Nanostructured materials are of upmost importance among advanced technologies thanks to their unique properties, which depends strongly on crystal size. Being able to tailor and engineer such materials, so that new devices with superior properties can be obtained, is a hot topic at the present. Functional nanocomposites are a brand new class of materials which possess features that traditional materials can't present; such features, for example opto-electronic properties, strongly depend on the nanoscale dimension of the system. Mixing different types of materials, i.e. creating a nanocomposite based on metallic and/or semiconductor nanocrystals, open the path to develop new devices for biomedics, energy application and environmental sensors, among others. Semiconductor nanoparticles (NPs) are obtained in several ways; we focused on the colloidal synthesis since it is a cheap, easy to set-up, environment-friendly technique that can lead to valuable results in terms of reliability and repeatability. The kinetics was one of the main factors to be fully understood to be able to control the process, aiming at reaching a certain NPs final dimension and distribution. A first study focused indicated how a higher concentration of oleic acid, the ligands for Cd atoms, causes a slower NPs growth rate, due to the increase of system viscosity. A modification of the classical homogeneous nucleation theory (CNT) has been developed to take in consideration the decrease of monomer concentration during crystal formation and growth. The new free energy curve, now, shows not only the typical maximum associated to nuclei critical size, but also an absolute minimum relative to the mean crystal size at equilibrium between solidus and liquidus (due to the solubility of the system). The trends of crystal size as function of OA concentration estimated using the modified CNT are in perfect agreement with experimental data. A novel strategy aimed to the extension of the validity of a recently proposed technique to create core-shell structure has been investigated: the colloidal atomic layer deposition. The goal was to apply such technique, applied only to CdSe-CdS core-shell structures, to metallic systems and investigate different shell compositions and crystal shapes. Based on the results of c-ALD, NCs phase transfer was performed changing the organic solvent (hexane) with a polar one (n-methylformamide) and stabilizing the nanoparticles with a surface layer of sodium sulfide. Once NCs are completely stripped out of organic ligands, but keep preserving stability and their emission properties, the solution has been drop casted on 0.2x0.2 mm2 microscope glass substrate and sealed in a vacuum chamber overnight for the evaporation of the solvent. A kinetic study has been performed on the films, investigating the effect of annealing time and temperature on the photoluminescence spectrum of samples. When excited with a wavelength (633 nm) larger than the gap of the barrier material (CdS, Egap=2.4 eV), annealed films presented a weak emission peak at 520 nm, associated to the band-edge emission of the shell. We proved that the peak at 520 nm is not a Raman peak, since there is no evidence of its Stokes counterpart located at 3400 cm-1, and it is no anti-Stokes PL. In this latter case, a multi-photon absorption would be requested, and the emission intensity shall present a non-linear dependence with the laser power. In our samples, on the contrary, a linear behavior is observed, indicating the absorption of one single photon per transition.

I materiali nanostrutturati ricoprono un ruolo di estrema prominenza nel panorama delle tecnologie avanzate grazie alle loro proprietà funzionali, le quali dipendono fortemente dalla dimensione dei cristalli. Ne consegue che l’ingegnerizzazione di tali materiali sia un argomento di primario interesse, al fine di poter ottenere nuovi dispositivi dalle proprietà superiori. Combinando diverse tipologie di materiali, quali ad esempio materiali metallici e/o materiali semiconduttori, è possibile concepire nuovi dispositivi per applicazioni biomediche, sensoristica ambientale e applicazioni energetiche, tra le varie. Le nanoparticelle di materiale semiconduttore sono ottenute in diversi modi; in questo lavoro verrà presentata la sintesi colloidale poiché è una tecnica economica, facile da configurare, ecologica e può portare a risultati molto interessanti in termini di affidabilità e ripetibilità. La cinetica è uno dei principali fattori a dover essere pienamente compreso per controllare il processo, avendo come obiettivo il raggiungimento di una specifica dimensione e distribuzione dei nuclei. Lo studio della cinetica di sintesi è il motivo per cui è stato investigata l’influenza del rapporto delle concentrazioni dei precursori: si è ottenuta evidenza che una maggiore concentrazione di acido oleico, molecola che si lega agli atomi di cadmio per stabilizzarli, provoca una velocità di crescita delle nanoparticelle più lenta. . Basandosi sui dati cinetici della sintesi di CdSe, è stata sviluppata una modifica della Teoria Classica della Nucleazione omogenea (CNT) nella quale si consideri il consumo di monomero durante la crescita di cristalli. Con questa modifica, nella classica curva di equilibrio il cui massimo rappresenta il raggio critico di nucleazione, appare un nuovo minimo assoluto, il quale rappresenterebbe la dimensione di equilibrio tra fase solida e fase liquida (dovuto al limite di solubilità). A conferma del modello di equilibrio di reazione proposto, risulta che gli andamenti sperimentali del raggio medio delle particelle a fine crescita corrispondono con quelli simulati tramite la modifica della CNT. Una strategia efficace per ottenere strutture complesse core-shell si basa sull’estensione della validità della tecnica colloidale Atomic Layer Deposition (c-ALD), proposta da Ithurria e Talapin ma studiata soltanto per sistemi a base di CdSe-CdS. Sono stati investigati il processo e la sua applicabilità a sistemi metallici (nello specifico core di Au, per il suo alto contrasto al TEM con la shell di CdS) e a base di semiconduttore, ma inserendo nuove configurazioni nella shell (molteplici combinazioni di CdS e ZnS). In base a quanto investigato per la c-ALD, un trasferimento di fase di nanocristalli da soluzione apolare a polare è stato sviluppato. Una volta ottenuta una soluzione stabile che presenti buona fotoluminescenza, i NCs sono stati depositati su substrati di silice e lasciati essiccare in camera a vuoto. È stato svolto uno studio cinetico delle trasformazioni che avvengono durante l’invecchiamento dei campioni in funzione della temperatura e del tempo, monitorando l’evoluzione del picco di fotoluminescenza della struttura. Tuttavia, il risultato più eclatante lo si è osservato eccitando la struttura con un laser a lunghezza d’onda inferiore rispetto l’energia richiesta dal band-gap del materiale barriera (ECdS=2.4 eV). Utilizzando il laser a 633 nm (E=1.96 eV), si è osservato un picco a 520 nm (E=2.38 eV). Sono state eseguite le opportune verifiche, scartando l’ipotesi che si tratti di un picco Raman (non era presente il relativo picco Stokes a 3400 cm-1) e nemmeno fotoluminescenza anti-Stokes, in quanto quest’ultima richiede una dipendenza non lineare con la potenza del laser, mentre il picco osservato ha dipendenza lineare.

Functional heterostructure based on colloidal nanocrystals / Slejko, EMANUELE ALBERTO. - (2018 Mar 08).

Functional heterostructure based on colloidal nanocrystals

SLEJKO, EMANUELE ALBERTO
2018-03-08

Abstract

Nanostructured materials are of upmost importance among advanced technologies thanks to their unique properties, which depends strongly on crystal size. Being able to tailor and engineer such materials, so that new devices with superior properties can be obtained, is a hot topic at the present. Functional nanocomposites are a brand new class of materials which possess features that traditional materials can't present; such features, for example opto-electronic properties, strongly depend on the nanoscale dimension of the system. Mixing different types of materials, i.e. creating a nanocomposite based on metallic and/or semiconductor nanocrystals, open the path to develop new devices for biomedics, energy application and environmental sensors, among others. Semiconductor nanoparticles (NPs) are obtained in several ways; we focused on the colloidal synthesis since it is a cheap, easy to set-up, environment-friendly technique that can lead to valuable results in terms of reliability and repeatability. The kinetics was one of the main factors to be fully understood to be able to control the process, aiming at reaching a certain NPs final dimension and distribution. A first study focused indicated how a higher concentration of oleic acid, the ligands for Cd atoms, causes a slower NPs growth rate, due to the increase of system viscosity. A modification of the classical homogeneous nucleation theory (CNT) has been developed to take in consideration the decrease of monomer concentration during crystal formation and growth. The new free energy curve, now, shows not only the typical maximum associated to nuclei critical size, but also an absolute minimum relative to the mean crystal size at equilibrium between solidus and liquidus (due to the solubility of the system). The trends of crystal size as function of OA concentration estimated using the modified CNT are in perfect agreement with experimental data. A novel strategy aimed to the extension of the validity of a recently proposed technique to create core-shell structure has been investigated: the colloidal atomic layer deposition. The goal was to apply such technique, applied only to CdSe-CdS core-shell structures, to metallic systems and investigate different shell compositions and crystal shapes. Based on the results of c-ALD, NCs phase transfer was performed changing the organic solvent (hexane) with a polar one (n-methylformamide) and stabilizing the nanoparticles with a surface layer of sodium sulfide. Once NCs are completely stripped out of organic ligands, but keep preserving stability and their emission properties, the solution has been drop casted on 0.2x0.2 mm2 microscope glass substrate and sealed in a vacuum chamber overnight for the evaporation of the solvent. A kinetic study has been performed on the films, investigating the effect of annealing time and temperature on the photoluminescence spectrum of samples. When excited with a wavelength (633 nm) larger than the gap of the barrier material (CdS, Egap=2.4 eV), annealed films presented a weak emission peak at 520 nm, associated to the band-edge emission of the shell. We proved that the peak at 520 nm is not a Raman peak, since there is no evidence of its Stokes counterpart located at 3400 cm-1, and it is no anti-Stokes PL. In this latter case, a multi-photon absorption would be requested, and the emission intensity shall present a non-linear dependence with the laser power. In our samples, on the contrary, a linear behavior is observed, indicating the absorption of one single photon per transition.
8-mar-2018
LUGHI, VANNI
30
2016/2017
Settore ING-IND/22 - Scienza e Tecnologia dei Materiali
Università degli Studi di Trieste
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11368/2919633
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