Polaron Transport and Thermoelectric Behavior in La‐Doped SrTiO3 Thin Films with Elemental Vacancies

The electrodynamic properties of La‐doped SrTiO₃ thin films with controlled elemental vacancies are investigated using optical spectroscopy and thermopower measurement. In particular, a correlation between the polaron formation and thermoelectric properties of the transition metal oxide (TMO) thin films is observed. With decreasing oxygen partial pressure during the film growth (P(O₂)), a systematic lattice expansion is observed along with the increased elemental vacancy and carrier density, experimentally determined using optical spectroscopy. Moreover, an absorption in the mid‐infrared photon energy range is found, which is attributed to the polaron formation in the doped SrTiO₃ system. Thermopower of the La‐doped SrTiO₃ thin films can be largely modulated from –120 to –260 μV K^?1, reflecting an enhanced polaronic mass of ≈3 < m _polron/m < ≈4. The elemental vacancies generated in the TMO films grown at various P(O₂) influences the global polaronic transport, which governs the charge transport behavior, including the thermoelectric properties.     

Tailoring Electron‐Transfer Barriers for Zinc Oxide/C60 Fullerene Interfaces

The interfacial electronic structure between oxide thin films and organic semiconductors remains a key parameter for optimum functionality and performance of next‐generation organic/hybrid electronics. By tailoring defect concentrations in transparent conductive ZnO films, we demonstrate the importance of controlling the electron transfer barrier at the interface with organic acceptor molecules such as C₆₀. A combination of electron spectroscopy, density functional theory computations, and device characterization is used to determine band alignment and electron injection barriers. Extensive experimental and first principles calculations reveal the controllable formation of hybridized interface states and charge transfer between shallow donor defects in the oxide layer and the molecular adsorbate. Importantly, it is shown that removal of shallow donor intragap states causes a larger barrier for electron injection. Thus, hybrid interface states constitute an important gateway for nearly barrier‐free charge carrier injection. These findings open new avenues to understand and tailor interfaces between organic semiconductors and transparent oxides, of critical importance for novel optoelectronic devices and applications in energy‐conversion and sensor technologies.     

Charge Transfer to LaAlO3/SrTiO3 Interfaces Controlled by Surface Water Adsorption and Proton Hopping

Electronic properties of low dimensional systems are particularly sensitive to surface adsorbates. Clear understanding of such phenomena can lead to highly effective and nondestructive material engineering techniques. In this work, water adsorption at the surface of LaAlO₃/SrTiO₃ heterostructures is systematically studied. The saturation of surface dangling bonds by spontaneous water chemisorptions is found to be a main enabler of the formation of the interface 2D electron gas. In particular, when imbalanced distributions of water based ions, namely protons and hydroxyls, are generated, interface electron doping or depletion becomes surface adsorbates dominant and independent of the LaAlO₃ layer thickness. The investigations also reveal the importance of hydrogen bonding through molecular water layers, which provides an energetically feasible pathway for manipulating the surface‐bond protons and thus the interface electrical characteristics.     

Metal/Metal‐Oxide Interfaces: How Metal Contacts Affect the Work Function and Band Structure of MoO3

When transition metal oxides are used in practical applications, such as organic electronics or heterogeneous catalysis, they often must be in contact with a metal. Metal contacts can affect an oxide's chemical and electronic properties within the first few nanometers of the contact, resulting in changes to an oxide's chemical reactivity, conductivity, and energy‐level alignment properties. These effects can alter an oxide's ability to perform its intended function. Thus, the choice of contacting metal becomes an important design consideration when tailoring the properties of transition‐metal oxide thin films or nanoparticles. Here, metal/metal‐oxide interfaces involving a widely used oxide in organic electronics, MoO₃, are examined. It is demonstrated that metal contacts tend to reduce the Mo⁶⁺ cation to lower oxidation states and, consequently, alter MoO₃’s valence electronic structure and work function when the oxide layer is very thin (less than 10 nm). MoO₃ becomes semimetallic and has a lower work function near metal contacts. The observed behavior is attributed to two causes: 1) charge transfer from the metal Fermi level into MoO₃’s low‐lying conduction band and 2) an oxidation‐reduction reaction between the metal and MoO₃ that results in oxidation of the metal and reduction of MoO₃. These results illustrate how interfaces are important to an oxide's ability to provide energy‐level alignment.     

Reconstruction of Low Dimensional Electronic States by Altering the Chemical Arrangement at the SrTiO3 Surface

Developing reliable methods for modulating the electronic structure of the 2D electron gas (2DEG) in SrTiO₃ is crucial for utilizing its full potential and inducing novel properties. Herein, it is shown that relatively simple surface preparation reconstructs the 2DEG at the SrTiO₃ (STO) surface, leading to a Lifshitz-like transition. Combining experimental methods, such as angle-resolved photoemission spectroscopy (ARPES) and X-ray photoemission spectroscopy with ab initio calculations, that the modulation of the surface band structures can be effectively achieved via transforming the chemical composition at the atomic scale is found. In addition, ARPES experiments demonstrate that vacuum ultraviolet light can be efficiently employed to alter the band renormalization of the 2DEG system and control the electron-phonon interaction . This study provides a robust and straightforward route to stabilize and tune the low-dimensional electronic structure via the chemical degeneracy of the STO surface.     

Oxygen Stoichiometry Effect on Polar Properties of LaAlO3/SrTiO3

Discovery of a ferroelectric‐like behavior of the LaAlO₃/SrTiO₃ (LAO/STO) interfaces provides an attractive platform for the development of nanoelectronic devices with functionality that can be tuned by electrical or mechanical means. However, further progress in this direction critically depends on deeper understanding of the physicochemical mechanism of this phenomenon. In this report, this problem by testing the electronic properties of the LAO/STO heterostructures with oxygen stoichiometry used as a variable is addressed. Local probe measurements in conjunction with interface electrical characterization allow to establish the field‐driven reversible migration of oxygen vacancies as the origin of the ferroelectric‐like behavior in LAO/STO. In addition, it is shown that oxygen deficiency gives rise to the formation of micrometer‐long atomically sharp boundaries with robust piezoelectricity stemming from a significant strain gradient across the boundary region. These boundaries are not ferroelectric but they can modulate the local electronic characteristics at the interface. The obtained results open a possibility to design and engineer electromechanical functionality in a wide variety of nominally nonpolar and non‐piezoelectric complex oxide heterostructures and thin films.     

The Origin of Magnetism in Mn‐Doped SrTiO3

An analysis of Mn substitution in SrTiO₃ is performed in order to understand the origin of reported spin coupling in lightly Mn‐doped SrTiO₃. The spin glass state magnetoelectrically coupled to the dipolar glass state has previously been reported for SrTiO₃ substituted with only 2% of Mn on the B‐site. An analysis of the substitution mechanism for A‐ and B‐site doping shows a strong influence of processing conditions, such as processing temperature, oxygen partial pressure, and off‐stoichiometry. The required conditions for a site‐selective substitution are defined, which yield a single‐phase and almost defect‐free perovskite. Magnetic measurements show no magnetic anomalies resulting from spin coupling and only a simple paramagnetic behavior. Magnetic anomalies are observed only for the samples in which Mn is misplaced within the cation sublattice of the SrTiO₃ perovskite. This occurs due to improper material processing, which causes initially unpredicted changes in the valence state of the Mn and results in the formation of structural defects and irregularities associated with segregation and nucleation of the magnetic species. Previously reported spin coupling in Mn‐doped SrTiO₃ is not an intrinsic phenomenon and cannot be treated as a spin glass.     

Ultra‐Flexible Boron‐Oxygen 3D Solid‐State Networks

The existence of ultra‐flexible low‐energy forms of boron oxides (B₂O₃ and BO) is demonstrated, in particular structures in which B₃O₃ or B₄O₂ six‐membered rings are linked by single B‐O‐B bridges. The minima in the energy landscapes are remarkably broad; the variation in the internal energies is very small over a very large range of volumes. Such volume changes may even exceed 200%. This remarkable behavior is attributed predominantly to the pronounced angular flexibility of the B‐O‐B bridges linking the rings, which is unusual for a covalent bond. At larger volumes, the structures are nanoporous; the pores collapse upon compression with negligible change in energy, making these suitable as guest‐host materials. In marked contrast, in other materials where low density frameworks have been reported or predicted, such low‐density phases are considerably higher in energy. The flexibility of the structures also offers a resolution of the long‐standing controversy reconciling the structure and density of vitreous B₂O₃.     

Selective Defect‐Patching of Zeolite Membranes Using Chemical Liquid Deposition at Organic/Aqueous Interfaces

The elimination of possible defects is indispensable in making zeolite membranes popular in process industries. A novel counter‐diffusion chemical liquid deposition (CLD) technique is proposed and developed for selective defect‐patching of zeolite membranes. Dodecyltrimethoxysilane (DMS) is employed as the silane coupling agent, forming a protective layer on the membrane surface so that intracrystalline pores can be kept intact in the subsequent reparation step. By using tetraethoxy orthosilicate (TEOS) and (3‐chloropropyl)triethoxysilane (3CP‐TES), co‐hydrolysis and co‐condensation at the organic/aqueous interface fabricate the silsesquioxane/silicate hybrid on macro‐, meso‐ and even microdefects. The silicalite‐1 membrane before and after reparation is characterized using contact‐angle measurements, Fourier transform IR spectroscopy, and electron probe microanalysis. Permporometry is conducted to study the pore‐size distribution of the membrane before and after reparation. It is found that the silsesquioxane/silicate hybrid is only deposited at the pore‐mouth of the defects, and the defects can be plugged to less than 1.3 nm pores after patching. After reparation, the separation factor of a 50/50 n/i‐butane‐gas mixture through the membrane can be increased to 35.8 from 4.4, and the separation factor of a CO₂/N₂ gas mixture through the membrane can be increased to around 15 from 1, while keeping the two‐thirds CO₂ permeation flux of the synthesized membrane.     

High‐TC Interfacial Ferromagnetism in SrMnO3/LaMnO3 Superlattices

Heterostructures of strongly correlated oxides demonstrate various intriguing and potentially useful interfacial phenomena. LaMnO₃/SrMnO₃ superlattices are presented showcasing a new high‐temperature ferromagnetic phase with Curie temperature, T_C ≈360 K, caused by electron transfer from the surface of the LaMnO₃ donor layer into the neighboring SrMnO₃ acceptor layer. As a result, the SrMnO₃ (top)/LaMnO₃ (bottom) interface shows an enhancement of the magnetization as depth‐profiled by polarized neutron reflectometry. The length scale of charge transfer, λ_TF ≈2 unit cells, is obtained from in situ growth monitoring by optical ellipsometry, supported by optical simulations, and further confirmed by high resolution electron microscopy and spectroscopy. A model of the inhomogeneous distribution of electron density in LaMnO₃/SrMnO₃ layers along the growth direction is concluded to account for a complex interplay between ferromagnetic and antiferromagnetic layers in superlattices.     

Tunable Oxygen Diffusion and Electronic Conduction in SrTiO3 by Dislocation‐Induced Space Charge Fields

Plastic strain engineering was applied to induce controllable changes in electronic and oxygen ion conductivity in oxides by orders of magnitude, without changing their nominal composition. By using SrTiO₃ as a model system of technological importance, and by combining electrical and chemical tracer diffusion experiments with computational modeling, it is revealed that dislocations alter the equilibrium concentration and distribution of electronic and ionic defects. The easier reducibility of the dislocation cores increases the n‐type conductivity by 50 times at oxygen pressures below 10^?5 atm at 650 °C. At higher oxygen pressures the p‐type conductivity decreases by 50 times and the oxygen diffusion coefficient reduces by three orders of magnitude. The strongly altered electrical and oxygen diffusion properties in SrTiO₃ arise because of the existence of overlapping electrostatic fields around the positively charged dislocation cores. The findings and the approach are broadly important and have the potential for significantly impacting the functionalities of electrochemical and/or electronic applications such as thin film oxide electronics, memristive systems, sensors, micro‐solid oxide fuel cells, and catalysts, whose functionalities rely on the concentration and distribution of charged point defects.     

Size Dependence of Resistive Switching at Nanoscale Metal‐Oxide Interfaces

Size dependent variations in resistive switching using a metal‐semiconducting oxide model to examine the underlying mechanisms are reported. In the range of 20 nm to 200 nm, Au nanoparticle/SrTiO₃ interface transport properties are size dependent. The size dependence is attributed to the combination of geometric scaling and size‐dependent Schottky properties. After electroforming, the observed “eight‐wise” bipolar resistive hysteresis loop is modulated by trap/detrap process. The size‐dependent high resistance state is consistent with changes in both the interfacial area and Schottky properties. The low resistance state exhibits size independent resistance through the dominant fast conductive path. Detrapping requires more work for smaller interfaces due to the associated larger built‐in electric field.     

Impact of Bi Deficiencies on Ferroelectric Resistive Switching Characteristics Observed at p‐Type Schottky‐Like Pt/Bi1–δFeO3 Interfaces

This work reports a resistive switching effect observed at rectifying Pt/Bi_1–δFeO₃ interfaces and the impact of Bi deficiencies on its characteristics. Since Bi deficiencies provide hole carriers in BiFeO₃, Bi‐deficient Bi_1–δFeO₃ films act as a p‐type semiconductor. As the Bi deficiency increased, a leakage current at Pt/Bi_1–δFeO₃ interfaces tended to increase, and finally, rectifying and hysteretic current–voltage (I–V) characteristics were observed. In I–V characteristics measured at a voltage‐sweep frequency of 1 kHz, positive and negative current peaks originating from ferroelectric displacement current were observed under forward and reverse bias prior to set and reset switching processes, respectively, suggesting that polarization reversal is involved in the resistive switching effect. The resistive switching measurements in a pulse‐voltage mode revealed that the switching speed and switching ratio can be improved by controlling the Bi deficiency. The resistive switching devices showed endurance of >10⁵ cycles and data retention of >10⁵ s at room temperature. Moreover, unlike conventional resistive switching devices made of metal oxides, no forming process is needed to obtain a stable resistive switching effect in the ferroelectric resistive switching devices. These results demonstrate promising prospects for application of the ferroelectric resistive switching effect at Pt/Bi_1–δFeO₃ interfaces to nonvolatile memory.     

Ultrathin Amorphous Silica Membrane Enhances Proton Transfer across Solid‐to‐Solid Interfaces of Stacked Metal Oxide Nanolayers while Blocking Oxygen

A large jump of proton transfer rates across solid‐to‐solid interfaces by inserting an ultrathin amorphous silica layer into stacked metal oxide nanolayers is discovered using electrochemical impedance spectroscopy and Fourier‐transform infrared reflection absorption spectroscopy (FT‐IRRAS). The triple stacked nanolayers of Co₃O₄, SiO₂, and TiO₂ prepared by atomic layer deposition (ALD) enable a proton flux of 2400 ± 60 s^?1 nm^?2 (pH 4, room temperature), while a single TiO₂ (5 nm) layer exhibits a threefold lower flux of 830 s^?1 nm^?2. Based on FT‐IRRAS measurements, this remarkable enhancement is proposed to originate from the sandwiched silica layer forming interfacial SiOTi and SiOCo linkages to TiO₂ and Co₃O₄ nanolayers, respectively, with the O bridges providing fast H⁺ hopping pathways across the solid‐to‐solid interfaces. Together with the complete O₂ impermeability of a 2 nm ALD‐grown SiO₂ layer, the high flux for proton transport across multi‐stack metal oxide layers opens up the integration of incompatible catalytic environments to form functional nanoscale assemblies such as artificial photosystems for CO₂ reduction by H₂O.     

X‐Ray Spectroscopic Investigation of Chlorinated Graphene: Surface Structure and Electronic Effects

Chemical doping of graphene represents a powerful means of tailoring its electronic properties. Synchrotron‐based X‐ray spectroscopy offers an effective route to investigate the surface electronic and chemical states of functionalizing dopants. In this work, a suite of X‐ray techniques is used, including near edge X‐ray absorption fine structure spectroscopy, X‐ray photoemission spectroscopy, and photoemission threshold measurements, to systematically study plasma‐based chlorinated graphene on different substrates, with special focus on its dopant concentration, surface binding energy, bonding configuration, and work function shift. Detailed spectroscopic evidence of C–Cl bond formation at the surface of single layer graphene and correlation of the magnitude of p‐type doping with the surface coverage of adsorbed chlorine is demonstrated for the first time. It is shown that the chlorination process is a highly nonintrusive doping technology, which can effectively produce strongly p‐doped graphene with the 2D nature and long‐range periodicity of the electronic structure of graphene intact. The measurements also reveal that the interaction between graphene and chlorine atoms shows strong substrate effects in terms of both surface coverage and work function shift.     

Doping of Organic Semiconductors Using Molybdenum Trioxide: a Quantitative Time‐Dependent Electrical and Spectroscopic Study

Doping of organic semiconductors (OSCs) with transition metal oxides such as molybdenum trioxide (MoO₃) has been used as a powerful method to overcome common issues such as contact resistance and low conductivity, which are limiting factors in organic optoelectronic devices. In this study, the mechanism and efficiency of MoO₃‐induced p‐type doping in OSCs are investigated by means of simultaneous electrical and spectroscopic measurements on lateral diodes. It is demonstrated that energetic changes in the MoO₃ energy levels outside vacuum can limit charge‐transfer doping and device performance. It is shown and investigated that these changes crucially depend on the OSC. The time evolution of important OSC parameters such as induced charge density, doping concentration and efficiency, conductivity and mobility, is deduced. Moreover, the energetic and chemical changes in MoO₃ are investigated via ultraviolet and x‐ray photoemission spectroscopy. Combining these experiments, important conclusions are drawn on the time‐dependence and stability of MoO₃‐doping of OSCs, as well as on the processing conditions and device architectures suitable for high‐performance devices.     

Atomic‐Level Coupled Interfaces and Lattice Distortion on CuS/NiS2 Nanocrystals Boost Oxygen Catalysis for Flexible Zn‐Air Batteries

The exploration of highly efficient nonprecious metal bifunctional electrocatalysts to boost oxygen evolution reaction and oxygen reduction reaction is critical for development of high energy density metal‐air batteries. Herein, a class of CuS/NiS₂ interface nanocrystals (INs) catalysts with atomic‐level coupled nanointerface, subtle lattice distortion, and plentiful vacancy defects is reported. The results from temperature‐dependent in situ synchrotron‐based X‐ray absorption fine spectroscopy and electron spin resonance spectroscopy demonstrate that the lattice distortion of 14.7% in CuS/NiS₂ caused by the strong Jahn–Teller effect of Cu, the strong atomic‐level coupled interface of CuS and NiS₂ domains, and distinct vacancy defects can provide numerous effective active sites for their excellent bifunctional performance. A liquid Zn‐air battery with the CuS/NiS₂ INs as air electrode displays a large peak power density (172.4 mW cm^?2), a high specific capacity (775 mAh g_Zn^?1), and long cycle life (up to 83 h), making the CuS/NiS₂ INs among the best bifunctional catalysts for Zn‐air battery. More remarkably, the flexible CuS/NiS₂ INs‐based solid‐state Zn‐air batteries can power the LED after twisting, making them be promising in portable and wearable electronic devices.     

Hydride Anion Substitution Boosts Thermoelectric Performance of Polycrystalline SrTiO3 via Simultaneous Realization of Reduced Thermal Conductivity and High Electronic Conductivity

The development of environmentally benign thermoelectric materials with high energy conversion efficiency (ZT) continues to be a long-standing challenge. So far, high ZT has been achieved using heavy elements to reduce lattice thermal conductivity (κ_lat). However, it is not preferred to use such elements because of their environmental load and high material cost. Here a new approach utilizing hydride anion (H⁻) substitution to oxide ion is proposed for ZT enhancement in thermoelectric oxide SrTiO₃ bulk polycrystals. Light element H⁻ substitution largely reduces κ_lat from 8.2 W/(mK) of SrTiO₃ to 3.5 W/(mK) for SrTiO_3−xH_x with x = 0.216. The mass difference effect on phonon scattering is small in the SrTiO_3−xH_x, while local structure distortion arising from the distributed Ti−(O,H) bond lengths strongly enhances phonon scattering. The polycrystalline SrTiO_3−xH_x shows high electronic conductivity comparable to La-doped SrTiO₃ single crystal because the H⁻ substitution does not form a grain boundary potential barrier and thus suppresses electron scattering. As a consequence, SrTiO_3−xH_x bulk exhibits maximum ZT = 0.11 at room temperature and the ZT value increases continuously up to 0.22 at T = 657 K. The H⁻ substitution idea offers a new approach for ZT enhancement in thermoelectric materials without utilizing heavy elements.     

Ideal Energy‐Level Alignment at the ZnO/P3HT Photovoltaic Interface

Hybrid semiconductor‐polymer nanostructured solar cells hold the promise of photovoltaic energy conversion based on abundant and nontoxic materials and scalable manufacturing processes. After a decade of intense research activity, hybrid solar cells still exhibit low short‐circuit currents and moderate open‐circuit voltages. These bottlenecks call for a detailed understanding of the physics underlying the device operation at the nanoscale. Using first‐principles calculations the ideal energy‐level alignment of hybrid solar cell interfaces based on the wide bandgap semiconductor ZnO and the polymer poly(3‐hexylthiophene) (P3HT) is investigated. The interfacial charge transfer is quantified and it is shown that this effect increases the ideal open‐circuit voltage with respect to the electron‐affinity rule by as much as 0.5 V. The results of this work suggests that there is significant room for optimizing this class of excitonic solar cells by tailoring the semiconductor/polymer interface at the nanoscale.