Chromium mask for plasma-chemical etching of Al x Ga1 − x N layers

The influence of the mask material used for formation of GaN mesa-structures using plasma-chemical etching in a BCl₃ : Ar : N₂ atmosphere is studied. It is shown that a bilayer SiO₂/Cr mask in which the thickness of the chromium layer is six to seven times smaller than a desired etch depth provides effective protection of the surface and allows the formation of structures with a flat surface for an etch depth of up to 2.5 μm. When the mask is produced by conventional lithography, the slope of the side walls is no more than 10° and decreases when liftoff lithography is applied.     

Unexpected C-H activation of Ru(II)-dithiomaltol complexes upon oxidation

Thione-substituted derivatives of maltol are of interest in several applications of metal-based drugs. In order to investigate the effect of the oxygenation on such thione chelates, Ru complexes of 3-hydroxy-2-methyl-4-thiopyrone (thiomaltol or Htma) and 3-hydroxy-2-methyl-4H-thiopyran-4-thione (dithiomaltol or Httma), [Ru(bpy)2(tma)](+), 1, and [Ru(bpy) 2(ttma)] (+), 2, were synthesized as diamagnetic PF6(-) salts. Peroxidation of 2 unexpectedly generated products of C-H activation at its pendant methyl group; an air-stable aldehyde [Ru(bpy)2(ttma-aldehyde)](+), 4, was the major product. In addition, an intermediate oxidation product [Ru(bpy) 2(ttma-alcohol)](PF6), 3, was characterized. Both 3 and 4 are also formed by reaction of 2 with outersphere oxidants (e.g., Na2IrCl6) and by bulk electrolysis under anaerobic conditions. Similar oxidations of the analogous [Ru(bpy)2(ettma)](+), 2' , complex (3-hydroxy-2-ethyl-4H-thiopyran-4-thione; ethyl dithiomaltol or Hettma) formed the corresponding ketone, [Ru(bpy)2(ettma-ketone)](PF6), 4', by oxidation at the same position adjacent to the conjugated ring. The structures of the aldehyde 4 and starting materials 1 and 2 have been confirmed by X-ray crystallography, and all complexes have been characterized by UV-vis, (1)H NMR, and IR spectroscopies. Initial mechanistic investigations are discussed.     

Interaction of nitrate, barium, strontium and cadmium ions with fused quartz/water interfaces studied by second harmonic generation

Inorganic anions and cations are ubiquitous in environmental chemistry. Here, we use second harmonic generation to track the interaction of the environmentally important metal cations barium, strontium, and cadmium and the nitrate anion with fused quartz/water interfaces at pH 7. Using a dynamic flow system, we assess the extent of reversibility in the binding process and report the absolute number density of adsorbed cations, their charge densities, and their free energies of adsorption. We also present resonantly enhanced second harmonic generation experiments that show that nitrate is surface active and report the free energies and binding constants for the adsorption process. The second harmonic generation spectrum of surface-bound nitrate shows a new adsorption band that cuts further into the solar spectrum than nitrate in the aqueous or solid state. The results that we obtain for all four inorganic ions and the implications for tropospheric and aquatic chemistry as well as geochemistry are discussed in the context of fundamental science as well as pollutant transport models.     

Investigation of binding event perturbations caused by elevated QCM-D oscillation amplitude

We report measurements with the quartz crystal microbalance with dissipation monitoring (QCM-D) technique, with focus on how the shear oscillation amplitude of the sensor surface influences biorecognition binding events. Technically, this is made as reported recently (M. Edvardsson, M. Rodahl, B. Kasemo, F. H??k, Anal. Chem., 2005, 77(15), 4918-4926) by operating the QCM in dual frequency mode; one harmonic (n = n1) is utilized for continuous excitation of the QCM-D sensor at resonance at variable driving amplitudes (1-10 V), while the second harmonic (n not equaln(1)) is used for combined f and D measurements. By using one harmonic as a "probe" and the other one as an "actuator", elevated amplitudes can be used to perturb - or activate - binding reactions in a controlled way, while simultaneously maintaining the possibility of probing the adsorption and/or desorption events in a non-perturbative manner using combined f and D measurements. In this work we investigate the influence of oscillation amplitude variations on the binding of NeutrAvidin-modified polystyrene beads (slashed circle approximately 200 nm) to a planar biotin-modified lipid bilayer supported on an SiO2-modified QCM-D sensor. These results are further compared with data on an identical system, except that the NeutrAvidin-biotin recognition was replaced by fully complementary DNA hybridization. Supported by micrographs of the binding pattern, the results demonstrate that there exists, for both systems, a unique critical oscillation amplitude, A(c), below which binding is unaffected by the oscillation, and above which binding is efficiently prevented. Associated with A(c), there is a critical crystal radius, r(c), defining the central part of the crystal where binding is prevented. From QCM-D data, A(c) for the present system was estimated to be approximately 6.5 nm, yielding a value of r(c) of approximately 3 mm--the latter number was nicely confirmed by fluorescent- and dark-field micrographs of the crystal. Furthermore, the fact that A(c) is observed to be identical for the two types of biorecognition reactions suggests that it is neither the strength, nor the number of contact points, that determine the amplitude at which binding is prevented. Rather, particle size seems to be the determining parameter.     

Effective mode area and its optimization in silicon-nanocrystal waveguides

We revisit the problem of the optimization of a silicon-nanocrystal (Si-NC) waveguide, aiming to attain the maximum field confinement inside its nonlinear core and to ensure optimal waveguide performance for a given mode power. Using a Si-NC/SiO2 slot waveguide as an example, we show that the common definition of the effective mode area may lead to significant errors in estimation of optical intensity governing the nonlinear optical response and, as a result, to poor strength evaluation of the associated nonlinear effects. A simple and physically meaningful definition of the effective mode area is given to relate the total mode power to the average field intensity inside the nonlinear region and is employed to study the optimal parameters of Si-NC slot waveguides.     

Finite- and Infinite-Dimensional Representations of the Lorentz Group

Finite- and infinite-dimensional representations of the Lorentz group are discussed and various topics in which this group is currently in use are mentioned. The infinitesimal approach of finding representations is reviewed and all finite-dimensional spinor representations of the Lorentz group are obtained. Infinite-dimensional representations are then discussed, including the principal, complementary, and complete series of representations. A generalized Fourier transformation is introduced which enables one to use the global approach to representation theory so as to express infinite-dimensional representations in terms of matrices. This method is shown to lead to a generalization of the spinor form of finite-dimensional representation to the infinite-dimensional case. However, whereas the usual spinor representations are nonunitary, the obtained new form describes both unitary and non-unitary representations, depending on the choice of certain parameters appearing in the representation formula.