Oliver Fenwick

Thermochemical nanopatterning of organic semiconductors

Patterning of semiconducting polymers on surfaces is important for various applications in nanoelectronics and nanophotonics. However, many of the approaches to nanolithography that are used to pattern inorganic materials are too harsh for organic semiconductors, so research has focused on optical patterning and various soft lithographies. Surprisingly little attention has been paid to thermal, thermomechanical and thermochemical patterning. Here, we demonstrate thermochemical nanopatterning of poly(p-phenylene vinylene), a widely used electroluminescent polymer, by a scanning probe. We produce patterned structures with dimensions below 28 nm, although the tip of the probe has a diameter of 5 microm, and achieve write speeds of 100 microm s(-1). Experiments show that a resolution of 28 nm is possible when the tip-sample contact region has dimensions of approximately 100 nm and, on the basis of finite-element modelling, we predict that the resolution could be improved by using a thinner resist layer and an optimized probe. Thermochemical lithography offers a versatile, reliable and general nanopatterning technique because a large number of optical materials, including many commercial crosslinker additives and photoresists, rely on chemical mechanisms that can also be thermally activated.

Large Work Function Shift of Gold Induced by a Novel Perfluorinated Azobenzene-Based Self-Assembled Monolayer

Tune it with light! Self-assembled monolayers on gold based on a chemisorbed novel azobenzene derivative with a perfluorinated terminal phenyl ring are prepared. The modified substrate shows a significant work function increase compared to the bare metal. The photo-conversion between trans and cis isomers chemisorbed on the surface shows great perspectives for being an accessible route to tune the gold properties by means of light.

Photoinduced work function changes by isomerization of a densely packed azobenzene-based SAM on Au: a joint experimental and theoretical study.

Responsive monolayers are key building blocks for future applications in organic and molecular electronics in particular because they hold potential for tuning the physico-chemical properties of interfaces, including their energetics. Here we study a photochromic SAM based on a conjugated azobenzene derivative and its influence on the gold work function (Φ(Au)) when chemisorbed on its surface. In particular we show that the Φ(Au) can be modulated with external stimuli by controlling the azobenzene trans/cis isomerization process. This phenomenon is characterized experimentally by four different techniques, kelvin probe, kelvin probe force microscopy, electroabsorption spectroscopy and ultraviolet photoelectron spectroscopy. The use of different techniques implies exposing the SAM to different measurement conditions and different preparation methods, which, remarkably, do not alter the observed work function change (Φ(trans)-Φ(cis)). Theoretical calculations provided a complementary insight crucial to attain a deeper knowledge on the origin of the work function photo-modulation.

Near‐Infrared Polymer Light‐Emitting Diodes Based on Low‐Energy Gap Oligomers Copolymerized into a High‐Gap Polymer Host

Near-infrared (NIR) polymer light-emitting diodes (PLEDs) based on a fluorene-dioctyloxyphenylene wide-gap host material copolymerized with a low-gap emitter are presented. Various loadings (1, 2.5, 10, 20 mol%) of the low-gap emitter are studied, with higher loadings leading to decreased efficiencies likely due to aggregation effects. While the 10 mol% loading resulted in almost pure NIR emission (>99.6%), the 1 mol% loading yielded optimum device performance, which is among the best reported to date for a unblended single-layer pure polymer emitter, with an external quantum efficiencies of 0.04% emitting at 909 nm. The high spectral purity of the PLEDs combined with their performance support the methodology of copolymerization as an effective strategy for developing NIR PLEDs.

Polymorphism, Fluorescence, and Optoelectronic Properties of a Borazine Derivative

We have prepared a new borazine derivative that bears mesityl substituents at the boron centers and displays exceptional chemical stability. Detailed crystallographic and solid-state fluorescence characterizations revealed the existence of several polymorphs, each of which showed different emission profiles. In particular, a bathochromic shift is observed when going from the lower- to the higher-density crystal. Computational investigations of the conformational dynamics of borazine 1 in both the gas phase and in the solid state using molecular dynamics (MD) simulations showed that the conformation of the peripheral aryl groups significantly varies when going from an isolated molecule (in which the rings are able to flip over the 90° barrier at RT) to the crystals (in which the rotation is locked by packing effects), thus generating specific nonsymmetric intermolecular interactions in the different polymorphs. To investigate the optoelectronic properties of these materials by fabrication and characterization of light-emitting diodes (LEDs) and light-emitting electrochemical cells (LECs), borazine 1 was incorporated as the active material in the emissive layer. The current and radiance versus voltage characteristics, as well as the electroluminescence spectra reported here for the first time are encouraging prospects for the engineering of future borazine-based devices.

Optical probing of sample heating in scanning near-field experiments with apertured probes

We have used the inherent thermochromism of conjugated polymers to investigate substrate heating effects in scanning near-field experiments with metal-coated “apertured” probes. Chemically etched and pulled fibers were used to provide near-field excitation of fully converted films of poly(p-phenylene vinylene), PPV, and of poly(4,4′-diphenylene diphenylvinylene). We detect no significant blueshift of the photoluminescence spectra generated with near-field excitation, in comparison to those collected with far-field excitation. We conclude that polymer heating in the region contributing to the luminescence is less than 40K. We also demonstrate that thermolithography of the PPV precursor is not significant by comparing UV (325nm) and red (670nm) illumination.

Efficient red electroluminescence from diketopyrrolopyrrole copolymerised with a polyfluorene

We report the synthesis, characterization, and device incorporation of copolymers based on a common green-emitting polyfluorene but containing a small proportion of a low energy gap donor-acceptor-donor unit for red emission in photo- and electro-luminescence. At just 1%–3% random incorporation, the low-gap unit is not present on all chains, yet we demonstrate that efficient charge and energy transfer can yield electroluminescent devices with 1% quantum efficiency and a color that can be tuned by adjusting the density of low-gap units to achieve primary red (National Television System Committee). The high current density tail off in the efficiency is reduced by replacing the hole-injection layer with a photochemically cross-linked electron‑blocking layer.

Shape dependent thermal effects in apertured fiber probes for scanning near-field optical microscopy

Metal-coated, “pulled,” and conically shaped fiber probes used in scanning near-field optical microscopy (SNOM) typically undergo a thermal expansion when injected with laser light, due to partial energy absorption by the metallic film. Here, we report investigations into the thermal behavior of fiber probes produced by selective chemical etching that in our experience provide high light throughputs (10−3–10−4 vs 10−6 for the pulled fibers). Unexpectedly, we find a shortening of such probes in response to “high-power” laser injection (>1mW). Thermal stress due to prolonged high-power laser injection (∼9mW at 325nm; compared to powers <1mW often used in SNOM experiments) determines permanent alterations of the probes, after which their thermomechanical behavior reverts to the commonly observed elongation in response to laser injection. Scanning electron microscopy after high-power irradiation on such probes shows partial detachment of the metallic coating near the fiber termination. This, however, does not...

Observation of tip-to-sample heat transfer in near-field optical microscopy using metal-coated fiber probes

Metal-coated scanning near-field optical microscopy fiber probes can undergo significant heating due to partial absorption of the coupled light by the metallic film covering the apical zone. In this letter we report experimental evidence of tip-to-sample heat transfer on a 7,7′,8,8′-tetracyanoquinodimethane molecular crystal. Local melting is observed at nanometric tip–sample distances, when increasing the laser power injected into the fiber above a threshold of 8.8mW. Hole formation and material displacement are observed, as well as failure of the shear-force-based imaging process, due to partial sticking of the melted material to the probe.

Luminescent properties of a water-soluble conjugated polymer incorporating graphene-oxide quantum dots.

We report the incorporation of graphene-oxide quantum dots (GOQDs) into films, diluted solutions, and light-emitting diodes (LEDs) as part of a water-soluble derivative of poly(p-phenylene vinylene), or PDV.Li, to investigate their impact on the light-emission properties of this model conjugated polymer. Despite the well-known ability of graphene and graphene oxide to quench the photoluminescence of nearby emitters, we find that the addition of GOQDs to diluted solutions of PDV.Li does not significantly affect the photoluminescence (PL) dynamics of PDV.Li, bringing about only a modest quenching of the PL. However, loading the polymer with GOQDs led to a substantial decrease in the turn-on voltage of LEDs based on GOQD-PDV.Li composites. This effect can be attributed to either the improved morphology of the host polymer, resulting in an increase in the charge mobility, or the enhanced injection through GOQDs near the electrodes.