Atomic-resolution Transmission Electron Microscopy Research Articles

Highly Air‐Stable Single‐Crystalline β‐CsPbI3 Nanorods: A Platform for Inverted Perovskite Solar Cells

AbstractThe synthesis of single‐crystalline β‐CsPbI3 perovskite nanorods (NRs) using a colloidal process is reported, exhibiting their improved photostability under 45–55% humidity. The crystal structure of CsPbI3 NRs films is investigated using Rietveld refined X‐ray diffraction (XRD) patterns to determine crystallographic parameters and the phase transformation from orthorhombic (γ‐CsPbI3) to tetragonal (β‐CsPbI3) on annealing at 150 °C. Atomic resolution transmission electron microscopy images are utilized to determine the probable atomic distribution of Cs, Pb, and I atoms in a single β‐phase CsPbI3 NR, in agreement with the XRD structure and selected area electron diffraction pattern, indicating the growth of single crystalline β‐CsPbI3 NR. The calculation of the electronic band structure of tetragonal β‐CsPbI3 using density functional theory (DFT) reveals a direct transition with a lower band gap and a higher absorption coefficient in the solar spectrum, as compared to its γ‐phase. An air‐stable (45–55% humidity) inverted perovskite solar cell, employing β‐CsPbI3 NRs without any encapsulation, yields an efficiency of 7.3% with 78% enhancement over the γ‐phase, showing its potential for future low cost photovoltaic devices.

Interdiffusion in Bimetallic Au-Fe Nanowhiskers Controlled by Interface Mobility

Nanoscale interdiffusion at the heterophase interfaces is the elementary process that controls solid-solid phase transformations and intermixing in multilayers. Here, we provide the first experimental proof that the nanoscale multiphase interdiffusion is controlled by the interface mobility. We observed the anisotropic diffusion intermixing in Au-Fe bimetallic nanowhiskers, where the diffusion penetration depth of Fe into single crystalline Au nanowhisker across the incoherent Au-Fe interface was much greater than across its coherent counterpart. By applying a simple kinetic model to the results of diffusion measurements, we obtained the absolute values of mobilities of coherent and incoherent Fe-Au interfaces, and the Arrhenius parameters describing their temperature dependence. Atomic resolution transmission electron microscopy confirmed that the lower mobility of the coherent interface is associated with the difficulties of the nucleation of interface disconnections. We proposed that the sluggish movement of the latter represents a kinetic bottleneck which determines the rate of intermixing. Our results indicate that the anisotropy of interface mobilities is a leading factor determining the anisotropic shape of precipitates formed in many heterogeneous solid-solid phase transformations.

Atomic reconstruction in twisted bilayers of transition metal dichalcogenides.

Van der Waals heterostructures form a unique class of layered artificial solids in which physical properties can be manipulated through controlled composition, order and relative rotation of adjacent atomic planes. Here we use atomic-resolution transmission electron microscopy to reveal the lattice reconstruction in twisted bilayers of the transition metal dichalcogenides, MoS2 and WS2. For twisted 3R bilayers, a tessellated pattern of mirror-reflected triangular 3R domains emerges, separated by a network of partial dislocations for twist angles θ < 2°. The electronic properties of these 3R domains, featuring layer-polarized conduction-band states caused by lack of both inversion and mirror symmetry, appear to be qualitatively different from those of 2H transition metal dichalcogenides. For twisted 2H bilayers, stable 2H domains dominate, with nuclei of a second metastable phase. This appears as a kagome-like pattern at θ ≈ 2°, transitioning at θ → 0 to a hexagonal array of screw dislocations separating large-area 2H domains. Tunnelling measurements show that such reconstruction creates strong piezoelectric textures, opening a new avenue for engineering of 2D material properties.

Creating thin magnetic layers at the surface of Sb2Te3 topological insulators using a low-energy chromium ion beam

The surfaces of Sb2Te3 topological insulator crystals were implanted using a 40 keV chromium ion beam. To facilitate uniform doping, the Sb2Te3 was passivated with a thin TiO2 film before the implantation step. The resulting chemical structure was studied using atomic-resolution transmission electron microscopy. A fluence of 7 × 1015 ions/cm2 at 40 keV lead to amorphization of the Sb2Te3 surface, with chromium predominantly incorporated in the amorphous layer. Heating to 200 °C caused the amorphous region to recrystallize and led to the formation of a thin chromium-rich interfacial layer. Near-edge x-ray absorption spectroscopy indicates a uniform valence state of Cr3+ throughout, with no evidence of metallic clustering. High-temperature superparamagnetic behavior was detected up to 300 K, with an increased magnetic moment below 50 K.

High performance UV photodetector based on MoS2 layers grown by pulsed laser deposition technique

Highly efficient ultraviolet (UV) photodetector based on Molybdenum Disulfide (MoS 2 ) layers has been fabricated using pulsed laser deposition (PLD) technique. Systematic layer dependent photoresponse studies have been performed from single layer to 10 layers of MoS 2 by varying the laser pulses to see the effect of the number of layers on the photoelectrical measurements. Raman and Photoluminescence studies have been carried out to ensure the growth of high-quality MoS 2 layers. Layers of MoS 2 grown at 100 pulses were found to exhibit the characteristic Raman phonon modes i.e. E 1 2g and A 1g at 383.8 cm −1 and 405.1 cm −1 respectively and Photoluminescence (PL) spectra show B exciton peak for MoS 2 at around 625 nm suggesting the growth of high-quality MoS 2 layers. Atomic force microscopy (AFM) thickness profiling and cross sectional-high resolution transmission electron microscopy (HRTEM) analysis gives the thickness of grown MoS 2 to be 2.074 nm and 1.94 nm, respectively, confirming the growth of trilayers of MoS 2 . X-ray photoelectron spectroscopy (XPS) spectra of the grown trilayer sample show characteristic peaks corresponding to Molybdenum and Sulphur doublet (Mo 4+ 3d 5/2,3/2 and S 2p 3/2 , 1/2 ) confirming the chemical state of pure MoS 2 phase without the presence of any Molybdenum oxide state. Dynamic photoelectrical studies with Indium Tin Oxide (ITO) as contact electrode upon UV laser illumination show superior responsivity of 3 × 10 4 A/W at 24 μW optical power of the incident laser (λ = 365 nm) and very high detectivity of 1.81 × 10 14 Jones at a low applied bias of 2 V. The obtained results are highly encouraging for the realization of low power consumption and highly efficient UV photodetectors based on MoS 2 layers. • Optimization of high-quality large area MoS2 layers utilizing the PLD technique. • Demonstration of low power operated UV photodetector on optimized MoS2 layers. • Explained the Physics of the UV photodetector using energy band diagrams. • Work justifies the development of efficient photodetectors using PLD technique.

Defect evolution in Mg ions implanted GaN upon high temperature and ultrahigh N2 partial pressure annealing: Transmission electron microscopy analysis

Defects in Mg ion-implanted GaN epitaxial layers formed after annealing at 1573 K and at 1753 K were analyzed by transmission electron microscopy. Degradation of the GaN surface, which occurs at temperatures higher than about 1573 K, was avoided by ultra-high-pressure annealing under a N2 atmosphere at 1 GPa. Annealing for damage recovery in ion-implanted compound semiconductors generally requires temperatures at about two-thirds of their melting point, which is reportedly 2518 K or higher for GaN. Thus, defect analysis in ion-implanted GaN annealed at temperatures higher than 1573 K is necessary to understand the defect recovery. Atomic-resolution transmission electron microscopy analysis showed that interstitial-type extended defects and inversion domains with Mg segregation were formed during the annealing at 1573 K. These defects were not observed in a sample annealed at 1753 K; instead, vacancy-type extended defects were formed. Such evolution of defects can be explained by previous theoretical studies that indicated that the migration energy of vacancy-type defects is higher than that of interstitial-type defects. Moreover, the formation of vacancy-type extended defects implies a reduction in the concentrations of vacancies and their complexes. Since the vacancies and their complexes can compensate for Mg acceptors, their reduced concentration should increase the acceptor activation efficiency. Also, the disappearance of Mg segregation during high-temperature annealing should increase the activation efficiency since the segregated Mg atoms are electrically inactive. It is thus concluded that the evolution of defects caused by high-temperature annealing above 1573 K increases the activation efficiency of acceptors in Mg ion-implanted GaN.

Investigation of the structure and chemical nature of Pd fission product agglomerations in irradiated TRISO particle SiC

Tristructural-isotropic (TRISO)-coated fuel particles are used in high-temperature gas-cooled nuclear reactors. Although the polycrystalline 3C–SiC layer acts as the main barrier to fission product release, post-irradiation examinations have shown that certain metallic fission products are found on the outside of the TRISO coated particle, with no observable evidence of micro-cracks or mechanical failure. In this study, an atomic resolution transmission electron microscopy investigation of a SiC layer from a neutron irradiated (19.38% fissions per initial metal atom average burnup; 1072 °C time-averaged temperature) TRISO-coated particle from the first fuel irradiation experiment as part of the US DOE Advanced Gas Reactor Fuel Qualification and Development (AGR) Program, was conducted. The fission product Pd was found to be present at dislocation cores associated with twins, stacking faults, and their intersections as atomic scale silicide inclusions. EELS elemental quantification of the silicide region showed a reduction of C and increase of Si in the Pd containing inclusion. Pd was also observed as distributed along grain boundaries down to the atomic scale. The local corrosion and dissociation of C from Pd containing grain boundary agglomerations was observed at the atomic level. Elemental quantification of atomically dispersed Pd grain boundary regions using EELS spectrum imaging similarly showed a measurable decrease in C concentration at Pd regions and an increase in Si concentration.

Polarization Screening Mechanisms at La0.7Sr0.3MnO3-PbTiO3 Interfaces.

The structural, electronic, and magnetic properties of interfaces between epitaxial La0.7Sr0.3MnO3 and PbTiO3 have been explored via atomic resolution transmission electron microscopy of a functional multiferroic tunnel junction. Measurements of the polar displacements and octahedral tilting show the competition between the two distortions at the interface and demonstrate strong dependence on the polarization orientation. The density functional theory provides information on the electronic and magnetic properties, where the interface termination plays a crucial role in the screening mechanisms.

Forming indium-carbon (In–C) bonds at the edges of graphitic nanoplatelets

Indium (In), one of the soft and malleable post-transition metals, was introduced along the broken edges of graphitic nanoplatelets (GnPs) by mechanochemically ball-milling graphite in the presence of solid state In beads. After completely leaching off unreacted In using royal water (aqua regia), the formation of In–C bonds in the resulting In-doped graphitic nanoplatelets (InGnPs) was confirmed using various analytical techniques, including atomic-resolution transmission electron microscopy (AR-TEM). Scanning TEM (STEM) image shows that In elements instead of In clusters were uniformly distributed in the InGnPs, suggesting the formation of In–C bonds. The content of In in the InGnPs was 0.34 at% (3.01 wt%), as determined by X-ray photoelectron spectroscopy (XPS). The mechanochemically induced chemical reaction was powerful enough to form In–C bonds. Further, the InGnPs demonstrated catalytic activity toward the oxygen reduction reaction (ORR) comparable to commercial Pt/C catalysts, as well as excellent durability and tolerance against impurities (methanol and CO) in alkaline medium.

Improved photochromic stability in less deficient cesium tungsten bronze nanoparticles

Hexagonal cesium tungsten bronze (Cs0.32WO3) nanoparticles are well known as near-infrared (NIR) shielding materials. However, one of the critical issues for industrial applications involves a photochromic instability, i.e., a blue color change of this material under ultraviolet (UV) irradiation. This color change is attributed to the formation of HxWO3 phase through doping of H+ ions into Cs-deficient sites present at the surface of Cs0.32WO3 nanoparticles. Therefore, a new approach was adopted to prevent the color change by synthesizing a Cs0.32WO3 nanoparticle with less Cs-deficient sites using spray pyrolysis followed by heat treatment and short-time milling. The Cs0.32WO3 nanoparticles of spray pyrolysis route exhibited an improved photochromic stability under UV irradiation compared to those synthesized through a solid-state-reaction route. Detailed analysis using atomic resolution transmission electron microscopy revealed that the Cs0.32WO3 nanoparticles of spray pyrolysis route retain Cs ions up to a depth of less than 1 nm from the surface. This work demonstrates that the less-Cs-deficient Cs0.32WO3 nanoparticles are beneficial to improve the photochromic stability.

Dynamic imaging of Ostwald ripening in copper oxide nanoparticles by atomic resolution transmission Electron microscope

Structural evolution of copper oxide nanoparticles is examined, especially with respect to Ostwald ripening under electron beam irradiation. Dissolution of the smaller particles into the larger one was clearly observed at the atomic scale using advanced transmission electron microscope.

Understanding solid solution strengthening at elevated temperatures in a creep-resistant Mg–Gd–Ca alloy

The present work studies the strengthening mechanisms of a creep-resistant Mg-0.5Gd-1.2Ca (at.%) alloy at both room and elevated temperatures. Although peak-ageing (T6) at 180 °C for 32 h led to a significant increase in room temperature strength due to the precipitation strengthening by three types of precipitates (Mg2Ca, Mg5Gd on prismatic planes and a new type of Mg–Gd–Ca intermetallic compound on the basal plane), the as-solid solution treated (T4) alloy exhibited better resistance to temperature softening during compression and to stress relaxation at 180 °C and better creep resistance at 210 °C/100 MPa. The Gd–Ca co-clusters with short-range order in the Mg solid solution, which was verified, at the first time, by atom probe tomography (APT) analysis and atomic-resolution high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), were responsible for the solid solution hardening, offering a more effective strengthening effect through local order-strengthening. Such solid solution strengthening increased the thermal stability of the alloy structure at elevated temperatures, at least at early stage of the creep. Subsequently, dynamic precipitation started contributing to the creep resistance due to the formation of higher density of precipitates. However, in the T6 alloy, creep testing at elevated temperatures, particularly at 210 °C that was higher than the ageing temperature, led to coarsening of the precipitates, which acted as over ageing. As a result of such over ageing, the resistance of the T6 alloy to heat-induced softening was weakened, leading to lower creep resistance than the T4 alloy.

Atomic Structure and Dynamics of Epitaxial Platinum Bilayers on Graphene.

Platinum atomic layers grown on graphene were investigated by atomic resolution transmission electron microscopy (TEM). These TEM images reveal the epitaxial relationship between the atomically thin platinum layers and graphene, with two optimal epitaxies observed. The energetics of these epitaxies influences the grain structure of the platinum film, facilitating grain growth via in-plane rotation and assimilation of neighbor grains, rather than grain coarsening from the movement of grain boundaries. This growth process was enabled due to the availability of several possible low-energy intermediate states for the rotating grains, the Pt-Gr epitaxies, which are minima in surface energy, and coincident site lattice grain boundaries, which are minima in grain boundary energy. Density functional theory calculations reveal a complex interplay of considerations for minimizing the platinum grain energy, with free platinum edges also having an effect on the relative energetics. We thus find that the platinum atomic layer grains undergo significant reorientation to minimize interface energy (via epitaxy), grain boundary energy (via low-energy orientations), and free edge energy. These results will be important for the design of two-dimensional graphene-supported platinum catalysts and obtaining large-area uniform platinum atomic layer films and also provide fundamental experimental insight into the growth of heteroepitaxial thin films.

Active‐Site Imprinting: Preparation of Fe–N–C Catalysts from Zinc Ion–Templated Ionothermal Nitrogen‐Doped Carbons

AbstractAtomically dispersed Fe–N–C catalysts are considered the most promising precious‐metal‐free alternative to state‐of‐the‐art Pt‐based oxygen reduction electrocatalysts for proton‐exchange membrane fuel cells. The exceptional progress in the field of research in the last ≈30 years is currently limited by the moderate active site density that can be obtained. Behind this stands the dilemma of metastability of the desired FeN4 sites at the high temperatures that are believed to be a requirement for their formation. It is herein shown that Zn2+ ions can be utilized in the novel concept of active‐site imprinting based on a pyrolytic template ion reaction throughout the formation of nitrogen‐doped carbons. As obtained atomically dispersed Zn–N–Cs comprising ZnN4 sites as well as metal‐free N4 sites can be utilized for the coordination of Fe2+ and Fe3+ ions to form atomically dispersed Fe–N–C with Fe loadings as high as 3.12 wt%. The Fe–N–Cs are active electocatalysts for the oxygen reduction reaction in acidic media with an onset potential of E0 = 0.85 V versus RHE in 0.1 m HClO4. Identical location atomic resolution transmission electron microscopy imaging, as well as in situ electrochemical flow cell coupled to inductively coupled plasma mass spectrometry measurements, is employed to directly prove the concept of the active‐site imprinting approach.

Extrinsic Green Photoluminescence from the Edges of 2D Cesium Lead Halides.

Since the first report of the green emission of 2D all-inorganic CsPb2 Br5 , its bandgap and photoluminescence (PL) origin have generated intense debate and remained controversial. After the discovery that PL centers occupy only specific morphological structures in CsPb2 Br5 , a two-step highly sensitive and noninvasive optical technique is employed to resolve the controversy. Same-spot Raman-PL as a static property-structure probe reveals that CsPbBr3 nanocrystals are contributing to the green emission of CsPb2 Br5 ; pressure-dependent Raman-PL with a diamond anvil cell as a dynamic probe further rules out point defects such as Br vacancies as an alternative mechanism. Optical absorption under hydrostatic pressure shows that the bandgap of CsPb2 Br5 is 0.3-0.4 eV higher than previously reported values and remains nearly constant with pressure up to 2 GPa in good agreement with full-fledged density functional theory (DFT) calculations. Using ion exchange of Br with Cl and I, it is further proved that CsPbBr3- x Xx (X = Cl or I) is responsible for the strong visible PL in CsPb2 Br5- x Xx . This experimental approach is applicable to all PL-active materials to distinguish intrinsic defects from extrinsic nanocrystals, and the findings pave the way for new design and development of highly efficient optoelectronic devices based on all-inorganic lead halides.

Transition in layer structure of atomic/molecular layer deposited ZnO-zincone multilayers

In this work, atomic-resolution transmission electron microscopy studies show that in atomic/molecular layer deposited ZnO-zincone [(ZnO)a(Zn-O-C6H4-O)b] multilayer thin films, ZnO crystals grow vertically and continuously through lateral zincone monolayers. When the thickness of zincone is increased from one to few monolayers, the growth of the ZnO crystals becomes confined, leading to the formation of discrete ZnO/zincone interfaces accompanied by a decrease in roughness. Apart from providing understanding on the formation of interfaces and on the structure of hybrid multilayers, these findings are relevant, e.g., for understanding thermoelectric interface phenomena at inorganic-organic interfaces and x-ray reflectometry of hybrid multilayer thin films.

Comparative effect of Mn/Ti solute atoms and TiC/Ni3(Al,Ti) nano-particles on work hardening behaviour in Ni[sbnd]Cu alloys fabricated by wire arc additive manufacturing

Two NiCu alloys with various contents of Mn, Ti, Al and C were deposited in a shape of single-bead multi layered walls using wire arc additive manufacturing technology. To modify solute atom concentrations and particle number density values, the as-welded alloys were subjected to annealing at 1100 °C and age-hardening heat treatment in the 610-480 °C temperature range. Microstructure characterisation was carried out using optical, scanning, conventional transmission and atomic resolution transmission electron microscopy. Work hardening behaviour was studied using tensile testing. For similar deposition and heat treatment conditions, an alloy with higher C and Al, and lower Mn contents exhibited a higher number density of >20 nm TiC particles, higher number density of <20 nm γ′-Ni3(Al, Ti) particles, and, associated with these, superior hardness, tensile strength, strain hardening rate and toughness. The comparative effect of solid solution and precipitation strengthening on work hardening behaviour and fracture mode is discussed.

Double-Spiral Hexagonal Boron Nitride and Shear Strained Coalescence Boundary.

Among the different growth mechanisms for two-dimensional (2D) hexagonal boron nitride (h-BN) synthesized using chemical vapor deposition, spiraling growth of h-BN has not been reported. Here we report the formation of intertwined double-spiral few-layer h-BN that is driven by screw dislocations located at the antiphase boundaries of monolayer domains. The microstructure and stacking configurations were studied using a combination of dark-field and atomic resolution transmission electron microscopy. Distinct from other 2D materials with single-spiral structures, the double-spiral structure enables the intertwined h-BN layers to preserve the most stable AA' stacking configuration. We also found that the occurrence of shear strains at the boundaries of merged spiral islands is dependent on the propagation directions of encountering screw dislocations and presented the strained features by density functional theory calculations and atomic image simulations. This study unveils the double-spiral growth of 2D h-BN multilayers and the creation of a shear strain band at the coalescence boundary of two h-BN spiral clusters.

Study of the Q′ (Q)-phase precipitation in Al–Mg–Si–Cu alloys by quantification of atomic-resolution transmission electron microscopy images and atom probe tomography

The precipitation mechanism of the Q phase in Al–Mg–Si–Cu alloys has long been the subject of ambiguity and debate since its metastable phase (Q′) has the same crystal structure and similar lattice parameters as its equilibrium counterparts. In the present work, the evolution of the Q′ (Q) phase during aging is studied by combination of quantitative atomic-resolution scanning transmission electron microscopy and atom probe tomography. It was found that the transformation from the Q′ to the Q phase involves changes of the occupancy of Al atoms in atomic columns of the Q′ (Q) phase. The Al atoms incorporated in the Cu, Si and Mg columns are gradually released into the Al matrix, while mixing between Cu and Si atoms occurs in the Si columns. This transformation process is mainly attributed to the low lattice misfit of the equilibrium Q phase. Besides, the formation of various compositions of the Q phase is due to the different occupancy in the atomic columns of the Q phase. The occupancy changes in the columns of the Q phase are kinetically controlled and are strongly influenced by the alloy composition and aging temperature.

The core structure of 60° mixed basal dislocation in alumina (α-Al2O3) introduced by in situ TEM nanoindentation

The dislocation associated with the (0001)1/3<112¯0> basal slip in alumina plays an important role on its plastic deformation at elevated temperatures. A 60° mixed basal dislocation was preferentially introduced by in situ transmission electron microscopy (TEM) nanoindentation, and characterized by atomic-resolution scanning TEM (STEM). It was revealed that the 60° mixed basal dislocation dissociates into two partial dislocations via self-climb with a stacking fault on the 11¯00 plane. The dissociation mechanisms of the basal dislocation have been discussed in terms of its core structure and atomic diffusion process.