Colloidal inorganic nanocrystal based nanocomposites: Functional materials for micro and nanofabrication

The unique size- and shape-dependent electronic properties of nanocrystals (NCs) make them extremely attractive as novel structural building blocks for constructing a new generation of innovative materials and solid-state devices. Recent advances in material chemistry has allowed the synthesis of colloidal NCs with a wide range of compositions, with a precise control on size, shape and uniformity as well as specific surface chemistry. By incorporating such nanostructures in polymers, mesoscopic materials can be achieved and their properties engineered by choosing NCs differing in size and/or composition, properly tuning the interaction between NCs and surrounding environment. In this contribution, different approaches will be presented as effective opportunities for conveying colloidal NC properties to nanocomposite materials for micro and nanofabrication. Patterning of such nanocomposites either by conventional lithographic techniques and emerging patterning tools, such as ink jet printing and nanoimprint lithography, will be illustrated, pointing out their technological impact on developing new optoelectronic and sensing devices. © 2010 by the authors

Biomimetic route to hybrid nano-Composite scaffold for tissue engineering

Hydroxyapatite-poly(vinyl) alcohol-protein composites have been prepared by a biomimetic route at ambient conditions, aged for a fortnight at 30±2°C and given a shape in the form of blocks by thermal cycling. The structural characterizations reveal a good control over the morphology mainly the size and shape of the particles. Initial mechanical studies are very encouraging. Three biocompatibility tests, i.e., hemocompatibility, cell adhesion, and toxicity have been done from Shree Chitra Tirunal, Trivandrum and the results qualify their standards. Samples are being sent for more biocompatibility tests. Optimization of the blocks in terms of hydroxyapatite and polymer composition w.r.t the applications and its affect on the mechanical strength have been initiated. Rapid prototyping and a β-tricalcium – hydroxyapatite combination in composites are in the offing

State-of-the-Art of Quantum Dot System Fabrications

The book "State-of-the-art of Quantum Dot System Fabrications" contains ten chapters and devotes to some of quantum dot system fabrication methods that considered the dependence of shape, size and composition parameters on growth methods and conditions such as temperature, strain and deposition rates. This is a collaborative book sharing and providing fundamental research such as the one conducted in Physics, Chemistry, Material Science, with a base text that could serve as a reference in research by presenting up-to-date research work on the field of quantum dot systems

Luminescence in sulfides : a rich history and a bright future

Sulfide-based luminescent materials have attracted a lot of attention for a wide range of photo-, cathodo- and electroluminescent applications. Upon doping with Ce3+ and Eu2+, the luminescence can be varied over the entire visible region by appropriately choosing the composition of the sulfide host. Main application areas are flat panel displays based on thin film electroluminescence, field emission displays and ZnS-based powder electroluminescence for backlights. For these applications, special attention is given to BaAl2S4:Eu, ZnS:Mn and ZnS:Cu. Recently, sulfide materials have regained interest due to their ability (in contrast to oxide materials) to provide a broad band, Eu2+-based red emission for use as a color conversion material in white-light emitting diodes (LEDs). The potential application of rare-earth doped binary alkaline-earth sulfides, like CaS and SrS, thiogallates, thioaluminates and thiosilicates as conversion phosphors is discussed. Finally, this review concludes with the size-dependent luminescence in intrinsic colloidal quantum dots like PbS and CdS, and with the luminescence in doped nanoparticles

Investigation of charge carriers in nanocrystal-based aerogels and the influence of micro- and nanostructuring

The transformation of intricate nanocrystals (NCs) available through colloidal chemical synthesis into branched network structures (referred to as gels) presents a promising pathway to macroscopic solids exhibiting properties of their NC building blocks. In this work, the possibilities to influence these properties at various stages during the gel synthesis sequence will be explored and expanded on. Additionally, the nanoscopic (opto-)electronic properties of networks build up from semiconducting, photoluminescent (PL) NCs will be investigated to gather insights into the electronic structure of such networks and the dynamics of excited charge carriers therein. In chapter 5 the investigation of gel networks based on CdSe/CdS dot/rod NC building blocks is shown. The PL properties of such structures are analysed in regards to their temperature and time dependent emission (down to 4K). It is shown that at cryogenic temperature the emission splits into two discernible processes correlating to dark and bright exciton states. While this observation is comparable to the results obtained for individual NCs, at all temperatures gel networks exhibit a longer PL decay most likely caused by increased electron delocalization. The investigation also includes a look at differently sized CdSe cores used for synthesis of the CdSe/CdS dot/rod building blocks. It is found that the combination of different core sizes and therefore different emission color building blocks allows for straight-forward color tuning of the final networks. The modification of NC-based gels with thin metal oxide shells is shown in chapter 6. Importantly, NC-based aerogels can be influenced not only at the point of NC building block synthesis but also after the gel formation (post-gelation). In this work, the application of wet-chemical methods based on colloidal seeded-growth processes to a NC-based gel network substrate is presented. This results in uniform, thin shells over the whole of the gel network. The approach can be shown to work on different gel substrates (semiconducting and noble metal) and different shell materials (silica and titania) with only minor modifications. The mechanical stability which is one of the common problems in the applicability of NC-based gels is considerably improved by this shell reinforcement. Building on these developed processes the optical properties of modified CdSe/CdS dot/rod gels are investigated in chapter 7. The silica modification of either the individual NC building blocks before gel assembly or the continuous gel after assembly yield two comparable systems only differing in one crucial point. In gels based on silica modified building blocks, these NC building blocks are isolated and thereby decoupled in the final gel network. For a modification after the gel assembly the building blocks are connected first and therefore coupled in the final gel. The crucial influence on charge carrier dynamics in such networks is shown by time-resolved PL spectroscopy and supported by theoretical calculations. Driving this process further, by incorporation of ZnS at the tip of the CdSe/CdS dot/rod building blocks using a cation exchange procedure a large bandgap material of variable length is introduced to the connecting point of the building blocks. Through the length of the ZnS segment separating theCdSe/CdS building blocks the interaction between these building blocks can not simply be coupled and decoupled but even influenced in its strength as also shown by optical spectroscopy.Die Transformation von komplexen Nanokristallen (NC), hergestellt durch kolloid-chemische Synthese, in verzweigte Netzwerkstrukturen (Gele genannt) repräsentiert einen vielversprechenden Weg zu makroskopischen Feststoffen, die die Eigenschaften ihrer NC Baueinheiten zeigen. In dieser Arbeit werden Möglichkeiten untersucht und erweitert, die Eigenschaften dieser Netzwerke an verschiedenen Punkten ihrer Herstellung zu beeinflussen. Zudem werden die (opto-)elektronischen Eigenschaften von Netzwerken basierend auf halbleitenden, lumineszenten NC untersucht, um ein tieferes Verständnis der elektronischen Struktur dieser Netzwerke und der Vorgänge der angeregten Ladungsträger zu generieren. Die Untersuchung von Gelnetzwerken aus stäbchenförmigen CdSe/CdS NC-baueinheiten ist in Kapitel 5 beschrieben. Die Photolumineszenz-(PL-)Eigenschaften dieser Strukturen werden temperaturabhängig (bis zu 4K) in Hinblick auf ihre zeitaufgelöste Emission untersucht. Dabei wird gezeigt, dass im kryogenen Bereich zwei Prozesse zur Emission beitragen, die sich hellen und dunklen Excitonen zuordnen lassen, ein von einzelnen NC bekanntes Verhalten. Die PL-lebenszeit ist allerdings bei allen Temperaturen in Netzwerkstrukturen länger, vermutlich aufgrund einer stärkeren Delokalisierung der angeregten Elektronen. Zusätzlich ist die Untersuchung von Netzwerken, bei denen die Bausteine auf unterschiedlichen Größen an CdSe Quantenpunkten in den CdSe/CdS Stäbchen basieren, gezeigt. Aus unterschiedlichen Größen resultieren unterschiedliche PL-Farben und ein unkomplizierter Weg zur Farbeinstellung in diesen Netzwerkstrukturen wird aufgezeigt. Die Modifikation von Nanokristallgelen mit dünnen Metalloxidschalen wird in Kapitel 6 gezeigt. NC-basierte Aerogele können nicht nur über die gezielte Synthese der Baueinheiten beeinflusst werden, sondern auch nach der Netzwerkbildung (post-Gelierung). In diesem Abschnitt wird die Adaption bekannter nasschemischer Prozesse zur Anwendung an Nanokristallgelen als Substrat beschrieben. Dabei werden Gelnetzwerke vollständig mit einer einheitlichen, dünnen Schale ummantelt. Dieser Prozess kann auf verschiedene Substrate (Gele aus Halbleiter oder Edelmetall NC) und für verschiedenen Schalenmaterialien (Siliziumdioxid und Titandioxid) mit minimalen Änderungen angepasst werden. Die mechanische Stabilität, eine bedeutende Hürde in der Anwendbarkeit von Nanokristallgelen, wird durch die Verstärkung mit einer solchen Schale deutlich erhöht. Darauf aufbauend werden die PL Eigenschaften modifizierter Netzwerke aus CdSe/CdS Stäbchen in Kapitel 7 untersucht. Die Modifikation mit einer Silikatschale wird entweder an einzelnen kolloiden Nanokristallbaueinheiten vor der Netzwerkbildung oder an dem bereits verbundenen Netzwerk durchgeführt. Werden zuerst die Baueinheiten mit einer Silikatschale umschlossen, so sind diese im finalen Netzwerk isoliert bzw. entkoppelt voneinander, während Baueinheiten, die zuerst in ein Netzwerk überführt werden, auch nach dem Wachstum der Silikatschale miteinander kontaktiert bzw. gekoppelt sind. Der entscheidende Einfluss dieser Verbindung auf die Ladungsträgervorgänge wird durch zeitaufgelöste PL-spektroskopie gezeigt und von theoretischen Berechnungen unterstützt. Dieser Ansatz wird durch den gezielten Einbau von ZnS an die Spitzen der CdSe/CdS Stäbchen weiter verfolgt. Durch einen Kationenaustauschprozess ist es möglich, verschiedene Mengen ZnS (ein Material mit großer Bandlücke) so in die Baueinheiten einzubringen, dass es sich im Netzwerk an den Kontaktpunkten befindet. Über die Länge dieser ZnS Abschnitte zwischen den CdSe/CdS Einheiten, können die Einheiten nicht nur gekoppelt und entkoppelt werden, sondern die Stärke der Wechselwirkung zwischen den Baueinheiten, welche sich in den optischen Eigenschaften widerspiegelt, gesteuert werden.ERC/Horizon 2020/714429/E

Semiconductor Nanocrystals: From Quantum Dots to Quantum Disks

The bottom-up colloidal synthesis opened up the possibility of finely tuning and tailoring the semiconductor nanocrystals. Numerous recipes were developed for the preparation of colloidal semiconductor nanocrystals, especially the traditional quantum dots. However, due to the lack of thorough understanding to those systems, the synthesis chemistry is still on the empirical level. CdS quantum dots synthesis in non-coordinating solvent were taken as a model system to investigate its molecular mechanism and formation process, ODE was identified as the reducing agent for the preparation of CdS nanocrystals, non-injection and low-temperature synthesis methods developed. In this model system, we not only proved it\u27s possible to systematically study the formation procedure of semiconductor nanocrystals, the insight learned during the research but also enhanced our understanding to this delicate system and promoted the development of synthetic chemistry. Although quantum dots could be routinely prepared in the lab with mature recipes, the colloidal semiconductor quantum well type materials are still hard to fathom. CdSe quantum disks structure was thoroughly analyzed with polar axes as the growth direction along the thickness direction, with both basal planes ended with Cd atom layer, which was coordinated with carboxylate ligands. Besides, four different thickness CdS quantum disks were prepared, its size-dependent lattice dilation, extremely sharp band-edge emission, and two-order of magnitude faster photoluminescence decay compared to quantum dots was investigated

Aqueous-Mediated Synthesis of Group IIB-VIA Semiconductor Quantum Dots: Challenges and Developments

Quantum dots (QDs) of group IIB-VIA are one of the most promising materials for various advanced technological applications in the field of optoelectronics, photovoltaic solar cells and biomedicine. Recent developments have suggested the incorporation of aqueous-mediated synthesis for the QDs, as it is greener, environment friendly, cost-effective and reproducible. However, the process involves several challenges, which ought to be met in order to produce stable, consistent and sustainable product formation. The present review discusses the significance of semiconducting QDs, their synthesis through various processes, their pros and cons, and above all the advantage of aqueous-mediated, atom economic and energy-saving methodologies

Group 12 metal chalcogenides as single source molecular precursors for the preparation of metal sulfide nanoparticles

Twelve Zn(II), Cd(II) and Hg(II) complexes of mixed ligands: alkyl thiourea, 1-ethoxylcarbonyl-1-cyanoethylene-2,2-dithiolate and tetramethylthiuram disulfide were synthesized by the reaction between the ligands and the respective metal salts. The compounds were characterized by elemental analysis, infrared (IR), 1H- and 13C-NMR spectroscopy. Four coordinate geometries were proposed for the compounds based on elemental and spectroscopic analyses. The metal complexes were at best sparingly soluble in polar coordinating solvents such as DMSO and DMF and insoluble in most organic solvents. This makes it practically impossible to grow single crystals suitable for X-ray crystallographic analysis and also resulted in extremely poor 13C-NMR spectra for some of the complexes. Thermogravimetric analysis on some of the complexes showed that they decomposed to their respective metal sulfides and thus suitable as single molecular precursors for the preparation of metal sulfide nanoparticles. Nine of the complexes with good yield were thermolysed and used as single source precursors to synthesized hexadexylamine capped metal sulfide nanoparticles. The optical and structural properties of the nanoparticles were studied using UV-Visible, photoluminescence (PL), scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and energy dispersive spectroscopy (EDX). The absorption and emission spectra of the nanoparticles show quantum confinement. The SEM showed the morphology of the particles as nearly spherical, the EDX spectra revealed peaks corresponding to respective metal and sulfur with traces of contaminants being phosphorus from tri-n-octylphosphine (TOP).Thesis (MSc) -- Faculty of Science and Agriculture, 201

Environmental friendly InP/ZnS nanocrystals

Ankara : The Department of Materials Science and Nanotechnology, Bilkent University, 2012.Thesis (Master's) -- Bilkent University, 2012.Includes bibliographical references leaves 80-89.Semiconductor nanocrystals are nanometer scale fluorescent crystallites with tunable optical properties, which can be controlled by the material composition and particle size. They can be prepared using various synthesis techniques and find applications in many different areas ranging from life sciences to electronics. In this thesis, indium phosphide based nanocrystals are studied for LED applications. The thesis research work focuses on the colloidal synthesis method and material characterization of these nanocrystals. Using one pot synthesis method, the indium phosphide/zinc sulfide (InP/ZnS) core/shell nanocrystal structures are synthesized. This synthesis technique allows for a reproducible and tunable preparation method. The material characterization techniques used in this thesis include UV-Vis spectroscopy, photoluminescence spectroscopy, transmission electron microscopy (TEM), X-ray diffraction, X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-mass spectrometry (ICP-MS), and Zeta Sizer (or DLS). These properties make InP/ZnS nanocrystals comparable to their cadmium (Cd) containing counterparts with respect to their optical properties, and InP/ZnS nanocrystals offer the potential to replace them because of environmental concerns in LED applications.Coşkun, YaseminM.S

Embedding of QDs into Ionic Crystals:: Methods, Characterization and Applications

Colloidal semiconductor quantum dots (QDs) have gained substantial interest as adjustable, bright and spectrally tunable fluorophores in the past decades. Besides their in-depth analyses in the scientific community, first industrial applications as color conversion and color enrichment materials were implemented. However, stability and processability are essential for their successful use in these and further applications. Methods to embed QDs into oxides or polymers can only partially solve this challenge. Recently, our group introduced the embedding of QDs into ionic salts, which holds several advantages in comparison to polymer or oxide-based counterparts. Both gas permeability and environmental-related degradation processes are negligible, making these composites an almost perfect choice of material. To evaluate this new class of QD-salt mixed crystals, a thorough understanding of the formation procedure and the final composites is needed. The present work is focused on embedding both aqueous-based and oil-based metal-chalcogenide QDs into several ionic salts and the investigations of their optical and chemical properties upon incorporation into the mixed crystals. QDs with well-known, reproducible and high-quality synthetic protocols are chosen as emissive species. CdTe QDs were incorporated into NaCl as host matrix by using the straightforward "classical" method. The resulting mixed crystals of various shapes and beautiful colors preserve the strong luminescence of the incorporated QDs. Besides NaCl, also borax and other salts are used as host matrices. Mercaptopropionic acid stabilized CdTe QDs can easily be co-crystallized with NaCl, while thioglycolic acid as stabilizing agent results in only weakly emitting powder-like mixed crystals. This challenge was overcome by adjusting the pH, the amount of free stabilizer and the type of salt used, demonstrating the reproducible incorporation of highest-quality CdTe QDs capped with thioglycolic acid into NaCl and KCl salt crystals. A disadvantage of the "classical" mixed crystallization procedure was its long duration which prevents a straightforward transfer of the protocol to less stable QD colloids, e.g., initially oil-based, ligand exchanged QDs. To address this challenge, the "Liquid-liquid-diffusion-assisted-crystallization" (LLDC) method is introduced. By applying the LLDC, a substantially accelerated ionic crystallization of the QDs is shown, reducing the crystallization time needed by one order of magnitude. This fast process opens the field of incorporating ligand-exchanged Cd-free QDs into NaCl matrices. To overcome the need for a ligand exchange, the LLDC can also be extended towards a two-step approach. In this modified version, the seed-mediated LLDC provides for the first time the ability to incorporate oil-based QDs directly into ionic matrices without a prior phase transfer. The ionic salts appear to be very tight matrices, ensuring the protection of the QDs from the environment. As one of the main results, these matrices provide extraordinary high photo- and chemical stability. It is further demonstrated with absolute measurements of photoluminescence quantum yields (PL-QYs), that the PL-QYs of aqueous CdTe QDs can be considerably increased upon incorporation into a salt matrix by applying the "classical" crystallization procedure. The achievable PL enhancement factors depend strongly on the PL-QYs of the parent QDs and can be described by the change of the dielectric surrounding as well as the passivation of the QD surface. Studies on CdSe/ZnS in NaCl and CdTe in borax showed a crystal-induced PL-QY increase below the values expected for the respective change of the refractive index, supporting the derived hypothesis of surface defect curing by a CdClx formation as one main factor for PL-QY enhancement. The mixed crystals developed in this work show a high suitability as color conversion materials regarding both their stability and spectral tunability. First proof-of-concept devices provide promising results. However, a combination of the highest figures of merit at the same time is intended. This ambitious goal is reached by implementing a model-experimental feedback approach which ensures the desired high optical performance of the used emitters throughout all intermediate steps. Based on the approach, a white LED combining an incandescent-like warm white with an exceptional high color rendering index and a luminous efficacy of radiation is prepared. It is the first time that a combination of this highly related figures of merit could be reached using QD-based color converters. Furthermore, the idea of embedding QDs into ionic matrices gained considerable interest in the scientific community, resulting in various publications of other research groups based on the results presented here. In summary, the present work provides a profound understanding how this new class of QD-salt mixed crystal composites can be efficiently prepared. Applying the different crystallization methods and by changing the matrix material, mixed crystals emitting from blue to the near infrared region of the electromagnetic spectrum can be fabricated using both Cd-containing and Cd-free QDs. The resulting composites show extraordinary optical properties, combining the QDs spectral tunability with the rigid and tight ionic matrix of the salt. Finally, their utilization as a color conversion material resulted in a high-quality white LED that, for the first time, combines an incandescent-like hue with outstanding optical efficacy and color rendering properties. Besides that, the mixed crystals offer huge potential in other high-quality applications which apply photonic and optoelectronic components