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A quantitative comparison between experiment and theory is given for the constant height mode imaging of metal nanoclusters in dynamic scanning force microscopy. We explain the fundamental mechanisms in the contrast formation with the help of the system Pd/MgO(001). The comparison shows that the shape and size of nanoclusters are precisely imaged due to the sharpness of the tip’s last nanometer. This quantitative comparison proves our previously proposed model for the contrast formation. [ABSTRACT FROM AUTHOR]

Abstract: We have studied the elongation of Au nanoclusters embedded in amorphous SiO2 using MD simulations. The effect of swift heavy ions (SHI) was implemented using instantaneous energy deposition with a radial profile that was obtained from the inelastic thermal spike model. During the first impact on the cluster, the clusters (d =9–11nm) gained about 20% in length due to thermal stress within the cluster, which is released by a longitudinal flow of Au atoms to the ion track in silica. Our simulations also show that high temperatures at the track core lead to the formation of silica vapor, whose pressure may flatten the cluster. [Copyright &y& Elsevier]

Understanding the way in which ice forms is of great importance to many fields of science. Pure water droplets in the atmosphere can remain in the liquid phase to nearly -40 ºC. Crystallization of ice in the atmosphere therefore typically occurs in the presence of ice nucleating particles (INPs), such as mineral dust or organic particles, which trigger heterogeneous ice nucleation at clearly higher temperatures. Such active INPs can also be used for rain enhancement. Experiments have shown in great detail what is the IN activity of different types of compounds, and recently also clarified the importance of small surface features such as surface defects. The molecular-scale processes responsible for ice nucleation are still not well known, however. In recent years, several computational studies have advanced our understanding of the details of ice nucleation in many materials, and also the role of defects. Recently simulations showed enhanced ice nucleation efficiency in confined geometry such as wedges or pits (Bi, Cao and Li, 2017). We are studying these topics by utilizing the monatomic water model (Molinero and Moore, 2009) for unbiased molecular dynamics (MD) simulations, where a system including a defected surface, such as pyramidal pits, steps or surface cracks, immersed in water, is cooled continuously below the melting point over tens of nanoseconds of simulation time and crystallization is followed.Results of simulations on pyramidal pits on Si (100) surfaces, an experimentally realizable system, show a clear (ΔT > 10 ºC) enhancement of ice nucleation compared to flat Si (100) or Si (111) surfaces, in agreement with initial experimental findings of preference of ice to nucleate at these sites. Understanding the enhanced activity in such confined geometry may lead to characterization of active sites on some ice nucleating materials, or lead to designs of new artificial materials optimized for cloud seeding applications.With a combination of finite difference calculations and molecular dynamics simulations we also show that the release of latent heat from ice growth has a noticeable effect on both the ice growth rate and the initial structure of the forming ice. However, latent heat is found not to be as critically important in controlling immersion nucleation as it is in vapor-to-liquid nucleation [Tanaka et al. 2017].This work was supported by the National Center for Meteorology (NCM), Abu Dhabi, UAE, under the UAE Research Program for Rain Enhancement Science, by the Academy of Finland Center of Excellence programme (grant no. 307331) and ARKTIKO project 285067 ICINA, by University of Helsinki, Faculty of Science ATMATH project, as well as ERC Grant 692891-DAMOCLES. Supercomputing resources were provided by CSC–IT Center for Science, Ltd, Finland.ReferencesBi, Y., B. Cao and T. Li (2017). Nat. Commun. 8, 15372. Molinero, V. and E. B. Moore (2009). J. Phys. Chem. B 113, 4008. Tanaka, K. K et al. (2017). Phys. Rev. E 96, 022804. [ABSTRACT FROM AUTHOR]

The structure and ion-conducting properties of the defect-fluorite ring structure formed around amorphous ion-tracks by swift heavy ion irradiation of Gd2Ti2O7 pyrochlore are investigated. High angle annular dark field imaging complemented with ion-track molecular dynamics simulations show that the atoms in the ring structure are disordered, and have relatively larger cation-cation interspacing than in the bulk pyrochlore, illustrating the presence of tensile strain in the ring region. Density functional theory calculations show that the non-equilibrium defect-fluorite structure can be stabilized by tensile strain. The pyrochlore to defect-fluorite structure transformation in the ring region is predicted to be induced by recrystallization during a melt-quench process and stabilized by tensile strain. Static pair-potential calculations show that planar tensile strain lowers oxygen vacancy migration barriers in pyrochlores, in agreement with recent studies on fluorite and perovskite materials. In view of these results, it is suggested that strain engineering could be simultaneously used to stabilize the defect-fluorite structure and gain control over its high ion-conducting properties. [ABSTRACT FROM AUTHOR]

While the interaction of energetic ions with solids is well known to result in inelastic energy loss to electrons and elastic energy loss to atomic nuclei in the solid, the coupled effects of these energy losses on defect production, nanostructure evolution and phase transformations in ionic and covalently bonded materials are complex and not well understood due to dependencies on electron-electron scattering processes, electron-phonon coupling, localized electronic excitations, diffusivity of charged defects, and solid-state radiolysis. Here we show that a colossal synergy occurs between inelastic energy loss and pre-existing atomic defects created by elastic energy loss in single crystal strontium titanate (SrTiO3), resulting in the formation of nanometer-sized amorphous tracks, but only in the narrow region with pre-existing defects. These defects locally decrease the electronic and atomic thermal conductivities and increase electron-phonon coupling, which locally increase the intensity of the thermal spike for each ion. This work identifies a major gap in understanding on the role of defects in electronic energy dissipation and electron-phonon coupling; it also provides insights for creating novel interfaces and nanostructures to functionalize thin film structures, including tunable electronic, ionic, magnetic and optical properties. [ABSTRACT FROM AUTHOR]

While the interaction of energetic ions with solids is well known to result in inelastic energy loss to electrons and elastic energy loss to atomic nuclei in the solid, the coupled effects of these energy losses on defect production, nanostructure evolution and phase transformations in ionic and covalently bonded materials are complex and not well understood due to dependencies on electron-electron scattering processes, electron-phonon coupling, localized electronic excitations, diffusivity of charged defects, and solid-state radiolysis. Here we show that a colossal synergy occurs between inelastic energy loss and pre-existing atomic defects created by elastic energy loss in single crystal strontium titanate (SrTiO3), resulting in the formation of nanometer-sized amorphous tracks, but only in the narrow region with pre-existing defects. These defects locally decrease the electronic and atomic thermal conductivities and increase electron-phonon coupling, which locally increase the intensity of the thermal spike for each ion. This work identifies a major gap in understanding on the role of defects in electronic energy dissipation and electron-phonon coupling; it also provides insights for creating novel interfaces and nanostructures to functionalize thin film structures, including tunable electronic, ionic, magnetic and optical properties. [ABSTRACT FROM AUTHOR]

Abstract: Ion irradiation of α-quartz renders the crystal SiO2 structure amorphous. The enormous amount of structural defects produced after ion irradiation give a chance for photoactive intrinsic defects to be formed. These may be responsible for the photoluminescence in irradiated α-quartz. On the other hand, the radiation defects are not stable, and thus, an alternative structure where the defects of interest can be stabilized is required. The stabilization of the defects can be achieved in the structures of amorphous silica with embedded Si nanocrystals (NC), thanks to the unique structure of the formed interface. By means of Molecular Dynamics (MD), we analyze defects in both amorphized α-quartz and Si-NC/a-SiO2 interfaces formed by 1.1, 2.4 and 4nm diameter NC’s. In the simulation, we employ a classical interatomic potential and a potential, which takes into consideration a charge transfer between Si and O atoms. We show that although the number of silanone bonds Sidiated quartz is higher, they are also found in a Si-NC/a-SiO2 interface without the necessity of preceding irradiation of the sample. We also compare the defects in irradiation-amorphized quartz and the three sizes of Si-NC/a-SiO2 interfaces. Analysis of the charges showed that the charge state of coordination defects depends on the type of atoms in the near neighborhood. [ABSTRACT FROM AUTHOR]

The structure and ion-conducting properties of the defect-fluorite ring structure formed around amorphous ion-tracks by swift heavy ion irradiation of Gd2Ti2O7 pyrochlore are investigated. High angle annular dark field imaging complemented with ion-track molecular dynamics simulations show that the atoms in the ring structure are disordered, and have relatively larger cation-cation interspacing than in the bulk pyrochlore, illustrating the presence of tensile strain in the ring region. Density functional theory calculations show that the non-equilibrium defect-fluorite structure can be stabilized by tensile strain. The pyrochlore to defect-fluorite structure transformation in the ring region is predicted to be induced by recrystallization during a melt-quench process and stabilized by tensile strain. Static pair-potential calculations show that planar tensile strain lowers oxygen vacancy migration barriers in pyrochlores, in agreement with recent studies on fluorite and perovskite materials. In view of these results, it is suggested that strain engineering could be simultaneously used to stabilize the defect-fluorite structure and gain control over its high ion-conducting properties. [ABSTRACT FROM AUTHOR]

For the first time, high quality images of metal nanoclusters which were recorded in the constant height mode of a dynamic scanning force microscope (dynamic SFM) are shown. Surfaces of highly ordered pyrolytic graphite (HOPG) were used as a test substrate since metal nanoclusters with well defined and symmetric shapes can be created by epitaxial growth. We performed imaging of gold clusters with sizes between 5 and 15 nm in both scanning modes, constant Δf mode and constant height mode, and compared the image contrast. We notice that clusters in constant height images appear much sharper, and exhibit more reasonable lateral shapes and sizes in comparison to images recorded in the constant Δf mode. With the help of numerical simulations we show that only a microscopically small part of the tip apex (nanotip) is probably the main contributor for the image contrast formation. In principle, the constant height mode can be used for imaging surfaces of any material, e.g. ionic crystals, as shown for the system Au/NaCl(001).