Chemisorption of barrelene on Si(100) from first principles calculations

The chemisorption of C8H8 bicyclo[2.2.2]-2,5,7-octatriene (barrelene) on the Si(100) surface is studied from first principles calculations. We find that, in the most stable configuration, barrelene is bonded to Si(100) through four Si-C bonds, with the C-C bonds which are orthogonal to the underlying Si dimers. The chemisorption reaction responsible for this structure is driven by the biradical nature of the Si-Si dimer bond. Two others, slightly less stable configurations, exist which are also characterized by four Si-C bonds but have a different orientation or location with respect to the Si(100) surface. The properties of these and other, less stable configurations have been investigated. For the most stable structures, the effect of different surface coverages has been also studied, showing a tendency to easily form complete monolayers of barrelene on the Si surface. On the basis of energetic and kinetic considerations, we expect that chemisorption of barrelene monolayers on the Si(100) surface will be characterized however by a certain amount of disorder. Finally, several possible reaction pathways, leading from one stable structure to another of lower energy or from a molecule in the gas phase to a chemisorbed configuration, have been investigated in detail and estimates of the relative energy barriers are given.     

Chlorination-methylation of the hydrogen-terminated silicon(111) surface can induce a stacking fault in the presence of etch pits

Recently, we reported STM images of the methylated Si(111) surface [prepared through chlorination-alkylation of the Si(111)-H surface] taken at 4.7 K, indicating that the torsion angle of the methyl group with respect to the subsurface silicon layer is phi = 23 +/- 3 degrees . Repulsions between H atoms in adjacent methyl groups are minimized at 30 degrees , while repulsions between H atoms and second layer Si atoms are minimized at 60 degrees . The experimental result of 23 degrees is surprising because it suggests a tendency of the methyl group toward the eclipsed configuration (0 degrees ) rather than staggered (60 degrees ). In contrast, extensive fully periodic quantum mechanical Density Functional Theory studies of this surface give an equilibrium torsion angle of 37.5 degrees , indicating a tendency toward the staggered configuration. This discrepancy can be resolved by showing that the CH3 on the step edges and etch pits interacts repulsively with the CH3 on the surface terraces unless a stacking fault is introduced between the first and second silicon layers of the Si(111)-CH3 surface terraces. We propose that this could occur during the chlorination-alkylation of the Si(111)-H surface. This stacking fault model predicted phi = 22.5 degrees measured with respect to the bulk (corresponding to phi = 37.5 degrees with respect to the second layer Si atoms). This model can be tested by measuring the orientation of the CH3 within the etch pits, which should have phi = 37.5 degrees , or by making a surface without etch pits, which should have phi = 37.5 degrees .     

Removal Mechanism Investigation of Ultraviolet Induced Nanoparticle Colloid Jet Machining

Ultraviolet induced nanoparticle colloid jet machining is a new ultra-precision machining technology utilizing the reaction between nanoparticles and the surface of the workpiece to achieve sub-nanometer ultra-smooth surface manufacturing without damage. First-principles calculations based on the density functional theory (DFT) were carried out to study the atomic material removal mechanism of nanoparticle colloid jet machining and a series of impacting and polishing experiments were conducted to verify the mechanism. New chemical bonds of Ti-O-Si were generated through the chemical adsorption between the surface adsorbed hydroxyl groups of the TiO₂ cluster and the Si surface with the adsorption energy of at least −4.360 eV. The two Si-Si back bonds were broken preferentially and the Si atom was removed in the separation process of TiO₂ cluster from the Si surface realizing the atomic material removal. A layer of adsorbed TiO₂ nanoparticles was detected on the Si surface after 3 min of fixed-point injection of an ultraviolet induced nanoparticle colloid jet. X-ray photoelectron spectroscopy results indicated that Ti-O-Si bonds were formed between TiO₂ nanoparticles and Si surface corresponding to the calculation result. An ultra-smooth Si workpiece with a roughness of Rq 0.791 nm was obtained by ultraviolet induced nanoparticle colloid jet machining.     

Gold as hydrogen. An experimental and theoretical study of the structures and bonding in disilicon gold clusters Si2Au(n)- and Si2Au(n) (n = 2 and 4) and comparisons to Si2H2 and Si2H4

In a previous communication, we showed that a single Au atom behaves like H in its bonding to Si in a series of Si-Au clusters, SiAu(n) (n = 2-4) (Kiran et al. Angew. Chem., Int. Ed. 2004, 43, 2125). In this article, we show that the H analogy of Au is more general. We find that the chemical bonding and potential energy surfaces of two disilicon Au clusters, Si(2)Au(2) and Si(2)Au(4), are analogous to Si(2)H(2) and Si(2)H(4), respectively. Photoelectron spectroscopy and ab initio calculations are used to investigate the geometrical and electronic structures of Si(2)Au(2)(-), Si(2)Au(4)(-), and their neutral species. The most stable structures for both Si(2)Au(2) and Si(2)Au(2)(-) are found to be C(2)(v), in which each Au bridges the two Si atoms. For Si(2)Au(4)(-), two nearly degenerate dibridged structures in a cis (C(2)(h)) and a trans (C(2)(v)) configuration are found to be the most stable isomers. However, in the neural potential energy surface of Si(2)Au(4), a monobridged isomer is the global minimum. The ground-state structures of Si(2)Au(2)(-) and Si(2)Au(4)(-) are confirmed by comparing the computed vertical detachment energies with the experimental data. The various stable isomers found for Si(2)Au(2) and Si(2)Au(4) are similar to those known for Si(2)H(2) and Si(2)H(4), respectively. Geometrical and electronic structure comparisons with the corresponding silicon hydrides are made to further establish the isolobal analogy between a gold atom and a hydrogen atom.     

Ab initio surface reaction energetics of SiH4 and Si2H6 on Si(001)-(2 x 2)

First-principles pseudopotential calculations, within a simple dynamically constrained scheme, have been performed to investigate the reaction of 0.25 ML coverage of SiH4 and Si2H6 with the Si(001)-(2 x 2) surface. The silane molecule (SiH4) is adsorbed on to the surface at a number of different sites (on dimer, interrow, or intrarow) with varying barrier heights. Two distinct structures, which are similar in energy, arise from the initial dissociative reaction SiH4-->SiH3(silyl) + H, where the dissociated species are adsorbed either on the same dimer components or on adjacent dimer components. Several further decays of silyl from SiH4 are presented in two separate regimes of high and low ambient hydrogen coverages. The decomposition of silyl can form two different bridging structures: an on top or an intrarow bridging structure in both of the two hydrogen coverage regimes. The disilane molecule (Si2H6) is also adsorbed upon this surface with varying energy barriers, resulting in a dissociation reaction where two SiH3 species are adsorbed on one dimer or in an adjacent dimer configuration. Plausible energy reaction paths for the above models are presented. The stability of the SiH2 species is also discussed.     

On the electronic origins of barriers to methyl rotation: CNDO/2 calculations on (CH3)2XHn (X = C,Si, N,P, O,S) molecules

CNDO calculations are performed for the homogeneous series of (CH₃)₂XH_n compounds (X = C, Si, N, P, O, S) in order to determine the molecular equilibrium configuration. In agreement with available experimental data, for all investigated molecules, the theoretical energy minimum is found for the (θ = 60°, ψ = 60°) conformation in which one hydrogen of each methyl group is in the X heavy atoms plane but pointing outside the CXC angle. A partitioning of total energy shows that the variations of this quantity are completely reflected by the variations of the interaction energy between non-bonded terms. A more detailed analysis reveals, only for the third-row compounds, the essential role of the interactions between central atom and methyl hydrogens.     

Silaacetylene: A possible target for experimental studies

The equilibrium geometries and transition states for interconversion of the CSiH₂ isomers in the singlet electronic ground state are optimized at the MP2 and CCSD(T) levels of theory using a TZ2P basis set. The heats of formation, vibrational frequencies, infrared intensities, and rotational constants are also predicted. There are three energy minima on the CSiH₂ potential energy surface. Energy calculations at CCSD(T)/TZ2P(fd) + ZPE predict that the global energy minimum is silavinylidene (1), which is 34.1 kcal mol⁻¹ lower in energy than trans-bent silaacetylene (2) and 84.1 kcal mol⁻¹ more stable than the vinylidene isomer (3). The barrier for rearrangement 2→1 is calculated at the same level of theory to be 5.1 kcal mol⁻¹, while for the rearrangement 3→2 a barrier of 2.7 kcal mol⁻¹ is predicted. The natural bond orbital (NBO) population scheme indicates a clear polarization of the C(SINGLE BOND)Si bonds toward the carbon end. A significant ionic contribution to the C(SINGLE BOND)Si bonds of 1 and 2 is suggested by the NBO analysis. The C(SINGLE BOND)Si bond length of trans-bent silaacetylene (2) is longer than previously calculated [1.665 Å at CCSD(T)/TZ2P)]. The calculated carbon-silicon bond length of 2 is in the middle between the C(SINGLE BOND)Si double bond length of 1 (1.721 Å) and the C(SINGLE BOND)Si triple bond of the linear form HCSiH (4), which is 1.604 Å. Structure 4 is a higher-order saddle point on the potential energy surface. © 1996 by John Wiley & Sons, Inc.     

Semiconductor surface-induced 1,3-hydrogen shift: the role of covalent vs zwitterionic character

X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) are used to compare the reaction of 1,2-cyclohexanedione (1,2-CHD) with Si(001) and diamond(001) surface dimers under ultra-high-vacuum conditions. 1,2-CHD is known to undergo a keto-enol tautomerization, with the monoenol being the primary equilibrium species in the solid and gas phases. XPS and FTIR data demonstrate that 1,2-CHD reacts with diamond(001) through the OH group of the monoenol, resulting in only one O atom being bonded to the surface. In contrast, XPS and FTIR data suggest that both oxygen atoms in the 1,2-CHD molecule bond via Si-O-C linkages to the Si(001) surface dimer, and that the molecule undergoes an intramolecular 1,3-H shift. While the Si(001) and diamond(001) surfaces are both comprised of surface dimers, the diamond(001) dimer is symmetric, with little charge separation, whereas the Si(001) dimer is tilted and exhibits zwitterionic character. The different reaction products that are observed when clean Si(001) and diamond(001) surfaces are exposed to 1,2-CHD demonstrate the importance of charge separation in promoting a 1,3-H shift and provide new mechanistic insights that may be applicable to a variety of organic reactions.     

A theoretical investigation of atmospheric sulfur chemistry. 1. The HSO/HOS energy separation and the heat of formation of HSO, HOS, and HS2

The energy separation between the ground-state structures of HSO and HOS has been determined by using two independent ab initio methods. In the first method, the optimized geometry of all species was obtained at the HF/6-31G(d) level, as were harmonic vibrational frequencies for zero-point energy corrections. The energies were calculated by using fourth-order Moller-Plesset perturbation theory and a 6-31G(d,p) basis set. After corrections for extrapolation of the Moller-Plesset series to infinite order and extension of the basis set to include diffuse sp-, extra d-, and f-type Gaussian functions, the predicted energy separation, including zero-point vibrational effects, is 2.5 kcal/mol. HOS is the more stable isomer. The second method uses a double-zeta basis augmented with an extra set of p functions and two sets of d functions on the sulfur and oxygen atoms and a double-zeta + p basis on hydrogen. With this basis, equilibrium structures of HSO and HOS were obtained from MCSCF calculations; the energy separation between these structures was corrected by using large scale configuration interaction. In good agreement with the first method, HOS is the more stable isomer by 3.1 kcal/mol. Through calculation of the energy change in the reaction HO2 + XY --> O2 + HXY, the first method predicts the heats of formation of HXY = HSO, HOS, and HS2 to be -0.4, -2.9, and 26.7 kcal/mol, respectively.     

Peculiarities of the silica surface center structure in rigid dehydroxidation conditions

A model of molecular claster and nonempirical calculation schemes MP2/6‐31G*, MP4/6‐31G*, and MP2/DZVP2 were applied to study electronic and energy characteristics of the surface centers of different SiO₂ modifications in rigid dehydroxidation conditions. It was established that depending on an angle α = Si′O′Si″ (where O′ is an oxygen atom of the nearest to the surface layer), O* atom corresponding with two surface Si atoms, could (i) form a chemical bond with one of the Si′ or Si″‐centers (an asymmetric configuration, α > 118° ÷ 120°); or (ii) be common for these centers (a symmetric configuration, α < 118° ÷ 120°). It was also established that when α > 132° ÷ 133°, the basic state was triple and at α < 132° ÷ 133°, it was single. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Adhesion between a charged particle in an electrolyte solution and a charged substrate: Electrostatic and van der Waals interactions

The equilibrium separation between a charged particle in an electrolyte solution and a substrate with an initially uniform surface charge density is obtained using the classical Derjaguin-Landau-Verwey-Overbeek theory. The electrostatic free energy is obtained by coupling the electric response of the substrate with the electric potential obtained from the solution of the Debye-Hückel equation. The van der Waals free energy is calculated by integrating the 6-12 Lennard-Jones potential. Metallic, dielectric, and semiconducting substrates are considered in turn. At low ionic strength, our results demonstrate a distinct response to the charged particle in each case. For example, in the case of a metallic substrate, the attached state (corresponding to equilibrium separation at short range) is always close to the van der Waals energy minimum. In addition, the application of a surface charge of sign opposite to that of the particle facilitates the transition from the detached state (corresponding to large separation at which the interaction between the particle and the substrate is negligible) to the attached state but scarcely changes the equilibrium separation. In the case of a dielectric substrate, the attached state is located at a distance of around two orders of magnitude larger than that for a metallic substrate and this equilibrium separation decreases as the (opposing) surface charge increases. A semiconducting substrate can behave either like a metal or like a dielectric, depending on the ratio of its Debye length to that of the electrolyte solution.     

Theoretical investigation of the structure and coverage of the Si(111)-OCH3 surface

The surface structure, strain energy, and charge profile of the methoxylated Si(111) surface, Si(111)-OCH3, has been studied using quantum mechanics, and the results are compared to those obtained previously for Si(111)-CH3 and Si(111)-C2H5. The calculations indicate that 100% coverage is feasible for Si(111)-OCH3 (similar to the methylated surface), as compared to only approximately 80% coverage for the ethylated surface. These differences can be understood in terms of nearest-neighbor steric and electrostatic interactions. Enthalpy and free energy calculations indicate that the formation of the Si(111)-OCH3 surface from Si(111)-H and methanol is favorable at 300 K. The calculations have also indicated the conditions under which stacking faults can emerge on Si(111)-OCH3, and such conditions are contrasted with the behavior of Si(111)-CH3 and Si(111)-CH2CH3 surfaces, for which stacking faults are calculated to be energetically feasible when etch pits with sufficiently long edges are present on the surface.     

Molecular quantum cellular automata cells. Electric field driven switching of a silicon surface bound array of vertically oriented two-dot molecular quantum cellular automata

The amine functionality of the linker on the dinuclear complex [trans-Ru(dppm)(2)(Ctbd1;CFc)(NCCH(2)CH(2)NH(2))][PF(6)] reacts with Si-Cl bonds of a chlorinated, highly B doped Si (111) surface to yield Si-N surface-complex bonds. The surface bound complex is constrained to a near vertical orientation by the chain length of the linker as confirmed by variable angle XPS. Oxidation of the dinuclear complex with ferrocenium ion or electrochemically generates a stable, biased Fe(III)-Ru(II) mixed-valence complex on the surface. Characterization of the array of surface bound complexes with spectroscopic as well as electrochemical techniques confirms the presence of strongly bound, chemically robust, mixed-valence complexes. Capping the flat array of complexes with a minimally perturbing mercury electrode permits the equalization of the Fe and Ru energy wells by an applied electric field. The differential capacitance of oxidized and unoxidized bound complexes is compared as a function of voltage applied between the Hg gate and the Si. The results show that electron exchange between the Fe and Ru sites of the array of dinuclear mixed-valence complexes at energy equalization generates a fluctuating dipole that produces a maximum in the capacitance versus voltage curve for each complex-counterion combination present. Passage through the capacitance maximum corresponds to switching of the molecular quantum cellular automata (QCA) cell array by the electric field from the Fe(III)-Ru(II) configuration to the Fe(II)-Ru(III) configuration, thereby confirming that molecules possess an essential property necessary for their use as elements of a QCA device.     

The basis set dependence of structures and energies of various states of cyclodisiloxane

Several common basis sets, ranging from minimal to double-zeta, are applied to study the neutral singlet and triplet as well as positive- and negative-ion doublet states of cyclodisiloxane. The effect of d-polarization function exponents on the equilibrium geometries and energies is analyzed. The d-type functions seem to be essential in the basis set of silicon, whereas their presence on oxygen is less critical. The optimum exponents (with respect to SCF energy) are determined to be 0.45 for Si and 0.60 for O, very close to those recommended for the 6–31G^** basis set. The best structural predictions are obtained with the 6–31G(2d, p) basis set, which contains two sets of d functions on the heavy atoms. The predicted Si? O bond length is 166 pm; the Si? Si and O? O distances are 237 and 232 pm, respectively, which correspond to an O—Si? O angle of 88.6°. The ground state is found to be a singlet. All higher states have longer Si? O bonds and Si—Si distances, whereas O—O distances are shorter. The energy separation between the singlet and other states is modified by electron correlation (MP treatment) by only a few kcal/mol.     

Acidification of three-dimensional emeraldine polymers: Search for minimum energy paths from base to salt

We present a numerical simulation of the HCl acidification process of a three-dimensional semiconducting emeraldine base (EB) polymer leading to the corresponding metallic emeraldine salt form. We have searched minimum energy paths connecting the initial configuration, composed of two EB polymer chains per cell each one attached by two HCl molecules, with the Pc2a polaronic configuration which is the final state of the acidification process. For this aim, the variational nudged elastic band method has been adopted. We provide a pictorial representation of the acidification process at T=0 K, monitoring the EB protonation and the evolution of the polymeric chains and of the positions of the Cl(-) counterions on the lowest potential energy surface. To include also temperature effects, we have explored the potential energy surface around the final equilibrium configuration, heating the system and following its dynamics by the Car-Parrinello procedure.     

Interactions between radical growth precursors on plasma-deposited silicon thin-film surfaces

We present a detailed analysis of the interactions between growth precursors, SiH3 radicals, on surfaces of silicon thin films. The analysis is based on a synergistic combination of density functional theory calculations on the hydrogen-terminated Si(001)-(2x1) surface and molecular-dynamics (MD) simulations of film growth on surfaces of MD-generated hydrogenated amorphous silicon (a-Si:H) thin films. In particular, the authors find that two interacting growth precursors may either form disilane (Si2H6) and desorb from the surface, or disproportionate, resulting in the formation of a surface dihydride (adsorbed SiH2 species) and gas-phase silane (SiH4). The reaction barrier for disilane formation is found to be strongly dependent on the local chemical environment on the silicon surface and reduces (or vanishes) if one/both of the interacting precursors is/are in a "fast diffusing state," i.e., attached to fivefold coordinated surface Si atoms. Finally, activation energy barriers in excess of 1 eV are obtained for two chemisorbed (i.e., bonded to a fourfold coordinated surface Si atom) SiH3 radicals. Activation energy barriers for disproportionation follow the same tendency, though, in most cases, higher barriers are obtained compared to disilane formation reactions starting from the same initial configuration. MD simulations confirm that disilane formation and disproportionation reactions also occur on a-Si:H growth surfaces, preferentially in configurations where at least one of the SiH3 radicals is in a "diffusive state." Our results are in agreement with experimental observations and results of plasma process simulators showing that the primary source for disilane in low-power plasmas may be the substrate surface.     

Atomic structure and formation mechanism of identically sized Au clusters grown on Si(111)-(7×7) surface

Identically sized Au clusters are grown on the Si(111)-(7×7) surface by room temperature deposition of Au atoms and subsequent annealing at low-temperature. The topographical images investigated by in situ scanning tunneling microscopy show a bias-dependent feature. The current-voltage properties measured by scanning tunneling spectroscopy indicate some semiconducting characteristics of the Au adsorbed surface, which is attributable to the saturation of Si dangling bonds. These experimental results, combined with the simulated scanning tunneling microscopy images and the first-principles adsorption energy calculations, show that the Au cluster is most likely to have a Au(6)Si(3) structure. In the Au(6)Si(3) cluster, three adsorbed Au atoms replace the three Si center adatoms, forming a hollow triangle, while the replaced Si atoms and other three Au atoms connect into a hexagon locating within the triangle. The formation mechanism of this atomic configuration is intimately associated with the complicated chemical valences of Au and the specific annealing conditions.     

Nanometer-scale wetting of the silicon surface by its equilibrium oxide

Despite the extremely broad technical applications of the Si/SiO2 structure, the equilibrium wetting properties of silicon oxide on silicon are poorly understood. Here, we produce new results in which a solid-state buffer method is used to systematically titrate oxygen activity about the Si/SiO2 coexistence value. The equilibrium morphology at the Si(001) surface over >8 decades of PO2 about coexistence is revealed to be a uniform sub-stoichiometric SiOx film of sub-nanometer thickness, coexisting with secondary island structures which coarsen with annealing time. A new thermodynamic method using chemical potential to stabilize and control surficial oxides in nanoscale devices is suggested.     

Tuning Optical Properties of Si Quantum Dots by π‐Conjugated Capping Molecules

The absorption and photoluminescence (PL) properties of silicon quantum dots (QDs) are greatly influenced by their size and surface chemistry. Herein, we examined the optical properties of three Si QDs with increasing σ–π conjugation length: octyl‐, (trimethylsilyl)vinyl‐, and 2‐phenylvinyl‐capped Si QDs. The PL photon energy obtained from as‐prepared samples decreased by 0.1–0.3 eV, while the PL excitation (PLE) extended from 360 nm (octyl‐capped Si QDs) to 400 nm (2‐phenylvinyl‐capped Si QDs). A vibrational PL feature was observed in all samples with an energy separation of about 0.192±0.013 eV, which was explained based on electron–phonon coupling. After soft oxidization through drying, all samples showed blue PL with maxima at approximately 410 nm. A similar high‐energy peak was observed with the bare Si QD sample. The changes in the optical properties of Si QDs were mainly explained by the formation of additional states arising from the strong σ–π conjugation and QD oxidation.