Dielectric Continuum Model Research Articles

Surface optical phonon replica in photoluminescence spectroscopy of nitride nanostructures: Crystal structure and surface effects

The surface optical (SO) phonon replica in photoluminescence (PL) spectroscopy of nitride nanowires (NWs) was theoretically investigated in this study. The dispersive relationships of SO phonon mode in anisotropic wurtzite (WZ) and isotropic zinc-blende (ZB) crystal structure NWs with circular and square cross sections (CSs) were derived within the framework of the dielectric continuum model. Based on the energy and momentum conservation laws, a constraint relationship between the frequency and wave-number was constructed for SO phonon-assisted excitonic PL spectra in the NW structure. By combining the dispersive and constraint relationships, the frequency and wave-number of the SO phonon replica in the PL spectra could be determined. The WZ and ZB crystal structures of nitride semiconductor were considered. The influences of surface factors including the CS shape, dielectric medium, and environment temperature on the frequency and photon wavelength of the band-edge emission of the SO phonon replica were studied in detail. Numerical results reveal that the crystal structure, surface factors, and environment temperature greatly affect the frequency and photon wavelength of the band-edge emission of the SO phonon replica. The calculated results for the photon wavelength agree well with the experimental values of the SO phonon replica in AlN NWs. The results of the dielectric effect obtained here are also supported by previous experimental and theoretical results for nitrides and other semiconductor NWs. The present theoretical scheme and numerical results can be used to analyze and design the SO phonon replica in PL spectra of nanostructures.

A computational study of a light-driven artificial device: a third generation rotational photo-molecular motor in dilute solutions.

A third-generation artificial photo-molecular motor, featuring two photo-switchable rotating moieties in connection with a pseudoasymmetric molecular centre, is investigated by combining quantum-mechanics (QM) algorithms with classical molecular dynamics (MD) propagators. In particular, in the present contribution we have addressed such a molecular motor in different rotational isomers following the experimental observations arising from the application of multiple spectroscopic techniques in dilute solutions. At first, we focused our attention on the reproduction of the UV/Vis absorption spectrum in two solvents (acetonitrile and cyclohexane) with different gradient-corrected density functional theory (B3LYP, Cam-B3LYP, PBE, PBE0) functionals in conjunction with the conductor-like and polarizable continuum model (C-PCM). Furthermore, we refined the absorption signals by combining a classical MD sampling at room-temperature with DFT-based electronic degrees of freedom to compute perturbed excitation wavelengths driven by thermal fluctuation and solvation effects. In this respect, we have modelled the investigated artificial motor within solution nanodroplets with solvent molecules treated contextually at atomistic level and via a dielectric and polarizable continuum model.

Optical phonons in quantum well with anisotropic permittivity

The dielectric continuum model has been developed to describe the optical phonons in quantum well with anisotropic permittivity. The developed model is used to calculate the spectra and potentials of the optical phonons in a CdHgTe/HgTe heterostructure with narrow-gap quantum wells.

A generalized Kirkwood implicit solvent for the polarizable AMOEBA protein model.

Computational simulation of biomolecules can provide important insights into protein design, protein-ligand binding interactions, and abinitio biomolecular folding, among other applications. Accurate treatment of the solvent environment is essential in such applications, but the use of explicit solvents can add considerable cost. Implicit treatment of solvent effects using a dielectric continuum model is an attractive alternative to explicit solvation since it is able to describe solvation effects without the inclusion of solvent degrees of freedom. Previously, we described the development and parameterization of implicit solvent models for small molecules. Here, we extend the parameterization of the generalized Kirkwood (GK) implicit solvent model for use with biomolecules described by the AMOEBA force field via the addition of corrections to the calculation of effective radii that account for interstitial spaces that arise within biomolecules. These include element-specific pairwise descreening scale factors, a short-range neck contribution to describe the solvent-excluded space between pairs of nearby atoms, and finally tanh-based rescaling of the overall descreening integral. We then apply the AMOEBA/GK implicit solvent to a set of ten proteins and achieve an average coordinate root mean square deviation for the experimental structures of 2.0 Å across 500ns simulations. Overall, the continued development of implicit solvent models will help facilitate the simulation of biomolecules on mechanistically relevant timescales.

Dispersive response of polar optical phonon modes on the dielectric and size effects in a nitride nanostructure

Within the framework of the macroscopic dielectric continuum model, the dispersive spectra of polar optical phonon modes of III-nitride based nanostructures with different inner and outer dielectric media are investigated in this paper. The dielectric and quantum size effects on the dispersive frequencies of phonon modes are focused and emphasized. In the case of GaN quantum rings (QRs), we found that the dielectric constant of the inner/outer medium could significantly influence the dispersive spectra of the surface optical (SO) phonon mode as the high/low-frequency SO phonon modes are mainly localized at the inner/outer surface of QRs, respectively. Moreover, such effects associated with different crystallographic structures of GaN, i.e., the wurtzite (WZ) and zinc blende (ZB) crystal structures, are also studied in detail, showing the different degenerating behavior and electron–phonon coupling strength. The results obtained in this study provide a beneficial opportunity to independently adjust dispersive spectra of the two branches of SO modes in GaN nanostructures; thus, it could be used to design and develop phonon-based detectors and sensors.

Quantum Chemistry in Solution: Fiftieth Anniversary.

In this perspective, we briefly present the historical context in which, fifty years ago, dielectric continuum models were developed to incorporate solvent effects into quantum mechanical calculations. Since the first self-consistent-field equations including the solvent electrostatic potential (or reaction field) were reported in 1973, continuum models have become extremely popular in the computational chemistry community and are routinely used in a very wide range of applications.

Exciton binding energy and radiation lifetime in CdTe/Cd1−xZnxS strained core/shell spherical quantum dots under pressure

In this paper, the exciton binding energy and radiation lifetime in type I and II structures of CdTe/Cd[Formula: see text]ZnxS strained core/shell spherical quantum dots under the hydrostatic pressure have been studied by using the variational method within the continuous dielectric model and effective mass approximation. The results show that for these two structures, the exciton binding energies with and without strain decrease with the increasing core radius but increase with the increasing pressure. The exciton binding energy with strain is smaller than that without strain. For type I structure, the effect of strain is small, and the radiation lifetime decreases monotonically with the increasing pressure. By contrast, for type II structure, the effects of shell radius and strain on the exciton binding energy are obvious, and the radiation lifetime increases first and then decreases with the increasing pressure.

Hydration shell effects in ac-driven single-molecule junctions

We propose a quantum-mechanical model to calculate the current through a single molecular junction immersed in a solvent and surrounded by a thin shell of bound water under an applied ac voltage. The solvent plus hydration shell are captured by a dielectric continuum model for which the resulting spectral density is determined. Here the dielectric properties, e.g., the Debye relaxation time and the dielectric constant, of the bulk solvent and the hydration shell as well as the shell thickness directly enter. We determine the charge current through the molecular junction under an ac voltage in the sequential tunneling regime where we solve a quantum master equation by a real-time diagrammatic technique. Interestingly, the Fourier components of the charge current show an exponential-like decline when the hydration shell thickness increases. Finally, we apply our findings to binary solvent mixtures with varying volume fractions and find that the current is highly sensitive to both the hydration shell thickness as well as the volume fraction of the solvent mixture, giving rise to possible applications as shell and concentration sensors on the molecular scale.

Modified quantum mechanical approach for the estimation of single-electron reduction potential of nitroaromatic compounds in aqueous medium

The midpoint single-electron reduction potential of nitroaromatic compounds in aqueous medium at pH 7.0 (potential of ArNO2/ArNO2 · – couple, E m7) frequently determines their therapeutic and/or toxic properties. However, its estimation remains a complex problem. We propose a modified method of E m7 estimation by quantum mechanical calculations, based on the use of the dielectric continuum model together with a certain number of H2O molecules at the vicinity of nitro group. The optimal number of H2O molecules corresponds to a minimal difference between the experimentally determined and calculated values of E m7, and/or the most negative value of calculated E m7. This enabled us to calculate the E m7 values for a number of ArNO2 (n = 19) with the average deviation of 0.027 V from the experimentally determined ones. Apart from nitrobenzene derivatives, the application of this approach for the representatives of nitropyridines, nitrofurans, nitrothiophenes, and nitrothiazoles was demonstrated. In this case, nitroimidazole derivatives are an exception, evidently due to a strong proton accepting properties of N3 atom of their free radicals.

What is the Optimal Dipole Moment for Nonpolarizable Models of Liquids?

In classical nonpolarizable models, electrostatic interactions are usually described by assigning fixed partial charges to interaction sites. Despite the multitude of methods and theories proposed over the years for partial charge assignment, a fundamental question remains─what is the correct degree of polarization that a fixed-charge model should possess to provide the best balance of interactions (including induction effects) and yield the best description of the potential energy surface of a liquid phase? We address this question by approaching it from two separate and independent viewpoints: the QUantum mechanical BEspoke (QUBE) approach, which assigns bespoke force field parameters for individual molecules from ab initio calculations with minimal empirical fitting, and the Polarization-Consistent Approach (PolCA) force field, based on empirical fitting of force field parameters with an emphasis on transferability by rigorously accounting for polarization effects in the parameterization process. We show that the two approaches yield consistent answers to the above question, namely, that the dipole moment of the model should be approximately halfway between those of the gas and the liquid phase. Crucially, however, the reference liquid-phase dipole needs to be estimated using methods that explicitly consider both mean-field and local contributions to polarization. In particular, continuum dielectric models are inadequate for this purpose because they cannot account for local effects and therefore significantly underestimate the degree of polarization of the molecule. These observations have profound consequences for the development, validation, and testing of nonpolarizable models.

Dispersive Spectra of Phonon Modes and Fröhlich Electron–Phonon Coupling Properties in AlGaN Quantum Rings: Wurzite versus Zinc‐Blende Structure

The influence of crystal structure on the phonon modes and electron–phonon coupling properties in nitride quantum ring (QR) structures is investigated based on the macroscopic dielectric continuum model. It is found that three types of phonon modes including the surface optical (SO) phonon modes, the exact confined (EC) modes, and quasiconfined (QC) modes can coexist in the wurtzite nitride QR. However, only the SO modes and EC modes may appear in zinc‐blende nitride QR, and the QC modes cannot exist in zinc‐blende QRs in general. Numerical calculation on AlN QR shows that the SO modes of zinc‐blende QR are more dispersive than those of wurtzite QR. Furthermore, the degenerative behavior of QC modes to SO modes in wurtzite QRs is clearly observed. Moreover, the electron–phonon coupling functions in zinc‐blende QRs are larger than those in wurtzite QRs. These results reveal that the crystal structures greatly affect not only the type of phonon mode, but also the coupling strength of electron–phonon interaction. The present theoretical scheme and the numerical results are quite useful and important for further understanding and investigating the polaronic effect and related physical properties in wurtzite and zinc‐blende nitride QRs.

Role of Confined Optical Phonons in Exciton Generation in Spherical Quantum Dot.

Optical control of excitonic states in semiconducting quantum dots has enabled it to be deployed as a qubit for quantum information processing. For self-assembled quantum dots, these excitonic states couple with phonons in the barrier material, for which the previous studies have shown that such exciton—phonon coupling can also lead to the generation of exciton, paving the way for their deployment in qubit-state preparation. Previous studies on self-assembled quantum dots comprising polar materials have considered exciton—phonon coupling by treating phonon modes as bulk acoustic modes only, owing to nearly the same acoustic property of the dot and barrier material. However, the dimensional confinement leads to significant modification phonon modes, even though acoustic confinement is weak but optical confinement cannot be overlooked. In this paper, we investigate for the first time the exciton—optical phonon coupling using dielectric continuum model duly accounting for the dimensional confinement leading to exciton generation. We report that at low temperatures (below 10 K), the exciton creation rate attributed to confined optical phonon is approximately 5.7 times (~6) slower than bulk acoustic phonons, which cannot be ignored, and it should be accounted for in determining the effective phonon assisted exciton creation rate.

Fröhlich electron–phonon interaction Hamiltonian and potential distribution of a polar optical phonon mode in wurtzite nitride triangular nanowires

Polar optical phonon modes of wurtzite triangular nanowires (NWs) with three different cross sections, including the hemi-equilateral triangle (HET), the isosceles right triangle (IRT), and the equilateral triangle (ET), are deduced and analyzed using the dielectric continuum model. The exact and analytical phonon states of exactly confined (EC) modes in nitride NWs with HET, IRT, and ET cross sections are derived. The characteristic frequency of EC phonon modes in the triangular nitride NW systems is specified. Fröhlich electron–phonon interaction Hamiltonians in wurtzite NWs with three types of triangular cross sections are obtained. It is found from the numerical results that, among the three types of GaN NWs, the electron–phonon coupling of EC modes in NWs with an HET cross section is the weakest one, that in NWs with an ET cross section is the strongest one, and that in NWs with an IRT cross section is in the middle. The electrostatic potentials of EC modes in HET NWs are neither symmetric nor antisymmetric. The potential functions of EC modes in the ET NW structures have one (three) symmetric axis (axes) as the quantum numbers p and q take fractions (integers). The potential functions of EC modes in IRT NWs behave either symmetrically or anti-symmetrically, which are closely dependent on the parities of the quantum numbers p and q. With the increase of order-number of EC modes, the electron–phonon coupling becomes weaker and weaker. This reveals that cross-sectional morphology of quantum structures has an important influence on the symmetries of phonon modes and electron–phonon coupling strengths in low-dimensional quantum systems.

Optical phonons in core-shell semiconductor prism nanowires affected by the cross-section shape

As building blocks of modern optoelectronic devices, semiconductor core-shell nanowires are usually prisms. The Raman spectra of the nanowires can vary with the cross-section shape and size. In the framework of the dielectric continuum and Loudon's uniaxial crystal models, we investigate the influence of cross-section shape on the optical phonons in prism core-shell nanowires. It is found that while the layer components decide the modes contained in the optical phonon spectra, the shape of a cross-section mainly affects the linewidth of a spectrum. Our calculations show that the electrostatic potentials of phonons have symmetry with the cross-section, while the dispersion relationships extend to bands in prism nanowires due to the cross-section's anisotropy, in contrast with the single dispersion lines in cylindrical ones. The anisotropy of optical phonons becomes apparent in the nanowires with fewer cross-section sides. Increasing the core area can narrow the dispersion bands, approaching the single lines of cylindrical nanowires as the area gets infinitely large. Reducing the gap between the layers' components can make the anisotropy of optical phonons weaker but not disappear. The theoretical results help to interpret the variation of the Raman spectra with the cross-section shape in the nanowires and related optical and electrical properties.

The dipole moment of alcohols in the liquid phase and in solution

Understanding polarization effects in condensed phases, like liquids and solutions, requires computational methods that can accurately predict dipole moments and energy of polarized molecules. In this paper, we report an improvement and extension of our recently developed Self-Consistent Electrostatic Embedding (SCEE) method, and apply it to determine the dipole moment of pure liquid alcohols, as well as of methanol dissolved in a variety of solvents (namely, other alcohols, water and hexadecane). We observe that the dipole moments of pure liquid alcohols are enhanced by ∼0.9 D over their gas phase values, which is similar to the dipole enhancement previously observed for water, and much higher than what is predicted by dielectric continuum models. Our results demonstrate the importance of accounting for local solvation effects, namely the formation of hydrogen bonds, when calculating the extent of liquid phase polarization. In fact, we argue that the dipole enhancement upon solvation can be explained as a superposition of two effects: bulk screening described by the solvent dielectric constant and local solvation that requires a discrete molecular-level description of the system. SCEE is able to account for both effects simultaneously, and is thus a powerful tool to estimate polarization effects in liquids and solutions.

Potential distribution of polar optical phonons and their electron–phonon coupling properties of a wurtizite GaN-based trianglar quantum dot

Polar optical phonon modes of a wurtzite nitride trianglar quantum dot (QD) are deduced and analyzed by using the dielectric continuum model. Based on a method of nonseparable variables, the Laplace equation of electristatic potential of the structure is solved, and the exact and analytical phonon states of exact confined (EC) modes are derived. It is found that the frequency of EC modes in the nitride trianglar QD systems can take both the longitudinal optical phonons in [Formula: see text]-direction [Formula: see text] and the longitudinal optical phonons in [Formula: see text]-plane [Formula: see text], which is obviously different from the cases of EC modes in wurtzite GaN quantum wells and quantum wires due to their different confined dimensions. The polarization eigenvectors and their orthogonality relations for the EC phonon modes are derived, which could form a set of orthogonal and completed basis vectors. By using the orthogonality relations of polarization eigenvector and field quantization method, the free phonon field of EC modes and the corresponding Fröhlich electron–phonon interaction Hamiltonian are obtained. Numerical calculations on a wurtztie GaN trianglar QD are performed. The spatial distributions of phonon potentials of EC phonons both in [Formula: see text]-plane and in [Formula: see text]-direction as well as the electron–phonon coupling strength are plotted and discussed. It is observed that the potential of these EC phonon modes is strictly confined within the trianglar QD ranges. The dependence of symmetries and electron–phonon coupling strength on the quantum numbers [Formula: see text] and [Formula: see text] in [Formula: see text]-plane, and the quantum number [Formula: see text] in [Formula: see text]-direction are analyzed in detail. The relationship of the total number of peaks and troughs with these quantum numbers are also discussed and induced.

COSMO‐RS as an effective tool for predicting the physicochemical properties of fragrance raw materials

AbstractA predictive tool called COSMO‐RS, Conductor‐Like Screening Model for Real Solvents, based on quantum chemistry, dielectric continuum models, electrostatics surface interactions, and statistical thermodynamics, has been used to predict five key physicochemical properties of raw materials used in perfumery industries. The prediction of boiling point (BP), octanol‐water partition coefficient (logP), vapor pressure (VP), water solubility (WS), and Henry's law constant (HLC) of fragrance molecules has been validated with a reference data set of 166 organic compounds. Knowing these properties for a fragrance molecule is essential and being able to predict them precisely can be particularly useful in the development of new molecules or in the replacement of controversial molecules regarding safety issues without compromising the overall hedonic accord. Finally, mapping the vapor pressureversusthe Henry's law constant and the octanol‐water partition coefficient consistently predicts the note class of the molecules useful for release comparison of captives.

Effects of electron-phonon interaction and pressure on optical properties of wedge-shaped quantum dots

The energy levels and lattice vibrational modes in quantum dots (QDs) become discrete due to 3D quantum confinement. Therefore, the influence of electron-phonon (e-p) interaction on physical properties of QDs is an important subject to study. In this work, the effects of e-p interaction and pressure in a GaAs wedge-shaped (WS) quantum dot are considered. In first step, using the dielectric continuum model, the Hamiltonian of longitudinal optical (LO) phonons and the e-p interaction Hamiltonian are analytically derived. In the next step, we have studied the e-p effect on energy eigenvalues, magnetic susceptibility and optical properties of the WS quantum dot. We have also investigated the pressure effect on optical properties of the system in the presence of the e-p interaction. It is found that the polaronic energy shift reduces when the QD size increases. The magnetic susceptibility enhances with raising the QD size. The e-p effect on magnetic susceptibility is strong for smaller QD sizes. The absorption coefficient shows two peaks in the presence of the e-p interaction. The refractive index change enhances and moves to higher energies under the e-p interaction. We found that the pressure and e-p interaction, simultaneously, have important effects on absorption coefficient and refractive index changes.

Quadrupole-mediated dielectric response and the charge-asymmetric solvation of ions in water.

Treating water as a linearly responding dielectric continuum on molecular length scales allows very simple estimates of the solvation structure and thermodynamics for charged and polar solutes. While this approach can successfully account for basic length and energy scales of ion solvation, computer simulations indicate not only its quantitative inaccuracies but also its inability to capture some basic and important aspects of microscopic polarization response. Here, we consider one such shortcoming, a failure to distinguish the solvation thermodynamics of cations from that of otherwise-identical anions, and we pursue a simple, physically inspired modification of the dielectric continuum model to address it. The adaptation is motivated by analyzing the orientational response of an isolated water molecule whose dipole is rigidly constrained. Its free energy suggests a Hamiltonian for dipole fluctuations that accounts implicitly for the influence of higher-order multipole moments while respecting constraints of molecular geometry. We propose a field theory with the suggested form, whose nonlinear response breaks the charge symmetry of ion solvation. An approximate variational solution of this theory, with a single adjustable parameter, yields solvation free energies that agree closely with simulation results over a considerable range of solute size and charge.

New Cationic fac-[Re(CO)3(deeb)B2]+ Complex, Where B2 Is a Benzimidazole Derivative, as a Potential New Luminescent Dye for Proteins Separated by SDS-PAGE.

Sodium-dodecyl-sulfate polyacrylamide gel electrophoresis (SDS-PAGE) can be used to separate proteins based mainly on their size such as in denaturing gels. Different staining methods have been reported to observe proteins in the gel matrix, where the most used dyes are generally anionic. Anionic dyes allow for interactions with protonated amino acids, retaining the dye in the proteins. Fluorescent staining is an alternative technique considered to be sensitive, safe, and versatile. Some anionic complexes based on d6 transition metals have been used for this purpose, where cationic dyes have been less explored in this context. In this work, we synthesized and characterized a new monocationic rhenium complex fac-[Re(CO)3(deeb)B2]+ (where deeb is 4,4′-bis(ethoxycarbonyl)-2,2′-bpy and B2 is 2,4-di-tert-butyl-6-(3H-imidazo[4,5-c]pyridine-2-yl)phenol). We carried out a structural characterization of this complex by MS+, FTIR, 1H NMR, D2O exchange, and HHCOSY. Moreover, we carried out UV-Vis, luminescence, and cyclic voltammetry experiments to understand the effect of ligands on the complex’s electronic structure. We also performed relativistic theoretical calculations using the B3LYP/TZ2P level of theory and R-TDDFT within a dielectric continuum model (COSMO) to better understand electronic transitions and optical properties. We finally assessed the potential of fac-[Re(CO)3(deeb)B2]+ (as well as the precursor fac-Re(CO)3(deeb)Br and the free ligand B2) to stain proteins separated by SDS-PAGE. We found that only fac-[Re(CO)3(deeb)B2]+ proved viable to be directly used as a luminescent dye for proteins, presumably due to its interaction with negatively charged residues in proteins and by weak interactions provided by B2. In addition, fac-[Re(CO)3(deeb)B2]+ seems to interact preferentially with proteins and not with the gel matrix despite the presence of sodium dodecyl sulfate (SDS). In future applications, these alternative cationic complexes might be used alone or in combination with more traditional anionic compounds to generate counterion dye stains to improve the process.