In case of charge-transfer complex of TTF:TCNQ lot of work had previously been done on single crystals and thin films to study various interesting properties including phase transitions which were attributed to Peierls instability. But as seen from the review of molecular wires it is clear that apart from synthesis of TTF:TCNQ in molecular wire form, not much was known about the behavior of these wires at low temperatures. There were some open questions listed below, which we tried to address in the thesis Can nanowires of TTF:TCNQ be grown across prefabricated electrodes which are separated by gaps < 1 μm. Can the nanowires grown in such smaller gaps, show Peierls transition, which is the signature of quasi one dimensional conduction. As the size and length of the grown wires are small it was expected that they will have less staking disorder as compared to the thin films. What will be conduction mechanism at low temperatures in such single/few nanowire samples. If the nanowires show Peierls transition and CDW formation at low temperatures, can nonlinear conduction be seen due to motion of CDW, if so how well do they compare with the reported results for TTF:TCNQ single crystals. In case of Cu:TCNQ it can be noted from the above review that even though much advances have been made on synthesizing good quality Cu:TCNQ films and incorporating them in novel device structures, there has been much controversy regarding conduction mechanism. There were many conflicting results in literature regarding switching in these devices. In this thesis work we wanted to address the feasibility of switching in Cu:TCNQ under reduced size of top electrodes and also address few other issues like To grow Cu:TCNQ nanowires by using vapor phase evaporation method Can resistive switching be induced in Cu:TCNQ by using a local probe STM tip (Pt-Rh) operated in high vacuum. Since the measurement will be done in high vacuum what will be the effect of environment (absence of oxygen, water vapor) on reproducibility of resistive switching. Will localized switching depend on the top electrode material. This has been probed by coating different metals on the C-AFM tip and using them as top electrode in conducting mode. With what contact force will we get reproducible resistive switching. Can a device structure be made with an array of top electrode in the form of metal dots (< 10 μm) and study switching using C-AFM. This thesis is divided into seven main chapters and two appendix chapters, which are listed below: In the present chapter 1, a detailed overview and literature survey of charge-transfer complexes TTF:TCNQ and Cu:TCNQ which were relevant to our present study was presented. This was followed by our motivation in undertaking the present work. In chapter 2 the various experimental techniques developed during the course of the thesis work such as e-beam lithography, design of the vacuum chamber for deposition of organic molecules, design of ultra high vacuum scanning tunneling microscope (UHV-STM chamber along with the STM head, modification of conducting AFM for obtaining the switching data have been described. In chapter 3 we describe the preparation of TTF:TCNQ molecular wires across prefabricated electrodes and different measurements done on the samples. In particular the observation Peierls transition in the grown nanowires of TTF:TCNQ and the nonlinear conduction mechanism involved at low temperatures will be discussed in detail. In chapter 4 we describe the preparation of Cu:TCNQ nanowires on Cu substrate using vapor phase technique. Resistive switching measurements done on the Cu:TCNQ nanowires in high vacuum with Pt-Rh tip as top electrode will be discussed in detail. In chapter 5 we describe the resistive switching measurements performed on Cu:TCNQ nanowires with different metal coated C-AFM tips as well as FIB deposited platinum dots as top electrodes. In chapter 6 we make a few comments about possible switching mechanism involved, when STM tip, C-AFM induced as well as platinum coated dots were used as top electrodes. In chapter 7 we conclude this thesis by summarizing the main results. Also we point out the scope for future work that can be based upon the results presented in this work. In Appendix A a brief review of self assembled monolayer (SAM) of alkane thiols is presented followed by details about experiments done for insitu study of growth of SAMs of decanethiol and octadecanethiol on silver substrates using ellipsometry and force-displacement spectroscopy. In Appendix B a brief description of work done to grow isolated nanowires of Cu:TCNQ, between two metal electrodes in planar geometry and in anodic alumina membranes is given.

In disordered alloys, atoms belonging to the same chemical element will exhibit different environments. This leads to variations in the atoms’ local electronic structures, which in turn leads to variations in the binding energies of their core levels. These binding energies can be measured experimentally using core level X-ray photoelectron spectroscopy (XPS). Therefore, in theory at least, core level XPS can be used to resolve different environments in alloys. However, to make this a reality one must understand how an atom’s local electronic structure, and hence the binding energies of its core levels, are affected by local environment. In this thesis, two simple phenomenological models are explored which purport to correctly describe the local electronic structure of disordered alloys. The first model which we consider has its roots in chemical intuition; specifically, the notion that pairs of unlike atoms, i.e. atoms belonging to different chemical elements, transfer a certain quantity of charge, while like atoms do not. Using this model - known as the optimised linear charge model (OLCM) - the relationship between an atom’s local electronic structure, core level binding energies, and its environment is explored in detail, both in the bulk of disordered alloys and near their surfaces. As well as ‘homogeneous’ disordered alloys, in which the concentrations of the alloy’s constituent elements are the same throughout the entire alloy, various ‘inhomogeneous’ disordered alloy systems are considered. These include alloys exhibiting surface segregation - in which the concentrations at the surface differ from those in the bulk - as well as interfaces between two metals with various levels of intermixing. The results of our investigation of bulk inhomogeneous alloys are compared to analogous ab initio results, which confirms the model’s viability as a tool for rationalising the relationship between local electronic structure, core level binding energies, and environment. More generally, our results also reveal a number of interesting new phenomena. Firstly, the widths of spectra in inhomogeneous disordered alloys are significantly larger in some cases than is possible in any analogous homogeneous disordered alloy. Secondly, differences between the concentrations of each element at the surface and deep within the bulk cause a shift in the work function of the alloy under consideration. The latter results in qualitatively different trends than one would expect if this phenomenon was ignored, and prompts an alternative interpretation of the results of a recent experimental study. The second model which we consider is a particular case of the charge-excess functional model, in which the realised charges on all atoms are those which minimise a particular expression for the total energy of the system, and whose accuracy has been well established. The underlying assumptions and properties of this model are explored in detail, adding insight into the nature of the screening and inter-atomic interactions in disordered alloys. The model is shown to be equivalent to the OLCM for the case of binary alloys, and can therefore be considered to be the generalisation of the OLCM for alloys containing more than two chemical elements. The model is also used to derive analytical expressions for various physical quantities for any alloy, including the width of core level XPS spectra and the Madelung energy. These expressions are then used to investigate how the physical quantities to which they pertain vary with the concentrations of each element in a homogeneous disordered alloy consisting of three elements. Among other things, it was observed that the width of the core level XPS spectra is maximised when the concentrations of the two elements in the alloy with the largest electronegativity difference have equal concentrations, while the remaining element has a vanishing concentration.

Electrospinning is a method of producing nano structured material from a polymer solution or melt using high strength electric fields. It is a process that has yet to find extensive industrial application yet shows promise if obstacles such as low rate of production overcome perhaps by more complete theoretical modelling. This work examines the effects of adding an ionic salt to a solution of poly(vinyl alcohol) in water. The direct effect was an increase the charge density and electric current. It was found that an increase in charge density decreases the mass deposition rate and forms a thinner initial jet. When the sign of the charge on the polymer solution was changed from positive to negative the charge density increased and the initial jet diameter and mass deposition rate also decreased. It was proposed that a smaller radius of curvature is formed by the Taylor cone at higher charge densities resulting in a smaller “virtual orifice”. The extent of the bending instability was explored and it was found that adding ionic salt results in a decrease in the bending instability resulting in thicker fibres. Changing the sign of the charge on the polymer solution from positive to negative resulted in an increase in the bending instability and resulted in thinner fibres. The charge transfer mechanisms used in different electrospinning models are explored and some assumptions not explicitly stated are discussed. From this discussion a generalized equation describing the charge transport mechanisms is proposed.

Electron transfer to and between the redox centers of milk xanthine oxidase was investigated by laser flash-photolysis. Evidence is presented for slow equilibration of electrons (k < 38 s⁻¹) between the various redox centers of the enzyme. The enzyme-bound flavin and the heme moieties of the flavoprotein and cytochrome subunits of p-cresol methyl hydroxylase from Pseudomonas putida are both reduced rapidly in a second order manner by 5-dRF generated by the laser flash, followed by slower first order intramolecular electron transfer (k = 220 s⁻¹) from the protein-bound neutral flavin radical to the oxidized cytochrome. Complex formation between spinach ferredoxin:NADP⁺-reductase (FNRₒᵪ), spinach ferredoxin (Fdₒᵪ), rubredoxin (Rdₒᵪ) from Clostridium pasteurianum, two homologous HIPIP's from Ectothiorhodospira halophila (iso-1 and iso-2) and two homologous cytochromes (cytochromes-c₂ from Paracoccus denitrificans and Rhodospirrilum rubrum) have been investigated. Evidence is presented supporting the formation of 1:1 complexes that are stabilized by attractive electrostatic interactions at low ionic strength. Kinetic studies of the above-mentioned complexes provide evidence for extremely rapid to relatively slower intracomplex electron transfer rates (k 7000 s⁻¹ to 4 s⁻¹). In addition the effect of complexation on the degree of accessibility of the various redox centers of the respective complexes to reduction by small reductants such as 5-dRF· and LfH· generated by the laser flash has been evaluated. The effect of both pH and ionic strength on the second order rate of reduction and the intracomplex rates in the respective complexes have also been investigated. The results have been interpreted in terms of redox potential differences, electrostatic and structural features that influence the electron transfer rates in these systems.

Strong electron-electron correlations and lattice frustration are two physical interactions that pose serious challenges to condensed matter physics. A variety of exotic physical phenomena, for example, charge ordering, spin liquid, and unconventional superconductivity, are believed to arise from the interplay of the two interactions. In this dissertation, I examine two families of systems which exhibit both electron-electron correlations and lattice frustration – charge transfer solids and layered cobaltates. The half-filled band Hubbard model on the triangular lattice has been proposed by mean-field theories as the minimal model for the superconductivity in the charge transfer solids. In the first part of this dissertation, by using exact calculations, I prove the absence of superconductivity in this model. This result calls for a new theoretical approach to describe the rich physics in charge transfer solids. In the second part of this dissertation, I study charge transfer solids by focusing on its real bandfilling ¼. I show that a new kind of insulating phase, paired electron crystal, emerges from antiferromagnetism as the frustration is increased. The paired electron crystal state can explain the various insulating states adjacent to the superconducting state, thus provides a new avenue towards the understanding of the unconventional superconductivity in charge transfer solids and other ¼ filled systems. In the third part of this dissertation, I investigate the carrier concentration-dependent electronic behavior in layered cobaltates. I provide a natural yet simple explanation for this behavior. I show that it can be described within correlated-electron Hamiltonians with finite on-site and significant nearest neighbor hole-hole Coulomb repulsions. I also point out the similarities between organic charge transfer solids and layered cobaltates, which may involve superconductivity.

Interfacial charge transfer between metal oxides and organic semiconductors has been found to limit the efficiency of organic optoelectronic devices. Although a number of investigations of inorganic/organic systems exist, very few generally applicable rules for oxide/organic interfaces have been developed and many questions about these systems remain unanswered. Thus the studies presented in this dissertation were designed to improve the understanding of the fundamental interface physics of metal oxide/organic systems. Single molecule fluorescence microscopy was employed to determine the charge transfer mechanism while photoelectron spectroscopy was used to determine the energy level alignment of model systems. Additional computational studies allowed the examination of the properties of the charged organic molecules involved in charge transfer and modeling of the molecule-surface interaction. Calculations of the ground state properties and excited state transitions of the neutral and singly charged states of a modified perylene molecule were performed to provide insight into the orbitals of the initial and final states involved in the interfacial charge transfer process. The design and implementation of a novel UHV single molecule microscope is described. This microscope was used to observe the excited state charge transfer between a modified perylene molecule and Al₂O₃ (0001). The charge transfer mechanism was identified as involving activated trapping and detrapping of the defect derived states within the Al₂O₃ band gap, which resulted in the observation of strongly distributed kinetics for this system. The influence of defects and adsorbates on the electronic structure of ZnO and its interface with organic semiconductors was determined from photoelectron spectroscopy. Modified perylene molecules were found to have strong chemisorptive interactions with the ZnO surface involving charge transfer from defect derived ZnO states to the LUMO, while magnesium phthalocyanine molecules appear to have only weak physisorptive interactions with the ZnO surface. The interfacial investigations of the organic/oxide systems demonstrate the rich defect structure present in metal oxides. In both cases, defects were found to control the interfacial interactions between the metal oxide surface and the modified perylene molecules. Thus the manipulation of these defects states is of fundamental importance for optoelectronic device design.

No description available.

The energy levels of a hydrogen Rydberg atom approaching a metallic structure are perturbed by the image-charge interaction with the surface. At small atom-surface separations surface ionisation of the Rydberg electron can occur, whereby the electron is transferred to a metal-localised state. In previous studies investigating surface ionisation at bulk metallic surfaces, this state has been part of a conduction band; however this thesis focuses on metallic and structured surfaces where the Rydberg electron transfers into a discrete image-state or hybrid 'well-image state'. The surface ionisation of hydrogen Rydberg atoms at a Cu(100) projected band-gap surface is investigated experimentally and theoretically. Experimentally, the surface ionisation of an incident beam of hydrogen Rydberg atoms is measured by extraction of the resulting ions. Resonance-enhanced charge transfer is seen for hydrogen Rydberg states that are degenerate with copper-localised image-states. A wavepacket propagation study shows that for on-resonance states the maximum in the surface-ionisation probability is shifted away from the surface by decreasing the collisional velocity. The discrete hybrid 'well-image states' localised along the surface normal of a thin-film change energy with thin-film thickness. The interaction of hydrogen Rydberg atoms with iron thin films deposited on an insulating substrate is investigated. The preference for electron penetration along the surface normal is seen by the resonance-enhancement of charge transfer at energies where the Rydberg state and well-image state are degenerate. By changing the thickness of the thin film, by in situ depositions, the energies of the well-image state are altered and the Rydberg n-values at which resonances occur, change. At a thickness of 30-monolayer the energetic spacings between the well-image states and the Rydberg states become comparable, and the single well-image state resolution is lost. A wavepacket-propagation study investigates the interaction of a nanoparticle and low-n hydrogen Rydberg atoms. The nanoparticle has a fully confined potential which at small radii yields well-spaced, fully discrete well-image states. Resonance-enhanced charge transfer occurs when the Rydberg state and the nanoparticle well-image state energy levels are degenerate. However, when there is poor energy matching between the nanoparticle well-image state and the Rydberg atom, no charge transfer is seen i.e. surface ionisation does not occur. Overall, the work presented here demonstrates the capability of Rydberg-surface studies to identify discrete, high-lying energy levels at specific surfaces.

Charge transfer salts of Tetracyanoquinodimethane (TCNQ) were synthesised and their electrical and magnetic properties were investigated. These salts show unusual electrical and magnetic behaviour in contrast to conventional organic compounds. These salts have crystal structures which in general consist of TCNQ radical ions stacked in chains, isolated from each other by the diamagnetic cations. They are thus regarded as "one-dimensional" electrical and magnetic systems. The ESR spectra of these salts are attributed to triplet excitons showing that the spin-spin and electronelectron correlation effects are important. In the ESR spectra (Chapter III) of some TCNQ salts dipolar splitting is observed confirming the spin-spin interaction. These triplet excitons are regarded as bound electron-hole pairs. The experimentally determined dipolar splitting tensors are presented in Chapter III and the intensity data in Chapter IV. A large number of fine structure lines are observed in the ESR spectra of Pyridinium-TCNQ and 4-Aminopyridinium-TCNQ apart from regular triplet exciton lines (Chapter III). These lines are attributed to the trapping of excitons on an extended formula finit (TCNQ2 )n. In Chapter IV the temperature dependent magnetic susceptibilities are discussed in terms of Heisenberg antiferromagnetism and Pauli paramagnetism. In Chapter V temperature dependent behaviour of electrical conductivity is discussed in terms of an exciton band model, the lattice structure of the salts and one-dimensional lattice consisting of defects giving rise to high and low conducting segments. Low temperature electrical and magnetic phases are discussed (Chapters IV and VII) in terms of a band and hopping mechanisms.In Chapter VI self consistent field calculations are made with reference to the tight binding one electron band theory using simplified Roothaan equations considering CNDO approximations. Theoretical results are related to experimental band gaps, spinspin interactions and charge alteration.

Le dopage non covalent de nanostructures carbonées par transfert de charge depuis/vers des molécules donneuses ou acceptrices (EDA) ou bien par des molécules d’acide sulfurique H2SO4, est considéré comme potentiellement intéressant pour de nombreuses applications. Parmi celles-ci on peut citer: capteur chimique, transistor à effet de champ, et d’autre l'électronique. Cependant, d'un point de vue théorique, on en sait peu au sujet de ces processus de transfert de charge par électrons ou par trous.Dans un premier temps, nous nous sommes intéressés à l’interaction entre des molécules d’acide sulfurique et des nanostructures modèles, car elles sont capables de doper des nanotubes, de s’intercaler dans le graphite et même d’aligner les tubes dans une phase nématique, ce qui pourrait mener à la création de matériaux composites à forte valeur ajoutée.Bien que certaines études théoriques DFT ont été menées récemment, leurs résultats restent source de confusion. Par exemple, même s’il est rapporté un transfert de charge entre une molécule de H2SO4 et un plan de graphène, tous nos efforts pour reproduire ces calculs ont été infructueux. Nous proposons dans ce travail de thèse, un mécanisme de réaction qui expliquent la "protonation" des parois du tube, tel que proposé dans la littérature. Enfin nous proposons un scénario possible pour une meilleure compréhension de la structuration à grande échelle des molécules d'acide autour de points d'ancrage, telles que des défauts, de la structure carbonée / Non-covalent doping of carbon nanostructures by charge transfer from/to donor/acceptor molecules (EDA) or by H2SO4 molecules, be it with holes or electrons, is usually thought as potentially interesting for many applications of carbon based nano-devices. However, from a theoretical point of view, little is known about such “charge transfer” processes.Employing first-principles method based on Density Functional Theory (DFT), we have studied in details, and proposed a model to rationalize, the interaction between a prototypical donor molecule the tetrathiafulvalene (TTF), a standard acceptor organic molecule, tetracyanoethylene (TCNE) and carbon nanostructures: graphene layer and SWNTs with various chiral indices. Main results concern structural and thermodynamic aspects including dispersion forces effects, and evidently electronic structure modifications of the nanostructures. Various adsorption modes and concentration effects have been investigated. At very low coverage values, we have reported a charge transfer between graphene and TCNE or TTF. Moreover, we have shown that the charge transfer can be enhanced by increasing the concentration of those two EDA molecules, as it has been demonstrated experimentally. Those results are beneficial for comprehending the nonchemical doping mechanism in graphene structure by means of charge transfers. Considering the interaction between these prototypical molecules and carbon nanotubes, we have found that charge transfers tend to decrease while the curvature of nanotube is increasing. Besides, a strong influence of the metallic/semi-conductor character of the SWNTs can be observed and be explained by the change of polarisability of the curved carboneous substrates. Additionally, we have studied the adsorption properties of sulfuric acid molecules, in its non-hydrated form, on carboneous nanostructures. Against the common believe, no charge transfer is observed in the H2SO4@graphene or H2SO4@CNTs cases, even at very high concentrations. Instead, in order to elucidate the origin of p-doping observed experimentally, we have proposed that molecule is responsible of the reversible doping. Besides we have shown that a proton transfer could cause the experimental phenomenon of crystallization of H2SO4 molecules on SWNT’s surface. Finally in such process, defects like vacancy are of first importance, since they could provide anchorage points for hydrogen atoms. The results of the present work will certainly help to understand the charge transfer and doping mechanism of carbon nanostructures by means of non-covalent functionalization, which is a promising method for their future applications