Ultrasmall-sized light-emitting diodes fabricated by ion implantation based on GaN epitaxial wafers with fully activated or unactivated p-GaN.

A key challenge in realizing ultrahigh-resolution displays is the efficient preparation of ultrasmall-sized (USS) light-emitting diodes (LEDs). Today, GaN-based LEDs are mainly prepared through dry etching processes. However, it is difficult to achieve efficient and controllable etching of USS LED with high aspect ratios, and LED sidewalls will appear after etching, which will have a negative impact on the device itself. Herein, a method for preparing USS LED based on GaN epitaxial wafers is reported (on two types of wafers, i.e., with p-GaN fully activated and unactivated). F-ions are injected into the intentionally exposed areas on the two types of wafers to achieve device isolation. The area under the micro-/nano-sized protective masks (0.5, 0.8, 1, 3, 5, 7, 9, and 10 µm wide Ni/Au stripes) are the LED lighting areas. The LED on the p-GaN unactivated wafer (UAW) requires further activation. The Ni/Au mask not only serves as the p-electrode of LED but also Ni as a hydrogen (H) removing metal covering the surface of p-GaN UAW that can desorb H from a Mg element in the film at relatively low temperatures, thereby achieving the selective activation of LED lighting areas. Optoelectronic characterization shows that micro-/nano-sized LED arrays with individual-pixel control were successfully fabricated on the two types of wafers. It is expected that the demonstrated method will provide a new way toward realizing ultrahigh-resolution displays. Analyzing the changes in the current flowing through LED (before and after selective activation) on the F-injected p-GaN UAW, it is believed that depositing H removing metal on p-GaN UAW could possibly realize the device array through the selective activation only (i.e., without the need for ion implantation), offering a completely new insight.

Design and fabrication of hybrid MLAs/gratings for the enhancement of light extraction efficiency and distribution uniformity of OLEDs.

Extracting light from organic light-emitting diodes (OLEDs) and improving the angular distribution are essential for their commercial applications in illumination and displays. In this work, hybrid microlens arrays (MLAs) and gratings with periods and depths in the scale of submicron have been designed and incorporated on the lighting surface of OLEDs for simultaneous enhancement of light outcoupling efficiency and angular distribution improvement. It is found that the augmentation of light extraction efficiency is mainly attributed to the MLAs, while the gratings can improve the viewing angle by increasing the angular distribution uniformity. A novel approach was proposed by combining photoresist thermal reflow, soft-lithography and plasma treatments on polydimethylsiloxane (PDMS) surfaces synergistically to realize gratings on the wavy surface of MLAs. It has been proved that with the hybrid MLAs/gratings, the external quantum efficiency (EQE) of the OLED can reach up to 22.8%, which increased by 24% compared to that of bare OLED. Moreover, the OLED with the hybrid MLAs/gratings showed an obvious lateral enhancement at wider viewing angle.

Fabrication of a micro-lens array for improving depth-of-field of integral imaging 3D display.

In this paper, we present a new, to the best of our knowledge, structure of double pinhole/micro-lens array (DP/MLA) with two center-depth planes, used for improving the depth-of-field (DOF) of integral imaging (II), which can be fabricated by a combination of lithography and inkjet printing. The results show that a black circular groove array prepared by lithography can be used for micro-lens location and reduce the stray light for II. By controlling the parameters of the inkjet printing system, DP/MLA with high precision, high alignment, and good focusing ability can be achieved. When the fabricated DP/MLA is applied in the II system, the reconstructed image has a better three-dimensional (3D) image with higher DOF than that by traditional MLA and higher quality than that by ordinary double-layer MLA.

Multi-primary-color quantum-dot down-converting films for display applications.

We propose and fabricate a multi-primary-color (MPC) quantum-dot down-converting film (QDDCF). A four-primary-color QDDCF composed of red (R), yellowish green (YG), bluish green (BG), and blue (B) subpixels was fabricated via totally five rounds of photolithographic processes. A verification platform was built up using a laser projector, and the measured results show that the QD film can expand display color gamut to 118.60% of Rec. 2020 and can cover the entire Pointer's gamut. The issues of blue light absorption and film thickness are analyzed in detail. The combination of MPC technology and QDDCF is a potential strategy to realize ultra wide color gamut for emerging display technologies.

Design and fabrication of bi-functional TiO2/Al2O3 nanolaminates with selected light extraction and reliable moisture vapor barrier performance.

Bi-functional thin film with both selected light extraction and reliable moisture vapor barrier was proposed for simultaneous light management and encapsulation in the fields of lighting and display. Atomic layer deposition (ALD) was employed to obtain TiO2 and Al2O3 films with high uniformity, forming distributed Bragg reflector (DBR) structure. The DBRs exhibited excellent and tunable optical properties, as well as reliable moisture barrier performance. With increasing the DBR layers, the transmittances decreased obviously. The transmittance in the blue light region was as low as 0.66% for DBR with 6.5 pairs and the water vapor transmission rates value was 3.06 × 10-5 g m-2 d-1 for DBR with 4.5 pairs. These DBRs were integrated in the red quantum dot (QD) based color converters excited by blue LED, enabling an obvious increase in red emission and a strong decrease in blue light transmittance. Furthermore, these DBRs can prolong the lifetime of QDs evidently by isolating the QDs from the moisture (oxygen) vapor. These results highlight the potentials for the exploitation of DBRs fabricated using ALD in the application of lighting and display devices based on QD photoluminescence and electroluminescence.

Electron Oscillation-Induced Splitting Electroluminescence from Nano-LEDs for Device-Level Encryption.

Data security is a major concern in digital age, which generally relies on algorithm-based mathematical encryption. Recently, encryption techniques based on physical principles are emerging and being developed, leading to the new generation of encryption moving from mathematics to the intersection of mathematics and physics. Here, device-level encryption with ideal security is ingeniously achieved using modulation of the electron-hole radiative recombination in a GaN-light-emitting diode (LED). When a nano-LED is driven in the non-carrier injection mode, the oscillation of confined electrons can split what should be a single light pulse into multiple pulses. The morphology (amplitude, shape, and pulse number) of those history-dependent multiple pulses that act as carriers for transmitted digital information depends highly on the parameters of the driving signals, which makes those signals mathematically uncrackable and can increase the volume and security of transmitted information. Moreover, a hardware and software platform are designed to demonstrate the encrypted data transmission based on the device-level encryption method, enabling recognition of the entire ASCII code table. The device-level encryption based on splitting electroluminescence provides an encryption method during the conversion process of digital signals to optical signals and can improve the security of LED-based communication.

Memory-electroluminescence for multiple action-potentials combination in bio-inspired afferent nerves.

The development of optoelectronics mimicking the functions of the biological nervous system is important to artificial intelligence. This work demonstrates an optoelectronic, artificial, afferent-nerve strategy based on memory-electroluminescence spikes, which can realize multiple action-potentials combination through a single optical channel. The memory-electroluminescence spikes have diverse morphologies due to their history-dependent characteristics and can be used to encode distributed sensor signals. As the key to successful functioning of the optoelectronic, artificial afferent nerve, a driving mode for light-emitting diodes, namely, the non-carrier injection mode, is proposed, allowing it to drive nanoscale light-emitting diodes to generate a memory-electroluminescence spikes that has multiple sub-peaks. Moreover, multiplexing of the spikes can be obtained by using optical signals with different wavelengths, allowing for a large signal bandwidth, and the multiple action-potentials transmission process in afferent nerves can be demonstrated. Finally, sensor-position recognition with the bio-inspired afferent nerve is developed and shown to have a high recognition accuracy of 98.88%. This work demonstrates a strategy for mimicking biological afferent nerves and offers insights into the construction of artificial perception systems.

Silver Nanowire Networks with Moisture-Enhanced Learning Ability.

The human brain possesses a remarkable ability to memorize information with the assistance of a specific external environment. Therefore, mimicking the human brain's environment-enhanced learning abilities in artificial electronic devices is essential but remains a considerable challenge. Here, a network of Ag nanowires with a moisture-enhanced learning ability, which can mimic long-term potentiation (LTP) synaptic plasticity at an ultralow operating voltage as low as 0.01 V, is presented. To realize a moisture-enhanced learning ability and to adjust the aggregations of Ag ions, we introduced a thin polyvinylpyrrolidone (PVP) coating layer with moisture-sensitive properties to the surfaces of the Ag nanowires of Ag ions. That Ag nanowire network was shown to exhibit, in response to the humidity of its operating environment, different learning speeds during the LTP process. In high-humidity environments, the synaptic plasticity was significantly strengthened with a higher learning speed compared with that in relatively low-humidity environments. Based on experimental and simulation results, we attribute this enhancement to the higher electric mobility of the Ag ions in the water-absorbed PVP layer. Finally, we demonstrated by simulation that the moisture-enhanced synaptic plasticity enabled the device to adjust connection weights and delivery modes based on various input patterns. The recognition rate of a handwritten data set reached 94.5% with fewer epochs in a high-humidity environment. This work shows the feasibility of building our electronic device to achieve artificial adaptive learning abilities.

Working Mechanisms of Nanoscale Light-Emitting Diodes Operating in Non-Electrical Contact and Non-Carrier Injection Mode: Modeling and Simulation.

Non-electrical contact and non-carrier injection (NEC&NCI) mode is an emerging driving mode for nanoscale light-emitting diodes (LEDs), aiming for applications in nano-pixel light-emitting displays (NLEDs). However, the working mechanism of nano-LED operating in NEC&NCI mode is not clear yet. In particular, the questions comes down to how the inherent holes and electrons in the LED can support sufficient radiation recombination, which lacks a direct physical picture. In this work, a finite element simulation was used to study the working process of the nano-LED operating in the NEC&NCI mode to explore the working mechanisms. The energy band variation, carrier concentration redistribution, emission rate, emission spectrum, and current-voltage characteristics are studied. Moreover, the effect of the thickness of insulating layer that plays a key role on device performance is demonstrated. We believe this work can provide simulation guidance for a follow-up study of NEC&NCI-LED.

Low-voltage driving high-resistance liquid crystal micro-lens with electrically tunable depth of field for the light field imaging system.

Light field imaging (LFI) based on Liquid crystal microlens array (LC MLAs) are emerging as a significant area for 3D imaging technology in the field of upcoming Internet of things and artificial intelligence era. However, in scenes of LFI through conventional MLAs, such as biological imaging and medicine imaging, the quality of imaging reconstruction will be severely reduced due to the limited depth of field. Here, we are proposed a low-voltage driving LC MLAs with electrically tunable depth of field (DOF) for the LFI system. An aluminum-doped zinc oxide (AZO) film was deposited on the top of the hole-patterned driven-electrode arrays and used as a high resistance (Hi-R) layer, a uniform gradient electric field was obtained across the sandwiched LC cell. Experimental results confirm that the proposed LC MLAs possess high-quality interference rings and tunable focal length at a lower working voltage. In addition, the focal lengths are tunable from 3.93 to 2.62 mm and the DOF are adjustable from 15.60 to 1.23 mm. The experiments demonstrated that the LFI system based on the proposed structure can clearly capture 3D information of the insets with enlarged depths by changing the working voltage and driving frequency, which indicates that the tunable DOF LC MLAs have a potential application prospects for the biological and medical imaging.

In-Well Ionization from Monolayer Quantum Dots for Non-Carrier-Injection Electroluminescence.

Quantum dot (QD) light-emitting devices operating in non-carrier-injection (NCI) mode have attracted intense interest. Revealing the source of carriers that support the periodic electroluminescence is important because there is no injection of carriers from the external electrode. Electrons/holes generated by well-to-well multiple ionization in adjacent QDs are generally recognized as the carrier source for electroluminescence, and the stacked QD layers are necessary. In this work, NCI electroluminescence (NCI-EL) from monolayer QDs is successfully demonstrated, which cannot be properly explained by the previously proposed mechanism of multiple ionization. A working mechanism related to periodic in-well ionization is proposed, in which electrons tunnel directly from the valence band of QDs to the conduction band to form free electrons and holes. The effects of driving voltage amplitude, frequency, and QD size on the NCI-EL performance are investigated. Finite element simulation is used to clarify the ionization process. We believe this work can extend the working mechanism model of NCI-EL from QDs and provide guidance for promoting QD-based light-emitting device performance.

Omnidirectional Triboelectric Nanogenerator for Wide-Speed-Range Wind Energy Harvesting.

The environmentally friendly harvesting of wind energy is an effective technique for achieving carbon neutrality and a green economy. In this work, a core-shell triboelectric nanogenerator (CS-TENG) for harvesting wind energy is demonstrated and the device structure parameters are optimized. The core-shell structure enables the CS-TENG to respond sensitively to wind from any direction and generate electrical output on the basis of the vertical contact-separation mode. A single device can generate a maximum power density of 0.14 W/m3 and can power 124 light-emitting diodes. In addition, wind energy can be harvested even at a wind speed as low as 2.3 m/s by paralleling CS-TENGs of different sizes. Finally, a self-powered water quality testing system that uses the CS-TENG as its power supply is built. The CS-TENG exhibits the advantages of a simple structure, environmentally friendly materials, low cost, and simple fabrication process. These features are of considerable significance for the development of green energy harvesting devices.

Hybrid Device of Blue GaN Light-Emitting Diodes and Organic Light-Emitting Diodes with Color Tunability for Smart Lighting Sources.

A lighting device with a wide color-tunable range is still a challenge for lighting based on either organic light-emitting diodes (OLEDs) or inorganic LEDs. In this work, we first proposed a novel hybrid device of organic LEDs and inorganic blue GaN LEDs to achieve full white and other colors. Organic LEDs were stacked with green and red emissive layers and connected with blue GaN LEDs in parallel but in opposite polarity voltage. Under the alternate-current (AC) driving, the hybrid structure can be controlled independently by applying timing variable opposite voltages to emit the light from either blue LEDs or the stacked OLEDs for forming mixed colors. The hybrid device can generate white light, varying in a wide range by changing the amplitude and duty ratio (DR) of AC-driving signals, from cold white to standard white and to warm white (3668-11 833 K). When an AC voltage of (4.80 V, -2.45 V) was applied, the device has a high color gamut of 95.24% National Television System Committee (NTSC) and a high color rendering index (R a) of 92.4%. The novel hybrid device with the blue LED and OLED in opposite polarity exhibits potential applications in smart solid-state lighting, display, and light communication.

Microfluidic devices with disposable enzyme electrode for electrochemical monitoring of glucose concentrations.

This article describes the fabrication of tube-like microchannels made of UV curable polymer on a glass substrate and the device assembling with a disposable enzyme-working electrode for high-sensitivity electrochemical detection. While both reference and counter electrodes are patterned on the surface of the glass substrate, the working electrode is flipped on the top of the channel with an open access, providing a face-to-face probing configuration. When the enzyme electrode is contaminated or degraded, it can be easily replaced by a new one, keeping the main body of the device and the detection schema unchanged. Using glucose oxidase-coated gold electrodes, we were able to determine a linear amperometry response to the glucose concentrations in the range of 2-16 mM. By replacing the as-prepared working electrode by the one after thermal treatments, we showed a much more degraded enzyme electrode activity, enabling efficient determination of the electrode quality as well as the whole process optimization.

Anisotropic wet etched silicon substrates for reoriented and selective growth of ZnO nanowires and enhanced hydrophobicity.

Herein we report the fabrication of ZnO nanowires on anisotropic wet etched silicon substrates by selective hydrothermal growth. <100> oriented silicon wafers were first patterned by anisotropic wet etch with a KOH solution, resulting in V-shaped stripes of different periods. Then, a thin layer of gold was deposited and annealed to promote the hydrothermal growth of ZnO nanowires. It was found that the growth rate of ZnO nanowires on <111> surfaces was much higher than that on <100> surfaces. As a first application of such micro- and nanostructured surfaces, we show enhanced wetting properties by measuring the contact angle of water droplets on the samples obtained under different patterning and growth conditions. Our results also demonstrated the possibility of tuning the contact angle of the sample in the range between 115° and 155°, by changing either the pattern of the silicon template or the hydrothermal growth conditions.

Emerging Nanopixel Light-Emitting Displays: Significance, Challenges, and Prospects.

The requirement for increased resolution has created the concept of displays with nanoscale pixels; that is, each subpixel consists of multiple or even a single nanolight source, which is considered the ultimate light source for light field, near-eye, and implantable displays. However, related research is still at an early stage, and further insights into this future display concept should be provided. In this Perspective, we provide our proposed term for this future display, namely, nanopixel light-emitting display (NLED). We present an overview of nanolight-emitting diodes, which are considered the core component of NLEDs. Then, a roadmap to realize NLEDs from the view of material design is provided. Finally, we introduce our proposed operation mode (nonelectrical contact and noncarrier injection mode) for NLEDs and recommend possible nanopixel-level drive approaches. We hope that this Perspective will be helpful in designing innovative display technologies.

Spontaneous Formation of Random Wrinkles by Atomic Layer Infiltration for Anticounterfeiting.

Continuous developments of innovative anticounterfeiting strategies are vital to restrain the fast-growing counterfeit markets. Physical unclonable function (PUF)-based taggants allow for a practical solution to provide irreproducible codes for strong authentication. Herein, an advanced anticounterfeiting strategy with multiple security levels was successfully developed using screen printing and atomic layer infiltration (ALI) techniques. Macroscale poly(dimethylsiloxane) (PDMS) patterns were fabricated for primary verification. Spontaneous formation of random wrinkles with size in the micrometer scale was achieved on the top surface of screen-printed PDMS patterns due to the anisotropic relief and redistribution of extra compressive stress after Al2O3 infiltration, which can be used for senior authentication by image identification using the artificial intelligence (AI) technique. Furthermore, the complexity and security level of a code, which are proportional to the minutia density, can be adjusted by the morphology of the wrinkles in terms of amplitude and wavelength via the degree of Al2O3 permeation depending on ALI conditions. These spontaneously formed random wrinkles were demonstrated for validation and decoding with AI, exhibiting the merits of being unclonable, nondestructive, universally adaptable, environmentally stable, and mass-producible, and sufficiently adaptable for an industry-suitable authentication strategy.

Reversed cell imprinting, AFM imaging and adhesion analyses of cells on patterned surfaces.

Cell adhesion and motility depend strongly on the interactions between cells and cell culture substratum. To observe the cell morphology at the interface between cells and artificial substratum or patterned surfaces, we have developed a technique named reversed cell imprinting. After culture and chemical fixation of the cells on a patterned hole array, a liquid polymer was poured on and UV cured, allowing taking off the cell-polymer assembly for a direct observation of the underside cell surface using atomic force microscopy. As expected, we observed local deformation of the cell membrane in the hole area with a penetration depth strongly dependent on the size and depth of the hole as well as the culture time. Quantitative analyses of Hela cells on patterned surfaces of polydimethylsiloxane (PDMS) revealed that the penetration was also position dependent over the cell attachment area due to the non-homogeneous distribution of the membrane stress. With the increase of the culture time, the penetration depth was reduced, in a close correlation with the increase of the cell spreading area. Nevertheless, both cell seeding and adhesion efficiency on high density hole arrays could be significantly increased comparing to that on a smooth surface. Patterned substrates are increasingly required to produce and interrogate new biomaterials for therapeutic benefit. Overall, this work suggests a strategy to endow conventional imaging methods with added functionality to enable easy observation of the underside cell morphology on topographic patterns.

Large-scale microlens arrays on flexible substrate with improved numerical aperture for curved integral imaging 3D display.

Curved integral imaging 3D display could provide enhanced 3D sense of immersion and wider viewing angle, and is gaining increasing interest among discerning users. In this work, large scale microlens arrays (MLAs) on flexible PMMA substrate were achieved based on screen printing method. Meanwhile, an inverted reflowing configuration as well as optimization of UV resin's viscosity and substrate's surface wettability were implemented to improved the numerical aperture (NA) of microlenses. The results showed that the NA values of MLAs could be increased effectively by adopting inverted reflowing manner with appropriate reflowing time. With decreasing the substrate's wettability, the NA values could be increased from 0.036 to 0.096, when the UV resin contact angles increased from 60.1° to 88.7°. For demonstration, the fabricated MLAs was combined to a curved 2D monitor to realize a 31-inch curved integral imaging 3D display system, exhibiting wider viewing angle than flat integral imaging 3D display system.

Electroluminescence from µLED without external charge injection.

Stable electroluminescence from micro-pixelated light-emitting diode (µLED) occurs when electrons and holes are continuously injected from external electrodes. Different from the general recognition, in this work, µLED works in an operation mode, namely, non-electrical contact and non-carrier injection mode, and can be 'wirelessly' lit up without external charge injection, which is different from the general recognition. Inherent holes and electrons in µLEDs can provide sufficient carriers for radiative recombination under alternating-current electric field. A possible model related to the diffusion of majority carrier and the drift of minority carrier in µLED was proposed, which is further confirmed by the employment of a 'carrier pump'. Finally, the intrinsic characteristics of the device-in-capacitor, namely, self-protection against electrical breakdown, were discussed. This work demonstrates a new device configuration and an alternative operating mode for µLED and provides a research manner to obtain advanced µLED-based technology.