Search

Organic-inorganic hybrid perovskites are promising materials for the next generation photovoltaics and optoelectronics; however, their practical application has been hindered by poor structural stability mainly caused by ion migration and external stimuli. Understanding the mechanism(s) of ion migration and structure decomposition is thus critical. Here we observe the sequence of structural changes at the atomic level that precede structural decomposition in the technologically important Cs 1-x FA x PbI 3 using ultralow dose transmission electron microscopy. We find that these changes differ, depending upon the A-site composition. Initially, there is a random loss of FA + , complemented by the loss of I - . The remaining FA + and I - ions then migrate, unit cell by unit cell, into an ordered and more stable phase with a √2 x √2 superstructure. Further ion loss is accompanied by A-site dependent octahedral tilt modes and associated tetragonal phases with different stabilities. These observations of the loss of FA + /I - ion pairs, ion migration, octahedral tilt modes, and the role of the A-cation, provide insights into the atomic-scale structural mechanisms that drive and block ion loss and ion migration, opening pathways to inhibit ion loss, migration and improve structural stability., (© 2023. The Author(s).)

Diamond's unique properties on the nanoscale make it one of the most important materials for use in biosensors and quantum computing and for components that can withstand the harsh environments of space. We synthesize oriented, faceted diamond particles by flash laser heating of glassy carbon at 16 GPa and 2300 K. Detailed transmission electron microscopy shows them to consist of a mosaic of diamond nanocrystals frequently joined at twin boundaries forming microtwins. Striking 3-fold translational periodicity was observed in both imaging and diffraction. This periodicity was shown to originate from nanodimensional wedge-shaped overlapping regions of twinned diamond and not from a possible 9R polytype, which has also been reported in other group IVa elements and water ice. Extended bilayers of hexagonal layer stacking were observed, forming lonsdaleite nanolaminates. The particles exhibited optical fluorescence with a rapid quench time (<1 ns) attributed to their unique twinned microstructure.

Integrated, on-chip lasers are vital building blocks in future optoelectronic and nanophotonic circuitry. Specifically, III-V materials that are of technological relevance have attracted considerable attention. However, traditional microcavity laser fabrication techniques, including top-down etching and bottom-up catalytic growth, often result in undesirable cavity geometries with poor scalability and reproducibility. Here, we utilize the selective area epitaxy method to deterministically engineer thousands of microring lasers on a single chip. Specifically, we realize a catalyst-free, epitaxial growth of a technologically critical material, InAsP/InP, in a ring-like cavity with embedded multi-quantum-well heterostructures. We elucidate a detailed growth mechanism and leverage the capability to deterministically control the adatom diffusion lengths on selected crystal facets to reproducibly achieve ultrasmooth cavity sidewalls. The engineered devices exhibit a tunable emission wavelength in the telecommunication O-band and show low-threshold lasing with over 80% device efficacy across the chip. Our work marks a significant milestone toward the implementation of a fully integrated III-V materials platform for next-generation high-density integrated photonic and optoelectronic circuits.

The waveguide modes in chemically-grown silver nanowires on silicon nitride substrates are observed using spectrally- and spatially-resolved cathodoluminescence (CL) excited by high-energy electrons in a scanning electron microscope. The presence of a long-range, travelling surface plasmon mode modulates the coupling efficiency of the incident electron energy into the nanowires, which is observed as oscillations in the measured CL with the point of excitation by the focused electron beam. The experimental data are modeled using the theory of surface plasmon polariton modes in cylindrical metal waveguides, enabling the complex mode wavenumbers and excitation strength of the long-range surface plasmon mode to be extracted. The experiments yield insight into the energy transfer mechanisms between fast electrons and coherent oscillations in surface charge density in metal nanowires and the relative amplitudes of the radiative processes excited in the wire by the electron., (© 2023 The Authors. Small published by Wiley-VCH GmbH.)

Scanning transmission electron microscopy (STEM) has a broad range of applications in materials characterization, including real-space imaging, spectroscopy, and diffraction, at length scales from the micron to sub-Ångström. The recent development and adoption of high-speed, direct electron STEM detectors has enabled diffraction patterns to be collected at each probe position, generating four-dimensional STEM (4D-STEM) datasets and opening new imaging modalities. However, the limited pixel numbers in these detectors enforce a tradeoff between angular resolution and maximum collection angle. In this paper, we describe a straightforward method for quantifying 4D-STEM data by utilizing the full flux of the electron beam, including electrons scattered beyond the limits of the detector. This enables significantly increased experimental flexibility, including the synthesis of quantitative, high-contrast complementary annular dark field (cADF) STEM images from low-angle diffraction patterns whilst maintaining high angular resolution; as well as the optimization of electron dose and the more effective use of low dynamic range detectors., Competing Interests: Declaration of Competing Interest None., (Copyright © 2022 Elsevier B.V. All rights reserved.)

We introduce an image-contrast mechanism for scanning transmission electron microscopy (STEM) that derives from the local symmetry within the specimen. For a given position of the electron probe on the specimen, the image intensity is determined by the degree of similarity between the exit electron-intensity distribution and a chosen symmetry operation applied to that distribution. The contrast mechanism detects both light and heavy atomic columns and is robust with respect to specimen thickness, electron-probe energy, and defocus. Atomic columns appear as sharp peaks that can be significantly narrower than for STEM images using conventional disk and annular detectors. This fundamentally different contrast mechanism complements conventional imaging modes and can be acquired simultaneously with them, expanding the power of STEM for materials characterization., Competing Interests: The authors declare no competing interest.

We present spatially and spectrally resolved emission from nanowires with a thin radial layer of GaAs embedded in AlGaAs barriers, grown radially around taper-free GaAs cores. The GaAs layers are thin enough to show quantization, and are quantum wells. Due to their shape, they are referred to as quantum well tubes (QWTs). We have investigated three different nominal QWT thicknesses: 1.5, 2.0, and 6.0 nm. They all show average emission spectra from the QWT with an energy spread corresponding to a thickness variation of ±30%. We observe no thickness gradient along the length of the nanowires. Individual NWs show a number of peaks, corresponding to different QW thicknesses. Apart from the thinnest QWT, the integrated emission from the QWTs shows homogeneous emission intensity along the NW. The thinnest QWTs show patchy emission patterns due to the incomplete coverage of the QWT. We observe a few NWs with larger diameters. The QWTs in these NWs show spatially resolved variations across the NW. An increase in the local thickness of the QWT at the corners blocks the diffusion of carriers from facet to facet, thereby enabling us to visualise the thickness variations of the radial quantum wells.

Nanoparticle dimers composed of different metals or metal oxides, as well as different shapes and sizes, are of wide interest for applications ranging from nanoplasmonic sensing to nanooptics to biomedical engineering. Shaped nanoparticles, like triangles and nanorods, can be particularly useful in applications due to the strong localized plasmonic hot-spot that forms at the tips or corners. By placing catalytic, but traditionally weakly- or non-plasmonic nanoparticles, such as metal oxides and metals like palladium, in these hot-spots, an enhanced function for sensing, photocatalysis or optical use is predicted. Here, we present an electrostatic colloidal assembly strategy for nanoparticles, incorporating different sizes, shapes and metal or metal oxide compositions into heterodimers with smaller gaps than are achievable using nanofabrication techniques. This versatile method is demonstrated on 14 combinations, including a variety of shaped gold nanoparticles as well as palladium, iron oxide, and titanium oxide nanoparticles. These colloidal nanoparticles are stabilized with traditional surfactants, such as citrate, CTAB, PVP and oleic acid/oleylamines, indicating the wide applicability of our approach. Heterodimers of gold and palladium are further analyzed using cathodoluminescence to demonstrate the tunability of these "plasmonic molecules". Since systematically altering the absorption and emission of the plasmonic nanoparticles dimers is crucial to extending their functionality, and small gap sizes produce the strongest hot-spots, this method indicates that the electrostatic approach to heterodimer assembly can be useful in creating new nanoparticle dimers for many applications.

Electronic phase separation in complex oxides is the inhomogeneous spatial distribution of electronic phases, involving length scales much larger than those of structural defects or nonuniform distribution of chemical dopants. While experimental efforts focused on phase separation and established its correlation with nonlinear responses under external stimuli, it remains controversial whether phase separation requires quenched disorder for its realization. Early theory predicted that if perfectly "clean" samples could be grown, both phase separation and nonlinearities would be replaced by a bicritical-like phase diagram. Here, using a layer-by-layer superlattice growth technique we fabricate a fully chemically ordered "tricolor" manganite superlattice, and compare its properties with those of isovalent alloyed manganite films. Remarkably, the fully ordered manganite does not exhibit phase separation, while its presence is pronounced in the alloy. This suggests that chemical-doping-induced disorder is crucial to stabilize the potentially useful nonlinear responses of manganites, as theory predicted., Competing Interests: The authors declare no competing interest.

Nanosizing of metal oxide particles is a common strategy for improving materials properties; however, small particles often take structures different from the bulk material. MoO 2 nanoparticles show a structure that is distinct from the bulk distorted rutile structure and which has not yet been determined. Here, we present a model for nanostructured MoO 2 obtained through detailed atomic pair distribution function analysis combined with high-resolution electron microscopy. Defects occur in the arrangement of [MoO 6 ] octahedra, in both large (40-100 nm) nanoparticles, where the overall distorted rutile structure is preserved, and in small nanoparticles (<5 nm), where a new nanostructure is formed. The study provides a piece in the puzzle of understanding the structure/properties relationship of molybdenum oxides and further our understanding of the origin of structural changes taking place upon nanosizing in oxide materials.

Lighting accounts for one-fifth of global electricity consumption 1 . Single materials with efficient and stable white-light emission are ideal for lighting applications, but photon emission covering the entire visible spectrum is difficult to achieve using a single material. Metal halide perovskites have outstanding emission properties 2,3 ; however, the best-performing materials of this type contain lead and have unsatisfactory stability. Here we report a lead-free double perovskite that exhibits efficient and stable white-light emission via self-trapped excitons that originate from the Jahn-Teller distortion of the AgCl 6 octahedron in the excited state. By alloying sodium cations into Cs 2 AgInCl 6 , we break the dark transition (the inversion-symmetry-induced parity-forbidden transition) by manipulating the parity of the wavefunction of the self-trapped exciton and reduce the electronic dimensionality of the semiconductor 4 . This leads to an increase in photoluminescence efficiency by three orders of magnitude compared to pure Cs 2 AgInCl 6 . The optimally alloyed Cs 2 (Ag 0.60 Na 0.40 )InCl 6 with 0.04 per cent bismuth doping emits warm-white light with 86 ± 5 per cent quantum efficiency and works for over 1,000 hours. We anticipate that these results will stimulate research on single-emitter-based white-light-emitting phosphors and diodes for next-generation lighting and display technologies.

III-V semiconductor multi-quantum-well nanowires (MQW NWs) via selective-area epitaxy (SAE) is of great importance for the development of nanoscale light-emitting devices for applications such as optical communication, silicon photonics, and quantum computing. To achieve highly efficient light-emitting devices, not only the high-quality materials but also a deep understanding of their growth mechanisms and material properties (structural, optical, and electrical) are extremely critical. In particular, the three-dimensional growth mechanism of MQWs embedded in a NW structure by SAE is expected to be different from that of those grown in a planar structure or with a catalyst and has not yet been thoroughly investigated. In this work, we reveal a distinctive radial growth evolution of InGaAs/InP MQW NWs grown by the SAE metal organic vapor-phase epitaxy (MOVPE) technique. We observe the formation of zinc blende (ZB) QW discs induced by the axial InGaAs QW growth on the wurtzite (WZ) base-InP NW and propose it as the key factor driving the overall structure of radial growth. The role of the ZB-to-WZ change in the driving of the overall growth evolution is supported by a growth formalism, taking into account the formation-energy difference between different facets. Despite a polytypic crystal structure with mixed ZB and WZ phases across the MQW region, the NWs exhibit high uniformity and desirable QW spatial layout with bright room-temperature photoluminescence at an optical communication wavelength of ∼1.3 μm, which is promising for the future development of high-efficiency light-emitting devices.

Under almost all circumstances, electron diffraction patterns contain information about the phases of structure factors, a consequence of the short wavelength of an electron and its strong Coulombic interaction with matter. However, extracting this information remains a challenge and no generic method exists. In this work, a set of simple analytical expressions is derived for the intensity distribution in convergent-beam electron diffraction (CBED) patterns recorded under three-beam conditions. It is shown that these expressions can be used to identify features in three-beam CBED patterns from which three-phase invariants can be extracted directly, without any iterative refinement processes. The octant, in which the three-phase invariant lies, can be determined simply by inspection of the indexed CBED patterns ( i.e. the uncertainty of the phase measurement is ±22.5°). This approach is demonstrated with the experimental measurement of three-phase invariants in two simple test cases: centrosymmetric Si and non-centrosymmetric GaAs. This method may complement existing structure determination methods by providing direct measurements of three-phase invariants to replace 'guessed' invariants in ab initio phasing methods and hence provide more stringent constraints to the structure solution.

Organic-inorganic hybrid perovskites, such as CH 3 NH 3 PbI 3, have shown highly promising photovoltaic performance. Electron microscopy (EM) is a powerful tool for studying the crystallography, morphology, interfaces, lattice defects, composition, and charge carrier collection and recombination properties at the nanoscale. Here, the sensitivity of CH 3 NH 3 PbI 3 to electron beam irradiation is examined. CH 3 NH 3 PbI 3 undergoes continuous structural and compositional changes with increasing electron dose, with the total dose, rather than dose rate, being the key operative parameter. Importantly, the first structural change is subtle and easily missed and occurs after an electron dose significantly smaller than that typically applied in conventional EM techniques. The electron dose conditions under which these structural changes occur are identified. With appropriate dose-minimization techniques, electron diffraction patterns can be obtained from pristine material consistent with the tetragonal CH 3 NH 3 PbI 3 phases determined by X-ray diffraction. Radiation damage incurred at liquid nitrogen temperatures and using Ga + irradiation in a focused ion beam instrument are also examined. Finally, some simple guidelines for how to minimize electron-beam-induced artifacts when using EM to study hybrid perovskite materials are provided., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We investigate the transformation of single crystal gold nanorod surface morphology over extended growth times. After initial rapid anisotropic growth and disappearance of {111} bridging facets, the aspect ratios converge across AgNO 3 concentrations. The surface morphology transitions from faceted to curved. These observations imply the final aspect ratio has little dependence on the AgNO 3 concentration, consistent with primary control of the AgNO 3 over aspect ratio occurring at the symmetry breaking point.

The phenomenon of symmetry breaking-in which the order of symmetry of a system is reduced despite manifest higher-order symmetry in the underlying fundamental laws-is pervasive throughout science and nature, playing a critical role in fields ranging from particle physics and quantum theory to cosmology and general relativity. For the growth of crystals, symmetry breaking is the crucial step required to generate a macroscopic shape that has fewer symmetry elements than the unit cell and/or seed crystal from which it grew. Advances in colloid synthesis have enabled a wide variety of nanocrystal morphologies to be achieved, albeit empirically. Of the various nanoparticle morphologies synthesized, gold nanorods have perhaps been the most intensely studied, thanks largely to their unique morphology-dependent optical properties and exciting application potential. However, despite intense research efforts, an understanding of the mechanism by which a single crystal breaks symmetry and grows anisotropically has remained elusive, with many reports presenting seemingly conflicting data and theories. A fundamental understanding of the symmetry breaking process is needed to provide a rational framework upon which future synthetic approaches can be built. Inspired by recent experimental results and drawing upon the wider literature, we present a mechanism for gold nanorod growth from the moments prior to symmetry breaking to the final product. In particular, we describe the steps by which a cuboctahedral seed particle breaks symmetry and undergoes anisotropic growth to form a nanorod. With an emphasis on the evolving crystal structure, we highlight the key geometrical and chemical drivers behind the symmetry breaking process and factors that govern the formation and growth of nanorods, including control over the crystal width, length, and surface faceting. We propose that symmetry breaking is induced by an initial formation of a new surface structure that is stabilized by the deposition of silver, thus preserving this facet in the embryonic nanorod. These new surfaces initially form stochastically as truncations that remove high-energy edge atoms at the intersection of existing {111} facets and represent the beginnings of a {011}-type surface. Crucially, the finely tuned [HAuCl 4 ]:[AgNO 3 ] ratio and reduction potential of the system mean that silver deposition can occur on the more atomically open surface but not on the pre-existing lower-index facets. The stabilized surfaces develop into side facets of the nascent nanorod, while the largely unpassivated {111} facets are the predominant site of Au atom deposition. Growth in the width direction is tightly controlled by a self-sustaining cycle of galvanic replacement and silver deposition. It is the [HAuCl 4 ]:[AgNO 3 ] ratio that directly determines the particle size at which the more open atomic surfaces can be stabilized by silver and the rate of growth in the width direction following symmetry breaking, thus explaining the known aspect ratio control with Ag ion concentration. We describe the evolving surface faceting of the nanorod and the emergence of higher-index facets. Collectively, these observations allow us to identify facet-size and edge-atom effects as a simple fundamental driver of symmetry breaking and the subsequent development of new surfaces in the presence of adsorbates.

Recent advances in the ability to synthesize metallic nanoparticles with tailored geometries have led to a revolution in the field of plasmonics. However, studies of the important complementary system, an inverted nanostructure, have so far been limited to two-dimensional sphere-segment voids or holes. Here we reveal the localized surface plasmon resonances (LSPRs) of nanovoids that are topologically enclosed in three-dimensions: an "anti-nanoparticle". We combine this topology with the favorable plasmonic properties of aluminum to observe strongly localized field enhancements with LSPR energies in the extreme UV range, well beyond those accessible with noble metals or yet achieved with aluminum. We demonstrate the resonance tunability by tailoring the shape and size of the nanovoids, which are truncated octahedra in the 10-20 nm range. This system is pristine: the nanovoid cavity is free from any oxide or supporting substrate that would affect the LSPRs. We exploit this to infer LSPRs of pure, sub-20-nm Al nanoparticles, which have yet to be synthesized. Access to this extreme UV range will allow applications in LSPR-enhanced UV photoemission spectroscopy and photoionization.

We demonstrate experimentally an efficient electron axicon lens using a magnetic vortex. We show that naturally occurring magnetic vortices with circular magnetic moment distributions in a soft-magnetic thin film create conical phase shifts for fast electrons. Such radially symmetric linear phase ramps are equivalent to ideal light optical axicons. We apply this lens to generate efficient nondiffracting electron Bessel beams, which we observe experimentally in through-focus Lorentz images as well as in propagated off-axis electron holograms. This highlights the potential for using magnetic nanostructures as highly efficient and flexible phase plates for crafting desired electron beam shapes.

Ionic codoping offers a powerful approach for modifying material properties by extending the selection of potential dopant ions. However, it has been a major challenge to introduce certain ions that have hitherto proved difficult to use as dopants (called "difficult-dopants") into crystal structures at high concentrations, especially through wet chemical synthesis. Furthermore, the lack of a fundamental understanding of how codopants are incorporated into host materials, which types of defect structures they form in the equilibrium state, and what roles they play in material performance, has seriously hindered the rational design and development of promising codoped materials. Here we take In 3+ (difficult-dopants) and Nb 5+ (easy-dopants) codoped anatase TiO 2 nanocrystals as an example and investigate the doping mechanism of these two different types of metal ions, the defect formation, and their associated impacts on high-pressure induced structural transition behaviors. It is experimentally demonstrated that the dual mechanisms of nucleation and diffusion doping are responsible for the synergic incorporation of these two dopants and theoretically evidenced that the defect structures created by the introduced In 3+ , Nb 5+ codopants, their resultant Ti 3+ , and oxygen vacancies are locally composed of both defect clusters and equivalent defect pairs. These formed local defect structures then act as nucleation centers of baddeleyite- and α-PbO 2 -like metastable polymorphic phases and induce the abnormal trans-regime structural transition of codoped anatase TiO 2 nanocrystals under high pressure. This work thus suggests an effective strategy to design and synthesize codoped nanocrystals with highly concentrated difficult-dopants. It also unveils the significance of local defect structures on material properties.

Asymmetric nanoparticle trimers composed of particles with increasing diameter act as "plasmonic lenses" and have been predicted to exhibit ultrahigh confinement of electromagnetic energy in the space between the two smallest particles. Here we present an electrostatic self-assembly approach for creating gold nanoparticle trimers with an assembly yield of over 60%. We demonstrate that the trimer assembly leads to characteristic red-shifts and show the localization of the relevant plasmon modes by means of cathodoluminescence and electron energy loss spectroscopy. The results are analyzed in terms of surface plasmon hybridization.

Organic-inorganic hybrid perovskites are exciting candidates for next-generation solar cells, with CH 3 NH 3 PbI 3 being one of the most widely studied. While there have been intense efforts to fabricate and optimize photovoltaic devices using CH 3 NH 3 PbI 3 , critical questions remain regarding the crystal structure that governs its unique properties of the hybrid perovskite material. Here we report unambiguous evidence for crystallographic twin domains in tetragonal CH 3 NH 3 PbI 3 , observed using low-dose transmission electron microscopy and selected area electron diffraction. The domains are around 100-300 nm wide, which disappear/reappear above/below the tetragonal-to-cubic phase transition temperature (approximate 57 °C) in a reversible process that often 'memorizes' the scale and orientation of the domains. Since these domains exist within the operational temperature range of solar cells, and have dimensions comparable to the thickness of typical CH 3 NH 3 PbI 3 films in the solar cells, understanding the twin geometry and orientation is essential for further improving perovskite solar cells.

In this Letter, we explore the nature of exciton localization in single GaAs/AlGaAs nanowire quantum well tube (QWT) devices using photocurrent (PC) spectroscopy combined with simultaneous photoluminescence (PL) and photoluminescence excitation (PLE) measurements. Excitons confined to GaAs quantum well tubes of 8 and 4 nm widths embedded into an AlGaAs barrier are seen to ionize at high bias levels. Spectroscopic signatures of the ground and excited states confined to the QWT seen in PL, PLE, and PC data are consistent with simple numerical calculations. The demonstration of good electrical contact with the QWTs enables the study of Stark effect shifts in the sharp emission lines of excitons localized to quantum dot-like states within the QWT. Atomic resolution cross-sectional TEM measurements and an analysis of the quantum confined Stark effect of these dots provide insights into the nature of the exciton localization in these nanostructures.

Self-assembled nanostructures with periodic phase separation hold great promise for creating two- and three-dimensional superlattices with extraordinary physical properties. Understanding the mechanism(s) driving the formation of such superlattices demands an understanding of their underlying atomic structure. However, the nanoscale structural fluctuations intrinsic to these superlattices pose a new challenge for structure determination methods. Here we develop an optimized atomic-level imaging condition to measure TiO6 octahedral tilt angles, unit-cell-by-unit-cell, in perovskite-based Li(0.5-3x)Nd(0.5+x)TiO3, and thereby determine the mathematical formula governing this nanoscale superstructure. We obtain a direct real-space correlation of the octahedral tilt modulation with the superstructure geometry and lattice-parameter variations. This reveals a composition-dependent, self-ordered octahedral superlattice. Amazingly, we observe a reversible annihilation/reconstruction of the octahedral superlattice correlated with the delithiation/lithiation process in this promising Li-ion conductor. This approach to quantify local octahedral tilt and correlate it with strain can be applied to characterize complex octahedral behaviours in other advanced oxide systems.

The interpretation of angular symmetries in electron nanodiffraction patterns from thin amorphous specimens is examined. It is found that in general there are odd symmetries in experimental electron nanodiffraction patterns. Using simulation, it is demonstrated that this effect can be attributed to dynamical scattering, rather than other divergences from the ideal experimental conditions such as probe-forming lens aberrations and camera noise. The departure of opposing diffracted intensities from Friedel's law in the phase grating formalism is calculated using a general structure factor for disordered materials. On the basis of this, a simple correction procedure is suggested to recover the kinematical angular symmetries, and thus readily interpretable information that reflects the symmetries of the original projected object. This correction is numerically tested using both the phase object and multislice calculations, and is demonstrated to fully recover all the kinematical diffracted symmetries from a simulated atomic model of a metallic glass.

We use low-temperature photoluminescence, photoluminescence excitation, and photoluminescence imaging spectroscopy to explore the optical and electronic properties of GaAs/AlGaAs quantum well tube (QWT) heterostructured nanowires (NWs). We find that GaAs QWTs with widths >5 nm have electronic states which are delocalized and continuous along the length of the NW. As the NW QWT width decreases from 5 to 1.5 nm, only a single electron state is bound to the well, and no optical excitations to a confined excited state are present. Simultaneously, narrow emission lines (fwhm < 600 μeV) appear which are localized to single spatial points along the length of the NW. We find that these quantum-dot-like states broaden at higher temperatures and quench at temperatures above 80 K. The lifetimes of these localized states are observed to vary from dot to dot from 160 to 400 ps. The presence of delocalized states and then localized states as the QWTs become more confined suggests both opportunities and challenges for possible incorporation into quantum-confined device structures.

Metal nanocrystals can be grown in a variety of shapes through the modification of surface facet energies via surfactants. However, the surface facets are only a few atoms wide, making it extremely challenging to measure their geometries and energies. Here, we locate and count atoms in Au nanorods at successive time intervals using quantitative scanning transmission electron microscopy. This enables us to determine the atomic-level geometry and the relative stability of the facets and to expound their relationship to the overall three-dimensional nanocrystal shape and size. We reveal coexisting high- and low-index facets with comparable stability and dimensions and find the geometry of the nanorods is remarkably stable, despite significant atom movements. This information provides unique insights into the mechanisms that govern growth kinetics and nanocrystal morphology.

Formation of anisotropic nanocrystals from isotropic single-crystal precursors requires an essential symmetry breaking event. Single-crystal gold nanorods have become the model system for investigating the synthesis of anisotropic nanoparticles, and their growth mechanism continues to be the subject of intense investigation. Despite this, very little is known about the symmetry breaking event that precedes shape anisotropy. In particular, there remains limited understanding of how an isotropic seed particle becomes asymmetric and of the growth parameters that trigger and drive this process. Here, we present direct atomic-scale observations of the nanocrystal structure at the embryonic stages of gold nanorod growth. The onset of asymmetry of the nascent crystals is observed to occur only for single-crystal particles that have reached diameters of 4-6 nm and only in the presence of silver ions. In this size range, small, asymmetric truncating surfaces with an open atomic structure become apparent. Furthermore, {111} twin planes are observed in some immature nanorods within 1-3 monolayers of the surface. These results provide direct observation of the structural changes that break the symmetry of isotropic nascent nanocrystals and ultimately enable the growth of asymmetric nanocrystals.

Thin-film photovoltaics based on alkylammonium lead iodide perovskite light absorbers have recently emerged as a promising low-cost solar energy harvesting technology. To date, the perovskite layer in these efficient solar cells has generally been fabricated by either vapor deposition or a two-step sequential deposition process. We report that flat, uniform thin films of this material can be deposited by a one-step, solvent-induced, fast crystallization method involving spin-coating of a DMF solution of CH3NH3PbI3 followed immediately by exposure to chlorobenzene to induce crystallization. Analysis of the devices and films revealed that the perovskite films consist of large crystalline grains with sizes up to microns. Planar heterojunction solar cells constructed with these solution-processed thin films yielded an average power conversion efficiency of 13.9±0.7% and a steady state efficiency of 13% under standard AM 1.5 conditions., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We introduce off-axis chromatic scanning confocal electron microscopy, a technique for fast mapping of inelastically scattered electrons in a scanning transmission electron microscope without a spectrometer. The off-axis confocal mode enables the inelastically scattered electrons to be chromatically dispersed both parallel and perpendicular to the optic axis. This enables electrons with different energy losses to be separated and detected in the image plane, enabling efficient energy filtering in a confocal mode with an integrating detector. We describe the experimental configuration and demonstrate the method with nanoscale core-loss chemical mapping of silver (M4,5) in an aluminium-silver alloy and atomic scale imaging of the low intensity core-loss La (M4,5@840  eV) signal in LaB6. Scan rates up to 2 orders of magnitude faster than conventional methods were used, enabling a corresponding reduction in radiation dose and increase in the field of view. If coupled with the enhanced depth and lateral resolution of the incoherent confocal configuration, this offers an approach for nanoscale three-dimensional chemical mapping.

Quantitative analysis of lattice resolved images generated by scanning transmission electron microscopy (STEM) requires specification of probe characteristics, such as defocus, aberration and source distribution. In this paper we show that knowledge of such characteristics is unnecessary for quantitative interpretation, if the signal is integrated over a unit cell. Such a condition, whether the result of experimental setup or post-processing of lattice resolved images, reduces the intensity distribution to that of channelling contrast, where the signal for plane wave incidence is averaged over the angular range of the probe, and the result is independent of the probe characteristics. We use a Bloch wave model to show analytically how this applies to all forms of STEM imaging, such as that formed by annular dark field or backscatter detection, as well as characteristic X-ray fluorescence or electron energy loss. As a specific example, we consider how the signal from an annular dark field detector can be used to determine specimen thickness via a transfer curve for the zone axis and scattering geometries employed. This method has advantages over matching lattice images with calculations since these are sensitive to probe coherence and aberration, and saturation of the on-column intensity is approached more rapidly., (© 2013 Published by Elsevier B.V.)

We report on the synthesis and characterization of single-crystal caged gold nanorods (CGNRs) which exhibited broadband plasmonic resonances. Redshift of plasmon resonance can be realized by increasing the length, whereas blueshift can be achieved by increasing either the overall width of the cage or the thickness of nanocage walls.

A century has passed since Bragg solved the first atomic structure using diffraction. As with this first structure, all atomic structures to date have been deduced from the measurement of many diffracted intensities using iterative and statistical methods. We show that centrosymmetric atomic structures can be determined without the need to measure or even record a diffracted intensity. Instead, atomic structures can be determined directly and quickly from the observation of crystallographic phases in electron diffraction patterns. Furthermore, only a few phases are required to achieve high resolution. This represents a paradigm shift in structure determination methods, which we demonstrate with the moderately complex α-Al2O3. We show that the observation of just nine phases enables the location of all atoms with a resolution of better than 0.1 Å. This level of certainty previously required the measurement of thousands of diffracted intensities.

AlGaAs/GaAs quantum well heterostructures based on core-multishell nanowires exhibit excellent optical properties which are acutely sensitive to structure and morphology. We characterize these heterostructures and observe them to have 3-fold symmetry about the nanowire axis. Using aberration-corrected annular dark field scanning transmission electron microscopy (ADF-STEM), we measure directly the polarity of the crystal structure and correlate this with the shape and facet orientation of the GaAs core, quantum wells and cap, and the width of radial Al-rich bands. We discuss how the underlying polarity of the crystal structure drives the growth of these heterostructures with a 3-fold symmetry resulting in a nonuniform GaAs quantum well tube and AlGaAs shell. These observations suggest that the AlGaAs growth rate is faster along the [112] B compared to the [112] A directions and/or that there is a polarity driven surface reconstruction generating AlGaAs growth fronts inclined to the {110} planes. In contrast, the observations suggest that the opposite is true for the GaAs growth, with the preferred surface reconstruction plane being parallel to {110} and an apparent faster growth rate along the [112] A. This two-dimensional layer growth of the nanowire multishells strongly depends on the surface energies and surface reconstruction of the facets which are related to the crystal polarity and lead to the 3-fold symmetry observed here.

The electronic properties of thin, nanometer scale GaAs quantum well tubes embedded inside the AlGaAs shell of a GaAs core-multishell nanowire are investigated using optical spectroscopies. Using numerical simulations to model cylindrically and hexagonally symmetric systems, we correlate these electronic properties with structural characterization by aberration-corrected scanning transmission electron microscopy of nanowire cross sections. These tubular quantum wells exhibit extremely high quantum efficiency and intense emission for extremely low submicrowatt excitation powers in both photoluminescence and photoluminescence excitation measurements. Numerical calculations of the confined eigenstates suggest that the electrons and holes in their ground states are confined to extremely localized one-dimensional filaments at the corners of the hexagonal structure which extend along the length of the nanowire.

A method to measure the thickness of a single-crystal nanoparticle in the direction parallel to the incident beam from annular dark field scanning transmission electron microscope (ADF-STEM) images is reported, providing a map of thickness versus position across the nanoparticle--a 'thickness profile' image. The method is rapid and hence suitable for surveying large numbers of nanoparticles. The method measures the intensity scattered to a characterised ADF detector and compares this to the incident beam intensity, to obtain a normalized ADF image. The normalised intensity is then converted to thickness via dynamical ADF image simulations. The method is accurate within 10% and the precision is dominated primarily by 'shot noise'. Merits and limitations of this method are discussed. A method to calibrate the response function of the ADF detector without external equipment is also described, which is applicable to the entire range of gain and background settings., (Copyright © 2012 Elsevier B.V. All rights reserved.)

Ptychographic coherent diffractive imaging (CDI) has been extensively applied using both x rays and electrons. The extension to atomic resolution has been elusive. This Letter demonstrates ptychographic electron diffractive imaging at atomic resolution, permitting identification of structure in a boron nitride helical cone at a resolution of order 1 Å, beyond that of comparative Z-contrast images. A scanning transmission electron microscope is used to create a diverging illumination in a defocused Fresnel CDI geometry, providing a robust strategy leading to a unique solution.

We present an alternative atomic resolution incoherent imaging technique derived from scanning transmission electron microscopy (STEM) using detectors in real space, in contrast to conventional STEM that uses detectors in diffraction space. The images obtained from various specimens have a resolution comparable to conventional high-angle annular dark-field (HAADF) STEM with good contrast, which seems to be very robust with respect to thickness, focus and imaging conditions. The results of the simulations are consistent with the experimental results and support the interpretation of the real-space STEM image contrast as being a result of aberration-induced displacements of the high-angle scattered electrons., (© 2011 International Union of Crystallography)

The intensity distribution in three-beam CBED patterns from centrosymmetric crystals can be inverted analytically to enable the direct measurement of crystal structure amplitudes and three-phase invariants. The accuracy of the measurements depends upon the accuracy and precision with which specific loci within the discs can be identified. The present work exploits the equivalence in form of the intensity distribution along these loci to provide an algorithm for their automated location, enabling the rapid and unequivocal identification of their position. Moreover, it demonstrates how the loci can be used to determine directly the relative magnitudes of structure amplitudes with superior accuracy and without recourse to complex pattern-matching calculations., (Copyright © 2011 Elsevier B.V. All rights reserved.)

It has recently become possible to focus a beam of high-energy electrons to a spot smaller than an atom, with vast potential for the study of condensed matter. We devise an experiment that can image, with subangstrom resolution, the distribution of such an electron beam as it scatters within an atomic lattice. Our experiments reveal the acute sensitivity of the scattered electron distribution to 0.1 Å shifts of the impact point. Scattering due to plasmon excitations is also examined. Implications for the study of the atomic and electronic structure of condensed matter are discussed.

Aluminum is considered to approach an "ideal" metal or free electron gas. The valence electrons move freely, as if unaffected by the presence of the metal ions. Therefore, the electron redistribution due to chemical bonding is subtle and has proven extremely difficult to determine. Experimental measurements and ab initio calculations have yielded substantially different results. We applied quantitative convergent-beam electron diffraction to aluminum to provide an experimental determination of the bonding electron distribution. Calculation of the electron distribution based on density functional theory is shown to be in close agreement. Our results yield an accurate quantitative correlation between the anisotropic elastic properties of aluminum and the bonding electron and electrostatic potential distributions.

We determine the three-dimensional shape, to within 1 nm resolution, of single-crystal gold nanorods grown in the presence of silver ions using electron tomography and thickness profile measurements. We find that, contrary to the current literature, the octagonal side-facets are sparsely packed atomic planes all belonging to the same symmetry-equivalent family, {0 5 12}. Furthermore, the rod ends terminate in a pyramid with slightly different facets, and each pyramid is connected to the sides by four small {0 5 12} "bridging" facets.

The practical details for the direct measurement of structural phases and magnitudes from centrosymmetric crystals using three-beam convergent beam electron diffraction patterns are given. These details are presented as a step-by-step guide for easy implementation. Each step is illustrated with worked examples from alpha-Al(2)O(3). Twenty-nine measurements of 6 independent three-phase invariants and 87 measurements of 10 independent structure magnitudes have been made.

The structure of a periodic object, such as a crystal, may be described by an infinite series of Fourier coefficients and phases. In associating this with scattering theory appropriate to any radiation, a classic problem arises, namely, the determination of phases from the resulting discrete diffraction pattern. The solution to this phase problem is presented in this paper in which the first direct measurement of structural phase by inspection of convergent-beam electron diffraction patterns is described.

This paper reports a structural and microstructural investigation of the hexagonal perovskite Ba(3)Ti(2)MnO(9) using electron microscopy and diffraction. Convergent-beam electron diffraction (CBED) revealed the structure has the non-centrosymmetric space group P6(3)mc (186) at room temperature and at approximately 110 K. Compared with the centrosymmetric parent structure BaTiO(3), with space group P6(3)/mmc, this represents a break in mirror symmetry normal to the c axis. This implies the Ti and Mn atoms are ordered on alternate octahedral sites along the 0001 direction in Ba(3)Ti(2)MnO(9). Using high-resolution electron microscopy (HREM), we observed occasional 6H/12R interfaces on (0001) planes, however, no antiphase boundaries were observed, as were seen in Ba(3)Ti(2)RuO(9). Using powder X-ray Rietveld refinement we have measured the lattice parameters from polycrystalline samples to be a = 5.6880 +/- 0.0005, c = 13.9223 +/- 0.0015 A at room temperature.

We have used electron microscopy and diffraction to refine the structure and investigate the microstructure of Ba(3)Ti(2)RuO(9). The parent compound is hexagonal BaTiO(3) with the space group P6(3)/mmc. Using convergent-beam electron diffraction (CBED) combined with electron-sensitive image plates we have found that the space group of Ba(3)Ti(2)RuO(9) is the non-centrosymmetric group P6(3)mc at room temperature and at approximately 110 K. This is consistent with the Ru and Ti atoms occupying alternate face-sharing octahedral sites in the 0001 direction. This maintains the c-glide, but breaks the mirror normal to the c axis and consequently removes the centre of symmetry. Using powder X-ray diffraction, we have measured the lattice parameters from polycrystalline samples to be a = 5.7056 +/- 0.0005, c = 14.0093 +/- 0.0015 A at room temperature. Using high-resolution electron microscopy (HREM) we observed highly coherent, low-strain {10\bar 10} grain boundaries intersecting at 60 and 120 degrees . From CBED we deduce that adjacent grains are identical but for the relative phase of the Ti and Ru atom ordering along the c axis. HREM also revealed occasional stacking faults, normal to the c-axis.