Alan S. Lea

Clusters and magnetism in epitaxial Co-doped TiO2 anatase

We show that under certain conditions, highly Co-enriched TiO2 anatase clusters nucleate on epitaxial TiO2 anatase grown on LaAlO3(001) by oxygen plasma assisted molecular beam epitaxy. In the most extreme cases, virtually all incident Co segregates to the clusters, yielding a nanoscale ferromagnetic phase that is not ferromagnetic in homogeneous films of the same Co concentration. The nucleation of this phase simultaneous with continuous epitaxial film growth must be carefully monitored in order to avoid drawing false conclusions about the film structure.

Role of extracellular polymeric substances in bioflocculation of activated sludge microorganisms under glucose-controlled conditions

Extracellular polymeric substances (EPS) secreted by suspended cultures of microorganisms from an activated sludge plant in the presence of glucose were characterized in detail using colorimetry, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. EPS produced by the multi-species community were similar to literature reports of pure cultures in terms of functionalities with respect to C and O but differed subtly in terms of N and P. Hence, it appears that EPS produced by different microorganisms maybe homologous in major chemical constituents but may differ in minor components such as lipids and phosphodiesters. The role of specific EPS constituents on microbial aggregation was also determined. The weak tendency of microorganisms to bioflocculate during the exponential growth phase was attributed to electrostatic repulsion when EPS concentration was low and acidic in nature (higher fraction of uronic acids to total EPS) as well as reduced polymer bridging. However, during the stationary phase, polymeric interactions overwhelmed electrostatic interactions (lower fraction of uronic acids to total EPS) resulting in improved bioflocculation. More specifically, microorganisms appeared to aggregate in the presence of protein secondary structures including aggregated strands, beta-sheets, alpha- and 3-turn helical structures. Bioflocculation was also favored by increasing O-acetylated carbohydrates and overall C-(O,N) and O=C-OH+O=C-OR functionalities.

Epitaxial growth and properties of MBE-grown ferromagnetic Co-doped TiO2 anatase films on SrTiO3(001) and LaAlO3(001)

Abstract We have investigated the heteroepitaxial growth and materials properties of pure and Co-doped TiO 2 anatase on SrTiO 3 (001) and LaAlO 3 (001), grown by oxygen plasma assisted molecular beam epitaxy. This material is a promising new diluted magnetic semiconductor that shows large magnetization and a Curie temperature well above room temperature. We have found that epitaxial films with the highest crystalline quality and most uniform distribution of Co result when a rather slow growth rate (∼0.01 nm/s) is used over a substrate temperature range of 550–600 °C on LaAlO 3 (001). These conditions result in layer-by-layer growth of single-crystal films and a very low density of extremely small nanocrystalline inclusions. In contrast, growth at a higher rate (∼0.04 nm/s) leads to extensive formation of secondary-phase rutile nanocrystals to which Co diffuses and segregates. The rutile nanocrystals nucleate on the evolving anatase film surface in such a way that lattice strain between the two phases is minimized. Cobalt appears to substitute for Ti in the lattice and exhibits a +2 formal oxidation state. Both pure and Co-doped films can be grown as n-type semiconductors by controlled incorporation of oxygen atom vacancies. Free electrons are required to couple the Co(II) spin to a ferromagnetic state.

Structure of the cleaved CaCO3(101̄4) surface in an aqueous environment

Abstract The surface structure of the cleaved (1014) face of calcite single crystal in an aqueous environment has been studied using atomic force microscopy. Force images acquired under different solutions and forces show structures consistent with only slight relaxation of the structure of the terminated bulk crystal. Atomic scale images are interpreted in terms of the 1 × 1 symmetry from the top-lying surface oxygen atoms. A variation of atomic corrugations within the surface unit cell suggests different relaxations of carbonate groups on the surface.

Comparison of the sputter rates of oxide films relative to the sputter rate of SiO2

There is a growing interest in knowing the sputter rates for a wide variety of oxides because of their increasing technological importance in many different applications. To support the needs of users of the Environmental Molecular Sciences Laboratory, a national scientific user facility, as well as our research programs, the authors made a series of measurements of the sputter rates from oxide films that have been grown by oxygen plasma-assisted molecular beam epitaxy, pulsed laser deposition, atomic layer deposition, electrochemical oxidation, or sputter deposition. The sputter rates for these oxide films were determined in comparison with those from thermally grown SiO2, a common reference material for sputter rate determination. The film thicknesses and densities for most of these oxide films were measured using x-ray reflectivity. These oxide films were mounted in an x-ray photoelectron or Auger electron spectrometer for sputter rate measurements using argon ion sputtering. Although the primary objec...

Spectroscopic characterization of extracellular polymeric substances from Escherichia coli and Serratia marcescens: suppression using sub-inhibitory concentrations of bismuth thiols.

Free and bound (or capsular) EPS produced by suspended cultures of Escherichia coli and Serratia marcescens were characterized in detail using colorimetric analysis of total proteins and polysaccharides, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) in the presence and absence of bismuth-based antifouling agents. Subtle differences in the chemical composition of free and bound EPS were observed for both bacteria in the absence of bismuth. Total polysaccharides and proteins in free and bound EPS decreased upon treatment with subinhibitory concentrations of lipophilic bismuth thiols (bismuth dimercaptopropanol, BisBAL; bismuth ethanedithiol, BisEDT; and bismuth pyrithione, BisPYR), with BisBAL being most effective. Bismuth thiols also influenced acetylation and carboxylation of polysaccharides in EPS from S. marcescens. Extensive homology between EPS samples in the presence and absence of bismuth was observed with proteins, polysaccharides, and nucleic acids varying predominantly only in the total amount produced. Second derivative analysis of the amide I region of FTIR spectra revealed decreases in protein secondary structures in the presence of bismuth thiols. Hence, antifouling properties of bismuth thiols appear to originate in their ability to suppress O-acetylation and protein secondary structure formation in addition to free and bound EPS secretion.

Growth and properties of molecular beam epitaxially grown ferromagnetic Fe-doped TiO2 rutile films on TiO2(110)

We have grown epitaxial Fe-doped TiO2 rutile films on rutile TiO2(110) substrates, and have explored the resulting compositional, structural, morphological and magnetic properties. Clusters of mixed TiO2 rutile and Fe3O4 form on the surface of a continuous rutile epitaxial film during growth. Room-temperature ferromagnetism is observed, and is associated with the formation of secondary phase Fe3O4 rather than a true diluted magnetic oxide semiconductor.

Heteroepitaxial growth of a manganese carbonate secondary nano-phase on the (1 0 1̄ 4) surface of calcite in solution

Heteroepitaxy of a manganese carbonate phase with nanometer dimensions on the (101{sub 4}) surface of calcite (CaCO3) using an AFM has been observed in solution during dissolution of calcite when the ion activity product of Mn2? and CO32- nears the solubility limit of MnCO3. Growth-rate observations at different Mn concentrations, coupled with XPS and EPR measurements, suggest that the resulting phase is Mn05Ca05CO3. These islands, while growing many microns in length along the[22{sub 1}] direction, have a uniform width in the range of 120-240 nm and a uniform height of approximately 2.7 nm, corresponding to nine atomic layers. The islands cease growing when they encounter step edges and have been observed to dissolve when undercut by a growing etch pit.

Co-doped anatase TiO2 heteroepitaxy on Si(001)

Pure anatase TiO2 and CoxTi1−xO2(0.01<x<0.04) epitaxial thin films were deposited by oxygen-plasma-assisted molecular beam epitaxy on Si(001) for evaluation as a potential dilute magnetic semiconductor material suitable for Si-based spintronic devices. Epitaxial growth on Si(001) was facilitated by the deposition of 1∕4 or 1∕2ML Sr metal on the clean Si(001) surface to form an oxidation resistant silicide layer, followed by deposition of a thin SrTiO3 buffer layer. Using 1∕2ML Sr metal to form the silicide allowed the deposition of 10 ML SrTiO3 without oxidation of the Si interface. Epitaxial anatase could be grown on this heterostructure, although use of the oxygen plasma during deposition resulted in significant SiO2 formation. Pure anatase films consisted of epitaxial anatase surface particles on a continuous anatase film. For Co-doped films, Co segregation to surface particles of epitaxial anatase was observed by Auger electron spectroscopy and transmission electron microscopy (TEM); faceting of the p...

Molecular Depth Profiling of Sucrose Films: A Comparative Study of C60n+ Ions and Traditional Cs+ and O2+ Ions

Depth profiling of sucrose thin films was investigated with time-of-flight secondary ion mass spectrometry (TOF-SIMS) using 10 keV C(60)(+), 20 keV C(60)(2+), and 30 keV C(60)(3+), and 250, 500, and 1000 eV Cs(+) and O(2)(+) as sputtering ions. With C(60)(n+) ions, the molecular ion signal initially decreases and reaches a steady state that is about 38-51% of its original intensity, depending on the energy of the C(60)(n+) ions. In contrast, with Cs(+) and O(2)(+) sputtering, molecular ion signals decrease quickly to the noise level, even using very low-energy (250 eV) ions. In addition, the measured width of the sucrose/Si interface is much narrower using C(60)(+) ions than that using Cs(+) or O(2)(+) ions. To understand the mechanisms of sputtering-induced damage by these ions, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to characterize the bottoms of these sputter craters. XPS data show very little chemical change in the C(60)(+) sputter crater, while considerable amorphous carbon was found in the O(2)(+) and Cs(+) sputter craters, indicating extensive decomposition of the sucrose molecules. AFM images show a very flat bottom in the C(60)(+) sputter crater, while the bottoms of the Cs(+) and O(2)(+) sputter craters are significantly rougher. We used the sputtering model developed by Wucher and co-workers to quantitatively analyze our C(60)(1-3+) data. The results show that low energy C(60)(+) ions generate a relatively thin damage layer with a high molecular ion signal, suggesting that low energy C(60)(+) may be the optimal choice for molecular depth profiling of sucrose films.