Vibrational characterization and electronic properties of long range-ordered, ideally hydrogen-terminated Si(111)

The use of a well defined, long range ordered surface is of fundamental interest for the determination of surface-specific intrinsic physical parameters. Ideally hydrogen terminated Si(111) surfaces are prepared by wet chemical treatment in basic HF solutions. The mechanism of formation is based on preferential etching of defects, leading to an ideal hydrogen terminaison of the (111) plane, without any reconstruction and with a high degree of perfection. Infrared spectroscopy is used to probe the quality of the surfaces by quantifying the extent of the perfect domains.

High resolution photoemission spectroscopy of such highly homogeneous surfaces shows exceptional narrow features in both the valence band and the core level regions. The valence band levels and their dispersion are well described by first-principles calculations using a quasi particle self-energy approach within the Heidin's GW approximation.

Two surfaces core level states are evidenced, arising from the silicon surface atoms and the backbonds. A crystal field effect splits the surface Si 2_p3/2 component. Their position and relative intensity find a satisfactory agreement with recent calculations using first principles perturbation theory.     

Processing of semicrystalline polymers by high-stress extrusion

A study has been conducted on the solid-state extrusion of three semicrystalline polymers:poly-propylene (PP), poly(vinylidene fluoride) (PVDF), and high-density polyethylene (HDPE). HDPE has been extruded in continuous lengths with area reductions up to 25× at temperatures substantially below the melting region. Such extrusion has been identified as a solid-state process, since measurements of the temperature of the polymer during extrusion indicate the absence of significant heating due to deformation. In contrast, continuous lengths of PP and PVDF could not be obtained substantially below their melting temperatures, indicating that crystallization during extrusion is an important process for these polymers. Under severe extrusion conditions (low temperatures, high area reductions. etc.), all three polymers failed within the tapered region of the extrusion die. Two modes of failure have been identified, brittle fracture and, surprisingly, necking. Grid-line distortion patterns and a highly simplified upper-bound plasticity analysis both indicate that shear deformations are a major factor during high-stress extrusion.     

Nucleation kinetics of sodium disilicate

The kinetics of crystal nucleation in Na₂O · 2SiO₂ have been determined over the range of undercoolings between 173 and 373°C. The plot of log(I_v?) versus $\text{1}\text{ΔT}{}^2{}_{\mathrm{r}}\text{T}{}^3{}_{\mathrm{r}}$ is a straight line of negative slope over some 13 orders of magnitude in I_v. The slope of this relation indicates a nucleation barrier of about 45 kT at ΔT_r = 0.2, and the intercept at $\text{1}\text{ΔT}{}^2{}_{\mathrm{r}}\text{T}{}^3{}_{\mathrm{r}}\text{= 0}$ is 10²⁶ cm^-3 sec^-1. poise. The results are in good agreement with predictions of the theory of homogeneous nucleation, even in the pre-exponential factor.     

Role of humidity on indium and tin migration in organic photovoltaic devices

The stability of a common interface used in organic photovoltaic cells, between the transparent electrode of Indium Tin Oxide (ITO) and a buffer layer of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is strongly influenced by the presence of humidity during processing, leading to significant migration of indium and tin species into the PEDOT:PSS layer. The interface was studied using neutral impact collision ion scattering spectroscopy (NICISS) and X-ray photoelectron spectroscopy (XPS), to determine migration of indium and tin into the polymer layer. It was found that the migration starts almost instantly after spin coating of the aqueous PEDOT:PSS solution and it reaches a saturation level within twenty four hours. The indium and tin were found always uniformly distributed over the sampling depth of almost one-third of the thickness of the PEDOT:PSS layer. Exposure to humidity following annealing resulted in the highest concentration (1.8 × 10(-3) mol cm(-3)) of indium or tin species, corresponding to about one indium or tin moiety per 4.7 monomer units in the PEDOT:PSS. The maximum bulk concentration of indium is about two orders of magnitude higher after exposure to humid conditions compared to vacuum dried conditions. XPS measurements confirm the presence of both indium and tin in the PEDOT:PSS and the formation of salts with the metal ions as cations.     

Solvent‐Dependent Truxene‐Based Nanostructures

Three truxene derivatives functionalized with alkyl chains, either attached directly or distanced by linking phenyl or ethynyl groups, self‐assemble in solution and induce the gelation of different solvents in spite of not being endowed with groups able to establish strong directional interactions. A ¹H NMR study points to face‐to‐face alternating π‐stacked motifs at the origin of nucleation. Solvents play an important role in modulating the aggregation of these derivatives giving rise to fibrous or spherical superstructures. Analysis of the influence of different solvents on the morphology of the aggregates provides a better understanding of the various stages of the hierarchical self‐assembly. The way in which alkyl chains are attached to the central core also strongly affects the self‐assembling properties and gelation ability of this series. Phenyl spacers present the highest association constants in solution and give rise to gelation in a broader range of solvents. This behavior has been rationalized by means of ¹H NMR spectroscopy, X‐ray powder diffraction, SEM, and photophysical measurements. Interestingly, it was found that these compounds in the gel state exhibit unusual emission properties most likely arising from the formation of excimers, which evidences that π–π interactions also occur in the excited state.     

Microwave-assisted synthesis of naphthalenemonoimides and N-desymmetrized naphthalenediimides

Naphthalenemonoimides and N-desymmetrized naphthalenediimides were synthesized using a stepwise microwave-assisted protocol. The steric and electronic properties of aliphatic amines determined the outcome of the reactions, while in the amino acid series their ability to solubilize the naphthalene dianhydride starting material was crucial. Molecular modeling was used to rationalize the observed selectivity.     

Zeta functions that hear the shape of a Riemann surface

To a compact hyperbolic Riemann surface, we associate a finitely summable spectral triple whose underlying topological space is the limit set of a corresponding Schottky group, and whose “Riemannian” aspect (Hilbert space and Dirac operator) encode the boundary action through its Patterson–Sullivan measure. We prove that the ergodic rigidity theorem for this boundary action implies that the zeta functions of the spectral triple suffice to characterize the (anti-)complex isomorphism class of the corresponding Riemann surface. Thus, you can hear the complex analytic shape of a Riemann surface, by listening to a suitable spectral triple.