Dielectric Continuum Approach Research Articles

Modeling environmental effects in two-photon circular dichroism calculations

Accounting for solvent effects in theoretical predictions of spectroscopic properties may be of significant importance since a solvent—and on a more general basis any environment—may influence key spectroscopic parameters in nontrivial ways—especially in relation to calculation of nonlinear optical properties. At the simplest level, solvent effects may be included into quantum chemistry calculations by use of a dielectric continuum approach; however, such a description may fail in addressing correctly environment anisotropies as well as specific interactions such as hydrogen bonding. On the other hand, discrete embedding methods allow for a more correct description of an environment since such methods keep the atomistic nature of the environment intact. In this paper, two-photon circular dichroism (TPCD) spectra will be calculated for two different biaryl derivatives introducing solvent effects by two different discrete embedding schemes, i.e., polarizable embedding based on induced dipoles (PE) or the fluctuating charge (FQ) model. While we find inclusion of solvent effects on this molecular property to be important, we conclude at the same time that the influence of the solvent on the TPCD rotatory strength is accounted for in an overall rather equivalent manner by either embedding methods.

Optical vibration modes in multi-layer quantum dots of polar ternary mixed crystals

Within the framework of the dielectric continuum approach and the modified random-element isodisplacement model, the optical vibration modes as well as the corresponding electron-phonon interactions in spherical multi-layer quantum dots (QDs) consisting of ternary mixed crystals (TMCs) are investigated in detail. The dispersion relation and electron-phonon interaction Hamiltonian are obtained. As applications of the present theory, the numerical calculations for the ZnS/ZnxCd1-xS/ZnS and GaAs/AlxGa1-xAs/GaAs QDs are performed and discussed. Considering the “one-mode” and “two-mode” behaviors of the TMCs, the results show that there are 5 and 7 branches of interface/surface optical (IO/SO) phonon modes in the two systems, respectively. It is found that the effects of TMCs have a remarkable influence on the optical phonon frequencies and the electron-phonon interactions. The results also reveal that the presence of the cap layer introduces significant influence on the TMCs effects of optical phonon in multi-layer QDs. We hope that our theoretical results can stimulate further investigations of related photoelectric properties, as well as device applications in multi-layer QDs consisting of TMCs.

Effects of ternary mixed crystals on interface/surface optical phonon in spherical core-shell quantum dots

Within the framework of the dielectric continuum approach and modified random-element-isodisplacement model, the optical vibration mode in a spherical core-shell quantum dot (CSQD) consisting of ternary mixed crystals (TMCs) are investigated. The dispersion relation and electron–phonon interaction Hamiltonian are derived. As a typical case, the numerical results for [Formula: see text] and [Formula: see text] CSQDs are obtained and discussed. Taking the one- and two-mode behaviors of TMCs into account, the effects of TMCs on interface/surface optical (IO/SO) phonon show that there are 3 and 5 branches of IO/SO phonon modes in [Formula: see text] and [Formula: see text] CSQDs for a given component of TMC, respectively. It is also found that the IO/SO phonon frequencies and electron–phonon interactions are strongly dependent on the component of TMCs and the size of CSQDs. We hope this work would be useful for the study of the phonon-related photoelectric properties in CSQDs consisting of TMCs.

Modeling magnetic circular dichroism within the polarizable embedding approach

Magnetic circular dichroism (MCD) is defined as the differential absorption of left and right circularly polarized light in a sample subjected to an external magnetic field. In order to interpret the results of MCD measurements, theoretical predictions of key MCD parameters can be of utmost importance. From an experimental point of view, MCD spectra of molecules are often measured in an environment and most notably in a solution. Thus, it may be very important that the method used to predict the MCD parameters is able to correctly account for medium effects. In this paper, we investigate the quality of MCD calculations within the polarizable embedding approach, which represents a fully atomistic and polarizable representation of an environment surrounding a smaller region treated using quantum mechanics. Furthermore, we compare the performance of the polarizable embedding scheme to the use of the more conventional dielectric continuum approach. Results are presented for cytosine and hypoxanthine solvated in water.

Resonant Raman scattering from CdTe/ZnTe self-assembled quantum dot structures

Abstract We report resonant Raman scattering results of CdTe/ZnTe self-assembled quantum dot (QD) structures. Photoluminescence spectra reveal that the band gap energies of the CdTe QDs decrease with the increase of CdTe thickness from 2.0 to 3.5 monolayers, which indicates that the size of the QDs increases. When the CdTe/ZnTe QD structures are excited by non-resonant excitation, a longitudinal optical (LO) phonon response from the ZnTe barrier material is observed at 206 cm−1. In contrast, when the CdTe/ZnTe QD structures are resonantly excited near the band gap energy of the QDs, additional phonon modes emerge at 167 and 200 cm−1, while the ZnTe LO phonon response completely disappears. The 167 cm−1 mode corresponds to the LO phonon of the CdTe QDs. A spatially resolved Raman scattering from the cleaved edge of the QD sample reveals that the 200 cm−1 mode is strongly localized at the interface between the CdTe QDs and ZnTe cap layer. This phonon mode is attributed to the interface optical (IO) phonon. The analytically calculated value of the IO phonon energy using a dielectric continuum approach, assuming a spherical dot boundary, agrees well with the experimental value.

The importance of orientation of exocyclic groups in a naphthoxyloside: A specific rotation calculation study

2-Naphthyl β-d-xylopyranoside (XylNap) inhibits β-1,4-galactosyltransferase 7 (β4GalT7) and thereby growth of tumor cells both in vitro and in vivo. The binding pocket of β4GalT7 has a defined orientation of hydrogen bond acceptors and hydrophobic moiety. Knowing the orientation of the hydroxyl and naphthyl groups of this molecule would help in the development of more efficient inhibitors. In this work, we have tried, for the first time, to determine the exocyclic hydroxyl and aglycon groups orientation of XylNap, using ab initio descriptions, and calculation of the specific rotation values, in methanol solutions, using 2 different solvent descriptions: a dielectric continuum approach (polarizable continuum model [PCM]) and a microsolvated + continuum approach (MS + PCM). In the PCM approach, [α]D = −59 deg/(dm(g/cm3)) whereas for the MS + PCM approach [α]D = −29 deg/(dm(g/cm3)). The latter is in excellent agreement with the experimentally determined value in methanol solution, viz, [α]D = −30 deg/(dm(g/cm3)). This agreement allows us to say that the hydroxyl groups have similar orientations in xylose and XylNap, and the naphthyl group has a very well-defined ψ dihedral angle value in the most abundant conformations.

Efficient modeling of liquid phase photoemission spectra and reorganization energies: Difficult case of multiply charged anions.

An efficient approach for quantitative modeling of liquid phase photoelectron spectra, reorganization energies, and redox potentials with DFT-based molecular dynamics simulations is presented. The method is based on a large scale cluster-continuum approach combined with the so-called reflection principle (RP). Finite size clusters of solute molecules with solvating water molecules are at first generated using either classical molecular dynamics or molecular dynamics with a quantum thermostat which accounts for nuclear quantum effects. In the next step, the electron binding energies are calculated. Finite-size corrections for (i) positions of electron binding energies and (ii) width of the spectrum are evaluated via a dielectric continuum approach. The performance of such a reflection principle with additional broadening approach (RP-AB) for oxidation of multiply charged iron anions, [Fe(CN)6 ]4- and [Fe(CN)6 ]3- is demonstrated. The role of nuclear quantum effects is discussed as well as the relation between spectroscopic data and electrochemical quantities. Results are compared with recent liquid photoemission experiments, explaining the obstacles for applying liquid phase photoemission spectroscopy as a direct method for obtaining absolute redox potentials and suggesting a way to overcome them. © 2017 Wiley Periodicals, Inc.

The influence of size effect on interface phonons in core-shell quantum dot: a resonant Raman study

In this paper, the interface phonons in a core-shell quantum dot are theoretically studied by a resonant Raman scattering (RRS) process. Fröhlich electron–phonon interaction is considered in the framework of the dielectric continuum approach. The Raman peaks are found to be sensitive to the size of strongly confined shell. The shift of the Raman resonant peaks is a consequence of the change of observed dispersion of the phonon frequency. The Raman intensity changes in the system with shell thickness, originating from the competition between the spacial distribution of electron wave function and the number of phonons joining in the RRS process. The analysis of the Raman spectra gives a physical explanation to the size-selective nature of the Raman process and some experimental results.

Electron-phonon interaction effect on the energy levels and diamagnetic susceptibility of quantum wires: Parallelogram and triangle cross section

In this paper, optical phonon modes are studied within the framework of dielectric continuum approach for parallelogram and triangular quantum wires, including the derivation of the electron-phonon interaction Hamiltonian and a discussion on the effects of this interaction on the electronic energy levels. The polaronic energy shift is calculated for both ground-state and excited-state electron energy levels by applying the perturbative approach. The effects of the electron-phonon interaction on the expectation value of r2 and diamagnetic susceptibility for both quantum wires are discussed.

Theoretical study on quasi-confined optical phonon modes in a free-standing cylindrical GaN nanowire with ring geometry: A resonant Raman scattering

• The DCS of QC phonons are sensitive to the wire size. • The high-frequency of QC phonons make main contribution to the DCS. • The lower orders of the QC phonons are, the larger the DCS are in the GaN NWs. • The (DCS) max has a peak as the anisotropic effect of wurtzite nitride crystal. We have investigated quasi-confined (QC) optical resonant Raman scattering in GaN nanowires (NWs) with ring geometry. We consider the Fröhlich electron–phonon interaction in the framework of the dielectric continuum approach. The selection rules are studied. For small ring radius, our results show that the main contribution to the differential cross section (DCS) comes from high-frequency of the QC phonons, with a minor contribution from the LO phonons. The peak of the DCS is mainly attributed to the electron-QC coupling strength can get the maximum value as the anisotropic effect of wurtzite nitride crystal. Moreover, the lower orders of the QC phonons, the larger the DCS.

OPTICAL VIBRATION MODES IN SPHERICAL CORE-SHELL QUANTUM DOTS

Using a dielectric continuum approach, the optical vibration modes in a spherical core-shell quantum dots (QDs) imbedded in a host nonpolar material are studied. The dispersion relation and the corresponding electron–phonon interaction Hamiltonian are derived. The numerical calculations for the CdSe/ZnS system are performed. The results reveal that there are three branches frequencies of interface/surface optical phonon in the system. A detailed discussion of the combined effects of the spatial confinement and dielectric mismatch between the dot and the host medium is given.

One-phonon resonant electron Raman scattering in multilayer coaxial cylindrical Alx Ga1-x As/GaAs quantum cables

We have presented a theoretical calculation of the differential cross section (DCS) for the electron Raman scattering (ERS) process associated with the interface optical (IO) and surface optical (SO) phonons in multilayer coaxial cylindrical AlxGa1−x As/GaAs quantum cables (QC). We consider the Fröhlich electron-phonon interaction in the framework of the dielectric continuum approach. The selection rules for the processes are studied. Singularities are found to be sensitively size-dependent and by varying the size of the QC, it is possible to control the frequency shift in the Raman spectra. A discussion of the phonon behavior for the QC with different size is presented. The numerical results are also compared with those of experiments.

Optical vibration modes in spherical multilayer nanostructures

Abstract Using the dielectric continuum approach, we study the optical vibration modes in a polar spherical multilayer nanostructure hosted in a non-polar material. The dispersion relation and the corresponding electron–phonon interaction Hamiltonians for interface optical (IO) or surface optical (SO) phonons are derived. As a typical case, the numerical results for the CdSe/ZnS/CdSe spherical multilayer nanostructure are given. The frequency and electric potential characteristics of 5 branches of IO/SO phonon modes are analyzed. The results show that the electron–phonon coupling related to the IO modes with higher frequencies and the SO modes are more significant.

One-phonon resonant electron Raman scattering in a cylindrical GaAs/AlAs quantum dot

We have presented a theoretical calculation of the differential cross section for the electron Raman scattering process associated with the interface optical phonon modes in cylindrical GaAs quantum dots (QDs) with a AlAs matrix. We consider the Fröhlich electron–phonon interaction in the framework of the dielectric continuum approach. The selection rules for the processes are studied. Singularities are found to be sensitively size-dependent, and, by varying the size of the QDs, it is possible to control the frequency shift in the Raman spectra. A discussion of the phonon behavior for QDs with different size is presented. Copyright © 2013 John Wiley & Sons, Ltd.

First-order Raman scattering in a free-standing GaN nanowire with ring geometry

Abstract We have investigated one phonon resonant Raman scattering in GaN nanowires (NWs) with ring geometry. We consider the Frohlich electron–phonon interaction in the framework of the dielectric continuum approach. The selection rules are studied. For the GaN NWs with small radius, results reveal that the main contribution to the differential cross-section (DCS) stems from the surface optical (SO) phonons especially from the high-frequency of SO phonons, with a minor contribution from the longitudinal optical (LO) phonons. Meanwhile, dispersions of the two branches of the SO phonon modes are obvious when the wire is thin. Moreover, compared to GaAs NWs, the GaN NWs make more contribution to the DCS in the small quantum size.

Interface/surface optical phonon in onion-like quantum dots

We calculate the interface/surface phonon in onion-like quantum dots by using dielectric continuum approach. The form of the phonon potential of the onion-like quantum dots and the electron–phonon interaction Hamiltonian are obtained. Core/shell quantum dots and quantum dot quantum well are studied in detail. The interface/surface phonon modes and corresponding frequencies as a function of the sizes of the structure are investigated. We observe that the asymptotic behavior is depended on the host medium. The sensitivity of the interface/surface phonon frequencies are decreasing with the increasing of the shell thickness. The contribution of different angular momentum quantum numbers to the electron–phonon interaction are given for the onion-like quantum dots. Numerical calculations are performed on the CdS/ZnS and CdS/ZnS/CdS heterostructure which is embedded in the polyethylene.

Association constants and distribution functions for ion pairs in binary solvent mixtures: Application to a cyanine dye system

The computations of the association constants K(ass) were performed at the microscopic level for the ion pair Cy(+)I(-) composed of the complex cyanine dye cation Cy(+) coupled to the negative iodine counterion. The wide array of K(ass) values is arranged by a variation of the composition of the binary solvent mixtures toluene/dimethylsulfoxide with the accompanying change of the solvent polarity. The potentials of mean force (PMFs) are calculated for a set of interionic separations R in the Cy(+)I(-) by a methodology which combines the quantum-chemical techniques for the treatment of the electronic structure of the Cy(+)I(-) system with the recent dielectric continuum approach which accounts for the solvation effects. For a given solute/solvent system the probability function P(R), which describes the distribution of interionic separations, is constructed in terms of the PMFs and implemented for the evaluation of the K(ass).

Surface optical phonon-assisted electron Raman scattering in a free-standing quantum wire with ring geometry

We present a theoretical calculation of the differential cross section for the electron Raman scattering process associated with surface optical phonon modes in semiconductor quantum wires (QWs) with ring geometry. Electron states are considered to be confined within the QWs. We consider the Fröhlich electron–phonon interaction in the framework of the dielectric continuum approach. Some singularities for the ring with various sizes in the Raman spectra are found and interpreted. A discussion of the phonon behavior for the QWs with large and small sizes is presented. The numerical results are also compared with those of experiments.

Solvated potential energy surfaces for MePC

A conformational study of methylphosphocholine (MePC) was performed from a conformational map calculated considering the dihedral angles that most affected the energy of the system. This conformational map was obtained from interpolations of energy values found from calculations that took into account the presence of solvent through a dielectric continuum approach. We used a quantum–mechanical method based on the Density Functional Theory (DFT) and the 6-31G(d,p) basis set. Conformational samplings were performed in stability regions and the most stable conformers were identified. Comparisons of samplings in solvated and non-solvated conformational maps were performed, and although very different from each other, they furnish equivalent final conformational sets. Both the sets reproduced satisfactorily the experimental shift observed for the vibrational modes of –[P(O2)]−– and –CH2– methylene groups of MePC when the system was solvated.

Optical phonons in spherical core/shell semiconductor nanoparticles: Effect of hydrostatic pressure

By applying a phenomenological macroscopic approach we have studied polar optical phonons in core-shell semiconductor hybrid nanoparticles with spherical shape. The coupling of electromechanical oscillations is taken into account within the long-wavelength limit. A detailed analysis of the optical-phonon modes is presented with emphasis on the phonon-dispersion laws. Different kinds of II-VI and III-V semiconductor compounds are discussed, showing the differences resulting from their bulk phonon frequency dependence on the wave vector, and on the shell-to-core radius ratio. We make a systematic application of the theory for different nanostructures and report all possible polar optical-phonon modes. The effects of hydrostatic pressure on confined and surface-optical-phonon mode optical vibrations are considered. Also, we provide results for the usual dielectric continuum approach as a function of the applied pressure, where just the electric aspect of the oscillations is addressed. The effects of pressure on confined LO- and TO-like, and on surface-optical-phonon modes in core/shell InP/CdS nanoparticles are explored by Raman scattering. The observed pressure-induced shifts of the Raman spectra are well described by the theoretical calculations, and by applying a fitting procedure to the data we may extract the value for the core diameter and the shell-to-core radius ratio.