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Depth profiles of 1H and 2D in rapidly grown KD2xH2(1−x)PO4 single crystals are studied by elastic recoil detection analysis. Results show that, under ambient conditions, deuteration in the first ∼500 nm from the sample surface significantly decreases within the first several days after D2O surface polishing. This effect is attributed to deuterium–hydrogen exchange. The effective diffusion coefficient of this process is strongly dependent on both the degree of deuteration and the sample growth conditions. Physical mechanisms of D/H exchange are discussed.

The intense, ultrafast electronic excitation of clean silicon (100)–(2×1) surfaces leads to the formation of silicon nanostructures embedded in silicon, which photoluminescence at ∼560 nm wavelength (∼2 eV band gap). The silicon surfaces were irradiated with slow, highly charged ions (e.g., Xe44+ and Au53+) to produce the electronic excitation. The observation of excitonic features in the luminescence is particularly unusual for silicon nanostructures. The temperature dependence and the measurement of the triplet–singlet splitting of the emission strongly support the excitonic assignment.

The ‘‘opening’’ of C60 on Si(111)‐(7×7) has been directly observed by scanning tunneling microscopy and correlated with Auger electron spectroscopy (AES) and temperature‐programmed desorption spectroscopy. Isolated C60 clusters are observed on Si(111)‐(7×7) after annealing the surface to 620 K. Annealing the surface to 1020 K causes the C60 cages to open and thereby cover more of the surface. Also evident is that the opened clusters agglomerate on the surface. The opening of the C60 cage is correlated with an increase in the amount of silicon—carbon bonding and with the increase in the carbon‐to‐silicon AES peak height ratio with increasing annealing temperature. The ratio increases since the opened cages cover more of the substrate silicon atoms, reducing AES emission from the substrate.

We observe a strong twin-orientation dependent hardness and strain rate sensitivity (m) in nanotwin copper. A highest m value of 0.059 ± 0.004 and an activation volume (V) of ∼10b3 are measured when deformation is predominately vertical to twin boundaries (TBs), whereas a much smaller m and larger V are observed for the direction parallel to TBs. Dislocation density is found to have a stronger impact on m and V in nanotwin materials, compared to that in coarse-grained materials.

The relation between stress evolution and surface morphology during deposition of sputtered films is examined by combining kinetic Monte Carlo simulations and stress measurements. We find that the surface morphology is susceptible to an instability, which transforms from layer-by-layer growth to the formation of pillarlike columns. The gaps between these columns prevent complete densification and can lead to a network of pores in the layer. We propose that the formation of this structure changes the stress in the growing layers from compressive to tensile.

We describe the fabrication of ultralow-density carbon nanotube (CNT) foams that simultaneously exhibit high electrical conductivities and robust mechanical properties. Our approach utilizes carbon nanoparticles as a binder to crosslink randomly oriented bundles of single-walled CNTs. The resulting CNT foams are the stiffest low-density nanoporous solids reported and exhibit elastic behavior up to strains as large as ∼80%. The use of the carbon binder also allows bulk electrical conductivity to be maintained at low densities.

We identify vapor-etched grain boundary grooves on the solid-vapor interface as the main source of surface roughness in the deuterium-tritium (D–T) fuel layers, which are solidified and then cooled. Current inertial confinement fusion target designs impose stringent limits to the cross-sectional area and total volume of these grooves. Formation of these grain boundaries occurs over time scales of hours as the dislocation network anneals and is inevitable in a plastically deformed material. Therefore, either cooling on a much shorter time scale or a technique that requires no cooling after solidification should be used to minimize the roughness.

We employ molecular dynamics simulations of defect-free nanopillars with realistic cylindrical geometries to obtain an atomic-level picture of their deformation behavior under compression. We find that dislocations are nucleated in the two outermost surface layers. Furthermore, plastic yield depends crucially on the particular arrangement of steps and facets at the surface of the nanopillars. We show that different facet orientations can differ dramatically in their response to external stresses. Freestanding nanopillars exhibit a highly nonuniform distribution of stresses along their height. This causes an elastic deformation that leads to a barrel-like shape attained by the nanopillars under compression. The stress concentration at the center of the pillars due to barreling causes dislocations to preferentially nucleate in this region.

The plastic deformation in copper-zirconium nanocrystalline-amorphous nanolaminates is investigated by means of stress-relaxation experiments at a range of initial stress levels. Progressive multistep relaxation cycles reveal that the onset of plastic deformation occurs at a much lower stress level in nanocrystalline-amorphous nanolaminate than in crystalline-crystalline nanolaminates or other nanostructured materials. The derived activation volumes and strain rate sensitivities imply interfacial dislocation mechanisms, consistent with the observations from postmorterm transmission electron microscopy. This indicates that the crystalline-amorphous interfaces may be the preferred source for dislocation nucleation and/or emission.

Although Er-doped Ge nanomaterials are attractive for photonic applications, very little is known about the basic properties of Er in Ge. Here, the authors study the annealing behavior of Ge implanted with keV Er ions to doses resulting in ≲1at.% of Er. Large redistribution of Er, with segregation at the amorphous/crystalline interface, starts at ≳500°C, while lower temperatures are required for material recrystallization. However, even at 400°C, Er forms precipitates. The concentration of Er trapped in the bulk after recrystallization decreases with increasing temperature but is independent of the initial bulk Er concentration for the range of ion doses studied here.

Control of spectral bandwidth, illumination angle, and degree of collimation of light passing through a conformable phase mask provides an experimentally simple route to nanostructured materials with well defined depth gradients in density (i.e., porosity). In this approach, the illumination conditions define spatial variations in visibility of the distributions of intensity. Exposure of photopolymers to these intensity distributions generates, after developing, solid structures with geometries that compare well with expectation based on modeling of the optics. A range of structures is demonstrated, some of which have potential applications ranging from targets for laser fusion to vehicles for controlled release of chemicals.

Nanocrystalline tantalum prepared through direct current magnetron sputtering exhibits a negative strain rate sensitivity behavior under nanoindentation; i.e., the hardness value of nanocrystalline tantalum decreases with increasing indentation loading rates. High resolution transmission electron microscopy reveals a beta- to alpha-phase transformation underneath the indents, suggesting that the pseudohexagonal to body-centered cubic structural transformation in nanocrystalline tantalum is likely pressure induced. Evidence of shear banding was documented along the edge of indents, consistent with the negative strain rate sensitivity observed.

Colloidal solutions of metal nanoparticles are currently among the most studied substrates for sensors based on surface-enhanced Raman scattering (SERS). However, such substrates often suffer from not being cost-effective, reusable, or stable. Here, we develop nanoporous Au as a highly active, tunable, stable, biocompatible, and reusable SERS substrate. Nanoporous Au is prepared by a facile process of free corrosion of AgAu alloys followed by annealing. Results show that nanofoams with average pore widths of similar to 250 nm exhibit the largest SERS signal for 632.8 nm excitation. This is attributed to the electromagnetic SERS enhancement mechanism with additional field localization within pores. (c) 2006 American Institute of Physics.

Potential of aerogels for technological applications is often limited by their poor mechanical properties. Here, we demonstrate that alumina aerogel monoliths with excellent mechanical properties can be made by controlling the crystallographic phase, shape, and size of nanoligaments. In particular, we show that thermal processing of aerogels with a morphology of interconnected nanoleaflets causes dehydration and associated curling of the nanoleaflets, resulting in a dramatic improvement of mechanical properties. This study shows an effective way to control mechanical properties of the nanoporous solids that can be synthesized with ligaments having a quasi-two-dimensional shape, such as platelets, ribbons, or leaflets.

We correlate topography and diffraction measurements to demonstrate that grain orientation profoundly influences polishing rates in polycrystalline diamond synthesized by chemical vapor deposition. Grains oriented with {111} or {100} planes perpendicular to the surface normal polish at significantly lower rates compared with grains of all other orientations when the surface is polished in continuously varying in-plane directions. These observations agree with predictions of the periodic bond chain vector model, developed previously for single crystals, and indicate that the polishing rate depends strongly on the number of periodic bond chain vectors that are within 10° of the exposed surface plane.

We report a sol-gel synthesis of monolithic three-dimensional GeO2 nanostructures (aerogels). Transmission electron microscopy shows that these ultralow-density (∼99.2% porous) nanostructures are formed by a continuous network of elongated amorphous GeO2 nanoligaments, ∼15–20nm in diameter and ∼50–200nm in length. As a result, monoliths exhibit a high surface area of ∼60m2g−1. A comparison of soft x-ray absorption spectra at GeL3 and OK edges with theoretical densities of states reveals that the electronic structure of aerogels is similar to that of hexagonal GeO2, indicating a (distorted) tetrahedral environment of Ge atoms in the amorphous nanostructure.

The authors apply a hybrid Monte Carlo-molecular dynamics code to the study of grain boundary motion upon annealing of pure Cu and Cu with low concentrations of Fe. The hybrid simulations account for segregation and precipitation of the low solubility Fe, together with curvature driven grain boundary motion. Grain boundaries in two different systems, a {Sigma}7+U-shaped half-loop grain and a nanocrystalline sample, were found to be pinned in the presence of Fe concentrations exceeding 3%.

Nanoporous metals have recently attracted considerable interest fueled by potential sensor and actuator applications. One of the key issues in this context is the synthesis of high yield strength materials. Nanoporous Au (np-Au) has been suggested as a candidate due to its monolithic character. The material can be synthesized by dealloying Ag-Au alloys, and exhibits an open sponge-like morphology of interconnecting Au ligaments with a typical pore size distribution on the nanometer length scale. Unfortunately, very little is known about the mechanical properties of np-Au besides a length-scale dependent ductile-brittle transition. A key question in this context is: what causes the macroscopic brittleness of np-Au? Is the normal dislocation-mediated plastic deformation suppressed in nanoscale Au ligaments, or is the brittleness a consequence of the macroscopic morphology? Here, we report on the fracture behavior of nanoporous Au studied by scanning electron microscopy. Specifically, we demonstrate the microscopic ductility of nanometer-sized Au ligaments. The observed fracture behavior seems to be general for nanoporous metals, and can be understood in terms of simple fuse networks.

We report the analysis of repeated transients to monitor the coupled evolution of dislocation velocity and mobile dislocation density in plastically deforming nanocrystalline Ni. The stress relaxation series allowed the determination of the physical activation volume, indicating a rate-controlling mechanism different from that in coarse-grained Ni. The mobile dislocation exhaustion observed is correlated with the unusually high apparent work-hardening rate during the early stage of straining.

We describe a hot-filament chemical vapor deposition process for growing freestanding nanostructured diamond films, ∼80μm thick, with residual tensile stress levels ≲90MPa. We characterize the film microstructure, mechanical properties, chemical bond distribution, and elemental composition. Results show that our films are nanostructured with columnar grain diameters of ≲150nm and a highly variable grain length along the growth direction of ∼50–1500nm. These films have a rms surface roughness of ≲200nm for a 300×400μm2 scan, which is about one order of magnitude lower than the roughness of typical microcrystalline diamond films of comparable thickness. Soft x-ray absorption near-edge structure (XANES) spectroscopy indicates a large percentage of sp3 bonding in the films, consistent with a high hardness of 66GPa. Nanoindentation and XANES results are also consistent with a high phase and elemental purity of the films, directly measured by x-ray and electron diffraction, Rutherford backscattering spectrometr...

We report on atomic layer deposition of an ∼2-nm-thick ZnO layer on the inner surface of ultralow-density (∼0.5% of the full density) nanoporous silica aerogel monoliths with an extremely large effective aspect ratio of ∼105 (defined as the ratio of the monolith thickness to the average pore size). The resultant monoliths are formed by amorphous-SiO2 core/wurtzite-ZnO shell nanoparticles which are randomly oriented and interconnected into an open-cell network with an apparent density of ∼3% and a surface area of ∼100m2g−1. Secondary ion mass spectrometry and high-resolution transmission electron microscopy imaging reveal excellent uniformity and crystallinity of ZnO coating. Oxygen K-edge and Zn L3-edge soft x-ray absorption near-edge structure spectroscopy shows broadened O p- as well as Zn s- and d-projected densities of states in the conduction band.