Multilayer Films Research Articles

Multi-layer heterojunction phase change thin films with extremely low resistance drift

This work prepared [GeSb9 (x nm)/Ga3Sb7 (16-x nm)]5 multi-layer phase change thin films and studied their related properties. Compared to traditional GST and GeSb9, [GeSb9 (6 nm)/Ga3Sb7 (10 nm)]5 multi-layer phase change thin films have a higher crystallization temperature (∼205 °C) and an extremely low resistance drift coefficient (0.0004). The crystalline behavior and bonding of thin films are analyzed to elucidate their intrinsic mechanisms, thermal stability, and interfacial effects. According to crystal mechanism analysis, the Avrami index (n) is 0.904, indicating that the [GeSb9(6 nm)/Ga3Sb7(10 nm)]5 multi-layer phase change thin films grow in one dimension. The presence of interfaces alters the crystallization behavior of crystals, leading to a growth mode that transitions from diffusion-controlled to interface-controlled and back to diffusion-controlled. This transformation significantly reduces the nucleation index (m ≤ 0.16), minimizing the inherent randomness of the film itself and achieving extremely low resistance drift. Research has demonstrated that [GeSb9 (6 nm)/Ga3Sb7 (10 nm)]5 multi-layer phase change materials have promising applications in embedded memory.

On the reduction of gas permeation through the glass windows of micromachined vapor cells using Al2O3 coatings

Stability and precision of atomic devices are closely tied to the quality and stability of the internal atmosphere of the atomic vapor cells on which they rely. Such an atmosphere can be stabilized by building the cell with low permeation materials such as sapphire or aluminosilicate glass in microfabricated devices. Recently, we have shown that permeation barriers made of Al2O3 thin-film coatings deposited on standard borosilicate glass could be an alternative for buffer gas pressure stabilization. In this study, we, hence, investigate how helium permeation is influenced by the thickness, ranging from 5 to 40 nm, of such Al2O3 thin films coated by atomic layer deposition. Permeation rates are derived from long-term measurements of the pressure-shifted transition frequency of a coherent population trapping (CPT) atomic clock. From thicknesses of 20 nm onward, a significant enhancement of the cell hermeticity is experienced, corresponding to two orders of magnitude lower helium permeation rate. In addition, we test cesium vapor cells filled with neon as a buffer gas and whose windows are coated with 20 nm of Al2O3. As for helium, the permeation rate of neon is significantly reduced, thanks to alumina coatings, leading to a fractional frequency stability of 4×10−12 at 1 day when the cell is used in a CPT clock. These features outperform the typical performances of uncoated Cs–Ne borosilicate cells and highlight the significance of Al2O3 coatings for buffer gas pressure stabilization.

Regulation of nanoscale Mg/V incoherent interface interactions to enhance hydrogen storage properties in Mg/V multilayer films

Magnesium-based materials offer a promising, cost-effective, and high-capacity solution for hydrogen storage. However, their slow kinetics and elevated absorption/desorption temperatures limit their large-scale application. In this study, a series of Mg1-x/Vx (x = 0.05, 0.10, 0.15, 0.20) multilayer films were designed and fabricated using an ultra-high vacuum magnetron sputtering technique. Transmission electron microscopy (TEM) analysis revealed the presence of a discontinuous nanoscale V single-phase interlayer between the Mg layers, confirming the successful formation of Mg/V multilayer films. The surface of the deposited films predominantly consists of hexagonal particles of varying sizes stacked upon one another. Upon hydrogenation, tetragonal rutile structures of MgH2 and V2H were formed. The Mg0.95V0.05 film absorbed 6.40 wt% hydrogen within 15 min at 150 °C and desorbed 0.34 wt% hydrogen within 1.8 h at the same temperature, with an initial dehydrogenation temperature of 79 °C. The activation energies for hydrogen absorption and desorption were estimated to be 60 ± 7 and 140 ± 10 kJ/mol H2, respectively, significantly lower than the corresponding values of 100 and 160 kJ/mol H2 for conventional pure Mg/MgH2. The enhanced hydrogen absorption performance was attributed to the catalytic effect of nanocrystalline V at the Mg/V incoherent interface, while the improved desorption performance was due to the synergistic catalytic effect of nanocrystalline V2H at the MgH2/V2H incoherent interface. DFT calculations indicated that the interaction at the Mg/V incoherent interface promoted H atom adsorption and diffusion, significantly enhancing the hydrogenation and dehydrogenation performance of the multilayer films.

Toward Mass Production of Transition Metal Dichalcogenide Solar Cells: Scalable Growth of Photovoltaic-Grade Multilayer WSe2 by Tungsten Selenization.

Semiconducting transition metal dichalcogenides (TMDs) are promising for high-specific-power photovoltaics due to their desirable band gaps, high absorption coefficients, and ideally dangling-bond-free surfaces. Despite their potential, the majority of TMD solar cells to date are fabricated in a nonscalable fashion, with exfoliated materials, due to the lack of high-quality, large-area, multilayer TMDs. Here, we present the scalable, thickness-tunable synthesis of multilayer WSe2 films by selenizing prepatterned tungsten with either solid-source selenium at 900 °C or H2Se precursors at 650 °C. Both methods yield photovoltaic-grade, wafer-scale WSe2 films with a layered van der Waals structure and superior characteristics, including charge carrier lifetimes up to 144 ns, over 14× higher than those of any other large-area TMD films previously demonstrated. Simulations show that such carrier lifetimes correspond to ∼22% power conversion efficiency and ∼64 W g-1 specific power in a packaged solar cell, or ∼3 W g-1 in a fully packaged solar module. The results of this study could facilitate the mass production of high-efficiency multilayer WSe2 solar cells at low cost.

Transparent vanadium doped titania-silica films: Structural characterization and self-cleaning properties

Vanadium is one of the most promising dopants for titanium dioxide, as it has different oxidation states and comparable atomic sizes to titanium cation. In this article, vanadium-modified TiO2 was prepared at low temperatures and immobilized on glass for self-cleaning and superhydrophilic purposes. The synthesis was carried out at 80 °C under reflux for 48 hours and only 150 °C was required for complete immobilization on the glass surface using silica binder, which in turn formed 120 nm thick films. Such multilayered (3) films showed enhanced transparency in the visible region of the spectrum and exhibited strong superhydrophilic properties (water contact angle, WCA < 5°) even when not illuminated. Vanadium was present in interstitial regions, while the unbound vanadium formed V2O5 nanoparticles at higher concentrations. The presence of vanadium did not improve the photocatalytic properties for the degradation of fatty layers composed of stearic acid, which could be attributed to reduced migration of electrons and holes due to the trapping effects of vanadium species in such samples.

Understanding vapor phase growth of hexagonal boron nitride.

Hexagonal boron nitride (hBN), with its atomically flat structure, excellent chemical stability, and large band gap energy (∼6 eV), serves as an exemplary 2D insulator in electronics. Additionally, it offers exceptional attributes for the growth and encapsulation of semiconductor transition metal dichalcogenides (TMDCs). Current methodologies for producing hBN thin films primarily involve exfoliating multi-layer or bulk crystals and thin film growth via chemical vapor deposition (CVD), which entails the thermal decomposition and surface reaction of molecular precursors like ammonia boranes (NH3BH3) and borazine (B3N3H6). These molecular precursors contain pre-existing B-N bonds, thus promoting the nucleation of BN. However, the quality and phase purity of resulting BN films are greatly influenced by the film preparation and deposition process conditions that remain a substantial concern. This study aims to comprehensively investigate the impact of varied CVD systems, parameters, and precursor chemistry on the synthesis of high-quality, large scale hBN on both catalytic and non-catalytic substrates. The comparative analysis provided new insights into most effective approaches concerning both quality and scalability of vapor phase grown hBN films.

Assessing Energy Savings: A Comparative Study of Reflective Roof Coatings in Four US Climate Zones

Assessing Energy Savings: A Comparative Study of Reflective Roof Coatings in Four US Climate Zones

Perovskite manganese oxide-based superposed multilayer film with high emittance tunability for smart radiation devices

Smart radiation devices (SRDs) with the ability to switch emittance according to the radiator surface’s temperature are significant for spacecraft thermal control on exploration missions with drastic changes in thermal environments. However, it is quite urgent to improve the performance of the intelligent thermal control films in SRDs due to the limitations such as low emittance tunability and inappropriate phase transition temperature. Consequently, a film with self-adaptive infrared emittance based on anti-reflective design is proposed in this paper. The main structure is a superposed multilayer film based on a doped perovskite manganese oxide La0.825Sr0.175MnO3 (LSMO) and BaF2 stacks. The emittances of the designed multilayer film are 0.18 and 0.83 at the temperatures of 100 K and 295 K, and therefore the emittance tunability achieves 0.65. Compared with single-layer LSMO, the emittance tunability has increased 25 % approximately. The enhancement in emittance tunability can be explained by the fact that multilayer films with staggered low and high refractive index dielectric layers may cause destructive interference and suppress the Fresnel reflections at high temperatures. In addition, the thickness of dielectric part is carefully designed to avoid the formation of high emissivity Fabry-Perot (FP) resonance when LSMO switches to metallic state at low temperatures. This perovskite manganese oxide-based multilayer film exhibits remarkable intelligent thermal control properties. It provides a significant opportunity to improve the performance of SRDs and a promising potential for maintaining the optimal operating temperature of the spacecraft.

Confinement Effect in Multilayer Films Made from Semicrystalline and Bio-Based Polyamide and Polylactic Acid.

Bio-based multilayer films were prepared by using the innovative nanolayer coextrusion process to produce films with a number of alternating layers varying from 3 to 2049. For the first time, a semicrystalline polymer was confined by another semicrystalline polymer by nanolayering in order to develop high barrier polyamide (PA11)/polylactic acid (PLA) films without compromising thermal stability and mechanical behavior. This process allows the preparation of nanostratified films with thin layers (down to nanometric thicknesses) in which a confinement effect can be induced. The stratified structure has been investigated, and the layer thicknesses have been measured. Barrier properties were successfully correlated to the microstructure, as well as the thermal behavior, and mechanical properties. The layer continuity was fully achieved for most of the films, but some layer breakups have been observed on the film with the thinnest PLA layer (2049-layers film). Coextruding PLA with PA11 has induced an increase in PLA crystallinity (from 4 to 16%) along with an increase in thermal stability of the multilayer films without impacting PA11 properties. Gas barrier properties were driven by the PLA confined layers due to the microstructural rearrangement by increasing crystallinity, whereas water barrier properties were governed by the PA11 confining layers due to its lower water affinity. As a consequence, a decrease of water permeability (up to 11 times less permeable for the 6M film) but an increase of gas barrier properties (barrier improvement factor (BIF) of 66% for the 0M film for N2 and BIF of 36% for the 6M film for CO2 for instance) were evidenced as the layer number was increased. This study paves the way for the development of ecofriendly materials with outstanding barrier performances and highlights the importance of nonmiscible polymers adhesion at melt state and additives presence.

Molecular dynamics study of the groove-forming mechanism of mechanically scratched grating multilayer aluminum films

For a long time, the forming quality of large area diffraction gratings is restricted by the one-time coating quality and uniformity of large thickness and large area aluminum films. Large thickness coatings, low accuracy of large depth grooves and diamond tool wear problems were the main reasons for the failure of the test to inscribe large area diffraction gratings. Therefore, the use of layered coating process can effectively ensure the uniformity and consistency of large-area and large-thickness grating blanks coated with aluminum film, and improve the high precision of large depth diffraction grating slot type. In order to reveal the mechanism of the groove-forming process of multilayer aluminum films, the microstructure and mechanical properties of different aluminum films were observed. In this article, the removal behavior and mechanism of nanomechanically scratched multilayer aluminum film materials are investigated using a molecular dynamics approach, and the characteristics of multilayer aluminum films scratched into grooves are analyzed in terms of surface morphology, scratching force, structural evolution, dislocation distribution, von Mises stress and shear strain. The results indicate that in the scratching simulation process, multilayer aluminum film has a higher number of removed atoms and a larger atomic displacement compared to single layer aluminum film, which is conducive to precise shape control of the groove. The average tangential force and normal force of multilayer aluminum film show a decreasing trend compared to single layer aluminum film, which helps to scratch grooves and reduce tool wear. The total length of the dislocation lines of the multilayer aluminum film is large, and the distribution area is large, which improves the precision of scratching the grooves. Therefore, an appropriate increase in the number of aluminum film layers has a precise control effect on the groove shape. This study has certain reference significance for analyzing the complex grooving deformation law of grating aluminum film, improving the diffraction efficiency of grating aluminum film, and further improving the manufacturing process of grating aluminum film.

Bioinspired multilayer mulch film integrating mud repulsion, adhesion reduction, and antifouling for high-efficiency resource recycling

Resource recycling is a promising solution to the current problem of residual film pollution. However, the challenge that remains to be addressed in the resourcefulness of mulch films lies in mitigating heavy water and power consumption during the cleaning process. Herein, inspired by the unique structure of typical soil-dwelling insects, the mass production of high-strength multilayer mulch (Bio-HAM) films is achieved through a sequential combination of multilayer coextrusion, blow molding, roll-to-roll imprinting, and spray coating. The regular micro-nano hierarchical structure on the Bio-HAM films surface enables it to effectively inherit the antifouling, slurry repulsion, and adhesion reduction functions of the prototypes, even when exposed to ultraviolet radiation, deformation, and liquid or solid impact. Benefiting from the aforementioned properties, the Bio-HAM films exhibit low impurities adhering and soil wrapping during the entire planting cycle. The impurity content of the recovered Bio-HAM films wrapping is approximately 12.7 g/m2, which is a 67.4% reduction in impurity content compared to the basic films. Moreover, the recovered film also possesses favorable mechanical and thermal properties, thereby ensuring water and energy conservation as well as high-value regeneration. The proposed approach is expected to address the excessive consumption and persistent ecological pollution of residual film recycling.

Wideband optically transparent absorber based on multilayer ITO films with high absorption rate

In this paper, a wideband optically transparent absorber based on multilayer indium tin oxide (ITO) films is designed, fabricated, and measured. The structure uses polyethylene terephthalate (PET) and ITO films to replace the dielectric substrate and metal pattern of the traditional absorber to achieve optical transparency characteristics. The absorption of the proposed absorber is above 90% from 4.92 to 24.55 GHz and exceeding 95% in the range of 5.74 ∼ 21.85 GHz, respectively, under normal transverse electric (TE) and transverse magnetic (TM) polarized incidences. Moreover, it shows outstanding stability with various incident angles over a wide range. A prototype of the proposed absorber is fabricated and experimentally verified, with good measurement results. The optical transparency and great absorption performance make the absorber a promising candidate for transparent electromagnetic compatibility.

Significantly enhanced energy storage performance in multi-layer polyimide films with nano dielectric layer

Polymer-based composites with multilayer structure were emerging as an effective way to solve the contradiction between high breakdown strength and large permittivity in past decades. However, the incorporating layers in conventional multilayer composite films are usually inorganic thick films, which is not beneficial for the retention of excellent mechanical properties of the polymer matrix. In this work, the multilayer films consisting of micrometer polyimide (PI) and nanoscale TiO2 layer were prepared by spin-coating and atomic layer deposition (ALD), respectively. Carrier migration in PI-TiO2 multilayer films was hindered effectively by the interfacial barrier, as indicated by the results of the Pulsed Electro-Acoustic (PEA) tests. Compared to pure PI, both the permittivity, dielectric loss and breakdown strength were optimized in the PI-TiO2 multilayer films. A satisfactory discharge energy density of 6.77 J cm−3 (efficiency >90 %) was obtained in the multilayer films, which is about 5 times higher than that of pure PI. The strategy of constructing heterogeneous interfaces by incorporated nanoscale TiO2 layers is promising for designing high performance capacitors in practical application.

Determination of the significance of atomic concentration on surface properties of Ba x Mg1-x F2 alloy coatings via microscopic and spectroscopic techniques.

Both BaF2 and MgF2 compounds and Ba x Mg1-x F2 alloy thin films were deposited on glass and silicon (Si) substrates in nanometric sizes (100 ± 10 nm) in a high vacuum environment by radio frequency (rf) magnetron sputtering. Using BaF2 (99% purity) and MgF2 (99% purity) target materials and adjusting the power levels applied to these targets, Ba x Mg1-x F2 alloy coatings at different atomic concentrations were formed under the same vacuum conditions. The microstructure and surface characteristics of the samples were analysed with the help of spectroscopic and microscopic methods. For the surface characterization, with scanning acoustic microscopy (SAM), 2-dimensional surface acoustic images of the samples were mapped, the surface acoustic impedance values were determined, and information about the micro hardness of the materials was obtained. Surface roughness values and grain sizes were obtained by taking 3-dimensional surface images of investigated materials using atomic force microscopy (AFM). Average nanometric particle sizes were determined for each sample with scanning electron microscopy (SEM), therefore, information about surface homogeneity was obtained. For the microstructural characterization, quantitative elemental analysis was performed using scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDS), and stoichiometric ratios of atomic compositions were identified. By evaluating the data obtained from the microscopic and spectroscopic measurements, the effect of the atomic concentration parameter on the morphological properties of the material was determined. The usability of the produced binary fluoride alloy thin film coatings is promising for emerging optoelectronic, ceramic industry, biomedical and surface acoustic wave applications.

Diazonium-Functionalized Silicon Hybrid Photoelectrodes: Film Thickness and Composition Effects on Photoelectrochemical Behavior.

Aryl diazonium electrografting is a powerful method for imparting molecular functionality onto various substrates by forming a stable carbon-surface covalent bond. While the high reactivity of the aryl radical intermediate makes this method fast and reliable, it can also lead to the formation of an insulating and disordered multilayer film. These thick films affect electrochemical performance, especially for semiconductor substrates used in photoelectrochemical applications. We studied the effects of film thickness and composition by electrografting in situ-generated aminobenzene diazonium salts onto both n-type and p-type silicon electrodes at fixed potentials. Next, we attached ferrocene to the amine-terminated films and probed their (photo)electrochemical behavior. Cyclic voltammetry measurements showed decreased electrochemical reversibility with increasing diazonium film thickness; this reversibility was restored when ferrocene was incorporated throughout the film with a layer-by-layer deposition process. Finally, we compared the behavior of dark p-type electrodes to n-type photoelectrodes and observed differences in the electrochemical reversibility that we attribute to the change in potential drop across the two interfaces.

Nano-multilayered ZrN-Ag/Mo-S-N film design for stable anti-frictional performance at a wide range of temperatures

A multilayer film, composed by ZrN-Ag (20 nm) and Mo-S-N (10 nm) layers, combining the intrinsic lubricant characteristics of each layer was deposited using DC magnetron sputtering system, to promote lubrication in a wide-range of temperatures. The results showed that the ZrN-Ag/Mo-S-N multilayer film exhibited a sharp interface between the different layers. A face-centered cubic (fcc) dual-phases of ZrN and Ag co-existed in the ZrN-Ag layers, whilst the Mo-S-N layers displayed a mixture of hexagonal close-packed MoS2 (hcp-MoS2) nano-particles and an amorphous phase. The multilayer film exhibited excellent room temperature (RT) triblogical behavior, as compared to the individual monolayer film, due to the combination of a relative high hardness with the low friction properties of both layers. The reorientation of MoS2 parallel to the sliding direction also contributed to the enhanced anti-frictional performance at RT. At 400 °C, the reorientation of MoS2 as well as the formation of MoO3 phase were responsible for the lubrication, whilst the hard t-ZrO2 phase promoted abrasion and, consequently, led to increasing wear rate. At 600 °C, the Ag2MoO4 double-metal oxide was the responsible for the low friction and wear-resistance; furthermore, the observed transformation from t-ZrO2 to m-ZrO2, could also have contributed to the better tribological performance.

Study on the Performance of Deep Red to Near-Infrared pc-LEDs by the Simulation Method Considering the Distribution of Phosphor Particles.

Effectively utilizing deep red to near-infrared (DR-NIR) phosphors to achieve the optimal performance of NIR phosphor-converted white LEDs (DR-NIR pc-wLEDs) is currently a research hotspot. In this study, an optical model of DR-NIR pc-wLEDs with virtual multilayer fluorescent films was established based on the Monte Carlo ray-tracing method. Different gradient distributions of the particles were assigned within the fluorescent film to explore their impact on the optical performance of pc-LEDs. The results show that, for the case with single-type particles, distributing more DR-NIR particles far from the blue LED chip increased the overall radiant power. The distribution of more DR-NIR particles near the chip increased the conversion ratio from blue to DR-NIR light. The ratio of the 707 nm fluorescence emission intensity to the 450 nm excitation light intensity increased from 1:0.51 to 1:0.28. For multiple-type particles, changes in the gradient distribution resulted in dual-nature changes, leading to a deterioration in the color rendering index and an increase in the correlated color temperature, while also improving the DR-NIR band ratio. The reabsorption caused by the partial overlap between the excitation band of the DR-NIR particles and the emission band of the other particles enhanced the radiant power at 707 nm. Distributing DR-NIR phosphor particles closer to the chip effectively amplified this effect. The proposed model and its results provide a solution for the forward design of particle distributions in fluorescent films to improve the luminous performance of DR-NIR pc-wLEDs.

Effects of voltage schemes on the conductance modulation of artificial synaptic device based on 2D hBN memristor: Its applications for pattern classifications

Artificial synaptic devices utilizing nonvolatile memristors with 2D materials show potential for neuromorphic computing systems due to their intriguing electrical properties, simple structure, and capability to emulate biological synaptic behaviors. Here, we fabricated nonvolatile memristors based on 2D multilayer hBN film and demonstrated their analog memory performance along with biological synaptic characteristics, including paired-pulse facilitation and depression (PPF and PPD), and synaptic plasticity. We utilized different voltage scheme algorithms to analyze the impact of synaptic functions on image classification tasks across various pattern datasets through simulations. Our findings reveal that pattern recognition outcomes varied due to the modulation of synaptic plasticity by the applied voltage schemes. We also investigated the relationship between conductance update variations and classification accuracy to highlight the significance of optimized synaptic plasticity in improving image recognition capabilities in large-scale neural networks. Our experimental results contribute to the development of high-performance neuromorphic computing for diverse pattern classification tasks using artificial synaptic devices based on 2D hBN.

Multi-objective scintillator shape optimization for increased photodetector light collection

Inorganic scintillators often use exotic, expensive materials to increase their light yield. Although material chemistry is a valid way to increase the light collection, these methods are expensive and limited to the material properties. As such, alternative methods such as the use of specific reflective coatings and crystal optical shapes are critical for the scintillator crystal design procedure. In this paper, we explore the modeling of a scintillator and silicon-photomultiplier (SiPM) assembly detector using GEANT4. GEANT4, an open-source software for particle–matter interaction based on ray-tracing, allows the modeling of a scintillator-based detector while offering methods to simplify and study the computational requirements for a precise calculation of the light collection. These studies incorporate two different geometries compatible with the barrel timing layer (BTL) particle detector that is being built for the compact muon solenoid (CME) experiment at CERN. Furthermore, the geometry of our model is parameterized using splines for smoother results and meshed using GMSH to perform genetic numerical optimization of the crystal shape through genetic algorithms, in particular non-dominated sorting genetic algorithm II (NGSAII). Using NSGA-II, we provide a series of optimized scintillator geometries and study the trade-offs of multiple possible objective functions including the light output, light collection, light collection per energy deposited, and track path length. The converged Pareto results according to the hypervolume indicator are compared to the original simplified design, and a recommendation towards the use of the light collection per energy deposition and track path length is given based on the results. The results provide increases in this objective of up to 18% for a constant volume for a geometry compatible with the current design of the BTL detector.

Self-assembled rectorite films with remarkable mechanical performance: preparation, structural characterization, and properties

Cohesive flexible rectorite clay films with good mechanical performance were prepared by a simple casting method through the self-assembly of exfoliated natural clay from aqueous dispersions. The multi-layered microstructure of the films consisted of continuous layers of aligned clay platelets parallel to the casting surface. Layers overlap randomly in the lateral direction (plane) and join vertically in an irregular manner by edge-to-face cross-linkages (bridging) to form coherent multi-layered nanostructured films with platelet-void microstructure. The films with the highest mechanical properties had thicknesses below 30 µm. Overall films from rectorite clay with monovalent interlayer content exhibited a higher experimental tensile strength ranging up to 44 MPa and Young’s modulus up to 56 GPa. The corresponding experimental values for films with divalent interlayer cations were 23 MPa for strength and 25 GPa for modulus. The highest experimental values for strength and modulus for neat Na–Ca–rectorite films were 25 MPa and 50 GPa respectively. These two mechanical property values of the best rectorite-based clay films compare favorably with values featured by polymer films typically used for packaging applications.