In-plane Correlation Research Articles

Effect of self-affine fractal characteristics of surfaces on wetting

In this letter, we show experimentally that the wetting property of a solid surface crucially depends on the surface morphological parameters such as; (1) root mean square (rms) roughness σ, (2) in-plane roughness correlation length ξ, and (3) roughness exponent α of the self-affine surface. We have shown that the contact angle monotonically decreases with the increase in the rms local surface slope ρ (∝σ/ξα). We have shown that the same solid surface can be made hydrophobic or hydrophilic by merely tuning these self-affine surface morphological parameters.

Kinetics of alloy formation at the interfaces in a Ni-Ti multilayer: X-ray and neutron reflectometry study

Vacuum-deposited Ni/Ti multilayers on annealing at different temperatures exhibit formation of ordered alloy layers at interfaces. We have studied in detail the formation of alloy at interfaces of a Ni/Ti multilayer on annealing at $300\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and $400\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ using powder x-ray diffraction, x-ray reflectometry, polarized neutron reflectometry, and off-specular x-ray reflectometry techniques. Correlation among structural, magnetic, and morphological properties in as-deposited Ni/Ti multilayer as well as in a sample annealed at $300\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and $400\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ has been studied. X-ray diffraction technique was used to obtain crystal structure of the sample. Specular x-ray reflectometry and neutron reflectometry were used to determine the growth of alloy layers from density profile, as a function of depth in the sample. From off-specular x-ray reflectivity we found that the in-plane correlation lengths at the interfaces grew with annealing. This is associated with alloy crystallite growth obtained from x-ray reflectometry. Detailed magnetic-moment density profile of as-deposited as well as annealed multilayer sample has been determined by polarized neutron reflectometry measurements, which was used to determine the magnetic nature of the alloy layers. The results show formation of nonmagnetic alloy layers at the interfaces on annealing. From the Bragg-peak intensities of x-ray reflectivity and polarized neutron reflectivity measurements, we have estimated the diffusion lengths after annealing at $300\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and $400\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$.

Structure of quasi-one-dimensional ribbon colloid suspensions

We report the results of an experimental study of a colloid fluid confined to a quasi-one-dimensional (q1D) ribbon channel as a function of channel width and colloid density. Our findings confirm the principal predictions of previous theoretical studies of such systems. These are (1) that the density distribution of the liquid transverse to the ribbon channel exhibits stratification; (2) that even at the highest density the order along the strata, as measured by the longitudinal pair correlation function, is characteristic of a liquid; and (3) the q1D pair correlation functions in different strata exhibit anisotropic behavior resembling that found in a Monte Carlo simulation for the in-plane pair correlation function of a hard sphere fluid in a planar slit.

Influence of polymer porogens on the porosity and mechanical properties of spin coated Ultra Low k dielectrics

Structural properties deduced from Fourier Transform Infrared spectroscopy, ellipsometric porosimetry (EP), X-ray reflectivity and grazing incidence small-angle X-ray scattering (GISAXS) are correlated with dielectric and mechanical properties of Ultra Low k layers prepared by spin coating. Different sacrificial polymers were used as porogens and different porogen loadings were tested. After crosslinking and porogen removal by thermal treatment, the GISAXS images indicate a fairly isotropic distribution, with a slight in-plane correlation of the pores. Dielectrics having the same dielectric constant close to 2.2, have almost the same pore volume fraction ~ 40%, but the azimuthally averaged GISAXS profiles display porogen-dependent shapes, leading to different sizes and inter-pore correlations. For a given porogen, the increase of the volume fraction from ~ 10% to ~ 50% causes k to decrease from 2.7 to 1.9, with a slight increase in the pore size. Thermal curing, supplemented or not by UV illumination, hardly changes the porous structure, as observed by EP and GISAXS. Finally, the influence of the pore size on the elastic properties appears to be weak in the range studied (2.5–10 nm), while changes in the mechanical properties are governed mainly by the pore volume fraction and skeleton cross-linking.

Gold Nanoparticles at the Liquid−Liquid Interface: X-ray Study and Monte Carlo Simulation

The behavior of mixed-ligand-coated gold nanoparticles at a liquid-liquid interface during compression has been investigated. The system was characterized by measuring pressure-area isotherms and by simultaneously performing in situ X-ray studies. Additionally, Monte Carlo (MC) simulations were carried out in order to interpret the experimental findings. With this dual approach it was possible to characterize and identify the different stages of compression and understand what happens microscopically: first, a compression purely in-plane, and, second, the formation of a second layer when the in-plane pressure pushes the particles out of the plane. The first stage is accompanied by the emergence of an in-plane correlation peak in the scattering signal and a strong increase of the pressure in the isotherm. The second stage is characterized by the weakening of the correlation peak and a slower increase in pressure.

Probing the Effect of Structural Roughness on Domain Wall Formation in Spin Valves Using the Offspecular Reflectivity Technique

A previous polarized neutron reflectivity study of IrMn-based spin valves demonstrated that domain wall formation during training is influenced by the thickness of the ferromagnetic pinned layer. However, magnetoresistance measurements and x-ray reflectivity of spin valves with the same nominal structure revealed that sample roughness may also affect the characteristics of the exchange bias, and thus the type of domain walls that develop. To probe the interplay between structural roughness and domain formation, we measured diffuse neutron scattering from a sample with a thick pinned layer (15 nm) and extracted the field evolution of the in-plane correlation length. At saturation, the correlation length was estimated to be 750 plusmn 50 nm and originates from magnetic roughness. The correlation length at intermediate fields decreases to 340 nm or less and signifies the formation of in-plane domains. These results suggest that structural roughness enhances the formation of perpendicular domain walls during training.

The Role of Preoxidation on the Interface Structure of Co/MgO Multilayers

It has previously been reported that preoxidation of the bottom electrode prior to deposition of the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> or MgO barrier in magnetic tunnel junctions acts to suppress orange-peel coupling between Co electrodes. Here, we present specular and diffuse X-ray scattering measurements from Co/MgO multilayer repeat structures which suggest that this drop arises from a reduction in the in-plane correlation length of the roughness at the electrode-barrier interface rather than reduction in roughness amplitude.

Raman and FTIR Spectroscopy Investigations of Carbon-Coated LixFePO4 Materials

Raman and Fourier transform infrared (FTIR) spectroscopy investigations were performed on carbon-coated materials differing by the temperature of their thermal treatments (575 and ) and by their electrochemical performance, with that obtained at a higher temperature showing larger reversible capacity and better capacity retention at high rates. Raman spectra gave information on the carbon located at the surface of the particles, which was shown for the two samples to be highly disordered with small in-plane correlation lengths . A UV Raman study has shown that these carbon coatings contain almost no -type carbon hybridization. This study has also highlighted again that the -type -type C ratio cannot be determined straightforwardly from Raman spectra recorded with visible excitation (such as ), and thus that no direct correlation can be done between the Raman band intensity ratio and the -type -type C ratio; a UV Raman study is necessary to get the true information on the -type C contribution. The baseline and absolute intensity of the FTIR spectra were shown to be sensitive to changes in the electronic conductivity of the samples. Furthermore, good crystallinity was maintained for materials upon cycling, showing good reversibility of the lithium deintercalation∕intercalation reaction.

Raman spectroscopy studies of carbide derived carbons

The Raman spectra of a number of carbide derived carbons (CDCs) synthesised from TiC at 700, 800, 900, 1000, 1100 and 1200 °C and from VC, WC, TaC, NbC, HfC and ZrC made at 1000 °C have been recorded using laser excitation wavelengths of 514 and 785 nm. The spectra show two main features, the D- and G-peaks situated around 1350 cm −1 and 1600 cm −1, respectively. The peak positions, their intensities ( I D/ I G) and full width at half maximum (FWHM), as well as their wavelength dependent dispersion, were used to obtain information about the degree of disorder in CDCs. The increasing ordering with synthesis temperature was confirmed by lower FWHM values obtained from CDCs made at higher synthesis temperatures. However, this parameter was not very sensitive to variation in ordering in CDCs made at 1000 °C from different carbides. The I D/ I G was used for determination of the in-plane correlation length, which has shown to be independent of synthesis temperature and more sensitive to the choice of the precursor carbide. However, the changes in in-plane correlation length were small and barely accounted for the size of one sixfold ring.

Translational order in a nanoconfined dipolar fluid

Using Monte Carlo simulations in a constant-parallel pressure ensemble we investigate the translational structure in strongly coupled dipolar soft sphere fluids confined to a film of nanoscopic thickness. To this end we compute density profiles, in-plane correlation functions, and bond order parameters at different pressures corresponding to globally isotropic and orientationally ordered states. We then compare these quantities to those evaluated for a corresponding non-dipolar fluid, that is, soft spheres at the same average density (and temperature). We find that the dipolar forces, and the resulting tendency to form chained and orientationally ordered states, strongly change the “underlying” soft-sphere structure in the whole density range considered.

In-plane field-induced vortex liquid correlations in underdopedBi2Sr2CaCu2O8+δ

The effect of a magnetic field component parallel to the superconducting layers on longitudinal Josephson plasma oscillations in the layered high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ is shown to depend on the thermodynamic state of the underlying vortex lattice. Whereas the parallel magnetic field component depresses the Josephson Plasma Resonance (JPR) frequency in the vortex solid phase, it may enhance it in the vortex liquid. There is a close correlation between the behavior of microwave absorption near the JPR frequency and the effectiveness of pancake vortex pinning, with the enhancement of the plasma resonance frequency occurring in the absence of pinning, at high temperature close to the vortex melting line. An interpretation is proposed in terms of the attraction between pancake vortices and Josephson vortices, apparently also present in the vortex liquid state.

Artificial introduction of defects into vertically aligned multiwalled carbon nanotube ensembles: Application to electrochemical sensors

Carbon nanotubes (CNTs) inevitably contain defects that exert a significant influence on their physical, electrical, and electrochemical properties. In this study, we subject vertically aligned multiwalled CNT ensembles to argon and hydrogen ion irradiation, to artificially introduce defects into the structure. Subsequently, Raman spectroscopy in conjunction with electrochemical analyses was used to characterize the amount and nature of disorder within the CNTs. While an increased disorder with ion irradiation was generally observed, argon and hydrogen exhibit different effects on the Raman intensity spectra. Argon irradiation seems to cause charged defects, e.g., in the form of dangling bonds, and increases the in-plane correlation length (La), while hydrogen irradiation passivates residual defects and decreases the La. It was noted that hydrogen treated CNTs could serve as electrochemical sensors with quicker response time.

Numerical studies of free exciton and trion wave functions in high magnetic fields

Wave functions of excitons and negative and positive trions confined in GaAs quantum wells and subject to high magnetic fields are studied by exact numerical diagonalization in Haldane spherical geometry. Finite width of the quantum well and its asymmetry caused by one-sided doping are both fully taken into account by using self-consistent subband wave functions in the integration of Coulomb matrix elements and by inclusion of multiple electron and hole subbands and Landau levels in the configuration-interaction basis. The main results regard the interaction-induced mixing of electron and hole single-particle levels, normal charge density profiles, in-plane correlation functions, and optical oscillator strengths. These characteristics are calculated for the exciton and different trion states, in terms of their dependence on the quantum-well width, carrier concentration, and magnetic field.

Self-ordering of random intercalates in thin films of cuprate superconductors: Growth model and x-ray diffraction diagnosis

We propose a simple model for the nucleation of random intercalates during the growth of high-temperature superconductor (HTSC) films by pulsed laser deposition (PLD). The model predicts a very particular spatial distribution of defects: a Markovian-like sequence of displacements along the growth direction ($c$ axis), as well as a two-component in-plane correlation function, characteristic of self-organized intercalates. A model for x-ray diffraction (XRD) on such structures is also developed and accounts for both $c$-axis and in-plane anomalies observed in XRD experiments. The method presented in this work constitutes a useful characterization tool in the optimization of deposition parameters for the growth of HTSC films.

Neutron and X-ray Scattering on the Monolayer Structure of a Lecithin Fullerene-Derivative

Using neutron reflectivity with contrast variation, X-ray reflectivity, and grazing incident small-angle X-ray scattering (GISAXS), we have characterized the in-depth and in-plane structural characteristics of the Langmuir and Langmuir-Blodgett (LB) films formed by a novel lecithin C60-derivative, FPTL, of three phospholipids jointly bonded on one single olefinic moiety of a C60 cage. Based on the neutron reflectivity measured, we have proposed a monolayer structure, with the C60 cages of FPTL lifted into the air and hydrophilic phospholipid heads immersed in water, for the FPTL Langmuir layer formed on water. On the other hand, the LB film of FPTL prepared on mica exhibits clear Kiessig fringes in the X-ray reflectivity profile, indicating a 27 angstroms monolayer film with less molecular orientation. With GISAXS, we have extracted an in-plane correlation length of about 210 A for a possible in-plane aggregation of C60 of FPTL in the LB monolayer. We have also demonstrated the highly ordered monolayer structures of a lecithin lipid, in elucidating the positive effect of the attached functional group-phospholipids on the monolayer formation of the lecithin C60-derivative.

Time-resolved grazing-incidence small-angle X-ray scattering studies of lipid multibilayers with the insertion of amyloid peptide during the swelling process

The β-amyloid peptide (Aβ) (1–40) is one of the major components that form Alzheimer's amyloid deposits. Studies of the membrane insertion of amyloid showed that amyloid is surface active and can insert into lipid monolayers [Ji et al. (2002). Biochemistry (Moscow), 67, 1283–1288; Ege & Lee (2004). Biophys. J. 87, 1732–1740]. The interaction between the peptide and a lipid monolayer or bilayer is critical to the understanding of the formation of amyloid peptide deposits on the membrane. In this paper, we have studied the structural transition of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) multibilayers with the insertion of amyloid peptide 1–40 by grazing-incidence small-angle X-ray scattering at different bilayer hydration levels (changing the relatively humidity). We mixed the DPPC and amyloid peptide in an organic solvent at a 10 to 1 weight ratio, then cast it onto a silicon wafer to form the mixed lipid multibilayer film. In this study of the pure DPPC multibilayer film and the DPPC multibilayer film inserted with amyloid peptide, it was found that the hydration process was bimodal with a better-hydrated top layer and a less-hydrated bottom layer. The gel-to-ripple phase transition suffers a strong confinement effect due to the presence of the solid substrate. With the insertion of the amyloid peptide, the ripple phase of the membrane bilayers was suppressed at high humidity and the whole film can be swollen more uniformly at lower incubation time than the pure DPPC film supported on a silicon wafer. This means water vapor can penetrate more easily into the DPPC bilayers inserted with Aβ than into the pure DPPC bilayers. Amyloid peptides were found to form clusters in the bilayer and possess in-plane correlation. From analyzing the diffuse scattering around the Bragg peak in the lateral direction, the amyloid peptides are found to form clusters in the bilayer with a radius of about 9 nm. It is also estimated that the number of Aβ molecules in one cluster is about 12 and on average each Aβ molecule occupies an interface area of about 22 nm2. As the relative humidity exceeds about 94%, the Aβ clusters seem to develop an ordered structure with a spacing of about 300 Å.

Contact angle influence on the pull-in voltage of microswitches in the presence of capillary and quantum vacuum effects

Capillary condensation between the electrodes of microswitches influences the effective pull-in voltage in a manner that depends on the contact angle of the capillary meniscus and the presence of plate surface roughness. Indeed, surface roughening is shown to have a stronger influence on the pull-in potential for relatively small contact angles with respect to that of a flat surface when capillary condensation takes place. For long wavelength roughness ratios w∕ξ⪡1 with w the rms roughness amplitude and ξ the in-plane correlation length, the pull-in voltage increases with increasing theoretical contact angle θ0 for flat surfaces. With decreasing correlation length ξ (increasing roughness), the pull-in potential decreases faster for smaller contact angles θ0 In addition, with decreasing roughness exponent H (0&amp;lt;H&amp;lt;1), which characterizes short wavelength roughness fluctuation at short length scales (&amp;lt;ξ), the pull-in potential shows a steeper decrease with decreasing correlation length ξ. Finally, with increasing relative humidity, the sensitivity of the pull-in voltage at small correlation lengths attenuates significantly with increasing contact angle θ0.

Electric-Field-Effect Modulation of the Transition Temperature, Mobile Carrier Density, and In-Plane Penetration Depth ofNdBa2Cu3O7−δThin Films

We explore the relationship between the critical temperature T(c), the mobile areal carrier density n(2D), and the zero-temperature magnetic in-plane penetration depth lambda(ab)(0) in very thin underdoped NdBa(2)Cu(3)O(7-delta) films near the superconductor to insulator transition using the electric-field-effect technique. Having established consistency with a Kosterlitz-Thouless transition, we observe that T(KT) depends linearly on n(2D), the signature of a quantum superconductor to insulator transition in two dimensions with znu(over)=1, where z is the dynamic and nu is the critical exponent of the in-plane correlation length.

Orbital ordering of layered manganites from resonant soft X-ray scattering

We present measurements of Mn L -edge resonant soft X-ray scattering on single-layered manganites with different sizes of cations. Orbital ordering of Pr 0.5 Ca 1.5 MnO 4 exhibits a stronger three-dimensional character and a dramatically enhanced transition temperature, as compared with those of La 0.5 Sr 1.5 MnO 4 . The c -axis correlation length of orbital ordering in Pr 0.5 Ca 1.5 MnO 4 is about half of the in-plane correlation length. Our results indicate that reduction in one-electron bandwidth and quenched disorder strongly enhances the stabilization of charge–orbital ordering.

Capillary condensation and quantum vacuum effects on the pull-in voltage of electrostatic switches with self-affine rough plates

In this work, we study the influence of capillary forces in combination with electrostatic and quantum vacuum generated forces on the pull-in voltage of microswitches having self-affine rough surfaces. This type of roughness is described by the rms roughness amplitude w, the in-plane correlation length ξ, and the roughness exponent H that quantifies the degree of surface irregularity at short length scales (&amp;lt;ξ). It is shown that an attractive capillary force decreases more the effective pull-in voltage when the plate surfaces are rougher. The latter corresponds to smaller roughness exponents H and/or larger long wavelength roughness ratios w∕ξ. Notably, the capillary contribution increases the sensitivity of the effective pull-in voltage on the roughness exponent H. This behavior takes place for values of H close to its experimental accuracy.