Porosity Of Thin Films Research Articles

Tuning Thin Film Thickness and Porosity with Layer‐by‐Layer Submicronic Particles Assembly

AbstractPorous NiO films were prepared using Layer‐by‐Layer assembly of submicronic β‐Ni(OH)2 nanoplatelets and size‐controlled poly(methyl methacrylate) (PMMA) particles. β‐Ni(OH)2/PMMA thin films were elaborated using different PMMA particle sizes and concentrations. The elaborated films were then heated at a temperature of 325 °C in air for one hour to form porous NiO films by calcining β‐Ni(OH)2 nanoplatelets. PMMA polymeric particles were also calcined during this process, leading to film porosity. Thermal treatment experiments clearly showed a relationship between PMMA particle size and film properties i. e., porosity and thickness. NiO films exhibited huge pores, around 1 μm, and matter loss after calcination when PMMA particles with a diameter of 190±43 nm were employed. Partial collapsing of the films was also denoted. Experiments were then carried out by decreasing PMMA particle size (100±19 nm) leading to cohesive NiO films. Finally, the specific surface area (SSA) of the NiO films prepared with PMMA particles of 100 nm was determined by adsorption‐desorption of nitrogen gas using the BET (Brunauer–Emmet–Teller) method. This showed an increase in SSA with PMMA particle concentration.

In-plane structural and electronic anisotropy of nanoporous Pt films formed by oblique angle deposition

Nanoporous Pt films fabricated by oblique angle deposition hold potential as electrocatalysts in various energy-related fields owing to their high surface area, structural stability, and adequate conductivity. In this study, we investigated the morphology, porosity, and electrical conductivity of nanoporous Pt thin films and systematically studied their interrelationships. Specifically, we revealed an in-plane anisotropy in the electrical conductivity that correlates with the surface morphology of the film. This anisotropy was evident in the resistance measurements along the in-plane lateral and vertical directions, which aligned well with our simple model. The results emphasize the significance of film morphology in determining the film’s electrical properties. This study contributes to the understanding of the physical properties of Pt films fabricated via oblique angle deposition and offers valuable insights for designing nanoporous films for various applications.

Conformal TiO2 Aerogel-Like Films by Plasma Deposition: from Omniphobic Antireflective Coatings to Perovskite Solar Cell Photoelectrodes.

The ability to control the porosity of thin oxide films is a key factor determining their properties. Despite the abundance of dry processes for synthesizing oxide porous layers, a high porosity range is typically achieved by spin-coating-based wet chemical methods. Besides, special techniques such as supercritical drying are required to replace the pore liquid with air while maintaining the porous network. In this study, we propose a new method for the fabrication of ultraporous titanium dioxide thin films at room or mild temperatures (T ≤ 120 °C) by a sequential process involving plasma deposition and etching. These films are conformal to the substrate topography even for high-aspect-ratio substrates and show percolated porosity values above 85% that are comparable to those of advanced aerogels. The films deposited at room temperature are amorphous. However, they become partly crystalline at slightly higher temperatures, presenting a distribution of anatase clusters embedded in the sponge-like open porous structure. Surprisingly, the porous structure remains after annealing the films at 450 °C in air, which increases the fraction of embedded anatase nanocrystals. The films are antireflective, omniphobic, and photoactive, becoming superhydrophilic when subjected to ultraviolet light irradiation. The supported, percolated, and nanoporous structure can be used as an electron-conducting electrode in perovskite solar cells. The properties of the cells depend on the aerogel-like film thickness, which reaches efficiencies close to those of commercial mesoporous anatase electrodes. This generic solvent-free synthesis is scalable and applicable to ultrahigh porous conformal oxides of different compositions, with potential applications in photonics, optoelectronics, energy storage, and controlled wetting.

Effect of Ag incorporation on the microstructure and properties of ZnS thin films

In this study, ZnS thin films doped with different content of Ag were deposited on quartz glass substrates by RF magnetron sputtering technique and were annealed at 600 °C in sulfur vapor. The crystal structure, grain size, surface morphology, composition, optical properties and defects of the doped ZnS thin films were analyzed by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), UV–Vis spectroscopy and positron annihilation doppler broadening spectroscopy (DBS). The results showed that all films exhibited the cubic sphalerite structure with a preferred orientation along the (111) crystal plane, and the composition of the doped films were more consistent with the stoichiometric ratio compared to the undoped film. The grain size gradually increased as the Ag content grew, indicating the improvement of crystallinity of the ZnS thin films. Besides, the bandgap values of these ZnS films with good optical transparency in the visible region were decreased from 3.87eV to 3.53eV as the Ag content grew. DBS results also showed that the defect concentration of ZnS films gradually decreased from the surface to the inner layer, and the film defects decreased with the increase of Ag content. However, the types of defects changed when the Ag content increased. Furthermore, the results showed Ag also has an effect on the porosity of ZnS thin films.

Challenges in porosity characterization of thin films: Cross-evaluation of different techniques

The review article provides an overview of the most important and popular techniques for evaluating the porosity of thin films developed for various applications. These methods include ellipsometric porosimetry (EP), positron annihilation (lifetime) spectroscopy (PAS/PALS), and grazing incidence small-angle x-ray scattering (GISAXS). Special attention is given to the challenges associated with interpreting the measured data and the inherent limitations of each method. It is demonstrated that EP, GISAXS, and PALS are all informative for studying the pore structure in thin films, with each method offering unique insights. GISAXS, in particular, allows for the evaluation of three-dimensional mesostructures, including pore arrangement, pore spacing, and structural order. On the other hand, PALS has a unique advantage in its capability to analyze extremely small isolated pores (free volume). The advantage of EP lies in its simplicity and the possibility to analyze multiple properties from the same set of measurements. The cross-evaluation of different methods offers important insights into the complex pore structure of materials, highlighting the significance of appropriate modeling and interpretation of data.

Enhanced electrochemical performance of WO3 thin films prepared from polyvinyl alcohol-modified nanoparticle ink

Tungsten oxide (WO3) is a potential material in catalysis, sensors, electrochromic devices, etc. In this research work, porous WO3 thin films are prepared on FTO-glass employing spin coating of WO3 nanoparticle's ink modified with polyvinyl alcohol (PVA) in the concentration range of 0 to 25.0 wt%. This study explored the change in structure and morphology of WO3 thin films with varying PVA concentration in ink composition and their effect on the ability to intercalate and de-intercalate Li+ ions as a function of applied bias. The wettability of ink on FTO-glass improved by raising its PVA content. All WO3 thin films exhibited a monoclinic structure. Scanning electron microscopy images indicated that the porosity of WO3 thin films gradually increased with increasing PVA concentration up to 25 wt% but the thin film became highly nonuniform at a very high concentration of additive. WO3 thin film prepared with ink having 10 wt% of PVA had optimum porosity and uniformity, therefore, yielded better electrochemical performance compared with all other samples. The addition of an optimum amount of PVA to ink is found to be an effective approach to modulating ink's characteristics and inducing porosity in thin films which eventually improves their redox reaction characteristics.

Vapor Deposited Zeolitic Imidazolate Framework-8 Derived from Porous ZnO Thin Films

In recent years, the vapor deposition of zeolitic imidazolate framework-8 (ZIF-8) has gained high attraction due to its good scalability, conformality, and thickness control. The present study provides new fundamental insights regarding the vapor deposition of ZIF-8 from zinc oxide (ZnO). During synthesis, ZnO thin films with different percentages of open porosity (14.5%–24%) were subjected to a 2-methylimidazole vapor for different conversion times (20 min–24 h). For the first time, the impact of the porosity of ZnO thin films onto the converted ZIF-8 is investigated. Grazing incidence X-ray diffraction reveals randomly oriented crystallites of ZIF-8, which already appear after 20 min of conversion. The thickness, roughness, and average particle height of the ZIF-8 layers increase with the conversion time, reaching values up to (172 ± 20) nm, (29 ± 3) nm, and (113 ± 8) nm, respectively, for ZIF-8 obtained from ZnO with 14.5% open porosity. At long conversion times (i.e., 24 h), the results hint at greater precursor porosities resulting in lower thicknesses of ZIF-8, as the thickness, roughness, and average particle height for ZIF-8 obtained from 24%-porous ZnO show values of (132 ± 20) nm, (25 ± 3) nm and (80 ± 8) nm, respectively. Additionally, the potential of the ZIF-8 layers as a photocatalyst for the degradation of the organic dye methylene blue was studied. The ZIF-8 enhances the degradation by approximately 8% when compared to degradation without a photocatalyst.

Chemical Structure, Optical and Dielectric Properties of PECVD SiCN Films Obtained from Novel Precursor

A phenyl derivative of hexamethyldisilazane—bis(trimethylsilyl)phenylamine—was first examined as a single-source precursor for SiCN film preparation by plasma enhanced chemical vapor deposition. The use of mild plasma (20 W) conditions allowed the preparation of highly hydrogenated polymeric-like films. The synthesis was carried out under an inert He atmosphere or under that of NH3 with the deposition temperature range from 100 to 400 °C. The chemical bonding structure and elemental composition were characterized by Fourier-transform infrared spectroscopy, energy-dispersive X-ray analysis and X-ray photoelectron spectroscopy. The surface morphology was investigated by scanning electron microscopy. Ellipsometric porosimetry, a unique high-precision technique to investigate the porosity of thin films, was applied to examine the porosity of SiCN samples. The films were found to possess a morphologically homogenous dense defect-free structure with a porosity of 2–3 vol.%. SiCN films were studied in terms of their optical and dielectric properties. Depending on the deposition conditions the refractive index ranged from 1.53 to 1.78. The optical bandgap obtained using UV-Vis spectroscopy data varied from 2.7 eV for highly hydrogenated polymeric-like film to 4.7 eV for cross-linked nitrogen-rich film. The dielectric constant was found to decrease from 3.51 to 2.99 with the rise of hydrocarbon groups’ content. The results obtained in this study were compared to the literature data to understand the influence of precursor design to the optical and electrical properties of the films.

(Invited) Effect of Anode Thickness on Thin-Film Solid Oxide Fuel Cells Deposited on Nanoporous Substrates

In this study, low-temperature solid oxide fuel cells having different thin-film anode thicknesses were fabricated, and their characteristics were studied. Since the low-temperature solid oxide fuel cells (LT-SOFCs) have the advantages of a low-cost fuel cell system and a wide selection of components’ materials, many researchers have made efforts to lower the operating temperature range. Naturally, thin-film electrolyte technology has been improved to reduce the exponentially increasing ohmic resistance of oxygen conduction when the working temperature is lowered. Various substrates or even no substrate were used to fabricate thin-film solid oxide fuel cells (TF-SOFCs). However, free-standing TF-SOFCs have poor mechanical strength, and the active area was limited to tens of square micrometers. A nanoporous substrate such as Anodic aluminum oxide (AAO) has been a great platform on which TF-SOFCs were stably deposited and operated. AAO has well-arranged and straight pores from its bottom to the top but no electrical conductivity. Due to the resistance characteristics of TF-SOFCs based on AAO, the anode’s thickness and the nanostructure significantly influence the overall performance of the fuel cell. According to previous studies, the performance tends to increase as the thickness increases with well-maintained porosity in the thin-film anode. To confirm this tendency and further improve cell performance, AAO substrates with 250nm-size pores and relatively rough surfaces were used. On the AAO substrate, nickel gadolinium-doped-ceria (Ni-GDC) mixed ionic and electronic conductor (MIEC) was introduced as the anode material, which increased hydrogen oxidation reaction (HOR) reactivity at the anode. In addition, the glancing angle deposition (GLAD) of the sputtering method was applied to maintain porosity better than the conventional method. With this method, thickening the thin-film anode improved the current collecting performance while the pores were well connected to the anode-electrolyte interface. Following that, the yttria-stabilized zirconia (YSZ) electrolyte was densely deposited, and the porous deposited platinum cathode completed the entire cell. The fabricated cells were tested at 500C and electrochemically analyzed by i-V-P curves and the EIS method. Also, the anode’s nanostructures were revealed by various analyzing methods such as surface FESEM, FIB cross-sectional SEM, TEM-EDS, and 4-probe conductivity measurement. Each cell’s stable structure and the uniformity of some fuel cell components such as the electrolyte and the cathode were confirmed. We found that the electrochemical performance increases as the anode thickness increases while its porosity is well-maintained. About 550mW/cm2 was measured as the highest maximum power density, which is more than a 20% increased result than the other fabricated cells. Those investigations also enabled the anode nanostructure to be identified, such as the anode thin-film porosity, conductivity, and three-dimensional morphology. The correlation between the anode nanostructure and the fuel cell performance was discussed, and the optimized cell structure was found. This study enlarged the understanding of nanoporous substrate-based TF-SOFC manufacturing and their expandability.

Tuning the Porosity of Piezoelectric Zinc Oxide Thin Films Obtained from Molecular Layer-Deposited "Zincones".

Porous zinc oxide (ZnO) thin films were synthesized via the calcination of molecular layer-deposited (MLD) “zincone” layers. The effect of the MLD process temperature (110 °C, 125 °C) and of the calcination temperature (340 °C, 400 °C, 500 °C) on the chemical, morphological, and crystallographic properties of the resulting ZnO was thoroughly investigated. Spectroscopic ellipsometry reveals that the thickness of the calcinated layers depends on the MLD temperature, resulting in 38–43% and 52–56% of remaining thickness for the 110 °C and 125 °C samples, respectively. Ellipsometric porosimetry shows that the open porosity of the ZnO thin films depends on the calcination temperature as well as on the MLD process temperature. The maximum open porosity of ZnO derived from zincone deposited at 110 °C ranges from 14.5% to 24%, rising with increasing calcination temperature. Compared with the 110 °C samples, the ZnO obtained from 125 °C zincone yields a higher porosity for low calcination temperatures, namely 18% for calcination at 340 °C; and up to 24% for calcination at 500 °C. Additionally, the porous ZnO thin films were subjected to piezoelectric measurements. The piezoelectric coefficient, d33, was determined to be 2.8 pC/N, demonstrating the potential of the porous ZnO as an, e.g., piezoelectric sensor or energy harvester.

Mass and density determination of porous nanoparticle films using a quartz crystal microbalance

A method is presented to directly measure the mass output of an impaction printer coupled with a spark ablation generator. It is based on a quartz crystal microbalance and shown to be reliable in quantifying mass deposition rate. Here, the method is demonstrated with an Au nanoparticle aerosol synthesized under several spark ablation and deposition settings. Changes in the deposition rate in response to changed synthesis conditions follow the spark ablation models on generation rate made in previous studies, validating this novel measurement method. In combination with the volume of a deposit, a good estimate of the film porosity can be made. The Au nanoparticle films synthesized here have a low porosity of 0.18 due to extensive restructuring and compaction on impact with the substrate. The porosity is found to be insensitive to deposition settings and is constant throughout the film. The simplicity and low cost of a quartz crystal microbalance setup make this an accessible method to determine porosity in porous thin films.

Controlled Porosity in Ferroelectric BaTiO3 Photoanodes.

The use of ferroelectric polarization to promote electron–hole separation has emerged as a promising strategy to improve photocatalytic activity. Although ferroelectric thin films with planar geometry have been largely studied, nanostructured and porous ferroelectric thin films have not been commonly used in photo-electrocatalysis. The inclusion of porosity in ferroelectric thin films would enhance the surface area and reactivity, leading to a potential improvement of the photoelectrochemical (PEC) performance. Herein, the preparation of porous barium titanate (pBTO) thin films by a soft template-assisted sol–gel method is reported, and the control of porosity using different organic/inorganic ratios is verified by the combination of scanning electron microscopy and ellipsometry techniques. Using piezoresponse force microscopy, the switching of ferroelectric domains in pBTO thin films is observed, confirming that the ferroelectric polarization is still retained in the porous structures. In addition, the presence of porosity in pBTO thin films leads to a clear improvement of the PEC response. By electrochemical poling, we also demonstrated the tuning of the PEC performance of pBTO thin films via ferroelectric polarization. Our work offers a simple and low-cost approach to control the morphology optimization of ferroelectric thin films, which could open up the development of materials with great potential for PEC applications.

Solution route processed and organically-Capped Quinary CuIn1-xGax(SySe1-y)2 (CIGSSe) inks for use in low-cost photovoltaics

Multicomponent chalcogenides are a class of derived materials that present a compelling vision that allows for the realization of benefits that are limited in their pristine counterparts. CIGSSe is one such material that constitutes the dual role of prominent chalcogens, i.e., sulfur and selenium. The benefits possessed due to both sulfur and selenium's presence influence the improvement in properties compared to their other counterparts. The dual role offered by both the constituents is noteworthy as one of them, i.e., sulfur, aids in enhancing the material's bandgap. Simultaneously, the other, i.e., selenium, is responsible for eliminating the porosity in thin-films of these chalcogenides. Comparing the properties of individual CIGSe and CIGS with quinary CIGSSe can lead to the discernment of a material with properties particularly desirable for the potential photovoltaic absorber. To the best of the author's knowledge, the quinary CIGSSe nanocrystals' colloidal synthesis employs organic ligands-TOP(tri octyl phosphine)/TOPO (tri octyl phosphine oxide) has been reported for the first time. Technologically, this solution-based synthesis's main benefit is the ease of formation of the colloidal-ink for a scalable film formation on various compatible substrates (rigid/flexible) via the printing route for photovoltaics application.

Subtractive Low-Temperature Preparation Route for Porous SiO2 Used for the Catalyst-Assisted Growth of ZnO Field Emitters.

The possibility to gradually increase the porosity of thin films facilitates a variety of applications, such as anti-reflective coatings, diffusion membranes, and the herein investigated tailored nanostructuring of a substrate for subsequent self-assembly processes. A low-temperature (<160 °C) preparation route for porous silicon oxide (porSiO2) thin films with porosities of about 60% and effective refractive indices down to 1.20 is tailored for bulk as well as free-standing membranes. Subsequently, both substrate types are successfully employed for the catalyst-assisted growth of nanowire-like zinc oxide (ZnO) field emitters by metal organic chemical vapor deposition. ZnO nanowires can be grown with a large aspect ratio and exhibit a good thermal and chemical stability, which makes them excellent candidates for field emitter arrays. We present a method that allows for the direct synthesis of nanowire-like ZnO field emitters on free-standing membranes using a porSiO2 template. Besides the application of porSiO2 for the catalyst-assisted growth of nanostructures and their use as field emission devices, the herein presented general synthesis route for the preparation of low refractive index films on other than bulk substrates—such as on free-standing, ultra-thin membranes—may pave the way for the employment of porSiO2 in micro-electro-mechanical systems.

A review on the prominence of porosity in tungsten oxide thin films for electrochromism

Transition metal oxides have gained substantial attention in the previous decade by virtue of their distinctive physical and chemical properties. Amidst all, tungsten oxide (WO3) is emerging as distinct metal oxide on account of its outstanding electrochromic performance. Furthermore, optical-switching speed has topmost importance in the actual device. The sole approach to enhance this attribute is to escalate surface area of the electrochromic coatings, which can be done by instigating porosity within bulk material. Porosity can be considered as one of the pivotal facets of functionalized electrochromic thin films. Cations can be made to get transported into the depth of the host with the presence of high porosity in a structure. This featured article is attempting to pivot the path as how porosity is upgrading the electrochromic attribute of WO3 films. Furthermore, various methods to achieve optimum porosity of WO3 films leading to best electrochromic performance are also discussed.

Control of porosity and optical properties of slanted columnar Ni thin films

Abstract In this work we deposited nickel thin films by Glancing Angle Deposition at two different angles (65° and 85°) onto glass substrates. The structure of the films, thicknesses between 50 and 200 nm, was studied by scanning electron microscopy, atomic force microscopy and X-ray diffraction, while the chemical properties were analyzed using X-ray photoelectron spectroscopy. According to the obtained results it can be seen that the deposition angle has influence on porosity, crystallinity and surface roughness. Optical properties were investigated by spectroscopic ellipsometry and electrical properties were measured by four point probe. Spectroscopic ellipsometry revealed that the refractive index and extinction coefficient varied with thickness and deposition angle, which can be correlated with changes in microstructure and porosity of Ni films. The observed variations in resistivity could be attributed to the changes in the width of the columns, as well as to the amount of oxide present in the samples.

Effects of Y Doping on Dielectric and Varistor Properties of CaCu3Ti4O12 Thin Films

Ca1−3x/2YxCu3Ti4O12 (x = 0, 0.05, 0.1, 0.15, and 0.2) (CCTO) thin films were prepared by a modified sol–gel method. The effects of Y doping on the microstructure and electrical properties of the CCTO thin films were investigated. According to x-ray diffraction analysis, the main phase of all Y-doped CCTO thin films is cubic Im $$ \bar{3} $$ . Meanwhile, some secondary phases were defected. Scanning electron microscopy analysis showed that Y doping can easily lead to a smaller grain size and larger porosity in CCTO thin films. Room-temperature dielectric analysis showed that the dielectric constant of different Y doping samples at 1 kHz was 4213, 4523, 4974, 4824, and 4777, and the dielectric losses were 0.053, 0.051, 0.062, 0.067, and 0.049, respectively. The study of Y-doped CCTO thin films showed that Y doping increased the dielectric constant of the films at low frequencies, but at the same time it increased their dielectric loss and decreased their dielectric frequency and temperature stability range. Y-doping had little effect on the nonlinearity of the films and did not significantly change their nonlinear coefficients.

Filling of Nanometric Pores with Polymer by Initiated Chemical Vapor Deposition.

The integration of porous thin films using microelectronic compatible processes sometimes requires the protection of the interior of the pores during the critical integration steps. In this paper, the polymerization of neo-pentyl methacrylate (npMA) is performed via initiated chemical vapor deposition (iCVD) on a porous organosilicate (SiOCH) and on a dense SiOCH. The characterizations by Fourier-transform infrared spectroscopy, spectroscopic ellipsometry, and time-of-flight secondary ion mass spectrometry of the different stacks show that iCVD is a powerful technique to polymerize npMA in the nanometric pores and thus totally fill them with a polymer. The study of the pore filling for very short iCVD durations shows that the polymerization in the pores is complete in less than ten seconds and is uniform in depth. Then, the poly(npMA) film growth continues on top of the filled SiOCH layer. These characteristics make iCVD a straightforward and very promising alternative to other infiltration techniques in order to fill the porosity of microporous thin films.

Graphitic Nanoporous Carbon Thin Films: Fabrication Method, Structural, Electrical and Gas Sensor Properties

New method (magnetron plasma enhanced CVD) is suggested to obtain record values of porosity of thin carbon films. Further high temperature annealing results in graphitization of the carbon film and reduction of their resistivity down to 102 Ohm×cm. Plasma treatment in forming gas leads to modification both of a surface and bulk thin nanoporous graphitic film that decreases resistivity of the film, reduces sensor sensitivity to water vapour and enhances sensor sensitivity to ammonia gas.

Optical properties of gyrotropic polygonal chiral thin films

The optical response of gyrotropic and non-gyrotropic dielectric polygonal chiral thin films were investigated using transfer matrix method. The influence of gyrotropy factor, porosity and incident angle of light on Bragg regimes, selective reflection and transmission were reported. The results revealed that the Fabry-Perot oscillations were affected by gyrotropy factor and porosity of polygonal chiral thin films.