Local Phase Transition Research Articles

Lateral Phase Heterojunction for Perovskite Microoptoelectronics.

Perovskite heterojunction engineering is the prerequisite but still a deficiency in the fabrication of micro-optoelectronic devices, where the present top-down or bottom-up techniques mainly focus on preparing the vertical heterojunction stacks. Perovskite lateral heterojunction structures generally rely on epitaxial growth, which cannot meet the demands of mass production of micro-devices. Here, a contact diffusion lithography technique is proposed to demonstrate a perovskite lateral phase heterojunction (LPH) polycrystalline film by ion-driven local phase transition. Under the guidance of thermodynamic simulations, methylamine contact and migration collectively promote in situ formation of α-phase formamidine-based perovskite patterns surrounded by δ-phase polymorphs. Spontaneous type-I heterojunction alignment between α- and δ-phases establishes energy funnels in the LPH film to facilitate carrier utilization and radiative recombination. The wide-bandgap δ-phase also serves as the coplanar isolator to achieve local anti-leakage for device integration. Based on the bright and stable LPH pattern layer, the near-infrared microscale perovskite light-emitting diode (micro-PeLED) with impressive device performance is achieved by following conventional device fabrication protocol. The proposed LPH enriches the perovskite heterojunction family, creates a new optoelectronic processing platform, and advances its versatile applications in micro-optoelectronics and photonics.

Microstructure evolution of CdZnTe crystals irradiated by heavy ions

Consideration of irradiation effects is important for in-orbit operation and radiation protection in aerospace applications. The configuration of irradiation defects and their impact on deteriorating the carrier transport properties of CZT crystals need to be clarified. In this study, the microdefect evolution mechanism of CdZnTe (CZT) crystals under 10 MeV Chlorine (Cl) ion irradiation at flux of 1×1010∼1×1012 n·cm-2 are investigated. The stacking faults first appear in the CZT crystal at 4.5×10-4 displacement per atom (dpa), which are categorized into slip-type, gap-type and vacancy-type. In order to hinder the further expansion of stacking fault, S-shaped stacking fault dipole appears. When the dpa reaches 4.5×10-2, the local phase transition occurs in the damage zone to form CuPt-A ordered phase with ZnTe/CdTe periodic arrangement because the fluctuation in local composition and strain caused by cascade collisions. The deterioration of electric field distribution and charge collection caused by irradiation damage layer leads to the decrease of γ-ray detection ability. The energy resolution (ER) of the CZT detector for γ-ray can be maintained at 30.6% when the ion flux accumulates to 1×1012 n·cm-2, revealing high irradiation hardness. Consequently, these findings provide a theoretical basis for the radiation damage recovery of CZT detectors, further demonstrating that CZT crystals are the most competitive new generation of room-temperature nuclear radiation detection materials, and can provide radiation protection strategies for similar compound semiconductor materials.

Reversible Single‐Pulse Laser‐Induced Phase Change of Sb2S3 Thin Films: Multi‐Physics Modeling and Experimental Demonstrations

AbstractPhase change materials (PCMs) have gained a tremendous interest as a means to actively tune nanophotonic devices through the large optical modulation produced by their amorphous to crystalline reversible transition. Recently, materials such as Sb2S3 emerged as particularly promising low loss PCMs, with both large refractive index modulations and transparency in the visible and near‐infrared. Controlling the local and reversible phase transition in this material is of major importance for future applications, and an appealing method to do so is to exploit pulsed lasers. Yet, the physics and limits involved in the optical switching of Sb2S3 are not yet well understood. Here, the reversible laser‐induced phase transition of Sb2S3 is investigated, focusing specifically on the mechanisms that drive the optically induced amorphization, with multi‐physics considerations including the optical and thermal properties of the PCM and its environment. The laser energy threshold for reversibly changing the phase of the PCM is determined through both theoretical analysis and experimental investigation, not only between fully amorphous and crystalline states but also between partially recrystallized states. Then, the non‐negligible impact of the material's polycrystallinity and anisotropy on the power thresholds for optical switching is revealed. Finally, the challenges related to laser amorphization of thick Sb2S3 layers are addressed, as well as strategies to overcome them. These results enable a qualitative and quantitative understanding of the physics behind the optically‐induced reversible change of phase in Sb2S3 layers.

Fantastic Photons and Where to Excite Them: Revolutionizing Upconversion with KY3F10-Based Compounds

This review delves into the forefront of upconversion luminescence (UCL) research, focusing on KY3F10-based compounds, particularly their cubic α-phase. These materials are renowned for their exceptional luminescent properties and structural stability, making them prime candidates for advanced photonic applications. The synthesis methods and structural characteristics of the existing works in the literature are meticulously analyzed alongside the transformative effects of various doping strategies on UCL efficiency. Incorporating rare earth (RE) sensitizer ions such as Yb3+, along with activator ions like Er3+, Ho3+, Nd3+, or Tm3+, researchers have achieved remarkable enhancements in emission intensity and spectral control. Recent and past breakthroughs in understanding the local structure and phase transitions of single-, double-, and triple-RE3+-doped KY3F10 nanocrystals are highlighted, revealing their pivotal role in fine-tuning luminescent properties. Furthermore, the review underscores the untapped potential of lesser-known crystal structures, such as the metastable δ-phase of KY3F10, which offers promising avenues for future exploration. By presenting a comprehensive analysis and proposing innovative research directions, this review aims to inspire continued advancements in the field of upconversion materials, unlocking new potentials in photonic technologies.

Effects of preexisting dislocations on phase transition and deformation of NiTi shape memory alloy: An in situ multi-scale study

Phase transition and deformation in cold-rolled and heat-treated Ni50.6Ti49.4 alloys are explored by in situ multi-scale measurements with simultaneous X-ray diffraction and optical digital image correlation. Macroscale stress−strain curves, mesoscale strain fields and microscale X-ray diffraction peaks are obtained. Electron backscatter diffraction and transmission electron microscopy are also conducted on samples before and after tension as complements. For cold-rolled samples, since martensite nucleation is strongly associated with the widely distributed preexisting dislocations, homogeneous martensite phase transition takes place, in contrast with the localized mode in heat-treated samples. For localized phase transition, the intense growth of martensite results in clear Lüders bands. The beginning and the end of phase transition plateau on the stress−strain curve are associated with the initiation and complete propagation of Lüders bands. For the homogeneous phase transition, broad growth of martensite leads obscure Lüders bands and uniform strain fields, without any phase transition plateau. Given the initial {110}〈110〉 and {111}〈110〉 sheet textures in the heat-treated samples, different martensite variants with different preferred orientations are activated with respect to the loading directions.

Applying network-free renormalization and clustering algorithms to reveal the crack evolution laws of laterally loaded composite T-joints

Delamination or crack are standard failure patterns of composite T-joints. Most existing studies focus on pull-off loaded composite T-joint cracking but little focus on adverse conditions such as lateral loading. The mainstream composite T-joint research predicts the macroscale failure based on the composite’s microscale/mesoscale crack starting and evolution. However, the cracking process is within the microscale, mesoscale, and macroscale, making detecting its starting based on phenomena complicated. This work attempts to directly model the macroscale deformation distribution from a thermodynamic perspective to reveal the cracking/failure evolution of a laterally loaded composite T-joint. Firstly, we conducted eight laterally loaded composite T-joint tests with different molding processes. Then, network-free renormalization was applied to construct Matrices (Modes) and Hamiltonians (Characteristic parameters) that can characterize their cracking evolution process. Further, a clustering algorithm was applied to reveal the cracking starting points (phase transition loads) embedded in the macroscale deformation distribution. We can verify its rationality by comparing the composite T-joints localized and systematic phase transition loads based on Wilson’s phase transition theory. In summary, this work applies network-free renormalization and clustering algorithms from a thermodynamic perspective to reveal the cracking starting point embedded in the macroscale deformation distribution of the laterally loaded composite T-joints.

Observation of stacking engineered magnetic phase transitions within moiré supercells of twisted van der Waals magnets

Recent demonstrations of moiré magnetism, featuring exotic phases with noncollinear spin order in the twisted van der Waals (vdW) magnet chromium triiodide CrI3, have highlighted the potential of twist engineering of magnetic (vdW) materials. However, the local magnetic interactions, spin dynamics, and magnetic phase transitions within and across individual moiré supercells remain elusive. Taking advantage of a scanning single-spin magnetometry platform, here we report observation of two distinct magnetic phase transitions with separate critical temperatures within a moiré supercell of small-angle twisted double trilayer CrI3. By measuring temperature-dependent spin fluctuations at the coexisting ferromagnetic and antiferromagnetic regions in twisted CrI3, we explicitly show that the Curie temperature of the ferromagnetic state is higher than the Néel temperature of the antiferromagnetic one by ~10 K. Our mean-field calculations attribute such a spatial and thermodynamic phase separation to the stacking order modulated interlayer exchange coupling at the twisted interface of moiré superlattices.

Atomistic insights into hydrogen-enhanced strain-induced vacancy in α - iron across varied strain rates

Dynamic behavior of α-iron in the presence of hydrogen (H) under triaxial tension is investigated by molecular dynamic simulations. The process of hydrogen enhanced strain induced vacancy is directly captured at the atomic scale in a wide range of strain rates. Significant strain rate effects and H concentration dependence are observed in the hydrogen embrittlement of α-iron. When the concentration of H (CH) is substantially high, the maximum tensile stress of iron falls. More local phase transitions are observed at the aggregation sites of H atoms, which plays a crucial role in ensuring that the material does not lose its load bearing capacity immediately after reaching the maximal stress. Our study sheds light on the intricate interplay of H, strain rate, phase transition (PT) in α-iron and provides valuable insights into the complex dynamics of H embrittlement.

In-plane distribution of huge local permittivity of KTa1-xNbxO3 estimated from local phase transition temperatures and spatially averaged permittivity.

Potassium tantalate niobate (KTa1-xNbxO3, KTN) single crystals have a very large relative permittivity εr (>104) just above the paraelectric to ferroelectric phase transition temperature (TC). The quadratic electro-optic coefficient and the electro-strictive coefficient are also very large because of their proportionality to εr2. However, the local relative permittivity can easily vary spatially due to the incongruently melting nature of KTN. In this study, we quantitatively estimated the in-plane distribution of the huge local relative permittivity of KTN. First, we measured the spatial distribution of TC using scanning nonlinear dielectric microscopy, then deposited the electrodes and measured the temperature dependence of the spatially averaged permittivity using an LCR meter. Following that, we evaluated the spatial distribution of the huge local permittivity from the combination of the spatial distribution of TC and the spatially averaged permittivity. Finally, we measured the deflection angle of light to confirm the validity of the εr estimation procedure. The maximum error for the estimated permittivity was estimated to be around 3.3%.

Modeling the Solvolysis of Composite Materials of Wind Turbine Blades

A computational model of the depolymerization of composites during solvolysis of wind turbine blades is developed. The model is based on a phenomenological approach, describing the dissolution of an epoxy matrix as a local phase transition influenced by temperature, solvent diffusion, and local microstructures. The model is implemented in the finite element code Abaqus, using the user‐defined field and heat flux subroutines. Parametric studies are carried out to study the influence of defects and heterogeneities on the depolymerization of composite materials. The results show that variations of the solvent diffusivity in the vicinity of fiber/matrix lead to nonhomogeneity of depolymerization, and smaller diffusivity may explain matrix residues remaining on the fibers. Fiber volume density and distribution influence the polymer dissolution rate because the resistance to solvent diffusion arises between fibers. Further, voids in the polymer may lead to a local acceleration of the polymer dissolution. This work also looks into the contributions of diffusion and reaction to depolymerization, and the former dominates. The rate and homogeneity of depolymerization, therefore, depend significantly on the manufacturing quality of composites.

Unveil the origin of voltage oscillation for sodium-ion batteries operating at −40 °C

Voltage oscillation at subzero in sodium-ion batteries (SIBs) has been a common but overlooked scenario, almost yet to be understood. For example, the phenomenon seriously deteriorates the performance of Na3V2(PO4)3 (NVP) cathode in PC (propylene carbonate)/EC (ethylene carbonate)-based electrolyte at -20 °C. Here, the correlation between voltage oscillation, structural evolution, and electrolytes has been revealed based on theoretical calculations, in-/ex-situ techniques, and cross-experiments. It is found that the local phase transition of the Na3V2(PO4)3 (NVP) cathode in PC/EC-based electrolyte at -20 °C should be responsible for the oscillatory phenomenon. Furthermore, the low exchange current density originating from the high desolvation energy barrier in NVP-PC/EC system also aggravates the local phase transformation, resulting in severe voltage oscillation. By introducing the diglyme solvent with lower Na-solvent binding energy, the voltage oscillation of the NVP can be eliminated effectively at subzero. As a result, the high capacity retentions of 98.3% at -20 °C and 75.3% at -40 °C are achieved. The finding provides insight into the abnormal SIBs degradation and brings the voltage oscillation behavior of rechargeable batteries into the limelight.

Effect of Ce2O3 phase transition on the catalytic oxidation for toluene over CeO2 catalysts

Cerium oxide was a potential replacement for precious metals as a catalyst for VOCs. Here, we prepared CeOx-350, CeOx-450, CeOx-550 and CeOx-650 catalysts by controlling the calcination temperature, the occurrence of Ce2O3 localized phase transitions at CeOx-350 and CeOx-550 was confirmed by XRD, HRTEM and TGA. This localized phase transition caused the change in specific surface area and crystallinity, and prolonged the Ce-O-Ce bond lengths at the heterogeneous interfaces, enhancing the redox capacity and activity of oxygen species. CeOx-550 exhibited the optimal low-temperature activity towards toluene due to the highest Ce3+ content, oxygen vacancy content and the best activity of reactive oxygen species. The oxidation of toluene experienced intermediates of benzyl alcohol, benzaldehyde, benzoate, maleic acid and carbonates. Among them, benzoic acid was the decisive step in the toluene oxidation pathway. Combined with the reaction pathway of toluene, the reaction stages of different oxygen species were determined. The active oxygen species located at the heterogeneous interface would serve as an adsorption site for toluene. The Ce2O3 phase would participate in the deep oxidation of toluene as an active site, which was equivalent to the oxygen vacancies. This study was conducted to explore the effect of local phase transition on catalytic activity, which provided a new strategy for the synthesis of highly active cerium oxide catalysts.

Nano-scale corrosion mechanism of T91 steel in static lead-bismuth eutectic: A combined APT, EBSD, and STEM investigation

T91 steel is a candidate material for structural components in lead-bismuth-eutectic (LBE) cooled systems, for example fast reactors and solar power plants [1]. However, the corrosion mechanisms of T91 in LBE remain poorly understood. In this study, we have analysed the static corrosion of T91 in liquid LBE using a range of characterisation techniques at increasingly smaller scales. A unique pattern of liquid metal intrusion was observed that does not appear to correlate with the grain boundary network. Upon closer inspection, electron backscatter diffraction (EBSD) reveals a change in the morphology of grains at the LBE-exposed surface, suggesting a local phase transition. Energy dispersive X-ray (EDX) maps show that Cr is depleted in the T91 material near the LBE interface. Furthermore, we observed the dissolution of all Cr-enriched precipitates in this region. Although the corrosion is conducted in an oxygen deficient environment, both scanning transmission electron microscopy (STEM) and atom probe tomography (APT) reveal a thin surface oxide layer (presumably wüstite) at the LBE-steel interface. Using electron energy loss spectroscopy (EELS) in the STEM, as well as APT, the atomic scale elemental redistribution and 3D morphology of the corrosion interface is investigated. By combining results from these different techniques, several types of oxide phases and structures can be identified. Based on this detailed nano-scale information, we propose potential mechanisms of T91 corrosion in LBE.

First-order localization and quantum phase transition induced by quasicrystal imaginary domain

The non-Hermitian extension of quasicrystals provides a highly tunable system for exploring novel material phases. While extended-localized phase transitions have been observed in one dimension, quantum phase transitions in higher dimensions and various system sizes remain unexplored. Here, we show the discovery of a new critical phase and first-order structural transition induced by imaginary zeros in the two-dimensional Haldane model with a quasicrystal potential on the upper boundary. Initially, we illustrate a phase diagram that evolves with the amplitude and phase of the quasicrystal potential, which is divided into three distinct phases by two critical boundaries: phase (I) with extended wave functions, parity-time-restore phase (II) with localized wave functions, and a critical phase (III) with multifunctional wave functions. To characterize the wave functions in these distinct phases, we introduce a low-energy approximation theory and an effective two-chain model. Additionally, we uncover a first-order quantum phase transition induced by quasicrystal domains. As we increase the size of the potential boundary, we observe the partitioning of the critical phase into regions in proportion to the growing number of quasicrystal domains. Importantly, these observations are consistent with ground state fidelity and energy gap calculations. Our research advances the understanding of phase diagrams associated with high-dimensional quasicrystal potentials and marks the first discovery of a first-order quantum phase transition induced by quasicrystal domains in non-Hermitian systems. Published by the American Physical Society 2024

The role of Ca/Zr ratio on the local structure and phase transitions in lead-free (Ba,Ca)(Zr,Ti)O3

In this study, the structural phase transitions are investigated as a function of composition and temperature for polycrystalline x(Ba0.7Ca0.3)TiO3-(1 − x)Ba(Zr0.2Ti0.8)O3 (x = 0.40, 0.45, 0.50, 0.55, and 0.60) through a combination of Raman spectroscopy, synchrotron X-ray diffraction, and dielectric spectroscopy. The aim is to gain insight into the complex phase boundary region responsible for the excellent electromechanical properties. The results demonstrate the correlation between local site substitutions based on the stoichiometric variations to the microstructure and dielectric properties. The dielectric response has been correlated with the BCT/BZT content displaying a maximum depending on the phase content. Additionally, in situ temperature-dependent Raman, permittivity, and pair distribution function (PDF) studies were performed from −60 °C to 130 °C to highlight the structure and phase evolution. In particular, the in situ temperature-dependent Raman measurements reveal sudden discontinuities in the vibrational modes that correspond to the structural changes in the perovskite structure. Therefore, the results of the permittivity response, based on the average of a large volume, are consistent with the local structural changes obtained by other techniques. Indeed, correlations of Raman, X-ray diffraction, and PDFs obtained from synchrotron X-ray total scattering data, along with permittivity measurements allowed the identification of the discrete ferro- to paraelectric phase transitions and a more robust characterization. The rotation and distortion of the octahedral caused by oxygen displacement are driving forces behind symmetry changes and phase transitions, explaining the mechanism of polymorphic phase transition based on A- and B-site substitution. Therefore, this work provides a comprehensive understanding of temperature and composition-dependent phase transitions in BCZT.

Improving electron mobility in MoS2 field-effect transistors by optimizing the interface contact and enhancing the channel conductance through local structural phase transition

A structural phase incorporation strategy was proposed to improve the electron mobility of MoS2 FETs up to 237 cm2 V−1 s−1.

CO<sub>2</sub>-mediated conversion of physical pressure into chemical pressure to realize lattice strain

<p>Lattice strains often lead to novel phenomena, and they are used in a variety of applications, such as thermoelectricity, ferroelectricity, ferromagnetic, skyrmion and superconductivity. In this paper, we briefly summarize the novel phenomena brought about by lattice strain. An efficient method to realize lattice strain using supercritical CO<sub>2</sub> (SC CO<sub>2</sub>) is proposed. When CO<sub>2</sub> molecules shuttle through the confined space of the crystal structure, it can generate huge energy subsequently. Specifically, CO<sub>2</sub> carries MPa-level external physical pressure inside the crystal structure, forming localized defects or phase transitions. This alters the original chemical coordination environment and generates internal chemical pressures up to the GPa level. The crystal is stripped from three-dimensional to two-dimensional under high pressure, and lattice strain is realized at the same time. This work will provide new insights into the realization of lattice strain and give valuable guidance to broaden its application.</p>

(Invited) Interlayer Excitons in Two-Dimensional Perovskite/Monolayer Transition Metal Dichalcogenide Heterostructures

The emergence of two-dimensional (2D) materials has intrigued a great deal of research on novel physical phenomena and various functional applications due to their particular crystal structures and reduced dimensionality. Unlike 3D materials, bulks of 2D materials can be easily thinned to atomic thickness by mechanical exfoliation and atomically thin 2D materials can be arbitrarily stacked to form vertical van der Waals (vdW) heterostructures, which could inherit the unique characteristics of the constituent layers and even exhibit new properties not possessed by them. Particularly, when type-Ⅱ vdW heterostructures are excited, the positive and the negative charges would reside in the different layers after the charge transfer but be limited within a short distance due to the strong quantum confinement effect of 2D materials, leading to strong Coulomb interaction and the formation of interlayer excitons (IXs). IXs are generally equipped with orientated dipole moment and long lifetime, making them ideal media for the future interconnects between optical transmission and electronic computation. Currently, studies on IXs are mainly focused on the vdW heterostructures formed by monolayers of transition-metal dichalcogenides (TMDs). Here, I will first talk about vdW heterostructures formed by 2D perovskites and TMDs for studying IXs. Stacking different kinds of 2D perovskites and TMDs, formation of IXs is confirmed by excitation-power-, temperature-, electric-field-dependent and time-resolved photoluminescence (PL) studies. Notably, robust IX emission can be observed regardless of the stacking sequence and geometric alignment of the constituent layers, showing great advantages over the TMD/TMD vdW heterostructures, which require special twist-angle and thermal annealing. Then, I would like to give a brief introduction on widely tuning the IX emission energy by changing the layer number of the 2D perovskite or the TMD, which shed light on the application of 2D perovskite/TMD vdW heterostructures in broad-spectrum optoelectronics. Furthermore, I would like to next talk about how the selection of organic chains in 2D perovskites influences the properties of the IXs. By using chiral 2D perovskites, the IX emission shows substantial circular polarization and the polarization direction is only related to the chirality of the molecules regardless of the excitation scheme or any other external field, which could open up new passages for controlling valley- or spin-polarization of IXs. By introducing molecules with different dielectric constants, the dielectric screening strength in the vdW heterostructure is changed and hence the binding energy of the IXs is also modified, which offers great opportunities for exploiting tunable excitonic devices and studying exciton condensation. Finally, I will also introduce IX as a non-destructive tool for probing the local phase transition at the surface of the 2D perovskite (BA)2PbI4 flakes. By spatially PL mapping of the (BA)2PbI4/WSe2 heterostructure, two different IX species can be observed to distinguish the low-temperature and the high-temperature phase respectively.

Critical Insight into Pretransitional Behavior and Dielectric Tunability of Relaxor Ceramics.

This model discussion focuses on links between the unique properties of relaxor ceramics and the basics of Critical Phenomena Physics and Glass Transition Physics. It indicates the significance of uniaxiality for the appearance of mean-field type features near the paraelectric-to-ferroelectric phase transition. Pretransitional fluctuations, that are increasing up to the size of a grain and leading to inter-grain, random, local electric fields are responsible for relaxor ceramics characteristics. Their impact yields the pseudospinodal behavior associated with "weakly discontinuous" local phase transitions. The emerging model redefines the meaning of the Burns temperature and polar nanoregions (PNRs). It offers a coherent explanation of "dielectric constant" changes with the "diffused maximum" near the paraelectric-to-ferroelectric transition, the sensitivity to moderate electric fields (tunability), and the "glassy" dynamics. These considerations are challenged by the experimental results of complex dielectric permittivity studies in a Ba0.65Sr0.35TiO3 relaxor ceramic, covering ca. 250 K, from the paraelectric to the "deep" ferroelectric phase. The distortion-sensitive and derivative-based analysis in the paraelectric phase and the surrounding paraelectric-to-ferroelectric transition reveal a preference for the exponential scaling pattern for ε(T) changes. This may suggest that Griffith-phase behavior is associated with mean-field criticality disturbed by random local impacts. The preference for the universalistic "critical & activated" evolution of the primary relaxation time is shown for dynamics. The discussion is supplemented by a coupled energy loss analysis. The electric field-related tunability studies lead to scaling relationships describing their temperature changes.

Plastic Avalanches in Metal-Organic Framework Crystals Due to the Dynamic Phase Mixing.

The compressive properties of metal-organic framework (MOF) crystals are not only crucial for their densification process but also key in determining their performance in many applications. We herein investigated the mechanical responses of a classic crystalline MOF, HKUST-1, using in situ compression tests. A serrated flow accompanied by the unique strain avalanches was found in individual and contacting crystals before their final flattening or fracture with splitting cracks. The plastic flow with serrations is ascribed to the dynamic phase mixing due to the progressive and irreversible local phase transition in HKUST-1 crystals, as revealed by molecular dynamics and finite element simulations. Such pressure-induced phase coexistence in HKUST-1 crystals also induces a significant loading-history dependence of their Young's modulus. The observation of plastic avalanches in HKUST-1 crystals here not only expands our current understanding of the plasticity of MOF crystals but also unveils a novel mechanism for the avalanches and plastic flow in crystal plasticity.