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Silvija Gradečak. Brewster, Megan Marie Massachusetts Institute of Technology. Dept. of Materials Science and Engineering. Massachusetts Institute of Technology. Department of Materials Science and Engineering 2012-03-16T14:41:45Z 2012-03-16T14:41:45Z 2011 2011 http://hdl.handle.net/1721.1/69662 777362084 Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2011. This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. Cataloged from PDF version of thesis. Vita. Includes bibliographical references (p. 163-174). Semiconductor nanostructures exhibit distinct properties by virtue of nano-scale dimensionality, allowing for investigations of fundamental physics and the improvement of optoelectronic devices. Nanoscale morphological variations can drastically affect overall nanostructure properties because the investigation of nanostructure assemblies convolves nanoscale fluctuations to produce an averaged result. The investigation of individual nanostructures is thus paramount to a comprehensive analysis of nanomaterials. This thesis focuses on the study of individual GaAs, AlGaAs, and ZnO nanostructures to understand the influence of morphology on properties at the nanoscale. First, the diameter-dependent exciton-phonon coupling strengths of individual GaAs and AlGaAs nanowires were investigated by resonant micro-Raman spectroscopy near their direct bandgaps. The one-dimensional nanowire architecture was found to affect exciton lifetimes through an increase in surface state population relative to volume, resulting in Fröhlich coupling strengths stronger than any previously observed. Next, ZnO nanowire growth kinetics and mechanisms were found to evolve by altering precursor concentrations. The cathodoluminescence of nanowires grown by reaction-limited kinetics were quenched at the nanowire tips, likely due to point defects associated with the high Zn supersaturation required for reaction-limited growth. Further, cathodoluminescence was quenched in the vicinity of Au nanoparticles, which were found on nanowire sidewalls due to the transition in growth mechanism, caused by excited electron transfer from the ZnO conduction band to the Au Fermi level. Finally, ZnO nanowalls were grown by significantly increasing precursor flux and diffusion lengths over that of the ZnO nanowire growth. Nanowall growth began with the Au-assisted nucleation of nanowires, whose growth kinetics was a combination of Gibbs- Thomson-limited and diffusion-limited, followed by the domination of non-assisted film growth to form nanowalls. Nanoscale morphological variations, such as thickness variations and the presence of dislocations and Au nanoparticles, were directly correlated with nanoscale variations in optical properties. These investigations prove unequivocally that nanoscale morphological variations have profound consequences on optical properties on the nanoscale. Studies of individual nano-objects are therefore prerequisite to fully understanding, and eventually employing, these promising nanostructures. by Megan Marie Brewster. Ph.D. 174 p. eng Massachusetts Institute of Technology M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission. http://dspace.mit.edu/handle/1721.1/7582 Materials Science and Engineering. The interplay of structure and optical properties in individual semiconducting nanostructures Thesis application/pdf false Publication unknown unknown <Publication xmlns="https://www.openaire.eu/cerif-profile/1.1/" id="1298185a-24b7-4cb2-a8db-6e5d67e30c9e"> <Type xmlns="https://www.openaire.eu/cerif-profile/vocab/COAR_Publication_Types">http://purl.org/coar/resource_type/c_1843</Type> <Language>eng</Language> <Title>The interplay of structure and optical properties in individual semiconducting nanostructures</Title> <PublishedIn> <Publication> </Publication> </PublishedIn> <PublicationDate>2011</PublicationDate> <Authors> <Author> <DisplayName>Brewster, Megan Marie</DisplayName> <Affiliation> <OrgUnit> </OrgUnit> </Affiliation> </Author> </Authors> <Editors> </Editors> <Publishers> <Publisher> <DisplayName>Massachusetts Institute of Technology</DisplayName> <OrgUnit /> </Publisher> </Publishers> <License>http://dspace.mit.edu/handle/1721.1/7582</License> <Keyword>Materials Science and Engineering.</Keyword> <Abstract>Semiconductor nanostructures exhibit distinct properties by virtue of nano-scale dimensionality, allowing for investigations of fundamental physics and the improvement of optoelectronic devices. Nanoscale morphological variations can drastically affect overall nanostructure properties because the investigation of nanostructure assemblies convolves nanoscale fluctuations to produce an averaged result. The investigation of individual nanostructures is thus paramount to a comprehensive analysis of nanomaterials. This thesis focuses on the study of individual GaAs, AlGaAs, and ZnO nanostructures to understand the influence of morphology on properties at the nanoscale. First, the diameter-dependent exciton-phonon coupling strengths of individual GaAs and AlGaAs nanowires were investigated by resonant micro-Raman spectroscopy near their direct bandgaps. The one-dimensional nanowire architecture was found to affect exciton lifetimes through an increase in surface state population relative to volume, resulting in Fröhlich coupling strengths stronger than any previously observed. Next, ZnO nanowire growth kinetics and mechanisms were found to evolve by altering precursor concentrations. The cathodoluminescence of nanowires grown by reaction-limited kinetics were quenched at the nanowire tips, likely due to point defects associated with the high Zn supersaturation required for reaction-limited growth. Further, cathodoluminescence was quenched in the vicinity of Au nanoparticles, which were found on nanowire sidewalls due to the transition in growth mechanism, caused by excited electron transfer from the ZnO conduction band to the Au Fermi level. Finally, ZnO nanowalls were grown by significantly increasing precursor flux and diffusion lengths over that of the ZnO nanowire growth. Nanowall growth began with the Au-assisted nucleation of nanowires, whose growth kinetics was a combination of Gibbs- Thomson-limited and diffusion-limited, followed by the domination of non-assisted film growth to form nanowalls. Nanoscale morphological variations, such as thickness variations and the presence of dislocations and Au nanoparticles, were directly correlated with nanoscale variations in optical properties. These investigations prove unequivocally that nanoscale morphological variations have profound consequences on optical properties on the nanoscale. Studies of individual nano-objects are therefore prerequisite to fully understanding, and eventually employing, these promising nanostructures.</Abstract> <Access xmlns="http://purl.org/coar/access_right" > </Access> </Publication>