Atomic vibrations control all thermally activated processes in materials including ionic, atomic and electron diffusion, heat transport, phase transformations and surface chemical reactions. The jump frequency characterizing thermally activated processes is of great practical importance and is determined by the local phonon and molecular vibrational modes of the system. Atomic and molecular heterogeneities and defects such as vacancies, interstitials, dislocations and grain boundaries often regulate kinetic pathways and are associated with vibrational modes which are substantially different from bulk modes. High spatial resolution vibrational spectroscopy is required to probe these defect modes.Recent developments in aberration corrected, monochromated, scanning transmission electron microscopy (STEM) have enabled nanoscale probing of vibrational modes via electron energy-loss spectroscopy (EELS) 1,2 . Nanoscale vibrational spectroscopy is already impacting a wide range of important scientific problems such as measurement of surface and bulk vibrational excitations in MgO nanocubes 3 , probing hyperbolic phonon polaritons in nanoflakes of hBN 4 , measuring temperature in nanometer-sized areas with 1°K precision 5,6 and determining phonon dispersion in nanoparticles 7 . The delocalized nature of certain vibrational signals allows damagefree nanoscale detection for a variety of organic and inorganic material-systems 8-11 . This progress has been impressive, however, to date there have been no experimental methods to spectroscopically probe individual vibrational modes in materials with atomic resolution. Theoretical treatments have explored the question of spatial resolution 12,13 with some treatments suggesting that atomic resolution vibrational EELS should be possible [14][15][16] . Here we demonstrate atomic resolution vibrational spectroscopy in STEM for signals predominantly excited by impact scattering. The resulting order of magnitude advance in spatial resolution will

Palladium is one of the few metals capable of forming hydrides, with the catalytic properties being dependent on the elemental composition and spatial distribution of H atoms in the lattice. Herein, we report a facile method for the complete transformation of Pd nanocubes into a stable phase made of PdH0.706 by treating them with aqueous hydrazine at a concentration as low as 9.2 mM. Using formic acid oxidation (FAO) as a model reaction, we systematically investigated the structure–catalytic property relationship of the resultant nanocubes with different degrees of hydride formation. The current density at 0.4 V was enhanced by four times when the nanocubes were completely converted from Pd to PdH0.706. On the basis of a set of slab models with PdH(100) overlayers on Pd(100), we conducted density functional theory calculations to demonstrate that the degree of hybrid formation could influence both the activity and selectivity toward FAO by modulating the relative stability of formate (HCOO) and carboxyl (COOH) intermediates. This work provides a viable strategy for augmenting the performance of Pd-based catalysts toward various reactions without altering the loading of this scarce metal.

High-resolution monochromated electron energy-loss spectroscopy has the potential to map vibrational modes at nanometer resolution. Using the SiO2/Si interface as a test case, we observe an initial drop in the SiO2 vibrational signal when the electron probe is 200 nm from the Si due to long-range nature of the Coulomb interaction. However, the distance from the interface at which the SiO2 integrated signal intensity drops to half its maximum value is 5 nm. We show that nanometer resolution is possible when selecting the SiO2/Si interface signal which is at a different energy position than the bulk signal. Calculations also show that, at 60 kV, the signal in the SiO2 can be treated non-relativistically (no retardation) while the signal in the Si, not surprisingly, is dominated by relativistic effects. For typical transmission electron microscope specimen thicknesses, surface coupling effects must also be considered.

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Specially designed instrumentation for electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) makes it possible to probe very low-loss excitations in matter with a focused electron beam. Here we study the nanoscale interaction of fast electrons with optical phonon modes in silica. In particular we analyze the spatial dependence of EEL spectra in two geometrical arrangements: a free-standing truncated slab of silica and a slab with a junction between silica and silicon. In both cases, we identify different loss channels, involving polaritonic and non-polaritonic contributions to the total electron energy loss, and obtain the corresponding energyfiltered maps. Furthermore, we present a comparison of the theoretical simulations for a silica-silicon junction with experimental results, and discuss the spatial resolution attainable from the energyfiltered map considering optical phonon excitations in a conventional experimental arrangement.

SUMMARYNumerical simulations of a heaving airfoil undergoing non-sinusoidal motions in an incompressible viscous ow is presented. In particular, asymmetric sinusoidal motions, constant heave rate oscillations, and sinusoidal motions with a quiescent gap, are considered. The wake patterns, thrust force coe cients, and propulsive e ciency at various values of non-dimensional heave velocity are computed. These have been compared with those of corresponding sinusoidal heaving motions of the airfoil. It is shown that for a given non-dimensional heave velocity and reduced frequency of oscillation, asymmetric sinusoidal motions give better thrust and propulsive e ciencies in comparison to pure harmonic motion. On the other hand, constant rate heave motion do not compare favourably with harmonic motion. A train of sinusoidal pulses separated by a quiescent gap compares favourably with a pure sinusoidal motion, but with the notable exception that the quiescent gap induces a discontinuity that induces large impulses to the wake pattern.

Abstract-Greedy algorithms for subgraph pattern matching operations are often sufficient when the graph data set can be held in memory on a single machine. However, as graph data sets increasingly expand and require external storage and partitioning across a cluster of machines, more sophisticated query optimization techniques become critical to avoid explosions in query latency. In this paper, we introduce several query optimization techniques for distributed graph pattern matching. These techniques include (1) a System-R style dynamic programming-based optimization algorithm that considers both linear and bushy plans, (2) a cycle detection-based algorithm that leverages cycles to reduce intermediate result set sizes, and (3) a computation reusing technique that eliminates redundant query execution and data transfer over the network. Experimental results show that these algorithms can lead to an order of magnitude improvement in query performance.