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Characterization of CdSe-CdxZn1-xS core-shell QDs as active materials for compact micro-cavity lasers
Full text linkInnovation within the field of nanophotonics is fostering the progress in diverse technological fields, spanning light emitting, communication technologies, renewable energies, medical diagnostic and therapy. Among the different classes of nanomaterials that are contributing to such evolution, semiconductor nanocrystals, a.k.a Quantum Dots (QDs), are the most versatile ones. QDs are inorganic semiconductor nanostructures, whose outstanding light emitting performances make them promising competitors to more “conventional” bulk solid-state materials in many commercial applications. The interest on developing QD-based devices spread on a large scale with the development of colloidal synthesis methods. The colloidal approach expedites their processability and integration in light emitting devices with dimensions ranging from the macro- to the nano-scale. In particular, colloidal QDs are suitable active media for the fabrication of compact and flexible solid-state laser sources.
Optical properties of QDs are ruled by the Quantum Confinement (QC) regime. The latter occurs when the size of the material is reduced to levels comparable with the exciton Bohr radius. QC is a size-effect and consequently leads to size-dependent absorption and emission properties. Thanks to QC, QDs exhibit well-defined electronic levels, which enable molecular-like optically allowed absorption transitions. At the same, high absorption cross-sections and stabilities typical of bulk semiconductor materials are preserved.
In this thesis work an emerging class of colloidal QDs, namely CdSe-CdXZn1-XS core-shell QDs, is investigated. The attention is mainly focused on the optical gain, which represents one of the most inspected and promising applications for QDs. By investigating the Amplified Spontaneous Emission (ASE) of different series of CdSe-CdXZn1-XS heterostructures, this work demonstrates that key properties such as the ASE activation threshold and photo-stability can be optimized by a careful design of the core-shell heterostructure. Guidelines for the synthesis of such best performing optical gain QDs are drawn by means of optical spectroscopy, which provides insights into the correlation between the excitation and relaxation dynamics with the shell thickness, composition and, ultimately, the structure.
Basic parameters such as QD dimensions, size dispersion and photoluminescence quantum yield (QY) can be easily extracted from steady-state absorption and emission spectra. Steady-state absorption and phtoluminescence studies on CdSe-CdXZn1-XS QDs were employed as preliminary tools to prove that different shell materials induce distinct exciton confinement, size dispersion and QY.
In a second step, Surface Enhanced Raman Scattering (SERS) technique has been employed, for the first time, as a local probe for the study of the core-shell interfaces. SERS permits the analysis of the nanocrystals with the same structural features and lattice dynamics present when the QDs are employed as emitters in photonic devices. Results of this study revealed that the composition of the CdXZn1-XS shell entails a significant structural difference at the core-shell interface. This structural difference modifies the electronic structure within the QDs, since it directly tailors the QC of the electrons and holes.
The effect of the core-shell interface on optical properties has been unambiguously detected with the use of transient optical spectroscopy. In this thesis work, transient absorption (TA) and transient PL (tPL) techniques were employed to probe the exciton generation and recombination dynamics. The evolution of the exciton population was compared with kinetic models. Differently from steady-state techniques, transient techniques are sensitive to the nature and time-scales of the different radiative and non-radiative relaxation paths, whose control is crucial for guiding the heterostructure engineering process. The kinetic rates obtained revealed a clear dependence on the core-shell interface and the correlation with SERS results is discussed.
The correlation between structure and dynamics was detected from the nanosecond (tPL analysis) down to the sub-nanosecond time scales (TA analysis). A secondary mission of this thesis was also to find a global interpretation of the dynamics of all signals present in TA spectra of the different CdSe-CdXZn1-XS QD series. Pump fluence, shell thickness and composition are the coordinates along which the global analysis has been developed. This step is of pivotal importance in order to identify the mechanisms involved in the optical gain process, whose temporal evolution for QDs systems spans from the picosecond to the few nanosecond time-scale.
From the discussion of the results obtained from the different characterization techniques, it emerges that the most efficient way to boost the optical properties of CdSe QDs is the realization of a “graded” CdXZn1-XS shell, with Zn concentration (and confinement potential) gradually increasing along the radial direction. In a single entity, this solution should provide suitable confinement of the charge-carriers from the defective outer surface, prevent defect formation at the core-shell interface due the mismatch between the different materials and, eventually, limit the dot dimensions. Low QD dimensions increase the packing density and limits the scattering losses when QDs are included in a thin film and/or in a solid-state matrix. Such aspects have to be taken into serious consideration in order to increase the performances of a QD-based optical amplifier.
Finally, the validity of the hypothesis formulated is experimentally verified by characterizing the bi-exciton radiative recombination, which represents the photo-physical origin of ASE and thus defines the optical gain performances of differently engineered nano-heterostructures. As predicted, best optical gain performances have been achieved from ASE experiments by using CdSe QDs covered with a graded CdS-Cd0.5Zn0.5S-ZnS shell. Therefore, the results obtained from the spectroscopic characterization provide a guideline for the engineering of new synthetic approaches, addressed to the preparation of highly stable core-shell QDs with minimal optical gain activation threshold. Moreover, the rationalization of the dynamics involved in exciton and multi-exciton generation and recombination in core-shell QDs expedites their application in all types of light emitting devices
Bridging Energetics and Dynamics of Exciton Trapping in Core-Shell Quantum Dots
Full text linkThe widespread application of quantum dots greatly profits from their broad absorption band. However, the variable nature of excitations within these bands is expected to result in undesired excitation energy dependence of steady state emission properties. We demonstrate the different role played by hot and cold carrier trapping in determining fluorescence quantum yields. Our analysis relates the energetic parameters with the available knowledge on the dynamics of charge trapping. It turns out that detrapping processes play a pivotal role in determining steady state emission properties. We studied excitation dependent photoluminescence quantum yields (PLQY) in different CdSe/CdxZn1–xS (x = 0, 0.5, and 1) quantum dots to identify best performing heterostructures in terms of shell thickness and composition. Our rationalization of the observed behavior is focused on the modulation of trapping and detrapping rates. The combination of experimental results and PLQY kinetics modeling reveals the need to consider hot-carrier trapping, supporting recent dynamics observations. This work provides a deeper insight into the trapping process in quantum dots, relating its energetics and dynamics
Role of Core-Shell Interfaces on Exciton Recombination in CdSe-CdxZn1-xS Quantum Dots
No full textThe shell thickness and composition of CdSe-Cd(x)Zn(1-x)S core-shell quantum dots (QDs) are defining parameters for the efficiency of such materials as light emitters. In this work we present a detailed study into the optical absorption and fluorescence properties of CdSe-CdS, CdSe-Cd0.5Zn0.5S, and CdSe-ZnS QDs as a function of shell thickness. Moreover, the single-exciton recombination dynamics of these systems are analyzed by means of a time-correlated single-photon counting technique and directly related to the specific core-shell interfaces of the various QDs studied using a phenomenological kinetic model. The findings from this model highlight the strong role of the core-shell interface on both steady state photoluminescence and exciton recombination dynamics in these systems
Investigation into the Heterostructure Interface of CdSe-Based Core–Shell Quantum Dots Using Surface-Enhanced Raman Spectroscopy
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Influence of core-shell interfaces on exciton and multi-exciton dynamics of CdSe-CdxZn1-xS quantum dots. Nanophotonic Materials XI
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