Ultraviolet Photoelectron Spectroscopy Analysis Research Articles

Nanoscale interface engineering for enhanced performance in light-emitting-diode devices: ITO/Ag-SRCOOH

In light-emitting diode (LED) devices, the hole-injection layer (HIL) reduces the hole injection barrier (HIB) between the anode and organic layers to obtain favorable conditions for hole transport, which considerably affects device efficiency and performance. However, the fabrication of ultrathin silver (Ag) films for electron and hole transport layers remains challenging. We demonstrated the design of a nanoscale HIL interface with chemisorption-bonded carboxylic acid-terminated alkanethiol self-assembled layers on ultrathin Ag. The device structure consisted of indium tin oxide (ITO)/Ag-mercaptan acid (Ag-SRCOOH)/N, Nj-Di (1-naphthyl)-N,Nj-diphenyl-(1,1-biphenyl)-4,4-diamine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq3)/ETL(LiF/Al. Notably, a chemical bond peak was not detected between ITO and ultrathin Ag. However, X-ray photoelectron spectroscopy (XPS) measurements confirmed Ag binding to the thiol group of 3-mercaptopropionic acid (MPA) and 11-mercaptoundecanoic acid (MUA) on the Ag island surface, which is evident from -S-Ag- and -RS-Ag- association peaks. Ultraviolet photoelectron spectroscopy analysis results revealed a reduction in the HIB of device from 0.68 to 0.19 eV. Consequently, the luminance and current density, and current density of the AgNP–MPA device increased by 63-fold, 5.8-fold, and 1.1-fold, respectively. Similarly, the AgNP-MUA device exhibited enhanced performance with a 61-fold increase in luminance, 43-fold increase in current density, and a 1.4-fold increase in current efficiency, respectively. The results of this study not only opened novel avenues for future applications, including operation requiring flexibility and transparency of LEDs, but also proposed the optimization of nanoscale interfaces to enhance LED performance.

Red-Emitting CsPbI3/ZnSe Colloidal Nanoheterostructures with Enhanced Optical Properties and Stability.

Producing heterostructures of cesium lead halide perovskites and metal-chalcogenides in the form of colloidal nanocrystals can improve their optical features and stability, and also govern the recombination of charge carriers. Herein, the synthesis of red-emitting CsPbI3/ZnSe nanoheterostructures is reported via an insitu hot injection method, which provides the crystallization conditions for both components, subsequently leading to heteroepitaxial growth. Steady-state absorption and photoluminescence studies alongside X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy analysis evidence on a type-I band alignment for CsPbI3/ZnSe nanoheterostructures, which exhibit photoluminescence quantum yield of 96% due to the effective passivation of surface defects, and an enhancement in carrier lifetime. Furthermore, the heterostructure growth of ZnSe domains leads to significant improvement in the stability of the CsPbI3 nanocrystals under ambient conditions and against thermal and UV irradiation stress.

Effect of UV‐ozone treatment for KCl interlayer in perovskite solar cells

AbstractThe performance of perovskite solar cells (PSCs) is significantly governed by the interface of perovskite layer. Therefore, a great deal of attention has been paid to the interface engineering of perovskite layer to improve the performance of PSC. In the meantime, KCl is one of the popular molecules being utilized for the interface treatment between SnO2 and perovskite. In this study, we investigate the effect of UV‐ozone (UVO) treatment of KCl interlayer on the photovoltaic performance. A device employing UVO‐treated KCl shows a higher power conversion efficiency mainly based on an increased open‐circuit voltage (VOC). Ultraviolet photoelectron spectroscopy analysis indicates that the UVO treatment induces a rearrangement of energy level at the interface, being responsible for the increase in VOC. Accordingly, an increased charge recombination resistance is evidenced by impedance spectroscopy owing to the inhibited recombination at the SnO2 and perovskite interface by the aid of the aligned energy level.

Experimental and theoretical study on the facet‐dependent SERS activity of α‐Fe2O3 films

AbstractSemiconductors have attracted great attention for surface‐enhanced Raman scattering (SERS) applications due to their rich variety, adjustable band structure, good chemical stability, and biocompatibility. However, their intrinsic weak SERS activity limited the further development of semiconductor substrates. Here, the α‐Fe2O3 films with high {} and {} facet exposure ratios were fabricated through an electrodeposition method with the existence of NH4Cl and CH3COONa. The α‐Fe2O3{} and α‐Fe2O3{} films show excellent SERS properties with enhancement factor of 4.155×103 and 5.481×103, as well as the relatively low limit of detection down to 2 × 10−7 M for 4‐nitrobenzenethiol. Meanwhile, the lower relative standard deviation values (<10%) confirm the well uniformity of as‐prepared substrates. Ultraviolet photoelectron spectroscopy analysis reveals that the α‐Fe2O3{} possesses the lower work function, which could guarantee the efficient interfacial charge transfer between substrates and probe molecules. Moreover, first principles calculations further indicates that the charge transfer efficiency on {} facet can be effectively improved, and thus significantly enhance the SERS activity of α‐Fe2O3. The current study provides a strategy for the fabrication and application of semiconductor‐based SERS substrates.

“Coupling-conversion” effect induced by interface-local electric field to improve oxygen reaction kinetics in zinc-air batteries

Practical application of rechargeable zinc-air batteries is restricted by slow oxygen reaction kinetics of their bifunctional catalysts. This study is based on the ability of interface-localized electric fields to enhance charge transfer and accelerate electrochemical reactions. Fe and VN metal clusters formed on the foam carbon were used to form local electric field-orientated heterogeneous interfaces, which exhibited effective adsorption and catalytic performance for ORR/OER. Theoretical simulations and ultraviolet photoelectron spectroscopy analysis have confirmed that an interface electric field close to the Fermi level can improve the electronic structure of metals, enhancing their adsorption/desorption performance in ORR and OER. The resulting FeACs-VNNCs (1:1)/NFC material exhibited “coupling-conversion” effect and demonstrated outstanding bifunctional ORR/OER activity (E1/2 = 0.87 V, Ej=10 = 1.476 V). The rechargeable zinc-air battery, assembled using the newly prepared catalyst, displayed a high specific capacity of 779 mAh g−1 and was stable after 3104 operation cycles. The study substantiates the possibility of ion diffusion and charge transfer acceleration by interface-localized electric fields induced in zinc-air battery electrodes.

Interdigitated electrodes-based Au-MoS2 hybrid gas sensor for sensing toxic CO and NH3 gases at room temperature

In the quest to create effective sensors that operate at room temperature, consume less power and maintain their stability over time for detecting toxic gases in the environment, molybdenum disulfide (MoS2) and MoS2-based hybrids have emerged as potent materials. In this context, the current work describes the fabrication of Au-MoS2 hybrid gas sensor fabricated on gold interdigitated electrodes (GIEs) for sensing harmful CO and NH3 gases at room temperature. The GIEs-based Au-MoS2 hybrid sensors are fabricated by decorating MoS2 nanoflowers (MNF) with varying size of Au nanoparticles using an inert gas evaporation technique. It is observed that by varying the size of Au nanoparticles, the crystallinity gets modified, as confirmed by x-ray diffraction and Micro-Raman spectroscopy (μRS). The gas sensing measurements revealed that the best sensing response is found from the Au-MoS2 hybrid (with an average particle size of 10 nm). This particular hybrid shows a 79% response to CO exposure and a 69% response to NH3 exposure. The measurements are about 3.5 and 5 times higher than the bare MoS2 when exposed to CO and NH3 at room temperature, respectively. This enhancement in sensing response is attributed to the modified interfacial interaction between the Au nanoparticles and MNF gets improved, which leads to the formation of a Schottky barrier, as confirmed using x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy analysis. This enables the development of efficient gas sensors that respond quickly to changes in the gas around them.

Comparative investigation on defect-engineered Bi2S3-xSex for boosting photocatalytic Cr(VI) reduction and in-situ H2O2 generation: Mechanistic and kinetic studies

Heterogeneous photocatalysis has been demonstrated as a highly effective approach in addressing the difficulties encountered by conventional technologies in environmental remediation. Herein, for the first time, a novel hierarchical photocatalyst of selenium-doped Bi2S3 (Bi2S3-xSex) was successfully synthesized through a one-spot hydrothermal route followed by a vacancy engineering process (V-Bi2S3-xSex). The photocatalytic reduction of Cr(VI) and in-situ generation of hydrogen peroxide (H2O2) under simulated solar-light irradiation were performed to evaluate the catalytic activity of the as-prepared samples. The catalytic activity of as-prepared samples was evaluated toward the photocatalytic reduction of Cr(VI) and in-situ generation of H2O2 under simulated solar-light irradiation. Notably, V-Bi2S3-xSex (V-BSSe-5, as optimum sample) exhibited a photo-reduction of Cr(VI) at a rate of 97.04% during 150 min, which was 1.53- and 1.39-fold higher than those of pure Bi2S3 and Bi2Se3, respectively. Interestingly, the V-Bi2S3-xSex photocatalyst not only harvested more incident light in the UV–vis and near-infrared (NIR) regions but also supplied many active sites, improving the promotion of photo-generated charge-carriers, inhibiting charge recombination, and thus enhancing the photocatalytic activity. In addition, V-BSSe-5 showed greater photocatalytic efficiency for H2O2 generation, which was 15.69, 10.07, and 1.79 times higher than those of Bi2S3, Bi2Se3, and BSSe-5, respectively. The charge-carrier migration pathway and possible photocatalytic mechanisms were systematically discussed by assisting the electron spin resonance and ultraviolet photoelectron spectroscopy analyses. The findings of this study demonstrate that doping and defect engineering strategies have the potential to be a significant advancement in the development of visible- and NIR-light responsiveness photocatalysts, thereby providing a solution to current environmental and energy challenges.

In-depth insight into the photocatalytic and electrocatalytic mechanisms of Mg3V2O8@Zn3V2O8@ZnO ternary heterostructure toward linezolid: Experimental and DFT studies

Heterogeneous catalysis has been extensively studied as a potential means of overcoming the challenges associated with the detection and elimination of pharmaceutical pollutants in wastewater. In the present study, a novel ternary Mg3V2O8@Zn3V2O8@ZnO (MVO@ZVO@ZnO) composite catalyst was synthesized in three steps using a hydrothermal approach for the first time. The catalytic performance of the composite was investigated for the photodegradation of linezolid (LIZ) under visible light. The LIZ degradation rate of MVO@ZVO@ZnO (the photocatalyst with the optimal composition) was ∼12, 6.5, 7.8, and 2.6 times higher than those of ZnO, ZVO, MVO, and ZVO@ZnO, respectively. This significantly higher photocatalytic efficiency was attributed to synergism between its constituents, a narrow bandgap, improved light-harvesting ability, and greater charge-carriers separation and migration. Reactive-radical trappi ng experiments demonstrated that hydroxyl and superoxide radicals were largely responsible for LIZ degradation. Assisted by the ultraviolet photoelectron spectroscopy analyses and density functional theory calculations, the charge-carriers pathway and corresponding photocatalytic mechanism were thoroughly discussed in double S-scheme system. Furthermore, the MVO@ZVO@ZnO composite was used as an electrocatalyst to detect small amounts of LIZ in an aqueous medium and exhibited a detection limit of ∼0.33 μM. We believe that our findings encourage efforts to develop vanadate-based photocatalysts for use in environmental decontamination and detection applications.

Highly Reproducible Heterosynaptic Plasticity Enabled by MoS2/ZrO2-x Heterostructure Memtransistor.

Electrically tunable resistive switching of a polycrystalline MoS2-based memtransistor has attracted a great deal of attention as an essential synaptic component of neuromorphic circuitry because its switching characteristics from the field-induced migration of sulfur defects in the MoS2 grain boundaries can realize multilevel conductance tunability and heterosynaptic functionality. However, reproducible switching properties in the memtransistor are usually disturbed by the considerable difficulty in controlling the concentration and distribution of the intrinsically existing sulfur defects. Herein, we demonstrate reliable heterosynaptic characteristics using a memtransistor device with a MoS2/ZrO2-x heterostructure. Compared to the control device with the MoS2 semiconducting channel, the Schottky barrier height was more effectively modulated by the insertion of the insulating ZrO2-x layer below the MoS2, confirmed by an ultraviolet photoelectron spectroscopy analysis and the corresponding energy-band structures. The MoS2/ZrO2-x memtransistor accomplishes dual-terminal (drain and gate electrode) stimulated multilevel conductance owing to the tunable resistive switching behavior under varying gate voltages. Furthermore, the memtransistor exhibits long-term potentiation/depression endurance cycling over 7000 pulses and stable pulse cycling behavior by the pulse stimulus from different terminal regions. The promising candidate as an essential synaptic component of the MoS2/ZrO2-x memtransistors for neuromorphic systems results from the high recognition accuracy (∼92%) of the deep neural network simulation test, based on the training and inference of handwritten numbers (0-9). The simple memtransistor structure facilitates the implementation of complex neural circuitry.

Design of a Highly Selective Benzimidazole-Based Derivative for Optical and Solid-State Detection of Zinc Ion.

A novel series of benzimidazole-based molecules mimicking biological receptors, which exhibit selective coordination with zinc ions, were designed and synthesized. The photochromic behavior of these derivatives with various metal ions suggests a selective interaction of one of the receptors 2-(pyridin-2-yl)-4,7-di(thiophen-2-yl)-3H-benzo[d]imidazole (2c) with zinc ion. The lower limit of detection by photoluminescence quenching was determined to be 16 nM. The mechanism of selective complexation was elucidated by 1H nuclear magnetic resonance titrations and dynamic light scattering analysis. The stoichiometry of the formation of the Zn(2c)2 complex was evaluated by single-crystal X-ray diffraction and mass spectral techniques and calculated to be 2:1 (L:M). A change in the electronic energy levels on the sensor analyte interaction was observed by both ultraviolet photoelectron spectroscopy analysis and by density functional theory calculations, suggesting an electroactive semiconductor behavior. A symmetric Schottky structured sensor device was fabricated using the receptor 2c as the active sensing layer. A distinct change in current-voltage characteristics between the receptor and the complex suggests that the fabricated device could be used as a solid-state sensor for detecting zinc ion.

Enhanced Electrical Properties of Metal‐Organic Chemical Vapor Deposition‐Grown MoS2 Thin Films through Oxygen‐Assisted Defect Control

AbstractAlthough the synthesis of MoS2 thin film with a large area and excellent uniformity has been achieved through advanced synthesis techniques, such as metal–organic chemical vapor deposition (MOCVD), intrinsic defects such as vacancies and grain boundaries which degrade electrical performance still inevitably result from the process. In this paper, a method for controlling intrinsic defects in MOCVD‐grown MoS2 thin film to achieve enhanced electrical performance is reported. After applying the defect‐control process, high‐resolution transmission electron microscopy confirms that the MoS2 thin film maintains a hexagonal lattice structure without any destruction or distortion, indicating that this is a nondestructive method. In addition, the MoS2 thin film subjected to the defect‐control process exhibits enhanced n‐type characteristics in the photoluminescence and ultraviolet photoelectron spectroscopy analyses. Field‐effect transistors using the defect‐controlled MoS2 as the channel also show enhanced electrical performance, arising from reductions in sheet and contact resistances of 21% and 46%, respectively. This improvement in the resistances leads to an increase in field‐effect mobility from 3.2 up to 11.8 cm2 V−1 s−1.

Mechanism and quantum efficiency of yellow emission at the organic-inorganic F8T2/p-GaN interface

An organic/inorganic F8T2/gallium nitride (GaN) hybrid light-emitting device is fabricated and found to emit white light. The white light emission is determined to be the combination of the blue emission of the GaN light-emitting diode and the yellow emission of the F8T2 layer. Analysis of the results of this work reveals the following: (1) the yellow emission of the F8T2 layer in the F8T2/GaN light-emitting device is caused by both electroluminescence and photoluminescence and (2) the origin of the majority of the yellow emission is the electroluminescence mechanism.A potential barrier in the valence band at the F8T2/p-GaN interface has been observed and determined to be 1.05 eV by ultraviolet photoelectron spectroscopy analysis. The number of hole carriers flowing over the potential barrier (1.05 eV) into the valence band of GaN; multiple quantum wells (MQWs) governs the quantum efficiency of the yellow emission. Because the percentage of the overflowed hole carriers in the valence band increases with the current input, the quantum efficiency of the active region in the F8T2 layer increases with the current input; this observed behavior is opposite to that of existing solid-state light-emitting devices.

Effect of H2 annealing on SnS thin films grown by thermal evaporation and their transfer characteristics with Ti, W, and Mo electrodes

Tin monosulfide (SnS) is a promising absorber layer material in solar cell devices. In addition to its outstanding electrical and optical properties, SnS does not contain rare heavy-metal atoms. Thus, many researchers have focused on fabricating highly efficient SnS-based solar cell devices. However, the effect H2 annealing on thermally evaporated SnS thin films and their transfer characteristics with various metals have not been thoroughly studied yet. In this paper, SnS was deposited by thermal evaporation and annealed in a H2 gas ambient at 400 °C. X-ray diffraction (XRD), scanning electron microscopy, Hall measurements, and Rutherford backscattering spectrometry revealed that our SnS thin films were single-phase with enhanced physical and electrical properties. Furthermore, the electrical band structure of SnS was investigated by UV-visible and ultraviolet photoelectron spectroscopy analyses to predict the electrical band alignment when in contact with Ti, W, and Mo electrodes. XRD and atomic force microscopy analysis confirmed that SnS was well grown on these metal electrodes. Finally, SnS with Mo electrodes showed the most conductive transfer characteristics.

Full-spectrum-responsive Bi2S3@CdS S-scheme heterostructure with intimated ultrathin RGO toward photocatalytic Cr(VI) reduction and H2O2 production: Experimental and DFT studies

Heterostructure-based photocatalysis offers significant potential for developing ultraviolet–visible (UV–Vis) to near-infrared (NIR) light-responsive catalysts with abundant beneficial physicochemical properties to boost environmental remediation upon solar-light irradiation. In this study, for the first time a novel ternary heterostructure photocatalysts containing Bi2S3@CdS@RGO (BCG) were rationally constructed using the hydrothermal approach. The resultant ternary composite with 5 wt% of CdS and 20 wt% of reduced graphene oxide (RGO) contents (BCG-5) was adopted as an optimal sample for photo-reduction of Cr(VI) under simulated solar-light irradiation. The apparent rate constant of the photo-reduction process over BCG-5 was ~ 13, 4, and 3 times higher than those of Bi2S3, CdS, and BC-5, respectively, within 150 min. The enhanced photocatalytic activity of ternary composite could be linked predominantly to the formation of heterostructural, synergistic behavior between the components, hierarchical morphology, the formation of n-n type high-low junctions, efficient interfacial charge-transfer capability, large specific surface area, full-spectrum light-absorption, and outstanding photo-stability. Electron spin resonance and reactive radical-scavenging results demonstrated that the hydroxyl and superoxide active species were primarily responsible for Cr(VI) removal. Furthermore, the photocatalytic activity of BCG-5, as an optimal sample, was further assessed regarding the photocatalytic production of H2O2, with 1.37 and 15.14 times higher efficiency than binary and bare samples, respectively. Assisted by the density functional theory calculations, ultraviolet photoelectron spectroscopy analyses, the charge-carriers pathway and possible photocatalytic mechanism were systematically discussed in S-scheme heterojunction. We expect that our findings will open new horizons for significant applications of bismuth-rich-based heterostructures under both visible- and NIR-light irradiation to address environmental and energy issues.

Metal–Organic Framework for Efficient Electron Injection

AbstractA 3D metal–organic framework (MOF) [(H3O)Zn(HCOO)3]∞ is developed as a highly efficient and air‐stable solution‐processed electron injection layer (s‐EIL). The [(H3O)Zn(HCOO)3]∞ film can be easily obtained by annealing a spin‐coated [Zn(HCOO)2(H2O)2]∞ film at 120 °C in air, which is facilely prepared by the reaction of ZnO powder with formic acid and the aqueous solution of ammonium. The un‐encapsulated electron‐only device with [(H3O)Zn(HCOO)3]∞ as the s‐EIL exhibits significantly enhanced electrical property and air stability, compared to that with vacuum‐evaporated Cs2CO3 as EIL. Such improvements can also be observed in inverted organic light‐emitting diodes. The device with [(H3O)Zn(HCOO)3]∞ s‐EIL demonstrates an operating voltage of 6.8 V at 1000 cd m−2, much lower than that with Cs2CO3 EIL (8.4 V). And the power efficiency is increased about 18%. The enhanced electron injection property of [(H3O)Zn(HCOO)3]∞‐modified indium–tin oxide (ITO) is attributed to its reduced work function compared to bare ITO, which is confirmed by ultraviolet photoelectron spectroscopy analysis. This approach is anticipated to be applied in other optoelectronic devices for efficient electron injection.

N-Type host materials based on nitrile and triazine substituted tricyclic aromatic compounds for high-performance blue thermally activated delayed fluorescence devices

Novel n-type host materials based on tricyclic aromatic compounds, dibenzo[b,d]furan and dibenzo[b,d]thiophene (2Trz6CNDBF and 2Trz6CNDBT), have been successfully synthesized and characterized for high-performance blue thermally activated delayed fluorescence (TADF) organic light-emitting devices (OLEDs). Dibenzo[b,d]furan and dibenzo[b,d]thiophene were utilized as central molecular building blocks to achieve excellent thermal stability and high triplet energy (ET). Nitrile and diphenyltriazine functional groups were introduced at the 6- and 2- positions of the central building blocks to achieve low-lying lowest unoccupied molecular orbital (LUMO) energy levels, high ET, and excellent electron transport properties. UV–Vis absorption, low-temperature photoluminescence, and ultraviolet photoelectron spectroscopy analysis showed that 2Trz6CNDBF and 2Trz6CNDBT possessed high ET (2.95 and 2.88 eV) and low-lying LUMO energy levels (−3.43 and −3.16 eV) that were well-matched with a blue TADF emitter, 5CzCN. Moreover, the electron-only device (EOD) result revealed that 2Trz6CNDBF and 2Trz6CNDBT had excellent electron transport properties. Blue TADF OLEDs fabricated with a p-type host material, mCBP, and n-type host, 2Trz6CNDBF or 2Trz6CNDBT, exhibited lower driving voltages (3.34 and 3.26 V, respectively) than a TADF OLED with only a p-type host material, mCBP (3.83 V). Blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT exhibited superior external quantum efficiency (ηext, 15.6 and 14.7%), current efficiency (ηce, 33.8 and 32.7 cd A−1), and power efficiency (ηpe, 25.6 and 25.7 lm W−1), respectively. The ηext and ηce of blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT increased by more than 70% and ηpe by approximately 150% compared to those of the TADF OLED with a single p-type host, mCBP. In addition, the device lifetimes of blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT increased by more than 1000%. The efficient electron injection by the low-lying LUMO energy level, effective exciton confinement by high ET, and high thermal stability of the film morphology provided the enhanced efficiency and lifetime for blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT.

High Photosensitive Indium–Gallium–Zinc Oxide Thin-Film Phototransistor with a Selenium Capping Layer for Visible-Light Detection

Visible light can be detected using an indium-gallium-zinc oxide (IGZO)-based phototransistor, with a selenium capping layer (SCL) that functions as a visible light absorption layer. Selenium (Se) exhibits photoconductive properties as its conductivity increases with illumination. We report an IGZO phototransistor with an SCL (SCL/IGZO phototransistor) that demonstrated optimal photoresponse characteristics when the SCL was 150 nm thick. The SCL/IGZO phototransistor exhibited a photoresponsivity of 1.39 × 103 A/W, photosensitivity of 4.39 × 109, detectivity of 3.44 × 1013 Jones, and external quantum efficiency of 3.52 × 103% when illuminated by green light (532 nm). Ultraviolet-visible spectroscopy and ultraviolet photoelectron spectroscopy analysis showed that Se has a narrow energy band gap, in which visible light is absorbed and forms a p-n junction with IGZO so that photogenerated electron-hole pairs are easily separated, which makes recombination more challenging. We show that electrons generated in the SCL flow through the IGZO layer, which enables the phototransistor to detect visible light. Furthermore, the SCL/IGZO phototransistor exhibited excellent durability and reversibility owing to the constant light and dark current and the time-dependent photoresponse characteristics over 8000 s when a red light (635 nm) source was turned on and off at a frequency of 0.1 Hz.

Creation of a two-dimensional conducting path based on the engineering oxidation state of WO3 with Al2O3 at the interface

We report the dramatic manipulation of an oxide interface in Al2O3/WO3 from insulating to metallic state by creating a two-dimensional conducting path. The WO3 thin film is deposited on the SiO2 grown Si substrate using radio frequency magnetron sputtering and the amorphous Al2O3 layer is formed by atomic layer deposition on the WO3 layer. After rapid vacuum annealing at 400 °C, the interfacial conducting path of Al2O3/WO3 reveals an abnormally high sheet carrier density of 4.91 × 1016 cm−2 compared with <109 cm−2 for bare WO3. The X-ray photoelectron spectroscopy (XPS) shows that WO3 is partially reduced; and a W5+ oxidation state is observed at the interface. The schematic electronic band diagram of the interfacial region obtained by combined XPS and ultraviolet photoelectron spectroscopy analyses indicates that the downward band bending at the surface of WO3 induces a confined electron accumulation at the interface. Synergetic effects between the increased unpaired W 5d electrons in the W5+ oxidation state and the accumulative charge confinement at the Al2O3/WO3 interfaces lead to a two-dimensional conducting path. The metallic conduction is distinct from the insulating bulk properties. This study demonstrates an example of a short-range-ordered two-dimensional conducting path at two insulating oxide interfaces formed at low temperature, which is applicable to flexible oxide conductors with great conduction path stability.

Engineering the work function of solution-processed electrodes of silver nanocrystal thin film through surface chemistry modification

A solution-processable electrode is a key component in flexible electronics. Despite recent studies on silver nanocrystal (Ag NC)-based electrodes with high conductivity, the absence of a strategy to control the work function has limited the development of low-cost and high-performance electronic devices. In this report, we introduce a chemical route to manipulate the work function of solution-processed Ag NC-based electrodes. The structural, chemical, optical, and electronic properties, as well as the work functions, of the Ag NC thin films treated with three types of halide ligands (Cl−, Br−, and I−) were investigated. Ultraviolet photoelectron spectroscopy analysis shows that the work functions are shifted to 4.76, 4.43, and 4.04 eV when the ligands are changed to Cl−, Br−, and I−, respectively. The trend in the shift induced by the halide ligands matches the results of atomistic density functional theory calculations and scales with the strength of the dipoles formed by the electron transfer at the NC/ligand interfaces. To demonstrate the impact of our strategy in device applications, we fabricated all-NC-based thin-film transistors and complementary metal-oxide-semiconductor inverters. This study provides a fundamental understanding of the surface states of nanomaterials and also offers technological benefits for the construction of low-cost, high-performance electronic devices.

Electrochemical behaviors and passive film properties of Fe-based bulk metallic glass in Cl−-containing acetic acid solutions under high temperature

This study aimed to investigate the electrochemical behavior and passive film properties of Fe-based bulk metallic glass (BMG) formed in Cl−-containing acetic acid solutions under high temperature. Electrochemical tests showed that Fe-based BMG exhibited excellent pitting resistance and stable passivation characteristics. However, the uniform corrosion resistance of Fe-based BMG is deteriorated in the presence of Cl− anions. Mott-Schottky, X-ray photoelectron spectroscopy depth profiling, and ultra-violet photoelectron spectroscopy analyses revealed that the passive films possess a duplex membrane structure. High-valence cations and Cl− anions relatively localized in the external layer, whereas low-valence cations relatively localized in the internal layer. Moreover, Cl− exposure increases the porosity and decreases the compactness of the passive film by modifying stoichiometry, improving acceptor density, reducing work function, and enhancing the state density of valence electrons around the Fermi level. Results implied that Fe-based BMGs are potential substitutes for conventional stainless steel components that are exposed to organic manufacturing environments.