Probing pi-conjugated polymers with circularly polarized light

In this thesis, we use circular polarized light to study aggregation in p-conjugated polymers at various structural levels. The importance of this study stems from the necessity to understand polymer packing and aggregation in thin films, used as active layer in optoelectronic devices like solar cells and light-emitting diodes. A systematic study on understanding polymer aggregation would provide freedom to tune the opto-electronic properties of the polymer in desired fashion. p-conjugated polymer have a backbone consisting of alternating single and double bonds, which gives these polymers semiconducting properties and creates optical transitions in visible wavelength region. These opto-electronic properties depend strongly on the conformation of the polymer backbone and on interchain contacts arising from aggregation of polymer chains. Here we use circularly polarized light to probe the optical, conformational and electronic properties of p-conjugated polymers. This thesis focuses on chiral p-conjugated polymers. Chirality serves here as a spectroscopic ‘label’ allowing us to investigate aggregation and chiroptical properties of the polymer chains using circular dichroism (CD) spectroscopy. CD spectroscopy allows us to study differential absorption between left and right handed circularly polarized light. The dissymmetry ratio in absorbance is often termed as gabs. The gabs can be independent of the amount of the material, in which case the gabs can be regarded as intensive property of the material. On the other hand, if it is dependent on the amount of the material, gabs can be associated as the extensive property. Chapters 2 and 3 utilize these definitions to study the intensive and extensive properties of chiral polythiophene and chiral polyfluorene respectively. Chapter 3 divulges further into the origins of the intensive and extensive nature of gabs, hence outlining the polymer aggregation at different structural levels. On investigating poly[3-((3S)-3,7-dimethyloctyl)thiophene] (PT811), gabs was found to be independent of the film thickness (Chapter 2). Thus the gabs in this case can be regarded as intensive, originating from a chiral molecular organization on a length scale <50 nm. Having understood the intensive nature of gabs, a detailed study on aggregation of poly[9,9-bis((3S)- 3,7-dimethyloctyl)-2,7-fluorene] (PF811) was carried out (Chapter 3). It was found that pristine films of PF811 also show gabs that is independent of the film thickness and hence can be regarded as intensive in nature. AFM reveals that the polymer chains collapse into fibrillous aggregates upon spin coating the solutions of PF811. Similar CD effects were also observed in solution by inducing aggregation in polymer chains by adding a non-solvent (MeOH) to molecularly dissolved solutions of PF811 in chloroform. These aggregates can be transferred from solution to film. These films also show similar fibrillous aggregates. This suggests that the intensive gabs is probably related to chiral arrangement of a few polymer chains within a fibril. Interestingly, it was found that thermally annealed films of PF811 show gabs dependent of the thickness of the film, indicating that here the CD effects arise on a larger length scale (>50 nm). The AFM shows retention of fibrils, suggesting long range ordering of fibrils, probably cholesteric in nature. Chapters 2 and 3 dealt with the differential absorbance of circularly polarized light in transmission mode. However, information about polymer aggregation can also be extracted from reflection and scattering of circularly polarized light, which is emphasized in Chapter 4. It was observed that the annealed films of PF811 show a CD effect for > 450nm, even though polymer does not absorb in this wavelength range. This anomalous behavior was investigated via a tailor-made setup and it was found that this apparent CD is a result of selectivity in scattering of left and right circular polarized light by the polymer film. Interestingly CD effects are also observed in triplet-triplet photoinduced absorbance of the PF811 polymer (Chapter 5). The large CD observed here most likely arises from interaction of the linearly polarized photoinduced transition T1-Tn with the surrounding birefringent matrix of thermally annealed polymer. Besides being an important blue light-emitting polymer, polyfluorenes (PFs) are also interesting because they can occur in a and ß aggregates, each having its own optoelectronic properties. Processing conditions are of paramount importance in promoting the aggregation/phase. Through temperature dependent CD studies of PF811 in 1-octanol, these aggregates could be identified by their different CD spectra for the first time (Chapter 6). It was discovered that dilute solutions often yielded ¿-aggregates, whereas slow controlled cooling of concentrated solutions led to formation of ¿-aggregates. Remarkably though, pi- aggregates were found to appear only via formation of a precursor involving cooperative folding of two or more chains. Rod-like behavior of PFs is well known and anisotropy in achiral PF films has already been established by their different in-plane and out-of-plane optical constants obtained by spectroscopic ellipsometry (SE). Here one essential measures the change of linearly polarized light into elliptically polarized light upon reflection from the polymer film. The anisotropic dielectric tensor of chiral polyfluorene is determined via variable angle spectroscopic ellipsometry and circular dichroism spectroscopy (Chapter 7). The anisotropy was found to be influenced by high in-plane alignment of polymer chains in the thin film. Chirality of the polymer results in small but non-zero off diagonal matrix elements in the dielectric tensor. Subsequently, optical rotation and ellipticity in reflection are predicted for pristine film of chiral polyfluorene. In all previous chapters circular polarized light has been mostly employed to investigate optical and conformational aspects of ¿-conjugated polymers. Chapter 8 utilizes polarized light to study electronic properties of ¿-conjugated polymer in p-n junction device involving poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and ZnO. The Moss- Burstein shift, i.e. an apparent increase of the optical band gap, is observed upon UV illumination of a layer of 5 nm ZnO nanoparticles in O2-free atmosphere using spectroscopic ellipsometry. This shift is caused by populating the conduction band with excess free charge carriers and depleting the valence band. The free charge carrier density was determined to be 2 ¿ 10-19 /cm3, about one carrier per particle. The shift can be reversed by introduction of O2. For junctions of ZnO nanoparticle layers with a pH neutral PEDOT:PSS conducting polymer layer a depletion width of 5 nm in ZnO is observed under UV illumination in O2-free atmosphere. At the junction free charge carriers in ZnO are transferred to neutral-PEDOT:PSS causing a slight reduction of the latter

Percutaneous Method of Management of Simple Bone Cyst

Introduction. Simple bone cyst or unicameral bone cysts are benign osteolytic lesions seen in metadiaphysis of long bones in growing children. Various treatment modalities with variable outcomes have been described in the literature. The case report illustrates the surgical technique of minimally invasive method of treatment. Case Study. A 14-year-old boy was diagnosed as active simple bone cyst proximal humerus with pathological fracture. The patient was treated by minimally invasive percutaneous curettage with titanium elastic nail (TENS) and allogenic bone grafting mixed with bone marrow under image intensifier guidance. Results. Pathological fracture was healed and allograft filled in the cavity was well taken up. The patient achieved full range of motion with successful outcome. Conclusion. Minimally invasive percutaneous method using elastic intramedullary nail gives benefit of curettage cyst decompression and stabilization of fracture. Allogenic bone graft fills the cavity and healing of lesion by osteointegration. This method may be considered with advantage of minimally invasive technique in treatment of benign cystic lesions of bone, and the level of evidence was therapeutic level V

Regulación de la expresión del gen homeodominio Alx3 por ácido fólico y su participación en el cierre del tubo neural craneal durante el desarrollo embrionario

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid. Facultad de Medicina. Departamento de Bioquímica . Fecha de lectura: 19 de Enero de 200

Valor pronóstico de la recuperación de la inmunoparesia en el mieloma múltiple.

La inmunoparesia, definida como una disminución de las inmunoglobulinas policlonales, es un hallazgo muy frecuente en el mieloma múltiple al diagnóstico. Una parte de estos pacientes recuperan los niveles de inmunoglobulinas al ser tratados. Nosotros analizamos el valor pronóstico de esta recuperación de la inmunoparesia midiendo supervivencia global (SG), supervivencia libre de progresión (SLP) y supervivencia libre de enfermedad (SLE) en una muestra de pacientes homogénea tratados dentro del ensayo clínico GEM2010MAS65 del grupo cooperativo de trabajo GEM/PETHEMA que habían alcanzado respuesta completa o muy buena respuesta parcial. Observamos que los pacientes que recuperaron la inmumoparesia tenían mejor SG, SLE y SLP que los que no la recuperaron, constituyendo un factor pronóstico independiente en el análisis multivariante. Además observamos que la recuperación podía ocurrir durante el tratamiento o al finalizar el mismo, con el mismo valor pronóstico. También constatamos que la recuperación de la inmunoparesía podía ser un factor pronóstico más potente que la profundidad de la respuesta convencional en algunos pacientes

Mergers and acquisitions’s impact on financial performance: an evaluation with perspective of time

Mergers and Acquisitions (M&A) is the most prefered technique of the corpoates from diversed areas across the globe for achieiving inorganic growth. In Indian context, M&A has a graceful history from pre liberalization to post librelization period wherein the companies have used this process in different scenarios to accomplish various objectives. Though this method have enormous benefits but at the same time are firms are able to convert these qualitative aspects into quantitative form and if yes than do they see an immediate impact or it takes considerable time to reflect same in their financial performance. The present research work with the help of Du Pont Return on Assets (ROA) framework assess the success of M&A in the long run by taking a sample of 24 companies that have acquired companies in financial year 2005- 2006.Keywords: mergers and acquisitions; du pont analysis; long run; financial performance; return on asset

Probing the switching mechanism in ZnO nanoparticle memristors

We investigate the resistance switching mechanism in memristors based on colloidal ZnO nanoparticles using electroabsorption (EA) spectroscopy. In this EA experiment, we incorporate a small amount of low-bandgap polymer, poly(9,9-dioctylfluorene-cobenzothiadiazole) (F8BT), as a probe molecule in ZnO-nanoparticle memristors. By characterizing this polymer, we can study the change of built-in potential (VBI) in the device during the resistance switching process without disturbing the resistance state by the EA probe light. Our results show that VBI increases when the device is switched to the high resistance state, suggesting a shift of effective workfunction of the electrode. Thus, we attribute the resistance switching to the field-dependent migration of oxygen vacancies associated with the adsorption and desorption of oxygen molecules at the Al/ZnO interface. This process results in the modulation of the interfacial injection barrier which governs the resistance state of the device.This work was supported by the Engineering and Physical Sciences Research Council [Grant Number EP/G060738/1]Copyright 2014 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. This article "Probing the switching mechanism in ZnO nanoparticle memristors" has been accepted by Journal of Applied Physics. After it is published, it will be found at http://scitation.aip.org/content/aip/journal/ja

Safe Water – Is it really Safe?

Entering into the new field of business is not an easy go. One has to do market research, to identify the gap, to identify the product/service to be offered, and to target the right customer segment. After doing the required research, the next step is to make decisions on plant location, layout and then set up.  Assessment of funds required for capital as well as revenue expenditure, procurement of machinery and other materials also take substantial amount of efforts. Searching for vendors and finalizing the right one are few other issues. After having worked successfully for more than 30 years in Bali in the field of sesame seeds and oil business Harsh was standing on a crossroad in 2011. Rising prices of sesame oil resulted in shift in customers’ taste and preferences towards refined oil. Moreover entry of big players at a large scale of operations and rise in price of raw material (sesame seeds) made survival tough. He had a number of options, one to continue with existing business at different scale of operations. Two, enter another business? Three, what business, Four, which place? The case talks about Harsh’s journey to new business, his choice and then the result

In situ optical measurement of charge transport dynamics in organic photovoltaics.

We present a novel experimental approach which allows extraction of both spatial and temporal information on charge dynamics in organic solar cells. Using the wavelength dependence of the photonic structure in these devices, we monitor the change in spatial overlap between the photogenerated hole distribution and the optical probe profile as a function of time. In a model system we find evidence for a buildup of the photogenerated hole population close to the hole-extracting electrode on a nanosecond time scale and show that this can limit charge transport through space-charge effects under operating conditions.This work was supported by the EPSRC [Grant number EP/ G060738/1].This is the author accepted manuscript. The final published version is available at http://pubs.acs.org/doi/abs/10.1021/nl503687u

A model for exciton-polaritons in uniaxial molecular crystals describing spatial dispersion, refraction and reflection

Propagation of light through a uniaxial material is studied using field theoretical methods. The materials is modeled by cubic lattice of oriented classical Lorentz oscillators. A two-step coarse graining approach is applied. At the bulk level, excitations of the coupled light-matter system, or polaritons, are described by a Proca-type equation for massive vector bosons. On the microscopic level, multiple scattering is used to relate the sub-luminal speed of the polaritons to the polarizability of the Lorentz oscillators. For each direction of propagation of the polaritons, three independent polarizations exist, consistent with the integer spin of massive vector bosons. Reflection and refraction are calculated by imposing the requirement of a uniform gauge for the electromagnetic vector potential across the interface of the uniaxial molecular material and vacuum. Reflectance spectra near the resonance frequency are calculated. The spectra feature a characteristic minimum in middle of the reflection band, in agreement with experiment. An incident unpolarized light beam is predicted to refract into three different rays. The model supports surface bound excitations and predicts a Goos-Haenchen shift of the reflected beam upon reflection of light incident from vacuum onto the material.Comment: 55 pages, 9 figure