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Abstract: Halide perovskites exhibit superior optoelectronic properties but lack precise thickness and band offset control in heterojunctions, which is critical for modular multilayer architectures such as multiple quantum wells. We demonstrate vapor-phase, layer-by-layer heteroepitaxial growth exemplified by CsPbBr3 deposition on single crystals of PEA2PbBr4 (PEA: 2-phenylethylammonium). Angstrom-level thickness control and subangstrom smooth layers enable quantum-confined photoluminescence of CsPbBr3 from monolayer, bilayer, and through to bulk. The interfacial structure controls the electronic structure from a Cs‒PEA-terminated interface (type II heterojunction) to a PEA‒PEA-terminated interface (type I heterojunction), with a layer-tunable band offset shift exceeding 0.5 electron volts. Electron transfer from CsPbBr3 to PEA2PbBr4 for a type II Cs‒PEA heterojunction results in delayed electron-hole recombination beyond 10 microseconds. Precise quantum confinement control and large band offset tunability unlock perovskite heterojunctions as platforms for scalable, superlattice-based optoelectronic applications.

Open Access

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Abstract: Insulating nanomaterials have large energy gaps and are only electrically accessible under extreme conditions, such as high-intensity radiation and high temperature, pressure or voltage1,2. Lanthanide-doped insulating nanoparticles (LnNPs) are widely studied owing to their exceptional luminescence properties, including bright, narrow-linewidth, non-blinking and non-bleaching emission in the second near-infrared (NIR-II) range3,4. However, it has not been possible to electrically generate excited states in these insulating nanomaterials under low biases and, therefore, not possible to fabricate optoelectronic devices from these systems. Here we report an electrical excitation pathway to obtain emission from LnNPs. By forming LnNP@organic molecule nanohybrids, in which the recombination of electrically injected charges on the organic molecule is followed by efficient triplet energy transfer (TET) to the LnNP, it is possible to turn on LnNPs under a low operating bias. We demonstrate this excitation pathway in light-emitting diodes (LEDs), with low turn-on voltages of about 5 V, very narrow electroluminescence (EL) spectra and a peak external quantum efficiency (EQE) greater than 0.6% in the NIR-II window5. Our LnNP-based LEDs (LnLEDs) also allow for widely tunable EL properties, by changing the type and concentration of lanthanide dopants. These results open up a new field of hybrid optoelectronic devices and provide new opportunities for the electrically driven excitation sources based on lanthanide nanomaterials for biomedical and optoelectronic applications.

Open Access

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Abstract: Superlattices of lead chalcogenide colloidal quantum dots hold promise to revolutionise the field of infrared optoelectronics due to their unique combination of optical and transport properties. However, the main challenge remains to form a homogeneous thin-film with long-range order avoiding cracking upon ligand exchange. To overcome these issues, we introduce an approach where external lateral pressure is applied during the self-assembly and ligand exchange, thus avoiding the formation of cracks due to volume shrinking. The formed monolayer superlattices are crack-free over several millimetres square. Transport measurements in an ionic gel-gated field-effect transistor reveal that increasing the external pressure during the superlattice formation leads to higher electron mobilities above 25 cm2V−1s−1 thanks to better compactness, high ordering, and a higher number of nearest neighbours. These results demonstrate that colloidal quantum dot superlattices with high charge mobility can be fabricated over large areas with important implications for technological applications.

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Abstract: Interfacial electrochemistry is fundamental to the development of electrocatalytic renewable energy technologies such as fuel cells, batteries and carbon capture systems. Although vast amounts of resources are applied to the fabrication and characterisation of electrocatalysts, for technologies such as these to be viable on a large scale and mitigate the effects of anthropogenic climate change, a better fundamental understanding of the atomic and electronic structure of the electrochemical interface is required. This thesis presents the results of a series of experiments employing surface X-ray diffraction (SXRD) of the electrochemical interface of fcc(111) metal surfaces. First presented is an SXRD study of Cu(111) and Ag(111) surfaces in varying concentrations of aqueous acetonitrile-containing electrolyte. The study shows evidence of a combination of potential-induced dissolution and re-adsorption of surface metal layers and formation of surface metal oxides, and a roughening of the metal surface with increasing concentrations of acetonitrile, demonstrating the dramatic effect that acetonitrile can have on transition metal surfaces without being specifically adsorbed. A study of thin cobalt films electrodeposited on Au(111) surfaces is also presented, where energy-dispersive resonant SXRD measurements reveal subtle changes in the RSXRD spectrum at the cobalt K edge depending on the thickness of the thin film and the applied electric field, providing foundational information on the underlying physical mechanism behind the favourable magneto-electric properties of thin Co films. Lastly presented is a study of the cation contribution to the structure of the electrochemical interface, consisting of measurements of Pt(111) and Au(111) in aqueous cesium hydroxide (CsOH) electrolyte. The study employs SXRD to construct a model of the electrochemical interface, combined with resonant SXRD and XAS measurements. Results suggest a hydrated Cs layer ordered in the plane of the surface on Pt(111), while measurements on Au(111) suggest a more disordered Cs layer. The experimental work detailed in this thesis, performed over a series of experimental sessions at synchrotron facilities (the i07 beam line at Diamond Light Source, Didcot, UK, the XMaS beamline at the ESRF, Grenoble, and the Advanced Photon Source, Illinois), represents a significant contribution to our understanding of the electrochemical interface, demonstrating the power and versatility of SXRD and providing foundational information for the development and refinement of vital electrochemical technologies.

Open Access

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Abstract: Mechanisms for surface pattern formation from evaporation of a reactive nanofluid sessile drop are not well understood. In contrast to the coffee-ring effect from inert particles, rapid chemical and morphological transformation of reactive nanoparticles upon rapid evaporative drying are challenging to probe experimentally. Here, using grazing-incidence X-ray surface scattering, the nanostructure of nascent surface patterns has been probed as a ZnO nanofluid sessile drop rapidly dries. The high temporal resolution enabled by the high flux of synchrotron X-rays allows the observation of the emergence of Zn(OH)2 surface crystals from the onset of evaporation and their rapid evolution into the final residual surface pattern, via transient layered complexes evident from the temporary appearance of X-ray diffraction peaks preceding Zn(OH)2 formation. The results offer mechanistic insights of morphogenesis of surface patterns from evaporation-induced self-assembly and self-organization of reactive nanofluids, previously untenable using other experimental methods.