This thesis is dealing with nanowires of zinc oxide and of cesium lead bromide. Main goal was a preparation of ZnO nanowires using MBE. This goal was partially achieved and growth of needle-like structures was observed. Another goal was characterization of already prepared ZnO nanowires. We successfully determined polarity of their surfaces, examined the influence of lattice defects and assessed the result of their doping. Side goal of this work was characterization of nanostructures of cesium lead bromide. However, preparation of cesium lead bromide nanowires remains an open issue.

The thesis is focused on the study of properties of GaN nanocrystals and Ga structures on the surface of silicon and graphene substrate. In the theoretical part of this thesis, the basic properties of Ga/GaN and graphene are described, as well as their applications or connection of both structures together in different devices. The ability of metal nanoparticles to enhance not only photoluminescence, due to the interaction of the material with surface plasmons, is also shown in several examples. The experimental part of the work first deals with the production and characterization of graphene sheets prepared by Chemical Vapor Deposition. Ga/GaN growth on both types of substrates was performed in a UHV chamber using an effusion cell for Ga deposition and an atomic ion source for nitridation. Prepared structures were characterized using various methods (XPS, SEM, AFM, Raman spectroscopy or photoluminescence). In the last step, GaN nanocrystals were coated with Ga islands to study the photoluminescence enhancement.

Colloidal semiconductor nanocrystal quantum dots (QDs) have been extensively studied for applications in optoelectronic devices, biosensing, and imaging. Recent interest has turned to heavy metal-free compositions such as indium phosphide as an alternative to cadmium- and lead-based materials. Photoluminescence emission from InP QDs is size-tunable over a wide spectral range, providing superior color tuning compared to traditional CdSe QD but their optical properties and chemical synthesis is less well established. This study examines how InP-based heterostructures can be engineered to enhance their utility as heavy metal-free fluorophores emitting throughout the visible and near infrared (NIR) wavelength ranges by addressing three fundamental materials design and synthesis issues. First, the bandgap engineering of InP-based QDs is achieved by varying the core size, shell composition, and shell thickness of a core/shell heterostructures, generating emitters spanning 500 – 1100 nm. Second, the brightness mismatch between small blue/green emitters and large red-emitting QDs is addressed by tuning the absorption cross-section and extinction coefficient by synthesizing a series of QDs with a combination of core sizes, shell thicknesses, and shell compositions, resulting in a rainbow of brightness-matched InP emitters. Finally, the synthesis of inverted InP heterostructures, producing the reddest-emitting InP QDs ever reported by generating photoluminescence from a quantum confined InP shell, was significantly improved. The non-toxic nature of InP in conjunction with its unique optical properties render it an excellent candidate for use in in vitro and in vivo clinical or commercial settings.

Silicon nanocrystals (SiNCs) are nanometer-sized semiconducting materials. Their small size endows them with unique photophysical properties. Efficient photoluminescence (PL) from silicon nanocrystal (SiNC) composites has important implications for emerging solar-energy collection technologies, yet a detailed understanding of PL relaxation in non-colloidal SiNCs is still materializing. In this dissertation, we examine the photophysical properties of silicon nanocrystal/off-stoichiometry thiol-ene composites (SiNCs/OSTE hybrids). The dissertation begins with an introduction to the photophysical properties of SiNCs, their photophysical properties, how SiNC/polymer composites are made, the various SiNC preparation techniques, and the most likely application areas for these nanocrystals. A description of experimental methods such as PL spectroscopy and transmission electron microscopy (TEM) follows, and SiNC/OSTE polymer preparation methods are then explained in detail. In the first study, TEM and photophysical characterization were performed on selected polydisperse SiNCs samples. These samples were synthesized in a nonthermal plasma reactor, using Si6H12 as precursor, and functionalized with R (where R is 1-dodecene). These SiNCs were dispersed in mesitylene:1-dodecene (5:1) as a colloid. Optical absorption, quantum efficiency, and PL lifetime of SiNCs were then investigated, as well as the relationship between quantum yield, lifetime, and PL peak. In the second study, we selected samples for size separation via the density gradient ultracentrifugation method (DGU). We successfully applied this technique to separate silicon nanocrystals with sizes from 2 nm to 4 nm from the ensemble samples using an engineered density medium layer stack, and photophysical characterization was performed on the DGU size–separated SiNCs. Lastly, we explored details of PL relaxation in photo-polymerized off-stoichiometric polymer/nanocrystal hybrids. We found time- and air-stable emission from dilute composites with up to 70% QY, and we investigated PL relaxation in the parameter space of nanocrystal size and temperature. In light of previous work, our results reveal similarities between the impacts of crosslinking and cooling to cryogenic temperature, but of which are characterized by a relative reduction in the available of phonons.

Metal halide perovskites have emerged as photoactive materials in solution-processed devices thanks to their unique properties such as high absorption coefficient, sharp absorption edge, long carrier diffusion lengths, and tunable bandgap, together with ease of fabrication. The single-junction perovskite solar cells have reached power conversion efficiencies of more than 25%. Although the efficiency of perovskite devices has increased tremendously in a very short time, the efficiency is still limited by carrier recombination at defects and interfaces. Thus, understanding these losses and how to reduce them is the way forward towards the Shockley-Queisser limit. This thesis aims to apply ultrafast optical spectroscopy techniques to investigate the recombination pathways in halide perovskites, and understand the charge extraction from perovskite to transport layers and nonradiative losses at the interface. The first part focuses on perovskite solar cells with planar n–i–p device architecture which offers significant advantages in terms of large scale processing, the potential use of flexible substrates, and applicability to tandems. In addition to the optimization of MAPbI3 solar cell fabrication using a modified sequential interdiffusion protocol, the photophysics of perovskites exposed to humid air and illumination are discussed. The MAPbI3 film processed with the addition of glycol ethers to the methylammonium iodide solution results in the control of PbI2 to perovskite conversion dynamics, thus enhanced morphology and crystallinity. For samples exposed to humid air and illumination, the formation of sub-bandgap states and increased trap-assisted recombination are observed, using highly-sensitive absorption and time-resolved photoluminescence measurements, respectively. It appears that such exposure primarily affects the perovskite surface. The second part discusses the hole extraction from Cs0.07Rb0.03FA0.765MA0.135PbI2.55Br0.45 to the polymeric hole transport layer and interfacial recombination using ultrafast transient absorption spectroscopy technique. To illustrate this, PDPP-3T was used as HTL, since its ground state absorption is red-shifted compared to the perovskite’s photobleach, thereby allowing direct probing of the interfacial hole extraction and recombination. Moreover, carrier diffusion is investigated by varying the perovskite film thickness, and carrier mobility is found to be 39 cm2V-1s-1. Finally, hole extraction is found to be one order of magnitude faster than the recombination at the interface.

Upconverted light from nanostructured metal surfaces can be produced by harmonic generation and multi-photon luminescence; however, these are weak processes and require extremely high field intensities to produce a measurable signal. Here we report on bright emission, five orders of magnitude greater than harmonic generation, that can be seen from metal tunnel junctions due to light-induced inelastic tunneling. Like inelastic tunneling light emission, which was recently reported to have 2% conversion efficiency per tunneling event, the emission wavelength recorded varies with the local electric field applied; however, here the field is from a 1560 nm femtosecond pulsed laser source. Finite-difference time-domain simulations of the experimental conditions show the local field is sufficient to generate tunneling-based inelastic light emission in the visible regime. This phenomenon is promising for producing ultrafast upconverted light emission with higher efficiency than conventional nonlinear processes. / Graduate

Photon avalanching (PA), one of the more unique nonlinear optical processes due to its combination of efficiency and extreme response, first attracted attention from the optics community more than four decades ago. But interest waned as researchers found that it did not provide immediately useful features observed in other nonlinear optical systems, such as amplified coherent light generation from lasing or optoelectronic amplification and transduction afforded by light-stimulated electron avalanching. The material systems supporting PA were also found to be rather limited, with reports concentrating on fragile, bulk lanthanide-doped crystals. However, the inter-ionic energy transfer mechanisms responsible for PA and its extreme nonlinearity are, in principle, realizable in objects with dimensions at the nanoscale. Further, new applications for PA in nanomaterials including simple super-resolution microscopy have recently been proposed. These factors motivated my research on the development of the first-ever lanthanide-doped nanoparticles capable of supporting PA behavior. In this thesis, the optical properties of Tm³⁺-doped NaYF₄ nanocrystals are investigated with photoluminescence microscopy, spectroscopy and differential rate equation model simulations. First, the photon avalanching behavior of Tm³⁺-doped NaYF₄ nanocrystals is studied. Specifically, the excitation-power-dependent luminescence of 1%, 4%, 8%, 20%, and 100% Tm³⁺-doped NaYF₄ is measured. The slopes of log-log excitation intensity versus emission intensity plots show that photon avalanche is realized in the nanocrystals when Tm³⁺ content is 8% and above. Time-resolved luminescence and rate equation model fitting to the experimental data validate the existence of photon avalanche, showing luminescence rise times > 600 ms, and the ratio of the ³F₄-to-³F₃ excited state absorption to the ³H₆-to-³F₄ ground state absorption is > 10⁴, which are signatures of photon avalanche. The design-dependent shift of the photon avalanching threshold also shows that photon avalanche is the main excitation scheme for the nanocrystals and implies potential applications for ultra-sensitive nano-sensing with the help of extreme nonlinearity. Additionally, the steep nonlinearity leads to super-resolution microscopy of single 8% Tm³⁺-doped nanocrystals with resolution down to <70 nm using conventional confocal microscopy without sophisticated techniques. In the second part of the thesis, the photodarkening effect of Tm³⁺-doped NaYF₄ nanocrystals is studied. We have found that photodarkening behavior is observed in Tm³⁺-doped nanocrystals that exhibit the photon avalanche effect. Power-dependent luminescence of a single 8% Tm3+-doped nanocrystal reveals that photodarkened nanocrystals still support photon avalanche behavior, but the avalanching threshold is shifted to a higher value. A photodarkening mechanism is proposed based on the concentration-dependent and power-dependent luminescence properties, and optical spectroscopic data. Notably, photodarkened nanocrystals are found to recover their original brightness and behavior under Vis-NIR optical illumination. This so-called “photobrightening” allows novel photoswitching of the inorganic nanocrystals, which has never before been achieved. We observe robust single nanocrystal photoswitching over 1000 cycles without permanent photodegradation. In addition, rewritable photolithography of multiple patterns using NIR lasers at 700 nm and 1064 nm is demonstrated.

京都大学 / 0048 / 新制・課程博士 / 博士(人間・環境学) / 甲第18361号 / 人博第674号 / 新制||人||162(附属図書館) / 25||人博||674(吉田南総合図書館) / 31219 / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 田部 勢津久, 教授 加藤 立久, 教授 杉山 雅人 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DGAM

京都大学 / 0048 / 新制・課程博士 / 博士(人間・環境学) / 甲第18380号 / 人博第693号 / 新制||人||166(附属図書館) / 25||人博||693(吉田南総合図書館) / 31238 / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 田部 勢津久, 教授 加藤 立久, 教授 杉山 雅人, 教授 森本 芳則, 教授 山本 行男 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DGAM

Photoluminescence spectroscopy is one of the most efficient and sensitive non-contact techniques used to investigate defects in SiC. In this work, room temperature photoluminescence mapping is employed to identify different defects that influence material properties. The correlation of the distribution of these defects in n-type 4H-SiC substrates with electronic properties of SiC revealed connection between the deep levels acting as efficient recombination centers and doping in the substrate. Since deep levels are known to act as minority carrier lifetime killers, the obtained knowledge may contribute to our ability to control important characteristics such as minority carrier lifetime in SiC. In semi-insulating (SI) 6H-SiC, the correlation between room temperature infrared photoluminescence maps and the resistivity maps is used to identify deep defects responsible for semi-insulating behavior of the material. Different defects were found to be important in different families of SI SiC substrates, with often more than one type of defect playing a significant role. The obtained knowledge is expected to enhance the yield of SI SiC fabrication and the homogeneity of the resistivity distribution across the area of large SiC substrates.