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Register a new userX-ray phase contrast topography to measure the surface stress and bulk strain in a silicon crystal
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The measurement of the Si lattice parameter by x-ray interferometry assumes the use of strain-free crystals, which might not be true because of intrinsic stresses due to surface relaxation, reconstruction, and oxidation. We used x-ray phase-contrast topography to investigate the strain sensitivity to the finishing, annealing, and coating of the interferometer crystals.We assessed the topography capabilities by measuring the lattice strain due to films of copper deposited on the interferometer mirror-crystal. A byproduct has been the measurement of the surface stresses after complete relaxation of the coatings.
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Strain engineering is one of the most effective approaches to manipulate the physical state of materials, control their electronic properties, and enable crucial functionalities. Because of their rich phase diagrams arising from competing ground states, quantum materials are an ideal playground for on-demand material control, and can be used to develop emergent technologies, such as adaptive electronics or neuromorphic computing. It was recently suggested that complex oxides could bring unprecedented functionalities to the field of nanomechanics, but the possibility of precisely controlling the stress state of materials is so far lacking. Here we demonstrate the wide and reversible manipulation of the stress state of single-crystal WO3 by strain engineering controlled by catalytic hydrogenation. Progressive incorporation of hydrogen in freestanding ultra-thin structures determines large variations of their mechanical resonance frequencies and induces static deformation. Our results demonstrate hydrogen doping as a new paradigm to reversibly manipulate the mechanical properties of nanodevices based on materials control.
We report the initial demonstrations of the use of single crystals in indirect x-ray imaging for x-ray phase contrast imaging at the Washington University in St. Louis Computational Bioimaging Laboratory (CBL). Based on single Gaussian peak fits to the x-ray images, we observed a four times smaller system point spread function (21 {mu}m (FWHM)) with the 25-mm diameter single crystals than the reference polycrystalline phosphors 80-{mu}m value. Potential fiber-optic plate depth-of-focus aspects and 33-{mu}m diameter carbon fiber imaging are also addressed.
Here, we study the role of stress state and stress gradient in whisker growth in Sn coatings electrodeposited on brass. The bulk stress in Sn coatings was measured using a laser-optics based curvature setup, whereas glancing angle X-ray diffraction was employed to quantify the surface stress; this also allowed studying role of out-of-plane stress gradient in whisker growth. Both bulk stress and surface stress in Sn coating evolved with time, wherein both were compressive immediately after the deposition, and thereafter while the bulk stress monotonically became more compressive and subsequently saturated with aging at room temperature, the stress near the surface of the Sn coating continually became more tensile with aging. These opposing evolutionary behaviors of bulk and surface stresses readily established a negative out-of-plane stress gradient, required for spontaneous growth of whiskers. The importance of out-of-plane stress gradient was also validated by externally imposing widely different stress states and stress gradients in Sn coatings using a 3-point bending apparatus. It was consistently observed that whisker growth was more in the coatings under external tensile stress, however, with higher negative out-of-plane stress gradient. The results conclusively indicate the critical role of negative out-of-plane stress gradient on whisker growth, as compared to only the nature (i.e., sign and magnitude) of stress.
The resolution function of a spectrometer based on a strongly bent single crystal (bending radius of 10 cm or less) is evaluated. It is shown that the resolution is controlled by two parameters, (i) the ratio of the lattice spacing of the chosen reflection to the crystal thickness and (ii) a single parameter comprising crystal thickness, its bending radius, and anisotropic elastic constants of the chosen crystal. Diamond, due to its unique elastic properties, can provide notably higher resolution than silicon. The results allow to optimize the parameters of bent crystal spectrometers for the hard X-ray free electron laser sources.
We investigate a novel hybrid system composed of an ensemble of room temperature rare-earth ions embedded in a bulk crystal, intrinsically coupled to internal strain via the surrounding crystal field. We evidence the generation of a mechanical response under resonant light excitation. Thanks to an ultra-sensitive time- and space-resolved photodeflection setup, we interpret this motion as the sum of two resonant optomechanical backaction processes: a conservative, piezoscopic process induced by the optical excitation of a well-defined electronic configuration, and a dissipative, non-radiative photothermal process related to the phonons generated throughout the atomic population relaxation. Parasitic heating processes, namely off-resonant dissipative contributions, are absent. This work demonstrates an unprecedented level of control of the conservative and dissipative relative parts of the optomechanical backaction, confirming the potential of rare-earth-based systems as promising hybrid mechanical systems.
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