Terahertz photonics

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.

Abstract:

Effective charge-carrier transport is a key requirement of next-generation thin-film materials developed for solar cells. Perovskite-inspired materials (PIMs), including metal chalcohalides, show great promise as lead-free solar absorbers. However, intrinsic charge-carrier localization processes have frequently been reported to severely limit their transport properties. Recent research has thus focused on developing a rational understanding of this localization process and identifying strategies to eliminate it. Mixed-metal chalcohalides (A₂BCh₂X₃) may offer promising solutions, combining enhanced chemical stability with promising optoelectronic properties. Here, we demonstrate how charge-carrier localization can be overcome through judicious chemical substitution in this family of materials. Upon changing the M(II) cation on the A-site, the lattice symmetry shifts from the lower-symmetry monoclinic P2₁/c phase in Pb₂SbS₂I₃ to the higher-symmetry orthorhombic Cmcm phase in Sn₂SbS₂I₃. Crucially, a rapid localization of charge carriers within the first few picoseconds of their generation is observed only for Pb₂SbS₂I₃, whereas Sn₂SbS₂I₃ maintains a longer-lived nanosecond photoconductivity. We attribute this observation to the higher electronic dimensionality of the Cmcm Sn₂SbS₂I₃ structure, whose more symmetric lattice suppresses the charge-carrier localization dominating in the lower-dimensional P2₁/c Pb-analogue. These findings establish a direct link between structural and optoelectronic properties in metal chalcohalides, demonstrating how facile chemical tuning can be harnessed to overcome charge-carrier localization in PIM absorbers for solar energy harvesting.

Abstract:

Abstract Coupling CO₂ reduction with selective oxidation of biomass-derived molecules enables simultaneous carbon utilisation and value-added chemical synthesis, offering a sustainable pathway when driven by solar energy. Here, we report a bias-free photoelectrochemical platform that co-produces formate from CO₂ and 5-formyl-2-furoic acid, a key precursor to bio-based plastics. The system integrates a Co₃O₄/WO₃/g-C₃N₄ heterojunction photoanode on carbon paper, which selectively oxidises 5-hydroxymethyl-2-furoic acid to 5-formyl-2-furoic acid with a Faradaic efficiency of 95%, and a tin nanoparticle-decorated carbon paper cathode, which reduces CO₂ to formate with a Faradaic efficiency of 93% under solar irradiation and without external bias. The WO₃/g-C₃N₄ heterojunction promotes efficient Z-scheme charge transfer, enabling spatially separated oxidation and reduction reactions using earth-abundant materials and scalable electrode architectures. Life cycle assessment demonstrates low greenhouse gas emissions and low water consumption potential of the CO₂-to-formate pathway, highlighting the environmental sustainability of the process. By directly coupling CO₂ reduction with the upgrading of biomass-derived furanics into polymer-relevant intermediates, this work establishes a practically meaningful and scalable co-production strategy for solar-driven carbon utilisation and sustainable plastic precursor synthesis.