149 research outputs found
Effect of the lattice misfit on the equilibrium shape of strained islands in Volmer-Weber growth
We have studied the effect of the misfit on the equilibrium shape of
three-dimensional pyramidal islands grown on a foreign substrate in the case of
incomplete wetting (Volmer-Weber mode of growth). We have found that tensile
islands have smaller aspect ratios compared with compressed islands owing to
its better adhesion to the substrate. The average strains of consecutive layers
decrease faster with thickness in compressed than in tensile islands. The
strains decrease rapidly with thickness, with the consequence that above a
certain height, the upper layers of the pyramid become practically unstrained
and does not contribute to a further reduction in the upper base. As a result,
the truncated pyramids are not expected to transform into full pyramids. Our
results are in good agreement with experimental observations in different
systems.Comment: 6 pages, 7 figures. Accepted version, minor change
Second-layer nucleation in coherent Stranski-Krastanov growth of quantum dots
We have studied the monolayer-bilayer transformation in the case of the
coherent Stranski-Krastanov growth. We have found that the energy of formation
of a second layer nucleus is largest at the center of the first-layer island
and smallest on its corners. Thus nucleation is expected to take place at the
corners (or the edges) rather than at the center of the islands as in the case
of homoepitaxy. The critical nuclei have one atom in addition to a compact
shape, which is either a square of i*i or a rectangle of i*(i-1) atoms, with
i>1 an integer. When the edge of the initial monolayer island is much larger
than the critical nucleus size, the latter is always a rectangle plus an
additional atom, adsorbed at the longer edge, which gives rise to a new atomic
row in order to transform the rectangle into the equilibrium square shape.Comment: 6 pages, 4 figures. Accepted version, minor change
Nucleation of amyloid fibrils
We consider nucleation of amyloid fibrils in the case when the process occurs
by the mechanism of direct polymerization of practically fully extended protein
segments, i.e. beta-strands, into beta-sheets. Applying the classical
nucleation theory, we derive a general expression for the work to form a
nanosized amyloid fibril (protofilament) constituted of successively layered
beta-sheets. Analysis of this expression reveals that with increasing its size,
the fibril transforms from one-dimensional into two-dimensional aggregate in
order to preserve the equilibrium shape corresponding to minimal formation
work. We determine the size of the fibril nucleus, the fibril nucleation work
and the fibril nucleation rate as explicit functions of the concentration and
temperature of the protein solution. The results obtained are applicable to
homogeneous nucleation which occurs when the solution is sufficiently pure
and/or strongly supersaturated
Nucleated polymerisation in the presence of pre-formed seed filaments
We revisit the classical problem of nucleated polymerisation and derive a
range of exact results describing polymerisation in systems intermediate
between the well-known limiting cases of a reaction starting from purely
soluble material and for a reaction where no new growth nuclei are formed
Coherent Stranski-Krastanov growth in 1+1 dimensions with anharmonic interactions: An equilibrium study
The formation of coherently strained three-dimensional islands on top of the
wetting layer in Stranski-Krastanov mode of growth is considered in a model in
1+1 dimensions accounting for the anharmonicity and non-convexity of the real
interatomic forces. It is shown that coherent 3D islands can be expected to
form in compressed rather than in expanded overlayers beyond a critical lattice
misfit. In the latter case the classical Stranski-Krastanov growth is expected
to occur because the misfit dislocations can become energetically favored at
smaller island sizes. The thermodynamic reason for coherent 3D islanding is the
incomplete wetting owing to the weaker adhesion of the edge atoms. Monolayer
height islands with a critical size appear as necessary precursors of the 3D
islands. The latter explains the experimentally observed narrow size
distribution of the 3D islands. The 2D-3D transformation takes place by
consecutive rearrangements of mono- to bilayer, bi- to trilayer islands, etc.,
after exceeding the corresponding critical sizes. The rearrangements are
initiated by nucleation events each next one requiring to overcome a lower
energetic barrier. The model is in good qualitative agreement with available
experimental observations.Comment: 12 pages text, 15 figures, Accepted in Phys.Rev.B, Vol.61, No2
Room Temperature Electrochemical Synthesis of Hg-1212 Superconducting Thin Films
In the present investigation, the novel two-step electrochemical process of
room temperature synthesis of Hg-1212 superconducting films has been developed
and reported first time. Electrochemical parameters were optimized by studying
cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry
(CA) for the deposition of Hg-Ba-Ca-Cu alloy at room temperature. Current time
transient showed progressive growth with hemispheriodal granules, which were
then revealed by scanning electron microscopy (SEM). Stoichiometric
electrocrystallization to get Hg1Ba2Ca1Cu2O6+x (Hg-1212) was completed by
electrochemically intercalating oxygen species into Hg-Ba-Ca-Cu alloy at room
temperature. The oxygen content in the samples was varied by varying the
electrochemical oxidation period and the changes in the crystal structure,
microstructure, and superconducting transition temperature (Tc) and critical
current density (Jc) were recorded. The films oxidized for 28 min showed Tc =
104.7 K with Jc = 1.437 x 103 A/cm2. The dependence of superconducting
parameters on oxygen content is correlated with structure property relations
and reported in this paper.Comment: 39 pages, 17 figures. Submitted to Physica
- …