Atomic Layer Deposited Aluminum Oxide Research Articles

Microwaves induced epitaxial growth of urchin like MIL-53(Al) crystals on ceramic supports

Shaping Metal–Organic Frameworks (MOFs) poses a significant challenge for their widespread application on a large scale. In particular, a precise control over crystal orientation and arrangement on substrates are expected to provide exiting opportunities for novel materials with customized characteristics and enhanced performance in catalysis, gas storage, sensing, optics and electronics. Here we demonstrated for the first time that microwave irradiation can induce well controlled epitaxial growth of urchin-like MIL-53(Al) crystals via the hydrothermal conversion of Atomic Layer Deposition alumina layers on SiC foams. The resulting large, ordered crystals feature specific size, homogeneity, dispersion, and quantity that strongly correlate with the nature of the ceramic support and its ability to absorb microwaves. Furthermore, the supported MIL-53(Al) urchins were considered as templates for generating nanostructured alumina fibers on SiC foams, providing attractive catalyst carriers with high specific surface areas.

Enhanced Thermal Stability of Conductive Mercury Telluride Colloidal Quantum Dot Thin Films Using Atomic Layer Deposition.

Colloidal quantum dots (CQDs) are valuable for their potential applications in optoelectronic devices. However, they are susceptible to thermal degradation during processing and while in use. Mitigating thermally induced sintering, which leads to absorption spectrum broadening and undesirable changes to thin film electrical properties, is necessary for the reliable design and manufacture of CQD-based optoelectronics. Here, low-temperature metal-oxide atomic layer deposition (ALD) was investigated as a method for mitigating sintering while preserving the optoelectronic properties of mercury telluride (HgTe) CQD films. ALD-coated films are subjected to temperatures up to 160 °C for up to 5 h and alumina (Al2O3) is found to be most effective at preserving the optical properties, demonstrating the feasibility of metal-oxide in-filling to protect against sintering. HgTe CQD film electrical properties were investigated before and after alumina ALD in-filling, which was found to increase the p-type doping and hole mobility of the films. The magnitude of these effects depended on the conditions used to prepare the HgTe CQDs. With further investigation into the interaction effects of CQD and ALD process factors, these results may be used to guide the design of CQD-ALD materials for their practical integration into useful optoelectronic devices.

A channel-homogenized micro gas chromatography column for light alkane separation with uniform HKUST-1 stationary phase

BackgroundThe micro gas chromatography column (μGCC) is one of the key components of the miniaturized gas chromatography system. However, light alkanes are difficult to be separated by a micro gas chromatography column, especially for methane and ethane, because the length of μGCC is limited by the area of a silicon substrate. More importantly, the heterogeneous microchannel surface formed by silicon glass bonding causes uneven stationary phase coating and the forces between the untreated microchannel surfaces and the stationary phase materials are weak, which will prevent the improvement of separation performance. ResultsIn this paper, a micro gas chromatography column (μGCC) with uniform HKUST-1 stationary phase is reported. Significantly, an alumina film prepared by the atomic layer deposition (ALD) technique is used to homogenize the heterogeneous microchannels. The alumina is a hydrophilic material and the alumina made by the ALD technique is uniform. The forces between hydrophilic alumina film and HKUST-1 are strong, which can greatly improve the coating uniformity of the hydrophilic stationary phase HKUST-1. The test results show that the μGCC could baseline separate the light alkane mixtures (CH4, C2H6, C3H8, and C4H10) at the high testing temperature of 120 °C. The maximum resolution of the difficult-separated methane and ethane reached 19.2, which is 108 % higher than the μGCC using the same stationary phase without homogenizing the microchannel inner surface. SignificanceThe μGCC uses ALD alumina film to homogenize the microchannel inner surface; meanwhile, hydrophilic ALD alumina has a strong electrostatic attraction with the hydrophilic stationary phase HKUST-1. Homogeneous microchannel surface and strong electrostatic attraction are favorable to obtain uniform stationary phase which greatly improves the separation performance, resulting in a large resolution for methane and ethane. The μGCC has broad application prospects in light alkane separation.

Double Passivation Using Ultra‐Thin Insulators for Improved Stability and Efficiency of Quasi‐2D Perovskite Light Emitting Diodes

AbstractInterface engineering is a critical parameter for optimal optoelectronic device performance. In quasi‐2D perovskite light emitting diodes (PeLEDs), traditional poly (3,4‐ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS) hole injection layer (HIL) is replaced by self‐assembled monolayer (SAM) [2‐(3,6‐dibromo‐9H‐carbazol‐9‐yl) ethyl] phosphonic acid (Br‐2PACz) HIL. The deep highest occupied molecular orbital (HOMO @ −5.73 eV) of Br‐2PACz facilitates the efficient hole‐injection along with a reduction in leakage current by 4–5 orders of magnitude as compared to PEDOT:PSS (HOMO @ −5.00 eV) HIL based optimized PeLEDs. Ultra‐thin SAM‐based Br‐2PACz HIL presents two challenges: i) device reproducibility is hindered by spatial inhomogeneity, and ii) it fails to regulate nonradiative recombination at the perovskite/HIL interface. To address these challenges, the ultra‐thin interlayer of atomic layer deposited aluminum oxide (Al2O3) is utilized between Br‐2PACz, and perovskite which aids in passivating the interfacial states at the HIL/perovskite interface. With the optimized thickness of Al2O3 interlayer (3 nm), the device's external quantum efficiency (EQE) improved from 9.47% to 13.14%, accompanied by luminous efficiency of 42.8 cd A−1. An ultra‐thin interlayer of lithium fluoride (LiF) (2 nm) is implemented on the top side of the perovskite, which can provide 1.5 times increase in the device's LT50 stability.

Scalable Large-Area 2D-MoS2/Silicon-Nanowire Heterostructures for Enhancing Energy Storage Applications.

Two-dimensional (2D) transition-metal dichalcogenides have shown great potential for energy storage applications owing to their interlayer spacing, large surface area-to-volume ratio, superior electrical properties, and chemical compatibility. Further, increasing the surface area of such materials can lead to enhanced electrical, chemical, and optical response for energy storage and generation applications. Vertical silicon nanowires (SiNWs), also known as black-Si, are an ideal substrate for 2D material growth to produce high surface-area heterostructures, owing to their ultrahigh aspect ratio. Achieving this using an industrially scalable method paves the way for next-generation energy storage devices, enabling them to enter commercialization. This work demonstrates large surface area, commercially scalable, hybrid MoS2/SiNW heterostructures, as confirmed by Raman spectroscopy, with high tunability of the MoS2 layers down to the monolayer scale and conformal MoS2 growth, parallel to the silicon nanowires, as verified by transmission electron microscopy (TEM). This has been achieved using a two-step atomic layer deposition (ALD) process, allowing MoS2 to be grown directly onto the silicon nanowires without any damage to the substrate. The ALD cycle number accurately defines the layer number from monolayer to bulk. Introducing an ALD alumina (Al2O3) interface at the MoS2/SiNW boundary results in enhanced MoS2 quality and uniformity, demonstrated by an order of magnitude reduction in the B/A exciton photoluminescence (PL) intensity ratio to 0.3 and a reduction of the corresponding layer number. This high-quality layered growth on alumina can be utilized in applications such as for interfacial layers in high-capacity batteries or for photocathodes for water splitting. The alumina-free 100 ALD cycle heterostructures demonstrated no diminishing quality effects, lending themselves well to applications that require direct electrical contact with silicon and benefit from more layers, such as electrodes for high-capacity ion batteries.

Reducing the Thermal Effects during Coating of Superconducting Radio-Frequency Cavities: A Case Study for Atomic Layer Deposition of Alumina with a Combined Numerical and Experimental Approach

Reducing the Thermal Effects during Coating of Superconducting Radio-Frequency Cavities: A Case Study for Atomic Layer Deposition of Alumina with a Combined Numerical and Experimental Approach

Influence of AlOx Interlayers on LeTID Kinetics in Ga-Doped Cz-Si

Light and elevated temperature-induced degradation (LeTID) is causing a reduction in efficiency especially in p-type silicon based solar cells. It is assumed to be strongly influenced by the hydrogen content in the bulk material. The presented work focuses on the impact of differently thick (5-25 nm) atomic layer-deposited aluminum oxide (AlOx) interlayers underneath the hydrogen-rich silicon nitride (SiNy:H) capping layer. The interlayer acts as a diffusion barrier for H during the firing step. It is demonstrated that the AlOx interlayer has a comparable effect on the LeTID kinetics in Ga-doped Cz-Si (Cz-Si:Ga) as it is observed in B-doped Cz-Si (Cz-Si:B). Additionally, it substantially minimizes lifetime degradation in the Cz-Si:Ga sample. With a determined ratio of electron to hole capture cross sections k=26(3), the degradation phenomena are attributed to the LeTID kinetics. Deposition of AlOx barrier layers exceeding 10 nm in thickness does not yield additional positive effects. Resistivity measurements revealed that the change in hole concentration correlates with the defect density for varying AlOx layer thicknesses. The doping concentration seems to influence the change in maximum defect density for varying AlOx layer thicknesses.

Restricting Anion Migrations by Atomic Layer-Deposited Alumina on Perovskite Nanocrystals while Preserving Structural and Optical Properties

Restricting Anion Migrations by Atomic Layer-Deposited Alumina on Perovskite Nanocrystals while Preserving Structural and Optical Properties

Analyzing the Li-Al-O Interphase of Atomic Layer-Deposited Al2O3 Films on Layered Oxide Cathodes Using Atomistic Simulations.

Alumina surface coatings are commonly applied to layered oxide cathode particles for lithium-ion battery applications. Atomic layer deposition (ALD) is one such surface coating technique, and ultrathin alumina ALD films (<2 nm) are shown to improve the electrochemical performance of LiNixMnyCo1-x-yO2 materials, with groups hypothesizing that a beneficial Li-Al-O product is being formed during the alumina ALD process. However, the atomic structure of these films is still not well understood, and quantifying the interface of ultrathin (∼1 nm) ALD films is an arduous experimental task. Here, we perform molecular dynamics simulations of amorphous alumina films of varying thickness in contact with the (0001) LiCoO2 (LCO) surface to quantify the film nanostructure. We calculate elemental mass density profiles through the films and observe that the Li-Al-O interphase extends ∼2 nm from the LCO surface. Additionally, we observe layering of Al and O atoms at the LCO-film interface that extends for ∼1.5 nm. To access the short-range order of the amorphous film, we calculated the Al coordination numbers through the film. We find that while [4]Al is the prevailing coordination environment, significant amounts of [6]Al exist at the interface between the LiCoO2 surface and the film. Taken together, these principal findings point to a pseudomorphic Li-Al-O overlayer that approximates the underlying layered LiCoO2 lattice but does not exactly replicate it. Additionally, with sufficient thickness, the Li-Al-O film transitions to an amorphous alumina structure. We anticipate that our findings on the ALD-like, Li-Al-O film nanostructure can be applied to other layered LiNixMnyCo1-x-yO2 materials because of their shared crystal structure with LiCoO2. This work provides insight into the nanostructure of amorphous ALD alumina films to help inform their use as protective coatings for Li-ion battery cathode active materials.

Additive Influence of Top Metal Contact and Alumina Deposition on the Threshold Voltage of Suspended Carbon Nanotube Field-Effect Transistors.

One-dimensional nanostructures such as carbon nanotubes offer excellent properties useful for applications in gas sensors, piezoresistive devices, and radio frequency resonators. Considering their nanoscale form factor, carbon nanotubes (CNTs) are highly sensitive to surface adsorbents. This study presents the fabrication flow of CNT devices with extended passivated areas around electrical contacts between the CNT and source and drain electrodes. These types of structures could help in understanding the intrinsic CNT response by eliminating the analyte impact on the Schottky barrier regions of the CNT field-effect transistors (CNTFETs). The influence of multiple processing conditions on the electronic properties of CNTFETs with a suspended individual CNT used as the CNTFET channel is presented. Our findings show a threshold voltage shift in CNT ISD-Vg characteristics following the metal deposition and alumina atomic layer deposition.

Optimum temperature of atomic layer deposition of alumina on CsPbBr3 quantum-dot for optical performance and environmental stability

Atomic layer deposition (ALD) plays a pivotal role in raising the long-term stability of perovskite quantum dot (PQD). The deposition temperature of excellent alumina film prepared by ALD is generally higher than 200 °C. In such a temperature environment, the lattice morphology of PQD is seriously damaged. In this work, to effectively protect PQD, the optimum temperature for depositing alumina on the surface of CsPbBr3 PQD is investigated. The CsPbBr3 PQD is deposited on a transparent glass substrate. Subsequently, an alumina layer is deposited on the surface of PQD via ALD at a deposition temperature of 40–120 °C. In our experiment, the significant degradation of PQD is observed when the deposition temperature of ALD is set to 80 °C and above. Lowering the deposition temperature can effectively improve the optical performance of PQD. The samples with alumina deposited at 75 °C show excellent stability after undergoing aging tests, and therefore 75 °C is considered the best ALD temperature. Besides, white light-emitting diode (W-LED) is produced by a method that a stack structure consisting of CsPbBr3 coated via ALD at 75 °C and CdSe QD is excited using blue Micro-LED, and the color gamut of W-LED accounts for the wide area of 94.00% NTSC (1953) and 86.20% Rec.2020 displayed wide color gamut features. The experiment provides practical guidance for selecting the optimal deposition temperature for ALD-packaged PQD-based devices.

Understanding the multilevel phenomena that enables inorganic atomic layer deposition to provide barrier coatings for highly-porous 3-D printed plastic in vacuums

The potential of 3-D printing polymers to achieve low-weight space flight hardware has seen increasing interest. Additionally, robust 3-D printed polymer vacuum equipment would provide highly attractive low-cost alternatives to conventional industrial-scale tools for the scientific community. Inorganic barrier coatings of plastic parts can be used to minimize outgassing and damage to polymers from extreme vacuum environments. Among different coating technologies in this area of research, atomic layer deposition (ALD) has shown the most promise. Nevertheless, the exact formation morphology of ALD coatings on 3-D printed polymers under vacuum are not yet well understood, which hinders use of 3-D printed polymers in vacuum environments. In this study, the film formation mechanisms of ALD alumina on porous 3-D printed polymers are investigated via SEM, EDS, XRD, ATR-FTIR, and XPS. ALD alumina is deposited on 3-D printed pigmented and un-pigmented acrylonitrile butadiene styrene, polycarbonate, commercially available polypropylene, and pure polypropylene. The results reveal that the formation of the ALD barrier layer, its thickness, and diffusion of ALD precursor materials into the polymer substrate is a multi-scale phenomenon, and that substrate porosity and polymer functionality both dominate film formation behavior. Additionally, this study demonstrates that during ALD processes on 3-D printed polymers a vapor phase infiltration (VPI) growth mode also occurs, especially where porosity is present.

Comprehensive Study of the Chemical, Physical, and Structural Evolution of Molecular Layer Deposited Alucone Films during Thermal Processing

Thermal processing of molecular layer deposited (MLD) hybrid inorganic–organic alucone thin films produced porous and low-k materials. Alucone films were deposited by MLD using trimethyl aluminum and ethylene glycol at 120 °C. Changes in the film density and thickness during annealing were monitored using in-situ X-ray reflectivity and were compared to atomic layer deposited (ALD) alumina films. The chemical evolution of the as-deposited and annealed alucone films during post-deposition heating with and without UV was probed using infrared spectroscopy, Rutherford backscattering, nuclear reaction analysis, and 15N spectroscopy, providing a detailed understanding of the induced changes. The concentration of OH groups decreased after depositing 1 nm of alumina capping layer as a barrier to moisture uptake, which also decreased the etch rate in CF4/O2 plasma. The lowest dielectric constant of the processed alucone films (kmin = 4.75) was 25% lower than the lowest values measured in ALD alumina counterparts (kmin = 6.7). Large thickness decreases for alucone films were observed at ∼200 °C of anneal temperatures. Removal of retained organic components by thermal processing of MLD films is demonstrated to be a promising and versatile route to porous thin films for a wide range of applications including low dielectric constant materials.

Sinter‐Resistant Nickel Catalyst for Lignin Hydrogenolysis Achieved by Liquid Phase Atomic Layer Deposition of Alumina

AbstractLignin hydrogenolysis is a key step in the sustainable production of renewable bio‐based chemicals and fuels. Heterogeneous metal catalysts have led to high yields but they rapidly deactivate, notably due to nanoparticle sintering and carbonaceous deposit formation. While these deposits can be removed by regeneration, sintering is irreversible and a significant barrier to commercialization. Here, simple liquid phase atomic layer deposition is used to deposit an alumina layer to protect nickel particles from sintering. In the gas phase, it is proved that alumina can prevent sintering during reduction up to 600 °C. This catalyst for hydrogenolysis of extracted lignin in batch and continuous operation is used. In batch, the overcoated catalyst maintains high monomer yields with little sintering over four cycles of reuse while the yield obtained with the catalyst without an overcoat reduces to half and severe sintering occurs. In a continuous flow reactor, deactivation rates are three times lower for the catalyst with the alumina overcoat. Microscopy images confirm that the alumina overcoat largely preserves nickel particle sizes after ten days of operation. The results demonstrate that catalyst overcoating with metal oxides substantially slows irreversible deactivation during lignin hydrogenolysis, which could facilitate the development of continuous lignin upgrading.

Atomic layer deposition for surface area determination of solid oxide electrodes

A method to measure the total surface area of porous electrodes is developed, leveraging the conformal atomic layer deposition of alumina over internal features, with example structures measured from the field of solid oxide electrochemical cells.

Chemical Stability of Atomic Layer Deposited (ALD) Alumina Thin Films in Aqueous Solutions

Aluminum oxide (alumina) thin films deposited through atomic layer deposition (ALD) are of great interest in chemical barrier and corrosion protection applications. However, the stability of ALD alumina in aqueous solutions is still not fully understood. Due to its metastable amorphous phase, the hydration and degradation behavior of ALD alumina films behaves differently from its crystalline Al2O3 counterpart. A full understanding of why these films hydrate and/or dissolve requires the exploration of different deposition conditions and ion content in solutions used. This talk will discuss efforts to further elucidate the hydration and dissolution behavior of ALD alumina films. For this study, alumina thin films were ALD deposited onto silicon substrate at 150 °C using trimethylaluminum (TMA) and H2O. These films were then studied in Type 1 DI water, 2M NaCl, .5M NaCl, and .1M NaCl solutions at room temperature. Films were gently dried using a nitrogen gun and thickness was measured using a Cauchy ellipsometry model. After 15 days of immersion in Type 1 DI water, significant thickness growth is observed at twice (27 days) and 2.5 times (33 days) the normalized thickness. Similar hydration is not observed in salt-containing aqueous solutions nor upon exposure to air. We will discuss the possible effects of CO2 dissolution and carbonate formation as well as ionic species on the hydration and dissolution processes of these alumina films. Finally, we will show how a post-ALD plasma treatment to the film’s surface alters the hydration and dissolution behavior, improving stability in several aqueous conditions.

Atomic layer deposited aluminium oxide membranes for selective hydrogen separation through molecular sieving

Ultra-thin membranes have significant potential for the gas separation. However, the fabrication of ultra-thin membrane with high selectivity and permeance remains a challenge. Herein, we develop a high-performance hydrogen separation membrane using atomic layer deposition (ALD) method to deposit ultra-thin Al2O3 layer (ALD-Al2O3) on a porous anodic aluminum oxide (AAO) support. The pore openings of the AAO substrate were fully covered, but not sealed, by the Al2O3 overlayer after 260 ALD cycles. Due to the low deposition temperature (50 °C), angstrom-scale pores were formed within the Al2O3 overlayer. Single gas permeance results revealed that the size of the pores to be smaller than 0.364 nm, but larger than the kinetic diameter of molecular H2 (0.289 nm). This membrane at room temperature, exhibits high H2 permeance of ∼3.03 × 10−7 mol m−2 s−1 Pa−1 and high H2 selectivity for H2/CO2 (5148.2), H2/O2 (120.7), H2/N2 (161.5) and H2/CH4 (219.5), showing great potential for industrial H2 purification and recovery.

Measuring Local Atomic Structure Variations through the Depth of Ultrathin (&lt;20 nm) ALD Aluminum Oxide: Implications for Lithium-Ion Batteries

Understanding the atomic structure of ultrathin (<20 nm) atomic layer deposition (ALD) coatings is critical to establish structure–property relationships and accelerate the application of ALD films to stabilize battery interfaces. Previous studies have measured the atomic structure of nanoscale ALD films using cryogenic electron diffraction with a large (∼200 nm) beam diameter. However, for ultrathin ALD coatings, these measurements provide only ensemble average structural information and cannot be used to directly measure differences in atomic structure through the depth of the ALD film. In this study, we localize the electron beam to a small (∼5 nm) spot size using cryogenic scanning transmission electron microscope (STEM), and we collect electron diffraction data at multiple points along the depth of a 12 nm thick ALD AlOx film deposited onto a carbon nanotube (CNT) substrate without a contribution from the substrate. We couple these diffraction measurements with pair distribution function (PDF) analysis and iterative reverse Monte Carlo-molecular statics (RMC-MS) modeling to compare atomic structure metrics at different positions in the film depth. We interpret the modeling results considering the three-dimensional (3D) concentric cylindrical sample geometry of a CNT with uniform AlOx coating. These measurements confirm a two-phase bulk/interface structural model proposed previously for ALD AlOx and indicate that the interfacial layer at the CNT–AlOx interface is 2.5 nm thick─5 times larger than previously reported. This report demonstrates direct measurement of atomic structural variations across nanoscale material interfaces that is of broad interest for electrochemical applications and will help inform the use of ALD coatings to stabilize lithium-ion battery interfaces.

Surface oxidation of hydrophobic ZnSe for enhanced growth of atomic layer deposited aluminum oxide

The growth of atomic layer deposited (ALD) Al2O3 on planar ZnSe substrates is studied using in situ spectroscopic ellipsometry. An untreated ZnSe surface requires an incubation period of 27 cycles of ALD Al2O3 before film growth is observed. Pretreating the surface with an ultraviolet generated ozone lowers the incubation to 17 cycles, whereas a plasma-enhanced ALD Al2O3 process can further lower the incubation period to 13 cycles. The use of ozone or plasma-activated oxygen species on ZnSe is found to create ZnO and SeO2, which are responsible for converting ZnSe from a hydrophobic to a hydrophilic surface. The interfacial layer between Al2O3 and ZnSe is mapped using high-resolution transmission electron microscopy and scanning transmission electron microscopy/energy dispersive spectroscopy. SeO2 is volatile and leaves a zinc-rich interface, which is 4.3 nm thick for the ultraviolet generated ozone pretreated sample and 2.5 nm for the plasma-enhanced ALD process.

Is Coating Oxide on Porous Metal Thin-Film for Low-Temperature Solid Oxide Fuel Cell Cathode a Panacea for Performance Enhancement?

The Pt thin-film cathode thermo-mechanically reinforced by using atomic layer-deposited (ALD) alumina overcoat for low-temperature solid oxide fuel cells was investigated on electrochemical and electrical performances and its area specific resistance (ASR) with respect to cell dimension was evaluated. The nanoporous Pt thin-film cathode was fabricated by direct current magnetron sputtering with a deposition pressure of 50 mTorr. And alumina overcoat was deposited with thermal ALD 30 cycles onto the nanoporous Pt thin-film cathode. Compared to bare Pt thin-film cathode, ALD alumina-overcoated Pt thin-film cathode provided ~ 3-times lower activation ASR and ~ 2-times lower effective electrical conductivity after 4-h operation at 550 °C. Despite increases in the thermo-mechanical stability of Pt thin-film cathode by ALD alumina overcoating, it was evaluated that the total ASR of ALD alumina-overcoated Pt thin-film cathode was lower than that of the bare Pt thin-film cathode only if cell dimension is below 0.95 cm2 because of its lower current collecting performance in the in-plane direction.