Activity Report 2010 - CNRS

CRISTAL PHASE
TRANSITIONS IN
PRESSURIZED SILICON
NANOWIRES
The possibility to synthesize new
crystalline phases in single silicon
nanowires recently attracted attention
owing to the potential tuning of the opto
electronic properties that can be expected
when the crystal structure undergoes
phase transitions. In particular, it is
predicted that the wurtzite phase which is
metastable at room pressure and
temperature is an indirect semiconductor
with a 0.85 eV band gap. Unlike most of
its III-V compounds counterparts where
axial phase switching between cubic and
hexagonal structure is routinely observed,
direct crystal growth and characterization
of wurtzite silicon nanowires is still a
challenging task.
In the frame of the NEP-IV project, we
initiated a structural study of phase
transitions in Si NWs, based on a
“pressure engineering” approach that uses
diamond (phase I) Si NWs as a starting
material instead of the direct synthesis of
modified wires, still under controversy.
Indeed, in the case of bulk material, it was
shown in the 60’s and 70’s that a highpressure
loading and unloading cycle leads
to high-pressure intermediate metallic or
semimetallic phases that relax upon
pressure release into the metastable Si III
phase (body centred cubic, semimetallic)
and Si IV phase (wurtzite, semiconductor)
under a slight annealing of Si III at room
pressure. In our experiment, we used a
diamond-anvil cell to monitor the
pressure-induced phase changes, which
we followed by confocal micro-Raman
spectroscopy. Size calibrated Si NWs were
synthesized using 50 nm gold colloids as
seeds for the vapour liquid solid growth.
The sample was sonicated in a 4:1
methanol-ethanol mixture and the
resulting enriched solution was drop
casted in the pressure cell for Raman
investigation. This resulted in the
formation of visible Si NW bundles at
some preferential places in the cell that
permitted precise excitation of the same
group of wires all along the pressure rise
and release cycle. The results are reported
in Fig. 1 and show the characteristic
Stokes Raman spectrum observed in bulk
silicon I under increasing hydrostatic
pressures up to 16-18 GPa where a first
phase transition occurs, presumably
towards the Si II phase (body centred
tetragonal, metallic), which has no sharp
Raman response. After completion of the
phase transition and transformation of all
Si I into Si II, pressure is progressively
released. This does not lead to Si I phase
retrieval but instead, and like in the bulk
material, Si II remains stable down to the
5-10 GPa range where a phase transition
towards Si III phase takes place. The
characteristic Raman peaks of Si III are
detected together with two broader bands
corresponding to amorphous Si but no
contribution from Si I is found. Upon
aperture of the cell, as the pressure is
fully released to room pressure, the
alcohol evaporates and Si I phase is
immediately recovered, without any
transit via Si IV. This is most likeky due to
the lack of heat sink in the wire
surroundings and subsequent intense laser
annealing of Si III. Further experiments
are necessary to fully understand the
diameter dependence of the phase
transitions and special care will be given
to the low-pressure domain upon pressure
release where observation of the Si IV
phase is expected.
Fig. 1: Raman spectra obtained on a bundle of
50 nm diameter Si NWs for increasing
pressures (top) and decreasing pressures
(bottom). The different phases are labeled I, II
and III
CONTACTS
nicolas.pauc@cea.fr
pierre.bouvier@grenoble-inp.fr
mael.guennou@grenoble-inp.fr
10
HIGHLIGHT : NANOMATERIALS, NANOASSEMBLY AND NANOSTRUCTURATION