Nanoscale density fluctuations in swift heavy ion irradiated amorphous SiO2

We report on the observation of nanoscale density fluctuations in 2 μm thick amorphous SiO₂ layers irradiated with 185 MeV Au ions. At high fluences, in excess of approximately 5 × 10¹² ions/cm², where the surface is completely covered by ion tracks, synchrotron small angle x-ray scattering measurements reveal the existence of a steady state of density fluctuations. In agreement with molecular dynamics simulations, this steady state is consistent with an ion track “annihilation” process, where high-density regions generated in the periphery of new tracks fill in low-density regions located at the center of existing tracks.The authors acknowledge the Australian Research Council and the Australian Synchrotron Research Program for financial support and thank the staff at the ANU Heavy Ion facility for their continued technical assistance. O.P., F.D., and K.N. acknowledge financial support from the Academy of Finland under its Centre of Excellence program as well as the OPNA project, and grants of computer capacity from CSC

First-principles study of adsorption, diffusion, and charge stability of metal adatoms on alkali halide surfaces

In this work we have performed first-principles calculations based on the spin-polarized density-functional theory for the adsorption and diffusion of Au, Ag, and Pb atoms on NaCl(001), KCl(001), and KBr(001) surfaces. We consider also the influence of adatom charge on the adsorption and diffusion. In order to characterize the different systems we explicitly calculate charge transfer between surface and adatom and consider the relative stability of the various charge states. Our results show that in general, apart from positively charged systems, the adatoms are weakly bound to the surface via orbital polarization and ionic interactions, and relatively little charge transfer occurs. Au and Ag adatoms are highly mobile on all surfaces, although they can be pinned by removal of an electron. In contrast, Pb adatoms are fairly immobile, and their mobility increases upon charging. Analysis of the charge stability suggests that Ag offers the potential of charge controlled mobility on insulators.Peer reviewe

High-resolution scanning force microscopy of gold nanoclusters on the KBr (001) surface

In this study we use a combination of dynamic scanning force microscopy experiments and first-principles simulations to study the imaging process of gold nanoclusters adsorbed on the (001) surface of KBr. In previous experiments atomic resolution was readily obtained on the KBr substrate. However, it was not possible to resolve atoms within the clusters themselves. This correlates with imaging simulations we present here using several different probable tip models: measurable contrast was readily achieved on the KBr surface and on the gold (001) surface, but simulations on the clusters demonstrated poor contrast for all tips. We further consider the role of cluster charging in the tip-surface interaction, and the role that surface defects play in the properties of adsorbed clusters.Peer reviewe

The so-called dry laser cleaning governed by humidity at the nanometer scale

Illumination with single nanosecond pulses leads to the detachment of silica particles with 250nm radii from siliconsurfaces. We identify two laser-energy dependent cleaning regimes by time-of-flight particle-scattering diagnostics. For the higher energies, the ejection of particles is produced by nanoscale ablation due to the laser field enhancement at the particle-surface interface. The damage-free regime at lower energy is shown to be governed by the residual water molecules, which are inevitably trapped on the materials. We discuss the great importance that the humidity plays on the cleaning force and on the adhesion in the experiments.Peer reviewe

Simulating atomic force microscopy imaging of the ideal and defected TiO2 (110) surface

In this study we simulate noncontact atomic force microscopy imaging of the TiO2 (110) surface using first-principles and atomistic methods. We use three different tip models to investigate the tip-surface interaction on the ideal surface, and find that agreement with experiment is found for either a silicon tip or a tip with a net positive electrostatic potential from the apex. Both predict bright contrast over the bridging oxygen rows. We then study the interaction of this tip with a bridging oxygen vacancy on the surface, and find that the much weaker interaction observed would result in vacancies appearing as dark contrast along the bright rows in images.Peer reviewe

Role of van der Waals forces in the adsorption and diffusion of organic molecules on an insulating surface

The adsorption and diffusion of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules on a nanostructured KBr (001) surface were investigated by combining noncontact atomic force microscopy (NC-AFM) and first-principles calculations. Atomically resolved measurements demonstrate trapping of PTCDA molecules in intentionally created rectangular monolayer-deep substrate pits and a preferential adsorption at kink sites. In order to understand the experimental results, we found that it was essential to include a first-principles treatment of the van der Waals interactions. We show that at some sites on the surface, 85% of the molecular binding is provided by van der Waals interactions, and in general it is always the dominant contribution to the adsorption energy. It also qualitatively changes molecular diffusion on the surface. Based on the specificity of the molecular interaction at kink sites, the species of the imaged ionic sublattice in the NC-AFM measurements could be identified.Peer reviewe

Clinical and microbiological characterization of Aerococcus urinae bacteraemias at Helsinki metropolitan area, Finland

Our objective was to assess the incidence of bacteraemic Aerococcus urinae cases at Helsinki metropolitan area, Finland, from a 6-year study period (2013 to 2018) and to further characterize available cases. The study evaluates the outcome of commonly used cefuroxime treatment and determinate a set of A. urinae in vitro antimicrobial susceptibilities for benzylpenicillin, cefuroxime, and ceftriaxone. Clinical records of A. urinae bacteraemic patients were reviewed retrospectively. Antimicrobial susceptibility testing was performed by disk diffusion, gradient test, and broth microdilution for 139-141 clinical A. urinae isolates. Clinical data of 72/77 patients were combined with the in vitro susceptibilities. We found an increasing number of bacteraemic A. urinae cases within 6-year study period (p = 0.01). The patients were mainly elderly males, and all suffered from underlying conditions. A total of 27.3% of cases (21/77) showed polymicrobial blood cultures. Thirty-day mortality was 22.1%. Cefuroxime was the initial empiric antimicrobial agent given for 66/76 of the patients and treatment outcome was favorable for 20/22 patients who received cefuroxime at least up to day 5. All isolates were susceptible to benzylpenicillin and cefuroxime interpreted by EUCAST breakpoints for Aerococci and PK-PD breakpoints, respectively. MIC determinations gave variable results for ceftriaxone, 2.1-2.9% of the isolates were resistant. To conclude, it seems that the number of bacteraemic Aerococcus urinae cases is increasing at Helsinki metropolitan area, Finland, reflecting the growing blood culture sampling. Clinical A. urinae isolates were susceptible to cefuroxime in vitro. Treatment data indicate that empirical cefuroxime started for possibly urinary tract -derived community-acquired bacteraemia covers A. urinae.Peer reviewe

Atomic dynamics in evaporative cooling of trapped alkali atoms in strong magnetic fields

We investigate how the nonlinearity of the Zeeman shift for strong magnetic fields affects the dynamics of rf field induced evaporative cooling in magnetic traps. We demonstrate for the 87-Rb and 23-Na F=2 trapping states with wave packet simulations how the cooling stops when the rf field frequency goes below a certain limit (for the 85-Rb F=2 trapping state the problem does not appear). We examine the applicability of semiclassical models for the strong field case as an extension of our previous work [Phys. Rev. A 58, 3983 (1998)]. Our results verify many of the aspects observed in a recent $^{87}$Rb experiment [Phys. Rev. A 60, R1759 (1999)].Comment: 9 pages, RevTex, eps figures embedde

An XPS study of CrO x on a thin alumina film and in alumina supported catalysts

Abstract We have investigated chromium layers evaporated onto a thin alumina film at room temperature. The oxidation and reduction behavior of this model catalyst was compared to atomic layer deposition (ALD) and impregnated alumina supported catalysts using X-ray photoelectron spectroscopy (XPS) with a detailed analysis method utilizing asymmetric peak shapes to represent both metallic and oxidic states. The ALD and impregnated catalysts were measured after calcination in air and after reduction with several gases at 850 K. Both catalysts show Cr 3+ and Cr 6+ species after calcination and mostly Cr 3+ after reduction. The chromium layers deposited in vacuum show initially small partial oxidation due to the interaction with the oxygen terminated alumina film. These model catalysts can be oxidized in vacuum to Cr 3+ species but not to higher oxidation states. The model catalysts were also subjected to calcination and reduction treatments after deposition in vacuum. Under these conditions the model systems exhibit similar oxidation/reduction behavior as the supported catalysts. Photoreduction of Cr 6+ during the measurements was also studied and found to be very slow having a negligible effect on the results. # 2005 Elsevier B.V. All rights reserved. 82.80.P

Towards chemical identification in atomic-resolution noncontact AFM imaging with silicon tips

In this study we use ab initio calculations and a pure silicon tip to study the tip-surface interaction with four characteristic insulating surfaces: (i) the narrow gap TiO2 (110) surface, (ii) the classic oxide MgO (001) surface, (iii) the ionic solid CaCO3 (101¯4) surface with molecular anion, and (iv) the wide gap CaF2 (111) surface. Generally we find that the tip-surface interaction strongly depends on the surface electronic structure due to the dominance of covalent bond formation with the silicon tip. However, we also find that in every case the strongest interaction is with the highest anion of the surface. This result suggests that, if the original silicon tip can be carefully controlled, it should be possible to immediately identify the species seen as bright in images of insulating surfaces. In order to provide a more complete picture we also compare these results to those for contaminated tips and suggest how applied voltage could also be used to probe chemical identity.Peer reviewe