Low Tilt Angle Research Articles

Critical behaviour of the smectic A*–C* phase transition and electroclinic effect of ferroelectric liquid crystals with terphenyl rigid core

The SmA*–SmC* phase transition was studied by measuring the temperature and electric field dependences of the optical tilt angle, the electric polarisation and the dielectric spectra collected in a wide frequency range. Critical behaviour of the phase transition was analysed by varying the length of the fluorinated part of the alkyl terminal chain and by differing fluorine substitution in the terphenyl core. Both tilt and polarisation show tricritical mean-field behaviour for all homologues with n > 2. Almost all coefficients that describe the SmA*–SmC* transition in the frame of the Landau theory were derived for homologue series. Double fluorine substitution in the central ring of the core seems to promote the ‘de Vries'-type smectic A*–C* phase transition with a little layer shrinkage. These well correspond with the lower tilt angle and smaller changes of the birefringence at the phase transition compared to the other homologues.

Rational design of monolayers for improved water evaporation mitigation

Seven chemically designed monolayer compounds were synthesized and investigated with comparison to the properties and water evaporation suppression ability of 1-hexadecanol and 1-octadecanol. Increasing the molecular weight and polarity of the compound headgroup drastically altered the characteristics and performance of the monolayer at the air/water interface. Contrary to the common expectation the monolayer's lifetime on the water surface decreased with increasing number of ethylene oxy moieties, thus optimal performance for water evaporation suppression was achieved when only one ethylene oxy moiety was used. Replacing the hydroxyl headgroup with a methyl group and with multiple ethylene oxy moieties resulted in a loss of suppression capability, while an additional hydroxyl group provided a molecule with limited performance against water evaporation. Theoretical molecular simulation demonstrated that for exceptional performance, a candidate needs to possess a high equilibrium spreading pressure, the ability to sustain a highly ordered monolayer with a stable isotherm curve, and low tilt angle over the full studied range of surface pressures by simultaneously maintaining H-bonding to the water surface and between the monolayer chains.

Changes in the electro-optical behaviour of ferroelectric liquid crystal mixture via silica nanoparticles doping

We investigated the dynamic properties of a ferroelectric liquid crystal mixture doped with silica nanoparticles using polarization current reversal method. The effect of temperature and bias field on the switching responses of silica–ferroelectric liquid crystal composites with different silica concentrations (0.01, 0.02, 0.05 and 0.1wt%) were studied and compared. Various electro-optic properties of ferroelectric liquid crystal mixture changed according to silica nanoparticles concentration. We observed about 14% decrease in spontaneous polarization and up to 17% increase in switching response for 0.01wt% silica dispersion, whereas a minor increase in the isotropic temperature of composites. The rotational viscosity and optical contrast increased with increasing silica concentration. At a 0.1wt% silica nanoparticle doping concentration, the ferroelectric liquid crystal cell showed the best electro-optic properties, such as low threshold and driving voltage, better optical contrast and low tilt angle. Our experimental results show that silica doped ferroelectric liquid crystal composites are promising materials for better optical contrast in electro-optical applications.

High-throughput subtomogram alignment and classification by Fourier space constrained fast volumetric matching

Cryo-electron tomography allows the visualization of macromolecular complexes in their cellular environments in close-to-live conditions. The nominal resolution of subtomograms can be significantly increased when individual subtomograms of the same kind are aligned and averaged. A vital step for such a procedure are algorithms that speedup subtomogram alignment and improve its accuracy to allow reference-free subtomogram classifications. Such methods will facilitate automation of tomography analysis and overall high throughput in the data processing. Building on previous work, here we propose a fast rotational alignment method that uses the Fourier equivalent form of a popular constrained correlation measure that considers missing wedge corrections and density variances in the subtomograms. The fast rotational search is based on 3D volumetric matching, which improves the rotational alignment accuracy in particular for highly distorted subtomograms with low SNR and tilt angle ranges in comparison to fast rotational matching of projected 2D spherical images. We further integrate our fast rotational alignment method in a reference-free iterative subtomogram classification scheme, and propose a local feature enhancement strategy in the classification process. As a proof of principle, we can demonstrate that the automatic method can successfully classify a large number of experimental subtomograms without the need of a reference structure.

Superhydrophilic–superoleophobic coatings

A novel superhydrophilic and superoleophobic nanocomposite coating is fabricated by spray casting nanoparticle–polymer suspensions on various substrates. Water droplets can spread over the coating completely; meanwhile, oil droplets can roll off the coating at low tilt angles without any penetration. Besides overcoming oil-fouling problems for the hydrophilic coating, the superhydrophilic–superoleophobic coating applied to the stainless steel mesh can be used for the separation of oil and water.

Uncertainty studies of topographical measurements on steel surface corrosion by 3D scanning electron microscopy

Pitting corrosion is a damage mechanism quite serious and dangerous in both carbon steel boiler tubes for power plants which are vital to most industries and stainless steels for orthopedic human implants whose demand, due to the increase of life expectation and rate of traffic accidents, has sharply increased. Reliable methods to characterize this kind of damage are becoming increasingly necessary, when trying to evaluate the advance of damage and to establish the best procedures for component inspection in order to determine remaining lives and failure mitigation. A study about the uncertainties on the topographies of corrosion pits from 3D SEM images, obtained at low magnifications (where errors are greater) and different stage tilt angles were carried out using an in-house software previously developed. Additionally, measurements of pit depths on biomaterial surfaces, subjected to two different surface treatments on stainless steels, were carried out. The different depth distributions observed were in agreement with electrochemical measurements.

Odd–Even Effects on the Structure, Stability, and Phase Transition of Alkanethiol Self-Assembled Monolayers

Molecular dynamics simulations were conducted to predict the structural properties and phase transition temperatures of n-alkanethiols CH(3)(CH(2))(n-1)SH (Cn, 4 ≤ n ≤ 22) self-assembled monolayers (SAMs) on Au (111) surfaces. We studied the effects of chain length on the structural properties, including tilt and orientation angles, and on phase transition temperature. We found clear dependence of the structural properties, on both the number of carbon atoms, n; and on n being odd or even. Alkanethiols with n ≤ 7 show liquid-like behavior and large rotational mobility, whereas those with n ≥ 12 are well-ordered and stable. For 12 ≤ n ≤ 15, odd-even effects are observed, where for n = odd, larger tilt angles, oriented in the direction of their next next nearest neighbor (NNNN), and for n = even, lower tilt angles, mostly tilted toward next nearest neighbor (NNN), were observed. For 15 ≤ n ≤ 19, we find tilt angle and orientation to be independent of n. For all alkanethiols, a gradual decrease of the tilt angle occurred by increasing the temperature from 300 to 420 K. Order-disorder phase transitions occurred at a certain temperature. This was signified by abrupt instabilities in the tilt orientation angle. This transition temperature showed an enhancement of ∼67-100 °C over the melting point of the corresponding n-alkane bulk system. This enhancement depended on n, and was larger for n = odd. Overall, we found that odd alkanethiols show better structural and thermal stability, and smaller gauche defects.

Subthreshold performance of pocket-implanted silicon-on-insulator CMOS devices and circuits for ultra-low-power analogue/mixed-signal applications

Analogue circuits based on subthreshold operation of the devices are very attractive as they show significantly better performance in terms of both power dissipation and voltage gain. A systematic investigation, with the help of extensive process and device simulations, of the effects of halo doping [both double-halo and single-halo or lateral asymmetric channel (LAC)] on the subthreshold analogue performance of 100 nm silicon-on-insulator CMOS devices and circuits is reported for the first time. Although the halo doping is found to improve the subthreshold performance in general, the improvement is significant for a low tilt angle of the halo implant. The optimisation of the analogue performance is made for the halo devices by varying the tilt angle of the halo implant. CMOS amplifiers made with the halo-implanted devices are found to have higher voltage gain over their conventional counterpart, and a more than 70% improvement in the voltage gain is observed when both n- and p-channel transistors in the amplifier are LAC devices.

Experimental Characterization of High-Performance Miniature Auto-Focusing VCM Actuator

When designing an auto-focusing actuator for use in cell phone cameras, it is important to maximize the positioning repeatability while simultaneously minimizing the device size, the power consumption, the response time, and the dynamic tilt angle. In a previous study (Liu and Lin, Opt. Express, vol. 17, pp. 9754-9763, 2009), the current group designed a miniature auto-focusing voice coil motor (VCM) actuator in which the lens was held in position using a magnetoconductive plate and a closed-loop control mechanism. The aim of this study is to characterize the performance of this device using an experimental approach. The results show that the proposed actuator has a high positioning repeatability, a low dynamic tilt angle, a rapid response, and a low power consumption.

HCl Etching Behavior on Low-Tilt-Angle and Step-Free 4H-SiC Surfaces

We report on new observations made, when 4H-SiC, Si-face substrate mesas, having either low tilt-angle (< 1°) with steps or step-free top surfaces, were exposed to three separate HCl etching conditions for five minutes at temperatures of 1130°C, 1240°C and 1390°C. We observed that HCl was ineffective at 1130°C, as etching was incomplete with abundant surface contamination. At 1240°C, screw dislocations were aggressively etched by HCl, while multiple shallow flat-bottomed etch pits were formed on step-free mesa surfaces. At 1390°C, step-flow etching dominated as large etch pits were formed at screw dislocations and previously step-free surfaces etched inward from mesa edges to form parallel rows of organized steps.

Study of gate dielectric permittivity variation with different equivalent oxide thickness on channel engineered deep sub-micrometer n-MOSFET device for mixed signal applications

The impact of high permittivity gate dielectrics with different equivalent oxide thickness (EOT) for conventional, low and high tilt angle halo implants on the performance of 100 nm n-MOSFETs device is studied using device simulator Synopsys ISE-TCAD. In this paper, we systematically increase the value of gate dielectric (3.9–50) and investigate its effects on conventional, low angle of tilt (10 o) and high angle of tilt (50 o) halo implants for different device parameters of 100 nm n-MOSFETs using two different EOT viz. 1.5 nm and 2.0 nm. The impact of gate dielectric permittivity along with the different angles of halo implants on short channel performance contributing to the DIBL, the subthreshold swing, I ON/ I OFF ratio, and the threshold voltage V T are studied for two different EOT thicknesses. The device has been investigated for digital performance parameters like the variation of substrate–body voltage on DIBL, I OFF, I ON and the threshold voltage V T for sub 100 nm technology generation. It has also been investigated for analog performance like trans-conductance generation factor (gm/ I D) and overall gain (gm R 0).

Broadband Planar Antenna Combining Monopole Element with Electromagnetic Bandgap

New structure of broadband planar antenna, combining monopole elements with electromagnetic bandgap (EBG) structures, is proposed. The antenna has a simple single layer structure and unique beam pattern. Antenna is fabricated on a surface of a single layer dielectric substrate and back side of the substrate is covered with metal layer. At the center of the substrate, an inverted L monopole strip is fabricated and on both sides of this monopole, EBG unit cells are placed. By tuning monopole length and EBG bandgap frequency, the monopole resonates even if metal layer exists close to the monopole radiator. Three types of EBG, one dimensional (1D), square two dimensional (2D) and hexagonal 2D, are tested. By combining monopole strip with hexagonal 2D-EBG, the bandwidth of prototype antenna, whose return loss is less than 10dB, is 840MHz in 5GHz band. To control beam patterns of antenna, parasitic elements are placed close to the monopole radiator and EBGs. These parasitic elements work as directors of quasi Yagi-Uda antenna and radiation gain at lower tilt angles is improved.

Switching Dynamics of a Post-Aligned Bistable Nematic Liquid Crystal Device

The operation of a post-aligned bistable nematic (PABN) liquid crystal device is simulated using a dynamic finite-element implementation of the Landau-de Gennes theory in 3D. Two topologically distinct stable director profiles of higher and lower tilt angles are identified. The switching process between these two states is simulated while monitoring the variation in total free energy of the liquid crystal material. Maxima in the free energy, acting as energy barriers separating the two stable states, are shown to coincide with the formation of -1/2 defect lines in the director field.

Modeling Spatio-Temporal Dynamics of Optimum Tilt Angles for Solar Collectors in Turkey.

Quantifying spatial and temporal variations in optimal tilt angle of a solar collector relative to a horizontal position assists in maximizing its performance for energy collection depending on changes in time and space. In this study, optimal tilt angles were quantified for solar collectors based on the monthly global and diffuse solar radiation on a horizontal surface across Turkey. The dataset of monthly average daily global solar radiation was obtained from 158 places, and monthly diffuse radiation data were estimated using an empirical model in the related literature. Our results showed that high tilt angles during the autumn (September to November) and winter (December to February) and low tilt angles during the summer (March to August) enabled the solar collector surface to absorb the maximum amount of solar radiation. Monthly optimum tilt angles were estimated devising a sinusoidal function of latitude and day of the year, and their validation resulted in a high R2 value of 98.8%, with root mean square error (RMSE) of 2.06°.

Wetting study of patterned surfaces for superhydrophobicity

Superhydrophobic surfaces have considerable technological potential for various applications due to their extreme water-repellent properties. A number of studies have been carried out to produce artificial biomimetic roughness-induced hydrophobic surfaces. In general, both homogeneous and composite interfaces are possible on the produced surface. Silicon surfaces patterned with pillars of two different diameters and heights with varying pitch values were fabricated. We show how static contact angles vary with different pitch values on the patterned silicon surfaces. Based on the experimental data and a numerical model, the trends are explained. We show that superhydrophobic surfaces have low hysteresis and tilt angle. Tribological properties play an important role in many applications requiring water-repellent properties. Therefore, it is important to study the adhesion and friction properties of these surfaces that mimic nature. An atomic/friction force microscope (AFM/FFM) is used for surface characterization and adhesion and friction measurements.

A fully 3-dimensional thermal model of a comet nucleus

A 3-D numerical model of comet nuclei is presented. An implicit numerical scheme was developed for the thermal evolution of a spherical nucleus composed of a mixture of ice and dust. The model was tested against analytical solutions, simplified numerical solutions, and 1-D thermal evolution codes. The 3-D code was applied to comet 67P/Churyumov-Gerasimenko; surface temperature maps and the internal thermal structure was obtained as function of depth, longitude and hour angle. The effect of the spin axis tilt on the surface temperature distribution was studied in detail. It was found that for small tilt angles, relatively low temperatures may prevail on near-pole areas, despite lateral heat conduction. A high-resolution run for a comet model of 67P/Churyumov-Gerasimenko with low tilt angle, allowing for crystallization of amorphous ice, showed that the amorphous/crystalline ice boundary varies significantly with depth as a function of cometary latitude.

Assessment of cardiac autonomic modulation during graded head-up tilt by symbolic analysis of heart rate variability

Two symbolic indexes, the percentage of sequences characterized by three heart periods with no significant variations (0V%) and that with two significant unlike variations (2UV%), have been found to reflect changes in sympathetic and vagal modulations, respectively. We tested the hypothesis that symbolic indexes may track the gradual shift of the cardiac autonomic modulation during an incremental head-up tilt test. Symbolic analysis was carried out over heart period variability series (250 cardiac beats) derived from ECG recordings during a graded head-up tilt test (0, 15, 30, 45, 60, 75, and 90 degrees ) in 17 healthy subjects. The percentage of subjects showing a significant linear correlation (Spearman rank-order correlation) with tilt angles was utilized to evaluate the performance of symbolic analysis. Spectral analysis was carried out for comparison over the same series. 0V% progressively increased with tilt angles, whereas 2UV% gradually decreased. The decline of 2UV% was greater than the increase of 0V% at low tilt angles. Linear correlation with tilt angles was exhibited in a greater percentage of subjects for 0V% and 2UV% than for any spectral index. Our findings suggest that symbolic analysis performed better than spectral analysis and, thus, is a suitable methodology for assessment of the subtle changes of cardiac autonomic modulation induced by a graded head-up tilt test. Moreover, symbolic analysis indicates that the changes of cardiac sympathetic and vagal modulations observed during this protocol were reciprocal but characterized by different absolute magnitudes.

Polymer-Stabilized Method for Vertically Aligned Supertwisted Nematic Liquid Crystal Displays

To achieve stable and uniform tilt angles below 85° for vertical liquid crystal (LC) alignment, a polymer-stabilized method with a bias voltage (BPS) that is combined with a photoalignment method has been proposed. An initially tilted vertically aligned LC medium is fabricated by a photoalignment method followed by a BPS method to further decrease tilt angle; the resulting low tilt angle of about 80° shows excellent uniformity and stability. This BPS method is applied to the fabrication of vertically aligned supertwisted nematic liquid crystal displays (LCDs), and we succeed in obtaining very steep voltage and transmittance characteristics with excellent temperature stability, which are essential for high-multiplexing passive matrix LCDs.

Using Vapour-Liquid-Solid Mechanism for SiC Homoepitaxial Growth on on-axis α-SiC (0001) at Low Temperature

The vapour-Liquid-Solid mechanism was used for growing epitaxial SiC layers on onaxis 6H-SiC and 4H-SiC substrates. By feeding Al70Si30 melts with propane, homoepitaxial growth was demonstrated down to 1100°C on both polytypes. At this temperature, the surface morphology is rough and non uniform with spiral growth forming large hillocks at the places where screw dislocations emerge from the substrate. Raman spectroscopy confirms the absence of the 3C-SiC polytype and shows the high Al doping of the layers. This growth temperature of 1100°C is the lowest one ever reported for growing homoepitaxial layers on low tilt angle SiC substrates. Increasing the temperature to 1200°C eliminates these hillocks but creates other morphological features due to fast substrate etching at this high temperature before growth starts.

Low-temperature homoepitaxial growth of α-SiC on on-axis (0 0 0 1) substrate by vapor–liquid–solid mechanism

Vapor–liquid–solid mechanism was used for growing epitaxial SiC layers on on-axis 6H–SiC and 4H–SiC substrates. By feeding Al 70Si 30 melts with propane, homoepitaxial growth was demonstrated on both polytypes down to 1100 °C, which is the lowest temperature ever reported for growing homoepitaxial layers on low tilt angle SiC substrates. However, at this temperature, the surface morphology is rough and non-uniform with spiral growth, forming large hillocks at the places where screw dislocation emerges from the substrate. Raman spectroscopy confirms the absence of 3C–SiC polytype and shows the high Al doping of the layers. Increasing temperature to 1200 °C eliminates these hillocks but creates other morphological features due to fast substrate etching at this high temperature before growth starts. These defects were reduced in size by introducing propane at 1100 °C before ramping to 1200 °C and completely eliminated by increasing the Si content of the melt to 50 at%.