Maximizing Photoelectrochemical Performance in Metal-Oxide Hybrid Composites via Amorphous Exsolution-A New Exsolution Mechanism for Heterogeneous Catalysis.

Exsolution generates metal nanoparticles anchored within crystalline oxide supports, ensuring efficient exposure, uniform dispersion, and strong nanoparticle-perovskite interactions. Increased doping level in the perovskite is essential for further enhancing performance in renewable energy applications; however, this is constrained by limited surface exsolution, structural instability, and sluggish charge transfer. Here, hybrid composites are fabricated by vacuum-annealing a solution containing SrTiO3 photoanode and Co cocatalyst precursors for photoelectrochemical water-splitting. In situ transmission electron microscopy identifies uniform, high-density Co particles exsolving from amorphous SrTiO3 films, followed by film-crystallization at elevated temperatures. This unique process extracts entire Co dopants with complete structural stability, even at Co doping levels exceeding 30%, and upon air exposure, the Co particles embedded in the film oxidize to CoO, forming a Schottky junction at the interface. These conditions maximize photoelectrochemical activity and stability, surpassing those achieved by Co post-deposition and Co exsolution from crystalline oxides. Theoretical calculations demonstrate in the amorphous state, dopant─O bonds become weaker while Ti─O bonds remain strong, promoting selective exsolution. As expected from the calculations, nearly all of the 30% Fe dopants exsolve from SrTiO3 in an H2 environment, despite the strong Fe─O bond's low exsolution tendency. These analyses unravel the mechanisms driving the amorphous exsolution.

Epitaxial growth of 1D GaN-based heterostructures on various substrates for photonic and energy applications.

GaN is an important III-V semiconductor for a variety of applications owing to its large direct band gap. GaN nanowires (NWs) have demonstrated significant potential as critical building blocks for nanoelectronics and nanophotonic devices, as well as integrated nanosystems. We present a comprehensive analysis of the vapor-liquid-solid (VLS) as a general synthesis technique for NWs on a variety of substrates, the morphological and structural characterization, and applications of GaN NWs in piezoelectric nanogenerators, light-emitting diodes, and solar-driven water splitting. We begin by summarizing the overall VLS growth process of GaN NWs, followed by the growth of NWs on several substrates. Subsequently, we review the various uses of GaN NWs in depth.

Technological Breakthroughs in Chip Fabrication, Transfer, and Color Conversion for High-Performance Micro-LED Displays.

The implementation of high-efficiency and high-resolution displays has been the focus of considerable research interest. Recently, micro light-emitting diodes (micro-LEDs), which are inorganic light-emitting diodes of size <100 µm2 , have emerged as a promising display technology owing to their superior features and advantages over other displays like liquid crystal displays and organic light-emitting diodes. Although many companies have introduced micro-LED displays since 2012, obstacles to mass production still exist. Three major challenges, i.e., low quantum efficiency, time-consuming transfer, and complex color conversion, have been overcome with technological breakthroughs to realize cost-effective micro-LED displays. In the review, methods for improving the degraded quantum efficiency of GaN-based micro-LEDs induced by the size effect are examined, including wet chemical treatment, passivation layer adoption, LED structure design, and growing LEDs in self-passivated structures. Novel transfer technologies, including pick-up transfer and self-assembly methods, for developing large-area micro-LED displays with high yield and reliability are discussed in depth. Quantum dots as color conversion materials for high color purity, and deposition methods such as electrohydrodynamic jet printing or contact printing on micro-LEDs are also addressed. This review presents current status and critical challenges of micro-LED technology and promising technical breakthroughs for commercialization of high-performance displays.

High Performance, Stable, and Flexible Piezoelectric Nanogenerator Based on GaN:Mg Nanowires Directly Grown on Tungsten Foil.

Rapid development of micro-electromechanical systems increases the need for flexible and durable piezoelectric nanogenerators (f-PNG) with high output power density. In this study, a high-performance, flexible, and highly stable f-PNG is prepared by directly growing the Mg-doped semi-insulating GaN nanowires (NWs) on a 30-µm-thick tungsten foil using vapor-liquid-solid growth mechanism. The direct growth of NWs on metal foil extends the overall lifetime of the f-PNG. The semi-insulating GaN NWs significantly enhance the piezoelectric performance of the f-PNG by reducing free electron density. Additionally, the direct integration of NWs on the tungsten foil improves the conductivity, resulting in current enhancement (2.5 mA) with an output power density of 13 mW cm-2 . The piezoelectric performance of the f-PNG is investigated under several bending angles, actuation frequencies, continuous vibrations, and airflow velocities. The maximum output voltage exhibited by the f-PNG is 20 V at a bending angle of 155°. The f-PNG is connected to the backside of an index finger to monitor finger bending behavior by changing the current density. Depending on its flexibility and sensitivity, the f-PNG can be used as a health-monitoring sensor to be mounted on joints (fingers, hands, elbows, and knees) to monitor their repeated bending and relaxation.

Antinociceptive effect of intrathecal P7C3 via GABA in a rat model of inflammatory pain.

The recently identified molecule P7C3 has been highlighted in the field of pain research. We examined the effect of intrathecal P7C3 in tissue injury pain evoked by formalin injection and determined the role of the GABA system in the activity of P7C3 at the spinal level. Male Sprague-Dawley rats with intrathecal catheters implanted for experimental drug delivery were studied. The effects of intrathecal P7C3 and nicotinamide phosphoribosyltransferase (NAMPT) administered 10 min before the formalin injection were examined. Animals were pretreated with bicuculline, a GABA-A receptor antagonist; saclofen, a GABA-B receptor antagonist; L-allylglycine, a glutamic acid decarboxylase (GAD) blocker; and CHS 828, an NAMPT inhibitor; to observe involvement in the effects of P7C3. The effects of P7C3 alone and the mixture of P7C3 with GABA receptor antagonists on KCl-induced calcium transients were examined in rat dorsal root ganglion (DRG) neurons. The expression of GAD and the concentration of GABA in the spinal cord were evaluated. Intrathecal P7C3 and NAMPT produced an antinociceptive effect in the formalin test. Intrathecal bicuculline, saclofen, L-allylglycine, and CHS 828 reversed the antinociception of P7C3 in both phases. P7C3 decreased the KCl-induced calcium transients in DRG neurons. Both bicuculline and saclofen reversed the blocking effect of P7C3. The levels of GAD expression and GABA concentration decreased after formalin injection and were increased by P7C3. These results suggest that P7C3 increases GAD activity and then increases the GABA concentration in the spinal cord, which in turn may act on GABA receptors causing the antinociceptive effect against pain evoked by formalin injection.

GaN Nanowire Growth Promoted by In-Ga-Au Alloy Catalyst with Emphasis on Agglomeration Temperature and In Composition.

The crystallographic orientation control of GaN nanowires (NWs) has been widely investigated by varying the V-III ratio. Here, we report the tuning of crystallographic orientation of GaN NWs by varying the composition of indium (In) in gallium-gold (Ga-Au) alloy catalyst using metal-organic chemical vapor deposition (MOCVD). The c-plane GaN thin film and sapphire substrate are used as growth templates. We found that the substrates of same orientation have a negligible influence on the orientation of the GaN NWs. The catalyst composition and the dimensions of alloy droplets determine the morphology of the NWs. The density of the NWs was controlled by tuning the droplet size of the alloy catalysts. With the constant V/III ratio, the crystallographic orientation of the GaN NWs was tuned from m- to c-axis by increasing the In composition inside alloy catalyst.

Nanostructured CuO with a thin g-C3N4 layer as a highly efficient photocathode for solar water splitting.

A g-C3N4/CuO nanostructure featuring improved photoelectrochemical properties was successfully prepared using a facile and cost-effective method involving electrodeposition and thermal oxidation. The improved photoelectrochemical properties were mainly ascribed to the increased surface area and improved charge transportation of the g-C3N4/CuO photocathode. This photocathode can be used in novel strategies for resolving problems associated with low-efficiency CuO photocathodes.

Epitaxial Growth of GaN Core and InGaN/GaN Multiple Quantum Well Core/Shell Nanowires on a Thermally Conductive Beryllium Oxide Substrate.

Beryllium oxide (BeO) belongs to a very unique material family that exhibits the divergent properties of high thermal conductivity and high electrical resistivity. BeO has the same crystal structure as GaN, and the absolute difference in the lattice constants is less than 17%. Here, the growth of GaN nanowires (NWs) on the polycrystalline BeO substrate is reported for the first time. The NWs are grown by a vapor-liquid-solid approach using a showerhead-based metal-organic chemical vapor deposition. The growth direction of NWs is along the m-axis on all planes of the substrate, and it is confirmed by transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns. The vertical and tilted growth of NWs is due to the different planes of the substrate such as the m-plane, a-plane, and semipolar planes and is confirmed by X-ray diffraction. Subsequently, the GaN shell and InGaN/GaN multiple quantum wells (MQWs) are coaxially grown using a vapor-solid approach in the same reactor. A very high crystal quality is verified by TEM and SAED and is also confirmed by measuring the photoluminescence. The optical emission is tuned for the entire visible spectrum by increasing the indium incorporation in InGaN quantum wells. The conformal growth of InGaN/GaN MQW shells and the defect-free nature of the structure are confirmed from spatially resolved cathodoluminescence. This study will provide a platform for researchers to grow GaN NWs on the BeO substrate for a range of optical and electrical applications.

RGB Arrays for Micro-Light-Emitting Diode Applications Using Nanoporous GaN Embedded with Quantum Dots.

The multiple light scattering of nanoporous (NP) GaN was systematically studied and applied to the color down-conversion for micro-light-emitting diode (LED) display applications. The transport mean free path (TMFP) in NP GaN is 660 nm at 450 nm (light wavelength), and it decreases with a decreasing wavelength. It was observed that the short TMFP of the NP GaN increased the light extinction coefficient at 370 nm by 11 times. Colloidal QDs were loaded into a half 4″ wafer scale NP GaN, and 96 and 100% of light conversion efficiencies for green and red were achieved, respectively. By loading green and red QDs selectively into NP GaN mesas, we demonstrated the RGB microarrays based on the blue-violet pumping light with green and red color converting regions.

Three-dimensional hierarchical semi-polar GaN/InGaN MQW coaxial nanowires on a patterned Si nanowire template.

We have demonstrated for the first time the hybrid development of next-generation 3-D hierarchical GaN/InGaN multiple-quantum-well nanowires on a patterned Si nanowire-template. The patterned Si nanowire-template is fabricated using metal-assisted chemical-etching, and the conformal growth of the GaN/InGaN multiple-quantum-well (MQW) coaxial nanowires is conducted using metal-organic-chemical-vapor-deposition by the two-step growth approach of vapor-liquid-solid for the GaN core and vapor-solid for the GaN/InGaN MQW shells. The growth directions of the GaN nanowires are confirmed by transmission electron microscopy and selected area electron diffraction patterns. The emission of the GaN/InGaN MQW nanowire is tuned from 440 nm to 505 nm by increasing the InGaN quantum-well thickness. The carrier dynamics were evaluated by performing temperature-dependent time-resolved photoluminescence measurement, and the radiative lifetime of photogenerated electron-hole pairs was found to range from 30 to 35 ps. A very high IQE of 56% was measured due to the suppressed quantum-confined Stark effect which was enabled by the semi-polar growth facet of the GaN/InGaN MQWs. The demonstration of the growth of the hybrid 3-D hierarchical GaN/InGaN MQW nanowires provides a seamless platform for a broad range of multifunctional optical and electronic applications.

Enhanced stability of piezoelectric nanogenerator based on GaN/V2O5 core-shell nanowires with capacitive contact.

Enhanced stability of a piezoelectric nanogenerator (PNG) was demonstrated using c- and m-axis GaN/V2O5 core-shell nanowires (NWs) by analyzing the capacitive coupling of the PNG's output. The NW array grown on GaN thin film was embedded in polydimethylsiloxane (PDMS) matrix, following which the matrix was transferred to an indium (In)-coated PET substrate for achieving superior flexibility of the PNG. The stability of the PNG was enhanced by holding the NW PDMS composite with a PDMS polymer as a bonding material on the PET substrate. The inserted PDMS layer improved the lifetime of the PNG, however, because of the insulating nature of PDMS, the piezoelectric output of GaN NWs was coupled capacitively to In contact on PET substrate and it resulted in a slight degradation of piezoelectric output due to the voltage drop across the bottom capacitive contact. The maximum piezoelectric current was 64 nA and output voltage was 11.9 V from the PNG with c-axis NWs. While the PNG with direct bottom contact exhibited 57% output reduction after 72 000 operation cycles, the PNG with capacitive contact did not show any degradation in stability even after 150 000 cycles.

Transferred monolayer MoS2 onto GaN for heterostructure photoanode: Toward stable and efficient photoelectrochemical water splitting.

Solar-driven photoelectrochemical water splitting (PEC-WS) using semiconductor photoelectrodes is considered a promising solution for sustainable, renewable, clean, safe and alternative energy sources such as hydrogen. Here, we report the synthesis and characterization of a novel heterostructure MoS2/GaN to be used as a photoanode for PEC-WS. The heterostructure was synthesized by metal-organic chemical vapor deposition of single crystalline GaN onto a c-plane sapphire substrate, followed by the deposition of a visible light responding MoS2 monolayer (Eg = 1.9 eV) formed by a Mo-sulfurization technique. Our experimental results reveal that MoS2/GaN photoanode achieved efficient light harvesting with photocurrent density of 5.2 mA cm-2 at 0 V vs Ag/AgCl, which is 2.6 times higher than pristine GaN. Interestingly, MoS2/GaN exhibited a significantly enhanced applied-bias-photon-to-current conversion efficiency of 0.91%, whereas reference GaN yielded an efficiency of 0.32%. The superior PEC performance of the MoS2/GaN photoelectrode is mainly related to the enhanced light absorption due to excellent photocatalytic behavior of MoS2, which reduces charge transfer resistance between the semiconductor and electrolyte interface, and the improvement of charge separation and transport. This result gives a new perspective on the importance of MoS2 as a cocatalyst coated onto GaN to synthesize photoelectrodes for efficient solar energy conversion devices.

Ultrafast carrier dynamics of conformally grown semi-polar (112[combining macron]2) GaN/InGaN multiple quantum well co-axial nanowires on m-axial GaN core nanowires.

The growth of semi-polar (112[combining macron]2) GaN/InGaN multiple-quantum-well (MQW) co-axial heterostructure shells around m-axial GaN core nanowires on a Si substrate using MOCVD is reported for the first time. The core GaN nanowire and GaN/InGaN MQW shells are grown in a two-step growth sequence of vapor-liquid-solid and vapor-solid growth modes. The luminescence and carrier dynamics of GaN/InGaN MQW coaxial nanowires are studied by photoluminescence, cathodoluminescence, and low temperature time-resolved photoluminescence (TRPL). The emission is tuned from 430 nm to 590 nm by increasing the InGaN QW thickness. The non-single exponential decay measured by low-temperature TRPL was attributed to the indium fluctuations in the InGaN QW. The ultrafast radiative lifetime was measured from 14 ps to 26 ps with different emission wavelengths at a very high internal quantum efficiency up to 68%. An ultrafast carrier lifetime was assigned to the growth of the InGaN QW on semi-polar (112[combining macron]2) growth facet and the improved carrier collection efficiency due to the radial growth of the GaN/InGaN MQW shells. Such an ultrafast carrier dynamics of NWs provides a meaningful active medium for high speed optoelectronic applications.

Solar hydrogen generation: feature introduction.

The expected depletion of fossil fuel reserves and its severe environmental impact have brought about the need for sustainable and clean energy resources. Solar hydrogen generation via photoelectrochemical (PEC) water splitting techniques, which combine sunlight, water, and semiconductor materials, are promising alternatives to conventional fossil fuels. Solar-hydrogen fuel produced using PEC methods are renewable, sustainable and environmentally friendly.

Type-II ZnO/ZnS core-shell nanowires: Earth-abundant photoanode for solar-driven photoelectrochemical water splitting.

A core-shell structure, formed in a nanostructured photoanode, is an effective strategy to achieve high solar-to-hydrogen conversion efficiency. In this study, we present a facile and simple synthesis of a unique vertically aligned ZnO/ZnS core-shell heterostructure nanowires (NWs) on a Si substrate. Well-aligned ZnO NWs were grown on Si (100) substrates on a low-temperature ZnO buffer layer by metal-organic chemical vapor deposition. The ZnO NWs were then coated with various thicknesses of ZnS shell layers using atomic layer deposition. The structural characterizations exhibit the well-developed ZnO/ZnS core-shell NWs heterostructure. The as-prepared ZnO/ZnS core-shell NWs was applied as photoanode for photoelectrochemical (PEC) water splitting. This unique ZnO/ZnS core-shell NWs photoanode shows photocurrent density of 1.21 mA cm-2, which is 8.5 times higher than bare ZnO NWs. The PEC performance and the applied-bias-photon-to-current conversion efficiency of ZnO/ZnS core-shell NWs photoanode are further improved with the optimized ZnS shell. The type-II band alignment of the heterostructure photoanode is the key factor for their excellent PEC performance. Importantly, this type of core-shell NWs heterostructure provides useful insights into novel electrode design and fabrication based on earth abundant materials for low-cost solar fuel generation.

Enhanced photoelectrochemical stability of GaN photoelectrodes by Al2O3 surface passivation layer.

Photoelectrochemical (PEC) water splitting is one of the most promising hydrogen production methods because of its high efficiency, renewable resources and harmless by-products. Gallium nitride (GaN) is suitable for PEC water splitting because it has excellent stability in electrolyte and band gap energy which straddles the redox potential of water (Vredox = 1.23 V). These characteristics allow this material to split water stably without external bias. However, the stability of GaN is still not sufficient for practical applications. In this study, we investigated the properties of GaN photoelectrodes with aluminum oxide (Al2O3) thin film as a protection layer for increasing stability. In a long-term stability test, Al2O3-coated GaN showed more stable photocurrent than that of bare GaN. The total hydrogen production amount was also improved in Al2O3-coated samples than bare GaN. These results indicate that the Al2O3 protection layer significantly enhances stability and hydrogen production.

Stable and High Piezoelectric Output of GaN Nanowire-Based Lead-Free Piezoelectric Nanogenerator by Suppression of Internal Screening.

A piezoelectric nanogenerator (PNG) that is based on c-axis GaN nanowires is fabricated on flexible substrate. In this regard, c-axis GaN nanowires were grown on GaN substrate using the vapor-liquid-solid (VLS) technique by metal organic chemical vapor deposition. Further, Polydimethylsiloxane (PDMS) was coated on nanowire-arrays then PDMS matrix embedded with GaN nanowire-arrays was transferred on Si-rubber substrate. The piezoelectric performance of nanowire-based flexible PNG was measured, while the device was actuated using a cyclic stretching-releasing agitation mechanism that was driven by a linear motor. The piezoelectric output was measured as a function of actuation frequency ranging from 1 Hz to 10 Hz and a linear tendency was observed for piezoelectric output current, while the output voltages remained constant. A maximum of piezoelectric open circuit voltages and short circuit current were measured 15.4 V and 85.6 nA, respectively. In order to evaluate the feasibility of our flexible PNG for real application, a long term stability test was performed for 20,000 cycles and the device performance was degraded by less than 18%. The underlying reason for the high piezoelectric output was attributed to the reduced free carriers inside nanowires due to surface Fermi-level pinning and insulating metal-dielectric-semiconductor interface, respectively; the former reduced the free carrier screening radially while latter reduced longitudinally. The flexibility and the high aspect ratio of GaN nanowire were the responsible factors for higher stability. Such higher piezoelectric output and the novel design make our device more promising for the diverse range of real applications.

Solvent Additive-Assisted Anisotropic Assembly and Enhanced Charge Transport of π-Conjugated Polymer Thin Films.

Charge transport in π-conjugated polymer films involves π-π interactions within or between polymer chains. Here, we demonstrate a facile solution processing strategy that provides enhanced intra- and interchain π-π interactions of the resultant polymer films using a good solvent additive with low volatility. These increased interactions result in enhanced charge transport properties. The effect of the good solvent additive on the intra- and intermolecular interactions, morphologies, and charge transport properties of poly(3-hexylthiophene) (P3HT) films is systematically investigated. We found that the good solvent additive facilitates the self-assembly of P3HT chains into crystalline fibrillar nanostructures by extending the solvent drying time during thin-film formation. As compared to the prior approach using a nonsolvent additive with low volatility, the solvent blend system containing a good solvent additive results in enhanced charge transport in P3HT organic field-effect transistor (OFET) devices [from ca. 1.7 × 10-2 to ca. 8.2 × 10-2 cm2 V-1 s-1 for dichlorobenzene (DCB) versus 4.4 × 10-2 cm2 V-1 s-1 for acetonitrile]. The mobility appears to be maximized over a broad spectrum of additive concentrations (1-7 vol %), indicative of a wide processing window. Detailed analysis results regarding the charge injection and transport characteristics of the OFET devices reveal that a high-boiling-point solvent additive decreases both the contact resistance ( Rc) and channel resistance ( Rch), contributing to the mobility enhancement of the devices. Finally, the platform presented here is proven to be applicable to alternative good solvent additives with low volatility, such as chlorobenzene (CB) and trichlorobenzene (TCB). Specifically, the mobility enhancement of the resultant P3HT films increases in the order CB (bp 131 °C) < DCB (bp 180 °C) < TCB (bp 214 °C), suggesting that solvent additives with higher boiling points provide resultant films with preferable molecular ordering and morphologies for efficient charge transport.

One-Pot in Situ Hydrothermal Growth of BiVO4/Ag/rGO Hybrid Architectures for Solar Water Splitting and Environmental Remediation.

BiVO4 is ubiquitously known for its potential use as photoanode for PEC-WS due to its well-suited band structure; nevertheless, it suffers from the major drawback of a slow electron hole separation and transportation. We have demonstrated the one-pot synthesis of BiVO4/Ag/rGO hybrid photoanodes on a fluorine-doped tin oxide (FTO)-coated glass substrate using a facile and cost-effective hydrothermal method. The structural, morphological, and optical properties were extensively examined, confirming the formation of hybrid heterostructures. Ternary BiVO4/Ag/rGO hybrid photoanode electrode showed enhanced PEC performance with photocurrent densities (J ph ) of ~2.25 and 5 mA/cm2 for the water and sulfate oxidation, respectively. In addition, the BiVO4/Ag/rGO hybrid photoanode can convert up to 3.5% of the illuminating light into photocurrent, and exhibits a 0.9% solar-to-hydrogen conversion efficiency. Similarly, the photocatalytic methylene blue (MB) degradation afforded the highest degradation rate constant value (k = 1.03 × 10-2 min-1) for the BiVO4/Ag/rGO hybrid sample. It is noteworthy that the PEC/photocatalytic performance of BiVO4/Ag/rGO hybrid architectures is markedly more significant than that of the pristine BiVO4 sample. The enhanced PEC/photocatalytic performance of the synthesized BiVO4/Ag/rGO hybrid sample can be attributed to the combined effects of strong visible light absorption, improved charge separation-transportation and excellent surface properties.

Transparent, Flexible Piezoelectric Nanogenerator Based on GaN Membrane Using Electrochemical Lift-Off.

A transparent and flexible piezoelectric nanogenerator (TF PNG) is demonstrated based on a GaN membrane fabricated by electrochemical lift-off. Under shear stress on the TF PNG by finger force (∼182 mN), the GaN membrane effectively undergoes normal stress and generates piezoelectric polarization along the c-axis, resulting in the generation of piezoelectric output from the TF PNG. Although the GaN layer is 315 times thinner than the flexible polyethylene terephthalate (PET) substrate, the low Young's modulus of PET allows the GaN membranes to absorb ∼41% of the applied strain energy, which leads to their large lattice deformation under extremely low applied stress. Maximum output voltage and current values of 4.2 V and 150 nA are obtained, and the time decay of the output voltage is discussed.