Silicon Center Research Articles

Rabi oscillation and electron-spin-echo envelope modulation of the photoexcited triplet spin system in silicon

We report on a pulsed electron paramagnetic resonance (EPR) study of the photoexcited triplet state ($S=1$) of oxygen-vacancy centers in silicon. Rabi oscillations between the triplet sublevels are observed using coherent manipulation with a resonant microwave pulse. The Hahn echo and stimulated echo decay profiles are superimposed with strong modulations known as electron-spin-echo envelope modulation (ESEEM). The ESEEM spectra reveal a weak but anisotropic hyperfine coupling between the triplet electron spin and a ${}^{29}$Si nuclear spin ($I=1/2$) residing at a nearby lattice site, that cannot be resolved in conventional field-swept EPR spectra.

Facile Access to Silicon‐Functionalized Bis‐Silylene Titanium(II) Complexes

A series of unprecedented bis-silylene titanium(II) complexes of the type [(η(5)-C(5)H(5))(2)Ti(LSiX)(2)] (L=PhC(NtBu)(2); X=Cl, CH(3), H) has been prepared using a phosphane elimination strategy. Treatment of the [(η(5)-C(5)H(5))(2)Ti(PMe(3))(2)] precursor (1) with two molar equivalents of the N-heterocyclic chlorosilylene LSiCl (2), results in [(η(5)-C(5)H(5))(2)Ti(LSiCl)(2)] (3) with concomitant PMe(3) elimination. The presence of a Si-Cl bond in 3 enabled further functionalization at the silicon(II) center. Accordingly, a salt metathesis reaction of 3 with two equivalents of MeLi results in [(η(5)-C(5)H(5))(2)Ti(LSiMe)(2)] (4). Similarly, the reaction of 3 with two equivalents of LiBHEt(3) results in [(η(5)-C(5)H(5))(2)Ti(LSiH)(2)] (5), which represents the first example of a bis-(hydridosilylene) metal complex. All complexes were fully characterized and the structures of 3 and 4 elucidated by single-crystal X-ray diffraction analysis. DFT calculations of complexes 3-5 were also carried out to assess the nature of the titanium-silicon bonds. Two σ and one π-type molecular orbital, delocalized over the Si-Ti-Si framework, are observed.

Copper centers in copper-diffused n-type silicon measured by photoluminescence and deep-level transient spectroscopy

While photoluminescence observed the continuous change of the 1.014-eV copper center (CuPL center) intensity with diffusion temperature, deep-level transient spectroscopy (DLTS) measured a considerable number of independent copper-related peaks in silicon samples copper-diffused at different temperatures. There was no DLTS peak whose intensity showed the same diffusion-temperature dependence as that of the CuPL center intensity, which led the conclusion that the CuPL center has no electrically active energy level in the upper-half of the band-gap in silicon. All the DLTS peaks observed were attributed to originate from the variety of copper clusters or small copper precipitates.

An Acyclic Imino‐Substituted Silylene: Synthesis, Isolation, and its Facile Conversion into a Zwitterionic Silaimine

A new type of Si(II): A novel silylene stabilized by a Cp* and an imidazolin-2-iminato ligand has been prepared using two different methods. The X-ray crystallographic structure shows that the silicon(II) center is coordinated to an η(2)-Cp* ligand and the nitrogen atom of an imidazolin-2-iminato ligand. This silylene easily reacts with B(C(6)F(5))(3) to give a stable borane adduct having a zwitterionic resonance structure.

Light-harvesting bio-nanomaterial using porous silicon and photosynthetic reaction center

Porous silicon microcavity (PSiMc) structures were used to immobilize the photosynthetic reaction center (RC) purified from the purple bacterium Rhodobacter sphaeroides R-26. Two different binding methods were compared by specular reflectance measurements. Structural characterization of PSiMc was performed by scanning electron microscopy and atomic force microscopy. The activity of the immobilized RC was checked by measuring the visible absorption spectra of the externally added electron donor, mammalian cytochrome c. PSi/RC complex was found to oxidize the cytochrome c after every saturating Xe flash, indicating the accessibility of specific surface binding sites on the immobilized RC, for the external electron donor. This new type of bio-nanomaterial is considered as an excellent model for new generation applications of silicon-based electronics and biological redox systems.

Stoichiometric Reaction Chemistry of Cationic Ruthenium Silylene Complexes toward Polar and Nonpolar Organic Substrates

Reactions of the cationic ruthenium silylene complexes [Cp*(PiPr3)Ru(H)2(═SiRR′)][B(C6F5)4] (R = Mes, R′ = H, 1; R = R′ = Ph, 2) with alkenes, alkynes, ketones, and Lewis bases were explored. Addition of 1-hexene, 3,3-dimethylbut-1-ene, styrene, and cyclopentene to 1 afforded the disubstituted silylene products [Cp*(PiPr3)Ru(H)2(═SiMesR)][B(C6F5)4] (R = Hex, 3; R = CH2CH2tBu, 4; R = CH2CH2Ph, 5; R = C5H9, 6). Analogous reactions with 2-butyne and 3,3-dimethylbut-1-yne yielded the vinyl-substituted silylene complexes [Cp*(PiPr3)Ru(H)2(═Si(CR═CHR′)Mes)][B(C6F)4] (R = R′ = Me, 7; R = H, R′ = tBu, 8). Complex 1 undergoes reactions with ketones to give the heteroatom-substituted silylene complexes [Cp*(PiPr3)Ru(H)2(═Si(OCHPhR)Mes)][B(C6F)4] (R = Ph, 9; R = Me, 10). Interestingly, complexes 3–8 display a weak interaction between the hydride ligands and the silicon center, while 9 and 10 exhibit a relatively large interaction (as determined by 2JSiH values). The reaction of isocyanates with 1 resulted in the silyl complexes [Cp*(PiPr3)Ru(H)2(Si(Mes)[κ2-O(CH)(NC6H4R)][B(C6F5)4] (R = H, 11; R = CF3, 12), and an intermediate in this transformation is observed. Complex 2 was subjected to various Lewis bases to yield the base-stabilized silylene complexes [Cp*(PiPr3)Ru(H)2(SiPh2·L)][B(C6F)4] (L = DMAP, 13; L = Ph2CO, 14; L = PhCONH2, 15; L = NHMePh, 16, L = tBuSONH2, 18), and the reaction of 1 with NHMePh gave [Cp*(PiPr3)Ru(H)2(SiHMes·NHMePh)][B(C6F)4].

Formation and Annealing Behavior of Copper Centers in Silicon Crystal Measured by Photoluminescence and Deep-Level Transient Spectroscopy

Formation and annealing behavior of the 1.014-eV copper center and its dissociation product (center) in silicon are characterized by photoluminescence (PL) and deep-level transient spectroscopy (DLTS) measurements. On the basis of the findings reported in this study, the structures of the centers are discussed.

ChemInform Abstract: Chemistry of Functionalized Silylenes

AbstractReview: 194 refs.

Lifetime-Degrading Boron-Oxygen Centres in p-types and n-type Silicon

not Available.

ChemInform Abstract: Siloles. Part 2. Silaindenes (Benzosiloles) and Silafluorenes (Dibenzosiloles): Synthesis, Characterization, and Applications

AbstractReview: 170 refs.

Iodine(III)-Mediated Cyclization of Unsaturated O-Alkyl Hydroxamates: Silyl-Assisted Access to α-Vinyl and α-(2-Silylvinyl) Lactams

The embodiment of lactam rings within a wealth of physiologically active natural products and pharmaceutical agents ensures that the development of synthetic methods, which facilitate the preparation of these saturated N<i>-</i>heterocycles, is of critical importance. Herein the development of a versatile method for the synthesis of 4 to 8-membered α-vinyl and α-(2-silylvinyl) lactams involving the iodine(III)-mediated oxidative cyclization of unsaturated <i>O</i>-alkyl hydroxamates, which encompass an allylsilane, is reported. Importantly, the outcome of this transformation can be effectively controlled through variation of the substitution pattern at the silicon center. While allyltrimethylsilanes undergo ring closure with desilylation to form α-vinyl lactams, the corresponding triisopropyl and triphenylsilanes cyclize without loss of the larger silyl group to form <i>E-</i>vinylsilanes with excellent stereoselectivity. From a mechanistic standpoint, it is proposed that this reaction proceeds via concerted alkene addition of a singlet nitrenium ion (or its equivalent) to form a bicyclic <i>N</i>-acyl-<i>N</i>-alkoxyaziridinium ion, which undergoes eliminative ring opening.

The synthesis of N-heterocyclic carbene–silica nano-particles and its catalytic activity in the cyclization of glycerol

1-Butylimidazole (BIm) was successfully immobilized onto rice husk ash via 3-chloropropyltriethoxysilane (CPTES) to produce RHABIm-Cl. The chlorine was easily replaced with sulphate and phosphate anion at room temperature, and these modified catalysts were labeled as RHABIm-HSO4 and RHABIm-H2PO4. The percentage replacement of chlorine was 57% in RHABIm-HSO4. The RHABIm-HSO4 had a specific surface area of 194m2g−1. The 29Si MAS NMR solid-state spectrum showed the presence of Q4, Q3, T3 and T2 silicon centers were present in RHABIm-HSO4 and RHABIm-H2PO4. RHABIm-H2PO4 was used to show the importance of the ionizable proton in the acid catalysis. It showed a higher catalytic activity and selectivity than RHABIm-HSO4 in the cyclization of glycerol (Gly) to cyclic acetals. The catalytic activities of RHABIm-H2PO4 and RHABIm-HSO4 were determined to be 80 and 75% respectively. The benzaldehyde (Benz) conversion over RHABIm-HSO4 was achieved in 6h with high selectivity toward the five-member ring isomer. The kinetic study over RHABIm-HSO4 showed the reaction obeyed the pseudo-first order rate law. RHABIm-HSO4 was stable and re-used several times with only minor loss in catalytic activity due to possible leaching of the organic ligand.

Investigating minority carrier trapping in n-type Cz silicon by transient photoconductance measurements

Minority carrier trapping was investigated in n-type Cz silicon by means of transient-photoconductance (PCD). A simplified Hornbeck and Haynes model was developed for fitting results from transient-PCD to calculate trap density, and it was found to be identical to the model developed for quasi-steady-state photoconductance technique. This indicates that the model can be applied to all photoconductance techniques for lifetime measurement. The results revealed that the trap density is dependent on the concentration of interstitial oxygen and thermal donors, indicating a good agreement with reported results and the results from annealing experiments in this work. Meanwhile, a deep trap energy level was revealed, probably implying that traps also act as recombination centers in n-type silicon. By studying detrapping processes, the concentration of the trapped holes was found to decrease exponentially with time, resulting in a detrapping constant of 167 s.

Synthesis and Characterization of a Singlet Delocalized 2,4‐Diimino‐1,3‐disilacyclobutanediyl and a Silylenylsilaimine

The synthesis and characterization of a singlet delocalized 2,4-diimino-1,3-disilacyclobutanediyl, [LSi(μ-CNAr)(2)SiL] (2, L: PhC(NtBu)(2), Ar: 2,6-iPr(2) C(6) H(3)), and a silylenylsilaimine, [LSi(=NAr)-SiL] (3), are described. The reaction of three equivalents of the disilylene [LSi-SiL] (1) with two equivalents of ArN=C=NAr in toluene at room temperature for 12 h afforded [LSi(μ-CNAr)(2)SiL] (2) and [LSi(=NAr)-SiL] (3) in a ratio of 1:2. Compounds 2 and 3 have been characterized by NMR spectroscopy and X-ray crystallography. Compound 2 was also investigated by theoretical studies. The results show that compound 2 possesses singlet biradicaloid character with an extensive electronic delocalization throughout the Si(2)C(2) four-membered ring and exocyclic C=N bonds. Compound 3 is the first example of a silylenylsilaimine, which contains a low-valent silicon center and a silaimine substituent. A mechanism for the formation of 2 and 3 is also proposed.

Coherent Storage of Photoexcited Triplet States UsingSi29Nuclear Spins in Silicon

Pulsed electron paramagnetic resonance spectroscopy of the photoexcited, metastable triplet state of the oxygen-vacancy center in silicon reveals that the lifetime of the m(s)=±1 sublevels differs significantly from that of the m(s)=0 state. We exploit this significant difference in decay rates to the ground singlet state to achieve nearly ~100% electron-spin polarization within the triplet. We further demonstrate the transfer of a coherent state of the triplet electron spin to, and from, a hyperfine-coupled, nearest-neighbor (29)Si nuclear spin. We measure the coherence time of the (29)Si nuclear spin employed in this operation and find it to be unaffected by the presence of the triplet electron spin and equal to the bulk value measured by nuclear magnetic resonance.

NHC-Catalyzed Chemo- and Regioselective Hydrosilylation of Carbonyl Derivatives

The hydrosilylation of carbonyl derivatives has been explored by the activation of diphenylsilane in the presence of a catalytic amount of an N-heterocyclic carbene (NHC). Presumably, a hypervalent silicon intermediate featuring strong Lewis acid character allows dual activation of both the carbonyl moiety and the hydride at the silicon center. Reduction under mild conditions could be accomplished using this organocatalytic process. Some interesting selectivities have been encountered.

Redistribution of Trialkyl Silanes Catalyzed by Iridium Silyl Complexes

[Ir(COE)2Cl]2 reacts with Et3SiH at 23 °C to form a binuclear iridium complex (Et3Si)2(H)2Ir(μ-Cl)2Ir(H)2(SiEt3)2, 1. Complex 1 reacts further with Et3SiH at 60 °C to form a second binuclear iridium complex, Et3Si(H)2Ir(μ-SiEt2)2Ir(H)2SiEt3, 2, containing bridging Et2Si groups. Activation of 2 with H2 produces trace quantities of a very highly reactive but unobservable species which rapidly and efficiently catalyzes alkyl redistribution reactions of silanes, RR′R″SiH. D2 and silane exchange experiments establish reactivity features of both 2 and the reactive intermediate. The intermediate cannot be observed, but it is likely a monomeric iridium silyl silylene complex that catalyzes alkyl scrambling via silane exchanges coupled with 1,3-alkyl migrations between silicon centers. DFT calculations support such a mechanism.

Luminescent properties of MBE-grown Si:Er/SOI structures

Here we report on luminescent properties of multilayer Si:Er structures grown by sublimation molecular-beam epitaxy on “silicon-on-insulator” substrates. We demonstrate formation in such structures of a unique erbium-related center Er-1. This optical complex stands out among other known erbium-related centers in silicon for its record narrow luminescent line (<10μeV) and largest absorption cross-section and, therefore, provides the best conditions to achieve practically significant amplification and stimulated emission in erbium-doped silicon structures.

Chemistry of functionalized silylenes

Recent years have witnessed important developments in organosilicon chemistry, courtesy to the high yield access of stable monomeric chlorosilylene (LSiCl, L = PhC(NtBu)2) and N-heterocyclic carbene (NHC) stabilized dichlorosilylene (L1SiCl2, L1 = 1,3-bis(2,6-iPr2C6H3)imidazol-2-ylidene) by dehydrochlorination technique using strong bases such as N-heterocyclic carbenes or LiN(SiMe3)2 as HCl scavenger. The chemistry of functionalized silylenes is markedly different from the previously reported dicoordinate silylenes. Their utility as synthons is well documented which paves the way for several striking compounds, e.g. monosilaoxiranes, CSi2P derivative, CSi3P cation, a dimer of silaisonitrile (Si2N2) with dicoordinate Si atoms etc. This study also unravels that these functionalized silylenes are proficient at activating phosphorus, and the functional group attached to the α-position of the silicon(II) centre plays a key role to determine the final product. The reaction of P4 with LSiCl affords a zwitterionic Si2P2 ring, while with LSiN(SiMe3)2 it gives an acyclic Si2P4 derivative. Another recent achievement in this field is the successful isolation of a valence isomer of disilyne known as the inter-connected bis-silylene. This perspective portrays the chemistry of functionalized silylenes which will attract great attention, heading for the further development of organosilicon chemistry.

Silylboranes as New Sources of Silyl Radicals for Chain‐Transfer Reactions

Various silylboranes, which were outfitted with a catecholborane moiety at one end and a (Me(3)Si)(3)Si moiety at the other end of a carbon chain, were prepared through the hydroboration of the corresponding unsaturated silanes. The C-centered radical species generated from these silylboranes efficiently cyclized to provide, through a 5-exo intramolecular homolytic substitution at the silicon center, the corresponding silacycle and a Me(3)Si radical that was subsequently trapped by sulfonyl acceptors. These cyclizations proceeded at unprecedented rates, due, in part, to a strong gem-dialkyl effect that was attributable to the presence of bulky substituents on a quaternary center located on the chain. In parallel, we designed arylsilylboranes that produced silyl radicals through a 1,5-hydrogen transfer. Such silyl radicals may be valuable radical chain carriers, for instance, in oximation reactions of alkyl halides. Finally, computational studies allowed calculation of activation barriers of the homolytic substitution step and additionally illustrated that the overall reaction mechanism involved a transition state in which the attacking carbon center, the central silicon atom, and the Me(3)Si leaving group were collinear.