Electrodeposition of nanostructured Pt films from lyotropic liquid crystalline phases on α-Al2O3 supported dense Pd membranes

Using the lyotropic liquid crystalline templating strategy, the nanostructured platinum film was electrochemically deposited on the α-Al₂O₃ supported dense palladium membrane. The XRD and TEM results of the Pt film revealed a hexagonal array of cylindrical pores with a uniform pore diameter of ca. 3.8 nm and a pore-to-pore separation of ca. 7.6 nm. The structure parameters of the Pt film were almost the same as those of the hexagonal liquid crystalline template. Based on SEM observations, the Pt film was featureless, smooth, and tightly adherent to the dense Pd membrane. The specific surface area of the Pt film, measured by using cyclic voltammetry, was ca. 13.8 m² g⁻¹, which was in accord with the theoretical value of 14.5 m² g⁻¹ for a perfect hexagonal nanostructure with the same structure parameters. By combining the dense Pd membrane for selective permeation to hydrogen with the Pt film of high specific surface area for catalysis, the as-synthesized two-layer film will be a promising catalytic membrane to intensify hydrogen-related reaction processes.     

Coarsening process in one-dimensional surface growth models

Surface growth models may give rise to instabilities with mound formation whose typical linear size L increases with time (coarsening process). In one dimensional systems coarsening is generally driven by an attractive interaction between domain walls or kinks. This picture applies to growth models for which the largest surface slope remains constant in time (corresponding to model B of dynamics): coarsening is known to be logarithmic in the absence of noise ( L(t) ∼ ln t) and to follow a power law ( L(t) ∼t ^1/3) when noise is present. If the surface slope increases indefinitely, the deterministic equation looks like a modified Cahn-Hilliard equation: here we study the late stages of coarsening through a linear stability analysis of the stationary periodic configurations and through a direct numerical integration. Analytical and numerical results agree with regard to the conclusion that steepening of mounds makes deterministic coarsening faster : if α is the exponent describing the steepening of the maximal slope M of mounds ( M ^α∼L) we find that L(t) ∼t ⁿ: n is equal to for 1≤α≤2 and it decreases from to for α≥2, according to n = α/(5α - 2). On the other side, the numerical solution of the corresponding stochastic equation clearly shows that in the presence of shot noise steepening of mounds makes coarsening slower than in model B: L(t) ∼t ^1/4, irrespectively of α. Finally, the presence of a symmetry breaking term is shown not to modify the coarsening law of model α = 1, both in the absence and in the presence of noise. Received 28 September 2001 and Received in final form 21 November 2001     

Folding of lattice protein chains with modified Gō potential

We propose a modified Gō model in which the pairwise interaction energies vary as local environment changes. The stability difference between the surface and the core is also well considered in this model. Thermodynamic and kinetic studies suggest that this model has improved folding cooperativity and foldability in contrast with the Gō model. The free energy landscape of this model has broad barriers and narrow denatured states, which is consistent with that of the two-state folding proteins and is lacked for the Gō model. The role of non-native interactions in protein folding is also studied. We find that appropriate consideration of the contribution of the non-native interactions may increase the folding rate around the transition temperature. Our results show that conformation-dependent interaction between the residues is a realistic representation of potential functions in protein folding. Received 10 April 2002 / Received in final form 20 August 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: wangwei@nju.edu.cn     

Overlap distribution in random and designed heteropolymers

We study the overlap between low-energy states in lattice models of heteropolymers with contact interactions. The overlap distribution gives information on the degree of correlation in the energy landscape. Designed sequences have rather correlated energy landscapes, which favor fast folding kinetics. Chains with random interactions have much less correlated energy landscapes. It is indeed believed that the mean-field theory for this model coincides with the Random Energy Model, whose different low-energy states are completely unrelated. This picture has been supported by numerical studies of maximally compact configurations. Without applying this constraint, we find that the overlap distribution is indeed bimodal as expected, but it has a broad peak at large overlap, indicating a non-vanishing width for the valleys of low-energy states. This feature probably plays an important role in the kinetics of the model. It is not evident that the range of such correlations shrinks to zero for large systems. The range of the correlations seems to be influenced by the number of contacts per residue in the ground state: the smaller this quantity, the larger the correlations. Received 16 August 2000     

Surface partition of ion energy during the growth of TiNx thin films

A simple evaluation of ion-deposited energy during surface displacement of adatoms has been presented for physical vapor deposition technology using an appropriate interaction model. The rf reactive magnetron sputtering deposition of titanium nitride (TiN_x) thin films was taken as evidence supporting the theoretical calculation. The evolution of crystallite morphology dependent on bias (or input power) illustrates that surface and subsurface microstructure of growing films can be optimized by increasing the mobility of adatoms through ion-assistance.     

Tautomery and H-bonding characteristics of 2-aminopurine: a combined experimental and theoretical study

An experimental and theoretical RHF, MP2 and DFT/6-31++G^** study is described of the matrix FT-IR spectra of monomer 2-aminopurine and H-bonded complexes of 2-aminopurine with water. 2-aminopurine occurs in Ar predominantly as the amino-N9H tautomer, but small amounts of the amino-N7H tautomer are also present. An approximate K_T value for this tautomeric equilibrium is found to be 0.016 (RHF) and 0.015 (DFT) using the infrared intensity measurement. Four H-bonded complexes of the abundant amino-N9H form with water are detected in the experimental FT-IR spectrum by their characteristic predicted absorptions, i.e. the three closed complexes N3 ^... H-O ^... H-N9, N1 ^... H-O ^... H-NH, N3 ^... H-O ^... H-NH and the open complex N7 ^... H-OH. From the experimental results, the proton affinity of the N7 atom in 2-aminopurine can be estimated. The dependence of the H-bond strength on the H-bond linearity is demonstrated by a correlation between the N ^... H distance and the N ^... H-O angle in closed N ^... H-O ^... H-N complexes. Received 10 December 2001 Published online 13 September 2002     

A class of stable spectral methods for the Cahn–Hilliard equation

In this work, the initial-boundary value problem of two-dimensional Cahn–Hilliard equation is considered. A class of fully discrete dissipative Fourier spectral schemes are proposed. Moreover, semi-implicit prediction–correction schemes are presented. The numerical simulations are performed to demonstrate the effectiveness of the proposed schemes.     

On the observation of a huge lattice contraction and crystal habit modifications in LiMn2O4 prepared by a fuel assisted solution combustion

Two batches of poly-crystalline lithium manganate were prepared by a fuel assisted solution combustion method. LiMn₂O₄(S) was prepared using starch as the fuel and LiMn₂O₄(P) was prepared using poly vinyl alcohol (PVA) as the fuel. XRD studies indicated a significant and consistent shift in the 2θ values of all the hkl peaks to higher values in LiMn₂O₄(P) compared to LiMn₂O₄(S) indicating a lattice contraction in the former. TG/DTA studies indicated a higher formation temperature (∼25 °C higher) for LiMn₂O₄(P). The higher formation temperature most likely promotes the oxidation of some Mn³⁺ to Mn⁴⁺ with a lower ionic radius causing a lattice contraction. This hypothesis is confirmed through XPS studies which indicated the presence of a higher fraction of Mn⁴⁺ in LiMn₂O₄(P) than that present in LiMn₂O₄(S). A crystal shape algorithm was used to generate the crystal habits of lithium manganate from their XRD data leading to an understanding on the exposed hkl planes in these materials. From the atomic arrangement on the exposed hkl planes it is predicted that LiMn₂O₄(P) would be less prone to manganese dissolution and hence would possess a higher cycle life when compared to LiMn₂O₄(S).     

Dielectric tunability and conduction mechanisms of?nanostructured (Pb1?xSrx)TiO3 thin films

The effect of nanosize grains to enhance dielectric tunability in chemically prepared (Pb_1−x Sr _x )TiO₃ (PST) (x=0.1 to 0.5) thin films has been observed. The grain size is evaluated from X-ray diffraction patterns and atomic force microscopy. The average grain size lies in the range of 80–23 nm with varying Sr content. The nanosize grains in the PST films control the dielectric behavior up to the higher frequency region and exhibit large tunability with low loss factor at room temperature. The current–voltage characteristics show a large tunability as electron transport takes place within a highly resistive Fermi gap.     

Thermally activated dewetting of organic thin films: the case of pentacene on SiO<Subscript>2</Subscript> and gold

The morphology of pentacene organic thin films deposited on SiO₂ and Au(111) surfaces using organic molecular beam deposition (OMBD) has been characterized by a multi-technique approach. Among the techniques applied were X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM) and thermal desorption spectroscopy (TDS). Our rather detailed studies reveal that on both substrates the growth is strongly influenced by dewetting and islanding phenomena, yielding very rough surfaces. Surprisingly, substantial changes in the morphology were observed also after deposition on room-temperature samples on a time scale of several hours. The rather extensive set of in situ XPS data was analyzed in the framework of a simple model, which allows us to derive rather detailed information on the roughness parameters.