Improved heat dissipation in gallium nitride light-emitting diodes with embedded graphene oxide pattern

Hierarchical Incorporation of Reduced Graphene Oxide into Anisotropic Cellulose Nanofiber Foams Improves Their Thermal Insulation

Anisotropic cellulose nanofiber (CNF) foams represent the state-of-the-art in renewable insulation. These foams consist of large (diameter >10 μm) uniaxially aligned macropores with mesoporous pore-walls and aligned CNF. The foams show anisotropic thermal conduction, where heat transports more efficiently in the axial direction (along the aligned CNF and macropores) than in the radial direction (perpendicular to the aligned CNF and macropores). Here we explore the impact on axial and radial thermal conductivity upon depositing a thin film of reduced graphene oxide (rGO) on the macropore walls in anisotropic CNF foams. To obtain rGO films on the foam walls we developed liquid-phase self-assembly to deposit rGO in a layer-by-layer fashion. Using electron and ion microscopy, we thoroughly characterized the resulting rGO-CNF foams and confirmed the successful deposition of rGO. These hierarchical rGO-CNF foams show lower radial thermal conductivity (λr) across a wide range of relative humidity compared to CNF control foams. Our work therefore demonstrates a potential method for improved thermal insulation in anisotropic CNF foams and introduces versatile self-assembly for postmodification of such foams.

Transparent radiative cooling cover window for flexible and foldable electronic displays

Transparent radiative cooling holds the promise to efficiently manage thermal conditions in various electronic devices without additional energy consumption. Radiative cooling cover windows designed for foldable and flexible displays could enhance cooling capacities in the ubiquitous deployment of flexible electronics in outdoor environments. However, previous demonstrations have not met the optical, mechanical, and moisture-impermeable criteria for such cover windows. Herein, we report transparent radiative cooling metamaterials with a thickness of 50 microns as a cover window of foldable and flexible displays by rational design and synthesis of embedding optically-modulating microstructures in clear polyimide. The resulting outcome not only includes excellent light emission in the atmospheric window under the secured optical transparency but also provides enhanced mechanical and moisture-impermeable properties to surpass the demands of target applications. Our metamaterials not only substantially mitigate the temperature rise in heat-generating devices exposed to solar irradiance but also enhance the thermal management of devices in dark conditions. The light output performance of light-emitting diodes in displays on which the metamaterials are deployed is greatly enhanced by suppressing the performance deterioration associated with thermalization.

Highly Thermoconductive, Strong Graphene-Based Composite Films by Eliminating Nanosheets Wrinkles

Graphene-based thermally conductive composites have been proposed as effective thermal management materials for cooling high-power electronic devices. However, when flexible graphene nanosheets are assembled into macroscopic thermally conductive composites, capillary forces induce shrinkage of graphene nanosheets to form wrinkles during solution-based spontaneous drying, which greatly reduces the thermal conductivity of the composites. Herein, graphene nanosheets/aramid nanofiber (GNS/ANF) composite films with high thermal conductivity were prepared by in-plane stretching of GNS/ANF composite hydrogel networks with hydrogen bonds and π-π interactions. The in-plane mechanical stretching eliminates graphene nanosheets wrinkles by suppressing inward shrinkage due to capillary forces during drying and achieves a high in-plane orientation of graphene nanosheets, thereby creating a fast in-plane heat transfer channel. The composite films (GNS/ANF-60 wt%) with eliminated graphene nanosheets wrinkles showed a significant increase in thermal conductivity (146 W m-1 K-1) and tensile strength (207 MPa). The combination of these excellent properties enables the GNS/ANF composite films to be effectively used for cooling flexible LED chips and smartphones, showing promising applications in the thermal management of high-power electronic devices.

Applications of remote epitaxy and van der Waals epitaxy

Epitaxy technology produces high-quality material building blocks that underpin various fields of applications. However, fundamental limitations exist for conventional epitaxy, such as the lattice matching constraints that have greatly narrowed down the choices of available epitaxial material combinations. Recent emerging epitaxy techniques such as remote and van der Waals epitaxy have shown exciting perspectives to overcome these limitations and provide freestanding nanomembranes for massive novel applications. Here, we review the mechanism and fundamentals for van der Waals and remote epitaxy to produce freestanding nanomembranes. Key benefits that are exclusive to these two growth strategies are comprehensively summarized. A number of original applications have also been discussed, highlighting the advantages of these freestanding films-based designs. Finally, we discuss the current limitations with possible solutions and potential future directions towards nanomembranes-based advanced heterogeneous integration.

Tailoring Dense, Orientation-Tunable, and Interleavedly Structured Carbon-Based Heat Dissipation Plates

The controllability of the microstructure of a compressed hierarchical building block is essential for optimizing a variety of performance parameters, such as thermal management. However, owing to the strong orientation effect during compression molding, optimizing the alignment of materials perpendicular to the direction of pressure is challenging. Herein, to illustrate the effect of the ordered microstructure on heat dissipation, thermally conductive carbon-based materials are fabricated by tailoring dense, orientation-tunable, and interleaved structures. Vertically aligned carbon nanotube arrays (VACNTs) interconnected with graphene films (GF) are prepared as a 3D core-ordered material to fabricate compressed building blocks of O-VA-GF and S-VA-GF. Leveraging the densified interleaved structure offered by VACNTs, the hierarchical O-VA-GF achieves excellent through-plane (41.7 W m^-1 K^-1 ) and in-plane (397.9 W m^-1 K^-1 ) thermal conductivities, outperforming similar composites of S-VA-GF (through-plane: 10.3 W m^-1 K^-1 and in-plane: 240.9 W m^-1 K^-1 ) with horizontally collapsed carbon nanotubes. As heat dissipation plates, these orderly assembled composites yield a 144% and 44% enhancement in the cooling coefficient compared with conventional Si₃ N₄ for cooling high-power light-emitting diode chips.

Graphene oxide for photonics, electronics and optoelectronics

Graphene oxide (GO) was initially developed to emulate graphene, but it was soon recognized as a functional material in its own right, addressing an application space that is not accessible to graphene and other carbon materials. Over the past decade, research on GO has made tremendous advances in material synthesis and property tailoring. These, in turn, have led to rapid progress in GO-based photonics, electronics and optoelectronics, paving the way for technological breakthroughs with exceptional performance. In this Review, we provide an overview of the optical, electrical and optoelectronic properties of GO and reduced GO on the basis of their chemical structures and fabrication approaches, together with their applications in key technologies such as solar energy harvesting, energy storage, medical diagnosis, image display and optical communications. We also discuss the challenges of this field, together with exciting opportunities for future technological advances.

Thermal Transport of AlN/Graphene/3C-SiC Typical Heterostructures with Different Crystallinities of Graphene

It is proven that introduction of graphene into typical heterostructures can effectively reduce the high interfacial thermal resistance in semiconductor chips. The crystallinity of graphene varies greatly; thus, we have investigated the effects of single-crystal and polycrystalline graphene on the thermal transport of AlN/graphene/3C-SiC heterostructures by molecular dynamics. The results show that polycrystalline graphene contributes more to the interfacial thermal conductance (ITC) inside the chip with a maximum increase of 75.09%, which is further confirmed by the energy transport and thermal relaxation time. Multiple analyses indicate that grain boundaries lead to the increase in C-Si covalent bonds, and thus, strong interactions improve the ITC. However, covalent bonding further causes local tensile strain and wrinkles in graphene. The former decreases the ITC, and the latter leads to the fluctuation of the van der Waals interaction at the interface. The combined effect of various influential factors results in the increase in the ITC, which are confirmed by phonon transmission with 0-18 THz. In addition, wrinkles and covalent bonding lead to increased stress concentration in polycrystalline graphene. This leads to a maximum reduction of 19.23% in the in-plane thermal conductivity, which is not conducive to the lateral diffusion of hot spots within the chip. The research results would provide important guidance in designing for high thermal transport performance high-power chips.

Recent progress of heterostructures based on two dimensional materials and wide bandgap semiconductors

Recent progress in the synthesis and assembly of two-dimensional (2D) materials has laid the foundation for various applications of atomically thin layer films. These 2D materials possess rich and diverse properties such as layer-dependent band gaps, interesting spin degrees of freedom, and variable crystal structures. They exhibit broad application prospects in micro-nano devices. In the meantime, the wide bandgap semiconductors (WBS) with an elevated breakdown voltage, high mobility, and high thermal conductivity have shown important applications in high-frequency microwave devices, high-temperature and high-power electronic devices. Beyond the study on single 2D materials or WBS materials, the multi-functional 2D/WBS heterostructures can promote the carrier transport at the interface, potentially providing novel physical phenomena and applications, and improving the performance of electronic and optoelectronic devices. In this review, we overview the advantages of the heterostructures of 2D materials and WBS materials, and introduce the construction methods of 2D/WBS heterostructures. Then, we present the diversity and recent progress in the applications of 2D/WBS heterostructures, including photodetectors, photocatalysis, sensors, and energy related devices. Finally, we put forward the current challenges of 2D/WBS heterostructures and propose the promising research directions in the future.

Fabrication of a Microcavity Prepared by Remote Epitaxy over Monolayer Molybdenum Disulfide

Advances in epitaxy have enabled the preparation of high-quality material architectures consisting of incommensurate components. Remote epitaxy based on lattice transparency of atomically thin graphene has been intensively studied for cost-effective advanced device manufacturing and heterostructure formation. However, remote epitaxy on nongraphene two-dimensional (2D) materials has rarely been studied even though it has a broad and immediate impact on various disciplines, such as many-body physics and the design of advanced devices. Herein, we report remote epitaxy of ZnO on monolayer MoS₂ and the realization of a whispering-gallery-mode (WGM) cavity composed of a single crystalline ZnO nanorod and monolayer MoS₂. Cross-sectional transmission electron microscopy and first-principles calculations revealed that the nongraphene 2D material interacted with overgrown and substrate layers and also exhibited lattice transparency. The WGM cavity embedding monolayer MoS₂ showed enhanced luminescence of MoS₂ and multimodal emission.

Lattice Polarity Manipulation of Quasi-vdW Epitaxial GaN Films on Graphene Through Interface Atomic Configuration

Quasi van der Waals epitaxy, a pioneering epitaxy of sp³ -hybridized semiconductor films on sp² -hybridized 2D materials, provides a way, in principle, to achieve single-crystal epilayers with preferred atom configurations that are free of substrate. Unfortunately, this has not been experimentally confirmed in the case of the hexagonal semiconductor III-nitride epilayer until now. Here, it is reported that the epitaxy of gallium nitride (GaN) on graphene can tune the atom arrangement (lattice polarity) through manipulation of the interface atomic configuration, where GaN films with gallium and nitrogen polarity are achieved by forming CONGa(3) or COGaN(3) configurations, respectively, on artificial CO surface dangling bonds by atomic oxygen pre-irradiation on trilayer graphene. Furthermore, an aluminum nitride buffer/interlayer leads to unique metal polarity due to the formation of an AlON thin layer in a growth environment containing trace amounts of oxygen, which explains the open question of why those reported wurtzite III-nitride films on 2D materials always exhibit metal polarity. The reported atomic modulation through interface manipulation provides an effective model for hexagonal nitride semiconductor layers grown on graphene, which definitely promotes the development of novel semiconductor devices.

Transfer-Enabled Fabrication of Graphene Wrinkle Arrays for Epitaxial Growth of AlN Films

Formation of graphene wrinkle arrays can periodically alter the electrical properties and chemical reactivity of graphene, which is promising for numerous applications. However, large-area fabrication of graphene wrinkle arrays remains unachievable with a high density and defined orientations, especially on rigid substrates. Herein, relying on the understanding of the formation mechanism of transfer-related graphene wrinkles, the graphene wrinkle arrays are fabricated without altering the crystalline orientation of entire graphene films. The choice of the transfer medium that has poor wettability on the corrugated surface of graphene is proven to be the key for the formation of wrinkles. This work provides a deep understanding of formation process of transfer-related graphene wrinkles and opens up a new way for periodically modifying the surface properties of graphene for potential applications, including direct growth of AlN epilayers and deep ultraviolet light emitting diodes.

Beyond homogeneous dispersion: oriented conductive fillers for high κ nanocomposites

Rational design of structures for regulating the thermal conductivities (κ) of materials is critical to many components and products employed in electrical, electronic, energy, construction, aerospace, and medical applications. As such, considerable efforts have been devoted to developing polymer composites with tailored conducting filler architectures and thermal conduits for highly improved κ. This paper is dedicated to overviewing recent advances in this area to offer perspectives for the next level of future development. The limitations of conventional particulate-filled composites and the issue of percolation are discussed. In view of different directions of heat dissipation in polymer composites for different end applications, various approaches for designing the micro- and macroscopic structures of thermally conductive networks in the polymer matrix are highlighted. Methodological approaches devised to significantly ameliorate thermal conduction are categorized with respect to the pathways of heat dissipation. Future prospects for the development of thermally conductive polymer composites with modulated thermal conduction pathways are highlighted.

Assessment of the optical and electrical properties of light-emitting diodes containing carbon-based nanostructures and plasmonic nanoparticles: a review

Light-emitting diodes (LED) are widely employed in display applications and lighting systems. Further research on LED that incorporates carbon nanostructures and metal nanoparticles exhibiting surface plasmon resonance has demonstrated a significant improvement in device performance. These devices offer lower turn-on voltages, higher external quantum efficiencies, and luminance. De facto, plasmonic nanoparticles, such as Au and Ag have boosted the luminance of red, green, and blue emissions. When combined with carbon nanostructures they additionally offer new possibilities towards lightweight and flexible devices with better thermal management. This review surveys the diverse possibilities to combine various inorganic, organic, and carbon nanostructures along with plasmonic nanoparticles. Such combinations would allow an enhancement in the overall properties of LED.

In situ fabrication of silver/polyimide composite films with enhanced heat dissipation

In this study, silver/polyimide (Ag/PI) composite films with enhanced heat dissipation properties were prepared. Ag was formed in situ by reducing AgNO₃ at various locations according to the reduction method. Two different types of soluble PIs capable of solution processing were used, namely Matrimid and hydroxy polyimide (HPI). Unlike Matrimid with bulky substituents, HPI with polar hydroxy groups formed ion-dipole interactions with Ag ions to form Ag particles with uniform size distribution. The location and distribution of Ag particles affect the heat emission characteristics of the composite films, resulting in better heat dissipation properties with the thermally and photochemically reduced Ag/HPI films having more Ag particles distributed inside of the films than the chemically reduced films.

Enhanced Heat Dissipation in Gallium Nitride-Based Light-Emitting Diodes by Piezo-phototronic Effect

As a new generation of light sources, GaN-based light-emitting diodes (LEDs) have wide applications in lighting and display. Heat dissipation in LEDs is a fundamental issue that leads to a decrease in light output, a shortened lifespan, and the risk of catastrophic failure. Here, the temperature spatial distribution of the LEDs is revealed by using high-resolution infrared thermography, and the piezo-phototronic effect is proved to restrain efficaciously the temperature increment for the first time. We observe the temperature field and current density distribution of the LED array under external strain compensation. Specifically, the temperature rise caused by the self-heating effect is reduced by 47.62% under 0.1% external strain, which is attributed to the enhanced competitiveness of radiative recombination against nonradiative recombination due to the piezo-phototronic effect. This work not only deepens the understanding of the piezo-phototronic effect in LEDs but also provides a novel, easy-to-implement, and economical method to effectively enhance thermal management.

Experimental and Modeling Investigations of Miniaturization in InGaN/GaN Light-Emitting Diodes and Performance Enhancement by Micro-Wall Architecture

The recent technological trends toward miniaturization in lighting and display devices are accelerating the requirement for high-performance and small-scale GaN-based light-emitting diodes (LEDs). In this work, the effect of mesa size-reduction in the InGaN/GaN LEDs is systematically investigated in two lateral dimensions (x- and y-directions: parallel to and perpendicular to the line where p-n directions are) both experimentally and numerically. The role of the lateral size-reduction in the x- and y-directions in improving LED performance is separately identified through experimental and modeling investigations. The narrowed dimension in the x-direction is found to cause and dominate the alleviated current crowding phenomenon, while the size-reduction in the y-direction has a minor influence on that. The size-reduction in the y-orientation induces an increased ratio of perimeter-to-area in miniaturized LED devices, which leads to improved thermal dissipation and light extraction through the sidewalls. The grown and fabricated LED devices with varied dimensions further support this explanation. Then the effect of size-reduction on the LED performance is summarized. Moreover, three-micro-walls LED architecture is proposed and demonstrated to further promote light extraction and reduce the generation of the Joule heat. The findings in this work provide instructive guidelines and insights on device miniaturization, especially for micro-LED devices.

Graphene Oxide for Integrated Photonics and Flat Optics

With superior optical properties, high flexibility in engineering its material properties, and strong capability for large-scale on-chip integration, graphene oxide (GO) is an attractive solution for on-chip integration of 2D materials to implement functional integrated photonic devices capable of new features. Over the past decade, integrated GO photonics, representing an innovative merging of integrated photonic devices and thin GO films, has experienced significant development, leading to a surge in many applications covering almost every field of optical sciences such as photovoltaics, optical imaging, sensing, nonlinear optics, and light emitting. This paper reviews the recent advances in this emerging field, providing an overview of the optical properties of GO as well as methods for the on-chip integration of GO. The main achievements made in GO hybrid integrated photonic devices for diverse applications are summarized. The open challenges as well as the potential for future improvement are also discussed.

Nanoscale structural and electrical properties of graphene grown on AlGaN by catalyst-free chemical vapor deposition

The integration of graphene (Gr) with nitride semiconductors is highly interesting for applications in high-power/high-frequency electronics and optoelectronics. In this work, we demonstrated the direct growth of Gr on Al_0.5Ga_0.5N/sapphire templates by propane (C₃H₈) chemical vapor deposition at a temperature of 1350 °C. After optimization of the C₃H₈ flow rate, a uniform and conformal Gr coverage was achieved, which proved beneficial to prevent degradation of AlGaN morphology. X-ray photoemission spectroscopy revealed Ga loss and partial oxidation of Al in the near-surface AlGaN region. Such chemical modification of a ∼2 nm thick AlGaN surface region was confirmed by cross-sectional scanning transmission electron microscopy combined with electron energy loss spectroscopy, which also showed the presence of a bilayer of Gr with partial sp²/sp³ hybridization. Raman spectra indicated that the deposited Gr is nanocrystalline (with domain size ∼7 nm) and compressively strained. A Gr sheet resistance of ∼15.8 kΩ sq^-1 was evaluated by four-point-probe measurements, consistently with the nanocrystalline nature of these films. Furthermore, nanoscale resolution current mapping by conductive atomic force microscopy indicated local variations of the Gr carrier density at a mesoscopic scale, which can be ascribed to changes in the charge transfer from the substrate due to local oxidation of AlGaN or to the presence of Gr wrinkles.

Enhancing the Heat-Dissipation Efficiency in Ultrasonic Transducers via Embedding Vertically Oriented Graphene-Based Porcelain Radiators

Ultrasonic transducers with large output power have attracted extensive attentions due to their widespread applications in sonar, acoustic levitation, ultrasonic focusing, and so forth. However, the traditional transducer has almost no heat-dissipation capability itself, strictly relying on the assistant coolant system. Introducing high-performance heat-dissipation component is thus highly necessary. Herein, an embedded porcelain radiator component was designed by combining the excellent thermal conductivity of vertically oriented graphene (VG) with the outstanding heat-dissipation characteristics of thermosensitive ceramics, and a new-type transducer with an embedded VG/ceramic-hybrid radiator was constructed to show high heat-dissipation efficiency (up to ∼5 °C/min). Remarkably, prominent heat-dissipation effectiveness (temperature decline of ∼12 °C), enhanced amplitude and vibration uniformity were also achieved for the new-type transducer along with stabilized operating states. This research should pave ways for extending the applications of VG/ceramic hybrids to heat-dissipation scenarios and provide newfangled thoughts for the performance upgrade of multitudinous high-power devices.

Van der Waals Epitaxy of III-Nitride Semiconductors Based on 2D Materials for Flexible Applications

III-nitride semiconductors have attracted considerable attention in recent years owing to their excellent physical properties and wide applications in solid-state lighting, flat-panel displays, and solar energy and power electronics. Generally, GaN-based devices are heteroepitaxially grown on c-plane sapphire, Si (111), or 6H-SiC substrates. However, it is very difficult to release the GaN-based films from such single-crystalline substrates and transfer them onto other foreign substrates. Consequently, it is difficult to meet the ever-increasing demand for wearable and foldable applications. On the other hand, sp² -bonded two-dimensional (2D) materials, which exhibit hexagonal in-plane lattice arrangements and weakly bonded layers, can be transferred onto flexible substrates with ease. Hence, flexible III-nitride devices can be implemented through such 2D release layers. In this progress report, the recent advances in the different strategies for the growth of III-nitrides based on 2D materials are reviewed, with a focus on van der Waals epitaxy and transfer printing. Various attempts are presented and discussed herein, including the different kinds of 2D materials (graphene, hexagonal boron nitride, and transition metal dichalcogenides) used as release layers. Finally, current challenges and future perspectives regarding the development of flexible III-nitride devices are discussed.

High-Power GaN-Based Vertical Light-Emitting Diodes on 4-Inch Silicon Substrate

We demonstrate high-power GaN-based vertical light-emitting diodes (LEDs) (VLEDs) on a 4-inch silicon substrate and flip-chip LEDs on a sapphire substrate. The GaN-based VLEDs were transferred onto the silicon substrate by using the Au-In eutectic bonding technique in combination with the laser lift-off (LLO) process. The silicon substrate with high thermal conductivity can provide a satisfactory path for heat dissipation of VLEDs. The nitrogen polar n-GaN surface was textured by KOH solution, which not only improved light extract efficiency (LEE) but also broke down Fabry-Pérot interference in VLEDs. As a result, a near Lambertian emission pattern was obtained in a VLED. To improve current spreading, the ring-shaped n-electrode was uniformly distributed over the entire VLED. Our combined numerical and experimental results revealed that the VLED exhibited superior heat dissipation and current spreading performance over a flip-chip LED (FCLED). As a result, under 350 mA injection current, the forward voltage of the VLED was 0.36 V lower than that of the FCLED, while the light output power (LOP) of the VLED was 3.7% higher than that of the FCLED. The LOP of the FCLED saturated at 1280 mA, but the light output saturation did not appear in the VLED.

Enhancement of Heat Dissipation in Ultraviolet Light-Emitting Diodes by a Vertically Oriented Graphene Nanowall Buffer Layer

For III-nitride-based devices, such as high-brightness light-emitting diodes (LEDs), the poor heat dissipation of the sapphire substrate is deleterious to the energy efficiency and restricts many of their applications. Herein, the role of vertically oriented graphene (VG) nanowalls as a buffer layer for improving the heat dissipation in AlN films on sapphire substrates is studied. It is found that VG nanowalls can effectively enhance the heat dissipation between an AlN film and a sapphire substrate in the longitudinal direction because of their unique vertical structure and good thermal conductivity. Thus, an LED fabricated on a VG-sapphire substrate shows a 37% improved light output power under a high injection current (350 mA) with an effective 3.8% temperature reduction. Moreover, the introduction of VG nanowalls does not degrade the quality of the AlN film, but instead promotes AlN nucleation and significantly reduces the epilayer strain that is generated during the cooling process. These findings suggest that the VG nanowalls can be a good buffer layer candidate in III-nitride semiconductor devices, especially for improving the heat dissipation in high-brightness LEDs.

A graphene-based smart thermal conductive system regulated by a reversible pressure-induced mechanism

Thermal dissipation and thermal insulation are important for maintaining the normal operation of devices, extending the service life of instruments, ensuring efficient energy utilization, and improving temperature-related human comfort. Yet it is difficult to achieve both the functions of thermal dissipation and thermal insulation in a single material with a specific thermal conductivity under specific conditions. In this work, based on the huge difference in thermal conductivity between air and reduced graphene oxide (rGO), a pressure-induced mechanism is used to regulate the amount of air inside an rGO foam, so that a periodic reversible change of thermal conductivity can be realized, achieving the dual functions of thermal dissipation and thermal insulation to meet the requirements of different application scenarios. Further fitting calculations suggest that the thermal conductivity of rGO foam is positively and negatively associated with the applied pressure and temperature, respectively, and it can be calculated for given pressure and temperature conditions. The pressure-induced reversible regulation of thermal conductivity in rGO foam provides a new design construct for smart thermal-management devices, and a new direction of application for 2D materials.

Highly efficient GaN-based high-power flip-chip light-emitting diodes

High-power flip-chip light-emitting diodes (FCLEDs) suffer from low efficiencies because of poor p-type reflective ohmic contact and severe current crowding. Here, we show that it is possible to improve both the light extraction efficiency (LEE) and current spreading of an FCLED by incorporating a highly reflective metallic reflector made from silver (Ag). The reflector, which consists of an Ag film covered by three pairs of TiW/Pt multilayers, demonstrates high reflectance of 95.0% at 460 nm at arbitrary angles of incidence. Our numerical simulation and experimental results reveal that the FCLED with Ag-based reflector exhibits higher LEE and better current spreading than the FCLED with indium-tin oxide (ITO)/distributed Bragg reflector (DBR). As a result, the external quantum efficiency (EQE) of FCLED with Ag-based reflector was 6.0% higher than that of FCLED with ITO/DBR at 750 mA injection current. Our work also suggests that the EQE of FCLED with the Ag-based reflector could be further enhanced 5.2% by replacing the finger-like n-electrodes with three-dimensional (3D) vias n-electrodes, which spread the injection current uniformly over the entire light-emitting active region. This study paves the way towards higher-performance LED technology.

Improved Epitaxy of AlN Film for Deep-Ultraviolet Light-Emitting Diodes Enabled by Graphene

The growth of single-crystal III-nitride films with a low stress and dislocation density is crucial for the semiconductor industry. In particular, AlN-derived deep-ultraviolet light-emitting diodes (DUV-LEDs) have important applications in microelectronic technologies and environmental sciences but are still limited by large lattice and thermal mismatches between the epilayer and substrate. Here, the quasi-van der Waals epitaxial (QvdWE) growth of high-quality AlN films on graphene/sapphire substrates is reported and their application in high-performance DUV-LEDs is demonstrated. Guided by density functional theory calculations, it is found that pyrrolic nitrogen in graphene introduced by a plasma treatment greatly facilitates the AlN nucleation and enables fast growth of a mirror-smooth single-crystal film in a very short time of ≈0.5 h (≈50% decrease compared with the conventional process), thus leading to a largely reduced cost. Additionally, graphene effectively releases the biaxial stress (0.11 GPa) and reduces the dislocation density in the epilayer. The as-fabricated DUV-LED shows a low turn-on voltage, good reliability, and high output power. This study may provide a revolutionary technology for the epitaxial growth of AlN films and provide opportunities for scalable applications of graphene films.

Thermal Characterization of Low-Dimensional Materials by Resistance Thermometers

The design, fabrication, and use of a hotspot-producing and temperature-sensing resistance thermometer for evaluating the thermal properties of low-dimensional materials are described in this paper. The materials that are characterized include one-dimensional (1D) carbon nanotubes, and two-dimensional (2D) graphene and boron nitride films. The excellent thermal performance of these materials shows great potential for cooling electronic devices and systems such as in three-dimensional (3D) integrated chip-stacks, power amplifiers, and light-emitting diodes. The thermometers are designed to be serpentine-shaped platinum resistors serving both as hotspots and temperature sensors. By using these thermometers, the thermal performance of the abovementioned emerging low-dimensional materials was evaluated with high accuracy.

Tracking ultrafast hot-electron diffusion in space and time by ultrafast thermomodulation microscopy

The ultrafast response of metals to light is governed by intriguing nonequilibrium dynamics involving the interplay of excited electrons and phonons. The coupling between them leads to nonlinear diffusion behavior on ultrashort time scales. Here, we use scanning ultrafast thermomodulation microscopy to image the spatiotemporal hot-electron diffusion in thin gold films. By tracking local transient reflectivity with 20-nm spatial precision and 0.25-ps temporal resolution, we reveal two distinct diffusion regimes: an initial rapid diffusion during the first few picoseconds, followed by about 100-fold slower diffusion at longer times. We find a slower initial diffusion than previously predicted for purely electronic diffusion. We develop a comprehensive three-dimensional model based on a two-temperature model and evaluation of the thermo-optical response, taking into account the delaying effect of electron-phonon coupling. Our simulations describe well the observed diffusion dynamics and let us identify the two diffusion regimes as hot-electron and phonon-limited thermal diffusion, respectively.

Single step synthesis of highly conductive room-temperature stable cation-substituted mayenite electride target and thin film

Novel approaches to synthesize efficient inorganic electride [Ca24Al28O64]4+(e-)4 (thereafter, C12A7:e-) at ambient pressure under nitrogen atmosphere, are actively sought out to reduce the cost of massive formation of nanosized powder as well as compact large size target production. It led to a new era in low cost industrial applications of this abundant material as Transparent Conducting Oxides (TCOs) and as a catalyst. Therefore, the present study about C12A7:e- electride is directed towards challenges of cation doping in C12A7:e- to enhance the conductivity and form target to deposit thin film. Our investigation for cation doping on structural and electrical properties of Sn- and Si-doped C12A7:e- (Si-C12A7:e, and Sn-C12A7:e-) reduced graphene oxide (rGO) composite shows the maximum achieved conductivities of 5.79 S·cm-1 and 1.75 S·cm-1 respectively. On the other hand when both samples melted, then rGO free Sn-C12A7:e- and Si-C12A7:e- were obtained, with conductivities ~280 S.cm-1 and 300 S·cm-1, respectively. Iodometry based measured electron concentration of rGO free Sn-C12A7:e- and Si-C12A7:e-, 3 inch electride targets were ~2.22 × 1021 cm-3, with relative 97 ± 0.5% density, and ~2.23 × 1021 cm-3 with relative 99 ± 0.5% density, respectively. Theoretical conductivity was already reported excluding any associated experimental support. Hence the above results manifested feasibility of this sol-gel method for different elements doping to further boost up the electrical properties.

A self-assembled graphene nanomask for the epitaxial growth of nonplanar and planar GaN

Herein, we demonstrated the fabrication of architectural GaN nanostructures by the self-assembly NSAG (SNSAG) technology using multilayer graphene (MLG) as a nanomask.

Green-amber emission from high indium content InGaN quantum wells improved by interface modification of semipolar (112̄2) GaN templates

Green-amber-emitting high indium content InGaN quantum wells improved by interface modification of semipolar (112̄2) GaN templates.

Effect of Dielectric Distributed Bragg Reflector on Electrical and Optical Properties of GaN-Based Flip-Chip Light-Emitting Diodes

We demonstrated two types of GaN-based flip-chip light-emitting diodes (FCLEDs) with distributed Bragg reflector (DBR) and without DBR to investigate the effect of dielectric TiO₂/SiO₂ DBR on optical and electrical characteristics of FCLEDs. The reflector consisting of two single TiO₂/SiO₂ DBR stacks optimized for different central wavelengths demonstrates a broader reflectance bandwidth and a less dependence of reflectance on the incident angle of light. As a result, the light output power (LOP) of FCLED with DBR shows 25.3% higher than that of FCLED without DBR at 150 mA. However, due to the better heat dissipation of FCLED without DBR, it was found that the light output saturation current shifted from 268 A/cm² for FCLED with DBR to 296 A/cm² for FCLED without DBR. We found that the use of via-hole-based n-type contacts can spread injection current uniformly over the entire active emitting region. Our study paves the way for application of DBR and via-hole-based n-type contact in high-efficiency FCLEDs.

Direct van der Waals Epitaxy of Crack-Free AlN Thin Film on Epitaxial WS₂

Van der Waals epitaxy (vdWE) has drawn continuous attention, as it is unlimited by lattice-mismatch between epitaxial layers and substrates. Previous reports on the vdWE of III-nitride thin film were mainly based on two-dimensional (2D) materials by plasma pretreatment or pre-doping of other hexagonal materials. However, it is still a huge challenge for single-crystalline thin film on 2D materials without any other extra treatment or interlayer. Here, we grew high-quality single-crystalline AlN thin film on sapphire substrate with an intrinsic WS₂ overlayer (WS₂/sapphire) by metal-organic chemical vapor deposition, which had surface roughness and defect density similar to that grown on conventional sapphire substrates. Moreover, an AlGaN-based deep ultraviolet light emitting diode structure on WS₂/sapphire was demonstrated. The electroluminescence (EL) performance exhibited strong emissions with a single peak at 283 nm. The wavelength of the single peak only showed a faint peak-position shift with increasing current to 80 mA, which further indicated the high quality and low stress of the AlN thin film. This work provides a promising solution for further deep-ultraviolet (DUV) light emitting electrodes (LEDs) development on 2D materials, as well as other unconventional substrates.

Facile synthesis of tin-doped mayenite electride composite as a non-noble metal durable electrocatalyst for oxygen reduction reaction (ORR)

In this study, we synthesized nanosized Sn-doped C12A7:e- (C12Al7-xSnx:e-, where x = 0.20 to 1) composite with high surface area of 244 m2 g-1. An increasing trend in conductivity of Sn-doped C12A7:e- composites was observed at 300 K: 24 S cm-1, 68 S cm-1, 190 S cm-1 and 290 S cm-1, at doping levels of x = 0.20, 0.40, 0.80, and 1, respectively. Sn-doped C12A7:e-, with and without reduced graphene oxide (rGO), acts as a less expensive and highly active and durable electrocatalyst in the oxygen reduction reaction (ORR) for fuel cells. In the case of C12A7-xSnx:e- (where x = 1), calculated onset potential and current density were comparable to the commercially available 20% Pt/C electrode. Moreover, significant improvement was observed for Sn-doped C12A7:e- (doping level x = 1) with rGO composite. The ORR current density was about 5.9 mA cm-2, which was higher than that of Pt/C (5.2 mA cm-2). Our investigation of the effect of cation doping on structural and electrical properties of Sn-doped C12A7:e- composites shows that these results manifested the feasibility of this sol-gel method for different element doping. Furthermore, the as-prepared promising non-noble metal catalysts (NNMCs), viz., Sn-doped C12A7:e- composite materials, possess intrinsic long-time stability and excellent methanol resistance toward ORR in alkaline media and may serve as a promising alternative to Pt/C materials for ORR in its widespread implementation in fuel cells.

Monolithic Flexible Vertical GaN Light-Emitting Diodes for a Transparent Wireless Brain Optical Stimulator

Flexible inorganic-based micro light-emitting diodes (µLEDs) are emerging as a significant technology for flexible displays, which is an important area for bilateral visual communication in the upcoming Internet of Things era. Conventional flexible lateral µLEDs have been investigated by several researchers, but still have significant issues of power consumption, thermal stability, lifetime, and light-extraction efficiency on plastics. Here, high-performance flexible vertical GaN light-emitting diodes (LEDs) are demonstrated by silver nanowire networks and monolithic fabrication. Transparent, ultrathin GaN LED arrays adhere to a human fingernail and stably glow without any mechanical deformation. Experimental studies provide outstanding characteristics of the flexible vertical μLEDs (f-VLEDs) with high optical power (30 mW mm^-2 ), long lifetime (≈12 years), and good thermal/mechanical stability (100 000 bending/unbending cycles). The wireless light-emitting system on the human skin is successfully realized by transferring the electrical power f-VLED. Finally, the high-density GaN f-VLED arrays are inserted onto a living mouse cortex and operated without significant histological damage of brain.

High-Brightness Blue Light-Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer

Single-crystalline GaN-based light-emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid-state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high-quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high-brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal-organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and In_x Ga_1-x N/GaN multiple quantum well structures. The as-fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one-step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high-brightness LEDs.

Facile synthesis of a cationic-doped [Ca24Al28O64]4+(4e-) composite via a rapid citrate sol-gel method

One of the greatest challenges in the enhancement of the electrical properties of conductive mayenite [Ca₂₄Al₂₈O₆₄]⁴⁺(4e^-) (hereinafter C12A7:e^-) is the design of a more suitable/simple synthesis strategy that can be employed to obtain the required properties such as excellent stable electrical conductivity, a high electron concentration, outstanding mobility, and an exceptionally large surface area. Therefore, to synthesize C12A7:e^- in the metallic state, we proposed a facile, direct synthesis strategy based on an optimized sol-gel combustion method under a nitrogen gas environment using the low-cost precursors Ca(NO₃)₂·4H₂O and Al(NO₃)₃·9H₂O. Using this developed strategy, we successfully synthesized moderately conductive nanoscale C12A7:e^- powder, but with unexpected carbon components (reduced graphene oxide (rGO) and/or graphene oxide (GO)). The synthesized C12A7:e^- composite at room temperature has an electrical conductivity of about 21 S cm^-1, a high electron concentration of approximately 1.5 × 10²¹ cm^-3, and a maximum specific surface area of 265 m² g^-1. Probably, the synthesized rGO was coated on nanocage C12A7:e^- particles. In general, the C12A7:e^- electride is sensitive to the environment (especially to oxygen and moisture) and protected by an rGO coating on C12A7:e^- particles, which also enhances the mobility and keeps the conductivity of C12A7:e^- electride stable over a long period. Doped mayenite electride exhibits a conductivity that is strongly dependent on the substitution level. The conductivity of gallium-doped mayenite electride increases with the doping level and has a maximum value of 270 S cm^-1, which for the first time has been reported for the stable C12A7:e^- electride. In the case of Si-substituted calcium aluminate, the conductivity has a maximum value of 222 S cm^-1 at room temperature.

Ultrahigh lattice thermal conductivity in topological semimetal TaN caused by a large acoustic-optical gap

Topological semimetals may have potential applications such as in topological qubits, spintronics and quantum computations. Efficient heat dissipation is a key factor for the reliability and stability of topological semimetal-based nano-electronics devices, which is closely related to high thermal conductivity. In this work, the elastic properties and lattice thermal conductivity of TaN are investigated using first-principles calculations and the linearized phonon Boltzmann equation within the single-mode relaxation time approximation. According to the calculated bulk modulus, shear modulus and C ₄₄, TaN can be regarded as a potential incompressible and hard material. The room-temperature lattice thermal conductivity is predicted to be 838.62 [Formula: see text] along the a axis and 1080.40 [Formula: see text] along the c axis, showing very strong anisotropy. It is found that the lattice thermal conductivity of TaN is several tens of times higher than other topological semimetals, such as TaAs, MoP and ZrTe, which is due to the very longer phonon lifetimes for TaN than other topological semimetals. The very different atomic masses of Ta and N atoms lead to a very large acoustic-optical band gap, and then prohibit the scattering between acoustic and optical phonon modes, which gives rise to very long phonon lifetimes. Calculated results show that isotope scattering has little effect on lattice thermal conductivity, and that phonons with mean free paths larger than 20 (80) [Formula: see text] along the c direction at 300 K have little contribution to the total lattice thermal conductivity. This work implies that TaN-based nano-electronics devices may be more stable and reliable due to efficient heat dissipation, and motivates further experimental works to study lattice thermal conductivity of TaN.

High-performance flexible supercapacitors based on electrochemically tailored three-dimensional reduced graphene oxide networks

A simple approach for growing porous electrochemically reduced graphene oxide (pErGO) networks on copper wire, modified with galvanostatically deposited copper foam is demonstrated. The as-prepared pErGO networks on the copper wire are directly used to fabricate solid-state supercapacitor. The pErGO-based supercapacitor can deliver a specific capacitance (C_sp) as high as 81±3 F g⁻¹ at 0.5 A g⁻¹ with polyvinyl alcohol/H₃PO₄ gel electrolyte. The C_sp per unit length and area are calculated as 40.5 mF cm⁻¹ and 283.5 mF cm⁻², respectively. The shape of the voltammogram retained up to high scan rate of 100 V s⁻¹. The pErGO-based supercapacitor device exhibits noticeably high charge-discharge cycling stability, with 94.5% C_sp retained even after 5000 cycles at 5 A g⁻¹. Nominal change in the specific capacitance, as well as the shape of the voltammogram, is observed at different bending angles of the device even after 5000 cycles. The highest energy density of 11.25 W h kg⁻¹ and the highest power density of 5 kW kg⁻¹ are also achieved with this device. The wire-based supercapacitor is scalable and highly flexible, which can be assembled with/without a flexible substrate in different geometries and bending angles for illustrating promising use in smart textile and wearable device.

Numerical simulation and experimental investigation of GaN-based flip-chip LEDs and top-emitting LEDs

We demonstrate a GaN-based flip-chip LED (FC-LED) with a highly reflective indium-tin oxide (ITO)/distributed Bragg reflector (DBR) ohmic contact. A transparent ITO current spreading layer combined with Ta₂O₅/SiO₂ double DBR stacks is used as a reflective p-type ohmic contact in the FC-LED. We develop a strip-shaped SiO₂ current blocking layer, which is well aligned with a p-electrode, to prevent the current from crowding around the p-electrode. Our combined numerical simulation and experimental results revealed that the FC-LED with ITO/DBR has advantages of better current spreading and superior heat dissipation performance compared to top-emitting LEDs (TE-LEDs). As a result, the light output power (LOP) of the FC-LED with ITO/DBR was 7.6% higher than that of the TE-LED at 150 mA, and the light output saturation current was shifted from 130.9  A/cm² for the TE-LED to 273.8  A/cm² for the FC-LED with ITO/DBR. Owing to the high reflectance of the ITO/DBR ohmic contact, the LOP of the FC-LED with ITO/DBR was 13.0% higher than that of a conventional FC-LED with Ni/Ag at 150 mA. However, because of the better heat dissipation of the Ni/Ag ohmic contact, the conventional FC-LED with Ni/Ag exhibited higher light output saturation current compared to the FC-LED with ITO/DBR.

Boron nitride nanotubes as a heat sinking and stress-relaxation layer for high performance light-emitting diodes

High-density threading dislocations, the presence of biaxial compressive strain, and heat generation are the major limitations obstructing the performance and reliability of light emitting diodes (LEDs). Herein, we demonstrate a facile epitaxial lateral overgrowth (ELOG) method by incorporating boron nitride nanotubes (BNNTs) on a sapphire substrate by spray coating to resolve the above issues. Atomic force microscopy, X-ray diffraction, micro-Raman, and photoluminescence measurements confirmed the growth of a high quality GaN epilayer on the BNNT-coated sapphire substrate with reduced threading dislocations and compressive strain owing to the ELOG process. GaN LEDs fabricated using this approach showed a significant enhancement in the internal quantum efficiency and electroluminescence intensity compared to conventional LEDs grown on sapphire. Moreover, reduced efficiency droop and surface temperature at high injection currents were achieved due to the excellent thermal stability and conductivity of BNNTs. Based on our findings we infer that the BNNTs would be a promising material for high power devices vulnerable to self-heating problems.

Mixed-dimensional van der Waals heterostructures

The isolation of a growing number of two-dimensional (2D) materials has inspired worldwide efforts to integrate distinct 2D materials into van der Waals (vdW) heterostructures. Given that any passivated, dangling-bond-free surface will interact with another through vdW forces, the vdW heterostructure concept can be extended to include the integration of 2D materials with non-2D materials that adhere primarily through non-covalent interactions. We present a succinct and critical survey of emerging mixed-dimensional (2D + nD, where n is 0, 1 or 3) heterostructure devices. By comparing and contrasting with all-2D vdW heterostructures as well as with competing conventional technologies, we highlight the challenges and opportunities for mixed-dimensional vdW heterostructures.

Long-term stability improvement of light-emitting diode using highly transparent graphene oxide paste

A highly transparent paste adhesive is successfully fabricated by introducing graphene oxide (GO) to silicone paste adhesive by using a solvent-exchange method. The GO incorporated in the paste adhesive has a significant role in improving thermal conductivity, transparency and adhesive strength. The GO-embedded silicone paste is applied as a die-attach paste to light-emitting diodes (LEDs) in order to enhance the optical quality of the LEDs. The presence of GO in the die-attach layer of the LEDs gives rise to the enhancement of luminous intensity, effective heat dissipation, improvement of moisture barrier property as well as high adhesive strength. Consequently, the LEDs with the GO-embedded die-attach paste exhibit enhanced long-term stability. This novel approach provides a feasible and effective strategy for improving LED performance.

Reduced graphene oxide enwrapped phosphors for long-term thermally stable phosphor converted white light emitting diodes

The long-term instability of the presently available best commercial phosphor-converted light-emitting diodes (pcLEDs) is the most serious obstacle for the realization of low-cost and energy-saving lighting applications. Emission from pcLEDs starts to degrade after approximately 200 h of operation because of thermal degradation of the phosphors. We propose a new strategy to overcome this thermal degradation problem of phosphors by wrapping the phosphor particles with reduced graphene oxide (rGO). Through the rGO wrapping, we have succeeded in controlling the thermal degradation of phosphors and improving the stability of fabricated pcLEDs. We have fabricated pcLEDs with long-term stability that maintain nearly 98% of their initial luminescence emission intensity even after 800 h of continuous operation at 85 °C and 85% relative humidity. The pcLEDs fabricated using SrBaSi₂O₂N₂:Eu²⁺ phosphor particles wrapped with reduced graphene oxide are thermally stable because of enhanced heat dissipation that prevents the ionization of Eu²⁺ to Eu³⁺. We believe that this technique can be applied to other rare-earth doped phosphors for the realization of highly efficient and stable white LEDs.

Synthesis, Characterization, and Tribological Evaluation of TiO2-Reinforced Boron and Nitrogen co-Doped Reduced Graphene Oxide Based Hybrid Nanomaterials as Efficient Antiwear Lubricant Additives

The microwave-synthesized reduced graphene oxide (MRG), boron-doped reduced graphene oxide (B-MRG), nitrogen-doped reduced graphene oxide (N-MRG), boron-nitrogen-co-doped reduced graphene oxide (B-N-MRG), and TiO2-reinforced B-N-MRG (TiO2-B-N-MRG) nanomaterials have been synthesized and characterized by various state-of-the-art techniques, like Raman spectroscopy, powder X-ray diffraction, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. Furthermore, the tribological properties of prepared nanomaterials as antiwear additives in neutral paraffin oil have been evaluated using a four-ball machine at an optimized additive concentration (0.15% w/v). The tribological parameters, like mean wear scar diameter, coefficient of friction, and wear rates, revealed that these nanomaterials have potential to be developed as environmentally friendly sulfated-ash-, phosphorus-, and sulfur-free antiwear lubricant additives. The friction- and wear-reducing behavior of MRG increased upon successive doping of nitrogen, boron, and both nitrogen and boron. Among these additives, B-N-co-doped MRG shows superior tribological behavior in paraffin base oil. Besides this, the load-carrying properties of B-N-co-doped MRG have significantly improved after its reinforcement with TiO2 nanoparticles. A comparative study of the surface morphology of a lubricated track in the presence of various additives has been assessed by SEM and contact-mode atomic force microscopy. The X-ray photoelectron spectroscopy studies have proved that the excellent lubrication properties of TiO2-B-N-MRG are due to the in situ formation of a tribofilm composed of boron nitride, adsorbed graphene layers, and tribosintered TiO2 nanoparticles during the tribocontact. Being sulfur-, halogen-, and phosphorus-free, these graphene-based nanomaterials act as green antiwear additives, protecting interacting surfaces significantly from wear and tear.