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Transition metal dichalcogenides are 2D structures with remarkable electronic, chemical, optical and mechanical properties. Monolayer and crystal properties of these structures have been extensively investigated, but a detailed understanding of the properties of their few-layer structures are still missing. In this work we investigated the mechanical differences between monolayer and multilayer WSe2 and MoSe2, through fully atomistic molecular dynamics simulations (MD). It was observed that single layer WSe2/MoSe2 deposited on silicon substrates have larger friction coefficients than 2, 3 and 4 layered structures. For all considered cases it is always easier to peel off and/or to fracture MoSe2 structures. These results suggest that the interactions between first layer and substrate are stronger than interlayer interactions themselves. Similar findings have been reported for other nanomaterials and it has been speculated whether this is a universal-like behavior for 2D layered materials. We have also analyzed fracture patterns. Our results show that fracture is chirality dependent with crack propagation preferentially perpendicular to W(Mo)-Se bonds and faster for zig-zag-like defects.
Phase relations along the Li2O⋅2B2O3-Yb2O3⋅B2O3 polythermal section of the Li2O –B2O3–Yb2O3 system were investigated by differential thermal analysis, x-ray diffraction, and microstructural analysis. The state phase diagram of the Li2O⋅2B2O3-Yb2O3⋅B2O3 section is an eutectic system with invariant eutectic point corresponding to ∼0.2 mole fraction of Yb2O3⋅B2O3 and 800 °C. According to physico-chemical analysis, the Li2O⋅2B2O3-Yb2O3⋅B2O3 polythermal section is quasi-binary, allowing us to partially triangulate the Li2O-B2O3-Yb2O3 system. The borders of the glass formation region were defined in the Li2O⋅2B2O3-B2O3-Yb2O3⋅B2O3 concentration triangle. The vitreous samples showed a semiconducting nature.
Piezoelectric materials find important applications in micro- and nano-electromechanical systems (MEMS/NEMS). Pb(Zrx,Ti1-x)O3 (PZT) is currently the most widely used composition for such applications but due to environmental concerns over the toxicity of lead, lead free alternative materials are required. K0.5Na0.5NbO3 (KNN) is considered as a potential lead free piezoelectric but the current generation of monolithic ceramics has inferior electromechanical properties as compared to PZT. Consequently, there is great interest in improving the piezoelectric properties of KNN ceramics and various methods such as doping, hot-pressing and texturing are currently being studied. KNN single crystals like lead based single crystals have shown better electromechanical properties as compared to their ceramic counterparts. In addition, the behavior of a ferroelectric is largely dependent on its local domain response to an applied electrical or mechanical loading. Therefore, to understand better the material’s macroscopic properties, it is essential to access local ferroelectric domains behavior which collectively determines the electromechanical performance.
Anomalous origin of the left coronary artery from the pulmonary trunk is rare, occurring at an incidence of 1 in 300 000. If not diagnosed and treated early, it is life-threatening. Children with the anomaly usually present in infancy with congestive cardiac failure, and are occasionally referred for cardiac transplant. We investigated the medium term outcome for patients following creation of a two-coronary arterial circulation.
Between 1992 and 2007, we diagnosed 15 patients seen at our Institution as having anomalous origin of the left coronary artery from the pulmonary trunk. Over a period of 13 years, aortic reimplantation was undertaken in 12 of these patients, who form the studied cohort.
Direct reimplantation was performed in 5 patients. In 3 cases, a tension-free anastomosis was created using a caudally based flap. In another 3 cases, an extended flap was used, while a patch arterioplasty was fashioned in the final patient. There were no deaths. Left ventricular function recovered in all but one of the patients, and all patients had a reduction in the degree of mitral regurgitation.
Among the variety of surgical techniques, transfer of the anomalous left coronary artery to the aorta is the ideal method for long-term patency and adequate blood supply. This can be achieved by creating flaps based on the walls of the pulmonary trunk and aorta, producing a dual coronary arterial supply with no mortality and low morbidity.
The interface between a highly-crosslinked polymer film and a thin silicon nitride layer can be regulated using adhesion promoting molecules. This work compares the effects of both indirect polymer/inorganic interface chemistry modification by blending organosilane adhesion promoting molecules into the polymer layer, and direct modification by confining the organosilane molecules to the substrate surface. Of particular interest are the effects of these modifications on the occurrence of an anomalous subcritical debonding phenomenon previously observed for the unmodified interface. While significantly different adhesion values were measured, the influence of the blended organosilanes was limited to moderating moisture diffusion through the polymer layer, which correllates with moderated near-threshold growth rates. Conversely, nanoscale confinement of the adhesion promoting molecules did not result in expected universal increases in adhesion energy but did inhibit anomalous near-threshold behavior.
We report on the integration of flowable oxide based Fresnel microlenses with AlGaN based 280 nm light emitting diodes (LED). The lenses were fabricated on the back side of the LED sapphire substrates using direct electron beam writing. Ten concentric rings with different width and variable thickness were designed for 360 degree phase correction. Within each ring the thickness was varied in five steps to approximate a linear profile. The width of each thickness step varied from 100 nm to several microns. Outer diameter of the lens was 65μm. A focal distance of 68 μm was measured for the fabricated microlenses. At the focal plane a FWHM of intensity profile as small as 14 μm was measured for lenses integrated with 30 μm diameter UV LEDs . The maximum intensity at focal plane exceeded the background radiation by a factor of 50. Comparison of the LED performance before and after the lens fabrication did not reveal any degradation of integral efficiency of devices. These results demonstrate the feasibility of using flowable oxide Fresnel microlenses in optical systems based on micro-pixel deep UV AlGaN LEDs.
Exposure of poly(dimethylsiloxane) (PDMS) to oxygen plasma creates a thin, stiff surface-modified layer that reaches a submicron depth. Due to a significant modulus mismatch between the stiff surface-modified layer and the compliant bulk PDMS the surface-modified layer forms intricate patterns of surface buckles when under compressive stress and nano-cracks when under tensile stress. It is desirable to be able to design patterns of nano-cracks, or at least to have an understanding of them. Among the properties necessary to do this are the thickness and elastic modulus of the surface-modified layer. Due to the very small length scale of the surface-modified layer, it is a significant challenge to measure these properties. In this proceedings paper, a two-step method is described for determining the thickness and elastic modulus of the surface-modified layer using the atomic force microscope (AFM). First, nanoindentation is performed from which the bending stiffness of the surface-modified layer is calculated. Second, the surface-modified layer thickness is determined by using phase imaging on the cross-section of oxidized PDMS to map the region of the relatively stiffer surface-modified layer.
This paper presents a bilayer model to account for surface effects on the wrinkling of ultrathin polymer films. Assuming a surface layer of finite thickness, effects of surface properties on the critical strain, the equilibrium wavelength, and the wrinkle amplitude are discussed in comparison with conventional analysis. Experimental measurements of wrinkling in polymer films with thickness ranging from 200 nm to 5 nm are conducted. The bilayer model provides a consistent understanding of the experiments that deviate from conventional analysis for thickness less than 30 nm. A set of empirical surface properties is deduced from the experimental data.
It was demonstrated, on general thermodynamic grounds, that, in non-hydrostatically stressed elastic systems, phase and grain interfaces undergo morphological destabilization due to different mechanisms of “mass rearrangement”. Destabilization of free surfaces due to the combined action of mass rearrangement in the presence of electrostatic field has been well known since the end of the 19th century. Currently, morphological instabilities of thin solid films with electro-mechanical interactions have found various applications in physics and engineering. In this paper, we investigate the combined effects of the stress driven rearrangement instabilities and the destabilization due to the electro-mechanical interactions. The paper presents relevant results of theoretical studies for ferroelectric thin films. Theoretical analysis involves highly nonlinear equations allowing analytical methods only for the initial stage of unstable growth. At present, we are unable to explore analytically the most important, deeply nonlinear regimes of growth. To avoid this difficulty, we developed numerical tools facilitating the process of solving and interpreting the results by means of visualization of developing morphologies.
The advancement of imprint lithography as a method for fabricating nanostructures is impeded by a lack of effective tools for characterizing mechanical properties and geometry at the nanoscale. In this report, we describe the development of methods for determining elastic moduli and cross sectional dimensions of imprinted nanolines from Brillouin light scattering (BLS) measurements using finite-element (FE) and Farnell-Adler models for the vibrational modes. An array of parallel nanoimprinted lines of polymethyl methacrylate (PMMA) with widths of ∼65 nm and heights of ∼140 nm served as a model specimen. Several acoustic modes were observed with BLS in the low-gigahertz frequency range, and the forms of the vibrational displacements were identified through correlation with calculations using measured bulk-PMMA moduli and density as input. The acoustic modes include several flexural, Rayleigh-like, and Sezawa-like modes. Fitting of Farnell-Adler calculations to the measured dispersion curves was explored as a means of extracting elastic moduli and nanoline dimensions from the data. Some values obtained from this inversion analysis were unrealistic, which suggests that geometric approximations in the model introduce significant systematic errors. In forward calculations, the frequencies determined with the FE method are found to more closely match measured frequencies. This suggests that the FE approach may be more accurate for inversion analysis. Initial estimates of uncertainties in the FE calculations support this conclusion.
We report a study on the plasma-enhanced chemical vapor deposition of silicon carbonitride, as well as the resonant behavior of nanomachined SiCN structures. Films with thicknesses of 1 um, and 200 nm were deposited at varying gas ratios using ammonia (NH3), nitrogen (N2), and diethylsilane (DES) as precursors. X-ray photoelectron spectroscopy revealed high nitrogen and low carbon content in films deposited at high NH3:DES gas flow ratios. Selected samples annealed at varying temperatures experienced shifts in stress towards tensile of Δσ = 235 MPa, 432 MPa, 724 MPa, and 1140 MPa, at annealing temperatures of T = 400 °C, 500 °C, 600 °C, and 700 °C respectively. Infrared spectroscopy reported a loss of incorporated hydrogen as a mechanism of stress modulation. Resonant assaying of cantilevers fabricated from 200 nm-thick SiCN yielded root-modulus-over-density values of √(E/ρ) = 6.95 × 103 m/s and √(E/ρ) = 8.35 × 103 m/s, comparable to those of silicon.
A comprehensive computational analysis is presented of the atomistic mechanisms of strain relaxation over a wide range of applied biaxial tensile strain in free-standing Cu thin films. The analysis is based on large-scale isothermal-isostrain MD simulations using slab supercells with cylindrical voids normal to the film plane and extending throughout the film thickness. Our analysis has revealed various regimes in the film's mechanical response as the applied strain level increases. Following an elastic response at low strain (≶ 2%), plastic deformation occurs accompanied by emission of screw dislocations from the void surface and threading dislocations from the film surfaces, in parallel with generation of vacancies due to slip of jogged dislocations. At the lower strain range following the elastic-to-plastic deformation transition (⋚ 6%), void growth is the major strain relaxation mechanism, while at higher levels of applied strain (≥ 8%), a subsequent transition leads to a new plastic deformation regime where void growth plays a negligible role in the film strain relaxation.
The evolution of stress during the MOCVD growth of AlN thin films on sapphire substrates under both low and high temperature conditions has been evaluated. The final stress state of the films is assumed to consist of the summation of stresses from three different sources: (1) the stress which arises from residual lattice mismatch between film and substrate i.e. that which persists after partial relaxation by misfit dislocation formation. The extent of relaxation is determined from High Resolution TEM analysis of the substrate/film interface; (2) the stress arising from the coalescence of the 3D islands nucleated in this high mismatch epitaxy process. This requires knowledge of the island sizes just prior to coalescence and this was provided by AFM studies of samples grown under the conditions of interest; and (3) the stress generated during post-growth cooling which arises from the differences in thermal expansion coefficient between AlN and sapphire. The final resultant stress, comprising the summation of stresses arising from these three sources, is found to be tensile in the sample grown at lower temperature and compressive in the sample grown at higher temperature. These results are in general qualitative agreement with results of TEM and High resolution X-ray diffraction (HRXRD) studies, which show evidence for tensile and compressive stresses in the low temperature and high temperature cases, respectively.
The strong ion bombardment, applied during sputter deposition of diamond-like carbon films (DLC), which is needed to promote the growth of the sp3-bonded hard phase, inevitably is accompanied by compressive stress generation, thus limiting their maximum thickness. Gradient coatings with gradients in composition, constitution or properties are a well-known concept to manage such stress problems. The stepwise graded layer concept adjusts a graded constitution of the growing carbon film by a stepwise increase of the ion energy, i.e. the substrate bias voltage, during magnetron sputtering. To study the influence of the layer thickness on the expansion of the interface regions between the layers deposited with different bias voltage, samples with increasing deposition time of the top layer and thus thickness ratio were investigated by using the small angle cross-section nanoindentation method (SACS). It was revealed that the thickness of the interface regions is linearly dependent on the thickness ratio of the graded layers, which might be an evidence for stress-induced diffusion and relaxation processes in the carbon network. By using microindentation with a Berkovich indenter and ex-situ AFM-imaging it was found that all graded films exhibited higher Berkovich thresholds for crack development and thus better crack resistance than the hardest single-layer film and kept a high hardness value of about 4000 HV0.005.