Gianluca Latini

Charge separation in semicrystalline polymeric semiconductors by photoexcitation: is the mechanism intrinsic or extrinsic?

We probe charge photogeneration and subsequent recombination dynamics in neat regioregular poly(3-hexylthiophene) films over six decades in time by means of time-resolved photoluminescence spectroscopy. Exciton dissociation at 10K occurs extrinsically at interfaces between molecularly ordered and disordered domains. Polaron pairs thus produced recombine by tunnelling with distributed rates governed by the distribution of electron-hole radii. Quantum-chemical calculations suggest that hot-exciton dissociation at such interfaces results from a high charge-transfer character. PACS numbers: 72.20.Jv, 78.47.jd, 78.55.Kz, 78.66.Qn

Polyfluorene-based light-emitting diodes with an azide photocross-linked poly(3,4-ethylene dioxythiophene):(polystyrene sulfonic acid) hole-injecting layer

We used a water-soluble bis(fluorinated phenyl azide) to cross-link a poly(ethylene dioxythiophene):poly(styrene sulphonic acid) (PEDOT:PSS), hole-injection layer, with a view to its future use with water-soluble emitters. To enable direct comparison with conventional PEDOT:PSS, we studied the cross-linked films in diodes incorporating the organic-solvent soluble polymer poly(9,9′-dioctylfluorene-alt-benzothiadiazole). Kelvin probe characterization of the PEDOT:PSS and electroabsorption measurements of the devices consistently show a 0.2eV increase of the PEDOT:PSS work function upon cross-linking. We also observe a 70-fold reduction in resistivity, an increase of the current above threshold and a decrease of the “leakage” current below threshold.

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.

Thermal processes in metal-coated fiber probes for near-field experiments

We have used a ray optics model to calculate the optical power absorbed in the metal coating of apertured probes for scanning near-field optical microscopy. We have then introduced the absorbed power profile into the heat balance equation to calculate the temperature of the probe as a function of the distance from the apex. By comparing our results with available experimental data, we demonstrate accurate prediction of both the temperature profile along the probe, and the temperature increase per mW of power launched into the fiber (60.7 versus 60K∕mW at 25μm from the apex).

Enhanced luminescence properties of highly threaded conjugated polyelectrolytes with potassium counter-ions upon blending with poly"ethylene oxide…

The photophysics and electroluminescence (EL) of thin films of unthreaded and cyclodextrin-encapsulated poly(4,4′-diphenylenevinylene) (PDV) with potassium countercations, blended with poly(ethylene oxide) (PEO) are investigated as a function of the PEO concentration. We show that three main factors contribute to increasing the photoluminescence (PL) quantum efficiency as a result of suppressed intermolecular interactions, namely: the high degree of encapsulation of the polyrotaxanes, the relatively large countercation (e.g., compared to lithium), and the complexation of the rotaxanes with PEO. By facilitating cationic transport to the negative electrodes, PEO also leads to devices with enhanced electron injection and improved charge balance, whose operation therefore resembles that of “virtually unipolar” light-emitting electrochemical cells. This effect, together with the enhanced PL efficiency, leads to higher EL efficiency for both polyrotaxanes and unthreaded polymers, upon addition of the PEO. We sh...

Self-assembled monolayers of protonated poly(amidoamine) dendrimers on indium tin oxide

We have investigated the change of work function of indium tin oxide (ITO) anodes induced by adsorption of positively charged poly(amidoamine) (PAMAM) dendrimers. Kelvin probe characterization of the functionalized ITO films and electroabsorption measurements on polymer light-emitting diodes incorporating poly(9,9-dioctylfluorene) active layers revealed an abrupt (0.55eV) lowering of the effective work function upon addition of the adsorbed layer and a weak dependence on the PAMAM generation. We interpret our results with an electrostatic model accounting for both positively charged amines and for possible contaminations providing compensating negative charges.

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.

Scanning force microscopy and optical spectroscopy of phase-segregated thin films of poly(9,9′-dioctylfluorene-alt-benzothiadiazole) and poly(ethylene oxide)

We describe the formation of ordered phase-segregated domains at the hundred nm scale in thin films prepared from two molecular systems of interest as active materials in light-emitting electrochemical cells, i.e. the ion-transport polymer poly(ethylene oxide), PEO, and the well-known electron-transport macromolecule poly(9,9′-dioctylfluorene-alt-benzothiadiazole), F8BT. Scanning force microscopy investigations revealed signs indicative of self-organization processes taking place during film deposition, and characterized by the formation of PEO crystalline lamellae arranged either face-on or as intertwined fibres assembled in an edge-on fashion surrounding large and disordered F8BT grains. This self-segregated architecture, prepared by a single step co-deposition of the two components from a chloroform solution, provides unambiguous evidence for the poor miscibility of the two polymers. Fluorescence titration studies in solution and measurements of the photoluminescence quantum efficiency of thin solid state films showed little change of the optical properties upon addition of PEO to F8BT, thus confirming the modest interaction, at the molecular level, between the PEO and F8BT macromolecular strands. The results obtained are significant to the understanding of the parameters contributing to the interfacial and intermolecular interactions.

STM topographic and barrier imaging of self-assembled InAs/GaAs dots

Abstract In this paper we present scanning tunneling microscopy (STM) images of self-assembled InAs dots on GaAs(001) surfaces. TheSTM was operating in air at room temperature using both constant-current as well as gap-modulated modes. The conductivityand surface band bending have been modified by irradiating the surface by an Ar + laser beam during the scan. In this manner,photoconductivity images have been obtained in an attempt to have a contrast between the dots and the InAs wetting layer. Ourresults reveal a clear difference between STM topographies and ‘barrier’ method. While topographic images show dome-shapeddots, the barrier ones resemble sharp pyramids even if they are acquired on the same place and at the same time. The generationof STM images is considered for both modes of operation and the concepts of topography and barrier height are revised andcompared with a numerical simulation of Qdot surfaces.