Stress inversion from initial tensile to compressive side during ultrathin oxide growth of the Si(100) surface.

We report the real-time observation of the stress change during sub-nanometer oxide growth on the Si(100) surface. Oxidation initially induced a rapid buildup of tensile stress up to -1.9 × 10(8) N m(-2) with an oxide thickness of 0.25 nm, followed by gradual compensation by a compressive stress. The compressive stress saturated at 5 × 10(7) N m(-2) for an oxide thickness of 1.2 nm. The analysis, assisted by theoretical study, indicates that the observed initial tensile stress is caused by oxygen bridge-bonding between the Si dimers. Atomistic model calculations considering mutually orthogonal orientations of the Si(100) surface structure reproduce the stress inversion from the tensile to the compressive side.

Polyoxometalate (POM)-based, multi-functional, inorganic-organic, hybrid compounds: syntheses and molecular structures of silanol- and/or siloxane bond-containing species grafted on mono- and tri-lacunary Keggin POMs.

Using 3-mercaptopropyltrimethoxysilane (HS(CH2)3Si(OMe)3) as a silane-coupling agent (SCA), mono- and tri-lacunary Keggin polyoxometalate (POM)-based, multi-functional, inorganic-organic, hybrid compounds, (Et4N)3[α-PW11O39{(HS(CH2)3Si)2O}] EtN-1 (the 1 : 2 complex of a POM unit and organosilyl groups), (Bu4N)3[A-PW9O34(HS(CH2)3SiOH)3] BuN-2 (the 1 : 3 complex) and (Bu4N)3[A-α-PW9O34(HS(CH2)3SiO)3(Si(CH2)3SH)] BuN-3 (the 1 : 4 complex) were synthesized and unequivocally characterized by elemental analysis, thermogravimetric and differential thermal analyses (TG/DTA), FTIR, solid-state (²9Si and ³¹P) CPMAS NMR, solution (²9Si, ³¹P, ¹H and ¹³C) NMR, and X-ray crystallography. [Note: The moieties of their polyoxoanions are abbreviated simply as 1-3, respectively.] The X-ray molecular structures of EtN-1 and BuN-3 were determined. In EtN-1, two organic groups connected through a siloxane bond (-Si-O-Si- bond) were grafted on a mono-lacunary site of a Keggin POM, whereas in BuN-3 four organic groups connected through siloxane bonds were grafted on a tri-lacunary site of a Keggin POM. In BuN-2, three organic groups were grafted in the form of silanol (-SiOH) on a tri-lacunary site, i.e., in BuN-2 there was no siloxane bond. BuN-3 was synthesized as BuN-3a and BuN-3b by two methods, respectively; (1) BuN-3a was obtained by a 1 : 1 molar-ratio reaction of BuN-2 and an SCA in CH3CN, and (2) BuN-3b was prepared by a direct 1 : 4 molar-ratio reaction of a tri-lacunary Keggin POM and SCA in water-CH3CN. X-Ray crystallography revealed that BuN-3a is the same as BuN-3b. It is probable that BuN-2 is an intermediate in the formation of BuN-3. Terminal -SH groups in 1-3, as well as -OH groups in 2, can be utilized for immobilization of POMs and, also, as building blocks for the formation of novel hybrid compounds.

Cyclic oligomer of oxide clusters through a siloxane bond. Synthesis and structure of reaction products of alpha2-mono-lacunary Dawson polyoxometalate with tetrachlorosilane and tetraethoxysilane.

The title compound with the formula (Me2NH2)13H5[{alpha2-P2W17O61(Si2O)}3(micro-O)3].39H2O Me2NH(2)-1 was obtained in 12.8% (0.258 g scale) yield by a 1:2 molar ratio reaction of alpha2-mono-lacunary Dawson polyoxometalate (POM) [alpha2-P2W17O61]10- with SiCl4 in water and unequivocally characterized by complete elemental analysis, TG/DTA, FTIR, (solution and solid-state) 31P NMR, solid-state 29Si NMR and X-ray crystallography. [Note: the moieties of the polyoxoanions in Me2NH(2)-1 and Me2NH(2)-2 are abbreviated simply as 1 and 2, respectively.] X-Ray crystallography revealed that the molecular structure of polyoxoanion 1 was a cyclic trimer consisting of three alpha2-mono-lacunary Dawson POM subunits constituted through intra- and intermolecular siloxane bonds. The compound resulted in an approximate Cs symmetry, but not the idealized C3v symmetry, because one Dawson unit was unequivalently incorporated into the cyclic framework. This compound is the first example of a siloxane-bonding cyclic trimer of POMs and its formation suggests that the oxygen atoms in the lacunary site of POM are reactive just like the surface oxygen atoms of silica. The formation of the cyclic trimer was also clarified by two control experiments: (1) the reaction of [alpha2-P2W17O61]10- with Si(OEt)4 under acidic conditions (pH 1), and (2) oligomerization under 6 M aqueous HCl acidic conditions (pH approximately 0) of the reactive monomer units (RMUs: tentatively assigned as "[alpha2-P2W17O61{O(SiOH)2}]6-"), derived from the cleavage of Ph-Si bonds by the thermal degradation of [alpha2-P2W17O61{O(SiPh)2}]6-. On the other hand, the reactions of alpha2-mono-lacunary Dawson POMs with GeX4 (X=OEt, Cl) gave the monomeric species of mono-germanium-substituted Dawson POM (Me2NH2)7[alpha2-P2W17O61(GeOH)].5H2O Me2NH(2)-2, which was characterized by CHN analysis, TG/DTA, FTIR, X-ray crystallography, and (solid-state and solution) 31P NMR spectroscopy. X-ray crystallography revealed that compound Me2NH(2)-2 was a monomeric species, but the Ge site per formula unit was not determined due to disorder of alpha-Dawson structure.