WOOD-BASED NANOTECHNOLOGY FOR OIL−WATER SEPARATION The ability to separate oil and water effectively and efficiently is useful both for the treatment of industrial oil-containing wastewater and for cleaning up marine oil-spill accidents. Three-dimensional cellulosic materials have shown promise in this area, offering a low-cost and eco-friendly way to purify oil− water mixtures. However, preparing these materials typically takes many time- and energy-consuming steps, such as mechanical disintegration, chemical modification, sol−gel transformation, and careful drying. Thus, alternatives are urgently needed. Toward this end, Fu et al. (DOI: 10.1021/acsnano.8b00005) developed a material for oil−water separation based on a wood template. Starting with balsa wood as the raw material, the researchers first extracted lignin, leaving behind a honeycomblike cell wall architecture with micron-scale pores in the middle lamella and cell wall corners and numerous pits in the fiber walls. This template was then impregnated with an epoxy/amine/ acetone solution, cured, and then purified. Epoxy coated the lumen walls in the resulting material, covering and blocking these pits and pores in a thin layer. The epoxy not only boosted the specific modulus and specific yield strength of this biocomposite higher than did cellulosic materials investigated for oil−water separations but also made the wood template both hydrophobic and oleophilic. Experiments showed that this material readily absorbed oil both above and below water surfaces. The authors suggest that the mechanical and other properties of this woodbased structure could make it a favorable alternative to cellulosic materials for oil−water separation. In addition, the functionality of this material could be further tailored by chemical modification, giving it potential for applications in structural materials, photonics, sensors, and optical devices.
for achieving well-defined structural organization at the nanoscale, nature attains complex hierarchical structures from the bottom-up assembly of biomolecules. The mussel byssus, or beard, is a prime example of this phenomenon. Used to anchor these bivalve mollusks in wave-battered habitats, the byssus is tough, self-healing, and provides wet adhesion. This material is assembled from collagenous protein building blocks called preCols into a semicrystalline organization within minutes following secretion, mediated via the N- and C-terminal histidine-rich domains of the preCols. This high histidine content both makes the byssus stiffen with the pH change between acidic secretory vesicles and basic seawater and endows it with the capacity to coordinate transition metal ions in highly defined complexes. Taking advantage of these properties, Jehle et al. (DOI: 10.1021/acsnano.7b07905) used a natural amino acid sequence from byssal thread proteins to develop free-standing peptide films with complex hierarchical organization across length scales with properties that can be controlled by including transition metal ions. Like the mussel byssus, film formation was triggered by pH changes. Microscopy and mechanical analysis showed that films produced in the presence of Zn ions possessed both higher hierarchical order as well as 10-fold increases in stiffness. The authors suggest that this work provides a paradigm for creating tunable polymeric materials with multiscale organizational structure that depends on the processing and inclusion of metal ions.
GO-ANYWHERE GRAPHENE FOR FOOD, CLOTHES, PAPER Porous three-dimensional graphene-based materials have unique physical and chemical properties that give them promise for use in a variety of applications. Recently, researchers reported a straightforward method of synthesizing laser-induced graphene (LIG) using polyimide, also known as Kapton, as a starting material. This method has thus far been put to use in applications ranging from supercapacitors, to electrocatalysts for water splitting, to electrochemical biosensors. Although this approach for producing LIG has proven useful, it is limited by polyimide being the only precursor. Other recent experiments used wood as a starting material, yet these required an inert atmosphere.
FILMS INSPIRED FROM MUSSEL BEARDS Structural hierarchy from the nanoscale through the macroscale imbues many biological polymers with their unique and useful functions. Although traditional synthetic polymer production relies on top-down assembly processes, which present challenges © 2018 American Chemical Society
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Published: March 27, 2018 2084
DOI: 10.1021/acsnano.8b01867 ACS Nano 2018, 12, 2084−2087
In Nano
Cite This: ACS Nano 2018, 12, 2084−2087
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Chyan et al. (DOI: 10.1021/acsnano.7b08539) report a method to synthesize LIG with no need for an inert atmosphere on a wide variety of carbon precursors, including polymers, natural materials, food, and nonpolymeric materials such as activated carbon or anthracite coal. The researchers’ technique relies on multiple lasing, or multiple passes with a laser over the same surface. For these materials, a single pass converted the exposed surface to amorphous carbon, while multiple passes converted the substrate to graphene, confirmed with Raman and transmission electron microscopy. Experiments showed nearly any substrate that could first be thermally, photochemically, or chemically carbonized without ablating could then be made into LIG. Additional tests showed that defocusing the laser beam, thus essentially lasing each spot many additional times with a single pass of the laser, could significantly reduce the processing speed. The authors suggest that this technique could be applied in making flexible or even biodegradable or edible electronics.
SHEDDING LIGHT ON IMPROVING COMPUTING POWER As the demand for more powerful computational systems increases, so does the importance of introducing additional functionalities on semiconductor microchips. One approach that is being actively investigated is the use of massless photons instead of electrons for data transfer on-and off-chip, which holds the promise of higher speed at lower power consumption. Realizing coherent on-chip laser sources will be key to enabling these fully integrated electronic−photonic integrated circuits. However, accomplishing this goal at room temperature has been challenging thus far. Wirths et al. (DOI: 10.1021/acsnano.7b07911) show that room temperature lasing on Si is possible from monolithically integrated GaAs microdisks. The researchers fabricated these lasers by etching holes in a thermally oxidized Si(001) wafer to provide a high reactive index mismatch between the GaAs and the surrounding medium. They then deposited and patterned a sacrificial layer of amorphous Si over top, which they covered with SiO2 using atomic layer deposition and plasma-enhanced chemical vapor deposition. After making four template openings into the oxide, they removed the sacrificial amorphous Si using XeF2. Finally, they filled the hollow SiO2 cavities with GaAs using a template-assisted selective epitaxy technique. Analysis with scanning transmission electron microscopy and energy dispersive X-ray spectroscopy revealed monocrystalline growth of the GaAs. Photoluminescent spectra provided a clear signature of room-temperature lasing with this device with thresholds between 2 and 18 pJ/pulse depending on the cavity size. The authors suggest that these results show promise for merging electronics with photonics to reduce power consumption and to enrich the functionality of future Si integrated circuits.
INTRODUCING ORDER IN GRAPHENE NANORIBBON HETEROJUNCTIONS Graphene nanoribbons (GNRs), narrow strips of two-dimensional carbon sheets, show significant promise for high-speed digital nanodevices because their bandgaps increase as their widths narrow. Thus, GNR heterojunctions could be used in devices such as molecular-scale tunneling field-effect transistors and resonant tunneling diodes. These materials are so sensitive to structural variations that their feature sizes need to be well controlled down to the atomic scale, a feat achievable with bottom-up fabrication techniques. Bottom-up GNR heterojunctions have been synthesized by combining molecular precursors with different heteroatom doping patterns or different widths, or have been fabricated from a single molecular precursor designed with sacrificial ligands that can be removed or chemically altered to create abrupt variations in bandgap profile along the GNR axis. However, the number and placement of heterojunctions produced by these methods has so far been random, limiting their practical use in nanodevices. Bronner et al. (DOI: 10.1021/acsnano.7b08658) describe a hierarchical fabrication strategy of GNRs with a single heterojunction interface rather than a random statistical sequence of junctions to solve this problem. The researchers accomplish this structure by functionalizing one precursor with iodine and a second precursor with bromine, which have different dissociation energies. A linker molecule added to the mix promotes growth between the two. By gradually ramping up the reaction temperature, the researchers created fully extended GNRs with a single in-line heterojunction. The researchers suggest that this method could be used both for basic research and for developing functional GNR heterojunction nanodevices.
THE ALL-DIELECTRIC METASURFACE OF MANY COLORS Colors can be produced by light scattering, a phenomenon known as structural coloration. Inspired by stained glass from the middle ages, which incorporated metallic nanoparticles to produce colors, researchers have generated structural colors using plasmonic resonances in metal films with a subwavelength 2085
DOI: 10.1021/acsnano.8b01867 ACS Nano 2018, 12, 2084−2087
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anedithiols had a strong predominance of divalent structures, at 98% for 1O2 and 99% for 9O12. Under acidic conditions, 102 showed a slightly higher concentration of monovalent species than at neutral conditions. None of these manipulations changed the lattice structures or molecular densities of the monolayers. The authors suggest that this type of manipulation adds to the repertoire of controllable interactions at surface−molecule, molecule−molecule, and molecule−environment interfaces.
nanohole array. To reduce intrinsic losses and cost, more recent work has used dielectric metasurfaces instead of metallic nanostructures. Many of these studies have focused on silicon due to its complementary metal−oxide−semiconductor compatibility and high refractive index, producing many colors in the visible range. In other studies, TiO2 metasurfaces have been utilized to produce distinct colors across the visible spectrum. However, each of these all-dielectric nanostructures faces a severe limitation: once fabricated, their optical performances are usually static or change slowly, preventing their use in advanced displays. Sun et al. (DOI: 10.1021/acsnano.7b07121) developed an alldielectric metasurface to produce structural colors that is reconfigurable using microfluidics to address this issue. The system is composed of a trapezoidal TiO2 nanoblock array on an indium tin oxide glass substrate. This array alone produces a static structural color that is based on lattice size. However, using the microfluidic system to inject solutions with different refractive indices changes this color. This system can be used to produce distinct colors with a transition time as small as 16 ms, which is orders of magnitude faster than current techniques involving other all-dielectric metasurfaces for structural color. The authors suggest that placing this system on a stretchable platform could produce displays that span the color spectrum. Applications, they add, could include anti-counterfeiting, banknote security, and biomedical sensing.
SELF-ASSEMBLED MONOLAYERS SEE THE LIGHT Self-assembled monolayers (SAMs) of thiolated molecules on metals have proven to be a facile and versatile way to functionalize nanomaterials. For example, a wide variety of biofunctional and inorganic molecules have or can be modified to include a thiol group. Consequently, thiols have become workhorse molecular linkers in many nanoscientific fields, including molecular electronics, catalysis, sensing, and medical therapy. Given the importance of these surface chemistry processes, being able to control them down to the nanometer scale can have a tremendous impact on nanoscience and nanotechnology. However, there are no ways to control the self-assembly process down to the sub-nanoparticle level in situ easily. Simoncelli et al. (DOI: 10.1021/acsnano.7b08563) seeking a way to control self-assembly of thiolated molecules at the nanoscale, looked to light. Working with a monolayer of reactive thiol molecules self-assembled on electron-beam-patterned gold nanoantenna arrays, the researchers illuminated this structure with a wavelength-tunable linearly polarized laser pulse. An imaging technique called metallic DNA-PAINT showed that this method selectively detached only the thiol molecules localized on the resonantly excited region of the nanoantenna. With new openings on the gold nanoantennae, the researchers were able to create bifunctionalization in a subdiffraction area by adding a different type of thiol-labeled docking strand. Further investigation showed that the mechanism behind the Au−S bond breaking is most likely localized distribution of hot electrons, rather than uniformly distributed lattice heating. The authors suggest that this method can be used to displace any thiolated molecule selectively, opening the door to creating multifunctionalized nanoparticle arrays.
TWEAKING SELF-ASSEMBLED MONOLAYER VALENCE BY A SIMPLE PH SWITCH The most studied self-assembled monolayers (SAMs) are nalkanethiolates on Au{111}. These molecules contain a single thiol group available for substrate binding, and their formation is typically governed by simple reactions in which an acid thiol group is deprotonated to a thiolate or a disulfide bond is cleaved on reactive surfaces. Being able to control the properties of both exposed and buried interfaces of these SAMS has broad implications for molecular devices and lithographic patterning, among other applications. Seeking ways to manage and to manipulate this class of SAMs, Thomas et al. (DOI: 10.1021/acsnano.7b09011) investigated the effects of acid−base chemistry on valency, the molecule− substrate bond density, and subsequent monolayer formation. As models, the researchers used two different isomers of carboranedithiol, 9,12-carboranedithiol (9O12) and 1,2-carboranedithiol (1O2). At neutral pH, scanning tunneling microscopy showed two distinct binding states for each compound on Au{111}, supported by density functional theory calculations: 1O2 showed about 21% coverage of singly bound (monovalent) structures, with the remainder being doubly bound (divalent); 9O12 showed a strong preference for monovalent binding, with 98% monovalent and 2% divalent. However, under basic conditions, either before deposition in solution or during deposition, these modes dramatically changed. Both carbor2086
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THE WRITE STUFF: LASER TUNING NANOPARTICLE PATTERNS Surface-patterned periodic nanostructures are key elements in a variety of applications, including electronics and spintronics, chemical catalysts, plasmonic and photonic devices, memory devices, and sensing devices. Some of these applications require high customization of specific optical and electrical signals. However, being able to control pattern complexity and variation precisely remains challenging. Although pattern-guided lasers, thermal dewetting, and liquid-assembly can produce periodic metallic and dielectric nanostructures with a good combination of fidelity and low cost, these methods do not offer flexibility for complex and tunable patterning. Optical field patterning holds promise in this area, but no method reported has shown the ability to pattern and to tune the particle number, size, and placement in a scalable fashion. Wang et al. (DOI: 10.1021/acsnano.8b00198) show this type of tuning is possible using laser-induced modulated assembly (LiMA), a method that relies on the modulation of local laser absorption due to near-field optical coupling. Using prepatterned amorphous silicon films with large and small nanoparticles arranged around nanoholes, the researchers successfully modulated this arrangement to change particle number, size, and location with the laser pulse energy and polarization. In addition, using laser-induced phase switching (LiPS), they also converted crystalline Si nanoparticles to an amorphous state. The authors suggest that combined use of LiMA and LiPS could be used to fabricate Mie resonator arrays with programmed numbers, resonance peaks, and dielectric constants. This technology, they add, could eventually be used in optical metasurfaces, multidimensional optical storage devices, and objects with structural color.
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