Vibrational dynamics of the bending mode of water interacting with ions

We studied the vibrational relaxation dynamics of the bending mode (ν(2)) of the H(2)O water molecules in the presence of different salts (LiCl, LiBr, LiI, NaI, CsI, NaClO(4), and NaBF(4)). The linear and nonlinear spectra of the bending mode show distinct responses of water molecules hydrating the anions. We observe that the bending mode of water molecules that are hydrogen-bonded to an anion exhibits much slower relaxation rates (T(1)~1ps) than water molecules that are hydrogen-bonded to other water molecules (T(1)=400 fs). We find that the effect of the anion on the absorption spectrum and relaxation time constant of the water bending mode is not only determined by the strength of the hydrogen-bond interaction but also by the shape of the anion.     

Capillary Stokes drift: a new driving mechanism for mixing in AC-electrowetting

We studied the flow fields generated inside sessile drops that oscillate periodically between states of high and low contact angle under the influence of alternating electric fields of variable frequency and amplitude. Following the motion of dye patches, we show that the number of oscillation cycles required to achieve mixing scales logarithmically with the Péclet number as expected for chaotic mixing. High speed movies reveal an asymmetry of the drop shape between the spreading and receding phase of the oscillations. This results in net internal flow fields that we characterize by tracing the motion of colloidal seed particles. The strength and frequency dependence of the flow are explained in terms of Stokes drift driven by capillary waves that emanate from the oscillating contact line.     

Vibrational and orientational dynamics of water in aqueous hydroxide solutions

We report the vibrational and orientational dynamics of water molecules in isotopically diluted NaOH and NaOD solutions using polarization-resolved femtosecond vibrational spectroscopy and terahertz time-domain dielectric relaxation measurements. We observe a speed-up of the vibrational relaxation of the O-D stretching vibration of HDO molecules outside the first hydration shell of OH(-) from 1.7 ± 0.2 ps for neat water to 1.0 ± 0.2 ps for a solution of 5 M NaOH in HDO:H(2)O. For the O-H vibration of HDO molecules outside the first hydration shell of OD(-), we observe a similar speed-up from 750 ± 50 fs to 600 ± 50 fs for a solution of 6 M NaOD in HDO:D(2)O. The acceleration of the decay is assigned to fluctuations in the energy levels of the HDO molecules due to charge transfer events and charge fluctuations. The reorientation dynamics of water molecules outside the first hydration shell are observed to show the same time constant of 2.5 ± 0.2 ps as in bulk liquid water, indicating that there is no long range effect of the hydroxide ion on the hydrogen-bond structure of liquid water. The terahertz dielectric relaxation experiments show that the transfer of the hydroxide ion through liquid water involves the simultaneous motion of ~7 surrounding water molecules, considerably less than previously reported for the proton.     

Structure models for β-MnGa2S4 as derived from electron diffraction and high-resolution electron microscopy

A study by high-resolution electron microscopy and electron diffraction on the tetrahedrally coordinated compound β-MnGa₂S₄ is reported. The structure-imaging technique enabled a new structure model to be proposed for the accommodation of deviations from stoichiometry in compounds of the type $\text{x}\text{MnS}\text{y}\text{Ga}{}_2\text{S}{}_3$. The model is based on the Ga₂S₃ structure wherein by the substitution of MnS layers chemical twin planes are formed. Depending on the spacing of the periodic twin planes a homologous series of compounds is expected and a number of members of this series was identified mainly by electron diffraction. In the β-MnGa₂S₄ compound, two structure modifications are observed mainly differing in the orientation of the twin planes (coherent or incoherent). This conclusion is substantiated by structure image calculations for the interpretation of the high-resolution results.     

Hydrodynamic description of femtosecond transient thermoreflectivity

Femtosecond transient reflectivity curves are computed using hydrodynamic approximation for describing the behaviour of the material in the collisionless free-electron domain. Two cases, the dominant electron-impurity scatterings and the dominant electron-phonon relaxations are considered to describe the heat diffusion in non-equilibrated lattice and electronic subsystems.     

An efficient step size control for continuation methods

In solving a nonlinear equation by the use of a continuation method one of the crucial problems is the choice of the step sizes. We present a model for the total computational cost of a standard numerical continuation process and solve the problem of optimal step size control for this model. Using the theoretical results as a basis, we develop an adaptive step size algorithm for Newton's method. This procedure is computationally inexpensive and it gives quite satisfactory results compared to some other numerical experiments found in the literature.     

The (twin) composition plane as an extended defect and structure-building entity in crystals

The burden of this paper is the recognition of the composition plane as an extended (planar) “defect” in twinned crystals, together with the notion that new structure types may be generated from simple prototype structures by regular and frequent repetition of a twinning operation, usually on a very fine scale. Less regular application of the operation leads to disorder and variation in stoichiometry, without point defects being necessary.

The range of compounds whose structures may be described in these terms is very wide, from metal alloys through metal carbides and borides etc. and inorganic materials, such as metal oxides and sulphides, to minerals, including silicates and the “sulphosalts” of the heavy metals (Pb, Bi etc.). Isostructural compounds frequently span this whole gamut of substances.

The main reasons for the twinning appear to be () the generation of new types of interstices (not present in the prototype) of appropriate size for some of the constituent atoms, and/or (b) a variation in the concentration of appropriate interstices, i.e. in the stoichiometric ratio anion/cation.

After a general introduction to the relevant aspects of twinning, attention is successively concentrated on reflection-twinned h.c.p., then c.c.p. and finally mixed c.p. arrays: first with “interstitial” atoms only in the composition planes; and then with other interstices also occupied. In each case lamellar twinning is first considered, and then cyclic twinning. Glide-reflection-twinning of similar arrays is then considered.

Attention is also drawn to small topological distortions that, for example, convert the octahedra in a c.c.p. array (twinned or even un-twinned) to arrays of capped trigonal prisms (or to cubes). In this way the apparently complex structures of a number of compounds are derived from those of much simpler twinned c.c.p. arrays.

Finally, the not-very-large amount of available, relevant, experimental evidence is reviewed. As far as it goes, this confirms that the notion of the composition plane as an extended defect is well founded. But more experimental data are needed.

[In most cases specific literature references are provided for each structure. For a few, rather common, structures they are not. In such cases reference may be made to standard texts, e.g. for alloys, to Schubert⁹⁹ or Pearson¹³³ and, for more ionic structures, to Wyckoff¹³⁴ or Povarennykh¹³⁵. Diagrams are to scale: 1 cm = 4.0 Å.]     

Influence of a TiCl4 post-treatment on nanocrystalline TiO2 films in dye-sensitized solar cells

In this study, the influence of the TiCl(4) post-treatment on nanocrystalline TiO(2) films as electrodes in dye-sensitized solar cells is investigated and compared to nontreated films. As a result of this post-treatment cell efficiencies are improved, due to higher photocurrents. On a microscopic scale TiO(2) particle growth on the order of 1 nm is observed. Despite a corresponding decrease of BET surface area, more dye is adsorbed onto the oxide surface. Although it seems trivial to match this finding with the improved photocurrent, this performance improvement cannot be attributed to higher dye adsorption only. This follows from comparison between incident photon to current conversion efficiency (IPCE) and light absorption characteristics. Since the charge transport properties of the TiO(2) films are already more than sufficient without treatment, the increase in short circuit current density J(SC) cannot be related to improvements in charge transport either. Transient photocurrent measurements indicate a shift in the conduction band edge of the TiO(2) upon TiCl(4) treatment. It is concluded that the main contribution to enhanced current originates from this shift in conduction band edge, resulting in improved charge injection into the TiO(2).     

Measuring charge transport from transient photovoltage rise times. A new tool to investigate electron transport in nanoparticle films

Charge transport rate at open-circuit potential (V(oc)) is proposed as a new characterization method for dye-sensitized (DS) and other nanostructured solar cells. At V(oc), charge density is flat and measurable, which simplifies quantitative comparison of transport and charge density. Transport measured at V(oc) also allows meaningful comparison of charge transport rates between different treatments, temperatures, and types of cells. However, in typical DS cells, charge transport rates at V(oc) often cannot be measured by photocurrent transients or modulation techniques due to RC limitations and/or recombination losses. To circumvent this limitation, we show that charge transport at V(oc) can be determined directly from the transient photovoltage rise time using a simple, zero-free-parameter model. This method is not sensitive to RC limitation or recombination losses. In trap limited devices, such as DS cells, the comparison of transport rates between different devices or conditions is only valid when the Fermi level in the limiting conductor is at the same distance from the band edge. We show how to perform such comparisons, correcting for conduction band shifts using the density of states (DOS) distribution determined from the same photovoltage transients. Last we show that the relationship between measured transport rate and measured charge density is consistent with the trap limited transport model.