Дисертації з теми "Cadium Nanostructures"

Nanocrystallin CdS films with controlled stoichiometry deposited by CSVS were investigated by meth- od of the current-voltage characteristics in ITO/CdS /In structures. It was shown that in the case of cadmi- um excess (S Cd) charge flow mechanism is deter- mined by monomolecular recombination. In the band gap of CdS with excess of cadmium there was detected localized states with energy Et = 0.514 ± 0.026 eV, while in the material with Excess sulfur there are two localized states with energy Et1 = 0.514 ± 0.026 eV and Et2 = 0.700 ± 0.026 eV. Full concentration of localized states is more than 2·1021 m-3 – 5·1022 m-3. Dependence of injection in parameters and nature of injection in the structures based on nanostructured CdS films on their stoichiometry was determined. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/33901

Nanocrystallin CdS films with controlled stoichiometry deposited by CSVS were investigated by meth-od of the current-voltage characteristics in ITO/CdS /In structures. It was shown that in the case of cadmi-um excess (S Cd) charge flow mechanism is deter-mined by monomolecular recombination. In the band gap of CdS with excess of cadmium there was detect-ed localized states with energy Et = 0.514 ± 0.026 eV, while in the material with Excess sulfur there are two localized states with energy Et1 = 0.514 ± 0.026 eV and Et2 = 0.700 ± 0.026 eV. Full concentration of lo-calized states is more than 2·1021 m-3 – 5·1022 m-3. Dependence of injection in parameters and nature of in-jection in the structures based on nanostructured CdS films on their stoichiometry was determined When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35240

Thesis (Ph. D.)--University of California, Riverside, 2009.
Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed March 12, 2010). Includes bibliographical references. Also issued in print.

Deux méthodes de synthèse chimique, relevant de l'approche bottom-up, sont mises en oeuvre pour élaborer des nanocristaux (NC) de sulfure de cadmium (CdS) : les croissances (i) par source unique de précurseur et (ii) par voie micro-ondes. Ces deux techniques, complémentaires dans la gamme de tailles obtenues, permettent la réalisation de NC de petites tailles (2,8 nm - 5,2 nm) en seulement (i) 120 min et (ii) quelques minutes. Un protocole de caractérisation par techniques conjointes est mis au point pour étudier ces NC. La spectrométrie de masse (SM) couplée à des sources d'ionisation douce contrôle la pureté et la stabilité des précurseurs et permet d'estimer la taille et la distribution en taille des NC. Ces estimations sont confirmées par microscopie électronique en transmission (MET). La confrontation des résultats de SM et de MET suggère une géométrie des NC (i) sphérique et (ii) ellipsoïdale. La diffraction des rayons X montre l'état cristallin des nanoparticules en structures (i) würtzite et (ii) zinc blende. La spectrométrie optique à température ambiante (absorption et photoluminescence - PL) témoigne des effets de confinement quantique par le glissement de la réponse excitonique dans le domaine bleu proche UV en fonction de la taille des NC, s'inscrivant dans la correspondance connue énergie-taille. Dans la perspective d'applications optoélectroniques, le dépôt en couches minces de polymère (PMMA) contenant des NC de CdS est entrepris par spin coating. Le même protocole de caractérisations, enrichi de techniques adaptées aux couches minces, montre que les NC conservent leur intégrité et leurs propriétés de PL après inclusion dans la couche
Two chemical methods are developed to synthesize cadmium sulfide (CdS) nanocrystals (NC) in bottom-up approach: (i) single-source precursor methodology and (ii) microwave synthetic route. These two growth techniques are complementary in the size range obtained and allow production of small NC (2.8 nm - 5.2 nm) in only (i) 120 min and (ii) some minutes. A joint technique characterization protocol is developed to study the synthesized NC. Mass spectrometry (MS) coupled to soft ionization sources allows to control the purity and stability of the precursors and to estimate the NC size and size distribution. These estimations are confirmed by transmission electron microscopy (TEM). Comparison between SM and TEM results suggests that NC have (i) spherical and (ii) prolate shapes. X-ray diffraction reveals nanoparticle crystalline structure in (i) wurtzite and (ii) zinc blende symmetries. Room temperature optical spectrometry (absorption and photoluminescence - PL) evidences quantum confinement effects by the shift of the excitonic response as a function of the NC size, in the blue-UV spectral range. These results are consistent with the well-known empirical energy-size correspondence. For optoelectronic application purpose, thin film deposition of polymer (PMMA) containing CdS NC is initiated by spin coating. The previous characterization protocol, extended to techniques dedicated to thin film studies, shows that NC maintain their integrity and PL properties after inclusion in PMMA layer

Traditional sources of electrical energy are finite and can produce significant pollution. Solar cells produce clean energy from incident sunlight, and will be an important part of our energy future. A new nanostructured extremely thin absorber solar cell with 0.98% power conversion efficiency and maximum external quantum efficiency of 61% at 650 nm has been fabricated and characterized. This solar cell is composed of a fluorine-doped tin oxide base layer, n-type aluminum doped zinc oxide nanowires, a cadmium selenide absorber layer, poly(3-hexylthiophene) as a p-type layer, and thermally evaporated gold as a back contact. Zinc oxide nanowire electrodeposition has been investigated for different electrical environments, and the role of a zinc oxide thin film layer has been established. Cadmium selenide nanoparticles have been produced and optimized in-house and compared to commercially produced nanoparticles. Argon plasma cleaning has been investigated as a method to improve electronic behavior at cadmium selenide interfaces. The thermal anneal process for cadmium selenide nanoparticles has been studied, and a laser anneal process has been investigated. It has been found that the most efficient solar cells in this study are produced with a zinc oxide thin film, zinc oxide nanowires grown under constant -1V bias between the substrate material and the anode, cadmium selenide nanoparticles purchased commercially and annealed for 24 hours in the presence of cadmium chloride, and high molecular weight poly(3-hexylthiophene) spin-coated in a nitrogen environment.

Nous presentons dans ce travail la mise au point des conditions de croissance epitaxiale permettant d'elaborer de maniere auto-organisee des nanostructures 1d et 0d a base de semiconducteurs ii-vi de la famille des tellurures. Les boites quantiques sont realisees en epitaxie par jets atomiques dans le systeme cdte/znte (desaccord de maille de 6,2%), et nous montrons que dans ce cas la croissance n'est pas de type stranski-krastanow. Ce sont des poches riches cd dans une matrice de znte qui sont responsables du comportement 0d. En particulier, l'emission excitonique d'une boite unique a pu etre observee. Les fils quantiques (cdte/cdmn (mg)te) sont realises en epitaxie par jets moleculaires sur des substrats vicinaux permettant de travailler en regime d'avancee de bords de marches : nous obtenons ainsi un superreseau de fils, dont l'inclinaison est determinante dans le caractere 1d du systeme. Un calcul complet de la structure de bandes est confronte aux resultat experimentaux. Une etude de spectroscopie optique sur l'ensemble de ces nanostructures est presentee. En particulier les experiences de temps de declin de la luminescence sont caracteristiques du type de systeme etudie : le comportement du temps de declin avec la temperature est la signature de la dimensionnalite de nos systemes.

Les phosphates de zirconium et d'etain (forme alpha) et le phosphate de zirconium (forme gamma) pontes par des fragments de silice sont des solides poreux avec une distribution de dimensions des pores entre 8 et 15 a. Tous ces materiaux presentent des proprietes d'echange ionique et les isothermes d'adsorption a 25c pour les ions de zinc et de cadmium divalents sont reportes. Les solides echanges ont ete utilises comme precurseurs pour la formation in situ de nanoparticules de sulfures metalliques (reaction avec le sulfure d'hydrogene). Les spectres optiques sont compatibles avec la presence de particules semi-conductrices de dimension 6a dans la direction perpendiculaire aux feuillets et inferieurs a 20 a dans les deux directions paralleles aux feuillets. La structure de ces agregats a ete mise en evidence par spectroscopie d'absorption x. Elle est basee sur des tetraedres de zinc comportant un atome de soufre au centre dans le cas des clusters du sulfure de zinc et sur des agregats cubiques de cadmium, d'oxygene et de soufre dans le cas des composes contenant le cadmium

La premiere partie de ce travail presente des resultats de magnetooptique obtenus sur des heterostructures de semiconducteurs semimagnetiques cdte/(cd,mn)te. Sous l'action d'un champ magnetique, le decalage des bandes de conduction et de valence dans les deux materiaux varie, du a un effet zeeman geant dans les barrieres semimagnetiques. Pour un champ magnetique critique on peut observer la transition type i-type ii. Nous presentons des resultats de magnetoreflectivite obtenus sur des puits quantiques etroits. La transition excitonique e#1hh#1 suit un comportement normal en champ magnetique. Une structure additionnelle, attribuee a une transition excitonique e#1hh#3 est observee pour un champ superieur a trois tesla. Elle n'est pas observee sur des puits plus larges. La comparaison avec un modele variationnel dans l'approximation de la fonction enveloppe nous permet de determiner un offset relatif de bande de valence de l'ordre de 23%. L'influence du choix du modele, en particulier de la fonction d'essai bi- ou tridimensionnelle, sur la valeur de l'offset est discutee. La seconde partie est consacree a la fabrication et a la caracterisation de points quantiques. Les techniques employees sont la lithographie par faisceau d'electrons et la gravure ionique reactive dans un melange methane/hydrogene/oxygene. Des colonnes presentant un facteur d'aspect eleve ont ete obtenues. L'influence des parametres de la gravure ionique reactive sur la qualite des structures realisees est examinee. Nous presentons egalement les premiers resultats de photoluminescence sur ces echantillons. Un deplacement vers les hautes energies des transitions excitoniques dans les points quantiques est observe par rapport au materiau de reference. L'origine de ce deplacement est discute en termes d'interdiffusion et/ou relaxation de la contrainte

Les nanostructures de semiconducteurs II-VI ont de nombreuses applications en microélectronique, optoélectronique et photonique. Notamment, les boites quantiques II-V peuvent servir de source de photons uniques. Dans cette étude, nous nous sommes intéressés à la caractérisation chimique et structurale des nanostructures de semiconducteurs II-VI (boites quantiques (BQs) auto-organisées, nanofils II-VI et III-V …) par sonde atomique tomographique (SAT). Dans un premier temps, nous avons optimisé les conditions d’analyse des semiconducteurs III-V et II-VI par SAT. Ensuite, nous avons étudié les compositions chimiques des interfaces II-VI/III-V en montrant la formation d’un composé Ga2.7Se3 à l’interface ZnSe/GaAs et un mélange de cations (Ga, Zn) à l’interface ZnTe/InAs. Les mesures de compositions chimiques et des tailles des boites quantiques en trois dimensions par SAT ont permis de faire une corrélation avec les mesures optiques. Nous nous sommes aussi intéressés à l’étude des mécanismes de croissance des nanofils GaAs et ZnTe ainsi que des BQs (CdTe) insérés dans des nanofils ZnTe en analysant la composition chimique des catalyseurs, les BQs dans les nanofils aussi que la base des nanofils. Ces mesures montrent que les boites quantiques sont formées d’un fort mélange CdxZn1-xTe. Un scénario basé sur la diffusion de surface a été proposé pour expliquer la croissance ainsi que le mélange entre Zn/Cd pour les BQs insérées dans les nanofils
Nanostructures of II-VI nanostructure have many applications in microelectronics, optoelectronics and photonics. For example, II -V quantum dots have shown the ability to be a source of single photons. In this work, we performed in the chemical and structural characterization of nanostructures of II-VI semiconductors (self- organized quantum dots (QDs), nanowires II-VI and III- V ...) by atom probe tomography (APT). Firstly, the analysis conditions of III-V and II- VI semiconductors by APT were optimized. Then, we studied the chemical composition of II-VI/III-V interfaces and showed the formation of a Ga2.7Se3 compound at the ZnSe/GaAs interface and the (Ga, Zn) cations mixing at the ZnTe/InAs interface. The measurements of the chemical composition and the sizes of quantum dots in three dimensions by APT allowed making a correlation with optical measurements. We studied also growth mechanisms of GaAs, ZnTe nanowire and the CdTe QDs inserted in ZnTe nanowires by analyzing the chemical composition of the catalysts QDs and nanowires basis. These measurements show that the quantum dots are formed of a strong mixing of CdxZn1-xTe. A scenario based on surface diffusion has been proposed to explain the growth and the mixing between Zn/Cd for the QDs

O presente trabalho descreve a síntese e caracterização de nanopartículas de hidróxido de níquel, puras e aditivadas por cobalto e cádmio, empregando a radiação sonoquímica. Foram obtidas partículas de aproximadamente 5 nm de diâmetro, sendo caracterizadas por HRTEM, espectroscopia Raman e no Infravermelho, Termogravimetria, ICP-OES e Difração de Raios-X. A imobilização das nanopartículas sobre o eletrodo transparente condutor foi efetuada pela técnica de deposição de camadas eletrostáticas e pela deposição eletroforética. Foram obtidos eletrodos eletrocrômicos de alto desempenho, apresentando altos valores de eficiência eletrocrômica e baixos tempos de resposta, evidenciando a arquitetura nanométrica do hidróxido de níquel. A incorporação de aditivos às nanopartículas conferiu uma grande melhora nas propriedades do hidróxido de níquel. A adição de cobalto levou ao deslocamento dos picos redox para valores de potenciais menos positivos, se afastando da reação de desprendimento de oxigênio. A incorporação de cádmio levou a uma diminuição da repulsão eletrostática entre as lamelas do hidróxido, impedindo o chamado efeito , conferindo ao eletrodo uma alta durabilidade.
The present work describes the synthesis and characterization of pure Nickel Hydroxide nanoparticles, and with cobalt and cadmium as additives, by applying ultrasound radiation. Were obtained nanoparticles of about 5 nm, being characterized by HRTEM, Raman and Infrared spectroscopies, thermogravimetric, ICP-OES and X-ray diffraction. Nanoparticles immobilization onto conducting glass substrates was performed by adsorption of electrostatic layers and by electrophoretic deposition. High performant electrochromic electrodes were obtained, showing high electrochromic efficiencies and low response times, evidencing the nickel hydroxide nanometric architecture. Additives incorporation on nickel hydroxide nanoparticles provided high improvements on electrochromic properties. Cobalt addition shifted the redox peaks to lower potentials, avoiding by this way the oxygen reaction. Cadmium addition diminished the electrostatic repulsion between layers, avoiding the so-called effect, providing a high durability to the electrochromic electrode.

Le but de ce travail est de developper des methodes de croissance pour fabriquer in-situ des nanostructures - en particulier des reseaux de fils quantiques - a base de semiconducteurs ii-vi de la famille des tellurures. Pour ce faire, nous avons fait appel a l'epitaxie par jets moleculaires alternes (envoi alterne des elements ii et des elements vi), technique permet un controle ultime des phenomenes d'incorporation des atomes en surface. Nous avons obtenu pour cdte deux regimes d'autoregulation, le premier a 0. 5 monocouches (mc) de cdte deposees par cycle d'ejma entre 260c et 290c, le second a 1 mc/cycle entre 230c et 240c. Pour mgte, un regime d'autoregulation a 0. 7mc/cycle d'ejma a ete mis en evidence entre 260c et 300c. Pour mnte, par contre, tous les atomes de mn incidents s'incorporent a 280c. Une etude systematique de super-reseaux cdte/mnte et cdte/mgte nous a permis d'une part d'evaluer et d'optimiser la morphologie des interfaces, d'autre part d'avoir acces a des parametres physiques importants de mgte (parametre de maille 6. 42 a, energie de bande interdite a l'ambiante 3. 5 ev, rapport des constantes elastiques 2c#1#2/#c#1#1=1. 06) comme de mnte (2c#1#2/#c#1#1=1. 12). En particulier, un modele de segregation a ete utilise pour decrire quantitativement les interfaces directes et inverses des super-reseaux cdte/mnte. Enfin, nous avons obtenu en croissance par avancees de marches sur surfaces vicinales des super-reseaux inclines (sri) cdte/mnte. Une grande regularite de la periode et de l'angle d'inclinaison de ces sri a ete demontree, avec une modulation laterale de la composition en mn de l'ordre de10%. Ce potentiel lateral de confinement a ete mis en evidence en optique a l'aide de mesures d'anisotropie de la polarisation.

Un certain nombre de techniques ont ete developpees ces dernieres annees pour fabriquer des nanostructures quantiques, principalement dans le domaine des semiconducteurs iii-v. Nous avons realise des fils et des boites quantiques de semiconducteurs ii-vi, a partir de puits quantiques cdte/cdznte, soit par nanolithographie et gravure de structures 2d, soit directement par epitaxie par jets moleculaires avec une croissance en deux etapes. Les proprietes optiques de ces nanostructures ont ete etudiees en photoluminescence a basse temperature (2k). Dans la premiere approche, les nanostructures ont ete obtenues par lithographie electronique et gravure seche. Nous avons developpe une etape de gravure supplementaire consistant en une oxydation anodique de la couche de surface. Cette etape permet de reduire la taille des fils et des boites et d'eliminer la couche de defauts introduite en surface par la premiere gravure. Les etudes optiques ont prouve l'existence de cette couche de defauts d'une epaisseur de 30 nm environ. Elle contient des centres radiatifs qui localisent les excitons et permet d'augmenter le rendement radiatif des structures de largeur superieure a 150 nm. Pour des tailles inferieures, les recombinaisons sur les defauts non radiatifs de surface font chuter l'intensite de luminescence. Pour s'affranchir des problemes dus a la gravure (fluctuations de largeur, defauts non radiatifs), nous avons developpe une autre approche basee sur une croissance epitaxiale directe. Un puits quantique cdte/cdznte est depose sur la face clivee 110 d'un superreseau contraint. Celui-ci induit une modulation de contrainte dans le plan de la recroissance, et donc un confinement lateral dans le puits 110. La luminescence de ces fils quantiques est decalee vers le rouge par rapport a celle du puits 2d. Ce decalage depend de la densite d'excitation, ce qui est explique par le champ piezoelectrique lateral dans les fils

The thesis investigates the optical, photo-physical, and electrical properties of CdTe and HgCdTe nano- and micro-structures synthesized by hydrothermal/solvothermal technique. Application oriented studies like fluorescence resonance energy transfer (FRET), bio-sensing, two photon absorption are carried out on colloidal CdTe QDs. Defect related and temperature dependent luminescence studies are carried out in detail on HgCdTe nano and micro-crystals. The electronic device application-oriented studies of nano- Schottky diode and electrical bistability are carried out on colloidal CdTe QDs. This work is presented in seven chapters including summary and directions for future work. Chapter 1 provides a brief introduction to optical, electrical, and photo-physical properties of semiconductor QDs and hydrothermal/solvothermal technique in preparation of quantum nanostructures. A review of CdTe and HgCdTe nanostructures and its technological applications are discussed. Chapter 2 provides the experimental techniques used in this work. First, the hydrothermal/ solvothermal synthesis of CdTe, HgCdTe nano- and micro-structures, and secondly, the characterization tools used in this work are briefly presented. Also, we presented hydrothermal/solvothermal synthesis of few other nanostructures such as CdSe, PbTe and Au for future work. Chapter 3 describes, the interaction of CdTe QDs with biomolecules and the energy transfer phenomena between two different size CdTe QDs in aqueous media with the use of steady-state PL spectroscopy. The structural and optical properties of the QDs were characterized by transmission electron microscopy, photoluminescence and UV-visible spectroscopy and their formation mechanism is discussed. The hydrothermally grown highly luminescent 3-MPA capped CdTe QDs shows good stability in aqueous media even after 45 days under natural ambient and room conditions. The presence of thioalkyl acid groups in 3-MPA helps the QDs to become bio-compatible. The quenching of photoluminescence intensity of CdTe QDs in the presence of l-cysteine and DNA confirms its bio-compatibility nature for bio-sensing. The overlapping in absorption and emission spectra of two different size CdTe QDs is described here as one of the reasons for energy transfer in aqueous media. Chapter 4 describes the growth and PL properties of NIR emitting Hg1−xCdxTe (MCT) nanostructures with different compositions (x = 0.1 - 0.8) synthesized by solvothermal method, which is a facile, cost effective and solution growth approach to the large-scale preparation of MCT at relatively low temperature ( 180 ◦C) using an air stable and water soluble Te source. The room temperature FTIR transmission and low temperature PL studies for a composition of x = 0.8 gives a band gap of 1.1 eV and a broad emission in NIR region (0.5 - 0.9 eV) respectively. The temperature dependent PL study is understood by the configuration-coordinate model that give insight on the competition between radiative recombination through localized states and non-radiative recombination process which involves phonon emission. Hence, it is suggested that the observed luminescence bands are related to defect states originating from the compositional disorder in MCT nanostructures. Chapter 5 describes non-linear absorption studies in 3-mercaptopropioninc acid capped water-soluble CdTe QDs using open z-scan technique in near resonant regime. The origin of optical limiting is predominantly effective two photon absorption mechanism which varies with QD size. The effective two photon absorption coefficient (β) was observed to be 0.55 cm/GW for 2 nm size QDs which is about 10 times higher than the value reported in off-resonant region. Because of their excellent nonlinear optical properties, they are promising materials for all-optical switching and optical limiting devices. Chapter 6 describes the current-voltage characteristics of MPA capped CdTe QDs in different device geometries such as planar and sandwiched using Pt, Ag, and ITO as metal electrodes. In particular, nano-Schottky diodes of CdTe QDs with platinum metal electrodes in metal-semiconductor-metal planar configuration fabricated by drop-casting shows an asymmetry and non-linear I-V characteristics between forward and reverse directions, which has been explained in detail in terms of size distributions of QDs. It also describes the observation of electrical bistability in CdTe QD/polymer heterostructures in sandwich device geometry. Chapter 7 presents the summary and directions for the future work.

Lam, Ngai Sze.
Thesis (M.Phil.)--Chinese University of Hong Kong, 2008.
Includes bibliographical references.
Text in English; abstracts in English and Chinese.
Lam, Ngai Sze.
Abstract --- p.i
Acknowledgements --- p.iii
Table of contents --- p.iv
List of Figures --- p.viii
List of Tables --- p.xiii
Chapter Chapter 1 --- Introduction --- p.1
Chapter 1.1 --- Background --- p.1
Chapter 1.1.1 --- Properties of CdSe --- p.1
Chapter 1.1.2 --- Synthesis of CdSe one-dimensional (ID) nanostructure --- p.5
Chapter 1.1.3 --- Application of CdSe nanostructures --- p.8
Chapter 1.1.4 --- Significance of surface engineering --- p.10
Chapter 1.1.4.1 --- Surface passivation --- p.11
Chapter 1.1.4.2 --- Surface functionalization --- p.11
Chapter 1.1.4.3 --- Modulation of optical/electrical properties --- p.12
Chapter 1.2 --- Present study --- p.14
Chapter 1.2.1 --- Objective --- p.14
Chapter 1.2.2 --- General methodology --- p.14
Chapter Chapter 2 --- Instrumentation --- p.19
Chapter 2.1 --- Introduction --- p.19
Chapter 2.2 --- Setup of Sample Preparation --- p.19
Chapter 2.2.1 --- Synthesis --- p.19
Chapter 2.2.1.1 --- Thermal evaporation apparatus --- p.19
Chapter 2.2.1.2 --- Microwave assisted chemical synthesis --- p.21
Chapter 2.2.2 --- Sample handling --- p.22
Chapter 2.2.3 --- Other treatments --- p.22
Chapter 2.3 --- X-ray photoelectron spectrometer (XPS) --- p.22
Chapter 2.3.1 --- Basic Principle --- p.22
Chapter 2.3.2 --- Instrumentation --- p.24
Chapter 2.3.3 --- Charging problem --- p.27
Chapter 2.3.4 --- Qualitative analysis --- p.27
Chapter 2.3.5 --- Quantitative analysis --- p.28
Chapter 2.3.5.1 --- Curve fitting --- p.28
Chapter 2.3.5.2 --- Atomic percentage --- p.29
Chapter 2.3.5.3 --- Thickness determination --- p.29
Chapter 2.4 --- Photoluminescence --- p.30
Chapter 2.4.1 --- Basic principle --- p.30
Chapter 2.4.2 --- Instrumentation --- p.31
Chapter 2.5 --- Other equipments --- p.32
Chapter Chapter 3 --- Synthesis of CdSe Nanorods --- p.34
Chapter 3.1 --- Introduction --- p.34
Chapter 3.2 --- Thermal evaporation --- p.34
Chapter 3.2.1 --- Experimental procedures --- p.34
Chapter 3.2.2 --- Characterization --- p.35
Chapter 3.3 --- Microwave assisted method --- p.41
Chapter 3.3.1 --- Experimental procedures --- p.41
Chapter 3.3.2 --- Characterization --- p.42
Chapter 3.4 --- Summary --- p.47
Chapter Chapter 4 --- Surface Treatment of CdSe Nanorods --- p.49
Chapter 4.1 --- Introduction --- p.49
Chapter 4.2 --- Experimental procedures --- p.50
Chapter 4.3 --- Results and Discussion --- p.51
Chapter 4.3.1 --- Formation of Se-coated CdSe NRs --- p.51
Chapter 4.3.2 --- Desorption and thinning --- p.56
Chapter 4.3.3 --- Surface degradation --- p.67
Chapter 4.4 --- Summary --- p.69
Chapter Chapter 5 --- Surface Capping of CdSe Nanorods --- p.73
Chapter 5.1 --- Introduction --- p.73
Chapter 5.2 --- Experimental procedures --- p.73
Chapter 5.3 --- Results and Discussion --- p.74
Chapter 5.3.1 --- Capping of thiol with halo-functional group --- p.74
Chapter 5.3.1.1 --- Compositional analysis --- p.75
Chapter 5.3.1.2 --- PL analysis --- p.79
Chapter 5.3.2 --- Capping of DNA --- p.81
Chapter 5.3.2.1 --- Compositional analysis --- p.81
Chapter 5.3.2.2 --- PL analysis --- p.83
Chapter 5.4 --- Summary --- p.92
Chapter Chapter 6 --- Conclusions and Future Work --- p.94
Chapter 6.1 --- Conclusions --- p.94
Chapter 6.2 --- Future work --- p.95

by Wang Yu = 一維納米結構硫化鎘的生長和表面 / 王瑜.
Thesis (M.Phil.)--Chinese University of Hong Kong, 2005.
Includes bibliographical references.
Text in English; abstracts in English and Chinese.
by Wang Yu = Yi wei na mi jie gou liu hua ge de sheng chang he biao mian / Wang Yu.
Acknowledgements --- p.i
Abstract --- p.ii
摘要 --- p.iii
Table of contents --- p.iv
List of figures --- p.vi
List of tables --- p.ix
Chapter Chapter 1 --- Introduction
Chapter 1.1 --- Background --- p.1-1
Chapter 1.1.1 --- One-dimensional (1 -D) nanostructures --- p.1-1
Chapter 1.1.2 --- "Characteristics, properties, and applications of 1 -D nanostructures" --- p.1-1
Chapter 1.1.3 --- Synthesis and growth mechanisms --- p.1-3
Chapter 1.1.3.1 --- Vapor-Liquid-Solid (VLS) mechanism --- p.1-4
Chapter 1.1.3.2 --- Vapor-Solid (VS) mechanism --- p.1-5
Chapter 1.2 --- II-VI semiconductor nanomaterials --- p.1-5
Chapter 1.2.1 --- Cadmium sulfide (CdS) --- p.1-6
Chapter 1.2.1.1 --- Characteristics and potential applications --- p.1-6
Chapter 1.2.1.2 --- Works performed by others --- p.1-7
Chapter 1.3 --- Objectives and approaches in this work --- p.1-8
Chapter 1.4 --- Thesis layout --- p.1-9
References --- p.1-10
Figures --- p.1-13
Tables --- p.1-14
Chapter Chapter2 --- Methodology and instrumentation
Chapter 2.1 --- Experimental setup --- p.2-1
Chapter 2.1.1 --- Substrates --- p.2-1
Chapter 2.1.2 --- Experimental settings --- p.2-2
Chapter 2.2 --- Growth parameters --- p.2-2
Chapter 2.3 --- Characterization methods --- p.2-3
Chapter 2.3.1 --- "Phase, morphology and microstructure analysis" --- p.2-3
Chapter 2.3.1.1 --- X-ray diffractometry (XRD) --- p.2-3
Chapter 2.3.1.2 --- Scanning electron microscopy (SEM) --- p.2-4
Chapter 2.3.1.3 --- Transmission electron microscopy (TEM) --- p.2-4
Chapter 2.3.1.4 --- High-resolution transmission electron microscopy (HRTEM)
Chapter 2.3.2 --- Cathodoluminescence (CL) --- p.2-5
Chapter 2.3.2.1 --- Principles of CL --- p.2-5
Chapter 2.3.2.2 --- Advantages of CL --- p.2-6
Chapter 2.3.2.3 --- CL settings --- p.2-6
References --- p.2-7
Figures --- p.2-8
Chapter Chapter3 --- Results and discussions part I - Growth of CdS nanobelts
Chapter 3.1 --- Characterization in general --- p.3-1
Chapter 3.2 --- Morphology and microstructure --- p.3-1
Chapter 3.2.1 --- Nanobelt with Au droplet at the tip --- p.3-2
Chapter 3.2.2 --- Nanobelt without Au droplet at the tip --- p.3-2
Chapter 3.3 --- Effect of Au catalyst --- p.3-3
Chapter 3.4 --- Growth models --- p.3-3
Chapter 3.5 --- Samples sintered at different temperatures --- p.3-5
Chapter 3.6 --- Samples at different deposition zones --- p.3-6
Chapter 3.7 --- Cathodoluminescence --- p.3-6
Chapter 3.7.1 --- Blue shift in the deep level emission --- p.3-7
Chapter 3.7.2 --- Intensity of the emission --- p.3-8
References --- p.3-10
Figures --- p.3-11
Tables --- p.3-24
Chapter Chapter4 --- Results and discussions part II - Asymmetric growth on the CdS ribbons
Chapter 4.1 --- Surface polarization --- p.4-1
Chapter 4.2 --- One sided saw-teeth ribbons --- p.4-2
Chapter 4.3 --- Two-sided comb-like ribbons --- p.4-3
Chapter 4.4 --- Growth models for the asymmetric growth --- p.4-5
References --- p.4-7
Figures --- p.4-8
Chapter Chapter5 --- Conclusions and future studies
Chapter 5.1 --- Conclusions --- p.5-1
Chapter 5.2 --- Future studies --- p.5-2
References --- p.5-4

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.

Ho, Yee Man Martina = 硒化鎘三維納米結構之製作及其感應脫氧核糖核酸之應用潛能 / 何綺雯.
Thesis (M.Phil.)--Chinese University of Hong Kong, 2009.
Includes bibliographical references.
Abstract also in Chinese.
Ho, Yee Man Martina = Xi hua ge san wei na mi jie gou zhi zhi zuo ji qi gan ying tuo yang he tang he suan zhi ying yong qian neng / He Qiwen.
Chapter Chapter 1 --- Introduction
Chapter 1 --- Photovoltaic properties of CdSe --- p.1
Chapter 1.1 --- Quantum size effect --- p.1
Chapter 1.2 --- Synthesis of CdSe nanostructures --- p.3
Chapter 1.3 --- Electrochemical sensing of CdSe nanostructures --- p.4
Chapter 1.3.1 --- Surface passivation and functionalization of CdSe nanostructures --- p.5
Chapter 1.4 --- Electronic properties of nanocrystalline semiconductor electrode --- p.6
Chapter 1.4.1 --- Band alignment --- p.6
Chapter 1.4.2 --- Interfacial charge transfer process --- p.9
Chapter 1.4.3 --- Surface traps and adsorbed molecules --- p.10
Chapter 1.4.4 --- DNA molecules as a capping group --- p.11
Chapter 1.5 --- Literatures review in DNA sensing --- p.12
Chapter 1.6 --- Present study --- p.14
Chapter 1.6.1 --- Objective --- p.14
Chapter 1.6.2 --- General methodology --- p.15
Chapter Chapter 2 --- Experimental
Chapter 2.1 --- Introduction into the instrumentation of this project --- p.21
Chapter 2.2 --- CHI Electrochemical workstation --- p.22
Chapter 2.2.1 --- Linear sweep voltammetry --- p.24
Chapter 2.2.2 --- Cyclic voltammetry --- p.24
Chapter 2.2.3 --- Multiple potential step --- p.25
Chapter 2.3 --- CEM Microwave-assisted chemical synthesizer --- p.27
Chapter 3.1 --- Morphological examination by scanning electron microscopy --- p.28
Chapter 3.2 --- Elemental analysis by energy dispersive x-ray spectroscopy --- p.30
Chapter 3.3 --- Crystal structure analysis by x-ray diffraction --- p.31
Chapter 3.4 --- Surface compositional analysis by x-ray photoelectron spectroscopy --- p.32
Chapter 3.5 --- Transmission electron microscopy --- p.34
Chapter Chapter 3 --- Synthesis of 3D nanostructured CdSe multipod electrodes
Chapter 3.1 --- Introduction into the synthesis of CdSe MP electrode --- p.35
Chapter 3.2 --- Recipe for the synthesis of CdSe NPs --- p.36
Chapter 3.3 --- The synthesis of CdSe MPs --- p.37
Chapter 3.3.1 --- Tuning the experimental parameters: Reaction temperature --- p.37
Chapter 3.3.2 --- Tuning the experimental parameters: Reaction hold time --- p.46
Chapter 3.3.3 --- Tuning in experimental parameters: Precursor molar ratio --- p.50
Chapter 3.4 --- The fabrication of MP CdSe on a conductive substrate --- p.54
Chapter 3.4.1 --- The electrodeposition of CdSe thin films on ITO/glass substrates --- p.55
Chapter 3.4.2 --- The growth of CdSe MPs on CdSe/ ITO/glass --- p.57
Chapter 3.5 --- The characterization of MP CdSe electrode --- p.57
Chapter Chapter 4 --- Electrical and opto-electric characteristics of CdSe MP electrodes and their applications as platforms for the DNA recognition
Chapter 4.1 --- Introduction to the property characterization of CdSe MP electrodes --- p.62
Chapter 4.2 --- DNA surface attachment --- p.64
Chapter 4.2.1 --- Mechanism of DNA surface anchoring --- p.65
Chapter 4.3 --- I-V characterization in PBS --- p.69
Chapter 4.3.1 --- Experimental procedures of the I-V tests in PBS --- p.70
Chapter 4.3.2 --- Results and discussions of I-V tests in PBS --- p.72
Chapter 4.3.2.1 --- Exercising as-prepared CdSe MP electrode --- p.74
Chapter 4.3.2.2 --- I-V characteristics of CdSe MP electrodes before and after ssDNA attachment --- p.75
Chapter 4.3.2.3 --- I-V characteristics of CdSe MP electrodes before and after the dsDNA attachment --- p.76
Chapter 4.3.2.4 --- "Photo-response of bare CdSe MP, ssDNA/CdSe MP and dsDNA/CdSe electrodes" --- p.77
Chapter 4.4 --- "Photovoltaic I-V measurement in I3""/I"" redox electrolyte" --- p.79
Chapter 4.4.1 --- Experimental procedures --- p.79
Chapter 4.4.2 --- Results and discussions --- p.80
Chapter 4.5 --- Possible application implied by the results --- p.88
Chapter 4.5.1 --- DNA base pair mismatch identification --- p.91
Chapter 4.5.2 --- Field-assisted DNA hybridization acceleration process --- p.92
Chapter Chapter 5 --- Conclusions
Chapter 5.1 --- Conclusions --- p.95

This thesis explores colloidal semiconductor nanocrystal solutions as a feedstock for creating thin film semiconductor materials through printing processes. This thesis will span the synthesis of nanocrystals, ligand exchange chemistry, solution phase characterization methods, thin film device fabrication, thin film characterization methods, and device characteristics. We will focus extensively relating the structure of nanocrystals in solution and in thin films to their chemistry, optical properties and electronic properties. By way of introduction, the origin and nature of semiconductor nanocrystals will be explored. This discussion will place semiconductor nanocrystals in their historical context, namely the oil-shocks of the 1970s. The interest in II-VI semiconductor materials stemmed from a desire find photochemical synthetic routes to reduce the use of fossil fuels. As a result, II-VI semiconductor nanocrystal are far more developed synthetically. Additionally, our understanding of II-VI semiconductor nanocrystals is couched in the language of solid state physics rather than chemistry. This will lead into a discussion of their electronic structure and the iterative nature of nanocrystal synthetic development and our theoretical understanding of nanocrystals. The first chapter will discuss nanocrystal synthetic methods in a broad context, finally narrowing in on the synthesis chosen for this work. Following a description of the synthesis, we will then describe the ligand chemistry and the reactions which may be performed in the ligand shell. The final sections of the chapter will describe the synthetic routes to the three nanocrystal materials used in the rest of this work, namely CdSe-CdCl2/PBu3, CdSe-CdCl2/NH2Bu, and CdSe/NH2Bu. The second chapter will introduce the crystal structure of II-VI semiconductor nanocrystals and describe how the structure is measured. This will lead in to a discussion of pair distribution function analysis of X-ray data and examples of its application to the solution phase structure of semiconductor nanocrystals. Some size dependent structural properties, namely stain, will be demonstrated by PDF. At the end evidence for surface reconstruction in solution as ligands are removed will be presented. In the final chapter, techniques for film formation and ligand dissolution with be presented. Annealing of films produces electronic and structural changes which can be observed in the absorbance spectrum, electron microscopy, and X-ray scattering. I propose a three phase annealing model which includes 1) reversible desorption of the organic ligands, 2) irreversible particle fusion, and 3 ripening of grains. The temperature at which ripening occurs depends sensitively on the sample content, which increase chloride concentration decreasing the temperature at which ripening occurs. The ripening process is found to correlate with a phase transition from zinc blende to wurtzite, which indicates that grain boundary mobility is an important part of the ripening process. Finally thin film transistors are characterized electronically. Fused grains show superior electron mobility as high as 25 cm2/(Vs) and on/off ratios of 10\up5 and less than 0.5 V hysteresis in threshold voltage without the addition of indium. Surprisingly, the ripened grains show poorer transport characteristics. The manuscript concludes by noting the importance of the sintering process in achieving conductivity in thin films and discussing future directions to build upon this work.

碩士
國立清華大學
化學工程學系
95
The present research focuses on the preparation and applications of the one-dimensional (1-D) nanostructures of cadmium selenide (CdSe) and zirconium dioxide (ZrO2) fabricated via the vapor phase based deposition processes. First, for the preparation of the CdSe nanowires, we developed a PVD process with the VLS growth mechanism. SEM, XRD, TEM, PL, and UV-vis analyses were conducted and the plausible growth mechanism of the 1-D CdSe nanowires was proposed and discussed. Second, for the application of the cadmium selenium nanowires, a low molar ratio of Mn was doped into the CdSe nanowires to produce CdSe based diluted magnetic semiconductor (DMS) nanowires. The Mn content (0.4、0.8 and 1.4 mol%) was found to affect the magnetic property of the present DMS nanostructure. For example, the blocking temperature (TB) and the coercivity of the ferro- magnetism state increase with increasing Mn doping concentration. For the preparation of ZrO2 nanorod array, we developed an MOCVD process. When the furnace temperature was above 700℃,the monoclinic and tetragonal phases would both exist in the product nanorod array. Lastly, the additive Y2O3 was doped into the ZrO2 nanorod array to prepare yttria-stabilized-zirconia (YSZ) nanorod arrays. The Y content of 0.8(monoclinic)、13(tetragonal)、20 and 45 mol % was determined by TEM-EDS. The oxygen ion conductivity of the YSZ nanorod array with Y2O3 content of 10 mol% is only 18-31% for the empirical equation 8- YSZ nano-powders because the activation energy of the YSZ nanorod array is larger than the empirical equation. The more compact YSZ nanorod structure, the compact sinter between Pt electrode and YSZ nanorod structure and optimal co-doping system can be used to improve the oxygen ion conductivity in the future.

Thesis (Ph. D.)--University of Notre Dame, 2003.
Thesis directed by Albert E. Miller for the Department of Chemical Engineering. "December 2003." Includes bibliographical references (leaves 245-258).

M.Sc.
The contamination of water by toxic compounds is one of the most serious environmental problems today. These toxic compounds mostly originate from industrial effluents, agriculture runoff, natural sources (e.g. heavy metals in water from rocks and soil erosion) and human waste. The contamination, which is both “organic” and “inorganic” has an impact on the environment and human health. The demand for water and the pressure to re-use this valuable resource has increased the need for improved techniques and materials to remove pollutants from water. The Nanomaterials Science research group at the University of Johannesburg has focused on developing synthetic polymers that can be employed in water treatment and pollutant monitoring. Recently, cyclodextrins (CD) and carbon nanotubes (CNTs) have been included in polymers for this application. For example, CD-co-hexamethylene-/toluene-diisocyanate polyurethanes and CNT-modified equivalents have been developed and have been successfully applied in removing organic contaminants from water to very low levels.1 Calixarenes are synthetic analogues of cyclodextrins that can be exploited via chemical modification to express a range of properties. In the present study, calixarenes, thiacalixarenes and carbon nanotube-based polymeric materials incorporating these molecules have been synthesised, characterised and tested for removing both organic pollutants (such as p-nitrophenol) and inorganic pollutants (Cd2+, Pb2+) from water. Lead(II) and Cadmium(II) are a threat in South Africa because of their toxicity, and while p-nitrophenol is much less of a problem it represents a useful model organic pollutant. The absorption capacity of the polymers towards heavy metals and organic contaminants was tested by mixing the polymer with synthetic water containing known concentration of the contaminants at about 10 mg/L. Atomic absorption spectrometry (AAS) and ultraviolet-visible spectrometry (UV-vis) were used to determine the levels of heavy metals and organic contaminants, respectively. The target pollutants (Cd2+, 1 see KL Salipira MTech dissertation, University of Johannesburg 2008 Pb2+ and p-nitrophenol) were all successfully removed from water by the various polymers, however the degree of removal and loading capacities of the polymers differed. This information gives some insight into what functional components are needed for making successful adsorbents. It was observed, for example, that ptert- butylcalix[8]arene/hexamethylene diisocyanate (C8A/HMDI) had a higher adsorption capacity towards p-nitrophenol and Pb2+ than towards Cd2+, and also a higher capacity than the corresponding calix[4]arene polymers with smaller calixarene cavities.

This thesis details the synthesis and characterization of anisotropic cadmium and lead sulfide nanostructures from single-source molecular precursors. Six new precursors were synthesized for cadmium and lead sulfide each, by the reaction of the appropriate metal acetate with picolinic (HPic), 2,6-dipicolinic (H2dipic) or salicylic acid (H2sal) followed by the addition of thiourea (th) or thiosemicarbazide (ths). The precursors for CdS are [Cd(Hsal)2(tu)2] (Cd1a), [Cd(Hsal) 2(ths)2]·nH2O (Cd1b), [Cd(pic) 2(tu)2]·0.5H2O (Cd2a), [Cd(pic) 2(ths)2]·2H2O (Cd2b), [Cd(dipic)(tu) 2] (Cd3a) and [Cd(dipic)(ths)2(H2O)]·2H 2O (Cd3b) and the precursors for PbS are [Pb(Hsal) 2(th)2] (Pb1a), [Pb(Hsal)2(ths) 2] (Pb1b), [Pb(pic)2(th)2] ( Pb2a), [Pb(pic)2(ths)2] (Pb2b), [Pb(dipic)(th)(H2O)]2·2H2O ( Pb3a) and [Pb(dipic)(ths)2]·H2O ( Pb3b). All of the compounds were characterized spectroscopically and by elemental analysis. Cd1a, Cd2a, Cd2b, Cd3a, Cd3b, Pb2b Pb3a and Pb3b formed well-defined crystals and were characterized by single crystal X-ray diffraction. The precursors were decomposed at or around 170°C using n-cetyltrimethylammonium bromide (CTAB), sodium dodecylsulphate (SDS), ethylenediamine, oleic acid, oleylamine, trioctylamine or hexadecylamine as surfactants. Systematic variations of surfactants gave small spherical nanoparticles, micro-sized flowers, multipods and nanorods for CdS and nanocubes, truncated nanocubes, hexapods, octahedrons and dendritic stars for PbS. From XRPD studies it was found that most of the CdS nanostructures were of the stable hexagonal phase. However, in two cases the nanostructures were found to be predominantly of a metastable orthorhombic phase. For PbS system, all the decompositions yielded pure crystalline galena. For CdS system, TEM studies revealed planar defects (such as polysynthetic and multiplet twinning) in the nanocrystals, which gave an explanation for mechanism of growth. For PbS system, in order to elucidate the effect of single source precursors on the mechanism of growth of nanoparticles, the decomposition results were compared with PbS nanostructures synthesized from multiple-source precursors, lead acetate and thiourea or thiosemicarbazide. It was found that in the reactions of multiple source precursors, acidic components in the reaction mixture (oleic acid, acetic acid) led to etching and crystal splitting, which played a crucial role in the formation of anisotropic nanostructures.

Using two different model systems, this thesis considers the old, but fascinating question: how do atoms or particles possessing a particular set of individual characteristics combine to form assemblies with quite distinct, ensemble characteristics, and how do those characteristics evolve as a function of the size of the assembly? For the last thirty years, numerous experiments studying the emergence of collective material properties have focused on a class of semiconducting, colloidal nanocrystals commonly known as quantum dots, which are notable for the size-dependence of their optical properties. Despite years of effort, even the most uniform quantum dot samples possess some heterogeneity in size, shape, and composition, which has prevented complete structure determination and hindered understanding of structure-property relationships. Chapter 1 of this thesis presents an approach to overcoming this challenge and reports the synthesis of a set of four, new, atomically precise cadmium selenide nanocrystal samples, which we call CdSe(350 nm), CdSe(380 nm), CdSe(408 nm), and CdSe(435 nm) after their lowest energy absorption features. We determine their structures and formulas through a combination of single crystal and powder X-ray diffraction measurements, elemental analysis, and spectroscopy. We also describe the optical properties of these samples and their sensitivity to ligand coverage, compare them to other previously reported cadmium selenide nanomaterials, and discuss ongoing experiments. Because CdSe(350 nm), CdSe(380 nm), CdSe(408 nm), and CdSe(435 nm) are atomically precise, they allow us to correlate specific structural features with material properties, which is the focus Chapter 2. Here we present a series of Raman scattering experiments designed to probe the evolution of vibrational structure with size. We find that the Cd-Se stretching region of the Raman spectra exhibits two peaks, which are assigned to primarily surface-derived and interior-derived atomic motions using density functional theory calculations. By performing variable temperature measurements, we discover that the smallest sample, CdSe(350 nm), exhibits behavior that can be well-described using a model developed for small molecules while the vibrations of the largest measured cluster, CdSe(408 nm), are better described by a model developed for bulk materials. This observation is evidence that the transition to a more bulk-like vibrational structure occurs relatively rapidly when cadmium selenide materials are approximately 2 nm in size. The emergence of collective material properties is also the subject of Chapter 3, but the topic is approached from a different perspective. Instead of focusing on a series of atomically precise clusters that differ in size, Chapter 3 presents a series of molecules composed of atomically precise clusters. We prepare octahedral hexaruthenium carbonyl clusters, [Ru₆C(CO)₁₆]²⁻, and use them as building blocks to assemble oligomers linked by single metal atom bridges. We synthesize and structurally characterize a set of compounds varying in length (from monomer to trimer) and linker atom identity (cadmium and mercury) and study the effect on electronic structure using infrared and UV-Visible absorption spectroscopies and density functional theory calculations. With increasing oligomer length, the UV-Vis absorption profile changes and shifts to lower energy, which we attribute in part to the development of coupling between neighboring clusters. Our calculations show that the infinite polymer composed of [Ru₆C(CO)₁₆]²⁻ linked by Hg²⁺ would be a one-dimensional semiconductor with a 1.5 eV direct band-gap. More detailed abstracts can be found at the beginning of each chapter.

The motivation of this thesis is to understand the physical properties of semiconductor quantum dots (QDs) and to get insight on the basic physics of charge separation in composites made from QDs with graphene oxide (GO)/organic semiconductors. The flexion phonon interactions is one of fundamental issues in solid state physics, which has a significant effect on both electrical and optical properties of solid state materials. This thesis investigates the physical properties of aqueous grown QDs through exciton-phonon coupling and non-radiative relaxation of excited carriers which have been carried out by temperature dependent photoluminescence spectroscopy. Several e orts have been made in order to understand the basic physics of photo induced charge separation in the hybrid systems made from QDs with graphene oxide and organic semiconductors. Investigations on the photoconductivity of the devices made from these hybrid composites have been carried out keeping the motive of its application in nanotechnology. This thesis work is presented in six chapters inclusive of summary and directions for future work. Chapter 1 discusses the background knowledge and information of the general properties of semiconductor nanostructures, QDs and their hybrid nanocomposites. Chapter 2 deals with the sample preparation and experimental techniques used in this thesis. Chapter 3 elaborates the exciton-phonon scattering and nonradiative relaxations of excited carriers in visible emitting cadmium telluride QDs with help of temperature and size dependent photoluminescence. Chapter 4 presents the investigations on time resolved photoluminescence dynamics and temperature dependent photoluminescence properties of near infrared (NIR) emitting mercury cadmium telluride (CdHgTe). Chapter 5 discusses the importance of NIR emitting silver sulphide (Ag2S) QDs and gives insight of nonradiative recombinations through defect/trap states. Chapter 6 investigates the excited state interactions between CdHgTe QDs and GO. Chapter 7 focuses on the understanding of basic physics of charge separation/transfer between poly (3hexylthiophene) and Ag2S QDs. Chapter 1: Semiconductor nanostructures have attracted significant scientific attention due to their fundamental physical properties and technological interests. Quasi zero dimensional nanocrystals or quantum dots (QDs) have shown unique optical and electrical properties compared to its bulk counterpart. These QDs show discrete energy levels due to the quantum confinement effect hence known as arti cial atoms. Large surface to volume ratio in these QDs is expected to play a crucial role in determing the photo-physical properties. Temperature dependent photoluminescence is a powerful tool for understanding the role of the large surface area on exciton recombination process in QDs. Inorganic QDs combined with different materials like graphene oxide or organic semiconductors forms an exciting class of synthetic materials which integrates the properties of organic and inorganic semiconductors. It is quite important to understand the basic physics of electronic interactions in these composites for its future application in many elds. Chapter 2: Synthesis of the inorganic QDs, graphene oxide, composites and fabrication of devices is an important and integral part of this thesis. Hydrothermal and three necked ask technique is adopted to get highly dispersible colloidal quantum dots in solvents. Synthesis of graphene oxide from graphite through oxidation and ultrasonication has been carried out to obtain homogenous dispersed graphene oxide in water. Structural properties have been studied by techniques like X ray diffraction, Raman spectroscopy, X ray photoelectron spectroscopy and high resolution transmission electron microscopy. Morphological properties are studied by atomic force microscopy and transmission electron microscopy. Optical properties are investigated by absorption spectroscopy, steady state and time resolved photoluminescence spectroscopy. Photoconductivity characteristics are analyzed to understand the basics of enhanced current in the various devices made from QDs composites. Chapter 3:Investigations on exciton phonon coupling and nonradiative relaxations in various sizes of visible light emitting cadmium telluride (CdTe) QDs size have been presented. Due to the large surface area, QDs are prone to have defect/trap states which can affect the exciton relaxation. Hence, understanding the role of such defect/trap states on photoluminescence is very essential for achieving the optimum optical properties. Temperature dependent (15 300 K) photoluminescence has been used to understand nonradiative relaxation of excited carriers. Thermally activated processes and multiple phonons scattering is thoroughly investigated to understand the quenching of photoluminescence with temperature. The strength of exciton-phonon coupling is investigated which determines the variation in energy bandgap of QDs with temperature. Role of exciton phonon scattering is also discussed to understand the basic physics of photoluminescence line width broadening in QDs. Chapter 4 and 5: This part of thesis focuses on the size and temperature pho-toluminescence properties of near infra red emitting ternary alloyed CdHgTe and Ag2S QDs. Near infrared emitting semiconductor quantum dots (QDs) have attracted significant scientific and technological interests due to their potential applications in the fields of photosensor, solar energy harvesting cells, telecommunication and biological tissue imaging etc. Structural and photophysical properties of CdHgTe QDs have been analyzed by high resolution transmission electron microscopy, X rayphotoelectron microscopy, photoluminescence decay kinetics and low temperature photoluminescence. Investigations on the nonradiative recombinations through trap/defects states and exciton phonon coupling are carried out in colloidal Ag 2S QDs which emits in the range of 1065 1260 nm. Particularly, the photoluminescence quenching mechanism with increasing temperature is analyzed in the presence of multiple nonradiative relaxation channels, where the excited carriers are thermally stimulated to the surface defect/trap states of QDs. Chapter 6 and 7: The aim of these chapters is to understand the basic physics of photo induced charge separation in the hybrid systems made from the inorganic QDs with graphene oxide and organic semiconductors. In chapter 6, CdHgTe QDs are decorated on graphene oxide sheets through physisorption. The excited state electronic interactions have been studied by optical and electrical characterizations in these CdHgTe QDs GO hybrid systems. In chapter 7, investigations are carried out for understanding the basic physics of charge separation in the composites of Ag2S QDs and poly (3hexylthiophene 2,5 diyl)(P3HT). These composites of inorganic organic materials are made by simple mixing with help of ultrasonication technique. Steady state and time resolved photoluminescence measurements are used as powerful technique to gain insight of energy/charge transfer process between P3HT and Ag2S QDs. Furthermore, investigations have been carried out on the photoconductivity of the devices made from these hybrid composites keeping the motive of its application in nanotechnology. Chapter 8: The conclusions of the work presented in this thesis are coherently summarized in this chapter. Thoughts and prospective for future directions are also summed up.

The motivation of this thesis is to understand the physical properties of semiconductor quantum dots (QDs) and to get insight on the basic physics of charge separation in composites made from QDs with graphene oxide (GO)/organic semiconductors. The flexion phonon interactions is one of fundamental issues in solid state physics, which has a significant effect on both electrical and optical properties of solid state materials. This thesis investigates the physical properties of aqueous grown QDs through exciton-phonon coupling and non-radiative relaxation of excited carriers which have been carried out by temperature dependent photoluminescence spectroscopy. Several e orts have been made in order to understand the basic physics of photo induced charge separation in the hybrid systems made from QDs with graphene oxide and organic semiconductors. Investigations on the photoconductivity of the devices made from these hybrid composites have been carried out keeping the motive of its application in nanotechnology. This thesis work is presented in six chapters inclusive of summary and directions for future work. Chapter 1 discusses the background knowledge and information of the general properties of semiconductor nanostructures, QDs and their hybrid nanocomposites. Chapter 2 deals with the sample preparation and experimental techniques used in this thesis. Chapter 3 elaborates the exciton-phonon scattering and nonradiative relaxations of excited carriers in visible emitting cadmium telluride QDs with help of temperature and size dependent photoluminescence. Chapter 4 presents the investigations on time resolved photoluminescence dynamics and temperature dependent photoluminescence properties of near infrared (NIR) emitting mercury cadmium telluride (CdHgTe). Chapter 5 discusses the importance of NIR emitting silver sulphide (Ag2S) QDs and gives insight of nonradiative recombinations through defect/trap states. Chapter 6 investigates the excited state interactions between CdHgTe QDs and GO. Chapter 7 focuses on the understanding of basic physics of charge separation/transfer between poly (3hexylthiophene) and Ag2S QDs. Chapter 1: Semiconductor nanostructures have attracted significant scientific attention due to their fundamental physical properties and technological interests. Quasi zero dimensional nanocrystals or quantum dots (QDs) have shown unique optical and electrical properties compared to its bulk counterpart. These QDs show discrete energy levels due to the quantum confinement effect hence known as arti cial atoms. Large surface to volume ratio in these QDs is expected to play a crucial role in determing the photo-physical properties. Temperature dependent photoluminescence is a powerful tool for understanding the role of the large surface area on exciton recombination process in QDs. Inorganic QDs combined with different materials like graphene oxide or organic semiconductors forms an exciting class of synthetic materials which integrates the properties of organic and inorganic semiconductors. It is quite important to understand the basic physics of electronic interactions in these composites for its future application in many elds. Chapter 2: Synthesis of the inorganic QDs, graphene oxide, composites and fabrication of devices is an important and integral part of this thesis. Hydrothermal and three necked ask technique is adopted to get highly dispersible colloidal quantum dots in solvents. Synthesis of graphene oxide from graphite through oxidation and ultrasonication has been carried out to obtain homogenous dispersed graphene oxide in water. Structural properties have been studied by techniques like X ray diffraction, Raman spectroscopy, X ray photoelectron spectroscopy and high resolution transmission electron microscopy. Morphological properties are studied by atomic force microscopy and transmission electron microscopy. Optical properties are investigated by absorption spectroscopy, steady state and time resolved photoluminescence spectroscopy. Photoconductivity characteristics are analyzed to understand the basics of enhanced current in the various devices made from QDs composites. Chapter 3:Investigations on exciton phonon coupling and nonradiative relaxations in various sizes of visible light emitting cadmium telluride (CdTe) QDs size have been presented. Due to the large surface area, QDs are prone to have defect/trap states which can affect the exciton relaxation. Hence, understanding the role of such defect/trap states on photoluminescence is very essential for achieving the optimum optical properties. Temperature dependent (15 300 K) photoluminescence has been used to understand nonradiative relaxation of excited carriers. Thermally activated processes and multiple phonons scattering is thoroughly investigated to understand the quenching of photoluminescence with temperature. The strength of exciton-phonon coupling is investigated which determines the variation in energy bandgap of QDs with temperature. Role of exciton phonon scattering is also discussed to understand the basic physics of photoluminescence line width broadening in QDs. Chapter 4 and 5: This part of thesis focuses on the size and temperature pho-toluminescence properties of near infra red emitting ternary alloyed CdHgTe and Ag2S QDs. Near infrared emitting semiconductor quantum dots (QDs) have attracted significant scientific and technological interests due to their potential applications in the fields of photosensor, solar energy harvesting cells, telecommunication and biological tissue imaging etc. Structural and photophysical properties of CdHgTe QDs have been analyzed by high resolution transmission electron microscopy, X rayphotoelectron microscopy, photoluminescence decay kinetics and low temperature photoluminescence. Investigations on the nonradiative recombinations through trap/defects states and exciton phonon coupling are carried out in colloidal Ag 2S QDs which emits in the range of 1065 1260 nm. Particularly, the photoluminescence quenching mechanism with increasing temperature is analyzed in the presence of multiple nonradiative relaxation channels, where the excited carriers are thermally stimulated to the surface defect/trap states of QDs. Chapter 6 and 7: The aim of these chapters is to understand the basic physics of photo induced charge separation in the hybrid systems made from the inorganic QDs with graphene oxide and organic semiconductors. In chapter 6, CdHgTe QDs are decorated on graphene oxide sheets through physisorption. The excited state electronic interactions have been studied by optical and electrical characterizations in these CdHgTe QDs GO hybrid systems. In chapter 7, investigations are carried out for understanding the basic physics of charge separation in the composites of Ag2S QDs and poly (3hexylthiophene 2,5 diyl)(P3HT). These composites of inorganic organic materials are made by simple mixing with help of ultrasonication technique. Steady state and time resolved photoluminescence measurements are used as powerful technique to gain insight of energy/charge transfer process between P3HT and Ag2S QDs. Furthermore, investigations have been carried out on the photoconductivity of the devices made from these hybrid composites keeping the motive of its application in nanotechnology. Chapter 8: The conclusions of the work presented in this thesis are coherently summarized in this chapter. Thoughts and prospective for future directions are also summed up.