Measuring phonon dispersion at an interface.

The breakdown of translational symmetry at heterointerfaces leads to the emergence of new phonon modes localized at the interface1. These modes have an essential role in thermal and electrical transport properties in devices, especially in miniature ones wherein the interface may dominate the entire response of the device2. Although related theoretical work began decades ago1,3-5, experimental research is totally absent owing to challenges in achieving the combined spatial, momentum and spectral resolutions required to probe localized modes. Here, using the four-dimensional electron energy-loss spectroscopy technique, we directly measure both the local vibrational spectra and the interface phonon dispersion relation for an epitaxial cubic boron nitride/diamond heterointerface. In addition to bulk phonon modes, we observe modes localized at the interface and modes isolated from the interface. These features appear only within approximately one nanometre around the interface. The localized modes observed here are predicted to substantially affect the interface thermal conductance and electron mobility. Our findings provide insights into lattice dynamics at heterointerfaces, and the demonstrated experimental technique should be useful in thermal management, electrical engineering and topological phononics.

Dislocation-tuned ferroelectricity and ferromagnetism of the BiFeO3/SrRuO3 interface.

Misfit dislocations at a heteroepitaxial interface produce huge strain and, thus, have a significant impact on the properties of the interface. Here, we use scanning transmission electron microscopy to demonstrate a quantitative unit-cell-by-unit-cell mapping of the lattice parameters and octahedral rotations around misfit dislocations at the BiFeO3/SrRuO3 interface. We find that huge strain field is achieved near dislocations, i.e., above 5% within the first three unit cells of the core, which is typically larger than that achieved from the regular epitaxy thin-film approach, thus significantly altering the magnitude and direction of the local ferroelectric dipole in BiFeO3 and magnetic moments in SrRuO3 near the interface. The strain field and, thus, the structural distortion can be further tuned by the dislocation type. Our atomic-scale study helps us to understand the effects of dislocations in this ferroelectricity/ferromagnetism heterostructure. Such defect engineering allows us to tune the local ferroelectric and ferromagnetic order parameters and the interface electromagnetic coupling, providing new opportunities to design nanosized electronic and spintronic devices.

Atomic-scale probing of heterointerface phonon bridges in nitride semiconductor.

Interface phonon modes that are generated by several atomic layers at the heterointerface play a major role in the interface thermal conductance for nanoscale high-power devices such as nitride-based high-electron-mobility transistors and light-emitting diodes. Here we measure the local phonon spectra across AlN/Si and AlN/Al interfaces using atomically resolved vibrational electron energy-loss spectroscopy in a scanning transmission electron microscope. At the AlN/Si interface, we observe various interface phonon modes, of which the extended and localized modes act as bridges to connect the bulk AlN modes and bulk Si modes and are expected to boost the phonon transport, thus substantially contributing to interface thermal conductance. In comparison, no such phonon bridge is observed at the AlN/Al interface, for which partially extended modes dominate the interface thermal conductivity. This work provides valuable insights into understanding the interfacial thermal transport in nitride semiconductors and useful guidance for thermal management via interface engineering.

Resolving Multielement Semiconductor Nanocrystals at the Atomic Level: Complete Deciphering of Domains and Order in Complex CuαZnßSnγSeδ (CZTSe) Tetrapods.

Semiconductor nanocrystals (NCs) with high elemental and structural complexity can be engineered to tailor for electronic, photovoltaic, thermoelectric, and battery applications etc. However, this greater complexity causes ambiguity in the atomic structure understanding. This in turn hinders the mechanistic studies of nucleation and growth, the theoretical calculations of functional properties, and the capability to extend functional design across complementary semiconductor nanocrystals. Herein, we successfully deciphered the atomic arrangements of 4 different nanocrystal domains in CuαZnßSnγSeδ (CZTSe) nanocrystals using crucial zone axis analysis on multiple crystals in different orientations. The results show that the essence of crystallographic progression from binary to multielemental semiconductors is actually the change of theoretical periodicity. This transition is caused by decreased symmetry in the crystal instead of previously assumed crystal deformation. We further reveal that these highly complex crystalline entities have highly ordered element arrangements as opposed to the previous understanding that their elemental orderings are random.

Atomic-scale observations of electrical and mechanical manipulation of topological polar flux closure.

The ability to controllably manipulate complex topological polar configurations such as polar flux-closures via external stimuli may allow the construction of new electromechanical and nanoelectronic devices. Here, using atomically resolved in situ scanning transmission electron microscopy, we find that the polar flux-closures in PbTiO3/SrTiO3 superlattice films are mobile and can be reversibly switched to ordinary single ferroelectric c or a domains under an applied electric field or stress. Specifically, the electric field initially drives movement of a flux-closure via domain wall motion and then breaks it to form intermediate a/c striped domains, whereas mechanical stress first squeezes the core of a flux-closure toward the interface and then form a/c domains with disappearance of the core. After removal of the external stimulus, the flux-closure structure spontaneously recovers. These observations can be precisely reproduced by phase field simulations, which also reveal the evolutions of the competing energies during phase transitions. Such reversible switching between flux-closures and ordinary ferroelectric states provides a foundation for potential electromechanical and nanoelectronic applications.

Colossal Room-Temperature Ferroelectric Polarizations in SrTiO3/SrRuO3 Superlattices Induced by Oxygen Vacancies.

Artificial superlattices have demonstrated many unique phenomena not found in bulk materials. For this investigation, SrTiO3/SrRuO3 paraelectric/metallic superlattices with various stacking periods were synthesized via pulsed laser deposition. A robust room-temperature ferroelectric polarization (∼46 µC/cm2) was found in the superlattices with 2 unit cell (u.c.) thick SrRuO3 layers, despite the fact that neither SrTiO3 nor SrRuO3 is inherently ferroelectric. Results obtained from atomically resolved elemental mapping and X-ray photoelectron spectroscopy verified that oxygen vacancies accumulated at the SrTiO3/SrRuO3 interfaces, causing lattice distortions and increased tetragonality (c/a). The observed ferroelectric responses can be mainly attributed to the broken spatial inversion symmetry induced by the ordered distribution of oxygen vacancies at the SrTiO3/SrRuO3 interfaces, coupled with the triggering of external electric field. The resulting polarization mechanism induced by oxygen vacancies suggests viable ways for improving the electrical properties of ferroelectric materials, with the goal of expanding the functionality of a range of electronic devices.

Flexoelectric Domain Walls Originated from Structural Phase Transition in Epitaxial BiVO4 Films.

Polar domain walls in centrosymmetric ferroelastics induce inhomogeneity that is the origin of advantageous multifunctionality. In particular, polar domain walls promote charge-carrier separation and hence are promising for energy conversion applications that overcome the hurdles of the rate-limiting step in the traditional photoelectrochemical water splitting processes. Yet, while macroscopic studies investigate the materials at the device scale, the origin of this phenomenon in general and the emergence of polar domain walls during the structural phase transition in particular has remained elusive, encumbering the development of this attractive system. Here, it is demonstrated that twin domain walls arise in centrosymmetric BiVO4 films and they exhibit localized piezoelectricity. It is also shown that during the structural phase transition from the tetragonal to monoclinic, the symmetry reduction is accompanied by an emergence of strain gradient, giving rise to flexoelectric effect and the polar domain walls. These results not only expose the emergence of polar domain walls at centrosymmetric systems by means of direct observation, but they also expand the realm of potential application of ferroelastics, especially in photoelectrochemistry and local piezoelectricity.

Dynamics of Polar Skyrmion Bubbles under Electric Fields.

Room-temperature polar skyrmions, which have been recently discovered in oxide superlattice, have received considerable attention for their potential applications in nanoelectronics owing to their nanometer size, emergent chirality, and negative capacitance. For practical applications, their manipulation using external stimuli is a prerequisite. Herein, we study the dynamics of individual polar skyrmions at the nanoscale via in situ scanning transmission electron microscopy. By monitoring the electric-field-driven creation, annihilation, shrinkage, and expansion of topological structures in real space, we demonstrate the reversible transformation among skyrmion bubbles, elongated skyrmions, and monodomains. The underlying mechanism and interactions are discussed in conjunction with phase-field simulations. The electrical manipulation of nanoscale polar skyrmions allows the tuning of their dielectric permittivity at the atomic scale, and the detailed knowledge of their phase transition behaviors provides fundamentals for their applications in nanoelectronics.

High-Mobility Flexible Oxyselenide Thin-Film Transistors Prepared by a Solution-Assisted Method.

Two-dimensional (2D) semiconductors hold great promise in flexible electronics because of their intrinsic flexibility and high electrical performance. However, the lack of facile synthetic and subsequent device fabrication approaches of high-mobility 2D semiconducting thin films still hinders their practical applications. Here, we developed a facile, rapid, and scalable solution-assisted method for the synthesis of a high-mobility semiconducting oxyselenide (Bi2O2Se) thin film by the selenization and decomposition of a precursor solution of Bi(NO3)3·5H2O. Simply by changing the rotation speed in spin-coating of the precursor solution, the thicknesses of Bi2O2Se thin films can be precisely controlled down to few atomic layers. The as-synthesized Bi2O2Se thin film exhibited a high Hall mobility of ∼74 cm2 V-1 s-1 at room temperature, which is much superior to other 2D thin-film semiconductors such as transition metal dichalcogenides. Remarkably, flexible top-gated Bi2O2Se transistors showed excellent electrical stability under repeated electrical measurements on flat and bent substrates. Furthermore, Bi2O2Se transistor devices on muscovite substrates can be readily transferred onto flexible polyvinyl chloride (PVC) substrates with the help of thermal release tape. The integration of a high-mobility thin-film semiconductor, excellent stability, and easy transfer onto flexible substrates make Bi2O2Se a competitive candidate for future flexible electronics.

Thickness-dependent band gap of α-In2Se3: from electron energy loss spectroscopy to density functional theory calculations.

α-In2Se3 has attracted increasing attention in recent years due to its excellent electrical and optical properties. Especially, attention has been paid to its peculiar ferroelectric and piezoelectric properties which most other two-dimensional (2D) materials do not possess. This paper presents the first measurement of the thickness-dependent band gaps of few-layer α-In2Se3 by electron energy loss spectroscopy (EELS). The band gap increases with decreasing film thickness which varies from 1.44 eV in a 48 nm thick area to 1.64 eV in an 8 nm thick area of the samples. Further, by combining the improved exchange-correlation potential and proper screening of the internal electric field in an advanced 2D electronic structure technique, we have been able to obtain the structural dependence of the band gap within density functional theory up to hundreds of atoms. This is also the first calculation of a similar type for 2D ferroelectric materials. Both experiment and theory suggest that the variation of the band gap of α-In2Se3 fits well with the quantum confinement model for 2D materials.

Probing Far-Infrared Surface Phonon Polaritons in Semiconductor Nanostructures at Nanoscale.

Phonon polaritons hold potential prospects of nanophotonic applications at the mid- and far-infrared wavelengths. However, their experimental investigation in the far-infrared range has long been a technical challenge due to the lack of suitable light sources and detectors. To obviate these difficulties, here we use an electron probe with sub-10 meV energy resolution and subnanometer spatial resolution to study far-infrared surface phonon polaritons (∼50-70 meV) in ZnO nanostructures. We observe ultraslow propagation and interference fringes of propagating surface phonon polaritons and obtain their dispersion relation through measurements in the coordinate space. By mapping localized modes in nanowires and flakes, we reveal their localized nature and investigate geometry and size effects. Associated with simulation, we show that surface phonon polariton behaviors can be well described by the local continuum dielectric model. Our work paves the way for spatial-resolved investigation of surface phonon polaritons by electron probes and forwards polaritonics in the far-infrared range.

Preparation and Performance Evaluation of Ionic Liquid Copolymer Shale Inhibitor for Drilling Fluid Gel System.

An inhibitor that can effectively inhibit shale hydration is necessary for the safe and efficient development of shale gas. In this study, a novel ionic liquid copolymer shale inhibitor (PIL) was prepared by polymerizing the ionic liquid monomers 1-vinyl-3-aminopropylimidazolium bromide, acrylamide, and methacryloyloxyethyl trimethyl ammonium chloride. The chemical structure was characterized using fourier transform infrared spectroscopy (FT-IR) and hydrogen-nuclear magnetic resonance (H-NMR), and the inhibition performance was evaluated using the inhibition of slurrying test, bentonite flocculation test, linear expansion test, and rolling recovery test. The experimental results showed that bentonite had a linear expansion of 27.9% in 1 wt% PIL solution, 18% lower than that in the polyether amine inhibitor. The recovery rate of shale in 1 wt% PIL was 87.4%. The ionic liquid copolymer could work synergistically with the filtrate reducer, reducing filtration loss to 7.2 mL with the addition of 1%. Mechanism analysis showed that PIL adsorbed negatively charged clay particles through cationic groups, which reduced the electrostatic repulsion between particles. Thus, the stability of the bentonite gel systems was destroyed, and the hydration dispersion and expansion of bentonite were inhibited. PIL formed a hydrophobic film on the surface of clay and prevented water from entering into the interlayer of clay. In addition, PIL lowered the surface tension of water, which prevented the water from intruding into the rock under the action of capillary force. These are also the reasons for the superior suppression performance of PIL.

Conceptual Model, Experiment and Numerical Simulation of Diaphragm Wall Leakage Detection Using Distributed Optical Fiber.

Leakage in the diaphragm wall is difficult to detect in deep foundation pits. In this study, the conceptual model of active and passive thermal leak detection methods was proposed according to the occurrence of temperature field anomalies caused by seepage. Experiments were performed using a heating system and an optical fiber temperature measurement system to verify the thermal leakage detection systems. Numerical simulations were performed to understand the mechanism of the detecting method. Results indicated that the optical cable could detect the low-temperature anomaly in the active temperature field leak detection. The arrangement method of the leakage detection system was also presented in actual engineering.

Mechanism of Detecting the Construction Quality of a Diaphragm Wall by an Infrared Thermal Field and Engineering Application.

During underground space exploitation in the urbanization process, numerous foundation pits were constructed where a diaphragm wall was often used as a retaining structure and waterproof curtain. Due to complicated engineering geological conditions or improper construction, diaphragm walls and wall joints often exhibit quality defects. Groundwater leaked from these quality defects to foundation pits during excavation, endangering the safety of the pit and surrounding facilities. The current leakage identification of the underground retaining structure was performed by artificial visual detection, which cannot satisfy the engineering requirement. The temperature field in the leakage area of the diaphragm wall was different from other areas. The leakage wall imaging system using a thermal imager was efficient in visualizing leaking, which was not visible to the naked eye. In this study, infrared thermal imaging technology was introduced in potential leakage detection for the diaphragm wall of a foundation pit. The infrared radiation characteristics of the diaphragm wall leakage and the potential leakage parts were studied through laboratory simulation tests and on-site detection methods. The maximum temperature appeared at the water outlet and the surface of the defect with hidden defect, and the temperature field was symmetrically distributed along the cross-section direction. In the potential leakage area, the temperature difference at the penetration point was 23.4 °C when the initial water pressure was 10 kPa. The temperature difference at the penetration point was 21.8 °C when the initial water pressure was 30 kPa. In the field test, the maximum temperature difference between the leakage area and the surrounding wall was 4.5 °C. The study can provide a reference for similar engineering.

A Novel Amphoteric Ion-Modified, Styrene-Based Nano-Microsphere and Its Application in Drilling Fluid.

With the gradual depletion of shallow oil and gas, deep oil and gas has become the focus of development. However, deep formations generally face the challenge of high-temperature and high-salinity, and drilling fluid agents are prone to failure, leading to drilling fluid intrusion into the formation that can cause serious drilling accidents such as well bore collapse. For this, a styrene-based nano-microsphere (SSD) modified with amphoteric ions was developed, with a particle size of 228 nm which could resist temperatures up to 200 °C and sodium chloride (NaCl) up to saturation. SSD has significant salt-responsive properties and its aqueous dispersion becomes transparent with increasing salinity. The SSD provided superior plugging performance in solutions containing NaCl, with a core plugging efficiency of 95.2%, and it was significantly better than the anion-modified microspheres. In addition, in drilling fluids under high temperature and high-salinity conditions, the SSD promotes particle gradation of drilling fluids and improves the zeta potential through its own plugging and synergistic effect with clay, which significantly improves the comprehensive performance of drilling fluids, such as stability, rheological performance, and filtration reduction performance. The development of SSD provides a new idea for research of high-temperature and high-salinity-resistant drilling fluid agents.

Novel Modified Styrene-Based Microspheres for Enhancing the Performance of Drilling Fluids at High Temperatures.

Ensuring wellbore stability is of utmost importance for safety when drilling in deep formations. However, high temperatures severely disrupt the drilling fluid gel system, leading to severe stability issues within ultra-deep formations containing micropores. This study focused on the development of a polymer-based plugging material capable of withstanding high temperatures up to 200 °C. A kind of microsphere, referred to as SST (styrene-sodium styrene sulfonate copolymer), was synthesized with a particle size of 322 nm. Compared to polystyrene, the thermal stability of SST is greatly improved, with a thermal decomposition temperature of 362 °C. Even after subjecting SST to hot rolling at 200 °C for 16 h, the particle size, elemental composition, and zeta potential remained stable within an aqueous dispersion system. The results of core displacement and NMR tests demonstrate that SST considerably reduces the pore diameter with a remarkable plugging efficiency of 78.9%. Additionally, when drilling fluids reach 200 °C, SST still enhances drilling fluid suspension and dispersion, and reduces fluid loss by over 36% by facilitating the dispersion of clay particles, improving the gel structure of the drilling fluid, resisting clay dehydration, and promoting plugging. The development of SST provides valuable insights into the preparation of high-temperature-resistant microspheres and the formulation of effective plugging agents for deep-well drilling fluids.

Development and Performance Evaluation of an Intelligent Sand Control Screen Based on Polyphenylene Sulfide.

Open hole combined with a sand filter pipe is the main development mode of an unconsolidated sandstone horizontal well, which causes problems such as shaft wall collapse, sand filter pipe blockage, and even damage. At present, the polyurethane intelligent sand control screen pipe can effectively solve these problems, but its application scope is limited by the downhole temperature and material strength. Therefore, the high-strength shape memory polyphenylene sulfide (SMPPPS) was synthesized by an indoor experiment, then the shape memory performance was analyzed, and the variation law of pore size with composition and processing technology was obtained. Finally, the performance of the SMPPPS intelligent screen was evaluated. It is found that (1) with the increase of the cross-linking agent content, the melt index and melt peak temperature of SMPPPS decrease rapidly at first and then remain unchanged; the cross-linking agent content is set at 4.5 wt %. (2) The shape memory performance of SMPPPS is good, and the recovery ratio can reach 99.8% at 110 °C in 11.8 min. (3) The relationship between the pore size distribution proportion of SMPPPS and the foaming agent content, pore enhancer content, temperature, and pressure is determined through experiments; the main distribution range of pore size increases with the increase of the foaming agent content and temperature and decreases with the increase of the pore enhancer content and pressure. When the foaming agent content is 0.5 wt %, the pore enhancer concentration is 10 wt %, the molding temperature is 320 °C, the molding pressure is 10 MPa, and the main distribution range of pore size is 80-320 µm. (4) The permeability of recovery SMPPPS with a compression ratio of 300% first increases slightly and then decreases slightly with the increase of flow, with a variation range of 420.5-463 mD; the compressive strength range is 9.5-11.4 MPa; the SMPPPS has good adaptability to downhole fluid; the sand retaining accuracy is higher than that of the screen sand filter pipe, but the blockage is more serious.

Switching magnon chirality in artificial ferrimagnet.

Chirality, an intrinsic degree of freedom, has been barely exploited as the information carriers in data transmission, processing, computing, etc. Recently the magnons in antiferromagnets were proposed to carry both right-handed and left-handed chiralities, shedding a light on chirality-based spintronics in which chirality-based computing architectures and chiral magnonic devices may become feasible. However, the practical platform for chirality-based spintronics remains absent yet. Here we report an artificial ferrimagnetic Py/Gd/Py/Gd/Py/Pt multilayer by which the switching, reading, and modulation of magnon chirality are demonstrated. In particular, the coexisting resonance modes of ferromagnetic and antiferromagnetic characteristics permit the high adjustability and easy control of magnon chirality. As a main result, we unambiguously demonstrated that Py precessions with opposite chiralities pump spin currents of opposite spin polarizations into the Pt layer. Our result manifests the chirality as an independent degree of freedom and illustrates a practical magnonic platform for exploiting chirality, paving the way for chirality-based spintronics.

Photoenhanced Electroresistance at Dislocation-Mediated Phase Boundary.

Ferroelectric tunneling junctions have attracted intensive research interest due to their potential applications in high-density data storage and neural network computing. However, the prerequisite of an ultrathin ferroelectric tunneling barrier makes it a great challenge to simultaneously implement the robust polarization and negligible leakage current in a ferroelectric thin film, both of which are significant for ferroelectric tunneling junctions with reliable operating performance. Here, we observe a large tunneling electroresistance effect of ∼1.0 × 104% across the BiFeO3 nanoisland edge, where the intrinsic ferroelectric polarization of the nanoisland makes a major contribution to tuning the barrier height. This phenomenon is beneficial from the artificially designed tunneling barrier between the nanoscale top electrode and the inclined conducting phase boundary, which is located between the rhombohedral-island and tetragonal-film matrix and arranged with the dislocation array. More significantly, the tunneling electroresistance effect is further improved to ∼1.6 × 104% by the introduction of photoinduced carriers, which are separated by the flexoelectric field arising from the dislocations.

Interpenetrated N-rich MOF derived vesicular N-doped carbon for high performance lithium ion battery.

High-performance lithium ion batteries (LIBs) juggling high reversible capacity, excellent rate capability and ultralong cycle stability are urgently needed for all electronic devices. Here we report employing a vesicle-like porous N-doped carbon material (abbr. N/C-900) as a highly active anode for LIBs to balance high capacity, high rate and long life. The N/C-900 material was fabricated by pyrolysis of a designed crystal MOF LCU-104, which exhibits a graceful two-fold interpenetrating structural feature of N-rich nanocages {Zn6(dttz)4} linked through an N-donor ligand bpp (H3dttz = 4,5-di(1H-tetrazol-5-yl)-2H-1,2,3-triazole, bpp = 1,3-bis(4-pyridyl)propane). The features of LCU-104 combine high N content (35.1%), interpenetration, and explosive characteristics, which endow the derived N/C material with optimized N-doping for tuning its chemical and electronic structure, a suitably thicker wall to enhance its stability, and a vesicle-like structure to improve its porosity. As an anode material for LIBs, N/C-900 delivers a highly reversible capacity of ca. 734 mA h g-1 at a large current density of 1 A g-1 until the 2000th cycle, revealing its ultralong cycle stability and excellent rate capability. The unique structure and preferential interaction between abundant pyridinic N active sites and Li atoms are responsible for the improved excellent lithium storage capacity and durability performances of the anode according to analysis of the results of computational modeling.