Thèses sur le sujet « Nanocrystals Synthesis »

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2001.
Includes bibliographical references.
Although several theoretical models for the behavior of magnetic crystals smaller than a single domain size were developed in the 1950's and 60's, they have hardly been verified experimentally because of the lack of appropriate material systems. This thesis is an attempt to develop such a system using metallic cobalt as a magnetic material and to verify its magnetic behavior in the context of a Stoner-Wohlfarth model of coherent rotation. The problem of preparing crystals of a desired shape and the effect of the crystal shape on its magnetic properties is also addressed. Cobalt nanocrystals are prepared by thermal decomposition of dicobalt octacarbonyl in solution and in the presence of suitable surfactants and coordinating ligands, which influence the shape of the resulting crystals as well as their internal structure. The presence of trialkylphosphines in the growth solution leads to the formation of spherical nanocrystals with mixed fcc-hcp structure, where as trioctylphosphine oxide leads to a newly discovered structure of [epsilon]-cobalt. The final size of the crystals is controlled by the precursor-to-ligand ratio, and low polydispersity is achieved by the separation of nucleation and growth stages. Size-selective precipitation is used to further reduce the size variation of the samples. As a result, cobalt nanocrystals in the size range of 4-12 nm in diameter can be routinely produced with size distributions as small as 6%. The study of magnetic properties reveals the superparamagnetic nature of cobalt nanocrystals of this size range at room temperature. At low temperatures, a good qualitative agreement with the theoretical (Stoner-Wohlfarth) model is found,
(cont.) although quantitative results are strongly influenced by the presence of an oxide shell around each nanocrystal. The presence of two surfactants (trialkylphosphines and sodium carboxylates) during the growth leads to the formation of a significant number of triangular and rod-shaped nanocrystals. Unlike disordered spherical particles, these nanocrystals have pure fcc structure without visible defects. The length of the rods is roughly controlled by the concentration of carboxylates in the growth solution and can be changed within a 40-400 nm range. Unlike spherical crystals of comparable volume, the rods are ferromagnetic even at room temperature due to an added effect of shape anisotropy. A growth mechanism for the formation of nanorods with cubic structure is also proposed.
by Dmitry P. Dinega.
Ph.D.

The present PhD thesis focuses on two main classes of semiconductor colloidal nanocrystals, i.e. lead halide perovskite and copper chalcogenides. The former class of semiconductor NCs are promising materials for many high performance optoelectronics applications, as they exhibit a tunable band gap in the range of 1.4 to 2.9 eV and an efficient photoluminescence characterized by narrow emission linewidths and have been explored the most in the last years. Following the standard hot injection based synthesis and selecting a combination of short chain acid (octanoic acid or hexanoic acid) together with alkyl amines (octylamine and oleylamine) we prepared strongly fluorescent CsPbBr3 perovskite nanowires with tuneable width, in the range from 20 nm (exhibiting no quantum confinement, hence emitting in the green) to 3 nm (in the strong quantum-confinement regime, emitting in the blue) for the first time. However the main limitation of the colloidal synthesis protocols that was followed in aforementioned case including the ligand assisted reprecipitation routes which is the second most frequently used method for preparation of LHPs, is that they employ PbX2 (X= Cl, Br, or I) salts as both lead and halide precursors which consequently limit the precise tunability of the amount of reaction species such as metals or halides precursors and are not applicable to entire family of APbX3 (A=FA, MA and Cs; X=Cl, Br, I). To overcome this issue we developed benzoyl halide based colloidal synthesis route i.e broadly applicable to the entire family of LHP NCs and not only ensures the independent tunability of reaction precursors but also maintain the overall integrity of the NCs such as phase purity and high PLQY. Despite the significant advances in synthesis procedures, the control over size monodispersity, shape and phase purity remains another long standing challenge. This is in fact due to the tendency of primary alkyl amine in the form of alkylammonium ions that could compete with Cs+ ions and leads to the anisotropic growth such as NPLs or their use in excess permotes the Pb-depleted Cs4PbX6 phases. We develop here a strategy to achieve size, shape and phase pure CsPbBr3 nanocubes by substituting primary alkyl amines with secondary alkyl amines. We attributed this excellent control over the shape and phase purity to the inability of secondary amines to find the right steric conditions at the surface of the nanocrystals which consequently limits the formation of low dimensional structures. The shape purity and narrow size distribution leads to their ease of self-assembly in superlattices reaching up to 50 microns in lateral dimensions, which are the largest dimensions reported to date for superlattices of LHP NCs. The second class of materials studied here, i.e. copper chalcogenides, are mainly attractive due to their tunable composition via post synthesis chemical transformations, plasmonic properties, low toxicity and environmental friendliness. Taking the advantage of colloidal synthesis and using Cu2S as a template we develop a strategy to obtain novel AuCuS-Cu2S heterostructure through cation exchange, which cannot be realized through conventional synthesis approaches. We further investigated the stability of Cu2S NCs with different dimensionalities and their thermal evolution subsequent to the metal decoration. Interestingly the presence of additional metallic NCs, such as Au and Pt not only improves their thermal stability but also leads to the formation of bi-metallic alloys semiconductor heterostructure.

Aim of this work was, to investigate the preparation of Si NC memories by sputter deposition. The milestones are as follows: - Review of relevant literature. - Development of processes for an ultrathin tunnel-oxide and high quality sputtered SiO2 for use as control-oxide. - Evaluation of methods for the preparation of an oxygen-deficient silicon oxide inter-layer (the precursor of the Si NC layer). - Characterization of deposited films. - Establishment of techniques capable of probing the phase separation of SiOx and the formation of Si NC. - Establishment of annealing conditions compatible with the requirements of current CMOS technology based on experimental results and simulations of Si NC formation. - Preparation Si NC memory capacitors using the developed processes. - Characterization of these devices by suitable techniques. Demonstration of their memory functionality.

Aim of this work was, to investigate the preparation of Si NC memories by sputter deposition. The milestones are as follows: - Review of relevant literature. - Development of processes for an ultrathin tunnel-oxide and high quality sputtered SiO2 for use as control-oxide. - Evaluation of methods for the preparation of an oxygen-deficient silicon oxide inter-layer (the precursor of the Si NC layer). - Characterization of deposited films. - Establishment of techniques capable of probing the phase separation of SiOx and the formation of Si NC. - Establishment of annealing conditions compatible with the requirements of current CMOS technology based on experimental results and simulations of Si NC formation. - Preparation Si NC memory capacitors using the developed processes. - Characterization of these devices by suitable techniques. Demonstration of their memory functionality.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2007.
Vita.
Includes bibliographical references.
Several microfluidic reactors were designed and applied to the synthesis of colloidal semiconductor nanocrystals (NCs). Initially, a simple single-phase capillary reactor was used for the synthesis of CdSe NCs. Precursors were delivered into a section of the capillary maintained at high temperature where they decomposed and reacted to form NCs. Monodisperse, bright CdSe NC samples were prepared over a significant range of average sizes. The excellent stability and reproducibility of the continuous flow system was also demonstrated. However, a limitation of the single-phase flow approach was that slow mixing and large residence time distributions can be detrimental to the overall quality (eg. size distribution) of the NC samples produced. These limitations were overcome by designing and fabricating in silicon a gas-liquid segmented flow microreactor. In contrast to the single-phase flow, recirculation within the liquid segments provides a mechanism to exchange fluid elements located near the channel walls with those in the center. This recirculation has the dual of effect of reducing axial dispersion and greatly improving the mixing efficiency - factors which have a strong influence on the ultimate size and size distribution of NCs produced.
(cont.) Compared to single-phase operation, preparation of CdSe NCs in segmented flow resulted in superior reactor throughput and narrower size distributions. Finally, the segmented flow method was extended in a microreactor designed for the synthesis of more complicated NC architectures. The design incorporated multiple inlet channels, which allowed for continuous injection of multiple precursor streams. This reactor was used to synthesize several core/shell NC structures - CdSe/ZnS, CdSe/ZnSe, and CdSe/CdxZnl-xSe.
by Brian K.H. Yen.
Ph.D.

Colloidal nanocrystals offer new and improved performance in applications as well as less environmental impact when compared to traditional device fabrication methods. The important properties that enable improved applications are a direct result of nanocrystal structure. While there have been many great advances in the production of colloidal nanocrystals over the past three decades, precise, atomic-level control of the size, composition, and structure of the inorganic core remains challenging. Rather than dictate these material aspects through traditional synthetic routes, this dissertation details the development and exploitation of a colloidal nanocrystal synthetic method inspired by polymerization reactions. Living polymerization reactions offer precise control of polymer size and structure and have tremendously advanced polymer science, allowing the intuitive production of polymers and block co-polymers of well-defined molecular weights. Similarly, living nanocrystal synthetic methods allow an enhanced level of structural control, granting the synthesis of binary, doped, and core/shell nanocrystals of well-defined size, composition, and structure. This improved control in turn grants enhanced nanocrystal property performance and deepens our understanding of structure/property relationships. This dissertation defines living nanocrystal growth and demonstrates the potential of the living methods in the colloidal production of oxide nanocrystals. After a brief introduction, living growth is defined and discussed in the context of synthetic prerequisites, attributes, and outcomes. Living growth is also compared to more traditional colloidal nanocrystal synthetic methods. The following chapters then demonstrate the precise control living approaches offer in three separate studies; the first highlights sub-nanometer control of nanocrystal size from 2-22+ nm in diameter. Next the improvement in nanocrystal composition is illustrated using several transition metal dopants into an oxide nanocrystal matrix at near thermodynamically allowed compositions. Additionally, precise radial dopant placement is demonstrated, which has striking implications for material properties. The radial position of tin in tin-doped indium oxide nanocrystals and the resulting differences on the localized surface plasmon resonance are discussed. Finally, future opportunities are reviewed. This dissertation includes previously published co-authored material.

Metal halide perovskites (particularly doped perovskites and lead free double perovskites) are starting to generate great interest in the scientific community due to their unique electronic and structural properties, such as high photoluminescent quantum yields (PLQY, up to 90%), chemical diversity in terms of elements employed and tunable optical properties. Consequently, their application in optoelectronic devices gained attention. During these three years, I intensively worked on the synthesis and characterization of inorganic perovskites nanocrystals, starting from lead halide perovskites (LHPs) in 3D and 0D structure and then moving to the double perovskites (DPs). Generally, the aim of these studies is to replace Pb with less toxic elements, producing materials more stable to atmosphere conditions and with good optical properties. Thus, synthesis and optimization are the key words of this part of the work. Pb has been replaced with a monovalent (Ag, Na) and a trivalent (In, Bi), or a bivalent (Cu, Mn) and a trivalent (Sb) metal cation, leading to DPs (e.g. Cs2AgInCl6) and layered perovskites (e.g. Cs4CuSb2Cl12), respectively. However, perovskites are not the only promising candidate for optoelectronic devices, in particular considering the increasing interest in studying NIR emitting materials. In this field, my work on silicates takes place. In fact, Cu - based silicates (e.g. CaCuSi4O10) possess a high emission in NIR region (900–1000 nm). Moreover, their high Stokes shift, which limits re-absorbance phenomenon, and the high stability to ambient condition and sun irradiation, suggest their use in solar absorbing devices. During my PhD I performed deep structural investigation using synchrotron radiation after an optimization of the material synthesis; then, I worked on their exfoliation leading to the formation of very homogeneous nanosheets.

New approaches for the synthesis of highly luminescent InP and InP/ZnS nanocrystals were developed by stepwise systematic investigation of the parameters during the reaction stages. The parameters, including solvents, precursors, ligands, capping agents, protic agents, Lewis acids and bases and temperature were discussed in detail in different chapters. The investigation processes helped increase understanding of understand the reaction and surface passivation mechanisms and to develop convenient synthesis approaches. Highly luminescent InP NCs were prepared with an in-situ indium chloride complex in the presence of zinc carboxylates or zinc dithiocarbamates - convenient nucleation initiators and stabilisers. The nanocrystals prepared covered a wide photoluminescence emission range from blue to the near infra-red. This synthesis method also allowed the in-situ growth of highly luminescent InP/ZnS core-shell nanocrystals as well. The principles of selection of reagents are also applicable in the synthesis of other nanocrystals. The as-prepared high-quality InP and InP/ZnS nanocrystals have been exploited in applications in the fields of light emitting diodes (LEOs) and bioimaging.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Vita.
Includes bibliographical references.
Nanoscale materials, including nanocrystals and carbon nanotubes, exhibit an appealing array of physical properties, and provide an interesting prospect for research both from a fundamental as well as a technological perspective. The current emerging themes in nanoscale research are: controlled synthesis with well defined sizes and geometries; unraveling their fundamental physical properties; and assembly of these nanoscale building blocks into functional devices. Although several approaches for producing the nanoparticles have been reported in the past decade, a general, large scale method for controlled synthesis of well-defined nanoparticles in the 1-5 nm size regimes is yet to be found. A general method that enables both syntheses of nanoparticles and their assembly on substrates is critical towards furthering technological applications. The work described here involved developing a method that utilized principles of self assembly in conjunction with inorganic and organic synthetic chemistry for the controlled synthesis of ordered arrays of nanocrystals. A unique attribute of this technique is it combined themes one and three, aforementioned, into a single step. First, uniform arrays of various mono- and hetero-bimetallic nanoparticles with sizes in the range of 1-5 nm were synthesized on various substrates using PS-P4VP block copolymer (BCP) templates. These arrays of monodisperse nanoparticles were employed as catalysts for the diameter-controlled growth of SWNTs.
(cont.) Comparisons on their catalytic activities provided valuable insight on the catalyst-assisted growth of SWNTs. Alternate ways to improve the catalytic yield of nanotubes employing bi-metallic nanoparticles as well as novel catalysts for nanotube growth are also being reported for the first time. Importantly, a combinatorial approach involving BCPs and gas phase reactions was designed that enabled us in addressing some of the long standing problems associated with the syntheses of semiconductor III-Nitride nanocrystals. Finally, versatility of this synthesis method was further demonstrated by syntheses of ternary nitrides as well as rare earth ions doped GaN. While the investigations on the latter aspects are still in there infancy, initial results show significant promise and pave an exciting prospect for future studies.
by Sreekar Bhaviripudi.
Ph.D.

This thesis primarily focuses on perovskite nanocrystals (NCs), addressing synthesis of the materials with tailored optical and electrical properties, morphology, stability for applications in optoelectrical devices and catalytic reactions. In particular, the methods of solvent engineering and compositional engineering were applied to tackle the toxicity and stability issues of the metal halide perovskite NCs. Advanced measurements were used to investigate the fundamental properties of the metal halide perovskite NCs. The developed perovskite NCs were successfully employed in highly efficient light emitting devices and catalytic applications.

A main driving force behind the recent years’ immense interest in nanoscience and nanotechnology is the possibility of achieving new material properties and functionalities within, e.g., material physics, biomedicine, sensor technology, chemical catalysis, energy storing systems, and so on. New (theoretical) possibilities represent, in turn, a challenging task for chemists and physicists. An important feature of the present nanoscience surge is its strongly interdisciplinary character, which is reflected in the present work. In this thesis, nanocrystals and thin films of magnetic and ferroelectric metal oxides, e.g. RE2O3 (RE = Y, Gd, Dy), GdFeO3, Gd3Fe5O12, Na0.5K0.5NbO3, have been prepared by colloidal and sol-gel methods. The sizes of the nanocrystals were in the range 3-15 nm and different carboxylic acids, e.g. oleic or citric acid, were chemisorbed onto the surface of the nanoparticles. From FT-IR measurements it is concluded that the bonding to the surface takes place via the carboxylate group in a bidentate or bridging fashion, with some preference for the latter coordination mode. The magnetic properties of nanocrystalline Gd2O3 and GdFeO3 were measured, both with respect to magnetic resonance relaxivity and magnetic susceptibility. Both types of materials exhibit promising relaxivity properties, and may have the potential for use as positive contrast enhancing agents in magnetic resonance imaging (MRI). The nanocrystalline samples were also characterised by transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), and quantum chemical calculations. Thin films of Na0.5K0.5NbO3, GdFeO3 and Gd3Fe5O12 were prepared by sol-gel methods and characterized by x-ray powder diffraction (XRPD) and scanning electron microscopy (SEM). Under appropriate synthesis conditions, rather pure phase materials could be obtained with grain sizes ranging from 50 to 300 nm. Magnetic measurements in the temperature range 2-350 K indicated that the magnetization of the perovskite phase GdFeO3 can be described as the sum of two contributing terms. One term (mainly) due to the spontaneous magnetic ordering of the iron containing sublattice, and the other a susceptibility term, attributable to the paramagnetic gadolinium sublattice. The two terms yield the relationship M(T)=M0(T)+χ(T)*H for the magnetization. The garnet phase Gd3Fe5O12 is ferrimagnetic and showed a compensation temperature Tcomp ≈ 295 K.

In this report a novel low temperature synthesis approach of CdS nanocrystals is described starting from well known precursors, Cd(SA) and TOP-S, in a ligand system of aliphatic long chain alcohols. A one-pot synthesis approach is applied using a laboratory microwave heating source. The resulting CdS nanocrystals exhibit an absorbance with a pronounced fine-structure, a photoluminescence with a very high ratio between the band gap peak and the defect peak and a fluorescence quantum yield of 33%. Different synthesis approaches have been investigated by changing heating rate, temperature, precursor concentration and chain length of the aliphatic alcohol ligand as well as chain length of the Cadmium precursor. It was found that small changes in the heating rate do not affect the reaction. Changing the reaction temperature between 200°C and 160°C has no visible effects on the quality of the resulting CdS nanocrystals. At 140°C the nanoparticles experience a significant drop in quality, probably because there is a major change in the growth mechanism of the nanocrystals at that low temperature. At 100°C and 120°C the creation of so-called CdS nanoclusters is observed, and a growth mechanism towards nanocrystals based on cluster aggregation is suggested. For the synthesis of high quality nanoparticles it was found that a ratio of 1:25 between precursor and aliphatic alcohol is preferable as well as a ratio of 1:1 between the two precursors. If the chain length of both the precursors and the alcohol is short, the reaction rate is enhanced. If the chain length is too short the nanocrystals grow very fast and the size distribution gets broad, the photoluminescence intensity decreases and the ratio between band gap luminescence and defect luminescence decreases. The best Cd-precursor was found to be Cd-Laurate and the most suitable ligand evaluated was Tetradecanol.

Semiconductor nanocrystals exhibit strongly size-dependent absorption spectra and consequently are of great interest for use in nanoscale devices. As the excited states of these crystals are short-lived, spectroscopic investigation of the nature and size dependence of their energy levels requires femtosecond laser pulses. We have synthesized cadmium selenide nanocrystals and constructed a femtosecond instrument around a commercial laser system from Coherent to study them.
The instrument is capable of measuring the autocorrelation and crosscorrelation of laser pulses and can measure the transient absorption dynamics of nanocrystals via pump-probe spectroscopy. A novel means of simultaneously measuring the dynamics associated with two different excitation wavelengths has also been employed.
I have written a LabVIEW program for data acquisition from this instrument, capable of reducing measurement noise through averaging and rejection of bad measurements. We have successfully used this software in probing specific transitions in cadmium selenide nanocrystals.

Parmi les différents nanocatalyseurs, ceux constitués de nanoparticules de métaux nobles méritent une attention particulière en raison de leurs propriétés électroniques, chimiques et même optiques (dans le cas de transformations renforcées par les plasmons). Le platine ou le palladium sont bien connus pour leurs remarquables propriétés catalytiques, mais ils sont chers et leurs ressources sont limitées. En outre, les nanocatalyseurs monométallique ne peuvent conduire qu'à une gamme limitée de réactions chimiques. Ainsi, notre stratégie a été de développer des nanocatalyseurs bimétalliques composés de deux éléments métalliques qui peuvent présenter des effets synergiques entre leurs propriétés physicochimiques et une activité catalytique accrue. Nous avons ainsi conçu des nanocatalyseurs bimétalliques de type cœur-coquille composés d'un cœur en argent et d'une coquille en platine. L'intérêt est de combiner les activités catalytiques élevées et efficaces de la coquille de platine avec le cœur d'argent hautement énergétique, capable de renforcer les activités de la coquille grâce à ses propriétés plasmoniques. En outre, ces nanoparticules bimétalliques présentent souvent une activité catalytique supérieure en raison de la modification de la distance inter-atomique Pt-Pt (c'est-à-dire l'effet de contrainte). Dans ce travail de thèse, les nanoparticules Ag@Pt ont été synthétisées via un processus en deux étapes utilisant d'une part des nanoparticules d'Ag synthétisées chimiquement comme germes et d'autre part des complexes platine-oleylamine qui sont ensuite réduits à la surface des germes à une température contrôlée. Différentes tailles de germes d'Ag de 8 à 14 nm avec une très faible distribution de taille (<10%) ont été obtenues en ajustant le temps de réaction, la rampe de température, la concentration en précurseur d'Ag et la température finale pendant la synthèse. Différentes épaisseurs de coquille (de 1 à 6 couches atomiques) ont été obtenues en ajustant le rapport entre les concentrations de précurseur de platine et de germe d'argent. L'activité catalytique des nanoparticules Ag@Pt a été testée en considérant une réaction modèle de réduction du 4-nitrophénol en 4-aminophénol par NaBH4 en phase aqueuse. Nous avons observé que l'épaisseur de la coquille de Pt et la taille du noyau d'Ag influençaient les propriétés catalytiques et conduisaient à une activité catalytique accrue par rapport à l'argent ou au platine pur. Ceci a été attribué à des effets synergiques. De plus, nous avons observé une augmentation de l'activité catalytique des nanoparticules Ag et Ag@Pt sous irradiation lumineuse. Ce phénomène a été corrélé à la génération d'électrons chauds dans les noyaux d'Ag. Afin de développer une plateforme de nanocatalyse supportée, nous avons fabriqué des auto-assemblages 3D appelés aussi supercristaux composés de nanoparticules d'Ag@Pt obtenus spontanément après dépôt sur un substrat solide en raison de leur distribution de taille étroite et de leur forme homogène. L'activité catalytique de ces supercristaux pour la réaction d'évolution de l’hydrogène (HER) a été étudiée en suivant in situ par microscopie optique la production de nanobulles de gaz H2. Trois comportements distincts dans l'activité photo-catalytique (activité, activité intermittente et non-activité) ont été observés sur les supercristaux dans la même région d'intérêt. En outre, 50 % des assemblages ont été déterminés comme étant actifs pour l'HER qui a été démontrée comme étant accompagnée par une corrosion oxydative de l’argent
Among several nanocatalysts, those based on noble metal NPs deserve particular attention because of their electronic, chemical and even optical properties (in the case of plasmonic-enhanced transformations). Platinum or palladium are well known for their remarkable catalytic properties, but they are expensive and their resources are limited. In addition, single component nanocatalysts can only lead to a limited range of chemical reactions. Thus, our strategy was to develop bimetallic nanocatalysts composed of two metal elements that can exhibit synergistic effects between their physicochemical properties and enhanced catalytic activity. We have thus designed bimetallic nanocatalysts of the core-shell type composed of a silver core and a platinum shell. The interest is to combine the high and efficient catalytic activities of the platinum shell surface with the highly energetic silver core capable of enhancing the activities of the shell through its plasmonic properties. In addition, these bimetallic NPs often exhibit superior catalytic activity due to the modification of the Pt-Pt atomic bonding distance (i.e. the strain effect). In this thesis work, Ag@Pt NPs have been synthesized via a two-step process using chemically synthesized spherical Ag NPs as seeds on the one hand and platinum complexes with oleylamine on the other hand which are then reduced on the surface of the seeds at a controlled temperature. Different Ag seed sizes from 8 to 14 nm with a very low size distribution (<10%) have been obtained by adjusting the reaction time, temperature ramp, Ag precursor concentration and final temperature during the synthesis. The control of the shell thicknesses (from 1 to 6 atomic layers) has been possible by adjusting the ratio of platinum precursor to silver seed concentrations. The catalytic activity of the core-shell Ag@Pt NPs was tested by a model reaction of reduction of 4-nitrophenol to 4-aminophenol by NaBH4 in aqueous phase. We have observed that the thickness of the Pt shell and the size of the Ag core influence the catalytic properties and led increased catalytic activity compared to pure silver or platinum. This was attributed to synergistic effects. Furthermore, we have observed an enhancement of the catalytic activity of Ag and Ag@Pt NPs under light irradiation. This is correlated to the generation of hot electrons in the Ag core. Finally, in order to develop a supported nanocatalysis platform, 3D self-assemblies also called supercrystals composed of Ag@Pt nanoparticles have been spontaneously obtained after deposition on a solid substrate due to their narrow size distribution and homogeneous shape. The catalytic activity of these supercrystals for the hydrogen evolution reaction (HER) has been studied by following in situ by optical microscopy the production of H2 gas nanobubbles. Three distinct behaviors in photo-catalytic activity (activity, intermittent activity and non-activity) have been observed on the supercrystals in the same region of interest. In addition, 50% of the assemblies were determined to be active for HER which was shown to be accompanied by oxidative corrosion of silver

The work presented in the thesis is focused on the synthesis of diverse colloidal semiconductor NCs in organic media, their surface design with tiny inorganic and hybrid capping species in solution phase, and subsequent assembling of these NC building units into two-dimensional close-packed thin-films and three-dimensional non-ordered porous superstructures.

This research work reports a series of new colloidal approaches for the synthesis of uniform Zn-based chalcogenide semiconductor nanocrystals, including 0D and 1D ZnS, ZnSe, ZnSxSe1-x nanocrystals. The influence of precursor activity on nanoparticles growth was investigated by experiment and theoretical calculation. Novel Au-ZnSe hybrid structures with controlled Au domain growth were obtained and used for photocatalytic H2 generation.

This thesis is a comprehensive study of the synthesis of nanomaterials. It explores the synthetic methods on the control of the size, shape and phase of semiconductor nanocrystals. A number of important conclusions, including the mechanism behind crystal growth and the structure-relationship, have been drawn through the experimental and theoretical investigation. The synthesized nanocrystals have been tested for applications in gas sensing, photocatalysis and solar cells, which exhibit considerable commercialization potential.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2010.
"April 29, 2010." Vita. Cataloged from PDF version of thesis.
Includes bibliographical references.
The primary focus of this thesis is the synthesis and applications of semiconductor nanocrystals, or quantum dots (QDs). Novel synthetic routes to ternary 1-III-VI QDs are presented, and we report the first highly luminescent Cu-In-Se QDs spanning the red to near-infrared region. The synthetic method is modular and is extended to Ag-In-Se, Cu-In-Zn-S, and Ag-In-Zn-S QDs, luminescent from the blue to near-infrared. The development of new core-shell InAs(ZnCdS) QDs is discussed in the context of making highly fluorescent, stable biological probes in the near-infrared region. Applications in biological systems from cellular labeling to sentinel lymph node mapping are demonstrated. In addition, we present new methods for doping InAs QDs in order to control carrier type through the introduction of acceptor defects such as cadmium. The synthesis and characterization of n and p type InAs QDs is discussed. In order to understand the differences in size distributions with current III-V QD synthetic procedures and II-VI and IV-VI QD syntheses we have explored the molecular mechanisms that lead to the formation of InP and InAs QDs. We find that current III-V QD syntheses result in the depletion of molecular precursors immediately following nucleation, preventing growth from molecular precursors, thus failing to meet the a key criterion for a monodisperse colloidal synthesis in the Dinegar and LaMer model. In the conclusion of this thesis, we explore the electrically controlled solution-liquid-solid (EC-SLS) synthesis of InP nanowires. Using the EC-SLS method, we are able to controllably place n type InP nanowires into field effect transistor geometries.
by Peter M. Allen.
Ph.D.

In this report we establish cyclohexasilane (CHS) as a reliable precursor for non-thermal plasma synthesis of high quality photoluminescent silicon nanocrystals (SiNCs). We demonstrate that this synthesis approach can produce high quality, size tunable silicon quantum dots with quantum yields exceeding 60% as synthesized (subsequent work in our group has measured over 70% quantum yield after density gradient ultracentrifugation size purification).After a brief background on non-thermal plasma synthesis, the characterization methods used in this study, and an overview of CHS, we report at length on our development of the apparatus used, and our exploration of the controllable processing parameters of the synthesis method. We describe our successes and challenges with size tuning, sample collection, and passivation. Finally, we discuss preliminary studies we performed to identify promising future research areas. Novel reactor designs, blue light passivation, and magnetic confinement of plasma are described briefly to entice future researchers.

Magnetic semiconductor nanocrystals are being studied for their potential application in the field of spintronics as spin-injectors for spin-based transistors and spin-based storage elements for nonvolatile memories. They also have a number of biomedical engineering applications including contrast enhancing agents for magnetic resonance imaging (MRI). In this study, we present a synthesis route to grow colloidal II-VI magnetic nanoparticles at room temperature with easily handled, relatively non-toxic source materials. CoSe and CrSe nanocrystals were synthesized in an aqueous solution where gelatin is used to retard the reaction. Characterization of the nanocrystals was done through transmission electron microscope (TEM) imaging and UV-Vis absorption spectroscopy. Spin-carrier relaxation times were determined using a superconducting quantum interference device (SQUID) magnetometer.
Master of Science

En resumen, esta tesis presenta algunas rutas sintéticas para diseñar nanopartículas dispersables en agua, biocompatibles y evalúa su interacción con entidades biológicas. Para lograr este propósito, utilicé la síntesis asistida por microondas para obtener SPIONs, nanopartículas de oro (Au NPs) y Au – SPIONs nanocomposites. Posteriormente, me centré en la funcionalización superficial de SPIONs con diferentes estabilizantes electrostáticos y BSA para hacerlas dispersables y estables en los medios de cultivo celular. Así, investigamos la interacción entre SPIONs y BSA mediante el uso de técnicas diferentes. Determinamos constantes de interacción, cambios estructurales y la termodinámica de la interacción de BSA con SPIONs. Finalmente, trabajando con otros miembros del grupo, analizamos la influencia de la capa de BSA en la toxicidad, la absorción y la localización intracelular de SPIONs en dos tipos de líneas celulares in vitro. Los efectos biológicos de recubrimiento BSA en SPIONs también se investigaron en el modelo simple in vivo C. elegans. Esta tesis se organiza en siete capítulos: • El capítulo 1 introduce: i) los conceptos básicos de la nanociencia, nanomedicina y el magnetismo de las nanopartículas de óxido de hierro, ii) los métodos sintéticos generales para la producción de nanopartículas de óxido de hierro y nanopartículas de oro, y sus aplicaciones biomédicas, iii) la interacción de nanopartículas con proteínas, y el efecto del recubrimiento de proteína en las respuestas biológicas de las nanopartículas. • El capítulo 2 presenta: i) la síntesis asistida por microondas de SPIONs y nanopartículas de Au, ii) la funcionalización superficial de SPIONs con polivinil pirrolidona, hidróxido de tetrametilamonio y citrato trisódico hidratado y iii) la síntesis a gran escala de SPIONs funcionalizadas con citrato mediante el uso de un Microondas multi-modo. • El capítulo 3 extiende el uso del método de síntesis asistida por microondas de Au-SPIONs híbridas. Se ha establecido una ruta fácil asistida por microondas utilizando polioles de forma rápida y verde para sintetizar nanotriángulos (NT) de oro (Au) decorados con nanopartículas de óxido de hierro superparamagnéticas (SPIONs) con un alto rendimiento. El rendimiento de Au-NTs puede ser controlado ajustando los parámetros de síntesis. • El capítulo 4 muestra la funcionalización superficial de SPIONs con BSA. La estabilidad de las BSA-SPIONs funcionalizadas se ha investigado en varias soluciones biológicamente relevantes. El mecanismo de adsorción, la termodinámica y la conformación estructural del BSA sobre los SPIONs se analiza en detalle. • El capítulo 5 presenta la modificación del comportamiento de SPIONs en entornos biológicos después de su recubrimiento con BSA. En particular, se evalúa la degradación de SPIONs con y sin recubrimiento de BSA en una solución tampón de citrato (pH 4,6), imitando el ambiente ácido lisosomal. También se investigó la citotoxicidad, la captación y localización de SPIONs con y sin recubrimiento de BSA en dos tipos de células y los efectos biológicos del recubrimiento BSA en el modelo in vivo C. elegans. • El capítulo 6 enumera las principales conclusiones derivadas de esta tesis doctoral y algunas sugerencias para futuros trabajos en el campo. • El capítulo 7 recoge información sobre el autor y sus publicaciones durante el período de realización de esta tesis doctoral.
Briefly, this thesis presents some synthetic ways to engineer water dispersible, biocompatible nanoparticles and evaluation of their behaviors in biological environments. To achieve this purpose, I first used a microwave-assisted method to synthesize superparamagnetic iron oxide nanoparticles (SPIONs), Gold nanoparticles (Au NPs) and Au nanotriangles-SPIONs nanocomposites (Au NTs-SPIONs). I then focused on the surface functionalization of SPIONs with different electrostatic stabilizers and BSA to make them dispersible and stable in cell media. Thereafter, the interaction between SPIONs and BSA was investigated by using several different techniques. Binding behaviors, structural changes and thermodynamics of BSA upon interaction with SPIONs have been elucidated. Finally, working with other group members, we evaluated the effects of the BSA coating on the toxicity, uptake and intracellular localization of SPIONs on two types of in vitro cell lines (MDA MB 231 cell line and HL 60 cell line). Biological effects of BSA coating on SPIONs' surface were also investigated on a simple in vivo model of C. elegans. This thesis is organized in seven chapters. • Chapter 1 introduces: i) basic concepts of nanoscience, nanomedicine and magnetism of iron oxide nanoparticles, ii) general synthetic methods for producing iron oxide and gold nanoparticles, and their biomedical applications, iii) the interaction of nanoparticles with proteins, and the effect of protein coating on the biological responses of nanoparticles. • Chapter 2 presents: i) microwave-assisted synthesis of SPIONs and Au nanoparticles, ii) surface functionalization of SPIONs with polyvinyl pyrrolidone, tetramethylammonium hydroxide and trisodium citrate dihydrate and iii) large scale-up synthesis of citrate functionalized SPIONs by using a multi-mode MW apparatus. • Chapter 3 further explores the use of microwave assisted method in the synthesis of Au-SPIONs hybrid nanoparticles. A facile, fast and bio-friendly microwave-assisted polyol route was established to synthesize high yield of gold (Au) nanotriangles (NT) decorated with superparamagnetic iron oxide nanoparticles (SPIONs). The yield of AuNTs could be controlled by adjusting synthetic parameters. • Chapter 4 shows the further surface functionalization of SPIONs by BSA. The stability of the BSA functionalized SPIONs has been investigated in several biologically relevant media. The adsorption mechanism, thermodynamics and structure conformation of BSA upon adsorption on SPIONs were also revealed in detail. • Chapter 5 reports the effects of BSA coating on the behaviors of SPIONs in biological environments. In particular, degradation of SPIONs with and without BSA coating in citrate buffer (pH 4.6), mimicking the lysosomal acidic environment, was evaluated. We also investigated the cytotoxicity, uptake and localization of SPIONs with and without BSA coating on two types of cells. Furthermore, biological effects of BSA coating were also evaluated on the in vivo model C. elegans.. • Chapter 6 includes the general conclusions extracted from the PhD work. Some suggestions for the future work are also included. Chapter 7 provides the author’s CV and the publications that have resulted from the thesis.

This dissertation contains six chapters detailing recent advances that have been made in the synthesis and characterization of metal-semiconductor hybrid nanocrystals (HNCs), and the applications of these materials. Primarily focused on the synthesis of well-defined II-VI semiconductor nanorod (NR) and tetrapod (TP) based constructs of interest for photocatalytic and solar energy applications, the research described herein discusses progress towards the realization of key design rules for the synthesis of functional semiconductor nanocrystals (NCs). As such, a blend of novel synthesis, advanced characterization, and direct application of heterostructured nanoparticles are presented. Additionally, for chapters two through six, a corresponding appendix is included containing supporting data pertinent to the experiments described in the chapter. The first chapter is a review summarizing the design, synthesis, properties, and applications of multicomponent nanomaterials composed of disparate semiconductor and metal domains. By coupling two compositionally distinct materials onto a single nanocrystal, synergistic properties can arise that are not present in the isolated components, ranging from self-assembly to photocatalysis. While much progress was made in the late 1990s and early 2000s on the preparation of a variety of semiconductor/metal hybrids towards goals of photocatalysis, comprehensive understanding of nanoscale reactivity and energetics required the development of synthetic methods to prepare well-defined multidimensional constructs. For semiconductor nanomaterials, this was first realized in the ability to tune nanomaterial dimensions from 0-D quantum dot (QD) structures to cylindrical (NR) and branched (TP) structures by exploitation of advanced colloidal synthesis techniques and understandings of NC facet reactivities. Another key advance in this field was the preparation of "seeded" NR and TP constructs, for which an initial semiconductor QD (often CdSe) is used to "seed" the growth of a second semiconductor material (for example, CdS). These advances led to exquisite levels of control of semiconductor nanomaterial composition, shape, and size. Concurrently, many developments were made in the functionalization of these NCs with metallic nanoparticles, allowing for precise tuning of metal nanoparticle deposition position on the surface of preformed semiconductor NCs. To date, photoinduced and thermally induced methods are most widely used for this, providing access to metal-semiconductor hybrid structures functionalized with Au, Pt, Ag2S, Pd, Au/Pt, Ni, and Co nanoparticles (to name a few). With colloidal nanomaterial preparation becoming analogous to traditional molecular systems in terms of selectivity, property modulation, and compositional control, the field of nanomaterial total synthesis has thus emerged in the past decade. With a large toolbox of reactions which afford selectivity at the nanoscale developed, to date it is possible to design a wider array of materials than ever before. Only recently (the past ~ 5 years), however, has the transition from design of model systems for fundamental characterization to realization of functional materials with optimized properties begun to be demonstrated. The emphasis of chapter 1 is thus on the key advances in the preparation of metal-semiconductor hybrid nanoparticles made to date, with seminal synthetic, characterization, and application milestones being highlighted. The second chapter is focused on the synthesis and characterization of well-defined CdSe-seeded-CdS (CdSe@CdS) NR systems synthesized by overcoating of wurtzite (W) CdSe quantum dots with W-CdS shells. 1-dimensional NRs have been interesting constructs for applications such as solar concentrators, optical gains, and photocatalysis. In each of these cases, a critical step is the localization of photoexcited excitons from the light-harvesting CdS NR "antenna" into the CdSe QD seed, from which emission is primarily observed. However, effects of seed size and NR length on this process remained unexplored prior to this work. Previous work had demonstrated that, for core@shell CdSe@CdS systems, small CdSe seed sizes (< 2.8 nm in diameter) resulted in quasi-type II alignment between semiconductor components (with photoexcited electrons delocalized across the structure and holes localized in the CdSe seed), and large seed sizes (> 2.8 nm) resulted in type I alignment (with photoexcited electrons and holes localized in the CdSe seed). Through synthetic control over CdSe@CdS NR systems, materials with small and large CdSe seeds were prepared, and for each seed size, multiple NR lengths were prepared. Through transient absorption studies, it was found that band alignment did not affect the efficiency of charge localization in the CdSe core, whereas NR length had a profound effect. This work indicated that longer NRs resulted in poor exciton localization efficiencies owing to ultrafast trapping of photoexcited excitons generated in the CdS NR. Thus, with increasing rod length, poorer efficiencies were observed. This work served to highlight the ideal size range for CdSe@CdS NR constructs targeted towards photocatalysis, with ~ 40 nm NRs exhibiting the best rod-to-seed localization efficiencies. Additionally, it served to expand the understanding of exciton trapping in colloidal NC systems, allowing development of a predictive model to help guide the preparation of other nanorod based photocatalytic systems. The third chapter describes the synthesis of Au-tipped CdSe NRs and studies of the effects of selective metal nanoparticle deposition on the band edge energetics of these model photocatalytic systems. Previous studies had demonstrated ultrafast localization of photoexcited electrons in Au nanoparticles (AuNP) (and PtNP) deposited at the termini of CdSe and CdSe@CdS NR constructs. Also, for similar systems, the hydrogen evolution reaction (HER) had been studied, for which it was found that noble metal nanoparticle tips were necessary to extract photoexcited electrons from the NR constructs and drive catalytic reactions. However, in these studies, energetic trap states, generally ascribed to surface defects on the NC surface, are often cited as contributing to loss of catalytic efficiency. In this study, we found that the literature trend of assuming the band-edge energetics of the parent semiconductor NC applies to the final metal-functionalized catalyst did not present a complete picture of these systems. Through a combination of ultraviolet photoelectron spectroscopy and waveguide based spectroelectrochemistry on films of 40 nm long CdSe NRs before and after AuNP functionalization, we found that metal deposition resulted in the formation of mid-gap energy states, which were assigned as metal-semiconductor interface states. Previously these states had only been seen in single particle STS studies, and their identification in this study from complementary characterization techniques highlighted a need to further understand the nature of the interface between metal/semiconductor components for the design of photoelectrochemical systems with appropriate band alignments for efficient photocatalysis. The fourth chapter transitions from NR constructs to highly absorbing CdSe@CdS TP materials, for which a single zincblende (ZB) CdSe NC is used to seed the growth of four identical CdS arms. These arms act as highly efficient light absorbers, resulting in absorption cross sections an order of magnitude greater than for comparable NR systems. In the past, many studies have been published on the striking properties of TP nanocrystals, such as dual wavelength fluorescence, multiple exciton generation, and inherent self-assembly owing to their unique geometry. Nonetheless, these materials have not been exploited for photocatalysis, primarily owing to challenges in preparing TP from ultrasmall ZB-CdSe seed size (owing to phase instability of the zincblende crystal structure), thus preventing access to quasi-type II structures necessary for efficient photocatalysis. In this study, we successfully break through the type I/quasi-type II barrier for TP NCs, reclaiming lost ground in this field and demonstrating for the first time quasi-type II behavior in CdSe@CdS TPs through transient absorption measurements. This was enabled by new synthetic protocols for the synthesis and stabilization of ultrasmall (1.8 – 2.8 nm) ZB-CdSe seeds, as well as for the synthesis of CdSe@CdS TPs with arm lengths of 40 nm. Easily scalable, TPs were prepared on gram scales, and the quasi-type II systems showed dramatically enhanced rates of selective photodeposition of AuNP tips under ultraviolet and solar irradiation. These are promising materials for photocatalytic and solar energy applications. The fifth chapter continues with the study of CdSe@CdS TPs, and elaborates on a new method for the selective functionalization of the highly symmetrical TP construct. Previous studies had demonstrated that access to single noble metal NP tips was vital for efficient photocatalytic HER from NR constructs. However, TP materials have been notoriously difficult to selectively functionalize, owing to their symmetric nature. Using a novel photoinduced electrochemical Ostwald ripening process, we found that initially randomly deposited AuNPs could be ripened to a single, large (~ 7 nm) AuNP tip at the end of one arm of a type I CdSe@CdS TP with 40 nm arms. To demonstrate the selectivity of this tipping process, dipolar cobalt was selectively overcoated onto the AuNP tips of these TPs, resulting in dipolar Au@Co-CdSe@CdS TP nanocrystals. These particles were observed to spontaneous self-assemble into 1-D mesoscopic chains, owing to pairing of N-S dipoles of the ferromagnetic CoNPs, resulting in the first example of “colloidal polymers” (CPs) bearing bulky, tetrapod ("giant t-butyl") pendant groups. The sixth chapter elaborates further on the preparation of colloidal polymers, further extending the analogy between molecular and colloidal levels of synthetic control. One challenge in the field of colloidal science is the realization of new modes of self-assemble for compositionally distinct nanoparticles. In this work, it was found that Au@Co nanoparticle dipole strength could be systematically varied by tuning of AuNP size on CdSe@CdS nanorods/tetrapods. In the first example of a colloidal analogue to reactivity ratios observed for traditional chain growth polymerization systems, highly disparate AuNP tip sizes (and thus final Au@Co NP dipole strength) were found to result in segmented colloidal copolymers upon dipolar self-assembly, whereas similar AuNP tip sizes ultimately led to random dipolar assemblies. Clearly visualized through incorporation of NR and TP sidechains into these colloidal polymers, this study presented a compelling case for continued exploration of colloidal analogues to traditional molecular levels of synthetic control.

Semiconductor nanocrystals with sizes comparable to the corresponding bulk excitonic diameter exhibit unique size-dependent electronic and optical properties resulting from quantum confinement effect. Such nanocrystals not only allow the study of evolution of bulk properties from the molecular limit providing important fundamental understandings, but also have great technological implications, leading to intense research over the past several years. Besides tuning the crystal size in the nm regime to obtain novel properties, an additional route to derive new functionalities has been to dope transition metal ions into a semiconductor host. Thus, transition metal doped nanocrystals are of great interest since it allows two independent ways to functionalize semiconductor materials, one via the tunability of properties by size variation and other due to properties of such dopants. Chapter 1 of the thesis provide a general introduction to the subject matters dealt in with this thesis, while the necessary methodologies have been discussed in chapter 2. Chapters 3 and 4 of this thesis deal with nanocrystal doping. Following suggestions in previous literatures that the doping of nanocrystal depends strongly upon the crystal structure of the synthesized host nanocrystal, we have studied the phase-transformation between the somewhat zinc-blende and the usual wurtzite structures for CdS and CdSe nanocrystals in chapter 5. In chapter 6 we have pointed out that a gradient structure is essential to achieve nearly ideal photoluminescence efficiency using heterostructured nanocrystals and also achieved strong two-photon absorptions, adding optical bifunctionality to these nanocrystals. Finally, in chapter 7, we establish different approaches to generate white-light using nanocrystals and their unique advantages, as a first step to realizing white light emitting devices. Chapter 1 provides a brief introduction to various interesting properties and concepts relevant for the studies carried out in the subsequent chapters of this thesis. The present status of the research in the field of semiconductor nanocrystals with an emphasis on synthesizing high quality nanocrystals, doping of nanocrystals and exciting optical properties exhibited by these nanocrystals has been discussed. We have discussed the existing theories and practices of colloidal synthesis that allow us to prepare high quality semiconductor nanocrystals with required size and very narrow size distribution. Optical properties, covering excitonic fine structure, photoluminescence, auger recombination and two-photon absorption have been discussed. We have described heterostructured nanocrystals of different types, particularly in the light of enhancing photoluminescence quantum yield. The difficulty in doping Mn2+ ion in semiconductor nanocrystals and the recent developments in this field have been addressed. Chapter 2 describes experimental and theoretical methodologies that have been employed to study different nanocrystal systems reported in this thesis. The topics covered in this chapter include UV-visible absorption spectroscopy, steady-state and time-resolved luminescence spectroscopy, X-ray diffraction, transmission electron microscopy, electron spin resonance spectroscopy, photoemission spectroscopy, two-photon absorption and least-squared-error fitting. Chapter 3 presents a detailed study of water soluble Mn2+-doped CdS nanocrystals synthesized using colloidal routes. Earlier efforts to dope Mn2+ ion into CdS nanocrystals and therefore, obtain the characteristic orange emission, have been largely impeded by the strong overlap of surface state emission of the host and Mn2+ d-emission. We are the first ones to obtain a distinct Mn2+ d-related emission at around 620 nm, well-separated from the surface state emission with its maximum near 508 nm. In spite of using very high (~30%) concentration of Mn2+ precursor, only ~1% Mn2+ was found in the final product, which is consistent with previous literatures, where Mn2+ doping in such nanocrystals was found to be extremely difficult. Most interestingly, present results establish that Mn2+ ion is found to be incorporated preferentially in the relatively larger sized nanocrystals compared to the smaller sized ones even within the narrow size distribution achieved for a specific reaction condition. We found that 55 oC is the optimum reaction temperature to synthesize Mn2+-doped CdS nanocrystals, at higher reaction temperatures, Mn2+ ions get annealed out of the substitutional sites, leading to a lower level of doping in spite of the formation of larger sized particles. Additionally, we could tune the color of the Mn2+ d- emission from red (620 nm) to yellow (580 nm) by increasing the reaction temperature from 55 oC to 130 oC. Another important aspect is that the synthesized nanocrystals readily dissolve in water without any perceptible effect on the Mn2+ d emission intensity. Chapter 4 discusses the outstanding problem that a semiconductor host in the bulk form can be doped to a large extent, while the same host in the nanocrystal form resist any appreciable level of doping. We first describe two independent models available in literatures to explain this baffling phenomenon. In one, it was suggested that the doping of Mn2+ ion in such nanoclusters is invariably an energetically unfavorable state, thus, Mn2+ ions get annealed out from the host nanocrystal and an increase in reaction temperature facilitate such annealing, a phenomenon known as self-purification. In the second model, it was suggested that the ease of initial adsorption of Mn2+ ions on specific surfaces of a growing nanocrystal, kinetically controls the extent of impurity doping. Specifically, it is easier to dope zinc-blende nanocrystals compared to their wurtzite counterpart. In contrast, the main claim of this chapter is neither crystal structure nor self-purification is as important in nanocrystal doping as lattice mismatch between the dopant and host lattice. To support this claim, we have doped Mn2+ ions into alloyed ZnxCd1-xS nanocrystals. Ionic radius of Mn2+ ion being in between those of Zn2+ and Cd2+ ions, the lattice mismatch between the host ZnxCd1-xS nanocrystal and MnS could be tuned in either side by tuning the composition “x”. It was gratifying to observe that there is an evident maximum of manganese content for Zn0.49Cd0.51S host nanocrystals that has no lattice mismatch with MnS, and the manganese content decreases systematically with increasing compressive as well as tensile lattice mismatches. Based on lattice parameter tuning, we could dope an extraordinarily higher amount of ~7.5% manganese for x = 0.49, at a reaction temperature as high as 310 oC and in a nanocrystal that exhibit wurtzite structure, which was previously suggested unfavorable for doping. These results prove our hypothesis that the strain fields generated because of the lattice mismatch between the dopant and host, are necessarily long range, much longer than typical nanocrystal dimensions and it tends to relieve itself by ejecting the dopant to the surface of nanocrystals, thus, resisting doping in such nanocrystals. High temperature synthesis, on the other hand, leads to a very high photoluminescence efficiency of ~25%. Chapter 5 deals with the phase-control of CdS and CdSe nanocrystals synthesized employing colloidal routes. CdS nanocrystals exhibit a very sensitive phase transformation from zinc-blende to wurtzite structure by increasing the reaction temperature from 280 to 310 oC, which is also accompanied by an increase in particle size from 6 to 6.8 nm, respectively. More importantly, just by changing the S precursor, it has been possible to change the crystal structure of the CdS nanocrystals at a given synthesis temperature of 310 oC. En route, we have synthesized >12 nm zinc-blende CdS nanocrystal, which is the largest one known in literature and that too employing the highest (310 oC) reaction temperature. Thus, our results contradict with the suggestions already in literatures that low reaction temperature and small crystal size favors zinc-blende structure. Also, we could tune crystal structure between zincblende and wurtzite at a given pressure of the reaction vessel and for a given solvent, just by changing the S-precursor, which is again in contradiction to previously made suggestions in literatures that high pressure or noncoordinating solvents favors the formation of zinc-blende nanocrystals. Instead, we believe that the surface energy might be crucial in stabilizing the usually rare zinc-blende structure for such nanocrystals. Chapter 6 is divided into two sections and deals with optically active heterostructured nanocrystals exhibiting high photoluminescence efficiency and strong two-photon absorption. In section-I, we probe the internal structure of extraordinarily luminescent (quantum yield = 85%) CdSeS nanocrystals making a somewhat unconventional use of Photoelectron spectroscopy, using the tunability of the photon energy from the third generation synchrotron radiation source as well as the traditional Mg Kα and Al Kα photon sources. CdSeS nanocrystals synthesized with Se:S precursor ratios 1:5 and 1:50, emitting red and green light have CdSe/CdSeS/CdS core/gradient-shell/shell and CdSeS/CdS gradient-core/shell structure, respectively. Gradient interface/core tunes the lattice parameters continuously between that of CdSe and CdS minimizing the interface related defects which in turn increases the photoluminescence efficiency even beyond that obtained from traditional core/shell nanocrystals, as evidenced by the nearly single exponential photoluminescence decay dynamics exhibited by these nanocrystals. Quantum mechanical calculations further show that a graded-core/shell structure leads to a remarkable spatial collapse and consequently a stronger overlap of the HOMO and LUMO wavefunctions towards the core region and thereby, making these luminescent beyond the traditional core/shell limit. In section-II, we have synthesized hetero-structured nanocrystals with CdSe rich core and CdS-ZnS hybrid shell using a simple single-step reaction. These nanocrystals exhibit a very rare example of an optically bi-functional material, simultaneously exhibiting high (~65%) photoluminescence efficiency and strong two-photon absorption cross-section of 1923 GM. Open-aperture z-scan technique was used to measure two-photon absorptions. Chapter 7 is divided into two sections and deals with the generation of white-light emitting nanophosphors. Section-I addresses the white-light emission from a blend of blue, green and red emitting CdSeS nanocrystals. Different shades of the emitted white-light were achieved by tailoring the composition of the blende. Chromaticity of the emitted light of a particular blend is independent of excitation wavelength. Section-II discusses a new approach to generate white-light by combining surface-state emission of nanocrystalline host and d-electron transitions from dopant centres, with an example of Mn2+-doped CdS nanocrystals. Relative contributions from both surface-state emission and Mn2+ d-emission can be tuned by controlling the dopant concentration to generate white lights of different shades. Similar to section-I, here again the chromaticity of the emitted light is independent of the excitation wavelength; but this approach offers additional advantages. Since the surface state emission as well as the Mn2+ d-emission are relatively less sensitive to a size variation compared to the band-edge emission, the chromaticity of the emitted light is not critically dependent on the particle size. Most importantly, these nanocrystals exhibit a huge stokes shift between the absorption and emission spectra resulting in a complete absence of the well-known self-absorption problem, thus, chromaticity of the white-light emitted by these nanocrystals remains unchanged both in dilute dispersion form as well as in solid state. Also there are two appendices in the thesis. Appendix A discusses the preparation of InP nanocrystals using a novel solvothermal route. Appendix B contains the equations explaining photoemission intensity ratios between Se and S (ISe/IS) for a model nanocrystal with a given internal structure.

Semiconductor nanocrystals with sizes comparable to the corresponding bulk excitonic diameter exhibit unique size-dependent electronic and optical properties resulting from quantum confinement effect. Such nanocrystals not only allow the study of evolution of bulk properties from the molecular limit providing important fundamental understandings, but also have great technological implications, leading to intense research over the past several years. Besides tuning the crystal size in the nm regime to obtain novel properties, an additional route to derive new functionalities has been to dope transition metal ions into a semiconductor host. Thus, transition metal doped nanocrystals are of great interest since it allows two independent ways to functionalize semiconductor materials, one via the tunability of properties by size variation and other due to properties of such dopants. Chapter 1 of the thesis provide a general introduction to the subject matters dealt in with this thesis, while the necessary methodologies have been discussed in chapter 2. Chapters 3 and 4 of this thesis deal with nanocrystal doping. Following suggestions in previous literatures that the doping of nanocrystal depends strongly upon the crystal structure of the synthesized host nanocrystal, we have studied the phase-transformation between the somewhat zinc-blende and the usual wurtzite structures for CdS and CdSe nanocrystals in chapter 5. In chapter 6 we have pointed out that a gradient structure is essential to achieve nearly ideal photoluminescence efficiency using heterostructured nanocrystals and also achieved strong two-photon absorptions, adding optical bifunctionality to these nanocrystals. Finally, in chapter 7, we establish different approaches to generate white-light using nanocrystals and their unique advantages, as a first step to realizing white light emitting devices. Chapter 1 provides a brief introduction to various interesting properties and concepts relevant for the studies carried out in the subsequent chapters of this thesis. The present status of the research in the field of semiconductor nanocrystals with an emphasis on synthesizing high quality nanocrystals, doping of nanocrystals and exciting optical properties exhibited by these nanocrystals has been discussed. We have discussed the existing theories and practices of colloidal synthesis that allow us to prepare high quality semiconductor nanocrystals with required size and very narrow size distribution. Optical properties, covering excitonic fine structure, photoluminescence, auger recombination and two-photon absorption have been discussed. We have described heterostructured nanocrystals of different types, particularly in the light of enhancing photoluminescence quantum yield. The difficulty in doping Mn2+ ion in semiconductor nanocrystals and the recent developments in this field have been addressed. Chapter 2 describes experimental and theoretical methodologies that have been employed to study different nanocrystal systems reported in this thesis. The topics covered in this chapter include UV-visible absorption spectroscopy, steady-state and time-resolved luminescence spectroscopy, X-ray diffraction, transmission electron microscopy, electron spin resonance spectroscopy, photoemission spectroscopy, two-photon absorption and least-squared-error fitting. Chapter 3 presents a detailed study of water soluble Mn2+-doped CdS nanocrystals synthesized using colloidal routes. Earlier efforts to dope Mn2+ ion into CdS nanocrystals and therefore, obtain the characteristic orange emission, have been largely impeded by the strong overlap of surface state emission of the host and Mn2+ d-emission. We are the first ones to obtain a distinct Mn2+ d-related emission at around 620 nm, well-separated from the surface state emission with its maximum near 508 nm. In spite of using very high (~30%) concentration of Mn2+ precursor, only ~1% Mn2+ was found in the final product, which is consistent with previous literatures, where Mn2+ doping in such nanocrystals was found to be extremely difficult. Most interestingly, present results establish that Mn2+ ion is found to be incorporated preferentially in the relatively larger sized nanocrystals compared to the smaller sized ones even within the narrow size distribution achieved for a specific reaction condition. We found that 55 oC is the optimum reaction temperature to synthesize Mn2+-doped CdS nanocrystals, at higher reaction temperatures, Mn2+ ions get annealed out of the substitutional sites, leading to a lower level of doping in spite of the formation of larger sized particles. Additionally, we could tune the color of the Mn2+ d- emission from red (620 nm) to yellow (580 nm) by increasing the reaction temperature from 55 oC to 130 oC. Another important aspect is that the synthesized nanocrystals readily dissolve in water without any perceptible effect on the Mn2+ d emission intensity. Chapter 4 discusses the outstanding problem that a semiconductor host in the bulk form can be doped to a large extent, while the same host in the nanocrystal form resist any appreciable level of doping. We first describe two independent models available in literatures to explain this baffling phenomenon. In one, it was suggested that the doping of Mn2+ ion in such nanoclusters is invariably an energetically unfavorable state, thus, Mn2+ ions get annealed out from the host nanocrystal and an increase in reaction temperature facilitate such annealing, a phenomenon known as self-purification. In the second model, it was suggested that the ease of initial adsorption of Mn2+ ions on specific surfaces of a growing nanocrystal, kinetically controls the extent of impurity doping. Specifically, it is easier to dope zinc-blende nanocrystals compared to their wurtzite counterpart. In contrast, the main claim of this chapter is neither crystal structure nor self-purification is as important in nanocrystal doping as lattice mismatch between the dopant and host lattice. To support this claim, we have doped Mn2+ ions into alloyed ZnxCd1-xS nanocrystals. Ionic radius of Mn2+ ion being in between those of Zn2+ and Cd2+ ions, the lattice mismatch between the host ZnxCd1-xS nanocrystal and MnS could be tuned in either side by tuning the composition “x”. It was gratifying to observe that there is an evident maximum of manganese content for Zn0.49Cd0.51S host nanocrystals that has no lattice mismatch with MnS, and the manganese content decreases systematically with increasing compressive as well as tensile lattice mismatches. Based on lattice parameter tuning, we could dope an extraordinarily higher amount of ~7.5% manganese for x = 0.49, at a reaction temperature as high as 310 oC and in a nanocrystal that exhibit wurtzite structure, which was previously suggested unfavorable for doping. These results prove our hypothesis that the strain fields generated because of the lattice mismatch between the dopant and host, are necessarily long range, much longer than typical nanocrystal dimensions and it tends to relieve itself by ejecting the dopant to the surface of nanocrystals, thus, resisting doping in such nanocrystals. High temperature synthesis, on the other hand, leads to a very high photoluminescence efficiency of ~25%. Chapter 5 deals with the phase-control of CdS and CdSe nanocrystals synthesized employing colloidal routes. CdS nanocrystals exhibit a very sensitive phase transformation from zinc-blende to wurtzite structure by increasing the reaction temperature from 280 to 310 oC, which is also accompanied by an increase in particle size from 6 to 6.8 nm, respectively. More importantly, just by changing the S precursor, it has been possible to change the crystal structure of the CdS nanocrystals at a given synthesis temperature of 310 oC. En route, we have synthesized >12 nm zinc-blende CdS nanocrystal, which is the largest one known in literature and that too employing the highest (310 oC) reaction temperature. Thus, our results contradict with the suggestions already in literatures that low reaction temperature and small crystal size favors zinc-blende structure. Also, we could tune crystal structure between zincblende and wurtzite at a given pressure of the reaction vessel and for a given solvent, just by changing the S-precursor, which is again in contradiction to previously made suggestions in literatures that high pressure or noncoordinating solvents favors the formation of zinc-blende nanocrystals. Instead, we believe that the surface energy might be crucial in stabilizing the usually rare zinc-blende structure for such nanocrystals. Chapter 6 is divided into two sections and deals with optically active heterostructured nanocrystals exhibiting high photoluminescence efficiency and strong two-photon absorption. In section-I, we probe the internal structure of extraordinarily luminescent (quantum yield = 85%) CdSeS nanocrystals making a somewhat unconventional use of Photoelectron spectroscopy, using the tunability of the photon energy from the third generation synchrotron radiation source as well as the traditional Mg Kα and Al Kα photon sources. CdSeS nanocrystals synthesized with Se:S precursor ratios 1:5 and 1:50, emitting red and green light have CdSe/CdSeS/CdS core/gradient-shell/shell and CdSeS/CdS gradient-core/shell structure, respectively. Gradient interface/core tunes the lattice parameters continuously between that of CdSe and CdS minimizing the interface related defects which in turn increases the photoluminescence efficiency even beyond that obtained from traditional core/shell nanocrystals, as evidenced by the nearly single exponential photoluminescence decay dynamics exhibited by these nanocrystals. Quantum mechanical calculations further show that a graded-core/shell structure leads to a remarkable spatial collapse and consequently a stronger overlap of the HOMO and LUMO wavefunctions towards the core region and thereby, making these luminescent beyond the traditional core/shell limit. In section-II, we have synthesized hetero-structured nanocrystals with CdSe rich core and CdS-ZnS hybrid shell using a simple single-step reaction. These nanocrystals exhibit a very rare example of an optically bi-functional material, simultaneously exhibiting high (~65%) photoluminescence efficiency and strong two-photon absorption cross-section of 1923 GM. Open-aperture z-scan technique was used to measure two-photon absorptions. Chapter 7 is divided into two sections and deals with the generation of white-light emitting nanophosphors. Section-I addresses the white-light emission from a blend of blue, green and red emitting CdSeS nanocrystals. Different shades of the emitted white-light were achieved by tailoring the composition of the blende. Chromaticity of the emitted light of a particular blend is independent of excitation wavelength. Section-II discusses a new approach to generate white-light by combining surface-state emission of nanocrystalline host and d-electron transitions from dopant centres, with an example of Mn2+-doped CdS nanocrystals. Relative contributions from both surface-state emission and Mn2+ d-emission can be tuned by controlling the dopant concentration to generate white lights of different shades. Similar to section-I, here again the chromaticity of the emitted light is independent of the excitation wavelength; but this approach offers additional advantages. Since the surface state emission as well as the Mn2+ d-emission are relatively less sensitive to a size variation compared to the band-edge emission, the chromaticity of the emitted light is not critically dependent on the particle size. Most importantly, these nanocrystals exhibit a huge stokes shift between the absorption and emission spectra resulting in a complete absence of the well-known self-absorption problem, thus, chromaticity of the white-light emitted by these nanocrystals remains unchanged both in dilute dispersion form as well as in solid state. Also there are two appendices in the thesis. Appendix A discusses the preparation of InP nanocrystals using a novel solvothermal route. Appendix B contains the equations explaining photoemission intensity ratios between Se and S (ISe/IS) for a model nanocrystal with a given internal structure.

碩士
國立中央大學
化學工程與材料工程研究所
94
Abstract A procedure was developed in our laboratory to synthesis non-agglomerated silicalite nanocrystals within days, instead of weeks as required in the literatures. This was achieved by concentrating the precursor sol, aging at 80oC for about a day, followed by a short hydrothermal reaction at 175oC. In this study, the same procedure has been extended to the synthesis of aluminum containing ZSM-5 zeolite nanocrystals. However, before applying the method to ZSM-5 system, some details of the previous procedures were further investigated to clarify the controlling parameters. For example, the proper way to concentrate the precursor sol, the selection of temperature and duration in the first aging stage, as well as that in the second hydrothermal reaction stage. After clarifying the controlling parameters, the method was applied to the synthesis of ZSM-5. In addition to the Si/Al ratio, the effect of TPAOH ratio in the starting recipe was also investigated. It was found that the size of the resulted nanozeolite increases with the amount of aluminum incorporated. On the other hand, there exists an optimum TPAOH/SiO2 ratio leading to the smallest particles size. Under appropriate conditions,non-agglomerated ZSM-5 (silicalite) nanocrystals of 140, 110 and 50 nm for Si/Al of 50, 100 and infinite, respectively, were achieved with a total synthesis time shorted than 2 days.

As nanotechnology becomes ever more pervasive in everyday life, the ability to control the bottom-up synthesis of nanomaterials is of great technological interest. In particular, colloidal semiconducting nanocrystals, or quantum dots, are beginning to find applications in biological imaging, solar cells, and as down-converters for LED-driven light bulbs and displays. Herein we report various experimental endeavors that explore the role of the precursors used to make these nanocrystals, with a special interest in the kinetics of their reactivity. In doing so, we highlight the influence that precursor conversion rate has on the size of nanocrystals, which is essential to optimizing their performance
in optoelectronic devices, catalysis, and imaging applications. After surveying the history, applications, and theoretical models describing the synthesis of semiconductor nanocrystals, we focus on phosphine-based precursors. We describe the synthesis of cadmium bis(diphenyldithiophosphinate) (Cd(S₂PPh₂)₂) from secondary phosphine sulfides and its conversion to cadmium sulfide nanocrystals. Heating Cd(S₂PPh₂)₂ and
 cadmium tetradecanoate to 240 °C results in complete
conversion of Cd(S₂PPh₂)₂ to cadmium sulfide nanocrystals with tetradecanoate surface termination. The nanocrystals have a narrow size distribution that is evident from the line width of the
lowest energy absorption feature and
display bright photoluminescence. Monitoring the reaction with ³¹P NMR, UV-Visible, and infrared absorption spectroscopies shows that the production of cadmium diphenylphosphinate (Cd(O₂PPh₂)₂) and tetradecanoic anhydride co-products is coupled with the formation of cadmium sulfide. From these measurements we propose a balanced chemical equation for the conversion reaction and use it to optimize a synthesis that affords CdS nanocrystals in quantitative yield. Interestingly, the final diameter is insensitive to the reaction conditions, including the total concentration of precursors, which we attribute to a first-order rate of precursor conversion. Using CdS nanocrystals synthesized from Cd(S₂PPh₂)₂ as a model system, we demonstrate that metal carboxylate complexes (L− Cd(O₂CR)₂, R = oleyl, tetradecyl) are
readily displaced from carboxylate-terminated nanocrystals. Removal of up to 90% of surface-bound Cd(O₂CR)₂ from the CdS nanocrystals is possible with N,N,N’,N’-tetramethylethylenediamine (TMEDA), decreasing the photoluminescence quantum yield (PLQY) from 20% to <1% and broadening the 1Sₑ-2S_(3/2)h absorption feature. These changes are partially reversed upon rebinding of M(O₂CR)₂ at room temperature (∼60%) and fully reversed at elevated temperature. A model is proposed in which electron-accepting M(O₂CR)₂ complexes (Z-type ligands) reversibly bind to nanocrystals, leading to a range of stoichiometries for a given core size. The results demonstrate that nanocrystals lack a single chemical formula, and are instead dynamic structures with concentration-dependent compositions. Following the precursor reactivity and rate study undertaken on Cd(S₂PPh₂)₂, we establish a novel method of controlling the number and size of nanocrystals produced from a reaction through the use of a precursor library. We report a library of thioureas whose substitution pattern tunes their conversion reactivity over more than five orders of magnitude and demonstrate that faster thiourea conversion kinetics increases the extent of crystal nucleation. Tunable kinetics thereby allows the nanocrystal concentration to be adjusted and a desired crystal size to be prepared at full conversion. Controlled precursor reactivity and quantitative conversion improve the batch-to-batch consistency of the final nanocrystal size at industrially relevant reaction scales and open up new synthetic routes towards commercially-relevant core/shell heterostuctures. The ability to tune reaction rate independent of the reaction conditions for the first time also enables new studies of the underlying mechanisms of nanocrystal synthesis.

碩士
國立中央大學
化學工程與材料工程研究所
96
Zeolite BEA, with large pore size of 12-membered ring pore openings, is an industrial important catalyst for the transalkylation of toluene and C9 aromatics to produce xylene. It is also expected that if beta zeolite is made into nanocrystals, the higher external surface area may reduce the diffusion length and make the catalyst more effective. Consequently, the synthesis of beta zeolite nanocrystals has been the objectives of many researches. To synthesize smaller zeolite nanoparticles, one can either increase the ratio of organic-template or reduce the hydrothermal temperature. However, the consequence is longer hydrothermal time and lower zeolite yield. Based on our experience in the synthesis of MFI zeolite, a pre-concentrating step before low temperature hydrothermal seemed to accelerate the formation of crystal nuclei and speed up the crystallization. The objective of this research is to applied the experience on MFI zeolite to beta zeolite system and verify if we can improve the zeolite yield without sacrifice the particle size. 1SiO2:0.04AIP:0.36 TEAOH:25H2O was found to be a better recipe. At first the EtOH was removed to force complete hydrolysis and the zeolite beta precursor would be concentrated to speed up the aggregation. Finally, the precursor was transferred to 90℃ for nucleation and crystallization. The formation of zeolite from precursor and its aggregation and growth was monitored to understand the kinetics, based on which the final zeolite particle size could be controlled. Finally, Beta zeolite nanoparticles with uniform particle size distribution (＜50 nm ) are successfully prepared from zeolite precursor. The yield of centrifugation could be 60 wt%.

碩士
南台科技大學
奈米科技研究所
98
In the study, we use Polyol and Thermo decomposition method to preparation for this greigite (Fe3S4) and pyrrhotite (Fe7S8). We can control the size of particles by different temperature, time, concentration of reactants. The results of experimental also show that greatest impact is to different concentration of reactants. Fe3S4 nanocrystals were prepared by the polyol process, the reaction solvent is diethylene glycol (DEG), take iron (II) acetate ((FeC4H6O4)) and N2H4CS added into the DEG, then heated temperature to 180 ℃, finally join the polyvinylpyrrolidone powder (PVP) as a surfactant for oxidation. Fe7S8 nanocrystals were prepared by the thermo decomposition method, take FeCl2‧4H2O into the oleyamine(OLA) and heated temperature to 180 ℃, then join the N2H4CS and heated temperature to 280 ℃. AS OLA is a good surfactant so do not add other surfactant. We use XRD patterns of the synthesized Fe3S4 and Fe7S8 nanocrystals with various mean crystallite sizes, and can notice that range of size is form 9 nm to 26 nm. The magnetic analysis is used of VSM, find that particle of different concentration have different result. Further we use VSM of high temperature measurement Fe7S8, find it is change from the ferrimagnetic to the ferromagnetic state at 400 K and magnetic moment disordering takes place at the Curie temperature, 580 K, at which ferromagnetic state transforms to paramagnetic structure.

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Synthesis and processing of optically active silicon nanocrystals are explored from an aerosol science perspective. Spark ablation, laser ablation and thermal evaporation in inert atmospheres are employed alternatively as vapor phase sources of nanocrystals. Nanocrystals generated employing these techniques comprise a highly polydisperse and morphologically diverse aerosol. After collection on a solid substrate, samples of these nanocrystals exhibit wide-band visible photoluminescence. A system for size classification of the initial polydisperse nanocrystal aerosol is demonstrated employing differential mobility analysis. Working at low nanocrystal concentrations (around [...]) size control within 15% to 20% is achieved in the 2 to 10 nm size regime with a radial differential mobility analyzer at the expense, however, of low throughputs which make optical studies challenging. Seeking higher throughputs, the physics of aerosol size classification by this technique is investigated in detail by self-consistent numerical simulations of the particle transport inside the differential mobility analyzer. Our results lead to the identification of critical design characteristics required to maximize the analyzer performance from the viewpoint of semiconductor nanocrystal synthesis. With the guidance of these theoretical predictions, an optimized differential mobility analyzer design is suggested. This instrument has its parameters chosen to perform high resolution, high throughput size classification of nanocrystals in the 0.5 to 10 nm range. Optical characterization studies on polydisperse and size-classified silicon nanocrystal samples are performed. Results suggest that at least two mechanisms for light emission are at work in aerosol synthesized silicon nanocrystals. X-ray photoelectron measurements on size-classified silicon nanocrystals reveal that an oxide layer with thickness in excess of several nanometers forms on the silicon nanocrystals within a few minutes of air exposure. In order to preserve and control the surface chemistry of the nanocrystals, a system for anaerobic transfer of the size- classified silicon nanocrystals is designed and built. The system couples the nanocrystal synthesis experiment with the ultra high vacuum chamber of a surface analysis system via a load lock high vacuum chamber. Optical characterization capabilities are also installed. Preliminary results on nanocrystal synthesis and characterization using this in situ setup are presented and discussed.

碩士
國立彰化師範大學
物理學系
95
Highly monodispersive CdSe nanocrystals (NCs) were synthesized in HDA-TOPO-TOP mixture. The crystal structure and optical properties of the samples were characterized by taking X-ray diffraction (XRD) and photoluminescence (PL) measurements. From the XRD data, wurtzite structure of the CdSe NCs was confirmed. The PL peaks showed a blue shift as the size of NCs decreased. The carrier dynamics of the CdSe NCs was studied by taking time-resolved photoluminescence (TRPL) measurements. Carrier lifetime was determined from the TRPL spectra. The temperature-dependence of the energy gap Eg(T)、integrated intensity and FWHM of CdSe NCs was studied by taking temperature-dependent PL measurements.

碩士
國立成功大學
材料科學及工程學系碩博士班
100
In the present study, the synthesis of Cu2CdSnSe3 (CCTSe) nanocrystals by two solvotherml processes as a function of the solvent, the molar ratio of precursors, temperature and time were explored. Meanwhile, the optical and thermoelectric properties of CCTSe and Cu-doped CCTSe nanocrystals were also studied. On synthesis in an autoclave, the addition of hydrazine to the ethylenediamine solvent speeded up the formation of pure CCTSe and Cu-doped CCTSe nanocrystals at 190˚C for 72 h. Without addition of hydrazine, some impurity phases such as CdSe and Cu2SnSe3 still remained in the synthesized powders after growth at 190˚C for 72 h. The dimensional reduction of metal chalcogenides in the solvothermal reaction by hydrazine enhanced the growth of the CCTSe and Cu-doped CCTSe nanocrystals. On synthesis in the oleylamine solvent in N2 at 250˚C for 72 h, pure CCTSe and Cu-doped CCTSe nanocrystals could be acquired. The bandgaps of CCTSe and Cu-doped CCTSe nanocrystals were determined to be about 1.1 eV by UV-vis spectroscopy, revealing that the Cu doping had no significant effect on the bandgap of the CCTSe crystals.

碩士
國立臺灣大學
化學研究所
100
Herein, the main theme of our study focuses in two parts as follows. First, a record high PCE of up to 3.2% demonstrates that the efficiency of hybrid solar cells (HSCs) can be boosted by utilizing a unique mono-aniline end group of poly[(4,4''-bis(2- ethylhexyl)-dithieno[3,2-b:2'',3''-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT-NH2) as a strong anchor to attach to CdTe nanocrystal surfaces and by simultaneously exploiting benzene-1,3-dithiol solvent-vapor annealing to improve the charge separation at the donor/acceptor interface, which leads to efficient charge transportation in the HSCs. Second, a method lying addition of oleylamine ligand to trigger the reaction for synthesis of highly emissive (ZnS)x-Cu0.1InS1.55(ZCIS)/ZnS core/shell NCs in one-pot reaction is developed using the low toxic and commercial precursors (CuI, In(CH3COO)3, Zn(S2CNEt2)2). The as-prepared ZCIS core is able to maintain the compositional homogeneity and similar optical properties during a long period of reaction time (≥ 60 min), so that subsequent cationic exchange can be carried out to form the ZnS shell. The resulting core/shell NCs exhibit high quantum yield (> 40%) and emission is tunable from green to red depending on elemental composition. This surfactant induced one-pot reaction also offers scale-up advantage, with 1.6 (green), 2.27 (yellow) and 2.52 (red) grams per reaction being successfully made.