Here we present direct investigation of the lasing behaviour by performing gain spectroscopy of solution-based CQDs enabled via in-situ tuning of the feedback wavelength of an open-access hemispherical microcavity. The investigation is performed on two different types of CQDs, namely spherical CdSe/CdS core-shell CQDs and nanopletelets (NPs). The lasing threshold and the differential gain/slope efficiency of the fundamental cavity mode are measured as a function of their spectral position over a spectral range of ∼ 32 nm and of ∼ 42 nm for the spherical CQDs and NPs, respectively. The results of the gain spectroscopy are described using theoretical models, providing insights into the mechanism governing the observed lasing behaviour. Furthermore, the open-access cavity architecture provides a very convenient way of producing in-situ tunable lasing, and single-mode lasing of the fundamental cavity mode over a spectral range of ∼ 25 nm and ∼ 37 nm is demonstrated using spherical CQDs and NPs, respectively. In addition, the stability of laser emission is investigated, with the lasing intensity of the fundamental cavity mode remaining constant over a time period of almost 6 mins. It is hoped that the results will provide a detailed understanding of the lasing behaviour of CQDs. This information can be fed back into the design of CQDs in which the lasing threshold can be reduced to the point where useful devices can be constructed, and in the design of resonant optical feedback structures for which the appropriate wavelength must be carefully selected.

In this thesis type II colloidal quantum dots (CQDs) with zinc blende crystal structure were investigated. The optical properties were characterized by steady state absorption and photoluminescence (PL) spectroscopy for all the samples, and the PL quantum yield was measured for selected samples by using both absolute and relative methods. Exciton dynamics and interactions were investigated by time-resolved PL (TRPL).The exciton-exciton interaction energy for CdSe, CdSe/CdTe and CdSe/CdTe/CdS CQDs was investigated using TRPL spectroscopy, an established method. The TRPL results were compared with previous results from ultrafast transient absorption (TA) measurements and theoretical predictions. The discrepancies between the TRPL and TA results and the theoretical calculations suggest that TRPL data has been misinterpreted in the literature. The single exciton recombination dynamics for CdSe, CdSe/CdTe and CdSe/CdTe/CdS CQDs were investigated. The effects of non-radiative recombination were identified from the PL transients by using a theoretically-calculated radiative lifetime as a fitting parameter. The combined rate of the non-radiative processes thus found was consistent with the localisation of holes into shallow traps by an Auger-mediated process. A rate equation analysis also showed how shallow trapping can give rise to the tri-exponential PL dynamics observed experimentally. Chloride passivation of CdTe CQDs resulted in a near-complete suppression of surface traps, producing a significant enhancement of the optical properties. PL quantum yield (PLQY) and PL lifetime in particular benefit from the chloride treatment. The radiative recombination rate that now could be extracted from PL transients for chloride treated samples was used to calculate the non-radiative recombination rate for the untreated samples. In addition, a study of the effects of air exposure on the PL, observed for both treated and untreated samples was undertaken and revealed the importance of the influence of the dielectric environment surrounding the traps states on recombination dynamics.

CdTe QDs capped with mercaptopropionic acid (MPA) and thioglycolic acid (TGA) were covalently linked to zinc and indium tetraaminophthalocyanines (TAPcs) using N-ethyl-N(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxy succinimide (NHS) as the coupling agents. The results presented give evidence in favour of formation of an amide bond between the MTAPc and CdTe QDs. Both the linked ZnTAPc–QD complexes and the mixture of QDs and ZnTAPc (without chemical linking) showed Förster resonance energy transfer (FRET), though the linked showed less FRET, whereas the QD interactions with InTAPc yielded no evidence of FRET. Both MTAPcs quenched the QDs emission, with quenching constants of the order of 103–104M−1, binding constants of the order of 108-1010M-1 and the number of binding sites for the MTAPc upon the QD being 2. High energy transfer efficiencies were obtained (in some cases as high as 93%), due to the low donor to acceptor distances. Lastly, both MTAPc were shown to be poor optical limiters because their imaginary third-order susceptibility (Im[χ(3)]) was of the order of 10-17-10-16 (optimal range is 10-9-10-11), the hyperpolarizability (γ) of the order of 10-37-10-36 (optimal range is 10-29-10-34) and the k values were above one but below ten.

In this report we detail our work fabricating and measuring graphene quantum dots. We investigate a technique, relatively widely used in several other materials but not yet well investigated in graphene, known as Atomic Force Microscope Lithography (AFML). We then use AFML to fabricate graphene quantum dot systems. Transport measurements are carried out on our graphene quantum dots at low temperatures and high parallel magnetic fields and we try to understand the behaviour of spins in graphene. In our initial investigations into AFML we use graphene samples electrically contacted using standard electron-beam lithography. We were able to cut the graphene lattice by applying a negative voltage to the AFM tip and moving the tip across a grounded graphene surface. We have shown, by measuring the current through the AFM tip during lithography, that cutting of graphene is not current driven. Using a combination of transport measurements and scanning electron microscopy we show that , while indentations accompanied by tip current appear in the graphene lattice for a range of tip voltages, real cuts are characterized by a strong reduction of the tip current above a threshold voltage. The flexibility of the technique was then demonstrated by the fabrication, measurement, modification and re-measurement of graphene nanodevices with resolution down to 15 nm. We subsequently developed a shadow-masking technique to electrically contact graphene samples thus eliminating the use of chemical resists and the associated contamination of the graphene surface. With these pristine samples we were able to oxidise and hydrogenate the graphene using AFML. A graphene quantum dot was then fabricated using AFML oxidation. We also fabricated a graphene quantum dot using e-beam lithography in combination with oxygen plasma etching. We studied electron spin physics in these structures by J:1pplying large parallel magnetic fields at low temperatures and performing electrical transport measurements. We do not find an ordered filling sequence of spin states, which we assign to edge disorder and surface charge impurities.

Quantum dots (QDs) have the potential to produce more than one exciton per incident photon, if the photon energy is greater than twice the band gap energy. This process of multiple exciton generation (MEG) has the potential to lead to a step change in the efficiency of solar panels, by utilising energy commonly wasted as heat in conventional solar cells. A wide range of CdSe/CdTe and CdTe/CdSe quantum dots with and without a CdS shell were synthesised with varying core sizes and shell thicknesses. The excited state dynamics of these samples were studied with transient absorption and photoluminescence studies, with their MEG efficiencies obtained. Record MEG efficiencies were obtained with values reaching 142 ± 9 % achieved. The charge separation afforded by the type-II electronic configuration, allowed the first attractive biexciton interaction for a type-II QD system, with the potential for reducing the creation energy for a second exciton this affords. Efficient surface passivation of QDs was achieved through the reaction of CdCl2 with CdTe QDs, with near unity photoluminescence quantum yields (PLQYs) achieved. The suppression of surface trap states resulted in mono-exponential photoluminescence decay traces, with a resultant increase in exciton lifetime. Further CdCl2 treatment was carried out on CdSe/CdTe quasi-type-II QDs with alternating ‘Cd rich’ and ‘Te rich’ surfaces to elucidate the processes involved in surface treatment. It is shown that Te surface atoms are preferentially etched upon treatment, with the reaction being more aggressive when ‘Te rich’ surfaces are treated. The importance of surface composition is studied with trap states associated with chalcogen dangling bonds more prevalent and hence the increased requirement for their passivation is outlined. Control of the core/shell interface is also shown to be important in reducing trap states and ultimately increasing PLQYs, which is desirable for many optoelectronic applications.

M.Sc. / This report illustrates the synthesis and characterization of CdS and CdSe nanoparticles in TOPO, HDA and hexamethylenediamine. The prepared nanoparticles were characterized using UV-visible and photoluminescence spectrophotometers for optical properties, transmission electron microscopy for shapes and sizes as well as powder X-ray diffractometer for structural analysis. The effect of monomer concentration and temperature were investigated on the growth of nanocrystals. The monomer concentration was varied by changing the amount of stabilizer. The particle sizes increased with an increase in monomer concentration. Higher monomer concentration resulted in polydispersed nanoparticles due to faster uncontrolled growth. Increasing the temperature resulted in a faster growth thus increasing the size of the particles. The growth also affected the shapes of the particles as the particles tend to grow anisotropical ly at higher monomer concentration and high temperatures. The formation of tretrapods at high temperatures was due to a kinetically driven reaction as a result of increased temperature. Hexamethylenediamine was found to be a poor capping agent for the prepared CdS nanoparticles. The particles prepared in the compound agglomerated at all temperatures used in preparation. Such results were associated with lower steric hindrance due to a shorter molecular chain. The polymer nanofibres were fabricated via electrospinning technique while varying the concentrations of the polymer solutions. Solutions of low viscosity gave beaded fibres as mixtures of droplets and fibres due to the collection of wet fibres. An increase in the concentration (viscosity) of the solutions resulted in the deposition of solid fibres with bigger diameters. The TGA results show that PMMA electrospun fibres demonstrate a significant increase in thermal stability compared to the powder polymer. However the changes were very minimal on the PEO fibres. CdS and CdSe nanoparticles were incorporated into PMMA and PEO and electrospun to fabricate composite fibres. The incorporation of the quantum dots caused an increase in the viscosity of the solutions and resulted in the collection of fibres with spiral morphology. However this increase of concentration caused an increase in the diameters of the composite fibres as evaluated from the SEM analysis. The EDS analysis showed the presence of Cd, S, and Se elements in the composite fibres due to the presence of CdS and CdSe. The XRD analysis of the composite showed no effect of the quantum dots on the amorphous peak of the PMMA. However on the PEO it showed a decrease in the intensity as the peaks as they become broader due to the decrease of crystallinty. The FTIR spectra showed that the presence of the quantum dots in the polymers on both PMMA and PEO. The optical analysis showed absorption and emissions peaks on the composites fibres due to the showed incorporated light emitter. These peaks were not affected by any change in the concentrations as a result of increased wt % of the quantum dots. Thermal analysis of the composite fibres demonstrates an increase in the thermal stability of the polymers after the incorporation of the quantum dots. Very small changes were observed for the quantum dots doped-PEO material compared to the doped PMMA. DSC analysis showed an increase in the glass transition temperature of the PMMA with increasing wt % of the CdS and CdSe. The addition of CdS and CdSe nanoparticles into PEO caused a decrease in the melting temperature of the polymer due to a decrease in the polymer crystallinity.

This dissertation explores the effect of nanometer-scale changes in structure on the energetics of photocatalytic and photovoltaic materials. Of particular interest are semiconductor nanocrystals (NCs), which have interesting chemical properties that lead to novel structures and applications. Chief among these properties are quantum confinement and the high surface area-to-volume ratio, which allow for chemical tuning of the energetics and structure of NCs. This tunable energetic landscape has led to increasing application of NCs in various areas of research, including solar energy conversion, light-emitting diode technologies, and photocatalysis. However, spectroscopic methods to determine the energetics of NCs have not been well developed, due to chemical complexities of relevant NCs such as polydispersity, capping ligand effects, core-shell structures, and other chemical modifications. In this work, we demonstrate and expand the utility of photoelectron spectroscopy (PES) to probe the energetics of NCs by considering the physical processes that lead to background and secondary photoemission to enhance photoemission from the sample of interest. A new methodology for the interpretation of UP spectra was devised in order to emphasize the minute changes to the UP spectra line shape that arise from nanoscopic changes to the NCs. We applied various established subtractions that correct for photon source satellites, secondary photoelectrons, and substrate photoemission. We then investigated the effect of ligand surface coverage on the surface chemistry and density of states at the top of valence band (VB). We systematically removed ligands by increasing numbers of purification steps for two diameters of NCs and found that doing so increased photoemission density at the top of the VB, which is due to undercoordinated surface atoms. Deeper VB structure was also altered, possibly due to reorganization of the atoms in the NC. Using the new UPS interpretation methodology, we examined the evolution of the valence band energy (EVB) of CdSe NCs as it was modified from spherical NC to rod to Au-NP tipped nanorod (NR). We also employed potential-modulated attenuated total reflectance spectroscopy (PM-ATR) to probe the conduction band energy (ECB) of the series. The EVB decreased with each modification, which is predicted with a band-bending model. This trend was also observed in the ECB, as revealed by spectroelectrochemistry, along with the appearance of new metal-semiconductor states in the band gap. UPS was finally used to investigate the even more complex Pt-NP tipped CdSe@CdS core@shell NR heterostructure. The addition of the CdS shell decreases the EVB relative to CdSe, as expected from common cation II-VI compounds. The Pt-NC increases the EVB, which, like the Au-CdSe NR, is predicted by employing a band-bending model. XPS revealed that PtSx-like chemical states were formed near the CdS-Pt interface. These experiments, along with the improved UP spectra interpretation methodology, demonstrate the wealth of information regarding surface chemistry and energetics that can be obtained with PES which can be applied to not only NCs, but also to metal oxide or molecular thin films.

The emergence of optical applications, such as lasers, fiber optics, and semiconductor based sources and detectors, has created a drive for smaller and more specialized devices. Nanophotonics is an emerging field of study that encompasses the disciplines of physics, engineering, chemistry, biology, applied sciences and biomedical technology. In particular, nanophotonics explores optical processes on a nanoscale. This dissertation presents nanophotonic applications that incorporate various forms of the organic polymer N-isopropylacrylamide (NIPA) with inorganic semiconductors. This includes the material characterization of NIPA, with such techniques as ellipsometry and dynamic light scattering. Two devices were constructed incorporating the NIPA hydrogel with semiconductors. The first device comprises a PNIPAM-CdTe hybrid material. The PNIPAM is a means for the control of distances between CdTe quantum dots encapsulated within the hydrogel. Controlling the distance between the quantum dots allows for the control of resonant energy transfer between neighboring quantum dots. Whereby, providing a means for controlling the temperature dependent red-shifts in photoluminescent peaks and FWHM. Further, enhancement of photoluminescent due to increased scattering in the medium is shown as a function of temperature. The second device incorporates NIPA into a 2D photonic crystal patterned on GaAs. The refractive index change of the NIPA hydrogel as it undergoes its phase change creates a controllable mechanism for adjusting the transmittance of light frequencies through a linear defect in a photonic crystal. The NIPA infiltrated photonic crystal shows greater shifts in the bandwidth per ºC than any liquid crystal methods. This dissertation demonstrates the versatile uses of hydrogel, as a means of control in nanophotonic devices, and will likely lead to development of other hybrid applications. The development of smaller light based applications will facilitate the need to augment the devices with control mechanism and will play an increasing important role in the future.

Orientador: Guillermo Gerardo Cabrera Oyarzún / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-27T21:31:38Z (GMT). No. of bitstreams: 1 Fernandes_ImaraLima_D.pdf: 2572980 bytes, checksum: 17e1019655bf7dc13c5d5180196e89db (MD5) Previous issue date: 2015 / Resumo: Esta tese de doutorado abordou principalmente o estudo teórico das propriedades de transporte dependente do spin em nanoestruturas magnéticas. As principais estruturas estudadas foram junções magnéticas de tunelamento e sistemas compostos por um arranjo de pontos quânticos acoplados a eletrodos ferromagnéticos. Com o intuito de obter propriedades físicas do sistema como, a corrente elétrica, a corrente de spin, a densidade local de estados, a ocupação média nos pontos quânticos e a corrente induzida por spin transfer-torque utilizamos o formalismo de funções de Green de não-equilíbrio. Na primeira parte deste trabalho, estudamos os efeitos da inversão do spin nas propriedades de transporte em junções de tunelamento. Para estes sistemas, o fenômeno da magnetorresistência tem origem na densidade de estados dos elétrons de condução dos eletrodos e mostramos que ela é fortemente afetada pela inversão do spin no tunelamento. Além disso, foi possível observar que a inversão do spin induz um spin-torqueadicional ao sistema. Na segunda parte deste trabalho, investigamos as propriedades de transporte em um sistema composto por dois pontos quânticos em forma de T acoplados a eletrodos ferromagnéticos. Com a mesma metodologia empregada anteriormente, encontramos que o aparecimento da ressonância de Fano e a formação de estados ligados dependem fortemente dos parâmetros do sistema. Explorando em detalhes o sistema, foi observado que é possível controlar a condutância elétrica do sistema através de um potencial de gate. Em particular, ao variar a posição do nível de energia do ponto quântico central é possível inverter a ressonância de Fano. Na última parte, apresentamos os resultados numéricos para o sistema considerando um ponto quântico com dois níveis de energia acoplado a dois eletrodos ferromagnéticos, e no interior do ponto quântico levamos em conta a interação e-e e a interação spin-órbita de Rashba. A interação de Rashba introduz a transição entre os níveis de energia com a inversão do spin, o que originou interessantes propriedades no transporte dependente do spin. Em particular, para eletrodos não magnéticos obtivemos que o acoplamento spin-órbita resultou na criação de corrente de spin polarizada / Abstract: In this work, we have studied the spin-dependent quantum transport in magnetic nanostructures. The main structures studied are magnetic tunneling junctions and systems composed of an arrangement of quantum dots coupled to ferromagnetic electrodes. Using the nonequilibrium Green's function techinique, we were able to calculate selected properties of the systems such as the electric current, the spin current, local density of states and the current-induced spin-transfer torque. In the first system, we have observed the effects of the spin-flip scattering in the transport properties considering tunneling junctions composed by an insulating layer between two ferromagnetic electrodes. The results obtained for this system showed that the magnetoresistance is related to density of state effects at the ferromagnetic electrodes, and we have found that it is strongly affected by the spin-flip scattering. Besides, we also observed that the spin-flip scattering gives rise to an additional spin-torque to the system. For the system composed of two quantum dots T-shaped electrodes coupled to ferromagnetic electrodes, we investigated the spin-dependent properties. We have observed that the appearance of the Fano resonance and the formation of bound states rely strongly on system parameters. Another interesting finding is the possibility to control the electrical conductance via a gate voltage. We figured out that changing the energy level of the central dot affected directly the Fano resonance peak intensities. Lastly, we have showed that for the system composed by a quantum-dot with two energy levels coupled to two ferromagnetic electrodes must be modified when the \textit{e-e} and the Rashba spin-orbit interaction are taken into account. The Rashba interaction induces level transitions with spin-flip resulting in interesting properties in the spin-dependent transport. We have found out that the spin-orbit interaction strongly contributes to the spin current even for non-magnetic electrodes / Doutorado / Física / Doutora em Ciências

This work is focused on examining photoluminescent properties of InAs quantum dots (QDs) on GaAs substrate covered by GaAs1-xSbx strain reducing capping layer (SRL) prepared by Stranski-Krastanow method. We measured luminescence decay time of two samples with different concentration of Sb in this layer. We investigated the influence of temperature, intensity and wavelength of the excitation pulse on the luminescent decay time. We also compared the properties of the samples after excitation by 760 nm pulse and 850 nm pulse - the former one is energetically above the substrate band gap; in the second case we excited only the QDs and the wetting layer (WL). We consequently derived recombination and relaxation processes occurring inside InAs QDs and also the transport of charge carriers from the substrate and the WL into QDs. One part of this diploma thesis was to learn about the methods of measuring ultrafast photoluminescence and build the experimental set-up.