Academic literature on the topic 'Gas sensing; Plasmonic applications'

My research activity during the Ph. D. period has been focused on the simulation and the experimental characterization of Surface Plasmon Polaritons (SPP). Surface Plasmon Polaritons are evanescent electromagnetic waves that propagate along a metal/dielectric interface. Since their excitation momentum is higher than that of the photons inside the dielectric medium, they cannot be excited just by lighting the interface, but they need some particular coupling configurations. Among all the possible configurations the Kretschmann and the grating are those largely widespread. When the SPP coupling conditions are reached, abrupt changes of some components of the light reflected or transmitted at the metal/dielectric interface appear. Usually this resonances are characterized by a minimum of the reflectance acquired as a function of the incident angle or light wavelength. Several experimental methods are available to detect these SPP resonances, for instance by monitoring the light intensity, its polarization or its phase. Changes in the physical conditions of the metal/dielectric interface produce some changes of the SPP coupling constant, and consequently a shift in the resonance position. If these changes derive from a molecular detection process, it is possible to correlate the presence of the target molecules to the resonance variations, thus obtaining a dedicated SPP sensor. I focused the first part of my Ph.D. activity on the simulation of SPP resonances by using several numerical techniques, such as the Rigorous Coupled Wave Analysis method, the Chandezon method, and the Finite Element Method implemented through Comsol v3.5. I simulated the SPP resonance in the Kretschmann coupling configuration for plane and nano-grating structured metal/dielectric interfaces. Afterward, I calculated the SPP resonance behaviour for grating and bi-dimensional periodic structures lighted in the conical configuration. Furthermore, I analysed the correlations between the grating coupling method and the Kretschamann coupling method. Through all these simulations, I studied the sensitivity of the different SPP resonances to the refractive index variation of the dielectric in contact with the metal. In this way, I was able to find a new parameter suitable for describing the SPP resonance, i.e., the azimuthal angle. By considering this particular angle, the sensitivity of the SPP resonances could be properly set according to the experimental needs and, even more important, noticeably increased to high values. Experimentally I used two opto-electronic benches, one for the Kretschmann configuration and one for the conical mounting configuration. I have performed experimental measurements, in order to compare the experimental data with the simulations. In particular the following conditions were tested: • Plane interface, Kretschmann configuration • Nanostructured grating, Kretschmann configuration • Nanostructured grating, Conical configuration I focused my attention on the nano-structured grating in conical mounting configuration. I found an innovative way to characterize its SPP resonances, by measuring the transmitted signal as a function of the incident and azimuthal angles. The transmittance and the azimuthal sensitivities were characterized with the gratings in both air and water. In order to study the experimental azimuthal sensitivity, I changed the liquid refractive index in contact with the grating by using different water/glycerol solutions. Moreover, I functionalized the surface by using thiolated molecules that form Self Assembled Monolayer onto the metallic layer. In this way, I was able to change the SPP coupling constants and detect the corresponding azimuthal resonance shifts. I also detected the immobilization of an antibody layer onto the metallic surface of the plasmonic interface. All the devices I used in the experimental measurements were produced by the University spin off Next Step Engineering.<br>Durante il mio periodo di dottorato in Scienza e Tecnologia dell’Informazione l’attività di ricerca principale è stata focalizzata sulla caratterizzazione, simulativa e sperimentale, dei plasmoni di superficie. I plasmoni di superficie sono onde elettromagnetiche evanescenti che si propagano all’interfaccia tra un mezzo metallico ed un mezzo dielettrico. Il loro vettore d’onda è più elevato rispetto a quello della luce nel mezzo dielettrico. Per poter quindi generare l’eccitazione si devono utilizzare particolari tecniche di accoppiamento. I due metodi più diffusi sono l’accoppiamento Kretschmann e l’accoppiamento tramite reticolo. Una volta raggiunte le condizioni di accoppiamento dei plasmoni di superficie, si realizza il fenomeno della risonanza plasmonica, la quale si manifesta attraverso brusche variazioni nelle componenti della luce riflessa o trasmessa dalla superficie. Tipicamente si può registrare un minimo della riflettanza in funzione dell’angolo di incidenza della luce sulla superficie. Esistono, tuttavia, anche altre modalità per registrare e misurare queste risonanze, come ad esempio monitorando intensità, polarizzazione o fase della luce trasmessa e riflessa dalla superficie, in funzione della sua lunghezza d’onda o dei sui angoli di incidenza. Le variazioni chimico/fisiche che avvengono all’interfaccia metallo/dielettrico, modificando la costante di accoppiamento plasmonica, cambiano le condizioni di risonanza. Nel caso in cui le variazioni all’interfaccia siano dovute ad un processo di riconoscimento molecolare è possibile rilevare le molecole d’interesse valutando i cambiamenti della risonanza plasmonica, fornendo così l’opportunità per l’implementazione di sensori specifici. L’attività di dottorato è stata focalizzata innanzitutto sullo studio teorico del comportamento della risonanza plasmonica, utilizzando varie tecniche di simulazione numerica: il metodo RCWA (Rigorous Coupled Wave Analysis), Il metodo di Chandezon ed il metodo agli elementi finiti, implementato tramite Comsol v3.5. Ho poi affrontato lo studio, tramite simulazioni, delle risonanze di superficie in configurazione Kretschmann, sia per interfacce metallo/dielettrico piane sia per interfacce nano-strutturate. Considerando una configurazione conica, ho simulato le risonanze di superficie per nano-strutture reticolari e per nano-strutture bi-dimensionali periodiche. Inoltre ho analizzato il legame tra le modalità di accoppiamento grating e Kretschmann. Tramite queste simulazioni mi è stato possibile valutare e studiare la sensibilità delle varie risonanze plasmoniche alla variazione di indice di rifrazione, quando essa avviene all’interfaccia metallo/dielettrico. È stato così possibile identificare un nuovo parametro per descrivere la risonanza plasmonica e la sua sensibilità, ossia l’angolo azimutale, definito come l’angolo tra il vettore del grating ed il piano di scattering della luce. Considerando questo particolare angolo, la sensibilità del sensore può essere controllata con un’opportuna regolazione degli altri parametri coinvolti nell’eccitazione plasmonica, consentendole di raggiungere valori molto elevati. Successivamente, grazie all’utilizzo di due banchi, uno per la configurazione Kretschmann ed uno per la misura di reticoli nano-strutturati in configurazione conica, ho realizzato delle campagne di misure sperimentali. E’ stato così possibile confrontare i risultati sperimentali con le simulazioni numeriche per le seguenti condizioni: • Interfaccia piana, configurazione Kretschmann • reticolo nano-strutturato, configurazione Kretschmann • reticolo nano-strutturato, configurazione conica L’attività sperimentale si è particolarmente focalizzata sul reticolo nano-strutturato, sia per l’innovativa modalità di caratterizzazione delle sue risonanze plasmoniche (valutazione del segnale trasmesso in funzione dell’angolo di incidenza e dell’angolo azimutale), sia per l’elevata sensibilità ottenuta valutando la variazione dell’angolo azimutale. La caratterizzazione è stata effettuata sia per il reticolo esposto all’aria che per il reticolo immerso in un liquido (tipicamente acqua). Per poter verificare il comportamento della sensibilità azimutale ho variato l’indice di rifrazione del liquido in contatto con la superficie utilizzando soluzioni miste di acqua e glicerolo. Inoltre, tramite tecniche di funzionalizzazione della superficie, ovvero applicando delle molecole thiolate che vengono adsorbite sulla parte metallica dell’interfaccia, mi è stato possibile variare le costanti di accoppiamento plasmonico, in modo da verificare la capacità del dispositivo di rilevare l’avvenuta creazione di uno strato molecolare sulla superficie. Inoltre ho positivamente verificato la capacità di immobilizzare uno strato di anticorpi sulla superficie plasmonica. Tutte le misure sperimentali che ho svolto in questa tesi sono state effettuate su sensori con superfici piane o nano-strutturate prodotte dallo spin-off universitario Next Step Engineering, con il quale ho collaborato durante il percorso di ricerca.

Plasmonics, the science of the excitation of surface plasmon polaritons (SPP) at the metal-dielectric interface under intense beam radiation, has been studied for its immense potential for developing numerous nanophotonic devices, optical circuits and lab-on-a-chip devices. The key feature, which makes the plasmonic structures promising is the ability to support strong resonances with different behaviors and tunable localized hotspots, excitable in a wide spectral range. Therefore, the fundamental understanding of light-matter interactions at subwavelength nanostructures and use of this understanding to tailor plasmonic nanostructures with the ability to sustain high-quality tunable resonant modes are essential toward the realization of highly functional devices with a wide range of applications from sensing to switching. We investigated the excitation of various plasmonic resonance modes (i.e. Fano resonances, and toroidal moments) using both optical and terahertz (THz) plasmonic metamolecules. By designing and fabricating various nanostructures, we successfully predicted, demonstrated and analyzed the excitation of plasmonic resonances, numerically and experimentally. A simple comparison between the sensitivity and lineshape quality of various optically driven resonances reveals that nonradiative toroidal moments are exotic plasmonic modes with strong sensitivity to environmental perturbations. Employing toroidal plasmonic metasurfaces, we demonstrated ultrafast plasmonic switches and highly sensitive sensors. Focusing on the biomedical applications of toroidal moments, we developed plasmonic metamaterials for fast and cost-effective infection diagnosis using the THz range of the spectrum. We used the exotic behavior of toroidal moments for the identification of Zika-virus (ZIKV) envelope proteins as the infectious nano-agents through two protocols: 1) direct biding of targeted biomarkers to the plasmonic metasurfaces, and 2) attaching gold nanoparticles to the plasmonic metasurfaces and binding the proteins to the particles to enhance the sensitivity. This led to developing ultrasensitive THz plasmonic metasensors for detection of nanoscale and low-molecular-weight biomarkers at the picomolar range of concentration. In summary, by using high-quality and pronounced toroidal moments as sensitive resonances, we have successfully designed, fabricated and characterized novel plasmonic toroidal metamaterials for the detection of infectious biomarkers using different methods. The proposed approach allowed us to compare and analyze the binding properties, sensitivity, repeatability, and limit of detection of the metasensing devices

Different types of plasmonic nanostructures are proposed and examined experimentally and theoretically, with a view towards sensing applications. First, a self-assembly approach was developed to create arrays of well-ordered glass-supported gold nanoparticles (AuNPs) with controllable particle size and inter-particle spacing. Then, a periodic array of gold nano-disks (AuNDs) supported by a Bragg reflector was proposed and examined in a search for Fano resonances in its optical response. Arrays of heptamer-arranged nanoholes (HNH) in a thin gold film were also proposed and explored theoretically and experimentally, revealing a very rich spectrum of resonances, several exhibiting a Fano lineshape. A commercial implementation of the vectorial finite element method (FEM) was used to model our plasmonic structures. Taking advantage of the periodic nature of the structures, a unit cell containing a single element was modelled. The transmittance, reflectance or absorbance spectra were computed, and the associated electromagnetic fields were obtained by solving the vector wave equations for the electromagnetic field vectors throughout the structures, subject to the applicable boundary conditions, and the applied source fields. The sensing performance of the structures, based on the bulk sensitivity, surface sensitivity and figure of merit (FOM) was calculated. First, a novel bottom-up fabrication approach was applied (by our collaborators) to form a periodic array of AuNPs with controllable size over large areas on SiO2 substrates. In this method, self-assembly of block copolymer micelles loaded with metal precursors was combined with a seeding growth route to create ordered AuNPs of desired size. It was shown that this new fabrication method offers a new approach to tune the AuNP size and edge-to-edge inter-particle spacing while preserving the AuNP ordering. The optical characteristics of the AuNP arrays, such as their size, interparticle spacing, localized surface plasmon resonance (LSPR) wavelength, and bulk sensitivity, were examined, numerically and experimentally. This proposed novel fabrication method is applicable for low-cost mass-production of large-area arrays of high-quality AuNPs on a substrate for sensing applications. Then, we proposed and examined the formation of Fano resonances in a plasmonic-dielectric system consisting of uncoupled gold nano-disk (AuND) arrays on a quarter-wave dielectric stack. The mechanism behind the creation of Fano resonances was explained based on the coherent interference between the reflection of the Bragg stack and the LSPPs of the AuNDs. Fano parameters were obtained by fitting the computational data to the Fano formula. The bulk sensitivities and figure of merit of the Fano resonances were calculated. This plasmonic structure supports Fano resonances with a linewidth around 9 nm which is much narrower than the individual AuND LSPP bandwidth ( 80 nm) and the Bragg stack bandwidth ( 100 nm). Supporting Fano resonances with such a narrow linewidth, the structure has a great potential to be used for sensing applications. Also, this metallic-dielectric nanostructure requires no near-field coupling between AuNDs to generate the Fano resonances. So, the AuNDs can be located far enough from each other to simplify the potential fabrication process. The optical properties of HNH arrays on an SiO2 substrate were investigated, numerically and experimentally. Helium focused ion beam (HeFIB) milling was applied (by Dr. Choloong Hahn) to fabricate well-ordered and well-defined arrays of HNHs. Transmittance spectra of the structures were obtained as the optical response, which exhibits several Fano resonances. Then, the mechanism behind the formation of the Fano resonances was explained, and the sensing performance of the structure was inspected by measuring the bulk sensitivities. This array of nanohole cluster is exciting because it supports propagating SPPs and LSPPs, and also Wood’s anomaly waves, which makes the optical response very rich in excitations and spectral features. Also, as a periodic array of sub-wavelength metallic nanoholes, the system produces extraordinary optical transmission - highly enhanced transmission through (otherwise) opaque metallic films at specific wavelengths, facilitating measurement acquisition in transmission.

Thesis (Ph.D.)--Boston University<br>PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.<br>The control of light on the nano-scale has driven the development of novel optical devices such as biosensors, antennas and guiding elements. These applications benefit from the distinctive resonant properties of noble metal thin films and nanoparticles. Many optimization parameters exist in order to engineer nanoparticle properties for spectroscopy and sensing applications: for example, the choice of metal, the particle morphology, and the array geometry. By utilizing various designs from simple monomer gratings to more complex engineered arrays, we model and characterize plasmonic arrays for sensing applications. In this thesis, I have focused on the novel paradigm of photonic-plasmonic coupling to design, fabricate, and characterize optimized nanosensors. In particular, nanoplasmonic necklaces, which consist of circular loops of closely spaced gold nanoparticles, are designed using 3D finite-difference time-domain (FDTD) simulations, fabricated with electron-beam lithography, and characterized using dark-field scattering and surface-enhanced Raman spectroscopy (SERS) of p-mercaptoaniline (pMA) monolayers. I show that such necklaces are able to support hybridized dipolar scattering resonances and polarization-controlled electromagnetic hot-spots. In addition, necklaces exhibit strong intensity enhancement when the necklace diameter leads to coupling between the broadband plasmonic resonance and the circular resonator structure of the necklace. Hence, these necklaces lead to stronger field intensity enhancement than nanoparticle monomers and dimers, which are also carefully studied. Furthermore, by embedding a dimer into one or more concentric necklace resonators, I am able to efficiently couple radiation into the dimer hot-spot by utilizing first- and second-order far-field coupling. This nanolensing leads to an order of 6-18 times improvement in Raman enhancement over isolated dimers, which is a promising platform for compact on-chip sensors. Additionally, I have fabricated and experimentally characterized devices that were designed in my group for SERS of pMA using an optimization algorithm. The algorithm confirms that the best arrangement of nanoparticles to increase near-field intensity enhancement in a single hot-spot is to embed a dimer into particles that couple light into the hot-spot via far-field photonic radiation. These genetically optimized nanoantennas show improvement in Raman enhancement 10 times that of nanoparticle dimers, and 100 times the enhancement of optimized two-dimensional monomer diffraction gratings.<br>2031-01-02

In this Thesis I have investigated the use of nanostructured films as sensing and contact layers for field effect gas sensors in order to achieve high sensitivity, selectivity, and long term stability of the devices in corrosive environments at elevated temperatures. Electrochemically synthesized Pd and Au nanoparticles deposited as sensing layers on capacitive field effect devices were found to give a significant response to NOx with small, or no responses to H2, NH3, and C3H6. Pt nanoparticles incorporated in a TiC matrix are catalytically active, but the agglomeration and migration of the Pt particles towards the substrate surface reduces the activity of the sensing layer. Magnetron sputtered epitaxial films from the Ti-Si-C and the Ti-Ge-C systems were grown on 4H-SiC substrates in order to explore their potential as high temperature stable ohmic contact materials to SiC based field effect gas sensors. Ti3SiC2 thin films deposited on 4H-SiC substrates were found to yield ohmic contacts to n-type SiC after a high temperature rapid thermal anneal at 950 ºC. Investigations on the growth mode of Ti3SiC2 thin films with varying Si content on 4H-SiC substrates showed the growth to be lateral step-flow with the propagation of steps with a height as small as half a unit cell. The amount of Si present during deposition leads to differences in surface faceting of the films and Si-supersaturation conditions gives growth of Ti3SiC2 films with the presence of TiSi2 crystallites. Current-voltage measurements of the as-deposited Ti3GeC2 films indicate that this material is also a promising candidate for achieving long term stable contact layers to 4H-SiC for operation at elevated temperatures in corrosive environments. Further investigations into the Ti-Ge-C system showed that the previously unreported solid solutions of (Ti,V)2GeC, (Ti,V)3GeC2 and (Ti,V)4GeC3 can be synthesized, and it was found that the growth of these films is affected by the nature of the substrate.

Chemistry<br>Ph.D.<br>The localized surface plasmon resonance (LSPR), arising due to the collective oscillation of free electrons in metal nanoparticles, is a sensitive probe of the nanostructure and its surrounding dielectric medium. Synthetic strategies for developing surfactant free nanoparticles using ultrafast lasers providing direct access to the metallic surface that harvest the localized surface plasmons will be discussed first followed by the applications. It is well known that the hot carriers generated as a result of plasmonic excitation can participate and catalyze chemical reactions. One such reaction is the dissociation of hydrogen. By the virtue of plasmonic excitation, an inert metal like Au can become reactive enough to support the dissociation of hydrogen at room temperature, thereby making it possible to optically detect this explosive gas. The mechanism of sensing is still not well understood. However, a hypothesis is that the dissociation of hydrogen may lead to the formation of a metastable gold hydride with optical properties distinct from the initial Au nanostructures, causing a reversible increase in transmission and blue shift in LSPR. It will also be shown that by tracking the LSPR of bare Au nanoparticles grown on a substrate, the adsorption of halide ions on Au can be detected exclusively. The shift in LSPR frequency is attributed to changes in electron density rather than the morphology of the nanostructures, which is often the case.<br>Temple University--Theses

This work is aimed to evaluate the optical gas sensing properties of carbon nanomaterial. In particular it is focused on two materials, Carbon Nanotubes (CNTs) and Graphene Oxide (GO). The comprehension of the mechanisms of interaction of these materials with the gas molecules is fundamental for a future application of these materials as sensors targeted to a specific specie or capable to distinctly detect several dangerous species. On this purpose nanostructures based on GO and CNTs have been produced and tested as optical gas sensors toward oxidizing/reducing gases (H2, CO, NO2) and aromatic volatile Organic Compounds (benzene, toluene, xylene). Gold nanoparticles (Au NPs) have been used as optical probe thanks to the peculiar Localized Surface Plasmon Resonance feature in the visible range, which is extremely sensitive to the variation in optoelectronic properties of the surrounding media, such as refractive index and the variation in charge carrier involved in plasmonic excitation in the Au NPs. Not only amplify the Au NPs the variation in optoelectronic properties of the layer of carbon nanomaterial, but also the electromagnetic coupling with carbon nanomaterials may induce an enhancement in response and a lowering of the limit of detection of the sensors to the target species. Moreover, the GO and CNTs are provided of a large possibility of functionalization, which can be used to tailor the gas sensing properties of the nanostructures toward specific species. CNTs have been combined with the Au NPs, Pd NPs, Ni NPs and fullerenes. Pd and Au NPs increase the response toward H2 , meanwhile Ni NPs and fullerenes appear specific to CO. It is also suggested the opportunity to monitor the features of the absorbance plot of fullerenes and CNT in the NIR as optical probes, with the carbon nanomaterials playing both the role of sensing element and optical probe. The presence of the different functional groups in GO was investigated. The increase in sp2conjugation has a profitable effect for the sensing of H2. Instead, the removal of the oxidized functional groups hinder the response of the films toward CO and NO2. The reduction and functionalization of the GO with para- Phenylene Diamine induces the detection of NH3without Au NPs as optical probe. The sensors produced are characterized by high transparency in the visible range and may be incorporated as non-invasive sensors on transparent surfaces. Most of the sensors worked at 150°C and 300°C. Test of gas sensing have been conducted at low temperatures, at 80°C for CNTs in fullerene matrix and good results were achieved. The possibility of sensors active at room temperature is suggested by the positive tests conducted with CMG, paving the way for future developments in active optical material sensitive to gases at room temperature.<br>Il presente lavoro è focalizzato sullo studio di sensori ottici basati su nanomateriali di carbonio, nell’ottica di un’applicazione di questi materiali come sensori di gas. Il lavoro prende in analisi due materiali, i nanotubi di carbonio (CNTs) e il grafene ossido (GO). La comprensione dei meccanismi di interazione di questi materiali con le molecole di gas è fondamentale per le applicazioni future di questi materiali nel rilevamento di diverse specie nocive di gas. A tal proposito, nanostrutture a base di GO e CNTs sono state sviluppate e studiate come sensori ottici verso gas ossidanti-riducenti (H2, CO, NO2) e nei contronti di composti volatili organici aromatici (benzene, toluene, xylene). Le nanoparticelle di oro sono state utilizzate come sonde ottiche grazie alla loro peculiare caratterista di risonanza plasmonica di superficie localizzata, la quale è estremamente sensibile alle variazioni di proprietà ottico-elettroniche del mezzo che le circonda, come l’indice di rifrazione, e alle variazione di densità di portatori di carica che sono coinvolti nell'eccitazione plasmonica nelle nanoparticelle di oro. Quindi, le nanoparticelle di oro, non solo amplificano le variazioni optoelettroniche del film di nanomateriali di carbonio a cui sono state accoppiate, ma interagiscono con questi inducendo un miglioramento della risposta ai gas e un abbassamento del limite di rilevamento ai gas in analisi. Inoltre, GO e CNTs presentano una vasta gamma di possibili funzionalizzazioni, che, possono essere sfruttate per una progettazione mirata delle proprietà di gas sensing delle nanostrutture di carbonio. I CNTs sono stati abbinati a nanoparticelle di Au, Pd, Ni e a fullereni. Pd e Au portano ad un miglioramento delle prestazioni dei sensori verso il gas H2, nanoparticelle di Ni e fullereni sembrano avere un’azione specifica verso il gas CO. In questo lavoro viene anche suggerita la possiblità di monitorare le proprietà di assorbanza di fullereni e CNTs nel range del vicino IR. I CNTs, in tal caso, avrebbero la duplice funzione di sonde ottiche e di materiale sensibile. Oltre all'effetto delle nanoparticelle di oro sulle proprietà di gas sensing del GO, è stata valutata l’influenza dei diversi gruppi funzionali. L’estensione dei domini sp2 sembra favorire il rilevamento di H2, mentre una forte rimozione di gruppi funzionali inibisce la risposta del GO verso CO e NO 2.