Br Si Research Articles

Reactivity of Pt(0) bromosilylene complexes towards ethylene.

The base-free carbazolyl bromosilylene RSiBr (R = 1,8-bis(3,5-di-tert-butyl-phenyl)-3,6-di-tert-butyl-carbazolyl) reacts with (η2-C2H4)Pt(PPh3)2 and Pt(PCy3)2 to form platinasilacyclobutane R(Br)Si(C2H4)Pt(PPh3)2 (1) and silylene platinum complex R(Br)SiPt(PCy3)2 (2), respectively. When silylene complex 2 is treated with C2H4, the six-membered metallasilacycle R(Br)Si(C2H4)2Pt(PCy3)2 (3) is obtained. All compounds are characterised by XRD and multinuclear NMR spectroscopy.

The stability of Cl-, Br-, and I-passivated Si(100)-(2 × 1) in ambient environments for atomically-precise pattern preservation

Atomic precision advanced manufacturing (APAM) leverages the highly reactive nature of Si dangling bonds relative to H- or Cl-passivated Si to selectively adsorb precursor molecules into lithographically defined areas with sub-nanometer resolution. Due to the high reactivity of dangling bonds, this process is confined to ultra-high vacuum (UHV) environments, which currently limits its commercialization and broad-based appeal. In this work, we explore the use of halogen adatoms to preserve APAM-derived lithographic patterns outside of UHV to enable facile transfer into real-world commercial processes. Specifically, we examine the stability of H-, Cl-, Br-, and I-passivated Si(100) in inert N2 and ambient environments. Characterization with scanning tunneling microscopy and x-ray photoelectron spectroscopy (XPS) confirmed that each of the fully passivated surfaces were resistant to oxidation in 1 atm of N2 for up to 44 h. Varying levels of surface degradation and contamination were observed upon exposure to the laboratory ambient environment. Characterization by ex situ XPS after ambient exposures ranging from 15 min to 8 h indicated the Br– and I–passivated Si surfaces were highly resistant to degradation, while Cl–passivated Si showed signs of oxidation within minutes of ambient exposure. As a proof-of-principle demonstration of pattern preservation, a H–passivated Si sample patterned and passivated with independent Cl, Br, I, and bare Si regions was shown to maintain its integrity in all but the bare Si region post-exposure to an N2 environment. The successful demonstration of the preservation of APAM patterns outside of UHV environments opens new possibilities for transporting atomically-precise devices outside of UHV for integrating with non-UHV processes, such as other chemistries and commercial semiconductor device processes.

Linearly Two-Coordinated Silicon: Transition Metal Complexes with the Functional Groups M≡Si—M and M═Si═M

A detailed experimental and theoretical analysis is presented of unprecedented molybdenum complexes featuring a linearly coordinated, multiply bonded silicon atom. Reaction of SiBr2(SIdipp) (SIdipp = C[N(C6H3-2,6- iPr2)CH2]2) with Na[Tp'Mo(CO)2(PMe3)] (Na-1) in the ratio 1:2 afforded the reddish-brown metallasilylidyne complex [Tp'(CO)2Mo≡Si-Mo(CO)2(PMe3)Tp'] (Tp' = κ3- N, N', N″-hydridotris(3,5-dimethylpyrazolyl)borate) (2), in which an almost linearly coordinated silicon atom (∠(Mo1-Si-Mo2) = 162.93(7)°) is bridging the 15VE metal fragment Tp'Mo(CO)2 with the 17VE metal fragment Tp'Mo(CO)2(PMe3) via a short Mo1-Si bond (2.287(2) Å) and a considerably longer Mo2-Si bond (2.438(2) Å), respectively. The reddish-orange silylidyne complex [Tp'(CO)2Mo≡Si-Tbb] (3) was also prepared from Na-1 and the 1,2-dibromodisilene ( E)-Tbb(Br)Si═Si(Br)Tbb (Tbb = C6H2-2,6-[CH(SiMe3)2]2-4- tBu) and contains as 2 a short Mo-Si bond (2.2614(9) Å) to an almost linearly coordinated Si atom (∠(Mo-Si-CTbb) = 160.8(1)°). Cyclic voltammetric studies of 2 in diglyme revealed an irreversible reduction of 2 at -1.907 V vs the [Fe(η5-C5Me5)2]+/0 redox couple. Two-electron reduction of 2 with potassium graphite yielded selectively the 1,3-dimetalla-2-silaallene dianion [Tp'(CO)2Mo═Si═Mo(CO)2Tp']2- (42-), which was isolated as the bright yellow dipotassium salt [K(diglyme)]2-4. Single crystal X-ray diffraction analysis revealed a centrosymmetric structure of 42-. The Mo-Si bond length of 42- (2.3494(2) Å) compares well with those of Mo-Si double bonds and lies in-between the Mo1-Si triple bond and Mo2-Si single bond length of 2. Compounds 2, 3 and [K(diglyme)2]-4 were characterized by elemental analyses, IR and multinuclear NMR spectroscopy. Comparative ELF (electron localization function), NBO (natural bond orbital) and NRT (natural resonance theory) analyses of 2, 3 and 42- shed light into the electronic structures of these compounds.

Bromine as a Preferred Etchant for Si Surfaces in the Supersaturation Regime: Insights from Calculations of Atomic Scale Reaction Pathways

Etching of semiconductors by halogens is of vital importance in device manufacture. A greater understanding of the relevant processes at the atomistic level can help determine optimal conditions for etching to be carried out. Supersaturation etching is a seemingly counterintuitive process where the coverage of the etchant molecules on the surface to be etched is >1. Here we use density functional theory computations of reaction pathways and barriers to suggest that supersaturation etching of Si(001) by Br2 should be more effective than conventional etching by Br2, as well as both conventional and supersaturation etching by Cl2. Analysis of our results shows that this is due in part to the larger size of bromine atoms, and partly due to Br–Si bonds being weaker than Cl–Si bonds. We also show that, for both conventional and supersaturation etching, the barrier for the rate-limiting step of desorption of SiX2 units is lower when the halogen X is Br rather than Cl. This contributes to the overall reaction bar...

SiSi Double Bonds: Synthesis of an NHC‐Stabilized Disilavinylidene

An efficient two-step synthesis of the first NHC-stabilized disilavinylidene (Z)-(SIdipp)Si=Si(Br)Tbb (2; SIdipp=C[N(C6H3-2,6-iPr2)CH2]2, Tbb=C6H2-2,6-[CH(SiMe3)2]2-4-tBu, NHC=N-heterocyclic carbene) is reported. The first step of the procedure involved a 2:1 reaction of SiBr2(SIdipp) with the 1,2-dibromodisilene (E)-Tbb(Br)Si=Si(Br)Tbb at 100 °C, which afforded selectively an unprecedented NHC-stabilized bromo(silyl)silylene, namely SiBr(SiBr2Tbb)(SIdipp) (1). Alternatively, compound 1 could be obtained from the 2:1 reaction of SiBr2(SIdipp) with LiTbb at low temperature. 1 was then selectively reduced with C8K to give the NHC-stabilized disilavinylidene 2. Both low-valent silicon compounds were comprehensively characterized by X-ray diffraction analysis, multinuclear NMR spectroscopy, and elemental analyses. Additionally, the electronic structure of 2 was studied by various quantum-chemical methods.

SiSi Double Bonds: Synthesis of an NHC‐Stabilized Disilavinylidene

AbstractAn efficient two‐step synthesis of the first NHC‐stabilized disilavinylidene (Z)‐(SIdipp)SiSi(Br)Tbb (2; SIdipp=C[N(C6H3‐2,6‐iPr2)CH2]2, Tbb=C6H2‐2,6‐[CH(SiMe3)2]2‐4‐tBu, NHC=N‐heterocyclic carbene) is reported. The first step of the procedure involved a 2:1 reaction of SiBr2(SIdipp) with the 1,2‐dibromodisilene (E)‐Tbb(Br)SiSi(Br)Tbb at 100 °C, which afforded selectively an unprecedented NHC‐stabilized bromo(silyl)silylene, namely SiBr(SiBr2Tbb)(SIdipp) (1). Alternatively, compound 1 could be obtained from the 2:1 reaction of SiBr2(SIdipp) with LiTbb at low temperature. 1 was then selectively reduced with C8K to give the NHC‐stabilized disilavinylidene 2. Both low‐valent silicon compounds were comprehensively characterized by X‐ray diffraction analysis, multinuclear NMR spectroscopy, and elemental analyses. Additionally, the electronic structure of 2 was studied by various quantum‐chemical methods.

Reactions of diaryldibromodisilenes with N-heterocyclic carbenes: formation of formal bis-NHC adducts of silyliumylidene cations.

Reactions of stable 1,2-dibromodisilenes ((E)-Ar(Br)Si=Si(Br)Ar) with N-heterocyclic carbenes (NHC) afforded NHC-arylbromosilylene adducts or bromide salts of the corresponding bis-NHC adducts of the formal arylsilyliumylidene cations ([ArSi:](+)). In some cases, an NHC was able to replace a bromide anion in the coordination sphere of the arylbromosilylene-NHC adduct.

Mechanisms for the Reactions of Group 10 Transition Metal Complexes with Metal–Group 14 Element Bonds, Bbt(Br)E═M(PCy3)2(E = C, Si, Ge, Sn, Pb; M = Pd and Pt)

The electronic structures of the Bbt(Br)E═M(PCy(3))(2) (E = C, Si, Ge, Sn, Pb and M = Pt, Pd) complexes and their potential energy surfaces for the formation and water addition reactions were studied using density functional theory (B3LYP/LANL2DZ). The theoretical evidence suggests that the bonding character of the E═M double bond between the six valence-electron Bbt(Br)E: species and the 14 valence-electron (PCy(3))(2)M complexes has a predominantly high s-character. That is, on the basis of the NBO, this theoretical study indicates that the σ-donation from the E element to the M atom prevails. Also, theoretical computations suggest that the relative reactivity decreases in the order: Bbt(Br)C═M(PCy(3))(2) > Bbt(Br)Si═M(PCy(3))(2) > Bbt(Br)Ge═M(PCy(3))(2) > Bbt(Br)Sn═M(PCy(3))(2) > Bbt(Br)Pb═M(PCy(3))(2), irrespective of whether M = Pt or M = Pd is chosen. Namely, the greater the atomic weight of the group 14 atom (E), the larger is the atomic radius of E and the more stable is its Bbt(Br)E═M(PCy(3))(2) doubly bonded species toward chemical reactions. The computational results show good agreement with the available experimental observations. The theoretical results obtained in this work allow a number of predictions to be made.

Isolation of a Stable, Acyclic, Two-Coordinate Silylene

The synthesis and characterization of a stable, acyclic two-coordinate silylene, Si(SAr(Me(6)))(2) [Ar(Me(6)) = C(6)H(3)-2,6(C(6)H(2)-2,4,6-Me(3))(2)], by reduction of Br(2)Si(SAr(Me(6)))(2) with a magnesium(I) reductant is described. It features a V-shaped silicon coordination with a S-Si-S angle of 90.52(2)° and an average Si-S distance of 2.158(3) Å. Although it reacts readily with an alkyl halide, it does not react with hydrogen under ambient conditions, probably as a result of the ca. 4.3 eV energy difference between the frontier silicon lone pair and 3p orbitals.

Synthesis of an Arylbromosilylene–Platinum Complex by Using a 1,2-Dibromodisilene As a Silylene Source

Kinetically stabilized 1,2-dibromodisilene, Bbt(Br)Si═Si(Br)Bbt (Bbt = 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl) (2), was treated with [Pt(PCy3)2] to afford the corresponding arylbromosilylene–Pt complex, Bbt(Br)Si═Pt(PCy3)2 (1), as stable orange crystals. The molecular structure of 1 was elucidated by spectroscopic and X-ray crystallographic analyses. The nature of the Si–Pt bond in 1 was investigated with the aid of DFT calcualtions.

Rare earth (Eu 3+, Tb 3+) mesoporous hybrids with calix[4]arene derivative covalently linking MCM-41: Physical characterization and photoluminescence property

MCM-41 mesoporous silica has been functionalized with two kinds of macrocylic calixarene derivatives Calix[4] and Calix[4]Br (Calix[4]=P- tert-butylcalix[4]arene, Calix[4]Br=5.11,17.23-tetra- tert-butyl-25.27-bihydroxy-26.28-bibromopropoxycalix[4]arene) through condensation approach of tetraethoxysilane (TEOS) in the presence of the cetyltrimethylammonium bromide (CTAB) surfactant as a template. Novel organic–inorganic mesoporous luminescent hybrid containing RE 3+ (Eu 3+, Tb 3+) complexes covalently attached to the functionalized ordered mesoporous MCM-41, which are designated as RE-Calix[4]-MCM-41 and RE-Calix[4]Br-MCM-41, respectively, are obtained by sol–gel process. It is found that they all have high surface area, uniform in the mesostructure and good crystallinity. Measurement of the photoluminescence properties show the mesoporous material covalently bonded Tb 3+ complexes (Tb-Calix[4]-MCM-41 and Tb-Calix[4]Br-MCM-41) exhibit the stronger characteristic emission of Tb 3+ and longer lifetime than the corresponding Eu-containing materials Eu-Calix[4]-MCM-41 and Eu-Calix[4]Br-MCM-41 due to the triplet state energy of modified organic ligands Calix[4]-Si and Calix[4]Br–Si match with the emissive energy level of Tb 3+ very well.

Synthesis and structure of stable 1,2-diaryldisilyne

A novel 1,2-diaryldisilyne, BbtSi≡SiBbt (Bbt = 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl), was synthesized as a stable compound by reduction of the corresponding 1,2-dibromodisilene, Bbt(Br)Si=Si(Br)Bbt. The spectral and structural features of this first stable 1,2-diaryldisilyne are revealed, and the Si≡Si triple-bond character is evaluated with the aid of detailed theoretical calculations. The triple-bond characters of BbtSi≡SiBbt and BbtGe≡GeBbt are compared based on experimental and theoretical results.

Synthesis and Reactions of a Stable 1,2-Diaryl-1,2-dibromodisilene: A Precursor for Substituted Disilenes and a 1,2-Diaryldisilyne

Synthesis and isolation of the stable diaryldibromodisilene, Bbt(Br)SiSi(Br)Bbt, has been accomplished for the first time. The dibromodisilene underwent substitution reactions with organometallic reagents on the low-coordinated silicon atom to afford the corresponding substituted disilenes. Furthermore, the reaction of 1 with t-BuLi afforded the corresponding 1,2-diaryldisilyne, BbtSi[triple bond]SiBbt, the characters of which were revealed by spectroscopic and crystallographic analyses.

Electron-stimulated desorption from an unexpected source: Internal hot electrons for Br–Si(1 0 0)-(2 × 1)

The desorption of Br adatoms from Br-saturated Si(1 0 0)-(2 × 1) was studied with scanning tunneling microscopy as a function of dopant type, dopant concentration, and temperature for 620–775 K. Analysis yields the activation energies and prefactors for desorption, and the former correspond to the energy separation between the Fermi level and Si–Br antibonding states. Thus, electron capture in long-lived states results in Br expulsion via a Franck–Condon transition. Analysis of the prefactors reveals that optical phonons provide the energy needed for the electronic excitation. These results show that desorption induced by an electronic transition can occur in closed system without external stimulus, and they indicate that thermally-excited charge carriers may play a general role in surface reactions.

Ag Multilayer island growth on Br–Si(0 0 1)-(2 × 1) and Ag-induced nano-pitting

Low temperature scanning tunneling microscopy was used to study the formation of single crystal nanostructures of Ag on Br-terminated Si(0 0 1)-(2 × 1) and their interactions with the surface at elevated temperature. Multilayer nanostructures were formed at 640 K. They were facetted, and they exhibited highly-ordered surfaces that were imaged with atomic resolution to identify the 〈0 1 1〉 and 〈1 1 2〉 directions on Ag(1 1 1)-(1 × 1). Heating above 700 K induced reactions involving Ag and Br and the desorption of AgBr. At these temperatures, contact between the Ag islands and Si(0 0 1) also led to Si dissolution in the islands, and three dimensional pits bounded by {1 1 3} facets were produced.

Imprinting Br-atoms at Si(1 1 1) from a SAM of CH 3Br(ad), with pattern retention

The formation of molecular-scale patterns at surfaces as a self-assembled monolayer (SAM) demands adsorbate mobility, whereas devices require stability. We describe a method of photo- or electron-imprinting a SAM of CH 3Br(ad) as covalently-bound Br–Si(s) at Si(1 1 1)7 × 7, with pattern-retention. This imprinting process, it is proposed, involves charge-transfer to the adsorbate, followed by downward recoil of Br − to give chemical attachment of Br at the reactive Si-atom beneath the parent physisorbed methyl bromide. The electron may subsequently be returned to the substrate along with the excess energy, accounting for the localised Br-imprint.

Parent- and daughter-mediated halogenation reactions modeled for 1,2- and 1,4-dibromobenzene at Si(1 1 1)-7 × 7

We report a first attempt, using AM1, to model the thermal reaction of 1,2- and 1,4-dibromobenzene with a Si(1 1 1)-7 × 7 surface, a reaction recently studied experimentally by scanning tunneling microscopy (STM) [S. Dobrin et al., Surf. Sci. 561 (2004) 11]. As postulated on the basis of the experimental findings, two generalizable types of reaction pathways are found to be open. The indirect, ‘Parent-mediated’, dynamics involves prior attachment of the parent molecule by (in the present case) its benzene ring to the surface, followed by detachment of one or two bromine atoms from the ring, R, which most often remains attached to the surface. This results in formation of a single Br–Si or a pair or Br–Si with, in each case, an adjacent feature, R–Si. The alternate ‘Daughter-mediated’ pathway involves the direct approach of the reactive daughter Br-atoms to the Si surface to form the same products without, generally, leaving an organic residue, R–Si, at the surface. The calculated activation energies for successive bromination of the surface by either the Parent-mediated (P) or Daughter-mediated (D) pathways were found to lie in the range 0.7–1.27 eV, consistent with the observation of thermal reaction of the title molecules to give both single Br–Si and pairs of Br–Si at room temperature by either mechanism. Activation energies for bromination by P or D are less than the bond-dissociation energies for the C–Br bond being broken, since Br–Si bond-formation occurs concurrently with bond breaking. The old C–Br bond extends to form the new Br–Si, so that Br–Si imprints at the surface have a wider mean separation than do the Br substituents in the parent, as well as a wider mean separation for Br–Si when they come from 1,4- than from 1,2 dibromobenzene. This agrees with observation. In Daughter-mediated reaction the adsorbate is postulated to approach laterally, ‘stamping’ a characteristic narrow Br⋯Br separation or ‘rocking’ to imprint the wider separations observed.

An STM study of the localized atomic reaction of 1,2- and 1,4-dibromobenzene at Si(111)-7×7

A comparative study is reported of the thermal reaction of 1,2- and 1,4-dibromobenzene (1,2- and 1,4-diBrPh) on Si(1 1 1)-7 × 7, investigated by STM. Some results are given for the intermediate case of 1,3-diBrPh. The STM images gave evidence of a different pattern of reaction to yield pairs of Br–Si for 1,2-, 1,3- and 1,4-diBrPh. The ratio of pairs of Br–Si to single bromination events was 1:2 for 1,2-diBrPh and 1:3 for 1,4-diBrPh. In many cases organic residue from the bromination reaction, R–Si, was evident in the STM image. The products R–Si and Br–Si were found to be bound to adjacent Si, for both 1,2- and 1,4-diBrPh. The mean Br⋯Br pair separation at the surface depended on the parent molecule, being 7.6 Å for 1,2-diBrPh, 10.3 Å for 1,3-diBrPh, and 11.3 Å for 1,4-diBrPh. These separations are, in each case, about 4 Å greater than the separation of the Br-atoms in the intact parent molecule, which increases systematically down the series. There was a marked decrease in the percentage of R–Si accompanying the Br–Si in going down the series, decreasing from 70% for 1,2- to 20% for 1,4-diBrPh; this was interpreted as being due to a decrease in the percentage of `benzene-mediated' reaction dynamics, in which the benzene ring was bound to the surface. At moderately increased surface temperature (45 °C) the reaction of 1,2- and also 1,4-diBrPh no longer resulted in R–Si formation, suggesting that the dynamics had altered from benzene-mediated to `bromine-mediated'.

Shape variation in epitaxial microstructures of gold silicide grown on Br-passivated Si(1 1 1) surfaces

Kinetic Monte Carlo simulations for growth on substrates of three-fold symmetry predict the growth of islands of various shapes depending on the growth temperature [Phys. Rev. Lett. 71 (1993) 2967]. On Br–Si(1 1 1) substrates growth of epitaxial gold silicide islands of equilateral triangular and trapezoidal shapes have earlier been observed by annealing at the Au–Si eutectic temperature, 363 °C [Phys. Rev. B 51 (1995) 14330]. We carried out annealing with temperature variation within a small window––(363 ± 30) °C. This has led to island growth of additional shapes like regular hexagon, elongated hexagon, walled hexagon and dendrite. Some of the observed island shapes have not been predicted.

1,3-Migration of Trimethylsilyl Group from Carbon to Oxygen in Highly Sterically Hindered Silanol of the Type (Me 3 Si) 3 CSi(C 6 H 4 Me- p )MeOH and Reaction of the Related Iodide with Iodine Monochloride and Iodine Monobromide

The silanol (Me 3 Si) 3 CSi(C 6 H 4 Me- p )MeOH has been shown to isomerize to (Me 3 Si) 2 CHSi(C 6 H 4 Me- p )(Me)(OSiMe 3 ) when it was kept at room temperature for 10 h in 0.2 M NaOMe/MeOH. Corresponding isomerizations of the above silanol (to give (Me 3 Si) 2 CHSi(C 6 H 4 Me- p ) (Me)(OSiMe 3 )) are complete after 26 h under reflux in pyridine. The reaction involve 1,3-migration from carbon to oxygen within a silanolate ion to give a carbanion, which rapidly acquires a proton from the solvent. Treatment of (Me 3 Si) 3 CSi(C 6 H 4 Me- p )MeOH with MeLi in Et 2 O/THF give, by the same rearrangement, the organolithium reagent (Me 3 Si) 2 CLiSi(C 6 H 4 Me- p )(Me)(OSiMe 3 ) which on treatment with Me 2 SiHCl gives (Me 3 Si) 2 C(SiMe 2 H)Si(C 6 H 4 Me- p )(Me)(OSiMe 3 ) and (Me 3 Si) 2 CHSi(C 6 H 4 Me- p )(Me)(OSiMe 3 ). When the experiment was repeated, but with Me 3 SiCl in place of Me 2 SiHCl, it gives exclusively (Me 3 Si) 2 CHSi(C 6 H 4 Me- p )(Me)(OSiMe 3 ). Treatment of the organolithium reagent (Me 3 Si) 2 CLiSi(C 6 H 4 Me- p )(Me)(OSiMe 3 ) with Mel gives exclusively (Me 3 Si) 2 CMeSi(C 6 H 4 Me- p )(Me)(OSiMe 3 ). The related iodide (Me 3 Si) 3 CSi(C 6 H 4 Me- p )Mel reacts with ICI and IBr to give rearranged (Me 3 Si) 2 C(SiMe 2 X)Si(C 6 H 4 Me- p )Me 2 and unrearranged products (Me 3 Si) 3 CSi(C 6 H 4 Me- p )MeX, (X = Cl, Br) respectively. The rearranged bromide (Me 3 Si) 2 C(SiMe 2 Br)Si(C 6 H 4 Me- p )Me 2 reacts with a range of silver [I] salts AgY (Y = OOCCH 3 , SO 4 2 m ) and Mercury [II] salt HgY 2 (Y = OOCCH 3 , SO 4 2 m ) in glacial CH 3 COOH to give the corresponding species (Me 3 Si) 2 C(SiMe 2 OOCCH 3 )Si(C 6 H 4 Me- p )Me 2 . The reaction of the bromide with AgBF 4 in MeOH or i -PrOH give the corresponding rearranged products (Me 3 Si) 2 C(SiMe 2 Y)Si(C 6 H 4 Me- p )Me 2 (Y = --OMe, --OPr i ).