Anna Bystrova

Molecular modeling of the piezoelectric effect in the ferroelectric polymer poly(vinylidene fluoride) (PVDF)

In this work, computational molecular modeling and exploration was applied to study the nature of the negative piezoelectric effect in the ferroelectric polymer polyvinylidene fluoride (PVDF), and the results confirmed by actual nanoscale measurements. First principle calculations were employed, using various quantum-chemical methods (QM), including semi-empirical (PM3) and various density functional theory (DFT) approaches, and in addition combined with molecular mechanics (MM) methods in complex joint approaches (QM/MM). Both PVDF molecular chains and a unit cell of crystalline β-phase PVDF were modeled. This computational molecular exploration clearly shows that the nature of the so-called negative piezo-electric effect in the ferroelectric PVDF polymer has a self-consistent quantum nature, and is related to the redistribution of the electron molecular orbitals (wave functions), leading to the shifting of atomic nuclei and reorganization of all total charges to the new, energetically optimal positions, under an applied electrical field. Molecular modeling and first principles calculations show that the piezoelectric coefficient d33 has a negative sign, and its average values lies in the range of d33 ~ −16.6 to −19.2 pC/N (or pm/V) (for dielectric permittivity ε = 5) and in the range of d33 ~ −33.5 to −38.5 pC/N (or pm/V) (for ε = 10), corresponding to known data, and allowing us to explain the reasons for the negative sign of the piezo-response. We found that when a field is applied perpendicular to the PVDF chain length, as polarization increases the chain also stretches, increasing its length and reducing its height. For computed value of ε ~ 5 we obtained a value of d31 ~ +15.5 pC/N with a positive sign. This computational study is corroborated by measured nanoscale data obtained by atomic force and piezo-response force microscopy (AFM/PFM). This study could be useful as a basis for further insights into other organic and molecular ferroelectrics.

Computational and experimental studies of size and shape related physical properties of hydroxyapatite nanoparticles

In this work, the properties of hydroxyapatite (HAP) nanoparticles (NPs) have been studied both theoretically and experimentally focusing on computational analysis. HAP is widely used to fabricate implants, for drug delivery, etc. The physical properties of the nanosized HAP particles play an important role in the interaction with cells in the human body and are of great interest. Computer simulation was employed to understand the properties of HAP clusters (Ca(5)(PO(4))(3)OH) including formation energies, dipole moments and polarization (surface charges) by molecular mechanics (MM + , OPLS) and mostly by quantum semi-empirical Hartree-Fock (PM3) methods. The size of the simulated cluster is found to affect its dipole moment, polarization, and, finally, the electron work function- ϕ. These parameters depend on the concentration of hydrogen atoms H (or protons) at the surface. Values of ϕ were experimentally estimated via photoelectron emission measurements. The magnitude of ϕ was demonstrated to have a positive correlation on sizes. The NPs demonstrated a capability to be gathered within conglomerates. This property is confirmed by the calculated data for various sizes. Their sizes have a positive correlation on ϕ by the native particles. The main results show that the distributions of dipole moments have very different space orientations (along the OX, OY and OZ axes, the OZ axis is oriented along the OH column) and change with the addition of hydrogen atoms, which saturate the broken hydrogen bonds. This electrical property of NP leads to different behaviors and motions with consequent aggregation: (1) for the case of NPs having dipole moment oriented preferably perpendicular to the OZ axis (with more hydrogen bonds saturated by added H)-the HAP NP aggregates with hexagonal orientation and forms a wider and more spherical shape (sphere-like or bundle-like); (2) for the case of NPs having dipole moment oriented along the OZ axis (as is the case in the absence of added protons or non-saturated hydrogen bonds)-the NPs firstly rotated and oriented along this axis to form the most elongated cylindrical shape (rod-like).

Polarization of poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) thin films revealed by emission spectroscopy with computational simulation during phase transition

The electronic structure and self-polarization of P(VDF-TrFE) Langmuir-Blodgett nanofilms were analyzed under temperature-driven phase transitions, according to their thickness, composition, and structural conformation. Both thermo-stimulated exoelectron emission (TSEE) spectroscopy and computational simulation, including quantum-chemical calculations from first principles, were carried out. PVDF and composite P(VDF-TrFE) (70:30) molecular chains as Trans and Gauche conformers, as well as crystal cells, were modeled for these TSEE analyses. The quantum-chemical calculations and the computational simulation were based on the density functional theory (DFT) as well as semi-empirical (PM3) methods. It was demonstrated that the energy of electron states, as well as the total energies of the studied P(VDF-TrFE) molecular clusters during phase transformation, is influenced by electron work function and electron affinity. Analysis was performed by combining TSEE experimental data with the computational data of t...

Computational study of hydroxyapatite structures, properties and defects

Hydroxyapatite (HAp) was studied from a first principle approach using the local density approximation (LDA) method in AIMPRO code, in combination with various quantum mechanical (QM) and molecular mechanical (MM) methods from HypemChem 7.5/8.0. The data obtained were used for studies of HAp structures, the physical properties of HAp (density of electronic states—DOS, bulk modulus etc) and defects in HAp. Computed data confirmed that HAp can co-exist in different phases—hexagonal and monoclinic. Ordered monoclinic structures, which could reveal piezoelectric properties, are of special interest. The data obtained allow us to characterize the properties of the following defects in HAp: O, H and OH vacancies; H and OH interstitials; substitutions of Ca by Mg, Sr, Mn or Se, and P by Si. These properties reveal the appearance of additional energy levels inside the forbidden zone, shifts of the top of the valence band or the bottom of the conduction band, and subsequent changes in the width of the forbidden zone. The data computed are compared with other known data, both calculated and experimental, such as alteration of the electron work functions under different influences of various defects and treatments, obtained by photoelectron emission. The obtained data are very useful, and there is an urgent need for such analysis of modified HAp interactions with living cells and tissues, improvement of implant techniques and development of new nanomedical applications.

Modified Hydroxyapatite Structure and Properties: Modeling and Synchrotron Data Analysis of Modified Hydroxyapatite Structure

First principle modeling and calculations of hydroxyapatite both native and surface modified and having various defects (OH vacancies, H inter-nodes) were performed. Local Density Approximation method used with calculations of Density of States allows us to analyze the experimental obtained work function data. Molecular modeling was confirmed by photo-electron measurements up to 6.5 eV and photoluminescence experimental data from synchrotron DESY up to 30 eV. Brief analysis of the influence of heating, microwave radiation, hydrogenation, and synchrotron radiation on hydroxyapatite surface is presented in this work. New data on the structure of modified hydroxyapatite are obtained.

Computational study of hydroxyapatite properties and surface interactions

The results of computational modeling for Hydroxyapatite (HAP) nanostructures and surface interactions properties are presented in this work. HAP were studied from first principles approaches using Local Density Approximation (LDA) method in combination with various quantum-chemical (QM), including Density Functional Theory (DFT) methods, and molecular mechanical (MM, BIO CHARM) methods from HypemChem 7.5/8.0 package. Obtained data then were used for studies of interactions of HAP clusters with various species (citrates, carbon nanotubes, living cells - osteoblasts, etc.).

Modeling of switching and piezoelectric phenomena in polyvinylidenefluoride (PVDF)

The molecular models of ferroelectric polymer polyvinylidene fluoride (PVDF) film, consisting from one, two chains [-CH2-CF2-]n and for PVDF unit cell are investigated in this work. HyperChem 7.5/8.0 for all modelings as for quantum calculations as well for molecular mechanics and molecular dynamic simulations (MD) were used. The first-principle approach is applied to the switching and kinetics of this model. Kinetics of polarization switching show a homogeneous critical behavior with a critical point at Landau-Ginzburg-Devonshire (LGD) coercive field E = EC. Two types of behavior were established for 2 PVDF chains: simultaneous and sequential rotation in low and high electric field. For a one-chain model we obtained a hysteresis loop for PVDF with an LGD intrinsic coercive field of EC ~ 1 GV/m, while for a two-chain model EC ~ 2 GV/m). These data are related with arisen of the PVDF piezoelectric effects, especially of the negative piezoelectric coefficients. Both PVDF molecular chains and a unit cell of crystalline β-phase PVDF were modeled. Molecular modeling and first principles calculations show that the piezoelectric coefficient d33 has negative values in the range d33 ~ -16.5..-33.5 pC/N (pm/V), corresponding to known data, and allowing us to explain the reasons for the negative sign of the piezoresponse. For value of ε ~ 5, we obtained a value of another oriented coefficient d31 ~ +15.5 pC/N. This computational study is corroborated by measured nanoscale data obtained by atomic force and piezoresponse force microscopy (AFM / PFM).

Ferroelectric PVDF films and graphene-based composites

Advantages in the studies of new composite nanomaterials based on polymer ferroelectrics and graphene are presented. We analyzed the main results of the computational molecular modeling of nanostructures and the pyroelectric properties of the composites from polyvinylidene fluoride (PVDF) films and graphene layers. The pyroelectric effect was modelled and pyroelectric coefficients were calculated for several models using molecular dynamics simulation with quantum-chemical semi-empirical PM3 method from HyperChem tool. The results obtained provide important insights into our understanding of the mechanisms of pyroelectricity in the new nanocomposites, give us new prospective for further studies of the ferroelectric polymer–graphene multifunctional nanomaterials.

Surface modified hydroxyapatites with various functionalized nanostructures: Computational studies of the vacancies in HAp

ABSTRACT Hydroxyapatite (HAp) has structural features that define its basic physical properties, which have an important role at the surface, and it is one of the most used materials in bone implants. In this work, we present a density functional modeling (DFT) study of HAp both as bulk and with special HAp models with various defects, especially oxygen vacancies in HAp surface layers, which can also determine photocatalytic properties, confirmed experimentally. The first-principles calculations of bulk and modified HAp were carried out using local basis (AIMPRO) and plane-wave (VASP) codes. Data obtained are analyzed using both approaches, and compared.

HAP nanoparticle and substrate surface electrical potential towards bone cells adhesion: Recent results review

Nanostructured hydroxyapatite (HAP) and its nanoparticles are widely used for implantation into the human organism. The biocompatibility of the implants depends very much on the interaction between the implant and the cells regenerating tissue to be connected to the implant. An implant surface electrical charged density plays an important role in these processes. Possible instruments managing the surface electrical potential of HAP are in the focus of this paper. Both theoretical and experimental results evidence that: - the surface electrical charge density of the nanoparticle depends on its size and shape; - the electrical charge density of HAP could be engineered by contact less technique because of deposition of the electrical charge from the external radiation source, surface couples reconstruction.