A NEW SPIN ON MOS2 PHOTOLUMINESCENCE MoS2 monolayers have attracted attention in recent years for their unusual qualities that might eventually play roles in nextgeneration optoelectronic devices, including a direct band gap, flexibility, and tunable optical emission. However, their singleatom thickness leads to weak light−matter interactions, which pose significant problems in evoking photoluminescence (PL) emission at practical levels for applications. Researchers have attempted to enhance PL emission in this material using a variety of methods, including by placing photonic microcavities and plasmonic nanostructures on the surface. However, precisely fabricating these features to have the desired effect has its own challenges. Trying a different approach, Li et al. (DOI: 10.1021/ acsnano.6b06834) modified the MoS2 exciton−plasmon interaction by optical spin−orbit coupling. Working with MoS2 film placed on an SiO2 layer with a mirror layer of Au film underneath, the researchers used electron-beam lithography to pattern a series of spiral rings with different numbers of turns. Using MoS2 patterned with two-turn rings under the excitation of left-handed circular polarized light, the researchers achieved a maximum PL enhancement of the MoS2 monolayer that was about 10 times higher than pure MoS2. In contrast, under right-handed circular polarized light, PL signals are barely enhanced, suggesting that polarization can tailor the exciton−plasmon interactions. Similarly, PL also depended on the strength of laser power and the number of rings. The authors suggest that this tactic might eventually be used to enhance MoS2 PL for the development of spin-dependent nanophotonic devices.
as gold nanorods (AuNRs). Such systems have successfully been used to craft reconfigurable photonic nanostructures with two AuNRs. In a recent study, Zhan et al. (DOI: 10.1021/acsnano.6b06861) added to that number, reporting a reconfigurable plasmonic tripod whose three arms can change the angles at which they are connected based on the structure of the DNA connecting them. Each arm of the tripod contains 14 DNA duplexes packed on a honeycomb lattice to hold three AuNRs, which are held in place at 30°, 60°, or 90° by “locking” strands. By adding complementary “releasing” strands and then new locking strands, the researchers could change the angle at which the AuNRs connect. Tests showed that these tripods could be reconfigured repeatedly for multiple cycles, with different configurations changing the plasmonic properties of these 3D structures. The authors suggest that DNA origami could be used to create various dynamic plasmonic nanostructures for a variety of optical applications.
SYNTHESIZING A CESIUM LEAD HALIDE PEROVSKITE LOVE TRIANGLE Lead halide perovskites have become a popular focus for research due to their exceptional characteristics, including long electron− hole diffusion lengths, tunable bandgap, high carrier mobility, and low trap-state density. All-inorganic perovskites hold definite advantages over their organic counterparts, including resistance to moisture, heat, oxygen, light, and electrical fields. Similarly, vapor-phase deposition holds advantages over traditional solution synthesis routes, such as the potential to achieve axial nanowire heterostructures and band gap grade semiconductor nanostructures and the ability to fabricate structures directly on the growth substrates. While vapor-phase growth of singlecomposition perovskite nanowires of CsPbI3, CsPbBr3, and CsPbCl3 have thus far been reported, vapor-phase growth of composition-tuned all-inorganic cesium lead halide perovskite nanowires has not yet been achieved. In a recent study, Zhou et al. (DOI: 10.1021/acsnano.6b07374) showed that this feat is possible, reporting vapor-phase growth of high-quality cesium lead halide micro/nanorods with tunable compositions. By heating CsX and PbX2 powders, the researchers grew one-dimensional perovskite micro/nanorods on a SiO2/Si substrate. Most of the resulting structures had triangular cross sections with well-defined and smooth surfaces. Further examination showed that these materials showed strong photoluminescence that could be tuned by varying the halide composition and that they functioned as effective Fabry−Perot
A DIFFERENT ANGLE ON PLASMONIC NANOSTRUCTURES DNA origami has led to a revolution in structural nanotechnology, with this technique being used to construct a wide range of nanostructures including closely packed three-dimensional (3D) shapes, hollow boxes, and wireframe nanostructures with multi-arm junction vertices. More recently, DNA nanostructures have served as templates to bind nanoscale objects and molecules, including chromophores, quantum dots, carbon nanotubes, and proteins, into well-defined geometries. Using this technique, researchers have fabricated two- and threedimensional photonic nanostructures using building blocks such © 2017 American Chemical Society
Published: February 28, 2017 1127
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DETECTION OF GENETIC BIOMARKERS: CHARGING AHEAD Researchers have investigated dielectrophoresis as a way to collect, to enrich, and to sort pathogenic cells and viruses and to manipulate proteins and DNA. When these polarizable particles and molecules are subject to a nonuniform electric field, their induced dipoles move them along or against the field gradient. However, this technology is not capable of discriminating different target molecules that have similar and small induced dipole moments. Similarly, although nanopores have shown promise for sensing genetic, epigenetic, and proteomic biomarkers and drug compounds, the target detection accuracy of this technology is hampered by nontarget molecules that disrupt ionic current signatures when they enter the nanopore. Seeking a better way to detect genetic biomarkers, Tian et al. (DOI: 10.1021/acsnano.6b07570) combined these two technologies into one. The researchers engineered a small polycationic nanocarrier that induces a prescribed dipole moment onto a target nucleic acid. Using a nanopore adapted from α-hemolysin, the researchers applied an electric field gradient at the nanopore entrance. Their results showed that target molecules bound to the nanocarrier are driven into the nanopore by dielectrophoretic forces, whereas nontarget molecules without a bound carrier are electrophoretically repelled. Further experiments showed that this technique was effective for singleor double-stranded DNA or RNA of any length as well as detection of multiple target biomarkers. The authors suggest that this approach could be used not only for biomedical applications, but also for plant science, foodborne detection, and other fields where rapid genetic testing is required.
cavities for nanoscale lasers. Using these micro/nanorods, the researchers achieved room temperature optically pumped tunable lasing that covered almost the entire visible spectral region with low lasing thresholds and high quality factors. The authors suggest that these materials could be promising candidates for applications in photonic and optoelectronic devices.
SLOW RELEASE GOES AGRICULTURAL Urea [CO(NH2)2] is one of the most common nitrogen-based fertilizers used worldwide. However, the action of water, volatilization, and urease enzyme cause its premature decomposition before much of its nitrogen can be adsorbed by plants. Because the cost of fertilizers can be significant in developing countries, this waste represents a major challenge for global agriculture and a threat for future food security. One potential solution is developing nanoparticle-based slow-release formulations for urea and other fertilizers. However, while this approach has been widely exploited for drug delivery, little work has been done to advance similar technology for delivering nutrients to crops. In a recent study, Kottegoda et al. (DOI: 10.1021/ acsnano.6b07781) adapted slow release to agriculture by incorporating urea into environmentally friendly nanoparticles. Using hydroxyapatite (HA) [(Ca10(PO4)6(OH)2], the key constituent of animal hard tissues, as the base, the researchers created a hybrid nanoparticle by dripping a solution of H3PO4 into a suspension containing Ca(OH)2 and urea. After flash drying, these nanohybrids carry a 6:1 ratio of urea to HA. Rapid water release tests showed that the nanohybrids released nitrogen ∼12 times slower than pure urea. Evaluations in rice as a model crop at the Rice Research and Development institute of Sri Lanka and additional field trials confirmed agricultural applicability, showing that the slow-release nanohybrids increased crop yield significantly more than the recommended amount of urea. The authors suggest that this technology might be used to develop other slow-release fertilizers that can improve crop yield while reducing the amount of applied chemicals.
STUCK ON YOU: ADHESION ENERGIES OF METAL NANOPARTICLES ON OXIDE SURFACES Many catalysts and electrocatalysts are composed of metal nanoparticles scattered across oxide supports. These hybrid materials are widely used to produce and to use fuels and chemicals, as well as in pollution abatement. The structure of these catalysts significantly affects their activity and lifetimes. Previous studies have shown that the chemical reactivity of the surface metal atoms in these supported catalysts correlates with the chemical potential of the metal atoms in these particles. This chemical potential, in turn, increases with decreasing particle size below 6 nm and decreases with the adhesion energy (Eadh) between the metal and its support. Thus, knowing the Eadh between a metal and oxide is critical for understanding and tailoring catalytic performance. In a recent study, Hemmingson and Campbell (DOI: 10.1021/acsnano.6b07502) investigated Eadh for a variety of different metal nanoparticles and oxide supports. Using singlecrystal adsorption calorimetry on cleaned, single-crystal oxide surfaces in ultrahigh vacuum, the researchers found that Eadh for metal nanoparticles on an oxide surface correlates with the 1128
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Probing its phototransport, the researchers found a large photogain that results from photogating. The authors suggest that these results confirm that tunnel spectroscopy of a single, narrow band gap QDot can be reliably measured.
oxide’s heat of formation, a measure of oxophilicity. Thus, metal−oxygen bonds appear to dominate the bonding between the metal surface and the surface of the oxide lattice. Consequently, the researchers are able to order Eadh between any given metal nanoparticle and the oxides they tested in a series: MgO(100) ≈ TiO2(110) < α-Al2O3(0001) < CeO2(111) ≈ Fe3O4(111). The authors suggest that these Eadh trends can be used to clarify structure−function relationships in catalysis, maximizing the performance of future catalysts.
ILLUMINATING THE INNER WORKINGS OF ORGANIC LIGHT-EMITTING DIODES Organic light-emitting diodes (OLEDs) have become the material of choice for forming the basis of screens in televisions, computer monitors, and mobile phones. To produce visible displays, OLEDs rely on the electroluminescence of thin organic films. The electroluminescence characteristics of these devices has been well explored on the macroscopic and mesoscopic levels; however, little is known about light excitation in OLEDs at the level of individual molecules or at the submolecular level. Consequently, it has been unknown whether the macroscopically measurable properties of these devices are determined by their bulk, surface, or defects. In a recent study, Große et al. (DOI: 10.1021/acsnano.6b08471) used low-temperature scanning tunneling microscopy (STM) to probe the nanoscopic electroluminescence characteristics of C60 films of less than 10 monolayers on Ag(111) and Au(111) substrates. Like an OLED, the metal substrate and STM noble metal tip inject complementary charge carriers that can combine within the molecular film to induce electroluminescence. However, because this charge injection is atomically defined by the STM tip, this technique enables mapping of the local electroluminescence down to the submolecular level. Using this technique, the researchers found that the radiative recombination in this organic thin film stems from structural defects, including sites with orientational disorder, near chemical impurities, screw dislocations, or domain boundaries. They observed only weak emission from tip-induced plasmons, which were distinguishable from the intrinsic radiative recombination of electron−hole pairs by the molecular orbital patterns visible in the generated electroluminescence maps. The authors suggest that these techniques can be applied to study the electroluminescence of other organic films on the molecular scale.
SHINING A LIGHT ON SINGLE, SELF-DOPED NANOCRYSTALS Researchers have used tunnel spectroscopy on single nanoparticles to study a variety of fundamental electron transportrelated phenomena. This technique offers a complementary approach to optical measurements to probe the electronic structure of confined semiconductor nanostructures. Zerodimensional structures such as quantum dots (QDots) have attracted significant interest in this area. Thus far, most tunnel spectroscopy efforts on QDots have focused on wide band gap materials, where quantum confinement only marginally affects the electron structure. In contrast, in narrow band gap semicondutors or semimetals such as HgSe, quantum confinement leads to a dramatic modification of the electron spectrum. The band gap of these materials corresponds with optical features in the mid-infrared (IR) region. However, although broadening of the spectrum is best addressed using single nanoparticle optical measurements, these experiments have proven challenging to accomplish in this spectral range. To address this problem, Wang et al. (DOI: 10.1021/ acsnano.6b07898) developed a technique to perform tunnel spectroscopy in a single HgSe QDot. Using an electrospray method, the researchers trapped a single QDot within a nanogap on a chip circuit. Upon tuning the gate bias, the researchers found two energy gaps of different sizes in the spectrum. Further investigation showed that the wider gap results form the interband gap, while the narrower gap results from intraband transitions. Observation of this narrow gap at near-zero gate voltage confirmed the self-doped character of the HgSe nanoparticle.
CONSCIOUS COUPLING: ELECTRONS AND PHONONS IN PD NANOSTRUCTURES Pd nanostructures have spurred increasing interest as plasmonic materials over the past decade. These materials also represent potential alternatives to Au and Ag nanostructures as plasmonic sensing materials and as photothermal agents. Many efforts have focused on controlling the localized surface plasmon resonance (LSPR) in Pd nanostructures with geometric design, or with various morphologies, such as nanoparticles, nanoplates, nanocubes, and nanopolyhdrons, synthesized with coordination of specific molecules. Although the development of synthetic techniques and their applications have been the focus of substantial research, studies on the interaction between photons and Pd nanostructures have been sparse. 1129
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In a recent study, Wang et al. (DOI: 10.1021/acsnano.6b07082) used transient absorption spectroscopy to investigate the relaxation dynamics of hot electrons through electron− phonon coupling and the resulting coherent acoustic phonon vibration in hexagonal Pd nanosheets and Pd nanoplates sandwiched between Ag layers of varying thickness. Their measurements show that the electron−phonon coupling constant of the Pd nanosheets is larger than that of bulk Pd. The effective coupling constant of the sandwich nanoplates decreases with increasing Ag shell thickness, approaching that of bulk Ag at the thickest construction. Further investigation showed that the coherent acoustic phonon vibration for the Pd nanosheets fits a fundamental breathing mode, similar to the vibration of benezene, while the phonon vibrations of the sandwich nanoplates fits a quasi-extensional mode. The authors suggest that these results will be useful in applications involving nanosized Pd and its hybrid nanostructures, such as plasmonic catalysts and nanomechanical resonators.
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