Table of contents

The failure and fracture properties of hydrogels and hydrogel composites are considered in the contexts of applicable fracture mechanics and biomaterials engineering. Distinction is made between material failure properties, characterized by a work of failure independent of mechanism, and fracture properties, characterized by fracture resistance that requires clear identification of crack propagation. Although advanced hydrogels can exhibit very large works of failure relative to conventional single-network hydrogels, they do so only at large strains and are not well characterized by fracture properties alone. The large failure strains are not obviously relevant to many biomaterial applications such as cartilage replacement, for which the engineering requirements involve limited physiological strains. An example is given of fiber-reinforced hydrogel composites that demonstrate increased work of failure at small strains.

Soft tissue engineering has been gaining increasing interest as an approach to overcome the limitations posed by current clinical procedures such as invasiveness of the surgery, post-operative complications and volume loss. Soft tissue damage occurs either due to congenital malformation, trauma/disease or surgical resection. Through the use of autologous cells, such as mesenchymal stem cells, combined with a biomaterial acting as a support, biological substitutes can be developed. A promising pathway in terms of delivery of these engineered constructs is the use of an injectable system, able to provide a minimally invasive approach. Advances have been made in the development of biocompatible biomaterials able to induce soft tissue regeneration. The present review provides an overview of fillers used in the clinic as well as a non-exhaustive overview of all injectable systems reported for soft tissue engineering. A particular focus is placed on the benefits and drawbacks of the biomaterials and the underlying polymerisation strategy. Furthermore, focus is placed on the mechanical properties of the systems.

Solid oxide fuel cells (SOFC) are highly efficient energy conversion device, but its high operating temperature (800∼1000 °C) restricts industrial commercialization. Reducing the operating temperature to <800 °C could broaden the selection of materials, improve the reliability of the system, and lower the operating cost. However, traditional perovskite cathode could not both attain the high catalytic activity towards the oxygen reduction reaction and good durability at medium and low temperature range. In contrast to the conventional perovskites, Ruddlesden–Popper perovskites exhibit fast oxygen surface exchange kinetic and excellent stability at medium and low temperatures, and excel both in oxide-conducting fuel cells (O-SOFC) and proton-conducting fuel cells (H-SOFC). In this paper, we try to relate its prominent performance with the crystal structure, main physical properties, and transport mechanism of oxygen ions and protons. We also summarize the current strategy in improving its application in O-SOFC and H-SOFC. Finally, we discuss the challenges and outlook for the future development of RP perovskites in SOFC.

Artificial intelligence (AI) technologies are accelerating the rapid innovations of multifunctional micro/nanosystems for boosting significant applications in flexible electronics, human healthcare, advanced robotics, autonomous control, and human–machine interfaces. III-nitride semiconductors, e.g. GaN, AlN, InN, and their alloys, exhibit superior device characteristics in high-performance opto-/electronics, due to the unique polarization effects in the non-central-symmetric crystal. Piezotronics, coupled with piezoelectric polarization and semiconductor properties, can provide a novel approach for controlling charge carrier transport across the interfacial Schottky barrier or p–n junction in these piezoelectric semiconductors. It means constructing a direct, real-time, seamless interaction between human/machine and environment, which indicates great potential in emerging AI systems. In this article, we review the research progress of piezotronics on III-nitride semiconductors, summarize the fundamental theory of piezotronics, illustrate flexible device process, present emerging piezotronic intelligent GaN-based devices, and provide innovative supports for building adaptive and interactive AI systems.

Serious challenges in energy and the environment require us to find solutions that use sustainable processes. There are many sustainable electrocatalytic processes that might provide the answers to the above-mentioned challenges, such as the oxygen reduction reaction (ORR), water splitting, the carbon dioxide reduction reaction (CO₂RR), and the nitrogen reduction reaction (NRR). These reactions can enhance the value added by producing hydrogen energy through water splitting or convert useless CO₂ and N₂ into fuels and NH₃. These electrocatalytic reactions can be driven by high-performance catalysts. Therefore, the exploration of novel electrocatalysts is one of the important electrocatalytic fields. In this paper, we aim to systematically discuss a variety of electrocatalysts used for sustainable processes and to give further insights into their status and associated challenges. We invited many famous research groups to write this roadmap with topics including platinum (Pt) and its alloys for ORR, oxides for ORR, chalcogenides for ORR, carbon-based hollow electrocatalysts for ORR, carbides for ORR, atomically dispersed Fe–N–C catalysts for ORR, metal-free catalysts for ORR, single-atom catalysts (SACs) for ORR, metal boride (MB) electrocatalysts for water splitting, transitional metal carbides (TMCs) for water splitting, transition metal (TM) phosphides for water splitting, oxides for water splitting, sulfides for water splitting, layered double hydroxides for water splitting, carbon-based electrocatalysts for water splitting, Ru-based electrocatalysts for water splitting, metal oxides for CO₂RR, metal sulfides for CO₂RR, metals for CO₂RR, carbon for CO₂RR, SACs for CO₂RR, heterogeneous molecular catalysts for CO₂RR, oxides for NRR, chalcogenides for NRR, C₃N₄ for NRR, SACs for NRR, etc. Their contributions enabled us to compile this 2020 roadmap on electrocatalysts for green catalytic processes and provide some suggestions for future researchers.

Ion-gated transistors are attracting significant attention due to their low operating voltage (<1 V) and modulation of charge carrier density by ion-gating media. Here we report flexible organic ion-gated transistors based on the high mobility donor–acceptor conjugated copolymer poly[4-(4,4-dihexadecyl 4H-cyclopenta[1,2-b:5,4-b']-dithiophen-2-yl)-alt[1,2,5]thiadiazolo[3,4c]pyridine](PCDTPT) and the ionic liquid [1-ethyl-3 methylimidazolium bis(trifluoromethylsulfonyl)imide] as the ion-gating medium. Electrical characteristics of devices made on both [rigid (SiO₂/Si) and flexible (polyimide (PI))] substrates showed very similar values of hole mobility (∼1 cm² V⁻¹ s⁻¹) and ON–OFF ratio (∼10⁵). Flexible ion-gated transistors showed good mechanical stability at different bending curvature radii and under repetitive bending cycles. The mobility of flexible ion-gated transistors remained almost unchanged upon bending. After 1000 bending cycles the mobility decreased by 20% of its initial value. Flexible photodetectors based on PCDTPT ion-gated transistors showed photosensitivity and photoresponsivity values of 0.4 and 93 AW⁻¹.

Monolayer tungsten disulfide (WS₂) has recently attracted a great deal of interest as a promising material for advanced electronic and optoelectronic devices such as photodetectors, modulators, and sensors. Since these devices can be integrated in a silicon (Si) chip via back-end-of-line (BEOL) processes, the stability of monolayer WS₂ in BEOL fabrication conditions should be studied. In this work, the thermal stability of monolayer single-crystal WS₂ at typical BEOL conditions is investigated; namely (a) heating temperature of 300 °C, (b) pressures in the medium- (10⁻³ mbar) and high- (10⁻⁸ mbar) vacuum range; (c) heating times from 30 minutes to 20 hours. Structural, optical and chemical analyses of WS₂ are performed via scanning electron microscopy, Raman spectroscopy, photoluminescence and X-ray photoelectron spectroscopy. It is found that monolayer single-crystal WS₂ is intrinsically stable at these temperature and pressures, even after 20 h of thermal treatment. The thermal stability of WS₂ is also preserved after exposure to low-current electron beam (12 pA) or low-fluence laser (0.9 mJ μm⁻²), while higher laser fluencies cause photo-activated degradation upon thermal treatment. These results are instrumental to define fabrication and inline monitoring procedures that allow the integration of WS₂ in device fabrication flows without compromising the material quality.

The partial replacement of Cu by Ag in Cu(In,Ga)Se₂ thin-film solar cells is strategically interesting to achieve smooth devices with high conversion efficiencies. Yet, the industrial exploitation requires further understanding of the deposition process and control of the absorber layer properties. In this study, three-stage co-evaporation of (Ag,Cu)(Ga,In)Se₂ films with [Ag]/([Ag] + [Cu]) contents up to 0.2 was investigated. Deep crevices and voids, sometimes extending down to the rear contact, were found. They mainly occur for high Ag contents and excessive group-I richness during the second stage of the deposition. The formation of cavities is attributed to the segregation of Ag–Se phases and slow Ag diffusion into the chalcopyrite during the deposition. Another identified challenge is the flattening of the desired bandgap grading which is correlated with the Ag content. Optimized process conditions allow fabrication of smooth (Ag,Cu)(Ga,In)Se₂ films in a manufacturing-like inline deposition with cell efficiencies up to 20.5%.

The new iron-based mixed polyanionic material Na₂Fe(C₂O₄)(HPO₄) has been synthesized by a hydrothermal technique. The compound contains four Na sites with a three-dimensional crystal structure. This compound shows promising reversible Li and Na insertion properties as a cathode material. The redox potentials observed were ∼3.2 V (vs Li⁺/Li) for the Li-ion cell and ∼3.1 V (vs Na⁺/Na) for the Na-ion cell with Fe^2+/3+ redox reactions. The reversible electrode operation was found to deliver 71 and 104 mAh g⁻¹ specific capacities in Li and Na half cells, respectively. This present study reveals promising performance from a mixed oxalate-phosphate based polyanionic material and opens up further possibilities for materials discovery.

Development of next generation batteries is predicated on the design and discovery of new, functional materials. Divalent cations are promising options that go beyond the canonical Li-based systems, but the development of new materials for divalent ion batteries is hindered due to difficulties in promoting divalent ion conduction. We have developed a family of cathode materials based on the divalent cation conductor ZnPS₃. Substitution of V for Zn in the lattice concomitant with vacancy introduction yields isostructural but redox-active materials that can reversibly store Zn^2 + in the vacancies. A range of voltammetry and galvanostatic cycling experiments along with x-ray photoelectron spectroscopy support that redox is indeed centered on V and that capacity is dependent on the V content. The voltage of the materials is limited by the irreversible decomposition of the $[\textrm{P}_2\textrm{S}_6]^{4-}$ polyanion above 1.4 V vs. Zn/Zn^2 +. The reversible capacity before anion decomposition is limited to half the vacancies and is due to the relative ratios of oxidized and reduced V centers. Such observations provide useful design rules for cathode materials for divalent cation based battery technologies, and highlight the necessity for a holistic interpretation of physical and electronic structural changes upon cycling.

Single-atom catalysts have attracted widespread attention in recent years due to their high atom utilization and excellent catalytic performance, particularly, in oxygen electrocatalysis. Herein, we report to use ZIF-8 as precursor, which was annealed and subsequently mixed with melamine and transition metal salt to finally synthesize Cu-atoms modified N-doped carbon (NC) catalysts, which exhibited excellent oxygen reduction reaction activity and kinetic performance in alkaline solution. The constructed rechargeable Zn-air battery using Cu-NC and benchmark Pt/C catalyst supported on carbon fiber paper as the cathode and anode, respectively, exhibited super good long-term stability with remaining voltaic efficiency of 56% at a charge/discharge current density of 5 mA cm⁻² after running 250 cycles (ca. 42 h). Moreover, the power density was as high as 104.5 mW cm⁻² at 0.65 V, and such three Zn-air batteries connected in series could light up 34 LED bulbs (2 V and 1.5 W for each) used for constructing a figure of 'USTL' for at least 12 h. This research provides a facile method on the synthesis of efficient and cost-effective ORR electrocatalysts in renewable energy storage and conversion systems.

The electrochemical performance of negative active materials employed in sodium-ion batteries is dependent on the amount of Na⁺ available in the test cells. As such, electrodes that exhibit long cycle-life and high coulombic efficiency (CE) in half-cells could suffer from fast capacity fading in full-cells as a result of unstable solid electrolyte interphase (SEI) and mechanical degradation leading to loss of active materials. In this work, the performance of Sb–graphite composite active materials prepared by extended ball-milling was evaluated in sodium half-cells and various types of symmetric cells (SCs). In half-cell tests, the composite electrodes provided specific capacities in the range 350–600 mAh g⁻¹ at C/20 with initial CE of 82%. A stable capacity of 380 mAh g⁻¹ was observed in the subsequent 100 cycles with the CE increasing to nearly 99%. However, self-discharge tests on half-cells and galvanostatic cycling of SCs revealed poor capacity retention as a result of parasitic reaction occurring through the SEI layer. Contrary to half-cells, the SCs revealed that Sb electrodes suffered from sharp capacity losses when a limited amount of Na⁺ ions was available in the cells. This is also characteristic of full-cells in which the sodium ions are supplied by the positive electrode.

Electron energy loss (EEL) spectroscopy carried out within a (scanning) transmission electron microscope can provide chemical and bonding information with atomic resolution. The information that lies within the spectrum can be difficult to extract, and often reference spectra are used to identify atomic bonding environments. First principles simulations are able to relate features in spectra to atomistic models and are particularly important in the interpretation of spectra where there are no appropriate bulk references, such as those from nanomaterials or interfaces. This paper reviews the recent developments in first principles simulations of EEL spectra and highlights the potential for advances in our understanding of materials.

We characterise the local mechanical properties of two-dimensional colloidal crystals with hexagonal symmetry assembled at the flat interface between oil and water. Our experiments elucidate the conditions under which the material behaves isotropically, as opposed to those where the microstructure plays a major role. Brownian fluctuations are used to extract the stiffness of the lattice under the continuum approximation, whereas at larger displacements, obtained by optically driving one particle through the structure, the mechanical resistance of the lattice depends on both the area fraction and the direction of the applied force. Remarkably, the minimum resistance does not necessarily correspond to a probe being driven between neighbours, i.e. at 30° with respect to the crystal axes.

Mechanically flexible electronics are devices designed to operate under significant physical deformations such as bending, twisting, and stretching. While the materials systems and devices compatible with flexible substrates have been extensively studied, the mathematical framework for analysis remains identical to that of traditional planar silicon-based electronics. However, the non-planar and dynamic form factors desired from flexible electronics invalidate assumptions made in these models. For electronic devices to be predictable and ultimately commercially viable, they must be understood in any physical form. Here we employ the method of moments to calculate the capacitance between two electrical conductors of arbitrary shape. Combined with a model for source–drain current in thin-film transistors (TFTs) on the surface of a cylinder, we are able to calculate the current–voltage characteristics in curved TFTs as a function of bending angle. We demonstrate how deformations to device geometry are expected to lead to non-negligible changes in current–voltage characteristics. This work represents the first step towards a new framework for understanding and characterizing electronics with any physical form factor, ultimately bringing flexible electronics closer to commercial viability.

The inhibition of Nb₃Sn grain growth in the presence of ZrO₂ nanoparticles appears to be one of the most promising method for pushing the critical current densities of Nb₃Sn superconducting wires to levels that meet the requirements set for the Future Circular Collider. We have investigated the effect of ZrO₂ nanoparticles formed by the internal oxidation of Zr on the superconducting properties and microstructure of Nb₃Sn formed from Nb-1 wt%Zr, Nb-7.5 wt%Ta, Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr alloys. A monofilamentary wire configuration was used, with a 0.22 mm outer diameter Nb-alloy tube containing a core of powdered metal oxide (SnO₂, CuO or MoO₃) as oxygen source and successive deposits of Cu, Sn and Cu on the outer surface. As determined from inductive measurements, the layer critical current densities of the samples based on Nb alloys with internally oxidized Zr were superior to those based on Nb-7.5 wt%Ta. The samples based on Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr showed higher critical current densities at high magnetic fields (above 10–15 T), and upper critical fields exceeding 28.5 T at 4.2 K (99% normal state resistivity criterion). A record value of 29.2 T of the upper critical field at 4.2 K was obtained on samples based on Nb-7.5 wt%Ta-2 wt%Zr. Hypotheses are proposed and discussed for explaining this unexpected increase of the upper critical field, by considering the possible effects of non-oxidized Zr on the superconducting properties of Nb₃Sn and of the oxidized Zr on the formation and microchemistry of Nb₃Sn. Regardless of sample type the Nb₃Sn grains observed in our samples have an aspect ratio of 1.5–1.7. When compared in the short axis direction, the mean distance between grain boundary intercepts (lineal intercept method) is ∼40% smaller in the samples with internally oxidized Zr than in the reference samples based on Nb-7.5 wt%Ta. In the long axis direction the reduction is of 20%–30%.

Alloying enables engineering of the electronic structure of semiconductors for optoelectronic applications. Due to their similar lattice parameters, the two-dimensional semiconducting transition metal dichalcogenides of the MoWSeS group (MX₂ where M = Mo or W and X = S or Se) can be grown as high-quality materials with low defect concentrations. Here we investigate the atomic and electronic structure of Mo_(1−x)W_xS₂ alloys using a combination of high-resolution experimental techniques and simulations. Analysis of the Mo and W atomic positions in these alloys, grown by chemical vapour transport, shows that they are randomly distributed, consistent with Monte Carlo simulations that use interaction energies determined from first-principles calculations. Electronic structure parameters are directly determined from angle resolved photoemission spectroscopy measurements. These show that the spin–orbit splitting at the valence band edge increases linearly with W content from MoS₂ to WS₂, in agreement with linear-scaling density functional theory predictions. The spin–orbit splitting at the conduction band edge is predicted to reduce to zero at intermediate compositions. Despite this, polarisation-resolved photoluminescence spectra on monolayer Mo_0.5W_0.5S₂ show significant circular dichroism, indicating that spin-valley locking is retained. These results demonstrate that alloying is an important tool for controlling the electronic structure of MX₂ for spintronic and valleytronic applications.

The lead-free double perovskite material (viz. Cs₂AgBiCl₆) has emerged as an efficient and environmentally friendly alternative to lead halide perovskites. To make Cs₂AgBiCl₆ optically active in the visible region of solar spectrum, band gap engineering approach has been undertaken. Using Cs₂AgBiCl₆ as a host, band gap and optical properties of Cs₂AgBiCl₆ have been modulated by alloying with M(I), M(II), and M(III) cations at Ag-/Bi-sites. Here, we have employed density functional theory (DFT) with suitable exchange-correlation functionals in light of spin–orbit coupling (SOC) to determine the stability, band gap and optical properties of different compositions, that are obtained on Ag–Cl and Bi–Cl sublattices mixing. On analyzing 64 combinations within Cs₂AgBiCl₆, we have identified 19 promising configurations having band gap sensitive to solar cell applications. The most suitable configurations with Ge(II) and Sn(II) substitutions have spectroscopic limited maximum efficiency (SLME) of 32.08% and 30.91%, respectively, which are apt for solar cell absorber.