The far infrared complex reflectivity of GaTe

The room temperature polarized complex (amplitude and phase) infra-red reflection spectra of single crystal monoclinic GaTe have been measured between 20 and 400 cm^?1 using a dispersive Fourier transform spectrometer. The measurements allow the experimental zone centre transverse and longitudinal phonon modes and optical constants to be determined without recourse to oscillator fit models or Kramers-Kronig analysis. Infrared power transmission measurements have allowed all fifteen predicted modes to be identified and the results are compared to recent Raman work.     

Low‐Symmetry Monoclinic Phases and Polarization Rotation Path Mediated by Epitaxial Strain in Multiferroic BiFeO3 Thin Films

A morphotropic phase boundary driven by epitaxial strain has been observed in lead‐free multiferroic BiFeO₃ thin films and the strain‐driven phase transitions have been widely reported as iso‐symmetric Cc‐Cc by recent works. In this paper, it is suggested that the tetragonal‐like BiFeO₃ phase identified in epitaxial films on (001) LaAlO₃ single crystal substrates is monoclinic M_C. This M_C phase is different from the M_A type monoclinic phase reported in BiFeO₃ films grown on low mismatch substrates, such as SrTiO₃. This is confirmed not only by synchrotron X‐ray studies but also by piezoresponse force microscopy measurements. The polarization vectors of the tetragonal‐like phase lie in the (100) plane, not the (11 0) plane as previously reported. A phenomenological analysis is proposed to explain the formation of M_C Phase. Such a low‐symmetry M_C phase, with its linkage to M_A phase and the multiphase coexistence open an avenue for large piezoelectric response in BiFeO₃ films and shed light on a complete understanding of possible polarization rotation paths and enhanced multiferroicity in BiFeO₃ films mediated by epitaxial strain. This work may also aid the understanding of developing new lead‐free strain‐driven morphotropic phase boundary in other ferroic systems.     

Template‐Assisted Synthesis of Metallic 1T′‐Sn0.3W0.7S2 Nanosheets for Hydrogen Evolution Reaction

Crystal phase control still remains a challenge for the precise synthesis of 2D layered metal dichalcogenide (LMD) materials. The T′ phase structure has profound influences on enhancing electrical conductivity, increasing active sites, and improving intrinsic catalytic activity, which are urgently needed for enhancing hydrogen evolution reaction (HER) activity. Theoretical calculations suggest that metastable T′ phase 2D Sn_1?xW_xS₂ alloys can be formed by combining W with 1T tin disulfide (SnS₂) as a template to achieve a semiconductor‐to‐metallic transition. Herein, 2D Sn_1?xW_xS₂ alloys with varying x are prepared by adjusting the molar ratio of reactants via hydrothermal synthesis, among which Sn_0.3W_0.7S₂ displays a maximum of concentration of 81% in the metallic phase and features a distorted octahedral‐coordinated metastable 1T′ phase structure. The obtained 1T′‐Sn_0.3W_0.7S₂ has high intrinsic electrical conductivity, lattice distortion, and defects, showing a prominently improved HER catalytic performance. Metallic Sn_0.3W_0.7S₂ coupled with carbon black exhibits at least a 215‐fold improvement compared to pristine SnS₂. It has excellent long‐term durability and HER activity. This work reveals a general phase transition strategy by using T phase materials as templates and merging heteroatoms to achieve synthetic metastable phase 2D LMDs that have a significantly improved HER catalytic performance.     

Long‐Term Structural Evolution of an Intercalated Layered Semiconductor

Intercalated molecules can dramatically modify the electronic band structure of layered semiconductors, significantly altering the optical properties of the material. In the layered monochalcogenide Gallium Telluride (GaTe), exposure to air induces a nearly 1 eV reduction of its band gap due to the interlayer diffusion and chemisorption of oxygen. The effect of oxygen chemisorption at the Te‐terminated surfaces on the structure of GaTe, however, is much less known. To better understand the structure–property relationship of intercalated GaTe, a systematic, long‐term, X‐ray diffraction study has been performed on GaTe exfoliated crystals exposed to ambient conditions. It is observed that the structural changes are not limited to a previously observed short‐term increase in lattice expansion. Over the course of months and even years after exfoliation, the oxygen adsorbates continue to modify the structure of GaTe, inducing significant disorder and grain reorientation. It is estimated that approximately one out of every two grains is slightly displaced by the intercalating oxygen, demonstrating a significant increase in grain mosaicity, while still maintaining the original {?2 0 1} out‐of‐plane texture. Correlating these structural transformations to observed changes in electrical and optical properties will enable capitalization of the use of adsorbates to engineer novel properties in these layered materials.     

Anomalous Dimensionality-Driven Phase Transition of MoTe2 in Van der Waals Heterostructure

Phase transition in nanomaterials is distinct from that in 3D bulk materials owing to the dominant contribution of surface energy. Among nanomaterials, 2D materials have shown unique phase transition behaviors due to their larger surface-to-volume ratio, high crystallinity, and lack of dangling bonds in atomically thin layers. Here, the anomalous dimensionality-driven phase transition of molybdenum ditelluride (MoTe₂) encapsulated by hexagonal boron nitride (hBN) is reported. After encapsulation annealing, single-crystal 2H-MoTe₂ transformed into polycrystalline T_d-MoTe₂ with tilt-angle grain boundaries of 60°-glide-reflection and 120°-twofold rotation. In contrast to conventional nanomaterials, the hBN-encapsulated MoTe₂ exhibit a deterministic dependence of the phase transition on the number of layers, in which the thinner MoTe₂ has a higher 2H-to-T_d phase transition temperature. In addition, the vertical and lateral phase transitions of the stacked MoTe₂ with different crystalline orientations can be controlled by inserted graphene layers and the thickness of the heterostructure. Finally, it is shown that seamless T_d contacts for 2H-MoTe₂ transistors can be fabricated by using the dimensionality-driven phase transition. The work provides insight into the phase transition of 2D materials and van der Waals heterostructures and illustrates a novel method for the fabrication of multi-phase 2D electronics.     

Interlayer Transition in a vdW Heterostructure toward Ultrahigh Detectivity Shortwave Infrared Photodetectors

Van der Waals (vdW) heterostructures of 2D atomically thin layered materials (2DLMs) provide a unique platform for constructing optoelectronic devices by staking 2D atomic sheets with unprecedented functionality and performance. A particular advantage of these vdW heterostructures is the energy band engineering of 2DLMs to achieve interlayer excitons through type‐II band alignment, enabling spectral range exceeding the cutoff wavelengths of the individual atomic sheets in the 2DLM. Herein, the high performance of GaTe/InSe vdW heterostructures device is reported. Unexpectedly, this GaTe/InSe vdWs p–n junction exhibits extraordinary detectivity in a new shortwave infrared (SWIR) spectrum, which is forbidden by the respective bandgap limits for the constituent GaTe (bandgap of ≈1.70 eV in both the bulk and monolayer) and InSe (bandgap of ≈1.20–1.80 eV depending on thickness reduction from bulk to monolayer). Specifically, the uncooled SWIR detectivity is up to ≈10¹⁴ Jones at 1064 nm and ≈10¹² Jones at 1550 nm, respectively. This result indicates that the 2DLM vdW heterostructures with type‐II band alignment produce an interlayer exciton transition, and this advantage can offer a viable strategy for devising high‐performance optoelectronics in SWIR or even longer wavelengths beyond the individual limitations of the bandgaps and heteroepitaxy of the constituent atomic layers.     

Soft Nanostructured Films for Actuated Surface‐Based siRNA Delivery

Substrate‐mediated gene delivery is an emerging technology that enables spatial control of gene expression and localized delivery. This is of particular interest for siRNA where surface‐based release can greatly impact the fields of stem‐cell reprograming, wound healing, and medical device coatings in general. However, reports on the use of siRNA for substrate‐mediated delivery are scarce and have suffered from low efficiency. Here, an alternative strategy is reported by designing self‐assembled substrates that experience stimuli‐responsive topological transformations. Specifically, a methodology is established to promote the molecular organization of lipid films having 3D‐bicontinuous cubic, 2D‐inverted hexagonal, or 1D‐lamellar nanostructures encapsulating siRNA. In response to a compositional, temperature, or humidity stimulus, the nanostructures evolve from 1D‐lamellar or 2D‐hexagonal to 3D‐cubic resulting in efficient siRNA release to human cell cultures. Grazing incidence X‐ray diffraction reveals that film nanostructures are highly ordered and preferentially aligned. The results indicate that film structure substantially affects siRNA delivery, with the supported 3D‐bicontinuous cubic phase yielding the most effective reduction of gene expression. Subsequent studies suggest this enhanced performance arises due to the ability of this phase to cross cell membranes, particularly those of endocytic compartments. This work underpins that nanostructure tuning is decisive to the performance of therapeutic films.     

Dual Phase‐Controlled Synthesis of Uniform Lanthanide‐Doped NaGdF4 Upconversion Nanocrystals Via an OA/Ionic Liquid Two‐Phase System for In Vivo Dual‐Modality Imaging

A novel OA/ionic liquid two‐phase system combining the merits of thermal decomposition method, the IL‐based strategy, and the two‐phase approach is introduced to synthesize high‐quality lanthanide‐doped NaGdF₄ upconversion nanocrystals with different crystal‐phases in OA‐phase and IL‐phase through a one‐step controllable reaction. Oil‐dispersible cubic‐phase NaGdF₄:Yb, Er (Ho, Tm) nanocrystals with ultra‐small size (～5 nm) and monodispersity are obtained in the OA phase of the two‐phase system via an IL‐based reaction. More importantly, water‐soluble hexagonal‐phase NaGdF₄:Yb, Er nanocrystals are obtained in the same system simply by adopting an extremely facile method to complete the dual phase‐transition (crystal‐phase transition and OA‐phase to IL‐phase transition) simultaneously. The synthesized lanthanide‐doped NaGdF₄ upconversion nanocrystals are effective for dual‐mode UCL imaging and CT imaging in vivo.     

Significantly Increased Raman Enhancement on MoX2 (X = S,Se) Monolayers upon Phase Transition

2D transition metal dichalcogenide (TMD) materials have been recognized as active platforms for surface‐enhanced Raman spectroscopy (SERS). Here, the effect of crystal structure (phase) transition is shown, which leads to altered electronic structures of TMD materials, on the Raman enhancement. Using thermally evaporated copper phthalocyanine, solution soaked rhodamine 6G, and crystal violet as typical probe molecules, it is found that a phase transition from 2H‐ to 1T‐phase can significantly increase the Raman enhancement effect on MoX₂ (X = S, Se) monolayers through a predominantly chemical mechanism. First‐principle density functional theory calculations indicate that the significant enhancement of the Raman signals on metallic 1T‐MoX₂ can be attributed to the facilitated electron transfer from the Fermi energy level of metallic 1T‐MoX₂ to the highest occupied molecular orbital level of the probe molecules, which is more efficient than the process from the top of valence band of semiconducting 2H‐MoX₂. This study not only reveals the origin of the Raman enhancement and identifies 1T‐MoSe₂ and 1T‐MoS₂ as potential Raman enhancement substrates, but also paves the way for designing new 2D SERS substrates via phase‐transition engineering.     

On‐Demand Nanoscale Manipulations of Correlated Oxide Phases

Controlling material properties at the nanoscale is a critical enabler of high performance electronic and photonic devices. A prototypical material example is VO₂, where a structural phase transition in correlation with dramatic changes in resistivity, optical response, and thermal properties demonstrates particular technological importance. While the phase transition in VO₂ can be controlled at macroscopic scales, reliable and reversible nanoscale control of the material phases has remained elusive. Here, reconfigurable nanoscale manipulations of VO₂ from the pristine monoclinic semiconducting phase to either a stable monoclinic metallic phase, a metastable rutile metallic phase, or a layered insulating phase using an atomic force microscope is demonstrated at room temperature. The capability to directly write and erase arbitrary 2D patterns of different material phases with distinct optical and electrical properties builds a solid foundation for future reprogrammable multifunctional device engineering.     

Growth behaviour of straight crystalline copper sulphide nanowires

We examine the growth process of the Cu₂S nanowires on an oxidized copper surface. The nanowire growth is shown to be favorable on the copper surface with an appropriate degree of oxidation. We also found the presence of oxygen to be essential during the growth of the Cu₂S nanowires. H₂S exposure time affects primarily the length of the anaowires, and it affects the nanowire thickness to a much less extent. Ultrasound treatment converts the monoclinic Cu₂S to tetragonal Cu₂S on the nanowire surface, forming a core/shell structure. On heating, the monoclinic phase changes to the hexagonal phase at a temperature, which is lower than that of bulk. Copyright © 2000 John Wiley & Sons, Ltd.     

Large‐Scale Growth of Two‐Dimensional SnS2 Crystals Driven by Screw Dislocations and Application to Photodetectors

2D SnS₂ crystals are attracting increasing attention owning to the huge potential for electronic and optoelectronic applications. However, batch production of 2D SnS₂ crystals via a simple vapor process remains challenging by far. Moreover, the growth mechanism for vapor growth of 2D SnS₂ is not well documented as well. Herein, a simple approach is presented for preparation of large‐scale 2D SnS₂ crystals on mica sheets and it is demonstrated that these 2D crystals follow a screw‐dislocation‐driven (SDD) spiral growth process. The synthesized 2D crystals show hexagonal and truncated triangular shapes with the lateral size ranging from a few micrometers to dozens of micrometers. Observations of key features for screw dislocations, such as helical fringes, dislocation hillocks, and herringbone contours, solidly confirm the SDD spiral growth behavior of the SnS₂. Possible mechanism is proposed in this work to show the generation and propagation of screw dislocations. Furthermore, in order to explore the optoelectronic property of the SnS₂, photodetectors based on 2D SnS₂ crystals are fabricated. The resulting device shows excellent operating characteristics, including good photo‐stability and reproducibility as well as a fast photoresponse time (≈42 ms), which enable the SnS₂ a promising candidate for photodetectors.     

Li2O-B2O3复合掺杂对BaAl2Si2O8陶瓷结构与介电性能的影响

采用传统固相反应工艺,按质量分数合成BaO-Al2O3-SiO2-5%(xLi2O-yB2O3)(x=0. 2~0. 6,y=0. 8~0. 4)陶瓷。研究xL-yB烧结助剂对BAS系微波介质陶瓷的结构和介电性能的影响。通过Clausius-Mossotti公式计算讨论了BAS理论与实验介电常数的差异。研究结果表明:xL-yB烧结助剂中Li+进入钡长石Ba2+位,并产生了O2-空位,促进BAS六方相向单斜相转变。添加适当比例的xL-yB烧结助剂后,BAS陶瓷的烧结温度从1400℃降低到925℃,同时BAS陶瓷样品密度、品质因数(Q×f)值以及谐振频率温度系数(τf)得到改善。当烧结助剂为0. 5L-0. 5B,烧结温度为925℃时,可获得综合性能相对较好的BAS陶瓷,其介电性能:εr=6. 74,Q×f=26670 GHz,τf=-21. 09×10-6℃-1。     

Synthesis of Hexagonal‐Phase NaYF4:Yb,Er and NaYF4:Yb,Tm Nanocrystals with Efficient Up‐Conversion Fluorescence

IR‐to‐visible up‐conversion fluorescent nanocrystals of hexagonal‐phase NaYF₄:20 %Yb,2 %Er and NaYF₄:20 %Yb,2 %Tm have been synthesized by decomposition of multiprecursors of CF₃COONa, (CF₃COO)₃Y, (CF₃COO)₃Yb, and (CF₃COO)₃Er/(CF₃COO)₃Tm in oleylamine at 330 °C. The average particle size is 10.5 ± 0.7 nm (from random measurements of 200 particles from five transmission electron microscopy images) and 11.1 ± 1.3 nm (from dynamic‐light‐scattering measurements). The up‐conversion fluorescence intensity of the hexagonal nanocrystals in this work is much higher than that of other cubic‐phase NaYF₄:Yb,Er nanocrystals, including the ones in this work (by a factor of 7.5). Mechanisms for nucleation and growth of the hexagonal‐phase nanoparticles are proposed. These nanocrystals are easily dispersed in organic solvents, producing a transparent colloidal solution. The hydrophobic surfaces of the particles are made hydrophilic using a bipolar surfactant. These nanoparticles and their dispersions in various media have potential applications in optical nanodevices and bioprobes.     

Stabilization of Isolated Hydrous Amorphous and Tetragonal Zirconia Nanoparticles Through the Formation of a Passivating Alumina Shell

In this work, we explore the high‐temperature phase stability of isolated, alumina‐coated zirconia nanocrystals with a goal of understanding how interfacial energy affects phase stability. Isolated tetragonal and hydrous amorphous zirconia colloids were synthesized and coated with alumina through the hydrolysis of aluminum isopropoxide. Alumina‐coated samples exhibited phase behavior that was markedly different from that of the uncoated analogs. Uncoated tetragonal particles transformed to the monoclinic phase at 1100 °C while alumina‐coated tetragonal particles did not transform until 1400 °C. Uncoated hydrous amorphous particles crystallized to the tetragonal phase after heating at 600 °C and transformed to the monoclinic phase after heating at 800 °C. Alumina‐coated hydrous amorphous particles crystallized only after heating at 1050 °C, and transformed to the monoclinic phase after heating at 1400 °C. Differences in phase behavior are postulated to depend on the zirconia–alumina interface, which must be disrupted before zirconia particles can fuse and facilitate the tetragonal‐to‐monoclinic phase transition. By coating the nanocrystals with a thin alumina shell and studying the resultant phase stability, we explore the effect of reproducibly modified interfacial chemistry on phase behavior in nanoscale ceramic composites.     

Room‐Temperature Partial Conversion of α‐FAPbI3 Perovskite Phase via PbI2 Solvation Enables High‐Performance Solar Cells

The two‐step conversion process consisting of metal halide deposition followed by conversion to hybrid perovskite has been successfully applied toward producing high‐quality solar cells of the archetypal MAPbI₃ hybrid perovskite, but the conversion of other halide perovskites, such as the lower bandgap FAPbI₃, is more challenging and tends to be hampered by the formation of hexagonal nonperovskite polymorph of FAPbI₃, requiring Cs addition and/or extensive thermal annealing. Here, an efficient room‐temperature conversion route of PbI₂ into the α‐FAPbI₃ perovskite phase without the use of cesium is demonstrated. Using in situ grazing incidence wide‐angle X‐ray scattering (GIWAXS) and quartz crystal microbalance with dissipation (QCM‐D), the conversion behaviors of the PbI₂ precursor from its different states are compared. α‐FAPbI₃ forms spontaneously and efficiently at room temperature from P₂ (ordered solvated polymorphs with DMF) without hexagonal phase formation and leads to complete conversion after thermal annealing. The average power conversion efficiency (PCE) of the fabricated solar cells is greatly improved from 16.0(±0.32)% (conversion from annealed PbI₂) to 17.23(±0.28)% (from solvated PbI₂) with a champion device PCE > 18% due to reduction of carrier recombination rate. This work provides new design rules toward the room‐temperature phase transformation and processing of hybrid perovskite films based on FA⁺ cation without the need for Cs⁺ or mixed halide formulation.     

New Ternary Europium Aluminate Luminescent Nanoribbons for Advanced Photonics

Developing novel one‐dimensional (1D) luminescent nanostructures (e.g., nanowires and nanoribbons) is highly desired for enabling progress in nanophotonics and other emerging optical technologies. Previous studies on 1D luminescent nanostructures were mostly focused on elemental and binary semiconductor materials, the light emission of which originates from the radiative recombination of electrons and holes via either intrinsic states or extrinsic defect states. Herein, three kinds of ternary europium aluminate nanoribbons are reported that have localized Eu²⁺ luminescent centers and exhibit new compositions, new crystal lattice structures, and new luminescence properties and mechanisms. These three europium aluminate nanoribbons are: blue luminescent EuAl₆O₁₀ with a new composition and a new tetragonal lattice structure, green luminescent EuAl₂O₄ with a monoclinic lattice structure, and orange luminescent EuAl₂O₄ with a new hexagonal lattice structure and extremely large band width and Stokes shift of emission. These materials have promising applications as nanometer‐scale light generators and waveguides in nanophotonics and as light converting phosphors in warm white light‐emitting diodes.     

The Roles of the Ge‐Te Core Network and the Sb‐Te Pseudo Network During Rapid Nucleation‐Dominated Crystallization of Amorphous Ge2Sb2Te5

Ge₂Sb₂Te₅ (GST) has demonstrated its outstanding importance among rapid phase‐change (PC) materials, being applied for optical and electrical data storage for over three decades. The mechanism of nanosecond phase change in GST, which is vital for its application, has long been disputed: various, quite diverse scenarios have been proposed on the basis of various experimental and theoretical approaches. Nevertheless, one central question still remains unanswered: why is amorphous GST stable at room temperature for long time while it can rapidly transform to the crystalline phase at high temperature? Here it is revealed for the first time, by modelling the amorphous structure based on synchrotron radiation anomalous X‐ray scattering data, that germanium and tellurium atoms form a “core” Ge‐Te network with ring formation. It is also suggested that the Ge‐Te network can stabilize the amorphous phase at room temperature and can persist in the crystalline phase. On the other hand, antimony does not contribute to ring formation but constitutes a “pseudo” network with tellurium, in which the characteristic Sb–Te distance is somewhat longer than the covalent Sb–Te bond distance. This suggests that the Sb‐Te pseudo network may act as a precursor to forming critical nuclei during the crystallization process. The findings conclude that the Ge‐Te core network is responsible for the outstanding stability and rapid phase change of the amorphous phase while the Sb‐Te pseudo network is responsible for triggering critical nucleation.     

Strong Band Bowing Effects and Distinctive Optoelectronic Properties of 2H and 1T′ Phase‐Tunable MoxRe1–xS2 Alloys

Structure and energy band engineering of 2D materials via selective doping or phase modulation provide a significant opportunity to design them for optoelectronic devices. Here, the synthesis of high‐quality Mo_xRe_1–xS₂ alloys with tunable composition and phase structure via chemical vapor deposition growth is reported, and their novel energy band structures and optoelectronic properties are explored. The phase separation and structure reconstruction, which are found to be two serious problems in the synthesis of these alloys, are successfully suppressed through tuning their growth thermodynamics. As a result, the obtained Mo_xRe_1–xS₂ alloys have uniform composition, phase structure, and crystal orientation. Together with X‐ray photoelectron spectroscopy analysis and first‐principle calculation, the Re/Mo doping‐induced Fermi level up‐shift/down‐shift, new electronic states, and “sub‐gap” formation in Mo_xRe_1–xS₂ alloys are revealed. Especially, a strong band bowing effect is discovered in the Mo_xRe_1–xS₂ alloys with structure transition between 1T′ and 2H phases. Furthermore, these alloys reveal tunable conduction behavior from n‐type to bipolar and p‐type in 1T′ phase, as well as novel “bipolar‐like” electron conduction behavior in 2H alloys. The results highlight the unique alloying effects, which do not exist in the single‐phase 2D alloys, and provide the feasibility for potential applications in building novel electronic and optoelectronic devices.     

2D Layered Dipeptide Crystals for Piezoelectric Applications

2D piezoelectric materials such as transition metal dichalcogenides are attracting significant attention because they offer various benefits over bulk piezoelectrics. In this work, the fabrication of layered biomolecular crystals of diphenylalanine (FF) obtained via a co-assembly of l,l - and d,d - enantiomers of FF monomers is reported. Their crystal structure, thermal and chemical stabilities, and piezoelectric properties are investigated. Single crystal X-ray diffraction results show that FF enantiomers are arranged in the form of bilayers consisting of monomers with alternating chirality packed into a tape-like monoclinic structure belonging to a polar space group P2₁. Each bilayer ( ≈ 1.5 nm thick) demonstrates strong out-of-plane piezoelectricity (d₃₃  ≈  20 pm V⁻¹) that is almost an order of magnitude higher than in the archetypical piezoelectric material quartz. The grown crystals demonstrate better thermal and chemical stabilities than self-assembled hexagonal FF nanotubes studied in the past. Piezoelectric bilayers, being held via weak aromatic interaction in the bulk crystals, can be exfoliated by mechanical or chemical methods, thus resulting in a 2D piezoelectric material, which can find various applications in biocompatible and ecologically friendly electromechanical microdevices, such as sensors, actuators, and energy harvesting elements used in implantable and wearable electronics.