Interface States Research Articles

Sign reversal and amplitude enhancement of unidirectional magnetoresistance in CoFe2O4/Pt heterostructures due to spin canting

We report the observation of unidirectional magnetoresistance (UMR) in the ferrimagnetic insulator CoFe2O4(CFO)/Pt heterostructures, which stem from the giant interfacial Rashba–Edelstein effect. Furthermore, UMR has been found to show sign reversal and amplitude enhancement characteristics with decrease in temperature. We have ascribed it to the modulated distortion of Fermi contours due to pronounced spin canting at low temperatures. The presence of spin canting induced interfacial magnetic state has also been demonstrated by spin Hall magnetoresistance in CFO/Pt/Co/Al2O3 films. Our work reveals the interfacial magnetic state modulated UMR in CFO/Pt bilayers, thereby paving the way for extending its applications in ferrimagnet-based spintronic devices.

Simultaneous electric dipoles and flat-band voltage modulation in 4H-SiC MOS capacitors through HfO2/SiO2 interface engineering

We report an approach to simultaneously tune the electric dipoles and flat-band voltage (V FB) of 4H-silicon carbide (SiC) metal-oxide-semiconductor (MOS) capacitors through high-k oxide dielectric interface engineering. With an additional HfO2 thin layer on atomic layer deposition (ALD) of SiO2 film, a dipole layer was formed at the HfO2/SiO2 interface, leading to a small positive shift of the V FB of 0.3 V in 4H-SiC MOS capacitors. The Kelvin probe method was used to examine the dipole layers induced at the direct-contact oxide/4H-SiC interfaces. It was found that a minor difference of 0.3 V in the contact potential difference (V CPD) is observed between the SiO2/4H-SiC and HfO2/SiO2/4H-SiC stacks, which signifies the presence of a weak interface dipole layer at the interface of HfO2 and SiO2. Additionally, investigation of the interface state density reveals that ALD of the HfO2 process had a negligible impact on the quality of the SiO2/4H-SiC interface, suggesting that the observed small positive V FB shift originated from the HfO2/SiO2 interface rather than the SiO2/4H-SiC interface.

Controlling the TE-TM splitting of topological photonic interface states by precise incident angle adjustment

Topological phases in photonic systems have garnered significant attention, often relying on precise structural design for generating non-trivial topological phases. However, this dependency on fixed structures limits their adaptability. This study systematically explores incident angle-induced topological phase transitions in a one-dimensional photonic crystal (PC). Both TE and TM polarized modes undergo topological phase transitions at the same critical transition angles. Additionally, the TM-polarized mode undergoes a unique topological phase transition at the Brewster angle. When these two kinds of transition angles coincide, even if the band structure of the TM-polarized mode undergoes an open-close-reopen process, the topological properties of the corresponding bandgap remain unchanged. Based on theoretical analysis, we design the composite PCs comprising two interfaced PCs having common bandgaps but different topological properties. By tuning the incident angle, we theoretically and experimentally achieve TE-TM splitting of topological interface states in the visible region, which may have potential applications in optical communications, optical switching, photonic integrated circuits, and so on.

Experimental investigation on lateral resistance capacity enhancement effect of a self-centering shape factor in self-centering pier systems

In self-centering (SC) pier systems, a rocking interface is typically employed at the bottom of a pier to achieve a gap-opening mechanism. However, compared with the full-section bending state of the bottom section of a ductile pier (DP), the local compressive state of the rocking interface in the SC pier inevitably leads to a significant reduction in the lateral resistance capacity. Although a prestressing tendon (PT) force can improve the lateral resistance capacity, it also induces local damage. Moreover, its enhancement effect is limited, particularly in the case of a column with a high axial compression ratio. Therefore, this study proposes an innovative measure to significantly improve the hysteretic performance of an SC pier without inducing additional damage, which is to raise the location of the rocking interface. First, the SC shape factor ξSC is innovatively defined to quantify the above-mentioned measure, and its working mechanism is investigated in detail in this study. Second, a high-lateral-resistance SC pier system (HLRSCP), which applies the concept of ξSC, is proposed and introduced for its advantages. Third, hysteretic tests of the DP and HLRSCP with high axial compression ratios are performed and analyzed. Finally, the enhancement effect of ξSC on the lateral resistance capacity is parametrically examined using a finite element model, and the feasible region of ξSC is inferred and presented. The results validate the effectiveness and feasibility of the concept of the SC shape factor in improving the hysteretic performance of the SC pier system.

Robust high capacity in-plane elastic wave transport in 2D chiral metastructures

Novel 2D tri-chiral metastructures with mass inclusion are proposed in this work. Compared to conventional 2D honeycomb metastructures, these superior metastructures have a wide in-plane low-frequency bandgap (BG) and single Dirac cone (DC) simultaneously. Ligament width and inclusion density are both key factors for tuning the DC and low-frequency BGs. Due to the superior dispersion properties, metamolecules analog of quantum spin Hall effects (QSHEs) are built by the band folding method, and topological phase transition is obtained by shrinking/expanding distance between the mass inclusion and metamolecule center. Topological interface states (TISs) are observed between the two domains with distinct topological properties. To further enhance energy capacity of in-plane elastic wave transport, a 2D heterostructure is constructed by doping waveguiding layer at the topological interface. As expected, robust high capacity in-plane elastic wave transport is realized, named as topological waveguide states (TWSs). While TWS velocity remains unaffected, an increasing number of waveguiding layers additionally leads to a reduced bandgap width and transition from TWSs to conventional edge states (CESs). Average transmitted energy is also observed to increase almost linearly with the thickness of waveguide layer. By virtue of the robust high-capacity wave transport, two potential applications for energy focusing and beam splitting are clearly demonstrated. Besides, the temperature field is introduced into the 2D topological heterostructure to widen the operating frequency of TWSs. Fortunately, TWSs can be tuned to the lower frequency range by increasing temperature, and retain gapless and high-capacity characteristics. Last but not least, we demonstrate that temperature can be used as a switch for in-plane topological wave transport. The proposed 2D chiral metastructures have great potentials to serve as building blocks for multifunctional topological devices.

Effect of Bi-Er-O addition on the electrical properties of ZnBiCoMnSb based varistor

In order to restrain the breakdown voltage gradient E1 mA at a low level and simultaneously improve nonlinear coefficient α, the rare earth element Er was doped into ZnBiCoMnSb based varistor ceramic in the form of Bi-Er-O phase. As a result, the excellent electrical performances including low E1 mA of 190 V/mm, high α of 55.6 as well as low leakage current IL of 2.5 μA/cm2 are obtained in 3 wt% Bi-Er-O phase modified ZnBiCoMnSb based varistor ceramic. It is found that the concentration of Er at grain boundaries is slightly higher than that in ZnO grains, while it is uniformly distributed throughout the bulk ceramic. This characteristic of element distribution mainly contributes to the enhancement of α by forming interface state densities and deep bulk traps at grain boundaries, rather than E1 mA. The reported herein strategy to restrain E1 mA and improve α at the same time on ZnBiCoMnSb based varistor ceramic can act as a reference for further fabrication of low-E1 mA varistor ceramics with excellent nonlinear performances by rare earth elements doping.

Ab Initio Electronic, Magnetic, and Optical Properties of Fe Phthalocyanine on Cr2O3(0001).

The organic molecules adsorbed on antiferromagnetic surfaces can produce interesting interface states, characterized by charge transfer mechanisms, hybridization of molecular-substrate orbitals, as well as magnetic couplings. Here, we apply an ab initio approach to study the adsorption of Fe phthalocyanine on stoichiometric Cr2O3(0001). The molecule binds via a bidentate configuration forming bonds between two opposite imide N atoms and two protruding Cr ones, making this preferred over the various possible adsorption structures. In addition to the local modifications at these sites, the electronic structure of the molecule is weakly influenced. The magnetic structure of the surface Cr atoms shows a moderate influence of molecule adsorption, not limited to the atoms in the close proximity of the molecule. Upon optical excitation at the onset, electron density moves toward the molecule, enhancing the ground state charge transfer. We investigate this movement of charge as a mechanism at the base of light-induced modifications of the magnetic structure at the interface.

Utilizing topological invariants for encoding and manipulating chiral phonon devices

As a fundamental degree of freedom, phonon chirality is expected to promote the development of quantum information technology just like electron spin. Currently, central to this area is the realization of efficient transmission and control of chiral information. In this paper, we propose an approach by integrating topological theory, leveraging topologically invariant Chern numbers, to encode hexagonal lattice systems. Our investigation reveals the presence of topologically protected chiral interface states within the shared band gaps of both trivial and non-trivial system units. By precisely modulating the magnetic field distribution within the encoding system, we can effectively manipulate the topological pathways. Building upon this framework, we design and implement a chiral phonon three-port device. Through dynamic calculations, we demonstrate the transmission process of chiral information, showcasing the chiral phonon switching effect and logical OR operation. Our findings not only establish a fundamental mechanism for the manipulation and control of phonon chiral information but also provide a promising direction for research in harnessing chirality degrees of freedom in practical applications.

Van der Waals Schottky Junction Photodetector with Ultrahigh Rectifying Ratio and Switchable Photocurrent Generation.

Metal-semiconductor junctions play an important role in the development of electronic and optoelectronic devices. A Schottky junction photodetector based on two-dimensional (2D) materials is promising for self-powered photodetection with fast response speed and large signal-to-noise ratio. However, it usually suffers from an uncontrolled Schottky barrier due to the Fermi level pinning effect arising from the interface states. In this work, all-2D Schottky junctions with near-ideal Fermi level depinning are realized, attributed to the high-quality interface between 2D semimetals and semiconductors. We further demonstrate asymmetric diodes based on multilayer graphene/MoS2/PtSe2 with a current rectification ratio exceeding 105 and an ideality factor of 1.2. Scanning photocurrent mapping shows that the photocurrent generation mechanism in the heterostructure switches from photovoltaic effect to photogating effect at varying drain biases, indicating both energy conversion and optical sensing are realized in a single device. In the photovoltaic mode, the photodetector is self-powered with a response time smaller than 100 μs under the illumination of a 405 nm laser. In the photogating mode, the photodetector exhibits a high responsivity up to 460 A/W originating from a high photogain. Finally, the photodetector is employed for single-pixel imaging, demonstrating its high-contrast photodetection ability. This work provides insight into the development of high-performance self-powered photodetectors based on 2D Schottky junctions.

Hexagon MXene based heterojunction interface with high ion adsorption and electron transfer efficiency for soft-package potassium-ions aqueous supercapacitors

Potassium-ion hybrid capacitors (PIHC) are considered as ideal large-scale rechargeable energy storage devices due to their low-cost, high-power density and environmental protection. However, a low energy density is the main factor restricting the practical application of PIHC. The interface is formed on the surface of electrode material of PIHC through strong correlation to construct heterojunction, which can significantly improve the performance of ion energy storage. However, how to reveal the influence of the interfacial state of the heterojunction on the adsorption and electron transmission of energy storage ions at the atomic level is still one of the key scientific problems in this field. In this work, metal ion intercalation and microwave-assisted in-situ etching are used to construct the Hexagon MXene Ti3C2 heterojunction with TiOHO strong correlation. At the interface of heterojunction, TiOHO highway for electron transmission is developed to improve the rate performance of PIHC. Through experimental and theoretical calculation, the optimum adsorption position and maximum adsorption amount of potassium-ion at the single interface of heterojunction are obtained, and the specific energy density of PIHC is increased. This lays a foundation for the practical application of high-performance soft-package PIHC.

Low Barriers and Faster Electron/Ion Transport Rates through the Ga2O3/MnCO3 Anode with a Heterojunction Structure for Lithium-Ion Batteries.

Electrode stability can be controlled to a large extent by constructing suitable composite structures, in which the heterojunction structure can affect the transport of electrons and ions through the effect of the interface state, changed band gap width, and the electric field at the interface. As a promising electrode material, the Ga-based material has a conversion between solid and liquid phases in the electrochemical reaction process, which endows it with self-healing properties with the structure and morphology. Based on these, the Ga2O3/MnCO3 composite was successfully synthesized with a heterogeneous structure by introducing a Ga source in the hydrothermal process. Benefitting from the acceleration effect of the internal electric field and the narrower band gap at the interface, a high-capacity Ga2O3/MnCO3 composite electrode (1112 mAh·g-1 after 225 cycles at 0.1 A·g-1 and 457.1 mAh·g-1 after 400 cycles at 1 A·g-1) can be achieved for lithium-ion batteries. The results can provide a reference for the research and preparation of electrode materials with high performance.

Study on optoelectronic properties of CuInSe2/CuInSSe films and PN junctions

CuInSe2 (CIS) is a semiconductor with a direct bandgap of about 1.04 eV. Its potential applications in the field of flexible thin-film solar cells are vast and promising. Improving cell performance through the sulfidation of the CIS absorber layer is a vital method. Furthermore, the bandgap can be adjusted through ideal element doping and interfacial state recombination can be decreased. In this study, the mixed source vacuum evaporation is used to prepare CIS, CuInS1.5Se0.5 and CuInS0.5Se1.5 films, followed by annealing under N2, S, and Se atmospheres. Subsequently, Mo/CIS/CdS PN junction and Mo/CuInSxSe2-x (x = 0.5 or 1.5)/CdS PN junctions are fabricated. The results demonstrate that Se-annealing of CIS films reduces hetero-phase formation, improves crystallization quality and yields high intensities of transmittance and reflectance. S-annealing of CISSe films results in enhanced intensities of transmittance and reflectance. The intensities of the primary XRD peaks of CuInS0.5Se1.5 film are greater than those of the CuInS1.5Se0.5 film. The morphologies of the CISSe films become flatter and smoother as annealing in S-atmosphere. The CuInS1.5Se0.5 film shows a higher photocurrent compared to the CuInS0.5Se1.5 film, suggesting that a greater proportion of S can enhance photo carrier concentration of CISSe films. Additionally, the carrier transport in S-annealed CuInS0.5Se1.5/CdS PN junction is predominantly governed by tunneling current mechanism, while carrier transports of other PN junctions are regulated by leakage current mechanism.

Unlocking Dynamic Solvation Chemistry and Hydrogen Evolution Mechanism in Aqueous Zinc Batteries.

Understanding the interfacial hydrogen evolution reaction (HER) is crucial to regulate the electrochemical behavior in aqueous zinc batteries. However, the mechanism of HER related to solvation chemistry remains elusive, especially the time-dependent dynamic evolution of the hydrogen bond (H-bond) under an electric field. Herein, we combine in situ spectroscopy with molecular dynamics simulation to unravel the dynamic evolution of the interfacial solvation structure. We find two critical change processes involving Zn-electroplating/stripping, including the initial electric double layer establishment to form an H2O-rich interface (abrupt change) and the subsequent dynamic evolution of an H-bond (gradual change). Moreover, the number of H-bonds increases, and their strength weakens in comparison with the bulk electrolyte under bias potential during Zn2+ desolvation, forming a diluted interface, resulting in massive hydrogen production. On the contrary, a concentrated interface (H-bond number decreases and strength enhances) is formed and produces a small amount of hydrogen during Zn2+ solvation. The insights on the above results contribute to deciphering the H-bond evolution with competition/corrosion HER during Zn-electroplating/stripping and clarifying the essence of electrochemical window widened and HER suppression by high concentration. This work presents a new strategy for aqueous electrolyte regulation by benchmarking the abrupt change of the interfacial state under an electric field as a zinc performance-enhancement criterion.

The role of interface energetics in Sb2Se3 thin film solar cells

We comprehensively simulated the interface energetics at the Sb2Se3/CdS interfaces and showed its impact on device performance. The interface discontinuity, band bending at interface and energy level alignment generates interfaces issues and must be optimized for an optimal device performance. The design parameters for controlling interface. Metal contact work function preferably higher than electron affinity (EA) and Fermi level (EF) combined (EA + EF), should result in near Ohmic behaviour of contact. Secondly electron affinity of buffer could be tuned to achieve small positive conduction bandoffset (spike barrier) at absorber/buffer interface which lowers the chances of recombination through interface states. A pn + configuration with highly doped buffer layer, as compared to p-absorber, is favourable as it will extend depletion in absorber, providing additional drift to photo-generated carriers. Lastly, acceptor defect at Sb2Se3-CdS interface generate surface inversion and detrimental to performance. Donor defects occupying interface states are preferred condition for optimal device performance. We have compiled the optimal ranges for these controlling parameters, to achieve theoretically ideal values of energy level alignment and energetics, leading to optimal performance.

The electrical transport property and gas detection behaviour of polyvinyl alcohol and γ-Fe2O3 nanocomposite film

This paper presents the electrical and gas detection behaviour of polyvinyl alcohol (PVA)/γ-Fe2O3 nanocomposite films. The physical characterization includes X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), and UV–Visible spectroscopy. The analysis of both the dc resistivity and ac conductivity using Correlated Barrier Hoping (CBH) conduction process shows that the conductivity enhances with the decreasing of activation energy in case of the PVA/γ-Fe2O3 nanocomposite film which is the indication of interfacial defect states. The current voltage study at various temperatures follows the thermoionic emission model of Schottky Barrier diode. Moreover, the dielectric constant enhances for PVA/γ-Fe2O3 nanocomposite film, which indicates homogeneous distribution of γ-Fe2O3 nanoparticles within PVA matrix and the aggregation of trapped charges found in interface develops an interfacial polarization. As a result, the gas adsorption capacity of the nanocomposite film increases which improves the signal-to-noise ratio, leading to higher gas detection sensitivity.

Vapor Deposition of Bilayer 3R MoS2 with Room-Temperature Ferroelectricity.

Two-dimensional ultrathin ferroelectrics have attracted much interest due to their potential application in high-density integration of non-volatile memory devices. Recently, 2D van der Waals ferroelectric based on interlayer translation has been reported in twisted bilayer h-BN and transition metal dichalcogenides (TMDs). However, sliding ferroelectricity is not well studied in non-twisted homo-bilayer TMD grown directly by chemical vapor deposition (CVD). In this paper, for the first time, experimental observation of a room-temperature out-of-plane ferroelectric switch in semiconducting bilayer 3R MoS2 synthesized by reverse-flow CVD is reported. Piezoelectric force microscopy (PFM) hysteretic loops and first principle calculations demonstrate that the ferroelectric nature and polarization switching processes are based on interlayer sliding. The vertical Au/3R MoS2/Pt device exhibits a switchable diode effect. Polarization modulated Schottky barrier height and polarization coupling of interfacial deep states trapping/detrapping may serve in coordination to determine switchable diode effect. The room-temperature ferroelectricity of CVD-grown MoS2 will proceed with the potential wafer-scale integration of 2D TMDs in the logic circuit.

Interfacial Charge Transfer in Photoexcited QD-Molecule Composite of Tetrahedral CdSe Quantum Dot Coupled with Carbazole.

Photoexcited charge transfer dynamics in CdSe quantum dots (QDs) coupled with carbazole were explored to model QD-molecule systems for light-harvesting applications. The absorption spectra of QDs with different sizes, i.e., Cd35Se20X30L30 (T1), Cd56Se35X42L42 (T2), and Cd84Se56X56L56 (T3) were simulated with quantum dynamical methods, which qualitatively match the reported experimental spectra. The carbazole is attached with a 3-amino group at the apex position of T1 (namely T1-3A-Cz), establishing proper electronic communication between T1 and carbazole. The spectra of T1-3A-Cz is 0.22 eV red-shifted compared to T1. A time-dependent perturbation was applied in tune with the lowest energy peak (3.63 eV) of T1-3A-Cz to investigate the charge transfer dynamics, which revealed an ultrafast charge separation within the femtosecond time scale. The electronic structure showed a favorable energy alignment between T1 and carbazole in T1-3A-Cz. The LUMO of carbazole was situated below the conduction band of the QD, while the HOMO of carbazole mixed perfectly with the top of the valence band of the QD, developing the interfacial charge transfer states. These states promoted the photoexcited electron transfer directly from the CdSe core to carbazole. A rapid and enhanced charge separation occurred with the laser field strength increasing from 0.001 to 0.005 V/Å. However, T1 connected to the other positions of carbazole did not show charge separation effectively. The photoinduced charge transfer is negligible in the case of T2-carbazole systems due to poor electronic coupling, and it is not observed in T3-carbazole systems. So, the T1-3A-Cz model acts as a perfect donor-acceptor QD-molecule nanocomposite that can harvest photon energy efficiently. Further enhancement of charge transfer can be achieved by coupling more carbazoles to the T1 QD (e.g., T1-3A-Cz2) due to the extension of hole delocalization between T1 and the carbazoles.

Experimental demonstration of an electroacoustic transistor

We experimentally demonstrate a topologically protected electroacoustic transistor. We construct a reconfigurable phononic analog of the quantum valley-Hall insulator composed of electrically shunted piezoelectric disks bonded to a patterned plate forming a monolithic structure. The device can be dynamically reconfigured to host one or more topological interface states via breaking inversion symmetry through selective powering of shunt circuits. Above a threshold, the amplitude of wave energy at a chosen location in one topological interface creates a second interface by dynamically switching power between two groups of shunts using relays. This enables the flow of wave energy between two locations in the reconfigured interface analogous to the voltage-controlled electron flow in a field effect transistor. The amplitude of wave energy in the second interface is used for bit abstraction to implement acoustic logic. We illustrate the various states of the transistor and experimentally demonstrate wave-based switching. The proposed electroacoustic transistor is envisioned to find applications in wave-based devices and edge computing in extreme environments and inspire novel technologies leveraging acoustic logic.

Metastability of α/β interfaces in titanium: Implications for interface migration

The interfacial structure of α/β interfaces significantly impacts the mechanical and kinetic properties of titanium and its alloys. This work employs density functional theory to investigate the coherent α/β interface in Ti, focusing on two stable/metastable interface structures, i.e., the Hollow and Bridge (experimentally observed). The relative stability of these two interface states is highly sensitive to in-plane strain. This study also examines the impact of structural metastability on interface migration behavior during the β→α phase transformation. We observe that the Bridge α/β interface migrates easily; the Hollow interface does not migrate; rather, the ω phase forms at the interface. This work also explores the influence of alloying, specifically with vanadium, on the metastability and energy barrier between the two interface states. We demonstrate manipulating α/β interface migration in titanium through strain and doping.

Localized thermal emission from topological interfaces.

The control of thermal radiation by shaping its spatial and spectral emission characteristics plays a key role in many areas of science and engineering. Conventional approaches to tailoring thermal emission using metamaterials are hampered both by the limited spatial resolution of the required subwavelength material structures and by the materials' strong absorption in the infrared. In this work, we demonstrate an approach based on the concept of topology. By changing a single parameter of a multilayer coating, we were able to control the reflection topology of a surface, with the critical point of zero reflection being topologically protected. The boundaries between subcritical and supercritical spatial domains host topological interface states with near-unity thermal emissivity. These topological concepts enable unconventional manipulation of thermal light for applications in thermal management and thermal camouflage.