Neste trabalho apresentamos um estudo sistemático das propriedades ópticas e estruturais de super-redes de pontos quânticos auto-organizados de lnAs. As superredes foram crescidas por epitaxia de feixes moleculares sobre substratos de GaAs orientados na direção (100) com diferentes números de camadas de pontos quânticos e diferentes valores do espaçamento entre elas. As propriedades estruturais das super-redes foram observadas em seção transversal por microscopia eletrônica de transmissão convencional e de alta resolução. Os resultados permitem determinar a evolução da altura, do diâmetro e da densidade dos pontos quânticos em função da modificação da espessura da camada de espaçamento. Também observamos que pontos quânticos empilhados muito próximos tendem a relaxar através da formação de defeitos estruturais identificados como micromaclas. As propriedades ópticas foram investigadas por meio de fotoluminescência a baixa temperatura, bem como variando-se a potência de excitação e a temperatura da amostra. Reportamos um novo comportamento da posição do pico de fotoluminescência com a redução da espessura da camada de espaçamento. Interpretamos este comportamento em termos de modificação do tamanho dos pontos quânticos, acoplamento eletrônico, relaxamento parcial da tensão e formação de centros de recombinação não-radiativa. Usando essas interpretações, calculamos os espectros de fotoluminescência das super-redes, que ajustam muito bem os dados experimentais. As interpretações propostas são também sustentadas pela influência da espessura da camada de espaçamento na intensidade integrada de fotoluminescência e nas energias de ativação / In this work we present a comprehensive and systematic study of the optical and structural properties of self-organized InAs quantum dots superlattices. The superlattices were grown by molecular beam epitaxy on GaAs (100) substrates with different number of quantum dot layers and different thicknesses between these layers. Their structural properties have been observed by conventional and highresolution cross-sectional transmission electron microscopy. The results allow us to sketch the evolution of the dot height, diameter and density when the spacer layer thickness is modified in a wide range. We also observe that closely stacked quantum dots tend to relax through the formation of structural defects identified as microtwins. The optical properties have been investigated by means of conventional, power dependent- and temperature dependent photoluminescence. We report for the first time on an unusual behavior of the photoluminescence peak position when the spacer layer thickness is reduced. We interpret this behavior in terms of quantum dot size modification, electronic coupling, partial strain relaxation and non-radiative recombination centers formation. Using these interpretations, we then produce simulated photoluminescence spectra that fit very well the experimental data. These interpretations are further supported by the spacer layer thickness influence upon photoluminescence integrated intensity and activation energies.

No description available.

Recentemente, uma implementação de um conjunto universal de portas lógicas de um e dois qubits para computação quântica usando estados de spin de pontos quânticos de um único elétron foi proposta. Estes resultados nos motivaram a desenvolver um estudo teórico formal do correspondente modelo de dois spins colocados em um campo magnético externo e acoplados por uma interação mútua de Heisenberg dependente do tempo. Nós então consideramos a assim chamada equação de dois spins, a qual descreve sistemas quânticos de quatro níveis de energia. Uma útil propriedade dessa equação é que o correspondente problema para o caso de campos magnéticos externos paralelos pode ser reduzido ao problema de um único spin em um campo externo efetivo. Isso nos permite gerar uma série de soluções exatas para a equação de dois spins a partir de soluções exatas já conhecidas da equação de um spin. Com base neste fato, nós construímos e apresentamos neste estudo uma lista de novas soluções exatas para a equação de dois spins para diferentes configurações de campos externos e de interação entre as partículas. Utilizando algumas destas soluções obtidas, estudamos a dinâmica da entropia de emaranhamento dos respectivos sistemas considerando diferentes estados de spins inicialmente separáveis. / Recently, an implementation of a universal set of one- and two-qubit logic gates for quantum computing using spin states of single-electron quantum dots was proposed. These results motivated us to develop a formal theoretical study of the corresponding model of two spins placed in an external magnetic field and coupled by a time-dependent mutual interaction of Heisenberg. We then consider the so-called two-spin equation, which describes four-level quantum systems. A useful property of this equation is that the corresponding problem for the case of parallel external magnetic fields can be reduced to the problem of a single spin in an effective external field. This allows us to generate a series of exact solutions for the two-spin equation from the already known exact solutions of the one-spin equation. Based on this fact, we construct and present in this study a list of new exact solutions for the two-spin equation for different configurations of external fields and interaction between particles. Using some of these solutions obtained, we study the dynamics of the entropy of entanglement of the respective systems considering different initially separable spins states.

Carbon dots (CDs) have attracted much attention as an excellent gene/drug delivery and biological imaging agent for early cancer theranostics. In this study, we prepared two series of nanoparticles (NPs), which are composed of (CDs) with a targeting agent, folic acid (FA), and a chemotherapeutic agent Doxorubicin (Dox). All the NPs and their intermediates were characterized using ultraviolet-visible spectroscopy (UV-vis), fluorescence spectroscopy, and Fourier transform-infrared spectroscopy (FT-IR). The drug loading capacity (DLC) and drug loading efficiency (DLE) of two series of FA-CDs-Dox were assessed using UV-vis absorption spectroscopy at the wavelength of 485 nm. Both showed good DLE and DLC results when compared to literature data. In addition, the cumulative release property of Dox from the FA-CDs-Dox complexes were investigated in a pH solution of 7.4.

Harnessing the Sun’s energy via the conversion of solar photons to electricity has emerged as a sustainable energy source to fulfill our future demands. In this regard, solution-processable, size-tunable PbS quantum dots (QDs) have been identified as a promising active materials for photovoltaics (PVs). Yet, there are still serious challenges that hinder the full exploitation of QD materials in PVs. This dissertation addresses two main challenges to aid these QDs in fulfilling their tremendous potential in PV applications. First, it is essential to establish a large-scale synthetic technique which maintains control over the reaction parameters to yield QDs with well-defined shape, size, and composition. Rigorous protocols for cost-effective production on a scale are still missing from literature. Particularly, previous reports of record-performance QD-PVs have been based on small-scale, manual, batch syntheses. One way to achieve a controlled large-scale synthesis is by reducing the reaction volume to ensure uniformity. Accordingly, we design a droplet-based continuous-flow synthesis of PbS QDs. Only upon separating the nucleation and growth phases, via a dual-temperature-stage reactor, it was possible to achieve high-quality QDs with high photoluminescence quantum yield (50%) in large-scale. The performance of these QDs in a PV device was comparable to batch-synthesized QDs, thus providing a promise in utilizing automated synthesis of QDs for PV applications. Second, it is crucial to study and control the charge transfer (CT) dynamics at QD interfaces in order to optimize their PV performance. Yet, the CT investigations based on PbS QDs are limited in literature. Here, we investigate the CT and charge separation (CS) at size-tunable PbS QDs and organic acceptor interfaces using a combination of femtosecond broadband transient spectroscopic techniques and steady-state measurements. The results reveal that the energy band alignment, tuned by the quantum confinement, is a key element for efficient CT and CS processes. Additionally, the presence of interfacial electrostatic interaction between the QDs and the acceptors facilitates CT from large PbS QD (bandgap < 1 eV); thus enabling light-harvesting from the broad near-infrared solar spectrum range. The advances in this work – from automated synthesis to charge transfer studies – pave new pathways towards energy harvesting from solution-processed nanomaterials.

The main objectives of this thesis are to develop viable process and/or device technologies for bandgap tuning of 1300-nm InGaAs/GaAs quantum-dot (QD) laser structures, and broad linewidth 1300-nm InGaAsP/InP quantum well (QW) superluminescent diode structures. The high performance bandgap-engineered QD laser structures were achieved by employing quantum-dot intermixing (QDI) based on impurity free vacancy diffusion (IFVD) technique for eventual seamless active-passive integration, and bandgap-tuned lasers. QDI using various dielectric-capping materials, such as HfO2, SrTiO3, TiO2, Al2O3 and ZnO, etc, were experimented in which the resultant emission wavelength can be blueshifted to ∼ 1100 nm ─ 1200 nm range depending on process conditions. The significant results extracted from the PL characterization were used to perform an extensive laser characterization. The InAs/GaAs quantum-dot lasers with QDs transition energies were blueshifted by ~185 nm, and lasing around ~1070 – 1190 nm was achieved. Furthermore, from the spectral analysis, a simultaneous five-state lasing in the InAs/InGaAs intermixed QD laser was experimentally demonstrated for the first time in the very important wavelength range from 1030 to 1125 nm. The QDI methodology enabled the facile formation of a plethora of devices with various emission wavelengths suitable for a wide range of applications in the infrared. In addition, the wavelength range achieved is also applicable for coherent light generation in the green – yellow – orange visible wavelength band via frequency doubling, which is a cost-effective way of producing compact devices for pico-projectors, semiconductor laser based solid state lighting, etc. [1, 2] In QW-based superluminescent diode, the problem statement lies on achieving a flat-top and ultra-wide emission bandwidth. The approach was to design an inhomogeneous active region with a comparable simultaneous emission from different transition states in the QW stacks, in conjunction with anti-reflection coating and tilted ridge-waveguide device configuration. In this regard, we achieved 125 nm linewidth from InGaAsP/InP multiple quantum well (MQW) superluminescent diode with a total output power in excess of 70 mW with an average power spectral density of 0.56 mW/nm, and a spectral ripple of ≤1.2 ± 0.5 dB. The high power and broadband SLD with flat-top emission spectrum is a desirable as optical source for noninvasive biomedical imaging techniques employing low coherence interferometry, for instance, optical coherence tomography (OCT).

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22876号 / 理博第4642号 / 新制||理||1667(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 寺西 利治, 教授 島川 祐一, 教授 長谷川 健 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Colloidal semiconductor nanocrystals exist at the interface of inorganic chemistry, solid-state physics, and materials applications. The highly tunable and size-dependent properties position them as prime candidates for advancing a range of technologies, including improving efficiency in solid-state lighting devices and high color-purity displays. To be successful in these endeavors, quantum dots require excellent optical properties, such as bright emission. Optimization of a zinc sulfide coating is widely regarded as a key requirement to achieving these necessary performances. Even so, zinc sulfide nanocrystal chemistry remains underdeveloped. This dissertation addresses these shortcomings and provides comprehensive synthetic and analytical tools to harness the potential of zinc sulfide containing nanocrystals. Chapter 1 introduces semiconductor nanocrystals, also referred to as quantum dots, and begins with a description of the size-dependent optical properties. Factors that lead to poorer emission properties, such as undercoordinated surface atoms are discussed. Methods to alleviate these issues, including controlling the surface coordination environment, and design and growth of heterostructures are introduced. Lastly, synthetic approaches and nanocrystal formation mechanisms are described. Chapter 2 covers the synthesis and size-dependent optical properties of zinc sulfide nanocrystals. We find that commonly used solvents in nanocrystal reactions lead to the formation of polymeric byproducts that are challenging to purify away, and thus design the zinc sulfide synthesis such that these can be avoided. Leveraging a library of rate tunable thioureas the final nanocrystal size can be carefully controlled. The reactions follow a thermally activated growth process, with larger zinc sulfide nanocrystals accessible at higher temperatures. Most relevantly for later chapters, the surface coordination environment is highly important; bulkier zinc carboxylate ligands that cannot achieve high surface coverages result in higher growth rates. These results represent the most tunable size controls reported for zinc sulfide nanocrystals. Chapter 3 uses high resolution electron microscopy techniques to study the shape (morphology) of zinc sulfide nanocrystals, synthesized using the methods developed in the second chapter. Irregular, anisotropic growth is commonly seen in zinc sulfide shell growth and is attributed to core/shell interfacial strain. We find that this growth also occurs in the binary zinc sulfide system. Synthetic conditions favoring fast growth result in unselective, isotropic growth of spherical zinc sulfide. Conversely, slower conditions can lead to irregular, anisotropic shapes. The shape is also highly dependent on the coordination environment during growth. Small, sterically unencumbered ligands stabilize specific crystal facets, leading to selective, anisotropic growth. These findings are translated to shelling procedures in Chapter 6, and further emphasize the need to understand and characterize zinc sulfide surfaces. Chapter 4 establishes an empirical relationship between the band gap energy of a zinc sulfide nanocrystal and its diameter. The literature reports a wide spread of diameters for a given energy, meaning zinc sulfide sizes could not previously be easily calculated from their optical properties. Leveraging the size- and shape-control discussed in Chapters 2 and 3, we assess the utility of a range of nanocrystal characterization techniques for accurately sizing quantum confined zinc sulfide. Using electron microscopy and X-ray scattering methods we present an updated energy-size (“sizing curve”) relationship for zinc sulfide. These results represent the most comprehensive zinc sulfide nanocrystal sizing study and enable the rapid size characterization of zinc sulfide from its absorbance spectrum. This provided crucial insight into the reaction progressions described in Chapter 2. Chapter 5 covers our endeavors to characterize and quantify the zinc sulfide nanocrystal surface chemistry, which we believe is imperative to improving shelling procedures and optical properties in zinc sulfide heterostructures. With no published extinction coefficient, the surface coverages of zinc sulfide cannot be obtained. Using the size- and shape-controlled syntheses, in conjunction with optical absorption spectroscopy and elemental analysis, we calculate extinction coefficients for a range of zinc sulfide nanocrystal sizes. The size-dependence is well described by a power law, and this represents the first reported extinction coefficient for zinc sulfide. Using this, we report the first surface coverages of zinc sulfide nanocrystals and assess the binding affinity of zinc carboxylates to the surface by monitoring their displacement by L-type ligands. Chapter 6 widens the zinc sulfide synthetic methods developed in earlier chapters to deposit zinc sulfide shells onto blue-emitting II-VI and red-emitting III-V nanocrystals. The reaction shows versatility, shelling nanocrystals over a wide range of temperatures. We demonstrate morphology control over the zinc shell by altering the deposition kinetics and coordination environment. Usually, thick, homogenous shells are desired by the nanocrystal field. However, by correlating the shell morphology to its optical properties, we see that the anisotropic shells generally achieve higher photoluminescence quantum yields (PLQYs). We also report progress towards cadmium-free quantum dot downconverters for use in solid-state lighting applications. Among other things, the photoluminescence intensity evolution throughout the shelling procedure is highly dependent on the initial surface termination of the nanocrystal core. Application of surface treatments allows brighter zinc sulfide shelled III-V heterostructures to be accessed.

This study aims to prepare theranostic nanoparticles (NPs) that are expected to increase cancer diagnostics and therapeutic efficacy. We prepared the NPs constituting carbon dots (CDs) as an imaging agent, folic acid as a targeting agent, doxorubicin (DOX), or gemcitabine (GEM) as chemotherapy agents. The NPs include noncovalent FA-CDs-DOX, covalent CDs-FA-DOX, and covalent FA-CDs-GEM. Through ultraviolet-visible spectroscopy, fluorescence spectroscopy, and Fourier transform-infrared spectroscopy, the fabrication of these NPs was confirmed. It was discovered that the high drug loading efficiency is the noncovalent series while the high drug loading capacity is the covalent series The in-vitro pH-dependent drug release data indicate the NPs release more drugs at around pH 5.0 than at pH 7.4. The NPs sizes are between 2-5 nm. The Cell viability was investigated using the Alamar Blue assay and the three NPs complexes exhibited strong therapeutic efficacy against MDA-MB-468 breast cancer cells as compared with CDs-drug.

Colloidal quantum dots (CQDs) are potentially low-cost, solution-processable semiconductors which are endowed, through their nanoscale dimensions, with strong absorption, band gap tunability, high dielectric constants and enhanced stability. CQDs are contenders as a standalone PV technology as well as a potential back layer for augmenting established photovoltaic (PV) technologies, such as Si. However, owing to their small size (ca. few nanometers), CQDs are prone to surface trap states that inhibit charge transport and threaten their otherwise wonderful optoelectronic properties. Surface traps have also, indirectly, impeded scalable and industry-compatible fabrication of these solar cells, as all of the reports, to date, have relied on spin-coating with sophisticated and tedious ligand exchange schemes, some of which need to be performed in low humidity environments. In this thesis, we posit that an in-depth understanding of the process-structure-property-performance relationship in CQDs can usher in fresh insights into the nature and origin of surface traps, lead to novel ways to mitigate them, and finally help achieve scalable fabrication. To this end, we probe the CQD surfaces and their interactions with process solvents, linkers, and ambient environment employing a suite of spectroscopic techniques. These fundamental insights help us develop facile chemical and physical protocols to mitigate surface traps such as solvent engineering, remote molecular doping, and oxygen doping, directly leading to better-performing solar cells. Our efforts finally culminate in the realization of >10% efficient, air-stable CQD solar cells scalably fabricated in an ambient environment of high, uncontrolled R.H. (50-65%). As-prepared solar cells fabricated in high humidity ambient conditions are found to underperform, however, an oxygen-doping recipe is devised to mitigate the moisture-induced surface traps and recover device performances. Importantly, these solar cells are fabricated at coating speeds of >15 m min-1 with roll-to-roll compatible techniques such as blade and bar coating requiring 1/25th the CQD material consumed by the standard spin-coated devices, overcoming the two major challenges of manufacturability and scalability faced by CQD PV.