Bilayer Thin Films Research Articles

Nanostructured PEDOT-based multilayer thin films with high thermoelectric performances

Intrinsically conductive polymers are considered as one of the most promising candidates for thermoelectric (TE) materials due to their outstanding properties. Prior studies have been primarily focused on conducting polymers such as polyaniline, polypyrrole, polythiophene, and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). In particular, currently available cationic water-soluble conjugated polymers are limited because of difficulties in synthetic routes. Herein, a positively charged PEDOT nanoparticles (PEDOT:NPs) dispersed in water are synthesized to serve as a structure directing agent for the development of hierarchical architectures with high flexibility and TE performances. A completely organic composite is fabricated by alternately depositing layers of aqueous solutions of PEDOT:NPs and double-walled carbon nanotubes (DWNT) stabilized with PEDOT:PSS via a layer-by-layer methodology. A 20 bilayer thin film (≈ 2.1 µm thick), comprised of a PEDOT:NPs/DWNT-PEDOT:PSS repeating sequence, exhibits high electrical conductivities of up to 744 S cm−1 and a high Seebeck coefficient up to 83 µV K−1. As a result, the multilayer thin films achieve a power factor of 512 µW m−1 K−2. These excellent TE properties can be attributed to the formation of a three-dimensional conjugated network with a highly ordered structure, which facilitates carrier transport within the film. This organic composite, based on a newly synthesized conducting polymer, with high flexibility and power factor offers a promising route to utilize efficient thermoelectric devices on flexible surfaces.

Surface and interface properties of TiO2/CuO thin film bilayers deposited by rf reactive magnetron sputtering

Thin film bilayers of TiO2/CuO with intentionally varied thickness of the top layer of TiO2(25–150 nm) were grown by means of sequential rf magnetron reactive sputtering from Cu and Ti metallic targets in a flow rate-controlled Ar + O2 gas mixture. XRD diffraction in GID configuration, SEM top and cross-sectional images, EDS combined with SEM, XRR profiles, UV–VIS-NIR spectrophotometry and synchrotron soft radiation XAS in TEY mode were used to characterize crystal structure, morphology, the mode of growth, chemical composition, density and roughness as well as the optical and electronic properties of the synthesized thin film bilayers. CuO bottom layer turned out to crystallize in a monoclinic structure while TiO2 top layer remained amorphous. Film roughness of CuO decreased from about 12 nm with the increasing thickness of TiO2 thin film to about 1–2 nm. X-ray absorption spectroscopy demonstrated a presence of Cu+ and Cu2+ in TiO2 top layer of TiO2/CuO heterostructure. The photoelectrochemical measurements revealed that thin film bilayers of TiO2/CuO behaved as photocathodes in the process of water splitting. The photocurrent density was found to be an increasing function of the top TiO2 layer thickness.

Elucidating the performance benefits enabled by YSZ/Ni-YSZ bilayer thin films in a porous anode-supported cell architecture.

Increasing the performance and improving the stability of solid oxide cells are critical requirements for advancing this technology toward commercial applications. In this study, a systematic comparison of anode-supported cells utilizing thin films with those utilizing conventional screen-printed yttria-stabilized zirconia (YSZ) is performed. High-resolution secondary ion mass spectrometry (SIMS) imaging is used to visualize, for the first time, the extent of Ni diffusion into screen-printed microcrystalline YSZ electrolytes of approximately 2-3 μm thickness, due to the high temperature (typically >1300 °C) used in the conventional sintering process. As an alternative approach, dense YSZ thin films and Ni(O)-YSZ nanocomposite layers are prepared using pulsed laser deposition (PLD) at a relatively low temperature of 750 °C. YSZ thin films exhibit densely packed nanocrystalline grains and a remarkable suppression of Ni diffusion, which are further associated with some reduction in the ohmic resistance of the cell, especially in the low temperature regime. Moreover, the use of a Ni-YSZ nanocomposite layer resulted in improved contact at the YSZ/anode interface as well as a higher density of triple phase boundaries due to the nanoscale Ni and YSZ grains being homogeneously distributed throughout the structure. The cells utilizing the YSZ/Ni-YSZ bilayer thin films show excellent performance in fuel cell operation and good durability in short-term operation up to 65 hours. These results provide insights into ways to improve the electrochemical performance of SOCs by utilizing innovative thin film structures in conjunction with commercially viable porous anode-supported cells.

Instability and rupture of surfactant-laden bilayer thin liquid films.

The stability of surfactant-laden bilayer thin films, where the top layer is subject to van der Waals driven breakup, is of particular relevance to applications where one thin liquid layer is spread on another, such as film-forming firefighting foams and multilayer coatings. Although there has been much prior modeling work on the stability of thin liquid bilayers, additional physical effects and assumptions were incorporated in those studies, making it difficult to isolate the influence of surfactant on the rupture of the top layer. The present work addresses this issue through application of the lubrication approximation to derive a coupled system of nonlinear evolution equations describing the perturbations to the liquid-liquid and liquid-air interfaces and the surfactant interfacial concentrations. The surfactant is assumed to be insoluble and can be present at each interface. Linear stability analysis suggests, and nonlinear simulations confirm, that by using surfactant that adsorbs to both interfaces, the rupture time can be increased by an order of magnitude relative to the surfactant-free case. However, we find it crucial to have the right amount of surfactant to generate strongly stabilizing Marangoni stresses without reducing the interfacial tension too much. Nonlinear simulations and linear stability analysis provide insight into the mechanisms of the delayed rupture and show how the direction and strength of the Marangoni stresses strongly depend on the viscosity ratio of the layers. These results can help guide the choice and design of surfactants to achieve more effective firefighting foams and more stable liquid coatings.

Highly Sensitive, Selective and Stable NO2 Gas Sensor Based on Porous Influenced SnO2 Bilayer Thin Films

In this work, an attempt has been made to fabricate SnO2 porous films using automated nebulizer spray pyrolysis technique with the influence of a porogen. Pure tin dioxide and porous tin dioxide (SnO2/SnO2) thin films were prepared from a precursor solution composed of SnCl2·2H2O and polyethylene glycol as a porogen. The structural, morphological, optical and gas sensing performance of the prepared thin films were characterized. The inclusion of porogen significantly improved the sensing property of porous SnO2 bilayer thin films and it was confirmed by structural, morphological and gas sensing performance studies. The optimized spray process parameter was determined finally in this work as SnO2 precursor strength of 0.2 M for porous SnO2 layer. Under this condition, the increased lattice constant, lattice defect and increased pores diameter were achieved, which exhibited good gas sensitivity and selectivity towards NO2 gas at 250 ºC. The response and recovery time is decreased as 50%, the deduction limit is 0.9 ppm. In addition, during the five weeks test, the response level was observed nearly constant, which indicated that the SnO2 bilayer sensor has long-term stability.

Epitaxial bilayer La0.7Sr0.3MnO3/Ba0.7Sr0.3TiO3 thin films obtained by polymer assisted deposition

In this work bilayer structures, composed of ferromagnetic manganite and ferroelectric titanate layers, were obtained by solution deposition technique. The first step in preparation of the bilayer thin films was deposition of manganite (LaMnO3 or La0.7Sr0.3MnO3) layer by water-based polymer assisted deposition (PAD). Polycrystalline structures were obtained when manganite films were deposited by spin coating on commercial Pt/TiO2/SiO2/Si type substrate, whereas epitaxial films were grown on a single crystal SrTiO3 with (001) orientation substrate. The second ferroelectric titanate (BaTiO3 or Ba0.7Sr0.3TiO3) layer was deposited by spin coating using sol-gel method. The obtained bilayer structures have thickness below 100 nm, and epitaxial growth of the Ba0.7Sr0.3TiO3 film on the La0.7Sr0.3MnO3 surface was obtained, due to the similar lattice parameters between these two crystal structures.

IR photodetector capabilities of p-NiO/n-ZnO heterojunction structure

A p-NiO/n-ZnO bilayer thin film device was deposited by a simple, low temperature spray pyrolysis technique on the glass substrate at 250 OC. The deposited thin films are characterized by X-ray diffraction (XRD), and UV–Vis spectroscopy, which show a good crystalline nature of NiO thin film with the existence of interfacial defects sites. Further, p-NiO/n-ZnO bilayer thin film-based photodetector reveals promising IR detection induced due to the interfacial states. As a result, the heterojunction device exhibited high responsivity, photocurrent gain, and good reliability for IR illumination. Therefore, the heterojunction based p-NiO/n-ZnO photodetector showing promising results for IR detection.

Towards lithium-free solid-state batteries with nanoscale Ag/Cu sputtered bilayer electrodes.

Enhancing the reversible Li growth efficiency in "Li-free" solid-state batteries is key for the deployment of this technology. Here, we demonstrate a nanoscale material design path that enables the reversible cycling of a lithium-free solid-state battery, using Li7La3Zr2O12 (LLZO) electrolyte. By means of nanometric Ag-Cu bilayers, directly sputtered onto the LLZO, we can effectively control Li deposition. The robust thin film bilayer, which is compatible with LLZO, enables stable cycling, accommodating the volume changes without the need for extra external pressure.

Equilibration and thermal reversibility in mixtures of model OPV small-molecules and polymers

Following annealing at sufficiently high temperatures, well-controlled thin-film bilayers exhibit thermal changes in phase-composition, phase-volume and interfacial roughness that are reversible, irrespective of the starting layer compositions.

Electrical and optical enhancement of ITO/Mo bilayer thin films via laser annealing.

ITO/Mo bilayer thin films were sputtered on n-type silicon and glass substrates and annealed with a Nd:YAG pulsed laser. The structural results show that both the as-deposited and the annealed ITO/Mo thin films have a polycrystalline structure, and that the annealing treatment enhanced the crystallinity of samples. Moreover, the XRD patterns exhibited a cubic structure preferentially oriented along the (222) and (400) planes. The AFM analysis shows that grain size and RMS roughness increased from 16.02 to 36.19 nm and 0.4 to 2.6 nm, respectively, when the laser energy was increased to 120 mJ. The as-deposited sample has an optical transmittance of nearly 80% in the 300-800 nm range. The laser annealing yielded a higher transmittance of 94% and increased the bandgap energy from 2.76 to 2.88 eV at 120 mJ. The annealing treatment decreased the resistivity from 15.63 × 10-4 to 1.73 × 10-4 Ω/cm-1. Additionally, the figure of merit of the ITO/Mo structure improved significantly from 6.63 × 10-4 Ω-1 of the as-deposited sample to 17.6 × 10-3 Ω-1 of the the annealed structure. The results indicate that the laser annealing could improve the efficiency of the transparent conductive layer, which can be potentially applied in optoelectronic devices.

Extraction of Optical Constants and Dispersion Parameters of CdTe/CdSe Bi-Layer Thin Films

The current study focuses on the annealing-induced changes in the optical constants and dispersion parameters of the CdTe/CdSe bi-layer films deposited onto glass substrates by thermal evaporation technique. The slight shift in transmission threshold towards higher wavelength region with increasing annealing temperature revealed the systematic reduction in optical energy band gap (1.92 to 1.37 eV) of the films. The dispersion curve of the refractive index shows an anomalous dispersion in the absorption region and a normal dispersion in the transparent region. It was observed that the dispersion data obeyed the single oscillator of the Wemple-Didomenico model, from which the dispersion parameters and dielectric constants were evaluated. The increase in the refractive index with the annealing temperature modified the other linear parameters such as optical density, dissipation factor, volume, and surface energy loss function. The increase in the optical conductivity value is good for the CdSe film to be used as a semitransparent solar cells absorbing layer.We have made an attempt to discuss and correlate these results with the annealing temperature of possible mechanisms underlying the phenomena.

Encapsulation of bacteria in bilayer Pluronic thin film hydrogels: A safe format for engineered living materials

In engineered living materials (ELMs) non-living matrices encapsulate microorganisms to acquire capabilities like sensing or biosynthesis. The confinement of the organisms to the matrix and the prevention of overgrowth and escape during the lifetime of the material is necessary for the application of ELMs into real devices. In this study, a bilayer thin film hydrogel of Pluronic F127 and Pluronic F127 acrylate polymers supported on a solid substrate is introduced. The inner hydrogel layer contains genetically engineered bacteria and supports their growth, while the outer layer acts as an envelope and does not allow leakage of the living organisms outside of the film for at least 15 days. Due to the flat and transparent nature of the construct, the thin layer is suited for microscopy and spectroscopy-based analyses. The composition and properties of the inner and outer layer are adjusted independently to fulfil viability and confinement requirements. We demonstrate that bacterial growth and light-induced protein production are possible in the inner layer and their extent is influenced by the crosslinking degree of the used hydrogel. Bacteria inside the hydrogel are viable long term, they can act as lactate-sensors and remain active after storage in phosphate buffer at room temperature for at least 3 weeks. The versatility of bilayer bacteria thin-films is attractive for fundamental studies and for the development of application-oriented ELMs.

Refractory Metals and Oxides for High-Temperature Structural Color Filters.

Refractory metals have recently garnered significant interest as options for photonic applications due to their superior high-temperature stability and versatile optical properties. However, most previous studies only consider their room-temperature optical properties when analyzing these materials' behavior as optical components. Here, we demonstrate structural color pixels based on three refractory metals (Ru, Ta, and W) for high-temperature applications. We quantify their optical behavior in an oxygenated environment and determine their dielectric functions after heating up to 600 °C. We use in situ oxidation, a fundamental chemical reaction, to form nanometer-scale metal oxide thin-film bilayers on each refractory metal. We fully characterize the behavior of the newly formed thin-film interference structures, which exhibit vibrant color changes upon high-temperature treatment. Finally, we present optical simulations showing the full range of hues achievable with a simple two-layer metal oxide/metal reflector structure. All of these materials have melting points >1100 °C, with the Ta-based structure offering high-temperature stability, and the Ru- and W-based options providing an alternative for reversible color filters, at high temperatures in inert or vacuum environments. Our approach is uniquely suitable for high-temperature photonics, where the oxides can be used as conformal coatings to produce a wide variety of colors across a large portion of the color gamut.

Spin pumping at interfaces with ferro- and paramagnetic Fe60Al40 films acting as spin source and spin sink

We present a study of spin pumping efficiency and determine the spin mixing conductance and spin diffusion length in thin bilayer films based on 3d transition metal alloy Fe60Al40. Due to its magnetostructural phase transition, Fe60Al40 can be utilized as a ferromagnetic (FM) or paramagnetic (PM) material at the same temperature depending on its structural order; thus a thin Fe60Al40 film can act as a spin source or a spin sink when interfaced with a paramagnet or a ferromagnet, respectively. Ferromagnetic resonance measurements were performed in a frequency range of 5–35 GHz on bilayer films composed of FM–Fe60Al40/Pd and PM–Fe60Al40/Ni80Fe20 (permalloy). The increase in damping with the thickness of the paramagnetic layer was interpreted as a result of spin pumping into the paramagnet. We determine the spin mixing conductance gPd↑↓=(3.8±0.5)×1018m−2 at the FM–Fe60Al40/Pd interface and the spin diffusion length λPd=9.1±2.0nm in Pd. For the PM–Fe60Al40/permalloy interface, we find a spin mixing conductance gFeAl↑↓=(2.1±0.2)×1018m−2 and a spin diffusion length λFeAl=11.9±0.2nm for PM–Fe60Al40. The demonstrated bi-functionality of the Fe60Al40 alloy in spin pumping structures may be promising for spintronic applications.

Investigation on transport properties of GdMnO3 based bilayer thin films

Two heterostructures (Al-ZnO/GdMnO3/SrTiO3 and ZnO/GdMnO3/SrTiO3) were fabricated using Pulsed Laser Deposition System. The lattice parameters of the hexagonal (Al-ZnO and ZnO) and orthorhombic (GdMnO3) films are determined from the analysis of the x- ray diffraction patters. To study the surface morphology of prepared heterostructures Atomic Force Microscopy (AFM) were performed which revels that both heterostructures have homogenous island type of growth. To understand the transport behaviour of semiconductor-ferromagnetic (S-F) bilayers/films the resistivity measurement and field dependent dielectric measurements show and one Variation/Comparison in resistivity and dielectric behaviour of both heterostructures discussed based on Variable Rang Hopping (VRH) model and Universal dielectric response (UDR) Model.

Phase Transition and Optical Properties of VO2 and Al: ZnO/VO2 Thin Films

Thermochromic Vanadium dioxide (VO2) has strong potential for smart window applications but its commercial scale usage is limited due to low visible light transmission. To address this issue, aluminum doped zinc oxide (AZO) anti-reflecting layer is integrated with VO2 layer in the present work. VO2 single layer and AZO/VO2 bilayer thin film samples were deposited by sputtering technique on quartz substrate. The single-phase growth of VO2 and AZO in single layer and bilayer thin film samples is confirmed by X-ray diffraction measurements. Monoclinic M1 phase of VO2 is detected in VO2 and AZO/VO2 thin film samples at room temperature. Monoclinic to rutile structural phase transition (SPT) in these samples is studied by performing temperature-dependent X-ray diffraction measurements. SPT in VO2 thin film samples is close to 68 °C and SPT temperature appears slightly lower in AZO/VO2 sample as compared to VO2 sample. Spectral transmittance measurement at room temperature showed significant improvement in the visible transmittance of AZO bilayer film than that of single layer VO2 thin film. These results demonstrate the possibility of integration of anti-reflecting layers such as AZO with VO2 layer for better visible transmittances suitable for large-scale smart window applications.

Solid-solution and precipitation softening effects in defect-free faceted Nickel-Iron nanoparticles

It is well known that metals are strengthened by alloying additions or finely dispersed precipitates of a second phase. Here we show that alloying pristine, defect-free single crystalline nickel nanoparticles with iron results in a counter-intuitive softening due to randomly distributed solute (Fe) atoms and nano-size precipitates of the ordered Ni3Fe or Fe-rich phases. The Ni-Fe particles with Fe concentration of 0-50 at.% were synthesized by solid-state dewetting of Ni-Fe bilayer thin films deposited on a sapphire substrate. Ni-27Fe and Ni-50Fe particles exhibited a bimodal size distribution with small (111) and large (100) oriented particles. The solid solution softening was observed in all particles. The precipitation softening was observed in (100) oriented Ni-27Fe particles with uniformly distributed ordered Ni3Fe (L12) precipitates. Fe-rich precipitates were found on the surfaces and near the edges of the highly alloyed Ni-50Fe particles, leading to even greater softening. Molecular dynamic simulations of particle deformation have demonstrated that the softening effect is associated with premature dislocation nucleation at sites with a local stress concentration caused by the randomly distributed solute atoms. This work illustrates how the classical hardening mechanisms operating in bulk materials can be manipulated and even reversed in defect-free single-crystalline metal nanoparticles whose plastic deformation is controlled by dislocation nucleation.

A Simple Method for the Measurement of Young's Moduli of Bilayer Thin Films Based on the Electrostatic Drive Approach.

This paper presents a simple method for the in situ determination of Young's moduli of surface-micromachined bilayer thin films. The test structure consists of a cantilever, a bottom drive electrode located near the anchor, and a bottom contact electrode placed below the free end of the cantilever. The cantilever is driven by applying a voltage sweep between the cantilever and the drive electrode, and bends due to the electrostatic force. A novel theoretical model is derived to relate Young's modulus with the applied voltage and structure dimensions. The theoretical model is validated by finite element simulation. Test structures for Au/polysilicon thin films are fabricated by the PolyMUMPsand tested with the current-voltage measurement system. The measured Young modulus of polysilicon ranges from 152.344 GPa to 154.752 GPa, and the measured Young modulus of Au ranges from 71.794 GPa to 74.880 GPa. Compared with existing extraction methods, the proposed method is featured with simple operation, good repeatability, relatively high precision, and low requirements for equipment. It can be used alongside the application of a process control monitor (PCM) in surface-micromachining process lines.

Structural, Electrical and Optical Properties of Mg-Ni Thin Films for Hydrogen Storage Applications

Using the magnetron sputtering technique, Mg-Ni (150nm) bilayer thin films were deposited onto the simple, and ITO coated glass substrates. To obtain proper mixing and inter diffusion of thin film structure, annealing of the prepared thin films was made at constant temperature 523 K. The surface morphology of thin films confirms the uniform deposition of the Mg-Ni thin films on substrates. The Raman spectroscopy of thin films shows that the intensity of Raman peaks is decreased as the hydrogen pressure increases and variation in the intensity peak confirms the existence of hydrogen in the prepared film. The d.c. conductivity of thin films were observed at different hydrogen pressure (10 - 40 psi) for both the as-deposited and annealed thin films and the variation in the conductivity after hydrogenation was found out. In Uv-Vis spectroscopy, it was found that the optical bandgap of thin films increases with the increase of hydrogen pressure. The deviations in the structural, electrical and optical properties after hydrogenation indicate that Mg-Ni thin films may be used for solid state hydrogen storage applications. HIGHLIGHTS Preparation of Mg-Ni bilayer thin films using magnetron sputtering technique Finding the opportunity to use Mg-Ni bilayer thin film as solid-state hydrogen storage materials Raman spectroscopy for confirming the presence of hydrogen in Mg-Ni bilayer thin films Effect of hydrogen on conductivity and optical energy bandgap of Mg-Ni bilayer thin films GRAPHICAL ABSTRACT

Fabrication and analysis of Mg3Bi2 thin films by post annealing Mg/Bi bilayer thin films

Mg-based thermoelectric materials have attracted significant attention because of the lightweight, low cost, and abundance of the raw materials. We successfully fabricated randomly oriented hexagonal Mg3Bi2 polycrystalline thin films by the post-annealing treatment of Mg/Bi bilayer thin films deposited by magnetron sputtering. Irregular clusters were observed on the surface of the underlying Bi layer; thus, the Mg-Bi thin film surface exhibited an uneven structure with irregular protrusions. Further, two-step post annealing (523 + 673 K) was performed to obtain a homogeneous chemical composition; the thermoelectric power factor of the Mg3Bi2 film was found to be 0.89 μW/cmK2 at 565 K.