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Thermal atomic layer deposition (ALD) is used for the first time to deposit iron‐nickel oxides onto carbon nanotubes in a ternary process to produce a wide range of mixed oxide thin films. When using ferrocene (FeCp2) and nickelocene (NiCp2) with ozone (O3) as metals and oxygen sources, respectively, a competition between the metal precursors and the growth modes is observed. Indeed, while ferrocene promotes a 2D‐growth, nickelocene prefers a 3D‐growth. Although both precursors are homoleptic metallocenes, they behave differently in the ALD of their respective metal oxide, leading to unexpected atomic ratios and films morphologies of the iron‐nickel oxides. The 2Fe:1Ni sample displays the best performances in the electrochemical water oxidation (oxygen evolution reaction) exhibiting an overpotential of 267 mV at a current density of 10 mA cm−1, a Tafel slope of 36.8 mV dec−1, as well as a good stability after 15 h of continuous operation. [ABSTRACT FROM AUTHOR]

Tungsten disulfide (WS2) layers with different thicknesses were deposited on carbon nanotubes (CNTs) by atomic layer deposition (ALD) to obtain CNT/WS2 core–shell heteronanostructures. WS2 conformally grows like small platelet-flakes on the CNTs initially at low ALD cycles, while, with increasing the number of ALD cycles, WS2 platelets further grow to form a continuous film. The electrical and electrochemical properties of the synthesized CNT/WS2 hierarchical heterostructures were evaluated for sensing and electrocatalysis applications. First, a CNT/WS2 conductometric sensor was developed for gas-sensing monitoring of nitrogen dioxide (NO2). CNT/WS2 heteronanostructures were also tested as effective electrochemical probes for detecting riboflavin (vitamin B6) and as electrocatalysts for CO2 reduction. The results of gas sensing and electrochemical tests were discussed and correlated to the morphology and surface coverage of WS2 shell on CNT core. CNT/WS2 with 200–300 ALD cycles demonstrated the best performances because of the optimal catalytic WS2 properties and the formation of CNT/WS2 junctions, allowing the sensitive and selective detection of NO2 gas and riboflavin with a limit of detection (LOD) of 70 ppb and 0.4 μM, respectively, and formate production in the electrocatalytic reduction of CO2. [ABSTRACT FROM AUTHOR]

Atomic layer deposition (ALD) is a deposition technique well‐suited to produce high‐quality thin film materials at the nanoscale for applications in transistors. This review comprehensively describes the latest developments in ALD of metal oxides (MOs) and chalcogenides with tunable bandgaps, compositions, and nanostructures for the fabrication of high‐performance field‐effect transistors. By ALD various n‐type and p‐type MOs, including binary and multinary semiconductors, can be deposited and applied as channel materials, transparent electrodes, or electrode interlayers for improving charge‐transport and switching properties of transistors. On the other hand, MO insulators by ALD are applied as dielectrics or protecting/encapsulating layers for enhancing device performance and stability. Metal chalcogenide semiconductors and their heterostructures made by ALD have shown great promise as novel building blocks to fabricate single channel or heterojunction materials in transistors. By correlating the device performance to the structural and chemical properties of the ALD materials, clear structure–property relations can be proposed, which can help to design better‐performing transistors. Finally, a brief concluding remark on these ALD materials and devices is presented, with insights into upcoming opportunities and challenges for future electronics and integrated applications. [ABSTRACT FROM AUTHOR]

A thermal atomic layer deposition (ALD) process to fabricate MoS2 thin films is successfully demonstrated by using cycloheptatriene molybdenum tricarbonyl (C7H8Mo(CO)3) and H2S as precursors at an ALD temperature below 300 °C. The process is systematically investigated, showing a typical self‐limiting characteristic within an ALD temperature window of 225–285 °C and a high growth‐per‐cycle of 0.11 nm. The as‐deposited films are amorphous while they can be crystallized in situ by sulfurization with H2S at a low temperature of 300 °C. A prototypical application of the developed ALD process is demonstrated by constructing a MoS2/TiO2 heterostructure through depositing MoS2 onto anodized TiO2 nanotubes for photoelectrochemical water splitting. The MoS2/TiO2 heterostructures exhibit approximately three times superior photoelectrochemical performance than the pristine TiO2 nanotubes. This is attributed to an enhanced visible light‐harvesting ability of MoS2 and an improved separation of the photo‐generated charge carriers at the heterostructure interface, which is affirmed by a staggering gap (type II) between MoS2 and TiO2 as probed by ultraviolet photoelectron spectroscopy. [ABSTRACT FROM AUTHOR]

Metal carbides are promising materials for electrocatalytic reactions such as water electrolysis. However, for application in catalysis for the oxygen evolution reaction (OER), protection against oxidative corrosion, a high surface area with facile electrolyte access, and control over the exposed active surface sites are highly desirable. This study concerns a new method for the synthesis of porous tungsten carbide films with template‐controlled porosity that are surface‐modified with thin layers of nickel oxide (NiO) to obtain active and stable OER catalysts. The method relies on the synthesis of soft‐templated mesoporous tungsten oxide (mp. WOx) films, a pseudomorphic transformation into mesoporous tungsten carbide (mp. WCx), and a subsequent shape‐conformal deposition of finely dispersed NiO species by atomic layer deposition (ALD). As theoretically predicted by density functional theory (DFT) calculations, the highly conductive carbide support promotes the conversion of Ni2+ into Ni3+, leading to remarkably improved utilization of OER‐active sites in alkaline medium. The obtained Ni mass‐specific activity is about 280 times that of mesoporous NiOx (mp. NiOx) films. The NiO‐coated WCx catalyst achieves an outstanding mass‐specific activity of 1989 A gNi−1 in a rotating‐disc electrode (RDE) setup at 25 °C using 0.1 m KOH as the electrolyte. [ABSTRACT FROM AUTHOR]

SnO2 is one of the most employed n‐type semiconducting metal oxide in chemo‐resistive gas‐sensing although it presents serious limitations due to a low selectivity. Herein, the authors introduce 1D SnO2‐SiO2 core‐shell nanowires (CSNWs). The amorphous SiO2‐shell layer with varying thicknesses (1.8–10.5 nm) is grown onto the SnO2 nanowires (NWs) by atomic layer deposition (ALD). SiO2‐coated SnO2 CSNWs show a dramatic improvement of the selectivity towards hydrogen. Moreover, the sensing‐response is strongly correlated to the thickness of the SiO2‐shell and the working temperature. The SnO2‐SiO2 CSNWs sensor with a 4.8‐nm SiO2 shell thickness exhibits the best selectivity and sensitivity, having ca. 7‐fold higher response toward hydrogen compared to bare‐SnO2 NWs. The selectivity and enhanced sensing‐response are related to the masking effect of the SiO2 shell and an increase in the width of the electron‐depletion‐layer due to a strong electronic coupling between the SnO2 core and SiO2 coating, respectively. [ABSTRACT FROM AUTHOR]

Nanocomposites of TiO2 and carbon nanotubes (CNTs) have been extensively studied in photocatalysis, sensing, and energy conversion and storage over the last decade. The unique properties of these nanocomposites are greatly dependent on the morphology, crystallinity, and homogeneity of the TiO2 coating. However, a fine control of the film microstructure is still challenging due to limited understanding of early stages of the TiO2 growth. The presence of an intermediate buffer layer can induce remarkable changes in the morphological and structural characteristics of the coatings. Here, TiO2 films deposited by atomic layer deposition (ALD) on CNTs without and with different intermediate layers (Al2O3 and ZnO) have been systematically investigated. Compared to bare CNTs, it is suggested that these two intermediate layers with higher surface energy can lead to a delay of the TiO2 crystallization, ultimately resulting in the growth of conformal and crystalline TiO2 films. This study demonstrates a strategy to tailor the microstructure and the properties of thin films via ALD by applying intermediate layers and provides information about the role of surface energy of the substrate in crystallization and growth behavior of ALD thin films. [ABSTRACT FROM AUTHOR]

Crystalline hematite/polycrystalline nickel oxide (α-Fe2O3/NiO) core/shell nanofibers are prepared by electrospinning and calcination, followed by a varying number (100–1150) of atomic layer deposition cycles of NiO. The deposition of the conformal NiO layer leads to the passivation of the surface states and the appearance of a photoluminescence band in the micro-Raman spectra excited by 532 nm laser. As a continuous NiO layer is formed, a peak, possibly arising from a two-magnon mode, appears at 1585 cm−1. The detection of the peak, which is not observed in the spectra excited by a 633 nm laser, is assisted by the surface plasmon at around 510 nm introduced by the polycrystalline NiO layer, due to the electron doping induced by coordination-defects at its edge-rich surface. [ABSTRACT FROM AUTHOR]

Hierarchical core–shell (C–S) heterostructures composed of a NiO shell deposited onto stacked‐cup carbon nanotubes (SCCNTs) are synthesized by atomic layer deposition (ALD). A film of NiO particles (0.80–21.8 nm in thickness) is uniformly deposited onto the inner and outer walls of the SCCNTs. The electrical resistance of the samples is found to increase of many orders of magnitude with the increasing of the NiO thickness. The response of NiO–SCCNT sensors toward low concentrations of acetone and ethanol at 200 °C is studied. The sensing mechanism is based on the modulation of the hole‐accumulation region in the NiO shell layer upon chemisorption of the reducing gas molecules. The electrical conduction mechanism is further studied by the incorporation of an Al2O3 dielectric layer at NiO and SCCNT interfaces. The investigations on NiO–Al2O3–SCCNT, Al2O3–SCCNT, and NiO–SCCNT coaxial heterostructures reveal that the sensing mechanism is strictly related to the NiO shell layer. The remarkable performance of the NiO–SCCNT sensors toward acetone and ethanol benefits from the conformal coating by ALD, large surface area of the SCCNTs, and the optimized p‐NiO shell layer thickness followed by the radial modulation of the space‐charge region. [ABSTRACT FROM AUTHOR]

Vertically aligned TiO2/ZnO nanotube (NT) arrays were developed for application to photoanodes in mesoscopic solar cells. By a two-step anodic oxidation, vertically aligned TiO2 NT arrays with highly ordered surface structure were prepared, followed by deposition of a ZnO shell with a precisely controlled thickness using atomic layer deposition (ALD). When applied to a photoanode of dye-sensitized solar cells (DSSCs), the photovoltage is gradually enhanced as the ZnO shell thickness of the TiO2/ZnO NT electrodes is increased. Furtheremore, the electron lifetime in photoanodes is significantly enhanced due to the ZnO shell, which is examined by open-circuit voltage decay (OCVD) measurement. Photocurrent density-voltage (J-V) curves under the dark condition and OCVD spectra reveal that a negative shift in TiO2 conduction band potential and an energy barrier effect owing to the ZnO shell concurrently contribute to the enhancement of VOC and electron lifetime. [ABSTRACT FROM AUTHOR]

Nanocomposites made of stacked‐cup carbon nanotubes coated with NiO (NiO/SCCNTs) via atomic layer deposition (ALD) were synthesized in order to obtain a material exhibiting enhanced and optimized electrochemical performance towards detections of glucose. The structure and morphology were characterized by transmission electron microscopy (TEM) and X‐ray diffraction (XRD). NiO deposited as nanocrystalline particles in the cubic modification, were well dispersed and directly anchored on SCCNTs forming a smooth particulate thin film, which becomes more dense with the increase of the number of ALD cycles. The NiO/SCCNTs samples with various thicknesses of the NiO coating (0.8 nm, 1.7 nm, 4.0 nm, 6.5 nm, 14.0 nm and 21.8 nm) were applied for enzyme‐free glucose sensing. Their electrochemical performance strongly depends on the thickness of the deposited NiO thin film. The best performing glucose sensors respond over a wide concentration range from 2 μM to 2.2 mM (R2=0.9979) with remarkably enhanced sensitivity (1252.3 μA cm−2 mM−1), with a limit of detection (LOD) of 0.10 μM (S/N=3) and with a fast response time (lower than 2 s). The significant performance improvement can be attributed to the conformal NiO coating, high surface to volume ratio and to the optimized thickness of the NiO thin film. The advantage of our sensors is also associated with the conductive supporting material (SCCNTs), simplicity of fabrication, high sensitivity, selectivity, stability and reproducibility for the rapid quantification of glucose. Ain't it sweet? Conductive stacked‐cup carbon nanotubes are conformally coated with NiO of variable thicknesses using atomic layer deposition (ALD). Their electrochemical performance towards glucose oxidation strongly depends on the thickness of the deposited NiO thin film. The NiO/SCCNTs electrode with 4.0 nm thickness of NiO exhibits the highest sensitivity and stability. The very thin NiO layer also protects the underneath SCCNT substrate from corrosion. [ABSTRACT FROM AUTHOR]

Abstract: The stabilization of crystal phases and nanostructured morphologies is an essential topic in application‐driven design of mesoporous materials. Many applications, e.g. catalysis, require high temperature and humidity. Typical metal oxides transform under such conditions from a metastable, low crystalline material into a thermodynamically more favorable form, i.e. from ferrihydrite into hematite in the case of iron oxide. The harsh conditions induce also a growth of the crystallites constituting pore walls, which results in sintering and finally collapse of the porous network. Herein, a new method to stabilize mesoporous templated metal oxides against sintering and pore collapse is reported. The method employs atomic layer deposition (ALD) to coat the internal mesopore surface with thin layers of either alumina or silica. The authors demonstrate that silica exerts a very strong influence: It shifts hematite formation from 400 to 600 °C and sintering of hematite from 600 to 900 °C. Differences between the stabilization via alumina and silica are rationalized by a different interaction strength between the ALD material and the ferrihydrite film. The presented approach allows to stabilize mesoporous thin films that require a high crystallization temperature, with submonolayer quantity of an ALD material, and to apply mesoporous materials for high temperature applications. [ABSTRACT FROM AUTHOR]

Abstract: Transition metal oxides have been proposed as a possible replacement of platinum for the electrocatalytic oxygen reduction reaction (ORR). However, as a result of the low intrinsic conductivity, an application in electrocatalysis is challenging. In this work, we demonstrate that atomic layer deposition (ALD) is capable to overcome this problem by coating conductive carbon nanotubes substrates with a conformal Mn3O4 layer of just few‐nm in thickness. The deposition parameters have been optimized in terms of thickness and crystallite sizes to produce a material exhibiting catalytic efficiency close to the one of carbon‐supported Pt particles and low polarization costs. The current densities recorded in linear sweep voltammetry prove that the Mn3O4 coating leads to a substantial increase of the catalytic efficiency, compared to uncoated carbon nanotubes, and was also higher than other manganese‐based catalysts reported so far. The sample prepared from only 50 ALD cycles (e. g. coating thickness of ∼2 nm) shows the best compromise between catalytic efficiency, with an onset potential at 0.867 V (vs. RHE), and good conductivity of the electrode materials minimizing polarization. Indeed, the Tafel plots exhibit a similar slope than Pt/C demonstrating that the Mn3O4/CNTs reduce oxygen in a one‐step four electrons mechanism and with a similar kinetics as Pt‐based electrocatalysts. Moreover, the current density keeps at 80 % even after 12 h at 0.58 V displaying a higher stability than Pt‐based catalysts. These findings are attributed to the few nm‐thick conformal and catalytic active coating obtained by atomic layer deposition, which also protects the underneath CNT substrate from corrosion. [ABSTRACT FROM AUTHOR]

A novel atomic layer deposition process of manganese oxide from methylcyclopentadienyl manganese tricarbonyl and ozone has been developed. It is used to coat vertically aligned carbon nanotubes (VACNTs) grown on a conductive Inconel substrate in order to produce a 3D array of metal oxide-carbon nanotube electrode. Electron microscopy studies show that a conformal polycrystalline film can be deposited on the VACNTs arrays. X-ray diffraction, small area electron diffraction, and X-ray photoelectron spectroscopy analysis determine the formation of hausmannite (Mn3O4). The electrochemical properties of the as-prepared VACNTs/Mn3O4 nanocomposite electrodes are studied using cyclic voltammetry, galvanostatic charge and discharge cycling in 1 m Na2SO4 aqueous electrolyte. Capacitances as high as 78.62 mF cm−2 at 5 mV s−1 are demonstrated which is one order of magnitude higher than that of pristine VACNTs. In addition, the as-prepared VACNTs/Mn3O4 nanocomposite electrode showed a good reversibility and cycling stability. [ABSTRACT FROM AUTHOR]

Atomic layer deposition is a thin film deposition technique based on self-terminated surface reactions. Contrarily to most of the thin film deposition techniques, it is not a line of sight deposition technique due to the sequential introduction of the gaseous precursors and because the reactants can only react with surface species. The precursors can thus diffuse into porous structures and the conformal coating of high aspect ratio structures can be achieved. Because of these peculiarities, atomic layer deposition is an attractive technique for fabricating materials to be applied in resistive gas sensors. This article focuses on materials for resistive gas sensor devices in which the sensing material is elaborated using atomic layer deposition, in at least one step of the fabrication. It will be shown that atomic layer deposition has proven to be well-suited for the elaboration of compact thin films, nanostructures, and heterostructures to be applied for the detection of a variety of analytes such as toxic compounds, pollutants, explosives, etc. The chemical and physical properties of the sensing layers will be discussed in parallel to the gas sensing mechanisms in an attempt to develop clear structure-property correlations [ABSTRACT FROM AUTHOR]

Abstract: Carbon-based nanomaterials demonstrated to be highly suitable as support for the elaboration of heterostructures. Atomic layer deposition (ALD) proved to be a technique of choice for the coating of nanostructured carbon materials. These heterostructures find applications in various areas such as electronics, sensors and energy storage and conversion. Because the chemical inertness of the graphitic carbon inhibits the initiation of ALD film growth, numerous surface functionalization approaches have been investigated in order to provide the required nucleation sites. The different strategies employed for the ALD onto carbon nanotubes, graphene, graphite and other nanostructured carbon materials (e.g. carbon black, fibers) are reviewed. The peculiarity of ALD for tailoring the chemical, structural and morphological properties of the deposited material are discussed. Finally, in order to highlight the importance of this class of materials, possible applications in catalysis and gas sensing devices are also reviewed. [Copyright &y& Elsevier]

MnO2/carbon nanotube composite electrodes for Li-ion battery application were directly coated with ultrathin thicknesses of aluminum oxide film by atomic layer deposition (ALD). The non-reactive Al2O3 layer not only provides a stable film to protect the manganese oxide and carbon nanotubes from undesirable reaction with the electrolyte but also restrains the volume change strain of manganese oxide during cycling. The first cycle Coulombic efficiency of coated samples was increased to different extents depending on the coating thickness. In the following cycles, the coated electrodes denote high specific capacity, good capacity retention ability, and perfect rate charge/discharge performance. [ABSTRACT FROM AUTHOR]

Low-temperature ozone-assisted atomic layer deposition (ALD) of SiO2 with four silane derivatives (3-aminopropyl)triethoxysilane (APTES), bis(diethylamino)silane (BDEAS), diphenylaminosilane (DPAS), and triethylsilane on carbon nanotubes (CNTs) leads to the one step formation of SiO2 nanotubes. In the process, CNTs act as templates and are removed during the ongoing deposition. From transmission electron microscopy images, the formation of a void between the CNTs surface and the SiO2 coating was observed, indicating an unexpected removal of carbon from the CNTs. This gap grows as the number of ALD cycles is increased, eventually leading to SiO2 nanotubes almost free of carbon. ATR-IR and EELS spectra proved the SiO2 formation. Depending on the CNTs templates used in this process, different morphologies of one-dimensional SiO2 nanostructures are obtained, including simple nanotubes, hollow wall nanotubes, tube-in-tube structures, and SiO2 nanowires. The application of this process on vertically aligned CNTs (VACNTs) templates allows the formation of a perfect SiO2 replica of the VACNTs. From experiments with different oxygen and silicon precursors, it is proposed that peroxides and oxygen-based radicals, which can be formed from the reaction of surface Si–H species with ozone, are the main reactive species leading to the unexpected etching of carbon from the CNTs during silica ALD. [ABSTRACT FROM AUTHOR]

The mechanisms of growth of TiO2thin films by atomiclayer deposition (ALD) using either acetic acid or ozone as the oxygensource and titanium isopropoxide as the metal source are investigatedby in situ Fourier transform infrared spectroscopy (FTIR) and ex situX-ray photoelectron spectroscopy. The FTIR study of the acetic acid-basedprocess clearly shows a ligand exchange leading to the formation ofsurface acetate species (vibrational bands at 1527 and 1440 cm–1) during the acetic acid pulse. Their removal duringthe metal alkoxide pulse takes place via the elimination of an esterand the formation of Ti–O–Ti bonds. These findings confirmthe expected ester elimination condensation mechanism and demonstratethat the reaction proceeds without intermediate surface hydroxyl species.The in situ FTIR study of the O3-based ALD process demonstratessimilarities with the process described above, with formation of surfaceformate and/or carbonate species upon exposure of the surface titaniumalkoxide species to ozone. These surface species are removed by thesubsequent titanium isopropoxide pulse, leading to the formation ofTi–O–Ti bonds. [ABSTRACT FROM AUTHOR]