Table of contents

Metal halide perovskite-based nanostructures, nanosheets and nanoparticles at the forefront, show attractive optoelectronic properties, suitable for photovoltaics and light emission applications. Achieving a sounded understanding of these basic electronic and optical properties represents therefore a crucial step for the full technological exploitation of this class of semiconductors. The rapidly expanding chemical engineering and their unusual structural diversity is fascinating but also challenging for a rational description on par with those well-known for conventional semiconductors. In this sense, group theory-based symmetry analyses offer a general and rigorous approach to understand the properties of various bulk perovskites and perovskite-based nanostructures. In this work, we review the electronic and optical response of metal halide perovskite semiconductors using symmetry analysis from group theory, recalling the main results for the prototypical cubic Pm-3m lattice of AMX₃ bulk perovskites (where A is cation, M metal and X halide), then extending the analysis to three cases of technological interest: AMX₃ nanoparticles, A₄MX₆ isolated octahedra, A₂MX₄ layered systems, and recently introduced deficient halide perovskites (d-HP). On the basis of symmetry arguments, we will stress analogies and differences in the electronic and optical properties of these materials, as induced by the spatial confinement and dimensionality. Meanwhile, we will take advantage of this analysis to discuss recent results and debates from the literature, as the energetics of dark/bright states in the band-edge exciton fine structure of perovskite nanoparticles and nanosheets. From the present work, we also anticipate that the band-edge exciton fine structure of d-HP does not present optically dark states, in striking contrast to AMX₃ nanoparticles and layered perovskites, a fact that can have important consequences on the photophysics of these novel perovskitoids.

Nano-sized advanced reference materials (RMs) based on synthetic polymers were developed using supercritical fluid chromatography (SFC) and certain size measurement methods. These RMs have reference values of accurate molecular size and molecular weight. One of the RMs investigated was poly(ethylene glycol) (PEG) with no distribution in its degree of polymerization, i.e., its absolute degree of polymerization was determined as 23. SFC was used to separate molecularly uniform polymers from a commercial sample with wide polydispersity in its degree of polymerization. Because of the polydispersity, the average molecular size of the commercial polymer sample showed a distribution. This PEG 23mer RM can provide a uniform molecular size as the degree of polymerization is determined to be precisely 23.

In the past few years, considerable progress has been made on the controlled synthesis of bilayer van der Waals (vdW) materials such as graphene and transition metal dichalcogenides (TMDs), which are of interest due to their attractive optical and electronic properties. A variety of methods have been developed to synthesize bilayer vdW materials. This review is devoted to recent advances in the properties and synthesis of bilayer graphene (BLG) and TMDs. We emphasize the intriguing properties of BLG and TMDs, depending on their composition, stacking configurations, and twisting angles. The large-scale chemical vapor deposition (CVD) growth of BLG and TMDs with large domain size, high quality, and strong interlayer coupling is highlighted. We then expand the discussion to the current understanding of the growth mechanisms of BLG by CVD and synthetic methods of bilayer TMDs. Finally, the crucial challenges of BLG synthesis and our perspective of the future of bilayer TMDs will be laid out for potential applications of vdW materials.

Currently, rechargeable sodium-ion batteries (SIBs) with high voltage and high energy density have attracted considerable attention. However, compared with lithium-ion batteries (LIBs), there are many urgent challenges that need to be solved to achieve the practical application of SIBs. Due to the similar physicochemical properties of sodium and lithium, the study of SIBs is based on LIBs. However, the radius of Na⁺ is larger than that of Li⁺, a limited number of LIBs electrode materials can be used in SIBs, especially anode materials. Graphite can store sodium ions if an ether-based electrolyte is being used. The storage capacity of graphite for sodium is low (∼35 mAh g⁻¹) when traditional carbonate-based electrolyte is used. Therefore, it is vital that anode materials with splendid rate capability, outstanding cycling performance and low cost are developed rapidly. Among all types of anode materials, metal sulfides (MS_x) with higher theoretical specific capacity and lower cost are an ideal practical anode material. Here, a summaryof the recent research advances on MS_x of SIBs is provided. The crystal structures, sodium storage mechanism and optimization strategies for high performance batteries are summarized. this paper hopes to provide inspiration for the development of MS_x to assist the development of the next generation of rechargeable battery applications.

In this work, we present a review of quantum dot (QD) material systems that allow us to obtain light emission in the telecom C-band at 1.55 µm. These epitaxial semiconductor nanostructures are of great technological interest for the development of devices for the generation of on-demand quanta of light for long-haul communication applications. The material systems considered are InAs QDs grown on InP, metamorphic InAs/InGaAs QDs grown on GaAs, InAs/GaSb QDs grown on Si, and InAsN QDs grown on GaAs.

In order to provide a quantitative comparison of the different material systems, we carried out numerical simulations based on envelope function approximation to calculate the strain-dependant energy band profiles and the associated confined energy levels. We have also derived the eigenfunctions and the optical matrix elements for confined states of the systems.

From the results of the simulations, some general conclusions on the strengths and weaknesses of each QD material system have been drawn, along with useful indications for the optimization of structural engineering aiming at single-photon emission in the telecom C-band.

In recent years, the notion of 'Quantum Materials' has emerged as a powerful unifying concept across diverse fields of science and engineering, from condensed-matter and coldatom physics to materials science and quantum computing. Beyond traditional quantum materials such as unconventional superconductors, heavy fermions, and multiferroics, the field has significantly expanded to encompass topological quantum matter, two-dimensional materials and their van der Waals heterostructures, Moiré materials, Floquet time crystals, as well as materials and devices for quantum computation with Majorana fermions. In this Roadmap collection we aim to capture a snapshot of the most recent developments in the field, and to identify outstanding challenges and emerging opportunities. The format of the Roadmap, whereby experts in each discipline share their viewpoint and articulate their vision for quantum materials, reflects the dynamic and multifaceted nature of this research area, and is meant to encourage exchanges and discussions across traditional disciplinary boundaries. It is our hope that this collective vision will contribute to sparking new fascinating questions and activities at the intersection of materials science, condensed matter physics, device engineering, and quantum information, and to shaping a clearer landscape of quantum materials science as a new frontier of interdisciplinary scientific inquiry. We stress that this article is not meant to be a fully comprehensive review but rather an up-to-date snapshot of different areas of research on quantum materials with a minimal number of references focusing on the latest developments.

As spintronic devices become more and more prevalent, the desire to find Pt-free materials with large spin Hall effects is increasing. Previously it was shown that β-W, the metastable A15 structured variant of pure W, has charge-spin conversion efficiencies on par with Pt, and it was predicted that β-W/Ta alloys should be even more efficient. Here we demonstrate the enhancement of the spin Hall ratio (SHR) in A15-phase β-W films doped with Ta (W_4−xTa_x where x = 0.34 ± 0.06) deposited at room temperature using DC magnetron co-sputtering. In close agreement with theoretical predictions, we find that the SHR of the doped films was ∼9% larger than pure β-W films. We also found that the SHR's in devices with Co₂Fe₆B₂ were nearly twice as large as the SHR's in devices with Co₄Fe₄B₂. This work shows that by optimizing deposition parameters and substrates, the fabrication of the optimum W₃Ta alloy should be feasible, opening the door to commercially viable, Pt-free, spintronic devices.

Crystallography employing conventional large-volume diffraction has enabled the firm connections between structure and properties and structure and function that have solved many of the most difficult problems in materials science and biology. Disordered materials possess a large variety of local structural arrangements and pose a special challenge for crystallography. Often the local structures and symmetries that are responsible for observed phenomena in structurally complex disordered materials cannot be distinguished from conventional diffraction alone. In this article, we review analytical approaches for understanding local structure and symmetry from angular correlations in limited-volume diffraction patterns of amorphous materials, with a special focus on electron nano-diffraction. We discuss how these angular correlations can be interpreted in the context of dense, disordered, three-dimensional materials probed in a projection geometry and highlight the experimental challenges and considerations. New developments in this field are described whereby these angular correlations are statistically analyzed to probe the symmetry and variety of local structures, transformed to a real-space function that contains the 2-, 3- and 4-body particle correlations, and employed to develop reverse Monte Carlo models with more realistic higher-order correlations.

To control rheological properties and accomplish perfect sensory properties and mouthfeel, polysaccharides are added to milk-based beverages. However, in contrast to expectations, it is often found that adding low concentrations of xanthan gum or guar gum to milk provokes phase separations of unclear physical origin. From this observation, questions arise regarding the interaction of added polysaccharides and the proteins present in milk – caseins and whey proteins. The focus of this study is to investigate such systems and to understand the basic interactions of caseins and whey proteins with different hydrocolloids. The hydrocolloids used in this study are xanthan gum, guar gum, gellan gum as well as iota-carrageenan, which were dissolved in pasteurized, non-homogenized, skimmed milk. The methods used for the examinations are light microscopy, measurement of zeta potential, atomic force microscopy and measurement of particle sizes. It was found for the case of xanthan gum dissolved in milk that the xanthan gum molecules and some of the whey proteins are found in the upper phase whereas the casein micelles as well as whey proteins are in the lower phase. For the case of guar gum dissolved in milk, the guar gum molecules are present in the upper phase and the casein micelles are present in the lower phase. This phase separation is probably caused by depletion interaction. Whey proteins are found in both phases. For the cases of iota-carrageenan, respectively, gellan gum dissolved in milk no macroscopic phase separation is observed and the measurements suggest the formation of complexes between the hydrocolloid and whey proteins.

Among the existing two-dimensional materials, MXenes, i.e. transition metal carbides, nitrides and/or carbonitrides, stand out for their excellent electrochemical properties. Due to their high charge storage capacity, metal-like conductivity, biocompatibility as well as hydrophilicity, Ti₃C₂T_x MXene-based inks hold great potential for scalable production of skin conformable electronics via direct printing methods. Herein, we develop an aqueous MXene ink and inkjet-print MXene films on freestanding, flexible, and conducting polymer-based substrates. These skin-adherent MXene electrodes detect electrocardiography signals with high signal-to-noise ratio while exhibiting preserved electrical performance after 1000 cycles of bending with a 50 d long shelf life in ambient conditions. We show that printed MXene films can be further functionalized to perform as multifunctional biosensing units. When integrated with a sodium (Na⁺) ion selective membrane, MXene electrodes detect Na⁺ in artificial sweat with a sensitivity of 40 mV per decade. When the films are functionalized with antibodies, they generate an electrical signal in response to a pro-inflammatory cytokine protein (interferon gamma) with a sensitivity of 3.9 mV per decade. Our findings demonstrate how inkjet-printed MXene films simplify the fabrication of next-generation wearable electronic platforms that comprise multimodal sensors.

Engineering of atomically thin transition metal dichalcogenides (TMDs) is highly sought after for novel optoelectronic and spintronic devices. With the limited number of naturally existing TMDs, chalcogen based alloying has become a viable solution for developing TMDs for optical modulators and photovoltaics. Here, we report on detailed optical and microscopic studies of ternary TMD alloys of molybdenum, sulfur, and selenium grown via a single step method. The developed material has tunable band gaps in a broad range 1.5–1.9 eV with the variation in sulfur compositions. Further, the existence of trions, bi-excitons, and defect bound excitons are shown using temperature dependent (4 K−300 K) photoluminescence spectroscopy. A detailed analysis on MoS_1.34Se_0.66 alloy system shows the evidence of new types of defect bound excitons originating at low temperatures along with the presence of bi-excitons having a binding energy of ∼41 meV. The prospects of defect induced quasiparticles are observed in scanning transmission electron microscope assisted analyses and verified using density functional theory calculations. The thermal conductivity values, calculated using micro-Raman studies, of MoS₂, MoSe₂, and MoS_1.34Se_0.66 are found to be 69(±2) W m⁻¹ K⁻¹, 33(±2) W m⁻¹ K⁻¹ and 17(±2) W m⁻¹ K⁻¹ respectively, in agreement with the theoretical predictions. Tunable optical properties of these ternary atomic layers along with moderate thermal conductivity reveal the potential of these layers in modern opto-electronic devices and sensors.

Au-based protein nanoclusters (PNCs) represent an emerging class of fluorescence probes that are inherently biocompatible and combine the functionality of proteins and optical properties of Au nanoclusters. Here we report on a methodology to create conjugated Au PNCs using amino acid coupling strategies from a series of common laboratory proteins. We discover that the host protein and the specific conjugation chemistry has a profound influence on the resulting fluorescence properties. Synchrotron analyses showcase local Au NC aggeration upon PNC conjugation, which causes local environment changes to invoke differences in fluorscence properties. The observed aggeration does not give rise to plasmonic properties nor signifigant fluorescence quenching, strongly indicating the PNCs are still in a near-native cluster state. Our methodology and findings here could open new pathways for tuning PNC fluorescence properties in a rational fashion, having a potential impact in host of biomedical and sensing applications.

This study investigates the magnetisation process of a martensitic Ni–Mn–Ga thin film with microstructure optimized to obtain a unidirectional and reversible magnetisation jump. The study has been realised by a thorough vector magnetometry characterisation and supported by micromagnetic modelling, considering different orientations of the applied field with respect to the symmetry directions of the sample. The model has been built on the film microstructure and experimental characteristics.

The main features of the magnetisation curves measured along the film symmetry directions can be well reproduced by the micromagnetic model, that is, neglecting structural and magnetostructural contributions to the free energy. The model also well reproduces the field-dependent behaviour of the transverse magnetisation components. The agreement demonstrates that the spatial organisation of magnetocrystalline anisotropy axes due to martensitic twinning has a dominant effect on the magnetisation process, giving rise to magnetisation jumps when the magnetic field is applied along the alignment direction of the twin boundaries. When a reverse field is applied along this direction, simulations show that magnetisation reversal proceeds through the formation and three-dimensional expansion of magnetic domains, passing around the zero field through a closed-flux domain configuration, with the domain walls showing perpendicular orientation of the magnetic moments.

Carbon nanotubes (CNTs) along with reduced graphene oxide (RGO) are synthesized using modest methods and their composites with the polymers PEDOT:PSS and P3HT are prepared using an easy solution method. An attractive improvement in the composites' physical properties with wt% increase of the filler material is observed, encouraging their applications in the fabrication of organic solar cells (OSCs). Using the composites in appropriate layers of the device architecture, OSCs have been fabricated by spin coating, and the incorporation of filler CNTs and RGO has been observed to result in considerable improvement in the power conversion efficiency (PCE) of all OSCs. To study the stability of the devices, the electrical properties of the OSCs have been periodically investigated in two different environments to understand the impact of both intrinsic and extrinsic degradation. The incorporation of filler carbon nanomaterials has been noticed to be successful in significantly prolonging the stability of the OSCs while maintaining the augmentation in PCE. For the best performing devices, the incorporation of CNTs and RGO has enhanced the PCE by 12.52% and 13.21% and improved the device lifetime by 37.31% and 43.23%, respectively, compared to the reference device. The results discussed in this report are greatly promising for the large scale consideration of a pioneering role of organic materials in numerous optoelectronic devices from a new and innovative perception assisted by the application of carbon-based nanomaterials.