Development of ALD Coatings for Harsh Environment Applications

Enhanced Corrosion Resistance in Aluminum-Based Electrolyzer Components via Stoichiometry Tuned Atomic Layer-Deposited TiOx Films

Titanium (Ti) is widely used as anode current collectors in proton exchange membrane (PEM)-based water electrolyzers due to its self-passivated oxide layer, which protects it from corrosion in acidic solutions. However, the cost of the material and machining process for Ti is high. A wider utilization of water electrolyzers to produce hydrogen could be favored by the use of less expensive coated aluminum (Al) substrates, which could potentially replace high-cost Ti-based components. It is shown here by depositing a pinhole-free oxygen vacancy-rich titanium oxide (TiOx) protection layer by atomic layer deposition (ALD), the corrosion resistance of Al substrates in acidic environments at oxygen evolution potentials can be enhanced. The optimization of the oxygen vacancy concentration is accomplished by tuning the ALD parameters to achieve ideal stoichiometry and conformal coating on rough substrates. The robustness of the coatings was evaluated at high potentials (2.4 V vs NHE = normal hydrogen electrode) in low pH conditions. A low TiOx dissolution rate of the order of ∼6 nm year-1 was observed. By testing under industrially relevant conditions, i.e., high applied voltages (2.4 V) and low pH, an Al loss at around the zero ppb level was achieved using optimized ALD parameters. It is proposed that a 40 nm TiOx coating on Al may be adequate to provide 60,000 h of durability in a PEM water electrolyzer anode current collector.

Ultralong-Term Durable Anticorrosive Coatings by Integration of Double-Layered Transfer Self-Healing Ability, Fe Ion-Responsive Ability, and Active/Passive Functional Partitioning

The application of self-healing polymers in corrosion protection is often limited by their slow and nonautonomous healing ability and poor long-term durability. In this paper, we propose a double-layered transfer self-healing coating constructed by soft and rigid polymer layers. The soft polymer has a fast self-healing rate of 10 min to repair, which was found to accelerate the self-healing of the upper rigid layer. The rigid polymer provided relatively high barrier ability while preserving certain self-healing ability owing to the shear-thinning effect. In this way, the double-layered coating combined rapid self-healing (∼1 h) and high impedance modulus |Z|f-0.01 Hz of 2.58 × 1010 Ω·cm2. Furthermore, the introduction of pyridine groups in B-PEA and polyacrylate-grafted-polydimethylsiloxane (PEA-g-PDMS) induced the Fe ion-responsive ability and shortened the self-healing time to 40 min (100 ppm Fe). Finally, barrier and anode sacrificed layers were introduced to produce multilayered architecture with active/passive anticorrosion performance. In the presence of scratches, the |Z|f-0.01 Hz can be preserved at 1.03 × 1010 Ω·cm2 after 200 days. The created anticorrosive coating technology combines long-term durability with room temperature autonomous rapid self-healing capability, providing a broad prospect for anticorrosive applications.

A flexible protruding microelectrode array for neural interfacing in bioelectronic medicine

Recording neural signals from delicate autonomic nerves is a challenging task that requires the development of a low-invasive neural interface with highly selective, micrometer-sized electrodes. This paper reports on the development of a three-dimensional (3D) protruding thin-film microelectrode array (MEA), which is intended to be used for recording low-amplitude neural signals from pelvic nervous structures by penetrating the nerves transversely to reduce the distance to the axons. Cylindrical gold pillars (Ø 20 or 50 µm, ~60 µm height) were fabricated on a micromachined polyimide substrate in an electroplating process. Their sidewalls were insulated with parylene C, and their tips were optionally modified by wet etching and/or the application of a titanium nitride (TiN) coating. The microelectrodes modified by these combined techniques exhibited low impedances (~7 kΩ at 1 kHz for Ø 50 µm microelectrode with the exposed surface area of ~5000 µm²) and low intrinsic noise levels. Their functionalities were evaluated in an ex vivo pilot study with mouse retinae, in which spontaneous neuronal spikes were recorded with amplitudes of up to 66 µV. This novel process strategy for fabricating flexible, 3D neural interfaces with low-impedance microelectrodes has the potential to selectively record neural signals from not only delicate structures such as retinal cells but also autonomic nerves with improved signal quality to study neural circuits and develop stimulation strategies in bioelectronic medicine, e.g., for the control of vital digestive functions.

Optical properties of black silicon structures ALD-coated with Al2O3

Atomic layer deposited (ALD) Al₂O₃coatings were applied on black silicon (b-Si) structures. The coated nanostructures were investigated regarding their reflective and transmissive behaviour. For a systematic study of the influence of the Al₂O₃coating, ALD coatings with a varying layer thickness were deposited on three b-Si structures with different morphologies. With a scanning electron microscope the morphological evolution of the coating process on the structures was examined. The optical characteristics of the different structures were investigated by spectral transmission and reflection measurements. The usability of the structures for highly efficient absorbers and antireflection (AR) functionalities in the different spectral regions is discussed.

Atmospheric atomic layer deposition of SnO2 thin films with tin(II) acetylacetonate and water

Due to its unique optical, electrical, and chemical properties, tin dioxide (SnO₂) thin films attract enormous attention as a potential material for gas sensors, catalysis, low-emissivity coatings for smart windows, transparent electrodes for low-cost solar cells, etc. However, the low-cost and high-throughput fabrication of SnO₂ thin films without producing corrosive or toxic by-products remains challenging. One appealing deposition technique, particularly well-adapted to films presenting nanometric thickness is atomic layer deposition (ALD). In this work, several metalorganic tin-based complexes, namely, tin(IV) tert-butoxide, bis[bis(trimethylsilyl)amino] tin(II), dibutyltin diacetate, tin(II) acetylacetonate, tetrakis(dimethylamino) tin(IV), and dibutyltin bis(acetylacetonate), were explored thanks to DFT calculations. Our theoretical calculations suggest that the three last precursors are very appealing for ALD of SnO₂ thin films. The potential use of these precursors for atmospheric-pressure spatial atomic layer deposition (AP-SALD) is also discussed. For the first time, we experimentally demonstrate the AP-SALD growth of SnO₂ thin films using tin(II) acetylacetonate (Sn(acac)₂) and water. We observe that Sn(acac)₂ exhibits efficient ALD activity with a relatively large ALD temperature window (140-200 °C), resulting in a growth rate of 0.85 ± 0.03 Å per cyc. XPS analyses show a single Sn 3d_5/2 characteristic peak for Sn⁴⁺ at 486.8 ± 0.3 eV, indicating that a pure SnO₂ phase is obtained within the ALD temperature window. The as-deposited SnO₂ thin films are in all cases amorphous, and film conductivity increases with the deposition temperature. Hall effect measurements confirm the n-type nature of SnO₂ with a free electron density of about 8 × 10¹⁹ cm^-3, electron mobility up to 11.2 cm² V^-1 s^-1, and resistivity of 7 × 10^-3 Ω cm for samples deposited at 270 °C.

Re-examination of the Aqueous Stability of Atomic Layer Deposited (ALD) Amorphous Alumina (Al2O3) Thin Films and the Use of a Postdeposition Air Plasma Anneal to Enhance Stability

Amorphous aluminum oxide (alumina) thin films are of interest as inert chemical barriers for various applications. However, the existing literature on the aqueous stability of atomic layer deposited (ALD) amorphous alumina thin films remains incomplete and, in some cases, inconsistent. Because these films have a metastable amorphous structure─which is likely partially hydrated in the as-deposited state─hydration and degradation behavior likely deviate from what is expected for the equilibrium, crystalline Al₂O₃ phase. Deposition conditions and the aqueous solution composition (ion content) appear to influence the reactivity and stability of amorphous ALD alumina films, but a full understanding of why these alumina films hydrate, solvate, and/or dissolve in near-neutral pH = 7 conditions, for which crystalline Al₂O₃ is expected to be stable, remains unsolved. In this work, we conduct an extensive X-ray photoelectron spectroscopy investigation of the surface chemistry as a function of water immersion time to reveal the formation of oxyhydroxide (AlOOH), hydroxide (Al(OH)₃), and possible carbonate species. We further show that brief postdeposition exposures of these ALD alumina films to an air plasma anneal can significantly enhance the film's stability in near-neutral pH aqueous conditions. The simplicity and effectiveness of this plasma treatment may provide a new alternative to thermal annealing and capping treatments typically used to promote aqueous stability of low-temperature ALD metal oxide barrier layers.

Bayesian Machine Learning for Efficient Minimization of Defects in ALD Passivation Layers

Atomic layer deposition (ALD) is an enabling technology for encapsulating sensitive materials owing to its high-quality, conformal coating capability. Finding the optimum deposition parameters is vital to achieving defect-free layers; however, the high dimensionality of the parameter space makes a systematic study on the improvement of the protective properties of ALD films challenging. Machine-learning (ML) methods are gaining credibility in materials science applications by efficiently addressing these challenges and outperforming conventional techniques. Accordingly, this study reports the ML-based minimization of defects in an ALD-Al₂O₃ passivation layer for the corrosion protection of metallic copper using Bayesian optimization (BO). In all experiments, BO consistently minimizes the layer defect density by finding the optimum deposition parameters in less than three trials. Electrochemical tests show that the optimized layers have virtually zero film porosity and achieve five orders of magnitude reduction in corrosion current as compared to control samples. Optimized parameters of surface pretreatment using Ar/H₂ plasma, the deposition temperature above 200 °C, and 60 ms pulse time quadruple the corrosion resistance. The significant optimization of ALD layers presented in this study demonstrates the effectiveness of BO and its potential outreach to a broader audience, focusing on different materials and processes in materials science applications.

Toolbox for atomic layer deposition process development on high surface area powders

Atomic layer deposition (ALD) is an industrially applied technique for thin film deposition. The vast majority of processes target flat substrates rather than powders. For ALD on powders, new processes are needed, as different reaction conditions are required. Here, two setups are described in detail, which enhance the ALD process development for powders. The first setup described is capable of directly measuring the vapor pressure of a given precursor by a capacitance diaphragm gauge. Promising precursors can be pre-selected, and suitable precursor saturation temperatures can be determined. The second setup consists of four parallel reactors with individual temperature zones to screen the optimal ALD temperature window in a time efficient way. Identifying the precursor saturation temperature beforehand and subsequently performing the first ALD half cycle in the parallel setup at four different reactor temperatures simultaneously will drastically reduce process development times. Validation of both setups is shown for the well-known ALD precursors, trimethylaluminum to deposit aluminum oxide and diethyl zinc to deposit zinc oxide, both on amorphous silica powder.

Atomic layer deposited films of Al2O3 on fluorine-doped tin oxide electrodes: stability and barrier properties

Al₂O₃ layers were deposited onto electrodes by atomic layer deposition. Solubility and electron-transport blocking were tested. Films deposited onto fluorine-doped tin oxide (FTO, F:SnO₂/glass) substrates blocked electron transfer to redox couples (ferricyanide/ferrocyanide) in aqueous media. However, these films were rapidly dissolved in 1 M NaOH (≈100 nm/h). The dissolution was slower in 1 M H₂SO₄ (1 nm/h) but after 24 h the blocking behaviour was entirely lost. The optimal stability was reached at pH 7.2 where no changes were found up to 24 h and even after 168 h of exposure the changes in the blocking behaviour were still minimal. This behaviour was also observed for protection against direct reduction of FTO.

In Situ X-ray Diffraction and Spectro-Microscopic Study of ALD Protected Copper Films

In many applications of copper in industry and research, copper migration and degradation of metallic copper to its oxides is a common problem. There are numerous ways to overcome this degradation with varying success. Atomic layer deposition (ALD) based encapsulation and passivation of the metallic copper recently emerged as a serious route to success owing to the conformality and density of the ALD films. So far, the majority of the studies have been focused on corrosion protection of copper in a variety of chemical environments, mostly at ambient temperature. An investigation of the stability of the ALD film stacks and copper's interaction with them at elevated temperatures has been lacking. Here, we study the mitigation of copper oxidation and migration in 50 nm thick Al₂O₃/TiO₂ and Al₂O₃/SiO₂ bilayer ALD stacks. First, the corrosion dynamics were investigated via in situ X-ray diffraction (XRD) at 350 °C under atmospheric conditions, and second, the interaction of copper with the passivation layers have been examined post factum using detailed spectro-microscopic investigations. According to the XRD results, both ALD films exhibited excellent oxidation protection. In contrast, bare Cu immediately started to oxidize at 350 °C and transformed entirely to its known oxide phases in 4 h. Spectro-microscopic studies revealed that there are structural and chemical changes on the top surface and within the film stacks. The TiO₂ layer was crystallized during annealing, while the SiO₂ layer stayed in the amorphous phase, which was analyzed by grazing incidence XRD and transmission electron microscopy. According to scanning electron microscopy and X-ray photoelectron spectroscopy analysis, copper was detected on the surface with a higher amount in Al₂O₃/TiO₂ than Al₂O₃/SiO₂, 5.2 at.% and 0.7 at.%, respectively. Based on the surface and cross-sectional analysis, copper migration was observed on both layers, albeit more substantially in Al₂O₃/TiO₂. In the case of Al₂O₃/SiO₂, the bulk of the copper was captured at the interface of the two oxides.

Seeded Growth of Large-Area Arrays of Substrate Supported Au Nanoparticles Using Citrate and Hydrogen Peroxide

While seeded growth of quasi-spherical colloidal Au nanoparticles (NPs) has been extensively explored in the literature, the growth of surface supported arrays of such particles has received less attention. The latter scenario offers some significant challenges, including the attainment of sufficient particle-substrate adhesion, growth-selectivity, and uniform mass-transport. To this end, a reaction system consisting of HAuCl₄, citrate, and H₂O₂ is here investigated for the growth of supported arrays of 10 nm Au seeds, derived via block copolymer (BCP) lithography. The effects of the reagent concentrations on the properties of the resultant NPs are evaluated. It is found that inclusion of citrate in the growth medium causes substantial particle desorption from Si surfaces. However, the presence of citrate also yields NPs with more uniformly circular top-view cross sections ("quasi-circular"), motivating the exploration of particle immobilization methods. We demonstrate that atomic layer deposition (ALD) of a single cycle of HfO₂ (∼1 Å), after the seed particle formation, promotes adhesion sufficiently to enable the use of citrate without the added oxide noticeably affecting the shape of the resultant NPs. The presented ALD-based approach differs from the conventional sequence of depositing the adhesion layer prior to the seed particle formation and may have advantages in various processing schemes, such as when surface grafting of brush layers is required in the BCP lithography process. A proof-of-concept is provided for the growth of large-area arrays of supported "quasi-circular" Au NPs, in a rapid one-step process at room temperature.