Thesis (Ph. D.)--Michigan State University. Dept. of Chemistry, 2006. / Title from PDF t.p. (viewed on June 19, 2009) Includes bibliographical references. Also issued in print.

Thesis (Ph. D.)--Michigan State University. Dept. of Chemistry, 2008. / Title from PDF t.p. (viewed Sept. 10, 2009). Includes bibliographical references. Also issued in print.

No description available.

Metal chalcogenides are one of the most important class of compounds in the field of Inorganic Chemistry. A wide variety of chalco-anion building blocks provides excellent opportunities to synthesize new compounds with unique structure and properties, essential drives in maximizing technological impact. In this dissertation, the exploratory synthesis of new mixed-metal chalcogenide compounds is carried out. The novel phases were characterized using a wide spectrum of techniques, and their properties were investigated. The project started by investigating the synthesis of zeolite-like chalcogenides using a solid-state reaction. As a result, the thioaluminogermanate Na(AlS2)(GeS2)4 was synthesized with successful insertion of Al3+ cations into the chalcogenogermanate framework. This effectively extended the structural chemistry for this family of materials and approximated them to the aluminosilicate zeolites. The crystal structure of Na(AlS2)(GeS2)4 displayed a [(AlS2)(GeS2)4]1- 3D polyanionic framework, in which Al and Ge atoms share atomic positions and Na cations occupy the channels in-between. At room temperature and in a solvent medium, this compound exhibits a unique cation-exchange property with monovalent Ag+ and Cu+ ions, resulting in the formation of the isostructural compounds Ag(AlS2)(GeS2)4 and Cu(AlS2)(GeS2)4. The replacement of Na+ in the parent compound with Ag+ or Cu+ results in enhanced properties such as higher stability in air and narrower bandgap energies. The completeness of the ion-exchange reactions was confirmed using various analytical tools including single crystal XRD, EDX, and 23Na NMR. Following this initial success, a systematic study was carried out to synthesize unknown phases of transition and main group mixed-metal chalcogenides. As a result, the first example of an alkali/transition metal thioaluminate compound K2Cu3AlS4 was synthesized. For this, a solid-state reaction with K2S acting as a self-flux was used. The crystal structure of K2Cu3AlS4 consists of [Cu3AlS4]2- polyanionic anti-PbO type layers, in which Al and Cu atoms share the atomic positions, separated by K+ cations. The coordination environments of the Al and K cations were confirmed by solid-state 27Al and 39K NMR spectroscopies. The optical property and thermal stability of this new quaternary compound were also studied. The mixed-metal chalcogenides class is not restricted only to purely inorganic components; it can also be extended to inorganic-organic hybrid materials. In an attempt to synthesize main group chalcogenides mixed with transition metal complexes, the new compound [Ni(en)3]GeS2(OH)2•H2O was obtained. In the complex cation [Ni(en)3]2+, the ethylenediamine (en) ligands are bidentate to the Ni2+ through the N atoms resulting in a distorted octahedral geometry which is charge balanced by the rarely observed [GeS2(OH)2]2- tetrahedral anion. In agreement with single crystal data, the solid-state 1H NMR spectrum exhibits four signals corresponding to the -CH2 and NH2 protons of the (en) in addition to the H2O and -OH protons. This compound exhibits a paramagnetic response, studied by EPR spectroscopy and ZFC/FC magnetization measurements. The optical properties including UV-Vis absorption and photoluminescence emission were also measured. Knowing that it was possible to synthesize various types of mixed-metal chalcogenides, the focus was shifted to the production of those with interesting functional properties. In this way, Na2BiSbQ4 (Q = S, Se, Te) compounds were synthesized by reacting Bi and Sb in the corresponding Na2Q flux. The three phases obtained are isostructural and crystallize with NaCl-type structure. The unique feature of these structures is the existence of only one crystallographic metal site in the unit cell (where Bi, Sb and Na share the same atomic position). These mix of position sites provide the desirable lattice complexity with a totally random distribution of Na, Bi and Sb atoms. As expected, extremely low thermal conductivities at room temperature have been observed for the studied phases. The optical properties, solid-state 27Na NMR spectra, chemical and thermal stabilities are discussed.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第24180号 / 理博第4871号 / 新制||理||1697(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 寺西 利治, 教授 島川 祐一, 教授 倉田 博基 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

A dissertation submitted to the Faculty of Science University of the Witwatersrand, in partial fulfilment of the requirements for the degree of master of science (MSc) School of physics, University of Witwatersrand, 2017. / Intensive study on structural, electronic and optical properties of bulk transition metal dichalcogenides and dipnictogenides (MX2; where M = V, Nb and X = S, Se, Te, P) was undertaken. A relative stability test was done to determine the most stable ground state configuration via calculation of total ground state energy and volume which was fitted to the third order Birch-Murnaghan equation of state to extract lattice parameters. Cohesive energies of the above mentioned MX2 compounds and their elemental solids were then computed from which formation energies were acquired based on their respective equations of reaction between reactants and product. Its significance was to aid in determining if a material is energetically stable. Elastic constants were predicted from which mechanical properties i.e bulk, Young’s and shear moduli and consequently Poisson’s ratio were resolved by feeding the stiffness matrix onto online elastic tensor analysis tool which facilitated verification of their mechanical stability based on the well-known Born stability conditions which varies from one crystal system to another, at a later stage phonon dispersion curves were plotted after performing phonon calculation based on phonopy code to verify if the materials of concern are dynamically stable. After a material had fulfilled all the above stability tests, its structural study was initiated using various functionals. Functional that described best the structural properties of each individual compound considered was then applied in exploring its electronic and optical properties whose motivation was to find out the most stable phase as well as gauge if these materials could be used in various fields that suits their mechanical and optical properties. Furthermore, from carefully calculated optical spectra, plasma frequencies were analyzed which indicated the possibility of applying a material in plasmonic related fields. In addition to above, other factors to be considered when selecting a given electrode material that are crucial for optoelectronics are good chemical and thermal stabilities, high transparency and excellent conductivity. / XL2018

The materials Fe���(Si,Ge)(S,Se)���, Cu���PS[subscript 4-x]Se[subscript x] (0 ��� x ��� 4), and Cu���PxAs[subscript 1-x]S��� (0 ��� x ��� 1) have been synthesized and studied as new earth-abundant absorbers for single and multijunction photovoltaic cells as well as solar fuel generation. The synthesis, single-crystal growth, and optical and electrical properties of these materials are described and discussed in Chapter 2, 3, and 4. Inspired by the photovoltaic absorber Cu���ZnSnS���, the new compound CuZnPS��� has been discovered; its structure and properties are discussed in Chapter 5, including a comparative analysis to Cu���PS���. The compound YZrF��� (Chapter 6) has been synthesized and evaluated for the first time as a new optical host for green up-conversion. / Graduation date: 2013

No description available.

Based on rst principles calculations the structural, mechanical stability and electronic properties of molybdenum chalcogenides 2H-MoX2 (X = S, Se, Te) have been studied using density functional theory (DFT). The generalized gradient approximation (GGA) proposed by Perdew, Burke and Ernzerhof (PBE) was employed together with the projector augmented plane wave (PAW) method. The van der Waals interactions in the Grimme (DFT-D2) and Lundqvist and Langreth (vdW-DF) approximations have been added on top of PBE or revised PBE in order to take into account the weak interactions between layers of 2H-MoX2 (X = S, Se, Te). The structural properties include the equilibrium lattice parameters, the cohesive energy and the formation energy. Besides we have studied the mechanical stability of these compounds by examining the elastic constants using the PBE, vdW-DF, and DFT-D2 approximations. Other quantities related to the mechanical properties such as the Young's modulus, the Poisson's ratio and the bulk modulus were also calculated. Electronic properties of bulk 2H-MoX2 (X = S, Se, Te) have been investigated using density functional theory (via band structure and projected density of state), and a partially self-consistent GW (GW0) approximation. We have investigated electronic properties ( band structure) for monolayers 1H-MoX2 (X = S, Se, Te) using a single shot GW (G0W0) approximation. Optical properties for monolayer 1H-MoX2 (X = S, Se, Te) were studied using the Bethe-Salpeter equation (BSE).

A Thesis submitted to the Faculty of Science, School of Chemistry at University of the Witwatersrand, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2015. / Photovoltaic cells offer a good alternative to the fossil fuels. Several approaches are being analysed in order to have solar cells that are capable to conquer the energy market all around the world. Quantum dots (QDs) have already proven features that can be taken into account to improve the properties of solar cells. Metal selenide nanoparticles (NPs) possess semiconducting behaviours that can vary with their structural and optical properties evolving from their synthesis. The reaction parameters such as the method, time, solvent and precursors can affect the growth and nucleation of particles and thus impose on the properties of the synthesized materials. The performance of solar cells made of the synthesized metal selenides will then be dependent upon the properties of the NPs used as active layer. Furthermore, the electrical current generation also depends on the structure of the deposited active layer and its interface with other films to be assembled for the device. The binary copper selenide, ternary copper indium selenide (CISe), quaternary copper indium gallium selenide (CIGSe) and quinary copper zinc tin sulphur selenide (CZTSSe) NPs were synthesized via conventional colloidal method (CCM) and microwave assisted method (MAM). The MAM has a particular interest as it is less time consuming and can easily be a large scale synthesis. Photovoltaic devices were fabricated from the synthesized materials as proof of concept for photovoltaic activities. The CCM was used to optimize various parameters for the synthesis of each type of the chalcogenide materials as this is easily controllable than the ones from the sealed vessel from MAM. The dependency of properties of all copper chalcogenide NPs on the time, precursor concentration, temperature and solvent of synthesis have been demonstrated via various characterization techniques including ultraviolet-visible-near infrared spectroscopy, photoluminescence spectroscopy, X-ray diffractometry and transmission electron microscopy. The binary copper selenide was first synthesized and considered as a template for evaluation of the use of copper chalcogenide materials in solar cells. Relatively smaller copper selenide NPs with average sizes of 4.5 and 6.0 nm were obtained from conventional colloidal and microwave assisted methods respectively. The sample yielded from the microwave assisted method possessed less polydispersed NPs. The later had better crystallinity in which prevailed a single cubic Cu2Se phase. To the best of our knowledge this is the first evidence of defined shapes and nearly single phase of small sized copper selenide NPs synthesized by mean of the MAM. The copper selenide particles synthesized via this method were used to fabricate a Schottky device. The conditions of copper selenide synthesis were optimized to 250 oC, 30 min of CCM synthesis using oleylamine (OLA) and a Cu/Se ratio of 1:1. Nearly hexagonal facets with blue-shifted absorption band edge of monodispersed NPs sizing 4-8 nm in diameter were obtained. The synthesized copper selenide showed better crystallinity with a single cubic Cu2Se phase. A Schottky device using MAM synthesized copper selenide NPs as the semiconducting layer was fabricated at room temperature. The diode effect was demonstrated with the electrical parameters such as the ideality factor, barrier height and the series resistances extracted from the experimental current-voltage data using the thermionic theory and Cheung’s modification. The thermionic theory resulted in the ideality factor of 4.35 and the barrier height of 0.895 eV whilst the Cheung’s method resulted in the ideality factor, barrier height and series resistance of 1.04, 2.59 10-3 eV and 0.870 Ω respectively. The ternary copper indium selenide NPs showed that the MAM allowed the formation of copper rich NPs alongside secondary products. The synthesis of the ternary sample via CCM was optimized using uncapped precursors (no TOP was added) in OLA at 220 oC for 30 min. The synthesized CuInSe2 NPs possessed a large blue-shift in their absorption band edges and emission peaks. The nearly stoichiometric CuInSe2 particles with diameter sizes of 5-9 nm were found in tetragonal crystalline orientation. The cyclic voltametry (CV) and the absorption spectra showed a large blue-shifted energy gap, about 0.95 eV, an increase from the bulk, proving the quantum confinement effects of synthesized copper indium selenide quantum dots. The CuInSe2 NPs were thus used as absorbing materials in the quantum dot sensitized solar cell devices (QDSSCs). The QDSSC devices were assembled via treatment of the titanium oxide, quantum dot layers and their interface. This was done by the treatment of copper indium selenide surface with mercapto-propionic acid (MPA) and ethanedithiol (EDT) during the deposition of the quantum dots onto TiO2 films. The MPA treatment did not reveal positive effects on copper indium selenide thin film and the assembled device under our optimized working conditions. However the use of EDT allowed the improvement of electron transport. The short circuit current (Jsc), open circuit voltage (Voc) and fill factor (FF) obtained from the current-voltage (J-V) curves reached the values of 324 μA cm-2, 487 mV and 43% respectively, indicating that the investigated quantum dots possess electrical properties. For the quaternary copper indium gallium selenide, relatively small sized NPs were synthesized via CCM and MAM. The CCM synthesized CIGSe NPs were less agglomerated with a shorter tailing in absorption than those from MAM. The stoichiometric CuIn0.75Ga0.25Se2 showed less agglomerated and highly crystalline particles with a large blueshifted absorption band edge and a smaller full width at halth maximum (FWHM) of the emission peak compared to CuIn0.5Ga0.5Se2 and CuIn0.25Ga0.75Se2. The use of OLA as solvent of synthesis improved the growth and dispersivity of copper indium gallium selenide NPs. The particles with a large blue-shifted absorption band edge, a lattice of tetragonal phase, more monodispersed CIGSe and possessing an average size of 6.5 nm were obtained from CCM synthesis using OLA. The OLA as-synthesized CIGSe NPs were used in thin film for the assembly of QDSSC. The device exhibited electrical properties with the Jsc, Voc and FF of 168 μA cm-2, 162 mV and 33% respectively. The overall device performance was poor but may further be improved for further photovoltaic application. The quinary CZTSSe NPs possessed large blue-shifted absorption band edges of 450-460 nm than the bulk material (827 nm). The emission peak at 532 nm and similar FWHM of less than 50 nm were observed in samples from both CCM and MAM. More monodispersed crystals were obtained with both methods whilst the average particle sizes of 10 and 9 nm were yielded from MAM and CCM respectively. The nanoparticles crystallized in tetragonal lattices between copper zinc tin sulphide and copper zinc tin selenide crystals. However, the MAM gave more crystalline phases. The CV and the absorption spectra showed a blue shifted energy gap, about 0.21 eV increase from the buk which is located at 1.51 eV. This is indicative of the quantum confinement effects of synthesized NPs. The evidence of electrical properties was also shown in the QDSSCs fabricated using the MAM synthesized quinary QDs. This was done following the same treatments as for copper indium selenide devices. The Jsc, Voc and FF were found at the maxima of 258 μA cm-2, 395 mV and 38% respectively. The MPA and EDT treatments did not improve the device performance under our working conditions. Nevertheless, the electrical properties observed in the assembled device were indicative of promising efficient solar cells from synthesized CZTSSe NPs.