Doctor of Philosophy

dissertationThe optoelectronic properties of nanoscale metal and semiconductor material systems are notably sensitive to their corresponding physical structure. Contemporary synthesis techniques enable careful control of nanoparticle con figurations and therefore provide a wide array of systems where the eff ects of physical morphology on the interaction between nanoscale materials and light can be carefully probed. The investigated properties are immediately relevant to light-harvesting and ultra-sensitive trace-analysis and sensing applications. In this work, the structure-property relationships of both individual semiconductor nanocrystal heterostructures and aggregates of plasmonic silver nanoparticles in rough metal fi lms are probed. The semiconductor heterostructures behave as model light-harvesting systems where optical energy absorbed by one portion of the structure is funneled, on the nanoscale, to a model light-harvesting center, in analogy to photosynthesis. In the plasmonic silver nanostructures, collective optical excitation of the conduction electrons - plasmons - con es electromagnetic radiation to well beyond the traditional diraction limit of light in nanoscale regions called "hot spots." Within these hot spots, light-matter interactions are greatly enhanced and thus enable trace-sensing applications such as Raman scattering from a single molecule. Thorough application of relatively simple single particle spectroscopy techniques is combined with high resolution electron microscopy to elucidate the subtle details on how physical structure controls the optical properties of both material systems. There are four main results of this work. (1) The linear and nonlinear optical response of rough silver fi lms is shown to be enhanced by the excitation of surface plasmon polaritons. (2) The enhanced nonlinear response of rough metal films is conjectured to originate from metal clusters, and the observation of stark fluctuations in their efficiency of second-harmonic generation is reported for the fi rst time. (3) The presence of and enhanced emission from silver clusters of only a few atoms plays an important role in the intrinsic optical response of the silver films with considerable implications for surface-enhanced Raman scattering. (4) The e ffects of physical anisotropy on the electronic states of semiconductor nanocrystals are explicitly identifi ed through correlated optical and electron microscopy of single particles. These eff ects are shown to have important rami cations in the internal energy-transfer process of single nanocrystals

Exciton storage in CdSe/CdS tetrapod semiconductor nanocrystals: Electric field effects on exciton and multiexciton states

CdSe/CdS nanocrystal tetrapods are interesting building blocks for excitonic circuits, where the flow of excitation energy is gated by an external stimulus. The physical morphology of the nanoparticle, along with the electronic structure, which favors electron delocalization between the two semiconductors, suggests that all orientations of a particle relative to an external electric field will allow for excitons to be dissociated, stored, and released at a later time. While this approach, in principle, works, and fluorescence quenching of over 95% can be achieved electrically, we find that discrete trap states within the CdS are required to dissociate and store the exciton. These states are rapidly filled up with increasing excitation density, leading to a dramatic reduction in quenching efficiency. Charge separation is not instantaneous on the CdS excitonic antennae in which light absorption occurs, but arises from the relaxed exciton following hole localization in the core. Consequently, whereas strong electromodulation of the core exciton is observed, the core multiexciton and the CdS arm exciton are not affected by an external electric field

Intermittency in second-harmonic radiation from plasmonic hot spots on rough silver films

Journal ArticleSurface enhancement of electromagnetic fields in plasmonic hot spots formed on rough silver films enables the observation of second-harmonic generation (SHG) from single metal nanoparticles. Nonlinear light scattering from these particles exhibits blinking in analogy to luminescence from single quantum dots, molecules and atoms; and fluctuations in single molecule surface-enhanced Raman scattering. Hot spots also display multiphoton white light emission besides SHG. In contrast to SHG, white light emission is stable with time, demonstrating that it is not the plasmonic field enhancement which fluctuates but the nonlinear polarizability (x(2)) of the emitting species

Mid-infrared Variability from the Spitzer Deep Wide-field Survey

We use the multi-epoch, mid-infrared Spitzer Deep Wide-Field Survey to investigate the variability of objects in 8.1 deg^2 of the NOAO Deep Wide Field Survey Boötes field. We perform a Difference Image Analysis of the four available epochs between 2004 and 2008, focusing on the deeper 3.6 and 4.5 μm bands. Out of 474, 179 analyzed sources, 1.1% meet our standard variability selection criteria that the two light curves are strongly correlated (r > 0.8) and that their joint variance (σ_(12)) exceeds that for all sources with the same magnitude by 2σ. We then examine the mid-IR colors of the variable sources and match them with X-ray sources from the XBoötes survey, radio catalogs, 24 μm selected active galactic nucleus (AGN) candidates, and spectroscopically identified AGNs from the AGN and Galaxy Evolution Survey (AGES). Based on their mid-IR colors, most of the variable sources are AGNs (76%), with smaller contributions from stars (11%), galaxies (6%), and unclassified objects, although most of the stellar, galaxy, and unclassified sources are false positives. For our standard selection criteria, 11%-12% of the mid-IR counterparts to X-ray sources, 24 μm AGN candidates, and spectroscopically identified AGNs show variability. The exact fractions depend on both the search depth and the selection criteria. For example, 12% of the 1131 known z>1 AGNs in the field and 14%-17% of the known AGNs with well-measured fluxes in all four Infrared Array Camera bands meet our standard selection criteria. The mid-IR AGN variability can be well described by a single power-law structure function with an index of γ ≈ 0.5 at both 3.6 and 4.5 μm, and an amplitude of S _0 ≃ 0.1 mag on rest-frame timescales of 2 yr. The variability amplitude is higher for shorter rest-frame wavelengths and lower luminosities

MAINTENANCE OF A MOUNTAIN VALLEY COLD POOL AND THERMAL BELT: A NUMERICAL STUDY

A mountain valley cold pool was simulated with the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model version 5 (MM5) to determine the effects of snow cover, planetary boundary layer (PBL) parameterizations, spin-up time, vertical and horizontal resolution, and horizontal diffusion on the maintenance of a cold pool. The simulation was of a cold pool that remained in the Yampa Valley of northwestern Colorado throughout 10 January 2004. Results of model runs were verified by a mesonetwork of weather stations located on the western slope of the valley. The presence of snow cover improved the simulation results, but was not sufficient to retain the cold pool in the valley. Increasing the model spin-up time, vertical resolution, and the PBL parameterization had little effect on the model results. However, increasing the horizontal resolution from 1 km to 100 m did improve the results and retained a weak inversion in the valley. Using the horizontal diffusion scheme of Zängl (2002) had an effect similar to that achieved by increasing the horizontal resolution

A polarizing situation: Taking an in-plane perspective for next-generation near-field studies

This mini-review provides a perspective on recent progress and emerging directions aimed at utilizing and controlling in-plane optical polarization, highlighting key application spaces where in-plane near-field tip responses have enabled recent advancements in the understanding and development of new nanostructured materials and devices

Further multiwavelength observations of the SSA22 Ly_alpha emitting `blob'

We present new follow-up observations of the sub-mm luminous Ly_alpha-emitting object in the SSA22 z=3.09 galaxy overdensity, referred to as `Blob 1' by Steidel et al.(2000). In particular we discuss high resolution Hubble Space Telescope optical imaging, Owens Valley Radio Observatory spectral imaging, Keck spectroscopy, VLA 20cm radio continuum imaging, and Chandra X-ray observations. We also present a more complete analysis of the existing James Clerk Maxwell Telescope sub-mm data. We detect several optical continuum components which may be associated with the core of the submillimeter emitting region. A radio source at the position of one of the HST components (22:17:25.94, +00:12:38.9) identifies it as the likely counterpart to the submillimeter source. We also tentatively detect the CO(4-3) molecular line, centered on the radio position. We use the CO(4-3) intensity to estimate a limit on the gas mass for the system. The optical morphology of sources within the Ly_alpha cloud appears to be filamentary, while the optical source identified with the radio source has a dense knot which may be an AGN or compact starburst. We obtain a Keck-LRIS spectrum of this object, despite its faintness (R=26.8). The spectrum reveals weak Ly_alpha emission, but no other obvious features, suggesting that the source is not an energetic AGN (or that it is extremely obscured). We use non-detections in deep Chandra X-ray images to constrain the nature of the `Blob'. Although conclusive evidence regarding the nature of the object remains hard to obtain at this redshift, the evidence presented here is at least consistent with a dust-obscured AGN surrounded by a starburst situated at the heart of this giant Ly_alpha cloud.Comment: 8 pages, 9figs (low res), to appear in ApJ, for higher res figures, http://www.submm.caltech.edu/~schapman/sa22_sept4.ps.g

Hydrophosphinylation of Styrenes Catalysed by Well‐Defined s‐Block Bimetallics

Advancing the applications of s-block heterobimetallic complexes in catalysis, we report the use of potassium magnesiate (PMDETA)2K2Mg(CH2SiMe3)4 [PMDETA=N,N,N’,N’,N’’-pentamethyldiethylenetriamine] for the catalytic hydrophosphinylation of styrenes under mild conditions. Exploiting chemical cooperation, this bimetallic approach offers greater catalytic activity and chemoselectivity than the single-metal components KCH2SiMe3 and Mg(CH2SiMe3)2. Stoichiometric studies between (PMDETA)2K2Mg(CH2SiMe3)4 and Ph2P(O)H help to elucidate the constitution of the active catalytic species, and illustrate the influence of donors on the potassium cation coordination, and how this may impact catalytic activity. Mechanistic investigations support that the rate determining step is the insertion of the olefinic substrate