Showing papers in "Materials Science & Engineering R-reports in 2020"

3D printing of hydrogels: Rational design strategies and emerging biomedical applications

Abstract: 3D printing alias additive manufacturing can transform 3D virtual models created by computer-aided design (CAD) into physical 3D objects in a layer-by-layer manner dispensing with conventional molding or machining. Since the incipiency, significant advancements have been achieved in understanding the process of 3D printing and the relationship of component, structure, property and application of the created objects. Because hydrogels are one of the most feasible classes of ink materials for 3D printing and this field has been rapidly advancing, this Review focuses on hydrogel designs and development of advanced hydrogel-based biomaterial inks and bioinks for 3D printing. It covers 3D printing techniques including laser printing (stereolithography, two-photon polymerization), extrusion printing (3D plotting, direct ink writing), inkjet printing, 3D bioprinting, 4D printing and 4D bioprinting. It provides a comprehensive overview and discussion of the tailorability of material, mechanical, physical, chemical and biological properties of hydrogels to enable advanced hydrogel designs for 3D printing. The range of hydrogel-forming polymers covered encompasses biopolymers, synthetic polymers, polymer blends, nanocomposites, functional polymers, and cell-laden systems. The representative biomedical applications selected demonstrate how hydrogel-based 3D printing is being exploited in tissue engineering, regenerative medicine, cancer research, in vitro disease modeling, high-throughput drug screening, surgical preparation, soft robotics and flexible wearable electronics. Incomparable by thermoplastics, thermosets, ceramics and metals, hydrogel-based 3D printing is playing a pivotal role in the design and creation of advanced functional (bio)systems in a customizable way. An outlook on future directions of hydrogel-based 3D printing is presented.

Three-dimensional interconnected networks for thermally conductive polymer composites: Design, preparation, properties, and mechanisms

Abstract: With the development of science and technology, microelectronic components have evolved to become increasingly integrated and miniaturized. As a result, thermal management, which can seriously impact the function, reliability, and lifetime of such components, has become a critical issue. Recently, the use of polymer-based thermal interface materials (TIMs) in thermal management systems has attracted considerable attention in view of the superior comprehensive properties of the former. Compared with designing and fabricating a polymer with an intrinsically high thermal conductivity, a more effective and widely used strategy for improving the heat conductivity is to fill a polymer matrix with a thermally conductive filler. Specifically, three-dimensional (3D) interconnected heat-conductive networks can increase the thermal conductivity (k) of polymers more effectively than dispersed fillers can, owing to their intrinsic continuous structures. In this review, we first introduce the heat conduction mechanisms and the problems associated with polymer-based TIMs fabricated using engineering polymer chains and traditional filling methods. Next, we discuss the advantages and mechanisms of 3D interconnected heat-conductive networks for preparing thermally conductive polymer-based composites. In addition, we highlight new advancements in the design and fabrication of 3D thermally conductive networks as well as their application in improving the k of polymers. Our exhaustive review of 3D interconnected networks includes graphene, carbon nanotubes, boron nitride, metal and other 3D hybrid architectures. The key structural parameters and control methods for improving the thermal properties of polymer composites are outlined. Finally, we summarize some effective strategies and possible challenges for the development of polymer-based thermally conductive composites via integration with 3D interconnected networks.

Fusing electrochromic technology with other advanced technologies: A new roadmap for future development

Abstract: Electrochromism is the phenomenon of certain materials reversibly changing their colours or optical properties through redox reactions under an applied electric field, which has found applications in smart windows, displays and so on. The past four decades have witnessed the rapid development of electrochromic technology; however, it remains severely developmentally challenged due to its limited practical applications. Predictably, as a colour control technology that gives visual information readable by the naked eye, electrochromism should have much wider applications by applying the visualization technique to various functional devices. Indeed, some recent research findings show that integrating electrochromic technology with other advanced technologies has imparted a new strong impetus for the further development of electrochromic technology. However, a systematic review of the literature on the fusing of electrochromic technology with other advanced technologies is still lacking, demonstrating the urgent need for further studies, although there are some review papers on electrochromic energy storage devices. In this review, we systematically discuss the recent advances in the fusing of electrochromic technology with other advanced technologies, including wearable technology, thermal control technology, energy storage technology, energy harvesting technology and sensing technology. The integration modes, design principles and performance optimization for different types of interdisciplinary electrochromic devices are highlighted. Finally, an outlook concerning future trends and issues in the research field is also provided.

Materials development and potential applications of transparent ceramics: A review

Abstract: Transparent ceramics have various potential applications such as infrared (IR) windows/domes, lamp envelopes, opto-electric components/devices, composite armors, and screens for smartphones and they can be used as host materials for solid-state lasers. Transparent ceramics were initially developed to replace single crystals because of their simple processing route, variability in composition, high yield productivity, and shape control, among other factors. Optical transparency is one of the most important properties of transparent ceramics. In order to achieve transparency, ceramics must have highly symmetric crystal structures; therefore, the majority of the transparent ceramics have cubic structures, while tetragonal and hexagonal structures have also been reported in the open literature. Moreover, the optical transparency of ceramics is determined by their purity and density; the production of high-purity ceramics requires high-purity starting materials, and the production of high-density ceramics requires sophisticated sintering techniques and optimized sintering aids. Furthermore, specific mechanical properties are required for some applications, such as window materials and composite armor. This review aims to summarize recent progress in the fabrication and application of various transparent ceramics.

Reviews of wearable healthcare systems: Materials, devices and system integration

Abstract: Wearable health monitoring systems are considered as the next generation of personal portable devices for telemedicine practice. These systems are based on monitoring different kinds of biological signals released by human beings through saliva, urine, breathing and epidemic skin perspiration. However, the development and commercialization of wearable healthcare equipment is still at a relatively slow pace, because the construction method of ordinary semiconductor equipment is no longer applicable. Despite these challenges, advances in materials science, chemical analysis techniques, equipment design and manufacturing methods have laid the foundation for a completely different wearable technology, which has led to the continuous evolution of wearable systems over the years. Hence, a comprehensive review in this field is required for the rational design and development of wearable systems for healthcare applications. In this regard, we like to summarize all the major components in wearable healthcare systems and discuss the progress made in this field, as well as the challenges that lie ahead. We think that this review will have a great implication for the wider scientific community which will contribute to the rational design of human health monitoring devices and accelerate the development of flexible electronics.

Broad-band emission in metal halide perovskites: Mechanism, materials, and applications

Abstract: Metal halide perovskites (MHPs) are in a blossoming status where their inherent optoelectronic properties are being revisited from the perspective of both fundamental science and practical application. In an attempt to boost the manipulating photoluminescence performance of MHPs, it is timely and vital to review the relation between structural attributes and the photo-physical phenomena for their unique broad-band emissions. In this review, we highlight the luminescent mechanisms of MHPs from dopants, self-trapped excitons (STEs) and defects, and some progresses with an emphasis on how multi-color broad-band emitters can be designed in various classes of MHPs frameworks. We also summarize the integration of MHPs into optoelectronic devices including light-emitting diodes, X-ray scintillators, fluorescence sensors and thermometers. This review aims to provide an in-depth insights into the structure-luminescence relationships from mechanism, materials, and applications, and further pave a way to discuss the current challenges and future promising prospects in MHPs.

Towards high-power-efficiency solution-processed OLEDs: Material and device perspectives

Abstract: Solution-processed organic light-emitting diodes (s-OLEDs) have received a great deal of interest owing to the huge market application potentials as large-size, flexible, high-quality self-luminous display panels and lighting sources. It is anticipated that those electronic products can be easily manufactured by modern wet-processing techniques, e.g. ink-jet printing and ‘roll-to-roll’ coating methods. However, issues related to power efficiency (PE) are highly hampering the progress of s-OLEDs towards real applications. Herein, we will demonstrate current development of s-OLEDs targeting for high PE with emphasis on introducing (i) theoretical and practical significance in simultaneously achieving close-to-unity (∼100 %) exciton emission and low driving voltage realized by advanced interface modification, bipolar-transporting-type host, all-exciton-harvesting emissive material and customized device architectures to integrate their functions, (ii) novel low-driving-voltage techniques for phosphorescent and thermally activated delayed fluorescence (TADF) s-OLEDs, i.e. barrier-free exciplex host or bipolar co-host scaffold, and charge-trapping- or charge-scattering-free emissive layer (EML) structures by matching the frontier molecular orbitals (FMOs) between host and dopant emitters, (iii) a variety of tactics to effectively alleviate the efficiency roll-off issue at the practically high luminance value, e.g. removing or largely restraining exciton-quenching in the EML and/or interfaces, the utilization of novel emitters with fast radiative decay rate and/or the EML architectures with prompt and efficient Forster energy transfer process.

Thermal conductivity of graphene-based polymer nanocomposites

Abstract: As a material possessing extremely high thermal conductivity, graphene has been considered as the ultimate filler for fabrication of highly thermally conductive polymer composites. In the past decade, graphene and its derivatives were demonstrated in many studies to be very effective in enhancing the thermal conductivity of various polymers. This paper reviews current progress in the development of graphene/polymer composites with high thermal conductivity. We began with the effects of isotopes, defects/doping, edges and substrate, polycrystallinity, functionalization, size and layer number, and folding/twisting on the thermal conductivity of graphene. We then modelled the thermal conductivity of graphene/polymer composites and, through molecular dynamics (MD) simulations, demonstrated its dependence on interfacial thermal conductance as well as size, dispersion and volume fraction of graphene. After a critique of recent studies on thermally conductive graphene/polymer composites and their potential applications, we identified several outstanding issues, new challenges and opportunities for future endeavours.

Polymers for supercapacitors: Boosting the development of the flexible and wearable energy storage

Abstract: The ever-growing demand of portable and wearable electric devices requires energy storage devices with flexibility and wearable compatibility while not sacrificing the performance too much. This requires the electrode and electrolyte materials becoming robust and durable under mechanical deformations, which grants the polymers with inherent advantages. Therefore, polymers for supercapacitors, either as substrate/matrix or active materials have received prime concerns. Viewing from the requirements of the flexible and wearable supercapacitors, this review provides an essential guidance to choose and/or design the proper polymer materials for both electrode and electrolyte. More importantly, supercapacitors with additional functions that are achieved by utilizing polymer materials are also specifically highlighted.

Laser engineering of biomimetic surfaces

Abstract: The exciting properties of micro- and nano-patterned surfaces found in natural species hide a virtually endless potential of technological ideas, opening new opportunities for innovation and exploitation in materials science and engineering. Due to the diversity of biomimetic surface functionalities, inspirations from natural surfaces are interesting for a broad range of applications in engineering, including phenomena of adhesion, friction, wear, lubrication, wetting phenomena, self-cleaning, antifouling, antibacterial phenomena, thermoregulation and optics. Lasers are increasingly proving to be promising tools for the precise and controlled structuring of materials at micro- and nano-scales. When ultrashort-pulsed lasers are used, the optimal interplay between laser and material parameters enables structuring down to the nanometer scale. Besides this, a unique aspect of laser processing technology is the possibility for material modifications at multiple (hierarchical) length scales, leading to the complex biomimetic micro- and nano-scale patterns, while adding a new dimension to structure optimization. This article reviews the current state of the art of laser processing methodologies, which are being used for the fabrication of bioinspired artificial surfaces to realize extraordinary wetting, optical, mechanical, and biological-active properties for numerous applications. The innovative aspect of laser functionalized biomimetic surfaces for a wide variety of current and future applications is particularly demonstrated and discussed. The article concludes with illustrating the wealth of arising possibilities and the number of new laser micro/nano fabrication approaches for obtaining complex high-resolution features, which prescribe a future where control of structures and subsequent functionalities are beyond our current imagination.

Tailoring PEDOT properties for applications in bioelectronics

Abstract: The authors would like to thank Brett Moore, Adam Williamson, and Xenofon Strakosas for edits.

High-Efficiency Silicon Heterojunction Solar Cells: Materials, Devices and Applications

Abstract: Photovoltaic (PV) technology offers an economic and sustainable solution to the challenge of increasing energy demand in times of global warming. The world PV market is currently dominated by the homo-junction crystalline silicon (c-Si) PV technology based on high temperature diffused p-n junctions, featuring a low power conversion efficiency (PCE). Recent years have seen the successful development of Si heterojunction technologies, boosting the PCE of c-Si solar cells over 26%. This article reviews the development status of high-efficiency c-Si heterojunction solar cells, from the materials to devices, mainly including hydrogenated amorphous silicon (a-Si:H) based silicon heterojunction technology, polycrystalline silicon (poly-Si) based carrier selective passivating contact technology, metal compounds and organic materials based dopant-free passivating contact technology. The application of silicon heterojunction solar cells for ultra-high efficiency perovskite/c-Si and III-V/c-Si tandem devices is also reviewed. In the last, the perspective, challenge and potential solutions of silicon heterojunction solar cells, as well as the tandem solar cells are discussed.

Device characteristics and material developments of indoor photovoltaic devices

Abstract: Indoor photovoltaics (IPVs), which convert the indoor light energy into direct electricity, have attracted research attention due to their potential use as an excellent amicable solution of sustainable power source to drive low-power-needed sensors for the internet of things (IoT) applications. Our daily life adopts various indoor light sources, such as indirect sunlight, incandescent lamps, halogen lamps, fluorescent lamps, and LED bulbs, that typically deliver lower light intensity (200–1000 lux) as compared to that of sun light (∼100,000 lx). In this review, we firstly classified the indoor lights depending on their working mechanism and resulting emission spectrum. Because the indoor light intensities are rather low that may lead to overestimate/underestimate the power conversion efficiency (PCE) of IPV devices, then, the cautious points for correctly measuring the indoor light intensity as well as the device characteristics are summarized. Several light sources with various light intensities are reported so far, but for lack of common or standard calibration meter that induces a ambiguity in PCE determination, so we suggest/propose to use a universal LED lux meter with NIST-traceable calibration (e.g. Extech LT40-NIST) and also recommended the device results are expressed in maximum power point Pmax along with PCE values. It is generally believed that the materials play key roles on the performance of the IPV devices. Since the indoor light intensity is much weaker as compared to that of outdoor irradiation, the typical inferior photo-stability of organic materials under sunlight may not be as crucial as we considered to harvest indoor light energy, opening a great room for organic IPV material developments. In principle, all materials for outdoor PVs may also be useful for IPVs, but the fundamental material requirement for IPVs which needs sufficiently covering the absorption range between the 350–700 nm with high molar extinction coefficient should be primarily concerned. In order to get the thorough knowledge of materials for achieving better efficient IPVs, the reported IPVs were collected and summarized. According to these reports, the materials utilized for IPVs have been classified into two major groups, inorganic and organic materials, then divided them into several sub-classes, including (1) silicon and III-V semiconductor photovoltaics, (2) dye-sensitized photovoltaics, (3) organic photovoltaics, and (4) perovskite-based photovoltaics, depend on their structural nature and device working principle. For every individual class, the structure-property-efficiency relationship of the materials was analyzed together with the highlights on the best efficiency material, challenge and perspective. For inorganic IPV materials, III-V semiconductor GaAs-based IPVs performed a very impressive PCE (28%). For dye sensitizers, there are more flexible strategies to modulate the absorption profiles of organic materials. A high efficiency dye-sensitized solar cell (DSSC)-based IPV with a PCE up to 32% has been successfully realized with co-sensitized dyes. For organic solar cell (OSC)-based IPVs, fullerene-based acceptors are advantageous for their well-matching desired absorption range and superior electron transport features. A recent OSC-based IPV with the active layer composed of dithienobenzene-based donor and fullerene acceptor was reported to deliver a PCE of 28%. Among these emerging photovoltaic materials, it is no doubt that perovskites (e.g. CH3NH3PbI3) are superior for solar energy conversion due to the crystallinity for good charge transport, better spectral coverage and the low exciton binding energy. Until very recent, a perovskite-based IPV with a PCE of 35% was reported with good stability by the incorporation of an ionic liquid for effectively passivating the surface of the perovskite film, indicating the bright prospect of perovskite for IPV application. Overall, the review on these reports implies the essential criteria of materials suitable for IPVs that may trigger new ideas for developing future champion materials for various devices and the realization of practical IPV applications.

Recent progress of all-polymer solar cells – From chemical structure and device physics to photovoltaic performance

Abstract: Single junction organic solar cells (OSCs) have now achieved power conversion efficiencies (PCEs) exceeding 17 %. Most of these high performance OSCs contain fullerene acceptors (FAs) and non-fullerene small-molecule acceptors (NFSMAs). In contrast, there are very limited usages of polymer acceptors. Recently, there are escalating recognition among perylene-diimide/naphthalene-diimide (PDI/NDI) and B⟵N-unit n-type polymers as electron acceptors in the all-polymer solar cells. FAs like PC71BM suffer from multiple limitations. They include restricted energy level tuning, weak absorptions in visible region, narrow spectral breadth, and morphological instability. In contrast to FAs, NFSMAs offer numerous advantages. They include strong and broad absorption in the visible and even the NIR region, tunable energy levels, and simple synthesis and purification procedures. Despite these advantages, the long-term device stability and large-area roll-to-roll (R2R) fabrication remain the major issues for the commercialization for NFSMA-based OSCs. All-polymer solar cells, on the other hand, largely address the problems of device stability and large-area film processing. Many all-polymer solar cells have been demonstrated to possess long-term thermal, photo and mechanical stability. Meanwhile, the precursor solutions for all-polymer solar cells enjoy superior control in the solution viscosity, which is an important factor for the solution processing of large-scale OSCs. Before 2015, all-polymer solar cells received little attention due to their disappointing device performance. Afterwards, PCEs of all-polymer solar cells are picking up. Currently, the best cells have achieved PCEs in excess of 11 %. Here, we provide a systematic review on the evolution of n-type polymeric acceptors used in OSCs. In addition, we summarize the morphological and charge carrier transport properties of all-polymer solar cells and compare with their small molecule acceptor counterparts. The outstanding properties of all-polymer solar cells are discussed from the perspectives of morphology and electron transport in bulk heterojunctions (BHJs). The concept of electron percolation in all-polymer BHJs is introduced and correlated with the excellent device stability. This review should have a broad appeal and enable researchers in comprehending the achievements, challenges, and future directions of all-polymer solar cells.

Dynamic crosslinked rubbers for a green future: A material perspective

Abstract: Conventional rubber products, such as tires, seals, tubing, and damping systems are manufactured via a vulcanization process, which forms covalently crosslinked network structures and ensures mechanical robustness, thermal stability and chemical resistance. However, the covalent networks are permanent and these products cannot be reprocessed or reshaped, which makes vulcanised rubbers one of the major challenges facing waste management and the circular economy. To reduce waste pollution for products such as tires, conventional vulcanised rubbers must be replaced with reversibly crosslinked structures which are able to achieve mechanical robustness and chemical stability, whilst also being able to be reprocessed, reshaped, reused and recycled. State-of-the-art developments in supramolecular chemistry have shed light on a new generation of reprocessable elastomers and rubbers, which have the potential to tackle the long-standing issue of waste tire pollution. The introduction of dynamic covalent bonds or supramolecular interactions in traditional elastomers can produce reversibly crosslinked structures, where the synergy between the dynamic bonds in the network are carefully optimised to balance the ease of processing, mechanical properties, and structural stability. Furthermore, dynamic covalent bonds and supramolecular interactions can provide ‘living’ functions to elastomers, such as self-healing and stimuli-responsiveness. These properties can be further enhanced by the addition of nanofillers with tailored surface chemistry to provide a dual role as a dynamic crosslinker and reinforcing element. To create reprocessable and recyclable elastomers, the coupling of multiple dynamic interactions provides unlimited possibilities to optimise the structure and properties of recyclable rubbers. Here we critically overview the applications of dynamic chemistry in rubbers, with a focus on macromolecular design and strategies to balance the mechanical, functional (e.g. self-healing) and reprocessing properties.

How far are we from attaining 10-year lifetime for metal halide perovskite solar cells?

Abstract: Metal halide perovskite solar cells (PSCs) have attracted considerable attention from both academia and industry as a promising next-generation technology to harvest solar energy with high power conversion efficiencies (PCEs) at low-cost. At the current stage, efficiencies of these cells have been improved to an impressive level, but the instability issue remains as a major obstacle impeding further commercialization of this technology. In this review, we start with examining the status about the technological aspects of PSC stability research, e.g., stability measurement protocols, their relevance to the realistic operational lifetime, and where we are from the viewpoint of 10-year lifetime. These pressing challenges are correlated with the investigations on the fundamental aspects emphasizing the comprehensive physicochemical understanding of degradation mechanisms in perovskite materials. We examine the various extrinsic and intrinsic factors influencing stability of perovskite materials with different compositions (mixed cations and / or mixed anions, double perovskites, etc.), dimensionality (3D perovskites, 2D perovskites, 2D/3D mixed dimensional perovskites), instability induced by functional layers other than the perovskite layer, and the interactions at various interfaces. On the basis of the analyses of these multiple degradation factors with an emphasis on the holistic microscopic views acquired above, we discuss strategies to improve PSC stability. We finish the review by outlining future research directions that can help achieve long-term stability of PSCs.

Technological challenges and progress in nanomaterials plasma surface modification – A review

Abstract: Nanoscale particulate materials draw great interest in an increasing number of applications, such as electronics, energy storage, automotive, health or environment. In particular, the addition of nanofillers in a polymer matrix can significantly improve the thermal, mechanical, electrical, optical, and biological or corrosion protection properties of a nanocomposite, provided that the fillers exist as discrete entities and strongly adhere to the matrix. Nanocomposite synthesis generates major technological challenges, due to the natural tendency of nanomaterials to agglomerate and to their poor compatibility with polymeric materials. The main approach to tackle these issues consists in modifying the fillers surface to enhance their affinity with the matrix and produce repulsive interactions between the particles. In this paper, after a brief review of the conventional “wet” methods used to modify the surface of nanomaterials, we highlight the numerous technical, environmental and economic advantages provided by dry and versatile plasma treatments. Then, we present the different plasma reactor configurations designed so far, for powders surface functionalization. In particular, we spotlight the advantages and drawbacks of each system regarding particle mixing, powder yields and up-scaling possibilities. Finally, we introduce the main characterization tools generally used to analyze modified nanopowders. In this last part, we underline the main results and achievements obtained up to now in terms of treatment uniformity, functionalization degree, dispersibility/stability enhancement and improvement of nanocomposite performances.

Recent progress in group III-nitride nanostructures: From materials to applications

Abstract: Group-III-nitride semiconductors, including AlN, GaN, InN and their ternary, quaternary compounds, are promising electronic and optoelectronic materials for the applications in light emitting diodes, lasers, field emitters, photodetectors, artificial photosynthesis, and solar cells. Owing to their direct bandgaps ranging from near infra-red to deep ultraviolet. In recent years, the growth of group-III nitride nanostructures has been extensively explored. Herein, we provide a comprehensive review on the rational synthesis, fundamental properties and promising applications of group-III nitride nanostructures. Group-III nitride nanostructures with diverse morphologies, their corresponding synthesis methods and formation mechanisms involved are systematically compared and discussed, as well as the detailed factors that influence the optical and electrical properties of the nanostructures. The recent achievements gained in the fields of III-nitride nanostructures are highlighted, including light emitting diodes, laser diodes, photodetectors, solar cells, artificial photocatalysis, nanosensors, and nanogenerators. Finally, some perspectives and outlook on the future developments of III-nitride nanostructures are commented.

Nonlinear optoelectronic processes in organic optoelectronic devices: Triplet-triplet annihilation and singlet fission

Abstract: Organic semiconductors with optoelectronic properties have attracted intensive interests in the fields of organic light-emitting diodes, organic photovoltaics and organic photodetectors. In these functional devices, exciton evolution is the key process. Generally, linear process is dominant that one photon is created from each injected electron-hole pair, vice versa. This linear process restricts the internal quantum efficiency to be 100% in optoelectronic devices. Alternatively, nonlinear optoelectronic processes, like triplet-triplet annihilation (TTA) and singlet fission (SF), can theoretically break the limitation stated in linear optoelectronic processes. TTA and SF are two dynamic reversible processes, which link one high-lying energy singlet with two low-lying energy triplet excitons on two neighboring chromophores. These nonlinear processes bring the possibility of novel functions and application scenarios, and hence have attracted more and more attentions recently. This review will systematically and comparatively summarize the basic properties of TTA and SF processes, and their applications in optoelectronic devices in the view of device physics. It starts with a concise introduction of basic knowledge of TTA and SF. Then, the characteristics used to identify these processes are described. Subsequently, implementing them in OLEDs are summarized, followed by the application examples in OPVs. Emphasis is laid on the triplet quenching mechanisms in working devices, since both TTA and SF processes highly rely on triplet excitons. These discussions on the recent important progresses aim to gain some insight into the device physics for device engineering. Finally, a perspective is presented.

Non-noble-metal-based organic emitters for OLED applications

Abstract: Organic light-emitting diodes (OLEDs) employing purely organic functional materials indicate a low-cost manufacturing route towards the next-generation display and solid-state lighting owing to the avoidance of noble heavy metal complex phosphorescent emitters. In recent years, several mechanisms have been proposed to design high performance purely organic emitters. This new generation of purely organic emitters shed light on the realization of both low-cost and high performances. The main idea of this paper is to review how to use purely organic semiconductors to realize high-efficiency OLEDs. This guides us to pay special attention to two aspects: 1) how to break the efficiency bottleneck resulting from exciton spin-statistics, which is critical to determine internal quantum efficiency; 2) how to enhance out-coupling efficiency by molecular designs, which eventually influences on external quantum efficiency. Several significant material design strategies are thus introduced, and the relevant mechanisms are classified as triplet-triplet annihilation, thermally activated delayed fluorescence, hot excitons, room temperature phosphorescence, and luminescent radicals. Then, device strategies by employing organic heterojunctions as the main luminescent center towards high-performance fluorescent OLEDs were introduced. Finally, we outline the progress of enhancing out-coupling efficiency by tuning the dipole orientation of emitters and the operational stability of OLEDs excluding noble heavy metal complex phosphorescent emitters.

Advances in finite element modelling of graphene and associated nanostructures

Abstract: Graphene and its associated nanostructures (GANS) have been widely investigated by means of experimental and numerical approaches over the last decade. GANS and GANS reinforced composite materials show exceptional promise towards superior mechanical and thermal properties along with limitless opportunity to tailor, control, design, modify and manipulate such properties. These attributes make graphene and its associated nanostructures as one of the most important future material technologies in aerospace, automotive, medical, civil and military sectors of the 21st century. Among the various numerical methods used to analyse GANS and GANS reinforced composite materials, the finite element method (FEM) plays a prominent role. The FEM has been the standard analysis and simulation method for conventional structural and mechanical problems over the past half a century. However, its growing role and impact in atomistic-scale numerical simulation in general, and GANS, in particular, is not well known within the wider scientific and engineering modelling and simulation research community. There is a compelling need to document the expansive use of the finite element method, its advantages, shortcomings, relevance and purpose in a way which is pertinent to both material science and numerical simulation researchers. This paper serves this need by discussing the current state of the art of finite element methodologies available to study GANS and GANS reinforced composites in the most comprehensive manner. A detailed description of the popular space frame based numerical simulation strategy widely used to represent GANS is given. An extensive survey is conducted on more than 600 research papers in order to examine the finite element predictions of the mechanical and thermal properties of graphene and its associated composite materials. These properties are selected in view of their direct relevance to crucial future technologies, such as high-performance automotive components, aerospace and bioengineering systems, energy technologies, and advanced therapeutic and surgical devices. Omissions of some fundamental mechanical and thermal modelling issues for GANS have been identified and insightful guidance towards future research directions to comprehensively address them is given. By reviewing a significant breadth of publications across several academic disciples, a large scatter in the numerical predictions of essential material constants arising from the differences in fundamental assumptions and approximations has been reported. The origin of such discrepancies has been identified, analysed and established. The paper further focuses on the idealization of nanostructures and nanocomposites by means of representative volume elements (RVEs). The need for this multiscale modelling strategy to mature in order to include the simultaneous description of different material length scales within multiphysics simulation problems has been discussed. This paper will serve as standalone reference material for future research works and will pave the way for novel investigations in the context of atomistic simulations and their potential applications to the development of next-generation engineering devices and cutting-edge technological applications.

Materials for blood brain barrier modeling in vitro

Abstract: Brain homeostasis relies on the selective permeability property of the blood brain barrier (BBB). The BBB is formed by a continuous endothelium that regulates exchange between the blood stream and the brain. This physiological barrier also creates a challenge for the treatment of neurological diseases as it prevents most blood circulating drugs from entering into the brain. In vitro cell models aim to reproduce BBB functionality and predict the passage of active compounds through the barrier. In such systems, brain microvascular endothelial cells (BMECs) are cultured in contact with various biomaterial substrates. However, BMEC interactions with these biomaterials and their impact on BBB functions are poorly described in the literature. Here we review the most common materials used to culture BMECs and discuss their potential impact on BBB integrity in vitro. We investigate the biophysical properties of these biomaterials including stiffness, porosity and material degradability. We highlight a range of synthetic and natural materials and present three categories of cell culture dimensions: cell monolayers covering non-degradable materials (2D), cell monolayers covering degradable materials (2.5D) and vascularized systems developing into degradable materials (3D).

New opportunities in transmission electron microscopy of polymers

Abstract: Recent advances in instrumentation for transmission electron microscopy have pushed the resolution limit, leading to remarkable instruments capable of imaging at 0.5 A. But, when imaging soft materials, the resolution is often limited by the amount of dose the material can handle rather than the instrumental resolution. Despite the strong constraints placed by radiation sensitivity, recent developments in electron microscopes have the potential to advance polymer electron microscopy. For example, the focused ion beam creates opportunities for site-specific imaging, recently developed sample holders enable liquid TEM, monochromated sources lead to spectroscopy and imaging based on the valence electronic structure, and direct electron detectors minimize the required dose for imaging. Transmission electron microscopy has transformed the field of polymer science, and it is poised to do so again in the near future.

Towards a predictive thermodynamic description of sorption processes in polymers: The synergy between theoretical EoS models and vibrational spectroscopy

Abstract: Understanding and predicting sorption thermodynamics of low molecular weight compounds in rubbery and glassy polymers is of great relevance to elucidate important phenomena in areas at the interface of various scientific branches, such as the colloid and interface science, membrane science, polymer foaming, tissue engineering, scaffolding, microcellular materials, aerogels, and for the implementation of technological applications. The development of thermodynamic models for polymer-based mixtures, applicable over a wide range of conditions, remains an active and fascinating research area. Recent advances in statistical thermodynamics and a better understanding of intra- and inter-molecular interactions, thanks to accurate experimental measurements and molecular simulations using realistic force fields, have contributed significantly to this end. In fact, sorption thermodynamics in polymers plays a relevant role in describing phase equilibria of polymer mixtures, (hydro)gel swelling, intramolecular association, hydrogen-bonding cooperativity and polymer degradation and stability, in assessing durability of polymers exposed to aggressive environments, in predicting penetrant induced crystallization and plasticization phenomena in polymers, in designing polymer-based separation processes, in tailoring polymer foaming processes, in improving gas and vapor barrier properties of polymer packaging, in modelling devolatilization of polymer solutions and migration phenomena of additives, in designing drug delivery systems, to mention a few. In the last decades, models have been introduced rooted on Equation of State theories, some of them based on compressible lattice frameworks. Notably, these models have been structured to specifically account for non-random distribution of molecular species and for dealing with several kinds of self-interactions that establish between polymer molecules and between penetrant molecules as well as cross-interactions that establish between moieties present on polymer backbone and penetrants. These models have been built to describe the behaviour of both rubbery polymers and out-of-equilibrium glassy polymers. Towards the further development of these approaches to gain an increased predictive capability of this thermodynamic description, recently have been also introduced approaches aimed at the estimation of relevant parameters based on molecular descriptors for calculations of properties of pure-components bulk phases and solutions. Such a quantitative description of the sorption process by use of advanced thermodynamic theories invariably relies on a molecular-level characterization of the system under scrutiny to validate and support the theoretical framework. Information is required on the molecular aggregates formed in the system, their structure, stoichiometry and, whenever possible, their population. In this respect, vibrational spectroscopy (FTIR, Raman) has demonstrated to be among the most powerful techniques, due to its sensitivity towards H-bonding detection and to its sampling flexibility, which allows the development of in-situ, time-resolved measurements. In the last ten years, significant advancements have occurred in terms of both experimental approaches and data analysis techniques, which considerably contributed to deepening the interpretation of the molecular interactions scenario. In particular, Two-dimensional correlation spectroscopy (2D-COS), Difference spectroscopy (DS) and first-principles quantum chemistry calculations have made a strong impact on the amount and quality of the acquired information. In view of the progress in this rapidly advancing and technologically relevant subject, this review article summarizes the state of the art on sorption thermodynamics modelling and on synergic combination with the wealth of information recently made available thanks to advanced vibrational spectroscopy techniques.

All-dielectric materials and related nanophotonic applications

Abstract: Generally, nanophotonics is associated with plasmonic materials and structures made of noble metals such as gold or silver. However, conventional plasmonic materials have several disadvantages restricting their applications. First, plasmonic materials like gold and silver suffer from high optical loss at optical frequencies. Second, noble metals are rare and not proper for large scale fabrication. Third, as a nanoantenna, plasmonic nanoparticles only hold electric dipole-like resonance which cannot tailor and direct the optical field as we want. Therefore, those have driven the intense search for all-dielectric materials (ADMs) which offer unique opportunities for reduced dissipative losses and large resonant enhancement of both electric and magnetic near-fields beyond plasmonic materials. There are usually three types of ADMs, i.e., firstly high-index ADMs such as silicon, germanium and gallium arsenide, secondly mid-index ADMs such as titanium dioxide, silicon carbide and boron and thirdly low-index ADMs such as silicon dioxide and polymer. ADMs with different refractive indexes bring a lot of freedom to design nanostructures with different optical properties. For example, ADMs with high refractive index and low loss can generate strong Mie resonances for far-field related applications, while ADMs with high absorption and plasmonic-like properties are favorable for near-field applications. Although many efforts have been devoted to prepare ADMs and study the related nanophotonic applications, researchers still face fundamental challenges: how to control phase, size and shape of building blocks in the synthesis of ADMs, how to fabricate functional nanostructures by using these building blocks, and further how to achieve the transformation from simple nanoparticles synthesis to functional nanostructures fabrication. To address these issues, we have developed a series of unique techniques based on laser ablation in liquids (LAL) for all-dielectric nanomaterials preparation and all-dielectric nanostructures fabrication in recent years. Using LAL, we have prepared a series of high quality all-dielectric nanomaterials. Meanwhile, optical nanoantennas and devices based on ADMs by LAL have demonstrated appreciable figures-of-merit, manifesting great potential for nanophotonics and the next generation optoelectronics. In this review, we will introduce the latest progresses of ADMs prepared via top-down and bottom-up methods and related applications in nanophotonics. Firstly, we will present the basic optical properties of ADMs. For example, a new branch of nanophotonics has emerged that seeks to manipulate the strong, optically induced electric and magnetic Mie resonances in nanoparticles based on high refractive index ADMs. Then, we will introduce various approaches for the fabrication of ADM-based nanostructures and their merits and demerits, in which we will demonstrate that LAL technique is more suitable to produce different kinds of ADMs compared with traditional top-down and bottom-up fabrication methods. Secondly, we will summarize the nanophotonic applications of ADMs, including the utilization of unique resonant modes in silicon nanoparticles, enhancement of both linear and nonlinear optical signals, biosensing, light trapping and harvesting, and the enhancement of light matter interaction. Finally, we will discuss several strategies for improvement of nanophotonic performances of ADMs. For example, through designing specific ADM nanostructures, we can obtain higher Q factor and better near-field feature. Additionally, recent progresses on active tuning of ADM-based nanodevices are presented which make contributions to the practical use of ADMs. Overall, ADMs have actually opened a window toward building highly efficient nanophotonic devices from their unique attributes. (i) ADMs contain a group of materials which have varied optical properties. This diversity provides us freedom and possibilities of functional structural design. (ii) Most of ADMs are compatible with mature semiconductor processing technology and have much lower cost compared with plasmonic materials of noble metals. (iii) ADM-based nanostructures can generate intriguing resonant modes, such as magnetic dipole, toroidal, anapole modes and others.

Direct observation of electric and magnetic fields of functional materials

Abstract: Electron holography is a useful technique to directly visualize the electromagnetic fields in and around various functional materials at the nanometer scale. The precision and resolution of this technique have been drastically improved by using various specialized instruments. From in situ experiments applying electric current and voltage and using these specialized instruments, conductivity changes in composite materials can be understood through their microstructure changes and electric field variations. On the other hand, the magnetization processes of various functional magnetic materials can also be clarified by applying a magnetic field and changing the temperature. As an extension of electron holographic techniques, the stationary orbits and collective motions of electrons around various charged insulators can be directly observed by detecting the electric field fluctuation due to these motions. Furthermore, by applying an external magnetic field to the electrons around charged insulators, the electron spin polarization can be observed. Thus, by combining the electron behaviors and electromagnetic fields of advanced functional materials under the application of electric current and external magnetic fields, we expect the development of new features in sensing techniques for clarifying the interactions between electrons and material surfaces. These studies offer a promising contribution of novel approaches for both development and characterization of advanced functional materials and devices based on them.