Atomic Layer Deposition Coatings Research Articles

Stabilizing the Li Metal Interface: Molecular Layer Deposition for Advanced Next-Generation Energy Storage Systems

The Li metal anode is sought-after for its high theoretical capacity (3860 mAh g-1) and low electrochemical potential (-3.04 V versus the standard hydrogen electrode). Nevertheless, the intrinsic instability of the Li metal towards both solid and liquid electrolytes leads to unstable solid electrolyte interphase (SEI) formation and dendritic lithium, causing safety concerns and rapid performance degradation [1]. Among the potential techniques used to protect the Li metal surface and inhibit dendrite growth, molecular layer deposition (MLD) has proven to be an invaluable tool for the development of nanoscale interfacial coatings with unique properties [2]. The layer-by-layer assembly of molecular fragments and conformal coating abilities of MLD yield the ability to tune the chemical and mechanical properties of the Li metal interface. It is believed that the exploration and adoption of new MLD films will open up new avenues for stabilizing the Li metal anode. Herein, we show several examples of high-performance Li metal anodes in liquid and solid-state systems achieved by polymeric coatings synthesized through MLD techniques [3-8]. New hybrid organic-inorganic coatings belonging to the metalcone family are shown to dramatically enhance the Coulombic efficiency and rate capability of Li metal anodes. Furthermore, the rational design and ordering of these coatings coupled with inorganic atomic layer deposition coatings reveal the importance of bilayer type structures in promoting enhanced mechanical properties for suppressing dendrite growth. The stabilizing properties of these advanced thin films are further extended to several next-generation battery systems including Li-S and Li-O2, proving effective when coupled with high energy density cathode materials. Moreover, advanced characterization techniques such as in-situ X-ray absorption spectroscopy, nanoindentation measurements, Rutherford backscattering spectrometry, and time-of-flight secondary ion mass spectrometry are used to reveal the mechanisms behind the electrochemical lithiation processes as well as cycling stability.

Atomic layer deposition of titanium oxide and nitride on vertically aligned carbon nanotubes for energy dense 3D microsupercapacitors

This work demonstrates the use of ALD coatings in a full-cell 3D microsupercapacitor (MSC) device for the first time. The novel device design described herein has potential to be exploited for rapid and scalable manufacturing of on-chip MSCs integrated with other electronic devices in standard semiconductor fabs or foundries. In this study, atomic layer deposition (ALD) was used to deposit conformal pseudocapacitive coatings of titanium oxide (TiO2) and titanium nitride (TiN) onto CVD-grown vertically aligned carbon nanotubes (VACNTs) to increase the area-specific capacitance (by 240× and 74× at 0.1 Vs−1 versus the as-deposited VACNT electrode and full-cell VACNT device, respectively) and thus energy density for microscale energy storage applications. The study indicated that combining TiO2 and TiN ALD layers resulted in a multilayered composite coating rich in oxygen vacancies with enhanced capacitance relative to a single TiO2 or TiN layer due to a dynamic surface chemistry that developed during charge-discharge cycling. A key component of this multilayered coating was the formation of titanium oxynitride (TiON) which we attributed to the improved performance. Further, we demonstrated that controlling the thickness and stoichiometric ratio of the TiN-TiO2 composite coating was key to enabling functionality and optimizing the symmetric 3D MSC device performance.

Probing the Atomic-Scale Structure of Amorphous Aluminum Oxide Grown by Atomic Layer Deposition.

Atomic layer deposition (ALD) is a well-established technique for depositing nanoscale coatings with pristine control of film thickness and composition. The trimethylaluminum (TMA) and water (H2O) ALD chemistry is inarguably the most widely used and yet to date, we have little information about the atomic-scale structure of the amorphous aluminum oxide (AlOx) formed by this chemistry. This lack of understanding hinders our ability to establish process-structure-property relationships and ultimately limits technological advancements employing AlOx made via ALD. In this work, we employ synchrotron high-energy X-ray diffraction (HE-XRD) coupled with pair distribution function (PDF) analysis to characterize the atomic structure of amorphous AlOx ALD coatings. We combine ex situ and in operando HE-XRD measurements on ALD AlOx and fit these experimental data using stochastic structural modeling to reveal variations in the Al-O bond length, Al and O coordination environment, and extent of Al vacancies as a function of growth conditions. In particular, the local atomic structure of ALD AlOx is found to change with the substrate and number of ALD cycles. The observed trends are consistent with the formation of bulk Al2O3 surrounded by an O-rich surface layer. We deconvolute these data to reveal atomic-scale structural information for both the bulk and surface phases. Overall, this work demonstrates the usefulness of HE-XRD and PDF analysis in improving our understanding of the structure of amorphous ALD thin films and provides a pathway to evaluate how process changes impact the structure and properties of ALD films.

Understanding Pore Formation in ALD Alumina Overcoats.

AlOX thin films deposited by atomic layer deposition (ALD) have previously been used to increase both stability and selectivity of supported palladium catalysts and are known to develop nanoscale porosity upon heating. Understanding the factors that affect ALD thin-film porosityenables future design of layered catalytic structures with tunable nanoscale features on industrially-relevant high-surface-area materials. In this study, porous and nonporous aluminum oxide supports with and without palladium nanoparticles were overcoated with thin films of 2-7 nm AlOX by ALD deposited at temperatures of 100, 200, and 300 °C. Hydroxyl loss and changes in surface chemistry were observed upon heating the films, and changes in surface area and pore volume of the annealed films were correlated to AlOX deposition temperature and the presence of Pd. Crystallization of the overcoat to γ-Al2O3 is shown to occur separately from hydroxyl loss and pore formation. A mechanistic understanding of pore formation in AlOX ALD films is obtained by reference to studies of the structural transformations accompanying the formation of transition aluminas from hydroxide precursors. Additionally, a direct and tunable correlation is established between pore development and the overall hydroxyl content of AlOX ALD coatings.

Discovery of an Unexpected Metal Dissolution of Thin‐Coated Cathode Particles and Its Theoretical Explanation

AbstractThe degree of metal dissolution of cathode materials is a critical parameter in determining the performance of lithium‐ion batteries (LIBs). Ultra‐thin coated cathode particles, fabricated via atomic layer deposition (ALD), exhibit superior battery performance over that of bare particles. Therefore, it is generally believed that a coating layer protects the particles from metal dissolution of active materials, which is a critical cathode degradation mechanism. However, it is observed that ultra‐thin CeO2 coating intensified the Mn dissolution of LiMn2O4 (LMO) during cycling of LIBs, whereas ultra‐thin Al2O3 coating tended to inhibit Mn dissolution. A detailed density functional theory (DFT) study is carried out to explain these experimental observations by analyzing interaction of Mn atoms with neighboring electrode atoms in terms of energetic and structural aspect. All atomic and electronic analyses are consistent with the experimental observations. Several common materials are investigated as possible ALD coatings for LIBs to provide general insight, and it is found that Mn dissolution can be suppressed or accelerated depending on the material selection. This is the first report finding that depending on the coating material, metal dissolution can be accelerated, providing new insights into the impact of ALD coating materials on metal dissolution in cathode materials.

Antireflection Coating on PMMA Substrates by Atomic Layer Deposition

Antireflection coatings (ARC) are essential for various optical components including such made of plastics for high volume applications. However, precision coatings on plastics are rather challenging due to typically low adhesion of the coating to the substrate. In this work, optimization of the atomic layer deposition (ALD) processes towards conformal optical thin films of Al2O3, TiO2 and SiO2 on poly(methyl methacrylate) (PMMA) has been carried out and a five-layer ARC is demonstrated. While the uncoated PMMA substrates have a reflectance of nearly 8% in the visible (VIS) spectral range, this is reduced below 1.2% for the spectral range of 420–670 nm by applying a double-side ARC. The total average reflectance is 0.7%. The optimized ALD coatings show a good adhesion to the PMMA substrates even after the climate test. Microscopic analysis on the cross-hatch areas on PMMA after the climate test indicates very good environmental stability of the ALD coatings. These results enable a possible route by ALD to deposit uniform, crack free, adhesive and environmentally durable thin film layers on sensitive thermoplastics like PMMA.

Atomic Layer Deposition Coating of TiO2 Nano-Thin Films on Magnesium-Zinc Alloys to Enhance Cytocompatibility for Bioresorbable Vascular Stents.

Background and purposeA coronary stent is a well-known cardiovascular medical device implanted to resolve disorders of the circulatory system due to bloodstream narrowing. Since the implanted device interacts with surrounding biological environments, the surface properties of a typical implantable stent play a critical role in its success or failure. Endothelial cell adhesion and proliferation are fundamental criteria needed for the success of a medical device. Metallic coronary stents are commonly used as biomaterial platforms in cardiovascular implants. As a new generation of coronary stents, bioresorbable vascular scaffolds have attracted a great deal of attention among researchers and studies on bioresorbable materials (such as magnesium and zinc) remain a target for further optimization. However, additional surface modification is needed to control the biodegradation of the implant material while promoting biological reactions without the use of drug elution.MethodsHerein, precise temperature and thickness controlled atomic layer deposition (ALD) was utilized to provide a unique and conformal nanoscale TiO2 coating on a customized magnesium-zinc stent alloy.ResultsImpressively, results indicated that this TiO2 nano-thin film coating stimulated coronary arterial endothelial cell adhesion and proliferation with additional features acting as a protective barrier. Data revealed that both surface morphology and surface hydrophilicity contributed to the success of the ALD nanoscale coating, which further acted as a protection layer inhibiting the release of harmful degradation products from the magnesium-zinc stent.ConclusionOverall, the outcome of this in vitro study provided a promising ALD stent coating with unique nano-structural surface properties for increased endothelialization, and as a result, ALD should be further studied for numerous biomedical applications.

A Critical Review: The Impact of the Battery Electrode Material Substrate on the Composition and Properties of Atomic Layer Deposition (ALD) Coatings

AbstractThe applications of atomic layer deposition (ALD) technology in the Li‐ion field are numerous: from protective coatings to high‐surface area active materials and composite electrodes to manufacturing and production of battery materials. The synergy between electrode materials and ALD coatings has been proven repeatedly; however, extensive knowledge of the reasons behind these benefits is needed. The lack of such know‐how is partly due to the common deposition and characterization methods applied in the field, where the deposition of films is performed on so‐called “simple substrates” (such as copper, silicon, or stainless steel) as opposed to the intended substrate material, which is either a complicated matrix of metal oxides or porous/inactive surfaces. In fact, real interactions between the electrode materials and the ALD coating vary greatly and depend on the substrate material identity. Here, the authors provide the reader with insight and an eye‐opening review of the effect of battery electrode substrates on the composition and quality of ALD coatings. They also report the importance of the suitable and most applicable characterization methods for ALD coatings and ALD film‐electrode interfaces.

Corrosion protection of Cu by atomic layer deposition

Atomic layer deposition (ALD) is a vapor phase technique that is able to deposit uniform, conformal thin films with an excellent thickness control at the atomic scale. 18 nm thick Al2O3 and TiO2 coatings were deposited conformaly and pinhole-free onto micrometer-sized Cu powder, using trimethylaluminum and tetrakis(dimethylamido)titanium(IV), respectively, as a precursor and de-ionized water as a reactant. The capability of the ALD coating to protect the Cu powder against corrosion was investigated. Therefore, the stability of the coatings was studied in solutions with different pH in the range of 0–14, and in situ raman spectroscopy was used to detect the emergence of corrosion products of Cu as an indication that the protective coating starts to fail. Both ALD coatings provide good protection at standard pH values in the range of 5–7. In general, the TiO2 coating shows a better barrier protection against corrosion than the Al2O3 coating. However, for the most extreme pH conditions, pH 0 and pH 14, the TiO2 coating starts also to degrade.

(Invited) Atomic Layer Deposition for Stabilizing Surface-Bound Molecules for Photoelectrochemistry and Catalysis

Molecularly sensitized devices that rely on surface bound molecules to impart functionality are keenly susceptible to device degradation via hydrolysis of anchor group chemistries and subsequent molecule detachment. Recent commercialization success in dye-sensitized solar cells is largely possible due to brute-force encapsulation engineering. However, using molecular sensitizers in aqueous environments as water oxidation catalysts or biological sensors poses an even greater challenge. This talk will highlight advances in the use of low temperature atomic layer deposition (ALD) applied after surface functionalization to drastically improve molecular attachment in aqueous environs. We have shown that ultrathin (< 1 nm) ALD coatings of inorganic oxides "surrounding" the molecular sensitizers improve attachment by orders of magnitude. These “coated” molecules still function as light absorbers, electron transfer agents, and catalytic species. The current state-of-the-science will be discussed including what has been learned about the changes in chemical structure upon ALD processing via in-situ FTIR spectroscopy and ex situ sum frequency generation spectroscopy. Finally, we will show the utility of this approach in creating new hybrid catalysts that combine the selectivity of homogeneous molecular catalysts with the recoverability of heterogeneous inorganic catalysts.

Probing the Atomic-Scale Structure of Thin Films Grown By Atomic Layer Deposition

The ability to grow conformal films with Ångstrom resolution makes atomic layer deposition (ALD) an attractive means of protecting and/or chemically functionalizing surfaces for catalysis, energy storage, photoelectrochemistry, and other applications. However, the performance of ALD coatings frequently falls short of expectations, largely because the atomic-scale structure (and therefore the chemical and physical properties) of ALD films deviate from idealized preconceptions. Single-crystal materials are only rarely achieved by ALD. More often, ALD films form amorphous, polycrystalline, or defective structures. ALD process conditions such as precursor selection(s), precursor exposure(s), and reactor temperature are known to impact the properties of resulting films. The substrate also affects the properties of ALD films, especially for ultrathin film thicknesses. To date, the ALD community has had difficulty connecting the atomic-scale structure of ALD films with their performance, largely because of limited availability of techniques to determine the atomic-scale structure of the ALD films. In this work, we demonstrate the ability to probe the structure of ALD films (e.g. Al2O3) using various advanced characterization techniques including both ex situ and in situ high-energy X-ray diffraction (HE-XRD) coupled with pair distribution function (PDF) analysis, electron diffraction (ED), and neutron scattering (NS). We evaluate the effect of ALD process variables (e.g. growth temperature and substrate) on atomic-scale structure, and connect the measured ALD structural features to observed electrochemical properties. The techniques we demonstrate can be applied broadly to probe the local structure of ALD films, while the fundamental understanding gained from this work will help tailor ALD for electrochemical applications. Figure 1

(Invited) Atomic Layer Deposition Coatings for Medical Devices

Implantable medical devices must withstand the corrosive environment inside the human body for prolonged periods of time. Hermetic sealing of the device to protect it from the corrosive environment of human body and vice versa is a key step to enable long life time for the smart medical devices. Atomic layer deposition (ALD) is an advanced deposition technique belonging to the family of chemical vapor deposition (CVD) techniques. In ALD the substrate to be coated is exposed sequentially to precursor chemicals that react with the reactive surface sites on the substrate and form a chemically bonded adsorption layer. This growth mode of the film enables extremely conformal and uniform films to be coated on three-dimensional objects having any shape. Even the smallest, nanometer sized features on the objects can be coated with ease, making the technique extremely useful when hermetic sealing of complex shaped objects are required. The advantages of ALD over other deposition techniques makes it powerful method for applications where sensitive substrate materials combine with extreme demands on coating quality and temperature/chemical resistance, such as those often seen in the medical applications. In order to make a robust protective sealing for the medical device, the film properties must fulfill certain key criteria. First and foremost, the film material needs to be non-toxic for human cells in order to be biocompatible. Secondly, the film needs to function as an highly efficient diffusion barrier against the ions and molecules present in body fluids. In addition, the adhesion of the film with the device surface needs to be strong enough to prevent the delamination of the film. In our recent work [1] we have shown that SiO2-HfO2 coated sub-millimeter-sized microelectronic chiplets for wireless body implants can last without degradation and electronic failure in accelerated aging tests in saline at T = 87 °C for more than 180 days. This corresponds an equivalent lifetime of more than 10 years at human body equivalent temperature of 37 °C. Latest results on ALD films (deposited with Picosun® R-200 Advanced ALD reactor) for medical applications will also be discussed. Biocompatibility tests for Al2O3, TiO2, HfO2, SiO2, Ta2O5 films using the ISO standard Cytotoxicity of Medical Device (ISO 10993-5) procedure will be presented. In addition, we will show results of corrosion tests in PBS solution at elevated temperature (87 °C) for a number of ALD oxides and their nanolaminates, highlighting a stress control method that can improve the long-term delamination resistance of the coatings. Furthermore, we will show some results on the diffusion barrier properties of Al2O3, TiO2, HfO2, SiO2, Ta2O5 and their nanolaminates on different electrode materials (Al, Au, Pt, TiN) when exposed to PBS solution. [1] Conformal Hermetic Sealing of Wireless Microelectronic Implantable Chiplets by Multilayered Atomic Layer Deposition (ALD) by J. Jeong, F. Laiwalla, J. Lee, R. Ritasalo, M. Pudas, L. Larson, V. Leung, and A. Nurmikko, https://doi.org/10.1002/adfm.201806440. Acknowledgements: This work is part of the ULIMPIA project funded by PENTA under grant number PENTA-2017-Call2-16101-ULIMPIA

Conformal High-K Dielectric Coating of Suspended Single-Walled Carbon Nanotubes by Atomic Layer Deposition.

As one of the highest mobility semiconductor materials, carbon nanotubes (CNTs) have been extensively studied for use in field effect transistors (FETs). To fabricate surround-gate FETs— which offer the best switching performance—deposition of conformal, weakly-interacting dielectric layers is necessary. This is challenging due to the chemically inert surface of CNTs and a lack of nucleation sites—especially for defect-free CNTs. As a result, a technique that enables integration of uniform high-k dielectrics, while preserving the CNT’s exceptional properties is required. In this work, we show a method that enables conformal atomic layer deposition (ALD) of high-k dielectrics on defect-free CNTs. By depositing a thin Ti metal film, followed by oxidation to TiO2 under ambient conditions, a nucleation layer is formed for subsequent ALD deposition of Al2O3. The technique is easy to implement and is VLSI-compatible. We show that the ALD coatings are uniform, continuous and conformal, and Raman spectroscopy reveals that the technique does not induce defects in the CNT. The resulting bilayer TiO2/Al2O3 thin-film shows an improved dielectric constant of 21.7 and an equivalent oxide thickness of 2.7 nm. The electrical properties of back-gated and top-gated devices fabricated using this method are presented.

Resonant Tunneling Induced Enhancement of Electron Field Emission by Ultra-Thin Coatings

The emission of electrons from the surface of a material into vacuum depends strongly on the material’s work function, temperature, and the intensity of electric field. The combined effects of these give rise to a multitude of related phenomena, including Fowler-Nordheim tunneling and Schottky emission, which, in turn, enable several families of devices, ranging from vacuum tubes, to Schottky diodes, and thermionic energy converters. More recently, nanomembrane-based detectors have found applications in high-resolution mass spectrometry measurements in proteomics. Progress in all the aforementioned applications critically depends on discovering materials with effective low surface work functions. We show that a few atomic layer deposition (ALD) cycles of zinc oxide onto suspended diamond nanomembranes, strongly reduces the threshold voltage for the onset of electron field emission which is captured by resonant tunneling from the ZnO layer. Solving the Schroedinger equation, we obtain an electrical field- and thickness-dependent population of the lowest few subbands in the thin ZnO layer, which results in a minimum in the threshold voltage at a thickness of 1.08 nm being in agreement with the experimentally determined value. We conclude that resonant tunneling enables cost-effective ALD coatings that lower the effective work function and enhance field emission from the device.

A Discovery of an Unexpected Metal Dissolution of Thin-Coated Cathode Particles: Its Theoretical and Experimental Explanations

The degree of metal dissolution of cathode materials is a critical parameter in determining Li-ion battery performance. Ultra-thin coated LiMn2O4 (LMO) cathode particles via Atomic Layer Deposition (ALD) exhibit superior battery performance over bare LMO particles. However, we have observed that ultra-thin CeO2 coating intensifies Mn dissolution of LMO during cycling of lithium-ion battery, whereas ultra-thin Al2O3 coating tends to inhibit Mn dissolution. A detailed study of Density Functional Theory (DFT) has been carried out to illustrate the experimental observations. First, the manganese vacancy formation energy is calculated, along with the bonding strengths of Mn-O of uncoated, Al2O3 coated, and CeO2 coated particles via Crystal Orbital Overlap Population (COOP) calculations. Further, the projected density of state calculation of Mn is used to confirm the electronic occupancy of Mn atom for each case. Surprisingly, the atomic analysis is consistent with the experimental observations. This is the first report that coatings can accelerate metal dissolution, which can show new insights into ALD coatings and their impact on metal dissolution in cathode materials.

Cooperating Effects of Conformal Iron Oxide ALD Coating and Post-Annealing on Li-Rich Layered Cathod

Li-rich layered cathode materials draw a wide attention due to their superior Li-storage capability and high energy density, but they suffer capacity degradation and voltage decay due to phase transition and metal dissolution during cycling. To address these problems, iron oxide atomic layer deposition (ALD) was performed on Li-rich layered cathode powders in a fluidized bed reactor, followed by an annealing process for a further improvement. The annealing of iron oxide-coated particles led to the diffusion of iron cation into the structure and a higher structure ordering. After 100 cycles of charge-discharge at a 1C rate and 55 °C, the iron oxide-coated cathode material delivered an initial capacity of ~196 mAh g-1 with a capacity retention of ~49%, while the annealing process improved it to ~221 mAh g-1 and a capacity retention of ~73%. The Fe doping on the surface of cathode facilitated Li+ transport and stabilized cyclic behavior. On the surface, the annealed-coated cathodes suppressed decomposition of electrolyte salt and formed a more stable solid permeable interface layer. In the bulk, a stable spinel phase formed during cycling contributed to the stability of structure; metal dissolution was also suppressed due to the annealing plus coating. These effects enabled a stable- and high-performance Li-ion battery cathode.

Effect of Low-Temperature Al2O3 ALD Coating on Ni-Rich Layered Oxide Composite Cathode on the Long-Term Cycling Performance of Lithium-Ion Batteries

Conformal coating of nm-thick Al2O3 layers on electrode material is an effective strategy for improving the longevity of rechargeable batteries. However, solid understanding of how and why surface coatings work the way they do has yet to be established. In this article, we report on low-temperature atomic layer deposition (ALD) of Al2O3 on practical, ready-to-use composite cathodes of NCM622 (60% Ni), a technologically important material for lithium-ion battery applications. Capacity retention and performance of Al2O3-coated cathodes (≤10 ALD growth cycles) are significantly improved over uncoated NCM622 reference cathodes, even under moderate cycling conditions. Notably, the Al2O3 surface shell is preserved after cycling in full-cell configuration for 1400 cycles as revealed by advanced electron microscopy and elemental mapping. While there are no significant differences in terms of bulk lattice structure and transition-metal leaching among the coated and uncoated NCM622 materials, the surface of the latter is found to be corroded to a much greater extent. In particular, detachment of active material from the secondary particles and side reactions with the electrolyte appear to lower the electrochemical activity, thereby leading to accelerated capacity degradation.

Atomic Layer Deposited TiO₂ and Al₂O₃ Thin Films as Coatings for Aluminum Food Packaging Application.

Titanium dioxide (TiO2) and aluminum oxide (Al2O3) coatings have been investigated in a wide range of bio-applications due to their biodegradation and biocompatibility properties, that are key parameters for their use in the food packaging and biomedical devices fields. The present study evaluates and compares the electrochemical behavior of the non-coated, commercial resin-coated, TiO2-coated and Al2O3-coated aluminum in commercial beer electrolyte. For this, TiO2 and Al2O3 thin films were deposited on aluminum (Al) substrates using atomic layer deposition (ALD). The evaluation of the corrosion barrier layer properties was performed by linear sweep voltammetry (LSV) during 10 min and electrochemical impedance spectroscopy (EIS). In addition, profilometry, grazing incidence X-ray diffractometry (GIXRD), scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FT-IR) analyses were performed to investigate the physical and chemical properties of the pristine and / or corroded samples. TiO2 and Al2O3 films presented an amorphous structure, a morphology that follows Al substrate surface, and a thickness of around 100 nm. Analysis of LSV data showed that ALD coatings promoted a considerable increase in corrosion barrier efficiency being 86.3% for TiO2-coated Al and 80% for Al2O3-coated Al in comparison with 7.1% of commercial resin-coated Al. This is mainly due to the lower electrochemical porosity, 11.4% for TiO2-coated Al and 20.4% for Al2O3-coated Al in comparison with 96% of the resin-coated Al, i.e. an increase of up to twofold in the protection of Al when coated with TiO2 compared to Al2O3. The EIS results allow us to complement the discussions about the reduced corrosion barrier efficiency of the Al2O3 film for beer electrolyte once SEM and FT-IR analyzes did not show drastic changes in both investigated ALD films after the corrosion assays. The above results indicate that ALD TiO2 and Al2O3 films may be a viable alternative to replace the synthetic resin coatings frequently used in aluminum cans of use in the food industry.

Preparation and Characterization of Microsphere ZnO ALD Coating Dedicated for the Fiber-Optic Refractive Index Sensor.

We report the fabrication of a novel fiber-optic sensor device, based on the use of a microsphere conformally coated with a thin layer of zinc oxide (ZnO) by atomic layer deposition (ALD), and its use as a refractive index sensor. The microsphere was prepared on the tip of a single-mode optical fiber, on which a conformal ZnO thin film of 200 nm was deposited using an ALD process based on diethyl zinc (DEZ) and water at 100 °C. The modified fiber-optic microsphere was examined using scanning electron microscopy and Raman spectroscopy. Theoretical modeling has been carried out to assess the structure performance, and the performed experimental measurements carried out confirmed the enhanced sensing abilities when the microsphere was coated with a ZnO layer. The fabricated refractive index sensor was operating in a reflective mode of a Fabry–Pérot configuration, using a low coherent measurement system. The application of the ALD ZnO coating enabled for a better measurement of the refractive index of samples in the range of the refractive index allowed by the optical fiber. The proof-of-concept results presented in this work open prospects for the sensing community and will promote the use of fiber-optic sensing technologies.

TiO2 ALD Coating of Amorphous TiO2 Nanotube Layers: Inhibition of the Structural and Morphological Changes Due to Water Annealing.

The present work presents a strategy to stabilize amorphous anodic self-organized TiO2 nanotube layers against morphological changes and crystallization upon extensive water soaking. The growth of needle-like nanoparticles was observed on the outer and inner walls of amorphous nanotube layers after extensive water soakings, in line with the literature on water annealing. In contrary, when TiO2 nanotube layers uniformly coated by thin TiO2 using atomic layer deposition (ALD) were soaked in water, the growth rates of needle-like nanoparticles were substantially reduced. We investigated the soaking effects of ALD TiO2 coatings with different thicknesses and deposition temperatures. Sufficiently thick TiO2 coatings (≈8.4 nm) deposited at different ALD process temperatures efficiently hamper the reactions between water and F− ions, maintain the amorphous state, and preserve the original tubular morphology. This work demonstrates the possibility of having robust amorphous 1D TiO2 nanotube layers that are very stable in water. This is very practical for diverse biomedical applications that are accompanied by extensive contact with an aqueous environment.