Utilizando a Dinâmica Molecular Tight Binding (TBMD), parametrizada para sistemas de carbono e hidrogênio, simulamos com condições periódicas de contorno e modelos de fatia, superfícies de diamante (100) puras e hidrogenadas em modelos de reconstruções ideais usualmente presentes na literatura, analisando o seu comportamento geométrico e eletrônico. Em seguida abordamos o comportamento morfológico e eletrônico, em simulações com temperaturas que variam entre 100K e 2000K de dois modelos de superfícies monohidrogenadas, que apresentam dois domínios em torno de uma estrutura de depressão local, característica de filmes de alta rugosidade. Em oposição à grande estabilidade térmica exibida pelo modelo monohidrogenado ideal e pelas colunas contínuas de dímeros, os modelos com depressão apresentaram significativa migração de átomos de hidrogênio para regiões subsuperficiais. Em nossas simulações os átomos de hidrogênio ficaram confinados nas regiões subsuperficiais, introduzindo uma desordem morfológica na superfície e nas regiões internas à fatia, induzindo estados eletrônicos nesta região, que levam ao fechamento do gap, passando a caracterizar uma fase quase-metálica. / By using the Tight Binding Molecular Dynamics (TBMD), parametrized to describe carbon and hydrogen atoms composed of systems, we apply periodic boundary conditions, slab models in order to simulate (100) clean and hydrogenated diamond surfaces. We study first the standard models used in the literature, analyzing their geometrical and eletronic behavior. We then focus on the morphological and electronic properties, in simulations under finite temperature dynamics ranging from 100K up to 2000K, of two distinct models of monohydride surfaces; Each model exhibits two distincts domains in the surface pattern characterized by a local depression, characteristic of rough surfaces. In opposition to the high thermal stability observed for ideal monohydrogenated surfaces and the extended dimer rows, these models showed an expressive hydrogen migration to the subsurface regions. In our simulations the hydrogen atoms remain in the subsurface regions, but introduce morphological disorder at the surface and in the slab internal regions. These hydrogen atoms induce electronic states mostly localized in the subsurface region, which are responsible for closing the gap, and leading the system to exhibit a quasi-metallic phase.

Utilizando a Teoria do Funcional da Densidade junto com o formalismo do pseudopotencial de primeiros princípios, realizamos um estudo sistemático do processo de adsorção da molécula de oxigênio sobre a superfície livre de CdTe (110) nas reconstruções 1x1, 1x2 e 2x1. Este estudo consistiu na determinação das adsorções energeticamente favoráveis e na viabilidade de suas formações através da análise das barreiras de ativação. Nossas análises indicam que apenas uma molécula de oxigênio adsorve sobre a superfície livre em uma reconstrução 1x1 e que não ocorre a quebra da molécula durante o processo de adsorção. As estruturas formadas foram divididas nos regimes de baixas e altas temperaturas. Do estudo das barreiras de ativação verificamos que no regime de baixas temperaturas a molécula de oxigênio liga-se exclusivamente ao Cd da primeira camada através de ligações Cd-O-O ou Cd-O2. A configuração da superfície de CdTe com a molécula adsorvida, se assemelha a configuração do cristal. As estruturas de faixas de energia neste regime apresentam um estado característico de defeito duplo aceitador. Para o regime de altas temperaturas, a molécula adsorve entre o Cd da primeira camada e o Te da segunda camada, quebrando esta ligação Cd-Te e também quebrando a ligação que o Cd da segunda camada realiza com o Te da terceira camada. O complexo formado apresenta ligações Cd-O, Cd-O2, Te-O e O-O e as estruturas de faixas de energia apresentam um gap indireto entre os pontos gama e X de 1.30 eV. / Density Functional Theory with first-principles pseudopotential formalism have been used to a systematic research of the oxygen molecule adsorption on the free surface of CdTe(110) in the 1x1, 1x2 and 2x1 reconstructions. This research was based on the determination of the adsorptions energetically favorables and the viability of each formation through their activation barriers analysis. This analysis indicates that only one oxygen molecule adsorbs over the free surface of the CdTe(110) in the 1x1 reconstruction and that the dissociation of the molecule does not occur during the adsorption process. The structures were divided on the high and low temperature regimes. From the activation barriers study it was verified that on the regime of low temperatures the oxygen molecule bind exclusively to the Cd of the first layer through the Cd-O-O or Cd-O$_2$ bonds. The CdTe surface configuration with the adsorbed molecule looks like a crystal configuration. The energy band structure, on this regime, shows a characteristic state of double acceptor defect. For the high temperature regime the molecule adsorbs between the Cd of the first layer and the Te of second layer, breaking the Cd-Te bond between them and also breaking the bonds that the Cd of the second layer does with the third layer. The complex shows Cd-O, Cd-O2, Te-O and O-O bonds and the energy band structure shows an indirect gap between the gamma and X points of 1.30eV.

A espectroscopia foto-térmica tem sido largamente usada na investigação de propriedades térmicas e ópticas de materiais sólidos. Neste trabalho, desenvolvemos novas câmaras foto-térmicas as quais foram adaptadas para os estudos de materiais poliméricos. Estendemos o modelo desenvolvido por Mandelis para a espectroscopia fotopiroelétrica (PPES), incluindo nele um termo devido à reflexão de luz na interface amostra-detetor. A aplicação da técnica PPES em filmes de Poli(3-Butiltiofeno) não dopado nos permitiu obter sua condutividade e difusividade térmicas, como também seu gap de energia. Também aplicamos a técnica fotoacústica, com um flash de laser He-Ne, ao estudo de filmes de polímeros transparentes. Finalmente, desenvolvemos um método semi-empírico o qual significa a análise do sinal foto-térmico, quando ele é, principalmente, devido à difusão térmica. / Photothermal spectroscopies have been largely used in the investigation of thermal and optical properties of solids materials. In this work we developed new photothermal cells which were adapted for the study of polymerics materials. We also extended the model developed by Mandelis for the Photopyroeletric Spectroscopy (PPES), including in it a term due to the reflected light in the sample-detector interface. The application of the PPES technique in films of undopedpoly(3-butylthiophene) allowed us to obtain its thermal conductivity and diffusivity, as well as its gap energy. We also applied the photoacoustic technique, with a flash of He-Ne laser to study of transparent films of polymers. Finally we developed a semi-empiric method which simplifies the analysis of the photothermal signal, when it is mainly due to the thermal diffusion.

Les cellules solaires organiques à hétérojonction en volume (BHJ en anglais) font l'objet d'un grand intérêt car elles représentent une source d'énergie bon marché et renouvelable. Cependant, à cause des rendements généralement bas, ce type de cellule peine à intégrer le marché. Afin d’en augmenter le rendement, contrôler la morphologie des semi-conducteurs dans la BHJ représente un élément clé. Dans ce contexte, il apparaît, dans la littérature, que les additifs solvant permettent de contrôler cette morphologie et d'augmenter les rendements.Cette thèse a pour but de fournir une étude complète sur l'utilisation des additifs. Le couple de semi-conducteurs étudié est le poly(3-hexylthiophene) (P3HT)/[6,6]-phényl-C61butanoate de méthyle (PC61BM).Une première partie présente une méthode développée pour guider la sélection d'additifs parmi une liste de solvants. Cette méthode emploie les paramètres de solubilité de Hansen des semi-conducteurs. Elle est appliquée au système P3HT/PC61BM et résulte en l'identification de trois nouveaux additifs performants. Ensuite, des caractérisations structurales, électriques et optiques sont menées sur la BHJ et permettent d'identifier les effets des additifs. Les effets de ces additifs se révèlent être différents en fonction de l'architecture des dispositifs. L'origine de telles différences est corrélée aux variations de mobilités des porteurs de charge causées par les additifs. Des tests de photo-stabilité ont été menés et montrent que les additifs sont capables d'augmenter la stabilité des cellules solaires. L’origine de telles améliorations est étudiée. Enfin, l'étude est étendue à deux autres nouveaux polymères semi-conducteurs. / Polymeric bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted significant interest as a low cost and renewable technology to harvest solar energy. However, their generally low efficiencies are a barrier for their movement into commercial application. Controlling the BHJ morphology is a key step in the pursuit of higher OSC efficiencies. Processing additives have emerged as effective components for optimizing the BHJ morphology. This thesis provides a comprehensive study on the introduction of additives in the formulation of semiconductors. The semiconductor system studied is based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61 BM). First, a method was developed to guide the selection of additives from a large range of solvents. This method employs the Hansen solubility parameters of the semiconductors and was successfully applied to the P3HT/PC61 BMsystem. It resulted in the identification of three new efficient additives. Next, the mechanistic role of additives in influencing the BHJ morphology is investigated by performing structural, electrical and optical characterizations. Also, the effect of additives on OSC performance was found to depend on the type of the OSC architecture. Such differences were correlated to the variations in charge carrier mobilities caused by the additive. Furthermore, photo-stability tests, performed on different types of OSCs, showed that processing additives can improve the photo-stability. The origin of such improvement is investigated. Finally, the scope of this study is extended to two other donor semiconducting polymers.

Interchain interactions between π-systems have a strong effect on the properties of conjugated organic materials that find application in devices such as light emitting diodes (OLEDs), organic photovoltaics (OPVs), and field effect transistors (FETs). We have prepared covalently-stacked oligo(1,4-phenylene ethynylene)s and oligo(1,4-phenylene vinylene)s to study the influence of chain-chain interactions on the electronic structure of closely packed conjugated units. These serve as models for segments of conjugated materials in thin film devices. Extension of this concept has allowed us to prepare multi-tiered systems that display the influence of pi-stacking. The stacked architectures were prepared by multi-step synthesis of the scaffolds, followed by metal-catalyzed cross coupling reactions (Sonogashira, Heck, Suzuki couplings) to incorporate the conjugated oligomers. The optical and electrochemical properties of these stacked compounds and polymers were compared to their unstacked linear counterparts. These studies provide a platform for the exploration of the nature of charge carriers and excitons in a broad class of materials that have significant potential in addressing challenges in power generation, lighting and electronics.

Semiconductor nanocrystals with sizes comparable to the corresponding bulk excitonic diameter exhibit unique size-dependent electronic and optical properties resulting from quantum confinement effect. Such nanocrystals not only allow the study of evolution of bulk properties from the molecular limit providing important fundamental understandings, but also have great technological implications, leading to intense research over the past several years. Besides tuning the crystal size in the nm regime to obtain novel properties, an additional route to derive new functionalities has been to dope transition metal ions into a semiconductor host. Thus, transition metal doped nanocrystals are of great interest since it allows two independent ways to functionalize semiconductor materials, one via the tunability of properties by size variation and other due to properties of such dopants. Chapter 1 of the thesis provide a general introduction to the subject matters dealt in with this thesis, while the necessary methodologies have been discussed in chapter 2. Chapters 3 and 4 of this thesis deal with nanocrystal doping. Following suggestions in previous literatures that the doping of nanocrystal depends strongly upon the crystal structure of the synthesized host nanocrystal, we have studied the phase-transformation between the somewhat zinc-blende and the usual wurtzite structures for CdS and CdSe nanocrystals in chapter 5. In chapter 6 we have pointed out that a gradient structure is essential to achieve nearly ideal photoluminescence efficiency using heterostructured nanocrystals and also achieved strong two-photon absorptions, adding optical bifunctionality to these nanocrystals. Finally, in chapter 7, we establish different approaches to generate white-light using nanocrystals and their unique advantages, as a first step to realizing white light emitting devices. Chapter 1 provides a brief introduction to various interesting properties and concepts relevant for the studies carried out in the subsequent chapters of this thesis. The present status of the research in the field of semiconductor nanocrystals with an emphasis on synthesizing high quality nanocrystals, doping of nanocrystals and exciting optical properties exhibited by these nanocrystals has been discussed. We have discussed the existing theories and practices of colloidal synthesis that allow us to prepare high quality semiconductor nanocrystals with required size and very narrow size distribution. Optical properties, covering excitonic fine structure, photoluminescence, auger recombination and two-photon absorption have been discussed. We have described heterostructured nanocrystals of different types, particularly in the light of enhancing photoluminescence quantum yield. The difficulty in doping Mn2+ ion in semiconductor nanocrystals and the recent developments in this field have been addressed. Chapter 2 describes experimental and theoretical methodologies that have been employed to study different nanocrystal systems reported in this thesis. The topics covered in this chapter include UV-visible absorption spectroscopy, steady-state and time-resolved luminescence spectroscopy, X-ray diffraction, transmission electron microscopy, electron spin resonance spectroscopy, photoemission spectroscopy, two-photon absorption and least-squared-error fitting. Chapter 3 presents a detailed study of water soluble Mn2+-doped CdS nanocrystals synthesized using colloidal routes. Earlier efforts to dope Mn2+ ion into CdS nanocrystals and therefore, obtain the characteristic orange emission, have been largely impeded by the strong overlap of surface state emission of the host and Mn2+ d-emission. We are the first ones to obtain a distinct Mn2+ d-related emission at around 620 nm, well-separated from the surface state emission with its maximum near 508 nm. In spite of using very high (~30%) concentration of Mn2+ precursor, only ~1% Mn2+ was found in the final product, which is consistent with previous literatures, where Mn2+ doping in such nanocrystals was found to be extremely difficult. Most interestingly, present results establish that Mn2+ ion is found to be incorporated preferentially in the relatively larger sized nanocrystals compared to the smaller sized ones even within the narrow size distribution achieved for a specific reaction condition. We found that 55 oC is the optimum reaction temperature to synthesize Mn2+-doped CdS nanocrystals, at higher reaction temperatures, Mn2+ ions get annealed out of the substitutional sites, leading to a lower level of doping in spite of the formation of larger sized particles. Additionally, we could tune the color of the Mn2+ d- emission from red (620 nm) to yellow (580 nm) by increasing the reaction temperature from 55 oC to 130 oC. Another important aspect is that the synthesized nanocrystals readily dissolve in water without any perceptible effect on the Mn2+ d emission intensity. Chapter 4 discusses the outstanding problem that a semiconductor host in the bulk form can be doped to a large extent, while the same host in the nanocrystal form resist any appreciable level of doping. We first describe two independent models available in literatures to explain this baffling phenomenon. In one, it was suggested that the doping of Mn2+ ion in such nanoclusters is invariably an energetically unfavorable state, thus, Mn2+ ions get annealed out from the host nanocrystal and an increase in reaction temperature facilitate such annealing, a phenomenon known as self-purification. In the second model, it was suggested that the ease of initial adsorption of Mn2+ ions on specific surfaces of a growing nanocrystal, kinetically controls the extent of impurity doping. Specifically, it is easier to dope zinc-blende nanocrystals compared to their wurtzite counterpart. In contrast, the main claim of this chapter is neither crystal structure nor self-purification is as important in nanocrystal doping as lattice mismatch between the dopant and host lattice. To support this claim, we have doped Mn2+ ions into alloyed ZnxCd1-xS nanocrystals. Ionic radius of Mn2+ ion being in between those of Zn2+ and Cd2+ ions, the lattice mismatch between the host ZnxCd1-xS nanocrystal and MnS could be tuned in either side by tuning the composition “x”. It was gratifying to observe that there is an evident maximum of manganese content for Zn0.49Cd0.51S host nanocrystals that has no lattice mismatch with MnS, and the manganese content decreases systematically with increasing compressive as well as tensile lattice mismatches. Based on lattice parameter tuning, we could dope an extraordinarily higher amount of ~7.5% manganese for x = 0.49, at a reaction temperature as high as 310 oC and in a nanocrystal that exhibit wurtzite structure, which was previously suggested unfavorable for doping. These results prove our hypothesis that the strain fields generated because of the lattice mismatch between the dopant and host, are necessarily long range, much longer than typical nanocrystal dimensions and it tends to relieve itself by ejecting the dopant to the surface of nanocrystals, thus, resisting doping in such nanocrystals. High temperature synthesis, on the other hand, leads to a very high photoluminescence efficiency of ~25%. Chapter 5 deals with the phase-control of CdS and CdSe nanocrystals synthesized employing colloidal routes. CdS nanocrystals exhibit a very sensitive phase transformation from zinc-blende to wurtzite structure by increasing the reaction temperature from 280 to 310 oC, which is also accompanied by an increase in particle size from 6 to 6.8 nm, respectively. More importantly, just by changing the S precursor, it has been possible to change the crystal structure of the CdS nanocrystals at a given synthesis temperature of 310 oC. En route, we have synthesized >12 nm zinc-blende CdS nanocrystal, which is the largest one known in literature and that too employing the highest (310 oC) reaction temperature. Thus, our results contradict with the suggestions already in literatures that low reaction temperature and small crystal size favors zinc-blende structure. Also, we could tune crystal structure between zincblende and wurtzite at a given pressure of the reaction vessel and for a given solvent, just by changing the S-precursor, which is again in contradiction to previously made suggestions in literatures that high pressure or noncoordinating solvents favors the formation of zinc-blende nanocrystals. Instead, we believe that the surface energy might be crucial in stabilizing the usually rare zinc-blende structure for such nanocrystals. Chapter 6 is divided into two sections and deals with optically active heterostructured nanocrystals exhibiting high photoluminescence efficiency and strong two-photon absorption. In section-I, we probe the internal structure of extraordinarily luminescent (quantum yield = 85%) CdSeS nanocrystals making a somewhat unconventional use of Photoelectron spectroscopy, using the tunability of the photon energy from the third generation synchrotron radiation source as well as the traditional Mg Kα and Al Kα photon sources. CdSeS nanocrystals synthesized with Se:S precursor ratios 1:5 and 1:50, emitting red and green light have CdSe/CdSeS/CdS core/gradient-shell/shell and CdSeS/CdS gradient-core/shell structure, respectively. Gradient interface/core tunes the lattice parameters continuously between that of CdSe and CdS minimizing the interface related defects which in turn increases the photoluminescence efficiency even beyond that obtained from traditional core/shell nanocrystals, as evidenced by the nearly single exponential photoluminescence decay dynamics exhibited by these nanocrystals. Quantum mechanical calculations further show that a graded-core/shell structure leads to a remarkable spatial collapse and consequently a stronger overlap of the HOMO and LUMO wavefunctions towards the core region and thereby, making these luminescent beyond the traditional core/shell limit. In section-II, we have synthesized hetero-structured nanocrystals with CdSe rich core and CdS-ZnS hybrid shell using a simple single-step reaction. These nanocrystals exhibit a very rare example of an optically bi-functional material, simultaneously exhibiting high (~65%) photoluminescence efficiency and strong two-photon absorption cross-section of 1923 GM. Open-aperture z-scan technique was used to measure two-photon absorptions. Chapter 7 is divided into two sections and deals with the generation of white-light emitting nanophosphors. Section-I addresses the white-light emission from a blend of blue, green and red emitting CdSeS nanocrystals. Different shades of the emitted white-light were achieved by tailoring the composition of the blende. Chromaticity of the emitted light of a particular blend is independent of excitation wavelength. Section-II discusses a new approach to generate white-light by combining surface-state emission of nanocrystalline host and d-electron transitions from dopant centres, with an example of Mn2+-doped CdS nanocrystals. Relative contributions from both surface-state emission and Mn2+ d-emission can be tuned by controlling the dopant concentration to generate white lights of different shades. Similar to section-I, here again the chromaticity of the emitted light is independent of the excitation wavelength; but this approach offers additional advantages. Since the surface state emission as well as the Mn2+ d-emission are relatively less sensitive to a size variation compared to the band-edge emission, the chromaticity of the emitted light is not critically dependent on the particle size. Most importantly, these nanocrystals exhibit a huge stokes shift between the absorption and emission spectra resulting in a complete absence of the well-known self-absorption problem, thus, chromaticity of the white-light emitted by these nanocrystals remains unchanged both in dilute dispersion form as well as in solid state. Also there are two appendices in the thesis. Appendix A discusses the preparation of InP nanocrystals using a novel solvothermal route. Appendix B contains the equations explaining photoemission intensity ratios between Se and S (ISe/IS) for a model nanocrystal with a given internal structure.

Dendritic encapsulation of poly(phenyleneethynylene)s or PPEs has been shown to enhance photoluminescent quantum efficiency and facilitate energy transfer by funneling photonic energy absorbed on the dendron periphery efficiently to the conjugated polymeric core. The research presented herein focuses on incorporating degradable dendron onto PPEs, examining whether or not similar benefits were conferred upon the proposed system and controlling polymer luminescence through the elimination of the insulating macromolecules. PPEs appended with disassembling dendrons of various generation sizes were synthesized and their optical properties studied. Polymer luminescence was then quenched via chemical degradation of the disassembling dendrons. Furthermore, the macromolecules resulting from disassembly exhibited tunable luminescence properties upon manipulation of pH. Consequently, it was determined that polymer luminescence could be controlled upon forming phenolic moieties along the PPE backbone. Tunable emission was later realized in the thin film as well through the integration of crosslinkable dendrons onto the polymer core.Recently, helical synthetic linear polymers have demonstrated the ability to facilitate stereoselective processes such as catalysis, recognition and separation. Consequently, it has become increasingly desirable to develop new platforms capable of imparting asymmetry. The work presented herein describes the synthesis of a series of polymers based upon chiral hydrobenzoin and the subsequent conformational analysis performed on these materials. It was envisioned that these polymeric materials might inherently possess conformational asymmetry and as result could be able to impart configurationally chirality by introducing a diastereomeric bias for the formation of one enantiomer over the other during the course of the reaction.

A novel UHV microcalorimeter has been used to study the interaction between calcium and three polymers: MEH-PPV, MEH-PPP and P3HT. All three polymers behave differently in their reaction kinetics with calcium. On MEH-PPV we measure 45 μJ/cm² of heat generated in excess of the heat of bulk metal growth, 120 μJ/cm² for MEH-PPP, and 100 μJ/cm² for P3HT. Comparison of the MEH-PPV and MEHPPP data indicate that the initial reaction of calcium with MEH-PPV occurs at the vinylene group. We propose, based on hypothetical models, that calcium reacts with the vinylene groups of MEH-PPV with a reaction heat of 360 kJ/mol and at a projected surface density of 1.7 sites/nm², while it reacts with the phenylene groups of MEH-PPP in a two-step process with reaction heats of 200 and 360 kJ/mol respectively, at a projected surface density of 3.5 sites/nm². Optical absorption experiments, using either a 1.85 eV diode laser or a xenon lamp coupled to a scanning monochromator, have also been performed using the same calorimeter sensor. In the case of MEH-PPV, using the laser we find an optical absorption cross-section of 3E-¹⁷ cm² per incident calcium atom at low coverages. The change in absorptance at higher coverages correlates perfectly with the population of reacted Ca atoms determined calorimetrically. The size of the absorbance cross-section, and its position just within the band gap of the polymer, are consistent with the reaction being one of polaron formation. Calcium does not appear to dope P3HT, while the photon energy range of 1.5 to 3.75 eV used in these experiments is likely too small for probing polaronic energy states in MEH-PPP.

No description available.

No description available.