Valence Force Field Research Articles

Building semiconductor nanostructures atom by atom

We present an atomistic tight-binding approach to calculating the electronic structure of semiconductor nanostructures. We start by deriving the strain distribution in the structure using the valence force field model. The strain field is incorporated into the tight-binding electronic structure calculation carried out in the frame of the effective bond orbital model and the fully atomistic sp3d5s* approach. We apply the method to a vertically coupled self-assembled double-dot molecule. Using the effective mass approach, we establish the existence of electronic bonding and antibonding molecular orbitals for electrons and holes, whose probability density is shared equally between the dots. In the atomistic calculation we recover the molecular character of electron orbitals, but find that structural and atomistic details of the sample modify the hole orbitals, leading to a strongly asymmetric distribution of the probability density between the dots.

Effect of post-growth annealing on the optical properties of InAs/GaAs quantum dots: A tight-binding study

We present an atomistic study of the strain field, the one-particle electronic spectrum and the oscillator strength of the fundamental optical transition in chemically disordered InxGa1−xAs pyramidal quantum dots (QDs). Interdiffusion across the interfaces of an originally “pure” InAs dot buried in a GaAs matrix is simulated through a simple model, leading to atomic configurations where the abrupt heterointerfaces are replaced by a spatially inhomogeneous composition profile x. Structural relaxation and the strain field calculations are performed through the Keating valence force field model, while the electronic and optical properties are determined within the empirical tight-binding approach. We analyze the relative impact of two different aspects of the chemical disorder, namely: (i) the effect of the strain relief inside the QD, and (ii) the purely chemical effect due to the group-III atomic species interdiffusion. We find that these effects may be quantitatively comparable, significantly affecting the electronic and optical properties of the dot. Our results are discussed in comparison with recent luminescence studies of intermixed QDs.

The Gradient Curves Method: An Improved Strategy for the Derivation of Molecular Mechanics Valence Force Fields from ab Initio Data.

A novel force-field development strategy is proposed that tackles the well-known difficulty of parameter correlations arising in a conventional least-squares optimization. In the first step of the new gradient curves method (GCM), continuity criteria are imposed to transform the raw multidimensional ab initio training data to distinct sets of one-dimensional data, each associated with an individual energy term. In the second step, the transformed data suggest suitable analytical expressions, and the parameters in these expressions are fitted to the transformed data; that is, one does not have to postulate a priori analytical expressions for the force-field energy terms. This approach facilitates the derivation of valence terms. Benchmarks have been performed on a set of small molecules. The results show that the new method yields physically acceptable energy terms exactly when a conventional parametrization would suffer from parameter correlations, that is, when an increasing number of redundant internal coordinates is used in the force-field model. The generic treatment of parameter correlations in the proposed method facilitates an intuitive physical interpretation of the individual terms in the force-field expression, which is a prerequisite for the transferability of force-field models.

Phonon Dispersion in Polyadenylic Acid

A study of the normal modes of vibration and their dispersion in polyadenylic acid based on the Urey–Bradley force field is reported. It gives a better interpretation of FTIR spectra as compared with the valence force field. Characteristic features of dispersion curves such as regions of high density‐of‐states, repulsion, and character mixing of dispersive modes are discussed. Predictive valuzes of heat capacity as a function of temperature are reported.

Effects of a Shell on the Electronic Properties of Nanowire Superlattices

We compute the structural and electronic properties of core-shell InAs/GaAs nanowire superlattices using Keating's valence force field and a sp3d5s* tight-binding model. We show that the GaAs shell limits strain relaxation but homogenizes the hydrostatic strain distribution in the InAs layers. This prevents the formation of a strain-induced well in the conduction band at the surface of the nanowires, which was shown to trap the electrons in thin InAs layers (Phys. Rev. B 2006, 74, 155304). The shell, however, enhances the piezoelectric field, which increases the separation between the electrons and holes in thick InAs layers. These results emphasize the intricate links between the structural and electronic properties of strained nanowire heterostructures.

Valley splitting in strained silicon quantum wells modeled with 2° miscuts, step disorder, and alloy disorder

Valley splitting (VS) in strained SiGe∕Si∕SiGe quantum wells grown on (001) and 2° miscut substrates is computed in a magnetic field. Calculations of flat structures significantly overestimate, while calculations of perfectly ordered structures underestimate experimentally observed VS. Step disorder and confinement alloy disorder raise the VS to the experimentally observed levels. Atomistic alloy disorder is identified as the critical physics, which cannot be modeled with analytical effective mass theory. NEMO-3D is used to simulate up to 106 atoms, where strain is computed in the valence-force field and electronic structure in the sp3d5s* model.

Ab initio parametrized MM3 force field for the metal‐organic framework MOF‐5

A new valence force field has been developed and validated for a particular class of coordination polymers known as nanoporous metal-organic frameworks (MOFs), introduced recently by the group of Yaghi. The experimental, structural, and spectroscopic data in combination with density functional theory calculations on several model systems were used to parametrize the bonded terms of the force field, which explicitly treats the metal-oxygen interactions as partially covalent as well as distinguishes different types of oxygens in the framework. Both the experimental crystal structure of MOF-5 and vibrational infrared spectrum are reproduced reasonably well. The proposed force field is believed to be useful in atomistic simulations of adsorption/diffusion of guest molecules inside the flexible pores of this important class of MOF materials.

Electronic Structure of Si/InAs Composite Channels

AbstractElectronic structure calculations on composite channels, consisting of indium arsenide grown on the technologically relevant (001), (011) and (112)-orientated silicon surfaces are reported. The calculations are performed with NEMO 3-D, where atoms are represented explicitly in the sp3d5s* tight-binding model. The Valence Force Field (VFF) method is employed to minimize the strain. NEMO 3-D enables the calculation of localized states in the quantum well (QW) and their dispersion in the quantum well plane. From this dispersion, the bandgap, its direct or indirect in character, and the associated effective masses of the valence and conduction band can be determined. Such composite bandstructure calculations are demonstrated here for the first time. The numerical results presented here can then be included in empirical device models to estimate device performance. Pure InAs QWs create a direct bandgap material, with a relatively small gap and effective masses of about one order of magnitude smaller than for pure Si QW of equivalent thickness. Si, on the other hand, has a larger bandgap, superior thermal and mechanical properties, and a heavily invested industry. Thus heteroepitaxy of both components is expected to yield a highly optimized overall system. For samples grown along the (001) direction, Si is a direct bandgap material, and deposition of an InAs 3nm layer reduces substantially the hole effective mass and slightly the electronic mass, decreasing the magnitude of the gap. Along the (011) and (112)-growth direction, Si QWs are indirect bandgap material, and deposition of a few InAs layers suffies to make the new material a direct-bandgap heterostructure, decreasing significantly the electronic effective mass. (011) and (112) are the experimentally most relevant growth directions since they prevent heterointerface dipoles.

General Elasticity Theory for Graphene Membranes Based on Molecular Dynamics

ABSTRACTWe have studied the mechanical properties of suspended graphene membranes using molecular dynamics (MD) and generalized continuum elasticity theory (GE) in order to develop and assess a continuum description for graphene. The MD simulations are based on a valence force field model which is used to determine the deformation and the elastic energy of the membrane (EMD) as a function of external forces. For the continuum description, we use the expression Econt = Estretching + Ebending for the elastic energy functional. The elastic parameters (tensile rigidity and Poisson ratio) entering Econt are determined by requiring that Econt = EMD for a set of deformations.Comparisons with the MD results show excellent agreement. We find that the elastic energy of a supported graphene sheets is typically dominated by the nonlinear stretching terms whereas a linear description is valid only for very small deflections. This implies that in some applications, i.e. NEMS, a linear description is of limited applicability.

Molecular dynamics simulation of hydroxyapatite–polyacrylic acid interfaces

Polymer–hydroxyapatite (HAP) composites are widely studied as potential bone replacement materials. The HAP–polymer interfacial molecular interactions have significant role on the mechanical response of composite systems. We have used molecular dynamics (MD) simulations to evaluate the nature of these interfaces in polyacrylic acid–hydroxyapatite composites. We have obtained the parameters for monoclinic hydroxyapatite in CVFF (consistent valence force field) from the known potential energy function of apatites. Our simulations indicate that potential sites for chelation and hydrogen bond formation between HAP and polyacrylic acid (PAAc) exist. Earlier, we have synthesized in situ HAP–polymer composites wherein intimate interaction between HAP and polymer is enabled through participation of polymer during HAP mineralization. Our simulations indicate that for in situ HAP, the most favorable orientation of PAAc for attachment with HAP is along the c-axis of HAP aligned parallel to polymer chains. Also, binding energy for ex situ HAP composites is found to be lower as compared to that of in situ HAP.

Assignment of theG+andG−Raman bands of metallic and semiconducting carbon nanotubes based on a common valence force field

We propose a model for calculating the phonon frequencies of any $(n,m)$ nanotube by considering explicitly the curvature of the system and the effects of long-range interactions determined by $\ensuremath{\pi}$ electrons. We show that the electron-phonon coupling strength is directly related to the electronic structure of the nanotube and it is the main interaction responsible for the different behavior shown in the Raman spectra by metallic and semiconducting nanotubes. On the basis of the vibrational valence force field derived in this work we propose an assignment of the $G$ band. Moreover, based on symmetry-selection rules, we can state that the LO modes of armchair nanotubes and the TO modes of zigzag nanotubes at ${\mathbf{\ensuremath{\Gamma}}}_{0}$ are silent in the Raman spectra.

Material Development for Improved 1eV(GaIn)(NAs) Solar Cell Structures

The dilute nitride (GaIn)(NAs) material system grown lattice matched to GaAs or Ge with a 1eV band gap is an interesting material for the use in four-junction solar cells with increased efficiencies. As a result of its metastability, several challenges exist for this material system, which up to now limits the device performance. We performed nanostructural analysis in combination with photoluminescence characterization to optimize the metal organic vapor phase growth as well as the annealing conditions for the quaternary solar cell material. The optimum annealing conditions depend strongly on the In content of the quaternary material. Valence force field calculations of stable N environments in the alloy support the model that the N moves from a Ga rich environment realized during growth into an In rich environment upon annealing. Simultaneously, N induced strain fluctuations, which are detected in the N containing material upon growth, are dissolved and the device properties are improved.

Strain-induced shift of phonon modes in [formula omitted] alloys

A theoretical approach to the lattice dynamics of Si 1 - x Ge x alloys within a valence force field framework is discussed. A modified Keating model, the anharmonic Keating, was employed to perform supercell calculations in order to investigate accurately the three Raman active optical phonon modes. Theoretical results on both relaxed and strained SiGe samples are shown. A detailed study of the biaxial strain-induced shift of phonon modes of epitaxial SiGe alloys has been performed in all the composition range. Furthermore, a comparison of experimental data with the model results on phonon strain shift coefficients will be also discussed.

Heterocyclic aromatic amines in H-bonded complexes (1:1 and 1:2 composition) in solutions with various proton acceptors

The influence of the position and number of the nitrogen atoms in the aromatic ring on the proton donation ability in intermolecular H-bond, spectral, geometrical, force, electrooptic and energetic characteristics of the amino group was investigated for free and H-bonded (1:1 and 1:2, with various proton acceptors) molecules of 2-aminopyrazine (I) and 3-amino-1,2,4-triazine (II). Acetonitrile, dioxane, tetrahydrofourane, dimethylformamide, dimethylsulfoxide and hexamethylphosphoramide were used as proton acceptors. Their proton accepting properties vary in a wide range. In the range of the amino group stretching and deformation vibrations, within the temperature interval 285–330 K, IR spectra of free and H-bonded (1:1) molecules of compounds I, II in CCl 4 were studied. Complexes of 1:2 composition were studied in the undiluted aprotic solvents. The following absorption band spectral characteristics were determined: M (0) – zero spectral moment (integrated intensity B), M (1) – first spectral moment (the centre of gravity of the band), M (2) – second central moment related to the effective half width (Δ ν 1/2) eff = 2(M (2)) 1/2. It was shown that within the explored temperature interval the absorption band spectral characteristics of the monomers and complexes are well approximated by the linear functions Y = aT + b ( Y = B, M (1), 2( M (2)) 1/2). The linear regression parameters for these equations were determined. The vibrational and electro-optic problems for free and H-bonded molecules of compounds I, II were solved in approximation of the R-NH 2 valence force field model. Valence angles γ (HNH), force constants K (NH), electro-optic parameters ∂ μ/∂ q (derivative of the dipole moment by the length of the corresponding bond) and ∂ μ/∂ q′ (derivative of the dipole moment by the length of the neighboring bond) were determined. It was shown that the absorption band spectral characteristics of the amino group stretching vibrations of the examined compounds in CCl 4 are most sensitive to the o-position of nitrogen atoms in the aromatic ring relatively to the amino group. Increasing the number of the nitrogen atoms in the ring leads to increasing the force constants K (NH), valence angle γ (HNH), and electrooptic parameters ∂ μ/∂ q and ∂ μ/∂ q′. The linear correlations were obtained between spectral, geometrical, force and electrooptic characteristics of amino group of free and H-bonded (1:1 and 1:2) molecules. The comparative analysis was performed on the dependence of the correlation relations on the position and number of the nitrogen atoms in the aromatic ring. The dependence of the monomer-complex (1:1) equilibrium constants on the temperature was examined. The thermodynamical characteristics of the complex: entropy and enthalpy of the complex formation were obtained on the basis of the Vant–Hoff equation. The H-bond enthalpy in complexes of 1:2 composition was calculated using A.V. Iogansen’s “intensities rule”. The basic regularities observed in the spectra of free molecules of compounds I, II are confirmed by the quantum mechanical calculations in the DFT-B3LYP/6-31G** approximation and ab initio MP2/6-31G**.

Probing molecular interactions in bone biomaterials: Through molecular dynamics and Fourier transform infrared spectroscopy

Polymer-hydroxyapatite (HAP) composites are widely investigated for their potential use as bone replacement materials. The molecular interactions at mineral polymer interface are known to have significant role of mechanical response of the composite system. Modeling interactions between such dissimilar molecules using molecular dynamics (MD) is an area of current interest. Molecular dynamics studies require potential function or force field parameters. Some force fields are described in literature that represents the structure of hydroxyapatite reasonably well. Yet, the applicability of these force fields for studying the interaction between dissimilar materials (such as mineral and polymer) is limited, as there is no accurate representation of polymer in these force fields. We have obtained the parameters of consistent valence force field (CVFF) for monoclinic hydroxyapatite. Validation of parameters was done by comparing the computationally obtained unit cell parameters, vibrational spectra and atomic distances with XRD and FTIR experiments. Using the obtained parameters of HAP, and available parameters of polymer (polyacrylic acid), interaction study was performed with MD simulations. The MD simulations showed that several hydrogen bonds may form between HAP and polyacrylic acid depending upon the exposed surface of HAP. Also there are some favourable planes of HAP where polyacrylic acid is most likely to attach. We have also simulated the mineralization of HAP using a “synthetic biomineralization”. These modeling studies are supported by photoacoustic spectroscopy experiments on both porous and non porous composite samples for potential joint replacement and bone tissue engineering applications.

Band-edge alignment ofSiGe∕Siquantum wells andSiGe∕Siself-assembled islands

We report on the energy band gap and band lineup of $\mathrm{SiGe}∕\mathrm{Si}$ heterostructures either in the case of coherently strained quantum wells or in the case of $\mathrm{SiGe}∕\mathrm{Si}$ self-assembled islands. We take into account the strain field and the quantum confinement effects through an accurate description of the conduction band including the $\ensuremath{\Delta}$ and $L$ bands. The strain field is calculated using a microscopic valence force field theory. The conduction-band diagram and energies are obtained from a 30-band $\mathbf{k}∙\mathbf{p}$ Hamiltonian accounting for the strain through the Bir-Pikus Hamiltonian. The band-edge description is first given for biaxially strained pseudomorphic $\mathrm{SiGe}$ layers. In $\mathrm{SiGe}$ quantum wells grown on relaxed silicon, the band line-up switches from type I to type II depending on the value of the average valence band offset. Applying the 30-band formalism to the case of heterostructures grown on relaxed silicon germanium buffer layers indicates that a better agreement with experimental data is obtained for a valence-band offset value $\mathrm{\ensuremath{\Delta}}{E}_{v}=0.54x$ where $x$ is the Ge composition. For this parameter, a type-II band lineup is thus expected for all compositions of pseudomorphic $\mathrm{SiGe}$/relaxed Si heterostructures. For $\mathrm{GeSi}∕\mathrm{Si}$ islands, we take into account the strain relaxation in the surrounding Si matrix. A type-II band lineup is predicted for all Ge compositions. The near-infrared interband recombination energy of the islands is calculated as a function of their $\mathrm{SiGe}$ composition.

FT-IR and FT-RAMAN investigations of nicotinaldehyde

The Fourier transform Raman and infrared spectra of nicotinaldehyde were recorded and the observed frequencies were assigned to various modes of vibration in terms of fundamentals by assuming Cs point group symmetry. A normal coordinate analysis was also carried out using a simple valence force field. A complete vibrational analysis is presented here for this molecule and the results are briefly discussed.

Phonon dispersion in poly(dimethylsilane)

In the present communication we report normal modes and their dispersion in polydimethylsilane (PDMS) [–Si(CH 3) 2–] n using Urey–Bradley force field, which in addition to valence force field accounts for the non-bonded interactions in the gem and cis configurations and tension terms. The partially deuterated PDMS (PDMS-d 3), i.e., (SiCH 3CD 3) n and fully deuterated PDMS (PDMS-d 6), i.e., (SiCD 3CD 3) n are also studied to check the assignments and validity of the force field. Dispersion curves show two interesting features: (1) a divergence of dispersion curves following repulsion of species belonging to the same symmetry; (2) crossing between the two modes. In addition, heat capacity as a function of temperature via density-of-states is evaluated and some of the modes left unassigned by the earlier workers have been assigned.

Relaxed state of GexSi1−x islands embedded in Si

The process of capping Ge islands with Si overlayers is known to have a strong influence on their composition and shape. In this work we have investigated Ge islands on Si produced by chemical vapor deposition covered with Si layers of different thickness. The structural characterization was carried out by X-ray absorption spectroscopy at the Ge-K edge. A noticeable Si uptake by the islands is evident upon capping. Bond length for the first three shells have been analyzed by comparison with models based on the valence force field method. The results evidence that the islands have, on the average, a relaxed state with presumably strained parts in contact with the Si matrix.

Normal coordinate analysis and crystal structure of N, N-dimethyl- N′-(2-chloro-benzoyl)thiourea

In the present paper, the crystal structural and vibrational analysis of the N, N-dimethyl- N′-(2-chloro-benzoyl)thiourea molecule is reported. The molecular structure of the title compound was determined by single crystal X-ray diffraction method. The compound crystallizes monoclinic, space group P2 1/ c with a = 5.6601(13) Å, b = 10.748(2) Å, c = 17.778(4) Å, β = 94.266(5)°, and V = 1078.5(4) Å 3 with Z = 4 for d calc = 1.495 g/cm 3. Calculations of the vibrational frequencies have been carried out on the basis of normal coordinate analysis using simple general valence force field in Wilson's GF matrix method with the SPSIM computer program. With the help of this modern technique we were able to complete the assignment of the vibrational spectrum of the title compound.