Terahertz photonics

Abstract:

Abstract Controlling excitation transport is central to the operation of light-harvesting and light-emitting devices based on nanocrystal films. Exciton diffusion is typically described as hopping through a homogeneous medium; however, even highly monodisperse nanocrystal ensembles exhibit energetic disorder arising from size variation. We demonstrate that such disorder couples exciton diffusivity in real space to energetic relaxation within the density of states. Using spectrally and temporally resolved photoluminescence reabsorption spectroscopy of CsPbBr3 nanocrystal films, we separately track exciton diffusion and energetic relaxation across a range of temperatures and nanocrystal sizes. We find that at low temperature, excitons preferentially migrate downhill toward larger, lower-energy nanocrystals, causing a transient collapse of exciton diffusivity that is particularly pronounced for smaller nanocrystals exhibiting higher energetic disorder. At room temperature, these effects are effectively countered by thermally activated uphill transfer, restoring normal diffusion. Nanocrystal solids thus offer tunable transport properties governed by the interplay between disorder and thermal energy.

Abstract:

Reliable predictions of long-term stability are essential as halide perovskite solar cells approach widespread commercial deployment. However, extrapolation from aging tests requires knowledge of material degradation dynamics, which remain unclear for halide perovskites. Here, we reveal the structural and optoelectronic evolution of formamidinium-caesium lead-halide perovskite films under heat and/or light using in situ X-ray diffraction and photoluminescence. Heat and light combined induce much faster degradation than either stressor alone, with 1 sun illumination degrading material 53 times faster at 85 °C than room temperature. For all compositions investigated the degradation rate is highly nonlinear in time and follows a sigmoidal decay, suggestive of interface-driven dynamics. Degradation proceeds in three steps: halide segregation (for mixed-halide perovskites), A-site cation diffusion triggered by heterogeneity and strain, and finally iodine and formamidinium loss. Our findings provide a quantitative framework for lifetime predictions and highlight the necessity of combined heat and light ageing tests.

Abstract:

Artificial intelligence (AI) is accelerating materials prediction and design by enabling efficient exploration of chemical and structural spaces, with particular promise for novel materials discovery. However, novelty in materials discovery encompasses chemical plausibility, structural distinctiveness, property relevance and experimental realisability, making AI-driven novelty claims difficult to substantiate. We introduce a materials property hierarchy, from intrinsic, composition-determined properties to extrinsic, processing-dependent performance, to clarify deployment constraints and distinguish structural, physical and deployment novelty. This framework motivates an evidence-based view of multimodal materials data spanning chemical composition, microstructure, processing, and testing and characterisation, showing that current evidence remains concentrated in composition and idealised structure while heterogeneous, under-represented and weakly integrated modalities limit support for physical and deployment novelty. It also highlights the limitations of benchmarks based mainly on computational labels and proxy novelty criteria. Community-wide standards for data collection, modality alignment and evidence synthesis are needed to support multimodal data construction, process-aware multimodal modelling, feasibility-first generative modelling and deployment-aware benchmarking, so that generative and multimodal AI can design experimentally realisable materials with defensible scientific and practical novelty.

Abstract:

Electrophoretic deposition (EPD) offers a scalable, solution-processable and binder-free route for assembling functional electrode films, yet its applicability across chemically diverse materials remains largely unexplored. In this study, we present a facile EPD platform for the rapid fabrication of electrodes and photoelectrodes from more than 20 functional materials, including carbon nanostructures, polymeric semiconductors, metal oxides, chalcogenides, nitrides, porous metal-organic frameworks, covalent organic frameworks and hybrid composites. Optimised deposition recipes are established for each material by controlling suspension chemistry, charging agents, applied voltage and deposition time. For photoactive materials, the photocurrent response is used as a practical descriptor to evaluate film quality and guide loading optimisation. Structural characterisation by scanning electron microscopy and transmission electron microscopy confirms uniform coatings, controlled morphologies, and intimate contact between deposited materials and conductive substrates. The versatility of the method is further validated by conformal deposition on both indium tin oxide (ITO) and porous carbon paper substrates, highlighting its compatibility with diverse electrode architectures. We also clarify the role of iodine and citric acid as charging agents for stabilising suspensions and ensuring reproducible deposition. Overall, this work demonstrates EPD as a versatile and scalable approach for integrating a wide range of functional materials into binder-free electrodes for photoelectrochemical and catalytic applications.

Abstract:

High-performance ultrafast terahertz devices such as modulators, photoconductive emitters, and detectors rely on an ultrafast change in conductivity. Semiconductors are suitable for this purpose since their charge-carrier density can be varied by external factors, such as incident photons or an applied electrical potential. High-performance ultrafast terahertz devices require semiconductors with high charge-carrier mobility but low charge-carrier lifetimes. The trade-off between charge-carrier mobility and the short charge-carrier lifetimes of standard semiconductors limits the signal-to-noise ratio of current terahertz devices. Hence, it is essential to develop high-performance materials to overcome these limitations and enable ultrafast terahertz technology. Low-temperature GaAs (LT-GaAs) has shown excellent performance in overcoming this limitation by providing a high charge-carrier mobility and, at the same time, a low charge-carrier lifetime. In this study, we have conducted terahertz time-domain spectroscopy (THz-TDS) and terahertz optical-pump terahertz-probe spectroscopy (OPTPS) on 2 µm-thick LT-GaAs thin films bonded with benzocyclobutene (BCB) to sapphire substrates. With these techniques, we demonstrate excellent sub-picosecond charge-carrier lifetimes across different infrared pump beam fluences. The measured samples showed a charge-carrier lifetime in LT-GaAs of 0.46 ps. We discuss observed saturation effects and how they can influence the shape of the photoconductivity spectrum. We explain the origin of this effect in the context of the experimental technique and charge-carrier dynamics in the material.