Showing papers on "Layer by layer published in 2020"

Layer-by-layer growth of complex-shaped three-dimensional nanostructures with focused electron beams

Abstract: The fabrication of three-dimensional (3D) nanostructures is of great interest to many areas of nanotechnology currently challenged by fundamental limitations of conventional lithography. One of the...

Layer-by-Layer Printing of Photopolymers in 3D: How Weak is the Interface?

Abstract: Additive manufacturing or, as also called, three-dimensional (3D) printing is considered as a game-changer in replacing traditional processing methods in numerous applications; yet, it has one intrinsic potential weakness related to bonding of layers formed during the printing process. Prior to finding solutions for improvement, a thorough quantitative understanding of the mechanical properties of the interface is needed. Here, a quantitative analysis of the nanomechanical properties in 3D printed photopolymers formed by digital light processing (DLP) stereolithography (SLA) is shown. Mapping of the contact Young's modulus across the layered structure is performed by atomic force microscopy (AFM) with a submicrometer resolution. The peakforce quantitative nanomechanical mapping (PF-QNM) mode was employed in the AFM experiments. The layered specimens were obtained from an acrylate-based resin (PR48, Autodesk), containing also a light-absorbing dye. We observed local depressions with values up to 30% of the maximum stiffness at the interface between the consecutively deposited layers, indicating local depletion of molecular cross-link density. The thickness values of the interfacial layers were approximately 11 μm, which corresponds to ∼22% of the total layer thickness (50 μm). We attribute this to heterogeneities of the photopolymerization reaction, related to (1) atmospheric oxygen inhibition and (2) molecular diffusion across the interface. Additionally, a pronounced stiffness decay was observed across each individual layer with a skewed profile. This behavior was rationalized by a spatial variation of the polymer cross-link density related to the variations of light absorption within the layers. This is caused by the presence of light absorbers in the printed material, resulting in a spatial decay of light intensity during photopolymerization.

An anisotropic layer-by-layer carbon nanotube/boron nitride/rubber composite and its application in electromagnetic shielding

Abstract: Multifunctional polymer composites with anisotropic properties are attracting interest as they fulfil the growing demand of multitasking materials. In this work, anisotropic polymer composites have been fabricated by combining the layer-by-layer (LBL) filtration method with the alternative assembling of carbon nanotubes (CNTs) and hexagonal boron nitride flakes (hBN) on natural rubber latex particles (NR). The layered composites exhibit anisotropic thermal and electrical conductivities, which are tailored through the layer formulations. The best composite consists of four layers of NR modified with 8 phr (parts per Hundred Rubber) CNTs (∼7.4 wt%) and four alternate layers with 12 phr hBN (∼10.7 wt%). The composites exhibit an electromagnetic interference (EMI) shielding effectiveness of 22.41 ± 0.14 dB mm−1 at 10.3 GHz and a thermal conductivity equal to 0.25 W m−1 K−1. Furthermore, when the layered composite is used as an electrical thermal heater the surface reaches a stable temperature of ∼103 °C in approx. 2 min, with an input bias of 2.5 V.

High-Performance Flexible Electrochemical Heavy Metal Sensor Based on Layer-by-Layer Assembly of Ti3C2Tx/MWNTs Nanocomposites for Noninvasive Detection of Copper and Zinc Ions in Human Biofluids.

Abstract: A flexible electrochemical heavy metal sensor based on a gold (Au) electrode modified with layer-by-layer (LBL) assembly of titanium carbide (Ti3C2Tx) and multiwalled carbon nanotubes (MWNTs) nanoc...

Auto-controlled fabrication of a metal-porphyrin framework thin film with tunable optical limiting effects.

Abstract: Metal–organic frameworks (MOFs) with third-order nonlinear optical (NLO) properties are still in their infancy but are very important. In this work, we first develop a layer by layer autoarm immersion method for preparing porphyrin-based MOF (PIZA-1) thin films with third-order NLO properties. By precisely controlling the thickness, the nonlinear absorption of PIZA-1 thin films can be switched continuously between reverse saturable absorption (RSA) and saturable absorption (SA) by using the Z-scan technique. In addition, the optical limiting effect could be further optimized by loading C60 in the pores of the PIZA-1 thin film. These findings not only open a new route for the exploitation of third-order NLO thin film materials, but also offer an insightful understanding of porphyrin-based MOF thin films for future broad practical applications.

Wire arc additive manufacturing Ti6Al4V aeronautical parts using plasma arc welding: Analysis of heat-treatment processes in different atmospheres

Abstract: PAW (Plasma Arc Welding), a WAAM (Wire Arc Additive Manufacturing) technology with high deposition rates, can produce metallic components, layer by layer, of varied sizes, from different alloys, yielding high mechanical performance. Two Ti6Al4V walls are manufactured in an inert argon atmosphere using WAAM-PAW to analyze the deposition process in terms of growth in height per layer, deposition process temperature, and cooling times. The properties of the walls are compared with the values obtained from a thermo-mechanical simulation and both the microstructural and mechanical properties of the annealed WAAM-PAW wall are studied. Moreover, the effect of the media on the oxidation layer and on the mechanical properties are also analyzed throughout the heat treatment process, as well as the microstructure of Ti6Al4V. Stable deposition rates were achieved for a high deposition ratio of Ti6Al4V at 2 kg/h, restricting the oxygen levels to under 100 ppm. No significant differences were found in either the microstructural or the mechanical properties following heat treatments in a vacuum, in air or in argon. All the heat-treated samples met the AMS4928 standard for Yield Strength (YS) and Ultimate Tensile Strength (UTS).

Enhanced four-wave-mixing with 2D layered graphene oxide films integrated with CMOS compatible micro-ring resonators

Abstract: Layered 2D graphene oxide (GO) films are integrated with microring resonators (MRRs) to experimentally demonstrate enhanced nonlinear optics in the form of four wave mixing (FWM). Both uniformly coated and patterned GO films are integrated on CMOS compatible doped silica MRRs using a large area, transfer free, layer by layer GO coating method together with photolithography and lift off processes, yielding precise control of the film thickness, placement, and coating length. The high Kerr nonlinearity and low loss of the GO films combined with the strong light matter interaction within the MRRs results in a significant improvement in the FWM efficiency in the hybrid MRRs. Detailed FWM measurements are performed at different pump powers and resonant wavelengths for the uniformly coated MRRs with 1 to 5 layers of GO as well as the patterned devices with 10 to 50 layers of GO. The experimental results show good agreement with theory, achieving up to 7.6 dB enhancement in the FWM conversion efficiency (CE) for an MRR uniformly coated with 1 layer of GO and 10.3 dB for a patterned device with 50 layers of GO. By fitting the measured CE as a function of pump power for devices with different numbers of GO layers, we also extract the dependence of the third-order nonlinearity on layer number and pump power, revealing interesting physical insights about the evolution of the layered GO films from 2D monolayers to quasi bulk like behavior. These results confirm the high nonlinear optical performance of integrated photonic resonators incorporated with 2D layered GO films.

PEG@ZIF-8/PVDF Nanocomposite Membrane for Efficient Pervaporation Desulfurization via a Layer-by-Layer Technology.

Abstract: The desulfurization property of conventional mixed matrix membranes (MMMs) cannot meet the necessary demand due to particles aggregation and interface defects. Here, we put forward a layer-by-layer (LBL) approach to make a novel PEG@ZIF-8/poly(vinylidene difluoride)(PVDF) composite membrane for pervaporation desulfurization. In this way, a ZIF-8 layer is covered on the surface of the PVDF porous membrane via an in situ growth method. Then, a PEG layer is covered on the ZIF-8 layer by a casting method. Compared with pristine PEG membranes, the separation performance of the ZIF-8@PEG/PVDF nanocomposite membrane increased significantly. This can be attributed to the homogeneous ZIF-8 particle layer and better compatibility between the poly(ethylene glycol) (PEG) matrix and ZIF-8 particles. The membrane achieves a maximum total flux of 3.08 kg·m-2·h-1 at the third in situ growth cycles of ZIF-8 particles and a maximum sulfur enrichment factor of 7.6 at the sixth in situ growth cycles of ZIF-8 particles.

A Review on the Pathways of the Improved Structural Characteristics and Photocatalytic Performance of Titanium Dioxide (TiO2) Thin Films Fabricated by the Magnetron-Sputtering Technique

Abstract: Titanium dioxide (TiO2) thin films are used for a broad range of applications such as wastewater treatment, photocatalytic degradation activity, water splitting, antibacterial and also in biomedical applications. There is a wide range of synthesis techniques for the deposition of TiO2 thin films, such as chemical vapor deposition (CVD) and physical vapor deposition (PVD), both of which are well known deposition methods. Layer by layer deposition with good homogeneity, even thickness and good adhesive nature is possible by using the PVD technique, with the products being used for photocatalytic applications. This review studies the effects of magnetron sputtering conditions on TiO2 films. This innovative technique can enhance the photocatalytic activity by increasing the thickness of the film higher than any other methods. The main purpose of this article is to review the effects of DC and RF magnetron sputtering conditions on the preparation of TiO2 thin films for photocatalysis. The characteristics of TiO2 films (i.e., structure, composition, and crystallinity) are affected significantly by the substrate type, the sputtering power, the distance between substrate and target, working pressure, argon/oxygen ratio, deposition time, substrate temperature, dopant types, and finally the annealing treatment. The photocatalytic activity and optical properties, including the degree of crystallinity, band gap (Eg), refractive index (n), transmittance (T), and extinction coefficient (k), of TiO2 films are dependent on the above- mentioned film characteristics. Optimal TiO2 films should have a small particle size, a strong degree of crystallinity, a low band gap, a low contact angle, a high refractive index, transmittance, and extinction coefficient. Finally, metallic and nonmetallic dopants can be added to enhance the photocatalytic activity of TiO2 films by narrowing the band gap.

Precise Tuning of Coordination Positions for Transition-Metal Ions via Layer-by-Layer Assembly To Enhance Solar Hydrogen Production

Abstract: Finely tuning the charge transfer constitutes a central challenge in photocatalysis, yet exquisite control of the directional charge transfer to the target reactive sites is hindered by the rapid charge recombination. Herein, dual separated charge transport channels were fabricated in a one-dimensional transition-metal chalcogenide (TMC)-based system via an elaborate layer-by-layer (LbL) self-assembly approach, for which oppositely charged metal-ion-coordinated branched polyethylenimine (BPEI) and MoS2 quantum dots (QDs) were alternately integrated to fabricate the multilayered TMC@(BPEI/MoS2 QDs)n heterostructures with controllable interfaces. Photocatalytic hydrogen generation performances of such ternary heterostructures under visible light irradiation were evaluated, which unravels that the BPEI layer not only behaves as "molecule glue" to enable the electrostatic LbL assembly with MoS2 QDs in an alternate stacking fashion on the TMC frameworks but also acts as a unidirectional hole-transfer channel. More significantly, transition-metal ions (Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) coordinated on the outmost BPEI layer are able to function as interfacial electron transfer mediators for accelerating the interfacial cascade electron transport efficiency. These simultaneously constructed dual high-speed electron and hole-transfer channels are beneficial for boosting the charge separation and enhancing the photocatalytic hydrogen evolution performances.

Effect of energy density on laser powder bed fusion built single tracks and thin wall structures with 100 µm preplaced powder layer thickness

Abstract: Laser Powder Bed Fusion (LPBF) is one of the advanced manufacturing technologies used for fabricating near net shaped components directly from CAD model data by selectively melting pre-placed layer of powder in layer by layer fashion. LPBF process is widely researched with layer thickness up to 60 µm and is now commercially deployed for many metallic materials. However, very limited literature is available in public domain for LPBF with layer thickness >60 µm as the process in this window has many challenges in geometry control and reproducibility due to inherent process instability. However, higher layer thickness with larger beam diameter can bring better productivity and shorter built time with limited compromise on minimum feature size. The present work focuses on a systematic parametric study on single track and thin wall fabrication using LPBF at layer thickness of 100 µm by varying laser power (150–450 W) and scan speed (0.02–0.08 m/s) using SS 316L powder. For the range of parameters under investigation, process window yielding stable tracks (regular and uniform) is obtained for energy density between 87.5 and 140 J/mm3. An analytical model for predicting the width of the track and a regression model for the depth of re-melted zone in the substrate subsurface and track area during single track fabrication is developed in terms of energy density. The average difference in predicted and experimental values for width and area of the track are 3.18% and 7.61%, respectively within the process window. Width of thin walls built at the same parameters is measured and the variation between width of thin wall and track is estimated in terms of energy density. The width of thin walls fabricated are observed to be larger than that of single track built at the same combinations of process parameters primarily due to preheating effect. For the range of parameters under investigation, the highest values of width of thin wall and its difference from corresponding width of track is observed at 112.5 J/mm3 in the process window. The study paves a way in understanding the effect of higher layer thickness on the geometry of LPBF built components.

Surface-coordinated metal–organic framework thin films (SURMOFs) for electrocatalytic applications

Abstract: The design and development of highly efficient electrocatalysts are very important in energy storage and conversion. As a kind of inorganic organic hybrid material, metal-organic frameworks (MOFs) have been used as electrocatalysts in electrocatalytic reactions due to their structural diversities and fascinating functionalities. Particularly, MOF thin films are coordinated on substrate surfaces by a liquid phase epitaxial (LPE) layer by layer (LBL) growth method (called surface-coordinated MOF thin films, SURMOFs), and recently have been studied in various applications due to their precisely controlled thickness, preferred growth orientation and homogeneous surface. In this review, we will summarize the preparation and electrocatalysis of SURMOFs and their derived thin films (SURMOF-D). The SURMOF based thin films possess diverse topological structures and flexible properties, providing abundant catalytically active sites and fast charge transfer for efficient electrocatalytic performance in the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CRR), supercapacitors, tandem electrocatalysis and so on. The research challenges and problems of SURMOFs for electrocatalytic applications are also discussed at the end of the review.

Highly hydrophobic and UV protective properties of cotton fabric using layer by layer self-assembly technique

Abstract: Electrostatic self-assembly layer by layer technique was used to immobilize ZnO/SiO2 nanocompsite on cationized cotton fabric. This occurs via the sequential dipping of cotton fabric in dilute solutions of poly (diallyldimethylammonium chloride) (PDDA) and ZnO/SiO2 colloidal suspension nanocomposite of different concentration ratios 1:0, 0:1, 1:1, 2:1, 1:2, and 2:2. The formation of multilayer thin film on cotton fabric creates different functional properties. UV protection properties were monitored at the ratio of (Zn/Si) as well as the number of layers. In the case of 1(Bilayer)BL and 5(Bilayer)BL, increasing the ratio of (ZnO/SiO2) within the nano composite (ZnO/SiO2) ratio, the UPF increases and the results indicate that the best ultraviolet protection factor is obtained when the Zn/Si ratio is 2. Additionally, dyeing the treated fabric often enhanced protection against ultra violet rays. FTIR spectra were utilized to distinguish the existence of effective groups on the surface of the treated cotton. Scanning electron microscopy studies confirmed successful deposition of the PDDA/(ZnO/SiO2) nanocomposite. Moreover, cotton fibers connected together because of the increased coating density and their surface become rougher. Post treatment by stearic acid rendered the fabric water repellent property. Other physical properties such as tensile strength as well as breathability of the cotton fabric were investigated.

3D Layer-By-Layer Pd-Containing Nanocomposite Platforms for Enhancing the Performance of Hydrogen Sensors

Abstract: Herein, a nanowelding technique is adopted to fabricate three-dimensional layer-by-layer Pd-containing nanocomposite structures with special properties. Nanowires fabricated from noble metals (Pd, ...

Layer-by-layer fabrication of 3D hydrogel structures using open microfluidics.

Abstract: Patterned deposition and 3D fabrication techniques have enabled the use of hydrogels for a number of applications including microfluidics, sensors, separations, and tissue engineering in which form fits function. Devices such as reconfigurable microvalves or implantable tissues have been created using lithography or casting techniques. Here, we present a novel open-microfluidic patterning method that utilizes surface tension forces to form hydrogel layers on top of each other, into a patterned 3D structure. We use a patterning device to form a temporary open microfluidic channel on an existing gel layer, allowing the controlled flow of unpolymerized gel in device-regions. After layer gelation and device removal, the process can be repeated iteratively to create multi-layered 3D structures. The use of open-microfluidic and surface tension-based methods to define the shape of each individual layer enables patterning to be performed with a simple pipette and with minimal dead-volume. Our method is compatible with unmodified (native) biological hydrogels, and other non-biological materials with precursor fluid properties compatible with capillary flow. With our open-microfluidic layer-by-layer fabrication method, we demonstrate the capability to build agarose, type I collagen, and polymer-peptide 3D structures featuring asymmetric designs, multiple components, overhanging features, and cell-laden regions.

Tunable layer-by-layer films containing hyaluronic acid and their interactions with CD44

Abstract: We report on the development of layer-by-layer (LbL) constructs whose viscoelastic properties and bioactivity can be finely tuned by using polyanions of different size and/or crosslinking. As a polyanion we used hyaluronic acid (HA) - a multi-signaling biomolecule whose bioactivity depends on its molecular weight. We investigated the interplay between the mechanical properties of the LbL systems built using HA of different sizes and the specific HA-mediated biochemical interactions. We characterized the assembled materials and their interactions with CD44, the main HA receptor, by Quartz Crystal Microbalance with Dissipation (QCM-D), Surface Plasmon Resonance (SPR) and Atomic Force Microscopy (AFM). We observed that the presence of CD44 resulted in the disruption of the non-crosslinked multilayers, while crosslinked films remain stable and bind CD44 in a HA molecular weight and charge specific fashion.

Anisotropic layer-by-layer carbon nanotubes/boron nitride/rubber composite and its application in electromagnetic shielding

Abstract: Multifunctional polymer composites with anisotropic properties are attracting interests as they fulfil the growing demand of multitasking materials. In this work, anisotropic polymer composites are fabricated by combining the layer-by-layer (LBL) filtration method with the alternative assembling of carbon nanotubes (CNTs) and hexagonal boron nitride flakes (hBN) on natural rubber latex particles (NR). The layered composites exhibit anisotropic thermal and electrical conductivities, which are tailored through the layer formulations. The best composite consists of four layers of NR modified with 8 phr (parts per Hundred Rubber) CNTs (~7.4 wt%) and four alternated layers with 12 phr hBN (~10.7 wt%). The composites exhibit an electromagnetic interference (EMI) shielding effectiveness of 22.41+-0.14 dB mm-1 at 10.3 GHz and a thermal conductivity equal to 0.25 W/(mK). Furthermore, when the layered composite is used as an electrical thermal heater the surface reaches a stable temperature of ~103 C in approx. 2 min, with an input bias of 2.5 V.

Continuous ultrathin UiO-66-NH2 coatings on a polymeric substrate synthesized by a layer-by-layer method: a kind of promising membrane for oil–water separation

Abstract: In this paper, we successfully controllably synthesize continuous nanothickness MOF coatings (NTMCs) by a layer-by-layer method on a polymeric substrate. The polymeric substrate was pretreated with high energy γ-irradiation to induce a high surface density of living reactive groups, which ensure the formation of continuous surface-integrated NTMCs. SEM, FT-IR spectroscopy and XPS were used to characterize NTMCs. The thickness and morphology were tuned by the LBL cycles, and NTMCs with a thickness of ∼44 nm were obtained. The chemical bonds between the NTMCs and polymeric substrate were confirmed by XPS and EDS. Moreover, the NTMCs exhibit good performance for oil–water separation. We believe that our work will promote the design and precise synthesis of high-performance MOF based membranes for multiple practical applications in future.

Fabrication and Imaging of Monolayer Phosphorene with Preferred Edge Configurations via Graphene-Assisted Layer-by-Layer Thinning

Abstract: Phosphorene, a monolayer of black phosphorus (BP), is an elemental two-dimensional material with interesting physical properties, such as high charge carrier mobility and exotic anisotropic in-plan...

Layer-by-Layer Stacked (NH4)2V4O9·0.5H2O Nanosheet Assemblies with Intercalation Pseudocapacitance for High Rate Aqueous Zinc Ion Storage

Abstract: Rechargeable aqueous zinc-ion batteries have become an emerging candidate for large-scale energy storage due to their low cost and high safety, the lack of suitable advanced cathode materials with ...

Layer-by-layer spray coating of a stacked perovskite absorber for perovskite solar cells with better performance and stability under a humid environment

Abstract: Perovskite is an emerging material for high performance solar cell application with low-cost solution-processable fabrication. As an ink, perovskite composition can be easily modified to create semi-transparent solar cells for window replacement. To enable scalable large-scale production, the spray process is one of the major candidates. In this work, we developed sequential spray deposition (SSD) to create double layer absorbers from different dimensional perovskites. SSD, for the first time, achieves layer-by-layer deposition of different perovskite materials for stacked architecture. To demonstrate the benefits, we spray-coated lower dimension, more stable perovskite onto high performance yet sensitive 3D semi-transparent perovskite. SSD performed under a humid environment (40 - 50% RH) brings about better film stability and retains good performance of 3D perovskite. Sequential spray deposition opens new routes for various stacking designs and large-scale production under economical ambient conditions.