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TOPOLOGICAL INSULATOR Bi2Te3: SYNTHESIS, OPTICAL
PROPERTIES AND ENERGY APPLICATIONS
ZHAO MENG
NATIONAL UNIVERSITY OF SINGAPORE
2014
TOPOLOGICAL INSULATOR Bi2Te3: SYNTHESIS, OPTICAL
PROPERTIES AND ENERGY APPLICATIONS
ZHAO MENG
B.Sc. (Chemistry)
Shandong University, China
A THESIS SUBMITTED
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
DEPARTMENT OF CHEMISTRY
NATIONAL UNIVERSITY OF SINGAPORE
2014
I
Declaration
I, hereby declare that this thesis is my original work and it has been written by me in its
entirety, under the supervision of Prof. Loh Kian Ping, Chemistry Department, National
University of Singapore, between Aug’ 2009 and Jan’ 2014.
I have duly acknowledged all the sources of information which have been used in this thesis.
This thesis has also not been submitted for any degree in any university previously.
---------------------------------------
Zhao Meng
24 Jan 2014
II
Acknowledgements
This dissertation would not have been possible without the opportunity given to me
by the Department of Chemistry, NUS and the constant support and inspiration given to me
by the following people.
First and foremost, I sincerely thank my supervisor Prof. Loh Kian Ping, who has
always been an earnest motivator since I joined his group in 2009. His philosophy in pursuing
excellent, thinking critically and passion for high-impact research are inspiring. It is truly a
privilege to work on cutting-edge fields in science under his supervision.
I am really appreciated to my collaborators, Dr. Michel Bosman at IMRE, Dr.
Qiaoliang Bao and his group members at Soochow University, China, Dr. Elbert E. M. Chia
and his group members at NTU, Dr. Andrivo Rusidy and his group members at SSLS, Dr.
Chu Hong Son at IHPC, Prof. Antonio H. Castro Neto at GRC. All of them are always nice to
discuss and promote my work.
My heartfelt thanks extend to the colleagues in GRC. I would like to give special
thanks to Dr. Lu Jiong, Dr. Priscilia Ang, Dr. Wang Yu, Dr. Zheng Yi, Dr. Goh Bee Min, Dr.
Lena Tang, Dr. Candy Lim, Dr. Alison Tong, Dr. Xu Hai, Nai Chang Tai, Janardhan
Balapanuru, Liu Yanpeng, Tang Wei, Dr. Liu Bo, Dr. Li Linjun.
Words are not enough to thank my parents, my sister and my boyfriend, whose
unconditional love and encouragement always inspire me to overcome difficulties and enjoy
life.
III
Table of Contents
Declaration................................................................................................................................ I
Acknowledgements ................................................................................................................. II
Table of Contents ................................................................................................................... III
Summary….... ..................................................................................................................... VIII
List of Tables .......................................................................................................................... IX
List of Figures .......................................................................................................................... X
List of Schemes .................................................................................................................. XVII
List of Abbreviations and Symbols ................................................................................ XVIII
List of Publications ........................................................................................................... XXII
Chapter 1: Introduction
1.1 Development of topological insulator ............................................................................. 1
1.2 Properties of topological insulator .................................................................................. 2
1.2.1 Dissipation-less charge transport ....................................................................... 4
1.2.2 Robustness of surface states ............................................................................... 6
1.2.3 Optical properties in terahertz regime ................................................................ 6
1.2.4 Plasmonics properties ......................................................................................... 8
1.2.4.1 General properties of surface plasmons ............................................... 8
1.2.4.2 Excitation and detection methods ........................................................ 9
1.2.4.3 Surface plasmon of topological insulators ......................................... 12
1.3 Synthesis of topological insulator nanostructures ......................................................... 14
1.3.1 Physical synthesis ............................................................................................ 15
IV
1.3.2 Chemical synthesis............................................................................................. 17
1.4 Applications of topological insulator .............................................................................. 18
1.5 Overview of objectives and work scope ......................................................................... 18
1.6 References ....................................................................................................................... 20
Chapter 2: Experimental Techniques
2.1 Introduction ................................................................................................................... 38
2.2 Microscopy ................................................................................................................... 38
2.2.1 Scanning electron microscopy (SEM) .............................................................. 39
2.2.2 Transmission electron microscopy (TEM) ....................................................... 40
2.2.3 Atomic force microscopy (AFM) ..................................................................... 42
2.2.4 Scanning near-field optical microscopy (SNOM) ............................................ 44
2.3 Spectroscopy ................................................................................................................. 48
2.3.1 X-ray diffraction (XRD) ................................................................................... 48
2.3.2 Electron energy-loss spectroscopy .................................................................... 50
2.3.3 Terahertz time domain spectroscopy (THz-TDS) ............................................. 53
2.5 References ..................................................................................................................... 54
Chapter 3: Controlled Synthesis of Single Crystalline Bi2Te3 Hexagonal Nanoplates via
Solvothermal Method
3.1 Introduction ................................................................................................................... 57
3.2 Materials and methods .................................................................................................. 59
3.2.1 Chemicals .......................................................................................................... 59
3.2.2 Synthesis ........................................................................................................... 60
3.2.3 Characterizations............................................................................................... 60
V
3. 3 Results and discussion .................................................................................................. 61
3.3.1 Characterizations of Bi2Te3 hexagonal nanoplates ........................................... 61
3.3.2 Influences of reaction parameters ..................................................................... 64
3.3.2.1 Influence of reaction temperature ...................................................... 65
3.3.2.2 Influence of NaOH ............................................................................. 66
3.3.2.3 Influence of PVP ................................................................................ 69
3.3.2.4 Influence of concentration of precursors ........................................... 71
3.3.2.5 Influence of solvent............................................................................ 74
3.3.3 Evolution of morphology and growth mechanisms .......................................... 77
3.4 Conclusion .................................................................................................................... 84
3.5 References ..................................................................................................................... 85
Chapter 4: Localized Surface Plasmon Resonance of Topological Insulator Bi2Te3
4.1 Introduction ................................................................................................................... 90
4.2 Materials and methods .................................................................................................. 92
4.2.1 EELS spectra and mapping ............................................................................... 92
4.2.2 Spectroscopic ellipsometry ............................................................................... 93
4.2.3 SNOM imaging ................................................................................................. 94
4.3 Results and discussion .................................................................................................. 95
4.3.1 EELS spectra and mapping ............................................................................... 95
4.3.1.2 Bi2Te3 hexagonal plate with high symmetry ..................................... 95
4.3.1.2 Bi2Te3 hexagonal plate with low symmetry ..................................... 101
4.3.3 SNOM imaging ............................................................................................... 104
4.4 Conclusion .................................................................................................................. 106
4.5 References ................................................................................................................... 107
VI
Chapter 5: Bi2Te3 Enhanced High-Performance Silicon / PEDOT:PSS Hybrid Solar Cell
5.1 Introduction ................................................................................................................. 111
5.2 Materials and methods ................................................................................................ 113
5.2.1 Chemicals ........................................................................................................ 113
5.2.2 Preparation of silicon substrates ..................................................................... 113
5.2.3 Device fabrication and characterization ......................................................... 114
5.3 Results and discussion ................................................................................................ 115
5.3.1 Performance of Bi2Te3 incorporated Si/PEDOT:PSS hybrid solar cell .......... 115
5.3.2 Role of Bi2Te3 in enhanced Si/PEDOT:PSS hybrid solar cell ........................ 119
5.4 Conclusion .................................................................................................................. 125
5.5 References ................................................................................................................... 125
Chapter 6: Terahertz Response of Topological Insulator Bi2Te3
6.1 Introduction ................................................................................................................. 130
6.2 Materials and methods ................................................................................................ 132
6.2.1 Preparation of Bi2Te3 and Bi2Te3/graphene films ........................................... 132
6.2.2 Fabrication of Bi2Te3/grapheme THz modulator ............................................ 133
6.2.3 THz-TDS measurements ............................................................................. 133
6.3 Results and discussion ................................................................................................ 134
6.3.1 Transmission, complex refractive index and complex conductivity .............. 135
6.3.2 Physical model fitting of the data of Bi2Te3 ................................................... 141
6.3.3 The interaction between Bi2Te3 and graphene ................................................ 144
6.4 Conclusion .................................................................................................................. 148
6.5 References ................................................................................................................... 148
VII
Chapter 7: Conclusion and outlook
7.1 Conclusion .................................................................................................................. 154
7.1 Challenges and outlook ............................................................................................... 156
VIII
Summary
Topological insulators are new quantum states of matters that exhibit gapless surface
states and insulating bulk states. It has been shown that topological insulators possess many
peculiar properties, such as suppressed backscattering of electrons on the surface and
robustness of surface states against modification. Bi2Te3 is a typical 3D topological insulator
with a small bulk band gap of around 0.2 eV.
In this thesis, a kinetically-controlled synthesis method is developed to obtain high-
quality single crystal of Bi2Te3 hexagonal nanoplates and the growth mechanism is carefully
investigated. Optical properties of Bi2Te3 nanoplates are studied in the visible and terahertz
range. In the visible range, we observe and image the surface plasmons resonance of single
nanoplate in real-space with both electron energy-loss spectroscopy and scanning near-field
optical microscopy for the first time. The surface plasmons of Bi2Te3 can cover almost the
whole visible range, making it promising in the photoenergy harvesting. In the terahertz
range, temperature dependent optical conductivity has been probed spectroscopically and we
observe large bulk resistivity of Bi2Te3, which addresses a long standing issue in the study of
topological insulators. We also demonstrate that in the terahertz region, Bi2Te3 can improve
the modulation depth of graphene under back gating. In view of its plasmonic properties in
the visible range and unique surface state as topological insulator, the Bi2Te3 nanoplates are
incorporated in silicon/polymer hybrid solar cell and we find that a high PCE of 11.6% can
be attained, which is comparable to the highest value achieved to date for state of the art
silicon / polymer hybrid solar cell.
IX
List of Tables
Table 3.1 Size distribution of Bi2Te3nanoplates as a function of concentration of precursors.
Table 5.1 Detailed photovoltaic properties of devices with and without Bi2Te3.
X
List of Figures
Figure 1.1 Surface states in 2D (a) and 3D (b) topological insulators. The right image is the
corresponding band structure. Local density of states of Bi2Se3 (c) and Bi2Te3 (d) on the [111]
surface, with surface states (red lines) can be clear seen around Γpoint. The Dirac point of
Bi2Se3 falls in the bulk gap while that of Bi2Te3 is embedded in the bulk valence band. (a) and
(b) Reprinted with permission from Macmillan Publishers Ltd., Ref. 16, copyright (2011). (c)
and (d) Reprinted with permission from Macmillan Publishers Ltd., Ref. 11, copyright (2009).
Figure 1.2 (a) Light reflection on the surface of antireflection coating, (b) Two possible paths
taken by an electron on a QSH edge when scattered by a nonmagnetic impurity. Reprinted
Fig. 5 with permission from Ref. 2, copyright (2011) by the American Physical Society.
Figure 1.3 (a) Kretschmann and (b) Otto configuration of prism coupling, (c) grating
coupling and (d) near-field excitation.
Figure 1.4 Theoretically proposed spin plasmons (red arrows) excited on the surface of a
patterned topological insulator. Reprinted with permission from Macmillan Publishers Ltd.,
Ref. 90, copyright (2013).
Figure 1.5 (a) Layered crystal structure of Bi2Te3. (b) Mechanical exfoliation of Bi2Se3
nanoribbons using AFM tip. (c) SEM image of Bi2Se3 nanoribbons grown by VLS method. (d)
SEM image of Bi2Te3 nanoplates grown by catalyst-free VS method. (b) and (d) Reprinted
with permission from Ref. [98] and [112], respectively, copyright (2010), American
Chemical Society. (c) Reprinted by permission from Macmillan Publishers Ltd., Ref. [110],
copyright (2010).
XI
Figure 2.1 Schematic representation of the instrumentation of SEM. Reprinted from
Reference [1].1
Figure 2.2 Configuration of typical TEM.
Figure 2.3 Schematic illustration of AFM.
Figure 2.4 (a) Schematic illustration of Synge’s idea for achieving spatial resolution with
sub-diffraction limit, by illuminating the sample through a small hole with size of sub-
wavelength. (b) Schematically representation of the different zones together with the optical
wave’s characteristics. Reprinted from Reference [8]
Figure 2.5 Basic SNOM arrangements including illumination unit (a), collection unit (b), and
detection unit (c).
Figure 2.6 Different configurations of SNOM. Reprinted from Reference [9].
Figure 2.7 Schematic illustration of the diffraction pattern of X-ray on the surface of crystal
structure.
Figure 2.8 Schematic of the transition processes in some of the spectroscopic techniques that
can probe electronic structure.
Figure 2.9 Three procedures for TEM-based energy-loss spectroscopy: (a) Conventional
TEM with a magnetic-prism spectrometer below the viewing screen, (b) TEM incorporating
an in-column imaging filter and (c) Scanning-transmission (STEM) system. Reprinted from
reference [18].18
Figure 2.10 Schematic diagram of a standard THz-TDS spectrometer.
XII
Figure 3.1 Morphology of the as-synthesized Bi2Te3 deposited on SiO2/Si substrate: SEM
images of nanoplates with low (a) and high (b) magnification. AFM images with overview of
three uniform nanoplates (c) and a single flake (d). The height profile corresponds to the
dashed line in the topography image of the single Bi2Te3nanoplate. (e) Thickness distribution
of the Bi2Te3nanoplates.
Figure 3.2 (a) Powder XRD spectrum of as-synthesized Bi2Te3 with indexed peaks. (b) Low-
magnification TEM image of several Bi2Te3 nanoplates on cooper grid with lacey carbon
support film. (c) TEM image of a single Bi2Te3 nanoplate with its corresponding SAED
pattern (d) and HRTEM image (e).
Figure 3.3 SEM images of products synthesized at (a) 150 ℃, (b) 180 ℃, (c) 195 ℃, (d) 210 ℃
for 4 h with 0.3g PVP in 0.5M NaOH.
Figure 3.4 SEM images of products synthesized at 210 ℃ for 4 h with 0.3g PVP in (a) 0M,
(b) 0.05M, (c) 0.1M, (d) 0.2M, (e) 0.5M, (f) 1M NaOH.
Figure 3.5 SEM image of the product synthesized at 210 ℃ without NaOH (a) for 4 h, (b) for
24 h.
Figure 3.6 SEM images of products synthesized at 210 ℃ for 4 h in 0.5M NaOH with (a) 0g,
(b) 0.05g, (c) 0.1g, (d) 0.2g, (e) 0.3g, (f) 0.8g PVP.
Figure 3.7 SEM images of products synthesized at 210 ℃ for 4 h in 0.5 M NaOH with 0.3 g
(a) CTAB and (b) SDBS.
Figure 3.8 SEM images of products synthesized at 210 ℃ for 12 h in 0.5 M NaOH with 0.3 g
PVP and different concentration of precursor of (a) 1 SC, (b) 1/2 SC, (c) 1/3 SC, (d) 1/5 SC,
(e) 1/10 SC, (f) 1/20 SC.
XIII
Figure 3.9 Solvent effect of IPA. SEM images of products synthesized at 210℃ for 4 h in
0.5M NaOH with 0.3 g PVP and (a) 15%, (b) 30%, (c) 50%, (d) 100% IPA replaced EG.
Inserted in (a) is the high-magnification image shows ultra-large plates.
Figure 3.10 Solvent effect of glycerol. SEM images of products synthesized at 210 ℃ for 4 h
in 0.5M NaOH with 0.3 g PVP and (a) 15%, (b) 30%, (c) 50%, (d) 100% glycerol replaced
EG. Inserted in (d) is the SEM image with high-magnification.
Figure 3.11 SEM images of products synthesized at 210 ℃ in 0.5 M NaOH with 0.3 g PVP
for (a) 1 h, (b) 2 h, (c) 4 h and (d) 20 h. Inserted in Figure 11b is the high-magnification
image of a Bi2Te3-Te heterostructure.
Figure 3.12 (a) TEM image of one Te-Bi2Te3 heterostructure. Blue and red spots show the
beam position where the EDS spectra were taken. (b) The corresponding EDS spectrum of
blue spot. (c) The corresponding EDS spectrum of red spot. The Cu peaks in both spectra
originate from the copper TEM grid.
Figure 3.13 SEM images of products synthesized at 195 ℃ for (a) 2 h, (b) 4 h, (c) 8 h and at
180 ℃ for (d) 2 h, (e) 4 h, (f) 8 h in 0.5 M NaOH with 0.3 g PVP.
Figure 3.14 SEM images of Bi2Te3 nanoplates with hole in the centre synthesized at 210℃ in
0.2 M NaOH with 1/3 SC and 0.3 g PVP for 4 h.
Figure 4.1 (a) TEM image of the first studied hexagonal Bi2Te3 nanoplate, (b) High
resolution TEM (HRTEM) image of the same nanoplate and the corresponding SAED pattern
(inset), (c) EELS spectra of the nanoplate when electron beam was positioned at the edge
(black line) and center (red line) of the nanoplate and (d) EELS mapping of the nanoplate at
different energies corresponding to the three peaks in (c).
XIV
Figure 4.2 Real part ( 1 ) and imaginary part ( 2 ) of the dielectric function. (b) Carrier
density of Bi2Te3 calculated from ellipsometry results.
Figure 4.3 (a) TEM image of the second studied hexagonal Bi2Te3 nanoplate, (b)
Corresponding SAED pattern of the center of the nanoplate, (c) EELS spectra of the
nanoplate when electron beam was positioned at the edge (black line) and center (red line) of
the nanoplate and (d) EELS mapping of the nanoplate at different energies corresponding to
the three peaks in (c).
Figure 4.4 (a) High-angle annular dark field (HAADF) STEM image of the asymmetrical
hexagonal Bi2Te3 nanoplate, the thickness of left half and right half are 86 and 34 nm,
respectively. (b) HRTEM image of the nanoplate. (c) EELS spectra of the nanoplate when the
electron beam was positioned at three different spots (A= middle of top corner, B= middle of
right short edge, C= middle of right long edge). (d) A series of EELS spectra when the
electron beam was positioned at different spots along the long edge of nanoplate is indicated
in (a). (e) EELS intensity difference shows the intensity difference of mapping at the peak
energy of 1.36 eV and 1.93 eV.
Figure 4.5 (a) Two dimensional and (b) three dimensional topography images, (c) two
dimensional and (d) three dimensional near-field amplitude distribution, and (e) height and
field profile of a hexagonal Bi2Te3 nanoplate along the line trace (white dash line) indicated
in (c).
Figure 5.1 Solar cell performances as a function of Bi2Te3 concentration.
Figure 5.2 Current density-Voltage (J-V) characteristics of devices with and without Bi2Te3.
XV
Figure 5.3 (a) IPCE spectra of the solar cell devices with and without Bi2Te3; (b) IPCE
increasement of the solar cell device incorporated with Bi2Te3 and (c) EELS spectrum of
Bi2Te3 as a function of wavelength.
Figure 5.4 (a) Charge carrier life time as a function of light intensity obtained by transient
photovoltage decay; (b) Charge carrier concentration as at different open circuit voltage
obtained by transient photocurrent decay.
Figure 5.5 Nyquist plots of the devices measured by EIS under one sun illumination.
Figure 6.1 Time-domain terahertz signal transmitted through sample and reference substrate.
(Inset: magnified peak region of main pulse)
Figure 6.2 Frequency dependent transmission amplitude and phase, refractive index and
extinction coefficient of Bi2Te3, graphene and Bi2Te3/graphene at 300K.
Figure 6.3 Frequency-domain spectra of the real part and imaginary part of the optical
conductivity of Bi2Te3, graphene and Bi2Te3/graphene at 300K.
Figure 6.4 (a) Real part and (b) imaginary part of optical conductivity of Bi2Te3 at 100K, dot
line = experimental data, red line = fitting curve, green line = Drude contribution, dark blue
line = first Lorentz contribution, light blue line = second Lorentz contribution and purple line
= third Lorentz contribution.
Figure 6.5 Real part and imaginary part of the optical conductivity of Bi2Te3 at temperature
of 60, and 140K (dot line) and the corresponding fitting curve (solid line) with Drude-Lorentz
model.
XVI
Figure 6.6 (a) Real conductivity at 0.22 THz, (b) Drude plasma frequency, (c) scattering rate
and (d) carrier density of Bi2Te3 extracted from Drude-Lorentz model at different temperature.
Figure 6.7 (a) Time-domain THz spectra, (b) frequency-domain transmission of graphene, (c)
time-domain THz spectra and (d) frequency-domain transmission of Bi2Te3/graphene at
different back gating voltage.
XVII
List of Schemes
Scheme 3.1 (a) Crystal structure of Bi2Te3 with each QL formed by five atomic layers
stacked in the sequence of Te-Bi-Te-Bi-Te.(b) Schematic illustration of the growth
mechanism which includes both homogeneous and heterogeneous nucleation process.
Scheme 5.1 (a) Schematic illustration of the device structure of the Bi2Te3 incorporated
Si/PEDOT:PSS hybrid solar cell. (b) Energy diagram of Si/PEDOT:PSS hybrid solar cell.
Scheme 5.2 Band diagram of the Schottky barrier formed at the interface of Si and
PEDOT:PSS.
Scheme 6.1 Illustrations of device structure and operating principle. (a) Schematic of the
proof-of-concept Bi2Te3/graphene terahertz modulator. (b) Band structure of graphene and its
optical processes under THz. Red arrows represents the intraband transitions. Applying
negative voltage on p-doped graphene can lead to the increasing of Fermi level, for example,
from EF1 to EF2.
XVIII
List of Abbreviations and Symbols
2D Two-dimensional
3D Three-dimensional
AFM Atomic force microscope
AM Air mass
APD Avalanche photodiode detector
ARPES Angle-resolved photoemission spectroscopy
CB Chlorobenzene
CCD Charge-coupled device
CTAB Cetyltrimethylammonium bromide
DI water Deionized water
DMSO Dimethyl sulfoxide
EELS Electron energy-loss spectroscopy
EFTEM Energy-filtered transmission electron microscopy
EG Ethylene glycol
EIS Electrochemical impedance spectroscopy
FESEM Field emission scanning electron microscopy
FF Fill factor
HAADF High-angle annular dark field
IPA Isopropanol
IPCE Incident photon to charge carrier efficiency
Liq 8-hydroxyquinolinolato-lithium
LSPR Localized surface plasmons resonance
MBE Molecular beam epitaxy
XIX
MOCVD Metal-organic chemical vapor deposition
NA Numerical aperture
n-Si n - type silicon
PCE Power conversion efficiency
PEDOT:PSS Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)
PVD Physical vapour deposition
PVDF Polyvinylidene difluoride
PVP Polyvinylpyrrolidone
QL Quintuple layer
QSH Quantum spin hall
SAED Selected area electron diffraction
SC Standard concentration
SDBS Sodium dodecyl benzene sulphonate
SdH Shubnikov-de Haas
SEM Scanning electron microscopy
SNOM Scanning near-field microscopy
SPPs Surface plasmons polaritons
STEM Scanning transmission electron microscopy
STS Scanning tunneling spectroscopy
TEM Transmission electron microscopy
THF Tetrahydrofuran
THz Terahertz
THz-TDS Terahertz time domain spectroscopy
VLS Vapour–liquid–solid
VS Vapour–solid
XX
XRD X-ray diffraction
SBH
Schottky barrier height
(t)E Transmitted pulse
T( ) Transmission coefficient
n Complex refractive index
( )n
Refractive index
( )k
Extinction coefficient
1( )
Real part of optical conductivity
2 ( )
Imaginary part of optical conductivity
0 Wavelength
n Refractive index
pD
Drude plasma frequency
Scattering rate
Correction factor to the value of dielectric constant
1p 2p 3p
Plasma frequency of Lorentz terms
01 02 03
resonance frequency
Effective electron mass
E0 Standard reduction potentia
J Current density
Jsc Short-circuit current density
nd Doping concentration
Q Quality factor
XXI
Rct Charge transfer resistance
V Voltage
Vbi Built-in potential
Voc Open-circuit voltage
Wp Work function
χSi Electron affinity of silicon
XXII
List of Publications
(1) Meng Zhao, Shuai Wang, Qiaoliang Bao, Yu Wang, Priscilla Kailian Ang and Kian
Ping Loh*, “A simple, high yield method for the synthesis of organic wires from
aromatic molecules using nitric acid as the solvent”, Chemical communications, 2011,
47, 4153–4155
(2) Shuai Wang, Manga Kiran Kumar; Meng Zhao, Qiaoliang Bao and Kian Ping Loh,
“Wrapping graphene sheets around organic wires for making memory devices”,
Small, 2011, 7, 2372-2378
(3) Meng Zhao, Qiaoliang Bao and Kian Ping Loh, “Surface plasmon resonance of
topological insulator Bi2Te3 and its role in high performance silicon/Bi2Te3-
PEDOT:PSS schottky junction solar cell”, Manuscript under preparation
(4) Meng Zhao, Huanxin Xia, Chan La-o-vorakiat, Chia Ee Min, Elbert and Kian Ping
Loh, “Terahertz response of topological insulator Bi2Te3”, Manuscript under
preparation
(5) Bee Min Goh, M.V. Reddy, Chengxin Peng, Meng Zhao and Kian Ping Loh,
“Graphene as electrode material for Li-ion batteries”, Manuscript under preparation
1
Chapter 1
Introduction
Topological insulators are recently discovered phase of quantum matter with insulating bulk
and gapless surface states which are protected by spin-orbit coupling and time-reversal
symmetry.1-5
Since the first theoretical predication of topological insulator6 and experimental
observation of such system in HgTe quantum wells,7,8
dramatically increased attention has
been devoted to this area. Thereafter, a number of materials such as Bi2Te3, Sb2Te3, and
Bi2Se3 were discovered to be topological insulators.9-11
Due to their unique surface states,
topological insulators have manifested themselves as a fantastic platform for both
fundamental studies in condensed-matter physics and applied research.
1.1 Development of topological insulator
To form topological insulator, spin-orbit coupling must be strong enough to modify
electronic structures significantly, which indicates that small-bandgap semiconductors
consisted of heavy elements are the most promising candidates. Based on this principle,
Zhang SC proposed two dimensional (2D) HgTe quantum wells to be topological insulator
for the first time in 2006.6 This prediction was soon verified experimentally.
8 The search of
topological insulators has witnessed multiple breakthrough in the past years, especially the
discovery of three dimensional (3D) topological insulators, which possess fully gapped bulk
states covered with metallic surfaces.12,13
3D topological insulator was first observed with
semiconducting alloy Bi1-xSbx,14,15
followed by three binary chalcogenides, Bi2Te3, Sb2Te3,
2
and Bi2Se3,9-11
which boosted the research in topological insulators and became the materials
of choice in experiments because of the following advantages. First, they have a single Dirac
cone inside a relatively large bandgap (~0.15 to 0.3 eV), leading to the access to the
remarkable properties of surface state up to room temperature. Second, they are traditional
well-known binary compounds with long history of study on synthesis, and high-quality
single crystals with appropriate stoichiometry are easier to be obtained as compared with
alloys and other complex compounds with multi-element.16
Apart from those binary alloys, several new families of materials have been
demonstrated to be 3D topological insulators, such as ternary chalcogenide TlBiQ2 and
TlSbQ2 (Q=Te, Se and S),17,18
and complex material Bi2-xSbxTe3-ySey, which have been
proven to be able to approach topological insulator regime by tuning the value of x and y.19-21
The search for new topological insulator is still a field that promises exciting discoveries and
it may finally lead to the emergence of candidates for practical use.
1.2 Properties of topological insulator
The origin of the unique behavior of topological insulator is conduction and valence
band inversion induced by the spin-orbital coupling.7 As a result, highly conducting metallic
states are formed on the surface, in which the spin of electrons is oriented perpendicularly to
the orbital momentum, termed as spin-momentum locking. In 2D topological insulator, which
is also known as the quantum spin Hall state,22
electrons with opposite spin directions (spin-
up and spin-down) counter-propagate along the edge and wire-like surface states are formed,
3
while the surface states can cover the entire material in 3D topological insulators, as shown in
Figure 1.1a and 1.1b. The surface states of the typical 3D topological insulator Bi2Se3 and
Bi2Te3 are shown in Figure 1.1c and 1.1d.
Figure 1.1 Surface states in 2D (a) and 3D (b) topological insulators. The right image is the
corresponding band structure. Local density of states of Bi2Se3 (c) and Bi2Te3 (d) on the [111]
surface, with surface states (red lines) can be clear seen around Γ point. The Dirac point of
Bi2Se3 falls in the bulk gap while that of Bi2Te3 is embedded in the bulk valence band. (a) and
(b) Reprinted with permission from Macmillan Publishers Ltd., Ref. 16, copyright (2011). (c)
and (d) Reprinted with permission from Macmillan Publishers Ltd., Ref. 11, copyright (2009).
4
Spin-orbit coupling is vital for the unique properties of 3D topological insulators. Two
essential properties, dissipation-less charge transport and robustness of surface states are
introduced in the following. As a result of those unique properties, topological insulators are
expected to show promising application potential in various areas, including energy-efficient
electronics,5,23
thermoelectronics,23,24,16
catalysis,25
near-infrared transport electrodes,26
and
quantum computing.27
1.2.1 Dissipation-less charge transport
During the transport of electrons in metallic materials, collision of electrons with
impurities and/or defects happens and results in random scattering of the electrons. This
scattering process would count against the flow of electrons and degrade conductivity
(Anderson localization28
). The adverse effect of electron scattering on conductivity is
especially significant at low temperature. Backscattering refers to the process where the
direction of electrons changes 180 degree after collision with impurities and defects, which
would degrade current flow much more significantly.
One of the special features of topological insulators is the completely suppressed
backscattering on the surface,29
which means charge transport on the surface of topological
insulators is a dissipation-less or low dissipation process. The absence of backscattering on
the surface states of topological insulator is analogous to the operation mechanism of
antireflection coating.30
Figure 1.2 (a) illustrates how the interference happens on the surface
of antireflection coating. On the surface of topological insulator, all possible backscattering
5
paths of surface electrons interfere with each other in destructive manner, resulting in
completely suppressed backscattering. As depicted in Figure 1.2 (b), due to the spin-
momentum lock of surface states, a forward-moving spin-up electron can only turn clockwise
or counterclockwise to travel around the scattering center. The scattered backward-moving
electron must be spin-down, which makes a phase difference of π for the forward-moving
electron and backward-moving electron and they interfere destructively.
Figure 1.2 (a) Light reflection on the surface of antireflection coating, (b) Two possible paths
taken by an electron on a QSH edge when scattered by a nonmagnetic impurity. Reprinted
Fig. 5 with permission from Ref. 2, copyright (2011) by the American Physical Society.
It is believed that time reversal symmetry of surface states plays a key role in the
suppressed backscattering,2 which suggests that Anderson localization effect does not happen
on the surface states of topological insulators even with strong crystal disorder31
as long as
the time reversal symmetry is not broken. This unique feature makes topological insulator a
promising candidate in energy-efficient electronics that suffer to a lesser extent from heat
diffusion problem, which had impeded further progress in conventional electronics.
6
1.2.2 Robustness of surface states
Robust surface state that is stable against modification and defects represents another
peculiar property of topological insulators. In normal materials, surface properties are largely
dependent on active sites of surface, such as dangling bond and reconstruction sites, which
indicates that surface properties can be altered by modifying the active sites with chemical or
physical methods. However, the metallic surface states of topological insulators stem from
topological invariants, which will not change as long as the materials remain insulating. It
was reported that the Shubnikov-de Haas (SdH) oscillations of Bi1.5Sb0.5Te1.7Se1.3 could be
observed even after long exposure to air for one month.32
The magnetic field dependence of
SdH oscillation indicates the 2D nature of surface states, and proves the robustness of the
surface states against aging. The robustness of surface states makes topological insulators
suitable for practical applications under certain environment.
1.2.3 Optical properties in terahertz regime
Terahertz (THz) electromagnetic wave lies between the end of far infrared and the start
of microwave. It usually consists of electromagnetic waves with the frequency ranging from
0.3 THz to 3THz, corresponding to an energy range from 1.24 to 12.4 meV. THz radiation is
a good probe to study the electronic structures of a system close to its equilibrium states
because of the weak perturbation imposed by the low-energy radiation.
7
THz spectroscopy is a powerful technique to investigate optical and electronic properties
of various materials, including semiconductor,33,34
carbon materials,35,36
superconductors37
and so on. Temperature dependent terahertz conductivity of topological insulator
Bi1.5Sb0.5Te1.7Se1.3 has been measured by THz-TDS.38
For most topological insulators, their
optical conductivity, refractive index and carrier dynamics are still unrevealed. Detailed
research work is necessary to obtain these data to better understand topological insulators.
For the study of topological insulators, another very important advantage of THz
spectroscopy is that it can detect topological surface states and distinguish the surface states
from bulk states. As described before, the fantastic properties of topological insulators lie in
the peculiar surface states that protected by time-reversal symmetry. Thus, detection of the
unique surface states of topological insulators has been a major goal for the community. Most
signatures of topological nontrivial surface states have been detected by surface techniques,
such as angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling
spectroscopy (STS).9,10,14,39,40
Measuring charge transport properties is another method that has been used in many
studies. For 3D topological insulators, charge transport properties are expected dominated by
surface carriers if chemical potential lies in the bulk of bandgap. However, it is not suitable
for most synthesized topological insulators, whose surface carriers are significantly
outnumbered by the bulk carriers41-44
which dominate the charge transport properties of the
materials. In order to measure the charge transport on the surface, effective suppression of
bulk conductance and improvement of surface conductance are needed, and this proves to be
serious challenges. There have been many efforts to suppress bulk carriers, and the carrier
density and type have been tuned with chemical doping,39,45-48
electrical gating,44,46,49,50
nanostructure with high surface-to-volume ratio.51-53
8
THz-TDS provides a non-contact, facile and more direct way to probe surface states
compared with the two methods mentioned above. For example, terahertz Kerr measurement
was first used to prove the surface states of topological insulator Bi2Se3.54,55
More study to
probe the terahertz response of other topological surface states at different temperature would
be needed to establish a solid database for this method.
1.2.4 Plasmonics properties
Surface plasmons are collective oscillation of conduction electrons in metals.56-59
Surface plasmons of metallic structures have been extensively investigated because of its
ability to confine light into sub-wavelength volume,60
thus facilitate development of
nanophotonics and electronics beyond diffraction limit.61-64
Excitation of surface plasmons
represents one of the most intriguing optical properties of topological insulators because of
the helical states of surface electrons. In this subsection, a general introduction is given
before discussing the surface plasmons of topological insulators.
1.2.4.1 General properties of surface plasmons
The great application value of surface plasmons comes from its superior properties. First
of all, a relaxation time on the order of 10 femtoseconds of plasmons leads to the ultrafast
dynamics of surface plasmons, which is important in next generation information processing
9
and optical memory. Another three important features of surface plasmons are the high
quality factor (Q), high oscillator frequency and ultra-small modal volume. Quality factor is
an index used to characterize the oscillation process that happen before the energy of surface
plasmons is substantially decayed. On the other hand, quality factor can also be used to
measure the local field enhancement. For noble metal, Q varies between 10 and 100, which
results in local field enhancement on the order of 100 to 10000. The oscillator strength of
surface plasmons is equal to the number of conduction electrons, which is on the order of 105
for metallic nanostructures. It is well known that the absorption cross section is proportional
to oscillator strength and the scattering cross section is proportional to square of oscillator
strength. The super high oscillator strength of surface plasmons compared with
semiconductors and dye molecules (about 1) indicates that they are much more efficient in
improving light-matter interaction by acting as scatters and absorbers, resulting in widely
studied surface plasmons enhanced photon-electron conversion and sensing process. Finally,
the ultra-small modal volume of surface plasmons can overcome the diffraction limit of light
which limits traditional photonic elements. With those crucial properties, surface plasmons
can be applied in various fields, including photovoltaic devices,65
nanoscale sensing,66-68
subwavelength waveguiding,69-71
enhanced spectroscopy72,73
and so on.
1.2.4.2 Excitation and detection methods
In order to study the plasmonic properties of a certain sample, like Bi2Te3 in our case,
techniques to excite and detect surface plasmons must be well chosen. Surface plasmons can
be excited by both photons and electrons, whose energy and momentum can be transferred at
10
the same time during their interaction with surface plasmons. A quasiparticle termed as
surface plasmons polaritons (SPPs) is formed when surface plasmons couple with photons.
SPPs can propagate along the interface where it is formed and attenuate exponentially via
radiative and non-radiative decay. Due to the momentum mismatch between dispersion of
light line and surface plasmons, SPPs cannot be excited directly. There are several methods to
overcome the problem of momentum mismatch.74
Figure 1.3 shows three methods that are
most widely used: prism coupling, grating coupling and near-field excitation. In prism
coupling, attenuated total internal reflection happens in the prism and the fields of excitation
beam tunnel into the metal/dielectric interface. The momentum of the tunneling radiation
increases and become sufficient to excite surface plasmons. Grating coupling overcome the
mismatch problem by patterning the metal surface with shallow grating of grooves or holes.
For the near-field excitation method, great momentum enhancement can be achieved at the
apex of the tip to fulfill the conditions for momentum matching.
Figure 1.3 (a) Kretschmann and (b) Otto configuration of prism coupling, (c) grating
coupling and (d) near-field excitation.
11
Localized surface plasmons can confine electromagnetic energy into volumes much
smaller than the diffraction limit of3
0( / 2 )n , where n is the refractive index of
surrounding medium. The excellent energy confinement would lead to a concomitant local
field enhancement around the boundary region. Localized surface plasmons resonance (LSPR)
occurs when the frequency of incident light or electrons matches natural oscillation frequency
of valence electrons against restoring force. There is no momentum match requirement to
realize LSPR, which can be achieved by direct excitation of incident photos and electrons.
Detection methods represent another aspect in the study of surface plasmons. So far,
surface plasmons of various structures have been investigated mainly with three techniques:
spectroscopy,75,76
electron energy-loss spectroscopy,77-81
and scanning near-field optical
microscopy.82,83
Imaging of surface plasmons can be achieved with near-field optical
microscopy, fluorescence based imaging, leakage radiation based imaging, scattered light
based imaging,74
as well as cathodoluminescence imaging.84
Among these methods, near-
field optical microscopy is the most effective and the only method to image surface plasmons
with sub-wavelength resolution, which is of great significance to characterize confinement
and propagation properties of samples.
Electron energy-loss spectroscopy is another widely used method to study plasmonics.
While near-field microscopy mainly provides real space imaging of surface plasmons, EELS
is powerful in providing direct measurement of the wavelength of surface plasmons.
Combination of EELS and near-field microscopy are powerful tools for the study of surface
plasmons. In this thesis, surface plasmons of Bi2Te3 are carefully investigated with both
techniques.
12
1.2.4.3 Surface plasmon of topological insulators
Surface plasmons induced by oscillation of charge density are the subject of a plethora
of research activities. With the advent of 3D topological insulators, novel phenomena are
likely to be discovered due to the unique surface states. Electrons on the surface of 3D
topological insulator obey Dirac-like equation for massless particles, analogous to electrons
in graphene. Surface plasmons caused by oscillation of Dirac fermions are termed
specifically as Dirac plasmons, which possess peculiar properties compared with surface
plasmons caused by massive electrons in metals. First, density of Dirac fermions of graphene
and topological insulators can be easily tuned by gating, which in turn changes the frequency
of plasmons.85
Second, Dirac plasmons dispersion can renormalized even in the long
wavelength limit due to exchange and correlation effects.86
Third, coupling of Dirac
plasmons and light can be easily achieved with the aid of an extremely sharp tip, which can
compensate the momentum mismatch. A much longer lifetime of Dirac plasmons compared
with plasmons of metals and semiconductors also earns it intensive research interest in
electronic applications.87
However, at variance with graphene, spin direction of electron in topological insulators
is strictly determined by its momentum, resulting in the so called “helical” states. Due to the
strict spin-momentum locking, collective excitation of electrons on the surface of topological
insulators is predicated to manifest itself as coupled charge density and spin density wave,88,89
termed as spin plasmons. Figure 1.4 shows the spin plasmons formation on the surface of
topological insulators.90
13
Figure 1.4 Theoretically proposed spin plasmons (red arrows) excited on the surface of a
patterned topological insulator. Reprinted with permission from Macmillan Publishers Ltd.,
Ref. 90, copyright (2013).
Surface plasmons of topological insulators have been investigated recently in the theory
perspective. S. Raghu et al built a theory model to study the coupling between spin and
charge excitation on the surface of Bi2Se3-Bi2Te3 family topological insulators, indicating
that the spin plasmon mode can propagate along the surface.89
Andreas Karch studied surface
plasmons on the interfaces between topological insulator and metal in three dimensions by
adopting effective field theory, and found that the magnetic polarization of surface plasmons
was rotated out of the interface plane.91
Generating, modulating surface plasmons experimentally would be of great significance,
especially in the areas of optoelectronics and spintronics. Until now, surface plasmons of
topological insulator have not been studied a lot experimentally, with very limited work on
Bi2Se3 published.92,93
By fabricating micro-ribbon array of Bi2Se3 with different size, Pietro et
14
al studied their optical response in the terahertz range and found that it is consistent with the
behavior of Dirac plasmons. Albeit with the first evidence of Dirac plasmons of topological
insulator, many questions remain to be elusive regarding to the fundamental properties of
surface plasmons. First, the evidence presented above was based on spectroscopy, which is
not a direct and most conclusive evidence to prove the existence of surface plasmons of the
topological surface states. Direct imaging of surface plasmons of topological insulators is
highly desirable. Second, the fundamental properties of surface plasmons of topological
insulators in relation to the size, shape and spatial dependence, propagation length, and
energy confinement capability are not understood well. In this thesis, these problems are
properly answered with Bi2Te3 as the research objective.
1.3 Synthesis of topological insulator nanostructures
Akin to the study of other exciting materials, methods to synthesis high quality materials
are indispensable for both fundamental and applied study of 3D topological insulators.
Compared with bulk materials, nanostructures of topological insulator are superior in
investigating the fundamental nature of the surface states and making functional devices. The
most important advantage is the enhanced surface effects induced by the large surface-to-
volume ratio, which makes the topological insulator nanostructures a good platform to study
the surface states. The unique morphology is another advantage of nanostructured topological
insulator. Surface electrons in topological insulator nanostructures are forced to travel in
well-defined path, making them promising for interference-type experiments such as
Aharonov-Bohm oscillations.94
15
Many endeavors have been devoted to produce topological insulator nanostructures,
such as nanowires, nanorribons and nanoplates. Various synthesis methods have been
developed, which are generally categorized as either physical synthesis or chemical synthesis.
1.3.1 Physical synthesis
The physical synthesis methods are mainly divided into “top-down” and “bottom up”
approaches. The top-down approach refers to the mechanical exfoliation of bulk topological
insulator materials into thin flakes, achieved by the so-called Scotch-tape method49,95
that is
widely used to produce graphene from graphite.96,97
Some topological insulators, such as
Bi2Te3, Bi2Se3 and Sb2Te3, are layered materials, and the crystal structure of Bi2Te3 is shown
in Figure 1.5a. Each quintuple layer (QL) is built up of five atomic layers stacked in the
sequence of Te (1) – Bi – Te (2) – Bi – Te (1). The bonding between each of the 5 layers is
covalent while it is weaker van der Waals between each QL. Therefore, thin flakes can be
cleaved from the bulk by breaking van der Waals interaction using an adhesive tape.
However, the products suffer from irregular shape with rough surface and low yield.
Furthermore, atomic force microscope (AFM) tip can be used to obtain ultra-thin
nanomaterials with smooth surface by breaking both covalent and van der Waals bonding, as
shown in Figure 1.5b.98
The bottom up approach includes multiple methods such as molecular beam epitaxy
(MBE), vapor–liquid–solid (VLS), vapor–solid (VS), and metal-organic chemical vapor
deposition (MOCVD). They were used to grow topological insulator nanostructures with high
16
Figure 1.5 (a) Layered crystal structure of Bi2Te3. (b) Mechanical exfoliation of Bi2Se3
nanoribbons using AFM tip. (c) SEM image of Bi2Se3 nanoribbons grown by VLS method. (d)
SEM image of Bi2Te3 nanoplates grown by catalyst-free VS method. (b) and (d) Reprinted
with permission from Ref. 98 and 112, respectively, copyright (2010), American Chemical
Society. (c) Reprinted by permission from Macmillan Publishers Ltd., Ref. 110, copyright
(2010).
crystallinity and well-defined morphology. MBE has been widely used to grow high quality
film such as Bi2Se3,99-103
Bi2Te3,104,105
and Sb2Te3.106
Usually, lattice matching between
substrates and target materials is crucial for the epitaxial growth of high quality crystals.
While MBE is suitable for growth of thin film with precise control of thickness, other
methods such as VLS, VS and MOCVD have also been applied to produce nanoribbons107-110
and nanoplates111-113
, as shown in Figure 1.5c and 1.5d. Those aforementioned methods are
mainly used by physicists to conduct fundamental studies of topological insulators and are
limited by their low yield.
17
1.3.2 Chemical synthesis
To develop topological insulator for practical applications, the large scale synthesis of
high-quality crystals are desired. Therefore, various chemical synthesis methods, such as
solvothermal,114-116
hydrothermal,117,118
reflux,119, 120
electrodeposition,121-123
colloidal,124-126
photochemical,127
and microwave-assisted synthesis,128
have been developed. Besides large-
scale production, chemical synthesis is much easier and lower cost compared to physical
synthesis, and chemical modification can be achieved more readily. For example, topological
insulators are sensitive to air and moisture and the formed oxidation layer could dramatically
affect the surface property.48,129
In-situ coating of a surfactant layer during the chemical
synthesis can effectively prevent degradation of the surface from environment exposure.130
Controllable doping can also be easily realized by chemical synthesis to tune the property of
topological insulator, for instance, sodium doped Bi2Te3 with tunable Fermi level has been
demonstrated.47
So far, synthesis of topological insulators with different shapes and sizes
have been achieved using chemical methods, including quantum dots,131
nanoribbons,132
nanoplates,114,133
nanowires,134-137
nanotubes,138,139
nanorods,124
nanofolks,140
nanorings,141
and heteostructures.142
In addition to these bottom-up methods, top-down approach can be realized by chemical
exfoliation. Lithium intercalation is a generic way to produce large quantities of topological
insulator thin flakes by intercalating lithium into the layers and exfoliating them through
oxidation of lithium.143,144
Common solvents have also been used to produce single-layer
flakes of Bi2Te3 successfully.145
18
1.4 Applications of topological insulator
Electronics, optoelectronics and spintronics are among the most promising applications
of topological insulators. At present stage, most study of topological insulators involves
fundamental physical research. The applications of topological insulators have not been
exploited widely. Although the applications of Bi2Te3 as thermoelectric materials have been
studied for decades, there is no specific application that utilizes the properties of topological
nontrivial surface states. It is reported that the topological surface states can facilitate surface
reaction by promoting different directions of static electron transfer in theory.25
The enhanced
surface reaction suggests that topological insulators may be used as high-performance
catalyst. Zhang et al proposed application of Bi2Se3 in wide band-width, high performance
photodetector that works from the terahertz to infrared range.146
Peng et al fabricated near-
infrared transparent flexible electrode that show low resistance and high transparency with
Bi2Se3 grown on mica.147
The promise to take advantage of the unique surface electrons of
topological insulators in the energy utilization is tempting. The effect of topological
insulators on the performance of various energy devices remains to be studied.
1.5 Overview of objectives and work scope
As a typical 3D topological insulator, Bi2Te3 has not been widely studied and its
fundamental properties and applications still remain unrevealed. We choose nanostructured
Bi2Te3 instead of its bulk counterpart because the residual conductance in the bulk stemming
from unintentional doping and inevitable defects can be better suppressed in these
19
nanocrystals, which are thought to be as one of the main obstacles against the practical
utilization of topological insulators.148,149
Based on these considerations, we synthesized
uniform Bi2Te3 hexagonal nanoplates using a solution-phase approach, studied its surface
plasmons and utilized it in solar cells. The terahertz response of Bi2Te3 was also measured
and comprehensive analysis of its terahertz conductivity was conducted, with the expectation
to exploit understanding of the peculiarity of topological insulators.
Various techniques have been applied in the characterizations of Bi2Te3. Several key
experimental techniques involved in the study are introduced in Chapter 2.
The controlled synthesis of hexagonal Bi2Te3 nanoplates is detailed in Chapter 3, with
systematic studies on its growth mechanism and the effects of various conditions on the
morphology and size of Bi2Te3. High-quality Bi2Te3 single crystals with high yield and
uniform morphology can be obtained, which provides a good platform for the study of
surface plasmons, the results are presented in Chapter 4. Electron energy-loss spectroscopy
(EELS) and scanning near-field microscopy (SNOM) are employed to investigate surface
plasmons. Size and shape dependent plasmonic behavior of Bi2Te3 nanoplates are carefully
analyzed.
Leveraging on the surface plasmon of Bi2Te3 in the visible range and its unique surface
state, we report in Chapter 5 the application of Bi2Te3 in photovoltaics for the first time. We
fabricated Bi2Te3 incorporated silicon/PEDOT:PSS hybrid solar cell. The performance of the
solar cell is significantly enhanced with the addition of Bi2Te3 nanoplates and possible
mechanisms contributing to that are discussed. In Chapter 6, the dielectric properties of
Bi2Te3 have been studied with terahertz time-domain spectroscopy (THz-TDS), which allows
the contact-free measurement of the surface states of topological insulators.
20
In summary, the findings presented in this thesis are important for understanding the
material growth and optical properties of topological insulators. This work also explores the
utilization of topological insulators Bi2Te3 in energy conversion, which may bring the
research focus of topological insulators from fundamental study to applications.
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(139) Xiao, F.; Yoo, B.; Lee, K. H.; Myung, N. V.: Synthesis of Bi2Te3 Nanotubes by
Galvanic Displacement. Journal of the American Chemical Society 2007, 129, 10068-10069.
(140) Shi, S.; Cao, M.; Hu, C.: Controlled Solvothermal Synthesis and Structural
Characterization of Antimony Telluride Nanoforks. Crystal Growth & Design 2009, 9, 2057-
2060.
(141) Liang, Y.; Wang, W.; Zeng, B.; Zhang, G.; He, Q.; Fu, J.: Influence of NaOH on the
formation and morphology of Bi2Te3 nanostructures in a solvothermal process: From
hexagonal nanoplates to nanorings. Materials Chemistry and Physics 2011, 129, 90-98.
(142) Wang, W.; Goebl, J.; He, L.; Aloni, S.; Hu, Y.; Zhen, L.; Yin, Y.: Epitaxial Growth of
Shape-Controlled Bi2Te3−Te Heterogeneous Nanostructures. Journal of the American
Chemical Society 2010, 132, 17316-17324.
(143) Zeng, Z.; Sun, T.; Zhu, J.; Huang, X.; Yin, Z.; Lu, G.; Fan, Z.; Yan, Q.; Hng, H. H.;
Zhang, H.: An Effective Method for the Fabrication of Few-Layer-Thick Inorganic
Nanosheets. Angewandte Chemie International Edition 2012, 51, 9052-9056.
(144) Ding, Z.; Viculis, L.; Nakawatase, J.; Kaner, R. B.: Intercalation and Solution
Processing of Bismuth Telluride and Bismuth Selenide. Advanced Materials 2001, 13, 797-
800.
(145) Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.;
Gaucher, A.; De, S.; Smith, R. J.; Shvets, I. V.; Arora, S. K.; Stanton, G.; Kim, H.-Y.; Lee, K.;
Kim, G. T.; Duesberg, G. S.; Hallam, T.; Boland, J. J.; Wang, J. J.; Donegan, J. F.; Grunlan, J.
C.; Moriarty, G.; Shmeliov, A.; Nicholls, R. J.; Perkins, J. M.; Grieveson, E. M.; Theuwissen,
37
K.; McComb, D. W.; Nellist, P. D.; Nicolosi, V.: Two-Dimensional Nanosheets Produced by
Liquid Exfoliation of Layered Materials. Science 2011, 331, 568-571.
(146) Zhang, X.; Wang, J.; Zhang, S.-C.: Topological insulators for high-performance
terahertz to infrared applications. Physical Review B 2010, 82, 245107.
(147) Peng, H.; Dang, W.; Cao, J.; Chen, Y.; Wu, D.; Zheng, W.; Li, H.; Shen, Z.-X.; Liu, Z.:
Topological insulator nanostructures for near-infrared transparent flexible electrodes. Nature
Chemistry 2012, 4, 281-286.
(148) Kong, D.; Koski, K. J.; Cha, J. J.; Hong, S. S.; Cui, Y.: Ambipolar Field Effect in Sb-
Doped Bi2Se3 Nanoplates by Solvothermal Synthesis. Nano Letters 2013, 13, 632-636.
(149) Cha, J. J.; Claassen, M.; Kong, D.; Hong, S. S.; Koski, K. J.; Qi, X.-L.; Cui, Y.: Effects
of Magnetic Doping on Weak Antilocalization in Narrow Bi2Se3 Nanoribbons. Nano Letters
2012, 12, 4355-4359.
38
Chapter 2
Experimental Techniques
2.1 Introduction
This chapter introduces several techniques that are applied in the study of topological
insulators Bi2Te3 nanoplates. Scanning electron microscopy (SEM), transmission electron
microscopy (TEM) and atomic force microscopy (AFM) are utilized to characterize
topography information of different samples. X-ray diffraction (XRD) is used to investigate
crystal structure of samples. Electron energy-loss spectroscopy (EELS) and scanning near-
field optical microscopy (SNOM) are used to study the surface plasmons of Bi2Te3. Terahertz
time domain spectroscopy (THz-TDS) is useded to study the optical properties of Bi2Te3 in
the terahertz range. Techniques involved in device fabrication include electron beam
evaporation, thermal evaporation.
2.2 Microscopy
Nowadays, microscopy has become an indispensable technique for the studies of various
nanostructures because of its ability to visualize structures at dimensions as low as nanometer
scale. Usually, microscopy is used to study the morphology of the subjects and different
resolution can be achieved with different microscopy techniques. Basically, microscopy can
be classified into three categories according to the working mechanism: optical microscopy,
electron microscopy and probe microscopy. The three most widely used microscopy
39
techniques are scanning electron microscopy (SEM), transmission electron microscopy
(TEM), and atomic force microscopy (AFM). While SEM, TEM and AFM all represent far-
field microscopy, scanning near-field microscopy (SNOM) is based on near-field properties
of samples and can defeat the diffraction limit.
2.2.1 Scanning electron microscopy (SEM)
SEM acquires information of the sample’s surface topography and composition by
scanning the sample with a focused beam of electrons. Figure 2.1 shows the configuration of
a typical SEM. Electrons with energy of tens of KeV are first emitted from electron gun and
then focused by condenser lens to form an electron beam with a diameter of several
nanometer. Electron beam is subsequently deflected by the deflection coils so that it can
raster scan the specimen. When the incident electron beam interacts with specimen,
secondary electrons can be excited, the number of which is a function of specimen density,
composition, the angle between the surface and the beam, thus can be used to obtain
topography information by collecting secondary electrons.
SEM has large depth of field due to the narrow electron beam with high energy and
produce three-dimensional images. A resolution of 10 nm scale can be achieved with most
SEM. SEM can also be used to perform element analysis by collecting back-scattered
electrons, the intensity of which is strongly related to the atomic number included in sample.
Furthermore, the distribution of different elements can be studied by zone scan. To obtain
the SEM image, samples are required to be electrically conductive, which is usually achieved
by sputtering of gold on surface for insulators. SEM has been widely used thanks to the ease
of sample preparation and its wide applicability as long as the composition does not damage
the equipment.
40
Figure 2.1 Schematic representation of the instrumentation of SEM. Reprinted from
Reference [1].1
2.2.2 Transmission electron microscopy (TEM)
TEM is widely used in nanoscience and nanotechnology to study chemical identity,
crystal orientation, electronic structure and morphology of sample. Figure 2.2 shows
configuration of a typical TEM. Electron gun connected with high voltage is used to emit
electrons with energy as high as 100-200 KeV. Electrons beam are formed when electrons
pass through condenser lens, after which, the high-energy electron beam would interact with
the specimen. Due to the high energy of electron beam and low thickness of specimen, part of
41
electrons can transmit the specimen. Those transmitted electrons are collected and can be
used to form an image, which is then magnified and focused onto a phosphor screen.
Figure 2.2 Configuration of typical TEM.
Apart from acquiring topography information, TEM is also widely used to study
crystallographic properties of specimen in the form of selected area electron diffraction
(SAED). In this configuration, some electrons are diffracted at certain angles and forming
particular spots. Crystal structure of specimen is closely related to the diffraction pattern.
This SEAD technique can be used to study crystal structure of selected area down to
hundreds nanometers, instead of large area that is investigated with X-ray diffraction.
42
Scanning TEM (STEM) differentiate from conventional TEM by focusing electron beam
into narrow spot and scanning sample in a raster way.
TEM can achieve a much higher resolution (sub-nanometers) than optical microscopy
due to the much shorter de Broglie wavelength of electrons. In order to characterize with
TEM, sample must be thin enough to allow electrons to transmit, which means specimens are
required to be hundreds of nanometers thick at most. The thickness requirement makes
sample preparation very complex for TEM.
2.2.3 Atomic force microscopy (AFM)
AFM has been one of the most important tools for imaging, measuring and manipulating
matter at nanoscale with resolution as high as sub-nanometer. As a member of scanning
probe microscopy, AFM acquires information of surface by measuring the interaction force
between sample surface and tip while raster - scanning the tip across the surface. It is
noteworthy that interatomic repulsive force dominates the interaction between AFM tip and
surface when it comes to extremely short range and makes mapping of surface topography
down to atomic dimension possible.
A typical AFM is consisted of five components: (1) a sharp tip mounted on a soft
cantilever; (2) a detection system to monitor the deflection of the cantilever; (3) a
piezoelectric translator to move the sample relative to the tip; (4) a feedback system to keep
the deflection constant by keep adjusting height of the probe; (5) a imaging system to analyse
force-distance relationship and convert it into image of the surface. In a typical process, a
sample is placed on the piezoelectric translator and raster scanning relative to the tip. The tip-
surface interactive force is recorded for each scanning point and can be converted into
topography information of the surface. AFM can operate in two modes: constant force mode
43
and constant height mode. In constant force mode, the interactive force between the tip and
surface is kept constant during raster scanning by adjusting the height of sample. In constant
height mode, the vertical height of sample and tip is kept constant, and the cantilever
deflection varies at different scanning points and can be recorded and converted into
topography information.
Apart from resolution as high as sub-nanometer, working in ambient condition with little
requirement on sample represents another predominant advantage of AFM. In contrary to
SEM and TEM that work in high vacuum, AFM work well in ambient condition and can
measure almost any sample without modification that is necessary for SEM and TEM.
Figure 2.3 Schematic illustration of AFM.
44
2.2.4 Scanning near-field optical microscopy (SNOM)
Scanning near-field optical microscopy enables investigation of topography and optical
properties simultaneously at sub-wavelength scale. SNOM has arisen more and more
attention in imaging, sensing and modification because of its ability to achieve spatial
resolution beyond the classical diffraction limit.2 A probe with a sub-wavelength apex is
scanned over the sample while maintaining a probe-sample distance of a few nanometers and
obtains optical information of the sample.
In conventional microscopy, the maximal resolution is determined by diffraction limit,
which is given by equation
00.61( / nsin )d ------------ (1)
where 0 is the wavelength of incident light, n is the refractive index of the medium in which
the light travels, is the light convergence angle for the focusing element and nsin is also
known as the numerical aperture (NA) for the objective. Nowadays, high-quality objective
lenses can achieve NA of about 1.3, which means the theoretic maxima resolution is about
half of the incident light. The highest resolution available in conventional microscopy is
about 200-300 nm due to the existence of diffraction limit.3
Since early of the 20th
century,4 much endeavour has been devoted to realize resolution
beyond diffraction limit. It was not until 1980s that the initial ideal was realized in the
SNOM,5,
6 which can beat the diffraction limit by working in the optical near field zone
where the evanescent waves are predominant. Different optical wave’s characteristics are
associated with different zones, as shown in Figure 2.4. In the near filed zone, the feature of
evanescent field carries optical information of the sample and can be collected in several
ways. Access to the evanescent waves allows achieving high optical resolution.7
45
Figure 2.4 (a) Schematic illustration of Synge’s idea for achieving spatial resolution with
sub-diffraction limit, by illuminating the sample through a small hole with size of sub-
wavelength. (b) Schematically representation of the different zones together with the optical
wave’s characteristics. Reprinted from reference 7.
Most SNOM mainly includes three components, which are illumination unit,
collection and adjustment unit, and detection unit, as shown in Figure 2.5. Incident light
source, which usually is laser, is coupled into the near-field aperture. In order to achieve near-
field resolution, the distance between the aperture and sample must be precisely controlled
down to 10 nm, which is realised by utilizing the control and feedback system of AFM. The
46
detection unit composed of a CCD camera coupled with a spectrography can record the near-
field optical properties of samples.8
Figure 2.5 Basic SNOM arrangements including illumination unit (a), collection unit (b), and
detection unit (c).
SNOM works in the following way: laser light is first directed into an optical fiber. The
sample is scanned below the SNOM tip and fluorescence is collected from below with a high
numerical aperture objective. The fluorescence is filtered and detected with an avalanche
photodiode detector (APD). To maintain the tip–sample gap, a shear-force feedback
mechanism is implemented. In the shear-force feedback method, the NSOM probe is dithered
laterally with respect to the sample surface and the amplitude of the motion is detected by
shining a laser across the tip and onto a split photodiode. The dither amplitude is detected by
47
a lock-in amplifier, the output of which creates an error signal that is used to maintain a
constant gap between the tip and the sample.
Figure 2.6 Different configurations of SNOM. Reprinted from reference 8.
The aperture represents one of the most important components of SNOM, based on
which SNOM can be classified into aperture based and aperture-less configuration. Figure 2.6
(a)-(d) shows four configurations of aperture based SNOM. Figure 2.6(a) represents
transmission mode, in which incident laser illuminates through an aperture and the
transmitted signal is collected by external far-field optics. Figure 2.6(b) shows illumination
and collection mode, in which incident illumination and signal collection are conducted
through the same aperture. Figure 2.6(c) elaborates reflection mode, in which the aperture is
used for illumination and the reflected signal is collected by external far-field optics for
analyse. What shown in Figure 2.6 (d) is the photon scanning tunnelling mode, in which
evanescent field at the surface of the prism generated by total internal reflection is collected
through the aperture. Figure 2.6(e) and (f) shows the aperture-less SNOM, scattering
48
evanescent excitation mode and scattering-type SNOM, respectively. In both aperture-less
modes, the sharp tip induces significant field enhancement and is used as a sub-wavelength
light source.
With the power to obtain topographic and optical information simultaneously at ultra-
high resolution, SNOM has manifested itself as a powerful technique in the study of
plasmonic structures.9,
10,
11,
12
On one hand, surface plasmons can be readily excited by
SNOM tip due to the greatly enhanced in-plane wave vector, which otherwise is not enough
to excite surface plasmons. On the other hand, SNOM can provide real-space imaging of field
distribution, from which spatial distribution, energy confinement capability and propagation
length of surface plasmons can be derived. Single molecule detection by observing
fluorescence is another field that SNOM has made a great contribution. Combined with
SNOM, distribution of fluorescence label at single molecule level can be achieved.13, 14
2.3 Spectroscopy
Spectroscopy is a set of techniques that study the interaction between and
electromagnetic waves and matters. Composition, crystal structure, electronic structure et al
can be obtained by comprehensive analysis of spectroscopy data.
2.3.1 X-ray diffraction (XRD)
X-ray diffraction is a well-established technique to study structure and composition of
various crystals. Incident X-ray would diffract into a region with specific angles and
intensities after interact with atoms on the surface of crystals, as shown in Figure 2.7. By
49
collecting diffracted X-ray and measuring their density at different angles, the
crystallographic and compositional information of the sample can be obtained.
Figure 2.7 Schematic illustration of the diffraction pattern of X-ray on the surface of crystal
structure.
The diffraction behaviour of X-ray can be described with Bragg’s law:
2 sinn d ------------ (2)
where is the wavelength of incident X-ray, d is the distance of two lattice planes, is the
angle between incident X-ray beam and lattice plane. n is any integer.
In common case, powder or film sample is mounted on a goniometer, which can rotate
relative to the incident X-ray beam. As the sample is bombarded with X-ray, the goniometer
gradually rotates and measures intensity of diffracted X-ray at different angles. Peak intensity
would be measured when the angle is consistent with the spacing between two neighbouring
lattice planes, i.e. the value of d when n is unit.
50
2.3.2 Electron energy-loss spectroscopy
Electron energy-loss spectroscopy (EELS) is a technique that measures the energy loss
of electrons scattered into a specific angular range after interacting with the target specimen,
and it is usually equipped in TEM. The electronic structure (i.e. bonding), chemical
composition and circumstance of the specimen can be acquired through comprehensive
analysis of the EELS spectra. EELS is widely used in surface science because of its high
spatial (<1 nm) and energy (<0.2 eV) resolution.15
Figure 2.8 Schematic of the transition processes in some of the spectroscopic techniques that
can probe electronic structure.
51
The interaction of electrons and specimen in TEM can be classified into transmission,
reflection, scattering, and excitation, as shown in Figure 2.8. The variety of process during
electron-specimen interaction makes it an excellent platform to investigate the sample, as
different information about the sample can be acquired based on different interaction
process.16
For the EELS technique described here, transmitted electrons are collected and
duly characterized. Those electrons can transmit the specimen by elastic scattering as well as
inelastic scattering. During elastic scattering, the kinetic energy of electrons keeps unchanged
after interacting with specimen. Elastic scattering usually involves the interaction of incident
electrons and atomic nucleus, where the energy exchange is usually too small to measure
because nucleus mass greatly exceeds mass of electron. During inelastic scattering, electrons
loss part of its kinetic energy after interacting with specimen. Coulomb interaction, plasmons
excitation and single particle excitation would give rise to inelastic scattering.
EELS is mainly used to study surface plasmons of the sample in our study. Excitation of
surface plasmons arises from collective oscillation of free electrons. Surface plasmons can be
generated in EELS in the following fashion: when high energy electrons pass through a thin
specimen, Coulomb repulsion would displace the electrons and form correlated holes that
trails behind electrons. As a result, space containing positive and negative charge would
appear alternatively along the electron trajectory. Pseudoparticles known as plasmons are
generated in the process. The backward attractive force between the positive correlated holes
and electrons would cause energy loss as the electrons move through specimen. By
measuring the energy loss, the frequency of plasmons can be derived.
Three types of TEM based EELS are widely used and shown in Figure 2.9. EELS in
conventional TEM is the simplest form and require little modification to TEM. By tilting the
TEM screen to a vertical position, electrons would enter into the spectrometer and they are
dispersed according to their kinetic energy. A spectrometer entrance aperture with a diameter
52
varying from 1 to 5mm would limit the range of entrance angles and ensure adequate energy
resolution. The second type is EFTEM based EELS, in which a spectrometer is incorporated
into the TEM imaging column. In this configuration, only electrons within a small energy
window are allowed to pass a narrow slit, producing an energy-filtered (EFTEM) image on
the TEM screen or the CCD camera. The third type EELS is the one that we used for our
study, which is based on scanning-TEM (STEM). Electrons emitted from the field-emission
source would form a small probe that can be raster-scanned across the specimen after passing
Figure 2.9 Three procedures for TEM-based energy-loss spectroscopy: (a) Conventional
TEM with a magnetic-prism spectrometer below the viewing screen, (b) TEM incorporating
an in-column imaging filter and (c) Scanning-transmission (STEM) system. Reprinted from
reference 17.17
53
strong electromagnetic lenses. Electrons scattered through smaller angles enter a single prism
spectrometer, which produces an energy-loss spectrum for a given position of the probe on
the specimen. The whole spectrum is read out at each probe position, resulting in a large
spectrum-image data set that can be processed and analysed carefully.
2.3.3 Terahertz time domain spectroscopy (THz-TDS)
THz-TDS is a non-contact technique that measure transmitted or reflected time-domain
wave, which can be further used to study dielectric response, complex conductivity,
refractive index of solid materials in the far-infrared range without resort to Karmers-Kronig
analysis.18
Thanks to the development of methods to generate THz radiation, THz-TDS has
been widely used to study various materials, including semiconductor19
, ferroelectrics20
,
superconductors, photonic crystals. The popularity of this technique is based on two facts.
First, both amplitude and phase of single-cycle oscillation of the THz electric field can be
measured. Second, the same order of range of THz frequency and carrier scattering time
makes THz spectroscopy a suitable technique which provides more accurate modelling of
data.
THz-TDS works in the following fashion: optical pulse that emitted from the Ti :
sapphire laser can excite terahertz radiation from the photoconductive antenna. After passing
through the off-axis paraboloidal reflector, the THz radiation is collimated and passes
through sample or reference placed in the optical circuit. The transmitted radiation is then
focused onto another photoconductive antenna by another paraboloidal reflector. The current
generated in the photoconductive antenna detector is triggered by gate pulses after passing
through a time-delay circuit. The electric field of coherent THz pulses transmitted through the
sample and reference is measured by scanning the optical delay stage and recorded as (t)sE
54
Figure 2.10 Schematic diagram of a standard THz-TDS spectrometer. Adopted from
reference 21.
and (t)rE , respectively.18
By taking Fourier transform, (t)sE and (t)rE can be converted to
frequency dependent ( )sE and ( )rE , respectively, the ratio of which can be used to
characterize the complex transmission coefficient through the formula: ( ) ( ) / ( )s rT E E .
Starting from the complex transmission coefficient, dielectric function, complex conductivity
and refractive index can be derived.
2.5 References
(1) Scanning electron microscope. [Art]. In Encyclopædia Britannica; Retrieved from
http://www.britannica.com/EBchecked/media/110970/Scanning-electron-microscope, 2013.
55
(2) Stockman, M. I.: Nanoplasmonics: The physics behind the applications. Physics Today
2011, 64, 39-44.
(3) Dunn, R. C.: Near-Field Scanning Optical Microscopy. Chemical Reviews 1999, 99,
2891-2928.
(4) Synge, E. H.: A suggested method for extending microscopic resolution into the ultra-
microscopic region. Philosophical Magazine 1928, 6, 356-362.
(5) Pohl, D. W.; Denk, W.; Lanz, M.: Optical stethoscopy: Image recording with resolution
λ/20. Applied physics letters 1984, 44, 651.
(6) Lewis, A.; Isaacson, M.; Harootunian, A.; Muray, A.: Development of a 500 Å spatial
resolution light microscope: I. light is efficiently transmitted through λ/16 diameter apertures.
Ultramicroscopy 1984, 13, 227-231.
(7) Lereu, A. L.; Passian, A.; Dumas, P.: Near field optical microscopy: a brief review.
International Journal of Nanotechnology 2012, 9, 488-501.
(8) Richter, M.; Deckert, V.: Scanning Near-Field Optical Microscopy (SNOM). In Surface
and Thin Film Analysis; Wiley-VCH Verlag GmbH & Co. KGaA, 2011; pp 481-497.
(9) Marti, O.; Bielefeldt, H.; Hecht, B.; Herminghaus, S.; Leiderer, P.; Mlynek, J.: Near-field
optical measurement of the surface-plasmon field. Optics Communications 1993, 96, 225-228.
(10) Adams, P. M.; Salomon, L.; Defornel, F.; Goudonnet, J. P.: Determination of the spatial
extension of the surface-plasmon evanescent field of a silver film with a photon scanning
tunneling microscope. Physical Review B 1993, 48, 2680-2683.
(11) Passian, A.; Lereu, A. L.; Wig, A.; Meriaudeau, F.; Thundat, T.; Ferrell, T. L.: Imaging
standing surface plasmons by photon tunneling. Physical Review B 2005, 71.
(12) Passian, A.; Wig, A.; Lereu, A. L.; Meriaudeau, F.; Thundat, T.; Ferrell, T. L.: Photon
tunneling via surface plasmon coupling. Applied Physics Letters 2004, 85, 3420-3422.
56
(13) Betzig, E.; Chichester, R. J.: Single molecules observed by near-field scanning optical
microscopy. Science 1993, 262, 1422-1425.
(14) de Lange, F.; Cambi, A.; Huijbens, R.; de Bakker, B.; Rensen, W.; Garcia-Parajo, M.;
van Hulst, N.; Figdor, C. G.: Cell biology beyond the diffraction limit: near-field scanning
optical microscopy. Journal of cell science 2001, 114, 4153-4160.
(15) Ibach, H.: Eelectron-energy-loss spectroscopy : the vibration spectroscopy of surfaces.
Surface Science 1994, 299, 116-128.
(16) Schneider, R.: Electron Energy-Loss Spectroscopy (EELS) and Energy-Filtering
Transmission Electron Microscopy (EFTEM). In Surface and Thin Film Analysis; Wiley-
VCH Verlag GmbH & Co. KGaA, 2011; pp 67-91.
(17) Egerton, R. F.: Electron energy-loss spectroscopy in the TEM. Reports on Progress in
Physics 2009, 72.
(18) Hangyo, M.; Tani, M.; Nagashima, T.: Terahertz time-domain spectroscopy of solids: A
review. International Journal of Infrared and Millimeter Waves 2005, 26, 1661-1690.
(19) Zou, X.; Luo, J.; Lee, D.; Cheng, C.; Springer, D.; Nair, S. K.; Cheong, S. A.; Fan, H. J.;
Chia, E. E. M.: Temperature-dependent terahertz conductivity of tin oxide nanowire films.
Journal of Physics D-Applied Physics 2012, 45.
(20) Silwal, P.; La-o-Vorakiat, C.; Chia, E. E. M.; Kim, D. H.; Talbayev, D.: Effect of growth
temperature on the terahertz-frequency conductivity of the epitaxial transparent conducting
spinel NiCo2O4 films. Aip Advances 2013, 3.
(21) THz-TDS spectrometer: http://www.riken.jp/lab-www/THz-img/English/annual_gas.htm.
57
Chapter 3
Controlled synthesis of single crystalline Bi2Te3 hexagonal nanoplates
via solvothermal method
Abstract
Bi2Te3 hexagonal nanoplates with high yield were synthesized via solvothermal method. The
role of reaction parameters such as reaction time, temperature, concentration of precursors
and NaOH, surfactant and solvent have been systematically studied in order to derive insights
into the growth mechanism. Two important processes, homogeneous nucleation of Bi2Te3 and
heterogeneous nucleation of Bi2Te3 on the tips of Te nanorods subsequently followed by
direct reaction of Bi and Te nanorods via Kirkendall effect, were demonstrated to be the main
growth mechanism. By fine-tuning the reaction conditions, uniformly-shaped hexagonal,
single-crystalline nanoplates can be obtained, the size and shape of these nanoplates can also
be controlled.
3.1 Introduction
Bi2Te3 and its alloys are well-known as the best thermoelectric materials at room
temperature due to its very high figure of merit for several decades.1-3
Recently, this
traditional semiconductor material has generated intense interests since it was discovered to
58
be a three dimensional (3D) topological insulator, which is insulating in the bulk but
conductive on the surface with a single Dirac cone.4-6
There is a need to develop synthesis
strategies for Bi2Te3 nanostructures with various dimensional control since its enhanced
surface effect and unique morphology can facilitate both fundamental research and
application studies.7
Conventional synthesis methods for Bi2Te3nanomaterials are broadly categorized as dry,
physical synthesis or wet, chemical synthesis. Compared to physical synthesis such as
molecular beam epitaxy (MBE),8 metal-organic chemical vapor deposition (MOCVD),
9
vapor–solid (VS),10
and vapor–liquid–solid (VLS)11
methods, chemical synthesis requires
only mild conditions and is lower cost. In addition, control of morphology and chemical
modification on the surface can be achieved. Bi2Te3 nanostructures of different morphologies
have been successfully prepared by chemical method, such as nanotubes,12
nanowires,13,14
nanorods,15
and nanoplates.16-18
However, in most of the reported routes, toxic and dangerous chemicals were involved,
such as reducing agent hydrazine and sodium borohydride. Although a new route with the
environment-friendly ethylene glycol (EG) acting as both reducing agent and solvent was
developed to synthesize Bi2Te3nanoplates recently, the reaction time was too long18
and the
detailed growth mechanism was not fully investigated.19
Besides, the controlled synthesis of
Bi2Te3 nanoplates such as size and shape control was not reported. It is valuable to develop a
simpler and more effective route and to carry out systematic study of the growth progress in
order to control the shape and size of the products.
59
In this chapter, we present a facile route to synthesize hexagonal Bi2Te3 nanoplates with
high yield and uniform morphology using solvothermal method. The growth progress was
carefully investigated as a function of reaction time, temperature, concentration, surfactant
and so on. In addition to the homogenous nucleation, the heterogeneous nucleation of Bi2Te3
on the tips of Te nanorods followed by direct reaction of Bi and Te nanorods via Kirkendall
effect was demonstrated to be another growth mechanism. Different sizes and shapes of
Bi2Te3 nanoplates can be obtained, which is very useful for the fundamental study of its
properties such as plasmonic effect. The systematic work described here lays solid foundation
for the design of solution synthesis of A2VB3
VI-type nanostructures such as Bi2Se3, Sb2Te3 and
their alloys.
3.2 Materials and methods
3.2.1 Chemicals
Bismuth oxide (Bi2O3, 99.999%) and tellurium dioxide (TeO2, 99.995%) were purchased
from Alfa Aesar. Ethylene glycol (EG), isopropanol (IPA), glycerol, sodium hydroxide
(NaOH), and polyvinylpyrrolidone (PVP, molecular weight 55 000), were purchased from
Sigma-Aldrich. All the chemicals were of analytical grade and directly used as received
without further purification.
60
3.2.2 Synthesis
Bi2Te3 nanoplates were synthesized by solvothermal method. In a typical process, 0.3 g
PVP was dissolved in 18 mL of EG, and stirred vigorously to form a clear solution. Bi2O3
(0.2298 g, 0.5 mmol), TeO2 (0.2394 g, 1.5 mmol) and 2 mL of NaOH solution (5 mol/L) were
then added into the solution and kept stirring for 30 minutes. The resulting suspension was
transferred into a stainless steel autoclave with inner lining and heated to 210 ℃ in an oven
for 4 h. After cooling to room temperature, the synthesized products were collected by
centrifugation and washed several times with distilled water, absolute ethanol and IPA.
To study the growth mechanism and control the synthesis, almost all the reaction
parameters such as the reaction time and temperature, the concentration of precursors, NaOH
and PVP, and the kind of solvents were systematically varied.
3.2.3 Characterizations
The morphology of the products was investigated using a JEOL 6701 field emission
scanning electron microscopy (FESEM). A Bruker Fast Scan atomic force microscopy (AFM)
was used to measure the thickness. For SEM and AFM measurements, the samples were
prepared by drop-casting Bi2Te3 suspension on SiO2/Si substrates.
The phase structure of the products was checked by X-ray diffraction (XRD) using a
Siemens D5005 X-ray diffractometer with Cu Kα (λ = 1.5406 Å) as the incident beam.
Vacuum-dried products were prepared for XRD.
61
Transmission electron microscopy (TEM) analysis including high-resolution TEM
(HRTEM), and selected area electron diffraction (SAED) were performed with JEOL JEM
3010 microscope at an acceleration voltage of 300 kV. Gatan Digital-Micrograph (software)
was used to analyze TEM results. The sample was prepared by drop-casting dilute Bi2Te3
suspension on to TEM copper grid with lacey carbon film.
3. 3 Results and discussion
3.3.1 Characterizations of Bi2Te3 hexagonal nanoplates
The morphology of the as-synthesized products was studied by SEM and AFM. As
shown in SEM images (Figure 3.1a), uniform Bi2Te3 nanoplates with regular hexagonal
morphology and high yield are clearly identified. In general, the nanoplates possess edge to
edge length of 550-800 nm with flat surfaces and sharp edges, which is clearly shown in the
high-magnification image in Figure 3.1b. The thickness was accurately measured by AFM, as
shown in Figure 3.1c. The AFM topography image of Bi2Te3 nanoplates shows the
characteristic hexagonal shape, a typical nanoplate with thickness of 16.85 nm is shown in
Figure 3.1d. It is noteworthy that the nanoplates have very flat surface indicated by the
uniform height in the AFM images and height profile. The thickness distribution was
determinated by randomly selected 70 nanoplates and shown in Figure 3.1e, which indicates
that nanoplates with thickness between 10 and 20 nm are major products. We also observed
62
some ultra-thin nanoplates whose thickness was within 5 quintuple layers (QLs synchronous),
considering that the thickness of each QL of Bi2Te3 is about 1 nm.20
Figure 3.1 Morphology of the as-synthesized Bi2Te3 deposited on SiO2/Si substrate: SEM
images of nanoplates with low (a) and high (b) magnification. AFM images with overview of
three uniform nanoplates (c) and a single flake (d). The height profile corresponds to the
dashed line in the topography image of the single Bi2Te3 nanoplate. (e) Thickness distribution
of the Bi2Te3 nanoplates.
63
To shed light on the crystal structure of the Bi2Te3 nanoplates, XRD and TEM studies
were performed. All diffraction peaks in the Bi2Te3 XRD pattern can be indexed to a single
rhombohedral lattice phase with the lattice constants a = 4.395 Å and c = 30.44 Å (JCPDS
Card Number 82-0358), no remarkable diffractions of other phases can be found, which
indicates that pure Bi2Te3 has been obtained. Six main peaks can be easily indexed to (015),
(1010), (110), (205), (0210) and (1115) planes, which is shown in Figure 3.2a.
Figure 3.2 (a) Powder XRD spectrum of as-synthesized Bi2Te3 with indexed peaks. (b)
Low-magnification TEM image of several Bi2Te3 nanoplates on copper grid with lacey
carbon support film. (c) TEM image of a single Bi2Te3 nanoplate with its corresponding
SAED pattern (d) and HRTEM image (e).
64
The TEM image in Figure 3.2b shows the morphology of several hexagonal Bi2Te3
nanoplates while Figure 3.2c shows a typical nanoplate. The ripple-like patterns are due to
the strain resulting from the bending of ultra-thin nanoplates. The sharp and well-organized
diffraction spots in the corresponding SAED pattern (Figure 3.2d) indicates the high quality
of the single-crystallinity, and the pattern can be identified as the [0001] zone axis projection
of the hexagonal Bi2Te3 reciprocal lattice. The top and bottom surfaces and the six side
surfaces are (0001) facets, (1120) facets, respectively. It can be concluded that as-prepared
Bi2Te3 nanoplates are single crystal dominated by (0001) facets and (1120) is the growth
direction. The defect-free single crystal nature of the as-synthesized Bi2Te3 nanoplates was
furthered confirmed by the corresponding HRTEM image (Figure 3.2e). The measured
spacing between the lattice fringes in the HRTEM images is 0.22 nm, which is consistent
with the lattice spacing of (1120) planes.
3.3.2 Influences of reaction parameters
Common to chemical synthesis, the structure and morphology of the product are
strongly affected by many reaction parameters, such as reaction temperature and time,
concentration of precursors, as well as the solvent and surfactant, etc. In this part, we studied
the effects of all the major factors of the reaction systematically, which would be helpful to
understand the growth process and synthesize target products on demand.
65
3.3.2.1 Influence of reaction temperature
Products synthesized at a series of temperatures (120, 150, 180, 195 and 210 ℃) are
compared, with all the other reaction parameters kept constant. There is no precipitate and
only orange solution was obtained at low temperature (120 ℃). When the temperature is
increased to 150 ℃, precipitate is acquired and the major products are small Te nanorods with
only a little amount of Bi2Te3 nanoplates, as shown in Figure 3.3a. Bi2Te3 nanoplates are the
major product at higher temperature (180 ℃) and become the only product when temperature
reaches 195 ℃, which are clearly shown in Figure 3.3b and 3.3c, respectively. However, the
morphology of the products is not uniform and many irregular and incomplete flakes are
observed. Increasing temperature to 210 ℃ improves the quality of the nanoplates a lot and
Figure 3.3d clearly indicates that regular hexagonal nanoplates with uniform morphology and
high yield could be achieved. This study demonstrates that an appropriate temperature of 210 ℃
is most ideal to obtain uniform Bi2Te3nanoplates in 4 hours.
The temperature-dependent morphology is mainly attributed to the thermal reduction
ability of EG. Its reduction ability is very weak at low temperature and is enhanced at higher
temperature. On the other hand, the solubility of the reactants in EG is not good at low
temperature, and there is insufficient energy to overcome the activation barrier for crystal
growth. Although the reduction ability of EG is weak at low temperature, the reaction time
can be extended to get Bi2Te3 nanoplates as the dominant products, which is shown in the
discussion part of reaction time.
66
Figure 3.3 SEM images of products synthesized at (a) 150℃, (b) 180℃, (c) 195℃, (d) 210℃
for 4 h with 0.3g PVP in 0.5M NaOH.
3.3.2.2 Influence of NaOH
Products synthesized with a series of NaOH concentrations (0, 0.05, 0.1, 0.2, 0.5 and 1M)
are compared, while keeping all the other reaction parameters constant. As indicated in
Figure 3.4a, no Bi2Te3 plates are obtained when there is no NaOH in the reaction. Bi2Te3
plates emerge when the concentration of NaOH is as low as 0.05 M, together with some Te
nanorods and irregular and random Bi-Te alloys (Figure 3.4b). The percentage of Bi2Te3
67
Figure 3.4 SEM images of products synthesized at 210℃ for 4 h with 0.3g PVP in (a) 0M, (b)
0.05M, (c) 0.1M, (d) 0.2M, (e) 0.5M, (f) 1M NaOH.
nanoplates increases with the increase of concentration of NaOH and become dominant
products with regular shape when the concentration of NaOH reaches 0.2 M, as shown in
Figure 3.4c-d. When the concentration of NaOH is raised to 0.5 M, only uniform hexagonal
68
nanoplates could be found (Figure 3.4e). It is noteworthy that the nanoplates become much
thinner so that they are prone to deformation and tend to aggregate when the concentration of
NaOH is as high as 1M, which is shown in Figure 3.4f.
The importance of NaOH in the solvothermal synthesis of Bi2Te3 has been mentioned in
previous reports. Y. Zhang argued that the alkaline environment was needed for suppressing
the hydrolysis of Bi3+
.18
However, the product of the hydrolysis of Bi3+
in their experiment
was BiOCl which can be further reduced to Bi, and they can react with Te to form Bi2Te3.21
So the hydrolysis of Bi3+
is favorable and adding NaOH should favor its synthesis instead of
suppressing it. Zhang et al reported that only Te nanowires were obtained without NaOH and
the role of NaOH was to influence the dissolution rate of Te nanowire and its recrystallization
into Bi-Te alloys.19
In our experiments, only sparse quantities of Te nanorods/nanowires are
obtained in 4 hours without NaOH and Bi2Te3 could be obtained even without adding NaOH
if the reaction time is long enough. Figure 3.5a and 3.5b show the close-up SEM image of the
product synthesized at 210 ℃ for 4 hours and 24 hours without NaOH, respectively.
Figure 3.5 SEM image of the product synthesized at 210℃ without NaOH (a) for 4 h, (b) for
24 h.
69
Accordingly, we propose the reactions as followings:
(i) Reduction of Bi2O3 and TeO2:
4Bi2O3 + 3HOCH2CH2OH <=> 8Bi + 3HOOCCOOH + 6H2O ----- (1)
2TeO2 + HOCH2CH2OH <=> 2Te + HOOCCOOH + 2H2O ----- (2)
(ii) Formation of Bi2Te3:
2Bi + 3Te = Bi2Te3 ----- (3)
EG is oxidized to oxalic acid during the reduction of Bi and Te precursors. By adding
NaOH, the reduction is significantly enhanced as oxalic acid would react with NaOH. Thus
more Bi and Te nanocrystals can be generated, which leads to more nucleation of Bi2Te3.
Therefore, the main role of NaOH is to drive the reduction of the precursor to provide
elemental precursors for the formation of Bi2Te3. Without NaOH, Bi2Te3 still can be
generated but the reaction rate is very slow. Increasing the pH drives the reaction forward
according to Le Chatelier’s principle and when the concentration of NaOH is very high, i.e. 1
M in our experiment, the generated Bi2Te3 monomer concentration is so high that the
anisotropic growth is facilitated thus the products become thinner.
3.3.2.3 Influence of PVP
PVP is widely used as the surfactant to obtain nanostructures with regular shape. To
study the effect of PVP on the morphology of Bi2Te3, products synthesized with different
amount of PVP (0, 0.05, 0.1, 0.2, 0.3, and 0.8 g) are compared, while all the other reaction
parameters remain the same. It is clearly shown in Figure 3.6a that without PVP, the shape of
70
the products is not regular and the size distribution is very broad. More uniform hexagonal
Bi2Te3 nanoplates could be generated with increasing amounts of PVP added, as shown in
Figure 3.6b-e. These results indicate that PVP play a critical role in determining the
hexagonal morphology of Bi2Te3 nanoplates.
Figure 3.6 SEM images of products synthesized at 210 ℃ for 4 h in 0.5 M NaOH with (a) 0 g,
(b) 0.05 g, (c) 0.1 g, (d) 0.2 g, (e) 0.3 g, (f) 0.8 g PVP.
71
PVP molecules can adsorb on the hexagonal Bi2Te3 nuclei and control the growth rate of
different faces kinetically to result in the formation of 2-D hexagonal nanoplates.22
Further
increasing the amount of PVP to 0.8g would not change the shape of the nanoplates but the
size of the crystals become relatively smaller, due to the passivation of growth front by the
PVP molecules adsorbed.
Other conventional surfactants such as cetyltrimethylammonium bromide (CTAB) and
sodium dodecyl benzene sulphonate (SDBS) do not work as well in producing uniformly
shaped or sized flakes, as shown in Figure 3.7.
Figure 3.7 SEM images of products synthesized at 210℃ for 4 h in 0.5 M NaOH with 0.3 g
(a) CTAB and (b) SDBS.
3.3.2.4 Influence of concentration of precursors
Setting the initial concentration of precursors (0.5 mmol Bi2O3 and 1.5 mmol TeO2) as
the standard concentration (SC), a series of products were synthesized with different
concentrations, as shown in Figure 3.8. Note that the reaction time were all extended to 12
72
hours since it require longer time for the crystal growth with lower concentration, and the
time effect will be discussed in the part of formation mechanism. The size of the Bi2Te3
nanoplates synthesized with 1 SC is in the range of 550-800 nm.
Figure 3.8 SEM images of products synthesized at 210 ℃ for 12 h in 0.5 M NaOH with 0.3 g
PVP and different concentration of precursor of (a) 1 SC, (b) 1/2 SC, (c) 1/3 SC, (d) 1/5 SC,
(e) 1/10 SC, (f) 1/20 SC.
73
When the concentration of precursors decrease to 1/2 and 1/3 SC, there is not much
change in the morphology, except that the size of the nanoplates slightly decreases to 500-750
nm and 400-600 nm, respectively, as shown in Figure 3.8b and 3.8c. Much smaller crystals
with size in the range of 200-300 nm can be obtained with decreased concentration of 1/5 SC
(Figure 3.8d). Although the size of the nanoplates can be further decreased, the morphology
is not as uniform as that of product synthesized with higher concentrations. Figure 3.8e and
3.8f show the products synthesized at 1/10 and 1/20 SC, which implies that there are few
larger crystals within the major products with size of 100-250 nm. The size of the products
synthesized with different concentrations of precursors is summarized in Table 1. It
demonstrates a mass-limiting effect where smaller crystals were obtained with lower
concentration of precursors.
Table 3.1 Size distribution of Bi2Te3 nanoplates as a function of concentration of precursors.
74
3.3.2.5 Influence of solvent
In most reported synthesis of Bi2Te3 nanomaterials, hydrazine and sodium
borohydride,12,23
which are toxic and dangerous chemicals, were used as the reducing agents.
In our experiments, EG was used as both solvent and reducing agent. EG was chosen because
it is much less toxic and environmental friendly. To study the influence of solvent on the
morphology and size of the products, IPA and glycerol were chosen to mix with EG at
different percentage (15%, 30%, 50%, and 100%). For example, normally 18 ml of EG was
used to synthesize Bi2Te3nanoplates. When we studied the effect of IPA, 9 ml IPA was mixed
with 9 ml EG to achieve a 50% IPA replaced EG solution. The reason why IPA and glycerol
were selected is that they are both alcohols with different reducing ability, and are miscible
with each other.
Figure 3.9 shows the solvent effect of IPA. If 15% EG is replaced by IPA, the
morphology of the products remain the same but the size become larger, compared to the
products with size around 550-800 nm synthesized in pure EG under the same conditions. As
shown in Figure 3.9a, the percentage of the nanoplates with size around 1 μm is around 50%,
and some ultra-large nanoplates can be obtained as well. The inserted image in Figure 3.9a
shows two big nanoplates with size over 3.5 μm. Further increasing the IPA percentage to
30%, larger nanoplates with size over 1 μm become the major product, as shown in Figure
3.9b. However, when the percentage of IPA reaches 50%, the products are not uniform and
the shape is not regular. If EG is replace by IPA completely, no more nanoplates can be
generated and the products become big clusters.
75
Figure 3.9 Solvent effect of IPA. SEM images of products synthesized at 210 ℃ for 4 h in
0.5 M NaOH with 0.3 g PVP and (a) 15%, (b) 30%, (c) 50%, (d) 100% IPA replaced EG.
Inserted in (a) is the high-magnification image shows ultra-large plates.
In contrast, when EG is replaced with glycerol, the nanoplates become smaller, as shown
in Figure 3.10. The size of the nanoplates is around 450-700 nm when 15% EG is replaced by
glycerol and it is getting smaller and smaller with increasing percentage of glycerol, finally
reaching the smallest size that is only 50-100 nm when the products are synthesized with pure
glycerol. It is worth noticing that unlike IPA, adding glycerol would not change the
morphology of the products but only size.
76
Figure 3.10 Solvent effect of glycerol. SEM images of products synthesized at 210 ℃ for 4 h
in 0.5 M NaOH with 0.3 g PVP and (a) 15%, (b) 30%, (c) 50%, (d) 100% glycerol replaced
EG. Inserted in (d) is the SEM image with high-magnification.
The difference can be explained by the different reducing ability of the solvents. A
stronger reducing agent promotes a faster reaction rate, gives rise to a higher density of nuclei
and produces smaller crystals.24
The reducing ability of the three solvent can be described as
IPA < EG < glycerol, which is determined by the number of the hydroxyl groups. When EG is
replaced by glycerol, the reducing ability of the solvents is strengthened and results in a very
high super-saturation. Hence, a larger number of nuclei is generated in pure EG solution,
which would lead to a smaller size of the final nanoplates. On the contrary, IPA can weaken
77
the reducing ability of the mixed solution and reduce the reaction rate, thus relatively larger
nanoplates can be generated. At the same time, a slow reaction rate can also lead to wider size
distribution, contributing to the continuous formation of the new nuclei or secondary nuclei.
Therefore, some ultra-large and relatively small nanoplates are shown in Figure 9. When IPA
fully substituted for EG, no nanoplates can be generated since the reducing ability of IPA is
too weak.
3.3.3 Evolution of morphology and growth mechanisms
The possible mechanism for the formation of nanoplates with hexagonal morphology
can be described as followings: First, in the nucleation process, the shape of the Bi2Te3
crystal seeds is determined by its intrinsic crystal property.25,26
Bi2Te3 crystals possess a
rhombohedral phase with hexagonal unit cells.27
Thus hexagonal or triangular shaped crystal
seeds are favorable. Second, the unit cell contains 3 formula units that consist of 15 layers
stacked together along c-axis. As shown in Scheme 3.1a, each QL is built up of 5 atomic
layers stacked in the sequence of Te (1) – Bi – Te (2) – Bi – Te (1). The bonding within single
QL is covalent bonding while it is weaker van der Waals interaction between each QL. To
minimize the surface energy when growing, the crystal facets tend to develop on the
low-index planes thus the growth rate along the a-b crystal planes should be larger than that
along the perpendicular c-axis, due to the anisotropic crystal characteristics.26
Third, PVP
molecules could adsorb on the surface of the crystal seeds which would further lead to larger
energetic difference between the top/bottom facets and the side facets. The steric effect of
78
PVP is weak since there is no large functional groups.28
Therefore, the growth along a-b
planes is further larger than that of c-axis and the products end with morphology of
hexagonal nanoplates.
Scheme 3.1 (a) Crystal structure of Bi2Te3 with each QL formed by five atomic layers
stacked in the sequence of Te-Bi-Te-Bi-Te.(b) Schematic illustration of the growth
mechanism which includes both homogeneous and heterogeneous nucleation process.
To further investigate the growth mechanism, a series of time-dependent experiments
were carried. It is interesting to find that two growth paths exist, one involves the
homogeneous nucleation of Bi2Te3 and the other involves the heterogeneous nucleation of
Bi2Te3 on the tips of Te nanorods followed by direct reaction of Bi and Te nanorods via
Kirkendall effect. Figure 3.11 demonstrates the evolution of the obtained nanostructures. At 1
79
h, the products exhibit disordered structure with irregular shapes. After 2 h, the products
consist of hexagonal nanoplates, nanorods as well as some heterostructures. The inserted
high-magnification image of the heterostructure clearly shows that a single nanoplate
attached onto the tips of a nanorod. Hexagonal nanoplates with regular shape and sharp edges
can be obtained at 4 h and longer reaction time (up to 20 hours) does not change the
morphology significantly.
Figure 3.11 SEM images of products synthesized at 210 ℃ in 0.5 M NaOH with 0.3 g PVP
for (a) 1 h, (b) 2 h, (c) 4 h and (d) 20 h. Inserted in Figure 11b is the high-magnification
image of a Bi2Te3-Te heterostructure.
80
To shed light on the composition of the heterostructure, EDS (X-ray Energy Dispersive
Spectroscopy) analysis was carried out, and the results are shown in Figure 3.12. Strong
peaks from both Bi and Te with atomic ratio of 39.62 : 60.38 are detected in the plate area as
clearly shown in Figure 3.12b, suggesting the composition of Bi2Te3. While there are only Te
peaks in the EDS spectrum taken from rod area (Figure 3.12c), indicating the pure
composition of Te. These EDS results strongly support the conclusion that the heterostructure
is comprised of Te nanorod and Bi2Te3 nanoplates.
Figure 3.12 (a) TEM image of one Te-Bi2Te3 heterostructure. Blue and red spots show the
beam position where the EDS spectra were taken. (b) The corresponding EDS spectrum of
blue spot. (c) The corresponding EDS spectrum of red spot. The Cu peaks in both spectra
originate from the copper TEM grid.
81
To confirm the observation, the time-dependent experiments were carried out at lower
temperatures as well. Figure 3.13 is the SEM images of the products synthesized for different
time (2, 4, and 8 h) at 195 ℃ and 180 ℃. Note that the reaction time was extended to
complete the reaction since the reaction rate is slower at lower temperature. The results
confirm that there are multi-morphologies at the initial stage of the reaction, which consist of
the Bi2Te3 nanoplates, Te nanorods and Bi2Te3/Te heterostructures. The lower the temperature,
the higher the concentration of Te nanorods and heterostructures, which disappeared with
longer time. The final products after completing reaction are uniform Bi2Te3 hexagonal
nanoplates.
Figure 3.13 SEM images of products synthesized at 195 ℃ for (a) 2 h, (b) 4 h, (c) 8 h and at
180 ℃ for (d) 2 h, (e) 4 h, (f) 8 h in 0.5 M NaOH with 0.3 g PVP.
82
Scheme 3.1b presents the growth process of the Bi2Te3 nanoplates. The standard
reduction potential of TeO2 is E0
TeO2/Te = 0.593 V while that of Bi2O3 is E0Bi2O3/Bi = -0.46 V,
29
indicating that the reduction of TeO2 is spontaneous but extra energy is needed to promote the
reduction of Bi2O3. Thus the reduction rate of TeO2 to Te is faster than that of Bi2O3.
Therefore, the concentration of Te is higher than that of Bi in the initial stage of the reaction.
When super-saturation is reached, nucleation of Te crystals as well as homogeneous
nucleation of Bi2Te3 occurs, which grow larger to form Te nanorods and Bi2Te3 nanoplates. At
the same time, the nucleation of Bi2Te3 also can locate on the tips of as-formed Te nanorods
due to the close lattice match between the two. Te possesses hexagonal crystal structure with
a=4.457Å and c=5.929Å (JCPDS Card Number 36-1452) while Bi2Te3 has rhombohedral
crystal structure with a=4.395Å and c=30.44Å (JCPDS Card Number 82-0358). The
formation of Te nanorods by favorable growth along the c-axis has been well-discussed in
literatures.30,31
As the c-axis of Bi2Te3 nanoplate is parallel to that of Te rod in the
heterostructure, the lattice mismatch between Te (0001) and Bi2Te3 (0001) can then be
calculated to be 1.39% according to the formula｜(aTe– aBi2Te3)/ aTe｜. The lattice mismatch
is so little that the epitaxial growth of Bi2Te3 nanoplates on the ends of Te nanorods is
favorable. However, the lattice mismatch on the side surface is much higher.26
Besides the
tips of the Te nanorods, the nucleation of Bi2Te3 on the side surface could also occur and
subsequent growth of these nuclei results in the formation of a bunch of Bi2Te3 nanoplates
listed perpendicular to the Te rod, which is clearly shown in Figure 3.13a.
After nucleation on the surface of Te nanorods, the Bi2Te3 nanoplates can be grown via
Kirkendall effect, which has been utilized to synthesize various nanostructures.12,32,
The
83
Kirkendall effect refers to a nonreciprocal mutual diffusion process through the interface of
two materials, and vacancy diffusion occurs to compensate for the inequality of the material
flow.33
It has been proven that Bi2Te3 hollow structure could be obtained via Kirkendall effect
by outward diffusion of Te atoms to the interface.12,23
In our process, Te atoms could diffuse
to the interface of Bi2Te3 nanoplate and Te nanorod, react with Bi atom to form Bi2Te3
subsequently, and create voids inside the Te nanorod. As the Bi2Te3 nanoplate grow larger, Te
nanorod is consumed and then a Bi2Te3 hollow structure with a hole in the centre of the
nanoplate can be produced, which is clearly shown in Figure 3.14. Such Bi2Te3 nanoplate
would further grow completely in the solution with longer time and sufficient elemental
precursors, and uniform hexagonal nanoplates could be obtained finally.
Figure 3.14 SEM images of Bi2Te3 nanoplates with hole in the centre synthesized at 210 ℃
in 0.2 M NaOH with 1/3 SC and 0.3 g PVP for 4 h.
84
3.4 Conclusion
Single crystal Bi2Te3 hexagonal nanoplates with uniform morphology were synthesized
with solvothermal method with Bi2O3 and TeO2 as precursors, PVP as surfactant, EG as the
solvent and reducing agent and NaOH as additive. The effects of reaction temperature, time
and each starting material on the final products were systematically studied and concluded as
follows: (1) Reaction temperature as high as 200℃ is required to obtain high quality Bi2Te3
hexagonal nanoplates; (2) NaOH facilitates the formation of Bi2Te3 hexagonal nanoplates by
shifting the chemical equilibrium; (3) PVP plays a critical role in determining the hexagonal
morphology of Bi2Te3 nanoplates by absorbing on the surface and adjusting growth rate; (4)
The concentration of precursors affects the size of final products, with lower concentration
results in smaller nanoplates, while the morphology is not much affected; (5) Solvent with
different reducing ability would affect both the size and morphology of final products, with
stronger reducing agent results in smaller and more uniform hexagonal nanoplates.
In addition, time-dependent experiments revealed that heterogeneous nucleation on the
tips of Te nanorods followed by direct reaction of Bi and Te nanorods via Kirkendall effect
dominated the growth, although homogeneous nucleation of Bi2Te3 existed as well.
85
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Template-Free Synthesis of Single-Crystal Bismuth Telluride Nanorods. Advanced Materials
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(16) Mehta, R. J.; Zhang, Y. L.; Karthik, C.; Singh, B.; Siegel, R. W.; Borca-Tasciuc, T.;
Ramanath, G.: A new class of doped nanobulk high-figure-of-merit thermoelectrics by
scalable bottom-up assembly. Nature Materials 2012, 11, 233-240.
(17) Son, J. S.; Choi, M. K.; Han, M.-K.; Park, K.; Kim, J.-Y.; Lim, S. J.; Oh, M.; Kuk, Y.;
Park, C.; Kim, S.-J.; Hyeon, T.: n-Type Nanostructured Thermoelectric Materials Prepared
from Chemically Synthesized Ultrathin Bi2Te3 Nanoplates. Nano Letters 2012, 12, 640-647.
(18) Zhang, Y.; Hu, L. P.; Zhu, T. J.; Xie, J.; Zhao, X. B.: High Yield Bi2Te3 Single Crystal
Nanosheets with Uniform Morphology via a Solvothermal Synthesis. Crystal Growth &
Design 2013, 13, 645-651.
(19) Zhang, G.; Wang, W.; Lu, X.; Li, X.: Solvothermal Synthesis of V−VI Binary and
Ternary Hexagonal Platelets: The Oriented Attachment Mechanism. Crystal Growth &
Design 2008, 9, 145-150.
(20) Kong, D.; Dang, W.; Cha, J. J.; Li, H.; Meister, S.; Peng, H.; Liu, Z.; Cui, Y.: Few-Layer
Nanoplates of Bi2Se3 and Bi2Te3 with Highly Tunable Chemical Potential. Nano Letters 2010,
10, 2245-2250.
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Semiconductor Nanocrystals. Advanced Materials 2003, 15, 459-463.
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(23) Wang, W.; Goebl, J.; He, L.; Aloni, S.; Hu, Y.; Zhen, L.; Yin, Y.: Epitaxial Growth of
Shape-Controlled Bi2Te3−Te Heterogeneous Nanostructures. Journal of the American
Chemical Society 2010, 132, 17316-17324.
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a Metastable InS Crystal Structure as Colloidal Crystals. Journal of the American Chemical
Society 2000, 122, 3562-3563.
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nanoplatelets and their two-step epitaxial growth. Journal of the American Chemical Society
2005, 127, 10112-10116.
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bismuth - tellurium system. Materials Research Bulletin 1993, 28, 591-596.
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low-dimensional structured Bi2Te3 nanocrystals with various morphologies. Journal of Alloys
and Compounds 2010, 497, 57-61.
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single-molecule analysis at micromolar concentrations. Nature Nanotechnology 2013, 8,
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Chapter 4
Localized Surface Plasmon Resonance of Topological Insulator Bi2Te3
Abstract
Localized surface plasmons of Bi2Te3 single crystal hexagonal nanoplates are excited with
both electron beam and infrared light. Spatial distribution of the surface plasmons mode is
investigated by electron energy-loss spectroscopy and mapping. Different plasmon modes are
observed in the center and edge of the nanoplate, which are determined to be crystal
geometry- dependent. The plasmons modes are confirmed with spectroscopic ellipsometry by
measuring the complex dielectric function of Bi2Te3 thin film. Furthermore, scanning near-
field microscopy provides the real space imaging of surface plasmons of topological
insulators for the first time. The decay distance between the maxima and 1/e of maxima of
the electric field is used to characterize the energy confinement of the edge mode of Bi2Te3.
Excellent light confinement capability by the surface plasmons of Bi2Te3 is observed with a
decay distance of about 45 nm, which is much smaller than the wavelength of the incident
infrared light.
4.1 Introduction
Surface plasmons are electromagnetic wave arising from collective oscillation of
electrons. They can confine light into subwavelength region and propagate along the surface.
91
Surface plasmons not only reveal fundamental insights into light-matter interaction, but also
enable applications that are restricted by classical diffraction limits. Localized resonance of
surface plasmons with the driving electromagnetic field occurs when they have the same
frequency and is termed as localized surface plasmons resonances (LSPR). Under LSPR
condition, the electromagnetic field around the surface is strongly enhanced, which enables a
variety of applications, such as photovoltaic1, sensing
2,3,4 and waveguide
5,6. Surface plasmons
are usually studied on nanostructures of noble metals because of their high free electron
density (1023
cm-3
).7,8
At the same time, surface plasmons of semiconductor with appreciable
free carrier densities have also been observed.9 Recently, surface plasmons of two-
dimensional graphene have attracted interests because of its tunability and high confinement
in terahertz and mid-infrared range.10,11-14
As another two-dimensional electron system, topological insulators with metallic
surface states and an insulating band gap in bulk have attracted a lot of attention for its
strength to study fundamental physical phenomena and electronic applications.15
As the
second generation three dimensional topological insulator, Bi2Te3 exhibits large bulk energy
gap and stable surface state protected by time reversal symmetry.16,17
The surface states obey
two-dimensional Dirac equation for massless particles, similar to electrons in graphene18
, but
related to real spin of electrons, rather than sublattice pseudospin in graphene. However,
surface plasmons of topological insulators have rarely been investigated. A recent
experimental evidence of Dirac plasmons in Bi2Se3 thin film was obtained by coupling IR
light into patterned Bi2Se3 gratings and observed with infrared spectroscopy.19
To date, there
is no direct imaging of the surface plasmons of topological insulator in real space and no
report of surface plasmons of these two dimensional electron systems in visible range.
Furthermore, many intrinsic properties related to surface plasmons of topological insulators
have not been revealed, such as size, shape dependence, energy confinement ability.
92
Although there are several reports of plasmons in graphene and topological insulator,
patterning the surface of the material with subwavelength grating is necessary in most of the
studies. This result from the fact that the momentum of surface plasmon is always larger than
that of a photon of the same frequency, thus surface plasmon cannot be directly excited by
incident light and additional momentum must be provided to generate surface plasmons.20
Here, we obtain surface plasmons of Bi2Te3 nanoplate directly without any gratings, by
performing both electron energy-loss spectroscopy (EELS) and scanning near-field optical
microscopy (SNOM). The size of the hexagonal Bi2Te3 plate is in the nanometer range, thus
localized plasmon modes can be supported on the boundary edges. Besides, surface plasmons
can be readily excited either by SNOM tip due to the greatly enhanced momentum or the high
energy electron beam in EELS. Previously reported surface plasmons for two-dimensional
electron system are mainly in the IR and terahertz range. Herein, we managed to observe
Bi2Te3 surface plasmons in the visible range, with spatial-dependent distribution.
Furthermore, real-space image and field profile of the surface plasmons of Bi2Te3 are also
obtained.
4.2 Materials and methods
4.2.1 EELS spectra and mapping
The dilute suspension of Bi2Te3 in IPA was sonicated for 10 minutes before preparing
samples for EELS measurements. 10 µL of the suspension was drop-casted onto the 30 nm
silicon nitride membrane with a silicon support frame and dried in air. The as-prepared
93
sample was subsequently annealed in CVD furnace with Ar/H2 atmosphere to remove
impurities and oxidized layer.
EELS measurements were carried out using an FEI Titan TEM equipped with a
Schottky electron source in STEM mode, operated at 80 kV. A convergence semiangle of 13
mrad was used and the electron beam was focused to a diameter of approximately 1 nm. A
Wien-type monochromator was used to disperse the electron beam in energy, from which a
monochrome electron beam was selected using a narrow energy-selecting slit. This resulted
in an energy resolution of 70 meV as full-width at half-maximum (FWHM) value, and 0.9 eV
as full width at 1/1000 of the maximum value. A Gatan Tridiem ER EELS detector was used
for EELS mapping and spectroscopy, using a collection semiangle of 12 mrad. Spectra were
collected using the binned gain averaging method.21
EELS mapping was carried out by
scanning a rectangular raster of pixels with the 1 nm electron probe, while from each pixel an
EELS spectrum was acquired and stored. EELS maps were obtained by mapping the EELS
counts in each pixel using an energy window of 0.1 eV centered around a specific peak in the
spectrum.
4.2.2 Spectroscopic ellipsometry
Bi2Te3 film was prepared on single-side-polished quartz for ellipsometry
measurements. The film covered only half of the quartz while the other half part was left as
reference. The measurements were carried out with Sentech SE850 ellipsometer which is
equipped with three different light sources-deep UV (deuterium), UV/VIS source (Xe-lamp)
and the NIR source (Halogen lamp of the FT-IR spectrometer), resulting in a wide energy
94
range from 0.5 eV to 6.3 eV. Spectroscopic ellipsometry data for (amplitude ratio) and Δ
(phase difference) between the p- and s- polarized light waves were taken at several spots on
the samples with an incident angle of 80o.
To extract the dielectric function and other optical properties of Bi2Te3/quartz
structure, multilayer modelling was performed which took into account reflections at each
interface through Fresnel coefficients. The software Reffit written by Dr. A. Kuzmenko has
been used for graphical fitting of experimental data using this model.22
4.2.3 SNOM imaging
The dilute suspension of Bi2Te3 in IPA was sonicated for 10 minutes before preparing
samples for SNOM measurements. 40 µL of the suspension was drop-casted onto the clean
SiO2/Si substrate with gold markers and dried in air. The as-prepared sample was
subsequently annealed in CVD furnace with Ar/H2 atmosphere to remove impurities and
oxidized layer. The locations of the individual nanoplates were determined by a JEOL 6701
FESEM first, thus SNOM measurements could be carried out on the target nanoplate easily.
Scattering-type SNOM was used for the measurements, which is a commercial
system equipped with CO2 laser (Access Laser Company) covering a wavelength range
of 10.680 - 11.310 μm, mid-infrared quantum lasers with the wavelength range of 4.47 - 4.89
μm and 6.87 - 7.70 μm and solid-state crystal laser with the wavelength of 1.55 μm. AFM
based pseudo-heterodyne interferometric detection module was used with the
tapping frequency of about 250 kHz and the tip radius less than10 nm.
95
4.3 Results and discussion
4.3.1 EELS spectra and mapping
4.3.1.2 Bi2Te3 hexagonal plate with high symmetry
Individual nanoplate was characterized with scanning TEM (STEM) equipped with
monochromated electron energy-loss spectrometer, which is a powerful method to probe
surface plasmons because of its high spatial (<1 nm) and energy (<0.2 eV) resolution. Figure
4.1a shows a hexagonal Bi2Te3 nanoplate with regular shape and sharp edges. The edge-to-
edge distance was measured to be 700 nm. Subsequent analysis shows that it is single crystal
with high crystallinity, indicated by the sharp and well-organized diffraction spots in the
corresponding SAED pattern and clear hexagonal lattice arrangements in the HRTEM image,
as shown in Figure 4.1b. Experimental EELS spectra of the Bi2Te3 nanoplate are shown in
Figure 4.1c. When the electron beam is positioned at the edge of the nanoplate, an energy
loss peak locate at 1.6 eV is observed. Moving the electron beam from edge to the center of
the nanoplate, a broad EELS spectrum with two energy loss peaks identified at 2.1 eV and
3.1 eV is obtained.
To further confirm the physical origin of the EELS peaks, STEM-EELS mapping is
performed, which has been used to probe surface plasmons in noble metals23
and
semiconductor.24
Figure 4.1d shows the intensity maps measured at different resonance
energies of 1.6 eV, 2.1 eV and 3.1 eV, corresponding to the three peaks shown in Figure 4.1c,
respectively. Three different resonance modes can be clearly identified under different
energies. As shown in Figure 4.1d (1), the resonance mode with energy of 1.6 eV occurs only
at the edges of the nanoplate, with obvious evanescent wave field decaying into vacuum,
96
Figure 4.1 (a) TEM image of the first studied hexagonal Bi2Te3 nanoplate, (b) High
resolution TEM (HRTEM) image of the same nanoplate and the corresponding SAED pattern
(inset), (c) EELS spectra of the nanoplate when electron beam was positioned at the edge
(black line) and center (red line) of the nanoplate and (d) EELS mapping of the nanoplate at
different energies corresponding to the three peaks in (c).
which is the characteristic of surface plasmons.25
The sharp corners of nanoplate can support
strong localized mode due to the “lightning-rod” effect,26
which is adopted to described the
enhanced electric filed at shape features of objects. The intensity map of the resonance mode
with energy of 2.1 eV shows higher intensity at both center and edges, which is reasonable
since it is close to the energy used to excite the edge mode. In contrast, the resonance mode at
97
3.1 eV is localized only in the center of the nanoplate, suggesting that it has a different origin
from the lower energy modes. None of these three modes can be assigned to bulk plasmon,
excitation of which would lead to homogeneously distributed EELS intensity in the entire
nanoplate. The EELS map indicates that we have excited three different surface plasmons
modes and the mode with energy of 2.1 eV is akin to the combination of the other two modes.
Figure 4.2 (a) Real part ( 1 ) and imaginary part ( 2 ) of the dielectric function. (b) Carrier
density of Bi2Te3 calculated from ellipsometry results.
The surface plasmons modes are further confirm with spectroscopic ellipsometry from
the aspect of dielectric function and carrier density, which are the most important two
parameters related to plasmonic excitation. In order to support surface plasmons at the
interface between metallic structure and dielectric layer, the real part of dielectric function ɛ1
must be negative.27
Figure 4.2a shows that the real part of dielectric function is equal to zero
when the photon energy is 1.64 eV and it becomes negative when the photon energy is
greater than 1.64 eV. The turning point of ɛ1 at 1.64 eV suggests that the surface plasmons of
Bi2Te3 should have resonance energy greater than 1.64 eV. Considering that the dielectric
98
function is obtained by fitting the spectroscopic ellipsometry data of a film consisted of a lot
of Bi2Te3 nanoplates, while the EELS measurements are performed on individual nanoplate,
the slight difference between the turning point of the dielectric function (1.64 eV) and the
resonance mode with lowest energy of 1.6 eV in EELS is acceptable. Thus, the negative real
part at photon energy greater than 1.64 eV are consistent with EELS spectra, where most
resonance modes occur near or above 1.64 eV. The agreement between EELS spectra and
dielectric function strongly supports that the resonance modes observed in EELS are caused
by surface plasmon resonances.
It is worth noting that the plasmons modes we investigated here are located in the
visible range (1.6 - 3.1 eV). Previously the bulk plasmons modes of Bi2Se3 crystals are
observed in the high-energy range from 7 eV to 30 eV28
and the Dirac plasmons of Bi2Se3
thin film gratings have been observed in the terahertz range.19
The plasmon frequency is
primarily determined by the carrier density of the material, which must be high enough to
generate plasmons in the visible range. The most common plasmonic materials, such as gold
and silver nanoparticles, possess high electron density in the range of 1022
-1023
cm-3
so that
their plasmon peaks are located around 400-600 nm.29
We obtain the carrier density of Bi2Te3,
sampled at different photon energies in ellipsometry studies, as shown in Figure 4.2b. The
carrier density of Bi2Te3 is on the order of 1022
cm-3
, which is a comparable to that of metals.
Since the effective mass of carrier in Bi2Te3 is 0.15 times that of standard electron mass in
metals,30
and the surface plasmon frequency is inversely proportional to the square root of
effective mass of carrier, it is reasonable that the surface plasmon generated in Bi2Te3 is in
the visible range. We also note that the carrier density of Bi2Te3 increases fast in lower
energy range and peaks at 1.6 eV, then gradually decreases. This is in good agreement with
our EELS results, in which the plasmon peak with lowest energy occurs at 1.6 eV. The
different resonance modes in the Bi2Te3 single nanoplate span most of the visible range, this
99
means that Bi2Te3 nanoplates absorb visible light very efficiently and thus hold promising
application in solar energy utilization.
In the excitation of edge mode, the electron beam is positioned a few nanometers
away from the edge and only the surface of the edge can be excited by the electron beam. As
a topological insulator, the surface states of Bi2Te3 exhibit gapless metallic behaviour. Thus,
the edge mode observed in the EELS spectra can be caused by the collective excitation of
surface electrons and the surface states should have major contribution to the interaction with
electron beam. On the other hand, electron beam goes through the whole nanoplate in the
excitation of center mode. Therefore, both surface carrier and bulk carriers contribute to the
interaction with the electron beam. More than one plasmon modes can be excited and there
may be hybridization between different plasmon modes. As a consequence, the EELS
spectrum of center mode presents broad feature with two major peaks identified.
One hypothesis for the realization of edge mode could be the different electron
oscillation direction between the edge and center of the single nanoplate. Unlike metal
nanoparticles, in which the free electron gas is present in the whole volume of the particle
and all conduction electrons in the particles are involved in the surface plasmon resonances as
long as the dimension of the metal particle is smaller than twice the skin depth, the very thin
conductive surface layer of topological insulators means that the surface plasmons are
restricted to the surface strictly. In the edge mode, the conducting surface electrons oscillate
along the edges of the nanoplate, which can be called “out of plane”, while the other observed
modes in the center of the nanoplate could then be in-plane oscillation, parallel to the flat
surface of the Bi2Te3 nanoplate.
Similar measurements carried out on a second nanoplate of similar shape and size to
confirm the reproducibility of the edge and center modes. Figure 4.3a shows the TEM image
100
of the second nanoplate, whose edge-to-edge distance is 660 nm, which is quite similar to the
first nanoplate. The electron diffraction pattern in Figure 4.3b confirms that this nanoplate is
also single crystal. The EELS spectra are shown in Figure 4.3c with different resonance
modes observed at the edge and center of the nanoplate. The reproducible EELS spectra
suggests that the surface plasmon resonances are common to the Bi2Te3 nanoplates. Note that
Figure 4.3 (a) TEM image of the second studied hexagonal Bi2Te3 nanoplate, (b)
Corresponding SAED pattern of the center of the nanoplate, (c) EELS spectra of the
nanoplate when electron beam was positioned at the edge (black line) and center (red line) of
the nanoplate and (d) EELS mapping of the nanoplate at different energies corresponding to
the three peaks in (c).
101
an amorphous corner appears in this nanoplate as identified by the TEM image. In the EELS
mapping, diminished intensity is observed at this area, which is expected and attributed to the
reduced free carrier concentration at the amorphous corner.
4.3.1.2 Bi2Te3 hexagonal plate with low symmetry
For common plasmonic nanostructures, the spatial distribution and wavelength of
LSPR modes are affected by the size, shape, composition and surrounding environment.31,32
To conduct investigation on how the surface plasmons of Bi2Te3 are affected by these factors,
Bi2Te3 hexagonal nanoplate with low symmetry is selected and studied with EELS. High-
angle annular dark field (HAADF) STEM image of the investigated nanoplate is shown in
Figure 4.4a. The nanoplate exhibits unequal thickness for the left (86 nm) and right half (34
nm), with different edge lengths. HRTEM image shown in Figure 4.4b indicates that this
asymmetrical nanoplate is highly crystallized. Interestingly, a distribution of edge mode
along the edges is obtained.
Figure 4.4c shows the EELS spectra at three distinct positions (A= top corner, B=
middle of right short edge, C= middle of right long edge) on the right half of the nanoplate.
Significant location dependence of the EELS spectra is observed while resonance energy of
1.6, 2.3 and 1.9 eV is identified for the top corner, middle of short edge and middle of long
edge, respectively. This is attributed to the different resonant length of the plasmons. The
longer the resonant length, the weaker the restoring force, and the lower the plasmon
energy.26 In the Bi2Te3 hexagonal with high symmetry, the length of six edges is equal and
edge-to-edge distance is the same, hence the generated plasmon at different edges can
102
Figure 4.4 (a) High-angle annular dark field (HAADF) STEM image of the asymmetrical
hexagonal Bi2Te3 nanoplate, the thickness of left half and right half are 86 and 34 nm,
respectively. (b) HRTEM image of the nanoplate. (c) EELS spectra of the nanoplate when the
electron beam was positioned at three different spots (A= middle of top corner, B= middle of
right short edge, C= middle of right long edge). (d) A series of EELS spectra when the
electron beam was positioned at different spots along the long edge of nanoplate is indicated
in (a). (e) EELS intensity difference shows the intensity difference of mapping at the peak
energy of 1.36 eV and 1.93 eV.
103
resonate harmonically. For a nanoplate with low symmetry as shown in Figure 4.4a, the
distance between top corner and bottom corner is the longest, thus the plasmon of top corner
has the lowest energy, while the plasmon of position B (middle of right short edge) is a little
stronger. Following this theory, the plasmon of position C (middle of right long edge) should
have the highest energy due to the much smaller resonant distance. Unfortunately, it is not
detectable due to the limitation of our detector since our EELS measurements are carried out
in a low and narrow energy range. However, a much stronger intensity is indeed detected at
position C at 5.5 eV other than 1.9 eV.
Edge mode dispersion can be observed by studying the variation of EELS feature
along the long edge. A series of EELS spectra are shown in Figure 4.4d, corresponding to the
colourful dots indicated in Figure 4.4a. The long edge was split into three regions according
to the energy of resonance mode, with the top and bottom region occurring at 1.4 eV and
middle region occurring at 1.9 eV. This is possibly due to the interaction of the plasmons in
the top and bottom region with the low-energy plasmons modes in their neighbour edges.
Figure 4.4e maps the intensity difference between EELS mapping at the peak energy of 1.4
and 1.9 eV. Alternating increase and decrease of EELS intensity is observed and this may be
the result of interference among various surface plasmons propagating along the edge and
formation of standing waves.
104
4.3.3 SNOM imaging
In EELS study, electron beam with high energy can penetrate through the sample,
which will inevitably excite the bulk plasmons. To obtain the signature of surface plasmon
modes, SNOM is employed as it is sensitive to the field distribution on the surface of sample.
With this technique, the surface plasmon resonance modes are visualized directly. Figure 4.5a
and 4.5b show the regular-shaped nanoplate with edge-to-edge distance of 720 nm and
thickness of 18.7 nm. The nanoplate has very flat and clean surface as indicated by the
uniform height in both 2D and 3D of the AFM images, so any image contrast observed in
SNOM is not a result of topographical variation. As shown in Figure 4.5c and 4.5d, real-
space field image reveals that the near-field amplitude exhibits maxima at the center of the
nanoplate and decreases gradually away from the center. A significant field enhancement is
observed along the edge of the nanoplate with evanescent decay into the air, which provides
direct evidence of the existence of localized surface plasmon resonance, which is consistent
with the previous EELS results. Surface plasmons decay exponentially in dielectric layer and
the decay length of the field perpendicular to the interface is determined by the distance in
which the field intensity decreases to 1/e of the maxima. Surface plasmons are well known
for confining light into subwavelength volume and the decay length can be used as an
indication of the light confinement of surface plasmon. Smaller decay length means that the
energy of incident light is confined into a smaller volume, which is of great significance for
the trapping of light by plasmonic structures.27,20
Figure 4.5d shows the field profile along a
line trace on the nanoplate. The two intensity peaks at the left and right of the nanoplate can
be assigned to the excitation of edge modes. By measuring the distance between the highest
peak and the point where its intensity decays to 1/e, the average decay length determined by
105
Figure 4.5 (a) Two dimensional and (b) three dimensional topography images, (c) two
dimensional and (d) three dimensional near-field amplitude distribution, and (e) height and
field profile of a hexagonal Bi2Te3 nanoplate along the line trace (white dash line) indicated
in (c).
106
the left resonance mode and right resonance mode is about 45 nm. Compared to the
wavelength of incident infrared light (11.086 µm), the decay length is much smaller, meaning
that the energy of incident light is confined into ultra-small volume by the surface plasmon of
Bi2Te3. The excellent confinement of edge mode surface plasmon of Bi2Te3 hexagonal
nanoplate indicates excellent properties for light harvesting, which is verified by the
experiments in Chapter 5.
4.4 Conclusion
In conclusion, localized surface plasmons resonance of two dimensional electron
systems in the visible range from 1.6 to 3.1 eV has been observed in Bi2Te3. The plasmons
modes of Bi2Te3 nanoplate are demonstrated to be dependent on crystal geometry and
different spatially dispersed modes have been observed. The edge mode observed by EELS
and SNOM is believed to arise from surface plasmon resonance, while the center mode may
be ascribed to the hybridization of surface and bulk plasmon. The turning point of the real
part of dielectric function at 1.64 eV confirms that the edge resonance modes are surface
plasmons resonance. Real-space images of the localized modes have been recorded by
SNOM and excellent energy confinement of the localized mode is observed. These
observations pave the way for the comprehensive study of plasmonic properties of
topological insulators. Further research is needed to explain the origin of different localized
modes and to establish the relationship between the structure and plasmonic properties in
topological insulators.
107
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Chapter 5
Bi2Te3 Enhanced High-Performance Silicon/PEDOT:PSS Hybrid Solar Cell
Abstract
Plasmon excitations in the visible range and surface state-mediated low-dissipation charge
transport in Bi2Te3, a 3D topological insulator, suggest that it can be a promising material in
solar energy harvest and transport. Using a Schottky barrier type polymer (PEDOT:PSS)–
silicon (Si) solar cell as the test bed, we incorporated Bi2Te3 into the polymer matrix to
fabricate a Si/Bi2Te3–PEDOT:PSS hybrid solar cell. The solar cell performance is improved
after introducing Bi2Te3 into the device, leading to an efficiency enhancement of about 26%
and achieving power conversion efficiency (PCE) as high as 11.6%. Among various possible
factors, the plasmonic effect of Bi2Te3 in the visible range is believed to be the major factor in
enhancing light absorption.
5.1 Introduction
Si/PEDOT:PSS hybrid solar cells have been widely studied due to its ease of
fabrication using solution phase methods.1,2,3,4
Efforts to improve the PCE of Si/PEDOT:PSS
solar cells include structuring silicon to form nanostructures such as nanowires5,6,7
and
nanocones1, which have stronger light absorption than that of planar Si. Cui et al achieved a
PCE of 11.1% with Si nanocone/PEDOT:PSS solar cells. However, this kind of solar cell
usually suffers from large shunt leakage and poor coverage of metal electrodes over high
aspect-ratio nanotextured Si surface. On the other hand, effects of the passivation of Si
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surface8 as well as the property of PEDOT:PSS film
9 have also been investigated, and higher
PCE could be achieved by reducing charge recombination, increasing barrier height and
facilitating charge transfer. At present, improving the light harvesting and Schottky junction
property are the main strategy to achieve high performance for planar Si/PEDOT:PSS hybrid
solar cells.
Bi2Te3 has drawn dramatically increased research interest since its discovery as three-
dimensional (3D) topological insulators,10,11,12
a new quantum matter with an insulating bulk
gap and conductive massless Dirac fermions on their surface.13,14
The distinct and unique
properties of topological insulators have brought great opportunities and challenges to
physicist, chemists and materials scientists since their discovery.15
However, researches have
mainly focused on the fundamental studies16,17
as well as potential applications in novel
electronics, spintronics and quantum information. There are few studies on its plasmonic
properties and applications in energy conversion field. Recently, we have successfully
observed surface plasmon of Bi2Te3 in the visible range, which makes it a potential material
for solar energy harvest and transport. To the best of our knowledge, there is no report on
applying Bi2Te3 in Si based solar cells, which are currently the most promising photovoltaic
devices for commercial use.
In this study, we synthesized Bi2Te3 nanoplates and dispersed it into PEDOT:PSS
film to fabricate hybrid solar cell with planar n-type silicon (n-Si). Plasmonic effect of Bi2Te3
in the visible range is confirmed to strongly enhance light absorption of silicon, resulting in
substantial increase of short circuit current. The unique surface states of Bi2Te3 could increase
the barrier height and the built-in potential of the hybrid solar cell to reduce charge
recombination and also facilitate charge transfer, leading to a remarkable open circuit voltage
of 0.59 V and high fill factor of 0.70. The Bi2Te3 incorporated Si/PEDOT:PSS hybrid solar
cell achieves a high PCE of 11.6%, with an improvement of 26% as compared to that of 9.2%
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for the reference device.
5.2 Materials and methods
5.2.1 Chemicals
Bismuth oxide (Bi2O3, 99.999%) and tellurium dioxide (TeO2, 99.995%) were
purchased from Alfa Aesar. Ethylene glycol (EG), isopropanol (IPA), glycerol, sodium
hydroxide (NaOH), and polyvinylpyrrolidone (PVP, molecular weight 55000) were
purchased from Sigma-Aldrich. Highly conductive PEDOT:PSS (Clevios PH1000) was
purchased from Heraeus Precious Metals GmbH & Co. KG, in which the PEDOT:PSS ratio
is 1 : 2.5 (by weight) and the specific conductivity is 850 S/cm. All the chemicals were
analytical grade and directly used as received without further purification. Synthesis of
Bi2Te3 nanoplates can be referred to section 3.2.1.
5.2.2 Preparation of silicon substrates
The n-type Si (100) wafer (resistivity of 5 Ω/cm) was sequentially cleaned in acetone,
ethanol and deionized (DI) water for 20 minutes at room temperature, this is followed by
cleaning in the mixture of concentrated H2SO4 and H2O2 for 1 hour at 110 ℃.
The chemical passivation of silicon substrate was carried out in the following ways. First,
the substrate was chlorinated by immersing it in an aqueous solution of HF (4.8 M) for 30
minutes at room temperature and then dipped into the saturated solution of PCl5 in
chlorobenzene (CB) at 120 ℃ under inert atmosphere for 1 h, after which the Si–H bond
became Si–Cl bond. The substrate was then rinsed sequentially with CB and tetrahydrofuran
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(THF). Second, the substrate was immersed in the solution of CH3MgCl (1 M) in THF for at
least 5 h at 80 ℃ to complete methylation process, then rinsed with THF. Finally, the
substrate was immersed into diluted hydrochloric acid for 1 h and rinsed with DI water.
5.2.3 Device fabrication and characterization
PEDOT:PSS solution was filtered through a 0.45 um PVDF filter, and then mixed with a
certain amount of Bi2Te3 nanoplates as well as a wetting agent of 1wt% Trion X-100 and 5
wt% DMSO, which help to decrease the surface energy of hydrophilic PEDOT:PSS on
hydrophobic silicon substrate and increase the conductivity of the film. After stirring at room
temperature and under inert atmosphere for half an hour, the Bi2Te3/PEDOT:PSS blend film
was deposited on planar silicon substrate by spin-coating process with a spin velocity of 1800
rpm per minute. The substrate was annealed at 115 ℃ for 15 minutes in ambient atmosphere.
200-nm-thick silver top grid contacts were deposited by electron beam evaporation. The
metal covered part consists of 10% of the whole area of top surface. Aluminum back
electrode was deposited by thermal evaporation.
The characterizations of the devices were performed in ambient atmosphere. Simulated
solar spectrum irradiation source was generated by Newport 91160 solar simulator with a 300
W Xenon lamp and an air mass (AM) 1.5 filter. The irradiation intensity was 100 mW/cm2,
and calibrated by a Newport standard Si solar cell 91150. Incident photon to charge carrier
efficiency (IPCE) measurements were performed by Newport monochromator 74125 and
power meter 1918 with Si detector 918D. The data were recorded by Keithley 2612.
Transient electric output characteristics measurements were carried out by connecting the
devices to a digital oscilloscope with an input impedance of 1 MΩ, and the Voc of the devices
was controlled by the intensity of white light. A laser with wavelength of 532 nm was used as
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the optical perturbation, and the pulse duration was set to be 1 μs while frequency was 100
Hz. The transient photocurrent was measured with an external resistor (50 Ω) while all the
other parameters such as the frequency, pulse duration and light intensity were kept the same.
Electrochemical impedance spectroscopy (EIS) was performed under one sun illumination by
Autolab electrochemical station.
5.3 Results and discussion
5.3.1 Performance of Bi2Te3 incorporated Si/PEDOT:PSS hybrid solar cell
The as–synthesized Bi2Te3 nanoplates were blended with PEDOT:PSS solution to form
a composite, which was then spin-coated on a treated planar n-Si. Silver top grid electrode
and aluminum back electrode were deposited by electron beam evaporation and thermal
evaporation, respectively. The schematic illustration of the device structure of fabricated
solar cell is shown in Scheme 5.1a. In Si/PEDOT:PSS hybrid solar cell, a Schottky junction is
formed at the interface between Si and PEDOT:PSS film. Schottky barrier, which is the
difference between the work function of PEDOT:PSS and the conduction band of n-Si,
provides a built-in potential across the interface. Therefore the generated charge carriers can
be separated under the internal electrical field and holes are collected and transferred by
PEDOT:PSS film while electrons go to aluminum electrode directly, as shown in Scheme
5.1b.
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Scheme 5.1 (a) Schematic illustration of the device structure of the Bi2Te3 incorporated
Si/PEDOT:PSS hybrid solar cell. (b) Energy diagram of Si/PEDOT:PSS hybrid solar cell.
The PEDOT:PSS film not only acts as hole transportation layer, but also plays an
important role in collecting and transporting the generated charge carriers since the silver grid
top electrode covers only 10% of the surface area. To enhance the charge carrier separation,
the n-Si substrate is pre-treated to form a surface passivation layer by methyl termination.
The covalent bonding between methyl group and silicon means the dangling bonds on the
surface of silicon are dramatically reduced, resulting in efficient suppressing of the surface
charge recombination.20,21
To optimize the performance of the Bi2Te3-incorporated Si/PEDOT:PSS solar cell, a
series of test devices were prepared with different Bi2Te3 weight percentages of 0, 0.44%,
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0.52%, 0.78%, 1.04% and 1.30%, respectively. The performance of the fabricated devices
was measured under one sun illumination, and the results are summarized in Figure 5.1. It is
clear that the device performance can be significantly improved by incorporating Bi2Te3 into
the polymer film. The PCE increases with increasing Bi2Te3 weight fraction, reaching the
highest value of 11.6% with 1.04 wt% Bi2Te3 added. However, further increasing the weight
fraction of Bi2Te3 to 1.3 wt% results in a deterioration of the performance, which is caused by
the aggregation of Bi2Te3 nanoplates in the PEDOT:PSS film.
Figure 5.1 Solar cell performances as a function of Bi2Te3 concentration.
The device with 1.04 wt% Bi2Te3 nanoplates was further investigated. Figure 5.2 shows
the current density-voltage (J-V) characteristics of devices with and without Bi2Te3 under
illumination of AM 1.5 with an intensity of 100 mW/cm2. The detailed photovoltaic
properties are summarized in Table 5.1. The performance of Si/PEDOT:PSS hybrid solar cell
with Bi2Te3-incorporated shows marked improvement in all parameters. The device with
Bi2Te3 exhibits an open-circuit voltage (Voc) of 0.59 V, while the Voc of reference solar cell is
0.55 V. The short-circuit current density (Jsc) of hybrid solar cell also shows an increase from
26.80 mA/cm2
to 28.02 mA/cm2 after addition of Bi2Te3 nanoplates into the PEDOT:PSS
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composite film. In addition to the improvement of Voc and Jsc, the Bi2Te3-enhanced solar cell
showed a higher fill factor (FF) of 0.70, compared with that of reference solar cell (0.63). All
these improvements contribute to a much higher PCE of 11.6%, compared with that of the
reference solar cell (9.2%), indicating an increase of 26% due to the presence of Bi2Te3. It is
also worth noting that the PCE we achieved here is comparable to the highest value for
Si/polymer hybrid solar cell to date. Sun et. al. achieved the record PCE of 12.2% for
Si/PEDOT:PSS hybrid solar cell by adding 8-hydroxyquinolinolato-lithium (Liq), which is an
organic semiconductor , to enhance the Schottky barrier height and built-in potential.22
Figure 5.2 Current density-Voltage (J-V) characteristics of devices with and without Bi2Te3.
Table 5.1 Detailed photovoltaic performance of devices with and without Bi2Te3.
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5.3.2 Role of Bi2Te3 in enhanced Si/PEDOT:PSS hybrid solar cell
In order to investigate the role of the Bi2Te3 in enhancing the performance of
Si/PEDOT:PSS solar cell, a series of measurements such as IPCE, transient photovoltage and
EIS were performed. The results show that the plasmonic property of Bi2Te3, the increased
Schottky barrier height, the reduced charge recombination and facilitated charge transfer
induced by Bi2Te3, are responsible for the enhanced performance.
The incorporation of Bi2Te3 leads to a marked improvement in the Jsc of the solar cell,
corresponding well with the significant enhanced IPCE as shown in Figure 5.3a. The
enhancement of IPCE is observed for the Bi2Te3 incorporated solar cell throughout the whole
wavelength range compared with that of reference solar cell. The IPCE increasement as a
function of wavelength shown in Figure 5.3b clearly shows that there is a 30% enhancement
within the wavelength range of 400-700 nm and a relatively smaller enhancement within the
wavelength range of 800-1000 nm. We attribute this to the plasmonic absorption effects due
to the plasmon resonance modes originating from the center and edges of Bi2Te3 hexagonal
plates, respectively. As shown in Figure 5.3c, TEM-EELS results reveals that the center and
edges of Bi2Te3 nanoplates exhibit localized surface plasmon resonance (LSPR) frequency at
400-700nm and 800-1000 nm, respectively. The plasmonic effect of Bi2Te3 affects the
performance of solar cell in two ways: first, plasmonic light scattering increases the optical
path length of incident light in the Si layer, which will result in more intense light absorption
and excitons generation; second, LSPR enhances optical electrical field concentration and
thus improves light absorption. Under LSPR condition, the scattering cross-section of Bi2Te3
nanoplates can be many times its geometric cross-section.23
The stronger light absorption of
Si layer induced by Bi2Te3 can contribute to the improved short circuit current density of
Bi2Te3 incorporated solar cell. LSPR could also increase the exciton dissociation probability
120
as a result of local field enhancement.24
Higher exciton dissociation mean lower
recombination rate, which would lead to the higher Jsc and FF observed in the Bi2Te3
incorporated solar cell.
Figure 5.3 (a) IPCE spectra of the solar cell devices with and without Bi2Te3; (b) IPCE
increasement of the solar cell device incorporated with Bi2Te3 and (c) EELS spectrum of
Bi2Te3 as a function of wavelength.
Besides light absorption, the quality of Schottky junction formed at the interface of Si
and polymer play an important role in determining the performance of the hybrid solar cell.
Larger barrier height and built-in potential can effectively reduce charge recombination and
enhance open circuit voltage, resulting in a higher efficiency of the device. Over the past few
121
years, many studies have been taken to modify the Schottky junction by adding certain
additive, such as graphene oxide,25
green tea modified multiple wall carbon nanotube,26
Zonyl fluorosurfactant,27
leading to notable improvement of IPCE.
The distinct surface states of Bi2Te3 can be utilized to modify the Schottky junction. The
lower work function of Bi2Te3 (-5.3 eV) compared with that of PEDOT:PSS (-5.0 eV),
coupled with the metallic surface states of Bi2Te3, suggest that its addition may increase the
work function of PEDOT : PSS. According to Schottky – Mott model,28
the work function of
PEDOT:PSS is related to the built-in potential and Schottky barrier height, as listed below:
1 ln( / )...... (1)
...... (2)
SBH bi c d
SBH p Si
V e kT n n equation
W equation
Where SBH is the Schottky barrier height, Vbi is the built-in potential, k is the Boltzman
constant, T is the absolute temperature, nc is the effective density of states of Si, nd is the
doping concentration of Si, Wp is the work function of PEDOT:PSS and χSi is the electron
affinity of Si. The band diagram of the Schottky barrier at the interface of Si and
PEDOT:PSS is shown in Scheme 5.2.
Scheme 5.2 Band diagram of the Schottky barrier formed at the interface of Si and
PEDOT:PSS.
122
The increased work function of PEDOT : PSS is indeed confirmed by the barrier height
of Schottky junction, which is determined using the following equation29
* 2
0[exp( ) 1] exp( )[exp( ) 1]...... (3)SBHeV eVJ J A AT equation
nkT kT nkT
Where A is the contact area , *A is the effective Richardson constant (252 A cm-2
K-2
for n-Si) , e is the electronic charge, n is the diode ideality factor, SBH is the barrier height, V
is the applied potential, 0J is the reverse leakage current density, J is the current density. The
SBH of reference solar cell and Bi2Te3 enhanced solar cell was calculated as 0.79 eV and 0.85
eV, respectively. Thus, Bi2Te3 could increase the Schottky barrier height and this in turn
increase the open circuit voltage. Therefore, the Bi2Te3 incorporated solar cell show a much
higher open circuit voltage (0.59 V) than that of the reference device (0.55 V). The larger
barrier height also facilitates the separation of generated electron-hole pairs, which can
partially explain the increased Jsc and FF of Bi2Te3 incorporated solar cell device.
At the same time, the higher FF and Jsc can also result from the reduced charge
recombination and facilitated charge transfer in the device with Bi2Te3. To investigate the
effect of Bi2Te3 on charge recombination, transient photovoltage and photocurrent
measurements were performed. In the transient photovoltage measurement, the device was
exposed to white bias light to generate a photovoltage, while an extra green light pulse gave a
small increase to the photovoltage. After switching off the light pulse, the increased
photovoltage began to decay due to charge recombination and the decay rate of the
photovoltage reflects the charge carrier lifetime.30,31
As shown in Figure 5.4a, the carrier
lifetime of the device with Bi2Te3 is longer than that of reference device, indicating a reduced
charge recombination due to the introduction of Bi2Te3.
123
Figure 5.4 (a) Charge carrier life time as a function of light intensity obtained by transient
photovoltage decay; (b) Charge carrier concentration as at different open circuit voltage
obtained by transient photocurrent decay.
In addition, transient photocurrent characterization was used to determine the residual
carrier concentration in the device.32,33
In the measurement, Voc of device under the white
bias light was offset by a constant voltage with the same value but opposite polarity. A green
light pulse generated a current flow through the external load and the current decay on the
external resistor was recorded. Figure 5.4b shows that residual carrier concentration at
different open circuit voltage and it is clear that the carrier concentration in the device with
Bi2Te3 is much lower, indicating that the charge recombination is less and a larger portion of
charge carrier is transported to external circuit compared with reference solar cell. Both
transient photovoltage and photocurrent results suggest that the charge recombination in the
Bi2Te3 incorporated solar cell device is suppressed, giving a higher FF and Jsc.
The metallic surface states of Bi2Te3 can also facilitate charge transfer within the solar
cell. On the surface of topological insulators Bi2Te3, backscattering is suppressed and
electrons transport in low-dissipation fashion.14
As a result, more electrons can be collected
with the addition of Bi2Te3 and the efficiency of solar cell is enhanced. To further confirm the
124
effect of Bi2Te3 on the charge transfer, EIS was carried out under one sun illumination. The
Nyquist plots of the devices in Figure 5.5 show well-defined semicircles, of which a smaller
diameter indicates a smaller charge transfer resistance Rct.34,35
The Rct for reference device
and Bi2Te3-incorporated device calculated from EIS were 59.5 Ω and 33.7 Ω, respectively,
indicating that the incorporated Bi2Te3 has significantly improved the charge transfer rate,
which is in good agreement with the transient photocurrent results.
Figure 5.5 Nyquist plots of the devices measured by EIS under one sun illumination.
Despite the impressive efficiency we achieved with addition of Bi2Te3 nanoplates, there
should still be space for further improvement, for which goal, we suggest the following routes
can be investigated. First and most important, solutions that can achieve better dispersion and
film forming properties of Bi2Te3 in PEDOT:PSS may lead to promising results and can be
realized with proper surface modification. Second, an effective way to tune the size and shape
distribution of Bi2Te3 nanoplates can also show positive role when the surface plasmon
resonance occur within wider range. In this case, the absorption of solar energy would be
enhanced for wider spectrum and thus improve the power conversion efficiency.
125
5.4 Conclusion
In conclusion, taking advantage of the metallic surface state of Bi2Te3 and its plasmonic
properties, we have fabricated hybrid solar cell composed of planar n-Si and Bi2Te3
incorporated PEDOT:PSS. A PCE of 11.6% was achieved, which was a 26% improvement to
a control sample without Bi2Te3. The roles of Bi2Te3 are multifold, it not only enhances the
light absorption of Si, but also increases the Schottky barrier height, reduces charge
recombination and facilitates charge transfer in the device. This study identifies the potential
application of topological insulators in enhancing the performance of optoelectronic devices.
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130
Chapter 6
Terahertz response of topological insulator Bi2Te3
Abstract
The optical properties in the terahertz (THz) regime of topological insulator Bi2Te3 has been
investigated by terahertz time-domain spectroscopy (THz-TDS). Complex refractive index
and complex optical conductivity of solution-synthesized Bi2Te3 nanoplates at room
temperature are extracted and compared with graphene. Drude-Lorentz model is used to fit
complex optical conductivity of Bi2Te3 at low temperature (60-140K). Temperature
dependence of real conductivity, scattering rate, carrier density of Bi2Te3 are studied. At room
temperature, Bi2Te3 is more transparent to THz radiation as compared with graphene. Large
bulk resistivity is observed for Bi2Te3 nanoplates by the THz response of real conductivity,
which is further demonstrated to be dominated by carrier density instead of scattering rate. In
an electrically gated Bi2Te3/graphene composite film, the modulation depth of graphene is
greatly improved because of electron injection from Bi2Te3 to graphene.
6.1 Introduction
Topological insulators have drawn tremendous research interests because of the
unique combination of insulating bulk and metallic surface states.1,2
The surface states of 3D
131
topological insulators are protected by time reversal symmetry and exhibit peculiar properties,
including spin helicity,3,4
immunity to backscattering5 and robustness to modification.
6,7 Due
to these unique properties, topological insulators are believed to possess wide applications in
optoelectronics, spintronics and electronics.8,9
Terahertz electromagnetic wave supports variety of applications ranging from
communication, spectroscopy to imaging. 3D Topological insulators were predicated to have
promising applications as broad-band, high-performance photodetector working in a wide
spectrum ranging from THz to infrared.10
To realize such optoelectronic applications, the
optical response and corresponding electronic structures of topological insulators must be
well understood first.
Terahertz time-domain spectroscopy (THz-TDS) is a non-contact technique that can
probe low-energy excitations of strongly correlated electron systems by recording the
interaction of time-domain terahertz radiation through films. It has been adopted to measure
the dielectric properties, optical conductivity of semiconductors,11
superconductivity,12
spinel,13
graphene14
and so on. For topological insulators, THz response will be dominated by
the gapless surface states because the interband transition in bulk state is not possible as a
result of large gap and low energy of radiation. The THz response of Bi2Se3,15,16
(Bi1-xInx)2Se317
and Bi1.5Sb0.5Te1.8Se1.218
have been studied in the past. Thickness independent
surface conductivity and thickness linearly dependent bulk conductivity was observed for
Bi2Se3 film grown by MBE16
. Aging study showed that THz response of surface states of
Bi2Se3 exhibited robustness for a long period.15
Combined with theoretic modeling, this
technique is capable of acquiring carrier density, scattering rate of topological insulator
132
separately and investigating their temperature and frequency dependence.16,18,19
As another
typical topological insulator, optical properties of Bi2Te3 in THz range have not been fully
revealed and its optical conductivity remains elusive.
In this study, we carried out THz-TDS measurement of topological insulator Bi2Te3 at
300K (near room temperature) and extracted its complex refractive index and complex
conductivity with Fourier transformation. THz-TDS measurement of graphene and
Bi2Te3/graphene composite film were performed as well for comparison. Temperature
dependence of optical conductivity of Bi2Te3 was investigated in the temperature range from
60 to 140K and the conductivity was fitted with Drude-Lorentz model, from which carrier
density, scattering rate of Bi2Te3 were calculated. Two dimensional Dirac fermions exist on
the surface of Bi2Te3 and their interaction with Dirac fermions in graphene was studied by
comparing the modulator performance of two films with gate-controlled measurements.
6.2 Materials and methods
6.2.1 Preparation of Bi2Te3 and Bi2Te3/graphene films
High quality single-layer graphene was grown on copper by chemical vapor
deposition method and transferred onto clean SiO2/Si substrate.20
The lightly-doped p-type
SiO2/Si substrate with a doping concentration of ~1×1015
cm−3
and thickness of SiO2 layer as
300 nm was selected to facilitate terahertz detection since the terahertz beams will be
attenuated severely in a highly-doped substrate.
133
Bi2Te3 was prepared using solvothermal method at 210℃ for 4 hours, and then
dissolved in isopropyl alcohol to form concentrated suspension. The suspension was
sonicated for 10 minutes before drop-casting onto SiO2/Si substrate (with and without
graphene) in a nitrogen-filled glove box, to form Bi2Te3 film and Bi2Te3/graphene composite
film, respectively. The thickness of all the films was measured with Bruke FastScan AFM.
6.2.2 Fabrication of Bi2Te3/grapheme THz modulator
The device structure is illustrated in Scheme 6.1. The SiO2/Si substrate with lightly
p-doped Si was cut into small pieces of 0.9 cm×1.2 cm. The back side of the p-doped Si was
polished using sandpaper to remove the oxide layer. A circle of silver layer was then
deposited as back contact by thermal evaporation. Single-layer CVD graphene was
transferred onto the center of the clean top side of the substrate, with size around 0.8×0.8 cm,
followed by deposition of two gold electrodes as top contact. After THz measurements of the
graphene, Bi2Te3 nanoplates were deposited on to the graphene layer and the device was
tested again.
6.2.3 THz-TDS measurements
THz transmission through samples was measured with the TeraView Spectra 3000
THz-TDS system incorporated with a Janis ST-100-FTIR cryostat. In this system, THz signal
was generated and detected by photoconductive antennae, which was fabricated on low
134
temperature-grown GaAs films. The time-domain electric field of the THz pulse transmitted
through sample and reference were denoted as (t)SE and (t)RE , respectively. THz signal
through sample and reference were collected respectively by moving the sample holder back
and forth with a vertical motorized stage that can achieve a high resolution as 2.5 micron. For
each measurement, 1800 THz traces were collected in 60s. By changing sample and adjusting
temperature, time-domain terahertz spectra of different sample at different temperature were
obtained. For graphene and Bi2Te3/graphene THz modulator, all the measurements were
carried out at 300K. The terahertz data collecting and fitting were done in Prof. Chia Elbert’s
lab, with the help of Mr. Xia Huanxin and Dr. Chan La-o-vorakiat, Nanyang Technological
University.
6.3 Results and discussion
The sample of Bi2Te3 film on SiO2/Si substrate was measured while bare Si substrate
was used as the reference. Figure 6.1 shows a typical time-domain terahertz spectroscopy of
sample and reference. The main transmitted pulse located at about 228 ps was followed by
etalon pulses, which were caused by the multiple reflections at the top and bottom surfaces of
substrate. In the subsequent data analysis, etalon pulses were truncated and not considered to
avoid complication in data analysis. The difference between the terahertz signal transmitted
through sample and reference can be resolved, as shown in the inset of Figure 6.1. Due to the
higher absorption and longer light path caused by Bi2Te3 film, a lower transmission and larger
time delay is observed.
135
Figure 6.1 Time-domain terahertz signal transmitted through sample and reference substrate.
(Inset: magnified peak region of main pulse)
6.3.1 Transmission, complex refractive index and complex conductivity
Frequency-domain transmission through sample and reference are obtained by
performing Fast Fourier Transform (FFT) of the time-domain THz signal, which are denoted
as ( )SE and ( )RE , respectively. Transmission coefficient T( ) is calculated as the ratio
between the sample and reference spectra,( )
T( )( )
S
R
E
E
. In this way, frequency
dependence of transmission amplitude and phase are obtained for different samples.
We first studied the THz transmission through Bi2Te3 at 300K and compared with
graphene, considering that room temperature THz response would be most relevant to
applications and both graphene and the surface of Bi2Te3 are composed of Dirac fermions.
136
Figure 6.2 Frequency dependent transmission amplitude and phase, refractive index and
extinction coefficient of Bi2Te3, graphene and Bi2Te3/graphene at 300K.
Figure 6.2 (a), (c) and (e) show the frequency dependent transmission behavior of
Bi2Te3, graphene and Bi2Te3/graphene composite film, respectively. Bi2Te3 exhibits high
137
transmission to the THz electromagnetic wave, with 80%-90% of the radiation transmitted
through the film. There was a slight decrease of transmission with the increase of frequency.
The lower transmission at higher frequency indicates higher absorption and this may due to
the more intensive intraband transition at higher frequency. It is noteworthy that a
transmission valley is observed at 1.72 THz, which is attributed to a typical phonon mode in
Bi-related materials. It is consistent with the 1
1gA longitudinal optical phonon in Bi2Se3
observed in Raman21
and pump-probe22
studies. Similar phonon mode of Bi2Te3 has also been
reported to occur at 1.84 THz,23
which is in compliance with our observation, in view of the
different sample quality. The low-frequency phonon mode dominates carrier scattering and
electrical conductivity of sample at this range.24
Graphene shows a higher transmission
compared with Bi2Te3, with a transmission value of about 0.96, which is constant in the range
from 0.2 to 3.0 THz. Frequency independent transmission of graphene is also consistent with
previous report.25
Considering that the thickness of Bi2Te3 and graphene used in our study is
about 120 nm and 0.3 nm, respectively, it is reasonable to suggest that Bi2Te3 can achieve
better THz transmission when normalized to the same thickness. This is believed to have
promising application in terahertz range by integrating into devices such as
photodetectors.26,27
For the Bi2Te3/graphene composite film, the transmission behavior is
dominated by Bi2Te3.
The complex refractive index n of the sample is extracted from the following
equation describing a transmission light frequency ω through a film grown on top of a
substrate:28
138
2 ( 1)exp[ ( 1) / ]exp[ ( 1) / ]( )
(1 )( ) ( 1)( )exp[2 / ]
sub sub
sub sub
n n i d n c i L n cT
n n n n n n i dn c
Where n and subn are the complex refractive index of sample and substrate, respectively,
d is the thickness of the sample film, L is the thickness difference between reference
substrate and sample substrate, c is the light speed in vacuum. Multiple reflections inside
the sample are considered in this equation, but the multiple reflections in the substrates are
not considered because the etalon pulse is eliminated before analysis. By numerical iteration,
the complex refractive index can be extracted and expressed as ( ) ( ) ( )n n ik , where
( )n is the real refractive index and ( )k is the extinction coefficient. The refractive index
and extinction coefficient of three samples are shown in Figure 6.2 (b), (d) and (f).
In the complex refractive index, the real part ( )n is related to the phase speed and
the imaginary part ( )k is related to the absorption loss when the electromagnetic wave
passes through the material. The peak observed in refractive index and extinction coefficient
is consistent with the transmission valley. Both refractive index and extinction coefficient of
graphene decrease monotonically with the increase of frequency. It is noteworthy that the
extinction coefficient of graphene is much larger than Bi2Te3, although it shows higher
transmission, suggesting that graphene absorb THz radiation much more efficiently compared
with Bi2Te3.
The complex optical conductivity 1 2( ) ( ) ( )i can be obtained from
equations:
1 0( ) 2nk 2 2
2 0( ) ( 1)n k
where Ɛ0 is the permittivity of free space.
139
The real part 1( ) and imaginary part 2 ( ) of optical conductivity of the three samples
are shown in Figure 6.3.
Figure 6.3 Frequency-domain spectra of the real part and imaginary part of the optical
conductivity of Bi2Te3, graphene and Bi2Te3/graphene at 300K.
140
Physically, the conductivity of the sample describes the contributions from the
scatting of THz by phonons and free carriers. The phonon contribution of Bi2Te3 was obvious
from a peak around 1.72 THz as the peak behavior was expected at the resonance frequency
of a phonon oscillator. Moreover, higher phonon modes also contribute to our conductivity
data as suggested by the increasing trend of the conductivity on THz frequency. However the
resonance frequency is higher than our instrumental bandwidth (>3.5 THz) and it is not
observable. Therefore, multiple Lorentz terms are used to fit the conductivity data, which is
discussed in the next part.
The contribution due to free carriers can be described by the Drude model.29
In the
Drude model, 1( ) increases with the decrease of frequency and achieves the maximum
value at zero frequency, while 2 ( ) is positive with the maximum value achieved at the
frequency of carrier scattering rate. Upturn at zero frequency of 1( ) is not obvious for
Bi2Te3, indicating a suppression of Drude term in the conductivity, which suggests large bulk
resistivity of Bi2Te3 at room temperature. Similar results have been reported for topological
insulator Bi1.5Sb0.5Te1.8Se1.218
and Bi2Te2Se.30
Large bulk resistivity is one of the most
important pursue in the study of topological insulators because it would highlight the
signature of the peculiar surface states. The upturn at zero frequency in the real conductivity
of graphene indicates that carrier dynamics are dominated by free electrons and can be
described with Drude model. For the Bi2Te3/Graphene composite film, it shows similar
behaviour with Bi2Te3, but with larger conductivity value. This may be attributed to higher
conductivity of graphene and the large thickness difference between Bi2Te3 and graphene.
141
6.3.2 Physical model fitting of the data of Bi2Te3
After acquiring THz response of Bi2Te3 at room temperature, the temperature
dependence of the optical conductivity of Bi2Te3 is studied at lower temperature range from
60 to140K and the data are fitted with Drude-Lorentz model:
2 2 2
0 0 1 0 22 2 2 2
01 02
2
0 3 02 2
03
1
( ) ( )
( 1)( )
pD p p
p
i i i
ii
where the first term is Drude term used to describe free electron behavior, pD is the Drude
plasma frequency, is the scattering rate. The second, third and fourth term are Lorentz
terms used to describe resonance behaviors. 1p , 2p and 3p are the plasma frequency of
the Lorentz terms describing the strength of phonon oscillators scattering. is the damping
factor of phonon oscillator, 01 , 02 and 03 are the resonance frequency. The final term
involves a parameter
, which is a correction factor to the value of dielectric constant at
our experimental frequency bandwidth.
As shown in Figure 6.4, the experimental data of the optical conductivity of Bi2Te3 at
100K are well fitted to the model with the contributions of each term indicated, which
demonstrates that there are three phonon modes generated in Bi2Te3, located at 0.77 THz,
1.65 THz and 2.7 THz, respectively. Comparing with the common reported Raman data for
Bi2Te3, these phonon modes can be indexed into Eg1
mode, A1g1 mode and Eg
2 mode,
respectively. The Drude term had a major contribution to the real conductivity of Bi2Te3 in
the very low frequency range of 0.2-0.4 THz, suggesting that THz response of Bi2Te3 at low
frequency is dominated by the free carriers. The fitting of optical conductivity at 60 and 140K
142
are shown in Figure 6.5 and exhibits similar response.
Figure 6.4 (a) Real part and (b) imaginary part of optical conductivity of Bi2Te3 at 100K, dot
line = experimental data, red line = fitting curve, green line = Drude contribution, dark blue
line = first Lorentz contribution, light blue line = second Lorentz contribution and purple line
= third Lorentz contribution.
Figure 6.5 Real part and imaginary part of optical conductivity of Bi2Te3 at 60, and 140K
(dot line) and the corresponding fitting curve (solid line) with Drude-Lorentz model.
143
Total free carrrier density n can be obtained via the fitting parameter of Drude plasma
frequency 𝜔𝑝𝐷2
𝑛 = 𝜔𝑝𝐷
2 𝜀0𝑚∗
𝑒2
Where m* is the effective electron mass, e is the charge of electron. Figure 6.6 shows the
temperature dependence of real conductivity, Drude plasma frequency, scattering rate and
carrier density of Bi2Te3. Low frequency (0.22THz) real conductivity is extracted to study the
temperature dependence because data obtained at low frequency is closest to the equilibrium
state and is dominated by the free carriers. Real conductivity increases with the increase of
temperature, indicating that more free carriers are generated at higher temperature.
Temperature induced increase of free carrier density is caused by thermal excited transition of
bounded states to electronic continuum.30
In conventional metals, temperature dependent
conductivity is a result of temperature dependent electron scattering rate, not carrier density.
However, in Bi2Te3, both the carrier density and scattering rate increase with temperature.
Higher carrier density generated at higher temperature indicates that the carrier-carrier
scattering rate will also increase with temperature. Electron-phonon scattering also occurs at
higher temperature. The increased real conductivity with temperature suggests that the
conductivity of Bi2Te3 near its equilibrium state is dominated by carrier density instead of
scattering rate.
144
Figure 6.6 (a) Real conductivity at 0.22 THz, (b) Drude plasma frequency, (c) scattering rate
and (d) carrier density of Bi2Te3 extracted from Drude-Lorentz model at different
temperature.
6.3.3 The interaction between Bi2Te3 and graphene
2D electron system (2DES) has drawn extensive research interest due to its peculiar
properties. Both graphene and Bi2Te3 can be considered as 2DES, where massless Dirac
fermions exist on the basal plane of graphene and surface of Bi2Te3. It will be very interesting
to study the combination of the two Dirac systems. Gate-controlled conductivity of Dirac
fermions in graphene by THz-TDS has been demonstrated.31
However, the interaction
145
between two Dirac systems has not been investigated. Here, we construct Dirac interface by
depositing thin film of Bi2Te3 nanoplates on CVD graphene. The bare graphene and
graphene/ Bi2Te3 composite film was electrically gated in order to tune the Fermi level of
graphene. The unique surface states of Bi2Te3 are expected to affect the gating-dependent
THz behavior of graphene.
Scheme 6.1 Illustrations of device structure and operating principle. (a) Schematic of the
proof-of-concept Bi2Te3/graphene terahertz modulator. (b) Band structure of graphene and its
optical processes under THz. Red arrows represents the intraband transitions. Applying
negative voltage on p-doped graphene increases the Fermi level from EF1 to EF2.
Previous studies on optical absorptions in graphene have shown that intraband
transitions dominate in terahertz range while interband transitions are dominant in short
wavelengths range, such as infrared and visible range.32,33
Modulation of terahertz absorption
146
in graphene can be realized by tuning its electrical conductivity or Fermi level. Scheme 6.1a
shows the scheme of the graphene terahertz modulator. The back gate is used to tune the
Fermi level in graphene while the top contacts can monitor the graphene conductivity. Note
that the as-prepared CVD graphene on SiO2/Si is p-doped, thus the Fermi level is below
Dirac point. When a negative bias is applied, electrons will be introduced into the graphene
layer, which makes the Fermi level move towards Dirac point, resulting in decreased carrier
density and increased terahertz transmittance.
Figure 6.7 (a) shows the time-domain spectrum of bare graphene, in which obvious
change in the intensity of transmitted THz signal is observed in the negative bias range, while
no significant change is observed when positive bias is applied. This is attributed to the high
doping level of the as-prepared CVD graphene. The positive bias will introduce holes to the
p-doped graphene and shift down the Fermi level. Previous reports suggested that positive
voltage as high as 90 V was needed to significantly modulate the transmitted THz signal.14
High voltage would easily destroy the devices, thus, we studied the THz response of
graphene and Bi2Te3 modified graphene under negative voltage for simplicity.
The carrier density is primarily responsible for the transmission of THz radiation.31
By controlling the carrier density with back gating, THz transmission can be modulated. The
modulation depth represents one of the most significant indexes of terahertz wave
modulator.34
The modulation depth, defined as the ratio of transmittance change (with and
without gating) to the transmittance under bias and termed as |(T(V) – T0)/T(V)|, in which T(V)
is the transmittance at certain voltage and T0 is the transmittance at 0V. Figure 6.7b presents
the transmittance of graphene after removing the free carrier absorption and cavity effect
147
Figure 6.7 (a) Time-domain THz spectra, (b) frequency-domain transmission of graphene, (c)
time-domain THz spectra and (d) frequency-domain transmission of Bi2Te3/graphene at
different back gating voltage.
induced by the substrate, as a function of frequency with negative voltage. As expected, the
terahertz transmittance increases significantly with higher negative voltage, and a modulation
depth of 1.3% is achieved at -20V. Bi2Te3 nanoplates are deposited onto the same device and
measured again, with the results shown in Figure 6.7c and 6.7d. Greatly enhanced modulation
depth is achieved with the Bi2Te3/graphene composite film, with a value of 3.2% observed,
which is around 1.5 times larger than that of bare graphene. The results suggest that there are
electron injections from Bi2Te3 to graphene, resulting in the higher modulation depth.
148
6.4 Conclusion
The THz response of Bi2Te3, graphene and Bi2Te3/graphene films have been measured
and adopted to extract their optical parameters. Despite that both graphene and surface of
topological insulator Bi2Te3 can be considered as 2D electron gas, the THz response Bi2Te3
and graphene exhibited significant difference. Highly effective THz transmission of Bi2Te3
film is demonstrated compared with graphene, suggesting its promising application as THz
transparent electrode at room temperature. The THz response of Bi2Te3 indicates its large
bulk resistivity, which is highly desirable for the study of topological insulators. The real
conductivity of Bi2Te3 increases with temperature as a result of free carrier generation due to
thermal excitation. Within the temperature range from 60 to 140K, the real conductivity is
dominated by carrier density, instead of the scattering rate that dominated the conductivity of
common metals. Bi2Te3 was demonstrated to be able to improve the modulation depth of
graphene by electron injection and thus indicating its applications in THz wave modulator.
This study of terahertz response of topological insulator Bi2Te3 not only uncovers its intrinsic
electronic structures, but also paves way for its applications in THz region.
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Chapter 7
Conclusion and outlook
7.1 Conclusion
The uniqueness of Bi2Te3 as an archtypical 3D topological insulator stimulates many
research efforts on it. This thesis begins with the controlled synthesis of hexagonal Bi2Te3
nanoplates, which is followed by the spectroscopic investigation of the optical properties in
the visible and THz range, before finally exploring its application in solar cells.
By carefully controlling the growth conditions such as temperature, solvent, pH,
surfactant and reaction time during the solvothermal synthesis of Bi2Te3, uniform single
crystals of Bi2Te3 hexagonal nanoplates are successfully synthesized. Growth mechanism has
been systematically studied and it is discovered that the heterogeneous nucleation on the tips
of Te nanorods followed by direct reaction of Bi and Te nanorods via Kirkendall effect
dominate the growth. These findings help to build quantitative foundation for future synthesis
of Bi2Te3 and similar materials.
The metallic surface of Bi2Te3 allows collective oscillation of surface electrons, which
can lead to surface plasmons. In this thesis, surface plasmons of Bi2Te3 were excited with
both electron beam and light, using EELS and SNOM, respectively. Unlike previously
reported resonance modes in infrared and terahertz region for Bi2Se3 and graphene, we
observe Bi2Te3 surface plasmons occur in the visible range, and spatial-dependent distribution
of resonance frequency is observed in the hexagonal nanoplates. The surface plasmons mode
of a single Bi2Te3 nanoplate can cover almost the whole visible range, which is extremely
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useful in highly efficient solar energy utilization. To better understand the intrinsic features of
surface plasmons of Bi2Te3, we have also investigated its energy confinement capability by
studying the decay length of the edge mode.
Plasmonic properties represent the response of Bi2Te3 under strong external
perturbation. To obtain the intrinsic properties close to the equilibrium states, THz response
of Bi2Te3 was measured as only small perturbation would be imposed on sample by the low-
energy THz radiation. It is discovered that Bi2Te3 is highly transparent to THz
electromagnetic wave (more transparent than graphene), which makes it a promising
candidate for THz transparent electrode material. Furthermore, we measured large bulk
resistivity from Bi2Te3 sample, which is one of the most important pursuits in the study of
topological insulators, since the large residue bulk carrier will decrease the contribution of the
surface states. Optical conductivity is well-fitted with Drude-Lorentz model and the results
show that the real conductivity of Bi2Te3 is dominated by carrier density, in contrast with
conventional metal whose temperature dependence of conductivity is dominated by scattering
rate. Besides, we demonstrate that electron injection from Bi2Te3 to graphene on the interface
improves the modulation depth of Bi2Te3/graphene composite film compared to bare
graphene film under back gating condition.
Finally, we demonstrate that Bi2Te3 can improve the PCE of silicon/polymer hybrid
solar cell by as much as 26%, achieving an efficiency of 11.6% that is comparable to the state
of the art for silicon/polymer hybrid solar cell. Gapless surface states and plasmonics
properties of Bi2Te3 are the major contributors to the enhanced performance. In the energy
utilization field, 3D topological insulators are potentially useful since the low-dissipation
charge transport on their surface can facilitate efficient charge collection. To further improve
the e performance of solar cells, the effect of size and morphology should be studied.
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7.2 Challenges and outlook
There are still many questions and challenges to be addressed in the study of
topological insulator Bi2Te3. For the synthesis, one of major challenges is that plates larger
than several micrometers are very difficult to be obtained. In the study of fundamental
properties of topological insulators, a method to isolate the surface contribution and bulk
contribution is needed as many peculiar properties are associated with the surface states.
Disentangling the surface states and bulk states with THz spectroscopy is a promising method
but more evidence are needed to reach a general conclusion about the THz response of
topological surface states. In terms of surface plasmons of topological insulators, one of the
most important and exciting issue is the detection of charge density wave and spin wave
separately, through which more in-depth knowledge can be acquired about the unique spin
plasmons and possible applications can be proposed. A better understanding of the properties
of topological insulator allows one to consider it for applications in electronics,
optoelectronics or spintronics.