Dissertations / Theses on the topic 'Semiconductors - Electrochemistry'

Organic conducting polymers are efficient materials for a wide range of applications, ranging from energetics and electronics (bulk-heterojunction solar cells, dye-sensitized solar cells, organic light-emitting diodes, organic field effect transistors) to sensoristics, offering the advantage of being light-weight, flexible, low-cost compounds, thus providing a valid and interesting alternative to traditional inorganic semiconductors. Electrochemistry plays an important role in the study of these smart materials, being a powerful tool to determine the mechanisms of the electron-transfer processes occurring during the reduction/oxidation cycles of these molecules when, deposited as films on electrodic surfaces. Cyclic voltammetry is an essential tool for the experimental evaluation of the HOMO and LUMO levels and of the HOMO-LUMO gap, to be compared with spectroscopical and theoretical values, in order to better understand the relationships between structure and electronic properties and to evaluate the most suitable application for the new molecules. In addition, the combination of cyclic voltammetry with electrochemical quartz crystal microbalance EQCM technique and electrochemical impedance spectroscopy (EIS) affords further information about the mechanisms of the electrochemical coupling of polymerogenic units and,the resitance of the polymer films to mass and charge transfers. In the present PhD thesis, innovative thiophene-based organic semiconductors have been designed and characterized by cyclic voltammetry, in combination with many other electrochemical and spectroscopic techniques (EQCM, EIS, in situ UV-Vis-NIR, ESR and circular dichroism spectroscopy, AFM and SEM imaging) in order to obtain a complete insight into the new materials, to better understand their intrinsic properties for a more efficient target-oriented design. Particular attention was devoted to three-dimensional molecules (i.e., the so called “genetically-modified” spider-like oligothiophenes, derived from a previous work on branched all-thiophene molecules)[1,2] that fulfil the requirements of branching (which provides the polymers with a remarkable solubility in common organic apolar solvents, ensuring an easy processability), of a significative effective conjugation and of the possibility of fine tuning the HOMO and LUMO levels, by suitable structural modifications in the light of possible applications in organic photovoltaics. Another class of three-dimensional multithiophene compounds is that of the inherently chiral molecules. It would be a highly innovative result to find a material which would combine the potentialities of chirality (i.e., the ordered spontaneous chain assembling induced by chirality, the noncentrosymmetry associated to chiral materials, which is a prerequisite for second order nonlinear optical applications, the ability of chiral molecules to discriminate between antipodes, as required in sensors designed for the detection of chiral analytes, the possibility for a chiral semiconductor to be employed in asymmetric electrosynthesis) with the advantages typical of the conducting polymers (i.e., electrical conductivity, redox and pH switching capability, electrochromism, low cost, easy processability, light weight). According to the literature, the most common strategy to obtain chiral conducting polymers is to attach chiral pendants (i.e. natural sugar and aminoacids, or manmade designed for specific applications) to the conjugated electroactive backbone. The presence of carbon stereocenters invariably characterizes the chiral substituents. Only in few cases, however, significant chirality manifestations have been found in polymers designed according to this strategy, also because the experimental conditions (i.e., solvent, pH, temperature) strongly affect the chirality manifestation of the polymers. In the present case, instead, chirality is due to a tailored torsion internally produced along the conjugated backbone, that does not completely interrupt the conjugated sequence, thus ensuring the conductivity of the material in the doped state. The stereogenic element is an atropisomeric bithiophene or bipyrrole scaffold, introduced into the conjugated backbone of the monomer. The polymerization sites of the monomers are homotopic, thus granting the constitutional regularity of the polymers. This requirement is satisfied only by molecules belonging to the C2 point group which guarantees that all the products of the polymerization, from olygomers to polymers, are C2 symmetric as well. Finally, the monomers are properly and easily functionalized in different positions in order to tailor their properties to specific applications. In the present work, a complete characterization of the monomers (and of the oligomers derived from them by electroxidation) has been performed: particularly interesting is the circular dichroism behaviour of the enantiopure films electrodeposited on ITO electrodes, detected in situ during cyclic voltammetry scans. Upon polarization (p-doping), the maximum intensity in the CD signal decreases (while another band increases at higher wavelength, corresponding to the formation of the polaronic state) possibly because of a partial flattening of the atropisomeric structure (coherent with the enhanced conductivity in the doped state). This phenomenon is completely reversible, and chirality is fully recovered switching the potential back to the neutral state. In conclusion, this work on different families of molecules has given evidence of how electrochemistry is an essential tool in modern materials science, being a fast and reliable method to determine the crucial parameters for applications in the most advanced technological fields, affording a deeper understanding of the relationships between molecular structure and electronic properties and effectively assisting the target-oriented molecular design. In addition, the class of the inherently chiral monomers opens up a new path to the applications of chiral organic semiconductors in many different fields, both as racemates (active layers in bulk-heterojunction solar cells, on the basis of the preliminary knowledge of the HOMO and LUMO gaps and levels in these molecules; the cavities present in the polymers could be tailored to comfortably host bulky fullerene units for the preparation of donor-acceptor blends, for the construction of traditional or Molecularly Imprinted Polymers sensors) and as enantiopure materials (preparation of sensors for the recognition of biorelevant chiral analytes, preparation of chiral electrodes for performing electrochemical enantioselective oxo-reduction processes, exploitation of the inherent chirality and of the highly ordered solid-state structure in photoelectrochemical applications). [1] T. Benincori, M. Capaccio, F. De Angelis,L. Falciola, M. Muccini, P. Mussini, A. Ponti, S. Toffanin, P. Traldi, F. Sannicolò, Chem. Eur. J., 2008, 14, 459 – 471; [2] T. Benincori, V. Bonometti, F. De Angelis, L. Falciola, M. Muccini, P. R. Mussini, T. Pilati, G. Rampinini, S. Rizzo, S. Toffanin, F. Sannicolò, Chem. Eur. J., 2010, 16, 9086 – 9098

The high balance-of-system costs of photovoltaic installations indicate that reductions in absorber cost alone are likely insufficient for photovoltaic electricity to reach grid parity unless energy conversion efficiency is also increased. Technologies which both yield high-efficiency cells (>25%) and maintain low costs are needed. GaAs and related III-V semiconductors are used in the highest-efficiency single- and multi-junction photovoltaics, but the technology is too expensive for non-concentrated terrestrial applications. This is due in part to the limited scalability of traditional syntheses, which rely on expensive reactors and employ toxic and pyrophoric gas-phase precursors such as arsine and trimethyl gallium. This work describes GaAs films made by close-spaced vapor transport, a potentially scalable technique which is carried out at atmospheric pressure and requires only bulk GaAs, water vapor, and a temperature gradient to deposit crystalline films with similar electronic properties to GaAs prepared using traditional syntheses. Although close-spaced vapor transport of GaAs was first developed in 1963, there were few examples of GaAs photovoltaic devices made using this method in the literature at the onset of this project. Furthermore, it was unclear whether close-spaced vapor transport could produce GaAs films appropriate for use in photovoltaics. The goal of this project was to create and study GaAs devices made using close-spaced vapor transport and determine whether the technique could be used for production of grid-connected GaAs photovoltaics. In Chapter I the design of the vapor transport reactor, the chemistry of crystal growth, and optoelectronic characterization techniques are discussed. Chapter II focuses on compositional measurements, doping, and improved electronic quality in CSVT GaAs. Chapter III describes several aspects of the interplay between structure and electronic properties of photoelectrochemical devices. Chapter IV addresses heteroepitaxial growth of GaAs on "virtual" Ge-on-Si substrates. This is a topic of importance for the broader III-V community as well as the photovoltaic community, as Si is the substrate of choice in many areas of industry. This dissertation includes unpublished and previously published co-authored material.

Nos travaux concernent la synthèse et la caractérisation d'oxydes dopés par la méthode d'électrodéposition sous champ magnétique.L'enjeu d'une telle recherche est double puisqu'il associe une étude de synthèses électrochimiques et l'obtention de matériaux associant des propriétés semi-conductrices et magnétiques.Les oxydes étudiés sont l'oxyde de cuivre (I) dopé par le manganèse ou par le cobalt, et l'oxyde de zinc dopé par le cuivre.Notre objectif est l'élaboration sous champ magnétique d'oxydes de type DMS (semi-conducteurs magnétiques dilués), et leurs caractérisations physiques et chimiques.En effet, l'addition du dopage et celui du champ magnétique appliqué pendant l'électrodéposition génèrent des effets sur les matériaux électrodéposés.Nous avons ainsi mis en évidence des modifications au niveau de la morphologie, de la texture, de la composition, et des propriétés optiques ou magnétiques des matériaux obtenus
Our work focuses on the synthesis and characterization of doped oxides by electrodeposition method under magnetic field superimposition.The goal of this research presents two challenges, because it combines a study of electrochemical synthesis and obtaining materials with optical and magnetic properties. The materials which have been studied are manganese or cobalt doped copper (I) oxide on the one hand, and the copper doped zinc oxide in the other hand.Our goal is the elaboration of diluted magnetic oxides, and the study of their physical and chemical characterizations.Indeed, the effects of doping and of the magnetic field applied during the electrodeposition can provide interesting changes in morphology, texture, composition and optical and magnetic properties of the obtained materials

In this work, the use of modified diamond as an electrode material with superlative physical and electrochemical properties was investigated in a number of electrochemical applications. The surface chemistry of three differing forms of diamond, namely boron-doped microcrystalline diamond, boron-doped diamond powder and detonation nanodiamond powder was modified utilising such strategies as hydrogen plasma treatment, reactive ion plasma etching along with various chemical treatments. The surface and functional properties of the modified diamond electrodes were studied using a wide spectrum of techniques. The electrochemical activity of these materials was concomitantly investigated in order to expand the knowledge of diamond electrochemistry and to establish an understanding of how the surface chemistry of these materials impacts their electrochemical performance. In the first study, the nanostructuring strategies of boron-doped diamond surface with platinum nanoparticles were developed. In particular, two types of diamond nanostructures were produced: one consisting of platinum particles located on the top of diamond nanorods, the other with platinum particles located in the bottom of diamond nanopits. For the first time, the experimental evidence proving the mechanism of the diamond nanostructuring process was reported. The electrochemical activity of these nanostructured diamond electrodes with regard to the electrochemical oxidation of glucose and methanol was investigated. In the second study, the relationship between the surface chemistry of three differing forms of diamond, including microcrystalline boron-doped diamond, boron-doped diamond powder as well as detonation nanodiamond powder, and the electrode fouling in the result of the adsorption processes in methyl viologen and anthraquinonedisulfonate solutions was investigated. The influence of two dissimilar surface terminations: hydrophobic H-terminated and hydrophilic O-terminated on the electrode performance was studied in detail. This work provides a useful insight on the likely reasons for the undesirable adsorption occurrence which may be experienced in many electroanalytical applications that utilise solid and powdered forms of diamond. The third project extends the discussion on the study of the diamond electrodes, modified with detonation nanodiamond and boron-doped diamond powders and investigates the electrochemical behaviour of these materials. In this work, charge transport within the diamond powder films, partition coefficients of different redox mediators along with heterogeneous electron transfer constants were identified. The chemical modification of these electrodes with platinum nanoparticles along with the mechanism of nucleation and growth of the latter were studied. The enhanced electrode performance with regard to methanol electrooxidation reaction was demonstrated. The fourth study investigates the preparation of nickel modified boron-doped diamond electrodes and ascertains the relationship between the surface chemistry of the modified diamond and the associated electrocatalytic performance of nickel nanoparticles in hydrogen peroxide and glucose electrooxidation. The fifth study reports on the development of a novel surface functionalization strategy, based on porphyrin and amide coupling chemistry, which allows the creation of hybrid biomimetic diamond interface that was used as the artificial β-alanine receptor.

The discovery of semi-conducting organic materials has opened new possibilities for electronic devices and systems because of their solution processibility, lightweight and flexibility compared to inorganic semiconductors. The combination of semiconductors with electrolytes, and more especially organic semiconductors and solid electrolytes has attracted the attention of researchers because of the multiple phenomena originating from the simultaneous motion of electrons and ions. This thesis deals with organic-based devices whose working mechanism involves electrolytes. By measuring electrochromism induced by the field in isolated segments of conjugated polymer films, which is in contact with an electrolyte, the direction and the magnitude of the electric field along an electrolyte is quantified (paper I). In addition, using a polyanionic proton conductor in organic field-effect transistor (OFET) as gate dielectric results in low operation voltage and fast response thanks to the high capacitance of the electric double layer (EDLC) that is formed at organic semiconductor/ polyelectrolyte interface (paper III). Because an electrolyte is used as a gate insulator, the effect of the ionic currents on the performance of an EDLC-OFET has been investigated by varying the relative humidity of the device ambience (paper IV). Since the EDLC-OFET and the electrochromic display cell both are operated at low voltages, the transistor has been monolithically integrated with an electrochromic pixel, i.e. combining a solid state device and an electrochemical device (paper V). Further, a theoretical study of the electrostatic potential within a so called pen-heterojunction made up of two semi-infinite, doped semiconductor media separated by an electrolyte region is reported (paper II).

Adherent Cu films were electrodeposited onto polycrystalline W foils from purged solutions of 0.05 M CuSO4 in H2SO4 supporting electrolyte and 0.025 M CuCO3∙Cu(OH)2 in 0.32 M H3BO3 and corresponding HBF4 supporting electrolyte, both at pH = 1. Films were deposited under constant potential conditions at voltages between -0.6 V and -0.2 V versus Ag/AgCl. All films produced by pulses of 10 s duration were visible to the eye, copper colored, and survived a crude test called "the Scotch tape test", which involves sticking the scotch tape on the sample, then peeling off the tape and observing if the copper film peels off or not. Characterization by scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) and X-ray photon spectroscopy (XPS) confirmed the presence of metallic Cu, with apparent dendritic growth. No sulfur impurity was observable by XPS or EDX. Kinetics measurements indicated that the Cu nucleation process in the sulfuric bath is slower than in the borate bath. In both baths, nucleation kinetics does not correspond to either instantaneous or progressive nucleation. Films deposited from 0.05 M CuSO4/H2SO4 solution at pH > 1 at -0.2 V exhibited poor adhesion and decreased Cu reduction current. In both borate and sulfate baths, small Cu nuclei are observable by SEM upon deposition at higher negative overpotentials, while only large nuclei (~ 1 micron or larger) are observed upon deposition at less negative potentials. Osmium metal has been successfully electrodeposited directly onto p-Si (100) from both Os3+ and Os4+ in both sulfuric and perchloric baths. This electrochemical deposition of osmium metal can provide sufficient amount of osmium which overcome ion beam implantation limitations. The deposited metal can undergo further processing to form osmium silicides, such as Os2Si3, which can be used as optical active materials. The higher osmium concentration results in large deposition currents and more negative peak potential due to larger transfer coefficient. No matter which supporting electrolyte is used, no stripping peak exists in this study. The oxidation ability of anion plays an important role in osmium electrodeposition because it will change the silicon substrate conductivity. In our case, perchloric acid oxidized silicon surface severely. Os4+ seems more favorable for reduction but has a stronger oxidization ability to lower the conductivity. The microscopic images verified osmium is deposited on silicon and forms cluster sizes of < 1 µm to > 10 µm. The Rutherford backscattering spectroscopy (RBS) data indicate osmium can diffuse into the silicon as far as 500 nm and the Si crystal structure is unchanged by the process. This means that the Si does not disassociate and migrate into deposited Os. Osmium is distributed randomly throughout the lattice interstitially. It appears field assisted diffusion can significantly drive the Os into Si (100). This finding is very valuable but needs further study.

Oxidation is a fundamental process in the fabrication of microelectronic devices and in order to promote the incorporation of SiGe into Si technology we have investigated the thermal oxidation of Si1-yGey alloy (y≈0.5) at high temperatures (mainly 900°C and 1000°C) using Rutherford backscattering spectroscopy, infrared transmission spectroscopy and X-ray photoelectron spectroscopy. It has been observed that three distinct regions form during oxidation of Si0.5Ge0.5 alloy, which are (I) a mixed oxide layer Si0.5Geg0.5O2, (II) a pure SiO2 layer and (III) a Si1-yGey (y≠0.5) alloy layer. These are formed during both wet and dry oxidation when the sample is not preheated, whilst only two regions (II) and (III) form when the sample is preheated in a non oxygen ambient up to the oxidation temperature prior to oxidation. Enhancement of the rate of wet oxidation of SiGe compared with bulk Si and the accumulation of Ge in region III just below SiO2 layer (region II) has also been observed. This behaviour is in good agreement with other results, however, some abnormal behaviour during wet and dry oxidation has been observed. The rate of oxidation during short wet oxidations (15 minutes) of Si0.5 Ge0.5 decreases as the oxidation temperature increases from 800°C to 1000°C for 15 minutes, which has not been previously reported, A reduced rate of oxidation of Si1-yGey (y≈0.5) in a dry environment has been observed and is discussed. In order to investigate the thermodynamics of the oxidation process new experiments have been carried out which involve the synthesis of a buried oxide layer, by O+ implantation followed by a high temperature anneal. The implanted oxygen atoms preferentially bond to silicon atoms and the oxygen atoms are found to bond to germanium atoms only after all of the silicon atoms are fully oxidised. Germanium tends to be rejected from the growing oxide during a subsequent higher temperature ( > 900°C) anneal. The different behaviour of the Si and Ge atoms during both thermal and internal oxidations is described in terms of the thermodynamics and kinetics of the SiGe alloy system. There are three important conclusions which emerge from these analyses: (i) in order to adequately control the composition of thermal oxides (T > 800°C) grown on Si0.5Ge0.5 material it is necessary to preheat the samples in a non oxidising atmosphere prior to oxidation. By so doing the entrapment of Ge in the near surface layer (region I) is inhibited; (ii) during the initial stage of dry oxidation the process is described by the reaction Si+O2?Si2 and we suggest that the rate is controlled by the availability of oxygen atoms and is numerically the same as for bulk Si; (iii) in contrast, during the initial stage of wet oxidation, the process is described by the reaction Si+2OH→SiO2+H2 and the rate is controlled by the areal density of Si atoms, which we propose is high due to the weak Si-Ge binding energy and thus an enhanced oxidation rate occurs.

Thesis(Ph.D.)--Case Western Reserve University, 2010
Title from PDF (viewed on 2009-12-30) Department of Chemistry Includes abstract Includes bibliographical references and appendices Available online via the OhioLINK ETD Center

In spite of many efforts given during the last decades to find new alternative photocatalytic materials, titanium dioxide (TiO2) still represents the most widely employed semiconductor for photocatalytic applications, being photoactive, cheap, easily available, non-toxic, inert and, most of all, chemically stable. However, although exhibiting a powerful combination of extraordinary and attractive physico-chemical properties, it suffers from some issues, common to all semiconductors, related to the dynamic of the charge carriers. Precisely, trapping and recombination of valence band holes and conduction band electrons occur at a certain extent, anyway resulting in a drop of the process efficiency. Therefore, in view of limiting these detrimental phenomena, the charge transfer and the electric conductivity of a semiconductor can be enhanced, thus leading to an overall improvement of the photocatalyst performance. In the first part of the work, homemade and commercial TiO2 powders were studied as photocatalysts for different applications, including the liquid-phase photocatalytic oxidation of ammonia and formic acid, and the H2 production through photocatalytic reforming of water-methanol vapors. In this context, a dopant (NH4F) was used during the sol-gel synthesis of the semiconductor to stabilize the formation of the TiO2 anatase phase (typically more active than rutile TiO2 because of its higher electron mobility), especially when crystallization of the amorphous oxides was performed at high temperature (700 ˚C). Furthermore, the effects induced by noble metal nanoparticles deposition on TiO2 anatase powders were also investigated. In situ electron spin resonance spectroscopy was employed to determine the amount of electrons and holes trapping centers formed under irradiation, in the absence and in the presence of noble metal co-catalysts at the surface of TiO2, hence assessing also the ability of Au and Pt nanoparticles in trapping conduction band electrons. The results were of great usefulness not only to interpret the different H2 production rates but also to understand some mechanistic aspects concerning the selectivity towards the different oxidation products in the methanol photo-steam reforming reaction. In the second part of the work, the nanostructuring of the semiconductor was explored by fabricating TiO2 nanotube arrays through electrochemical anodization. The anodic oxides were employed for both photocatalytic and photo-electrochemical H2 production. In view of large-scale application, the anodization approach was studied on wide Ti substrate surfaces, in order to assess the feasibility of the scale up. Moreover, TiO2 nanotubes were also grown on Ti-based alloys. When fabricating the nanotubes under optimized conditions on Ti-Ta alloys, highly photoactive Ta-doped TiO2 nanotubes were obtained, exhibiting superior water splitting ability. When anodizing Ti-Au alloys, the TiO2 nanotubes resulted decorated with Au nanoclusters. These Au-decorated TiO2 nanotube arrays were used as efficient photocatalyst for H2 production from ethanol-water solutions. Finally, the fabrication of short TiO2 nanotube layers exhibiting an unprecedented level of self-ordering was achieved through an innovative anodization approach. The highly ordered topography allowed the subsequent self-ordering dewetting of Au, leading to Au nanoparticles of controllable size and distribution. These short, Au nanoparticles-filled TiO2 nanotubes exhibited advanced photoactivity ascribed to their reaction vessel-like geometry, fulfilling the requirements in terms of solid state charge carriers diffusion and liquid phase diffusion of oxidizing radicals.

This thesis addresses the observed fluorescence intermittency of single semiconductor nanocrystals, so called Quantum Dots (QDs), which is also referred to as blinking. Despite continuous excitation their fluorescence is randomly interrupted by dark periods that can last over several minutes. Especially the extraction of power law dwell time statistics in bright and dark states indicates very complex underlying processes that are not fully understood to date. Here two approaches are followed to reveal the nature of the blinking mechanism. One addresses the common threshold method for extraction of power law dwell times. Its performance is tested with simulations to a broad range of experimentally determined parameters. Strong deviations are found between input and extracted statistics dependent on input parameters themselves. A comparison with experimental data does not support the assignment of power law statistics for the bright state and indicates the existence of distinct blinking mechanisms. The second approach directly aims at the nature of the dark state, which is mostly attributed to charges in the QD or trap states in its vicinity. A method is developed to detect charging processes on single QDs with their fluorescence. Electrochemistry is combined with confocal microscopy also allowing evaluations of excited state lifetimes and emission spectra. Reduction and oxidation of the QD bands are successfully observed as a quenching of QD fluorescence. Single QD observations identify two independent blinking mechanisms, that are assigned to positive and negative charging. Positive charging is not only observed after hole injection but also the extraction of excited electrons. Three additional quenching mechanisms are identified, two of which are assigned to trap relaxation. Differences between two substrate electrodes demonstrate the importance of the substrate material.

This thesis presents a mathematical model of the nanoporous anode within a dyesensitised solar cell (DSC). The main purpose of this work is to investigate interfacial charge transfer and charge transport within the porous anode of the DSC under both illuminated and non-illuminated conditions. Within the porous anode we consider many of the charge transfer reactions associated with the electrolyte species, adsorbed dye molecules and semiconductor electrons at the semiconductor-dye- electrolyte interface. Each reaction at this interface is modelled explicitly via an electrochemical equation, resulting in an interfacial model that consists of a coupled system of non-linear algebraic equations. We develop a general model framework for charge transfer at the semiconductor-dye-electrolyte interface and simplify this framework to produce a model based on the available interfacial kinetic data. We account for the charge transport mechanisms within the porous semiconductor and the electrolyte filled pores that constitute the anode of the DSC, through a one- dimensional model developed under steady-state conditions. The governing transport equations account for the diffusion and migration of charge species within the porous anode. The transport model consists of a coupled system of non-linear differential equations, and is coupled to the interfacial model via reaction terms within the mass-flux balance equations. An equivalent circuit model is developed to account for those components of the DSC not explicitly included in the mathematical model of the anode. To obtain solutions for our DSC mathematical model we develop code in FORTRAN for the numerical simulation of the governing equations. We additionally employ regular perturbation analysis to obtain analytic approximations to the solutions of the interfacial charge transfer model. These approximations facilitate a reduction in computation time for the coupled mathematical model with no significant loss of accuracy. To obtain predictions of the current generated by the cell we source kinetic and transport parameter values from the literature and from experimental measurements associated with the DSC commissioned for this study. The model solutions we obtain with these values correspond very favourably with experimental data measured from standard DSC configurations consisting of titanium dioxide porous films with iodide/triiodide redox couples within the electrolyte. The mathematical model within this thesis enables thorough investigation of the interfacial reactions and charge transport within the DSC.We investigate the effects of modified cell configurations on the efficiency of the cell by varying associated parameter values in our model. We find, given our model and the DSC configuration investigated, that the efficiency of the DSC is improved with increasing electron diffusion, decreasing internal resistances and with decreasing dark current. We conclude that transport within the electrolyte, as described by the model, appears to have no limiting effect on the current predicted by the model until large positive voltages. Additionally, we observe that the ultrafast injection from the excited dye molecules limits the interfacial reactions that affect the DSC current.

Apres présentation de certains éléments théoriques sur l'interface semi-conducteur-électrolyte, étude du comportement à l'obscurité des matériaux du type N et du type P en milieu aqueux, à l'aide de mesures d'impédance. Etudes, aussi, du comportement sous éclairement de CDTE-N en solution aqueuse. Données sur l'influence de l'adsorption d'espèces redox en solution et sur le comportement de l'interface en présence de décomposition cathodique.

This thesis presents a mathematical model of the nanoporous anode within a dyesensitised solar cell (DSC). The main purpose of this work is to investigate interfacial charge transfer and charge transport within the porous anode of the DSC under both illuminated and non-illuminated conditions. Within the porous anode we consider many of the charge transfer reactions associated with the electrolyte species, adsorbed dye molecules and semiconductor electrons at the semiconductor-dye- electrolyte interface. Each reaction at this interface is modelled explicitly via an electrochemical equation, resulting in an interfacial model that consists of a coupled system of non-linear algebraic equations. We develop a general model framework for charge transfer at the semiconductor-dye-electrolyte interface and simplify this framework to produce a model based on the available interfacial kinetic data. We account for the charge transport mechanisms within the porous semiconductor and the electrolyte filled pores that constitute the anode of the DSC, through a one- dimensional model developed under steady-state conditions. The governing transport equations account for the diffusion and migration of charge species within the porous anode. The transport model consists of a coupled system of non-linear differential equations, and is coupled to the interfacial model via reaction terms within the mass-flux balance equations. An equivalent circuit model is developed to account for those components of the DSC not explicitly included in the mathematical model of the anode. To obtain solutions for our DSC mathematical model we develop code in FORTRAN for the numerical simulation of the governing equations. We additionally employ regular perturbation analysis to obtain analytic approximations to the solutions of the interfacial charge transfer model. These approximations facilitate a reduction in computation time for the coupled mathematical model with no significant loss of accuracy. To obtain predictions of the current generated by the cell we source kinetic and transport parameter values from the literature and from experimental measurements associated with the DSC commissioned for this study. The model solutions we obtain with these values correspond very favourably with experimental data measured from standard DSC configurations consisting of titanium dioxide porous films with iodide/triiodide redox couples within the electrolyte. The mathematical model within this thesis enables thorough investigation of the interfacial reactions and charge transport within the DSC.We investigate the effects of modified cell configurations on the efficiency of the cell by varying associated parameter values in our model. We find, given our model and the DSC configuration investigated, that the efficiency of the DSC is improved with increasing electron diffusion, decreasing internal resistances and with decreasing dark current. We conclude that transport within the electrolyte, as described by the model, appears to have no limiting effect on the current predicted by the model until large positive voltages. Additionally, we observe that the ultrafast injection from the excited dye molecules limits the interfacial reactions that affect the DSC current.

Die vorliegende Arbeit behandelt die Herstellung geordneter C60-Schichten, ihre elektrochemische Nanostrukturierung in wässrigen Lösungen und ionischen Flüssigkeiten und den Einsatz geordneter und nanostrukturierter Fullerenschichten in organischen Dünnschichttransistoren. Geordnete C60-Schichten wurden durch thermische Verdampfung im Hochvakuum hergestellt. Als Substratmaterial wurden HOPG (Graphit), Glimmer und einkristallines Silizium verwendet. Die größten einkristallinen Bereiche werden auf HOPG-Substraten erhalten. Die laterale Ausdehnung der C60-Kristallite parallel zu den Graphitstufen kann bis zu 50 µm erreichen, orthogonal zu den Stufen ist das Wachstum durch die Graphitstufen begrenzt. Die elektrochemische Reduktion von C60 -Schichten in wässriger Lösung ist elektrochemisch irreversibel. Die geflossene Ladung beträgt ein Vielfaches der theoretisch möglichen Menge. Durch die Reduktion tritt eine Nanostrukturierung der Schichtoberfläche ein, die Größe der gebildeten Cluster beträgt 20 nm bis 50 nm. Fullerenpolymere und hydriertes C60 sind die chemischen Hauptprodukte der elektrochemischen Nanostrukturierung in wässriger Lösung. Die Reduktion von Fullerenschichten in ionischen Flüssigkeiten ist aufgrund der geschlossenen Schichtoberfläche und des starken Potentialabfalls in der Fullerenschicht zunächst kinetisch gehemmt und setzt erst bei negativeren Potentialen im Bereich der Reduktion zum C60-Dianion ein. Die Reduktion der Fullerenschichten ist elektrochemisch irreversibel, zum Teil aber chemisch reversibel. Es konnte erstmals der Einsatz nanostrukturierter C60 -Schichten als aktives Halbleitermaterial in Feldeffekt-Transistoren gezeigt werden. Für die Verwendung nanostrukturierter Fullerenschichten in Feldeffekt-Transistoren wurde 11-(3-Thienyl-)undecyl-trichlorosilan als Haftvermittler eingesetzt. Die gezeigten Ergebnisse von C60 -Transistoren mit hoher Ladungsträgerbeweglichkeit und der erfolgreichen Verwendung nanostrukturierter Fullerenschichten in Transistorstrukturen zeigen die Möglichkeiten des C60 als aktives Halbleitermaterial auf.

Die vorliegende Arbeit behandelt die Herstellung geordneter C60-Schichten, ihre elektrochemische Nanostrukturierung in wässrigen Lösungen und ionischen Flüssigkeiten und den Einsatz geordneter und nanostrukturierter Fullerenschichten in organischen Dünnschichttransistoren. Geordnete C60-Schichten wurden durch thermische Verdampfung im Hochvakuum hergestellt. Als Substratmaterial wurden HOPG (Graphit), Glimmer und einkristallines Silizium verwendet. Die größten einkristallinen Bereiche werden auf HOPG-Substraten erhalten. Die laterale Ausdehnung der C60-Kristallite parallel zu den Graphitstufen kann bis zu 50 µm erreichen, orthogonal zu den Stufen ist das Wachstum durch die Graphitstufen begrenzt. Die elektrochemische Reduktion von C60 -Schichten in wässriger Lösung ist elektrochemisch irreversibel. Die geflossene Ladung beträgt ein Vielfaches der theoretisch möglichen Menge. Durch die Reduktion tritt eine Nanostrukturierung der Schichtoberfläche ein, die Größe der gebildeten Cluster beträgt 20 nm bis 50 nm. Fullerenpolymere und hydriertes C60 sind die chemischen Hauptprodukte der elektrochemischen Nanostrukturierung in wässriger Lösung. Die Reduktion von Fullerenschichten in ionischen Flüssigkeiten ist aufgrund der geschlossenen Schichtoberfläche und des starken Potentialabfalls in der Fullerenschicht zunächst kinetisch gehemmt und setzt erst bei negativeren Potentialen im Bereich der Reduktion zum C60-Dianion ein. Die Reduktion der Fullerenschichten ist elektrochemisch irreversibel, zum Teil aber chemisch reversibel. Es konnte erstmals der Einsatz nanostrukturierter C60 -Schichten als aktives Halbleitermaterial in Feldeffekt-Transistoren gezeigt werden. Für die Verwendung nanostrukturierter Fullerenschichten in Feldeffekt-Transistoren wurde 11-(3-Thienyl-)undecyl-trichlorosilan als Haftvermittler eingesetzt. Die gezeigten Ergebnisse von C60 -Transistoren mit hoher Ladungsträgerbeweglichkeit und der erfolgreichen Verwendung nanostrukturierter Fullerenschichten in Transistorstrukturen zeigen die Möglichkeiten des C60 als aktives Halbleitermaterial auf.

Increasing useof nanomaterials in consumer products and biomedical applications creates the possibilities of intentional/unintentional exposure to humans and the environment. Beyond the physiological limit, the nanomaterialexposure to humans can induce toxicity. It is difficult to define toxicity of nanoparticles on humans as it varies by nanomaterialcomposition, size, surface properties and the target organ/cell line. Traditional tests for nanomaterialtoxicity assessment are mostly based on bulk-colorimetric assays. In many studies, nanomaterials have found to interfere with assay-dye to produce false results and usually require several hours or days to collect results. Therefore, there is a clear need for alternative tools that can provide accurate, rapid, and sensitive measure of initial nanomaterialscreening. Recent advancement in single cell studies has suggested discovering cell properties not found earlier in traditional bulk assays. A complex phenomenon, like nanotoxicity, may become clearer when studied at the single cell level, including with small colonies of cells. Advances in lab-on-a-chip techniques have played a significant role in drug discoveries and biosensor applications, however, rarely explored for nanomaterialtoxicity assessment. We presented such cell-integrated chip-based approach that provided quantitative and rapid response of cellhealth, through electrochemical measurements. Moreover, the novel design of the device presented in this study was capable of capturing and analyzing the cells at a single cell and small cell-population level. We examined the change in exocytosis (i.e. neurotransmitterrelease) properties of a single PC12 cell, when exposed to CuOand TiO2 nanoparticles. We found both nanomaterials to interfere with the cell exocytosis function. We also studied the whole-cell response of a single-cell and a small cell-population simultaneously in real-time for the first time. The presented study can be a reference to the future research in the direction of nanotoxicity assessment to develop miniature, simple, and cost-effective tool for fast, quantitative measurements at high throughput level. The designed lab-on-a-chip device and measurement techniques utilized in the present work can be applied for the assessment of othernanoparticles' toxicity, as well.

The sun is a vast source of renewable energy, which can potentially be used to satisfy the world's increasing energy demand. Yet many material challenges need to be overcome before solar energy conversion can be implemented on a larger scale. This dissertation focuses on materials used for solar energy conversion through photo-electrochemical (PEC) processes. It discusses methods for improving PEC materials, namely mixed metal oxide semiconductors, by nanostructuring, incorporation of additional elements, and application surface electrocatalysts. In this dissertation several material synthesis techniques are detailed. A high vacuum synthesis process known as reactive ballistic deposition (RBD) is used to synthesize nanostructured bismuth vanadate (BiVO₄), which is studied for PEC water oxidation. Additionally, ballistic deposition (BD) is used to incorporate Mo and W into nanostructured BiVO₄ to improve the PEC activity. An array dispenser and scanner system is used to incorporate metals into copper oxide (CuO) and copper bismuth oxide (CuBi₂O₄) and over 3,000 unique material compositions are tested for cathodic photoactivity. The system is also used to test 35 elements as single component metal oxides, mixed metal oxides, and dopants for titanium dioxide (TiO₂) for use in dye-sensitized solar cells (DSCs). Lastly, RBD is used to deposit tungsten semicarbide (W₂C) onto p-type silicon (p-type) substrates as an electrocatalyst for PEC proton reduction. In many cases, the synthesis techniques and new material combinations presented in this dissertation result in improved PEC performance. The materials are thoroughly assessed and characterized to gain insights into their nanostructure, chemical composition, light absorption, charge transport properties, catalytic activity, and stability.
text

The purpose of my PhD thesis was to synthesize new, low and high molecular weight semiconduc-tors, to determine their spectroscopic, electrochemical, spectroelectrochemical and structural properties and to verify their applicability in test organic devices, namely light emitting diodes, photovoltaic cells and field effect transistors. The common element of all synthesized compounds was the diketo-pyrrolopyrrole (DPP) acceptor (A) further functionalized with donor (D) units consisting of differently substituted thiophene or oligothiophene derivatives. In particular four series of low molecular weight DAD compounds were prepared in which the D units consisted of alkyl derivative of mono-, bi- or terthi-ophene (T1, T2 and T3 series, respectively). The potential of the diketopyrrolopyrrole unit reduction to a radical anion was influenced neither by the type of alkyl substituents at nitrogen atoms in lactam units of the DPP moiety, nor by the position of alkyl substituents in terminal thiophene rings of the D seg-ment. It however increased with increasing length of the D segment from ca.-1.66 V vs Fc/Fc+ in T1 compounds to ca. -1.55 V vs Fc/Fc+ in T3 ones. The oxidation potential of the studied compounds to radical cations was even more strongly affected by the D segment length decreasing from 0.51 - 0.52 V in T1 compounds to 0.25 - 0.26 V vs Fc/Fc+ in T3 ones. It was also dependent on the position of the alkyl solubilizing substituent in the thiophene ring. The electrochemical data were found in a perfect agreement with the spectroscopic ones as demonstrated by very close values of the optically and elec-trochemically determined band gaps. All these trends were reproduced in DFT calculations which yield-ed ionization potentials (IP) and electron affinity (EA) values very similar to those measured experimen-tally. The highest photoluminescence quantum yield (QY), approaching 80 %, was measured for T1 com-pounds whereas for T3 ones its value dropped below 20 %. Time resolved photoluminescence studies consistently showed in turn shorter emission lifetimes and larger non-radiative rate constants for com-pounds of lower QY values. Guest/host-type single layer light emitting diodes were fabricated from the most luminescent compounds (T1 series) molecularly dispersed (1 wt.%) in a matrix consisting of 70 wt.% of poly(N-vinylcarbazole) and 30 wt.% of 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole. Appropri-ate alignment of the matrix components energy levels and those of the T1 compounds resulted in effec-tive electroluminescence of the guest molecules. The fabricated diodes showed luminance exceeding 2600 cd/m2 with the luminous efficiency of 0.7 cd/A. These were the first OLEDs reported for this type of DPP derivatives. All compounds of T2 and T3 series showed strong tendency to self-organize in monolayers deposit-ed on HOPG, as evidenced by STM investigations. The resulting 2D organization depended on the par-ticular topology of the investigated molecules, however in the majority of cases the determined 2D unit cell parameters were consistent with the corresponding parameters of the 3D unit cells determined on the basis of the X-ray data. In search of low molecular weight semiconductors of a very narrow band gap, a D1-A-D2-A-D1 com-pound consisting of two dithienyldiketopyrrolopyrrole units joined together by dialkoxythiophene (DPP-Th-DPP) was synthesized. Spectroscopic and electrochemical investigations indicated a narrow band gap of 1.62 eV and a good match of the absorption spectrum with the solar spectrum i.e. perfect suita-bility of the synthesized compound for the application in bulk heterojunction solar cells. Unfortunately, the fabricated test devices showed a rather modest power conversion efficiency (PCE): 0.5 %, indicat-ing the necessity of further device optimization. Similarly, application of DPP-Th-DPP in test p-channel FETs resulted in modest hole mobility, below 10-5 cm2/Vs. In addition to low molecular weight semiconductors, three solution processable -(DA)-copolymers, consisting of DPP units alternating with alkoxy (or alkyl) derivatives of ter-, quater- and pentathiophene ones (P1-P3) were synthesized. For all prepared copolymers the measured XPS spectra (C1s, S2p, N1s and O1s) were in a very good agreement with the expected chemical constitution. Spectroscopic studies showed that their optical band gaps were governed by the presence of the alkoxy substituents, whose electron donating properties led to additional gap narrowing yielding semiconductors with band gaps of the width inferior to 1.3 eV. The same trend was observed in the electrochemically determined band gaps. Vibrational model was calculated for the copolymer with 3’,4’-dioctyloxy-2,2’:5’,2’’-terthiophene D segment with the goal to unequivocally attribute the observed Raman modes and to support the interpretation of the spectral changes induced by the electrochemical oxidation. It was es-tablished that the charge imposed on the oligothiophene segment delocalize towards the diketo-pyrrolopyrrole unit. These findings found out to be in perfect agreement with the UV-vis-NIR spectroe-lectrochemistry data which showed metallic-like delocalization of charge carriers in this polymer. There-fore, this polymer seems to be very interesting not only as a potential low band gap component of or-ganic photovoltaic cells but also, in the doped state, as an electronic conductor of the metallic charac-ter. The choice of diketopyrrolopyrrole (DPP) derivatives as the main subject of my thesis can be ra-tionalized as follows. Among the plethora of recently developed organic semiconductors DPP seemed especially interesting as a building block in the synthesis of donor-acceptor (DA) or donor-acceptor-donor (DAD) compounds and their high molecular weight counterparts (-(DA)-, -(DAD)-). Mono- or oli-gothiophenes were selected in my research as D segments since with these units donor-acceptor inter-actions could be precisely tuned via changing the length of the D-segment and the position as well as the nature (alkyl vs alkoxy) of the solubilizing substituent. To the best of my knowledge the presented dissertation is the most comprehensive study of DPP-oligothiophene semiconductors involving their detailed electrochemical, spectroelectrochemical, struc-tural and spectroscopic characterization carried out with the goal of their application in organic electronic devices. Special emphasis is put on the determination of their optoelectronic properties which led to the fabrication of efficient light emitting diodes, never previously reported for this family of compounds.
Celem mojej pracy doktorskiej była synteza nowych polimerów przewodzących oraz półprzewodni-ków organicznych o małej masie cząsteczkowej, zbadanie ich właściwości elektrochemicznych, spek-troskopowych i strukturalnych, a następnie ich zastosowanie do konstrukcji urządzeń testowych (diody elektroluminescencyjne, ogniwa fotowoltaiczne, tranzystory polowe). Elementem wspólnym dla wszystkich zsyntezowanych pochodnych była elektronoakceptorowa (A) jednostka diketopirolopirolowa (DPP), która została następnie sfunkcjonalizowana pochodnymi tiofenu o właściwościach elektronodo-norowych (D). W pierwszej części badań, zsyntezowano cztery serie półprzewodników typu DAD, w których jed-nostki donorowe składają się z alkilowych pochodnych mono-, di- i tertiofenu (odpowiednio serie T1, T2 i T3). Potencjał redukcji diketopirolopirolu do anionorodnika wzrasta wraz ze wzrostem długości części donorowej, od ok. -1,66 V vs Fc/Fc+ dla serii T1 do ok. -1,55 V vs Fc/Fc+ dla serii T3, ale jest niezależ-ny od rodzaju podstawnika alkilowego przy atomie azotu jednostki DPP oraz sposobu podstawienia terminalnego pierścienia tiofenowego w segmencie donorowym. Potencjał utleniania zsyntezowanych pochodnych do kationorodników również zależy od długości segmentu donorowego i zmniejsza się od 0,51 – 0,52 V vs Fc/Fc+ dla serii T1 do 0,25 -0,26 V vs Fc/Fc+ dla serii T3. Jego wartość zależy również od pozycji podstawienia łańcucha alkilowego w terminalnym pierścieniu tiofenowym. Doskonała zgod-ność danych elektrochemicznych z danymi spektroskopowymi przejawia się bardzo zbliżonymi warto-ściami przerw energetycznych wyznaczonych metodami elektrochemicznymi i optycznymi. Wartości potencjału jonizacji (IP) i powinowactwa elektronowego (EA) wyznaczone eksperymentalnie zostały potwierdzone za pomocą metod teoretycznych (DFT). Najwyższą wartość kwantowej wydajności fluorescencji (QY), 80 %, wyznaczono dla pochodnych serii T1, dla pochodnych z serii T3 wartość ta spadła do 20 %. Pochodne o niższych wartościach QY charakteryzują się krótszym czasem emisji i większymi stałymi przejść bezpromienistych. Do kon-strukcji diod elektroluminescencyjnych typu gość/gospodarz wykorzystano związki o najlepszych wła-ściwościach luminescencyjnych (T1) rozproszonych (1 wt.%) w matrycy składającej się z poliwinylo-karbazolu (70 wt.%) i 2-tert-butylofenylo-5-bifenylo-1,3,4-oksadiazolu (30 wt.%). Odpowiednie wyrówna-nie poziomów energetycznych wszystkich składników mieszaniny spowodowało wydajną elektrolumi-nescencję molekuł „gościa” (seria T1). Skonstruowane urządzenia wykazują luminancję przekraczającą 2600 cd/m2 z wydajnością luminescencji równą 0,7 cd/A i są pierwszymi organicznymi diodami elektro-luminescencyjnymi wykorzystującymi pochodne DPP. Badania przeprowadzone za pomocą skaningowej mikroskopii tunelowej wskazują, że wszystkie pochodne serii T2 i T3 wykazują zdolność do uporządkowania w struktury dalekiego zasięgu w mono-warstwach osadzonych na powierzchni HOPG, a sposób ich organizacji zależy od budowy cząsteczek. W drugiej części badań, zsyntezowano małocząsteczkowy półprzewodnik typu D1-A-D2-A-D1 za-wierający dwie jednostki ditienylodiketopirolopirolowe połączone cząsteczką dialkoksytiofenu (DPP-Th-DPP). Niewielka szerokość przerwy energetycznej (1,62 eV) oraz dobra zgodność widma absorpcyjne-go DPP-Th-DPP z widmem promieniowania słonecznego wskazują na możliwość jego wykorzystania w objętościowych ogniwach fotowoltaicznych. Niestety, skonstruowane urządzenia charakteryzowały się niewielką wartością wydajności konwersji energii (PCE): 0,5 %, wskazując na konieczność dalszej optymalizacji budowy i sposobu konstrukcji urządzenia. Podobnie, tranzystory polowe typu p oparte o DPP-Th-DPP charakteryzują się niewielkimi wartościami ruchliwości ładunków: poniżej 10-5 cm2/Vs. Ostatnią, trzecią, część badań przeprowadzonych w ramach pracy doktorskiej stanowiła synteza trzech rozpuszczalnych kopolimerów typu -(DA)- składających się z jednostek DPP oddzielonych przez alkilowe bądź alkoksylowe pochodne łańcuchów oligotiofenowych (P1-P3). Widma XPS (C1s, S2p, N1s i O1s) zarejestrowane dla wszystkich polimerów potwierdziły oczekiwany skład chemiczny. Na szero-kości przerw energetycznych wyznaczonych metodami spektroskopowymi wpływa obecność łańcu-chów alkoksylowych, których właściwości elektronodonorowe prowadzą do jej dodatkowego zmniej-szenia (1,3 eV). Tę samą tendencję zaobserwowano dla przerw energetycznych wyznaczonych za pomocą technik elektrochemicznych. W celu jednoznacznego przypisania pasm obserwowanych w widmach Ramana jako wynik elektrochemicznego utlenienia i potwierdzenia słuszności ich interpretacji obliczono model wibracyjny dla segmentu 3’,4’-dioktyloksy-2,2’:5’,2’’-tertiofenowego. Ustalono, że ładu-nek wprowadzony do łańcucha oligotiofenowego ulega delokalizacji na jednostki elektronoakceptorową i elektronodonorową. Dane te są zgodne z wynikami uzyskanymi metodą spektroelektrochemii UV-vis-NIR. Dlatego też, polimer ten wydaje się być interesujący jako potencjalny składnik ogniw fotowoltaicz-nych. Wybór pochodnych diketopirolopirolu (DPP) na główny temat mojej pracy doktorskiej nie był przy-padkowy. Spośród opisanych dotychczas półprzewodników organicznych, DPP wydaje się być szcze-gólnie interesujący jako element strukturalny w syntezie półprzewodników organicznych typu donor-akceptor (DA) oraz donor-akceptor-donor (DAD) i ich odpowiedników o wysokiej masie cząsteczkowej (-(DA)-, -(DAD)-). Grupy mono- oraz oligotiofenowe zostały wybrane do moich badań jako segmenty elektronodonorowe, ponieważ oddziaływania donor-akceptor mogą być precyzyjnie modyfikowane po-przez zmianę długości segmentu D oraz charakteru podstawnika (alkilowy vs alkoksylowy). Według mojej najlepszej wiedzy prezentowana Rozprawa w sposób najbardziej wszechstronny pre-zentuje półprzewodniki łączące molekułę DPP i łańcuchy oligotiofenowe: ich szczegółowe właściwości elektrochemiczne, spektroskopowe, spektroelektrochemiczne i strukturalne, a także zastosowanie w urządzeniach. Szczególny nacisk w pracy położono na ustalenie ich właściwości optoelektronicznych, które doprowadziły do produkcji wydajnych diod elektroluminescencyjnych, zaprezentowanych po raz pierwszy dla tej grupy związków.

Thesis (Ph. D.)--Florida State University, 2003.
Advisor: Dr. Peng Xiong, Florida State University, College of Arts and Sciences, Dept. of Physics. Title and description from dissertation home page (viewed Apr. 9, 2004). Includes bibliographical references.

The present thesis is focused on rational design and synthesis of π-conjugated donoracceptor-donor (D-A-D) type oligomers and D-A type copolymers. Thesis is organized in seven chapters, apart from introduction remaining six chapters are grouped into two parts (A and B). Part A deals with Chapters 2, 3, 4 and Part B contains chapters 5, 6 and 7. A brief discussion on the content of individual chapters is provided below. Chapter 1 discusses the introduction to organic solar cell with operating principles and effect of spinodal decomposition on stability of the devices is presented. The status and literature related to the improvement of life time of the organic solar cells by self-assembly approach has been explored. In addition, design and synthesis of the fluorine substituted π-conjugated organic semiconductors for n-type OFETs and OLED has been discussed. Part A This part of the thesis attempt to address some of the challenges listed below (1) Investigation of miscibility of binary components in bulk heterojunction solar cells through H-bonding approach. (2) Synthesis of new low band gap molecular semiconductors having H-bonding sites. (3) Fabrication of bulk heterojunction solar cell devices using these new molecules and exploring the photovoltaics performance. Chapter 2, donor-acceptor-donor (D-A-D) concept has been employed to design low band gap oligomers named as TTB. Barbiturate functional group has been utilized to explore the concepts of supramolecular chemistry. It is shown that, TTB molecule self-organizes via intermolecular H-bonding between barbituric acid units. Interactions between the oligothiophene subunits were also found to be important, affording nanoribbons that were observed by atomic force and transmission electron microscopy. The applicability of TTB for organic electronic applications was investigated by fabricating organic field-effect transistors (OFETs) and organic photovoltaic device. The crystalline nanoribbons were beneficial in understanding the phase morphology of PCBM and TTB blend. Chapter 3, the self-assemble property of TTB was disrupted by the substitution of methyl group on the nitrogen of the barbituric acid moiety. The optical and electrochemical properties of the new derivative have been investigated by UV-Visible spectroscopy, photoluminescence spectroscopy and cyclic voltammetry. Further investigations on the effect of self-assembly on organic solar cells were carried out by fabricating BHJ and OFET. The results proved that the self-assembly within the donor moieties led to complete phase separation between the donor and acceptor which had an adverse effect on the photovoltaic performance. Chapter 4, the conjugation of TTB was extended by the synthesis of two new copolymers by polymerizing with two oliogothiophene (terthiophene and benzobithiophene) derivatives with different donating strength. The investigation of photophysical and electrochemical properties of copolymers were studied by varying the donating strength. As we increase the donating strength of oligothiophenes, the intramolecular charge transfer band of DA copolymers was red shifted. Further, density functional theory (DFT) calculation of these materials was carried out to get insight into their photophysical properties. Part B This part of the thesis attempt to address some of the challenges listed below (1) Investigation of fluorine substituted organic semiconductos like 2,2’ bithiazole and pheanthroimidazole. (2) Synthesis of pentafluoro phenyl appended derivatives of 2,2’ bithiazole and pheanthroimidazole. (3) Fabrication of OFETs and OLEDs using these new molecules and elucidated the device performance with molecular structure. Chapter 5, pentafluorophenyl appended 2,2’-bithiazole derivatives were synthesized. The single crystal x-ray diffraction studies shows the unusual strong type-II F•••F interactions within the distance of 2.668 Å, at an angle of 89.14° and 174.15°. It also shows the usual type-I F•••F interaction within the distance of 2.825Å, at an angle of 137.38° and 135.93°. Upon bromination type-II Br•••Br interaction was observed and the packing was further stabilized by S•••Br interactions. The conjugation was further extended with different aromatic and heteroaromatic substituents and synthesized the star shaped structure. The band gap as well as the electronic energy levels was tuned by substituting various aromatic and heteroaromatic substituents. These star shaped derivatives shows electron mobilities in the order of 10-4 to 10-3cm2/Vs. Chapter 6, Novel D-A copolymers were synthesized by Stille condensation of electron acceptor fluorinated phenanthroimidazole with electron donors like terthiophene and benzobithiophene. Prior to that insoluble pentafluoro phenyl phenanthroimidazole was Nalkylated in presence of DMF which concurrently resulted in C-F activation of the pentafluoro phenyl moiety. As we increase the donor strength from benzobithiophene to terthiophene the absorbance spectra was red shifted from 446 nm to 482 nm in solution and 455 nm to 484 nm in solid state. The band gap of these copolymers was found to be 2.4 eV for PIBDT and 2.2 eV for PIDHTT from the absorbance spectra. The photoluminescence data shows that these materials are promising for the yellow colour as well as orange colour displays, of narrow wavelength range (FWHM 40 nm for PIBDT and 35 nm for PIDHTT), which can be achieved just by the manipulation of donor moieties in the copolymers. The preliminary electroluminiscence data shows high brightness of 888cd/m2 (orange luminescence) for PIDHTT and 410cd/m2 (yellow luminescence) for PIBDT. Chapter 7, Acenaphtho[1,2-b]quinoxaline based donor–acceptor type low band gap conjugated copolymers were synthesized by Stille coupling reaction with the corresponding oligothiophene derivatives. The optical properties of the copolymers were characterized by ultraviolet-visible spectrometry while the electrochemical properties were determined by cyclic voltammetry. The band gap of these polymers was found to be in the range of 1.8-2.0 eV as calculated from the optical absorption band edge. The intense charge transfer band in absorption spectra shows the significant effect of acceptor in the copolymers. X-ray diffraction measurements show weak π–π stacking interactions between the polymer chains. The OFET devices fabricated using these co-polymers showed dominant p-channel transistor behavior with the highest mobility of 1×10-3cm2/Vs.

The present thesis is focused on rational design and synthesis of π-conjugated donoracceptor-donor (D-A-D) type oligomers and D-A type copolymers. Thesis is organized in seven chapters, apart from introduction remaining six chapters are grouped into two parts (A and B). Part A deals with Chapters 2, 3, 4 and Part B contains chapters 5, 6 and 7. A brief discussion on the content of individual chapters is provided below. Chapter 1 discusses the introduction to organic solar cell with operating principles and effect of spinodal decomposition on stability of the devices is presented. The status and literature related to the improvement of life time of the organic solar cells by self-assembly approach has been explored. In addition, design and synthesis of the fluorine substituted π-conjugated organic semiconductors for n-type OFETs and OLED has been discussed. Part A This part of the thesis attempt to address some of the challenges listed below (1) Investigation of miscibility of binary components in bulk heterojunction solar cells through H-bonding approach. (2) Synthesis of new low band gap molecular semiconductors having H-bonding sites. (3) Fabrication of bulk heterojunction solar cell devices using these new molecules and exploring the photovoltaics performance. Chapter 2, donor-acceptor-donor (D-A-D) concept has been employed to design low band gap oligomers named as TTB. Barbiturate functional group has been utilized to explore the concepts of supramolecular chemistry. It is shown that, TTB molecule self-organizes via intermolecular H-bonding between barbituric acid units. Interactions between the oligothiophene subunits were also found to be important, affording nanoribbons that were observed by atomic force and transmission electron microscopy. The applicability of TTB for organic electronic applications was investigated by fabricating organic field-effect transistors (OFETs) and organic photovoltaic device. The crystalline nanoribbons were beneficial in understanding the phase morphology of PCBM and TTB blend. Chapter 3, the self-assemble property of TTB was disrupted by the substitution of methyl group on the nitrogen of the barbituric acid moiety. The optical and electrochemical properties of the new derivative have been investigated by UV-Visible spectroscopy, photoluminescence spectroscopy and cyclic voltammetry. Further investigations on the effect of self-assembly on organic solar cells were carried out by fabricating BHJ and OFET. The results proved that the self-assembly within the donor moieties led to complete phase separation between the donor and acceptor which had an adverse effect on the photovoltaic performance. Chapter 4, the conjugation of TTB was extended by the synthesis of two new copolymers by polymerizing with two oliogothiophene (terthiophene and benzobithiophene) derivatives with different donating strength. The investigation of photophysical and electrochemical properties of copolymers were studied by varying the donating strength. As we increase the donating strength of oligothiophenes, the intramolecular charge transfer band of DA copolymers was red shifted. Further, density functional theory (DFT) calculation of these materials was carried out to get insight into their photophysical properties. Part B This part of the thesis attempt to address some of the challenges listed below (1) Investigation of fluorine substituted organic semiconductos like 2,2’ bithiazole and pheanthroimidazole. (2) Synthesis of pentafluoro phenyl appended derivatives of 2,2’ bithiazole and pheanthroimidazole. (3) Fabrication of OFETs and OLEDs using these new molecules and elucidated the device performance with molecular structure. Chapter 5, pentafluorophenyl appended 2,2’-bithiazole derivatives were synthesized. The single crystal x-ray diffraction studies shows the unusual strong type-II F•••F interactions within the distance of 2.668 Å, at an angle of 89.14° and 174.15°. It also shows the usual type-I F•••F interaction within the distance of 2.825Å, at an angle of 137.38° and 135.93°. Upon bromination type-II Br•••Br interaction was observed and the packing was further stabilized by S•••Br interactions. The conjugation was further extended with different aromatic and heteroaromatic substituents and synthesized the star shaped structure. The band gap as well as the electronic energy levels was tuned by substituting various aromatic and heteroaromatic substituents. These star shaped derivatives shows electron mobilities in the order of 10-4 to 10-3cm2/Vs. Chapter 6, Novel D-A copolymers were synthesized by Stille condensation of electron acceptor fluorinated phenanthroimidazole with electron donors like terthiophene and benzobithiophene. Prior to that insoluble pentafluoro phenyl phenanthroimidazole was Nalkylated in presence of DMF which concurrently resulted in C-F activation of the pentafluoro phenyl moiety. As we increase the donor strength from benzobithiophene to terthiophene the absorbance spectra was red shifted from 446 nm to 482 nm in solution and 455 nm to 484 nm in solid state. The band gap of these copolymers was found to be 2.4 eV for PIBDT and 2.2 eV for PIDHTT from the absorbance spectra. The photoluminescence data shows that these materials are promising for the yellow colour as well as orange colour displays, of narrow wavelength range (FWHM 40 nm for PIBDT and 35 nm for PIDHTT), which can be achieved just by the manipulation of donor moieties in the copolymers. The preliminary electroluminiscence data shows high brightness of 888cd/m2 (orange luminescence) for PIDHTT and 410cd/m2 (yellow luminescence) for PIBDT. Chapter 7, Acenaphtho[1,2-b]quinoxaline based donor–acceptor type low band gap conjugated copolymers were synthesized by Stille coupling reaction with the corresponding oligothiophene derivatives. The optical properties of the copolymers were characterized by ultraviolet-visible spectrometry while the electrochemical properties were determined by cyclic voltammetry. The band gap of these polymers was found to be in the range of 1.8-2.0 eV as calculated from the optical absorption band edge. The intense charge transfer band in absorption spectra shows the significant effect of acceptor in the copolymers. X-ray diffraction measurements show weak π–π stacking interactions between the polymer chains. The OFET devices fabricated using these co-polymers showed dominant p-channel transistor behavior with the highest mobility of 1×10-3cm2/Vs.

Indiana University-Purdue University Indianapolis (IUPUI)
In this research thesis, copper indium gallium selenium (CIGS) nanoparticles were synthesized from metal chlorides, functionalized to disperse in water, and further used in layer by layer (LbL) nanoassembly of CIGS films. CIGS nanoparticles were synthesized through the colloidal precipitation in an organic solvent. The peak and average sizes of the synthesized particles were measured to be 68 nm and 75 nm in chloroform, and 30 nm and 115 nm in water, respectively. Two methods were used to disperse the particle in water. In the first method the stabilizing agent oleylamine (OLA) was removed through multiple cleaning processes, and in the second method ligand exchange was performed with polystyrene sulfonate (PSS). Zeta potential of CIGS nanoparticles dispersed in water was measured to be +61 mV. The surface charge of the nanoparticles was reversed by raising the pH of the solution, which was measured to be −43.3 mV at 10.5 pH. In a separate process, the CIGS nanoparticles dispersed in water were coated with PSS. The resulting dispersion was observed to be stable and the surface charge was measured to be −56.9 mV. The LbL deposition process of CIGS nanoparticles was characterized by depositing thin films on quartz crystal microbalance (QCM). LbL depositions was conducted using (i) oppositely charged CIGS nanoparticles, (ii) positively charged CIGS nanoparticles and PSS, and (iii) PSS-coated CIGS (CIGS-PSS) and polyethyleneimine (PEI). The average thickness of each bi-layer of the above mentioned depositions were measured to be 2.2 nm, 1.37 nm, and 10.12 nm, respectively. The results from the QCM have been observed to be consistent with the film thickness results obtained from atomic force microscopy (AFM). Various immersion times versus thickness of the film were also studied. For electrical characterization, the CIGS films were deposited on indium tindioxide (ITO)-coated glass substrates. Current versus voltage (I/V) measurements were carried out for each of the films using the Keithley semiconductor characterization instruments and micromanipulator probing station. It was observed that the conductivity of the films was increased with the deposition of each additional layer. The I/V characteristics were also measured under the light illumination and after annealing to study the photovoltaic and annealing effects. It was observed that under light illumination, the resistivity of a 12-layer CIGS film decreased by 93% to 0.54 MΩ.m, and that of the same number of layers of PSS-coated CIGS and PEI film decreased by 60% to 0.97 MΩ.m under illumination. The resistivity of an 8-layer CIGS and PSS film decreased by 76.4% to 0.1 MΩ.m, and that of the same layers of PSS-coated CIGS and PEI decreased by 87% to 0.07 MΩ.m after annealing. The functionalized nanoparticles and the LbL CIGS films were implemented in the solar cell devices. Several configurations of CIGS films (p-type), and ZnO and CdS films (n-type) were considered. Poly(3,4-ethylenedioxythiophene) (PEDOT), molybdenum (Mo), and ITO were used as back contacts and ITO was used as front contact for all the devices. The devices were characterized the Keithley semiconductor characterization instruments and micromanipulator probing station. For a CIGS and n-ZnO films device with PEDOT as back contact and ITO as front contact, the current density at 0 V and under light illumination was measured to be 60 nA/cm2 and the power density was measured to be 0.018 nW/cm2. For a CIGS and CdS films device with ITO as both back and front contact, the current density at 0 V and under light illumination was measured to be 50 nA/cm2 and the power density was measured to be 0.01 nW/cm2. For a drop-casted CIGS and CdS films device with Mo as back contact and ITO as front contact, the current density of 50 nA/cm2 at 0 V and power density of 0.5 nW/cm2 under light illumination was measured. For the LbL CIGS and chemical bath deposited CdS films device with ITO as both back and front contact, the current density of 0.04 mA/cm2 at 0 V and power density of 1.6 μW/cm2 under light illumination was measured. Comparing to Device-III, an increase by 99% in the power density was observed by using the CIGS LbL film in the device structure. The novel aspects of this research include, (i) functionalization of the CIGS nanoparticles to disperse in water including coating with PSS, (ii) electrostatic LbL deposition of CIGS films using oppositely charged nanoparticles and polymers, and (iii) the utilization of the fabricated LbL CIGS films to develop solar cells. In addition, the n-type cadmium sulfide film (CdS) and zinc oxide (ZnO) buffer layer were also deposited through LbL process after the respective particles were functionalized with PSS coating in separate experiments.

Materials for studies related to nanoscience and nanotechnology have gained tremendous attention owing to their unique physical, chemical and electronic properties. Among various anisotropic nanostructures, one dimensional (1D) materials have received immense interest in numerous fields ranging from catalysis to electronics. Imparting multi-functionality to nanostructures is one of the major areas of research in materials science. In this direction, use of nanosized materials in energy systems such as fuel cells has been the subject of focus to achieve improved performance. Tuning the morphology of nanostructures, alloying of catalysts, dispersing catalytic particles onto various supports (carbon nanotubes, carbon nanofibers, graphene, etc.) are some of the ways to address issues related to electrochemical energy systems. It is worth mentioning that highly stable and corrosion resistant electrodes are mandatory as electrochemical cells operate under aggressive environments. Additionally, carbon, which is often used as a support for catalysts, is prone to corrosion and is subsequently implicated in reduced performance due to poor adherence of catalyst particles and loss in electrochemically active area. Hence, there is a quest for the development of stable and durable electrocatalysts / supports for various studies including fuel cells. The present thesis is structured in exploring the multi-functional aspects of titanium carbide (TiC), an early transition metal carbide. TiC, a fascinating material, possesses many favorable properties such as extreme hardness, high melting point, good thermal and electrical conductivity. Its metal-like conductivity and extreme corrosion resistance prompted us to use this material for various electrical and electrochemical studies. The current study explores the versatility of TiC in bulk as well as nanostructured forms, in electrical and electrochemical studies towards sensing, electrocatalytic reactions and active supports. 1D TiC nanowires (TiC-NW) are prepared by simple solvothermal method without use of any template and are characterized using various physico-chemical techniques. The TiC-NW comprise of 1D nanostructures with several µm length and 40 ± 15 nm diameter (figure 1). Electrical properties of individual TiC-NW are probed by fabricating devices using focused ion beam deposition (FIB) technique. The results depict the metallic nature of TiC-NW (figure 2). Figure 1. (a) SEM, (b) TEM and (c) HRTEM images of TiC-NW prepared by solvothermal method. Figure 2. (a) SEM image and (b) I-V characteristics of TiC-NW - based device as a function of temperature. The contact pads are made of Pt. Subsequently, oxidized TiC nanowires are prepared by thermal annealing of TiC-NW, leading to carbon - doped TiO2 nanowires (C-TiO2-NW) (figure 3). Photodetectors are fabricated with isolated C-TiO2-NW and the device is found to respond to visible light (figure 3) radiation with very good responsivity (20.5 A/W) and external quantum efficiency (2.7 X 104). The characteristics are quite comparable with several reported visible light photodetectors based on chalcogenide semiconductors. Figure 3. (a) HRTEM, (b) EDAX, (c) Scanning TEM-DF images of C-TiO2-NW along with (d) Ti (e) O and (f) C mapping. (g) Current – voltage curves of single C-TiO2-NW recorded in dark (black) and in presence of visible light radiation (red) of intensity 57.7 mW/cm2 at 25oC. Inset of (g) shows the SEM image of the device (top) and schematic illustration of fabricated photodetector (bottom). The next chapter deals with the electrochemical performance of TiC demonstrated for studies involving oxygen reduction and borohydride oxidation reactions. Electrochemical oxygen reduction reaction (ORR) reveal that TiC-NW possess high activity for ORR and involves four electron process while it is a two electron reduction for bulk TiC particles (figure 4). The data has been substantiated by density functional theory (DFT) calculations that reveal different modes of adsorption of oxygen on bulk and nanowire morphologies. Stable performance is observed for several hundreds of cycles that confirm the robustness of TiC. The study also demonstrates excellent selectivity of TiC for ORR in presence of methanol and thus cross-over issue can be effectively addressed in direct methanol fuel cells. In the chapter on borohydride oxidation, bare TiC electrode is explored as a catalyst for the oxidation of borohydride. One of the major issues in direct borohydride fuel cells (DBFC) is the hydrolysis of borohydride that happens on almost all electrode materials leading to low efficiency. The present study reveals that TiC is a very good catalyst for borohydride oxidation with little or no hydrolysis of borohydride [figure 5 (a)] under the experimental conditions studied. Further, shape dependant activity of TiC has been studied and fuel cell performance is followed [figure 5 (b)]. Polarization data suggests that the performance of TiC is quite stable under fuel cell experimental conditions. Figure 4. (a) Linear sweep voltammograms for ORR recorded using (i) bulk TiC particles and (ii) TiC-NW in O2-saturated 0.5 M KOH at 1000 rpm. Scan rate used is 0.005 Vs-1. (b) Variation of number of electrons with DC bias. Black dots correspond to TiC bulk particles while red ones represent nanowires. Figure 5. (a) Cyclic voltammograms of borohydride oxidation on TiC coated GC electrode in 1 M NaOH containing 0.1 M NaBH4. Scan rate used is 0.05 Vs-1. (b) Fuel cell polarization data at 70oC for DBFC assembled with (i) bulk TiC particles and (ii) TiC-NW as anode catalysts and 40 wt% Pt/C as cathode. Anolyte is 2.1 M NaBH4 in 2.5 M NaOH, and catholyte is 2.2 M H2O2 in 1.5 M H2SO4. Anode loading is 1.5 mg cm-2 and cathode loading is 2 mg cm-2. The corrosion resistance nature of TiC lends itself amenable to be used as an active support for catalytic particles (Pt and Pd) for small molecules oxidation reactions. In the present study, electro-oxidation of methanol, ethanol and formic acid have been studied. As shown in figure 6 (a), the performance of Pd loaded TiC (Pd-TiC) is found to be higher than that of Pd loaded carbon (Pd-C) suggesting the active role of TiC. The catalytic activities of TiC-based supports are further improved by tuning their morphologies. Figure 6 (c) reveals that the activities are higher in case of Pd-TiC-NW than that of Pd-TiC. Figure 6. (a) Cyclic voltammograms of Pd-TiC and Pd-C for ethanol oxidation, (b) T EM image of Pd-TiC-NW and (c) voltammograms of Pd-TiC-NW in N2-saturated 1 M ethanol in 1 M KOH medium, scan rate used is 0.05 Vs-1. The next aspect explored, is based on the preparation of C-TiO2 and its use as a substrate for surface enhanced Raman spectroscopy (SERS). Carbon doped titanium dioxide is prepared by thermal annealing of TiC. It is observed that the amount of dopant (carbon content) is dependent on the experimental conditions used. SERS studies using 4¬mercaptobenzoic acid (4-MBA) as the analyte, indicates that C-TiO2 [figure 7 (a)] enhances Raman signals based on chemical interactions between the analyte and the substrate. Raman signal intensities can be tuned with the amount of carbon content in C¬TiO2. Enhancement factors are calculated to be (7.7 ± 1.2) x 103 (for 4-MBA) and (1.7 ± 1.2) x 103 (for 4-nitrothiophenol). The SERS substrates are found to be surface renewable using visible light, a simple strategy to re-use the substrate [figure 7 (b)]. The regeneration of SERS substrates is based on self cleaning action of TiO2 that produces highly reactive oxygen containing radicals known to degrade the molecules adsorbed on TiO2. Thus, the versatility of TiC has been demonstrated with various studies. In addition to using TiC-based materials, nanoparticles of Rh, Ir and Rh-Ir alloy structures have also been used for borohydride oxidation reaction. This is explained in the last section. In Appendix-I, preliminary studies on the preparation of TiC-polyaniline (PANI) composites using liquid-liquid interfacial polymerization is explained. Raman spectroscopy results suggest that the presence of TiC-NW makes PANI to assume preferential orientation in the polaronic (conducting) form. Appendix-II discusses the role of TiC-NW as a fluorescence quencher for CdS semiconductor nanoparticles.

Materials for studies related to nanoscience and nanotechnology have gained tremendous attention owing to their unique physical, chemical and electronic properties. Among various anisotropic nanostructures, one dimensional (1D) materials have received immense interest in numerous fields ranging from catalysis to electronics. Imparting multi-functionality to nanostructures is one of the major areas of research in materials science. In this direction, use of nanosized materials in energy systems such as fuel cells has been the subject of focus to achieve improved performance. Tuning the morphology of nanostructures, alloying of catalysts, dispersing catalytic particles onto various supports (carbon nanotubes, carbon nanofibers, graphene, etc.) are some of the ways to address issues related to electrochemical energy systems. It is worth mentioning that highly stable and corrosion resistant electrodes are mandatory as electrochemical cells operate under aggressive environments. Additionally, carbon, which is often used as a support for catalysts, is prone to corrosion and is subsequently implicated in reduced performance due to poor adherence of catalyst particles and loss in electrochemically active area. Hence, there is a quest for the development of stable and durable electrocatalysts / supports for various studies including fuel cells. The present thesis is structured in exploring the multi-functional aspects of titanium carbide (TiC), an early transition metal carbide. TiC, a fascinating material, possesses many favorable properties such as extreme hardness, high melting point, good thermal and electrical conductivity. Its metal-like conductivity and extreme corrosion resistance prompted us to use this material for various electrical and electrochemical studies. The current study explores the versatility of TiC in bulk as well as nanostructured forms, in electrical and electrochemical studies towards sensing, electrocatalytic reactions and active supports. 1D TiC nanowires (TiC-NW) are prepared by simple solvothermal method without use of any template and are characterized using various physico-chemical techniques. The TiC-NW comprise of 1D nanostructures with several µm length and 40 ± 15 nm diameter (figure 1). Electrical properties of individual TiC-NW are probed by fabricating devices using focused ion beam deposition (FIB) technique. The results depict the metallic nature of TiC-NW (figure 2). Figure 1. (a) SEM, (b) TEM and (c) HRTEM images of TiC-NW prepared by solvothermal method. Figure 2. (a) SEM image and (b) I-V characteristics of TiC-NW - based device as a function of temperature. The contact pads are made of Pt. Subsequently, oxidized TiC nanowires are prepared by thermal annealing of TiC-NW, leading to carbon - doped TiO2 nanowires (C-TiO2-NW) (figure 3). Photodetectors are fabricated with isolated C-TiO2-NW and the device is found to respond to visible light (figure 3) radiation with very good responsivity (20.5 A/W) and external quantum efficiency (2.7 X 104). The characteristics are quite comparable with several reported visible light photodetectors based on chalcogenide semiconductors. Figure 3. (a) HRTEM, (b) EDAX, (c) Scanning TEM-DF images of C-TiO2-NW along with (d) Ti (e) O and (f) C mapping. (g) Current – voltage curves of single C-TiO2-NW recorded in dark (black) and in presence of visible light radiation (red) of intensity 57.7 mW/cm2 at 25oC. Inset of (g) shows the SEM image of the device (top) and schematic illustration of fabricated photodetector (bottom). The next chapter deals with the electrochemical performance of TiC demonstrated for studies involving oxygen reduction and borohydride oxidation reactions. Electrochemical oxygen reduction reaction (ORR) reveal that TiC-NW possess high activity for ORR and involves four electron process while it is a two electron reduction for bulk TiC particles (figure 4). The data has been substantiated by density functional theory (DFT) calculations that reveal different modes of adsorption of oxygen on bulk and nanowire morphologies. Stable performance is observed for several hundreds of cycles that confirm the robustness of TiC. The study also demonstrates excellent selectivity of TiC for ORR in presence of methanol and thus cross-over issue can be effectively addressed in direct methanol fuel cells. In the chapter on borohydride oxidation, bare TiC electrode is explored as a catalyst for the oxidation of borohydride. One of the major issues in direct borohydride fuel cells (DBFC) is the hydrolysis of borohydride that happens on almost all electrode materials leading to low efficiency. The present study reveals that TiC is a very good catalyst for borohydride oxidation with little or no hydrolysis of borohydride [figure 5 (a)] under the experimental conditions studied. Further, shape dependant activity of TiC has been studied and fuel cell performance is followed [figure 5 (b)]. Polarization data suggests that the performance of TiC is quite stable under fuel cell experimental conditions. Figure 4. (a) Linear sweep voltammograms for ORR recorded using (i) bulk TiC particles and (ii) TiC-NW in O2-saturated 0.5 M KOH at 1000 rpm. Scan rate used is 0.005 Vs-1. (b) Variation of number of electrons with DC bias. Black dots correspond to TiC bulk particles while red ones represent nanowires. Figure 5. (a) Cyclic voltammograms of borohydride oxidation on TiC coated GC electrode in 1 M NaOH containing 0.1 M NaBH4. Scan rate used is 0.05 Vs-1. (b) Fuel cell polarization data at 70oC for DBFC assembled with (i) bulk TiC particles and (ii) TiC-NW as anode catalysts and 40 wt% Pt/C as cathode. Anolyte is 2.1 M NaBH4 in 2.5 M NaOH, and catholyte is 2.2 M H2O2 in 1.5 M H2SO4. Anode loading is 1.5 mg cm-2 and cathode loading is 2 mg cm-2. The corrosion resistance nature of TiC lends itself amenable to be used as an active support for catalytic particles (Pt and Pd) for small molecules oxidation reactions. In the present study, electro-oxidation of methanol, ethanol and formic acid have been studied. As shown in figure 6 (a), the performance of Pd loaded TiC (Pd-TiC) is found to be higher than that of Pd loaded carbon (Pd-C) suggesting the active role of TiC. The catalytic activities of TiC-based supports are further improved by tuning their morphologies. Figure 6 (c) reveals that the activities are higher in case of Pd-TiC-NW than that of Pd-TiC. Figure 6. (a) Cyclic voltammograms of Pd-TiC and Pd-C for ethanol oxidation, (b) T EM image of Pd-TiC-NW and (c) voltammograms of Pd-TiC-NW in N2-saturated 1 M ethanol in 1 M KOH medium, scan rate used is 0.05 Vs-1. The next aspect explored, is based on the preparation of C-TiO2 and its use as a substrate for surface enhanced Raman spectroscopy (SERS). Carbon doped titanium dioxide is prepared by thermal annealing of TiC. It is observed that the amount of dopant (carbon content) is dependent on the experimental conditions used. SERS studies using 4¬mercaptobenzoic acid (4-MBA) as the analyte, indicates that C-TiO2 [figure 7 (a)] enhances Raman signals based on chemical interactions between the analyte and the substrate. Raman signal intensities can be tuned with the amount of carbon content in C¬TiO2. Enhancement factors are calculated to be (7.7 ± 1.2) x 103 (for 4-MBA) and (1.7 ± 1.2) x 103 (for 4-nitrothiophenol). The SERS substrates are found to be surface renewable using visible light, a simple strategy to re-use the substrate [figure 7 (b)]. The regeneration of SERS substrates is based on self cleaning action of TiO2 that produces highly reactive oxygen containing radicals known to degrade the molecules adsorbed on TiO2. Thus, the versatility of TiC has been demonstrated with various studies. In addition to using TiC-based materials, nanoparticles of Rh, Ir and Rh-Ir alloy structures have also been used for borohydride oxidation reaction. This is explained in the last section. In Appendix-I, preliminary studies on the preparation of TiC-polyaniline (PANI) composites using liquid-liquid interfacial polymerization is explained. Raman spectroscopy results suggest that the presence of TiC-NW makes PANI to assume preferential orientation in the polaronic (conducting) form. Appendix-II discusses the role of TiC-NW as a fluorescence quencher for CdS semiconductor nanoparticles.

En esta tesis se estudió la reacción de reducción de oxígeno (RRO) sobre electrodos de óxido de Ti modificado con Pt, en una solución de 0,010M HClO4 a 25°C. El óxido de Ti fue crecido potenciodinámicamente sobre un sustrato de vidrio/Ti (electrodo vidrio/Ti/TiO2). Se empleó la técnica de electrodeposición sobre el sustrato de vidrio/Ti previo al crecimiento del óxido, con el método de potencial constante y a diferentes tiempos de deposición (1 ≤ td ≤ 10 segundos). La mejor respuesta electroquímica se obtuvo con el electrodo de Ti/Pt/TiO2 (td = 10 segundos), que indicó un potencial de inicio de la RRO de 0,86V vs ENH (electrodo normal de hidrógeno). Los resultados obtenidos por voltamperometría cíclica revelan la estabilidad del óxido de Ti en presencia del Pt, bajo las condiciones experimentales establecidas en la presente tesis.
This thesis studies the oxygen reduction reaction (ORR) on Ti oxide electrodes modified by Pt in 0.010 M HClO4 at 25°C. The anodic oxide was potentiodynamically grown from glass/Ti substrates (glass/Ti/TiO2 electrode). Pt was electrodeposited on glass/Ti before the anodic oxide growth at constant potential and at deposition times (1 ≤ td ≤ 10 seconds). The highest electrochemical activity was obtained for Ti/Pt/TiO2 (td = 10 seconds) electrodes, with an ORR-current onset of 0.86V vs. NHE. The results obtained from cyclic voltammperometry showed Ti oxide remains stable in presence of Pt, under the specific experimental conditions reported in these studies.

No
The objective of this review is to provide an overview concerning what the authors believe to be the most important photoelectrochemical techniques for the study of semiconductor nanoparticles. After a short historical background and a brief introduction to the area of photoelectrochemistry, the working principles and experimental setups of the various static and dynamic techniques are presented. Experimental details which are of crucial importance for their correct execution are emphasized, and applications of the techniques as found in the recent research literature as applied to semiconductor nanoparticles are illustrated.

One third of world’s total energy is used in production of electricity and one fifth of the total electricity produced in the world is used in lighting. Hence, the materials that have high potential in the field of photovoltaic’s and photoluminescence have recently drawn special attention to meet the ever increasing energy demands. In this thesis, we have studied a few materials that hold tremendous promises in fabricating photovoltaics and photoluminescent devices. Any ferroelectric material is an efficient solar energy converter as it contains an the intrinsic dipolar field which can effectively separate the photo excited electron and hole. We have developed a few materials which possess inherent polarization efficiently absorb over a wide portion of the solar spectrum and hence can find application in the field of photovoltaics. Secondly, we also dealt with semiconductor nonmaterial’s which are technologically very important owing to their improved photoluminescence properties. We tried to improve their light emitting efficiency by engineering crystal structure in nanometer length scales. The thesis deals with such advanced energy materials and is divided in seven chapters. Chapter 1 provides a brief introduction to the fundamental concepts that are relevant in the subsequent chapters. The chapter is started with a brief scenario of current status of energy production and its usage. Next, we have discussed the prospects of ferroelectric materials in photovoltaic devices. This is followed by a brief background on ferroelectricity and related properties which we have studied subsequently. At the end of this chapter a brief overview of photoluminescence properties in semiconductor nonmaterial’s is presented. In this section we have addressed the particular issues that need to be taken care of in order to improve their light emission properties. Chapter 2 describes different experimental and theoretical methods that have been employed to carry out different studies presented in the thesis. Chapter 3 addresses the possibility of employing BaTiO3 (BTO) based composite perovskite oxides as a potent photovoltaic material. It is known that BTO can produce photocurrent upon excitation with suitable light source. However, inability of BTO to absorb sufficient sunlight owing to its near UV band gap prevents to make use of this material in photovoltaic devices. In order to reduce the band gap we have tried to tune the electronic structure at the band edge by doping non-d0 transition metal ions at Ti site. As it is known in the literature an isovalent substitution of Ti4+ stabilizes non-polar phase of BTO we employed a co-doping strategy to substitute tetravalent Ti with equal percentage of a trivalent and a pentavalent metal ion. Keeping in mind off-centering of Ti4+ is primary reason behind the large ferroelectric polarization of BTO, a judicious choice of co-dopant was necessary to minimize reduction of polarization due to replacement of Ti. We have found at least two pairs of co-dopants, namely Mn3+-Nb5+ and Fe3+-Nb5+ which at low doping concentration ( < 10%) effectively reduces the band gap of BTO without affecting its polarization to a large extent. We systematically increase the doping concentration of both the pair of dopants and found Mn3+-Nb5+ pair is more efficient over Fe3+-Nb5+ both in terms of reducing band gap and retaining the polarization of BTO. We have characterized the ferroelectric nature of all the doped compositions with the help of dielectric, polarization and pyroelectric measurements. We have also performed first principle density functional theory (DFT) calculations for an equivalent doped composition and addressed the nature of modulations of electronic structure at the band edges which is responsible for such large reduction of band gap. Chapter 4 deals with composite perovskite materials which posses large tetragonal distortions with reduced optical band gaps. Here we have exploited Cu-Nb and Cu-Ta pair which upon complete substitution of Ti of BTO leads to composite perovskites with enhanced tetragonal distortion of the perovskite lattice. For two resultant compositions, namely BaCu1/3Nb2/3O3 and BaCu 1/3Ta2/3O3 we have characterized the optical and ferroelectric properties. We found though these material possess small band gap (∼ 2 eV), these are not ferroelectric in nature. Results of second harmonic generation measurements and refinement of powder X-ray diffraction both establish Centro symmetric nature of these materials. We infer from these results that presence of large tetragonal distortion is a result of symmetric Jahn-Teller type distortion of Cu2+ and not due to off-centering of any of the metal ions in their MO6 octahedral geometries. In Chapter 5, we have considered the material SrTiO3 (STO) which is stable in cubic paraelectric phase at room temperature. But at the same time this material is considered as an incipient ferroelectric due to presence of an active polar vibrational mode which does not become completely soft even at temperature close to 0 K. While this polar vibrational mode can easily be frozen by making substitution at Sr site, a similar attempt by making substitution at Ti site failed earlier. Keeping in mind Ti is easier to substitute than Sr we employed same co-doping strategy that we have considered in Chapter 3. We found Mn- Nb and Mn-Ta co-dopants at low doping concentration are extremely useful in transforming incipient ferroelectric STO into a dipolar glass. We have characterized the glassy dipolar property of doped STO with the help of tem-perature dependent dielectric response of these material. At the same time we found these co-doped STO possess enhanced static dielectric constant at room temperature with favourable dielectric loss values in comparison to pure STO. We have also ad-dressed the origin of a glassy dipolar state with the help of DFT calculation performed on equivalent doped composition that we have considered for our experiments. In Chapter 6, we have considered another incipient ferroelectric material TiO2 in rutile phase which also possess polar vibrational mode at temperature close to 0 K. A lattice strain along the polar vibrational mode make symmetric non-polar structure unstable with respect to the distorted polar structure. In this context, we found two particular compositions FeTiTaO6 and CrTiTaO6 that are also stable in rutile phases at room temperature but possess similar strain due to presence of larger Fe or Cr and Ta in rutile lattice. Considering the fact these two composite rutile oxides are relaxer ferroelectric in nature, we critically evaluated the effect of the particular kind of strain that these materials introduce in rutile lattice. We also characterized relaxor ferroelectric property and optical band gap of these materials and commented on the potential of these materials in exploiting them in photovoltaic devices. Chapter 7 presents a unique strategy of making use of crystal defects in improving photoluminescent properties of semiconductor nanocrystals. We have shown defects when introduced in nanocrystals in a controlled protected manner efficiently overcome the problem of self absorption which is known to reduce quantum efficiency of emit-ted light. Controlling synthesis conditions we separately prepared CdS nanocrystals with and without intergrowth defects. We characterized the presence of the intergrowth defect with the help of high resolution transmission electron microscope (HRTEM) image. We have also characterized Stokes’ shifted PL emission and ultrafast charge carrier dynamics of these NCs with intergrowth defects. To support these experimental findings we have computed the electronic structures of model nanoclusters possessing similar intergrowth defects that has been observed in HRTEM images. We find that the presence of defects in a nanocluster particularly affect the position of the band edge. However our joint density of state calculation shows that contribution of these defect states to an absorption spectra is negligible. Thus presence of defect states at band edge ensures a Stokes’ shifted emission without affecting the position of absorption. In a separate section of this chapter we have shown apart from intergrowth defects presence of twin boundary also provide similar mid-gap states that can alter its’ optical proper-ties to large extent. In summary, we have studied a few bulk and nano-materials which can show improved photovoltaic and photoluminescence property. We investigated effect of external dopant ions on a classical ferroelectric material BaTiO3 and two incipient ferroelectric materials SrTiO3 and rutile TiO2. We have also shown that efficient defect engineering could be extremely useful in improving photoluminescent property of CdS nanocrystals which is a prototype of II-VI semiconductor nanomaterials. In a separate Appendix Chapter, we have shown an easy and efficient way to suppress coffee ring effect which takes place universally when a drop of colloidal suspension is dried on a solid substrate. We have shown temporary modification of hydropho-bicity of a glass substrate not only can suppress the coffee ring effect but also leaves the particle in a highly ordered self-assembled phase after completion of drying process

One third of world’s total energy is used in production of electricity and one fifth of the total electricity produced in the world is used in lighting. Hence, the materials that have high potential in the field of photovoltaic’s and photoluminescence have recently drawn special attention to meet the ever increasing energy demands. In this thesis, we have studied a few materials that hold tremendous promises in fabricating photovoltaics and photoluminescent devices. Any ferroelectric material is an efficient solar energy converter as it contains an the intrinsic dipolar field which can effectively separate the photo excited electron and hole. We have developed a few materials which possess inherent polarization efficiently absorb over a wide portion of the solar spectrum and hence can find application in the field of photovoltaics. Secondly, we also dealt with semiconductor nonmaterial’s which are technologically very important owing to their improved photoluminescence properties. We tried to improve their light emitting efficiency by engineering crystal structure in nanometer length scales. The thesis deals with such advanced energy materials and is divided in seven chapters. Chapter 1 provides a brief introduction to the fundamental concepts that are relevant in the subsequent chapters. The chapter is started with a brief scenario of current status of energy production and its usage. Next, we have discussed the prospects of ferroelectric materials in photovoltaic devices. This is followed by a brief background on ferroelectricity and related properties which we have studied subsequently. At the end of this chapter a brief overview of photoluminescence properties in semiconductor nonmaterial’s is presented. In this section we have addressed the particular issues that need to be taken care of in order to improve their light emission properties. Chapter 2 describes different experimental and theoretical methods that have been employed to carry out different studies presented in the thesis. Chapter 3 addresses the possibility of employing BaTiO3 (BTO) based composite perovskite oxides as a potent photovoltaic material. It is known that BTO can produce photocurrent upon excitation with suitable light source. However, inability of BTO to absorb sufficient sunlight owing to its near UV band gap prevents to make use of this material in photovoltaic devices. In order to reduce the band gap we have tried to tune the electronic structure at the band edge by doping non-d0 transition metal ions at Ti site. As it is known in the literature an isovalent substitution of Ti4+ stabilizes non-polar phase of BTO we employed a co-doping strategy to substitute tetravalent Ti with equal percentage of a trivalent and a pentavalent metal ion. Keeping in mind off-centering of Ti4+ is primary reason behind the large ferroelectric polarization of BTO, a judicious choice of co-dopant was necessary to minimize reduction of polarization due to replacement of Ti. We have found at least two pairs of co-dopants, namely Mn3+-Nb5+ and Fe3+-Nb5+ which at low doping concentration ( < 10%) effectively reduces the band gap of BTO without affecting its polarization to a large extent. We systematically increase the doping concentration of both the pair of dopants and found Mn3+-Nb5+ pair is more efficient over Fe3+-Nb5+ both in terms of reducing band gap and retaining the polarization of BTO. We have characterized the ferroelectric nature of all the doped compositions with the help of dielectric, polarization and pyroelectric measurements. We have also performed first principle density functional theory (DFT) calculations for an equivalent doped composition and addressed the nature of modulations of electronic structure at the band edges which is responsible for such large reduction of band gap. Chapter 4 deals with composite perovskite materials which posses large tetragonal distortions with reduced optical band gaps. Here we have exploited Cu-Nb and Cu-Ta pair which upon complete substitution of Ti of BTO leads to composite perovskites with enhanced tetragonal distortion of the perovskite lattice. For two resultant compositions, namely BaCu1/3Nb2/3O3 and BaCu 1/3Ta2/3O3 we have characterized the optical and ferroelectric properties. We found though these material possess small band gap (∼ 2 eV), these are not ferroelectric in nature. Results of second harmonic generation measurements and refinement of powder X-ray diffraction both establish Centro symmetric nature of these materials. We infer from these results that presence of large tetragonal distortion is a result of symmetric Jahn-Teller type distortion of Cu2+ and not due to off-centering of any of the metal ions in their MO6 octahedral geometries. In Chapter 5, we have considered the material SrTiO3 (STO) which is stable in cubic paraelectric phase at room temperature. But at the same time this material is considered as an incipient ferroelectric due to presence of an active polar vibrational mode which does not become completely soft even at temperature close to 0 K. While this polar vibrational mode can easily be frozen by making substitution at Sr site, a similar attempt by making substitution at Ti site failed earlier. Keeping in mind Ti is easier to substitute than Sr we employed same co-doping strategy that we have considered in Chapter 3. We found Mn- Nb and Mn-Ta co-dopants at low doping concentration are extremely useful in transforming incipient ferroelectric STO into a dipolar glass. We have characterized the glassy dipolar property of doped STO with the help of tem-perature dependent dielectric response of these material. At the same time we found these co-doped STO possess enhanced static dielectric constant at room temperature with favourable dielectric loss values in comparison to pure STO. We have also ad-dressed the origin of a glassy dipolar state with the help of DFT calculation performed on equivalent doped composition that we have considered for our experiments. In Chapter 6, we have considered another incipient ferroelectric material TiO2 in rutile phase which also possess polar vibrational mode at temperature close to 0 K. A lattice strain along the polar vibrational mode make symmetric non-polar structure unstable with respect to the distorted polar structure. In this context, we found two particular compositions FeTiTaO6 and CrTiTaO6 that are also stable in rutile phases at room temperature but possess similar strain due to presence of larger Fe or Cr and Ta in rutile lattice. Considering the fact these two composite rutile oxides are relaxer ferroelectric in nature, we critically evaluated the effect of the particular kind of strain that these materials introduce in rutile lattice. We also characterized relaxor ferroelectric property and optical band gap of these materials and commented on the potential of these materials in exploiting them in photovoltaic devices. Chapter 7 presents a unique strategy of making use of crystal defects in improving photoluminescent properties of semiconductor nanocrystals. We have shown defects when introduced in nanocrystals in a controlled protected manner efficiently overcome the problem of self absorption which is known to reduce quantum efficiency of emit-ted light. Controlling synthesis conditions we separately prepared CdS nanocrystals with and without intergrowth defects. We characterized the presence of the intergrowth defect with the help of high resolution transmission electron microscope (HRTEM) image. We have also characterized Stokes’ shifted PL emission and ultrafast charge carrier dynamics of these NCs with intergrowth defects. To support these experimental findings we have computed the electronic structures of model nanoclusters possessing similar intergrowth defects that has been observed in HRTEM images. We find that the presence of defects in a nanocluster particularly affect the position of the band edge. However our joint density of state calculation shows that contribution of these defect states to an absorption spectra is negligible. Thus presence of defect states at band edge ensures a Stokes’ shifted emission without affecting the position of absorption. In a separate section of this chapter we have shown apart from intergrowth defects presence of twin boundary also provide similar mid-gap states that can alter its’ optical proper-ties to large extent. In summary, we have studied a few bulk and nano-materials which can show improved photovoltaic and photoluminescence property. We investigated effect of external dopant ions on a classical ferroelectric material BaTiO3 and two incipient ferroelectric materials SrTiO3 and rutile TiO2. We have also shown that efficient defect engineering could be extremely useful in improving photoluminescent property of CdS nanocrystals which is a prototype of II-VI semiconductor nanomaterials. In a separate Appendix Chapter, we have shown an easy and efficient way to suppress coffee ring effect which takes place universally when a drop of colloidal suspension is dried on a solid substrate. We have shown temporary modification of hydropho-bicity of a glass substrate not only can suppress the coffee ring effect but also leaves the particle in a highly ordered self-assembled phase after completion of drying process

Celem pracy doktorskiej było otrzymanie na drodze syntezy elektrochemicznej materiałów kompozytowych składających się z półprzewodnika nieorganicznego - selenku kadmu, oraz polimeru przewodzącego z grupy polianilin. Selenek kadmu będący materiałem fotoaktywnym o właściwościach odpowiednich do zastosowania tego związku w układach fotoelektrochemicznych (prosta przerwa energetyczna o wartości 1,7 eV; wysoki współczynnik absorpcji światła; przewodnictwo typu n), ulega w roztworze wodnym fotokorozji. Pokrycie tego związku polimerową warstwą przewodzącą, miało na celu zabezpieczenie CdSe przed fotorozkładem oraz stworzenie heterozłącza p-n. W pracy autor skupił się na takich zagadnieniach jak: • Dobór odpowiedniego podłoża do osadzania CdSe i polimeru • Dobór odpowiedniego polimeru przewodzącego z grupy pochodnych polianiliny • Dobór odpowiednich warunków syntezy • Sposób elektroosadzania polimeru, który nie doprowadzi do rozkładu CdSe • Wybór najbardziej fotoaktywnego i stabilnego układu kompozytowego Zarówno półprzewodnik jak i polimer przewodzący był otrzymywany za pomocą techniki woltamperometrii cyklicznej w środowisku wodnym. Pierwszym etapem badań był dobór elektrody pracującej na, której osadzany był półprzewodnik i polimer przewodzący. Spośród elektrod Pt, Au i HOPG (pirolityczny grafit) optymalnym podłożem do osadzania okazał się grafit. Nadpotencjał osadzania polimeru na wcześniej osadzonym CdSe na HOPG jest przesunięty w stronę katodową bardziej niż nadpotencjał rozpuszczania CdSe, co eliminuje ryzyko rozpuszczania CdSe w trakcie nanoszenia polimeru. Polimery, które znajdowały się w obszarze zainteresowań tej pracy badawczej należą do grupy polianilin. W badaniach pokrywano CdSe polianiliną (PANI), poli-o-anizydyną (POMA) i poli-o-etoksyaniliną (POEA). Na podstawie wyników badań wykazano, że poli-o-anizydyna i poli-o-etoksyanilina są polimerami, które można osadzić na powierzchni osadzonego CdSe. Polianilina osadza się w zakresie potencjałów, w których rozpuszcza się CdSe. W badaniach wykorzystano poli-o-anizydynę, ponieważ w porównaniu do poli-o-etoksyaniliny wykazuje lepsze właściwości przewodzące. Potencjały elektropolimeryzacji obu polimerów są do siebie zbliżone. Spośród trzech syntezowanych grup układów kompozytowych (CdSe/POMA – polimer osadzony na CdSe, POMA/CdSe – CdSe osadzony na półprzewodniku, POMA(CdSe) – struktury półprzewodnika rozproszone w polimerze), optymalnym pod kątem fotoelektrochemicznym układ to CdSe/POMA, ze względu na korzystne położenie pasm obu materiałów i potencjału siarczynów jako akceptorów ładunku wytworzonego w procesie fotoelektrochemicznym. Teoretyczne założenia (położenie pasm) zostały potwierdzone badaniami fotoelektrochemicznymi, spektroskopowymi i mikroskopowymi. Na podstawie badań spektroskopii ramanowskiej potwierdzono obecność CdSe i POMA jedynie w przypadku układu CdSe/POMA, co zostało także potwierdzone badaniami fotoelektrochemicznymi. Układ o największej wartości fotoprądów to CdSe/POMA 20c_5c (CdSe otrzymany po 20 cyklach nanoszenia techniką woltamperometrii cyklicznej i POMA otrzymana po 5 cyklach nanoszenia techniką woltamperometrii cyklicznej), ze względu na odpowiednią grubość (150 nm) POMA, która zapewnia optymalną separację ładunku i transport dziur do granicy faz polimer/roztwór. W pracy doktorskiej wykorzystano techniki mikroskopowe do zbadania morfologii powierzchni otrzymanych układów. Za pomocą badań AFM in situ przedstawiono kolejne etapy elektrosyntezy wszystkich trzech grup układów kompozytowych CdSe/POMA, POMA/CdSe oraz POMA(CdSe). Między badaniami in situ AFM a ex situ AFM istnieją znaczące różnice morfologiczne, które wynikają z obecności roztworu, który w przypadku badań roztworu wnika w porowatą strukturę przypowierzchniową i zmienia tym samym obrazowanie powierzchni Badania mikroskopowe potwierdziły ziarnistą strukturę osadzonego CdSe i zwartą warstwę POMA. Opracowana na potrzeby tej pracy metodyka litografii, pozwoliła na oszacowanie grubości poszczególnych warstw kompozytu. Grubość CdSe w układzie CdSe/POMA 20c_Xc (X – ilość cykli nanoszenia POMA) wynosi 100nm natomiast grubość POMA nie przekracza 300 nm. Omówiono również wyniki badań spektroskopii XPS, dyfraktometrii XRD, spektroskopii prądu tunelowania STS. Wyniki badań spektroskopii XPS potwierdziły obecność wszystkich związków oraz obecność pierwiastkowego selenu.Wyniki badań dyfraktometrycznych XRD potwierdziły obecność CdSe i Se dla optymalnego układu CdSe/POMA 20c_5c. Nie zidentyfikowano obecności POMA ze względu na amorficzny charakter struktury oligomeru. Techniką STS i spektroelektrochemiczną wyznaczono przerwy energetyczne CdSe i układu kompozytowego, które wynoszą odpowiednio 1,7 eV i 2,2 eV co jest zgodne danymi literaturowymi. Układ tworzy tym samym heterozłącze. Dodatkowo wykonano badania podstawowe dotyczące mechanizmu osadzania selenu. Za pomocą metody elektrody wirującego dysku RDE, wyznaczono współczynnik dyfuzji dla jonów selenianowych, stałą szybkości reakcji redukcji jonów selenianowych, współczynnik przeniesienia ładunku oraz prąd wymiany. Wykonane badania potwierdziły wcześniej stawiane tezy dotyczące 6-elektronowego procesu redukcji jonów selenianowych do selenowodoru. Jest to zagadnienie istotne nie tylko przy badaniach elektrochemicznego osadzania selenu, ale również selenku kadmu.
The aim of the Ph.D. thesis was to electrochemically sythetize composite materials which contain inorganic semiconductor cadmium selenide and conductive polymer (polyaniline derivatives). Cadmium selenide is a photoactive n-typ compound with direct bandgap (1.7 eV) and high absorption coefficient. Mentioned features make it proper material as a photoelectrode in photogalvanic cells. Despites advantages CdSe is susceptible to photocorrosion in water. Therefore it is crucial to cover the surface of CdSe with conductive polymer layer. It protects semiconductor againts corrosion and creates p-n heterojunction. In this thesis the author was focused on the following issues: • The choice of the proper electrode to deposite CdSe and polymer • The choice of the proper conductive polymer from polyaniline derivatives • The choice of the proper sythesis conditions • The methodology of the polymer electrosythisis on CdSe layer • The choice of the most stable and the most photoactive composite Both semiconductor and conductive polymer were sythetised cyclic voltammetry technique in water solutions. The first step of the survey was the choice of the working electrode to electrodeposite CdSe and polymer. The optimal electrode from Pt, Au and HOPG (Highly Oriented Pyrolitic Graphite) is the last one. The overpotential of polymerization on the modified HOPG with CdSe is shifted to more cathodic potentials than overpotential of CdSe dissolution. This eliminates risk of paralel CdSe dissolution during electropolymerisation. Polyaniline, Poly-o-methoxyaniline (POMA) and poly-o-ethoxyanilne (POEA) were chosen to coat semiconductor. According to results POMA and POEA are those polymers which can be electrodeposited on CdSe. Although potentials of electropolymerization are almost the same for POEA and POMA, the last one has better conductive features. Three types of composites were sythetised (CdSe/POMA – polymer on CdSe; POMA/CdSe – CdSe on polymer; POMA(CdSe) – CdSe particles inside polymer structure). The most photoactive type of composite is CdSe/POMA, because of the location of bands both materials and redox potential of sulfites as acceptor of photogenerated chargé. 2 Photoelectrochemical experiments with raman spectroscopy and confirm both CdSe and polymer presence only in CdSe/POMA composite. The highest photocurrent of CdSe/POMA was achieved for 20c_5c composite ( CdSe obtained after 20 CV cycles and POMA obtained after 5 CV cycles). Is it related with thicknes of the POMA layer, which provides optimal charge separation and hole transport between polymer/solution phases. Microscopic techniques were engaged to describe morphology of the composite surface. Using in situ AFM every stage of electrosythesis was described. There are differences between ex situ and in situ AFM experimental results. They arise from the solution presence, which penetrate the structure near the surface. Microscopic pictures confirmed the grain structure of electrodeposited CdSe and dense layer of POMA. Lithography technique was engaged to determine the thicness of CdSe and POMA layer. CdSe thickness is 100 nm after 20 CV cycles. The maximum thickness of POMA reaches 300 nm. XPS spectroscopy, XRD diffractometry, Scanning Tunneling Spectroscopy was used to determine presence of both compounds, and width of bandgaps. Additional fundamental research was made to describe the mechanizm of selenium electrodeposition which is important to understand mechanism of CdSe electrodeposition. Rotating Disc Electrode experiments were engaged to determine diffusion coefficient, rate constant of selenous ions reduction, charge transfer coefficient and exchange current. Results of those experiments proved that mechanism involes 6-electron reduction of selenus ions to hydrogen selenide.