Magnesium Precursors Research Articles

Polydentate Amines as CVD Precursor Ancillary Ligands. Epitaxial MgO Thin-Film Growth Using a Highly Volatile, Thermally and Air-Stable Magnesium Precursor

A superior monomeric MOCVD precursor for epitaxial MgO growth is described.The complex (see Figure) can be readily prepared using commercially available reagents via a simple aqueous route under ambient laboratory conditions. Clean and complete volatilization with no loss of coordinated ligands is observed by TGA and high-quality epitaxial films were grown using the precursor.

Growth and microstructure of MgO thin films on Si(100) substrates by metal–organic molecular beam epitaxy

MgO thin films have been grown on Si(100) substrates at low temperatures of 500–850°C by metal–organic molecular beam epitaxy (MOMBE) using the solid precursor magnesium acetylacetonate. Oxygen plasma is required to achieve deposition. The composition of the films was determined by Auger electron spectroscopy (AES). The as-deposited films are phase-pure, stoichiometric, crystalline MgO with a [100] texture. Carbon contamination of the film resulting from precursor decomposition was not observed within detection limits. The deposition rate depended superlinearly on the plasma source power.

Epitaxial thin films of MgO on Si using metalorganic molecular beam epitaxy

Epitaxial cubic MgO thin films have been deposited on single crystal Si(001) substrates by metalorganic molecular beam epitaxy. The Mg source was the solid precursor magnesium acetylacetonate and a rf excited oxygen plasma was the oxidant. The growth process involved initial formation of an epitaxial β-SiC interlayer followed by direct deposition of a MgO overlayer. The films were characterized by in situ reflection high energy electron diffraction, x-ray diffraction, conventional and high resolution transmission electron microscopy, atomic force microscopy, Auger electron spectroscopy, and Fourier transform infrared spectroscopy. The β-SiC interlayer had an epitaxial relationship such that SiC(001)∥Si(001) and SiC [110]∥Si [110]. The SiC interlayer showed a columnar grain structure with planar defects including twin bands and stacking faults. The MgO overlayer showed an epitaxial relationship given by MgO(001)∥Si(001) and MgO[110]∥Si[110]. No evidence of twins in the MgO layers was observed.

An XRD, XPS, and EPR Study of Li/MgO Catalysts: Case of the Oxidative Methylation of Acetonitrile to Acrylonitrile with CH4

The overall purpose of this study was to investigate the effects of the magnesium and lithium precursors and the catalyst surface properties on the catalytic performance in the oxidative methylation of acetonitrile. The performance of the catalysts for the oxidative methylation of acetonitrile to acrylonitrile was significantly affected by the Li precursor, where catalysts prepared with LiCl and LiOH on MgO had the best performance for this reaction. The catalysts' activity was virtually unaffected by the source and surface area of the MgO. In contrast, relatively high BET surface areas appeared to have a negative affect on the catalysts' performance by producing less acrylonitrile and more COx. XPS and XRD analyses of Li/MgO-based catalysts indicated that the lithium salts used in the catalyst synthesis predominantly formed mixtures with MgO. It was found that the Li:Cl atomic ratio was 1:1 in catalysts prepared with LiCl, even after calcination at 650°C for 14 h in air. XRD analysis of LiCl/MgO catalysts calcined at 650°C suggests that some amount of LiCl precursor on MgO decomposes into Li+O− (Li2O2). The latter species are believed to be responsible for the effective transformation of methane and acetonitrile to acrylonitrile. The binding energy of Li 1s ranges between 57.0 and 52.3 eV. Very interestingly, the binding energy of the Li 1s peak observed at 57.0 eV in LiCl/MgO (C1–C6) and LiOH/MgO (C9) catalysts corresponds to Li+O− (Li2O2). This peak was negligible for Li2CO3/MgO (C7 and C8) and LiNO3/MgO (C10) catalysts. Our proposal for this XPS peak of Li+O− species is supported by the EPR peak at g⊥=2.0544. To the best of our knowledge there is no previous communication of the Li 1s peak which corresponds to Li+O− determined by XPS. Catalytic experiments for the oxidative methylation of acetonitrile to acrylonitrile over the aforesaid catalysts indicate that indeed the catalysts synthesized with LiCl and LiOH promote the desired reaction, or equivalently they are associated with Li+O− species. A direct relation between the EPR peak at g⊥=2.0544 and the lack of CO was established. Although the LiCl/MgO catalysts' performance was better than that of other catalysts prepared in this study, it was also the least stable. Observations made during these studies suggest that lithium sublimation occurs, thus resulting in catalyst deactivation. It is remarkable to note that catalysts synthesized with LiNO3 demonstrated superior thermal stability, even under severe calcination conditions.

Metalorganic Molecular Beam Epitaxy of Magnesium Oxide on Silicon

ABSTRACTEpitaxial cubic MgO thin films were deposited on single crystal Si (001) substrates by metalorganic molecular beam epitaxy (MOMBE) using the solid precursor magnesium acetylacetonate as the source and an RF excited oxygen plasma as the oxidant. The growth process involved initial formation of an epitaxial β-SiC interlayer followed by direct deposition of a MgO overlayer. The film structure was characterized by X-ray diffraction as well as conventional and high-resolution transmission electron microscopy. Both the MgO overlayer and β-SiC interlayer had an epitaxial relationship such that MgO (001) (or SiC (001)) // Si (001) and MgO [110] (or SiC [110])// Si [110]. No evidence of an amorphous layer was observed at either the MgO/SiC or SiC/Si interface. Dielectric properties of the epitaxial MgO thin films on Si (001) were evaluated from capacitance-voltage (C-V) characteristic of metal-oxide-semiconductor (MOS) structures. The C-V measurements indicated an interface trap density at midgap as low as 1011 to 1012 cm−2 eV−1 and fixed oxide charge of the order of 1011/ cm2, respectively. These results indicate that epitaxial MgO deposited by MOMBE has potential as a gate insulator.

A Cobalt Aluminophosphate with Encapsulated Ethylenediamine: Co0.65Al0.35PO4.0.5NH2.65CH2CH2NH2.65

The title compound, cobalt aluminium phosphate 0.5-ethylenediamine, is a further example of the partial replacement of aluminium by cobalt during hydrothermal-templated synthesis. The four-connected framework has the zeolite gismondine topology, being similar to the MAPO-43 precursor magnesium aluminophosphate (Al6,Mg2)P8O32.2NC6H16 [Pluth et al. (1989). J. Am. Chem. Soc. 111, 1692–1698]. The structure can be described as an occupancy modulation of an I41/a parent structure; the ethylenediamine disorder is modelled successfully in monoclinic space group I2/c.

MOVPE growth of magnesium cadmium sulphide: rocksalt or sphalerite?

Abstract Epitaxial MgS has been grown by the MOVPE process using bis(methylcyclopentadienyl)magnesium and tertiarybutylthiol precursors on both GaP and GaAs substrates. For substrate temperatures between 450 and 550°C epitaxial layers possessing the rocksalt (NaCl) crystal form were produced. At lower temperature polycrystalline layers resulted. The absence of the previously reported sphalerite (zincblende) phase can be attributed to a combination of the high substrate temperatures employed and the choice of sulphur precursor rather than the effect of substrate mismatch. Additionally dimethyl cadmium was used to produce magnesium cadmium sulphide layers. Low concentrations of cadmium (x ≤ 0.08) resulted in homogeneous Mg1−xCdxS alloy epilayers. For x ≥ 0.14 polycrystalline material was produced. Similar isovalent Mg doping of hexagonal Cd1−yMgyS produced an upper limit of y ≤ 0.18 for single crystal alloy growth.

Crystallographic Orientations of MgO Films Prepared by Plasma-Enhanced Metalorganic Chemical Vapor Deposition

Thin films of magnesium oxide (MgO) were prepared on glass substrates by plasma-enhanced metalorganic chemical vapor deposition (PE-MOCVD) and the relationship between deposition conditions and crystallographic orientations were studied. Highly crystalline and highly (100)-oriented MgO film was obtained at the vaporizing temperature of 210° C, O2 flow rate of 200 cm3· min-1, rf power of 400 W and substrate temperature of 400° C. The orientation of MgO films is changed from (100) to (110) upon increasing the vaporizing temperature from 210 to 240° C or upon decreasing the O2 flow rate from 200 to 50 cm3· min-1. These results indicated that the films deposited at the high arrival ratios of oxygen to magnesium precursor (O/Mg) onto the substrate had a (100) orientation, while those deposited at the low arrival ratios of O/Mg had a (110) orientation. The intrinsic stress of the (100) oriented film was tensile and the magnitude was 0.25 GPa.

The relationship between catalyst morphology and performance in the oxidative coupling of methane

A detailed study of the oxidative coupling of methane over magnesium oxide and lithium-doped MgO catalysts is presented. The morphology of a number of different catalysts has been examined by detailed transmission electron microscopy and the results have been correlated with catalyst performance, in particular selectivity to C 2 hydrocarbons. Three samples of magnesium oxide have been prepared by different methods. Magnesium oxides prepared from the hydroxide, (ex OH), and from burning magnesium ribbon in air show similar morphology, exposing largely {100} planes; they also show very similar catalytic selectivity and specific activity. The ribbon residue material, however, has a cube length which is greater than that of the ex OH material by a factor of 5–10. Steps and corner sites are therefore present in much greater density on the ex OH sample than on the ribbon residue, and, since catalyst performance is unchanged, it is clear that these sites play no significant part in the catalysis over these materials. The active site is therefore located on the planar {100} surfaces. The most selective magnesium oxide catalyst was prepared by thermal decomposition of magnesium hydroxycarbonate and exposed a greater proportion of higher index mean crystal planes, e.g., {l11}, than the less selective forms of magnesium oxide. It is suggested that an additional selective site is present in this form of magnesium oxide, with density related to morphology but not directly to surface area, perhaps a “bottom step” site. The morphology/performance relationship has also been examined for lithium-doped magnesium oxide catalysts. In agreement with previous studies, addition of lithium causes a loss of surface area and of the morphology characteristic of the precursor magnesium oxide; the grain size also increases, grain boundary dislocations become evident, and dislocations are also observed in the bulk of the grains. These are immobile and of the type 1 2 〉110〈 , pinned by the presence of lithium ions. The emergence of a dislocation at the surface of the crystallite provides a locus for [Li +O −] centres, thought to be the active site in methyl radical generation in methane coupling. Similar dislocations are observed in Au/MgO catalysts, which are much less selective to C 2 hydrocarbons than is pure MgO.

A Decrease in Cysteine Levels Causes the Glutathione Deficiency of Aging in the Mosquito

Our previous results indicated that a glutathione (GSH) deficiency is a determinant of the aging process in many tissues and organisms. Correction of this deficiency in the aging mosquito by feeding the cysteine (Cys) precursor magnesium thiazolidine carboxylic acid (MgTc) suggested that the cause could be a lack of Cys. Adult mosquitoes (Aedes aegypti) were fed either a control diet or a diet supplemented with MgTC and then were analyzed for their Cys, cystine, GSH, and glutathione disulfide contents with our HPLC method. The life span profile of Cys levels paralleled that of GSH in the control group with high levels in the young that decreased during maturity and aging. Cystine and glutathione disulfide were undetectable. The causal relationship between the Cys and the GSH deficiencies was shown in the MgTC-supplemented group with an 83% increase in Cys and a 39% increase in GSH relative to control values. Further the conversion steps of MgTC to Cys and then to GSH were verified by use of buthionine sulfoximine. These results demonstrate that a Cys deficiency occurs in the aging mosquito and is the cause of the GSH deficiency.