Activity Report 2010 - CNRS

NANO-ORDERING &
DEEP UNDERCOOLING
DRIVE THE SILICON
NANOWIRE GROWTH
Deep undercooling gives rise to a peculiar
state of matter in which a liquid does not
solidify even far below the normal
freezing point. A good example of this
phenomenon is found every day in
meteorology: clouds in high altitude are
an accumulation of undercooled droplets
of water below their freezing points due
to the high purity of the atmosphere at
these altitudes.
temperatures. Such a property is
employed to manufacture high-purity Si
nanowires through a vapour-liquid-solid
growth mechanism. Very recently, the
origin of the deep eutectic point was
shown to be related to the presence of a
well-defined chemical short-range order
that enhances AuSi interactions in the
liquid phase, in contrast with the solid
mixture and with the occurrence of an
important icosahedral ordering in the
undercooled region [3].
HIGHLIGHT : THEORY AND NANOSIMULATION
CONTACTS
alain.pasturel@grenoble.cnrs.fr
noel.jakse@grenoble-inp.fr
FURTHER READING
[1] N. Jakse et al., Physical Review Letters,
91, p205702, (2003); 93, p207801, (2004)
[2] T.U. Schulli et al., Nature 464, p1174,
(2010)
[3] A. Pasturel et al., Phys. Rev. B 81,
p140202R (2010)
Undercooling was discovered in 1724 by
Fahrenheit while observing that water
droplets stay liquid below 0°C. However
numerous questions about the underlying
mechanisms remain nowadays still open.
In the 1950’s, theoricians postulated the
structure at the atomic level to be
incompatible with crystallization. This led
to the speculation that the atoms in the
liquid could locally arrange in icosahedra
characterized by a five-fold symmetry
which is incompatible with the long-range
periodicity of the crystalline solid.
Fig. 1: Icosahedron
and pentagonal
rings
Fifty years later, ab initio molecular
dynamics simulations revealed for the
first time five-fold coordinated clusters
(pentagons) in pure liquid metals as well
as liquid metallic alloys, some of them
being known to form quasicrystalline
phases or bulk metallic glasses upon
rapid solidification [1].
Using ab initio molecular dynamics
simulations, a new remarkable
undercooling phenomenon has been
explained [2], namely an undercooling as
deep as 350°C for Gold-Silicon (Au 81 Si 19 )
eutectic alloy in contact with a specially
decorated silicon (111) surface where the
outermost layer of the solid featured
pentagonal atomic arrangements. This
alloy is characterized by an unusually
deep eutectic temperature, 359°C, that is
hundreds of degrees below the melting
points of Au (1063°C) and Si (1412°C)
and guarantees a very high mobility of
the Si atoms at relatively low
Fig. 2: Pentagonal arrangements in the goldsilicon
eutectic liquid are stabilized at the
interface. The close-packing of 7 atoms leads
to build the five-fold ring.
In order to understand the effect of the
silicon surface on the local structure of
the eutectic alloy and its undercooling
properties, we carried out calculations of
solid / eutectic liquid interfaces as a
function of different silicon surfaces. The
Silicon (111) surface forces the presence
of local pentagonal arrangements in the
liquid phase at the interface (see Figure
2) The main consequence is that the
alloy’s atoms near this interface display a
local order that increases the stability of
the supercooled phase of the liquid
instead of triggering heterogeneous
nucleation. It is also observed that the
pentagonal decorated silicon (111) [2]
surface influences the short-range order
and the metastability of the liquid
eutectic alloy, favouring the increase of
pentagons in the liquid phase.
This result has wide implications, not only
for fundamental studies of freezing, but
also for practical control of the phase
transition. For instance, it should lead to
important technological applications in
the field of nanowire growth for which the
eutectic alloy act as a catalyst. It is also
speculated that the containerless
techniques required today to obtain
undercooling could be in the future be
replaced by icosahedrally coated solid
containers.
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