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Single-crystalline gallium arsenide (GaAs) grown by various techniques can exhibit hillock defects on the surface when sub-optimal growth conditions are employed. The defects act as nonradiative recombination centers and limit solar cell performance. In this paper, we applied near-field transport imaging to study hillock defects in a GaAs thin film. On the same defects, we also performed near-field cathodoluminescence, standard cathodoluminescence, electron-backscattered diffraction, transmission electron microscopy, and energy-dispersive X-ray spectrometry. We found that the luminescence intensity around the hillock area is two orders of magnitude lower than on the area without hillock defects in the millimeter region, and the excess carrier diffusion length is degraded by at least a factor of five with significant local variation. The optical and transport properties are affected over a significantly larger region than the observed topography and crystallographic and chemical compositions associated with the defect.
Despite being one of the oldest phenomena known to mankind and its vast use, there still are open questions about the frictional process between two surfaces, especially at the nanometer scale, such as the energy dissipation mechanism, the influence of the crystallographic orientation and the correlation between macroscopic and microscopic scales. In this work, we analyze the interaction between a sharp tip and graphene by friction force microscopy. The graphene surface roughness and adhesion forces with the microscope tip were measured. Neither roughness nor adhesion were observed to influence the friction forces. The scanning velocity dependence of friction was also measured for a different number of layers. The friction forces were observed to increase with the scanning velocity until a critical velocity is achieved by which we have estimated the effective interaction potential between the tip and the graphene surface.
Atomic force microscopy (AFM) and nanoindentation were used to characterize poly (methyl methacrylate) (PMMA) films with a wide distribution of pores. Pores with diameters ranging from tens of nanometers to few micrometers were measured by AFM and cross-section scanning electron microscopy (SEM). Atomic force acoustic microscopy (AFAM) mapping of the elastic modulus were correlated with the samples topography and pore distribution. The elastic moduli of the samples were additionally measured by nanoindentation.
Although the galaxies in our sample are heterogenous in their optical classifications (LLAGN: M51, NGC 3147, NGC 4258; LINER: NGC 3079, NGC 3310, NGC 3998, NGC 4579, NGC 4594; starburst: M82, NGC 253, NGC 3628, NGC 6946), they are fit well by a “canonical” spectrum with a hot, optically-thin thermal component with T ~ 8 × 106 K and an absorbed (NH ~ 1022 cm−2) power-law with an energy index α ~ 0.7–0.8. Both the “soft” component, most likely due to SN or superwind-heated ISM, and the “hard” power-law, most likely due to a micro-AGN and/or blackhole candidates, appear to be common in low-activity galaxies. If the soft component is associated with a superwind outflow, than ~ 10% of the X-ray emission is due to “swept-up” ISM rather than superwind emission. The abundance of Fe relative to α-process elements tends to be sub-solar, possibly due to dust-depletion and/or type-II SN enrichment. The lack of short-term variability in the hard component suggests that if it is due to an AGN, then the mode of accretion is probably fundamentally different from “normal” Seyfert galaxies.
We present X-ray observations of LINERs with ASCA. We detected a hard point-like source of X-ray luminosity of 1040–1041 erg s−1 at the nucleus. Their hard X-ray continuum is well represented by power-law of photon index ~ 1.8. The X-ray to Hα luminosity ratio LX/LHα is quite similar to Seyfert galaxies and strongly support the presence of low luminosity AGNs.
We present ASCA spectra of low luminosity AGNs with X-ray luminosities of 1040-41 ergs s−1. Their X-ray continua are very similar to Seyfert galaxies. Although iron K emissin centered at 6.4 keV or 6.7 keV is detected from a few objects, iron K emission is weak on average. Possible reasons for variety of iron line properties are discussed.
Lipids found associated with skeletons represent an important avenue of study in molecular paleontology, because the chemical nature of these compounds means they are often biogenically specific and can survive in the geologic record over millions of years. Though it is clear that these skeletal compounds originate from the mineralizing organism, our understanding of the distributions and nutritional sources of lipids among invertebrates is still limited, requiring that baseline studies of modern taxa be completed to evaluate the potential of skeletal lipids in paleoecological studies.
In this study, skeletal lipids from nine modern invertebrate taxa (five mollusks, a brachiopod, an echinoid, a coral, and a barnacle) were extracted and analyzed. Though the major classes of lipids found in the skeletons are remarkably consistent, the sterol profiles from these organisms contain both dietary and environmental information. The skeletons of filter feeders generally have two to three times the variety of sterols as compared with carnivores and grazers. In addition, specific steroidal contributions from several plant and algal groups (including dinosterol, 22-dehydrocholesterol, and β- and γ-sitosterol) can be identified. The limited nature and distribution of sterols in the carnivore skeleton implies that both sterol-recycling and some degree of molecular selectivity may be occurring in this taxon. Taxa living in nearshore shallow marine habitats have significant sterol contributions from higher plants, while deepwater organisms have relatively little. Since the geologically stable form of sterols often retains the three-dimensional structure necessary to recognize the biological precursor, these compounds may be used for determining the ecology of fossil invertebrates.
Surface reconstructions during homoepitaxial growth of GaN (0001) are studied using reflection high-energy electron diffraction and scanning tunneling microscopy. In agreement with previous workers, a distinct transition from rough to smooth morphology is seen as a function of Ga to N ratio during growth. However, in contrast to some prior reports, no evidence for a 2×2 reconstruction during GaN growth is observed. Observations have been made using four different nitrogen plasma sources, with similar results in each case. A 2×2 structure of the surface can be obtained, but only during nitridation of the surface in the absence of a Ga flux.
The operating regimes of two rf-plasma sources, an Oxford CARS-25 and an EPI Unibulb, have been extensively characterized. By changing the exit aperture configuration and using an electrostatic deflector, the Oxford source could produce either primarily atomic nitrogen, atomic nitrogen mixed with low energy ions, or a large flux of higher energy ions (>65 eV) as the active species in a background of neutral molecular nitrogen. The EPI source produced a significant flux of metastable molecular nitrogen as the active species with a smaller atomic nitrogen component. Nitridation of sapphire using each source under the various operating conditions indicate that the reactivity was different for each type of active nitrogen. Boron contamination originating from the pyrolytic boron nitride plasma cell liner was observed.
Photoconductance decay and spectral photoconductance measurements were made on a set of ten undoped layers of GaN grown by rf-plasma MBE. The layers, also characterized by Hall, photoluminescence and reflectance measurements, represented a wide variety in electrical and optical properties, and several were grown under atomic hydrogen. Spectral photoconductance indicated transitions at 1.0-1.1, 1.92, 2.15, 3.08 and 3.2-3.4 eV. All layers exhibited persistent photoconductivity to some degree. In contrast with previous reports, a clear correlation was not observed between persistent photoconductivity and yellow luminescence or, indeed, with any measurement made. Analysis of photoconductance decay indicates that more than one type of persistent photoconductivity may be present.
Evidence is presented that nitrogen plasma sources utilizing a pyrolytic boron nitride liner may be a significant source of B contamination during growth and processing. Auger electron spectroscopy analysis performed during nitridation of sapphire indicate the resulting layers contain a significant amount of BN. The formation of Al1-xBxN would explain the observation of a lattice constant several percent smaller than AlN as measured by reflection high-energy electron diffraction. The presence of cubic inclusions in layers grown on such a surface may be related to the segregation of BN during the nitridation into its cubic phase.
A synthesis of the upper Moscovian sedimentological and palaeontological record of terrestrial habitats across the Variscan foreland and adjacent intramontane basins (an area which is referred to here as Variscan Euramerica) suggests a contraction and progressive westward shift of the coal swamps. These changes can be correlated with pulses of tectonic activity (tectonic phases) resulting from the northwards migration of the Variscan Front. This tectonic activity caused disruption to the landscapes and drainage patterns where the coal swamps were growing, which became less suitable to growth of the dominant plants of the swamps, the arborescent lycopsids. They were progressively replaced by vegetation dominated by marattialean ferns, which through a combination of slower growth and larger canopies resulted in less evapo-transpiration. This in turn caused localised reductions in rainfall, which further affected the ability of the lycopsids to dominate the swamp vegetation. These changes were initially localised and where the coal swamps were able to survive the lycopsids and pteridosperms show little change in either species diversity or biogeography, indicating that at this time there was minimal regional-scale climate change taking place. By Asturian times, however, the process had accelerated and the swamps in Variscan Euramerica became progressively replaced by predominantly conifer and cordaite vegetation that favoured much drier substrates. Except in localised pockets in intramontane basins of the Variscan Mountains, the last development of coal swamps in Variscan Euramerica was of early Cantabrian age. Further west, lycopsid-dominated coal swamps persisted for a little longer. The last remnants of the lycopsid-dominated coal swamps in the Illinois Basin disappeared probably by middle-late Cantabrian times, as the cycle of contracting wetlands and regional reductions in rainfall generated its own momentum, and no longer needed the impetus of tectonic instability. This tectonically-driven decline in the Euramerican coal swamps was probably responsible for an annual increase in atmospheric CO2 of c. 0.37 ppm, and may have been implicated in the marked increase in global temperatures near the Moscovian – Kasimovian boundary, and the onset of the Late Pennsylvanian interglacial.