Dissertations / Theses on the topic 'Transition metal Chalcogenides'

Atomically thin group-VI transition metal dichalcogenides (TMDC) has been emerging as a family of intrinsic 2-dimensional (2D) crystals with a sizeable bandgap in the visible and near infrared range, satisfying numerous requirements for ultimate electronics and optoelectronics. This intrinsic 2D crystal also provides a perfect platform for physics study in 2D semiconductors. The characteristic inversion symmetry breaking presented in monolayer TMDCs leads to non-zero but contrasting Berry curvatures and orbital magnetic moments at K/K’ valleys located at the corners of the first Brillouin zone. These features provide an opportunity to manipulate electrons’ additional internal degrees of freedom, namely the valley degree of freedom, making monolayer TMDC a promising candidate for the conceptual valleytronics. Besides, the strong spin-orbit interactions and the subsequent spin-valley coupling demonstrated in 2D TMDCs open potential new routes towards quantum manipulation. In this thesis, I give a brief review on the background and our progress of the physics study in 2D TMDCs (MoS2, WS2) via optical spectroscopy. Particularly, our experimental approach on the excitonic effect, valley dependent circular dichroism, and the spin-valley coupling in monolayer and bilayer TMDCs are elaborated in individual chapters.
published_or_final_version
Physics
Doctoral
Doctor of Philosophy

An atomically thin film of semiconducting transition metal dichalcogenides (TMDCs) is emerging as a class of key materials in chemistry and physics due to their remarkable chemical and electronic properties. The TMDCs are layered materials with weak out-of-plane van der Waals (vdW) interaction and strong in-plane covalent bonding enabling scalable exfoliation into two-dimensional (2D) layers of atomic thickness. The growth techniques to prepare these 2D TMDC materials in high yield and large scale with high crystallinity have attracted intensive attention recently because of the new properties and potentials in nano-elctronic, optoelectronic, spintronic and valleytronic applications. In this thesis, I develop methods for the chemical synthesis of 2D TMDCs films. The relevant growth mechanism and material characteristics of these films are also investigated. Molybdenum disulfide (MoS2) is synthesized by using molybdenum trioxide (MoO3) and sulfur (S) powder as the precursor. The films are formed on substrate pre-treated with reduced graphene oxide as the catalyst. However, this method cannot be extended to other TMDC materials such as molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2) because reduced graphene oxide (rGO) reacts with selenium to form alloy materials rather than TMDC films. At the same time, the conversion of MoO3 to MoSe2 or that of tungsten trioxide (WO3) to WSe2 without the assistance of hydrogen in the chemical reaction is not thermodynamically feasible because the oxygen in the metal oxide cannot be replaced by selenium due to lower reactivity of the latter. On the other hand, I demonstrate that MoSe2 film can be synthesized directly by using MoSe2 and Se powder. Furthermore, the method of sulfurization or selenization of pre-deposited metal film can be promising due to precise thickness/size controls. Finally, some perspectives on the engineering challenges and fabrication methods of this family of materials will be given.
published_or_final_version
Physics
Master
Master of Philosophy

In this thesis, we present results of studies of the electronic band structures and related electronic properties of some layered transition metal chalcogenides and their intercalation complexes. The materials investigated include group VIIc transition metal dichalcogenides, and 2H-TaS₂ and its lithium-, lead-, and tin-intercalated complexes, as well as dihafnium sulphide and selenide. Both experimental measurements and theoretical elect'onic band structure calculations have been carried out. The types of measurements conducted consist of reflectivity measurements in the energy range from 0.5 eV to 4.5 eV using the home-made reflectivity spectrometer, and electron energy loss measurements in the energy range up to 100 eV using the scanning transmission electron microscope as well as some characterization experiments (structural, chemical composition and thermal properties). The experimental investigations were restricted to the layered group VIIc metal dichalcogenides. All the electronic band structures are calculated using the linearized muffin-tin orbital (LMTO) method, and are reported for the first time except PdTe₂ and 2H-TaS₂. The obtained electronic band structures for the Ni-group metal dichalcogenides, and the semiconductor-metal shift in progression from PtS₂ through PtSe₂ to PtTe₂ are discussed in terms of the binding energies of the atomic valence orbitals of the constituent atoms, the local coordination of the metal atoms and the symmetry of the crystals as well as the charge transfer effects. A superlattice structural phase transition is proposed for PtSe₂, which may possibly explain the anomaly observed in the previous transport measurement. The previous photoemission spectra from NiTe₂, PdTe₂ and PtTe₂, and dHvA measurement on PdTe₂ are compared with their band structures in details, and a good agreement is found. Other available experimental data including the previous transport, optical and magnetic susceptibility measurements as well as the reflectivity and electron energy loss spectra measured in this work are also discussed in terms of these electronic structures. The band structure calculations for dihafnium chalcogenides predict that these materials are metals. They also suggest that there is a strong bonding between Hf atoms in the adjacent layers, thus giving rise to the rigidity in the c-direction which may preclude the intercalation of these materials. The results for 2H-TaS₂ and its intercalation complexes show that the rigid band model is essentially correct for 2H-LiTaS₂ but is an oversimplication for the post-transition metal intercalation compounds. Changes in the electronic structure upon intercalation are discussed in terms of the intercalant-host charge transfer and the hybridisation between the host states and the intercalation valence orbitals. Electrical conduction in 2H-PbTaS₂ and SnTaS₂ is found to be largely due to the p-valence electrons from the intercalant Pb (Sn) layers, resulting in the considerable increase in the superconducting transition temperature following intercalation. The results are also compared with the observed optical and transport properties and a broad agreement is found. The band structures and the electronic properties of other layered transition metal dichalcogenides and their intercalation complexes, as well as the band structure calculation techniques for the layered compounds are also reviewed in this thesis.

This habilitation thesis describes the syntheses and characterizations of mostly new metal-rich cluster compounds of early transition elements. It is devided into 4 parts. The first part describes the syntheses and solid-state X-ray structures of a total of 14 mixed-halide (iodide-chloride) zirconium cluster phases, which crystallize in 9 different (some novel) structure types. All these cluster phases contain octahedral Zr6Z-units that are centered by an interstitial atom Z. Phase widths and relations between the differend structure types are discussed. The second part deals with molecular (soluble) zirconium cluster phases which are excised from solid-state precursors. It is shown that liquid mixtures of 1,3-dialkylimidazolium bromide and AlBr3 as well as molten 18-crown-6 are useful solvents for molecular zirconium cluster compounds. The syntheses and single crystal X-ray structures of 3 new materials are detailed. Furthermore, from the reaction of an iron containing Zr-cluster phase with KSCN crystals have been obtained from which a single crystal X-ray structure analysis shows that they contain a high symmetry Fe-cubane cluster. The third part of this thesis is concerned with theoretical investigations (Extended-Hckel band structure calculations) of a series of monoclinic phases which contain double-octahedral chains of rare earth metal clusters of the general formula RE3I3Z (RE = rare-earth metal atoms). The results of the electronic band structure calculations show that the observed structural variations within this series depend largely on the difference of the orbital energies of the rare earth metals and the interstitials. In the last part the phase pure synthesis of Nb21S8 and measurements of physical properties (electrical conductivity and magnetical susceptibility) as well as results from electronic band structure calculations (LMTO) are described. The property measurements show that this metal rich sulfide becomes superconducting below 3.7(2) K. The results from the electronic band structure calculations indicate the existence of an arrangement of electronic levels ("fingerprint") which favors the formation of superconducting Cooper pairs.

Recentemente foi descoberto que diversos calcogenetos de metais de transição podem ter o estado charge density waves (CDW) suprimido a partir de pressão hidrostática e dopagens, e, por conseguinte, o estado supercondutor emerge. Nesse contexto, este trabalho apresenta um estudo sistemático de propriedades físicas de amostras poli e monocristalinas de dois compostos do sistema Nb-Te, NbTe2 e NbTe4. Com relação ao composto NbTe2, os resultados aqui apresentados demonstram que esse composto é mais um exemplo de material que exibe ambos os estados a pressão atmosférica e sem dopagens. No que tange as propriedades do NbTe4, este trabalho demonstra que amostras deficientes em telúrio tem a anomalia na curva de resistividade elétrica relacionada a formação do estado CDW amplificada e, a deficiência em telúrio é também capaz de fazer emergir supercondutividade em 5.5 K. Este trabalho também sugere algumas mudanças no diagrama de equilíbrio de fases Nb-Te publicado na base de dados da sociedade americana de metalurgia (ASM). As fases Nb5Te4 e Nb3Te4 foram identificadas como sendo fases de altas temperaturas que são formadas a partir de reações eutetóides. Além do mais, nossos resultados demonstram que a região entre as fases NbTe2 e NbTe4 consiste, na verdade, de uma região bifásica. Durante a realização deste trabalho, outro composto foi investigado, o NiTe2. Nesse composto, nossos resultados demonstram que a intercalação de Ti faz emergir um estado supercondutor em 4.5 K e cuja temperatura de transição parece insensível a pressão hidrostática. Cálculos de estrutura de bandas sugerem fortemente que o composto NiTe2 intercalado com Ti pode ser mais um exemplo de supercondutor com aspectos topológicos em sua superfície de Fermi.
Recently was demonstrated that it is possible to suppress the charge density waves (CDW) ground states while, simultaneously, a superconductor state emerges in several transition metal chalcogenides (TMC), by means of hydrostatic pressure or chemical doping. Within this context, this work presents a systematic study on physical properties of two Nb chalcogenides, NbTe2 and NbTe4. Our results demonstrate that NbTe2 is another example of a TMC which exhibit both stabilities at atmospheric pressure and without doping. Regarding the physical properties of NbTe4, we have demonstrated that Te deficiency increases significantly the anomaly in the electrical resistivity as function of temperature behavior related with the CDW formation. At the same time, Te deficiency can also cause a SC state to emerge at 5.5 K. This work also presents a review of the binary phase diagram, Nb-Te, and some changes are proposed. Nb5Te4 and Nb3Te4 were identified as high temperature phases originated from eutectoid reactions. Furthermore, our results demonstrated that the region between the phases NbTe2 and NbTe4 are, in fact, a two-phase region, differently from what is proposed in the actual version of the phase diagram. Also, during this work, another chalcogenide was investigated, NiTe2. Our results demonstrate that Ti can be intercalted between the Van der Waals gaps of the structure and consequently a superconductor state emerges at 4.5 K. The critical temperature is found to be insensitive to hydrostatic pressure. Band structure strongly suggests that NiTe2 could be another example of a superconductor with topological aspects in its Fermi surface.

Attempts have been made to synthesise thio-fluoride species of tantalum, osmium and iridium. The reaction of tantalum thiotrichloride and tri- bromide with an excess of anhydrous HF yields [SH3]+ [Ta2F11]-. When anhydrous HF is added to a solution of TaSX3 (X =C1 or Br) in aceto- nitrile an oil is formed which contains =NH and ENH+. The thermal reactions of the hexafluorides of osmium and iridium with zinc and boron sulphides yield the adducts SF4.MF5 (M=Os or Ir). Infra-red and X-ray powder diffraction studies indicate that they have contributions to the bonding from the ionic formulations [SF3]+ [MF6]-. The reaction of MF6 with antimony sulphide in anhydrous HF gives only lower oxidation-state fluorides. The reaction of the alkali metal fluorides with tungsten thiotetra-fluoride in anhydrous HF has yielded the first examples of solids containing [W2S2F9]- and [WSF5]-, viz. M+[W2S2F9]- (M=Li, Na, K, Rb or Cs) and M+[WSF5]- (M=Rb or Cs). Nitrosyl fluoride reacts rapidly with tungsten thiotetrafluoride to yield [NO]+ [WOF5]-, [NO]2+ [WF8]2- and sulphuryl fluoride. However, a low-temperature n.m.r. study has shown that the reaction initially yields [W2S2F9]- and [WSF5]-. Tungsten oxidetetrafluoride reacts with an excess of sulphur tetra-fluoride to give [SF3]+ [W2O2F9]-. X-ray powder diffraction, infra-red and n.m.r. studies have shown that fluorine bridging between [SF3]+ and [W2O2F9]- in the solid state or in solution in sulphur dioxide is minimal. The reaction between tungsten thiotetrafluoride and sulphur tetrafluoride yields only tungsten hexafluoride and sulphur. Xenon difluoride reacts violently with tungsten thiotetrafluoride in the solid state to yield tungsten hexafluoride, xenon and fluorides of sulphur. When the reaction is conducted in sulphuryl chloride fluoride at low-temperature a red-brown solution is formed. This has been shown to consist of tungsten hexafluoride and the radical cation, S8+. by n.m.r. and e.s.r. spectroscopy. The standard enthalpy of formation of tungsten thiotetrafluoride has been determined by hydrolysis in alkaline media.

This thesis discusses the synthesis of transition metal chalcogenide nanostructures (where the chalcogen is either sulfur, selenium or tellurium) through the use of standard chemical vapour transport (CVT) and chemical vapour deposition (CVD) techniques. The resultant structures are characterised with a variety of methods and comparisons of their properties are made with their bulk counterparts. A discussion into how some of these structures form during the reaction is also given. Highly symmetrical, isotropic, nickel disulfide (NiS2) nanocubes have been synthesised via a Physical Vapour Transport (PVT) method in which sulfur vapour generated in situ is reacted with nickel-coated silica substrates. Systematic studies demonstrate the effect of the reactant ratio, substrate, metal layer thickness and reaction temperature on the synthesis and growth process. The evolution of structure and composition has been followed by diffraction and scanning electron microscopy (SEM). The size of the NiS2 cubes can be varied from below 200 nm to 1 -2 1m across. Magnetic properties of the disulfide nanomaterials have been determined using superconducting quantum interference device (SQUID) magnetometry. Initial experiments also demonstrate that related CVT techniques can be exploited to produce alternative compositions in the Ni-S system with varying morphologies that can be controlled via chemical and physical reaction parameters. Surface Assisted Chemical Vapour Transport (SACVT) methods have been employed to grow flower-like nanostructures of titanium disulfide (TiS2) and titanium trisulfide (TiS3) on titanium coated silica substrates. Systematic studies demonstrate the role of the reactant ratio and reaction temperature on the synthesis and growth process. The evolution of structure and composition has been followed by powder X-ray diffraction (PXD) and electron microscopy techniques such as and transmission electron microscopy (TEM). Magnetic properties of the disulfide nanomaterials have been determined using SQUID and Raman spectroscopy has been used to confirm the identity of the sulfides. Investigations into nanostructured materials of the group IV transition metals zirconium and hafnium resulted in the successful synthesis of nanostructures of zirconium trisulfide/selenide (ZrS3/Se3) and hafnium trisulfide/selenide (HfS3/Se3). The unusual effects on structure that can occur when reactant time and synthesis temperature are varied and when a balance between these two factors is successfully found, nanostructures other than tubes and wires can be formed. Each of these systems were characterised with a variety of techniques including, TEM, PXD and SQUID.

This thesis details the physical and electronic structure of several technologically important transition metal chalcogenides (TMCs) using a combination of transmission electron microscopy (TEM) and surface science experimental techniques. The materials studied include CuxTe and CdTe, which find application in high efficiency, low weight photovoltaic devices. CuxTe alloys are frequently used as an electrical back contact in high efficiency CdTe photovoltaics. Here, we examine the alloying of Te on the Cu(111), polycrystalline Cu and Cu(643) surfaces. Chapter 3 of this thesis shows that the alloying of Te and Cu(111) is facile at room temperature, contrary to previous reports. Two distinct surface phases exist, depending on Te surface concentration. Below a coverage of 0.33 monolayers (MLs) of Te a surface substitutional alloy (SSA) is found to exist, where a Te adatom substituted for a surface Cu atom. For Te coverages greater than 0.66 ML, an unusual Cu3Te2 alloy continually grows on the surface, stabilised by a good lattice match to the Cu(111) substrate. The surface alloying of the Cu-Te system displays an intriguing dependence on the surface termination of the Cu substrate. Of the three Cu substrates studied here, Cu(111), Cu(643)R and polycrystalline Cu, a 1 ML film of Te gave ordered alloy structures with stoichiometries of Cu3Te2, CuTe and Cu2Te, respectively. In chapter 4, the study of thin film photovoltaics is extended to the deposition of CdTe onto Cu and CuxTe substrates. CdTe is observed to grow three dimensionally on Cu(111), Cu3Te2 and Cu2Te. Cu+ diffusion, crucial for photovoltaic performance, is detected for CdTe thicknesses greater than 2 ML and is assigned, predominately, to Cu2Te crystallites forming within the CdTe layer, with a minor amount of Cu residing in interstitial sites in the host CdTe structure. Chapter 5 describes the alloying of Te with a intrinsically chiral surface, Cu(643)R, the first study of its kind. The results of this study reveal that step mediated alloying occurs between Cu and Te with significant faceting of the surface. Two ordered CuTe alloy phases were observed for sub-monolayer Te coverages. The low coverage alloy exists for Te coverages between 0.18 ML and 0.45 ML and has a chiral unit cell. The high coverage alloy exists for Te coverages between 0.45 ML and 1.5 ML and has an achiral unit cell. The atomic positions of these surface alloys are tentatively interpreted from the scanning tunnelling microscopy (STM) images. In contrast to the thin film experiments in chapters 3-5, chapter 6 describes a study of TaS3 nanoribbons. These studies reveal that the nanoribbons have a distinct core-shell type structure. Characterisation with surface science techniques shows that the shell is nonstoichiometric and amorphous while TEM shows a crystalline core to the material. Interestingly, the TaS3 are observed to be unstable when interfaced on a Au substrate, with the shell persistently losing S to the substrate, which have potential implications in device integration.

Etude experimentale de l'instabilite de la valence des atomes de tm en fonction de la concentration dans les composes du titre. On mesure la valence des atomes de tm par susceptibilite magnetique et absorption rx, et on met en evidence une divergence de resultats par ces deux methodes. On met en evidence une transition metal-semiconducteur pour x=0,84, un ordre antiferromagnetique a 1,5 k, une magnetoresistivite negative. L'etude des sulfures amene a interpreter les proprietes de transport dans le domaine y >ou= 0,08 pour t>15 k par l'effet kondo

The complete understanding of a clusters electronic structure, the primary mechanisms for its properties and stabilization is necessary in order to functionalize them for use as building blocks within novel materials. First principle theoretical studies have been carried out upon the electronic properties of CrxTey (x = 1 – 6, y = 0 – 8, x + y ≤ 14), as well as for the larger triethylphosphine (PEt3) ligated cluster system of Cr6Te8(PEt3)6. Together, we aim to use the information garnered from the smaller clusters to address the underlying behavior of the ligated Cr6Te8(PEt3)6. Additionally, the properties of this larger cluster will be used to further understand its role when paired with C60 within the binary cluster assembled material. The stability and macroscopic properties of the Cr6Te8(PEt3)6 cluster, have been found to be sensitive to type of passivating ligand. As will be shown, the ground state structures of Crn atoms are sensitive to both the number and position of bonded Te atoms. Moreover, that this sensitivity carries over into larger cluster sizes, and at several size intervals produces clusters with high magnetization. To this, we add the investigation into the manipulation of the Cr6Te8 cluster geometry and its properties through various ligands, such as PH3, CO, and CN. It will show, that in altering these ligands there is a modification to the clusters valence shell count, which in turn alters its ionization potential and electron affinity. Additionally, although the ionization potential and electron affinity have changed for the Cr6Te8(PEt3)6 cluster, it has been found that its high magnetization does not.

Etudes effectuees en vue de preciser la nature des electrons qui participent ala conduction et au magnetisme de ces composes. Les composes, caracterises par la presence d'amas tetraedriques des ions metalliques mo et nb dans les bas etats d'oxydation, revelent des proprietes de magnetisme itinerant repondant au modele de stoner-wohlfarth avec les densites d'etats les plus elevees observees dans des composes intermetalliques 3d ou 4d. La rpe a confirme que les raies observees correspondant aux ions metalliques dans un etat s = 3/2 (ions mo**(3+) et nb**(3+)); leur elargissement est d'origine dipolaire retrecie par echange et les valeurs des integrales d'echange obtenues sont en bon accord avec celles obtenues a partir des temperatures de curie

Ziel dieser Arbeit war es, Beziehungen zwischen den kristallchemischen Eigenschaften und dem beobachteten anomalen Verhalten im spezifischen elektrischen Widerstand (nicht-magnetischer Kondo-Effekt) aufzuzeigen und zusammenhängend zu interpretieren. Verbindungen, an denen dieser Effekt beobachtet wurde, werden aus einem Übergangs-, oder Actinidmetall mit je einem Vertreter der 15. (Pniktogene) und 16. Gruppe (Chalkogene) des Periodensystems gebildet und kristallisieren im PbFCl-Strukturtyp. Da zu ternären Actinidmetall-Pniktid-Chalkogeniden (z.B. ThAsSe, UPS) nur sehr wenige chemische und kristallographische Informationen existieren, wurden in dieser Arbeit umfassende Untersuchungen zur Kristallchemie ternärer Phasen aus den Systemen M-Pn-Q (M = Zr, Hf, La-Ce; Pn = As, Sb; Q = Se, Te durchgeführt. Der Schwerpunkt lag dabei auf der strukturellen Lokalisierung der beobachteten Widerstandsanomalie und der Erarbeitung chemisch-physikalischer Eigenschaftsbeziehungen. Die Darstellung der untersuchten ternären Phasen in Form von Einkristallen gelang über exothermen Chemischen Transport mit Jod. Da die erhaltenen Kristalle bis zu mehreren Millimetern groß sind, konnten an ein und demselben Kristallindividuum sowohl die stoffliche Charakterisierung (EDXS, WDXS, ICP-OES, LA-ICP-MS, CIC) und die strukturelle Charakterisierung, als auch die Messung der physikalischen Eigenschaften erfolgen. Es konnte u.a. gezeigt werden, dass ZrAs1,4Se0,5 und HfAs1,7Se0,2 ein ähnlich ungewöhnliches Verhalten im temperaturabhängigen elektrischen Widerstand zeigen, welches bereits an Thorium-Arsenid-Seleniden und Uran-Phosphid-Sulfiden beobachtet wurde. Desweiteren gelang es den beobachteten Verlauf im elektrischen Widerstand, mit seinem Minium bei etwa T = 15 K, auf intrinsische strukturelle Merkmale in der anionischen Arsen-Teilstruktur zurückzuführen
The aim of this work was, to evaluate and interpret a relationship between the crystal-chemical properties and the observed unusual behavior in the electrical resistivity (non-magnetic Kondo-effect). Compounds, which show such an effect, are formed by a transition- or actinide-metal with both a group 15 element and a group 16 element of the periodic table. All these compounds crystallizing in the PbFCl type of structure. Because of less crystallographic and chemical information about actinide-metal-pnictide-chalcogenides (i.e. ThAsSe, UPS), intensive investigation were made concerning the crystal-chemistry of ternary phases of the systems M-Pn-Q (M = Zr, Hf, La-Ce; Pn = As, Sb; Q = Se, Te. Our studies were focused on the structurally localization of the observed anomaly in the electrical resistivity and the evaluation of chemical-physical relations of properties. The synthesis of the investigated ternary phases was realized by exothermically Chemical Transport with iodine as transport agent. The dimension of the synthesized crystals allowed a chemical (EDXS, WDXS, ICP-OES, LA-ICP-MS, CIC) and structurally characterization, as well as a determination of the physical properties on one large single crystal. It could be shown, that ZrAs1,4Se0,5 and HfAs1,7Se0,2 reveal a similar unusual behavior in the temperature dependent electrical resistivity, as it was observed in thorium-arsenide-selenides and uranium-phosphide-sulphides. In conclusion, the non-magnetic Kondo-effect, which was found in the low-temperature range (about 15 K), arises from structurally features of the anionic sublattice with arsenic

In dieser Arbeit wird das Wachstum von dünnen quasi-kristallinen Ge-Sb-Te (GST) Schichten mittels Molekularstrahlepitaxie demonstriert, die zu einer geordneten Konfiguration von intrinsischen Kristallgitterfehlstellen führen. Es wird gezeigt, wie es eine Strukturanalyse basierend auf Röntgenstrahlbeugungssimulationen, Dichtefunktionaltheorie und Transmissionselektronenmikroskopie ermöglicht, eine eindeutige Beurteilung der Kristallgitterlückenanordnung in den GST-Proben vorzunehmen. Das Verständnis für die Ordnungsprozesse der Gitterfehlstellen erlaubt eine gezielte Einstellung des Ordnungsgrades selbst, der mit der Zusammensetzung und der Kristallphase des Materials in Zusammenhang steht. Auf dieser Basis wurde ein Phasendiagramm mit verschiedenen Wachstumsfenstern für GST erstellt. Des Weiteren wird gezeigt, dass man eine hohe Ordnung der Gitterfehlstellen in GST auch durch Ausheizprozesse und anhand von Femtosekunden-gepulster Laserkristallisation von amorphem Material erhält, das zuvor auf einem als Kristallisationsgrundlage dienenden Substrat abgeschiedenen wurde. Diese Erkenntnis ist bemerkenswert, da sie zeigt, dass sich kristalline GST Schichten mit geordneten Kristallgitterlücken durch verschiedene Herstellungsprozesse realisieren lassen. Darüber hinaus wurde das Wachstum von GeTe/Sb2Te3 Übergittern durchgeführt, deren Struktur die von GST mit geordneten Gitterfehlstellen widerspiegelt. Die Möglichkeit den Grad der Gitterfehlstellenordung in GST gezielt zu manipulieren wurde mit einer Studie der Transporteigenschaften kombiniert. Die Anwendung von großflächigen Charakterisierungsmethoden wie XRD, Raman und IR-Spektroskopie, erlaubte die Bestimmung der Phase und des Fehlstellenordnungsgrades von GST und zeigte eindeutig, dass die Fehlstellenordnung für den Metall-Isolator-Übergang (MIT) verantwortlich ist. Insbesondere wird durch das Vergleichen von XRD-Messungen mit elektrischen Messungen gezeigt, dass der Übergang von isolierend zu leitend erfolgt, sobald eine Ordnung der Kristallgitterlücken einsetzt. Dieses Phänomen tritt in der kubischen Kristallphase auf, wenn Gitterfehlstellen in GST von einem ungeordneten in einen geordneten Zustand übergehen. Im zweiten Teil des Kapitels wird eine Kombination aus FIR- und Raman-Spektroskopie zur Untersuchung der Vibrationsmoden und des Ladungsträgerverhaltens in der amorphen und der kristallinen Phase angewendet, um Aktivierungsenergien für die Elektronenleitung, sowohl für die kubische, als auch für die trigonale Kristallphase von GST zu bestimmen. Hier ist es wichtig zu erwähnen, dass, in Übereinstimmung mit Ergebnissen aus anderen Untersuchungen, das Auftauchen eines MIT beim Übergang zwischen der ungeordneten und der geordneten kubischen Phase beobachtet wurde. Schlussendlich wurden verschiedene sogenannte Pump/Probe Technik, bei der man das Material mit dem Laser anregt und die Röntgenstrahlung oder Terahertz (THz)-spektroskopie als Sonde nutzt, angewandt. Dies dient um ultra-schnelle Dynamiken zu erfassen, die zum Verständnis der Umschaltmechanismen nötig sind. Die Empfindlichkeit der THz-Messungen hinsichtlich der Leitfähigkeit, sowohl in GST, als auch in GeTe/Sb2Te3 Übergittern zeigte, dass die nicht-thermische Natur der Übergitterumschaltprozesse mit Grenzflächeneffekten zusammenhängt und . Der Ablauf wird mit beeindruckender geringer Laser-Fluenz erreicht. Dieses Ergebnis stimmt mit Berichten aus der Literatur überein, in denen ein Kristall-zu Kristallwechsel von auf Übergittern basierenden Speicherzellen für effizienter gehalten wird als GST Schmelzen, was zu einen ultra-schwachen Energieverbrauch führt.
The growth by molecular beam epitaxy of Ge-Sb-Te (GST) alloys resulting in quasi-single-crystalline films with ordered configuration of intrinsic vacancies is demonstrated. It is shown how a structural characterization based on transmission electron microscopy, X-ray diffraction and density functional theory, allowed to unequivocally assess the vacancy ordering in GST samples, which was so far only predicted. The understanding of the ordering process enabled the realization of a fine tuning of the ordering degree itself, which is linked to composition and crystalline phase. A phase diagram with the different growth windows for GST is obtained. High degree of vacancy ordering in GST is also obtained through annealing and via femtosecond-pulsed laser crystallization of amorphous material deposited on a crystalline substrate, which acts as a template for the crystallization. This finding is remarkable as it demonstrates that it is possible to create a crystalline GST with ordered vacancies by using different fabrication procedures. Growth and structural characterization of GeTe/Sb2Te3 superlattices is also obtained. Their structure resembles that of ordered GST, with exception of the Sb and Ge layers stacking sequence. The possibility to tune the degree of vacancy ordering in GST has been combined with a study of its transport properties. Employing global characterization methods such as XRD, Raman and Far-Infrared spectroscopy, the phase and ordering degree of the GST was assessed, and unequivocally demonstrated that vacancy ordering in GST drives the metal-insulator transition (MIT). In particular, first it is shown that by comparing electrical measurements to XRD, the transition from insulating to metallic behavior is obtained as soon as vacancies start to order. This phenomenon occurs within the cubic phase, when GST evolves from disordered to ordered. In the second part of the chapter, a combination of Far-Infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline phases, enabling to extract activation energies for the electron conduction for both cubic and trigonal GST phases. Most important, a MIT is clearly identified to occur at the onset of the transition between the disordered and the ordered cubic phase, consistently with the electrical study. Finally, pump/probe schemes based on optical-pump/X-ray absorption and Terahertz (THz) spectroscopy-probes have been employed to access ultrafast dynamics necessary for the understanding of switching mechanisms. The sensitivity of THz-probe to conductivity in both GST and GeTe/Sb2Te3 superlattices showed that the non-thermal nature of switching in superlattices is related to interface effects, and can be triggered by employing up to one order less laser fluences if compared to GST. Such result agrees with literature, in which a crystal to crystal switching of superlattice based memory cells is expected to be more efficient than GST melting, therefore enabling ultra-low energy consumption.

The ever increasing demand for energy due to over consumption of non-renewable fossil fuels has emphasized the need for alternate, sustainable and efficient energy conversion and storage systems. In this direction, electrochemical energy conversion and storage systems involving various fundamental electrochemical redox processes such as hydrogen evolution (HER), oxygen reduction (ORR), oxygen evolution (OER), hydrogen oxidation (HOR) reactions and others become highly important. Electrocatalysts are often used to accelerate the kinetics of these reactions. Platinum (Pt), ruthenium oxide and iridium oxide (RuO2 and IrO2) are known to be the state of the art catalysts for several of these reactions due to favouarable density of states (DOS) near the Fermi level, binding energy with the reactant species, chemical inertness etc. Apart from HER, OER and ORR, chlorine evolution reaction (Cl-ER) is another industrially important reaction associated with water purification, disinfection, bleaching, chemical weapons and pharmaceuticals. Dimensionally stable anodes (RuO2/IrO2 mixed with TiO2 on Ti) are the most commonly used catalysts for this process. Issues related to surface poisoning, corrosion and cost of the catalysts, in addition to selectivity and specificity towards a particular reaction are various aspects to be addressed. For example, Pt is not very specific for ORR in presence of methanol in addition to high cost and corrosion in certain media. On the other hand, DSA can efficiently catalyze both OER and Cl-ER, and hence there is overlap of the two processes in the potential range available. There is an on going search for efficient, cost-effective, stable catalysts that possess high specificity for a particular redox reaction. Towards this goal, the present study explores certain layered (phospho)chalcogenides for catalyzing HER, ORR, OER and Cl-ER. The present thesis is structured in two parts, where the first part explores the multi-functional catalytic aspects of new classes of compounds based on layered transition metal mixed chalcogenides (MoS2(1-x)Se2x) and ternary phosphochalcogenides (FePS3, FePSe3 and MoPS). In addition, lithium insertion and desinsertion has been studied with the aim of using the layered materials for rechargeable batteries. The second part of the thesis explores organic electrode materials with active carbonyl groups such as rufigallol, polydihydroxyanthrachene succinic anhydride (PDASA) as battery electrodes. Additionally, covalently functionalized transition metal phthalocyanines with reduced graphene oxide are studied as counter electrodes in dye sensitized solar cells (DSSCs). MoS2(1-x)Se2x (x = 0 to 1) compositions are solid solutions of MoS2 and MoSe2 in different ratios. They crystallize in hexagonal structure with space group P63/mmc (D6h4) having Mo in trigonal prismatic coordination like the pristine counterparts. X-Ray diffraction studies reveal that Vegard’s law (figure 1a) is followed and hence complete miscibility of MoS2 and MoSe2 is established. MoS2(1-x)Se2x (x = 0 to 1) are layered in nature and the layers are held together by long range, weak van der Waal’s forces. This gives us the flexibility of exfoliation to produce corresponding few-layer materials (figure 1b). Figure 1. (a) Variation of lattice parameter corresponding to (002) reflection of MoS2(1-x)Se2x with different x values. (b) Scanning electron micrograph of few-layer MoS2(1-x)Se2x (x = 0.5). The electrocatalytic activity of the few-layer sulphoselenides have been studied towards HER in aqueous 0.5 M H2SO4 and towards Cl-ER in 3 M aqueous NaCl (pH = 3) solution. The mixed chalcogenides exhibit very good activities for both HER and Cl-ER as compared to the activity of their pristine counter parts (i.e. MoS2 and MoSe2) (figures 2a and 2b). Electrocatalytic activity on different compositions reveal that MoS1.0Se1.0 exhibits the maximum activity. Additionally, it has been observed that MoS1.0Se1.0 shows high specificity for Cl-ER with negligible interference of OER. Figure 2. Voltammetric data for (a) hydrogen evolution reaction (in 0.5 M aqueous H2SO4) and (b) chlorine evolution reaction (in 3 M aqueous NaCl solution, pH = 3) on MoS2(1-x)Se2x (x = 0, 0.5, 1). Figure 3. (a) XRD pattern of MoS2(1-x)Se2x (x = 0.5) electrode after a cycle of Li insersion and deinsersion (red) along with as-synthesized material (black) (b) Cycling behaviour of rGO supported (black) and pristine (red) MoS2(1-x)Se2x (x = 0.5) as electrode in rechargeable lithium-ion battery. The equiatomic MoS1.0Se1.0 has also been studied as an anode material for rechargeable lithium batteries. The cyclic voltammogram and characterization after charge-discharge cycle (figure 3a) indicate intercalation of Li with in the layers followed by conversion type formation of Li-S and Li-Se type compounds. The pristine material shows continuous capacity fading while the composites of sulphoselenides functionalized with conducting carbon supports such as rGO, MWCNT, super P carbon, toray carbon show marked improvement in capacity as well as cycling behavior. The rGO functionalized MoS1.0Se1.0 reveals ~1000 mAh/g of stable specific discharge capacity for 500 cycles (figure 3b). In the next two chapters, new class of transition metal-based layered materials FePS3 and FePSe3, containing both P and chalcogen (S and Se) is indroduced for electrocatalysis. FePS3 crystallizes in monoclinic symmetry with an indirect band gap of ~1.55 eV while FePSe3 possesses rhombohedral crystal structure with comparatively low band gap (~1.3 eV) as shown in figure 4a. The FePS3 and FePSe3 have been exfoliated as has been done for MoS1.0Se1.0 (liquid exfoliation method) using acetone as the solvent. Stable colloids with few-layer nanosheets having lamellar morphology and lateral sizes of ~100 to 200 nm are obtained. Electrical characterization indicates that they are semiconducting and the conductivity of the Se analogue is ~50 times higher than that of the S analogue (figure 4b). Figure 4. (a) Catholuminescence of FePX3 ( X = S and Se) reveals the band gap of the material. Band gap of the S analogue is 1.52 eV and that of the Se analogue is 1.33 eV (b) Resistivity of FePX3 ( X = S and Se) as a function of temperature. The tri-functional electrocatalytic activities on rGO-few layer FePX3 (X = S and Se) have been evaluated for HER over a wide pH range (0.5 M H2SO4, 0.5 M KOH, phosphate Figure 5. Catalytic activity of rGO-few-layer FePX3 (X = S, Se) towards HER in (a) aqueous 0.5 M H2SO4 and (b) 3.5 wt % NaCl solutions. (c) ORR activity of the catalysts in oxygen saturated 0.5 M KOH (d) OER behaviour on the catalysts in 0.5 M KOH at a rotation speed of 1600 rpm. buffer, pH 7 and 3.5 % NaCl), ORR and OER in alkaline media (0.5 M KOH). The studies clearly reveal that both rGO-FePS3 and rGO-FePSe3 exhibit excellent HER activity in acidic media (figure 5a) with high stability. The HER studies in 3.5 wt % aqueous NaCl solution (figure 5b) suggests that the catalysts are effective in evolving hydrogen from sea-water environment. Studies on ORR activity (figure 5c) indicate that the rGO composites of both S and Se analogues follow 4-electron pathways to produce water as the final product. They are also found to be highly methanol tolerant. In the case of OER (figure 5d), XPS characterization of the electrodes after the voltammetric studies reveals the presence of very thin layer of Fe2O3 (not detectable by XRD). All the three reactions (HER, ORR and OER) catalyzed by the Se analogue are better than the S analogue (figure 5). This could be due to the low band gap and high conductivity of FePSe3 as compared to FePS3. The over potential to achieve 10 mAcm-2 current density is ~108 mV for rGO-few-layer FePS3 catalyst where in the case of rGO-few layer FePSe3, it is ~97 mV (table 1). Table 1. Catalytic activities of rGO-few layer FePS3 and rGO-few layer FePSe3 towards HER, ORR and OER. Reaction studied rGO-FePS3 rGO-FePSe3 HER (η @ 10mAcm-2) ~108 mV ~97 mV ORR (peak potential) ~0.81 V ~0.87 V OER (η @ 10mAcm-2) ~470 mV ~430 mV It is likely that there is a strong interaction between FePX3 (metal d-orbital) and rGO, as observed from the downward shift of Fe 2p peak in high resolution XPS studies. This interaction may extend the density of states of metal d-orbitals thereby improving the catalytic activities. The next chapter deals with molybdenum-based phosphosulphide compound (MoPS). Molybdenum-based phosphide catalysts have been explored recently as excellent catalysts for various electrochemical reactions such as HER. It is expected that the catalyst containing both S and P will show positive effects on catalytic activities due to the synergy between S and P. In the present study, P incorporated MoS2 is studied towards HER. The XRD pattern of the as-synthesized crystal suggests the presence of mixed phase of MoS2, MoP2 and MoP while the elemental mapping in microscopy indicates the ratio of Mo, P and S to be 1:1:1. The electrochemical HER in 0.5 M H2SO4 indicates that the activity is improved drastically as compared to bulk and few-layer MoS2. The next section explores the use of different organic electrode materials possessing active carbonyl groups for Li-storage studies. The advantage of the use of carbonyl-based compounds lies in the high reversible activity towards Li ion insersion and de-insersion. Rufigallol (figure 6a) exhibits very stable capacity of ~200 mAh/g (at C/20 rate) upto 500 Figure 6. (a) and (c) Schematic representation of rufigallol and poly-dihydroanthracene succinic anhydride (PDASA) respectively. (b) and (d) Cyclic behaviour of rufigallol (at C/20 rate) and PDASA (at 20 mAg-1 current rate) in Li-storage devices. (e) and (f) represent the coulombic efficiency of rufigallol (at C/20 rate) and PDASA (at 20 mAg-1 current rate) as a function of number of cycles. cycles along (figure 6b) and with very good rate capability. A triptycene-based mesoporous polymer, PDASA (figure 6c) is introduced and explored as efficient electrode material for Li-storage. PDASA exhibits very high capacity of ~1000 mAh/g at a current rate of 50 mA/g upto 1000 cycles (figure 6d). Even at very high current rates (3A/g) excellent cyclability is observed. The mechanistic details of lithium uptake and release are studied using various spectroscopic techniques. In both the cases the coulombic efficiency observed is ~80 to 90 % (figures 6e and f). Figure 7. (a) Digital photograph of the dye sensitized solar cell with rGO-Co-TAPc counter electrode. (b) Photoconversion efficiency of DSSCs with different counter electrodes as mentioned in the figure. (c) Photo conversion efficiency of Pt and rGO-Co-TAPc based DSSCs as function of storage time. (d) Schematic illustration of DSSC wherein the energy level of the counter electrodes and electrolyte are shown for different M-TAPcs. In a slightly different direction, metal phthalocyanine - rGO composites (rGO-M-TAPc; M = Co, Zn, Fe) have been explored as counter electrodes in DSSC. Figure 7a depicts the digital image of a DSSC constructed using rGO-Co-TAPc as the counter electrode. It has been observed that rGO-cobalt tetraamino phthalocyanine (rGO-Co-TAPc) counter electrode exhibits ~6.6 % of solar conversion efficiency (figure 7b) and is close to that of standard DSSC (Pt counter electrode) under identical experimental conditions and are highly stable (figure 7c). Other metal phthalocyanines show less efficiency and is analysed based on the relative positions of HOMO energy levels of the materials and the energy level of the redox system (I-/I3- system) as given in figure 7d. The thesis contains eight chapters on aspects discussed above along with summary and future perspectives given at the end. It is devided into various chapters in two sections, one comprising inorganic chalcogenide-based electrocatalysts and another comprising organic electrode materials. Appendix I discusses the Na-storage behaviour of MoS1.0Se1.0 and appendix II describes the Li-storage behaviour of rGO functionalized benzoquinone and diamino anthraquinone electrode materials.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, October, 2015.
Harvesting renewable energy and the miniaturisation of electronic components are among the major challenges of the 21st century. Transition metal chalcogenides (TMC) have interesting properties that are promising in meeting these challenges. It is therefore important to conduct a systematic theoretical study of the structural, electronic and optical properties of the transition metal chalcogenides as possible components of low dimensional transistors or as solar-energy harvesters. In this work, we present the detailed theoretical investigation of the structural, electronic and optical properties of transition metal chalcogenides MyXz, (where M = Hf, Zr, Tc or Re), (X = S, Se and Te), y and z are integers. The structural properties of TMCs were studied using energy-volume relationship (equation of states (EOS)), equilibrium structural lattice parameters, formation and cohesive energies were extracted from the EOS. Mechanical stability test based on elastic constants and phonon dispersion relation were carried out to determine the strengths of the TMCs against mechanical distortions. The most stable structural congurations were used to investigate electronic properties through partial density of states (PDOS) and band structure analysis. Optical properties (absorption coe cients, refractivity, re ectivity) of some of the TMCs were then computed. Our computations of the structural, electronic and the optical properties were based on density functional theory (DFT). Projector-Augmented wave (PAW) was used to mimic electron-ion interactions and generalised-gradient approximation was used in the exchange correlation functional. Van der Waal's correction terms proposed by Grimme (DFT-D2), Lundqvist and Langreth (vdW-DF) and Tkatchenko- Sche er (vdW-TS) were used to account for long range dispersion forces in addition to PBE and its modi ed version for solids PBEsol. Optical properties were investigated at the many body (GW) and Bethe-Salpeter equation (BSE) levels of approximations. Our results obtained are discussed within the theoretical frame work and compared with experimental and previous theoretical results where available.

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

Thermoelectric energy (TE) conversion can be used to create electricity from temperature gradients. Hence power can be generated from waste heat using TE materials, e.g. from the exhaust in automotives. This power in turn may lead to a reduction of gas consumption by reducing the alternator load on the engine. Because of the increasing demand and limited availability of energy sources, there is strong and renewed interest in advancing thermoelectric materials. Past research shows that the best TE materials are narrow band gap semiconductors composed of heavy elements, exhibiting a large Seebeck coefficient, S, combined with high electrical conductivity, σ, and low thermal conductivity, κ. Various research projects have been attempted during the past four years of my Ph.D. studies. These include the synthesis, crystal structure studies, electronic structure calculations and thermoelectric properties of transition metal oxides and thallium main group chalcogenides. Because of the good thermal stability, lack of sensitivity to the air, and non-toxicity, transition metal oxides are potential candidates for commercial thermoelectric applications. During the investigation of oxides for thermoelectric application, several interesting features of different transition metal oxides have been discovered: 1. A new quaternary layered transition-metal oxide, Na2Cu2TeO6, has been synthesized under air using stoichiometric mixtures of Na2CO3, CuO and TeO2. Na2Cu2TeO6 crystallizes in a new structure type, monoclinic space group C2/m with a = 5.7059(6) Å, b = 8.6751(9) Å, c = 5.9380(6) Å,  = 113.740(2)°, V = 269.05(5) Å3 and Z = 2, as determined by single crystal X-ray diffraction. The structure is composed of[Cu2TeO6] layers with the Na atoms located in the octahedral voids between the layers. Na2Cu2TeO6 is a green nonmetallic compound, in agreement with the electronic structure calculation and electrical resistance measurement. 2. An n-type narrow band gap semiconductor, LaMo8O14, exhibiting the high Seebeck coefficient of -94 μVK-1 at room temperature has been investigated. 3. Pb0.69Mo4O6 with a new modulated structure and stoichiometry was determined from single-crystal X-ray diffraction data. The compound crystallizes in the tetragonal super space group, P4/mbm(00g)00ss, with a = 9.6112(3) Å, c = 2.8411(1) Å, q = 0.25c*, which is different from the previously reported structure. As for the research of thermoelectric properties of thallium main group chalcogenides, three new ternary thallium selenides, Tl2.35Sb8.65Se14, Tl1.97Sb8.03Se13 and Tl2.04Bi7.96Se13, have been discovered. All three compounds crystallize in the same space group P21/m with different cell parameters, and in part different Wyckoff sites, hence different structure types. The three selenides with similar structures are composed of distorted edge-sharing (Sb,Bi)Se6 octahedra, while the distorted Tl/(Sb, Bi) sites are coordinated by 8 - 9 Se atoms. Electronic structure calculations and physical property measurements reveal they are semiconductors with high Seebeck coefficient but low electrical conductivity, and therefore not good thermoelectrics. On the other hand, our transport property measurements on the unoptimized Tl2SnTe3 sample show interesting thermoelectric properties of this known compound. Advanced thermoelectrics are dominated by antimonides and tellurides so far. The structures of the tellurides are mostly composed of NaCl-related motifs, hence do not contain any Te–Te bonds. All of the antimonide structures containing Sb–Sb bonds of various lengths are much more complex. The Sb atom substructures are Sb24– pairs in β-Zn4Sb3, linear Sb37– units in Yb14MnSb11, planar Sb44– rectangles in the skutterudites, e.g., LaFe3CoSb12, and Sb8 cubes interconnected via short Sb–Sb bonds to a three-dimensional network in Mo3Sb5Te2. The results of electronic structure calculations suggested that these interactions have a significant impact on the band gap size as well as on the effective mass around the Fermi level, which represent vital criteria for advanced thermoelectrics. The crystal structure and electronic structure investigation for the unique T net planar Sb–Sb interactions in Hf5Sb9 will be also presented, although Hf5Sb9 is metallic compound with poor thermoelectric performances.

A new approach to the discovery of high absorbing semiconductors for solar cells was taken by working under a set of design principles and taking a systemic methodology. Three transition metal chalcogenides at varying states of development were evaluated within this framework. Iron pyrite (FeS���) is well known to demonstrate excellent absorption, but the coexistence with metallic iron sulfides was found to disrupt its semiconducting properties. Manganese diselenide (MnSe���), a material heavily researched for its magnetic properties, is proposed as a high absorbing alternative to iron pyrite that lacks destructive impurity phases. For the first time, a MnSe��� thin film was synthesized and the optical properties were characterized. Finally, CuTaS���, a known but never characterized material, is also proposed as a high absorbing semiconductor based on the design principles and experimental results.
Graduation date: 2013

Atomically thin 2D layered semiconductor materials such as Transition Metal Di-Chalcogenides (TMDCs) have great potential for use as flexible, ultra-thin photovoltaic materials in solar cells due to their favorable photon absorption and electronic transport properties. In this dissertation, the electronic properties, such as band structure and bandgap, and optical absorption properties of a TMDC known as Tungsten Disulfide (WS2) were obtained from Density Functional Theory (DFT) calculations to design conventional and unconventional solar cells. Using these properties, a 1 μm thick heterojunction solar cell based on monolayer and bulk WS2 together with amorphous silicon (a-Si) was modeled using numerical calculations and simulations. The maximum efficiency of this cell is 23.3% with Voc = 0.84 V and Jsc = 33.5 mA/cm2 under the AM1.5G terrestrial solar spectrum. Next, a similar but even thinner solar cell with a thickness of 200 nm, together with a light trapping structure and an anti-reflection coating layer, was modeled under the AM0 space solar spectrum; similar device performance efficiencies around 21-23% were obtained. The performance of these solar cell models is comparable to many commercial cells in both terrestrial and space photovoltaics. As conventional photovoltaics approach the Shockley-Queisser limit, the need for unconventional materials and approaches has become more apparent. Hybrid alloys of TMDCs exhibit tunable direct bandgaps and significant dipole moments. Dark state protection induced by dipole-dipole interactions forms new bright and dark states in the conduction band that reduce radiative recombination and enhance photon-to-electron conversion, leading to significantly higher photocurrents. In our work, current enhancement of up to 35% has been demonstrated by modeling dark state protection in a solar cell composed of Tungsten Diselenide (WSe2) and Tungsten Sulfo-Selenide (WSeS), with the potential to exceed the Shockley-Queisser limit under ideal conditions.

In recent years there is an increasing awareness of the importance of chiral phosphorus ligands in transition metal organometallic chemistry because of the utility of such complexes in homogeneous catalytic reactions. This thesis deals with synthetic, spectroscopic and X-ray crystallographic studies on ruthenium complexes of chiral and achiral P-N-P type ligands, known as "diphosphazanes", with emphasis on ruthenium carbonyl clusters. Several ruthenium carbonyl clusters have been synthesized and characterized by elemental analyses, ER and NMR (lH, nC and 3lP) spectroscopic data. In several instances, the molecular structures of the clusters have been confirmed by single crystal X-ray diffraction studies. Chapter 1 provides a brief overview of various types of chiral phosphorus ligands and general synthetic routes to diphosphazanes. A brief review of the transition metal chemistry of diphosphazanes and diphosphazane chalcogenides (published since 1994) is presented A review of the literature on the carbonyl clusters of the group-8 transition metals (Fe, Ru, Os) bearing mono- and diphosphines is also included in this chapter The scope and aim of the present investigation is outlined at the end of this chapter. Chapter 2 provides the results obtained in the present investigation and a detailed discussion of the spectroscopic and crystallographic data. The essential feature of the work is summarized at the end of the chapter. Chapter 3 gives a detailed account of the experimental procedure for the synthesis of the compounds and spectroscopic and analytical measurements. The experimental details of X-ray structure determination are also given in this chapter. To save space, the coordinates of the H-atoms and the calculated and observed structure factor tables are not included. In some cases, reference to CCDC deposition number is included. The references of the literature are compiled at the end of the thesis and are indicated in the text by appropriate numbers appearing as superscripts. The compounds synthesized in the present study are represented by bold Arabic numerals and are listed in Appendix I. The abbreviations employed in the thesis conform to those generally used in Chemical Abstracts.

博士
國立成功大學
材料科學及工程學系碩博士班
95
Phosphor materials have been widely used in solid state lighting, displays and medical imaging by doping with transition or rare-earth metals. Here we studied the influence of transition metals (Cu, Mn, Cd) on the structure and optical properties of electroluminescent ZnS powders, the preparation of two-band emission of ZnS/ZnO:Mn film for the white light usage, and the Raman properties of ZnS, CdSe, CdS and Zn1-xCdxS nanoparticles (NPs). A series of ZnS:Cu,Cl powders with Cu additions ranging from 40 to 5000ppm were studied by firing at 750-1050oC for 2h in the atmosphere of 3%H2/Ar. X-ray diffraction (XRD) and Raman analyses showed that ZnS:Cu,Cl samples with Cu additions of ≥400ppm exhibited a structure transformation from hexagonal to cubic, which was supposed to result from the Cu incorporation or CuxS precipitation (470cm-1). The red shift of the longitudinal optical (LO) mode in ZnS:Cu,Cl with the increased Cu suggested that Cu ions entered ZnS lattice interstitially and created a tensile strain. The whole series of ZnS:Cu,Cl samples showed significant photoluminescence (PL) in the region of 400-600nm; however, only the samples with Cu additions of ≥400ppm revealed measurable electroluminescence (EL). This difference was supposed to be a result of nano-sized CuxS precipitation in ZnS during the firing treatment, where CuxS acted as the electron emission source to induce the activation of luminescent centers. Red electroluminescent phosphor powders of ZnS:Cu,Mn,Cl and Zn1-xCdxS:Cu,Cl (x=0-0.9) were also prepared by a solid state reaction at 900oC for 2h in a reducing atmosphere. XRD showed that the structure of Zn1-xCdxS:Cu,Cl (Cu:800ppm) transformed from cubic to hexagonal as x increased to 0.2. Similarly, the addition of Mn preferred to the formation of hexagonal ZnS. This unavailability of CuxS to induce the cubic structure transformation in ZnS:Cu,Mn,Cl and Zn1-xCdxS:Cu,Cl may be due to the fact that Mn tends to bond to S in the hexagonal and Zn1-xCdxS stabilizes in a hexagonal structure. EL measurements revealed broad emission bands at 583nm and 614nm for ZnS:Cu,Mn,Cl and Zn0.4Cd0.6S:Cu,Cl, respectively. ZnO:Mn capped with ZnS:Mn (ZnS/ZnO:Mn) phosphor layers were prepared by thermal sulfidation of ZnO:Mn films deposited on Si(100) substrate with the method of RF magnetron sputtering. The PL of the ZnS/ZnO:Mn layer showed a three-band emission, i.e. UV, blue (465nm) and orange (573nm) bands. The chromaticity coordinate of the two-component ZnS/ZnO:Mn phosphor layer, which was estimated from the peak and width of each emission band, showed the potential of producing white light emission. PL measurements revealed that ZnS:Mn powders fired with 1wt% ZnS NPs showed the optimal luminescence intensity when compared to those without or with more ZnS NPs (>1wt%). An appropriate amount of ZnS NPs (1wt%) acting as the flux in the firing process was inferred to avoid the inhomogeneous distribution of Mn2+ as well as the migration of excitation energy to quenching sites and therefore to result in the enhanced PL intensity. ZnS:Mn NPs (~3.1nm) of a cubic structure were prepared by a co-precipitation method and then dispersed with various weights (0.5-8g) into a solution containing ammonium hydroxide, ethanol and tetraethyl orthosilicate (TEOS) for surface coating. The optical properties of PL and diffuse reflection of the SiO2-coated ZnS:Mn powders were found to depend on the quantity of ZnS:Mn NPs in the coating solution. The increased PL intensity of ZnS:Mn at 590nm after SiO2 coating was proposed to result from the inhibition of excitation energy transfer to quenching centers or surface states based on the observation of the reduced absorption in the region of 370-450nm. CdSe, CdS and Zn1-xCdxS NPs were prepared by the TOP process and studied with a Raman spectrometer, revealing very homogeneous size distributions and comparable sizes with the transmission electron microscope (TEM) and UV-vis measurements.

In the last few years a renewed interest has reappeared in materials that were highly investigated in the 50s-70s, like manganese perovskites, spinel chalcogenides and vanadium oxides. The first two classes of materials are nowadays intensively studied due to the colossal magnetoresistance effect, which is the magnetoresistance associated with a ferromagnetic-paramagnetic transition. Vanadium oxides are known to form many compounds and most of them undergo metal-to-insulator phase transitions, with a high increase in the electrical conductivity (MIT). Many technological applications derive from the variation of the physical properties around the phase transition temperature. Although many efforts have been done in order to understand their electronic structures and to elucidate the MIT mechanisms, the vanadium oxides are still matter of debate in science.The present study has been performed in order to understand the electronic structure of these very intriguing materials. The role of different dopants that induce strong changes in the electronic and magnetic properties has been investigated making use of two spectroscopic techniques, namely X-ray photoelectron and X-ray emission spectroscopy.

碩士
國立臺灣大學
物理學研究所
88
Magneto-optical properties in several 3d transition metals, their compounds, and cerium chalcogenides have been studied by using the first principles spin-polarized relativistic linear muffin-tin orbit (SPR-LMTO) method or spin-polarized, orbital polarized relativistic linear muffin-tin orbit (SOPR-LMTO) method with local density approximation or generalized gradient approximation. The calculated Kerr rotation spectra in the FeAu, FePt, and CeTe systems are in good agreement with experiments. We also calculate magnetic moments and the magneto-crystalline anisotropy energy of these systems. We find that magneto-optical Kerr effect depends on the magnetization directions. Based on these results, we propose a new device of magneto-optical recording. In cerium chalcogenides, we use the exact formula to calculate the large Kerr rotation angles and get almost the same results.

In this work the electronic and magnetic structure of the crystals Sr2FeMoO6, Fe0.5Cu0.5Cr2S4, LuFe2O4 and the molecules FeStar, Mo72Fe30, W72Fe30 are investigated by means of X-ray spectroscopic techniques. These advanced materials exhibit very interesting properties like magnetoresistance or multiferroic behaviour. In case of the molecules they also could be used as spin model systems. A long standing issue concerning the investigation of these materials are contradicting results found for the magnetic and electronic state of the iron (Fe) ions present in these compounds. Therefore this work focuses on the Fe state of these materials in order to elucidate reasons for these problems. Thereby the experimental results are compared to multiplet simulations.

Technology scaling beyond Moore’s law demands cutting-edge solutions of the gate length scaling in sub-10 nm regime for low power high speed operations. Recently SOI technology has received considerable attention, however manufacturable solutions in sub-10 nm technologies are not yet known for future nanoelectronics. Therefore, to continue scalinginsub-10 nm region, new one(1D) and two dimensional(2D) “nano-materials” and engineering are expected to keep its pace. However, significant challenges must be overcome for nano-material properties in carrier transport to be useful in future silicon nanotechnology. Thus, it is very important to understand and modulate their electronic band structure and transport properties for low power nanoelectronics applications. This thesis tries to provide solutions for some problems in this area. In recent times, one dimensional Silicon nanowire has emerged as a building block for the next generation nano-electronic devices as it can accommodate multiple gate transistor architecture with excellent electrostatic integrity. However as the experimental study of various energy band parameters at the nanoscale regime is extremely challenging, usually one relies on the atomic level simulations, the results of which are at par with the experimental observations. Two such parameters are the band gap and effective mass, which are of pioneer importance for the understanding of the current transport mechanism. Although there exists a large number of empirical relations of the band gap in relaxed Silicon nanowire, however there is a growing demand for the development of a physics based analytical model to standardize different energy band parameters which particularly demands its application in TCAD software for predicting different electrical characteristics of novel devices and its strained counterpart to increase the device characteristics significantly without changing the device architecture. In the first part of this work reports the analytical modeling of energy band gap and electron transport effective mass of relaxed and strained Silicon nanowires in various crystallographic directions for future nanoelectronics. The technology scaling of gate length in beyond Moore’s law devices also demands the SOI body thickness, TSi0 which is essentially very challenging task in nano-device engineering. To overcome this circumstance, two dimensional crystals in atomically thin layered materials have found great attention for future nanolectronics device applications. Graphene, one layer of Graphite, is such 2D materials which have found potentiality in high speed nanoelectronics applications due to its several unique electronic properties. However, the zero band gap in pure Graphene makes it limited in switching device or transistor applications. Thus, opening and tailoring a band gap has become a highly pursued topic in recent graphene research. The second part of this work reports atomistic simulation based real and complex band structure properties Graphene-Boron nitride heterobilayer and Boron Nitride embedded Graphene nanoribbons which can improve the grapheme and its nanoribbon band structure properties without changing their originality. This part also reports the direct band-to-band tunneling phenomena through the complex band structures and their applications in tunnel field effect transistors(TFETs) which has emerged as a strong candidate for next generation low-stand by power(LSTP) applications due to its sub-60mV/dec Sub threshold slope(SS). As the direct band-to-band tunneling(BTBT) is improbable in Silicon(either its bulk or nanowire form), it is difficult to achieve superior TFET characteristics(i.e., very low SS and high ON cur-rent) from the Silicon TFETs. Whereas, it is explored that much high ON current and very low subthreshold slope in hybrid Graphene based TFET characteristics open a new prospect in future TFETs. The investigations on ultrathin body materials also call for a need to explore new 2D materials with finite band gap and their various nanostructures for future nanoelectronic applications in order to replace conventional Silicon. In the third part of this report, we have investigated the electronic and dielectric properties of semiconducting layered Transition metal dichalcogenide materials (MX2)(M=Mo, W;X =S, Se, Te) which has recently emerged as a promising alternative to Si as channel materials for CMOS devices. Five layered MX2 materials(exceptWTe2)in their 2D sheet and 1D nanoribbon forms are considered to study the real and imaginary band structure of thoseMX2 materials by atomistic simulations. Studying the complex dispersion properties, it is shown that all the five MX2 support direct BTBT in their monolayer sheet forms and offer an average ON current and subthresholdslopeof150 A/mand4 mV/dec, respectively. However, onlytheMoTe2 support direct BTBT in its nanoribbon form, whereas the direct BTBT possibility in MoS2 and MoSe2 depends on the number of layers or applied uniaxial strain. WX2 nanoribbons are shown to be non-suitable for efficient TFET operation. Reasonably high tunneling current in these MX2 shows that these can take advantage over conventional Silicon in future tunnel field effect transistor applications.