Search

We demonstrated that a well-designed nanopatterned cover improves photovoltaic efficiency across a wide range of incident angles (θ). A nanopatterned cover was created using an integrated ray-wave optics simulation to maximize the light absorption of the surface-textured Si photovoltaic device. A hexagonally arranged nanocone array with a 300 nm pitch was formed into a polymer using nanoimprinting, and the nanostructured polymer was then attached to a glass cover with an index-matching adhesive. Angle-resolved current density-voltage measurements on Si photovoltaic devices showed that the nanopatterned glass cover yielded a 2–13% enhancement in power conversion efficiency at θ = 0–60°, which accounted for its broadband antireflective feature. We performed all-season-perspective simulations based on the results of the integrated ray-wave optics simulations and solar altitude database of South Korea, which validated the sustainability of the developed nanopatterned cover during significant seasonal fluctuations.

The development of advanced hierarchical anode materials has recently become essential to achieving high-performance sodium-ion batteries. Herein, we developed a facile and cost-effective scheme for synthesizing graphene-wrapped, nitrogen-rich carbon-coated iron sulfide nanofibers (FeS@NCG) as an anode for SIBs. The designed FeS@NCG can provide a significant reversible capacity of 748.5 mAh g

A thick AlN epilayer with an approximately 1.25-mm thickness was grown on a sapphire substrate by using a mixed source (Al+Ga of very small amount) at around 1150 °C for 2 hours and a mixed-source hydride vapor phase epitaxy (HVPE) method in a simplified reactor interlinked in series with no separation between the source and the growth zones. The simplified reactor was designed to minimize the reaction between quartz and AlCl vapor species of a high partial pressure at around 1150 °C. Thegrowthofthe thickAlN epilayerseemed to be due to theveryhighgrowthrate (maximum value of 600 µm/h) resulting from the minimization of the response distance between the vapor species and the source gases caused by interlinking in series the edge of the source zone with the substrate in the growth zone. The characteristics of the grown thick AlN epilayer were investigated by using cross-sectional scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), Raman spectrometry and X-ray diffraction (XRD).

Four AlN nanowire-like structures were simultaneously grown directly without a buffer layer on four substrates-sapphire, quartz, Si(111), and 6H-SiC-via a mixed-source hydride vapor phase epitaxy (HVPE) method using a mixed source (Al+Ga) containing a small quantity of Ga at 1150 °C for 2 h. Deposition was carried out using a simplified reactor designed in series without any separation between the source and the growth zones. AlN nanostructures with hexagonal crystal structures were grown successfully and directly on thin, pre-grown AlN nucleation areas on the quartz substrates. Furthermore, AlN nanostructures were grown on the sapphire substrate without a buffer layer and on pre-grown epilayers on the Si (111) and the 6H-SiC substrates, respectively. The characteristics of the AlN nanowire-like structures grown on the four substrates were investigated using energy-dispersive X-ray spectrometry and field-emission scanning electron microscopy.

AIN epilayers of different thicknesses were grown directly on sapphire substrates without a buffer layer by using a mixed (Al+Ga) source containing 95 at% Al and a mixed-source hydride vapor phase epitaxy (HVPE) method at a temperature of around 1120°C. The grown epilayers consisted of an AlN alloy in the upper region and an AlGaN alloy in the nucleation region just above the sapphire substrate. The upper part of the epilayer gradually transformed from AlGaN into AlN owing to a decrease in the Ga content of the AlGaN alloy grown on the sapphire substrate with increasing growth thickness. The role of Ga in the mixed (Al+Ga) source in the growth of the epilayer directly on the sapphire substrate and the dependence of the growth mechanism of the epilayer with varying Ga contents on the growth thickness were investigated. We found that Ga in the mixed (Al+Ga) source only acted as an activation material that generated gaseous precursors rather than directly contributing to the growth of the epilayers. The mixed-source HVPE method appears suitable for the growth of thick AIN epilayers.

Li

We synthesized polycrystalline ZnFe2O4 nanofibers via a facile electrospinning and annealing process. The ZnFe2O4 nanofibers exhibit continuous frameworks and uniform porosity through the abrupt combustion of the polymer and crystallization of inorganic elements. This well-defined structure and morphology facilitates Li+ transport, enhances the effective contact area with the electrolyte, and offers abundant active sites. For the ZnFe2O4 nanofiber anode, the reversible capacity reaches 753 mAh g−1 after 200 cycles at the high current density of 5 A g−1 and the anode shows excellent rate performance. The enhanced electrochemical performance can be attributed to the one-dimensional nanostructure and shortened diffusion pathways, which ensure full conversion reactions during lithiation-delithiation between Zn, Fe, and Li+, relieve volume expansion, and prevent pulverization/aggregation upon prolonged cycling at high current densities. Thus, we believe that ZnFe2O4 nanofibers present great potential as anode materials for Li-ion batteries.

We synthesized nanoarchitectures comprising γ-Fe2O3@C nanofibers with one-dimensional rice-panicle-like morphologies via a facile electrospinning and annealing process for use as anodes in lithium-ion batteries (LIBs). A thin carbon layer grown on the surface of γ-Fe2O3 provides a synergistic effect to relieve the stress and alleviate the volume expansion occurring during the lithium-ion insertion/extraction process. The unique structure not only offers good electron transport routes, but also enhances the lithium-ion conductive channels, resulting in excellent electrochemical activity and electrical conductivity of the LIB anode material. A high reversible specific capacity of 1252 mAh g−1 was achieved after 200 cycles even at the current density of 10 A g−1. When examined at the scan rate of 5 mV s−1, a high capacitive contribution ratio of 88.5% was achieved. The short lithium-ion diffusion pathways avoid structural damage to the active material and provide excellent rate capacities. This work suggests a new method to improve the electrochemical performance of LIBs through the synergistic design of uniquely structured metal oxides.

An electrode-evaporation technology on both the top and bottom sides of the bare vertical-type single chip separated from the traditional substrate by cooling, was developed for III-nitride vertical-type single chip LEDs with thick GaN epilayer. The post-process of the cooling step was followed by sorting the bare vertical-type single chip LEDs into the holes in a pocket-type shadow mask for deposition of the electrodes at the top and bottom sides of bare vertical-type single chip LEDs without the traditional substrate for electrode evaporation technology for vertical-type single chip LEDs. The variation in size of the hole between the designed shadow mask and the deposited electrodes owing to the use of the designed pocket-type shadow mask is investigated. Furthermore, the electrical and the optical properties of bare vertical-type single chip LEDs deposited with two different shapes of n-type electrodes using the pocket-type shadow mask are investigated to explore the possibility of the e-beam evaporation method.

Li+ intercalates into a pure face-centered-cubic (fcc) C60 structure instead of being adsorbed on a single C60 molecule. This hinders the excess storage of Li ions in Li-ion batteries, thereby limiting their applications. However, the associated electrochemical processes and mechanisms have not been investigated owing to the low electrochemical reactivity and poor crystallinity of the C60 powder. Herein, a facile method for synthesizing pure fcc C60 nanoparticles with uniform morphology and superior electrochemical performance in both half- and full-cells is demonstrated using a 1 m LiPF6 solution in ethylene carbonate/diethyl carbonate (1:1 vol%) with 10% fluoroethylene carbonate. The specific capacity of the C60 nanoparticles during the second discharge reaches ≈750 mAh g-1 at 0.1 A g-1 , approximately twice that of graphite. Moreover, by applying in situ X-ray diffraction, high-resolution transmission electron microscopy, and first-principles calculations, an abnormally high Li storage in a crystalline C60 structure is proposed based on the vacant sites among the C60 molecules, Li clusters at different sites, and structural changes during the discharge/charge process. The fcc of C60 transforms tetragonal via orthorhombic Lix C60 and back to the cubic phase during discharge. The presented results will facilitate the development of novel fullerene-based anode materials for Li-ion batteries.

We report on crystallographic analyses of one-dimensional GaN nanoneedles grown on a n-GaN epilayer by using hydride vapor phase epitaxy. The nanoneedles were grown with a HCl:NH3 gas flow ratio of 1:38 at 600 °C. The growth time of the GaN nanoneedles affected their morphologies. As time progressed, GaN dots nucleated and then evolved as nanoneedles. The vertical growth rate of GaN nanoneedles was higher than the lateral growth rate under optimized growth conditions. X-ray pole figure measurements were carried out using a four-axis diffractometer. For the sample grown for 20 min, we obtained discrete patterns with six strong dots and weak dough-nut and cotton swab patterns, indicating that most of the nanoneedles were grown ideally, but partially, in the x-y plane with an azimuthal rotation angle ϕ = 15 ~ 45° rotated to the substrate, and a few GaN nanoneedles were tilted by ±4° or by more than 32° from the vertical c-axis.

In this study, Silicon nanoparticles (Si NPs) were homogeneously self-incorporated into melamine–formaldehyde resin microspheres using a facile hydrothermal technique, followed by annealing the precursors to form hollow nitrogen-doped carbon microspheres (hNC MSs) containing approximately 10.8 wt.% Si NPs (Si@hNC MSs). Melamine was directly polymerized with formaldehyde, providing N and C sources for Li-ion battery anodes. Electrochemical tests demonstrated that the Si@hNC MS anode delivered a high reversible capacity of 872 mAh g–1 at a current density of 200 mA g–1 after 360 cycles. The hNC MSs largely limited the Si volumetric changes caused by repeated charging and discharging, resulting in the high electrochemical stability of the Si anode. This work demonstrates that melamine–formaldehyde resins can be used directly as C and N sources and can effectively mitigate the Si volumetric expansion. Moreover, we believe that this unique resin, an effective C source, has promising potential for improving the electrochemical performance of other metal or metal-oxide anodes.

A hybrid device that combines the properties of organic and inorganic semiconductors was fabricated and studied. It incorporated poly[2-methoxy-5-(2-ethylhexyloxy)- 1,4-phenylenevinylene] (MEH-PPV) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as organic polymers and GaN nanoneedles as an inorganic semiconductor. Layers of the two polymers were spin coated on to the GaN nanoneedles. The one peak in the electroluminescence spectrum originated from the MEH-PPV layer owing to the different potential barriers of electrons and holes at its interface with the GaN nanoneedles. However, the photoluminescence spectrum showed peaks due to both GaN nanoneedles and MEH-PPV. Such hybrid structures, suitably developed, might be able to improve the efficiency of optoelectronic devices.

Gallium nitride (GaN) nanoneedles were synthesized by using the hydride vapor phase epitaxy method at various growth temperatures and times under a HCl:NH3 flow ratio of 1:38. The morphology of GaN nanostructures varied according to the growth temperatures, and nanoneedles were formed at 640°C temperature zone. Further, their heights and diameters increased with increasing growth time whereas the stems of the nanoneedles coalesced. X-ray diffraction patterns showed that the GaN nanoneedles were orientated along the [0001] direction and underwent biaxial compressive stress. Also, the Raman spectra exhibited the smallest compressive strain in the sample grown for 60 min compared to the other samples.

We prepared InGaN light-emitting diodes (LEDs) with the active layers grown from a mixed source of Ga–In–N materials on an n-type GaN substrate by a selective-area growth method and three fabrication steps: photolithography, epitaxial layer growth, and metallization. The preparation followed a previously developed experimental process using apparatus for mixed-source hydride vapor-phase epitaxy (HVPE), which consisted of a multi-graphite boat, for insulating against the high temperature and to control the growth rate of epilayers, filled with the mixed source on the inside and a radio-frequency (RF) heating coil for heating to a high temperature (T > 900 °C) and for easy control of temperature outside the source zone. Two types of LEDs were prepared, with In compositions of 11.0 and 6.0% in the InGaN active layer, and room-temperature electroluminescence measurements exhibited a main peak corresponding to the In composition at either 420 or 390 nm. The consecutive growth of InGaN LEDs by the mixed-source HVPE method provides a technique for the production of LEDs with a wide range of In compositions in the active layer.

High Al-composition AlGaN and AlN epilayers were grown directly on Si(111) substrate by a hydride vapor phase epitaxy (HVPE) method with a melted mixed source in a graphite boat set in a source zone with high temperatures of T = 700 and 800 °C, respectively. The presence of the Ga material in the mixed source of Ga and Al promoted the growth of AlN and AlGaN epilayers in the growth zone. When the temperature in the source zone was 800 °C, the crystalline quality of the AlN and AlGaN epilayers increased as the ratio of Ga to Al increased, and the optimum mix ratio of Ga to Al for the growth of AlN epilayers was approximately 0.35–0.42, obtained from a numerical fitting analysis of the X-ray diffraction (XRD) data for these epilayers. It appears that they can be grown directly by our melted-mixed-source HVPE method in a high-temperature source zone.