UCL University College London Eprints
UCL Eprints collects the work of UCL researchers and makes it freely available over the web, helping the worldwide scholarly community to discover UCL research. Institutional repositories like UCL Eprints complement the traditional academic publishing and scholarly communications processes. They raise the visibility of research and help to maximise its impact. UCL researchers are encouraged to deposit a copy of each journal article, conference paper, working paper, and any other research output, in the UCL Eprints at the earliest opportunity, ensuring that their research reaches as wide an audience as possible.
Structural and dynamical characterization of P3HT/PCBM blends - Paternó, G; Cacialli, F; García Sakai, V
We report data from X-ray and neutron scattering on the structure and dynamics of P3HT-PCBM solid blends cast from three different solvents, characterized by different boiling points, namely chloroform (CF, boiling point T = 61.2 °C), chlorobenzene (CB, T = 131 °C) and ortho-dichlorobenzene (ODCB, T = 180.5 °C). Whereas the blend cast from CB shows already a significant degree of order in its "pristine" (as cast) state, blends prepared from CF and ODCB develop their order only upon annealing at 160 °C. In addition, blending seems to frustrate the polymer dynamics. We propose that such change in dynamics might...
Suppression of green emission in a new class of blue-emitting polyfluorene copolymers with twisted biphenyl moieties - Cacialli, F; Lim, S-F; Friend, RH; Rees, ID; Li, J; Ma, Y; Robinson, K; Holmes, AB; Hennebicq, E; Beljonne, D
The color purity of polyfluorene-based blue-emitting polymers is often compromized by "long-wavelength" green emission bands, attributed to polymer interchain species first and more recently to formation of emissive fluorenone defects. Here, we study the nature and the suppression of such bands via characterization of a new class of polyfluorene derivatives modified by insertion of functional groups at the bridging points (position C9), so as to increase inter-ring torsion angles. We find that the solid-state photoluminescence spectra of random copolymers of the modified polyfluorenes and the homopolymer display a progressive decrease of the long-wavelength emission. Electroluminescence spectra also show efficient suppression...
Thermal effects in near-field optical microscopy experiments - Ambrosio, A; Allegrini, M; Micheletto, R; Gucciardi, PG; Kawakami, Y; Fenwick, O; Cacialli, F
We have investigated the heating phenomena occurring in near-field experiments using metal-coated apertured optical fibers. We demonstrate heat transfer from the probe to the sample surface and permanent modification of the probe parameters.
Influence of polymerization conditions on the properties of poly(4,4′-dialkyl-2,2′-bithiophenes) - Zagórska, M; Kulszewicz-Bajer, I; Proń, A; Barta, P; Cacialli, F; Friend, RH
4,4′-octyl-2,2′-bithiophenes (DOcBT) was polymerized for 24 h in a purified Ar atmosphere in the presence of FeCl . The polymerization proceeded in a heterogeneous medium at the interface of iron chloride suspensions and the solution of bithiophenes. Polymerization results indicate that the synthetic conditions influence some of the physical properties of the obtained polymers.
Photo and electroluminescent properties of a porphyrin-poly(p-phenylene vinylene) derivative - Morgado, J; Friend, RH; Cacialli, F; Iqbal, R; Moratti, SC; Holmes, AB; Yahioglu, G; Milgrom, LR
The solid state luminescent properties of new statistical copolymers of 2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene and a porphyrin-substituted phenylene vinylene are reported. The photoluminescence, PL, emission shows an efficient energy transfer to the porphyrin, in spite of the small absorption coefficient of the porphyrin in the wavelength range of the poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene], MEH-PPV, emission. At a porphyrin content of 0.25%, by weight, the emission is already dominated by the porphyrin. The PL efficiency of these copolymers is significantly reduced from the value of 13% for MEH-PPV down to 3% for the copolymer with 10.84% of porphyrin. Efficient energy transfer is also observed in...
Versatile syntheses of various homo- and copolymers of poly(1,4-arylene vinylene)s - Martin, RE; Chuah, BS; Geneste, F; Holmes, AB; Riehn, RH; Cacialli, F; Friend, RH; Morgado, J
The synthesis of poly(1,4-phenylene vinylene)s (PPVs) containing a 2,3-dialkoxy substitution pattern has been developed. Poly[2,3-bis(2-ethylhexyloxy)-1,4-phenylene vinylene)] (BEH-PPV, 4) was prepared and its optical properties compared with the recently discussed poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV, 1), which showed in contrast to classical 2,5-substituted PPV derivatives a notable blue-shift of the longest-wavelength absorption (λ ) and a considerable increase of the solid-state photoluminescence (PL) efficiency. The two statistical PPV copolymers 5 and 6 comprising 2,3-dibutoxy and dimethyloctylsilyl- or 2,5-bis(dimethyloctylsilyl) side-chains, respectively, were synthesized via the Gilch dehydrohalogenation route. Double-layer light-emitting devices (LEDs) were demonstrated to combine high electroluminescence efficiencies with low turn-on voltages.
Resummation of fluctuations near ferromagnetic quantum critical points - Pedder, CJ; Krueger, F; Green, AG
We present a detailed analysis of the nonanalytic structure of the free energy for the itinerant ferromagnet near the quantum critical point in two and three dimensions. We analyze a model of electrons with an isotropic dispersion interacting through a contact repulsion. A fermionic version of the quantum order-by-disorder mechanism allows us to calculate the free energy as a functional of the dispersion in the presence of homogeneous and spiraling magnetic order. We resum the leading divergent contributions to derive an algebraic expression for the nonanalytic contribution to free energy from quantum fluctuations. Using a recursion which relates subleading divergences...
Circularly Polarised X-rays Scattering Investigation of Spin-Lattice Coupling in TbMnO$_3$ in Crossed Electric and Magnetic Fields - Walker, HC; Fabrizi, F; Paolasini, L; Bergevin, FD; Prabhakaran, D; Boothroyd, AT; McMorrow, DF
We present a study of the magnetic and crystallographic structure of TbMnO$_3$ in the presence of crossed electric and magnetic fields using circularly polarised X-ray non-resonant scattering. A comprehensive account is presented of the scattering theory and data analysis methods used in our earlier studies, and in addition we present new high magnetic field data and its analysis. We discuss in detail how polarisation analysis was used to reveal structural information, including the arrangement of Tb moments which we proposed for $H = 0$ T, and how the diffraction data for $H