This study shows a detailed report on the morphological, structural and optical properties of CdSe QDs synthesised by the hot injection method. Cadmium acetate dihydrate and Se powder were used as cadmium and selenide precursors, respectively. Various QD sizes were achieved by synthesizing in temperature range of 150ºC, 175ºC, 200ºC, 225ºC, 250ºC, 275ºC and 300ºC, respectively. The as synthesized QDs by the hot injection method were cross-examined for their morphological, structural and optical using HRTEM, FTIR, XRD, RS, and UV-Vis spectroscopy techniques respectively. FTIR analysis has revealed vibrations at 738, 738, 738, 738, 735, 735 and 733 cm-1 for the QDs synthesized at various temperatures of 150, 175, 200, 225, 250, 275, and 300℃, respectively. The presence of the above mentioned peaks confirms the presence of Cd-Se bond in our samples. XRD analysis of CdSe QDs revealed diffraction peaks at 2 angles of 16.66 , 25.20 , 34.77 , 40.9 , 45.39 and 49.1 for 150 17.4 , 25.22 , 34.85 , 41.7 , 44.45 and 47.5 for the QDs synthesized at various temperatures of 175 17.07 , 25.19 , 34.85 , 41.34 , 44.41 and 48.86 for 200 ; 16.34 , 25.20 , 34.76 , 40.6 , 44.74 and 49.48 for 225 ; 17.44 , 25.17 , 34.19 , 41.7 , 44.45 , 49.24 for 250 ; 16.70 , 25.16 , 34.85 , 40.32 , 45.1 and 49.1 7 for 275 ;and 17.35 , 25.18 , 35.13 , 41.63 , 45.7 , 49.48 for 300 . These XRD peaks relate to crystal planes of (100), (002), (102), (220), (103) and (112) which belong to hexagonal Wurtzite CdSe crystal structure. Additionally XRD analysis has revealed a general peak shift to higher 2 values was observed for CdSe QDs. HRTEM analysis showed that the synthesised CdSe QDs have a spherical shape and are monodispersed. Moreover, HRTEM analysis has revealed CdSe QDs modal crystallite size of 1.79 nm, 1.81 nm, 2.06 nm, 2.08 nm, 2.11 nm, 3.10 nm and 3.12 nm for the QDs synthesized at various temperatures of 150ºC, 175ºC, 200ºC, 225ºC, 250ºC, 275ºC and 300ºC, respectively. HRTEM results were in mutual agreement with XRD results. Additionally, the SAED images showed intense electron diffraction rings, which confirmed that the as-synthesised CdSe QDs have a Wurtzite crystal structure. RS analysis showed that CdSe QDs have LO and 2LO vibrational modes which are characteristic peaks for CdSe. The presence of these peaks in Raman spectra further supports our previous observation from XRD analysis and HRTEM analysis that the synthesized CdSe QDs have a Wurtzite crystal structure. The effect of synthesis temperature Raman peak shift, FHWH and peak intensity has been cross examined in this work, Moreover, the effect of increasing temperature on the peak shift, FWHM and peak intensity is discussed in detail below. UV-Vis analysis revealed an absorbance of CdSe QDs in higher wavelengths as temperature was increased. Furthermore, the Yu et al 2003 relation was used to calculate QD size and band gap energy of CdSe QDs. The results showed that QD size increases with increasing synthesis temperature, which is in agreement with HRTEM and XRD results.

Submitted by Irene Nascimento (irene.kessia@ufpe.br) on 2016-06-28T16:52:16Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertação Denilson Digital.pdf: 4230195 bytes, checksum: 254e94141d3bf5cb07e451a017faacd5 (MD5) / Made available in DSpace on 2016-06-28T16:52:16Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertação Denilson Digital.pdf: 4230195 bytes, checksum: 254e94141d3bf5cb07e451a017faacd5 (MD5) Previous issue date: 2015-04-10 / FACEPE / A necessidade de metodologias sintéticas verdes vem aumentando significativamente nos últimos anos. O uso de materiais tóxicos e de metodologias caras vem sendo evitadas, sendo necessário o desenvolvimento de novas metodologias de síntese para Quantum Dots (QDs). Portanto, com o objetivo de baratear e diminuir a toxicidade dos QDs formados, devido à presença de agentes redutores presentes nas metodologias de síntese tradicionais, nesse trabalho foi desenvolvida uma metodologia de síntese eletroquímica para síntese de QDs de CdTe e CdSe estabilizados pelo ácido tioglicólico (TGA). A metodologia de síntese eletroquímica desenvolvida consiste na redução eletroquímica de alguns calcogênios (telúrio e selênio) em solução aquosa de NaOH 0,2 mol L-1 (pH = 13) e atmosfera inerte, para posterior reação com o precursor metálico,CdCl2/TGA. Os QDs obtidos a partir dessa metodologia apresentaram boa estabilidade e alta luminescência, com tamanhos médios de 3,9 nm para o CdTe e de 2,6 nm para o CdSe. Também foi avaliada a fluorescência dos QDs na presença do íon Hg2+, sendo observado que estas nanopartículas podem ser considerados promissores sensores químicos para determinação analítica de íons metálicos. / The demand for green synthetic methodologies has increased significantly in recent years. The use of toxic materials and expensive methodologies has been avoided, requiring the development of new methods of synthesis for Quantum Dots (QDs). Therefore, in order to cheapen and reduce the toxicity of QDs sintetizados, due to the need of reducing agents in the traditional synthetic methods, in this work we developed an electrochemical method for the synthesis of CdSe and CdTe QDs, stabilized by thioglycolic acid (TGA ). The electrochemical methodology developed involves the electrochemical reduction of some chalcogenes (tellurium and selenium) in aqueous solution (pH = 13) and inert atmosphere, for subsequent reaction with the metal precursor, CdCl2/TGA. The QDs obtained from this methodology showed good stability and high luminescence, with average sizes 3.9 nm for CdTe and 2.6 nm for CdSe. The fluorescence of the QDs in the presence of Hg2+ ion was also evaluated. It was observed that the electrochemically synthesized nanoparticles can be considered promising chemical sensors for analytical determination of metal ions.

This dissertation addresses the delicate problem of aging in complex systems characterized by non-Poissonian statistics. With reference to a generic two-states system interacting with a bath it is shown that to properly describe the evolution of such a system within the formalism of the continuous time random walk (CTRW), it has to be taken into account that, if the system is prepared at time t=0 and the observation of the system starts at a later time ta>0, the distribution of the first sojourn times in each of the two states depends on ta, the age of the system. It is shown that this aging property in the fractional derivative formalism forces to introduce a fractional index depending on time. It is shown also that, when a stationary condition exists, the Onsager regression principle is fulfilled only if the system is aged and consequently if an infinitely aged distribution for the first sojourn times is adopted in the CTRW formalism used to describe the system itself. This dissertation, as final result, shows how to extend to the non-Poisson case the Kubo Anderson (KA) lineshape theory, so as to turn it into a theoretical tool adequate to describe the time evolution of the absorption and emission spectra of CdSe quantum dots. The fluorescence emission of these single nanocrystals exhibits interesting intermittent behavior, namely, a sequence of "light on" and "light off" states, departing from Poisson statistics. Taking aging into account an exact analytical treatment is derived to calculate the spectrum. In the regime fitting experimental data this final result implies that the spectrum of the "blinking" quantum dots must age forever.

published_or_final_version / abstract / toc / Physics / Master / Master of Philosophy

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

Quantum dots (QDs) CdS and CdSe were synthesized in three different media including reverse micelles, aqueous solution, and trioctylphosphine oxide/trioctylphosphine (TOPO/TOP). Transmission electron microscope (TEM), X-ray diffraction (XRD), UV-VIS, fluorescence spectroscopy and microscopy were used to characterize the QDs and their nanoassemblies. CdS QDs synthesized in reverse micelles showed broad emission spectra. CdSe-CdS QDs with core shell structure synthesized in aqueous solution showed more than 30% emission quantum yield. CdSe-CdS QDs of different emission colors were prepared. CdSe and CdSe-ZnS QDs were also synthesized in TOPO/TOP media. This synthesis route produced highly luminescent CdSe QDs with over 50% emission quantum yield. The application of QDs as ions probes and methods to encapsulate QDs in nanospheres including micelles, glyconanospheres and silica nanospheres and the use of these nanospheres in bioassays are described. CdS QDs capped with different ligands such as thioglycerol, cysteine and polyphosphate showed different responses to biological relevant ions. The emission intensity of polyphosphate capped CdS QDs was affected by all the tested ions and did not show a selective response. On the other hand, the emission of thioglycerol capped CdS QDs was selectively quenched by copper ions while the emission of cysteine capped CdS QDs was selectively enhanced by zinc ions. Stern-Volmer equation was applied to correlate the emission intensity of the CdS QDs and the copper ion concentration. A Lagmuir isotherm binding equation was used to describe the relation between the emission intensity of cysteine capped CdS QDs and zinc ion concentration. The possible mechanism to explain the effects of capping ligands on CdS QDs responses to ions is also discussed. CdSe QDs synthesized in TOPO/TOP media were encapsulated in nanospheres for bioassay applications. The glyconanospheres contained a large number of glucose residues on their surface and showed high binding activity towards the lectinic protein Concanavalin A (Con A). Silica nanospheres containing hundreds of CdSe QDs were functionalized with thiol groups to enable the conjugation of streptavidin to the nanospheres. The streptavidin modified silica nanospheres were used as luminescent indicators in a sandwich immunoassay for the detection of antiprotein A antibody. The advantages and disadvantages of the nanospheres based bioassay are discussed.

Thesis advisor: Torsten Fiebig / This dissertation examines the chemistry and photophysics of semiconductor quantum dots with the intent of studying their capabilities and limitations as they pertain to photovoltaic technologies. Specifically, experiments are presented detailing the first time-resolved measurements of electron transfer in electronically coupled quantum rods. Electron transfer from the conduction band of CdTe was measured to occur on the 400 fs timescale (<italic>k_ET</italic> = 2.5 x 10<super>12</super> s<super>-1</super>), more than 500x faster than previously believed. Additionally, the direct optical promotion of an electron from the valence band of CdTe was observed, occurring on the timescale of the pump pulse (~50 fs). Based on the determined injection rates, a carrier separation efficiency of > 90% has been calculated suggesting these materials are sufficient for use in solar energy capture applications where efficient carrier separation is critical. To this end, model photovoltaic cells were fabricated, and their power conversion efficiency and photon-to-current generation efficiency characterized. In devices based of CdSe and heteromaterial quantum rods we observed fill-factors on the order of 10-20% though with power conversion efficiencies of < 0.02%. It was discovered that using a high temperature annealing step, while critical to get electrochemically stable photoelectrodes, was detrimental to quantum confinement effects and likely removed any hQR specific capabilities. Additionally, a detailed study on the role of nucleotide triphosphate chemistry in stabilizing emissive CdS nanoparticles is presented. Specifically it was observed that in a neutral pH environment, GTP selectively stabilizes CdS quantum dots with diameters of ~4 nm while the other naturally occurring ribonucleotides do not yield emissive product. The selectivity is dependent on the presence of the nucleophilic N-7 electrons near a triphosphate pocket for Cd<super>2+</super> complexation as well as an exocyclic amine to stabilize the resulting product particles. However, in an elevated pH environment, the nucleobase specificity is relaxed and all NTPs yield photo-emissive quantum dots with PLQEs as high as 10%. / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Thesis (MTech (Chemistry))--Cape Peninsula University of Technology, 2016. / The synthesis of water soluble CdTe, CdTe/CdSe and CdTe/CdSe/ZnSe nanoparticles (NPs) and their optical, cytotoxicity as well as imaging properties are presented. The synthesis was carried out under ambient conditions in the absence of an inert environment and involved the use of potassium tellurite (K2TeO3) and sodium selenosulphate (Na2SeSO4) as a stable tellurium and selenium precursor respectively, while mercaptopropanoic acid (MPA) was used as capping agents. In this method, the CdTe NPs were prepared by the addition of tellurium source solution to MPA-cadmium complex solution at different pH while keeping other parameters constant. The formation of the shell (CdSe) and multi shell (CdSe/ZnSe) were achieved by adding desired precursors to the growing CdTe core NPs at one hour interval. The temporal evolution of the optical properties and stability of the growing nanocrystals was monitored in detail by varying the refluxing time, pH and storing the NPs under ambient condition for several days. The as-prepared NPs were characterised using UV-Vis absorption and photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The formation of the shells was indicated by an immediate change in the colour of the reaction solutions after the addition of the desired precursor and the shift in the absorption wavelength towards red-region. The optical analyses showed an enhancement in the fluorescent intensity after the addition of the shell solution accompanied by red-shifting of the absorption and emission maximum. The stability study revealed an increase in the emission intensity as the ageing days increased. The stability study of the NPs in air at room temperature show highly improved stability of the core-shell NPs than the core. The TEM analysis showed that the materials are small, monodispersed, spherical and highly crystalline. The cytotoxicity of the NPs was investigated on LM 8 and KM-Luc/ GFP cell line using an MTT protocol at different concentrations. The cell viability show significant improvement after the shell formation with CdTe/CdSe/ZnSe core multi shell NPs having the highest cell viability at higher concentration (60 μg/mL). Furthermore a decrease in cytotoxicity is revealed with increase in reaction time, thus NPs prepared at longer (7 h) reaction time showed lower cytotoxicity compared with those prepared at shorter (0.5 h) reaction time. The confocal laser microscope image of the cells after the addition of the as-synthesised NPs confirmed the transfection of the NPs by KM-Luc/GFP cell line, indicating that the NPs have been endocytosis. This study demonstrates the great potential of the as-synthesised core-multi shell nanoparticles for biological and any applications that require efficiency, high fluorescence intensity and stability.

No description available.

<p>In this project we have used Fourier transform infrared spectroscopy to study the photoresponse of two different types of quantum dot-in-a-well infrared photodetectors (DWELL QDIPs). The basic task was to compare the photoresponse of these two detectors, and to study the influence of external resonant laser pumping on the photoresponse. Series of measurements were done at 77K. In the first measurements we investigated the photoresponse for different applied voltages at 77K. </p><p>In a second run of experiments, we used a 1064 nm infrared semiconductor laser to resonantly </p><p>pump the fundamental transition of the quantum dots. The results show that by using this </p><p>additional illumination the photoresponse was dramatically increased by creating additional </p><p>charge carriers in the quantum dots. This could be used to increase the sensitivity of infrared </p><p>detectors based on QDs.</p>