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1.
Angew Chem Int Ed Engl ; 59(2): 935-942, 2020 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-31670455

RESUMO

Photocatalytic overall water splitting has been recognized as a promising approach to convert solar energy into hydrogen. However, most of the photocatalysts suffer from low efficiencies mainly because of poor charge separation. Herein, taking a model semiconductor gallium nitride (GaN) as an example, we uncovered that photogenerated electrons and holes can be spatially separated to the nonpolar and polar surfaces of GaN nanorod arrays, which is presumably ascribed to the different surface band bending induced by the surface polarity. The photogenerated charge separation efficiency of GaN can be enhanced significantly from about 8 % to more than 80 % via co-exposing polar and nonpolar surfaces. Furthermore, spatially assembling reduction and oxidation cocatalysts on the nonpolar and polar surfaces remarkably boosts photocatalytic overall water splitting, with the quantum efficiency increased from 0.9 % for the film photocatalyst to 6.9 % for the nanorod arrays photocatalyst.

2.
Nanotechnology ; 31(14): 145713, 2020 Apr 03.
Artigo em Inglês | MEDLINE | ID: mdl-31860878

RESUMO

Seed-catalysed growth has been proved to be an ideal method to selectively tune the crystal structure of III-V nanowires along its growth axis. However, few results on relevant nitride NWs have been reported. In this study, we demonstrate the growth of epitaxial kinked wurtzite (WZ)/zinc-blende (ZB) heterostructure GaN NW arrays under the oxygen rich condition using hydride vapour-liquid-solid vapour phase epitaxy (VLS-HVPE). The typical GaN crystal includes WZ and ZB phases throughout the whole NW structure. A detailed structural analysis indicates that a stacking faults free zone was occasionally observed near the NW tips and in the relatively long kinked 〈11-23〉 directions segments (>200 nm). Furthermore, some NWs (<5%) develop phase boundaries, resulting in kinking and crystal phase evolution. A layer-by-layer growth mode was proposed to explain the crystal phase evolution along the phase boundaries. This study provides new insights into the controlled growth of wurtzite (WZ)/zinc-blende (ZB) heterostructure GaN NW.

3.
Opt Lett ; 44(17): 4155-4158, 2019 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-31465351

RESUMO

With the penetration of semiconductor lighting, GaN-based white-light-emitting diodes (WLEDs) with a high color-rendering index (CRI) and simultaneous high luminous efficiency (LE) are required, especially for high-quality indoor lighting. Here, by adopting metal nanoparticles (Ag and Au NPs) into hybridized color conversion material composed of broadband LuAG:Ce phosphor and narrowband CdSe/ZnS red quantum dots, we have fabricated AgAu-WLEDs with simultaneously increased LE (12% increment at 40 A/cm2) and CRI (maximum of 94.5), and decreased correlated color temperature (CCT, from CCT=6000 K to = 4800 K), compared with WLEDs without metal NPs. This improved performance of WLEDs is ascribed to increased color conversion efficiency brought from localized surface plasmon resonance and thus a strong resonant light scattering effect from the incorporated metal NPs. We believe the approach reported in this work will find its application in GaN WLEDs, thus advancing the development of high-efficiency and quality semiconductor lighting.

4.
Nanomaterials (Basel) ; 9(6)2019 Jun 06.
Artigo em Inglês | MEDLINE | ID: mdl-31174358

RESUMO

A 365-nm UV LED was fabricated based on embedded nanoporous AlGaN distributed Bragg reflectors (DBR) by electrochemical etching. The porous DBR had a reflectance of 93.5% at the central wavelength of 365 nm; this is the highest value of porous AlGaN DBRs below 370 nm which has been reported so far. An innovative two-step etching method with a SiO2 sidewall protection layer (SPL) was proposed to protect the n-AlGaN layer and active region of UV LED from being etched by the electrolyte. The DBR-LED with SPL showed 54.3% improvement of maximal external quantum efficiency (EQE) and 65.7% enhancement of optical power at 100 mA without any degeneration in electrical properties, compared with the un-etched standard LED sample. This work has paved the way for the application of electrically-pumped UV LEDs and VCSELs based on nanoporous AlGaN DBRs.

5.
Micromachines (Basel) ; 10(5)2019 May 13.
Artigo em Inglês | MEDLINE | ID: mdl-31086010

RESUMO

ß-Ga2O3, characterized with high n-type conductivity, little lattice mismatch with III-Nitrides, high transparency (>80%) in blue, and UVA (400-320 nm) as well as UVB (320-280 nm) regions, has great potential as the substrate for vertical structure blue and especially ultra violet LEDs (light emitting diodes). Large efforts have been made to improve the quality of III-Nitrides epilayers on ß-Ga2O3. Furthermore, the fabrication of vertical blue LEDs has been preliminarily realized with the best result that output power reaches to 4.82 W (under a current of 10 A) and internal quantum efficiency (IQE) exceeds 78% by different groups, respectively, while there is nearly no demonstration of UV-LEDs on ß-Ga2O3. In this review, with the perspective from materials to devices, we first describe the basic properties, growth method, as well as doping of ß-Ga2O3, then introduce in detail the progress in growth of GaN on (1 0 0) and (-2 0 1) ß-Ga2O3, followed by the epitaxy of AlGaN on gallium oxide. Finally, the advances in fabrication and performance of vertical structure LED (VLED) are presented.

6.
Adv Mater ; 31(29): e1901624, 2019 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-31140651

RESUMO

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.

7.
Adv Mater ; 31(23): e1807345, 2019 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-30993771

RESUMO

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.

8.
Nanotechnology ; 30(4): 045604, 2019 Jan 25.
Artigo em Inglês | MEDLINE | ID: mdl-30485254

RESUMO

Here, we demonstrate the growth of horizontal GaN nanowires (NWs) on silicon (111) by a surface-directed vapor-liquid-solid growth. The influence of the Au/Ni catalysts migration and coalescence on the growth of the NWs has been systematically studied. 2D root-like branched NWs were gown spontaneously through catalyst migration. Furthermore, a novel phenomenon that a catalyst particle is embedded in a horizontal NW was observed and attributed the destruction of growth steady state due to the catalysts coalescence. The transmission electron microscopy and photoluminescence, cathodoluminescence measurement demonstrated that the horizontal NWs exhibit single crystalline structures and good optical properties. Our work sheds light on the horizontal NWs growth and should facilitate the development of highly integrated III-V nanodevices on silicon.

9.
Materials (Basel) ; 11(12)2018 Dec 04.
Artigo em Inglês | MEDLINE | ID: mdl-30518146

RESUMO

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.

10.
Materials (Basel) ; 11(12)2018 Nov 26.
Artigo em Inglês | MEDLINE | ID: mdl-30486245

RESUMO

High density of defects and stress owing to the lattice and thermal mismatch between nitride materials and heterogeneous substrates have always been important problems and limit the development of nitride materials. In this paper, AlGaN light-emitting diodes (LEDs) were grown directly on a single-layer graphene-covered Si (111) substrate by metal organic chemical vapor deposition (MOCVD) without a metal catalyst. The nanorods was nucleated by AlGaN nucleation islands with a 35% Al composition, and included n-AlGaN, 6 period of AlGaN multiple quantum wells (MQWs), and p-AlGaN. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) showed that the nanorods were vertically aligned and had an accordant orientation along the [0001] direction. The structure of AlGaN nanorod LEDs was investigated by scanning transmission electron microscopy (STEM). Raman measurements of graphene before and after MOCVD growth revealed the graphene could withstand the high temperature and ammonia atmosphere in MOCVD. Photoluminescence (PL) and cathodoluminescence (CL) characterized an emission at ~325 nm and demonstrated the low defects density in AlGaN nanorod LEDs.

11.
J Am Chem Soc ; 140(38): 11935-11941, 2018 09 26.
Artigo em Inglês | MEDLINE | ID: mdl-30175921

RESUMO

We study the roles of graphene acting as a buffer layer for growth of an AlN film on a sapphire substrate. Graphene can reduce the density of AlN nuclei but increase the growth rate for an individual nucleus at the initial growth stage. This can lead to the reduction of threading dislocations evolved at the coalescence boundaries. The graphene interlayer also weakens the interaction between AlN and sapphire and accommodates their large mismatch in the lattice and thermal expansion coefficients; thus, the compressive strain in AlN and the tensile strain in sapphire are largely relaxed. The effective relaxation of strain further leads to a low density of defects in the AlN films. These findings reveal the roles of graphene in III-nitride growth and offer valuable insights into the efficient applications of graphene in the light-emitting diode industry.

12.
Opt Lett ; 43(18): 4366-4369, 2018 Sep 15.
Artigo em Inglês | MEDLINE | ID: mdl-30211865

RESUMO

Chip-scale mode-locked dissipative Kerr solitons have been realized on various materials platforms, making it possible to achieve a miniature, highly coherent frequency comb source with high repetition rates. Aluminum nitride (AlN), an appealing nonlinear optical material having both Kerr (χ3), and Pockels (χ2) effects, has immerse potential for comb self-referencing without the need for external harmonic generators. However, cavity soliton states have not yet been achieved in AlN microresonators. Here, we demonstrate mode-locked Kerr cavity soliton generation in a crystalline AlN microring resonator. By utilizing ultrafast tuning of the pump frequency through single-sideband modulation, in combination with an optimized wavelength scan and pump power-ramp patterns, we can deterministically elongate a ∼400 ns short-lived soliton to a time span as long as we wish to hold it.

13.
Bioresour Technol ; 268: 684-691, 2018 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-30144743

RESUMO

The process performance of a full-scale sludge process reduction activated sludge (SPRAS) system in long-term operation were investigated by inserting a micro-aerobic tank and a clarifier before conventional activated sludge process. The full-scale SPRAS for industrial park wastewater treatment achieved efficient pollutants removal and a low observed sludge yield of 0.074 g SS/g COD. Batch tests showed that influent feeding into the micro-aerobic tank favored sludge reduction, and obtained a sludge decay constant of 0.168 d-1. The SPRAS enriched slow growers and hydrolytic bacteria for sludge reduction, showed high simultaneous nitrification and denitrification efficiency in the micro-aerobic tank with abundant denitrifying bacteria, and improved sludge settleability by enriching floc-forming bacteria. Process configuration of the SPRAS was beneficial to enhance maintenance metabolism, cyclic micro-aerobic and anaerobic uncoupling, and lysis-cryptic growth for sludge reduction. Techno-economic analysis showed that the SPRAS greatly reduced sludge production with small footprint and low cost.


Assuntos
Esgotos/química , Reatores Biológicos , Desnitrificação , Hidrólise , Nitrificação , Esgotos/microbiologia , Eliminação de Resíduos Líquidos , Águas Residuárias
14.
Adv Mater ; 30(30): e1801608, 2018 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-29883036

RESUMO

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 Inx Ga1-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.

15.
Nanoscale ; 10(13): 5888-5896, 2018 Mar 29.
Artigo em Inglês | MEDLINE | ID: mdl-29521388

RESUMO

Recently, horizontal nanowires (NWs) have attracted much attention due to their increased compatibility with NW-based integrated nanoelectronic and nanophotonic systems. However, it still remains challenging to synthesize horizontal NWs efficiently. Here we introduce a novel method towards controllable growth of horizontal GaN NWs using HVPE with an Au/Ni thin film as the catalyst. By simply flipping the substrate, horizontal GaN NWs with various growth directions and cross sections have been obtained on a sapphire substrate with various facet orientations. Benefiting from the high decomposition frequency of GaCl precursors, the growth rate for the horizontal NWs is as fast as 400 µ h-1. Our results show that the facing orientation of the loaded substrate affects the flow of the local precursor, which determines the growth mode of the GaN NWs, i.e., no matter whether the substrate is facing downward or upwards. Photoluminescence measurements of the horizontal NWs show a finite blue shift of the band edge-related emission. It indicates the presence of compressed stress and is confirmed by the geometrical phase analysis (GPA) further. Our work opens up a new route and sheds light on the horizontal GaN NWs and will advance the development of horizontal NW-based nanoelectronic and nanophotonic devices and systems.

16.
Opt Express ; 25(19): 23267-23274, 2017 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-29041627

RESUMO

A demonstration of ultraviolet-B (UVB) communication link is implemented utilizing quadrature amplitude modulation (QAM) orthogonal frequency-division multiplexing (OFDM). The demonstration is based on a 294-nm UVB-light-emitting-diode (UVB-LED) with a full-width at half-maximum (FWHM) of 9 nm and light output power of 190 µW, at 7 V, with a special silica gel lens on top of it. A -3-dB bandwidth of 29 MHz was measured and a high-speed near-solar-blind communication link with a data rate of 71 Mbit/s was achieved using 8-QAM-OFDM at perfect alignment. 23.6 Mbit/s using 2-QAM-OFDM when the angle subtended by the pointing directions of the UVB-LED and photodetector (PD) is 12 degrees, thus establishing a diffuse-line-of-sight (LOS) link. The measured bit-error rate (BER) of 2.8 ×10-4 and 2.4 ×10-4, respectively, are well below the forward error correction (FEC) criterion of 3.8 ×10-3. The demonstrated high data-rate OFDM-based UVB communication link paves the way for realizing high-speed non-line-of-sight free-space optical communications.

17.
Opt Express ; 25(9): 9617-9627, 2017 May 01.
Artigo em Inglês | MEDLINE | ID: mdl-28468344

RESUMO

Based on the nanorod structure, we have fabricated GaN-based surface plasmon light-emitting diodes with Ag nanoparticles deposited laterally proximity to the multiple quantum wells (MQWs) region, which allows us to investigate the quantum well - surface plasmon (QW-SP) coupling effect. Our results show that the QW-SP coupling effect increases significantly when the SP resonant wavelength of Ag nanoparticles is close to the QW emission wavelength, especially by using a shorter wavelength light source, which will further enhance the spontaneous emission rate. Combined with the simulations, we find that the enhancement is due to the decreased excitation light penetration depth into the active region, which can modulate the carrier distribution and increase the proportion of SP-coupled carriers in the MQWs of LEDs. To increase the spontaneous emission rate for the electrical QW-SP coupled LEDs, we can use single QW or MQW structure to confine the carriers in the topmost QW, which will effectively increase the proportion of SP-coupled carriers. Our findings pave a way to design the ultrafast LED light source for the application of visible light communication (VLC).

18.
Nanoscale ; 9(21): 7021-7026, 2017 Jun 01.
Artigo em Inglês | MEDLINE | ID: mdl-28537630

RESUMO

We report here a new approach of Talbot effect based colloidal photolithography for fabricating nanostructure light emitting diodes (LEDs). By employing a rigid metal nanohole array template (RDT) as the diffraction grating and a polysiloxane-based spin on dielectric (SOD) as the thickness-controllable spacer layer, various InGaN/GaN nanostructure LEDs have been fabricated. Three-dimensional finite-difference time-domain (3D-FDTD) simulations have been conducted to verify the proposed approach. Photoluminescence (PL) and time-resolved photoluminescence (TRPL) have been performed to investigate the optical properties of the obtained nanostructure LEDs. Our report shows significance in fabricating two dimensional (2D) functional nanostructures and understanding the optical properties of various nanostructure InGaN/GaN LEDs.

19.
Nanoscale ; 9(24): 8142-8148, 2017 Jun 22.
Artigo em Inglês | MEDLINE | ID: mdl-28397909

RESUMO

Photodetectors for the ultraviolet (UV) range of the electromagnetic spectrum are in great demand for several technologies, but require the development of novel device structures and materials. Here we report on the high detectivity of UV photodetectors based on well-ordered laterally mesoporous GaN. The specific detectivity of our devices under UV-illumination reaches values of up to 5.3 × 1014 Jones. We attribute this high specific detectivity to the properties of the mesoporous GaN/metal contact interface: the trapping of photo-generated holes at the interface lowers the Schottky barrier height thus causing a large internal gain. High detectivity along with a simple fabrication process endows these laterally mesoporous GaN photodetectors with great potential for applications that require selective detection of weak optical signals in the UV range.

20.
Nanotechnology ; 28(11): 114003, 2017 Mar 17.
Artigo em Inglês | MEDLINE | ID: mdl-28103586

RESUMO

Gallium nitride-based nanopyramid light-emitting diodes are a promising technology to achieve highly efficient solid-state lighting and beyond. Here, periodic nanopyramid light-emitting diode arrays on gallium nitride/sapphire templates were fabricated by selective-area metalorganic chemical vapor deposition and multiple-exposure colloidal lithography. The electric field intensity distribution of incident light going through polystyrene microspheres and photoresist are simulated using finite-different time-domain method. Nitrogen as the carrier gas and a low V/III ratio (ratio of molar flow rate of group-V to group-III sources) are found to be important in order to form gallium nitride nanopyramid. In addition, a broad yellow emission in photoluminescence and cathodoluminescence spectra were observed. This phenomena showed the potential of nanopyramid light-emitting diodes to realize long wavelength visible emissions.

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