Ferroelectric incommensurate spin crystals.

Ferroics, especially ferromagnets, can form complex topological spin structures such as vortices1 and skyrmions2,3 when subjected to particular electrical and mechanical boundary conditions. Simple vortex-like, electric-dipole-based topological structures have been observed in dedicated ferroelectric systems, especially ferroelectric-insulator superlattices such as PbTiO3/SrTiO3, which was later shown to be a model system owing to its high depolarizing field4-8. To date, the electric dipole equivalent of ordered magnetic spin lattices driven by the Dzyaloshinskii-Moriya interaction (DMi)9,10 has not been experimentally observed. Here we examine a domain structure in a single PbTiO3 epitaxial layer sandwiched between SrRuO3 electrodes. We observe periodic clockwise and anticlockwise ferroelectric vortices that are modulated by a second ordering along their toroidal core. The resulting topology, supported by calculations, is a labyrinth-like pattern with two orthogonal periodic modulations that form an incommensurate polar crystal that provides a ferroelectric analogue to the recently discovered incommensurate spin crystals in ferromagnetic materials11-13. These findings further blur the border between emergent ferromagnetic and ferroelectric topologies, clearing the way for experimental realization of further electric counterparts of magnetic DMi-driven phases.

Defect Dynamics in Self-Catalyzed III-V Semiconductor Nanowires.

The droplet consumption step in self-catalyzed III-V semiconductor nanowires can produce material that contains a high density of line defects. Interestingly, these defects are often associated with twin boundaries and have null Burgers vector, i.e., no long-range strain field. Here, we analyze their stability by considering the forces that act on them and use in situ aberration corrected scanning transmission electron microscopy (STEM) to observe their behavior in GaAsP nanowires (NWs) using short annealing cycles. Their movement appears to be consistent with the thermally activated single- or double-kink mechanisms of dislocation glide, with velocities that do not exceed 1 nm s-1. We find that motion of individual defects depends on their size, position, and surrounding environment and set an upper limit to activation energy around 2 eV. The majority of defects (>70%) are removed by our postgrowth annealing for several seconds at temperatures in excess of 640 °C, suggesting that in situ annealing during growth at lower temperatures would significantly improve material quality. The remaining defects do not move at all and are thermodynamically stable in the nanowire.

Self-Formed Quantum Wires and Dots in GaAsP-GaAsP Core-Shell Nanowires.

Quantum structures designed using nanowires as a basis are excellent candidates to achieve novel design architectures. Here, triplets of quantum wires (QWRs) that form at the core-shell interface of GaAsP-GaAsP nanowires are reported. Their formation, on only three of the six vertices of the hexagonal nanowire, is governed by the three-fold symmetry of the cubic crystal on the (111) plane. In twinned nanowires, the QWRs are segmented, to alternating vertices, forming quantum dots (QDs). Simulations confirm the possibility of QWR and QD-like behavior from the respective regions. Optical measurements confirm the presence of two different types of quantum emitters in the twinned individual nanowires. The possibility to control the relative formation of QWRs or QDs, and resulting emission wavelengths of the QDs, by controlling the twinning of the nanowire core, opens up new possibilities for designing nanowire devices.

Towards a 3D GeSbTe phase change memory with integrated selector by non-aqueous electrodeposition.

We have recently reported a new method for the electrodeposition of thin film and nanostructured phase change memory (PCM) devices from a single, highly tuneable, non-aqueous electrolyte. The quality of the material was confirmed by phase cycling via electrical pulsed switching of both 100 nm nano-cells and thin film devices. This method potentially allows deposition into extremely small confined cells down to less than 5 nm, 3D lay-outs that require non-line-of-sight techniques, and seamless integration of selector devices. As electrodeposition requires a conducting substrate, the key condition for electronic applications based on this method is the use of patterned metal lines as the working electrode during the electrodeposition process. In this paper, we show the design and fabrication of a 2D passive memory matrix in which the word lines act as the working electrode and nucleation site for the growth of confined cells of Ge-Sb-Te. We will discuss the precursor requirement for deposition from non-aqueous, weakly coordinating solvents, show the transmission electron microscopy analysis of the electrodeposition growth process and elemental distribution in the deposits, and show the fabrication and characterisation of the Ge-Sb-Te memory matrix.

Exploration of the Smallest Diameter Tin Nanowires Achievable with Electrodeposition: Sub 7 nm Sn Nanowires Produced by Electrodeposition from a Supercritical Fluid.

Electrodeposition of Sn from supercritical difluoromethane has been performed into anodic alumina templates with pores down to 3 nm in diameter and into mesoporous silica templates with pores of diameter 1.5 nm. Optimized deposits have been characterized using X-ray diffraction, scanning electron microscopy, and scanning transmission electron microscopy (bright field, high-angle annular dark field, and energy-dispersive X-ray elemental mapping). Crystalline 13 nm diameter Sn nanowires have been electrodeposited in symmetric pore anodic alumina. Direct transmission electron microscopy evidence of sub 7 nm Sn nanowires in asymmetric anodic alumina has been obtained. These same measurements present indirect evidence for electrodeposition through 3 nm constrictions in the same templates. A detailed transmission electron microscopy study of mesoporous silica films after Sn deposition is presented. These indicate that it is possible to deposit Sn through the 1.5 nm pores in the mesoporous films, but that the nanowires formed are not stable. Suggestions of why this is the case and how such extreme nanowires could be stabilized are presented.

Nonradiative Step Facets in Semiconductor Nanowires.

One of the main advantages of nanowires for functional applications is their high perfection, which results from surface image forces that act on line defects such as dislocations, rendering them unstable and driving them out of the crystal. Here we show that there is a class of step facets that are stable in nanowires, with no long-range strain field or dislocation character. In zinc-blende semiconductors, they take the form of Σ3 (112) facets with heights constrained to be a multiple of three {111} monolayers. Density functional theory calculations show that they act as nonradiative recombination centers and have deleterious effects on nanowire properties. We present experimental observations of these defects on twin boundaries and twins that terminate inside GaAsP nanowires and find that they are indeed always multiples of three monolayers in height. Strategies to use the three-monolayer rule during growth to prevent their formation are discussed.

In situ annealing enhancement of the optical properties and laser device performance of InAs quantum dots grown on Si substrates.

The addition of elevated temperature steps (annealing) during the growth of InAs/GaAs quantum dot (QD) structures on Si substrates results in significant improvements in their structural and optical properties and laser device performance. This is shown to result from an increased efficacy of the dislocation filter layers (DFLs); reducing the density of dislocations that arise at the Si/III-V interface which reach the active region. The addition of two annealing steps gives a greater than three reduction in the room temperature threshold current of a 1.3 µm emitting QD laser on Si. The active region of structures grown on Si have a room temperature residual tensile strain of 0.17%, consistent with cool down from the growth temperature and the different Si and GaAs thermal expansion coefficients. This strain limits the amount of III-V material that can be grown before relaxation occurs.

Polarity-Driven Quasi-3-Fold Composition Symmetry of Self-Catalyzed III-V-V Ternary Core-Shell Nanowires.

A quasi-3-fold composition symmetry has for the first time been observed in self-catalyzed III-V-V core-shell nanowires. In GaAsP nanowires, phosphorus-rich sheets on radial {110} planes originating at the corners of the hexagonal core were observed. In a cross section, they appear as six radial P-rich bands that originate at the six outer corners of the hexagonal core, with three of them higher in P content along ⟨112⟩A direction and others along ⟨112⟩B, forming a quasi-3-fold composition symmetry. We propose that these P-rich bands are caused by a curvature-induced high surface chemical potential at the small corner facets, which drives As adatoms away more efficiently than P adatoms. Moreover, their polarity related P content difference can be explained by the different adatom bonding energies at these polar corner facets. These results provide important information on the further development of shell growth in the self-catalyzed core-shell NW structure and, hence, device structure for multicomponent material systems.

Lateral heterojunctions within monolayer MoSe2-WSe2 semiconductors.

Heterojunctions between three-dimensional (3D) semiconductors with different bandgaps are the basis of modern light-emitting diodes, diode lasers and high-speed transistors. Creating analogous heterojunctions between different 2D semiconductors would enable band engineering within the 2D plane and open up new realms in materials science, device physics and engineering. Here we demonstrate that seamless high-quality in-plane heterojunctions can be grown between the 2D monolayer semiconductors MoSe2 and WSe2. The junctions, grown by lateral heteroepitaxy using physical vapour transport, are visible in an optical microscope and show enhanced photoluminescence. Atomically resolved transmission electron microscopy reveals that their structure is an undistorted honeycomb lattice in which substitution of one transition metal by another occurs across the interface. The growth of such lateral junctions will allow new device functionalities, such as in-plane transistors and diodes, to be integrated within a single atomically thin layer.

InAsP quantum dot lasers grown by MOVPE.

We report on InAsP quantum dot lasers grown by MOVPE for 730-780 nm wavelength emission and compare performance with InP dot samples grown under similar conditions and with similar structures. 1-4 mm long, uncoated facet InAsP dot lasers emit between 760 and 775 nm and 2 mm long lasers with uncoated facets have threshold current density of 260 Acm(-2), compared with 150 Acm(-2) for InP quantum dot samples, which emit at shorter wavelengths, 715-725 nm. Pulsed lasing is demonstrated for InAsP dots up to 380 K with up to 200 mW output power. Measured absorption spectra indicate the addition of Arsenic to the dots has shifted the available transitions to longer wavelengths but also results in a much larger degree of spectral broadening. These spectra and transmission electron microscopy images indicate that the InAsP dots have a much larger degree of inhomogeneous broadening due to dot size variation, both from layer to layer and within a layer.

Mechanistic Insight into the Synthesis of Silica-Based "Matchstick" Colloids.

We report an insight into the synthesis of silica-based "matchstick"-shaped colloidal particles, which are of interest in the area of self-propulsion on small length scales. The generation of aqueous emulsion droplets dispersed in an n-pentanol-rich continuous phase and their use as reaction centers allows for the fabrication of siliceous microparticles that exhibit anisotropy in both particle morphology, that is, a "matchstick" shape, and chemistry, that is, a transition-metal oxide-enriched head. We provide a series of kinetic studies to gain a mechanistic understanding and unravel the particle formation and growth processes. Additionally, we demonstrate the ability to select the aspect ratio of the "matchstick" particle in a straightforward manner.

Ordered mesoporous silica films with pores oriented perpendicular to a titanium nitride substrate.

The production of thin mesoporous silica films with small (∼2-3 nm) pores oriented perpendicular to a titanium nitride growth surface is demonstrated using two methods. These are the growth from a Stöber silica solution with surfactant ordering at the surface of the electrode, and electrochemically assisted growth from an acidic sol achieved by polarisation of the electrode surface. The thickness, pore order and pore size that can be achieved with these two methods is contrasted. A number of methods to vary the pore size by using different surfactants and swelling agents are explored. The advantage of applying these growth methods on titanium nitride surfaces is that it provides access to a wider electrochemical window for nanowire growth and sensor applications with non-aqueous electrolytes whilst retaining good film growth and adhesion properties.

Wafer-scale fabrication of self-catalyzed 1.7 eV GaAsP core-shell nanowire photocathode on silicon substrates.

We present the wafer-scale fabrication of self-catalyzed p-n homojunction 1.7 eV GaAsP core-shell nanowire photocathodes grown on silicon substrates by molecular beam epitaxy with the incorporation of Pt nanoparticles as hydrogen evolution cocatalysts. Under AM 1.5G illumination, the GaAsP nanowire photocathode yielded a photocurrent density of 4.5 mA/cm(2) at 0 V versus a reversible hydrogen electrode and a solar-to-hydrogen conversion efficiency of 0.5%, which are much higher than the values previously reported for wafer-scale III-V nanowire photocathodes. In addition, GaAsP has been found to be more resistant to photocorrosion than InGaP. These results open up a new approach to develop efficient tandem photoelectrochemical devices via fabricating GaAsP nanowires on a silicon platform.

AgNb7O18: an ergodic relaxor ferroelectric.

AgNb7O18 is a relaxor ferroelectric with a Burns temperature of ~490 K and an incipient transition to the nonergodic state. The short-range structure is shown by convergent-beam electron diffraction to have the polar space group Im2m, but refinements against powder X-ray diffraction find the long-range structure to have the centrosymmetric space group Immm. Relaxor behavior in AgNb7O18 appears to originate from the partial occupation of large interstices by Ag(+) cations. Both cations and oxygen anions are displaced away from zones where NbO6 octahedra are edge-sharing.

High dynamic range electron imaging: the new standard.

Transmission electron microscopes regularly produce data which has a dynamic range that exceeds the capabilities of the recording media used, particularly in diffraction patterns. Hardware solutions such as readable phosphor imaging plates have existed since the 1990s, but in recent years the advent of robust CCD digital cameras capable of capturing high intensities in a transmission electron microscope has made image acquisition fast and straightforward. However, all CCD cameras have a saturation limit, making imaging of low intensities difficult when an image is dominated by strong features. Here we present a simple and effective tool to overcome this limitation through acquisition of multiple images and software processing to produce high dynamic range electron images.

Zinc Oxide Nanostructure Deposition into Sub-5 nm Vertical Mesopores in Silica Hard Templates by Atomic Layer Deposition.

Nanostructures synthesised by hard-templating assisted methods are advantageous as they retain the size and morphology of the host templates which are vital characteristics for their intended applications. A number of techniques have been employed to deposit materials inside porous templates, such as electrodeposition, vapour deposition, lithography, melt and solution filling, but most of these efforts have been applied with pore sizes higher in the mesoporous regime or even larger. Here, we explore atomic layer deposition (ALD) as a method for nanostructure deposition into mesoporous hard templates consisting of mesoporous silica films with sub-5 nm pore diameters. The zinc oxide deposited into the films was characterised by small-angle X-ray scattering, X-ray diffraction and energy-dispersive X-ray analysis.

An ultra high-endurance memristor using back-end-of-line amorphous SiC.

Integrating resistive memory or neuromorphic memristors into mainstream silicon technology can be substantially facilitated if the memories are built in the back-end-of-line (BEOL) and stacked directly above the logic circuitries. Here we report a promising memristor employing a plasma-enhanced chemical vapour deposition (PECVD) bilayer of amorphous SiC/Si as device layer and Cu as an active electrode. Its endurance exceeds one billion cycles with an ON/OFF ratio of ca. two orders of magnitude. Resistance drift is observed in the first 200 million cycles, after which the devices settle with a coefficient of variation of ca. 10% for both the low and high resistance states. Ohmic conduction in the low resistance state is attributed to the formation of Cu conductive filaments inside the bilayer structure, where the nanoscale grain boundaries in the Si layer provide the pre-defined pathway for Cu ion migration. Rupture of the conductive filament leads to current conduction dominated by reverse bias Schottky emission. Multistate switching is achieved by precisely controlling the pulse conditions for potential neuromorphic computing applications. The PECVD deposition method employed here has been frequently used to deposit typical BEOL SiOC low-k interlayer dielectrics. This makes it a unique memristor system with great potential for integration.

Experimental and theoretical evidence for unprecedented strong interactions of gold atoms with boron on boron/sulfur-doped carbon surfaces.

The 16e square-planar bis-thiolato-Au(iii) complexes [AuIII(1,2-dicarba-closo-dodecarborane-1,2-dithiolato)2][NBu4] (Au-1) and [AuIII(4-methyl-1,2-benzenedithiolato)2][NBu4] (Au-2) have been synthesized and fully characterized. Au-1 and Au-2 were encapsulated in the symmetrical triblock copolymer poloxamer (Pluronic®) P123 containing blocks of poly(ethylene oxide) and poly(propylene oxide), giving micelles AuMs-1 and AuMs-2. High electron flux in scanning transmission electron microscopy (STEM) was used to generate single gold atoms and gold nanocrystals on B/S-doped graphitic surfaces, or S-doped amorphous carbon surfaces from AuMs-1 and AuMs-2, respectively. Electron energy loss spectroscopy (EELS) data suggested strong interactions of gold atoms/nanocrystals with boron in the B/S-doped graphitic matrix. Density-functional theory (DFT) calculations, also supported the experimental findings, pointing towards strong Au-B bonds, depending on the charge on the Au-(B-graphene) fragment and the presence of further defects in the graphene lattice.

Film before aggregates: an operando GISAXS study on electrochemically assisted surfactant assembly.

The process of electrochemically assisted surfactant assembly was followed in real time by grazing incidence small angle X-ray scattering with the aim to deconvolute the formation of mesoporous silica film and unwanted porous particles. The X-ray technique proved to be useful for the characterisation of this process, as it takes place at a very dynamic, solid/liquid interface. This paper shows the electrochemically driven onset and evolution of silica/surfactant structures. Additional control experiments indicate the formation of vertically aligned structures without the use of an electric field, although it seems to be beneficial for increased pore ordering.

Modelling fine-sliced three dimensional electron diffraction data with dynamical Bloch-wave simulations.

Recent interest in structure solution and refinement using electron diffraction (ED) has been fuelled by its inherent advantages when applied to crystals of sub-micrometre size, as well as its better sensitivity to light elements. Currently, data are often processed with software written for X-ray diffraction, using the kinematic theory of diffraction to generate model intensities - despite the inherent differences in diffraction processes in ED. Here, dynamical Bloch-wave simulations are used to model continuous-rotation electron diffraction data, collected with a fine angular resolution (crystal orientations of ∼0.1°). This fine-sliced data allows a re-examination of the corrections applied to ED data. A new method is proposed for optimizing crystal orientation, and the angular range of the incident beam and the varying slew rate are taken into account. Observed integrated intensities are extracted and accurate comparisons are performed with simulations using rocking curves for a (110) lamella of silicon 185 nm thick. R1 is reduced from 26% with the kinematic model to 6.8% using dynamical simulations.