Atomic layer deposition in nanostructured photovoltaics: tuning optical, electronic and surface properties

One-Step Dry Coating of Hybrid ZnO-WO3 Nanosheet Photoanodes for Photoelectrochemical Water Splitting with Composition-Dependent Performance

In this study, the potential of zinc oxide (ZnO), tungsten oxide (WO3), and their composites (ZnO-WO3) as photoanodes for photoelectrochemical (PEC) water splitting was investigated. ZnO-WO3 nanocomposites (NCs) were deposited on fluorine-doped tin oxide substrates at room temperature using a one-step dry coating process, the nanoparticle deposition system, with no post-processes. Different compositions of ZnO-WO3 NCs were optimized to enhance the kinetics of the PEC water-splitting reaction. Surface morphology analysis revealed the transformation of microsized particle nanosheets (NS) powder into nanosized particle nanosheets (NS) across all photoanodes. The optical characteristics of ZnO-WO3 photoanodes were scrutinized using diffuse reflectance and photoluminescence emission spectroscopy. Of all the hybrid photoanodes tested, the photoanode containing 10 wt.% WO3 exhibited the lowest bandgap of 3.20 eV and the lowest emission intensity, indicating an enhanced separation of photogenerated carriers and solar energy capture. The photoelectrochemical results showed a 10% increase in the photocurrent with increasing WO3 content in ZnO-WO3 NCs, which is attributed to improved charge transfer kinetics and carrier segregation. The maximum photocurrent for a NC, i.e., 10 wt.% WO3, was recorded at 0.133 mA·cm-2 at 1.23V vs. a reversible hydrogen electrode (RHE). The observed improvement in photocurrent was nearly 22 times higher than pure WO3 nanosheets and 7.3 times more than that of pure ZnO nanosheets, indicating the composition-dependence of PEC performance, where the synergy requirement strongly relies on utilizing the optimal ZnO-WO3 ratio in the hybrid NCs.

Atomic-scale study of TiO2-GR nanohybrid formation by ALD: the effect of the gas phase precursor

In the present work, we report on a theoretical-computational study of the growth mechanism of the TiO2-Graphene nanohybrid by atomic layer deposition. Hydroxyl groups (OH) are anchoring sites for interacting with the main ALD titanium precursors (Tetrakis (dimethylamino) Titanium, Titanium Tetrachloride, and Titanium Isopropoxide). Results demonstrate that the chemical nature of the precursor directly affects the reaction mechanism in each ALD growth step. Tetrakis(dimethylamino)titanium is the precursor that presents a higher affinity (lower energy barriers for the reaction) to hydroxylated graphene in the growth process. A complete reaction mechanism for each precursor was proposed. The differences between precursors were discussed through the non-covalent interactions index. Finally, the water molecules help reduce the energy barriers and consequently favor the formation of the TiO2-graphene nanohybrid.

First-Principles Molecular Dynamics Simulations on Water-Solid Interface Behavior of H2O-Based Atomic Layer Deposition of Zirconium Dioxide

As an important inorganic material, zirconium dioxide (ZrO₂) has a wide range of applications in the fields of microelectronics, coating, catalysis and energy. Due to its high dielectric constant and thermodynamic stability, ZrO₂ can be used as dielectric material to replace traditional silicon dioxide. Currently, ZrO₂ dielectric films can be prepared by atomic layer deposition (ALD) using water and zirconium precursors, namely H₂O-based ALD. Through density functional theory (DFT) calculations and first-principles molecular dynamics (FPMD) simulations, the adsorption and dissociation of water molecule on the ZrO₂ surface and the water-solid interface reaction were investigated. The results showed that the ZrO₂ (111) surface has four Lewis acid active sites with different coordination environments for the adsorption and dissociation of water. The Zr atom on the surface can interacted with the O atom of the water molecule via the p orbital of the O atom and the d orbital of the Zr atom. The water molecules could be dissociated via the water-solid interface reaction of the first or second layer of water molecules with the ZrO₂ (111) surface. These insights into the adsorption and dissociation of water and the water-solid interface reaction on the ZrO₂ surface could also provide a reference for the water-solid interface behavior of metal oxides, such as H₂O-based ALD.

Reaction mechanism of atomic layer deposition of zirconium oxide using zirconium precursors bearing amino ligands and water

As a unique nanofabrication technology, atomic layer deposition (ALD) has been widely used for the preparation of various materials in the fields of microelectronics, energy and catalysis. As a high-κ gate dielectric to replace SiO₂, zirconium oxide (ZrO₂) has been prepared through the ALD method for microelectronic devices. In this work, through density functional theory calculations, the possible reaction pathways of ZrO₂ ALD using tetrakis(dimethylamino)zirconium (TDMAZ) and water as the precursors were explored. The whole ZrO₂ ALD reaction could be divided into two sequential reactions, TDMAZ and H₂O reactions. In the TDMAZ reaction on the hydroxylated surface, the dimethylamino group of TDMAZ could be directly eliminated by substitution and ligand exchange reactions with the hydroxyl group on the surface to form dimethylamine (HN(CH₃)₂). In the H₂O reaction with the aminated surface, the reaction process is much more complex than the TDMAZ reaction. These reactions mainly include ligand exchange reactions between the dimethylamino group of TDMAZ and H₂O and coupling reactions for the formation of the bridged products and the by-product of H₂O or HN(CH₃)₂. These insights into surface reaction mechanism of ZrO₂ ALD can provide theoretical guidance for the precursor design and improving ALD preparation of other oxides and zirconium compounds, which are based ALD reaction mechanism.

Iterative reverse Monte Carlo and molecular statics for improved atomic structure modeling: a case study of zinc oxide grown by atomic layer deposition

Reverse Monte Carlo (RMC) modeling is a common method to derive atomic structure models of materials from experimental diffraction data. However, conventional RMC modeling does not impose energetic constraints and can produce non-physical local structures within the simulation volume. Although previous strategies have introduced energetic constraints during RMC modeling, these approaches have limitations in computational cost and physical accuracy. In this work, we periodically introduce molecular statics (MS) energy minimizations during RMC modeling in an iterative RMC-MS approach. We test this iterative RMC-MS approach using diffraction data collected by in operando high energy X-ray diffraction during atomic layer deposition of ZnO as a sample case. For MS relaxations we employ ReaxFF pair potentials previously established for ZnO. We find that RMC-MS and RMC provide equivalent agreement with experimental data, but RMC-MS structures are on average 0.6 eV per atom lower in energy and are more consistent with known ZnO atomic structure features. The iterative RMC-MS approach we report can accommodate large systems with minimal additional computational burden beyond a standard RMC simulation and can leverage established pair potentials for immediate application to study a wide range of materials.

Applications of Atomic Layer Deposition in Design of Systems for Energy Conversion

There is a huge demand for clean energy conversion in all industries. The clean energy production processes include electrocatalytic and photocatalytic conversion of water to hydrogen, carbon dioxide reduction, nitrogen conversion to ammonia, and oxygen reduction reaction and require novel cheap and efficient photo- and electrocatalysts and their scalable methods of fabrication. Atomic layer deposition is a thin film deposition method that allows to deposit thin layers of catalysts on virtually any surface of any shape, size, and porosity in an even and easy to control manner. Here the state of the art in applications of atomic layer deposition in the clean energy production and the opportunities it represents for the whole field of the photo- and electrocatalysis for a sustainable future are reviewed.

Reaction mechanism of atomic layer deposition of aluminum sulfide using trimethylaluminum and hydrogen sulfide

Atomic layer deposition (ALD) is a nanopreparation technique for materials and is widely used in the fields of microelectronics, energy and catalysis. ALD methods for metal sulfides, such as Al2S3 and Li2S, have been developed for lithium-ion batteries and solid-state electrolytes. In this work, using density functional theory calculations, the possible reaction pathways of the ALD of Al2S3 using trimethylaluminum (TMA) and H2S were investigated at the M06-2X/6-311G(d, p) level. Al2S3 ALD can be divided into two consecutive and complementary half-reactions involving TMA and H2S, respectively. In the TMA half-reaction, the methyl group can be eliminated through the reaction with the sulfhydryl group on the surface. This process is a ligand exchange reaction between the methyl and sulfhydryl groups via a four-membered ring transition state. TMA half-reaction with the sulfhydrylated surface is more difficult than that with the hydroxylated surface. When the temperature increases, the reaction requires more energy, owing to the contribution of the entropy. In the H2S half-reaction, the methyl group on the surface can further react with the H2S precursor via a four-membered ring transition state. The orientation of H2S and more molecules have minimal effect on the H2S half-reaction. The reaction involving H2S through a six-membered ring transition state is unfavorable. In addition, the methyl and sulfhydryl groups on the surface can both react with the adjacent sulfhydryl group on the subsurface to form and release CH4 or H2S in the two half-reactions. Furthermore, sulfhydryl elimination occurs more easily than methyl elimination on the surface. These findings for the TMA and H2S half-reactions of Al2S3 ALD may be used for studying precursor chemistry and improvements in the preparation of other metal sulfides for emerging applications.

Atomic/molecular layer deposition for energy storage and conversion

Energy storage and conversion systems, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting, have played vital roles in the reduction of fossil fuel usage, addressing environmental issues and the development of electric vehicles. The fabrication and surface/interface engineering of electrode materials with refined structures are indispensable for achieving optimal performances for the different energy-related devices. Atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques, the gas-phase thin film deposition processes with self-limiting and saturated surface reactions, have emerged as powerful techniques for surface and interface engineering in energy-related devices due to their exceptional capability of precise thickness control, excellent uniformity and conformity, tunable composition and relatively low deposition temperature. In the past few decades, ALD and MLD have been intensively studied for energy storage and conversion applications with remarkable progress. In this review, we give a comprehensive summary of the development and achievements of ALD and MLD and their applications for energy storage and conversion, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting. Moreover, the fundamental understanding of the mechanisms involved in different devices will be deeply reviewed. Furthermore, the large-scale potential of ALD and MLD techniques is discussed and predicted. Finally, we will provide insightful perspectives on future directions for new material design by ALD and MLD and untapped opportunities in energy storage and conversion.

New development of atomic layer deposition: processes, methods and applications

Atomic layer deposition (ALD) is an ultra-thin film deposition technique that has found many applications owing to its distinct abilities. They include uniform deposition of conformal films with controllable thickness, even on complex three-dimensional surfaces, and can improve the efficiency of electronic devices. This technology has attracted significant interest both for fundamental understanding how the new functional materials can be synthesized by ALD and for numerous practical applications, particularly in advanced nanopatterning for microelectronics, energy storage systems, desalinations, catalysis and medical fields. This review introduces the progress made in ALD, both for computational and experimental methodologies, and provides an outlook of this emerging technology in comparison with other film deposition methods. It discusses experimental approaches and factors that affect the deposition and presents simulation methods, such as molecular dynamics and computational fluid dynamics, which help determine and predict effective ways to optimize ALD processes, hence enabling the reduction in cost, energy waste and adverse environmental impacts. Specific examples are chosen to illustrate the progress in ALD processes and applications that showed a considerable impact on other technologies.

Reaction Temperature and Partial Pressure Induced Etching of Methylammonium Lead Iodide Perovskite by Trimethylaluminum

Al₂O₃ atomic layer deposition (ALD), which uses trimethylaluminum (TMA) as the metal precursor, shows promise in improving the environmental stability of hybrid halide perovskites. However, it is not yet entirely clear how TMA, a strong Lewis acid, reacts with fresh perovskites and how the reaction affects the nucleation of ALD Al₂O₃. Here, the effects of reaction temperature and partial pressure of TMA on the mechanisms of TMA/CH₃NH₃PbI₃ reactions are investigated. Our real time mass gain data and in situ mass spectrometry data show that the TMA/CH₃NH₃PbI₃ reaction can either remove mass or accumulate mass onto CH₃NH₃PbI₃ substrates, depending strongly on the reaction temperature and partial pressure of TMA. The TMA/CH₃NH₃PbI₃ reaction probably generates TMA-CH₃NHx adduct compounds, which protects CH₃NH₃PbI₃ from TMA by forming a shell at 25 °C in the vacuum process. However, these adduct compounds decompose at higher temperatures (e.g., 75 °C). This product layer is much thicker than a monolayer, suggesting the interface formed between Al₂O₃ coating and CH₃NH₃PbI₃ is blurring and messy. These results have not yet, but should be, carefully considered to correctly interpret the effect of ALD Al₂O₃ treatment on optoelectronic properties of CH₃NH₃PbI₃.

Atomic Layer Deposition for Preparation of Highly Efficient Catalysts for Dry Reforming of Methane

In this article, the structural and chemical properties of heterogeneous catalysts prepared by atomic layer deposition (ALD) are discussed. Oxide shells can be deposited on metal particles, forming shell/core type catalysts, while metal nanoparticles are incorporated into the deep inner parts of mesoporous supporting materials using ALD. Both structures were used as catalysts for the dry reforming of methane (DRM) reaction, which converts CO2 and CH4 into CO and H2. These ALD-prepared catalysts are not only highly initially active for the DRM reaction but are also stable for long-term operation. The origins of the high catalytic activity and stability of the ALD-prepared catalysts are thoroughly discussed.

Atomic Layer Deposition of Nickel Carbide from a Nickel Amidinate Precursor and Hydrogen Plasma

A new atomic layer deposition (ALD) process for depositing nickel carbide (Ni₃C _x) thin films is reported, using bis( N, N'-di- tert-butylacetamidinato)nickel(II) and H₂ plasma. The process shows a good layer-by-layer film growth behavior with a saturated film growth rate of 0.039 nm/cycle for a fairly wide process temperature window from 75 to 250 °C. Comprehensive material characterizations are performed on the Ni₃C _x films deposited at 95 °C with various H₂ plasma pulse lengths from 5 to 12 s, and no appreciable difference is found with the change of the plasma pulse length. The deposited Ni₃C _x films are fairly pure, smooth, and conductive, and the x in the nominal formula of Ni₃C _x is approximately 0.7. The ALD Ni₃C _x films are polycrystalline with a rhombohedral Ni₃C crystal structure, and the films are free of nanocrystalline graphite or amorphous carbon. Last, we demonstrate that, by using this ALD process, highly uniform Ni₃C _x films can be conformally deposited into deep narrow trenches with an aspect ratio as high as 20:1, which thereby highlights the broad and promising applicability of this process for conformal Ni₃C _x film coatings on complex high-aspect-ratio 3D architectures in general.

Stabilization of Polyoxometalate Water Oxidation Catalysts on Hematite by Atomic Layer Deposition

Fast and earth-abundant-element polyoxometalates (POMs) have been heavily studied recently as water oxidation catalysts (WOCs) in homogeneous solution. However, POM WOCs can be quite unstable when supported on electrode or photoelectrode surfaces under applied potential. This article reports for the first time that a nanoscale oxide coating (Al₂O₃) applied by the atomic layer deposition (ALD) aids immobilization and greatly stabilizes this now large family of molecular WOCs when on electrode surfaces. In this study, [{Ru^IV₄(OH)₂(H₂O)₄}(γ-SiW₁₀O₃₄)₂]^10- (Ru₄Si₂) is supported on hematite photoelectrodes and then protected by ALD Al₂O₃; this ternary system was characterized before and after photoelectrocatalytic water oxidation by Fourier transform infrared, X-ray photoelectron spectroscopy, energy-dispersive X-ray, and voltammetry. All these studies indicate that Ru₄Si₂ remains intact with Al₂O₃ ALD protection, but not without. The thickness of the Al₂O₃ layer significantly affects the catalytic performance of the system: a 4 nm thick Al₂O₃ layer provides optimal performance with nearly 100% faradaic efficiency for oxygen generation under visible-light illumination. Al₂O₃ layers thicker than 6.5 nm appear to completely bury the Ru₄Si₂ catalyst, removing all of the catalytic activity, whereas thinner layers are insufficient to maintain a long-term attachment of the catalytic POM.

Low-Temperature Atomic Layer Deposition of CuSbS2 for Thin-Film Photovoltaics

Copper antimony sulfide (CuSbS₂) has been gaining traction as an earth-abundant absorber for thin-film photovoltaics given its near ideal band gap for solar energy conversion (∼1.5 eV), large absorption coefficient (>10⁴ cm^-1), and elemental abundance. Through careful in situ analysis of the deposition conditions, a low-temperature route to CuSbS₂ thin films via atomic layer deposition has been developed. After a short (15 min) postprocess anneal at 225 °C, the ALD-grown CuSbS₂ films were crystalline with micron-sized grains, exhibited a band gap of 1.6 eV and an absorption coefficient >10⁴ cm^-1, as well as a hole concentration of 10¹⁵ cm^-3. Finally, the ALD-grown CuSbS₂ films were paired with ALD-grown TiO₂ to form a photovoltaic device. This photovoltaic device architecture represents one of a very limited number of Cd-free CuSbS₂ PV device stacks reported to date, and it is the first to demonstrate an open-circuit voltage on par with CuSbS₂/CdS heterojunction PV devices. While far from optimized, this work demonstrates the potential for ALD-grown CuSbS₂ thin films in environmentally benign photovoltaics.

Atomic and molecular layer deposition: off the beaten track

ALD archetype and deviations from it.

Recent Development of Advanced Electrode Materials by Atomic Layer Deposition for Electrochemical Energy Storage

Electrode materials play a decisive role in almost all electrochemical energy storage devices, determining their overall performance. Proper selection, design and fabrication of electrode materials have thus been regarded as one of the most critical steps in achieving high electrochemical energy storage performance. As an advanced nanotechnology for thin films and surfaces with conformal interfacial features and well controllable deposition thickness, atomic layer deposition (ALD) has been successfully developed for deposition and surface modification of electrode materials, where there are considerable issues of interfacial and surface chemistry at atomic and nanometer scale. In addition, ALD has shown great potential in construction of novel nanostructured active materials that otherwise can be hardly obtained by other processing techniques, such as those solution-based processing and chemical vapor deposition (CVD) techniques. This review focuses on the recent development of ALD for the design and delivery of advanced electrode materials in electrochemical energy storage devices, where typical examples will be highlighted and analyzed, and the merits and challenges of ALD for applications in energy storage will also be discussed.

Tuning band alignment by CdS layers using a SILAR method to enhance TiO2/CdS/CdSe quantum-dot solar-cell performance

We report tuning band alignment by optimized CdS layers using a SILAR method to achieve the recorded best performance with about 6% PCE in TiO2/CdS/CdSe QDSSCs. Combining experimental and theoretical studies, we find that a better lattices match between CdS and TiO2 assists the growth of CdSe, and the combined effect of charge transfer and surface dipole moment at the TiO2/CdS/CdSe interface shifts the energy levels of TiO2 upward and increases Voc of the solar cells. More importantly, the band gap of CdS buffer layers is sensitive to the distortion induced by lattice mismatch and numbers of CdS layers. For example, the barrier for charge transfer disappears when there are more than 4 layers of CdS, facilitating the charge injection from CdSe to TiO2.

Photosensitization of ZnO Crystals with Iodide-Capped PbSe Quantum Dots

Lead selenide (PbSe) quantum dots (QDs) are an attractive material for application in photovoltaic devices due to the ability to tune their band gap, efficient multiple exciton generation, and high extinction coefficients. However, PbSe QDs are quite unstable to oxidation in air. Recently there have been multiple studies detailing postsynthetic halide treatments to stabilize lead chalcogenide QDs. We exploit iodide-stabilized PbSe QDs in a model QD-sensitized solar cell configuration where zinc oxide (ZnO) single crystals are sensitized using cysteine as a bifunctional linker molecule. Sensitized photocurrents stable for >1 h can be measured in aqueous KI electrolyte that is usually corrosive to QDs under illumination. The spectral response of the sensitization extended out to 1700 nm, the farthest into the infrared yet observed. Hints of the existence of multiple exciton generation and collection as photocurrent, as would be expected in this system, are speculated and discussed.

Atomic Layer Deposition of Bismuth Vanadates for Solar Energy Materials

The fabrication of porous nanocomposites is key to the advancement of energy conversion and storage devices that interface with electrolytes. Bismuth vanadate, BiVO4 , is a promising oxide for solar water splitting where the controlled fabrication of BiVO4 layers within porous, conducting scaffolds has remained a challenge. Here, the atomic layer deposition of bismuth vanadates is reported from BiPh3 , vanadium(V) oxytriisopropoxide, and water. The resulting films have tunable stoichiometry and may be crystallized to form the photoactive scheelite structure of BiVO4 . A selective etching process was used with vanadium-rich depositions to enable the synthesis of phase-pure BiVO4 after spinodal decomposition. BiVO4 thin films were measured for photoelectrochemical performance under AM 1.5 illumination. The average photocurrents were 1.17 mA cm(-2) at 1.23 V versus the reversible hydrogen electrode using a hole-scavenging sulfite electrolyte. The capability to deposit conformal bismuth vanadates will enable a new generation of nanocomposite architectures for solar water splitting.

Atomic Layer Deposition of MnS: Phase Control and Electrochemical Applications

Manganese sulfide (MnS) thin films were synthesized via atomic layer deposition (ALD) using gaseous manganese bis(ethylcyclopentadienyl) and hydrogen sulfide as precursors. At deposition temperatures ≤150 °C phase-pure γ-MnS thin films were deposited, while at temperatures >150 °C, a mixed phase consisting of both γ- and α-MnS resulted. In situ quartz crystal microbalance (QCM) studies validate the self-limiting behavior of both ALD half-reactions and, combined with quadrupole mass spectrometry (QMS), allow the derivation of a self-consistent reaction mechanism. Finally, MnS thin films were deposited on copper foil and tested as a Li-ion battery anode. The MnS coin cells showed exceptional cycle stability and near-theoretical capacity.