Michael J. Aziz

Ion-beam Sculpting at Nanometre Length Scales

Manipulating matter at the nanometre scale is important for many electronic, chemical and biological advances, but present solid-state fabrication methods do not reproducibly achieve dimensional control at the nanometre scale. Here we report a means of fashioning matter at these dimensions that uses low-energy ion beams and reveals surprising atomic transport phenomena that occur in a variety of materials and geometries. The method is implemented in a feedback-controlled sputtering system that provides fine control over ion beam exposure and sample temperature. We call the method “ion-beam sculpting”, and apply it to the problem of fabricating a molecular-scale hole, or nanopore, in a thin insulating solid-state membrane. Such pores can serve to localize molecular-scale electrical junctions and switches and function as masks to create other small-scale structures. Nanopores also function as membrane channels in all living systems, where they serve as extremely sensitive electro-mechanical devices that regulate electric potential, ionic flow, and molecular transport across cellular membranes. We show that ion-beam sculpting can be used to fashion an analogous solid-state device: a robust electronic detector consisting of a single nanopore in a Si3N4 membrane, capable of registering single DNA molecules in aqueous solution.

Model for Solute Redistribution During Rapid Solidification

A microscopic model for impurity uptake at a sharp crystal‐liquid interface during alloy solidification is presented in terms of the bulk properties of the liquid and solid phases. The results for stepwise growth and continuous growth at the same interface velocity differ quantitatively but exhibit the same qualitative features. A transition from equilibrium segregation to complete solute trapping occurs as the velocity surpasses the diffusive speed of solute in the liquid. The location of the transition varies little with equilibrium segregation coefficient, and a kinetic limit to solute trapping is found to be quite unlikely. Comparison is made with other models; critical differences are pointed out. Coupled with a growth velocity equation and with macroscopic heat‐ and solute‐diffusion equations, the model forms a complete description of one‐dimensional crystal growth. The steady‐state solution to this system is indicated for the case of a planar interface. The results are applied to describe regrowth ...

A metal-free organic–inorganic aqueous flow battery

As the fraction of electricity generation from intermittent renewable sources—such as solar or wind—grows, the ability to store large amounts of electrical energy is of increasing importance. Solid-electrode batteries maintain discharge at peak power for far too short a time to fully regulate wind or solar power output. In contrast, flow batteries can independently scale the power (electrode area) and energy (arbitrarily large storage volume) components of the system by maintaining all of the electro-active species in fluid form. Wide-scale utilization of flow batteries is, however, limited by the abundance and cost of these materials, particularly those using redox-active metals and precious-metal electrocatalysts. Here we describe a class of energy storage materials that exploits the favourable chemical and electrochemical properties of a family of molecules known as quinones. The example we demonstrate is a metal-free flow battery based on the redox chemistry of 9,10-anthraquinone-2,7-disulphonic acid (AQDS). AQDS undergoes extremely rapid and reversible two-electron two-proton reduction on a glassy carbon electrode in sulphuric acid. An aqueous flow battery with inexpensive carbon electrodes, combining the quinone/hydroquinone couple with the Br2/Br− redox couple, yields a peak galvanic power density exceeding 0.6 W cm−2 at 1.3 A cm−2. Cycling of this quinone–bromide flow battery showed >99 per cent storage capacity retention per cycle. The organic anthraquinone species can be synthesized from inexpensive commodity chemicals. This organic approach permits tuning of important properties such as the reduction potential and solubility by adding functional groups: for example, we demonstrate that the addition of two hydroxy groups to AQDS increases the open circuit potential of the cell by 11% and we describe a pathway for further increases in cell voltage. The use of π-aromatic redox-active organic molecules instead of redox-active metals represents a new and promising direction for realizing massive electrical energy storage at greatly reduced cost.

Continuous growth model for interface motion during alloy solidification

Abstract A model is developed that predicts the steady state velocity of a planar interface and the chemical composition of the growing phase in terms of the interface temperature and the composition of the parent phase at the interface. The model is applied to solidification of a two-component melt. Solute partitioning is treated by a previously developed continuous growth model for solute trapping. The interface velocity is found by generalizing the driving force in a velocity-vs-driving force function used for solidification of one-component melts. Two different ways of generalizing the driving force are used, with and without the inclusion of a “solute drag” term. Predictions are made both with and without solute drag for an ideal solution and for AgCu, a simple eutectic system in which the terminal phases have the same crystal structure. In both cases, a transition from diffusion-controlled to diffusionless solidification and a falling interface temperature occur as the interface velocity increases. In the model without solute drag, significantly less interfacial undercooling is predicted than in the model with solute drag. The relationship to previous theoretical work, especially to the continuum treatments of Baker and Cahn, and to pertinent experiments is discussed.

Comparison of structure and properties of femtosecond and nanosecond laser-structured silicon

We compare the optical properties, chemical composition, and crystallinity of silicon microstructures formed in the presence of SF6 by femtosecond laser irradiation and by nanosecond laser irradiation. In spite of very different morphology and crystallinity, the optical properties and chemical composition of the two types of microstructures are very similar. The structures formed with femtosecond (fs) pulses are covered with a disordered nanocrystalline surface layer less than 1 μm thick, while those formed with nanosecond (ns) pulses have very little disorder. Both ns-laser-formed and fs-laser-formed structures absorb near-infrared (1.1–2.5 μm) radiation strongly and have roughly 0.5% sulfur impurities.

Alkaline quinone flow battery

A solution for scalable-flow batteries Flow batteries, in which the redox active components are held in tanks separate from the active part of the cell, offer a scalable route for storing large quantities of energy. A challenge for their large-scale development is to avoid formulations that depend on toxic transition metal ions. Lin et al. show that quinones can be dissolved in alkaline solutions and coupled with ferricyanides to make a flow cell battery (see the Perspective by Perry). This gives scope for developing flow cells with very low costs, high efficiencies at practical power densities, simplicity of operation, and inherent safety. Science, this issue p. 1529; see also p. 1452 A flow battery is designed with low-toxicity, Earth-abundant materials. [Also see Perspective by Perry] Storage of photovoltaic and wind electricity in batteries could solve the mismatch problem between the intermittent supply of these renewable resources and variable demand. Flow batteries permit more economical long-duration discharge than solid-electrode batteries by using liquid electrolytes stored outside of the battery. We report an alkaline flow battery based on redox-active organic molecules that are composed entirely of Earth-abundant elements and are nontoxic, nonflammable, and safe for use in residential and commercial environments. The battery operates efficiently with high power density near room temperature. These results demonstrate the stability and performance of redox-active organic molecules in alkaline flow batteries, potentially enabling cost-effective stationary storage of renewable energy.

Thermodynamics of diffusion under pressure and stress: Relation to point defect mechanisms

A thermodynamic formalism is developed for illuminating the predominant point defect mechanism of self- and impurity diffusion in silicon and is used to provide a rigorous basis for point defect-based interpretation of diffusion experiments in biaxially strained epitaxial layers in the Si–Ge system. A specific combination of the hydrostatic and biaxial stress dependences of the diffusivity is ±1 times the atomic volume, depending upon whether the predominant mechanism involves vacancies or interstitials. Experimental results for Sb diffusion in biaxially strained Si–Ge films and ab initio calculations of the activation volume for Sb diffusion by a vacancy mechanism are in quantitative agreement with no free parameters. Key parameters are identified that must be measured or calculated for a quantitative test of interstitial-based mechanisms.

The energy penalty of post-combustion CO2 capture & storage and its implications for retrofitting the U.S. installed base

A review of the literature has found a factor of 4 spread in the estimated values of the energy penalty for post-combustion capture and storage of CO2 from pulverized-coal (PC) fired power plants. We elucidate the cause of that spread by deriving an analytic relationship for the energy penalty from thermodynamic principles and by identifying which variables are most difficult to constrain. We define the energy penalty for CCS to be the fraction of fuel that must be dedicated to CCS for a fixed quantity of work output. That penalty can manifest itself as either the additional fuel required to maintain a power plants output or the loss of output for a constant fuel input. Of the 17 parameters that constitute the energy penalty, only the fraction of available waste heat that is recovered for use and the 2nd-law separation efficiency are poorly constrained. We provide an absolute lower bound for the energy penalty of ∼11%, and we demonstrate to what degree increasing the fraction of available-waste-heat recovery can reduce the energy penalty from the higher values reported. It is further argued that an energy penalty of ∼40% will be easily achieved while one of ∼29% represents a decent target value. Furthermore, we analyze the distribution of PC plants in the U.S. and calculate a distribution for the additional fuel required to operate all these plants with CO2 capture and storage (CCS).

Pressure-Enhanced Crystallization Kinetics of Amorphous Si and Ge: Implications for Point-Defect Mechanisms

The effects of hydrostatic pressure on the solid‐phase epitaxial growth (SPEG) rate v of intrinsic Ge(100) and undoped and doped Si(100) into their respective self‐implanted amorphous phases are reported. Samples were annealed in a high‐temperature, high‐pressure diamond anvil cell. Cryogenically loaded fluid Ar, used as the pressure transmission medium, ensured a clean and hydrostatic environment. v was determined by in situ time‐resolved visible (for Si) or infrared (for Ge) interferometry. v increased exponentially with pressure, characterized by a negative activation volume of −0.46Ω in Ge, where Ω is the atomic volume, and −0.28Ω in Si. The activation volume in Si is independent of both dopant concentration and dopant type. Structural relaxation of the amorphous phases has no significant effect on v. These and other results are inconsistent with all bulk point‐defect mechanisms, but consistent with all interface point‐defect mechanisms, proposed to date.A kinetic analysis of the Spaepen–Turnbull inte...

Theory for the trapping of disorder and solute in intermetallic phases by rapid solidification

Abstract A theory is developed to predict the long range order parameter, composition and temperature at the interface of a chemically ordered phase as a function of interface velocity and liquid composition during rapid crystal growth. It extends the solute trapping theory of Aziz to a solid phase consisting of two sublattices. The engulfment of atoms randomly on the two sublattices by the rapidly moving liquid-solid interface is balanced against the interdiffusion across the interface that attempts to restore local equilibrium. With increasing interface velocity the theory predicts a progression from the solidification of a phase with equilibrium long range order parameter and with equilibrium solute partitioning to the solidification of a disordered crystalline phase with the same composition as the liqiud. Predictions for solids with free energy functions in which the order disorder transition is first or second order show that the decrease of order parameter to zero with increasing interface velocity will be discontinuous or continuous respectively. Also solute trapping can occur at either a higher or a lower growth rate than disorder trapping depending on the free energy function.