Theoretical Studies of Atomic Transport in Ternary Semiconductor Quantum Dots and Charge Transport in Organic Photovoltaic Active Layers

Ternary semiconductor quantum dots with thermodynamically stable structures are particularly important for achieving optimal performance in optoelectronic and photovoltaic applications. Ternary quantum dots (TQDs) are typically synthesized in the form of core/shell structures. However, misfit strain induced by the abrupt core/shell interface can change the nature of the TQDs dramatically, leading to unstable optoelectronic function. In this thesis, a transient species transport model is developed to predict species distributions in TQDs during their thermal annealing. Specifically, the interdiffusion kinetics is analyzed of group-VI species in ZnSe1-xSx and ZnSe1-xTex TQDs and of group-III species in InxGa1-xAs TQDs. The modeling results are used to interpret the evolution of near-surface species concentration during thermal annealing and predict the equilibrium species distribution as a function of TQD size and composition. A database of constituent species transport properties is generated for further design of post-growth processes that enables the development of thermodynamically stable TQD structures with optimal optoelectronic function grown through simple one-step colloidal synthesis techniques. Nanoparticle assemblies of organic semiconducting materials are particularly appealing for next-generation organic photovoltaic (OPV) devices because their low-cost aqueous synthesis reduces the usage of chlorinated solvents. Another class of novel semiconducting materials, organometallic halide perovskites, have emerged as promising materials for solar cells because of their high photo-absorption coefficient and high power conversion efficiency (PCE). Based on deterministic charge carrier transport models, this thesis presents a computational analysis of charge transport in photovoltaic devices with active layers of the above two types of materials and develops design protocols for improving photovoltaic device efficiency. Our results demonstrate that charge transport efficiencies in centrifuged organic nanoparticle assemblies are comparable with those in drop cast thin films. The effects on charge transport of excess stabilizing surfactant molecules and dispersion of insulating nanoparticles in the assemblies have been analyzed. The simulation results accurately reproduce experimental data and provide interpretations for the observed effects of the active layer nanostructure, i.e., nanoparticle size, ratio, and internal morphology, on charge transport and device PCE. Furthermore, the charge generation rate in the active layer is maximized and the device’s photovoltaic performance is optimized with respect to the OPV device parameters. For photovoltaic devices based on organometallic halide perovskites, the modeling results demonstrate quantitatively that incorporation of multi-walled carbon nanotubes (MWCNTs) into the perovksite layer reduces bimolecular recombination, thus increasing the device’s PCE. In addition, we find that electronic band offsets play an important role in determining the effects on device performance of the charge carrier mobilities and of majority doping in the electron and hole transporting layers (ETLs and HTLs). The modeling results provide guidelines for designing hybrid perovskite photovoltaic devices with enhanced photovoltaic performance

Functionalisation of colloidal transition metal sulphides nanocrystals: A fascinating and challenging playground for the chemist

Metal sulphides, and in particular transition metal sulphide colloids, are a broad, versatile and exciting class of inorganic compounds which deserve growing interest and attention ascribable to the functional properties that many of them display. With respect to their oxide homologues, however, they are characterised by noticeably different chemical, structural and hence functional features. Their potential applications span several fields, and in many of the foreseen applications (e.g., in bioimaging and related fields), the achievement of stable colloidal suspensions of metal sulphides is highly desirable or either an unavoidable requirement to be met. To this aim, robust functionalisation strategies should be devised, which however are, with respect to metal or metal oxides colloids, much more challenging. This has to be ascribed, inter alia, also to the still limited knowledge of the sulphides surface chemistry, particularly when comparing it to the better established, though multifaceted, oxide surface chemistry. A ground-breaking endeavour in this field is hence the detailed understanding of the nature of the complex surface chemistry of transition metal sulphides, which ideally requires an integrated experimental and modelling approach. In this review, an overview of the state-of-the-art on the existing examples of functionalisation of transition metal sulphides is provided, also by focusing on selected case studies, exemplifying the manifold nature of this class of binary inorganic compounds

Second harmonic generation spectroscopy of plasmonic nanostructures and metamaterials

Plasmonic nanostructures and metamaterials composed of those are promising tools for the manipulation and generation of light. Their key feature is an engeneerable optical response, which is based on the possibility to tailor their localized plasmon modes. In this thesis second harmonic generation (SHG) from plasmonic nanostructures and metamaterials is investigated by means of SHG spectroscopy. For this purpose a frequency tunable femtosecond light source is developed. This light source is based on optical parametric generation and amplification with a double-pass geometry in a single macroscopic lithium niobate crystal. By this, femtosecond pulses generated by a 42 MHz repetition rate passively mode-locked Yb:KGW oscillator are converted into more than two watts of tunable near-infrared radiation between 1370 nm and at least 1650 nm. Beside its high average output power this device shows a high long term stability and allows to achieve pulse durations down to below 200 fs. With the help of this system the interplay between the local intensity enhancement, inherent to plasmonic nanostructures and the nonlinear optical response of dielectric matter is investigated. By combining the results of linear extinction measurements and SHG spectroscopy it is revealed that an increase of the SHG efficiency of plasmonic nanoantennas obtained by filling their feed gaps with a dielectric nanoparticle is independent of the nonlinear properties of the dielectric. Furthermore it is shown that the SHG efficiency of plasmonic nanoantennas is several orders of magnitude higher than that of nonlinear dielectric nanoparticles. By comparing a series of nanoantenna arrays, owning spectral distinct plasmonic resonances, it is shown, that the SHG efficiency of these structures is strongly dependent and resonantly enhanced by two-photon resonances, i.e., resonances for the generated second harmonic light. This result is qualitatively and in part also quantitatively explained in a metamaterial picture, connecting the results of linear extinction spectroscopy with those of SHG spectroscopy measurements via an anharmonic oscillator model. Furthermore noncentrosymmtric nanostructures resonant for the generated light are studied. This study indicates that the general symmetry selection rules for second harmonic generation can be also applied to plasmonic nanostructures

Continuous flow platforms for the synthesis of high-quality semiconductor nanocrystals

Semiconductor nanocrystals are of great interest due to their unique optical and electronic properties that are intermediate between bulk semiconductors and molecules. These nanoparticles find use in various applications ranging from electronics (light emitting diodes, photovoltaics) to photocatalysis and biolabeling. Typically, these nanoparticles are produced via batch synthesis routes that suffer from various issues, including slow mixing, slow heating/cooling, and lack of batch-to-batch reproducibility. These issues escalate further when increasing the scale of the production, thereby hindering their application on a commercial scale. Continuous flow synthesis can be an alternative approach that may enable high throughput and superior control of particle size and quality. However, since its first application in the early 2000s, most of the literature remains focused on continuous flow synthesis of Cd-based dots. Recently, the use of Cadmium (Cd) has been banned for many applications owing to its toxicity. Therefore, there is an immediate need for robust continuous flow reactors that enable synthesis of high quality Cd-free semiconductor nanoparticles. The modular continuous flow reactor reported in this work enables multistep, high temperature (up to 750 °C), air-sensitive synthesis of semiconductor nanocrystals involving solid and/or viscous reactants. Additionally, the millifluidic dimensions of the reactor allow for high working flow rates (> 10 ml/min) that translate into a production rate of about 150 g/day of nanocrystals. This this configuration is well suited for scale-up. The developed continuous flow reactor is designed to achieve quick heating and cooling times (< 1 s), thereby providing superior control over reaction conditions compared to the level of control that can be achieved in conventional batch synthesis techniques. The flow reactor is composed of fracture-resistant material, stainless steel, which is compatible with a wide variety of solvents at high temperatures. Furthermore, the modular flow reactor allows for inline characterization of the product, through absorbance and fluorescence spectroscopy. To demonstrate the applicability of the modular continuous flow reactor, we used the reactor to synthesize multi-layered Cd-based core-shell dots, CdSe bipods and nanorods, ZnSe nanorods, and highly luminescent InP/ZnSeS core-shell dots. The need for superior size control, shape selectivity and high reproducibility has resulted in a shift from conventional batch synthesis techniques to alternate synthesis routes. In the wake of such tight requirements, continuous flow syntheses, especially those relying on microfluidics, have emerged as viable routes for the synthesis of high-quality semiconductor nanocrystals. In general, continuous flow syntheses provide higher control over reaction conditions, for example mixing and heating times. We started by identifying the right material of construction and fabrication technique for building a continuous flow reactor that could withstand high temperatures. Design and fabrication of a simple oil-bath based continuous flow reactor and its application to demonstrate proof-of-principle syntheses of multi-layered Cd-based core-shell nanocrystals is discussed in Chapter 2. Use of heating media such as oil or hot water limits the maximum temperature attained by the reactor and is not suited for scale-up. To obviate the use of oil as a heating medium, a new continuous flow reactor was developed that uses a solid-state heating technique. The reactor configuration was further modified and coupled with a Schlenk line to enable high temperature, air-sensitive synthesis of semiconductor nanocrystals. Chapter 3 describes the design, fabrication, and operation of the new continuous flow reactor setup to synthesize anisotropic semiconductor nanocrystals, both Cd-based (CdSe nanorods/bipods) and Cd-free (ZnSe nanorods). Next, an inline mixer and a second reactor were added to the setup to enable multistep synthesis of InP/ZnSeS dots. Furthermore, the reactor design was upgraded to minimize the residence time distribution effects by the effective use of static mixers inside the reactor modules. Two flow cells were installed downstream of the reactors to enable inline spectroscopic characterization of the product. The design, fabrication, and operation of this multistep reactor setup are discussed in Chapter 4. Effective and fast mixing is critical to obtain uniform nanocrystals. However, fast mixing comes at the expense of high pressure drop. To alleviate this problem, we designed a high-throughput millifluidic herringbone mixer which is discussed in Chapter 5. In summary, the modular continuous flow reactor developed here will pave the path for high-throughput synthesis of high-quality semiconductor nanocrystals. The described platform equipped with inline characterization capabilities can also be used to study the reaction kinetics of the aforementioned syntheses, which are not fully understood at present. Furthermore, the reactor also can be used for syntheses other than semiconductor nanocrystals, especially those that require stringent conditions, including high temperature, inert conditions, and fast mixing

Applied synthesis and characterisation of nanoparticles

This thesis covers three areas of development of nanomaterials synthesis; namely the synthesis of superhydrophobic polymer-nanoparticle composites (chapter 3), the synthesis of doped quantum dots for catalysis and photoluminescence enhancement (chapter 4) and the synthesis of magnetic iron oxide nanoparticles from inexpensive, readily available reagents (chapter 5). Details of characterisation and analytical techniques and synthetic methods used are given in chapter 2, and the thesis summarised in chapter 6. Superhydrophobic polymer-nanoparticle composites represent a class of material which combine the superhydrophobicity of the polymer with the functionality of incorporated nanoparticles. Reactive oxygen species generated by photocatalytic nanoparticles degrade organic matter, and thus degrade the polymer, resulting in a loss of superhydrophobicity. In this chapter, a general method for the incorporation of hydrophobically ligated nanoparticles into a superhydrophobic poly(dimethylsiloxane) polymer matrix via AACVD is demonstrated. This resulted in a highly effective, robust titania nanoparticle-poly(dimethylsiloxane) composite for photocatalysis, along with, to the best of the author's knowledge, the first superparamagnetic-superhydrophobic polymer composite. Chapter 4 deals with the synthesis and characterisation of a quantum dot based photoactivated catalyst vector which releases Cu+ via UV irradiation, the first of its kind. The catalytic activity was evaluated using “click” chemistry under UV irradiation, with quantum dots being recoverable and able to undergo several catalytic cycles. A mechanism for the photoluminescence and copper release is also postulated. The copper is incorporated into the shells of quantum dots via the decomposition of single source metal-dithiocarbamates. Chapter 5 details a method for the synthesis of iron oxide nanoparticles for magnetic hyperthermia, but from reagents obtained from the high street. A low cost synthesis was developed and the resulting nanoparticles functionalised with an amphiphilic polymer and tested for magnetic hyperthermia

Plasmonic Enhancement of Mn2+ Luminescence and Application of Temperature-Dependent Mn2+ Luminescence

Doping semiconductor nanocrystals, commonly known as quantum dots (QDs), has attracted significant attention from the scientific community due to the highly tunable nature of the physical properties, such as optical, electrical, opto-magnetic properties, with respect to both size and dopant type/concentration. In this dissertation, Mn-doped CdS/ZnS (core/shell) QDs were used as a model system to study the characteristics of dopant luminescence coupled with plasmonic metal nanoparticles (MNPs) and its application as a nano-thermometer using temperature dependent Mn luminescence. In the first part of this dissertation, plasmon-enhanced Mn luminescence from the Mn-doped CdS/ZnS QDs near plasmonic MNPs was studied. Rapid intraparticle energy transfer between exciton and Mn, occurring on a few picoseconds time scale, separates the absorber (exciton) from the emitter (Mn), whose emission is detuned far from the plasmonic absorption of the MNP. The rapid temporal separation of the absorber and emitter combined with the reduced spectral overlap between Mn and plasmonic MNP suppresses the quenching of the luminescence while taking advantage of the plasmon-enhanced excitation. The plasmon enhancement of exciton and Mn luminescence intensities in undoped and doped QDs were simultaneously compared as a function of the distance between MNP and QD layers in a multilayer structure to examine the expected advantage of the reduced quenching in the sensitized luminescence. At the optimum MNP-QD layer distance, Mn luminescence exhibits stronger net enhancement (ca. twice) than that of the exciton, which can be explained with a model incorporating fast sensitization along with reduced emitter-MNP spectral overlap. In the second part, ratiometric thermometry on Mn luminescence spectrum was performed using Mn-doped CdS/ZnS core/shell QDs that have a large local lattice strain on Mn site, which results in the enhanced temperature dependence of the bandwidth and peak position. Mn luminescence spectral lineshape is highly robust with respect to the change in the polarity, phase and pH of the surrounding medium and aggregation of the QDs, showing great potential in temperature imaging under chemically heterogeneous environment. The temperature sensitivity (ΔIR/IR = 0.5%/K at 293 K, IR = intensity ratio at two different wavelengths) is highly linear in a wide range of temperatures from cryogenic to above-ambient temperatures. Surface temperature imaging was demonstrated on a cryo-cooling device showing the temperature variation of ~200 K (77–260 K) by imaging the luminescence of the QD film formed by simple spin coating, taking advantage of the environment-insensitive luminescence

Targeted self-assembly of nanocrystal quantum dot emitters using smart peptide linkers on light emitting diodes

Ankara : The Department of Electrical and Electronics Engineering and the Institute of Engineering and Sciences of Bilkent University, 2008.Thesis (Master's) -- Bilkent University, 2008.Includes bibliographical references leaves 68-79.Semiconductor nanocrystal quantum dots find several applications in nanotechnology. Particularly in device applications, such quantum dots are typically required to be assembled with specific distribution in space for enhanced functionality and placed at desired spatial locations on the device which commonly has several diverse material components. In conventional approaches, self-assembly of nanocrystals typically takes place nonspecifically without surface recognition of materials and cannot meet these requirements. To remedy these issues, we proposed and demonstrated uniform, controlled, and targeted self-assembly of quantum dot emitters on multi-material devices by using cross-specificity of genetically engineered peptides as smart linkers and achieved directed immobilization of these quantum dot emitters decorated with peptides only on the targeted specific regions of our color-conversion LEDs. Our peptide decorated quantum dots exhibited 270 times stronger photoluminescence intensity compared to their negative control groups.Zengin, GülisM.S

A Review on Recent Patents and Applications of Inorganic Material Binding Peptides

Over the past decade, significant progress has been made in the identification of novel material binding peptides having affinity to a wide range of target materials and their use in nanobiotechnological innovations. These material binding peptides (MBPs), also known as solid/ substance binding peptides (SBPs) can be isolated using combinatorial display technologies such as phage display (PD), surface display (cell, bacterial, yeast, mRNA) exhibit material specific selectivity and affinity towards a range of inorganic and organic nanomaterial surfaces including metals, metal oxides, minerals, semiconductors and biomolecules. MBPs serve as mediators in bringing nanotechnology and biotechnology under one umbrella by linking solid nanoparticles with biomolecules including proteins, bioactive peptide motifs, bifunctional binding peptides, enzymes, antigens and antibody fragments. As the utilization and application of these inorganic binding peptides as molecular connectors, molecular assemblers and material specific synthesizers in nanotechnology has been expanding rapidly, so too has growing commercial interest in patenting such innovations. In this review, we present the past, current and future developments and applications of inorganic MBPs specific to nanomaterials and their applications

Construction of ZnO/ZnS core/shell nanotube arrays on AAO templates and relevant applications

Nanotechnologie ist eine multidisziplinäre Technologie, welche unterschiedliche Aspekte der Wissenschaft und Ingenieurwesen im Nanobereich umfasst. Es ist mehr als das Herstellen von sehr geordneten Nanostrukturen durch die gleichzeitige Verschmelzung von Nanomaterialien und es verlang nach gebrauchstauglichen Möglichkeiten einer präzisen Manipulation und Überwachung der entwickelten Nanostrukturen. Mit anderen Worten, die größte Herausforderung in der Nanotechnologie ist es, dass wir mehr über die Materialien und ihre Eigenschaften lernen und herausfinden müssen. Zinkoxid (ZnO) ist ein Halbleiter mit großer Bandlücke (3.37 eV) mit ausgezeichneten elektrischen, optischen, katalytischen und sensorischen Eigenschaften und hat eine Vielzahl von Verwendungsmöglichkeiten. Andererseits hat Zinksulfid (ZnS) eine hohe chemische Stabilität im alkalischen sowie schwach sauren Milieu. Die einzigartigen Eigenschaften der Kombination beider Materialien, ZnO und ZnS, können den Weg ebnen zur Realisierung von zukünftigen Devices (z.B. optoelektronische Bauteile, Sensoren, Wandler, Biomedizintechnik, usw.). Der Hauptbestandteil der in dieser Dissertation gezeigten Studien hat den Schwerpunkt des Designs von sehr geordneten Nanostrukturen aus ZnO und ZnO/ZnS Nanotubes die mithilfe von anodischen Aluminiumoxid (AAO) als feste Template hergestellt wurden. Die Dissertation bezieht sich besonders auf nanostruktur-basierte elektrochemische Sensoren und photoelektrochemische (PEC) Anwendungen zur Wasserspaltung bzw. Wasserstofferzeugung. In dieser Arbeit wurden ZnO/ZnS Nanotubes erfolgreich synthetisiert durch die Kombination von 3 Methoden: (i) AAO Template (ii) Atomlagenabscheidung (ALD) und (iii) schnelles thermischen Abscheiden. Es wurde festgestellt, dass AAO Template ohne weitere zusätzliche Behandlungen durch schnelles thermisches Abscheiden komplett während des Wachstums der ZnS-Ummantelung entfernt werden konnte. Die gleichmäßig angeordneten ZnO/ZnS Nanotube-Arrays mit hoher Kristallqualität zeigten eine verbesserte optische und elektrische Leistungsfähigkeit im Vergleich zu den ZnO Nanotubes. Somit erweist sich dies als kosteneffektive Möglichkeit für die Herstellung von röhrenartigen Core/Shell-Strukturen mit unterschiedlicher Zusammensetzung mittels AAO Template ohne weitere notwendige Prozesse zur Entfernung der Template. Im Gegensatz zu konventionellen Untersuchungen mit dem Fokus auf die Veränderung der optischen Absorptionsbandkante eines aus einen einzigen Material durch sog. Quantum Confinement Effects, wurden die optischen Absorptionseigenschaften von geordneten ZnO/ZnS Core/Shell Nanotubearrays, d.h. Quantum Confinement Effects über Materialgrenzen hinaus, untersucht. Die Daten zeigen, dass das Profil des Absorptionsspektrum der ZnO/ZnS Nanoarrays durch beide Komponenten und ihre geometrischen Parameter bestimmt wird. Beide Materialein zeigen eine Verringerung der optischen Bandlücke bei Erhöhung der ZnS Manteldicke und der Durchmesser der Nanotube-Arrays, was interessant ist bzgl. Der Erklärung in Bezug auf Aspekte des Materials. Nachfolgende Finite-Difference-Time-Domain (FDTD) Simulationen unterstützten die Beobachtungen und zeigten, dass die geometrischen und periodischen Parameter die optische Absorption der Core/Shell Nanostrukturarrays beeinflussen, sogar ohne Quanteneffekte. Diese Ergebnisse liefern eine neue Sichtweise auf die Verschiebung der optischen Bandlücke, was von Bedeutung für die Forschung in der Photoelektronik ist. Des Weiteren wurde der in dieser Arbeit hergestellte und charakterisierte Sensor angewandt um Veränderungen von chemischen und biochemischen Stoffen zu erkennen. Messungen mit dem Devices als primärere Sensoren wurden erfolgreich durchgeführt und zur Erkennung als Glukose-Biosensoren verwendet. Die Untersuchungen zeigen, dass die heterogene Elektronentransferratenkonstante (ks) von ZnO/ZnS gegenüber Glukose (1.69 s^-1) höher ist als die von reinem ZnO (0.95 s^-1), was für die Verbesserung der Leistungsfähigkeit und die höhere Empfindlichkeit verantwortlich ist. Zusätzlich haben Experimente eine Verbesserung der PEC Wasserstofferzeugung mit den hergestellten Nanostrukturen gezeigt, mit höheren Sättigungsphotostromdichten (1,02 mA/cm^2) und höheren Wirkungsgraden bei der Photokonversion (62%) bei ZnO/ZnS als bei den ZnO-Strukturen ohne jegliche Ummantelung (entsprechend 0,23mA/cm^2 und 55%).Nanotechnology is a multidisciplinary model that involves various fields of science and engineering assembled at the nanoscale level. It is not used merely to form/produce highly ordered nanostructures by using only amalgamation nanomaterials simultaneous. Also requires the capability to understand the precise manipulation, and surveillance of the developed nanostructures in a manageable way. On the other hand, the primary challenge that currently faces in nanotechnology, it needs to learn more about materials and their properties. Zinc oxide (ZnO) semiconductor has a relatively large direct band gap of 3.37 eV and exciton binding energy of 60 Mev, which exhibits excellent electrical, optical, catalytic and sensory properties. It has numerous applications in different fields. Furthermore, zinc sulfide (ZnS) has a high chemical stability in alkaline and weakly acidic environments. The unique properties of the combination of ZnO and ZnS can pave the way towards the realization of future devices (e. g. Optoelectronics, sensors, transducers and biomedical sciences, etc.). The major aim of the work presented in this dissertation focuses on designing highly ordered nanostructures of ZnO and ZnO/ZnS nanotubes by using anodic aluminum oxide (AAO) as a hard template. This dissertation relates specifically to these nanostructures-based electrochemical sensors and the photoelectrochemical (PEC) water splitting application. In this work, it successfully synthesized ZnO/ZnS nanotube arrays by combining three techniques: (i) AAO template (ii) atomic layer deposition (ALD) (iii) rapid thermal deposition. It was found that the AAO template could be removed completely without any further treatments by using a rapid thermal deposition during the growth of ZnS shell. The well–ordered ZnO/ZnS nanotube arrays with the great crystalline quality exhibited superior optical and electrical performances compared with the ZnO nanotube arrays. Thus, it provides a cost–effective platform for the fabrication of tubular core/shell structures with various compositions via AAO template without concerning additional template removal procedures. Unlike the conventional investigations that focus on the manipulation of the optical absorption band edge of a single componential material through quantum confinement effects. The optical absorption property of the well–ordered ZnO/ZnS core/shell nanotube arrays was studied beyond quantum confinement effects. The data showed that the profile of the absorbance spectrum of the modified nanotube arrays was determined by the two components and their geometrical parameters. The results demonstrated that both ZnO and ZnS showed a decrease in the optical band gap. With the increase of the ZnS shell thickness and the diameter of nanotube arrays, is interestingly inexplicable from the material aspect. The subsequent finite–difference-time-domain simulations (FDTD) supported such observations and illustrated that the geometrical and periodical parameters. It was showing the optical absorption of the core/shell nanostructure arrays could be influenced even without quantum effects. These results provided a new perspective shift of the optical band gap. This is of importance to the research in photoelectronics. Furthermore, a biosensor device was synthesized and characterized by the electrochemical approach. It was applied to detect the real changes of the chemical or biochemical species. The data successfully demonstrated the measurement of the obtained device as primary transducers/sensors for the determination of the glucose biosensor. The heterogeneous electron transfer rate constant (ks) of ZnO/ZnS towards glucose was (1.69 s-1) is higher than bare ZnO (0.95 s-1). Where ks responsible of the performance improvement and high sensitivity. In addition, the experiments showed an improved performance of PEC water splitting. The saturation photocurrent density (~ 1.02 mA/cm2) and photoconversion efficiency (62%) of ZnO/ZnS are higher than those of bare ZnO (~ 0.23mA/cm2 and 55%, respectively)

반도체 양자점과 양자점 올리고머의 합성을 통한 단일 입자 분광학 및 특성에 대한 연구

학위논문 (박사)-- 서울대학교 대학원 : 화학부 물리화학 전공, 2016. 2. Thomas Basché.콜로이드 상태의 반도체 양자점은 크기에 따라 변화하는 광학적 특성으로 인하여 과학 및 기술분야에서 많은 관심을 받고 있다. 본 학위논문의 첫번째 파트에서는CdSe/CdS/ZnS 코어/쉘/쉘 양자점들을 합성하고, 이를 구조체로 하여 양자점 이합체들을 제조하였다. 이합체화 과정에서 양자점들의 침전/재분산 과정을 통해 양자점 올리고머를 형성시키고 밀도 구배 초원심분리를 통하여 농축시켰다. 얻어진 양자점 단량체와 호모-이합체에 대해서는 295K과 4.5K의 온도 하에서 단일 입자 분광법을 시행하였다. 비슷한 크기를 갖지만 서로 다른 조성을 가지는 양자점들로 구성된 양자점 헤테로-이합체에 대해서는 원자 힘 현미경과 공초점 형광 현미경을 결합한 장치 구성을 통해 식별 및 분석을 진행하였다. 두번째 파트에서는 InP/ZnSeS 양자점들을 원형 구배의 조성과 구조를 갖도록 제조 및 분석하여 Se의 양이 광물리적 특성에 중대한 영향을 끼침을 확인하였다. 최선의 시료인 0.2 mmol Se의 경우 높은 형광 양자 효율(85%)과 강한 형광 깜박임 억제를 보이며 단일 분자 실험에서 평균 on-time이 85%에 달하였다. 이러한 깜박임 억제는 계면 변형 및 결함의 감소, 구배를 갖는 내부 구조로 인한 quasi-type II 전자 구조 및 완만한 밴드 끝단 에너지 포텐셜에 의한 것으로 여겨진다.Semiconductor colloidal quantum dots (QDs) gain scientific and technological interest due to their size-dependent optical properties. In this thesis, (1) core/shell/shell CdSe/CdS/ZnS QDs have been synthesized and used as building blocks to construct QD dimers. The dimerization includes assembly of QDs into oligomers by a precipitation/re-dispersion process and subsequent enrichment of the dimers by density gradient ultracentrifugation. Single particle spectroscopy of the obtained monomers and homo-dimers has been performed at 295 K and 4.5 K. QD hetero-dimers, whose components have with similar sizes but different compositions, could be identified and characterized by a combined setup of atomic force and confocal fluorescence microscopy. (2) InP/ZnSeS QDs with radial gradient composition and structure have been prepared and characterized, whereas the amount of Se has a significant influence on the photo physical properties. The best sample with 0.2 mmol Se exhibits a high fluorescence quantum yield (85%) and a strong fluorescence blinking suppression, with an average on time fraction of 85%. This blinking suppression is tentatively attributed to the reduction of the interfacial strain and defects, the quasi-type-II electronic structure and the smooth band edge energy potential with a soft confinement derived from the gradient internal structure.1 Introduction 1 2 Theoretical background 5 2.1 Semiconductor nanocrystal quantum dots (QDs) 5 2.1.1 Electronic states and optical properties 5 2.1.2 Synthesis, growth and crystal structures 13 2.1.3 Core/shell structure 19 2.1.4 InP based quantum dots 21 2.2 Preparation of quantumdot oligomers 23 2.2.1 Self-assembly and separation of nanoparticles 23 2.2.2 Density gradient ultracentrifugation (DGU) 28 2.3 Single particle spectroscopy of quantumdots 32 2.3.1 Confocal fluorescence microscopy 32 2.3.2 Detection of a single particle (QD) 35 2.3.3 Fluorescence blinking 36 2.3.4 Spectral diffusion 43 2.3.5 Single particle spectroscopy at cryogenic temperature 44 3 Experimentalmaterials and methods 51 3.1 Materials 51 3.2 Synthetic methods 53 3.2.1 Synthesis of CdSe/CdS/ZnS quantum dots 53 3.2.2 Assembly and enrichment of quantumdot oligomers 57 3.2.3 Synthesis of InP/ZnSeS quantumdots 57 3.3 Characterization techniques 58 3.3.1 Ensemble absorption and fluorescence spectroscopy 58 3.3.2 Transmission electron microscopy (TEM) 60 3.3.3 Energy dispersive X-ray spectroscopy (EDS) 60 3.3.4 X-ray diffraction (XRD) 60 3.3.5 Fluorescence lifetime measurements 61 3.4 Experimental setups for single particle investigations 61 3.4.1 Sample preparation 61 3.4.2 Excitation light sources 61 3.4.3 Confocal microscope for cryogenic temperature measurement 62 3.4.4 Confocal microscope for roomtemperature measurement 66 3.4.5 Experimental conditions for single particle measurements 67 4 Monomers, homo- and hetero- dimers of CdSe/CdS/ZnS QDs 69 4.1 Motivation and approaches 69 4.2 CdSe/CdS/ZnS QD monomers and homo-dimers 71 4.2.1 CdSe cores and CdSe/CdS/ZnS QDs 71 4.2.2 Assembly of CdSe/CdS/ZnS QDs 73 4.2.3 Enrichment and separation of QD oligomers 76 4.2.4 Fluorescence imaging of single QDs 83 4.2.5 Single particle spectroscopy of QD monomer and dimer samples at room temperature (T = 295 K) 85 4.2.6 Single particle spectroscopy of CdSe/CdS/ZnS QD monomers at cryogenic temperature (T = 4.5 K) 86 4.2.7 Single particle spectroscopy of QD dimer sample at cryogenic temperature (T = 4.5 K) 92 4.3 CdSe/CdS/ZnS QD hetero-dimers 96 4.3.1 Size dependent fluorescence of CdSe core particles 96 4.3.2 QD hetero-dimers whose QD components have different sizes 96 4.3.3 QD hetero-dimers whose QD components have similar sizes 103 4.3.4 Structure IIa: QD components with the same CdSe core 104 4.3.5 Structure IIb: QD components with different CdSe cores 105 4.4 Summary 111 5 Investigations of InP/ZnSeS QDs 113 5.1 Introduction and motivation 113 5.2 Sample description 115 5.3 Ensemble characterizations 115 5.3.1 HLim series 115 5.3.2 H series 116 5.3.3 L series 118 5.4 Single particlemeasurements at roomtemperature (T = 295 K) 127 5.4.1 HLim series 127 5.4.2 H and L series 134 5.5 Single particlemeasurements at cryogenic temperature (T = 4.5 K) 141 5.5.1 Typical emission spectrum of InP/ZnSeS QDs at 4.5 K 142 5.5.2 Phonons contributions 142 5.5.3 Categories of single particle spectra 146 5.5.4 Average optical phonon energies 148 5.5.5 Fluorescence blinking at 4.5 K 148 5.6 Comparison of the different samples 151 5.7 Possible mechanisms of the strong blinking suppression 153 5.8 Summary 156 Summary and outlook 159 References 163Docto