Leopoldo Contreras

SnO2 thin films prepared by ion beam induced CVD: preparation and characterization by X-ray absorption spectroscopy

Abstract Tin dioxide thin films have been prepared by ion beam induced chemical vapour deposition (IBICVD). The films, with a SnO2 stoichiometry as determined by X-ray photoelectron spectroscopy (XPS), are compact and homogeneous as revealed by SEM/TEM. Preparation was carried out on different substrates at 300 K (i.e. room temperature) and 673 K. Small differences were detected in the granular structure at these two temperatures. In the two cases the films were partially crystalline and depicted the cassiterite structure of SnO2. Small crystallographic domains between 30 and 50 A and a preferential growth of certain planes are deduced from the analysis of the X-ray diagrams. The UV–vis spectra are characterised by a oscillatory absorption behaviour typical of a transparent film with a refractive index of 1.9 deposited on a transparent substrate with a different refraction index. This is fairly close to the value of bulk SnO2. A selective deposition of the SnO2 films has been also carried out by using a mask between the accelerated ion beam used for IBICVD and the substrate. The paper also presents a structural characterisation of the SnO2 thin films and of their crystallisation behaviour by annealing at increasing temperatures. The original films were annealed in air and their structure studied by X-ray absorption spectroscopy (XAS) at different stages of the annealing treatment. XAS provides a description of the structure of the films even if they are partially or completely amorphous. The procedure involves the factor analysis (FA) of the X-ray absorption near edge structure (XANES) spectra after the different treatments and the study by Fourier transformation, fitting and FEFF simulation of the extended X-ray absorption fine structure (EXAFS) zone. The results have shown that the original films are partially amorphous and that their amorphisation degree decreases with the annealing temperature. Approximate quantification of the amorphisation degree, progression of the crystallisation upon annealing and description of the crystal domains in the original samples are some of the information obtained by the proposed method of analysis.

Optical and crystallisation behaviour of TiO2 and V/TiO2 thin films prepared by plasma and ion beam assisted methods

This paper reports the synthesis of TiO2 and V/TiO2 thin films, prepared by plasma enhanced and ion beam induced chemical vapour deposition procedures. Thin films with different contents of vanadium from a few to some tenth percents of this element have been prepared. Vanadium oxide thin films were also prepared for comparison. Morphological, structural and chemical characterisation of the samples was carried out by means of several techniques including scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. Optical properties of these samples were studied by ellipsometry and ultraviolet–visible absorption spectroscopy. It has been shown that the absorption threshold of V/TiO2 thin films shifts continuously towards the visible by increasing the V content. On the basis of this behaviour it is proposed the use of these thin films as optical filters in the UV–vis region of the spectrum. In the ‘as prepared’ samples vanadium is homogeneously distributed within an amorphous Ti–O structure. After annealing in air at T>673 K, both segregation of vanadium as vanadium oxide and crystallisation of the TiO2 into the rutile structure take place. It is realised that infrared spectroscopy can be a complementary technique of XRD to determine the crystallisation behaviour of the samples. The observed structural modifications are accompanied by a change in the light absorption properties of the films that can now be interpreted as due to the superposition of the absorption spectra of TiO2 and vanadium oxide. In the ion beam assisted films, only slight changes in their refraction index (n) are found as a function of the vanadium content and/or the annealing treatments. The changes were greater for the less compact films obtained by plasma deposition, indicating that the main parameter controlling the value of n is the density of the films.

Novel carbon dioxide and carbonyl carbonate complexes of molybdenum. The X-ray structures of trans-[Mo(CO2)2(HN(CH2CH2PMe2)2}(PMe3)] and [Mo3(μ2-CO3)(μ2-O)2(O)2(CO)2(H2O)(PMe3)6].H2O

A new bis(carbon dioxide) adduct of molybdenum containing the tridentate, bis(phosphine) ligand, HN(CH2CH2PMe2)2 (NP2), in a mer conformation has been synthesized and structurally characterized. Its carbonyl carbonate isomer [Mo(CO3)(CO)(NP2)(PMe3)] has also been prepared. In addition, the reactivity of the complex [Mo(CO3)(CO)(PMe3)4] toward CO has been studied, a transformation that has led to the formation of the dicarbonyl complex [Mo(CO3)(CO)2(PMe3)3]. This complex liberates CO2 upon heating under a carbon monoxide atmosphere by means of a CO–CO3 conproportionation reaction. The new trinuclear complex [Mo3(μ2-CO3)(μ2-O)2(O)2(CO)2(H2O)(PMe3)6]·H2O, resulting from the interaction of [Mo(CO3)(CO)(PMe3)4] and water, has been isolated and structurally characterized.

AlN thin films prepared by ion beam induced chemical vapour deposition

Abstract AlN thin films have been prepared by bombardment of a suitable substrate with a beam of accelerated N2+ ions (400 eV) while a flow of an organometallic precursor is directed onto its surface. This procedure has resulted in very compact and dense films of AlN obtained at room temperature. Since the films were amorphous by XRD, the formation of an AlN phase has been confirmed by different spectroscopies, while the morphology and optical properties of the films have been characterized by electron microscopy and ultraviolet-visible absorption.

Iron oxide thin films prepared by ion beam induced chemical vapor deposition: Structural characterization by infrared spectroscopy

Iron oxide thin films as hematite (α-Fe2O3) have been prepared by ion beam induced chemical vapor deposition. Very compact and dense films are obtained by this procedure. The thin films have been grown by bombardment of the substrate surfaces with O2+ ions or mixtures of O2+ and Ar+ ions, while a volatile precursor of iron [i.e., Fe(CO)5] is dosed onto the substrate surface. In the latter case, Ar atoms are incorporated within the iron oxide lattice. Atomic force microscopy, Rutherford backscattering spectroscopy, and x-ray photoelectron spectroscopy were utilized to characterize the films’ surface morphology, stoichiometry and chemical state. The film structure has been analyzed by grazing angle x-ray diffraction (XRD) and infrared spectroscopies. In particular, infrared spectroscopy has permitted a thorough structural characterization of the films, even in the cases where XRD does not provide information about the structure. Thus, when O2+ ions are used for the synthesis, iron oxide thin films grow with...

BIS- AND TRIS(PYRAZOLYL)BORATE COMPLEXES OF THE HEAVIER ALKALINE-EARTH ELEMENTS CA, SR, AND BA

Abstract Sandwich complexes of the composition MTp2* [Tp*hydrotris(3,5-dimethylpyrazolylborate ; MCa, 1 ; Sr, 2 ; Ba, 3] have been generated by the direct reaction of the iodides MI2 with KTp*. Attempts to prepare the half-sandwich compounds MTp*(I) have met with no success when MCa, Sr, but the reaction of BaI2 with 1 equiv. of KTp* in THF produced BaTp*(I)(THF)n (4a), which can be readily converted into the related adduct BaTp*(I)(HMPA)2 [4b, HMPAOP(NMe2)3]. The above compounds have been characterized by analytical and spectroscopic methods, and the structures proposed have been confirmed by X-ray studies carried out with the Ba derivatives 3 and 4b. Attempts to derivatize compounds 4 led mostly to undesired products, but treatment with KTp* and KBp* [Bp*dihydrobis(3,5-dimethylpyrazolyl)borate] resulted in the formation of 3 and BaTp*Bp*(THF), 6, respectively. Bis(pyrazolyl)borate derivatives of the composition M(Bp′)2(THF)n have also been obtained by the direct reaction of MI2 (MCa, Sr, Ba) with KBp* or TlBp† [Bp†dihydrobis(3-t-butyl-pyrazolyl)borate].

ACYL COMPLEXES OF MOLYBDENUM : STRUCTURAL AND REACTIVITY STUDIES

Abstract The following structural peculiarities of the agostic acyl structure 2R) (R = H, SiMe3) and some characteristic chemical reactivity of the M-η2-acyl and iminoacyl linkage are described. (i) A structural comparison of the bonding parameters within three agostic acetyl Mo complexes containing the dithioacid ligand, indicates that the agostic interaction strengthens upon increasing the electron-releasing properties of the S-chelating ligand. (ii) The acyl-xanthate complex Mo(C(O)Me)(S2COR)(CO)(PMe3)2 undergoes loss of a sulfur atom from the coordinated xanthate and coupling with the acyl ligand to form complexes containing coordinated alkoxythiocarbonyl and monothioacetate ligands. The latter can be metathetically replaced by KS2COR. (iii) Upon heating at 70°C η2-acyl-dicarbonyl bispirazolilborate complexes of molybdenum of the type Mo ( H 2 B ( pz ∗ ) 2 )(η 2 - C ( O ) Me )( CO ) 2 ( PMe 3 ) ( pz ∗ = 3,5- dimethyl - pyrazol -1- yl ) yield functionalized acyl ligands derived from the stereo- and regioselective intramolecular addition of one of the BH bonds of the H2B(pz∗)2 group across the C=O moiety of the η2-acyl group. (iv) The η2-acyl-isocyanide complexes { Mo }(η 2 - C ( O ) R )( CNR ′) ({ Mo } = Mo ( H 2 B ( pz ∗ ) 2 )( CO )( PMe 3 )) undergo irreversible thermal isomerization to the corresponding η2-iminoacyl-carbonyl derivatives {MO}(η2-C(NR′)R)(CO). This isomerization reaction follows first-order kinetics.

Synthesis and characterization of bis(xanthate) complexes of MoII and WII. Crystal structure of [W(S2COPri-S)(S2COPri-SS′)(CO)2(PMe3)2]

The reactions of [MoCl2(CO)2(PMe3)2] with potassium xanthates, K(S2COR), give the red crystalline compounds [Mo(S2COR)2(CO)(PMe3)2][R = Me (1), Et (2), Pri(3), or But(4)] in which the xanthate groups seem to act as η3-(S,S′,C) pseudo-allylic ligands. Formation of the dicarbonyl species [Mo(S2COR)2(CO)2(PMe3)] is not detected, but the isopropyl derivative [Mo(S2CO-Pri)2(CO)2(PMe3)](5) can be obtained by carbonylation of (3) under relatively forcing conditions. The tungsten complex [WCl2(CO)2(PMe3)3] reacts similarly with K(S2COR) but gives mixtures of [W(S2COR)2(CO)2(PMe3)n](n= 1 or 2). The monophosphine derivatives [R = Et (6) or Pri(7)] exhibit n.m.r. spectroscopic features similar to those of (1)–(5), but for the bis(phosphine) species a formulation containing uni- and bi-dentate xanthate groups, [W(S2COR-S)(S2COR-SS′)(CO)2(PMe3)2][R = Et (8) or Pri(9)] is proposed, and this has been confirmed by an X-ray crystal structure determination of complex (9). Formation of the dithiocarbonate complexes, [M(S2CO)(CO)2(PMe3)2][M = MO (11) or W (12)] during the reactions leading to the above xanthates has also been observed. Complex (9) is triclinic, space group P1, with unit-cell constants a= 8.775(2), b= 14.027(5), c= 11.806(5)A, α= 106.09(2), β= 75.74(3), γ= 109.06(4)°, and Z= 2. The structure was refined to an R value of 0.032 by using 2 878 independent observed reflections.

Reactivity studies of η2-acyl complexes of molybdenum: reactions with strong bases and with bidentate phosphines☆

Abstract New η 2 -isopropyl–acyl complexes of formula Mo(η 2 -C(O)CHMe 2 )(L)(CO)(PMe 3 ) 2 (L=S 2 CNC 4 H 4 , H 2 B(pz) 2 ; pz=1-pyrazolyl) have been prepared from the reaction of an excess of KH with Mo(η 2 -C(O)CH 3 )(L)(CO)(PMe 3 ) 2 and subsequent addition of an excess of MeI. At ambient temperature, solutions of the isopropyl–acyl derivatives gradually evolve to the hydrides MoH(L)(CO) 2 (PMe 3 ) 2 and propene. The η 2 -acyls Mo(η 2 -C(O)Me)(L)(CO)(PMe 3 ) 2 complexes (L=H 2 B(pz) 2 , Bp; H 2 B(3,5-Me 2 pz) 2 , Bp Me 2 ) as well as the related derivatives of dithioacid ligands, Mo(C(O)Me)(L)(CO)(PMe 3 ) 2 (L=Me 2 NCS 2 , i -PrOCS 2 ) react with the chelating phosphines R 2 PCH 2 CH 2 PR 2 (R=Me, dmpe; Et, depe) with replacement of the two PMe 3 by the bidentate diphosphine. A similar substitution is observed in the reaction of Mo(η 2 -C(O)Me)Cl(CO)(PMe 3 ) 3 with depe, whereas in this case dmpe gives a mixture of Mo(η 2 -C(O)Me)Cl(CO)(dmpe)(PMe 3 ) and the cationic complex [Mo(η 2 -C(O)Me)(CO)(dmpe) 2 ]Cl.

Synthesis and structural characterization of volatile poly(3,5-dimethyl-1-pyrazolyl)-borate-complexes of Ca, Sr, and Ba

Abstract The complexes [MTp * 2 ] (Tp* = HB(3,5-Me 2 pz) 3 ; M = Ca, 1 ; Sr, 2 ; Ba, 3 ) are produced when the iodides MI 2 react with KTp* in tetrahydrofuran (THF). If BaI 2 is treated with only 1 mol equiv. of KTp* in THF, the compound [Tp* BaI(THF) n ] 4 is produced. Some reactions of complex 4 have been studied, particularly adduct formation with hexamethylphosphoramide, HMPA, to furnish [Tp*BaI-(HMPA) 2 ] 5 and the reactions with KBp* (Bp* = H 2 B(3,5-Me 2 pz) 2 ) and KTp*, that yield [Tp* Ba(Bp*)(THF)] 6 and complex 3 , respectively.