Deposition Of Metal Oxides Research Articles

Area-Selective Atomic Layer Deposition of Metal Oxides on DNA Nanostructures and Its Applications

We demonstrate area-selective atomic layer deposition (ALD) of oxides on DNA nanostructures. Area-selective ALD of Al2O3, TiO2, and HfO2 was successfully achieved on both 2D and 3D DNA nanostructures deposited on a polystyrene (PS) substrate. The resulting DNA-inorganic hybrid structure was used as a hard mask to achieve deep etching of a Si wafer for antireflection applications. ALD is a widely used process in coating and thin film deposition; our work points to a way to pattern oxide materials using DNA templates and to enhance the chemical/physical stability of DNA nanostructures for applications in surface engineering.

The chemistry and energetics of the interface between metal halide perovskite and atomic layer deposited metal oxides

The chemistry of the interface between the metal halide perovskite absorber and the charge transport layer affects the performance and stability of metal halide perovskite solar cells (PSCs). The literature provides several examples of poor PSC conversion efficiency values, when electron transport layers (ETLs), such as SnO2 and TiO2, are processed by atomic layer deposition (ALD) directly on the perovskite absorber. In the present work, we shed light on the chemical modifications occurring at the perovskite surface, during ALD processing of SnO2 and TiO2, in parallel with the evaluation of the PSC cell performance. The ALD processes are carried out on a (Cs,FA)Pb(I,Br)3 perovskite by adopting tetrakis(dimethylamino)tin(IV) and tetrakis(dimethylamino)titanium(IV) as metal precursors and H2O as the coreactant for SnO2 and TiO2, respectively. Perovskite surface modification occurs in the form of an ultrathin PbBr2 layer. Furthermore, in the case of SnO2, halogen molecules are detected at the interface, in parallel with the initial growth of an oxygen-deficient SnO2. Subgap defect states just above the valence band maximum of SnO2 are also detected. These states act as hole traps at the perovskite/SnO2 interface, subsequently promoting charge recombination and deteriorating the performance of the cell. We hypothesize that a redox reaction between the perovskite, or its decomposition products, and the Sn metal center of the ALD precursor takes place: I− and Br− are oxidized to I2 and Br2, respectively, and Sn(IV) is reduced to Sn(II). In contrast, the Ti(IV) metal center does not undergo any redox process, and, as a result, a promising 11% power conversion efficiency is measured with TiO2 as the ETL. This result strongly suggests that TiO2 may be a more suitable ETL, when processed directly on the perovskite absorber.

Reaction Selectivity of IrO2-Based Nano/Amorphous Hybrid Oxide-Coated Titanium Anodes in Acidic Aqueous Solutions: Oxygen Evolution and Lead Oxide Deposition

The present study demonstrated that novel IrO2-Ta2O5/Ti anodes prepared by thermal decomposition at a low temperature of 380 °C have high reaction selectivity for oxygen evolution and metal oxide deposition in acidic aqueous solutions. Conventional IrO2-Ta2O5/Ti anodes are generally prepared at a temperature between 450 °C and 500 °C. However, if Pb(II) ions are present as an impurity in the electrolyte, the oxidation of Pb(II) ions occurs as an anodic side reaction, leading to deposition of PbO2 onto the conventional anodes. Herein, we prepared IrO2-Ta2O5/Ti anodes by thermal decomposition at the low temperature of 380 °C and investigated their crystallographic structure, surface morphology, and polarization behaviors for oxygen evolution as well as their suppression effect for anodic PbO2 deposition in sulfuric acid/lead sulfate solutions. SEM observations with low accelerated incident electrons revealed that the nano/amorphous hybrid structure consisting of nanosized IrO2 particles highly dispersed in the amorphous Ta2O5 matrix can be prepared at 380 °C. Oxygen evolution was accelerated with an increasing Ir ratio in the anodes prepared at 380 °C in sulfuric acid solutions. Anodic PbO2 deposition on the anodes prepared at 380 °C in lead sulfate solutions, which assumed the conditions of practical use, was completely suppressed.

Perovskite/perovskite planar tandem solar cells: A comprehensive guideline for reaching energy conversion efficiency beyond 30%

Perovskite/perovskite tandem solar cells (Pk/Pk TSCs) have a substantial potential to outperform the Shockley-Queisser limit of single-junction solar cells. However, optimum material bandgap selection and device processability impede the progress in acquiring efficient Pk/Pk TSCs. The choice of charge transport/contact materials additionally has a significant influence on the photovoltaic performance of Pk/Pk TSCs. Hence, the actual fabrication of a two-terminal Pk/Pk TSC becomes tricky, which requires a detailed understanding of the underlying optical and electrical properties of the device. In this study, a wide bandgap (~1.72 eV) lead iodine-bromide (Pb–I–Br) and a narrow bandgap (~1.16 eV) tin lead-iodide (Sn–Pb–I) perovskite absorbers are considered as potential sub-cells for realizing highly efficient planar Pk/Pk TSCs. Furthermore, energetically associated hole and electron selective contacts are prepared by atomic layer deposition (ALD) of metal oxides. The optics of solar cells is investigated by three-dimensional finite-difference time-domain (FDTD) optical simulations, and finite element method (FEM) electrical simulations are exploited to determine realistic photovoltaic performance parameters. A comprehensive study is carried out to provide a complete guideline for the realization of energy conversion efficiency exceeding 30% in Pk/Pk TSCs.

Atomic Layer Deposition of Metal Oxides in Perovskite Solar Cells: Present and Future

AbstractIn recent years, the development of perovskite solar cells (PSCs) is advancing along the way, and the efficiency is comparable to traditional silicon‐based solar cells. However, as crucial factors in the road to commercialization, stability and upscaling manufacture have not been fully investigated yet. To solve these problems, the exploration of charge transport layer (CTL) is clearly imminent, which is critical to the stability of PSCs. Among them, inorganic metal oxides have better stability than organic CTL. Particularly, the atomic layer deposition (ALD) process can fabricate dense and scalable metal oxides based on the self‐limiting surface reaction. This perspective focuses on the recent progress of ALD‐grown metal oxides in PSCs: both of electron and hole transport layer; connection layer in tandem architectures; application in semi‐transparent perovskite solar cells (ST‐PSCs); prospective of commercialization feasibility of the ALD‐grown metal oxides in ST‐PSCs.

Improved carbon disulfide conversion: Modification of an alumina Claus catalyst by deposition of transition metal oxides

Destruction of CS2 in the modified Claus process plays a vital role in achieving high sulfur recovery (>99.9%). Although it has lower catalytic activity for CS2 hydrolysis than titania, alumina has been widely used in Claus plants for conversion of sulfur species into elemental sulfur, because it is less expensive, highly active for the Claus reaction and mechanically robust. This research explores improving CS2 hydrolysis on Claus alumina catalyst by modification with 1 wt.% of either CuO, TiO2, Fe2O3, Co2O3, Sc2O3 or Na2O. These modified catalysts were tested using the first catalytic convertor conditions. While catalyst impregnation with most of the metals described above leads to minor improved CS2 conversion, the deposition of both Cu2+ and Ti4+ stood out for resulting 22.6 and 33.7% higher activity, respectively. Results suggest that the improved catalytic activity can be explained by reduced hydrophilicity (increased hydroxyl dispersion) and somewhat increased basicity on the surface of the catalysts.

Silver nanoparticles coupled ZnO nanorods array prepared using photo-reduction method for localized surface plasmonic effect study

Coupling of selected metal nanoparticles (NPs) on the surface of ZnO nanorods (NRs) array grown on conductive glass substrates is one of the approaches to tailor the optical properties of ZnO NRs. It is a metal oxide deposition process followed by an annealing treatment to convert the deposits to metal NPs. However, the conductive glass substrates could easily deform during this annealing process. This issue limits the application of ZnO NRs array that grown on glass substrates for the fabrication of ZnO based devices. Thus, the development of metal NPs deposition technique without the need of post-deposition annealing is highly desired. This work demonstrated a successful and controlled deposition of silver (Ag) NPs on ZnO NRs array grown on fluorine doped tin oxide (FTO) glass substrates without any post-deposition annealing and, the product has exhibited localized surface plasmonic resonance (LSPR) phenomenon. The ZnO NRs arrays were grown on FTO glass substrates that pre-seeded with ZnO layer using hydrothermal method. The controlled coupling of Ag NPs particles on ZnO NRs was performed by adjusting the duration of photo-reduction process. The optical bandgap ZnO NRs was widened because of Ag NPs coupling, i.e. from 3.16 to 3.22 eV; as estimated from the result of UV–Vis measurement. The shift of near band-edge emission (NBE) in room temperature photoluminescence (RTPL) suggests that the presence of Ag NPs on the surface of ZnO NRs could block the transitions of electrons in ZnO whereas the shift in defect-related emission indicates the generation of defects at the interfaces between Ag NPs and ZnO NRs. The 5-folds increment of IUV/Vis ratio of Ag NPs/ZnO NRs in RTPL, indicating the specimens experienced LSPR effect. The LSPR effect of Ag NPs/ZnO NRs could be used to improve the performance of ZnO-based devices such as solar cells, light-emitting diodes and photocatalysts applications.

Interface Matters: Enhanced Photoluminescence and Long-Term Stability of Zero-Dimensional Cesium Lead Bromide Nanocrystals via Gas-Phase Aluminum Oxide Encapsulation.

Cesium lead halide perovskite nanocrystals (PNCs), while possessing facile chemical synthesis routes and high photoluminescence (PL) properties, are still challenged by issues of instability and degradation. Although atomic layer deposition (ALD) of metal oxides has been one of the common encapsulation approaches for longer term stability, its application inevitably resulted in severe loss of emission efficiency and at times partial loss of structural integrity of perovskites, creating a bottleneck in its practical viability. We demonstrate a nondestructive modified gas-phase technique with codeposition of both precursors trimethylaluminum and water to dramatically enhance the PL emission in zero-dimensional (0D) Cs4PbBr6 PNCs via alumina encapsulation. X-ray photoelectron spectroscopy analysis of Cs4PbBr6 films reveals the alumina deposition to be accompanied by elemental composition changes, particularly by the reduction of the excessive cesium content. Ab initio density functional theory simulations further unfold that the presence of excess Cs on the surface of PNCs leads to decomposition of structural [PbBr6]4- octahedra in the 0D perovskite lattice, which can be prevented in the presence of added hydroxyl groups. Our study thus unveils the pivotal role of the PNC surface composition and treatment in the process of its interaction with metal oxide precursors to control the PL properties as well as the stability of PNCs, providing an unprecedented way to use the conventional ALD technique for their successful integration into optoelectronic and photonic devices with improved properties.

Voltage regulated electrophoretic deposition of silver nanoparticles on halloysite nanotubes

Halloysite nanotubes (HNTs) are naturally occurring clay nanotubes mined from abundant mineral deposits, making it an easily accessible nanomaterial. Surface functionalization of HNTs with different components including metals, antibiotics, and bioactive compounds is of increased importance for their potential use in biomedical devices, antimicrobial surface coatings, drug delivery systems, radiation absorptive composites, elastomer composites, electronic components, and as industrial catalysts. We used a simple method for the fabrication of HNT-supported metal nanoparticles. Here, we report our strategy for controlled weight deposition of positively charged metal ions on negatively charged HNTs dispersed in an aqueous medium where metallization can be controlled using changes in voltage, solvent medium, time and other electrolytic parameters without the use of any toxic chemicals, expensive reagents or lengthy pre-processing steps. Our method offers a one-step and low-cost process that offers many advantages, including, the deposition of different metal oxides (Ag, Cu, and Zn) or dual metal coatings. HNTs act as a nanocarrier with an ability for sustained drug release and as a nanofiller with a record for improving the physical properties of polymers. When combined with metal-coated HNTs, this may lead to the creation of multi-functional applications in drug delivery, regenerative medicine and, tissue engineering.

Plasma-enhanced pulsed laser deposition of copper oxide and zinc oxide thin films

Plasma-Enhanced Pulsed Laser Deposition (PE-PLD) is a technique for depositing metal oxide thin films that combines traditional PLD of metals with a low-temperature oxygen background plasma. This proof-of-concept study shows that PE-PLD can deposit copper oxide and zinc oxide films of similar properties to ones deposited using traditional PLD, without the need for substrate heating. Varying the pressure of the background plasma changed the stoichiometry and structure of the films. Stoichiometric copper oxide and zinc oxide films were deposited at pressures of 13 Pa and 7.5 Pa, respectively. The deposition rate was ∼5 nm/min and the films were polycrystalline with a crystal size in the range of 3 nm–15 nm. The dominant phase for ZnO was (110) and for CuO, they were (020) and (111¯), where (020) is known as a high-density phase not commonly seen in PLD films. The resistivity of the CuO film was 0.76 ± 0.05 Ω cm, in line with films produced using traditional PLD. Since PE-PLD does not use substrate heating or post-annealing, and the temperature of the oxygen background plasma is low, the deposition of films on heat-sensitive materials such as plastics is possible. Stoichiometric amorphous zinc oxide and copper oxide films were deposited on polyethylene (PE) and polytetrafluoroethylene (PFTE).

Scalable metal oxide functionalized GaN nanowire for precise SO2 detection

In this work, GaN nanowires have been formed on Si substrate using production standard stepper lithography and top-down approach. Three different functionalized devices were prepared by the deposition of metal oxides- ZnO, WO3 and SnO2 by optimized RF sputtering on nanowires followed by rapid thermal annealing. The elemental composition, crystallinity and surface topography of metal-oxide/GaN nanowires were fully characterized by EDS, XRD, AFM and SEM. The SO2 gas sensing data was collected and analyzed for all three sensors. The ZnO/GaN sensor was found to be the best candidate for precise SO2 detection. To examine the real-world applicability of ZnO/GaN sensor device, its additional sensing properties, including gas sensing adsorption and desorption rate, cross-sensitivity to interfering gases, and long-term stability at various environmental conditions were investigated. Furthermore, principal component analysis has been performed to address the cross-sensitive behavior of ZnO. The SO2 sensing mechanism on metal-oxide/GaN under UV irradiation was discussed as well. Results demonstrate that ZnO functionalized GaN nanowire can be employed as a high performance SO2 sensor.

Selective conversion of xylose to lactic acid over metal-based Lewis acid supported on γ-Al2O3 catalysts

Gamma-alumina (γ-Al2O3) is a low-cost amphoteric solid catalyst which can enhance the transformation of d-xylose into lactic acid. The deposition of metal oxides on γ-Al2O3 could further improve the yield of lactic acid from xylose. Therefore, in this work, the thermochemical catalytic conversion of d-xylose to lactic acid using the Cr, Cu, Co, Ni, and Sn oxides supported on γ-Al2O3 as heterogeneous catalysts was studied. The effects of metal oxides on the d-xylose conversion to lactic acid activity of the catalysts were investigated. It was found that, under identical testing conditions (170 °C, 4 h), Cr/Al2O3 is superior to the other catalysts with the 99 % conversion of d-xylose and the lactic acid yield of 74%. The outstanding activity could be attributed to its richness in Lewis-acid sites. Since the highly active Cr/Al2O3 catalyst is composed of the earth-abundant materials and can be prepared by a simple process, it has feasibility for industrial-scale application.

Rapid Scalable Processing of Tin Oxide Transport Layers for Perovskite Solar Cells.

The development of scalable deposition methods for perovskite solar cell materials is critical to enable the commercialization of this nascent technology. Herein, we investigate the use and processing of nanoparticle SnO2 films as electron transport layers in perovskite solar cells and develop deposition methods for ultrasonic spray coating and slot-die coating, leading to photovoltaic device efficiencies over 19%. The effects of postprocessing treatments (thermal annealing, UV ozone, and O2 plasma) are then probed using structural and spectroscopic techniques to characterize the nature of the np-SnO2/perovskite interface. We show that a brief “hot air flow” method can be used to replace extended thermal annealing, confirming that this approach is compatible with high-throughput processing. Our results highlight the importance of interface management to minimize nonradiative losses and provide a deeper understanding of the processing requirements for large-area deposition of nanoparticle metal oxides.

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

AbstractEncapsulation 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 (AlOx) 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 AlOx 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.

Photocatalytic Bi2O3/TiO2:N Thin Films with Enhanced Surface Area and Visible Light Activity

Bi2O3 nanocone films functionalized with an overlayer of TiO2 were deposited by d.c. reactive magnetron sputtering. The aforementioned nanocone structures were formed via a vapour-liquid-solid (VLS) growth, starting from a catalytic bismuth seed layer. The resultant nanocones exhibit an improved surface area, measured by atomic force microscopy, when compared to non-VLS deposition of the same metal oxide. X-ray diffraction texture analysis enabled the determination of the crystallographic β-phase of Bi2O3. A very thin TiO2 overlayer (6 nm thick), undoped and doped with nitrogen, was deposited onto the nanocones template, in order to functionalize these structures with a photocatalytic, self-cleaning, cap material. N-doped TiO2 overlayers increased the selective absorption of visible light due to nitrogen doping in the anatase cell, thus, resulting in a concomitant increase in the overall photocatalytic efficiency.

Passivation of Defects in Macroscopic Graphene Sheets Via Selective Atomic Layer Deposition of Metal Oxides

Graphene, with its high conductivity, specific surface area, and overall chemical inertness, has been heavily researched for applications in batteries, supercapacitors, and water-splitting devices. While graphene films can be synthesized over large areas (>1 cm2) through chemical vapor deposition, graphene generated through this route exhibits a high defect density that can inhibit performance and stability in electrochemical systems. Considerable effort has been devoted to passivating, repairing, and remediating these defects for graphene-based devices to match their theoretical potential.To this end, we have developed new atomic layer deposition (ALD) processes designed to selectively passivate high energy defect sites with metal oxides. This deposition chemistry targets dangling bonds and oxidized regions on the graphene sheet based on their higher reactivity relative to the pristine basal plane. We have also demonstrated analogous chemistry on bulk highly-ordered pyrolytic graphite as well as exfoliated few-layer graphene. The efficacy of the deposition process was examined as a function of precursor dose and temperature. The selectivity of this process and metal-oxide layer thicknesses was assessed using several electron microscopy techniques. This work further illustrates an effective scheme to ameliorate the interfacial chemistry of large-area graphene and demonstrate how targeted passivation can improve device performance.

Defect-Seeded Atomic Layer Deposition of Metal Oxides on the Basal Plane of 2D Layered Materials.

Atomic layer deposition (ALD) on mechanically exfoliated 2D layered materials spontaneously produces network patterns of metal oxide nanoparticles in triangular and linear deposits on the basal surface. The network patterns formed under a range of ALD conditions and were independent of the orientation of the substrate in the ALD reactor. The patterns were produced on MoS2 or HOPG when either tetrakis(dimethylamido)titanium or bis(ethylcyclopentadienyl)manganese were used as precursors, suggesting that the phenomenon is general for 2D materials. Transmission electron microscopy revealed the presence, prior to deposition, of dislocation networks along the basal plane of mechanically exfoliated 2D flakes, indicating that periodical basal plane defects related to disruptions in the van der Waals stacking of layers, such as perfect line dislocations and triangular extended stacking faults networks, introduce a surface reactivity landscape that leads to the emergence of patterned deposition.

Surface Initiated Polymer Thin Films for the Area Selective Deposition and Etching of Metal Oxides.

The area selective growth of polymers and their use as inhibiting layers for inorganic film depositions may provide a valuable self-aligned process for fabrication. Polynorbornene (PNB) thin films were grown from surface-bound initiators and show inhibitory properties against the atomic layer deposition (ALD) of ZnO and TiO2. Area selective control of the polymerization was achieved through the synthesis of initiators that incorporate surface-binding ligands, enabling their selective attachment to metal oxide features versus silicon dielectrics, which were then used to initiate surface polymerizations. The subsequent use of these films in an ALD process enabled the area selective deposition (ASD) of up to 39 nm of ZnO. In addition, polymer thickness was found to play a key role, where films that underwent longer polymerization times were more effective at inhibiting higher numbers of ALD cycles. Finally, while the ASD of a TiO2 film was not achieved despite blanket studies showing inhibition, the ALD deposition on polymer regions of a patterned film produced a different quality metal oxide and therefore altered its etch resistance. This property was exploited in the area selective etch of a metal feature. This demonstration of an area selective surface-grown polymer to enable ASD and selective etch has implications for the fabrication of both micro- and nanoscale features and surfaces.

Single-atom scale metal vacancy engineering in heteroatom-doped carbon for rechargeable zinc-air battery with reduced overpotential

Metal-air batteries have drawn great attention in the past decade due to their high energy density and low cost. However, several challenges remain in the development of such promising devices, including slow kinetics of the cathodic reaction and undesirable deposition of metal oxide on air cathode. Herein, as a proof of concept, we show single-atom scale metal vacancy engineering in heteroatom-doped carbon cathode to enable high-performance zinc-air battery with reduced overpotential. Density functional theory calculations indicate that metal vacancy-induced pyridinic-N can tailor the electronic structure and thus facilitate the catalytic reaction. The single-atom dispersed Fe–N–C catalyst with optimized Fe vacancies shows improved reactivity and the facilitated chemisorption of oxygen intermediates in the actual battery operation is directly observed by in-situ X-ray diffraction characterization. Moreover, the in-situ observation also indicates the absence of zinc oxide, which can be ascribed to strong Lewis basicity created by pyridinic-N that effectively prevents the access of zincate ions with negative charge and thus enables non-deposition of zinc oxide. Applying this strategy, the rechargeable zinc-air battery shows high reversibility and stability over 1000 cycles with negligible voltage gap change of only 0.04 V. This work not only affords valuable insights but also opens up new perspectives to develop high-performance metal-air batteries.

Nano-layer deposition of metal oxides via a condensed water film

Nanocoatings on solids can be achieved by various processes, including sol-gel and atomic layer deposition. However, challenges remain for achieving uniform nanocoatings on nanoscale substrates at a large scale. Here, we report a versatile and fundamentally different technique, termed condensed layer deposition, for depositing conformal metal oxide nanocoatings on nanoparticles and nanofibers. This approach involves water in liquid hydrocarbons condensing as a nanoscale water film on the substrate surface, enabled by interfacial tension between polar water and nonpolar liquid hydrocarbons. Chemical precursors are then added, which react with the condensed water film to form a metal oxide nanocoating. We demonstrate this for titania, alumina, and niobia on substrates including carbon nanotubes, iron oxide particles and carbon black. Condensed layer deposition can achieve oxide nanocoatings on a variety of substrates with tunable thickness, in one pass, at room temperature.