Effects Of Interfacial Defects Research Articles

Monitoring and characterizing the debonding in CFRP retrofitted RC beams using acoustic emission technology

Reinforced concrete (RC) beams are retrofitted in flexure using externally bonded CFRP sheets to enhance strength and ductility. However, the strength of the CFRP is not completely utilized because of the debonding failure which is abrupt in nature. Additionally, the presence of interfacial defects further exacerbates the bond capacity. To ensure structural safety, continuous monitoring of the retrofitted members is necessary. Acoustic emission technology is the most feasible option for monitoring because of its non-invasive nature and continuous monitoring capability. Therefore, the capability of acoustic emission technology in detecting the debonding mechanism has been experimentally investigated. Furthermore, the effect of interfacial defects on debonding has also been studied. Three RC beams tested in flexure and monitored using acoustic emissions. It was found that the complex debonding mechanism can be detected and characterized using acoustic emissions and the presence of interfacial defects contributes to the debonding mechanism.

Raman imaging of strained bubbles and their effects on charge doping in monolayer WS2 encapsulated with hexagonal boron nitride

Interfacial defects significantly affect the optical and electronic properties of two-dimensional (2D) materials. Particularly, bubbles inevitably formed during the layer-by-layer fabrication of 2D heterostructures can cause spatially inhomogeneous distributions in charge density and strain, leading to modifications in emission efficiency and electronic structures that are markedly different depending on which interface the bubbles form. Here, we report spatially resolved Raman results of a hBN/WS2/hBN heterostructure over a large area in which microbubbles are present. Spatial variations in the optical phonon characteristics of both WS2 and hBN reveal that the bubbles are formed at the interface between the top hBN layer and the underlying WS2 monolayer. The presence of the hBN bubbles results in a relatively higher electron density of the underlying WS2 than that of the WS2 in the bubble-free surrounding flat region, possibly due to the flexoelectric effect of the bent hBN layer. In addition, the Grüneisen parameter of hBN is obtained using the relationship between E2g phonon frequency shifts and corresponding strain profiles of the hBN bubble layer. Our work inspires a more comprehensive understanding of charge and strain distributions under the effect of interfacial defects.

Interfacial defect mediated charge carrier trapping and recombination dynamics in TiO2-based nanoheterojunctions

The construction of heterojunctions is an effective strategy to prevent the charge carrier recombination. Owing to the lattice mismatch, it is unavoidable that many defects would be introduced at the heterojunction interface, thus developing a quantitative description on effect of interfacial defects on carrier dynamics become very imminent. In this paper, we employ ultrafast transient absorption (TA) and time resolved photoluminescence (TRPL) spectroscopy to investigate the interfacial defects related carrier trapping, separation, and recombination dynamics in WS2/TiO2 heterojunction. The results reveal the interfacial defects act as the carrier trapping center, and the trapping of electrons possesses active effect, while the co-trapping of both electrons and holes exhibits passive effect on charge carriers transition and separation. Accordingly, both the population and lifetime of carriers are accelerated substantially when only electrons are trapped, whereas the lifetime of charge carriers cannot be prolonged effectively when both electrons and holes are trapped. This work provides a detailed understanding of the interfacial defects mediated charge carrier trapping and recombination dynamics, providing a significant guidance for the future design of high-performance photoactive nanoheterojunctions.

Effect of interfacial defects on the electronic properties of MoS2 based lateral T-H heterophase junctions.

The coexistence of semiconducting (2H) and metallic (1T) phases of MoS2 monolayers has further pushed their strong potential for applications in the next generation of electronic devices based on two-dimensional lateral heterojunctions. Structural defects have considerable effects on the properties of these 2D devices. In particular, the interfaces of two phases are often imperfect and may contain numerous vacancies created by phase engineering techniques, e.g. under an electron beam. Here, the transport behaviors of the heterojunctions with the existence of point defects are explored by means of first-principles calculations and non-equilibrium Green's function approach. While vacancies in semiconducting MoS2 act as scattering centers, their presence at the interface improves the flow of the charge carriers. In the case of VMo, the current has been increased by two orders of magnitude in comparison to the perfect device. The enhancement of transmission was explained by changes in the electronic densities at the T–H interface, which open new transport channels for electron conduction.

Interfacial bonding features of friction stir additive manufactured build for 2195-T8 aluminum-lithium alloy

Friction stir additive manufacturing (FSAM) was performed successfully using 2 mm thick sheets of 2195-T8 aluminum-lithium alloy. The influence of the tool pin shape and process parameters on the interfacial bonding features among the additive manufactured layers was discussed, and the effects of interfacial defects on the performances of the additive build were analyzed based on microstructures, hardness profiles, and mechanical property evaluations. It is shown that the shape of the tool pin is one of the key factors in influencing the bonding interface between two manufactured layers. The cylindrical pin and the conical pin with three flats are not suitable for the FSAM process since very poor material mixing features are produced along the bonding interface. Although the material mixing degree of bonding interface is obviously improved at the advancing side (AS) interface of the nugget zone (NZ) by using the convex featured pin or the pin with three concave arc grooves, the material mixing degree at the retreating side (RS) interface of the NZ is always insufficient. Meanwhile, the weak-bonding defects along the bonding interfaces could be formed, which are originated from the hooking defects on the RS. The weak-bonding defects are related to the oxides and impurities existing at the original bonding interfaces as well as the insufficient stirring action of the tool pin. The back and forth double passes welding is one of the effective methods to improve the material mixing in the whole NZ and eliminate the hooking defects extending into the NZ. The welding rotation speeds of 800, 900 and 1000 rpm for giving welding speed of 100 mm/min were used in the additive manufacturing processes of 2195-T8 aluminum-lithium alloy, in which the optimum microstructure is obtained with the rotation speed of 800 rpm. The soften degree for the multilayered build is obvious, and the hardness profiles across the different bonding interfaces are always uneven. Meanwhile, compared with the AS interface, the fluctuation of the hardness value at the RS interface is greater. The mechanical properties of the multilayered build are inhomogeneous, and the maximum tensile strength of the multilayered build is only reached the 56.6% of the base metal. The mechanical properties are closely associated with the soften tendency of the material and the degree of the amelioration of weak-bonding defect along the bonding interface.

Effect of interfacial defects on the electronic properties of graphene/g-GaN heterostructures.

To gain deep insights into their interactions, the effects of interfacial defects on the structural and electronic properties of graphene/g-GaN heterostructures were investigated by using first-principles calculations. The graphene/g-GaN-VGa heterostructure maintains a p-type Schottky contact in the spin-up channel and the Schottky barrier height (SBH) is decreased to 0.332 eV, but there is not a metal/semiconductor contact in the spin-down channel. However, the n-type SBH is negative for the graphene/g-GaN-VN heterostructure, indicating an ohmic contact. Furthermore, the SBH in the graphene/g-GaN heterostructure can be effectively modulated by the interlayer distance. The research could provide a strategy for the development and fabrication of efficient novel nanoelectronic devices.

Analysis by Numerical Simulation of the Effect of Interfacial Defects on CZTS Solar Cell

The performance of CZTS (Cu2ZnSnS4) hetero-junction solar cell is numerically simulated taking into account different type of defects as well as the series and shunt resistances. It was found when considering an ideal structure that the simulation differs greatly from the experimental results (Dhakal et al., Solar Energy, 100, 23–30, 2014). When volume defects are considered in the CZTS absorber layer, better results were obtained but not as expected; only the short circuit current is reproduced. On the other hand, interface states are considered between CZTS and CdS give good agreement of the open circuit voltage. The series and shunt resistance lead to a better agreement of the fill factor. Only when all these are considered together that experimental measurements are reproduced.

Carrier-Selective NiO/Si and TiO2/Si Contacts for Silicon Heterojunction Solar Cells

Carrier-selective contacts based on thin oxides of nickel and titanium are computationally investigated for heterojunction silicon solar cells. Replacing the standard amorphous/c-Si heterojunction with NiO/c-Si (front) and TiO2/c-Si (back), we explore the physical requirements to enhance the cell efficiency beyond the physical limits of the conventional structure. Under ideal conditions, a wider bandgap (>3 eV) of these metal oxides provides a high optical transparency, whereas a near-perfect alignment of their energy bands with silicon ensures a high fill factor (FF), which is often difficult to obtain in some of the other wide-bandgap alternatives, e.g., SiO x , due to imperfect band offsets that hinder carrier extraction. We explore the practical nonidealities that could possibly degrade cell efficiency below its ideal limit. In particular, effects of interfacial defects, Fermi-level pinning at c-Si/TiO2/metal contact, variability in the bandgap of NiO, and nonoptimized metal oxide doping density are investigated quantitatively. Using the reported experimental data for these nonideal effects, we highlight that the cell efficiency of $\sim {28}$ % could be achieved under AM1.5 illumination with an optimal cell design. These modeling insights provide useful guidelines for the future development of exploratory window layers for silicon solar cells using NiO (front) and TiO2 (back) heterojunctions.

MFM Study of a Sintered Nd–Fe–B Magnet: Analyzing Domain Structure and Measuring Defect Size in 3-D View

This paper describes the use of magnetic force microscopy (MFM) to investigate the domain structure of sintered Nd–Fe–B magnet in a thermally demagnetized state. Observations are made at both perpendicular and parallel surfaces to the alignment axis (easy axis). Clear magnetic contrast images in 3-D view with defined domain structures are presented. Mazelike magnetic domains and stripe magnetic domains are observed in the perpendicular and parallel surfaces, respectively. Superficial spike domains and reverse spikes are observed within the main domains and analyzed in 3-D view. MFM images illustrate the grains of Nd–Fe–B magnets, and indicate the multi-domain structures within individual grains. The effect of interfacial defects upon the domain structure is studied in 3-D, and evident reverse magnetic domains (nucleation sites) are observed on the imperfection region. The height and width of the defect region are determined using the XEI imaging software. In addition, the volume and surface area of nucleation region are evaluated and validated theoretically by developing an approach involving coercivity versus the nucleus volume. Defect regions have been identified by using Raman spectroscopy as an oxide in character rather than metallic. The average domain width and domain wall energy have been determined for both directions.

Structural and Electronic Properties of BaO/MgO(001)-type Interface Studied via Aberration-corrected Transmission Electron Microscopy and First-principles Calculations

The properties of BaO/MgO-type interface in the BaZrO3/MgO(001) heterostructure are studied by aberration-corrected high-resolution transmission electron microscopy combined with first-principles calculations. Experimental evidence demonstrates that cation displacement and vacancies occur at the interface. Our first-principle calculations show that cation displacement results from the electrostatic potential effect at the interface, and cation vacancies could lower the interfacial work of separation and enhance the interfacial stability of BaO/MgO-type interface. The results highlight that the effect of interfacial defects should be taken into account in understanding the film growth kinetics and properties in oxide heteroepitaxy.

Effect of interfacial defects on the electronic and magnetic properties of epitaxial CrAs/InAs and CrAs/CdSe half-metallic multilayers

We present an extended study of single impurity atoms at the interface between the half-metallic ferromagnetic zinc-blende CrAs compound and the zinc-blende binary InAs and CdSe semiconductors in the form of very thin multilayers. Contrary to the case of impurities in the perfect bulk CrAs studied in Galanakis and Pouliasis [J. Magn. Magn. Mater. 321 (2009) 1084] defects at the interfaces do not alter in general the half-metallic character of the perfect systems. The only exception are Void impurities at Cr or In(Cd) sites which lead, due to the lower-dimensionality of the interfaces with respect to the bulk CrAs, to a shift of the p bands of the nearest neighboring As(Se) atom to higher energies and thus to the loss of the half-metallicity. But Void impurities are Schottky-type and should exhibit high formation energies and thus we expect the interfaces in the case of thin multilayers to exhibit a robust half-metallic character.

Effects of interfacial defects on the performance of grazing-incidence synthetic multilayer reflectors for medium- and low-energy x rays

A method of calculating the effects of interfacial roughness on the grazing-incidence reflectivity and transmissivity of a synthetic multilayer x-ray reflector (SMR) is developed; this method makes it possible to investigate the dependence of an SMR’s performance on the spatial-frequency content of the roughness profile. The roughness is modeled by a superposition of volume diffraction gratings that are described mathematically by using a coupled-wave approach. The coupled-wave equations for both the electric and magnetic fields are derived without invoking the usual slowly varying envelope approximation, and exact analytical solutions are obtained for unslanted gratings. The effects of individual gratings with different spatial periods are investigated for 1.5-keV x rays, and it is found that the performance of the SMR is affected most strongly by short-period gratings, whereas gratings with longer periods have relatively little effect. At higher energies, guided wave resonances occur for gratings with short periods, giving rise to singular behavior of the equations describing the system; this behavior is discussed in some detail. The theory of random functions provides methods for constructing the superposition of many gratings, each with a different spatial frequency, so that a randomly rough surface is modeled. The reflectances and transmittances of typical SMR’s designed for x rays of 1.5, 8, and 17 keV are calculated for rough interfaces with different power spectra, and the results are compared with those of similar calculations for an ideal SMR. It is shown that the resonance effects are inconsequential for power spectra that characterize the interfacial roughness that is expected to occur in the fabrication of a typical SMR. The effects on SMR performance of interpenetration of the multilayer constituents at the layer interfaces are also calculated by using a simple averaging model, and it is shown that for these energies such effects are dominant in comparison with those of boundary roughness.