M. Ariu

Bio: M. Ariu is an academic researcher from Imperial College London. The author has contributed to research in topics: Fluorenone & Signal. The author has an hindex of 9, co-authored 13 publications receiving 540 citations.

Understanding the Origin of the 535 nm Emission Band in Oxidized Poly(9,9‐dioctylfluorene): The Essential Role of Inter‐Chain/Inter‐Segment Interactions

Abstract: We present a careful study of the effects of photo-oxidation on the emissive properties of poly(9,9-dioctylfluorene) (PFO) that addresses important issues raised by a recent flurry of publications concerning the degradation of blue light-emitting, fluorene-based homo- and copolymers. The photoluminescence (PL) spectra of thin PFO films oxidized at room temperature comprise two major components, namely a vibronically structured blue band and a green, structureless component, referred to hereafter as the ‘g-band’. These are common features in a wide range of poly(fluorene)s (PFs) and whilst the former is uniformly accepted to be the result of intra-chain, fluorene-based, singlet-exciton emission, the origin of the ‘g-band’ is subject to increasing debate. Our studies, described in detail below, support the proposed formation of oxidation-induced fluorenone defects that quench intra-chain, singlet-exciton emission and activate the g-band emission. However, whilst these fluorenone defects are concluded to be necessary for the g-band emission to be observed, they are considered not to be, alone, sufficient. We show that inter-chain/inter-segment interactions are required for the appearance of the g-band in the PL spectra of PFO and propose that the g-band is attributable to emission from fluorenone-based excimers rather than from localized fluorenone π–π* transitions as recently suggested.

Ellipsometric Characterization of the Optical Constants of Polyfluorene Gain Media

Abstract: Variable-angle spectroscopic ellipsometry (VASE) has been applied to five polyfluorene gain media. The ellipsometric data have been analyzed using an electronic model based on critical points of zero dimension, i.e., an exciton model. The optical constants of thin-film samples on spectrosil B substrates have been deduced and are used to characterize the waveguiding conditions in these asymmetric slab structures. The exciton model that we have used leads to a small correlation of the parameters and accurate fits. The good match with normal-incidence transmission spectrophotometry data and surface profilometry determinations of thickness gives further confidence in the suitability of this model. Based on our measurements, we calculate the cut-off thicknesses for the fundamental TE-guided modes in our silica–polymer–air structures to lie within the range of 40 nm to 70 nm (depending on the specific polymer), and demonstrate corresponding confinement factors (Γ) between 37 % and 63 %. The calculated thickness dependence of the cut-off wavelength agrees well with experimental data for the thickness dependence of the peak amplified spontaneous emission (ASE) wavelength and is, therefore, consistent with previous explanations of the ASE spectral shifts.

Understanding the Nature of the States Responsible for the Green Emission in Oxidized Poly(9,9‐dialkylfluorene)s: Photophysics and Structural Studies of Linear Dialkylfluorene/Fluorenone Model Compounds

Abstract: Here, the optical properties of a series of structurally well-defined model compounds for oxidatively degraded poly(dialkylfluorenes) (PFs) are reported. Specifically, linear compounds comprising one, two, or four dihexylfluorene (F) moieties together with one fluorenone (O) moiety placed either at the end or in the center of each chain (i.e., FO, FFO, FOF, FFOFF) are studied. The results support the recent observation that the photophysics of the fluorenone-centered "pentamer" (FFOFF) is most similar to that of oxidized PFs. They further demonstrate that molecule―molecule interaction is essential to activate the green emission band. Investigations by X-ray diffraction (XRD) identify the solid-state structure of a representative member of this class of compounds and reveal inter-molecular interaction through dipole―dipole coupling between neighboring fluorenone moieties.

Rib waveguide dye-doped polymer amplifier with up to 26 dB optical gain at 625 nm

Abstract: We report optical signal amplification in a solid-state dye-doped polymer with a rib waveguide structure. A 625nm pulsed signal and a collinear 575nm pump are coupled into a 1μm×120μm poly(methyl methacrylate) waveguide doped with 1% by weight Rhodamine 640 dye. Depending on the signal intensity, a maximum optical gain in the 21–26dB range is obtained from a 1.2-cm-long device, accompanied by a signal-to-noise ratio in the 9–16dB range.

Photophysics of charge transfer in a polyfluorene/violanthrone blend

Abstract: We present a study of the photophysical and photovoltaic properties of blends of violanthrone in poly[9,$9\text{\ensuremath{-}}\mathrm{bis}$(2-ethylhexyl)-fluorene-2,$7\text{\ensuremath{-}}\mathrm{diyl}$] $({\mathrm{PF}}_{2∕6})$ Photoluminescence quenching and photocurrent measurements show moderate efficiencies for charge generation, characteristic of such polymer/dye blends Pump-probe measurements on blend films suggest that while $\ensuremath{\sim}47%$ of the total exciton population dissociates within $4\phantom{\rule{03em}{0ex}}\mathrm{ps}$ of photoexcitation, only $\ensuremath{\sim}32%$ subsequently results in the formation of dye anions We attribute the discrepancy to the likely formation of complex species with long lifetimes, such as stabilized interface charge pairs or exciplexes This conclusion is supported by the appearance of a long lifetime component of $24\phantom{\rule{03em}{0ex}}\mathrm{ns}$ in the dynamics of the photoinduced absorption signal associated to polarons in photoinduced absorption bands centered at $560\phantom{\rule{03em}{0ex}}\mathrm{nm}$

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Abstract: A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends.

Abstract: Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends

An analytic model for the optical properties of gold.

Abstract: A simple analytic model for the optical properties of gold is proposed. The model includes a minimum set of parameters necessary to represent the complex dielectric function of gold in the visible and near-uv regions. Explicit values for the parameters to reproduce the Johnson and Christy data [Phys. Rev. B 6, 4370 (1972)] on the optical properties of gold are provided.

Microcavity effects and optically pumped lasing in single conjugated polymer nanowires

Abstract: Conjugated polymers have chemically tuneable opto-electronic properties and are easily processed, making them attractive materials for photonics applications. Conjugated polymer lasers, in a variety of resonator geometries such as microcavity, micro-ring, distributed feedback and photonic bandgap structures, have been fabricated using a range of coating and imprinting techniques. Currently, one-dimensional nanowires are emerging as promising candidates for integrated, subwavelength active and passive photonic devices. We report the first observation of optically pumped lasing in single conjugated polymer nanowires. The waveguide and resonator properties of each wire are characterized in the far optical field at room temperature. The end faces of the nanowire are optically flat and the nanowire acts as a cylindrical optical cavity, exhibiting axial Fabry-Perot mode structure in the emission spectrum. Above a threshold incident pump energy, the emission spectrum collapses to a single, sharp peak with an instrument-limited line width that is characteristic of single-mode excitonic laser action.

High refractive index organic–inorganic nanocomposites: design, synthesis and application

Abstract: Organic–inorganic nanocomposites with high refractive index (RI) are typically constructed by integrating high RI inorganic nanoscale building blocks into a processable, transparent organic matrix. These nanocomposites combine the numerous advantages of organic and inorganic components, and have many promising applications in optical design and advanced optoelectronic fabrication. A substantial amount of work has been performed in this area. This Feature article summarizes the general design principles and different fabrication approaches of high RI nanocomposites, and reviews recent research advances and some important optical applications of these nanocomposites.