In recent years, the control of the optical properties of materials has aroused great interest from the scientific community. A huge number of new technologies can be obtained by engineering materials that respond to electromagnetic radiation, so a great effort was put into the theoretical and experimental development of these structures. Photonic crystals (PhC) represent a class of materials that are used in a wide range of applications, from optoelectronics to biosensing, thanks to their ability to control the propagation of light. They are periodic structures with high symmetry, in which constituent materials with different refractive indices show a repetition along one, two or three dimensions. The periodicity of the dielectric permittivity (or refractive index) in one or more dimensions generates an interference of the incident and scattered electromagnetic waves, leading to the formation of the photonic band gap (PBG), i.e. a wavelengths band prohibited for photons. This leads to the generation of structural reflection colours, called structural colours, which are not generated by the presence of dyes or pigments but only by the periodicity of the structure; an effect widely present in nature, such as in opals, Morpho butterflies and marble berries. The spectral position of the PBG can be tuned by acting on the refractive index of the constituent materials or on the geometric parameters of the lattice. Therefore, the optical properties are easily modifiable by engineering these parameters, allowing to customize the development of light through the material. In this thesis work, I focused on the fabrication of one-dimensional photonic crystals, also called Bragg stacks or Distributed Bragg Reflectors (DBRs), and on the possibility of tuning their optical properties upon application of external stimuli. An active tuning of the DBR optical properties can take place through various triggers, but in this discussion I focused mainly on the electrical and biological ones. Through the selection of responsive materials featured by particular intrinsic properties, it was possible to actively tune the DBR optical response upon application of external stimuli. In the case of electrochromic 1D PhCs, the plasmonic properties of ITO (indium doped tin oxide) were exploited in order to achieve a device displaying a modulation of the PBG by applying an electric field. On the other hand, for the bacterial sensors, the antimicrobial properties of silver have been integrated into a SiO2/TiO2 multilayer to obtain a colorimetric detection of bacterial contaminants. The results obtained provided a proof of concept of the possibility of developing efficient, low-cost and easy to fabricate sensors. Their versatility, in terms of DBR functionalization and fabrication, makes these devices particularly attractive for integration into technological applications. Bragg stacks can provide complete control over the propagation of light, allowing us to shape their optical response in order to integrate them into daily objects.
Negli ultimi anni il controllo delle proprietà ottiche dei materiali ha suscitato grande interesse da parte della comunità scientifica. Una nuova frontiera tecnologica può essere raggiunta ingegnerizzando materiali stimolabili attraverso radiazioni elettromagnetiche. I cristalli fotonici (PhC) rappresentano una classe di materiali con un'ampia gamma di applicazioni, dall'optoelettronica alla bio-sensoristica, grazie alla loro capacità di modulare la propagazione della luce. Sono strutture periodiche ad alta simmetria, i cui materiali costituenti, aventi diversi indici di rifrazione, mostrano una ripetizione lungo una, due o tre dimensioni. La periodicità della permittività dielettrica (o indice di rifrazione) genera un'interferenza delle onde elettromagnetiche incidenti e diffuse, portando alla formazione del band gap fotonico (PBG), cioè una banda di lunghezze d'onda vietata ai fotoni. Questo porta alla generazione di colori strutturali riflessi, detti colori strutturali, i quali non sono prodotti dalla presenza di coloranti o pigmenti ma solo dalla periodicità della struttura; un effetto ampiamente presente in natura, come negli opali, nelle farfalle Morpho e in alcune bacche. La posizione spettrale del PBG può essere ottimizzata agendo sull'indice di rifrazione dei materiali costituenti o sui parametri geometrici del reticolo. Pertanto, le proprietà ottiche sono facilmente modificabili ingegnerizzando questi parametri, consentendo di adattare la propagazione della luce attraverso il materiale. In questo lavoro di tesi, mi sono concentrata sulla fabbricazione di cristalli fotonici monodimensionali, chiamati anche Bragg stacks o riflettori di Bragg distribuiti (DBR), e sulla possibilità di regolare le loro proprietà ottiche applicando stimoli esterni. Una variazione delle proprietà ottiche del DBR può avvenire attraverso vari trigger, ma questa discussione si focalizza principalmente su stimoli elettrici e biologici. Attraverso la selezione di materiali reattivi caratterizzati da particolari proprietà intrinseche, è stato possibile modificare attivamente la risposta ottica dei DBRs applicando di stimoli esterni. Nel caso di PhC 1D elettrocromici, sono state sfruttate le proprietà plasmoniche di ITO (ossido di stagno drogato con indio) per ottenere un dispositivo che mostra una modulazione del PBG attraverso un campo elettrico. Per i sensori batterici, invece, le proprietà antimicrobiche dell'argento sono state integrate in un multistrato SiO2/TiO2 per ottenere una rilevazione colorimetrica dei contaminanti batterici. I risultati ottenuti hanno fornito un proof of concept della possibilità di sviluppare sensori efficienti, economici e di facile fabbricazione. La loro versatilità, in termini di funzionalizzazione e fabbricazione, rende questi dispositivi particolarmente attraenti per la loro integrazione in applicazioni tecnologiche. I Bragg stacks possono fornire un controllo completo sulla propagazione della luce, consentendoci di modellare la loro risposta ottica per integrarli negli oggetti quotidiani.
Metal oxide nanoparticles based distributed Bragg reflectors for electro and bacterial-chromism
Moscardi, Liliana
2021/2022
Abstract
In recent years, the control of the optical properties of materials has aroused great interest from the scientific community. A huge number of new technologies can be obtained by engineering materials that respond to electromagnetic radiation, so a great effort was put into the theoretical and experimental development of these structures. Photonic crystals (PhC) represent a class of materials that are used in a wide range of applications, from optoelectronics to biosensing, thanks to their ability to control the propagation of light. They are periodic structures with high symmetry, in which constituent materials with different refractive indices show a repetition along one, two or three dimensions. The periodicity of the dielectric permittivity (or refractive index) in one or more dimensions generates an interference of the incident and scattered electromagnetic waves, leading to the formation of the photonic band gap (PBG), i.e. a wavelengths band prohibited for photons. This leads to the generation of structural reflection colours, called structural colours, which are not generated by the presence of dyes or pigments but only by the periodicity of the structure; an effect widely present in nature, such as in opals, Morpho butterflies and marble berries. The spectral position of the PBG can be tuned by acting on the refractive index of the constituent materials or on the geometric parameters of the lattice. Therefore, the optical properties are easily modifiable by engineering these parameters, allowing to customize the development of light through the material. In this thesis work, I focused on the fabrication of one-dimensional photonic crystals, also called Bragg stacks or Distributed Bragg Reflectors (DBRs), and on the possibility of tuning their optical properties upon application of external stimuli. An active tuning of the DBR optical properties can take place through various triggers, but in this discussion I focused mainly on the electrical and biological ones. Through the selection of responsive materials featured by particular intrinsic properties, it was possible to actively tune the DBR optical response upon application of external stimuli. In the case of electrochromic 1D PhCs, the plasmonic properties of ITO (indium doped tin oxide) were exploited in order to achieve a device displaying a modulation of the PBG by applying an electric field. On the other hand, for the bacterial sensors, the antimicrobial properties of silver have been integrated into a SiO2/TiO2 multilayer to obtain a colorimetric detection of bacterial contaminants. The results obtained provided a proof of concept of the possibility of developing efficient, low-cost and easy to fabricate sensors. Their versatility, in terms of DBR functionalization and fabrication, makes these devices particularly attractive for integration into technological applications. Bragg stacks can provide complete control over the propagation of light, allowing us to shape their optical response in order to integrate them into daily objects.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/183690