Flexible Emitter Research Articles

Shape-deformable Micro-LEDs for advanced displays and healthcare

Recently, flexible/stretchable micro-scale light-emitting diodes (LEDs), with dimensions significantly smaller than conventional diodes used for illuminations, have emerged for promising applications in areas such as deformable displays, wearable devices for healthcare, etc . For such applications, these devices must have some unusual features that common inorganic LEDs do not intrinsically own, including conformability, biocompatibility, mechanical flexibility, etc . This Perspective focuses on summarizing the most recent progress in developing such flexible emitters based on inorganic semiconductors, followed by reviewing their potential applications. Finally, major challenges and future research directions of deformable micro-scale LEDs are presented.

Purifying single photon emission from giant shell CdSe/CdS quantum dots at room temperature.

Giant shell CdSe/CdS quantum dots are bright and flexible emitters, with near-unity quantum yield and suppressed blinking, but their single photon purity is reduced by efficient multiexcitonic emission. We report the observation, at the single dot level, of a large blueshift of the photoluminescence biexciton spectrum (24 ± 5 nm over a sample of 32 dots) for pure-phase wurtzite quantum dots. By spectral filtering, we demonstrate a 2.3 times reduction of the biexciton quantum yield relative to the exciton emission, while preserving as much as 60% of the exciton single photon emission, thus improving the purity from g2(0) = 0.07 ± 0.01 to g2(0) = 0.03 ± 0.01. At a larger pump fluency, spectral purification is even more effective with up to a 6.6 times reduction in g2(0), which is due to the suppression of higher order excitons and shell states experiencing even larger blueshifts. Our results indicate the potential for the synthesis of engineered giant shell quantum dots, with further increased biexciton blueshifts, for quantum optical applications requiring both high purity and brightness.

AlGaN nanowires with inverse taper for flexible DUV emitters

Deep ultraviolet (DUV) AlGaN light-emitting diodes (LEDs) are promising alternatives for production of DUV light, offering many advantages over mercury arc lamps. In this work, AlGaN nanowires with an inverse taper profile were demonstrated through a wet etching process, enabling removal of the nanowires from the growth substrate in a novel peeling process to form flexible devices. AlGaN nanowires with taper angles of ∼22° were obtained following a 70 min etch in AZ400K. Nanowire taper angle was studied as a function of etch time and nanowire top diameter. Nanowires with inverse taper were then embedded in a flexible polymer layer and removed from their growth substrate, which could enable development of high-efficiency flexible micro-LEDs. Released nanowires embedded within the polymer liftoff layer exhibit strain relaxation induced redshift due to reduction in piezoelectric polarization electric field intensity. The inverse taper structure was found to promote enhanced light extraction from the nanowire. The demonstrated flexible DUV emitters with inverse taper are shown to improve the device efficiency and allow for realization of flexible emitters through a novel fabrication process for the first time.

Superior B-Doped SiC Nanowire Flexible Field Emitters: Ultra-Low Turn-On Fields and Robust Stabilities against Harsh Environments.

Low turn-on fields together with boosted stabilities are recognized as two key factors for pushing forward the implementations of the field emitters in electronic units. In current work, we explored superior flexible field emitters based on single-crystalline 3C-SiC nanowires, which had numbers of sharp edges, as well as corners surrounding the wire body and B dopants. The as-constructed field emitters behaved exceptional field emission (FE) behaviors with ultralow turn-on fields (Eto) of 0.94-0.68 V/μm and current emission fluctuations of ±1.0-3.4%, when subjected to harsh working conditions under different bending cycles, various bending configurations, as well as elevated temperature environments. The sharp edges together with the edges were able to significantly increase the electron emission sites, and the incorporated B dopants could bring a more localized state close to the Fermi level, which rendered the SiC nanowire emitters with low Eto, large field enhancement factor as well as robust current emission stabilities. Current B-doped SiC nanowires could meet all essential requirements for an ideal flexible emitters, which exhibit their promising prospect to be applied in flexible electronic units.

Extremely Stable Current Emission of P-Doped SiC Flexible Field Emitters.

Novel P-doped SiC flexible field emitters are developed on carbon fabric substrates, having both low Eto of 1.03-0.73 Vμm-1 up to high temperatures of 673 K, and extremely high current emission stability when subjected to different bending states, bending circle times as well as high temperatures (current emission fluctuations are typically in the range ±2.1%-3.4%).

Mechanical and thermal stability of plasmonic emitters on flexible polyimide substrates

We have fabricated Ag/SiO2/Ag plasmonic thermal infrared (IR) emitters with different hexagonal lattice periods on flexible polyimide (PI) membranes. The high temperature resistance and comparable coefficient of thermal expansion with metallic films enabled the fabrication of plasmonic IR emitters on PI films. We have investigated the effects of temperature and mechanical strain on the spectral and structural properties of the flexible emitter. The low heat conductivity and low thermal mass of thin free standing PI substrates enables IR emitters with low power consumption (<1 W) and fast switching time (in the order of ten milliseconds).

Electromechanical self-oscillations of carbon nanotube field emitter

We report the observation of the electromechanical self-oscillations excited in a vacuum diode with a field emission cathode made of single-wall carbon nanotubes. The mechanical oscillations of the cathode accompanied by the periodic variations of the emission current intensity have been observed under the action of a constant voltage applied between the cathode and the anode of the diode. An empirical model of the phenomenon is proposed. It provides a qualitative agreement of the experimental results and the theoretical predictions. The modeling assuming a common behavior of the electromechanical systems with nano-sized mechanically flexible field emitters has been performed. It has demonstrated a dependence of the oscillation frequency value on the geometrical size of flexible emitters and on the value of voltage applied. This phenomenon opens a way for generation of the high frequency electromagnetic waves by using the electromechanical systems with emitters of nanometer size.

Versatile transfer of aligned carbon nanotubes with polydimethylsiloxane as theintermediate

A simple technique to transfer aligned multi-walled carbon nanotubes (MWCNTs) isdemonstrated in this work. With polydimethylsiloxane (PDMS) as the transfer medium,as-grown or patterned MWCNT arrays are directly transferred onto a wide variety ofPt-coated substrates such as glossy paper, cloth, polymers, glass slides, and metal foils atlow temperatures. The surface of the transferred CNTs is cleaner with better alignment,compared with the as-grown one. Furthermore, the transferred CNTs show strong adhesionand good electric contact with the target substrates. A maximal current density of∼104 A cm−2 has been achieved from the CNT interconnects prepared with this technique.Because of the lower density and open-ended structures, improved field emissionperformance has been obtained from CNTs transferred on polymers, based onwhich flexible emitter devices can be fabricated. In addition, the surface oftransferred CNTs becomes more hydrophilic, with an averaged contact angle of93.4 ± 5.8°, in contrast to the super-hydrophobic as-grown CNT surface (contact angle151.6 ± 5.5°). With versatile properties and flexible applications, the technique provides a simple andcost-effective way towards future nanodevices based on CNTs.

Reliable and flexible carbon-nanofiber-based all-plastic field emission devices

The authors present the fabrication and electrical characterization of carbon nanofiber-based flexible field emitters prepared by an ion beam technique. The flexible emitters are extremely robust under various stress conditions and show no sign of degradation after 16h long lifetime test. An all-plastic flexible field emission device with excellent emission properties has also been demonstrated using phosphor-coated polyester as an anode.

X-ray generation using carbon-nanofiber-based flexible field emitters

Carbon nanofibers were grown on flexible polyimide substrates using an ion-beam sputtering technique. Field emission measurement showed a fairly low threshold voltage of 1.5V∕μm with a current density of 1μA∕cm2. The field enhancement factor was determined to be 4400. The emitter showed resilience when exploited as a high voltage electron source for x-ray generation. The x-ray generated by the flexible emitter is capable of delivering fine images of biological samples with superior sharpness, resolution, and contrast.