Jorge Morgado

Near-field optical lithography of a conjugated polymer

We report the fabrication of poly(p-phenylene vinylene) nanostructures by direct scanning near-field lithography of its soluble precursor. The technique is based on the spatially selective inhibition of the precursor solubility by exposure to the ultraviolet optical field present at the apex of scanning near-field fiber probes with aperture diameters between 40 and 80 nm (+/-5 nm). After development in methanol and thermal conversion under vacuum we obtain features with a minimum dimension of 160 nm. We demonstrate the use of the technique for the direct writing of two-dimensional photonic crystals with intentional defects and a periodicity relevant to applications in the visible range. Using a Bethe-Bouwkamp model, we then discuss the influence of probe size, tip-sample distance, and film thickness on the resolution of the lithographic process. We also discuss limitations to the resolution that can arise from physical properties of the lithographic medium

Improved efficiency of light-emitting diodes based on polyfluorene blends upon insertion of a poly(p-phenylene vinylene) electron- confinement layer

We report the improvement of the electroluminescence efficiency of light-emitting diodes (LEDs) based on polyfluorene blends, upon insertion of a thin film of poly(p-phenylene vinylene), PPV, between a hole-injection layer of poly(3,4-ethylene dioxythiophene), doped with polystyrene sulfonic acid, and the polyfluorenes emissive layer. For LEDs using a blend of poly(9,9′-dioctylfluorene), with 5 wt % of the green emitter poly(9,9′-dioctylfluorene-altbenzothiadiazole), and calcium cathodes, the efficiency increases from 2.1 to 4.1 cd/A upon insertion of such a PPV layer. We propose that such an improvement is mainly due to the electron-blocking effect of the PPV layer, leading to improved charge carriers balance within the emissive layer.

Synthesis and luminescence properties of three novel polyfluorene copolymers

We report on the synthesis and characterization (including structural, optical, electrochemical and electroluminescence properties) of three alternating F-alt-X copolymers, where F is 9,9-bis(2 0 -ethylhexyl)fluorene unit and the X comonomer varies from a phenylene, to a thiophene and to a thiophene-S,S-dioxide unit. Among these X comonomers, the phenylene group is at the origin of a blue-emitting copolymer with unitary luminescence efficiency in solution, while thiophene-S,S-dioxide promotes the highest electron affinity. These copolymers are also used in the fabrication of light-emitting diodes. q 2003 Elsevier Science Ltd. All rights reserved.

Neural stem cell differentiation by electrical stimulation using a cross-linked PEDOT substrate: Expanding the use of biocompatible conjugated conductive polymers for neural tissue engineering.

BACKGROUND The use of conjugated polymers allows versatile interactions between cells and flexible processable materials, while providing a platform for electrical stimulation, which is particularly relevant when targeting differentiation of neural stem cells and further application for therapy or drug screening. METHODS Materials were tested for cytotoxicity following the ISO10993-5. PEDOT PSS was cross-linked. ReNcellVM neural stem cells (NSC) were seeded in laminin coated surfaces, cultured for 4 days in the presence of EGF (20 ng/mL), FGF-2 (20 ng/mL) and B27 (20 μg/mL) and differentiated over eight additional days in the absence of those factors under 100Hz pulsed DC electrical stimulation, 1V with 10 ms pulses. NSC and neuron elongation aspect ratio as well as neurite length were assessed using ImageJ. Cells were immune-stained for Tuj1 and GFAP. RESULTS F8T2, MEH-PPV, P3HT and cross-linked PEDOT PSS (x PEDOT PSS) were assessed as non-cytotoxic. L929 fibroblast population was 1.3 higher for x PEDOT PSS than for glass control, while F8T2 presents moderate proliferation. The population of neurons (Tuj1) was 1.6 times higher with longer neurites (73 vs 108 μm) for cells cultured under electrical stimulus, with cultured NSC. Such stimulus led also to longer neurons. CONCLUSIONS x PEDOT PSS was, for the first time, used to elongate human NSC through the application of pulsed current, impacting on their differentiation towards neurons and contributing to longer neurites. GENERAL SIGNIFICANCE The range of conductive conjugated polymers known as non-cytotoxic was expanded. x PEDOT PSS was introduced as a stable material, easily processed from solution, to interface with biological systems, in particular NSC, without the need of in-situ polymerization.

De‐mixing of Polyfluorene‐Based Blends by Contact with Acetone: Electro‐ and Photo‐luminescence Probes

De-mixing of Polyfluorene Based Blends by Contact with Acetone: Electro-and Photoluminescence Probes

The [(DT‐TTF)2M(mnt)2] Family of Radical Ion Salts: From a Spin Ladder to Delocalised Conduction Electrons That Interact with Localised Magnetic Moments

Electrocrystallisation of the p-electron donor dithiopheno-tetrathia- fulvalene (DT-TTF) with maleonitrile dithiolate (mnt) - metal (M) complexes gives rise to the new family of radical ion salts ((DT-TTF)2M(mnt)2 )( MaAu, Pt, Ni), which are isostructural and crystal- lise in the monoclinic space group P21/n forming regular segregated stacks of donor and acceptor molecules along the b axis. The DT-TTF stacks are paired and interact strongly through S ··· S contacts in a ladder-like motif. The three salts have quite high room-temperature electrical conductivities (9, 40 and 40 S cm ˇ1 for Ma Au, Pt and Ni respec- tively) but their conductivity - tempera- ture dependencies differ. The Au salt has an activated conductivity at room temperature whereas the Ni and Pt salts are metal-like at room temperature and both exhibit a metal - insulator transi- tion around 120 K. These contrasting transport properties are accounted for by the differences in the transfer inte- grals along the DT-TTF stacks. The magnetic susceptibility of the salt with Ma Au, in which the (Au(mnt)2 ˇ ) anion is diamagnetic, can be fitted to a two- legged spin-ladder model. From diffuse X-ray scattering studies it is established that below 220 K the donors dimerise along the b stacking direction, and the spin carrier units in the ladder are identified as those formed by dimerised donors ((DT-TTF)2) a . . Observation in their EPR spectra of a single line which increases dramatically in width as the conductivity increases is evidence for the presence of two magnetic subsys- tems which interact in the salts (Ma Ni, Pt) with paramagnetic (M(mnt)2) ˇ ions.

Light-emitting devices based on a poly(p-phenylene vinylene) derivative with ion-coordinating side groups

We report the phase-separation and the optoelectronic properties of an alternating poly(p-phenylene vinylene)-based copolymer, to which poly(ethylene oxide), PEO, and or lithium triflate are added. The alternating units of this copolymer (DB-BTEM-PPV) are 2,3-dibutoxy-1,4-phenylene vinylene, and 2,5-bis(triethoxymethoxy)-1,4-phenylene vinylene, a moiety containing side groups allowing ion solvation and transport. Upon addition of the ion-transporting polymer PEO to DB-BTEM-PPV blended with lithium triflate, we have found a sizeable efficiency increase (from 0.9 to 1.5 cd/A), with a concomitant increase of the response time. We propose that this is due to solvation and complexation of lithium triflate by the PEO, which simultaneously reduces the quenching of photoluminescence (and electroluminescence) efficiency by the ionic charge, and the effectiveness of formation of highly doped, low-barrier, polymer/electrode interfaces. We discuss charge transport and injection in the copolymer and in the blend with ...

4-AcNI—a new polymer for light-emitting diodes

Abstract We report investigations on a 4-acetamido-substituted naphthalimide polymer (4-AcNI) as a component for light-emitting diodes (LEDs). Devices prepared with 4-AcNI as the active material are relatively inefficient (4 × 10−4%). Higher internal quantum efficiency values of 0.4% are achieved if 4-AcNI is used as an electron-transporting layer (ETL) in devices which use poly(p-phenylene vinylene) (PPV) as a hole transport layer. For Al cathodes, this material proves to be superior to similar devices which use 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) as the ETL. The results are used to shed some light on the function of ETLs in organic LEDs.

Environmental aging of poly(p-phenylenevinylene) based light-emitting diodes

Abstract We report the effect of environmental aging of poly( p -phenylenevinylene) (PPV) on the efficiency of light-emitting diodes (LEDs) with calcium and aluminium cathodes. We prepared both single- and double-layer structures with an electron-transporting/hole-blocking layer of 2-(4-biphenylyl)-5-butylphenyl-1,3,4-oxadiazole (PBD) blended with poly(methyl metacrylate), inserted between the PPV and the calcium cathode. PPV was prepared via the precursor route and aged prior to the cathode deposition by exposing it to nitrogen (in a glove box) or air, and either in the dark, or in visible daylight. We have found that the reduction of electroluminescence (EL) efficiency after illumination in air is large for the single-layer devices, but much smaller, and comparable to the reduction of the photoluminescence (PL) efficiency, for the double-layer diodes. Surprisingly, aging in air, in the dark, does not alter significantly the efficiency of the Ca devices, whereas it increases that of Al/PPV/ITO by almost an order of magnitude. We interpret the influence of the different aging conditions in terms of a combination of photooxidation and doping of PPV by oxygen and water vapor.

Ultraviolet–visible near-field microscopy of phase-separated blends of polyfluorene-based conjugated semiconductors

We have used fluorescence scanning near-field microscopy to characterize polymer blends for electroluminescent applications, and thereby identify compositional nonhomogeneities. In particular, we have focused on the binary system constituted by poly(9,9′-dioctylfluorenealt-benzothiadiazole) and poly(9,9′-dioctylfluorene) (PFO), known to give efficiencies of up to 22 cd/A in light-emitting devices with suitable electrodes. Our primary aim was the assignment of the morphological features revealed in shear-force and atomic-force images of spin-coated films, and suggestive of phase separation on a 300-nm-length scale. From analysis of the fluorescence images (325 and 488 nm excitation), and quantitative correlation of optical and topographic data, we identify the raised features with PFO-rich regions. However, the limited variation in fluorescence intensity reveals a high extent of mixing within each phase on the length scale accessible in our experiment, approximately 100 nm for our focused-ion-beam-processe...