Dissertations / Theses on the topic 'Nanocrystal Solids'

During the last decades, silicon nanocrystals have focused great attention due to the size-dependent physical properties they present, attributed to the quantum confinement effect. This, added to the bulk silicon compatibility with the well-established microelectronics technology and the low mining and manipulation costs this material presents, makes silicon a potential candidate for the growing photonics and optoelectronics fields. In particular, the tunnability of the electronic properties of silicon nanocrystals can be reached by controlling the nanocrystal size. This has been recently achieved by means of the superlattice approach, consisting of the alternated deposition of ultra-thin (2-4 nm) stoichiometric and silicon-rich layers of a given silicon-rich material. After a high-temperature annealing treatment, the silicon excess precipitates and crystallizes in the final form of nanocrystals, whose properties strongly depend on the fabrication process. Consequently, an ordered arrange of size-controlled nanocrystals (the superlattice) is obtained. In this Thesis Project, the structural, optical, electrical and electro-optical properties of silicon nanocrystal superlattices have been studied, using two different silicon-based materials as host matrices: silicon oxide and silicon carbide. The fabrication of these material systems has been carried out at different European institutions, specialists in the controlled deposition of nm¬thick films. Aiming at the nanocrystal superlattices characterization, different experimental techniques have been employed, which yield structural (transmission and scanning electron microscopies, X-ray diffraction), optical (optical absorption, photoluminescence and Raman scattering spectroscopies) and electrical / electro-optical (current versus voltage analysis in dark and under illumination, and electroluminescence, electro-optical response and light-beam induced photocurrent spectroscopies) information. From the material's point of view, the optimum structural properties that allow an almost perfect nanocrystal arrangement, size control and crystalline degree have been determined, always aiming at an optimum light emission and/or light absorption. Within this frame, fundamental studies have been performed to assess the crystalline degree of the nanostructures (confirming an atomic-thin transition layer between the crystalline nanocrystal core and the surrounding matrix), and to carefully inspect the controversial origin of luminescence within the nanocrystals when embedded in a silicon oxide matrix; as well, the structural conditions under which size-confinement of nanocrystals is reached when embedded in silicon carbide are reported. Once the best structural and optical properties from silicon nanocrystal superlattices were found, these material systems have been employed as active layers for light emitting and light converter (i.e. photovoltaic) devices. In oxide-based systems, the mechanisms that govern charge transport through the superlattices have been studied, and impact ionization has been hypothesized as the main electroluminescence excitation mechanism according to the experimental observations. In addition, the structural conditions (sublayer thicknesses, silicon-rich layer stoichiometry) that yield a maximum electroluminescence efficiency have been determined. Regarding silicon nanocrystals embedded in silicon carbide, a correlation has been established between the charge photogeneration and extraction when acting as an absorber material, which allowed assessing the structural conditions that maximize charge transport while minimizing the non-desirable recombination. Finally, via spectral response measurements, quantum confinement of excitons within silicon nanocrystals has been reported in silicon carbide matrix for the first time. In conclusion, the study on silicon nanocrystal superlattices developed within the present Thesis Project reveals the potential of silicon oxide as host matrix for silicon nanostructures to be used as light-emitting devices; instead, silicon carbide has proved a more suitable host material for photovoltaic applications, which sheds light to the future application of silicon nanocrystals as the top cell of an all-Si tandem cell.
Els nanocristalls de silici han esdevingut objecte d'estudi durant l'últim quart de segle, degut a què presenten, a causa de l'efecte de confinament quàntic, unes propietats físiques dependents de la seva mida. A més, la compatibilitat del silici massiu amb la ben establerta tecnologia microelectrònica juga en favor de la seva utilització i el seu desenvolupament per a futures aplicacions en el camp de la fotònica i l'optoelectrónica. El control del creixement de nanocristalls de silici es pot dur a terme mitjançant el dipòsit de superxarxes d'entre 2 i 4 nm de gruix, on capes de material estequiomètric basat en silici s'alternen amb altres de material ric en silici. Un posterior procés de recuit a alta temperatura permet la precipitació de l'excés de silici i la seva cristal.lització, tot originant una xarxa ordenada de nanocristalls de silici de mida controlada. En aquesta Tesi, s'han estudiat les propietats estructurals, òptiques, elèctriques i electro-òptiques de superxarxes de nanocristalls de silici embeguts en dues matrius diferents: òxid de silici i carbur de silici. Amb tal objectiu, s'han emprat tot un seguit de tècniques experimentals, que comprenen la caracterització estructural (microscòpia electrònica de transmissió i d'escombrat, difracció de raigs X), òptica (espectroscòpies d'absorció òptica, de fotoluminescència i dispersió Raman) i elèctrica / electro-òptica (caracterització intensitat-voltatge en foscor o sota il.luminació, electroluminescència, resposta electro-òptica), entre d'altres. Des del punt de vista del material, s'han estudiat les propietats estructurals òptimes per tal d'obtenir un perfecte ordenament en la xarxa de nanocristalls, una major qualitat cristal.lina i unes propietats d'emissió òptimes. L'optimització del material s'ha dut a terme en vistes a la seva utilització com a capa activa dins de dispositius emissors de llum i fotovoltaics, l'eficiència dels quals ha estat monitoritzada segons els diferents paràmetres estructurals (gruix de les capes nanomètriques involucrades, estequiometria, temperatura de recuit). Finalment, els nanocristalls de silici embeguts en òxid de silici han demostrat un major rendiment com a emissors de llum, mentre que una matriu de carbur de silici beneficia les propietats d'absorció i extracció (fotovoltaiques) del sistema.

Biomedicine has recently exploited many nanotechnology platforms for the detection and treatment of disease as well as for the fundamental study of cellular biology. A prime example of these successes is the implementation of semiconductor quantum dots in a wide range of biological and medical applications, from in vitro biosensing to in vivo cancer imaging. Quantum dots are nearly spherical nanocrystals composed of semiconductor materials that can emit fluorescent light with high intensity and a strong resistance to degradation. The aim of this thesis is to understand the fundamental physics of colloidal quantum dots, to engineer their optical and structural properties for applications in biology and medicine, and to examine the interaction of these particles with biomolecules and living cells. Toward these goals, new synthetic strategies for colloidal nanocrystals have been developed, implementing a cation exchange method for independent tuning of size and fluorescence, and a bandgap engineering technique that utilizes mechanical strain imposed by coherent shell growth. In addition, stable nanocrystals have been prepared with ultrathin coatings (< 2 nm), 'amphibious' solubility, and broadly tunable bioaffinity, induced by self-assembly with polyhistidine-sequences on recombinant proteins. Finally, colloidal quantum dots have been studied in biological fluids and living cells in order to elucidate their interactions with biological systems. It was found that these interactions are strongly dependent on the size of the nanocrystal, and cytotoxic effects of these particles are largely independent of their composition of heavy metal atoms, demonstrating that the rule book for toxicology must be rewritten for nanomaterials.

Kyoto University (京都大学)
0048
新制・課程博士
博士(農学)
甲第19038号
農博第2116号
新制||農||1031(附属図書館)
学位論文||H27||N4920(農学部図書室)
31989
京都大学大学院農学研究科森林科学専攻
(主査)教授 木村 恒久, 教授 西尾 嘉之, 教授 髙野 俊幸
学位規則第4条第1項該当

Solid lubricant coatings with controlled microstructures are good candidates in providing lubricity in moving mechanical assembly applications, such as orthopedics and bearing steels. Nanocrystalline ZnO coatings with a layered wurtzite crystal structure have the potential to function as a lubricious material by its defective structure which is controlled by sputter deposition. The interrelationships between sputtered ZnO, its nanocrystalline structure and its lubricity will be discussed in this thesis. The nanocrystalline ZnO coatings were deposited on silicon substrates and Ti alloys by RF magnetron sputtering with different substrate adhesion layers, direct current biases, and temperatures. X-ray diffraction identified that the ZnO (0002) preferred orientation was necessary to achieve low sliding friction and wear along with substrate biasing. In addition, other analyses such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) were utilized to study the solid lubrication mechanisms responsible for low friction and wear.

Puisque cette thèse présente deux études indépendantes sur les nanocristaux de cellulose, le résumé a été divisé en deux sections qui font référence aux chapitres II et III, respectivement.Investigation morphologique et structurelle des nanocristaux de cellulose I et II préparés par hydrolyse à l'acide sulfuriqueLe but du travail de recherche présenté dans le chapitre II était de produire, de caractériser et de comparer les CNC obtenus à partir de la pâte de bois d'eucalyptus en utilisant trois méthodes différentes: i) l'hydrolyse classique à l'acide sulfurique (CN-I), ii) l'hydrolyse acide de la cellulose précédemment mercerisée par traitement alcalin (MCN-II), et iii) la solubilisation de la cellulose dans l'acide sulfurique et la recristallisation subséquente dans l'eau (RCN-II). Les trois types de CNC préparés présentent des morphologies et des structures cristallines différentes. Lorsque les conditions d'hydrolyse acide sont mises en place de telle sorte que les domaines cristallins dans la pâte de bois initial et la cellulose mercerisée (WP et MWP, respectivement) sont préservés (60 wt% H2SO4, 45°C, 50 min), les nanocristaux résultants conservent la nature fibrillaire des fibres d’origine (c'est-à-dire que l'axe de la chaîne est parallèle au grand axe des particules aciculaires) et leur type allomorphe initial (I pour WP et II pour la MWP). Dans les deux cas, les particules sont principalement composées de quelques cristallites élémentaires liées latéralement. Les nanocristaux unitaires dans les CNC préparés à partir de cellulose mercerisée (MCN-II) sont plus courts, mais plus larges que ceux préparés à partir des fibres de cellulose I (CN-I). Si des conditions plus sévères sont considérées (64 wt% H2SO4, 40°C, 20 min), ce qui entraîne la dépolymérisation et la dissolution de la cellulose native, les chaînes courtes recristallisent en rubans de Cell-II lors de la régénération dans l'eau à température ambiante. Dans ces rubans tortueux, l'axe de la chaîne serait perpendiculaire au grand axe du nanocristal et parallèle à son plan basal.La structure moléculaire et cristalline unique des nano-rubans implique qu'un nombre plus élevé d'extrémités de chaîne réductrice sont situées à la surface des particules, ce qui peut être important pour des modifications chimiques subséquentes et pour de potentielles applications spécifiques telles que la biodétection et la bio-imagerie. Donc, cette étude permet de mieux comprendre la structure cristalline et la morphologie de la CNC obtenue par régénération à l'acide sulfurique.Propriétés mécaniques de nanocomposites de caoutchouc naturel renforcé avec des nanocristaux de cellulose à facteur de forme élevé extraits de la coque de sojaDans cette étude, les CNCs ont été isolés des coques de soja à partir d’un traitement par hydrolyse avec de l'acide sulfurique. Ces CNCSH ont été utilisés comme phase de renfort dans une matrice NR par casting à différents taux de charge, à savoir 1, 2.5 et 5% en poids. Les effets des CNCSH sur la structure ainsi que sur les propriétés thermiques et mécaniques du NR ont été étudiés. Par exemple, en ajoutant seulement 2,5% en poids de CNC, le module de conservation en traction du nanocomposite à 25 °C est environ 21 fois plus élevé que celui de la matrice NR non chargée. Cet effet de renfort est supérieur à celui observé pour les CNCs extraits d'autres sources. Il peut être attribué non seulement au facteur de forme élevé de ces CNCs, mais aussi à la rigidité du réseau percolant de nanoparticules formé au sein de la matrice polymère. De plus, il a été constaté que la sédimentation des CNC pendant la mise en œuvre du film nanocomposite par casting joue un rôle crucial sur les propriétés mécaniques. Une contribution importante de ce travail est de mettre en évidence l'importance de la sédimentation des CNCs, pendant l'étape d'évaporation sur les propriétés mécaniques des nanocomposites, ce qui est rarement mentionné dans la littérature
Since this thesis presents two independent studies on cellulose nanocrystals, the abstract was divided in two sections referring to chapters II and III, respectively.Comprehensive morphological and structural investigation of cellulose I and II nanocrystals prepared by sulfuric acid hydrolysisCellulose nanocrystals (CNCs) were produced from eucalyptus wood pulp using three different methods: i) classical sulfuric acid hydrolysis (CN-I), ii) acid hydrolysis of cellulose previously mercerized by alkaline treatment (MCN-II), and iii) solubilization of cellulose in sulfuric acid and subsequent recrystallization in water (RCN-II). The three types of CNCs exhibited different morphologies and crystal structures that were characterized using complementary imaging, diffraction and spectroscopic techniques. CN-I corresponded to the type I allomorph of cellulose while MCN-II and RCN-II corresponded to cellulose II. CN-I and MCN-II CNCs were acicular particles composed of a few laterally-bound elementary crystallites. In both cases, the cellulose chains were oriented parallel to the long axis of the particle, although they were parallel in CN-I and antiparallel in MCN-II. RCN-II particles exhibited a slightly tortuous ribbon-like shape and it was shown that the chains lay perpendicular to the particle long axis and parallel to their basal plane. The unique molecular and crystal structure of the RCN-II particles implies that a higher number of reducing chain ends are located at the surface of the particles, which may be important for subsequent chemical modification. While other authors have described nanoparticles prepared by regeneration of short-chain cellulose solutions, no detailed description was proposed in terms of particle morphology, crystal structure and chain orientation. Was provide such a description in the present document.Mechanical properties of natural rubber nanocomposites reinforced with high aspect ratio cellulose nanocrystals isolated from soy hullsCellulose nanocrystals (CNCs) were isolated from soy hulls by sulfuric acid hydrolysis. The resulting CNCs were characterized using TEM, AFM, WAXS, elemental analysis and TGA. The CNCs have a high crystallinity, specific surface area and aspect ratio. The aspect ratio (around 100) is the largest ever reported in the literature for a plant cellulose source. These CNCs were used as a reinforcing phase to prepare nanocomposite films by casting/evaporation using natural rubber as matrix. The mechanical properties were studied in both the linear and non-linear ranges. The reinforcing effect was higher than the one observed for CNCs extracted from other sources. It may be assigned not only to the high aspect ratio of these CNCs but also to the stiffness of the percolating nanoparticle network formed within the polymer matrix. Moreover, the sedimentation of CNCs during the evaporation step was found to play a crucial role on the mechanical properties

Les nanomatériaux, matériaux de structure nanocristalline, suscitent depuis quelques années un vif intérêt: leur forte densité d'interfaces leur confère des propriétés physico-chimiques originales. L’élaboration de ces matériaux nanocristallins peut se faire par différentes méthodes, dont la mécanosynthèse. L’étude de solutions solides nanocristallines vise à obtenir une meilleure connaissance des nanomatériaux, avec notamment la caractérisation des joints de grains. Quant à la seconde étude, elle porte sur la caractérisation du désordre induit par broyage. Le choix de systèmes et composés à base de fer est sous-tendu par le fait que nous utilisons comme technique principale d'analyse la spectrométrie Mössbauer (57Fe, 119Sn, 121Sb), particulièrement bien adaptée à ces études puisqu'elle est sensible aux phénomènes qui se passent à l'échelle du nanomètre. La synthèse des solutions solides nanocristallines est réalisée à partir d'un mélange de poudres de fer et d'étain (ou antimoine) par broyage à haute énergie dans des conditions expérimentales bien définies. Après une caractérisation de ces solutions solides (composition chimique, taille des grains), le travail primordial porte sur l'étude des joints de grains dans de tels matériaux: structure, largeur des joints de grains, propriétés magnétiques, évolution lors de traitements thermiques. Toutes ces informations conduisent à conclure à une structure peu désordonnée de ces joints de grains. Confrontées à d'autres résultats concernant des matériaux nanocristallins obtenus par des techniques différentes, il apparait que la nature des joints de grains est fortement dépendante du mode d'élaboration. L’étude du désordre induit par broyage est menée dans des composés intermétalliques ordonnes de type L12 et dans des stannures de fer. Nous suivons l'évolution de ces composés intermétalliques au cours du broyage et caractérisons l'équilibre dynamique atteint par les poudres dans des conditions de broyage données

Mestrado em Química - Química Inorgânica e Materiais
O aumento da emissão visível sob excitação no infravermelho-próximo (processo de conversão ascendente de energia) em nanopartículas de Y2O3: Yb3+/Er3+/Li+ é investigado e quantificado. Usam-se os métodos de co-precipitação e reação de estado sólido, para lograr a incorporação efetiva de Li+ na rede do hospedeiro Y2O3. Apesar de frequentemente reportado na literatura, o método de co-precipitação não permite incorporar Li+ nas nanopartículas, como revelado pela análise elementar. O método de reação de estado sólido permite uma dopagem efetiva das nanopartículas de Y2O3: Yb3+/Er3+ com Li+. As medidas de luminescência revelam que o rendimento quântico de emissão (q) das nanopartículas aumenta com o aumento da concentração de Li+ até 12,3% molar, sendo os valores q máximos observados para 4,8 e 12,3% molar. Os difractogramas de raios X (XRD) de pós mostram que as amostras são cristalinas, não contendo fases secundárias. O refinamento de Rietveld dos dados de XRD de pós não evidencia a incorporação de iões de Li+ na rede hospedeira. No entanto, a espectrometria de emissão atómica por plasma acoplado indutivamente (ICP-AES) e a espectroscopia de fotoeletrões excitados por raios-X (XPS) confirmam que as nanopartículas preparadas por via de reação de estado sólido contêm lítio. Por outro lado, a análise termogravimétrica não mostra uma alteração de massa significativa até 800 °C, o que contraria o argumento frequentemente utilizado segundo o qual o aumento da conversão ascendente de energia se deve à diminuição do número de grupos OH presentes na amostra. O tamanho e a forma das nanopartículas são avaliados por microscopia eletrónica de transmissão e estão de acordo com o tamanho de cristalites obtido por XRD usando a equação de Scherrer, sugerindo que as nanopartículas são monocristais. Verifica-se, ainda, que o tamanho das partículas aumenta com o aumento a concentração de Li+. Além do aumento dos rendimentos quânticos por dopagem com iões Li+, a relação de intensidades de emissão vermelho/verde pode ser ajustada. Estes materiais podem ser promissores para bio-aplicações e para sensores de temperatura.
Near infrared-to-visible up-conversion emission enhancement in Y2O3: Yb3+/Er3+/Li+ nanoparticles is investigated and quantified. Co-precipitation and solid-state reaction routes are investigated to achieve an effective incorporation of Li+ in the Y2O3 host lattice. Despite numerous reports in the literature, the co-precipitation method does not allow the Li+ incorporation in the nanoparticles, as revealed by elemental analysis. Solid-state reaction route is shown to be suitable for an effective Li+ doping of Y2O3: Yb3+/Er3+ nanoparticles. Luminescence measurements reveal that the emission quantum yield (q) of the nanoparticles increases with increasing Li+ content up to 12.3 mol%, with the highest q values observed for 4.8 and 12.3 mol%. Powder X-ray diffraction (XRD) patterns show that the samples are crystalline and do not contain secondary phases. Rietveld refinement of powder XRD data does not evidence the incorporation of Li+ in the host lattice. However, inductively coupled plasma atomic emission spectroscopy (ICP-AES) and X-ray photoelectron spectroscopy (XPS) confirm that the nanoparticles prepared by the solid-state reaction route contain lithium. In addition, thermogravimetric analysis shows no significant weight change up to 800 °C, which does not support the often used argument that the up-conversion photoluminescence enhancement is due to the decrease in the number of OH-groups present in the sample. The size and shape of the nanoparticles are assessed by transmission electron microscopy and are in accord with the crystallite size obtained from XRD using Scherrer’s equation, suggesting that the nanoparticles are single crystals. Moreover, the particle size increases with increasing Li+ concentration. In addition to the enhancement of quantum yields by Li+ doping, the red/green emission intensity ratio can be controlled. These materials may be promising for bio-application and for temperature sensors.

Dans le cadre de ce travail, une méthode chimique, la voie « polymère précéramique » a été mise en oeuvre pour générer des nanocomposites céramiques à base de silicium et contenant des métaux de transition (Ti, Zr, Hf) sous forme d'objets massifs. Cette thèse consiste tout d'abord en un premier chapitre de bibliographie décrivant la méthode de préparation mise en oeuvre dans ce manuscrit ainsi que les matériaux étudiés. L'étude consiste dans un deuxième chapitre à synthétiser puis à caractériser des polymétallosilazanes qui, par des traitements thermiques appropriés, conduisent à des nanocomposites dans lesquels des nanocristaux de nitrures métalliques (nc-MN avec M = Ti, Zr et Hf) sont dispersés dans une phase amorphe ou cristallisée de nitrure de silicium. Ces nanocomposites sont alors caractérisés par différentes techniques afin de sélectionner les paramètres opératoires, allant de la synthèse des polymères à leur conversion en céramique, conduisant aux nanocomposites souhaités (e.g. matrice amorphe de nitrure de silicium) avec les propriétés visées (e.g. coloration). Dans un troisième chapitre, l'étude vise à modifier chimiquement ces polymétallosilazanes afin qu'ils soient adaptés à la conception d'objets massifs par compactage à chaud de polymetallosilazanes puis traitement thermique des compacts polymères. Des techniques de caractérisation sont alors mises en oeuvre sur l'ensemble du procédé pour suivre la transformation du compact polymère en objet massif. Les propriétés mécaniques de ces derniers sont notamment discutées. Dans chapitre 4, nous présentons des résultats préliminaires sur i) l'utilisation de la technique Spark Plasma Sintering pour optimiser la qualité des objets massifs à partir des poudres élaborées dans le chapitre 2, ii) l'ajout d'un second métal au système ternaire Si/Ti/N suivant le procédé d'élaboration étudié dans les chapitres 2 et 3 et iii) l'élaboration de ces nanocomposites à travers une voie d'élaboration dont le coût de préparation est plus abordable
In the present work, a chemical approach called the « Polymer Derived Ceramics » route, has been applied to prepare monolithic ceramic nanocomposites of the type nc-MN/a-Si3N4 with nc, nanocrystals, M, Transition Metal (Ti, Zr, Hf) and a being amorphous. After a literature review in the first chapter, we have designed preceramic polymers of the type polymetallosilazanes to provide after pyrolysis nanocomposites in form of powders in the second chapter. Each step of the process has been studied using characterization tools such as molecular weight measurements, solid-state NMR, and infrared spectroscopy. The structure of the polymers has been proposed. The pyrolysis has been investigated by thermogravimetric analysis and the final materials have been characterized by X-ray diffraction and TEM to confirm the nature of the nanocomposites. In a third chapter, polymers have been tested with regard to warm-pressing as shaping process to form green compacts which have been treated under ammonia then nitrogen at high temperatures to produce the desired ceramic nanocomposites (e.g. amorphous silicon nitride matrix) with specified properties (e.g. decorative properties) in form of monolith. Structural, mechanical and decorative properties have been finally studied. In the fourth chapter, we presented preliminary results on i) the use of Spark Plasma Sintering technique on the powders prepared in chapter 2 to optimize the quality of the solid objects ii) the introduction of two types of transition metals in the same polymetallosilazane leading to a new type of nanocomposite according to the process described in chapters 2 and 3 and iii) the elaboration of these nanocomposites through a cost-effective “two-step” process by dispersing transition metal nanoparticles within polycarbosilazanes

Nanocrystal solids represent an exciting new class of materials. These are often referred to as artificial solids, in which the nanocrystals take the place of atoms in traditional solids. This thesis reports the utility of field-effect transistor measurements to elucidate charge transport parameters, such as charge carrier density and charge carrier mobility in a nanocrystal solid. The evolution of these parameters with chemical treatments is followed and correlated to improved performance in photovoltaic devices. Chemical treatments are demonstrated to simultaneously engineer interparticle spacing, doping and electronic coupling in nanocrystal solids. The nanocrystal solids are then utilized as building blocks for fabricating all nanocrystal heterostructure. A type-I nanocrystal heterostructure is fabricated to demonstrate efficient electroluminescent device in the infrared communications wavelength. The device emits at peak wavelength of 1.58 um with an effciency of 0.5%.

Synthesis of nanocrystals is one of the most rapidly advancing areas of nanoscience, and today nanocrystals can be produced with impressive control over their composition, size, shape, polydispersity, and surface chemistry. As such, they are ideal building blocks for fabricating hierarchical architectures with tailorable functionality on every level of the hierarchy. Here an evaporation-driven, template-assisted nanocrystal assembly (ETNA) technique is developed, providing a novel and general approach to fabricating freestanding, 3D, functional architectures using diverse combinations of colloidal nanocrystal species and porous templates of arbitrary geometry. Colloidal PbS (photoluminescent) and CoFe2O4 (superparamagnetic) nanocrystals are template-assembled to fabricate freestanding nanorods and inverse opals, which retain the size-dependent properties of their constituent building blocks while replicating the geometry and preserving the functionality of the templates. Further multifunctionality is demonstrated through mixed-nanocrystal architectures which exhibit the aggregate functionality of their building blocks.

碩士
國立交通大學
材料科學與工程系所
97
In this research, we use solution phase method to fabricate PbSe that has different first absorption peak. The PbSe thin films are treated chemically in a solution of butylamine in acetonitrile to remove the surface oleate and the films are annealed in nitrogen. Subsequently, blend P3HT:PCBM composite (1: 0.8 wt %) dissolved in chlorobenzene was spin -casted at top of the PbSe thin film to fabricate solar cell device. In the experiment, we discover that change the solution of butylamine concentration, the immersion time and the annealing temperature and time, will affect the power conversion efficiency of solar cell. The thermal treatments of PbSe NC films were analysis by FTIR, TGA, and XRD; the result shows that oleate can be removed at certain annealing temperature, causing NCs growth and increasing the crystallinity. FTIR spectrum of modified PbSe thin films indicate that the amount of removal oleate increase with higher modified solution concentration and the longer soaking time. As shown by SEM image, the resulting modified films exist cracks. These cracks size become larger with increasing modified solution concentration, indicate that removed oleate attribute to decrease the distance of particles. Analysis by FTIR spectrum reveal an increasing amount of removal oleate at certain annealing temperature. The blend PCBM:P3HT solution was then spin-cast on top of the PbSe thin film. Subsequently, by Near IR-UV-Visible spectroscopy, we obtained a broaden absorption region from UV-Visible to Near IR of PCBM:P3HT spin-cast on top of the PbSe thin film. According to the analysis above, PbSe/PCBM:P3HT devices shows a better infrared power conversion efficiency and PCE than the devices without PbSe thin films.We attribute this enhancement to increased infrared power conversion efficiency and PCE as a result of the deposition of PbSe; the increasing absorption from UV-Visible to Near IR region and increasing crystallinity result from modification and annealing treatment of removal oleate.

This thesis reports a passive method for Fermi level regulation in quantum dot assemblies through ground state transfer between QDs. Here, ZnTe/CdS, and PbSe/CdSe core/shell QDs were used as valence band electron donors, while Cu containing CdS or ZnSe acts as electron acceptor QDs. Prior to study of ground state charge transfer process, this report discusses the synthesis of ZnTe/CdS, and PbSe/CdSe core shell QDs, which are later used to study charge transfer. Since ZnTe QDs are unstable and prone to oxidation, a CdS coated ZnTe QDs were used. Growing a CdS shell on ZnTe core is difficult because high reduction potential of Te. To overcome this problem, partially reduced sulphur is used for the synthesis of ZnTe/CdS. The peculiar optical properties exhibited by ZnTe/CdS also have been discussed. Even though the synthesis of Lead chalcogenide nanoparticles has been investigated previously, certain inconsistencies between the behavior expected from known mechanisms and empirical observations. An anion exchange mechanism is proposed and demonstrated to be involved in PbSe formation. Both ZnTe and PbSe based QDs are extensively used to study hole injection and copper containing QDs were used as acceptors. The charge transfer has been studied using optical spectroscopy. The structure and composition of the assemblies was identified using powder crystallography, electron-microscopy and composition analysis. The unique physical and chemical properties of these materials are exciting both fundamentally as well as from the point of view of applications.

This thesis reports a passive method for Fermi level regulation in quantum dot assemblies through ground state transfer between QDs. Here, ZnTe/CdS, and PbSe/CdSe core/shell QDs were used as valence band electron donors, while Cu containing CdS or ZnSe acts as electron acceptor QDs. Prior to study of ground state charge transfer process, this report discusses the synthesis of ZnTe/CdS, and PbSe/CdSe core shell QDs, which are later used to study charge transfer. Since ZnTe QDs are unstable and prone to oxidation, a CdS coated ZnTe QDs were used. Growing a CdS shell on ZnTe core is difficult because high reduction potential of Te. To overcome this problem, partially reduced sulphur is used for the synthesis of ZnTe/CdS. The peculiar optical properties exhibited by ZnTe/CdS also have been discussed. Even though the synthesis of Lead chalcogenide nanoparticles has been investigated previously, certain inconsistencies between the behavior expected from known mechanisms and empirical observations. An anion exchange mechanism is proposed and demonstrated to be involved in PbSe formation. Both ZnTe and PbSe based QDs are extensively used to study hole injection and copper containing QDs were used as acceptors. The charge transfer has been studied using optical spectroscopy. The structure and composition of the assemblies was identified using powder crystallography, electron-microscopy and composition analysis. The unique physical and chemical properties of these materials are exciting both fundamentally as well as from the point of view of applications.

This thesis deals with the research work carried out on the properties and applications such as GaN nanoparticles, Graphene etc. Chapter 1 of the thesis gives introduction to nanomaterials and various aspects of the thesis. Chapter 2 of the thesis describes the synthesis of GaN nanocrystals and their use as white light sources and as room temperature gas sensors. It also discusses negative differential resistance above room temperature exhibited by GaN. Electroluminescence from GaN-polymer heterojunction forms the last section of this chapter. Chapter 3 demonstrates the role of defect concentration on the photodetecting properties of ZnO nanorods with different defects prepared at different temperatures. Chapter 4 presents remarkable infrared and ultraviolet photodetector properties of reduced graphene oxide and graphene nanoribbons. Chapter 5 presents the infrared detecting properties of graphene-like few-layer MoS2. The summary of the thesis is given at the end of the thesis.

This thesis deals with the research work carried out on the properties and applications such as GaN nanoparticles, Graphene etc. Chapter 1 of the thesis gives introduction to nanomaterials and various aspects of the thesis. Chapter 2 of the thesis describes the synthesis of GaN nanocrystals and their use as white light sources and as room temperature gas sensors. It also discusses negative differential resistance above room temperature exhibited by GaN. Electroluminescence from GaN-polymer heterojunction forms the last section of this chapter. Chapter 3 demonstrates the role of defect concentration on the photodetecting properties of ZnO nanorods with different defects prepared at different temperatures. Chapter 4 presents remarkable infrared and ultraviolet photodetector properties of reduced graphene oxide and graphene nanoribbons. Chapter 5 presents the infrared detecting properties of graphene-like few-layer MoS2. The summary of the thesis is given at the end of the thesis.

博士
國立臺灣大學
化學工程學研究所
100
In this dissertation, the main purposes are to enhance the charge transport in the semiconductor nanocrystal-based photoanodes, and improve the long-term stability of the dye-sensitized solar cells (DSSCs). In chapter 1, we make a short introduce of DSSCs, and also the completed introductions for semiconductor nanocrystal-based photoanodes of DSSCs and quasi-solid-state DSSCs (QSS-DSSCs). Their history and applications are also discussed here. The experimental section for all of our studies is shown in chapter 2. In chapter 3, we try to enhance the performance of a DSSC with the incorporation of titanium carbide (TiC) in the titania (TiO2) matrix. It is established that TiC was partially converted into anatase TiO2 when the TiC was sintered at 450 °C. With the incorporation of 3.0 wt% of the TiC in the TiO2 film, the solar-to-electricity conversion efficiency (η) of the cell reached to 7.56% from its value of 6.61% with a bare TiO2 film. “In situ” incorporation of this TiC/anatase TiO2 composite in the commercial TiO2 is considered as the basis for enhanced cell efficiency of the benefited cell. In chapter 4, a highly efficient ruthenium dye with an alkyl bithiophene group, designated as CYC-B1, is employed as the photosensitizer for a zinc oxide (ZnO)-based DSSC. The DSSC with a ZnO film (designated as ZN20) sensitized with this dye exhibited an η of 4.88%. Further, PMMA spheres with uniform sizes of ca. 300 nm are synthesized and incorporated to template the ZN20 film (designated as PMMA-ZN20); this PMMA-ZN20 film is used as an overlayer on the underlayer ZN20 film to make the photoanode film (ZN20/PMMA-ZN20) of a DSSC; the thus fabricated DSSC shows an η of 5.42%. This efficiency (5.42%) is highest ever for an all ZnO-based DSSC with a ruthenium-based photosensitizer. In chapter 5, we study on the favorable effects of titanium nitride (TiN) or its thermally-treated version in a polymer-gelled electrolyte for a QSS-DSSC. With an addition of 3 wt% TiN, the η of the DSSC reaches 5.33% from 4.15% of the cell without TiN. X-ray diffraction (XRD) spectra of thermally treated-TiN (tt-TiN) clearly shows the partial conversion of TiN into TiO2 with both anatase and rutile crystal phases. The DSSC with the incorporation of 3 wt% of tt-TiN into its electrolyte shows a further improved η of 5.68%, with reference to the η of TiN-incorporated DSSC. The cell with 3 wt% of tt-TiN also shows unfailing at-rest stability after more than 1,000 h. In chapter 6, we fabricate a QSS-DSSC by using a room-temperature ionic liquid (IL), 1-propyl-3-methylimidazolium iodide (PMII), and polyaniline-loaded carbon black (PACB) as the composite electrolyte without the addition of iodine. The η of 5.81% is achieved with this type of DSSC. At-rest durability of the QSS-DSSC with PMII/PACB composite electrolyte was studied at 70 °C and shows unfailing durability. In chapter 7, a solid IL crystal, 1-ethyl-3-methylimidazolium iodide (EMII), employed as a charge transfer intermediate (CTI) to fabricate an all-solid-state DSSC. In addition, single-walled carbon nanotubes (SWCNTs) were incorporated into EMII and achieved a higher η of 1.88%, as compared to that containing bare EMII (0.41%). Moreover, PMII, which acts simultaneously as a co-CTI and crystal growth inhibitor, was used to further improve η. The highest η (3.49%) is achieved using a hybrid SWCNT-binary CTI (EMII/PMII) and shows a durability of greater than 1,000 h.

Electronic and optical properties of semiconducting nanocrystals, that can be engineered and manipulated by various ways like varying size, shape, composition, structure, has been a subject of intense research for more than last two decades. The size dependency of these properties in semiconductor nanocrystals is direct manifestation of the quantum confinement effect. Study of electronic and optical properties in smaller dimensions provides a platform to understand the evolution of fundamental bulk properties in the semiconductors, often leading to realization and exploration of entirely new and novel properties. Not only of fundamental interests, the semiconductor nanocrystals are also shown to have great technological implications in diverse areas. Besides size tunable properties, introduction of impurities, like transition metal ions, gives rise to new functionalities in the semicon-ductor nanocrystals. These materials, termed as doped semiconductor nanocrystals, have been the subject of great interest, mainly due to the their interesting optical properties. Among different transition metal doped semiconductor nanocrystals, manganese doped systems have drawn a lot on attention due to their certain advantages over other dopants. One of the major advantages of Mn doped semiconductor nanocrystals is that they do not suffer from the problem of self-absorption of emission, which quite often, is consid-ered detrimental in their undoped counterparts. The doped nanocrystals are known to produce a characteristic yellow-orange emission upon photoexcitation of the host that is relatively insensitive to the surface degradation of the host. This emission, originating from an atomic d-d transition of Mn2+ ions, has been a subject of extensive research in the recent past. In spite of the spin forbidden nature of the specific d-d transition, namely 6A1 −4 T1, these doped nanocrystals yield intense phosphorescence. However, one major drawback of utilizing this system for a wide range application has been the substantial inability of the community to tune the emission color of Mn-doped systems in spite of an intense effort over the years; the relative constancy of the emission color in these systems has been attributed to the essentially atomic nature of the optical transition involving localized Mn d levels. Interestingly, however, the Mn emission has a very broad spectral line-width in spite of its atomic-like origin. While the long (∼ 1 ms) emission life-time of the de-excitation process is well-studied and understood in terms of the spin and orbitally forbidden nature of the transition, there is little known concerning the process of energy transfer to the Mn from the host in the excitation step. In this thesis, we have studied the ultrafast dynamic processes involved in Mn emission and addressed the issues related to its tunability and spectral purity. Chapter 1 provides a brief introduction to the fundamental concepts relevant to the studies carried out in the subsequent chapters of this thesis. This chapter is started with a small preview of the nanomaterials in general, followed by a discussion on semiconducting nanomaterials, evolution of their electronic structure with dimensions and size as well as the effect of quantum confinement on their optical properties. As all the semiconducting nanomaterials studied in the thesis are synthesized via colloidal synthesis routes, a separate section is devoted on colloidal semiconducting nanomaterials, describing various ways of modifying or tuning their optical properties. This is followed by an introduction to the important class of materials “doped semiconductor nanocrystals”. With a general overview and brief history of these materials, we proceed to discuss about various aspects of manganese doped semiconductor nanocrystals in great details, highlighting the origin of the manganese emission and the associated carrier dynamics as well as different reported synthetic strategies to prepare these materials. The chapter is closed with the open questions related to manganese doped semiconductor nanocrystals and the scope of the present work. Chapter 2 describes different experimental and theoretical methods that have been employed to carry out different studies presented in the thesis. It includes common experimental techniques like UV-Vis absorption spectroscopy, steady-state and time-resolved photoluminescence spectroscopy used for optical measurements, X-ray diffraction, trans-mission electron microscopy and atomic absorption spectroscopy used for structural and elemental analysis. Experimental tools to perform special studies like transient absorption and single nanocrystal spectroscopy are also discussed. Finally, theoretical fitting method used to analyse various spectral data has been discussed briefly. Chapter 3 deals with the dynamic processes involved in the photoexcitation and emission in manganese doped semiconductor nanocrystals. For this study, Mn doped ZnCdS alloyed nanocrystal has been chosen as a model system. There are various radiative and nonrdiative recombination pathways of the photogenerated carriers and they often compete with each other. We have studied the dynamics of all possible pathways of carrier relaxation, viz. excitonic recombination, surface state emission and Mn d-d transition. The main highlight of this chapter is the determination of the time-scale to populate surface states and the Mn d-states after the photoexcitation of the host. Employing femtosecond pump-probe based transient absorption study we have shown that the Mn dopant states are populated within sub-picosecond of the host excitation, while it takes a few picoseconds to populate the surface states. Keeping in mind the typical life-time of the excitonic emission (∼ a few ns), the ultra-fast process of energy transfer from the host to the Mn ions explains why the presence of Mn dopant ions quenches the excitonic as well as the surface state emissions so efficiently. Chapter 4 presents a study of manganese emission in ZnS nanocrystals of different sizes. By varying the size of the ZnS host nanocrystal, we show that one can tune the Mn emission over a limited range. In particular, with a decrease in host size, the Mn emission has been observed to red-shift. We have attributed this shift in Mn emission to the change in the ratio of surface to bulk dopant ions with the variation of the host size, noting that the strength of the ligand field at the Mn site should depend on the position of the Mn ion relative to the surface due to a systematic lattice relaxation in such nanocrystals. The ligand field affects the emission wavelength directly by controlling the splitting of the t2 and e levels of Mn2+ ions. The surface dopant ions experience a strong ligand field due to distorted tetrahedral environment which leads to larger splitting of these t2 and e states. We further corroborated these results by performing doping concentration dependent emission and life-time studies. In Chapter 5 addresses two fundamental challenges related to manganese photolumines-cence, namely the lack of a substantial emission tunability and presence of a very broad spectral width (∼ 180-270 meV). The large spectral width is incompatible with atomic-like manganese 4T1 −6 A1 transition. On the other hand, if this emission is atomic in nature, it should be relatively unaffected by the nature of the host, though it can be manipulated to some extent as discussed in Chapter 3. The lack of Mn emission tunability and spectral purity together seriously limit the usefulness of Mn doped semiconductor nanocrystals. To understand why the Mn emission tunability range is very limited (typically 565-630 nm) and to understand the true nature of this emission, we carried out single nanocrystal imaging and spectroscopy on Mn doped ZnCdS alloyed nanocrystals. This study reveals that Mn emission, in fact, can vary over a much wider range (∼ 370 meV) and exhibits widths substantially lower (∼ 60-75 meV) than reported so far. We explained the occur-rence of Mn emission in this broad spectral range in terms of the possibility of a large number of symmetry inequivalent sites resulting from random substitution of Cd and Zn ions that leads to differing extent of ligand field contributions towards the splitting of Mn d-levels. The broad Mn emission observed in ensemble-averaged measurements is the result of contribution from Mn ions at different sites of varying ligand field strengths inside the NC. Chapter 6 presents a synthetic strategy to strain-engineer a nanocrystal host lattice for a controlled tuning of the ligand field effect of the doped Mn sites. It is realized synthesizing a strained quantum dot system with the structure ZnSe/CdSe/ZnSe. A larger lattice parameter of CdSe compared to that of ZnSe causes a strain field that is maximum near the interface, gradually decreasing towards the surface. We control the positioning of Mn dopant ions at different distances from the interface, thereby doping Mn at different predetermined strain fields. With the help of this strain engineering, we are able to tune Mn emission across the entire range of the visible spectrum. This strain induced tuning of Mn emission is accompanied by life-times that is dependent on the emission energy which has been explained in terms of perturbation effect on the Mn center due to the strain generated inside the quantum dot. The spectacular emission tuning has been explained by modelling the quantum dot system as an elastic continuum containing three distinct layers under hydrostatic pressure. From this modelling, we found that the strain is max-imum at the interface and decreases continuously as one goes away from the interface. We also show that the Mn emission maximum red shifts with increasing distance of the dopants from the maximum strained region. In summary, we have performed a study on the photophysical processes in manganese doped semiconductor nanocrystals. We have emphasized in understanding of different dynamic processes associated with the manganese emission and tried to understand the true nature of manganese emission in a nanocrystal. This study has brought out some new aspects of manganese emission and opened up possibilities to tune and control manganese emission by proper design of the host material.

Electronic and optical properties of semiconducting nanocrystals, that can be engineered and manipulated by various ways like varying size, shape, composition, structure, has been a subject of intense research for more than last two decades. The size dependency of these properties in semiconductor nanocrystals is direct manifestation of the quantum confinement effect. Study of electronic and optical properties in smaller dimensions provides a platform to understand the evolution of fundamental bulk properties in the semiconductors, often leading to realization and exploration of entirely new and novel properties. Not only of fundamental interests, the semiconductor nanocrystals are also shown to have great technological implications in diverse areas. Besides size tunable properties, introduction of impurities, like transition metal ions, gives rise to new functionalities in the semicon-ductor nanocrystals. These materials, termed as doped semiconductor nanocrystals, have been the subject of great interest, mainly due to the their interesting optical properties. Among different transition metal doped semiconductor nanocrystals, manganese doped systems have drawn a lot on attention due to their certain advantages over other dopants. One of the major advantages of Mn doped semiconductor nanocrystals is that they do not suffer from the problem of self-absorption of emission, which quite often, is consid-ered detrimental in their undoped counterparts. The doped nanocrystals are known to produce a characteristic yellow-orange emission upon photoexcitation of the host that is relatively insensitive to the surface degradation of the host. This emission, originating from an atomic d-d transition of Mn2+ ions, has been a subject of extensive research in the recent past. In spite of the spin forbidden nature of the specific d-d transition, namely 6A1 −4 T1, these doped nanocrystals yield intense phosphorescence. However, one major drawback of utilizing this system for a wide range application has been the substantial inability of the community to tune the emission color of Mn-doped systems in spite of an intense effort over the years; the relative constancy of the emission color in these systems has been attributed to the essentially atomic nature of the optical transition involving localized Mn d levels. Interestingly, however, the Mn emission has a very broad spectral line-width in spite of its atomic-like origin. While the long (∼ 1 ms) emission life-time of the de-excitation process is well-studied and understood in terms of the spin and orbitally forbidden nature of the transition, there is little known concerning the process of energy transfer to the Mn from the host in the excitation step. In this thesis, we have studied the ultrafast dynamic processes involved in Mn emission and addressed the issues related to its tunability and spectral purity. Chapter 1 provides a brief introduction to the fundamental concepts relevant to the studies carried out in the subsequent chapters of this thesis. This chapter is started with a small preview of the nanomaterials in general, followed by a discussion on semiconducting nanomaterials, evolution of their electronic structure with dimensions and size as well as the effect of quantum confinement on their optical properties. As all the semiconducting nanomaterials studied in the thesis are synthesized via colloidal synthesis routes, a separate section is devoted on colloidal semiconducting nanomaterials, describing various ways of modifying or tuning their optical properties. This is followed by an introduction to the important class of materials “doped semiconductor nanocrystals”. With a general overview and brief history of these materials, we proceed to discuss about various aspects of manganese doped semiconductor nanocrystals in great details, highlighting the origin of the manganese emission and the associated carrier dynamics as well as different reported synthetic strategies to prepare these materials. The chapter is closed with the open questions related to manganese doped semiconductor nanocrystals and the scope of the present work. Chapter 2 describes different experimental and theoretical methods that have been employed to carry out different studies presented in the thesis. It includes common experimental techniques like UV-Vis absorption spectroscopy, steady-state and time-resolved photoluminescence spectroscopy used for optical measurements, X-ray diffraction, trans-mission electron microscopy and atomic absorption spectroscopy used for structural and elemental analysis. Experimental tools to perform special studies like transient absorption and single nanocrystal spectroscopy are also discussed. Finally, theoretical fitting method used to analyse various spectral data has been discussed briefly. Chapter 3 deals with the dynamic processes involved in the photoexcitation and emission in manganese doped semiconductor nanocrystals. For this study, Mn doped ZnCdS alloyed nanocrystal has been chosen as a model system. There are various radiative and nonrdiative recombination pathways of the photogenerated carriers and they often compete with each other. We have studied the dynamics of all possible pathways of carrier relaxation, viz. excitonic recombination, surface state emission and Mn d-d transition. The main highlight of this chapter is the determination of the time-scale to populate surface states and the Mn d-states after the photoexcitation of the host. Employing femtosecond pump-probe based transient absorption study we have shown that the Mn dopant states are populated within sub-picosecond of the host excitation, while it takes a few picoseconds to populate the surface states. Keeping in mind the typical life-time of the excitonic emission (∼ a few ns), the ultra-fast process of energy transfer from the host to the Mn ions explains why the presence of Mn dopant ions quenches the excitonic as well as the surface state emissions so efficiently. Chapter 4 presents a study of manganese emission in ZnS nanocrystals of different sizes. By varying the size of the ZnS host nanocrystal, we show that one can tune the Mn emission over a limited range. In particular, with a decrease in host size, the Mn emission has been observed to red-shift. We have attributed this shift in Mn emission to the change in the ratio of surface to bulk dopant ions with the variation of the host size, noting that the strength of the ligand field at the Mn site should depend on the position of the Mn ion relative to the surface due to a systematic lattice relaxation in such nanocrystals. The ligand field affects the emission wavelength directly by controlling the splitting of the t2 and e levels of Mn2+ ions. The surface dopant ions experience a strong ligand field due to distorted tetrahedral environment which leads to larger splitting of these t2 and e states. We further corroborated these results by performing doping concentration dependent emission and life-time studies. In Chapter 5 addresses two fundamental challenges related to manganese photolumines-cence, namely the lack of a substantial emission tunability and presence of a very broad spectral width (∼ 180-270 meV). The large spectral width is incompatible with atomic-like manganese 4T1 −6 A1 transition. On the other hand, if this emission is atomic in nature, it should be relatively unaffected by the nature of the host, though it can be manipulated to some extent as discussed in Chapter 3. The lack of Mn emission tunability and spectral purity together seriously limit the usefulness of Mn doped semiconductor nanocrystals. To understand why the Mn emission tunability range is very limited (typically 565-630 nm) and to understand the true nature of this emission, we carried out single nanocrystal imaging and spectroscopy on Mn doped ZnCdS alloyed nanocrystals. This study reveals that Mn emission, in fact, can vary over a much wider range (∼ 370 meV) and exhibits widths substantially lower (∼ 60-75 meV) than reported so far. We explained the occur-rence of Mn emission in this broad spectral range in terms of the possibility of a large number of symmetry inequivalent sites resulting from random substitution of Cd and Zn ions that leads to differing extent of ligand field contributions towards the splitting of Mn d-levels. The broad Mn emission observed in ensemble-averaged measurements is the result of contribution from Mn ions at different sites of varying ligand field strengths inside the NC. Chapter 6 presents a synthetic strategy to strain-engineer a nanocrystal host lattice for a controlled tuning of the ligand field effect of the doped Mn sites. It is realized synthesizing a strained quantum dot system with the structure ZnSe/CdSe/ZnSe. A larger lattice parameter of CdSe compared to that of ZnSe causes a strain field that is maximum near the interface, gradually decreasing towards the surface. We control the positioning of Mn dopant ions at different distances from the interface, thereby doping Mn at different predetermined strain fields. With the help of this strain engineering, we are able to tune Mn emission across the entire range of the visible spectrum. This strain induced tuning of Mn emission is accompanied by life-times that is dependent on the emission energy which has been explained in terms of perturbation effect on the Mn center due to the strain generated inside the quantum dot. The spectacular emission tuning has been explained by modelling the quantum dot system as an elastic continuum containing three distinct layers under hydrostatic pressure. From this modelling, we found that the strain is max-imum at the interface and decreases continuously as one goes away from the interface. We also show that the Mn emission maximum red shifts with increasing distance of the dopants from the maximum strained region. In summary, we have performed a study on the photophysical processes in manganese doped semiconductor nanocrystals. We have emphasized in understanding of different dynamic processes associated with the manganese emission and tried to understand the true nature of manganese emission in a nanocrystal. This study has brought out some new aspects of manganese emission and opened up possibilities to tune and control manganese emission by proper design of the host material.

The thesis entitled “Local structure-property relationship in some selected Solid State Materials” mainly focuses on two fundamental topics: (a) evaluation of some standard global structural concepts in terms of local structure to provide a unique description of the crystal structure, and (b) the role of the crystal structure at different length-scales in controlling the properties in some selected materials.

The thesis entitled “Local structure-property relationship in some selected Solid State Materials” mainly focuses on two fundamental topics: (a) evaluation of some standard global structural concepts in terms of local structure to provide a unique description of the crystal structure, and (b) the role of the crystal structure at different length-scales in controlling the properties in some selected materials.

Abstract The present Thesis discusses various Titanium dioxide (TiO2) - Cadmium Sulfide (CdS) assemblies for efficient harvesting of the solar photon. Inorganic semiconductor nanocrystals such as CdS have attracted considerable attention in the realm of solar photon harvesting mainly due to beneficial properties such as easy tunability of their optical, electrical, magnetic properties, functional stability i.e. non-degradability under atmospheric conditions, materials synthesis and device fabrication by benchtop methods. However, a major detrimental issue that prevails in semiconductor nanocrystals is charge recombination. Tailored semiconductor assemblies with favourable energetics can significantly alleviate the effect of charge recombination. Improved charge separation in an optimum semiconductor assembly may aid in decrease in charge recombination and hence, result in enhanced photoelectrochemical function. Owing to the band structure, CdS can harvest solar photon and when attached with wide band gap semiconductor TiO2. The photogenerated electron in the CdS conduction band can be injected at ultrafast timescales to the conduction band of the TiO2. The thesis discusses easy and cost-effective synthesis of various TiO2 and CdS assemblies and explores application of them in photovoltaics, photocatalysis and (photo conducting) image sensor. Various interactions and physical properties are also studied including the ultrafast photoinduced electron dynamics from CdS to TiO2. Sun is a great source of alternative energy especially, electrical energy. In this context, nanostructured semiconductor assemblies have demonstrated great potential towards efficient harvest of the solar photon. In Chapter 1, general properties and scope of nanostructured assemblies in the context of few applications namely liquid junction semiconductor sensitized solar cell (for solar photon conversion to electricity), visible light photocatalysis (to degrade pollutants using solar photon) and large area image sensor (sensitive to white light) are discussed. The Chapter also discusses the various characterization and quantification methods which not only provide detailed analysis of properties of the novel semiconductor assemblies but also throw light on the prospects for industrial applications. Chapters 2 to 5 comprises of discussions on the electronic and photovoltaic properties of various shaped semiconductor nanocrystals (average size  30 nm). In Chapter 2, cadmium sulfide (CdS) semiconductor nanocrystals of various shapes (tetrapod, tetrahedron, sphere and rod) obtained using an optimized solvothermal process exhibited a mixed cubic (zinc blende) and hexagonal (wurtzite) crystal structure. The various nanocrystal shapes obtained here are a consequence of the simultaneous presence of wurtzite and zinc blende phases in varying amounts. The simultaneous presence of the two crystal phases in varying amounts is observed to play a pivotal role in not only determining the final nanocrystal shape but also both the electronic and photovoltaic properties of the CdS nanocrystals. Light to electrical energy conversion efficiencies measured in two-electrode configuration laboratory solar cells remarkably decreased by one order in magnitude from tetrapod  tetrahedron  sphere  rod. The tetrapod-CdS nanocrystals, which displayed the highest light to electrical energy conversion efficiency, showed a favourable shift in position of the conduction band edge leading to highest rate of electron injection (from CdS to TiO2) and lowest rate of electron-hole recombination (higher free electron lifetimes). Chapter 2 successfully demonstrated that the photovoltaic (PV) efficiency of a device can be influenced by tuning the shape of the light harvester nanocrystal. While the light to electricity conversion efficiencies varied by one order in magnitude between the various nanocrystal shapes (average size  30 nm), the magnitude of the efficiencies was itself not very high. In Chapter 3, the same nanocrystal shapes are used to sensitize multi-layered Titania films and liquid junction solar cells are then fabricated using them. This optimization of the cell configuration showed tremendous enhancement in the light to electricity conversion efficiency by nearly one order in magnitude compared to the ones discussed in Chapter 2. The semiconductor-electrolyte interface is also studied in detail by performing ac-impedance spectroscopy on the full cell to estimate the electron lifetime of the device. The estimated recombination resistance and the electron lifetime are observed to follow the same trend as of the PV-performances of the cells composed of various shaped nanocrystals in the new configuration. The photoinduced electron transfer processes in a nano-heterostructure semiconductor assembly are complex and depend on various parameters of the constituents of the assembly. Chapter 4 discusses the ultrafast electron transfer characteristics of an assembly comprising of a wide band gap semiconductor, titanium dioxide (TiO2) attached to light harvesting cadmium sulfide (CdS) nanocrystals of varying crystallographic phase content. The nanocrystals employed here are the same as that discussed in Chapters 2 and 3. Quantitative analysis of synchrotron high resolution X-ray diffraction data of CdS nanocrystals precisely reveal the presence of both wurtzite and zinc blende phases in varying amounts. The biphasic nature of CdS influences directly the shape of the nanocrystal at long reaction times (as also highlighted in Chapters 2 and 3) as well as the transfer of the photo-excited electrons from the CdS to TiO2 as obtained from transient absorption spectroscopy. Higher amount of zinc blende phase is observed to be beneficial for fast electron transfer across the CdS-TiO2 interface. The electron transfer rate constant differs by one order in magnitude between the CdS nanocrystals and varies linearly with the fraction of the phases. Chapters 2-4 show that the electron recombination lifetime in a sensitized semiconductor assembly, which has a major impact on the performance in a solar cell, is greatly influenced by the crystal structure and geometric form of the light harvesting semiconductor nanocrystal. In Chapter 5, the final Thesis Chapter related to semiconductor assemblies for liquid junction based semiconductor sensitized solar cells, deals with the influence of downsizing of light harvester nanocrystals on the electron recombination lifetime and its eventual influence on the light to electricity conversion efficiency of the solar cell. The semiconductor (photoanode)-electrolyte interface in a liquid junction semiconductor sensitized solar cell which has a direct impact on the photovoltaic performance is probed here systematically. The light harvesting cadmium sulfide (CdS) nanocrystals (average size  6-12 nm) of distinctly different and controlled shapes are obtained using a novel and simple liquid-gas phase synthesis method performed at different temperatures involving very short reaction times. High resolution synchrotron X-ray diffraction and spectroscopic studies respectively exhibit different crystallographic phase content and optical properties. When assembled on a mesoscopic TiO2 film by a linker molecule, they exhibit remarkable variation in electron recombination lifetime by one order in magnitude, as determined by ac-impedance spectroscopy. This also drastically affects the photovoltaic efficiency of the differently shaped nanocrystals sensitized solar cells. In Chapter 6, focus shifts from liquid junction semiconductor sensitized solar cells to visible light photocatalysis. The possibility of harvesting light via a semiconductor assembly of the same chemical compositions (as in Chapters 2-5) however, in a different spatial configuration is again explored. An unprecedented morphology of titanium dioxide (TiO2) and cadmium sulfide (CdS) self-assembly obtained using a ‘truly’ one-pot and highly cost-effective method with a multi-gram scale yield is discussed here. The TiO2– CdS assembly comprised of TiO2 and CdS nanoparticles residing next to each other homogeneously self-assemble into ‘woollen knitting ball’ like microspheres. The microspheres exhibited remarkable potential as a visible light photocatalysts with high recyclability. Finally, in Chapter 7, a semiconductors assembly comprising of titanium dioxide (TiO2) rods sensitized by cadmium sulfide (CdS) nanocrystals for potential applications in large area electronics on three dimensional (3-D) substrates is discussed. Vertically aligned TiO2 rods are grown on a substrate using a 1500C process flow and then sensitized with CdS by SILAR method at room temperature. This structure forms an effective photoconductor as the photo-generated electrons are rapidly removed from the CdS (‘carpet’) via the TiO2 thereby permitting a hole rich CdS. Current-voltage characteristics are measured, and models illustrate space charge limited photo-current as the mechanism of charge transport at moderate voltage bias. With this stable assembly, high speed can be achieved. The frequency response with a loading of 10 pF and 9 M shows a half power frequency of 100 Hz.

One third of world’s total energy is used in production of electricity and one fifth of the total electricity produced in the world is used in lighting. Hence, the materials that have high potential in the field of photovoltaic’s and photoluminescence have recently drawn special attention to meet the ever increasing energy demands. In this thesis, we have studied a few materials that hold tremendous promises in fabricating photovoltaics and photoluminescent devices. Any ferroelectric material is an efficient solar energy converter as it contains an the intrinsic dipolar field which can effectively separate the photo excited electron and hole. We have developed a few materials which possess inherent polarization efficiently absorb over a wide portion of the solar spectrum and hence can find application in the field of photovoltaics. Secondly, we also dealt with semiconductor nonmaterial’s which are technologically very important owing to their improved photoluminescence properties. We tried to improve their light emitting efficiency by engineering crystal structure in nanometer length scales. The thesis deals with such advanced energy materials and is divided in seven chapters. Chapter 1 provides a brief introduction to the fundamental concepts that are relevant in the subsequent chapters. The chapter is started with a brief scenario of current status of energy production and its usage. Next, we have discussed the prospects of ferroelectric materials in photovoltaic devices. This is followed by a brief background on ferroelectricity and related properties which we have studied subsequently. At the end of this chapter a brief overview of photoluminescence properties in semiconductor nonmaterial’s is presented. In this section we have addressed the particular issues that need to be taken care of in order to improve their light emission properties. Chapter 2 describes different experimental and theoretical methods that have been employed to carry out different studies presented in the thesis. Chapter 3 addresses the possibility of employing BaTiO3 (BTO) based composite perovskite oxides as a potent photovoltaic material. It is known that BTO can produce photocurrent upon excitation with suitable light source. However, inability of BTO to absorb sufficient sunlight owing to its near UV band gap prevents to make use of this material in photovoltaic devices. In order to reduce the band gap we have tried to tune the electronic structure at the band edge by doping non-d0 transition metal ions at Ti site. As it is known in the literature an isovalent substitution of Ti4+ stabilizes non-polar phase of BTO we employed a co-doping strategy to substitute tetravalent Ti with equal percentage of a trivalent and a pentavalent metal ion. Keeping in mind off-centering of Ti4+ is primary reason behind the large ferroelectric polarization of BTO, a judicious choice of co-dopant was necessary to minimize reduction of polarization due to replacement of Ti. We have found at least two pairs of co-dopants, namely Mn3+-Nb5+ and Fe3+-Nb5+ which at low doping concentration ( < 10%) effectively reduces the band gap of BTO without affecting its polarization to a large extent. We systematically increase the doping concentration of both the pair of dopants and found Mn3+-Nb5+ pair is more efficient over Fe3+-Nb5+ both in terms of reducing band gap and retaining the polarization of BTO. We have characterized the ferroelectric nature of all the doped compositions with the help of dielectric, polarization and pyroelectric measurements. We have also performed first principle density functional theory (DFT) calculations for an equivalent doped composition and addressed the nature of modulations of electronic structure at the band edges which is responsible for such large reduction of band gap. Chapter 4 deals with composite perovskite materials which posses large tetragonal distortions with reduced optical band gaps. Here we have exploited Cu-Nb and Cu-Ta pair which upon complete substitution of Ti of BTO leads to composite perovskites with enhanced tetragonal distortion of the perovskite lattice. For two resultant compositions, namely BaCu1/3Nb2/3O3 and BaCu 1/3Ta2/3O3 we have characterized the optical and ferroelectric properties. We found though these material possess small band gap (∼ 2 eV), these are not ferroelectric in nature. Results of second harmonic generation measurements and refinement of powder X-ray diffraction both establish Centro symmetric nature of these materials. We infer from these results that presence of large tetragonal distortion is a result of symmetric Jahn-Teller type distortion of Cu2+ and not due to off-centering of any of the metal ions in their MO6 octahedral geometries. In Chapter 5, we have considered the material SrTiO3 (STO) which is stable in cubic paraelectric phase at room temperature. But at the same time this material is considered as an incipient ferroelectric due to presence of an active polar vibrational mode which does not become completely soft even at temperature close to 0 K. While this polar vibrational mode can easily be frozen by making substitution at Sr site, a similar attempt by making substitution at Ti site failed earlier. Keeping in mind Ti is easier to substitute than Sr we employed same co-doping strategy that we have considered in Chapter 3. We found Mn- Nb and Mn-Ta co-dopants at low doping concentration are extremely useful in transforming incipient ferroelectric STO into a dipolar glass. We have characterized the glassy dipolar property of doped STO with the help of tem-perature dependent dielectric response of these material. At the same time we found these co-doped STO possess enhanced static dielectric constant at room temperature with favourable dielectric loss values in comparison to pure STO. We have also ad-dressed the origin of a glassy dipolar state with the help of DFT calculation performed on equivalent doped composition that we have considered for our experiments. In Chapter 6, we have considered another incipient ferroelectric material TiO2 in rutile phase which also possess polar vibrational mode at temperature close to 0 K. A lattice strain along the polar vibrational mode make symmetric non-polar structure unstable with respect to the distorted polar structure. In this context, we found two particular compositions FeTiTaO6 and CrTiTaO6 that are also stable in rutile phases at room temperature but possess similar strain due to presence of larger Fe or Cr and Ta in rutile lattice. Considering the fact these two composite rutile oxides are relaxer ferroelectric in nature, we critically evaluated the effect of the particular kind of strain that these materials introduce in rutile lattice. We also characterized relaxor ferroelectric property and optical band gap of these materials and commented on the potential of these materials in exploiting them in photovoltaic devices. Chapter 7 presents a unique strategy of making use of crystal defects in improving photoluminescent properties of semiconductor nanocrystals. We have shown defects when introduced in nanocrystals in a controlled protected manner efficiently overcome the problem of self absorption which is known to reduce quantum efficiency of emit-ted light. Controlling synthesis conditions we separately prepared CdS nanocrystals with and without intergrowth defects. We characterized the presence of the intergrowth defect with the help of high resolution transmission electron microscope (HRTEM) image. We have also characterized Stokes’ shifted PL emission and ultrafast charge carrier dynamics of these NCs with intergrowth defects. To support these experimental findings we have computed the electronic structures of model nanoclusters possessing similar intergrowth defects that has been observed in HRTEM images. We find that the presence of defects in a nanocluster particularly affect the position of the band edge. However our joint density of state calculation shows that contribution of these defect states to an absorption spectra is negligible. Thus presence of defect states at band edge ensures a Stokes’ shifted emission without affecting the position of absorption. In a separate section of this chapter we have shown apart from intergrowth defects presence of twin boundary also provide similar mid-gap states that can alter its’ optical proper-ties to large extent. In summary, we have studied a few bulk and nano-materials which can show improved photovoltaic and photoluminescence property. We investigated effect of external dopant ions on a classical ferroelectric material BaTiO3 and two incipient ferroelectric materials SrTiO3 and rutile TiO2. We have also shown that efficient defect engineering could be extremely useful in improving photoluminescent property of CdS nanocrystals which is a prototype of II-VI semiconductor nanomaterials. In a separate Appendix Chapter, we have shown an easy and efficient way to suppress coffee ring effect which takes place universally when a drop of colloidal suspension is dried on a solid substrate. We have shown temporary modification of hydropho-bicity of a glass substrate not only can suppress the coffee ring effect but also leaves the particle in a highly ordered self-assembled phase after completion of drying process

One third of world’s total energy is used in production of electricity and one fifth of the total electricity produced in the world is used in lighting. Hence, the materials that have high potential in the field of photovoltaic’s and photoluminescence have recently drawn special attention to meet the ever increasing energy demands. In this thesis, we have studied a few materials that hold tremendous promises in fabricating photovoltaics and photoluminescent devices. Any ferroelectric material is an efficient solar energy converter as it contains an the intrinsic dipolar field which can effectively separate the photo excited electron and hole. We have developed a few materials which possess inherent polarization efficiently absorb over a wide portion of the solar spectrum and hence can find application in the field of photovoltaics. Secondly, we also dealt with semiconductor nonmaterial’s which are technologically very important owing to their improved photoluminescence properties. We tried to improve their light emitting efficiency by engineering crystal structure in nanometer length scales. The thesis deals with such advanced energy materials and is divided in seven chapters. Chapter 1 provides a brief introduction to the fundamental concepts that are relevant in the subsequent chapters. The chapter is started with a brief scenario of current status of energy production and its usage. Next, we have discussed the prospects of ferroelectric materials in photovoltaic devices. This is followed by a brief background on ferroelectricity and related properties which we have studied subsequently. At the end of this chapter a brief overview of photoluminescence properties in semiconductor nonmaterial’s is presented. In this section we have addressed the particular issues that need to be taken care of in order to improve their light emission properties. Chapter 2 describes different experimental and theoretical methods that have been employed to carry out different studies presented in the thesis. Chapter 3 addresses the possibility of employing BaTiO3 (BTO) based composite perovskite oxides as a potent photovoltaic material. It is known that BTO can produce photocurrent upon excitation with suitable light source. However, inability of BTO to absorb sufficient sunlight owing to its near UV band gap prevents to make use of this material in photovoltaic devices. In order to reduce the band gap we have tried to tune the electronic structure at the band edge by doping non-d0 transition metal ions at Ti site. As it is known in the literature an isovalent substitution of Ti4+ stabilizes non-polar phase of BTO we employed a co-doping strategy to substitute tetravalent Ti with equal percentage of a trivalent and a pentavalent metal ion. Keeping in mind off-centering of Ti4+ is primary reason behind the large ferroelectric polarization of BTO, a judicious choice of co-dopant was necessary to minimize reduction of polarization due to replacement of Ti. We have found at least two pairs of co-dopants, namely Mn3+-Nb5+ and Fe3+-Nb5+ which at low doping concentration ( < 10%) effectively reduces the band gap of BTO without affecting its polarization to a large extent. We systematically increase the doping concentration of both the pair of dopants and found Mn3+-Nb5+ pair is more efficient over Fe3+-Nb5+ both in terms of reducing band gap and retaining the polarization of BTO. We have characterized the ferroelectric nature of all the doped compositions with the help of dielectric, polarization and pyroelectric measurements. We have also performed first principle density functional theory (DFT) calculations for an equivalent doped composition and addressed the nature of modulations of electronic structure at the band edges which is responsible for such large reduction of band gap. Chapter 4 deals with composite perovskite materials which posses large tetragonal distortions with reduced optical band gaps. Here we have exploited Cu-Nb and Cu-Ta pair which upon complete substitution of Ti of BTO leads to composite perovskites with enhanced tetragonal distortion of the perovskite lattice. For two resultant compositions, namely BaCu1/3Nb2/3O3 and BaCu 1/3Ta2/3O3 we have characterized the optical and ferroelectric properties. We found though these material possess small band gap (∼ 2 eV), these are not ferroelectric in nature. Results of second harmonic generation measurements and refinement of powder X-ray diffraction both establish Centro symmetric nature of these materials. We infer from these results that presence of large tetragonal distortion is a result of symmetric Jahn-Teller type distortion of Cu2+ and not due to off-centering of any of the metal ions in their MO6 octahedral geometries. In Chapter 5, we have considered the material SrTiO3 (STO) which is stable in cubic paraelectric phase at room temperature. But at the same time this material is considered as an incipient ferroelectric due to presence of an active polar vibrational mode which does not become completely soft even at temperature close to 0 K. While this polar vibrational mode can easily be frozen by making substitution at Sr site, a similar attempt by making substitution at Ti site failed earlier. Keeping in mind Ti is easier to substitute than Sr we employed same co-doping strategy that we have considered in Chapter 3. We found Mn- Nb and Mn-Ta co-dopants at low doping concentration are extremely useful in transforming incipient ferroelectric STO into a dipolar glass. We have characterized the glassy dipolar property of doped STO with the help of tem-perature dependent dielectric response of these material. At the same time we found these co-doped STO possess enhanced static dielectric constant at room temperature with favourable dielectric loss values in comparison to pure STO. We have also ad-dressed the origin of a glassy dipolar state with the help of DFT calculation performed on equivalent doped composition that we have considered for our experiments. In Chapter 6, we have considered another incipient ferroelectric material TiO2 in rutile phase which also possess polar vibrational mode at temperature close to 0 K. A lattice strain along the polar vibrational mode make symmetric non-polar structure unstable with respect to the distorted polar structure. In this context, we found two particular compositions FeTiTaO6 and CrTiTaO6 that are also stable in rutile phases at room temperature but possess similar strain due to presence of larger Fe or Cr and Ta in rutile lattice. Considering the fact these two composite rutile oxides are relaxer ferroelectric in nature, we critically evaluated the effect of the particular kind of strain that these materials introduce in rutile lattice. We also characterized relaxor ferroelectric property and optical band gap of these materials and commented on the potential of these materials in exploiting them in photovoltaic devices. Chapter 7 presents a unique strategy of making use of crystal defects in improving photoluminescent properties of semiconductor nanocrystals. We have shown defects when introduced in nanocrystals in a controlled protected manner efficiently overcome the problem of self absorption which is known to reduce quantum efficiency of emit-ted light. Controlling synthesis conditions we separately prepared CdS nanocrystals with and without intergrowth defects. We characterized the presence of the intergrowth defect with the help of high resolution transmission electron microscope (HRTEM) image. We have also characterized Stokes’ shifted PL emission and ultrafast charge carrier dynamics of these NCs with intergrowth defects. To support these experimental findings we have computed the electronic structures of model nanoclusters possessing similar intergrowth defects that has been observed in HRTEM images. We find that the presence of defects in a nanocluster particularly affect the position of the band edge. However our joint density of state calculation shows that contribution of these defect states to an absorption spectra is negligible. Thus presence of defect states at band edge ensures a Stokes’ shifted emission without affecting the position of absorption. In a separate section of this chapter we have shown apart from intergrowth defects presence of twin boundary also provide similar mid-gap states that can alter its’ optical proper-ties to large extent. In summary, we have studied a few bulk and nano-materials which can show improved photovoltaic and photoluminescence property. We investigated effect of external dopant ions on a classical ferroelectric material BaTiO3 and two incipient ferroelectric materials SrTiO3 and rutile TiO2. We have also shown that efficient defect engineering could be extremely useful in improving photoluminescent property of CdS nanocrystals which is a prototype of II-VI semiconductor nanomaterials. In a separate Appendix Chapter, we have shown an easy and efficient way to suppress coffee ring effect which takes place universally when a drop of colloidal suspension is dried on a solid substrate. We have shown temporary modification of hydropho-bicity of a glass substrate not only can suppress the coffee ring effect but also leaves the particle in a highly ordered self-assembled phase after completion of drying process