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PRODUCING CIRCULARLY POLARIZED LUMINESCENCE WITH CHIRAL NANOTUBES Optical materials that produce circularly polarized luminescence (CPL) have a variety of promising applications, including optical probes and sensors, advanced microscopes, and three-dimensional displays. This type of light can be produced by using a linear polarizer and quarter-wave plate, but this method leads to a loss of power during the transition phase and requires specially designed plates for each wavelength. As an alternative, chirally luminescent materials can also produce CPL. Although many organic CPL-active nanostructures have been discovered, there are currently few known inorganic ones. Investigators have taken two approaches to construct inorganic chiral emissive materials: One method is by capping inorganic nanostructures with chiral reagents. Another less common approach is by placing luminescent achiral inorganic nanomaterials into a chiral host. Jin et al. (DOI: 10.1021/acsnano.8b08273) use this second tack with a twist, placing upconversion nanoparticles (UCNPs) into chiral nanotubes to produce CPL. The researchers formed these composites by mixing two different UCNPs, either NaYF4:Yb/Er or NaYF4:Yb/Tm with N,N′-bis(octadecyl)-Lglutamic diamide and its enantiomer, both gelators that form chiral nanotubes during the gelation process. Microscopy showed that the UCNPs assemble along the chiral nanotubes. Additional tests confirmed the encapsulation of the UCNPs in the co-gel system and showed that the two-photon upconversion was preserved. These composites displayed intense upconverted CPL ranging from ultraviolet to nearinfrared wavelengths. The researchers took advantage of this phenomenon, using the UV portion to initiate the enantioselective polymerization of diacetylene. The authors suggest that this method could be used to create other inorganic CPL-emissive materials, which could find a variety of applications in the future.
molecules from the catalysts. Thus far, platinum has been the most active catalyst discovered. However, its relative rarity and high cost make it an unattractive choice. Seeking to improve HER efficiency while potentially reducing cost, Sheng et al. (DOI: 10.1021/acsnano.8b07572) looked to iridium, an element that is theoretically a more efficient HER catalyst than platinum; however, experiments have shown significantly worse performance, potentially based on iridium atoms’ and clusters’ tendency to aggregate. To improve iridium’s catalytic activity, the researchers used it in a binary system, attaching small iridium particles on silicon nanowires. This system splits HER into three steps: proton adsorption on iridium and reduction to hydrogen atoms; hydrogen migration from iridium to silicon; and adsorbed hydrogen atoms combining to generate hydrogen molecules, which desorb. Tests show that this two-surface system performs significantly better than platinum in terms of activity per mass, per mole, and per dollar, with lower overpotentials, higher current densities, and mass activity for all bias potentials applied. The authors suggest that this catalyst could boost not only the efficiency of HER but other important catalytic industrial processes, as well.
HARNESSING THE HEART’S ENERGY TO POWER A PACEMAKER Two different serious heart conditions can be treated with implantable devices: Patients with bradyrhythmia, which causes the heart to beat too slowly, often require a pacemaker. Patients with supraventricular tachycardia or atrial fibrillation often need cardioversion devices. Although these devices can save lives, they are plagued by the need for replacement every 4−10 years due to their limited battery life. Surgeries to replace pacemakers and cardioverters are associated with several complications, including infection and bleeding. Thus, finding a way to prolong their lifespan is an important priority. Researchers have investigated several different energy-scavenging technologies to try to use residual energy from bodily
A HYDROGEN EVOLUTION REACTION CATALYST THAT OUTPERFORMS PLATINUM Hydrogen’s high energy density and lack of pollution upon combustion make it an important “green” source of energy. However, producing it by splitting water requires a significant investment of energy; thus, finding ways to improve the efficiency of the hydrogen evolution reaction (HER) is an important goal. This reaction is composed of two basic steps: the formation and adsorption of hydrogen atoms on the catalysts, and the production and desorption of hydrogen © 2019 American Chemical Society
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Published: March 26, 2019 2672
DOI: 10.1021/acsnano.9b01881 ACS Nano 2019, 13, 2672−2674
In Nano
Cite This: ACS Nano 2019, 13, 2672−2674
ACS Nano
In Nano
under flexural stress. The authors suggest that this material could offer strength, stiffness, and toughness for various applications.
processes to power these devices, including electromagnetic induction, electrostatic, piezoelectric generators, and triboelectric generators; however, they have shown several drawbacks, such as insufficient in vivo power generation, rigid structure, and uneasy miniaturization. Seeking a better performing alternative, Li et al. (DOI: 10.1021/acsnano.8b08567) developed a high-performance, implantable piezoelectric energy generator (iPEG). This device, shaped like the number 8, is constructed with an elastic skeleton made of polyethylene terephthalate and two piezoelectric composites consisting of a piezoelectric layer, beryllium−bronze foil, and Cr/Au electrodes. By repeatedly deforming and expanding, the iPEG generates a high output current when implanted in the apex of the pericardial sac. Tests in vivo in a porcine model show that this device can successfully power a cardiac pacemaker without triggering biocompatibility issues. The authors suggest that the iPEG might eventually be used to power both pacemakers and cardioverters, either eliminating batteries completely or reducing the frequency of replacing them.
ALIGNING THE STARSAND SUB-5 NM FEATURES Nanopatterns with single-nanometer-scale feature size are important for developing next-generation electronics. Sub-5 nm features are especially intriguing due to their strong electric field enhancement and quantum confinement. For many applications, these features must be aligned over centimeter scales, a feat that has proven difficult and expensive to accomplish. Thus, straightforward and inexpensive methods for uniaxially aligned nanopatterns with sub-5 nm pitch on centimeter scales are in demand. Getting closer to this goal, Hirota et al. (DOI: 10.1021/ acsnano.8b07714) report a method for developing uniaxially aligned silica nanogrooves with sub-5 nm periodicity using poly(dimethylsiloxane) (PDMS) stamps. Their protocol involved placing a PDMS stamp with a striped pattern on a cetyltrimethylammonium chloride (CTAC) thin film on a Si substrate. When exposed to an ammonia−water vapor, the hygroscopic CTAC molecules absorb water, changing from a lamellar phase to a two-dimensional hexagonal phase. Ammonia in the vapor caused the outermost surfaces of the aligned micelles facing the substrate to become templated with soluble silicate species, generated from the Si substrate under basic conditions. After peeling off the PDMS stamp and removing the CTAC thin film with washing, microscopy and grazing incidence small-angle X-ray scattering experiments showed that the micelles confined in the cavities of the PDMS stamp became aligned in the lengthwise direction of the guide pattern, producing uniaxially aligned nanogrooves. These nanogrooves extended over the entire 2 cm × 2 cm surface that the stamp covered. The authors suggest that this method could offer a simple and inexpensive way to produce uniaxially aligned nanopatterns with ultrafine pitch.
MIMICKING NATURE FOR ARTIFICIAL NACRE To develop lightweight, damage-resistant materials, researchers have often looked to nature. One natural material with these qualities is nacre, the iridescent inner shell layer of some mollusks. Nacre owes its exceptional strength and toughness to a layer-by-layer structure composed of inorganic platelets and organic matrices. Although several artificial composites with similar layered structures have been reported, few have been films composed of microscale inorganic platelets with organic polymers. Those developed thus far have relied on complicated procedures that are not suitable for large-scale engineering of the composite film. Looking for a way to create nacre-inspired films that integrate high strength, stiffness, and toughness, Ji and Kim (DOI: 10.1021/acsnano.8b06767) fabricated polymer composites using a hydrogel-film casting method that enables the building of uniformly layered organic/inorganic microstructures. Using alginate as the organic matrix, the researchers enhanced its mechanical properties by Ca2+ cross-linking. They used alumina microplatelets as a horizontally aligned inorganic phase, improving their alignment and interactions with the organic matrix by coating them with polyvinylpyrrolidone. Their experiments show that the resulting film enables both elastic and plastic deformation under tensile stress, exhibiting high stiffness and toughness. Further experiments show that each constituent is pivotal in achieving the exceptional properties of the composite. These properties were dependent on humidity, with a higher degree of strength and stiffness in low humidity conditions. A bulk composite made by laminating thin films showed high mechanical properties 2673
DOI: 10.1021/acsnano.9b01881 ACS Nano 2019, 13, 2672−2674
ACS Nano
In Nano
Frey et al. (DOI: 10.1021/acsnano.8b09201) combine these concepts by investigating the effects of small applied electric fields on transition metal carbide and nitride MXenes. The researchers hypothesized that surface terminations on noncentrosymmetric Janus Mn 2 N, Cr 2 C, V 2 C, and Ti 2 C introduced during synthesis could act as chemical dopants to influence the electronic structure, enabling controllable magnetic order. Their results show ground-state magnetic ordering for some Janus MXenes with asymmetric surface functionalization, where local structural and chemical disorder induces magnetic ordering in some systems that are nonmagnetic or weakly magnetic in their pristine form. For various combinations of O and F surface terminations, Janus Mn2N demonstrated ground-state ferromagnetism, while Janus Cr2C, V2C, and Ti2C were robustly antiferromagnetic. The magnetic states of these materials could be switched and stabilized by tuning the interlayer exchange couplings with small applied electric fields. In addition, bond directionality was enhanced by the Janus functionalization, leading to improved magnetic anisotropy, which is essential for stable 2D magnetic ordering. The authors suggest that these findings provide insight into obtaining robust, electrically controllable nanoscale magnetism and strong anisotropy in MXenes with surface functionalization.
GOING WITH THE GRAIN: PHOTOINDUCED CARRIER MOTION IN LAYERED PEROVSKITES Organic−inorganic halide perovskites have strong promise for use in a variety of applications, including photovoltaics and light-emitting diodes. To avoid moisture-related stability issues that affect three-dimensional (3D) perovskites, recent research has focused on two-dimensional (2D) forms of these materials, including Ruddlestein−Popper systems. The source of 2D perovskites’ relative stability and mixed performance compared to their 3D counterparts has been unclear. One possibility is that 2D materials have restricted ion mobility due to the planes of hydrophobic aliphatic chains parallel to the layering axis. However, ion motion across the layered planes has been demonstrated, even in single crystals. To improve understanding of light-induced dynamics in these thin films, Giridharagopal et al. (DOI: 10.1021/ acsnano.8b08390) studied local photoinduced ion motion in layered Ruddlestein-Popper perovskites, using (BA)2(MA)n−1PBnI3n+1 (BAPI) as a model system. The researchers probed ionic and electronic carrier dynamics using two complementary scanning probe methods: timeresolved general-mode Kelvin probe force microscopy (GKPFM) and fast feedback-free time-resolved electrostatic force microscopy (FF-trERM). Their results show that these layered perovskites exhibited faster surface charge buildup in grain centers rather than at grain boundaries, a phenomenon that could be due to a mixture of ionic and trap-mediated electronic transport. The researchers propose that this effect could be caused by a combination of ion migration occurring between PbI4 planes and electronic carriers traversing grain boundary traps, changing the time-dependent band unbending at grain boundaries. They suggest that these findings not only add insight to photoinduced ion motion in 2D perovskites but could also lead to the development of more stable materials and devices in the future.
TWO-DIMENSIONAL MATERIALS WITH A MAGNETIC PERSONALITY Recent studies demonstrating magnetism in two-dimensional (2D) crystals have prompted research into how to control magnetization in these materials using external means, which could offer the potential for rapid switching of magnetic ordering. These 2D magnetic systems require significant anisotropy to protect magnetic ordering against thermal disorder. Applied electric fields and doping could help achieve both these goals, potentially leading to controllable, roomtemperature nanoscale magnetism for voltage-controlled spintronic devices. 2674
DOI: 10.1021/acsnano.9b01881 ACS Nano 2019, 13, 2672−2674
