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Minotto, Alessandro (2014) Characterization of CdSe-CdxZn1-xS core-shell QDs as active materials for compact micro-cavity lasers. [Ph.D. thesis]

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Abstract (english)

Innovation 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.

Abstract (italian)

Il progresso in svariati settori tecnologici, a partire dai dispositivi emettitori di luce, passando per le aree delle telecomunicazioni e delle energie rinnovabili, fino alla diagnostica medica e alla terapia, è favorito dalla ricerca e dallo sviluppo nel campo della nanofotonica. Tra le diverse classi di nanomateriali che stanno contribuendo a questo avanzamento, i nanocristalli di materiale semiconduttore, alias Quantum Dots (QDs) o Punti Quantici, presentano le proprietà ottiche più versatili.
I QDs sono nanostrutture inorganiche di materiale semiconduttore le cui eccezionali prestazioni in termini di emissione di luce li rendono diretti concorrenti dei materiali a stato solido più "convenzionali" in molte applicazioni commerciali. L'interesse a sviluppare dispositivi basati su QDs si è diffuso su larga scala con lo sviluppo di metodi di sintesi di tipo colloidale. L'approccio colloidale facilita la processabilità e l'integrazione in dispositivi emittitori di luce con dimensioni che vanno dal micron a pochi nanometri. In particolare, i QDs colloidali si prestano alla realizzazione di sorgenti laser a stato solido compatte e su substrati flessibili.
Le proprietà ottiche dei QDs sono regolate dal confinamento quantistico (QC). Questo regime si instaura quando la dimensioni del materiale sono comparabili con il raggio eccitonico di Bohr. Il QC, in quanto effetto di taglia, rende le proprietà di assorbimento e di emissione di luce dipendenti dalle dimensioni. Grazie al QC, i QDs possiedono livelli elettronici ben definiti e interagiscono con la luce in maniera simile ai sistemi molecolari. Allo stesso tempo, i QDs dimostrano elevate sezioni d’urto di assorbimento e stabilità al danneggiamento, proprietà tipiche dei semiconduttori inorganici.
Questo lavoro di tesi è incentrato su una classe emergente di QDs colloidali, ossia QDs “core-shell” composti da CdSe-CdXZn1-XS, aventi cioè un nucleo (“core”) di CdSe, ricoperto da un guscio (“shell”) di CdXZn1-XS. L'attenzione è focalizzata principalmente sulle proprietà di guadagno ottico il quale rappresenta per i QDs una delle applicazioni più promettenti e maggiormente studiate. Attraverso la caratterizzazione dell'Emissione Spontanea Amplificata (ASE) di diverse serie di QDs di CdSe-CdXZn1-XS, questo lavoro dimostra che proprietà chiave come la soglia di attivazione ASE, nonché la stabilità all’irragiamento, possono essere ottimizzate mediante un’attenta progettazione dell’eterostruttura core-shell. Mediante diverse tecniche di spettrocopia ottica è possibile ricavare alcune linee guida per la sintesi di QDs con proprietà di guadagno ottico ottimali. Con queste tecniche è quindi possibile identificare la correlazione tra le dinamiche di eccitazione/rilassamento e la composizione, spessore e, in ultima analisi, struttura del materiale di shell.
Parametri di base come le dimensioni medie dei QDs, la dispersione di taglia e la resa quantica di luminescenza (QY) possono essere facilmente estratti dalle tecniche di assorbimento ed emissione in stato stazionario. Queste ultime sono state impiegate come strumenti preliminari per dimostrare che, variando la composizione e lo spessore del guscio esterno di CdXZn1-XS, si altera il grado di confinamento degli eccitoni nel nucleo di CdSe, la dispersione in dimensioni e la QY.
In una seconda fase, la tecnica SERS (Surface-Enhanced Raman Scattering o Scattering Raman amplificato da superfici) è stata impiegata per la prima volta come sonda locale per lo studio dell’interfaccia tra core e shell. La tecnica SERS permette la caratterizzazione dei nanocristalli nelle stesse condizioni strutturali e di dinamica reticolare presenti nei QDs quando impiegati come mezzi attivi in dispositivi fotonici. I risultati di questo studio hanno rivelato che la composizione del guscio di CdXZn1-XS comporta delle significative differenze strutturali all'interfaccia core-shell. Questa variazione strutturale modifica la struttura elettronica nei QDs in quanto influenza il grado di confinamento degli elettroni e delle lacune nel core.
L'effetto dell'interfaccia core-shell sulle proprietà ottiche è stato inequivocabilmente rilevato mediante l'uso di tecniche di spettroscopia ottica transiente. In particolare, in questo lavoro di tesi sono stati studiati sia l’assorbimento transiente (TA) sia la luminescenza transiente (tPL) ai fini di esaminare le dinamiche di generazione e di ricombinazione degli eccitoni. L’evoluzione della densità eccitonica è stata quindi confrontata con dei modelli cinetici. A differenza delle tecniche a regime stazionario, le tecniche transienti sono sensibili alla natura e ai tempi caratteristici relativi ai diversi percorsi di rilassamento, radiativi e non radiativi, il cui controllo è fondamentale ai fini dell’ingegnerizzazione dell’eterostruttura. I parametri cinetici ottenuti hanno rivelato una chiara dipendenza dall'interfaccia core-shell e la correlazione con i risultati ottenuti mediante SERS sono stati discussi.
La correlazione tra struttura e dinamica è stata rilevata a partire dalla scala temporale del nanosecondo (tPL) fino alla scala dei picosecondi (TA). Uno scopo secondario di questa tesi è anche quello di elaborare un'interpretazione globale delle dinamiche di tutti i segnali presenti negli spettri transienti per diverse serie di QDs CdSe-CdXZn1-XS. La densità di eccitazione, lo spessore del guscio e la sua composizione sono le coordinate lungo le quali si è sviluppata tale analisi globale. Questo passo è di cruciale importanza ai fini di identificare i parametri legati al processo di guadagno ottico, i cui tempi caratteristici in sistemi a base di QDs variano dai picosecondi fino a pochi nanosecondi.
Dalla discussione dei risultati ottenuti dalle diverse tecniche di caratterizzazione, emerge che il modo più efficace per aumentare le proprietà ottiche dei QDs di CdSe è la realizzazione di un guscio CdXZn1-XS a composizione graduale, in cui la concentrazione di Zn (e di conseguenza il potenziale di confinamento) aumenta gradualmente lungo la direzione radiale. In una sola entità, questa soluzione è in grado di fornire un adeguato confinamento dei portatori di carica dalla superficie esterna, limitare la formazione di difetti all'interfaccia e infine ridurre le dimensioni globali dei QDs. La minimizzazione delle dimensioni permette di aumentare la densità d’impaccamento e limita le perdite dovute allo scattering quando i QDs sono inclusi in una matrice solida e/o depositati come film sottile. Tali aspetti sono di fondamentale importanza ai fini di migliorare l’efficienza di un amplificatore ottico a quantum dots.
Infine, la validità delle ipotesi formulate è stata verificata sperimentalmente caratterizzando la ricombinazione radiativa bi-eccitonica, la quale rappresenta l’origine fotofisica dell’ASE e quindi definisce le prestazioni di guadagno ottico delle diverse nano-eterostrutture opportunamente ingegnerizzate. Come previsto, dalle misure di ASE le migliori performance dal punto di vista del guadagno ottico sono state raggiunte utilizzando QDs di CdSe ricoperti con uno shell a composizione graduata di CdS-Cd0.5Zn0.5S-ZnS. I risultati ottenuti mediante la caratterizzazione spettroscopica forniscono dunque una linea guida per la progettazione di nuove strategie di sintesi che siano orientate alla preparazione di QDs altamente foto-stabili e con una soglia di attivazione ASE minimale. In aggiunta, la razionalizzazione delle dinamiche coinvolte nella generazione e ricombinazione eccitonica e multi-eccitonica in QDs core-shell può accelerare la loro applicazione in tutti i tipi di dispositivi emettitori di luce.

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EPrint type:Ph.D. thesis
Tutor:Signorini, Raffaella
Ph.D. course:Ciclo 27 > scuole 27 > SCIENZA ED INGEGNERIA DEI MATERIALI
Data di deposito della tesi:23 December 2014
Anno di Pubblicazione:14 December 2014
Key Words:punto quantico/quantum dot, spettroscopia/spectroscopy, interfaccia/interface, laser/laser
Settori scientifico-disciplinari MIUR:Area 03 - Scienze chimiche > CHIM/02 Chimica fisica
Area 09 - Ingegneria industriale e dell'informazione > ING-IND/22 Scienza e tecnologia dei materiali
Struttura di riferimento:Dipartimenti > Dipartimento di Scienze Chimiche
Codice ID:7396
Depositato il:04 Dec 2015 12:59
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