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Scanning tunneling spectroscopy was performed at 4.2K on epitaxial thin-film heterostructures comprising YBa2Cu3O7 and La0.7Ca0.3MnO3, to study the microscopic effects of spin-polarized quasiparticle injection from the half-metallic ferromagnetic manganite on the high-Tc cuprate superconductor.
Abstract The discovery of polar vortices and skyrmions in ferroelectric‐dielectric superlattices [such as (PbTiO 3 ) n /(SrTiO 3 ) n ] has ushered in an era of novel dipolar topologies and corresponding emergent phenomena. The key to creating such emergent features has generally been considered to be related to counterpoising strongly polar and non‐polar materials thus creating the appropriate boundary conditions. This limits the utility these materials can have, however, by rendering (effectively) half of the structure unresponsive to applied stimuli. Here, using advanced thin‐film deposition and an array of characterization and simulation approaches, polar vortices are realized in all‐ferroelectric trilayers, multilayers, and superlattices built from the fundamental building block of (PbTiO 3 ) n /(Pb x Sr 1− x TiO 3 ) n wherein in‐plane ferroelectric polarization in the Pb x Sr 1− x TiO 3 provides the appropriate boundary conditions. These superlattices exhibit substantially enhanced electromechanical and ferroelectric responses in the out‐of‐plane direction that arise from the ability of the polarization in both layers to rotate to the out‐of‐plane direction under field. In the in‐plane direction, the layers are found to be strongly coupled during switching and when heterostructured with ferroelectric‐dielectric building blocks, it is possible to produce multistate switching. This approach expands the realm of systems supporting emergent dipolar texture formation and does so with entirely ferroelectric materials thus greatly improving their responses.
The interplay of lattice, electronic, and spin degrees of freedom at epitaxial complex oxide interfaces provides a route to tune their magnetic ground states. Unraveling the competing contributions is critical for tuning their functional properties. The relationship between magnetic ordering and magnetic anisotropy and the lattice symmetry, oxygen content, and film thickness in compressively strained LaMnO 3 (LMO)/LaCrO 3 (LCO) superlattices is investigated. Mn–O–Cr antiferromagnetic superexchange interactions across the heterointerface result in a net ferrimagnetic magnetic structure. Bulk magnetometry measurements reveal isotropic in-plane magnetism for as-grown oxygen-deficient thin samples due to equal fractions of orthorhombic a+a-c-, and a-a+c- twin domains. As the superlattice thickness is increased, in-plane magnetic anisotropy emerges as the fraction of the a+a-c- domain increases. On annealing in oxygen, the suppression of oxygen vacancies results in a contraction of the lattice volume, and an orthorhombic to rhombohedral transition leads to isotropic magnetism independent of the film thickness. The complex interactions are investigated using high-resolution synchrotron diffraction and X-ray absorption spectroscopy. These results highlight the role of the evolution of structural domains with film thickness, interfacial spin interactions, and oxygen-vacancy-induced structural phase transitions in tuning the magnetic properties of complex oxide heterostructures.
The focused ion beam (FIB) microscope is well established as a high-resolution machining instrument capable of site-selectively removing material down to the nanoscale. Beyond subtractive processing, however, the FIB can also add material via a technique known as focused ion beam induced deposition (FIBID). Using FIBID, the FIB can thus be employed for the direct-write of complex nanostructures. This work explores new directions in three-dimensional FIBID nanoprinting, harnessing unique features of helium and neon FIBs. In particular, the superior spatial resolution of these novel FIBs is leveraged to fabricate precise multimaterial architectures, an isotope effect is used to create satellite deposits, and dose-controlled implantation of the gaseous ions is used to engineer internal voids. In the context of voids, the fabrication of hollow nanopillars by helium-FIBID due to concurrent milling (as shown previously by others) is revisited. Insight into the chemical and structural composition of the nanostructures is obtained using advanced electron microscopy, accurately revealing buried interfaces, crystallite distributions, chemical compositions, and material transformations. Next-generation devices and technologies that could be enabled by the novel heterostructures demonstrated here are discussed, setting the stage for the potential evolution of FIBID into a versatile platform for functional nanomaterials design and fabrication.
Abstract To address the issues of limited ionic conductivity and poor interface stability at room and low temperatures in solid‐state electrolytes, a robust intrinsic ferroelectrolyte or nanoferroelectrolyte strategy for engineering solid‐state flexible ferroelectric composite electrolytes utilizing strongly coupled intrinsic ion conducting 2D/2D sodium‐rich anti‐perovskite (NaRAP)/ferroelectric perovskite heterostructures is introduced. Herein, highly scalable PVDF‐based metaferroelectrolytes with Na 2.99 Ba 0.005 OCl/Ca 2 Na 2 Nb 5 O 16 − (CNNO − ) nanosheets into a ferroelectric poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) matrix, through an in situ cross‐linking and spontaneous bridging method, for compact solid‐state sodium batteries (SSBs), are reported. Benefiting from unique well‐dispersed 3D ferroelectric coupled network and the Na 2.99 Ba 0.005 OCl/CNNO − ‐induced PVDF‐HFP ferroelectric β phase, the Na + flux is regulated, thereby inhibiting Na dendrite growth at the interface. Notably, the optimized PH‐5% NC metaferroelectrolyte exhibits rapid ion transport (1.11 × 10 −4 S cm −1 at 25 °C), a wide electrochemical window (> 4.8V), superior conformal mechanical compatibility, improved flexibility, good elasticity and flame retardancy. The solid‐state Na 3 V 2 (PO 4 ) 3 /PH‐5% NC/Na batteries present a stable cycling performance (remaining 56.4 mAh g −1 after 500 cycles at 1 C) even at 0 °C, potential for cost‐effective, safe, stable and compact SSB energy storage over 600 Wh L −1 , vastly surpassing 365 Wh L −1 of the current commercial sodium‐ion liquid‐electrolyte batteries.
In conventional semiconductors, electrical and thermal conductivity are typically coupled, posing a challenge in optimizing both simultaneously. Overcoming this inherent trade-off enables strategies for advancing electronic applications. Herein, a strategy is demonstrated to decouple electrical and thermal conductivity trade-off by creating heterostructures of highly conductive single-walled carbon nanotubes (SWCNTs) coated with low conductivity hybrid perovskites. Coating SWCNTs with methylammonium lead iodide perovskite results in an enhancement in electrical conductivity (408-1266 S cm-1) due to p-type doping followed by a threefold decrease of the in-plane thermal conductivity (3.3-1 W m-1 K-1), compared to pristine SWCNTs. Molecular dynamics simulations uncover phonon boundary scattering at the SWCNT/perovskite interface as well as localization of methylammonium-related and softening of the Pb-I-related phonon modes in methylammonium lead iodide perovskite decreasing the thermal conductivity.
Ternary nitride alloys based on wurtzite AlN are a promising platform to realize functional materials, particularly ferroelectrics and optical emitters, that can smoothly integrate with conventional microelectronics. Here, a strategic design is presented to enable multifunctional materials by substituting multiple elements into AlN to create quaternary nitride alloys. By combining computational predictions and combinatorial thin film synthesis, the phase diagram of these quaternary Al-Sc-Gd-N alloys (or pseudo-ternary heterostructural AlN-ScN-GdN alloys) is successfully predicted as a function of effective temperature, and we experimentally grow Al1-x-xScxGdyN thin films for the first time. It is revealed that Al1-x-xScxGdyN crystallizes in a wurtzite-derived structure for X + y <~ 0.35, consistent with the calculated phase diagram. The computational investigation explores whether co-substitution induces cooperative effects on these alloys' piezoelectric and ferroelectric properties, finding that it is beneficial for reducing the polarization switching barrier. We calculate that Al1-x-xScxGdyN thin films should display ferroelectric switching. This is supported by our experimental measurements of a high optical bandgap, enhanced piezoelectric coefficient, and a change in the calculated polarization switching mechanism, and we achieve preliminary ferroelectric switching that experimentally realizes the prediction. Overall, our work sets the foundation toward quaternary wurtzite-nitride-based multifunctional materials, including piezoelectrics, ferroelectrics, and possibly even multiferroics.
Abstract Epitaxy, a process to prepare crystalline materials in nanostructures and thin films, is the core technology for preparing high‐quality materials as a key enabler of next‐generation microelectronics and quantum information system. Progress in epitaxy has been expanding the choice of materials and their heterostructures beyond the combinations limited by materials compatibility. However, the improvement of material quality, physical implementation of materials with unique properties, and integration of incommensurate materials in an architecture have been the challenging issues. Emerging materials, including 2D materials and quantum materials, have opened opportunities to study epitaxy mechanisms and realize various functional devices. Acceleration of discovery and progress in epitaxy research should be accomplished by “understanding of epitaxy under various circumstances at multiple length scales” and “integration of experiments and models.” In the perspective, a basic summary of the status of epitaxially grown materials, the challenges in epitaxy research, and integration of modeling epitaxy and ultimate control of the epitaxy process with advanced characterization techniques are discussed.
This article reports monocrystalline CdSeTe/MgCdTe double‐heterostructure (DH) solar cells with varying Se compositions in the absorber layers that are grown on InSb substrates by using molecular beam epitaxy. The Se composition in the samples studied is determined to be 4%–11% through high‐resolution X‐ray diffraction (XRD) and photoluminescence measurements. Increased Se incorporation induces higher defect density attributed to increased lattice mismatch, and mixed‐phase formation due to the small difference in formation energies between the wurtzite and zinc blende phases of CdSe, which causes stacking faults and grain boundaries. Devices are fabricated by directly depositing an n‐type indium tin oxide (ITO) layer on the CdSeTe/MgCdTe DHs followed by Ag metal contacts. Reduced bandgap is observed in solar cells with increased Se composition. The devices with an absorber containing 4% Se exhibit an average open‐circuit voltage ( V OC ) of 0.925 V, a short‐circuit current density ( J SC ) of 23.4 mA/cm 2 , a fill factor ( FF ) of 0.650 and an efficiency of 14.1% without anti‐reflection coating. Absorbers with higher Se compositions result in poor device performance, mainly attributed to high defect density in the materials.
Nickel-rich (Ni≥90%) layered oxides materials have emerged as a promising candidate for next-generation high-energy-density lithium-ion batteries (LIBs). However, their widespread application is hindered by structural fatigue and lattice oxygen loss. In this work, an epitaxial surface rock-salt nanolayer is successfully developed on the LiNi 0.9 Co 0.1 O 2 sub-surface via heteroatom anchoring utilizing high-valence element molybdenum modification. This in-situ formed conformal buffer phase with a thickness of 1.2nm effectively suppresses the continuous interphase side-reactions, and thus maintains the excellent structure integrity at high voltage. Furthermore, theoretical calculations indicate that the lattice oxygen reversibility in the anion framework of the optimized sample is obviously enhanced due to the higher content of O 2p states near the Fermi level than that of the pristine one. Meanwhile, the stronger Mo–O bond further reduces cell volume alteration, which improves the bulk structure stability of modified materials. Besides, the detailed charge compensation mechanism suggests that the average oxidation state of Ni is reduced, which induces more active Li + participating in the redox reactions, boosting the cell energy density. As a result, the uniquely designed cathode materials exhibit an extraordinary discharge capacity of 245.4 mAh g −1 at 0.1 C, remarkable rate performance of 169.3 mAh g −1 at 10 C at 4.5V, and a high capacity retention of 70.5% after 1000 cycles in full cells at a high cut-off voltage of 4.4V. Further, this strategy provides an valuable insight into constructing distinctive heterostructure on high-performance Ni-rich layered cathodes for LIBs.
X-ray photoelectron spectroscopy is a powerful experimental technique that yields invaluable information on a range of phenomena that occur in solids, liquids, and gasses. The binding energy and shape of a photoemission peak is sensitive not only to the atomic number, valence and orbital from which the electron is ejected, but also to complex many-body effects that accompany photoemission. Provided the influences of these different drivers of spectral line shapes can be disentangled, a great deal can be learned about the electronic structures of specific atoms in the material of interest. In addition to these largely local effects, the long-range electrostatic environment and resulting electric potential at the emitting atom have a direct effect on the measured binding energies. Furthermore, this fact opens the door to extracting information about the dependence of the valence and conduction band minima on depth below the surface, which in turn allows both vertical and lateral electrical transport data to be better understood. One purpose of this Report is to describe how the different physical forces described above impact spectral properties of complex oxide epitaxial films. This class of materials typically incorporates transition metal cations in different valences and of all the elements, these exhibit the most complex core-level spectra. A second purpose is to show how a comprehensive understanding of local physical effects in x-ray photoemission allows one to extract detailed information on internal electric fields and band edge discontinuities in heterostructures involving complex oxides from core-level line shapes.
Concentric lateral CdSe/CdTe/CdSe heterostructures show bicolor photoluminescence from both a red charge transfer band of the CdSe/CdTe interface and a green fluorescence from CdSe. Further, this work uses visible and near-infrared transient spectroscopy measurements to demonstrate that the deviation from Kasha's rule arises from a hole relaxation bottleneck from CdSe to CdTe. Hole transfer can take up to 1 ns, which permits radiative relaxation of excitons remaining in CdSe. Simulations indicate that the hole relaxation bottleneck arises due to the sparse density of states and poor spatial overlap of hole states at energies near the CdSe band edge. The divergent kinetics of transfer for band edge and hot holes is exploited to vary the ratio of green and red photoluminescence with excitation wavelength, providing another knob to control emission color. These findings support the use of lateral heterojunctions as a method for slowing carrier relaxation in two-dimensional materials.
Despite the weak, van der Waals interlayer coupling, photoinduced charge transfer vertically across atomically thin interfaces can occur within surprisingly fast, sub-50 fs time scales. An early theoretical understanding of charge transfer is based on a noninteracting picture, neglecting excitonic effects that dominate optical properties of such materials. We employ an ab initio many-body perturbation theory approach, which explicitly accounts for the excitons and phonons in the heterostructure. Our large-scale first-principles calculations directly probe the role of exciton-phonon coupling in the charge dynamics of the WS 2 /MoS 2 heterobilayer. We find that the exciton-phonon interaction induced relaxation time of photoexcited excitons at the K valley of MoS 2 and WS 2 is 67 and 15 fs at 300 K, respectively, which sets a lower bound to the intralayer-to-interlayer exciton transfer time and is consistent with experiment reports. We further show that electron-hole correlations facilitate novel transfer pathways that are otherwise inaccessible to noninteracting electrons and holes.
Dynamically manipulating carriers in van der Waals heterostructures could enable solid-state quantum simulators with tunable lattice parameters. A key requirement is the formation of deep potential wells to reliably trap excitations. Here, we report the observation of nonlinear acoustoelectric transport and dynamic carrier modulation in boron nitride-encapsulated graphene devices coupled to intense surface acoustic waves (SAWs) on LiNbO 3 substrates. SAWs generate strong acoustoelectric current densities ( J AE ), transitioning from linear to nonlinear regimes with increasing SAW intensity. In the nonlinear regime, periodic carrier (electrons, holes, or their mixtures) stripes emerge. Using counter-propagating SAWs, we create standing SAWs (SSAWs) to dynamically manipulate charge distributions without static gates. The saturation of J AE , attenuation transitions, and tunable resistance peaks confirms strong carrier localization. Finally, these results establish SAWs as a powerful tool for controlling carrier dynamics in two-dimensional (2D) materials, paving the way for the development of time-dependent quantum systems and acoustic lattices for quantum simulation.
Hybridization between interlayer exciton (ILX) and intralayer excitons offers a powerful route to engineer light–matter interactions in transition-metal dichalcogenide (TMDC) heterostructures, yet the underlying mechanism and selection rules remain elusive. Here we probe exciton hybridization in MoTe2/MoSe2 heterobilayers under a tunable out-of-plane electric field and observe distinct avoided crossings in the electric-field dependent reflection spectra. We associate them with coupling between excited ILX states, including the 2p and 2s Rydberg states, and MoTe2 intralayer exciton. Supported by ab initio GW plus Bethe-Salpeter equation (GW-BSE) calculations, we identify the hybridizing states and establish optical selection rules based on the effective angular momentum of the exciton states. These findings highlight the role of excitonic binding in the observed hybridization, rather than that of simple band-mixing models, and provide a new pathway for brightening high-energy dark states.
The demand for ultranarrow spectral emission below 10 nm is critical for next-generation displays. Although current colloidal nanocrystals are successful in light down-conversion applications, they fall short in matching the stringent color purity requirements set by standards like Rec. 2020. Achieving such narrow linewidths through material engineering alone remains elusive, necessitating innovative light management strategies that avoid optical losses and maintain device efficiency. Here, in this work, we present a novel 2D colloidal heterostructure design, combining a CdSe core nanoplatelet, a CdS crown, and a CdZnS shell, to achieve bright bicolor emission in the solid state (green/red) emission with tunable intensity ratios controlled by excitation power (either optical or electrical). Integration of these nanoplatelets into a dielectric cavity further narrows the emission linewidth to sub-2 nm while enhancing the photoluminescence intensity by up to 200 times, meeting the Rec. 2020 color purity targets. This approach paves the way for a strategy avoiding complex laser-based solutions.
Quantum materials that combine magnetism with topological order are emerging as key platforms for next-generation spintronics and low-energy electronics. They enable the realization of emergent quantum phenomena, such as the quantum anomalous Hall effect and axion insulator states. The ferromagnetic insulator (FMI)/topological insulator (TI)/FMI sandwich structure of a single-septuple layer (1SL) MnBi 2 Te 4 /four-quintuple layer (4QL) Bi 2 Te 3 /1SL MnBi 2 Te 4 holds great potential to achieve such desirable quantum phenomena at an elevated temperature, owing to its large Dirac point band gap and high Curie temperature. Here, for this work, spin- and angle-resolved photoemission spectroscopy (spin-ARPES) is employed to directly verify that the band gap arises from broken time-reversal symmetry via proximity-driven magnetization. This study demonstrates direct control of the spin state via external magnetic fields and unambiguously confirms the exchange interaction as the gap-opening mechanism. The robust magnetic gap and controllable spin texture make this heterostructure a suitable candidate for spintronic applications and magnetic topological quantum phases.
Symmetry breaking in van der Waals materials enables the realization of quantum states and advanced device functionalities. Janus transition-metal dichalcogenides (TMDs) exhibit distinctive nonlinear optical properties due to their broken out-of-plane mirror symmetry. However, the dynamic control of second harmonic generation (SHG) anisotropy and resonance behavior via optical excitation remains elusive. Here, in this work, we investigate the SHG response of Janus MoSSe/MoS 2 heterostructures with 2H and 3R stackings. We can tune the SHG response by varying the incident photon wavelength from 800 to 1000 nm, which shows a resonance-dependent enhancement in intensity and a deviation from 6-fold symmetry, indicating wavelength-dependent anisotropy. The ratio between maximum and minimum intensity in the armchair directions, associated with the SHG anisotropy, reaches a value of 1.73 at the excitation wavelength of 1000 nm. Group theory analysis and first-principles calculations reveal that the observed anisotropy arises from optically induced strain. Our findings highlight the role of symmetry breaking and optical resonance contributing to the optomechanical tuning of SHG anisotropy, offering opportunities for developing Janus TMD-based photonic devices for frequency conversion, light generation, and optical switching.