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- All Subjects: Quantum Dots
- Creators: Smith, David
Description
This dissertation presents research findings regarding the exploitation of localized surface plasmon (LSP) of epitaxial Ag islands as a means to enhance the photoluminescence (PL) of Germanium (Ge) quantum dots (QDs). The first step of this project was to investigate the growth of Ag islands on Si(100). Two distinct families of Ag islands have been observed. “Big islands” are clearly faceted and have basal dimensions in the few hundred nm to μm range with a variety of basal shapes. “Small islands” are not clearly faceted and have basal diameters in the 10s of nm range. Big islands form via a nucleation and growth mechanism, and small islands form via precipitation of Ag contained in a planar layer between the big islands that is thicker than the Stranski-Krastanov layer existing at room-temperature.
The pseudodielectric functions of epitaxial Ag islands on Si(100) substrates were investigated with spectroscopic ellipsometry. Comparing the experimental pseudodielectric functions obtained for Si with and without Ag islands clearly identifies a plasmon mode with its dipole moment perpendicular to the surface. This observation is confirmed using a simulation based on the thin island film (TIF) theory. Another mode parallel to the surface may be identified by comparing the experimental pseudodielectric functions with the simulated ones from TIF theory. Additional results suggest that the LSP energy of Ag islands can be tuned from the ultra-violet to the infrared range by an amorphous Si (α-Si) cap layer.
Heterostructures were grown that incorporated Ge QDs, an epitaxial Si cap layer and Ag islands grown atop the Si cap layer. Optimum growth conditions for distinct Ge dot ensembles and Si cap layers were obtained. The density of Ag islands grown on the Si cap layer depends on its thickness. Factors contributing to this effect may include the average strain and Ge concentration on the surface of the Si cap layer.
The effects of the Ag LSP on the PL of Ge coherent domes were investigated for both α-Si capped and bare Ag islands. For samples with low-doped substrates, the LSPs reduce the Ge dot-related PL when the Si cap layer is below some critical thickness and have no effect on the PL when the Si cap layer is above the critical thickness. For samples grown on highly-doped wafers, the LSP of bare Ag islands enhanced the PL of Ge QDs by ~ 40%.
The pseudodielectric functions of epitaxial Ag islands on Si(100) substrates were investigated with spectroscopic ellipsometry. Comparing the experimental pseudodielectric functions obtained for Si with and without Ag islands clearly identifies a plasmon mode with its dipole moment perpendicular to the surface. This observation is confirmed using a simulation based on the thin island film (TIF) theory. Another mode parallel to the surface may be identified by comparing the experimental pseudodielectric functions with the simulated ones from TIF theory. Additional results suggest that the LSP energy of Ag islands can be tuned from the ultra-violet to the infrared range by an amorphous Si (α-Si) cap layer.
Heterostructures were grown that incorporated Ge QDs, an epitaxial Si cap layer and Ag islands grown atop the Si cap layer. Optimum growth conditions for distinct Ge dot ensembles and Si cap layers were obtained. The density of Ag islands grown on the Si cap layer depends on its thickness. Factors contributing to this effect may include the average strain and Ge concentration on the surface of the Si cap layer.
The effects of the Ag LSP on the PL of Ge coherent domes were investigated for both α-Si capped and bare Ag islands. For samples with low-doped substrates, the LSPs reduce the Ge dot-related PL when the Si cap layer is below some critical thickness and have no effect on the PL when the Si cap layer is above the critical thickness. For samples grown on highly-doped wafers, the LSP of bare Ag islands enhanced the PL of Ge QDs by ~ 40%.
ContributorsKong, Dexin (Author) / Drucker, Jeffery (Thesis advisor) / Chen, Tingyong (Committee member) / Ros, Robert (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2015
Description
The development of high efficiency III-V solar cells is needed to meet the demands of a promising renewable energy source. Intermediate band solar cells (IBSCs) using semiconductor quantum dots (QDs) have been proposed to exceed the Shockley-Queisser efficiency limit [1]. The introduction of an IB in the forbidden gap of host material generates two additional carrier transitions for sub-bandgap photon absorption, leading to increased photocurrent of IBSCs while simultaneously allowing an open-circuit voltage of the highest band gap. To realize a high efficiency IBSC, QD structures should have high crystal quality and optimized electronic properties. This dissertation focuses on the investigation and optimization of the structural and optical properties of InAs/GaAsSb QDs and the development of InAs/GaAsSb QD-based IBSCs.
In the present dissertation, the interband optical transition and carrier lifetime of InAs/GaAsSb QDs with different silicon delta-doping densities have been first studied by time-integrated and time-resolved photoluminescence (PL). It is found that an optimized silicon delta-doping density in the QDs enables to fill the QD electronic states with electrons for sub-bandgap photon absorption and to improve carrier lifetime of the QDs.
After that, the crystal quality and QD morphology of single- and multi-stack InAs/GaAsSb QDs with different Sb compositions have been investigated by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The TEM studies reveal that QD morphology of single-stack QDs is affected by Sb composition due to strain reducing effect of Sb incorporation. The XRD studies confirm that the increase of Sb composition increases the lattice mismatch between GaAs matrix and GaAsSb spacers, resulting in increase of the strain relaxation in GaAsSb of the multi-stack QDs. Furthermore, the increase of Sb composition causes a PL redshift and increases carrier lifetime of QDs.
Finally, the spacer layer thickness of multi-stack InAs/GaAsSb QDs is optimized for the growth of InAs/GaAsSb QD solar cells (QDSCs). The InAs/GaAsSb QDSCs with GaP strain compensating layer are grown and their device performances are characterized. The increase of GaP coverage is beneficial to improve the conversion efficiency of the QDSCs. However, the conversion efficiency is reduced when using a relatively large GaP coverage.
In the present dissertation, the interband optical transition and carrier lifetime of InAs/GaAsSb QDs with different silicon delta-doping densities have been first studied by time-integrated and time-resolved photoluminescence (PL). It is found that an optimized silicon delta-doping density in the QDs enables to fill the QD electronic states with electrons for sub-bandgap photon absorption and to improve carrier lifetime of the QDs.
After that, the crystal quality and QD morphology of single- and multi-stack InAs/GaAsSb QDs with different Sb compositions have been investigated by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The TEM studies reveal that QD morphology of single-stack QDs is affected by Sb composition due to strain reducing effect of Sb incorporation. The XRD studies confirm that the increase of Sb composition increases the lattice mismatch between GaAs matrix and GaAsSb spacers, resulting in increase of the strain relaxation in GaAsSb of the multi-stack QDs. Furthermore, the increase of Sb composition causes a PL redshift and increases carrier lifetime of QDs.
Finally, the spacer layer thickness of multi-stack InAs/GaAsSb QDs is optimized for the growth of InAs/GaAsSb QD solar cells (QDSCs). The InAs/GaAsSb QDSCs with GaP strain compensating layer are grown and their device performances are characterized. The increase of GaP coverage is beneficial to improve the conversion efficiency of the QDSCs. However, the conversion efficiency is reduced when using a relatively large GaP coverage.
ContributorsKim, Yeongho (Author) / Honsberg, Christiana (Thesis advisor) / Goodnick, Stephen (Committee member) / Faleev, Nikolai (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2015
Description
Photocatalytic water splitting is a promising technique to produce H2 fuels from water using sustainable solar energy. To better design photocatalysts, the understanding of charge transfer at surfaces/interfaces and the corresponding structure change during the reaction is very important. Local structural and chemical information on nanoparticle surfaces or interfaces can be achieved through characterizations on transmission electron microscopy (TEM). Emphasis should be put on materials structure changes during the reactions in their “working conditions”. Environmental TEM with in situ light illumination system allows the photocatalysts to be studied under light irradiation when exposed to H2O vapor. A set of ex situ and in situ TEM characterizations are carried out on typical types of TiO2 based photocatalysts. The observed structure changes during the reaction are correlated with the H2 production rate for structure-property relationships.
A surface disordering was observed in situ when well-defined anatase TiO2 rhombohedral nanoparticles were exposed to 1 Torr H2O vapor and 10suns light inside the environmental TEM. The disordering is believed to be related to high density of hydroxyl groups formed on surface oxygen vacancies during water splitting reactions.
Pt co-catalyst on TiO2 is able to split pure water producing H2 and O2. The H2 production rate drops during the reaction. Particle size growth during reaction was discovered with Z-contrast images. The particle size growth is believed to be a photo-electro-chemical Ostwald ripening.
Characterizations were also carried out on a more complicated photocatalyst system: Ni/NiO core/shell co-catalyst on TiO2. A decrease of the H2 production rate resulting from photo-corrosion was observed. The Ni is believed to be oxidized to Ni2+ by OH• radicals which are intermediate products of H2O oxidation. The mechanism that the OH• radicals leak into the cores through cracks on NiO shells is more supported by experiments.
Overall this research has done a comprehensive ex situ and in situ TEM characterizations following some typical TiO2 based photocatalysts during reactions. This research has shown the technique availability to study photocatalyst inside TEM in photocatalytic conditions. It also demonstrates the importance to follow structure changes of materials during reactions in understanding deactivation mechanisms.
A surface disordering was observed in situ when well-defined anatase TiO2 rhombohedral nanoparticles were exposed to 1 Torr H2O vapor and 10suns light inside the environmental TEM. The disordering is believed to be related to high density of hydroxyl groups formed on surface oxygen vacancies during water splitting reactions.
Pt co-catalyst on TiO2 is able to split pure water producing H2 and O2. The H2 production rate drops during the reaction. Particle size growth during reaction was discovered with Z-contrast images. The particle size growth is believed to be a photo-electro-chemical Ostwald ripening.
Characterizations were also carried out on a more complicated photocatalyst system: Ni/NiO core/shell co-catalyst on TiO2. A decrease of the H2 production rate resulting from photo-corrosion was observed. The Ni is believed to be oxidized to Ni2+ by OH• radicals which are intermediate products of H2O oxidation. The mechanism that the OH• radicals leak into the cores through cracks on NiO shells is more supported by experiments.
Overall this research has done a comprehensive ex situ and in situ TEM characterizations following some typical TiO2 based photocatalysts during reactions. This research has shown the technique availability to study photocatalyst inside TEM in photocatalytic conditions. It also demonstrates the importance to follow structure changes of materials during reactions in understanding deactivation mechanisms.
ContributorsZhang, Liuxian (Author) / Crozier, Peter (Thesis advisor) / Smith, David (Committee member) / Chan, Candace (Committee member) / Liu, Jingyue (Committee member) / Arizona State University (Publisher)
Created2015