Controlling the size and size distribution of colloidal nanoparticles have gained extraordinary attention as their physical and chemical properties are strongly affected by size. Ligands are widely used to control the size and size distribution of nanoparticles; however, their exact roles in controlling the nanoparticle size distribution and the way they affect the nucleation and growth kinetics are poorly understood. Therefore, understanding the nucleation and growth mechanisms and developing theoretical/modeling framework will pave the way towards controlled synthesis of colloidal nanoparticles with desired sizes and polydispersity. This dissertation focuses on identifying the possible roles of ligands and size on the kinetics of nanoparticle formation and growth using in-situ characterization tools such as small-angle X-ray scattering (SAXS) and kinetic modeling. The presented work further focuses on developing kinetic models to capture the main nucleation and growth reactions and examines how ligand-metal interactions could potentially alter the rate of nucleation and growth rates, and consequently the nanoparticle size distribution. Additionally, this work highlights the importance of using multi-observables including the concentration of nanoparticles, size and/or precursor consumption, and polydispersity to differentiate between different nucleation and growth models and extract accurate information on the rates of nanoparticle nucleation and growth. Specifically, during the formation and growth of colloidal nanoparticles, complex reactions are occurring and as such nucleation and growth can take place through various reaction pathways. Therefore, sensitivity analysis was applied to effectively compare different nucleation and growth models and identify the most important reactions and obtain a reduced model (e.g. a minimalistic model) required for efficient data analysis. In the following chapters, a more sophisticated modeling approach is presented (population balance model) capable of capturing the average-properties of nanoparticle size distribution. PBM allows us to predict the growth rate of nanoparticles of different sizes, the ligand surface coverage for each individual size, and the parameters involved in altering the size distribution. Additionally, thermodynamic calculations of nanoparticle growth and ligand-metal binding as a function of size and ligand surface coverage were conducted to further shed light on the kinetics of nanoparticle formation and growth. The combination of kinetic modeling, in-situ SAXS and thermodynamic calculations can significantly advance the understanding of nucleation and growth mechanisms and guide toward controlling size and polydispersity. / Doctor of Philosophy / The synthesis of colloidal metal nanoparticles with superior control over size and size distribution, and has attracted much attention given the wide applications of these nanomaterials in the fields of catalysis, photonics, and electronics. Obtaining nanoparticles with desired sizes and polydispersity is vital for enhancing the consistency and performance for specific applications (e.g., catalytic converters for automotive emission). Ligands are often employed to prevent agglomeration and also control the nanoparticle size and size distribution. Ligands can affect the precursor reactivity and therefore the reduction/nucleation by binding with the metal precursor. Nucleation refers to the assimilation of few atoms to form initial nuclei acting as templates for nanoparticle growth. Additionally, ligands can bind with the nanoparticle surface sites and change the rate of surface growth and therefore the final nanoparticle size. Despite strong effects of ligands in the colloidal nanoparticle synthesis, their exact role in the nucleation and growth kinetics is yet to be identified. Additionally, nucleation and growth models capable of unraveling the underlying mechanisms of nucleation and growth in the presence of ligands are still lacking in the literature. Therefore, obtaining nanoparticles with desired sizes and polydispersity mostly relies on trial-and-error approach making the synthesis costly and inefficient. As such, developing models capable of predicting suitable synthesis conditions is contingent upon understanding the chemistry and mechanism involved during nanoparticles formation. Therefore, in this study, novel kinetic models were developed to capture the nucleation and growth kinetics of colloidal metal nanoparticles under different synthetic conditions (different types of solvents, different concentrations of ligand and metal). In-situ SAXS was further employed to measure the average diameter, concentration of nanoparticles, and polydispersity during the synthesis and extract the nucleation and growth rates (evolution of concentration of nanoparticles and size). First, an average-property model was developed to account for ligand-metal bindings and capture the size and concentration of nanoparticles during the synthesis. Then, a more complex modeling approach; PBM, accompanied by the thermodynamic calculations of surface growth and ligand-nanoparticle binding enthalpies was implemented to capture the size distribution. As it will be shown later, the determination of the underlying mechanisms resulted in a highly predictive kinetic model capable of predicting the synthetic conditions to obtain nanoparticles with desired sizes. The proposed methodology can serve as a powerful tool to synthesize nanoparticles with specific sizes and polydispersity.

Materials in smaller scales exhibit promising properties that are useful for wide variety of applications. Semiconductor quantum wells and quantum dots are two main examples of low-dimensional systems, where the quantum wells act as two-dimensional systems and the quantum dots act as zero-dimensional systems. Alternatively, semiconductor nanowires act as one-dimensional materials, and they exhibit promising and device applicable properties. These nanowires are relatively new class of materials compared to the quantum wells and the quantum dots. The semiconductor nanowires are expected to be the building blocks for future nanoelectronic and nano-optoelectronic device technology. Vapor-liquid-solid (VLS) mechanism is a widely used mechanism for the growth of semiconductor nanowires, where metal nanoparticles are used as the catalysts for the nanowires growth. This growth mechanism offers a flexibility to control the size, morphology and location of the semiconductor nanowires. In the VLS growth, changing the chemical composition of vapor constituents produce consequent compositional modulation in each nanowire. The compositional modulation along the nanowire axis produces axial nanowire heterostructures and in radial direction produces radial nanowire heterostructures. Such compositional modulation within an individual nanowire enables the designing of band structure of a nanowire and thereby allows the fabrication of single nanowire devices. These nanowire heterostructures show many potential properties and consequent applications. Although the semiconductor nanowire heterostructures are promising semiconductor nanostructures, the fundamental growth mechanisms of axial and radial nanowire heterostructures have not been explored sufficiently due to their complex nature of the growth. In this regard, this PhD thesis addresses the fundamental issues associated with axial and radial nanowire heterostructures. For such fundamental investigations, this PhD work chooses InAs/GaAs nanowire heterostructure system due to its potential applications. In fact, InAs/GaAs nanowire heterostructures are the first reported axial nanowire heterostructure system. However, no detailed investigations were reported on this system so far. The detailed nucleation and growth mechanisms associated with InAs/GaAs nanowire heterostructure system are explored in this thesis using electron microscopy investigations. This objective is achieved in the following steps. • InAs/GaAs nanowire heterostructures are grown using Au nanoparticles and metal-organic chemical vapor deposition (MOCVD) method. To determine the axial and radial growth evolution of InAs on GaAs nanowires, different InAs/GaAs nanowire heterostructures are produced by depositing InAs for different durations on GaAs nanowires. The GaAs nanowires are initially grown for 30 min and then the InAs is deposited on these nanowires for 1, 3, 5 and 30 min. • These InAs/GaAs nanowire heterostructures are subjected to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations. These investigations determine that, in the initial stages of the InAs axial growth (1 min), the Au particles move sidewards and subsequently downwards by maintaining an interface with the GaAs nanowire. Such a movement of Au catalysts is attributed to lower Au/GaAs interfacial energy than Au/InAs. The detailed TEM investigations show that this Au movement depends upon the crystallographic nature of the GaAs nanowire. The Au particle is always tend to move towards {112}B sidewall of the GaAs nanowire rather than its {112}A sidewalls. Increase in InAs growth duration shows that InAs branches evolve from GaAs-InAs core-shell structures. Such evolution is observed in following steps: (1) the movement of Au particle terminates when it encounters the radially grown InAs on GaAs nanowires; (2) further growth of InAs leads to the InAs nanowire growth from those terminated Au nanoparticles in the form of branches. • The TEM observations of InAs/GaAs nanowire heterostructures show that, in the initial stages of InAs radial growth on GaAs nanowires, InAs nucleates preferentially in the concave regions of the non-planar sidewalls of the GaAs nanowire. The further growth of InAs leads to the preferential formation of InAs shell structure at the regions of concave regions. Such heterogeneous formation of shell structure resembles InAs nanoring structures around GaAs nanowire cores. InAs growth on the planar {112} sidewalls of GaAs nanowires with hexagonal cross sections shows different growth phenomena to the above described InAs nanorings formation. In this case, InAs preferentially nucleates on {112}A sidewalls of the GaAs nanowires and with further deposition of InAs, the complete shell structure of InAs form with {110} sidewalls on the GaAs nanowire cores. • In addition to the above mentioned investigations, to observe the growth evolution of GaAs on InAs nanowires, GaAs is grown for 3 and 30 min on InAs nanowires. The TEM investigations of these nanostructures show that the axial GaAs/InAs hetero-interface contains an InGaAs transition segment in contrast to the sharp InAs/GaAs (InAs on GaAs) hetero-interface. The different nature of hetero-interfaces is attributed to the different affinities between Au catalysts and Ga or In. The radial growth of GaAs on InAs nanowires show that the GaAs shell has grown in wurtzite structure around the wurtzite structured InAs nanowire cores. Overall, through the extensive SEM and TEM investigations, this PhD thesis addresses the fundamental issues related to the growth of axial and radial nanowire heterostructures. Such fundamental investigations are expected to advance the processing and application prospective of the semiconductor nanowires and their associated heterostructures.

Organic solar cells (OSC) are interesting as low cost alternative to conventional solar cells. Unsubstituted Metal-phthalocyanines (Pc) are excellent electron donating molecules for heterojunction OSC. Usually organic solar cells with Pcs are produced by vapor deposition, although solution based deposition (like spin casting) is cheaper and offers more possibilities to control the structure of the film. With solution based deposition several parameters (like temperature, solvent and etc.) affect the self-organized structure formation via nucleation and growth. The reason why vapor deposition is typically used is the poor solubility of the metal-phthalocyanines in most common solvents. Furthermore the process of nucleation and growth of Pc aggregates from solution is not well understood. For preparation of Pc films from solution, it is necessary to find the appropriate solvents, assess the solution deposition techniques, such as dip coating, and spin casting. It is necessary to understand the nucleation and growth process for aggregation/precipitation and to use this knowledge to produce nanostructures appropriate for OSC. This is important because the nanostructure of the films determines their performance. In this thesis, optical absorption and the stability of 8 different unsubstituted metal Pc’s were studied quantitatively in 28 different solvents. Among the several solution based deposited thin films produced based on this study, copper phthalocyanine (CuPc) dissolved in trifluoroacetic acid (TFA) is chosen as a model system for an in-depth study. CuPc has sufficient solubility and stability in TFA and upon solution processing forms appropriate structures for OSCs. CuPc molecules aggregate into layers of nanoribbons with a thickness of ~ 1 nm and an adjustable width and length. The morphology and the number of deposited layers in the thin films are controlled by different parameters, like temperature and solution concentration. Material properties of CuPc deposited from TFA are studied in detail via x-ray diffraction, UV-Vis and FT-IR spectroscopy. Atomic force microscopy was used to study the morphology of the dried film. The mechanism of the formation of CuPc nanoribbons from spin casted CuPc/TFA solution in ambient temperature is investigated and explained. The parameters (e.g. solution concentration profile) governing nucleation and growth are calculated based on the spin casting theory of a binary mixture of a nonvolatile solute and evaporative solvent. Based on this and intermolecular interactions between CuPc and substrate a nucleation and growth model is developed explaining the aggregation of CuPc in a supersaturated TFA solution. Finally, a solution processed thin film of CuPc is applied as a donor layer in a functioning bilayer heterojunction OSC and the influence of the structure on OSC performance is studied. / In den vergangenen Jahren wurden kosteneffiziente nasschemische Beschichtungsverfahren für die Herstellung organischer Dünnfilme für verschiedene opto-elektronische Anwendungen entdeckt und weiterentwickelt. Unter anderem wurden Phthalocyanin-Moleküle in photoaktiven Schichten für die Herstellung von Solarzellen intensiv erforscht. Aufgrund der kleinen bzw. unbekannten Löslichkeit wurden Phthalocyanin-Schichten durch Aufdampfverfahren im Vakuum hergestellt. Des Weiteren wurde die Löslichkeit durch chemische Synthese erhöht, was aber die Eigenschaften von Pc beeinträchtigte. In dieser Arbeit wurde die Löslichkeit, optische Absorption und Stabilität von 8 verschiedenen unsubstituierten Metall-Phthalocyaninen in 28 verschiedenen Lösungsmitteln quantitativ gemessen. Wegen ausreichender Löslichkeit, Stabilität und Anwendbarkeit in organischen Solarzellen wurde Kupferphthalocyanin (CuPc) in Trifluoressigsäure (TFA) für weitere Untersuchungen ausgewählt. Durch die Rotationsbeschichtung von CuPc aus TFA Lösung wurde ein dünner Film aus der verdampfenden Lösung auf dem Substrat platziert. Nach dem Verdampfen des Lösungsmittels, die Nanobändern aus CuPc bedecken das Substrat. Die Nanobänder haben eine Dicke von etwa ~ 1 nm (typische Dimension eines CuPc-Molekül) und variierender Breite und Länge, je nach Menge des Materials. Solche Nanobändern können durch Rotationsbeschichtung oder auch durch andere Nassbeschichtungsverfahren, wie Tauchbeschichtung, erzeugt werden. Ähnliche Fibrillen-Strukturen entstehen durch Nassbeschichtung von anderen Metall-Phthalocyaninen, wie Eisen- und Magnesium-Phthalocyanin, aus TFA-Lösung sowie auf anderen Substraten, wie Glas oder Indium Zinnoxid. Materialeigenschaften von aufgebrachten CuPc aus TFA Lösung und CuPc in der Lösung wurden ausführlich mit Röntgenbeugung, Spektroskopie- und Mikroskopie Methoden untersucht. Es wird gezeigt, dass die Nanobänder nicht in der Lösung, sondern durch Verdampfen des Lösungsmittels und der Übersättigung der Lösung entstehen. Die Rasterkraftmikroskopie wurde dazu verwendet, um die Morphologie des getrockneten Films bei unterschiedlicher Konzentration zu studieren. Der Mechanismus der Entstehung der Nanobändern wurde im Detail studiert. Gemäß der Keimbildung und Wachstumstheorie wurde die Entstehung der CuPc Nanobänder aus einer übersättigt Lösung diskutiert. Die Form der Nanobändern wurde unter Berücksichtigung der Wechselwirkung zwischen den Molekülen und dem Substrat diskutiert. Die nassverarbeitete CuPc-Dünnschicht wurde als Donorschicht in organischen Doppelschicht Solarzellen mit C60-Molekül, als Akzeptor eingesetzt. Die Effizienz der Energieumwandlung einer solchen Zelle wurde entsprechend den Schichtdicken der CuPc Schicht untersucht.

The graphene moiré superstructure formed on Ru(0001) (g/Ru(0001)) has shown the potential as a template to self-assemble super-lattices of metal nanoparticles as model catalysts. To explore the possibility of rational catalyst design on g/Ru(0001), detailed density functional theory (DFT) calculations have been performed to investigate the adsorption and diffusion of Rh and Au adatoms on g/Ru(0001). The consequences of different hopping rates for cluster nucleation have been explored by performing Monte Carlo-based statistical analysis, which suggests that diffusing species other than adatoms need to be taken into account to develop an accurate description of cluster nucleation and growth on this surface. DFT calculations have also been carried out to investigate the adsorption and diffusion of 18 4d (Y-Ag) and 5d (La-Au) transition metal adatoms on g/Ru(0001). Given the necessity to study larger diffusing species than adatoms, DFT calculations have been performed to study the adsorption and diffusion of Rh and Au dimers and trimers on g/Ru(0001). It was shown that the mobility of Rh clusters decreases with the increase of cluster size, while for Au, dimers diffuse faster than monomers and trimers on the moiré surface. We then used a genetic algorithm combined with DFT calculations to predict the lowest energy structure of a Au8 cluster on g/Ru(0001). Our prediction leads us to propose that Au clusters aggregates through Oswald ripening with Au dimer being the major diffusing species. Finally, we examined the morphology of a Cu19 cluster on g/Cu(111) using MD simulations with COMB3 potential. We also studied the mobility of Cu clusters on g/Cu(111) at elevated temperatures. The analysis suggests that g/Cu(111) may not be a suitable substrate for the formation and growth of isolated Cu clusters. All these calculation results have provided us a better understanding and useful insights into the nucleation and growth mechanism of metal clusters on graphene moiré.

Materials in smaller scales exhibit promising properties that are useful for wide variety of applications. Semiconductor quantum wells and quantum dots are two main examples of low-dimensional systems, where the quantum wells act as two-dimensional systems and the quantum dots act as zero-dimensional systems. Alternatively, semiconductor nanowires act as one-dimensional materials, and they exhibit promising and device applicable properties. These nanowires are relatively new class of materials compared to the quantum wells and the quantum dots. The semiconductor nanowires are expected to be the building blocks for future nanoelectronic and nano-optoelectronic device technology. Vapor-liquid-solid (VLS) mechanism is a widely used mechanism for the growth of semiconductor nanowires, where metal nanoparticles are used as the catalysts for the nanowires growth. This growth mechanism offers a flexibility to control the size, morphology and location of the semiconductor nanowires. In the VLS growth, changing the chemical composition of vapor constituents produce consequent compositional modulation in each nanowire. The compositional modulation along the nanowire axis produces axial nanowire heterostructures and in radial direction produces radial nanowire heterostructures. Such compositional modulation within an individual nanowire enables the designing of band structure of a nanowire and thereby allows the fabrication of single nanowire devices. These nanowire heterostructures show many potential properties and consequent applications. Although the semiconductor nanowire heterostructures are promising semiconductor nanostructures, the fundamental growth mechanisms of axial and radial nanowire heterostructures have not been explored sufficiently due to their complex nature of the growth. In this regard, this PhD thesis addresses the fundamental issues associated with axial and radial nanowire heterostructures. For such fundamental investigations, this PhD work chooses InAs/GaAs nanowire heterostructure system due to its potential applications. In fact, InAs/GaAs nanowire heterostructures are the first reported axial nanowire heterostructure system. However, no detailed investigations were reported on this system so far. The detailed nucleation and growth mechanisms associated with InAs/GaAs nanowire heterostructure system are explored in this thesis using electron microscopy investigations. This objective is achieved in the following steps. • InAs/GaAs nanowire heterostructures are grown using Au nanoparticles and metal-organic chemical vapor deposition (MOCVD) method. To determine the axial and radial growth evolution of InAs on GaAs nanowires, different InAs/GaAs nanowire heterostructures are produced by depositing InAs for different durations on GaAs nanowires. The GaAs nanowires are initially grown for 30 min and then the InAs is deposited on these nanowires for 1, 3, 5 and 30 min. • These InAs/GaAs nanowire heterostructures are subjected to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations. These investigations determine that, in the initial stages of the InAs axial growth (1 min), the Au particles move sidewards and subsequently downwards by maintaining an interface with the GaAs nanowire. Such a movement of Au catalysts is attributed to lower Au/GaAs interfacial energy than Au/InAs. The detailed TEM investigations show that this Au movement depends upon the crystallographic nature of the GaAs nanowire. The Au particle is always tend to move towards {112}B sidewall of the GaAs nanowire rather than its {112}A sidewalls. Increase in InAs growth duration shows that InAs branches evolve from GaAs-InAs core-shell structures. Such evolution is observed in following steps: (1) the movement of Au particle terminates when it encounters the radially grown InAs on GaAs nanowires; (2) further growth of InAs leads to the InAs nanowire growth from those terminated Au nanoparticles in the form of branches. • The TEM observations of InAs/GaAs nanowire heterostructures show that, in the initial stages of InAs radial growth on GaAs nanowires, InAs nucleates preferentially in the concave regions of the non-planar sidewalls of the GaAs nanowire. The further growth of InAs leads to the preferential formation of InAs shell structure at the regions of concave regions. Such heterogeneous formation of shell structure resembles InAs nanoring structures around GaAs nanowire cores. InAs growth on the planar {112} sidewalls of GaAs nanowires with hexagonal cross sections shows different growth phenomena to the above described InAs nanorings formation. In this case, InAs preferentially nucleates on {112}A sidewalls of the GaAs nanowires and with further deposition of InAs, the complete shell structure of InAs form with {110} sidewalls on the GaAs nanowire cores. • In addition to the above mentioned investigations, to observe the growth evolution of GaAs on InAs nanowires, GaAs is grown for 3 and 30 min on InAs nanowires. The TEM investigations of these nanostructures show that the axial GaAs/InAs hetero-interface contains an InGaAs transition segment in contrast to the sharp InAs/GaAs (InAs on GaAs) hetero-interface. The different nature of hetero-interfaces is attributed to the different affinities between Au catalysts and Ga or In. The radial growth of GaAs on InAs nanowires show that the GaAs shell has grown in wurtzite structure around the wurtzite structured InAs nanowire cores. Overall, through the extensive SEM and TEM investigations, this PhD thesis addresses the fundamental issues related to the growth of axial and radial nanowire heterostructures. Such fundamental investigations are expected to advance the processing and application prospective of the semiconductor nanowires and their associated heterostructures.

Materials in smaller scales exhibit promising properties that are useful for wide variety of applications. Semiconductor quantum wells and quantum dots are two main examples of low-dimensional systems, where the quantum wells act as two-dimensional systems and the quantum dots act as zero-dimensional systems. Alternatively, semiconductor nanowires act as one-dimensional materials, and they exhibit promising and device applicable properties. These nanowires are relatively new class of materials compared to the quantum wells and the quantum dots. The semiconductor nanowires are expected to be the building blocks for future nanoelectronic and nano-optoelectronic device technology. Vapor-liquid-solid (VLS) mechanism is a widely used mechanism for the growth of semiconductor nanowires, where metal nanoparticles are used as the catalysts for the nanowires growth. This growth mechanism offers a flexibility to control the size, morphology and location of the semiconductor nanowires. In the VLS growth, changing the chemical composition of vapor constituents produce consequent compositional modulation in each nanowire. The compositional modulation along the nanowire axis produces axial nanowire heterostructures and in radial direction produces radial nanowire heterostructures. Such compositional modulation within an individual nanowire enables the designing of band structure of a nanowire and thereby allows the fabrication of single nanowire devices. These nanowire heterostructures show many potential properties and consequent applications. Although the semiconductor nanowire heterostructures are promising semiconductor nanostructures, the fundamental growth mechanisms of axial and radial nanowire heterostructures have not been explored sufficiently due to their complex nature of the growth. In this regard, this PhD thesis addresses the fundamental issues associated with axial and radial nanowire heterostructures. For such fundamental investigations, this PhD work chooses InAs/GaAs nanowire heterostructure system due to its potential applications. In fact, InAs/GaAs nanowire heterostructures are the first reported axial nanowire heterostructure system. However, no detailed investigations were reported on this system so far. The detailed nucleation and growth mechanisms associated with InAs/GaAs nanowire heterostructure system are explored in this thesis using electron microscopy investigations. This objective is achieved in the following steps. • InAs/GaAs nanowire heterostructures are grown using Au nanoparticles and metal-organic chemical vapor deposition (MOCVD) method. To determine the axial and radial growth evolution of InAs on GaAs nanowires, different InAs/GaAs nanowire heterostructures are produced by depositing InAs for different durations on GaAs nanowires. The GaAs nanowires are initially grown for 30 min and then the InAs is deposited on these nanowires for 1, 3, 5 and 30 min. • These InAs/GaAs nanowire heterostructures are subjected to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations. These investigations determine that, in the initial stages of the InAs axial growth (1 min), the Au particles move sidewards and subsequently downwards by maintaining an interface with the GaAs nanowire. Such a movement of Au catalysts is attributed to lower Au/GaAs interfacial energy than Au/InAs. The detailed TEM investigations show that this Au movement depends upon the crystallographic nature of the GaAs nanowire. The Au particle is always tend to move towards {112}B sidewall of the GaAs nanowire rather than its {112}A sidewalls. Increase in InAs growth duration shows that InAs branches evolve from GaAs-InAs core-shell structures. Such evolution is observed in following steps: (1) the movement of Au particle terminates when it encounters the radially grown InAs on GaAs nanowires; (2) further growth of InAs leads to the InAs nanowire growth from those terminated Au nanoparticles in the form of branches. • The TEM observations of InAs/GaAs nanowire heterostructures show that, in the initial stages of InAs radial growth on GaAs nanowires, InAs nucleates preferentially in the concave regions of the non-planar sidewalls of the GaAs nanowire. The further growth of InAs leads to the preferential formation of InAs shell structure at the regions of concave regions. Such heterogeneous formation of shell structure resembles InAs nanoring structures around GaAs nanowire cores. InAs growth on the planar {112} sidewalls of GaAs nanowires with hexagonal cross sections shows different growth phenomena to the above described InAs nanorings formation. In this case, InAs preferentially nucleates on {112}A sidewalls of the GaAs nanowires and with further deposition of InAs, the complete shell structure of InAs form with {110} sidewalls on the GaAs nanowire cores. • In addition to the above mentioned investigations, to observe the growth evolution of GaAs on InAs nanowires, GaAs is grown for 3 and 30 min on InAs nanowires. The TEM investigations of these nanostructures show that the axial GaAs/InAs hetero-interface contains an InGaAs transition segment in contrast to the sharp InAs/GaAs (InAs on GaAs) hetero-interface. The different nature of hetero-interfaces is attributed to the different affinities between Au catalysts and Ga or In. The radial growth of GaAs on InAs nanowires show that the GaAs shell has grown in wurtzite structure around the wurtzite structured InAs nanowire cores. Overall, through the extensive SEM and TEM investigations, this PhD thesis addresses the fundamental issues related to the growth of axial and radial nanowire heterostructures. Such fundamental investigations are expected to advance the processing and application prospective of the semiconductor nanowires and their associated heterostructures.

Materials in smaller scales exhibit promising properties that are useful for wide variety of applications. Semiconductor quantum wells and quantum dots are two main examples of low-dimensional systems, where the quantum wells act as two-dimensional systems and the quantum dots act as zero-dimensional systems. Alternatively, semiconductor nanowires act as one-dimensional materials, and they exhibit promising and device applicable properties. These nanowires are relatively new class of materials compared to the quantum wells and the quantum dots. The semiconductor nanowires are expected to be the building blocks for future nanoelectronic and nano-optoelectronic device technology. Vapor-liquid-solid (VLS) mechanism is a widely used mechanism for the growth of semiconductor nanowires, where metal nanoparticles are used as the catalysts for the nanowires growth. This growth mechanism offers a flexibility to control the size, morphology and location of the semiconductor nanowires. In the VLS growth, changing the chemical composition of vapor constituents produce consequent compositional modulation in each nanowire. The compositional modulation along the nanowire axis produces axial nanowire heterostructures and in radial direction produces radial nanowire heterostructures. Such compositional modulation within an individual nanowire enables the designing of band structure of a nanowire and thereby allows the fabrication of single nanowire devices. These nanowire heterostructures show many potential properties and consequent applications. Although the semiconductor nanowire heterostructures are promising semiconductor nanostructures, the fundamental growth mechanisms of axial and radial nanowire heterostructures have not been explored sufficiently due to their complex nature of the growth. In this regard, this PhD thesis addresses the fundamental issues associated with axial and radial nanowire heterostructures. For such fundamental investigations, this PhD work chooses InAs/GaAs nanowire heterostructure system due to its potential applications. In fact, InAs/GaAs nanowire heterostructures are the first reported axial nanowire heterostructure system. However, no detailed investigations were reported on this system so far. The detailed nucleation and growth mechanisms associated with InAs/GaAs nanowire heterostructure system are explored in this thesis using electron microscopy investigations. This objective is achieved in the following steps. • InAs/GaAs nanowire heterostructures are grown using Au nanoparticles and metal-organic chemical vapor deposition (MOCVD) method. To determine the axial and radial growth evolution of InAs on GaAs nanowires, different InAs/GaAs nanowire heterostructures are produced by depositing InAs for different durations on GaAs nanowires. The GaAs nanowires are initially grown for 30 min and then the InAs is deposited on these nanowires for 1, 3, 5 and 30 min. • These InAs/GaAs nanowire heterostructures are subjected to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations. These investigations determine that, in the initial stages of the InAs axial growth (1 min), the Au particles move sidewards and subsequently downwards by maintaining an interface with the GaAs nanowire. Such a movement of Au catalysts is attributed to lower Au/GaAs interfacial energy than Au/InAs. The detailed TEM investigations show that this Au movement depends upon the crystallographic nature of the GaAs nanowire. The Au particle is always tend to move towards {112}B sidewall of the GaAs nanowire rather than its {112}A sidewalls. Increase in InAs growth duration shows that InAs branches evolve from GaAs-InAs core-shell structures. Such evolution is observed in following steps: (1) the movement of Au particle terminates when it encounters the radially grown InAs on GaAs nanowires; (2) further growth of InAs leads to the InAs nanowire growth from those terminated Au nanoparticles in the form of branches. • The TEM observations of InAs/GaAs nanowire heterostructures show that, in the initial stages of InAs radial growth on GaAs nanowires, InAs nucleates preferentially in the concave regions of the non-planar sidewalls of the GaAs nanowire. The further growth of InAs leads to the preferential formation of InAs shell structure at the regions of concave regions. Such heterogeneous formation of shell structure resembles InAs nanoring structures around GaAs nanowire cores. InAs growth on the planar {112} sidewalls of GaAs nanowires with hexagonal cross sections shows different growth phenomena to the above described InAs nanorings formation. In this case, InAs preferentially nucleates on {112}A sidewalls of the GaAs nanowires and with further deposition of InAs, the complete shell structure of InAs form with {110} sidewalls on the GaAs nanowire cores. • In addition to the above mentioned investigations, to observe the growth evolution of GaAs on InAs nanowires, GaAs is grown for 3 and 30 min on InAs nanowires. The TEM investigations of these nanostructures show that the axial GaAs/InAs hetero-interface contains an InGaAs transition segment in contrast to the sharp InAs/GaAs (InAs on GaAs) hetero-interface. The different nature of hetero-interfaces is attributed to the different affinities between Au catalysts and Ga or In. The radial growth of GaAs on InAs nanowires show that the GaAs shell has grown in wurtzite structure around the wurtzite structured InAs nanowire cores. Overall, through the extensive SEM and TEM investigations, this PhD thesis addresses the fundamental issues related to the growth of axial and radial nanowire heterostructures. Such fundamental investigations are expected to advance the processing and application prospective of the semiconductor nanowires and their associated heterostructures.

Materials in smaller scales exhibit promising properties that are useful for wide variety of applications. Semiconductor quantum wells and quantum dots are two main examples of low-dimensional systems, where the quantum wells act as two-dimensional systems and the quantum dots act as zero-dimensional systems. Alternatively, semiconductor nanowires act as one-dimensional materials, and they exhibit promising and device applicable properties. These nanowires are relatively new class of materials compared to the quantum wells and the quantum dots. The semiconductor nanowires are expected to be the building blocks for future nanoelectronic and nano-optoelectronic device technology. Vapor-liquid-solid (VLS) mechanism is a widely used mechanism for the growth of semiconductor nanowires, where metal nanoparticles are used as the catalysts for the nanowires growth. This growth mechanism offers a flexibility to control the size, morphology and location of the semiconductor nanowires. In the VLS growth, changing the chemical composition of vapor constituents produce consequent compositional modulation in each nanowire. The compositional modulation along the nanowire axis produces axial nanowire heterostructures and in radial direction produces radial nanowire heterostructures. Such compositional modulation within an individual nanowire enables the designing of band structure of a nanowire and thereby allows the fabrication of single nanowire devices. These nanowire heterostructures show many potential properties and consequent applications. Although the semiconductor nanowire heterostructures are promising semiconductor nanostructures, the fundamental growth mechanisms of axial and radial nanowire heterostructures have not been explored sufficiently due to their complex nature of the growth. In this regard, this PhD thesis addresses the fundamental issues associated with axial and radial nanowire heterostructures. For such fundamental investigations, this PhD work chooses InAs/GaAs nanowire heterostructure system due to its potential applications. In fact, InAs/GaAs nanowire heterostructures are the first reported axial nanowire heterostructure system. However, no detailed investigations were reported on this system so far. The detailed nucleation and growth mechanisms associated with InAs/GaAs nanowire heterostructure system are explored in this thesis using electron microscopy investigations. This objective is achieved in the following steps. • InAs/GaAs nanowire heterostructures are grown using Au nanoparticles and metal-organic chemical vapor deposition (MOCVD) method. To determine the axial and radial growth evolution of InAs on GaAs nanowires, different InAs/GaAs nanowire heterostructures are produced by depositing InAs for different durations on GaAs nanowires. The GaAs nanowires are initially grown for 30 min and then the InAs is deposited on these nanowires for 1, 3, 5 and 30 min. • These InAs/GaAs nanowire heterostructures are subjected to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations. These investigations determine that, in the initial stages of the InAs axial growth (1 min), the Au particles move sidewards and subsequently downwards by maintaining an interface with the GaAs nanowire. Such a movement of Au catalysts is attributed to lower Au/GaAs interfacial energy than Au/InAs. The detailed TEM investigations show that this Au movement depends upon the crystallographic nature of the GaAs nanowire. The Au particle is always tend to move towards {112}B sidewall of the GaAs nanowire rather than its {112}A sidewalls. Increase in InAs growth duration shows that InAs branches evolve from GaAs-InAs core-shell structures. Such evolution is observed in following steps: (1) the movement of Au particle terminates when it encounters the radially grown InAs on GaAs nanowires; (2) further growth of InAs leads to the InAs nanowire growth from those terminated Au nanoparticles in the form of branches. • The TEM observations of InAs/GaAs nanowire heterostructures show that, in the initial stages of InAs radial growth on GaAs nanowires, InAs nucleates preferentially in the concave regions of the non-planar sidewalls of the GaAs nanowire. The further growth of InAs leads to the preferential formation of InAs shell structure at the regions of concave regions. Such heterogeneous formation of shell structure resembles InAs nanoring structures around GaAs nanowire cores. InAs growth on the planar {112} sidewalls of GaAs nanowires with hexagonal cross sections shows different growth phenomena to the above described InAs nanorings formation. In this case, InAs preferentially nucleates on {112}A sidewalls of the GaAs nanowires and with further deposition of InAs, the complete shell structure of InAs form with {110} sidewalls on the GaAs nanowire cores. • In addition to the above mentioned investigations, to observe the growth evolution of GaAs on InAs nanowires, GaAs is grown for 3 and 30 min on InAs nanowires. The TEM investigations of these nanostructures show that the axial GaAs/InAs hetero-interface contains an InGaAs transition segment in contrast to the sharp InAs/GaAs (InAs on GaAs) hetero-interface. The different nature of hetero-interfaces is attributed to the different affinities between Au catalysts and Ga or In. The radial growth of GaAs on InAs nanowires show that the GaAs shell has grown in wurtzite structure around the wurtzite structured InAs nanowire cores. Overall, through the extensive SEM and TEM investigations, this PhD thesis addresses the fundamental issues related to the growth of axial and radial nanowire heterostructures. Such fundamental investigations are expected to advance the processing and application prospective of the semiconductor nanowires and their associated heterostructures.

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O presente trabalho apresenta um estudo sistemático sobre a adsorção e a eletrooxidação do CO sobre eletrodos monocristalinos de platina. A partir da análise das intensidades das bandas integradas e das freqüências do Pt(111)-CO, apresenta-se uma interpretação dos efeitos de acoplamento dipolo-dipolo e de interconversão do COads.. Assim, sobre a Pt(111) os espectros de FTIR in situ mostram que o aumento na razão da intensidade das bandas integradas ACOB/ACOL e nas freqüências do νCOB quando θCO,total diminue é devido à redução do acoplamento dipolo-dipolo entre as moléculas do CO em diferentes sítios e, adicionalmente, à interconversão das formas inclinadas dos COL e COB para a forma do COB. No sentido de explicar esta interconversão, propomos um mechanism baseado nas interações dos orbitais de fronteiras do CO e do metal, associado com a retrodoação de elétrons. Nesse modelo, os deslocamentos das formas inclinadas do COL e do COB em direção à forma do COB são favoráveis provavelmente porque a retrodoação de elétrons, Ptd → CO2π* (LUMO), aumenta quando θCO,total diminui. Experimentos potenciostáticos sugerem que a cinética de nucleação e crescimento é o melhor modelo para descrever a eletrooxidação do CO. Propomos que no potencial de oxidação, ECO oxi. pode existir uma via muito rápida de formação do precursores oxigenados e que este pode lateralmente colidir com as ilhas de CO, impedindo que ocorra a dissipação das ilhas do COads. no potencial de oxidação, ECO oxi.. Apresentamos a evolução do crescimento e da oxidação de sub-monocamada de CO sobre monocristais de platina facetados. Em baixo grau de recobrimento do CO foi observado que a adsorção dessa molécula ocorre sem ocupação preferencial de sítios quinas ou terraças. Assim, sugerimos que a adsorção é um processo randômico e que depois que as moléculas do CO são adsorvidas estas não apresentam apreciáveis deslocamentos a partir de CO-(111) em direção aos sítios CO-(110). Isto significa que depois da adsorção, as moléculas do CO têm um longo tempo de residência ou que apresentam um coeficiente de difusão muito baixo. Mas, para alto grau de recobrimento por CO, os resultados mostram que é possível que laterais interações desempanham importantes papéis na distribuição de ocupação dos sítios e observamos que durante a eletrooxidação, são liberados simultaneamente sítios quinas e sítios terraços. Quanto à pré-oxidação, foi observado que quatro condições experimentais precisam ser satisfeitas para que ela ocorra sobre os monocristais de platina: (i) alto grau de recobrimento por CO; (ii) que a superfície onde oncorre a oxidação do CO tenha defeitos, como sítios quinas (110); (iii) que a camada do CO seja formada sob potenciais mais negativos do que o potencial de carga total zero do metal; (iv) e que exista pequena quantidade de CO dissolvido. As condições (i) e (ii) precisam ser satisfeitas simultaneamente para promover a pré-oxidação do CO; as condições (iii) e (iv) essencialmente contribuem correspondendo à condição (i). Observamos que a magnitude do pre-pico aumenta com o aumento do grau de recobrimento por CO. Então, isto pode ser indicativo que a pré-oxidação não tem relação com a difusão do CO em superfície porque o aumento do grau de recobrimento reduz a probabilidade de difusão em superfície. O modelo de ilhas comprimidas parece ser mais apropriado para descrever a pré-oxidação do CO. / This work presents a systematic study on the CO adsorption and its oxidation at platinum single crystal electrodes. From analysis of integrated band intensity and band frequency position of the Pt(111)-CO interface in acid, it is presented an interpretation of the dipole-dipole coupling effect and surface site inter-conversions of COads.. Thus, on Pt(111), in situ FTIR data show that the increase in both ratio integrated band intensity ACOB/ACOL and frequency of νCOB when θCO,total reduces it is indicative of reduce in dipole-dipole coupling interactions between CO molecules in different surface active sites and a mechanism where the tilted COL and COB in CO pressed adlayer displace or inter-convert in favor of increase of COB concentration. In order to explain that CO interconversion, we propose a mechanism based in frontier molecular orbitals of CO and the orbitals of the metal associated with the electron back bond donation. Thus, the displacement of tilted COL and COB on the surface towards COB is more stable because probably the back bond electron donation, Ptd → CO2π* (LUMO), increase when θCO,total diminishes. Potentiostatic experiments suggest that the nucleation and growth is the better model to describe the CO oxidation. It is proposed here that close to ECO oxi. might there is a fast pathway toward formation of oxygenated species and it might reach the CO islands by side and this hinder the dissipation of COads. islands at ECO oxi.. We report also time evolution studies of low CO adsorption coverage and oxidative stripping on stepped platinum surfaces. In low CO coverage, it was observed that there is no preferential site occupancy for CO adsorption on step or terrace. It is proposed that CO adsorption onto these surfaces is a random process, and after CO adsorption there is no appreciable shift from CO-(111) to CO-(110) sites. This implies that after adsorption, CO molecules either have a very long residence time, or that the diffusion coefficient is much lower than previously thought. But, in high CO coverage, the results show that it is possible that the lateral interaction might play important role in CO site occupancy and it was observed that during the CO electrooxidation the sites released included both terrace (111) and step (110) orientations. Among the CO oxidation a clear CO preoxidation process also occurs. It was observed four experimental conditions which were verified to be fulfilled to promote CO pre-oxidation on platinum single crystal: (i) the CO coverage is should be higher than minimum threshold; (ii) the surface where CO oxidation take place should have defects, such as (110) steps; (ii) the CO monolayer should be formed at potentials below the potential of zero total charge; (iv) and in a small amount of dissolved CO should be present in the electrolyte solution. In both conditions (i) and (ii) are necessary to take place simultaneously to promote CO pre-oxidation, (iii) and (iv) essentially contribute in fulfilling condition (i). It was verified that the magnitude of pre-peak increases with the amount of CO coverage. Thus, this might indicate that the CO pre-oxidation is not having relationship with the CO diffusion on the surface, because the increase of CO coverage diminishes surface diffusion. A picture model of compressed CO islands seems the most to describe CO pre-oxidation.