Wrinkling of a sol-gel-derived thin film

Enhanced Photocatalytic Performance under Ultraviolet and Visible Light Illumination of ZnO Thin Films Prepared by Modified Sol-Gel Method

Semiconductor oxides are frequently used as active photocatalysts for the degradation of organic agents in water polluted by domestic industry. In this study, sol-gel ZnO thin films with a grain size in the range of 7.5-15.7 nm were prepared by applying a novel two-step drying procedure involving hot air treatment at 90-95 °C followed by conventional furnace drying at 140 °C. For comparison, layers were made by standard furnace drying. The effect of hot air treatment on the film surface morphology, transparency, and photocatalytic behavior during the degradation of Malachite Green azo dye in water under ultraviolet or visible light illumination is explored. The films treated with hot air demonstrate significantly better photocatalytic activity under ultraviolet irradiation than the furnace-dried films, which is comparable with the activity of unmodified ZnO nanocrystal powders. The achieved percentage of degradation is 78-82% under ultraviolet illumination and 85-90% under visible light illumination. Multiple usages of the hot air-treated films (up to six photocatalytic cycles) are demonstrated, indicating improved photo-corrosion resistance. The observed high photocatalytic activity and good photo-corrosion stability are related to the hot air treatment, which causes a reduction of oxygen vacancies and other defects and the formation of interstitial oxygen and/or zinc vacancies in the films.

Synthesis of ZnO sol-gel thin-films CMOS-Compatible

Zinc oxide (ZnO) is a II-VI group semiconductor with a wide direct bandgap and is an important material for various fields of industry and high-technological applications. The effects of thickness, annealing process in N2 and air, optical properties, and morphology of ZnO thin-films are studied. A low-cost sol-gel spin-coating technique is used in this study for the simple synthesis of eco-friendly ZnO multilayer films deposited on (100)-oriented silicon substrates ranging from 150 to 600 nm by adjusting the spin coating rate. The ZnO sol-gel thin-films using precursor solutions of molarity 0.75 M exhibit an average optical transparency above 98%, with an optical band gap energy of 3.42 eV. The c-axis (002) orientation of the ZnO thin-films annealed at 400 °C were mainly influenced by the thickness of the multilayer, which is of interest for piezoelectric applications. These results demonstrate that a low-temperature method can be used to produce an eco-friendly, cost-effective ZnO sol-gel that is compatible with a complementary metal-oxide-semiconductor (CMOS) and integrated-circuits (IC).

Engineering Quick- and Long-acting Naloxone Delivery Systems for Treating Opioid Overdose

Purpose

Opioids have been the main factor for drug overdose deaths in the United States. Current naloxone delivery systems are effective in mitigating the opioid effects only for hours. Naloxone-loaded poly(lactide-co-glycolide) (PLGA) microparticles were prepared as quick- and long-acting naloxone delivery systems to extend the naloxone effect as an opioid antidote.

Methods

The naloxone-PLGA microparticles were made using an emulsification solvent extraction approach with different formulation and processing parameters. Two PLGA polymers with the lactide:glycolide (L:G) ratios of 50:50 and 75:25 were used, and the drug loading was varied from 21% to 51%. Two different microparticles of different sizes with the average diameters of 23 μm and 50 μm were produced using two homogenization-sieving conditions. All the microparticles were critically characterized, and three of them were evaluated with β-arrestin recruitment assays.

Results

The naloxone encapsulation efficiency (EE) was in the range of 70–85%. The EE was enhanced when the theoretical naloxone loading was increased from 30% to 60%, the L:G ratio was changed from 50:50 to 75:25, and the average size of the particles was reduced from 50 μm to 23 μm. The in vitro naloxone release duration ranged from 4 to 35 days. Reducing the average size of the microparticles from 50 μm to 23 μm helped eliminate the lag phase and obtain the steady-state drug release profile. The cellular pharmacodynamics of three selected formulations were evaluated by applying DAMGO, a synthetic opioid peptide agonist to a μ-opioid receptor, to recruit β-arrestin 2.

Conclusions

Naloxone released from the three selected formulations could inhibit DAMGO-induced β-arrestin 2 recruitment. This indicates that the proposed naloxone delivery system is adequate for opioid reversal during the naloxone release duration.

Formulation composition, manufacturing process, and characterization of poly(lactide-co-glycolide) microparticles

Injectable long-acting formulations, specifically poly(lactide-co-glycolide) (PLGA) based systems, have been used to deliver drugs systemically for up to 6 months. Despite the benefits of using this type of long-acting formulations, the development of clinical products and the generic versions of existing formulations has been slow. Only about two dozen formulations have been approved by the U.S. Food and Drug Administration during the last 30 years. Furthermore, less than a dozen small molecules have been incorporated and approved for clinical use in PLGA-based formulations. The limited number of clinically used products is mainly due to the incomplete understanding of PLGA polymers and the various variables involved in the composition and manufacturing process. Numerous process parameters affect the formulation properties, and their intricate interactions have been difficult to decipher. Thus, it is necessary to identify all the factors affecting the final formulation properties and determine the main contributors to enable control of each factor independently. The composition of the formulation and the manufacturing processes determine the essential property of each formulation, i.e., in vivo drug release kinetics leading to their respective pharmacokinetic profiles. Since the pharmacokinetic profiles can be correlated with in vitro release kinetics, proper in vitro characterization is critical for both batch-to-batch quality control and scale-up production. In addition to in vitro release kinetics, other in vitro characterization is essential for ensuring that the desired formulation is produced, resulting in an expected pharmacokinetic profile. This article reviews the effects of a selected number of parameters in the formulation composition, manufacturing process, and characterization of microparticle systems. In particular, the emphasis is focused on the characterization of surface morphology of PLGA microparticles, as it is a manifestation of the formulation composition and the manufacturing process. Also, the implication of the surface morphology on the drug release kinetics is examined. The information described here can also be applied to in situ forming implants and solid implants.

On-Substrate Joule Effect Heating by Printed Micro-Heater for the Preparation of ZnO Semiconductor Thin Film

Fabrication of printed electronic devices along with other parts such as supporting structures is a major problem in modern additive fabrication. Solution-based inkjet printing of metal oxide semiconductor usually requires a heat treatment step to facilitate the formation of target material. The employment of external furnace introduces additional complexity in the fabrication scheme, which is supposed to be simplified by the additive manufacturing process. This work presents the fabrication and utilization of micro-heater on the same thermal resistive substrate with the printed precursor pattern to facilitate the formation of zinc oxide (ZnO) semiconductor. The ultraviolet (UV) photodetector fabricated by the proposed scheme was successfully demonstrated. The performance characterization of the printed devices shows that increasing input heating power can effectively improve the electrical properties owing to a better formation of ZnO. The proposed approach using the on-substrate heating element could be useful for the additive manufacturing of functional material by eliminating the necessity of external heating equipment, and it allows in-situ annealing for the printed semiconductor. Hence, the integration of the printed electronic device with printing processes of other materials could be made possible.

Memristive and Memory Impedance Behavior in a Photo-Annealed ZnO–rGO Thin-Film Device

An oxygen-rich ZnO-reduced graphene oxide (rGO) thin film was synthesized using a photo-annealing technique from zinc precursor (ZnO)–graphene oxide (GO) sol–gel solution. X-ray diffraction (XRD) results show a clear characteristic peak corresponding to rGO. The scanning electron microscope (SEM) image of the prepared thin film shows an evenly distributed wrinkled surface structure. Transition Metal Oxide (TMO)-based memristive devices are nominees for beyond CMOS Non-Volatile Memory (NVRAM) devices. The two-terminal Metal–TMO (Insulator)–Metal (MIM) memristive device is fabricated using a synthesized ZnO–rGO as an active layer on fluorine-doped tin oxide (FTO)-coated glass substrate. Aluminum (Al) is deposited as a top metal contact on the ZnO–rGO active layer to complete the device. Photo annealing was used to reduce the GO to rGO to make the proposed method suitable for fabricating ZnO–rGO thin-film devices on flexible substrates. The electrical characterization of the Al–ZnO–rGO–FTO device confirms the coexistence of memristive and memimpedance characteristics. The coexistence of memory resistance and memory impedance in the same device could be valuable for developing novel programmable analog filters and self-resonating circuits and systems.

Cumulative energy analysis of thermally-induced surface wrinkling of heterogeneously multilayered thin films

Wrinkling is a well-known example of instability-driven surface deformation that occurs when the accumulated compressive stress exceeds the critical value in multilayered systems. A number of studies have investigated the instability conditions and the corresponding mechanisms of wrinkling deformation. Force balance analysis of bilayer systems, in which the thickness of the capping layer is importantly considered, has offered a useful approach for the quantitative understanding of wrinkling. However, it is inappropriate for multilayer wrinkling (layer number > 3) consisting of heterogeneous materials (e.g. polymer/metal or inorganic), in which the thickness variation in the substrate is also crucial. Therefore, to accommodate the additive characteristics of multilayered systems, we thermally treated tri- or quad-layer samples of polymer/metal multilayers to generate surface wrinkles and used a cumulative energy balance analysis to consider the individual contribution of each constituent layer. Unlike the composite layer model, wherein the thickness effect of the capping layer is highly overestimated for heterogenously stacked multilayers, our approach precisely reflects the bending energy contribution of the given multilayer system, with results that match well with experimental values. Furthermore, we demonstrate the feasibility of this approach as a metrological tool for simple and straightforward estimation of the thermomechanical properties of polymers, whereby a delicate change in the Young's modulus of a thin polymeric layer near its glass transition temperature can be successfully monitored.

Competitive concurrence of surface wrinkling and dewetting of liquid crystalline polymer films on non-wettable substrates

Polymeric thin films coated on non-wettable substrates undergo film-instabilities, which are usually manifested as surface deformation in the form of dewetting or wrinkling. The former takes place in fluidic films, whereas the latter occurs in solid films. Therefore, there have rarely been reports of systems involving simultaneous deformations of dewetting and wrinkling. In this study, we propose polymeric thin films of liquid crystalline (LC) mesogens prepared on a non-wettable Si substrate and apply a treatment of plasma irradiation to form a thin polymerized layer at the surface. The resulting compressive stress generated in the surface region drives the formation of wrinkles, while at the same time, dipolar attraction between LC molecules induces competitive cohesive dewetting. Intriguing surface structures were obtained whereby dewetting-like hole arrays are nested inside the randomly propagated wrinkles. The structural features are readily controlled by the degree of surface cross-linking, hydrophilicity of the substrates, and the LC film thickness. In particular, dewetting of LC mesogens is observed to be restricted to occur at the trough regions of wrinkles, exhibiting the typical behavior of geometrically confined dewetting. Finally, wrinkling-dewetting mixed structures are separated from the substrate in the form of free standing films to demonstrate the potential applicability as membranes.

Wrinkle motifs in thin films

On length scales from nanometres to metres, partial adhesion of thin films with substrates generates a fascinating variety of patterns, such as ‘telephone cord’ buckles, wrinkles, and labyrinth domains. Although these patterns are part of everyday experience and are important in industry, they are not completely understood. Here, we report simulation studies of a previously-overlooked phenomenon in which pairs of wrinkles form avoiding pairs, focusing on the case of graphene over patterned substrates. By nucleating and growing wrinkles in a controlled way, we characterize how their morphology is determined by stress fields in the sheet and friction with the substrate. Our simulations uncover the generic behaviour of avoiding wrinkle pairs that should be valid at all scales.

Surface instability of confined elastic bilayers: Theory and simulations

The surface of a soft elastic film becomes unstable and forms a self-organized undulating pattern because of adhesive interactions when it comes in contact proximity with a rigid surface. For a single film, the pattern length scale lambda , which is governed by the minimization of the elastic stored energy, gives lambda approximately 3h , where h is the film thickness. Based on a linear stability analysis and simulations of adhesion and debonding, we consider the contact instability of an elastic bilayer, which provides greater flexibility in the morphological control of interfacial instability. Unlike the case of a single film, the morphology of the contact instability patterns, debonding distance, and debonding force in a bilayer can be controlled in a nonlinear way by varying the thicknesses and shear moduli of the films. Interestingly, the pattern wavelength in a bilayer can be greatly increased or decreased compared to a single film when the adhesive contact is formed by the stiffer or the softer of the two films, respectively. In particular, lambda as small as 0.5h can be obtained. This indicates a new strategy for pattern miniaturization in elastic contact lithography.

Dewetting of a sol-gel-derived thin film

We report on the dewetting of a thin film produced by the sol-gel method. In the early stages of dynamic morphological instability, the drying stress in the capillary wave model determines the linearly scaling behaviors of the characteristic wavelength with the initial film thickness and the square law dependence of the number density of the dewetted holes on the film thickness. These power law dependences are weaker than those observed in the case of the spinodal dewetting of a polymer thin film. The wavelength determined in the early stages also dominates the scaling behaviors of the average length of the sides and number density of the polygons and the diameter of the droplets of the dewetting pattern with the film thickness in the final stages of the dynamic instability. We also observed that further drying eventually induces wrinkles in the droplets, rim, and film, which have a characteristic wavelength that can be theoretically predicted.

Theoretical analysis of growth of ZnO nanorods on the amorphous surfaces

Semiconductor nanorod arrays on a substrate have a preferential alignment orientation that minimizes the excessive free energy of the system. In the case of wet chemically synthesized zinc oxide (ZnO) nanorod on the amorphous surfaces, the thermodynamic driving force determines the orientation to be normal to the surface. Among the various kinds of amorphous surfaces, the spherical seed layer composed of ZnO precursors gives isotropic radially aligned arrays. For other surfaces, such as wrinkled and planar ZnO precursor thin film, nanorod arrays are aligned to be perpendicular to the tangential line of the surface. The maximum value of the aspect ratio of the nanorod is determined by the thermodynamic relationship. The number density of nanorods per unit precursor particles decreases with increasing contact angle of the seed particles.