Curvature-enhanced localised emission from dark states in wrinkled monolayer WSe2 at room temperature

Localised emission from defect states in monolayer transition metal dichalcogenides is of great interest for optoelectronic and quantum device applications. Recent progress towards high temperature localised emission relies on the application of strain to induce highly confined excitonic states. Here we propose an alternative paradigm based on curvature, rather than in-plane stretching, achieved through free-standing wrinkles of monolayer tungsten diselenide (WSe2). We probe these nanostructures using tip-enhanced optical spectroscopy to reveal the spatial localisation of out-of-plane polarised emission from the WSe2 wrinkles. Based on the photoluminescence and Raman scattering signatures resolved with nanoscale spatial resolution, we propose the existence of a manifold of spin-forbidden excitonic states that are activated by the local curvature of the WSe2. We are able to access these dark states through the out-of-plane polarised surface plasmon polariton resulting in enhanced strongly localised emission at room temperature, which is of potential interest for quantum technologies and photonic devices

Molecular Weight Tuning of Organic Semiconductors for Curved Organic-Inorganic Hybrid X-Ray Detectors

Curved X-ray detectors have the potential to revolutionize diverse sectors due to benefits such as reduced image distortion and vignetting compared to their planar counterparts. While the use of inorganic semiconductors for curved detectors are restricted by their brittle nature, organic-inorganic hybrid semiconductors which incorporated bismuth oxide nanoparticles in an organic bulk heterojunction consisting of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) are considered to be more promising in this regard. However, the influence of the P3HT molecular weight on the mechanical stability of curved, thick X-ray detectors remains less well understood. Herein, high P3HT molecular weights (>40 kDa) are identified to allow increased intermolecular bonding and chain entanglements, resulting in X-ray detectors that can be curved to a radius as low as 1.3 mm with low deviation in X-ray response under 100 repeated bending cycles while maintaining an industry-standard dark current of mu C Gy(-1) cm(-2). This study identifies a crucial missing link in the development of curved detectors, namely the importance of the molecular weight of the polymer semiconductors used

Multi-scale characterisation of a ferroelectric polymer reveals the emergence of a morphological phase transition driven by temperature

[EN] Ferroelectric materials exhibit a phase transition to a paraelectric state driven by temperature - called the Curie transition. In conventional ferroelectrics, the Curie transition is caused by a change in crystal symmetry, while the material itself remains a continuous three-dimensional solid crystal. However, ferroelectric polymers behave differently. Polymeric materials are typically of semi-crystalline nature, meaning that they are an intermixture of crystalline and amorphous regions. Here, we demonstrate that the semi-crystalline morphology of the ferroelectric copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) strongly affects its Curie transition, as not only a change in crystal symmetry but also in morphology occurs. We demonstrate, by high-resolution nanomechanical measurements, that the semicrystalline microstructure in the paraelectric state is formed by crystalline domains embedded into a softer amorphous phase. Using in situ X-ray diffraction measurements, we show that the local electromechanical response of the crystalline domains is counterbalanced by the amorphous phase, effectively masking its macroscopic effect. Our quantitative multiscale characterisations unite the nano- and macroscopic material properties of the ferroelectric polymer P(VDF-TrFE) through its semi-crystalline nature.European Union’s Horizon 2020 research and Innovation programme under the Marie Skłodowska-Curie grant agreement number 721874 (SPM2.0). RG acknowledges funding from the European Research Council ERC–AdG–340177 (3DNanoMech). This work was supported by the UK government’s Department for Business, Energy and Industrial Strategy. The dynamic mechanical analysis was supported by T. Koch from the Institute of Materials Science and Technology, TU Wien. We gratefully thank A. Muhamedagić for the contribution of artworks to the figures (armindesign.li).Peer reviewe

Electrical and tip enhanced optical scanning probe microscopy for the functional characterisation of emerging electronic materials

Silicon is widely used in (opto-)electronics devices, despite displaying fundamental downfalls such as an indirect band gap. As a result, substantial research efforts have been placed in finding novel materials that could suitably replace silicon on various electronic device applications. These novel material systems, here labelled as emerging electronics, exhibit critical nanoscale structure-function relationships that introduce measurement challenges additional to those successfully employed in silicon-based metrology. One important aspect is the need to characterise nanostructural inhomogeneities, which can determine device performance as well as operational stability.The present research focuses on employing scanning probe microscopy as a versatile ‘one-instrument’ approach for reliable and quantitative measurements on emerging electronics. With the focus lying on employing both electrical and tip-enhanced optical spectroscopic modes, their practical implementation is discussed. Combined modes of scanning probe microscopy are subsequently applied to study the effect of grain boundaries on the functional performance in organic-inorganic halide perovskites for photovoltaic applications. Beneficial charge separating properties due to local band bending are found to compete with detrimental increased carrier recombination, highlighting the critical role of the microstructure. Following, the intrinsic instability and degradation pathways of perovskite films under simulated operational conditions are spatially resolved. Decomposition effects are locally resolved and attributed to inhomogeneous charge redistribution effects under operational stresses. Finally, moving towards the study of device architectures, the electrical contact in two-dimensional material heterostructures is characterised. Absence of Fermi level pinning is demonstrated alongside the presence of a vertical interface dipole, which governs charge transfer and carrier relaxation pathways.Results of this research contribute novel findings in the current understanding of important mechanisms limiting industry uptake of emerging electronics. In addition, the multi-parameter approach to scanning probe microscopy is demonstrated as a powerful tool for investigation of measurement challenges related to emerging electronic systems

Enhancing and quantifying spatial homogeneity in monolayer WS2

Controlling the radiative properties of monolayer transition metal dichalcogenides is key to the development of atomically thin optoelectronic devices applicable to a wide range of industries. A common problem for exfoliated materials is the inherent disorder causing spatially varying nonradiative losses and therefore inhomogeneity. Here we demonstrate a five-fold reduction in the spatial inhomogeneity in monolayer WS2, resulting in enhanced overall photoluminescence emission and quality of WS2 flakes, by using an ambient-compatible laser illumination process. We propose a method to quantify spatial uniformity using statistics of spectral photoluminescence mapping. Analysis of the dynamic spectral changes shows that the enhancement is due to a spatially sensitive reduction of the charged exciton spectral weighting. The methods presented here are based on widely adopted instrumentation. They can be easily automated, making them ideal candidates for quality assessment of transition metal dichalcogenide materials, both in the laboratory and industrial environments

3D Magnetic Field Reconstruction Methodology Based on a Scanning Magnetoresistive Probe

The present work provides a detailed description on quantitative 3D magnetic field reconstruction using a scanning magnetoresistance microscopy setup incorporating a 19.5 μm × 2.5 μm magnetoresistive sensor. Therefore, making use of a rotation stage, 11 nm thick ferromagnetic CoFe elements with 20 μm × 5 μm planar size were measured along different sensor axes and converted into cartesian coordinate magnetic field components by use of the analytical coordinate transform equations. The reconstruction steps were followed and validated by numerical simulations based on a field averaging model caused by a non-negligible sensor volume. Detailed in-plane magnetic component reconstruction with ability to reconstruct sub-micrometer features is achieved. A discussion on the limiting factors for optimal resolution is presented

Ion-driven nanograin formation in early-stage degradation of tri-cation perovskite films

The operational stability of organic–inorganic halide perovskite based solar cells is a challenge for widespread commercial adoption. The mobility of ionic species is a key contributor to perovskite instability since ion migration can lead to unfavourable changes in the crystal lattice and ultimately destabilisation of the perovskite phase. Here we study the nanoscale early-stage degradation of mixed-halide mixed-cation perovskite films under operation-like conditions using electrical scanning probe microscopy to investigate the formation of surface nanograin defects. We identify the nanograins as lead iodide and study their formation in ambient and inert environments with various optical, thermal, and electrical stress conditions in order to elucidate the different underlying degradation mechanisms. We find that the intrinsic instability is related to the polycrystalline morphology, where electrical bias stress leads to the build-up of charge at grain boundaries and lateral space charge gradients that destabilise the local perovskite lattice facilitating escape of the organic cation. This mechanism is accelerated by enhanced ionic mobility under optical excitation. Our findings highlight the importance of inhibiting the formation of local charge imbalance, either through compositions preventing ionic redistribution or local grain boundary passivation, in order to extend operational stability in perovskite photovoltaics

Facilitating uniform large-scale MoS₂, WS₂ monolayers and their heterostructures through van der Waals Epitaxy

The fabrication process for the uniform large-scale MoS2, WS2 transition metaldichalcogenides (TMDCs) monolayers and their heterostructures has been developed by Van der Waals epitaxy (VdWE) through the reaction of MoCl5 or WCl6 precursors and the reactive gas H2S to form MoS2 or WS2 monolayers, respectively. The heterostructures of MoS2/WS2 or WS2/MoS2 can be easily achieved by changing the precursor from WCl6 to MoCl5 once the WS2 monolayer has been fabricated or switching the precursor from MoCl5 to WCl6 after the MoS2 monolayer has been deposited on the substrate.These VdWE-grown MoS2, WS2 monolayers and their heterostructures have been successfully deposited on Si wafer with 300 nm SiO2 coating (300 nm SiO2/Si), quartz glass, fused silica and sapphire substrates using the protocol we have developed. We have characterized these TMDCs materials with a range of tools/techniques including scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), micro-Raman, photoluminescence (PL), atomic force microscopy (AFM), transmission electron microscope (TEM), energy-dispersive X-ray spectroscopy (EDX) and selected area electron diffraction (SAED). The band alignment and large-scale uniformity of MoS2/WS2 heterostructures have also been evaluated with PL spectroscopy.This process and resulting large-scale MoS2, WS2 monolayers and their heterostructures have demonstrated promising solutions for the applications in next generation nanoelectronics, nanophotonics and quantum technology