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To date, the in situ fabrication of the large-scale van der Waals multi-heterojunction transition metal dichalcogenides (multi-TMDs) is significantly challenging using conventional deposition methods. In this study, vertically stacked centimeter-scale multi-TMD (MoS2/WS2/WSe2 and MoS2/WSe2) thin films are successfully fabricated via sequential pulsed laser deposition (PLD), which is an in situ growth process. The fabricated MoS2/WS2/WSe2 thin film on p-type silicon (p-Si) substrate is designed to form multistaggered gaps (type-II band structure) with p-Si, and this film exhibits excellent spatial and thickness uniformity, which is verified by Raman spectroscopy. Among various application fields, MoS2/WS2/WSe2 is applied to the thin-film catalyst of a p-Si photocathode, to effectively transfer the photogenerated electrons from p-Si to the electrolyte in the photo-electrochemical (PEC) hydrogen evolution. From a comparison between the PEC performances of the homostructure TMDs (homo-TMDs)/p-Si and multi-TMDs/p-Si, it is demonstrated that the multistaggered gap of multi-TMDs/p-Si improves the PEC performance significantly more than the homo-TMDs/p-Si and bare p-Si by effective charge transfer. The new in situ growth process for the fabrication of multi-TMD thin films offers a novel and innovative method for the application of multi-TMD thin films to various fields.

Antiferromagnetic-paraelectric SrMnO3 (SMO) has aroused interest because of the theoretical strong coupling between the ferroelectric and ferromagnetic states with increasing epitaxial strain. In strained SMO films, the <110> polarized state and polar distortions have been observed, although high leakage currents and air degradation have limited their experimental verification. We herein provide a conclusive demonstration of room-temperature ferroelectricity and a high dielectric constant (εr = 138.1) in tensile-strained SMO by securing samples with insulating properties and clean surfaces using selective oxygen annealing. Furthermore, a paraelectricity and low dielectric constant (εr = 6.7) in the strain-relaxed SMO film have been identified as properties of the bulk SMO, which directly proves that the ferroelectricity of the tensile-strained SMO film is due to strain-induced polarization. We believe that these findings not only provide a cornerstone for exploring the physical properties of multiferroic SMO but also inspire new directions for single-phase multiferroics. Strain engineering: oxygen locks in room-temperature ferroelectricity Thin cracks and other defects which appear in multifunctional next-generation memory materials can be suppressed through a new synthetic technique. Compounds such as strontium manganese oxide (SrMnO3) transform from insulators into electrically and magnetically active materials when fabricated into thin films that strain the crystal's natural atom-atom separations. Sanghan Lee from the Gwangju Institute of Science and Technology, South Korea, and colleagues now report the first example of room-temperature electrical activity in strained SrMnO3 thin films. The team achieved this result by capping SrMnO3 with a layer of strontium ruthenium oxide (SrRuO3), and then annealing their new material in an oxygen-rich atmosphere. The team showed that the upper capping layer maintained the strained structure while also permitting oxygen to incorporate into the lower SrMnO3 film to eliminate vacancy defects. We provide a conclusive demonstration of strain-induced room-temperature ferroelectricity in SrMnO3 thin films by securing samples with insulating properties and clean surfaces using selective oxygen annealing. These findings will not only provide a cornerstone for exploring the physical properties of multiferroic SrMnO3 but will also inspire new directions for single-phase multiferroics. [ABSTRACT FROM AUTHOR]

Harvesting sustainable hydrogen through water-splitting requires a durable photoelectrode to achieve high efficiency and long lifetime. Dense, uniform CuBi2O4/NiO thin film photocathodes grown by pulsed laser deposition achieved photocurrent density over 1.5 mA cm−2 at 0.4 VRHE and long-term chronoamperometric stability for over 8 hours. [ABSTRACT FROM AUTHOR]

UV phototransistors based on ZnO, a material considered promising owing to its wide direct bandgap and high stability in harsh environments, have been intensively investigated. However, ZnO single-layer UV phototransistors, especially solution-processed devices, still exhibit poor electrical and UV photoresponse characteristics. Herein, we report the fabrication of a low-cost, large-area, and high-performance solution-processed ZnO/SnO2 bilayer UV phototransistor with improved electrical and UV photoresponse characteristics attained by inserting a SnO2 carrier transport layer, which is the actual path of the electrons. The photogenerated electrons are readily transferred from the ZnO UV-sensitive layer to the SnO2 carrier transport layer, owing to the lower conduction band of the SnO2 carrier transport layer than the ZnO UV-sensitive layer. In addition, the efficient extraction of photogenerated electrons from the ZnO UV-sensitive layer through the SnO2 carrier transport layer with high field effect mobility contributes to the improvement in the UV photoresponse characteristics of the ZnO/SnO2 bilayer UV phototransistor. The ZnO/SnO2 bilayer UV phototransistor exhibits high responsivity and detectivity as well as fast photoresponse. These results demonstrate that the solution-processed ZnO/SnO2 bilayer UV phototransistor developed in this study provides a novel approach for improving the performance of UV phototransistors with low-cost and large-area processing. [ABSTRACT FROM AUTHOR]

In photoelectrochemical (PEC) water splitting, charge separation and collection by the electric field in the photoactive material are the most important factors for improved conversion efficiency. Hence, ferroelectric oxides, in which electrons are the majority carriers, are considered promising photoanode materials because their high built-in potential, provided by their spontaneous polarization, can significantly enhance the separation and drift of photogenerated carriers. In this regard, the PEC properties of BiFeO thin-film photoanodes with different crystallographic orientations and consequent ferroelectric domain structures are investigated. As the crystallographic orientation changes from (001) via (110) to (111), the ferroelastic domains in epitaxial BiFeO thin films become mono-variant and the spontaneous polarization levels increase to 110 μC/cm. Consequently, the photocurrent density at 0 V vs. Ag/AgCl increases approximately 5.3-fold and the onset potential decreases by 0.180 V in the downward polarization state. It is further demonstrated that ferroelectric switching in the (111) BiFeO thin-film photoanode leads to an approximate change of 8,000% in the photocurrent density and a 0.330 V shift in the onset potential. This study strongly suggests that domain-engineered ferroelectric materials can be used as effective charge separation and collection layers for efficient solar water-splitting photoanodes. [ABSTRACT FROM AUTHOR]

Bismuth vanadate (BiVO4) has attracted significant attention as a promising photoanode material for hydrogen production via photoelectrochemical (PEC) water splitting because of its narrow optical band gap and suitable band edge positions for water oxidation. However, the actual photoactivity of BiVO4 is considerably limited by its poor electron transport and slow water oxidation kinetics. Although several studies have been carried out to improve its photo-efficiency via the enhancement of electron transport and water oxidation kinetics, only a few studies have reported the growth of epitaxial BiVO4 to explore the fundamental properties of BiVO4 for PEC water splitting because extremely flat epitaxial films exhibit poor photo-efficiency because of their low surface-active area. However, studies of epitaxial BiVO4 still have the potential to provide new routes for improving its photo-efficiency. In this study, the growth of epitaxial BiVO4 is investigated using a thin γ-WO3 template layer deposited on a SrTiO3(001) substrate covered by a SrRuO3 (SRO) bottom electrode using pulsed laser deposition. Consequently, at 1.23 V vs. the RHE (reversible hydrogen electrode), the photocurrent density of epitaxial BiVO4 on the γ-WO3 template layer (2.20 mA cm−2) is approximately 10 times that of bare BiVO4, related to the effective charge transfer by the γ-WO3 intermediate layers and the subsequent increase in the surface-active area of epitaxial BiVO4. These results strongly suggest that epitaxial BiVO4 grown using a template layer can be a cornerstone for the in-depth understanding of the fundamental properties of BiVO4 for PEC water splitting. [ABSTRACT FROM AUTHOR]

Unlike planar photoelectrodes, bendable and malleable photoelectrodes extend their application to mechanical flexibility beyond conventional rigid structures, which have garnered new attention in the field of photoelectrochemical water splitting. A bendable metal (Hastelloy), which has both bendability and compatibility with various oxide layers, allows high-temperature processes for crystallization; therefore it is far superior as a substrate than a conventional flexible polymer. In this study, we fabricate bendable BiVO 4 crystalline thin films on the metal substrates by employing template layers (SrRuO 3 /SrTiO 3 ) to reduce the structural misfits between BiVO 4 and the substrate. The crystallinities were verified through X-ray diffraction and transmission electron microscopy, and photocatalytic performances were examined. The crystallinity of BiVO 4 was significantly improved by utilizing similar lattice constants and affinities between BiVO 4 and the oxide template layers. We also formed a type II heterojunction by adding a WO 3 layer which complements the charge separation and charge transfer as a photoanode. The photocurrent densities of tensile-bent BiVO 4 /WO 3 thin films with a bending radius of 10 mm are comparable to those of pristine BiVO 4 /WO 3 thin film in various aqueous electrolytes. Moreover, photostability tests showed that the tensile-bent crystalline photoanodes retained 90% of their initial photocurrent density after 24 h, which proved their exceptional durability. Our work demonstrates that the bendable photoelectrodes with crystallinity hold great potential in terms of device structure for solar-driven water splitting.

Nonequilibrium deposition is a remarkable method for the in situ growth of unique nanostructures and phases for the functionalization of thin films. We introduce a distinctive structure of a mixed-phase, composed of BiVO 4 and β-Bi 2 O 3 , for photoelectrochemical water splitting. The mixed-phase is fabricated via nonequilibrium deposition by adjusted oxygen partial pressure. According to density functional theory calculations, we find that vanadium exsolution can be facilitated by introducing oxygen vacancies, enabling the fabrication of a nanostructured mixed-phase. These unique structures enhance charge migration by increasing the interfacial area and properly aligning the band offset between two crystalline phases. Consequently, the photocurrent density of the nanostructured mixed-phase thin films is about twice that of pristine BiVO 4 thin films at 1.23 V RHE . Our work suggests that nonequilibrium deposition provides an innovative route for the structural engineering of photoelectrodes for the understanding of fundamental properties and improving the photocatalytic performance for solar water splitting.

Harvesting sustainable hydrogen through water-splitting requires a durable photoelectrode to achieve high efficiency and long lifetime. Dense, uniform CuBi 2 O 4 /NiO thin film photocathodes grown by pulsed laser deposition achieved photocurrent density over 1.5 mA cm -2 at 0.4 V RHE and long-term chronoamperometric stability for over 8 hours.