Search

Abstract Size effects decisively influence the properties of materials at small length scales. In the context of mechanical properties, the trend of ‘smaller is stronger’ has been well established. This statement refers to an almost universal trend of increased strength with decreasing size. A strong influence of size on the elastic properties has also been widely reported, albeit without a clear trend. However, the influence of nanostructure shape on the mechanical properties has been critically neglected. Here, we demonstrate a profound influence of shape and size on the elastic properties of materials on the example of gold nanowires. The elastic properties are determined using in-situ mechanical testing in scanning and transmission electron microscopy by means of resonance excitation and uniaxial tension. The combination of bending and tensile load types allows for an independent and correlative calculation of the Young's modulus. We find both cases of softening as well as stiffening, depending critically on the interplay between size and shape of the wires. Graphic abstract

ACS nano 15(1), 362-376 (2021). doi:10.1021/acsnano.0c06480, Published by American Chemical Society, Washington, DC

A new in situ synthesis method for the growth of MoO2 nanowires via controlled thermal oxidation of MoS2 flakes is presented, going from a 2D transition metal chalcogenide to a transition metal oxide nanostructure. The wire growth is performed under an optical microscope using a heating stage with adjustable atmospheric conditions. In contrast to prevalent syntheses, this templated growth leads to highly directional wires along defined MoS2 crystallographic directions. We examine the growth kinetics of the wires in dependence of the process temperature. In the temperature regime from 650 °C to 710 °C high quality MoO2 nanowires are formed in a reaction-limited growth process with an activation energy of 596 kJ mol−1. The functional properties of the nanowires are studied by a combination of in situ electron microscopy techniques. Four point measurements in an SEM reveal outstanding metal-like behavior of the nanowires with resistivity values as low as 3.5 × 10−6 Ω m. Surprisingly, junctions between intergrown nanowires show hardly any increase in resistivity which can be attributed to the well-defined orientational relationship of the nanowires resulting from their templated growth on MoS2. Elastic properties of the nanowires are studied by complementary in situ bending and resonance measurements in SEM yielding consistent values of 383 GPa for the Young's modulus. Finally, field emission of single MoO2 nanowires is studied in situ inside the TEM, and an emission current of 500 nA is achieved. The combination of simple synthesis route with outstanding functional properties make the MoO2 nanowires promising candidates for functional devices in the field of novel 1D-oxide/2D-chalcogenide hybrids. The presented synthesis can be generalized and applied to other metal chalcogenides such as WS2, as well.

Avoiding and removing surface contamination is a crucial task when handling specimens in any scientific experiment. This is especially true for two-dimensional materials such as graphene, which are extraordinarily affected by contamination due to their large surface area. While many efforts have been made to reduce and remove contamination from such surfaces, the issue is far from resolved. Here we report on an in situ mechanical cleaning method that enables the site-specific removal of contamination from both sides of two dimensional membranes down to atomic-scale cleanliness. Further, mechanisms of re-contamination are discussed, finding surface-diffusion to be the major factor for contamination in electron microscopy. Finally the targeted, electron-beam assisted synthesis of a nanocrystalline graphene layer by supplying a precursor molecule to cleaned areas is demonstrated., Contamination of 2D materials adversely impacts device performance and calls for cleaning methods down to the atomic scale and over large areas. Here, the authors present a site-specific mechanical cleaning approach capable of cleaning both sides of suspended 2D membranes and achieving atomically clean areas of several μm2 within minutes.

Quantum size-confined CH3NH3PbX3 (X = Br and I) perovskite nanoplatelets with remarkably high photoluminescence quantum yield (up to 90%) were synthesized by ligand-assisted re-precipitation. Thickness-tunability was realized by varying the oleylamine and oleic acid ligand ratio. This method allows tailoring the nanoplatelet thickness by adjusting the number of unit cell monolayers. Broadly tunable emission wavelengths (450-730 nm) are achieved via the pronounced quantum size effect without anion-halide mixing.

The transition of a thin film into an energetically favorable set of particles at temperatures below the melting point of the bulk material is known as solid-state dewetting. In this work the dewetting behavior of 16 nm thick discontinuous Au thin films on amorphous silicon nitride membranes is quantitatively studied by complementary in situ transmission electron microscopy techniques taking advantage of the unique capabilities of a chip-based heating system. The combination of dedicated imaging and diffraction techniques is used to investigate the interplay of grain growth and texture evolution with the process of dewetting. The results show an initial coarsening of the microstructure preceding the other processes. Texture evolution is highly correlated to material retraction and agglomeration during the following dewetting process. In-plane grain rotation has been observed, acting as an additional mechanism for orientation changes. From a methodological perspective this work demonstrates the capabilities of today’s transmission electron microscopy in combination with state-of-the-art in situ instrumentation. In particular the combination of complementary information from different dedicated techniques in one and the same setup is demonstrated to be highly beneficial.

Semitransparent solar cells (SSCs) can open photovoltaic applications in many commercial areas, such as power-generating windows and building integrated photovoltaics. This study successfully demonstrates solution-processed small molecule SSCs with a conventional configuration for the presently tested material systems, namely BDTT-S-TR:PC70BM, N(Ph-2T-DCN-Et)3:PC70BM, SMPV1:PC70BM, and UU07:PC60BM. The top transparent cathode coated through solution processes employs a highly transparent silver nanowire as electrode together with a combination interface bilayer of zinc oxide nanoparticles (ZnO) and a perylene diimide derivative (PDINO). This ZnO/PDINO bilayer not only serves as an effective cathode buffer layer but also acts as a protective film on top of the active layer. With this integrated contribution, this study achieves a power conversion efficiency (PCE) of 3.62% for fully solution-processed SSCs based on BDTT-S-TR system. Furthermore, the other three systems with various colors exhibited the PCEs close to 3% as expected from simulations, demonstrate the practicality and versatility of this printed semitransparent device architecture for small mole­cule systems. This work amplifies the potential of small molecule solar cells for window integration.

Electron diffraction is a powerful characterization method that is used across different fields and in different instruments. In particular, the power of transmission electron microscopy (TEM) largely relies on the capability to switch between imaging and diffraction mode enabling identification of crystalline phases and in-depth studies of crystal defects, to name only examples. In contrast, while diffraction techniques have found their way into the realm of scanning electron microscopy (SEM) in the form of electron backscatter diffraction and related techniques, on-axis transmission diffraction is still in its infancy. Here we present a simple but versatile setup that enables a ‘diffraction mode’ in SEM using a fluorescent screen and a dedicated in vacuo camera. With this setup spot-like nano-beam diffraction patterns of thin samples can be acquired with electron energies as low as 500 eV. We therefore coin the name Low Energy Nano Diffraction (LEND). Diffraction patterns can be recorded from single positions on the sample or integrated over selected areas by adjustable scan patterns. Besides showing the principal application of the technique to standard materials such as gold and silicon we also explore the application to graphene and other 2D materials. Besides single pattern measurements, also full 4D-STEM diffraction mappings are demonstrated. Finally, we show how the integration of a versatile diffraction mode in SEM enables a thorough analysis performed with a single instrument.

Despite major technological advances within the field of cardiovascular engineering, the risk of thromboembolic events on artificial surfaces in contact with blood remains a major challenge and limits the functionality of ventricular assist devices (VADs) during mid- or long-term therapy. Here, a biomimetic blood-material interface is created via a nanofiber-based approach that promotes the endothelialization capability of elastic silicone surfaces for next-generation VADs under elevated hemodynamic loads. A blend fiber membrane made of elastic polyurethane and low-thrombogenic poly(vinylidene fluoride- co-hexafluoropropylene) was partially embedded into the surface of silicone films. These blend membranes resist fundamental irreversible deformation of the internal structure and are stably attached to the surface, while also exhibiting enhanced antithrombotic properties when compared to bare silicone. The composite material supports the formation of a stable monolayer of endothelial cells within a pulsatile flow bioreactor, resembling the physiological in vivo situation in a VAD. The nanofiber surface modification concept thus presents a promising approach for the future design of advanced elastic composite materials that are particularly interesting for applications in contact with blood.

A materials scientist’s dream come true: manipulating and controlling individual dislocations in situ., Topological defects in crystalline solids are of fundamental interest in physics and materials science because they can radically alter the properties of virtually any material. Of particular importance are line defects, known as dislocations, which are the main carriers of plasticity and have a tremendous effect on electronic and optical properties. Understanding and controlling the occurrence and behavior of those defects have been of major and ongoing interest since their discovery in the 1930s. This interest was renewed with the advent of two-dimensional materials in which a single topological defect can alter the functionality of the whole system and even create new physical phenomena. We present an experimental approach to directly manipulate dislocations in situ on the nanometer scale by using a dedicated scanning electron microscope setup. With this approach, key fundamental characteristics such as line tension, defect interaction, and node formation have been studied. A novel switching reaction, based on the recombination of dislocation lines, was found, which paves the way for the concept of switches made of a bimodal topological defect configuration.

We demonstrate an innovative solution-processing fabrication route for organic and perovskite solar modules via depth-selective laser patterning of an adhesive top electrode. This yields unprecedented power conversion efficiencies of up to 5.3% and 9.8%, respectively. We employ a PEDOT:PSS–Ag nanowire composite electrode and depth-resolved post-patterning through beforehand laminated devices using ultra-fast laser scribing. This process affords low-loss interconnects of consecutive solar cells while overcoming typical alignment constraints. Our strategy informs a highly simplified and universal approach for solar module fabrication that could be extended to other thin-film photovoltaic technologies.

UNLABELLED A scalable, hysteresis-free and planar architecture perovskite solar cell is presented, employing a flame spray synthesized low-temperature processed NiO (LT-NiO) as hole-transporting layer yielding efficiencies close to 18%. Importantly, it is found that LT-NiO boosts the limits of open-circuit voltages toward an impressive non-radiative voltage loss of 0.226 V only, whereas PEDOT PSS suffers from significant large non-radiative recombination losses.

The memory effect in organic thin film transistors is described with a dielectric hybrid material composed of selectively localized charge trapping titania nanoparticles in a polystyrene-b-polyethyleneoxide (PS-b-PEO) diblock copolymer film. Upon solvent annealing in toluene atmosphere, the films readily form nanophase-separated features of PEO cylinders in a matrix of PS. X-ray reflectivity (XRR) investigations reveal that the TiO2 particles are selectively distributed in the top half of the hybrid layers. Applied in organic thin film transistor devices with 2-tridecyl-[1]benzothieno[3,2-b][1]benzothiophen (mono-C13-BTBT) semiconductor, the hybrid layers induce clockwise drain current hysteresis with promising memory ratio, long-term stability, and endurance. The occurrence of nonvolatile charge trapping directly relates to the hierarchically assembled nanostructure of the nanoparticles in the block copolymer since reference devices with PS homopolymers blended with TiO2 are found to be hysteresis free. Morphological characterization of the hybrid dielectric layers by atomic force microscopy and XRR experiments is presented to shed light on the internal electron density distribution. The layers are subsequently tested in capacitor stacks and transistors.

Thin-film solar cell based on hybrid perovskites shows excellent light-to-power conversion efficiencies exceeding 22%. However, the mixed ionic-electronic semiconductor hybrid perovskite exhibits many unusual properties such as slow photocurrent instabilities, hysteresis behavior, and low-frequency giant capacitance, which still question us so far. This study presents a direct surface functionalization of transparent conductive oxide electrode with an ultrathin ≈2 nm thick phosphonic acid based mixed C60/organic self-assembled monolayer (SAM) that significantly reduces hysteresis. Moreover, due to the strong phosphonates bonds with indium tin oxide (ITO) substrates, the SAM/ITO substrates also exhibit an excellent recyclability merit from the perspective of cost effectiveness. Impedance studies find the fingerprint of an ion-based diffusion process in the millisecond to second regime for TiO2-based devices, which, however, is not observed for SAM-based devices at these low frequencies. It is experimentally demonstrated that ion migration can be considerably suppressed by carefully engineering SAM interfaces, which allows effectively suppressing hysteresis and unstable diode behavior in the frequency regime between ≈1 and 100 Hz. It is suggested that a reduced density of ionic defects in combination with the absence of charge carrier accumulation at the interface is the main physical origin for the reduced hysteresis.