Atomic Layer Deposited Aluminum Oxide Research Articles

Rapid Proton Transmission through Nafion | Graphene | ALD Alumina | Nafion Membranes

Recent reports from several laboratories indicate that proton transmission through graphene and other 2D materials in contact with proton-conducting electrolytes can occur rapidly with a much lower activation energy than expected from a wide range of computational studies on the proton / graphene interaction energy, and a much higher selectivity relative to transmission of other ions than would be expected based only on ion size. It would be desirable to be able to control transport through the graphene, and many approaches have been tried to rationally control transport rates through graphene via chemical treatments applied to the graphene. One such approach involves use of atomic layer deposition (ALD), in which single layers of inorganic materials, often oxides, are coated in a self-limiting way onto a surface by a series of pulsed reaction steps delivered in series. A common ALD implementation involves coatings of aluminum oxide, or alumina, created by sequential dosing of a surface with trimethyl aluminum and water vapor. We report here on use of the ALD method to coat single-layer CVD graphene supported on copper foil or on Nafion membranes with layers of alumina. Coatings resulting from 50 ALD cycles are found to be thick enough and homogeneous enough to block photoelectron emission from the underlying substrates (copper or Nafion) when the coatings are analyzed by X-ray photoelectron spectroscopy. Electrochemical hydrogen pump experiments were then performed to assess the degree to which ALD treatment of the graphene on Nafion, delivered prior to preparing Nafion | graphene | Nafion sandwich membranes, could change the rate of proton transmission through the graphene that is embedded within the Nafion. As little as two ALD treatment cycles are sufficient to reduce proton transmission currents through graphene by approximately half relative to graphene that is not treated by ALD. Surprisingly, fifty ALD cycles has almost the same effect as two cycles. This finding indicates that that ALD alumina coating is a relatively good proton conductor which is thought to reflect the action of transport pathways on the hydrated surface of the porous ALD alumina layer. The insensitivity of proton transmission to the number of ALD cycles applied suggests that the function of the ALD is to change the transport rates at interfaces, perhaps by creating a new ALD alumina / Nafion interface that protons must traverse in order to pass through the membrane.

Design of an active and stable catalyst for dry reforming of methane via molecular layer deposition

The dry reforming of methane (DRM) has been proposed as an efficient way to convert two greenhouse gases, namely CO2 and CH4 to syngas. However, most catalysts reported in the literature suffer from strong deactivation during the reforming reaction. The deactivation is caused by strong sintering of catalytically active nanoparticles and the formation of filamentous carbon. Herein a new synthesis procedure based on molecular layer deposition (MLD) is established to stabilize DRM catalysts under reaction conditions. Deactivation of a Ni/SiO2 reference catalyst was prevented by forming a defined porous net-like over-layer, which prevents the sintering and detachment of Ni nanoparticles by filamentous carbon. The MLD approach was further compared to the formation of an overlayer by atomic layer deposition (ALD), demonstrating the advantages of MLD forming hybrid organic-inorganic alucone layers over classical alumina ALD.

Evaluating Mechanical Properties of 100nm-Thick Atomic Layer Deposited Al2o3 as a Free-Standing Film

The mechanical properties of free-standing atomic layer-deposited alumina thin films were evaluated in this study. Stress-strain curves were obtained by measuring alumina films floating on the water surface. The Young's modulus, strength, and elongation of the free-standing films were analyzed at varying deposition temperatures of 80-200°C. The results show that the mechanical properties improve with increasing temperature. The microstructures of the films were examined to explain the change of the mechanical properties depending on the deposition conditions. Finally, the underlying causes of the differences in the mechanical properties were analyzed by evaluating the chemical composition of the thin films.

Surface modification of organic powders for enhanced rheology via atomic layer deposition

Some organic additive manufacturing feedstocks can be cohesive and tend to agglomerate in suspensions, which can lead to significant challenges in formulating solids-loaded fluids, like those used in many additive manufacturing processes. By depositing an ultra-thin, layer of a second material conformally with the surface of the feedstock powder, Atomic Layer Deposition (ALD) can be used to uniquely modify the surface and, thereby, cohesion of the powder material without changing the properties of the bulk material. This paper demonstrates low temperature (<115 °C), ALD of aluminum oxide on temperature-sensitive materials: first, on polyimide thin film flats, then, on melamine and nylon organic powder feedstocks with the goal of improving their powder rheology. The process produced amorphous aluminum oxide-hydroxide coatings that are both uniform and conformal to the powder’s surface. Aluminum oxide coatings on the nylon powders did not show a significant change in the flow properties of the powder, given the already low cohesivity of nylon. In contrast, the highly cohesive melamine powders exhibited significant improvements in flowability after being coated with a layer as thin as 20 nm, due to a reduction in inter-particle cohesivity. The basic flowability decreased from 199 to 185 mJ and the specific energy increased from 4.95 to 6.39 mJ/g.

Understanding Pore Formation in ALD Alumina Overcoats.

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

Thickness determination of porous Pt cathode thin film capped by atomic layer-deposited alumina for low-temperature solid oxide fuel cells

This study investigated the effects of the apparent thickness of porous Pt cathode thin films thermo-mechanically stabilized by an atomic layer-deposited (ALD) alumina capping agent on effective electrical conductivity and electrochemical performance. Four-point probe resistance measurements revealed that a 120 nm-thick porous Pt film exhibits a relatively high electrical conductivity of 9.2 × 103 S/cm. Cathode reaction resistance measurements by means of alternating current impedance spectroscopy (ACIS) analysis show that ALD alumina can act as a thermo-mechanical stabilizer for a 120 nm-thick porous Pt cathode film. The ACIS analysis results for solid oxide fuel cells (SOFCs) with porous Pt cathode thin films of various thicknesses showed that a 120 nm-thick porous Pt cathode film is thermo-mechanically stabilized by the conformal coating of an ALD alumina capping agent. The iR-free polarization curves show that an ALD alumina-capped 120 nm porous Pt cathode film-embedded SOFC delivers ~65% higher power density than an ALD alumina-capped 480 nm-thick porous Pt cathode film-embedded SOFC.

Atomic layer deposited aluminum oxide mitigates outgassing from fused filament fabrication–based 3-D printed components

Open-source scientific hardware based on affordable fused filament fabrication (FFF) 3-D printing has the potential to reduce the cost of research tools considerably. So far, development has focused on tools that do not require compatibility with vacuum environments. Highly porous 3-D printed plastics require surface treatments to mitigate their outgassing, and in this study we explored the outgassing reduction from 3-D printed black-colored acrylonitrile butadiene styrene (ABS) and polycarbonate (PC) using a commercial vacuum sealing resin as well as atomic layer deposited (ALD) aluminum oxide (AlOx). The outgassing properties of uncoated plastics could not be measured due to a too high level of outgassing, which was attributed to their high porosity and high specific surface area. However, both the commercial resin and the ALD coatings reduced the extent of outgassing from both ABS and PC, which enabled their comparison by residual gas analysis (RGA). Remarkably, the outgassing performance achieved with ALD AlOx was superior to the performance of the commercial vacuum resin across a temperature range of 40 to 100 °C for both plastics, despite the uneven coverage of the plastic surface with AlOx. Results indicated that both ABS and PC could be made compatible with at least moderate vacuums using ALD AlOx. Thus, the fabrication of laboratory vacuum tools can be realized with affordable 3-D printed plastics. However, further studies on the physical mechanisms behind the outgassing reduction and the durability of the coatings are required.

Mechanically and thermally stable, transparent electrodes with silver nanowires encapsulated by atomic layer deposited aluminium oxide for organic optoelectronic devices

Abstract Flexible conductive electrodes are essential components for organic optoelectronic devices (OODs). One of the main challenges in the development of flexible OODs is to achieve an optimal combination of photoelectrical properties, enhanced flexibility and stability in transparent conductive electrodes (TCEs). In this work, high-performance flexible nonfullerene organic solar cells (OSCs) and polymer light-emitting diodes (PLEDs) based on TCEs of silver nanowires (AgNWs) encapsulated with an ultra-thin atomic layer deposited aluminum oxide (Al2O3) have been demonstrated. The hybrid AgNWs/Al2O3 composite electrodes with enhanced thermal, ambient and mechanical stabilities enable an efficient flexible transparent electrode with high transmittance and conductivity, which can synergistically optimize the device performance of nonfullerene OSCs and PLEDs. The maximum power conversion efficiency value of 7.03%, as well as a current efficiency of 7.26 cd A−1 for flexible OSCs and PLEDs are achieved, respectively. Notably, excellent flexibility, long-term atmospheric and thermal stabilities have been systematically investigated and demonstrated. These results provide a new design platform for the fabrication of high-performance, flexible transparent electrodes, which can be further explored in a wide range of organic optoelectronics field.

A ferroelectric semiconductor field-effect transistor

Ferroelectric field-effect transistors employ a ferroelectric material as a gate insulator, the polarization state of which can be detected using the channel conductance of the device. As a result, the devices are of potential to use in non-volatile memory technology, but suffer from short retention times, which limits their wider application. Here we report a ferroelectric semiconductor field-effect transistor in which a two-dimensional ferroelectric semiconductor, indium selenide ({\alpha}-In2Se3), is used as the channel material in the device. {\alpha}-In2Se3 was chosen due to its appropriate bandgap, room temperature ferroelectricity, ability to maintain ferroelectricity down to a few atomic layers, and potential for large-area growth. A passivation method based on the atomic-layer deposition of aluminum oxide (Al2O3) was developed to protect and enhance the performance of the transistors. With 15-nm-thick hafnium oxide (HfO2) as a scaled gate dielectric, the resulting devices offer high performance with a large memory window, a high on/off ratio of over 108, a maximum on-current of 862 {\mu}A {\mu}m-1, and a low supply voltage.

Optical function of atomic layer deposited alumina (0.5–41.0 nm) from 191 to 1688 nm by spectroscopic ellipsometry with brief literature review

Atomic layer deposition (ALD) is widely used in the semiconductor industry to provide atomic level control over film thicknesses and layer conformality. Here, the authors report the thermal (332 °C) ALD of thin amorphous alumina films (0.5–41.0 nm) deposited using water and trimethylaluminum precursors. Alumina deposition was optimized by varying the dose and purge times for both precursors with an eye toward obtaining uniform film thicknesses and constant growth per cycle. Films were characterized by x-ray photoelectron spectroscopy and spectroscopic ellipsometry (from 191 to 1688 nm). The subsequent workup of the SE data was based on a multisample analysis. It considered Cauchy and Sellmeier functions and accounted for surface roughness. This modeling yielded an optical function for ALD alumina over the measured wavelength range. The authors recommend the optical function determined from the Sellmeier model.

Ιn situ N2-NH3 plasma pre-treatment of silicon substrate enhances the initial growth and restricts the substrate oxidation during alumina ALD

The initial substrate inhibiting island growth and the formation of an interfacial layer with uncontrollable characteristics are the two main drawbacks of the Atomic Layer Deposition (ALD) of high-k metal-oxide gate dielectrics on silicon (Si). In this paper, we investigate the ALD of Al2O3 films from trimethyl aluminum and H2O, on fluorhydric acid (HF) cleaned, as well as on HF-cleaned and in situ N2-NH3 plasma pretreated Si between 0 and 75 cycles. The films and their interface were characterized via Scanning Transmission Electron Microscopy coupled to Energy-Dispersive X-ray spectroscopy. The initial deposition is clearly increased on the pretreated surfaces, obtaining a linear ALD regime even after 5 ALD cycles, compared to several tens of cycles needed on HF-cleaned Si. Furthermore, a SixNy layer is formed by the N2-NH3 plasma pretreatment, which acts as a barrier layer, reducing the oxidation of the Si substrate beneath it. This analysis provides a general framework for the understanding and determination of adequate surface pretreatments, able to combat the substrate inhibited initial growth and the Si oxidation during metal-oxide ALD on Si.

Reactive Wetting of Atomic Layer Deposition Al2O3 By Molten Lithium Metal

Oxide coating presents a promising methodology to enable reactive wetting of solid-state garnet electrolyte by molten lithium metal. The resulting intimate contact allows effective lithium-ion migration at the garnet/lithium metal interface. Although changing in coating composition by the interfacial reaction has been previously revealed, the transition in chemistry, structure, topography and mechanical performance of the contact coating area remain unclear. Here, a combination of impedance spectroscopy, scanning electron microscope, time-of-flight secondary ion mass spectrometry and nanoindentation is devoted to illustrating and discussing the effect of the interfacial reaction in altering the properties of atomic layer deposition alumina coating layer. These findings provide unique insight into the mechanism of the reactive wetting of oxide coating by molten lithium metal, thereby, enabling design of more effective lithium metal-solid electrolyte interfaces.

Blocking Polysulfides in Graphene–Sulfur Cathodes of Lithium–Sulfur Batteries through Atomic Layer Deposition of Alumina

A lithium–sulfur (Li–S) battery is one of the post‐lithium‐ion battery chemistry candidates due to the high theoretical capacity of the sulfur cathode (1672 mAh g−1). However, low electronic conductivity of sulfur and severe capacity fading during the charge–discharge process limit the commercial realization of Li–S batteries. The origin of capacity fading is mainly due to the polysulfide shuttling effect, resulting from the dissolution of sulfur in the electrolyte solution. Herein, atomic layer deposition (ALD) (6–8 Å thickness) of alumina is presented as a strategy to suppress capacity degradation on the 2D graphene–sulfur hybrid electrode. Low‐temperature ALD prevents sulfur sublimation from the composite electrode. Despite the insulating property of alumina, atomic layer coating maintains good electrical conductivity, thereby yielding lower charge transfer resistance. Alumina‐coated graphene–sulfur hybrid electrodes (AGS) exhibit a high specific capacity of 960 mAh g−1 at a current density of 50 mA g−1 and retain 519 mAh g−1 after 100 galvanostatic cycles. Superior electrochemical performance is credited to the combination of the high electronic conductivity of multilayered graphene platelets and low charge transfer resistance, resulting from effective polysulfide blocking by the atomic scale Al2O3 coating.

Surface Depletion Layers in Plasmonic Metal Oxide Nanocrystals.

Strong infrared (IR) light-matter interaction and spectral tunability combine to make plasmonic metal oxide nanocrystals (NCs) a compelling choice for IR applications. In particular, visible transparency paired with strong, dynamically tunable IR absorption has motivated their implementation in electrochromic smart windows, but these NCs hold promise for a far broader range of plasmonically driven processes such as surface-enhanced infrared sensing, photothermal therapy, and enhanced photocatalysis. These unique properties result from localized surface plasmon resonance (LSPR) sustained by a relatively low free charge carrier concentration, which in turn requires consideration of distinct materials physics relative to traditional plasmonic materials (i.e., metals). Particularly important is the formation of insulating shells devoid of charge carriers (depletion layers) near the NC surface. Surface states as well as applied surface potentials can give rise to a potential difference between the NC surface and its core that depletes free charge carriers from the surface, forming an insulating shell that reduces the conductivity in NC films, lowers the dielectric sensitivity of the LSPR, and diminishes the incident electric field enhancement. In this Account, we report recent investigations of depletion layers in plasmonic metal oxide NCs that have advanced understanding of the semiconductor physics underlying the optoelectronic properties of these NCs and the electrochemical modulation of their LSPR, establishing a conceptual framework with which to broaden their applicability and optimize their performance. As a result of surface depletion, larger, highly doped NCs have improved dielectric sensitivity compared with their smaller, lightly doped counterparts. Concentrating dopants near the NC surface compresses the depletion layer, resulting in improved conductivity of NC films. Moreover, atomic layer deposition of alumina to infill NC films enhances the film conductivity by more than 2 orders of magnitude, ascribed to the elimination of depletion effects by reactive removal of surface water species. At the conclusion, we reflect on how our newfound understanding of surface depletion in plasmonic metal oxide NCs is quickly leading to rational material design. This insight is already resulting in significant performance improvements, and the same principles can be applied to new, exciting opportunities in hot carrier extraction and resonant IR energy transduction.

Comparative study and characterization of atomic layer deposition Al2O3 films as metal-insulator-metal capacitor dielectric for GaAs hetero-junction bipolar transistor technology

Atomic layer deposition (ALD) aluminum oxide (Al2O3) films have been deposited using trimethyl aluminum (TMA) and H2O, and using TMA and H2O with O3 as precursors, and were characterized and evaluated as a metal-insulator-metal (MIM) capacitor dielectric in GaAs hetero-junction bipolar transistor (HBT) technology. The results show that the capacitor with 60 nm of ALD Al2O3 capacitor dielectric films deposited using TMA and H2O and deposited using TMA, H2O, and O3 resulted in a capacitance density of 1.36 × 10−15–1.37 × 10−15 F/μm2, and a dielectric constant of 9.2–9.3. The capacitance density of the capacitor using these films increased by 2.3%–3.0%, when the temperature was increased from 25 to 125 °C. No significant change in capacitance density of these ALD films was observed when the applied voltage was varied from −5 to +5 V. However, the 60 nm ALD aluminum oxide film deposited using the TMA, H2O, and O3 has significantly higher breakdown voltage (46 V) compared to the ALD aluminum oxide film deposited using TMA and H2O only (38.4 V). Furthermore, the leakage current density of the ALD Al2O3 from TMA, H2O, and O3 is significantly lower compared to that of ALD Al2O3 from TMA and H2O (5.6 × 10−8 A/cm2 vs 8.9 × 10−6 A/cm2 at 5 MV/cm). As the temperature was increased from 25 to 125 °C, the breakdown voltage of both films decreased, while the leakage current marginally increased. Additionally, as the capacitor area is increased, the leakage current also increased, while no change in breakdown voltage is observed.

Passivation of Detector‐Grade Float Zone Silicon with Atomic Layer Deposited Aluminum Oxide

Silicon radiation and particle detectors are traditionally passivated with thermal silicon dioxide. It has been shown that aluminum oxide (Al2O3) films provide better surface passivation due to their high negative charge, but studies on Al2O3 surface passivation are usually performed on low‐resistivity substrates. Herein, the passivation of high‐resistivity, detector‐grade float zone silicon (FZ‐Si) with Al2O3 is studied, with a specific emphasis on the effect of post‐annealing temperature on carrier lifetimes and film properties. It is confirmed that Al2O3 provides excellent surface passivation also on high‐resistivity FZ‐Si substrates, with a low interface defect density of around (2–4) × 1011 cm−2 eV−1 and a high negative oxide charge of 1 × 1012 to 3 × 1012 q cm−2, when post‐annealed at temperatures of up to 450–500 °C. In addition, high‐resistivity samples are studied for the phenomenon of bulk lifetime degradation occurring at typical post‐annealing or metal sintering temperatures, which has been reported for low‐resistivity FZ‐Si. At post‐annealing temperatures of &gt;500 °C, reduced bulk lifetimes are observed if the substrates did not receive high‐temperature treatment prior to surface passivation. Furthermore, it is noticed that n‐type samples exhibit lower bulk lifetimes even when a high‐temperature treatment is performed, which indicates a connection between FZ‐Si bulk lifetime degradation and doping type.

Laser-written colours on silver: optical effect of alumina coating

Abstract In this paper we discuss the optical response of laser-written plasmonic colours on silver coated via the atomic layer deposition of alumina. These colours are due to nanoparticles distributed on a flat surface and on a surface with periodic topographical features (i.e. ripples). The colours are observed to shift with increasing alumina film thickness. The colours produced by surfaces with ripples recover their original vibrancy and hue after the deposition of film of thickness ~60 nm, while colours arising from flat surfaces gradually fade and never recover. Analysis of the surfaces identifies periodic topographical features to be responsible for this behaviour. Finite-difference time-domain simulations unravel the role played by the alumina thickness in colour formation and confirm the rotations and recovery of colours for increasing alumina thickness. The coloured surfaces were evaluated for applications in colourimetric and radiometric sensing showing large sensitivities of up to 3.06/nm and 3.19 nm/nm, respectively. The colourimetric and radiometric sensitivities are observed to be colour dependent.

Nanoengineering of Crystal and Amorphous Surfaces of Pharmaceutical Particles for Biomedical Applications.

The morphology, size, and surface properties of pharmaceutical particles form an essential role in the therapeutic performance of active pharmaceutical ingredients (APIs) and excipients as constituents in various drug delivery systems and clinical applications. Recent advances in methods for surface modification, however, rely heavily on liquid-phase-based modification processes and afford limited control over the thickness and conformality of the coating. Atomic layer deposition (ALD), on the other hand, enables the formation of conformal nanoscale films on complex structures with thickness control on the molecular level, while maintaining the substrate particle size and morphology. Moreover, this enables nanoengineering of surfaces of pharmaceutical particles also in the dry state. Successful nanoengineeering of crystal and amorphous surfaces of pharmaceutical particles is demonstrated in this study whereby functional properties, such as dissolution and dispersibility, were tailored for drug delivery applications. This expands on our initial work on ALD of alumina on pharmaceutical particles within the lower micro- to higher nanosize ranges to here probe both crystalline and amorphous lactose substrate surfaces (d50 = 3.5 and 21 μm). In addition, both water and ozone coreactants were evaluated, the latter having not been evaluated previously for pharmaceutical particles. The deposition process is carried out at ambient conditions in a fluidized bed reactor for a low number of cycles (i.e., from 4 to 14). Improved dissolution and extended release were achieved by the ALD nanoengineering of both crystalline and amorphous surfaces. This novel concept opens up exciting opportunities to produce more complex materials and structures using temperature- and moisture-sensitive drugs, e.g., targeting and drug delivery opportunities, as well as delivering new functionalities for novel applications in the pharmaceutical, medical, biological, and advanced materials fields. The prospects for advancing inhaled drug delivery are exemplified by the ALD surface nanoengineering concept.

Low-loss integrated photonics for the blue and ultraviolet regime

We present a low-loss integrated photonics platform in the visible and near ultraviolet regime. Fully-etched waveguides based on atomic layer deposition (ALD) of aluminum oxide operate in a single transverse mode with $<$3 dB/cm propagation loss at a wavelength of 371 nm. Ring resonators with intrinsic quality factors exceeding 470,000 are demonstrated at 405 nm, and the thermo optic coefficient of ALD aluminum oxide is estimated to be $2.75\times10^{-5}$ [RIU/$^\circ$C]. Absorption loss is sufficiently low to allow on-resonance operation with intra-cavity powers up to at least 12.5 mW, limited by available laser power. Experimental and simulated data indicates the propagation loss is dominated by sidewall roughness, suggesting lower loss in the blue and UV is achievable.

Probing the Electronic Properties of Monolayer MoS2 via Interaction with Molecular Hydrogen

AbstractThis work presents a detailed experimental investigation of the interaction between molecular hydrogen (H2) and monolayer MoS2 field effect transistors (MoS2 FET), aiming for sensing application. The MoS2 FET exhibits a response to H2 that covers a broad range of concentration (0.1–90%) at a relatively low operating temperature range (300–473 K). Most important, H2 sensors based on MoS2 FETs show desirable properties such as full reversibility and absence of catalytic metal dopants (Pt or Pd). The experimental results indicate that the conductivity of MoS2 monotonically increases as a function of the H2 concentration due to a reversible charge transferring process. It is proposed that such process involves dissociative H2 adsorption driven by interaction with sulfur vacancies in the MoS2 surface (VS). This description is in agreement with related density functional theory studies about H2 adsorption on MoS2. Finally, measurements on partially defect‐passivated MoS2 FETs using atomic layer deposited aluminum oxide consist of an experimental indication that the VS plays an important role in the H2 interaction with the MoS2. These findings provide insights for future applications in catalytic process between monolayer MoS2 and H2 and also introduce MoS2 FETs as promising H2 sensors.