Dipole Force Research Articles

D-A-π-A based organic dyes for efficient DSSCs: A theoretical study on the role of π-spacer

A series of novel D-A-π-A organic dyes with different π-spacers for dye-sensitized solar cells (DSSCs) are designed based on hetero-tri-arylamine donor-based dye DP2. The key parameters of all the dyes affecting short-circuit current (JSC) and open-circuit voltage (VOC) are theoretically investigated in detail using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to reveal structure-property relationships. The theoretical results indicate that extending π-spacer is beneficial to improve light-absorbing capacities with larger responsive range of spectra and molar extinction coefficients (ε). The designed dyes DP2-1–DP2-5 exhibit a good balance in various important properties including light harvesting efficiency (LHE), electronic injection driving force (ΔGinject), regeneration driving force (ΔGreg), reorganization energy (λtotal) and vertical dipole moment (μnormal) and number of photoinjected electrons (nc) compared to reference dye DP2. Among all the designed dyes, DP2-3–DP2-5 containing π-spacers with branched chain exhibit obvious red shifted absorption band, smaller regeneration driving force (ΔGreg) and larger excited state lifetime (τ0). This study is expected to provide new clues for the experimental synthesis of highly efficient metal-free organic dyes for DSSCs.

Coupled Self-Organized Hydrodynamics and Stokes Models for Suspensions of Active Particles

We derive macroscopic dynamics for self-propelled particles in a fluid. The starting point is a coupled Vicsek–Stokes system. The Vicsek model describes self-propelled agents interacting through alignment. It provides a phenomenological description of hydrodynamic interactions between agents at high density. Stokes equations describe a low Reynolds number fluid. These two dynamics are coupled by the interaction between the agents and the fluid. The fluid contributes to rotating the particles through Jeffery’s equation. Particle self-propulsion induces a force dipole on the fluid. After coarse-graining we obtain a coupled Self-Organised Hydrodynamics–Stokes system. We perform a linear stability analysis for this system which shows that both pullers and pushers have unstable modes. We conclude by providing extensions of the Vicsek–Stokes model including short-distance repulsion, finite particle inertia and finite Reynolds number fluid regime.

Surface energy and surface stress on vicinals Cu(1 1 n)

The step energy and the step stresses are calculated as a function of the distance L between steps for vicinals Cu(1 1 n). In addition to the well-known Marchenko–Parshin model where the steps behave as dipole forces, we show that the steps are displaced in the direction parallel to the terrace with respect to the dipolar displacements. This results from the application at the step of a monopole force, Fb, whose modulus decreases as 1/L. The extra displacement due to Fb does not modify the step energy with respect to the MP model but is linked to the presence of the interaction term in the step stresses, that vary as 1/L. Because the step stress is calculated with respect to the nominal surface stress, we calculate the diagonal surface stresses in both the vicinal system (x, y, z) where z is normal to the vicinal and the projected system (x, b, c) where c is normal to the nominal terrace. Moreover, we calculate the surface stresses by using two methods: the first called the ‘Force’ method, from the surface pressure forces and the second called the ‘ΔE’ method, by homogeneously deforming the vicinal in the parallel direction, x or y, and by calculating the surface energy excess proportional to the applied strain. We confirm that the variation of the step stress in the tensor direction ‘xx’ is the same between the two methods while it is different in the direction ‘yy’. In the ‘Force’ method, the step stress in the direction ‘yy’ is the sum of the two step stresses in the directions ‘bb’ and ‘cc’. In the ‘ΔE’ method, the step stress in the direction ‘yy’ equals this calculated in the direction ‘bb’ (parallel to the terrace) in the ‘Force’ method, this in the normal direction ‘cc’ being excluded.

Atom-subjected optical dipole force exerted by femtosecond laser field

<sec> In 2011, Kumar et al. (2011 <i>Phys. Rev. A</i> <b>84</b> 043402) studied the light force acting on a beam of neutral two-level atoms superimposed on a few-cycle-pulse Gaussian laser field under both resonant and off-resonant conditions by solving the optical Bloch equation beyond the rotating-wave approximation, and they found that under resonant condition the transverse component of the light force shows oscillatory behavior but vanishes when a time average is taken, and the time averaged longitudinal force is nonzero only when the Rabi frequency is smaller than the resonant frequency and vanishes when the Rabi frequency is equal to or larger than the resonant frequency.</sec><sec> In this paper, we investigate further the strong nonlinear optical interaction between a two-level atomic system and a femtosecond Gaussian laser pulse by solving numerically the full-wave optical Bloch equations through using the predictor-corrector method. It is found that the light forces and the light potentials are sensitive to the value of the Rabi frequency and the detuning of the laser field. Under the resonant condition, the instant light forces induced by the femtosecond laser pulse change their signs as a function of time. The instant longitudinal light force changes its sign at twice the Rabi frequency, while the instant transverse light force changes its sign at twice the light carrier-wave frequency. However, none of the time-averaged light forces is zero, showing periodical oscillation characters as a function of Rabi frequency. Both of the time-averaged longitudinal and transverse light forces oscillate at the Rabi frequency corresponding to the pulse area of 2<inline-formula><tex-math id="Z-20190110084840-1">\begin{document}${\text{π}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20182016-e_Z-20190110084840-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20182016-e_Z-20190110084840-1.png"/></alternatives></inline-formula>. The time-averaged transverse light force shows also a trend of enhancement with Rabi frequency increasing, and the time-averaged longitudinal light force shows also a saturation trend with the increase of the Rabi frequency. The optical potential depends strongly on the detuning. It changes gradually from repulsive potential to attractive potential when the detuning defined here changes from negative to positive detuning. When the field is nearly resonant, the optical potential then oscillates between repulsive and attractive potentials. Therefore, neutral atoms can be focused, defocused, trapped, splitted or steered by the femtosecond laser field with appropriate detuning and Rabi frequency. </sec>

Polarizability effect in strong-field ionization: Quenching of the low-energy structure in C60

The low-energy photoelectron spectra from strong-field ionization of ${\mathrm{C}}_{60}$ fullerenes and noble gases (xenon and krypton) with $3\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ laser pulses are measured and compared. It is found that the low-energy structure (LES), a universal spikelike feature in the strong-field limit of atoms and small molecules, is significantly suppressed in the ${\mathrm{C}}_{60}$ photoelectron distribution. We propose that the large polarizability of the ${\mathrm{C}}_{60}$ core disrupts the corresponding electron trajectories. In particular, the induced dipole force repels the electron, which opposes the focusing and bunching due to the Coulomb potential that is responsible for the LES, thus leading to its reduction.

Laser cooling and magneto-optical trapping of molecules analyzed using optical Bloch equations and the Fokker-Planck-Kramers equation

We study theoretically the behavior of laser-cooled calcium monofluoride (CaF) molecules in an optical molasses and magneto-optical trap (MOT), and compare our results to recent experiments. We use multi-level optical Bloch equations to estimate the force and the diffusion constant, followed by a Fokker-Planck-Kramers equation to calculate the time-evolution of the velocity distribution. The calculations are done in three-dimensions, and we include all the relevant energy levels of the molecule and all the relevant frequency components of the light. Similar to simpler model systems, the velocity-dependent force curve exhibits Doppler and polarization-gradient forces of opposite signs. We show that the temperature of the MOT is governed mainly by the balance of these two forces. Our calculated MOT temperatures and photon scattering rates are in broad agreement with those measured experimentally over a wide range of parameters. In a blue-detuned molasses, the temperature is determined by the balance of polarization gradient cooling, and heating due to momentum diffusion, with no significant contribution from Doppler heating. In the molasses, we calculate a damping rate similar to the measured one, and steady-state temperatures that have the same dependence on laser intensity and applied magnetic field as measured experimentally, but are consistently a few times smaller than measured. We attribute the higher temperatures in the experiments to fluctuations of the dipole force which are not captured by our model. We show that the photon scattering rate is strongly influenced by the presence of dark states in the system, but that the scattering rate does not go to zero even for stationary molecules because of the transient nature of the dark states.

Fabrication of a patterned Pt counter electrode for dye-sensitized solar cells using neutralized H2PtCl6·6H2O paste

Patterned platinum counter electrodes for dye-sensitized solar cells (DSSCs) were formed using neutralized platinum precursor pastes and a screen-printing method. Hydrogen hexachloroplatinate (IV) hexahydrate was used as a platinum precursor. Hexylene glycol was used as a solvent for the precursor and lithium hydroxide dissolved in ethylene glycol monomethyl ether was used as a neutralizing agent to prepare neutralized pastes. The results suggest that approximation of the energy from dipolar intermolecular forces between molecules, as represented by one of the Hansen solubility parameters, is an important factor for avoiding liquid–liquid phase separation in this case. The surface of the platinum layers exhibited a two-dimensional structure of fine particles (5–10 nm) with a ∼99.5% coverage rate. Consequently, the photoelectric conversion performance of DSSCs fabricated using the printed platinum counter electrodes was nearly equivalent to that fabricated using a sputtered platinum counter electrode, which is often used as a reference electrode.

Modelling and particle based simulation of electro-coalescence of a water-in-oil emulsion

Electrocoalescence is an industrially relevant process, especially in oil exploration and petroleum refining industries, and is the working principle of the process equipment, an electrocoalescer. In this work, electrocoalescence is modelled using dipolar electrostatic forces which bring the droplets together but are resisted by Stokesian drag forces. We demonstrate that hydrodynamic interactions are important to model electrocoalescence. Similarly, a multi box methodology is necessary for getting improved statistics for drop size distribution, since any coagulation/coalescence process, typically results in the loss of droplets. Moreover, the process of chaining can be modelled by assuming a “wait-time” on contact of the interacting drops, and n-mer formation can be predicted. Our results show a reasonable agreement between model predictions and experimental data. The simulations also show that an increase in volume fraction, average droplet size, and polydispersity enhance the dynamics of the coalescence process.

Roton in a few-body dipolar system

We solve numerically exactly the many-body 1D model of bosons interacting via short-range and dipolar forces and moving in the box with periodic boundary conditions. We show that the lowest energy states with fixed total momentum can be smoothly transformed from the typical states of collective character to states resembling single particle excitations. In particular, we identify the celebrated roton state. The smooth transition is realized by simultaneous tuning short-range interactions and adjusting a trap geometry. With our methods we study the weakly interacting regime as well as the regime beyond the range of validity of the Bogoliubov approximation.

The role of NaOH and papaya latex bio-activator during production of carbon nanoparticle from rice husks

This research explains the role of NaOH and bio-activator from papaya latex in the synthesis of carbon nanoparticles from rice husks in a water medium to produce fullerene-like polytype of SiC. The process begins with the pyrolysis and then the high energy milling (HEM) process followed by dispersion of NaOH and coagulation of bio-activator. The HEM process creates residual stress which produces nanocrack. NaOH dissolved into ions which activate dipole force to create dipole moment on the tip of nanocrack and become the trigger of nanocrack dispersion. The crack dispersion breaks carbon into nanoparticles having polar electrical charge so that they are dispersed homogeneously inside the water. Bio-activator then continues to perform catalytic building molecular chain as a bridge connecting pole sides of carbon nanoparticles with the bonds of van der Walls. Catalytic performance causes the coagulation process of a nanoparticle to occur which resembles the morphological structure of fullerene-like polytype of SiC with the size of 20 up to 100 nm.

Control of multiferroic order by magnetic field in frustrated helimagnet MnI2. Theory

We provide a theoretical description of frustrated multiferroic $\rm MnI_2$ with a spiral magnetic ordering in magnetic field $\bf h$. We demonstrate that subtle interplay of exchange coupling, dipolar forces, hexagonal anisotropy, and the Zeeman energy account for the main experimental findings observed recently in this material (Kurumaji, et al., Phys.\ Rev.\ Lett.\ {\bf 106}, 167206 (2011)). We describe qualitatively the non-trivial evolution of electric polarization $\bf P$ upon $\bf h$ rotation, changing $\bf P$ direction upon $h$ increasing, and disappearance of ferroelectricity at $h>h_c$, where $h_c$ is smaller than the saturation field.

Spiral plane flops in frustrated helimagnets in external magnetic field

We discuss theoretically frustrated Heisenberg spiral magnets in magnetic field $\bf H$. We demonstrate that small anisotropic spin interactions (single-ion biaxial anisotropy or dipolar forces) select the plane in which spins rotate (spiral plane) and can lead to the spiral plane flop upon in-plane field increasing. Expressions for the critical fields $H_{flop}$ are derived. It is shown that measuring of $H_{flop}$ is an efficient and simple method of quantifying the anisotropy in the system (as the measurement of spin-flop fields in collinear magnets with axial anisotropy). Corresponding recent experiments are considered in spiral magnets some of which are multiferroics of spin origin.

Entangling power of holonomic gates in atom-based systems

Entanglement is one of the main resources of quantum computation, and entangling power of a quantum system is a crucial element in the universality and efficiency of a proposed architecture for realization of quantum processing. Our goal here is to study the entangling power of holonomic gates in some particular systems. We explore the holonomy-induced entanglement, by means of nonadiabatic quantum holonomies, through different types of interactions in atom-based systems, namely, the tripod-type interaction induced by the quantum Zeno effect between three-level atoms, as well as the Λ-type interaction arising from dipole–dipole or van der Waals forces between high-lying states of two-level atoms in systems consisting of N optically trapped identical atoms. Our analysis shows that although the two schemes provide completely separate classes of entangling gates, both schemes permit for full entangling power and also in the sense of quantum efficiency both families of entanglers consist of holonomic gates that have the same efficiency in quantum algorithms. Besides, we observe that holonomy-induced entanglement characteristics remarkably depend on the interaction configuration of the system.

Regulating spin reversal in dipolar systems by the quadratic Zeeman effect

A mechanism is advanced suggesting the resolution of the dichotomy of long-lived spin polarization storage versus fast spin reversal at the required time. A system of atoms or molecules is considered interacting through magnetic dipolar forces. The constituents are assumed to possess internal structure allowing for the generation of the alternating-current quadratic Zeeman effect, whose characteristics can be efficiently regulated by quasiresonant dressing. The sample is connected to an electric circuit producing a feedback field acting on spins. By switching on and off the alternating-current quadratic Zeeman effect it is possible to realize spin reversals with a required delay time. The suggested technique of regulated spin reversal can be used in quantum information processing and spintronics.

Extensile motor activity drives coherent motions in a model of interphase chromatin

The 3D spatiotemporal organization of the human genome inside the cell nucleus remains a major open question in cellular biology. In the time between two cell divisions, chromatin-the functional form of DNA in cells-fills the nucleus in its uncondensed polymeric form. Recent in vivo imaging experiments reveal that the chromatin moves coherently, having displacements with long-ranged correlations on the scale of micrometers and lasting for seconds. To elucidate the mechanism(s) behind these motions, we develop a coarse-grained active polymer model where chromatin is represented as a confined flexible chain acted upon by molecular motors that drive fluid flows by exerting dipolar forces on the system. Numerical simulations of this model account for steric and hydrodynamic interactions as well as internal chain mechanics. These demonstrate that coherent motions emerge in systems involving extensile dipoles and are accompanied by large-scale chain reconfigurations and nematic ordering. Comparisons with experiments show good qualitative agreement and support the hypothesis that self-organizing long-ranged hydrodynamic couplings between chromatin-associated active motor proteins are responsible for the observed coherent dynamics.

Substructure imaging of heterogeneous nanomaterials with enhanced refractive index contrast by using a functionalized tip in photoinduced force microscopy

ABSTRACTThe opto-mechanical force response from light-illuminated nanoscale materials has been exploited in many tip-based imaging applications to characterize various heterogeneous nanostructures. Such a force can have two origins: thermal expansion and induced dipoles. The thermal expansion reflects the absorption of the material, which enables one to chemically characterize a material at the absorption resonance. The induced dipole interaction reflects the local refractive indices of the material underneath the tip, which is useful to characterize a material in the spectral region where no absorption resonance occurs, as in the infrared (IR)-inactive region. Unfortunately, the dipole force is relatively small, and the contrast is rarely discernible for most organic materials and biomaterials, which only show a small difference in refractive indices for their components. In this letter, we demonstrate that refractive index contrast can be greatly enhanced with the assistance of a functionalized tip. With the enhanced contrast, we can visualize the substructure of heterogeneous biomaterials, such as a polyacrylonitrile-nanocrystalline cellulose (PAN-NCC) nanofiber. From substructural visualization, we address the issue of the tensile strength of PAN-NCC fibers fabricated by several different mixing methods. Our understanding from the present study will open up a new opportunity to provide enhanced sensitivity for substructure mapping of nanobiomaterials, as well as local field mapping of photonic devices, such as surface polaritons on semiconductors, metals and van der Waals materials.

Localized optical micromotors based on twin laser cavity solitons

It has recently been shown that vertical-cavity surface-emitting lasers with an intracavity saturable absorber are capable of forming binary localized structures called twin laser cavity solitons (LCSs). Apart from their asymmetric intensity distribution, they can spontaneously rotate about their center of mass with a frequency that is controllable via a bifurcation parameter representing the ratio of carrier lifetimes in the amplifier and in the absorber. Moreover, circles of maximum phase are found to form around the center of the binary structure which start rotation once the twin LCSs begin to do so. In this paper, we propose to use the fact that these phase circles continually shrink on the center of mass to trap microparticles at the center and subsequently rotate them through the dipole force provided by the asymmetric intensity distribution. This finding can be regarded as a first step in the realization of a localized microdimensional optical motor potentially useful in the manipulation of microparticles and nanofluids.

Multicomponent Solvent-Free Synthesis, Antibacterial Evaluation and QSAR Study of 2-(Bis(benzylthio)methylene) malononitriles

Multicomponent reaction of malononitrile, carbon disulphide and various benzyl halides was developed as an efficient strategy for the synthesis of 2-(bis(benzylthio)methylene)malononitrile derivatives via two different procedures: (a) in the presence of K2CO3 as a base in acetonitrile and (b) under solvent-free conditions in the presence of triethylamine. Higher yields with shorter reaction times were obtained from the latter procedure. Inhibitory activity of all derivatives was evaluated against 22 pathogenic bacteria including both Gram-negative and Gram-positive strains. Thioether 4b showed broad-spectrum antibacterial activities according to the antibiogram tests. DFT calculations (B3LYP/6-311++G**) were performed to determine the type of drug-receptor interactions. It was found that reversible dipole-dipole forces play a key role in most interactions.

An Improved Numerical Computation of Hull Pressure Fluctuations Due to Unsteady Sheet Cavitation of a Propeller

An operating propeller is the main source of vibration on the ship hull, especially on the stern. In this article, an improved numerical scheme is presented to predict the pressure fluctuations on the ship hull due to the unsteady sheet cavitation of a propeller. The calculated results are compared with the published results and experimental data carried out in the hydrodynamics and cavitation tunnel of Hamburgische Schiffbau-Versuchsanstalt to verify the improvement. The present method is based on a two-cycle iterating scheme which satisfies the boundary integral equation in time domain. The hull pressure fluctuations calculated by the first-cycle iterations are treated as the initial values for the second-cycle iterations. The solid angles of the elements will deviate from the standard value, .5, as the dramatic variation in geometry appears, and accumulate numerical errors in the calculating process. During the second-cycle iterations, a filter based on the solid angles on hull elements is proposed to minimize the iterating error. A container ship is treated as the computing sample in this study, and evidence is offered regarding the 16% improvement achieved by the present method. 1. Introduction A propeller operating in a nonuniform wake field in the ocean is one of the prime sources of vibration and noise affecting the ship. The prediction of hull pressure fluctuations caused by propellers is an important consideration for many vessels and worthy of being researched. Hull pressure fluctuation caused by the marine propeller has been widely investigated since the 1980s. Huse and Guoqiang (1982) developed a semi-empirical prediction method, in which the cavitation volume on the propeller blade surface can be estimated using experimental data. Breslin et al. (1982) proposed a model based on the potential flow theory, which simulates the ship hull by means of a simple panel method and the propeller by means of a lifting surface vortex lattice method. Kinns and Bloor (2004) used an acoustic boundary element (BE) model to simplify the problem of hull vibration excitation due to propeller sources and dipoles. The convergence of results for a cruise liner model was demonstrated with different element distributions. Kehr and Kao (2004) derived the incident blade rate pressure induced by unsteady sheet cavitation (monopole) and unsteady forces (dipole) of an operating propeller, for calculating the hull pressure fluctuations. Brouwer (2005) applied a stationary set of rings of monopole and dipole sources in the frequency domain to solve the propeller-induced noise and vibrations. Lee et al. (2006) integrated computational fluid dynamics (CFD) and the finite difference method for the computation of propeller-induced hull vibration. It was recommended that the phase difference of the propeller-induced pressure should be considered for preventing overprediction. Kao and Kehr (2006) developed a time domain iteration method to calculate the hull pressure fluctuations induced by the operating propeller. The phase difference is included in the iterating scheme, and the computation is robustly convergent. Seol and Moon (2009) derived the governing equation of pressure fluctuation induced by sheet cavitation according to the Ffowcs Williams approach. In the study by van Wijngaarden (2011), a potential flow boundary element method (BEM) with measured hull pressure data as input was applied to solve the hull pressure fluctuations, and the model experiments were also carried out. It was concluded that the computed hull pressure fluctuations due to non-cavitating propellers is reasonably accurate compared with the experimental data. Kehr and Kao (2011) calculated the pressure fluctuations on ship hull due to propeller sheet cavitation by using the method presented in Kao and Kehr (2006). The numerical result is higher than the experimental data provided by the hydrodynamics and cavitation tunnel (HYKAT) of HSVA (Wiemer 1999) by approximately 27%. The pressure fluctuations on ship stern induced by cavitating propellers were solved by Kanemaru and Ando (2011) with a surface panel method. The maximum amplitude appears in the first blade frequency and overestimates the experimental data by about 30%. Kim et al. (2012) predicted the hull pressure fluctuations induced by marine propeller sheet cavitation, and the Doppler effect was considered at the same time. Wei and Wang (2013) employed CFD to simulate the propulsion of a submarine. The finite element method and BEM were then combined to solve the submarine's structure and acoustic responses under the propeller excitations. The matched-field inversion technique applied in Lee et al. (2014) uses the fluctuating hull pressure field measured by receivers and the acoustic field calculated by BEM to define the sheet cavitation noise, source strengths, source positions, and number of sources. The equivalent source model for the propeller can result in more accurately extrapolated hull pressure distribution. In Wei et al. (2016), the unsteady forces of a submarine propeller were predicted by CFD, and the hull pressure fluctuations due to the non-cavitation propeller were calculated by the method proposed in Kao and Kehr (2006).

Doubly dressed states for near-field trapping and subwavelength lattice structuring

We propose a scheme to tailor nanostructured trapping potentials for ultracold atoms. Our trapping scheme combines an engineered extension of repulsive optical dipole forces at short distances and attractive Casimir-Polder forces at long distances between an atom and a nanostructured surface. This extended dipole force takes advantage of excited-state dressing by plasmonically-enhanced intensity to doubly dress the ground state and create a strongly repulsive potential with spatially tunable characteristics. In this work, we show that, under realistic experimental conditions, this method can be used to trap Rubidium atoms close to surfaces (tens of nanometers) or to realize nanostructured lattices with subwavelength periods. The influence of the various losses and heating rate mechanism in such traps is characterized. As an example we present a near-field optical lattice with 100nm period and study the tunability of lattice and trapping depths. Such lattices can enhance energy scales with interesting perspectives for the simulation of strongly-correlated physics. Our method can be extended to other atomic species and to other near-field hybrid systems where a strong atom-light interaction can be expected.