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Thick epitaxial BaTiO3 films ranging from 120 nm to 1 μm were grown by off-axis RF magnetron sputtering on SrTiO3-templated silicon-on-insulator (SOI) substrates for use in electro-optic applications, where such large thicknesses are necessary. The films are of high quality, rivaling those grown by molecular beam epitaxy (MBE) in crystalline quality, but can be grown 10 times faster. Extraction of lattice parameters from geometric phase analysis of atomic-resolution scanning transmission electron microscopy images revealed how the in-plane and out-of-plane lattice spacings of sputtered BaTiO3 changes as a function of layer position within a thick film. Our results indicate that compared to molecular beam epitaxy, sputtered films retain their out-of-plane polarization (c-axis) orientation for larger thicknesses. We also find an unusual re-transition from in-plane polarization (a-axis) to out-of-plane polarization (c-axis), along with an anomalous lattice expansion, near the surface. We also studied a method of achieving 100% a-axis-oriented films using a two-step process involving amorphous growth and recrystallization of a seed layer followed by normal high temperature growth. While this method is successful in achieving full a-axis orientation even at low thicknesses, the resulting film has a large number of voids and misoriented grains. Electro-optic measurement using a transmission setup of a sputtered BTO film grown using the optimized conditions yields an effective Pockels coefficient as high as 183 pm/V. A Mach–Zehnder modulator fabricated on such films exhibits phase shifting with an equivalent Pockels coefficient of 157 pm/V. These results demonstrate that sputtered BTO thick films can be used for integrated electro-optic modulators for Si photonics.

Each 2D material has a distinct structure for its grain boundary and dislocation cores, which is dictated by both the crystal lattice geometry and the elements that participate in bonding. For the class of noble metal dichalcogenides, this has yet to be thoroughly investigated at the atomic scale. Here, we examine the atomic structure of the dislocations and grain boundaries (GBs) in two-dimensional PtSe2, using atomic-resolution annular dark field scanning transmission electron microscopy, combined with density functional theory and empirical force field calculations. The PtSe2 we study adopts the 1T phase in large-area polycrystalline films with numerous planar tilt GB distinct dislocations, including 5|7+Se and 4|4|8+Se polygons, in tilt-angle monolayer GBs, with features sharply distinguished from those in 2H-phase TMDs. On the basis of dislocation cores, the GB structures are investigated in terms of pathways of dislocation chain arrangement, dislocation core distributions in different misorientation angles, and 2D strain fields induced. Based on the Frank-Bilby equation, the deduced Burgers vector magnitude is close to the lattice constant of 1T-PtSe2, building the quantitative relationship of dislocation spacings and small GB angles. The 30° GBs are most frequently formed as a stitched interface between the armchair and zigzag lattices, constructed by a string of 5|7+Se dislocations asymmetrically with a small deviation angle. Another special angle GB, mirror twin 60° GB, is also mapped linearly by metal-condensed asymmetric or Se-rich symmetric dislocations. This report gives atomic-level insights into the GBs and dislocations in 1T-phase noble metal TMD PtSe2, which is a promising material to underpin extending properties of 2D materials by local structure engineering.

As the push for inexpensive vehicle electrification grows, high-energy-density cathodes for lithium-ion batteries, such as high-nickel layered oxides, have received a great deal of attention in both industry and academia. These materials, however, suffer from severe residual lithium formation, which causes slurry gelation during electrode fabrication and gas evolution during cycling. Herein, a novel cobalt hydroxide coating method on wet-CO2 gas-treated LiNi0.91Mn0.03Co0.06O2 (Co-CO2-NMC91) is presented. Notably, the wet-CO2 treatment prior to a dry cobalt hydroxide coating plays a critical role in improving the coating uniformity and ultimately decreases the effective residual lithium content. Furthermore, full cells of Co-CO2-NMC91 exhibit excellent capacity retention of 91% after 200 cycles. This study highlights how a wet-CO2 treatment can be used to improve a typical dry coating and provides new insights toward the development of cathodes for high-energy-density LIBs without severe slurry gelation or gas evolution.

High-nickel layered oxide cathodes suffer seriously from the formation of residual lithium on the surface, which causes notorious issues, such as slurry gelation and gas evolution. Due to the use o...

Standardisation of animal handling procedures for a wide range of preclinical imaging scanners will improve imaging performance and reproducibility of scientific data. Whilst there has been significant effort in defining how well scanners should operate and how in vivo experimentation should be practised, there is little detail on how to achieve optimal scanner performance with best practices in animal welfare. Here, we describe a system-agnostic, adaptable and extensible animal support cradle system for cardio-respiratory-synchronised, and other, multi-modal imaging of small animals. The animal support cradle can be adapted on a per application basis and features integrated tubing for anaesthetic and tracer delivery, an electrically driven rectal temperature maintenance system and respiratory and cardiac monitoring. Through a combination of careful material and device selection, we have described an approach that allows animals to be transferred whilst under general anaesthesia between any of the tomographic scanners we currently or have previously operated. The set-up is minimally invasive, cheap and easy to implement and for multi-modal, multi-vendor imaging of small animals.

Magnesium-ion batteries have been regarded as a promising alternative to the lithium-ion batteries due to their high theoretical capacity, relatively high potential, and magnesium abundance. However, the contradiction between the plating/stripping of Mg2+ and the electrolytes’ oxidative stability has hampered the Mg-ion battery's development for energy storage applications. Here, we designed an amorphous MgO-wrapped Zn-skeleton as a unique current collector for an anode-free Mg battery to allow reversible Mg2+ plating/stripping in oxidatively stable electrolytes. The significant lattice mismatch between hexagonal Zn and MgO induces dislocations, leading to a highly defective interphase. This layer behaves as a mixed ionic-electronic conductor, rendering Mg nanoparticles upon electroplating. Combined with a large surface area, the proposed current collector considerably improved the charge transfer kinetics and lowered the cell impedance for Mg2+ plating/stripping by 1/20 of the typical Mg metal. Moreover, the Mg2+ interphase conduction was two orders of magnitude higher (∼10−11 S cm−1) compared to the widely known passivating layer (

Molybdenum trioxide (MoO

Two-dimensional (2D) photodetectors based on photovoltaic effect or photogating effect can hardly achieve both high photoresponsivity and large linear dynamic range at the same time, which greatly limits many practical applications such as imaging sensors. Here, the conductive-sensitizer strategy, a general design for improving photoresponsivity and linear dynamic range in 2D photodetectors is provided and experimentally demonstrated on vertically stacked bilayer WS2/GaS0.87 under a parallel circuit mode. Owing to successful band alignment engineering, the isotype type-II heterojunction enables efficient charge carrier transfer from WS2, the high-mobility sensitizer, to GaS0.87, the low-mobility channel, under illumination from a broad visible spectrum. The transferred electron charges introduce a reverse electric field which efficiently lowers the band offset between the two materials, facilitating a transition from low-mobility photocarrier transport to high-mobility photocarrier transport with increasing illumination power. We achieved a large linear dynamic range of 73 dB as well as a high and constant photoresponsivity of 13 A/W under green light. X-ray photoelectron spectroscopy, cathodoluminescence, and Kelvin probe force microscopy further identify the key role of defects in monolayer GaS0.87 in engineering the band alignment with monolayer WS2. This work proposes a design route based on band and interface modulation for improving performance of 2D photodetectors and provides deep insights into the important role of strong interlayer coupling in offering heterostructures with desired properties and functions.

Two-dimensional (2D) molybdenum disulfide (MoS2) has excellent electrocatalytic behavior for the hydrogen evolution reaction (HER), where the catalysis of 2H phase originates from its edges, defects, and strains. Most synthetic methods to activate the electrochemically inert basal planes with catalytic active metals are completed by sequential steps. However, this is extremely time-consuming and lacks production scalability. Herein, we develop a one-step strategy to achieve efficient electrocatalyst of Pt:MoS2 hybrid utilizing atmospheric pressure chemical vapor deposition synthesis on a conductive glassy carbon (GC) plate that can be directly employed as the working electrode in the HER. The monolayer thickness ensures decreased interlayer electron hopping and increased efficiency of the charge transfer from the electrode. We tune the domain morphology by controlling the precursor flux to enter kinetic or thermodynamic growth regime, delivering dendritic or triangular shape. The materials chemistry undertaken provides fundamental insights into the instability of Pt as metal substitutional dopants in the MoS2 lattice, and instead the stable configuration observed is with Pt as highly dispersed small nanocrystals and single atoms bound to the MoS2 surface. The Pt functionalization at a reduced loading level modulates the favorable HER pathway and triggers synergies in the cocatalyst, which exhibits an onset potential of 48 mV, a Tafel slope of 46 mV dec–1, and an exchange current density of 110 μA cm–2. The enriched edges and defects of dendrite endow it superiority to the triangle, with regard to the density of catalytic sites and synergistic effects along with electrical resistance. These underpin the positive role of a large dendritic MoS2 monolayer as a Pt scaffold in water electrolysis.

Monolayer 2H-phase MoS2-based photodetectors exhibit high photon absorption but suffer from low photoresponse, which severely limits their applications in optoelectronic fields. The metallic 1T pha...

We present a transition-metal complex containing a Pt center, large aromatic ligand, and Mo-binding terminal group, designed to maximize the dispersion of individual molecules across the 2D MoS2 su...

We show interlayer stacking shifts occur in transition metal dichalcogenides (TMD) bilayers due to the strain introduced during sample heating, and attributed to rippling of one layer relative to the other. The atomic structure of the interlayer stacking is studied using annular dark field scanning transmission electron microscopy with an in situ heating holder. Before heating, bilayers show uniform interlayer stacking of AA′ and AB. When heated, contrast change is seen and associated with interlayer stacking changes at the atomic scale due to ripples. When cooled down to room temperature, these contrast features disappear, confirming it is a reversible process that is not related to defects or vacancies. Because the bottom layer is attached to the in situ heating chip made from Si3N4 and the top layer is in contact with the underlying TMD layer with weak van der Waals interaction, the two layers experience different forces during thermal expansion.

Heterophase homojunction formation in atomically thin 2D layers is of great importance for next-generation nanoelectronics and optoelectronics applications. Technologically challenging, controllable transformation between the semiconducting and metallic phases of transition metal chalcogenides is of particular importance. Here, we demonstrate that controlled laser irradiation can be used to directly ablate PdSe2 thin films using high power or trigger the local transformation of PdSe2 into a metallic phase PdSe2–x using lower laser power. Such transformations are possible due to the low decomposition temperature of PdSe2 and a variety of stable phases compared to other 2D transition metal dichalcogenides. Scanning transmission electron microscopy is used to reveal the laser-induced Se-deficient phases of PdSe2 material. The process sensitivity to the laser power allows patterning flexibility for resist-free device fabrication. The laser-patterned devices demonstrate that a laser-induced metallic phase PdSe2–x is stable with increased conductivity by a factor of about 20 compared to PdSe2. These findings contribute to the development of nanoscale devices with homojunctions and scalable methods to achieve structural transformations in 2D materials.

We show controlled production of atomically thin nanowells in bilayer WS2 using an in situ heating holder combined with a focused electron beam in a scanning transmission electron microscope (STEM). We systematically study the formation and evolvement mechanism involved in removing a single layer of WS2 within a bilayer region with 2 nm accuracy in location and without punching through to the other layer to create a hole. Best results are found when using a high temperature of 800 °C, because it enables thermally activated atomic migration and eliminates the interference from surface carbon contamination. We demonstrate precise control over spatial distributions with 5 nm accuracy of patterning and the width of nanowells adjustable by dose-dependent parameters. The mechanism of removing a monolayer of WS2 within a bilayer region is different than removing equivalent sections in a monolayer film due to the van der Waals interaction of the underlying remaining layer in the bilayer system that stabilizes the excess W atom stoichiometry within the edges of the nanowell structure and facilitates expansion. This study offers insights for the nanoengineering of nanowells in two-dimensional (2D) transitional metal dichalcogenides (TMDs), which could hold potential as selective traps to localize 2D reactions in molecules and ions, underpinning the broader utilization of 2D material membranes.

Atomic resolution studies of 2D layered PtSe2 crystals under in situ heating conditions are performed with an aberration corrected scanning transmission electron microscope (STEM). We reveal the te...

Nickel is well recognized for its high catalytic activity in hydrogen production and hydrotreating, but it experiences severe deactivation upon coke formation. The development of a nickel-based catalyst that is resilient to deactivation while maintaining its signature high activity is therefore desirable. We demonstrate a simple but effective technique for the synthesis of nickel phosphide catalysts by impregnating nickel into carbonized natural cellulose fibers (NCFs) that naturally contain phosphorus. At a sufficient annealing temperature (1100 °C), the nickel particles react with phosphorus, forming Ni2P and Ni5P4 nanoparticles. Higher annealing temperatures (>1500 °C) promote the formation of single atom nickel, which greatly supplements the catalytic performance. Our nickel phosphide catalyst exhibits a markedly superior activity and stability in the synthesis of hydrogen by C2H2 decomposition and the dry reforming of methane (DRM) compared to the Ni/Al2O3 model catalyst. The sustained activity and stability exhibited by the developed catalyst suggest its promise for other hydrotreating reactions and the hydrogen evolution reaction.

We use annular dark-field scanning transmission electron microscopy (ADF-STEM) to study how solution-deposited molecules bind to the edges and surface regions around nanopores in MoS2 monolayers. N...

Two-dimensional (2D) materials are atomically thick and without out-of-plane dangling bonds. As a result, they could break the confinement of lattice matching, and thus can be freely mixed and matched together to construct vertical van der Waals heterostructures. Here, we demonstrated an asymmetrical vertical structure of graphene/hexagonal boron nitride (h-BN)/tungsten disulfide (WS

Two-dimensional (2D) photodetectors based on photovoltaic effect or photogating effect can hardly achieve both high photoresponsivity and large linear dynamic range at the same time, which greatly limits many practical applications such as imaging sensors. Here, the conductive-sensitizer strategy, a general design for improving photoresponsivity and linear dynamic range in 2D photodetectors is provided and experimentally demonstrated on vertically stacked bilayer WS

Thick epitaxial BaTiO

Each 2D material has a distinct structure for its grain boundary and dislocation cores, which is dictated by both the crystal lattice geometry and the elements that participate in bonding. For the class of noble metal dichalcogenides, this has yet to be thoroughly investigated at the atomic scale. Here, we examine the atomic structure of the dislocations and grain boundaries (GBs) in two-dimensional PtSe

Platinum single-site catalysts (SSCs) are a promising technology for the production of hydrogen from clean energy sources. They have high activity and maximal platinum-atom utilisation. However, the bonding environment of platinum during operation is poorly understood. In this work, we use operando, synchrotron-X-ray absorption spectroscopy to study the platinum bonding in SSCs. First, we synthesise an atomically dispersed platinum complex with aniline and chloride ligands onto graphene and characterise it with ex-situ electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, X-ray absorption near edge structure spectroscopy (XANES), and extended X-ray absorption fine structure spectroscopy (EXAFS). Then, by operando EXAFS and XANES, we show that as a negatively biased potential is applied, the Pt-N bonds break first followed by the Pt-Cl bonds. The platinum is reduced from platinum(II) to metallic platinum(0) by the onset of the hydrogen-evolution reaction at 0 V. Furthermore, we observe an increase in Pt-Pt bonding, indicating the formation of platinum agglomerates. Together, these results indicate that while aniline is used to prepare platinum SSCs, the single-site complexes are decomposed and platinum agglomerates at operating potentials. This work is an important contribution to the understanding of the bonding environment and the evolution of the molecular structure of platinum complexes in SSCs.

Triangular nanovoids in 2D materials transition metal dichalcogenides have vertex points that cause stress concentration and lead to sharp crack propagation and failure. Here, the atomistic mechanics of back folding around triangular nanovoids in monolayer WS

Artificially introduced small twist angles at the interfaces of vertical layered heterostructures (VLHs) have allowed deterministic tuning of electronic and optical properties such as strongly correlated electronic phases and Moiré excitons. But creating a Moiré twist in van der Waals (vdWs) systems by manual stacking is challenging in reproducibility, uniformity, and accuracy of the twist angle, which hinders future studies. Here, it is demonstrated that contrary to the commonly believed 0°-orientation in vdWs epitaxy, these VLHs show small twist angles controlled by the low-order commensurate phase with low energy and local atomic relaxation. A commensurate multilevel map is proposed to predict possible orientations. Remarkably, high-mismatch VLHs show discrete and sometimes non-zero twist angles dependent on their natural mismatch value. Such framework is experimentally confirmed in five epitaxially grown VLHs under high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and can provide significant insights for large-scale engineering of twist angle in VLHs.

Sustainability has become a critical concern in the semiconductor industry as hazardous wastes released during the manufacturing process of semiconductor devices have an adverse impact on human beings and the environment. The use of hazardous solvents in existing fabrication processes also restricts the use of polymer substrates because of their low chemical resistance to such solvents. Here, we demonstrate an environmentally friendly mechanical, bilayer lithography that uses just water for development and lift-off. We show that we are able to create arbitrary patterns achieving resolution down to 310 nm. We then demonstrate the use of this technique to create functional devices by fabricating a MoS2 photodetector on a polyethylene terephthalate (PET) substrate with measured response times down to 42 ms.

Simultaneous imaging of individual low and high atomic number atoms using annular dark field scanning transmission electron microscopy (ADF-STEM) is often challenging due to substantial differences in their scattering cross sections. This often leads to contrast from only the high atomic number species when imaged using ADF-STEM such as the Mo and 2S sites in monolayer MoS2 crystals, without detection of lighter atoms such as C, O, or N. Here, we show that by capturing an array of convergent beam electron diffraction patterns using a 2D pixelated electron detector (2D-PED) in a 4D STEM geometry enables identification of individual low and high atomic number atoms in 2D materials by multicomponent imaging. We have used ptychographic phase reconstructions, combined with angular dependent ADF-STEM reconstructions, to image light elements at lateral (nanopores) and vertical interfaces (surface dopants) within 2D monolayer MoS2. Differential phase contrast imaging (Div(DPC)) using quadrant segmentation of the 2D pixelated direct electron detector data not only qualitatively matches the ptychographic phase reconstructions in both resolution and contrast but also offers the additional potential for real time display. Using 4D-STEM, we have identified surface adatoms on MoS2 monolayers and have separated atomic columns with similar total atomic number into their relative combinations of low and high atomic number elements. These results demonstrate the rich information present in the data obtained during 4D-STEM imaging of ultrathin 2D materials and the ability of this approach to extract unique insights beyond conventional imaging.

Direct imaging of single molecules has to date been primarily achieved using scanning probe microscopy, with limited success using transmission electron microscopy due to electron beam damage and low contrast from the light elements that make up the majority of molecules. Here, we show single complex molecule interactions can be imaged using annular dark field scanning TEM (ADF-STEM) by inserting heavy metal markers of Pt atoms and detecting their positions. Using the high angle ADF-STEM Z1.7 contrast, combined with graphene as an electron transparent support, we track the 2D monolayer self-assembly of solution-deposited individual linear porphyrin hexamer (Pt-L6) molecules and reveal preferential alignment along the graphene zigzag direction. The epitaxial interactions between graphene and Pt-L6 drive a reduction in the interporphyrin distance to allow perfect commensuration with the graphene. These results demonstrate how single metal atom markers in complex molecules can be used to study large scale packing and chain bending at the single molecule level.