3 research outputs found
Brightening of Long, Polymer-Wrapped Carbon Nanotubes by sp Functionalization in Organic Solvents
The functionalization of semiconducting single-walled carbon nanotubes
(SWNTs) with sp defects that act as luminescent exciton traps is a
powerful means to enhance their photoluminescence quantum yield (PLQY) and to
add optical properties. However, the synthetic methods employed to introduce
these defects are so far limited to aqueous dispersions of surfactant-coated
SWNTs, often with short tube lengths, residual metallic nanotubes and poor film
formation properties. In contrast to that, dispersions of polymer-wrapped SWNTs
in organic solvents feature unrivaled purity, higher PLQY and are easily
processed into thin films for device applications. Here, we introduce a simple
and scalable phase-transfer method to solubilize diazonium salts in organic
nonhalogenated solvents for the controlled reaction with polymer-wrapped SWNTs
to create luminescent aryl defects. Absolute PLQY measurements are applied to
reliably quantify the defect-induced brightening. The optimization of defect
density and trap depth results in PLQYs of up to 4 % with 90 % of photons
emitted through the defect channel. We further reveal the strong impact of
initial SWNT quality and length on the relative brightening by sp
defects. The efficient and simple production of large quantities of
defect-tailored polymer-sorted SWNTs enables aerosol-jet printing and
spin-coating of thin films with bright and nearly reabsorption-free defect
emission, which are desired for carbon nanotube-based near-infrared
light-emitting devices
QUANTUM EFFECTS ON ENERGY TRANSPORT IN 2D HETERO-INTERFACES AND LEAD HALIDE PEROVSKITE QUANTUM DOTS
Photovoltaics are leading devices in green energy production. Understanding the fundamental physics behind energy transport in candidate materials for future photovoltaic and optoelectronic devices is necessary to both realize material limitations and improve efficiency. Excitons, which are bound electron-hole pairs, are central to determining how energy propagates throughout semiconductors. Exciton transport is greatly influenced by material dimensionality. In highly ordered quantum dot (QD) systems, electronic coupling between individual QDs can lead to coherent exciton transport, whereas in two-dimensional heterostructures, excitons can form at the interface of a heterojunction, creating charge-transfer excitons.This dissertation is dedicated to summarizing the studies of exciton transport and behavior in two systems: perovskite QD superlattices and transition metal dichalcogenide (TMDC)/polyacene heterostructures. Chapter 1 provides readers with details on these materials in addition to information on the fundamental concepts (i.e., excitons, phonons, energy transfer) needed to best appreciate further chapters. Chapter 2 summarizes the spectroscopic techniques (photoluminescence and transient absorption spectroscopy and microscopy) used to examine exciton behavior. Next, the effects of disorder and dephasing pathways on the ability of perovskite QDs to coherently couple is investigated through the lens of superradiance in Chapter 3. After this, the temperature-dependent exciton transport within perovskite QD superlattices is imaged with high spatial and temporal resolutions in Chapter 4. The experimental transport data on these superlattices provides evidence for environment-assisted quantum transport, which, until this study, had yet to be realized in solid-state systems. In Chapter 5, attention is switched to verifying the existence and deepening the understanding of the behavior of several spatially separated interlayer excitons in a tungsten disulfide/tetracene heterostructure. Finally, Chapter 6 summarizes the preliminary results obtained through transient absorption spectroscopy on other TMDC/polyacene heterostructures where separation of the triplet pair state is attempted. It is this author’s hope that this dissertation will not only summarize their graduate work but will also serve as inspiration for others to continue learning and contribute to the advancement of the energy research field.</p
Superradiance and Exciton Delocalization in Perovskite Quantum Dot Superlattices
Achieving superradiance in solids is challenging due
to fast dephasing
processes from inherent disorder and thermal fluctuations. Perovskite
quantum dots (QDs) are an exciting class of exciton emitters with
large oscillator strength and high quantum efficiency, making them
promising for solid-state superradiance. However, a thorough understanding
of the competition between coherence and dephasing from phonon scattering
and energetic disorder is currently unavailable. Here, we present
an investigation of exciton coherence in perovskite QD solids using
temperature-dependent photoluminescence line width and lifetime measurements.
Our results demonstrate that excitons are coherently delocalized over
3 QDs at 11 K in superlattices leading to superradiant emission. Scattering
from optical phonons leads to the loss of coherence and exciton localization
to a single QD at temperatures above 100 K. At low temperatures, static
disorder and defects limit exciton coherence. These results highlight
the promise and challenge in achieving coherence in perovskite QD
solids