investigation of thermal transport in hybrid silicene nanoribbons

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 germanene is found to be 2.44 Å.

Width of ribbon taken to be 21 for 3p configuration.


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:



Width of ribbon taken to be 22 for 3p+1 configuration.


Calculators used for Simulation:For thermal conductivity (), Classical ATK Tersoff_SiGeO_2013.


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:






For thermal conductivity, Classical ATK Tersoff_SiGeO_2013.


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:






For thermal conductivity, Classical ATK Tersoff_SiGeO_2013.


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

YOU

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