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NONLINEAR OPTICAL PROCESSES IN MOLECULAR SYSTEMS AT THE INTERFACE WITH METAL NANOSTRUCTURES
Full text linkSurface plasmon excitations, i.e. collective oscillations of the conduction electrons, strongly influence the optical properties of metal nanostructures. Furthermore, under resonance conditions, a giant local electromagnetic field develops on the nanostructure surface. By controlling the size and shape of the nanostructures, it is possible to control the surface plasmon resonance frequency and so the frequency at which the local field will be amplified. If a molecule is located at the interface with metal nanostructures, its optical properties will be affected by the local field and can be enhanced or suppressed by the metal.
Here, the effect on the optical properties of molecules due to the interaction with metal nanostructures is investigated by different techniques. Emphasis is placed on the synthesis and the study of the nonlinear properties of nude and functionalized gold nanoshells (NSs).
Different shape gold nanoparticles are synthesized: spherical particles in water and in toluene, rod like particles in water subsequently transferred in organic solvents, core-shells in water. Gold nanostructures have been functionalized with spiropyran, PEP-C3-SH and PEP-C12-SH (two heterocyclic dyes), Rhodamine B.
Fluorescence decay times for PEP-C3-SH and PEP-C12-SH free and bound to nanorods (NRs) have been measured to study the interaction between the dye and the nanostructure and to evaluate the effects of the chain length. The PEP moieties present a two exponential decay times: a short one of 6.2 ± 0.5 ns and a long one of 61 ± 2 ns. When the dye is bound to NRs, the interaction between the molecules and the nanoparticles provoked a change in the PEP dynamics. Both PEP-C3 and PEP-C12 bound to NRs, decay faster, indicating quenching of fluorescence.
Nanoshells functionalized with Rhodamine B have been characterized with Pump and Probe technique. The dye clearly modified the dynamics of naked NSs and shortened the electron-phonon time decay from 4.9 ps (naked NSs) to 3.7 ps (RhB-NSs) introducing an additional channel by which hot electrons can decay.
SERS measurements of NSs, deposited on glass substrates, covered by benzenethiol were performed. Incomplete shells with sharp tips may turn out to be quite interesting for SERS analysis due to the strong local field that take place at tip apexes. Enhancement of the Raman signal has been clearly observed because the concentration of benzenethiol is too low to give Raman signal except in case of strong enhancement field effects.
Z-scan measurements, with fs laser pulses, were performed at 20 and 200 Hz and at different energy to study the nonlinear response of naked NRs and NSs. Both nanostructures behaved like saturable absorber and reshaped during laser pulse illumination. NSs z-scan data, fitted with Sheik-Bahae model, yielded an effective value of the non linear absorption coefficient β = (–1.1 ÷ –9.6) X 10-11 cmW-1 dependent on the pulse energy and repetition rate. The absolute value of the nonlinear absorption coefficient increased both with energy and pulse repetition rate. The effect of increasing the repetition rate is likely related to thermal effects, also confirmed by an increase of the beam waist radius used for fitting the z-scan data. The dependence on the energy, instead, indicated that the process is not a third order phenomenon, but higher order contributions should be taken into account.
Together with the plasmonic effects, the NSs deposition onto a transparent electrode potentially allows for improved efficiency both for light collection and charge collection. With this motivation, work function measurements on NSs deposited ITO substrates have been performed by the Kelvin probe method. They showed that by selectively choosing the shell thickness, ITO pre-functionalization and nanoparticle coverage, it is possible to fine-tune the WF of the transparent electrode within the range of 3.9 – 4.4 eV.L’eccitazione dei plasmoni di superficie, cioè dell’oscillazione collettiva degli elettroni di conduzione, influenza fortemente le proprietà ottiche delle nanostrutture metalliche. Inoltre, in condizioni di risonanza, si sviluppa, sulla superficie della nanostruttura, un fortissimo campo elettrico localizzato. Controllando le dimensioni e la forma delle nanostrutture, è possibile controllare la frequenza di risonanza del plasmone superficiale e, di conseguenza, la frequenza alla quale il campo elettrico locale verrà amplificato. Se una molecola è posizionata all’interfaccia con la nanostruttura metallica, le sue proprietà ottiche risentiranno del campo locale e potranno essere amplificate o ridotte dal metallo.
In questo lavoro di tesi, sono stati studiati, con diverse tecniche, gli effetti, sulle proprietà ottiche di molecole, dell’interazione con nanostrutture metalliche. In particolare, ci si è concentrati sulla sintesi e sullo studio delle proprietà non lineari di nanoshell (NSs) nude e funzionalizzate.
Sono state sintetizzate nanoparticelle di oro di forma diversa: particelle sferiche in acqua e toluene, a forma di rod in acqua (successivamente trasferite in solventi organici), core-shell in acqua. Le nanostrutture d’oro sono state funzionalizzate con spiropirani, PEP-C3-SH e PEP-C12-SH (due molecole eterocicliche), Rodamina B.
I tempi di decadimento di fluorescenza per PEP-C3-SH e PEP-C12-SH liberi e legati alle nanorods (NRs) sono stati misurati per studiare l’interazione tra il colorante e la nanostruttura e per valutare gli effetti della diversa lunghezza della catena alchilica. Il colorante, indipendentemente dalla lunghezza della catena, presenta un decadimento biesponenziale con un tempo breve di 6.2 ± 0.5 ns e uno lungo di 61 ± 2 ns. In seguito alla funzionalizzazione, l’interazione tra la molecola e le NRs provoca un cambiamento nella dinamica di decadimento del PEP. Sia il PEP-C3 che il PEP-C12 legati alle NRs, decadono più rapidamente, indicando un’attenuazione della fluorescenza.
Le nanoshell funzionalizzate con la Rodamina B sono state caratterizzate con la tecnica Pump and Probe. Il colorante modifica la dinamica delle NSs e riduce il tempo di decadimento dell’accoppiamente elettrone-fonone da 4.9 ps (NS nude) a 3.7 ps (RhB-NSs), introducendo un canale addizionale attraverso cui gli elettroni caldi possono decadere.
Sono state effettuate delle misure SERS su benzentiolo attaccato a nanoshell, depositate su substrati di vetro. Shell incompleti caratterizzati da punte acuminate possono risultare molto interessanti per la Spettroscopia SERS per i forti campi locali che si sviluppano sulle punte. Un aumento del segnale Raman è stato chiaramente osservato, dato che la concentrazione di benzentiolo era troppo bassa per dare segnale Raman eccetto in caso di effetti di campo locale.
Sono state effettuare misure Z-scan con impulsi di durata di centinaia di femtosecondi, a frequenza di ripetizione di 20 e 200 Hz e a diverse energie, per studiare la risposta non lineare di NRs e NSs non funzionalizzate. Entrambe le nanostrutture si comportano come assorbitori saturabili e subiscono delle modifiche morfologiche per effetto dell’illuminazione con impulsi laser.
I dati z-scan per le nanoshell, fittati con il modello Sheik-Bahae, producono un valore effettivo del coefficiente di assorbimento non lineare β = (–1.1 ÷ –9.6) X 10-11 cmW-1 che dipende sia dall’energia dell’impulso che dalla frequenza di ripetizione. Il valore assoluto del coefficiente di assorbimento non lineare aumenta sia con l’energia che con la frequenza di ripetizione. L’aumento con la frequenza di ripetizione è dovuto a effetti termici, confermati anche da un aumento del raggio del beam waist usato per fittare i dati z-scan. Invece, la dipendenza dall’energia indica che il processo non è un fenomeno al terzo ordine ma si deve tenere conto di contributi di ordine superiore.
Insieme agli effetti plasmonici, le nanoshell depositate su elettrodi trasparenti, come l’ITO (Indium Tin Oxide), potrebbero potenzialmente migliorare l’efficienza sia dell’assorbimento della luce che della separazione di carica. Per questa ragione, è stata misurata la funzione lavoro di nanoshell depositate su substrati di ITO con la tecnica Kelvin probe. Scegliendo opportunamente lo spessore dello shell, se pre-funzionalizzare l’ITO e il livello di ricoprimento di particelle, è possibile variare la funzione lavoro dell’elettrodo nell’intervallo 3.9 – 4.4 eV
Femtosecond nonlinear absorption of gold nanoshells at surface plasmon resonance
No full textThe nonlinear optical absorption of gold nanoshells (Au NSs) of different size, in water, was investigated using open aperture z-scan technique with femtosecond laser pulses at 806 nm. It is found that, in general, NSs behave as saturable absorbers. The level of saturation depends on the Au NSs structure and precisely on the ratio between the core size and shell thickness. The measured values of the nonlinear absorption coefficient show a dependence on both the repetition rate and the pulse energy. An average value of the nonlinear absorption coefficient beta = -4.5 +/- 1.0 x 10(-11) cm W(-1) is obtained from z-scan data of core-shell particles of inner and outer radius 95 and 110 nm, respectively, measured at 20 Hz repetition rate in the energy range 120-300 nJ
Saturable Absorption of Femtosecond Laser Pulses at Surface Plasmon Resonance in Gold Nanoshells
No full textSaturable absorption of femtosecond laser pulses at surface plasmon resonance in gold nanoshells
No full textIn this work, we present an investigation of the nonlinear optical properties of nanoshells of different size in solutions and deposited on glass substrates using a single beam z-scan method at a wavelength of 806 nm with laser duration of 170 fs. Structural properties of gold nanoshells of about 150 and 220 nm, prepared in water, are characterized by AFM and TEM microscopy. It is found that, in general, they behave as saturable absorbers. The level of saturation depends on the relative overlap between the plasmon resonance of nanoshells and the laser excitation wavelengths. The nonlinear absorption coefficient, obtained by fitting the experimental results, is of the order of -10-11 cmW-1
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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