Academic literature on the topic 'Plasmonic metal nanostructures'

Plasmonic, including of located surface Plasmon resonance (LSPR) and surface plasmon polariton (SPP), is a special kind of electromagnetic mode in nanometer scale. Plasmonic nanostructures can be generated to improving the conversion efficiency of photovoltaic devices. In the paper, the concepts of plasmonic and their influences by different metal nanostructure were introduced. Then the different principles of light utilization of plasmonic nanostructure in thin film photovoltaic cell was analyzed.

The magnetic response of most natural materials, characterized by magnetic permeability, is generally weak. Particularly, in the optical range, the weakness of magnetic effects is directly related to the asymmetry between electric and magnetic charges. Harnessing artificial magnetism started with a pursuit of metamaterial design exhibiting magnetic properties. The first demonstration of artificial magnetism was given by a plasmonic nanostructure called split-ring resonators. Engineered circulating currents form magnetic plasmons, acting as the source of artificial magnetism in response to external electromagnetic excitation. In the past two decades, magnetic plasmons supported by plasmonic nanostructures have become an active topic of study. This Perspective reviews the latest studies on magnetic plasmons in plasmonic nanostructures. A comprehensive summary of various plasmonic nanostructures supporting magnetic plasmons, including split-ring resonators, metal–insulator–metal structures, metallic deep groove arrays, and plasmonic nanoclusters, is presented. Fundamental studies and applications based on magnetic plasmons are discussed. The formidable challenges and the prospects of the future study directions on developing magnetic plasmonic nanostructures are proposed.

Plasmonic-active nanomaterials are of high interest to scientists because of their expanding applications in the field for medicine and energy. Chemical and biological sensors based on plasmonic nanomaterials are well-established and commercially available, but the role of plasmonic nanomaterials on photothermal therapeutics, solar cells, super-resolution imaging, organic synthesis, etc. is still emerging. The effectiveness of the plasmonic materials on these technologies depends on their stability and sensitivity. Preparing plasmonics-active nanostructured thin films (PANTFs) on a solid substrate improves their physical stability. More importantly, the surface plasmons of thin film and that of nanostructures can couple in PANTFs enhancing the sensitivity. A PANTF can be used as a transducer for any of the three plasmonic-based sensing techniques, namely, the propagating surface plasmon, localized surface plasmon resonance, and surface-enhanced Raman spectroscopy-based sensing techniques. Additionally, continuous nanostructured metal films have an advantage for implementing electrical controls such as simultaneous sensing using both plasmonic and electrochemical techniques. Although research and development on PANTFs have been rapidly advancing, very few reviews on synthetic methods have been published. In this review, we provide some fundamental and practical aspects of plasmonics along with the recent advances in PANTFs synthesis, focusing on the advantages and shortcomings of the fabrication techniques. We also provide an overview of different types of PANTFs and their sensitivity for biosensing.

Thanks to their tunable and strong interaction with light, plasmonic nanostructures have been investigated for a wide range of applications. In most cases, controlling the electric field enhancement at the metal surface is crucial. This can be achieved by controlling the metal nanostructure size, shape, and location in three dimensions, which is synthetically challenging. Electrochemical methods can provide a reliable, simple, and cost-effective approach to nanostructure metals with a high degree of geometrical freedom. Herein, we review the use of electrochemistry to synthesize metal nanostructures in the context of plasmonics. Both template-free and templated electrochemical syntheses are presented, along with their strengths and limitations. While template-free techniques can be used for the mass production of low-cost but efficient plasmonic substrates, templated approaches offer an unprecedented synthetic control. Thus, a special emphasis is given to templated electrochemical lithographies, which can be used to synthesize complex metal architectures with defined dimensions and compositions in one, two and three dimensions. These techniques provide a spatial resolution down to the sub-10 nanometer range and are particularly successful at synthesizing well-defined metal nanoscale gaps that provide very large electric field enhancements, which are relevant for both fundamental and applied research in plasmonics.

Optical properties of metal nanostructures, governed by the so-called localised surface plasmon resonance (LSPR) effects, have invoked intensive investigations in recent times owing to their fundamental nature and potential applications. LSPR scattering from metal nanostructures is expected to show the symmetry of the oscillation mode and the particle shape. Therefore, information on the polarisation properties of the LSPR scattering is crucial for identifying different oscillation modes within one particle and to distinguish differently shaped particles within one sample. On the contrary, the polarisation state of light itself can be arbitrarily manipulated by the inverse designed sample, known as metamaterials. Apart from polarisation state, external stimulus, e.g., magnetic field also controls the LSPR scattering from plasmonic nanostructures, giving rise to a new field of magneto-plasmonics. In this review, we pay special attention to polarisation and its effect in three contrasting aspects. First, tailoring between LSPR scattering and symmetry of plasmonic nanostructures, secondly, manipulating polarisation state through metamaterials and lastly, polarisation modulation in magneto-plasmonics. Finally, we will review recent progress in applications of plasmonic and magneto-plasmonic nanostructures and metamaterials in various fields.

AbstractMetallic nanostructures have received great attention due to their ability to generate surface plasmon resonances, which are collective oscillations of conduction electrons of a material excited by an electromagnetic wave. Plasmonic metal nanostructures are able to localize and manipulate the light at the nanoscale and, therefore, are attractive building blocks for various emerging applications. In particular, hollow nanostructures are promising plasmonic materials as cavities are known to have better plasmonic properties than their solid counterparts thanks to the plasmon hybridization mechanism. The hybridization of the plasmons results in the enhancement of the plasmon fields along with more homogeneous distribution as well as the reduction of localized surface plasmon resonance (LSPR) quenching due to absorption. In this review, we summarize the efforts on the synthesis of hollow metal nanostructures with an emphasis on the galvanic replacement reaction. In the second part of this review, we discuss the advancements on the characterization of plasmonic properties of hollow nanostructures, covering the single nanoparticle experiments, nanoscale characterization via electron energy-loss spectroscopy and modeling and simulation studies. Examples of the applications, i.e. sensing, surface enhanced Raman spectroscopy, photothermal ablation therapy of cancer, drug delivery or catalysis among others, where hollow nanostructures perform better than their solid counterparts, are also evaluated.

The inherent thinness of two-dimensional 2D materials limits their efficiency of light-matter interactions and the high loss of noble metal plasmonic nanostructures limits their applicability. Thus, a combination of 2D materials and plasmonics is highly attractive. This review describes the progress in the field of 2D plasmonics, which encompasses 2D plasmonic materials and hybrid plasmonic-2D materials structures. Novel plasmonic 2D materials, plasmon-exciton interaction within 2D materials and applications comprising sensors, photodetectors and, metasurfaces are discussed.

The field of plasmonics — the study of collective electron excitation in nanostructured metal and other conductors — is currently highly active with research foci in a number of related fields, including plasmon-enhanced spectroscopies and plasmon-mediated photochemical and photocatalytic processes through which the energy stored temporarily as plasmons can be used to enable and (or) accelerate photochemistry. This enhancement is accomplished either by the action of the large optical fields produced in the vicinity of plasmonic nanostructures or mediated by the energetic electrons and holes surviving transiently following the dephasing of the plasmon. This article traces the early contributions to the foundation of the current field of plasmonics by two scientists working in Canada in the early 1970s, J. P. Marton at McMaster University and Welwyn Corporation and the current author while he was at the University of Toronto.

In contrast to conventional photovoltaic devices which rely on bulk semiconductor material absorption and separation of electron-hole pairs, surface plasmon-based solar energy harvesting employs rectifying metal/dielectric interfaces to capture light and separate charges. Here, we describe the requirements for efficient hot electron extraction in plasmonic photovoltaic devices and demonstrate a new scalable and environmentally friendly electroless deposition method for single-crystal epitaxial noble metals films and nanostructures. The method produces ultra-smooth, low loss, single-crystal noble metal films ideal for subtractive patterning of nanostructures through ion beam milling, and high definition, sub-wavelength single crystal nanostructures through lithographic patterning methods. We describe the nucleation and growth of these metal films and nanostructures in the absence and presence of anionic shape-control agents and examine the role of specific anions in determining the resulting film and nanostructure morphologies via scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). These effects have been exploited to yield large area patterned, and shape-controlled nanoarrays of single crystal metal nanostructures for plasmonic and metamaterial applications. These approaches offer new and cost effective routes to achieve crystalline, shape-controlled surface nanostructure to enable efficient hot electron extraction for energy harvesting and catalysis applications and new noble metal alloys for improved electrocatalysis.

AbstractThe interaction of light with free electrons in a gold or silver nanostructure can give rise to collective excitations commonly known as surface plasmons. Plasmons provide a powerful means of confining light to metal/dielectric interfaces, which in turn can generate intense local electromagnetic fields and significantly amplify the signal derived from analytical techniques that rely on light, such as Raman scattering. With plasmons, photonic signals can be manipulated on the nanoscale, enabling integration with electronics (which is now moving into the nano regime). However, to benefit from their interesting plasmonic properties, metal structures of controlled shape (and size) must be fabricated on the nanoscale. This issue of MRS Bulletin examines how gold and silver nanostructures can be prepared with controllable shapes to tailor their surface plasmon resonances and highlights some of the unique applications that result, including enhancement of electromagnetic fields, optical imaging, light transmission, colorimetric sensing, and nanoscale waveguiding.

This thesis is dedicated to the study of linear and nonlinear properties of closely spaced gold nanoparticle dimers, so-called nanoantennas, and hybrid nanoantenna devices consisting of metals and semiconductors. Coupled nanoparticles are of particular interest for nanophotonics because of their ability to focus light into subwavelength volumes and the associated large field enhancement in the gap. The samples used in this thesis are gold rectangles designed by electron-beam lithography, with both symmetric and asymmetric arms, as well as symmetric closely spaced 100 nm disk dimers which were fabricated by colloidal lithography in combination with angle-dependent evaporation. We investigate the linear interplay of modes in the two arms with Spatial Modulation Microscopy, an experimental technique which results in a measure directly proportional to the extinction cross-section. We find a variety of constructive and destructive interference between different order modes, which we can better understand by comprehensive simulations of antennas, varying the parameter space of gap size (coupling strength) and length-length ratio using advanced numerical methods such as the Fourier Domain Time Difference and the Boundary Element Method. We find that the presence of nonradiative modes is made visible by Electromagnetically Induced Transparency. In order to probe the nonlinear properties of the antennas and their interaction with Indium Tin Oxide substrates, a pump-probe setup is used to get an insight into ultrafast nonlinear response with picosecond resolution. These measurements (and corresponding fits using numerical simulations) lead us to identify a new energy transfer mechanism where fast electrons are injected from the nanoparticles into the semiconductor, resulting in a refractive index change due to heating of the surroundings. In follow-up experiments, we find this mechanism to be universal (and versatile) for other types of transparent conductive oxides. These results open new avenues towards application of nanoantennas for ultrafast switching.

Resumen en Español Las nanoestructuras metálicas están siendo objeto de gran atención dada su capacidad para generar resonancias plasmónicas, que son oscilaciones colectivas de electrones alojados en la banda de conducción en un metal excitado por efecto de un campo electromagnético. El creciente interés entorno a las nanoestructuras metálicas como fuentes de plasmones, ha resultado en el desarrollo de un nuevo campo, la plasmónica, definida como la ciencia y tecnología de la generación, control y manipulación de las excitaciones resultantes de las interaciones de la luz con la materia. Las nanoestructuras plasmónicas encuentran aplicaciones en diversos campos que cubren biología, física, química, ingeniería y medicina. Por ejemplo, son ampliamente usados en sensores, espectroscopía Raman aumentada por la superficie (SERS), celdas solares potenciadas con plasmones, fotodetectores, sistemas de transporte de medicamentos en el cuerpo y terapia de cáncer, así como nanoláseres, capas de invisibilidad y computación cuántica. Es bien sabido que las propiedades plasmónicas de las nanoestructuras metálicas se ven muy afectadas por diferentes parámetros, como el tamaño, la forma, la composición y las condiciones ambientales. Por tanto, entender y manipular las propiedades de los plasmones en la escala nanométrica es imprescindible para fabricar dispositivos con las características deseadas. En este manuscrito de tesis, presentamos un detallado estudio de caracterización de las propiedades plasmónicas de nanoestructuras huecas de AuAg, empleando técnicas espectroscópicas de pérdida de energía electrónica (en inglés, electron energy-loss spectroscopy, EELS). Se sabe que las nanoestructuras huecas muestran propiedades plasmónicas mejoradas si se comparan con las mismas estructuras macizas, debido al acoplamiento de las resonancias plasmónicas internas y externas. Este estudio incluye los primeros ejemplos de mapeo de plasmones resueltos espacialmente en nanoestructuras huecas de AuAg, tales come nanocajas y nanotubos, en 2 y 3D. Este manuscrito de tesis está divido en seis capítulos. El Capítulo 1 es la introducción, que incluye las bases teóricas de la resonancia de plasmones de superficie, revisiones de los diferentes parámetros que afectan a las propiedades plasmónicas de nanoestructuras metálicas, las áreas de aplicación de las nanoestructuras plasmónicas y las técnicas de caracterización usadas para determinar estas propiedades. En el Capítulo 2 se presentan los detalles metodológicos. Los resultados experimentales acompañados de simulaciones se presentan en los Capítulos 3, 4 y 5, donde realizamos caracterizaciones detalladas y estudios de modelaje de complejas nanoestructuras metálicas. Finalmente, el Capítulo 6 recoge las conclusiones generales de la tesis completa, así como los proyectos relacionados empezados o planeados a corto plazo.
Metallic nanostructures have received great attention due to their ability to generate surface plasmon resonances, which are the collective oscillations of conduction band electrons in a metal excited by an electromagnetic field. Ever-increasing interest in plasmonic metal nanostructures has emerged into the field of plasmonics, which can be defined as the science and technology of generation, control and manipulation of excitations resulted by the light-matter interactions. Plasmonic nanostructures have been used in many different applications spanning over the fields of biology, physics, chemistry, engineering and medicine. For instance, they are widely used in sensing, surface enhanced Raman spectroscopy (SERS), plasmon-enhanced solar cells, photodetectors, drug delivery and cancer therapy as well as nanolasers, invisibility cloaks and quantum computing. It is very-well known that plasmonic properties of metallic nanostructures are greatly affected by different parameters such as the size, shape, composition and local environment. Thus, understanding and manipulating the plasmonic properties at the nanoscale is essential to fabricate devices with the desired features. In this thesis manuscript, we present a detailed characterization study on the plasmonic properties hollow AuAg nanostructures by using electron energy-loss spectroscopy (EELS) technique. Hollow nanostructures are known to have enhanced plasmonic properties compared to their solid counterparts due to the coupling of inner and outer plasmon resonances. This study involves the first examples of spatially resolved plasmon mapping in hollow AuAg nanostructures such as nanoboxes and nanotubes, both in 2D and 3D. This thesis manuscript is divided into six chapters. Chapter 1 is the introduction, which includes the theoretical background of surface plasmon resonances, the reviews of different parameters that affect the plasmonic properties of metal nanostructures, the application areas of plasmonic nanostructures and characterization techniques used to determine the plasmonic properties. In Chapter 2, details of the methodology are presented. Experimental results and accompanying simulations are presented in Chapters 3, 4 and 5, where we perform a detailed characterization and modeling studies on complex metal nanostructures. Finally, Chapter 6 includes the general conclusions of the whole thesis and some future works that are already on-going or planned to be done in the near future.

In this thesis the interaction of ultrashort laser pulses with metal nanostructures is investigated via two different phenomena: coherent acoustic oscillations of nanoparticles and generation of THz pulses on metal surfaces. Both of these effects rely on the collective oscillations of free conduction electrons in metal surfaces, plasmons. The field of plasmonics gained a great interest in the last twenty years due to the unique properties of these surface modes. It is the effects of the resonant response of plasmonic structures to incident electromagnetic wave, in particular, in visible and infrared bands and the concentration of the electromagnetic field in small subwavelength regions with significant enhancement of the incident field that make plasmonics so attractive for various applications, such as biochemical sensing, enhanced fluorescence, surface-enhanced Raman scattering, and second harmonic generation, amongst others. Investigation of the coherent particle vibrations is performed using the pump-probe technique which allows measurement of the transient transmission signals. The expansion and subsequent contraction of the nanoparticle following the ultrashort laser pulse excitation lead to a shift of the plasmon band which can be traced by transient spectroscopy. We have investigated the effect of the particle thickness on the frequency of the fundamental vibrational mode. In addition, we measured the vibrational particle response during the particle shape deformation, both symmetrical and asymmetrical. Exploration of the THz generation phenomena on plasmonic structures was performed using THz time-domain spectroscopy, the method which allows tracing of the generated THz field in the time-domain. We were able for the first time to measure the THz pulses generated from arrays of metal nanoparticles. Our observations verify the role of the particle plasmon mode in the generation of THz pulses. In addition, by exploring the dependence of the THz emission on the femtosecond pulse intensity we showed a high nonlinearity in the THz generation mechanism. The experimental results were assessed in the context of a recently proposed model where the THz radiation is generated via the acceleration of the ejected electrons by ponderomotive forces. To reveal another proposed mechanism of the THz generation from plasmonic structures, namely optical rectification, we investigated the THz generation and electron emission from the arrays of nanoparticles and nanoholes. Our results suggest that both mechanisms may contribute to generation of THz pulses from the same sample under different illumination conditions. In addition to periodic arrays of nanoparticles and nanoholes, THz generation from random metal-dielectric films was investigated. The microstructuring of such films allowed selective THz frequency generation which was explained by a model of dipole THz emitters. In addition, the effects of low temperature and pressure on the THz generation efficiency were investigated.

The field of Plasmonics deals with interaction processes between an electromagnetic radiation of appropriate wavelength and the conduction electrons of a metal. The induced collective oscillation of the electrons is called Plasmon Resonance. The Localized Surface Plasmon Resonance (LSPR) occur when the excitation involves surface electrons of nanostructures with dimensions less or comparable to the excitation wavelength. The excitation causes a strong enhancement of the local field around the metal nanostructure, which, combined with Raman Spectroscopy, could be very interesting for molecular sensing. The Raman technique is well known for providing a fingerprint spectrum of a given molecule, but has the great limitation of low sensibility. By adsorbing the analyte of interest on a plasmonic substrate in the region of enhanced local field, high detection sensitivity can be reached through Surface Enhanced Raman Spectroscopy (SERS). The first part of the present work is focused on the synthesis and characterization of gold and silver nanoparticles (Au and Ag NPs) and gold nanoshells (Au NSs) and their exploitation for the realization of SERS substrates, both in colloidal solutions and on solid supports. Different metal nanostructures give the possibility to exploit the LSPR in a wide spectral range, from the Vis to the near IR. Their optical and morphological characterization is carried out with conventional techniques, like TEM, AFM, UV-Vis absorption and Surface Enhanced Raman Spectroscopy, and with a new characterization technique, rarely used in this research field: the Photoacoustic Spectroscopy. It provides information about the absorption contribution to the total extinction of a plasmonic nanostructure. From a rigorous measurement of the SERS enhancement factor and from Photoacoustic Spectroscopy data at different excitation wavelengths, some considerations could be done concerning the relation of far field extinction and near field SERS properties. SERS EF profile measurements on liquid and solid SERS substrates demonstrated the presence of hot spots. The solid SERS substrates were chemically stable, homogeneous and reproducible and showed EF values of about 104-105. In colloidal solution, the EF values were about 103-106, depending on the metal nanostructure. Photoacoustic measurements performed on Au NSs in solution were in agreement with theoretical predictions found in literature. In the second part of the work, the plasmonic substrates, realized with Au NPs and Au NSs, were used for the realization of label free SERS sensors, to detect toxic aromatic chemical species and biological molecules. A sensor for toxic volatile compounds, based on Au NPs and Au NSs substrates coupled with a porous organic-inorganic hybrid sol-gel matrix, was realized. The matrix was specifically chosen for exhibiting a high-affinity interaction to aromatic hydrocarbons. The enhancement activity of the Au NPs and Au NSs substrates on the sol gel matrix alone was demonstrated. Some problems in the xylene detection process through SERS were probably due to the fast matrix regeneration under the laser radiation. Although, the enhanced SERS efficiency due to the detection design was demonstrated. Another application was based on the development of a novel label-receptor system, based on the cromophore 4-hydroxyazobenzene-2 carboxylic acid (HABA) and its specific antibody, to be used in bio-analytical applications. The interesting behaviour of the HABA dye relies in changing its tautomeric structure from an azo to a hydrazo form, thanks to the interaction with its antibody. This structural change can be exploited for SERS detection of the label-receptor interaction. Properly synthesized and characterized HABA derivatives were adsorbed onto SERS substrates, further incubated in the antibody solution. The HABA signals were well visible on both Au NSs and Au NPs substrates. No HABA change could be detected through SERS, because the antibodies extracted in vivo from two rabbits, do not cause the quantitative change of the HABA structure.
La Plasmonica si occupa dell’interazione di una radiazione elettromagnetica di opportuna lunghezza d’onda con gli elettroni di conduzione di un metallo. L’oscillazione collettiva degli elettroni, indotta da questa interazione, è chiamata appunto Risonanza Plasmonica. La risonanza plasmonica di superficie localizzata avviene quando gli elettroni coinvolti sono quelli di superficie di un metallo nanostrutturato con dimensioni minori o comparabili alla lunghezza d’onda di eccitazione. Da questa eccitazione deriva una forte amplificazione del campo elettromagnetico locale, localizzato nelle immediate vicinanze della nanostruttura metallica. Tale amplificazione, unita a una tecnica di rivelazione spettroscopica specifica, quale la spettroscopia Raman, può essere sfruttata per la realizzazione di sensori molecolari. La tecnica Raman è conosciuta come altamente specifica, perché in grado di fornire uno spettro caratteristico della singola molecola, identificandone univocamente la presenza e la costituzione. La sua maggiore limitazione, però, è la bassa sensibilità. Ponendo l’analita in prossimità di un substrato plasmonico, proprio nella regione di forte amplificazione del campo locale, la sensibilità di rivelazione viene fortemente aumentata, dando origine alla spettroscopia Raman amplificata da superfici (SERS). La prima parte del presente lavoro è focalizzata sulla sintesi e sulla caratterizzazione di nanoparticelle d’argento, d’oro e di nano gusci d’oro (chiamati nanoshell) e sul loro impiego per la realizzazione di substrati SERS, sia in soluzione colloidale che su substrato solido. L’utilizzo di differenti nanostrutture metalliche, dà la possibilità di sfruttare la risonanza plasmonica localizzata di superficie in un’ampia regione spettrale, che si estende dal visibile al vicino infrarosso. La caratterizzazione ottica e morfologica delle nanostrutture è stata effettuata con tecniche convenzionali, come la spettroscopia di assorbimento UV-visibile, il SERS, la microscopia elettronica a trasmissione e la microscopia a forza atomica. Ad esse è stata affiancata anche una tecnica raramente usata nell’ambito della plasmonica: la spettroscopia fotoacustica. Questa può fornire informazioni riguardanti il contributo di assorbimento, all’estinzione totale, di una nanostruttura plasmonica. Da una rigorosa misura dei fattori di amplificazione e delle proprietà di fotoacustica al variare della lunghezza d’onda, possono essere fatte alcune considerazioni riguardanti la possibile relazione tra l’estinzione (proprietà di campo lontano) e l’ amplificazione SERS (proprietà di campo vicino). Le misure dei profili di eccitazione SERS su substrati plasmonici in liquido e su supporto solido, hanno evidenziato la presenza di hot spots, ovvero di zone fortemente amplificate dall’interazione di due o più nanostrutture. I substrati SERS solidi sono risultati chimicamente stabili, omogenei e riproducibili; essi presentano valori di fattori di amplificazione attorno a 104-105. In soluzione colloidale, i fattori di amplificazione delle nanostrutture hanno raggiunto valori nell’intervallo 103-106, dipendentemente dal tipo di nanostruttura metallica investigata. Le misure di fotoacustica effettuate su soluzioni colloidali di nanoshell d’oro si sono rivelate in accordo con le predizioni teoriche di letteratura. Nella seconda parte del lavoro, i substrati plasmonici, realizzati principalmente con nanoparticelle e nanoshell d’oro, sono stati impiegati per la realizzazione di sensori SERS per la rivelazione di specie chimiche e biologiche. É stato realizzato un sensore di composti tossici aromatici volatili, accoppiando un substrato plasmonico con un film poroso di sol gel ibrido organico-inorganico. La componente organica della matrice sol gel è stata appositamente scelta per la sua alta affinità a composti aromatici, quali lo Xilene. È stata dimostrata l’amplificazione dei segnali della matrice da parte della componente plasmonica, ma si sono riscontrati alcuni problemi nella rivelazione delle molecole di analita attraverso il SERS. La difficoltà nella rivelazione è probabilmente dovuta al veloce deadsorbimento dello Xilene dalla matrice a causa del forte riscaldamento locale causato dalla radiazione laser. Nonostante questo, si è comunque dimostrata l’aumentata efficienza del sensore progettato, rispetto ai suoi componenti singoli. La seconda applicazione studiata ha riguardato la realizzazione di un sistema analita-accettore innovativo, che può essere utilizzato per diverse applicazioni bioanalitiche; esso è basato sull’interazione tra un cromoforo diazobenzenico (HABA) e il suo anticorpo specifico. Alla base dell’applicazione si trova una proprietà interessante del suddetto cromoforo, che è quella di cambiare la sua struttura molecolare, passando da una forma azo alla forma idrazo, dopo aver interagito con il suo anticorpo specifico. Questa variazione nella struttura molecolare può essere sfruttata per la rivelazione dell’avvenuta interazione analita-accettore, mediante SERS. Alcuni derivati di questo cromoforo sono stati sintetizzati e caratterizzati in modo da poter essere adsorbiti su un substrato SERS, che viene successivamente incubato in una soluzione di anticorpo. I segnali SERS della molecola di HABA sono risultati ben visibili sia sui substrati di nanoparticelle che di nanoshell d’oro. Purtroppo non è stato possibile rivelare la variazione strutturale del cromoforo, in quanto gli anticorpi, estratti in vivo da due coniglietti, inducono solo un parziale cambio di struttura, rendendo la rivelazione SERS alquanto difficile.

The purpose of the present thesis is the study of the interaction of plasmonic and pre-plasmonic nanostructures with an emitter in close proximity. The investigation was carried out following different approaches but always with the aim of inserting the experimental results in the frame- work of new or existing theoretical models in order to better understand the photophysical nature of the interaction. To this aim in the framework of this thesis different nanoarchitectures have been synthesised and coupled to Er-doped silica layers. The choice of Erbium as emitting source was driven by the great technological importance of this rare earth in photonics and optoelectronics, connected to the characteristic emission at 1540 nm that matches the window of minimum transmission loss for silica. For this reason the first step of the research activity was devoted to the optimization of the Erbium photoluminescent properties in silica. When an emitter is placed near an interface, its optical properties will be modified. To describe this variation different contributions have to be taken into account: the variation of the local density of state due to the reflection from the interface, the coupling of the emitted radiation with propagating surface plasmons on the metal-dielectric interface and the dissipation in the overlayer. All these aspects have been studied in detail for different overlayer materials demonstrating that the strong control of the excited state lifetime of the emitter can be obtained by tailoring the dielectric properties of the overlayer and the separation distance from the interface. Nanostructuring the overlayer offers further opportunities for changing the optical properties of a nearby emitter. Among different plasmonic nanostructures, nanohole arrays (NHAs) can represent the ideal candidate for this purpose due to their extraordinary optical transmission (EOT): at specific frequencies determined by the hole periodicity, the light transmitted through the NHA is orders of magnitude higher than the one predicted with the classical diffraction theory. When the EOT peak was tailored with the emission wavelength of the emitter strong plasmonic coupling was demonstrated, leading to lifetime shortening with almost no dissipation in the overlayer. The improvement of the optical performances of an emitter can be obtained not only acting on the decay from the excited state but also increasing the excitation efficiency. For this purpose, an interesting possibility that has been explored is the sensitization by of ultra-small molecular-like metal nanoclurters (NCs) produced by ion implantation. Noble metal NCs can indeed efficiently absorb light through broad-band interband transitions and transfer energy to a nearby emitter, acting as efficient nanoantennae for excitation of the emitter. Such interaction leads to the increase of the effective excitation cross-section by several orders of magnitude. Finally, all the obtained results allowed the development of predictive models that can be used in the design of novel devices for different photonic applications
Lo scopo del presente lavoro di tesi è l’analisi dell’interazione di nanostrutture plasmoniche e pre-plasmoniche con un emettitore. Lo studio è stato condotto seguendo diversi approcci, ma sempre con il fine di confrontare i risultati sperimentali con modelli teorici sia già noti che nuovi, in modo da comprendere appieno la natura foto-fisica dell’interazione. In questo senso nell’ambito della presente tesi diverse nano-architetture sono state sintetizzate ed accoppiate con film sottili di silice drogata con erbio. La scelta dell’erbio come emettitore è stata dettata dalla sua grande importanza tecnologica della terra rara nella fotonica e nell’optoelettronica, associata alla caratteristica emissione radiativa a 1540nm, che si trova nella finestra di minimo assorbimento ottico della silice. Per questa ragione il primo passo dell’attività di ricerca è stato volto all’ottimizzazione delle proprietà di fotoluminescenza dello ione erbio in silice. Quando un emettitore è posto in prossimità di un film sottile le sue proprietà ottiche vengono modificate. Per descrivere tale variazione è necessario tenere conto di contributi differenti: la variazione della densità locale degli stati dovuta alla riflessione all’interfaccia, l’accoppiamento della radiazione emessa con plasmoni di superficie propaganti sull’interfaccia metallo-dielettrico e infine la dissipazione nel film. Tutti questi aspetti sono stati studiati in dettaglio per film di diversi materiali, dimostrando che un ottimo controllo sul tempo di vita dello stato eccitato può essere ottenuto agendo sulle proprietà dielettriche del film e sulla distanza di separazione tra l’emettitore e l’interfaccia. La nanostrutturazione del film può offrire ulteriori opportunità nella modifica delle proprietà ottiche di un emettitore. Tra le diverse nanostrutture plasmoniche, i nanohole arrays (NHAs) possono essere visti come i candidati ideali per questo scopo grazie alla loro trasmissione ottica straordinaria (EOT): a determinate lunghezze d’onda definite dalla periodicità dei buchi e dalle proprietà dielettriche dei materiali coinvolti, la luce trasmessa attraverso il NHA è ordini di grandezza più grande rispetto a quella predetta dalla teoria classica della diffrazione. Quando il picco della EOT è risonante con la lunghezza d’onda di emissione dell’emettitore, è stato dimostrato un forte accoppiamento plasmonico che porta ad un marcato accorciamento del tempo di vita nella quasi assenza di dissipazione nella nanostruttura. Il miglioramento delle proprietà ottiche di un emettitore può essere ottenuto non solamente agendo sulla parte emissiva del processo, ma anche aumentando la probabilità di eccitazione. A questo scopo, una possibilità interessante è offerta dalla sensitizzazione da aggregati metallici ultra-piccoli ottenuti per impiantazione ionica. Cluster di metalli nobili composti da 10–20 atomi possono infatti assorbire efficientemente la radiazione di eccitazione attraverso transizioni interbanda e trasferire l’energia a un emettitore posto nelle vicinanze, agendo in questo modo da efficienti nanoantenne. Tale interazione può portare ad un aumento della sezione d’urto di eccitazione efficace di diversi ordini di grandezza. Infine, tutti questi risultati hanno permesso lo sviluppo di modelli predittivi che possono essere utilizzati nella progettazione di nuovi dispositivi per diverse applicazioni fotoniche

Research in nanoscience has gained noteworthy interest over the past three decades. As novel chemical and physical properties that are vastly different from extended solids are realized in nanosized materials, nanotechnology has become the center of attention for material in research community. Much to our amazement, investigations in the past two decades revealed that the nanocrystalline semiconductors are “THE PRIME CANDIDATES” to meet the growing energy demand, sensor development, cellular imaging and a number of other optoelectronic applications. Nonetheless, synthesis of nanostructures with control over physical parameters is not sufficient, yet assembling them into functional nanoarchitectures with unique and tunable physical properties is critical for device integration studies. Among bottom-up assembling methods, sol-gel method has received noteworthy interest to produce macroscopic nanostructures of metal and semiconductor NPs with no use of intervening ligands or supports. In 2005, condensation of pre-formed semiconductor NPs (CdSe, CdS, ZnS and PbS) into voluminous gels is reported via controlled destabilization of the surfactant ligands. The resultant chalcogenide aerogels are reported to exhibit extremely low density, high surface area and porosity, and quantum confined optical properties of the NP building blocks. More recently, this method has been extended for the assembly of metal NPs, where transparent and opaque nanostructures (aerogels) of Ag and Au/Ag NPs were produced. The aerogels produced by condensation of NPs are low dimensional (fractal) nanostructures and exhibit a physically connected network of colloidal NPs. Interactions between NPs in a gel structure are intermediate of those of the ligand stabilized NPs and core/shell hetero-nanostructures (e.g. Au@CdSe NPs) with the potential to couple chemically dissimilar systems. In this research study, NP condensation strategy has been utilized to efficiently integrate the plasmonic and excitonic properties of metal and semiconductor nanostructures to produce high-efficiency hybrids that exhibit unique tunable physical and photophysical properties. Two hybrid systems composed of spherical CdSe/Ag hollow NPs and rod shaped CdSe/Ag hollow NPs were investigated for the fabrication of metal-semiconductor hybrid aerogels. The first excitonic energy of spherical CdSe NPs is overlapped with the plasmonic energy of Ag hollow NPs at 515 - 530 nm. The second excitonic energy of rod shaped CdSe is overlapped with the plasmonic energy of Ag hollow NPs at 490 - 505 nm. The photophysical properties of both systems were thoroughly probed through UV-Visible absorption, photoluminescence (PL), and time-resolved (TR) PL spectroscopy. A novel hybrid emission emerged at 640 nm (for spherical CdSe/Ag hollow NPs) and 720 nm (for rod shaped CdSe/Ag hollow NPs) with ~0.2-1% Ag loading. TRPL studies revealed 685 ns and 689 ns PL decay times for hybrid emissions, which are vastly different from the band-edge and trap state emission of phase pure spherical and rod shaped CdSe aerogels respectively, supporting the generation of novel radiative decay pathways. Overall, synthesis of CdSe/Ag hybrid aerogels with novel/tunable photophysical properties will add to the toolbox of semiconductor aerogels with the potential application in future light harvesting technologies.

The development of nanotechnology has provided a variety of noble metal nanostructures with unique optical properties that are useful for different application fields. Metal nanoparticles present strongly enhanced optical properties associated with localized surface plasmon resonance (LSPR): here, the effect on the optical properties of metal nanostructures is investigated by different techniques. The large AuNPs absorption cross section coupled with fast nonradiative decay rate and low radiative decay efficiency make them perfect converter of light into heat: the high temperatures reached can be used for photothermal terapy, light conversion in thermal and photovoltaic devices, but our interest has been focused on optical limiting application against cw laser. The study of the thermal conversion of incoming light could be useful for the protection of the human eye from accidental or intentional damage. A good protection device should be a “smart material” able to activate the protection at high energy with a large dynamic range and in a wide wavelength interval. The last property is especially required in the case of military use, for protection against laser pointing devices or blinding weapons of unpredictable emission wavelength. In this case, passive filters, commonly used for specific wavelengths, are useless because of their selectivity and lack of tuning properties. The irradiation of an optical limiting material with a focused cw laser beam induces energy absorption rapidly converted into a local heating and a temperature gradient corresponding to a refractivity index variation across the sample. In this way, even a flat sample acts as a focusing or defocusing lens and spreads the laser beam. We have studied different aspects of the phenomenon, as described below, to achieve the application in a solid state device with a broadband range of activity and a fast response time. In the first experimental part of this thesis different nanostructures have been synthesized, starting from gold nanoparticles, nanoshells and nanorods with different aspect ratio, in order to obtain plasmonic resonances in a wide range of the visible spectrum. Nanostructures has been then manipulated for the functionalization with a thiolated-fulleropyrrolidine (FULP-SH) to combine the thermal relaxation process with a faster one. A useful material for protection devices should preferably be in the solid state, so a thorough study has been centered on polycarbonate (PC) as matrix because of its good optical qualities. Film production and nanoparticles embedding require a specific study of the functionalization and transfer of nanostructures synthesized in aqueous solvent. We characterized the morphology and their linear optical properties with conventional techniques: transmission electron microscopy (TEM) gives information about the dimension of nanostructures to implement the synthesis, UV-Vis spectroscopy correlates structures with extinction properties and surface enhanced Raman spectroscopy (SERS) of the nanosystems defines the correct functionalization with organic molecules. In the second part of the project we studied and tried to improve the nonlinear optical response of these promising systems in order to obtain different characteristics. Using z-scan technique we define the nature of the defocusing mechanism, confirming the self-defocusing behavior and giving nonlinear efficiency parameters to compare different systems. Optical power limiting measurements give direct information on the protection ability of these systems. Thanks to the easy functionalization of nanostructures we figured out promising properties for a solid state protection device. First we have studied the optical limiting properties of gold nanoparticle solutions identifying a thermal response as the main mechanism. We have then compared these results with those obtained by coupling gold nanoparticles with a thiolated-fulleropyrrolidine. In this way we wanted to combine the thermal process with a faster one, to permit a stronger reduction of transmittance and a better limiting efficiency. Such a strategy has been proved to be effective for improving OL through a quite different mechanism that is activated in a much shorter time. Optical limiting measurements have been conducted on gold nanoparticles embedded in polycarbonate with good results that have been compared to the colloidal solutions. The study of a different matrix for optical limiting studies has been attempt: silk fibroin. This matrix has been selected because of the easier nanoparticles embedding. Furthermore it can be applied for instance in controlled release of drugs, thanks to the biocompatibility and gradual solubility of silk matrix. Preliminary studies discourage the use of this system for optical limiting but different application could be considered. The fibroin-nanoparticles solution can be easily transform to obtain a porous structure: the idea is to employ this matrix as a sensor for liquid samples with SERS characterization, taking advantage of the high porosity and the presence of plasmonic structures. In the last part we tried to compare thermal properties revealed by our systems through cw laser excitation to nonlinear optical properties classically expressed by pulsed laser excitation. Optical limiting related to photoacoustic measurements allow us to discriminate the contribution of the absorption and to choose the best system with higher linear transmittance and lower threshold for nonlinear behavior
Lo sviluppo delle nanotecnologie ha fornito una varietà di nanostrutture metalliche con proprietà ottiche uniche utili per diverse applicazioni. Le nanoparticelle metalliche presentano una forte amplificazione delle proprietà ottiche associate al plasmone di risonanza superficiali (LSPR): in questo lavoro abbiamo studiato le proprietà ottiche di nanoparticelle d’oro (AuNPs) con diverse tecniche. La grande cross section di assorbimento delle AuNPs accoppiata con la rapido decadimento non radiativo e la scarsa efficienza di decadimento rendono efficace la conversione di luce in calore: le alte temperature raggiunte possono essere utilizzate per terapia fototermica, conversione luminosa in dispositivi fotovoltaici, ma il nostro interesse si è focalizzato sull’applicazione nella limitazione ottica contro laser in continuo (cw). Lo studio della conversione termica della luce incidente può essere utilizzato per la realizzazione di dispositivi per la protezione dell’occhio contro danni accidentali o intenzionali. Un buon dispositivo di protezione dovrebbe essere un materiale intelligente in grado di attivarsi sopra una certa soglia di intensità, con un ampio intervallo di attività e a diverse lunghezze d’onda. Quest’ultima proprietà è di particolare interesse in ambito militare per la protezione contro dispositivi laser di puntamento o armi accecanti di lunghezze d’onda non note a priori. In questo caso sono i filtri passivi per specifiche lunghezze d’onda attualmente utilizzati risultano inefficaci data la loro alta selettività e scarsa versatilità. L’irraggiamento di un limitatore ottico con un raggio laser cw focalizzato induce un assorbimento dell’energia che viene rapidamente convertito in un riscaldamento locale e la formazione di un gradiente di temperatura che corrisponde ad una variazione di indice di rifrazione attraverso il campione. In questo modo anche un campione piatto agisce come una lente focalizzante o defocalizzante e diffonde la luce. Abbiamo studiato diversi aspetti del fenomeno, come descritto in seguito, per ottenere un dispositivo a stato solido con un ampio intervallo di attività e una risposta rapida. Nella prima parte sperimentale di questa tesi sono state sintetizzate diverse nanostrutture, a partire da nanoparticelle d’oro, nanoshells e nanorods con aspect ratio differenti, al fine di ottenere risonanze plasmoniche in un ampio intervallo dello spettro visibile. Le nanostrutture sono state in seguito funzionalizzate con molecole di fulleropirrolidina tiolata (FULP-SH) per combinare il processo di rilassamento termico con uno più rapido. Un limitatore ottico per un dispositivo di protezione deve essere preferibilmente solido, e quindi lo studio delle proprietà ottiche è stato effettuato anche in matrice, in particolare in polycarbonato (PC), scelto per le sue ottime qualità ottiche. La produzione dei film e l’inglobamento delle nanoparticelle ha richiesto degli studi sulla funzionalizzazione e la stabilizzazione delle nanostrutture sintetizzate in solvente acquoso. Abbiamo caratterizzato la morfologia e le proprietà ottiche lineari con tecniche convenzionali: microscopia a trasmissione elettronica (TEM), che fornisce informazioni sulle dimensioni e la forma delle nanostrutture al fine di implementarne la sintesi, spettroscopia UV-Visibile che correla le strutture con le proprietà di estinzione, e la spettroscopia Raman che ha verificato l’effettiva funzionalizzazione dei sistemi con le molecole organiche. Nella seconda parte del progetto abbiamo studiato le risposte ottiche non lineari di questi promettenti sistemi per poterne modulare le proprietà. Attraverso la tecnica Z-scan siamo stati in grado di definire la natura del meccanismo di defocalizzazione e di ottenere i parametri non lineari che ci hanno permesso di confrontare i nostri risultati con quelli attualmente presenti in letteratura. Misure di limitazione ottica hanno dato informazioni sull’efficacia di protezione dei nostri sistemi. Grazie alla semplicità di funzionalizzazione delle nanoparticelle abbiamo individuato delle nuove e promettenti proprietà per un dispositivo di protezione a stato solido. In primo luogo abbiamo studiato le proprietà di limitazione ottica di nanoparticelle in soluzione per identificare la tipologia di funzionamento. In seguito i risultati sono stati confrontati con quelli ottenuti con nanoparticelle funzionalizzate con FULP-SH. In questo modo abbiamo tentato di associare al processo di rilassamento termico un meccanismo più rapido, in modo da ridurre maggiormente la trasmittanza e migliorare l’efficienza di limitazione. Abbiamo quindi verificato l’efficacia della strategia utilizzata evidenziando un miglioramento della limitazione ottica in un tempo inferiore. Le misure di limitazione ottica eseguite su nanoparticelle in matrice di PC hanno dato ottimi risultati, paragonabili a quelli ottenuti in soluzione. Un primo di studio di matrici differenti si è concentrato sulla fibroina della seta, scelta per la semplicità di inglobamento delle nanoparticelle. Inoltre questo sistema AuNPs-fibroina potrebbe trovare sbocco anche in diverse applicazioni: grazie alla biocompatibilità della matrice ed alla sua solubilità graduale in acqua potrebbe essere usato per il rilascio controllato di farmaci. Studi preliminari scoraggerebbero l’utilizzo di questo sistema nella limitazione ottica ma possono essere comunque considerate altre applicazioni. Le nanoparticelle in fibroina possono infatti essere facilmente trasformate in strutture porose: un’idea potrebbe essere quella di utilizzarle come sensori per campioni in soluzione con caratterizzazione Raman amplificata (SERS), combinando l’alta porosità e la presenza di strutture plasmoniche. Nell’ultima parte abbiamo confrontato le proprietà termiche dei nostri sistemi attraverso studi di fotoacustica che ci hanno permesso di discriminare il contributo assorbitivo dall’estinzione totale e di scegliere il sistema migliore con alta trasmittanza lineare e basse soglie di attivazione nonlineari

This thesis focuses on the design, synthesis and development of dynamic multivalent nanostructures such as supramolecular dendrimers, liposomes and gold-functionalized nanostructures. These structures can be used for drug delivery and molecular sensing applications. This thesis is divided into three parts: In part one, a general introduction to self-assembly, dynamic systems, metalligand exchange, nanostructured dendritic scaffolds, liposomes and gold nanostructures is given. In part two, a microwave approach is presented as an efficient method for the regioselective deuteration of bipyridine scaffolds. Dynamic systems based on transition metal-bipyridine coordination complexes were investigated. The compositional self-adaptation and kinetics of these dynamic systems were successfully assessed by ESI-MS. Based on this amphiphilic dendrimers/metallodendrimers were also designed and synthesized via  a convergent strategy. Their ability to self-assemble into supramolecular assemblies and their controlled disassembly was effectively demonstrated. In part three, two types of drug delivery systems based on dynamic multivalent nanostructures of glycodendrimers/metalloglycodendrimers and drugpresenting liposomes were developed. The dynamic self-assembly of these architectures into supramolecular nanostructures with site-specific functionality through interacting carbohydrate or cholesterol moieties was assessed. The host-guest interaction/encapsulation and controlled release with external stimuli were studied using a fluorescent probe, as well as selected drug molecules. The antibacterial property of the drug delivery systems was also evaluated, demonstrating an enhanced bactericidal activity. A new, rapid and simple approach for the functionalization of plasmonic gold nanostructured surfaces was also developed. The optical performance and light-specific sensitivity of the fluorescent probe on the resulting nanostructures were also presented.

QC 20151119

Thesis (M. S.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2008.
Committee Chair: El-Sayed, Mostafa A.; Committee Member: Lyon, L. Andrew; Committee Member: Sherrill, C. David; Committee Member: Wang, Zhong Lin; Committee Member: Whetten, Robert L.

Plasmonic nanostructures have attracted growing interest over the last decades due to their efficiency in improving the performance in various application fields such as catalysis, photovoltaics, (opto-)electronic devices, and biomedicine. The behavior of a specific metal plasmonic system depends on many factors such as the material, the size, the shape, and the dielectric environment. The geometry, that is, size and shape of both single plasmonic elements and patterned arrays of plasmonic nanostructures, plays an essential role, and it provides considerable freedom to tune the plasmonic properties of a single plasmonic nanostructure or any combination of nanostructures. This freedom is mainly used in the application fields of surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS). In this context, the chapter encompasses how the geometry of the SERS-active plasmonic nanostructures and tips with/without metal substrates used in TERS influences the localized surface plasmon resonances of the plasmonic systems.

Plasmon-induced photocatalyst has found its application in the clean and renewable energy issue due to its combination of the large absorption and resonance in the visible region for plasmonic nanostructures with the ability of producing the electron-hole pairs in the ultraviolet range for semiconductors (e.g., TiO2). The Schottky barrier at the interface between metals and semiconductors could assist in separating electrons and holes, and increase the photocatalytic efficiency because the Fermi levels of plasmonic metals are lower than semiconductors. Several mechanisms have been proposed for different systems, including plasmonic heating, plasmonic-excited charge transfer, resonant energy transfer, and plasmonic-enhanced scattering, but none could perfectly explain all the phenomena to date [1]. In this study, Au and Ag nanotriangles synthesized by reduction of metal precursor were deposited on the surface of nanobranched TiO2 arrays, which were prepared by hydrothermal methods. The photoactivity enhancement was evaluated using three-electrode system illuminated with the solar simulator. Finite-difference time-domain (FDTD) method was performed to investigate the electric field enhancement at the interface between Au (or Ag) nanoparticles and TiO2 arrays upon illumination.

As the next-generation technology moves below 100 nm mark, the need arises for a capability of manipulation and positioning of light on the scale of tens of nanometers. Plasmonic optics opens the door to operate beyond the diffraction limit by placing a sub-wavelength aperture in an opaque metal sheet. Recent experimental works [1] demonstrated that a giant transmission efficiency (>15%) can be achieved by exciting the surface plasmons with artificially displaced arrays of sub-wavelength holes. Moreover the effectively short modal wavelength of surface plasmons opens up the possibility to overcome the diffraction limit in the near-field lithography. This shows promise in a revolutionary high throughput and high density optical lithography. In this paper, we demonstrate the feasibility of near-field nanolithography by exciting surface plasmon on nanostructures perforated on metal film. Plasmonic masks of hole arrays and “bull’s eye” structures (single hole surrounded by concentric ring grating) [2] are fabricated using Focused Ion Beam (FIB). A special index matching spacer layer is then deposited onto the masks to ensure high transmissivity. Consequently, an I-line negative photoresist is spun on the top of spacer layer in order to obtain the exposure results. A FDTD simulation study has been conducted to predict the near field profile [3] of the designed plasmonic masks. Our preliminary exposure test using these hole-array masks demonstrated 170 nm period dot array patterns, well beyond the resolution limit of conventional lithography using near-UV wavelength. Furthermore, the exposure result obtained from the bull’s eye structures indicated the characteristics of periodicity and polarization dependence, which confirmed the contribution of surface plasmons.