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1.
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.

2.
Nanomaterials (Basel) ; 9(4)2019 Apr 06.
Artigo em Inglês | MEDLINE | ID: mdl-30959885

RESUMO

Transparent heating devices are widely used in daily life-related applications that can be achieved by various heating materials with suitable resistances. Herein, high-performance vertically-oriented graphene (VG) films are directly grown on soda-lime glass by a radio-frequency (rf) plasma-enhanced chemical vapor deposition (PECVD) method, giving reasonable resistances for electrothermal heating. The optical and electrical properties of VG films are found to be tunable by optimizing the growth parameters such as growth time, carrier gas flow, etc. The electrothermal performances of the derived materials with different resistances are thus studied systematically. Specifically, the VG film on glass with a transmittance of ~73% at 550 nm and a sheet resistance of ~3.9 KΩ/□ is fabricated into a heating device, presenting a saturated temperature up to 55 °C by applying 80 V for 3 min. The VG film on the glass at a transmittance of ~43% and a sheet resistance of 0.76 KΩ/□ exhibits a highly steady temperature increase up to ~108 °C with a maximum heating rate of ~2.6 °C/s under a voltage of 60 V. Briefly, the tunable sheet resistance, good adhesion of VG to the growth substrate, relative high heating efficiency, and large heating temperature range make VG films on glass decent candidates for electrothermal related applications in defrosting and defogging devices.

3.
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.

4.
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.

5.
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.

6.
Chem Commun (Camb) ; 54(47): 5960-5963, 2018 Jun 08.
Artigo em Inglês | MEDLINE | ID: mdl-29736502

RESUMO

Carbon dots (CDs) with an absolute fluorescence quantum yield of 87% are facilely prepared via irradiation of self-assembled terthiophene amphiphile TTC4L in aqueous solution by mild visible light. Visible light irradiation of TTC4L triggers the production of superoxide radicals in water, which oxidize the closely packed terthiophene group into carbon dots. Our results reveal that the molecular self-assembly may act as important precursor for the generation of single molecule-like carbon dots; this method paves the way for the fabrication of CDs of high quality.

7.
Adv Mater ; 30(8)2018 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-29318672

RESUMO

Plasma-enhanced chemical vapor deposition (PECVD) is an applicable route to achieve low-temperature growth of graphene, typically shaped like vertical nanowalls. However, for transparent electronic applications, the rich exposed edges and high specific surface area of vertical graphene (VG) nanowalls can enhance the carrier scattering and light absorption, resulting in high sheet resistance and low transmittance. Thus, the synthesis of laid-down graphene (LG) is imperative. Here, a Faraday cage is designed to switch graphene growth in PECVD from the vertical to the horizontal direction by weakening ion bombardment and shielding electric field. Consequently, laid-down graphene is synthesized on low-softening-point soda-lime glass (6 cm × 10 cm) at ≈580 °C. This is hardly realized through the conventional PECVD or the thermal chemical vapor deposition methods with the necessity of high growth temperature (1000 °C-1600 °C). Laid-down graphene glass has higher transparency, lower sheet resistance, and much improved macroscopic uniformity when compare to its vertical graphene counterpart and it performs better in transparent heating devices. This will inspire the next-generation applications in low-cost transparent electronics.

8.
Adv Mater ; 29(47)2017 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-29076181

RESUMO

Aluminum (Al) foil, as the most accepted cathode current collector for lithium-ion batteries (LIBs), is susceptible to local anodic corrosions during long-term operations. Such corrosions could lead to the deterioration or even premature failure of the batteries and are generally believed to be a bottleneck for next-generation 5 V LIBs. Here, it is demonstrated that Al foil armored by conformal graphene coating exhibits significantly reinforced anodic corrosion resistance in both LiPF6 and lithium bis(trifluoromethanesulphonyl) imide (LiTFSI) based electrolytes. Moreover, LiMn2 O4 cells using graphene-armored Al foil as current collectors (LMO/GA) demonstrate enhanced electrochemical performance in comparison with those using pristine Al foil (LMO/PA). The long-term discharge capacity retention of LMO/GA cell after ≈950 h straight operations at low rate (0.5 C) reaches up to 91%, remarkably superior to LMO/PA cell (75%). The self-discharge propensity of LMO/GA is clearly relieved and the rate/power performance is also improved with graphene mediations. This work not only contributes to the long-term stable operations of LIBs but also might catalyze the deployment of 5 V LIBs in the future.

9.
Angew Chem Int Ed Engl ; 55(31): 9036-40, 2016 07 25.
Artigo em Inglês | MEDLINE | ID: mdl-27272178

RESUMO

Herein, we report label-free detection of single-molecule DNA hybridization dynamics with single-base resolution. By using an electronic circuit based on point-decorated silicon nanowires as electrical probes, we directly record the folding/unfolding process of individual hairpin DNAs with sufficiently high signal-to-noise ratio and bandwidth. These measurements reveal two-level current oscillations with strong temperature dependence, enabling us to determine the thermodynamic and kinetic properties of hairpin DNA hybridization. More importantly, successive, stepwise increases and decreases in device conductance at low temperature on a microsecond timescale are successfully observed, indicating a base-by-base unfolding/folding process. The process demonstrates a kinetic zipper model for DNA hybridization/dehybridization at the single base-pair level. This measurement capability promises a label-free single-molecule approach to probe biomolecular interactions with fast dynamics.


Assuntos
Técnicas Biossensoriais , DNA/análise , Hibridização de Ácido Nucleico , Termodinâmica , Técnicas Biossensoriais/instrumentação , Nanofios/química , Silício/química
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