Making sense of nanocrystal lattice fringes

The orientation-dependence of thin-crystal lattice fringes can be gracefully quantified using fringe-visibility maps, a direct-space analog of Kikuchi maps. As in navigation of reciprocal space with the aid of Kikuchi lines, fringe-visibility maps facilitate acquisition of 3D crystallographic information in lattice images. In particular, these maps can help researchers to determine the 3D lattice parameters of individual nano-crystals, to ``fringe fingerprint'' collections of randomly-oriented particles, and to measure local specimen-thickness with only modest tilt. Since the number of fringes in an image increases with maximum spatial-frequency squared, these strategies (with help from more precise goniometers) will be more useful as aberration-correction moves resolutions into the subangstrom range.Comment: 12 pages, 15 figures, 2 tables, 60 refs, RevTex4, notes http://www.umsl.edu/~fraundor/help/imagnxtl.ht

Atomic mechanism of metal crystal nucleus formation in a single-walled carbon nanotube

Knowing how crystals nucleate at the atomic scale is crucial for understanding, and in turn controlling, the structure and properties of a wide variety of materials. However, because of the scale and highly dynamic nature of nuclei, the formation and early growth of nuclei are very difficult to observe. Here, we have employed single-walled carbon nanotubes as test tubes, and an ‘atomic injector’ coupled with aberration-corrected transmission electron microscopy, to enable in situ imaging of the initial steps of nucleation at the atomic scale. With three different metals we observed three main processes prior to heterogeneous nucleation: formation of crystal nuclei directly from an atomic seed (Fe), from a pre-existing amorphous nanocluster (Au) or by coalescence of two separate amorphous sub-nanometre clusters (Re). We demonstrate the roles of the amorphous precursors and the existence of an energy barrier before nuclei formation. In all three cases, crystal nucleus formation occurred through a two-step nucleation mechanism

Comparison of atomic scale dynamics for the middle and late transition metal nanocatalysts

Catalysis of chemical reactions by nanosized clusters of transition metals holds the key to the provision of sustainable energy and materials. However, the atomistic behaviour of nanocatalysts still remains largely unknown due to uncertainties associated with the highly labile metal nanoclusters changing their structure during the reaction. In this study, we reveal and explore reactions of nm-sized clusters of 14 technologically important metals in carbon nano test tubes using time-series imaging by atomically-resolved transmission electron microscopy (TEM), employing the electron beam simultaneously as an imaging tool and stimulus of the reactions. Defect formation in nanotubes and growth of new structures promoted by metal nanoclusters enable the ranking of the different metals both in order of their bonding with carbon and their catalytic activity, showing significant variation across the Periodic Table of Elements. Metal nanoclusters exhibit complex dynamics shedding light on atomistic workings of nanocatalysts, with key features mirroring heterogeneous catalysis

Atomic-Scale Time-Resolved Imaging of Krypton Dimers, Chains and Transition to a One-Dimensional Gas

Single-atom dynamics of noble-gas elements have been investigated using time-resolved transmission electron microscopy (TEM), with direct observation providing for a deeper understanding of chemical bonding, reactivity, and states of matter at the nanoscale. We report on a nanoscale system consisting of endohedral fullerenes encapsulated within single-walled carbon nanotubes ((Kr@C60)@SWCNT), capable of the delivery and release of krypton atoms on-demand, via coalescence of host fullerene cages under the action of the electron beam (in situ) or heat (ex situ). The state and dynamics of Kr atoms were investigated by energy dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS). Kr atom positions were measured precisely using aberration-corrected high-resolution TEM (AC-HRTEM), aberration-corrected scanning TEM (AC-STEM), and single-atom spectroscopic imaging (STEM-EELS). The electron beam drove the formation of 2Kr@C120 capsules, in which van der Waals Kr2 and transient covalent [Kr2]+ bonding states were identified. Thermal coalescence led to the formation of longer coalesced nested nanotubes containing more loosely bound Krn chains (n = 3–6). In some instances, delocalization of Kr atomic positions was confirmed by STEM analysis as the transition to a one-dimensional (1D) gas, as Kr atoms were constrained to only one degree of translational freedom within long, well-annealed, nested nanotubes. Such nested nanotube structures were investigated by Raman spectroscopy. This material represents a highly compressed and dimensionally constrained 1D gas stable under ambient conditions. Direct atomic-scale imaging has revealed elusive bonding states and a previously unseen 1D gaseous state of matter of this noble gas element, demonstrating TEM to be a powerful tool in the discovery of chemistry at the single-atom level

Testing various facets of the equivalence principle using lunar laser ranging

Abstract More than 42 years of lunar laser ranging provides an excellent basis for the determination of various parameters of the Earth-Moon system and for tests related to gravitational physics. In this paper, we focus on tests of the equivalence principle with the Earth and Moon as test bodies in the gravitational field of the Sun as well as a test of a possible violation of the equivalence principle as a result of the coupling of the galactic dark matter with the ordinary matter of the Earth and the Moon. Tests were carried out for three different data sets with observations from 1969 to 2011, 1986 to 2011, and one set with data only from the Apache Point Observatory. No significant deviation from the predictions of general relativity was found within the reached accuracy of 3.6 × 10 −4 for the Nordtvedt parameter η and 1.6 × 10 −13 for the mass ratio (m g /m i ) EM of gravitational and inertial masses between the Earth and the Moon. PACS numbers: 95.55. Pe, 04.80.Cc, 95.35.+

Lunar laser ranging test of the Nordtvedt parameter and a possible variation in the gravitational constant

Context. Forty years of lunar laser ranging (LLR) data provide an excellent basis to determine various parameters of the Earth-Moon system as well as parameters related to gravitational physics. Aims. We update the Institut für Erdmessung (IfE) LLR model taking the effect of a fluid lunar core into consideration. The temporal variation in the gravitational constant Ġ/G0 and the strong equivalence principle, parameterized by the Nordtvedt parameter η, are investigated. Methods. A set of LLR observations from 1970 to 2009 was analysed and the parameters were determined by a least squares adjustment in two runs. After solving for classical Newtonian parameters (e.g. initial conditions for the lunar orbit and rotation) in the first run, relativistic parameters were determined in the second run. Results. The upper limits to the gravitational constant and the Nordtvedt parameter were found to be Ġ/G0 = (−0.7 ± 3.8) × 10-13  yr-1 and η = (−0.6 ± 5.2) × 10-4