Mass Redistribution Causes the Structural Richness of Ion-Irradiated Surfaces

We show that the “sputter patterning” topographical instability is determined by the effects of ion impact-induced prompt atomic redistribution and that erosion—the consensus predominant cause—is essentially irrelevant. We use grazing incidence small angle x-ray scattering to measure in situ the damping of noise or its amplification into patterns via the linear dispersion relation. A model based on the effects of impact-induced redistribution of those atoms that are not sputtered away explains both the observed ultrasmoothening at low angles from normal incidence and the instability at higher angles.Engineering and Applied Science

Time-resolved measurements of nanoscale surface pattern formation kinetics in two dimensions on ion-irradiated Si

The nanoscale kinetics of surface topography evolution on silicon surfaces irradiated with 1 keV Ar+ ions is examined in both directions perpendicular and parallel to the projection of the ion beam on the surface. We use grazing incidence small angle x-ray scattering to measure in situ the evolution of surface morphology via the linear dispersion relation. We study the transition from surface ultra-smoothening at low angles of deviation from normal ion incidence to a pattern-forming instability at high incidence angles. A model based on the effects of impact-induced redistribution of those atoms that are not sputtered away explains both the observed ultra-smoothening at low angles from normal ion incidence and the instability at higher angles and accounts quantitatively for the measured two-dimensional dispersion relation and its dependence on incidence angle.Physic

Nanoscale surface structuring during ion bombardment of elemental semiconductors

Thesis (Ph.D.)--Boston University PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at [email protected]. Thank you.Nano-patterning of surfaces with uniform ion bombardment yields a rich phase-space of topographic patterns. Particle irradiation can cause surface ultra-smoothing or selforganized nanoscale pattern formation in surface topography. Topographic pattern formation has previously been attributed to the effects of the removal of target atoms by sputter erosion. In this thesis, the surface morphology evolution of Si(100) and Ge(100) during low energy ion bombardment of Ar+ and Kr+ ions, respectively, is studied. Our facilities for studies of surface processes at the National Synchrotron Light Source (NSLS) allow in-situ characterization of surface morphology evolution during ion bombardment using grazing incidence small angle x-ray scattering (GISAXS). This technique is used to measure in reciprocal space the kinetics of formation or decay of correlated nanostructures on the surface, effectively measuring the height-height correlations. A linear model is used to characterize the early time kinetic behavior during ion bombardment as a function of ion beam incidence angle. The curvature coefficients predicted by the widely used erosive model of Bradley and Harper are quantitatively negligible and of the wrong sign when compared to the observed effect in both Si and Ge. A mass-redistribution model explains the observed ultra-smoothing at low angles, exhibits an instability at higher angles, and predicts the observed 45° critical angle separating these two regimes in Si. The Ge surface evolution during Kr+ irradiation is qualitatively similar to that observed for Ar+ irradiation of Si at the same ion energy. However, the critical angle for Ge cannot be quantitatively reproduced by the simple mass redistribution model. Crater function theory, as developed by Norris et al., incorporates both mass redistributive and erosive effects, and predicts constraining relationships between curvature coefficients. These constraints are compared to experimental data of both Si and Ge. There is good agreement at low incidence angles; however, at higher angles the data disagrees with the predictions of the crater function formalism. This calls into question the ability of crater function theory to model the surface morphology evolution during ion bombardment

Model-independent test of the truncated crater function theory of surface morphology evolution during ion bombardment

A broad class of “local response” theories seeks to predict morphology evolution during energetic particle irradiation in terms of average surface height response to individual impacts—an approach that has been generalized by the crater function formalism of Norris et al. [J. Phys.: Condens. Matter 21, 224017 (2009); Nat. Commun. 2, 276 (2011)]. Keeping only the terms in the crater function formalism associated with the response of a flat surface has facilitated the use of molecular dynamics simulations of individual ion impacts to predict the stability or instability of a flat surface to ion bombardment. Here we report a sensitive experimental test of this truncated crater function theory that is independent of any a priori knowledge of the crater function itself. Existing measurements for 1 keV Ar+/Si and Kr+/Ge are inconsistent with the predictions of truncated crater function theory, for any conceivable crater function, at high bombardment angles. The failure of the theory suggests that the prediction of surface evolution from simulations of single-ion impacts will be more challenging than had been assumed.Engineering and Applied Science

Distinguishing physical mechanisms using GISAXS experiments and linear theory: the importance of high wavenumbers

In this work we analyze GISAXS measurements of the structure factor of Si surfaces evolving during 1 keV Ar+ ion bombardment. Using newly-developed methods sensitive to the full range of experimentally-available wavenumbers q, we extract the linear amplification rate R(q) governing surface stability over a range of wavenumbers 4–5 times larger than has previously been obtained. Comparing with theoretical models also retaining full wavenumber-dependence, we find an excellent fit of the experimental data over the full range of irradiation angles and wavenumbers. Moreover, the fitted parameter values represent experimental evaluation of the magnitudes of most physical mechanisms currently believed to be important to the pattern-formation process. In all cases, the extracted values agree well with direct observations or atomistic simulations of the same quantities, suggesting that GISAXS analysis may allow more powerful comparison between experiment and theory than had previously been thought

Oxygen-atom transfer chemistry and thermolytic properties of a di-tert-butylphosphate-ligated Mn4O4 cubane.

[Mn4O4{O2P(OtBu)2}6] (1), an Mn4O4 cubane complex combining the structural inspiration of the photosystem II oxygen-evolving complex with thermolytic precursor ligands, was synthesized and fully characterized. Core oxygen atoms within complex 1 are transferred upon reaction with an oxygen-atom acceptor (PEt3), to give the butterfly complex [Mn4O2{O2P(OtBu)2}6(OPEt3)2]. The cubane structure is restored by reaction of the latter complex with the O-atom donor PhIO. Complex 1 was investigated as a precursor to inorganic Mn metaphosphate/pyrophosphate materials, which were studied by X-ray absorption spectroscopy to determine the fate of the Mn4O4 unit. Under the conditions employed, thermolyses of 1 result in reduction of the manganese to Mn(II) species. Finally, the related butterfly complex [Mn4O2{O2P(pin)}6(bpy)2] (pin = pinacolate) is described

Discovering Ce-rich oxygen evolution catalysts, from high throughput screening to water electrolysis

Discovering improved electrocatalysts for the oxygen evolution reaction (OER) is of great importance for efficient solar fuels generation, electrowinning of metals, regenerative fuel cells, and recharging metal air batteries. The slow kinetics of the 4- electron OER requires large overpotentials to drive water oxidn. at appreciable current densities. Among the numerous compns. investigated, mixed metal oxides in the (Ni-Fe)Ox and (Ni-Co)Ox compn. spaces are among the most active and most studied OER catalysts. Although this technol. important reaction has been studied for more than 50 years, many of the mechanistic details remain under investigation. Lacking a robust fundamental understanding of the basic science and mechanistic details of multi-electron heterogeneous electrocatalysis, an efficient high-throughput synthesis and property screening methodol. is well-suited to discovering the requisite new catalytic materials. We have established high throughput methods to systematically investigate the performance of pseudo-quaternary material libraries as OER electrocatalysts. We report a new Cerich family of active catalysts composed of earth abundant elements, which was discovered using high-throughput methods to produce 5456 discrete compns. in the (Ni-Fe-Co-Ce)Ox compn. space. The activity and stability of this new OER catalyst was verified by re-synthesis and extensive electrochem. testing of samples in a std. format in 1.0 M NaOH, as well as by operation in a photovoltaic-powered electrolyzer for more than 100 h. The most interesting variations in activity lie in a pseudoternary crosssectional plane contg. 665 compns. Our detailed investigation of this psuedoternary cross-section has revealed systematic trends in Tafel slopes and electrochem. signals with compn., which provide a connection between the previously known Ni-Fe and newly discovered Ni-Co-Ce catalysts. Characterization of selected compns. by XRD, XPS, SEM, TEM, EDS, XRF mapping, and EXAFS, both as-synthesized and after electrochem. testing, reveal important differences in nanostructure and stability along with the obsd. differences in electrochem. performance under OER conditions