Electronic Structure and Band Alignment at the NiO and SrTiO3 p-n Heterojunctions

Thermoelectric Performance of Hybrid Inorganic/Organic Composites Based on PEDOT:PSS/Tin(II) Oxide

PEDOT

PSS(poly(3,4-ethylenedioxylthiophene):poly(styrenesulfonate))-based composites often exhibit remarkable characteristics regarding high electrical conductivity and great processability, being a suitable candidate for thermoelectric (TE) applications. To increase its performance, PEDOT:PSS is commonly blended with scarce and toxic inorganic compounds based on Se, Te or Bi. In this work we propose the use of one p-type metal oxide semiconductor (MOs): tin(II) oxide (SnO), motivated by its abundance and low toxicity. Hybrid PEDOT:PSS/SnO composites were obtained by firstly blending Ethylene glycol (EG) with PEDOT:PSS and then by adding p-type SnO, previously synthesized by a chemical route. The mixture was deposited via spin-coating onto glass substrates. The Power Factor (PF) of the composites increased by a factor of 300 with the combined EG/SnO composition.

Fabrication of Ga2O3 Schottky Barrier Diode and Heterojunction Diode by MOCVD

In this article, we reported on a Ga₂O₃-based Schottky barrier diode and heterojunction diode from MOCVD. The Si-doped n-type Ga₂O₃ drift layer, grown by MOCVD, exhibited high crystal quality, flat surfaces, and uniform doping. The distribution of unintentional impurities in the films was studied. Then nickel Schottky barrier diode and p-NiO/n-Ga₂O₃ heterojunction diode were fabricated and measured. Without any electric field management structure, the Schottky barrier diode and heterojunction diode have specific resistances of 3.0 mΩ·cm² and 6.2 mΩ·cm², breakdown voltages of 380 V and 740 V, thus yielding power figures of merit of 48 MW·cm^-2 and 88 MW·cm^-2, respectively. Besides, both devices exhibit a current on/off ratio of more than 10¹⁰. This shows the prospect of MOCVD in power device manufacture.

Fabrication Strategies and Optoelectronic Applications of Perovskite Heterostructures

Metal halide perovskites (MHPs) are emerging low‐cost and multifunctional semiconductor materials. They have been widely used in optoelectronic devices such as perovskite solar cells, light‐emitting diodes, photodetectors, memristors, and lasers. Developing new MHPs, defects passivation, optimizing device structures, and packaging techniques are all effective methods to improve photoelectric performance and stability of perovskite devices. Particularly, the fabrication of perovskite/perovskite heterostructures (PPHSs) is a novel and arresting method to obtain stable and high‐performing optoelectronic perovskite devices since it can passivate defects, regulate energy gaps, and provide new carrier transmission modes of MHPs for multiple semiconductor applications. In this paper, representative fabrication strategies of PPHSs including films and single‐crystal heterostructures are reviewed, and their applications in optoelectronic devices are summarized. Furthermore, the challenges and prospects of PPHSs are discussed based on the current status.

Ternary Oxide CuWO4/BiVO4/FeCoOx Films for Photoelectrochemical Water Oxidation: Insights into the Electronic Structure and Interfacial Band Alignment

Photoelectrochemical (PEC) water oxidation using ternary oxide systems has been considered a promising approach for investigating the effective utilization of sunlight and the production of green fuel. Herein, we report a ternary-oxide-based CuWO4/BiVO4/FeCoOx film deposited entirely by RF-magnetron sputtering using homemade ceramic targets. Our CuWO4/BiVO4 photoanode exhibits a significant photocurrent density of 0.82 mA cm-2 at 1.23 V vs RHE under AM 1.5G illumination, which is a record 382% increase compared to that of the bare CuWO4 film. To further boost the PEC performance, we deposited an ultrathin layer of amorphous FeCoOx cocatalyst, resulting in a triple CuWO4/BiVO4/FeCoOx heterojunction with a significant reduction in onset potential and a 500% increase in the photocurrent density of bare CuWO4. Experimental and theoretical approaches were used to provide insights into the interfacial band alignment and photoinduced charge carrier pathway across heterojunctions. Our results reveal noticeable interface potential barriers for charge carriers at the CuWO4/BiVO4 heterojunction, potentially limiting its application in tandem systems. Conversely, the deposition of the FeCoOx ultrathin layer over the CuWO4/BiVO4 heterojunction induces a p-n junction on the BiVO4/FeCoOx interface, which, when combined with the abundant FeCoOx oxygen vacancies, results in improved charge separation and transport as well as enhanced photoelectrochemical stability. Our study provides a feasible strategy for producing photocatalytic heterojunction systems and introduces simple tools for investigating interface effects on photoinduced charge carrier pathways for PEC water splitting.

Flexible zirconium doped strontium titanate nanofibrous membranes with enhanced visible-light photocatalytic performance and antibacterial activities

Constructing flexible perovskite structured ceramic fibrous materials would potentially facilitate applications of photocatalysis, wearable devices, and energy storage. However, current perovskite structured ceramic fibrous materials were fragile with small deformation resistance, which have limited their wide applications. Herein, flexible zirconium doped strontium titanate (ZSTO) nanofibrous membranes were fabricated via combining sol-gel and electrospinning methods. The microstructures (pore and crystal) of ZSTO nanofibers were affected by zirconium doping contents and closely relevant to flexibility of resultant membranes. The probable mechanism for flexibility of ZSTO nanofibrous membranes was presented. Furthermore, the silver phosphate modified ZSTO (AZSTO) exhibited superior photocatalytic performance towards tetracycline hydrochloride (TCHC) and antibacterial performance towards Gram-negative and Gram-positive bacteria with visible-light irradiation, including 85% degradation towards TCHC within 60 min, >99.99% inhibition rate and > 3 mm inhibition zone against Gram bacteria. Furthermore, the·superoxide free radical (O₂^-) and holes played significant roles in the degradation of TCHC that verified by radical scavenger experiment. Additionally, the membranes exhibited good reusability over five cycles without tedious recycling operations needed for micro/nanoparticle-based catalysts. The successful fabrication of ZSTO nanofibrous membranes would provide a new insight into photocatalysts, antibacterial materials, and wearable device.

The origins of charge separation in anisotropic facet photocatalysts investigated through first-principles calculations

It was recently discovered that the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) can be completed on the {110} and {001} facets, respectively, of a 18-facet SrTiO₃ mono-crystal. The effective charge separation is attributed to the facet junction at the interface between two arbitrary anisotropic crystal planes. Theoretical estimation of the built-in potential at the facet junction can greatly improve understanding of the mechanism. This work employs density functional theory (DFT) calculations to investigate such potential at the (110)/(100) facet junction in SrTiO₃ crystals. The formation of the facet junction is verified by a calculated work function difference between the (110) and (100) planes, which form p-type and n-type segments of the junction, respectively. The built-in potential is estimated at about 2.9 V. As a result, with the ultra high built-in potential, electrons and holes can effectively transfer to different anisotropic planes to complete both photo-oxidative and photo-reductive reactions.

Designed Construction of SrTiO3 /SrSO4 /Pt Heterojunctions with Boosted Photocatalytic H2 Evolution Activity

Efficient separation of photogenerated electron-hole pairs is a crucial factor for high-performance photocatalysts. Effective electron-hole separation and migration could be achieved by heterojunctions with suitable band structures. Herein, a porous SrTiO₃ /SrSO₄ heterojunction is prepared by a sol-gel method at room temperature followed by an annealing process. XRD characterization suggests high crystallinity of the heterostructure. A well-defined interface between the two phases is confirmed by high-resolution (HR)TEM. The photocatalytic H₂ evolution productivity of the SrTiO₃ /SrSO₄ heterojunction with Pt as co-catalyst reaches 396.82 μmol g^-1  h^-1 , which is 16 times higher than that of SrTiO₃ /Pt. The boosted photocatalytic activity of SrTiO₃ /SrSO₄ /Pt can be ascribed to the presence of SrSO₄ , which promotes the transfer and migration of photogenerated carriers by forming the heterojunction and porous structure, which provides a large amount of active sites. This novel porous heterostructure brings new ideas for the development of high-efficiency photocatalysts for H₂ release.

Optimizing the Electronic Structure of In2O3 through Mg Doping for NiO/In2O3 p-n Heterojunction Diodes

In₂O₃ is a wide bandgap oxide semiconductor, which has the potential to be used as an active material for transparent flexible electronics and UV photodetectors. However, the high concentration of unintentional background electrons existing in In₂O₃ makes it hard to be modulated by the electric field or form p-n heterojunctions with a sufficient band-bending width at the interface. In this work, we report the reduction of the background electrons in In₂O₃ by Mg doping (Mg-In₂O₃) and thereby improve the device performance of p-n diodes based on the NiO/Mg-In₂O₃ heterojunction. In particular, Mg doping compensates the free electrons in In₂O₃ and reduces the electron concentration from 1.7 × 10¹⁹ cm^-3 without doping to 1.8 × 10¹⁷ cm^-3 with 5% Mg doping. Transparent p-n heterojunction diodes were fabricated based on p-type NiO and n-type Mg-In₂O₃. The device performance was considerably enhanced by Mg doping with a high rectification ratio of 3 × 10⁴ and a remarkable high breakdown voltage of >20 V. High-resolution X-ray photoelectron spectroscopy was used to investigate the interfacial electronic structure between NiO and Mg-In₂O₃, revealing a type II band alignment with a valence band offset of 1.35 eV and a conduction band offset of 2.15 eV. A large built-in potential of 0.98 eV was found for the undoped In₂O₃ but decreased to 0.51 eV for 5% Mg doping of In₂O₃. The NiO/Mg-In₂O₃ diodes with an improved rectification ratio and wider depletion region provide the possibility of achieving photodetectors with rapid photoresponse.

Electronic Structure and Interface Energetics of CuBi2O4 Photoelectrodes

CuBi₂O₄ exhibits significant potential for the photoelectrochemical (PEC) conversion of solar energy into chemical fuels, owing to its extended visible-light absorption and positive flat band potential vs the reversible hydrogen electrode. A detailed understanding of the fundamental electronic structure and its correlation with PEC activity is of significant importance to address limiting factors, such as poor charge carrier mobility and stability under PEC conditions. In this study, the electronic structure of CuBi₂O₄ has been studied by a combination of hard X-ray photoemission spectroscopy, resonant photoemission spectroscopy, and X-ray absorption spectroscopy (XAS) and compared with density functional theory (DFT) calculations. The photoemission study indicates that there is a strong Bi 6s-O 2p hybrid electronic state at 2.3 eV below the Fermi level, whereas the valence band maximum (VBM) has a predominant Cu 3d-O 2p hybrid character. XAS at the O K-edge supported by DFT calculations provides a good description of the conduction band, indicating that the conduction band minimum is composed of unoccupied Cu 3d-O 2p states. The combined experimental and theoretical results suggest that the low charge carrier mobility for CuBi₂O₄ derives from an intrinsic charge localization at the VBM. Also, the low-energy visible-light absorption in CuBi₂O₄ may result from a direct but forbidden Cu d-d electronic transition, leading to a low absorption coefficient. Additionally, the ionization potential of CuBi₂O₄ is higher than that of the related binary oxide CuO or that of NiO, which is commonly used as a hole transport/extraction layer in photoelectrodes. This work provides a solid electronic basis for topical materials science approaches to increase the charge transport and improve the photoelectrochemical properties of CuBi₂O₄-based photoelectrodes.

Template Engineering of CuBi2 O4 Single-Crystal Thin Film Photocathodes

To develop strategies for efficient photo-electrochemical water-splitting, it is important to understand the fundamental properties of oxide photoelectrodes by synthesizing and investigating their single-crystal thin films. However, it is challenging to synthesize high-quality single-crystal thin films from copper-based oxide photoelectrodes due to the occurrence of significant defects such as copper or oxygen vacancies and grains. Here, the CuBi₂ O₄ (CBO) single-crystal thin film photocathode is achieved using a NiO template layer grown on single-crystal SrTiO₃ (STO) (001) substrate via pulsed laser deposition. The NiO template layer plays a role as a buffer layer of large lattice mismatch between CBO and STO (001) substrate through domain-matching epitaxy, and forms a type-II band alignment with CBO, which prohibits the transfer of photogenerated electrons toward bottom electrode. The photocurrent densities of the CBO single-crystal thin film photocathode demonstrate -0.4 and -0.7 mA cm^-2 at even 0 V_RHE with no severe dark current under illumination in a 0.1 m potassium phosphate buffer solution without and with H₂ O₂ as an electron scavenger, respectively. The successful synthesis of high-quality CBO single-crystal thin film would be a cornerstone for the in-depth understanding of the fundamental properties of CBO toward efficient photo-electrochemical water-splitting.

Perovskite Transparent Conducting Oxide for the Design of a Transparent, Flexible, and Self-Powered Perovskite Photodetector

Transparent and flexible electronic devices are highly desired to meet the great demand for next-generation devices that are lightweight, flexible, and portable. Transparent conducting oxides (TCOs), such as indium-tin oxide, serve as fundamental components for the design of transparent and flexible electronic devices. However, indium is rare and expensive. Herein, we report the fabrication of low-cost perovskite SrVO₃ TCO films on transparent and flexible mica substrates and further demonstrate their utilization as a TCO electrode for building a transparent, flexible, and self-powered perovskite photodetector. Superior stable optical transparency and electrical conductivity are retained in SrVO₃ after bending up to 10⁵ cycles. Without an external power source, the constructed all-perovskite photodetector exhibits a high responsivity (42.5 mA W^-1), fast response time (3.09/1.23 ms), and an excellent flexibility and bending stability after dozens of cycles of bending at an extreme 90° bending angle. Our results demonstrate that low-cost and structure-compatible transition metal-based perovskite oxides, such as SrVO₃, as TCO electrodes have huge potential for building high-performance transparent, flexible, and portable smart electronics.

Revealing the Adsorption and Decomposition of EP-PTCDI on a Cerium Oxide Surface

Cerium oxide has constantly attracted intense attention during the past decade both in research and industry as an appealing catalyst or a noninert support for catalysts, for instance, in the water-gas shift reaction and hydrogenation of the ketone group. Herein, the cerium oxide surface has been chosen to investigate the adsorption and decomposition behaviors of the N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxdiimide (EP-PTCDI) molecule by photoelectron spectroscopy. As expected, EP-PTCDI molecules self-assemble on the cerium oxide surface comprising both trivalent and tetravalent cerium at room temperature. Interestingly, the EP-PTCDI molecule exhibits selective adsorption on cerium oxide after the heating treatment. It was found that the ketone group of EP-PTCDI first undergoes hydrogenation after annealing to 400 °C, which is probably related to the fact that high temperature annealing provides sufficient thermal energy to trigger the reaction between the ketone group and trivalent cerium. Furthermore, EP-PTCDI molecules are discovered to start to decompose hierarchically on the ceria substrate from annealing at 400 °C due to the strong molecule-substrate interaction and the effective catalysis by the trivalent cerium, whereas the decomposition sequence of functional groups is revealed to be, first, the ethyl propyl group (-C₅H₉), followed by the hydrogenated ketone (alcohols) group. Finally, our study may provide a new platform for the fundamental understanding of complex organic reactions on the cerium oxide surface.

All-Oxide Nanocomposites to Yield Large, Tunable Perpendicular Exchange Bias above Room Temperature

In all-oxide-based spintronic devices, large exchange bias effect with robustness against temperature fluctuation and compatibility with perpendicular magnetic recording is highly desired. In this work, rock-salt antiferromagnetic NiO with a Néel temperature ( T_N) of ∼525 K and spinel ferrimagnetic NiFe₂O₄ with a high Curie temperature, T_C, ≈ 790 K and T_C > T_N were chosen as compatible materials to form a well-phase-separated, vertically aligned nanocomposite thin film. In this nanoengineered thin film, an exchange bias effect with a blocking temperature far above room temperature has been achieved. A large perpendicular exchange bias field of up to 0.91 kOe with an interfacial exchange energy density of 0.11-0.34 erg/cm² was obtained at room temperature. It was also demonstrated that the exchange bias effect can be easily tuned by changing the alignment of the magnetic moments in the NiO phase using substrates of different crystalline orientations and by changing the microstructure of the film with substrates of different lattice parameters. The results demonstrate that proper choice of the phases (including use of nonperovskite compositions) and careful strain engineering and nanostructure engineering makes oxide nanocomposites strong potential candidate systems for next generation spintronic devices.

Impact of Strain and Morphology on Magnetic Properties of Fe₃O₄/NiO Bilayers Grown on Nb:SrTiO₃(001) and MgO(001)

We present a comparative study of the morphology and structural as well as magnetic properties of crystalline Fe₃O₄/NiO bilayers grown on both MgO(001) and SrTiO₃(001) substrates by reactive molecular beam epitaxy. These structures were investigated by means of X-ray photoelectron spectroscopy, low-energy electron diffraction, X-ray reflectivity and diffraction, as well as vibrating sample magnetometry. While the lattice mismatch of NiO grown on MgO(001) was only 0.8%, it was exposed to a lateral lattice mismatch of −6.9% if grown on SrTiO₃. In the case of Fe₃O₄, the misfit strain on MgO(001) and SrTiO₃(001) amounted to 0.3% and −7.5%, respectively. To clarify the relaxation process of the bilayer system, the film thicknesses of the magnetite and nickel oxide films were varied between 5 and 20 nm. While NiO films were well ordered on both substrates, Fe₃O₄ films grown on NiO/SrTiO₃ exhibited a higher surface roughness as well as lower structural ordering compared to films grown on NiO/MgO. Further, NiO films grew pseudomorphic in the investigated thickness range on MgO substrates without any indication of relaxation, whereas on SrTiO₃ the NiO films showed strong strain relaxation. Fe₃O₄ films also exhibited strong relaxation, even for films of 5 nm thickness on both NiO/MgO and NiO/SrTiO₃. The magnetite layers on both substrates showed a fourfold magnetic in-plane anisotropy with magnetic easy axes pointing in 100 directions. The coercive field was strongly enhanced for magnetite grown on NiO/SrTiO₃ due to the higher density of structural defects, compared to magnetite grown on NiO/MgO.

Design of a Vertical Composite Thin Film System with Ultralow Leakage To Yield Large Converse Magnetoelectric Effect

Electric field control of magnetism is a critical future technology for low-power, ultrahigh density memory. However, despite intensive research efforts, no practical material systems have emerged. Interface-coupled, composite systems containing ferroelectric and ferri-/ferromagnetic elements have been widely explored, but they have a range of problems, for example, substrate clamping, large leakage, and inability to miniaturize. In this work, through careful material selection, design, and nanoengineering, a high-performance room-temperature magnetoelectric system is demonstrated. The clamping problem is overcome by using a vertically aligned nanocomposite structure in which the strain coupling is independent of the substrate. To overcome the leakage problem, three key novel advances are introduced: a low leakage ferroelectric, Na_0.5Bi_0.5TiO₃; ferroelectric-ferrimagnetic vertical interfaces which are not conducting; and current blockage via a rectifying interface between the film and the Nb-doped SrTiO₃ substrate. The new multiferroic nanocomposite (Na_0.5Bi_0.5TiO₃-CoFe₂O₄) thin-film system enables, for the first time, large-scale in situ electric field control of magnetic anisotropy at room temperature in a system applicable for magnetoelectric random access memory, with a magnetoelectric coefficient of 1.25 × 10^-9 s m^-1.

Facile synthesis of a ZnO-BiOI p-n nano-heterojunction with excellent visible-light photocatalytic activity

In this paper, an efficient method to produce a ZnO/BiOI nano-heterojunction is developed by a facile solution method followed by calcination. By tuning the ratio of Zn/Bi, the morphology varies from nanoplates, flowers to nanoparticles. The heterojunction formed between ZnO and BiOI decreases the recombination rate of photogenerated carriers and enhances the photocatalytic activity of ZnO/BiOI composites. The obtained ZnO/BiOI heterostructured nanocomposites exhibit a significant improvement in the photodegradation of rhodamine B under visible light (λ ≥ 420 nm) irradiation as compared to single-phase ZnO and BiOI. A sample with a Zn/Bi ratio of 3:1 showed the highest photocatalytic activity (≈99.3% after 100 min irradiation). The photodegradation tests indicated that the ZnO/BiOI heterostructured nanocomposites not only exhibit remarkably enhanced and sustainable photocatalytic activity, but also show good recyclability. The excellent photocatalytic activity could be attributed to the high separation efficiency of the photoinduced electron-hole pairs as well as the high specific area.