INTERIOR DESIGN AND FIRE SAFETY IN ONE Wallpapers have resurged in popularity in recent years. These wall coverings, typically made of plant cellulose fibers or synthetic polymers, are lightweight, flexible, and inexpensive for interior decoration. However, they are often highly flammable and can promote the spread of fires. Making wallpapers fireresistant could help mitigate fire disasters. Some efforts toward this end have focused on adding fire retardants to conventional wallpaper materials or using nonflammable inorganic materials as the substrate. However, the resulting wallpapers have problems with environmental safety, flexibility, mechanical robustness, and color. In addition, these wallpapers have not included fire alert mechanisms to protect people and property. To address these issues, Chen et al. (DOI: 10.1021/ acsnano.8b00047) created fire alarm wallpaper using a paper substrate made of hydroxyapatite, the major inorganic component of bones and teeth in vertebrates. By creating long nanowires, they made this typically brittle but fire-resistant material highly flexible with good mechanical properties. The team then installed a thermosensitive sensor on this paper by drop-casting graphene oxide (GO) aqueous ink and connecting it to external electrodes. Tests showed that when this material was exposed to flame, it resisted burning significantly longer than conventional wallpapers. In addition, the high temperature stripped the GO of oxygen-containing groups and converted it from an insulator into a conductor, a process that triggered an alarm lamp and buzzer. Further modification with polydopamine improved the sensitivity and flame retardancy of this system. The authors suggest that this smart fire alarm wallpaper has promise in high-safety interior decoration of houses.
droplets into structural color pixels. However, maintaining alignment over large areas has proven challenging with this technique, with misalignment leading to incorrect colors being printed, a phenomenon called “color leaking”. To solve this problem, Jiang and Kaminska (DOI: 10.1021/ acsnano.7b08580), who developed the MIONS technique, report a method in which MIONS-printed substrates are stacked to produce different colors. The researchers inkjet-printed to TiO2 nanoparticles onto transparent polymers prepatterned with nanostructured gratings then laminated these substrates with a transparent index-matching cover film, creating red, green, and blue structurally colored surfaces. They were then able to generate different colors by layering these substrates. This technique was able to generate images as large as 10 cm by 10 cm, with the problem of color leaking eliminated. As proof of principle, the researchers used thismultilayer MIONS technique to generate customizable transparent color optical variable devices and for a “print and paste” transparent color image display panel. The authors suggest that this technique can eventually be used for industrial-scale customized manufacturing of structural colors with high speed, low cost, and large image size.
GIVING BATTERIES A TWIST An increasing interest in flexible and wearable electronics has driven an accompanying focus on highly deformable, durable, and wearable energy storage devices. One-dimensional fiber or yarn-based energy devices could offer a solution. These lightweight, small-volume, and structurally varied energy storage devices can easily be integrated with commercial textiles, offering a more practical way to store energy in wearable applications than thin film devices. Yarn-based energy devices should ideally be able to maintain their electrochemical functions under different conditions, such as being bent, stretched, cut, or even washed in water. However, current research in yarn batteries or power-type supercapacitors falls far behind these goals. Li et al. (DOI: 10.1021/acsnano.7b09003) significantly advance this field with a yarn zinc ion battery (ZIB) that is elastic, washable, and tailorable. MnO2-coated carbon nanotube (CNT) yarn served as the cathode, and zinc-coated CNT yarn
STACKABLE STRUCTURAL COLOR To make structural color printing practical for industrial applications, particularly for customized manufacturing, production techniques must be economical and scalable. Consequently, inkjet-based techniques offer a potential solution to meet these demands. Recently, researchers introduced an inkjet-based molded-ink-on-nanostructured-surface (MIONS) printing technique, which implements a nanostructured surface composed of pixilated nanocone arrays for printing full-color images. They demonstrated that silver nanoparticle ink droplets could be accurately printed onto nanostructured surfaces that molded the © 2018 American Chemical Society
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served as the anode. These two were twisted onto an elastic fiber in parallel, then coated with a polyacrylamide-based electrolyte and encapsulated in Eco-flex silicone and water repellant. Tests showed that this yarn ZIB delivered high specific capacity and volumetric energy density and had excellent cycling stability. It retained these properties well after knitting, stretching, and washing. In addition, it was tailorable, with pieces continuing to function well after cutting. A proof-of-principle test showed that this battery could be used to power a long flexible belt embedded with 100 LEDs and a 100 cm2 flexible electroluminescent panel. The authors suggest that these yarn ZIBs could serve as promising and reliable energy storage devices for flexible and wearable applications.
PHOTONIC FLUIDS AND CRYSTALS FROM THE BLOCK Photonic crystals, materials that exhibit vibrant structural colors, are found in many natural systems. Due to their significant advantages over chemical dyes and pigments, such as nonbleaching and the ability to respond to stimuli, researchers have pursued various approaches to form these materials synthetically. Polymer-based particles, in the range of 150−300 nm required for photonic crystals, have been synthesized through top-down methods. However, with the exception of very large core−shell virus particles, thus far no synthetic particles intrinsically selfassembled from the bottom-up have been used for this purpose. Even though block copolymer micelles have been widely studied in various fields, these bottom-up-synthesized particles have not been investigated for structural color, mainly due to the stringent length requirements of photonic lattices. Upending this idea, Poutanen et al. (DOI: 10.1021/ acsnano.7b09070) created photonic fluids and crystals with block copolymer micelles (BCMs) with superstretched coronas but narrow size distributions. With polystyrene-block-poly(2vinylpyridine), the researchers synthesized micelles with a polystyrene core and a poly(2-vinylpyridine) corona. Quaternizing the corona with methyl iodide introduced charges that dramatically increased the corona size. These BCMs formed freeflowing micellar photonic fluids with an average interparticle distance of 150−300 nm due to electrosteric repulsion from the corona. Under quiescent conditions, millimeter-sized micellar photonic crystals formed within the fluid. The colors of these crystals ranged across the visible spectrum, with the intensity and position of the reflected wavelength tunable with the micelles’ core size, concentration, and the salt concentration of the solution. The authors suggest that this approach offers a facile way to generate tunable micellar photonic structures.
MAKING CERAMIC AEROGELS COMPRESSIBLE Ceramic aerogels have potential in a wide variety of applications, such as high-temperature thermal insulators, catalyst supports, and filters, due to their bevy of useful characteristics, including low density, high porosity, large surface area, and excellent thermal and chemical stability. However, conventional ceramic aerogels are usually based on oxide ceramic nanoparticles, which limits practical applications due to these materials’ brittle nature and volume shrinkage at high temperatures. Although silicon carbide (SiC) aerogels have excellent high-temperature chemical stability and better heat resistance than oxide ceramic aerogels, their brittleness remains a limitation. To overcome this drawback, Su et al. (DOI: 10.1021/ acsnano.7b08577) developed an aerogel from SiC nanowires, which display outstanding elasticity, flexibility, high tensile strength, and high Young’s modulus, in contrast to typical SiC ceramics. The researchers prepared these nanowire aerogels by decomposing siloxane xerogel in a graphite crucible in a furnace, creating a freestanding paper-like aerogel on the crucible lid. This material, composed of an interwoven and highly porous threedimensional network of SiC nanowires, was light enough to stand on a dandelion seed head without compressing it and flexible enough to be rolled and unrolled without breakage. Tests confirmed high-temperature chemical and thermal stability and excellent thermal insulation performance. In addition, the nanowire aerogel exhibited an adsorption selectivity of lowviscosity organic solvents with high absorption capacity. The authors suggest that this material could be an inspiration for the design and fabrication of other compressible and multifunctional ceramic nanowire aerogels.
STRUCTURAL COLORATION THROUGH ATOMIZATION Structural colors, generated when photonic nanostructures interact with visible light, have recently garnered increasing 3057
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MgAl2O4 ceramics. These materials had average grain sizes ranging from 37.5 nm down to 3.6 nm. Hardness measurements showed that a conventional Hall−Petch relationship holds for ceramics composed of grain sizes above 5 nm, with maximum hardness at a grain size of 18.4 nm. However, below 5 nm, the hardness decreases as the grain size decreases, confirming the existence of a Hall−Petch breakdown. At the smallest grain sizes, the effect of this breakdown plateaus and becomes insensitive to grain size change. Further investigations showed that the mechanism behind the breakdown, negative, and plateau behaviors is not diffusion-based but rather is consistent with structural changes. The authors suggest that these findings offer insight that could be used to tailor nanocrystalline ceramics’ strength and dissipative properties.
attention due to some significant advantages over chemical pigments and dyes, such as highly bright, fadeless, and environmentally friendly characteristics. The short-rangeordered amorphous photonic structures (APSs) are particularly attractive for applications such as paints, cosmetics, textiles, and displays due to their ability to generate angle-independent noniridescent structural colors. Self-assembling colloidal nanoparticles are the most commonly used bottom-up strategy to fabricate artificial APSs. Although APSs have been applied to surfaces using a variety of methods, including drop casting, spin coating, spray coating, and electrophoretic deposition, these methods lack fine control of the APS coatings on surfaces, hindering their use on irregular or highly curved surfaces. Li et al. (DOI: 10.1021/acsnano.7b08259) solved this challenge by applying colloidal nanoparticles through atomization deposition. Working with silica colloidal nanoparticles with poly(vinyl alcohol) (PVA) as the additive, the researchers used a commercial mesh nebulizer to generate an aerosol. They then placed various substrates into this mass of foggy vapor composed of microdroplets. Using this method, the researchers were able to apply structural color on a variety of irregular surfaces, such as papers, resins, metal plates, and ceramics. Experiments showed that 4 wt % of PVA was the optimal concentration to achieve the formation of homogeneous APSs. By layering blue, green, and red nanoparticles, the researchers could attain different shades. This method could even produce colorfast patterns on silk. The authors suggest that their technique has potential for developing environmentally friendly coloring for the textile industry.
USEFUL TIP: CONTROLLING OXIDATION AND SPIN WITH SCANNING TUNNELING MICROSCOPY Surface-anchored metalloporphyrins have been used as model systems in numerous studies to explore the interplay between ligand geometry, oxidation state, magnetism, and spin transport. Most scanning tunneling microscopy (STM) experiments in these systems assume that the microscope’s tip is a noninvasive tool. However, the tip can affect molecular properties in close proximity to the molecule. For example, it can affect the coupling of a molecule to the surface or induce small conformational changes within a molecule, even far before it makes contact. Heinrich et al. (DOI: 10.1021/acsnano.8b00312) took advantage of this phenomenon by using the force field of an STM tip to control the oxidation and spin state reversibly in a single Fe-porphyrin molecule. The researchers first deposited iron(III) octaethyl-porphyrin chloride (Fe-OEP-Cl) on an atomically clean Pb(111) surface. The Fe center for these molecules lies in a +3 oxidation state with a spin of 5/2. When the researchers approached single molecules with the STM tip, the force exerted by the tip leads to a deformation of the molecule, elongating the Fe−Cl bond length in particular. Consequently, this affects the magnetocrystalline anisotropy of the 5/2 spin state. At a critical bond length, both the oxidation state and the spin state suddenly change, with the spin state moving from a half-integer state to an integer state. These changes, which were fully reversible, lead to a distinctly different fingerprint in the excitation spectra. Density functional theory and wave functional theory simulations confirm these findings. The authors suggest that these findings could open pathways for controlling magnetic properties by external forces.
WHAT HAPPENS BELOW THE HALL−PETCH LIMIT Many studies in metals have shown that as the grain size decreases, the strength of the bulk material increases, a phenomenon described by the Hall−Petch relationship. However, this association breaks down at a particular size, leading to a relationship in which smaller grain sizes have decreasing strength. This phenomenon is often attributed to diffusion-based mechanisms that enable grain boundary activities, such as sliding, and grain activity, such as rotation, in conjunction with dislocation mechanisms. Hall−Petch relationships have also been observed in nanocrystalline ceramics, but the few studies examining the breakdown of these relationships have had contradictory results. Even the existence of the breakdown in these materials has been uncertain. To understand the Hall−Petch relationship in ceramics, Ryou et al. (DOI: 10.1021/acsnano.7b07380) performed instrumented indentation studies on fully dense nanocrystalline 3058
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IMPERFECT PAIRING: HOW PBTE NANOCRYSTAL PAIRS ATTACH Nanocrystals are often free of nonequilibrium crystal defects, such as dislocations and impurities. Leveraging these defect-free individual particles as building blocks to create larger crystals could lead to higher quality materials for applications. Nanocrystals can be epitaxially joined through oriented attachment, a process in which particles spontaneously organize to share a common crystallographic orientation and then fuse to create a crystallographically coherent interface. Ideally, oriented attachment produces perfect interfaces. However, dislocations can arise, affecting crystal growth and properties. Having a better understanding of defect removal mechanisms and the size dependence of these mechanisms will be necessary to realize perfect epitaxial assemblies of these materials. Toward this goal, Ondry et al. (DOI: 10.1021/ acsnano.8b00638) used in situ high-resolution transmission electron microscopy to study the structure and dynamics of welldefined edge dislocations in imperfectly attached PbTe nanocrystals. The researchers found that attachment of these nanocrystals on both {100} and {110} facets gives rise to a Burger vector b = a/2⟨110⟩ edge dislocations. Defects in particles attached on {100} facets are able to move through glide planes that lie at 45° to the surface, resolving within seconds. However, for {110} attached particles, the glide plane is collinear with the attachment direction, leading to a complex and slower trajectory for resolution that involves a dislocation climb and conversion to a screw dislocation. In addition, results showed that dislocations closer to the surface were removed faster, consistent with the strong dislocation-free-surface attractive force. The authors suggest that further understanding in this area could lead to ever-increasing material quality.
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