Küçük altın ​​nanotozlarının X-ışını kırınımı analizi

Powder X-ray diffraction is a powerful method to characterize structure of materials in a non-destructive way. Conventionally, diffraction pattern of powder material consisting of crystallites is analyzed using Rietveld analysis, which provides structural parameters such as lattice parameter, atomic displacement parameter, and crystallite size. The technique is proven to work well when crystallite sizes are in micrometer-scale, that can be considered as a bulk with surface to volume ratio negligible. Therefore, infinite atomic plane assumption in Rietveld algorithm is satisfied. However, when crystallite size is in nanometer-scale, number of atomic planes is limited due to finite dimension of nanocrystal. Moreover, realistic nanocrystals exhibit different properties on surface region compared to core region, indicating non-periodic structure. We showed that the inconsistency between the algorithm and nanocrystal structure causes errors in extracted structural parameters. These errors are orders of magnitude higher than precision of lab diffractometers. The computational work was extended to Pair Distribution Function (PDF) analysis, which also showed similar trends. All of these errors were size-dependent and became negligible with increasing particle size or decreasing surface to volume ratio. Finally, we measured X-ray diffraction pattern of 14 nm gold nanocrystal using laboratory diffractometer. The measured PDF suffered from low X-ray energy accessible by such diffractometers. However, we showed that PDF analysis can still extract structural parameters using Attenuated Crystal Modeling (ACM). The better fit quality was achieved when realistic nanocrystals obtained from Molecular Dynamics (MD) simulation were modeled as polydispersed system.