Sequential Infiltration Synthesis of Silicon Dioxide in Polymers with Ester Groups-Insight from In Situ Infrared Spectroscopy

New strategies to synthesize nanometer-scale silicon dioxide (SiO2) patterns have drawn much attention in applications such as microelectronic and optoelectronic devices, membranes, and sensors, as we are approaching device dimensions shrinking below 10 nm. In this regard, sequential infiltration synthesis (SIS), a two-step gas-phase molecular assembly process that enables localized inorganic material growth in the targeted reactive domains of polymers, is an attractive process. In this work, we performed in situ Fourier transform infrared spectroscopy (FTIR) measurements during SiO2 SIS to investigate the reaction mechanism of trimethylaluminum (TMA) and tri(tert-pentoxy) silanol (TPS) precursors with polymers having ester functional groups (poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), polycaprolactone (PCL), and poly(t-butyl methacrylate) (PBMA)), for the purpose of growing patterned nanomaterials. The FTIR results show that for PMMA and PEMA, a lower percentage of functional groups participated in the reactions and formed weak and unstable complexes. In contrast, almost all functional groups in PCL and PBMA participated in the reactions and showed stable and irreversible interactions with TMA. We discovered that the amount of SiO2 formed is not directly correlated with the number of interacting functional groups. These insights into the SiO2 SIS mechanism will enable nanopatterning of SiO2 for low-dimensional applications.

Protein-activated atomic layer deposition for robust crude-oil-repellent hierarchical nano-armored membranes

Atomic layer deposition (ALD) offers unique capabilities to fabricate atomically engineered porous materials with precise pore tuning and multi-functionalization for diverse applications like advanced membrane separations towards sustainable energy-water systems. However, current ALD technique is inhibited on most non-polar polymeric membranes due to lack of accessible nucleation sites. Here, we report a facile method to efficiently promote ALD coating on hydrophobic surface of polymeric membranes via novel protein activation/sensitization. As a proof of concept, TiO2 ALD-coated membranes activated by bovine serum albumin exhibit remarkable superhydrophilicity, ultralow underwater crude oil adhesion, and robust tolerance to rigorous environments including acid, alkali, saline, and ethanol. Most importantly, excellent cyclable crude oil-in-water emulsion separation performance can be achieved. The mechanism for activation/sensitization is rooted in reactivity for a particular set of amino acids. Furthermore, the universality of protein-sensitized ALD is demonstrated using common egg white, promising numerous potential usages in biomedical engineering, environmental remediation, low-carbon manufacturing, catalysis, and beyond.

Vibrational Probe at the Electrochemical Interface: Dependence on Plasmon Coupling and Potential of the Lineshape in Two-Dimensional Infrared Spectroscopy

Two-dimensional infrared spectroscopy of vibrational probes at an electrode surface shows promise for studying the structural dynamics at an active electrochemical interface. This interface is a complex environment where the solution structures in response to the applied potential. A strategy for achieving the necessary monolayer sensitivity is to use a plasmonically active electrode, which enhances the electromagnetic fields that produce the spectroscopic response. Here, we show how the coupling between the plasmon and the vibrations of the molecular monolayer impacts the FTIR and 2D IR spectroscopy, with an emphasis on the electrochemical potential difference spectra. We show how mixing between the vibrational and plasmonic states gives rise to the distortions that are observed in these measurements. This provides an important step toward 2D IR measurements of vibrational probes at the electrochemical interface as a tool for probing the structural dynamics in the double layer.

Montmorillonite Membranes with Tunable Ion Transport by Controlling Interlayer Spacing

Membranes incorporating two-dimensional (2D) materials have shown great potential for water purification and energy storage and conversion applications. Their ordered interlayer galleries can be modified for their tunable chemical and structural properties. Montmorillonite (MMT) is an earth-abundant phyllosilicate mineral that can be exfoliated into 2D flakes and reassembled into membranes. However, the poor water stability and random interlayer spacing of MMT caused by weak interlamellar interactions pose challenges for practical membrane applications. Herein, we demonstrate a facile approach to fabricating 2D MMT membranes with alkanediamines as cross-linkers. The incorporation of diamine molecules of different lengths enables controllable interlayer spacing and strengthens interlamellar connections, leading to tunable ion transport properties and boosted membrane stability in aqueous environments.

Ceramic thin-film composite membranes with tunable subnanometer pores for molecular sieving

Ceramic membranes are a promising alternative to polymeric membranes for selective separations, given their ability to operate under harsh chemical conditions. However, current fabrication technologies fail to construct ceramic membranes suitable for selective molecular separations. Herein, we demonstrate a molecular-level design of ceramic thin-film composite membranes with tunable subnanometer pores for precise molecular sieving. Through burning off the distributed carbonaceous species of varied dimensions within hybrid aluminum oxide films, we created membranes with tunable molecular sieving. Specifically, the membranes created with methanol showed exceptional selectivity toward monovalent and divalent salts. We attribute this observed selectivity to the dehydration of the large divalent ions within the subnanometer pores. As a comparison, smaller monovalent ions can rapidly permeate with an intact hydration shell. Lastly, the flux of neutral solutes through each fabricated aluminum oxide membrane was measured for the demonstration of tunable separation capability. Overall, our work provides the scientific basis for the design of ceramic membranes with subnanometer pores for molecular sieving using atomic layer deposition.

Multifunctional Coatings on Sulfide-Based Solid Electrolyte Powders with Enhanced Processability, Stability, and Performance for Solid-State Batteries

Sulfide-based solid-state electrolytes (SSEs) exhibit many tantalizing properties including high ionic conductivity and favorable mechanical properties for next-generation solid-state batteries. Widespread adoption of these materials is hindered by their intrinsic instability under ambient conditions, which makes them difficult to process at scale, and instability at the Li||SSE and cathode||SSE interfaces, which limits cell performance and lifetime. Atomic layer deposition is leveraged to grow thin Al₂ O₃ coatings on Li₆ PS₅ Cl powders to address both issues simultaneously. These coatings can be directly grown onto Li₆ PS₅ Cl particles with negligible chemical modification of the underlying material and enable exposure of powders to pure and H₂ O-saturated oxygen environments for ≥4 h with minimal reactivity, compared with significant degradation of the uncoated powder. Pellets fabricated from coated powders exhibit ionic conductivities up to 2× higher than those made from uncoated material, with a simultaneous decrease in electronic conductivity and significant suppression of chemical reactivity at the Li-SSE interface. These benefits result in significantly improved room temperature cycle life at high capacity and current density. It is hypothesized that this enhanced performance derives from improved intergranular properties and improved Li metal adhesion. This work points to a completely new framework for designing active, stable, and scalable materials for next-generation solid-state batteries.

Thermal Atomic Layer Etching of MoS2 Using MoF6 and H2O

Two-dimensional (2D) layered materials offer unique properties that make them attractive for continued scaling in electronic and optoelectronic device applications. Successful integration of 2D materials into semiconductor manufacturing requires high-volume and high-precision processes for deposition and etching. Several promising large-scale deposition approaches have been reported for a range of 2D materials, but fewer studies have reported removal processes. Thermal atomic layer etching (ALE) is a scalable processing technique that offers precise control over isotropic material removal. In this work, we report a thermal ALE process for molybdenum disulfide (MoS₂). We show that MoF₆ can be used as a fluorination source, which, when combined with alternating exposures of H₂O, etches both amorphous and crystalline MoS₂ films deposited by atomic layer deposition. To characterize the ALE process and understand the etching reaction mechanism, in situ quartz crystal microbalance (QCM), Fourier transform infrared (FTIR), and quadrupole mass spectrometry (QMS) experiments were performed. From temperature-dependent in situ QCM experiments, the mass change per cycle was -5.7 ng/cm² at 150 °C and reached -270.6 ng/cm² at 300 °C, nearly 50× greater. The temperature dependence followed Arrhenius behavior with an activation energy of 13 ± 1 kcal/mol. At 200 °C, QCM revealed a mass gain following exposure to MoF₆ and a net mass loss after exposure to H₂O. FTIR revealed the consumption of Mo-O species and formation of Mo-F and MoF _x =O species following exposures of MoF₆ and the reverse behavior following H₂O exposures. QMS measurements, combined with thermodynamic calculations, supported the removal of Mo and S through the formation of volatile MoF₂O₂ and H₂S byproducts. The proposed etching mechanism involves a two-stage oxidation of Mo through the ALE half-reactions. Etch rates of 0.5 Å/cycle for amorphous films and 0.2 Å/cycle for annealed films were measured by ex situ ellipsometry, X-ray reflectivity, and transmission electron microscopy. Precisely etching amorphous films and subsequently annealing them yielded crystalline, few-layer MoS₂ thin films. This thermal MoS₂ ALE process provides a new mechanism for fluorination-based ALE and offers a low-temperature approach for integrating amorphous and crystalline 2D MoS₂ films into high-volume device manufacturing with tight thermal budgets.

Tunable Ion Transport with Freestanding Vermiculite Membranes

Membranes integrating two-dimensional (2D) materials have emerged as a category with unusual ion transport and potentially useful separation applications in both aqueous and nonaqueous systems. The interlayer galleries in these membranes drive separation and selectivity, with specific transport properties determined by the chemical and structural modifications within the inherently different interlayers. Here we report an approach to tuning interlayer spacing with a single source material─exfoliated and restacked vermiculite with alkanediamine cross-linkers─to both control the gallery height and enhance the membrane stability. The as-prepared cross-linked 2D vermiculite membranes exhibit ion diffusivities tuned by the length of the selected diamine molecule. The 2D nanochannels in these stabilized vermiculite membranes enable a systematic study of confined ionic transport.

Tunable Solid Acid Catalyst Thin Films Prepared by Atomic Layer Deposition

Solid acid catalysts, including zeolites and amorphous silica-aluminas (ASAs), are industrially important materials widely used in the fuel and petrochemical industries. The versatility of zeolites is due to the Brønsted acidity of the bridging hydroxyl and shape selectivity that can be tailored during and after synthesis. This is in contrast to amorphous silica-alumina, where tailoring acidity is a major challenge as the Brønsted acid structure in ASA is still debated. In both cases, however, the pore size and acidity cannot be tuned independently, and this is particularly limiting in the application of biomass conversion, where zeolite pores are too small for the molecules of interest. Herein, we present a method using atomic layer deposition (ALD) to prepare thin films of solid acid materials where the ratio of Brønsted to Lewis acid sites can be tuned precisely. This capability, combined with the sub-nm pore size control afforded by ALD yields a powerful and flexible method for synthesizing solid acid catalysts inside virtually any mesoporous host. We demonstrate the utility of these materials in two acid-catalyzed reactions relevant to biomass conversion: (1) Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction and dehydration of fructose and (2) cascade reaction of glucose to 5-hydroxymethylfurfural. Finally, we propose a plausible structure for the Brønsted acid sites in our materials based on infrared spectroscopy and solid-state nuclear magnetic resonance measurements and density functional theory calculations and argue that this same structure might apply to conventional ASAs as well.

Atomic-Scale View of Redox Induced Changes for Monolayer MoOx on α-TiO2(110) with Chemical-State Sensitivity

Supported molybdenum oxide (MoOx) plays an important role in catalytic transformations from alcohol dehydrogenation to transesterification. During these reactions, molybdenum and oxygen surface species undergo structural and chemical changes. A detailed, chemical-state specific, atomic-scale structural analysis of the catalyst under redox conditions is important for improving catalytic properties. In this study, a monolayer of Mo grown on α-TiO2(110) by atomic-layer deposition is analyzed by X-ray standing wave (XSW) excited X-ray photoelectron spectroscopy (XPS). The chemical shifts for Mo 2p3/2 and O 1s peaks are used to distinguish Mo6+ from Mo4+ and surface O from bulk O. Excitation of XPS by XSW allows pinpointing the location of these surface species relative to the underlying substrate lattice. Measured 3D composite atomic density maps for the oxidized and reduced interfaces compare well with our density functional theory models and collectively create a unique view of the redox-driven dynamics for this complex catalytic structure.

Structural Changes during the Conversion Reaction of Tungsten Oxide Electrodes with Tailored, Mesoscale Porosity

In-plane tungsten oxide nanostructures, including hexagonally patterned cylinders and holes in a matrix, were fabricated via sequential infiltration synthesis (SIS) on self-assembled block copolymer templates. Using the tailored morphology and porosity of these model electrodes with in situ grazing incidence small-angle X-ray scattering, the intrinsic structural change of nanoscale active materials during the conversion reaction of WO₃ + 6Li ↔ W + 3Li₂O was investigated at controlled electrochemical conditions. Reversible electrode volume expansion and contraction was observed during lithiation and delithiation cycles, respectively. The potential where the electrode's thickness expansion started was ∼1.6 V, which is close to the thermodynamically expected one for the conversion reaction of WO₃ with lithium (1.65 V). The temporal evolution of the electrode volume at constant electrode potentials revealed high overpotential for bulk lithiation and slow conversion reaction kinetics, despite the tailored porosity of the SIS electrodes. Oxide cylinders showed a smaller overall electrode thickness change, likely due to unconstrained lateral volume change, as compared to a matrix with holes. On the other hand, better connectivity and guided volume change of the latter electrode morphology provided improved cycling stability. In addition, heterogeneity in an electrode, from internal pores and density gradients, was found to aggravate the fragmentation of the electrode during the conversion reaction. Insights into oxide conversion reaction kinetics and the relationship between electrode mesostructure and cycling behavior obtained from this study can help guide the more rational design of conversion electrodes for high-performing batteries.

Atomic-Scale Structure of Chemically Distinct Surface Oxygens in Redox Reactions

During redox reactions, oxide-supported catalytic systems undergo structural and chemical changes. Improving subsequent catalytic properties requires an understanding of the atomic-scale structure with chemical state specificity under reaction conditions. For the case of 1/2 monolayer vanadia on α-TiO₂(110), we use X-ray standing wave (XSW) excited X-ray photoelectron spectroscopy to follow the redox induced atomic positional and chemical state changes of this interface. While the resulting XSW 3D composite atomic maps include the Ti and O substrate atoms and V surface atoms, our focus in this report is on the previously unseen surface oxygen species with comparison to density functional theory predictions.

Surface Zeta Potential of ALD-Grown Metal-Oxide Films

Membranes are among the most promising technologies for energy-efficient and highly selective separations, and the surface-charge property of membranes plays a critical role in their broad applications. Atomic layer deposition (ALD) can deposit materials uniformly and with high precision and controllability on arbitrarily complex and large substrates, which renders it a promising method to tune the electrostatics of water/solid interfaces. However, a systematic study of surface-charge properties of ALD-grown films in aqueous environments is still lacking. In this work, 17 ALD-grown metal-oxide films are synthesized, and a comprehensive study of their water stability, wetting properties, and surface-charge properties is provided. This work represents a resource guide for researchers and ultimately for materials and process engineers, seeking to tailor interfacial charge properties of membranes and other porous water treatment components.

Advanced Materials for Energy-Water Systems: The Central Role of Water/Solid Interfaces in Adsorption, Reactivity, and Transport

The structure, chemistry, and charge of interfaces between materials and aqueous fluids play a central role in determining properties and performance of numerous water systems. Sensors, membranes, sorbents, and heterogeneous catalysts almost uniformly rely on specific interactions between their surfaces and components dissolved or suspended in the water-and often the water molecules themselves-to detect and mitigate contaminants. Deleterious processes in these systems such as fouling, scaling (inorganic deposits), and corrosion are also governed by interfacial phenomena. Despite the importance of these interfaces, much remains to be learned about their multiscale interactions. Developing a deeper understanding of the molecular- and mesoscale phenomena at water/solid interfaces will be essential to driving innovation to address grand challenges in supplying sufficient fit-for-purpose water in the future. In this Review, we examine the current state of knowledge surrounding adsorption, reactivity, and transport in several key classes of water/solid interfaces, drawing on a synergistic combination of theory, simulation, and experiments, and provide an outlook for prioritizing strategic research directions.

Intelligent Agents for the Optimization of Atomic Layer Deposition

Atomic layer deposition (ALD) is a highly controllable thin film synthesis approach with applications in computing, energy, and separations. The flexibility of ALD means that it can access a massive chemical catalogue; however, this chemical and process diversity results in significant challenges in determining processing parameters that result in stable and uniform film growth with minimal precursor consumption. In situ measurements of the ALD growth per cycle (GPC) can accelerate process development but it still requires expert intuition and time-consuming trial and error to identify acceptable processing parameters. This procedure is made more difficult by the presence of experimental noise in the GPC values and the complexity of ALD surface chemistries. A need exists for efficient optimization approaches capable of autonomously determining processing conditions resulting in optimal ALD film growth. In this work, we present the development of three optimization strategies and compare their performance in optimizing four simulated ALD processes. Furthermore, the effect of noise in the GPC measurements on optimization convergence is studied.

Understanding KOtBu in atomic layer deposition - in situ mechanistic studies of the KNbO3 growth process

Functional coatings based on alkali metals have become increasingly attractive in the current shift towards sustainable technologies. While lithium-based compounds have a natural impact on batteries, other alkali metal compounds are important as replacements for toxic materials in a range of electronic devices. This is especially true for potassium, being a major component in e.g. KxNa1-xNbO3 (KNN) and KTaxNb1-xO3 (KTN), with hope to replace Pb(ZrxTi1-x)O3 (PZT) in piezo-/ferroelectric and electrooptic devices. ALD facilitates functional conformal coatings at deposition temperatures far below what is reported using other techniques and with excellent compositional control. The ALD growth of potassium-containing films using KOtBu has, however, been unpredictable. Untraditional response to the pulse composition and precursor dose, severe reproducibility issues, and very high growth per cycle are some of the puzzling features of these processes. In this article, we shed light on the growth behavior of KOtBu in ALD by in situ quartz crystal microbalance and Fourier transform infrared spectroscopy studies. We study the precursor's behavior in the technologically interesting KNbO3-process, showing how the potassium precursor strongly affects the growth of other cation precursors. We show that the strong hygroscopic nature of the intermediary potassium species has far-reaching implications throughout the growth. This helps not only to enhance the understanding of alkali metal containing compounds' growth in ALD, but also to provide the means to control the growth of novel sustainable technological materials.

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 (AlO_x) formed by this chemistry. This lack of understanding hinders our ability to establish process-structure-property relationships and ultimately limits technological advancements employing AlO_x 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 AlO_x ALD coatings. We combine ex situ and in operando HE-XRD measurements on ALD AlO_x 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 AlO_x 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.

Synergetic effect on catalytic activity and charge transfer in Pt-Pd bimetallic model catalysts prepared by atomic layer deposition

Pt-Pd bimetallic nanoparticles were synthesized on TiO₂ support on the planar substrate as well as on high surface area SiO₂ gel by atomic layer deposition to identify the catalytic performance improvement after the formation of Pt-Pd bimetallic nanoparticles by surface analysis techniques. From X-ray absorption near edge spectra of Pt-Pd bimetallic nanoparticles, d-orbital hybridization between Pt 5d and Pd 4d was observed, which is responsible for charge transfer from Pt to Pd. Moreover, it was found from the in situ grazing incidence X-ray absorption spectroscopy study that Pt-Pd nanoparticles have a Pd shell/Pt core structure with CO adsorption. Resonant photoemission spectroscopy on Pt-Pd bimetallic nanoparticles showed that Pd resonant intensity is enhanced compared to that of Pd monometallic nanoparticles because of d-orbital hybridization and electronic states broadening of Pt and Pd compared monometallic catalysts, which results in catalytic performance improvement.

Enrichment and Distribution of Pb2+ Ions in Zwitterionic Poly(cysteine methacrylate) Brushes at the Solid-Liquid Interface

Cysteine-based polyzwitterionic brushes have been prepared via a two-step route. First, poly(allyl methacrylate) (PAMA) brushes have been grown from the surface of silicon substrates using surface-initiated atom transfer radical polymerization. The obtained PAMA brushes with free pendant vinyl groups were further modified via radical thiol-ene addition reaction to attach l-cysteine moieties. Surface ζ potential investigations on pH-responsiveness of these poly(cysteine methacrylate) (PCysMA) brushes confirm their zwitterionic character at intermediate pH range, while at pH values either below pH 3.50 or above pH 8.59, they exhibit polyelectrolyte character. Under acid (pH < 3.50) or base (pH > 8.59) conditions, they possess either cationic or anionic character, respectively. In the zwitterionic region, these PCysMA brushes show positive surface ζ potential in the presence of Pb(CH₃COO)₂ solutions of various concentrations. The results are in line with microscopic investigations using anomalous X-ray reflectivity (AXRR) carried out along the absorption edge of Pb²⁺ ions. When the photon energies were varied around the absorption L3 edge of lead (13037 eV), the Pb²⁺ concentration normal to the silicon substrates, as a function of depth inside PCysMA brushes, could be revealed at the nanoscale. Both ζ potential and AXRR measurements confirm the enrichment of Pb²⁺ ions inside PCysMA brushes, indicating the potential of PCysMA to be used as a water purification material.

The chemical physics of sequential infiltration synthesis-A thermodynamic and kinetic perspective

Sequential infiltration synthesis (SIS) is an emerging materials growth method by which inorganic metal oxides are nucleated and grown within the free volume of polymers in association with chemical functional groups in the polymer. SIS enables the growth of novel polymer-inorganic hybrid materials, porous inorganic materials, and spatially templated nanoscale devices of relevance to a host of technological applications. Although SIS borrows from the precursors and equipment of atomic layer deposition (ALD), the chemistry and physics of SIS differ in important ways. These differences arise from the permeable three-dimensional distribution of functional groups in polymers in SIS, which contrast to the typically impermeable two-dimensional distribution of active sites on solid surfaces in ALD. In SIS, metal-organic vapor-phase precursors dissolve and diffuse into polymers and interact with these functional groups through reversible complex formation and/or irreversible chemical reactions. In this perspective, we describe the thermodynamics and kinetics of SIS and attempt to disentangle the tightly coupled physical and chemical processes that underlie this method. We discuss the various experimental, computational, and theoretical efforts that provide insight into SIS mechanisms and identify approaches that may fill out current gaps in knowledge and expand the utilization of SIS.

Crude-Oil-Repellent Membranes by Atomic Layer Deposition: Oxide Interface Engineering

Crude oil fouling on membrane surfaces is a persistent, crippling challenge in oil spill remediation and oilfield wastewater treatment. In this research, we present how a nanosized oxide coating can profoundly affect the anti-crude-oil property of membrane materials. Select oxide coatings with a thickness of ∼10 nm are deposited conformally on common polymer membrane surfaces by atomic layer deposition to significantly mitigate fouling during filtration processes. TiO₂- and SnO₂-coated membranes exhibited far greater anti-crude-oil performance than ZnO- and Al₂O₃-coated ones. Tightly bound hydration layers play a crucial role in protecting the surface from crude oil adhesion, as revealed by molecular dynamics simulations. This work provides a facile strategy to fabricate crude-oil-resistant membranes with negligible impact on membrane structure, and also demonstrates that, contrary to common belief, excellent crude oil resistance can be achieved easily without implementation of sophisticated, hierarchical structures.

Novel ALD Chemistry Enabled Low-Temperature Synthesis of Lithium Fluoride Coatings for Durable Lithium Anodes

Lithium metal anodes can largely enhance the energy density of rechargeable batteries because of the high theoretical capacity and the high negative potential. However, the problem of lithium dendrite formation and low Coulombic efficiency (CE) during electrochemical cycling must be solved before lithium anodes can be widely deployed. Herein, a new atomic layer deposition (ALD) chemistry to realize the low-temperature synthesis of homogeneous and stoichiometric lithium fluoride (LiF) is reported, which then for the first time, as far as we know, is deposited directly onto lithium metal. The LiF preparation is performed at 150 °C yielding 0.8 Å/cycle. The LiF films are found to be crystalline, highly conformal, and stoichiometric with purity levels >99%. Nanoindentation measurements demonstrate the LiF achieving a shear modulus of 58 GPa, 7 times higher than the sufficient value to resist lithium dendrites. When used as the protective coating on lithium, it enables a stable Coulombic efficiency as high as 99.5% for over 170 cycles, about 4 times longer than that of bare lithium anodes. The remarkable battery performance is attributed to the nanosized LiF that serves two critical functions simultaneously: (1) the high dielectric value creates a uniform current distribution for excellent lithium stripping/plating and ultrahigh mechanical strength to suppress lithium dendrites; (2) the great stability and electrolyte isolation by the pure LiF on lithium prevents parasitic reactions for a much improved CE. This new ALD chemistry for conformal LiF not only offers a promising avenue to implement lithium metal anodes for high-capacity batteries but also paves the way for future studies to investigate failure and evolution mechanisms of solid electrolyte interphase (SEI) using our LiF on anodes such as graphite, silicon, and lithium.

Directly Formed Alucone on Lithium Metal for High-Performance Li Batteries and Li-S Batteries with High Sulfur Mass Loading

Lithium metal is considered the "holy grail" of next-generation battery anodes. However, severe parasitic reactions at the lithium-electrolyte interface deplete the liquid electrolyte and the uncontrolled formation of high surface area and dendritic lithium during cycling causes rapid capacity fading and battery failure. Engineering a dendrite-free lithium metal anode is therefore critical for the development of long-life batteries using lithium anodes. In this study, we deposit a conformal, organic/inorganic hybrid coating, for the first time, directly on lithium metal using molecular layer deposition (MLD) to alleviate these problems. This hybrid organic/inorganic film with high cross-linking structure can stabilize lithium against dendrite growth and minimize side reactions, as indicated by scanning electron microscopy. We discovered that the alucone coating yielded several times longer cycle life at high current rates compared to the uncoated lithium and achieved a steady Coulombic efficiency of 99.5%, demonstrating that the highly cross-linking structured material with great mechanical properties and good flexibility can effectively suppress dendrite formation. The protected Li was further evaluated in lithium-sulfur (Li-S) batteries with a high sulfur mass loading of ∼5 mg/cm². After 140 cycles at a high current rate of ∼1 mA/cm², alucone-coated Li-S batteries delivered a capacity of 657.7 mAh/g, 39.5% better than that of a bare lithium-sulfur battery. These findings suggest that flexible coating with high cross-linking structure by MLD is effective to enable lithium protection and offers a very promising avenue for improved performance in the real applications of Li-S batteries.

Mechanistic understanding of tungsten oxide in-plane nanostructure growth via sequential infiltration synthesis

Tungsten oxide (WO_3-x) nanostructures with hexagonal in-plane arrangements were fabricated by sequential infiltration synthesis (SIS), using the selective interaction of gas phase precursors with functional groups in one domain of a block copolymer (BCP) self-assembled template. Such structures are highly desirable for various practical applications and as model systems for fundamental studies. The nanostructures were characterized by cross-sectional scanning electron microscopy, grazing-incidence small/wide-angle X-ray scattering (GISAXS/GIWAXS), and X-ray absorption near edge structure (XANES) measurements at each stage during the SIS process and subsequent thermal treatments, to provide a comprehensive picture of their evolution in morphology, crystallography and electronic structure. In particular, we discuss the critical role of SIS Al₂O₃ seeds toward modifying the chemical affinity and free volume in a polymer for subsequent infiltration of gas phase precursors. The insights into SIS growth obtained from this study are valuable to the design and fabrication of a wide range of targeted nanostructures.

Lithium Self-Discharge and Its Prevention: Direct Visualization through In Situ Electrochemical Scanning Transmission Electron Microscopy

To understand the mechanism that controls low-aspect-ratio lithium deposition morphologies for Li-metal anodes in batteries, we conducted direct visualization of Li-metal deposition and stripping behavior through nanoscale in situ electrochemical scanning transmission electron microscopy (EC-STEM) and macroscale-cell electrochemistry experiments in a recently developed and promising solvate electrolyte, 4 M lithium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane. In contrast to published coin cell studies in the same electrolyte, our experiments revealed low Coulombic efficiencies and inhomogeneous Li morphology during in situ observation. We conclude that this discrepancy in Coulombic efficiency and morphology of the Li deposits was dependent on the presence of a compressed lithium separator interface, as we have confirmed through macroscale (not in the transmission electron microscope) electrochemical experiments. Our data suggests that cell compression changed how the solid-electrolyte interphase formed, which is likely responsible for improved morphology and Coulombic efficiency with compression. Furthermore, during the in situ EC-STEM experiments, we observed direct evidence of nanoscale self-discharge in the solvate electrolyte (in the state of electrical isolation). This self-discharge was duplicated in the macroscale, but it was less severe with electrode compression, likely due to a more passivating and corrosion-resistant solid-electrolyte interphase formed in the presence of compression. By combining the solvate electrolyte with a protective LiAl_0.3S coating, we show that the Li nucleation density increased during deposition, leading to improved morphological uniformity. Furthermore, self-discharge was suppressed during rest periods in the cycling profile with coatings present, as evidenced through EC-STEM and confirmed with coin cell data.

Effect of Nanostructured Domains in Self-Assembled Block Copolymer Films on Sequential Infiltration Synthesis

There are broad interests in selective and localized synthesis in nanodomains of self-assembled block copolymers (BCPs) for a variety of applications. Sequential infiltration synthesis (SIS) shows promise to selectively grow a controllable amount of materials in one type of nanodomain of a self-assembled BCP film. However, the effects of nanostructured domains in a BCP film and SIS cycles on the material growth behavior of SIS are rarely studied. In this work, we investigated the growth behavior of TiO₂ SIS within self-assembled polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) films and the two corresponding pure homopolymer films (PS and PMMA) by using in situ quartz crystal microbalance (QCM). According to the experimental results, reactant purge steps are essential to enable a high selectivity of SIS in PMMA nanodomains in the BCP films by eliminating the undesired homogeneous reactions. The continuous PS nanodomain acts as the main channel in transporting reactants to PMMA nanodomains in the self-assembled PS-b-PMMA BCP films. The segregated nanoscale PMMA nanodomains in the BCP films show dramatically different TiCl₄ diffusion/reaction behavior than a continuous PMMA film. The mass gain per SIS cycle within PMMA nanodomains decreases quickly with increasing cycle number. After 7 TiO₂ SIS cycles, TiO₂ SIS can only take place at the interface between PS and PMMA nanodomains in the BCP film. The TiO₂ SIS process can uniformly modify PMMA nanodomains throughout a self-assembled PS-b-PMMA film up to the diffusion depth owing to the unique nanostructure-enabled diffusion. SIS cycle number and chemistry of a BCP will strongly affect the material growth behavior of a SIS chemistry on the BCP film and, therefore, the final morphology of the resulting nanomaterial. Detailed studies are warranted for a SIS process on a self-assembled BCP film of different chemistry.

Atomic Layer Deposition of Al-W-Fluoride on LiCoO2 Cathodes: Comparison of Particle- and Electrode-Level Coatings

Atomic layer deposition (ALD) of the well-known Al₂O₃ on a LiCoO₂ system is compared with that of a newly developed AlW _x F _y material. ALD coatings (∼1 nm thick) of both materials are shown to be effective in improving cycle life through mitigation of surface-induced capacity losses. However, the behaviors of Al₂O₃ and AlW _x F _y are shown to be significantly different when coated directly on cathode particles versus deposition on a composite electrode composed of active materials, carbons, and binders. Electrochemical impedance spectroscopy, galvanostatic intermittent titration techniques, and four-point measurements suggest that electron transport is more limited in LiCoO₂ particles coated with Al₂O₃ compared with that in particles coated with AlW _x F _y . The results show that proper design/choice of coating materials (e.g., AlW _x F _y ) can improve capacity retention without sacrificing electron transport and suggest new avenues for engineering electrode-electrolyte interfaces to enable high-voltage operation of lithium-ion batteries.

Sequential Infiltration Synthesis for the Design of Low Refractive Index Surface Coatings with Controllable Thickness

Control over refractive index and thickness of surface coatings is central to the design of low refraction films used in applications ranging from optical computing to antireflective coatings. Here, we introduce gas-phase sequential infiltration synthesis (SIS) as a robust, powerful, and efficient approach to deposit conformal coatings with very low refractive indices. We demonstrate that the refractive indices of inorganic coatings can be efficiently tuned by the number of cycles used in the SIS process, composition, and selective swelling of the of the polymer template. We show that the refractive index of Al₂O₃ can be lowered from 1.76 down to 1.1 using this method. The thickness of the Al₂O₃ coating can be efficiently controlled by the swelling of the block copolymer template in ethanol at elevated temperature, thereby enabling deposition of both single-layer and graded-index broadband antireflective coatings. Using this technique, Fresnel reflections of glass can be reduced to as low as 0.1% under normal illumination over a broad spectral range.

Water Oxidation by Size-Selected Co27 Clusters Supported on Fe2 O3

The complexity of the water oxidation reaction makes understanding the role of individual catalytic sites critical to improving the process. Here, size-selected 27-atom cobalt clusters (Co₂₇ ) deposited on hematite (Fe₂ O₃ ) anodes were tested for water oxidation activity. The uniformity of these anodes allows measurement of the activity of catalytic sites of well-defined nuclearity and known density. Grazing incidence X-ray absorption near-edge spectroscopy (GIXANES) characterization of the anodes before and after electrochemical cycling demonstrates that these Co₂₇ clusters are stable to dissolution even in the harsh water oxidation electrochemical environment. They are also stable under illumination at the equivalent of 0.4 suns irradiation. The clusters show turnover rates for water oxidation that are comparable or higher than those reported for Pd- and Co-based materials or for hematite. The support for the Co₂₇ clusters is Fe₂ O₃ grown by atomic layer deposition on a Si chip. We have chosen to deposit a Fe₂ O₃ layer that is only a few unit cells thick (2 nm), to remove complications related to exciton diffusion. We find that the electrocatalytic and the photoelectrocatalytic activity of the Co₂₇ /Fe₂ O₃ material is significantly improved when the samples are annealed (with the clusters already deposited). Given that the support is thin and that the cluster deposition density is equivalent to approximately 5 % of an atomic monolayer, we suggest that annealing may significantly improve the exciton diffusion from the support to the catalytic moiety.

Towards ALD thin film stabilized single-atom Pd1 catalysts

Supported precious metal single-atom catalysts have shown interesting activity and selectivity in recent studies. However, agglomeration of these highly mobile mononuclear surface species can eliminate their unique catalytic properties. Here we study a strategy for synthesizing thin film stabilized single-atom Pd1 catalysts using atomic layer deposition (ALD). The thermal stability of the Pd1 catalysts is significantly enhanced by creating a nanocavity thin film structure. In situ infrared spectroscopy and Pd K-edge X-ray absorption spectroscopy (XAS) revealed that the Pd1 was anchored on the surface through chlorine sites. The thin film stabilized Pd1 catalysts were thermally stable under both oxidation and reduction conditions. The catalytic performance in the methanol decomposition reaction is found to depend on the thickness of protecting layers. While Pd1 catalysts showed promising activity at low temperature in a methanol decomposition reaction, 14 cycle TiO2 protected Pd1 was less active at high temperature. Pd L3 edge XAS indicated that the low reactivity compared with Pd nanoparticles is due to the strong adsorption of carbon monoxide even at 250 °C. These results clearly show that the ALD nanocavities provide a basis for future design of single-atom catalysts that are highly efficient and stable.

High Thermal Stability of La2O3- and CeO2-Stabilized Tetragonal ZrO2

Catalyst support materials of tetragonal ZrO2, stabilized by either La2O3 (La2O3-ZrO2) or CeO2 (CeO2-ZrO2), were synthesized under hydrothermal conditions at 200 °C with NH4OH or tetramethylammonium hydroxide as the mineralizer. From in situ synchrotron powder X-ray diffraction and small-angle X-ray scattering measurements, the calcined La2O3-ZrO2 and CeO2-ZrO2 supports were nonporous nanocrystallites that exhibited rectangular shapes with a thermal stability of up to 1000 °C in air. These supports had an average size of ∼ 10 nm and a surface area of 59-97 m(2)/g. The catalysts Pt/La2O3-ZrO2 and Pt/CeO2-ZrO2 were prepared by using atomic layer deposition with varying Pt loadings from 6.3 to 12.4 wt %. Monodispersed Pt nanoparticles of ∼ 3 nm were obtained for these catalysts. The incorporation of La2O3 and CeO2 into the t-ZrO2 structure did not affect the nature of the active sites for the Pt/ZrO2 catalysts for the water-gas shift reaction.

Atomic Layer Deposition of MnS: Phase Control and Electrochemical Applications

Manganese sulfide (MnS) thin films were synthesized via atomic layer deposition (ALD) using gaseous manganese bis(ethylcyclopentadienyl) and hydrogen sulfide as precursors. At deposition temperatures ≤150 °C phase-pure γ-MnS thin films were deposited, while at temperatures >150 °C, a mixed phase consisting of both γ- and α-MnS resulted. In situ quartz crystal microbalance (QCM) studies validate the self-limiting behavior of both ALD half-reactions and, combined with quadrupole mass spectrometry (QMS), allow the derivation of a self-consistent reaction mechanism. Finally, MnS thin films were deposited on copper foil and tested as a Li-ion battery anode. The MnS coin cells showed exceptional cycle stability and near-theoretical capacity.

Design and synthesis of model and practical palladium catalysts using atomic layer deposition

We investigated the “one-batch” synthesis of model and practical palladium catalysts using atomic layer deposition (ALD).

A modular reactor design for in situ synchrotron x-ray investigation of atomic layer deposition processes

Synchrotron characterization techniques provide some of the most powerful tools for the study of film structure and chemistry. The brilliance and tunability of the Advanced Photon Source allow access to scattering and spectroscopic techniques unavailable with in-house laboratory setups and provide the opportunity to probe various atomic layer deposition (ALD) processes in situ starting at the very first deposition cycle. Here, we present the design and implementation of a portable ALD instrument which possesses a modular reactor scheme that enables simple experimental switchover between various beamlines and characterization techniques. As first examples, we present in situ results for (1) X-ray surface scattering and reflectivity measurements of epitaxial ZnO ALD on sapphire, (2) grazing-incidence small angle scattering of MnO nucleation on silicon, and (3) grazing-incidence X-ray absorption spectroscopy of nucleation-regime Er2O3 ALD on amorphous ALD alumina and single crystalline sapphire.

New Insights into Sequential Infiltration Synthesis

Sequential infiltration synthesis (SIS) is a process derived from ALD in which a polymer is infused with inorganic material using sequential, self-limiting exposures to gaseous precursors. SIS can be used in lithography to harden polymer resists rendering them more robust towards subsequent etching, and this permits deeper and higher-resolution patterning of substrates such as silicon. Herein we describe recent investigations of a model system: Al₂O₃ SIS using trimethyl aluminum (TMA) and H₂O within the diblock copolymer, poly(styrene-block-methyl methacrylate) (PS-b-PMMA). Combining in-situ Fourier transform infrared absorption spectroscopy, quartz-crystal microbalance, and synchrotron grazing incidence small angle X-ray scattering with high resolution scanning transmission electron microscope tomography, we elucidate important details of the SIS process: 1) TMA adsorption in PMMA occurs through a weakly-bound intermediate; 2) the SIS kinetics are diffusion-limited, with desorption 10× slower than adsorption; 3) dynamic structural changes occur during the individual precursor exposures. These findings have important implications for applications such as SIS lithography.

Towards a microchannel-based X-ray detector with two-dimensional spatial and time resolution and high dynamic range

X-ray detectors that combine two-dimensional spatial resolution with a high time resolution are needed in numerous applications of synchrotron radiation. Most detectors with this combination of capabilities are based on semiconductor technology and are therefore limited in size. Furthermore, the time resolution is often realised through rapid time-gating of the acquisition, followed by a slower readout. Here, a detector technology is realised based on relatively inexpensive microchannel plates that uses GHz waveform sampling for a millimeter-scale spatial resolution and better than 100 ps time resolution. The technology is capable of continuous streaming of time- and location-tagged events at rates greater than 10(7) events per cm(2). Time-gating can be used for improved dynamic range.

Characterizing the Three-Dimensional Structure of Block Copolymers via Sequential Infiltration Synthesis and Scanning Transmission Electron Tomography

Understanding and controlling the three-dimensional structure of block copolymer (BCP) thin films is critical for utilizing these materials for sub-20 nm nanopatterning in semiconductor devices, as well as in membranes and solar cell applications. Combining an atomic layer deposition (ALD)-based technique for enhancing the contrast of BCPs in transmission electron microscopy (TEM) together with scanning TEM (STEM) tomography reveals and characterizes the three-dimensional structures of poly(styrene-block-methyl methacrylate) (PS-b-PMMA) thin films with great clarity. Sequential infiltration synthesis (SIS), a block-selective technique for growing inorganic materials in BCPs films in an ALD tool and an emerging technique for enhancing the etch contrast of BCPs, was harnessed to significantly enhance the high-angle scattering from the polar domains of BCP films in the TEM. The power of combining SIS and STEM tomography for three-dimensional (3D) characterization of BCP films was demonstrated with the following cases: self-assembled cylindrical, lamellar, and spherical PS-b-PMMA thin films. In all cases, STEM tomography has revealed 3D structures that were hidden underneath the surface, including (1) the 3D structure of defects in cylindrical and lamellar phases, (2) the nonperpendicular 3D surface of grain boundaries in the cylindrical phase, and (3) the 3D arrangement of spheres in body-centered-cubic (BCC) and hexagonal-closed-pack (HCP) morphologies in the spherical phase. The 3D data of the spherical morphologies was compared to coarse-grained simulations and assisted in validating the simulations' parameters. STEM tomography of SIS-treated BCP films enables the characterization of the exact structure used for pattern transfer and can lead to a better understating of the physics that is utilized in BCP lithography.

Atomic layer deposition of metal sulfide materials

CONSPECTUS: The field of nanoscience is delivering increasingly intricate yet elegant geometric structures incorporating an ever-expanding palette of materials. Atomic layer deposition (ALD) is a powerful driver of this field, providing exceptionally conformal coatings spanning the periodic table and atomic-scale precision independent of substrate geometry. This versatility is intrinsic to ALD and results from sequential and self-limiting surface reactions. This characteristic facilitates digital synthesis, in which the film grows linearly with the number of reaction cycles. While the majority of ALD processes identified to date produce metal oxides, novel applications in areas such as energy storage, catalysis, and nanophotonics are motivating interest in sulfide materials. Recent progress in ALD of sulfides has expanded the diversity of accessible materials as well as a more complete understanding of the unique chalcogenide surface chemistry. ALD of sulfide materials typically uses metalorganic precursors and hydrogen sulfide (H2S). As in oxide ALD, the precursor chemistry is critical to controlling both the film growth and properties including roughness, crystallinity, and impurity levels. By modification of the precursor sequence, multicomponent sulfides have been deposited, although challenges remain because of the higher propensity for cation exchange reactions, greater diffusion rates, and unintentional annealing of this more labile class of materials. A deeper understanding of these surface chemical reactions has been achieved through a combination of in situ studies and quantum-chemical calculations. As this understanding matures, so does our ability to deterministically tailor film properties to new applications and more sophisticated devices. This Account highlights the attributes of ALD chemistry that are unique to metal sulfides and surveys recent applications of these materials in photovoltaics, energy storage, and photonics. Within each application space, the benefits and challenges of novel ALD processes are emphasized and common trends are summarized. We conclude with a perspective on potential future directions for metal chalcogenide ALD as well as untapped opportunities. Finally, we consider challenges that must be addressed prior to implementing ALD metal sulfides into future device architectures.

Synthesis of palladium nanoparticles on TiO2(110) using a beta-diketonate precursor

Combined STM and DFT studies reveal the adsorption sites of Pd precursors and nucleation of Pd nanoparticles on TiO₂surface.

Vapor-phase atomic-controllable growth of amorphous Li2S for high-performance lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries hold great promise to meet the formidable energy storage requirements of future electrical vehicles but are prohibited from practical implementation by their severe capacity fading and the risks imposed by Li metal anodes. Nanoscale Li(2)S offers the possibility to overcome these challenges, but no synthetic technique exists for fine-tailoring Li(2)S at the nanoscale. Herein we report a vapor-phase atomic layer deposition (ALD) method for the atomic-scale-controllable synthesis of Li(2)S. Besides a comprehensive investigation of the ALD Li(2)S growth mechanism, we further describe the high performance of the resulting amorphous Li(2)S nanofilms as cathodes in Li-S batteries, achieving a stable capacity of ∼ 800 mA · h/g, nearly 100% Coulombic efficiency, and excellent rate capability. Nanoscale Li(2)S holds great potential for both bulk-type and thin-film high-energy Li-S batteries.

Palladium nanoparticle formation on TiO₂(110) by thermal decomposition of palladium(II) hexafluoroacetylacetonate

Palladium nanoparticles were synthesized by thermal decomposition of palladium(II) hexafluoroacetylacetonate (Pd(hfac)2), an atomic layer deposition (ALD) precursor, on a TiO2(110) surface. According to X-ray photoelectron spectroscopy (XPS), Pd(hfac)2 adsorbs on TiO2(110) dissociatively yielding Pd(hfac)(ads), hfac(ads), and adsorbed fragments of the hfac ligand at 300 K. A (2 × 1) surface overlayer was observed by scanning tunneling microscopy (STM), indicating that hfac adsorbs in a bidentate bridging fashion across two Ti 5-fold atoms and Pd(hfac) adsorbs between two bridging oxygen atoms on the surface. Annealing of the Pd(hfac)(ads) and hfac(ads) species at 525 K decomposed the adsorbed hfac ligands, leaving PdO-like species and/or Pd atoms or clusters. Above 575 K, the XPS Pd 3d peaks shift toward lower binding energies and Pd nanoparticles are observed by STM. These observations point to the sintering of Pd atoms and clusters to Pd nanoparticles. The average height of the Pd nanoparticles was 1.2 ± 0.6 nm at 575 K and increased to 1.7 ± 0.5 nm following annealing at 875 K. The Pd coverage was estimated from XPS and STM data to be 0.05 and 0.03 monolayers (ML), respectively, after the first adsorption/decomposition cycle. The amount of palladium deposited on the TiO2(110) surface increased linearly with the number of adsorption/decomposition cycles with a growth rate of 0.05 ML or 0.6 Å per cycle. We suggest that the removal of the hfac ligand and fragments eliminates the nucleation inhibition of Pd nanoparticles previously observed for the Pd(hfac)2 precursor on TiO2.

Fabrication of transparent-conducting-oxide-coated inverse opals as mesostructured architectures for electrocatalysis applications: a case study with NiO

Highly ordered, and conductive inverse opal arrays were made with silica and subsequently coated with tin-doped indium oxide (ITO) via atomic layer deposition (ALD). We demonstrate the utility of the resulting mesostructured electrodes by further coating them with nickel oxide via ALD. The NiO-coated arrays are capable of efficiently electrochemically evolving oxygen from water. These modular, crack-free, transparent, high surface area, and conducting structures show promise for many applications including electrocatalysis, photocatalysis, and dye-sensitized solar cells.