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Abstract: Two-dimensional (2D) transition metal dichalcogenides have emerged as a promising platform for next-generation optoelectronic and spintronic devices. Mechanical exfoliation using adhesive tape remains the dominant method for preparing 2D materials of highest quality, including transition metal dichalcogenides, but always results in small-sized flakes. This limitation poses a significant challenge for investigations and applications where large scale flakes are needed. To overcome these constraints, we explored the preparation of 2D WS 2 and WSe 2 using a recently developed kinetic in situ single-layer synthesis method (KISS). In particular, we focused on the influence of different substrates, Au and Ag, and chalcogen atoms, S and Se, on the yield and quality of the 2D films. The crystallinity and spatial morphology of the 2D films were characterized using optical microscopy and atomic force microscopy, providing a comprehensive assessment of exfoliation quality. Low-energy electron diffraction verified that there is no preferential orientation between the 2D film and the substrate, while optical microscopy revealed that WSe 2 consistently outperformed WS 2 in producing large monolayers, regardless of the substrate used. Finally, X-ray diffraction and X-ray photoelectron spectroscopy demonstrate that no covalent bonds are formed between the 2D material and the underlying substrate. These results identify KISS method as a non-destructive method for a more scalable approach of high-quality 2D transition metal dichalcogenides.

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Abstract: This study provides a comprehensive analysis of the electronic structure, reflectivity, and luminescent spectra of the organic-inorganic, metal-halide MAPbCl3 perovskite, which has considerable potential for various optoelectronic applications. Using density functional theory (DFT) calculations, we investigated the electronic structure of MAPbCl3 and interpreted the key features of its reflectivity spectra across a wide energy range from 3 to 10 eV. The reflectivity spectra reveal prominent excitonic features at 3.22 eV near the absorption edge and additional optical transitions at higher energies, highlighting the material’s intricate electronic structure. Furthermore, we examined the temperature dependence of radiative decay dynamics under high-energy radiation through X-ray luminescence spectra and decay time measurements. We observe emission from free and bound excitons with an exceptionally short decay time (≤ 1 ns) and significant thermal quenching at low temperatures (100 K) in the 385–430 nm range. These findings underline the importance of continued exploration of optoelectronic properties of the material to enhance its performance in practical applications.

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Abstract: The development of analog resistive non-volatile memory elements is of great interest for future information storage technology. These devices can be used to emulate the electronic behaviour of biological synapses and neurons for neuromorphic computing or sensing applications. Hafnia is a promising material to use in resistive memory systems due to its high industrial compatibility. Both the migration of oxygen ions and ferroelectricity are two mechanisms actively being explored to drive resistance change in hafnia based devices. This thesis explores the interplay between interfacial redox chemistry and ferroelectricity and how these influence the resistance switching behaviour in epitaxial hafnia films. A device stack is engineered to elicit large resistance changes through both ferroelectricity and interfacial redox reactions. Ultra-thin epitaxial Y doped HfO2 (YHO) is grown with a controlled oxygen stoichiometry. To stabilise the polar phase in YHO and provide a resistance-tuneable bottom interface, YHO is grown on La0.66Sr0.33MnO3 (LSMO) buffered Nb doped SrTiO3 substrates. In-depth X-Ray diffraction based structural analysis identified the polar nature of the film and a strain induced rhombohedral distortion. Ferroelectricity in the film was confirmed by piezoresponse force microscopy. In device configuration, using an Au top electrode with an ultra-thin Ti adhesion layer, a resistance switching mode with large on-off ratio was observed. The pristine device was stabilised in an intermediate resistance state and did not need an electro-forming process. The mode reversibly switched between a purely capacitive (Schottky) and resistive (Ohmic) state, a Schottky-to-Ohmic transition (SOT). A series of voltage pulse trains were used to explore the intermediate resistance states of the system. Both analog and integration behaviour was demonstrated, depending on the pulse profile. The SOT may therefore find application as both a synapse and a neuron respectively in neuromorphic applications. Capacitance-voltage measurements were employed to correlate the ferroelectric polarisa- tion reversal with the SET process and not the RESET process. Therefore, ferroelectricity was indicated to only be partially responsible for the observed resistance change. Electrode area dependent current measurements suggested that in the low resistance state, current transport occurred through a localised conductive filament. A detailed analysis of the inter- face chemistry suggested the importance of precisely designed oxygen scavenging at both interfaces to stabilise the SOT. Furthermore, interface stoichiometry changes were observed during resistance switching by a combined hard photo-electron spectroscopy and electron energy loss spectroscopy study. Impedance spectroscopy measurements were used to correlate the stoichiometry changes at the LSMO|YHO interface to resistance changes during the SOT. The conductive pristine resistance state was shown to predominantly be limited by the LSMO|YHO interface and was proposed to occur due to oxygen scavenging at the top electrode, by-passing through-grain conduction. Analysis of the SOT analog switching behaviour showed that both the YHO layer and the LSMO|YHO interface switch separately during the RESET operation, but simultaneously during the SET process. Furthermore, stack modifications demonstrated the importance of using an ultra-thin LSMO to stabilise the SOT. It was hypothesised to occur due to interface strain enhanced defect formation, oxygen transfer or symmetry changes at the LSMO|YHO interface.

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Abstract: Multi-walled carbon nanotubes (MWCNTs), synthesized using the microwave plasma-enhanced chemical vapor deposition (MPCVD) technique, have been examined to elucidate their electronic and magnetic structures through near-edge X-ray absorption fine structure (NEXAFS) and X-ray magnetic circular dichroism (XMCD) spectroscopy. NEXAFS analysis at the Fe and Co L-edges reveals the presence of Fe-metal nanoparticles embedded within the CNT lattice, along with divalent Co ions coordinated to the matrix in an octahedral symmetry. Furthermore, the appearance of two distinct NEXAFS peaks between the π* and σ* transitions indicates 1s to sp3 hybridization, attributed to the interaction of Fe and Co2+ ions with the carbon nanotube structure. Additionally, XMCD spectra confirm that MWCNTs exhibit room temperature ferromagnetism, primarily driven by Fe–C and Co–C bonding within the nanotubes. This intrinsic ferromagnetic behavior, along with the high aspect ratio and unique electronic properties of MWCNTs, highlights their promising potential for applications in spintronic storage devices.

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Abstract: Multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters have drawn significant interest for use in organic lighting-emitting diodes (OLEDs) as they typically have bright and narrowband emission. However, their rigid, planar structures result in poor solubility in organic solvents and a tendency to aggregate. This usually results in severe aggregation-caused quenching (ACQ), which hinders in particular, their application in solution-processed OLEDs. Here, a solution-processable MR-TADF emitter 2,7-tBuCzNB has been designed, synthesized and studied. The presence of eight tert-butyl groups and the use of second-generation donor dendrons help to enhance its solubility and to suppress the ACQ. 2,7-tBuCzNB exhibits narrowband green emission at 493 nm, with full-width at half maximum of 32 nm, and a high photoluminescence quantum yield (PL) of 93% in toluene. The PL values in 1–10 wt% doped films in mCP are slightly lower yet are as high as 80%. The solution-processed OLEDs using this emitter showed maximum external quantum efficiencies (EQEmax) of 11.4 and 10.6% at 5 and 10 wt% doping concentration, respectively. This work demonstrates a strategy to synthesize MR-TADF emitters that are solution processable for use in solution-processed OLEDs.