Atomic and Molecular Layer Deposition as Surface Engineering Techniques for Emerging Alkali Metal Rechargeable Batteries

Alkali metals (lithium, sodium, and potassium) are promising as anodes in emerging rechargeable batteries, ascribed to their high capacity or abundance. Two commonly experienced issues, however, have hindered them from commercialization: the dendritic growth of alkali metals during plating and the formation of solid electrolyte interphase due to contact with liquid electrolytes. Many technical strategies have been developed for addressing these two issues in the past decades. Among them, atomic and molecular layer deposition (ALD and MLD) have been drawing more and more efforts, owing to a series of their unique capabilities. ALD and MLD enable a variety of inorganic, organic, and even inorganic-organic hybrid materials, featuring accurate nanoscale controllability, low process temperature, and extremely uniform and conformal coverage. Consequently, ALD and MLD have paved a novel route for tackling the issues of alkali metal anodes. In this review, we have made a thorough survey on surface coatings via ALD and MLD, and comparatively analyzed their effects on improving the safety and stability of alkali metal anodes. We expect that this article will help boost more efforts in exploring advanced surface coatings via ALD and MLD to successfully mitigate the issues of alkali metal anodes.     

Atomic-Scale Designing of Zeolite Based Catalysts by Atomic Layer Deposition

Zeolite-supported catalysts have been widely used in the field of heterogeneous catalysis. Atomic-scale governing the metal or acid sites on zeolites still encounters great challenge in controllable synthesis and developing of novel catalysts. Atomic layer deposition (ALD), owing to its unique character of self-limiting surface reactions, becomes one of the most promising and controllable strategies to tailor the metallic deposition sites in atomic scale precisely. In this review, we present a comprehensive summary and viewpoint of recent research in designing and engineering the structural of zeolite-based catalysts via ALD method. A prior focus is laid on the deposition of metals on the zeolites with emphasis on the isolated states of metals, followed by introducing the selected metals into channels of zeolites associates with identifying the location of metals in and/or out of the channels. Subsequently, detailed analysis of tailoring the acid sites of different zeolites is provided. Assisted synthesis of zeolite and the regioselective deposition of metal on special sites to modify the structures of zeolites are also critically discussed. We further summarize the challenges of ALD with respect to engineering the active sites in heterogeneous zeolite-based catalysts and provide the perspectives on the development in this field.     

Inherently Area-Selective Atomic Layer Deposition of Manganese Oxide through Electronegativity-Induced Adsorption

Manganese oxide (MnO_x) shows great potential in the areas of nano-electronics, magnetic devices and so on. Since the characteristics of precise thickness control at the atomic level and self-align lateral patterning, area-selective deposition (ASD) of the MnO_x films can be used in some key steps of nanomanufacturing. In this work, MnO_x films are deposited on Pt, Cu and SiO₂ substrates using Mn(EtCp)₂ and H₂O over a temperature range of 80–215 °C. Inherently area-selective atomic layer deposition (ALD) of MnO_x is successfully achieved on metal/SiO₂ patterns. The selectivity improves with increasing deposition temperature within the ALD window. Moreover, it is demonstrated that with the decrease of electronegativity differences between M (M = Si, Cu and Pt) and O, the chemisorption energy barrier decreases, which affects the initial nucleation rate. The inherent ASD aroused by the electronegativity differences shows a possible method for further development and prediction of ASD processes.     

Atomic Layer Deposition Overcoating: Tuning Catalyst Selectivity for Biomass Conversion

The terraces, edges, and facets of nanoparticles are all active sites for heterogeneous catalysis. These different active sites may cause the formation of various products during the catalytic reaction. Here we report that the step sites of Pd nanoparticles (NPs) can be covered precisely by the atomic layer deposition (ALD) method, whereas the terrace sites remain as active component for the hydrogenation of furfural. Increasing the thickness of the ALD‐generated overcoats restricts the adsorption of furfural onto the step sites of Pd NPs and increases the selectivity to furan. Furan selectivities and furfural conversions are linearly correlated for samples with or without an overcoating, though the slopes differ. The ALD technique can tune the selectivity of furfural hydrogenation over Pd NPs and has improved our understanding of the reaction mechanism. The above conclusions are further supported by density functional theory (DFT) calculations.     

Nanoscale Structuring of Surfaces by Using Atomic Layer Deposition

Controlled structuring of surfaces is interesting for a wide variety of areas, including microelectronic device fabrication, optical devices, bio(sensing), (electro‐, photo)catalysis, batteries, solar cells, fuel cells, and sorption. A unique feature of atomic layer deposition (ALD) is the possibility to form conformal uniform coatings on arbitrarily shaped materials with controlled atomic‐scale thickness. In this Minireview, we discuss the potential of ALD for the nanoscale structuring of surfaces, highlighting its versatile application to structuring both planar substrates and powder materials. Recent progress in the application of ALD to porous substrates has even made the nanoscale structuring of high‐surface‐area materials now feasible, thereby enabling novel applications, such as those in the fields of catalysis and alternative energy.     

Enhanced Electrochemical Performance of Supercapacitors via Atomic Layer Deposition of ZnO on the Activated Carbon Electrode Material

Fabricating electrical double-layer capacitors (EDLCs) with high energy density for various applications has been of great interest in recent years. However, activated carbon (AC) electrodes are restricted to a lower operating voltage because they suffer from instability above a threshold potential window. Thus, they are limited in their energy storage. The deposition of inorganic compounds’ atomic layer deposition (ALD) aiming to enhance cycling performance of supercapacitors and battery electrodes can be applied to the AC electrode materials. Here, we report on the investigation of zinc oxide (ZnO) coating strategy in terms of different pulse times of precursors, ALD cycles, and deposition temperatures to ensure high electrical conductivity and capacitance retention without blocking the micropores of the AC electrode. Crystalline ZnO phase with its optimal forming condition is obtained preferably using a longer precursor pulse time. Supercapacitors comprising AC electrodes coated with 20 cycles of ALD ZnO at 70 °C and operated in TEABF₄/acetonitrile organic electrolyte show a specific capacitance of 23.13 F g⁻¹ at 5 mA cm⁻² and enhanced capacitance retention at 3.2 V, which well exceeds the normal working voltage of a commercial EDLC product (2.7 V). This work delivers an additional feasible approach of using ZnO ALD modification of AC materials, enhancing and promoting stable EDLC cells under high working voltages.     

十八烷基键合硅胶的原子层沉积法封尾及其在碱性化合物分离中的应用

采用原子层沉积法对十八烷基键合硅胶进行端基封尾.以六甲基二硅氮烷为封端试剂,在250 ℃下与键合硅胶样品反应6 h,制备了对碱性化合物具有高惰性的反相色谱填料.分别以吡啶/苯酚和萘/阿米替林为分子探针,考察了经原子层沉积法和传统液相有机溶剂法封尾处理的十八烷基键合硅胶的色谱性能,并与商品十八烷基硅胶Zorbax SB-C18和Kromasil C18进行对比.结果表明,原子层沉积法封尾的十八烷基硅胶对碱性化合物的分离特性明显优于传统液相法,其色谱性能已超过Zorbax SB-C18,与Kromasil C18相当.本方法无需有机溶剂,操作简便,反应耗时短,易于放大生产,封尾反应完全,应用前景良好.     

Deposition Mechanism and Properties of Plasma-Enhanced Atomic Layer Deposited Gallium Nitride Films with Different Substrate Temperatures

Gallium nitride (GaN) is a wide bandgap semiconductor with remarkable chemical and thermal stability, making it a competitive candidate for a variety of optoelectronic applications. In this study, GaN films are grown using a plasma-enhanced atomic layer deposition (PEALD) with trimethylgallium (TMG) and NH₃ plasma. The effect of substrate temperature on growth mechanism and properties of the PEALD GaN films is systematically studied. The experimental results show that the self-limiting surface chemical reactions occur in the substrate temperature range of 250–350 °C. The substrate temperature strongly affects the crystalline structure, which is nearly amorphous at below 250 °C, with (100) as the major phase at below 400 °C, and (002) dominated at higher temperatures. The X-ray photoelectron spectroscopy spectra reveals the unintentional oxygen incorporation into the films in the forms of Ga₂O₃ and Ga-OH. The amount of Ga-O component decreases, whereas the Ga-Ga component rapidly increases at 400 and 450 °C, due to the decomposition of TMG. The substrate temperature of 350 °C with the highest amount of Ga-N bonds is, therefore, considered the optimum substrate temperature. This study is helpful for improving the quality of PEALD GaN films.     

Stabilization of Copper Catalysts for Liquid‐Phase Reactions by Atomic Layer Deposition

Atomic layer deposition (ALD) of an alumina overcoat can stabilize a base metal catalyst (e.g., copper) for liquid‐phase catalytic reactions (e.g., hydrogenation of biomass‐derived furfural in alcoholic solvents or water), thereby eliminating the deactivation of conventional catalysts by sintering and leaching. This method of catalyst stabilization alleviates the need to employ precious metals (e.g., platinum) in liquid‐phase catalytic processing. The alumina overcoat initially covers the catalyst surface completely. By using solid state NMR spectroscopy, X‐ray diffraction, and electron microscopy, it was shown that high temperature treatment opens porosity in the overcoat by forming crystallites of γ‐Al₂O₃. Infrared spectroscopic measurements and scanning tunneling microscopy studies of trimethylaluminum ALD on copper show that the remarkable stability imparted to the nanoparticles arises from selective armoring of under‐coordinated copper atoms on the nanoparticle surface.     

Low‐Temperature Atomic Layer Deposition of Copper Metal Thin Films: Self‐Limiting Surface Reaction of Copper Dimethylamino‐2‐propoxide with Diethylzinc

A uniform, conformal, pure copper metal thin film was grown at very low substrate temperatures (100–120 °C) on Si(100) substrates by atomic layer deposition involving the ligand exchange of [Cu(OCHMeCH₂NMe₂)₂] with Et₂Zn (see scheme). Patterned copper thin films of Cu nanotubes (diameter 150 nm, length 12 μm) were fabricated.

     

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

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

Atomic Layer Deposition of Cobalt Phosphide for Efficient Water Splitting

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

Multiply Confined Nickel Nanocatalysts Produced by Atomic Layer Deposition for Hydrogenation Reactions

To design highly efficient catalysts, new concepts for optimizing the metal–support interactions are desirable. Here we introduce a facile and general template approach assisted by atomic layer deposition (ALD), to fabricate a multiply confined Ni‐based nanocatalyst. The Ni nanoparticles are not only confined in Al₂O₃ nanotubes, but also embedded in the cavities of Al₂O₃ interior wall. The cavities create more Ni–Al₂O₃ interfacial sites, which facilitate hydrogenation reactions. The nanotubes inhibit the leaching and detachment of Ni nanoparticles. Compared with the Ni‐based catalyst supported on the outer surface of Al₂O₃ nanotubes, the multiply confined catalyst shows a striking improvement of catalytic activity and stability in hydrogenation reactions. Our ALD‐assisted template method is general and can be extended for other multiply confined nanoreactors, which may have potential applications in many heterogeneous reactions.     

Role of Anionic Backbone in NHC-Stabilized Coinage Metal Complexes: New Precursors for Atomic Layer Deposition**

Cu and Ag precursors that are volatile, reactive, and thermally stable are currently of high interest for their application in atomic-layer deposition (ALD) of thin metal films. In pursuit of new precursors for coinage metals, namely Cu and Ag, a series of new N-heterocyclic carbene (NHC)-based Cu^I and Ag^I complexes were synthesized. Modifications in the substitution pattern of diketonate-based anionic backbones led to five monomeric Cu complexes and four closely related Ag complexes with the general formula [M(^tBuNHC)(R)] (M=Cu, Ag; ^tBuNHC=1,3-di-tert-butyl-imidazolin-2-ylidene; R=diketonate). Thermal analysis indicated that most of the Cu complexes are thermally stable and volatile compared to the more fragile Ag analogs. One of the promising Cu precursors was evaluated for the ALD of nanoparticulate Cu metal deposits by using hydroquinone as the reducing agent at appreciably low deposition temperatures (145–160 °C). This study highlights the considerable impact of the employed ligand sphere on the structural and thermal properties of metal complexes that are relevant for vapor-phase processing of thin films.     

Rational Development of Guanidinate and Amidinate Based Cerium and Ytterbium Complexes as Atomic Layer Deposition Precursors: Synthesis,Modeling, and Application

Owing to the limited availability of suitable precursors for vapor phase deposition of rare-earth containing thin-film materials, new or improved precursors are sought after. In this study, we explored new precursors for atomic layer deposition (ALD) of cerium (Ce) and ytterbium (Yb) containing thin films. A series of homoleptic tris-guanidinate and tris-amidinate complexes of cerium (Ce) and ytterbium (Yb) were synthesized and thoroughly characterized. The C-substituents on the N-C-N backbone (Me, NMe₂, NEt₂, where Me=methyl, Et=ethyl) and the N-substituents from symmetrical iso-propyl (iPr) to asymmetrical tertiary-butyl (tBu) and Et were systematically varied to study the influence of the substituents on the physicochemical properties of the resulting compounds. Single crystal structures of [Ce(dpdmg)₃] 1 and [Yb(dpdmg)₃] 6 (dpdmg=N,N'-diisopropyl-2-dimethylamido-guanidinate) highlight a monomeric nature in the solid-state with a distorted trigonal prismatic geometry. The thermogravimetric analysis shows that the complexes are volatile and emphasize that increasing asymmetry in the complexes lowers their melting points while reducing their thermal stability. Density functional theory (DFT) was used to study the reactivity of amidinates and guanidinates of Ce and Yb complexes towards oxygen (O₂) and water (H₂O). Signified by the DFT calculations, the guanidinates show an increased reactivity toward water compared to the amidinate complexes. Furthermore, the Ce complexes are more reactive compared to the Yb complexes, indicating even a reactivity towards oxygen potentially exploitable for ALD purposes. As a representative precursor, the highly reactive [Ce(dpdmg)₃] 1 was used for proof-of-principle ALD depositions of CeO₂ thin films using water as co-reactant. The self-limited ALD growth process could be confirmed at 160 °C with polycrystalline cubic CeO₂ films formed on Si(100) substrates. This study confirms that moving towards nitrogen-coordinated rare-earth complexes bearing the guanidinate and amidinate ligands can indeed be very appealing in terms of new precursors for ALD of rare earth based materials.     

Encapsulation of Homogeneous Catalysts in Mesoporous Materials Using Diffusion‐Limited Atomic Layer Deposition

The heterogenization of homogeneous metal complex catalysts has attracted great attention. The encapsulation of metal complexes into nanochannels of mesoporous materials is achieved by coating metal oxides at/near the pore entrance by diffusion‐limited atomic layer deposition (ALD) to produce a hollow plug. The pore size of the hollow plug is precisely controlled on the sub‐nanometer scale by the number of ALD cycles to fit various metal complexes with different molecular sizes. Typically, Co or Ti complexes are successfully encapsulated into the nanochannels of SBA‐15, SBA‐16, and MCM‐41. The encapsulated Co and Ti catalysts show excellent catalytic activity and reusability in the hydrolytic kinetic resolution of epoxides and asymmetric cyanosilylation of carbonyl compounds, respectively. This ALD‐assisted encapsulation method can be extended to the encapsulation of other homogeneous catalysts into different mesoporous materials for various heterogeneous reactions.     

Potential Precursor Alternatives to the Pyrophoric Trimethylaluminium for the Atomic Layer Deposition of Aluminium Oxide

New precursor chemistries for the atomic layer deposition (ALD) of aluminium oxide are reported as potential alternatives to the pyrophoric trimethylaluminium (TMA) which is to date a widely used Al precursor. Combining the high reactivity of aluminium alkyls employing the 3-(dimethylamino)propyl (DMP) ligand with thermally stable amide ligands yielded three new heteroleptic, non-pyrophoric compounds [Al(NMe₂)₂(DMP)] ( 2 ), [Al(NEt₂)₂(DMP)] ( 3 , BDEADA) and [Al(NiPr₂)₂(DMP)] ( 4 ), which combine the properties of both ligand systems. The compounds were synthesized and thoroughly chemically characterized, showing the intramolecular stabilization of the DMP ligand as well as only reactive Al−C and Al−N bonds, which are the key factors for the thermal stability accompanied by a sufficient reactivity, both being crucial for ALD precursors. Upon rational variation of the amide alkyl chains, tunable and high evaporation rates accompanied by thermal stability were found, as revealed by thermal evaluation. In addition, a new and promising plasma enhanced (PE)ALD process using BDEADA and oxygen plasma in a wide temperature range from 60 to 220 °C is reported and compared to that of a modified variation of the TMA, namely [AlMe₂(DMP)] (DMAD). The resulting Al₂O₃ layers are of high density, smooth, uniform, and of high purity. The applicability of the Al₂O₃ films as effective gas barrier layers (GBLs) was successfully demonstrated, considering that coating on polyethylene terephthalate (PET) substrates yielded very good oxygen transmission rates (OTR) with an improvement factor of 86 for a 15 nm film by using DMAD and a factor of 25 for a film thickness of just 5 nm by using BDEDA compared to bare PET substrates. All these film attributes are of the same quality as those obtained for the industrial precursor TMA, rendering the new precursors safe and potential alternatives to TMA.     

Optimizing the Atomic Layer Deposition of Alumina on Perovskite Nanocrystal Films by Using O2 As a Molecular Probe

Encapsulation methods have shown to be effective in imparting improved stability to metal-halide perovskite nanocrystals (NCs). Atomic layer deposition (ALD) of metal oxides is one of the promising approaches for such encapsulation, yet better control on the process parameters are required to achieve viable lifetimes for several optoelectronic and photocatalytic applications. Herein, we optimize the ALD process of amorphous aluminum oxide (AlO_x) as an encapsulating layer for CsPbBr₃ NC thin films by using oxygen (O₂) as a molecular diffusion probe to assess the uniformity of the deposited AlO_x layer. When O₂ reaches the NC surface, it extracts the photogenerated electrons, thus quenching the PL of the CsPbBr₃ NCs. As the quality of the ALD layer improves, less quenching is expected. We compare three different ALD deposition modes. We find that the low temperature/high temperature and the exposure modes improve the quality of the alumina as a gas barrier when compared with the low temperature mode. We attribute this result to a better diffusion of the ALD precursor throughout the NC film. We propose the low temperature/high temperature as the most suitable mode for future implementation of multilayered coatings.     

Atomic Layer Deposition of the Metal Pyrites FeS2, CoS2, and NiS2

Atomic layer deposition (ALD) of the pyrite‐type metal disulfides FeS₂, CoS₂, and NiS₂ is reported for the first time. The deposition processes use iron, cobalt, and nickel amidinate compounds as the corresponding metal precursors and the H₂S plasma as the sulfur source. All the processes are demonstrated to follow ideal self‐limiting ALD growth behavior to produce fairly pure, smooth, well‐crystallized, stoichiometric pyrite FeS₂, CoS₂, and NiS₂ films. By these processes, the FeS₂, CoS₂, and NiS₂ films can also be uniformly and conformally deposited into deep narrow trenches with aspect ratios as high as 10:1, which thereby highlights the broad and promising applicability of these ALD processes for conformal film coatings on complex high‐aspect‐ratio 3D architectures in general.