15 research outputs found
Quantum Photon Sources in WSe<sub>2</sub> Monolayers Induced by Weakly Localized Strain Fields
Quantum emitters in semiconductor transition metal dichalcogenide
(TMD) monolayers hold great promise for many quantum optics applications
due to the intriguing properties afforded by the host materials. The
creation of localized excitonic states in two-dimensional semiconductors
is also fundamentally interesting. Local strain engineering of TMD
monolayers has been attested to be a viable approach for creating
quantum emitters. However, despite the ubiquitous existence of local
topography variations in the structures used to create strain gradients
in the TMD monolayers, an understanding of their influence on the
strain fields and exciton trapping is notably lacking, especially
on the nanoscale. In this study, we investigate WSe2 monolayers
deposited on the edges of as-fabricated trenches, which are deemed
to induce 1D delocalized strain profiles in the monolayers, and observe
optical signatures of weakly confined excitonic states supporting
biexciton emission. Our numerical simulations of the strain distributions
suggest that the quantum emitters originate from quasi-1D like localized
strain profiles induced by local topography variations at the trench
edges. These findings have strong implications toward the controlled
creation of quantum emitters in TMD monolayers and their efficient
coupling to photonic structures
Metal-Lined Semiconductor Nanotubes for Surface Plasmon-Mediated Luminescence Enhancement
Highly
efficient solid-state light-emitting devices require semiconductor
architectures equipped with high quantum efficiency and integratability
on conductive substrates. Surface plasmon (SP)-mediated luminescence
enhancement has been considered as one of the most promising solutions,
because SP resonance can greatly improve the radiative recombination
rate and be achieved using metal entities compatible with the electrode
fabrication process. Nevertheless, metal/semiconductor heterostructures
have had several fabrication-compatible issues due to metal as a potential
contaminant of the semiconductor. We present here a simple fabrication
scheme for a metal-lined semiconductor nanotube heterostructure, in
which a metal layer is selectively formed on the inner wall of the
semiconductor nanotube. The Ag-lining process in a ZnO nanotube resulted
in 7.5-fold enhancement of the photoluminescence intensity at 11 K.
This SP fabrication technique looks promising for highly efficient
solid-state lighting based on semiconductor nanostructures without
detrimental effects
Anisotropic Cracking of Nanocrystal Superlattices
The
synthesis colloidal nanocrystals in nonpolar organic solvents
has led to exceptional size- and shape-control, enabling the formation
of nanocrystal superlattices isostructural to atomic lattices built
with nanocrystals rather than atoms. The long aliphatic ligands (e.g.,
oleic acid) used to achieve this control separate nanocrystals too
far in the solid state for most charge-transporting devices. Solid-state
ligand exchange, which brings particles closer together and enhances
conductivity, necessitates large changes in the total volume of the
solid (compressive stress), which leads to film cracking. In this
work, truncate octahedral lead selenide nanocrystals are shown to
self-assemble into body-centered cubic superlattices in which the
atomic axes of the individual nanocrystals are coaligned with the
crystal axes of the superlattice. Due to this coalignment, upon ligand
exchange of the superlattices, cracking is preferentially observed
on āØ011ā© superlattice directions. This observation is
related to differences in the ligand binding to exposed {100} and
{111} planes of the PbSe nanocrystal surfaces. This result has implications
for binary and more complex structures in which differential reactivity
of the constituent elements can lead to disruption of the desired
structure. In addition, cracks in PbSe superlattices occur in a semiregular
spacings inversely related to the superlattice domain size and strongly
influenced by the presence of twin boundaries, which serve as both
emission centers and propagation barriers for fractures. This work
shows that defects, similar to behavior in nanotwinned metals, could
be used to engineer enhanced mechanical strength and electrical conductivity
in nanocrystal superlattices
Influence of Exciton Dimensionality on Spectral Diffusion of Single-Walled Carbon Nanotubes
We study temporal evolution of photoluminescence (PL) spectra from individual single-walled carbon nanotubes (SWCNTs) at cryogenic and room temperatures. Sublinear and superlinear correlations between fluctuating PL spectral positions and line widths are observed at cryogenic and room temperatures, respectively. We develop a simple model to explain these two different spectral diffusion behaviors in the framework of quantum-confined Stark effect (QCSE) caused by surface charges trapped in the vicinity of SWCNTs. We show that the wave function properties of excitons, namely, localization at cryogenic temperature and delocalization at room temperature, play a critical role in defining sub- and superlinear correlations. Room temperature PL spectral positions and line widths of SWCNTs coupled to gold dimer nanoantennas on the other hand exhibit sublinear correlations, indicating that excitonic emission mainly originates from nanometer range regions and excitons appear to be localized. Our numerical simulations show that such apparent localization of excitons results from plasmonic confinement of excitation and an enhancement of decay rates in the gap of the dimer nanoantennas
Electronic Structure and Chemical Nature of Oxygen Dopant States in Carbon Nanotubes
We performed low temperature photoluminescence (PL) studies on individual oxygen-doped single-walled carbon nanotubes (SWCNTs) and correlated our observations to electronic structure simulations. Our experiment reveals multiple sharp asymmetric emission peaks at energies 50ā300 meV red-shifted from that of the <i>E</i><sub>11</sub> bright exciton peak. Our simulation suggests an association of these peaks with deep trap states tied to different specific chemical adducts. In addition, oxygen doping is also observed to split the <i>E</i><sub>11</sub> exciton into two or more states with an energy splitting <40 meV. We attribute these states to dark states that are brightened through defect-induced symmetry breaking. While the wave functions of these brightened states are delocalized, those of the deep-trap states are strongly localized and pinned to the dopants. These findings are consistent with our experimental observation of asymmetric broadening of the deep trap emission peaks, which can result from interaction between pinned excitons and one-dimensional phonons. Exciton pinning also increases the sensitivity of the deep traps to the local dielectric environment, leading to a large inhomogeneous broadening. Observations of multiple spectral features on single nanotubes indicate the possibility of different chemical adducts coexisting on a given nanotube
Anisotropic Photoluminescence from Isotropic Optical Transition Dipoles in Semiconductor Nanoplatelets
Many important light-matter
coupling and energy-transfer processes
depend critically on the dimensionality and orientation of optical
transition dipoles in emitters. We investigate individual quasi-two-dimensional
nanoplatelets (NPLs) using higher-order laser scanning microscopy
and find that absorption dipoles in NPLs are isotropic in three dimensions
at the excitation wavelength. Correlated polarization studies of the
NPLs reveal that their emission polarization is strongly dependent
on the aspect ratio of the lateral dimensions. Our simulations reveal
that this emission anisotropy can be readily explained by the electric
field renormalization effect caused by the dielectric contrast between
the NPLs and the surrounding medium, and we conclude that emission
dipoles in NPLs are isotropic in the plane of the NPLs. Our study
presents an approach for disentangling the effects of dipole degeneracy
and electric field renormalization on emission anisotropy and can
be adapted for studying the intrinsic optical transition dipoles of
various nanostructures
Photoluminescence Dynamics of Aryl sp<sup>3</sup> Defect States in Single-Walled Carbon Nanotubes
Photoluminescent
defect states introduced by sp<sup>3</sup> functionalization
of semiconducting carbon nanotubes are rapidly emerging as important
routes for boosting emission quantum yields and introducing new functionality.
Knowledge of the relaxation dynamics of these states is required for
understanding how functionalizing agents (molecular dopants) may be
designed to access specific behaviors. We measure photoluminescence
(PL) decay dynamics of sp<sup>3</sup> defect states introduced by
aryl functionalization of the carbon nanotube surface. Results are
given for five different nanotube chiralities, each doped with a range
of aryl functionality. We find that the PL decays of these sp<sup>3</sup> defect states are biexponential, with both components relaxing
on time scales of ā¼100 ps. Exciton trapping at defects is found
to increases PL lifetimes by a factor of 5ā10, in comparison
to those for the free exciton. A significant chirality dependence
is observed in the decay times, ranging from 77 ps for (7,5) nanotubes
to >600 ps for (5,4) structures. The strong correlation of time
constants
with emission energy indicates relaxation occurs <i>via</i> multiphonon decay processes, with close agreement to theoretical
expectations. Variation of the aryl dopant further modulates decay
times by 10ā15%. The aryl defects also affect PL lifetimes
of the free <i>E</i><sub>11</sub> exciton. Shortening of
the <i>E</i><sub>11</sub> bright state lifetime as defect
density increases provides further confirmation that defects act as
exciton traps. A similar shortening of the <i>E</i><sub>11</sub> dark exciton lifetime is found as defect density increases,
providing strong experimental evidence that dark excitons are also
trapped at such defect sites
Thermal Hysteresis and Ordering Behavior of Magnetic Skyrmion Lattices
The physics of phase transitions in two-dimensional (2D)
systems
underpins research in diverse fields including statistical mechanics,
nanomagnetism, and soft condensed matter. However, many aspects of
2D phase transitions are still not well understood, including the
effects of interparticle potential, polydispersity, and particle shape.
Magnetic skyrmions are chiral spin-structure quasi-particles that
form two-dimensional lattices. Here, we show, by real-space imaging
using in situ cryo-Lorentz transmission electron
microscopy coupled with machine learning image analysis, the ordering
behavior of NeĢel skyrmion lattices in van der Waals Fe3GeTe2. We demonstrate a distinct change in the
skyrmion size distribution during field-cooling, which leads to a
loss of lattice order and an evolution of the skyrmion liquid phase.
Remarkably, the lattice order is restored during field heating and
demonstrates a thermal hysteresis. This behavior is explained by the
skyrmion energy landscape and demonstrates the potential to control
the lattice order in 2D phase transitions
Thermal Hysteresis and Ordering Behavior of Magnetic Skyrmion Lattices
The physics of phase transitions in two-dimensional (2D)
systems
underpins research in diverse fields including statistical mechanics,
nanomagnetism, and soft condensed matter. However, many aspects of
2D phase transitions are still not well understood, including the
effects of interparticle potential, polydispersity, and particle shape.
Magnetic skyrmions are chiral spin-structure quasi-particles that
form two-dimensional lattices. Here, we show, by real-space imaging
using in situ cryo-Lorentz transmission electron
microscopy coupled with machine learning image analysis, the ordering
behavior of NeĢel skyrmion lattices in van der Waals Fe3GeTe2. We demonstrate a distinct change in the
skyrmion size distribution during field-cooling, which leads to a
loss of lattice order and an evolution of the skyrmion liquid phase.
Remarkably, the lattice order is restored during field heating and
demonstrates a thermal hysteresis. This behavior is explained by the
skyrmion energy landscape and demonstrates the potential to control
the lattice order in 2D phase transitions
Thermal Hysteresis and Ordering Behavior of Magnetic Skyrmion Lattices
The physics of phase transitions in two-dimensional (2D)
systems
underpins research in diverse fields including statistical mechanics,
nanomagnetism, and soft condensed matter. However, many aspects of
2D phase transitions are still not well understood, including the
effects of interparticle potential, polydispersity, and particle shape.
Magnetic skyrmions are chiral spin-structure quasi-particles that
form two-dimensional lattices. Here, we show, by real-space imaging
using in situ cryo-Lorentz transmission electron
microscopy coupled with machine learning image analysis, the ordering
behavior of NeĢel skyrmion lattices in van der Waals Fe3GeTe2. We demonstrate a distinct change in the
skyrmion size distribution during field-cooling, which leads to a
loss of lattice order and an evolution of the skyrmion liquid phase.
Remarkably, the lattice order is restored during field heating and
demonstrates a thermal hysteresis. This behavior is explained by the
skyrmion energy landscape and demonstrates the potential to control
the lattice order in 2D phase transitions