Laser ablation of silver nanoparticles (NPs) was studied with laser post-ionization (LPI) time-of-flight mass spectrometry (TOF MS). Silver NPs containing similar to 15 000 Ag atoms (4 nm radius) were deposited by soft landing (energy 3 eV/atom) onto indium tin oxide (ITO)/glass substrates. Laser ablation was performed using frequency-doubled Ti:sapphire nanosecond pulsed laser irradiation at three different wavelengths (371, 401, and 421 nm), whereas for post-ionization, pulses from an F-2 laser were used. Laser fluences and time delay dependencies of Ag and In signals were obtained. Using these data, the temperature of the desorption source as well as its time duration were calculated. It was found that the peak-temperature of NPs was above their melting point and they cooled down slowly, with temperature decay time of several hundreds of nanoseconds. This anomalous behavior was explained based on a model where the semiconducting ITO substrate is initially transparent to the desorption laser radiation but starts to adsorb it due to the temperature increase arising from heat exchange with NPs. Poor heat conduction in the ITO film creates conditions for long-lived hot spots on the surface and initiates further optical damage of the substrate. No difference in the ablation process due to plasmon resonance was detected, likely due to thermal expansion and melting of NPs during laser irradiation, which then broadens the plasmon absorption band enough to cover all wavelengths used. These results clearly demonstrate that the process of NP interaction with laser radiation is governed not only by initial optical and thermophysical parameters of NPs and the surrounding media, but also by their alteration due to temperature increases during the irradiation process.

Ionized and neutral clusters were desorbed from spangold, a polycrystalline ternary alloy with composition Au7Cu5Al4, using both a femtosecond laser beam and an energetic ion beam and the resulting time of flight mass spectra compared. Neutral clusters containing up to 7 atoms were ejected by the 15 key Ar+ beam whereas only smaller positively and negatively charged clusters were observed from the laser ablated spangold surface.Laser ionization mass spectrometry (LIMS) positive ion spectra were dominated by Al containing cluster ions whereas Au containing ions dominated the negative LIMS spectrum. An odd-even variation in LIMS cluster yield was observed, consistent with previous results and due to fragmentation of photoionized clusters. The laser sputtered neutral mass spectrometry (laser SNMS) spectrum showed that larger desorbed clusters were gold rich. The cluster signals also followed a power law dependence with cluster size with the exponent value of 6-7.6 for sputtered mixed clusters being greater than that found from sputtering of pure elements, similar to the result found previously in the Cu-Au system. (C) 2014 Elsevier B.V. All rights reserved.

We have used the surface sensitivity of laser sputter neutral mass spectrometry to make measurements of clusters sputtered from AuAl alloys surfaces with high dynamic range. Polycrystalline AuAl4 and Au4Al were bombarded with 15keV Ar+ at 60° incidence, and the resulting secondary neutral yield distributions were measured using laser postionization mass spectrometry. Neutral clusters containing up to 28 atoms were observed and exhibited an odd–even variation in signal dependent on the stability of the photoion. Clusters sputtered from Au4Al were gold rich compared to the substrate and the yield of neutral clusters containing n atoms, Yn, was found to follow a power in n, i.e. Yn∝n−δ, where the exponent δ was approximately 3.4.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Polycrystalline Cu, Cu20Au80, Cu40Au60, Cu80Au20 and Au samples were bombarded with 15keV Ar+, and the resulting secondary neutral yield distribution was studied by non-resonant laser post-ionisation mass spectrometry. Neutral clusters containing up to 15 atoms were observed for the targets. The yield of neutral clusters, CumAun−m, containing n atoms, Yn, was found to follow a power in n, i.e. Yn∝n-δ, where the exponent δ varied from 5.2 to 10.1. For a fixed n, the cluster yields showed a variation with number of copper atoms, m, much greater than expected for a binomial distribution suggesting that the clusters are not formed randomly above the surface and a component of preformed cluster emission occurs. In addition, the cluster compositions from the sputtered alloys were indicative of sputtering from a copper rich surface.

Polycrystalline Au was bombarded with 15keV Ar+, and the resulting secondary neutral cluster yield distribution was measured by laser postionisation mass spectrometry. Neutral Aun clusters containing up to 20 atoms were observed. The yield of Aun clusters, Yn, was found to follow a power in n, Yn∝n−3.4, but the yield of individual clusters depended on whether n was even or odd. This odd–even yield variation was caused by fragmentation of the cluster photoions. Simulation of photoion trajectories within the TOF spectrometer shows that the fragmentation dominantly occurs before the photoions enter the reflectron part of the spectrometer.

Atomic Layer Deposition (ALD) is a process that synthesizes materials in successive monolayers, at rates up to 1 micron/hour. We have been using this technique at Argonne as a possible way to improve superconducting radio frequency (SCRF) cavities performances. Initial experiments using tunneling spectroscopy and ALD have led to a new model for dissipation mechanisms occurring at the surface and news ways of controlling SCRF surfaces, as well as suggesting ways to significantly improve the operating gradients of superconducting cavities. Initial measurements of ALD treated samples show significant improvement over untreated cavity-grade Nb samples. We will report these results.

The results of chemical etching of industrially produced SiC ceramic seals in chlorine-contained gas mixtures with varying Cl 2 and H 2 concentrations, temperatures and treatment durations are reported. The etching progress was followed by XPS, which allowed monitoring the concentration and chemical state of the main elements present in the experimental samples. Due to preferable reaction of Cl 2 with Si rather than C, selective etching of the SiC surface can take place and a surface layer consisting of different carbon polytrophic forms was created. MicroRaman spectroscopy and scanning electron microscopy (SEM) were also used for analysis of the films. Depth profiling of the created films showed a smooth change of C/Si ratio deep into the film and the absence of a sharp edge that would indicate a reaction interface. The adhesion tests of created films were conducted and demonstrated very high contact force between film and substrate.

Tunneling spectroscopy was performed on Nb pieces prepared by the same processes used to etch and clean superconducting radio frequency (SRF) cavities. Air exposed, electropolished Nb exhibited a surface superconducting gap Δ=1.55meV, which is characteristic of a clean, bulk Nb. However, the tunneling density of states (DOS) was significantly broadened. The Nb pieces, which were treated with the same mild baking used to improve the Q slope in SRF cavities, reveal a sharper DOS. Good fits to the DOS were obtained by using the Shiba theory, suggesting that magnetic scattering of quasiparticles is the origin of the gapless surface superconductivity and a heretofore unrecognized contributor to the Q-slope problem of Nb SRF cavities.

A method to treat the surface of Nb is described, which potentially can improve the performance of superconducting rf cavities. We present tunneling and x-ray photoemission spectroscopy measurements at the surface of cavity-grade niobium samples coated with a 3 nm alumina overlayer deposited by atomic layer deposition. The coated samples baked in ultrahigh vacuum at low temperature degraded superconducting surface. However, at temperatures above 450 C, the tunneling conductance curves show significant improvements in the superconducting density of states compared with untreated surfaces.

The generation of the acoustic vibration of laser back-irradiated thin metal foils and their influence on desorption of organic molecules from the foil's front surface were studied. The possible mechanisms of this phenomenon are discussed.

Acoustic waves, generated in solids by irradiation of a surface with powerful laser pulses, are widely used to study mechanical, thermal and elastic properties of materials. Application of this technique to MEMS technology will open new insights into fabrication and characterization but will require understanding of acoustic wave generation in small-sized objects. To that end, acoustic wave generation was studied in thin (10-50 μm) metal and semiconductor foils (including Mo, Si, W, Ni, Ta, Au) back-side irradiated by nanosecond IR and UV laser pulses over a range of peak intensities. Both interferometric techniques and capacitance transducers were employed for detection of surface displacements in the foils. By varying the peak laser power over a wide range of intensities (1-500 MW/cm2) detection of the transition from a thermoelastic to a laser-plasma driven shock-wave mechanism for acoustic wave generation was possible. Measurements show that this transition is accompanied by a dramatic change in the waveform of the generated shock-wave and that this waveform differs for various materials and foil thicknesses. Since thin foils were studied, the longitudinal and shear waves were experimentally indistinguishable, making the observed waveform very complex. Moreover, at higher peak laser powers, mechanical vibrations at resonance frequencies of the thin foils can occur and further complicate the analysis. Nevertheless, the observed phenomena can be described in the framework of a simplified theoretical model and can be used for materials testing in different applications.

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

The modification of the silicon carbide (4H-SiC) single-crystal surface in a chlorine-containing gas mixture at high temperature (800–1000°C) and ambient pressure was investigated. The results of silicon carbide chlorination are found to strongly depend on the hexagonal surface orientation. Due to the thermodynamically more favorable reaction of chlorine with silicon rather than carbon, the C-terminated side (0001¯) clearly undergoes considerable changes, resulting in coverage by a black-colored carbon film, whereas the Si-side (0001) surprisingly remains visually untouched. With using X-ray photoelectron spectroscopy (XPS), angle-resolved XPS and SEM it is shown that this drastic change in behavior is associated with a different structure of oxicarbide/silicate adlayer formed on the C- and Si-terminated sides of silicon carbide surface during experimental pre-treatment and air exposure. The presence of oxygen bridges connecting the silicate adlayer with the bulk SiC in the case of Si-side inhibits the chlorination reaction and makes this surface strongly resistant to chlorine attack. Only some places on the Si-terminated side demonstrate traces of chlorine etching in the form of hexagonal-shaped voids, which are possibly initiated by distortion of the initial crystalline structure by micropipes. In contrast, a thin carbon layer resulted on the C-terminated side as a consequence of the chlorination process. XPS, ARXPS, SEM and Raman spectroscopy study of created film allows us to argue that it consists mainly of sp2-bonded carbon, mostly in the form of nanoscale graphene sheets. The absence of a protective oxygen bridge between the silicate adlayer and the bulk silicon carbide crystal leads to unlimited growth of carbon film on the SiC(0001¯) side.

A NiAl(111) single crystal was bombarded with 15keV Ar+, and the resulting secondary neutrals were analysed by laser postionisation secondary neutral mass spectrometry. By measuring the individual cluster photoion intensity as a function of laser power, the sputter yields of 33 individual clusters were determined. The yield of Aln clusters sputtered from NiAl falls with increasing cluster nuclearity as n−8.7 while Nin and Alm−nNin yields are proportional to n−5.9 and n−5.2, respectively. The distribution of thee yields of mixed Alm−nNin clusters with n and m is found to diverge significantly from the expected distribution based on a random combinatorial approach, indicating that the energetics due to the chemical bonding in the clusters plays a significant role during cluster formation in the sputtering process.

Chemical tags such as anthracene can be attached to a molecular analyte and serve as chromophores for 7.87eV laser postionization by lowering the overall ionization potential of the tagged molecular complex. Fluorescein and tryptophan are demonstrated as two new tags for 7.87eV laser postionization of various amino acids and peptides. Other molecular species that are efficient fluorescence probes should also serve as tags for 7.87eV postionization since they display highest occupied molecular orbitals with extended π-conjugation that lead to ionization potentials below this photon energy and an ability to stabilize the net positive charge of the radical cations. This technique is demonstrated here for laser desorbed species, but is also applicable to keV ion sputtered neutrals. Overall, 7.87eV laser postionization of derivatized species promises to expand the capabilities of mass spectrometric surface analysis.

In anticipation of the return of comet (and ISM?) dust grains by the Stardust mission [1] in mid-January next year, Academia Sinica (AS) and Argonne National Laboratory (ANL) have entered into a collaboration to develop instrument and method for the isotopic analysis of these samples. We need to achieve the highest possible sensitivity so that we can analyze individual grains one at a time to the smallest possible size. Only by doing so can we hope to reach one of the main science goals of the mission, namely the recognition of those isotopically distinct grains each carrying the characteristic signature of a particu-lar nucleosynthetic stage of its parent star. In order to facilitate the interpretation of these grains the second requirement of our method is that the measurements must be made over the widest possible mass range before samples exhaustion. For instance, the thermo-nuclear fusion reactions that produced the isotopes of various major elements of a wide mass range required drastically different temperatures. Therefore their abundances could constrain the conditions at greatly varying depth inside the source star hence its structure and evolution. For high sensitivity, we decided to select Secondary Neutral Mass Spectrometry (SNMS) with post-ionization by VUV laser because in the secondary particles released from the sample surface bombarded by primary micro-beam of ions or photons, the neutrals outnumber the ions by a factor of 100-1000. Using a new design of the extraction optics, the ANL team has been able to demonstrate experimentally that the useful yield (i.e. ion detected over atoms consumed) of their improved SNMS with post-ionization over a large vol-ume (3x3x4 mm

A new generation of secondary neutral mass spectrometry (SNMS) instruments has been developed that is especially designed for laser post-ionization (LPI). These instruments combine high useful yield and high background discrimination. Results presented here demonstrate that these instruments can detect one in every four atoms removed from a samples surface – a greater than one order of magnitude improvement over current large frame secondary ion mass spectrometry instruments. Because of their high sensitivity, these new LPI-SNMS instruments are especially amenable to analysis of samples of limited size and rare one-of-a-kind samples. Such an application is analysis of samples returned to Earth from space by the Genesis and Stardust Discovery missions of NASA.

When toluene solutions of the tetrametallic compound, (Al{µ-OEt)2AlMe2}3 )( 1), are stirred in air at room temperature, pure amorphous nanoparticulate Al2O3 precipitates. The average particle size is 17.7 ± 7.4 nm. The tetrametallic core of 1, having the correct stoichiometry for alumina (2Al2O3) appears to act as a template or nucleating site for the formation of alumina rather than the expected products of formula, AlOOH. The stability of the Al4O6 core of 1 was tested by combining it with six equivalents