Crystal-Site Engineering of Novel Na3KMg7(PO4)6-x(BO3)x:Eu2+ Phosphors for Full-Spectrum Lighting.

The "cyan gap" is the bottleneck problem in violet-driven full-spectrum white-light-emitting diodes (wLEDs) in healthy lighting. Accordingly, we develop a novel broadband-blue-cyan emission Na3KMg7(PO4)6-x(BO3)x:Eu2+ (NKMPB:Eu2+) phosphor via crystal-site engineering. This phosphor is derived from the Na3KMg7(PO4)6:Eu2+ phosphor, which shows desired abundant cyan emissive components. A comparative study is conducted to reveal the microstructure-property relationship and the key influential factors to its spectrum distribution. It can be found that the introduced (BO3)3- units can manipulate the site-selective occupation of Eu2+ activators, asymmetrically broadening the emission spectrum in NKMPB:Eu2+. Considering detailed luminescence performance analysis and the density functional theory calculations, a new substitution pathway of Eu2+ is created by substituting (PO4)3- with (BO3)3- units, making partial Eu2+ ions enter the Mg2+ (CN = 5, CN = 6) crystallographic sites, and yielding an extra emission band at 600 nm (16667 cm-1) and especially 501 nm (19960 cm-1). Meanwhile, a high-color-quality full-spectrum-emitting wLEDs was fabricated, upon 100 mA forward-bias current driven. Due to the achieved extra cyan emissive components of NKMPB:Eu2+, the constructed NKMPB:Eu2+-based wLEDs show better color rendering ability (∼90.9) than that of Na3KMg7(PO4)6:Eu2+-based wLEDs (∼86.3), and also demonstrate its great potential in full-spectrum healthy lighting.

K+-doping-induced highly efficient red emission in CsPb(Br,I)3 quantum dot glass toward Rec. 2020 displays.

It is demonstrated that the incorporation of K+ into CsPb(Br,I)3 perovskite quantum dot glass leads to the simultaneous increases of quantum efficiency and phase stability. The latent mechanism is analyzed via the microstructural and spectroscopic studies. The constructed prototype white-light-emitting diode device yields an ultra-wide color gamut attaining 96% Rec. 2020 standard.

Toward high-power-density laser-driven lighting: enhancing heat dissipation in phosphor-in-glass film by introducing h-BN.

In this work,  hexagonal boron nitride (h-BN) nanocrystals as functional additives in a phosphor-in-glass film are shown to substantially increase the luminous performance driven by a blue laser. Microstructural and spectroscopic studies reveal that h-BN particles distributed over the whole glass matrix build in situ a local heat conductive path which effectively accelerates heat dissipation and so greatly relieves the "thermal run-away effect". The developed composite material with fine thermal manipulation may be a promising phosphor color converter for high-power-density laser-driven lighting.

Van der Waals contacted MoOx staked ZnO/GaN vertical heterostructured ultraviolet light emitting diodes.

Since the discovery of two dimensional (2D) materials, there has been a gold rush for van der Waals integrated 2D material heterostructure based optoelectronic devices. Van der Waals integration involves the physical assembly of the components of the device. In the present work, we extended van der Waals integration from 2D materials to three-dimensional (3D) materials, and herein we uniquely designed a van der Waals contacted light emitting diode based on MoOx staked ZnO/GaN heterostructure. The presence of the MoOx layer between n-type ZnO and p-type GaN leads to the confinement of electrons and an increase in the electron charge density at n-type ZnO. The n-type MoOx, a well-known hole injection layer, favors the availability of holes at the ZnO site, leading to the efficient recombination of electrons and holes at the ZnO site, which results in predominant high-intensity UV-EL emission around 380 nm in both forward and reverse bias.

A synergetic enhancement of localized surface plasmon resonance and photo-induced effect for graphene/GaAs photodetector.

Photodetectors based on graphene/GaAs heterostructure were fabricated and demonstrated for application in self-powered photodetection. Then, Si quantum dots (QDs) were spin-coated onto the surface of the devices to enhance the built-in field by photo-induced doping, because of the tunable Fermi level (E F) of graphene and shallow junction of the heterojunction. Additionally, Au nanoparticles working as a light trapping structure were used to the enhance quantum efficiency of the Si QDs and the optical absorption of the heterojunction, benefitting from localized surface plasmon resonance. Therefore, a large-area photodetector under self-powered conditions achieved a high performance i.e. responsivity (1.81 × 105 V W-1), detectivity (2.0 × 1012 Jones), fast response speed (<0.04 ms), and on-off ratio (6 × 103). The high voltage responsivity opens a promising pathway to ultra-weak light detection, and facilities the development of novel sensors.

Gate tunable surface plasmon resonance enhanced graphene/Ag nanoparticles-polymethyl methacrylate/graphene/p-GaN heterostructure light-emitting diodes.

By combining the surface plasmon enhancement technique with gating effect, a tunable blue lighting emitting diode (LED) based on graphene/Ag nanoparticles (NPs)-polymethyl methacrylate (PMMA)/graphene/p-GaN heterostructure has been achieved. The surface plasmon enhancement is introduced through spin-coating Ag nanoparticles on graphene/p-GaN heterostructure while the gating effect is demonstrated through a graphene/PMMA/graphene sandwich structure, where the top graphene layer acts as the gate electrode. Compared with initial graphene/p-GaN heterostructure LEDs, the electroluminescence (EL) emission intensity of Ag NPs/graphene/p-GaN heterostructure LEDs has been largely enhanced, attributing to the surface plasmon resonance (SPR) of Ag nanoparticles. The EL emission intensity of graphene/Ag NPs-PMMA/graphene/p-GaN heterostructure LEDs can further be gate-tunable effectively through exerting a static voltage between the sandwich structure, which tunes the Fermi level of graphene contacting with p-GaN. These results indicate that through sophisticated design, graphene/Ag NPs-PMMA/graphene/p-GaN heterostructure LEDs can be a potential candidate for many essential electronic and optoelectronic applications.

Graphene/h-BN/GaAs sandwich diode as solar cell and photodetector.

In graphene/semiconductor heterojunction, the statistic charge transfer between graphene and semiconductor leads to decreased junction barrier height and limits the Fermi level tuning effect in graphene, which greatly affects the final performance of the device. In this work, we have designed a sandwich diode for solar cells and photodetectors through inserting 2D hexagonal boron nitride (h-BN) into graphene/GaAs heterostructure to suppress the static charge transfer. The barrier height of graphene/GaAs heterojunction can be increased from 0.88 eV to 1.02 eV by inserting h-BN. Based on the enhanced Fermi level tuning effect with interface h-BN, through adopting photo-induced doping into the device, power conversion efficiency (PCE) of 10.18% has been achieved for graphene/h-BN/GaAs compared with 8.63% of graphene/GaAs structure. The performance of graphene/h-BN/GaAs based photodetector is also improved with on/off ratio increased by one magnitude compared with graphene/GaAs structure.

ZnO quantum dot-doped graphene/h-BN/GaN-heterostructure ultraviolet photodetector with extremely high responsivity.

A ZnO quantum dot  photo-doped graphene/h-BN/GaN-heterostructure ultraviolet photodetector with extremely high responsivity of more than 1915 A W-1 and detectivity of more than 1.02 × 1013 Jones (Jones = cm Hz1/2 W-1) has been demonstrated. The interfaced h-BN layer increases the barrier height at the graphene/GaN heterojunction, which decreases the dark current and improves the on/off current ratio of the device. The photo-doping effect increases the barrier height and carrier concentration at the graphene/h-BN/GaN heterojunction, thus the responsivity is improved from 1473 A W-1 to 1915 A W-1 and the detectivity is improved from 5.8 × 1012 to 1.0 × 1013 Jones. Moreover, all of the responsivity and detectivity values are the highest values among all the graphene-based ultraviolet photodetectors.

Graphene/h-BN/ZnO van der Waals tunneling heterostructure based ultraviolet photodetector.

We report a novel ultraviolet photodetector based on graphene/h-BN/ZnO van der Waals heterostructure. Graphene/ZnO heterostructure shows poor rectification behavior and almost no photoresponse. In comparison, graphene/h-BN/ZnO structure shows improved electrical rectified behavior and surprising high UV photoresponse (1350AW(-1)), which is two or three orders magnitude larger than reported GaN UV photodetector (0.2~20AW(-1)). Such high photoresponse mainly originates from the introduction of ultrathin two-dimensional (2D) insulating h-BN layer, which behaves as the tunneling layer for holes produced in ZnO and the blocking layer for holes in graphene. The graphene/h-BN/ZnO heterostructure should be a novel and representative 2D heterostructure for improving the performance of 2D materials/Semiconductor heterostructure based optoelectronic devices.

Atomically thin spherical shell-shaped superscatterers based on a Bohr model.

Graphene monolayers can be used for atomically thin three-dimensional shell-shaped superscatterer designs. Due to the excitation of the first-order resonance of transverse magnetic (TM) graphene plasmons, the scattering cross section of the bare subwavelength dielectric particle is enhanced significantly by five orders of magnitude. The superscattering phenomenon can be intuitively understood and interpreted with a Bohr model. In addition, based on the analysis of the Bohr model, it is shown that contrary to the TM case, superscattering is hard to achieve by exciting the resonance of transverse electric (TE) graphene plasmons due to their poor field confinements.

Graphene coated ZnO nanowire optical waveguides.

We report the fabrication and characterization of freestanding graphene coated ZnO nanowires (GZNs) for optical waveguiding. The GZNs are fabricated using a tape-assist transfer under micromanipulation. Owing to the deep-subwavelength diameter and high index contrast of the ZnO nanowire waveguide, light-graphene interaction is significantly enhanced by the strong surface optical fields, resulting in a linear absorption as high as 0.11 dB/µm in a 606-nm-diameter GZN at 1550-nm wavelength. Launched by 1550-nm-wavelength femto-second pulses, a 475-nm-diameter GZN with a graphene coating length of merely 24 µm exhibits evident nonlinear saturable absorption with a peak power threshold down to 1.3 W. In addition, we also demonstrate a transmission modulation for 1550-nm-wavelength signal with a 590-nm-diameter GZN, showing the possibility of using GZN waveguides as nanoscale bulding blocks for nanophotonic devices.

Free-space carpet cloak using transformation optics and graphene.

Free-space carpet cloak designed with transformation optics requires materials exhibiting simultaneously anisotropic properties and plasma-like behaviors, but materials that simultaneously meet these requirements are rarely found in nature. The recently discovered graphene has shown unique anisotropic plasma-like behavior benefitting from its natural two-dimensional structure and in-plane ultrahigh electron mobility, and therefore, can be a good candidate for the free-space carpet cloak design. In this Letter, we theoretically propose a novel free-space carpet cloak by using periodically stacking layered graphene for the first time. Simulation results show an object under the graphene-based carpet cloak becomes invisible in the THz frequencies. By exploiting the large tunability of graphene's conductivity, we also demonstrate the working frequency of the designed cloak is continuously tunable in a wide spectrum.

A Novel PiGF@Diamond Color Converter with a Record Thermal Conductivity for Laser-Driven Projection Display.

High-brightness laser lighting is confronted with crucial challenges in developing laser-excitable color converting materials with effective heat dissipation and super optical performance. Herein, a novel composite of phosphor-in-glass film on transparent diamond (PiGF@diamond) is designed and fabricated via a facile low-temperature co-sintering strategy. The as-prepared La3Si6N11:Ce3+ (LSN:Ce) PiGF@diamond with well-retained optical properties of raw phosphor shows a record thermal conductivity of ≈599 W m-1 K-1, which is about 60 times higher than that of currently well-used PiGF@sapphire (≈10 W m-1 K-1). As a consequence, this color converter can bear laser power density up to 40.24 W mm-2 and a maximum luminance flux of 5602 lm without luminescence saturation due to efficient inhibition of laser-induced heat accumulation. By further supplementing red spectral component of CaAlSiN3:Eu2+ (CASN:Eu), the PiGF@diamond based white laser diode is successfully constructed, which can yield warm white light with a high color rendering index of 89.3 and find practical LD-driven applications. The findings will pave the way for realizing the commercial application of PiGF composite in laser lighting and display.

Oat ß-glucan repairs the epidermal barrier by upregulating the levels of epidermal differentiation, cell-cell junctions and lipids via Dectin-1.

BACKGROUND AND PURPOSE: Oat ß-glucan could ameliorate epidermal hyperplasia and accelerate epidermal barrier repair. Dectin-1 is one of the receptors of ß-glucan and many biological functions of ß-glucan are mediated by Dectin-1. Dectin-1 promotes wound healing through regulating the proliferation and migration of skin cells. Thus, this study aimed to investigate the role of oat ß-glucan and Dectin-1 in epidermal barrier repair. EXPERIMENTAL APPROACH: To investigate the role of Dectin-1 in the epidermal barrier, indicators associated with the recovery of a damaged epidermal barrier, including histopathological changes, keratinization, proliferation, apoptosis, differentiation, cell-cell junctions and lipid content were compared between WT and Dectin-1-/- mice. Further, the effect of oat ß-glucan on the disruption of the epidermal barrier was also compared between WT and Dectin-1-/- mice. KEY RESULTS: Dectin-1 deficiency resulted in delayed recovery and marked keratinization, as well as abnormal levels of keratinocyte differentiation, cell-cell junctions and lipid synthesis during the restoration of the epidermal barrier. Oat ß-glucan significantly reduces epidermal hyperplasia, promotes epidermal differentiation, increases cell-cell junction expression, promotes lipid synthesis and ultimately accelerates the recovery of damaged epidermal barriers via Dectin-1. Oat ß-glucan could promote CaS receptor expression and activate the PPAR-γ signalling pathway via Dectin-1. CONCLUSION AND IMPLICATIONS: Oat ß-glucan promote the recovery of damaged epidermal barriers through promoting epidermal differentiation, increasing the expression of cell-cell junctions and lipid synthesis through Dectin-1. Dectin-1 deficiency delay the recovery of epidermal barriers, which indicated that Dectin-1 may be a potential target in epidermal barrier repair.

Twisted moiré conductive thermal metasurface.

Extensive investigations on the moiré magic angle in twisted bilayer graphene have unlocked the emerging field-twistronics. Recently, its optics analogue, namely opto-twistronics, further expands the potential universal applicability of twistronics. However, since heat diffusion neither possesses the dispersion like photons nor carries the band structure as electrons, the real magic angle in electrons or photons is ill-defined for heat diffusion, making it elusive to understand or design any thermal analogue of magic angle. Here, we introduce and experimentally validate the twisted thermotics in a twisted diffusion system by judiciously tailoring thermal coupling, in which twisting an analog thermal magic angle would result in the function switching from cloaking to concentration. Our work provides insights for the tunable heat diffusion control, and opens up an unexpected branch for twistronics -- twisted thermotics, paving the way towards field manipulation in twisted configurations including but not limited to fluids.

Self-Driven Photo-Polarized Water Molecule-Triggered Graphene-Based Photodetector.

Flowing water can be used as an energy source for generators, providing a major part of the energy for daily life. However, water is rarely used for information or electronic devices. Herein, we present the feasibility of a polarized liquid-triggered photodetector in which polarized water is sandwiched between graphene and a semiconductor. Due to the polarization and depolarization processes of water molecules driven by photogenerated carriers, a photo-sensitive current can be repeatedly produced, resulting in a high-performance photodetector. The response wavelength of the photodetector can be fine-tuned as a result of the free choice of semiconductors as there is no requirement of lattice match between graphene and the semiconductors. Under zero voltage bias, the responsivity and specific detectivity of Gr/NaCl (0.5 M)W/N-GaN reach values of 130.7 mA/W and 2.3 × 109 Jones under 350 nm illumination, respectively. Meanwhile, using a polar liquid photodetector can successfully read the photoplethysmography signals to produce accurate oxygen blood saturation and heart rate. Compared with the commercial pulse oximetry sensor, the average errors of oxygen saturation and heart rate in the designed photoplethysmography sensor are ~1.9% and ~2.1%, respectively. This study reveals that water can be used as a high-performance photodetector in informative industries.

High Efficient Solar Cell Based on Heterostructure Constructed by Graphene and GaAs Quantum Wells.

Despite the fascinating optoelectronic properties of graphene, the power conversion efficiency (PCE) of graphene based solar cells remains to be lifted up. Herein, it is experimentally shown that the graphene/quantum wells/GaAs heterostructure solar cell can reach a PCE of 20.2% and an open-circuit voltage (Voc ) as high as 1.16 V at 90 K. The high efficiency is a result of carrier multiplication (CM) effect of graphene in the graphene/GaAs heterostructure. Especially, the external quantum efficiency (EQE) in the ultraviolet wavelength can be improved up to 72.2% based on the heterostructure constructed by graphene/In0.15 Ga0.85 As/GaAs0.75 P0.25 quantum wells/GaAs. The EQE increases as the light wavelength decreases, which indicates more carriers can be effectively excited by the higher energy photons through CM effect. Owing to these physical characters, the graphene/GaAs heterostructure solar cell will provide a possible way to exceed Shockley-Queisser (S-Q) limit.

Hybrid thermionic-photovoltaic converter with graphene-on-semiconductor heterojunction anode for efficient electricity generation.

Thermionic energy converters are solid-state heat engines to produce electricity with significant potential, whereas the output voltage is constrained by the work function difference between cathode and anode. In this work, we originally apply a graphene-on-semiconductor heterojunction anode to a thermionic-photovoltaic (TIPV) converter to output additional voltage. Thermionic electrons are injected into the graphene layer and then recombined with photogenerated holes. Photogenerated electrons are extracted from the conduction band and reinjected into the cathode through an external load. A proof-of-concept demonstration of the TIPV converter is developed with barium surface-engineered cathode and anode. Open-circuit voltage is increased from ∼0.9 to ∼1.9 V by comparing with an identical configuration without graphene layer. The TIPV converter yields a power generation density of 2.7 kW/m2 with an electronic efficiency of ∼27%. This work paves the way for the development of TIPV converter toward high power density.

Broadband Insulator-Based Dynamic Diode with Ultrafast Hot Carriers Process.

The excitation, rebound, and transport process of hot carriers (HCs) inside dynamic diode (DD) based on insulators has been rarely explored due to the original stereotyped in which it was thought that the insulators are nonconductive. However, the carrier dynamics of DD is totally different from the static diode, which may bring a subverting insight of insulators. Herein, we discovered insulators could be conductive under the framework of DD; the HC process inside the rebounding procedure caused by the disappearance and reestablishment of the built-in electric field at the interface of insulator/semiconductor heterostructure is the main generation mechanism. This type of DD can response fast up to 1 µs to mechanical excitation with an output of ~10 V, showing a wide band frequency response under different input frequencies from 0 to 40 kHz. It can work under extreme environments; various applications like underwater communication network, self-powered sensor/detector in the sea environment, and life health monitoring can be achieved.

Hot Carrier Transport and Carrier Multiplication Induced High Performance Vertical Graphene/Silicon Dynamic Diode Generator.

Dynamic semiconductor diode generators (DDGs) offer a potential portable and miniaturized energy source, with the advantages of high current density, low internal impedance, and independence of the rectification circuit. However, the output voltage of DDGs is generally as low as 0.1-1 V, owing to energy loss during carrier transport and inefficient carrier collection, which requires further optimization and a deeper understanding of semiconductor physical properties. Therefore, this study proposes a vertical graphene/silicon DDG to regulate the performance by realizing hot carrier transport and collection. With instant contact and separation of the graphene and silicon, hot carriers are generated by the rebounding process of built-in electric fields in dynamic graphene/silicon diodes, which can be collected within the ultralong hot electron lifetime of graphene. In particular, monolayer graphene/silicon DDG outputs a high voltage of 6.1 V as result of ultrafast carrier transport between the monolayer graphene and silicon. Furthermore, a high current of 235.6 nA is generated due to the carrier multiplication in graphene. A voltage of 17.5 V is achieved under series connection, indicating the potential to supply electronic systems through integration design. The graphene/silicon DDG has applications as an in situ energy source for harvesting mechanical energy from the environment.