrobial mechanism was suggested. Among the uncoated and coated leathers, AgOx modified samples have higher CoF indicating that this coating can permit higher immobilization of the foot in relation to the skin. The AgOx coating has also impressive antibacterial and antifungal activity without causing any cytotoxic effect, suggesting the potential application of AgOx modified leathers to diabetic specialized footwear.

Design and Analytical Evaluation of a High Resistance Sensitivity Bolometer Sensor Based on Plasmonic Metasurface Structure

The bolometric effect (BE) allows to electrically monitor spectral characteristics of plasmonic sensors, but trade-off between sensitivity, response time, and strong light absorption is an important challenge. Here, the BE for a high resistance sensitivity plasmonic sensor is proposed and studied based on plasmonic multilayer structure at 26 °C. By generating strong coupling between incident wave and surface plasmon polaritons (SPPs), a very narrow absorption spectra with high figure of merit (FoM) is achieved, that heats the silver thin film, leads to variation in temperature and supports TE surface mode. The temperature variation rectifies the resistance of the metal thin film by the BE. Optical characteristics of the proposed metasurface bolometer sensor (MBS), are calculated by TMM and 3D-FDTD, in that maximum quality factor is 17.2 RIU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , sensitivity is 530 and FoM is 434.5. Finally, we analytically simulate the temperature coefficient of resistance in terms of wavelength and refractive index of analyte that can be monitored by an external electric model. The proposed thermal, optical, and electric plasmonic MBS can be used in biophysics, biology, and environmental science because of its very compact footprint with high resistance sensitivity and FoM in comparison with any previous reports.

Effect of Temperature on the Surface Plasmon Resonance (SPR) Mode along a Metal - Liquid Crystal Interface

The effect of temperature on the surface plasmon mode parameters for surface plasmons (SPs) excited by prism coupling along the interface of a thin silver film and homeotropically aligned liquid crystals (LCs), 5CB and UCF25 are theoretically studied. The change in the SPR angle of dip with temperature is found to be almost linear below the clearing temperature of the LCs. The temperature sensitivity is found to be high in the temperature range where the LC behaves as an anisotropic material. The sensitivity is found to be higher when SPR reflectance at a fixed incident angle is used as the variable parameter. The shift in SPR angle of dip is found to be linear with the change in birefringence of the LC due to the change in temperature. The change in SPR parameters with temperature is found to be much lesser in the temperature range where the LC behaves as an isotropic material.

Simulation and fabrication of butane gas sensor based on surface plasmon resonance phenomenon

In this study, a butane (C4H10) gas sensor was simulated and fabricated using the surface plasmons resonance phenomenon. In this research, a thin layer of silver covered by a thin layer of magnesium fluoride (MgF2) was used to create surface plasmons. The most suitable thickness of silver and magnesium fluoride thin films for fabrication of butane gas sensor was determined by using computer simulations as well as laboratory experiments of 56 nm and 13 nm, respectively. With the aid of a BK7 prism and the Kretschmann configuration, surface plasmons were generated in the silver layer. Nitrogen gas was used to dilution of butane gas concentration. The fabricated sensor was tested and evaluated at different concentrations of butane gas. The sensor test results showed that this sensor is very sensitive to the concentration of butane gas, and changes in the concentration of this gas cause significant displacement at the peak location of the creation of surface plasmons.

The Fabrication of Photonic Crystal Microchip with Controllable Wettability and SERS Activity based on Surface Roughness for Trace Organic Compounds Determination

AbstractInspired by the fog collection structure on the back of the desert beetle, the hydrophobic photonic crystal microchip (HPCM) with dual functions of hydrophobic enrichment and surface‐enhanced Raman spectroscopy (SERS) activity is fabricated by depositing thin Ag film on the photonic crystals (PC) surface, and this is the first time that the wettability and SERS activity are controlled simultaneously by adjusting surface roughness. A 5 nm thin silver film is deposited on the PC surface of hydrophilic SiO2 to increase surface roughness so that the wettability is transformed from hydrophilicity to hydrophobicity. The hydrophobic surface not only reduces the influence of the “coffee ring” effect but also enriches the samples during analysis. Moreover, due to the sputtering of Ag film, the “hot spots” on the PC surface are increased while the periodic structure of PC remains unchanged. By coupling the photonic bandgap (PBG) of HPCM with Raman laser wavelength, the enhancement factor (EF) reaches 2.3 × 106. The outstanding performance of HPCM is attributed to the synergy between the rough surface and the PBG of PC. The designed HPCM can not only quickly, efficiently, and sensitively detect organic compounds determination, but also has potential application in environmental monitoring.

Silver Nanoparticle Arrays onto Glass Substrates Obtained by Solid-State Thermal Dewetting: A Morphological, Structural and Surface Chemical Study.

Silver nanoparticles (NPs) on glass substrates were obtained by a solid-state thermal dewetting (SSD) process using vacuum-evaporated-silver precursor layers. An exhaustive investigation of the morphological, structural, and surface chemistry properties by systematically controlling the precursor film thickness, annealing temperature, and time was conducted. Thin silver films with thicknesses of 40 and 80 nm were deposited and annealed in air by applying a combined heat-up+constant temperature–time program. Temperatures from 300 to 500 °C and times from 0 to 50 min were assayed. SSD promoted the morphological modification of the films, leading to the Ag NPs having a discrete structure. The size, shape, surface density, and inter-nanoparticle distance of the nanoparticles depended on the initial film thickness, annealing temperature, and time, exhibiting a cubic silver structure with a (111) preferred crystallographic orientation. The prepared NPs were found to be highly enriched in the Ag{111}/Ag{110}/Ag{100} equilibrium facets. SSD not only promotes NP formation but also promotes the partial oxidation from Ag to AgO at the surface level. AgO was detected on the surface around the nanoparticles synthesized at 500 °C. Overall, a broad framework has been established that connects process factors to distinguish resultant Ag NP features in order to develop unique silver nanoparticles for specific applications.

Mesoporous Ag–TiO2 based nanocage like structure as sensitive and recyclable low-cost SERS substrate for biosensing applications

We fabricated a porous Ag–TiO2 film with a nanocage (NC) like structure as a sensitive, recyclable, and low-cost SERS substrate using facile methods. The developed nanocage structure of TiO2 film was first coated on a glass plate by doctor-blade method, and then thin film of silver with an optimum thickness of 10 nm was deposited onto it using thermal evaporation method. The unique mesoporous cage-like structure of TiO2 film leads to the exceptional and non-uniform growth of Ag film. The developed Ag nanostructure is also porous, caps spherical in shape, and the average separation between two consecutive nanostructures is 10 nm. These closely decorated nanostructures of Ag produce hotspots by localising the incident electric field within the minimal volume. The unique nanocage structure of TiO2 film provides a larger effective surface area of adsorption to the analytes. Thus, the synergistic effects of Ag–TiO2 NC structure exhibit significant enhancement of 108 considering rhodamine-6g (R6G) molecule as a probe using a portable handheld Raman spectrometer. The amount of electromagnetic enhancement was further investigated by 3D FDTD simulation that matches well with the experiments. The recyclability of the developed SERS substrate was explored by placing the SERS substrate under UV illumination for 130 min. The enhanced photocatalytic activity of Ag–TiO2 NC SERS film degrades the dye molecules under UV radiation multiple times successively. Further, the Ag–TiO2 SERS substrate is highly sensitive to detect various concentrations of urea up to 1 mM covering the vital range of blood urea level. Thus, the optimised, low-cost, reusable SERS substrate can be used as level free biosensors for routine analysis of biomolecules in clinical applications.

Nanostructured silver substrates produced by cluster-assisted gas jet deposition for surface-enhanced Raman spectroscopy

The gas jet deposition (GJD) technique is a very promising method for synthesis of nanostructured films allowing fabrication of uniform thin layers over large areas with high growth rates. In this work, thin silver films of various morphology with the thickness in the range 10–50 nm are produced by the GJD method using a supersonic jet of silver vapor with helium as a carrier gas. The film morphology transformation with increasing thickness is shown to occur from individual nanostructures to a continuous film. The jet is investigated by time-of-flight mass spectrometry and small silver clusters in the deposited flow are observed which are supposed to play a key role in the nanostrtucture formation. The GJD-produced films demonstrate excellent uniformity over a ∼100 cm2 deposited area and exhibit remarkable plasmonic properties. The surface-enhancement Raman scattering (SERS) activity of the films to R6G is investigated and the enhancement factor of about 105 is obtained with the detection limit of 10−8 M R6G. The influence of morphology on plasmonic properties of the films is discussed.

Surface resonance properties of thin silver films with nanoparticles induced by pulsed-laser interference dewetting process

Surface resonance properties of thin silver films with nanoparticles induced by pulsed-laser interference dewetting process

Para-hexaphenylene (p-6P) nanofibers grown on a silver surface for polarization-insensitive surface plasmon polariton excitation

Surface plasmon polariton excitation was demonstrated by fluorescent light from organic para-hexaphenylene (p-6P) nanofibers grown directly on a silver thin film surface. We show results of both photoluminescence lifetime measurements and angle-resolved leakage radiation spectroscopy in the excitation range 420–620 nm, which corresponds to the material photoluminescence band. The optical characteristics of randomly oriented p-6P nanofibers assembled directly on the metal surface are remarkably comparable to those of fibers grown on crystalline substrate surfaces. In addition, compared to mica-transferred fibers, the photoluminescence lifetime is longer. More importantly, they show polarization-insensitive plasmonic activity and are entirely consistent with theoretical predictions. Both analytical and numerical finite-difference time-domain simulations support the experimental data. Our findings open a new avenue for incorporating organic nanofibers into optoelectronic applications by directly depositing the p-6P material onto selected substrates. This direct deposition significantly reduces the deformation of the fiber structure and the impurity introduction, usually implied by the transfer process.

Spectral Phase Measurements of Heterodyne Detection in Interfacial Broadband Electronic Spectroscopy

A spectral phase is vital to retrieve both the real and imaginary parts of both second-order vibrational and electronic sum frequency generation spectra. Despite recent efforts, phase measurements in electronic spectra at interfaces are lacking. In particular, references are not well established for frequency-dependent spectral phases in interfacial electronic spectroscopy. In this work, we present broadband heterodyne detection (HD) of interfacial electronic spectroscopy for spectral phases of references. Such a broadband HD method was based upon a broadband short-wave IR laser source from 1100 to 2200 nm. A collinear geometry method was adopted for passive phase stability. We further compared often-used reference samples, including gold (Au) thin films, silver (Ag) thin films, p-type GaP (100), p-type GaAs (100), as well as left-handed and right-handed z-cut α-quartz crystals. The phases of the second-order susceptibilities for Ag thin films, the GaP crystal, the GaAs crystal, and the quartz crystals are independent of frequency from 630 to 730 nm. More importantly, it was found that the often-used Au thin film in the literature is not a good phase reference for frequency-dependent phases.

The Effects of Stresses and Interfaces on Texture Transformation in Silver Thin Films.

Thin metal films are critical elements in nano- and micro-fabricated technologies. The texture orientation of thin films has a significant effect on applied devices. For Face-Centered Cubic (FCC) metal thin films, when the critical thickness is reached, the texture orientation can transform from (111) to (100) based on the model related to the balance between interfacial energy and strain energy. This research focused on the texture transformation of thin films under two conditions: (1) with or without an adhesion layer in the thin film and (2) with or without initial stress applied through a four-point bending load. In the experiment, two samples (silicon/silver and silicon/titanium/silver) were used to apply different initial stress/strain values and different annealing times. After annealing, an X-ray Diffractometer (XRD) was used to ascertain the preferred orientation of the thin films and the percentage of (111) and (100). Finally, Electron Back-Scattered Diffraction (EBSD) was used to observe the grain size of the thin films. The results showed that, regardless of the existence of an adhesion layer, texture transformation occurred, and this was relatively significant with Ti adhesion layers. Further, the initial stress was found to be small compared to the internal stress; thus, the initial stress imposed in the tests in this research was not significantly influenced by the texture transformation.

Aluminum Doping Effect on Surface Structure of Silver Ultrathin Films.

Ultrathin silver films with low loss in the visible and near-infrared spectrum range have been widely used in the fields of metamaterials and optoelectronics. In this study, Al-doped silver films were prepared by the magnetron sputtering method and were characterized by surface morphology, electrical conductivity, and light transmittance analyses. Molecular dynamics simulations and first-principles density functional theory calculations were applied to study the surface morphologies and migration pathway for the formation mechanisms in Al-doped silver films. The results indicate that the migration barrier of silver on a pristine silver surface is commonly lower than that of an Al-doped surface, revealing that the aluminum atoms in the doping site decrease the surface mobility and are conducive to the formation of small islands of silver. When the islands are dense, they coalesce into a single layer, leading to a smoother surface. This might be the reason for the observably lower 3D growth mode of silver on an Al-doped silver surface. Our results with electronic structure insights on the mechanism of the Al dopants on surface morphologies might benefit the quality control of the silver thin films.

Effect of sputtered silver thin film thickness towards morphological and optical properties of black silicon fabricated by two-step silver-assisted wet chemical etching for solar cells application

Effect of sputtered silver (Ag) thin film thickness towards morphological and optical properties of black silicon (b-Si) fabricated by two-step silver-assisted wet chemical etching for solar cells applications is investigated. The method involves low temperature annealing of crystalline silicon (c-Si) coated with Ag thin films of 10 nm, 15 nm, and 25 nm. This is followed by an etching in a solution of HF:H2O2:DI H2O (1:5:10 volume ratio) at room temperature for 70 s. Dense and spherical Ag NPs with an average diameter of 203 ± 17.8 nm and surface coverage of about 72.5% are achieved on a sample with Ag film thickness of 15 nm prior to the annealing process. After the etching, the average nanopores’ height of ~420 nm with an average diameter of ~200 nm owing to denser Ag NPs on the c-Si surface before the etching are obtained. Optical absorption enhancement due to low weight average reflection (WAR) within wavelength region of 300–1100 nm is observed on the b-Si wafers. Sample with 15 nm of Ag thin film prior to annealing, demonstrates WAR of 7.7% compared 40.0% of the reference planar c-Si. The low WAR is due to the efficient light coupling effect of the b-Si nanopores. The fabricated b-Si nanopores can be used in b-Si solar cells for enhanced optical absorption and high photocurrent in the future.

Morphological features of micro- and nanoporous silver and copper films synthesized by the substitution reaction method

The results of a study of the structural features of thin silver and copper films synthesized by the chemical substitution reaction are presented. Silver films were obtained by immersing copper substrates in a solution of silver nitrate. Copper films were synthesized by immersing iron and tin-plated iron substrates in a copper sulfate solution. The study of the morphology and composition of the synthesized layers was carried out using a scanning electron microscope. It has been shown that metal nanoporous layers up to 1 μm thick are formed on the substrates as early as 2-3 s after the start of the reaction. The layers consist of microcrystalline hexagonal plates and micro- and nanodendrites. As the reaction time increases, the layers become denser. In this case, the minimum pore size is 20 nm. The synthesized nanoporous films can be used for photocatalytic decomposition of water and enhancement of Raman scattering. Keywords: morphology, porous film, silver, copper, substitution reaction.

Морфологические особенности микро- и нанопористых пленок из серебра и меди, синтезированных методом реакции замещения

The results of a study of the structural features of thin silver and copper films synthesized by the chemical substitution reaction are presented. Silver films were obtained by immersing copper substrates in a solution of silver nitrate. Copper films were synthesized by immersing iron and tin-plated iron substrates in a copper sulfate solution. The study of the morphology and composition of the synthesized layers was carried out using a scanning electron microscope. It has been shown that metal nanoporous layers up to 1 μm thick are formed on the substrates as early as 2–3 s after the start of the reaction. The layers consist of microcrystalline hexagonal plates and micro- and nanodendrites. As the reaction time increases, the layers become denser. In this case, the minimum pore size is 20 nm. The synthesized nanoporous films can be used for photocatalytic decomposition of water and enhancement of Raman scattering.

Surface Plasmon resonance implemented silver thin film PCF sensor with multiple – Hole microstructure for wide ranged refractive index detection

This paper aims to demonstrate the sensing of different materials based on their refractive indices. The Surface Plasmon Resonance(SPR) phenomenon shown by Photonic Crystal Fiber(PCF) is proving to be quite useful among the sensor markets and thus, it becomes really important to design a flexible and affordable sensor of this kind with high sensitivity. SPR based PCF sensor with plasmonic material, silver embedded on it has been designed with the implementation of the Finite element method. We have modeled SPR sensors with multiple air holes and silica is the background material. The basic principle of using the analyte is that it binds the immobile molecules of silver to its own molecules which get mobilized in presence of the incident light and finally cause the variation of the refractive index. When the refractive indices of the analyte are varied from 1.25 to 1.30, a range of variation in the loss curves is seen. The core and Surface Plasmon Polaritons modes are matched based on their phases and it is done by plotting the refractive indices of core and SPP modes against the wavelength in µm along with the confinement loss against wavelength. The sensor that is obtained is highly sensitive with a wavelength sensitivity of around 1932.09 RIU−1. The resolution of the sensor is 3 × 10-5 RIU. On comparison with the literature, it is found that the sensitivity of the sensor so achieved is highly suitable for the detection purposes in the field of pharmacy and this Ag-based photonic crystal fiber sensor proves to be a revolutionary sensor in the field of biosensors.

Hydrothermal Synthesis of Ag Thin Films and Their SERS Application.

In this article, a facile, one-step method for the formation of silver thin-film nanostructures on the surface of Al2O3 substrates using the hydrothermal method is proposed. The dependence of the SERS effect intensity of the formed films during the detection of methylene blue (MB) low concentrations on the synthesis conditions, additional temperature treatment, and laser radiation wavelength (532 and 780 nm) in comparison with similar dye films on commercial SERS substrates is shown. The detection limit of the analyte used for the indicated lasers is estimated. The effect of the synthesis temperature on the particle size, crystal structure, and microstructure features of the obtained thin films based on silver nanoparticles is demonstrated. Using spreading resistance microscopy, the interface between the substrate and Ag particles is studied, and the dependence of the size of the corresponding gap between them and the nature of microstructural defects on the parameters of hydrothermal treatment of reaction systems in the presence of Al2O3 substrates is shown. As a result of the study, the factors associated with the properties of the obtained SERS substrates and the parameters of recording the spectra, which affect the amplification factor of the spectral lines intensity of the analyte, are revealed.