Porous Thin Films Research Articles

A sensing method based on InSb grating coupled terahertz surface plasmon polariton resonance

A grating-coupled terahertz (THz) surface plasmon polariton (SPP) resonant biochemical sensing structure is designed with simulation, which can be easily prepared by etching a submillimeter grating on the surface of indium antimonide (InSb) substrate. The simulation results based on the phase matching equation show that when the TM-polarized broadband terahertz collimated beam is incident on the InSb grating at a 30° angle, the low-frequency SPP and high-frequency SPP with opposite propagation directions can be simultaneously excited by the –1st and +1st order diffraction beams of the grating, respectively. Since the low-frequency SPP is easy to accurately measure with a commercial THz time-domain spectroscopy devices, the dependence of the resonance characteristics and sensing characteristics of low-frequency SPP on the grating structure parameters is systematically simulated in this paper. The simulation results show that the refractive-index sensitivity of the InSb grating-coupled THz-SPP resonant sensor chip decreases with the increase of the grating period, and is 1.05 THz/RIU at a grating period of 120 μm and an incident angle of 30°. Under these conditions, the sensor chip cannot make a detectable response to the monolayer adsorption of biomolecules, because the evanescent field penetration depth of the low-frequency SPP is much greater than the biomolecular size, resulting in insufficient field-biomolecular interaction at the surface. In order to detect biomolecules, a sensitivity enhancement method based on porous thin films is proposed and analyzed with simulation. The porous films enable not only to enrich biomolecules, but also to extend the interaction between THz-SPP and biomolecules from the molecular size to the entire film thickness, thereby improving the sensitivity of the sensor to biomolecular adsorption. Taking tyrosine adsorption as an example, the simulation results show that when the InSb grating is covered with a porous polymethyl methacrylate (PMMA) film with a thickness of 120 μm and a porosity of 0.4, the sensor sensitivity to tyrosine adsorption is 0.39 THz/unit volume fraction.

Stable polydisperse free-standing porous films made by mechanical deformation.

Using molecular dynamics simulations, we show that the methodology of making thin stable nanoporous monodisperse films by biaxial mechanical expansion and subsequent cooling into the glassy state, also works for polydisperse films. To test this, a bidisperse polymer system of an equal number of very long (≈72 entanglements) and short (≤4 entanglements) chains with a polydispersity index of 1.80 is considered. The void formation and the development of the local morphology upon expansion, relaxation, and cooling are investigated. As for the monodisperse case, long chains in thin porous polydisperse films extend over several pores, stabilizing the whole morphology. The short chains do not fill up the pores but tend to aggregate inside the polymer matrix and to avoid surface areas and reduce conformational constraints imposed by the surrounding, a scenario very similar to strain-induced segregation between the strained long and relaxed short chains.

Design, synthesis and characterization of hierarchical porous stacked thin films based on MOFs and ordered mesoporous oxides

Integration of mesoporous thin films and metal–organic frameworks as stacked bilayers, towards hierarchical porous 1D photonic crystals.

Viologen-based solution-processable ionic porous polymers for electrochromic applications.

Electrochromic porous thin films are promising for applications in smart windows and energy-efficient optical displays. However, their generally poor processing ability and excessive processing times remain grand challenges. Herein, we report the design and convenient synthesis of core-altered N-arylated viologens with aldehyde groups (πV-CHO) as new building blocks to prepare soluble, viologen-embedded ionic porous polymers. We also demonstrate that these polymers can be easily solution-processed by drop-coating to fabricate high-quality electrochromic films with tunable optoelectronic properties in a cost-effective fashion. The prepared films exhibit excellent electrochromic performance, including a low driving voltage (1.2-1.4 V), fast switching times (0.8-1.7 s), great coloration efficiency (73-268 cm2 C-1), remarkably high optical contrast up to 95.6%, long cycling stability, and tunable oxidation and reduction colors. This work sheds important light on a new molecular engineering approach to produce redox-active polymers with combined properties of intrinsic porosity, reversible and tunable redox activity, and solution processability. This provides the materials with an inherently broad utility in a variety of electrochemical devices for energy storage, sensors, and electronic applications.

Manipulating the Nanophase Separation of a Polymer-Salt Microfluid Generates an Advanced In Situ Separator for Component-Integrated Energy Storage Devices.

A polymer separator plays a pivotal role in battery safety, overall electrochemical performance, and cell assembly process. Traditional separators are separately produced from the electrodes and dominated by porous polyolefin thin films. In spite of their commercial success, today's separators are facing growing challenges with the increasing demand on the device safety and performance. As an attempt to address this urgent need, here, we propose a concept of in situ separator technology by manipulating the two-dimensional (2D) microfluid nanophase separation (2D-MFPS) of a poly(vinylidene difluoride)/lithium salt solution during drying. Particularly, nanophase separation is effectively regulated by low humidity, salt type, and compositions. For application studies, this 2D-MFPS is directly performed onto commercial electrodes under drying conditions with low humidity to fabricate a high-performance in situ separator with thickness and porous structures comparable to those of commercial Celgard separators. This in situ separator shows superior performance in high-temperature stability and wetting capability to a variety of liquid electrolytes. Finally, pouch cells with this in situ separator technology are successfully assembled with an extremely simplified separator-stacking-free process and demonstrate stable cycle performance due to the well-controlled porous structures and electrode-separator interface.

Electrodeposition Behavior of Al-Au Alloys in AlCl3-NaCl-KCl-Aucl Molten Salt

Quartz Crystal Microbalance (QCM) has been used as a highly sensitive mass sensor. This study aims to form porous gold thin films on QCM elements and apply them to highly sensitive gas sensors by attaching selective media. Physical vapor deposition and chemical etching are known methods for forming porous Au thin films. In this study, the electrochemical formation of porous gold films in AlCl3-NaCl-KCl molten salts containing AuCl was investigated. There are few reports about electrodeposition and dissolution potentials of Au in the molten salt electrolyte. As a first step in this study, voltammogram measurements and constant potential electrolysis were performed in the electrolyte, and electrodeposition potentials of Au and Al-Au alloys were discussed.An electrolyte in this study is a mixture of 61 mol% AlCl3- 26 mol% NaCl- 13 mol% KCl molten salt. The mixture salt was melted at 443 K and AuCl was added to the molten salt. A glassy carbon plate (GC) and a pure Al wire were used as the working electrode and reference electrode, respectively. The GC plate and Au mesh were used as the counter electrode for voltammogram measurement and constant potential electrolysis, respectively. Voltammogram measurements were carried out at potential range from -0.1 to 2.4 V vs. Al/Al(Ⅲ). Constant potential electrolysis was performed in the range from -0.2 to 0.3 V with an electric charge density of 40 C cm-2. The electrodeposits were observed by scanning electron microscopy (SEM) and determined the composition of Al and Au by energy dispersive X-ray spectroscopy (EDS).Au ions concentration in the electrolyte was 2.5 mmol dm-3. In voltammogram measurements, a small cathodic current was observed at about 0.8 V, then gradually increased from 0.2 V. In the potential lower than 0 V, a drastical increase of cathodic current was observed, which was attributed to Al electrodeposition. When the potential was reversed, large anodic current flowed from 0 V and a small current peak was also observed at 0.2 V. These were considered to correspond to the dissolution of Al and Al-Au alloys, respectively. Furthermore, a relatively large anodic current peak was observed at about 2 V, which was expected to be due to the dissolution of Au. From the results of constant potential electrolysis, the Au concentration in the electrodeposits at -0.1 V and -0.2 V was less than 1 at%. At 0 V, the Au concentration in the electrodeposit increased sharply to 84 at%, and at 0.3 V it increased moderately to 89 at%. From SEM observation, the morphology of the electrodeposits at -0.1 V and -0.2 V was dense, while that at potentials more than 0 V was sponge-like.AcknowledgementsThis work was supported by JST SICORP and JPMJSC2108.

Quantitative Analysis of CO2 Evolution in an Alkaline Electrolyte Solution By Differential Electrochemical Mass Spectroscopy

CO2 is involved in various main and side reactions of electrochemical energy devices, such as carbon corrosion reactions and alcohol oxidation reactions. Differential electrochemical mass spectroscopy (DEMS) has been intensively applied in acidic electrolyte solutions to analyze these reactions. DEMS is the in-situ electrochemical measurement technique to analyze volatile species with the mass spectrometer (MS). However, the applications of DEMS to alkaline electrolyte solutions have been limited due to the high solubility of CO2 in alkaline solutions. While "conventional DEMS cells," where porous thin-film working electrodes are formed onto polytetrafluoroethylene (PTFE) membrane interface, can quantitatively analyze CO2 that has not dissolved into the electrolyte yet [1], the working electrode materials are limited to those that can be deposited on porous PTFE membrane as porous thin-films. Recently, Möller et al. have developed the DEMS system, which can detect CO2 from general planer electrodes coated by catalyst powders by combining a neutralizer and "dual thin-layer flow cell" [2]. However, due to the relatively low detection limit, quantitative analysis of evolved CO2 has not been accomplished. In this study, we attempt to use high surface area electrodes to enhance the MS signal of CO2 to enable quantitative analysis of evolved CO2. To suppress the in-plane reaction distribution in the flow cell, we integrated an anion exchange membrane into the cell and arranged the counter electrode opposing the working electrode [3] (Fig. 1 (a)).CO-stripping voltammetry was performed to evaluate the CO2 detectability of the constructed DEMS system. The DEMS cell was composed of a commercial Pt/C electrode as the working electrode, a commercial Hg/HgO electrode as the reference electrode, a Pt mesh as the reference electrode, 1 mol dm–3 KOH aq as the electrolyte solution, and 1 mol dm–3 H2SO4 aq as the neutralizer. Figure 1 (b) shows the CO-stripping voltammogram and corresponding MS signal of CO2 (m/z = 44). MS signal of CO2 accompanied by the anodic CO oxidation current is clearly observed from 0.6 to 1.2 V vs. RHE. The calibration constant, which correlates the MS signal of CO2 to partial current for CO2 evolution reaction, is successfully obtained as 2.2 × 10–10, which shows that the DEMS system established in this study can quantitatively analyze CO2. Applications to the carbon corrosion reactions are also presented at the site.

Hydrous Iridium Oxide Thin Films: Revealing the Structure of a Highly Active Model Catalyst By in-Situ Ir L 3-Edge XANES and EXAFS

Currently, only iridium oxide shows favourable activity and stability under the harsh oxygen evolution reaction (OER) conditions for application in proton exchange membrane water electrolysis (PEM-WE) [1-3]. There is a well established inverse relationship between OER activity and stability of iridium oxide-based OER catalysts given its degree of crystallinity and hydration [4-6]. Crystalline, anhydrous iridium oxides are more stable but have diminished OER activity compared to amorphous, hydrous iridium oxides, which are more active. Different OER mechanisms have been theorized to occur on these materials, explaining their difference in performance [6]. Several spectroscopic investigations have been performed to explore the properties of such different iridium oxides, in particular to identify the chemical and electronic state of their active sites. However, a consensus regarding the active site structure and OER mechanism has yet to be reached [7-10].Using scanning electron microscopy, x-ray photoelectron spectroscopy, and combining in-situ Ir L 3-edge x-ray absorption spectroscopy with density functional theory (DFT) calculations and ab initio thermodynamics, we have investigated in-situ electrochemically grown porous hydrous iridium oxide thin films (HIROF) as a model system to examine the chemical and electronic structure of the highly active, hydrous iridium oxide species. In-situ extended x-ray absorption fine structure(EXAFS) results show that HIROF grows preferentially in a form most often associated with the OER catalytically active site of hydrous iridium oxides. Calculations over different possible structures allowed us to identify a unique structural group with enhanced hydrogenation that best fits the EXAFS data. In-situ x-ray absorption near edge structure (XANES) results reveal a lower onset potential for the redox behaviour of HIROF compared to rutile IrO2. Based on this study, we propose a new structural model explaining the high activity and poorer stability of hydrous iridium oxides compared to crystalline IrO2.

Fabrication of ZnO-encapsulated porous alumina structure utilizing a two-step anodizing technique for memristor applications

Highly ordered porous alumina thin films with various aspect ratios were fabricated by a two-step anodizing technique. The dependence of the high resistance state to low resistance state ratio on the aspect ratio of the pores was observed in the memristors utilizing porous alumina. The reduction of threshold voltage and improvement in stability were achieved by properly encapsulating i-ZnO inside the porous alumina as the switching medium. A significant improvement in the ratio of high resistance state to low resistance state was achieved by i-ZnO encapsulation in porous alumina compared to the conventional planer switching layer of ZnO alone.

Wireless Deep Brain Stimulation by Ultrasound-Responsive Molecular Piezoelectric Nanogenerators.

Implantable neural stimulation devices are becoming prevalent in bioelectronic medicine for the precise treatment of various clinical diseases. Nevertheless, the limited lifespan and buckling size of the implanted devices remain significant obstacles for chronic clinical application. In this study, we developed an ultrasound-driven battery-free neurostimulator based on a high-performance mini-sized nanogenerator and demonstrated its successful application for the deep-brain-stimulation (DBS) therapy of Parkinson's disease in a rat model. This soft piezoelectric-triboelectric hybrid nanogenerators (PTNG) are made of porous thin-films of molecular piezoelectric materials, which have great advantages of facile, scalable, low-temperature, and flexible processing. Without any bucky accessory control circuits, the subcutaneously implanted soft PTNG can function as a wirelessly powered neurostimulator, allowing for the adjustment of stimulation parameters through external programmable ultrasound pulses. This DBS electroceutical application of energy-harvesting thin-film devices based on molecular piezoelectric materials provides valuable insight into the development of a soft high-performance bioelectronic device.

Quasi‐hexagonal nanopatterned porous cobalt ferrite thin films prepared via block copolymer self‐assembly

AbstractWe present a novel method for fabricating dense and nanopatterned porous cobalt ferrite (CoFe2O4) thin films with a remarkable thickness of 65 nm, achieved through direct block copolymer self‐assembly. By decomposing the block copolymer and inducing crystallization of the spinel phase, we successfully produce films exhibiting quasi‐hexagonal arrays of pores with an average diameter ranging from 60 to 80 nm. The resulting nanopatterned porous thin films demonstrate exceptional orderliness, as the pores are intricately designed by the cobalt ferrite grains, and the platinum substrate offers complete accessibility in the pore area. Our findings reveal high magnetization values, comparable to those of bulk CoFe2O4, in the dense film. Additionally, in the case of the nanopatterned porous film, we observed a distinctive contribution of perpendicular magnetization. This innovative approach holds great promise for advanced magnetization and thin‐film engineering applications.

On the Measurements of the Surface-Enhanced Raman Scattering Spectrum: Effective Enhancement Factor, Optical Configuration, Spectral Distortion, and Baseline Variation

In this paper, a comprehensive theoretical framework for understanding surface-enhanced Raman scattering (SERS) measurements in both solution and thin-film setups, focusing on electromagnetic enhancement principles, was presented. Two prevalent types of SERS substrates found in the literature were investigated: plasmonic colloidal particles, including spherical and spheroid nanoparticles, nanoparticle diameters, and thin-film-based SERS substrates, like ultra-thin substrates, bundled nanorods, plasmonic thin films, and porous thin films. The investigation explored the impact of analyte adsorption, orientation, and the polarization of the excitation laser on effective SERS enhancement factors. Notably, it considered the impact of analyte size on the SERS spectrum by examining scenarios where the analyte was significantly smaller or larger than the hot spot dimensions. The analysis also incorporated optical attenuations arising from the optical properties of the analyte and the SERS substrates. The findings provide possible explanations for many observations made in SERS measurements, such as variations in relative peak intensities during SERS assessments, reductions in SERS intensity at high analyte concentrations, and the occurrence of significant baseline fluctuations. This study offers valuable guidance for optimizing SERS substrate design, enhancing SERS measurements, and improving the quantification of SERS detection.

Humidity induced resistive switching and negative differential resistance in α-Fe2O3 porous thin films

Humidity induced resistive switching and negative differential resistance in α-Fe2O3 porous thin films

Spray deposited pristine and Mo doped WO3 thin films for acetaldehyde gas sensing at room temperature

In this paper, pure WO3 and molybdenum-incorporated WO3 were directly deposited in the form of thin films on the glass substrate. The films were deposited through the spray pyrolysis technique, and their gas sensing properties, structural, optical, electrical, and morphological properties were altered by adding a dopant to the pristine WO3. A direct, one-step approach is used to deposit metal oxide semiconductor materials as thin films, which enhances the ability to detect gas. A novel gas sensing response for acetaldehyde gas detection was observed from Molybdenum doped WO3 thin film at 25 °C. On investigating the sensing performance of both films for various volatile organic compounds (VOC), a high response (54.55 %) towards acetaldehyde was achieved for 2 % molybdenum-doped WO3 film. The XRD analysis demonstrated that the synthesized films are crystalline with triclinic phase. Decrease in crystallite size increases the grain boundaries that enhance the sensing response. The absorption spectra of UV–Visible spectroscopy showed an apparent change of increase in the absorbance and decrease in the optical band gap on addition of the dopant to the pure WO3 thin film. Filamentous structured porous thin films are discovered to have larger voids and greater surface roughness from FE-SEM and AFM, which aids in the adsorption of gas molecules. Mo-doped WO3 thin films showed a strong sensing response due to larger grain boundary scattering and wide pores abetting for more gas adsorption.

Effect of Oxalic Acid Additives on Aluminum Anodizing in Formic Acid Containing Ammonium Heptamolybdate

This paper reports a systematic study of the role of oxalic acid additives in aluminum anodizing in formic acid containing ammonium heptamolybdate. Adding oxalic acid in a concentration range of 5–20 mM to the 0.4 M formic acid solution containing 0.03 M ammonium heptamolybdate improves anodic film growth, increasing the film thickness and smoothing strongly wavy interface between the film and aluminum, and adding 100 mM of oxalic acid results in an almost complete block of the regular anodic film formation. In the case of aluminum anodizing in formic acid, the ammonium heptamolybdate additive prevents aluminum dissolution more effectively than only oxalic acid. The role of oxalic acid in this process is only to improve film growth and morphology. However, ammonium heptamolybdate improves film growth by increasing its thickness. Linear sweep voltammetry studies combined with SEM investigations of alumina growth show that in heptamolybdate-containing electrolytes, a thin porous alumina film is formed at the beginning of the process. Then, when the electrolyte oxidation potential is reached, the thin film on the surface breaks, resulting in a significant increase in the anodizing surface, and anodic oxide begins to grow rapidly.

Inducing Circularly Polarized Luminescence by Confined Synthesis of Ultrasmall Chiral Carbon Nanodot Arrays in Pyrene-Based MOF Thin Film.

Combining the features of the host-guest system and chirality is an efficient strategy to achieve circularly polarized luminescence (CPL). Herein, well-defined chiral carbon nanodot (chirCND) arrays were confined-synthesized by low-temperature calcination of a chiral amino acid loaded metal-organic framework (MOF) to induce high CPL. An achiral porous pyrene-based MOF NU-1000 thin film as the host template was prepared by a liquid-phase epitaxial layer-by-layer fashion, and chiral amino acids as the carbon sources could be confined in the porous MOF and carbonized to homogeneous and ultrasmall chirCND arrays, resulting in a chirCNDs@NU-1000 thin film (l-CNDsx@NU-1000; x = l-cysteine (cys), l-serine, l-histidine, l-glutamic acid, and l-pyroglutamic acid). The results show the pristine chirCNDs by directly carbonizing chiral amino acids hardly endow them with a CPL property. By contrast, benefiting from the arrayed confinement and coordination interaction between chirCNDs and NU-1000, the chirality transfer on the excited state of chirCNDs@NU-1000 is enabled, leading to strong CPL performance (a high luminescence dissymmetry factor glum of l-CNDscys@NU-1000 thin film reached 1.74 × 10-2). This study of chirCNDs encapsulated in fluorescent MOF thin films provides a strategy for developing uniform chiral carbon nanoarrays and offers chiral host-guest thin-film materials for optical applications.

Fabrication of BixY3-xFe5O12 thin films by thermal decomposition of metal oleates

Metal organic decomposition (MOD) for the metal oxides thin films fabrication has been used in various studies over the past 30 years. MOD is a simple and inexpensive method for manufacturing optical grade polycrystalline thin films, including bismuth-substituted yttrium iron garnet. We present a variant of the previously described MOD method and discuss a chemical route for fabricating thin porous oxide films with a thicknesses of more than 100 nm, which non-epitaxially crystallized on fused silica substrates into BixY3-xFe5O12 layers (x = 0.5 and 1.5). Studies of the structural and magneto-optical properties of the layers show that the garnet layers crystallized under optimal conditions are characterized by high values of the specific Faraday rotation. Importantly, crystallization can be done locally by laser irradiation, and wet etching makes possible to remove non-crystallized areas, thus extending microfabrication capabilities.

PdRh-functionalized porous In2O3 films to boost NO2 detection at room temperature and mechanistic insights by DFT

To achieve high and fast response at room temperature (RT) has long been a challenge for metal oxide semiconductors (MOS) sensors. Compared to single-metal functionalization, dual-metal modification of MOS offers a viable avenue to high-performance RT sensors. Herein, we employ atomic layer deposition (ALD) technology to deposit Pd and Rh metals on porous In2O3 thin films and investigate their sensitization effects on the NO2 sensing properties. Notably, the Pd/Rh catalysts enable the In2O3 sensor to have a remarkable 13-fold response (31.2) to 10 ppm NO2 at RT compared to that (2.4) of pure In2O3, which is superior to those reported In2O3-based sensors. Meanwhile, the In2O3/Pd/Rh sensor has a very fast response speed of 6.6 s and a low detection limit of 62 ppb. Density functional theory (DFT) calculations reveal that the Pd/Rh catalysts dramatically improve the interaction and charge transfer between NO2 and In2O3. This work will further promote the exploration of RT sensor sensitization strategy.

Porphyrin-Based MOF Thin Film on Transparent Conducting Oxide: Investigation of Growth, Porosity and Photoelectrochemical Properties.

Synthesizing metal-organic frameworks (MOFs) composites with a controlled morphology is an important requirement to access materials of desired patterning and composition. Since the last decade, MOF growth from sacrificial metal oxide layer is increasingly developed as it represents an efficient pathway to functionalize a large number of substrates. In this study, porphyrin-based Al-PMOF thin films were grown on conductive transparent oxide substrates from sacrificial layers of ALD-deposited alumina oxide. The control of the solvent composition and the number of atomic layer deposition (ALD) cycles allow us to tune the crystallinity, morphology and thickness of the produced thin films. Photophysical studies evidence that Al-PMOF thin films present light absorption and emission properties governed by the porphyrinic linker, without any quenching upon increasing the film thickness. Al-PMOF thin films obtained through this methodology present a remarkably high optical quality both in terms of transparency and coverage. The porosity of the samples is demonstrated by ellipsometry and used for Zn(II) insertion inside the MOF thin film. The multifunctional transparent, porous and luminescent thin film grown on fluorine-doped tin oxide (FTO) is used as an electrode capable of photoinduced charge separation upon simulated sunlight irradiation.

Thin Film Fabrication and Characterization of LSM-YSZ/YSZ Half-Cell for Solid Oxide Electrolysis Cell via Screen-Printing

Green or renewable energy is important to attain a sustainable low carbon economy and the use of hydrogen has been an alternative solution for the unreliable continuous supply of these energy sources. Solid oxide electrochemical cells is a promising technology for hydrogen generation in which the LSM-YSZ is one of the important component materials for this application. In this study, LSM-YSZ will be deposited on the YSZ substrate via screen-printing. Screen-printing is an easy and simple set-up that offers film quality control. Specifically, the study aimed to deposit a porous LSM-YSZ thin film, as well as to determine the effect of the particle size of the starting powders on the quality of the deposited film. Two synthesis methods for the powders were utilized before screen printing, the solid-state reaction (SSR) ball milling, and the glycine nitrate process (GNP). Results showed that a porous and even thin film with uniform distribution of LSM and YSZ was obtained after the screen printing. Moreover, the GNP powders produced a smaller particle size, a more porous morphology, and a thinner film when screen-printed compared to the SSR ball milled.