Binary ionic porphyrin self-assembly: Structures, and electronic and light-harvesting properties

The increasing demand of efficient optoelectronic devices such as photovoltaics has created a great research interest in methods to manipulate the electronic and optical properties of all the layers of the device. Tin dioxide (SnO2), due to his charge transport capability, high stability and easy fabrication is the main electron transport layer in modern photovoltaics which have achieved a record efficiency. While the wide band gap of SnO2 makes it an effective electron transport layer, its potential for other energy applications such as photocatalysis is limited. To further improve is conductivity and reduce its bandgap, doping or co-doping with various elements has been proposed. In the present density functional theory (DFT) study, we focus on the investigation of vanadium (V) and tantalum (Ta) doped SnO2 both in the bulk and the surface. Here we focus on interstitial and substitutional doping aiming to leverage these modifications to enhance the density of states for energy application. These changes also have the potential to influence the optical properties of the material, such as absorption, and make SnO2 more versatile for photovoltaic and photocatalytic applications. The calculations show the formation of gap states near the band edges which are beneficial for the electron transition and in the case of Ta doping the lowest bandgap value is achieved. Interestingly, in the case of Ta interstitial, deep trap states are formed which depending of the application could be advantageous. Regarding the optical properties, we found that V doping significantly increases the refractive index of SnO2 while the absorption is generally improved in all the cases. Lastly, we investigate the electronic properties of the (110) surface of SnO2, and we discuss possible other applications due to surface doping. The present work highlights the importance of V and Ta doping for energy applications and sensor applications.

The family of transition metal chalcogenides, particularly barium titanium trisulphide ( ), is one of the classes of materials that have garnered a lot of interest recently, owing to their unique combination of structural versatility, environmental stability, and tunable electronic properties. However, controlled doping of these materials, as is necessary for customizing their electrical and electronic properties, is one of the several challenges still being faced by researchers. Studies on dopant incorporation processes on these materials are yet to be comprehensively determined. In this research paper, two new materials ( and ) were modelled from the and their electronic and transport properties investigated using density functional theory calculations as implemented in the Quantum Espresso package. The Perdew-Burke-Ernzerhof exchange-correlation functional was utilized. It was found out that the substitution resulted in lowering the electrical conductivities, carrier mobilities, and formation energies compared to the original sample, while their densities increased. The results also pointed to the adjustment of the materials' band gaps from 0.71 eV for the original sample to as low as 0.312 eV in the sample. Regardless of the changes, the Hall coefficients of the samples turned out to be p-type, whose values increased with the substitutions. The above findings underline the significance of compositional adjustment in maximizing the performance of and its derivatives in photovoltaic, thermoelectric, and sensor applications.

Two-dimensional layered materials such as graphene and transition metal dichalcogenides have gained a lot of attention because of their distinctive chemical, optical, and electronic properties. Transition metal dichalcogenides can have a tunable bandgap in the range of 1 − 3 eV (visible), which enables their applications in ultrathin field effect transistors, photovoltaics, sensors, and optoelectronic devices. They exhibit an indirect-to-direct band gap transition when thinned down from bulk to a single layer. They show strong photoluminescence, electron–photon interaction, valley pseudospin, nonlinear optical response, and lack of dangling bonds. Many exotic phenomena appear when materials are thinned to nanoscale size because of the quantum confinement effect and the symmetry changes in lower dimensions. The stacking of two-dimensional transitional metal dichalcogenides with a controlled twist angle creates “van der Waals” heterostructures, which has opened a pandora box that allows us to explore new quantum properties. These “van der Waals” structures can host Mott insulating states, topological states, and correlated electron physics. In this dissertation, to explore these intriguing properties, I focused on fabricating bilayers of tungsten disulfide (WS2) with a controlled twist angle. The new transfer method can minimize trapped interfacial impurities, chemical residue cracks, or mechanical damage during the transfer process. Furthermore, I studied the electronic and optical properties of stacked bilayers and the role of twist angle in tuning these properties.

The creation of anisotropic nanostructures with precise size control is desirable for new properties and functions, but it is challenging for ionic self-assembly (ISA) because of the non-directional electrostatic interactions. Herein, the formation of size-controllable tetragonal nanoprisms is reported via crystallization-directed ionic self-assembly (CDISA) through evaporating a micellar solution on solid substrates. First, ISA is designed with a crystalline polyethylene oxide (PEO) containing cationic polymer poly(2-(2-guanidinoethoxy)ethyl methacrylate)-b-poly(ethyleneoxide)-b-poly(2-(2-guanidinoethoxy)-ethylmethacrylate) (PGn -PEO230 -PGn ) and an anionic 5,10,15,20-Tetrakis(4-sulfonatophenyl) porphyrin (TPPS) to form micelles in aqueous solution. The PG segments binds excessive TPPS with amplenet chargeto form hydrophilic corona, while the PEO segments are unprecedentedly dehydrated and tightly packed into cores. Upon naturally drying the micellar solution on a silicon wafer, PEO crystallizationdirects the micelles to aggregate into square nanoplates, which are further connected to nanoprisms. Length and width of the nanoprisms can be facilely tuned by varying the initial concentration. In this hierarchical process, the aqueous self-assembly is prerequisite and the water evaporation rate is crucial for the formation of nanostructures, which provides multiple factors for morphology regulating. Such precise size-control strategy is highly expected to provide a new vision for the design of advanced materials with size controllable anisotropic nanostructures.

A set of problems pertaining to quantum information processing in semiconductors is investigated. Two schemes for implementing electronic qubits in strong and weak three dimensional quantum confinement regimes are studied along with their related electronic properties. Recent experiments motivated us to calculate electronic properties and g factors for nanowhisker quantum dots. These calculations were done using 8 band strain dependent k.p theory on a 3D grid and are in excellent agreement with experiment. It has been observed that the growth conditions cause the nanowhiskers to crystallize in wurtzite(WZ) form instead of their stable-phase zinc-blende bulk structure. Very little is known about the WZ phase of non-nitride III-V semiconductors as they do not naturally occur. We have therefore also predicted the electronic bandstructure and optical properties of nine III-V semiconductors in the WZ phase using transferable empirical pseudopotentials. Apart from quantum dots, the spin of an electron bound to an atomic impurity is an attractive candidate for quantum information processing as they do not suffer from structural uncertainties. This makes spin of an electron bound to a hydrogenic impurity an attractive candidate for a qubit as it possess the biggest radii of any ionic bound states in the solid and is a natural two state system. We have calculated the electric and magnetic field dependent modulation of the g tensor for a single Silicon donor embedded in a GaAs substrate. The spin dynamics of the weakly bound electron exhibits an unusual nonlinear behavior, which is not seen in structures with strong quantum confinement such as quantum dots. Abstract Approved: Thesis SupervisorApproved: Thesis Supervisor Title and Department

ABSTRACTThe multifunctionality of hybrid organic-inorganic materials has been clearly demonstrated in recent years. Their application in solar- related devices is a growing research area with important technological implications. Our interest is centered on the interplay between the light-harvesting and hole-conducting properties of conjugated polymers and the wide band gap values observed from different inorganic semiconductor oxides. The materials applied in this work are the combination of semiconductor oxides like TiO2, ZnO, Nb2O5, CeO2 and TiO2-CeO2, and different conjugated polymers like poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) or poly(3,4-ethylenedioxythiophene) (PEDOT). Semiconductor oxides have been applied in different configurations like dense thin films, nanostructured electrodes or as nanoparticles. The conjugated polymers have been chosen depending on their light-harvesting properties (e.g. MEH-PPV) or due to their good electrical conductivity and hole conducting properties (e.g. PEDOT). We have characterized our devices in terms of Voc, Jsc, IV-curves, effect of different atmospheres and device lifetime under simulated sunlight irradiation. Tuning the different device parameters such as type of oxide applied, active layer thickness, starting materials concentration, effect of different atmospheres, effect of UV irradiation, etc., permit the fabrication of devices with well-defined properties. A brief discussion and comparison of hybrid solar cells (HSC) with solid-state Dye Sensitized Solar Cells (ss-DSC) applying nanocomposite materials based on TiO2 and PEDOT, as the electron and hole conducting materials respectively, is also included.

Bismuth oxyhalides: Recent progress and its applications in photocatalysis, hydrogen production, antibacterial studies, and sensors

TiO2 has extensive applications in the fields of renewable energy and environmental protections such as being used as photocatalysts or electron transport layers in solar cells. To achieve highly efficient photocatalytic and photovoltaic applications, ongoing efforts are being devoted to developing novel TiO2-based material structures or compositions, in which a first-principles computational approach is playing an increasing role. In this review article, we discuss recent computational and theoretical studies of structural, energetic, electronic, and optical properties of TiO2-based nanocluster, bulk, and material interface for photocatalytic and photovoltaic applications. We conclude the review with a discussion of future research directions in the field.

We present the specific ab-initio calculations that detail the variations of perovskite BaZrO3 caused by in-plane strain. Specifically, the internal relaxation, which was not captured in the widely used biaxial strain model, was included in a complementary manner to lattice relaxation. Density functional theory as well as a hybrid functional method based on a plane wave basis set was employed to calculate the lattice structure, elastic constants, electronic properties and optical properties of perovskite BaZrO3. The lattice parameter c exhibited a clear linear dependence on the imposed in-plane strain, but the Poisson’s ratio caused by internal relaxation was smaller than the elastic deformation, indicating an “inelastic” or “plastic” relaxation manner caused by the introduction of internal relaxation. As a result, the related electronic and optical properties of perovskite BaZrO3 were also strongly affected by the in-plane strain, which revealed an effective way to adjust the properties of perovskite BaZrO3 via internal relaxation.

We report the results of a computational study of TiO2 nanoclusters of various sizes as well as of complex systems with various molecules adsorbed onto the clusters to set the ground for the modeling of charge transfer processes in hybrid organic–inorganic photovoltaics or photocatalytic degradation of pollutants. Despite the large number of existing computational studies of TiO2 clusters and in spite of the higher computing power of the typical available hardware, allowing for calculations of larger systems, there are still studies that use cluster sizes that are too small and not appropriate to address particular problems or certain complex systems relevant in photovoltaic or photocatalytic applications. By means of density functional theory (DFT) calculations, we attempt to find acceptable minimal sizes of the TinO2n+2H4 (n = 14, 24, 34, 44, 54) nanoclusters in correlation with the size of the adsorbed molecule and the rigidity of the backbone of the molecule to model systems and interface processes that occur in hybrid photovoltaics and photocatalysis. We illustrate various adsorption cases with a small rigid molecule based on coumarin, a larger rigid oligomethine cyanine dye with indol groups, and the penicillin V antibiotic having a flexible backbone. We find that the use of the n = 14 cluster to describe adsorption leads to significant distortions of both the cluster and the molecule and to unusual tridentate binding configurations not seen for larger clusters. Moreover, the significantly weaker bonding as well as the differences in the density of states and in the optical spectra suggest that the n = 14 cluster is a poor choice for simulating the materials used in the practical applications envisaged here. As the n = 24 cluster has provided mixed results, we argue that cluster sizes larger than or equal to n = 34 are necessary to provide the reliability required by photovoltaic and photocatalytic applications. Furthermore, the tendency to saturate the key quantities of interest when moving from n = 44 to n = 54 suggests that the largest cluster may bring little improvement at a significantly higher computational cost.

Finely tuned energy gaps in host-guest complexes: Insights from belt[14]pyridine and fullerene-based nano-Saturn systems

The size-dependent stiffness variations in nanocrystalline anatase, a leading material for applications in photovoltaics, photocatalysis, photoelectrochromics, sensors, and optical coatings, were determined using in situ synchrotron x-ray diffraction and Raman scattering. An unusual, abrupt change in the compression curve at approximately 10 GPa and subtle breaks in the pressure shifts of the intense E(g) Raman band at approximately 10 and approximately 15 GPa have been correlated with approximately 2 A-scale disordering of nanocrystalline anatase structure that fully amorphizes under high compression.

Chalcogenide perovskites have attracted growing interest as inorganic materials with tunable electronic and optical properties and enhanced chemical stability compared to halide counterparts. In this work, we present a first-principles density functional theory (DFT) investigation of the structural, electronic, and optical properties of the cesium-based chalcogenide perovskites CsNbS3 and CsVS3. Structural optimizations are performed within the generalized gradient approximation (PBE-GGA) to establish equilibrium geometries within the ABX3 perovskite framework. The electronic properties are analyzed primarily using PBE-GGA to ensure internal consistency between band structures, density of states, and derived optical spectra. Both compounds exhibit narrow indirect band gaps at the PBE-GGA level, with valence-band maxima dominated by S-3p states and conduction-band minima arising mainly from Nb-4d and V-3d orbitals. Hybrid-functional (HSE06) calculations are additionally employed to assess the sensitivity of the electronic structure to exchange–correlation effects, revealing pronounced band-edge renormalization near the Fermi level rather than serving as a basis for electronic classification. Optical properties, including the dielectric function, absorption coefficient, reflectivity, and optical conductivity, are calculated consistently from the PBE-GGA electronic structures and show strong optical activity in the visible to near-infrared energy range, originating from S-p to transition-metal d interband transitions. A model-based thickness-dependent efficiency analysis is further presented to compare relative optoelectronic trends, without making predictive device-performance claims. Overall, this study provides a coherent and internally consistent description of the electronic and optical behaviour of CsNbS3 and CsVS3, clarifies the functional dependence of their electronic structures, and highlights the importance of careful methodological interpretation in first-principles studies of transition-metal chalcogenide perovskites.

The present study deals with the structural, electronic and optical properties of CaCN2 ternary compound. For the computation of structural, electronic and optical properties of the CaCN2, the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) exchange correlation within the framework of density functional theory as available in Wien2k code is used. Wien2k code is based on full potential linearized augmented plane wave method. The electronic properties are investigated in terms of energy band structure, partial and total density of states. Our investigation reveals the direct band gap nature of CaCN2 with band gap of 1.66 eV. The optical behaviour CaCN2 is explained through dielectric tensor, absorption, reflection and refraction spectra which show the utility of this compound in photovoltaic applications.

This work presented theoretical studies on the one-dimensional (1D) nanowires constructed from fullerene C20 cages based on first-principle calculations. The relative energies, electronic, charge transport, and mechanical properties of the 1D nanowires were investigated systemically and in detail. It is found that formations of the C20 nanowires built from isolated cages were all energetically favorable. They also exhibit high kinetic stability according to molecular dynamics simulations. Although they were all constructed with C20 cages as building blocks, NW-2–NW-6, and NW-9 are semiconductors, whereas NW-1, NW-7, and NW-8 exhibit metallic property. Thus the metallic/semiconducting properties of the 1D C20 nanowires can be mainly determined by the connecting patterns. High charge mobility was revealed for the 1D C20 nanowires based on the deformation potential theory and effective mass approach. Further understanding of the charge mobility is achieved with the aid of crystal orbital analyses. Moreover, the mechanical property of the 1D C20 nanowires was also studied based on the results of Young’s modulus.