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

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

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

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

Surface plasmon delocalization in silver nanoparticle aggregates revealed by subdiffraction supercontinuum hot spots

The plasmonic resonances of nanostructured silver films produce exceptional surface enhancement, enabling reproducible single-molecule Raman scattering measurements. Supporting a broad range of plasmonic resonances, these disordered systems are difficult to investigate with conventional far-field spectroscopy. Here, we use nonlinear excitation spectroscopy and polarization anisotropy of single optical hot spots of supercontinuum generation to track the transformation of these plasmon modes as the mesoscopic structure is tuned from a film of discrete nanoparticles to a semicontinuous layer of aggregated particles. We demonstrate how hot spot formation from diffractively-coupled nanoparticles with broad spectral resonances transitions to that from spatially delocalized surface plasmon excitations, exhibiting multiple excitation resonances as narrow as 13 meV. Photon-localization microscopy reveals that the delocalized plasmons are capable of focusing multiple narrow radiation bands over a broadband range to the same spatial region within 6 nm, underscoring the existence of novel plasmonic nanoresonators embedded in highly disordered systems

Electrically driven photon emission from individual atomic defects in monolayer WS2.

Quantum dot-like single-photon sources in transition metal dichalcogenides (TMDs) exhibit appealing quantum optical properties but lack a well-defined atomic structure and are subject to large spectral variability. Here, we demonstrate electrically stimulated photon emission from individual atomic defects in monolayer WS2 and directly correlate the emission with the local atomic and electronic structure. Radiative transitions are locally excited by sequential inelastic electron tunneling from a metallic tip into selected discrete defect states in the WS2 bandgap. Coupling to the optical far field is mediated by tip plasmons, which transduce the excess energy into a single photon. The applied tip-sample voltage determines the transition energy. Atomically resolved emission maps of individual point defects closely resemble electronic defect orbitals, the final states of the optical transitions. Inelastic charge carrier injection into localized defect states of two-dimensional materials provides a powerful platform for electrically driven, broadly tunable, atomic-scale single-photon sources

Long-Range Exciton Diffusion in Two-Dimensional Assemblies of Cesium Lead Bromide Perovskite Nanocrystals

F\"orster Resonant Energy Transfer (FRET)-mediated exciton diffusion through artificial nanoscale building block assemblies could be used as a new optoelectronic design element to transport energy. However, so far nanocrystal (NC) systems supported only diffusion length of 30 nm, which are too small to be useful in devices. Here, we demonstrate a FRET-mediated exciton diffusion length of 200 nm with 0.5 cm2/s diffusivity through an ordered, two-dimensional assembly of cesium lead bromide perovskite nanocrystals (PNC). Exciton diffusion was directly measured via steady-state and time-resolved photoluminescence (PL) microscopy, with physical modeling providing deeper insight into the transport process. This exceptionally efficient exciton transport is facilitated by PNCs high PL quantum yield, large absorption cross-section, and high polarizability, together with minimal energetic and geometric disorder of the assembly. This FRET-mediated exciton diffusion length matches perovskites optical absorption depth, opening the possibility to design new optoelectronic device architectures with improved performances, and providing insight into the high conversion efficiencies of PNC-based optoelectronic devices