Laser-assisted field evaporation is studied in a large number of compounds, including amorphous and crystalline phase change materials employing atom probe tomography. This study reveals significant differences in field evaporation between amorphous and crystalline phase change materials. High probabilities for multiple events with more than a single ion detected per laser pulse are only found for crystalline phase change materials. The specifics of this unusual field evaporation are unlike any other mechanism shown previously to lead to high probabilities of multiple events. On the contrary, amorphous phase change materials as well as other covalently bonded compounds and metals possess much lower probabilities for multiple events. Hence, laser-assisted field evaporation in amorphous and crystalline phase change materials reveals striking differences in bond rupture. This is indicative for pronounced differences in bonding. These findings imply that the bonding mechanism in crystalline phase change materials differs substantially from conventional bonding mechanisms such as metallic, ionic, and covalent bonding. Instead, the data reported here confirm a recently developed conjecture, namely that metavalent bonding is a novel bonding mechanism besides those mentioned previously.

Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphous-to-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular Beam Epitaxy, thermal annealing and application of femtosecond laser pulses. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows.

Carbon nanothreads are likely the most attracting new materials produced under high pressure conditions. Their synthesis is achieved by compressing crystals of different small aromatic molecules also exploiting the applied...

Dislocations have been considered to be an efficient source for scattering midfrequency phonons, contributing to the enhancement of thermoelectric performance. The structure of dislocations can be resolved by electron microscopy whereas their chemical composition and decoration state are scarcely known. Here, we correlate transmission Kikuchi diffraction and (scanning) transmission electron microscopy in conjunction with atom probe tomography to investigate the local structure and chemical composition of dislocations in a thermoelectric Ag-doped PbTe compound. Our investigations indicate that Ag atoms segregate to dislocations with a 10-fold excess of Ag compared with its average concentration in the matrix. Yet the Ag concentration along the dislocation line is not constant but fluctuates from ∼0.8 to ∼10 atom % with a period of about 5 nm. Thermal conductivity is evaluated applying laser flash analysis, and is correlated with theoretical calculations based on the Debye-Callaway model, demonstrating that these Ag-decorated dislocations yield stronger phonon scatterings. These findings reduce the knowledge gap regarding the composition of dislocations needed for theoretical calculations of phonon scattering and pave the way for extending the concept of defect engineering to thermoelectric materials.

A hexagonal phase in the ternary Ge-Se-Te system with an approximate composition of GeSe Te has been known since the 1960s but its structure has remained unknown. We have succeeded in growing single crystals by chemical transport as a prerequisite to solve and refine the Ge Se Te structure. It consists of layers that are held together by van der Waals type weak chalcogenide-chalcogenide interactions but also display unexpected Ge-Ge contacts, as confirmed by electron microscopy analysis. The nature of the electronic structure of Ge Se Te was characterized by chemical bonding analysis, in particular by the newly introduced density of energy (DOE) function. The Ge-Ge bonding interactions serve to hold electrons that would otherwise go into antibonding Ge-Te contacts.

Liquid-phase exfoliation is the most suitable platform for large-scale production of two-dimensional materials. One of the main open challenges is related to the quest of green and bioderived solvents to replace state-of-the-art dispersion media, which suffer several toxicity issues. Here, we demonstrate the suitability of methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate (Rhodiasolv Polarclean) for sonication-assisted liquid-phase exfoliation of layered materials for the case-study examples of WS 2 , MoS 2 , and graphene. We performed a direct comparison, in the same processing conditions, with liquid-phase exfoliation using N -methyl-2-pyrrolidone (NMP) solvent. The amount of few-layer flakes (with thickness <5 nm) obtained with Polarclean is increased by ∼350% with respect to the case of liquid-phase exfoliation using NMP, maintaining comparable values of the average lateral size, which even reaches ∼10 μm for the case of graphene produced by exfoliation in Polarclean, and of the yield (∼40%). Correspondingly, the density of defects is reduced by 1 order of magnitude by Polarclean-assisted exfoliation, as evidenced by the I (D)/ I (G) ratio in Raman spectra of graphene as low as 0.07 ± 0.01. Considering the various advantages of Polarclean over state-of-the-art solvents, including the absence of toxicity and its biodegradability, the validation of superior performances of Polarclean in liquid-phase exfoliation paves the way for sustainable large-scale production of nanosheets of layered materials and for extending their use in application fields to date inhibited by toxicity of solvents (e.g., agri-food industry and desalination), with a subsequent superb impact on the commercial potential of their technological applications.

We report first-principles GW results on the dispersion of the bulk band-gap edges in the three-dimensional topological insulator Sb 2 Te 3 . We find that, independently of the reference density-functional-theory band structure and the crystal-lattice parameters used, the one-shot GW corrections enlarge the fundamental band gap, bringing its value in close agreement with experiment. We conclude that the GW corrections cause the displacement of the valence-band maximum (VBM) to the point, ensuring that the surface-state Dirac point lies above the VBM. We extend our study to the analysis of the electron-energy-loss spectrum (EELS) of bulk Sb 2 Te 3 . In particular, we perform energy-filtered transmission electron microscopy and reflection EELS measurements. We show that the random-phase approximation with the GW quasiparticle energies and taking into account virtual excitations from the semicore states leads to good agreement with our experimental data.

The burgeoning interest in two-dimensional (2D) black phosphorus (bP) contributes to the expansion of its applications in numerous fields. In the present study, 2D bP is used as a support for homogeneously dispersed palladium nanoparticles directly grown on it by a wet chemical process. Electron energy loss spectroscopy–scanning transmission electron microscopy analysis evidences a strong interaction between palladium and P atoms of the bP nanosheets. A quantitative evaluation of this interaction comes from the X-ray absorption spectroscopy measurements that show a very short Pd–P distance of 2.26 Å, proving for the first time the existence of an unprecedented Pd–P coordination bond of a covalent nature. Additionally, the average Pd–P coordination number of about 1.7 reveals that bP acts as a polydentate phosphine ligand toward the surface of the Pd atoms of the nanoparticles, thus preventing their agglomeration and inferring with structural stability. These unique properties result in a superior performance in the catalytic hydrogenation of chloronitroarenes to chloroanilines, where a higher chemoselectivity in comparison to other heterogeneous catalyst based on palladium has been observed.