A convolutional neural network for defect classification in Bragg coherent X-ray diffraction

Coherent diffraction imaging enables the imaging of individual defects,such as dislocations or stacking faults,in materials.These defects and their surrounding elastic strain fields have a critical influence on the macroscopic properties and functionality of materials.However,their identification in Bragg coherent diffraction imaging remains a challenge and requires significant data mining.The ability to identify defects from the diffraction pattern alone would be a significant advantage when targeting specific defect types and accelerates experiment design and execution.Here,we exploit a computational tool based on a three-dimensional(3D)parametric atomistic model and a convolutional neural network to predict dislocations in a crystal from its 3D coherent diffraction pattern.Simulated diffraction patterns from several thousands of relaxed atomistic configurations of nanocrystals are used to train the neural network and to predict the presence or absence of dislocations as well as their type(screw or edge).Our study paves the way for defect-recognition in 3D coherent diffraction patterns for material science.

Huge metastable axial strain in ultrathin heteroepitaxial vertically aligned nanowires

Strain engineering is a powerful tool to tailor the physical properties of materials coherently stacked in an epitaxial heterostructure. Such an approach, applied to the mature field of planar heteroepitaxy, has yielded a variety of new phenomena and devices. Recently, heteroepitaxial vertically aligned nanocomposites have emerged as alternatives to planar structures. Owing to the peculiar geometry of such nanoarchitectures, efficient strain control can be achieved, opening the way to novel functionalities. In this paper, we report a very large tensile axial strain in epitaxial transition metal nanowires embedded in an oxide matrix. We show that axial strains in excess of 1.5% can be sustained over a large thickness （a few hundred nanometers） in epitaxial nanowires having ultrasmall diameters （-3-6 nm）. The axial strain depends on the diameter of the nanowires, reflecting its epitaxial nature and the balance of interface and elastic energies. Furthermore, it is experimentally shown that such strain is metastable, in agreement with the calculations performed in the framework of the Frenkel-Kontorova model. The diameter dependence and metastability provide effective ways to control the strain, an appealing feature for the design of functional nanoarchitectures.

Vasarely painting at the nanoscale on sapphire crystals

We demonstrate that convenient thermal treatment of a specific sapphire vicinal surface can induce the formation of a fully two-dimensional(2D)ordered surface made of a periodic assembly of(006)facets.The simiarity between the resuting surface topography and pattemns represented in the hexagon series"of paintings by Vasarely is really striking!We thus propose to call these surfaces as"nanoscaled Vasarely surfaces".We also show that the self-organization process,which is driven by the minimization of the free energy of a closed system,results in a quasi-linear isothermal growth of the facets'surface area over time.