Molecular Rotors as Switches

En este articulo describen un switch compuesto de rotores moleculares. El rotor se compone de una monocapa de compuestos de cobre ligados con actividad redox intercalados entre un electrodo de oro y un P+Si altamente dopado. La velocidad de rotación de este tipo de rotor molecular se encuentra en los picosegundos, lo que proporciona un potencial muy prometedor para las aplicaciones de alta velocidad de conmutación. El análisis del dispositivo de características corriente vs voltaje, sugiere que la fuente de los efectos observados de conmutación es resultado de la rotación inducida por ligando redox alrededor del centro de metal de cobre y esta atribución de conmutación es consistente con la dependencia de la temperatura observada del comportamiento de conmutación, así como el diagrama de energía propuesto del dispositivo (disponible en el articulo).

La conmutación de la resistencia observada demuestra el potencial para futuras memorias no volátiles y la lógica de aplicaciones de dispositivos. Además de la descripción del switch, es interesante que se discuten los avances y la perspectiva de dispositivos moleculares para la nanoelectrónica y otras aplicaciones. 

 Mei, X., & Kang L., W. (2012). Molecular Rotors as Switches. Sensors (14248220), 12(9), 11612-11637. doi:10.3390/s120911612 

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New method to measure OLED molecule behavior while device is working.

Last week the National Institute of Advanced Industrial Science and Technology (AIST) presented at the IDTechEx, Asia 2012 event. AIST have developed a method that can selectively measure the behavior of specific molecules at the interfaces of organicas layers in a multilayered organic light emitting diode (OLED) device during light emission in collaboration with Chemical Materials Evaluation and Research Base (CEREBA). The researchers have succeeded for the first time in measuring the behavior of the electric charges in a device at the molecular level.

 

The method uses an advanced laser spectroscopic technique that has been improved to measure the molecular vibrational spectrum at the interface of a specific organic layer inside an OLED device. By employing a signal enhancement phenomenon that occurs at the interface with a concentrated electric field, the method can be used to evaluate the molecular condition of the organic layer during light emission without destroying the device.

 

The method is expected to be useful for elucidating the deterioration of materials and deterioration of interfaces in a device on the basis of molecular level information. Such elucidation is necessary for extending the life of OLED devices.

Conceptual diagram of laser spectroscopy of a multilayeredOLED device during light emission

OLED devices are composed of stacked organic layers with different properties sandwiched between two electrodes. The device emits light when a voltage is applied to the electrodes. In ordinary multilayered OLED devices, between three and fiveorganic layers are stacked on a transparent electrode with an overall thickness of only about 200 nm. To eliminate the influence of oxygen and water, the device is tightly sealed with a desiccant (Fig. 1). The multilayered OLED device used for the measurement was composed of six different organic substances and had a brightness half-life (the length of time required for the brightness to deteriorate to 50% of the initial value) of more than 13,000 hours at brightness of 1000 cd/m2 (candelas per square meter).

In this effect, the wavelength of the visible light is adjusted to match the absorption wavelength (color) of the target organic substance, thus allowing only the target organic substance to be selectively brought to a high-energy state. The use of this double resonance effect makes it possible to eliminate the influence of other organic layers within the device and to enhance and isolate the signals from the target organic layer. In addition, to minimize damage that could be caused to the device by the laser light, improvements were made to the SFG spectrometer so that it could maintain its measurement resolution even when the laser power was reduced to less than 1/100 of that in conventional SFG spectrometers.

Figure shows the structure of an actual multilayered OLED device. The visible and infrared laser lights used in SFG spectroscopy were incident from the transparent substrate side when measurements were made. Even though an operating OLED device emits strong light, the SFG light has a different wavelength from the emitted light. Moreover, because the beam-like SFG light is emitted in a certain direction, a filter and two spectrometers were used to clearly separate the SFG light from the strong emitted light and then measure it.

For OLED devices to be applied to next-generation flat screen televisions, smartphones, and flexible devices, the degradation progressing in practical devices must be investigated in detail. Using the developed method, the researchers plan to continue to investigate the molecular-level status in operating OLED devices and in devices that have been in operation for long hours. They aim at the molecular-level elucidation of the driving mechanism of OLED devices and the degradation mechanism of them which is indispensable to the extension of device life.

 

The researchers also aim to apply the method to the evaluation and analysis of other organic electronic devices such as organicsolar cells and organic transistors.

Reference:

www.printedelectronicsworld.com/articles/new-method-to-measure-oled-molecule-behavior-while-device-is-working-00004796.asp?sessionid=1

Highly Transparent and Flexible Nanopaper Transistors

Renewable and clean “green” electronics based on paper substrates is an emerging field with intensifying research and commercial interests, as the technology combines the unique properties of flexibility, cost efficiency, recyclability, and renewability with the lightweight nature of paper. Because of its excellent optical transmittance and low surface roughness, nanopaper can host many types of electronics that are not possible on regular paper. However, there can be tremendous challenges with integrating devices on nanopaper due to its shape stability during processing. Here we demonstrate for the first time that flexible organic field-effect transistors (OFETs) with high transparency can be fabricated on tailored nanopapers. Useful electrical characteristics and an excellent mechanical flexibility were observed. It is believed that the large binding energy between polymer dielectric and cellulose nanopaper, and the effective stress release from the fibrous substrate promote these beneficial properties. Only a 10% decrease in mobility was observed when the nanopaper transistors were bent and folded. The nanopaper transistor also showed excellent optical transmittance up to 83.5%. The device configuration can transform many semiconductor materials for use in flexible green electronics.



Nanopaper with optimal transmittance and surface smoothness was prepared based on nanostructured cellulose fibers. Nanopaper shows much lower surface roughness and much higher transparency than traditional paper. Highly transparent and flexible OFETs were successfully fabricated on the properly designed nanopaper. The nanopaper OFETs exhibit good transistor electrical characteristics. To demonstrate the flexibility of nanopaper OFETs, devices were measured before and during bending. Only a 10.2% and a 9.8% decrease in mobility were observed when the device was bent in the direction parallel to the conduction channel direction and vertical to the conduction channel direction, respectively. These excellent optical, mechanical, and electrical properties suggest the great potential of nanopaper FETs in next-generation of flexible and transparent electronics and in a broad range of other cost-efficient and practical applications.

Reference
Highly Transparent and Flexible Nanopaper Transistors
Jia Huang, Hongli Zhu, Yuchen Chen, Colin Preston, Kathleen Rohrbach, John Cumings, and Liangbing Hu
ACS Nano 2013 7 (3), 2106-2113

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