Spontaneous Self-Assembly of Silver Nanoparticles into Lamellar Structured Silver Nanoleaves

Uniform lamellar silver nanoleaves (AgNLs) were spontaneously assembled from 4 nm silver nanoparticles (AgNPs) with p-aminothiophenol (PATP) as mediator under mild shaking at room temperature. The compositions of the AgNLs were verified to be 1 nm Ag25 nanoclusters and PATP molecules in quinonoid model. The underlying assembly mechanism was systematically investigated and a two-step reaction process was proposed. First, the 4 nm AgNPs were quickly etched to 1 nm Ag25 nanoclusters by PATP in the form of [Ag25(PATP)n]n+ (n < 12), which were then further electrostatically or covalently interconnected by PATP to form the repeated unit cells of [Ag25(PATP)n−1](n−1)+–PATP–[Ag25(PATP)n−1](n−1)+ (abbreviated as Ag25–PATP–Ag25). Second, these Ag25–PATP–Ag25 complexes were employed as building blocks to construct lamellar AgNLs under the directions of the strong dipole–dipole interaction and the π–π stacking force between the neighboring benzene rings of PATP. Different reaction parameters including the types and concentrations of ligands, solvents, reaction temperature, ionic strength, and pH, etc., were carefully studied to confirm this mechanism. Finally, the preliminary investigations of the applications for AgNLs as “molecular junctions” and SERS properties were demonstrated. We expect that this convenient and simple method can be in principle extended to other systems, or even mixture system with different types of NPs, and will provide an important avenue for designing metamaterials and exploring their physicochemical properties.

Has been reported a convenient and simple approach to spontaneously assemble AgNPs into uniformly lamellar AgNLs with PATP molecule as mediator. The self-assembly mechanism has been experimentally elucidated as (1) the formation of uniform 1 nm Ag25 nanoclusters obtained from the etching of 4 nm AgNPs by PATP; (2) the formation of the quinonoid model of PATP with the electrostatical/covalent interaction between −HS/–NH2 and AgNP resulting in the Ag25–PATP–Ag25 complex as a building block; (3) the strong dipole–dipole interaction and the π–π stacking force between the neighboring rigid benzene skeleton of PATP induced the ordered organization of Ag25–PATP–Ag25 complexes into lamellar AgNLs. They expect that this methodology will provide an important avenue for designing new metamaterials and exploring their novel physicochemical properties.

Reference
Spontaneous Self-Assembly of Silver Nanoparticles into Lamellar Structured Silver Nanoleaves
Lun Li and Qiangbin Wang
ACS Nano Article ASAP

Nitrogen-Doped Partially Reduced Graphene Oxide Rewritable Nonvolatile Memory

As memory materials, two-dimensional (2D) carbon materials such as graphene oxide (GO)-based materials have attracted attention due to a variety of advantageous attributes, including their solution-processability and their potential for highly scalable device fabrication for transistor-based memory and cross-bar memory arrays. In spite of this, the use of GO-based materials has been limited, primarily due to uncontrollable oxygen functional groups. To induce the stable memory effect by ionic charges of a negatively charged carboxylic acid group of partially reduced graphene oxide (PrGO), a positively charged pyridinium N that served as a counterion to the negatively charged carboxylic acid was carefully introduced on the PrGO framework. Partially reduced N-doped graphene oxide (PrGODMF) in dimethylformamide (DMF) behaved as a semiconducting nonvolatile memory material. Its optical energy band gap was 1.7–2.1 eV and contained a sp2 C═C framework with 45–50% oxygen-functionalized carbon density and 3% doped nitrogen atoms. In particular, rewritable nonvolatile memory characteristics were dependent on the proportion of pyridinum N, and as the proportion of pyridinium N atom decreased, the PrGODMF film lost memory behavior. Polarization of charged PrGODMF containing pyridinium N and carboxylic acid under an electric field produced N-doped PrGODMF memory effects that followed voltage-driven rewrite-read-erase-read processes.

chemically synthesized N-doped semiconducting PrGOs were produced using a mild reducing agent, the polar aprotic solvent DMF, and yielded different band gap values. Reaction time-dependent GO reduction with DMF resulted in a variety of optical band gaps from 2.55 eV (GO) to 1.35 eV (120 min PrGODMF). In two-terminal memory devices with the N-doped semiconducting PrGOs sandwiched between top and bottom metal electrodes, the current hysteresis exhibited two conducting states in current–voltage characteristics. However, for fully reduced GO with a low oxygen-functionalized carbon density (small amount of a negatively charged oxygen) and highly oxidized GO with a high oxygen-functionalized carbon density (no positively charged pyridinium), the current hysteresis was negligible. As carefully designed, the semiconducting PrGODMF devices containing sp2 C═C frameworks with 2.7–3.2% nitrogen atoms and an oxygen-functionalized carbon density of 45–50% (especially 1.5% pyridinium N atoms) clearly displayed rewritable nonvolatile memory behaviors, suggesting that novel N-doped PrGODMF could form a charge transporting path by associating with oxygen/nitrogen-functional groups such as Hδ+N-PrGO–COOδ−/Hδ+N-PrGO–COOδ− in interlayers of PrGODMF sheets. As a whole, the voltage-induced polarization of the film allowed the switching of an OFF state to an ON state, producing a rewritable nonvolatile memory.

Reference

Nitrogen-Doped Partially Reduced Graphene Oxide Rewritable Nonvolatile Memory
Sohyeon Seo, Yeoheung Yoon, Junghyun Lee, Younghun Park, and Hyoyoung Lee
ACS Nano Article ASAP

Self-Assembly of Light-Harvesting Crystalline Nanosheets in Aqueous Media

A methodology leading to facile self-assembly of crystalline aromatic arrays in dilute aqueous solutions would enable efficient fabrication and processing of organic photonic and electronic materials in water. In particular, soluble 2D crystalline nanosheets may mimic the properties of photoactive thin films and self-assembled monolayers, covering large areas with ordered nanometer-thick material. We designed such solution-phase arrays using hierarchical self-assembly of amphiphilic perylene diimides in aqueous media. The assemblies were characterized by cryogenic transmission electron microscopy (cryo-TEM), revealing crystalline order and 2D morphology (confirmed by AFM studies). The order and morphology are preserved upon drying as evidenced by TEM and AFM. The 2D crystalline-like structures exhibit broadening and red-shifted absorption bands in UV–vis spectra, typical for PDI crystals and liquid crystals. Photophysical studies including femtosecond transient absorption spectroscopy reveal that two of the assemblies are superior light-harvesters due to excellent solar spectrum coverage and fast exciton transfer, in one case showing exciton diffusion comparable to solid-state crystalline systems based on perylene tetracarboxylic dianhidride (PTCDA).



They have designed a family of amphiphilic PDI derivatives that self-assemble into crystalline 2D structures in aqueous solutions. This assembly motif is generated by hierarchical mode of two distinct hydrophobic interaction types induced by an aromatic core and alkyl groups, suggesting a simple design strategy to obtain crystalline organic assemblies in water. The self-assembled materials largely preserve their ordered structure in the dry state. Assemblies based on 2 and 3 show promising light-harvesting characteristics, with 2 exhibiting exciton diffusion comparable to solid crystalline films. The light absorption properties, ordered 2D morphology and nanoscale thickness of the nanosheets appear to be useful for fabrication of light-harvesting systems. Remarkably, relatively simple molecular systems can be designed to undergo self-assembly into extended ordered assemblies with advantageous photonic properties using water as an assembling medium. The ability to rationally design and efficiently assemble aromatic crystalline systems in aqueous media may lead to water-based photonic and electronic materials employing facile, cost-efficient, and environmentally friendly fabrication and processing.

Reference:
Self-Assembly of Light-Harvesting Crystalline Nanosheets in Aqueous Media
Chen Shahar, Jonathan Baram, Yaron Tidhar, Haim Weissman, Sidney R. Cohen, Iddo Pinkas, and Boris Rybtchinski
ACS Nano Article ASAP

Ordered Polymer Nanofibers Enhance Output Brightness in Bilayer Light-Emitting Field-Effect Transistors

Polymer light emitting field effect transistors are a class of light emitting devices that reveal interesting device physics.Device performance can be directly correlated to the most fundamental polymer science. Control over surface properties of the transistor dielectric can dramatically change the polymer morphology, introducing ordered phase. Electronic properties such as carrier mobility and injection efficiency on the interface can be promoted by ordered nanofibers in the polymer. Moreover, by controlling space charge in the polymer interface, the recombination zone can be spatially extended and thereby enhance the optical output.




Figure 1. (a) Photo of the operating device; (b) schematic of the device architecture; (c) molecular structures of PFN+BIm4- CPE, Super Yellow, and PATBT

The physical structure between the transistors and the passivated dielectric provides energetic preference on the surface, guides semiconducting polymers and forms ordered polymer fibers. Order in the PATBT (poly(3,6-dialkylthieno[3,2-b]thiophene-co-bithiophene)) raises carrier mobility and injects more holes into the luminescent SY layer. More holes accumulate because of the energy barrier introduced by the CPE at the drain electrode, therefore, holes distribute into a wider region in SY. While the current density increases, the recombination zone width simultaneously increases and emits more photons. To improve the efficiency, asymmetric contacts are foreseen as a next step. We expect that still higher brightness and efficiency should be achievable from bilayer LEFETs with asymmetric contacts.

Referencia:
Ordered Polymer Nanofibers Enhance Output Brightness in Bilayer Light-Emitting Field-Effect Transistors
Ben B.Y. Hsu, Jason Seifter, Christopher J. Takacs, Chengmei Zhong, Hsin-Rong Tseng, Ifor D. W. Samuel, Ebinazar B. Namdas, Guillermo C. Bazan, Fei, Huang, Yong Cao, and Alan J. Heeger
ACS Nano 2013 7 (3), 2344-2351

Color of OLEDs Can Now at Last Be Predicted Thanks to New Modeling Technique

LEDs -- thin, light-emitting surfaces -- are regarded as the light sources of the future. White OLEDs consist of stacked, ultra-thin layers, each emitting its own light color, all together resulting in white light. Up to now it has been impossible to predict the exact light color produced by a white OLED; manufacturers had to rely on trial and error. Researchers at Eindhoven University of Technology, Philips Research, Dresden University of Technology and other institutes have now developed a method that allows the color of light produced by a specific OLED design to be calculated with high precision. They did this by modeling the complex processes in OLEDs on a molecular scale. This technique will allow manufacturers to greatly improve their OLED design processes and reduce the cost. At the same time the energy efficiency and lifetime of OLEDs can be increased.

A transparent OLED made at Philips Research Aachen, seen from the rear; light is emitted from the front. Researchers at TU/e and other institutes have developed a tool that allows the light color of OLEDs to be predicted precisely. (Credit: Bart van Overbeeke)

Revolution It looks as though OLEDs -- organic light emitting diodes -- will cause a revolution in the world of lighting. OLEDs are light-emitting surfaces, which means they are visually more attractive than point light sources such as conventional or LED lamps. They can also be flexible and transparent, which opens up all kinds of new opportunities. Plus -- and unlike normal LEDs -- OLEDs are made of very low-cost materials, of which only very thin layers are needed. As a result, the prices of OLEDs are expected to be low once mass production starts. This clip gives an impression of what's possible with OLEDs. To predict what kind of light an OLED design will produce, the researchers made computer models of the electronic processes in the OLED at the deepest level. These showed for example the injection of electrical charge, the creation and distribution of the 'excitons' -- pairs of positively charged electrons and holes in a bound state -- and the creation from these of individual photons, the light that is emitted. "At first we thought it would never be possible," says researcher Peter Bobbert of Eindhoven University of Technology. The main difficulty was that each change in the electrical charge also influences all the other charges, which makes the simulation extremely complex. But they succeeded by using Monte Carlo simulations with nanosecond steps. The results proved to correspond very well to measurements carried out at Philips on real OLEDs made at Dresden University of Technology. Factor of three One of the results is that the researchers can now predict where light is produced and lost in the ultra-thin layers. That makes it possible to optimize OLEDs so they produce the same amount of light using much less electric power. The researchers expect that the efficiency can still be increased by a factor of three. Manufacturers can also use this new knowledge to design OLEDs with specific colors. They can calculate in advance exactly how thick the different layers need to be, and how much pigment has to be added to the layers. The much shorter and less costly design process will allow the overall development costs to be reduced, leading to lower prices of the final products. "This has already been possible for a long time in the field of micro-electronics, with the ability to precisely predict the behavior of integrated circuits," says Bobbert. "Now we can do the same thing with OLEDs." The results were published online on Sunday 14 April 2013 in the journal Nature Materials.

Célula solar que consiste en una sola molécula

En la Universidad Técnica de Munich y la Universidad de Tel Aviv un grupo de científicos desarrollaron un método para medir las fotocorrientes de un único sistema funcionalizado de proteínas fotosintéticas. En el sistema se pudo demostrar que dicho sistema es capaz de integrarse y dirigirse de forma selectiva en arquitecturas de dispositivos fotovoltaicos artificiales conservando sus propiedades funcionales biomoleculares. Esto ocurre debido a que las proteínas actúan como bombas de electrones de una sola molécula inducidas por la luz y altamente eficientes, capaces de actuar como generadores de corriente en los circuitos eléctricos a escala nanométrica.

Se investigó que el centro de reacción del fotosistema-I es un complejo de proteína de clorofila localizado en las membranas de los cloroplastos de las cianobacterias. Las plantas, las algas y las bacterias utilizan la fotosíntesis para convertir la energía solar en energía química. Las etapas iniciales de este proceso en las que se absorbe la luz y se transfieren la energía y los electrones están mediadas por proteínas fotosintéticas compuestas de complejos de carotenoides y clorofila. Hasta ahora, ninguno de los métodos disponibles eran lo suficientemente sensibles como para medir las fotocorrientes generadas por una sola proteína El fotosistema-I exhibe excelentes propiedades optoelectrónicas que sólo se encuentran en los sistemas fotosintéticos. La dimensión de la nanoescala, además, hace del fotosistema-I una unidad prometedora para las aplicaciones en la optoelectrónica molecular.

El primer reto que los físicos tuvieron que amo fue el desarrollo de un método para ponerse en contacto eléctricamente moléculas individuales en fuertes campos ópticos. El elemento central del nanodispositivo son las proteínas fotosintéticas auto-ensamblado y unido covalentemente a un electrodo de oro a través de grupos de mutaciones de cisteína. La fotocorriente se midió por medio de una punta de vidrio recubierto de oro empleado en una exploración de campo cercano. Las proteínas fotosintéticas son ópticamente excitadas por un flujo de fotones guiado a través de la punta tetraédrica que al mismo tiempo proporciona el contacto eléctrico. Con esta técnica, los físicos fueron capaces de controlar la fotocorriente generada en las unidades individuales de la proteína.

El equipo interdisciplinario publicó los resultados en la revista Nature Nanotechnology, bajo el título "Photocurrent of a single photosynthetic protein"

Fotosistema-I (verde) es ópticamente excitado por un electrodo (en la parte superior). Un electrón es transferido a continuación, paso a paso, en sólo 16 nanosegundos. (Ilustración: Christoph Hohmann, Nanosistemas Iniciativa de Munich (NIM))

La investigación fue apoyada por la Fundación Alemana de Investigación (DFG) a través del SPP 1243 (subvenciones HO 3324/2 y RE 2592/2), la excelencia Clusters Munich Centro para la Fotónica avanzada y Nanosistemas Iniciativa de Munich, así como ERC Advanced GrantMolArt (no . 47299).

Fuentes: 

• http://www.nanowerk.com/news2/newsid=26855.php#ixzz2PQ37FyqS 
• Munich-Centre for Advanced Fotónica

Electrónica Plástica. Alineando Polímeros

Ingenieros de la Universidad de Michigan en conjunto con la compañía Samsung de Corea, desarrollaron un método para alinear un tipo de polímeros semiconductores, lo que fue comprobado por estudios de difracción de rayos-X y sugiere que es el principio de una electrónica plástica que se puede pintar, económica y "verde".  

De acuerdo con el científico de materiales Jingsang Kim, uno de los autores de este artículo de Nature Materials, "Esta es la primera capa de película delgada altamente alineada, conductora, para electrónica plástica de alto desempeño y que puede ser pintada". Los autores explican que desarrollaron el principio de un diseño molecular para hacer cristales líquidos liotrópicos de polímeros conjugados que son diferentes a los polímeros conjugados convencionales. Debido a su propiedad de cristal líquido liotrópico, estos polímeros conjugados tienen buenas propiedades de ensamblaje y al mismo tiempo, una buena movilidad.

El equipo de la Universidad de Michigan ha usado sus películas delgadas para crear un transistor simple pero funcional. Actualmente están desarrollando un método versátil de fabricación para realizar electrónica plástica de alto desempeño en varias escalas, desde nanómetros hasta metros.

Referencia: 

Kim BG, Jeong EJ, Chung JW, Seo S, Koo B, Kim J. A molecular design principle of lyotropic liquid-crystalline conjugated polymers with directed alignment capability for plastic electronics. Nature Materials, 2013.