Search

The aim of this study was to evaluate the possibility of reducing the radiation dose to paediatric patients undergoing computed tomography (CT) brain examination by using image-enhancing software. Artificial noise was added to the raw data collected from 20 patients aged between 1 and 10 y to simulate tube current reductions of 20, 40 and 60 mA. All images were created in duplicate; one set of images remained unprocessed whereas the other was processed with image-enhancing software. Three paediatric radiologists assessed the image quality based on their ability to visualise the high- and low-contrast structures and their overall impression of the diagnostic value of the image. For patients aged 6-10 y, it was found that dose reductions from 27 mGy (CTDI(vol)) to 23 mGy (15 %) in the upper brain and from 32 to 28 mGy (13 %) in the lower brain were possible for standard diagnostic CT examinations when using the image-enhancing filter. For patients 1-5 y, the results for standard diagnostics in the upper brain were inconclusive, for the lower brain no dose reductions were found possible.

A method based on the piezoelectric quartz crystal microbalance (QCM) technique for in vitro real-time characterization of cell attachment and spreading on surfaces has been developed. The method simultaneously measures the resonant frequency, f, and the dissipation energy, D, of the oscillating system. The QCM responses are sensitive to very small amounts (a few hundreds) of cells and highly specific to surface chemical properties. The first results from deposition of cells on two polystyrene surfaces of different wettability in serum-containing medium are reported. It has previously been shown that a decrease in f is related to the degree of cell spreading. In our data it appears that the extent or quality of cell attachment is reflected in an increase in D caused by adhering cells. The combined information from f and D measured by this technique might therefore be useful to probe cell-surface interactions for biomaterials.

We explore the potential of using a quartz crystal microbalance based technique for the characterization of living cells during the process of adhesion to a surface. The combined information from s...

Trichodesmium is the first described example of a filamentous cyanobacterium without heterocysts that contains cells specialised for nitrogen fixation. The ultrastructure of cells with and without nitrogenase were compared using primarilyTrichodesmium tenue Wille, but alsoT. thiebautii Gomont andT. erythraeum Ehrenberg et Gomont. Immunohistochemistry demonstrated that the cytoplasm of certain cells was densely labelled with antibodies against Fe-protein (dinitrogenase reductase). Comparative TEM-image analysis revealed that these cells were also distinguished by a denser thylakoid network, dividing the vacuole-like space into smaller units. The nitrogenase-containing cells also exhibited less extensive gas vacuoles as well as fewer and smaller cyanophycin granules compared to cells which lacked nitrogenase. Carboxysomes were present in both cell types in equal proportion. Longitudinal sections showed that cells with nitrogenase were arranged adjacent to each other, and that groups of cells with and without nitrogenase may coexist in the same trichome. The correlation between modifications in ultrastructure and the presence of nitrogenase suggests a new type of cyanobacterial cell specialisation related to nitrogen fixation. The results obtained also question the systematic affiliation of the genusTrichodesmium.

We report a combined theoretical and experimental study of the interaction between a series of transition metals (Ni, Cu, Zn) and acrylonitrile. We demonstrate experimentally that the transition metal has a selective role in the grafting of electropolymerized acrylonitrile. Nickel and copper substrates support the formation of a polyacrylonitrile film, while zinc does not. Quantum chemical calculations indicate that acrylonitrile molecules form π-d bonds with Ni and Cu atoms, but do not react chemically with a Zn atom, in qualitative agreement with the experiment. Computational results also show that the electron affinity is significantly increased upon chemisorption, promoting radical anion species at the metal surface as initiators for polymerization.

We have employed quantum chemical methods, at the local spin density approximation level, to study the interaction between an organic semiconductor, polythiophene, and potential metals for hole injecting contacts in devices: vanadium, chromium, copper, and gold. The results show that there is a strong interaction between vanadium and the thiophene ring, mainly due to covalent bond formation between the metal and the S and Cα atoms of the thiophene. Vanadium is therefore predicted to provide good conditions for chemisorption and mechanical stability at the polymer/contact interface. A similar, but considerably weaker, covalent interaction is found between chromium and all the conjugated atoms of the thiophene molecule. For both these metals, the interactions cause the thiophene ring to lose its aromaticity and planarity which, as a consequence, would interrupt the π‐electron system in a polymer and impair charge transport along the chains. In the case of copper, the metal is found to react only with the su...

The chemical nature and the electronic structure of metal/conjugated polymer interfaces are investigated in the context of polymer-based light-emitting diodes. We consider the interaction of low-workfunction metals (Al, Ca) with the surface of conjugated polymers or model oligomer molecules with a combined experimental and theoretical approach. The early stages of the interface formation are followed with X-ray and ultraviolet photoelectron spectroscopies and the experimental data are compared to the results of quantum chemical calculations. The reactions of Al and Ca with the organic surface are found to be fundamentally different: while the former forms new covalent bonds onto the polymer backbone, the latter tends to dope the conjugated system. Both types of reaction are expected to modify drastically the electronic properties of the polymer semiconductor.

The chemical and electronic structure of ultrathin molecular films of trimethylamine alane (TMAA), condensed in UHV at − 100°C, have been studied in the solid state, using both X-ray and ultraviolet photoelectron spectroscopy. The results are analyzed with the help of quantum chemical calculations at the ab initio Hartree-Fock 6-31G ∗ level. Based upon the good agreement between theory and experimental, it is determined that clean, oxygen-free, condensed molecular solid films consist of the 2:1 adduct of TMAA, which was previously uncertain. In addition, based upon the electronic structure results, it is clear that the mechanism of the photodecomposition of TMAA can be explained in terms of the wavefunction of electrons photoexcited into the first unoccupied molecular orbital.

Quantum chemical calculations are performed to theoretically study the interaction between low work function metal atoms and trans‐tetradecaheptaene, an extended model molecule for π‐conjugated polymers. We present characterizations of metal/polyene complexes obtained by the local spin density method. The results show that charge transfer reactions occur for Na and Ca and the formation of a covalent bond for Al. Mg atoms, however, are found not to react significantly with π‐conjugated systems. Differences in the bonding mechanism between the metals are discussed and the theoretical density of valence states are compared to experimental photoelectron spectroscopy (UPS) data. The results demonstrate that the local spin density method can be successfully used to interpret valence UPS spectra from early stages of metal/π‐conjugated interface formation.

The semi-empirical Austin Model 1 and the non-empirical pseudo-potential valence effective Hamiltonian (VEH) methods as well as the local spin density (LSD) approximation technique have been applied to the investigation of the doping-induced electronic and geometrical changes in some conjugated molecules related to poly(p-phenylene) and poly(p-phenylenevinylene) (PPV): biphenyl, stilbene and a phenyl-capped dimer of PPV. The theoretical results are compared with experimental valence band spectra, as recorded by ultraviolet photoelectron spectroscopy (UPS). The experimental UPS studies show that two ingap states are detected upon doping with alkali metals. The energy splitting between the two in-gap states increases as the molecule size decreases. The results of the LSD calculations agree very well with the experimental results, while the VEH method overestimates the energy splitting for the small molecules. The LSD modelling also indicates a destabilization of several high binding energy valence levels, due to the presence of counter-ions, in agreement with experiment.

The interactions between aluminum atoms and model molecules representing trans‐polyacetylene are studied quantum chemically by a local density functional method. We focus on the chemical and electronic structure of the organoaluminum complexes. Special emphasis is put on a comparison between results at the local spin density approximation and ab initio Hartree–Fock levels. In unmetallized polyenes, the density functional method provides a very good description of the carbon–carbon bond lengths of conjugated systems; in the case of hexatriene, it reproduces the bond dimerization in very good agreement with experimental measurements. Upon metallization, a strong covalent interaction between aluminum and carbon is found. The Al–C bond formation induces an interruption of the bond alternation pattern and reduces the π‐conjugation in the oligomer, in qualitative agreement with photoelectron spectroscopy data and previous theoretical results at the Hartree–Fock level. Notably, the π‐electron levels in the organoaluminum complexes maintain delocalization. In contrast to Hartree–Fock results where an aluminum atom binds to a single carbon, the interactions calculated with the local spin density approximation lead to (i) formation of multicenter aluminum–carbon bonding; (ii) near planarity of the polyene molecule; and (iii) a lower degree of charge transfer from the metal atom to the polymer.

The interactions between different low work function metals aluminium,calcium and sodium, and α,ω‐diphenyltetradecaheptaene, a model molecule for certain conjugated polymers, have been investigated using both x‐ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The spectra are interpreted with the help of the results of quantum chemical calculations performed within the local spin density (LSD) approximation methodology. The metals are found to interact with the conjugated system in very different ways. Aluminium forms a covalent bond, which strongly modifies the π‐electronic structure of the conjugated molecule, while both the sodium and the calcium atoms act as doping agents, inducing new states in the otherwise forbidden bandgap. These new gap states can be viewed as a soliton–antisoliton pair for the Na/DP7 and a bipolaronic‐like defect for Ca/DP7.

The chemical and electronic structure of the interface between aluminum and several proto-typical conjugated systems is investigated with a combined experimental and theoretical approach. The experiments consists of following the evolution of the polymer surface during the early stages of aluminum deposition, with X-ray and Ultraviolet Photoelectron Spectroscopies (XPS, UPS). In parallel, quantum chemical calculations are performed on model oligomer systems interacting with isolated Al atoms. Aluminum is found to interact strongly with the polymer chain. New covalent Al-carbon bonds are formed along the polymer backbone; the chain geometry is deeply modified and the π electron conjugation can be dramatically reduced.

New results on studies of the early stages of formation of the aluminum-poly(p−phenylenevinylene) interface are presented.

The interaction between Al atoms and trans-polyacetylene has been studied quantum chemically at the ab initio Hartree-Fock level using oligomeric model systems. Investigations of the Al-polyacetylene bonding and modifications to the chemical and electronic structure of model systems for polyacetylene upon interaction with Al atoms are reported. The density-of-states is calculated for a polyene chain interacting with a pair of Al atoms. The results are discussed in relation to photoelectron spectra taken during Al deposition on an oligomeric model for polyacetylene (a diphenylpolyene with 7 C=C bonds in the polyene segment).

The π-electronic structural changes in diphenylpolyene, α,ω-di[henyltetradecaheptaene, or DP7, have been studied upon gradually doping with sodium Xu using X-ray and Ultraviolet Photoelectron Spectroscopy, XPS and UPS. The spectra are interpreted with the help of the results from Austin Model 1 (AM1) and Valence Effective Hamiltonian (VEH) quantum chemical calculations. The results of the doping with sodium can be discussed in terms of two confined solitons on the polyene part of the molecule, which results in two new energy levels in the originally forbidden energy gap.

A molecular quantum chemical approach is used to study the aluminum on PPV (Poly(p-Phenylene Vinylene)) interface. We focus on modifications to the chemical and electronic structure of the polymer upon interaction with a submonolayer of aluminum. A model system, trans-stilbene, is taken to investigate the nature of the Al-PPV bonding. Energetically favorable conformations are then used as prototypes to study the evolution of the electronic structure as modified by the reaction with aluminum. Results at the ab initio Hartree-Fock level indicates that Al atoms react with the vinylene linkage rather than the phenyl groups at early stages of interface formation.

Recently, it has been shown that ordered overlayers of many organic and polymeric materials can be made on specially-prepared ultra-thin films of polytetrafluoroethylene, or PTFE [1]. We have prepared PTFE films, in the range of 15 to 35 A in thickness, by a slightly modified version of the hot-dragging method of Pooley and Tabor [2], and studied the electronic structure using ultraviolet and X-ray photoelectron spectroscopy, UPS and XPS. The electronic structure results are compared with new ab initio quantum chemical electronic structure calculations.

The interaction between aluminum and α-ω-diphenyltetradecaheptaenee (DP7), α-sexithienyl (6T), and poly( p -phenylenevinylene) (PPV), respectively have been studied using both X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS). The UPS valence band spectra, are interpreted with the help of quantum chemical calculations based upon Modified Neglect of Diatomic Overlap (MNDO), Valence Effective Hamitonian (VEH) and ab initio Hartree-Fock methods. DP7 is a model molecule for polyacetylene, while 6T is a model molecule (an oligomer) of polythiophene. The results indicate that aluminum reacts strongly with the surfaces of all of the materials studied. The π-electronic structure of each material was strongly modified. Furthermore, aluminum reacts preferentially with the polyene partof DP7, with the vinylene part of PPV, and with the α-carbons of the thiophene nits of 6T.

Molecular model systems are used to quantum chemically investigate the interface between aluminum and trans‐polyacetylene. Modifications to the chemical and electronic structure of trans‐polyacetylene oligomers upon interaction with a submonolayer of aluminum are studied at the semiempirical and ab initio Hartree–Fock levels. An aluminum atom is found to react strongly with a carbon atom of the trans‐polyacetylene chain to form a heteropolar covalent bond. In this process, the binding carbon evolves from an sp2‐ to an sp3‐hybridized electronic structure. Significant contributions from Al 3s and 3p atomic orbitals are found in the frontier molecular orbitals in aluminum/polyene complexes. This results in the fact that despite the presence of sp3 sites due to Al–C bonds, which reduces π conjugation along the chain, a large degree of delocalization in π levels is maintained. Our calculations are discussed in relation to experimental ultraviolet photoelectron spectra (UPS) taken during initial stages of aluminum deposition on oxygen‐free films of trans‐polyacetylene oligomers.

The advantages of using model systems for spectroscopic studies of conjugated polymers and interface formation, as well as for charge-induced electronic and geometric structural changes, are discussed. The electronic structure of a diphenylpolyene, α,ω-diphenyltetradecaheptaene, or DP7, is an example of a model molecular system studied using X-ray and Ultraviolet Photoelectron Spectroscopy, XPS, and UPS. The spectra are interpreted with the help of the results from MNDO, VEH and INDO/S-CI quantum chemical calculations. The frontier orbitals of DP7 are localized mostly on the polyene chain portion of the molecule, resulting in a high degree of separation of the phenyl and polyene parts of the π-system. The INDO calculations show two regions of shake-up features corresponding to a benzene-like part and a polyene-like part. The most important individual shake-up transitions, which contribute to the two observed shake-up spectral features, involve one-electron redistributions separable into contributions from the polyene chain and from the phenyl groups. The analysis indicates the extent to which many chemical and electronic properties of DP7 are expected to be similar to those of (at least short chain) trans -polyacetylene.

We have performed split valence ab initio Hartree—Fock calculations of the total binding energy, E B , of charged solitons to donor impurities in doped trans-polyacetylene. The total energy is calculated for a system with the soliton pinned to the impurity and for a system where the soliton is isolated from the impurity. E B is taken to be the difference between the optimized total energies of these two systems. For a sodium counterion we obtain a total binding energy of E B = 3.18 eV. This value reduces to 2.25 eV when Moller—Plesset second-order perturbation corrections are included. The energy of the soliton level relative to the valence band edge is found to decrease by 0.22 eV in the presence of a sodium counterion, a result which correlates well with optical absorption data.

The geometrical and electronic properties of a negatively charged soliton in trans -polyacetylene are calculated at the ab initio Hartree-Fock level. The optimized geometry of a system including a sodium counterion is compared with that of a negatively charged soliton with no counterion present. It is shown that in the presence of the dopant, the characteristics width of the soliton is reduced by a factor of 2.5. For higher doping levels, a single counterion does not represent the dopant potential accurately. Instead, the situation of high doping concentration is represented by a cluster of three polymer chains, carrying one solition each and surrounding a channel of three sodium ions. In our calculations, only one of these chains is treated quantum mechanically. Theother two chains are represented by an array of point charges located at the carbon sites of these chains. When the electrostatic interaction with these point charges is taken into account, as well as the full interaction with the sodium ions, the soliton width is increased as compared to the case of the soliton interacting with a single counterion. Due to the attractive potential of soliton-counterion complexes on neighboring chains, additional geometrical distortions in the soliton geometry occur. This effect must be considered when discussing the transition into a metallic state for highly doped trans -polyacetylene.

Biological substances in contact with solid, non-biological materials, is a situation of broad scientific interest and technological importance, and there is a growing need for new tools to study these interactions. Among many different properties of the biological films formed at the interfaces, the viscoelastic properties are of central interest, since these properties can be used as a discriminator in biosensing and in the study of polymer films. We have developed a sensor system based on the traditional quartz crystal microbalance (QCM) technique, but where both the resonant frequency (f) and the energy dissipation (D) are measured simultaneously for a non-driven (freely oscillating) sensor crystal. This provides accurate and precise measurements of f and D in the gaseous and liquid phases. The performance of this so called QCM-D system is illustrated by measurements of protein adsorption and antibody-antigen reactions, biomembrane formation on surfaces from vesicles in solution, cell attachment experiments, and polymer cross-linking kinetics.

Two features make research on the marine cyanobacterium Trichodesmium of particular relevance. The first, relates to its common occurrence and important role in a global perspective. Although Trichodesmium is restricted to coastal habitats and open oceans with temperatures above about 18–20°C, it is probably quantitatively one of the most common cyanobacterium in nature. Recent estimates also indicate that it supports the pelagic zone of the oligotrophic oceans with considerable amounts of fixed nitrogen (Carpenter & Romans 1991). Furthermore, Trichodesmium is unique in that it fixes nitrogen aerobically in light, a feature only known from cyanobacteria differentiating heterocysts, a specific cell type for the oxygen sensitive nitrogen-fixing enzyme nitrogenase.

Quantum-chemical calculations and ultraviolet photoelectron spectroscopy (UPS) measurements have been performed in order to study the interaction between sodium and oligothiophenes, with a focus on the origin of experimentally observed relaxation energy effects in alkali-metal-doped conjugated molecules. Upon doping of a -sexithienylene (α-6T) with sodium atoms, (1) a broad feature appears in the valence band, in an energy region corresponding to the band gap in pristine α-6T, and (2) certain structural features in the valence band shift towards lower binding energies in the doped material. In particular, upon doping, a structural peak related to electronic levels mainly localized to the sulfur and b-carbon atoms destabilizes to an energy corresponding to that of the valence-band edge in pristine α-6T. The results of ab initio Hartree-Fock and local-spin-density calculations on α-trithienylene and bithiophene are consistent with the experimental data, and allow for an assignment of these destabilization effects in terms of initial-state relaxations. We stress that similar destabi-lization effects, reported for other alkali-metal-doped conjugated systems, had previously been proposed to be associated with final-state electronic screening, i.e., a dynamic artifact within the UPS measurements; this is in contradiction to the results of our ab initio theoretical studies. Our present results show that all structural features in the UPS data are contained in the results of sufficiently complete quantum chemical calculations.

The electronic and geometric changes in polyenes induced by doping with sodium have been studied using X-ray and Ultraviolet Photoelectron Spectroscopy, and quantum chemical calculations. The molecular geometry changes induced by doping have been studied using the semiempirical Austin Model 1 method, the results of which has served as input parameters for Valence Effective Hamiltonian band structure calculations, which are compared with the experimental density-of-states data. The molecules studied are members of a series of diphenylpolyenes with 4, 5, 6 or 7 C=C double bonds in the polyene part of the molecule, i.e., the series DPx, with x = 4, 5, 6, or 7. Since the frontier orbitals of the diphenylpolyenes are localized mostly on the polyene chain portion of the molecule, there is a high degree of separation in energy of the phenyl and polyene parts of the π-system. Hence, many chemical and electronic properties of diphenylpolyenes are similar to those of (at least short chain) trans-polyacetylene. The present doping results indicate the charge is stored in short polyenes in the form of two confined solitons per molecule.

The \ensuremath{\pi}-electronic structural changes in a polyene molecule containing seven double bonds, \ensuremath{\alpha},\ensuremath{\omega}-diphenyltetradecaheptaene (DP7), have been studied upon gradually doping with sodium, using x-ray and ultraviolet photoelectron spectroscopies. The spectra are interpreted with the help of detailed quantum chemical calculations. Analysis of the evolution of the XPS and UPS spectra as a function of doping with sodium indicates that the extra charges are stored in the form of two charged solitons on the polyene part of the molecules, which results in two new energy levels in the originally forbidden energy gap.

We investigate the interface between aluminum and several prototypical conjugated systems with a combined experimental and theoretical approach. The experiments consist of following the evolution of the polymer surface during the early stages of aluminum deposition, with X-ray and Ultraviolet Photoelectron Spectroscopies (XPS, UPS). In parallel, we perform quantum chemical calculations on model oligomer systems interacting with one to four Al atoms. Aluminum is found to interact strongly with the polymer chain. Al-carbon covalent bonds are formed along the polymer backbone; the chain geometry is deeply modified and the tc electron conjugation can be dramatically reduced.

We have investigated the chemical nature and the electronic structure of the interface between a low work function metal, aluminum, and a conjugated polymer semiconductor, polythiophene. We have studied the initial stages of the interface formation by depositing the metal onto the surface of a polymer film. Charge transfer processes between the metal and the polymer are analyzed using core‐level x‐ray photoelectron spectroscopy (XPS); the evolution upon metallization of the valence electronic levels directly related to the polymer electronic structure is followed with ultraviolet photoelectron spectroscopy (UPS). With these techniques, we investigate the deposition of aluminum on two polythiophene systems (i) the alkyl‐substituted poly‐3‐octylthiophene and (ii) the α‐sexithiophene oligomer. The experimental data are compared to the results of a recent quantum chemical study on model systems consisting of thiophene oligomers (up to sexithiophene) interacting with a few Al atoms. The interaction of polythio...