Electrical activity at the AlN/Si Interface: identifying the main origin of propagation losses in GaN-on-Si devices at microwave frequencies.

AlN nucleation layers are the basement of GaN-on-Si structures grown for light-emitting diodes, high frequency telecommunication and power switching systems. In this context, our work aims to understand the origin of propagation losses in GaN-on-Si High Electron Mobility Transistors at microwaves frequencies, which are critical for efficient devices and circuits. AlN/Si structures are grown by Metalorganic Vapor Phase Epitaxy. Acceptor dopant in-diffusion (Al and Ga) into the Si substrate is studied by Secondary Ion Mass Spectroscopy and is mainly located in the first 200 nm beneath the interface. In this region, an acceptor concentration of a few 1018 cm-3 is estimated from Capacitance-Voltage (C-V) measurements while the volume hole concentration of several 1017 cm-3 is deduced from sheet resistance. Furthermore, the combination of scanning capacitance microscopy and scanning spreading resistance microscopy enables the 2D profiling of both the p-type conductive channel and the space charge region beneath the AlN/Si interface. We demonstrate that samples grown at lower temperature exhibit a p-doped conductive channel over a shallower depth which explains lower propagation losses in comparison with those synthesized at higher temperature. Our work highlights that this p-type channel can increase the propagation losses in the high-frequency devices but also that a memory effect associated with the previous sample growths with GaN can noticeably affect the physical properties in absence of proper reactor preparation. Hence, monitoring the acceptor dopant in-diffusion beneath the AlN/Si interface is crucial for achieving efficient GaN-on-Si microwave power devices.

Expression of AHI1 Rescues Amyloidogenic Pathology in Alzheimer's Disease Model Cells.

A hallmark of Alzheimer's disease (AD) pathogenesis is the accumulation of extracellular plaques mainly composed of amyloid-ß (Aß) derived from amyloid precursor protein (APP) cleavage. Recent reports suggest that transport of APP in vesicles with huntingtin-associated protein-1 (HAP1) negatively regulates Aß production. In neurons, HAP1 forms a stable complex with Abelson helper integration site-1 (AHI1), in which mutations cause neurodevelopmental and psychiatric disorders. HAP1 and AHI1 interact with tropomyosin receptor kinases (Trks), which are also associated with APP and mediate neurotrophic signaling. In this study, we hypothesize that AHI1 participates in APP trafficking and processing to rescue AD pathology. Indeed, AHI1 was significantly reduced in mouse neuroblastoma N2a cells expressing human Swedish and Indiana APP (designed as AD model cells) and in 3xTg-AD mouse brain. The AD model cells as well as Ahi1-knockdown cells expressing wild-type APP-695 exhibited a significant reduction in viability. In addition, the AD model cells were reduced in neurite outgrowth. APP C-terminal fragment-ß (CTFß) and Aß42 were increased in the AD cell lysates and the culture media, respectively. To investigate the mechanism how AHI1 alters APP activities, we overexpressed human AHI1 in the AD model cells. The results showed that AHI1 interacted with APP physically in mouse brain and transfected N2a cells despite APP genotypes. AHI1 expression facilitated intracellular translocation of APP and inhibited APP amyloidogenic process to reduce the level of APP-CTFß in the total lysates of AD model cells as well as Aß in the culture media. Consequently, AHI1-APP interactions enhanced neurotrophic signaling through Erk activation and led to restored cell survival and differentiation.

Ganglioside Hp-s1 Analogue Inhibits Amyloidogenic Toxicity in Alzheimer's Disease Model Cells.

Alzheimer's disease (AD) is characterized by extracellular deposition of amyloid plaques, which are predominantly composed of amyloid-ß (Aß) peptide derived from amyloid precursor protein (APP) cleavage. APP interacts with tropomyosin receptor kinase A, a neurotrophic receptor associated with gangliosides and mediating neuronal survival and differentiation through the extracellular signal-regulated protein kinase (ERK) pathway. The ganglioside Hp-s1's analogue Hp-s1A exerts neuritogenic activity; however, its effect on AD pathology remains unknown. To test the hypothesis that Hp-s1A is a potential candidate to treat AD, we established the AD-modeled cell line by expressing human Swedish and Indiana APP gene (APP-Swe/Ind) in N2a mouse neuroblastoma cells. The cells were treated with Hp-s1A or monosialoganglioside GM1 for comparison. The AD model cells expressing APP-Swe/Ind exhibited a significant reduction in viability, as well as neurite outgrowth rate, in comparison to the control cells expressing APP-695. APP C-terminal fragment-ß (CTFß) and Aß42 were increased in the AD cell lysates and the culture media, respectively. With the treatment of either Hp-s1A or GM1 at 1 µM, the AD model cells showed a significant increase in viability; however, only Hp-s1A reduced CTFß levels in these cells. Further analysis of the culture media revealed that Hp-s1A also reduced Aß42 production from AD model cells. The phosphorylation of ERK was elevated and the neurite outgrowth rate was restored with Hp-s1A treatment. In conclusion, the ganglioside analogue Hp-s1A inhibited amyloidogenic processing of APP and promoted neurotrophic activity and survival of AD model cells. Hp-s1A has great potential in AD therapeutic development.

Enhanced excitonic emission efficiency in porous GaN.

We investigate the optical properties of porous GaN films of different porosities, focusing on the behaviors of the excitonic features in time-integrated and time-resolved photoluminescence. A substantial enhancement of both excitonic emission intensity and recombination rate, along with insignificant intensity weakening under temperature rise, is observed in the porous GaN films. These observations are in line with (i) the local concentration of electric field at GaN nanoparticles and pores due to the depolarization effect, (ii) the efficient light extraction from the nanoparticles. Besides, the porosification enlarges the surface of the air/semiconductor interface, which further promotes the extraction efficiency and suppresses non-radiative recombination channels. Our findings open a way to increasing the emission efficiency of nanophotonic devices based on porous GaN.

A comparative study of micellization with fluorinated and hydrogenated diquaternary ammonium bolaamphiphiles.

Two new N(a),N(b)-Bis[3-(trimethylamino)propyl]-perfluoroalkanes diamide-Iodide with (a=1; b=10) and (a=1; b=12), differing by the fluorocarbon tail spacer (8 or 10 CF2), and the N(1),N(12)-Bis[3-(trimethylamino)propyl]-dodecanediamide-Iodide have been synthesized in two steps involving the appropriate dimethyl perfluoro or hydrogeno alkane dicarboxylate, N,N-dimethyl-1,3-propane diamine, and methyl iodide. Their surface properties were studied at 45°C in aqueous solutions using conductivity, surface tension, and dynamic light scattering (DLS) measurements. A comparison of the micellar parameters usually obtained by these techniques has been carried out. The critical micelle concentrations (CMCs), the geometric packing parameters, the surface excess concentration, the area/surfactant molecule at the interface, the micellar ionization degree ß and the free energies of micellization, the diffusion coefficient, and the hydrodynamic diameter were discussed. The effect of added KI on these parameters and on the size and on the kind of aggregates has been also investigated. The addition of 1,2-Dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) to the surfactant solutions, where the size of the complex DOPE-surfactant was determined by DLS over a wide range of concentrations, reveals a very different behavior comparing fluorinated and hydrogenated compounds.

Aggregation of new bipolar glucosyled and cationic fluoroamphiphiles in aqueous media.

A new series of N-[3-(trimethylamino)propyl]-perfluoro-N-glucosyl amide-Iodides, differing by the length of the fluorocarbon tail (7, 9, and 11), have been synthesized in three steps involving unprotected glucose, N,N-dimethyl-1,3-propane diamine, the appropriate methyl-perfluoroalcanoate, and methyl iodide. Their aggregation and surface properties were studied in aqueous solution using conductivity, surface tension, and dynamic light scattering measurements. The critical micelle concentrations (CMC), the micellar aggregation numbers, the geometric packing parameters, the area/surfactant molecule at the interface, the surface excess concentration, the micellar ionization degree ß, and the free energies of micellization have been investigated. DLS results show various morphologies of aggregates such as micelles and vesicles according to the increase in the hydrophobic chain length.