Last year's work

The Laboratory of Advanced Structural Studies (LASS) carries out research in three areas: carbon based materials, the theory of phase transformations and X-ray related methods. In the last year, we have reached significant results in all of these fields.

Carbon-based systems. — Lately, various carbon-based materials have become the center of intensive research. Earlier, we concentrated on fullerenes and related compounds whereas metal organic framework (MOF) materials, carbon nanotubes and nanotube-based hybrid systems are our center of interest currently.

Metal-organic frameworks. Metal-organic frameworks are porous coordination polymers with large inner cavities. The rigid, metal-containing clusters at the nodes (inorganic Secondary Building Units, inorganic SBUs) are joined by organic linkers. In our recent work, we have successfully prepared six new MOF structures with zinc-containing secondary building units and cubane-1,4-dicarboxylic acid linkers. This year, we have continued the study of this new family of materials. We scaled up the preparation in order to get homogeneous, high quality samples in large quantities. We have elaborated the safe storage of water-sensitive samples, and the activation of MOF structures. We started to examine the absorption properties of our new materials. We have characterized the structure of high-symmetry MOF-5 and analogue materials with various solvents in their voids by single-crystal X-ray diffraction. Recently, we have also prepared a new four-nuclear MOF with spiro[3.3]heptane-2,6-dicarboxylic acid linkers with similar structure to our highest symmetry frameworks.

Infrared spectroscopy on carbon-based systems: Our research concentrated on fullerenes, nanotubes, nanotube-based hybrids and two-dimensional systems. We introduced new methods: synchrotron-based infrared spectroscopy, near-field infrared microscopy and photoinduced spectroscopy. Materials studied were carbon and boron nitride nanotubes, both pristine and filled with small molecules, graphene and solar cell materials based on methylamine-PbI3 perovskite (Fig. 1) and carbon nanotubes. We also studied the luminescence properties of another prospective solar-cell material, silicon carbide.

Figure 1. Methylamine-lead iodide (MA-PbI3) deposited on carbon nanotube film.

Theory of phase transformations.We have investigated various aspects of crystalline freezing within atomistic and coarse-grained continuum models:

(1) We used two differently formulated orientation-field-based phase-field models (termed as KWC model and HMP model) to study polycrystalline solidification. First, we studied the grain coarsening process including the determination of the limiting grain-size distribution and compared the results to those from experiments on thin films to the models of Hillert and Mullins, and to predictions by multiphase-field theories. In contrast to the other models mentioned, the results of the orientation-field-based phase-field models were in agreement with the experiments. Then, as we did earlier with the KWC model, we extended the orientation field to the liquid state in the HMP model, and applied it to describe multi-dendritic solidification, polycrystalline growth, including the formation of “dizzy” dendrites disordered via the interaction with foreign particles (Fig. 2).

Figure 2. Interaction of a growing dendrite with foreign particles represented by 'orientation pinning centers' in the HMP model. From left to right: composition, phase-field, and orientation maps.

(2) We studied heteroepitaxy, two-step nucleation, and nucleation at the growth front within the framework of a simple dynamical density functional theory, the Phase-Field Crystal (PFC) model. We investigated the misfit dependence of the critical thickness in the Stranski-Krastanov growth mode in isothermal studies. The simulation results for stress release via the misfit dislocations fit better to the People-Bean model than to the one by Matthews and Blakeslee. Next, we investigated structural aspects of two-step crystal nucleation at high undercoolings, where an amorphous precursor forms in the first stage. Finally, we modelled the formation of new grains at the solid–liquid interface at high supersaturations/supercoolings, a phenomenon termed Growth Front Nucleation (Fig. 3).

X-ray related methods. — We have carried out X-ray diffraction experiments by inside X-ray sources. The atoms which we used as point sources were exited by a very intense focused synchrotron-generated X-ray beam. The diffraction pattern, which consists of lines (called Kossel lines) were detected by a 2D position-sensitive detector. Using the results of the dynamical diffraction of X-rays, we analyzed the line profiles of the Kossel lines and determined experimentally the phase of the structure factors of all the measured diffraction lines (Fig. 4). This opens the way to single-pulse structure determination by X-ray free-electron lasers.

Figure 3. Two-step nucleation in the PFC model. Time increases from left to right. Upper row: grey and red spheres corresponds to atoms with bcc-like and amorphous neighbourhood, respectively, while atoms in the liquid are transparent. Lower row: the corresponding`q4 vs.`q6 bond-order parameter maps. Solidification appears to start with the nucleation of amorphous domains.

Figure 4. Measured Kossel pattern (left panel) of GaAs. On the upper part three consecutive detector positions are shown, while in the lower part a selected region given on the upper part by the dashed rectangle.  Profiles of selected Kossel lines (right panel). The fitted curves are depicted by continuous lines and the measured and theoretically calculated phases are also given in the figure.