Elettra | ESCA Microscopy Revealing working principles of perovskite nanowire devices: ion migration and redox reactions Metal halide perovskites, such as CsPbBr3 or CsPbCl3, have emerged as highly promising materials for solar cells, light-emitting diodes (LEDs), photodetectors, and nextgeneration memory devices, owing to their exceptional optoelectronic performance, tunable bandgaps, and low-cost fabrication. Among them, one-dimensional perovskite nanowires offer additional advantages and provide the ability to engineer axial or radial heterojunctions. Their elongated geometry also confines ion transport along a well-defined axis, making them ideal platforms to study coupled electronic and ionic dynamics at the nanoscale. However, challenges remain in understanding their operational stability and the interplay of defects, mobile ions, and electrode interfaces under electrical bias. Surface sensitive x-ray techniques, especially X-ray Photoelectron Spectroscopy (XPS), can provide critical chemical and electronic insights. By focusing the x-ray beam to a spot size smaller than a µm, as provided by the ESCA Microscopy beamline at Elettra, it is possible to follow local changes along the heterostructure nanowire; and by individually contacting the nanowire electrodes and applying an electrical bias, local changes can be investigated operando, i.e. under device operation. However, metal halide perovskites are highly sensitive to radiation damage, which can occur quickly in a focused x-ray beam. To overcome this, in this study operando scanning photoelectron microscopy (SPEM) was employed under well controlled exposure conditions (<100 ms per pixel) at ESCA Microscopy. This allowed simultaneous chemical and electrical mapping of functional CsPb(Br1xClx)3 axial heterojunction nanowire devices under applied bias, enabling direct visualization of halide migration, defect dynamics, and redox processes in real-time. The fabricated nanowire devices featured well-defined Br-rich and Cl-rich segments, as shown in Fig. 1 (a), using anion exchange, and lithographically processed metal contacts to each end of the nanowire. Compositional measurements show Br/Cl contrast along the wire and quantitatively imply 71% Cl concentrations at the Clsegment, in accordance with photoluminescence measurements obtained on both segments of the nanowire. When an electrical bias is applied to the contacted nanowire in the SPEM setup, the binding energy of the XPS core-level will shift accordingly, as demonstrated in Fig. 1 (b). Electrical characterization of the nanowire region close to the contacted electrode using Br 3d corelevel shifts revealed Ohmic contacts of the devices, which is crucial both for reliable operando investigation and for perovskite devices in general. The main focus of the study was on dynamically mapping the Br- ions along the nanowire: The intensity of the Br 3d core-level XPS signal would highlight local Br composition, while the binding energy of the XPS peak indicates the local surface potential. The heterojunction nanowire with more Br-rich and more Cl-rich sections can be understood as a pn-junction, with an in-built potential even without applied bias and additional voltage drop upon electrical operation. Prior to biasing the device, an in-built potential of about 0.5 eV, associated with the heterojunction and local surface defects, was observed. This potential distribution followed an applied external bias of up to 1.5 V, while the potential drop over the NW became linear at 2 V, as shown in Fig. 1 (c), meaning that the in-build potential was removed. This irreversible change in potential gradient also denotes clear occurrence of halide ion migration within the device. By tracing the Br intensity using a spatially resolved Br 3d map, migration of Br⁻ and Cl⁻ ions, under applied voltages as low as 1 V, can be concluded. The measurements also suggest that the more electronegative Cl⁻ ions exhibit higher mobility than Br⁻, leading to faster migration. These ion movements were accompanied by a reduction in halide vacancies, indicating that these halide vacancies can be filled during device operation. A particularly compelling discovery was the reversible redox behavior of lead under applied electric fields. Operando measurements of the Pb 4f core-level and its oxidation state revealed a partially reversible transition between oxidized state Pb2+ and metallic state Pb0, depending on the bias polarity, as plotted in Fig. 1 (d). Here the oxidized state is expected for pristine CsPbBr3 or CsPbCl3, while lead in the metallic state indicates (surface) degradation of the perovskite material. The findings imply that decayed perovskite surfaces could, to some extent, be self-repaired via field-driven redox cycling. Notably, Pb in Br-rich and Cl-rich regions responded differently to biasing, reinforcing the role of local composition in governing redox dynamics. Furthermore, we observed evidence for an interface reaction between the perovskite nanowire and its metal contacts, including diffusing Br⁻ and Cl⁻ ions and Ti atoms of the electrodes. Our results demonstrate how nano-XPS and especially SPEM can reveal local composition and dynamic behavior of sensitive or partially instable nanostructure devices even under operando conditions, understanding physical and chemical effects at the nanoscale that can limit or enhance nanodevice performance and advancing both fundamental understanding and applied materials engineering. Although this study focused on axial heterojunction nanowires, the methodology is broadly applicable to thin films, multi-segment nanostructures, and radiation sensitive materials and devices. Elettra Highlights 2024–2025 76
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