Resolution Scanning Transmission Electron Microscopy Research Articles

Crystal structures of two new high-pressure oxynitrides with composition SnGe4N4O4, from single-crystal electron diffraction.

SnGe4N4O4 was synthesized at high pressure (16 and 20 GPa) and high temperature (1200 and 1500°C) in a large-volume press. Powder X-ray diffraction experiments using synchrotron radiation indicate that the derived samples are mixtures of known and unknown phases. However, the powder X-ray diffraction patterns are not sufficient for structural characterization. Transmission electron microscopy studies reveal crystals of several hundreds of nanometres in size with different chemical composition. Among them, crystals of a previously unknown phase with stoichiometry SnGe4N4O4 were detected and investigated using automated diffraction tomography (ADT), a three-dimensional electron diffraction method. Via ADT, the crystal structure could be determined from single nanocrystals in space group P63mc, exhibiting a nolanite-type structure. This was confirmed by density functional theory calculations and atomic resolution scanning transmission electron microscopy images. In one of the syntheses runs a rhombohedral 6R polytype of SnGe4N4O4 could be found together with the nolanite-type SnGe4N4O4. The structure of this polymorph was solved as well using ADT.

Application of the polyhedral template matching method for characterization of 2D atomic resolution electron microscopy images

High-throughput image segmentation of atomic resolution electron microscopy data poses an ongoing challenge for materials characterization. In this paper, we investigate the application of the polyhedral template matching (PTM) method, a technique widely employed for visualizing three-dimensional (3D) atomistic simulations, to the analysis of two-dimensional (2D) atomic resolution electron microscopy images. This technique is complementary with other atomic resolution data reduction techniques, such as the centrosymmetry parameter, that use the measured atomic peak positions as the starting input. Furthermore, since the template matching process also gives a measure of the local rotation, the method can be used to segment images based on local orientation. We begin by presenting a 2D implementation of the PTM method, suitable for atomic resolution images. We then demonstrate the technique's application to atomic resolution scanning transmission electron microscopy images from close-packed metals, providing examples of the analysis of twins and other grain boundaries in FCC gold and martensite phases in 304 L austenitic stainless steel. Finally, we discuss factors, such as positional errors in the image peak locations, that can affect the accuracy and sensitivity of the structural determinations.

Thermomechanical and isothermal fatigue properties of MAR-M247 superalloy

Thermomechanical low-cycle fatigue (TMF) tests were performed on polycrystalline cast nickel-based superalloy MAR-M247 under in-phase (IP) and out-of phase (OP) loading in the temperature range of 500–900 °C. A constant heating and cooling rate of 5 °C.s−1 was selected. Furthermore, isothermal low-cycle fatigue (ILCF) tests with the same cycle period as TMF cycle were carried out at the maximal temperature of TMF cycle (900 °C). All tests ran under strain control and fully reversed cycle on solid cylindrical specimens in laboratory air. High resolution scanning electron microscopy and transmission electron microscopy were utilized to characterize the damage mechanism occurring during TMF and ILCF loading. The cyclic deformation and lifetime behaviour as well as damage mechanisms dependent on loading regime were examined. Results show that the TMF loading has a detrimental effect on the lifetime of MAR-M247. Lifetime reduction is most significant under IP loading. The fracture surfaces are characterized by typical striation fields, where the largest spacing between striations was observed for IP loading with prevailing intercrystalline cracking followed by OP and isothermal ILCF loading with transcrystalline crack propagation. To improve discussion of obtained results, experimental data were compared with damage and life prediction models.

Seeing is believing: Correlating optoelectronic functionality with atomic scale imaging of single semiconductor nanocrystals.

A unique on-chip method for the direct correlation of optical properties, with atomic-scale chemical-structural characteristics for a single quantum dot (QD), is developed and utilized in various examples. This is based on performing single QD optical characterization on a modified glass substrate, followed by the extraction of the relevant region of interest by focused-ion-beam-scanning electron microscope processing into a lamella for high resolution scanning transmission electron microscopy (STEM) characterization with atomic scale resolution. The direct correlation of the optical response under an electric field with STEM analysis of the same particle allows addressing several single particle phenomena: first, the direct correlation of single QD photoluminescence (PL) polarization and its response to the external field with the QD crystal lattice alignment, so far inferred indirectly; second, the identification of unique yet rare few-QD assemblies, correlated directly with their special spectroscopic optical characteristics, serving as a guide for future designed assemblies; and third, the study on the effect of metal island growth on the PL behavior of hybrid semiconductor-metal nanoparticles, with relevance for their possible functionality in photocatalysis. This work, therefore, establishes the use of the direct on-chip optical-structural correlation method for numerous scenarios and timely questions in the field of QD research.

Cr5.7Si2.3P8N24-A Chromium(+IV) Nitridosilicate Phosphate with Amphibole-Type Structure.

The first nitridic analog of an amphibole mineral, the quaternary nitridosilicate phosphate Cr5.7Si2.3P8N24 was synthesized under high-pressure high-temperature conditions at 1400 °C and 12 GPa from the binary nitrides Cr2N, Si3N4 and P3N5, using NH4N3 and NH4F as additional nitrogen source and mineralizing agent, respectively. The crystal structure was elucidated by single-crystal X-ray diffraction with microfocused synchrotron radiation (C2/m, a=9.6002(19), b=17.107(3), c=4.8530(10) Å, β=109.65(3)°). The elemental composition was analyzed by energy dispersive X-ray spectroscopy. The structure consists of vertex-sharing PN4-tetrahedra forming zweier double chains and edge-sharing (Si,Cr)-centered octahedra forming separated ribbons. Atomic resolution scanning transmission electron microscopy shows ordered Si and Cr sites next to a disordered Si/Cr site. Optical spectroscopy indicates a band gap of 2.1 eV. Susceptibility measurements show paramagnetic behavior and support the oxidation state Cr+IV, which is confirmed by EPR. The comprehensive analysis expands the field of Cr-N chemistry and provides access to a nitride analog of one of the most prevalent silicate structures.

Cr5.7Si2.3P8N24—A Chromium(+IV) Nitridosilicate Phosphate with Amphibole‐Type Structure

AbstractThe first nitridic analog of an amphibole mineral, the quaternary nitridosilicate phosphate Cr5.7Si2.3P8N24 was synthesized under high‐pressure high‐temperature conditions at 1400 °C and 12 GPa from the binary nitrides Cr2N, Si3N4 and P3N5, using NH4N3 and NH4F as additional nitrogen source and mineralizing agent, respectively. The crystal structure was elucidated by single‐crystal X‐ray diffraction with microfocused synchrotron radiation (C2/m, a=9.6002(19), b=17.107(3), c=4.8530(10) Å, β=109.65(3)°). The elemental composition was analyzed by energy dispersive X‐ray spectroscopy. The structure consists of vertex‐sharing PN4‐tetrahedra forming zweier double chains and edge‐sharing (Si,Cr)‐centered octahedra forming separated ribbons. Atomic resolution scanning transmission electron microscopy shows ordered Si and Cr sites next to a disordered Si/Cr site. Optical spectroscopy indicates a band gap of 2.1 eV. Susceptibility measurements show paramagnetic behavior and support the oxidation state Cr+IV, which is confirmed by EPR. The comprehensive analysis expands the field of Cr−N chemistry and provides access to a nitride analog of one of the most prevalent silicate structures.

Latent ion tracks were finally observed in diamond

Injecting high-energy heavy ions in the electronic stopping regime into solids can create cylindrical damage zones called latent ion tracks. Although these tracks form in many materials, none have ever been observed in diamond, even when irradiated with high-energy GeV uranium ions. Here we report the first observation of ion track formation in diamond irradiated with 2–9 MeV C60 fullerene ions. Depending on the ion energy, the mean track length (diameter) changed from 17 (3.2) nm to 52 (7.1) nm. High resolution scanning transmission electron microscopy (HR-STEM) indicated the amorphization in the tracks, in which π-bonding signal from graphite was detected by the electron energy loss spectroscopy (EELS). Since the melting transition is not induced in diamond at atmospheric pressure, conventional inelastic thermal spike calculations cannot be applied. Two-temperature molecular dynamics simulations succeeded in the reproduction of both the track formation under MeV C60 irradiations and the no-track formation under GeV monoatomic ion irradiations.

Unraveling the mechanism of vanadium self-intercalation in 1T-VSe2: atomic-scale evidence for phase transition and superstructure model for intercalation compound

Self-intercalation is an efficient strategy for tailoring the property of layer structured materials like transition metal dichalcogenides (TMDCs), while the associated kinetics and mechanism remain scarcely explored. In this study, we investigate the atomic-scale dynamics and mechanism of vanadium (V) self-intercalation in multi-layer 1T-VSe2 using in situ high resolution scanning transmission electron microscopy. The results reveal that the self-intercalation of V induces structural transformation of pristine VSe2 into three V-enrich intercalated compounds, i.e. V5Se8, V3Se4 and VSe. The self-intercalated V follows an ordered arrangement of 2×2 , 2×1 , and 1×1 within the interlayer octahedral sites, corresponding to an intercalation concentration of 25%, 50% and 100% in V5Se8, V3Se4 and VSe, respectively. The V intercalants induced lattice distortions to the host 1T-VSe2 such as the dimerization of neighboring lattice V is observed experimentally, which are further supported by density functional theory (DFT) calculations. Finally, a superstructure model generalizing the possible structures of self-intercalated compounds in layered TMDCs is proposed and then validated by the DFT determined formation energy landscape. This study provides comprehensive insights on the kinetics and mechanism of the self-intercalation in layered TMDC materials, contributing to the precise control for the structure and stoichiometry of self-intercalated TMDC compounds.

Tuning the atomic and electronic structures of mirror twin boundaries in molecular beam epitaxy grown MoSe2 monolayers via rhenium doping

Interplay between defects like mirror twin boundaries (MTBs) and dopants may provide additional opportunities for furthering the research on two-dimensional monolayer (ML) transition metal dichalcogenides. In this work, we successfully dope rhenium (Re) into molecular beam epitaxy grown ML MoSe2 and confirm the formation of a new type of MTBs, named 4|4E-M (M represents metal, Mo/Re) according to the configuration. Data from statistic atomic resolution scanning transmission electron microscopy also reveals a preferable MTB enrichment of Re dopants, rather than intra-domain. In conjunction with density functional theory calculation results, we propose the possible routes for Re doping induced formation of 4|4E-M MTBs. Electronic structures of Re doped MTBs in ML MoSe2 are also predicted theoretically and then preliminarily tested by scanning tunneling microscopy and spectroscopy.

Ce Doped SnO2 Nanoparticcles: Investigation of Structural and Optical Properties

Tin oxide (SnO2 ) and (1 wt%, 3 wt%, 5wt %) Ce-doped SnO2 nanoparticles were synthesized by the Co-precipitation method. X-ray diffraction investigations have been confirmed that the synthesized nanoparticles are polycrystalline in nature with tetragonal rutile phase. The particle size is determined using Scherrer’s formula and it is found to increase with the “Ce” dopant. High resolution scanning electron microscope (HRSEM) and transmission electron microscopy (TEM) analysis showed spherical morphology composed of fine crystallites with diameters around ∼200 nm. Optical band gap was decreased by the doping confirming the direct energy transfer between f-electrons of rare earth ion and the SnO2 conduction or valence band. This study demonstrates the Ce doped SnO2 nanoparticles for applications in optoelectronic devices.

Unraveling the atomic mechanism of the disorder–order phase transition from γ-Ga2O3 to β-Ga2O3

In this paper, we employ in situ transmission electron microscopy to study the disorder–order phase transition from amorphous Ga2O3 to γ-Ga2O3 and then to β-Ga2O3. The in situ studies are complemented by ex situ annealing experiments, of which the results are analyzed by x-ray diffraction and high resolution (scanning) transmission electron microscopy. Amorphous Ga2O3 deposited at 100 °C by molecular beam epitaxy crystallizes at 470 °C in the γ phase (Fd3̄m), which undergoes a phase transition to the β phase above 500 °C. Between 500° and 900 °C, we find a mixture of γ-Ga2O3 and β-Ga2O3 coexisting. Above 950 °C, we find only β-Ga2O3. Through our analyses and by considering symmetry relations, we have constructed a coincidence site lattice of both structures containing a common fcc-type sublattice occupied by oxygen atoms, the cation sites of β-Ga2O3 common to both phases, and partially occupied cation sites in the γ phase corresponding to the interstitial sites in the β phase. We assign the atomic displacements within this lattice responsible for transforming the initially disordered spinel structure with partially occupied cation sites into the well-ordered lattice of β-Ga2O3. We identify this transition as a reconstructive disorder-to-order phase transition, mediated by the exchange of cations to next nearest neighbor sites. Our model not only explains recent observations of the formation of γ-Ga2O3 during implantation for n-type doping and the subsequent recovery of β-Ga2O3 following annealing but also holds potential for inspiring understanding in other materials with similar phase transitions.

Improved Thermal Boundary Conductance in Annealed Direct Wafer Bonded Si-Ge

The evolution of structural and thermal properties of wafer bonded Si|Ge with annealing is investigated in this study. Theoretical work suggests that the thermal boundary conductance between amorphous Si and Ge provide improved thermal properties as compared to a crystalline Si to Ge interface, and wafer bonded systems provides a fabrication method to experimentally test this theory [1]. EVG® ComBond® equipment was used for bonding under high vacuum (~10-8 mtorr) at room temperature to remove unwanted native surface oxides [2-4]. The bombardment of the surfaces prior to bonding produces an amorphous region at the bonded interface that has been seen in many other bonded systems [5,6]. In the as-bonded sample, high resolution scanning transmission electron microscopy revealed a ~1.2 nm amorphous region at the bonded interface. Subsequent annealing was done in an effort to recrystallize the amorphous interface. Previous work has shown that recrystallization between Si|Si wafer bonded samples occurred when annealed at 450 °C for 12 hours [3]. After annealing at 600 °C for 2 hours, the amorphous interface was observed to decrease to ~0.9 nm. Complete recrystallization is observed when the sample was annealed at 600 °C for 48 hours.Preliminary thermal results show that the thermal boundary conductance (TBC) of the as bonded sample is 47 ± 5 MW/(m2K). The TBC for the sample annealed at 600 °C for 48 hours is 94 ± 5 MW/(m2K), an improvement by a factor of two compared to the as-bonded interface. These results demonstrate that the TBC can be improved through annealing of the interface and that improvements due to an amorphous-amorphous interface do not dominate the thermal boundary conductance in this system. We consider that the improvements are due to recrystallization of the interface and/or to increased interdiffusion, which has been observed to increase TBC in epitaxially grown Si-Ge interfaces [7]. K. Gordiz, et al., J. Appl. Phys., 121(2), p.025102 (2017)V. Dragoi, et al., ECS Trans., 86(5), 23 (2018)C. Flötgen, et al., ECS Trans., 64(5), 103 (2014)M.E. Liao, et al., ECS Trans., 86(5), 55 (2018)Y. Xu, et al., Ceramics International, 45, 6552 (2019)F. Mu, et al., Appl. Surf. Sci., 416, 1007 (2017)Z. Cheng, et al., Nature communications, 12(1), p.6901 (2021) Figure 1

Stability of Interface Morphology and Thermal Boundary Conductance of Direct Wafer Bonded GaN|Si Heterojunction Interfaces Annealed at Growth and Annealing Temperatures

The properties of thin (~2 um) GaN templates on a silicon support substrate were studied to assess the stability of the direct wafer bonded GaN / Si interface. EVG® ComBond® equipment was used for bonding under high vacuum (~10-8 mtorr) at room temperature to remove unwanted native surface oxides [1,2]. For the bonded samples, the [1010] GaN edge was aligned parallel to the Si [110] edge. An X-ray diffraction reciprocal space map of the (004) Si and (0004) GaN revealed that there is a ~0.2° tilt between the GaN and Si layers and is simply due to the relative miscut between the two wafers. The treatment of the surfaces prior to bonding produces an amorphous region at the bonded interface that has been seen in many other bonded systems [3-5]. In the as-bonded sample, high resolution scanning transmission electron microscopy revealed a ~2 nm amorphous region on the Si side of the bonded interface, as confirmed by energy dispersive x-ray spectroscopy (EDX). Subsequent annealing was performed in an effort to recrystallize the amorphous interface. Previous work has shown that recrystallization between Si|Si wafer bonded samples occurred when annealed at 450 °C for 12 hours [3]. However, in the GaN|Si system, we found that the amorphous interface did not recrystallize when annealed under those conditions. Annealing at temperatures up to 450 °C and 120 hours showed only initial stages of interdiffusion and a stable interface. However, after annealing at 700 °C for 24 hours, high resolution EDX revealed the formation of amorphous SiN as well as the diffusion of gallium into silicon.Preliminary thermal results show that the thermal boundary conductance (TBC) of the as bonded sample is ~140 MW/(m2K). The TBC results of wafer bonded GaN|Si reported here is higher than previously reported TBC values of epitaxially grown interfaces such as GaN on Si [6], GaN on SiC [7], and GaN on diamond [8]. The TBC for the annealed interface is degraded by a factor of two compared to the as-bonded interface for the sample that was annealed at 700 °C for 24 hours. These results demonstrate that high TBC can be achieved through wafer bonding of GaN with materials such as silicon and that such interfaces are stable even up to device operation up to 300 °C. However, chemically rough interfaces formed due to high temperature annealing are detrimental to thermal transport across these interfaces. V. Dragoi, et al., ECS Trans., 86(5), 23 (2018)C. Flötgen, et al., ECS Trans., 64(5), 103 (2014)M.E. Liao, et al., ECS Trans., 86(5), 55 (2018)Y. Xu, et al., Ceramics International, 45, 6552 (2019)F. Mu, et al., Appl. Surf. Sci., 416, 1007 (2017)L. Yates, et al., ASME InterPACK (2015)J. Cho, et al., Phys. Rev. B, 89, 115301 (2014)H. Sun, et al., APL, 106(11), 111906 (2015) Figure 1

Gas-Phase CuPd Bimetallic Cluster-Modified Electrodes as Model Electrocatalysts for CO2 Conversion

Copper-based electrocatalysts possess the unique ability to convert CO2 to multicarbon products such as ethylene – a precursor in many industrial processes. However, their stability and product selectivity remain insufficient. A promising approach to overcome these shortcomings and design better CO2RR electrocatalysts is to tune Cu selectivity by forming bimetallic Cu-M systems [1] while establishing their detailed structure-selectivity relationship. To achieve this, we used laser ablation Cluster Beam Deposition (CBD) [2] to produce well-defined bimetallic cluster-modified electrodes [2]. More specifically, CuPd clusters with an original average size of 2.5 nm and mass loadings of 1-3 µg cm-2 were deposited onto a carbon support. To produce a model catalyst which allows the independent investigation of the electronic and geometric structural effects induced to Cu by the second element, a Cu0.9Pd0.1 composition was selected [3]. CuPd cluster-decorated electrodes were tested for CO2 electrolysis and methane (C1), and ethylene (C2) were found among the products. Observed from the electrocatalytic activity trends, the evolution of C2 products is correlated with the cluster mass loading. In addition, the cluster coverage has an effect on the onset of the competitive H2 evolution (HER); with the lowest cluster-loaded electrodes presenting more intense HER due to limited cluster coverage of the carbon substrate. High resolution (Scanning) Transmission Electron Microscopy ((S)TEM) coupled with Energy-dispersive X-ray spectroscopy (EDX), and X-ray Photoelectron Spectroscopy (XPS) analyses of the as-prepared cluster-modified electrodes, show that the clusters feature some degree of CuPd alloying in the ambient. CuPd cluster-based systems will then be investigated in-situ using X-ray absorption spectroscopy to unravel their structure-catalytic performance relationship.

Elucidation of the Interplay between the Features of the Precursor and the Physicochemical Properties of “Core-Shell” Hierarchical Carbon Nitride Electrocatalysts for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is a major bottleneck in the operation of proton-exchange membrane fuel cells (PEMFCs) since it introduces large overpotentials (no less than 250-300 mV), which degrade significantly the energy conversion efficiency of the device. In addition, the electrocatalysts (ECs) yielding the lowest ORR overpotentials in the acidic environment found at the PEMFC cathode are based on Pt, a very scarce critical raw material (CRM). Pt raises concerns as it might trigger serious supply bottlenecks in the event of a practical widespread implementation of PEMFCs in the framework of today’s energy transition. Hence, the development of high-performing and durable ECs for the ORR is a major stepping stone towards the large-scale rollout of PEMFCs.In our research group a new breakthrough approach has been devised for the synthesis of ORR ECs, that is completely different form the preparation routes that are commonly adopted in the state of the art [1, 2]. Briefly, the approach consists in the pyrolysis and subsequent treatment of a precursor obtained by coupling suitable carbon species/sacrificial supports with a zeolitic inorganic-organic polymer electrolyte (Z-IOPE). The latter comprises anionic complexes including the Pt “catalyst” and other “co-catalysts”, which are bridged by organic binders [1,2].On one hand, the proposed approach has already shown its potential to yield ECs exhibiting a performance and durability improved with respect to Pt/C benchmark ECs. On the other hand, the proposed approach is extremely flexible, especially with respect to the synthesis of the initial EC precursor. In principle, it is possible to modulate a number of crucial features of such EC precursor, with a particular reference to: (i) the chemical composition (e.g., in terms of Pt, other “co-catalysts” and N; the latter plays a crucial role to stabilize the active sites into “coordination nests” on the EC surface, raising durability); and (ii) the morphology, that can be “templated” on suitable species (e.g., carbon species, sacrificial supports). The very flexibility of the proposed synthetic process needs to be leveraged appropriately to maximize the performance of the final ORR ECs, at the same time minimizing the efforts wasted on approaches with a low development potential.Herein it is elucidated how the modulation in the parameters involved in the synthesis of the EC precursor (e.g., the binder and the approaches to couple the Z-IOPE with the support) impact the physicochemical and electrochemical properties of the final ECs, with a particular reference to the chemical composition, morphology and distribution of the ORR active sites. An extensive characterization of the final ECs is carried out. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and microanalysis are adopted to determine the bulk chemical composition. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is used to elucidate the surface chemical composition and the chemical states of the surface elements. Ultra-high resolution scanning electron microscopy (UHR-SEM) and transmission electron microscopy (TEM) allow for the elucidation of EC morphology. The porosimetric features are probed by automatic gas adsorption instrumentations. Wide-angle X-ray diffraction (WAXD) is crucial to resolve the crystal structure. Cyclic voltammetry with the rotating ring-disk electrode method (CV-TF-RRDE) is used to clarify the “ex-situ” electrochemical performance and ORR reaction mechanism. Finally, the most promising ECs are used in the fabrication of single PEMFCs, that are tested as a function of the operating conditions.It is revealed that, in comparison with the Pt/C benchamark, some ECs are characterized by a much higher intrinsic kinetic performance due to the electronic and bifunctional effects bestowed by the carbon nitride support and the «co-catalysts». Finally, the binder has a marked effect on the stoichiometry, size, and distribution of the PtMx aggregates bearing the active sites.

The effect of aluminium concentration on the resistance of Si3N4 to ion track formation

The role of Al impurities on the structural response of polycrystalline silicon nitride to swift Xe and Bi ions at single ion track and overlapped ion track fluences is investigated. Si3N4 with Al impurity concentrations of 3 at.%, 1 at.% and 0.1. at.% were examined by means of high resolution scanning transmission electron microscopy. The threshold electronic energy loss (Set) required to form amorphous tracks was found to decrease with increasing Al fraction from above 33 keV/nm (0.1 at.% Al) to below 22 keV/nm for specimens containing about 3 at.% Al. All specimens were partially amorphized at overlapping ion fluence and the amorphous fraction monotonically increased with increasing Al content at the same ion fluence.

Atomic scale volume and grain boundary diffusion elucidated by in situ STEM

Diffusion is one of the most important phenomena studied in science ranging from physics to biology and, in abstract form, even in social sciences. In the field of materials science, diffusion in crystalline solids is of particular interest as it plays a pivotal role in materials synthesis, processing and applications. While this subject has been studied extensively for a long time there are still some fundamental knowledge gaps to be filled. In particular, atomic scale observations of thermally stimulated volume diffusion and its mechanisms are still lacking. In addition, the mechanisms and kinetics of diffusion along defects such as grain boundaries are not yet fully understood. In this work we show volume diffusion processes of tungsten atoms in a metal matrix on the atomic scale. Using in situ high resolution scanning transmission electron microscopy we are able to follow the random movement of single atoms within a lattice at elevated temperatures. The direct observation allows us to confirm random walk processes, quantify diffusion kinetics and distinctly separate diffusion in the volume from diffusion along defects. This work solidifies and refines our knowledge of the broadly essential mechanism of volume diffusion.

Direct observation of intrinsic room-temperature ferroelectricity in 2D layered CuCrP2S6

Multiferroic materials have ignited enormous interest owing to their co-existence of ferroelectricity and ferromagnetism, which hold substantial promise for advanced device applications. However, the size effect, dangling bonds, and interface effect in traditional multiferroics severely hinder their potential in nanoscale device applications. Recent theoretical and experimental studies have evidenced the possibility of realizing two-dimensional (2D) multiferroicity in van der Waals (vdW) layered CuCrP2S6. However, the incorporation of magnetic Cr ions in the ferroelectric framework leads to antiferroelectric and antiferromagnetic orderings, while macroscopic spontaneous polarization is always absent. Herein, we report the direct observation of robust out-of-plane ferroelectricity in 2D vdW CuCrP2S6 at room temperature with a comprehensive investigation. Modification of the ferroelectric polarization states in 2D CuCrP2S6 nanoflakes is experimentally demonstrated. Moreover, external electric field-induced polarization switching and hysteresis loops are obtained in CuCrP2S6 down to ~2.6 nm (4 layers). By using atomically resolved scanning transmission electron microscopy, we unveil the origin of the emerged room-temperature ferroelectricity in 2D CuCrP2S6. Our work can facilitate the development of multifunctional nanodevices and provide important insights into the nature of ferroelectric ordering of this 2D vdW material.

Large bi-axial tensile strain effect in epitaxial BiFeO3 film grown on single crystal PrScO3

A BiFeO3 film is grown epitaxially on a PrScO3 single crystal substrate which imparts ~ 1.45% of biaxial tensile strain to BiFeO3 resulting from lattice misfit. The biaxial tensile strain effect on BiFeO3 is investigated in terms of crystal structure, Poisson ratio, and ferroelectric domain structure. Lattice resolution scanning transmission electron microscopy, precession electron diffraction, and X-ray diffraction results clearly show that in-plane interplanar distance of BiFeO3 is the same as that of PrScO3 with no sign of misfit dislocations, indicating that the biaxial tensile strain caused by lattice mismatch between BiFeO3 and PrScO3 are stored as elastic energy within BiFeO3 film. Nano-beam electron diffraction patterns compared with structure factor calculation found that the BiFeO3 maintains rhombohedral symmetry, i.e., space group of R3c. The pattern analysis also revealed two crystallographically distinguishable domains. Their relations with ferroelectric domain structures in terms of size and spontaneous polarization orientations within the domains are further understood using four-dimensional scanning transmission electron microscopy technique.

Nanoscale domain engineering in SrRuO3 thin films

We investigate nanoscale domain engineering via epitaxial coupling in a set of SrRuO3/PbTiO3/SrRuO3 heterostructures epitaxially grown on (110)o-oriented DyScO3 substrates. The SrRuO3 layer thickness is kept at 55 unit cells, whereas the PbTiO3 layer is grown to thicknesses of 23, 45, and 90 unit cells. Through a combination of atomic force microscopy, x-ray diffraction, and high resolution scanning transmission electron microscopy studies, we find that above a certain critical thickness of the ferroelectric layer, the large structural distortions associated with the ferroelastic domains propagate through the top SrRuO3 layer, locally modifying the orientation of the orthorhombic SrRuO3 and creating a modulated structure that extends beyond the ferroelectric layer boundaries.