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Two-Dimensional Materials for Nonlinear Optics
Comprehensive resource covering concepts, perspectives, and skills required to understand the preparation, nonlinear optics, and applications of two-dimensional (2D) materials
Bringing together many interdisciplinary experts in the field of 2D materials with their applications in nonlinear optics, Two-Dimensional Materials for Nonlinear Optics covers preparation methods for various novel 2D materials, such as transition metal dichalcogenides (TMDs) and single elemental 2D materials, excited-state dynamics of 2D materials behind their outstanding performance in photonic devices, instrumentation for exploring the photoinduced excited-state dynamics of the 2D materials spanning a wide time scale from ultrafast to slow, and future trends of 2D materials on a series of issues like fabrications, dynamic investigations, and photonic/optoelectronic applications. Powerful nonlinear optical characterization techniques, such as Z-scan measurement, femtosecond transient absorption spectroscopy, and microscopy, are also introduced.
Edited by two highly qualified academics with extensive experience in the field, Two-Dimensional Materials for Nonlinear Optics covers sample topics such as:
- Foundational knowledge on nonlinear optical properties, and fundamentals and preparation methods of 2D materials with nonlinear optical properties
- Modulation and enhancement of optical nonlinearity in 2D materials, and nonlinear optical characterization techniques for 2D materials and their applications in a specific field
- Novel nonlinear optical imaging systems, ultrafast time-resolved spectroscopy for investigating carrier dynamics in emerging 2D materials, and transient terahertz spectroscopy
- 2D materials for optical limiting, saturable absorber, second and third harmonic generation, nanolasers, and space use
With collective insight from researchers in many different interdisciplinary fields, Two-Dimensional Materials for Nonlinear Optics is an essential resource for materials scientists, solid state chemists and physicists, photochemists, and professionals in the semiconductor industry who are interested in understanding the state of the art in the field.
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
List of Abbreviations
1 Preparation of 2D Materials
1.1 Mechanical Exfoliation of 2D Materials
1.2 Liquid‐Phase Exfoliation of 2D Materials
1.3 Chemical Vapor Deposition Growth of 2D Materials
1.4 CVD Growth of Wafer‐Scale Single Crystal 2D Materials
1.5 Thickness Control in CVD Growth of 2D Materials
1.6 Phase Control in CVD Growth of 2D Materials
1.7 Summary and Prospect
References
2 An Introduction to the Nonlinear Optical Properties of 2D Materials
2.1 Introduction
2.2 Nonlinear Optics of 2D Materials
2.3 Application of 2D Nonlinear Materials
2.4 Prospect
Acknowledgment
References
3 Modulation and Enhancement of Optical Nonlinearity in 2D Materials
3.1 Introduction
3.2 Nonlinear Optics in 2D Materials
3.3 Nonlinearity Modulation in 2D Materials
Author Contributions
Notes
Acknowledgment
References
4 Characterizing the Nonlinear Optical Properties of 2D Materials by Double 4f Nonlinear Imaging System with Phase Object and Four‐Wave‐Mixing Microscopy
4.1 Introduction
4.2 Principle of NLO Measurement Technique
4.3 Characterizing NLO Response of 2D Materials via Double 4f Nonlinear Imaging System with Phase Object (NIT‐PO) Microscopy
4.4 Characterizing NLO Response of 2D Materials via Four‐Wave‐Mixing Microscopy
4.5 Outlook and Perspective
References
5 Ultrafast Carrier Dynamics in Emerging 2D Materials
5.1 Introduction
5.2 Ultrafast Time‐Resolved Spectroscopy
5.3 Ultrafast Optics in van der Waals 2D Materials
5.4 Ultrafast Optics in Emerging Quasi‐2D Materials
5.5 Perspectives on Ultrafast Optics for 2D Material
Author Contributions
Notes
Acknowledgment
References
6 Transient Terahertz Spectroscopy for 2D Materials
6.1 Introduction
6.2 Generation and Detection of THz Radiation
6.3 Nanoscale THz Scanning Probe Microscopy of 2D Materials
6.4 Perspectives
Acknowledgment
References
7 Graphene Glass for Nonlinear Optics
7.1 Light Absorption of Graphene
7.2 Nonlinear Optical Properties of Graphene
7.3 Nonlinear Optical Properties of Graphene Glass
7.4 Perspectives
Acknowledgment
References
8 2D Materials for Nonlinear Optical Limiting
8.1 Introduction
8.2 Nonlinear Optical Limiting Mechanism
8.3 2D Materials for Optical Limiting
8.4 Conclusions and Prospects
Acknowledgments
References
9 The Saturable Absorbers Based on 2D Materials
9.1 Introduction
9.2 The Fundaments in the 2D Materials‐Based Saturable Absorbers
9.3 The Family of 2D Material‐Based Saturable Absorbers
9.4 Applications
9.5 Perspectives and Outlook
Acknowledgment
References
10 Second‐Harmonic and Third‐Harmonic Generations in 2D Layered Materials
10.1 Introduction
10.2 Conclusions and Outlook
References
11 2D Perovskites for Nanolasers
11.1 Introduction
11.2 Laser Formation Principle and Performance Parameters
11.3 The Application of 2D Perovskites in Nanolasers
11.4 Prospect
References
12 2D Materials for Space Use
12.1 Introduction
12.2 Space Radiation and Two‐Dimensional Materials
12.3 2D Materials for Space Use
12.4 Perspectives
Acknowledgment
References
Index
End User License Agreement
List of Tables
Chapter 6
Table 6.1 The advantages and disadvantages of the three primary THz generat...
Table 6.2 Substrate materials.
Chapter 8
Table 8.1 A brief summary of representative 2D nonlinear optical limiting m...
Chapter 10
Table 10.1 Recent SHG and THG studies in graphene.
Table 10.2 Recent SHG studies in transition metal dichalcogenides.
Table 10.3 Recent SHG studies in other 2DLMs.
Table 10.4 Recent THG studies in transition metal dichalcogenides.
Table 10.5 Recent THG studies in other 2DLMs.
Chapter 12
Table 12.1 A summary of radiation source, typical 2D materials, and radiati...
List of Illustrations
Chapter 1
Figure 1.1 Mechanical exfoliation preparation of 2D materials. (a) Schematic...
Figure 1.2 Liquid‐phase exfoliation of 2D materials. (a) Schematic diagram o...
Figure 1.3 CVD growth of 2D materials. (a) Schematic diagram of the CVD setu...
Figure 1.4 CVD growth of wafer‐scale single‐crystal 2D materials. (a) Schema...
Figure 1.5 Thickness control for CVD growth of 2D materials. (a) Schematic d...
Figure 1.6 Phase‐tunable synthesis of 2D materials. (a) Schematic illustrati...
Chapter 2
Figure 2.1 Schematic illustration of various nonlinear optical phenomena and...
Figure 2.2 A summary of representative SHG, THG and HHG setups. (a) simultan...
Figure 2.3 (a) Typical experimental setup for FWM measurements in integrated...
Figure 2.4 (a) The simplified scheme of the ns Z‐scan setup.(b) The simp...
Figure 2.5 (a) The microscope system and Bright field, SRS, CARS, and SHG im...
Figure 2.6 THz s‐SNOM setup for imaging polaritons in Bi
2
Se
3
films.
Figure 2.7 (a) Z‐scan results of Cr at 355, 532, and 1064 nm and relationshi...
Figure 2.8 (a) Schematic diagram of a gate‐variable all‐fiber graphene devic...
Figure 2.9 (a) Coherent phonon dynamics of 2D magnetic topological insulator...
Chapter 3
Figure 3.1 (a) SHG and THG of MoS
2
with different layers, (b) TPA spectra of...
Figure 3.2 (a) SHG of bilayer with a twisted angle, (b) interference of the ...
Figure 3.3 (a) Valley index dependent SHG and (b) valley index dependent TPA...
Chapter 4
Figure 4.1 Nonlinear optical interactions available for 2D material research...
Figure 4.2 Schematic diagram of (a) the system setup of the 4f coherent imag...
Figure 4.3 Numerical simulation of the beam profile at the output plane of N...
Figure 4.4 Schematic diagram of the double 4f coherent imaging system with p...
Figure 4.5 Experimental results of an ultrathin organic molecule thin film m...
Figure 4.6 (a) Schematic layout of the experimental FWM microscopy system an...
Chapter 5
Figure 5.1 Principle of pump–probe measurement. (a) The concept of a pump–pr...
Figure 5.2 The experimental schematic configuration of ultrafast pump–probe ...
Figure 5.3 Fundamental mechanisms of recombination dynamics for semiconducto...
Figure 5.4 The ultrafast carrier dynamics in graphene. (a) The photoinduced ...
Figure 5.5 Ultrafast dynamics in 2D TMDC. (a) Ultrafast exciton valley relax...
Figure 5.6 (a) Photographs of the PtSe
2
films synthesized by the thermally a...
Figure 5.7 (a) Crystal structure of PdSe
2
. The blue and yellow balls represe...
Figure 5.8 (a) Micro‐transient absorption spectroscopy of bilayer BP. (b) Th...
Figure 5.9 (a) Crystal structures of four types of 2D Sn‐based perovskites. ...
Figure 5.10 Synthesis and Characterizations of PtS. (a) Topography and PiFM ...
Figure 5.11 Ultrafast carrier dynamics of PtS. (a) Transient and steady abso...
Chapter 6
Figure 6.1 Schematic of a typical PCA.
Figure 6.2 Schematic of titled pulse front technology of LiNbO
3
.
Figure 6.3 Schematic of EO crystal detection.
Figure 6.4 Pump‐fluence‐ and wavelength‐dependent THz generation in GaTe. (a...
Figure 6.5 Graphene THz light‐emitting metasurfaces. (a) Top‐view
scanning e
...
Figure 6.6 Schematic diagram of a THz‐TDS system for THz radiation generated...
Figure 6.7 (a) THz‐TDS and (b) corresponding Fourier transform spectroscopy ...
Figure 6.8 Schematic diagram of the DAC setup.
Figure 6.9 (a) Schematic of the DAC for in situ TRTS. (b) Morphology of the ...
Figure 6.10 (a) and (d) Normalized −Δ
T
/
T
...
Figure 6.11 (a) Time‐resolved and frequency‐resolved THz photoconductivities...
Figure 6.12 (a) Schematic of charge transport within the network of MXene fl...
Figure 6.13 (a) Typical scanning near‐field THz microscopy setup. (b) A meta...
Figure 6.14 Operational concept of ultrafast light wave‐driven THz‐STM. (a) ...
Figure 6.15 THz‐driven STM imaging of Si 111 surface.
Chapter 7
Figure 7.1 (a) Crystal structure of graphene. (b) Graphene band structure an...
Figure 7.2 (a) Schematic of ion‐gel‐gate controlled graphene field effect tr...
Figure 7.3 (a) High‐harmonic radiation spectrum of graphene.(b) Red line...
Figure 7.4 (a) Photographs of hybrid glasses doped with graphene quantum dot...
Figure 7.5 Comparison of strategies for the growth of graphene on Glass in t...
Figure 7.6 Direct growth of graphene on high‐temperature‐resistant Glass. (a...
Figure 7.7 Graphene grown on a molten soda lime glass surface. (a) Photograp...
Figure 7.8 Graphene grown with Metal vapor. (a) Schematic illustrations of t...
Figure 7.9 Segregation growth of graphene at the interface of a pre‐deposite...
Figure 7.10 Graphene growth on glass using the sacrificial metal coating app...
Figure 7.11 (a) A schematic of graphene growth on h‐BN/Cu foil using a nicke...
Figure 7.12 (a) A schematic for the direct growth of vertical graphene on gl...
Chapter 8
Figure 8.1 The illustration of processes of reverse saturable absorption (a)...
Figure 8.2 (a) Open‐aperture Z‐scan curves of pure rGO, Au‐Fe
2
O
3
, and Au‐Fe
2
Figure 8.3 (a) The normalized transmittance as a function of input laser int...
Figure 8.4 OA Z‐scan traces of h‐BN at 1064 nm (a) and fluence‐dependent non...
Figure 8.5 OA Z‐scan curves at 532 (a) and 1064 nm (b) and closed‐aperture Z...
Figure 8.6 (a) and (b) Lattice structure of Te. (c) OA Z‐scan results of tel...
Figure 8.7 (a) OA Z‐scan curve. (b) Inverse transmission as a function of pe...
Figure 8.8 (a) OA Z‐scan results (I), I‐scan results (II), and CA Z‐scan res...
Figure 8.9 (a) Circular dichroism for second harmonic generation waves obser...
Figure 8.10 (a) and (b) The intensity‐dependent Z‐scan profiles of graphene ...
Figure 8.11 (a) The F‐P cavity‐type electrochromic supercapacitor electrodes...
Chapter 9
Figure 9.1 (a) Schematic diagram of a light beam traveling through the mediu...
Figure 9.2 Elemental composition of 2D nanomaterials with saturable absorpti...
Figure 9.3 (a) Schematic diagram of the light Absorption in graphene. (b) No...
Figure 9.4 (a) The buckled honeycomb lattice of germanene nanosheet and its ...
Figure 9.5 (a) Schematic diagram of the crystalline structure of black phosp...
Figure 9.6 (a) Schematic diagram of the crystalline structure of antimonene....
Figure 9.7 (a) Schematic diagram of the atomic structure of the three main p...
Figure 9.8 (a) Simulated molecular structure of Cu1‐ and Cu2‐MOF. (b) Thickn...
Figure 9.9 (a) The Scotch tape exfoliation process for fabricating a TMD‐bas...
Figure 9.10 (a) Schematic diagram of the passive Q‐switched process. (b) Typ...
Figure 9.11 (a) Temporal evolution of optical power and losses in a passivel...
Chapter 10
Figure 10.1 Different nonlinear frequency conversion processes of SHG (a) an...
Figure 10.2 Selected family of 2D materials and their bandgaps. 2D materials...
Figure 10.3 SHG signal from monolayer graphene (a) and bilayer graphene (b) ...
Figure 10.4 (a) Schematic illustration of the graphene/SiO
2
/Si (001) sample ...
Figure 10.5 (a) SHG and THG maps of the MoS
2
flakes. Optical spectrum of the...
Figure 10.6 (a) Confocal laser scanning microscope image of layered GaSe fla...
Figure 10.7 (a) The average power of the third harmonic as a function of the...
Figure 10.8 (a) AFM image and height profile inset, (b) Raman spectrum, and ...
Figure 10.9 (a) Optical microscope image of BP flake. (b) Intensity map of t...
Figure 10.10 (a) Measured spectra of the generated light from three differen...
Chapter 11
Figure 11.1 Perovskite crystal structure. (a) 3D perovskite unit cell. (b) 3...
Figure 11.2 The basic process of lasers. (a) Stimulated absorption. (b) Spon...
Figure 11.3 The physical process of (a) three‐level system and (b) four‐leve...
Figure 11.4 Intensity and full width at half maximum (FWHM) of the emission ...
Figure 11.5 Different types of cavities. (a) A nanowire laser serving as Fab...
Figure 11.6 Lasing characterization of a CsPb
2
Br
5
microplate in F–P mode (a–...
Figure 11.7 (a) Schematic illustration of the synthesis of FA‐(N‐MPDA)PbBr
4
...
Figure 11.8 Both (a) and the upper right inset show the optical microscopy i...
Figure 11.9 (a) μ‐PL images of nanowire arrays...
Figure 11.10 (a–c) Template‐confined solution‐growth of microring arrays of ...
Figure 11.11 (a) Schematic diagram of cavity architecture with approximate t...
Figure 11.12 Pulsed and CW lasing properties of P2F8 and N2F8 films with DFB...
Figure 11.13 (a–d) Lasing device performance of the quasi‐2D perovskites. (a...
Chapter 12
Figure 12.1 The Van Allen radiation belt that features high‐energy particles...
Figure 12.2 (a) MoS
2
and graphene/MoS
2
/graphene heterostructure exposed to 8...
Figure 12.3 (a) The TEM images, (b) the open‐aperture fs Z‐scan data, and (c...
Figure 12.4 (a) Graphene irradiated by Ar
+
ions with various doses.(...
Figure 12.5 (a) Schematic diagram of the planar X‐ray detector based on 2D p...
Figure 12.6 (a) Transfer characteristics of the MoS
2
PSE. (b) Annealing‐indu...
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
List of Abbreviations
Begin Reading
Index
End User License Agreement
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Two‐Dimensional Materials for Nonlinear Optics
Fundamentals, Preparation Methods, and Applications
Edited by Qiang Wang and Hao‐Li Zhang
Editors
Prof. Qiang Wang
Lanzhou University
College of Chemistry and Chemical Engineering
No. 222 Tianshui South Road
730000 Lanzhou
China
Prof. Hao‐Li Zhang
Lanzhou University
College of Chemistry and Chemical Engineering
No. 222 Tianshui South Road
730000 Lanzhou
China
Cover Images: © Serg Myshkovsky/Getty Images; © Marco_de_Benedictis/Getty Images
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Preface
The development of two‐dimensional (2D) materials has garnered increasing interest due to their unique structure and versatile applications. For instance, in the optoelectronic and photonic fields, 2D materials have been broadly employed for optical modulation through optical limiting/mode locking, photodetectors, optical communications, miniaturized all‐optical devices, and so on. The ever‐expanding 2D family particularly offers abundant sources to the emerging new physical insights and phenomena in traditional nonlinear optics. Although previously there have been many excellent reviews on the applications of 2D materials in one specific field, systematic introduction integrating all of the key elements ranging from material fabrications to novel characterizing techniques and myriad nonlinear optical applications is still rare.
Hence, in this book, collective efforts through researchers from interdisciplinary fields focus on 2D materials for nonlinear optical applications to further the exciting territory toward a new horizon. Preparation methods for various 2D materials including graphene glass, transition metal dichalcogenides (TMDs), single elemental 2D materials from Group 15, and perovskites have been thoroughly summarized. Moreover, powerful nonlinear optical characterizing techniques, such as Z‐scan measurement, femtosecond transient absorption spectroscopy and microscopy are introduced. The origin of the unique optoelectronic properties of 2D materials is elucidated from various perspectives like excited‐state dynamics.
Specifically, Chapter 1 provides a comprehensive perspective on the fundamentals and preparation methods of 2D materials with nonlinear optical properties. Chapter 2 is an overall introduction to the nonlinear optical properties of 2D materials, and Chapter 3 elaborates on the modulation and enhancement of optical nonlinearity in 2D materials. Furthermore, Chapters 4–6 discuss the nonlinear optical characterization techniques for 2D materials and their applications in the specific field, including novel nonlinear optical imaging systems, ultrafast time‐resolved spectroscopy for investigating carrier dynamics in emerging 2D materials, transient terahertz spectroscopy, etc. Chapter 7 expounds on graphene glass for nonlinear optics while Chapters 8–12 emphasize the applications of 2D materials for optical limiting, saturable absorber, SHG and THG, nanolasers, and space use, respectively. The topics covered in this book are of vital importance in understanding the unique optoelectronic properties and applications of 2D materials and the physical insights behind them. We hope the book will be a handy toolkit for professionals, researchers, and graduate students working or interested in the field of nonlinear optics and materials.
The editors thank all the contributors of this book for their insightful chapters. We are grateful to the production editors at Wiley, Ms. Lifen Yang and Katherine Wong, for their diligent work and expertise in managing the process.
Lanzhou, People's Republic of China
Qiang Wang
Hao‐Li Zhang
List of Abbreviations
Number
2D
Two dimensional
2DLMs
2D layered materials
2DCS
Two‐dimensional coherent spectroscopy
1D
One dimensional
0D
Zero dimensional
2PA
Two‐photon absorption
3PA
Three‐photon absorption
3PM
Three‐photon microscopy
A
ABCD
Air‐biased coherent detection
AC‐STEM
Aberration‐corrected scanning transmission election microscopy
AIE
Aggregation‐induced emission
AFM
Atomic force microscope
ASE
Amplified spontaneous emission
B
Bi
Bismuthene
BC
Birefringence crystals
BP
Black phosphorus
BS
Beam splitter
BPDs
Balanced photodiodes
BBO
Beta barium borate
BN
Boron nitride
BLG
Bilayer graphene
BGR
Bandgap renormalization
C
CARS
Anti‐Stokes Raman scattering
CMOS
Complementary metal‐oxide semiconductor
CVD
Chemical vapor deposition
CSS
Chinese Space Station
COFs
Covalent organic frameworks
CB
Conduction bands
CW
Continuous wave
D
DAC
Diamond anvil cell
DFG
Difference‐frequency generation
DLC
Diamond‐like carbon
D‐A
Donor–acceptor
DBR
Distributed Bragg reflector
DSM
Dirac semimetal
DFWM
Degenerate FWM
DT
Differential transmission
DFB
Distributed feedback
DWP
Double‐wavelength wave plate
E
ESA
Excited‐state absorption
EDF
Erbium‐doped fiber
EBL
Electron beam lithography
ENZ
Epsilon near zero
ED
Electric dipole
EQ
Electric quadrupole
EO
Electro‐optic
EFISH
Electric‐field‐induced second harmonics
F
FWM
Four‐wave mixing
FWR
Four‐wave rectification
fs
Femtosecond
FCA
Free carrier absorption
FET
Field‐effect transistor
FTO
Fluorine‐doped tin oxide
FW
Fundament wavelength
F‐P
Fabry–Pérot cavity
FWHM
Full width at half maxima
G
Ge
Germanium
GO
Graphene oxide
GD
Graphdiyne
GEOs
Geostationary orbits
GSAM
Graphene saturable absorber mirror
H
HG
Harmonic generation
HHG
High‐harmonic generation
h‐BN
Hexagonal boron nitride
HEOs
Highly elliptical orbits
HAADF
High‐angle annular dark field
HT
High temperature
HP
Half‐wave plate
HEG
Hydrogen‐induced exfoliated graphene
h‐MO
Hexagonal metal oxides
I
ISS
International Space Station
IPA
Isopropylammonium
L
LEED
Low‐energy electron diffraction
LD
Laser diode
LEOs
Low‐Earth orbits
LiDAR
Light detection and ranging
LDOS
Local density of states
LPE
Liquid‐phase exfoliation
LAPCAs
Large‐aperture photoconductive antennas
M
M
Magnetization
MPL
Multiphoton lithography
MPEA
Chiral β‐methylphenethylammonium
MPA
Multiphoton absorption
MRRs
Micro‐ring resonators
MZI
Mach–Zehnder interferometer
MEOs
Medium‐Earth orbits
MOFs
Metal–organic frameworks
MOCVD
Metal–organic chemical vapor deposition
MRA
Metamaterial resonant absorber
MD
Magnetic dipole
MI
Multiphoton ionization
MSHG
Magnetization‐induced SHG
MAEF
Multiphoton‐absorption‐excited fluorescence
MXenes
Transition metal carbides and nitrides
N
NLO
Nonlinear optics
NLA
Nonlinear absorption
NLR
Nonlinear refraction
NLS
Nonlinear scattering
NWs
Nanowires
NRs
Nanorods
NDs
Nanodots
NSs
Nanosheets
NIT‐PO
Nonlinear imaging system with phase object
NMP
N‐methylpyrrolidone
O
OPA
Optical parametric amplifier
OM
Optical microscope
OL
Optical limiting
OC
Output coupler
OA
Open aperture
OR
Optical rectification
P
PMMA
Polymethyl methacrylate
PDMS
Polydimethylsiloxane
PEEM
Photoemission electron microscopy
Pt
Platinum
PC
Polarization controller
PD
Photodiode
PCAs
Photoconductive antennas
PO
Phase object
PI‐ISO
Polarization‐independent isolator
PSE
Polymer solid electrolyte
PVD
Physical vapor deposition
PVT
Physical vapor transport
PB
Photoinduced bleaching
PM
Parabolic mirrors
PMT
Photomultiplier tube
PIA
Photoinduced absorption
PtS
Platinum sulfide
PMT
Photomultiplier tube
Q
QWs
Quantum wells
QDs
Quantum dots
R
RSA
Reverse saturable absorption
rGO
Reduced graphene oxide
RIE
Reactive ion etching
S
SHG
Second‐harmonic generation
SRS
Stimulated Raman scattering
SA
Saturable absorption
SAE
Sonication‐assisted exfoliation
SEEs
Single‐event effects
SEU
Single‐event upset
SEFI
Single‐event functional interruption
SET
Single‐event transient
SEL
Single‐event latch‐up
SNSPD
Superconducting nanowire single‐photon detectors
SDI
spectral‐domain interferometry
SOI
Silicon on insulator
SEM
Scanning electron microscopy
SFM
Sum‐frequency mixing
SPM
Self‐phase modulation
SNOM
Scanning near‐field optical microscopy
STE
Self‐trapped exciton
SVEA
Slowly varying envelope approximation
SESAM
Semiconductor saturable absorber mirror
STM
Scanning tunneling microscopy
STS
Scanning tunneling spectroscopy
SECT
Slow evaporation at constant temperature
SNR
Signal‐to‐noise ratio
T
TPA
Two‐photon absorption
THG
Third‐harmonic generation
TPF
Two‐photon fluorescence
TPF
Tilted pulse front
THz
Terahertz
TFISH
THz‐field‐induced second harmonic
TPL
Two‐photon luminescence
TPEF
Two‐photon excitation fluorescence
TAM
Transient absorption microscopy
TAS
Transient absorption spectroscopy
TAC
Thermally assisted conversion
TCR
Transition‐Cherenkov radiation
TMDs
Transition metal chalcogenides
TMDCs
Transition metal dichalcogenides
TI
Tunnel ionization
TID
Total ionizing dose
tr‐SNOM
Time‐resolved scanning near‐field optical microscope
TRTS
Time‐resolved THz time‐domain spectroscopy
V
vdW
van der Waals
VB
Valence bands
W
W
Tungsten
WP
Wollaston prism
WDM
Wavelength division multiplexer
WGM
Whispering‐gallery mode
X
XUV
Extreme ultraviolet
1Preparation of 2D Materials
Yue Tang and Hua Xu
Shaanxi Normal University, Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Xi'an, 710119, P. R. China
Two‐dimensional (2D) materials, an emerging new class of nanomaterials with rich structures and remarkable properties, would bring many transformative technologies and applications [1]. Since the discovery of graphene for the first time in 2004, the 2D material family has expanded dramatically to include insulators (hexagonal boron nitride [h‐BN]), semiconductors (most transition metal dichalcogenides [TMDCs], black phosphorus [BP], and tellurium [Te]), semi‐metals (some TMDCs and graphene), metals (transition metal carbides and nitrides [MXenes]), superconductors (NbSe2), and topological insulators (Bi2Se3 and Bi2Te3) [2, 3]. The atomic thickness and dangling free surface of 2D materials, together with their superior optical, electrical, magnetism, thermal, and mechanical properties, endow them with great promise for applications in optical communication, electronics, optoelectronics, spintronics, memory, thermoelectric, and energy conversation and storage devices [4, 5].
As highlighted by the famous nanomaterial scientist Zhongfan Liu, “preparation determines the future” is an inexorable law for all materials. In the past decade, a series of preparation technologies have been developed to fabricate 2D materials for satisfying the requirements of their fundamental studies and various applications. In view of the layered structure of 2D materials, the primary preparation technologies can be divided into two major types: top‐down and bottom‐up approaches. In this chapter, we will introduce the recently developed preparation technologies for 2D materials, including two top‐down approaches (mechanical exfoliation and liquid exfoliation) and one bottom‐up approach (vapor phase growth). Here, we give more space to introduce single crystal growth, thickness control, and phase control in the vapor phase growth of 2D materials.
Figure 1.1 Mechanical exfoliation preparation of 2D materials. (a) Schematic diagram of mechanical cleavage process and optical micrograph of one of the graphene flakes on the SiO2/Si substrate with different thicknesses.
Source: Reproduced with permission from Yuan Huang et al. [7]/American Chemical Society.
(b) Schematic diagram of the Au‐assisted mechanical exfoliation process and corresponding optical microscope (OM) images of obtained vdW‐layered 2D materials.
Source: Yuan Huang et al. [8]/Springer Nature/CC BY 4.0.
1.1 Mechanical Exfoliation of 2D Materials
In 2004, Geim and Novosolov firstly prepared monolayer graphene by exfoliating bulk graphite using Scotch tape [6]. The schematic exfoliation method and the obtained 1‐4‐layer graphene are shown in Figure 1.1a [7]. Since then, mechanical cleavage, commonly referred to as the Scotch tape method and involving no chemical reactions, is considered to be the simplest and best approach to obtain large‐area, high‐quality 2D materials that retain their pristine structures and properties. Until 2010, Heinz's group firstly extended the mechanical cleavage method to prepare monolayer MoS2 and discovered the indirect to direct bandgap transition with the thickness changing from bulk to monolayer [9]. Then, A. Kis's group fabricated the field effect transistor based on the exfoliated monolayer MoS2 and achieved room‐temperature current on/off ratios of 108 and ultralow standby power dissipation [10]. Later, mechanical cleavage techniques were widely used to produce dozens of 2D transition metal dichalcogenides (TMDCs: WS2, WSe2, MoSe2, ReS2, etc.) [11]. However, with the increasing requirements on thickness and domain size of 2D materials for device fabrication and property exploration, cleavage technology pursues efficient preparation of 2D materials with large areas and high quality. Hence, many optimized mechanical cleavage techniques have been developed in recent years. For example, Huang et al. developed a universal Au‐assisted mechanical cleavage technique, as shown in Figure 1.1b [8]. Theoretical calculation indicates that Au and many 2D materials can form quasi‐covalent bonds, which are larger than van der Waals (vdW) interactions but smaller than covalent bonds. In the experiment, a thin Au layer was firstly deposited onto a substrate covered with a thin Ti or Cr adhesion layer, and a freshly cleaved bulk crystal on tape was brought into contact with the Au layer. Then the adhesive tape was placed on the outward side of the crystal, and gentle pressure was applied to establish a good crystal/Au contact. Finally, peel off the tape to remove the major portion of the crystal, leaving monolayer or few‐layer flakes on the Au surface. Using this approach, they obtained large‐area monolayer flakes on the Au surface, including MoS2, FeSe, PtSe2, PtTe2, PdTe2, and CrSiTe3. Using the above exfoliation methods, a series of new 2D‐layered materials have been successfully prepared, such as Fe3GeTe2, MnBi2Te4, CrOCl, NbSe2, and NbOCl2, [12–16] which exhibit superior ferromagnetic, superconduction and nonlinear optic properties. In a word, mechanical cleavage technology has been widely utilized to prepare 2D‐layered materials for studying their fundamental properties.
However, the methods mentioned above are restricted to 2D‐layered materials in which the interlayer interactions are dominated by weak vdW force, and thus these methods are not applicable to materials in which interlayer interactions are dominated by non‐vdW force. In view of this, Zhang et al. developed a new mechanical cleavage strategy [17]. The polished metallic surface is oxidized under a controlled environment to enable the growth of hexagonal metal oxides (h‐MO). These high‐crystalline h‐MO with layered structures, without ionic dopants or vacancies, can be easily exfoliated by stamping them onto the target substrates. This cleavage strategy was firstly applied to prepare the three‐dimensional (3D) transition metal‐based h‐MO (TiO2, Fe2O3, and Ni2O3), and it could be readily extended to prepare a variety of other metal oxides for exploring their novel 2D quantum properties. Most recently, Fengxia Geng's group reported a calendaring pretreatment mechanically exfoliate approach to prepare 2D materials from non‐vdW structures [18]. On the basis of the traditional scotch tape method, an external mechanical force was applied to the non‐vdW materials. This approach involves laterally sliding in the closely packed neighboring layers to transform the structure from a stable to a metastable phase, weakening the interlayer binding. Using this approach, a variety of 2D materials have been prepared, including metals (Bi, Sb), semiconductors (SnO, V2O5, Bi2O2Se), and superconducting compounds (KV3Sb5). This method for mechanically exfoliating non‐vdW materials increases the availability of 2D materials for the exploration of their physical characteristics and potential applications. To sum up, using the mechanical cleavage method can prepare most of the target 2D materials with high crystal quality, but the samples acquired via this approach also possess several problems, such as irregular morphology, uncontrollable thickness, small domain size, and low yield. Therefore, the present mechanical cleavage technology for 2D material preparation still faces great challenges, which limits its basic research to practical application. It is worth to further optimize the present cleavage methods and explore novel cleavage technologies to achieve 2D materials with controllable layers and sizes.
1.2 Liquid‐Phase Exfoliation of 2D Materials
As demonstrated above, the preparation of 2D materials via mechanical exfoliation is limited by the inevitable low yield and uncontrollable thickness, domain size, and morphology. To resolve these issues, liquid‐phase exfoliation (LPE) was developed to prepare 2D materials, which has the advantages of high efficiency, large scale, and better controllability [19–23]. The LPE of 2D materials can be divided into two primary approaches: direct exfoliation and chemical exfoliation [24]. Direct exfoliation methods include sonication‐assisted exfoliation [SAE] and shear exfoliation. Chemical exfoliation methods include chemical intercalation and electrochemical intercalation [25, 26].
For the SAE method, layered bulk crystal was firstly dispersed in a solvent, followed by material exfoliation via ultrasonic energy and removal of the non‐exfoliated material via centrifugation [27]. The dispersion is then centrifuged and purified to achieve 2D nanosheets with uniform domain size and thickness. Moreover, a shear exfoliation method has been widely used to disperse the pretreated layered crystals, which enables the production of high‐quality 2D nanosheets while avoiding SAE's fragile issues. However, the exfoliation is only suitable for 2D materials with weak interlayer force, while it is difficult to effectively exfoliate 2D materials with strong interlayer coupling.
Huang et al. developed an efficient and stable three‐stage sonication‐assisted liquid‐phase exfoliation (TS‐LPE) method for mass preparation of high‐structural‐integrity few‐ and single‐layer Fe3GeTe2 nanoflakes [28]. The three stages include slicing crystals, weakening interlayer vdW forces, and using ultrasonic cavitation (Figure 1.2a). At the first stage, ball milling is used to reduce the thickness and size of Fe3GeTe2 crystals. At the second stage, the shear of the solvent via stirring in N‐methylpyrrolidone (NMP) and the shock waves caused by the collision events in the container lead to expanding layer spacing and weakening interlayer vdW forces to facilitate the subsequent shedding of the nanoflakes. The third stage is ultrasonic cavitation to facilitate the exfoliation of nanoflakes. Finally, Fe3GeTe2 nanoflakes were extracted by centrifugation. The highest yield of Fe3GeTe2 nanoflakes is 22.3 wt%, with single layers accounting for 6%, and a maximum of 103 μm is available. The 200 mg level of output has overcome the limitations of mechanical exfoliation and molecular beam epitaxy in economically amplificated production.
Figure 1.2 Liquid‐phase exfoliation of 2D materials. (a) Schematic diagram of the TS‐LPE method.
Source: Reproduced with permission from Ma et al. [28]/American chemical society.
(b) Schematic illustrations of the electrochemical Li+ intercalation‐based exfoliation process.
Source: Reproduced with permission from Yang et al. [29]/Springer Nature.
Zeng et al. developed an electrochemical lithium‐ion intercalation‐based exfoliation method that can be easily conducted at room temperature within 26 hours (Figure 1.2b) [29]. This exfoliation method is relatively simple and straightforward. It offers a higher degree of control under mild conditions, and thus it was widely utilized to prepare various 2D materials on a large scale. The obtained 2D nanosheets possess high crystal quality, uniform thickness, and a relatively large domain size. However, during the lithium‐ion intercalation process, the insertion of each Li+ ion involves the injection of one electron into the host crystal. Thus, the intercalation of a large number of Li+ ions (one per formula unit in LiMoS2) would lead to massive electron injection into the MoS2 crystal, which would induce an undesired phase transition from the semiconducting 2H phase to the metallic 1T phase [30–32]. Theoretical studies suggest that this phase transition occurs only when the electron injection exceeds a certain threshold (0.29 electrons per MoS2 formula unit) [33, 34]. A possible way of reducing electron injection into the host 2D crystal and thus preventing the undesired phase transition is to replace the small Li+ ions (diameter d ∼2 Å) with larger cations, such as quaternary ammonium ions [32]. Duan et al. developed an electrochemical exfoliation method via intercalating large‐size tetramethyl ammonium bromide (THAB; d ∼20 Å) molecules into the MoS2 crystal [35]. Driven by the negative electrochemical potential, THA+ cations were inserted into the MoS2 layer, causing a substantial volume expansion. Then, the expanded MoS2 crystal was immediately sonicated in the organic solvent to disperse thin MoS2 nanosheets within several minutes. This approach can achieve high‐quality MoS2 nanosheets on a large scale with a relatively uniform thickness that have been applied to constructing large‐area thin‐film electronic devices, and this method has been extended to prepare other 2D TMDs [20, 36].
To sum up, the LPE method can prepare large‐scale 2D materials with monolayer and few‐layer, but it still needs to improve the controllability of thickness and morphology. In addition, high‐power ultrasonic energy, organic solvents, or chemical intercalators inevitably degenerate the intrinsic structure and properties of 2D materials, which hinders their high‐performance optical and electronic applications. Hence, the 2D materials prepared by LPE methods are favorable to some particular fields, such as supercapacitors, lithium‐ion batteries, catalysis, printer ink, and composite materials.
1.3 Chemical Vapor Deposition Growth of 2D Materials
Comparing with the above top‐down methods, bottom‐up synthesis by chemical vapor deposition (CVD) proves to be an effective method to grow large‐scale 2D materials with controllable domain size, morphology, thickness, and electronic‐grade quality, which shows great potential for fabricating high‐performance optical, electronic, and optoelectronic devices for commercial applications. The CVD method used to synthesize 2D materials refers to the gas‐vapor growth of material on a substrate within the tube furnace system (Figure 1.3a) [37, 41]. To be specific, two or more precursors are introduced into the high‐temperature reaction zone in quartz tube by carrier gas for chemical reaction, nucleation, growth, and final formation of 2D flakes or continuous films on the substrate.
In the past decades, CVD has been widely used in the preparation of various 2D materials, such as elemental graphene and BP, binary h‐BN, TMDs (MoS2, WSe2, MoTe2, VS2, TiS2, ReSe2, etc.), ternary chalcogenides, and their alloys and heterostructures [42–47]. Take the CVD growth of MoS2 as an example, Van der Zande et al. synthesized monolayer MoS2 nanosheets with uniform triangle morphology by using CVD growth with sulfur (S) powder and molybdenum trioxide (MoO3) as precursors [48]. The obtained MoS2 possesses high‐quality, uniform monolayer and large grain size up to 120 μm, which enables comparable electrical and photoelectric properties with those of mechanically exfoliated MoS2.
During the process of 2D material growth, many factors affect the quality and properties of the product, such as precursor, substrate, growth temperature, carrier gas flow, and additives [49–51]. Thus, great efforts have been made to optimize the CVD growth method to achieve the preparation of large‐area, high quality, homogeneous‐thickness 2D materials [49]. For instance, to achieve a stable and continuous supply of Mo source, electrochemically oxidized arched Mo foil was utilized as the precursor instead of the traditional powder, and the growth of MoS2 with controllable thickness was realized (Figure 1.3b) [38]. In addition, researchers have confirmed that the addition of synergistic additives can modulate the growth chemistry of 2D materials, facilitate the growth rate, and adjust the morphology and structure. For many 2D TMDs, the high melting point of metal precursors leads to a challenge for low‐temperature growth through CVD. In view of this, Zhou et al. developed a molten salt‐assisted CVD method and demonstrated that salt decreases the melting point of the reactants and increases the overall reaction rate, which is favorable for highly efficiently achieving various 2D TMDs (Figure 1.3c) [39]. Moreover, by introducing fluorine into the growth system, Liu et al. demonstrated that the growth rate of graphene, h‐BN, and WS2 could be greatly accelerated, which is beneficial for growing large domain‐size 2D materials [52].
Figure 1.3 CVD growth of 2D materials. (a) Schematic diagram of the CVD setup and the main growth process [37]. (b) Schematics of the CVD growth of MoS2 using arched oxidized Mo foil as precursor and optical images of the growth products with varied thickness.
Source: Reproduced with permission from Jingying Zheng et al. [38]/John Wiley & Sons.
(c) The transition metals and chalcogens used for molten salt‐assisted CVD growth and optical images of the different atomically thin TMCs.
Source: Reproduced with permission from Jiadong Zhou et al. [39]/Springer Nature.
(d) Schematic diagram of the multisource CVD setup and photograph of a monolayer MoS2 film at a 4 in. wafer scale.
Source: Reproduced with permission from Qinqin Wang et al. [40]/American Chemical Society.
Owing to the inherent difficulty in controlling the concentration of solid precursors during the entire growth time, it remains a challenge to prepare large‐area monolayer films with spatial homogeneity and structural continuity. Kang et al. adopted a metal‐organic chemical vapor deposition (MOCVD) technique, with Mo(CO)6, W(CO)6, (C2H5)2S, and H2 as independent gas‐phase precursors, to grow large‐area and uniform 2D TMD films [53]. They demonstrated that maintaining a low partial pressure at the Mo source by controlling the partial pressure is the key to the layer‐by‐layer growth and the formation of a wafer‐scale TMDs film. Furthermore, since the configuration of the CVD instrument may play an important role in 2D material growth, many studies have also focused on the optimization of the CVD system [54, 55]. For example, Wang et al. designed a multisource supply tube in the traditional CVD setup to ensure a sufficient and uniform precursor supply, and the c‐sapphire substrate was vertically placed in the growth chamber to avoid precursor concentration inhomogeneity along the gas flow direction. Using this CVD setup, they successfully obtained wafer‐scale MoS2 films with domain sizes up to 180 μm (Figure 1.3d) [40].
Although CVD‐growth of 2D materials has made great progress in the past decades, the controllable growth of 2D materials still faces great challenges in thickness control, wafer‐scale single‐crystal preparation, and phase modulation. Moreover, there is great development space to combine various 2D materials with distinct structures and properties to build 2D alloys and heterostructures to satisfy their various applications. The specific CVD growth strategies for the controllable preparation of 2D materials, including wafer‐scale single crystal, thickness control, and phase modulation, will be introduced in detail in the following sections.
1.4 CVD Growth of Wafer‐Scale Single Crystal 2D Materials
In most of the present works, amorphous SiO2/Si was primarily used as the substrate in CVD growth of 2D materials, which results in the random orientation of 2D grains [48]. Thus, CVD‐grown 2D films are usually polycrystalline with many grain boundaries, which seriously deteriorates their optical, electrical, mechanical, chemical, stability, etc. To improve these properties, it is highly desired to prepare 2D films with uniform thickness, high crystallinity, defect‐free, and limited grain boundaries, especially wafer‐scale single crystal 2D films [56]. Substrate‐guided epitaxial growth is widely utilized to achieve these 2D films. The idea is based on the effective control of the orientation of 2D materials grown on a substrate, where the grains merge seamlessly with the same (or small misorientation) angle, which is considered to be an essential way to decrease or eliminate the grain boundaries [57].
Most 2D TMDs have a threefold symmetry structure, thus the lattice‐induced epitaxial can be used to achieve wafer‐scale 2D films. So, the c‐plane sapphire, which has the same symmetry and compatible lattice constant with TMDs, has been widely used as a substrate to align the TMDs [40, 58]. However, the epitaxial relationship indicates that the [] direction of the c‐plane sapphire is perpendicular to the zig‐zag edge of TMD domains, which equalizes the two antiparallel domains and prevents the growth of single crystals. To solve the above problem, Wang et al. designed a c‐plane sapphire wafer with a major miscut angle toward the A axis (defined as C/A), which produced surface steps along direction for unidirectionally aligning MoS2, as shown in Figure 1.4a [59]. In principle, a good epitaxial relationship between MoS2 and c‐plane sapphire is formed (Figure 1.4b) when the armchair and zigzag directions of MoS2 are parallel to and of Al2O3, respectively. An OM image of as‐grown triangular MoS2 domains on c‐plane sapphire shows an obvious unidirectional alignment. Second harmonic generation (SHG) mapping of two adjacent MoS2 grains (Figure 1.4d) shows uniform intensity, suggesting the same grain orientations without an obvious grain boundary. By extending the growth time, they obtained wafer‐scale (2‐inch) monolayer MoS2 films that can be easily transferred to arbitrary substrates (Figure 1.4e,f). Zhang et al. reported the epitaxial growth of wafer‐scale MoS2 single crystal on sapphire with Mo foil and S powder as precursors [60]. The carbon cloth layer placed between Mo film and sapphire substrate ensures the homogenous and continuous Mo supply (Figure 1.4g), which enables uniform nucleation and growth of monolayer MoS2 film. In addition, the commensurability between MoS2 lattice and sapphire lattice and the guiding effect of sapphire step edges are proposed to direct the unidirectionally aligned growth of monolayer MoS2 single crystals. The aligned seed crystal can be seamlessly stitched to a continuous single crystal film by increasing growth time (Figure 1.4h). Moreover, Xu et al. reported an epitaxial growth of highly aligned MoS2 grains on a twofold symmetry a‐plane sapphire substrate [61]. The obtained MoS2 grains have an unusual rectangle shape with perfect orientation alignment along the direction of a‐plane sapphire. They found that high temperature is beneficial to the MoS2 seeds rotation to a favorable orientation, which is beneficial for the unidirectional alignment of MoS2.
Figure 1.4 CVD growth of wafer‐scale single‐crystal 2D materials. (a) Schematically show the epitaxial relationship between MoS2 grain and c‐plane sapphire with different step orientations.
Source: Taotao Li et al. [59]/Reproduced with permission of Springer Nature.
(b) The epitaxial relationship of MoS2 on c‐plane sapphire in atomic scale.
Source: Taotao Li et al. [59]/Reproduced with permission of Springer Nature.
(c) Unidirectional alignment of MoS2 domains on a C/A sapphire (0001) substrate.
Source: Taotao Li et al. [59]/Reproduced with permission of Springer Nature.
(d) Polarized SHG mapping of two merging MoS2 domains on the C/A sapphire (0001) substrate.
Source: Reproduced with permission from Taotao Li et al. [59]/Springer Nature.
(e, f) Photographs of 2‐inch monolayer MoS2 single crystal on C/A sapphire and SiO2/Si substrates.
Source: Taotao Li et al. [59]/Reproduced with permission of Springer Nature.
(g) Schematic illustration of the carbon‐cloth shield over sapphire template‐assisted growth of monolayer MoS2 single crystal.
Source: Pengfei Yang [60]/Reproduced with permission from John Wiley & Sons.
(h) Epitaxial growth of 2‐inch uniform monolayer MoS2 single crystal on sapphire.
Source: Reproduced with permission from Pengfei Yang [60]/John Wiley & Sons.
Many efforts have been devoted to grow wafer‐scale non‐centrosymmetric 2D materials by aligning unidirectional grains on single‐crystal substrates [62–64]. The aligned domains can be seamlessly stitched, and then the 2D film is formed. Liu et al. reported the growth of a wafer‐scale single crystal h‐BN monolayer on a vicinal Cu (110) substrate [64]. The Cu (110) vicinal surface, on which the presence of metal steps uniformly along the <211> direction leads to a C1 symmetry, is critical to prepare the unidirectional arrangement of the h‐BN monolayer. The step edges reduced the symmetry of the substrate, broke the energetical degeneracy of the antiparallel domains, and aligned the domain orientation. The h‐BN single crystal grains are nucleated near the step edges, with one edge of the grains tightly attached to the upward side of the step edge during the growth process, and the grains propagate rapidly on the plateau between neighboring step edges. Once one of its edges reached a neighboring step edge in the downward direction, the propagation of the edge was temporarily arrested, and finally spliced into a continuously single‐crystalline 2D film. Using this method, the unidirectional alignment of h‐BN grains can be achieved on a large scale.
Beside the substrate‐guided epitaxial growth strategy, a seed‐induced epitaxial growth strategy was also developed for controllable fabrication of wafer‐scale single crystal 2D TMDs films. For example, Ye et al. reported the synthesis of a large‐area 2H‐MoTe2 single crystal via solid‐to‐solid transition and recrystallization process, where a piece of 2H‐MoTe2 as a seed crystal was implanted into the 1T′‐MoTe2 film to trigger the in‐plane 2D‐epitaxy growth [65]. The resulting single‐crystalline film completely covered a 2.5‐cm wafer with excellent uniformity.
1.5 Thickness Control in CVD Growth of 2D Materials
As we all know, the properties of 2D materials are closely related to their layer numbers [9, 38]. For example, compared with the single layer, the bilayer TMDs have good electrostatic control, a smaller band gap, and higher mobility, which will improve the energy‐delay product of the transistor [66–69]. Hence, the preparation of high‐quality 2D films with controllable layers is another important project in this field. In the traditional CVD growth process, the continuous flow of carried gas in the same direction leads to the random growth of many small triangular grains (the second layer) on the surface of monolayer crystal (the first layer), which makes the controllable growth of uniform multiple TMD layers much more difficult [70]. Ostrikov et al. developed a reverse‐flow CVD growth strategy to prepare the uniform bilayer MoS2[71]. The detailed growth produced is shown in Figure 1.5a. Different from the traditional CVD method with constant gas flow direction during the entire growth process, here they introduced a reverse airflow in the variable temperature section (b–c section). This reverse gas flow reduces uncontrolled nucleation and promotes uniform epitaxy of the second monolayer from the active nucleation center on the first monolayer. This approach enables high‐quality, uniform bilayer MoS2 crystals with high yield, controllability, and reliability, and it provides a possible route for the subsequent large‐scale growth of 2D materials with controllable layers. Wu et al. successfully grew a controllable three‐layer MoS2 with high mobility and large single crystals on a sodium‐lime glass substrate by using a CVD strategy [75]. In addition, Zhang et al. used a similar reverse‐flow CVD growth strategy to inhibit the uncontrolled nucleation and thus achieved the highly robust epitaxial growth of various 2D heterogeneous structures and superlattices [51]. Li et al. reported the controlled growth of 2H‐stacked bilayer WSe2 by CVD growth on a c‐plane sapphire substrate with atomic steps, as shown in Figure 1.5b [72]. They demonstrate that the nuclei growth of bilayer WSe2 slides along the pronounced atomic steps induced by WSe2 crystals atop, resembling the graphoepitaxy mechanism.
Figure 1.5 Thickness control for CVD growth of 2D materials. (a) Schematic diagram of reverse‐flow CVD process for growing bilayer MoS2 and corresponding OM image of obtained bilayer MoS2.
Source: Xiumei Zhang [71]/Springer Nature/CC BY 4.0.
(b) Schematic diagram of the bilayer WSe2 epitaxy growth process on c‐plane sapphire and the corresponding AFM and OM images.
Source: Reproduced with permission from Ali Han et al. [72]/Reproduced with permission from Royal Society of Chemistry.
(c) Diagram of the CVD growth of wafer‐scale (4‐inch) MoSe2 in three‐temperature‐zone tube furnace.
Source: Jiawei Li et al. [73]/John Wiley & Sons/CC BY 4.0.
(d) Schematic diagram of layer‐by‐layer epitaxy of multilayer wafer MoS2 and corresponding photographs, STEM images (cross‐section), and Raman spectra line mapping images of the wafers.
Source: Qinqin Wang et al. [74]/Oxford University Press/CC BY 4.0.
Though great progress has been made in the growth of multilayer TMD grains, achieving wafer‐scale multilayer TMD films remains a big challenge. Wang et al. report the step‐induced uniform nucleation (>99%) of bilayer MoS2 on c‐plane sapphire [76]. According to DFT calculations, a bilayer nucleation with aligned edges is required before merging to obtain a uniform bilayer TMDs film, and the interfacial formation energy of bilayer MoS2 markedly decreases with step height. In an experiment, they explored the atomic terrace height on c‐plane sapphire to enable an edge‐nucleation mechanism and the coalescence of MoS2 domains into continuous, centimeter‐scale films. Moreover, Zhang's group devotes a great effect on the growth of wafer‐scale multilayer 2D TMDs. They firstly developed a multi‐channel source‐supply CVD strategy to grow wafer‐scale (4‐inch) single‐layer MoSe2 film in a three‐temperature zone tubular furnace [73]. In this work, they placed the sources in different temperature zones, with the sapphire substrate placed vertically in the third temperature zone. Especially, three small quartz tubes in the growth chamber act as containers for MoO3, each capable of independently transporting carrier gas. This multi‐channel design provides an even and continuous precursor supply, allowing uniform nucleation of MoSe2 across the entire wafer with high nucleation density and the ability to grow wafer‐scale monolayers in a short period of time. This work provides an important foundation for the subsequent wafer‐scale growth of multilayer TMD films. Zhang et al. report the layer‐by‐layer epitaxy process growth method for preparing high‐quality 4‐inch multilayer MoS2 wafers on sapphire substrate [74]. Firstly, monolayer MoS2 is prepared by domain‐domain coalescence in a multi‐channel oxygen‐enhanced CVD system. Then, additional epitaxial layers are grown on top of the first layer using the same technique to control the number of layers, resulting in multilayers of MoS2 with clean, sharp interface atoms. The layer‐by‐layer epitaxial growth process enables a well‐defined stack sequence with precise control over the number of layers, up to six. Furthermore, recent studies have demonstrated the successful synthesis of high‐quality wafer‐scale multilayer 2D films such as graphene and h‐BN by reasonably designing the CVD growth conditions and processes [77–79]. Above progresses on thickness‐controllable preparation of 2D materials provides the material foundation for exploring their thickness‐dependent properties and device applications. With the development of new CVD technology, it is believed that the thickness‐controllable preparation of many other 2D materials will be realized in the near future.
1.6 Phase Control in CVD Growth of 2D Materials
The phase multiplicity of 2D materials is pivotal for exploring their novel physical and chemical properties [80]. For example, most 2D TMDs (MoS2, WS2, WSe2, etc.) possess a stable 2H phase and a metastable 1T and 1T′ phase, which exhibit semiconducting and metallic properties, respectively [5]. Moreover, researchers found that the semiconducting 2H MoTe2 might be commendable in thermoelectricity [81], and the metallic 1T′ MoTe2 has extremely large magnetoresistance and quantum spin Hall effect [82, 83]. Interestingly, the hexagonal and tetragonal FeTe nanosheets were demonstrated to possess ferromagnetism and antiferromagnetism, respectively [84].
Phase engineering of 2D materials during the CVD growth process is much important to explore their various properties and device applications. One of the important approaches for phase engineering is to realize the phase transformation of existing 2D materials in a CVD system. Ye et al. reported a route for synthesizing wafer‐scale single‐crystalline 2H MoTe2 by in‐plane epitaxial tellurizing, which was triggered by a deliberately implanted single seed crystal [65]. Hu et al. realized a large‐scale selective growth of the 1T′/2H/1T′ MoTe2 multiphase structure, with the 1T′ and 2H phases seamlessly stitched [85]. The various phase transformation methods not only provide convenient and effective approaches readily applicable in many applications but also play critical roles in understanding the fundamentals of how crystal phases impact their properties.
Compared with the phase transformation method, the direct synthesis of 2D materials with distinct phase structures is more favorable to achieve a high‐purity phase structure. CVD growth has great potential in the phase‐controllable synthesis of 2D materials because of its diverse growth parameters (temperature, precursor, carrier gas, composition, etc.) [86]. For example, Jiao et al. reported the phase‐selective growth of 1T′ and 2H MoS2 monolayers and 1T′/2H heterophase bilayers using a potassium (K)‐assisted CVD method [87]. This was realized by using K2MoS4 as a precursor and tuning the concentration of K in the growth products to invert the stability of the 1T′ and 2H phases. In Figure 1.6a, Xu et al. developed a facile CVD method to synthesize high‐quality MoxRe1−xS2 alloys with tunable composition and phase structure [88]. The 1T′ phase MoxRe1−xS2 alloys were obtained for x in the range of 0–0.25, while the 2H phase MoxRe1−xS2 alloys were achieved for x in the range of 0.75–1. Liu et al. selectively synthesized the hexagonal phase and the tetragonal phase FeTe nanosheets on SiO2/Si by controlling the growth temperature during the CVD process (Figure 1.6b) [84]. The phase‐controllable growth of FeTe originates from the formation energy difference between the hexagonal and tetragonal phases, and maintaining a relatively high temperature is essential for obtaining the thermodynamically stable hexagonal phase, while a low temperature is favorable to the tetragonal phase. Most recently, Zhao et al. synthesized both pure β and β′ In2Se3 by means of controlling whether to add InSe into the In2O3 precursor [89]. Using DFT calculations and in situ TEM experiments, they confirm that the Se deficiency triggers the β → β′ phase transition, which effectively explains the seeding effect of InSe additive in CVD precursors for the β′‐phase growth. The above results demonstrate the feasibility of CVD synthesis of large‐area, highly crystalline 2D materials with controllable phase structures, which is highly desirable for their promising wide applications.
Figure 1.6 Phase‐tunable synthesis of 2D materials. (a) Schematic illustration of the structure, phase, and energy band engineering of MoxRe1−xS2 alloys, the atomic structure of 2H and 1T′ MoxRe1−xS2 alloys, and the ADF‐STEM images of 1T′ and 2H MoxRe1−xS2 alloys.
Source: Reproduced with permission from Qixin Deng et al. [88]/John Wiley & Sons.
(b) Schematic view for the temperature‐modulated phase selective growth process of FeTe, the OM images (scale bar: 20 μm) and atomic‐resolution STEM‐ADF images (scale bar: 5 nm) of tetragonal and hexagonal FeTe nanosheets.
Source: Lixing Kang et al. [84]/Springer Nature/CC BY 4.0.
1.7 Summary and Prospect
Over the past two decades, the preparation of 2D materials has made tremendous progress, which greatly promotes the fast development of the 2D field. To realize the high‐efficiency synthesis of 2D materials with controllable thickness, domain size, crystal quality, and phase structures, researchers have developed a series of preparation approaches, including mechanical exfoliation, LPE, and CVD. The various preparation methods possess their own advantages and disadvantages, and thus the obtained 2D materials exhibit distinct features that fulfill the requirements of their diverse applications. In brief, the mechanical cleavage method can prepare most of the target 2D materials with high crystal quality, but the samples acquired via this approach also possess several problems, such as irregular morphology, uncontrollable thickness, small domain size, and low yield. The corresponding 2D materials are primarily used to explore their fundamental properties. LPE is suitable for large‐scale production at a low cost, but precise control of size and layer number with preservation of pristine quality is still highly challenging. The obtained 2D materials exhibit prominent superiorities in some special applications, such as lithium‐ion batteries, catalysis, printer ink, and composite materials. Bottom‐up synthesis via CVD growth has emerged as a versatile and scalable approach enabling precise control over the thickness, morphology, crystallinity, and phase structure, which provides significant opportunities for exploring their fundamental physics and device applications.
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