Symbiotic plasmonic nanomaterials: Synthesis and properties

Metal particles of the dimensions of the order of 1 to 100\u27s of nanometers show unique properties that are not clearly evident in their bulk state. These nanoparticles are highly reactive and sensitive to the changes in the vicinity of the particle surface and hence find applications in the field of sensing of chemical and biological agents, catalysis, energy harvesting, data storage and many more. By synthesizing bimetallic nanoparticles, a single nanoparticle can show multifunctional characteristics. The focus of this thesis is to detail the synthesis and understand the properties of bimetallic nanomaterial systems that show interesting optical, chemical, and magnetic behaviors, some of which fall into the category of a symbiotic behavior. Symbiosis is the mutual sharing of resources between two individual organisms. The potential design considerations in the synthesis of such symbiotic nanomaterials include their position in the electrochemical series, thermodynamic immiscibility, and vastly contrasting properties, such as plasmonic (Ag) and ferromagnetic (Co). In addition to these aspects, nanostructure size, shape, and composition can also play an important role in the ensuing optical, magnetic and chemical behaviors. For this work, two different synthesis routes were utilized to make nanostructures of various shapes, size, composition and spacing. The second part of the thesis focused on to understand the relationship between the role of intrinsic and extrinsic factors on the optical and chemical properties of these bimetallic nanostructures. From measurements of the plasmonic resonance energy and bandwidth, we developed a quantitative picture of the dependence of oxidation stability, plasmon quality factor and the radiative quantum efficiency on size and energy. These results showed that the bimetal nanoparticles could have comparable or better quality factor and quantum efficiency than pure Ag. We also discovered a new class of thin film amorphous transparent semiconducting material. The semiconductor was made from a ternary oxide comprising of the metals Fe, Tb, and Dy. The combination of high visible light transparency, high conductivity and extraordinarily high mobility makes this material a potential candidate for use in thin film transistor and transparent conductor applications, and is a possible replacement for In-based materials

Nanoparticle Biofunctionalization for Self-Assembly and Energy Transfer Applications

Metal and semiconductor nanocrystals (NCs) have unique optical and physical properties that are dependent on size, composition and morphology. When NCs are coupled to biomolecules, their properties are combined to create unique materials with biomimetic capabilities that can function as biosensors, cellular imaging agents or drug delivery vehicles. Most NCs are synthesized in air free, non-polar conditions, so surface chemistries must be tuned to accommodate hydrophilic biomolecules. This can be achieved through ligand exchange or polymer encapsulation procedures. This work takes advantage of both phase transfer routes to functionalize gold nanoparticles (AuNPs), quantum dots (QDs), and quantum rods (QRs) with DNA and proteins for self-assembly, energy transfer and drug delivery applications. In the first project, we explored the ability to assemble QDs into clusters with a high degree of control through DNA-mediated interactions. The hydrophobic QDs were first transferred to buffers using a polymer encapsulation approach that used an amphiphilic polymer. The polymer encapsulated QDs were successfully functionalized with oligonucleotides through both EDC/NHS coupling and click chemistry. The final QD/DNA conjugates were assembled into multicolor QD clusters through a colloidal stepwise approach. One of the greatest challenges of this project was an inconsistent batch-to-batch QD/DNA coupling efficiency, which was attributed to the presence of excess polymer, QD aggregates and poor stoichiometry. Purifying QDs via ultracentrifugation in a sucrose density gradient removed excess polymer, leading to a decreased optical scattering and increased DNA loading that was beneficial for increasing coupling efficiency. In these clusters, a decrease in the QD donor emission and an increase in the QD acceptor emission indicated that QD-QD FRET occurred. One disadvantage to using QDs as energy acceptors is their broad absorption profile, which causes them to be coexcited with the donor. To overcome this limitation, a bioluminescent protein can be used to generate QD emission through bioluminescence resonance energy transfer (BRET) without external excitation. In the next project, CdSe/CdS quantum rods (QRs) were functionalized with the bioluminescent firefly protein, Photinus pyralis (Ppy). The aim of this project was to improve the long-term stability of the QR/Ppy conjugates. To make these conjugates, hydrophobic CdSe/CdS QRs are rendered hydrophilic through a ligand exchange with histidine (His) followed by an additional ligand exchange to conjugate hexahistagged Ppy proteins to QRs (QR/His/Ppy). In these conjugates, there was a decrease in the stability of the BRET over time. The retention of the BRET signal was significantly improved by changing the QR capping ligand prior to protein conjugation from His to glutathione (GSH). This is because the GSH ligands that remain on the QR surface after Ppy coupling are more highly charged than His, leading to more efficient electrostatic repulsions between QRs. To incorporate the improved QR/Ppy nanoconjugates into the QD/DNA clusters, the QR emission should be a result of non-radiative energy transfer contributions only to prevent simultaneous excitation of the energy donor and acceptor. To investigate the contribution from radiative energy transfer to the BRET signal, control experiments were performed that indicated that most of the BRET signal arises from non-radiative energy transfer from the Ppy to the QR. In the last project, DNA functionalized AuNPs were used as drug carriers for idarubicin (IDA), a clinically approved chemotherapeutic agent. To construct these conjugates, AuNPs are synthesized using a citrate reduction method and a ligand exchange is carried out to exchange the citrate capping molecules with thiol modified DNA and thermoresponsive polymers. Drug binding was investigated using DNA denaturation measurements and kinetic studies. An increase in duplex DNA melting temperature with drug loading verified IDA intercalation at the dsDNA. The kinetics of drug release were investigated at physiological temperature, where the presence of drug outside of a dialysis membrane was monitored through IDA fluorescence. The low drug release, small dissociation rate constant of 0.05 min-1 and high equilibrium constant of 3.0 x 108 M-1 demonstrates that these nanoconjugates can act as efficient vehicles for in vivo drug delivery

Laser-matter interactions, phase changes and diffusion phenomena during laser annealing of plasmonic AlN:Ag templates and their applications in optical encoding

Nanocomposite thin films incorporating silver nanoparticles are emerging as photosensitive templates for optical encoding applications. However, a deep understanding of the fundamental physicochemical mechanisms occurring during laser-matter interactions is still lacking. In this work, the photosensitivity of AlN:Ag plasmonic nanocomposites is thoroughly examined and a series of UV laser annealing parameters, such as wavelength, fluence and the number of pulses are investigated. We report and study effects such as the selective crystallization of the AlN matrix, the enlargement of the Ag nanoparticle inclusions by diffusion of laser-heated Ag and the outdiffusion of Ag to the film's surface. Detailed optical calculations contribute to the identification and understanding of the aforementioned physical mechanisms and of their dependency on the laser processing parameters. We are then able to predetermine the plasmonic response of processed AlN:Ag nanocomposites and demonstrate its potential by means of optically encoding an overt or covert cryptographic pattern

Surface modifications and growth of titanium dioxide for photo-electrochemical water splitting

This study investigates photo-anodes based on titanium dioxide (TiO2) that can be used to produce hydrogen by the photo-electrochemical decomposition of water. TiO2 is a wide band gap semiconductor that absorbs only the UV region of the solar spectrum. Sensitization of TiO2 to visible light by the addition of gold nanoparticles (AuNPs) was studied. AuNPs sustain localized surface plasmon resonance (LSPR) that results in the absorption of light at the resonant energy. The evidence for water splitting by Au-TiO2 systems is discussed critically. Fabrication of arrays of AuNPs was done by; annealing sputtered gold thin films, micellar nanolithography, and nano-sphere lithography. The optical characteristics and photo-electrochemical ‘water splitting’ performance of AuNP coated rutile (110) electrodes were determined. Nb-doped crystals coated in AuNPs of ca. 20 nm exhibited a small photocurrent that was not present with the bare rutile electrode. Reduced un-doped rutile (110) with AuNPs did not exhibit the ‘plasmonic photocurrent’. Some Nb-doped electrodes did not exhibit an effect. Batches of Nb-doped and reduced rutile were examined using voltammetry and impedance spectroscopy and it was found that the ‘inactive’ Nb-doped TiO2 was partially reduced. Thin films of TiO2 were fabricated by pulsed laser deposition (PLD) onto amorphous and single crystal substrates. The effect of growth conditions on the phase and orientation of the film were studied, and procedures to grow anatase films oriented with (100), (001), and (101) were developed. The temperature and heating regime of TiO2 films fused silica affected the orientation of film growing. Nb doping of the films also affected the temperature of the anatase-rutile phase transition and the orientation of the films, acting to stabilize anatase at higher temperatures. Surprisingly, highly doped films were found to be non-conductive. The importance of the oxygen partial pressure in producing conductive films for use as electrodes is discussed.Open Acces

Superfocusing, Biosensing and Modulation in Plasmonics

Plasmonics could bridge the gap between photonics and electronics at the nanoscale, by allowing the realization of surface-plasmon-based circuits and plasmonic chips in the future. To build up such devices, elementary components are required, such as a passive plasmonic lens to focus free-space light to nanometre area and an active plasmonic modulator or switch to control an optical response with an external signal (optical, thermal or electrical). This thesis partially focuses on designing novel passive and active plasmonic devices, with a specific emphasis on the understanding of the physical principles lying behind these nanoscale optical phenomena. Three passive plasmonic devices, designed by conformal transformation optics, are numerically studied, including nanocrescents, kissing and overlapping nanowire dimers. Contrary to conventional metal nanoparticles with just a few resonances, these devices with structural singularities are able to harvest light over a broadband spectrum and focus it into well-defined positions, with potential applications in high efficiency solar cells and nanowire-based photodetectors and nanolasers. Moreover, thermo-optical and electrooptical modulation of plasmon resonances are realized in metallic nanostructures integrated with either a temperature-controlled phase transition material (vanadium dioxide, VO2), or ferroelectric thin films. Taking advantage of the high sensitivity of particle plasmon resonances to the change of its surrounding environment, we develop a plasmon resonance nanospectroscopy technique to study the effects of sizes and defects in the metal-insulator phase transition of VO2 at the single-particle level, and even single-domain level. Finally, we propose and examine the use of two-dimensional metallic nanohole arrays as a refractive index sensing platform for future label-free biosensors with good surface sensitivity and high-throughput detection ability. The designed plasmonic devices have great potential implications for constructing nextgeneration optical computers and chip-scale biosensors. The developed plasmon resonance nanospectroscopy has the potential to probe the interfacial or domain boundary scattering in polycrystalline and epitaxial thin films

Investigating the Electron Transport and Light Scattering Enhancement in Radial Core-Shell Metal-Metal Oxide Novel 3D Nanoarchitectures for Dye Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have attained considerable attention during the last decade because of the potential of becoming a low cost alternative to silicon based solar cells. Electron transport is one of the prominent processes in the cell and it is further a complex process because the transport medium is a mesoporous film. The gaps in the pores are completely filled by an electrolyte with high ionic strength, resulting in electron-ion interactions. Therefore, the electron transport in these so called state-of-the-art systems has a practical limit because of the low electron diffusion coefficient (Dn) in this mesoporous film photoanode. This work focuses on the influence of the advanced core-shell nanoarchitecture geometry on electron transport and also on the influence of electron-ion interactions. In order to achieve the proposed goals, DSSCs based on ordered, highly aligned, 3D radial core-shell Au-TiO2 hybrid nanowire arrays were fabricated, using three different approaches. J-V, IPCE, and EIS characteristics were studied. The efficiency, light scattering and charge transport properties of the core-shell nanowire based devices were compared to TiO2 nanotube as well as TiO2 mesoporous film based DSSCs. The Au nanowires inside the crystalline TiO2 anatase nanoshell provided a direct conduction path from the TiO2 shell to the TCO substrate and improved transport of electrons between the TiO2 and the TCO. The optical effects were studied by IPCE measurement which demonstrated that Au-TiO2 nanowires showed an improved light harvesting efficiency, including at longer wavelengths where the sensitizer has weak absorption. The metal nanostructures could enhance the absorption in DSSCs by either scattering light enabling a longer optical path-length, localized surface plasmon resonance (LSPR) or by near-field coupling between the surface plasmon polariton (SPP) and the dye excited state. Rapid, radial electron collection is of practical significance because it should allow alternate redox shuttles that show relatively fast electron-interception dynamics to be utilized without significant sacrifice of photocurrent. A combination of improved electron transport and enhanced light harvesting capability make Au-TiO2 core-shell nanowire arrays a promising photoanode nanoarchitecture for improving photovoltaic efficiency while minimizing costs by allowing thinner devices that use less material in their construction