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 from laser‐induced melting. Preliminary comparison with experiment is made. The steady‐state solution for thermal and impurity transport is suggested for use whenever detailed computer calculations are unavailable or are unnecessarily involved.

/pdf/model-for-solute-redistribution-during-rapid-solidification-2dc0uhmnya.pdf

Model for solute redistribution during rapid solidification

Kinetics of solid phase crystallization in amorphous silicon

A parameterization for band-to-band Auger recombination in silicon at 300 K is proposed. This general parameterization accurately fits the available experimental lifetime data for arbitrary injection level and arbitrary dopant density, for both n-type and p-type dopants. We confirm that Auger recombination is enhanced above the traditional free-particle rate at both low injection and high injection conditions. Further, the rate of enhancement is shown to be less for highly injected intrinsic silicon than for lowly injected doped silicon, consistent with the theory of Coulomb-enhanced Auger recombination. Variations on the parameterization are discussed.

General parameterization of Auger recombination in crystalline silicon

As defined by the European Commission, nanomaterial is a natural, incidental or manufactured material containing particles in an unbound state or as an aggregate or agglomerate in which ≥ 50% of the particles in the number size distribution have one or more external dimensions in the size range 1 to 100 nm. In specific cases and where warranted by concerns for the environment, health, safety or competition, the number size distribution threshold of 50% may be replaced with a threshold between 1 and 50%.1 Engineered nanomaterials (ENMs) refer to man-made nanomaterials. Materials in the nanometer range often possess unique physical, optical, electronic, and biological properties compared with larger particles, such as the strength of graphene,2 the electronic properties of carbon nanotubes (CNTs),3 the antibacterial activity of silver nanoparticles4 and the optical properties of quantum dots (QDs).5 The unique and advanced properties of ENMs have led to a rapid increase in their application. These applications include aerospace and airplanes, energy, architecture, chemicals and coatings, catalysts, environmental protection, computer memory, biomedicine and consumer products. Driven by these demands, the worldwide ENM production volume in 2016 is conservatively estimated in a market report by Future Markets to be 44,267 tons or ≥ $5 billion.6

As the production and applications of ENMs rapidly expand, their environmental impacts and effects on human health are becoming increasingly significant.7 Due to their small sizes, ENMs are easily made airborne.8 However, no accurate method to quantitatively measure their concentration in air currently exists. A recently reported incident of severe pulmonary fibrosis caused by inhaled polymer nanoparticles in seven female workers obtained much attention.9 In addition to the release of ENM waste from industrial sites, a major release of ENMs to environmental water occurs due to home and personal use of appliances, cosmetics and personal products, such as shampoo and sunscreen.10 Airborne and aqueous ENMs pose immediate danger to the human respiratory and gastrointestinal systems. ENMs may enter other human organs after they are absorbed into the bloodstream through the gastrointestinal or respiratory systems.11,12 Furthermore, ENMs in cosmetics and personal care products, such as lotion, sunscreen and shampoo may enter human circulation through skin penetration.13 ENMs are very persistent in the environment and are slowly degraded. The dissolved metal ions from ENMs can also revert back to nanoparticles under natural conditions.14 ENMs are stored in plants, microbes and animal organs and can be transferred and accumulated through the food chain.15,16 In addition to the accidental entry of ENMs into human and biological systems, ENMs are also purposefully injected into or enter humans for medicinal and diagnostic purposes.17 Therefore, interactions of ENMs with biological systems are inevitable.

In addition to engineered nanomaterials, there are also naturally existing nanomaterials such as proteins and DNA molecules, which are key components of biological systems. These materials, combined with lipids and organic and inorganic small molecules, form the basic units of living systems –cells.18 To elucidate how nanomaterials affect organs and physiological functions, a thorough understanding of how nanomaterials perturb cells and biological molecules is required (Figure 1). Rapidly accumulating evidence indicates that ENMs interact with the basic components of biological systems, such as proteins, DNA molecules and cells.19-21 The driving forces for such interactions are quite complex and include the size, shape and surface properties (e.g., hydrophobicity, hydrogen-bonding capability, pi-bonds and stereochemical interactions) of ENMs.22-25



Figure 1

Interactions of nanoparticles with biological systems at different levels. Nanoparticles enter the human body through various pathways, reaching different organs and contacting tissues and cells. All of these interactions are based on nanoparticle-biomacromolecule ...



Evidence also indicates that chemical modifications on a nanoparticle’s surface alter its interactions with biological systems.26-28 These observations not only support the hypothesis that basic nano-bio interactions are mainly physicochemical in nature but also provide a powerful approach to controlling the nature and strength of a nanoparticle’s interactions with biological systems. Practically, a thorough understanding of the fundamental chemical interactions between nanoparticles and biological systems has two direct impacts. First, this knowledge will encourage and assist experimental approaches to chemically modify nanoparticle surfaces for various industrial or medicinal applications. Second, a range of chemical information can be combined with computational methods to investigate nano-biological properties and predict desired nanoparticle properties to direct experiments.29-31

The literature regarding nanoparticle-biological system interactions has increased exponentially in the past decade (Figure 2). However, a mechanistic understanding of the chemical basis for such complex interactions is still lacking. This review intends to explore such an understanding in the context of recent publications.



Figure 2

An analysis of literature statistics indicates growing concern for the topics that are the focus of this review. The number of publications and citations were obtained using the keywords “nanoparticles” and “biological systems” ...



A breakthrough technology cannot prosper without wide acceptance from the public and society; that is, it must pose minimal harm to human health and the environment. Nanotechnology is now facing such a critical challenge. We must elucidate the effects of ENMs on biological systems (such as biological molecules, human cells, organs and physiological systems). Accumulating experimental evidence suggests that nanoparticles interact with biological systems at nearly every level, often causing unwanted physiological consequences. Elucidating these interactions is the goal of this review. This endeavor will help regulate the proper application of ENMs in various products and their release into the environment. A more significant mission of this review is to direct the development of “safe-by-design” ENMs, as their demands for and applications continue to increase.

Chemical Basis of Interactions Between Engineered Nanoparticles and Biological Systems

https://aziz.seas.harvard.edu/files/mja026.pdf

Continuous growth model for interface motion during alloy solidification

The transition to single crystal of ion‐implanted amorphous Si and Ge layers is described in terms of a liquid‐phase epitaxy occurring during pulsing‐laser irradiation. A standard heat equations including laser light absorption was solved numerically to give the time evolution of temperature and melting as a function of the pulse energy density and its duration. The structure dependence of the absorption coefficient and the temperature dependence of the thermal conductivity were accounted for in the calculations. In this model the transition to single crystal occurs above a well‐defined threshold energy density at which the liquid layer wets the underlying single‐crystal substrate. Experiments were performed in ion‐implanted amorphous layers of thicknesses ranging between 500 and 9000 A. The energy densities of the Q‐switched ruby laser ranged between 0.2 and 3.5 J/cm2; time durations of 20 and 50 ns were used. The experimental data are in good agreement with the calculated values for the amorphous thickness–energy−density threshold. The model deals mainly with plausibility arguments and does not account for processes occuring in the near‐threshold region or below the melting temperature.

A melting model for pulsing‐laser annealing of implanted semiconductors

The time evolution of temperature and melting in amorphous silicon layers laser irradiated was calculated numerically. Experiments were performed in Si crystals implanted with 400‐keV As to a dose of 5×1015/cm2 and illuminated with 50‐ns‐duration Q‐switched ruby laser pulse in the energy range 1.0–3.0 J/cm2. Comparison between experimental and calculated results allows a quantitative understanding of the amorphous–to–single‐crystal transition. A good agreement was found between the experimental As profiles after laser irradiation and those calculated with a diffusion coefficient of 10−4 cm2/s for As in liquid silicon.

Arsenic diffusion in silicon melted by high‐power nanosecond laser pulsing

Interface segregation coefficients have been measured for Bi in Si for melt growth as a function of velocity for (111) and (100) crystals. Surface layers were melted by ruby laser irradiation and liquid‐solid interface velocities varied from 0.8 to 5 m/s by changing Si substrate temperatures or laser pulse length. Segregation coefficients are strongly dependent on velocity and orientation in this range.

Orientation and velocity dependence of solute trapping in Si

Cu-implanted Si crystals were irradiated with $Q$-switched ruby-laser single pulses. After irradiation with energy density in excess of 1.0 J/${\mathrm{cm}}^{2}$, the Cu atoms accumulate at the sample surface. Thermal annealing in the 500-800\ifmmode^\circ\else\textdegree\fi{}C range casues a migration of Cu inside the specimen, in agreement with diffusion coefficient and solid solubility values. The results indicate the formation of a liquid layer induced by laser irradiation. The solid-liquid interface movement during freezing qualitatively justifies the observed surface accumulation.

Segregation Effects in Cu-Implanted Si after Laser-Pulse Melting

Experiments are reported for Te and Ag implantation in silicon, as examples of slow and fast diffusers, after furnace or laser annealing. Slow diffusers are substitutionally located at concentrations in great excess of the maximum solid solubility after both processes. Fast diffusers inhibit the solid‐phase epitaxial regrowth or are rejected at the sample surface after laser irradiation. Although the epitaxial growth occurs with velocities which differ up to ten orders of magnitude after furnace or laser annealing, the supersaturation is interpreted as due to the same basic mechanism: solute trapping at the moving interface when the residence time is larger than the one monolayer regrowth time. This process is controlled by the diffusion coefficient in the two adjacent phases.

P. Baeri

Papers

A melting model for pulsing‐laser annealing of implanted semiconductors

Arsenic diffusion in silicon melted by high‐power nanosecond laser pulsing

Orientation and velocity dependence of solute trapping in Si

Segregation Effects in Cu-Implanted Si after Laser-Pulse Melting

Solute trapping by moving interface in ion‐implanted silicon