Imperfect Substrate Research Articles

Homogeneous system of brownian particles diffusing through an imperfect substrate: Rate and probability of jump

Homogeneous system of brownian particles diffusing through an imperfect substrate: Rate and probability of jump

Homogeneous System of Brownian Particles Diffusing Through an Imperfect Substrate: Rate and Probability of Jump

Homogeneous System of Brownian Particles Diffusing Through an Imperfect Substrate: Rate and Probability of Jump

POLED displays: Robust printing of pixels

The fabrication of a high-quality polymeric organic light emitting diode display requires the deposition of identical, uniform fluid films into a large number of shallow recessed regions that form a regular array of pixels on a display backplane. We determine the protocols required to achieve continuous liquid coverage of the entire pixel area for the case where equally spaced fluid droplets are sequentially printed along a straight line within a stadium-shaped pixel and explore how these protocols depend on the wetting properties of the pixel surface. Our investigation uses a combination of experiments and numerical modeling, based on the assumption of fluid redistribution via capillary spreading according to a Cox-Voinov law. We show that the model is able to predict quantitatively the evolution of the liquid deposited in a pixel and provides a computationally inexpensive design tool to determine efficient printing strategies that account for uncertainties arising from imperfect substrate preparation or printhead dysfunction.

Aggregate-driven reconfigurations of carbon nanotubes in thin networks under strain: in-situ characterization

This work focuses on the in-situ characterization of multi-walled carbon nanotube (CNT) motions in thin random networks under strain. Many fine-grain models have been devised to account for CNT motions in carbon nanotube networks (CNN). However, the validation of these models relies on mesoscopic or macroscopic data with very little experimental validation of the physical mechanisms actually arising at the CNT scale. In the present paper, we use in-situ scanning electron microscopy imaging and high-resolution digital image correlation to uncover prominent mechanisms of CNT motions in CNNs under strain. Results show that thin and sparse CNNs feature stronger strain heterogeneities than thicker and denser ones. It is attributed to the complex motions of individual CNTs connected to aggregates within thin and sparse CNNs. While the aggregates exhibit a collective homogeneous deformation, individual CNTs connecting them are observed to fold, unwind or buckle, seemingly to accommodate the motion of these aggregates. In addition, looser aggregates feature internal reconfigurations via cell closing, similar to foam materials. Overall, this suggests that models describing thin and sparse CNN deformation should integrate multiphase behaviour (with various densities of aggregates in addition to individual CNTs), heterogeneity across surface, as well as imperfect substrate adhesion.

Effects of substrate topography on current injection and light emission properties of organic light emitting devices

Substrate topography plays a critical role in the function of nano-scale materials and devices. We study small molecular organic light emitting devices (OLEDs) deposited onto non-planar substrates, where the substrate’s radius of curvature in some regions approaches the thickness of the active device layers. As a result, the electric field profile inside the organic charge transport layers is modified, influencing carrier injection, transport, and light emission properties. Experiments and numerical modeling suggest that charge balance and electroluminescence efficiency potentially can be improved in electron injection-limited OLED architectures via substrate geometry. These findings elucidate the optoelectronic behavior (and degradation) of OLEDs on imperfect substrates, and suggest a strategy based on substrate topography for controlling device behavior.

U2 snRNA is inducibly pseudouridylated at novel sites by Pus7p and snR81 RNP

All pseudouridines identified in RNA are considered constitutive modifications. Here, we demonstrate that pseudouridylation of Saccharomyces cerevisiae U2 snRNA can be conditionally induced. While only Ψ35, Ψ42 and Ψ44 are detected in U2 under normal conditions, nutrient deprivation leads to additional pseudouridylation at positions 56 and 93. Pseudouridylation at position 56 can also be induced by heat shock. Detailed analyses have shown that Pus7p, a single polypeptide pseudouridylase known to modify U2 at position 35 and tRNA at position 13, catalyses Ψ56 formation, and that snR81 RNP, a box H/ACA RNP known to modify U2 snRNA at position 42 and 25S rRNA at position 1051, catalyses Ψ93 formation. Using mutagenesis, we have demonstrated that the inducibility can be attributed to the imperfect substrate sequences. By introducing Ψ93 into log-phase cells, we further show that Ψ93 has a role in pre-mRNA splicing. Our results thus demonstrate for the first time that pseudouridylation of RNA can be induced at sites of imperfect sequences, and that Pus7p and snR81 RNP can catalyse both constitutive and inducible pseudouridylation.

Study on surface properties of PDMS microfluidic chips treated with albumin.

Electrokinetic properties and morphology of PDMS microfluidic chips intended for bioassays are studied. The chips are fabricated by a casting method followed by polymerization bonding. Microchannels are coated with 1% solution of bovine serum albumin (BSA) in Tris buffer. Albumin passively adsorbs on the PDMS surface. Electrokinetic characteristics (electro-osmotic velocity, electro-osmotic mobility, and zeta potential) of the coated PDMS channels are experimentally determined as functions of the electric field strength and the characteristic electrolyte concentration. Atomic force microscopy (AFM) analysis of the surface reveals a "peak and ridge" structure of the protein layer and an imperfect substrate coating. On the basis of the AFM observation, several topologies of the BSA-PDMS surface are proposed. A nonslip mathematical model of the electro-osmotic flow is then numerically analyzed. It is found that the electrokinetic characteristics computed for a channel with the homogeneous distribution of a fixed electric charge do not fit the experimental data. Heterogeneous distribution of the fixed electric charge and the surface roughness is thus taken into account. When a flat PDMS surface with electric charge heterogeneities is considered, the numerical results are in very good agreement with our experimental data. An optimization analysis finally allowed the determination of the surface concentration of the electric charge and the degree of the PDMS surface coating. The obtained findings can be important for correct prediction and possibly for robust control of behavior of electrically driven PDMS microfluidic chips. The proposed method of the electro-osmotic flow analysis at surfaces with a heterogeneous distribution of the surface electric charge can also be exploited in the interpretation of experimental studies dealing with protein-solid phase interactions or substrate coatings.

The Crystallographic Properties of Strained Silicon Measured by X-Ray Diffraction

Strained silicon represents a materials-based enhancement to further scaling of CMOS transistors. In epitaxial strained silicon substrates, strain is provided by a relaxed SiGe graded buffer layer that expands the in-plane lattice constant of silicon. Because this is accompanied by the introduction of crystalline defects, in the form of dislocations, mosaic structure, and lattice tilting, the deposition of strained silicon occurs on imperfect substrates. Therefore, there is a fundamental need to study the materials properties to ensure strained silicon substrates meet the rigorous criteria for CMOS processing. This paper focuses on the in-depth investigation of the crystallographic properties of epitaxial strained silicon and strained silicon on insulator (SSOI) substrates by X-ray diffraction (XRD). The results for both epitaxial strained silicon and bonded SSOI substrates are presented and contrasted, with particular emphasis on the effect of the layer transfer process used during the formation of SSOI substrates. Although the focus is on strained silicon, the X-ray diffraction techniques highlighted in this paper are readily extendable to other materials heterostructures, such as germanium on insulator, strained germanium, and III–V compound semiconductors on insulator, allowing characterization of many future microelectronic platforms.

Aberration-free lateral shear holographic interferometry

Lateral shear interferometry is considered as a possible means for optical correction of wave fronts reconstructed from singly and doubly exposed holograms. Optical aberrations are eliminated in a moire pattern by superposing two lateral shear interferograms using waves diffracted by original holograms. It is shown that the singly exposed hologram allows compensation for only optical inhomogeneities due to the substrate, while the doubly exposed hologram also excludes aberrations due to the recording system. The experimental results supporting the efficiency of the method as applied to imperfect substrates of holograms are presented.

Ellipsometric method for real time control of thin film deposition on imperfect substrates

In this work we present a method of the ellipsometric control for optical filter deposition. This control procedure uses the length of the real time ellipsometric trajectory as a control parameter with respect to the length of precalculated target trajectory. This control method has been adapted for substrates whose optical structure cannot be adequately modeled. In this case an effective refractive index and absorption coefficient is assigned to the substrate, which is determined by computing the length of the experimental trajectory from the deposition of a reference layer. This effective refractive index is used to compute the target trajectory. The control procedure has been applied to the deposition of a graded index antireflection coating on Corning and float glass by plasma enhanced chemical vapor deposition. Spectrophotometry transmission measurements show the high efficiency of the control method with respect to uncontrolled filter deposition.

Fluorocytosine causes uncoupled dissipation of the proton gradient and behaves as an imperfect substrate of the yeast cytosine permease.

At pH 5-6 ATP-depleted washed cell preparations of strain NC233-10b[pII4-9], in which the cytosine permease was overexpressed, absorbed cytosine, hypoxanthine or fluorocytosine stoichiometrically with, respectively, about 1, 1.4 and 5 proton equivalents. The cellular pH fell proportionately. The membrane depolarization caused by each compound was assayed in the presence of glucose with a voltage-sensitive dye and increased in the same order. Fluorocytosine significantly lowered the growth yield that a 'petite' strain of the yeast formed at limiting glucose concentrations. At pH 5.6 with extracellular [K+] below 1 mM, each of the three substrates was accumulated about 200-fold from a dilute solution at the expense of the proton gradient. This concentration ratio corresponds to a solute gradient (delta mu(s)) of 13 kJ mol-1. Raising [K+]o systematically lowered the substrate accumulation ratio and delta muH. The mean ratio delta mu(s)/delta muH was 0.82 for all three substrates. It was concluded that whereas the behaviour of cytosine approximated to that expected for a symport of unit proton stoichiometry, the absorption of protons with fluorocytosine and, to a lesser extent, hypoxanthine, was only partly conserved as useful work. A possible mechanism of this novel phenomenon is outlined.

Low-temperature homoepitaxial film growth of Si by reactive ion beam deposition

Homoepitaxial film growth maintaining primary surface structures of Si substrates was investigated by using the reactive ion beam deposition method proposed recently. This method uses ionized species of reactive SiH4 gas controlled in the low-energy region of less than 500 eV. At 100–150 eV, homoepitaxial film growth on Si(111) and Si(100) maintaining their primary 7×7 and two-domain 2×1 surface structures, respectively, can be achieved at the low temperatures of 650 and 600 °C, respectively. In addition, oxygen impurities on substrate surfaces, due to imperfect substrate cleaning and recontamination caused by residual gases in a growth chamber before film growth, were successfully reduced at 600 °C by irradiating the 100-eV controlled ionized species onto them.

Effects of substrate heterogeneity on adsorption isotherms near a two-dimensional gas-liquid critical point.

We calculate adsorption isotherms for a model of a two-dimensional (2D) fluid system obeying Griffiths's 2D Ising scaling equation of state, in equilibrium with a 3D ideal gas. We model the effects of an imperfect substrate using the theory of Dash and Puff. We compare the Ising-model results with experimental isotherms of Kr adsorbed on MgO smoke. The model isotherms are used to evaluate empirical methods for determining the critical temperature and the calculation of the critical exponents \ensuremath{\gamma} and \ensuremath{\beta} from experimental isotherms. We find that current methods overestimate ${T}_{c}$, presently available substrates preclude a direct measurement of \ensuremath{\gamma}, and the blunt shape of the phase boundary makes the precise determination of \ensuremath{\beta} difficult.

Crystal-glass phase transitions in the adatom layers

Phase transitions in the two-dimensional crystalline films adsorbed onto an imperfect substrate (with impurities and defects) are analyzed. It is shown that the phase transition from the commensurate long-range-order (LRO) crystal to the glass state occurs with increase of impurity concentration. The orientational correlation function is shown to tend algebraically to zero at large distances in the glass state. The influence of defects on the phase transition from the LRO-commensurate crystal to the disordered state at the definite temperature T m is considered.

Excitation of Waveguides for Integrated Optics with Laser Beams

We discuss in this paper the excitation of dielectric waveguides used for integrated optical circuitry. Thin optical films are usually excited by means of coupling prisms using frustrated total internal reflection. We propose instead excitation of thin-film waveguides directly by gaussian laser beams. Harmful effects of imperfect substrate edges can be avoided by letting the dielectric waveguide end inside of the substrate. The exciting laser beam is directed towards the end of the guide from the inside of the substrate material. The analysis is based on neglecting reflection at the end of the thin-film waveguide and on assuming that the thin film is infinitely extended in one dimension. The maximum excitation efficiency predicted with this model is 97 percent. It is expected that the excitation efficiency of more realistic guides for integrated optics can be as high as 90 percent.