Influence of tin oxide decoration on the junction conductivity of silver nanowires.

Flexible electrodes using nanowires (NWs) suffer from challenges of long-term stability and high junction resistance which limit their fields of applications. Welding via thermal annealing is a common strategy to enhance the conductivity of percolated NW networks, however, it affects the structural and mechanical integrity of the NWs. In this study we show that the decoration of NWs with an ultrathin metal oxide is a potential alternative procedure which not only enhances the thermal and chemical stability but, moreover, provides a totally different mechanism to reduce the junction resistance upon heat treatment. Here, we analyze the effect of SnOxdecoration on the conductance of silver NWs and NW junctions by using a four-probe measurement setup inside a scanning electron microscope. Dedicated transmission electron microscopy analysis in plan-view and cross-section geometry are carried out to characterize the nanowires and the microstructure of the junctions. Upon heat treatment the junction resistance of both plain silver NWs and SnOx-decorated NWs is reduced by around 80%. While plain silver NWs show characteristic junction welding during annealing, the SnOx-decoration reduces junction resistance by a solder-like process which does not affect the mechanical integrity of the NW junction and is therefore expected to be superior for applications.

Defect Engineering of Two-Dimensional Molybdenum Disulfide.

Two-dimensional (2D) molybdenum disulfide (MoS2 ) holds great promise in electronic and optoelectronic applications owing to its unique structure and intriguing properties. The intrinsic defects such as sulfur vacancies (SVs) of MoS2 nanosheets are found to be detrimental to the device efficiency. To mitigate this problem, functionalization of 2D MoS2 using thiols has emerged as one of the key strategies for engineering defects. Herein, we demonstrate an approach to controllably engineer the SVs of chemically exfoliated MoS2 nanosheets using a series of substituted thiophenols in solution. The degree of functionalization can be tuned by varying the electron-withdrawing strength of substituents in thiophenols. We find that the intensity of 2LA(M) peak normalized to A1g peak strongly correlates to the degree of functionalization. Our results provide a spectroscopic indicator to monitor and quantify the defect engineering process. This method of MoS2 defect functionalization in solution also benefits the further exploration of defect-free MoS2 for a wide range of applications.

Sensing Capabilities of Single Nanowires Studied with Correlative In Situ Light and Electron Microscopy.

Modern devices based on modular designs require versatile and universal sensor components which provide an efficient, sensitive, and compact measurement unit. To improve the space capacity of devices, miniaturized building elements are needed, which implies a turning away from conventional microcantilevers toward nanoscale cantilevers. Nanowires can be seen as high-quality resonators and offer the opportunity to create sensing devices on small scales. To use such a one-dimensional nanostructure as a resonant cantilever, a precise characterization based on the fundamental properties is needed. We present a correlative electron and light microscopy approach to characterize the pressure and environment sensing capabilities of single nanowires by analyzing their resonance behavior in situ. The high vacuum in electron microscopes enables the characterization of the intrinsic vibrational properties and the maximum quality factor. To analyze the damping effect caused by the interaction of the gas molecules with the excited nanowire, the in situ resonance measurements have been performed under non-high-vacuum conditions. For this purpose, single nanowires are mounted in a specifically designed compact gas chamber underneath the light microscope, which enables direct observation of the resonance behavior and evaluation of the quality factor with dependence of the applied gas atmosphere (He, N2, Ar, Air) and pressure level. By using the resonance vibration, we demonstrate the pressure sensing capability of a single nanowire and examine the molar mass of the surrounding atmosphere. Together this shows that even single nanowires can be utilized as versatile nanoscale gas sensors.

Low energy nano diffraction (LEND) - A versatile diffraction technique in SEM.

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.

Strain-activated light-induced halide segregation in mixed-halide perovskite solids.

Light-induced halide segregation limits the bandgap tunability of mixed-halide perovskites for tandem photovoltaics. Here we report that light-induced halide segregation is strain-activated in MAPb(I1-xBrx)3 with Br concentration below approximately 50%, while it is intrinsic for Br concentration over approximately 50%. Free-standing single crystals of CH3NH3Pb(I0.65Br0.35)3 (35%Br) do not show halide segregation until uniaxial pressure is applied. Besides, 35%Br single crystals grown on lattice-mismatched substrates (e.g. single-crystal CaF2) show inhomogeneous segregation due to heterogenous strain distribution. Through scanning probe microscopy, the above findings are successfully translated to polycrystalline thin films. For 35%Br thin films, halide segregation selectively occurs at grain boundaries due to localized strain at the boundaries; yet for 65%Br films, halide segregation occurs in the whole layer. We close by demonstrating that only the strain-activated halide segregation (35%Br/45%Br thin films) could be suppressed if the strain is properly released via additives (e.g. KI) or ideal substrates (e.g. SiO2).