Ultrathin Silver Film Research Articles

Antireflective glass nanoholes on optical lenses.

Antireflective structures, inspired from moth eyes, are still reserved for practical use due to their large-area nanofabrication and mechanical stability. Here we report an antireflective optical lens with large-area glass nanoholes. The nanoholes increase light transmission due to the antireflective effect, depending on geometric parameters such as fill factor and height. The glass nanoholes of low effective refractive index are achieved by using solid-state dewetting of ultrathin silver film, reactive ion etching, and wet etching. An ultrathin silver film is transformed into nanoholes for an etch mask in reactive ion etching after thermal annealing at a low temperature. Unlike conventional nanopillars, nanoholes exhibit high light transmittance with enhancement of ~4% over the full visible range as well as high mechanical hardness. Also, an antireflective glass lens is achieved by directly employing nanoholes on the lens surface. Glass nanoholes of highly enhanced optical and mechanical performance can be directly utilized for commercial glass lenses in various imaging and lighting applications.

Integrated structural color array enabled by ultrathin silver film via cavity-enhanced absorption

In this work, we experimentally demonstrate enhanced absorption resonance enabled by the deconstructive interference of the reflected light in a sandwiched nanostructures composing of the ultrathin silver layer, transparent SiO2 cavity, and the opaque metallic substrate (e.g., 300-nm Ag). Such cavity-enhanced absorption is tunable via modulating the thickness of the absorptive ultrathin silver layer or the embedded transparent SiO2 cavity, giving rise to tunable structural coloration. Through five deposition processes, a 32-channel colorful sample is successfully fabricated. We believe the concepts and results make it evident that such ultrathin absorptive film offers us unprecedented opportunities on developing various photonic applications such as labeling, visual arts, and optical filters.

Metamaterial-inspired compact optical coating for broadband polarization beam splitting.

A metal-dielectric is designed and fabricated as a compact polarization beam splitter. High p-polarized transmission is achieved by admittance matching, which is developed using a normalized admittance diagram. High s-polarized reflection is achieved with a metal-like equivalent s-polarized admittance which real part is much smaller than its imaginary part. An ultra-thin silver film with a thickness of around 11 nm is deposited to form an Ag-SiO2 multilayer. The polarization beam splitter requires only three or five layers of thin films to perform broadband beam splitting over wavelengths from 450 nm to 850 nm.

Optical constants of ultrathin silver films obtained by ellipsometry and spectrophotometer

Optical constants of ultrathin silver films obtained by ellipsometry and spectrophotometer

Measuring optical constants of ultrathin layers using surface-plasmon-resonance-based imaging ellipsometry.

A setup for surface-plasmon-resonance- (SPR) based imaging ellipsometry was developed, which gains from the sensitivities of both SPR and ellipsometry to ultrathin film parameters. It is based on Otto's configuration for prism-sample coupling and a wide-beam imaging ellipsometry. A set of ultrathin gold and silver films was measured to determine their optical constants and thicknesses. Coupling the sample using a prism with a convex surface enables us to capture images of generated SPR elliptical fringes, which correspond to different SPR amplitude values at different air gap thicknesses. Analysis of the images acquired at different polarizer and analyzer angles provides the ellipsometric functions Ψ and Δ versus thickness of air gap and hence the extraction of the optical constants of ultrathin metal films. The measured film thickness is in agreement with the results of x-ray reflectivity measurements.

Chemically imaging bacteria with super-resolution SERS on ultra-thin silver substrates

Plasmonic hotspots generate a blinking Surface Enhanced Raman Spectroscopy (SERS) effect that can be processed using Stochastic Optical Reconstruction Microscopy (STORM) algorithms for super-resolved imaging. Furthermore, by imaging through a diffraction grating, STORM algorithms can be modified to extract a full SERS spectrum, thereby capturing spectral as well as spatial content simultaneously. Here we demonstrate SERS and STORM combined in this way for super-resolved chemical imaging using an ultra-thin silver substrate. Images of gram-positive and gram-negative bacteria taken with this technique show excellent agreement with scanning electron microscope images, high spatial resolution at <50 nm, and spectral SERS content that can be correlated to different regions. This may be used to identify unique chemical signatures of various cells. Finally, because we image through as-deposited, ultra-thin silver films, this technique requires no nanofabrication beyond a single deposition and looks at the cell samples from below. This allows direct imaging of the cell/substrate interface of thick specimens or imaging samples in turbid or opaque liquids since the optical path doesn’t pass through the sample. These results show promise that super-resolution chemical imaging may be used to differentiate chemical signatures from cells and could be applied to other biological structures of interest.

Ефект зниження порогу перколяції у надтонких плівках срібла при квазирівноважній конденсації та дії плазми на ростову поверхню

Ефект зниження порогу перколяції у надтонких плівках срібла при квазирівноважній конденсації та дії плазми на ростову поверхню

The influence of tertiary butyl hydrazine as a co-reactant on the atomic layer deposition of silver

Ultra-thin conformal silver films are the focus of development for applications such as anti-microbial surfaces, optical components and electronic devices. In this study, metallic silver films have been deposited using direct liquid injection thermal atomic layer deposition (ALD) using (hfac)Ag(1,5-COD) ((hexafluoroacetylacetonato)silver(I)(1,5-cyclooctadiene)) as the metal source and tertiary butyl hydrazine (TBH) as a co-reactant. The process provides a 23°C wide ‘self-limiting’ ALD temperature window between 105 and 128°C, which is significantly wider than is achievable using alcohol as a co-reactant. A mass deposition rate of ∼20ng/cm2/cycle (∼0.18Å/cycle) is observed under self-limiting growth conditions. The resulting films are crystalline metallic silver with a near planar film-like morphology which are electrically conductive. By extending the temperature range of the ALD window by the use of TBH as a co-reactant, it is envisaged that the process will be exploitable in a range of new low temperature applications.

Optical performance of ultra-thin silver films under the attenuated total reflection mode

Ultra-thin silver films were deposited by thermal evaporation, and the dielectric functions of samples were simulated using Drude-Lorentz oscillators. When s-polarized incident light from the BK7 glass into thin silver film at 45° angle using attenuated total reflection (ATR) mode, we experimental observed that the reflection reach a minimum of 1.87% at 520 nm for thickness of d~6.3 nm silver film, and it reach a minimum of 10.1% at 500 nm for thickness of d~4.1 nm. Moreover, we simulated the absorption changes with incident angles at 520 nm for both p-polarized (TM wave) and s-polarized (TE wave) light using transfer matrix theory, and calculated the electric field distributions. The absorption as a function of incident angles of TM wave and TE wave showed different characteristics under ATR mode, TE wave reached the maximum absorption around the critical angle θc~41.1°, while TM wave reached the minimum absorption.

Development of plasma assisted thermal vapor deposition technique for high-quality thin film.

The novel technique of Plasma-Assisted Vapor Deposition (PAVD) is developed as a new deposition method for thin metal films. The PAVD technique yields a high-quality thin film without any heating of the substrate because evaporated particles acquire energy from plasma that is confined to the inside of the evaporation source. Experiments of silver thin film deposition have been carried out in conditions of pressure lower than 10-3 Pa. Pure silver plasma generation is verified by the measurement of the Ag-I peak using optical emission spectroscopy. A four point probe and a UV-VIS spectrophotometer are used to measure the electrical and optical properties of the silver film that is deposited by PAVD. For an ultra-thin silver film with a thickness of 6.5 nm, we obtain the result of high-performance silver film properties, including a sheet resistance <20 Ω sq-1 and a visible-range transmittance >75%. The PAVD-film properties show a low sheet resistance of 30% and the same transmittance with conventional thermal evaporation film. In the PAVD source, highly energetic particles and UV from plasma do not reach the substrate because the plasma is completely shielded by the optimized nozzle of the crucible. This new PAVD technique could be a realistic solution to improve the qualities of transparent electrodes for organic light emission device fabrication without causing damage to the organic layers.

Durability of Ultra-Thin Silver Films and Silver—Gold Alloy Films under UV Irradiation**Supported by the National Natural Science Foundation of China under Grant No 61308070.

Silver films (Ag) and silver—gold films (Ag—Au) with thickness ∼15 nm are coated on Bk7 glasses through thermal evaporation. After doping gold of 5.2%, the grain size of the Ag film reduces from 13.6 nm to 9.1 nm, also the surface roughness decreases from 1.45nm to 0.94nm. A UV lamp is used as the irradiation light source to accelerate the corrosion process in the atmosphere. After 17h irradiation, the pure silver film surface turns dark, and the transmittances reduce from 350 nm to 500 nm, while the Ag—Au film degrades much less, almost negligibly after UV radiation. Additional x-ray photoelectron spectroscopy and atomic force micrographs data are provided to show atomic content of films and their surface morphologies. It is suggested that small grain size and high packing density of alloy film prevent reaction of silver with oxygen in the atmosphere, which leads to high stability of the Ag—Au film.

Continuous ultrathin silver films deposited on SiO2 and SiNx using a self-assembled monolayer

In this letter, we study the deposition of ultrathin silver films on silicon oxide and nitride surfaces functionalized with self-assembled monolayers of (3-mercaptopropyl)-trimethoxysilane. First, we compare both solution and vapour-phase functionalization techniques and find the greatest improvement in electrical and optical properties using deposition from solution. Using X-ray photoelectron spectroscopy, we demonstrate that the formation of silver-sulfur covalent bonds is at the root of the improved wetting confirmed by ellipsometry, sheet resistance measurement, and atomic force microscopy. Second, we show that this technique can be extended to functionalize silicon nitride. Finally, we demonstrate a simple, but efficient, low-emissivity optical filter.

Bulk-like Al/Ag bilayer film due to suppression of surface plasmon resonance for high transparent organic light emitting diodes

We demonstrate the enhanced optical and electrical properties of an ultrathin silver (Ag) film by applying an aluminum (Al) seed layer between LiF and Ag as a transparent cathode for higher-transparency organic light-emitting diodes (OLEDs). Although the thickness ranges from 4 to 8 nm, the ultrathin Ag film is a continuous and uniform bulk-like film with an Al seed layer, which suppresses the surface plasmon absorption. Compared to an Ag-only cathode, the measured transmittance spectra were considerably increased, comparable with the theoretical calculations of a bulk Al/Ag bilayer film. The Al/Ag bilayer cathode has a transmittance of 87% at a 550 nm wavelength and a sheet resistance of 19.5 Ω/sq with a 4-nm-thick Ag layer. The transparent OLED devices that employed the Al/Ag cathode showed a transmittance of 72% at a 550 nm wavelength for an Ag thickness of 6 nm.

Highly Flexible Transparent Electrodes Containing Ultrathin Silver for Efficient Polymer Solar Cells

Transparent electrodes (TEs) having electrooptical trade‐offs better than state‐of‐the‐art indium tin oxide (ITO) are continuously sought as they are essential to enable flexible electronic and optoelectronic devices. In this work, a TiO2‐Ag‐ITO (TAI)‐based TE is introduced and its use is demonstrated in an inverted polymer solar cell (I‐PSCs). Thanks to the favorable nucleation and wetting conditions provided by the TiO2, the ultrathin silver film percolates and becomes continuous with high smoothness at very low thicknesses (3–4 nm), much lower than those required when it is directly deposited on a plastic or glass substrate. Compared to conventional ITO‐TE, the proposed TAI‐TE exhibits exceptionally lower electrical sheet resistance (6.2 Ω sq−1), higher optical transmittance, a figure‐of‐merit two times larger, and mechanical flexibility, the latter confirmed by the fact that the resistance increases only 6.6% after 103 tensile bending cycles. The I‐PSCs incorporating the TAI‐TE show record power conversion efficiency (8.34%), maintained at 96% even after 400 bending cycles.

Opto-Electrical Performance Improvement of Mono c-Si Solar Cells Using Dielectric–Metal–Dielectric (D-M-D) Sandwiched Structure-Based Plasmonic Anti-Reflector

Mono c-Si solar cells based on plasmonic anti-reflector, an ultra-thin silver (Ag) film sandwiched between silicon nitride (SiNx) layers on non-textured c-Si surface, have been explored. Prior to solar cell fabrication, the plasmonic anti-reflector structure was optimized separately by varying different thicknesses of top and bottom SiNx layers and the sandwiched ultra-thin Ag film. For broad wavelength range (300–1200 nm), minimum weighted reflectance was observed in the range of 8–9 %, for the top and bottom SiNx layers having 70–80-nm thickness. The refractive index of top and bottom SiNx layers was in the range of 1.77–1.80 and 1.92–1.96, respectively. A 180-μm non-textured mono c-Si solar cell is fabricated with plasmonic anti-reflector, having top and bottom SiNx layer thickness of 80-nm and 8–9-nm sandwiched ultra-thin Ag layer, resulted in 17 % solar cell efficiency. The measured efficiency was 1.8 % higher compared to 180-μm mono c-Si solar cell with standard 80 nm SiNx anti-reflector. The short circuit current density (J sc) extracted from external quantum efficiency (EQE) has shown 0.5 mA/cm2 enhancement for the dielectric–metal–dielectric (D-M-D) sandwiched structure (plasmonic anti-reflector)-based solar cell compared to standard SiNx anti-reflection coating (ARC) structure-based solar cell. EQE enhancement was seen for all wavelengths above 700 nm with maximum 25–26 % enhancement at around 1125 nm. Also, in UV wavelength region (300–400 nm), EQE enhancement was seen with maximum 25 % enhancement at around 370 nm. Possible phenomenon for improved anti-reflection and EQE in D-M-D-based plasmonic anti-reflector has been discussed.

Optical Properties and Surface Morphology of Thin Silver Films Deposited by Thermal Evaporation**Supported by the National Natural Science Foundation of China under Grant No 61308070.

Thin silver films with different thicknesses are deposited by thermal evaporation, and the Drude–Lorentz mode is used to describe the optical properties of samples measured by an ellipsometer, and the thickness uniqueness analysis and reflection spectrum testing are used to verify the simulated results. We obtain the surface morphologies of the samples using the scanning electron microscopy, and calculate the relationship of reflectance with film thickness at wavelength 800 nm using the Mathcad software. Moreover, the effective medium approximation mode is used to explain the differences of wavelength-dependent n and k as the thickness changes. As the silver film thickness decreases, the optical constant changes regularly, and the films show flat surface, small cracks, discontinuous and island distribution, respectively, while n and k of the island distributed silver film have an intersection point in the visible spectrum. Our experiments provide an in-depth research for the ultra-thin silver film under thermal evaporation deposition, and will be helpful for its application in multilayers and plasmonic devices.

Fabrication of Plasmonically Active Ag Thin Films in the Region of Quasi-amorphous to Nanocrystalline and its Application towards SERS

The transition from quasi-amorphous to nanocrystalline state of Ag film is studied on plasmonically active ultra-thin silver films. The vital mechanisms for thin film formation known as coalescence and Ostwald ripening have been discussed. Surface plasmon resonance (SPR) is robust in the region of highly disordered to less disordered phase of Ag structure. The surface oxidation of Ag nanoparticles is studied by confocal Raman spectroscopy. The effect of SPR on Surface Enhanced Raman Scattering (SERS) enhancement of Rhodamine-6G (Rh6G) molecules studied by altering crystallinity, interparticles distance, size distribution and number density of Ag nanoparticles.

Near‐Field Analysis of Bright and Dark Modes on Plasmonic Metasurfaces Showing Extraordinary Suppressed Transmission

Plasmonic metasurfaces are investigated that consist of a sub‐wavelength line pattern in an ultrathin (∼10 nm) silver film, designed for extraordinary suppressed transmission (EOST) in the visible spectral range. Measurements with a near‐field scanning optical microscope (NSOM) demonstrate that far‐field irradiation resonantly excites antenna‐like (bright) modes that are localized on the metal ridges. The re‐radiation of these modes into the far‐field interferes destructively with the transmitted wave, thus suppressing transmission almost completely. In contrast, a second type of mode, bound (dark) surface plasmon polaritons (SPPs) launched from an NSOM tip, propagates well across the metasurface, preferentially perpendicular to the grating lines.

Local fields in nanostructured silver films

Results of theoretical and experimental investigations of local fields in prepercolation ultrathin nanostructured silver films are presented. Numerical calculations based on the film structure obtained by atomic force microscopy imaging indicate that a substantial amplification of the initial exciting field is possible. The effect is experimentally confirmed by luminescence and Raman measurements.

Fabrication of partially oxidized ultra-thin nanocrystalline silver films: effect of surface plasmon resonance on fluorescence quenching and surface enhanced Raman scattering

Plasmonically active nanocrystalline silver (Ag) films of mass thickness ∼1 nm were fabricated using thermal evaporation followed by air annealing process temperature varying from 50 °C to 250 °C. The effect of air annealing on surface morphology and optical absorbance of quasi-amorphous and nanocrystalline Ag films was studied. The possibility of formation of silver oxide (AgO) at the surface level of Ag nanostructures due to air annealing was studied using a confocal Raman spectrometer. Surface enhanced Raman scattering (SERS) enhancement and its reverse mechanism known as fluorescence quenching corresponding to Rhodamine-6G (Rh6G) molecules were studied on Ag films through altering crystallinity, interparticles distance, size distribution and number density of Ag nanopartices (NPs).