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| Identifier |
000390000 |
| Title |
Theory and simulations of silicon nanocomposite systems |
| Alternative Title |
Θεωρία και προσομοιώσεις νανοσύνθετων συστημάτων πυριτίου |
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Author
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Κλεοβούλου, Κωνσταντίνος
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Thesis advisor
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Ζώτος, Ξενοφών
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Reviewer
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Κελίρης, Παντελής
Φλυτζάνης, Νικόλαος
Παπανικολάου, Νικόλαος
Ρεμεδιάκης, Ιωάννης
Κοπιδάκης, Γεώργιος
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| Abstract |
This
thesis
focuses
on
the
study
of
silicon
nanocomposite
systems
using
theory
and
computer
simulations.
The
major
part
is
dedicated
on
the
study
of
silicon
nanocrystals
(Si-NC)
embedded
in
amorphous
silicon
dioxide
(a-SiO2)
with
great
attention
given
towards
the
mechanical
properties
and
the
spatial
arrangement
of
the
system.
Such
analysis
is
crucial
not
only
for
the
stability
of
the
system
but
also
for
unveiling
the
origins
of
the
optoelectronic
properties.
The
latter
are
strongly
correlated
with
the
mechanical
response
of
the
system
to
spatial
and
structural
changes.
A
multiscale
approach
for
sensing
applications
is
developed
for
simulating
the
early
stage
dry
oxidation
of
Si(100)
cantilever
surface
that
will
ultimately
provide
a
frame-work
for
science-based
optimization
of
cantilever
sensors.
The
multiscale
formulation
couples
Monte
Carlo
stochastic
simulations
employed
for
capturing
the
oxidation
process
and
continuum
level
Finite
Element
simulations
for
calculating
the
cantilever
deflection.
Continuous-space
Monte
Carlo
simulations
using
the
empirical
potential
approach
are
employed.
For
the
interactions,
the
Tersoff
empirical
potential
parametrized
to
describe
SiO2
systems
is
used.
This
potential
describes
well
the
elemental
Si
properties,
silica
polymorphs,
and
phase
transitions
between
them,
as
well
as
the
structure
and
energetics
of
a-SiO2.
To
further
advocate
the
use
of
the
potential
for
the
current
application
and
ensure
the
validity
of
the
results
the
elastic
properties
of
the
system
and
its
bulk
components
are
calculated
and
compared
with
previous
theoretical
and
experimental
measurements.
The
stress
state
of
Si-NC/a-SiO2
is
investigated
and
its
nature
and
origins
are
unraveled
and
explained.
This
is
achieved
by
generating
detailed
stress
maps
and
by
calculating
the
stress
profile
as
a
function
of
the
NC
size.
For
normal
oxide
matrix
densities,
the
average
stress
in
the
NC
core
is
found
to
be
compressive
in
excellent
agreement
with
experimental
measurements.
It
drastically
declines
at
the
interface,
despite
the
existence
of
several
highly
strained
geometries.
Tensile
conditions
prevail
in
NC
embedded
in
densified
silica
matrices.
The
NC-NC
interplay
at
various
interparticle
distances
is
examined.
The
decrease
of
the
interparticle
distance
introduces
structural
and
chemical
deformation
on
the
matrix.
This
is
attributed
on
the
force
field
exerted
on
the
matrix
by
the
surrounding
NC,
which
amplifies
considerably
as
they
approach.
Nevertheless,
It
is
shown
that
the
system
is
stable
against
segregation
and
phase
separation
even
for
close
interparticle
distances.
The
ordering
of
3D
Si-NC
arrangements
is
investigated
by
comparing
a
conventional
cubic
and
a
hexagonal
arrangement.
For
this,
a
novel
computational
technique
for
generating
hexagonal
arrangements
whilst
minimizing
the
computational
effort
is
introduced.
This
study,
driven
by
the
structural
characteristics
and
the
energetics
of
the
system
in
thermodynamic
equilibrium
does
not
show
preferential
ordering
in
any
of
the
two
arrangements.
The
mechanical
response
of
the
system
to
the
NC
size
and
density
variation
is
studied.
The
problem
of
local
rigidity
is
investigated.
By
analyzing
the
elastic
(bulk)
modulus
field
into
atomic
contributions,
it
is
showed
that
it
is
highly
inhomogeneous.
It
consists
of
a
hard
region
in
the
interior
of
the
nanocrystals,
and
of
“superhard”
and
“supersoft”
regions
on
the
nanocrystal
periphery.
Overall,
the
nanocrystal
bulk
modulus
is
significantly
enhanced
compared
to
the
bulk,
and
its variation
with
size
accurately
follows
a
power-law
dependence
on
the
average
bond
length.
The
bulk
modulus
of
the
oxide
matrix
and
of
the
interface
region
is
nearly
constant
with
size.
The
average
optical
(homopolar)
gap
is
directly
linked
to
the
elastic
and
bond-length
variations.
The
MC
results
for
the
system’s
mechanical
response
against
the
increase
of
NC
density
are
in
good
match
with
the
Self
Consistent
micromechanics
model
predictions
denoting
the
great
importance
of
the
NC-NC
interplay.
The
early
dry
oxidation
of
Si(100)
is
simulated
using
a
2x1
dimer
reconstructed
c-Si
slab.
Tensile
surface
stresses
are
generated
due
to
oxidation
causing
the
cantilever
to
bent.
Penetration
of
stress
is
limited
to
the
first
four
layers.
The
oxygen
absorption
reaches
a
plateau
for
concentrations
higher
than
20%.
The
cantilever
deflection
is
calculated
by
the
finite
element
method
using
the
surface
stress
calculated
by
the
Monte
Carlo
simulations
as
a
boundary
(surface
traction)
condition.
It
is
anticipated
that
this
multiscale
formulation
will
ultimately
provide
a
framework
for
science-based
optimization
of
cantilever
sensors
with
improved
stability,
durability,
operating
range,
fictionalization
and
sensitivity.
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| Language |
English |
| Subject |
Cantilever sensors |
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Monte Carlo |
|
Nanocrystals |
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Ανιχνευτές προβολού |
|
Μόντε Κάρλο |
|
Νανοκρύσταλλοι |
|
Πυρίτιο |
| Issue date |
2014-12-22 |
| Collection
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School/Department--School of Sciences and Engineering--Department of Physics--Doctoral theses
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Type of Work--Doctoral theses
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| Permanent Link |
https://elocus.lib.uoc.gr//dlib/e/d/f/metadata-dlib-1423130740-145161-20288.tkl
|
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