Radek Fajgar

The equilibrium structure of silene H2C=SiH2 from millimeter wave spectra and from ab initio calculations

Silene, H2CSiH2, has been efficiently produced by pyrolysis of 5,6-bis(trifluoromethyl)- 2-silabicyclo[2.2.2]octa-5,7-diene (SBO). Seven isotopomers have been observed by millimeter- and submillimeter-wave spectroscopy. From the different sets of experimental molecular parameters and from ab initio calculations of the rovibrational interaction parameters, the equilibrium structure has been obtained by a least squares analysis of the rotational constants. The results are: re(Si=C)=1.7039(18) A, re(C–H)=1.0819(12) A, re(Si–H)=1.4671(9) A, ∠HCSi=122.00(4)°, and ∠HSiC=122.39(3)°. This experimental structure is in excellent agreement with the equilibrium geometry calculated at the CCSD(T) level of theory with a cc-pV(Q,T)Z basis set. This is the first experimental determination without any constraint of the Si=C double bond length in the parent compound of the silaalkene family. A lifetime of 30 ms has been observed for this molecule in the gas phase at low pressure.

Si/C phases from the IR laser-induced decomposition of silacyclobutane and 1,3-disilacyclobutane

CO 2 laser-induced infrared multiphoton decomposition (IRMPD) and SF 6 photosensitized decomposition (LPD) of silacyclobutane (SCB) and 1,3-disilacyclobutane (DSCB) in the gas phase results in the very efficient deposition of Si/C/H and SiC materials, and it is inferred that the process is dominated by formation of transient silene ; silene rearrangement to methylsilylene ; silene and methylsilylene dehydrogenation ; and polymerization of SiCH n (n < 4) species. The deposits are sensitive to oxygen. Decomposition and SiC formation are favoured with IRMPD experiments conducted with high-energy fluxes. The laser technique is promising for low-temperature chemical vapour deposition of amorphous SiC.

Laser-induced decomposition of 1,1-dichloro-1-silacyclobutane for gas-phase deposition of reactive solid polycarbosilane

Abstract Continuous-wave CO 2 laser photosensitized (SF 6 ) decomposition of 1,1-dichloro-1-silacyclobutane (DCSCB) leads to volatile ethene, methyltrichlorosilane, 1,1,3,3-tetrachloro-1,3-disilacyclobutane (TCDSCB) and a solid material. The reaction is assumed to be initiated by competitive (2 + 2) cycloreversion and dehydrochlorination. The amounts of depleted DCSCB and those of the ethene and CH 3 SiCl 3 that are formed, as well as detection of methyltrichlorosilane- d 1 and methyltrichlorosilane- d 2 in the decomposition of DCSCB carried out in an excess of DCl provide indirect evidence for the intermediacy of dichlorosilene Cl 2 Si=CH 2 and chlorosilyne ClSi-CH. The solid deposit is judged to be mostly poly(dichlorocarbosilane); it reacts with gaseous methanol, trifluoroethanol, trifluoroacetic acid and water.

Observation of secondary 2-butene ozonide in the ozonation of trans-2-butene in the gas phase

Abstract GC/MS detection of secondary 2-butene ozonide in the gas-phase ozonation of trans-2-butene provides the first conclusive evidence on the feasibility of the combination of Criegee intermediate with parent carbonyl compound, which is not facilitated by solvent.

Chemical vapour deposition of reactive organogermanium films by laser-induced decomposition of tetramethoxygermane

Laser-induced multiphoton and SF6-photosensitised decomposition of tetramethoxygermane has been used for chemical vapour deposition of reactive organooxogermanium polymers, which were characterised by IR, VIS, and UV spectra and ESCA and TEM techniques. The properties of the deposited layers differed depending on the mode of tetramethoxygermane decomposition. The films were reactive towards the ambient atmosphere; this is discussed in terms of the reaction of CH3OGe groups with air moisture.

Germanium-containing coatings by IR laser-induced decomposition of ethoxy(trimethyl) germane and tetramethylgermane

Abstract Different germanium-based materials are produced from ethoxy(trimethyl)germane (ETG) and tetramethylgermane (TMG) by CO 2 laser radiation. The CO 2 laser SF 6 -photosensitized decomposition of TMG affords germanium, the infrared multiphoton decomposition of ETG yields a material which is rich in Ge and contains small amounts of oxygen and carbon, and the SF 6 -photosensitized decomposition of ETG yields a deposit containing Ge and substantial quantities of C and O.

IR laser-induced thermolysis of (chloromethyl)silane: complex reaction involving H2Si:, H2C: and HClSi: transients and yielding nanostructured Si/C/H phases

Multi-pulse and single-pulse infrared laser induced thermolysis of gaseous (chloromethyl)silane, H3SiCH2Cl occurs via 1,1-HCl and H2C: elimination and dehydrogenation and yields nano-structured Si/C/H phases containing most of the Si and C atoms of the precursor. The identification of final volatile products and observation of H2Si:, H2C: and HClSi: transients by LIF spectroscopy suggest that intermediate silene isomers decompose into silylene and carbene. The deposited films are composed of SiC and polycarbosilane and their H content differs depending on the mode of the IR thermolysis.

Laser-induced decomposition of silacyclobutane: extensive H(Si)/H(C) scrambling via 1,2-H shift in silene and radical reactions

Abstract The mechanism of the silacyclobutane decomposition has been further refined through a study of the laser induced decomposition of 1,1-dideuterio-1-silacyclobutane. It is concluded that the identified volatile and solid products and the hydrogen and deuterium content in them are in accord with 1,2-H(D)-shift in intermediate silene and with radical reactions.

From shelled Ge nanowires to SiC nanotubes

Shelled germanium nanowires up to 100 nm in diameter and several micrometers in length were prepared by low pressure chemical vapor deposition (LPCVD) of tris(trimethylsilyl)germane (SiMe(3))(3)GeH. Vapors of the precursor were deposited on tantalum substrates in an oven at 365 degrees C. Subsequently, the products were annealed at 700 degrees C in vacuum. The wires consist of a crystalline Ge core surrounded by a two-layer jacket. The presence of hexagonal Ge in the core was documented in some of the nanowires. The inner jacket is formed by amorphous germanium, the outer part by an Si/C material. By annealing at 900 degrees C, germanium in the core is expelled and nanotubes formed by the Si/C material remain. The samples were studied by SEM, HRTEM, EDX, FTIR and Raman spectroscopy, and the XRD technique.

Optical properties of p–i–n structures based on amorphous hydrogenated silicon with silicon nanocrystals formed via nanosecond laser annealing

Silicon nanocrystals are formed in the i layers of p–i–n structures based on a-Si:H using pulsed laser annealing. An excimer XeCl laser with a wavelength of 308 nm and a pulse duration of 15 ns is used. The laser fluence is varied from 100 (below the melting threshold) to 250 mJ/cm2 (above the threshold). The nanocrystal sizes are estimated by analyzing Raman spectra using the phonon confinement model. The average is from 2.5 to 3.5 nm, depending on the laser-annealing parameters. Current–voltage measurements show that the fabricated p–i–n structures possess diode characteristics. An electroluminescence signal in the infrared (IR) range is detected for the p–i–n structures with Si nanocrystals; the peak position (0.9–1 eV) varies with the laser-annealing parameters. Radiative transitions are presumably related to the nanocrystal–amorphous-matrix interface states. The proposed approach can be used to produce light-emitting diodes on non-refractory substrates.