Ian Arthur Howard

Sub-unit motion within metal-organic frameworks (MOFs) offers unique opportunities for nanoscale sensing. However, achieving controlled partial rotation of bulky linkers remains a significant challenge. In this study, a 50-fold luminescence enhancement is observed from a MOF thin film when intra-pore solvent flow orients the linker chromophores. These MOF thin films can be prepared via a facile drop-casting method on various substrates. The MOFs structure consists of zinc-coordinated layers containing rotatable chromophores, separated by pillar molecules. Grazing-incidence wide-angle X-ray scattering analysis confirms the formation of highly oriented films. The deposition of a volatile organic compound, such as ethanol, triggers a significant enhancement in luminescence as the solvent nears complete evaporation. Photophysical characterization and quartz crystal microbalance measurements reveal that this phenomenon is driven by internal stress on the MOF’s pore level generated during the final stages of evaporation. This stress can result in a realignment of the MOF chromophores at the molecular scale. Consequently, this dynamic turn-on luminescence behavior establishes a foundation for nanoscale platforms capable of indicating solvent volatilization in real time.

The shift toward a circular economy for plastics requires innovative sorting technologies that improve recycling efficiency and material purity. Tracer‐based sorting (TBS) is a promising approach that employs luminescent tracers with distinct optical properties for the precise identification of plastics. This study presents the synthesis and characterization of europium‐doped calcium sulfide (CaS:Eu²⁺) luminescent materials as potential tracers for TBS. Synthesized via a flux‐assisted solid‐state reaction, these phosphors achieve photoluminescence quantum yields (PLQYs) of up to 65% with tunable luminescence decay lifetimes ranging from 0.05 to 0.3 seconds. Long‐term chemical stability under ambient conditions is confirmed over 18 months. The application of CaS:Eu²⁺ in a TBS prototype is demonstrated using a standard CMOS video camera to differentiate materials based on decay times. These results highlight the suitability of CaS:Eu²⁺ phosphors as stable, high‐performance tracers, offering a viable way to improve plastics recycling processes.

Metal halide perovskites hold promise for nonepitaxial laser diodes, yet, continuous‐wave (CW) optically pumped (photonic) lasing in CsPbBr ₃ remains elusive despite its superior thermal‐ and photo‐stability among the perovskite family. This work reports on CsPbBr ₃ vertical cavity surface emitting lasers with low lasing thresholds (1.3 µJ cm ^{− 2} ) at nanosecond pumping and remarkable lasing stability. Furthermore, the electrically switchable gain is achieved in CsPbBr ₃ electrically assisted optically pumped laser (EAOPL) devices by leveraging ion migration. Applying a small positive DC voltage to the EAOPL device significantly reduces the lasing threshold under nanosecond laser excitation and enhances the cavity mode intensity at CW laser excitation. These findings present a novel strategy, combining a small DC voltage with an electrical pulse, for exploring electrical injection lasing in CsPbBr ₃ perovskites.

Large‐scale fabrication and optimization of high‐quality polycrystalline perovskite thin films present significant challenges in scientific research and industry. Shifting from single‐spot measurements to imaging techniques facilitates the transition from laboratory‐scale to large‐scale processing. While single‐spot photoluminescence (PL) methods provide high‐depth insights into local optoelectronic characteristics, they are insufficient for assessing reliable information on homogeneity and spatial characteristics. Currently, no PL‐based imaging method delivers a comparable level of information depth on charge carrier dynamics to single‐spot PL methods. In response, this work introduces a non‐invasive imaging technique based on double‐pulse excitation. Varying the time delay between the pulses gives rise to spatial information on relative photoluminescence quantum yield (rPLQY) in thin films, yielding fundamental optoelectronic characteristics such as external radiative and effective non‐radiative recombination rates and charge‐carrier lifetime. Compared to traditional PL‐imaging and k‐imaging demonstrates the superiority of rPLQY‐imaging in revealing charge carrier dynamics. This technique proves applicable across various sample configurations, irrespective of the presence of electrodes and charge transport layers. In addition, the method can estimate surface recombination velocity and variations in escape and parasitic absorption probabilities. Overall, the rPLQY‐imaging method emerges as a valuable tool for scientific research aimed at characterizing and optimizing large‐area perovskite thin films.

Understanding the structure of thin films is essential for successful applications of metal-organic frameworks (MOFs), such as low k-dielectrics in electronic devices. This study focuses on the thin film formation of the 3D nanoporous MOF Cu₂(bdc)₂(dabco). The thin films are prepared by a layer-by-layer technique with varying deposition cycles (1 to 50). Thin film morphologies and crystallographic properties were investigated using atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, and grazing-incidence X-ray diffraction (GIXD). AFM revealed an island growth (Volmer-Weber) with plate-like shaped islands. FTIR and GIXD revealed that Cu₂(bdc)₂(dabco) crystals form already during the first preparation cycle. The heights of the islands do not increase linearly with the number of deposition cycles, suggesting multiple growth stages. X-ray diffraction pole figures uncover a uniplanar texture of the Cu₂(bdc)₂(dabco) crystals, together with randomly oriented crystallites. The fraction of uniplanar oriented crystals increases with each deposition cycle, reaching a maximum of 75% at ten deposition cycles, simultaneously achieving complete substrate coverage. However, already at five cycles, an additional phase of randomly oriented copper-terephthalate (Cu₂(bdc)) crystals appeared; this phase reaches a fraction of 22% at the largest film thickness (50 cycles). In summary, a detailed understanding of the thin film formation of an archetypal layer-pillar MOF is presented, elucidating how films grow in terms of their morphology and crystalline properties. Samples prepared by ten cycles show complete coverage of the substrate together with the highest degree of preferred crystal orientation. These results establish a deepened understanding of critical parameters for MOF thin film applications, such as complete substrate coverage and definition of the nanopores relative to the substrate surface.

Thin films of crystalline solids with substantial free volume built from organic chromophores and metal secondary building units (SBUs) are promising for engineering new optoelectronic properties through control of interchromophore coupling. Zn‐based SBUs are especially relevant in this case because they avoid quenching the chromophore's luminescence. We find that layer‐by‐layer spin‐coating using Zn acetate dihydrate and benzene‐1,4‐dicarboxylic acid (H₂BDC) and biphenyl‐4,4’‐dicarboxylic acid (H₂BPDC) linkers readily produces crystalline thin films. However, analysis of the grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) data reveals the structures of these films vary significantly with the linker, and with the metal‐to‐linker molar ratio used for fabrication. Under equimolar conditions, H₂BPDC creates a type of structure like that proposed for SURMOF‐2, whereas H₂BDC generates a different metal‐hydroxide‐organic framework. Large excess of Zn²⁺ ions causes the growth of layered zinc hydroxides, irrespective of the linker used. Density functional theory (DFT) calculations provide structural models with minimum total energy that are consistent with the experimentally observed diffractograms. In the broader sense, this work illustrates the importance in this field of careful structure determination, e. g., by utilizing GIWAXS and DFT simulations to determine the structure of the obtained crystalline metal‐organic thin films, such that properties can be rationally engineered and explained.

Laser-induced crystallization is a novel alternative to classical methods for crystallizing organic molecules but requires a judicious choice of experimental parameters for the onset of crystallization to be predictable. This study investigated the impact of the laser repetition rate on the time delay from the start of the pulsed laser illumination to the initiation of crystallization, the so-called induction time. A supersaturated urea solution was irradiated with near-infrared (λ = 1030 nm) laser pulses of pulse duration τ = 5 ps at a pulse energy of approximately E = 340 μJ while varying the repetition rate from 10 to 20,000 Hz. The optimal rate discovered ranged from 500 Hz to 1 kHz, quantified by the measured induction time (median 2-5 s) and the mean probability of inducing a successful crystallization event (5 × 10^-2%). For higher repetition rates (5-20 kHz), the mean probability dropped to 3 × 10^-3%. The reduced efficiency at high repetition rates is likely due to an interaction between an existing thermocavitation bubble and subsequent pulses. These results suggest that an optimized pulse repetition rate can be a means to gain further control over the laser-induced crystallization process.

A hybrid SURMOF with specific crystallographic [001] orientation is fabricated via step-by-step liquid-phase heteroepitaxial method. The preferential [001] orientation of Cu ₂ (BDC) ₂ (DABCO) is retained via Zn ₂ (BDC) ₂ (DABCO) to Zn ₂ (ADC) ₂ (BPy).

Metal–organic frameworks (MOFs) have emerged as promising hosts for lanthanide ions owing to their ability to sensitize the lanthanides and provide a rigid topology while still permitting interactions with guest species. Among these lanthanide MOFs, MOF‐76 is a noteworthy example composed of 1,3,5‐benzenetricarboxylic acid (H₃BTC) and lanthanide ions. In this study, a high photoluminescence quantum yield (Φ_PL) of 91% for Tb‐based MOF‐76 (Tb‐MOF) is reported, while the efficiency of Eu‐based MOF‐76 (Eu‐MOF) is over five times lower. A potential circumvention of the poor performance of Eu³⁺ is energy transfer from Tb³⁺ to Eu³⁺, but it is found that although energy transfer is efficient the Φ_PL of the Eu³⁺ remains low. The study investigates the cause of the lower quantum yield in Eu‐MOF by ruling out possible factors such as crystal size, guest solvent molecules, and energy transfer from or back to the linker. None of these are significant loss mechanisms, leading to conclude that the MOF‐76 ligand unfortunately leads to fast non‐radiative deactivation of the Eu³⁺ excited state. The results suggest that MOF‐76 is an excellent host for Tb³⁺ ions, but exploration is required for MOF linkers to simply access the potentially excellent emission efficiency of Eu³⁺ ions.

More complete recycling of plastic waste is possible only if new technologies that go beyond state-of-the-art near-infrared (NIR) sorting are developed. For example, tracer-based sorting is a new technology that explores the upconversion or down-shift luminescence of special tracers based on inorganic materials codoped with lanthanide ions. Specifically, down-shift tracers emit in the shortwave infrared (SWIR) spectral range and can be detected using a SWIR camera preinstalled in a state-of-the-art sorting machine for NIR sorting. In this study, we synthesized a very efficient SWIR tracer by codoping Li₃Ba₂Gd₃ (MoO₄)₈ with Yb³⁺ and Er³⁺, where Yb³⁺ is a synthesizer ion (excited near 976 nm) and Er³⁺ emits near 1550 nm. Fine-tuning of the doping concentration resulted in a tracer (Li₃Ba₂Gd_(3-x-y)(MoO₄)₈:xYb³⁺, yEr³⁺, where x = 0.2 and y = 0.4) with a high photoluminescence quantum yield for 1550 nm emission of 70% (using 976 nm excitation). This tracer was used to mark plastic objects. When the object was illuminated by a halogen lamp and a 976 nm laser, the three parts could be easily distinguished based on reflectance and luminescence spectra in the SWIR range: a plastic bottle made of polyethylene terephthalate, a bottle cap made of high-density polyethylene, and a label made of the tracer Li₃Ba₂Gd₃(MoO₄)₈:Yb³⁺, Er³⁺. Importantly, the use of the tracer in sorting may require only the installation of a 976 nm laser in a state-of-the-art NIR sorting system.