Well-oriented Layer Research Articles

Electric-Field-Induced Assembly of an Ionic Liquid-Water Interphase Enables Efficient Heavy Metal Electrosorption.

Controlling ion desolvation, transport, and charge transfer at the electrode-electrolyte interface (EEI) is critical to enable the rational design of the efficient and selective separation of targeted heavy metals and the decontamination of industrial wastewater. The main challenge is to sufficiently resolve and interrogate the desolvation of solvated metal ions and their subsequent electroreduction at the EEI and establish pathways to modulate these intermediate steps to achieve efficient energy transfer for targeted reactive separations. Herein, we obtained a predictive understanding of modulating the desolvation and electrosorption of Pb2+ cations using the hydrophobic ionic liquid 1-ethyl-3-methylimidazolium chloride (EMIMCl) in aqueous electrolyte. We revealed the formation of a compact interphase layer consisting of EMIMCl-Pb complexes under an applied electric field using operando electrochemical Raman spectroscopy, atomic force microscopy, and electrochemical impedance spectroscopy measurements combined with classical molecular dynamics simulations. A lower negative potential was shown to result in the formation of a well-oriented layer with the positive imidazolium ring of EMIMCl lying perpendicular to the electrode and the hydrophobic alkyl chain extending into the bulk electrolyte. This oriented layer, which formed from a dilute concentration of EMIMCl added to the electrolyte, was demonstrated to facilitate desolvation of incoming solvated Pb2+ cations and decrease the charge transfer resistance for Pb electrodeposition, which has important implications for the selective removal of Pb from contaminated mixtures. Overall, our findings open up new opportunities to modulate ion desolvation using hydrophobic ionic liquids in aqueous electrolytes for efficient heavy-metal separation.

Orientational order induced by a polymer network in the isotropic phase of liquid crystal.

We studied the paranematic ordering induced by a polymer network in the isotropic phase of a liquid crystal (LC) that occurs in polymer-stabilized cells with bend configuration of the LC director (π cells) fabricated via photopolymerization of photoreactive monomer RM 82 added in small concentrations (3-5 wt %) to a nematic LC [4-cyano-4'-pentylbiphenyl (5CB)] when low voltage was applied across the cell. The polymer network formed in the nematic phase of the LC consists of fine fibrils that are aligned along the LC director and thus mirror the bend deformation of the LC at the time of polymerization. When heated to temperatures above the nematic-to-isotropic (N-I) phase transition such highly ordered polymer network anchors LC molecules providing ordering of the LC around the fibrils which results in unusually high optical retardation of the cell, R_{cell}. We present a theoretical model that relates R_{cell} to the degree of order of the fibrils, the anchoring energy of the LC molecules on the surface of the polymer fibrils, and the fibril radius r_{0}. Fitting of the experimental R_{cell}(T) curves with the developed model reveals correlation of r_{0} with the nematic correlation length ξ_{0} which characterizes penetration of the nematic order in the isotropic phase of the LC. Accepting ξ_{0} as a material constant of about 1 nm leads to a very small radius of the fibrils, r_{0}∼1nm, which is also supported by other reported experimental data. High optical retardation and fast electro-optical response of the cells at the temperatures deep into the isotropic phase point toward the enhancement of the polymer-induced paranematic order by a well-oriented layer of LC molecules that are absorbed on the surface of fibrils. Application of high voltage at the isotropic phase temperatures results in high variations of the optical retardation of the cells. Characteristic on and off response times were about 10-100 μs, independent of the cell gap. Combination of large voltage-driven changes of the optical retardation occurring in the low-viscosity isotropic state with switching times that are at least two orders of magnitude shorter than the typical relaxation times of the cells operating in the nematic phase make such polymer-stabilized π cells very promising for application in fast electro-optical switches and light modulators.

Chlorinated silicon carbide CVD revisited for polycrystalline bulk growth

Silicon carbide is interesting for high power, high temperature and high frequency applications but its development is limited by the low quality and high cost of single-crystalline wafers. The transfer by wafer-bonding of single-crystalline SiC thin films to a polycrystalline SiC support is an attractive way for lowering the cost. This transfer requires high quality polycrystalline substrates fabricated with high growth rate, controlled morphology (columnar, dense, well-oriented layers), and very low surface roughness (RMS < 5 nm), to achieve tailored bulk properties. Compared to the Physical Vapour Transport technique which is limited by the low number of process parameters (temperature, pressure, crucible design), high temperature Chemical Vapour Deposition represents an easier and more efficient way of controlling high growth rate, morphology, crystalline quality via the gas phase composition (Si/C ratio, supersaturation…) and temperature. Typical deposition conditions using methyltrichlorosilane (MTS) has been revisited at higher temperature (above 1500 °C) to achieve high growth rate. Advantages of chlorine system vs. organosilane system have been highlighted by thermodynamic calculations in the range of 1000–2000 °C. Thermodynamic and thermal simulations have been performed to describe the methyltrichlorosilane decomposition. Finally, bulk silicon carbide layers are synthesized between 1200 °C and 1600 °C at various H 2/MTS ratios. The observed morphologies are correlated to calculated thermodynamic supersaturations.

Exfoliation of carbon fibers through intercalation compounds synthesized electrochemically

Carbon fibers, mesophase-pitch-based, PAN-based and vapor-grown carbon fibers, were exfoliated through their intercalation compounds by rapid heating to 1000°C. Intercalation into carbon fibers was electrochemically performed in concentrated nitric acid at 3 V for 15 min. Formation of intercalation compounds was confirmed from the fact that the interlayer spacing increased to about 0.78 nm, though a part of them was decomposed to the residual compounds after rinsing and drying. Textural and morphological changes in carbon fibers by exfoliation were studied by SEM and TEM. After rapid heating to 1000°C, a marked morphological change was observed, a single fiber being converted to a bundle of thin filaments split along the original fiber axis. Each of these filaments was concluded to consist of well-oriented layer planes from TEM analysis. By the present procedure, even long fibers, as long as 50 cm, and carbon fiber fabrics could be exfoliated. On PAN-based carbon fibers with low crystallinity, the exfoliation was observed after rapid heating, though the formation of intercalation compounds was not proved from X-ray diffraction.

The temperature behaviour of a ferroelectric smectic mixture and the detection of ferroelectric switching by NMR

Abstract The orientational order of a ferroelectric mixture has been measured by proton NMR. The alignment of the mesogenic units is reflected in the splitting of the NMR signal into a doublet. Up to a cell thickness of 200 μm, it was possible to produce well-oriented layers in the bookshelf geometry by magnetic orientation of the substances in the B 0-field. The angular dependences of the line width of the NMR signal on the tilt angle of the director have been calculated. A ferroelectric switching was detected by measuring the angular dependence of the line width in the switched state. The tilt angle and the orientational order parameter S of the bookshelf samples were estimated at various temperatures.

Infrared external reflection spectroscopy of self-assembled monolayer films on Si substrate with a buried metal layer (BML) structure

This paper reports a study of the signal behavior of IR reflection spectra on the flat Si substrate with a buried metal layer (BML) and MBE-grown Si overlayer. A self-assembled monolayer of octadecyltrichlorosilane on the surface was used as a typical sample of a well-oriented layer. The signal intensities strongly depend on the orientation of surface vibration, the polarization of IR radiation and the angle of incidence. The dependence is quite different from that seen on metal and normal (without BML) semiconductor surfaces and can be explained by local electric field intensity based on classical electromagnetic treatment. On a relatively thin Si overlayer (typically 700–1200Å), vibration modes only with the transition moment in the vertical direction were observed for p-polarization and modes only in the lateral direction were observed for s-polarization. On a relatively thick Si overlayer (typically 2000A˚), modes with lateral as well as vertical components show significant signals for p-polarization, and only lateral modes could be detected at significant intensity for s-polarization. These results show that the orientation of surface species can be determined by analyzing the spectral dependence on polarization, angle of incidence and the thickness of the Si overlayer.

Graphoepitaxy of platinum on Sawtooth profile gratings

The graphoepitaxy of platinum on sawtooth profile gratings was investigated as a means of fabricating (100) oriented Pt films on Si substrates. Successful alignment was achieved using electron beam evaporated Pt layers on substrates with pitches of 0.20, 0.24, 0.27, and 0.30 μm. In order to achieve smooth, well-oriented layers, the alignment and microstructures of these films were studied as a function of annealing time and temperature. The degree of graphoepitaxial alignment achieved was found to depend on the stabilization and growth of (100) grains before extensive mass motion of Pt was permitted. Remanent (111) grains could then be reoriented through mass motion and assimilation into growing (100) grains. An annealing profile fulfilling these requirements for a 700 Å thick evaporated film is presented.

Effect of Geometrical Factors on the Shape of Diffraction Profile for Natural Graphite and Petroleum Coke

Half maximum breadth (Bo) of a diffraction profile of carbon increased with the increase of sample thickness. When sample thickness was less than 0.2mm, Bo value came closely to the value obtained by the reflection only from the surface. When silicon powder was used as an internal standard, Bo value of (002) diffraction line was almost constant regardless of sample thickness. But Bo value of (004) and (110) lines showed still some broadness due to penetration of X-ray beam into sample, even if silicon powder was used as an internal standard. Therefore, it is concluded that it is preferable to use sample thickness less than 0.2mm and to use slit width as small as possible in order to reduce low absorption effect of the sample.Shape of (00l) diffraction profiles was also affected by the reflection from an internal well-oriented layer in the sample which might be produced during mounting process. If the internal well-oriented layer is made at a distance from the rotation axis, the reflection from the layer appears at displaced position, and the reflection overlaps the profile which is reflected from surface layer.