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Dissertation Entitled on
INVESTIGATION OF Thermal TRANSPORT IN hybrid
SILICENE-germanene NANO-RIBBONs to enhance its
thermoelectric performance
SUBMITTED BY:GAGANDEEP SINGH RANDHAWAM.TECH 4th SEM.2014ECB1006
INTRODUCTION TO NANOTECHNOLOGY:Nanotechnology has emerged as a new field of technology aiming to revolutionize
the mankind-materials relationship through the end of 20th century.
Nanotechnology studies materials at atomic scale and aims to manipulate them
with atomic accuracy for the benefit of the human kind.
National Nanotechnology Initiative (NNI) defines nanotechnology as:
Ability to control or manipulate on the atomic scale.
Research and technology development at the atomic, molecular, or
macromolecular levels, in the length scale of approximately 1 to 100-nanometer
range.
Creating and using structures, devices, and systems that have novel properties
because of their small or intermediate size.
INTRODUCTION TO SILICENE NANO-RIBBONS:
Study of nanomaterial took off some 40 years ago with the design
of so-called quasi-two-dimensional.
In 2004, the study of exact 2D solids became a renewed focus.
Graphene was isolated from graphite and its many fascinating
properties were demonstrated.
Silicene is a two-dimensional allotrope of silicon.
Has a hexagonal honeycomb structure similar to that of graphene.
CONTD.
Figure: Progress from Graphene to Silicene
CONTD.
Silicene nanoribbons (SiNRs), are strips of silicene with ultra-thin
width (<50 nm).
Silicene nanoribbons were introduced as a theoretical model by A.
Kara et al. and C. Lian et al. to examine the edge and nanoscale size
effect in silicene.
In 2015, a silicene field-effect transistor made its debut.
It opened up new opportunities for two-dimensional silicon for various
fundamental science studies and electronic applications.
ATOMIC STRUCTURE OF SILICENE NANO-RIBBONS:
Primary method for growing silicene is on silver substrate.
On Ag (111), silicene forms a continuous sheet, with at least three
distinct ordered phases, depending on the deposition conditions.
On the Ag (110) surface, one-dimensional silicene Nano ribbons
(NRs) can be grown.
Isolated NRs show a low reactivity to molecular oxygen.
Oxidation occurs only at the Si NRs terminations.
CONTD.
Figure: STM image of silicene NRs grown at 230◦ C on a Ag (110) surface (12.5x12.5 nm2, V = -80 mV, I = 2.2 nA)
INTRODUCTION TO GERMANENE NANO-RIBBONS:
Germanene is the least explored element of the group IV monolayers.First suggested alongside silicene in 1994.But its stability has been questioned by conflicting phonon dispersion results.
Figure: Structure of Germanene
CONTD.
G. Le Lay, et al. grow germanene on Au (111) surfaces that yielded
honeycomb-like patches.
Deposition of Ge on Ag surfaces results in Ge-Ge interaction leading
to the formation of Ge tetramers on Ag atoms and the Ge-Ag
interaction leadings to surface alloying.
Figure: STM image of Ge tetramers after Ge deposition on Ag (001)
surface
RESEARCH AIM AND OBJECTIVES:
The main objective of my thesis is to investigate the thermal transport in
hybrid Silicene-Germanene Nanoribbons to enhance its thermoelectric
performance using:
Molecular dynamics simulation technique.
Pristine SiNRS as a base reference.
Introducing concept of Interfaces.
Analysis of different configurations of hybrid SiGeNRs ( i.e. 3p, 3p+1
and 3p+2 configurations).
Effect of number of interfaces on the thermoelectric performance.
PROBLEM FORMULATION: Despite extensive study on the electric property of silicene, little research
has been devoted to the thermal (phonon) transport of silicene so far.
Many researches on improving thermal conductivity in silicene
nanoribbons has been conducted lately.
But a very little effort has been devoted to study the thermal conductivity
and figure of merit (ZT) of hybrid silicene interfaces.
Few researchers have investigated the thermal behavior of hybrid silicene-
graphene interfaces.
But hybrid silicene-graphene interfaces suffer from lattice mismatch
problems which may affect its accuracy due to unwanted strain.
SOLUTION TO THE PROBLEM: Lattice mismatch problem in hybrid CSiNRs led to an opening for
another novel nanomaterial, germanene-germanium based counterpart of
graphene.
The electronic properties of silicene and germanene have also been
studied theoretically.
Both materials being predicted to be gapless semiconductors with linear
energy dispersion relations near the K points, like graphene.
Also the lattice mismatch between silicene nanoribbons (SiNRs) and
germanene nanoribbons (GeNRs) has been reduced to greater extent
resulting in lesser strain effect.
RESEARCH METHODOLOGY:
EXPERIMENTAL METHODS
COMPUTATIONAL METHODS
TOOL USED:
Atomistix Virtual Nano Lab (VNL) is a commercial point-and-
click software for simulation and analysis of physical and chemical
properties of Nano scale devices.
Developed and sold commercially by Quantum Wise A/S.
Provides a user-friendly approach to atomic-scale modeling.
GRAPHICAL USER INTERFACE OF
ATOMISTIX VIRTUAL NANOLAB:
Allows the user to design Nano systems, to set up and execute
numerical calculations, and to visualize the results.
Combines density functional theory and non-equilibrium Green's
functions to ab-initio electronic-structure and transport
calculations.
Built on top of Python, NanoLanguage includes the same
functionality as Python and with the same syntax.
CONTD.
WORK DETAILS:In the first part of this thesis, thermal conductivity and figure of merit
of pristine silicene nanoribbons (SiNRs) are analyzed using the
molecular dynamics simulation technique.
In the second part of this thesis, the thermal conductivity and figure of
merit of hybrid silicene germanene nanoribbons (SiGeNRs) are analyzed
using the molecular dynamics simulation technique.
In the final part of this thesis, the effect of varying the number of
interfaces on the thermal conductivity and figure of merit (ZT) of hybrid
silicene germanene nanoribbons (SiGeNRs) is analyzed using the
molecular dynamics simulation technique.
ARMCHAIR PRISTINE SILICENE NANO-
RIBBONS:The various parameters used for calculating the thermal conductivity
and figure of merit of Armchair Pristine Silicene nanoribbon are:
Bond Length of silicene is found to be 2.25 Å.
Width of ribbon taken to be 21 atoms wide.
Length of ribbon is 10.125 nm.
Calculators used for Simulation:For thermal conductivity (), Classical ATK Tersoff_Si_2005.
For Electrical conductivity, Slater Koster is taken into account.
STRUCTURE OF PRISTINE SiNR:
CALCULATED RESULTS:
CONTD.
In the range of -2eV to +2 eV, maximum value of ZT was
demonstrated to be 0.1559.
Therefore PSiNR has a comparatively low figure of merit (ZT)
which needed to be improved.
In order to improve the performance of PSiNR in terms of thermal
conductivity and figure of merit, we decided to work out on the
concept of interfaces.
CONCEPT OF INTERFACING: To improve the performance of PSiNR in terms of thermal
conductivity and figure of merit, we decided to work out on the
concept of interfaces.
Hybrid structures consisting of silicene nanoribbon and germanene
nanoribbon were constructed .
Some further variations among the interfaces were studied to
improve figure of merit.
Si-Ge-Si-Ge-Si INTERFACE FOR 3P SERIES:The various parameters used for calculating the thermal conductivity and
figure of merit of hybrid Silicene-Germanene nanoribbons for 3p series are:
Bond Length of silicene is found to be 2.25 Å.
Bond Length of germanene is found to be 2.44 Å.
Width of ribbon taken to be 21 for 3p configuration.
Length of ribbon is 10.4 nm.
Calculators used for Simulation:
For thermal conductivity (), Classical ATK Tersoff_SiGeO_2013.
For electrical conductivity, Slater Koster is taken into account.
STRUCTURE OF Si-Ge-Si-Ge-Si INTERFACE FOR 3P SERIES:
CALCULATED RESULTS:
CONTD.
In the range of -2eV to +2 eV, maximum value of ZT was demonstrated
to be 0.3115.
It is clear that thermoelectric performance had improved drastically
when compared to the previous version i.e pristine SiNR.
Si-Ge-Si-Ge-Si INTERFACE FOR 3P+1 SERIES:
The various parameters used for calculating the thermal conductivity and
figure of merit of hybrid Silicene-Germanene nanoribbons for 3p+1 series are:
Bond Length of silicene is found to be 2.25 Å.
Bond Length of germanene is found to be 2.44 Å.
Width of ribbon taken to be 22 for 3p+1 configuration.
Length of ribbon is 10.4 nm.
Calculators used for Simulation:For thermal conductivity (), Classical ATK Tersoff_SiGeO_2013.
For electrical conductivity, Slater Koster is taken into account.
STRUCTURE OF Si-Ge-Si-Ge-Si INTERFACE FOR 3P+1 SERIES:
CALCULATED RESULTS:
CONTD.
In the range of -2eV to +2 eV, maximum value of ZT was
demonstrated to be 0.428.
It is clear that thermoelectric performance had improved
drastically when compared to the previous version i.e pristine SiNR
and hybrid SiGeNR for 3p series.
The figure of merit has the best value for this combination of
configuration and width of the nanoribbon.
Si-Ge-Si-Ge-Si INTERFACE FOR 3P+2 SERIES:
The various parameters used for calculating the thermal conductivity and figure
of merit of hybrid Silicene-Germanene nanoribbons for 3p+2 series are:
Bond Length of silicene is found to be 2.25 Å.
Bond Length of germanene is found to be 2.44 Å.
Width of ribbon taken to be 23 for 3p+2 configuration.
Length of ribbon is 10.4 nm.
Calculators used for Simulation:
For thermal conductivity, Classical ATK Tersoff_SiGeO_2013.
For electrical conductivity, Slater Koster is taken into account.
STRUCTURE OF Si-Ge-Si-Ge-Si INTERFACE FOR 3P+2 SERIES:
CALCULATED RESULTS:
EFFECT OF VARIATION OF NUMBER OF INTERFACES: Out of all the three configurations of hybrid SiGeNR, 3p+1 series
produced best thermoelectric performance.
After getting best results for 3p series, we decided to study the
effect of variation of number of interfaces on thermoelectric
performance.
We considered hybrid Si-Ge-Si nanoribbon for this purpose.
Si-Ge-Si INTERFACE FOR 3P+1 SERIES:The various parameters used for calculating the thermal conductivity and
figure of merit of hybrid Silicene-Germanene-Silicene nanoribbons for
3p+1 series are:
Bond Length of silicene is found to be 2.25 Å.
Bond Length of germanene is found to be 2.44 Å.
Width of ribbon taken to be 22 for 3p+1 configuration.
Length of ribbon is 10.4 nm.
Calculators used for Simulation:
For thermal conductivity, Classical ATK Tersoff_SiGeO_2013.
For electrical conductivity, Slater Koster is taken into account.
STRUCTURE OF Si-Ge-Si INTERFACE FOR 3P+2 SERIES:
CALCULATED RESULTS:
CONTD. Here we have decreased the number of interfaces in hybrid
SiGeNR i.e. Si-Ge-Si-Ge-Si to Si-Ge-Si.
From calculated results it is clear that on decreasing the number of
interfaces in hybrid SiGeNR, thermoelectric figure of merit also
decrease.
Hence it can be concluded that the thermoelectric figure of merit
is directly proportional to the number of interfaces used in hybrid
SiGeNR and vice-versa.
THANK
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