39 Search Results

Achieving multiple types of resistive switching in a single material with controlled ionic motion is a key challenge in neuromorphic computing, traditionally addressed by combining materials with distinct switching behaviors. Here, Cu_2-xS is identified as a promising candidate to overcome this limitation due to its hierarchical phase transitions. Using in situ biasing experiments, reversible and non-reversible phase transitions (and resistive switching) are demonstrated in γ-Cu₂S by controlling the compliance current. The formation of parallel high-digenite Cu_1.8S channels, orientated along the γ-Cu₂S [201] crystallographic direction, drives the nonvolatile resistive switching. These channels emerge via an intermediate δ-Cu₂S phase and are stabilizedmore » at room temperature by residual strains, alongside β-Cu₂S phase. In conclusion, the work clarifies the complex, electrically triggered phase transformations in γ-Cu₂S, and highlights the potential of Cu_2-xS as a versatile material for neuromorphic computing.« less

Volatile resistive switching in neuromorphic computing can be tuned by external stimuli such as temperature or electric-field. However, this type of switching is generally coupled to structural changes, resulting in slower reaction speed and higher energy consumption when incorporated into an electronic device. The vanadium dioxide (VO₂), which has near room temperature metal-insulator transition (MIT), is an archetypical volatile resistive switching system. Here, we demonstrate an isostructural MIT in an ultrathin VO₂ film capped with a photoconductive cadmium sulfide (CdS) layer. Transmission electron microscopy, resistivity experiments, and first-principles calculations show that the hole carriers induced by CdS photovoltaic effect aremore » driving the MIT in rutile VO₂. The insulating-rutile VO₂ phase has been proved and can remain stable for hours. Our finding provides a new approach to produce purely electronically driven MIT in VO₂, and widens its applications in fast-response, low-energy neuromorphic devices.« less

The Schottky barrier height can be greatly affected by the metal diffusion, reaction, and covalent bonding formation at the contact. Exploring novel methods and revealing the fundamental mechanisms for contact engineering are of vital importance for microelectronic devices. Here, in this study, the annealing induced interfacial reactions at Cu-ReSe₂ contact are dynamically revealed from the atomic scale. Accompanied by the diffusion of Se to Cu, ReSe₂ is gradually decomposed to a thin Re interlayer through a “chain-by-chain” manner. Theoretical calculations show that the Cu atoms can facilitate the chemical bond breaking of ReSe₂, significantly lowering the Se diffusion energy barriermore » toward Cu. The formed Re/ReSe₂ heterostructure presents a metal-like band structure, which underscores the critical role of Cu in altering the interfacial chemistry and promoting carrier transport across the interface. Our results can provide vital insights into the contact properties of ReSe₂ and provide a possible method for fabricating high-performance ReSe₂-based devices.« less

Cold welding of metals at the nanoscale has been demonstrated to play a significant role in bottom-up manufacturing and self-healing processes of nanostructures and nanodevices. However, the welding mechanism at the nanoscale is not well understood. In this study, a comprehensive demonstration of the cold welding process of gold nanorods with different modes is presented through in situ liquid cell transmission electron microscopy. The experimental results and molecular dynamics simulations reveal that the nanorods are welded through the facet-dependent atomic surface diffusion and rearrangement along {100} facets. The density functional theory calculations indicate that the preferred coalescence of two {100}more » surfaces is thermodynamically favorable. Unlike the prevalent “oriented attachment” in the nanoparticle coalescence, the misalignment of nanorod orientations and local stresses can induce grain boundaries and stacking faults in the welded interface.« less

Thermal treatment is a general and efficient way to synthesize intermetallic catalysts and may involve complicated physical processes. So far, the mechanisms leading to the size and composition heterogeneity, as well as the phase segregation behavior in Pt-Co nanoparticles (NPs) are still not well understood. Here, via in-situ environmental transmission electron microscopy, the formation dynamics and segregation behaviors of Pt-Co alloyed NPs during the thermal treatment were investigated. It is found that Pt-Co NPs on zeolitic imidazolate frameworks-67-derived nanocarbon (NC) are formed consecutively through both particle migration coalescence and the Ostwald ripening process. The existence of Pt NPs is foundmore » to affect the movement of Co NPs during their migration. With the help of theoretical calculations, the correlations between the composition and migration of the Pt and Co during the ripening process were uncovered. Herein, these complex alloying processes are revealed as key factors leading to the heterogeneity of the synthesized Pt-Co alloyed NPs. Under oxidation environment, the Pt-Co NPs become surface faceted gradually, which can be attributed to the oxygen facilitated relatively higher segregation rate of Co from the (111) surface. This work advances the fundamental understanding of design, synthesis, and durability of the Pt-based nanocatalysts.« less

Hexagonal LuFe₂O₄ is a promising charge order (CO) driven multiferroic material with high charge and spin-ordering temperatures. The coexisting charge and spin orders on Fe³⁺/Fe²⁺ sites result in magnetoelectric behaviors, but the coupling mechanism between the charge and spin orders remains elusive. Here, by tuning external pressure, we reveal three charge-ordered phases with suggested correlation to magnetic orders in LuFe₂O₄: (i) a centrosymmetric incommensurate three-dimensional CO with ferrimagnetism, (ii) a non-centrosymmetric incommensurate quasi-two-dimensional CO with ferrimagnetism, and (iii) a centrosymmetric commensurate CO with antiferromagnetism. Experimental in situ single-crystal X-ray diffraction and X-ray magnetic circular dichroism measurements combined with density functionalmore » theory calculations suggest that the charge density redistribution caused by pressure-induced compression in the frustrated double-layer [Fe₂O₄] cluster is responsible for the correlated spin-charge phase transitions. The pressure-enhanced effective Coulomb interactions among Fe-Fe bonds drive the frustrated (1/3, 1/3) CO to a less frustrated (1/4, 1/4) CO, which induces the ferrimagnetic to antiferromagnetic transition. Our results not only elucidate the coupling mechanism among charge, spin, and lattice degrees of freedom in LuFe₂O₄, but also provide a new way to tune the spin-charge orders in a highly controlled manner.« less

Abstract Achieving self‐assembling structures that exhibit emergent properties and functionalities is one of the holy grails in nanotechnology and holds great promise for a wide range of potential applications. Herein, a self‐assembled LuFeO ₃ –(1/3)LuFe ₂ O ₄ –LuFeO ₃ heterostructure that consists of two model multiferroics, i.e., hexagonal LuFeO ₃ and rhombohedral LuFe ₂ O ₄ , is reported in single‐layer epitaxial LuFeO ₃ thin films. Atomic‐scale electron microscopy investigations reveal the spontaneous organization of ferroelectric polarization in each building block, which forms a series of polar configurations. Specifically, the charge and lattice of neighboring LuFeO ₃ and LuFemore » ₂ O ₄ blocks are closely coupled with competing degrees of freedom. A combination of the quantitative analysis of atomically resolved images and spectroscopy with theoretical calculations determines how charge‐ordering‐induced polarization accommodates charge naturality across the interfaces to reduce the system energy and unravels the formation mechanism of this heterostructure. The results point to the possibilities of generation and control of multiferroic self‐assembled systems, which may provide a basis for engineering novel materials with emergent properties.« less

Oxide superlattices have drawn great attentions owing to the intriguing coupling among elastic, electrical, and magnetic orderings at the interfaces and the emerging of improper ferroelectricity. Here, superlattices composed of hexagonal LuFeO₃ (h-LuFeO₃) and LuFe₂O₄ are investigated via density functional theory calculations. h-LuFeO₃ is a well-known multiferroic material that is stable only in thin-film or doped bulk state, while LuFe₂O₄ is a charge ordered material where the existence of ferroelectricity is still in controversy. We have found that the charge ordering (CO) induced polarizations in LuFe₂O₄ layers coexist with the geometric polarizations in h-LuFeO₃ layers in the (LuFe₂O₄)_m/(LuFeO₃)_n superlattices withmore » different periodicities, and the ferroelectric states are generally preferred over the antiferroelectric states for LuFe₂O₄ in superlattices. The out-of-plane polarizations in h-LuFeO₃ and LuFe₂O₄ layers tend to be aligned in parallel, and the overall polarization increases with the ratio of h-LuFeO₃. The influence of layered polarizations on the local electrostatic potential is not significant except the detected small trend caused by the CO-induced polarization within a FeO bilayer. Besides, the local electronic structures show that the Fermi level position in a certain layer can be tuned by the valences of Fe in this layer and the polarization distributions in neighboring layers. LuFe₂O₄ layers sandwiched between thick h-LuFeO₃ layers are more susceptible. The calculated configurations of the superlattices are supported by atomic-resolution transmission electron microscopy experiments. Our results pave a way for tunable ferroelectricity in superlattice systems and create a new playground for manipulating the coupling between various degrees of freedom.« less

Significance Dynamically understanding the microscopic processes governing ordering transformations has rarely been attained. The situation becomes even more challenging for nanoscale alloys, where the significantly increased surface-area-to-volume ratio not only opens up a variety of additional freedoms to initiate an ordering transformation but also allows for kinetic interplay between the surface and bulk due to their close proximity. We provide direct evidence of the microscopic processes controlling the ordering transformation through the surface–bulk interplay in Pt–Fe nanoalloys and new features rendered by variations in alloy composition and chemical stimuli. These results provide a mechanistic detail of ordering transformation phenomena whichmore » are widely relevant to nanoalloys as chemical ordering occurs in most multicomponent materials under suitable environmental bias.« less

Engineering oxygen vacancy formation and distribution is a powerful route for controlling the oxygen sublattice evolution that affects diverse functional behavior. The controlling of the oxygen vacancy formation process is particularly important for inducing topotactic phase transitions that occur by transformation of the oxygen sublattice. Here we demonstrate an epitaxial nanocomposite approach for exploring the spatial control of topotactic phase transition from a pristine perovskite phase to an oxygen vacancy-ordered brownmillerite (BM) phase in a model oxide La_0.7Sr_0.3MnO₃ (LSMO). Incorporating a minority phase NiO in LSMO films creates ultrahigh density of vertically aligned epitaxial interfaces that strongly influence the oxygenmore » vacancy formation and distribution in LSMO. Combined structural characterizations reveal strong interactions between NiO and LSMO across the epitaxial interfaces leading to a topotactic phase transition in LSMO accompanied by significant morphology evolution in NiO. Using the NiO nominal ratio as a single control parameter, we obtain intermediate topotactic nanostructures with distinct distribution of the transformed LSMO-BM phase, which enables systematic tuning of magnetic and electrical transport properties. The use of self-assembled heterostructure interfaces by the epitaxial nanocomposite platform enables more versatile design of topotactic phase structures and correlated functionalities that are sensitive to oxygen vacancies.« less