Browsing by Subject "metal nanoparticles"
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Item Open Access Localized plasmon-coupled semiconductor nanocrystal emitters for innovative device applications(Bilkent University, 2007) Soğancı, İbrahim MuratQuantum confinement allows for the development of novel luminescent materials such as colloidal semiconductor quantum dots for a variety of photonic applications spanning from biomedical labeling to white light generation. However, such device applications require efficient photoluminescence. To this end, in this thesis we investigate the spontaneous emission characteristics of semiconductor nanocrystal emitters under different conditions and their enhancement and controlled modification via plasmonic resonance coupling, placing metallic nanoparticles in their proximity, for innovative device applications. We first present our theoretical and experimental work on the optical characterization of nanocyrstals (e.g., CdSe, CdS, and CdSe/ZnS) including absorption/photoluminescence, time-resolved luminescence, and excitation spectra measurements. Here we demonstrate very strong electromodulation (up to 90%) of photoluminescence and absorption of such nanocrystals (nanodots and nanorods) for optical modulator applications. Second, we present our electromagnetic modeling on the optical response of metal nanoparticles using finite-difference-time-domain method. For the first time, using localized plasmons of metal nanoisland films (nano-silver) carefully spectrally and spatially tuned for optimal coupling conditions, we report very significant controlled modifications of nanocrystal emission including the peak emission wavelength shift (by 14nm), emission linewidth reduction (by 10nm with 22% FWHM reduction), photoluminescence intensity enhancement (15.1- and 21.6-fold compared to the control groups of the same nanocrystals with no plasmonic coupling and those with identical nano-silver but no dielectric spacer in the case of non-radiative energy transfer, respectively), and selectable peaking of surface-state emission at desired wavelengths. Such localized plasmonic engineering of nanocrystal emitters opens new possibilities for our lightemitting and photovoltaic devices.Item Open Access Novel design-based complex nanostructures in hybrid core-shell architectures for high-efficiency light generation(Bilkent University, 2010) Özel, İlkem ÖzgeRecent developments in nanoscience and nanotechnology have given rise to the discovery of hybrid nanostructured multi-component materials that serve several tasks all at once. A very important and rapidly growing field of these materials is the development of highly efficient fluorophores to meet the urgent demand of low-energy consuming, high-quality light emitters for future solid-state lighting applications. Such hybrid nanomaterials are entailed to exhibit extraordinary optoelectronic properties compared to the bulk case of their single components such as enhanced quantum efficiency, tunable multi-color emission, and reduction of multiple processing steps. Herein, to address these requirements, we propose and demonstrate novel design-based complex nanomaterials in hybrid multi-shell architectures for high-efficiency light generation. These requirements are made possible by using the concept of hybrid core-shell-… nanostructures comprising at least two units, including hybrid metalcore/dielectric-shell nanoparticles furnished with an outer shell of semiconductor nanocrystals for enhanced emission and different conjugated polymers forming a single multi-polymer nanoparticle and emitting simultaneously at different wavelengths. In the first part of this thesis, we developed and demonstrated Au-silica core/shell nanoparticles that successfully assemble CdTe nanocrystals right on their silica shells for enhanced plasmonexciton interactions, while solving the common problems of lacking control in dielectric spacing and limited film thickness typically encountered in such plasmon-coupled nanocrystals. Here we present the synthesis and characterization results of this new set of multi-shell decorated nanoparticle composites with a tunable dielectric spacing thickness of silica shell precisely controlled by synthesis to optimize plasmon-exciton interactions for enhanced emission. Experimental data obtained from steady-state and time-resolved photoluminescence measurements together with extensive computational analysis clearly verify the strong plasmon-exciton interactions in these designbased multi-shell nanocomposites. In the second part, we construct bi-polymer nanoparticle systems in various architectures of core/shells, for each of which thorough investigations of the non-radiative energy transfer mechanisms are made. Here we present the synthesis and characterization results of these core/shell bi-polymer nanoassemblies. The flexibility of designing such bipolymer nanostructures allows for the optimization of maximum energy transfer efficiency. This concept of complex hybrid nanostructures for high-efficiency light generation opens up new paths for optoelectronic devices and nanophotonics applications including those in solid-state lighting.Item Open Access Plasmonics from metal nanoparticles for solar cell applications(Bilkent University, 2013) Günendi, Mehmet CanIn today’s economy, need for development in energy is essential. Solar energy is safe, and at the same time is one of the cleanest, cheapest choices of energy alternative to fossil fuels. In this perspective, using the sun light effectively is in fundamental importance. One of the problems, because of the indirect band gap of the material Si, is small energy conversion ratios of various solar cell structures and limited absorption of red light. Because of the material properties, Si cells cannot absorb red light, which contributes great amount of the sun light. One of the recent developed techniques to use red light is using metal nanoparticles (MNP) embedded in a semiconductor medium as sub-wavelength antennas or MNP scatterers, hence increasing the effective path length of light in the cell. Absorption and scattering are mostly in plasmon resonances. Shifting the plasmon resonance peaks is possible by changing various parameters of the system like the size of the MNPs. In this work, Finite-Difference Time-Domain (FDTD) method is used to analyze various systems worked. Mainly the MEEP package, developed at MIT, is used to simulate systems and other codes, related to analytical work, have also used to compare results. The plasmon resonances of various sizes of Ag MNPs embedded in different mediums at different positions are analyzed. Critical parameters like particle size, shape, dielectric medium, film thickness are discussed for improved solar cell applications.Item Open Access Probing hot-electron effects in plasmonic surfaces using X-ray photoelectron spectroscopy(Bilkent University, 2014) Çupallari, AndiHot-electron effects in plasmonic structures have been recently investigated as potential alternative mechanisms for solar energy harvesting and photodetection. [1][2][3] Hot-electron effects provide a semiconductor free route for the conversion of photons into electrical power. Here we investigate plasmonic hot electron effects in Metal-Insulator-Metal (MIM) structures using X-ray photoelectron spectroscopy (XPS). XPS has been previously used to investigate optoelectronic effects in semiconductors and nanocomposite surfaces. [4][5][6] Here, a similar approach is used to characterize the plasmonic and hot electron effects in MIM Junctions. Monochromatic Laser excitation with 450, 532 and 650 nm wavelengths are employed to illuminate the plasmonic surfaces fabricated using thermal evaporation, atomic layer deposition and electron beam lithography. The top metal of the MIM structures act as the plasmonic antenna (metal nanodiscs and gratings/stripes) that provide wavelength selective or wide band optical absorption. Plasmonic enhancement at the interface between the top metal and the insulator enhances the absorption of light in the device and leads to excitation of a larger number of hot electrons from the metal. Hot electron effects are characterized through studying the metal-insulator-metal junction and comparing shifts of binding energy belonging to the top metal islands for dark and illuminated conditions. XPS spectrum provides important information regarding the plasmonic and hot electron effects in the interface between top metal and the dielectric. A systematic study of the dependence of the XPS spectra on excitation wavelength, light intensity, polarization, insulator thickness and nanostructure geometry is presented. Effects of using different metals and insulator materials are also studied in symmetric and asymmetric tunnel junctions. Keywords:Item Open Access Selective plasmonic control of excitons and their non-radiative energy transfer in colloidal semiconductor quantum dot solids(Bilkent University, 2009) Özel, TuncayTo date extensive research has proved that semiconductors and metals exhibit extraordinary optical properties in nano-dimensions compared to their bulk counterparts. For example, an interesting effect is observed in metal nanostructures/nanoparticles (NPs) that we form to obtain localized plasmons, with their optical response highly tuneable using the size effect. Another field of interest at the nanoscale is the investigation of light generation and harvesting using colloidal semiconductor quantum dot nanocrystals (NCs) that we synthesize in few nanometers, with their emission and absorption excitonic peaks conveniently tuneable using the size effect. In this thesis, we proposed and demonstrated the first accounts of selectively plasmonically-controlled colloidal quantum dot emitters assembled in innovative architectures, with a control achieved either through spatial selection or spectral selection. In the first set of designs, we developed for the first time plasmonic NC-composites that rely on spatially-selected plasmon-coupled CdTe NC-monolayers interspaced with respect to Au NP-monolayers in a repeating three-dimensional layer-by-layer architecture. In these bottom-up designs of hybrid nanocomposites, the photoluminescence kinetics is strongly modified and a record quantum efficiency of 30% is achieved for such CdTe NC solids. In the second set of designs, we showed the first spectrally-selected plasmon-coupling of surfaceemitting CdS NCs using optimized Ag NP deposits. This architecture allowed for the surface-state emission to be selectively enhanced while the interband emission is simultaneously suppressed in the same plasmon-coupled NCs, leading to the strongest surface-state emission from such CdS NCs reported with respect to their interband emission (with a >12-fold enhancement). Yet another important proximity phenomenon effective among quantum dot emitters is the Förster-type non-radiative resonance energy transfer (ET), in which excitonic excitation energy of the donor-NCs is non-radiatively transferred to the acceptor-NCs via dipole-dipole coupling. In the third set of our designs, we combined two fundamental proximity mechanisms of plasmon coupling and non-radiative energy transfer in the same NC solids. In plasmonic ET, we reported for the first time selectively plasmon-coupling of NC-acceptors and then that of NC-donors in the ET pair, both of which result in substantial enhancement of the acceptor emission with respect to ET with no plasmon coupling (with a maximum of 2-fold enhancement) as verified by their steadystate and time-resolved photoluminescence. This concept of spectrally/spatiallyselective plasmon coupling in quantum dots paves a new path for devices and sensors in nanophotonics.