Atomic Defects Influenced Mechanics of II-VI Nanocrystals.

Mechanical properties of nanocrystals are influenced by atomic defects. Here, we demonstrate the effect of planar defects on the mechanics of ZnO nanorods using atomic force microscopy, high-resolution transmission electron microscopy, and large-scale atomistic simulation. We study two different conditionally grown single nanorods. One contains extended I1-type stacking fault (SF) and another is defect free. The SF containing nanorods show buckling behaviors with reduced critical loading, whereas the other kinds show linear elastic behavior. We also studied the size dependence of elastic modulus and yield strength. The elastic modulus in both nanorods is inversely proportional to their size. Similar trend is observed for yield strength in the SF containing nanorods; however, the opposite is observed in the SF-free nanorods. This first experimental and theoretical study will guide toward the development of reliable electromechanical devices.

Light-Controlled Interconversion between a Self-Assembled Triangle and a Rhombicuboctahedral Sphere.

Stimuli-responsive structural reorganizations play an important role in biological processes, often in combination with kinetic control scenarios. In supramolecular mimics of such systems, light has been established as the perfect external trigger. Here, we report on the light-driven structural rearrangement of a small, self-assembled Pd3L6 ring based on photochromic dithienylethene (DTE) ligands into a rhombicuboctahedral Pd24L48 sphere measuring about 6.4 nm across. When the wavelength is changed, this interconversion can be fully reversed, as confirmed by NMR and UV/Vis spectroscopy as well as mass spectrometry. The sphere was visualized by AFM, TEM, and GISAXS measurements. Due to dissimilarities in the photoswitch conformations, the interconversion rates between the two assemblies are drastically different in the two directions.

Photoluminescence of carbon nanodots: dipole emission centers and electron-phonon coupling.

Inorganic carbon nanomaterials, also called carbon nanodots, exhibit a strong photoluminescence with unusual properties and, thus, have been the focus of intense research. Nonetheless, the origin of their photoluminescence is still unclear and the subject of scientific debates. Here, we present a single particle comprehensive study of carbon nanodot photoluminescence, which combines emission and lifetime spectroscopy, defocused emission dipole imaging, azimuthally polarized excitation dipole scanning, nanocavity-based quantum yield measurements, high resolution transmission electron microscopy, and atomic force microscopy. We find that photoluminescent carbon nanodots behave as electric dipoles, both in absorption and emission, and that their emission originates from the recombination of photogenerated charges on defect centers involving a strong coupling between the electronic transition and collective vibrations of the lattice structure.

Composition and electronic structure of [Formula: see text]/[Formula: see text]/Al passivating carrier selective contacts on n-type silicon solar cells.

Carrier-selective and passivating SiO[Formula: see text]/TiO[Formula: see text] heterocontacts are an attractive alternative to conventional contacts due to their high efficiency potentials combined with relatively simple processing schemes. It is widely accepted that post deposition annealing is necessary to obtain high photovoltaic efficiencies, especially for full area aluminum metallized contacts. Despite some previous high-level electron microscopy studies, the picture of atomic-scale processes underlying this improvement seems to be incomplete. In this work, we apply nanoscale electron microscopy techniques to macroscopically well-characterized solar cells with SiO[Formula: see text]/TiO[Formula: see text]/Al rear contacts on n-type silicon. Macroscopically, annealed solar cells show a tremendous decrease of series resistance and improved interface passivation. Analyzing the microscopic composition and electronic structure of the contacts, we find that partial intermixing of the SiO[Formula: see text] and TiO[Formula: see text] layers occurs due to annealing, leading to an apparent thickness reduction of the passivating SiO[Formula: see text]. However, the electronic structure of the layers remains clearly distinct. Hence, we conclude that the key to obtain highly efficient SiO[Formula: see text]/TiO[Formula: see text]/Al contacts is to tailor the processing such that the excellent chemical interface passivation of a SiO[Formula: see text] layer is achieved for a layer thin enough to allow efficient tunneling through the layer. Furthermore, we discuss the impact of aluminum metallization on the above mentioned processes.

Seebeck effect in magnetic tunnel junctions.

Creating temperature gradients in magnetic nanostructures has resulted in a new research direction, that is, the combination of magneto- and thermoelectric effects. Here, we demonstrate the observation of one important effect of this class: the magneto-Seebeck effect. It is observed when a magnetic configuration changes the charge-based Seebeck coefficient. In particular, the Seebeck coefficient changes during the transition from a parallel to an antiparallel magnetic configuration in a tunnel junction. In this respect, it is the analogue to the tunnelling magnetoresistance. The Seebeck coefficients in parallel and antiparallel configurations are of the order of the voltages known from the charge-Seebeck effect. The size and sign of the effect can be controlled by the composition of the electrodes' atomic layers adjacent to the barrier and the temperature. The geometric centre of the electronic density of states relative to the Fermi level determines the size of the Seebeck effect. Experimentally, we realized 8.8% magneto-Seebeck effect, which results from a voltage change of about -8.7 µV K⁻¹ from the antiparallel to the parallel direction close to the predicted value of -12.1 µV K⁻¹. In contrast to the spin-Seebeck effect, it can be measured as a voltage change directly without conversion of a spin current.

Site-specific plan-view TEM lamella preparation of pristine surfaces with a large field of view.

Transmission electron microscopy has become a major characterization tool with an ever increasing variety of methods being applied in a wide range of scientific fields. However, the probably most famous pitfall in related workflows is the preparation of high-quality electron-transparent lamellae enabling for extraction of valuable information. Particularly in the field of solid state physics and materials science, it often required to study the surface of a macroscopic specimen with plan-view orientation. Nevertheless, despite tremendous advances in instrumentation, i.e. focused ion beam, the yield of existing plan-view lamellae preparation techniques is relatively low compared to cross-sectional extraction methods. Furthermore, techniques relying on mechanical treatments, i.e. conventional preparation, compromise site-specifity. In this paper, we demonstrate that by combining a mechanical grinding step prior to backside lift-out in the focused ion beam plan-view lamellae preparation becomes increasingly easy. The suggested strategy combines site-specifity with micrometer precision as well as possible investigation of pristine surfaces with a field of view of several hundred square micrometers.

Phase Transitions in a Perovskite Thin Film Studied by Environmental In Situ Heating Nano-Beam Electron Diffraction.

The rich phase diagram of bulk Pr1-x Cax MnO3 resulting in a high tunability of physical properties gives rise to various studies related to fundamental research as well as prospective applications of the material. Importantly, as a consequence of strong correlation effects, electronic and lattice degrees of freedom are vigorously coupled. Hence, it is debatable whether such bulk phase diagrams can be transferred to inherently strained epitaxial thin films. In this paper, the structural orthorhombic to pseudo-cubic transition for x = 0.1 is studied in ion-beam sputtered thin films and differences to the respective bulk system are pointed out by employing in situ heating nano-beam electron diffraction to follow the temperature dependence of lattice constants. In addition, it is demonstrated that controlling the environment during heating, that is, preventing oxygen loss, is crucial in order to avoid irreversible structural changes, which is expected to be a general problem of compounds containing volatile elements under non-equilibrium conditions.

Tailored synthetic polyamines for controlled biomimetic silica formation.

Organic compounds isolated from diatoms contain long-chain polyamines with a propylamine backbone and variable methylation levels and chain lengths. These long-chain polyamines are thought to be one of the important classes of molecules that are responsible for the formation of the hierarchically structured silica-based cell walls of diatoms. Here we describe a synthetic route based on solid-phase peptide synthesis from which well-defined long-chain polyamines with different chain lengths, methylation patterns, and subunits can be obtained. Quantitative silica precipitation experiments together with structural information about the precipitated silica structures gained by scanning and transmission electron microscopy revealed a distinct dependence of the amount, size, and form of the silica precipitates on the molecular structure of the polyamine. Moreover, the influence of the phosphate concentration was elucidated, revealing the importance of divalent anions for the precipitation procedure. We were able to derive further insights into the precipitation properties of long-chain polyamines as functions of their hydrophobicity, protonation state, and phosphate concentration, which may pave the way for better control of the formation of nanostructured silica under ambient conditions.

A posteriori synchronization of scanning transmission electron microscopy signals with kilopixel per second acquisition rates.

The stability and sensitivity of scanning transmission electron microscopes as well as detectors collecting e.g. electrons which suffered different scattering processes, or secondary radiation, have increased tremendously during the last decade. In order to fully exploit capabilities of simultaneously recording various signals with up to 1000 px/s acquisition rates the central issue is their synchronization. The latter is frequently a non-trivial problem without commercially available solution especially if detectors of different manufacturers are involved. In this paper, we present a simple scanning pattern enabling a posteriori synchronization of arbitrarily many signals being recorded entirely independently. We apply the approach to the simultaneous atomic-scale acquisition of signals from an annular dark-field detector and electron energy loss as well as energy-dispersive x-ray spectrometers. Errors emerging in scanning direction due to the independence of the respective processes are quantified and found to have a standard deviation of roughly half the pixel spacing. Since there are no intermediate waiting periods to maintain synchronicity, the proposed acquisition process is, in fact, demonstrated to be 12% faster than a commercial hardware-synchronized solution for identical sub-millisecond signal integration times and hence follows the trend in electron microscopy to extract more information per irradiating electron.

Detection of quantum well induced single degenerate-transition-dipoles in ZnO nanorods.

Quantifying and characterising atomic defects in nanocrystals is difficult and low-throughput using the existing methods such as high resolution transmission electron microscopy (HRTEM). In this article, using a defocused wide-field optical imaging technique, we demonstrate that a single ultrahigh-piezoelectric ZnO nanorod contains a single defect site. We model the observed dipole-emission patterns from optical imaging with a multi-dimensional dipole and find that the experimentally observed dipole pattern and model-calculated patterns are in excellent agreement. This agreement suggests the presence of vertically oriented degenerate-transition-dipoles in vertically aligned ZnO nanorods. The HRTEM of the ZnO nanorod shows the presence of a stacking fault, which generates a localised quantum well induced degenerate-transition-dipole. Finally, we elucidate that defocused wide-field imaging can be widely used to characterise defects in nanomaterials to answer many difficult questions concerning the performance of low-dimensional devices, such as in energy harvesting, advanced metal-oxide-semiconductor storage, and nanoelectromechanical and nanophotonic devices.

Current-voltage characteristics of manganite-titanite perovskite junctions.

After a general introduction into the Shockley theory of current voltage (J-V) characteristics of inorganic and organic semiconductor junctions of different bandwidth, we apply the Shockley theory-based, one diode model to a new type of perovskite junctions with polaronic charge carriers. In particular, we studied manganite-titanate p-n heterojunctions made of n-doped SrTi1- y Nb y O3, y = 0.002 and p-doped Pr1- x Ca x MnO3, x = 0.34 having a strongly correlated electron system. The diffusion length of the polaron carriers was analyzed by electron beam-induced current (EBIC) in a thin cross plane lamella of the junction. In the J-V characteristics, the polaronic nature of the charge carriers is exhibited mainly by the temperature dependence of the microscopic parameters, such as the hopping mobility of the series resistance and a colossal electro-resistance (CER) effect in the parallel resistance. We conclude that a modification of the Shockley equation incorporating voltage-dependent microscopic polaron parameters is required. Specifically, the voltage dependence of the reverse saturation current density is analyzed and interpreted as a voltage-dependent electron-polaron hole-polaron pair generation and separation at the interface.

Pattern recognition in high-resolution electron microscopy of complex materials.

Structural features like defects or heterointerfaces in crystals or amorphous phases give rise to different local patterns in high-resolution electron micrographs or object wave functions. Pattern recognition techniques can be used to identify these typical patterns that constitute the image itself, as was already demonstrated for compositional changes in isostructural heterostructures, where the patterns within unit cells of the lattice were analyzed. To extend such analyses to more complex materials, we examined patterns in small circular areas centered on intensity maxima of the image. Nonsupervised clustering, namely, Ward's clustering method, was applied to these patterns. In two examples, a highly defective ZnMnTe layer on GaAs and a tunnel magneto resistance device, we demonstrate how typical patterns are identified by this method and how these results can be used for a further investigation of the microstructural properties of the sample.