( 2019 2018 2017)
LASS carries out research in three areas: carbon based materials, computational material science and x-ray related methods. In the last year we have reached significant results in all of these fields.
Carbon based systems. — One of the unique applications of carbon nanotubes is their use as nanocontainers for various encapsulated species. We could succesfully encapsulate sexithiophene molecules in single-walled carbon nanotubes using two different methods: sublimation filling at 400 °C and nanoextraction from supercritical carbon dioxide at 50 °C. We observed thermally induced polymerization during sublimation filling, which increased the electronic conjugation length of the chains inside the nanotubes. Although the conjugation length in the thiophene chain is increasing upon heating, no Raman signatures of graphene nanoribbons are observed . These results complement our earlier studies, in which metal nanoclusters were encapsulated in the nanotubes.
An other type of the carbon based systems are fullerene cocrystals. Based on the analysis of the supramolecular interactions in several high-symmetry fullerene cocrystals, a consistent model has been worked out to predict the formation and stability of similar high symmetry molecular crystals . In one of the cocrystal built from metallofullerenes and highly energetic cubanes, an anomalous, irreversible, pressure-driven lattice expansion has been detected by applying high pressure. Based on detailed theoretical and experimental study of this system, a new strategy was suggested for the creation of composite materials with similar negative volume compressibilities with exciting potential applications in various fields .
Computational materials science. — In European collaboration, we investigated microstructure evolution during laser additive manufacturing (LAM) of hyperfine eutectic alloys in the Fe-Ti system, with the aim of designing materials for application in supercharger propeller (Bosch) and cloth cutting device (Procter and Gamble). Modelling of the microstructure was performed in the Wigner RCP using an orientation field-based phase field model, under heating and cooling conditions typical to LAM . A layered structure consisting of alternating thin globular layers and extended lamellar domains was observed in the experiments. Its origin was explained in terms of a repeated remelting, nucleation, and growth sequence taking place after adding a new layer during LAM, as indicated in Figs. 1 and 2.
Fig. 1: Snapshots of the composition map from the phase-field simulation of melting and solidification during a full heating/cooling cycle of laser additive manufacturing. Time elapses left to the right showing (a) the lamellar eutectic structure from the previous cycle and freshly melted powder on the top, (b) the remelting the topmost part of the lamellar structure during heating, (c) the epitaxial growth of the lamellae and the nucleation of the primary phase ahead and (d) the subsequent growth of elongated eutectic grains during cooling. At this point, the simulation domain is stepped up by half the sample height and a new heating/cooling cycle can be started with a configuration similar to (a).
Fig. 2: Composition map of the layered microstructure seen in LAM-produced eutectic alloys as predicted by the phase-field theory.
X-ray related methods. — Our research concentrated on the evaluation of Kossel patterns, which contain full crystallographic information on the sample, allowing unambiguous determination of the atomic structure. An autoindexing program was developed . It allows the automatic indexing of the Kossel lines without preliminary knowledge of the crystal lattice. This work is essential for single pulse imaging experiments planed at XFEL-s, which will facilitate the measurement of various short lived phases at extremely non-ambient conditions.
Carbon based systems. — One of the unique applications of carbon nanotubes is their use as nanocontainers for various encapsulated species. Nanoscale metal clusters present a special perspective in this regard as the tubes give both a natural constraint and an effective protection from the environment. Nickel nanoclusters grown inside single-walled carbon nanotubes were studied by infrared scattering-type scanning near-field optical microscopy (s-SNOM). The metal clusters give high local contrast enhancement in near-field phase maps caused by the excitation of free charge carriers. The experimental results are supported by calculations using the finite dipole model, approximating the clusters with elliptical nanoparticles. Compared to magnetic force microscopy, s-SNOM appears much more sensitive to detect metal clusters inside carbon nanotubes. The number of Ni atoms in these clusters were estimated to be less than 700 .
An other type of the carbon based systems are the metal organic frameworks (MOF). As a potential precursor of molecular pumps, a new MOF containing rotating cubylene units was prepared . Its high symmetry structure is an average of several lower-symmetry unit cells. The resulted crystal structure with rotating linkers has special dynamics that is called rotor-stator framework. Another member of cubane containing MOF family containing paddlewheel secondary building units has also been synthetized and totally characterized.
Computational material science. — Melt flow induced by gravity or other external fields can have a significant influence on the microstructures formed during solidification. To account for these effects, we have developed a new model, which combines the phase-field and lattice-Boltzmann methods. Mobile particles were handled by an overlapping multigrid scheme, in which each individual particle has its own moving grid with local fields attached to it. Using this approach, we were able to simulate simultaneous binary solidification, solute diffusion, melt flow, solid motion, the effect of gravity, and collision of the particles. The method has been applied for describing the columnar to equiaxed transition in the binary Al–Ti system (Fig. 1) . For more information please visit our homepage at https://phasefield.hu.
Figure 1. Snapshots of the velocity (upper row) and concentration fields (bottom row) during the columnar to equiaxed transition. The simulation shows that the growth of the columnar dendrites is blocked by the equiaxed dendrites nucleated in and falling from the upper part of the simulation domain.
X-ray related methods. — Our research concentrated on X-ray free electron laser (XFEL) related experiments. We carried out an X-ray holographic experiment at the European Synchrotron Radiation Facility (ESRF), which serves as a prototype for single pulse structure determination at XFEL-s. We showed that atomic resolution structural information can be obtained within 1 second at ESRF . Since the number of photons in the probe beam at ESRF is about the same as during a single pulse at an XFEL, this experiment indicates that it will be possible to determine the atomic structure by x-ray holography during a few femtoseconds at an XFEL. This will facilitate the measurement of various short lived phases at extremely non-ambient conditions.
LASS carries out research in three areas: carbon based materials, computational material science and x-ray related methods. In the last year, we have reached significant results in all of these fields.
Carbon based systems. — As a new category of solids, crystalline materials constructed with amorphous building blocks expand the structure categorization of solids. New amorphous carbon clusters are found by compressing C8H8/C60 cocrystals, in which the highly energetic cubane (C8H8) exhibits unusual role. The significant role of C8H8 is to stabilize the boundary interactions of the highly compressed or collapsed C60 clusters, which preserve their long‐range ordered arrangement up to 45 GPa. With increasing time and pressure, the gradual random bonding between C8H8 and carbon clusters, -due to “energy release” of highly compressed cubane, leads to the loss of the ability of C8H8 to stabilize the carbon cluster arrangement. Thus, a transition from short‐range disorder to long‐range disorder (amorphization) occurs. The spontaneous bonding reconstruction most likely results in a 3D network in the material, which can create ring cracks on diamond anvils.
Figure 1. (a) X-ray diffraction pattern of 3 particles, 2 Ar clusters and a Mimi virus. The diffraction pattern can be analogously analized as a hologram. (b) The icosahedral envelop of the mimi virus and its 2D SEM projection. (c) Enlarged part of (a) showing two fine line patterns, corresponding to the 3 particles in the beam.
Computational material science. — During biomineralization, such as during the formation of bones, teeth, or mollusc shells and coral skeletons, hierarchically structured organic-inorganic composites of unique properties form, where the unique properties originate from their microstructure. We used traditional computational materials science tools to model the formation of these complex microstructures. In collaboration with German experimental scientists, we have reported a spectacular agreement between the microstructure of mollusc shells determined by experimental methods (including electron microscopy, electron back-scattering diffraction (EBSD), and X-ray micro-tomography) and the microstructure predicted by the phase-field theory. Such a complex structure was addressed for the first time by phase-field modelling, which may open the way for the modelling of even more complex biomineralization processes.
X-ray related methods. — Our research concentrated on X-ray free-electron laser (XFEL) related experiments. We took part in a holographic experiment on free-flying nanoparticles. We showed that structural information of individual nanoparticles could be obtained using a single XFEL pulse only.
Further, we gave general guidelines for the treatment of experimental data from coherent diffractive imaging and incorporated particle symmetry into the orientation determination in single-particle imaging (Fig. 1).
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 became the center of intensive research. Earlier we concentrated on fullerenes and related compounds. Recently, metal organic framework materials (MOF), carbon nanotubes and nanotube-based hybrid systems are our center of interest.
Figure 1. Spatial distribution of the boron nitride phonon-polariton mode (left) and the infrared-active defect mode (right) in a multiwall boron nitride nanotube.
Metal-organic frameworks. — Metal organic frameworks are coordination polymers with high porosity. These crystalline, high-symmetry materials consist of metal-containing nodes and rigid organic linkers. Formerly, we developed a new MOF family with Zn-based secondary building units (SBUs) and 1,4-cubanedicarboxylate linkers. This year, we started a new family of MOF-s with the same Zn-containing nodes, but with a previously unknown spiroheptane-dicarboxylate linker. We prepared the organic precursor in a racemic form and demonstrated the formation of the new MOFs. We made ready the raw material for chromatographic separation of the enantiomers. The goal of the experiment is the preparation of chiral MOFs, suitable for separation of various racemic mixtures.
Infrared spectroscopy on carbon based systems. — We changed the focus of our research to include two new topics: near-field infrared spectroscopy and microscopy of various nanotubes and optoelectronic properties of organic perovskite-based solar cell materials. We succeeded in determining the semiconducting or metallic character of individual carbon nanotubes below 10 nm in diameter. In the case of boron nitride nanotubes, we mapped the defect distribution with a spatial resolution of a few nanometers (Fig. 1).
We combined methylamine lead iodide perovskite with carbon nanotubes to obtain hybrid structures for possible application as photovoltaic devices. We proved the charge transfer from the perovskite to the carbon nanotube layer upon illumination. These observations may lead to new solar cells with the perovskite as active layer and the carbon nanotubes as hole-transporting layer.
Theory of phase transformations
Hydrodynamic theory of freezing – Nucleation and polycrystalline growth. — Structural aspects of crystal nucleation in undercooled liquids are explored using a nonlinear hydrodynamic theory of crystallization we proposed recently, which is based on combining fluctuating hydrodynamics with the phase-field crystal (PFC) theory. We have shown that in our hydrodynamic approach not only homogeneous and heterogeneous nucleation processes are accessible, but also growth front nucleation, which leads to the formation of new (differently oriented) grains at the solid-liquid front in highly undercooled systems. Formation of dislocations at the solid-liquid interface and interference of density waves ahead of the crystallization front are responsible for the appearance of the new orientations at the growth front that lead to spherulite-like nanostructures (Fig. 2).
Figure 2. Polycrystalline growth in the hydrodynamic model of freezing. Snapshots of the orientation field (upper row), the Voronoi map (bottom row), and coarse-grained density (bottom row central panel: lighter colour denotes higher density) taken at dimensionless times t = 900, 2100, 2900, 3400, and 3900 are shown. Note the spatial variation of the orientation field due to the dislocations shown as red-blue pairs of dots (atoms of 7 and 5 neighbours) in the Voronoi map, and the small crystallite formed close to the interface in the 4th panel from the left. This indicates two mechanisms for growth front nucleation: (i) nucleation of dislocations at the interface, and (ii) crystal nucleation ahead of the growth front.
Grain coarsening in two-dimensional phase-field models with orientation field. — Contradictory results were published regarding the form of the long-time grain size distribution (LGSD) that characterizes grain coarsening in two-dimensional systems: While experiments and the PFC model indicate a log-normal distribution, other works including studies based on phase-field simulations that rely on coarse-grained fields, like the multi-phase-field and orientation field (OF) models, yield significantly different distributions. We investigated this problem, and demonstrated for the OF models that an insufficient resolution of the small-angle grain boundaries leads to a log-normal distribution close to those seen in the experiments. Our work also indicates that the LGSD is critically sensitive to the details of the evaluation, and raises the possibility that the differences among the LGSD results from different sources originate from differences in the detection of small-angle grain boundaries.
Topological defects in two-dimensional orientation-field models. — In 2D, a continuous scalar field is used to represent crystallographic orientation. The respective order parameter space is the unit circle, which is not simply connected. This property has important consequences for the multigrain structures: (i) trijunctions may be singular; (ii) for each pair of grains, there exist two different interfacial solutions that cannot be continuously transformed to each another; (iii) if both solutions appear along a grain boundary, a topologically stable singular point defect forms between them. While (i) can be interpreted in the classical picture of grain boundaries, (ii) and, therefore, (iii) cannot. To overcome these problems, we proposed two solutions. The first is based on a three-component unit vector field, while in the second we utilize a two-component vector field with an additional potential. In both cases, the additional degree of freedom makes the order parameter space simply connected, which removes the topological stability of these defect.
We have continued our studies on structure determination by inside x-ray sources. We have carried out a series of experiments at ESRF, and measured atomic resolution holograms and also Kossel line patterns. The atoms which we used as point sources were exited by a very intense, synchrotron-generated focused X-ray beam. The diffraction patterns and the holograms were detected by a new 2D position sensitive detector allowing the collection of higher quality data then in earlier measurements. The evaluation of the data is under way. This type of measurements open the way to single-pulse structure determination at X-ray free-electron lasers.