Changes In Effective Thickness Research Articles

Optical band gap, local work function and field emission properties of MBE grown β-MoO3 nanoribbons

Monoclinic molybdenum trioxide (β-MoO3) nanostructures (shaped like nanoribbons: NRs) were grown on Si(1 0 0), Si(5 5 1 2) and fluorine-doped tin oxide (FTO) by molecular beam epitaxy (MBE) technique under ultra-high vacuum (UHV) conditions. The dependence of substrate conditions and the effective thickness of MoO3 films on the morphology of nanostructures and their structural aspects were reported. The electron microscopy measurements show that the length and the aspect ratio of nanostructures increased by, 260% without any significant change in the width for a change in effective thickness from 5 nm to 30 nm. NRs are grown along 〈0 1 1〉 for all the effective thickness of MoO3 films. Similarly, when we increased the film thickness from 5 nm to 30 nm, the optical band gap decreased from 3.38 ± 0.01 eV to 3.17 ± 0.01 eV and the local work function increased from 5.397 ± 0.025 eV to 5.757 ± 0.030 eV. Field emission turn-on field decreased from 3.58 V/μm for 10 μA/cm2 to 2.5 V/μm and field enhancement factor increased from 1.1 × 104 to 5.9 × 104 for effective thickness variation of 5–30 nm β-MoO3 structures. The β-MoO3 nanostructures found to be much better than the α-MoO3 nanostructures due to low work function, low turn on field and high field enhancement factor, and are expected to be useful applications.

Detection of Sub-surface Delamination based on the Spatio-temporal Gradient Analysis over the A0-mode Lamb wave Fields

This study proposes a novel method of local phase velocimetry based on the spatio-temporal gradient analysis. Structural flaws such as sub-surface delamination, corrosion and fatigue cracks represent changes in effective thickness and local properties of materials, and therefore, measurement of phase velocity changes in elastic wave propagation can be employed to assess the integrity of material structures. Due to the dispersive character of the A0-mode Lamb wave, the phase velocity of Lamb wave is varied depends on the thickness of the plate. Hence, a measure of the local phase velocity of the A0-Lamb wave yields a measure of the sub-surface delaminations of the plate. Theoretical analysis and several numerical experiments show the principle of the proposed method in this paper.

Interference resisting design for guided wave tomography

RAPID (reconstruction algorithm for the probabilistic inspection of damage) is a new promising tomography approach for the detection and monitoring of critical areas in a structure. With the sensors permanently installed on or embedded in structures, changes in effective thickness and material properties caused by structural damage can be detected and mapped to the tomogram. However, in this method, the tomographic feature SDC (signal difference coefficient) captures the overall change of the received ultrasonic signals, which makes it sensitive to environmental factors (e.g. rain, changes in temperature and humidity). As a result, the approach is restricted in the laboratory environment. In this paper, the influence of measurement data length on the SDC and the tomogram are investigated, and a new strategy is established on how to choose the measurement data to obtain good reconstruction by matching the coverage zone of each transmitter–receiver pair with the corresponding affected zone. The proposed method is then applied to identify defects of the specimen in the presence of external sources of interference, such as water droplets and structural variations outside the critical area. The results demonstrate its capability of improved robustness in the presence of external sources of interference.

Electric field induced reversible tuning of resistance of thin gold films

The change in resistance of nanostructured metals with respect to an applied field is believed to be due to a change in carrier concentration and hence a linear variation of resistance with the surface charge is expected. In this article, we propose a different approach to explain the resistance variation based on a change in the effective thickness of the film due to a shift of the electron density profile resulting from the applied surface charge. The change in effective thickness together with its effect on surface scattering of electrons account for the majority of the observed variation in resistance. The thin film geometry with different thicknesses and hence a controlled variation of the surface-to-volume ratio allows a deep quantitative understanding and interpretation of the observed phenomena. The model presented in this work shows that a nominal nonlinear response of the resistance of a metal on electrochemically applied surface charge does not necessarily indicate an onset of a redox reaction.

Capacitive Detection in Ultrathin Chemosensors Prepared by Molecularly Imprinted Grafting Photopolymerization

The usual applications of capacitive detection in chemo- and biosensors are based on changes in effective thickness of insulating layers due to adsorption of analyte onto receptors. Ultrathin chemosensors based on molecularly imprinted polymerization enable a realization of another capacitive approach that exploits changes in electrical capacitance due to modification of the dielectric constant of the polymer. Such chemosensors were prepared by photografted molecularly imprinted polymerization on the surface of gold electrodes. An adsorbed layer of hydrophobic photoinitiator (benzophenone) provided grafted polymerization on the surface of the alkanethiol-modified gold electrode. The chemosensors were characterized by cyclic voltammetry, impedance spectroscopy, and scanning electron and atomic force microscopy. Binding of analyte was detected by measurements of electrical capacitance. The results indicate a decrease of the dielectric constant of the polymer layer due to analyte binding up to 20%.

Subsurface flaw localization based on the spatio-temporal gradient array processing

Lamb waves are guided ultrasonic waves capable of propagating relatively long distances in plates and laminated structures, such as airframe skins, storage tanks and pressure vessels. Their propagation properties in these media depend on the vibrational frequency as well as on the thickness and material properties of the structure. Structural flaws such as dis-bonds, corrosion and fatigue cracks represent changes in effective thickness and local material properties, and therefore measurement of variations in Lamb wave propagation can be employed to assess the integrity of these structures. In this study, the localization based on the spatio-temporal gradient array processing is derived theoretically. The proposed method has an ability to characterize the phase velocity of the Lamb-wave propagation through the linearity among the 4-dimensional vector which is composed by following components: (1) a vertical (z-directional) displacement, (2) its vertical particle velocity, (3) x-directional and (4) y-directional out-of-plane strains. This paper is organized as follows: the second section presents the process of the localization and the proposed algorithm, the third section evaluates the proposed velocimetry via numerical experiments.

Characterization of DNA immobilization and subsequent hybridization on a 2D arrangement of streptavidin on a biotin-modified lipid bilayer supported on SiO2.

We show how the water content (and effective density) of thin adsorbed films composed of biomolecules can be determined using combined quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR) analysis. In particular, these techniques, combined with theoretical treatment using a Voigt-based viscoelastic model, were applied to analyze the state of surface immobilized single stranded biotin-modified probe DNA (b-DNA) coupled via streptavidin to a biotin-doped supported phospholipid bilayer (b-SPB)). From a proper analysis, it is demonstrated how changes in effective thickness, delta(f), and the viscoelastic components (shear viscosity, eta(f), and shear elasticity, mu(f))) can be obtained during both DNA immobilization and hybridization with single stranded fully complementary target DNA. In particular, it is demonstrated how this type of analysis can be used to control the state of streptavidin arrangement for improved measurements of DNA hybridization kinetics. The latter is demonstrated by identifying a surface-coverage dependent viscoelastic behavior of immobilized b-DNA, which is shown to influence the hybridization efficiency.

Parallel projection and crosshole Lamb wave contact scanning tomography

Lamb waves are guided ultrasonic waves capable of propagating relatively long distances in thin plates and thin laminated structures, such as airframe skins, storage tanks, and pressure vessels. Their propagation properties in these media depend on the vibrational frequency as well as on the thickness and material properties of the structure. Structural flaws such as disbonds, corrosion, fatigue cracks, and voids represent changes in effective thickness and material properties, and therefore measurement of variations in Lamb wave propagation can be employed to assess the integrity of these structures. Lamb wave measurements can be made for a number of relative transducer positions (projections) and an image of the flawed region can be reconstructed tomographically. This paper presents a new technique in which two contact piezoelectric transducers are independently scanned along parallel lines in a fashion analogous to that commonly used in seismic crosshole tomography. These results are compared to those for parallel projection Lamb wave tomography data collected with an automated contact scanning apparatus. The advantages and drawbacks of these two methods in the development of automated tomographic Lamb wave scanners for quantitative mapping of thickness variation in platelike materials are discussed.

Contact scanning Lamb wave tomography

Lamb waves are guided ultrasonic waves capable of propagating relatively long distances in platelike structures such as airframe skins, storage tanks, and pressure vessels. Their propagation depends on frequency thickness and material properties, and, because structural flaws present changes in effective thickness and/or material properties, Lamb waves can be employed to assess the integrity of these structures. For structures with large surface areas, a full integrity evaluation can be a time-consuming operation, but with Lamb wave techniques, this evaluation can be performed with waves propagating along one dimension of the inspection area as the probing transducers are moved in the perpendicular dimension. Such an approach yields information about the presence of flaws within the scanned area. Then, in order to quantitatively characterize the flaws, Lamb wave measurements can be made for a number of projections and an image of the flawed region can be reconstructed tomographically. Contact scanning Lamb wave tomography for metallic and composite structures with flaws is discussed as a practical technique for quantitative nondestructive evaluation.