Submitted by Rosivalda Pereira (mrs.pereira@ufma.br) on 2017-10-02T19:59:47Z No. of bitstreams: 1 ThatyaraMonteiro.pdf: 3528798 bytes, checksum: 12d3da9115c45896b4f14c67dbe3d20e (MD5) / Made available in DSpace on 2017-10-02T19:59:47Z (GMT). No. of bitstreams: 1 ThatyaraMonteiro.pdf: 3528798 bytes, checksum: 12d3da9115c45896b4f14c67dbe3d20e (MD5) Previous issue date: 2017-09-01 / Two novel and pioneering photoelectrochemical sensors were developed for determination of tert-butyl hydroquinone (TBHQ) in biodiesel and edible oil samples. The former based on composite formed by TiO2 nanoparticles and lithium tetracyanethylene (LiTCNE), and the last based on the sensitization of CdSe/ZnS quantum dots with LiTCNE. In both cases, indium tin oxide (ITO) was used as the work electrode surface. The LiTCNE/TiO2/ITO sensor showed a TBHQ photocurrent about 28-fold higher than the TiO2 sensor. The same was observed for the CdSe/ZnS/LiTCNE/ITO sensor, which presented a photocurrent for TBHQ about 13-fold higher than that presented by the electrode modified with CdSe/ZnS. Both developed sensors showed lower resistance to charge transfer than their non-sensitized components. They also demonstrated high selectivity to TBHQ, with high photocurrent for this compound in comparison to photocurrent responses to other phenolic antioxidants. The experimental conditions optimized for both sensors were: 0.1 mol L-1 of phosphate buffer solution pH 7.0 and applied potential to the working electrode of 450 mV, for the LiTCNE/TiO2/ITO sensor, and 0.1 mol L-1 of phosphate buffer solution pH 6.0, and potential of 400 mV for the CdSe/ZnS/LiTCNE/ITO sensor. In these conditions, the sensors presented a linear range of TBHQ response between 0.4 and 500 μmol L-1 for LiTCNE/TiO2/ITO sensor and between 0.6 and 250 μmol L-1 for the CdSe/ZnS/LiTCNE/ITO sensor, with limits of detection of 0.10 and 0.21 μmol L-1, respectively. The LiTCNE/TiO2/ITO sensor was applied in biodiesel samples for determination of TBHQ using standard addition method, showing recovery values between 96.8 and 98.2%. The CdSe/ZnS/LiTCNE/ITO sensor was applied in edible oil samples to detect TBHQ using an external calibration method, with recovery values between 98.25 and 99.83%. The photoelectrochemical sensors were successfully used to determine the TBHQ antioxidant in real samples of biodiesel and vegetable oil. / Dois novos e pioneiros sensores fotoeletroquímicos foram desenvolvidos para determinação de tert-butil hidroquinona (TBHQ) em amostras de biodiesel e de óleo comestível. O primeiro baseado no compósito formado por nanopartículas de TiO2 e tetracianoetileneto de lítio (LiTCNE), e o segundo baseado na sensibilização de quantum dots CdSe/ZnS com o LiTCNE. Em ambos os casos utilizou-se como eletrodo de trabalho o óxido de índio e estanho (ITO) como superfície eletródica. O sensor à base de LiTCNE/TiO2/ITO apresentou uma fotocorrente para o TBHQ cerca de 28 vezes mais elevada que o sensor à base de TiO2. O mesmo foi observado para o sensor à base de CdSe/ZnS/LiTCNE/ITO, que apresentou fotocorrente para o TBHQ cerca de 13 vezes maior do que à apresentada pelo eletrodo modificado com CdSe/ZnS. Ambos os sensores desenvolvidos apresentaram baixa resistência à transferência de carga em comparação a seus componentes não sensibilizados. Também demonstraram grande seletividade ao TBHQ, com alta fotocorrente para esse composto em comparação às respostas de fotocorrente para outros antioxidantes fenólicos. As condições experimentais otimizadas para ambos os sensores desenvolvidos foram, respectivamente: 0,1 mol L-1 de solução tampão fosfato pH 7,0 e potencial aplicado ao eletrodo de trabalho de 450 mV, para o sensor LiTCNE/TiO2/ITO, e 0,1 mol L-1 de solução tampão fosfato pH 6,0, e potencial de 400 mV, para o sensor CdSe/ZnS/LiTCNE/ITO. Nessas condições, os sensores apresentaram faixa linear de resposta de TBHQ entre 0,4 a 500 µmol L-1 para sensor LiTCNE/TiO2/ITO e entre 0,6 a 250 µmol L-1 para o sensor CdSe/ZnS/LiTCNE/ITO, apresentando limites de detecção de 0,10 e 0,21 µmol L-1 , respectivamente. O sensor LiTCNE/TiO2/ITO foi aplicado em amostras de biodiesel para determinação de TBHQ usando método de adição de padrão, mostrando valores de recuperação entre 96,8 e 98,2%. Já o sensor CdSe/ZnS/LiTCNE/ITO foi aplicado em amostras de óleo comestível para detecção de TBHQ usando método de calibração externa, com valores de recuperação entre 98,25 e 99,83%. Os sensores fotoeletroquímicos foram empregados com sucesso para determinação de antioxidante TBHQ em amostras reais de biodiesel e óleo vegetal.

Blinking dynamics of CdSe/ZnS semiconductor quantum dots (QD) are characterized by (truncated) power law distributions exhibiting a wide dynamic range in probability densities and time scales both for off- and on-times. QDs were immobilized on silicon oxide surfaces with varying grades of hydroxylation and silanol group densities, respectively. While the off-time distributions remain unaffected by changing the surface properties of the silicon oxide, a deviation from the power law dependence is observed in the case of on-times. This deviation can be described by a superimposed single exponential function and depends critically on the local silanol group density. Furthermore, QDs in close proximity to silanol groups exhibit both high average photoluminescence intensities and large on-time fractions. The effect is attributed to an interaction between the QDs and the silanol groups which creates new or deepens already existing hole trap states within the ZnS shell. This interpretation is consistent with the trapping model introduced by Verberk et al. (R. Verberk, A. M. van Oijen and M. Orrit, Phys. Rev. B, 2002, 66, 233202).

A study combining infrared (IR) absorption spectroscopy and first-principles theory is presented for a sulfur–oxygen complex in CdSe characterized by IR absorption lines located at 1094, 1107, and 1126 cm-1 (10 K). The properties of the center are compared to a similar species occurring in CdTe that gives rise to two absorption lines at 1097 and 1108 cm-1 (10 K). Temperature- and polarization-sensitive measurements performed on 18O-enriched samples reveal that for both materials the IR absorption lines are due to split ν3 stretch vibrations of a distorted sulfate (SO4) tetrahedron, whereby the local point group of the SO4 complex is reduced to Cs and C3v in hexagonal CdSe and cubic CdTe, respectively. Measurements on the vibrational spectrum of the sulfate species in the spectral range of symmetric stretch (ν1), bend (ν4), and combinationmodes (ν1 þ ν3) are presented. The cation vacancy VCd is discussed as a likely site occupied by SO4 in CdSe.

Diese Dissertation besteht aus zwei Teilen von Studien. Der erste Teil demonstriert die Steuerung der Elektron-Kern-Spin-Systems in II-VI Halbleiter Quantum Dots (QDs) durch elektrische Ströme über Mikrospulen. Mikrometer-große Leiterschleifen sind auf der Oberseite von Heterostrukturen mit geladenen CdSe/ZnS QDs hergestellt worden. Eine Strominjektion erzeugt magnetische Felder im Bereich von einige 10 mT, welche stark genug sind, um die Hyperfeinwechselwirkung in CdSe QDs modulieren zu können. Der Durchmesser des Spulen im Mikrometer-Bereich ermöglicht die Generation von schnellen Feld transienten im Bereich von wenigen ns. Mit diesen Vorteilen der Mikrospulen werden die Steuerungs des Spins der residenten Elektronen sowie das Auslesen des Kernspinzustandes durch elektrische Impulse nachgewiesen. Der zweite Teil befasst sich mit der Ladungsträger-Spindynamik in ZnO Quantum Well (QW) Strukturen und Epitaxieschichten, die mittels des optischen Übergang von negativ geladenen Exzitonen X− beziehungsweise des am neutralen Donator gebunden Exziton D0X untersucht werden. Der Loch-Spin kann direkt über die zirkular polarisierten Photolumineszenz der beiden Komplexe zurückverfolgt werde. Die Spin-Relaxationszeit von QW und Epiplyer verfolgt werden. Der Spin des Donatorelektronens wird über die Ausbleichung des Spin-selektive Anregungprozesses nachgewiesen. Es werden longitudinale Loch-Spinrelaxationszeiten von 80 bis 140 ps für D0X und X− gefunden. Deutlich längere longitudinalen Elektronen-Spin-Relaxationszeiten in Bereich von mehreren 100 ns werden gefunden, wenn die Hyperfeinwechselwirkung durch ein geeignetes externes Magnetfeld unterdrückt wird. Eine Feldstärke von 2 mT ist groß genug. Dies zeigt den extrem kleinen Wert des Overhauser-Feldes in ZnO auf, der durch die sehr begrenzte Anzahl von magnetischen Kernen in Wechselwirkung mit dem Elektronen innerhalb des Volumens des Donators verursacht wird. / This work is composed of two parts of studies. The first part represents an electron-nuclear spin control in II-VI semiconductor quantum dots (QDs) by electrical currents via micro coils. Micrometer single turn coils are fabricated on top of heterostructures with charged CdSe/ZnSe QDs. Current injection creates magnetic fields in the range of some 10 mT which is strong enough to modulate the hyperfine interaction in CdSe. The micrometer-range diameter of coil allows for generation of fast field transient in the range of few ns. Using these advantages of micro coils, local control of the resident electron spin as well as read out of the nuclear spin state are demonstrated by electrical pulses. The second part presents charged carrier spin dynamics in ZnO quantum wells and epilayers using the optical transition of the negatively charged exciton X− and the neutral donor bound exciton D0X, respectively. The hole spin can be directly traced by the circular polarized photoluminescence of both complexes. The spin relaxation of the resident electrons and donor electrons is accessed via the bleaching of the spin selective excitation process. Longitudinal hole spin relaxation times of 80 and 140 ps are found for D0X and X−, respectively. Much longer longitudinal electron spin relaxation times in the several 100 ns range are uncovered if the hyperfine interaction is suppressed by a proper external magnetic field. A field strength of 2 mT is large enough proving that the extremely small value of the Overhauser field in ZnO caused by the very restricted number of magnetic nuclei interacting with the electron inside the donor volume.

Semiconductor nanocrystals are interesting candidates as new light-absorbing materials for photovoltaic (PV) devices. They can be dispersed in solvents and cheaply deposited at low-temperature on various substrates. Also, the nanocrystals have unique optical properties depending on their size due to the quantum size effect and moreover it is easy to uniformly control their stoichiometry. CdTe/CdSe/CdTe heterostructure nanorods and I-III-VI₂ nanocrystals were selected to synthesize and investigate in order to utilize the benefits of colloidal nanocrystals described above. Colloidal nanorods with linear CdTe/CdSe/CdTe heterojunctions were synthesized by sequential reactant injection. After CdTe deposition at the ends of initially formed CdSe nanorods, continued heating in solution leads to Se-Te interdiffusion across the heterojunctions and coalescence to decreased aspect ratio. The Se-Te interdiffusion rates were measured by mapping the composition profile using nanobeam energy dispersive X-ray spectroscopy (EDS). The rate of nanorod coalescence was also measured and compared to model predictions using a continuum viscous flow model. The synthetic method of monodisperse chalcopyrite (tetragonal) CuInSe₂ nanocrystals was also developed. The nanocrystals have trigonal pyramidal shape with one polar and three non-polar surface facets. When drop-cast onto carbon substrates, the nanocrystals self-assemble into close-packed monolayers with triangular (honeycomb) lattice structure. Moreover, the effect of excess Cu precursor (CuCl) was studied for the formation of monodisperse trigonal pyramidal CuInSe₂ nanocrystals. The formation mechanism of monodisperse trigonal pyramidal CuInSe₂ nanocrystals was suggested with regard to excess amount of CuCl precursor, based on the nucleationgrowth model of colloidal nanocrystal formation. A new wurtzite phase of CuInS₂, CuInSe₂, and Cu(InxGa1-x)Se₂ (CIGS) was observed in nanocrystals synthesized by heating metal precursors and Se-(or S-)urea in alkylamine. X-ray diffraction (XRD) showed the predominant phase to be wurtzite (hexagonal) instead of chalcopyrite (tetragonal). High resolution transmission electron microscopy (TEM), however, revealed polytypism in the nanocrystals, with the wurtzite phase interfaced with significant chalcopyrite domains. / text

Nous présentons dans ces travaux de thèse la caractérisation optique d'un nouveau type de boîtes quantiques : les boîtes quantiques de CdSe insérées dans des nanofils de ZnSe. <br />Cette étude comprend de la spectroscopie optique standard, où l'on mesure les raies d'émission d'une boite quantique unique en fonction de leur énergie, mais aussi de la spectroscopie résolue en temps, comme des mesures de temps de déclin ou des mesures de corrélation. Les expériences présentées nous ont permis d'étudier la structure fine de l'exciton avec la mesure de l'énergie entre l'exciton noir et l'exciton brillant et le temps de spin flip entre ces deux états. Puis les mesures de corrélation entre chacun des états brillants d'une boîte quantique unique nous ont permis de montrer que nos émetteurs de photons sont des sources de photons uniques de qualité jusqu'à haute température (220 K), et de proposer un modèle décrivant les différents états de cette boîte et leur dynamique de relaxation. <br />Pour finir, nous nous sommes intéressé à la largeur importante des raies d'émission et aux temps caractéristiques de la diffusion spectrale qui en est responsable. Pour cela, nous avons présenté une nouvelle méthode expérimentale basée sur des mesures de corrélation entre des bandes spectrales de la raie permettant de directement mesurer le temps caractéristique de la diffusion spectrale et d'en étudier le comportement en fonction de différents paramètres comme la puissance d'excitation et la température.

In this work we investigated fabrication of semiconductor nanomaterials and evaluated their potential for photo-chemical and photovoltaic applications. We investigated different II-VI semiconductor nanomaterial systems; (i) ZnO oriented nanowire arrays non-epitaxially grown from a substrate; and (ii) colloidal CdE (E=Te,Se,S) quantum structures synthesized by solution-based thermal decomposition of organo-metallic precursors. We have studied the synthesis of vertically aligned ZnO nanowire arrays (NWA), by a wet chemical process on various substrates. We have extended this method wherein nanofibers of poly-L-lactide act as a substrate for the radially oriented growth of ZnO nanowires. By combining the large surface area and the flexibility of the PLLA-ZnO hierarchical nanostructure we have shown the proof-of-principle demonstration of a ‘continuous-flow’ water treatment system to decompose known organic pollutants in water, as well as render common waterborne bacteria non-viable. We have studied synthesis of colloidal quantum dots (QD), and show size, morphology and composition tailored nanocrystals for CdE (E=S, Se, Te) compositions. We have studied the influence of crystal growth habits of the nanocrtsyals on the final morphology. Furthermore we have synthesized core-shell, CdSe-CdS QDs with spherical and tetrahedral morphologies by varying the reaction conditions. We show that these core-shell quantum dots show quasi-type II characteristics, and demonstrate with I-V measurements, the spatial localization of the charge carriers in these hetero-nanocrystals. For this purpose, we developed hybrid materials consisting of the core-shell quantum dots with electron acceptors (ZnO nanowires) and hole acceptors (polymeric P3HT nanofibers). In addition we have also compared the synthesis reaction when carried out with conventional heating and microwave-mediated heating. We find that the reaction is enhanced, and the yield is qualitatively better when using microwave induced heating. / QC 20120525

The research presented has been focused on understanding the surface effects on the optical and electronic properties of some metallic and semiconductor nanomaterials. When the particle sizes are on the nanometer length scale, a large fraction of atoms in the particles are on the surface. The bonding of the surface atoms being unsaturated could cause trapping and introduce defects that interact with the excited electrons. The effect of the surface on the optical and electronic properties of some semiconductor and metallic nanoparticles is investigated. When the size and shape of nanomaterials change, both the electron density of the excited electrons on the surface and the electronic structure change. Therefore, it becomes important to understand how these changes affect the electronic motion in the particles in order to exploit their full potential in a variety of applications. Semiconductor nanoparticles studied include cadmium selenide (CdSe) and cadmium sulfide (CdS). Effect of changing CdSe shape and size on optical and electronic properties has been investigated and the ability for the CdS nanoparticles to show optical gain (stimulated emission) in solution at room temperature is reported. Effect of surface phonon contribution on the exited electron relaxation in copper nanoparticles is investigated. For the particles size smaller than the mean free path of the electrons in the metal, electron-surface phonon coupling becomes an important factor (contribution) for hot electron relaxation. In the thesis presented, it is shown for the first time the size depended electronic relaxation in copper nanoparticles. Fluorescence due to surface plasmon field enhancement is observed for copper nanoparticles to be million times stronger than the fluorescence observed from bulk copper.

The synthesis of nanocrystals is unique compared to the formation of larger micron-sizesspecies as the final crystal sizes are not much larger than the primary nuclei. As a consequencethe final outcome of a nanocrystal synthesis i.e mean crystal size, concentrationand standard deviation is almost solely determined by the end of the nucleation phase. Directingthe growth of crystals beginning from aggregates of only tens of atoms into maturemonodisperse nanocrystals requires that the governing kinetics are strictly controlled at everymoment of the reaction. To effect this task various different ligands need to be employed,each performing a particular function during both nucleation and growth. (For complete abstract open document)

Deposition of noble metal and semiconductor nanocrystalline thin films has received much attention. CdS and CdSe are important semiconductors used in optical devices. A wet chemical route which uses the interface of two immiscible liquids to control the growth and deposition of nanocrystalline thin films forms the basis of the current study. In this method, a metal precursor dissolved in toluene or decane is held in contact with a water layer containing a reducing or sulphiding agent. The reaction proceeds at the interface of the liquids and results in deposits adhering to the interfacial region. The products of such reactions typically consist of nanocrystals forming a thin film. Stable sols of Au, Ag were found to metathesize on contact with alkylamine in oil to form monolayer films that spread across large areas at the water/oil interface. The nature and properties of interfacial thin films depend on the alkylamine. Nanocrystalline thin films consisting of CdS adhering to the interface starting with a polydispersed aqueous sol of crystallites and alkylamine were obtained. The optical band gaps of the films formed are dependent on the alkylamine chain length, with the shortest chain yielding the largest gap. A systematic increase in particle diameters following adsorption is responsible for changes in the electronic structure of films. The formation of nanocrystalline films of CdS adhering at the interface using a toluene solution of cadmium diethyldithiocarbamate and aqueous Na2S solution, in the presence of tetraoctylammonium bromide (TOAB) in the aqueous phase, was investigated under various reaction parameters, while CdSe was obtained using Na2SeSO3 solution and the influences of deposition temperature and solution concentration were studied. A ternary water/decane/2-butoxyethanol /salt system was used to grow deposits of CdSe and CdS. Nanostructured thin films were obtained at the upper interface of the ternary system, between the emulsive middle layer and oil rich top phase. The influence of deposition conditions such as precursor concentrations and temperature, as well as the nature of the medium on the properties of the deposits was studied. Deposits grown using the ternary system were compared with those obtained using water/decane and water/toluene systems. Reaction parameters such as temperature, solution concentration and the size of CdS and CdSe were controlled. A thin film of CdS and CdSe nanocrystals was formed at the interface. The grain size was found to be dependent on reaction temperature and solution concentration, with higher temperatures and solution concentration resulting in larger grains. The nature of thin films obtained at the interface of two immiscible liquids and of a water/decane/2-butoxyethanol/salt ternary system were studied using Scanning and Transmission electron microscopy, X-ray diffraction and UV-visible spectroscopy.