Quantitative Mapping of Chemical Defects at Charged Grain Boundaries in a Ferroelectric Oxide.

Polar discontinuities, as well as compositional and structural changes at oxide interfaces can give rise to a large variety of electronic and ionic phenomena. In contrast to earlier work focused on domain walls and epitaxial systems, this work investigates the relation between polar discontinuities and the local chemistry at grain boundaries in polycrystalline ferroelectric ErMnO3 . Using orientation mapping and scanning probe microscopy (SPM) techniques, the polycrystalline material is demonstrated to develop charged grain boundaries with enhanced electronic conductance. By performing atom probe tomography (APT) measurements, an enrichment of erbium and a depletion of oxygen at all grain boundaries are found. The observed compositional changes translate into a charge that exceeds possible polarization-driven effects, demonstrating that structural phenomena rather than electrostatics determine the local chemical composition and related changes in the electronic transport behavior. The study shows that the charged grain boundaries behave distinctly different from charged domain walls, giving additional opportunities for property engineering at polar oxide interfaces.

Focused ion beam lithography for position-controlled nanowire growth.

To exploit the promising properties of semiconductor nanowires and ensure the uniformity required to achieve device integration, their position on the growth substrate must be controlled. This work demonstrates the direct patterning of a SiO2/Si substrate using focused ion beam (FIB) patterning to control self-catalyzed GaAsSb nanowire growth in molecular beam epitaxy (MBE). Besides position control, FIB patterning parameters influence nanowire yield, composition and structure. Total ion dose per hole is found to be the most important parameter. Yield of single nanowires ranges from ≈34% to ≈83%, with larger holes dominated by multiple nanowires per hole. Areas exposed to low ion beam doses are selectively etched by routine pre-MBE HF cleaning, enabling patterning and nanowire nucleation with minimal damage to the Si substrate. The optical and electronic properties of nanowires are found to depend on the ion dose used during patterning, indicating the potential for FIB patterning to tune nanowire properties. These findings demonstrate the possibility for a FIB lithography protocol which could provide a rapid and direct patterning process for flexible controlled nanowire growth.

The Third Dimension of Ferroelectric Domain Walls.

Ferroelectric domain walls are quasi-2D systems that show great promise for the development of nonvolatile memory, memristor technology, and electronic components with ultrasmall feature size. Electric fields, for example, can change the domain wall orientation relative to the spontaneous polarization and switch between resistive and conductive states, controlling the electrical current. Being embedded in a 3D material, however, the domain walls are not perfectly flat and can form networks, which leads to complex physical structures. In this work, the importance of the nanoscale structure for the emergent transport properties is demonstrated, studying electronic conduction in the 3D network of neutral and charged domain walls in ErMnO3 . By combining tomographic microscopy techniques and finite element modeling, the contribution of domain walls within the bulk is clarified and the significance of curvature effects for the local conduction is shown down to the nanoscale. The findings provide insights into the propagation of electrical currents in domain wall networks, reveal additional degrees of freedom for their control, and provide quantitative guidelines for the design of domain-wall-based technology.

Composition Analysis by STEM-EDX of Ternary Semiconductors by Internal References.

A practical method to determine the composition within ternary heterostructured semiconductor compounds using energy-dispersive X-ray spectroscopy in scanning transmission electron microscopy is presented. The method requires minimal external input factors such as user-determined or calculated sensitivity factors by incorporating a known compositional relationship, here a fixed stoichiometric ratio in III­V compound semiconductors. The method is demonstrated for three different systems; AlGaAs/GaAs, GaAsSb/GaAs, and InGaN/GaN with three different specimen geometries and compared to conventional quantification approaches. The method incorporates absorption effects influencing the composition analysis without the need to know the thickness of the specimen. Large variations in absorption conditions and assumptions regarding the reference area limit the accuracy of the developed method.

Observation of Electric-Field-Induced Structural Dislocations in a Ferroelectric Oxide.

Dislocations are 1D topological defects with emergent electronic properties. Their low dimensionality and unique properties make them excellent candidates for innovative device concepts, ranging from dislocation-based neuromorphic memory to light emission from diodes. To date, dislocations are created in materials during synthesis via strain fields or flash sintering or retrospectively via deformation, for example, (nano)-indentation, limiting the technological possibilities. In this work, we demonstrate the creation of dislocations in the ferroelectric semiconductor Er(Mn,Ti)O3 with nanoscale spatial precision using electric fields. By combining high-resolution imaging techniques and density functional theory calculations, direct images of the dislocations are collected, and their impact on the local electric transport behavior is studied. Our approach enables local property control via dislocations without the need for external macroscopic strain fields, expanding the application opportunities into the realm of electric-field-driven phenomena.

Publisher Correction: Conductivity control via minimally invasive anti-Frenkel defects in a functional oxide.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Conductivity control via minimally invasive anti-Frenkel defects in a functional oxide.

Utilizing quantum effects in complex oxides, such as magnetism, multiferroicity and superconductivity, requires atomic-level control of the material's structure and composition. In contrast, the continuous conductivity changes that enable artificial oxide-based synapses and multiconfigurational devices are driven by redox reactions and domain reconfigurations, which entail long-range ionic migration and changes in stoichiometry or structure. Although both concepts hold great technological potential, combined applications seem difficult due to the mutually exclusive requirements. Here we demonstrate a route to overcome this limitation by controlling the conductivity in the functional oxide hexagonal Er(Mn,Ti)O3 by using conductive atomic force microscopy to generate electric-field induced anti-Frenkel defects, that is, charge-neutral interstitial-vacancy pairs. These defects are generated with nanoscale spatial precision to locally enhance the electronic hopping conductivity by orders of magnitude without disturbing the ferroelectric order. We explain the non-volatile effects using density functional theory and discuss its universality, suggesting an alternative dimension to functional oxides and the development of multifunctional devices for next-generation nanotechnology.

Epitaxially grown III-arsenide-antimonide nanowires for optoelectronic applications.

Epitaxially grown ternary III-arsenide-antimonide (III-As-Sb) nanowires (NWs) are increasingly attracting attention due to their feasibility as a platform for the integration of largely lattice-mismatched antimonide-based heterostructures while preserving the high crystal quality. This and the inherent bandgap tuning flexibility of III-As-Sb in the near- and mid-infrared wavelength regions are important and auspicious premises for a variety of optoelectronic applications. In this review, we summarize the current understanding of the nucleation, morphology-change and crystal phase evolution of GaAsSb and InAsSb NWs and their characterization, especially in relation to Sb incorporation during growth. By linking these findings to the optical properties in such ternary NWs and their heterostructures, a brief account of the ongoing development of III-As-Sb NW-based photodetectors and light emitters is also given.

Stabilisation of amorphous calcium phosphate in polyethylene glycol hydrogels.

Acellular polymer-calcium phosphate composites are promising bone graft materials. Hydrogels are suitable for providing a temporary matrix, while calcium phosphate minerals serve as ion depots for calcium and phosphate required for de novo bone formation. Crystalline calcium phosphates are stable under biological conditions and are commonly used in such scaffolds. However, the low solubility of these phases reduces the availability of free ions and potentially obstructs the remodelling necessary for the formation of mineralised tissue. Here, we investigate two different strategies to stabilise amorphous calcium phosphates in a synthetic polyethylene glycol-based hydrogel matrix. In vitro experiments mimicking an injectable application showed that amorphous calcium phosphate (ACP) of variable stability was formed in the hydrogel matrices. In additive-free composites, ACP transformed into brushite within minutes. Citrate or zinc additives were found to stabilise the formed ACP phase to different degrees. In the presence of citrate, ACP was stable for at least 2 h before it transformed into hydroxyapatite within 3-20 days. Partial calcium substitution with zinc (Zn/Ca = 10%) produced zinc-doped ACP of high stability that did not show signs of crystallisation for at least 20 days. The presented methods and findings open new possibilities for the design of novel injectable synthetic bone graft materials. The possibility to produce ACP with tailorable stability promises great potential for creating temporary scaffolds with good osteogenic properties. STATEMENT OF SIGNIFICANCE: Synthetic hydrogel-calcium phosphate (CaP) composites are promising biomaterials to replace human- and animal-derived bone scaffolds. Most reported hydrogel-CaP composite materials employ crystalline CaP phases that lack the osteoinductive properties of autograft. Stabilising amorphous calcium phosphates (ACP) could overcome this limitation, readily delivering calcium and phosphate ions and facilitating remodelling into new bone tissue. The design of synthetic hydrogel-ACP scaffolds, however, requires more understanding of the mineralisation processes in such matrices. This study presents a model system to characterise the complex mineral formation and transformation processes within a hydrogel matrix. We demonstrate a facile route to produce self-mineralising injectable synthetic hydrogels and prove two different strategies to stabilise ACP for different periods within the formed composites.

Single-Mode Near-Infrared Lasing in a GaAsSb-Based Nanowire Superlattice at Room Temperature.

Semiconductor nanowire lasers can produce guided coherent light emission with miniaturized geometry, bringing about new possibilities for a variety of applications including nanophotonic circuits, optical sensing, and on-chip and chip-to-chip optical communications. Here, we report on the realization of single-mode and room-temperature lasing from 890 to 990 nm, utilizing a novel design of single nanowires with GaAsSb-based multiple axial superlattices as a gain medium under optical pumping. The control of lasing wavelength via compositional tuning with excellent room-temperature lasing performance is shown to result from the unique nanowire structure with efficient gain material, which delivers a low lasing threshold of ∼6 kW/cm2 (75 µJ/cm2 per pulse), a lasing quality factor as high as 1250, and a high characteristic temperature of ∼129 K. These results present a major advancement for the design and synthesis of nanowire laser structures, which can pave the way toward future nanoscale integrated optoelectronic systems with superior performance.

Vertically Oriented Growth of GaN Nanorods on Si Using Graphene as an Atomically Thin Buffer Layer.

The monolithic integration of wurtzite GaN on Si via metal-organic vapor phase epitaxy is strongly hampered by lattice and thermal mismatch as well as meltback etching. This study presents single-layer graphene as an atomically thin buffer layer for c-axis-oriented growth of vertically aligned GaN nanorods mediated by nanometer-sized AlGaN nucleation islands. Nanostructures of similar morphology are demonstrated on graphene-covered Si(111) as well as Si(100). High crystal and optical quality of the nanorods are evidenced through scanning transmission electron microscopy, micro-Raman, and cathodoluminescence measurements supported by finite-difference time-domain simulations. Current-voltage characteristics revealed high vertical conduction of the as-grown GaN nanorods through the Si substrates. These findings are substantial to advance the integration of GaN-based devices on any substrates of choice that sustains the GaN growth temperatures, thereby permitting novel designs of GaN-based heterojunction device concepts.

In Situ Heat-Induced Replacement of GaAs Nanowires by Au.

Here we report on the heat-induced solid-state replacement of GaAs by Au in nanowires. Such replacement of semiconductor nanowires by metals is envisioned as a method to achieve well-defined junctions within nanowires. To better understand the mechanisms and dynamics that govern the replacement reaction, we performed in situ heating studies using high-resolution scanning transmission electron microscopy. The dynamic evolution of the phase boundary was investigated, as well as the crystal structure and orientation of the different phases at reaction temperatures. In general, the replacement proceeds one GaAs(111) bilayer at a time, and no fixed epitaxial relation could be found between the two phases. The relative orientation of the phases affects the replacement dynamics and can induce growth twins in the Au nanowire phase. In the case of a limited Au supply, the metal phase can also become liquid.

New Insights into the Origins of Sb-Induced Effects on Self-Catalyzed GaAsSb Nanowire Arrays.

Ternary semiconductor nanowire arrays enable scalable fabrication of nano-optoelectronic devices with tunable bandgap. However, the lack of insight into the effects of the incorporation of Vy element results in lack of control on the growth of ternary III-V(1-y)Vy nanowires and hinders the development of high-performance nanowire devices based on such ternaries. Here, we report on the origins of Sb-induced effects affecting the morphology and crystal structure of self-catalyzed GaAsSb nanowire arrays. The nanowire growth by molecular beam epitaxy is changed both kinetically and thermodynamically by the introduction of Sb. An anomalous decrease of the axial growth rate with increased Sb2 flux is found to be due to both the indirect kinetic influence via the Ga adatom diffusion induced catalyst geometry evolution and the direct composition modulation. From the fundamental growth analyses and the crystal phase evolution mechanism proposed in this Letter, the phase transition/stability in catalyst-assisted ternary III-V-V nanowire growth can be well explained. Wavelength tunability with good homogeneity of the optical emission from the self-catalyzed GaAsSb nanowire arrays with high crystal phase purity is demonstrated by only adjusting the Sb2 flux.

Rectifying Single GaAsSb Nanowire Devices Based on Self-Induced Compositional Gradients.

Device configurations that enable a unidirectional propagation of carriers in a semiconductor are fundamental components for electronic and optoelectronic applications. To realize such devices, however, it is generally required to have complex processes to make p-n or Schottky junctions. Here we report on a unidirectional propagation effect due to a self-induced compositional variation in GaAsSb nanowires (NWs). The individual GaAsSb NWs exhibit a highly reproducible rectifying behavior, where the rectifying direction is determined by the NW growth direction. Combining the results from confocal micro-Raman spectroscopy, electron microscopy, and electrical measurements, the origin of the rectifying behavior is found to be associated with a self-induced variation of the Sb and the carrier concentrations in the NW. To demonstrate the usefulness of these GaAsSb NWs for device applications, NW-based photodetectors and logic circuits have been made.

Rapid estimation of catalyst nanoparticle morphology and atomic-coordination by high-resolution Z-contrast electron microscopy.

Heterogeneous nanoparticle catalyst development relies on an understanding of their structure-property relationships, ideally at atomic resolution and in three-dimensions. Current transmission electron microscopy techniques such as discrete tomography can provide this but require multiple images of each nanoparticle and are incompatible with samples that change under electron irradiation or with surveying large numbers of particles to gain significant statistics. Here, we make use of recent advances in quantitative dark-field scanning transmission electron microscopy to count the number atoms in each atomic column of a single image from a platinum nanoparticle. These atom-counts, along with the prior knowledge of the face-centered cubic geometry, are used to create atomistic models. An energy minimization is then used to relax the nanoparticle's 3D structure. This rapid approach enables high-throughput statistical studies or the analysis of dynamic processes such as facet-restructuring or particle damage.

A story told by a single nanowire: optical properties of wurtzite GaAs.

The optical properties of the wurtzite (WZ) GaAs crystal phase found in nanowires (NWs) are a highly controversial topic. Here, we study high-quality pure WZ GaAs/AlGaAs core-shell NWs grown by Au-assisted molecular beam epitaxy (MBE) with microphotoluminescence spectroscopy (µ-PL) and (scanning) transmission electron microscopy on the very same single wire. We determine the room temperature (294 K) WZ GaAs bandgap to be 1.444 eV, which is ∼20 meV larger than in zinc blende (ZB) GaAs, and show that the free exciton emission at 15 K is at 1.516 eV. On the basis of time- and temperature-resolved µ-PL results, we propose a Γ(8) conduction band symmetry in WZ GaAs. We suggest a method for quantifying the optical quality of NWs, taking into consideration the difference between the room and low temperature integrated PL intensity, and demonstrate that Au-assisted GaAs/AlGaAs core-shell NWs can have high PL brightness up to room temperature.