In situ monitoring of atomic layer deposition in nanoporous thin films using ellipsometric porosimetry

Ellipsometric porosimetry (EP) is a handy technique to characterize the porosity and pore size distribution of porous thin films with pore diameters in the range from below 1 nm up to 50 nm and for the characterization of porous low-k films especially. Atomic layer deposition (ALD) can be used to functionalize porous films and membranes, e.g., for the development of filtration and sensor devices and catalytic surfaces. In this work we report on the implementation of the EP technique onto an ALD reactor. This combination allowed us to employ EP for monitoring the modification of a porous thin film through ALD without removing the sample from the deposition setup. The potential of in situ EP for providing information about the effect of ALD coating on the accessible porosity, the pore radius distribution, the thickness, and mechanical properties of a porous film is demonstrated in the ALD of TiO(2) in a mesoporous silica film.     

Controlled synthesis of self-assembled 3D nanostructures using metastable atomic layer deposition

An important feature of atomic layer deposition (ALD) is the fact that the coating that has been deposited is conformal to the substrate surface. Therefore, prepatterned substrates are usually used for the fabrication of 3D nanostructures using ALD. This article presents a new method to generate 3D silver-silica nanostructures using plasma-enhanced atomic layer deposition of silica with tri(dimethylamino)silane (TDMAS) and oxygen plasma as precursors. For this method, silver nanoparticles are used as templates, and during the deposition of silica, the repeatable process of the formation of metastable silver oxides and their decomposition is involved, leading to strong side reactions and the formation of 3D silver-silica hybrid nanostructures. This method is known as metastable atomic layer deposition (MS-ALD). Unlike the conventional ALD, the coating of MS-ALD is not conformal to the substrate surface. Rather, the 3D nanostructures are self-assembled because of side reactions. The geometry of the formed nanostructures can be easily adjusted by tuning the deposition parameters, such as dose time of both precursors and cycle numbers. In our study, we observed nanosponges with features sizes of up to 4 for less than 45 MS-ALD cycles. Nanowire-like silver-silica hybrid nanostructures are generated at higher cycle numbers with feature sizes of up to 10 μm. A similar trend could be observed for changing the dose time of both precursors of TDMAS and oxygen plasma. The height of the nanostructures increases with dose time of both precursors. In contrast to this trend, the surface coverage declines when the investigated parameters (number of cycles, TDMAS, and oxygen plasma dose time) are increased.     

Mechanisms of atomic layer deposition on substrates with ultrahigh aspect ratios

Atomic layer deposition (ALD) appears to be uniquely suited for coating substrates with ultrahigh aspect ratios (> or similar 10(3)), including nanoporous solids. Here, we study the ALD of Cu and Cu3N on the inner surfaces of low-density nanoporous silica aerogel monoliths. Results show that Cu depth profiles in nanoporous monoliths are limited not only by Knudsen diffusion of heavier precursor molecules into the pores, as currently believed, but also by other processes such as the interaction of precursor and reaction product molecules with pore walls. Similar behavior has also been observed for Fe, Ru, and Pt ALD on aerogels. On the basis of these results, we discuss design rules for ALD precursors specifically geared for coating nanoporous solids.     

A novel strategy to increase performance of solid‐phase microextraction fibers: Electrodeposition of sol‐gel films on highly porous substrate

In the present work, the effect of substrate porosity for preparation of solid‐phase microextraction (SPME) fibers was investigated. The fibers were prepared by electrodeposition of sol‐gel coatings using negative potentials on porous Cu wire and compared with previous reported technique for preparation of SPME fibers using positive potentials on smooth gold wire. Porous substrate was prepared by electrodeposition of a thin layer of Cu on a Cu wire. The extraction capability of prepared fibers was evaluated through extraction of some aromatic hydrocarbons from the headspace of aqueous samples. The effect of substrate porosity and some operating parameters on extraction efficiency was optimized. The results showed that extraction efficiency of SPME fibers highly depends on porosity of the substrate. The LOD ranged from 0.005 to 0.010 ng/mL and repeatability at the 1 ng/mL was below 12%. Electrodeposited films were characterized for their surface morphology and thermal stability using SEM and thermogravimetric analysis, respectively. SEM analysis revealed formation of porous substrate and subsequently porous coating on the wire surface and thermogravimetric analysis showed high thermal stability of the prepared fiber.     

Growth of silicon thin film by LPE on porous silicon bilayers

The fabrication of solar cells based on the transfer of a thin silicon film on a foreign substrate is an attractive way to realise cheap and efficient photovoltaic devices. The aim of this work is to realise a thin mono-crystalline silicon film on a double porous silicon layer in order to detach and transfer it on mullite. The first step is the fabrication of a double porous silicon layer by electrochemical anodisation using two different current densities. The low current leads to a low porosity layer and during annealing, the recrystallisation of this layer allows epitaxial growth. The second current leads to a high porosity which permits the transfer on to a low cost substrate. Liquid Phase Epitaxy (LPE) performed with indium (or In+Ga) in the temperature range of 950–1050°C leads to almost homogeneous layers. Growth rate is about 0.35 μm min⁻¹. Crystallinity of the grown epilayer is similar on porous silicon and on single crystal silicon. In this paper, we focus on the realisation of porous silicon sacrificial layer and subsequent LPE growth. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of surface temperature on the mechanism of atomic layer deposition of aluminum oxide using an oxygen plasma and ozone

We have examined the role of substrate temperature on the surface reaction mechanisms during the atomic layer deposition (ALD) of Al(2)O(3) from trimethyl aluminum (TMA) in combination with an O(2) plasma and O(3) over a substrate temperature range of 70-200 °C. The ligand-exchange reactions were investigated using in situ attenuated total reflection Fourier transform infrared spectroscopy. Consistent with our previous work on ALD of Al(2)O(3) from an O(2) plasma and O(3) [Rai, V. R.; Vandalon, V.; Agarwal, S. Langmuir 2010, 26, 13732], both -OH groups and carbonates were the chemisorption sites for TMA over the entire temperature range explored. The concentration of surface -CH(3) groups after the TMA cycle was, however, strongly dependent on the surface temperature and the type of oxidizer, which in turn influenced the corresponding growth per cycle. The combustion of surface -CH(3) ligands was not complete at 70 °C during O(3) exposure, indicating that an O(2) plasma is a relatively stronger oxidizing agent. Further, in O(3)-assisted ALD, the ratio of mono- and bidentate carbonates on the surface after O(3) exposure was dependent on the substrate temperature.     

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.     

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.     

Integration of porous layers in ordered pillar arrays for liquid chromatography

The present paper describes a method for the production of partly porous micro-pillars in columns suitable for use in liquid chromatography. These layers increase the available surface at least two orders of magnitude without destroying the huge benefits of the ordered nature of the system. A process flow was developed that enabled us to create a 550 nm thick porous layer on the pillar array in a sealed channel configuration, withstanding pressures up to at least 70 bar. Measuring band broadening under non-retained conditions, only a modest increase in plate height was observed in the porous pillar array as compared to that in a non-porous pillar array. The homogeneity of the layers was demonstrated using an optical microscope and SEM pictures and by monitoring peak velocities at constant pressures. The internal porosity was determined using particles with a diameter larger than the mesopores in combination with a dye that could penetrate into the pores.     

Facile Synthesis of Three‐Dimensional Pt‐TiO2 Nano‐networks: A Highly Active Catalyst for the Hydrolytic Dehydrogenation of Ammonia–Borane

Three‐dimensional (3D) porous metal and metal oxide nanostructures have received considerable interest because organization of inorganic materials into 3D nanomaterials holds extraordinary properties such as low density, high porosity, and high surface area. Supramolecular self‐assembled peptide nanostructures were exploited as an organic template for catalytic 3D Pt‐TiO₂ nano‐network fabrication. A 3D peptide nanofiber aerogel was conformally coated with TiO₂ by atomic layer deposition (ALD) with angstrom‐level thickness precision. The 3D peptide‐TiO₂ nano‐network was further decorated with highly monodisperse Pt nanoparticles by using ozone‐assisted ALD. The 3D TiO₂ nano‐network decorated with Pt nanoparticles shows superior catalytic activity in hydrolysis of ammonia–borane, generating three equivalents of H₂.     

Atomic Layer Deposition of Cobalt Phosphide for Efficient Water Splitting

Transition‐metal phosphides (TMP) prepared by atomic layer deposition (ALD) are reported for the first time. Ultrathin Co‐P films were deposited by using PH₃ plasma as the phosphorus source and an extra H₂ plasma step to remove excess P in the growing films. The optimized ALD process proceeded by self‐limited layer‐by‐layer growth, and the deposited Co‐P films were highly pure and smooth. The Co‐P films deposited via ALD exhibited better electrochemical and photoelectrochemical hydrogen evolution reaction (HER) activities than similar Co‐P films prepared by the traditional post‐phosphorization method. Moreover, the deposition of ultrathin Co‐P films on periodic trenches was demonstrated, which highlights the broad and promising potential application of this ALD process for a conformal coating of TMP films on complex three‐dimensional (3D) architectures.     

Surface chemistry in the atomic layer deposition of TiN films from TiCl4 and ammonia

The surface chemistry of atomic layer depositions (ALD) of titanium nitride films using alternate doses of TiCl4 and NH3 was characterized by using X-ray photoelectron spectroscopy. The nature of the species deposited by each half-reaction was explored first. Evidence was obtained for the partial loss of chlorine atoms and the reduction of the metal during the adsorption of the TiCl4. Subsequent ammonia treatment removes most of the remaining chlorine and leads to the formation of a nitride. Both half-reactions were proven self-limited, stopping after the deposition of submonolayer quantities of the materials. Repeated ALD cycles were shown to lead to the buildup of thick films. However, those films display a Ti3N4 layer on top of the expected TiN. The data suggest that the reduction of the Ti4+ species may therefore occur during the TiCl4, not NH3, dosing step. The incorporation of impurities in the films was also investigated. Chlorine is only deposited on the surface, and in negligible quantities. This Cl appears to originate from readsorption of the HCl byproduct, and could be removed by light sputtering, heating, or further ammonia treatment. Oxygen incorporation, on the other hand, was unavoidable and was determined to possibly come from diffusion from the underlying substrate.     

金属│固体电解质│含水凝胶│石墨体系的研究

应用点缺陷模型和双层模型分析了金属│固体电解质│含水凝胶│石墨体系贮藏和放电时金属表面氧化膜的变化.在固体电解质含有微量水分的情况下,微量水分有助于金属表面形成松层,但由于水分子与紧密钝化膜的持续作用,金属表面松层厚度会不断增长,孔率降低,使得电池的电压和放电电流下降.松层的孔率强烈影响膜溶解行为,与紧密钝化膜的电学特性一样,松层也是一种半导体.     

医用多孔NiTi合金表面溶胶-凝胶法制备TiO2涂层

采用溶胶-凝胶法结合浸渍提拉工艺在多孔NiTi合金表面制备出了结构均一的锐钛矿型TiO2涂层,并在溶胶中添加聚乙二醇(PEG)作为造孔剂,进而在多孔NiTi合金表面制备出内层致密、外层多孔的TiO2复合涂层。SEM分析结果表明,TiO2涂层均匀地覆盖了多孔NiTi合金基体的外表面以及孔的内表面。Hanks溶液中的阳极极化曲线结果表明,与未处理的多孔NiTi合金相比,具有致密TiO2涂层的多孔NiTi合金其耐腐蚀性能有了显著提高。而多孔TiO2复合涂层进一步增大了多孔NiTi合金的实际表面积,提高了材料表面的生物活性。     

Role of vacuum ultraviolet irradiation in plasma‐induced graft polymerization in the pore‐filling polymerization of porous materials

The role of vacuum ultraviolet (VUV) rays contained in the plasma during plasma‐induced graft polymerization in the pores of a porous high‐density polyethylene (HDPE) substrate was investigated through the separation of the VUV rays from the plasma with LiF, CaF₂, and quartz filters. Two characteristic phenomena, the effect of the solvent on the grafting rate and the graft polymerization in the pores of the porous substrate, were observed in the VUV‐induced graft polymerization process. These results provided clear evidence that VUV rays in the plasma played an important role in the formation of grafted layers in the pores of the HDPE substrate. The relationship between the penetration depth of the VUV rays and the distribution of the grafted layer inside the substrate was examined. The calculated penetration depth of the VUV rays (and hence the distribution of radicals) and the distribution of the grafted layer were not consistent. However, this inconsistency could be explained by the fact that the effective density of the radicals that could react with the monomer to grow the grafting polymer was very low because of the steric hindrance of the grafted chains. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2068–2074, 2005     

Concentration profiles retention—flux curves for composite membranes in reverse osmosis

The influence of a transition layer between the skin porous sublayer in asymmetric membranes was investigated, using a numerical integration of the Spiegler—Kedem equation. It is shown that the transition layer will only interfere if the resistance to solute diffusion within this layer is of the same order of magnitude as in the skin layer.Experiments with multi-layer membranes were performed by placing a highly selective asymmetric membrane on top of a membrane of lower selectivity, and vice versa. The resulting retention-flux curves can be simulated by a three-layer model: σ $\text{P}$/Δx for the two skin layers were determined from reverse osmosis data on the individual membranes whereas the values for the interjacent porous layer were correlated with the porosity and thickness of that layer.     

Thermogravimetric Study of Volatile Precursors For Chemical Thin Film Deposition. Estimation of vapor pressures and source temperatures

Normal pressure thermogravimetry (TG) measurements were used to study the sublimation behavior of several volatile metal compounds, used as metal precursors in thin film fabrication by chemical vapor phase methods, like atomic layer deposition (ALD) and chemical vapor deposition (CVD). The results indicated that dynamic TG measurements may be used to find correct source temperatures to be used in an ALD reactor: a good correlation between the source temperatures used in ALD and temperatures corresponding to mass losses of 10 and 50% in TG was verified. It was also found that isothermal TG measurements offer a simple way for the vapor pressure measurements which otherwise are not trivial for solids with only moderate volatility. This revised version was published online in July 2006 with corrections to the Cover Date.     

Porogen approach for the fabrication of plasma-polymerized nanoporous polysiloxane films

Nanoporous polysiloxane films were fabricated by plasma polymerization of hexamethyldisiloxane mixed with cyclohexane under different conditions. The pores were generated through the elimination of carbonaceous aggregates (porogen) by annealing at 600 degrees C. Results of spectroscopic ellipsometry, Fourier transform infrared spectroscopy, and positron annihilation lifetime spectroscopy suggest that not only film porosity but also average pore size depends on the amount of the decomposable porogen. The pore size was controllable in a range between 0.6 and 1.0 nm in radius by proper selection of the substrate temperature and precursor composition.     

Imaging of atomic layer deposited (ALD) tungsten monolayers on alpha-TiO2(110) by X-ray standing wave Fourier inversion

A single atomic layer of tungsten grown by atomic layer deposition (ALD) on a single-crystal rutile TiO2(110) support is studied by the X-ray standing wave (XSW) technique. The surface structural and chemical properties were also examined using atomic force microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction. The XSW measured set of hkl Fourier components for the W atomic distribution function are summed together to produce a model-independent 3D map of the W atoms relative to the rutile lattice. The 3D atomic image shows surface tungsten atoms equally occupying the two nonequivalent Ti sites with a slight outward displacement. This corresponds to the atop and bridge sites with respect to the underlying lattice oxygen atoms. These XSW measurements clearly show that ALD conformal layers can be highly coherent with respect to the substrate lattice.     

Flow of liquid around the encapsulated drop of another liquid

The problem of the infinite uniform flow of liquid around the spherical drop coated with the porous layer is solved. External liquid permeates into the porous layer but is not mixed with the liquid located in the internal cavity of a capsule. The flow inside the porous layer is described by the Brinkman equation; moreover, the viscosity of the Brinkman medium is assumed to be different than the viscosity of pure liquid. The boundary condition of the jump of tangential stresses at the liquid-porous medium interface is used. Velocity and pressure distributions are found and the hydrodynamic force acting on the capsule is calculated. Different limiting cases are considered.