Interface science has become a key aspect for fundamental research questions and for the understanding, design and optimization of urgently needed energy and information technologies. As the interface properties change during operation, e.g. under applied electrochemical stimulus, and because multiple bulk and interface processes coexist and compete, detailed operando characterization is needed. In this perspective, I present an overview of the state-of-the art and challenges in selected X-ray spectroscopic techniques, concluding that among others, interface-sensitivity remains a major concern in the available techniques. I propose and discuss a new method to extract interface-information from nominally bulk sensitive techniques, and critically evaluate the selection of X-ray energies for the recently developed meniscus X-ray photoelectron spectroscopy, a promising operando tool to characterize the solid-liquid interface. I expect that these advancements along with further developments in time and spatial resolution will expand our ability to probe the interface electronic and molecular structure with sub-nm depth and complete our understanding of charge transfer processes during operation.
Key technologies in energy conversion and storage, sensing and chemical synthesis rely on a detailed knowledge about charge transfer processes at electrified solid-liquid interfaces. However, these interfaces continuously evolve as a function of applied potentials, ionic concentrations and time. We therefore need to characterize chemical composition, atomic arrangement and electronic structure of both the liquid and the solid side of the interface under operating conditions. In this chapter, we discuss the state-of-the-art X-ray based spectroscopy and diffraction approaches for such 'operando'…
Operando spectro-ptychography reveals dynamical charge-storage and degradation pathways in redox-active electrodes
Stanford University（斯坦福大学）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；University of California, Los Angeles（加州大学洛杉矶分校）；SLAC National Accelerator Laboratory（SLAC国家加速器实验室）；University of California, Berkeley（加州大学伯克利分校）
Electrochemical energy systems rely on particulate porous electrodes to store or convert energies. While the three-dimensional porous structures were introduced to maximize the interfacial area for better overall performance of the system, spatiotemporal heterogeneities arose from materials thermodynamics localize the charge transfer processes onto a limited portion of the available interfaces. Here, we demonstrate a simple but precision method that can directly track and analyze the operando (i.e. local and reacting) interfaces at the mesoscale in a practical graphite porous electrode to obta…
Electrolyte-based transistors utilizing ionic liquids/gels have been highly successful in the study of charge-density-controlled phenomena, particularly in oxides. Experimental probes beyond transport have played a significant role, despite challenges to their application in electric double-layer transistors. Here, we demonstrate application of synchrotron soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) as operando probes of the charge state and magnetism in ion-gel-gated ferromagnetic perovskite films. Electrochemical response via oxygen vacancies at posi…
Transition metal dichalcogenide MoS2 is a two-dimensional material, attracting much attention for next-generation applications thanks to rich functionalities stemmed from the crystal structure. Many experimental and theoretical works have focused on the spin-orbit interaction which couples the valley and spin degree of freedom so that the spin-states can be electrically controllable. However, the spin-states of charge carriers and vacancies have not been yet elucidated directly from a microscopic viewpoint. We report the spin-states in thin-film electric double-layer transistors using operando…
In view of the long-standing controversy over the reversibility of transition metals in Sn-based alloys as anode for Li-ion batteries, an in situ real-time magnetic monitoring method was used to investigate the evolution of Sn-Co intermetallic during the electrochemical cycling. Sn-Co alloy film anodes with different compositions were prepared via magnetron sputtering without using binders and conductive additives. The magnetic responses showed that the Co particles liberated by Li insertion recombine fully with Sn during the delithiation to reform Sn-Co intermetallic into stannum richer phase…
Operando Charge Carrier Dynamics by Intensity-Modulated Photoluminescence: From Perovskite Thin Films to Solar Cells
Technische Universität Chemnitz（开姆尼茨工业大学）；Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden (IFW)（德累斯顿莱布尼茨固体与材料研究所）；Chair for Emerging Electronic Technologies, Technische Universität Dresden（德累斯顿工业大学新兴电子技术系）
Recent years have seen a rapidly escalating demand for battery technologies capable of storing more energy, charging more quickly and having longer usable lifetimes, driven largely by increased electrification of transport and by grid-scale energy storage systems. This has led to the development of many promising new electrode materials for high-rate lithium ion batteries. In order to rationalise and improve upon material performance, it is crucial to understand the fundamental ion-intercalation and degradation mechanisms occurring during realistic battery operation, on the nano- to meso-scale…
Mapping reaction mechanism during overcharge of a LiNiO2/Graphite-silicon lithium-ion battery: a correlative operando approach by simultaneous gas analysis and synchrotron scattering techniques
The interface properties of interest in multilayers include interfacial charge densities, dipole densities, band offsets, and screening-lengths, among others. Most such properties are inaccesible to direct measurements, but are key to understanding the physics of the multilayers. They are contained within first-principles electronic structure computations but are buried within the vast amount of quantitative information those computations generate. Thus far, they have been extracted from the numerical data by heuristic nanosmoothing procedures which do not necessarily provide results independe…
Spatially inhomogeneous delithiation in LiNiO2 positive electrode: the effect of X-rays dose
Synchrotron SOLEIL（Soleil 同步辐射光源）；Technische Universität München（慕尼黑工业大学）；Centre National de la Recherche Scientifique, CNRS UR1（法国国家科学研究中心）；Réseau sur le Stockage Electrochimique de l’Energie (RS2E), CNRS FR3459（电化学储能研究网络）；Sorbonne Université, CNRS, Physicochimie des Electrolytes et Nanosystèmes Interfaciaux, CNRS UMR8234（索邦大学）
Charge transfer (CT) at metal-graphene contacts induces a potential variation from the contact edges that extends to ~1 micrometer. Potential variations with a similar length should be observed around charge-transferring surface adsorbates. Thus, it is expected that a competition exists between these two CT sources when one source is within ~1 micrometer from the other. In this letter, weakly-coupled Ni contacts and 7,7,8,8-tetracyanoquinodimethan molecules are employed as the CT sources to investigate their possible competition. The CT from the molecules adsorbed only in the channel region ch…
Hybrid materials of MXenes (2D carbides and nitrides) and transition-metal oxides (TMOs) have shown great promise in electrical energy storage and 2D heterostructures have been proposed as the next-generation electrode materials to expand the limits of current technology. Here we use first principles density functional theory to investigate the interfacial structure, energetics, and electronic properties of the heterostructures of MXenes (Tin+1CnT2; T=terminal groups) and anatase TiO2. We find that the greatest work-function differences are between OH-terminated-MXene (1.6 eV) and anatase TiO2…
Interfacial Oxidation Enables Charge-Transfer Contacts and Degenerate n-Doping in Monolayer MoS$_2$
Elettra - Sincrotrone Trieste S.C.p.A（的里雅斯特电子同步加速器股份有限公司）；University of Udine（乌迪内大学）；University of Trieste（的里雅斯特大学）；Instituto de Ciencia de Materiales de Madrid (ICMM - CSIC)（马德里材料科学研究所（CSIC））；CNR - Istituto Officina dei Materiali (IOM)（国家研究委员会材料工坊研究所）；ArtemisProgram,U.K.Central Laser Facility, STFC Rutherford Appleton Laboratory（英国中央激光设施，STFC鲁瑟福阿普尔顿实验室）；School of Physics and Astronomy, University of St Andrews（圣安德鲁斯大学物理与天文学学院）；Department of Physics and Astronomy, Aarhus University（奥胡斯大学物理与天文学系）
We study the electronic structures of TiO2/Ti4O7 and Ta2O5/TaO2 interfaces using the screened exchange (sX-LDA) functional. Both of these bilayer structures are useful infrastructures for high performance RRAMs. We find that the system Fermi energies of both interfaces are just above the conduction band edge of the corresponding stoichiometric oxides. According to the charge transition levels of the oxygen vacancies in Ta2O5 and TiO2, the oxygen vacancies are stabilized at the -2 charged state. We propose to introduce interfacial dopants to shift the system Fermi energies downward so that the …
Excitonic insulators are long-sought-after quantum materials predicted to spontaneously open a gap by the Bose condensation of bound electron-hole pairs, namely, excitons, in their ground state. Since the theoretical conjecture, extensive efforts have been devoted to pursuing excitonic insulator platforms for exploring macroscopic quantum phenomena in real materials. Reliable evidences of excitonic character have been obtained in layered chalcogenides as promising candidates. However, owing to the interference of intrinsic lattice instabilities, it is still debatable whether those features, su…
We demonstrate that the interfacial dipole associated with bonding across the SrTiO3/Si heterojunction can be tuned through space charge, thereby enabling the band alignment to be altered via doping. Oxygen impurities in Si act as donors that create space charge by transferring electrons across the interface into SrTiO3. The space charge induces an electric field that modifies the interfacial dipole, thereby tuning the band alignment from type-II to type-III. The transferred charge, resulting in built-in electric fields, and change in band alignment are manifested in electrical transport and h…
Ionic liquids (IL) are promising electrolytes for electrochemical applications due to their remarkable stability and high charge density. Molecular dynamics simulations are essential for better understanding the complex phenomena occurring at the electrode-IL interface. In this work, we have studied the interface between graphene and 1-ethyl-3-methyl-imidazolium tetrafluoroborate IL, using density functional theory-based molecular dynamics simulations at variable surface charge densities. We have disassembled the electrical double layer potential drop into two main components: one involving at…
The structure and chemical composition are the key parameters influencing the properties of organic thin films deposited on inorganic substrates. Such films often display structures that substantially differ from the bulk, and the substrate has a relevant influence on their polymorphism. In this work, we illuminate the role of the substrate by studying its influence for para-benzoquinone on two different substrates, Ag(111) and graphene. We employ a combination of first principles calculations and machine learning to identify the energetically most favorable structures on both substrates and s…
Distinguishing Carrier Transport and Interfacial Recombination at Perovskite-Transport Layer Interfaces Using Ultrafast Spectroscopy and Numerical Simulation
Interfaces between correlated complex oxides are promising avenues to realize new forms of magnetism that arise as a result of charge transfer, proximity effects and locally broken symmetries. We report upon the discovery of a non-collinear magnetic structure in superlattices of the ferromagnetic metallic oxide La2/3Sr1/3MnO3 (LSMO) and the correlated metal LaNiO3 (LNO). The exchange interaction between LSMO layers is mediated by the intervening LNO, such that the angle between the magnetization of neighboring LSMO layers varies in an oscillatory manner with the thickness of the LNO layer. The…
This work explores the use of joint density-functional theory, a new form of density-functional theory for the ab initio description of electronic systems in thermodynamic equilibrium with a liquid environment, to describe electrochemical systems. After reviewing the physics of the underlying fundamental electrochemical concepts, we identify the mapping between commonly measured electrochemical observables and microscopically computable quantities within an, in principle, exact theoretical framework. We then introduce a simple, computationally efficient approximate functional which we find to …
When redox active molecules are covalently tethered to the surface of the n-doped silicon, light induces their oxidation by the semiconductor. To visualize this charge separation we have followed the formation and decay of the surface photovoltage by an auxiliary transparent electrode, which serves as a capacitive probe that picks up the transient surface photovoltage. This method has been used in electronics and biophysics but is novel for the surface redox chemistry. The probe provides both the direction of the charge transfer and the resolution of the dynamics. Its application is independen…
In organic bulk heterojunction solar cells, the donor/acceptor interfacial energy offset ($ΔE$) is found to provide the driving force for efficient charge separation which gives rise to high short circuit current density ($J_\mathrm{sc}$), but a high $ΔE$ inevitably undermines the open circuit voltage ($V_\mathrm{oc}$). In this paper, employing the device model method we calculated the steady state current density-voltage ($J-V$) and the $J_\mathrm{sc}-ΔE$ curves under two different charge separation mechanisms to investigate the optimum driving force required for achieving sizable $V_\mathrm{…
We report charge-transfer up to a single electron per interfacial unit cell across non-polar heterointerfaces from the Mott insulator LaTiO3 to the charge transfer insulator LaCoO3. In high-quality bi- and tri-layer systems grown using pulsed laser deposition, soft X-ray absorption, dichroism and STEM-EELS are used to probe the cobalt 3d-electron count and provide an element-specific investigation of the magnetic properties. The experiments prove a deterministically-tunable charge transfer process acting in the LaCoO3 within three unit cells of the heterointerface, able to generate full conver…
The possibility of controlling the interfacial properties of artificial oxide heterostructures is still attracting researchers in the field of materials engineering. Here, we used surface sensitive techniques and high-resolution transmission electron microscopy to investigate the evolution of the surface spin-polarization and lattice strains across the interfaces between La0.66Sr0.33MnO3 thin films and low-doped manganites as capping layers. We have been able to finely tune the interfacial spin-polarization by changing the capping layer thickness and composition. The spin-polarization was foun…
We propose a new method to calculate polarization induced interfacial charges in semiconductor heterostructures using classical electrostatics applied to real-space band diagrams from first principles calculations and apply it to GaN/AlN heterostructures with ultrathin AlN layers (4-6 monolayers). We show that the calculated electric fields and interfacial charges are independent of the exchange-correlation functionals used (local-density approximation and hybrid functionals). We also find the calculated interfacial charge of (6.8 +/- 0.4) x 10^13 cm-2 to be in excellent agreement with experim…
Charge transfer induced interfacial ferromagnetism and its impact on the exchange bias effect in La$_{0.7}$Sr$_{0.3}$MnO$_3$/NdNiO$_3$ correlated oxide heterostructures were investigated by soft x-ray absorption and x-ray magnetic circular dichroism spectra in a temperature range from 10 to 300 K. Besides the antiferromagnetic Ni$_3^+$ cations which are naturally part of the NdNiO$_3$ layer, Ni$_2^+$ ions are formed at the interface due to a charge transfer mechanism involving the Mn element of the adjacent layer. They exhibit a ferromagnetic behavior due to the exchange coupling to the Mn$_4^…
We present the effect of oxidative etching in FLG/TiO$_2$ hybrids on FLG edges chemistry/morphology addressing also the catalytic activity of TiO$_2$ NPs. According to the XPS and TGA analysis the oxidation degree strongly depends on TiO$_2$:FLG ratio. The preparation of two series of FLG/TiO$_2$ hybrids via sol-gel method and in-situ oxidation permitted to correctly describe and distinguish the chemical composition, core shift and charge distribution in C1s XPS with Tip peak as reference. Such analysis is challenging in carbon materials due the overlapping of C1s peaks from the sample and adv…
Novel phenomena appear when two different oxide materials are combined together to form an interface. For example, at the interface of LaAlO3/SrTiO3, two dimensional conductive states form to avoid the polar discontinuity and magnetic properties are found at such interface. In this work, we propose a new type of interface between two nonmagnetic and nonpolar oxides that could host a conductive state with magnetic properties, where it is the ferroelectric polarization discontinuity instead of the polar discontinuity that leads to the charge transfer, forming the interfacial conductive or magnet…
Tuning Electronic and Optical Properties of 2D/3D Construction based on Hybrid Perovskites through Interfacial Charge Transfer: Towards Higher Efficiency Solar Cells
Interfacial Reconstructions and Engineering in III-V@II-VI Core-Shell Quantum Dots
BCMaterials, Basque Center for Materials, Applications, and Nanostructures, UPV/EHU（巴斯克材料、应用与纳米结构中心，巴斯克地区大学）；Istituto Italiano di Tecnologia（意大利理工大学）；Ghent University（根特大学）；Ikerbasque Basque Foundation for Science（伊克尔巴斯克科学基金会）
We derive a connection between the intrinsic tribological properties and the electronic properties of a solid interface. In particular, we show that the adhesion and frictional forces are dictated by the electronic charge redistribution occurring due to the relative displacements of the two surfaces in contact. We define a figure of merit to quantify such charge redistribution and show that simple functional relations hold for a wide series of interactions including metallic, covalent and physical bonds. This suggests unconventional ways of measuring friction by recording the evolution of the …
Tuning the Coercive Field in Ferroelectric Hf0.5Zr0.5O2-Al2O3 Heterostructures via Interfacial Charge Dynamics
School of Materials Science and Engineering, Georgia Institute of Technology（佐治亚理工学院材料科学与工程学系）；School of Electrical and Computer Engineering, Georgia Institute of Technology（佐治亚理工学院电气与计算机工程学系）；Center for Nanophase Materials Sciences, Oak Ridge National Laboratory（橡树岭国家实验室纳米相材料科学中心）；The Institute for Matter and Systems, Georgia Institute of Technology（佐治亚理工学院物质系统研究所）
One prominent structural feature of ionic liquids near surfaces is formation of alternating layers of anions and cations. However, how this layering responds to applied potential is poorly understood. We focus on the structure of 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate (BMPY-FAP) near the surface of a strontium titanate (SrTiO$_3$) electric double-layer transistor. Using x-ray reflectivity, we show that at positive bias, the individual layers in the ionic liquid double layer thicken and the layering persists further away from the interface. We model the reflecti…
An attractive class of materials for photo(electro)chemical reactions are hybrids based on semiconducting metal oxides and nanocarbons (e.g. carbon nanotubes (CNT), graphene), where the nanocarbon acts as a highly-stable conductive scaffold onto which the nanostructured inorganic phase can be immobilised; an architecture that maximises surface area and minimises charge transport/transfer resistance. TiO$_{2}$/CNT photoanodes produced by atomic layer deposition on CNT fabrics are shown to be efficient for H$_{2}$ production ($0.07 μmol/min$ $H_{2}$ at $0.2V$ $vs Ag/AgCl$), nearly doubling the p…
The surface potential and the efficiency of interfacial charge transfer are extremely important for designing future semiconductor devices based on the emerging two-dimensional (2D) phosphorene. Here, we directly measured the strongly layer-dependent surface potential of mono- and few-layer phosphorene on gold, which confirms with the reported theoretical prediction. At the same time, we used an optical way - photoluminescence (PL) spectroscopy to probe the charge transfer in phosphorene-gold hybrid system. We firstly observed highly anisotropic and layer-dependent PL quenching in the phosphor…
Interfacial charge transfer plays an essential role in establishing the relative alignment of the metal Fermi level and the energy bands of organic semiconductors. While the details remain elusive in many systems, this charge transfer has been inferred in a number of photoemission experiments. We present electronic transport measurements in very short channel ($L < 100$ nm) transistors made from poly(3-hexylthiophene) (P3HT). As channel length is reduced, the evolution of the contact resistance and the zero-gate-voltage conductance are consistent with such charge transfer. Short channel conduc…
Interfacial charge separation and recombination at heterojunctions of monolayer transition metal dichalcogenides (TMDCs) are of interest to two dimensional optoelectronic technologies. These processes can involve large changes in parallel momentum vector due to the confinement of electrons and holes to the K-valleys in each layer. Since these high-momentum valleys are usually not aligned across the interface of two TMDC monolayers, how parallel momentum is conserved in the charge separation or recombination process becomes a key question. Here we probe this question using the model system of a…
(LaNiO3)n/(LaMnO3)2 superlattices were grown using ozone-assisted molecular beam epitaxy, where LaNiO3 is a paramagnetic metal and LaMnO3 is an antiferromagnetic insulator. The superlattices exhibit excellent crystallinity and interfacial roughness of less than 1 unit cell. X-ray spectroscopy and dichroism measurements indicate that electrons are transferred from the LaMnO3 to the LaNiO3, inducing magnetism in LaNiO3. Magnetotransport measurements reveal a transition from metallic to insulating behavior as the LaNiO3 layer thickness is reduced from 5 unit cells to 2 unit cells and suggest a mo…
Interfacial thermal transport between electrodes and polymer electrolytes can play a crucial role in the thermal management of solid-state lithium-ion batteries (SLIBs). Modifying the electrode surface with functional molecules can effectively increase the interfacial thermal conductance (ITC) between electrodes and polymers (e.g., electrolytes, separators); however, how they influence the interfacial thermal transport in SLIBs during charge/discharge remains unknown. In this work, we conduct molecular dynamics (MD) simulations to investigate the ITC between charged electrodes and solid-state …
We investigate the charge-transfer interfaces between metal and redox arylamine molecular films through studying the current-voltage characteristics of single-layer organic diodes with the aid of anode interfacial engineering method. The diode turn-on voltage is shown to be highly sensitive to the arylamine/metal charge-transfer interfaces and thus can serve as a probe in detecting such organic/metal interfaces. We show that the diode electrical performance could be tuned through engineering the arylamine/metal interfaces via controlling the factors of anode work function, arylamine substitute…
Heterostructures of 2D van der Waals semiconductor materials offer a diverse playground for exploring fundamental physics and potential device applications. In InSe/GaSe heterostructures formed by sequential mechanical exfoliation and stacking of 2D monochalcogenides InSe and GaSe, we observe charge transfer between InSe and GaSe due to the 2D van der Waals interface formation and a strong hysteresis effect in the electron transport through the InSe layer when a gate voltage is applied through the GaSe layer. A gate voltage dependant conductance decay rate is also observed. We relate these obs…
Investigation of the interfacial electronic properties of N,N'-bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10-bisdicarboximide (PDI8-CN2) organic semiconductor films grown on silicon dioxide is performed by polarization-resolved second harmonic generation optical spectroscopy, pointing out a spatial region where charge carriers distribution in the semiconductor lacks inversion symmetry. By developing a model for nonlinear susceptibility in the framework of Debye-Huckel screening theory, we show that the experimental findings can be interpreted as resulting from the presence of a net charge lo…
