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The early stage degradation of electrical properties in AlGaN/GaN high electron mobility transistors (HEMTs) under fast neutron irradiation is studied. After the 1 MeV neutron irradiation at a low fluence of 1 × 1014 neutrons/cm2, the reverse leakage current decreases while the output and transfer characteristics remain unchanged, which cannot be explained by the previously reported high-fluence degradation model. By employing temperature-dependent gate leakage current measurements, we show that the dislocation related Poole–Frenkel (PF) emission dominates the gate leakage mechanism before and after irradiation whereas the barrier height for electron emission to conductive dislocation increases after the neutron irradiation. A model with the evolution of dislocation from the VGa-decorated configuration to the pure configuration is proposed to describe the degradation of AlGaN/GaN HEMTs at the low-fluence early stage neutron irradiation. This model enriches the understanding of the degradation mechanism of neutron irradiated AlGaN/GaN HEMTs.

High quality factor optical microcavities have been employed in a variety of material systems to enhance nonlinear optical interactions. While single-crystalline aluminum nitride microresonators have recently emerged as a low loss platform for integrated nonlinear optics such as four wave mixing and Raman lasing, few studies have investigated this material for second-harmonic generation. In this letter, we demonstrate an optimized fabrication of dually resonant phase-matched ring resonators from epitaxial aluminum nitride thin films. An unprecendented second-harmonic generation efficiency of 17 000%/W is obtained in the low power regime, and pump depletion is observed at a relatively low input power of 3.5 mW. This poses epitaxial aluminum nitride as the highest efficiency second-harmonic generator among current integrated platforms.

We report on measurement of dielectric constant, mid-gap defect density, Urbach energy of tail states in CH3NH3PbIxCl1−x perovskite solar cells. Midgap defect densities were estimated by measuring capacitance vs. frequency at different temperatures and show two peaks, one at 0.66 eV below the conduction band and one at 0.24 eV below the conduction band. The attempt to escape frequency is in the range of 2 × 1011/s. Quantum efficiency data indicate a bandgap of 1.58 eV. Urbach energies of valence and conduction band are estimated to be ∼16 and ∼18 meV. Measurement of saturation capacitance indicates that the relative dielectric constant is ∼18.

Nanodielectric elastomer films consisting of a silicone rubber as the matrix and conductive polyaniline nanorods (PANI-NR) as the filler were fabricated and their electro-mechanical actuation performance was investigated. The nanodielectric elastomers displayed a typical insulator-to-conductor transition as the content of PANI-NR increased. The percolation threshold was observed to show a relatively low value of 5.2 vol. %, which should be mainly ascribed to the one-dimensional structure of the nanorods. As the concentration of PANI-NR approached to the percolation threshold, the electro-mechanical actuation strain of the elastomer films was significantly enhanced. The actuation coefficient of nanodielectric elastomer films with only 4.0 vol. % PANI-NR loading was about 5 times higher than that of pure silicone rubber matrix.

We present the design and realization of ultra-thin chiral metasurfaces with giant broadband optical activity in the infrared wavelength. The chiral metasurfaces consisting of periodic hole arrays of complementary asymmetric split ring resonators are fabricated by femtosecond laser two-photon polymerization. Enhanced transmission with strong polarization conversion up to 97% is observed owing to the chiral surface plasmons resulting from mirror symmetry broken. The dependence of optical activity on the degree of structural asymmetry is investigated. This simple planar metasurface is expected to be useful for designing ultra-thin active devices and tailoring the polarization behavior of complex metallic nanostructures.

We use molecular dynamics and phase field simulations to demonstrate that Cu-Nb multilayered nanocomposites with individual layer thicknesses above 2–4 nm remain morphologically stable when subjected to 100 keV collision cascades, characteristic of neutron or heavy ion irradiation. The probability of morphological instability rapidly increases as the layer thickness decreases to 1 nm, which we attribute to overlap of zones of liquid-like interdiffusion inside radiation-induced thermal spikes at neighboring interfaces in the multilayer.

The effect of n-AlGaN versus p-AlGaN electron-blocking layers (EBLs) on the performance of InGaN/GaN light-emitting diodes is studied in this work. Experimental results suggest that the n-type EBL leads to higher optical output power and external quantum efficiency, compared to the devices with p-AlGaN EBL, which is commonly used today. Numerical simulations on the carrier distribution and energy band diagram reveal that the n-AlGaN EBL is more efficient in preventing electron overflow, while not blocking the hole injection into the active region, hence leading to higher radiative recombination rate within the multiple quantum wells active region. © 2013 AIP Publishing LLC.

Ultrafast carrier dynamics and saturable absorption of few-layered graphene oxide, well-dispersed in organic solvent, are studied using femtosecond pump-probe and Z-scan techniques. The results demonstrate that few-layered graphene oxide has a fast energy relaxation of hot carriers and strong saturable absorption, which is comparable with that of reduced graphene oxide. Fast carrier relaxation combined with well solution processing capability arises from the large fraction of sp2 carbon atom inside the few-layered graphene oxide sheet together with oxidation mainly existing at the edge areas. This superiority of few-layered graphene oxide will facilitate potential applications of graphene for ultrafast photonics.

Surface emitting linearly polarized InGaN/GaN light emitting diode (LED) is demonstrated using a subwavelength metallic nanograting. The aluminum based grating with a period of 150 nm is fabricated on top of the p-contact layer in a conventional InGaN LED structure grown on (0001) oriented sapphire substrate. Polarization ratio can reach 7:1, the highest ever reported polarization ratio directly from a light emitting diode. The polarization characteristics are studied in details both experimentally and theoretically, suggesting an effective way to make polarized light emission devices.

The nonlinear optical properties of graphene oxide (GO) were investigated at 532 nm in nanosecond and picosecond regimes. Results show that two-photon absorption dominates nonlinear absorption process of GO in the case of picosecond pulses, while excited state nonlinearities play an important role in the case of nanosecond pulses. Additionally, we compared nonlinear optical properties of three different dimensional carbon-based materials (two-dimensional graphene, one-dimensional carbon nanotube, and zero-dimensional fullerene) in nanosecond and picosecond regimes, respectively. The nonlinear mechanism of GO is distinctly different from nonlinear scattering of carbon nanotube and excited state nonlinearity of fullerene.

We have fabricated well-ordered organic donor/acceptor nanojunction arrays comprising p-sexiphenyl (6P) and C60 via self-assembly of C60 on the molecular nanotemplate of 6P nanostripes on Ag(111). Our scanning tunneling microscopy measurements show that the preferential trapping of C60 triplets atop 6P single molecule leads to the formation of the C60∕6P vertical nanojunctions with well-defined two-dimensional arrangement. Annealing this binary molecular superstructure at 380K results in the insertion of C60 linear chains between the neighboring 6P nanostripes and thereby the appearance of the periodic C60∕6P lateral nanojunction arrays.

We present a low-temperature scanning tunneling microscopy investigation revealing the formation of a long range ordered α-sexithiophene (6T) monolayer nanostripe array on highly ordered pyrolytic graphite, with thiophene ring units oriented parallel to the surface. The 6T monolayer nanostripe is an effective molecular nanotemplate for the assembly of an ordered “zigzag” C60 chain array, accompanied by a slight structural modification of the underlying 6T nanostripes due to donor-acceptor intermolecular interactions between 6T and C60 molecules.

In situ low-temperature scanning tunneling microscopy and near-edge x-ray absorption spectroscopy measurements have been used to investigate the molecular orientation of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) thin films at the interface of organic heterojunctions of PTCDA on copper(II) phthalocyanine (CuPc). On the CuPc monolayer on highly oriented pyrolitic graphite, PTCDA molecules form a well-ordered in-plane herringbone structure with their molecular plane parallel to the substrate surface. The formation of multiple in-plane hydrogen bonding between neighboring PTCDA molecules is responsible for the flat-lying PTCDA on CuPc monolayer, and gives rise to the lying-down orientation of PTCDA thin films on both standing-up and lying-down CuPc thin films, as well as on Au(111) passivated by a self-assembled monolayer of octane-1-thiol.

Vertically aligned single-walled carbon nanotube (SWNT) forest was studied by using angular-dependent C K-edge x-ray absorption near edge structure (XANES) with linearly polarized x-ray beam. The XANES analysis found a crust of entangled nanotubes on top of the forest formed at the first stage of the forest growth, which shapes the morphology of the entire forest and constricts the nanotubes to grow to the same length. It indicates that this type of SWNT forest has a different growth mechanism from the multiwalled carbon nanotube forest.