Semiconductor colloidal nanocrystals (NCs) are solution-processable materials that have focused scientific and technological attention thanks to their tunable optical and electrical properties. Colloidal NCs have indeed wide applicative perspectives that span from light-emitting diodes, to lasers, from solar cells to luminescent solar concentrators, from bioimaging to quantum information. Such a large range of potential NCs technologies is warranted by the unique knowledge and control that has been achieved over the years about their electronic properties. Specifically, the optical and electric properties of these nanomaterials have been tuned by either controlling their size, composition and shape, or producing multicomponent heterostructures and introducing few atoms of a different chemical element, i.e. doping the NCs. Because of the vast gamut of possibilities that colloidal NCs offer, many questions on the elusive charge carrier dynamics underlying the macroscopic observations are still unanswered. In this picture, my work points toward three different sub-classes of NCs: i) interface engineered NCs; ii) doped NCs and iii) ‘electronic’ doped NCs. After a brief review about the ‘state of the art’ of the colloidal NC science (Chap. 1), in Chap. 2 I show a detailed investigation on the interaction between the photoexcited charge carriers and the engineered interface of Dot-in-Bulk core/shell NC, which are featured by radiative recombination from both the core and shell states. I demonstrate that their uncommon dual emission is due to the peculiar interface structure between the compositional domains and that a fine tuning of the optical properties can be also achieved by modifying the interfacial potential profile. In Chap. 3, I propose a novel synthetic approach to overcome the intrinsic Poisson distribution characteristic of the up-to-date NC doping strategies that are based on stochastic distribution of impurity ions in the NC ensemble. To this aim, I use monodispersed metal cluster as seeds for the NC nucleation in the synthesis reaction flask. By mean of combined optical and elemental analysis, I show that the copper clusters composed of exactly four atoms are indeed embedded in the semiconductor matrix, giving monodispersed doped NCs. Semiconductor doping can be further distinguished in ‘isovalent’ doping, in which the impurity has the same oxidation state of the host compound, and ‘electronic’ doping, given by ions which introduce a net charge in the surrounding matrix. The most known ‘isovalent’ dopant for II-VI NCs is Mn2+. Its d5 configuration is featured by unique magnetic properties that, in quantum confined nanomaterials lead to the formation of magnetic polarons. In Chap. 4, I reveal how polaron formation affects the exciton energy by mean of resonant PL measurements, offering a precise estimation of the intensity of the internal magnetic field generated by the Mn2+ spins. In Chap. 5, I report how the magnetic response typical of Mn2+ is reproduced by introducing silver, which is an electronic dopant for II-VI semiconductors, since it can only assume the +1 oxidation state. However, it introduces an electronic level in the forbidden energy gap of the host semiconductor that participates to the radiative recombination and therefore transiently switches to the paramagnetic +2 state. By mean of magnetic circular dichroism experiments I demonstrate that in NCs doped with nonmagnetic silver dopants, the paramagnetic response is completely optically activated. Finally, in Chap. 6 I focused the attention on non toxic, ternary CuInS2 colloidal NCs. The photophysical processes underlying their emission mechanism are, however, still under debate. To address this gap, I carried out temperature-controlled photoluminescence and spectro-electrochemical experiments to unravel the intrinsic and extrinsic charge carrier dynamics of this last-generation class of colloidal N

I nanocristalli colloidali a semiconduttore (NC) sono materiali processabili da soluzione che, dalla loro scoperta 30 anni fa, hanno attirato l’attenzione in campo scientifico e tecnologico per le loro proprietà ottiche ed elettriche. Infatti, i NC hanno un ampio range di potenziali applicazioni, che vanno dalle sorgenti luminose, alle celle solari, al bioimaging fino all’informazione quantistica. Ciò è dovuto alla profonda conoscenza e controllo delle loro proprietà elettroniche che si è raggiunto. Infatti, queste ultime si possono modificare controllando la dimensione, la composizione ma anche formando eterostrutture o introducendo impurezze, cioè drogando i NC. A causa dell’ampia varietà di NC che si possono sintetizzare, molti dubbi sui processi fotofisici sottostanti le proprietà ottiche macroscopiche rimangono ancora irrisolti. Dunque, mi sono focalizzato sullo studio di tre sotto-classi di NC: 1) a interfaccia ingegnerizzata; 2) drogati e 3) drogati elettronicamente. Dopo un breve ‘stato dell’arte’ della scienza dei NC colloidali (Capitolo 1), nel secondo Capitolo riporto una studio dettagliato dell’interazione fra i portatori di carica eccitati e l’interfaccia ingegnerizzata dei Dot-in-Bulk core/shell NC, che sono caratterizzati da emissione di fotoluminescenza (PL) sia dagli stati di core che da quelli di shell. Tramite misure di PL ultraveloce, dimostro che la caratteristica struttura all’interfaccia è la motivazione ultima da cui scaturisce la capacità di avere una doppia emissione radiativa, aggiungendo un ulteriore parametro nella chimica dei NC con il quale è possibile modificare le loro proprietà ottiche. Nel Capitolo 3, propongo una nuova strategia di sintesi che permetta di avere NC contenenti tutti un esatto numero di atomi droganti, evitando la distribuzione Poissoniana tipica dei contemporanei metodi di drogaggio. A questo scopo, uso cluster metallici monodispersi come semi di nucleazione per la sintesi dei NC e tramite analisi elementali ed ottiche mostro che effettivamente ogni NC sintetizzato contiene un solo cluster metallico e quindi un numero preciso di impurezze. Il drogaggio può essere considerato ‘isovalente’ nel caso in cui l’impurezza abbia lo stesso stato di ossidazione del semiconduttore, o ‘elettronico’ nel caso questa introduca una carica netta nella matrice ospitante. Il drogante isovalente più noto per i NC II-VI è il Mn2+. La sua configurazione elettronica d5 è caratterizzata da proprietà magnetiche uniche che, in strutture confinate quanticamente porta alla formazione di polaroni. Nel Capitolo 4, mostro come la formazione di polaroni tocca l’energia degli eccitoni tramite misure di PL risonante, ottenendo anche una stima precisa dell’intensità di campo magnetico generata solo dagli ioni Mn2+. Nel Capitolo 5, mostro come la risposta magnetica tipica del Mn2+ si può ottenere anche con l’argento, che è un drogante elettronico in quanto può assumere solo lo stato di ossidazione +1. L’argento però introduce uno stato nel gap energetico del semiconduttore ospitante che partecipa alla ricombinazione radiativa diventando, in modo transiente, un Ag2+ paramagnetico. Tramite misure di dicroismo circolare magnetico, dimostro che NC drogati con impurezze non magnetiche di argento possono assumere comportamenti paramagnetici attivati otticamente. Infine, nel Capitolo 6 ho focalizzato l’attenzione sui NC non tossici di CuInS2. I processi fotofisici alla base del meccanismo di emissione sono ancora dibattuti. A questo scopo, ho eseguito misure di PL risolta in temperatura e di spettroelettrochimica per studiare le dinamiche intrinseche ed estrinseche di questa classe di NC colloidali di ultima generazione.

(2018). Advanced Spectroscopy of Interface Engineered, Doped and “Electronically” Doped Colloidal Semiconductor Nanocrystals. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2018).

Advanced Spectroscopy of Interface Engineered, Doped and “Electronically” Doped Colloidal Semiconductor Nanocrystals

PINCHETTI, VALERIO
2018

Abstract

Semiconductor colloidal nanocrystals (NCs) are solution-processable materials that have focused scientific and technological attention thanks to their tunable optical and electrical properties. Colloidal NCs have indeed wide applicative perspectives that span from light-emitting diodes, to lasers, from solar cells to luminescent solar concentrators, from bioimaging to quantum information. Such a large range of potential NCs technologies is warranted by the unique knowledge and control that has been achieved over the years about their electronic properties. Specifically, the optical and electric properties of these nanomaterials have been tuned by either controlling their size, composition and shape, or producing multicomponent heterostructures and introducing few atoms of a different chemical element, i.e. doping the NCs. Because of the vast gamut of possibilities that colloidal NCs offer, many questions on the elusive charge carrier dynamics underlying the macroscopic observations are still unanswered. In this picture, my work points toward three different sub-classes of NCs: i) interface engineered NCs; ii) doped NCs and iii) ‘electronic’ doped NCs. After a brief review about the ‘state of the art’ of the colloidal NC science (Chap. 1), in Chap. 2 I show a detailed investigation on the interaction between the photoexcited charge carriers and the engineered interface of Dot-in-Bulk core/shell NC, which are featured by radiative recombination from both the core and shell states. I demonstrate that their uncommon dual emission is due to the peculiar interface structure between the compositional domains and that a fine tuning of the optical properties can be also achieved by modifying the interfacial potential profile. In Chap. 3, I propose a novel synthetic approach to overcome the intrinsic Poisson distribution characteristic of the up-to-date NC doping strategies that are based on stochastic distribution of impurity ions in the NC ensemble. To this aim, I use monodispersed metal cluster as seeds for the NC nucleation in the synthesis reaction flask. By mean of combined optical and elemental analysis, I show that the copper clusters composed of exactly four atoms are indeed embedded in the semiconductor matrix, giving monodispersed doped NCs. Semiconductor doping can be further distinguished in ‘isovalent’ doping, in which the impurity has the same oxidation state of the host compound, and ‘electronic’ doping, given by ions which introduce a net charge in the surrounding matrix. The most known ‘isovalent’ dopant for II-VI NCs is Mn2+. Its d5 configuration is featured by unique magnetic properties that, in quantum confined nanomaterials lead to the formation of magnetic polarons. In Chap. 4, I reveal how polaron formation affects the exciton energy by mean of resonant PL measurements, offering a precise estimation of the intensity of the internal magnetic field generated by the Mn2+ spins. In Chap. 5, I report how the magnetic response typical of Mn2+ is reproduced by introducing silver, which is an electronic dopant for II-VI semiconductors, since it can only assume the +1 oxidation state. However, it introduces an electronic level in the forbidden energy gap of the host semiconductor that participates to the radiative recombination and therefore transiently switches to the paramagnetic +2 state. By mean of magnetic circular dichroism experiments I demonstrate that in NCs doped with nonmagnetic silver dopants, the paramagnetic response is completely optically activated. Finally, in Chap. 6 I focused the attention on non toxic, ternary CuInS2 colloidal NCs. The photophysical processes underlying their emission mechanism are, however, still under debate. To address this gap, I carried out temperature-controlled photoluminescence and spectro-electrochemical experiments to unravel the intrinsic and extrinsic charge carrier dynamics of this last-generation class of colloidal N
BROVELLI, SERGIO
Nanocrystals,; Photophysics,; Semiconductor,; Spectroscopy,; Optics
Nanocrystals,; Photophysics,; Semiconductor,; Spectroscopy,; Optics
FIS/01 - FISICA SPERIMENTALE
English
19-mar-2018
SCIENZA E NANOTECNOLOGIA DEI MATERIALI - 79R
30
2016/2017
open
(2018). Advanced Spectroscopy of Interface Engineered, Doped and “Electronically” Doped Colloidal Semiconductor Nanocrystals. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2018).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/199097
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