Surface Relaxation Mechanisms in the Morphological Equilibration of Crystal Surfaces

The phenomenology of surface relaxation below the roughening temperature TR is shown via kinetic Monte Carlo simulation to exhibit extremely non-classical behavior depending on (1) initial morphology and (2) kinetic rate laws for atom motion. However, within the temperature and size scale of our simulations there are several different mechanisms that operate. The “pinch-off” mechanism is seen to be operative for ripple relaxation employing i-kinetics. For ripple relaxation employing ∆i-kinetics, pinch-off is seen at short times. At long times, these particular surfaces spontaneously island, and then reduction of line tension drives relaxation. Reduction of line tension also drives relaxation of dimpled surfaces.Engineering and Applied Science

Ion-Sputter Induced Rippling of Si(111)

The morphology of ion sputtered Si(111) surfaces at glancing angles and elevated temperatures was studied by AFM. Under particular ion beam and sample conditions, the morphology of the surfaces was found to be rippled, with roughly sinusoidal-shaped bumps. This observation is in agreement with predictions of Bradley and Harper, and is similar to experiments performed by Chason et al. on germanium. Observation of the temporal evolution by stopping growth and performing microscopy suggests that a steady-state rippled morphology develops by coalescence of oblong islands. The wavelength of the ripples can be controlled up to about 600 nm, with amplitudes up to 60 nm. This observation creates an opportunity to make nanostructured surfaces for the study of crystal surface dynamics.Engineering and Applied Science

Evolution of Nanoporosity in Dealloying

Dealloying is a common corrosion process during which an alloy is "parted" by the selective dissolution of the electrochemically more active elements. This process results in the formation of a nanoporous sponge composed almost entirely of the more noble alloy constituents . Even though this morphology evolution problem has attracted considerable attention, the physics responsible for porosity evolution have remained a mystery . Here we show by experiment, lattice computer simulation, and a continuum model, that nanoporosity is due to an intrinsic dynamical pattern formation process - pores form because the more noble atoms are chemically driven to aggregate into two-dimensional clusters via a spinodal decomposition process at the solid-electrolyte interface. At the same time, the surface area continuously increases due to etching. Together, these processes evolve a characteristic length scale predicted by our continuum model. The applications potential of nanoporous metals is enormous. For instance, the high surface area of nanoporous gold made by dealloying Ag-Au can be chemically tailored, making it suitable for sensor applications, particularly in biomaterials contexts.Comment: 13 pages, PDF, incl. 4 figures. avi movies of simulations available at http://www.deas.harvard.edu/matsci/downdata/downdata.htm

One and Two-Dimensional Pattern Formation on Ion Sputtered Silicon

The evolution of surface morphology during ion beam erosion of Si(111) at glancing ion incidence (60o from normal, 500 eV Ar+, 0.75 mA/cm2 collimated beam current) was studied over a temperature range of 500-730o Celsius. Keeping ion flux, incident angle, and energy fixed, it was found that one-dimensional sputter ripples with wavevector oriented perpendicular to the projected ion beam direction form during sputtering at the lower end of the temperature range. For temperatures above approximately 690o Celsius, growth modes both parallel and perpendicular to the projected ion beam direction contribute to the surface morphological evolution. This effect leads to the formation of bumps (“dots”) with nearly rectangular symmetry.Engineering and Applied Science

One and Two-Dimensional Pattern Formation on Ion Sputtered Silicon

ABSTRACT The evolution of surface morphology during ion beam erosion of Si(111) at glancing ion incidence (60 o from normal, 500 eV Ar + , 0.75 mA/cm 2 collimated beam current) was studied over a temperature range of 500-730 o Celsius. Keeping ion flux, incident angle, and energy fixed, it was found that one-dimensional sputter ripples with wavevector oriented perpendicular to the projected ion beam direction form during sputtering at the lower end of the temperature range. For temperatures above approximately 690 o Celsius, growth modes both parallel and perpendicular to the projected ion beam direction contribute to the surface morphological evolution. This effect leads to the formation of bumps ("dots") with nearly rectangular symmetry

A high-throughput technique for determining grain boundary character non-destructively in microstructures with through-thickness grains

Grain boundaries (GBs) govern many properties of polycrystalline materials. However, because of their structural variability, our knowledge of GB constitutive relations is still very limited. We present a novel method to characterise the complete crystallography of individual GBs non-destructively, with high-throughput, and using commercially available tools. This method combines electron diffraction, optical reflectance and numerical image analysis to determine all five crystallographic parameters of numerous GBs in samples with through-thickness grains. We demonstrate the technique by measuring the crystallographic character of about 1,000 individual GBs in aluminum in a single run. Our method enables cost- and time-effective assembly of crystallography–property databases for thousands of individual GBs. Such databases are essential for identifying GB constitutive relations and for predicting GB-related behaviours of polycrystalline solids.United States. Department of Energy. Office of Basic Energy Sciences (award no DE-SC0008926)MIT International Science and Technology InitiativesNational Science Foundation (U.S.) (grant DMR-1003901

A Comparative Plasmonic Study of Nanoporous and Evaporated Gold Films

Previously, we have reported that nanoporous gold (NPG) films prepared by a chemical dealloying method have distinctive plasmonic properties, i.e., they can simultaneously support localized and propagating surface plasmon resonance modes (l-SPR and p-SPR, respectively). In this study, the plasmonic properties of NPG are quantified through direct comparison with thermally evaporated gold (EG) films. Cyclic voltammetry and electrochemical impedance spectroscopy experiments reveal that the NPG films have 4–8.5 times more accessible surface area than EG films. Assemblies of streptavidin–latex beads generate p-SPR responses on both NPG and EG films that correlate well with the bead density obtained from scanning electron microscopy (SEM) images. A layer-by-layer assembly experiment on NPG involving biotinylated anti-avidin IgG and avidin, studied by l-SPR and SEM, shows that the l-SPR signal is directly linked to the accessibility of the interior of the NPG porosity, an adjustable experimental parameter that can be set by the dealloying condition and time