Semicontinuous Films Research Articles

Asymmetrical Absorption and Surface‐Enhanced Raman Scattering Enhancement by Silver Nanoflower Metasurface

A metasurface composed of silver nanoflower arrays, which exhibit asymmetrical absorption and surface‐enhanced Raman scattering (SERS) due to hybrid plasmonic effects, is reported. The silver nanoflowers are fabricated by oblique deposition of silver on a polymer nanohole array on a glass substrate, forming petal‐like semicontinuous thin films on the inner walls of the holes. Depending on the deposition angle, three‐ or five‐petal nanoflowers are obtained. The nanoflower arrays show strong reflection from the air side and broadband and wide‐angle absorption from the glass side, as a result of transmission surface plasmon resonance and localized surface plasmon resonance, respectively. The three‐petal structure, which absorbs most of the incident light from the glass side, induces a localized enhancement of electric field in the center of each nanohole, providing a high‐sensitivity SERS substrate. The SERS performance of the metasurface by direct measurement and near‐field simulation is demonstrated.

Recent Advances in the Plasma-Assisted Synthesis of Zinc Oxide Nanoparticles

An overview of recent work on the low-temperature plasma-assisted synthesis of zinc oxide (ZnO) nanoparticles is presented and interpreted in terms of gas-phase and surface reactions with illustrated examples. The thermodynamical nonequilibrium conditions allow the formation of chemically reactive species with a potential energy of several eV, which readily interact with the Zn precursors and initiate reactions leading to the formation of nanoparticles or nanowires. The high-quality nanowires were synthesized from Zn powders only upon interaction with moderately ionized plasma in a narrow range of plasma parameters. This technique is promising for the synthesis of large quantities of nanowires with aspect ratios well above 10, but the exact range of parameters remains to be determined. Apart from the ex situ techniques, the ZnO nanoparticles can be synthesized by depositing a film of precursors (often Zn salts or Zn-containing organometallic compounds) and exposing them to oxygen plasma. This technique is useful for the synthesis of well-adherent ZnO nanoparticles on heat-sensitive objects but requires further scientific validation as it often leads to the formation of a semicontinuous ZnO film rather than nanoparticles. Both low-pressure and atmospheric plasmas are useful in converting the precursor film into ZnO nanoparticles despite completely different mechanisms.

Influencing mechanism of inherent aluminum oxide film on coach peel performance of baked Al-Steel RSW

A traditional resistance spot weld process in combination with Multi-Ring Domed welding electrodes was applied to join AA5754-O to low carbon steel. The coach peel performance of welds was compared between the as-welded (unbaked) and post-paint bake condition (baked). The unbaked joints exhibited button pullout fracture mode, whereas the baked joints were severely weakened and demonstrated an interfacial fracture mode which is attributed to the baking process inducing thermal stress at the dissimilar material interface which cracked the semi-continuous oxide film inclusions at the edge of the solidified aluminum nugget to form a more continuous defect layer across the diameter of the aluminum nugget. These defects act as low energy crack paths under applied loads and reduce joint strength by promoting undesirable interfacial fracture in baked welds.

Gold semicontinuous thin-film-coated mesoporous TiO2 for SERS substrates

In this work, the preparation and thermal evolution of sputtered semicontinuous gold thin films obtained over ordered mesoporous TiO2 films are presented. The controlled surface topography of the ordered mesoporous TiO2 substrate was used as template for obtaining plasmonic hotspots for Surface Enhanced Raman Spectroscopy (SERS) applications. Plasmonic thin film was obtained by Au sputtering when film thickness was under circa 10 nm. Above this value, the porosity effects from the substrate were lost and Raman signal enhancement with it. Moreover, heat treatment was explored in order to obtain gold nanoislands that can also provide very efficient Raman enhancement. Hence, a very straightforward synthesis procedure is presented for sensing application, even for large scale production.

Precise control of hydrogen response of semicontinuous palladium film using piezoelectric resonance method

During deposition of metallic material on a substrate, a semicontinuous film composed of isolated and connected clusters is formed transiently at a few nanometers thickness. The surface electrical conductivity at this moment is governed by the tunneling conduction, and slight subsequent evolution of the film morphology changes the conductivity markedly because of the island connection. When the semicontinuous palladium film is exposed to hydrogen, its morphology changes because of absorption of hydrogen, changing the surface conductivity drastically. This phenomenon is applicable for hydrogen sensing. However, it has been significantly difficult to fabricate an optimum semicontinuous structure because it appears in a very short time during deposition. In the present study, we precisely control the palladium film morphology using the piezoelectric resonance method. In this method, an electric field is excited around the substrate surface using the resonant vibration of the piezoelectric material. The electric field generates the electrical current in the deposited material, and the vibrational energy of the piezoelectric material is spent on it. Because the energy loss depends on the electrical conductivity (morphology) of the deposited material, by measuring the change in the attenuation of the resonant vibration, evolution of the morphology is detectable. Using this method, palladium films with several morphologies were fabricated, and the conductivity change in hydrogen was evaluated. The change ratio of the conductivity significantly depended on the morphology, and the conductive sensitivity to the hydrogen detection with the optimum structure was larger than that reported in the previous study by a factor of 12.

Laser-induced color printing on semicontinuous silver films: red, green and blue

Coloration using semicontinuous metal films has been explored recently and has shown far-reaching interest as a result of being inexpensive, environment-friendly, and non-bleaching. In this paper, we demonstrate the generation of bright colors through laser modification of semicontinuous Ag films. A palette of colors is obtained from blue through green, orange, up to red, and the potential exists to obtain other hues through varying the scan speed, number of pulses, energy density, power, and exposure time. This unique process can be applied to the macroscopic, mesoscopic and nanoscopic printing of innovatory fade-free artistic images as one example of application.

Deposition of Semicontinuous Film on Silicon Substrate using Noncontacting Piezoelectric Resonance Method

Deposition of Semicontinuous Film on Silicon Substrate using Noncontacting Piezoelectric Resonance Method

Optical reconfiguration and polarization control in semi-continuous gold films close to the percolation threshold.

Controlling and confining light by exciting plasmons in resonant metallic nanostructures is an essential aspect of many new emerging optical technologies. Here we explore the possibility of controllably reconfiguring the intrinsic optical properties of semi-continuous gold films, by inducing permanent morphological changes with a femtosecond (fs)-pulsed laser above a critical power. Optical transmission spectroscopy measurements show a correlation between the spectra of the morphologically modified films and the wavelength, polarization, and the intensity of the laser used for alteration. In order to understand the modifications induced by the laser writing, we explore the near-field properties of these films with electron energy-loss spectroscopy (EELS). A comparison between our experimental data and full-wave simulations on the exact film morphologies hints toward a restructuring of the intrinsic plasmonic eigenmodes of the metallic film by photothermal effects. We explain these optical changes with a simple model and demonstrate experimentally that laser writing can be used to controllably modify the optical properties of these semi-continuous films. These metal films offer an easy-to-fabricate and scalable platform for technological applications such as molecular sensing and ultra-dense data storage.

Laser-Scanned Programmable Color Temperature of Electroluminescence from White Light-Emitting Electrochemical Cells.

Recently, the control of correlated color temperature (CCT) of artificial solid-state white-light sources starts to attract more attention since CTs affect human physiology and health profoundly. In this work, we proposed and demonstrated a method that can widely tune the CCTs of electroluminescence (EL) from white-light-emitting electrochemical cells (LECs) by employing plasmonic filters. These integrated on-chip plasmonic filters are composed of semicontinuous thin Ag film or Ag nanoparticles (NPs) both included in the indium tin oxide anode contact, which have different characteristics of plasmonic resonant absorptions that can tune the EL spectra of white LECs. The CCTs of EL from white LECs integrated with semicontinuous thin Ag film and randomly distributed Ag NPs are 5778 and 2350 K, respectively. A commercially available laser scanning system was used to locally thermal anneal the semicontinuous thin Ag film to form the randomly distributed Ag NPs on the scanned areas. Hence, these two kinds of filters can be integrated on the same chip of white LEC, giving more freedom to control the CCTs of white EL and more potential applications. In addition, the laser scanning system used here is quite often used in display manufactures so that our proposed method can be immediately adopted by the light-emitting diode industry.

Effect of W on tempering behaviour of a 3 %Co modified P92 steel

The tempering behaviour of two 9Cr–3Co–0.5Mo–0.2 V–0.05Nb–0.005B steels containing 2 and 3 wt% W was examined. The decomposition of retained austenite induces the formation of semi-continuous films of W-rich M23C6 carbides along boundaries at 525 °C. The increase in W content from 2 to 3 wt% modifies the normal carbide precipitation sequence in the vicinity of these boundaries. The formation of W segregation at prior austenite grain/interlath boundaries leads to formation of the thermodynamically metastable W-rich M6C carbides. Finally, these M6C carbides transform to a stable Laves phase. Two-phase separation of MX carbonitrides into Nb-rich and V-rich dispersoids appears in both steels after tempering at 750 °C. This separation is thermodynamically metastable and attributed to the precipitation of Nb-rich MX carbonitrides with a round shape on dislocations at T ≤ 450 °C and V-rich MX carbonitrides within ferritic matrix at 750 °C. Moreover, in the steel with 2 %W, the additional separation of V-rich MX carbonitrides into W-rich particles located in the vicinity of the prior austenite grain/lath boundaries and W-free ones located within the interiors of laths was observed.

Localized surface plasmon resonances dominated giant lateral photovoltaic effect observed in ZnO/Ag/Si nanostructure.

We report substantially enlarged lateral photovoltaic effect (LPE) in the ZnO/Ag/Si nanostructures. The maximum LPE sensitivity (55.05 mv/mm) obtained in this structure is about seven times larger than that observed in the control sample (7.88 mv/mm) of ZnO/Si. We attribute this phenomenon to the strong localized surface plasmon resonances (LSPRs) induced by nano Ag semicontinuous films. Quite different from the traditional LPE in PN junction type structures, in which light-generated carriers contributed to LPE merely depends on direct excitation of light in semiconductor, this work firstly demonstrates that, by introducing a super thin metal Ag in the interface between two different kinds of semiconductors, the nanoscale Ag embedded in the interface will produce strong resonance of localized field, causing extra intraband excitation, interband excitation and an enhanced direct excitation. As a consequence, these LSPRs dominated contributions harvest much more carriers, giving rise to a greatly enhanced LPE. In particular, this LSPRs-driven mechanism constitutes a sharp contrast to the traditional LPE operation mechanism. This work suggests a brand new LSPRs approach for tailoring LPE-based devices and also opens avenues of research within current photoelectric sensors area.

Optical control of fluorescence through plasmonic eigenmode extinction.

We introduce the concept of optical control of the fluorescence yield of CdSe quantum dots through plasmon-induced structural changes in random semicontinuous nanostructured gold films. We demonstrate that the wavelength- and polarization dependent coupling between quantum dots and the semicontinuous films, and thus the fluorescent emission spectrum, can be controlled and significantly increased through the optical extinction of a selective band of eigenmodes in the films. This optical method of effecting controlled changes in the metal nanostructure allows for versatile functionality in a single sample and opens a pathway to in situ control over the fluorescence spectrum.

Second harmonic generation enhancement at the percolation threshold

We systematically investigate the second harmonic generated (SHG) signal created by metallic semi-continuous thin films as a function of the effective layer thickness for a large number of samples. The control of the film thickness allows us to precisely map the region around the percolation threshold. We obtain a sharp and intense SHG peak at the critical thickness where the near static permittivity diverges. This happens in the regime of almost touching particles while the thin film does not behave as a metal yet. This large SHG signal is obtained with samples fabricated using a thin film deposition technique that is easily wafer scalable.

Aggregation Behaviors of Novel Amphiphilic Dendrimers at Solid-Liquid Interface

Generation 1 poly(amidoamine) dendrimer is modified with different alkyl tails. A series of novel quaternary ammonium with dendrimeric framework has been obtained, which is abbreviated as QPAMCm (m represents the carbon number of the alkyl tail). The modified dendrimers were deposited onto the mica substrate from the solution and the aggregation properties including the morphology and the dynamic process of the adsorption were studied by atomic force microscope (AFM), x-ray photoelectron spectroscopy (XPS), and contact angle measurement. Results show that the bulk concentration and the length of hydrophobic tails significantly affect the aggregate morphology at mica-aqueous interface. With the increasing of the bulk concentration and the adsorption time the sphere aggregates transform to semi-continuous film, which is very different from conventional surfactants and linear polymers. The interaction between the hydrophobic tails might be stronger and the alkyl chains may tilt or wind when the end tail is long enough. The contact angle measurement indicates that the hydrophobicity of the solid surface adsorbed with the dendrimers vary regularly. The contact angles rise with the increasing periphery alkyl chains length of the dendrimers.

Experimental evidence of nanometer-scale confinement of plasmonic eigenmodes responsible for hot spots in random metallic films

We report on the identification and nanometer scale characterization over a large energy range of random, disorder-driven, surface plasmons in silver semicontinuous films embedded in silicon nitride. By performing spatially resolved electron energy loss spectroscopy experiments, we experimentally demonstrate that these plasmons eigenmodes arise when the films become fractal, leading to the emergence of strong electrical fields (``hot spots'') localized over few nanometers. We show that disorder-driven surface plasmons strongly depart from those usually found in nanoparticles, being strongly confined and randomly and densely distributed in space and energy. Beyond that, we show that they have no obvious relation with the local morphology of the films, in stark contrast with surface plasmon eigenmodes of nanoparticles.

An interference coating of metamaterial as an ultrathin light absorber in the violet-to-infrared regime

A metamaterial with brief and ultrathin structure performs high efficiency in light absorption. An upright aluminum nanorod array (Al NRA) is obliquely deposited, measured, and analyzed its optical property. The Al NRA performs high efficiency of light absorption and low reflectance simultaneously. Based on the measured refractive index and impedances, the wave propagation through the Al NRA is traced to demonstrate the destructive interference that leads to antireflection. According to the analysis of wave tracing, an Al semicontinuous film with thickness of 15nm is introduced under an Al NRA with thickness of only 245nm as a brief and thin two-layered structure. The broadband and polarization-independent light absorption is measured over the violet-to-infrared regime.

A microscopic study of strongly plasmonic Au and Ag island thin films

Thin Au and Ag evaporated films (∼5 nm) are known to form island-like growth, which exhibit a strong plasmonic response under visible illumination. In this work, evaporated thin films are imaged with high resolution transmission electron microscopy, to reveal the structure of the semicontinuous metal island film with sub-nm resolution. The electric field distributions and the absorption spectra of these semicontinuous island film geometries are then simulated numerically using the finite difference time domain method and compared with the experimentally measured absorption spectra. We find surface enhanced Raman scattering (SERS) enhancement factors as high as 108 in the regions of small gaps (≤2 nm), which dominate the electromagnetic response of these films. The small gap enhancement is further substantiated by a statistical analysis of the electric field intensity as a function of the nanogap size. Areal SERS enhancement factors of 4.2 × 104 are obtained for these films. These plasmonic films can also enhance the performance of photocatalytic and photovoltaic phenomena, through near-field coupling. For TiO2 photocatalysis, we calculate enhancement factors of 16 and 19 for Au and Ag, respectively. We study the effect of annealing on these films, which results in a large reduction in electric field strength due to increased nanoparticle spacing.

Surface Plasmon States in Inhomogeneous Media at Critical and Subcritical Metal Concentrations

Semicontinuous metal-dielectric films are composed of a wide range of metal clusters of various geometries—sizes as well as structures. This ensures that at any given wavelength of incident radiation, clusters exist in the film that will respond resonantly, akin to resonating nanoantennas, resulting in the broad optical response (absorption) that is a characteristic of semicontinuous films. The physics of the surface plasmon states that are supported by such systems is complex and can involve both localized and propagating plasmons. This chapter describes near-field experimental and numerical studies of the surface plasmon states in semicontinuous films at critical and subcritical metal concentrations and evaluates the local field intensity statistics to discuss the interplay between various eigenmodes.

Electron back-scattered diffraction of crystallized vanadium dioxide thin films on amorphous silicon dioxide

Crystalline films and isolated particles of vanadium dioxide (VO2) were obtained through solid phase crystallization of amorphous vanadium oxide thin films sputtered on silicon dioxide. Electron back-scattered diffraction (EBSD) was used to study the crystals obtained in the thin films, to differentiate them from different vanadium oxide stoichiometries that may have formed during the annealing process, and to study their phase and orientation. EBSD showed that the crystallization process yielded crystalline vanadium dioxide thin films, semi-continuous thin films, and films of isolated particles, and did not show evidence of other vanadium oxide stoichiometries present. Indexing of the crystals for the orientation study was performed using EBSD patterns for the tetragonal phase of vanadium dioxide, since it was observed that EBSD patterns for the monoclinic and tetragonal phases of vanadium dioxide are not distinguishable by computer automated indexing. Using the EBSD patterns for the tetragonal phase of vanadium dioxide, orientation maps showed that all VO2 crystals that were measurable (approximately the thickness of the film) had a preferred orientation with the c-axis of the tetragonal phase parallel to the plane of the specimen.

Demonstration of nonlinear absorption in Au semi-continuous film by electrical measurement

We have demonstrated the nonlinear absorption at 532 nm wavelength in an Au semi-continuous film (SF) resulting from smearing of the Fermi distribution and diffusion of conduction electrons into the substrate. The Au SF was irradiated by a pulsed laser with 8 ns pulse width at 532 nm in near resonance with the interband transition of the Au. We determined the temperature increase in the SF for different intensities by electrical measurement. We calculated the temperature increase by using a 1D heat transport equation; comparing the results of the calculation with measured values for the temperature increase, revealed the nonlinear absorption in the Au SF. We employed this deviation from linear behaviour to determine the nonlinear absorption coefficient.