Search

Bonded (or valence) interactions, which directly determine the local structures of the molecules, are fundamental parts of molecular mechanics force fields (FFs). Most popular classical FFs adopt the simple harmonic models for bond stretching and angle bending and ignore cross-coupling effects among the valence terms. This may lead to less accurate vibrational properties and configurations in molecular dynamics (MD) simulations. AMOEBA models utilize an MM3(MM4)-style bonded interaction model, in which the vibrational anharmonicity, the coupling effects among different energy terms, and the out-of-plane bending for sp2-hybridized atoms are considered. In this work, we report the development of bonded interaction parameters for a wide range of chemistry based on quantum mechanics (QM). About 270 atomic types defined by SMARTS strings were used to model the valence interactions. Our results indicate that the resulting valence parameters produce accurate vibrational frequencies (RMSD from QM is less than ∼36.6cm−1) over a large set of molecules with diverse functional groups (445 molecules). By contrast, the harmonic models usually give an RMS error greater than 60cm−1. Meanwhile, this model accurately reflects the potential energy surface of the out-of-plane bending. Our model can generally be applied to the AMOEBA family and any MM3(MM4)-based molecular mechanics FFs.

Numerical simulation of mechanical properties of silicene samples are made in the valence force field model. The dependences of elastic modules on linear dimensions are found. The results are compared with those obtained in the of central force field model. Behavior of the corresponding graphs is similar. At the same time, a lag of values of the elastic modules received within the valence force field model in comparison with the central forces field model was observed. The lag in question accrues with growth of the sample sizes.

In this paper, we used the 8-band k$\cdot$p model with valence force field considerations to investigate the effect of size and shape on electronic and optical properties of cadmium selenide quantum dots. Major factors related to their properties including band mixing probabilities, spatial charge distributions, transition matrix elements and Fermi factors were studied. Volumetrically larger CdSe dots were found to have smaller band-gaps but higher transition matrix elements and Fermi factors. The maximum optical gain for dots was observed to have an initially positive and then negative correlation with their real-space size as a result of combined effects of various factors. For the shape effects, cubic dots were found to have smaller band-gaps, Fermi factors and transition matrix elements than spherical dots due to higher level of asymmetry and different surface effects. Consequently, cubic dots have lower emission energy, smaller amplification. The occurrence of near E1-H1 transition broadens the gain spectrum of cubic dots. Cubic and spherical dots are both proven to be promising candidates for optical devices under visible range. We have demonstrated that size and shape change could both effectively alter the properties of quantum dots and therefore recommend consideration of both when optimizing the performance for any desired application., Comment: Published in Optik - International Journal for Light and Electron Optics (8 pages, 10 figures), 2017

Molecular dynamics (MD) simulations were carried out to investigate the conformations of fourth generation polyamidoamine (G4 PAMAM) dendrimer-encapsulated Au nanoparticles (AuNPs) of different sizes at different pH conditions. The consistent valence force field (CVFF) was used to model the interaction between the atoms of G4 PAMAM dendrimer and water molecules as well as their interactions with the Au atoms of AuNP. The many-body tight-binding potential was adopted to describe the interaction between Au atoms. The simulation results show a single G4 PAMAM dendrimer can only partially cover the AuNP when its diameter is larger than 2.3 nm. According to the variations of radius of gyration ( R g ) values, the conformations of PAMAM dendrimers become more compact after covering the AuNP at neutral pH. According to the Stokes–Einstein (SE) relation and the diffusion coefficients obtained by the Einstein equation, it was proposed that both the PAMAM dendrimer conformation and the interaction strength between the PAMAM/AuNP and water molecules have significant influences on the diffusion behaviors of PAMAM/AuNP systems.

Band alignment transition from type-I to type-II of InP/In 0.48 Ga 0.52 P quantum dots (QDs) is observed by modifying the geometrical factors of the dome-shaped QDs. Strain distribution in InP/In 0.48 Ga 0.52 P QD is calculated using a valence force field (VFF) model. With the strain Hamiltonian from the VFF model, electrical structures of the QDs are obtained using 8-band k•p model. The results from this theoretical study show that the band alignment of the InP/In 0.48 Ga 0.52 P QDs can be tailored from type-I to type-II by controlling the size of the QDs, and flat valence band can be achieved at certain values of height and width of the QDs. In addition, InP/In 0.48 Ga 0.52 QDs of various sizes with the same effective bandgap but different band alignment is observed.

The dynamical and thermal behaviors of Au nanoparticle (AuNP) and fourth generation poly(amidoamine) dendrimer (G4 PAMAM) of the dendrimer-encapsulated Au nanoparticle system under the dry environment were investigated by the molecular dynamics (MD) simulation. The consistent valence force field (CVFF) was used to describe the interaction of the G4 PAMAM dendrimer and the interaction between the dendrimer and the AuNP. The many-body tight-binding potential was adopted to describe the atomic interaction between the Au atoms of an AuNP. Three AuNPs, Au116, Au201, and Au405, with the diameters of 1.45, 1.8, and 2.3 nm were concerned, respectively. Au116 and Au201 are nearly completely covered by G4 PAMAM dendrimer, but the branching chains of the dendrimer are no longer able to wrap around the Au405.The conformation of G4 PAMAM dendrimer still possesses its unique spherical structure after encapsulating the AuNP, and the irregularity of the outer surface decreases as the size of the AuNP increases. For the t...

The present work achieves an understanding of the interplay between morphology, electronic, and optical properties of semiconductor nanostructures for single-photon emission. The latter is important for various quantum information applications, including future quantum communication devices. In the first part of this thesis, III-V semiconductor quantum dots are investigated. These systems have emerged as promising candidates for deterministic single-photon sources due to high repetition rates, integrability into electrical devices, and tunable Emission energy. In particular, we analyze two different quantum dot systems, which are capable of single-photon emission in the technologically important telecom C-band. The electronic single-particle energies and wave functions are calculated using a nearest neighbour sp3s empirical tight-binding (TB) model. Strain arising from lattice mismatch of the constituent materials is calculated atomistically by employing a valence-force field method. The theoretical model is based on realistic quantum dot geometries and concentration profiles, which are obtained from transmission electron microscopy. When using the measured results for a representative quantum dot geometry as well as experimentally reconstructed alloy concentrations, a combination of strain-field and TB calculations is able to reproduce the quantum dot emission wavelength in agreement with the experimentally determined photoluminescence spectrum. The inhomogeneous broadening of the latter can be related to calculated variations of the emission wavelength for the experimentally deduced In-concentration fluctuations and size variations. The results provide a deeper understanding of the interplay between morphology, electronic, and optical properties of semiconductor quantum dots. In the second part of this thesis, we investigate atomically thin layers of transition metal dichalcogenides (TMDCs), which have recently emerged as a new class of optically active materials for opto-electronic applications. These systems offer a relative ease of fabrication and the potential for large-scale applications. Engineering of the local confinement Situation can lead to discretized states, which opens the possibility for deterministic single-photon generation. One possible platform are molybdenum disulfide nanobubbles that develop when air is enclosed during the stacking of layers. An analysis of strain and dielectric effects in nanostructures reveals that an interplay of these physical effects leads to the formation of confined quantum-dot-like single-particle states that give rise to single-photon emission. The confinement of these states is caused by a wrinkling of the material, that supports the formation of strain-induced pockets. The results provide a deeper understanding of the interplay between strain, dielectric effects, electronic, and optical properties of TMDC nanostructures.

By combining an atomistic valence-force field approach and calculations based on the effective-mass approximation we investigate the influence of strain effects on the band alignment and general excitonic properties of core/shell CdSe/CdS dot/rod nanoparticles. We find that the inclusion of strain effects leads to an increase in exciton energy as well as to a decrease in electron and hole wave function overlap. Importantly, the native type-I band alignment of the CdSe/CdS material system is preserved and does not change into an quasi-type-II or even type-II band offset for the nanoparticles. Furthermore, we analyze the impact of strain on a model in which the spectral diffusion of the fluorescence emission of these nanoparticles is explained by migrating surface charges. Our calculations show that the addition of strain effects leads to increased energy shifts as well as larger changes in the squared electron and hole wave function overlap, while the correlation of both also exhibits a steeper slope than for the unstrained system. For a given CdSe core size, an increase in CdS-shell thickness decreases the possible ranges of energy shift and squared wave function overlap without changing the slope of their correlation. On the other hand, for a given nanoparticle overall thickness, dot/rod systems with a small CdSe core exhibit the strongest influenceability by surface charges., Comment: This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in The Journal of Physical Chemistry C, copyright \copyright American Chemical Society after peer review. To access the final edited and published work see https://pubs.acs.org/doi/full/10.1021/acs.jpcc.8b12253

We parametrize the Stillinger-Weber potential for 156 two-dimensional atomic crystals. Parameters for the Stillinger-Weber potential are obtained from the valence force field model following the analytic approach (Nanotechnology 26, 315706 (2015)), in which the valence force constants are determined by the phonon spectrum. The Stillinger-Weber potential is an efficient nonlinear interaction, and is applicable for numerical simulations of nonlinear physical or mechanical processes. The supplemental resources for all simulations in the present work are available online in Ref. 1, including a fortran code to generate crystals' structures, files for molecular dynamics simulations using LAMMPS, files for phonon calculations with the Stillinger-Weber potential using GULP, and files for phonon calculations with the valence force field model using GULP.

We study the strain profiles of Type-I CdS/CdSe quantum ring (QR) using the Valence force field method with Keating potential. The strain affects its electronic bandstructure which is calculated for varying QR widths using an effective mass envelope function theory based 8-band k·p model. This is followed by an electron-hole charge distribution study of the QR.

While most existing theoretical studies on the borophene are based on first-principles calculations, the present work presents molecular dynamics simulations for the lattice dynamical and mechanical properties in borophene. The obtained mechanical quantities are in good agreement with previous first-principles calculations. The key ingredients for these molecular dynamics simulations are the two efficient empirical potentials developed in the present work for the interaction of borophene with low-energy triangular structure. The first one is the valence force field model, which is developed with the assistance of the phonon dispersion of borophene. The valence force field model is a linear potential, so it is rather efficient for the calculation of linear quantities in borophene. The second one is the Stillinger-Weber potential, whose parameters are derived based on the valence force field model. The Stillinger-Weber potential is applicable in molecular dynamics simulations of nonlinear physical or mechanical quantities in borophene.

In this work, a theoretical study of the elastic and electronic properties of III-N semiconductors is made. Particular attention is given to wurtzite quantum wells (QWs) based on the technologically important InxGa1_xN alloy system. An investigation of the effects of structural inhomogeneities in these systems is made; specifically, the impact of random alloy fluctuations and well width fluctuations on the electronic and optical properties is investigated. This investigation comprises atomistic tight binding calculations on large ( 80,000 atoms) supercells. The theoretical framework used accounts for local strain and polarisation fluctuations arising from random alloy effects and possesses the computational efficiency needed for the calculation of many electronic states for realistic QW sizes. An accurate account is thus given of the interplay between nanoscopic and atomistic features of InGaN quantum wells, which was beyond the reach of previous ab-initio or continuum studies. The analysis is performed for excited states as well as ground states for InGaN quantum wells of varying indium content and growth direction. The calculations reveal that random alloy fluctuations are sufficient to bring about carrier localisation effects in both c- and m-plane InGaN based QWs. For the case of zincblende InGaN, shortcomings in the currently used semi-empirical methods of strain and atomic relaxation are pointed out and quantified. The development of an improved framework for the analysis of the structural and elastic properties of zincblende InGaN systems is thus undertaken. Elastic properties are extracted from density functional theory (DFT) data, and a new valence force field (VFF) implementation is introduced which makes explicit use of elastic properties neglected by ubiquitous VFF models. Additionally, third order elastic properties are determined from the results of DFT calculations using finite-strain theory. Finally, the utilisation of these properties in a valence force field model is discussed, and an appropriate functional form is suggested.

Employing NPT molecular dynamics method with consistent valence force field, the dissociation processes of sI, sII and sH gas hydrates are simulated at different temperatures and at a constant pressure of 100 MPa. The dissociation mechanisms of gas hydrates are revealed by analysing the structural snapshots, radial distribution functions and diffusion coefficients at different temperatures. As temperature increases, the diffusion rates of water molecules and guest molecules increase; thus the clathrate skeleton formed by water molecules with hydrogen bonds distorts and breaks down; meanwhile the guest molecules encapsulated in the water cavities are released. The size of guest molecules affects the dissociation behaviour of gas hydrate. In addition, the dissociation behaviour also relies on the structural phase of gas hydrates.

Spinodal decomposition ranges of wurtzite and zinc blende Zn B VI x O 1− x ( B VI =S, Se, Te) alloys from 0 °C to 1000 °C are represented. The strain energy was estimated in the framework of the valence force field model. All considered alloys have the very extensive decomposition zones in spite of the coherency strain energy. Spinodal decomposition in wurtzite alloys should occur in planes with an orientation that depends on the lattice parameters, stiffness coefficients and composition. Zinc blende alloys posses the larger zones and decomposition should take place only in the {100} oriented planes. Spinodal decomposition ranges are 0.02 x x x x O 1− x , ZnSe x O 1− x and ZnTe x O 1− x , respectively, at RT.

We compare the simplified valence-force model for single-layer black phosphorus with the original model and recent ab initio results. Using an analytic approach and numerical calculations we find that the simplified model yields Young's moduli that are smaller compared to the original model and are almost a factor of two smaller than ab initio results. Moreover, the Poisson ratios are an order of magnitude smaller than values found in the literature.

A detailed comparison of continuum and valence force field strain calculations in quantum-dot structures is presented with particular emphasis to boundary conditions, their implementation in the finite-element method, and associated implications for electronic states. The first part of this work provides the equation framework for the elastic continuum model including piezoelectric effects in crystal structures as well as detailing the Keating model equations used in the atomistic valence force field calculations. Given the variety of possible structure shapes, a choice of pyramidal, spherical and cubic-dot shapes is made having in mind their pronounced shape differences and practical relevance. In this part boundary conditions are also considered; in particular the relevance of imposing different types of boundary conditions is highlighted and discussed.

The purpose of our study is to determine the influence of elastic energy on the spinodal decomposition ranges of InAlGaN alloys lattice-matched to GaN. The distinction in lattice constants of the binary compounds constituting these alloys gives rise to their deformation energies. Such energies provide the tendency to disintegration that can lead to an appearance of the thermodynamically unstable states with respect to the decomposition. The initial stage of spinodal decomposition of the InAlGaN quaternary alloys is accompanied by transfers of atoms over distances of a lattice parameter. Accordingly, these transfers produce a thin two-layer region with negligibly small differences in their compositions. Formation of these layers causes the elastic energy. The alloys are represented as quaternary regular solutions. The internal deformation energy is calculated with the interaction parameters estimated by the valence force field model. The spinodal decomposition ranges of these materials are demonstrated up to 580 °C (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have studied, by computer simulation, some static and vibrationnal proprieties of SiGe/Si islands. We have used a Valence Force Field combined to Monte Carlo technique to study the growth of Ge and SiGe on (001)Si substrates. We have focalised on the case of large pyramidal islands presenting (113) facets on the free (001)Si surface with various non uniform composition inside the islands. The deformation inside the islands and Raman spectroscopy are discussed.

The Raman frequency shifts and the depolarization ratio of the Raman lines have been determined for the compounds NaCl. AlCl3 and NOCl. AlCl3 (in the last case with qualitative polarization measurements only). The preparation of pure NOCl. AlCl3 is described in extenso. The properties of the compound finally obtained are different from those given in the literature. NaCl. AlCl3, has to be considered as Na+AlCl4−, and NOCl. AlCl3 as NO+AlCl4−. There is a perceptible interaction between nitrosyl and the chlorines of AlCl4− in the last compound. The properties of NOCl. AlCl3 are intermediate between those for a pure heteropolar compound (NO+AlCl4−) and a pure molecular compound NOCl. AlCl3, being more alike those of the heteropolar form. Force constants have been calculated for AlCl4;−, making use of the Lechner formulae and of those for a Urey-Bradley field and also of those for the orbital valence force field of Heath and Linnett.

A set of force constants for dimethyl sulfide and deuterated analogues CH3SCD3 and (CD3)2S has been calculated using a modified general valence force field. The calculated force constants reproduce the fundamental frequencies observed fairly well.

Powell c.s. deduced from an X-ray study that solid PCl5 has to be considered as [PCl4+] [PCl6−]. From the distances between P and Cl in PCl4+ and in PCl6− given by the authors, force constants fPCl4+ and fPCl6− have been calculated using Badger's rule. From these f values and by a comparison with the results of a similar calculation for SiCl4 it follows that the strongest lines of the Raman spectrum of PCl5, namely 358(15) and 451 (7), belong to v1(PCl6−) and v1(PCl4+) respectively. With these frequencies known, it is possible to make an estimate of the frequencies to be expected for the other Raman-active vibrations of both ions. A probable assignment of Raman shifts to vibrations of PCl4+ and PCl6− is given. Force constants have been calculated for both ions, using formulae based on Nath's force field and on the orbital valence force field introduced by Heath and Linnett, Probable frequency values: PCl4+ 173, 244, 451, 627; PCl6− 244, 285, 358.

The infrared and Raman spectra of CH3HNCOCOOCH3, and the nitrogen-deuterated derivative have been recorded; the fundamentals have been assigned and a valence force field has been calculated.

The potential energy constants of HNSO, DNSO and H 15NSO have been evaluated using Cyvin's W-matrix method employing the General Valence Force Field. Making use of these constants, the mean-square amplitudes of vibration, the Coriolis coupling constants and centrifugal distortion constants have also been evaluated and presented here.

In order to compute later all the force constants and mean-square amplitudes of the generalized valence force field, we are proposing a method for the determination of the U-matrix for XYn groups, and the U-, G-, F- and Σ matrices (Wilson's Method) for XY6, (D3h ) molecules.

The calculation of an approximate Simple symmetry Valence Force Field including determination of diagonal symmetry valence force constants is achieved by using an iterative process derived from the Stepwise Coupling Method (SCM) in the form of the Matrix Polynomial Expansion Method (M.P.E.M.) of Alix. The discussion on the method itself and the comparison with similar methods such as the Eigen-Vector Method (EVM) is given in detail. Numerical examples are given to illustrate the effectiveness of the method.