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At National Ignition Facility (NIF), yield amplification due to alpha particle heating approached ~3 in the highest performing inertial confinement fusion (ICF) implosions, while yield amplification of ~15-30 is needed for ignition. Hydrodynamic instabilities are a major factor in degradation of implosions while understanding and mitigation of the instabilities are critical to achieving ignition. This article describes recent and planned hydrodynamic instability experiments with several focused platforms that have been developed to directly measure these instabilities in all phases of ICF implosions. Measurements of ripple-shock generation at OMEGA laser have indicated initial seeds for the instabilities in three ablators - plastic (CH), beryllium, and high-density carbon (HDC). Hydrodynamic Growth Radiography (HGR) platform was used to measure instability growth at the ablation front in the acceleration phase of implosions. This platform used pre-imposed 2-D perturbations for growth factor measurements at different perturbation wavelengths and was also used to measure growth of “native roughness” modulations, fill tubes, and capsule support membranes or “tents”. Also, in the acceleration phase several new experimental platforms have been or are being developed to measure instability growth at the ablator-ice interface. In the deceleration phase of implosions, “self-emission” and “self-backlighting” platforms were developed to measure perturbations near peak compression. This article reviews recent progress and results.

Synthesis of nanoporous alumina monoliths with controlled morphology and density is a challenge. Here, we demonstrate mechanically robust alumina monoliths synthesized by conformal overcoating of graphitic nanoligaments of low-density carbon-nanotube-based aerogels (CNT-CAs) by using atomic layer deposition. Young’s modulus of resultant monoliths increases superlinearly with the monolith density with an exponent of ∼2.4, defined by the morphology and connectivity of the CNT-CA scaffold. As a result, for a given monolith density, alumina-carbon composites have moduli comparable to those of CNT-CAs and significantly superior to those of pure alumina aerogels reported previously.

We report on the grain size dependent morphological, physical and chemical properties of thick microwave-plasma assisted chemical vapor deposited (MPCVD) diamond films that are used as target materials for high energy density physics experiments at the Lawrence Livermore National Laboratory. Control over the grain size, ranging from several μm to a few nm, was achieved by adjusting the CH4 content of the CH4/H2 feed gas. The effect of grain size on surface roughness, morphology, texture, density, hydrogen and graphitic carbon content was systematically studied by a variety of techniques. For depositions performed at 35 to 45 mbar and 3000 W microwave power (power density ~ 10 W cm− 3), an abrupt transition from micro-crystalline diamond to nanocrystalline diamond was observed at 3% CH4. This transition is accompanied by a dramatic decrease in surface roughness, a six percent drop in density and an increasing content in hydrogen and graphitic carbon impurities. Guided by these results, layered nano-microhybrid diamond samples were prepared by periodically changing the growth conditions from nano- to microcrystalline.

We report on a general method to fabricate transition metal related defects in diamond. Controlled incorporation of Mo and W in synthetic CVD diamond was achieved by adding volatile metal precursors to the diamond chemical vapor deposition (CVD) growth process. Effects of deposition temperature, grain structure and precursor exposure on the incorporation efficiency were systematically studied, and doping levels of up to 0.25 at.% have been achieved. The metal atoms are uniformly distributed throughout the diamond grains without any indication of inclusion formation. These results are discussed in context of the kinetically controlled growth process of CVD diamond.

The growth of uniform thin films on foreign substrates is impeded by several morphological instabilities. Hill-and-valley structures are formed and enhanced during sputter deposition where surface height perturbations have an opportunity to grow to large amplitudes. We show via kinetic Monte Carlo (kMC) simulations that while surface roughness can be partially controlled by changing growth conditions, such as substrate temperature, the diffusion of particles over step edges plays a very important role in determining both surface roughness and the density of the films. Our kMC simulations provide a way to evaluate the strength of surface instabilities during sputter deposition of thin films.

Here, we present the synthesis and characterization of low-density single-walled carbon nanotube-based aerogels (SWNT-CA). Aerogels with varying nanotube loading (0–55 wt.%) and density (20–350 mg cm −3 ) were fabricated and characterized by four-probe method, electron microscopy, Raman spectroscopy and nitrogen porosimetry. Several properties of the SWNT-CAs were highly dependent upon nanotube loading. At nanotube loadings of 55 wt.%, shrinkage of the aerogel monoliths during carbonization and drying was almost completely eliminated. Electrical conductivities are improved by an order of magnitude for the SWNT-CA (55 wt.% nanotubes) compared to those of foams without nanotubes. Surface areas as high as 184 m 2 g −1 were achieved for SWNT-CAs with greater than 20 wt.% nanotube loading.

Many applications of low-density nanoporous dielectrics are limited by their poor mechanical properties. Although nanoindentation is often used to evaluate the mechanical behavior of such materials, the physical meaning of the inelastic parameters measured by various indentation methods is not clear. Here, we study low-density nanoporous silica monoliths (aerogels) by nanoindentation using the most common indenter geometries (spherical, pyramidal and flat punches) and discuss the parameters measured. Results suggest that the deformation of the nanoporous silica monoliths studied in this work is controlled by elastic bending and fracture of nanoligaments with no signs of plasticity. The contact pressure (Meyer hardness) increases with increasing strain and does not represent the foam “crushing pressure”. The critical load for Hertzian fracture obeys the Auerbach law, and the formation of radial cracks and brittleness are strongly suppressed by the presence of nanopores. We also discuss the choice of indenter geometry and provide recommendations that can be used to overcome some typical challenges of indentation studies of low-density nanoporous solids, including very low contact stiffness for indenters with small contact areas, large surface roughness inherent to this class of materials, spatial non-uniformity (skin layers on monoliths), viscoelasticity and elastic nonlinearity.

Nanocrystalline diamond (NCD) films with a thickness of {approx}6 {micro}m and with average grain sizes ranging from 60 to 9 nm were deposited on silicon wafers using a hot-filament chemical vapor deposition (HFCVD) process. These samples were then characterized with the goal to identify correlations between grain size, chemical composition and mechanical properties. The characterization reveals that our films are phase pure and exhibit a relatively smooth surface morphology. The levels of sp{sup 2}-bonded carbon and hydrogen impurities are low, and showed a systematic variation with the grain size. The hydrogen content increases with decreasing grain size, whereas the sp{sup 2} carbon content decreases with decreasing grain size. The material is weaker than single crystalline diamond, and both stiffness and hardness decrease with decreasing grain size. These trends suggest gradual changes of the nature of the grain boundaries, from graphitic in the case of the 60 nm grain size material to hydrogen terminated sp{sup 3} carbon for the 9 nm grain size material. The films exhibit low levels of internal stress and freestanding structures with a length of several centimeters could be fabricated without noticeable bending.

The investigation of orientation dependent crystal growth and etch processes can provide deep insights into the underlying mechanisms and thus helps to validate theoretical models. Here, we report on homoepitaxial diamond growth and oxygen etch experiments on polished, polycrystalline CVD diamond wafers by use of electron backscatter diffraction (EBSD) and white-light interferometry (WLI). Atomic force microscopy (AFM) was applied to provide additional atomic scale surface morphology information. The main advantage of using polycrystalline diamond substrates with almost random grain orientation is that it allows determining the orientation dependent growth (etch) rate for different orientations within one experiment. Specifically, we studied the effect of methane concentration on the diamond growth rate, using a microwave plasma CVD process. At 1% methane concentration a maximum of the growth rate near and a minimum near is detected. Increasing the methane concentration up to 5% shifts the maximum towards while the minimum stays at . Etch rate measurements in a microwave powered oxygen plasma reveal a pronounced maximum at . We also made a first attempt to interpret our experimental data in terms of local micro-faceting of high-indexed planes.

Materials synthesized by deposition techniques are often plagued by high levels of residual stress. While the origin and control of this stress in thin (sub-micron) films has been an active area of research, it is not clear how the results extrapolate with thickness. In the present work, in situ residual stress measurements are performed during the sputter deposition of beryllium, spanning the transition from thin to thick. Variables including sputtering gas pressure and substrate biasing are shown to strongly affect both the average and instantaneous stress levels measured during film growth. Detailed microstructural characterization is performed to assess the grain structure, surface morphology, and crystallographic growth texture of representative specimens. The microstructure is correlated with theoretical models of stress generation to interpret experimental measurements. A stress map is also constructed, generalizing the effects of processing and material parameters on stress state.

Laser experiments, such as those planned at the National Ignition Facility (NIF) and the Omega facility, use small targets with the goal of studying high-energy density physics and inertial confinement fusion. One particular application is a target with layers whose density changes in a carefully designed gradient (from 0.2 to 1.2 g/cm 3 ) for use in isentropic compression experiments (ICE). We are nondestructively determining the density of these layers using two X-ray microscopes. Because of the many interfaces that comprise the layers, a plethora of X-ray phase contrast fringes appear in the images, leading to many radiographic and tomographic artifacts which compromise the ability to infer the density of the layer. In this paper, we describe how we are attacking this problem with a variety of radiographic standards and through radiographic simulation using the HADES radiographic simulation code.

A comprehensive study on the relationship between yield strength, relative density and ligament sizes is presented for nanoporous Au foams. Depth-sensing nanoindentation tests were performed on nanoporous foams ranging from 20% to 42% relative density with ligament sizes ranging from 10 to 900 nm. The Gibson and Ashby yield strength equation for open-cell macrocellular foams is modified in order to incorporate ligament size effects. This study demonstrates that, at the nanoscale, foam strength is governed by ligament size, in addition to relative density. Furthermore, we present the ligament length scale as a new parameter to tailor foam properties and achieve high strength at low densities.

Repeated stress relaxation tests and strain rate jump tests have been carried out over a range of deformation temperatures (77–373 K) on electrodeposited nanocrystalline Ni with an average grain size of ∼30 nm. The strain rate sensitivity, the apparent and physical activation volume, and the activation energy have been determined. The magnitude observed for these characteristic deformation parameters, as well as their temperature-dependent behavior, is very different from those of coarse-grained Ni. This suggests that the thermally activated process in nanocrystalline Ni is different from the conventional forest dislocation cutting mechanism. It is concluded that grain boundary diffusion-controlled processes such as Coble creep and grain boundary sliding can be ruled out as dominant mechanisms for the grain sizes and temperature range studied. Instead, our experimental findings and analysis suggest that the deformation kinetics are controlled by the activities of dislocations. The dominant thermally activated mechanism is suggested to originate from three possible processes, all involving interactions of mobile dislocations with grain boundaries.

Depleted uranium samples were ablated using five nanosecond pulses from a Nd:YAG laser and produced films of ∼1600 A thickness that were deposited with an angular distribution typical of a completely thermal ablation (cos1 θ). The films remained contiguous for many months in vacuum but blistered due to tensile stress induced in the films several days after being brought into air. While under vacuum (2 × 10−10 Torr base pressure) the films were allowed to oxidize from the residual gases, of which water vapor was found to be the primary oxidizer. During the oxidation, the samples were monitored with both X-ray and ultraviolet photoemission spectroscopy (XPS and UPS) and were found to oxidize following Langmuir kinetics. That a 2D-surface growth model described the oxidation indicates that, even at these low pressures, oxygen accumulation on the surface is a much faster process than diffusion into the bulk. While bulk diffusion did occur, the oxygen present at the surface saturated the measurements taken using photoemission and diffusion was difficult to observe. A method for determining oxide concentration via photoemission from the valence level, as opposed to the more conventional core levels, is also presented.

The total electron emission yields following the interaction of “Slow (⩽2 keV/a.u.) Highly Charged Ions” (SHCI) (O 3+⋯7+ , Xe 12+⋯52+ , Au 54+⋯69+ ) with different target surfaces (highly-oriented pyrolytic graphite (HOPG), Au and SiO2) have been measured. The emission yields increase with charge state, and is found to be highest for carbon, the HOPG target, and lowest for the SiO 2 target. An empirical formula for the electron emission is including recent results from investigations of plasmon excitation following SHCI impact are used to interpret the results.

New emission-based MeV nuclear microscopies have been in the process of development for the past four years. These techniques all fall under the heading of ion-induced emission microscopy (I-IEM), and the first to be developed was ion-electron emission microscopy (IEEM). With I-IEM the ion beam is not focused, but instead, secondary particles emitted when a single-ion strikes the sample are projected at great magnification onto a high efficiency single particle detector generating position signals. These X and Y signals are then put into coincidence with other signals made by this same ion in a fashion completely analogous to traditional nuclear microprobe analysis. In this paper, we update the current state of I-IEMs, which currently includes IEEM and highly charged ion-secondary ion mass spectroscopy (HCI-SIMS or IIEM) and ion-photon emission microscopy. At the present time none of these microscopies have atomic resolution, but the potential exists for resolution adequate for many scientific and nanotechnology applications.

We compare deformation behavior of conventional carbon and carbon-nanotube (CNT) based aerogels with monolith densities of 30–300 mg cm −3 . Results show that CNT-based aerogels have superior elastic moduli, comparable failure stresses, and, hence, lower failure strains. The density scaling law exponents are statistically indistinguishable for both types of aerogels, suggesting the same ligaments connectivity. The superior elastic properties and lower failure strains of CNT-based aerogels are attributed to a higher stiffness of CNT-based ligaments, while comparable failure stresses are attributed to the common junction geometry. Practical implications of these findings are discussed.

Alternatives to traditional nuclear microprobe analysis (NMA) emerged two years ago with the invention of ion electron emission microscopy (IEEM). With nuclear emission microscopy (NEM) the ion beam is only partially focused so as to fill the field of view of a special emission particle microscope system fitted with a single particle position sensitive detector (PSD). When a single ion strikes the sample, the emitted secondaries (e.g. electrons, photons, ions, etc.) are projected at great magnification onto this PSD where position signals are generated. These X and Y signals are then put into coincidence with other signals made by this same ion in a fashion completely analogous to traditional nuclear microprobe analysis. In this paper, an update will be given on the state of NEMs, which currently includes IEEM and highly charged ion–secondary ion mass spectroscopy (HCI–SIMS). In addition, a new type of full-field nuclear imaging is proposed: ion photon emission microscopy or IPEM.

Electronic sputtering in the interaction of slow (v < vBohr), highly charged ions (SHCI) with solid surfaces has been subject of controversial discussions for almost 20 years. We review results from recent studies of total sputtering yields and discuss distinct microscopic mechanisms (such as defect mediated desorption, Coulomb explosions and eAects of intense electronic excitation) in the response of insulators and semiconductors to the impact of SHCI. We then describe an application of ions like Xe 44a and Au 69a as projectiles in time-of-flight secondary ion mass spectrometry for surface characterization of semiconductors. ” 2000 Elsevier Science B.V. All rights reserved.

The present article reviews recent advances in the studies of the interaction of slow (v < vBohr), very highly charged ions (such as Xe 52+ and Au 69+ ) with surfaces of metals, semiconductors, and insulators (including biological materials). After a brief summary of past developments, we describe key experimental techniques for studies of secondary particle emission and the de-excitation dynamics of the highly charged ions. Recent progress in measurement and determination of the mechanisms leading to secondary electron yields, secondary ion yields and total sputtering yields will be discussed. The deexcitation dynamics are addressed in experiments on projectile neutralization and energy loss in thin films of material. We review the theoretical concepts briefly and introduce theoretical models in the discussion of experimental results. Following the presentation of fundamental studies we will address emerging applications of slow, very highly charged ions in surface analysis and surface modification. # 1999 Published by Elsevier Science Ltd. All rights reserved.

The emission of secondary electrons and ions from clean Au, CxHyAu and SiO2 surfaces at impact of slow (v ≈ 0.3 vBohr) ions has been measured as a function of incident ion charge for 1 + ≤ q ≤ 75 +. Electron yields from thermal silicon dioxide films (150 nm on Si) are found to be lower than those from Au and CxHyAu for q > 3 +. Yields of negative secondary ions from SiO2 and CxHyAu were recorded in parallel with electron emission data and exhibit a qn, n ≈ 4, dependency on incident ion charge. A direct comparison of collisional and electronic contributions to secondary ion production from SiO2 films using a beam of charge state equilibrated Xeq=qeq (at 2.75 keV/u) shows positive and negative secondary ion yield increases with incident ion charge of > 400. Results are discussed in relation to key signatures of electronic sputtering by Coulomb explosions.

Epitaxial silicon carbide films are grown on Si(100) and Si(111) substrates at surface temperatures between 950 and 1250 K via c60 precursors. Films have been grown up to thicknesses greater than 1 μm. The growth rate of the SiC film is not limited by the surface reaction rate of C60 with silicon at these temperatures, rather by the arrival rate of the reactants Si (by diffusion) or C60. This results in rapid film growth. Films have been characterized by low energy electron diffraction, X-ray diffraction, and Auger depth profiling. X-ray diffraction suggests the growth of β-SiC in the temperature range investigated. Auger depth profiling shows the film is stoichiometric. Selective crystalline silicon carbide growth is achieved on patterned silicon-silicon oxide samples.

The interaction of C60 with aluminum was investigated using Auger electron spectroscopy (AES), temperature programmed desorption (TPD), photoluminescence and soft X-ray photoelectron spectroscopy. The interaction of C70 with aluminum was also probed with soft X-ray photoelectron spectroscopy. The bonding of C60 and C70 to the aluminum surface is strong such that after multilayer desorption at 570 and 620 K, respectively, a monolayer coverage of the fullerene remains intact, as seen in the valence band spectra, on the surface to 700 K. Above surface temperatures of 700 K the continued presence of a C60 photoluminescence signal and the decline in the surface carbon concentration suggests that the C60 dissolves into the aluminum bulk. The chemical shift in the binding energy of the Al 2p electrons also indicates strong interaction between C60 and aluminum. Multilayer and monolayer coverages of C60 on aluminum passivated the surface such that exposures of 340 L of water at room temperature led to no oxidation of the surface. Multilayer coverages of C60 on aluminum were protective against oxidation at ambient conditions.

The dissociation of C60 to C58 has been observed on surfaces of silica and highly oriented pyrolytic graphite by modulated molecular beam reaction spectroscopy. At 1200 K on both surfaces approximately 5% of the incident C60 dissociates and desorbs as C58. The activation energy for C58 production from C60 on the surface (≈ 2.5 eV) is considerably smaller than that required in the gas phase, possibly due to the stabilization of the C1 or C2 products on the surface. At surface temperatures between 1000 and 1200 K, carbon was detected on the SiO2 surface.

The atomic and electronic structure of clean and water-covered sapphire (1102) surfaces have been studied by low energy electron diffraction (LEED), electron energy loss spectroscopy (ELS), and temperature programmed desorption (TPD). A surface electronic state with a loss energy of 4.1 eV is measured in the bulk bandgap on the clean, reconstructed (2 × 1) surface. Exposure of the surface to water at room temperature (RT) causes the surface electronic state to shift to lower loss energies reaching a value of ∼ 3.6 eV at saturation coverage. In addition a higher energy loss feature appears at ∼ 7.5 eV loss and is attributed to a hydroxyl adlayer. Adsorption of water on the surface at RT was observed to proceed with near unity sticking probability. Two desorption states were seen. At low coverage a state appears with a peak temperature for desorption of 525 K. With increasing exposure the peak shifts to lower temperature, reaching 450 K near one monolayer, defined as one water molecule per (1 × 1) unit cell. At higher coverages a second state becomes populated. The water adlayer removed the surface reconstruction only after exposure to an electron beam, yielding a (1 × 1) LEED pattern. The laser-induced desorption of aluminum ions from clean and water-covered sapphire (1102) at laser wavelengths of 1064 nm (1.17 eV), 355 nm (3.51 eV), and 266 nm (4.66 eV) was investigated by time-of-flight mass spectrometry. Below the laser ablation threshold, predominately Al + ions desorbed with an average kinetic energy of 7.0 ± 0.7 eV at all three wavelengths from both clean and water-covered surfaces. For 1064 nm light at fluences just below the ablation threshold, AlO − desorbed with kinetic energy of 1.7 ± 0.5 eV from the clean (2 × 1) surface. The observed aluminum desorption at high kinetic energy is consistent with the previously proposed aluminum-localized exciton-mediated desorption mechanism. In sharp contrast to the laser-surface interactions, a 300 eV electron beam desorbed O + from the clean (2 × 1) surface and H + and OH + from the water-covered surface.

Upon heating to greater than 1300 K, diamond C(100)-(1 × 1) reconstructs, exhibiting a low energy electron diffraction (LEED) pattern with two domains of (2 × 1) symmetry. No evidence for higher order reconstructions (e.g., c(4 × 2)) is observed, ruling out correlated buckled dimers as a possible structure for the reconstruction. Hydrogen is found to play an important role in the transformation of the (1 × 1) surface during reconstruction. The presence of chemisorbed hydrogen on the (1 × 1) surface is monitored by electron-stimulated desorption time-of-flight spectroscopy (ESD-TOF) as a function of anneal temperature. Two distinct H+ ESD velocity distributions, fast and slow, are observed in the TOF spectra on the polished (1 × 1) surface. Upon heating, the fast component disappears with the onset of the (2 × 1) reconstruction. The slower of the two features remains even after annealing at temperatures of up to 1530 K, demonstrating that some hydrogen remains on the reconstructed (2 × 1) surface. Temperature-programmed desorption (TPD) of H2 from the diamond C(100)-(1 × 1) surface also correlates with the surface reconstruction and the loss of the fast proton peak in the ESD spectra. For the “as polished” surface, the TPD results yielded an integrated hydrogen desorption flux of up to ten monolayers (based on two hydrogen atoms per surface carbon atom, e.g. 3.14 × 1015 H cm−2) indicating that the near-surface region for the as-polished diamond C(100)-(1 × 1) contains a large quantity of absorbed hydrogen. At low hydrogen coverage, the desorption process appears to be first order with an activation energy of ∼ 37 kcal mol and a first order pre-exponential of ∼ 3 × 105s−1. At higher coverages the apparent order increases and the activation energy decreases. Ultraviolet photoelectron spectroscopy (UPS) of the diamond C(100)-(1 × 1) does not reveal any occupied states in the band gap from the valence band maximum (VBM) to the Fermi level (EF). In contrast, UPS for diamond C(100)-(2 × 1) exhibits occupied states in the band gap from the VBM to 1.5 eV above the VBM. No empty states could be found from 1.2 to 5.5 eV above EF for this surface using two-photon photoemission. By analogy to the Si and Ge(100) surfaces, the proposed model for the diamond C(100)-(2 × 1) surface is based on the formation of dimer pairs. The sharp two domain (2 × 1) LEED pattern, the presence of hydrogen on the reconstructed surface, and the observation of occupied surface states in the band gap suggest that symmetric dimers with monohydride termination at each carbon atom of the pair is the most likely structure.

The adsorption and reaction of acetonitrile (CH3CN) on clean and oxygen covered Ag(110) surfaces has been studied using temperature programmed reaction spectroscopy (TPRS), isotope exchange, chemical displacement reactions and high resolution electron energy loss spectroscopy (EELS). On the clean Ag(110) surface, CH3CN was reversibly adsorbed, desorbing with an activation energy of 10 kcal mol-1 at 166 K from a monolayer state and at 158 K from a multilayer state. Vibrational spectra of multilayer, monolayer and sub-monolayer CH3CN were in excellent agreement with that of gas phase CH3CN indicating that CH3CN is only weakly bonded to the clean Ag(110) surface. On the partially oxidized surface CH3CN reacts with atomic oxygen to form adsorbed CH2CN, OH and H2O in addition to forming another molecular adsorption state with a desorption peak at 240 K. This molecular state shows a CN stretching frequency of 1840 cm-1, which is indicative of substantial rehybridization of the CN bond and is associated with side-on coordination via the π system. The CH2CN species is stable up to 430 K, where C-H bond breaking and reformation begins, leading to the formation of CH3CN at 480 K and HCN at 510 K and leaving only carbon on the surface. In the presence of excess oxygen atoms C-H bond breaking and reformation is more facile leading to additional desorption peaks for CH3CN and H2O at 420 K. This destabilizing effect of O(a) on Ch2CN(a) is explained in terms of an anionic (CH2CN-1) species. Comparison of the vibrational spectra from CH2CN(a) and CD2CN(a) supports the following assignment for the modes of adsorbed CH2CN: ν(Ag-C) 215: δ(CCN) 545; ϱt(CH2) 695; ϱw(CH2) 850; ν(C-C) 960; ϱr(CH2) 1060; δ(CH2) 1375; ν(CN) 2075; and ν(CH2) 2940 cm-1. These results serve to further indicate the wide applicability of the acid-base reaction concept for reactions between gas phase Bronsted acids and adsorbed oxygen atoms on solver surfaces.