Browsing by Subject "Charge transfer"
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Item Electron transfer in sensitized TiO₂ systems studied by time resolved surface second hermonic generation(2012-05) Williams, Kenrick John; Zhu, Xiaoyang, 1963-; Bard, Allen J.; Rossky, Peter J.; Webb, Lauren J.; Korgel, Brain A.Obtaining abundant, clean, sustainable energy has become an increasingly large need globally. To date, solar cells have had a limited impact in meeting energy demands. This is primarily due to their relatively high cost and low power conversion efficiencies. Sensitized solar cells, or Grätzel cells, have the potential for being made with low cost materials, and achieving power conversion efficiency high enough to economically compete with fossil fuels. Understanding the dynamics of charge carriers as they separate at the interface of the light absorbing donor and their semiconducting acceptor becomes an important first step in the realization of an inexpensive and efficient sensitized solar cell. Presented is the theory of treating electrons at donor-acceptor interfaces, and why time-resolved surface second harmonic generation (TR-SHG) is used to probe the dynamics of charge carriers at these interfaces. A series of experiments are described where various preparations of thin films of sensitizers on single crystal titanium dioxide, a common acceptor in Grätzel cells, are prepared and studied. TR-SHG studies of thin films of colloidal PbSe and CdSe QDs showed remarkably different electron cooling and transfer dynamics. The electron cooling in PbSe is thermally activated in PbSe QDs. By cooling samples, electron transfer from higher excited “hot” states was observed. Contrary, for CdSe QDs electron transfer rates were dependent on the energy of the excited state. When higher states were excited, charge transfer rates decreased, indicating that only low energy, electrically “cold”, states participate in charge transfer. When carbon based grapheme QDs are used, the electron dynamics mimic PbSe QDs. In this system, increasing the pump energy leads to slower recombination rates, indicating that electrons have to drift further back to the interface.Item Exciton behaviour at organic solar cell interfaces(2015-12) Dinica, Olivia; Henkelman, Graeme; Rossky, Peter J; Vanden Bout, David A; Makarov, Dmitrii E; Mullins, Charles BOrganic photovoltaics (OPVs) have emerged as a promising class of materials in the production of flexible and cheap solar cells. Polymer OPVs are typically composed of a blend of a semiconducting electron donating poly- mer with an electron accepting fullerene derivative. This blend leads to a high donor-acceptor interfacial surface area where excitons are split apart to create free charges. The generation of free charges after photo-excitation is a main factor influencing solar cell efficiency. However, the mechanisms of charge transfer and the competing process of charge recombination at the interface are not completely clear. The understanding of these processes is essential for the rational design of materials that can maximize photovoltaic conversion efficiencies. The focus of this dissertation is on the influence that electric fields and chemical structure have on exciton dissociation and recombination at the interface of donor-acceptor materials. In particular, we use mixed quantum/classical dynamical simulations and electronic structure calculations to investigate several oligomer-fullerene systems. In order to study the potential energy surfaces guiding the dynamics of electron transfer, the nuclear and electron dynamics of large systems need to be simulated. To make these calculations computationally feasible, a mixed quantum classical molecular dynamics (MQCMD) approach was taken. This technique is based on the QCFF/PI formalism first described by Warshel and Karplus and was further developed by Lobaugh and Rossky for the simulation of betaine-30. This approach divides the conjugated system into a classical and a quantum subsystem. The quantum treatment is reserved for the π electronic system described by the Pariser-Parr-Pople (PPP) Hamiltonian. The classical potential describes a fully flexible molecular backbone and is modeled using an empirical molecular mechanics force field. In the first part of the dissertation we are examining the effect of an external electric field on charge transfer pathways and rates at sexithiophene/fullerene interfaces. In the second part, we develop a rigorous parametrization technique that allows us to model push-pull polymers. These polymers include PCDTBT and KP115 which have a more complex molecular structure than homo-polymers like the one considered in the first part. We use the QCFF/PI method as well as electronic structure calculations to investigate the influence of molecular structure and donor-acceptor orientation on charge transfer and recombination. The pathways linking exciton formation, charge transfer and thermal relaxation are explored, particularly in the context of dependence in the morphology of the donor molecules as well as the non-adiabatic coupling between excited states.Item Extension of tetrathiafulvalene conjugation through pyrrollic-based dyes : ExTTF porphyrin and ExTTF BODIPY(2013-12) Bill, Nathaniel Lloyd; Sessler, Jonathan L.The research and development of organic electron donors is essential in the discovery of photodynamic therapy photosensitizers and catalysts, as well as in the fabrication of organic-based electronic devices. Recently, [pi]-extended tetrathiafulvalenes (exTTFs) have emerged as important organic donors due to their superb electronic properties. However, in general, exTTFs lack significant absorption in the visible and near-infrared portions of the electromagnetic spectrum, thereby limiting their utility. This doctoral dissertation depicts the author's efforts to address this inherent drawback of exTTFs by extending the electronic conjugation of tetrathiafulvalene moieties through pyrrole-based chromophores. The reported findings describe the design, synthesis, properties and potential applications of exTTFs with greatly enhanced absorption profiles. The first Chapter provides a brief historical overview on the history and development of [pi]-extended tetrathiafulvalenes. The various conjugated linkers utilized in exTTF systems are reviewed. In the latter part of the Chapter, emphasis is given to the applications in which exTTFs find use. Chapter 2, as the major focus of the dissertation, details the synthesis and characteristics of a quinoidal porphyrin-bridged exTTF, termed MTTFP. Several metalated complexes, including the Zn, Co, Cu, and Ni derivatives of MTTFP are reported. Additionally, the electrochemical, photophysical, and structural properties of MTTFPs are discussed. We also detail our efforts to synthesize and characterize both the one- and two-electron oxidized forms of MTTFPs. Finally, we discuss our efforts to reversibly switch thermodynamic electron transfer from ZnTTFP to Li@C₆₀ through coordination of axial ligands. Chapter 3 describes the formation of a 2:1 supramolecular ionic porphyrin complex between the two-electron oxidized form of ZnTTFP and a tetranionic sulfonated porphyrin. The association constants and the X-ray crystal structure of the complex are reported. A brief discussion outlining the photophysical characteristics (performed in Prof. Shunichi Fukuzumi and Prof. Dongho Kim's group) of the porphyrin donor-acceptor complexes are included. Chapter 4 details the synthesis, photophysical properties, and spectroelectrochemistry of a difluoroboradiazaindacene (BODIPY) bridged exTTF. This compound is referred to as ex-BODIPY. A singlet oxygen generation study provides initial evidence that ex-BODIPY could potentially serve as a photosensitizer. All of the experimental procedures, characterization data, and X-ray crystallographic data tables are reported in Chapter 5.Item Monolithic integration of functional perovskite structures on Si(2014-08) Choi, Miri; Demkov, Alexander A.; Shih, Chih-KangFunctional crystalline oxides with perovskite structure have a wide range of electrical properties such as ferroelectric, ferromagnetic, and superconductive, as well as unique properties that make them suited for a wide variety of applications including electro-optics, high-k dielectrics, and catalysis. Therefore, in order to realize the potential of perovskite oxides it is desirable to integrate them with semiconductors. Due to the high surface energy of oxides compared to that of semiconductors and the low number of oxides that are thermodynamically stable against SiO₂ formation, it has been extremely difficult to integrate epitaxial oxides with Si directly. However, in 1998, McKee and co-workers finally succeeded in depositing SrTiO₃ on Si directly using a Sr template via molecular beam epitaxy. This breakthrough opened the possibility of integrating the perovskite oxides with Si to realize potential device applications. In this dissertation, alkaline earth metal (Sr and Ba) templates on semiconductors, which enable epitaxial growth of complex oxides on semiconductors, are investigated using molecular beam epitaxy (MBE) for growth and in-situ X-ray/ultraviolet photoemission spectroscopy (XPS/UPS) for the electronic structure analysis. An epitaxial layer of SrTiO₃ on Si using such alkaline earth templates is used as a pseudo-substrate for the integration of perovskite oxides on Si. Through the use of post-deposition annealing as a function of oxygen pressure and annealing time, the strain relaxation behavior of epitaxial SrTiO₃ films grown on Si is also investigated to determine how the SiO₂ interlayer thickness affects the SrTiO₃ lattice constant. This ability to control strain relaxation can be used as a way to manipulate the properties of other perovskite oxides grown on SrTiO₃/Si. Additionally, SrTiO₃ can be made conductive by doping with La. Conductive SrTiO₃ can be used as a thermoelectric, a transparent conductive layer, and a quantum metal layer in a quantum metal field-effect transistor (QMFET). The structural, electrical, and optical properties of strained conductive La-doped SrTiO₃ are studied in order to understand the relation between elastic strain and electrical properties for electronic device applications. Oxide quantum well systems based on LaAlO₃/SrTiO₃ are also investigated using spectroscopic ellipsometry to understand how the quantum well layer structure affects the electronic structure. Such quantum well systems are good candidates for the monolithic integration of functional perovskites on semiconductors. Oxides quantum wells can be used in various device applications such as in quantum well cascade lasers, laser diodes and high performance transistors. As part of the growth optimization for high quality complex oxide heterostructures, the surface preparation of SrTiO₃ substrates using several different methods was also extensively studied using angle-resolved photoemission spectroscopy (ARPES). We found that acid-free water-based surface preparation is actually more effective at removing SrOx̳ crystallites and leaving the surface TiO₂-terminated compared to the more commonly used acid-based methods.Item Monte Carlo simulation of charge transport in Si-based heterostructure transistors(2002) Wang, Xin; Banerjee, SanjayStrain and bandgap engineering of strained materials has emerged as an important technique for improving the device performance other than conventional scaling method. The purpose of this work is to develop a Monte Carlo simulation tool to investigate properties of these strained materials and carrier transport in deep submicron novel devices with heterostructures and strained materials. A general full-band Monte Carlo simulation tool with high flexibility about device structure and material profiles is developed for the first time. The transport model is based on energy-dependent scattering rates including inelastic acoustic phonon scattering with longitudinal and transverse modes, optical phonon scattering, impact ionization, surface roughness scattering, impurity scattering and alloy scattering. The full-band treatment for strained material model substantially advances the state-of-the-art method relying on simpler valley model for the scattering rates. The multi-material profiles in devices are treated with parameterization of band structure. The tunneling across a potential barrier is treated with Feynman’s effective potential scheme. An orthorhombically-strained silicon (OS-Si) is reported in this work. The six degenerate valleys in OS-Si near X points break into three pairs with different energy minima due to the orthorhombic strain. Thus the drift velocity is enhanced under an electric field transverse to the longitudinal-axis of the lowest valleys. The OS-Si grown on a compressively-strained Si0.6Ge0.4 sidewall has a mobility almost twice that of bulk Si and electron saturation velocity approximately 20% higher. For homogenous strained silicon on Si0.7Ge0.3 (001), in-plane mobility of 2670 cm2 /(Vs) for electrons is obtained, with enhancement by a factor of 1.8 compared to the unstrained case. Electron transport in a strained Si nMOSFET with 50 nm channel length is also investigated by full-band Monte Carlo. Strained silicon devices exhibit around 60% increase of drain current compared to unstrained silicon. Strained SiGe is also studied with full-band Monte Carol tool. A 90% enhancement in hole mobility is obtained for strained Si0.7Ge0.3 compared with bulk Si. The preliminary investigation of hole transport in vertical pMOSFETs with graded SiGe channel is also reported in this work.Item Syntheses of novel bis(alkylimino)acenaphthene (BIAN) and tetrakis(arylimino)pyracene (TIP) ligands and studies of their redox chemistry(2009-08) Vasudevan, Kalyan Vikram; Cowley, Alan H.; Jones, Richard A.; Holliday, Bradley J.; Bielawski, Christopher W.; Gordon, John C.The evolution of the present work began with the syntheses of novel bis(alkylimino)acenaphthene (BIAN) ligands. At the outset of this research, despite the presence of dozens of aryl-BIAN ligands in the literature, there were as of yet no reported BIAN ligands bearing alkyl substituents. Given the nearly ubiquitous use of transition metal complexes of alkyl diazabutadiene (DAB) ligands for e.g. catalysis and as ligands for carbene chemistry, interest was generated in developing this emerging field of synthetic chemistry. Initial studies focused on the synthesis of alkyl-BIAN ligands since the traditional synthetic approaches that had been developed for aryl-BIAN ligands were unsuccessful for the alkyl analogues. As an alternate synthetic route, it was decided to employ amino- and imino-alane transfer reagents which had previously proved successful for the conversion of C=O into C=N-R functionalities. While this transfer route had proved successful to synthesize moderate yields of highly fluorinated DAB ligands, it was unknown how or whether this methodology would apply in the case of alkylated BIAN systems. Over the past decade, there has been a surge of interest regarding lanthanide complexes that are capable of undergoing spontaneous electron transfer processes. There are several reports in the literature that describe the ability of Ln(II) ions to undergo spontaneous oxidation, thereby causing one-electron reduction of the coordinated ligand and generally resulting in the corresponding Ln(III) complex. The present work focused on an enhanced understanding of the electronic communication between the lanthanide and the attached ligand. Particular emphasis was placed on defining the resulting oxidation states and the manner in which delocalized electrons of the radical anion species travel over a conjugated system. This fundamental information was gleaned from single-crystal X-ray diffraction studies and magnetic moment measurements that were obtained using the Evans method. Additional insights stemmed from the use of more classical techniques such as IR and NMR spectroscopy. In favorable cases, the presence or absence of spectral peaks can permit assignment of the lanthanide oxidation state. Accordingly, the research plan was to synthesize a series of BIAN-supported decamethyllanthanocene complexes with the goal of learning how to control the spontaneous charge transfer that had been reported in the literature. A longer term goal was to develop a bifunctional ligand of the BIAN type that was capable of accommodating two lanthanide or main group element moieties. Systems with tunable electronic interactions between lanthanide or main group elements are of interest because they offer the prospect of extended delocalization of electron density. Systems of this type have potential applications as e.g. molecular wires and single-molecule magnets. Indeed, such systems have been investigated by using bis(bipyridyl) and bis(terpyridyl) ligands to support two redox-active moieties. However, in the present work, it was recognized that a bifunctional BIAN-type ligand might be of considerable interest as the supporting structure for studying the communication between lanthanide or main group element moieties. A synthesis of variously substituted tetrakis(imino)pyracene (TIP) ligands was therefore undertaken. The flat, rigid nature of the TIP ligands rendered them ideal scaffolds for studying the redox behavior and electronic communication between lanthanide or main group element centers. The new TIP ligand class also proved to be useful for the assembly of the first example of a metallopolymer based on a BIAN-type ligand.Item Ultrafast spectroscopic study on charge-transfer reactions in condensed phase(2002) Son, Dong Hee; Barbara, P. F.Ultrafast femtosecond spectroscopic investigations on the dynamics of electron transfer in the solution phase have been performed. The chemical systems selected for the studies are the hydrated electron and a ruthenium mixedvalence compound. The hydrated electron has been a prototypical system for investigating many dynamical processes in the solution phase, such as solvation, non-adiabatic electronic relaxation and charge transfer reaction. In the present study, pump-probe spectroscopy has been employed to investigate structural and chemical properties of the optically excited states of the hydrated electron. Measurement of spatial extent of the wavefunction and kinetics of the electron transfer scavenging of the excited states is discussed. The electron transfer in a mixed-valence metal compound is an important system, in which the charge transfer process is strongly coupled to both the intramolecular and solvent nuclear degrees of freedom. Here, investigations on the dynamics of back electron transfer after the optical excitation and subsequent energy relaxation on intramolecular and solvent nuclear coordinates are described.