As the UN summit in September 2015 addressed with the Sustainable Development Goals (SDG), our planet faces great challenges.[1] Not only since then has the role of synthetic materials chemistry been discussed in this context.[2–16] This not only concerns the development of new materials with outstanding properties such as catalysts, materials for energy conversion, and cost-efficient energy converting and storage materials, but also a reduction of the energy consumption of established functional material syntheses. Therefore, new approaches addressing the three main categories to promote the potential for energy and resource efficiency have been proposed: lowering the temperature of the synthetic processes, improving the yield and purity of the materials, and reducing the amount of waste materials. In this context a number of low-temperature processes have been established, in which mainly solvents, i.e. amines and alcohols, are used in combination with previously synthesised precursors as the solubility of the starting materials limits their usability and most elements are not soluble in these solvents. Novel solvents like ionic liquids (ILs) showcase growing interest as they are considered particularly resource-efficient.[17,18] ILs are defined as liquids that are comprised entirely of ions, with melting points below 100 °C. Continuing on from the reported work in this field, this thesis focuses on investigating the ability of ILs in the syntheses for known chalcogenides at lower temperatures and the synthesis of new materials. The main focus lies on conversions with high atom economy, especially by starting from the elements and completely recycling the IL afterwards, and mechanistic studies elucidating the intermediate dissolved species. Furthermore, as imidazolium based ILs, and their derived LEWIS-acidic ILs [BMIm]Cl/nAlCl3 mixtures, have proven to be good crystallisation media in inorganic syntheses, and the class of mostly room-temperature liquid ILs (RTILs) based on phosphonium cations [P66614]Cl showcased the ability to dissolve red phosphorous, Pred, and the heavier chalcogens S, Se, and Te, this work mainly focuses on these two IL systems. This was also chosen as an in-depth understanding of the activation and resource-efficient synthesis of these chalcogenides has still not been established. As they are RTILs, they also made the characterisation of the reactive and dissolved species by liquid state NMR, Raman, UV/Vis spectroscopy and electrochemical characterisation possible. This expands the knowledge of which main group elements and ore-like starting materials can be used in ionothermal synthesis. As a starting point the thermoelectrically interesting materials class of tellurides is addressed. The under normal conditions hard to dissolve element tellurium readily dissolves in phosphonium ILs with the cations [P66614]+ and [P4444]+. In ILs with carboxylate anions a deep purple hue of the IL already occurred by dissolving tellurium at temperatures of 60 °C. Investigations on the solutions in the acetate ILs revealed the formation of tellurium anions (Ten)2– with chain lengths up to at least n = 5, which are in a dynamic equilibrium with each other. Since external influences could be excluded and no evidence of an IL reaction was found, disproportionation of the tellurium is the only possible dissolution mechanism. However, the spectroscopic detection of tellurium cations in these solutions is difficult, but the coexistence of tellurium cations, such as (Te4)2+ and (Te6)4+, and tellurium anions could be proven by cyclic voltammetry and electrodeposition experiments. DFT calculations indicate that electrostatic interactions with the ions of the ILs are sufficient in stabilizing both types of tellurium ions in solution.[19] In contrast, the acetate ILs show insufficient conversion in reactions of coin metals (Cu, Ag, Au) with tellurium to the corresponding tellurides, especially at low temperatures, however the chloride ILs successfully synthesise Cu2–xTe, CuTe, AuTe2 and Ag2Te. As the synthesis of the tellurides in neat ILs at temperatures down to 60 °C was only sufficient for the system Ag-Te, with a full conversion of the elements to Ag2Te, this was chosen as a model system for further investigations. Even at room temperature, a quantitative yield was achieved by using either 2 mol% of [P66614]Cl in dichloromethane or a planetary ball mill. The unexpected finding that phosphane-free [P66614]Cl also allows the quantitative synthesis of Ag2Te at 60°C implies an additional activation mechanism independent from the phosphane, which is yet unknown.[20] Subsequently, the manifoldly-used lighter chalcogen sulphur is tested for the synthesis of sulphides. Direct synthesis of binary sulphides of B, Bi, Ge, Mo, Cu, Au, Sn, In, Ti, V, Fe, Co, Ga, Ni, Al, Zn, and Sb in [P66614]Cl was tested at 100 °C, i.e. below the melting point of sulphur. Under these conditions, substantial sulphide formation occurred only for nickel (Ni3S4, Ni3S2, NiS) and copper (Cu2S, CuS). Sb showed no formation of crystalline sulphide, but after addition of EtOH, an orange material precipitated which was identified as amorphous metastibnite.[21] As generating these elements from their ores is highly energy consuming, direct dissolution experiments of the crystalline stibnite in [P66614][OAc] and Cl– were investigated and resulted in yellow solutions, from which the amorphous form can be precipitated upon exposure to EtOH air without any sign of decomposition of the IL. In particular, follow-up investigations were conducted on the solubility of Sb2S3 for follow-up chemistry in the LEWIS-acidic IL [BMIm]Cl · 4.7 AlCl3 at 160 °C which resulted in the formation of the novel chloride-terminated [Sb13S16Cl2]5+ quadruple-heterocubane cation-containing compound [Sb13S16Cl2][AlCl4]5.[21] Addition of CuCl in a slightly modified reaction resulted in the formation of the layered semiconductor Cu(Sb2S3)[AlCl4]. From this the AlCl3 can be leached by treatment with 0.1 molar hydrochloric acid, yielding a compound with the presumed composition Cu(Sb2S3)Cl.[22] As ILs showed to be able to activate elements that are insoluble in common solvents, and the formation of Sb2S3 from reactions mixtures of the elements raised the question of whether only the sulphur forms a mobile species or if antimony could additionally activated, the synthesis of binary antimony compounds directly from elements was explored as they are highly discussed as replacements for silicon-based semiconductors. Therefore the 12 elements Ti–Cu, Al, Ga, In, and Te, which are known to form binary compounds with Sb, were reacted with Sb in [P66614]Cl under inert conditions in a simple closed glass flask with vigorous stirring for 16 h at 200 °C. This resulted in the formation of NiSb, InSb, Cu2Sb and Sb2Te3. The applied reaction temperature is several hundred degrees below the temperatures required for solvent-free conversions. Compared to reactions based on diffusions in the solid state, reaction times are much shorter. The IL is not consumed and can be recycled. Since the reaction with Cu showed almost complete conversion, the influences of reaction time, temperature and medium were further investigated. In a diffusion experiment, Cu2Sb formed on the copper, which indicates that antimony forms mobile species in these ILs. These systematic studies hence deliver a contribution to how ILs can help in the synthesis of new materials and how they can make a difference in the synthesis of inorganic materials as well in the context of “GREEN CHEMISTRY”. This can help in developing a more educated choice/toolbox of IL systems for reducing energy costs by reducing the temperature from high temperature inorganic syntheses to syntheses near room-temperature by using the elements as starting materials, with a high atom economy for the synthesis of known and new materials.

<p dir="ltr">With climate change creating the need for renewable energy to replace fossil fuels, solar energy technologies are primed to dominate the energy sector. And while photovoltaics have improved significantly in recent decades, continued evolution of this technology requires research into new fabrication techniques and new materials. The solution processing of metal chalcogenide semiconductors offers an opportunity to fabricate photovoltaics in a low-cost and high-throughput way. However, for this methodology to make a commercial impact a variety of challenges around the fundamental chemistry and materials science need to be addressed. Furthermore, while solution processing has been applied heavily to the Cu(In,Ga)(S,Se)₂ family of materials, these techniques can also open doors for emerging materials like Cu₂ZnSnSe₄, Ag₂ZnSnSe₄, and the chalcogenide perovskites.</p><p dir="ltr">In solution processed Cu(In,Ga)(S,Se)₂ devices, researcher have generally started with a Cu(In,Ga)S₂ film that is then selenized to form the final Cu(In,Ga)(S,Se)₂ material. However, this process has been connected to the formation of a problematic “fine-grain” layer. To solve this issue, the molecular precursors from amine-thiol chemistry were modified to produce soluble molecules with metal selenium bonding. This enabled direct solution deposition of CuInSe₂ films that could be processed without forming a fine grain layer.</p><p dir="ltr">Reactive dissolution chemistry (or “alkahest” chemistry) is useful for solution processing because it can enable the direct use of metal or metal chalcogenide precursors, bypassing the potential impurities from metal salt precursors. However, the commonly used amine-thiol reactive solvent system is better suited to making metal sulfides than metal selenides because the thiol acts as a sulfur source. To address this limitation, a new alkahest based on alkylammonium polyselenide solutions was developed which could reactively dissolve a wide range of metals, metal chalcogenides, and metal oxides. This generalizable chemistry enabled the synthesis of a wide range of binary and multinary metal chalcogenides including Cu(In,Ga)Se₂, Cu₂ZnSnSe₄, and Ag₂ZnSnSe₄.</p><p dir="ltr">Emerging metal chalcogenide semiconductors composed of earth-abundant and non-toxic elements that can exhibit strong optoelectronic properties and high stability are a target of significant interest. Chalcogenide perovskites like BaZrS₃ and BaHfS₃ are an intriguing option to satisfy these requirements but have rarely been studied because of synthesis difficulties, historically being made by solid-state reactions or the sulfurization of oxides around 1000 °C. Here a solution-based approach that only requires moderate temperatures of 550-575 °C was developed utilizing a hybrid ink containing soluble metal thiolates and nanoparticulate metal hydrides.</p><p dir="ltr">The hybrid ink was an important proof of concept that chalcogenide perovskites could be synthesized at these moderate temperatures. However, it relies on complex and difficult to handle precursors. A simpler route would be to use air-stable precursors to make an oxide perovskite and subsequently sulfurize the material. However, this route has historically used excessively high temperatures. Therefore, a new sulfurization step was conceived based on thermodynamic arguments that includes both sulfur and hafnium sulfide as an oxygen sink. This redesigned sulfurization enabled the conversion of BaZrO₃ into BaZrS₃ at temperatures around 575 °C.</p><p dir="ltr">Finally, an energy systems and economic analysis was performed to consider how photovoltaics might be incorporated into agricultural lands. This work showed that when compared with traditional photovoltaics or a PV Aglectric concept, using corn for ethanol is an inefficient way to generate both food and energy from a given unit of land.</p>

Ever since the discovery of van der Waals materials, the condensed matter community has developed a wide spectrum of techniques to probe various phases in these fascinating materials. Among these phases, correlated phenomena are of great importance to physicists, and recent progress on moiré heterostructures offers a highly flexible and tunable platform to study them. It has been established in previous works that twisted WSe₂, a type of semiconductor in the van der Waals family, has great potential in hosting a large number of correlated phases and phase transitions. However, it is believed that commensurability plays a critical role in the stability of correlations. In this thesis, we demonstrate correlated physics in twisted WSe₂ beyond commensurate fillings, as well as their magnetic field dependence, via electric transport measurements. At modest magnetic fields, a Stoner-like instability in the system near van Hove singularities causes a reconstruction of the Fermi surface. On the other hand, at extremely high magnetic fields, the system exhibits reentrant insulating behaviors that are possibly due to the presence of strong excitonic interactions. Furthermore, correlated topological states are observed away from half-filling in the imbalanced excitonic metallic regime. This wide range of tunability once again proves moiré heterostructures as a promising platform to simulate quantum correlation effects on a lattice.

The growing speed and bandwidth requirements of telecommunication systems demand all-optical on-chip solutions. Microphotonic devices can deliver low power nonlinear signal processing solutions. This thesis looks at the slow light photonic crystals in chalcogenide glasses to enhance low power nonlinear operation. I demonstrate the development of new fabrication techniques for this delicate class of materials. Both, reactive ion etching and chemically assisted ion beam etching are investigated for high quality photonic crystal fabrication. A new resist-removal technique was developed for the chemical, mechanical and light sensitive thin films. I have developed a membraning method based on vapor phase etching in combination with the development of a save and economical etching tool that can be used for a variety of vapour phase processes. Dispersion engineered slow light photonic crystals in Ge₃₃As₁₂Se₅₅ are designed and fabricated. The demonstration of low losses down to 21±8dB/cm is a prerequisite for the successful demonstration of dispersion engineered slow light waveguides up to a group index of around n[subscript(g)] ≈ 40. The slow light waveguides are used to demonstrate highly efficient third harmonic generation and the first advantages of a pure chalcogenide system over the commonly used silicon. Ge₁₁.₅As₂₄24Se₆₄.₅ is used for the fabrication of photonic crystal cavities. Quality factors of up to 13000 are demonstrated. The low nonlinear losses have enabled the demonstration of second and third harmonic generation in those cavities with powers up to twice as high as possible in silicon. A computationally efficient model for designing coupled resonator bandpass filters is used to design bandpass filters. Single ring resonators are fabricated using a novel method to define the circular shape of the rings to improve the fabrication quality. The spectral responses of the ring resonators are used to determine the coupling coefficient needed for the design and fabrication of the bandpass filters. A flat top bandpass filter is fabricated and characterized as demonstration of this method. A passive all-optical regenerator is proposed, by integrating a slow-light photonic crystal waveguide with a band-pass filter based on coupled ring resonators. A route of designing the regenerator is proposed by first using the dispersion engineering results for nonlinear pulse propagation and then using the filter responses to calculate the nonlinear transfer function.

Silver containing chalcogenide glasses have been extensively studied during the last few decades; the main interest in these materials being their electrical conductivity which changes by several orders of magnitude upon silver doping. Glassy chalcogenides doped with silver have applications in optical elements, gratings, micro-lenses, waveguides, bio & chemical sensors, solid electrolytes, batteries, etc. Chalcohalide glasses have become important in the recent times, from both scientific & technological points of view, due to the interesting properties exhibited by these glasses such as the transparency in the infrared region, the stability against devitrification, solubility of rare earth elements, etc. In this thesis work, the thermal properties and electrical switching behavior of certain silver and iodine doped chalcogenide glasses have been investigated The thesis contains five chapters: Chapter 1: This chapter is an introduction to the fundamental aspects of amorphous semiconductors with a particular reference to chalcogenide glasses. The advantages and applications of chalcogenide glasses are also described. Chapter 2: The methods of preparation and characterization of the glasses investigated are described in this chapter. Also, the details of the experiments undertaken, namely temperature modulated Alternating Differential Scanning Calorimetry (ADSC), electrical switching analysis, Photo-thermal Deflection Spectroscopy (PDS), etc, are outlined. Chapter 3: In this chapter, the thermal behavior and electrical switching of silver doped Ge-Se and As-Se chalcogenide glasses are described. Bulk, melt-quenched Se-rich Ge0.15Se0.85-xAgx glasses have been found to be microscopically phase separated and composed of Ag2Se clusters and GeSe2-Se network. When the silver concentration exceeds 10 atom %, the Ag2Se clusters embedded in the GeSe2-Se network percolate. The signature of this percolation threshold is clearly observed as the sudden appearance of two exothermic crystallization peaks in ADSC runs. Density, molar volume and micro hardness studies also strongly support the view of a percolation transition. The super-ionic conduction observed earlier in these glasses at higher silver proportions, is likely to be connected with the silver phase percolation. It has been found that Ge0.15Se0.85-xAgx glasses of lower silver concentration (x = 0.07 and 0.08) do not exhibit electrical switching at voltages up to 1100 V. A negative resistance behavior and threshold type electrical switching is seen in Ge0.15Se0.85-xAgx samples with x 0.09. Also, fluctuations are observed in the I-V characteristics of these samples, which have been attributed to the difference in thermal conductivities between the Ag2Se inclusions and the Ge-Se base glass. A sharp drop has been observed in the switching voltage with Ag concentration which is due to the more metallic nature of silver and the presence of Ag+ ions. Further, the saturation in the decrease of VT around x = 0.10, is related to silver phase percolation in these glasses. Bulk As20Se80-xAgx glasses (0 x 15) have been found to exhibit two endothermic glass transitions and two exothermic crystallization reactions on heating. Based on which it is suggested that As20Se80-xAgx glasses are also microscopically phase separated, containing Ag2Se phases embedded in an As-Se backbone. The occurrence of microscopic phase separation in As20Se80-xAgx glasses is also confirmed by SEM studies. With increasing silver concentration, the Ag2Se phase percolates in the As-Se matrix, with a well-defined percolation threshold at x = 8. This silver phase percolation is exemplified by sudden jumps in the composition dependence of the second crystallization peak and non-reversible heat-flow, Hnr obtained at the second glass transition reaction of As20Se80-xAgx glasses. The super-ionic conduction observed earlier in these glasses at higher silver proportions, is likely to be associated with the observed silver phase percolation. Like Ge0.15Se0.85-xAgx glasses, As20Se80-xAgx glasses also exhibit threshold type electrical switching with fluctuations in the I-V characteristics; these fluctuations have been attributed to the difference in thermal conductivities between the Ag2Se inclusions and the As-Se base glass. A sharp drop has been observed in the switching voltage with Ag concentration which is due to the more metallic nature of silver and the presence of Ag+ ions. Further, the saturation in the decrease of VT around x = 8, is found to be related to silver phase percolation in these glasses, which has been proposed on the basis of ADSC experiments. Chapter 4: The chapter 4 deals with thermal studies, electrical switching investigations and Photo-thermal Deflection Spectroscopic (PDS) measurements on certain Ge-Te-I and As-Te-I chalcohalide glasses. It has been found that the compositional variation of the glass transition temperature of Ge22Te78-xIx glasses, obtained by Alternating Differential Scanning Calorimetry (ADSC), exhibits a broad hump around 5 atom % of iodine. Further, a sharp minimum is seen in the composition dependence of non-reversing enthalpy (Hnr) of Ge22Te78-xIx glasses at x = 5, which is suggestive of a thermally reversing window at this composition. Electrical switching studies on Ge22Te78-xIx glasses indicate that these glasses exhibit memory type electrical switching. At lower iodine concentrations, a decrease is seen in switching voltages with an increase in iodine content (in comparison with the base Ge22Te78 glass), which is due to the decrease in network connectivity. The increase seen in switching voltages of Ge22Te78-xIx glasses at higher iodine contents, suggests that the influence of the metallicity is stronger at higher iodine proportions. It is also interesting to note that the composition dependence of the threshold voltages shows a slope change at x = 5, the inverse rigidity percolation threshold of the Ge22Te78-xIx system. . Further, it is found that the thermal diffusivities ( D) of Ge22Te78-xIx glasses decrease with the increase in iodine content, which has been understood on the basis of fragmentation of the Ge-Te network with the addition of iodine. Also, a cusp is seen in the composition dependence of thermal diffusivity at the composition x = 5 (average coordination number, r = 2.39), which has been identified to be the inverse rigidity percolation threshold of the system at which the network connectivity is lost. ADSC studies on As45Te55-xIx chalcohalide glasses (3 x 10) reveal that there is not much variation in the glass transition temperature of As45Te55-xIx glasses, even though there is a wide variation in r . Based on this observation we suggest that the variation in glass transition temperature of network glasses is dictated by the variation in average bond energy rather than the average coordination number. Further, the non-reversing enthalpy Hnr of As45Te55-xIx glasses is found to exhibit a sharp minimum at the composition x = 6. A broad hump is also seen in glass transition and crystallization temperatures in the composition range 5 x 7. These results indicate a narrow thermally reversing window in As45Te55-xIx glasses around the composition x = 6. As45Te55-xIx glasses have been found to exhibit a memory to threshold type change in switching behavior with iodine content (x 6), which has been understood on the basis of the sharp increase in thermal diffusivity above x = 6. It is also observed that the switching voltages do not change appreciably with composition/average coordination number. Though no pronounced signature of a stiffness transition is seen in the variation with composition of VT, fluctuations are seen in the switching voltages around x = 6, the composition corresponding to the sharp thermally revering window. PDS studies indicate that the thermal diffusivities () of As45Te55-xIx chalcohalide exhibit a sharp minimum at the composition x = 6. This result reasserts the presence of a sharp thermally reversing window in As45Te55-xIx glasses around the composition x = 6. Chapter 5: The significant results obtained in the present thesis work have been summarized in this chapter. Further, the scope for future work is also presented.

Availability of amorphous semiconductors in the form of high quality multilayers provide potential applications in the field of micro- and optoelectronics. Although the misfit problems in amorphous multilayers (AML) are considerably reduced compared to crystalline superlattices, there are still some physical processes (e.g. quantum confinement, diffusion) that are not well understood. Recently chalcogenide glass multilayers were prepared with high quality nanomodulation, which demonstrated their potential for tailoring the optical properties. Moreover studies on amorphous nanolayered chalcogenide structures (ANC) are still at the infant stage. These ANCs are similar to the crystalline superlattices, yet distinct from ideal crystalline superlattices produced by molecular beam epitaxy. The ANCs can be considered as well-correlated layers with good periodicity and smooth interface. They are attractive because of the prominent photoinduced effects, similar to those exhibited by uniform thin films. For example, photoinduced diffusion in short period multilayer systems is important because of its potential applications in holographic recording and fabrication of phase gratings. In spite of its practical usefulness, the mechanism of photoinduced interdiffusion is not properly understood. Since most structural changes are related to atomic diffusion, understanding of the structural transformation must be based on the diffusion process. Moreover, in AML, the process of interdiffusion is not well understood. The main aim of this thesis is to study the photoinduced interdiffusion in Se/As2S3 and Bi/As2S3 nanolayered films. In literature, there are reports about the photoinduced interdiffusion in Se/As2S3 and Bi/As2S3 nanolayered films, but the mechanisms of photoinduced interdiffusion of these elements are not properly understood. Raman scattering and infrared spectroscopy techniques were used to study the photoinduced interdiffusion in Se/As2S3 and Bi/As2S3 nanolayered films by Kikineshi et al, but the results were not satisfactory. The characteristic spectra of components in the multilayer and those of the mixed layer were rather similar. In the present thesis, photoinduced interdiffusion in Se/As2S3 and Bi/As2S3 nanolayered samples are studied by optical absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and Photoluminescence (PL). The detailed information about the distribution of electronic states in the absorption edge, localized states and the new bonds formed between the components due to photoinduced interdiffusion elucidated from the above studies will give more insight into the mechanism and kinetics of photoinduced interdiffusion. The thesis consists of six chapters. References are given at the end of each chapter. Various general and unique physical properties of amorphous chalcogenides are discussed in Chapter 1. This chapter summarizes the fundamental aspects of amorphous state, such as the structure and its models, electronic band structure, defects as well as the physical properties like d.c conductivity, a.c conductivity, optical absorption, photoconductivity and PL. A more detailed account of the various photoinduced effects are also discussed. Apart from this, similar photoinduced effects observed in other systems like a-Si:H, oxide glasses, Polymers etc are described in brief. Finally, the scope of present investigations is furnished. Chapter 2 has been devoted to photoinduced interdiffusion and related changes in optical properties of nanolayered Se/As2S3 films. It begins with a brief introduction followed by a survey of the earlier work done on these multilayered films. The theory of optical absorption and experimental procedures are discussed. Photoinduced interdiffusion was observed with above band gap light in nanolayered Se/As2S3 films. It is discussed in terms of the optical parameters such as bandgap, Urbach edge (Ee) and Tauc’s parameter (B1/2). From the analysis of the optical absorption spectra, it was concluded that the optical bandgap, Ee and B1/2 change with photoinduced interdiffusion. These changes in properties are ascribed to the solid solution formation due to the intermixing of adjacent layers. The photoinduced intermixing of the adjacent layers are obviously related to the photoinduced viscous flow and it depends on the number of excited chalcogen bridge atoms, which determine the local deformations due to the bond switching and displacements. Experimental data of B1/2 and Ee for as prepared samples do not show a clear correlation implied by the Mott-Davis model. It is also observed that the optical parameters can be changed with a change in the Se sublayer thickness. Variations of these optical parameters as a function of modulation period and photoinduced interdiffusion were discussed in terms of the quantum confinement effect and changes in the valence and conduction bands. Chapter 3 deals with the PL studies on as prepared and irradiated samples of Se/As2S3 nanolayered films. The theory of PL, experimental procedures and data analysis are discussed in detail. PL studies were carried out on as prepared and irradiated nanolayered samples of Se/As2S3 films. None of the samples showed PL at 77 K, which clearly indicate that there exists a competitive non-radiative mechanism. We observed a broad PL in the range of 0.8–1.2 eV for as prepared and irradiated samples at 4.2 K. The observed stoke shift in PL is discussed in terms of the strong electron-phonon coupling at the recombination centers. We found that the PL intensity can be increased by several orders of magnitude by irradiating the samples with appropriate wavelengths in the range of the absorption edge. The broadening of luminescence bands takes place either with a decrease in Se layer thickness or with irradiation. The former is due to the change in interface roughness while the latter is due to photoinduced interdiffusion. Deconvolution showed that the PL spectrum consists of five transitions. The deconvoluted peak PL intensity, PL quantum efficiency and full width at half maximum are varying according to the function of sublayer thickness and interdiffusion. All these results indicate the high impact of interdiffusion on the luminescence intensity in the given AML is due to changes of defect states, which in turn are not directly connected to the band structure, i.e., confinement effects are not essential for this type of luminescence. The whole picture is complex due to more complicated carrier relaxation and recombination process, possibly with several interconnected effects, which are not properly understood, but the possibility for tuning the optical parameters of the Se/As2S3 nanolayered films, including the low temperature luminescence, is established. Chapter 4 is on kinetics and chemical analysis of photoinduced interdiffusion in nanolayered Se/As2S3 films. The basic formalism of X-ray photoelectron spectroscopy and in situ optical absorption spectroscopy together with a brief description of the theory and data analysis adopted in the present studies are given. We have studied the kinetics of photoinduced interdiffusion in nanolayered Se/As2S3 film by in situ optical absorption measurements. All previous measurements were performed ex situ, i.e., a film exposed under light irradiation during the measurement was never studied. In situ changes in the transmission spectra were measured, but at a fixed wavelength. Since the measurements were done on a single wavelength, the kinetics of the variation of optical bandgap and Tauc parameter were missing. In short, information has been missing about the metastable changes in the multilayer structure during photoinduced interdiffusion. In situ changes in transmission spectra were recorded over the wavelength range λ=400-1000 nm, and also at fixed wavelengths to understand the changes in absorption coefficient, optical bandgap and Tauc parameter during photoinduced interdiffusion. The in situ optical absorption measurements reveal that the photo darkening in amorphous nanolayered Se/As2S3 film is followed by photoinduced interdiffusion. An increase in disorder during photodarkening and its subsequent decrease during photoinduced interdiffusion was also observed. The observation of photodarkening of Se at room temperature when confined between As2S3 layers suggests that the glass transition temperature of Se shifts to higher temperature. The analysis shows that the atoms, which take part in photodarkening, play a vital role in photoinduced interdiffusion. We used XPS to analyze the new bonds formed between the components due to photoinduced interdiffusion. The XPS results showed that there is a considerable decrease in the As-S, As-As and S-S bonds after photoinduced interdiffusion; As-O and some of the S-S homopolar bonds are retained. There was a considerable decrease in As-S bond followed by an increase in As-Se and S-Se bonds. XPS analysis also shows that during photodiffusion, heteropolar bonds replace homopolar bonds, i.e., the irradiated samples are chemically ordered than the corresponding as prepared samples. Chapter 5 is concerned with the photoinduced interdiffusion in Bi/As2S3 nanolayered films. A brief description about the photoinduced interdiffusion of metals such as Ag, Zn, etc is given in the introduction. The experimental procedures and data analysis are also given. Two sets of samples with different ratios of sublayer thickness (d), d-Bi/d-As2S3 = 1/12 and 1/6 prepared by cyclic thermal evaporation are employed for the present study. A pump probe optical absorption technique was used to study the photoinduced interdiffusion in Bi/As2S3 nanolayered samples. Photoinduced interdiffusion of Bi into As2S3 was observed in both the films. The XPS analysis shows that the as prepared samples contain a large number of wrong As–As bonds and some of the As-As bonds are converted to As-S bonds during irradiation. The XPS analysis also reveals that the Bi is forming only bond with S during photoinduced interdiffusion. Chapter 6 summarizes the essential features of the present work and also points a few possible directions along which further work can be carried out.

The thesis is divided in to five chapters. Chapter I: Synthesis of New thiolevomannosan derivatives In this chapter, a general and efficient method for the synthesis of conformationally locked thiosugars has been discussed. An unprecedented synthesis of a novel thioorthoester and its synthetic utility in glycosylation has been demonstrated. Chapter II: Studies on -Mannosidase Inhibitors The synthesis and evaluation of novel, conformationally locked glycomimic, thiolevomannosan and its analogs sulfoxide and sulfone starting from readily available D-mannose is discussed in this chapter. This is the first report of thiosugar derivatives with enhanced potency compared to kifunensine. Docking and biochemical studies have been discussed. Chapter III: Studies on Novel Cyclic Tetraselenides of Mannose In this chapter, the syntheses and structural properties of novel cyclic tetraselenides starting from mannose have been discussed. These tetraselenides are the first of their kind where all four selenium atoms are arranged in a cyclic manner as the backbone of mannose. X-ray structures have been reported for the tetraselenides. Chapter IV: Novel Chalcogenides of Uridine and Thymidine: Synthesis and Applications An efficient and simple method to synthesise disulfides and diselenides of thymidine and uridine derivatives has been demonstrated in this chapter. The utility of these disulfides in various ring opening reactions as well in Michael addition reactions has been demonstrated. Chapter V: Studies on New, Potent Urease Inhibitors In this chapter, a facile one-pot synthesis of thio and selenourea derivatives under mild conditions by the reaction of amines and commercially available Viehe’s iminium salt in the presence of benzyltriethylammonium tetrathiomolybdate as sulfur transfer reagent and tetraethylammonium tetraselenotungstate as selenium transfer reagent has been discussed. A few of the urea derivatives have shown potent inhibitor activity in the nanomolar range for jackbean urease.

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.

Magister Scientiae - MSc (Chemistry) / This work involves the synthesis of copper telluride-polypropylenimine tetra(5-(2-thienyl) salicylaldimine) (CuTe@PPI) and copper antimonide-polypropylenimine tetra(5-(2-thienyl) salicylaldimine) (CuSb@PPI) core-shell nanoparticles (NPs), using two-pots and one-pot synthesis methods, respectively. Their morphology was studied by X-ray diffraction spectroscopy (XRD), high resolution transmission electron microscopy (HRTEM) and high resolution scanning electron microscopy (HRSEM); while their structures were characterized by Fourier transform infrared spectroscopy (FTIR) and elemental analysis. Photophysical properties of the core-shell NPs were determined from ultraviolet-visible absorption spectroscopy (UV-Vis) and photoluminescence spectroscopy (PL). For core-shell NPs produced via two-pots method only CuTe@PPI exhibited ? ? ?* and n ? ?* which indicate that CuSb@PPI produced via two-pots method was unsuccessfully synthesized. The ? ? ?* and n ? ?* transitions indicate the presence of polypropylenimine tetra(5-(2-thienyl) salicylaldimine) (PPI) on the surface of CuTe NPs and CuSb NPs. FTIR confirmed coordination of PPI on the surface of CuTe NPs and CuSb NPs by showing a shift in wavenumber of C=N group bands from PPI. HR-TEM showed that the CuTe@PPI synthesized via one-pot method have a wide particles sizes distribution with an average particles size of 13.60 nm while for CuTe@PPI synthesized via two-pots it was impossible to determine the particles size due to aggregation. CuSb@PPI synthesized via twopots method and one-pot method has a wide particles sizes distribution with an average size of 7.98 nm and 11.61 nm respectively. The average particles sizes determined by HR-SEM were found to be 35.24 nm (CuTe@PPI two-pots method), 33.90 nm (CuTe@PPI one-pot method), 18.30 nm (CuSb@PPI two-pots method), and 16.18 nm (CuSb@PPI one pot method). / 2021-08-31

Parmi les différentes cellules photovoltaïques existantes, les technologies à base de CIGS représentent aujourd'hui une alternative sérieuse à celles basées sur le silicium. De même, les technologies organiques émergent en vue d'applications sur le marché de la faible puissance. Afin d'être parfaitement concurrentielle, ces cellules doivent s'affranchir au maximum de la présence d'indium (surcoût) au sein de leurs structures (TCO, couche absorbante), ou de matériaux toxiques comme le CdS utilisé en tant que couche tampon. Les chalcogénures de zinc tels que le Zn1-xMgxO ou le ZnOzS1-z peuvent être employées à la place du CdS grâce à leurs propriétés semi-conductrices. En dopant le Zn1-xMgxO par un ou plusieurs atomes métalliques trivalents, on peut également créer des électrodes transparentes (TCO) pouvant substituer les électrodes traditionnelles à base d'indium (ITO). Les couches minces synthétisées au cours de ce travail ont été réalisées par spray-CVD, une technique de dépôt hybride et innovante utilisant un mode de chauffage radiatif. Les améliorations apportées au réacteur expérimental et les avantages qu'elles dégagent en font une alternative crédible aux techniques traditionnelles. Les couches de Zn1-xMgxO ainsi synthétisées exhibent de très bonnes propriétés, dont une énergie de gap facilement ajustable, une forte mobilité électronique et une très bonne transparence. De même, des couches de ZnS ont été réalisées par l'usage d'un précurseur original, permettant de s'affranchir du ZnCl2 (corrosif) couramment utilisé en spray pyrolyse. Les différents TCO étudiés ont montré de faibles résistivités (10-3 Ω.cm) et ont pu être testés dans des cellules solaires organiques en structures inverses. / In the field of photovoltaic devices, organic and CIGS-based solar cells are both promising way to compete with silicon-based technologies for low and high power generation. In order to provide safe and cost-effective thin films for these devices, zinc chalcogenides layers represent interesting opportunities to replace indium (expensive) and cadmium-based (toxic) layers. Semiconductors like Zn1-xMgxO and ZnS had been synthesized using an infrared assisted spray-CVD apparatus. The interaction between an aerosol and the infrared radiation is the main innovation in this process and sparked off many advantages. With this simple, vacuum-free and chemical soft technique, Zn1-xMgxO thin films exhibit excellent optical transparency, high electrical conductivity and an easily band gap adjustment. The obtained properties, compared with those reported by other traditional techniques, classed infrared assisted spray-CVD as an interesting and promising alternative technique in order to deposit thin films for such applications. ZnS thin films had been prepared with an original chemical precursor which enable to work without ZnCl2, the traditional corrosive chemical precursor in spray pyrolysis. In addition to that, some transparent conductive oxides (TCO) had been investigated by doping ZnO and Zn1-xMgxO layers with aluminum and/or gallium. With a very high optical transparency and a resistivity as low as 10-3 Ω.cm, ZnO:Al exhibit workable properties as transparent electrodes. Indeed, inverted organic solar cells had been realized with those TCO and proved their well-functioning into such devices.