SILICATE GLASS’ AMAZING GRAPHENE DREAMCOAT Silicate glasses can corrode in water through an ionic diffusion and exchange process, roughening the surface of the glass and subsequently reducing light transmittance. More seriously, corrosion also reduces the glass strength. This phenomenon is a serious problem in many applications, including in the pharmaceutical, industrial, environmental, and optical fields. To prevent this process, researchers have explored applying protective coatings. The ideal coating should be thin, transparent, and provide a good diffusion barrier. In a recent study, Wang et al. (DOI: 10.1021/ acsnano.6b04363) demonstrate the effectiveness of graphene in this role. After growing single-layer graphene films through chemical vapor deposition, the researchers transferred one or two layers to each side of a piece of silicate glass. After leaving samples for up to 120 days in water and then removing the graphene coatings, they then used atomic force microscopy to investigate the samples’ surface topography. While samples without the coatings showed evidence of corrosion after 20 days, developing a dramatic increase in surface roughness that quickly evolved into pitting and erosion, those with the graphene coatings showed negligible corrosion. Similarly, the fracture strength of bare glass gradually deteriorated the longer it was immersed in water, but samples coated with graphene maintained the same strength as bare glass that was never immersed. These results suggest that graphene can effectively protect the glass surface, with little difference between the protective abilities of the single and bilayer coatings and little loss of light transmittance. These results showcase the promise of graphene coatings as an anticorrosion barrier for silicate glasses.
In a recent study, Zhou et al. (DOI: 10.1021/ acsnano.6b05776) detail a synthetic process that readily produces Ag nanocubes with sharp corners and edges with average edge lengths between 35 and 95 nm. The researchers first formed AgCl octahedral by mixing CF3COOAg with an aqueous cetyltrimethylammonium chloride solution. Then, in the presence of reducing agent ascorbic acid and FeCl3 and under the irradiation of an electron beam, Ag nanocubes grew from the octahedral precursors. Further research suggested that the Fe acts to remove multiply twinned seeds through oxidative etching, and the Cl− ions both react with Ag to form the initial octahedra and then act as capping agents on the subsequent nanocubes. The authors suggest that this method represents a more reproducible, environmentally friendly, and economical route than polyol reduction to synthesizing Ag nanocubes.
MINDING THE GAP WITH DNA ORIGAMI Two gold nanoparticles can act as plasmonic nanoantennas, concentrating light into a nanoscale volume and strongly enhancing the electric field in the nanoparticle gap through plasmonic coupling. The size and spatial distribution of this plasmonic “hot spot” largely depends on the particle size and separation distance. However, to benefit from this effect for surface-enhanced Raman spectroscopy (SERS), target molecules must be positioned precisely between the two particles to get the strongest signal enhancement. One way to accomplish this goal is through DNA origami, a platform that can be used to design three-dimensional structures with nanoscale accuracy. In a recent study, Simoncelli et al. (DOI: 10.1021/ acsnano.6b05276) report a DNA origami design that positions molecules of interest between two gold nanoparticles with a separation gap of 1−2 nm, a size that effectively boosts the field enhancement in the hot spot. The design relies on a stack of five DNA layers with built-in grooves that embed the nanoparticles, while the middle layer binds Raman reporter molecules. Using the dye molecules Cy3 and Cy3.5 as models, the researchers show that when this setup is irradiated with a laser for SERS, plasmonic heating shrinks the designed gap between the nanoparticles to the desired 1−2 nm. After two rounds of heating, they observed an approximate 2 orders of magnitude enhancement of the Raman scattering signal of single molecules placed in the middle of the DNA origami. The authors suggest that using a DNA origami approach could offer
SILVER IS HIP TO BE SQUARED Ag nanocrystals have found use in numerous applications, including localized surface plasmon resonance (LSPR), surfaceenhanced Raman scattering (SERS), metal-enhanced fluorescence, and catalysis. For each of these uses, the precise shape of the nanocrystals is key to their performance. Of the wide range of Ag nanocrystal shapes thus reported, nanocubes are particularly interesting due to their ability to serve as sacrificial templates for fabricating Au-based nanocages with tunable LSPR peaks and as substrates for optical sensing and SERS detection. Thus far, most Ag nanocubes have been synthesized using polyol reduction. However, this method has several drawbacks, including being extremely sensitive to impurities and producing nanocubes that tend to be rounded at the corners and edges. © 2016 American Chemical Society
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WATCHING HYBRID PEROVSKITE NANOPARTICLES GROW UP Hybrid organic−inorganic perovskites’ (HOIPs) unique chemical and physical properties, including their band gap, large absorption coefficients, and long-range ambipolar chargetransport character, position these materials as promising sensitizers for photovoltaic solar cells. High-performance absorption layers incorporating HOIPs can be prepared using a low-temperature, solution-based approach, making fabrication facile and low-cost. However, synthetic conditions can significantly influence film quality and affect the resulting device performance. Thus, understanding how HOIPs crystallize from solution could help researchers develop methods to maximize film quality. In a recent study, Qin et al. (DOI: 10.1021/ acsnano.6b04234) use liquid-cell transmission election microscopy (LCTEM), a technique that enables dynamic observations of liquid samples, to perform a systematic study of nucleation and growth kinetics of HOIP nanoparticles from solutions. Using CH3NH3PbI3 as a model HOIP, the researchers initiated nucleation by evaporating the solvent from a CH3NH3PbI3 solution using the electron beam. They observed a dramatic rise of the nanoparticle number in the first 7 s followed by leveling off, suggesting rapid nucleation from the supersaturated solution. Further investigation showed that CH3NH3PbI3 growth did not follow a diffusion-limited nor a reaction-limited growth mechanism. Complex dynamical behavior as the nanoparticles form and coalesce suggested that crystal orientation and chemical characteristics play important roles in the growth of these films. The authors suggest that LCTEM could be a useful method for comparatively studying a broad range of other HOIPs with different properties.
a path toward future sensing applications with single-molecule resolution.
ABSENCE MAKES THE MOS2 GROW STRONGER Gaining understanding of how fractures develop in materials can help researchers develop strategies to delay or to prevent fracture formation to preserve desired mechanical properties. Because crack tips with atomic sharpness are thought to play a key role in crack propagation, it is important to understand their behavior at the atomic level. However, studies on this topic have been complicated by material thickness. Using ultrathin monolayer two-dimensional (2D) materials could solve this problem, making a realistic study of crack propagation at the atomic level possible. To this end, Wang et al. (DOI: 10.1021/acsnano.6b05435) used aberration-corrected transmission electron microscopy (AC-TEM) on monolayer MoS2, a 2D material that has attracted attention in recent years for its interesting electrical and photoluminescent properties. After using a laser to introduce cracks on a strained 2D MoS2 membrane, the researchers investigated the cracks experimentally while using molecular dynamics simulations to gain further insight at the atomic level into these observations. Their results suggest that cracks occur predominantly along the zigzag lattice direction. After introducing sparse S vacancies into this material using the electron beam, the researchers found that these defects deflected the cracks. As the density of defects increased, this mechanism shifted the fracture mechanism from brittle to ductile as vacancies migrated throughout the strain field. Because of these vacancies, the toughness of defective MoS2 exceeds that of graphene. The authors suggest that these results highlight the utility of using 2D materials to study the fundamental aspects of fracture mechanics, which is not possible with thicker materials.
CONTROLLING MAGNETISM WITH VOLTAGE Researchers are investigating a variety of technologies to tackle the challenges of making next-generation memory materials energy-efficient, reliable, fast, and able to transfer sizable amounts of data. Promising technologies include magnetic random access memories (MRAM), which use ferromagnetic materials for data writing by switching the magnetic states of the material. However, MRAM has drawbacks including high energy consumption, slow writing speeds, and overheating due to the high amounts of current required to generate the magnetic fields. An alternate technology, ferroelectric random access memory (FeRAM), operates by switching polarization states in ferroelectric materials. However, these devices have slower reading rates and problems with scaling down. 9766
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Seeking to incorporate the best of both technologies while minimizing their disadvantages, Erdem et al. (DOI: 10.1021/ acsnano.6b05469) report an effective and facile method for synthesizing multiferroic composite thin films incorporating ferroelectric BaTiO3 and ferromagnetic CoFe2O4 nanoparticles. By mixing dispersions of these materials and spin-coating them onto substrates followed by calcination, the researchers created crack-free multiferroic thin films in various ratios. Tests showed that the ideal CoFe2O4 content is around 5 wt % for dual control of electric and magnetic orders. Although the two nanoparticle types remained phase-separated after heat treatment, electrical and magnetic studies showed simultaneous ferroelectric and ferromagnetic order. By varying the applied voltage, the researchers could control the magnetism of these films at room temperature. The authors suggest that beyond memory storage, these BaTiO3−CoFe2O4 nanocomposite thin films could find use in applications including magnetic field sensors and actuators, energy harvesting, memory elements, and microwave devices.
HITTING THE RIGHT DOSE TO IMAGE NANOPARTICLE SUPERLATTICES The ability for nanoparticles to self-assemble into complex architectures can lead to a wide variety of collective functionalities that are tunable and might even adapt in response to environmental changes. To predict assembly accurately, researchers must develop better understanding of internanoparticle interactions and structuring kinetics in liquids, which has thus far been a challenge. Most studies to examine nanoparticle self-assembly have used small-angle X-ray scattering (SAXS). However, the ensemble nature of this modality provides little insight into real-time motions and interactions of single nanoparticles. Although liquid-phase transmission electron microscopy (LP-TEM) has been used successfully to image single nanoparticles and their interactions in liquids, this modality also has limitations, including the propensity of the imaging electron beam to alter the liquid components through radiolysis, which can influence nanoparticle motion and assembly. In a recent study, Kim et al. (DOI: 10.1021/ acsnano.6b05270) overcame this issue by using low-dose LPTEM imaging conditions typically used for biomolecular imaging. As a model system, the researchers used this method to examine superlattices composed of evenly spaced gold triangular nanoprisms aligned face-to-face. They observed that within an optimized dose rate range, center-to-center spacing between the nanoprisms could be quantitatively modulated by the electron beam dose rate. Further investigation suggests that the electron beam irradiation induces ionic strength increases that cause these assemblies to contract. The authors suggest that these findings could serve as a guide for designing future LP-TEM studies to examine nanoparticle assembly structures and dynamics.
ELECTROCHROMIC DISPLAYS FIT TO PRINT Fabricating electronics by printing holds several advantages over conventional fabrication methods, such as the enabling additive patterning, solution-based processing at low temperatures suitable for printing on flexible polymers, facile scalability, and eliminating high-vacuum conditions. Consequently, printing has the potential to reduce electronics manufacturing costs while facilitating the development of large-area and flexible displays for applications such as disposable tags, single-use medical electronics, and smart home appliances. This technique holds particular promise for electrochromic displays, which reversibly change their reflectivity upon application of a bias voltage. However, past attempts to create fully printed electrochromic displays have included low mobility of organic thin-film transistors in the backplane and the need for multiple patterning techniques, which significantly increase the complexity of manufacturing. In a recent study, Cao et al. (DOI: 10.1021/ acsnano.6b05368) report fully printed, large area, and flexible active-matrix electrochromic displays. Using screen-printing as their only patterning modality, the researchers created backplanes with silver nanoparticles as the conductor, semiconducting single-walled carbon nanotubes as the conduction channel, and varium titanate as the insulator. Thin-film transistors in these backplanes showed high carrier mobility and current on−off ratios, as well as good uniformity. By combining these components with screen-printed electrochromic cells with a silver/electrolyte/PEDOT:PSS lateral structure, the investigators fabricated complete active-matrix electrochromic displays. Demonstrating the utility of these devices, they turned pixels on and off to display the letters “U”, “S”, and “C”. The authors suggest that these fully screenprinted active-matrix electrochromic displays could find use in a variety of applications. 9767
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