Short Si Research Articles

Three-Membered Ring or Open Chain Molecule − (F3C)F2SiONMe2 a Model for the α-Effect in Silicon Chemistry

(F3C)F2SiONMe2 was prepared from LiONMe2 and F3CSiF3. It was characterized by gas IR and multinuclear solution NMR spectroscopy and by mass spectrometry. Its structure was elucidated by single crystal X-ray crystallography and by gas electron diffraction. (It exists as a conformer mixture.) Important findings were extremely acute SiON angles [solid 74.1(1) degrees , gas anti 84.4(32) degrees and gauche 87.8(20) degrees] and short Si...N distances [solid 1.904(2) A]. The bending potential of the SiON unit was calculated at the MP2/6-311++G(3df,2dp) level of theory and appears very flat and highly asymmetric. The calculated atomic charges (NPA) are counterintuitive to the expected behavior for a classical Si-N dative bond, as upon formation of the Si...N bond electron density is transferred mainly from oxygen to nitrogen, while the silicon charge is almost unaffected. Despite the molecular topology of a three-membered ring, the topology of the electron density shows neither a bond critical point between Si and N atoms nor a ring critical point, but the electron density and Laplacian values are related to other hypercoordinate Si compounds. The electronic properties of (F3C)F2SiONMe2 were compared to those of the adduct (F3C)F2(MeO)Si-NMe3, whose properties and structure were also calculated. The charge distribution and Laplacian values along the Si-N vectors in both molecules are similar but not equivalent. (F3C)F2SiONMe2 contains thus a nonclassical Si...N bond, and its properties can be regarded as a new model for the explanation of the old postulate of an alpha-effect in silicon chemistry, explaining the behavior of compounds with geminal Si and N atoms.

Pseudo-pentacoordination in silylcarbamates: structure-based prediction of silylating power

N-substituted trimethylsilyl carbamates (NTSCs) are powerful silylating agents. A survey of eight available X-ray molecular structures reveals an unusually short Si⋯O C distance that seems to convey a pseudo-pentacoordinated character of the silicon atom. The molecular structure of some representatives of NTSCs and their carbon analogues have been optimized at the MP2/DZ(2)P level. The theoretical structures confirm the existence of a Si⋯O through-space interaction and the pseudo-pentacoordination at the silicon atom. A structure-based predictive model is developed in which the high reactivity of NTSCs in displacing mobile hydrogens (i.e. the silylating power) is explained by both the pseudo-pentacoordination at the silicon and the unusually long Si–O ester bond, that is, by the use of ground-state structural parameters. On this basis, O-silylcarbamimidates are predicted to show higher reactivity in nucleophilic reactions, as compared to NTSCs, due to the stronger Si⋯N C interaction. Beyond NTSCs, the current predictive model can be applied to a wider range of compounds. O-alkylcarbamimidates are predicted to be good alkyl donors in nucleophilic substitutions. This conclusion is in accordance with the fact that O-alkyl-isoureas are general alkylating agents. Preliminary experimental results on the rate constants of hydrolysis of N-silylated cyclic ureas show strong correlation with the shift toward pseudo-pentacoordination around silicon, proving the validity of the current predictive model.

Magnetoresistance characterization of nanometer Si metal-oxide-semiconductor transistors

We report on the high-field (up to 10T) magnetoresistance measurements performed on the short (down to 75-nm gate length) n-type Si metal-oxide-semiconductor field-effect transistors. The electron magnetoresistance mobility of these nanometer devices was determined for a wide range of the electron concentration (107–1013cm−2, i.e., from a weak to a strong inversion) and gate length (10μm–75nm). In the case of long samples, the magnetoresistance mobility was compared to the effective mobility obtained by the standard parameter extraction and the split C–V techniques. The results are discussed in terms of the scattering power-law two-dimensional transport analysis. The data clearly indicate a significant decrease of the mobility with the gate length reduction below 100nm.

Si epitaxial growth on Br-Si(100): How steric repulsive interactions influence overlayer development

Scanning tunneling microscopy results show the consequences of Si adatom deposition onto Br-saturated $\mathrm{Si}(100)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$. Those adatoms undergo an exchange reaction with Br and they are immobile at room temperature. In the low coverage regime, annealing to $650\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ leads to dimerization, limited ordering, and the formation of short Si chains. Adatom capture by those chains produces features of even and odd numbers of atoms. Annealing at $700\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ eliminates the odd chains, but diffusion is highly constrained by Br site blocking. With subsequent Si depositions, there is further nucleation of chains and chain growth. The local patterning of the Si chains reveals the influence of the Br steric repulsive interactions as out-of-phase structures were favored over in-phase structures around any given chain. Eventually, $(3\ifmmode\times\else\texttimes\fi{}2)$ adlayer patches develop. Second layer chains appear after the deposition of $\ensuremath{\sim}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$, with layer-2 nucleation at antiphase domain boundaries. Bromine loss was observed, even at $650\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and it is probably tied to the weaker bonding to single isolated Si adatoms.

Nonclassical titanocene silyl hydrides.

The titanocene silyl hydride complexes [Ti(Cp)2(PMe3)(H)(SiR3)] [SiR3=SiMePhCl (6), SiPh2Cl (7), SiMeCl2 (8), SiCl3 (9)] were prepared by HSiR3 addition to [Ti(Cp)2(PMe3)2] and were studied by NMR and IR spectroscopy, X-ray diffraction (for 6, 8, and 9), and DFT calculations. Spectroscopic and structural data established that these complexes exhibit nonclassical Ti-H-Si-Cl interligand hypervalent interactions. In particular, the observation of silicon-hydride coupling constants J(Si,H) in 6-9 in the range 22-40 Hz, the signs of which we found to be negative for 8 and 9, is conclusive evidence of the presence of a direct Si-H bond. The analogous reaction of [Ti(Cp)2(PMe3)2] with HSi(OEt)3 does not afford the expected classical silyl hydride complex [Ti(Cp)2(PMe3)(H)[Si(OEt)3]], and instead NMR-silent titanium (apparently TiIII) complex(es) and the silane redistribution product Si(OEt)4 are formed. The structural data and DFT calculations for the compounds [Ti(Cp)2(PMe3)(H)(SiR3)] show that the strength of interligand hypervalent interactions in the chlorosilyl complexes decreases as the number of chloro groups on silicon increases. However, in the absence of an Si-bound electron-withdrawing group trans to the Si-H moiety, a silane sigma complex is formed, characterized by a long Ti-Si bond of 2.658 A and short Si-H contact of 1.840 A in the model complex [Ti(Cp)2(PMe3)(H)(SiMe3)]. Both the silane sigma complexes and silyl hydride complexes with interligand hypervalent interactions exhibit bond paths between the silicon and hydride atoms in Atoms in Molecules (AIM) studies. To date a classical titanocene phosphane silyl hydride complex without any Si-H interaction has not been observed, and therefore titanocene silyl hydrides are, depending on the nature of the R groups on Si, either silane sigma complexes or compounds with an interligand hypervalent interaction.

Ordered Arrays of Be-Encapsulated Si Nanotubes on Si(111) Surface

Using submonolayer Be deposition onto the Si(111) 7 × 7 surface under ultrahigh vacuum conditions, highly ordered honeycomb-like nanostructure arrays have been obtained. Scanning tunneling microscopy analysis of the nanostructure building blocks has revealed that they have composition, size, and properties similar to those theoretically predicted for the short Be-encapsulated Si nanotubes.

Ternary silicides M2Cr 4Si 5 ( M=Ti, Zr, Hf): filled variants of the Ta 4SiTe 4 structure type

The isostructural ternary silicides M 2Cr 4Si 5 ( M=Ti, Zr, Hf) were prepared by arc-melting of the elemental components. The single-crystal structure of Zr 2Cr 4Si 5 was determined by X-ray diffraction (Pearson symbol oI44, orthorhombic, space group Ibam, Z=4, a=7.6354(12) Å, b=16.125(3) Å, c=5.0008(8) Å). Zr 2Cr 4Si 5 adopts the Nb 2Cr 4Si 5-type structure, an ordered variant of the V 6Si 5-type structure. It consists of square antiprisms that have Zr and Cr atoms at the corners and Si atoms at the centers; they share opposite faces to form one-dimensional chains ∞ 1[Zr 4/2Cr 4/2Si] surrounded by additional Si atoms and extending along the c direction. In a new interpretation of the structure, additional Cr atoms occupy interstitial octahedral sites between these chains, clarifying the relation between this structure and that of Ta 4SiTe 4. The formation of short Si–Si bonds in Zr 2Cr 4Si 5 is contrasted with the absence of Te–Te bonds in Ta 4SiTe 4. The compounds M 2Cr 4Si 5 ( M=Ti, Zr, Hf) exhibit metallic behavior and essentially temperature-independent paramagnetism. Bonding interactions were analyzed by band structure calculations, which confirm the importance of Si–Si bonding in these metal-rich compounds.

STM characterization of the Si-P heterodimer

We use scanning tunneling microscopy (STM) and Auger electron spectroscopy to study the behavior of adsorbed phosphine (PH$_{3}$) on Si(001), as a function of annealing temperature, paying particular attention to the formation of the Si-P heterodimer. Dosing the Si(001) surface with ${\sim}$0.002 Langmuirs of PH$_{3}$ results in the adsorption of PH$_{x}$ (x=2,3) onto the surface and some etching of Si to form individual Si ad-dimers. Annealing to 350$^{\circ}$C results in the incorporation of P into the surface layer to form Si-P heterodimers and the formation of short 1-dimensional Si dimer chains and monohydrides. In filled state STM images, isolated Si-P heterodimers appear as zig-zag features on the surface due to the static dimer buckling induced by the heterodimer. In the presence of a moderate coverage of monohydrides this static buckling is lifted, rending the Si-P heterodimers invisible in filled state images. However, we find that we can image the heterodimer at all H coverages using empty state imaging. The ability to identify single P atoms incorporated into Si(001) will be invaluable in the development of nanoscale electronic devices based on controlled atomic-scale doping of Si.

Carbon induced ( [formula omitted])R30° reconstruction on Si(1 1 1) surface: a theoretical study

Various atomic structural models of recently observed carbon induced ( 3 × 3 )R30° reconstruction on Si(1 1 1) surface are studied by first-principles calculations. It is found that among the four possible configurations C–T 4, C–H 3, C–S 5 and C–S 4, the most stable one is C–S 5, which consists of a substitutional carbon atom directly below the Si adatom at T 4 site. The stability of the C–S 5 configuration is attributed to the relief of the stress caused by the short Si–C bond. The electronic structures of thermodynamically stable configuration C–S 5 and the kinetically favored C–T 4 configuration are studied. The calculated electronic structures show that both C–S 5 and C–T 4 surfaces are metallic. Based on the calculated STM images at different biases and the calculated density of states, we provide a scheme to distinguish which model is observed in the experiments.

H2-assisted control growth of Si nanowires

Large-scale desired silicon nanowires without amorphous silicon oxide sheath have been synthesized by thermal chemical vapor deposition using SiH4 gas at 650°C in a flow mixture of H2 and N2, compared with the short and thick Si nanowires with amorphous SiOx coating obtained in N2. Scanning electron microscopy (SEM), Energy dispersive X-ray spectrometry (EDX) analysis, and high-resolution transmission electron microscopy (HRTEM) have been employed to characterize the Si nanowires. The effects of H2 gas on the catalytic particle size and on the formation of Si nanowires are discussed in detail. Photoluminescence (PL) characteristics further demonstrate the large differences between the H2-assisted grown Si nanowires and the Si nanowires grown in N2.

Silicon carbide surface structure investigated by synchrotron radiation-based x-ray diffraction

We use synchrotron radiation based x-ray diffraction at grazing incidence to study the atomic structure of Si-rich β-SiC(100) 3×2 surface reconstruction. The latter includes three different Si atomic planes, in qualitative agreement with the theoretical two adlayers asymmetric dimer model. The measurements provide an accurate determination of the atomic bond, indicating asymmetric Si dimers in the first plane, and an alternating long and short Si dimers subsurface organization in the second atomic plane responsible for the lack of dimers buckling in the first plane, unlike corresponding silicon or germanium surfaces.

Three-Membered Ring Formation by Si···N Interactions in Aminosulfenylsilanes

The reaction of dimethylaminosulfenylchloride Me2NSCl with trichlorosilane HSiCl3 and triethylamine gives elemental sulphur and Me2NSiCl3. The aminosulfenylsilane Me2NSSiCl3 is postulated to be an intermediate of this reaction. Ab initio calculations (MP2/6D311G(d,p)) on Me2N-S-SiH3, Me2N-S-SiH2Cl, Me2N-S-SiH2F, and Me2N-S-SiCl3 have been carried out, demonstrating the occurrence of acute valence angles at sulphur and short Si···N distances, which are indicative of NSSi three-membered rings. The strength of the Si · · · N interactions depends on the electronegativity of the substituent at silicon in anti-position to the nitrogen atom and is strongest in the anti conformer of Me2N-S-SiH2F (< NSSi 68°, Si···N 2.208 Å ). The coordination spheres of the nitrogen atoms in the Me2N-S-SiR3 molecules are steeply pyramidal, which is in contrast to the planarised N atoms in other SNMe2 compounds, such as Me2NSCl. The crystal structure of this compound has been determined, as well as that of the product of the above reaction, Me2NSiCl3. Both have planar coordination geometries at the nitrogen atoms.

Defect sites at the (001) surface of mordenite: An ab initio study

The mechanisms and energetics of the formation of various defects upon dehydration of the surface of pure-siliceous and Al-monosubstituted mordenite are investigated using a periodic ab initio density functional theory technique. An energetically favorable defect at the pure-siliceous surface is a strained two-membered Si–O ring (2MR) formed via elimination of a water molecule from a pair of neighboring terminal silanol groups. Assuming the formation of two-membered rings, the dehydration-energy of the (001) surface of pure-silica mordenite is 133 kJ/mol. A relatively high reaction barrier of 179 kJ/mol coincides with the experimental observation that these defects are formed at high temperatures >700 K. Despite a short Si–Si distance of 2.35 Å across the 2MR which is comparable to the bond length between Si atoms in silicon in diamond structure, the electron-localization function reveals no bonding interaction between Si atoms on the 2MR. In the Al-substituted surfaces, the dehydration proceeds via proton transfer from the Brønsted-acid site (BA) to a neighboring terminal hydroxyl group. The low values of two subsequent energetic barriers of dehydration of 13 and 10 kJ/mol suggest that the surface BA sites are likely to be destroyed at even modest temperatures. The most stable defects formed in this mechanism are ones containing a threefold-coordinated Al atom and a defect with both an Al atom and a bridging OH group located on a two-membered ring. The heat of reaction of only 9 kJ/mol and the activation energy of the transformation between these two configurations of 26 kJ/mol suggest that both defects occur with similar probability.

Water-Induced Diastereomeric Isomerization of Novel Disiloxane-1,3-diols Separated Manually

Diastereomeric disiloxane-1,3-diols, 1, with two asymmetric silicon centers and intramolecular coordination arms were synthesized and separated manually. They show distinguishing structural features such as a linear or bent Si−O−Si skeleton, a short Si−O bond length, and strong intramolecular hydrogen bonding. A trace of water induced the diastreomeric isomerization of 1 under noncatalytic conditions, while the anhydrous disiloxane-1,3-diols were stereochemically stable.

Very strong anionic homoaromaticity in ( deloc-1,3,4)-1-sila-3,4-diboracyclopentane-1-ides, the importance of the energy of the reference system for homoaromatic stabilization energies

( deloc-1,3,4)-1-Sila-3,4-diboracyclopentane-1-ide ( 3a) was prepared as solvent separated ion pair [Li(Et 2O)(thf) 3][ 3a] and as contact ion pair [Li(Et 2O)][ 3a]. Both were fully characterized by NMR spectroscopy as well as by X-ray structure analyses. Their five-membered rings are strongly distorted as seen from short transannular Si⋯B distances of 206 and 208 pm as compared with 276 pm in the undistorted 1-sila-3,4-diboracyclopentane ( 4c). This distortion is also found in the unsubstituted prototype 3u by geometry optimizations at the MP2/6-311+G** level of theory. In addition, computations show that a three-center-two-electron (3c2e) bond between the silicon and the two boron atoms is present in the distorted 3u. The planar classical reference molecule 3u* with a 2c2e π bond between the boron atoms is not accessible by computations. Therefore, the energy difference between 3u and 3u* was estimated by isodesmic equations to be about 80 kcal mol −1 (at MP4/6-311+G**), considerably larger than any homoaromatic stabilization energy (HSE) ever discussed. The origin of this huge HSE of 3u is strong electrostatic destabilization of the reference system 3u* due to intramolecular charge separation. The series of bishomoaromatic systems is thus extended by a borderline case of very large HSE. The very small HSE of 1u at the other end of the series is also due to the energy of the reference: 1u* is strongly stabilized by hyperconjugation.

A novel morphology of SiO x nanowires with a modified diameter

A novel morphology of SiOx (x=1–2) nanowires was fabricated via a thermal evaporation method by heating pure silicon powder at 1373 K followed by depositing silicon vapor on a quartz-glass substrate which was coated with a catalyst precursor from a 0.005 M Fe(NO3)3 aqueous solution. At the deposition temperature, the catalyst particles aggregated and formed quasi-plates, from which a short Si rod grew outward on one side and a great deal of SiOx nanowires projected outward on the other side, forming comet-like objects with long tails. The diameters of the nanowires gradually decreased with an increase in the distance from the catalyst plate.

Morphological Studies of Binary Mixtures of Block Copolymers. 2. Chain Organization of Long and Short Blocks in Lamellar Microdomains and Its Effect on Domain Size and Stability

We focus on chain organization of long and short block chains on lamellar microdomains developed in a series of binary mixtures denoted as as/si, which are composed of a long asymmetric polystyrene-block-polyisoprene (SI) and one of three short symmetric SI's denoted respectively as as and si (i = 1, 2, 3). We investigated the effect of chain organization on stability of the microdomain and on microdomain size as a function of blending composition and ratio of degree of polymerization of as to si. Comparisons of the results with some theoretical predictions indicated that the strong segregation theory of Birshtein, Zhulina, and Lyatskaya fits well with the experimental results.

The gas-phase molecular structure of 1,1-diethynylsilacyclobutane as determined by means of electron diffraction and ab initio calculations

As a continuation of our systematic investigation of the effect of substituents on the ring geometry and dynamics in silacyclobutanes and in order to explore the role of the silicon atom as a mediator for electronic interactions between the attached fragments, we studied the molecular structure of 1,1-diethynylsilacyclobutane (DESCB) by means of gas-phase electron diffraction and ab initio calculations. The structural refinement of the electron diffraction data yielded the following bond lengths (ra) and bond angles (uncertainties are 3σ): r(Si–C)=1.874(2)Å,r(Si–C)=1.817(1)Å,r(–CC–)=1.209(1)Å,r(C–C)=1.563(2)Å, ∠(C–Si–C)=79.2(6)°, ∠(C–Si–C)=106.5(6)°. The geminal Si–CC moieties were found to be bent outwards by 3.1(15)° and the puckering angle was determined to be 30.0(15)°. The evidently short Si–C bond length, which was also reproduced by the ab initio calculations, could be rationalized as being the consequence of the electronic interaction between the outer π charges of the triple bond and the 3pπ orbitals at the silicon atom. It is also likely that the conjugation of the geminal ethynyl groups leads to an enhancement of this bond contraction. Electrostatic interactions and the subsequent reduction of the covalent radius of the silicon atom may also contribute to this bond shortening. It has been found that the endocyclic Si–C bond length fits nicely within a scheme describing a monotonous decrease of the Si–C bond length with the increase of the electronegativity of the substituent in various geminally substituted silacyclobutanes.A series of related silacyclobutanes and acyclic diethynylsilanes have been studied by applying various ab initio methods and their optimized structures were compared to the structure of DESCB. Among these compounds are 1,1-dicyanosilacyclobutane (DCYSCB), which is isoelectronic to DESCB, 1,1-diethynylcyclobutane (DECB) which is isovalent to DESCB, monoethynylsilacyclobutane (MESCB) and monocyanosilacyclobutane (MCYSCB). Searching for reasonable support for the explanation of the structural results of DESCB we performed detailed natural population analysis as well as Mulliken population analysis (MPA) on DESCB and other related molecules. In contrast to the Mulliken charges, the natural atomic charges provided helpful information concerning the bonding properties in DESCB and the corresponding compounds. By varying the size of some basis sets, we could demonstrate the validity of the repeatedly discussed dependency of the Mulliken MPA on the basis set.For the performance of the quantum mechanical calculations we employed the following methods and basis sets: HF/6-31G(d,p), DFT/B3PW91/6-31G(d), DFT/B3PW91/6-311++G(d,p), MP2/6-31G(d,p) and MP2/6-311++G(d,p).

Synthesis of more stable MCM-41 under high-pressurized conditions

MCM-41 was synthesized under high-pressurized conditions. Thus synthesized molecular sieves were shown to have thicker pore wall, and shorter SiO bond, improving their thermal and hydrothermal stability.

Reaction chemistry, NMR spectroscopy, and X-ray crystallography of [Fe 2(μ-SiMe 2) 2(CO) 4] and [Fe 2(μ-SiMeCl) 2(CO) 4]. Electronic structure and bonding in Fe 2E 2 rings of [Fe 2(μ-ER 2) 2(CO) 4] binuclear complexes (E=C, Si, Ge, Sn, Pb)

The rings [Fe 2(μ-SiRR′) 2(CO) 8] (R=Me, R′=Me or Cl) react with HMPA to give the base-stabilized silylenes [Fe(SiRR′(HMPA))(CO) 4]. The reactions of [Fe 2(μ-SiMe 2) 2(CO) 8] with DABCO, THF and PMe 3 have also been examined. The crystal structures of both [Fe 2(μ-SiMe 2) 2(CO) 8] and [Fe 2(μ-SiMeCl) 2(CO) 8] show planar rings, CH⋯OC short contacts and relatively short but non-bonding SiSi distances. The possibility of through-ring MM or EE bonding in complexes of the type [M 2(μ-ER 2) 2(CO) 8] (M is a transition metal and E is a Group 14 element) is analyzed with the help of theoretical calculations based on density functional theory. For compounds with 20 ring electrons (or a framework electron count, FEC, of eight), regular M 2E 2 rings are expected, with no short through-ring distances. The framework electron counting rules, geometry optimization of several model complexes and a structural database analysis consistently indicate that through-ring bonding exists only when the FEC is less than eight. In that case, the isomer with a short metal–metal bond is found to be significantly more stable than that with a short EE distance.