Monolayer Level Research Articles

On the defect structure of grain boundaries in Ba 2YCu 3O 7– x

Grain growth habits and the structure of grain boundaries have been investigated using transmission electron microscopy and scanning Auger microscopy of bulk sintered samples of orthorhombic Ba 2YCu 3O 7– x . This material exhibits typical transport critical currents of 40–500 A/cm 2 at 77 K and R = 0 T c' s near 90 K. All grains of the correct phase have the characteristic reflection twin lamellar structure, but exhibit one of two growth habits: (1) small (1–5 μm diameter) equiaxed grains, and (2) larger (3–40 μm diameter) platelet-shaped grains with an aspect ratio between the twinned a – b plane dimension and the short c-axis dimension on the order of 6. Grain boundary structure falls into one of three classes: (1) tight boundaries with only a network of dislocations at the interface, characteristic of equiaxed grains and boundaries of platelet grains not involving the c-axis boundary, (2) basal-plane-faced boundaries of platelets involving introduction of dislocation loops on the c-axis side of the boundary only, with the depth of the penetration of the dislocation loops (50–1500 Å) varying with degree of local strain, and (3) basal-plane-faced boundaries of platelets where severe local strain has produced substantial plastic deformation and generation of small voids, again with the depth of the boundary region a function of local strain. We propose a model for these observations based on the highly anisotropic thermal contraction of the material during cooling from the high sintering temperatures. Scanning Auger microscopy of in situ fractured surfaces reveals segregation of carbon at the grain boundaries at 1–4 monolayer level. These structures span the spatial range likely to cause weak-link coupling between grains as the source of low transport J c in polycrystalline bulk material.

GaAs/AlGaAs quantum well lasers with active regions grown by atomic layer epitaxy

Atomic layer epitaxy (ALE) is a relatively new crystal growth technique which allows control of the growth process at the monolayer level through a self-limiting, surface-controlled growth mechanism. We report here the use of ALE to grow high-quality GaAs/AlGaAs quantum wells and the first successful demonstration of an injection laser with a quantum well active region grown by ALE. Room-temperature threshold current densities as low as 640 A/cm2 have been achieved in nonoptimized separate confinement structures.

MONTE-CARLO SIMULATION OF EPITAXIAL GROWTH : GexSi(1-x)/Si INTERFACES

MONTE-CARLO SIMULATION OF EPITAXIAL GROWTH : Ge_xSi_(1-x)/Si INTERFACES

Eelfs study of oxygen chemisorption on cobalt

The chemisorption of oxygen on the (112̄0) surface of a cobalt single crystal at monolayer level and the oxide formed on a polycrystalline sample was studied by EELFS. From the radial distribution functions the distances of the neighbors of the oxygen atom are derived and the reliability of the technique for the oxygen adsorption is discussed.

Adsorption of benzene on the fresh steel surface formed by cutting under high vacuum

The adsorption of benzene on a fresh steel surface was carried out in high vacuum at room temperature. The fresh surface was formed by cutting in a high vacuum chamber, and benzene was allowed to adsorb on the surface during cutting. The adsorption and the surface reaction of benzene was monitored with a quadrupole mass spectrometer. It was found that the cut surface was so active that the catalytic decomposition of benzene occured even at room temperature. The adsorption rate and the sticking probability of benzene were estimated. The adsorption rate linearly increased with cutting speed, that is, the formation rate of the fresh steel surface. It was found that the adsorbed amount was under the monolayer level, and that the adsorption activity of toluene on the fresh surface was about two times larger than that of benzene.

The chemisorption of chlorine on zirconium: Neutral and negative ion thermal desorption spectroscopy

Chloride growth on zirconium is investigated by means of the novel thermal desorption spectroscopy of both neutral and negatively ionized products, and interpreted in terms of a single ionic adsorption model. The monohalide desorption energy decreases from a low-coverage value of 320 kJ to 255 kJ mol −1 at the monolayer level, at which point trihalide growth continues indefinitely, indicative of a corrosion layer. In contrast, the desorption energy for the chloride ion remains essentially independent of coverage, at 293 kJ mol −1

Growth of microstructures by molecular beam epitaxy

Molecular beam epitaxy is the most widely currently used technique for the growth of semiconductor microstructures. Multilayers with thicknesses and smoothness controlled near the monolayer level are being produced, including, recently, quantum wells with special shapes, quantum wells to which electric fields may be applied, new structures with enhanced carrier mobilities, structures for tunneling injection of carriers, and possible structures for achievement of quantum wires and dots. New crystal systems and new growth techniques are extending the range of accessible microstructures.

EFFECT OF WATER ACTIVITY ON TEXTURE PROFILE PARAMETERS OF APPLE FLESH

ABSTRACTFresh apple with a water activity of approximately 0.99 was partially dehydrated, then equilibrated to nine water activity levels down to Aw 0.01. The water desorption isotherm, for apple is given. Instrumental texture profile analysis was performed at each Aw level. As the Aw decreased, hardness increased slowly, then rapidly at Aw 0.12 then decreased again at Aw 0.01; deformation first increased sharply then steadily decreased; springiness increased to Aw 0.33 then decreased; cohesiveness increased down to Aw 0.65, held steady to Aw 0.33 then decreased; gumminess and chewiness increased down to Aw 0.12 then decreased. Fracturability was present only at the extremes (Aw 0.99 and 0.01) and was absent at all other water activity, levels. Complex changes in textural properties occur as apple is dehydrated and most of these changes occur most rapidly near the BET monolayer level of water activity.

Monolayer and multilayer growth of Cu on the Ru(0001) surface

The adsorption and growth of Cu films on the Ru(0001) surface were studied by work function measurements, low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal programmed desorption (TPD). The results indicate that for submonolayer depositions at 100 K the Cu grows in a dispersed mode forming 2D islands pseudomorphic to the Ru(0001) substrate upon annealing to 300 K. This behavior is seen to continue to the 1 monolayer (ML) level. Additional Cu deposition to 2 ML shows a similar 2D island growth but with an epitaxial Cu(111) structure. Subsequent annealing in both these cases to 900 K enhances the 2D character of the films but does not affect the overall structure. AES and LEED results show that a 900 K anneal of Cu films in excess of 2 ML leads to three-dimensional Cu(111) island formation exposing areas of the surface covered by the original Cu bilayer — one pseudomorphic and one epitaxial. The effects of Cu on the chemisorptive properties of Ru(0001) toward CO were also studied by TPD. It was found that Cu attenuates the CO adsorption relative to the open Ru(0001) sites on approximately a one-to-one basis. In addition, at the 1 ML level the TPD spectrum shows features which are intermediate between those for the tightly bound CO/Ru system and the weakly bound CO/Cu case. A feature in the TPD spectra of CO on submonolayer Cu deposits is identified with mixed Cu/Ru sites, i.e. at the 2D Cu island edges, and allows an estimate of the 2D Cu island sizes to be made. The results and conclusions of this study differ markedly from previous single-crystal studies but are consistent with recent observations of Cu adsorbed onto an epitaxial Ru(0001) film grown on a Mo(110) surface.

Graphite precipitation on sulfur covered cobalt; Models and mechanism

The effect of an adsorbed monolayer of sulfur on the precipitation of graphite on the surface of cobalt has been quantitatively determined and interpreted. Several possible nucleation and growth models for the graphite islands are examined. The only model which is compatible with all the experimental results assumes a nucleation already complete at the beginning of the growth and a determinant role of carbon diffusion from the bulk. The sulfur effect may be correlated with the structural knowledge of carbon-sulfur interactions at the monolayer level.

Surface sensitive spectroscopic study of the interaction of oxygen with Al(111) - low temperature chemisorption and oxidation

The interaction of an atomically clean Al(111) surface with O 2 has been studied using a combination of electron energy loss spectroscopy (EELS) and Auger spectroscopy (AES). Oxygen dissociatively adsorbs and occupies both surface and subsurface binding sites under all exposure conditions in the temperature range 122–700 K. Surface sites are preferentially occupied at low exposures, while higher exposures increasingly favor population of subsurface sites. Studies of O 2 adsorption at temperatures as low as 131 K have shown that formation of Al 2O 3 occurs at high oxygen exposures. The Al 2O 3 produced exhibits a 54 eV Auger transition and a characteristic vibrational spectrum with loss features at 430, 645, and 880 cm −1. Argon ion bombardment of thin monolayer level Al 2O 3 layers leads to preferential loss of Al 2O 3 and a reduction in the subsurface-to-surface oxygen ratio. Electron bombardment of similar, thin Al 2O 3 layers is ineffective in inducing desorption of surface species, whereas thick Al 2O 3 layers are strongly influenced by electron bombardment, as judged from AES behavior. Qualitative models for O 2 adsorption, oxidative annealing, and damage by ion and electron bombardment are given.

Photochemical deposition of Sn for use in molecular beam epitaxy of GaAs

The suitability of several Sn-containing molecules (tetramethyltin, tetrabutyltin, dibutyltin dibromide, and stannic chloride) as gas phase sources of Sn for use in molecular beam epitaxy of GaAs was evaluated. In situ Auger electron spectroscopy showed that the Sn molecules absorbed at approximately the monolayer level at room temperature on GaAs (001) epilayer surfaces and pyrolytically decomposed before desorption. Capacitance–voltage and secondary ion mass spectrometry profiles demonstrated that the pyrolytic Sn deposits incorporated as an n-type dopant into newly regrown GaAs epilayers. Sn films were ultraviolet (UV) photolytically deposited on GaAs from each of these Sn-containing molecules and characterized by x-ray photoelectron spectroscopy.

The interface region in squeeze-infiltrated composites containing ?-alumina fibre in an aluminium matrix

This paper examines the nature of the interfaces formed between fine,δ-alumina fibres and aluminium-based matrices. Alloys containing magnesium and silicon have been employed, as well as high-purity aluminium. The fabrication process involved high-pressure melt infiltration of a fibrous preform. It has been found that no detectable interfacial reaction product is formed in these systems, although there is in some cases evidence of a limited exchange of magnesium between melt and fibre surface. It is concluded, both from measurements and from process analysis, that any distinct chemical reaction product formed at the interface between fibre and matrix must be limited in extent to the atomic monolayer level.

Surface structure imaging by electron microscopy

SUMMARY Reconstructed structures at monolayer level on ‘clean and well-defined’ surfaces can be imaged by transmission electron microscopy in fixed beam illumination mode. The specimens are cleaned in-situ in the electron microscope in ultra high vacuum. Transmission electron diffraction pattern intensities can give useful information for determining the surface unit cell size of the structure, and the atom positions (geometric arrangement of atoms in the unit cell) especially those with a large unit cell, since the diffraction intensities are interpreted kinematically. High resolution surface imaging which gives directly the atom positions is tested here for a single monolayer terrace on Ag (111) surface. The result shows the value of HREM for studies of surface crystallography.

Characterization of supported Vanadium oxide catalysts by low temperature oxygen chemisorption technique: I. The V 2O 5/γ-Al 2O 3 system

Low temperature oxygen chemisorption (LTOC) has been applied to characterize γ-Al 2O 3 supported vanadium oxide catalysts. The results strongly suggest the formation of a monolayer with high dispersion of V-oxide on the alumina surface. The monolayer coverage of the surface is completed when the V 2O 5 content of the catalysts reach 11.9% and beyond this loading crystallites of V 2O 5 start to form on the alumina surface. Within the monolayer level the active surface area and active site density are found to increase with increased vanadia loading, while those in the ‘post monolayer’ region show a sharp decline as the load increases. The dispersion of vanadia remains essentially constant within the monolayer level and decreases almost linearly as a function of V 2O 5 loading in the post monolayer region. Information gathered from other techniques, including ESR, IR and surface area measurements, substantiate the results of LTOC experiments. Thus, LTOC technique is found to be a valuable tool for characterizing supported oxidation catalysts which are identified with redox properties.

Adsorption of Lubricating Oil Components on the Cut Steel Surface

The adsorption of model compounds of lubricating oil components on the cut steel surfaces was carried out under high vacuum. The estimated amount of adsorption was less than a monolayer level. The adsorption of nitrogen and n-hexane was not observed. The other adsorbates used were divided into two groups according to their adsorption activity on the cut surface. Benzene, cyclohexene and diethyl disulfide exhibited a high adsorption activity, while, propionic acid, propyl amine and trimethyl phosphate exhibited comparatively low activity. It is noteworthy that the functional group of the latter compounds was characterized by a lower polarizability than that of the former. The results were discussed on the basis of HSAB (Hard and Soft Acids and Bases) principle.

Surface and interface nitridation of thin polycrystalline silicon films

Polycrystalline silicon thin films (about 800 Å thick) on silicon dioxide, annealed in ammonia at 1160 °C for various times, have been analyzed using Auger electron spectroscopy in combination with argon ion sputtering and Rutherford backscattering spectrometry. Nitridation results in the growth of surface nitride layers of the same thickness as on monocrystalline silicon. However, nitridation times longer than 30 min give rise to an additional (oxy)nitride formation at the polysilicon/silicon dioxide interface. The degree of coverage of the grain boundary surface by nitrogen is estimated to be well below monolayer level.

Preparation of an RuO 2-modified semiconductor oxide surface — an optimization procedure of interest in energy conversion systems

A simple procedure is outlined for modifying the surface of a semiconducting or insulating oxide powder (such as TiO 2) by the adsorption of RuCl 3 from isopropanol solution. Adsorption at monolayer and submonolayer level was readily achieved, 1 g of ruthenium being uniformly dispersed as the trichloride or dioxide over 9000 m 2 (corresponding, in the present case, to about 250 g of oxide powder) of the surface of a TiO 2 support. Heating of the chloride-coated powder in air at 425 °C resulted in the formation of an RuO 2-modified surface. The surface-bonded RuO 2 groups were shown to be active with respect to catalytic oxidation reactions, and repeated runs with the same catalyst confirmed the stability of the RuO 2 linkage to the oxide support. The ability to disperse noble metals in this simple manner, and to recover the catalyst after use by a simple filtration technique, is of considerable importance in solar energy conversion and catalytic processes.

Interfacial Solute Segregation Observed Using X-Ray Microanalysis in the TEM/STEM

Solute segregation occurring at grain boundaries or interphase interfaces can take the form of Gibbsian (equilibrium) segregation or segregation associated with phase transformations occurring at the interface. Gibbsian segregation results in monolayer level segregation and has been associated with the cause of temper embrittlement in metals and the formation of thin amorphous grain boundary films in powder-compacted ceramics. To date the most successful technique for its study has been Auger electron spectroscopy (AES) which possesses the necessary surface resolution but has the specific drawbacks of UHV operation, the need for intergranular fracture in-situ and poor image resolution. Solute segregation associated with grain boundary and interphase interface phase transformations occasionally occurs over distances > 1 μm and hence is detectable by conventional electron microprobe microanalysis. However the properties of alloys are more often than not governed by sub-micron precipitation and associated composition profiles. To date quantitative microanalysis of such phenomena has only been achieved using plasmon energy loss microanalysis(1) which unfortunately is only applicable to low Z alloys such as Al and Mg base with well defined plasmon loss spectra.

INFLUENCE OF WATER ACTIVITY ON PELLICLE CHAFFING, COLOR AND BREAKAGE OF WALNUT (JUGLANS REGIA) KERNELS

The moisture sorption charcteristics of walnut kernel at 27 and 37 degree C and that of the pellicle at 27 degree C were determined. The equilibrium moisture content of the pellicle at water activities studied was much higher than that of the kernel, however, its water activity at the BET monolayer moisture level was close to that of the kernel. Storage studies of the kernels equilibrated to different RH indicate that the kernel colour and flavour, as indicated by TBA value, were more stable at a moisture content of 3.2 to 3.6% rather than at the monolayer level of 2.2%. Vibration and drop tests on kernels equilibrated to different RH's indicated that pellicle chaffing and kernel breakage decreased with water activity of the kernel particularly above 58% RH. Pellicle chaffing which occurs mostly during handling and transportation of the kernels can be minimized by maintaining the kernels around 58% RH during handling and transportation. Subsequently the moisture content can be brought down to 3.2 to 3.6%, which is the optimum range for storage of kernels.