Takae Takeuchi

Site‐specific fragmentation following Si:2p core‐level photoexcitation of F3SiCH2Si(CH3)3 in the vapor phase

Site‐specific fragmentation following Si:2p photoexcitation of (trifluorosilyl) (trimethylsilyl)methane [F3SiCH2Si(CH3)3 (FSMSM)] has been studied by means of photoelectron‐photoion and photoion–photoion coincidence techniques. The total photoionization efficiency curve of FSMSM has only one broad peak near the Si:2p core‐ionization threshold and no evidence is obtained for the occurrence of selective excitation of each of the two Si atoms. From the results of ab initio calculations of FSMSM, it is found that the magnitude of the Coulomb interaction between the valence electrons and the Si:2p core electrons is very similar at the two Si sites. As a result, the difference in the chemical shift of the 2p core level between the two Si sites is very small. However, the evidence for site‐specific fragmentation of the molecule following the Si:2p core‐level photoexcitation is obtained by analyzing the photoion–photoion coincidence spectra; the relative yields of H+–SiF+ and CH+n–SiF+3 (n=1–3) ion pairs are enha...

Ion beam-induced chemical vapor deposition with hexamethyldisilane for hydrogenated amorphous silicon carbide and silicon carbonitride films

Abstract We report on the preparation of hydrogenated amorphous silicon carbide (a-SiC:H) film and silicon carbonitride (a-SiCN:H) film on Si (100) substrate by Ar ion beam-induced chemical vapor deposition (IBICVD) method attached with a bubbling system of hexamethyldisilane (HMDS). Ar ions and HMDS precursor with carrier gases of Ar or N 2 were introduced onto the substrate at room temperature. Fourier transform infrared (FT-IR) spectra revealed that an elimination of an organic compound and the formation of SiC, SiN and CN bonds can be promoted by increasing ion impact energy. Smooth surface with a lower roughness has been achieved when Ar ion energy increases from 50 to 300 eV.

Distinct Features of Matrix-Assisted 6 μm Infrared Laser Desorption/Ionization Mass Spectrometry in Biomolecular Analysis

Midinfrared-matrix-assisted laser desorption/ionization mass spectrometry (mid-IR-MALDI MS) with a laser emission in the 6 microm wavelength range, which utilizes energy absorption at the C=O double-bond stretch region, was applied to biomolecular analysis. The softness of IR-MALDI MS was evident in the negative ion mode yielding clean mass spectra of [M - H](-) ions for acidic biomolecules with sulfate, phosphate, or carboxylate groups, resulting in better sensitivity than ultraviolet (UV)-MALDI MS. There was no substantial loss of sialic acid due to the prompt fragmentation occurring in IR-MALDI of sialylated glycoconjugates such as gangliosides. Furthermore, the advantage of the low photon energy of IR is that, for the first time, intact protonated molecules of S-nitrosylated peptides can be detected by MALDI MS. In the analysis of redox-sensitive molecules including methylene blue and riboflavin, reductive hydrogenation was minimal, suggesting few hydrogen radicals to have formed in the plume, in contrast to UV-MALDI. In conjunction with a potent new matrix, oxamide, requiring smaller laser fluence, distinct features of the 6 microm IR wavelength range are anticipated to remove one of the limitations of MALDI MS for biomolecular analysis.

Theoretical study of electron impact mass spectrometry. II. ab initio MO study of the fragmentation of ionized 1-propanol

Abstract In order to elucidate qualitatively the fragmentation mechanism of 1-propanol following low energy electron bombardment, the potential energy curves have been calculated using ab initio MO methods (4-31G//ST0-3G), The present study indicates that H 2 O elimination proceeds via the formation of a five-membered ring intermediate/transition state. A hydrogen atom of the methyl group isshifted to form H 2 O, which is in line with the deuterium labelling experiment. In the simple bond cleavage process, it is mainly the C α C β bond which is expected to be broken.

Ionization of hexamethyldisilane for SiC deposition

Abstract We developed the SiC deposition technique by using ion beam induced deposition. In this technique, hexamethyldisilane was excited through impact with Ar ions and fragmented ions were deposited on the substrate. In this work the fragmentation mechanism of hexamethyldisilane is studied using Mass Spectrometry. To discuss the effect of excitation energy, hexamethyldisilane was excited by impact with electrons. With an impact of electrons in an energy range of 10–70 eV, two types of fragmentation, namely, the Si–Si bond dissociation and the methyl radical loss, were observed. The formation of these fragment ions, most probably trimethylsilyl cations, contributed to SiC deposition.

Effects of the silicon core structures on the hole mobility of star-shaped oligothiophenes

Star-shaped compounds with three or four oligothiophene units linked by an organosilicon core were prepared and their hole-transport capabilities were studied. A top-contact type thin film transistor (TFT) with a vapour-deposited film of tris[(ethylterthiophenyl)dimethylsilyl]methylsilane (3T(3)Si(4)) showed field-effect mobility (μ(FET)) of 4.4 × 10(-5) cm(2) V(-1) s(-1), while the device with the carbon centred analogue tris[(ethylterthiophenyl)dimethylsilyl]methane (3T(3)Si(3)C) showed no TFT activity. Intrinsic intramolecular hole mobility of 3T(3)Si(4) and 3T(3)Si(3)C was determined by time-resolved microwave conductivity measurements to be 8 × 10(-2) and 2 × 10(-2) cm(2) V(-1) s(-1), respectively, arising from higher degree of σ-π interaction in 3T(3)Si(4). To know more about the effects of the organosilicon core structures on the intermolecular hole mobility, we calculated internal reorganization energies for hole transfer at the (U)B3LYP/6-311+G(d,p)//(U)B3LYP/6-31G(d) level, which suggested smoother intermolecular charge transfer in the silicon derivative than the carbon and germanium analogues. Star-shaped compounds with quarterthiophene units behave in a different manner from the terthiophene derivatives and tris[(ethylquarterthiophenyl)dimethylsilyl]methane (4T(3)Si(3)C) showed higher TFT mobility of μ(FET) = 1.2 × 10(-3) cm(2) V(-1) s(-1) than its silicon analogue (4T(3)Si(4): μ(FET) = 5.4 × 10(-4) cm(2) V(-1) s(-1)). This is probably due to the more condensed packing of 4T(3)Si(3)C in the film, arising from the shorter Si-C bonding. Compounds with four terthiophene units were also prepared and tetrakis[(ethylterthiophenyl)-dimethylsilyl]silane (3T(4)Si(5)) showed the mobility of μ(FET) = 2.0 × 10(-4) cm(2) V(-1) s(-1), higher than that of 3T(3)Si(4), indicating the potential of tetrakis(oligothiophenyl) compounds as the TFT materials. Tetrakis[(ethylterthiophenyl)dimethylsilyl]germane (3T(4)Si(4)Ge) was less thermally stable and could not be processed to a film by vapour-deposition, but was found to be TFT active in the spin-coated film, although the mobility was rather low (μ(FET) = 7.7 × 10(-7) cm(2) V(-1) s(-1)).

Production of organosilicon ions for SiC epitaxy

The production of organosilicon ions in a Freeman-type ion source were studied for SiC heteroepitaxial growth on a Si wafer. One of the possibilities for SiC epitaxy is a low energy deposition of an organosilicon ion beam. The advantage of this technique is that the organosilicon ion already has a binding of Si and C. The organosilicon ion usually also has a dipole moment which is useful for atomic arrangement on a depositing surface. Methylsilane and dimethylsilane were introduced in a Freeman-type ion source and discharged for ionization. Because of fragmentation, methylsilylene ions and methylsilicenium ions were produced. The ions were accelerated and mass selected in order to create a well defined ion beam. The energy distribution, measured by a plasma monitor, was ±1 eV. By using this ion beam, heteroepitaxitial growth of 3C-SiC on Si was successfully created.

Evidence for a difference in the dissociation mechanisms of acetylene (HCCH) and vinylidene (H2CC:) from charge inversion mass spectrometry

Vinylidene and acetylene are the simplest hydrocarbon isomers, and vinylidene is the simplest unsaturated carbene. The charge inversion mass spectra of C2H2+ cations derived from acetylene using Na, K, Rb and Cs targets were found to be clearly different from those derived from vinylidenechloride (1,1-dichloroethylene). The process of formation of the negative ions in charge inversion mass spectrometry is via near-resonant neutralization followed by spontaneous dissociation, and then endothermic negative ion formation. The intensity of the C2− peak relative to the C2H− peak in these spectra increased with decreasing ionization potential of the targets for both of the isomeric C2H2+ cations. The formation of the C2− anion is proposed to result from the dissociation of excited C2H2 neutrals into C2 and H2. The dependence on target species of the intensities of the C2− peak relative to the C2H− peak for HCCH and H2CC: cannot be rationalized by the internal energy of the excited C2H2 neutrals. The differences indicate that the isomeric C2H2 neutrals dissociate into C2H and H prior to 1,2-hydrogen atom migration.

Theoretical study of electron impact mass spectrometry. I. Ab initio MO study of the fragmentation of n-butane

Abstract The fragmentation mechanism of n-butane by low-energy electron bombardment has been studied by means of the ab initio MO method. Optimized geometries of possible n-butane cation conformers, reaction intermediates and fragments have been calculated using the energy gradient technique. The results suggest that the fragmentation to C1 + C3 is more favorable than that to C2 + C2, when the electron impact energy is at most only a few eV above the ionization threshold. The base peak at m/z 43 has been calculated to be due to the 2-propyl cation. In the course of fragmentation to C1 + C3 proton tunneling is expected.

Preparation and conformational analysis of C-glycosyl β2- and β/β2-peptides

Ten C-glycosyl beta(2)- and beta/beta(2)-peptides with three to eight amino acid residues have been prepared. Solution and solid-phase peptide syntheses were employed to assemble beta(2)-amino acids in which C-glycosylic substituents are attached to the C-2 position of beta-amino acids. Conformational analysis of the C-glycosyl beta(2)-peptides using NMR and CD spectra indicates that the tripeptide can form a helical secondary structure. Besides, helix directions of the C-glycosyl beta/beta(2)-peptides are governed by the configuration at the alpha-carbon of the peptide backbone that originates from the stereocenter of the C-glycosyl beta(2)-amino acids.