Equilibrium Crystal Shapes: free and supported nanoparticles If the surface energy is isotropic (as for a liquid) the problem is simple to minimize the surface and the solution is a sphere. In crystalline solids the surface energy is anisotropic and the energy-minimizing shape is found using the limiting planes of the lowest possible surface energy. review : A8.Morphology of supported nanoparticles_Henry, D2
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Equilibrium Crystal Shapes:free and supported
nanoparticles
If the surface energy is isotropic (as for a liquid ) the problem is simple to minimize the surface and the
solution is a sphere. In crystalline solids the surface energy is
anisotropic and the energy-minimizing shape is found using the limiting planes of the lowest
possible surface energy.
review : A8.Morphology of supported nanoparticles_Henry, D2
For a crystal grown at equilibrium
See D2
Wulff Plot
See D2
In 1901 Wullf introduced, withour proving, a theorem where he said: for an equilibrium crystal there is a point in the interior such that its perpendicular distance hi from the ith face is proportional to the surface energy γi
Around (but below) their melting temperatures, crystals tend to have shapes which are pretty round: not a complete sphere, but with no regions which are flat (faceted). This is because at high temperature the atoms on the surface jiggle and wiggle more: they don't care so much which places are easier to sit because they have so much energy to spare. The facets appear at lower temperatures, as the crystal is cooled: the first temperature at which a facet occurs is called the roughening temperature.
Equilibrium shape at T ≠0 K:
Crystals grown at T>T rough
do not form facets
(e.g. most nanoparticles grown by solution methods at high T are spherical)
Equilibrium shape in the nanoworld
Several factors can change the equilibrium shape when going to nanometer size range:
♦First, both the surface energy and the surface stress increase.
♦ Second, different structures (e.g., icosahedral structure) can become more stable.
♦ Finally, the proportion of edges atoms becomes no longer negligible. Even if the crystal structure remains bulk-like, the equilibrium shape can change.
This is the situation for naked nanoparticles
What does it happen when they aresupported ?
Solid-solid interfaces
Energia di adesione
Supported particles: Wullf-Kaichew construction
the thermodynamic approach
with the hypothesis that there is no strain between parti cle and substrate.
i.e.: the more is the Eadh, the more the particle is truncated
s
aspect ratio:height/lateral size
The space around the particles no more isotropic !!
Deviations from Wullf-Kaichew previsions
For non-zero misfit, the height-to-width aspect ratio can change. As an example if there is a compressive strain,the particle grows faster in height than laterally.
The equilibrium shape then deviates from the Wulff–Kais chew case, giving larger aspect ratios (i.e., taller crystal).
Qualitatively, one can understand this evolution becaus e the crystal is strainedat the interface (it can relax more easily at the top), and therefore prefers to
decrease the interface area.
However, even for macroscopic supported crystals, several factors can modify the equilibrium shape:
-the adsorption on foreign atoms or molecules-the presence of strain at the interface due to a m isfit
between the lattices of the support and of the depo sited crystal.
In practice, when we grow a crystal we are not at t he equilibrium because the supersaturation is larger than one. The supersaturation S is equal to the ratio of the (actual) pressure around the growing crystal and the equilibrium pressure at the same te mperature.
If S is larger than one the crystal grows, and it e vaporates if S is smaller than one.
Kinetics effects
In general (especially at large supersaturations) t he shape of the crystal depends on the growth rate of the different facets.
See Struttura e dinamica delle Superfici
Conclusions:
the morphology of nanocrystals depends on both kine tic (i.e.,growth) and thermodynamic parameters.
If the growth takes place far from equilibrium cond itions (i.e., large supersaturation)
the growth shape is not unique and depends onmany parameters, such as: flux of growing material, structure of the
support (if it is present),presence of defects (dislocations, twins), presence of impurities,
confinement (i.e., template effect).
If we grow particles close to the thermodynamic eq uilibrium (i.e., low growth rate, high temperature,but not to o high to avoid Ostwald ripening)
we can approach the equilibrium shape of the crysta lline particles, which is unique for defined thermodynamic conditions.
In the case of supported crystals the equilibrium s hape is truncated in proportion to the adhesion energy (i.e., deposit/substrate int eraction). Thus, choosing
substrates with stronger adhesion energy will resul t in particles with smaller aspect ratios (height/lateral size).