1,354 research outputs found
The role of surface charge in the interaction of nanoparticles with model pulmonary surfactants
Inhaled nanoparticles traveling through the airways are able to reach the
respiratory zone of the lungs. In such event, the incoming particles first
enter in contact with the liquid lining the alveolar epithelium, the pulmonary
surfactant. The pulmonary surfactant is composed of lipids and proteins that
are assembled into large vesicular structures. The question of the nature of
the biophysicochemical interaction with the pulmonary surfactant is central to
understand how the nanoparticles can cross the air-blood barrier. Here we
explore the phase behavior of sub-100 nm particles and surfactant substitutes
in controlled conditions. Three types of surfactant mimetics, including the
exogenous substitute Curosurf, a drug administred to infants with respiratory
distress syndrome are tested together with aluminum oxide (Al2O3), silicon
dioxide (SiO2) and polymer (latex) nanoparticles. The main result here is the
observation of the spontaneous nanoparticle-vesicle aggregation induced by
Coulombic attraction. The role of the surface charges is clearly established.
We also evaluate the supported lipid bilayer formation recently predicted and
find that in the cases studied these structures do not occur. Pertaining to the
aggregate internal structure, fluorescence microscopy ascertains that the
vesicles and particles are intermixed at the nano- to microscale. With
particles acting as stickers between vesicles, it is anticipated that the
presence of inhaled nanomaterials in the alveolar spaces could significantly
modify the interfacial and bulk properties of the pulmonary surfactant and
interfere with the lung physiology.Comment: 20 pages, 6 figure
Tight-binding molecular-dynamics studies of defects and disorder in covalently-bonded materials
Tight-binding (TB) molecular dynamics (MD) has emerged as a powerful method
for investigating the atomic-scale structure of materials --- in particular the
interplay between structural and electronic properties --- bridging the gap
between empirical methods which, while fast and efficient, lack
transferability, and ab initio approaches which, because of excessive
computational workload, suffer from limitations in size and run times. In this
short review article, we examine several recent applications of TBMD in the
area of defects in covalently-bonded semiconductors and the amorphous phases of
these materials.Comment: Invited review article for Comput. Mater. Sci. (38 pages incl. 18
fig.
Amorphous silicon under mechanical shear deformations: shear velocity and temperature effects
Mechanical shear deformations lead, in some cases, to effects similar to
those resulting from ion irradiation. Here we characterize the effects of shear
velocity and temperature on amorphous silicon (\aSi) modelled using classical
molecular dynamics simulations based on the empirical Environment Dependent
Inter-atomic Potential (EDIP). With increasing shear velocity at low
temperature, we find a systematic increase in the internal strain leading to
the rapid appearance of structural defects (5-fold coordinated atoms). The
impacts of externally applied strain can be almost fully compensated by
increasing the temperature, allowing the system to respond more rapidly to the
deformation. In particular, we find opposite power-law relations between the
temperature and the shear velocity and the deformation energy. The spatial
distribution of defects is also found to strongly depend on temperature and
strain velocity. For low temperature or high shear velocity, defects are
concentrated in a few atomic layers near the center of the cell while, with
increasing temperature or decreasing shear velocity, they spread slowly
throughout the full simulation cell. This complex behavior can be related to
the structure of the energy landscape and the existence of a continuous
energy-barrier distribution.Comment: 10 pages, 17 figure
Binary continuous random networks
Many properties of disordered materials can be understood by looking at
idealized structural models, in which the strain is as small as is possible in
the absence of long-range order. For covalent amorphous semiconductors and
glasses, such an idealized structural model, the continuous-random network, was
introduced 70 years ago by Zachariasen. In this model, each atom is placed in a
crystal-like local environment, with perfect coordination and chemical
ordering, yet longer-range order is nonexistent. Defects, such as missing or
added bonds, or chemical mismatches, however, are not accounted for. In this
paper we explore under which conditions the idealized CRN model without defects
captures the properties of the material, and under which conditions defects are
an inherent part of the idealized model. We find that the density of defects in
tetrahedral networks does not vary smoothly with variations in the interaction
strengths, but jumps from close-to-zero to a finite density. Consequently, in
certain materials, defects do not play a role except for being thermodynamical
excitations, whereas in others they are a fundamental ingredient of the ideal
structure.Comment: Article in honor of Mike Thorpe's 60th birthday (to appear in J.
Phys: Cond Matt.
Biophysicochemical interaction of a clinical pulmonary surfactant with nano-alumina
We report on the interaction of pulmonary surfactant composed of
phospholipids and proteins with nanometric alumina (Al2O3) in the context of
lung exposure and nanotoxicity. We study the bulk properties of
phospholipid/nanoparticle dispersions and determine the nature of their
interactions. The clinical surfactant Curosurf, both native and extruded, and a
protein-free surfactant are investigated. The phase behavior of mixed
surfactant/particle dispersions was determined by optical and electron
microscopy, light scattering and zeta potential measurements. It exhibits broad
similarities with that of strongly interacting nanosystems such as polymers,
proteins or particles, and supports the hypothesis of electrostatic
complexation. At a critical stoichiometry, micron sized aggregates arising from
the association between oppositely charged vesicles and nanoparticles are
formed. Contrary to the models of lipoprotein corona or of particle wrapping,
our work shows that vesicles maintain their structural integrity and trap the
particles at their surfaces. The agglomeration of particles in surfactant phase
is a phenomenon of importance since it could change the interactions of the
particles with lung cells.Comment: 19 pages 9 figure
Activated sampling in complex materials at finite temperature: the properly-obeying-probability activation-relaxation technique
While the dynamics of many complex systems is dominated by activated events,
there are very few simulation methods that take advantage of this fact. Most of
these procedures are restricted to relatively simple systems or, as with the
activation-relaxation technique (ART), sample the conformation space
efficiently at the cost of a correct thermodynamical description. We present
here an extension of ART, the properly-obeying-probability ART (POP-ART), that
obeys detailed balance and samples correctly the thermodynamic ensemble.
Testing POP-ART on two model systems, a vacancy and an interstitial in
crystalline silicon, we show that this method recovers the proper
thermodynamical weights associated with the various accessible states and is
significantly faster than MD in the diffusion of a vacancy below 700 K.Comment: 10 pages, 3 figure
Dynamics of Lennard-Jones clusters: A characterization of the activation-relaxation technique
The potential energy surface (PES) of Lennard-Jones clusters is investigated
using the activation-relaxation technique (ART). This method defines events in
the configurational energy landscape as a two-step process: (a) a configuration
is first activated from a local minimum to a nearby saddle-point and (b) is
then relaxed to a new minimum. Although ART has been applied with success to a
wide range of materials such as a-Si, a-SiO2 and binary Lennard-Jones glasses,
questions remain regarding the biases of the technique. We address some of
these questions in a detailed study of ART-generated events in Lennard-Jones
(LJ) clusters, a system for which much is already known. In particular, we
study the distribution of saddle-points, the pathways between configurations,
and the reversibility of paths. We find that ART can identify all trajectories
with a first-order saddle point leaving a given minimum, is fully reversible,
and samples events following the Boltzmann weight at the saddle point.Comment: 8 pages, 7 figures in postscrip
Evolution of the potential-energy surface of amorphous silicon
The link between the energy surface of bulk systems and their dynamical
properties is generally difficult to establish. Using the activation-relaxation
technique (ART nouveau), we follow the change in the barrier distribution of a
model of amorphous silicon as a function of the degree of relaxation. We find
that while the barrier-height distribution, calculated from the initial
minimum, is a unique function that depends only on the level of distribution,
the reverse-barrier height distribution, calculated from the final state, is
independent of the relaxation, following a different function. Moreover, the
resulting gained or released energy distribution is a simple convolution of
these two distributions indicating that the activation and relaxation parts of
a the elementary relaxation mechanism are completely independent. This
characterized energy landscape can be used to explain nano-calorimetry
measurements.Comment: 5 pages, 4 figure
Traveling through potential energy landscapes of disordered materials: the activation-relaxation technique
A detailed description of the activation-relaxation technique (ART) is
presented. This method defines events in the configurational energy landscape
of disordered materials, such as a-Si, glasses and polymers, in a two-step
process: first, a configuration is activated from a local minimum to a nearby
saddle-point; next, the configuration is relaxed to a new minimum; this allows
for jumps over energy barriers much higher than what can be reached with
standard techniques. Such events can serve as basic steps in equilibrium and
kinetic Monte Carlo schemes.Comment: 7 pages, 2 postscript figure
Self-vacancies in Gallium Arsenide: an ab initio calculation
We report here a reexamination of the static properties of vacancies in GaAs
by means of first-principles density-functional calculations using localized
basis sets. Our calculated formation energies yields results that are in good
agreement with recent experimental and {\it ab-initio} calculation and provide
a complete description of the relaxation geometry and energetic for various
charge state of vacancies from both sublattices. Gallium vacancies are stable
in the 0, -, -2, -3 charge state, but V_Ga^-3 remains the dominant charge state
for intrinsic and n-type GaAs, confirming results from positron annihilation.
Interestingly, Arsenic vacancies show two successive negative-U transitions
making only +1, -1 and -3 charge states stable, while the intermediate defects
are metastable. The second transition (-/-3) brings a resonant bond relaxation
for V_As^-3 similar to the one identified for silicon and GaAs divacancies.Comment: 14 page
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