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Controlling the synthesis of silver nanostructures for plasmonic applications.

Liang, Hongyan (2014). Controlling the synthesis of silver nanostructures for plasmonic applications. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 89 p.

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Silver nanostructures have been paid significant attentions in many applications due to their unique physical and chemical properties, which the bulk material does not possess. Controlling Ag nanostructures’ size and shape to define their unique plasmonic features is important for practical applications. Generally, Ag nanostructures could be obtained through two main approaches, chemical and physical methods. Chemical method is the most widely used synthesis technique. That involves creation of Ag atoms from precursors by reduction and subsequent their growth into nanostructures with an aid of a stabilizer. Ag nanostructures with various well- controlled morphologies and sizes have been obtained from the solution phase. The size- and shape-dependent plasmonic features of Ag nanostructures have led to their wide applications in many fields, such as surface enhanced spectroscopy for sensors, catalysis and biological labeling. Driven by potential applications, the work performed in this thesis centers on the synthesis and properties of Ag nanostructures. In the first part, synthesis and characterization of one dimensional (1D) Ag nanostructures were performed. To do so, two kinds of 1D silver nanostructures, silver ‘nanorice’ structures and ‘nanocarrot’ structures were synthesized through a facile polyol method. Polyethylene glycol 600 was used as a solvent and as a reducing agent, and polyvinylpyrrolidone (PVP) worked as capping agent. The main factor leading to the different morphologies of products is that the precursor of silver nitrate AgNO3 was used for synthesis of silver nanorice structures while silver trifluoroacetate CF3COOAg for nanocarrot structures. Their structure details were characterized by transmission electron microscopy and X-ray diffraction. The optical properties were characterized by ultraviolet-Vis-near infrared (UV-Vis-NIR) optical extinction spectra and electron energy loss spectroscopy (EELS). Their potential for sensor applications was tested by studying the effect of the change of environmental refractive index on the surface plasmon resonance (SPR) peak location. In detail, Part I is divided into two sections based on two kinds of nanostructures (Section  and Section II). In Section I, the investigation was focused on studying the growth mechanism of silver nanorice structures and optimizing experimental conditions. By modifying synthesis parameters, Ag nanorice structures could be obtained on a large scale in high yield. Their growth process was monitored for understanding their growth mechanism. The results showed the seed selection process, based on the oxidative etching of the twinned crystals, was an indispensable step for the growth of the nanorice structures and oxygen played a critical role in this seed selection process. The major shape development stage of the nanorice structures was dominated by the oriented attachment along the <111> direction, which was directed by the non-uniform capping of PVP on different facets. The Ostwald ripening was responsible for the seed growth into the primary nanoparticles (NPs) and the lateral growth of the nanorice structures albeit it was not straightforwardly apparent in the early stage of the anisotropic growth of the nanorice structures. Slightly increasing temperature showed the acceleration effect on the 1D growth along the <111> direction, while further increase in temperature leaded to the disappearance of the 1D shape and induced the formation of highly faceted, two-dimensional, truncated triangular and hexagonal plates mainly bound by low energy faces of {111}. The growth mechanism of these two-dimensional plates was remarkably different from that of the nanorice structures, and their growth was controlled by diffusion and dictated by the twin plane. The longitudinal SPR of the nanorice structures synthesized herein was highly sensitive to the surrounding dielectric medium, with the refractive index sensitivity as high as 820 nm/RIU (RIU, per refractive index unit), which makes them highly promising for sensor applications. Furthermore, in addition to the longitudinal resonance, the multipolar resonances in individual nanorice structures were mapped in real space by using the high-resolution EELS technique. In Section II, we focused on the synthesis of asymmetric Ag nanocarrot structures and corresponding characterization of their crystal structures and optical properties. Asymmetric 1D silver nanocarrot structures were synthesized in high yield for the first time. Structure characterization showed that the face centered cubic dominated, crystalline silver nanocarrot structures feature mixed twins and stacking faults along the <111> longitudinal direction. The crystal structure of Ag nanocarrot structures was the same as that of silver nanorice structures. The SPR characteristics of the nanocarrot structures were revealed by UV-Vis-NIR optical extinction spectroscopy on particle ensembles and by nanoscale EELS on individual nanocarrot structures. The results from both techniques were further supported by calculations. Multipolar plasmon resonances, observed by EELS, showed an interesting asymmetric distribution over the length of the ‘nanocarrot’, in contrast to the symmetric distribution observed in the ‘nanorice’ structures. The longitudinal SPR peaks were red shifted and amplified in optical spectra with increasing nanocarrot length. Silver nanocarrot structures also showed high refractive index sensitivity of 890 ± 87 nm/RIU, which making them very attractive for sensor applications. In addition, these nanocarrot structures are also promising for, but not limited to, biological sample studies due to their tunable SPR in the NIR spectral range and optical waveguiding below the diffraction limit. Part II is focused on the surface enhanced spectroscopy investigations based on silver nanostructures and is divided into two sections, corresponding to surface enhanced Raman scattering (SERS) (Section III) and surface enhanced fluorescence (Section V), respectively. In Section III, we focused on investigating the SERS of flower-like silver mesoparticle dimers. The dimers were performed in a controlled manner by means of micro-manipulation. The measured SERS enhancements were found to be 10~100 times higher on dimers than that on individual mesoparticles. The observation of high dependence of SERS on incident polarization illustrates that, even though the surface roughness is dominant for SERS on the individual mesoparticles with rough surface topography, the coupling effect still gives significant additional SERS enhancement in their dimers. In addition, the use of the micro-manipulator allows us to achieve dimers with high SERS enhancement controllably and reproducibly. This work contributed to the understanding the SERS enhancement mechanism in the roughened mesoparticle dimer system, as well as to controllably realizing SERS substrates with large “hot spot” area for high enhancement. In Section V, we focused on the photoluminescence (PL) enhancement in the plasmon/fluorophore system consisting of lead chalcogenide quantum dots (QDs) and Ag NPs film to investigate the interaction between QDs and plasmonic structures. Ag NPs instead of Ag “nanorice” or “naoncarrot” structures were used here because the high stability of NPs makes it easier to claim to concept for completing the preliminary work. The Ag NP/QD films were fabricated by a layer-by-layer approach. The films exhibited significant PL enhancement, dominated by the excitation enhancement mechanism when the spectra of the Ag NPs did not match the emission band of QDs and the excitation wavelength was far away from the SPR peak in the NIR range. In contrast, both the excitation and emission enhancement mechanisms could contribute to the PL enhancement when the absorption/emission spectra of QDs matched the resonance wavelengths of Ag NPs. Moreover, by optimizing the Ag NP/QD system, we were able to reach a PL enhancement factor as high as 2.8, due to the strong coupling between the QDs and Ag NPs. Therefore, these Ag NP/QD films hold great promise for emerging QD devices for potential applications, such as highly efficient light-emitting diodes and biological sensors.

Type de document: Thèse
Directeur de mémoire/thèse: Ma, Dongling
Co-directeurs de mémoire/thèse: Rosei, Federico
Informations complémentaires: Résumé avec symboles
Mots-clés libres: nanostructures d'argent; Ag; silver nanorice structures; croissance; propriété optique; dispersion Raman de surface renforcée
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 09 juill. 2014 20:14
Dernière modification: 24 nov. 2015 16:17
URI: http://espace.inrs.ca/id/eprint/2151

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