Multilayer Thickness Research Articles

Competition Pathways of Energy Relaxation of Hot Electrons through Coupling with Optical, Surface, and Acoustic Phonons

The hot electrons in carbon-based materials exhibit interesting ballistic transport behaviors for designing high-performance single-electron transistors. However, the cooling of such hot electrons back to the equilibrium state may be slowed down by the excessively populated optical phonons, thus limiting their ballistic transport. Therefore, a thorough understanding of the coupling between hot electrons and optical phonons is of critical importance. Here, by varying the thickness of a multilayer graphene film on a supporting substrate, we investigated the competition pathways of the energy relaxation of the photoinduced hot electrons through coupling with the optical, surface, and acoustic phonons. The difference in the τ2 values indicates that thickness plays an important role in the optical phonon population. For the multilayer graphene film thickness less than 3 nm, the super-collision model describes the hot electron cooling dynamics that is strongly affected by the surface phonons from the supporting substrate. As the multilayer graphene film thickness increases from 3 to 20 nm, the accumulation of the optical phonons induces a hot optical phonon effect, resulting in a bottleneck of cooling of the hot electrons. As the multilayer graphene film thickness further increases from 20 to 40 nm, a direct coupling between the hot electrons and acoustic phonons starts to dominate. The surface states of the interfaces at the inner layers of the multilayer graphene film contribute to this direct coupling as an additional cooling channel. The quantitative understanding of the energy relaxation pathways of the hot electrons offers insights into designing high-performance single-electron transistors.

Ablation behavior of Mo/MSZ thick alternate multilayer fabricated by combinative processes of plasma spray and hot isostatic pressing

Thermal barrier and anti-ablation coatings play essential roles in protecting jet nozzles, gas turbine engines and other machines from high temperatures. In this work, a novel curved structure of Mo/MSZ alternate multilayer was designed and fabricated by air plasma spraying. Two steps of the hot isostatic pressing were applied to improve the mechanical strength and interlayer bonding. The alternate structure enabled thick MSZ (Magnesium oxide stabilized Zirconia) layers and show an excellent thermal insulation ability under high-temperature ablation (4.3 MW/m2). Due to the curved surface, the thermal and residual stress is larger at the outer layer and edge, leading to cracks at the most out layer during the ablation. Moreover, during the ablation, the surface suffered steps in the order of hot flush damaging, MSZ and Mo melting, mixing, and Mo oxidizing. This work provided design, reinforcing method and ablation behavior analysis of alternate multilayer of TBC, supporting a solution to increase the performance of thermal insulation and anti-ablation.

Evaluation of the difference in multilayer thickness of TiO2 thin film by laser speckle photography

TiO2 thin film thickness is measured after each layer of coatings by laser speckle photography. TiO2 thin film coatings are carried out on the steel substrate in a periodical way by the spin coating method. At the time t1, the first layer coating scatters the coherent laser and develops the self-interference pattern of an image called speckle, which is captured and auto-correlated. After time t2, the speckle image is recorded by CCD and cross-correlated with the previous one. There is pixel distance movement and it is computed by image processing. This speckle imaging practice gives 8 µm as the thickness of the second layer of Tio2 film coating. Hence, the shift of pixel distance gives the thickness of the multilayer sample coatings in a satisfactory manner.

ReS2 on GaN Photodetector Using H+ Ion-Cut Technology.

The wafer-scale single-crystal GaN film was transferred from a commercial bulk GaN wafer onto a Si (100) substrate by combining ion-cut and surface-activated bonding. Well-defined, uniformly thick, and large-scale wafer size ReS2 multilayers were grown on the GaN substrate. Finally, ReS2 photodetectors were fabricated on GaN and sapphire substrates, respectively, and their performances were compared. Due to the polarization effect of GaN, the ReS2/GaN photodetector showed better performance. The ReS2/GaN photodetector has a responsivity of 40.12 A/W, while ReS2/sapphire has a responsivity of 0.17 A/W. In addition, the ReS2/GaN photodetector properties have reached an excellent reasonable level, including a photoconductive gain of 447.30, noise-equivalent power of 1.80 × 10-14 W/Hz1/2, and detectivity of 1.21 × 1010 Jones. This study expands the way to enhance the performance of ReS2 photodetectors.

Proposal of two parameters to evaluate in-situ apparent toughness of interphase in fiber reinforced ceramic matrix composites: Three-dimensional finite element simulations

Interphase structure characters such as single-layer interfacial thickness or number of multilayers may have a great influence on mechanical properties of continuous fibers reinforced ceramic matrix composites (CFRCMCs). However, the determination of the optimal interphase is still unclear for the complex microstructures of CFRCMCs. In this study, a novel Finite Element Method (FEM) -based numerical method considering mean scalar stiffness degradation and mean damage dissipation energy are proposed to quantitatively assess the effect of above structure characters on in-situ toughness of interphase. The proposed method has successfully applied on single pyrolytic carbon (PyC) layer and alternating silicon carbide/pyrolytic carbon (SiC-PyC) n multilayer systems. The results suggest that thicker PyC interphase will prone cause brittle fracture with lower toughness, whilst more layers with the same total thickness will improve fracture toughness for (SiC-PyC) n system. On the contrary, more layers with the same sublayer thickness will decrease in situ apparent fracture toughness. The proposed method explains well the previous contradictory experimental observations.

Thin film growth and mechanical properties of CrFeCoNi/TiNbZrTa multilayers

Multilayers of high entropy alloys (HEA) are picking up interest due to the possibility of altering material properties by tuningcrystallinity, thickness, and interfaces of the layers. This study investigates the growth mechanism and mechanical properties of CrFeCoNi/TiNbZrTamultilayers grown by magnetron sputtering. Multilayers of bilayer thickness (Λ) from 5 nm to 50 nm were grown on Si(100) substrates. Images taken by transmission electron microscopy and energy-dispersive X-ray spectroscopy mapping revealed that the layers were well defined with no occurrence of elemental mixing. Multilayers with Λ < 20 nm exhibited an amorphous structure. As Λ increased, the CrFeCoNi layer displayed a higher crystallinity in comparison to the amorphous TiNbZrTa layer. Themechanical properties were influenced by thecrystallinity of the layers and stresses in the film. The film with Λ = 20 nm had the highest hardness of approximately 12.5 GPa owing grain refinement of the CrFeCoNi layer. An increase of Λ ≥ 30 nm resulted in a drop in the hardness due to the increase in crystal domains of the CrFeCoNi layer. Micropillar compression induced shear in the material rather than fracture, along with elemental intermixing in the core of the deformed region of the compressed micropillar.

Finite-element dynamic-matrix approach for propagating spin waves: Extension to mono- and multi-layers of arbitrary spacing and thickness

In our recent work [Körber et al., AIP Adv. 11, 095006 (2021)], we presented an efficient numerical method to compute dispersions and mode profiles of spin waves in waveguides with translationally invariant equilibrium magnetization. A finite-element method (FEM) allowed to model two-dimensional waveguide cross sections of arbitrary shape but only finite size. Here, we extend our FEM propagating-wave dynamic-matrix approach from finite waveguides to the important cases of infinitely extended mono- and multi-layers of arbitrary spacing and thickness. To obtain the mode profiles and frequencies, the linearized equation of the motion of magnetization is solved as an eigenvalue problem on a one-dimensional line-trace mesh, defined along the normal direction of the layers. Being an important contribution to multi-layer systems, we introduce interlayer exchange into our FEM approach. With the calculation of dipolar fields being the main focus, we also extend the previously presented plane-wave Fredkin–Koehler method to calculate the dipolar potential of spin waves in infinite layers. The major benefit of this method is that it avoids the discretization of any non-magnetic material such as non-magnetic spacers in multilayers. Therefore, the computational effort becomes independent of the spacer thicknesses. Furthermore, it keeps the resulting eigenvalue problem sparse, which, therefore, inherits a comparably low arithmetic complexity. As a validation of our method (implemented into the open-source finite-element micromagnetic package TETRAX), we present results for various systems and compare them with theoretical predictions and with established finite-difference methods. We believe this method offers an efficient and versatile tool to calculate spin-wave dispersions in layered magnetic systems.

Hydrogen‐bonded polymer multilayer coatings via dynamic layer‐by‐layer assembly

AbstractWe developed the dynamic assembly of the hydrogen‐bonded multilayers of (poly(N‐vinylpyrrolidone/poly(methacrylic acid)) (PVPON/PMAA)) and compared their properties to the static multilayers. We found that dynamic multilayers, wherein a planar substrate is shaken during polymer adsorption, leads to a 15‐time faster deposition of the planar coatings. The thicknesses and roughness of the dynamic coatings were found to be ⁓30% larger than those of static (no shaking) multilayer films as measured by spectroscopic ellipsometry and atomic force microscopy. We examined the film growth, mechanical properties, wettability, hydration, and pH stability of the planar static and dynamic multilayers and demonstrated that these properties were insignificantly affected by the assembly mode. Both static and dynamic coatings produced microporous films when exposed to pH = 5.9, close to the film critical dissolution pH = 6. We discovered that during the release of the multilayer films into a solution to produce free‐standing films either as planar membranes or multilayer capsule shells, the molecular chain rearrangements result in the decreased roughness for both static and dynamic multilayers and lead to a decreased thickness of the dynamic multilayers. Our findings can help develop a rapid synthesis of thicker nanostructured polymer coatings for sensing and controlled delivery applications.

Protective Cr Coatings with ZrO2/Cr Multilayers for Zirconium Fuel Claddings

This article described the protective properties of Cr coatings with a barrier layer composed of ZrO2/Cr multilayers deposited onto E110 zirconium alloy. The coatings with a ZrO2/Cr multilayer thickness of 100, 250, and 750 nm and single-layer (1.5 µm) ZrO2 barrier were obtained by multi-cathode magnetron sputtering in Ar + O2 atmosphere. Then, cracking resistance and oxidation behavior were studied under conditions of thermal cycling (1000 °C) in air and high-temperature oxidation at 1200–1400 °C in a water steam. The role of the ZrO2/Cr multilayers and multilayer thickness on cracking resistance of the experimental coatings and oxidation resistance of the coated E110 alloy was discussed. It was shown that the coatings with more quantity of the ZrO2/Cr multilayers have higher cracking resistance, but such types of samples have a large amount of coating spallation under thermal cycling. The high-temperature steam oxidation (1200–1400 °C) demonstrated that interfaces of the ZrO2/Cr multilayers can act as a source of cavities formed by the Kirkendall mechanism that results in accelerating Cr–Zr interdiffusion for Cr-coated E110 alloy.

High efficient solar cells through multi-layer thickness optimization using particle swarm optimization and simulated annealing

High efficient solar cells through multi-layer thickness optimization using particle swarm optimization and simulated annealing

UV Ozone based rapid low temperature fabrication of flexible dye sensitized solar cells

Herein, we have developed a multilayer TiO2 film thickness of around 4 µm at lower temperature (120 °C) by spin coating and doctor blade technique on polyethylene terephthalate (ITO-PET) substrate. Further, the additional processing step Ultravoilet-Ozone (UVO3) treatment was performed on TiO2 electron transport layer (ETL) to enhance the performance of FDSSCs. UVO3 treatment significantly improved the crystallinity and charge transport characteristic of ETL by removing the organic impurities. The photovoltaic devices based on 4 h of UVO3 treated TiO2 film exhibited superior device performance with an overall power conversion efficiency (PCE) of 2.53% compared to 1.43% (without UVO3 treatment). Electrochemical impedance measurement indicated that TiO2 layer treated with UVO3 decreased series resistance, electron recombination and improved electron life substantially. The improved device properties, simple preparation procedure and low processing cost, enable this FDSSCs fabrication method to be a credible alternative of conventional techniques.

ZnO Multilayer Thin Films as The Seed Layer for ZnO Nanorods: Morphology, Structural and Optical properties

The effect of zinc oxide (ZnO) multilayer thin film thicknesses, deposited via the sol-gel spin coating technique, on the morphology, structural and optical properties of ZnO nanorods (ZNR) grown on the ZnO thin films were explored in this investigation. The ZNR was grown using the chemical bath deposition method on the ZnO thin film seed layer (SL). We found that ZnO thin film SL morphology changes according to the number of layers based on the results. Eventually, these changes also influence the structures of ZNR. ZNR structures improved when the thickness of the seed layer increased. Besides the surface roughness, better crystalline quality films were obtained when more layers were deposited. This crystalline quality then influenced the optical characteristics of both ZnO and ZNR thin films. The optical properties from UV-Vis showed transmittance in the visible region, showing that the ZnO films produced were suitable to be applied to solar cells. ZNR-based solar cells have become one of the promising materials to be studied further due to the environment-friendly, low-cost, and well-abundant material for solar cell applications.

Surface morphology engineering of metal oxide-transition metal dichalcogenide heterojunction

ABSTRACT A tremendous effort has been made to develop 2D materials-based FETs for electronic applications due to their atomically thin structures. Typically, the electrical performance of the device can vary with the surface roughness and thickness of the channel layer. Therefore, a two-step surface engineering process is demonstrated to tailor the surface roughness and thickness of MoSe2 multilayers involving exposure of O2 plasma followed by dipping in (NH4)2S(aq) solution. The O2 plasma treatment generated an amorphous MoOx layer to form a MoOx/MoSe2 heterojunction, and the (NH4)2S(aq) treatment tailored the surface roughness of the heterojunction. The ON/OFF current ratio of MoSe2 FET is about 1.1 × 105 and 5.7 × 104 for bare and chemically etched MoSe2, respectively. The surface roughness of the chemically treated MoSe2 is higher than that of the bare, 4.2 ± 0.5 nm against 3.6 ± 0.5 nm. Conversely, a 1-hour exposure of the multilayer MoOx/MoSe2 heterostructure with the (NH4)2S(aq) solution removed the amorphous oxide layer and scaled down the thickness of MoSe2 from ~92.2 nm to ~38.9 nm. The preliminary study shows that this simple two-step strategy can obtain a higher surface-area-to-volume ratio and thickness engineering with acceptable variation in electrical properties.

Electromagnetic waves in layered nanoresonator systems including bi-isotropic and metamaterial layers: analytical model and characteristic features of proper waves

The interaction of electromagnetic waves with multilayers of subwavelength thicknesses (nanoresonators) including the main layer made of a bi-isotropic material or metamaterial sandwiched between pairs of dielectric, epsilon-near-zero or metallic spacer layers has been modeled. The analytical model and numerical analysis are based on the exact and general solution of the electromagnetic boundary problem. The effects of birefringence, dichroism, chirality, nonreciprocity and new possibilities of their control and amplification, the conditions of excitation and features of proper waves inside of the bi-isotropic layer for various parameters of the nanoresonator geometry and components (in particular, metamaterials) and incident radiation are considered. It is shown that bi-isotropic effects in the one-layer chiral and nonreciprocal system can be significantly modified and amplified using the investigated nanoresonators.

Stable Metal/Metal Hydroxide Multilayers with Controlled Nanoscale Thickness Prepared from Liquid Metal Droplets with Oxide Skin

The practical applicability of ultrathin films, which offer interesting and novel functionalities, is often hampered by difficulties in large-area deposition while maintaining homogeneous film properties. Here, we induce a breakup after forced wetting to produce the ultrathin film [Runde, S. Adv. Mater. Interfaces 2018, 5(16), 1800323] and apply this deposition method to selected liquid metals and alloys to produce electrically conductive films at ambient conditions on wafer-scaled areas. In addition to ultrathin monolayers, vertically stacked and heterostructured multilayers of metal and metal hydroxide can be built by repeating the deposition method. Structural analysis using X-ray reflectometry shows that the multilayer thickness is proportional to the number of deposition cycles, yielding a single layer thickness between 2.9 and 5.2 nm, depending on the material used. Every single layer consists of a complex heterostructure composed of a nanometer-thin metallic core surrounded by stabilizing metal (hydr)oxide skin layers. The crystallinity of the layers within the films was investigated with grazing incidence X-ray diffraction; X-ray amorphous materials were Ga, GaIn(1:1), and GaInSn(7:2:1), which also showed low optical absorbance and low electrical resistivity. Films made from InSn(1:1)- and Bi-containing alloys showed weak diffraction peaks, indicating partial crystallization. The electrical conductivity of all multilayers increases with the number of deposition cycles, allowing to fine-tune the sheet resistance. The preparation of ultrathin multilayers of metallic materials at the centimeter scale is attributed to the low melting temperature combined with the high surface tension and wettability of the liquid metals.

Metal-cation-mixed lead-less two-dimensional hybrid perovskites with high carrier mobility and promoted light adsorption

The metal cation mixing in hybrid perovskite has been suggested as an effective strategy not only to promote the performance and stability but also to reduce the content of toxic lead element for environmental and health concerns. However, such a strategy has yet been well demonstrated in two-dimensional (2D) hybrid perovskites with multilayered thickness (n > 1). Here, we theoretically investigated the lead-less and lead-free 2D hybrid perovskites with tin-lead (Sn–Pb) and germanium-tin (Ge–Sn) mixed metal cations, namely, the BA2MAn-1Snn/2Pbn/2I3n+1 and BA2MAn-1Gen/2Snn/2I3n+1 (BA: C4H9NH3+; n = 1–4), by means of first-principle calculations. It is found that the electronic structure and light absorption properties of the 2D hybrid perovskites can be efficiently tuned to the optimal range for solar cells by combining the metal cations mixing and the layer thickness (n) adjustment. Markedly, due to the enhanced in-plane elastic modulus but greatly reduced deformation potential constant, both the Ge–Sn and Sn–Pb mixed 2D perovskites show high carrier mobility (∼103−104 cm2V–1S−1), which are superior to that of their 3D analogues. Moreover, the Sn–Pb mixed 2D perovskites expose much stronger visible light absorption than the non-mixed Sn- or Pb-based 2D perovskites. In addition, the in-air stability of metal-cation-mixed 2D hybrid perovskites are greatly improved compared to the 3D halide perovskites. This work suggests the great potential of the metal-cation-mixed 2D hybrid perovskite for photovoltaic and optoelectronic applications with largely reduced lead content.

Role of Substrate Type in the Process of Polyelectrolyte Multilayer Formation.

Polyelectrolyte multilayers are coatings formed by the alternate deposition of polycations and polyanions on a charged surface. In this study we examined how the type of substrate affects a multilayer prepared from poly(allylamine hydrochloride) and poly(acrylic acid). Silicon and titanium wafers were used as substrates. Their properties were systematically studied using ellipsometry, tensiometry, atomic force microscopy and streaming potential measurements. Multilayers were built up at pH = 7 with tetramethylammonium chloride as the background salt. The growth of films was monitored by ellipsometry, while the morphology and surface roughness were determined by atomic force microscopy. It was found that the thickness of multilayers containing 10 layers on silicon is 10 nm, whereas the thickness of the same film on titanium is three times higher. It was shown that multilayers formed on silicon display a grain-like structure, which was not the case for a film formed on titanium. Such morphological properties are also reflected in the surface roughness. Finally, it was shown that, in addition to the electrostatic interactions, the hydrophobicity of the substrate also plays an important role in the polyelectrolyte multilayer formation process and influences its thickness and properties.

Characterizing heavy ions-irradiated Zr/Nb: Structure and mechanical properties

In this work, the radiation responses of Zr/Nb nanostructured metallic multilayers (NMMs) are studied. The nanostructures with different layer thicknesses were deposited on Si (111) substrate by using magnetron sputtering and were subjected to heavy-ion irradiation at room temperature with different fluences. Nanoindentation, XRD, DFT, SIMS, and Variable Energy Positron Annihilation Spectroscopy (VEPAS) techniques were used to study the type and distribution of defects, and strain within the material as well as the changes in the hardness of the structures as a function of damage. Our results suggest that the strain and the irradiation hardening are layer thickness- and damage-dependent while they are independent of the type of irradiated ions. The magnitude of hardening decreases with decreasing individual layer thickness indicating that the number of interfaces has a direct effect on the radiation tolerance enhancement. For thin layers with a periodicity of 27 nm (Zr/Nb27), a transition from hardening to softening occurs at high fluence, and a saturation point is reached in thick layers with a periodicity of 96 nm (Zr/Nb96). The as-deposited thin multilayers presented a significantly higher atomic-scale disorder which increases with ion irradiation compared to the thick multilayers. VEPAS reveals the vacancy defects before and after irradiation that contribute to the presented strain. Based on the findings, thin nanostructured Zr/Nb multilayered structures possess excellent radiation resistance due to the high density of interfaces that act as sinks for radiation-induced point defects.

Non-destructive, spectrophotometric analysis of the thickness of the cell-multilayered periosteal sheet

BackgroundAutologous tissue-engineered periosteal sheets, which have been clinically applied for periodontal regeneration, sinus lift, and alveolar ridge augmentation, are enriched with osteoblast precursor cells and the abundant deposition of collagen type I in the extracellular spaces. Their quality is inspected prior to clinical use; however, most criteria cannot be evaluated without sacrificing samples. To reduce such losses, we developed a non-destructive optical method that can quantitatively evaluate the thickness of the periosteal sheet.MethodsDispersed periosteal cells were inoculated into small pieces of collagen sponge (Terudermis®) and plated into 60-mm dishes for further explant culture using a conventional medium and a stem-cell culture medium. The thickness of periosteal sheets was evaluated using inverted microscopic, histological, labeling (CellVue®)-based imaging and spectrophotometric (Spectro-1®) methods.ResultsThe three-dimensional growth of periosteal sheets did not necessarily correlate with two-dimensional growth. The periosteal sheet prepared with the stem-cell medium formed cell multilayers, a phenomenon that could be observed qualitatively by inverted microscopy. The spectrophotometric analysis enabled the quantitative evaluation of the thickness of the cell multilayer without sacrificing the samples processed for scheduled cell therapy.ConclusionsThe growth of periosteal sheets is influenced by several major factors, including the basic quality of the individual original periosteal tissue segments, the technical expertise of doctors and operators involved in tissue harvesting and processing, and culture conditions. This newly developed spectrophotometric analysis can quantify the thickness of cell-multilayered periosteal sheets for quality assurance in a non-destructive manner, thereby contributing to better bone augmentation prior to implant therapy.

Hidden surface channel in two-dimensional multilayers

Numerous carrier scatterers, such as atomic defects, fixed oxide charges, impurities, chemical residues, and undesired surface adsorbates, including oxygen and water molecules, strongly degrade the carrier mobility of atomically thin two-dimensional (2D) materials. However, the effect of surface adsorbates and surface oxidation on the carrier density profile along the thickness of 2D multilayers is not well known, particularly for a substantial interruption in the formation of the top-surface channel. Here, we uncover a hidden surface channel in p-type black phosphorus and n-type rhenium disulfide multilayers originating from undesired ambient adsorbates and surface oxides that not only populate hole density (or reduce electron density) but also suppress carrier mobility. The absence of a second peak in the transconductance curve under ambient conditions indicates the disappearance of the top-surface channel inside the 2D multilayers, which is a possible indicator for the cleanliness of the top surface and can be used in gas sensor applications. Moreover, the negligible variation in the drain bias polarity-dependent turn-on voltage for the bottom channel under ambient conditions validates the exclusive contribution of surface adsorbates to the formation of the top channel in 2D multilayers. Our results provide a novel insight into the distinct carrier transport in 2D optoelectronic devices and diverse sensors.