11 research outputs found
PlasmonâExciton Interactions Probed Using Spatial Coentrapment of Nanoparticles by Topological Singularities
We study plasmonâexciton interaction by using topological singularities to spatially confine, selectively deliver, cotrap and optically probe colloidal semiconductor and plasmonic nanoparticles. The interaction is monitored in a single quantum system in the bulk of a liquid crystal medium where nanoparticles are manipulated and nanoconfined far from dielectric interfaces using laser tweezers and topological configurations containing singularities. When quantum dot-in-a-rod particles are spatially colocated with a plasmonic gold nanoburst particle in a topological singularity core, its fluorescence increases because blinking is significantly suppressed and the radiative decay rate increases by nearly an order of magnitude owing to the Purcell effect. We argue that the blinking suppression is the result of the radiative rate change that mitigates Auger recombination and quantum dot ionization, consequently reducing nonradiative recombination. Our work demonstrates that topological singularities are an effective platform for studying and controlling plasmonâexciton interactions
Charge Generation in PbS Quantum Dot Solar Cells Characterized by Temperature-Dependent Steady-State Photoluminescence
Charge-carrier generation and transport within PbS quantum dot (QD) solar cells is investigated by measuring the temperature-dependent steady-state photoluminescence (PL) concurrently during <i>in situ</i> currentâvoltage characterization. We first compare the temperature-dependent PL quenching for PbS QD films where the PbS QDs retain their original oleate ligand to that of PbS QDs treated with 1,2-ethanedithiol (EDT), producing a conductive QD layer, either on top of glass or on a ZnO nanocrystal film. We then measure and analyze the temperature-dependent PL in a completed QD-PV architecture with the structure Al/MoO<sub>3</sub>/EDT-PbS/ZnO/ITO/glass, collecting the PL and the current simultaneously. We find that at low temperatures excitons diffuse to the ZnO interface, where PL is quenched <i>via</i> interfacial charge transfer. At high temperatures, excitons dissociate in the bulk of the PbS QD film <i>via</i> phonon-assisted tunneling to nearby QDs, and that dissociation is in competition with the intrinsic radiative and nonradiative rates of the individual QDs. The activation energy for exciton dissociation in the QD-PV devices is found to be âŒ40 meV, which is considerably lower than that of the electrodeless samples, and suggests unique interactions between injected and photogenerated carriers in devices
Coupling between a Molecular Charge-Transfer Exciton and Surface Plasmons in a Nanostructured Metal Grating
The
interaction of molecular excitons in organic thin films with
surface plasmon polaritons (SPPs) in nanostructured metal electrodes
represents a unique opportunity for enhancing the properties of the
active layer of a photoconversion device. We present evidence of hybridization
between charge-transfer excitons (CTEs) in 3,4,9,10-perylenetetracarboxylic
dianhydride (PTCDA) and SPP modes in silver grating nanostructures.
Molecular and SPP absorption peaks exhibit avoided crossings in angle-dependent
reflectivity experiments, which are verified by electromagnetic-field
simulations of the PTCDA-grating structure. Photoluminescence measurements
indicate that the radiative decay of the CTE is enhanced. Besides
energetic resonance, selective coupling between the SPP and the exciton
in this unique case may be aided by the oriented nature of PTCDA into
1-D âmolecular stacksâ as well as the delocalized nature
of the CTE
Controlling Exciton/Exciton Recombination in 2âD Perovskite Using ExcitonâPolariton Coupling
In this paper, we
demonstrate that exciton/exciton annihilation
in the 2D perovskite (PEA)2PbI4 (PEPI)a
major loss mechanism in solar cells and light-emitting diodes, can
be controlled through coupling of excitons with cavity polaritons.
We study the excited state dynamics using time-resolved transient
absorption spectroscopy and show that the system can be tuned through
a strong coupling regime by varying the cavity width through the PEPI
layer thickness. Remarkably, strong coupling occurs even when the
cavity quality factor remains poor, providing easy optical access.
We demonstrate that the observed derivative-like transient absorption
spectra can be modeled using a time-dependent Rabi splitting that
occurs because of transient bleaching of the excitonic states. When
PEPI is strongly coupled to the cavity, the exciton/exciton annihilation
rate is suppressed by 1 order of magnitude. A model that relies on
the partly photonic character of polaritons explains the results as
a function of detuning
Silicon Photoelectrode Thermodynamics and Hydrogen Evolution Kinetics Measured by Intensity-Modulated High-Frequency Resistivity Impedance Spectroscopy
We
present an impedance technique based on light intensity-modulated
high-frequency resistivity (IMHFR) that provides a new way to elucidate
both the thermodynamics and kinetics in complex semiconductor photoelectrodes.
We apply IMHFR to probe electrode interfacial energetics on oxide-modified
semiconductor surfaces frequently used to improve the stability and
efficiency of photoelectrochemical water splitting systems. Combined
with current density-voltage measurements, the technique quantifies
the overpotential for proton reduction relative to its thermodynamic
potential in Si photocathodes coated with three oxides (SiO<sub><i>x</i></sub>, TiO<sub>2</sub>, and Al<sub>2</sub>O<sub>3</sub>) and a Pt catalyst. In pH 7 electrolyte, the flatband potentials
of TiO<sub>2</sub>- and Al<sub>2</sub>O<sub>3</sub>-coated Si electrodes
are negative relative to samples with native SiO<sub><i>x</i></sub>, indicating that SiO<sub><i>x</i></sub> is a better
protective layer against oxidative electrochemical corrosion than
ALD-deposited crystalline TiO<sub>2</sub> or Al<sub>2</sub>O<sub>3</sub>. Adding a Pt catalyst to SiO<sub><i>x</i></sub>/Si minimizes
proton reduction overpotential losses but at the expense of a reduction
in available energy characterized by a more negative flatband potential
relative to catalyst-free SiO<sub><i>x</i></sub>/Si
Semiconductor-to-Metal Transition in Rutile TiO<sub>2</sub> Induced by Tensile Strain
We
report the first observation of a reversible, degenerate doping
of titanium dioxide with strain, which is referred to as a semiconductor-to-metal
transition. Application of tensile strain to a âŒ50 nm film
of rutile TiO<sub>2</sub> thermally grown on a superelastic nitinol
(NiTi intermetallic) substrate causes reversible degenerate doping
as evidenced by electrochemistry, X-ray photoelectron spectroscopy
(XPS), and conducting atomic force microscopy (CAFM). Cyclic voltammetry
and impedance measurements show behavior characteristic of a highly
doped <i>n</i>-type semiconductor for unstrained TiO<sub>2</sub> transitioning to metallic behavior under tensile strain.
The transition reverses when strain is removed. Valence band XPS spectra
show that samples strained to 5% exhibit metallic-like intensity near
the Fermi level. Strain also induces a distinct transition in CAFM
currentâvoltage curves from rectifying (typical of an <i>n</i>-type semiconductor) to ohmic (metal-like) behavior. We
propose that strain raises the energy distribution of oxygen vacancies
(<i>n</i>-type dopants) near the conduction band and causes
an increase in carrier concentration. As the carrier concentration
is increased, the width of the depletion region is reduced, which
then permits electron tunneling through the space charge barrier resulting
in the observed metallic behavior
Built-in Potential and Charge Distribution within Single Heterostructured Nanorods Measured by Scanning Kelvin Probe Microscopy
The electrostatic potential distribution across single,
isolated,
colloidal heterostructured nanorods (NRs) with component materials
expected to form a <i>pân</i> junction within each
NR has been measured using scanning Kelvin probe microscopy (SKPM).
We compare CdS to bicomponent CdS-CdSe, CdS-PbSe, and CdS-PbS NRs
prepared via different synthetic approaches to corroborate the SKPM
assignments. The CdS-PbS NRs show a sharp contrast in measured potential
across the material interface. We find the measured built-in potential
within an individual NR to be attenuated by long-range electrostatic
forces between the sample substrate, cantilever, and the measuring
tip. Surface potential images were deconvoluted to yield built-in
potentials ranging from 375 to 510 meV in the heterostructured NRs.
We deduce the overall built-in potential as well as the charge distribution
across each segment of the heterostructured NRs by combining SKPM
data with simulations of the system
Shape-Dependent Oriented Trapping and Scaffolding of Plasmonic Nanoparticles by Topological Defects for Self-Assembly of Colloidal Dimers in Liquid Crystals
We demonstrate scaffolding of plasmonic nanoparticles
by topological
defects induced by colloidal microspheres to match their surface boundary
conditions with a uniform far-field alignment in a liquid crystal
host. Displacing energetically costly liquid crystal regions of reduced
order, anisotropic nanoparticles with concave or convex shapes not
only stably localize in defects but also self-orient with respect
to the microsphere surface. Using laser tweezers, we manipulate the
ensuing nanoparticle-microsphere colloidal dimers, probing the strength
of elastic binding and demonstrating self-assembly of hierarchical
colloidal superstructures such as chains and arrays
Shape-Dependent Oriented Trapping and Scaffolding of Plasmonic Nanoparticles by Topological Defects for Self-Assembly of Colloidal Dimers in Liquid Crystals
We demonstrate scaffolding of plasmonic nanoparticles
by topological
defects induced by colloidal microspheres to match their surface boundary
conditions with a uniform far-field alignment in a liquid crystal
host. Displacing energetically costly liquid crystal regions of reduced
order, anisotropic nanoparticles with concave or convex shapes not
only stably localize in defects but also self-orient with respect
to the microsphere surface. Using laser tweezers, we manipulate the
ensuing nanoparticle-microsphere colloidal dimers, probing the strength
of elastic binding and demonstrating self-assembly of hierarchical
colloidal superstructures such as chains and arrays
Recommended from our members
Revisiting the Valence and Conduction Band Size Dependence of PbS Quantum Dot Thin Films
We use a high signal-to-noise X-ray
photoelectron spectrum of bulk
PbS, GW calculations, and a model assuming parabolic bands to unravel
the various X-ray and ultraviolet photoelectron spectral features
of bulk PbS as well as determine how to best analyze the valence band
region of PbS quantum dot (QD) films. X-ray and ultraviolet photoelectron
spectroscopy (XPS and UPS) are commonly used to probe the difference
between the Fermi level and valence band maximum (VBM) for crystalline
and thin-film semiconductors. However, we find that when the standard
XPS/UPS analysis is used for PbS, the results are often unrealistic
due to the low density of states at the VBM. Instead, a parabolic
band model is used to determine the VBM for the PbS QD films, which
is based on the bulk PbS experimental spectrum and bulk GW calculations.
Our analysis highlights the breakdown of the Brillioun zone representation
of the band diagram for large band gap, highly quantum confined PbS
QDs. We have also determined that in 1,2-ethanedithiol-treated PbS
QD films the Fermi level position is dependent on the QD size; specifically,
the smallest band gap QD films have the Fermi level near the conduction
band minimum and the Fermi level moves away from the conduction band
for larger band gap PbS QD films. This change in the Fermi level within
the QD band gap could be due to changes in the Pb:S ratio. In addition,
we use inverse photoelectron spectroscopy to measure the conduction
band region, which has similar challenges in the analysis of PbS QD
films due to a low density of states near the conduction band minimum