Self-healing thermal annealing: surface morphological restructuring control of GaN nanorods

With advances in nanolithography and dry etching, top-down methods of nanostructuring have become a widely used tool for improving the efficiency of optoelectronics. These nano dimensions can offer various benefits to the device performance in terms of light extraction and efficiency, but often at the expense of emission color quality. Broadening of the target emission peak and unwanted yellow luminescence are characteristic defect-related effects due to the ion beam etching damage, particularly for III–N based materials. In this article we focus on GaN based nanorods, showing that through thermal annealing the surface roughness and deformities of the crystal structure can be “self-healed”. Correlative electron microscopy and atomic force microscopy show the change from spherical nanorods to faceted hexagonal structures, revealing the temperature-dependent surface morphology faceting evolution. The faceted nanorods were shown to be strain- and defect-free by cathodoluminescence hyperspectral imaging, micro-Raman, and transmission electron microscopy (TEM). In-situ TEM thermal annealing experiments allowed for real time observation of dislocation movements and surface restructuring observed in ex-situ annealing TEM sampling. This thermal annealing investigation gives new insight into the redistribution path of GaN material and dislocation movement post growth, allowing for improved understanding and in turn advances in optoelectronic device processing of compound semiconductors.

Transfert ultrarapide d’électron et transfert modéré d’énergie au sein d’assemblages supramoléculaires de colorants et d’un cluster de palladium

Résumé : Les transferts d’électrons photo-induits et d’énergie jouent un rôle primordial dans un grand nombre de processus photochimiques et photobiologiques, comme la respiration ou la photosynthèse. Une très grande quantité de systèmes à liaisons covalentes ont été conçus pour copier ces processus de transferts. Cependant, les progrès sont, en grande partie, limités par les difficultés rencontrées dans la synthèse de nouveaux couples de types donneurs-accepteurs. Récemment, des espèces utilisant des liaisons non-covalentes, comme les liaisons hydrogènes, les interactions [pi]-[pi], les liaisons de coordination métal-ligands ou encore les interactions électrostatiques sont le centre d’un nouvel intérêt du fait qu’ils soient plus faciles à synthétiser et à gérer pour obtenir des comportements de transferts d’électrons ou d’énergie plus flexibles et sélectifs. C’est dans cette optique que le travail de cette thèse a été mené, i.e. de concevoir des composés auto-assemblés avec des porphyrines et un cluster de palladium pour l’étude des transferts d’électrons photo-induits et d’énergie. Cette thèse se divise en quatre parties principales. Dans la première section, le chapitre 3, deux colorants porphyriniques, soit le 5-(4-carboxylphényl)-10, 15, 20-tristolyl(porphyrinato)zinc(II) (MCP, avec Na+ comme contre-ion) et 5, 15-bis(4-carboxylphényl)-15, 20-bistolyl(porphyrinato)zinc(II) (DCP, avec Na+ comme contre-ion) ont été utilisés comme donneurs d’électrons, et le [Pd3(dppm)3(CO)]2+ ([Pd32+], dppm = (Ph2P)2CH2, PF6‾ est le contre-ion) a été choisi comme accepteur d’électrons. La structure de l’assemblage [Pd32+]•••porphyrine a été élucidée par l’optimisation des géométries à l’aide de calculs DFT. La spectroscopie d’absorption transitoire (TAS) montre la vitesse de transferts d’électrons la plus rapide (< 85 fs, temps inférieurs à la limite de détection) jamais enregistrée pour ce type de système (porphyrine-accepteur auto-assemblés). Généralement, ces processus sont de l’ordre de l’échelle de la ps-ns. Cette vitesse est comparable aux plus rapides transferts d’électrons rapportés dans le cas de systèmes covalents de type porphyrine-accepteur rapide (< 85 fs, temps inférieurs à la limite de détection). Ce transfert d’électrons ultra-rapide (ket > 1.2 × 1013 s-1) se produit à l’état énergétique S1 des colorants dans une structure liée directement par des interactions ioniques, ce qui indique qu’il n’est pas nécessaire d’avoir de forts liens ou une géométrie courbée entre le donneur et l’accepteur. Dans une deuxième section, au chapitre 4, nous avons étudié en profondeur l’effet de l’utilisation de porphyrines à systèmes π-étendus sur le comportement des transferts d’électrons. Le colorant 9, 18, 27, 36-tétrakis-meso-(4-carboxyphényl)tétrabenzoporphyrinatozinc(II) (TCPBP, avec Na+ comme contre-ion) a été sélectionné comme candidat, et le 5, 10, 15, 20-tétrakis-meso-(4-carboxyphényl)porphyrineatozinc(II) (TCPP, avec Na+ comme contre-ion) a aussi été utilisé à des fins de comparaisons. TCPBP et TCPP ont, tous deux, été utilisés comme donneurs d’électrons pour fabriquer des assemblages supramoléculaires avec le cluster [Pd32+] comme accepteur d’électrons. Les calculs DFT ont été réalisés pour expliquer les structures de ces assemblages. Dans les conditions expérimentales, ces assemblages sont composés principalement d’une porphyrine avec 4 équivalents de clusters. Ces systèmes ont aussi été investigués par des mesures de quenching (perte de luminescence), par électrochimie et par d’autres techniques. Les transferts d’électrons (< 85 fs; temps inférieurs à la limite de détection) étaient aussi observés, de façon similaire aux assemblages MCP•••[Pd32+] et [Pd32+]•••DCP•••[Pd32+]. Les résultats nous indiquent que la modification de la structure de la porphyrine vers la tétrabenzoporphyrine ne semble pas influencer le comportement des cinétiques de transferts d’électrons (aller ou retour). Dans la troisième section, le chapitre 5, nous avons synthétisé la porphyrine hautement [pi]-conjuguée: 9, 18, 27, 36-tétra-(4-carboxyphényléthynyl)tétrabenzoporphyrinatozinc(II) (TCPEBP, avec Na+ comme contre-ion) par des fonctionnalisations en positions meso- et β, β-, qui présente un déplacement vers le rouge de la bande de Soret et des bandes Q. TCPEBP était utilisé comme donneur d’électrons pour fabriquer des motifs supramoléculaires avec le [Pd32+] comme accepteur d’électrons. Des expériences en parallèle ont été menées en utilisant la 5, 10, 15, 20-tétra-(4-carboxyphényl)éthynylporphyrinatozinc(II) (TCPEP, avec Na+ comme contre-ion). Des calculs DFT et TDDFT ont été réalisés pour de nouveau déterminer de façon théorique les structures de ces systèmes. Les constantes d’association pour les assemblages TCPEBP•••[Pd32+]x sont les plus élevées parmi tous les assemblages entre des porphyrines et le cluster de palladium rencontrés dans la littérature. La TAS a montré, encore une fois, des processus de transferts d’électrons dans des échelles de l’ordre de 75-110 fs. Cependant, les transferts de retour d’électrons sont aussi très rapides (< 1 ps), ce qui est un obstacle potentiel pour des applications en cellules solaires à pigment photosensible (DSSCs). Dans la quatrième section, le chapitre 6, les transferts d’énergie triplets (TET) ont été étudiés pour les assemblages MCP•••[Pd32+] et [Pd32+]•••DCP•••[Pd32+]. Les analyses spectrales des états transitoires dans l’échelle de temps de la ns-[mu]s démontrent de façon évidente les TETs; ceux-ci présentent des transferts d’énergie lents et/ou des vitesses moyennes pour des transferts d’énergie T1-T1 (3dye*•••[Pd32+] → dye•••3[Pd32+]*) opérant à travers exclusivement le mécanisme de Förster avec des valeurs de kET autour de ~ 1 × 105 s-1 selon les mesures d’absorption transitoires à 298 K. Des forces motrices non-favorables rendent ces types de processus non-opérants ou très lents dans les états T1. L’état T1 de [Pd32+] (~8190 cm-1) a été qualitativement déterminé par DFT et par la mise en évidence de l’émission S0 ← Tn retardée à 680-700 nm provenant de l’annihilation T1-T1, ce qui fait que ce cluster peut potentiellement agir comme un donneur à partir de ses états Tn, et accepteur à partir de T1 à l’intérieur de ces assemblages. Des pertes d’intensités de types statiques pour la phosphorescence dans le proche-IR sont observées à 785 nm. Ce travail démontre une efficacité modérée des colorants à base de porphyrines pour être impliquée dans des TETs avec des fragments organométalliques, et ce, même attachées grâce à des interactions ioniques. En conclusion, les assemblages ioniques à base de porphyrines et de clusters de palladium présentent des propriétés de transferts d’électrons S1 ultra-rapides, et des transferts d’énergie T1 de vitesses modérées, ce qui est utile pour de possibles applications comme outils optoélectroniques. D’autres études, plus en profondeur, sont présentement en progrès.

ESTUDIO ESTRUCTURAL DE SEMICONDUCTORES USADOS EN APLICACIONES FOTOVOLTAICAS

Se presenta un estudio de las propiedades estructurales de los semiconductores Bi2S3, SnS, SnS2, SnS:Bi, Cu3BiS3 y Cu(In,Ga)Se2 (CIGS) usados como capa absorbente en dispositivos optoelectrónicos. Todas las muestras fueron crecidas por procesos de co-evaporación de sus especies metálicas sobre sustratos de vidrio. El efecto de las condiciones de preparación sobre las propiedades estructurales y composición química han sido analizados y obtenidos a partir de difracción de rayos-X (XRD) y espectroscopia de electrones Auger (AES). Los resultados revelan que todos los compuestos crecen con estructura ortorrómbica, a diferencia del SnS2 y el CIGS, que crecen con estructura hexagonal y tetragonal, respectivamente. Los resultados composicionales revelaron que a partir de la deconvolución de sus picos se encontraron fases asociadas a Cu2Se y In2Se3

Development and optimization of fibre-shaped dye-sensitized solar cells employing an innovative fully organic sensitizer

The quality of human life depends to a large degree on the availability of energy. In recent years, photovoltaic technology has been growing extraordinarily as a suitable source of energy, as a consequence of the increasing concern over the impact of fossil fuels on climate change. Developing affordable and highly efficiently photovoltaic technologies is the ultimate goal in this direction. Dye-sensitized solar cells (DSSCs) offer an efficient and easily implementing technology for future energy supply. Compared to conventional silicon solar cells, they provide comparable power conversion efficiency at low material and manufacturing costs. In addition, DSSCs are able to harvest low-intensity light in diffuse illumination conditions and then represent one of the most promising alternatives to the traditional photovoltaic technology, even more when trying to move towards flexible and transparent portable devices. Among these, considering the increasing demand of modern electronics for small, portable and wearable integrated optoelectronic devices, Fibre Dye-Sensitized Solar Cells (FDSSCs) have gained increasing interest as suitable energy provision systems for the development of the next-generation of smart products, namely “electronic textiles” or “e-textiles”. In this thesis, several key parameters towards the optimization of FDSSCs based on inexpensive and abundant TiO2 as photoanode and a new innovative fully organic sensitizer were studied. In particular, the effect of various FDSSCs components on the device properties pertaining to the cell architecture in terms of photoanode oxide layer thickness, electrolytic system, cell length and electrodes substrates were examined. The photovoltaic performances of the as obtained FDSSCs were fully characterized. Finally, the metal part of the devices (wire substrate) was substituted with substrates suitable for the textile industry as a fundamental step towards commercial exploitation.

Defect states in MAPbBr3 by photo-induced current transient spectroscopy

In the past decade, perovskites have been under the spotlight as promising semicon- ductors with unique properties. Hybrid halide perovskites show excellent characteristic properties suitable for optoelectronic applications as tunable band gap, high absorption coefficient, large mobility and long carrier recombination lifetime. However, a complete understanding of environmental instability and the nature of defects in these materials is still lacking, hindering the development of perovskite-based technologies. In this work we studied MAPbBr3 single crystals, fabricated with Inverse Temperature Crystallization (ITC) technique, with Photo-Induced Current Transient Spectroscopy (PICTS). PICTS is a transient photocurrent measurement rarely employed for studying perovskites mate- rials, that allows for the defects characterization in high resistivity materials. We studied the samples under different conditions, such as negative and positive voltage biases, bias stress, different contact geometries and different illumination wavelengths, in order to study their effect on the material physical properties and to evaluate the trap activation energies and their behavior under different working conditions.

Novel hydroxystearoyl-, heterocyclic and carbocyclic derivatives: synthesis and applications in biological field

This PhD project has been mainly focused on the synthesis of novel organic compounds containing heterocyclic and/or carbocyclic scaffold and on the study of stearic acid derivatives and their applications in biological field. The synthesis of novel derivatives of 9-hydroxystearic acid (9-HSA) evidenced how the presence of substituents on C9, able to make hydrogen bonds is of crucial importance for the biological activity. Also the position of the hydroxy group along the chain of hydroxystearic acids was investigated: regioisomers with the hydroxy group bound to odd carbons resulted more active than those bearing the hydroxy group on even carbons. Further, the insertion of (R)-9-HSA in magnetic nanoparticles gave a novel material which characterization remarked its suitability for drug delivery. Structural hybrids between amino aza-heterocycles and azelaic acid have been synthesized and some of them showed a selective activity towards osteosarcoma cell line U2OS. Several Apcin analogues bearing indole, benzothiazole, benzofurazan moieties connected to tryptaminyl-, amino pyridinyl-, pyrimidinyl- and pyrazinyl ring through a 1,1,1-trichloroethyl group were synthesized. Biological tests showed the importance of both the tryptaminyl and the pyrimidinyl moieties, confirming the effectiveness against acute leukemia models. The SNAr between 2-aminothiazole derivatives and 7-chlorodinitrobenzofuroxan revealed different behaviour depending from amino substituent of the thiazole. The reaction with 2-N-piperidinyl-, 2-N-morpholinyl-, or 2-N-pyrrolidinyl thiazole gave two isomeric species derived from the attack on C-5 of thiazole ring. Thiazoles substituted with primary- or not-cyclic secondary amines reacted with the exocyclic amino nitrogen atom giving a series of compounds whose biological activity have highlighted as they might be promising candidates for further development of antitumor agents. A series of 9-fluorenylidene derivatives, of interest in medical and optoelectronic field as organic scintillators, was synthesized through Wittig or Suzuky reaction and will be analyzed to test their potential scintillatory properties.

Light & Electron beam - the perfect combination for the observation and application of photo active materials

The aim of the present PhD thesis is to investigate the properties of innovative nanomaterials for energy conversion. The materials have been deeply studied by means of a wide spectrum of different techniques based on both light and electron sources, in order to get an insight into the correlation between the properties of each material and the activity towards different energy conversion applications. The activity has been carried out in the framework of a collaboration between the “G.Ciamician” Chemistry Department of the University of Bologna and the CNR-IMM Bologna. Four main topics have been explored: in the first part, luminescent silicon nanocrystals (SiNCs) have been discussed, suggesting a new approach to improve their optical properties as active material in complementary optoelectronic devices and photovoltaic cells. The luminescence of SiNCs have been exploited to increase the efficiency of conventional photovoltaic cells by means of an innovative architecture. Specifically, SiNCs were shown to be very promising light emitters in luminescent solar concentrators (LSC). The second part of the work has been focused on the study of high phosphorescent molecular chromophores, suggesting a new approach in their use as optical sensors successfully applied to the field of polymeric materials. This is due to the enhanced emission of light that appears in rigid, constrained or crystalline state, that is commonly called: "Aggregation-Induced Emission (AIE)". Such phenomenon is characteristic for molecular structures such as persulfurated benzene chromophores, hereafter named asterisks. The last two parts were focused on conventional and in-situ Transmission Electron Microscopy (TEM) morphological and structural characterization of photoactive and catalytic materials for energetic applications and in particular water splitting.

Design and characterization of polymorphs and co-crystals of organic molecular semiconductors

In the last decades, organic semiconductors have attracted attention due to their possible employment in solution-processed optoelectronic and electronic devices. One of the advantages of solution processing is the possibility to process into flexible substrates at low cost. Organic molecular materials tend to form polymorphs, which can exhibit very different properties. In most cases, the control of the crystal structure is decisive to maximize the performance of the final device. Although organic electronics have progressed a lot, n-type organic semiconductors still lag behind p-type, presenting challenges such as air instability and poor solubility. NDI derivatives are promising candidates for applications in organic electronics due to their characteristics. Recently, the structure-properties relationship and the polymorphism of these molecules have gained attention. In the first part of this thesis, NDI-C6 thermal behavior was extensively explored which revealed two different behaviors depending on the annealing process. This study allowed to define the stability ranking of the NDI-C6 bulk forms and to determine the crystal structure of Form γ at 54°C. Additionally, the polymorphic and thermal behavior of thin films of NDI-C6 was also explored. It was possible to isolate pure Form α, Form β, Form γ and a new metastable Form ε. It was also possible to determine the stability ranking of the phases in thin films. OFETs were fabricated having different polymorphs as active layer, unfortunately the performance was not ideal. During the second part of this thesis, core-chlorinated NDIs with fluoroalkyl chains were studied. Initially, the focus was on the polymorphism of CF3-NDI that revealed a solvate form with a very interesting molecular arrangement suggesting the possibility to form charge transfer co-crystals. In the last part of the thesis, the synthesis and characterization of CT co-crystal with different NDI derivatives, and acceptor and as donor BTBT and ditBu-BTBT were explored.

Determination of local density of states in amorphous oxide semiconductors with Kelvin Probe Force Microscopy

Amorphous semiconductors are important materials as they can be deposited by physical deposition techniques on large areas and even on plastic substrates. Therefore, they are crucial for transistors in large active matrices for imaging and transparent wearable electronics. The most widely applied candidate for amorphous thin film transistors production is Indium Gallium Zinc Oxide (IGZO). It is attracting much interest because of its optical transparency, facile processing by sputtering deposition and notable improved charge carrier mobility with respect to hydrogenated amorphous silicon a-Si:H. Degradation of the device and long-term performance issues have been observed if IGZO thin film transistors are subjected to electrical stress, leading to a modification of IGZO channel properties and subthreshold slope. Therefore, it is of great interest to have a reliable and precise method to study the conduction band tail, and the density of states in amorphous semiconductors. The aim of this thesis is to develop a local technique using Kelvin Probe Force Microscopy to study the evolution of IGZO DOS properties. The work is divided into three main parts. First, solutions to the non-linear Poisson-Boltzmann equation of a metal-insulator-semiconductor junction describing the charge accumulation and its relation to DOS properties are elaborated. Second macroscopic techniques such as capacitance voltage (CV) measurements and photocurrent spectroscopy are applied to obtain a non-local estimate of band-tail DOS properties in thin film transistor samples. The third part of my my thesis is dedicated to the KPFM measurements. By fitting the data to the developed numerical model, important parameters describing the amorphous conduction band tail are obtained. The results are in excellent agreement with the macroscopic characterizations. KPFM result is comparable also with non-local optoelectronic characterizations, such as photocurrent spectroscopy.

Characterization of in-gap electronic states in two-dimensional single crystal PEA2PbBr4 perovskite for X-ray detection

Hybrid Organic-Inorganic Halide Perovskites (HOIPs) include a large class of materials described with the general formula ABX3, where A is an organic cation, B an inorganic cation and X an halide anion. HOIPs show excellent optoelectronic characteristics such as tunable band gap, high adsorption coefficient and great mobility life-time. A subclass of these materials, the so-called two- dimensional (2D) layered HOIPs, have emerged as potential alternatives to traditional 3D analogs to enhance the stability and increase performance of perovskite devices, with particular regard in the area of ionizing radiation detectors, where these materials have reached truly remarkable milestones. One of the key challenges for future development of efficient and stable 2D perovskite X-ray detector is a complete understanding of the nature of defects that lead to the formation of deep states. Deep states act as non-radiative recombination centers for charge carriers and are one of the factors that most hinder the development of efficient 2D HOIPs-based X-ray detectors. In this work, deep states in PEA2PbBr4 were studied through Photo-Induced Current Transient Spectroscopy (PICTS), a highly sensitive spectroscopic technique capable of detecting the presence of deep states in highly resistive ohmic materials, and characterizing their activation energy, capture cross section and, under stringent conditions, the concentration of these states. The evolution of deep states in PEA 2 PbBr 4 was evaluated after exposure of the material to high doses of ionizing radiation and during aging (one year). The data obtained allowed us to evaluate the contribution of ion migration in PEA2PbBr4. This work represents an important starting point for a better understanding of transport and recombination phenomena in 2D perovskites. To date, the PICTS technique applied to 2D perovskites has not yet been reported in the scientific literature.