Ellipsometry porosimetry (EP): thin film porosimetry by coupling an adsorption setting with an optical measurement, highlights on additional adsorption results

Ellipsometry porosimetry (EP) is an emerging technique that is well adapted to porous thin films analysis; it is non contact and non destructive. EP tools developed at SOPRA, allows us to obtain adsorption isotherms with many different adsorptives at an ambient temperature. EP leads to the same results as classical adsorption experiments (e.g. porosity, pore size distribution …), but it also has some particular features leading to new information. For instance, our optical setup (Spectroscopic Ellipsometry) allows us to determine the variation of the thickness of the samples during the adsorption experiment. It is also very sensitive to interfaces; it is thus possible to detect a porosity gradient or to study a bi-layer sample and plot the two corresponding adsorption isotherms at the same time (Bourgeois et al. 2004). For porous thin films with a non porous barrier layer deposited on top, it is also possible to study the lateral diffusion phenomenon in the film (see figure below). In this paper, we will demonstrate a part of the different features of EP for adsorption on porous thin films.     

Nanometer-thick conformal pore sealing of self-assembled mesoporous silica by plasma-assisted atomic layer deposition

On a porous substrate, regular atomic layer deposition (ALD) not only takes place on top of the substrate but also penetrates into the internal porosity. Here we report a plasma-assisted process in which the ALD precursors are chosen to be nonreactive unless triggered by plasma, so that ALD can be spatially defined by the supply of plasma irradiation. Since plasma cannot penetrate within the internal porosity, ALD has been successfully confined to the immediate surface. This not only gives a possible solution for sealing of porous low dielectric constant films with a conformal layer of nm-scale thickness but also enables us to progressively reduce the pore size of mesoporous materials in a sub-A/cycle fashion for membrane formation.     

Atomic layer deposition of metal-oxide thin films on cellulose fibers

Abstract

Atomic layer deposition (ALD) is a vapor-phase technique capable of producing inorganic thin films with precise control over the thickness of the film. The ALD method offers high precision in the design of advanced 3D nanostructures. In this article, silica and alumina thin films have been grown over fibers of cellulose by the ALD process. The morphology and the chemical composition of the fabricated thin films are characterized, as well as their thermal durability through elevated temperatures. Moreover, XPS is used to confirm the phases of the alumina nanofilms and to further understand the deposition process on the cellulose microfibers.     

Low Dielectric Constant Organosilicate Films Prepared by Sol-Gel and Templating Methods

Low dielectric constant organosilicate films with controllable microstructure have been successfully synthesized by multiple-step sol-gel process and templating method, which are the two basic methods to establish porous network in the films. Ultra-low dielectric constant (k) of around 2.0 can be achieved for both films. The microstructure such as porosity, pore interconnection and pore size of the two types of the films have been studied and compared. It has been found that the sol-gel films have a higher level of porosity comparing to the templating films to obtain the same k value. The sol-gel film has a majority of closed pores with pore size around 5 nm. The templating film has a closed pore structure with pore size around 10 nm. Preliminary results present a very positive prospective for intermetal dielectric applications.     

Tailoring nanoporous materials by atomic layer deposition

Atomic layer deposition (ALD) is a cyclic process which relies on sequential self-terminating reactions between gas phase precursor molecules and a solid surface. The self-limiting nature of the chemical reactions ensures precise film thickness control and excellent step coverage, even on 3D structures with large aspect ratios. At present, ALD is mainly used in the microelectronics industry, e.g. for growing gate oxides. The excellent conformality that can be achieved with ALD also renders it a promising candidate for coating porous structures, e.g. for functionalization of large surface area substrates for catalysis, fuel cells, batteries, supercapacitors, filtration devices, sensors, membranes etc. This tutorial review focuses on the application of ALD for catalyst design. Examples are discussed where ALD of TiO(2) is used for tailoring the interior surface of nanoporous films with pore sizes of 4-6 nm, resulting in photocatalytic activity. In still narrower pores, the ability to deposit chemical elements can be exploited to generate catalytic sites. In zeolites, ALD of aluminium species enables the generation of acid catalytic activity.     

Spectroscopic ellipsometry investigations of porous SiO2 films prepared by glancing angle deposition

We present the optical and the structural properties of porous SiO₂ films fabricated by using a glancing angle deposition technique. The influence of the glancing angle on the pseudorefractive index of porous SiO₂ films was studied by spectroscopic ellipsometry in the UV–visible region. The relationships among the pseudorefractive index, the porosity, and the glancing angle are determined. The results show that the pseudorefractive index decreases and the porosity increases with the increase of glancing angle. The minimum pseudorefractive index is found to be 1.11 at 532 nm for the porous SiO₂ film deposited at a glancing angle of 87°. The structural and surface morphology of these samples was also investigated by using a scanning electron microscope. The results indicate that the as‐deposited SiO₂ thin films are porous with a tilted‐columnar structure and low pseudorefractive index. Copyright © 2013 John Wiley & Sons, Ltd.     

Surface morphology and biological activity of protein thin films produced by electrospray deposition

Protein thin films were prepared by the electrospray deposition (ESD) method from aqueous solutions of alpha-lactalbumin (alpha-LA) at different concentrations, and their surface morphologies and biological activities were characterized. The surface morphologies of the deposited films were observed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM and AFM images showed that the film surfaces had a fine porous structure, in which the pore diameters ranged from 40 to 600 nm. The biological activities of the cross-linked protein films were tested by the mechanochemical method. The response to calcium ion (Ca(2+)) demonstrated that the biological activity of the films was preserved. These results indicate that the ESD method is potentially useful for the fabrication of active protein thin films. The freestanding protein thin films prepared by ESD and postdeposition cross-linking provide novel options for protein-based biomaterials.     

Fabrication of Ordered TiO2 Porous Thin Films by Sol-Dipping PS Template Method

Monolayer polystyrene spheres (∼400 nm) array templates were assembled orderly on clean glass substrates by dip-drawing method from emulsion of PS and porous TiO₂ thin films were prepared by using sol-dipping template method to fill TiO₂ sol into the interstices among the close-packed PS templates and then annealing to remove the PS templates. The effects of TiO₂ precursor sol concentration and dipping time in sol on the porous structure of the thin films were studied. The results showed pore size of the ordered TiO₂ porous thin film depended mainly on PS size and partly on TiO₂ sol concentration. The shrinkage of pore diameter was about 10% for 0.2 M and 20% for 0.4 M TiO₂ sol concentrations. X-ray diffraction (XRD) spectra indicated the porous thin film was anatase structure. The transmittance spectrum showed that optical transmittance of the porous thin film kept above 70% beyond the wavelength of 430 nm. Optical band-gap of the porous TiO₂ thin film (fired at 550^∘;C) was 3.12 eV.     

Influence of surface morphology on D2 desorption kinetics from amorphous solid water

The influence of surface morphology/porosity on the desorption kinetics of weakly bound species was investigated by depositing D2 on amorphous solid water (ASW) films grown by low temperature vapor deposition under various conditions and with differing thermal histories. A broad distribution of binding energies of the D2 monolayer on nonporous and porous ASW was measured experimentally and correlated by theoretical calculations to differences in the degree of coordination of the adsorbed H2 (D2) to H2O molecules in the ASW depending on the nature of the adsorption site, i.e., surface valleys vs surface peaks in a nanoscale rough film surface. For porous films, the effect of porosity on the desorption kinetics was observed to be a reduction in the desorption rate with film thickness and a change in peak shape. This can be partly explained by fast diffusion into the ASW pore structure via a simple one-dimensional diffusion model and by a change in binding energy statistics with increasing total effective surface area. Furthermore, the D2 desorption kinetics on thermally annealed ASW films were investigated. The main effect was seen to be a reduction in porosity and in the number of highly coordinated binding sites with anneal temperature due to ASW restructuring and pore collapse. These results contribute to the understanding of desorption from porous materials and to the development of correct models for desorption from and catalytic processes on dust grain surfaces in the interstellar medium.     

高长径比管式反应器内壁SiO2与TiO2钝化层的原子层沉积制备及抗积碳性能

采用原子层沉积技术(ALD)在不锈钢微通道管式反应器内壁沉积二氧化硅(SiO₂)和二氧化钛(TiO₂)薄膜, 以抑制碳氢燃料热裂解过程中由于金属催化作用导致的结焦. 使用石英晶体微天平(QCM)测得SiO₂和TiO₂薄膜的生长速率分别为0.15 nm/周期和0.11 nm/周期, 因此可以通过改变沉积周期数精确控制钝化层的厚度. 在结焦实验中, 当钝化膜层较薄时, 其抗积碳钝化作用较弱; 随着钝化薄膜厚度的增加, 其钝化作用逐渐增强, 微通道反应器的运行寿命显著延长. 实验表明, TiO₂薄膜的抗积碳钝化性能普遍优于SiO₂薄膜. 沉积周期数为1000的TiO₂膜层具有最佳的抗积碳钝化效果, 能够使反应器的运行时间延长4~5倍.     

Optical activity of chiral thin film and liquid crystal hybrids

Using the glancing angle deposition (GLAD) technique, we have fabricated porous, chiral thin films with optically anisotropic helical microstructures that exhibit optical phenomena such as wavelength specific rotation of linearly polarized light. Initial research has shown that the porosity of the films allows for the addition of nematic liquid crystals (NLCs) to the films for promising applications in dynamically switchable devices, while simultaneously enhancing the optical properties of the film. This study describes the fundamental optical behaviour of LC-filled chiral thin films in relation to material, porosity, structure and thickness. It was found that for SiO₂ films, the addition of NLCs increased the effective rotatory power by two-fold when compared with results from the film without added LCs. The rotatory power of Al₂O₃ and MgF₂ films, while being similarly increased by the addition of LCs, exhibited a reversal in sign, or direction of rotation, for the visible wavelength spectrum investigated. The effects of film porosity and structure were studied by varying the angle of incidence from 83° to 86°; it was found that the greater porosity of the films deposited at larger angles allowed for more filling by the LCs and thus a larger increase in rotatory power. Finally, the addition of LCs was observed to shift the wavelength of peak rotation towards smaller values.     

Composite thin films of poly(phenylene oxide)/poly(styrene) and PPO/silver via vapor phase deposition

We report fabrication of thin (100～300 nm) poly(phenylene oxide) (PPO) films and their composites with poly (styrene) (PS) and silver (Ag) nanoparticles using a one‐step electron beam‐assisted vapor phase co‐deposition technique. Surface morphology and the structure of the deposited polymer thin film composites were characterized by FTIR, Raman, X‐ray spectroscopy, and contact angle measurements. As‐deposited PPO films and PPO/Ag composites were of porous nature and contrary to solvent casting techniques were free from nodular growth. In the case of PPO/PS thin film polymer composites, however, film morphology displayed nodular growth of PPO with nodule diameters of about ～200 nm and height of approximately 50 nm. Unique morphological changes on the porous PPO thin film surface were noticed at different Ag filling ratios. Further, the capacitance of PPO/Ag composites (<16 wt%) were measured under radio‐frequency conditions and they were functional up to 100 MHz with an average capacitance density of about 2 nF/cm². The fabricated PPO‐based composite systems are discussed for their potential applications including embedded capacitor technology. Copyright © 2008 John Wiley & Sons, Ltd.     

Ordered porous ZnO thin films formed by dip-coating method using PS templates

Monolayer polystyrene spheres (∼400 nm) array templates were assembled orderly on clean glass substrates by dip-coating method from emulsion of polystyrene (PS). Porous ZnO thin films were also prepared by dip-coating method to fill the interstices among the close-packed PS templates with ZnO and annealing to remove the PS templates. Results showed that ZnO sol concentration and dipping time of PS templates in sol had great influences on the morphology of ordered porous ZnO thin films. There was a shrinkage ratio of about 30% from pore to PS. SEM observation showed that the PS array templates had face-centered cubic close-packing. X-ray diffraction (XRD) spectra showed the porous ZnO thin film was wurtzite structure. The optical transmittance decreased with decreasing wavelength of lights, but was kept above 80% beyond the wavelength of 550 nm. Optical band-gap of the porous ZnO thin film annealed at 500°C was 3.22 eV.     

Optical activity of chiral thin film and liquid crystal hybrids

Using the glancing angle deposition (GLAD) technique, we have fabricated porous, chiral thin films with optically anisotropic helical microstructures that exhibit optical phenomena such as wavelength specific rotation of linearly polarized light. Initial research has shown that the porosity of the films allows for the addition of nematic liquid crystals (NLCs) to the films for promising applications in dynamically switchable devices, while simultaneously enhancing the optical properties of the film. This study describes the fundamental optical behaviour of LC-filled chiral thin films in relation to material, porosity, structure and thickness. It was found that for SiO₂ films, the addition of NLCs increased the effective rotatory power by two-fold when compared with results from the film without added LCs. The rotatory power of Al₂O₃ and MgF₂ films, while being similarly increased by the addition of LCs, exhibited a reversal in sign, or direction of rotation, for the visible wavelength spectrum investigated. The effects of film porosity and structure were studied by varying the angle of incidence from 83° to 86°; it was found that the greater porosity of the films deposited at larger angles allowed for more filling by the LCs and thus a larger increase in rotatory power. Finally, the addition of LCs was observed to shift the wavelength of peak rotation towards smaller values.     

Template-assisted electrostatic spray deposition as a new route to mesoporous, macroporous, and hierarchically porous oxide films

A novel film coating technique, template-assisted electrostatic spray deposition (TAESD), was developed for the synthesis of porous metal oxide films and tested on TiO(2). Organic templates are codeposited with the titania precursor by electrostatic spray deposition and then removed during calcination. Resultant films are highly porous with pores casted by uniformly sized templates, which introduced a new level of control over the pore morphology for the ESD method. Employing the amphiphilic block copolymer Pluronic P123, PMMA latex spheres, or a combination of the two, mesoporous, macroporous, and hierarchically porous TiO(2) films are obtained. Decoupled from other coating parameters, film thickness can be controlled by deposition time or depositing multiple layers while maintaining the coating's structure and integrity.     

Rapid vapor-phase fabrication of organic-inorganic hybrid superlattices with monolayer precision

We report a new layer-by-layer growth method of self-assembled organic multilayer thin films based on gas-phase reactions. In the present molecular layer deposition (MLD) process, alkylsiloxane self-assembled multilayers (SAMs) were grown under vacuum by repeated sequential adsorptions of C=C-terminated alkylsilane and titanium hydroxide. The MLD method is a self- limiting layer-by-layer growth process, and is perfectly compatible with the atomic layer deposition (ALD) method. The SAMs films prepared exhibited good thermal and mechanical stability, and various unique electrical properties. The MLD method, combined with ALD, was applied to the preparation of organic-inorganic hybrid nanolaminate films in the ALD chamber. The organic-inorganic hybrid superlattices were then used as active mediums for two-terminal electrical bistable devices. The advantages of the MLD method with ALD include accurate control of film thickness, large-scale uniformity, highly conformal layering, sharp interfaces, and a vast library of possible materials. The MLD method with ALD is an ideal fabrication technique for various organic-inorganic hybrid superlattices.     

Atomic layer deposition: An efficient tool for corrosion protection

Atomic layer deposition (ALD) is now a widely implemented thin film growing method. It is currently used in industrial fabrication processes of microelectronics and luminescent display technologies. Since compact and conformal films can be grown with perfect control of the thickness, ALD is envisioned in numerous other applications fields such as energy, sensing, biomaterials, and photonics. Although few reports can be found on its application to corrosion protection, it has been shown that the qualities of ALD can be highly beneficial to this field. After a brief review of the principle of ALD and the effect of the main parameters on the properties of the films, this report attempts to show the interest of this technique to mitigate corrosion. Various examples of successful uses of ALD to protect metallic and non-metallic surfaces in different fields are reviewed.     

Atomic layer deposition chemistry: recent developments and future challenges

New materials, namely high-k (high-permittivity) dielectrics to replace SiO(2), Cu to replace Al, and barrier materials for Cu, are revolutionizing modern integrated circuits. These materials must be deposited as very thin films on structured surfaces. The self-limiting growth mechanism characteristic to atomic layer deposition (ALD) facilitates the control of film thickness at the atomic level and allows deposition on large and complex surfaces. These features make ALD a very promising technique for future integrated circuits. Recent ALD research has mainly focused on materials required in microelectronics. Chemistry, in particular the selection of suitable precursor combinations, is the key issue in ALD; many interesting results have been obtained by smart chemistry. ALD is also likely to find applications in other areas, such as magnetic recording heads, optics, demanding protective coatings, and micro-electromechanical systems, provided that cost-effective processes can be found for the materials required.     

Blocking the lateral film growth at the nanoscale in area-selective atomic layer deposition

Area-selective atomic layer deposition (ALD) allows the growth of highly uniform thin inorganic films on certain parts of the substrate while preventing the film growth on other parts. Although the selective ALD growth is working well at the micron and submicron scale, it has failed at the nanoscale, especially near the interface where there is growth on one side and no-growth on the other side. The reason is that methods so far solely rely on the chemical modification of the substrate, while neglecting the occurrence of lateral ALD growth at the nanoscale. Here we present a proof-of-concept for blocking the lateral ALD growth also at the nanoscale by combining the chemical surface modification with topographical features. We demonstrate that area-selective ALD of ZnO occurs by applying the diethylzinc/water ALD process on cicada wings that contain a dense array of nanoscopic pillars. The sizes of the features in the inorganic film are down to 25 nm which is, to the best of our knowledge, the smallest obtained by area-selective ALD. Importantly, our concept allows the synthesis of such small features even though the film is multiple times thicker.     

Preliminary studies in the electrodeposition of PbSe/PbTe superlattice thin films via electrochemical atomic layer deposition (ALD)

This paper concerns the electrochemical growth of compound semiconductor thin film superlattice structures using electrochemical atomic layer deposition (ALD). Electrochemical ALD is the electrochemical analogue of atomic layer epitaxy (ALE) and ALD, methods based on nanofilm formation an atomic layer at a time, using surface-limited reactions. Underpotential deposition (UPD) is a type of electrochemical surfaced-limited reaction used in the present studies for the formation of PbSe/PbTe superlattices via electrochemical ALD. PbSe/PbTe thin-film superlattices with modulation wavelengths (periods) of 4.2 and 7.0 nm are reported here. These films were characterized using electron probe microanalysis, X- ray diffraction, atomic force microscopy (AFM), and infrared reflection absorption measurements. The 4.2 nm period superlattice was grown after deposition of 10 PbSe cycles, as a prelayer, resulting in an overall composition of PbSe0.52Te0.48. The 7.0 nm period superlattice was grown after deposition of 100 PbTe cycle prelayer, resulting for an overall composition of PbSe0.44Te0.56. The primary Bragg diffraction peak position, 2theta, for the 4.2 superlattice was consistent with the average (111) angles for PbSe and PbTe. First-order satellite peaks, as well as a second, were observed, indicating a high-quality superlattice film. For the 7.0 nm superlattice, Bragg peaks for both the (200) and (111) planes of the PbSe/PbTe superlattice were observed, with satellite peaks shifted 1 degrees closer to the (111), consistent with the larger period of the superlattice. AFM suggested conformal superlattice growth on the Au on glass substrate. Band gaps for the 4.2 and 7.0 nm period superlattices were measured as 0.48 and 0.38 eV, respectively.