Plasmonics, the study of resonant interaction of electromagnetic waves with free electrons in condensed matter is an expanding research field. Metals such as gold and silver have demonstrated to be excellent choices for UV-Vis operation range. However, these metals have high optical losses approaching the infrared (IR) range. Therefore, Transparent Conductive Oxides (TCOs) have attracted increasing interest for plasmonic application in the IR. A promising candidate is Ta:TiO2 (TaTO), displaying high carrier density and good chemical stability. Metamaterials (artificial structured materials) exploit plasmonic effects to generate properties not observed in nature. The main interest lies in the possibility to obtain negative refraction, hence overcoming the diffraction limit of conventional lenses. Hyperbolic metamaterials (HMMs) are a sub-class that have gained particular attention due to the simple geometry and ease of fabrication. In fact, they can be obtain with multilayers structure, alternating metallic and dielectric layers or with metallic nanowires array in a dielectric matrix. Many are the example of Au and Ag combined with alumina and titania for the UV-Vis range, but the literature is still scarse for what concern the IR range. This thesis work aims to expand the knowledge about TaTO in the light of developing future plasmonic multilayers metamaterials. Multilayers were deposited by the pulsed laser deposition (PLD) method. PLD allows to deposit conductive and non-conductive layers using this material just via changing the deposition pressure. Exploiting this feature, a single step deposition can be performed without opening the vacuum chamber, drastically reducing the operating time. A preliminary study was conducted on the single components that were later used in the multilayers. Thin films (20 nm, 10 nm, 5 nm) of Ta(10%):TiO2 were deposited at 1 Pa in pure O2 on glass substrate in order to evaluate their crystallization and electrical properties. Later, these three films were deposited on polycrystalline stoichiometric titania substrate. Such substrate helped the crystallization process thus ensuring improved electrical properties. Then, the effects of higher deposition pressure were analyzed. Ta(10%):TiO2 films deposited at 3 Pa and 6 Pa in pure O2 resulted both non-conductive, but the former crystallized whereas the latter did not. Higher deposition pressures were explored on Ta(5%):TiO2 as well. Film at 6 Pa in pure O2 resulted amorphous and non-conductive as for Ta(10%):TiO2. Film deposited at 3 Pa in mixed background atmosphere (Ar:O2=5:1) turned out conductive and crystalline. Successively, several multilayers were deposited. Metallic layers were obtained depositing TaTO at 1 Pa in pure O2, whereas non-conductive ones were deposited at 3 Pa or 6 Pa in pure O2. Multilayers with both 5% and 10% Ta concentration were synthesized. Different single layer thicknesses were explored in order to observe differences in the electrical and optical properties. Lastly, an attempt to create a fully conductive metamaterials was tried. Ta(5%):TiO2 layers deposited at 1 Pa in pure O2 were alternated with layers at 3 Pa in mixed atmosphere (Ar:O2=5:1). Unfortunately, the result was not satisfying. This thesis work is an preliminary and exploratory study on the synthesis, the structural, morphological, electrical and optical properties of these multilayers in the Vis-NIR range. In the future, ellipsometric analysis should be performed in order to assess the hyperbolic regime, if present, of these multilayers. Moreover, the geometry dependence on the optical and electrical properties was not completely explored. The effect of the layers number has to be investigated too. Furhtermore, the combination of TaTO with ceTiO2 could be invastigated, as well as the combination of TCOs and metals for a fully conductive multilayers with hyperbolic dispersion in the Vis-NIR.
La plasmonica, lo studio dell'interazione risonante delle onde elettromagnetiche con gli elettroni liberi nella materia è un campo di ricerca in espansione. Metalli come oro e argento si sono dimostrati scelte eccellenti per il range UV-Vis. Tuttavia, questi metalli soffrono di grandi dissipazioni ottiche approcciando l'infrarosso (IR). Perciò, gli ossidi trasparenti conduttivi (TCO) hanno attratto un interesse crescente per applicazioni plasmoniche nell'IR. Un promettente candidato è Ta:TiO2 (TaTO) che possiede un'alta densità di portatori e buona stabilità chimica. I metamateriali (materiali artificialmente strutturati) sfruttano effetti plasmonici per generare proprietà non osservate in natura. L'interesse principale risiede nella possibilità di ottenere rifrazione negativa, quindi superare il limite di diffrazione delle lenti convenzionali. I metamateriali iperbolici (HMM) sono una sotto classe che ha ottenuto particolare attenzione a causa della loro geometria semplice e della facilità di fabbricazione. Infatti, possono essere ottenuti con una struttura a multilayer, alternando layer metallici e dielettrici, oppure con una disposizione ordinata di nanofili metallici in una matrice dielettrica. Molti sono gli esempi di Au o Ag combinati con alluminia o titania per il range UV-Vis, ma la letteratura è ancora scarsa per quanto riguarda l'IR. Questo lavoro di tesi punta a espandere la conoscenza del TaTO per uno sviluppo futuro di multilayer come metamateriale plasmonico. I multilayers sono stati depositati attraverso l'ablazione a laser pulsato (PLD). La PLD permette di depositare layer conduttivi e non di questo materiale, cambiando semplicemente la pressione di deposizione. Sfruttando questa caratteristica, è possibile depositare multilayer in un single-step, senza aprire la camera a vuoto, riducendo enormemente i tempi di operazione. Uno studio preliminare è stato condotto sulle singole componenti che sarebbero state poi utilizzate nel multilayer. Film sottili (20 nm, 10 nm, 5 nm) di Ta(10%):TiO2 sono stati depositati a 1 Pa in puro O2 su un substrato di vetro, per valutare il loro grado cristallino e le loro proprietà elettriche. In seguito, gli stessi tre film sono stati depositati su un substrato policristallino di titania stechiometrica. Questo substrato ha aiutato il processo di cristallizzazione migliorando le proprietà elettriche. Successivamente, l'effetto di pressioni di deposizioni più alte è stato analizzato. Film di Ta(10%):TiO2 depositati a 3 Pa e 6 Pa in puro O2 sono risultati non conduttivi, ma quello a 3 Pa ha cristallizzato mentre quello a 6 Pa no. Pressioni di deposizioni maggiori sono state esplorate anche per Ta(5%):TiO2. Film a 6 Pa in puro O2 sono risultati amorfi e non conduttivi come per Ta(10%):TiO2. Film depositati a 3 Pa in un atmosfera mista (Ar:O2=5:1) si sono rivelati conduttivi e cristallini. Successivamente, diversi multilayers sono stati prodotti. I layer metallici sono stati ottenuti depositando TaTO a 1 Pa in puro O2, mentre quelli non conduttivi sono stati depositati a 3 Pa o 6 Pa in puro O2. I multilayers sono stati prodotti con una concentrazione di Ta pari al 5% e al 10%. Diversi spessori dei singoli sono stati sperimentati in modo da osservare differenze nelle properietà elettriche e ottiche. Infine, è stata provata la realizzazione di un multilayer interamente conduttivo. Layer di Ta(5%):TiO2 depositati a 1 Pa in puro O2 sono stati alternati a layer depositati a 3 Pa in atmosfera mista (Ar:O2=5:1). Purtroppo, il risultato ottenuto non è stato soddisfacente. Questo lavoro di tesi è uno studio preliminare ed esplorativo sulla sintesi e le proprietà strutturali, morfologiche, elettriche ed ottiche di questi multilayer nel range Vis-NIR. Come prospettiva, un'analisi ellissometrica dovrebbe essere fatta in modo da scoprire il regime iperbolico, se presente, di questi multilayer. In più, la dipendenza geometrica sulle proprietà ottiche ed elettriche non è stata esplorata completamente. Gli effetti del numero di layer non sono stati investigati. Inoltre, anche la combinazione di TaTO con TiO2 potrebbe essere investigata come le combinazioni di altri TCO con metalli per un multilayer totalmente conduttivo con una dispersione iperbolica nel Vis-NIR.
Development of Ta-TiO2 plasmonic multilayer metamaterials
Stasi, Lorenzo
2019/2020
Abstract
Plasmonics, the study of resonant interaction of electromagnetic waves with free electrons in condensed matter is an expanding research field. Metals such as gold and silver have demonstrated to be excellent choices for UV-Vis operation range. However, these metals have high optical losses approaching the infrared (IR) range. Therefore, Transparent Conductive Oxides (TCOs) have attracted increasing interest for plasmonic application in the IR. A promising candidate is Ta:TiO2 (TaTO), displaying high carrier density and good chemical stability. Metamaterials (artificial structured materials) exploit plasmonic effects to generate properties not observed in nature. The main interest lies in the possibility to obtain negative refraction, hence overcoming the diffraction limit of conventional lenses. Hyperbolic metamaterials (HMMs) are a sub-class that have gained particular attention due to the simple geometry and ease of fabrication. In fact, they can be obtain with multilayers structure, alternating metallic and dielectric layers or with metallic nanowires array in a dielectric matrix. Many are the example of Au and Ag combined with alumina and titania for the UV-Vis range, but the literature is still scarse for what concern the IR range. This thesis work aims to expand the knowledge about TaTO in the light of developing future plasmonic multilayers metamaterials. Multilayers were deposited by the pulsed laser deposition (PLD) method. PLD allows to deposit conductive and non-conductive layers using this material just via changing the deposition pressure. Exploiting this feature, a single step deposition can be performed without opening the vacuum chamber, drastically reducing the operating time. A preliminary study was conducted on the single components that were later used in the multilayers. Thin films (20 nm, 10 nm, 5 nm) of Ta(10%):TiO2 were deposited at 1 Pa in pure O2 on glass substrate in order to evaluate their crystallization and electrical properties. Later, these three films were deposited on polycrystalline stoichiometric titania substrate. Such substrate helped the crystallization process thus ensuring improved electrical properties. Then, the effects of higher deposition pressure were analyzed. Ta(10%):TiO2 films deposited at 3 Pa and 6 Pa in pure O2 resulted both non-conductive, but the former crystallized whereas the latter did not. Higher deposition pressures were explored on Ta(5%):TiO2 as well. Film at 6 Pa in pure O2 resulted amorphous and non-conductive as for Ta(10%):TiO2. Film deposited at 3 Pa in mixed background atmosphere (Ar:O2=5:1) turned out conductive and crystalline. Successively, several multilayers were deposited. Metallic layers were obtained depositing TaTO at 1 Pa in pure O2, whereas non-conductive ones were deposited at 3 Pa or 6 Pa in pure O2. Multilayers with both 5% and 10% Ta concentration were synthesized. Different single layer thicknesses were explored in order to observe differences in the electrical and optical properties. Lastly, an attempt to create a fully conductive metamaterials was tried. Ta(5%):TiO2 layers deposited at 1 Pa in pure O2 were alternated with layers at 3 Pa in mixed atmosphere (Ar:O2=5:1). Unfortunately, the result was not satisfying. This thesis work is an preliminary and exploratory study on the synthesis, the structural, morphological, electrical and optical properties of these multilayers in the Vis-NIR range. In the future, ellipsometric analysis should be performed in order to assess the hyperbolic regime, if present, of these multilayers. Moreover, the geometry dependence on the optical and electrical properties was not completely explored. The effect of the layers number has to be investigated too. Furhtermore, the combination of TaTO with ceTiO2 could be invastigated, as well as the combination of TCOs and metals for a fully conductive multilayers with hyperbolic dispersion in the Vis-NIR.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/170547