35 research outputs found

    Electroluminescence in Aligned Arrays of Single-Wall Carbon Nanotubes with Asymmetric Contacts

    No full text
    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

    No full text
    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

    No full text
    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

    No full text
    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

    No full text
    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

    No full text
    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 techniquesurface 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

    No full text
    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

    No full text
    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

    No full text
    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

    No full text
    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
    corecore