Valence Force Field Research Articles

Effect of stress by dopants and nitrogen on grown-in defects in silicon

We analyze the defect density change caused by B and Sb doping and the determination of configuration of doped N in silicon from the viewpoint of stress accompanied by doping. The former is phenomenologically analyzed by the size-effect model and the threshold point defect concentration change for void density change is estimated. The latter is analyzed using the valence force field model and the atomic strain energy is calculated. The role of strain energy and unpaired electron in determining the structure is discussed.

シリコン中の窒素の構造決定

Nitrogen doping has attracted much attention because it dramatically reduces void defects in as-grown CZ Si. Various forms of nitrogen have been proposed but primary factor on determining these structures has not been well understood yet. We analyze the atomic stress effect by the valence force field model and calculated the atomic strain energy. It is found that the strain energy determines the stable structure of N2 and N2-V, whereas the presence of unpaired electron determines that of interstitial N and substitutional N.

Thermodynamic properties and valence—power field of thiyl radicals

The consistent valence—force field of alkylthiyl radicals (RS·) was determined for the first time by the solution of an inverse spectral problem. The vibrational spectra of 12 linear and branched RS· forms were calculated. The thermodynamic functions (enthalpy, entropy, heat capacity, and Gibbs free energy) were determined by methods of statistical mechanics in a temperature interval of 298—1500 K. Within the framework of the additive—group approach, the quantitative structure—property relationships were considered for the thermodynamic functions of the RS· radicals, and the parameters of these relationships were calculated.

Thermodynamic properties and valence-force field of alkanethiols and dialkyl sulfides

The consistent valence-force field of alkanethiols (AlkSH), dialkyl sulfides (AlkSAlk"), and alkanedithiols (HSRSH, R is alkylene) was determined for the first time by the solution of an inverse spectral problem. The vibrational spectra of 40 linear and branched molecules AlkSH, RSR", and HSRSH were calculated. Their thermodynamic functions (enthalpy, entropy, thermal capacity, and free Gibbs energy) were determined by methods of statistical mechanics in the 298—1500 E temperature interval. Within the framework of the additive-group approach, the quantitative relationships “structure—property” were considered for the thermodynamic functions of AlkSH, AlkSAlk", and HSRSH, and the parameters of the relationships were calculated. The results of calculations and experimental data were compared.

Theoretical vibrational spectroscopy and the structure of nitrosubstituted glucopyranosides

Coupled calculations have been carried out of normal vibration frequencies from the point of view of the valence-force field scheme and of absolute IR band intensities by the CNDO/2 method for the 2,3-di-O-nitro-methyl-β-d-glucopyranoside and 2,3,6-tri-O-nitro-methyl-β-d-glucopyranoside molecules. A good agreement with experiment has been achieved. A detailed interpretation of the observable IR spectra of both the nitro-glucopyranosides compounds considered is given. Regularities of formation of complex absorption bands of characteristic vibrations of nitro-groups have been established.

A Molecular Modelling Approach of the Structure of Cyclophosphazenes

Molecular modelling of hexachlorocyclotriphosphazene, N3 P3 Cl6 , was achieved by using softwares produced by Biosym Technologies. The CVFF (Consistent Valence ForceField) and ESFF (Extended Standard ForceField) parameters for all atoms including the

Normal Coordinate Analysis and Vibrational Spectra of Aromatic Carbon Nanostructures

Theoretical infrared absorption spectra of aromatic ring molecules having up to 102 carbon atoms and with various edge hydrogenations have been obtained using a classic mechanical model and a simplified valence force field. Force constants have been adapted from those available for smaller molecules. Spectral line intensities are calculated in a double harmonic approximation with effective atomic charges obtained using a Huckel tight-binding Hamiltonian. Vibrational modes of clusters of like aromatic ring molecules are obtained through introduction of an interlayer force constant. These modes are predicted to occur in the wavelength range between 80 and 400 μm. We explore the effect of dehydrogenation on these spectra as well as hydrogenation of terminal C atoms in either aromatic or aliphatic form. Many spectra exhibit a quasi-continuum between 6 and 9 μm that arises from the overlap of many vibrational modes in this region. With CH2 edge groups, we find a new spectral feature near 16 μm and the aliphatic CH2 symmetric stretch vibration is found to shift from 3.5 to 3.3 μm when these groups are present at terminal sites on aromatic carbon skeletons. The relevance of calculated spectra to those of astronomical sources is briefly discussed.

Spatial distribution of elastic deformations in Ge/Si structures with quantum dots

A method is developed for calculating the elastic deformation in coherently strained heterostructures on the basis of the valence force field (VFF) model using the Green’s function of the “atomistic” elastic problem. The spatial distribution of the elastic deformations in a Ge/Si system with pyramidal Ge quantum dots buried in a Si matrix is investigated theoretically. The deformation distribution in and around the pyramids is determined. Near quantum dots, the region near the tip of the pyramid is most strongly intensely. Inside quantum dots the region of the vertex is most relaxed, and the most strained section lies on the contour of the pyramid base. Compression occurs in the plane of the pyramid base inside quantum dots, and stretching occurs along the vertical direction. The picture is reversed near quantum dots: stretching occurs in the lateral direction and compression in the vertical direction. It is shown that the local deformations and their spatial distribution are essentially independent (to within the scaling) of the size of the quantum dots for 10–15 nm pyramid bases.

Analysis of Unstable Two-Phase Region in Wurtzite Group III Nitride Ternary Alloy Using Modified Valence Force Field Model

The Group III-nitride ternary system is studied with respect to an unstable two-phase region in the phase field. The unstable two-phase region is analyzed using a strictly regular solution model. The interaction parameter used in the analysis is obtained from a strain energy calculation using the valence force field model, modified for both wurtzite and zinc-blende structures to avoid overestimation of the strain energy. The structural deviation from an ideal wurtzite structure in InN, GaN, and AlN is also taken into account in our model. According to the calculated results of the interaction parameters, the critical temperature for wurtzite InGaN, InAlN, and GaAlN are found to be 1967 K, 3399 K, and 181 K, respectively. This suggests that, at a typical growth temperature of 800–1000°C a wide unstable two-phase region exists in both InGaN and InAlN. In order to show the validity of our calculation results, we compare the calculated results and the experimental results using the calculation of the interaction parameter for the InGaAs system. The calculated results agree well with the experimental results.

Vibrational Study of the FeCl3-Doped Dimer of Polyaniline; A Good Model Compound of Emeraldine Salt

In this paper, optical and vibrational properties of the FeCl3-doped dimer of polyaniline, a model compound of emeraldine salt are presented. This oligomer is synthesized according to the method reported by Zaghal et al.. A complete assignment of the fundamental in-plane Raman and infrared modes (1700−600 cm-1) of this oligoaniline is proposed. The observed frequencies of the dark blue microcrystals are compared with those acquired by vibrational calculations based on a valence force field model. To show, by a most efficient way, the evolution of the electronic configuration of the dimer upon doping, the experimental vibrational modes of the doped dimer and its related force constants are compared with those of its fully reduced and oxidized forms. The strongly aromatic and semiquinone-like characters of the rings present in the doped dimer is put in evidence by Resonance Raman Scattering and infrared absorption and confirmed by vibrational calculations. Considering the numerous similarities between the vibrational characteristics of the polymeric chain and its well-defined oligomer, this study also provides useful information about the electronic configuration of the protonated emeraldine, which remains under debate.

Surface Enhanced Raman Scattering and Normal Coordinate Analysis of 1,10-Phenanthroline Adsorbed on Silver Sols

The surface enhanced Raman scattering of 1,10-phenanthroline adsorbed on silver sol has been obtained and analyzed by using normal mode calculations based on a valence force field suitably determined for this molecule. Evidence has been found for Ag−N bond formation with the silver surface, supporting the validity of a chemisorption process. The experimental and calculated data suggest an adsorbate−substrate interaction with perpendicular or tilted orientation of the molecule with respect to the colloidal surface. The halide anion effect on the SER spectrum improves a charge-transfer enhancement process involving adsorbate and metal electronic states. The results obtained on colloids agree with those detected by measurements performed on silver electrode. The similarity between SERS and normal Raman spectra of the corresponding 1:2 Ag(I)-phenanthroline complex indicates that the molecule is perturbed upon adsorption as well as upon coordination in the argentous complex.

Theoretical analysis of unstable two-phase region and microscopic structure in wurtzite and zinc-blende InGaN using modified valence force field model

A model to predict material characteristics of the InGaN ternary system, which is useful for blue and green light emitting and laser diodes, with respect to an unstable two-phase region in the phase field and the first neighbor anion–cation bond length is developed. The unstable region is analyzed using a strictly regular solution model. The interaction parameter used in the analysis is obtained from a strain energy calculation using the valence force field (VFF) model, modified for both wurtzite and zinc-blende structures to avoid overestimation of the strain energy. The structural deviation from an ideal wurtzite structure in GaN and InN is also taken into account in our model. The critical temperatures found in our analysis for wurtzite InGaN and zinc-blende InGaN are 1967 and 1668 K, respectively. This suggests that, at typical growth temperatures around 800 °C, a wide unstable two-phase region exists in both wurtzite and zinc-blende structures. The modified VFF model can also predict the microscopic crystal structure, such as first neighbor anion–cation bond lengths. In order to validate our calculation results for zinc-blende structures, we compare the calculated and the experimental results in terms of the interaction parameter and the first neighbor anion–cation bond lengths in the InGaAs system. For the wurtzite structure, we compare the calculated and the experimental results for the first neighbor anion–cation bond lengths in the InGaN system. The calculated results agree well with the experimental results.

Spectroscopic Characterization of the G*G·C Triplet in Triple Helix Structures

Intramolecular triple helices have been obtained by twice folding back oligonucleotides formed by decamers bound by nonnucleotide linkers: 5‘d(G)10−L−d(C)10−L−d(G)10−3‘ and 5‘d(C)10−L−d(G)10−L−d(G)10−3‘ (L = pO(CH2CH2O)6p). We have thus prepared two triple helices with forced third strand orientation, respectively parallel and antiparallel with respect to the guanosine strand of the Watson−Crick duplex. The stability of the parallel triplex is found to be lower than that of the antiparallel, and the sugar conformations determined by infrared spectroscopy are different for both triplexes. Only S-type sugars are found in the antiparallel triplex, whereas both S- and N-type sugars are present in the parallel one. In-plane normal-mode frequencies have been calculated for G*G·C triplet by using a valence force field that incorporates transition dipole coupling for guanine CO groups in the Hoogsteen triplet structure. This coupling mechanism is shown to give a reasonable explanation of the νCO splitting observed for the Hoogsteen G*G·C triplet, unlike the νCO spectral profile of the reverse Hoogsteen structure, where this coupling is absent. It is, therefore, now possible to distinguish with confidence between these triplet structures, and their characteristic spectral features can be applied to get structural information of nucleic acid triplexes.

Energy transfer pathways in the collisional activation of peptides

Classical trajectory simulations were carried out to study details of the energy transfer mechanism for activation of polyglycine (gly) n and polyalanine (ala) n peptide ions via collisions with Ar atoms. The Amber valence force field was used to represent the peptide intramolecular potential and the argon–peptide interaction was modeled using parameters previously determined from high level ab initio calculations. Energy transfer was studied versus collision impact parameter b, the collision energy, and peptide temperature and structure. Energy transfer to rotation becomes important for extended peptides at large b, but with averaging over impact parameter is smaller than transfer to vibration. Specific pathways for vibrational energy transfer were distinguished by determining the efficiency of energy transfer with various combinations of low and high frequency modes constrained. With all stretching and bending modes constrained the efficiency of energy transfer is more than 80% of that without constraints, which illustrates the efficient excitation of the torsional modes. Varying the peptide structure has a significant effect on the energy transfer efficiency, with larger energy transfer for the folded structures. The efficiency of energy transfer increases with increase in collisional energy.

A vibrational study of various K2PdCl4−xBrx solid solutions

Infrared and Raman vibrational spectra (700–30 cm−1) have been recorded on various K2PdCl4−xBrx (0≤x≤4) powdered samples at 300 K, including both K2PdCl4 and K2PdBr4 compounds and ten solid solutions with a bromide content varying from 5–95%. Characteristic variations of the sectra clearly demonstrate a wide domain of existence of the solid solutions and the only possible additional existence, due to no IR–Raman coincidence, of the ‘trans’ planar PdCl4Br2 centrosymmetric anion. A comparison of the vibrational data with results of valence force field calculations allows us to characterize this new anion, in Raman by two signals at 286 and 225 cm−1 (v sym., Ag, PdCl2 and PdBr2, respectively), in Infrared by not only the two bands at 309 and 200 cm−1 (v asym. B3u+B2u, of PdCl2 and PdBr2) but also intense deformation modes in the 178–140 cm−1 wavenumber range. These results suggest slight modifications in the internal f(PdCl) and f(PdBr) force constans on going from the PdCl42− or PdBr42− anions to the ‘trans’ PdCl2Br22− anonic species.

Lattice dynamics and the ferroelectric phase transition inSn2P2S6

This paper presents a quasiharmonic model for the lattice dynamics of the ${\mathrm{Sn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$ crystal. The model, constructed as a combination of the valence force field and the polarizable ion model with a majority of potential parameters fixed by auxiliary Hartree-Fock calculations, enables us to calculate vibrational spectra for both the paraelectric and the ferroelectric structure. The calculated eigenvector of the lowest frequency optic mode corresponds to the order parameter of the ferroelectric phase transition and can be identified with the symmetry-breaking soft mode. Interpretation of the obtained results and their comparison with available experimental data is provided.

Strain distribution around the step edge of ZnSe/CdSe/ZnSe strained quantum well grown on vicinal GaAs substrate

The strain energy and strain element distribution of ZnSe/CdSe/ZnSe strained quantum well grown on vicinal GaAs(0 0 1) substrate are investigated by an atomic scale calculation based on valence-force field model. The strain distribution around the step edge is inhomogeneous, while that on the terrace is uniform and not different from just-oriented substrate. The largest strain energy is found at Cd atom at the step edge. The origin of the largest strain energy is shear deformation. The strain inhomogeneity extends for only 2 ML from the step edge in the vicinal direction. The lattice deformation extends to longer distance along the 〈1 1 1〉 direction than the 〈1 1 0〉 direction. Considerably large lattice-plane-tilting is caused by the shear strain in (1 1 0) plane. The tilt angle in CdSe lattice is more than 10 000 arcsec at the step edge, while that at terrace is less than 1000 arcsec. The lattice-plane-tilting also occurs on ZnSe layer top and below the step edge of CdSe, though the tilt angle is opposite to CdSe.

Force Field Parametrization for Gadolinium Complexes Based on ab Initio Potential Energy Surface Calculations

The recent design of new magnetic resonance imaging (MRI) contrast agents is oriented toward the synthesis of gadolinium(III) complexes with ligands presenting formally neutral (amidic or alcoholic) or anionic (phosphinic) oxygen donor atoms. This paper presents the molecular mechanics (MM) parametrization of Gd interactions with amidic, alcoholic and phosphinic oxygen donor atoms, with the aim of supporting experimental effort. The parametrization is performed on the basis of a previously developed procedure applied to the parametrization of Gd interactions with polyamino carboxylate (PAC) ligands. Within the framework of valence force fields, the parameters for Gd−ligand interactions are determined by fitting the empirical potential to the ab initio potential energy surface (PES) of [Gd·3·OH2]3+, [Gd·5b·OH2]3+, and [Gd·8a]1-. Ab initio calculations were performed at the restricted Hartree−Fock (RHF) level by using an effective core potential (ECP) that includes 4f electrons in the core, an optimized valence basis set for the metal, and the 3-21G basis set for the ligand. Sampling of the PES is performed by moving the ion into the frozen coordination cage of the ab initio optimized geometries. The energy and first derivatives, with respect to the Cartesian coordinates of the metal and donor atoms, were calculated for each generated structure. Two sets of parameters, with the electrostatic contribution turned on or off in the force fields, were determined. To test the quality of the derived parameters and their transferability to other Gd complexes, MM calculations were performed on several gadolinium complexes. The results show that both sets of parameters provide reliable molecular geometries, but it is necessary to include the electrostatic contribution in the force fields to correctly reproduce the conformational energies.

Normal coordinate analyses of three isomeric acetylpyridines and acetophenone

Normal coordinate analyses for 2-, 3- and 4-acetylpyridine and acetophenone were carried out employing a systematic procedure based on a modified valence force field that is comparable to those used previously for different aromatic molecules. The general valence force fields (GVFF) were determined using a set of 41 parameters (25 planar and 16 non-planar) for the above compounds to compare the observed wavenumbers with the calculated values satisfactorily with an average error of 7.0 cm−1 (0.82%) for in-plane and 9.2 cm−1 (1.60%) for out-of-plane modes. On the basis of the calculated potential energy distribution, certain revisions of the previous assignments were made. The decomposition of the normal mode wavenumbers into those related to different internal coordinates is discussed. Copyright © 2000 John Wiley & Sons, Ltd.

Vibrational Analysis of a Chemisorbed Polyatomic Molecule: Methoxy on Cu(100)

Infrared reflection absorption spectra were obtained from 900 to 3000 cm-1 for five C3v symmetry isotopomers of methoxy: H312C16O, H313C16O, H312C18O, D312C16O, D313C16O; and two Cs symmetry isotopomers: DH212C16O, and HD212C16O. Measured frequencies of the five C3v isotopomers were used to derive an empirical valence force field based on a C3v symmetry H3COCu3 model. Without further adjustment, this force field was used to calculate the harmonic frequencies of the two Cs symmetry isotopomers which were found to be in good agreement with the experimental frequencies. Comparison of a Cs symmetry H3COCu12 model with an adsorption site structure that accurately models the experimentally determined geometry, as well as with a C3v symmetry H3COCu model, shows that the internal vibrations of the methoxy unit are relatively insensitive to the adsorption site. Among other advantages, an accurate description of the normal modes of this molecule provides a quantitative understanding of the unusual intensity patte...