Lensless imaging of inorganic nanostructures

The high coherence of the SwissFEL radiation is also in high demand for the lensless imaging of inorganic nanostructures. Fur thermore, the shor t, bright SwissFEL pulses allow ultrafast dynamic processes to be followed at the atomic level – in the form of ensemble-averaged time-dependent correlation functions. Figure III.11a shows the measured coherent X-ray diffraction pattern of a single misfit dislocation, at the inter face between a Si (001) substrate and a 280 nm film of Ge0.3Si0.7, i.e., slightly above the critical film thickness for dislocation- free pseudomorphic growth [11]. The pattern is a reciprocal space map near the (202) Bragg reflection from the substrate. To collect these data, the incoming synchrotron X-ray beam was focused to a 1 × 1 μm2 spot and scanned along the inter face until an isolated dislocation was found. Numerical simulations of the diffraction pattern permitted the construction of a physical model for the local bonding arrangement at the dislocation core (see Figs. III.11b and c). An impor tant branch of material science, which is currently inaccessible to real-time, microscopic investigation, is the generation and evolution of defect structures upon irradiation. Such defects are responsible for the production and segregation of vacancies and self-interstitials, which ultimately limit the strength of, for example, the structural components of nuclear reactors. Numerical simulations of radiation defects can be performed for limited regions of space and over limited time periods using state-of-the-ar t molecular dynamics (MD) calculations. An example of such a simulation, of the aftermath of a 5 keV primary knock-on event in a 12 nm grain in nanocrystalline Ni, is shown in Figure III.12 [12]. The simulation cell encompasses 15 grains, containing a total of approximately 106 atoms. Already 0.3 ps after the knock-on, a locally melted region has formed, from which two replacement collision sequences (RCS) emerge. As time progresses, self-intersitital atoms (SIA) move to the periphery of the cascade, either individually or as SIA clusters, away from the central vacancies, eventually becoming permanently segregated from them at the grain boundary. To investigate such displacement cascades at the Swiss- FEL, many individual irradiation events will be generated – each one with a different local geometry, and each one followed, after a par ticular time delay, by an X-ray probe pulse. The SwissFEL will thus deliver important physical correlations, such as the average size and density of the central vacancy cluster or the diffusion rate of multi-atom interstitial clusters.

The ability to measure spatial and temporal correlations at the atomic scale, using coherent scattering from the SwissFEL, represents a power ful tool for calibrating the accuracy of atomistic simulations in materials. The length and time scales currently accessible to MD calculations (~100 nm, ∼1 ns) fit very well to the spatial and temporal scales made accessible with the SwissFEL (see Fig. III.13). An impor tant result of the comparison of theor y and measurement will be the validation of the approximations and parameter values used in MD. Of par ticular interest is the relation, at the shor test time and length scales, between paramaterized MD calculations and ab initio methods, which explicitly treat the quantum-mechanical behavior of the electrons. Correct formulation at this fundamental level is of utmost impor tance to provide accurate input paramenters for larger-scale modelling.