A structural analysis of ultrathin barrier (In)AlN/GaN heterostructures for GaN-based high-frequency power electronics

Metal–organic chemical vapor deposition (MOCVD) is one of the best growth methods for GaN-based materials as well-known. GaN-based materials with very quality are grown the MOCVD, so we used this growth technique to grow InAlN/GaN and AlN/GaN heterostructures in this study. The structural and surface properties of ultrathin barrier AlN/GaN and InAlN/GaN heterostructures are studied by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. Screw, edge, and total dislocation densities for the grown samples have been calculated by using XRD results. The lowest dislocation density is found to be 1.69 × 10⁸ cm⁻² for Sample B with a lattice-matched In_0.17Al_0.83N barrier. The crystal quality of the studied samples is determined using (002) symmetric and (102) asymmetric diffractions of the GaN material. In terms of the surface roughness, although reference sample has a lower value as 0.27 nm of root mean square values (RMS), Sample A with 4-nm AlN barrier layer exhibits the highest rough surface as 1.52 nm of RMS. The structural quality of the studied samples is significantly affected by the barrier layer thickness. The obtained structural properties of the samples are very important for potential applications like high-electron mobility transistors (HEMTs).     

Temperature dependent energy relaxation time in AlGaN/AlN/GaN heterostructures

The two-dimensional (2D) electron energy relaxation in Al_0.25Ga_0.75N/AlN/GaN heterostructures was investigated experimentally by using two experimental techniques; Shubnikov-de Haas (SdH) effect and classical Hall Effect. The electron temperature (T_e) of hot electrons was obtained from the lattice temperature (T_L) and the applied electric field dependencies of the amplitude of SdH oscillations and Hall mobility. The experimental results for the electron temperature dependence of power loss are also compared with the current theoretical models for power loss in 2D semiconductors. The power loss that was determined from the SdH measurements indicates that the energy relaxation of electrons is due to acoustic phonon emission via unscreened piezoelectric interaction. In addition, the power loss from the electrons obtained from Hall mobility for electron temperatures in the range T_e > 100 K is associated with optical phonon emission. The temperature dependent energy relaxation time in Al_0.25Ga_0.75N/AlN/GaN heterostructures that was determined from the power loss data indicates that hot electrons relax spontaneously with MHz to THz emission with increasing temperatures.     

MATHEMATICAL AND EXPERIMENTAL ESSENTIALS OF THE MULSIFICATION PROCESS: OPTIMAL PARAMETERS DETERMINATION

The evaluation of the procedure for the determination of optimal parameters for the preparation of 0/W emulsions by the mechanical homogenizer has been studied. The method is based on a general functional dependence of the emulsion droplet-size distribution parameters on various factors (homogenization time interval, emulsifi-er concentration, and homogenization intensity) Calculated parameters of the derived mathematical relations were used for characterization of the specific action and limiting capabilities of a particular emulsifier, as well as for the determination of the optimal emulsifica-tion conditions. Changes in emulsifier concentration cause remarkable effects on the emulsion droplet sizes, but only at lower emulsifier concentrations (below the optimal ones). If the homogenization intensity is insufficient, coarse and very unstable emulsions are obtained; if higher than the optimal one, the homogenization effects are not proportional to the energy expended.     

A new donor–acceptor double‐cable carbazole polymer with perylene bisimide pendant group: Synthesis,electrochemical, and photovoltaic properties

We report here electrochemical synthesis of novel soluble donor–acceptor (D–A) polymer with suitably functionalized perylenetetracarboxylic diimide dye derivative covalently linked to carbazole moiety (Cbz‐PDI). The band gap, E_g was measured using UV–Vis spectroscopy and compared with that obtained by cyclic voltammetry (CV). Efficient intramolecular electron transfer from carbazole‐donor to perynediimide‐acceptor leads to remarkable fluorescence quenching of the perylene core. Furthermore, spectroelectrochemical property and surface morphology of the polymer film were investigated. Characteristic monoanion and dianion radical bands on the UV–Vis absorption spectra attributed to the electrochemical reduction of the neutral polymer were observed. During the reduction process, red color of the film turned into blue and violet, respectively. Finally, the photovoltaic performance of the D–A double‐cable polymer was checked and nearly 0.1% electrical conversion efficiency is obtained under simulated AM 1.5 solar light with 100 mW/cm² radiation power. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6280–6291, 2009     

Electrochemical deposition of gold at liquid–liquid interfaces studied by thin organic film-modified electrodes

The unique physico-chemical properties of gold nanoparticles portrayed in their chemical stability, the size-dependent electrochemistry, and the unusual optical properties make them suitable modifiers of various surfaces used in the fields of optical devices, electronics, and biosensors. In this work we present two different methods to obtain metallic gold nanoparticles at a liquid–liquid interface, and to control their growth by adjusting the experimental conditions. Decamethylferrocene (DMFC), used as an oxidizable compound dissolved in an organic solvent that is spread as a thin film on the surface of graphite electrode, serves as a redox partner to exchange electrons across the liquid–liquid interface with the other redox counter-partner [AuCl₄]^? present in the conjoined water phase. The interfacial electron transfer between the DMFC and the [AuCl₄]^? ions leads to deposition of metallic gold nanoparticles at the liquid–liquid interface. The structure and features of the deposited Au nanoparticles were studied by means of microscopic and voltammetric techniques. The morphology of the Au deposit depends on the concentration ratio of redox partners and both electrode and liquid–liquid interfacial potential differences. Depending on whether the Au deposit was obtained by ex situ (at open circuit potential) or by “in situ” (by cycling of the electrode potential) approach, we observed quite different effects to the ion transfer reactions probed by the thin-film electrode set-up. The possible reasons for the different behavior of the Au nanoparticles are discussed in terms of the structure and the properties of the obtained Au deposit. In separate experiments, we have demonstrated catalytic effects of the Au nanoparticles towards enhancing the electron transfer between DMFC and two aqueous redox substrates, hexacyanoferrate and hydrogen peroxide.     

Co-metal-free enantioselective conjugate addition reactions of zinc reagents

Asymmetric conjugate addition of diethylzinc to cinnamaldehyde in a co-metal-free fashion by using N,O-ligands with planar and central chirality is described. Different modulations of the ligand structure, including several combinations of the chiral units, indicate that a [2.2]paracyclophane backbone is essential for the activity and the enantioselectivity of the generated active catalyst. By using the optimized ligand, an isolated yield of 90 % was obtained with up to 99 % ee.     

A new near‐infrared switchable electrochromic polymer and its device application

A new near‐infrared switchable electrochromic polymer containing carbazole pendant (poly‐SNSC), synthesized by electrochemical polymerization of 2,5‐bis‐dithienyl‐1H‐pyrrole (SNS) main chain, has been prepared. The electrochemical and optical properties of SNSC monomer and its polymer have been investigated. Because of having two different electro‐donor moieties; that is, carbazole and SNS, SNSC gave two separate electrochemical oxidation and also light brown color of the film in the neutral state turn into gray on oxidation. An electrochromic device, contructed in the sandwich configuration [indium tin oxide (ITO)‐coated glass/anodically coloring polymer (poly‐SNSC)//gel electrolyte//cathodically coloring polymer (PEDOT)/ITO‐coated glass] and exhibited a high coloration efficiency (1216 cm² C^–1), a very short response time (about 0.3 s), low driving voltage, and a high redox stability. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Determination of the LO phonon energy by using electronic and optical methods in AlGaN/GaN

The longitudinal optical (LO) phonon energy in AlGaN/GaN heterostructures is determined from temperature-dependent Hall effect measurements and also from Infrared (IR) spectroscopy and Raman spectroscopy. The Hall effect measurements on AlGaN/GaN heterostructures grown by MOCVD have been carried out as a function of temperature in the range 1.8-275 K at a fixed magnetic field. The IR and Raman spectroscopy measurements have been carried out at room temperature. The experimental data for the temperature dependence of the Hall mobility were compared with the calculated electron mobility. In the calculations of electron mobility, polar optical phonon scattering, ionized impurity scattering, background impurity scattering, interface roughness, piezoelectric scattering, acoustic phonon scattering and dislocation scattering were taken into account at all temperatures. The result is that at low temperatures interface roughness scattering is the dominant scattering mechanism and at high temperatures polar optical phonon scattering is dominant.     

Ammonium transporters achieve charge transfer by fragmenting their substrate

Proteins of the Amt/MEP family facilitate ammonium transport across the membranes of plants, fungi, and bacteria and are essential for growth in nitrogen-poor environments. Some are known to facilitate the diffusion of the neutral NH(3), while others, notably in plants, transport the positively charged NH(4)(+). On the basis of the structural data for AmtB from Escherichia coli , we illustrate the mechanism by which proteins from the Amt family can sustain electrogenic transport. Free energy calculations show that NH(4)(+) is stable in the AmtB pore, reaching a binding site from which it can spontaneously transfer a proton to a pore-lining histidine residue (His168). The substrate diffuses down the pore in the form of NH(3), while the excess proton is cotransported through a highly conserved hydrogen-bonded His168-His318 pair. This constitutes a novel permeation mechanism that confers to the histidine dyad an essential mechanistic role that was so far unknown.