35 research outputs found
Electroluminescence in Aligned Arrays of Single-Wall Carbon Nanotubes with Asymmetric Contacts
High quantum efficiencies and low current thresholds are important properties for all classes of semiconductor light emitting devices (LEDs), including nanoscale emitters based on single wall carbon nanotubes (SWNTs). Among the various configurations that can be considered in SWNT LEDs, two terminal geometries with asymmetric metal contacts offer the simplest solution. In this paper, we study, experimentally and theoretically, the mechanisms of electroluminescence in devices that adopt this design and incorporate perfectly aligned, horizontal arrays of individual SWNTs. The results suggest that exciton mediated electron–hole recombination near the lower work-function contact is the dominant source of photon emission. High current thresholds for electroluminescence in these devices result from diffusion and quenching of excitons near the metal contact
Nonimaging Optical Gain in Luminescent Concentration through Photonic Control of Emission Étendue
Luminescent and nonimaging optical
concentration constitute two
fundamentally different ways of collecting and intensifying light.
Whereas nonimaging concentrators based on reflective, refractive,
or diffractive optics operate most effectively for collimated light,
luminescent concentrators (LCs) rely on absorption, re-emission, and
waveguiding to concentrate diffuse light incident from any direction.
LCs have been explored in many different shapes and sizes but have
so far been unable to exploit the power of nonimaging optics to further
increase their concentration ratio because their emission is angularly
isotropic. Here, we use a luminescent thin film bilayer to create
sharply directed conical emission in an LC and derive a nonimaging
optical solution to leverage this directionality for secondary geometric
gain ranging up to an order of magnitude or higher. We demonstrate
this concept experimentally using a custom compound parabolic optical
element index-matched to the LC surface and show that it delivers
three times more luminescent power to an opposing GaAs photovoltaic
cell when the emission profile is conically directed than when it
is isotropic or the nonimaging optic is absent. These results open
up a significant and general opportunity to improve LC performance
for a variety of applications including photovoltaics, photobioreactors,
and scintillator-based radiation detection
Nanoimprinting Techniques for Large-Area Three-Dimensional Negative Index Metamaterials with Operation in the Visible and Telecom Bands
We report advances in materials, designs, and fabrication schemes for large-area negative index metamaterials (NIMs) in multilayer “fishnet” layouts that offer negative index behavior at wavelengths into the visible regime. A simple nanoimprinting scheme capable of implementation using standard, widely available tools followed by a subtractive, physical liftoff step provides an enabling route for the fabrication. Computational analysis of reflection and transmission measurements suggests that the resulting structures offer negative index of refraction that spans both the visible wavelength range (529–720 nm) and the telecommunication band (1.35–1.6 μm). The data reveal that these large (>75 cm<sup>2</sup>) imprinted NIMs have predictable behaviors, good spatial uniformity in properties, and figures of merit as high as 4.3 in the visible range
Quantitative Reflection Imaging for the Morphology and Dynamics of Live <i>Aplysia californica</i> Pedal Ganglion Neurons Cultured on Nanostructured Plasmonic Crystals
We describe a reflection imaging
system that consists of a plasmonic
crystal, a common laboratory microscope, and band-pass filters for
use in the quantitative imaging and in situ monitoring of live cells
and their substrate interactions. Surface plasmon resonance (SPR)
provides a highly sensitive method to monitor changes in physicochemical
properties occurring at metal–dielectric interfaces. Polyelectrolyte
thin films deposited using the layer-by-layer (LBL) self-assembly
method provide a reference system for calibrating the reflection contrast
changes that occur when the polyelectrolyte film thickness changes
and provide insight into the optical responses that originate from
the multiple plasmonic features supported by this imaging system.
Finite-difference time-domain (FDTD) simulations of the optical responses
measured experimentally from the polyelectrolyte reference system
are used to provide a calibration of the optical system for subsequent
use in quantitative studies investigating live cell dynamics in cultures
supported on a plasmonic crystal substrate. Live <i>Aplysia californica</i> pedal ganglion neurons cultured in artificial seawater were used
as a model system through which to explore the utility of this plasmonic
imaging technique. Here, the morphology of cellular peripheral structures
≲80 nm in thickness were quantitatively analyzed, and the dynamics
of their trypsin-induced surface detachment were visualized. These
results illustrate the capacities of this system for use in investigations
of the dynamics of ultrathin cellular structures within complex bioanalytical
environments
Interplay of Surface Energy and Bulk Thermodynamic Forces in Ordered Block Copolymer Droplets
The
wetting state of a simple liquid on a solid substrate, as summarized
by Young’s equation, is dictated by the interfacial energies
of the different phases that coexist in the system. For simple fluids,
rotational symmetry gives rise to symmetric droplets around the axis
perpendicular to the substrate. This is not the case for nanostructured
fluids, such as block copolymers, where the inherent thermodynamic
ordering forces compete with surface tension. This competition is
particularly important in nanoscale droplets, where the size of the
droplets is a small multiple of the natural periodicity of the block
copolymer in the bulk. In the nanoscale regime, droplet shape and
internal structure arise from a subtle interplay between interfacial
and bulk contributions to the free energy. In this work, we examine
the consequences of surface–polymer interaction energies on
droplet morphology through a concerted simulation and experimental
effort. When the block copolymer is deposited on a neutral substrate,
we find noncircular arrangements with perpendicular domains. However,
when a preferential substrate is used, the resulting morphology depends
on droplet size. In large droplets, we observe bottle-cap-shaped structures
with a ring of perpendicular domains along the perimeter, while small
droplets exhibit stripes of perpendicular domains
Quantitative Reflection Imaging of Fixed Aplysia californica Pedal Ganglion Neurons on Nanostructured Plasmonic Crystals
Studies
of the interactions between cells and surrounding environment
including cell culture surfaces and their responses to distinct chemical
and physical cues are essential to understanding the regulation of
cell growth, migration, and differentiation. In this work, we demonstrate
the capability of a label-free optical imaging techniquesurface
plasmon resonance (SPR)to quantitatively investigate the relative
thickness of complex biomolecular structures using a nanoimprinted
plasmonic crystal and laboratory microscope. Polyelectrolyte films
of different thicknesses deposited by layer-by-layer assembly served
as the model system to calibrate the reflection contrast response
originating from SPRs. The calibrated SPR system allows quantitative
analysis of the thicknesses of the interface formed between the cell
culture substrate and cellular membrane regions of fixed Aplysia californica pedal ganglion neurons. Bandpass
filters were used to isolate spectral regions of reflected light with
distinctive image contrast changes. Combining of the data from images
acquired using different bandpass filters leads to increase image
contrast and sensitivity to topological differences in interface thicknesses.
This SPR-based imaging technique is restricted in measurable thickness
range (∼100–200 nm) due to the limited plasmonic sensing
volume, but we complement this technique with an interferometric analysis
method. Described here simple reflection imaging techniques show promise
as quantitative methods for analyzing surface thicknesses at nanometer
scale over large areas in real-time and in physicochemical diverse
environments
Modulated Degradation of Transient Electronic Devices through Multilayer Silk Fibroin Pockets
The recent introduction
of transient, bioresorbable electronics
into the field of electronic device design offers promise for the
areas of medical implants and environmental monitors, where programmed
loss of function and environmental resorption are advantageous characteristics.
Materials challenges remain, however, in protecting the labile device
components from degradation at faster than desirable rates. Here we
introduce an indirect passivation strategy for transient electronic
devices that consists of encapsulation in multiple air pockets fabricated
from silk fibroin. This approach is investigated through the properties
of silk as a diffusional barrier to water penetration, coupled with
the degradation of magnesium-based devices in humid air. Finally,
silk pockets are demonstrated to be useful for controlled modulation
of device lifetime. This approach may provide additional future opportunities
for silk utility due to the low immunogenicity of the material and
its ability to stabilize labile biotherapeutic dopants
Transfer-Printing of Tunable Porous Silicon Microcavities with Embedded Emitters
Here
we demonstrate, via a modified transfer-printing technique,
that electrochemically fabricated porous silicon (PSi) distributed
Bragg reflectors (DBRs) can serve as the basis of high-quality hybrid
microcavities compatible with most forms of photoemitters. Vertical
microcavities consisting of an emitter layer sandwiched between 11-
and 15-period PSi DBRs were constructed. The emitter layer included
a polymer doped with PbS quantum dots, as well as a heterogeneous
GaAs thin film. In this structure, the PbS emission was significantly
redistributed to a 2.1 nm full-width at half-maximum around 1198 nm,
while the PSi/GaAs hybrid microcavity emitted at 902 nm with a sub-nanometer
full-width at half-maximum and quality-factor of 1058. Modification
of PSi DBRs to include a PSi cavity coupling layer enabled tuning
of the total cavity optical thickness. Infiltration of the PSi with
Al<sub>2</sub>O<sub>3</sub> by atomic layer deposition globally red-shifted
the emission peak of PbS quantum dots up to ∼18 nm (∼0.9
nm per cycle), while introducing a cavity coupling layer with a gradient
optical thickness spatially modulated the cavity resonance of the
PSi/GaAs hybrid such that there was an ∼30 nm spectral variation
in the emission of separate GaAs modules printed ∼3 mm apart
Facile Synthesis of Free-Standing Silicon Membranes with Three-Dimensional Nanoarchitecture for Anodes of Lithium Ion Batteries
We propose a facile method for synthesizing
a novel Si membrane
structure with good mechanical strength and three-dimensional (3D)
configuration that is capable of accommodating the large volume changes
associated with lithiation in lithium ion battery applications. The
membrane electrodes demonstrated a reversible charge capacity as high
as 2414 mAh/g after 100 cycles at current density of 0.1 C, maintaining
82.3% of the initial charge capacity. Moreover, the membrane electrodes
showed superiority in function at high current density, indicating
a charge capacity >1220 mAh/g even at 8 C. The high performance
of
the Si membrane anode is assigned to their characteristic 3D features,
which is further supported by mechanical simulation that revealed
the evolution of strain distribution in the membrane during lithiation
reaction. This study could provide a model system for rational and
precise design of the structure and dimensions of Si membrane structures
for use in high-performance lithium ion batteries
Facile Synthesis of Free-Standing Silicon Membranes with Three-Dimensional Nanoarchitecture for Anodes of Lithium Ion Batteries
We propose a facile method for synthesizing
a novel Si membrane
structure with good mechanical strength and three-dimensional (3D)
configuration that is capable of accommodating the large volume changes
associated with lithiation in lithium ion battery applications. The
membrane electrodes demonstrated a reversible charge capacity as high
as 2414 mAh/g after 100 cycles at current density of 0.1 C, maintaining
82.3% of the initial charge capacity. Moreover, the membrane electrodes
showed superiority in function at high current density, indicating
a charge capacity >1220 mAh/g even at 8 C. The high performance
of
the Si membrane anode is assigned to their characteristic 3D features,
which is further supported by mechanical simulation that revealed
the evolution of strain distribution in the membrane during lithiation
reaction. This study could provide a model system for rational and
precise design of the structure and dimensions of Si membrane structures
for use in high-performance lithium ion batteries