Oliver Fenwick - Academia.edu (original) (raw)

Papers by Oliver Fenwick

Small, 2011

Kelvin probe force microscopy (KPFM) is usually applied to map the local surface potential of nan... more Kelvin probe force microscopy (KPFM) is usually applied to map the local surface potential of nanostructured materials at surfaces and interfaces. KPFM is commonly defi ned as a 'surface technique', even if this assumption is not fully justifi ed. However, a quantifi cation of the surface sensitivity of this technique is crucial to explore electrical properties at the nanoscale. Here a versatile 3D model is presented which provides a quantitative explanation of KPFM results, taking into account the vertical structure of the sample. The model is tested on nanostructured fi lms obtained from two relevant semiconducting systems for fi eld-effect transistor and solar cell applications showing different interfacial properties, i.e., poly(3-hexylthiophene) (P3HT) and perylenebis-dicarboximide (PDI). These fi ndings are especially important since they enable quantitative determination of the local surface potential of conjugated nanostructures, and thereby pave the way towards optimization of the electronic properties of nanoscale architectures for organic electronic applications.

ABSTRACT Nanostructured optical microfiber tips are proposed and experimentally demonstrated to e... more ABSTRACT Nanostructured optical microfiber tips are proposed and experimentally demonstrated to efficiently confine light beyond the diffraction limit at high powers. Focused ion beam milling was used for the nanostructuring of gold-coated optical microfiber tips with both single-ramp and wedge geometries. Small apertures were formed by flat cutting or hole drilling and optical spot sizes of similar to lambda/10 with high transmission efficiency were achieved. Numerical simulations were carried out to optimize the device design with circularly polarized light. Enhanced transmission efficiencies (higher than 10(-2)) were recorded by optimizing the overall light throughput along the fiber tips. The tip thermal behavior was investigated by launching high powers into the device and recording the tip position in a scanning near-field optical microscopy set-up. This nanostructured optical microfiber tip has the potential for applications in optical recording, scanning near-field optical microscopy and lithography. (C) 2012 Elsevier B.V. All rights reserved.

Nanoscale, 2014

We describe the synthesis of a novel biphenyl azobenzene derivative exhibiting: (i) a protected t... more We describe the synthesis of a novel biphenyl azobenzene derivative exhibiting: (i) a protected thiol anchoring group in the a-position to readily form self-assembled monolayers (SAMs) on Au surfaces; and (ii) a terminal perfluorinated benzene ring in the u-position to modify the surface properties. The design of this molecule ensured both an efficient in situ photoswitching between the trans and cis isomers when chemisorbed on Au(111), due to the presence of a biphenyl bridge between the thiol protected anchoring group and the azo dye, and a significant variation of the work function of the SAM in the two isomeric states, induced by the perfluorinated phenyl head group. By exploiting the light responsive nature of the chemisorbed molecules, it is possible to dynamically modify in situ the work function of the SAM-covered electrode, as demonstrated both experimentally and by quantum-chemical calculations, revealing changes in work function up to 220 meV. These findings are relevant for tuning the work function of metallic electrodes, and hence to dynamically modulate charge injection at metalsemiconductor interfaces for organic opto-electronic applications.

Synthetic Metals, 2004

Scanning probe techniques such as atomic force microscopy and scanning tunnelling microscopy have... more Scanning probe techniques such as atomic force microscopy and scanning tunnelling microscopy have been used extensively for the study of surface properties on the nanoscale. A valuable, but less commonly used technique is scanning near-field optical microscopy (SNOM). This scanning probe technique allows the optical properties of supramolecular structures to be probed at resolutions better than allowed by the diffraction limit. This means that resolution better than 100 nm is achievable even with visible radiation. However, the geometry of the SNOM probe causes images obtained by this method to contain a mixture of optical and topographical information. Where there is no optical contrast on a surface, topography can introduce contrast to the image, which is obviously undesirable. Additionally, the topographical artifacts can cause apparent distortion or displacement of features in the optical image. To have confidence in results obtained by SNOM, the nature of topographical artifacts must be fully understood. Here we present one-dimensional numerical simulations of SNOM images and an investigation into several observable effects. In addition to the occurrence of topography-induced optical contrast we show that a low topographical feature near a higher one can be visible in an image but with low intensity and can appear displaced and distorted if it is approximately a probe diameter from the larger feature.

Synthetic Metals, 2008

Polyfluorene copolymers are functionalised with oxetane side groups which make them cross-linkabl... more Polyfluorene copolymers are functionalised with oxetane side groups which make them cross-linkable upon exposure to an acid. Upon addition of a photoacid they become photo-cross-linkable if exposed to UV light. Optical and electrochemical properties of these copolymers, both in the soluble and crosslinked forms are compared. We further show that the use of this type of copolymer allows the fabrication of multilayer structures without restriction of the organic solvent used to deposit an upper layer. In particular, we fabricate multilayer LEDs with enhanced luminous efficiency using one of the copolymers. The possibility to photo-cross-link these materials is also used to prepare micrometer size patterns, using a transmission electron microscope grid as a shadow mask. Furthermore, the use of a scanning near-field optical microscope allows submicron patterns to be prepared. We propose that this approach, which can be extended to a wide range of conjugated polymers, can be useful for the fabrication of optical, electronic and optoelectronic structures.

Small, 2011

Kelvin probe force microscopy (KPFM) is usually applied to map the local surface potential of nan... more Kelvin probe force microscopy (KPFM) is usually applied to map the local surface potential of nanostructured materials at surfaces and interfaces. KPFM is commonly defi ned as a 'surface technique', even if this assumption is not fully justifi ed. However, a quantifi cation of the surface sensitivity of this technique is crucial to explore electrical properties at the nanoscale. Here a versatile 3D model is presented which provides a quantitative explanation of KPFM results, taking into account the vertical structure of the sample. The model is tested on nanostructured fi lms obtained from two relevant semiconducting systems for fi eld-effect transistor and solar cell applications showing different interfacial properties, i.e., poly(3-hexylthiophene) (P3HT) and perylenebis-dicarboximide (PDI). These fi ndings are especially important since they enable quantitative determination of the local surface potential of conjugated nanostructures, and thereby pave the way towards optimization of the electronic properties of nanoscale architectures for organic electronic applications.

Physical Chemistry Chemical Physics, 2011

Responsive monolayers are key building blocks for future applications in organic and molecular el... more 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 (F Au ) when chemisorbed on its surface. In particular we show that the F 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 (F trans -F cis ). Theoretical calculations provided a complementary insight crucial to attain a deeper knowledge on the origin of the work function photo-modulation.

Nature Nanotechnology, 2009

Patterning of semiconducting polymers on surfaces is important for various applications in nanoel... more 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 1-3 and various soft lithographies 4 . Surprisingly little attention has been paid to thermal 5 , thermomechanical 6,7 and thermochemical 8-13 patterning. Here, we demonstrate thermochemical nanopatterning of poly( p-phenylene vinylene), a widely used electroluminescent polymer 14 , by a scanning probe. We produce patterned structures with dimensions below 28 nm, although the tip of the probe has a diameter of 5 mm, and achieve write speeds of 100 mm s 21 . Experiments show that a resolution of 28 nm is possible when the tip-sample contact region has dimensions of 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 .

Nano Letters, 2011

Here we report organic light-emitting diodes incorporating linear and cyclic porphyrin hexamers w... more Here we report organic light-emitting diodes incorporating linear and cyclic porphyrin hexamers which have red-shifted emission (λ(PL) = 873 and 920 nm, respectively) compared to single porphyrin rings as a consequence of their extended π-conjugation. We studied the photoluminescence and electroluminescence of blends with poly(9,9'-dioctylfluorene-alt-benzothiadiazole), demonstrating a high photoluminescence quantum efficiency of 7.7% for the linear hexamer when using additives to prevent aggregation and achieving high color purity near-infrared electroluminescence.

Nano Letters, 2008

Effective nanoscale control of intermolecular interactions in conjugated polymers is needed for t... more Effective nanoscale control of intermolecular interactions in conjugated polymers is needed for the optimal development and exploitation of the latter in low-cost, large-area consumer electronics items, such as light-emitting and photovoltaic diodes, or transistors. Here we report our investigations on insulated molecular wires constituted by conjugated polymers threaded into cyclodextrin rings. Until now, there has been no detailed quantitative understanding of the role of progressive cyclodextrin encapsulation (quantifiable by the so-called "threading ratio", TR, or number of cyclodextrins per repeat unit) in tailoring the photophysics of the conjugated polymeric wires. We combine spectroscopic, electrical and surface analysis techniques to elucidate how the TR of cyclodextrin-threaded molecular wires controls formation of interchain species and related physical properties (0 < TR < or = 2.3; the maximum theoretical TR for close-packed CDs is 2.8). Increasing TR enhances the solid-state photoluminescence (PL) and electroluminescence quantum efficiency. To unravel the effect of progressive encapsulation on the intrachain decay kinetics of the polymer backbone, we added an electron-accepting quenching agent, methyl viologen (MV), to the polymer solutions. MV predominantly quenches the aggregate PL, thus enabling measurement of the decay kinetics of the intrinsic exciton even for low-TR polyrotaxanes, for which the different contributions are otherwise difficult to disentangle.

Macromolecular Rapid Communications, 2013

Near-infrared (NIR) polymer light-emitting diodes (PLEDs) based on a fluorene-dioctyloxyphenylene... more 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.

Journal of Applied Physics, 2006

Metal-coated, "pulled," and conically shaped fiber probes used in scanning near-field optical mic... more Metal-coated, "pulled," and conically shaped fiber probes used in scanning near-field optical microscopy ͑SNOM͒ 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 fibers͒. Unexpectedly, we find a shortening of such probes in response to "high-power" laser injection ͑Ͼ1 mW͒. Thermal stress due to prolonged high-power laser injection ͑ϳ9 mW at 325 nm; compared to powers Ͻ1 mW often used in SNOM experiments͒ 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 appear to compromise the probe's performance in terms of light confinement outside the aperture area, suggesting that the detachment only affects the coating over the fiber cladding and confirming the operational robustness of these probes. In comparison, tube-etched, conical probes display substantial damage of the coating, up to several microns from the apex, after being injected with a comparable high-power laser beam ͑Ͼ10 mW at 633 nm͒. Although the vertical feedback mechanism of the microscope can compensate for dilations/contractions of the probes, these findings are of general importance to the field. More specifically they are significant for the achievement of a detailed understanding of apertured-SNOM operation, for the selection and operation of near-field probes, and for preventing potential artifacts in imaging and lithography, due to uncontrolled alteration of the probe properties and/or light leakage from cracks of the opaque coating induced by thermal fatigue. In addition, our results demonstrate that it is important for probe design to also consider the probe's thermal regime during operation, so as to prevent cracks in the functional parts of the coating and thus spurious, undesired sample illumination from regions other than the probe intended aperture.

Journal of Applied Physics, 2012

Facile large-area photolithography of periodic sub-micron structures using a self-formed polymer ... more Facile large-area photolithography of periodic sub-micron structures using a self-formed polymer mask Appl. Phys. Lett. 100, 233503 (2012) Controlled addressing of quantum dots by nanowire plasmons Appl. Phys. Lett. 100, 231102 Effects of tip-substrate gap, deposition temperature, holding time, and pull-off velocity on dip-pen lithography investigated using molecular dynamics simulation J. Appl. Phys. 111, 103521 (2012) Unbiased line width roughness measurements with critical dimension scanning electron microscopy and critical dimension atomic force microscopy J. Appl. Phys. 111, 084318 Metallic nanomesh electrodes with controllable optical properties for organic solar cells

Dyes and Pigments, 2013

Synthesis, isolation and characterization of diketopyrrolopyrrole (DPP) dimers are described. The... more Synthesis, isolation and characterization of diketopyrrolopyrrole (DPP) dimers are described. These derivatives were prepared starting from symmetric or asymmetric DPP monomers through Pd-catalyzed homocoupling, Yamamoto coupling or Suzuki coupling. The spectroscopic properties of these newly synthesized DPP dimers were investigated and were supported by quantum-chemical calculations.

Chemistry - A European Journal, 2013

Small, 2011

Kelvin probe force microscopy (KPFM) is usually applied to map the local surface potential of nan... more Kelvin probe force microscopy (KPFM) is usually applied to map the local surface potential of nanostructured materials at surfaces and interfaces. KPFM is commonly defi ned as a 'surface technique', even if this assumption is not fully justifi ed. However, a quantifi cation of the surface sensitivity of this technique is crucial to explore electrical properties at the nanoscale. Here a versatile 3D model is presented which provides a quantitative explanation of KPFM results, taking into account the vertical structure of the sample. The model is tested on nanostructured fi lms obtained from two relevant semiconducting systems for fi eld-effect transistor and solar cell applications showing different interfacial properties, i.e., poly(3-hexylthiophene) (P3HT) and perylenebis-dicarboximide (PDI). These fi ndings are especially important since they enable quantitative determination of the local surface potential of conjugated nanostructures, and thereby pave the way towards optimization of the electronic properties of nanoscale architectures for organic electronic applications.

ABSTRACT Nanostructured optical microfiber tips are proposed and experimentally demonstrated to e... more ABSTRACT Nanostructured optical microfiber tips are proposed and experimentally demonstrated to efficiently confine light beyond the diffraction limit at high powers. Focused ion beam milling was used for the nanostructuring of gold-coated optical microfiber tips with both single-ramp and wedge geometries. Small apertures were formed by flat cutting or hole drilling and optical spot sizes of similar to lambda/10 with high transmission efficiency were achieved. Numerical simulations were carried out to optimize the device design with circularly polarized light. Enhanced transmission efficiencies (higher than 10(-2)) were recorded by optimizing the overall light throughput along the fiber tips. The tip thermal behavior was investigated by launching high powers into the device and recording the tip position in a scanning near-field optical microscopy set-up. This nanostructured optical microfiber tip has the potential for applications in optical recording, scanning near-field optical microscopy and lithography. (C) 2012 Elsevier B.V. All rights reserved.

Nanoscale, 2014

We describe the synthesis of a novel biphenyl azobenzene derivative exhibiting: (i) a protected t... more We describe the synthesis of a novel biphenyl azobenzene derivative exhibiting: (i) a protected thiol anchoring group in the a-position to readily form self-assembled monolayers (SAMs) on Au surfaces; and (ii) a terminal perfluorinated benzene ring in the u-position to modify the surface properties. The design of this molecule ensured both an efficient in situ photoswitching between the trans and cis isomers when chemisorbed on Au(111), due to the presence of a biphenyl bridge between the thiol protected anchoring group and the azo dye, and a significant variation of the work function of the SAM in the two isomeric states, induced by the perfluorinated phenyl head group. By exploiting the light responsive nature of the chemisorbed molecules, it is possible to dynamically modify in situ the work function of the SAM-covered electrode, as demonstrated both experimentally and by quantum-chemical calculations, revealing changes in work function up to 220 meV. These findings are relevant for tuning the work function of metallic electrodes, and hence to dynamically modulate charge injection at metalsemiconductor interfaces for organic opto-electronic applications.

Synthetic Metals, 2004

Scanning probe techniques such as atomic force microscopy and scanning tunnelling microscopy have... more Scanning probe techniques such as atomic force microscopy and scanning tunnelling microscopy have been used extensively for the study of surface properties on the nanoscale. A valuable, but less commonly used technique is scanning near-field optical microscopy (SNOM). This scanning probe technique allows the optical properties of supramolecular structures to be probed at resolutions better than allowed by the diffraction limit. This means that resolution better than 100 nm is achievable even with visible radiation. However, the geometry of the SNOM probe causes images obtained by this method to contain a mixture of optical and topographical information. Where there is no optical contrast on a surface, topography can introduce contrast to the image, which is obviously undesirable. Additionally, the topographical artifacts can cause apparent distortion or displacement of features in the optical image. To have confidence in results obtained by SNOM, the nature of topographical artifacts must be fully understood. Here we present one-dimensional numerical simulations of SNOM images and an investigation into several observable effects. In addition to the occurrence of topography-induced optical contrast we show that a low topographical feature near a higher one can be visible in an image but with low intensity and can appear displaced and distorted if it is approximately a probe diameter from the larger feature.

Synthetic Metals, 2008

Polyfluorene copolymers are functionalised with oxetane side groups which make them cross-linkabl... more Polyfluorene copolymers are functionalised with oxetane side groups which make them cross-linkable upon exposure to an acid. Upon addition of a photoacid they become photo-cross-linkable if exposed to UV light. Optical and electrochemical properties of these copolymers, both in the soluble and crosslinked forms are compared. We further show that the use of this type of copolymer allows the fabrication of multilayer structures without restriction of the organic solvent used to deposit an upper layer. In particular, we fabricate multilayer LEDs with enhanced luminous efficiency using one of the copolymers. The possibility to photo-cross-link these materials is also used to prepare micrometer size patterns, using a transmission electron microscope grid as a shadow mask. Furthermore, the use of a scanning near-field optical microscope allows submicron patterns to be prepared. We propose that this approach, which can be extended to a wide range of conjugated polymers, can be useful for the fabrication of optical, electronic and optoelectronic structures.

Small, 2011

Kelvin probe force microscopy (KPFM) is usually applied to map the local surface potential of nan... more Kelvin probe force microscopy (KPFM) is usually applied to map the local surface potential of nanostructured materials at surfaces and interfaces. KPFM is commonly defi ned as a 'surface technique', even if this assumption is not fully justifi ed. However, a quantifi cation of the surface sensitivity of this technique is crucial to explore electrical properties at the nanoscale. Here a versatile 3D model is presented which provides a quantitative explanation of KPFM results, taking into account the vertical structure of the sample. The model is tested on nanostructured fi lms obtained from two relevant semiconducting systems for fi eld-effect transistor and solar cell applications showing different interfacial properties, i.e., poly(3-hexylthiophene) (P3HT) and perylenebis-dicarboximide (PDI). These fi ndings are especially important since they enable quantitative determination of the local surface potential of conjugated nanostructures, and thereby pave the way towards optimization of the electronic properties of nanoscale architectures for organic electronic applications.

Physical Chemistry Chemical Physics, 2011

Responsive monolayers are key building blocks for future applications in organic and molecular el... more 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 (F Au ) when chemisorbed on its surface. In particular we show that the F 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 (F trans -F cis ). Theoretical calculations provided a complementary insight crucial to attain a deeper knowledge on the origin of the work function photo-modulation.

Nature Nanotechnology, 2009

Patterning of semiconducting polymers on surfaces is important for various applications in nanoel... more 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 1-3 and various soft lithographies 4 . Surprisingly little attention has been paid to thermal 5 , thermomechanical 6,7 and thermochemical 8-13 patterning. Here, we demonstrate thermochemical nanopatterning of poly( p-phenylene vinylene), a widely used electroluminescent polymer 14 , by a scanning probe. We produce patterned structures with dimensions below 28 nm, although the tip of the probe has a diameter of 5 mm, and achieve write speeds of 100 mm s 21 . Experiments show that a resolution of 28 nm is possible when the tip-sample contact region has dimensions of 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 .

Nano Letters, 2011

Here we report organic light-emitting diodes incorporating linear and cyclic porphyrin hexamers w... more Here we report organic light-emitting diodes incorporating linear and cyclic porphyrin hexamers which have red-shifted emission (λ(PL) = 873 and 920 nm, respectively) compared to single porphyrin rings as a consequence of their extended π-conjugation. We studied the photoluminescence and electroluminescence of blends with poly(9,9'-dioctylfluorene-alt-benzothiadiazole), demonstrating a high photoluminescence quantum efficiency of 7.7% for the linear hexamer when using additives to prevent aggregation and achieving high color purity near-infrared electroluminescence.

Nano Letters, 2008

Effective nanoscale control of intermolecular interactions in conjugated polymers is needed for t... more Effective nanoscale control of intermolecular interactions in conjugated polymers is needed for the optimal development and exploitation of the latter in low-cost, large-area consumer electronics items, such as light-emitting and photovoltaic diodes, or transistors. Here we report our investigations on insulated molecular wires constituted by conjugated polymers threaded into cyclodextrin rings. Until now, there has been no detailed quantitative understanding of the role of progressive cyclodextrin encapsulation (quantifiable by the so-called "threading ratio", TR, or number of cyclodextrins per repeat unit) in tailoring the photophysics of the conjugated polymeric wires. We combine spectroscopic, electrical and surface analysis techniques to elucidate how the TR of cyclodextrin-threaded molecular wires controls formation of interchain species and related physical properties (0 < TR < or = 2.3; the maximum theoretical TR for close-packed CDs is 2.8). Increasing TR enhances the solid-state photoluminescence (PL) and electroluminescence quantum efficiency. To unravel the effect of progressive encapsulation on the intrachain decay kinetics of the polymer backbone, we added an electron-accepting quenching agent, methyl viologen (MV), to the polymer solutions. MV predominantly quenches the aggregate PL, thus enabling measurement of the decay kinetics of the intrinsic exciton even for low-TR polyrotaxanes, for which the different contributions are otherwise difficult to disentangle.

Macromolecular Rapid Communications, 2013

Near-infrared (NIR) polymer light-emitting diodes (PLEDs) based on a fluorene-dioctyloxyphenylene... more 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.

Journal of Applied Physics, 2006

Metal-coated, "pulled," and conically shaped fiber probes used in scanning near-field optical mic... more Metal-coated, "pulled," and conically shaped fiber probes used in scanning near-field optical microscopy ͑SNOM͒ 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 fibers͒. Unexpectedly, we find a shortening of such probes in response to "high-power" laser injection ͑Ͼ1 mW͒. Thermal stress due to prolonged high-power laser injection ͑ϳ9 mW at 325 nm; compared to powers Ͻ1 mW often used in SNOM experiments͒ 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 appear to compromise the probe's performance in terms of light confinement outside the aperture area, suggesting that the detachment only affects the coating over the fiber cladding and confirming the operational robustness of these probes. In comparison, tube-etched, conical probes display substantial damage of the coating, up to several microns from the apex, after being injected with a comparable high-power laser beam ͑Ͼ10 mW at 633 nm͒. Although the vertical feedback mechanism of the microscope can compensate for dilations/contractions of the probes, these findings are of general importance to the field. More specifically they are significant for the achievement of a detailed understanding of apertured-SNOM operation, for the selection and operation of near-field probes, and for preventing potential artifacts in imaging and lithography, due to uncontrolled alteration of the probe properties and/or light leakage from cracks of the opaque coating induced by thermal fatigue. In addition, our results demonstrate that it is important for probe design to also consider the probe's thermal regime during operation, so as to prevent cracks in the functional parts of the coating and thus spurious, undesired sample illumination from regions other than the probe intended aperture.

Journal of Applied Physics, 2012

Facile large-area photolithography of periodic sub-micron structures using a self-formed polymer ... more Facile large-area photolithography of periodic sub-micron structures using a self-formed polymer mask Appl. Phys. Lett. 100, 233503 (2012) Controlled addressing of quantum dots by nanowire plasmons Appl. Phys. Lett. 100, 231102 Effects of tip-substrate gap, deposition temperature, holding time, and pull-off velocity on dip-pen lithography investigated using molecular dynamics simulation J. Appl. Phys. 111, 103521 (2012) Unbiased line width roughness measurements with critical dimension scanning electron microscopy and critical dimension atomic force microscopy J. Appl. Phys. 111, 084318 Metallic nanomesh electrodes with controllable optical properties for organic solar cells

Dyes and Pigments, 2013

Synthesis, isolation and characterization of diketopyrrolopyrrole (DPP) dimers are described. The... more Synthesis, isolation and characterization of diketopyrrolopyrrole (DPP) dimers are described. These derivatives were prepared starting from symmetric or asymmetric DPP monomers through Pd-catalyzed homocoupling, Yamamoto coupling or Suzuki coupling. The spectroscopic properties of these newly synthesized DPP dimers were investigated and were supported by quantum-chemical calculations.

Chemistry - A European Journal, 2013