Synthesis and in-depth structure determination of a novel metastable high-pressure CrTe3 phase.

This study reports the synthesis and crystal structure determination of a novel CrTe3 phase using various experimental and theoretical methods. The average stoichiometry and local phase separation of this quenched high-pressure phase were characterized by ex situ synchrotron powder X-ray diffraction and total scattering. Several structural models were obtained using simulated annealing, but all suffered from an imperfect Rietveld refinement, especially at higher diffraction angles. Finally, a novel stoichiometrically correct crystal structure model was proposed on the basis of electron diffraction data and refined against powder diffraction data using the Rietveld method. Scanning electron microscopy-energy-dispersive X-ray spectrometry (EDX) measurements verified the targeted 1:3 (Cr:Te) average stoichiometry for the starting compound and for the quenched high-pressure phase within experimental errors. Scanning transmission electron microscopy (STEM)-EDX was used to examine minute variations of the Cr-to-Te ratio at the nanoscale. Precession electron diffraction (PED) experiments were applied for the nanoscale structure analysis of the quenched high-pressure phase. The proposed monoclinic model from PED experiments provided an improved fit to the X-ray patterns, especially after introducing atomic anisotropic displacement parameters and partial occupancy of Cr atoms. Atomic resolution STEM and simulations were conducted to identify variations in the Cr-atom site-occupancy factor. No significant variations were observed experimentally for several zone axes. The magnetic properties of the novel CrTe3 phase were investigated through temperature- and field-dependent magnetization measurements. In order to understand these properties, auxiliary theoretical investigations have been performed by first-principles electronic structure calculations and Monte Carlo simulations. The obtained results allow the observed magnetization behavior to be interpreted as the consequence of competition between the applied magnetic field and the Cr-Cr exchange interactions, leading to a decrease of the magnetization towards T = 0 K typical for antiferromagnetic systems, as well as a field-induced enhanced magnetization around the critical temperature due to the high magnetic susceptibility in this region.

Boosting Thermoelectric Performance of Bi2 Te3 Material by Microstructure Engineering.

Due to the intrinsic contradiction of electrical conductivity and Seebeck coefficient in thermoelectric materials, the enhancement for the power factor (PF) is limited. Since the PF decides the output power, strategies to the enhancement of PF are of paramount importance. In this work, Bi2 Te3 /Sb and Bi2 Te3 /W multilayer films are proposed to enhance the thermoelectric properties. Both systems possess extremely high conductivity of ≈5.6 × 105 S m-1 . Moreover, the electrical conductivity and Seebeck coefficient simultaneously increase as temperature rising, showing the overcome of the intrinsic contradiction. This results in ultrahigh PFs of 1785 µWm-1  K-2 for Bi2 Te3 /W and of 1566 µWm-1  K-2 for Bi2 Te3 /Sb at 600 K. Thermal heating of the Bi2 Te3 /Sb multilayer system shows compositional changes with subsequent formation of Bi-Te-Sb phases, Sb-rich Bi-Te precipitates, and cavities. Contrary, the multilayer structure of the Bi2 Te3 /W films is maintained, while Bi2 Te3 grains of high-crystalline quality are confined between the W layers. In addition, bilayer defects in Bi2 Te3 and smaller cavities at the interface to W layers are also observed. Thus, compositional and confinement effects as well as structural defects result in the ultrahigh PF. Overall, this work demonstrates the strategies on how to obtain ultrahigh PFs of commercial Bi2 Te3 material by microstructure engineering using multilayer structures.

Energetic Electron-Assisted Synthesis of Tailored Magnetite (Fe3O4) and Maghemite (γ-Fe2O3) Nanoparticles: Structure and Magnetic Properties.

Iron oxide nanoparticles with a mean size of approximately 5 nm were synthesized by irradiating micro-emulsions containing iron salts with energetic electrons. The properties of the nanoparticles were investigated using scanning electron microscopy, high-resolution transmission electron microscopy, selective area diffraction and vibrating sample magnetometry. It was found that formation of superparamagnetic nanoparticles begins at a dose of 50 kGy, though these particles show low crystallinity, and a higher portion is amorphous. With increasing doses, an increasing crystallinity and yield could be observed, which is reflected in an increasing saturation magnetization. The blocking temperature and effective anisotropy constant were determined via zero-field cooling and field cooling measurements. The particles tend to form clusters with a size of 34 nm to 73 nm. Magnetite/maghemite nanoparticles could be identified via selective area electron diffraction patterns. Additionally, goethite nanowires could be observed.

Growth of Highly c-Axis Oriented AlScN Films on Commercial Substrates.

In this work, we present a method for growing highly c-axis oriented aluminum scandium nitride (AlScN) thin films on (100) silicon (Si), silicon dioxide (SiO2) and epitaxial polysilicon (poly-Si) substrates using a substrate independent approach. The presented method offers great advantages in applications such as piezoelectric thin-film-based surface acoustic wave devices where a metallic seed layer cannot be used. The approach relies on a thin AlN layer to establish a wurtzite nucleation layer for the growth of w-AlScN films. Both AlScN thin film and seed layer AlN are prepared by DC reactive magnetron sputtering process where a Sc concentration of 27% is used throughout this study. The crystal quality of (0002) orientation of Al0.73Sc0.27N films on all three substrates is significantly improved by introducing a 20 nm AlN seed layer. Although AlN has a smaller capacitance than AlScN, limiting the charge stored on the electrode plates, the combined piezoelectric coefficient d33,f with 500 nm AlScN is only slightly reduced by about 4.5% in the presence of the seed layer.

Exploding and weeping ceramics.

The systematic tuning of crystal lattice parameters to achieve improved kinematic compatibility between different phases is a broadly effective strategy for improving the reversibility, and lowering the hysteresis, of solid-solid phase transformations1-11. (Kinematic compatibility refers to the fitting together of the phases.) Here we present an apparently paradoxical example in which tuning to near perfect kinematic compatibility results in an unusually high degree of irreversibility. Specifically, when cooling the kinematically compatible ceramic (Zr/Hf)O2(YNb)O4 through its tetragonal-to-monoclinic phase transformation, the polycrystal slowly and steadily falls apart at its grain boundaries (a process we term weeping) or even explosively disintegrates. If instead we tune the lattice parameters to satisfy a stronger 'equidistance' condition (which additionally takes into account sample shape), the resulting material exhibits reversible behaviour with low hysteresis. These results show that a diversity of behaviours-from reversible at one extreme to explosive at the other-is possible in a chemically homogeneous ceramic system by manipulating conditions of compatibility in unexpected ways. These concepts could prove critical in the current search for a shape-memory oxide ceramic9-12.

Strongly enhanced and tunable photovoltaic effect in ferroelectric-paraelectric superlattices.

Ever since the first observation of a photovoltaic effect in ferroelectric BaTiO3, studies have been devoted to analyze this effect, but only a few attempted to engineer an enhancement. In conjunction, the steep progress in thin-film fabrication has opened up a plethora of previously unexplored avenues to tune and enhance material properties via growth in the form of superlattices. In this work, we present a strategy wherein sandwiching a ferroelectric BaTiO3 in between paraelectric SrTiO3 and CaTiO3 in a superlattice form results in a strong and tunable enhancement in photocurrent. Comparison with BaTiO3 of similar thickness shows the photocurrent in the superlattice is 103 times higher, despite a nearly two-thirds reduction in the volume of BaTiO3 The enhancement can be tuned by the periodicity of the superlattice, and persists under 1.5 AM irradiation. Systematic investigations highlight the critical role of large dielectric permittivity and lowered bandgap.

Nanocomposites with Three-Dimensional Architecture and Impact on Photovoltaic Effect.

We demonstrate the synthesis of self-assembled three-dimensional nanocomposite thin films consisting of NiO nanocolumns in an layered Aurivillius phase matrix. The structures were grown on single-crystal SrTiO3 substrates via pulsed laser deposition (PLD) with single ceramic (PbTiO3)x(BiNi2/3Nb1/3O3)1-x targets. The nanocolumns, which are about 10 nm in diameter each, extend over the entire film thickness of up to 225 nm. We reveal the difference in electrical conduction properties of the nanocolumns and the surrounding matrix on the nanoscale via conductive atomic force microscopy. The nanocomposite thin films exhibit improved photovoltaic performance compared to both pure PbTiO3 and homogeneous Aurivillius phase thin films.

"Stickier"-Surface Sb2Te3 Templates Enable Fast Memory Switching of Phase Change Material GeSb2Te4 with Growth-Dominated Crystallization.

Ge-Sb-Te (GST)-based phase-change memory (PCM) excels in the switching performance but remains insufficient of the operating speed to replace cache memory (the fastest memory in a computer). In this work, a novel approach using Sb2Te3 templates is proposed to boost the crystallization speed of GST by five times faster. This is because such a GST/Sb2Te3 heterostructure changes the crystallizing mode of GST from the nucleation-dominated to the faster growth-dominated one, as confirmed by high-resolution transmission electron microscopy, which captures the interface-induced epitaxial growth of GST on Sb2Te3 templates in devices. Ab initio molecular dynamic simulations reveal that Sb2Te3 templates can render GST sublayers faster crystallization speed because Sb2Te3's "sticky" surface contains lots of unpaired electrons that may attract Ge atoms with less antibonding interactions. Our work not only proposes a template-assisted PCM with fast speed but also uncovers the hidden mechanism of Sb2Te3's sticky surface, which can be used for future material selection.

Structural Transitions in Ge2Sb2Te5 Phase Change Memory Thin Films Induced by Nanosecond UV Optical Pulses.

Ge-Sb-Te-based phase change memory alloys have recently attracted a lot of attention due to their promising applications in the fields of photonics, non-volatile data storage, and neuromorphic computing. Of particular interest is the understanding of the structural changes and underlying mechanisms induced by short optical pulses. This work reports on structural changes induced by single nanosecond UV laser pulses in amorphous and epitaxial Ge2Sb2Te5 (GST) thin films. The phase changes within the thin films are studied by a combined approach using X-ray diffraction and transmission electron microscopy. The results reveal different phase transitions such as crystalline-to-amorphous phase changes, interface assisted crystallization of the cubic GST phase and structural transformations within crystalline phases. In particular, it is found that crystalline interfaces serve as crystallization templates for epitaxial formation of metastable cubic GST phase upon phase transitions. By varying the laser fluence, GST thin films consisting of multiple phases and different amorphous to crystalline volume ratios can be achieved in this approach, offering a possibility of multilevel data storage and realization of memory devices with very low resistance drift. In addition, this work demonstrates amorphization and crystallization of GST thin films by using only one UV laser with one single pulse duration and one wavelength. Overall, the presented results offer new perspectives on switching pathways in Ge-Sb-Te-based materials and show the potential of epitaxial Ge-Sb-Te thin films for applications in advanced phase change memory concepts.

Direct Measurement of Crystal Growth Velocity in Epitaxial Phase-Change Material Thin Films.

Central to the use of Ge-Sb-Te based phase-change materials for data storage applications is their crystallization capability since it determines memory writing time. Although being intensively studied to identify intrinsic limits and develop strategies to enhance memory performance, the crystallization process in these materials is still not fully explored. Therefore, this study focuses on the determination of crystal growth dynamics in an epitaxial phase-change material thin film model system offering the advantage of high crystalline quality and application-relevant sizing. By introducing a method that combines time-resolved reflectivity measurements with high-resolution scanning transmission electron microscopy, crystal growth velocities upon fast cooling after single ns-laser pulse irradiation of the prototypical phase-change material Ge2Sb2Te5 are determined. As a result, an increase in crystal growth velocity from 0.4 to 1.7 m/s with increasing laser fluence is observed with a maximum rate of 1.7 m/s as the upper detectable limit of the studied material.

In situ observations of the reversible vacancy ordering process in van der Waals-bonded Ge-Sb-Te thin films and GeTe-Sb2Te3 superlattices.

Chalcogenide-based thin films are employed in data storage and memory technology whereas van der Waals-bonded layered chalcogenide heterostructures are considered to be a main contender for memory devices with low power consumption. The reduction of switching energy is due to the lowering of entropic losses governed by the restricted motion of atoms in one dimension within the crystalline states. The investigations of switching mechanisms in such superlattices have recently attracted much attention and the proposed models are still under debate. This is partially due to the lack of direct observation of atomic scale processes, which might occur in these chalcogenide systems. This work reports direct, nanoscale observations of the order-disorder processes in van der Waals bonded Ge-Sb-Te thin films and GeTe-Sb2Te3-based superlattices using in situ experiments inside an aberration-corrected transmission electron microscope. The findings reveal a reversible self-assembled reconfiguration of the structural order in these materials. This process is associated with the ordering of randomly distributed vacancies within the studied materials into ordered vacancy layers and with readjustment of the lattice plane distances within the newly formed layered structures, indicating the high flexibility of these layered chalcogenide-based systems. Thus, the ordering process results in the formation of vacancy-bonded building blocks intercalated within van der Waals-bonded units. Moreover, vacancy-bonded building blocks can be reconfigured to the initial structure under the influence of an electron beam, while in situ exposure of the recovered layers to a targeted electron beam leads to the reverse process. Overall, the outcomes provide new insights into local structure and switching mechanism in chalcogenide superlattices.

Ultrafast interfacial transformation from 2D- to 3D-bonded structures in layered Ge-Sb-Te thin films and heterostructures.

Two-dimensional van-der-Waals-bonded chalcogenide heterostructures have recently received a lot of attention due to promising applications in the fields of photonics, plasmonics and data storage. Of particular interest is the interfacial switching process inherent in these structures, which is assumed to occur locally at the van-der-Waals interfaces and thus represents an intracrystalline transition. However, detailed experimental studies on the underlying mechanism are still lacking. In this work, epitaxially grown thin films consisting of van-der-Waals-bonded Ge-Sb-Te and GeTe/Sb2Te3 based heterostructures are employed as a model system to investigate structural changes induced by a single ns-laser pulse. A combined approach using X-ray diffraction and advanced transmission electron microscopy is applied to study phase transitions within the Ge-Sb-Te-based thin films in detail. The results reveal ultrafast transitions from 2D-bonded layered structures to 3D-bonded structures via a transient molten phase. Moreover, the interface between the 2D- and 3D-bonded structures is well defined by a single van-der-Waals gap, suggesting that the transition can be controlled very precisely in its spatial extent by an appropriate choice of the laser fluence. Overall, the results of this work offer a new perspective on the switching mechanism in Ge-Sb-Te-based materials and demonstrate the potential of van-der-Waals-bonded Ge-Sb-Te compounds to be applied for novel phase-change memory concepts.

Shortening Nucleation Time to Enable Ultrafast Phase Transition in Zn1Sb7Te12 Pseudo-Binary Alloy.

Zn1Sb7Te12 thin films have been deposited by magnetron co-sputtering of ZnTe and Sb2Te3 targets. The microstructure, phase-change speed, optical cycling stability, and crystallization kinetics have been investigated during thermal annealing and laser irradiation. The thermal-annealed and laser-irradiated films give a clear evidence of the coexistence of trigonal Sb2Te3 and cubic ZnTe phases, which are homogeneously distributed in a single alloy as confirmed by advanced scanning transmission electron microscopy. The formation of both phases increases the initial nucleation sites, leading to the rapid phase-change speed in the Zn1Sb7Te12 film. The film has a minimum crystallization time of ∼3 ns at 70 mW with almost no incubation period for the formation of critical nuclei compared to Ge2Sb2Te5 and other Zn-based films. Moreover, the complete crystallization of Zn1Sb7Te12 thin films is achieved within 10 ns. The ultrafast two-dimensional nucleation and crystal growth speed in Zn1Sb7Te12 obtained from the laser-irradiated system is almost 7 times faster compared to that in Ge2Sb2Te5 film. Controlling the crystallization process through doping ZnTe into Sb2Te3 is thus promising for the development of high-speed optical switching technology.

Ion Beam Assisted Deposition of Thin Epitaxial GaN Films.

The assistance of thin film deposition with low-energy ion bombardment influences their final properties significantly. Especially, the application of so-called hyperthermal ions (energy <100 eV) is capable to modify the characteristics of the growing film without generating a large number of irradiation induced defects. The nitrogen ion beam assisted molecular beam epitaxy (ion energy <25 eV) is used to deposit GaN thin films on (0001)-oriented 6H-SiC substrates at 700 °C. The films are studied in situ by reflection high energy electron diffraction, ex situ by X-ray diffraction, scanning tunnelling microscopy, and high-resolution transmission electron microscopy. It is demonstrated that the film growth mode can be controlled by varying the ion to atom ratio, where 2D films are characterized by a smooth topography, a high crystalline quality, low biaxial stress, and low defect density. Typical structural defects in the GaN thin films were identified as basal plane stacking faults, low-angle grain boundaries forming between w-GaN and z-GaN and twin boundaries. The misfit strain between the GaN thin films and substrates is relieved by the generation of edge dislocations in the first and second monolayers of GaN thin films and of misfit interfacial dislocations. It can be demonstrated that the low-energy nitrogen ion assisted molecular beam epitaxy is a technique to produce thin GaN films of high crystalline quality.

Crystallization of Ge2Sb2Te5 thin films by nano- and femtosecond single laser pulse irradiation.

The amorphous to crystalline phase transformation of Ge2Sb2Te5 (GST) films by UV nanosecond (ns) and femtosecond (fs) single laser pulse irradiation at the same wavelength is compared. Detailed structural information about the phase transformation is collected by x-ray diffraction and high resolution transmission electron microscopy (TEM). The threshold fluences to induce crystallization are determined for both pulse lengths. A large difference between ns and fs pulse irradiation was found regarding the grain size distribution and morphology of the crystallized films. For fs single pulse irradiated GST thin films, columnar grains with a diameter of 20 to 60 nm were obtained as evidenced by cross-sectional TEM analysis. The local atomic arrangement was investigated by high-resolution Cs-corrected scanning TEM. Neither tetrahedral nor off-octahedral positions of Ge-atoms could be observed in the largely defect-free grains. A high optical reflectivity contrast (~25%) between amorphous and completely crystallized GST films was achieved by fs laser irradiation induced at fluences between 13 and 16 mJ/cm(2) and by ns laser irradiation induced at fluences between 67 and 130 mJ/cm(2). Finally, the fluence dependent increase of the reflectivity is discussed in terms of each photon involved into the crystallization process for ns and fs pulses, respectively.

Local atomic arrangements and lattice distortions in layered Ge-Sb-Te crystal structures.

Insights into the local atomic arrangements of layered Ge-Sb-Te compounds are of particular importance from a fundamental point of view and for data storage applications. In this view, a detailed knowledge of the atomic structure in such alloys is central to understanding the functional properties both in the more commonly utilized amorphous-crystalline transition and in recently proposed interfacial phase change memory based on the transition between two crystalline structures. Aberration-corrected scanning transmission electron microscopy allows direct imaging of local arrangement in the crystalline lattice with atomic resolution. However, due to the non-trivial influence of thermal diffuse scattering on the high-angle scattering signal, a detailed examination of the image contrast requires comparison with theoretical image simulations. This work reveals the local atomic structure of trigonal Ge-Sb-Te thin films by using a combination of direct imaging of the atomic columns and theoretical image simulation approaches. The results show that the thin films are prone to the formation of stacking disorder with individual building blocks of the Ge2Sb2Te5, Ge1Sb2Te4 and Ge3Sb2Te6 crystal structures intercalated within randomly oriented grains. The comparison with image simulations based on various theoretical models reveals intermixed cation layers with pronounced local lattice distortions, exceeding those reported in literature.

High quality reduced graphene oxide flakes by fast kinetically controlled and clean indirect UV-induced radical reduction.

This work highlights a surprisingly simple and kinetically controlled highly efficient indirect method for the production of high quality reduced graphene oxide (rGO) flakes via UV irradiation of aqueous dispersions of graphene oxide (GO), in which the GO is not excited directly. While the direct photoexcitation of aqueous GO (when GO is the only light-absorbing component) takes several hours of reaction time at ambient temperature (4 h) leading only to a partial GO reduction, the addition of small amounts of isopropanol and acetone (2% and 1%) leads to a dramatically shortened reaction time by more than two orders of magnitude (2 min) and a very efficient and soft reduction of graphene oxide. This method avoids the formation of non-volatile species and in turn contamination of the produced rGO and it is based on the highly efficient generation of reducing carbon centered isopropanol radicals via the reaction of triplet acetone with isopropanol. While the direct photolysis of GO dispersions easily leads to degradation of the carbon lattice of GO and thus to a relatively low electric conductivity of the films of flakes, our indirect photoreduction of GO instead largely avoids the formation of defects, keeping the carbon lattice intact. Mechanisms of the direct and indirect photoreduction of GO have been elucidated and compared. Raman spectroscopy, XPS and conductivity measurements prove the efficiency of the indirect photoreduction in comparison with the state-of-the-art reduction method for GO (hydriodic acid/trifluoroacetic acid). The rapid reduction times and water solvent containing only small amounts of isopropanol and acetone may allow easy process up-scaling for technical applications and low-energy consumption.

Water-Soluble Phosphine Capable of Dissolving Elemental Gold: The Missing Link between 1,3,5-Triaza-7-phosphaadamantane (PTA) and Verkade's Ephemeral Ligand.

We herein describe a tricyclic phosphine with previously unreported tris(homoadamantane) cage architecture. That water-soluble, air- and thermally stable ligand, 1,4,7-triaza-9-phosphatricyclo[5.3.2.1(4,9)]tridecane (hereinafter referred to as CAP) exhibits unusual chemical behavior toward gold and gold compounds: it readily reduces Au(III) to Au(0), promotes oxidative dissolution of nanocrystalline gold(0) with the formation of water-soluble trigonal CAP-Au(I) complexes, and displaces cyanide from [Au(CN)2](-) affording triangular [Au(CAP)3](+) cation. From the stereochemical point of view, CAP can be regarded as an intermediate between 1,3,5-triaza-7-phosphaadamantane (PTA) and very unstable aminophosphine synthesized by Verkade's group: hexahydro-2a,4a,6a-triaza-6b-phosphacyclopenta[cd]pentalene. The chemical properties of CAP are likely related to its anomalous stereoelectronic profile: combination of strong electron-donating power (Tolman's electronic parameter 2056.8 cm(-1)) with the low steric demand (cone angle of 109°). CAP can be considered as macrocyclic counterpart of PTA with the electron-donating power approaching that of strongest known phosphine electron donors such as P(t-Bu)3 and PCy3. Therefore, CAP as sterically undemanding and electron-rich ligand populates the empty field on the stereoelectronic map of phosphine ligands: the niche between the classic tertiary phosphines and the sterically undemanding aminophosphines.

An aberration-corrected STEM study of structural defects in epitaxial GaN thin films grown by ion beam assisted MBE.

Ion-beam assisted molecular-beam epitaxy was used for direct growth of epitaxial GaN thin films on super-polished 6H-SiC(0001) substrates. The GaN films with different film thicknesses were studied using reflection high energy electron diffraction, X-ray diffraction, cathodoluminescence and primarily aberration-corrected scanning transmission electron microscopy techniques. Special attention was devoted to the microstructural characterization of GaN thin films and the GaN-SiC interface on the atomic scale. The results show a variety of defect types in the GaN thin films and at the GaN-SiC interface. A high crystalline quality of the produced hexagonal GaN thin films was demonstrated. The gained results are discussed.

Mechanistic aspects of the radiation-chemical reduction of graphene oxide to graphene-like materials.

PURPOSE: The aim of the work was to investigate mechanistic details of the preparation of graphene-like materials (GLM) via reduction of graphene oxide (GO) in aqueous dispersions by electron beam (EB) generated reducing free radicals. MATERIALS AND METHODS: A 10 MeV linear accelerator was employed to irradiate aqueous GO dispersions at ambient temperatures. The kinetics of GO reduction was followed using UV-Vis spectroscopy. The resulting GLM were characterized by X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM), Raman spectroscopy and conductivity measurements. RESULTS: The reduction of GO could be afforded with high efficiency within minutes at room temperature via the reaction of GO with reducing radicals generated by EB irradiation. The detailed investigation of the reduction mechanism allowed a selection of the best reducing free radicals in terms of both their efficiency and environmental impact of their precursors and final products. CONCLUSIONS: The EB-treatment of aqueous GO dispersions is a highly efficient, environmentally friendly, cost-effective and easily up-scalable method for the preparation of GLM. The efficiency of the new reduction approach is comparable with the best existing methods.