Synthetic Metals (v.155, #2)
Preface by K.S. Narayan; S. Ramasesha (241).
Time-dependent Langevin-type bimolecular charge carrier recombination in regiorandom poly(3-hexylthiophene) by A. Pivrikas; R. Österbacka; G. Juška; K. Arlauskas; H. Stubb (242-245).
We have studied transport and recombination of photogenerated charge carriers in thin films of regiorandom poly(3-hexylthiophene) (RRaPHT). Time-of-Flight studies show space charge perturbed current transients with a saturation of current level at high light intensities as a result of diffusion controlled Langevin-type bimolecular charge carrier recombination. We have been able to measure the time-dependent charge carrier mobility by using the carrier extraction by linearly increasing voltage technique. We found that the hole mobility follows a power law dependence over a few orders of magnitude in time as μ = μ 0 t −0.42. By using the carrier extraction by pulsed voltage technique we measured the photogenerated charge carrier concentration decay as a function of delay time and found that the decay can be fitted using a dispersive time-dependent bimolecular recombination coefficient governed by the time-dependent hole mobility.
Keywords: Photoconductivity; Amorphous thin films; Semiconducting films; Polythiophene and derivatives; Solar cells;
Exciton relaxation in single wall carbon nanotubes by G. Lanzani; G. Cerullo; A. Gambetta; C. Manzoni; E. Menna; M. Meneghetti (246-249).
Ultrafast exciton dynamics is measured in films of functionalized single wall carbon nanotubes embedded in PMMA with time resolution down to sub-10-fs. Ultrafast bleaching decay and photoinduced absorption formation (depending on probe wavelength) are observed and assigned to decay of the second (or higher) exciton into the first exciton. The time constant of the relaxation process is ≈40 fs. The origin of the phenomenon is discussed within the one-dimensional semiconductor picture.
Keywords: Time-resolved fast spectroscopy; Photoinduced absorption spectroscopy; Fullerenes and derivatives;
Excitons in semiconducting single-walled carbon nanotubes by Hongbo Zhao; Sumit Mazumdar (250-253).
We report correlated-electron calculations of optically excited states in 10 semiconducting single-walled carbon nanotubes with a wide range of diameters. Optical excitation occurs to excitons whose binding energies decrease with the increasing nanotube diameter, and are smaller than the binding energy of an isolated strand of poly-(paraphenylene vinylene). The ratio of the energy of the second optical exciton polarized along the nanotube axis to that of the lowest exciton is smaller than the value predicted within single-particle theory. The experimentally observed weak photoluminescence is an intrinsic feature of semiconducting nanotubes, and is consequence of dipole-forbidden excitons occurring below the optical exciton. Excited states absorption calculations show photoinduced absorption energies are lower than or comparable to the binding energy of the lowest exciton.
Keywords: Carbon nanotubes; Semiempirical models and model calculations; Photoluminescence;
Ultrafast exciton dynamics in isolated single-walled nanotubes by C.X. Sheng; Z.V. Vardeny; A.B. Dalton; R.H. Baughman (254-257).
Ultrafast relaxation dynamics of photoexcitations in semiconducting single-walled carbon nanotubes (S-NT) isolated in D2O solution have been investigated using polarized pump–probe photomodulation (with 150 fs time resolution), and cw polarized photoluminescence (PL). Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands were observed, which also showed photoinduced dichroism. After taking into account the PB spectral shift, then the PA and PB bands decay together in time following a power law of the form (t)−a with a in the range of 0.7–1. The PL emission shows polarization degree that agrees with that of the transient photoinduced dichroism. We therefore conclude that the primary photoexcitations in S-NT are excitons that are confined along the NTs. From the average PL polarization degree and the transient polarization memory decay, we estimate the PL lifetime in isolated NTs in solution to be of the order of 500 ps. This relatively long PL lifetime is dominated by non-radiative decay processes, which when coupled with the minute PL emission quantum efficiency indicates a very small radiative recombination rate, in good agreement with recent theories that include electron correlation.
Keywords: Photoluminescence; Non-linear optical methods; Carbon nanotubes; Ultrafast dynamics;
Cathodoluminescence from organic bilayer induced by field electron emission of carbon nanotubes by Rashmi Nanjundaswamy; Christopher Williams; Sergey B. Lee; Anvar A. Zakhidov (258-261).
We report the observation and studies of cathodoluminescence (CL) of organic bilayer structures, such as tris-(8-hydroxyquinoline) aluminium (Alq3) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD) deposited on ITO-coated glass, with and without hole transport layer, N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-phenyl)-4,4′-diamine (α-NPD). To excite the CL of such bilayer organic heterostructure, low energy electrons field emitted by single walled carbon nanotube cathodes were utilized. The dependence of CL spectrum and intensity on voltage (V), current (I), type of transport layer and the cathode–anode geometry has been studied. We propose carbon nanotubes as efficient cathodes for stable CL emission from organic multilayer heterostructures. The role of both electron and hole transport layers on stability and spectrum of CL is also discussed.
Keywords: Cathodoluminescence; Field emission; Electroluminescence; Exciplex; Organic light-emitting diodes;
Excited state dynamics in alternating polyfluorene copolymers by Kim G. Jespersen; Arkady Yartsev; Thorbjörn Pascher; Villy Sundström (262-265).
We present the excited state dynamics of the low band gap alternating polyfluorene copolymer, DiO-PFDTBT. The copolymer structure is an alternating sequence of donor and acceptor units consisting of 9,9-dioctyl-9H-fluorene and 4,7-di-thiophen-2-yl-benzo[1,2,5]thiadiazolyl units. Calculations have predicted that the first excited state has charge transfer character, while the second strongly allowed excited state is a delocalized π-conjugated state. We show that the excited state dynamics involves internal conversion (IC) from the second strongly allowed excited state to the first excited state. Furthermore, excitation energy transfer (EET) on the ∼1–10 ps time scale is observed as well as a subsequent formation of a stabilized long-lived emissive state, which has a decay time of 3.25 ns. The origin of this stabilized state is discussed in relation to the intrinsic charge transfer nature of the DiO-PFDTBT.
Keywords: Low band gap conjugated polymers; Photoinduced absorption spectroscopy;
Apparent phonon side-band modes in π-conjugated systems: Polymers, oligomers and crystals by E. Ehrenfreund; C.C. Wu; Z.V. Vardeny (266-269).
The emission spectra of many π-conjugated polymers and oligomers contain side-band replicas with apparent frequencies that do not match the Raman active mode frequencies. Using a time dependent model we show that in such many mode systems, the increased damping of the time dependent transition dipole moment correlation function results in an effective elimination of the vibrational modes from the emission spectrum; subsequently causing the appearance of a regularly spaced progression at a new apparent frequency. We use this damping dependent vibrational reshaping to quantitatively account for the vibronic structure in the emission spectra of π-conjugated systems in the form of films, dilute solutions and single crystals. In particular, we show that by using the experimentally measured Raman spectrum we can account in detail for the apparent progression frequencies and their relative intensities in the emission spectrum.
Keywords: Optical absorption and emission spectroscopy; Photoluminescence; Raman spectroscopy; Conjugated and/or conducting polymers;
Role of electron–electron interactions on spin effects in electron–hole recombination in organic light emitting diodes by Mousumi Das; S. Ramasesha; S. Mazumdar (270-273).
We investigate the possibility that high-energy singlet and triplet excited states with large electron–hole separations are generated in electron–hole recombination process, over and above the lowest singlet and triplet excitons. Using a time-dependent calculation of the interchain/intermolecular charge-transfer within model Hamiltonians that explicitly include electron–electron interactions between the π-electrons, we show that there occurs a bifurcation of the electron–hole recombination path in each of the two spin channels that leads to the generation of both the lowest energy exciton and a specific-high-energy charge-transfer state, with the matrix elements favoring the lowest energy exciton and the energy difference factor favoring the higher energy state, resulting in an enhancement of the singlet:triplet yield ratio above the value of 0.25 predicted from statistical considerations.
Keywords: Electroluminescence; Organic light emitting diodes; Electron–electron interactions; Model Hamiltonians;
Significant improvements in the optical gain properties of oriented liquid crystalline conjugated polymer films by Ruidong Xia; Mariano Campoy-Quiles; George Heliotis; Paul Stavrinou; Katharine S. Whitehead; Donal D.C. Bradley (274-278).
We report significant improvements in the optical gain properties of highly oriented films of the fluorene-based conjugated polymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). We demonstrate amplified spontaneous emission (ASE) in optically pumped aligned F8BT planar asymmetric waveguides. The influence of the polymer chain orientation and the excitation polarisation on the optical properties of the materials has been investigated. By selecting specific optimised excitation configurations, high net gain coefficients can be obtained, namely <47 cm−1 compared with <25 cm−1 for standard spin-coated films. The loss coefficients also reduce from α = 7.6 cm−1 (spin-coated film) to 0.9 cm−1 (aligned film). Ellipsometry allows determination of the optical constants and reveals that the refractive indices of the materials are also greatly affected by the orientation of the polymer chains.
Keywords: Polyfluorenes; Oriented polymer; Waveguides; Amplified spontaneous emission; Laser; Optical gain properties;
Exploring the potential of ellipsometry for the characterisation of electronic, optical, morphologic and thermodynamic properties of polyfluorene thin films by M. Campoy-Quiles; P.G. Etchegoin; D.D.C. Bradley (279-282).
We have used spectroscopic ellipsometry to characterise thin films of poly(9,9-dioctylfluorene) (PFO) spin coated on different substrates. Analysis of the ellipsometry data suggests that a model based on critical points of zero order (i.e. excitons) provides an appropriate framework to describe the data. It is found to be very accurate and to result in low standard deviations and a low correlation among the fitting parameters compared to other models, such as the Harmonic Oscillator Approximation or the Model Dielectric Function.We have used this exciton model to study the dielectric function of PFO: uniaxial anisotropy was found with the optical axis perpendicular to the plane of the substrate. We demonstrate, however, that conventional variable angle spectroscopic ellipsometry (VASE) in reflection mode is not sensitive enough to accurately establish the film birefringence, and that other techniques, such as interference enhancement VASE need to be used.We also demonstrate the use of ellipsometry for measuring the glass and crystalline phase transition temperatures of PFO and report the dependence of the polymer refractive index on temperature: the large dn/dT of PFO could be used to add functionality to optoelectronic/photonic devices.
Keywords: Ellipsometry; Conjugated polymers; PFO; Anisotropy; Phase transitions; IEVASE;
Fullerene non-linear excited state absorption induced by gold nanoparticles light harvesting by V. Amendola; G. Mattei; C. Cusan; M. Prato; M. Meneghetti (283-286).
Au nanoparticles can be synthesized in solution by a laser ablation methodology which allows to obtain funtionalized metal nanoparticles with a disulfide fullerene derivative in a simple one step process. The supramolecular system is shown to be an efficient non-linear absorbers of 532 nm nanosecond laser pulses. The mechanism of the non-linear absorption is shown to proceed through a light harvesting step by the metal nanoparticles and an efficient energy transfer to the fullerene moieties which absorb in a non-linear regime through their triplet states.
Keywords: Non-linear optical methods; Fullerenes and derivatives; Models of non-linear phenomena;
Molecular orientation in ultrathin films of α-sexithiophene on silicon dioxide revealed by spatially resolved confocal spectroscopy by Enrico Da Como; Maria Antonietta Loi; Franco Dinelli; Mauro Murgia; Fabio Biscarini; Roberto Zamboni; Michele Muccini (287-290).
In organic semiconductors devices like thin film transistors (TFTs), the supra-molecular organization on the substrate is one of the most important parameters to control the charge transport. Unprecedented insights into the molecular orientation of vacuum sublimed ultrathin films of α-sexithiophene (T6) on silicon dioxide are revealed by confocal laser scanning microscopy (CLSM) and spectroscopy. By the cross correlation of confocal microscopy and atomic force microscopy measurements, we demonstrated that in films thinner than 2 nm, regions where molecules are oriented perpendicular to the substrate and regions where molecules are parallel to the substrate co-exist. By spatially resolved spectroscopy, we gain information about the supra-molecular organization in ultrathin films. Implications for charge transport in thin film transistors are considered and discussed.
Keywords: Sexithiophene; Thin film; Confocal microscopy; Photoluminescence; Atomic force microscopy;
Optical excitations of porphyrin J-aggregates by A. Tonizzo; M. Cerminara; G. Macchi; F. Meinardi; N. Periasamy; P. Sozzani; R. Tubino (291-294).
In this work, we will present a study of the optical properties of aggregated tetrakis(p-sulfonatophenyl)porphyrin (TSPP) in different environments which can have a dramatic influence on the spectroscopic features. We will show that when the porphyrins are free to self-organize (i.e. in saturated solutions) they show two distinct photoluminescence peaks, revealing the presence of two different species, with a relative population strongly dependent on the temperature. If aggregation occurs in an environment with restricted geometry (i.e. within the nanochannels of MCM-41) it is possible to modify the spontaneous self-assembly of the molecules and, as a consequence, the relative ratio of the two emissions intensity. We will report on the possibility to induce the preferential formation of the low-energy emitting species, aiming to tune in a controlled way the emission spectrum of the aggregate.
Keywords: Porphyrins; Photoluminescence; Collective phenomena; Inclusion compounds;
Modulating phase-separation in blends of MEHPPV-x with different conjugation lengths by K. Nagesh; D. Kabra; K.S. Narayan; S. Ramakrishnan (295-298).
We report here the phase-separation and energy transfer characteristics of segmented MEHPPV blends with two different distributions of conjugation lengths, namely MEHPPV-8 (LC, low conjugation length) and MEHPPV-100 (HC, high conjugation length). Thin films containing varying weight fractions of LC and HC were spin-cast from chloroform solutions. Photoluminescence studies of these thin films suggested that complete energy transfer from LC to HC occurred even though there was phase-separation. Selective chain-collapse of MEHPPV-100 by using a solution containing increasing volume fraction of cyclohexane for spin-coating allowed one to modulate the extent of energy transfer to some extent—typically the extent of energy transfer decreased in the presence of high volume fraction of cyclohexane.
Keywords: Segmented MEHPPV; MEHPPV-x blends; Confocal microscopy; UV–vis; Fluorescence of polymer blends;
Recombination studies in a polyfluorene copolymer for photovoltaic applications by Harri Aarnio; Markus Westerling; Ronald Österbacka; Mattias Svensson; Mats R. Andersson; Torbjörn Pascher; Jinxi Pan; Villy Sundström; Henrik Stubb (299-302).
We present detailed continuous wave (cw) and transient photoinduced absorption (PA) measurements in thin films of a novel alternating polyfluorene copolymer, poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3-benzo-thiadiazole)] (DiO-PFDTBT), and its blends with the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in weight ratios of 1:0, 4:1 and 1:4. We measure the frequency, intensity and temperature dependence of the PA signal in the frequency domain, and compare with the results obtained from the transient PA decay measurements in the time domain. In all blends, the PA spectrum shows a broad high energy PA band ranging from ∼1 eV to 2 eV as well as a low energy band peaking at ∼0.35 eV. We attribute the low energy band to the P1 transition of polarons and part of the high energy band to the correlated P2 transition of polarons. Both frequency and time domain measurements show that the high energy band has two decay components, a faster component in the microsecond time regime and a slower component in the millisecond time regime. The slow component is strongly dispersive, whereas the fast component is practically non-dispersive.
Keywords: Photoinduced absorption spectroscopy; Low band gap conjugated polymers; Fullerenes and derivatives; Solar cells;
Electroabsorption spectroscopic study of cyano-substituted phenylene vinylene polymer (CN-PPV) by G.S. Samal; A.K. Biswas; Swapnil Singh; Y.N. Mohapatra (303-305).
We report electroabsorption spectroscopy measurements on a single layer cyano-substituted phenylene vinylene (CN-PPV) polymer device and also contact potential difference (CPD) in the device structure ITO/PEDOT: PSS/CN-PPV/Al. The electroabsorption (EA) spectrum shows well developed structure of a peak and valley at 2.2 and 2.56 eV, respectively. It is demonstrated that the line shape more closely reflects second derivative of absorption spectra than corresponding first derivative. The origin of dominant features in EA spectra in the range of 1.5–3.2 eV are discussed qualitatively without taking recourse to fitting a linear combination of derivatives of linear absorption. The contact potential difference (CPD) measured using this technique shows a value 0.6 V that is exactly the work function difference between ITO and Al electrode used in this structure. These features in EA measurements of CN-PPV are in contrast to the ones observed in the case of PPV thin films.
Keywords: Poly(phenylene vinylene) and derivatives; Electroabsorption;
Electrochemical synthesis and characterization of conducting polymers using room temperature melt as an electrolyte by S. Geetha; D.C. Trivedi (306-310).
Aromatic monomers can be polymerised using the chloroaluminate room temperature melt obtained by mixing 1:2 ratio of cetyl pyridinium chloride and anhydrous aluminium chloride miscible in all proportions with organic solvents as an electrolyte. The chloroaluminate (AlCl4 −) anion generated in this melt having a tetrahedral symmetry with equal bond lengths and bond angles is the dopant to stabilize macrocation generated near the vicinity of anode to yield better conducting and better ordered electronically conducting free standing polymer film. In this communication, we discuss the polymers derived from benzene and pyrrole and their characterization by various techniques.
Keywords: Chloroaluminate; Room temperature melt; Conductivity; Polypyrrole; Polyparaphenylene;
Photoluminescence studies on RF plasma-polymerized thin films by S. Saravanan; C. Joseph Mathai; M.R. Anantharaman; S. Venkatachalam; D.K. Avasthi; F. Singh (311-315).
Conjugated polymers in the form of thin films play an important role in the field of materials science due to their interesting properties. Polymer thin films find extensive applications in the fabrication of devices, such as light emitting devices, rechargeable batteries, super capacitors, and are used as intermetallic dielectrics and EMI shieldings. Polymer thin films prepared by plasma-polymerization are highly cross-linked, pinhole free, and their permittivity lie in the ultra low k-regime. Electronic and photonic applications of plasma-polymerized thin films attracted the attention of various researchers. Modification of polymer thin films by swift heavy ions is well established and ion irradiation of polymers can induce irreversible changes in their structural, electrical, and optical properties. Polyaniline and polyfurfural thin films prepared by RF plasma-polymerization were irradiated with 92 MeV silicon ions for various fluences of 1 × 1011 ions cm−2, 1 × 1012 ions cm−2, and 1 × 1013 ions cm−2. FTIR have been recorded on the pristine and silicon ion irradiated polymer thin films for structural evaluation. Photoluminescence (PL) spectra were recorded for RF plasma-polymerized thin film samples before and after irradiation. In this paper the effect of swift heavy ions on the structural and photoluminescence spectra of plasma-polymerized thin films are investigated.
Keywords: Photoluminescence; Coatings; Polyaniline; Infrared and Raman spectroscopy; Chemical vapour deposition;
A correlated study of linear optical absorption in tetracene and pentacene by Priya Sony; Alok Shukla (316-319).
In this paper, we report large-scale configuration interaction (CI) calculations of linear optical absorption spectra of tetracene and pentacene. For the purpose, we used Pariser–Parr–Pople (P–P–P) model Hamiltonian and quadruple-CI (QCI) and multi-reference single–double CI (MRSDCI) approaches. The role of Coulomb parameters used in P–P–P Hamiltonian was also examined by considering standard parameters as well as screened set of parameters. Results are presented both for the long- and the short-axis-polarized components of the spectra and wave functions of various excited states have also been analysed.
Keywords: Semi-emprical models and model calculations; Other conjugated and/or conducting polymers; Organic semi-conductors based on conjugated molecules (not polymers) (anthracenes, perylenes, etc.);
Grafting functional handles on to MEHPPV—Possible application for sensing by K. Nagesh; S. Ramakrishnan (320-323).
MEHPPV precursors containing dithiocarbamate groups along with methoxy units were used to initiate polymerisation of monomers, such as methyl acrylate and t-butyl acrylate, to yield grafted precursor systems. Typically, the grafting was done by UV irradiation, and the molecular weight of the grafted precursor polymer was seen to increase with exposure time. The t-butyl acrylate-grafted precursors were converted to the conjugated MEHPPV derivative by acid-catalysed elimination of the methoxy groups, during which time the hydrolysis of the t-butyl groups also occurred resulting in a hydrophilic MEHPPV with poly(acrylic acid) graft chains. These grafted MEHPPV derivatives were soluble in aqueous base as well as alcohols. Fluorescence spectroscopic studies indicated that these acrylic acid-grafted MEHPPV derivatives were very sensitive to analyte like methyl viologen.
Keywords: MEHPPV graft copolymers; Iniferter; Fluorescence quenching; Conjugated polyelectrolyte;
Synthesis and characterization of fatty acids passivated silver nanoparticles—their interaction with PPy by Chepuri R.K. Rao; D.C. Trivedi (324-327).
Metal nanoparticles have applications in the area of catalysis, opto-electronics due to their size dependent optical, electrical and electronic properties. It is believed that when the size of the particle tuned to one nanometer, the quantum effect is expected in their optical properties. Silver nanoparticles exhibit size dependant absorptions in the UV–vis region and also show anti-bacterial properties. In the present paper, we report the synthesis of silver nanoparticles passivated by stearic, palmitic and lauric acids. Three reducing agents, which are moderate strong to mild, formaldehyde, dimethyl formamide and triethanol amine are used to synthesize the nanoparticles as solids. The solids are redispersible in non-polar solvents. The IR spectra showed the existence of adsorbed fatty acids. It is shown that the stability of the silver nanoparticles in toluene solvent is decreased when the chain length of the fatty acid is decreased. The toluene solution of these nanoparticles exhibited a surface plasmon resonanace in the range 420–462 nm. Results showed that use of triethanol amine is the best choice in obtaining lower particle sizes. The nanoparticles are charecterized by UV–vis, FTIR, XRD and AFM techniques. The interaction of these particles with the conducting polymer PPy is explored.
Keywords: Silver; Nanoparticle; Surfaceplasmon; Passivated; Fattyacids; Reduction;
Spectroscopic studies of photoinduced transport in polymer field effect transistors by S. Dutta; K.S. Narayan (328-331).
Polymer field effect transistors (FETs) exhibit several interesting features upon photo-excitation. We review recent results from our laboratory of these polymer photo-FETs and present spectroscopic evidence to verify proposed mechanisms of charge-carrier generation and transport. We present results, which suggest that charge generation and recombination processes depend on the operating mode of FET. The location of photoinduced charge generation is probed using the intensity modulated photocurrent spectrum as a function of gate voltage with light incident from either side of the indium tin oxide (ITO)-gated FET structure.
Keywords: Slow relaxation; Photocurrent spectrum; FET; P3AT; Red-shift;
Multilayer encapsulation of plastic photovoltaic devices by Pallavi Madakasira; Kanzan Inoue; Ross Ulbricht; Sergey B. Lee; M. Zhou; John P. Ferraris; Anvar A. Zakhidov (332-335).
We report the first results on conformal deposition of parylene on plastic solar cells, and study the effects of protection on performance characteristics of polymer solar cell. We show that single parylene coating layer is not a very effective barrier even for thicknesses as large as 10 μm. Our results support the experimental observations that the main degradation processes in poly(3-hexylthiophene) are the oxygen-assisted photochemical reactions, such as photobleaching and chain scission. Multilayer barriers made of parylene and aluminum oxide coatings are very promising for full protection of organic solar cell from degradation even under intense AM1.5 solar light.
Keywords: Solar cell; Polythiophene and derivatives; Fullerene and derivatives; Bi-continuous polymer network interfaces; Coatings;
On the origin of multilevel conductance and memory in ultrathin organic films by Biswanath Mukherjee; Amlan J. Pal (336-339).
Conductivity of certain organic molecules switch to a high-state via electroreduction. Different high-states or multilevel conductivity in organics has been due to different density of high-conducting molecules in a device. We have studied how the population distribution of reduced molecules changes in achieving different conductivity levels. In devices based on a few molecular layers, we have observed that the number of conductivity levels can exceed the number of Langmuir–Blodgett layers. The results showed that the distribution of high-conducting molecules did not increase layer-by-layer, but throughout the volume of the device enabling large number of conductivity levels for higher level (multibit) applications.
Keywords: Conductance switching; Data-storage; Electroreduction; Memory applications; Multilevel conductance; Organic semiconductors;
Effect of condensed phase environment on luminescent properties of MEH-PPV thin films by A.K. Biswas; Awnish Tripathi; Swapnil Singh; Y.N. Mohapatra (340-343).
We study sensitivity of photoluminescence emission properties to state of aggregation in MEH-PPV thin films, and its solution in chloroform and xylene (50:50) mixture from which these films are either spin-cast or drop-cast. The choice of composition of solvent is dictated by the ease of fabrication of devices. We demonstrate unambiguous correlation between relative peak heights of the two characteristic major peaks in steady state photoluminescence with processing parameters, such as spin speed, and show that their ratio can be used as a robust measure of degree of entangled packing both in films and solutions. In thin films, a small but discernible red shift of about 3–10 nm in PL peak positions, and increase in width of peaks is observed with changes in degree of packing. A comparison of PL decay dynamics at the 588 nm emission shows that the decay develops slower components with increase in entangled packing in the case of thin films as compared to the fast nearly exponential decay in the case of solution. The progressive slowing down of dynamics is attributed to shift in distribution of inter-chain species of excitons in thin films.
Keywords: Poly(phenylene vinylene) and derivatives; Photoluminescence;
Photoluminescence quantum efficiency (PLQE) and PL decay characteristics of polymeric light emitting materials by G.S. Samal; A.K. Tripathi; A.K. Biswas; Swapnil Singh; Y.N. Mohapatra (344-348).
We report photoluminescence quantum efficiency (PLQE) measurements on thin films of a variety of in-house synthesized conjugated polymers such as PPV, MEH-PPV and CN-PPV with improved techniques using integrating sphere and synchronous detection. Our method allows use of low level of excitation avoiding problems due to degradation during measurement. Time correlated single photon counting (TCSPC) has been used to study PL decay to obtain radiative lifetime controlling efficiency in these materials. The measured PL efficiency in PPV synthesized by xanthate precursor route is measured to be 0.07 ± 0.01, which is lower than reported for similar films owing to presence of intrinsic defects. The measured PL quantum efficiency of CN-PPV and MEH-PPV are 0.24 ± 0.07 and 0.17 ± 0.01, respectively. These values are comparable to the ones reported by others [N.C. Greenham, I.D. Samuel, G.R. Hayes, R.T. Phillips, Y.A.R.R. Kessener, S.C. Moratti, A.B. Holmes, R.H. Friend, Chem. Phys. Lett. 89 (1995) 241]. We suggest a reliable method of obtaining the radiative lifetime from PL decay curves using time domain spectroscopic technique.
Keywords: Poly(phenylene vinylene) and derivatives; Photoluminescence; Time-resolved fast spectroscopy;
Measurement of built-in voltage of organic semiconductor devices by Debdutta Ray; Meghan P. Patankar; N. Periasamy; K.L. Narasimhan (349-351).
The built-in voltage in organic semiconductors is usually measured by electroabsorption measurements. In this paper, we show that for tris-(8-hydroxy quinolato)aluminium (Alq) devices, modulated photocurrent spectroscopy provides a simple viable alternative to measuring the built-in voltage. This is possible because the quantum efficiency for photoconductivity in Alq is independent of the electric field. We also examine the limitations of modulated photocurrent spectroscopy in obtaining the built-in electric field.
Keywords: Photoconductivity; Organic semiconductors based on conjugated molecule; Metal/semiconductor interfaces; Schottky barrier; Detectors;
Carbon-skinned metallic wires and magnetic nanocrystals prepared from metal acetylides by Ken Judai; Junichi Nishijo; Chie Okabe; Osamu Ohishi; Hiroshi Sawa; Nobuyuki Nishi (352-356).
Cobalt acetylide (CoC2) is synthesized by ion exchange reaction of CaC2 with CoCl2 in acetonitrile under oxygen and water-free condition. CoC2 produces carbon-skinned nano-metallic crystals. Inclusion of 20% SmCl2 provides the particles with a larger cohesive force. Cu2C2 synthesized by a reported method with some modifications produces nanowires with diameters 5–20 nm. Excitation of these nano-objects by electron beam or ArF laser, or simple heating produces carbon-skinned metallic nanowires and nanocrystals stable in air.
Keywords: Metal–semiconductor magnetic heterostructures; Scanning transmission electron microscopy; Magnetic measurements; Phase-segregated composite interfaces;
Electronic and structural instabilities of mixed-stack organic charge-transfer salts by Zoltán G. Soos; Sharon A. Bewick; Anna Painelli; Alberto Girlando (357-364).
Electronic (valence) and structural (Peierls) instabilities occur either separately or together in organic charge-transfer (CT) crystals with mixed stacks of donors (D) and acceptors (A). A Peierls–Hubbard model, H CT, with site D, A energies and linear electron–phonon coupling in a harmonic lattice is shown to provide a unified microscopic description and to distinguish between thermal and quantum Peierls transitions. Vibrational, magnetic and structural data are interpreted as thermal Peierls transitions in two ionic CT salts and as dimerized ground states with spin solitons in other salts. Quantum Peierls transitions are identified in largely neutral salts, and a neutral–ionic crossover is discussed in a salt with dipolar disorder that suppresses the Peierls instability. The coincident Peierls and neutral–ionic transition of tetrathiafulvalene-chloranil (TTF-CA) is another special case of H CT.
Keywords: Semi-empirical and model calculations; Structural phase transitions; Other phase transitions; Organic semiconductors;
Morphological changes of Au nanoparticles in Au–polystyrene nanocomposites: A combined spectroscopy, atomic force microscopy and X-ray scattering study by Sudeshna Chattopadhyay; Alokmay Datta (365-367).
Au nanoparticles sputter deposited on polystyrene-coated Si and fused quartz substrates have been studied using optical spectroscopy, atomic force microscopy and X-ray reflectivity. Under the same deposition conditions, both spectroscopy and atomic force microscopy indicate clearly that the nanoparticles undergo a shape transition from near-spherical to progressively ellipsoidal as the polystyrene film becomes thinner than 4R g, R g being the radius of gyration of the polymer. There is a gradual increase in the in-plane ellipticity a/b, a(b) = semi-major (semi-minor) axis, with decrease in polystyrene film thickness from 230 to 20 nm, where b remains almost invariant for a particular deposition time while the semi-major axis increases in dimension. Electron density profiles along the depth of the films, extracted from X-ray reflectivity data, show that the Au nanoparticles sit on the top of the polystyrene film with c, the third ellipsoid axis having a dimension of about 3.0 nm irrespective of film thickness or deposition times used (7, 10, 12 and 15 s).
Keywords: Spin coating; Au nanoparticles; UV–vis–NIR absorption spectroscopy; Atomic force microscopy; X-ray diffraction;
Photoinduced absorption and nonlinear optical properties of disubstituted polyacetylenes: Theory by Alok Shukla; Priya Sony (368-371).
In this paper, we summarize results of our recent large-scale correlated calculations of nonlinear optical spectra and photoinduced absorption (PA) spectra of phenyl-disubstituted polyacetylenes (PDPA). Calculations were performed on oligomers of PDPA's using correlated-electron Pariser–Parr–Pople (P–P–P) model and configuration interaction (CI) methodology. Computed PA spectra are compared with the recent experiments of Korovyanko et al., and good agreement is obtained between the two.
Keywords: Semi-empirical models and model calculations; Organic semiconductors based on conjugated molecules; Other conjugated and/or conducting polymers;
On the field and temperature dependence of hole mobility in molecularly doped polymer by S. Raj Mohan; M.P. Joshi; A.K. Srivastava (372-375).
The charge transport properties of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) doped polycarbonate (PC) have been measured as a function of field and temperature. Hole mobility is observed to follow log μ ∝ E 1/2 (i.e. Poole–Frenkel behavior) at relatively high field strengths while at low field strengths mobility initially decreased with increase in field strength. Mobility value undergoes minima at a particular field value for a given temperature. Minima in mobility are observed to occur at higher field strength as the temperature is lowered. Mobility data and transport parameters are analyzed using the formalism of Gaussian disorder model (GDM). The mobility behavior at low field and shift of mobility minima with temperature suggests large disorder in the film. Investigation of film morphology using photoluminescence, X-ray diffraction and scanning electron microscopy suggests that large positional disorder in the film is probably due to film morphology and TPD–PC interaction.
Keywords: Amorphous thin films; Photoconductivity; Mobility;
Time domain spectroscopic study of PL decay in zinc benzothiazole suitable for white light emitting OLEDs by Samarendra P. Singh; Y.N. Mohapatra; M. Qureshi; S. Sundar Manoharan (376-379).
There has been a keen interest in metal benzothiazoles, and specifically for zinc benzothiazole for applications involving broad emission such as white light organic light emitting diodes. The broad emission characteristic is normally attributed to occurrence of exciplexes and is believed to further broaden in electroluminescence from a bilayer device. In order to understand possible role of molecular level interaction in photoluminescence (PL), we study PL decay dynamics of zinc benzothiazole powder, thin film and a virtual alloy with a hole transport material TPD (prepared by stacking very thin alternate layers of the two materials). We observe significant hastening of the PL decay from powder and ‘effective alloy’ to the thin film. To unravel the nature of changes in decay dynamics, we employ both least square fitting, and time domain spectroscopy known as time analyzed transient spectroscopy (TATS). The efficacy of the later procedure in obtaining insights is demonstrated. Our results indicate that inter molecular interaction in the condensed phase significantly influence the life time of the radiative species in this material.
Keywords: Organic semiconductors based on conjugated molecules; Photoluminescence; Time-resolved fast spectroscopy;
Role of vibrational optical phonons in the heat capacity of K3C60 by Dinesh Varshney; N. Kaurav; A. Dube; R.K. Singh (380-383).
The reported heat capacity C(T) data of alkali doped fulleride K3C60 is theoretically investigated in the temperature domain 5 ≤ T ≤ 25 K. Calculations of C(T) have been made within the two component scheme: one is the phonon (optic and acoustic) and the other is electronic contribution. We begin with the intercage interactions between the adjacent C60 cages and expansion of lattice due to the intercalation of alkali atoms based on the spring model to estimate vibrational optic and acoustic phonon frequencies from the dynamical matrix for the intermolecular alkali-C60 phonon mode. Lattice specific heat is well estimated from the Debye and Einstein approximation. Fermionic component as the electronic specific heat coefficient is deduced using the band structure calculations for metallic phase. Comparison of the coefficient of the normal state electron contribution to C with band structure calculations gives an estimate of the electron–phonon coupling strength. It is notice that electron correlations are essential to enhanced density of state over simple Fermi liquid approximation in the metallic phase. The present numerical analysis of specific heat shows dominating role of vibrational optical phonons.
Keywords: Fullerenes; Optical vibrational mode; Heat capacity; Coulomb correlations; Electron–phonon coupling;
Designing effective nonlinear optical (NLO) materials with chiral substituents by Daly Davis; K. Sreekumar; Swapan K. Pati (384-388).
The experimental strategies and theoretical understanding in transforming molecular conformations with good SHG response functions into polymers with the help of chiral groups are discussed. Chirality plays an important role in designing new types of stable second-order NLO materials. The presence of chiral molecules in the main chain of the polymer introduces acentric conformations and thereby increases the SHG efficiency. On the other hand, for strong donor and acceptor groups between a pi-bridge, large SHG coefficients can be realized at the molecular scale. Chiral molecules are noncentrosymmetric with a nonvanishing electric-dipole-allowed second-order response, which persists in bulk samples also. On the other hand, contributions of magnetic-dipole interactions, which can be strong in chiral materials, allow second-order processes in centrosymmetric materials. Also the polymers incorporated with chiral molecules showed higher response than the standard MNA. This could be due to the chiral component contribution to the electric and magnetic-dipole tensor.
Keywords: Nonlinear optics; Polymers; Ab initio quantum chemical methods and calculation; Semiempirical calculations; Laser;
Second order nonlinearity in oligoamides by Mily Bhattacharya; Puspendu K. Das (389-392).
We report the synthesis and the first hyperpolarizabilities of a series of water-soluble sodium salts of oligoamides of all the three isomeric aminobenzoic acids. The first hyperpolarizabilities of the ortho and para isomers with different number of repeat units are comparable but higher than that of the corresponding meta isomer. The β value increases with the number of repeat units until a point and then decreases. This unexpected behaviour is attributed to the aggregation due to the presence of a hydrophobic backbone containing a water-soluble polar head group. Dilution experiments using the HRS confirm the formation of aggregates in water.
Keywords: Coupling reactions; Water-soluble salts of oligoamides; Self-assembly; First hyperpolarizability;
Experimental and theoretical determination of the first-order hyperpolarizability of photo- and acidochromic indolino[2,1-b]oxazolidines by F. Castet; L. Ducasse; B. Champagne; L. Sanguinet; J.-L. Pozzo; F. Adamietz; V. Rodriguez (393-397).
We report first-order hyperpolarizability studies of a new series of photo- and acidochromic compounds based on the 10-(2-arylethenyl)indolino[2,1-b]oxazolidine unit combined with various styrylic residues, through polarized hyper-Rayleigh scattering measurements and quantum chemical calculations. The reversible photo- and acido-induced oxazolinic ring opening is accompanied by a major change in the conjugation of the π electrons and therefore NLO-phores are generated. Structure–property relationships concerning intrinsic molecular non-linearity of these multiaddressable molecular switches are presented.
Keywords: Hyperpolarizability; Oxazolinic ring; Non-linearity;
Structure and electronic properties of the Watson–Crick base pairs: Role of hydrogen bonding by Sairam S. Mallajosyula; Ayan Datta; Swapan K. Pati (398-401).
The hydrogen bonding patterns in the adenine–thymine (A–T) and guanine–cytosine (G–C) base pairs for B-DNA has been studied using the density functional theory. The H-bond for the crystal geometry is found to differ considerably from the geometry optimized structures for the free base pairs with larger deviation for the G–C pair compared to the A–T pair. Furthermore, the H-bonding patterns are found to be highly non-local and co-operative. For the N···H―N bond in the G–C pair, the proton hops in between two symmetric double-well potentials localized on the donor and the acceptor, due to cancellation of the local polarizations of the top and down N―H···O and O···H―N bonds, respectively. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) gap (HOMO–LUMO gap) for the free bases decreases with increase in the strength of the H-bonds in between the base pairs with larger decrease for the G–C pair. The adiabatic electron affinity (AEA) for the free bases which are found to be all negative become positive on the formation of the H-bonds and more so for the G–C pair.
Keywords: Ab-initio quantum chemical methods; Density functional calculations; Self-organization; Watson–Crick base pairs; H-bonding;
Proton-pump mechanism in retinal Schiff base: On the molecular structure of the M-state by Ayan Datta; Swapan K. Pati (402-405).
Theoretical characterizations of the various intermediates in the proton-pump cycle of the retinal Schiff base in the Halobacterium salinarium have been performed. Contrary to the general belief over the years that the most stable intermediate, the M-state, is a non-protonated cis-isomer, we find that the M-state is a polarized cis-isomer stabilized due to interactions of the dissociating proton with the π-electrons. The role of proton in the pump cycle is found to be profound leading to the stabilization or in certain cases destabilization of the intermediates. We propose the chemical structure of the M-state for the first time.
Keywords: Quantum chemical methods; Density functional calculations; Non-linear optical methods; Excitation spectra calculations;
Optimisation of photorefractive multiple quantum wells for biomedical imaging by P. Yu; S. Balasubramanian; T.Z. Ward; M. Chandrasekhar; H.R. Chandrasekhar (406-409).
A novel method has been proposed and tested to optimize the photorefractive multiple quantum wells device for biomedical imaging. The device acts as a dynamic holographic beam combiner in a holographic optical coherence imaging. By choosing an appropriate duty cycle of the external electric field, the Joule heat limit and saturation of exciton are decreased. We examine several characteristics that govern the use of the device.
Keywords: Quantum wells; Semiconductor films; Detectors;
Metabolic sensing using fluorescence by David J.S. Birch; Ashok Ganesan; Jan Karolin (410-413).
With a view to assessing the current boundaries of non-invasive sensing of metabolites we compare the photophysical properties of melanin nanoparticles in vivo, size selected in natural form from Sepia officinalis extracted from cuttlefish, and synthesised from l-dihydoxyphenylalanine (DOPA) in water pools of reverse micelles. The fluorescence emission spectra and excited state decay components correlate well with the particle size. Recent developments in optical technology are discussed in the context of the formidable challenges and opportunities for metabolic sensing presented by endogenous fluorophores.
Keywords: Self-organisation in macromolecules; Optical absorption and emission spectroscopy; Time-resolved fast spectroscopy;
Observation of ultrafast conformational changes in carboxy-myoglobin by time-resolved circular dichroism by Thibault Dartigalongue; François Hache (414-417).
A time-resolved circular dichroism experiment is carried out on carboxy-myoglobin. CD is measured with a sub-picosecond time resolution after ligand dissociation. We observe a decrease of the CD signal in a few picoseconds followed by a 100 ps relaxation towards the deoxy-myoglobin values. Thanks to a calculation developed after the polarizability theory, we are able to assign this signal to a global reorganization of the protein conformation.
Keywords: Optical absorption and emission spectroscopy (UV–vis-NIR absorption); Time-resolved fast spectroscopy;
Charge transport in DNA strands using fragment orbital theory by A. Fritsch; L. Ducasse; F. Castet; S. Ramasesha (418-421).
A semi-empirical Valence-Bond/Hartree–Fock (VB/HF) method is developed to calculate one- and two-electron interactions between molecular fragments in conducting supramolecular stacks. This fragment orbital-based formalism allows for the consistent extraction of an effective hamiltonian defined as a “frontier orbital” model. This hamiltonian quantitatively describes transfer and electrostatic interactions between conducting electrons while reducing the active space so dramatically that the electronic eigenstates of very large systems may be investigated. The VB/HF formalism is applied here to the derivation an effective model for conduction holes along doped DNA double strands. Transferable intra and intersite parameters are first evaluated from VB/HF calculations carried out on nucleoside pairs. From this interaction databank, the effective hamiltonian of any kind of nucleoside sequence can eventually be defined. The thermalized charge distribution for a single hole delocalized along DNA sequences is then calculated and compared to the experimental yields of damage revealed by photocleavage experiments.
Keywords: Semi-empirical models and model calculations; Many-body and quasi-particles theories; Computer simulations;
Probing of the microwave radiation effect on the green fluorescent protein luminescence in solution by Anan Copty; Fadi Sakran; Oleg Popov; Roy Ziblat; Tsafi Danieli; Michael Golosovsky; Dan Davidov (422-425).
Microwaves have a larger effect on the green fluorescent protein (GFP) fluorescence intensity than is observed by conventional thermal heating. Our measurements show that thermally heating a GFP solution from 7 to 40 °C results in a ∼1% decrease in fluorescence for every 1 °C. On the other hand, under 250 mW of localized microwave irradiation, the fluorescence can decrease by up to 3–10% with an accompanying temperature rise of only 1 °C.
Keywords: Special-purpose functionalized polymers (GFP); Photoluminescence;
Electrochemical and spectrophotometric studies on dyes and proteins labelled with dyes by B.S.B. Salomi; Chanchal K. Mitra; Lo Gorton (426-429).
Electrochemical and fluorometric studies on a few selected phenazine and phenoxazine dyes has been carried out in free conditions and when covalently attached to a protein. The dyes have been coupled using the conventional coupling agent carbodiimide (derivative). Neutral Red and Nile Blue A, show significant shifts in their emission peaks to shorter wavelengths on binding to bovine serum albumin (BSA). Phenazine dyes such as Safranine O and Neutral Red show a shift in their formal redox potential to more positive potential on binding to BSA along with the two electron transfers taking place in a single step unlike incase of the free dyes where the two electron transfers take place at different potentials. Nile Blue A shows a negative shift in the reduction peak on covalently binding to BSA. These dyes when covalently coupled to the proteins mediate intramolecular electron transfers.
Keywords: Covalent coupling; Labelled protein; Safranine O; Neutral Red; Nile Blue A;
Persistent currents with long-range hopping in 1D single-isolated-diffusive rings by Santanu K. Maiti; J. Chowdhury; S.N. Karmakar (430-433).
We show from exact calculations that a simple tight-binding Hamiltonian with diagonal disorder and long-range hopping integrals, falling off as a power μ of the inter-site separation, correctly describes the experimentally observed amplitude (close to the value of an ordered ring) and flux-periodicity ( h c / e ) of persistent currents in single-isolated-diffusive (SID) normal metal rings of mesoscopic size. Long-range hopping integrals tend to delocalize the electrons even in the presence of disorder resulting orders of magnitude enhancement of persistent current relative to earlier predictions.
Keywords: Model calculations; Magnetotransport;
Optical diffraction of second harmonic generation in SrBi2(Nb0.7V0.3)2O9 in the SrO–Bi2O3–0.7Nb2O5–0.3V2O5–Li2B4O7 glass system by B. Harihara Venkataraman; K.B.R. Varma; Vincent Rodriguez; Mario Maglione; J. Etourneau (434-438).
Transparent glasses in the system (100 − 3x)(Li2O–4B2O3)–x(SrO–Bi2O3–0.7Nb2O5–0.3V2O5) (where x = 10, 30 and 50, in molar ratio) embedded with nanocrystallites of SrBi2(Nb0.7V0.3)2O9 exhibited intense second harmonic signals in transmission mode when exposed to IR laser light at λ = 1064 nm. The second harmonic waves were found to undergo optical diffraction. The origin of optical diffraction in these samples was attributed to the self organised structures of fine crystallites of submicrometer size that were inscribed in-situ by the IR laser radiation. Laser Raman studies confirmed these crystallites to be vanadium doped strontium bismuth niobate.
Keywords: Melt processing; Crystalline/glass interfaces; Transmission electron microscopy; Glass transitions; Nonlinear optical methods;
Excited states absorptions in oligomers of PPV by Haranath Ghosh (439-442).
Recent experimental studies based on pump-probe spectroscopy of phenyl-based polymers reveal quite different relaxation dynamics of low and high energy even parity states reached by photoinduced absorption (PA). One class of even parity states (mAg) experiences ultrafast internal conversion to the lowest singlet state (1Bu), whereas the other class (kAg) in violation of the Vavilov–Kash's rule undergoes a different pathway. We therefore investigate theoretically the nature of higher excited states, both even as well as odd parity states, for various sizes of oligomers of poly(para-phenylene vinylene) (PPV). We study Third Harmonic Generation (THG) of oligomers of PPV within a rigid band correlated Pariser–Parr–Pople (P–P–P) model Hamiltonian, using the π-electron basis and the powerful multireference single and double configuration interaction (MRSDCI) method. We also use single–double–triple–quadruple configuration interaction (SDTQCI) in limited cases. We reconfirm the nature of (mAg) states, which are responsible for the low energy PA band. We also show that in THG three kinds of one-photon states 1Bu, jBu and nBu (where j < n) appear whereas two kinds of even parity states 2Ag and mAg appears. While mAg absorptions appear just after the nBu, the 2Ag absorption peak appears immediately after the jBu absorptions. The later absorption features were not seen in case of linear chain materials like polydiaccetylene (PDA).
Keywords: Poly(phenylene vinylene) and derivatives; Models of non-linear phenomena; Many body and quasi-particle theories;