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FEATURE ARTICLEwww.afm-journal.de
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1706587 (1
of 28)
2D Layered Material-Based van der Waals Heterostructures for
Optoelectronics
Xing Zhou, Xiaozong Hu, Jing Yu, Shiyuan Liu, Zhaowei Shu, Qi
Zhang, Huiqiao Li, Ying Ma, Hua Xu,* and Tianyou Zhai*
Van der Waals heterostructures (vdWHs) based on 2D layered
materials with selectable materials properties pave the way to
integration at the atomic scale, which may give rise to fresh
heterostructures exhibiting absolutely novel physics and
versatility. This feature article reviews the state-of-the-art
research activities that focus on the 2D vdWHs and their
optoelectronic applications. First, the preparation methods such as
mechanical transfer and chemical vapor deposition growth are
comprehen-sively outlined. Then, unique energy band alignments
generated in 2D vdWHs are introduced. Furthermore, this feature
article focuses on the applications in light-emitting diodes,
photodetectors, and optical modula-tors based on 2D vdWHs with
novel constructions and mechanisms. The recently reported novel
constructions of the devices are introduced in three primary
aspects: light-emitting diodes (such as single defect
light-emitting diodes, circularly polarized light emission arising
from valley polarization), photodetectors (such as
photo-thermionic, tunneling, electrolyte-gated, and broadband
photodetectors), and optical modulators (such as graphene
integrated with silicon technology and graphene/hexagonal boron
nitride (hBN) heterostructure), which show promising applications
in the next-generation optoelectronics. Finally, the article
provides some conclusions and an outlook on the future development
in the field.
DOI: 10.1002/adfm.201706587
Dr. X. Zhou, X. Z. Hu, J. Yu, Z. W. Shu, Dr. Q. Zhang, Prof. H.
Q. Li, Prof. Y. Ma, Prof. T. Y. ZhaiState Key Laboratory of
Material Processing and Die & Mould TechnologySchool of
Materials Science and EngineeringHuazhong University of Science and
Technology (HUST)Wuhan 430074, P. R. ChinaE-mail:
[email protected]. S. LiuState Key Laboratory of Digital
Manufacturing Equipment and TechnologyHuazhong University of
Science and Technology (HUST)Wuhan 430074, P. R. ChinaProf. H.
XuKey Laboratory of Applied Surface and Colloid ChemistryMinistry
of EducationSchool of Materials Science and EngineeringShaanxi
Normal UniversityXi’an 710119, P. R. ChinaE-mail:
[email protected]
The ORCID identification number(s) for the author(s) of this
article can be found under
https://doi.org/10.1002/adfm.201706587.
1. Introduction
Graphene has ignited intensive attention since it was
mechanically exfoliated in 2004[1] due to the high carrier
mobility,[1,2] ultralarge specific surface area,[3] high in-plane
thermal conductivity and rela-tively low out-of-plane value,[4–7]
and relatively low Young’s modulus[8] which inspire a wide range of
promising appli-cations such as ultrafast high-frequency
photodetectors,[9–11] transparent conduc-tive electrodes,[12] and
broadband optical modulators.[13] However, the research of graphene
has been severely hampered due to the absence of a bandgap, which
results in a small current on/off ratio for graphene
transistors.[14] Thus, other 2D layered materials (2DLMs) with
varying bandgaps[15] including semimetals (such as WTe2[16–18]),
topological insulators (such as Pb1−xSnxTe,[19] Bi2Te3[20,21]),
semiconduc-tors (such as black phosphorous (BP),[22–24]
MoS2,[25–31] WS2,[32–36] WSe2[37,38]), insula-tors (such as boron
nitride (BN)[39–42]). Dif-ferent from gapless graphene, these 2DLMs
possess bandgaps in a wide range and can
also be modulated with the changing thickness, which have
trig-gered tremendous interest in many fields such as field effect
transistors,[30,43–46] photodetectors,[47–53] flexible
devices.[54–58]
van der Waals heterostructures (vdWHs) based on these 2DLMs with
selectable materials properties pave the way to integration at the
atomic scale which may give rise to fresh heterostructures
exhibiting absolutely novel physics and versa-tility.[59–62]
Generally, these 2DLM-based vdWHs could be real-ized by mechanical
transfer or chemical vapor deposition (CVD) growth.[63–65] Compared
with the conventional semiconductor-based heterostructures which
require the severely similar lattice structures of the component
semiconductors, vdWHs can release the strict lattice mismatching
requirement due to the weak inter-action between the adjacent
layers.[66,67] Furthermore, the inter-face can be atomically sharp
and the thickness can be as thin as a few atomic layers, and the
stacking sequence can be artifi-cially arranged to obtain novel
physical properties. Thus, as the extending field of 2D materials,
vdWHs has been growing fast.
Herein, we review the recent progress of 2D vdWHs, and mainly
focus on the preparation methods, energy band alignments of two or
more stacked 2DLMs, and the optoelectronic
2D Materials
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applications. First, we summarize the preparation methods of
vdWHs including mechanical transfer and CVD growth. Then, the novel
energy band alignments of vdWHs are discussed including the
interlayer coupling and the exciton dynamics of interlayer
transition. The optoelectronic applications are dis-cussed in
detail for the 2D vdWHs-based light-emitting diodes (LEDs),
photodetectors, and optical modulators with different
constructions. Finally, the conclusions and outlook of 2D vdWHs are
presented.
2. Preparation Methods of vdWHs
Up to now, reliable preparation methods of 2D vdWHs are of great
significance for further investigation and applications. Mechanical
transfer[68–72] and CVD growth[63,65,73] are the most used methods
for preparing 2D vdWHs. In the following con-text, we will focus on
these two methods.
2.1. Mechanical Transfer
Mechanical transfer is one of the most commonly used methods to
fabricate the 2D vdWHs once the few-layer or monolayer mate-rials
were prepared by mechanical exfoliation from their bulk
counterparts or CVD growth. Generally, it is easy to construct the
different stacking orders of 2D vdWHs artificially as shown in
Figure 1a. First, the atomic layers prepared by mechanical
exfo-liation or CVD growth are transferred onto the targeted
substrate (such as SiO2/Si). Next, the second atomic layer can be
either dry exfoliated or wet transferred on a sacrificial polymer
such as poly(methyl methacrylate) (PMMA). Then, the atomic layer
with the PMMA is transferred onto a transparent stamp such as
poly(dimethylsiloxane) (PDMS), and is located on the desired
position employing the micromanipulators under the objective lens,
and lowered down until the two atomic layers contacting to form
vdWH. Then, the polymers can be directly dissolved in
sol-vents.[74] However, the dissolved polymers will leave the
residue on the surface of 2D materials, hindering further stacking.
Thus, to realize the complex multilayer stacking artificially
without res-idue between the individual layers, more strategies
such as pick-up have been reported.[75] First, a silicon substrate
is coated with poly(propylene carbonate) (PPC), and one kind of
target crystals is mechanically exfoliated onto the PPC film. Then,
the PPC film is transferred onto a piece of PDMS with the
exfoliated flake-side up. The PDMS is then fixed to a glass slide.
On the other hand, flakes of other target materials are exfoliated
on different silicon substrates. Then, the vdWHs can be realized by
picking up dif-ferent layers one by one assisted by the PPC film
without residue left between the individual layers. The glass slide
and PDMS can be separated from the vdWH with PPC after heating to
90 °C via softening the PPC,[75] with the 2D vdWH left on the
substrate. Finally, the PPC or PMMA can be removed in chloroform or
acetone, leaving the 2D vdWHs on the substrate. The advanced
pick-up method results in clean interfaces and allows the stacking
orders or crystal orientation of these 2DLMs to be adjusted
artifi-cially which may result in novel physical properties. For
example, a vdWH based on monolayer MoS2 and WSe2 is demonstrated in
Figure 1b, and the electron diffraction pattern of the
hetero-bilayer
Xing Zhou received his B.S. degree in inorganic non-metallic
materials from the Wuhan University of Science and Technology
(WUST) in 2012, and then received his Ph.D. degree in Materials
from the School of Materials Science and Engineering, Huazhong
University of Science and Technology (HUST) in 2017. Currently, he
is an assistant professor
in the School of Materials Science and Engineering at the HUST.
His research concentrates on the controllable syn-thesis of 2D
group IV–VI semiconductors and heterostruc-tures via CVD methods
for electronic and optoelectronic applications.
Hua Xu received his B.S. degree in chemistry from the Ningxia
University in 2007, and then received his Ph.D. degree in organic
chemistry from the Lanzhou University under the joint supervision
of Prof. Haoli Zhang and Prof. Jin Zhang (Peking University) in
2012. He then worked as an Associate Professor at the
School of Materials Science and Engineering, Shaanxi Normal
University (SNNU). His research interest is focused on the design,
synthesis, and characterization of 2D nanomaterials for promising
applications in electronic, optoelectronic, and new energy
devices.
Tianyou Zhai received his B.S. degree in chemistry from the
Zhengzhou University in 2003, and then received his Ph.D. degree in
physical chemistry from the Institute of Chemistry, Chinese Academy
of Sciences (ICCAS) in 2008. Afterward, he joined the National
Institute for Materials Science (NIMS) as a JSPS postdoctoral
fellow,
and then as an ICYS-MANA researcher within the NIMS. Currently,
he is a Chief Professor of School of Materials Science and
Engineering, Huazhong University of Science and Technology (HUST).
His research interests include the controlled synthesis and
exploration of fundamental physical properties of inorganic
functional nanomaterials, as well as their promising applications
in energy science, electronics, and optoelectronics.
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WSe2/MoS2 along the [001] zone axis in Figure 1c demonstrates
that in this specific hetero-bilayer structure, the two hexagonal
reciprocal lattices are rotated by 12.5° with respect to each layer
without obvious lattice strain, resulting in moiré fringes with a
spatial periodicity on the order of four to six times the lattice
con-stants of each layer.[76] The atomically sharp interface of the
het-erostructure can be obtained by this stacking process confirmed
by the high-resolution cross-sectional scanning transmission
elec-tron microscope image of the heterostructure (Figure 1d).
Fur-thermore, the complex 2D vdWHs with more stacking layers can be
realized by this mechanical transfer process.
Mechanical transfer process provides a lot of flexibility in
constructing diverse 2D vdWHs with various materials which may give
rise to fresh physical properties, it is not scalable, which is
imperative for further practical applications in elec-tronics and
optoelectronics. Alternatively, the bottom-up method such as direct
CVD synthesis of 2D vdWHs has been successful in synthesizing
graphene- or transition metal dichal-cogenide (TMD)-based
vdWHs,[77–79] which shows promising applications in scalable
production.
2.2. CVD Growth
CVD growth has shown booming development in the last dec-ades
such as the CVD growth of graphene[80,81] and TMDs,[82,83] and has
been employed for synthesizing 2D vdWHs recently.[84] The most used
method for CVD growth of 2D vdWHs is that evaporating the target
sources such as WS2, WSe2, MoS2,
MoSe2. Xu and co-workers[65] employed the mixture of WSe2 and
MoSe2 powder as sources and obtained MoSe2/WSe2 lat-eral
heterostructures at a growth temperature of 950 °C with the system
pressure maintained at ≈7 Torr. Then, Duan et al.[85] synthesized
WS2/WSe2 lateral heterostructures through evap-orating WS2 at 1057
°C and followed by evaporating WSe2 at 1190 °C while under ambient
pressure. However, these methods via evaporating the source
materials usually need much high temperatures, which may make the
reaction condi-tions uncontrollable. Ajayan and co-workers[73] used
S, W, MoO3 powders as S, W, Mo precursors, respectively, and with
the addition of Te powder to accelerate the melting of W powder
during the growth as shown in Figure 2a, thus the growth
tem-perature was decreased. Furthermore, the precise temperature
and the different nucleation and growth rates determine the final
products: vertical heterostructures dominate at ≈850 °C (Figure
2b,c), while lateral heterostructures are preferred at ≈650 °C
(Figure 2d,e). These different heterostructures modu-lated by
temperatures mainly are related with the nucleation and growth rate
of each layer. At low temperatures (650 °C), nucleation and growth
of WS2 are extremely difficult and slow. Attaching WS2 to the MoS2
edge with strong chemical bonding, however, results in much smaller
nucleation energy than on the surface of MoS2, which leads to
in-plane heterostructure, a kinetic product preferred at low
temperatures. At high tempera-tures (850 °C), the environment would
provide enough energy to overcome the nucleation barrier. In this
case, the kinetic effect would not be critical and the
thermodynamically more stable product becomes preferable. Thus, the
WS2/MoS2 bilayer
Adv. Funct. Mater. 2018, 28, 1706587
Figure 1. a) Schematic illustration of the transfer process for
2D vdWHs. b) Optical microscope image of a WSe2/MoS2 hetero-bilayer
on a Si/SiO2 substrate (260 nm SiO2). c) High resolution
transmission electron microscopy (HRTEM) images of a boundary
region of monolayer MoS2 and the hetero-bilayer, showing the
resulting Moiré pattern. d) The electron diffraction pattern of the
hetero-bilayer shown in (b), with the pattern of MoS2 and WSe2
indexed in green and blue colors, respectively. (b–d) Reproduced
with permission.[76] Copyright 2014, National Academy of Sciences
of the United States of America. e) High-resolution Scanning
transmission electron microscopy (STEM) image of the same
heterostructure, consisting of four layers of MoS2 and WSe2. Right:
Electron dispersion X-ray spectroscopy (EDS) mapping of the
heterostructure. Reproduced with permission.[133] Copyright 2015,
American Chemical Society.
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heterostructure is preferred at higher temperatures. These
different structures induce novel physical properties in the 2D
vdWHs as shown in Figure 2f. The photoluminescence (PL) spectra
acquired from the monolayer region (points 1 and 2 in Figure 2c)
exhibit a strong peak at the wavelength of 680 nm, indicating the
1.82 eV direct excitonic transition energy in monolayer MoS2, while
three main peaks are observed at wave-lengths of 630, 680, and 875
nm, respectively (points 3 and 4). The peaks at 630 nm (1.97 eV)
and 680 nm (1.82 eV) are attrib-uted to the direct excitonic
transition energies in WS2 and MoS2. The comparable intensity of
the peak at 875 nm to that of its individual monolayer components
observed in the bilayer sample indicates a possible direct
excitonic transition at this energy range, which suggests that the
coupling between WS2 and MoS2 results in the unprecedented direct
bandgap with reduced energy. Figure 2g shows the atomic-resolution
Z-con-trast image from a step edge of the vertical heterojunction.
The alternative bright and dark atomic column arrangement in the
hexagonal lattice indicates that the as-synthesized vertical
WS2/MoS2 heterostructure presents the 2H stacking, where the bright
W and dark Mo atoms are aligned with S2, respectively.
However, the W and Mo substitutions can be found in the other
side occasionally with this one-step CVD growth, means the big
discount of the sharp interface. Thus, there are several attempts
on two-step CVD growth of 2D vdWHs which may protect the atomically
sharp interface at the junction. The most studied are
graphene- and TMD-based 2D vdWHs realized by two-step CVD
growth.[77,79,86–94] Li and co-workers[63] realized the lateral
WSe2/MoS2 heterostructure with high quality and sharp interface,
where WSe2 is grown on substrates through vdW epitaxy fol-lowed by
the edge epitaxial growth of MoS2 along the W growth front as shown
in Figure 3a. Two-step growth promises precise control to the
atomically sharp interface. The as-synthesized WSe2/MoS2 lateral
heterostructures are clearly observed from the optical image in
Figure 3b, showing the clear domain and uniformity. Besides, the
atomically sharp interface between the WSe2 and MoS2 is observed in
Figure 3c. Furthermore, Zhai and co-workers[95] synthesized
SnSe2/MoS2 vertical heterostructures using monolayer MoS2 triangles
as templates, with the top SnSe2 nearly covering the bottom MoS2 as
shown in Figure 3d. As dem-onstrated in Figure 3e, the
atomic-resolution Z-contrast image from the edge of the
heterostructure shows the top SnSe2 and bottom MoS2 with highly
symmetric crystallographic directions matching well with the atomic
models in the inset of Figure 3e, which indicates the 2H phase of
both SnSe2 and MoS2. Then, the Raman spectroscopy is employed to
evaluate the crystal struc-tures and vibrational properties as
shown in Figure 3f. The main peaks of SnSe2 (Eg ≈ 110 cm−1, A1g ≈
185 cm−1) and MoS2 (E2g1 ≈ 385 cm−1, A1g ≈ 405 cm−1) are all
observed. However, the Eg peak of SnSe2 shows absolutely redshift
of 6 cm−1 (may be induced by the built-in strain resulting from the
large lattice mismatch of these two components), whereas the A1g
peak shifts only 1 cm−1
Adv. Funct. Mater. 2018, 28, 1706587
Figure 2. a) Schematic diagram of the synthesis for
heterostructures. b,c) Schematic, optical images of the vertically
stacked WS2/MoS2 heterostructure. d,e) Schematic, optical images of
the WS2/MoS2 in-plane heterostructure. f) PL spectra taken from the
four points marked in (c). g) Z-contrast image of the step edge of
the WS2/MoS2 bilayer. The green dashed line indicates the step
edge, and the two triangles indicate the orientation of the MoS2
(top part of image) and WS2 (bottom part) layers. Inset: Fast
Fourier transform of the Z-contrast image showing only one set of
diffraction patterns. Reproduced with permission.[73] Copyright
2014, Nature Publishing Group.
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(may be induced by the strong electron–phonon coupling at the
interface of the junction). These reported graphene- and TMD-based
heterostructures are limited to those materials with sim-ilar
superlattices, thus it is still in great challenge to synthesize a
variety of different vdWHs, especially for those possessing
incommensurate superlattices, which may lead to unique band
alignments or structures. For example, many nonlayered mate-rials
such as CdSe,[96] CdTe,[97] PbS[98–100] exhibit excellent
opto-electronic properties. Thus, combination of such nonlayered
materials with layered materials may construct a new type of 2D
vdWHs to offer fresh platform for applications in electronics and
optoelectronics.[101–103] Hu and co-workers[101] synthesized
CdS/MoS2 2D vdWHs through epitaxial growth with the CdS nanoplates
distributed on the MoS2 triangles uniformly. Fur-thermore, He and
co-workers[102,103] fabricated nonlayered PbS–graphene (Figure 3g)
and PbS–MoS2 heterostructures with edge contacts along the [110]
direction (Figure 3h,i). As shown in Figure 3j, the Mo atoms of the
zigzag edge of MoS2 are bonded with the S atoms chain exposed to
PbS (110). A high activity of
the edge area results from the abundant unsaturated Mo atoms
along the edge of MoS2, which increase the possibility that the PbS
nanoplates are primarily nucleated at the edge.
As mentioned above, the construction of vdWHs via mechan-ical
transfer process is not limited by conventional lattice-matching
constraints. Thus, it provides a great deal of flexibility in
fabri-cating various kinds of vdWHs integrated with diverse
materials artificially, which may induce disparate properties.
However, the thickness and size of the vdWHs fabricated by
mechanical transfer process are usually uncontrollable, and the
efficiency of constructing vdWHs fabricated by mechanical transfer
process is too low. Besides, residues at the interface of the vdWHs
fabri-cated by mechanical transfer process are usually unavoidable,
which may impede the properties of the vdWHs. Thus, it is not
suitable and scalable for industrial integration. In contrast, CVD
growth[104,105] has been proposed as an alternative way to
synthe-size single-crystalline 2D semiconductors, due to the
advantages over the precise control on morphology, defects, and
structure of final products, particularly on large-area growth of
2D materials
Adv. Funct. Mater. 2018, 28, 1706587
Figure 3. a) Schematic illustration of the sequential growth of
the monolayer WSe2–MoS2 in-plane heterostructure. b) Optical image
of the WSe2–MoS2 heterostructure. c) High-resolution STEM image
taken from the WSe2–MoS2 in-plane heterostructure. (a–c) Reproduced
with permission.[63] Copyright 2015, American Association for the
Advancement of Science. d) Schematic of SnSe2/MoS2 heterostructures
by epitaxial growth. e) An atomic-resolution Z-contrast image from
the edge of the triangle. f) Raman spectra of single SnSe2, MoS2,
and the SnSe2/MoS2 heterostructure. (d–f) Reproduced with
permission.[95] Copyright 2017, Institute of Physics. g) Scanning
electron microscopy (SEM) image of epitaxial PbS nanoplates
heterostructure on graphene/SiO2/Si substrate. The right image
shows that the PbS nanoplates grows along edge of graphene ribbon
with orientation as shown in the inset. (g) Reproduced with
permission.[102] Copyright 2016, John Wiley & Sons, Inc. h,i)
Schematic, optical image of the PbS nanoplates–MoS2
heterostructures. j) Schematic image of edge contact between the
MoS2 and PbS. Inset is a top view of the schematic. Reproduced with
permission.[103] Copyright 2016, American Chemical Society.
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such as graphene,[80,81] TMDs.[82,83] Also, it has been employed
for synthesizing 2D vdWHs recently.[84] The size, morphology, and
thickness can be precisely adjusted by the gas flux, mass of
precur-sors, temperature of reaction, substrate, and so
on.[106–110] Thus, CVD growth is highly promising for fabricating
vdWHs in large-scale. Whereas, CVD growth is usually limited by
highly sensitive growth conditions for each 2DLM, which makes it
difficult to mix and match high-quality atomic layers without
damaging the inter-face such as the atom diffusion. Thus, CVD
growth of vdWHs is still at the initial stage and remains a great
challenge for the industrial integration.
3. Energy Band Alignments of vdWHs
The vdW interactions between the adjacent layers at the
inter-face are weak, while their electron orbitals extend to each
other
and affect the electronic band structures in each
layer.[111–177] The interlayer coupling between two vdW-stacked 2D
layers can be modulated from noninteraction to strong interaction,
resulting in novel physical properties.
Graphene has ultrahigh carrier mobility (≈104 cm2 V−1 s−1)[1]
due to the linear dispersion of the Dirac electrons. However, the
applications in transistors have been impeded by the zero bandgap
of graphene.[49,178] Thus, the vdWHs based on gra-phene have leaded
many booming research fields, which may make up for the shortage of
the zero bandgap of graphene. For example, random rotational
orientation between the gra-phene and hBN lattices will be induced
when constructing gra-phene on hBN devices. This rotation between
the lattices and the longer lattice constant of hBN result in
topographic moiré patterns as shown in Figure 4a–c.[179] This moiré
pattern acts as a weak periodic potential and thereby results in
the emer-gence of a new set of Dirac points in Figure 4d.[179] This
result
Adv. Funct. Mater. 2018, 28, 1706587
Figure 4. a–c) Scanning tunneling microscopy (STM) topography
images showing 2.4 nm (a), 6.0 nm (b), and 11.5 nm (c). d)
Experimental dI = dV curves for two different moiré wavelengths,
9.0 nm (black) and 13.4 nm (red). The dips in the dI = dV curves
are marked by arrows. (a–d) Reproduced with permis-sion.[179]
Copyright 2012, Nature Publishing Group. e) Band alignment of
monolayer semiconducting TMDs and monolayer SnS2. Conduction band
min-imum (CBM) and Valence band maximum (VBM) calculated by
Perdew-Burke-Ernzerhof (PBE) spin-orbital coupling (SOC) are
indicated by the filled gray columns, with G: the Green function of
electron, W: the screened Coulomb potential (GW) corrected band
edges indicated by the narrower olive columns. Reproduced with
permission.[156] Copyright 2013, American Institute of Physics.
f,g) Charge densities of VBM (f) and CBM (g) states for the
monolayer WX2–MoX2 lateral heterostructures with common X.
Reproduced with permission.[157] Copyright 2013, American Institute
of Physics. h) Schematic, optical images of monolayer and bilayer
MoS2 with different twist angles. i) Calculated values for the
Kohn–Sham K-valley direct bandgap (orange) and indirect bandgap
(dark yellow) for the energetically favorable structures at each
twist angle. (h, i) Reproduced with permission.[181] Copyright
2014, Nature Publishing Group.
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indicates that the electronic structures of graphene can be
modulated by stacking it on hBN layers and the density of states in
graphene layer for the new Dirac point can be determined by the
mismatch between the graphene and hBN layers. The electronic
structures can also be modulated by the stacking of different TMDs.
Several groups have demonstrated that mon-olayer MoX2–WX2 (X = S,
Se, or Te)-based vdWHs have type-II band alignments by theoretical
calculations and experiments (Figure 4e).[117,157,173,180] Because
the optically active states of the conduction minimum and valence
maximum bands are local-ized on opposite layers, the lowest energy
electron–hole pairs are spatially divided, which is beneficial for
the applications in solar energy conversion and optoelectronics
(Figure 4f).[157] Li and co-workers[173] have experimentally
verified that the MoS2–WSe2 heterostructure shows the conduction
and valence band offsets of 0.76 and 0.83 eV, respectively, which
suggests a type-II band alignment. On the contrary, Cho and
co-workers[156] found that vdWHs based on monolayer n-type MX2 (M =
Mo, W; X = Se, Te) and p-type MX2 (M = Zr, Hf; X = S, Se) are
calculated to be promising couples realizing broken gap junctions
with excel-lent electron tunneling efficiencies, which is of great
interest for low-power logic devices. Monolayer 2D TMDs such as
MoSe2, MoS2, WSe2, WS2 are direct bandgap semiconductors,[118]
while it transforms to indirect bandgap semiconductors as the layer
number increases due to the Γ point to an intermediate state (Γ–Q)
becoming non-negligible.[181] Furthermore, the electronic
properties of the vdW-stacked bilayer homostructure can be
mod-ulated by changing the interlayer distances or twisting the
layers. Wang and co-workers[181] realized MoS2 bilayers with
different twist angles as shown in Figure 4h, and they found that
the indi-rect bandgap size varies evidently with the twist angles:
it shows the largest redshift for AB (S atoms on top of the S atoms
of the bottom layer)- and AA (S atoms on top of the Mo atoms of the
bottom layer)-stacked bilayers, while a significantly smaller and
constant redshift for all other twist angles (Figure 4i).
Raman, PL, and absorption spectra, and exciton dynamics are the
optimum methods to probe the optical properties of vdWHs due to the
efficient, accurate, and nondestructive measure-ments.[182–196]
Javey and co-workers[76] fabricated vdWH based on monolayer WSe2
and MoS2 and a distinguished PL peak at 1.55 eV (Figure 5a) was
observed, which is lower than both the exci-tonic PL peaks at 1.87
and 1.64 eV for the mono layer MoS2 and WSe2, respectively.
Furthermore, the absorption spectrum of the vdWH exhibits two
absorption peaks at 1.91 and 1.65 eV in accord-ance with the
absorption peaks of individual monolayer MoS2 and WSe2,
respectively (Figure 5b). Interestingly, the vdWH shows a prominent
shift of ≈100 meV between the absorbance and PL peaks in Figure 5b,
and this large Stokes-like shift confirms the spatially indirect
transition in a staggered gap (type-II) heterostruc-ture. When
illuminating light on the heterostructure, the photo-excited
excitons relax at the interface, driven by the band offset in
Figure 5c. The PL excitonic peak energy is lower than the excitonic
bandgaps of each material due to the energy lost to the band
offset. They also found that the interlayer coupling can be
effectively modulated by inserting dielectric hBN layers into the
vdW gap.
Furthermore, Shin and co-workers[187] systematically
investi-gated the interlayer coupling between WSe2- and MoSe2-based
vdWHs with different twist angles. As illustrated in Figure 5d, Se
atoms of top layer are on top of the Se atoms of the bottom
layer for θ = 0°, and Se atoms of top layer are on top of the
metal atoms of the bottom layer for θ = 60°. They found that the
intensities of the PL excitonic peaks of the hetero-bilayer are one
order of magnitude weaker than those of each mate-rial and the
peaks show slightly redshift. The PL quenching in the
hetero-bilayer may result from the decrease of PL quantum yield in
the case of bilayer systems and the redshift of the vdWH may be
attributed to the changes in the band structure. More
interestingly, a new peak at ≈1.35 eV was observed, which may be
related to interlayer excitons based on band alignment in Figure
5e. Generally, the d orbitals of W and Mo dominate the energy
levels of WSe2 and MoSe2. The energy of 4d orbital of Mo is lower
than that of 5d orbital of W, thus the valence band and conduction
band of MoSe2 are lower than those of WSe2, resulting in type-II
band alignment between the hetero-bilayer system. Under light
illumination, electron–hole bound pairs (excitons) are generated in
individual WSe2 and MoSe2. The energy levels of the excitons
located between the conduc-tion band and the valence band in each
layer are due to their less energy than the unbound electrons and
holes. Then, the photoinduced electrons and holes are separated,
and migrated to the conduction band of MoSe2 and the valence band
of WSe2, respectively. Consequently, the holes in the valence band
of WSe2 and the electrons in the conduction band of MoSe2
recombined to form interlayer excitons, resulting in interlayer
excitonic emission. Furthermore, they found that the PL inten-sity
of the interlayer excitonic emission reached the maximum at 0° and
60° and reduced at other twist angles (Figure 5f). The
hetero-bilayer system possesses highly symmetric stacking
con-struction with strong interlayer coupling at 0° and 60°, thus
high charge transfer efficiency could be realized due to the
minimum interlayer distance, resulting in the higher PL inten-sity.
This result provides a new degree of freedom to modulate the
optical properties of vdWHs with rich functionalities.
Many vdWHs based on TMDs form type-II band
alignments,[111,118,180] resulting in highly efficient
electron–hole separation, which is beneficial for light harvesting
and detecting.[197,198] Thus, exciton dynamics using pump–probe
technique is booming development to probe the charge transfer
process. Wang and co-workers[196] observed the ultrafast charge
transfer in the photoexcited monolayer MoS2/WS2 heterojunction by
employing both femtosecond pump–probe and PL mapping spectroscopy
(Figure 5g). They found dramatical quenching effect of PL spectrum
at the heterojunction compared with that at the individual single
layer material (Figure 5h), suggesting the high efficiency of
interlayer charge transfer. They further investigated the transient
absorption spectra of the MoS2–WS2 heterojunction from 2.0 to 2.5
eV (Figure 5i), and determined a hole transfer time of ≈50 fs from
MoS2 to WS2 layer. Such ultrafast charge transfer in vdWHs can
promise the applications in photodetectors and solar energy
conversion. Zhao and co-workers[199] confirmed the coherent nature
of interlayer charge transfer in a trilayer of MoS2–WS2–MoSe2
heterostructure. Excited electrons in MoSe2 transfer to MoS2 in 1
ps at room temperature without accumulation in the middle WS2
layer, which indicates a coherent electron transfer pro-cess. Not
only that, the WS2 layer separated the electron–hole pairs and
extended their lifetime to ≈1 ns. This trilayer vdWH configu-ration
with long carrier lifetime and efficient charge transfer may
provide new applications in electronic and optoelectronic
devices.
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4. Optoelectronic Applications of vdWHs
Since the tunable band alignments and strong light–matter
interactions, the vdWHs provide a new platform for the
appli-cations in optoelectronics.[200–207] In this part, we will
focus on those recently reported LEDs, photodetectors, and optical
modulators with novel constructions and mechanisms different from
those conventional devices.
4.1. Light-Emitting Diodes
Monolayer TMDs are promising candidates for light emit-ting due
to their direct bandgaps from visible to near-infrared
range.[208,209] Electroluminescence (EL) can be observed in
monolayer MoS2-based transistor, which may result from the same
excited state (exciton A).[210] However, the EL emission limited at
the metal contacts results in low quantum efficiency (10−5 for
monolayer MoS2). Constructing p–n diode (LED) is an effective way
to improve the EL efficiency. There are already lots of reports on
the LEDs based on 2D heterostructures, showing high performances as
summarized in Table 1. The lateral p–n junctions in monolayer TMDs
by the dual-gating tactic have been successfully demonstrated
recently with the active area localized at the depletion
region.[198,211–213] In contrast, vertical vdWHs have arised up for
efficient carrier injection in LED due to the large active area
over the whole overlapping junc-tion.[203,214,215] For example,
Duan and co-workers[203] have investigated the EL properties based
on p-WSe2/n-MoS2 diodes (Figure 6a). An EL image (Figure 6b)
obtained under a forward
Adv. Funct. Mater. 2018, 28, 1706587
Figure 5. a) PL spectra of single-layer WSe2, MoS2, and the
corresponding hetero-bilayer. b) Normalized PL (solid lines) and
absorbance (dashed lines) spectra of single-layer WSe2, MoS2, and
the corresponding hetero-bilayer. c) Band diagram of WSe2/MoS2
hetero-bilayer under photoexcitation. (a–c) Reproduced with
permission.[76] Copyright 2014, National Academy of Sciences of the
United States of America. d) Schematic front and side view of the
MoS2/WSe2 heterostructures with different twist angles. e)
Excitonic band alignment of the MoSe2/WSe2 heterostructures under
photoexcitation. f) Intensity of the interlayer exciton peak versus
the twist angle. (d–f) Reproduced with permission.[187] Copyright
2017, American Chemical Society. g) Schematic illustration of the
MoS2/WS2-based heterostructure. h) PL spectra of the isolated
MoS2-, WS2-, and MoS2/WS2-based heterostructure. i) 2D plots of
transient absorption spectra at 77 K from a MoS2/WS2
heterostructure and an isolated MoS2 monolayer upon excitation of
the MoS2 A-exciton transitions. (g–i) Reproduced with
permission.[196] Copyright 2014, Nature Publishing Group.
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bias of 3 V exhibits that the EL signal is mainly from the
over-lapping area near the metal electrodes, which is significantly
contrasting to the photocurrent mapping generated from the whole
overlapping area. For photocurrent mapping under small bias lower
than turn-on voltage, the resistance of the p–n junc-tion governs
the entire diode, and thus the photocurrent can be observed from
the whole overlapping area. However, EL is measured at a higher
bias exceeding the turn-on voltage of the diode, and the entire
resistance is gradually dominated by that of monolayer WSe2. Thus,
the most voltage drop generates across the heterojunction edge near
the electrodes. It is observed that the EL intensity increasing as
a function of injection current exhibits a distinct threshold.
Under small forward bias lower than the certain threshold, the
holes from WSe2 are injected into MoS2, while few electrons can
flow from MoS2 to WSe2 due to the barrier. Thus, the radiative
recombination in MoS2 is very weak due to the indirect bandgap of
the few-layer MoS2. With further increasing of the bias higher than
the threshold, both the electrons and the holes can go through the
junction and are injected into p-type and n-type regions due to the
upward shift of the conduction band of MoS2. Then, the EL is
dominated by the radiative recombination in WSe2 and increases
linearly with the injection current. Notably, fitting the EL
spectra via multiple Guassian functions, two hot electron
luminescence peaks at ≈546 and ≈483 nm are also observed, which
could be employed to probe the electron–orbital interaction in
WSe2. In order to reduce the leakage current in the vertical
stacking structures, functional stacking structures for light
emission are developed by inserting tunneling layers (such as hBN,
Al2O3) into the p–n junction and/or the electrode contacts, which
enables the long lifetime of excitons in TMD quantum
wells.[31,74,202,216–219] For example, planar EL from tunnel diodes
based on a metal–insu-lator–semiconductor vdWH consists of
few-layer graphene, hBN, and monolayer WS2, showing an excellent
quantum effi-ciency of ≈1%.[217] The light emission is realized by
the injection
of hot minority carriers to n-doped WS2 by Fowler–Nordheim
tunneling, with hBN blocking the hole- and electron-transport.
Therefore, Novoselov and co-workers[74] have created efficient LEDs
employing graphene as transparent conductive layers, hBN as
tunneling barriers, and different TMDs as quantum walls (QWs) as
shown in Figure 6c. In these devices, electrons and holes are
injected into the TMD layer from the two gra-phene electrodes.
These kinds of vertical heterostructures allow brighter LEDs
(Figure 6c) due to the reduced contact resistance and higher
current densities. For example, the PL of MoS2-based single QW is
dominated by the neutral A exciton at 1.93 eV at low bias. There
are also two weaker and broader peaks at 1.79 and 1.87 eV
attributed to bound excitons. However, the PL spectrum changes
dramatically and exhibits a new peak at 1.90 eV at a certain gate
voltage. This transition may be caused by the Fermi level of the
bottom graphene rising above the conduc-tion band of MoS2,
resulting in electrons flowing into the QW. The obtained quantum
efficiency can reach ≈10% which is ten times higher than that of
planar p–n diodes[198,211,212] and 100 times higher than that of
Schottky barrier diodes.[210] They further introduced multiple QWs
stacked in series to increase the probability for injected carriers
to radiatively recombine. Besides, monolayer TMDs are mostly direct
bandgap semicon-ductors and possess a large range of bandgaps,
which are prom-ising for atomically thin white LEDs. Chen and
co-workers[201] fabricated the white LED employing
n-MoS2/p-MoS2/p-GaN as the orange, green, and blue emitters,
respectively. White LED has promising applications in lighting and
display due to high brightness for low-power consumption and long
lifetimes for high-performance operation. Thus, a heterostructure
based on n-MoS2/p-MoS2/p-GaN has been fabricated with the EL
spectra at 642 nm (n-MoS2, orange), 525 nm (p-MoS2, green), 481 nm
(p-GaN, blue), showing the potential to fabricate atomically thin
light sources with white LED.[201] Besides, thermal light emis-sion
from graphene[220] and MoS2[221] has also been realized
Adv. Funct. Mater. 2018, 28, 1706587
Table 1. Summary of typical heterostructure-based LEDs (Gr:
graphene).
Device Vds [V]
Emission wavelength [nm]
Line width [meV]
Luminance [cd cm−2]
Quantum efficiency [%]
References
Gr/hBN/WSe2/hBN/Gr 2 759 0.2 [216]
hBN/WSe2/hBN 2.8 730 5 [218]
Gr/hBN/WSe2/hBN/Gr 720 20 [202]
Gr/hBN/WSe2/hBN/Gr 1.94 727 0.3 [204]
p-MgNiO/perovskite/n-MgZnO 5.5 522 3.8 × 103 2.39 [353]
Gr/WS2/hBN 640 80 [226]
p-WSe2/n-WSe2 733 5 [212]
n-MoS2/p-MoS2/p-GaN 4 White light 3.0 × 104 29 [201]
MoS2/Si 5.5 685 [215]
p-WSe2/i-WSe2/n-WSe2 750 10−2 [213]
p-WSe2/n-WSe2 800 0.1 [198]
p-WSe2/n-WSe2 2 750 0.1 [211]
Al2O3/MoS2/GaN 10−2 [31]
hBN/Gr/hBN/WSe2/hBN/MoS2/
hBN/Gr/hBN−2.3 750 5 [74]
Gr/hBN/WS2 7 612 1 [217]
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with enhanced bright light emission via suspended device, which
may pave the way toward the realization of commercially viable
large-scale, atomically thin, flexible, and transparent light
emitters and displays with low operation voltage.
Most vdWHs exhibit type-II band alignment, resulting in
efficient separation of electron and hole pairs. However, the
spatially isolated electrons and holes still undergo strong
Cou-lomb interaction due to the small interlayer isolation,
resulting in tightly bound interlayer exciton (XI).[69] Thus, Xu
and co-workers[222] electrostatically constructed lateral p–n diode
based on the MoSe2–WSe2 hetero-bilayer to probe the EL prop-erties
(Figure 6d). The PL spectrum in Figure 6e illustrates that many
peaks from 1.56 to 1.74 eV in both MoSe2 and WSe2 relate with the
intralayer A exciton, neutral, charged, and localized excitons. The
dominant emission below 1.4 eV is fingerprint of the interlayer
exciton. However, the EL spectrum is deter-mined by the interlayer
emission, with only a small intralayer signal at 1.62 eV, probably
the MoSe2 trion.[223] Thus, though the energy of XI is lower than
that of intralayer exciton due to
the small electron–hole wave function overlap in XI, XI
domi-nates the radiative recombination. In brief, most of the holes
in the valence band of WSe2 and the electrons injected into the
conduction band of MoSe2 meet at the junction, and bind to XI due
to the strong Coulomb interaction, and then recom-bine to emit
light. Quantum emitters resulting from quantum confined structures
(such as quantum defects and dots) may induce single photons, which
is crucial for applications in quantum information and
high-resolution metrology. TMD quantum emitters with a very sharp
photon emission spectrum have been successfully
fabricated.[224–228] For example, Xu and co-workers[204]
constructed the heterostructure composed of two stacked graphene
layers as electrodes, separated by thin WSe2 layer with hBN as
barriers, as shown in Figure 6f. The Fermi level of the graphene
layers lies between the bandgap of WSe2 under no external bias.
While, the Fermi level rises above the available subgap defect
states as the device biased. Conse-quently, electrons (holes) can
tunnel through the hBN barriers to WSe2 with increased bias and the
carriers are expected to
Adv. Funct. Mater. 2018, 28, 1706587
Figure 6. a) Schematic diagram of the WSe2/MoS2 heterostructure.
b) The false-color EL image of the heterojunction device under an
injection current of 100 µA. (a, b) Reproduced with
permission.[203] Copyright 2014, American Chemical Society. c)
Schematic of the single quantum wall heterostructure
hBN/graphene/2hBN/WS2/2hBN/graphene/hBN, and the optical image of
EL from the same device. (c) Reproduced with permission.[74]
Copyright 2015, Nature Publishing Group. d) Schematic of the
arrangement of the heterostructure. e) PL and EL spectra of the
heterostructure. Inset: Schematic illustration of the carriers’
transportation. (d, e) Reproduced with permission.[222] Copyright
2017, American Chemical Society. f) Schematic of the vertical
heterostructure LED operation. (f) Reproduced with permission.[204]
Copyright 2017, American Chemical Society. g) Schematic drawing of
the device. (g) Reproduced with permission.[205] Copyright 2016,
American Chemical Society.
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form bound excitons due to the strong Coulomb interactions.
Then, the excitons recombine, resulting in EL from intrinsic along
with defect-bound excitons. Furthermore, a broad peak (1.651–1.71
eV, marked as Xd) at lower energy than the intrinsic excitons
becomes dominant. They further demonstrated that the emission
originates from spatially localized regions of the sample, and the
EL spectra from single defects have a doublet with the
characteristic exchange splitting and linearly polar-ized selection
rules. Kis and co-workers[205] introduced spin injection from a
ferromagnetic electrode into a heterostructure based on monolayer
WSe2/MoS2 and the spin-polarized holes in WSe2 transporting
laterally, resulting in circularly polarized light emission that
can be modulated by external magnetic field (Figure 6g). Because
the energy of the valence band at Γ point is 0.5 eV lower than that
at the K–K′ point in monolayer WSe2, resulting in the spin
injection occurring at the K–K′ point. Then, those spin-polarized
holes recombine with unpolarized electrons from MoS2 at the
heterojunction area. Due to the breaking inversion symmetry, the
electronic states in the K and K′ valleys exhibit different
chiralities and the interband transi-tions at band edges involve σ+
and σ− polarized light. The tran-sition energy can also be tuned by
the external magnetic field through the valley Zeeman effect. These
ingenious strategies such as the bound interlayer excitons, single
defect emitters, spin injection employed to realize atomically thin
LEDs provide new opportunities for the next-generation LEDs.
4.2. Photodetectors
Photodetector is a fundamental building block of many devices in
our daily life such as environment monitoring, video imaging,
military, remote sensing, optical communica-tions, and so
on.[115,229] 2DLMs exhibit excellent properties such as high
transparency, strong light–matter interaction, flexibility, and
facile integration with current complementary
metal–oxide–semiconductor technology. Furthermore, vdWHs based on
diverse 2DLMs provide more tunability for the band alignments,
carrier densities, resulting in multifunctional het-erostructures
and show promising applications in high per-formance
photodetectors. Thus, diverse photodetectors based on 2D
heterostructures exhibiting high performances or novel
constructions have been reported (Table 2). This section briefly
introduces the photodetection mechanisms of the electron–hole
separation and the extended applications.
4.2.1. Photovoltaic Effect
In the photovoltaic effect, photoexcited electron–hole pairs are
separated by the built-in electric field which is generated at the
p–n junction. In this case, the Ids–Vds curves show non-linear
characteristics in dark. Under illumination and without external
bias (Vds = 0), the built-in electric field separates the
photogenerated electron–hole pairs, thus resulting in a meas-urable
photocurrent (short-circuit current, Isc). The carriers of opposite
polarities in distinct parts of the device accumulate with the
circuit opening, and thus a voltage is generated (open circuit
voltage, Voc).[197,230–233]
Photodiodes based on n-type MoS2, p-type BP and WSe2 have been
recently constructed.[70,203,234] However, most of them show poor
external quantum efficiencies (EQEs) such as 0.3% for BP/MoS2 and
12% for WSe2/MoS2, respectively. Then, He and co-workers[235]
constructed photodiodes based on p-GaTe/n-MoS2 (Figure 7a), and
acquired a high EQE of 61.68%. To fur-ther investigate the
transport mechanism, they have measured the low temperature
electrical properties. Thus, they found the interlayer
recombination dominated by the Shockley–Read–Hall (SRH)
recombination, and a negative temperature gradient of (dVoc/dT) ≈
−0.7 mV K−1. Kim and co-workers[214] fabricated vertical
heterostructures based on atomically thin MoS2 and WSe2 in Figure
7b. They found that most of the voltage drop at the vertical
junction, with negligible potential barriers along the lateral
transport direction under forward bias, contrary with potential
barriers resulting from band bending in the lateral direction under
reverse bias. Thus, the tunneling-assisted inter-layer
recombination may determine the current under forward bias. This
unusual interlayer recombination may be related to two physical
mechanisms or a merging of them: (a) SRH recombination assisted by
inelastic tunneling of majority car-riers into trap states in the
gap; (b) Langevin recombination by Coulomb interaction.
Furthermore, the large band offset for conduction bands (∆EC) and
valence bands (∆EV) at the junc-tion could promote the efficient
separation of the photoexcited electron and hole pairs. Generally,
the charge transfer processes in the ultrathin p–n junctions are
efficient and fast due to the forbidding of the exciton (or
minority carriers), which are con-firmed by the observation of the
PL quenching and the genera-tion of photocurrent in the p–n
junction. Besides, the inter-layer tunneling-assisted recombination
also acts as a key role in determining the photoresponse. Because
the photocurrent is dominated by the difference between the
gate-independent generation rate and the recombination rate, the
prominent peak of the photoresponse can be experimentally fitted by
both the SRH and Langevin recombinations (Figure 7c). Though
Langevin recombination may play a key role here due to the enhanced
Coulomb interaction between the electrons and the holes confined in
the 2D systems,[236] SRH recombination could still affect the
process due to the defects at the interface.
Duan and co-workers[237] illustrated a graphene/MoS2/gra-phene
vertical heterostructure (Figure 7d). The heterostructure shows
clear photoresponse with a short-circuit current of ≈2 µA and an
open-circuit voltage of ≈0.3 V (Figure 7e) and the photo-current
mapping of the vertical heterostructure suggests photo-current
generated over the entire heterostructure. The whole photocurrent
increases and the area of photoresponse also extends to the
overlapping area of the top and bottom graphene with the gate
decreasing, which indicates that the photoexcited carriers outside
the vertical junction can also contribute to the total photocurrent
and the diffusion length of the minority car-riers in this device
is at least on the micrometer scale. They also found that
generation, separation, and transport processes of the photoexcited
carriers in the stacked device can be modulated by the back-gate
voltage. The Schottky barrier height difference between top
graphene–MoS2 and bottom graphene–MoS2 con-tacts dominates the
original built-in potential. A monotonic band slope across the
whole vertical junction is formed through the merging of the top
and bottom Schottky barriers, due to the
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shorter channel length (≈50 nm) than the total depletion length
(≈140–170 nm). Thus, this band slope dominates the generation and
separation of the excited carriers. Herein, because of the p-type
doping of graphene by the substrate, the Schottky bar-rier height
in top graphene–MoS2 is lower than that in bottom graphene–MoS2,
which leads to the fact that a built-in potential propels
photoexcited electrons to the top graphene. The EQE of the device
can reach up to ≈27%, which is much higher than many
heterostructures based on TMDs.[70,203] A much larger and tunable
band offset leading to efficient charge separation may partially
explain the high EQE. On the other hand, Lauhon
and co-workers[238] employed an organic small molecular p-type
pentacene and n-type MoS2 for constructing type-II photovol-taic
devices (Figure 7f). Furthermore, they employed transient
absorption spectroscopy to probe the kinetics of the excited
carriers.[239] The results illustrate that the separation of MoS2
exci-tons occurs by hole transferring to pentacene in 6.7 ps, and
the charge dissociation extends to 5.1 ns, which is at least one
order of magnitude longer than the recombination lifetimes from
those 2D heterostructures reported previously.[69,196] They
dem-onstrated a concept of an organic–2D MoS2 heterostructure and
the semiconducting polymer that could offer high performance
Adv. Funct. Mater. 2018, 28, 1706587
Table 2. Summary of typical 2D vdWH-based photodetectors (GO:
graphene oxide).
Device Iph/Idark Responsivity [A W−1]
EQE [%]
Rise time [ms]
Specific detectivity [Jones]
References
BP/MoS2 0.4 0.3 [70]
WSe2/BP/MoS2 6.3 1.25 × 1011 [265]
MoS2/BP 103 22.3 1.5 × 10−2 3.1 × 1011 [354]
InSe/Gr 4 × 103 3.1 × 103 [355]
MoTe2/MoS2 0.3 85 [356]
PbS/Gr 2.5 × 106 24 [102]
SnS2/MoS2 50 1.3 264 [90]
MoS2/hBN/Gr 105 180 230 2.6 × 1013 [245]
WS2/MoS2 103 1090 6.9 3.5 × 1011 [357]
MoTe2/MoS2 2 0.06 1.6 × 1010 [149]
CdS/MoS2 3.9 100 [101]
GaSe/GaSb 0.1 50 3.2 × 10−2 2.2 × 1012 [358]
MoS2/GaAs 0.3 1.7 × 10−2 3.5 × 1013 [359]
WSe2/MoS2 12 [203]
WSe2/MoS2 1.1 × 10−2 1.5 [234]
Gr/hBN/MoTe2 610 3.3 × 1011 [166]
MoS2/hBN/Gr 6.6 3 × 10−4 10 [167]
MoS2/Gr/WSe2 4250 106 5.3 × 10−2 2.2 × 1012 [267]
GaTe/MoS2 21.8 61.6 8.4 × 1013 [235]
PbS/MoS2 130 4.5 × 104 7.8 3 × 1013 [103]
SnSe2/BP 2.4 × 10−4 [244]
Gr/MoS2 2 × 103 1.5 × 1010 [360]
ReSe2/MoS2 6.7 1.2 × 103 [361]
WSe2/GaSe 6.2 1.4 × 103 3 × 10−2 [207]
GaSe/InSe 103 9.3 2 × 10−3 [362]
WSe2/MoS2 0.1 2.4 [214]
hBN/WSe2/hBN 7.3 1.5 × 10−9 [363]
Gr/MoS2 5 × 108 80 [364]
Gr/MoS2/Gr 0.2 55 [237]
Gr/WS2/Gr 30 [64]
GO/Si 104 1.5 2 [365]
BiI3/WSe2 1.6 1.4 [231]
MoTe2/Gr 970.8 [366]
Gr/MoS2 45.5 [257]
SnSe2/MoS2 9.1 × 103 3.1 × 104 200 9.3 × 1010 [95]
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in large-scale devices, which is possibly compatible with the
technology of nanoelectronics and optoelectronics.
4.2.2. Photo-Thermoelectric Effect
Photo-thermoelectric effect appears with the nonuniform heating
by light-induced temperature gradients, resulting in a photocurrent
or photovoltage. Thus, the two ends of the semi-conductor show a
temperature difference, resulting in a voltage difference
(photo-thermoelectric voltage, VPTE) via the Seebeck effect. And,
the VPTE can be presented by: VPTE = (S1 − S2)∆T, where S1, S2 are
the Seebeck coefficients of the two matters, ∆T is the temperature
difference. Notably, the photo-thermoelectric effect is sparse in a
popular semiconductor due to negligible temperature
gradients.[229,240] Successful execution of this tac-tics needs a
broadband absorber where the interaction of car-riers themselves is
stronger than with phonons, along with energy-selective contacts to
acquire the excess electronic heat. Generally, the photoexcited
carriers (or hot carriers) transfer a Schottky barrier between a
semiconductor and an electrode, allowing detection of photons with
lower energy than the bandgap of the semiconductor, which promote
the applica-tions in the visible and near-infrared photodetectors.
However, if the photon energy is lower than the Schottky barrier,
the efficiency of this mechanism will drop and will be restricted
by the ability to extract the carriers before losing their
initial
energy. Exploiting the redundant thermal energy in the elec-tron
bath is a prospective way to conquer these restrictions. This
energy comes from the thermalization of photoexcited carriers,
resulting in a the hot carrier distribution with a well-determined
temperature Te. More carriers can break through the Schottky
barrier with increasing Te, generating a current via thermionic
emission. On this occasion, even photons with the energy lower than
the Schottky barrier could make the Te increase and thus carrier
emission. Koppens and co-workers[241] fabricated vertical
heterostructure based on graphene/WSe2/graphene to detect
low-energy photons (a wavelength up to 1500 nm) via
photo-thermionic emission (Figure 8a). Graphene absorbed the
photons creating electron–hole pairs. Then, the electron–hole pairs
quickly equilibrate into a thermalized car-rier distribution with
an increasing Te. Carrier in this spreading with higher energy than
the Schottky barrier height at the graphene/WSe2 interface could
transfer through the WSe2 layer and further move to the graphene
layer on the other side (Figure 8b). The photocurrent generated in
the sub-bandgap range illustrates a prominent superlinear
dependence on laser power (Figure 8c), which confirms the thermal
emission of car-riers through the Schottky barrier. They further
employed gate voltage to enhance the photocurrent via modulating
the height of graphene/WSe2 Schottky barrier through controlling
the Fermi level of graphene. Time-resolved photocurrent
measure-ments are employed to further confirm the sub-bandgap
photo-current coming from the photo-thermionic effect (Figure
8d).
Adv. Funct. Mater. 2018, 28, 1706587
Figure 7. a) Schematic of GaTe/MoS2 vdWH. (a) Reproduced with
permission.[235] Copyright 2015, American Chemical Society. b)
Bottom left: Sche-matic diagram of a vdW-stacked MoS2/WSe2
heterojunction device with lateral metal contacts. Top: enlarged
crystal structure, with purple, red, yellow, and green spheres
representing Mo, S, W, and Se atoms, respectively. Bottom right:
Optical image of the fabricated device. Scale bar: 3 µm. c)
Measured (circles and dashed curve) and simulated (green curve for
2D Langevin process and purple curve for SRH mechanism)
photocurrent at Vds = 0 V as a function of gate voltages. For the
fit, B = 4.0 × 10−13 m2 s−1 and τ = 1 µs are used for the 2D
Langevin (s = 1.2) and SRH mechanisms, respectively. (b, c)
Reproduced with permission.[214] Copyright 2014, Nature Publishing
Group. d) Schematic illustration of the device layout. e) I–V
curves under dark and illumination. (d, e) Reproduced with
permission.[237] Copyright 2013, Nature Publishing Group. f)
Schematic diagram of the device. (f) Reproduced with
permission.[238] Copyright 2016, American Chemical Society.
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The extracted characteristic decay time of 1.3 ps is on the
order of the cooling time of hot carriers in graphene.[242,243]
These results verify the photo-thermionic effect dominating the
gener-ation of photocurrent. Applying a positive gate voltage
effectively reduces the Schottky barrier height due to the bottom
graphene doped by electrons, resulting in a strikingly enhanced
photo-current. Thus, the photoresponsivity can reach 0.12 mA W−1 at
wavelength of 1500 nm, and the translated internal quantum
efficiency is 2%.
4.2.3. Tunneling Effect
Interband tunneling in adjacent semiconductors has attracted
intensive attention as a new kind of transistors owing to the
promising applications in low-power consumption devices. Xing and
co-workers[244] integrated p-BP and n-SnSe2 for constructing the
Esaki diode (Figure 9a). The accumulation of electrons in SnSe2 and
holes in BP occur around the junc-tion due to the large work
function difference (Figure 9b). Electrons flow from the conduction
band of n-SnSe2 into the empty valence band states of p-BP via
tunneling through the barrier under a small forward bias. This
tunneling current achieves its maximum value when the unoccupied
states of
valence band in BP have a maximal overlap with the occupied
states of conduction band in SnSe2. Further increase of the
for-ward bias results in the alignment of the forbidden bandgap of
BP with the occupied conduction band states. Though the tunneling
probability increases slightly due to a stronger elec-tric field,
the tunneling current decreases. The device current acquired its
minimum value dominated by a combination of phonon-assisted
tunneling and thermionic current, and then increases due to the
overriding thermionic current. They further probed the
photoresponse of the tunneling diode (Figure 9c). The photocurrent
and photovoltage are closely related to the energy band bending in
the heavily doped p+ and n+ regions near the junction due to the
perfect Ohmic contact near zero bias. The photocurrent will drive
the I–V curve to the second quadrant with the carriers accumulation
near the junction as shown in the inset of Figure 9c. Whereas, the
photocurrent will drive the I–V curve to the fourth quad-rant as a
typical p–n diode with the carrier depletion near the junction.
Thus, the responsivity of the device can be estimated to be ≈0.24
mA W−1.
Yu and co-workers[245] reported a highly sensitive
hetero-structure based on MoS2/hBN/graphene by introducing hBN as
the tunneling barrier (Figure 9d). The I–V curves in this device
can be fitted by the direct tunneling (DT) at low voltage
Adv. Funct. Mater. 2018, 28, 1706587
Figure 8. a) Schematic representation of the heterostructure, to
which a gate voltage (Vgs) is applied to modify the Fermi level of
the bottom graphene. An interlayer bias voltage between the top and
bottom graphene flakes can be applied. b) Simplified band diagram
of the photo-thermionic effect at a graphene/WSe2 interface. c)
Power dependence of the photocurrent for various values of photon
energy. d) Time-resolved photocurrent change. Inset: The same data
and fit in logarithmic scale. (a–d) Reproduced with
permission.[241] Copyright 2016, Nature Publishing Group.
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while Fowler–Nordheim tunneling (FNT) at high voltage (Figure
9e). DT and FNT can be presented by the following equations
I VA m q V
h d
m d
h( ) exp
4DT
B2
ds
2
*Bϕ π ϕ= −
(1)
I VAq mV
h d m
m d
hqV( )
8exp
8 2 *3
FNT
3ds2
B2 *
B3/2
dsπ ϕπ ϕ= −
(2)
where A, d, h, m, ϕB, m*, and q are the effective contact area,
the thickness of hBN, the Planck’s constant, the free electron
mass, the barrier height, the effective electron mass, and the
electron charge, respectively. Therefore, the barrier height at the
graphene–hBN and MoS2–hBN interfaces, can be calculated
from the I–V curves under dark and illumination, resulting in
the band alignments in Figure 9f. At forward bias under dark, the
DT of electrons is severely impeded by the high trapezoidal h-BN
barrier, leading to ultralow dark current. Although the tri-angular
hole barrier at the MoS2/hBN junction is lower than the trapezoidal
graphene/hBN barrier, a negligible FNT current occurs due to
lacking of minority hole carriers in MoS2. How-ever, when light is
illuminated on the device, large amounts of electron–hole pairs are
generated in MoS2, resulting in a dramatic increase in hole
tunneling across the triangular hole barrier at MoS2/hBN. Thus, the
tunneling mechanism domi-nated phototransistor can show a high
detectivity of 2.6 × 1013 Jones as well as a high responsivity of
180 A W−1. Duan and co-workers[246] studied the layer-dependent
photoresponse in graphene/MoS2/graphene-based vertical
heterostructures (Figure 9g,h). Interestingly, they found that the
photorespon-sivity of monolayer MoS2 is several times higher than
that in
Adv. Funct. Mater. 2018, 28, 1706587
Figure 9. a) Optical image of the BP/SnSe2 heterostructure. b)
Id–Vds curves at 80 and 300 K in a linear scale. c) Id–Vds curves
under dark and illumi-nation with different laser powers. (a–c)
Reproduced with permission.[244] Copyright 2015, American Chemical
Society. d) Schematic diagram of the electron–hole pair generation
and tunneling across the barrier. e) I–V characteristics of the
device under dark and various illumination intensities of the 405
nm laser. f) Energy band diagrams of the MoS2/hBN/graphene
heterostructure at flat band model. (d–f) Reproduced with
permission.[245] Copyright 2017, American Chemical Society. g,h)
Schematic images of the graphene/1-layered MoS2/graphene and
graphene/7-layered MoS2/graphene heterostructures with SiO2
substrate and air environment, respectively. i) Electrostatic
potentials of the graphene/1L-MoS2/graphene heterostructures
including environmental condition. (g–i) Reproduced with
permission.[246] Copyright 2016, Nature Publishing Group.
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seven-layer MoS2. In this device, the doping difference of the
top and bottom graphene induces built-in voltage, resulting in
asymmetric barrier height between the top and bottom junctions
(Figure 9i). Thus, the photoinduced electrons can effectively
tunnel to lower barrier at the interface between the top graphene
and MoS2, whereas the electrons tunneling are blocked by the higher
barrier at the interface between the bottom graphene and MoS2. Such
asymmetric tunneling induces photocurrent in the vertical
heterostructure. The intro-duced quantum mechanical-based tunneling
mechanism pro-vides a new view to probe the interaction in vdWHs
and to fab-ricate the next-generation optoelectronics.
4.2.4. Electrolyte Gate
Electrolyte gate can modulate the carrier density of the device
to a higher upper limit compared with that tuned by solid state
dielectric (up to 1013 cm−2). When the gate is applied on the
electrolyte solution, the ions in the electrolyte will move to the
surface of the semiconductors and thus generate the elec-tric
double layer (EDL). Generally, the thickness of the EDL is
extraordinarily thin (
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4.3. Optical Modulators
The optical modulator is employed for modulating the proper-ties
such as phase and intensity of the incident light,[269] and is one
of the most crucial operations in photonics, showing prom-ising
applications in optical interconnect, security, and medicine. 2DLMs
provide prospective opportunities for various multifunc-tional
photonics, which may be totally different from those based on
conventional bulk materials.[270,271] Optical modulation effects in
2DLMs have been intensively explored recently. Consequently,
massive prototypes of optical modulators with different modula-tion
mechanisms (such as all-optical, electro-optic, thermo-optic
modulations, and other modulation approaches) have been
dem-onstrated showing exciting performance.
4.3.1. All-Optical Modulators
All-optical modulation based on 2DLMs has been intensively
investigated with the signal processing realized in photonic
system, including polarization controllers,[272] optical
limiters,[273] wavelength convertors,[274] and saturable
absorbers.[275] Graphene enables the existence of resonant
electron–hole pair with broad-band spectral range from visible to
far-infrared. Because of the interaction between ultrafast optical
pulses and charge carriers, a nonequilibrium carrier population in
valence and conduction bands relaxes on an ultrafast
timescale,[276] which enables wide-band and ultrafast saturable
absorption from Pauli blocking. How-ever, graphene’s application at
the end of the spectrum has been impeded by the tremendous
saturation fluence at wavelengths shorter than the near-infrared
spectral region.[277] Different from graphene, TMDs[278] and
BP[279] demonstrate bandgaps for resonant light absorption in the
visible and mid-infrared, respectively.[280–283] Abundant attempts
on 2DLM saturable absorbers have exhibited exciting improvement of
performance, especially for ultrafast pulse generation.[277] For
example, most reports have successfully demonstrated ultrafast
pulse genera-tion, improving pulse repetition rates up to 10
GHz,[284] pulse widths down to sub-100 fs.[285] External cavity
optical pro-cessing is also an effective way to improve the
performance
Adv. Funct. Mater. 2018, 28, 1706587
Figure 10. a) Device schematic with electrolyte gate. b)
Scanning photocurrent map of a large-area device in short-circuit
configuration. c) Ids–Vgs curves under dark and illumination. d)
Schematic image of charge transfer at the WS2/graphene interface.
(a–d) Reproduced with permission.[256] Copyright 2017, John Wiley
& Sons, Inc. e) Schematic of the graphene/MoS2 heterostructure
with electrolyte gate. f–h) Schematic band diagram of polyethylene
(PE)-gated graphene/monolayer MoS2 Photodetector (PD) at (f) zero,
(g) negative, and (h) positive Vgs. (e–h) Reproduced with
permission.[257] Copyright 2016, American Chemical Society.
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such as the pulse duration, the wavelength accessibility, and
the output power and pulse energy.[277,285] Combining passive and
active modulation function in 2DLMs is also a promising way to
further improve the ultrafast laser performance.[286] An
all-optical modulator with a single-mode microfiber coated with
graphene has been realized, exhibiting ≈2.2 ps response time and
38% modulation depth.[287] In free-space set-ups, graphene–silicon
heterostructure-based modulator shows a wideband (0.2–2.0 THz)
terahertz light modulation with a maximum modulation depth of 99%
via exploring the optical doping effect.[288] Because of the fast
response of the third-order nonlinear susceptibility in
graphene,[274,289] wavelength modulators based on atomically thin
nonlinear optics is highly promising for ultrafast all-optical
information processing such as all-optical wavelength conversion.
Whereas, several reports on 2DLMs demonstrated that it is
indispensible to improve the light–matter interaction due to the
sub-nanometer thickness of 2DLMs and optical damage due to the high
excitation power. Recently, many strategies have been proved to be
helpful for improving light–matter nonlinear optical interaction in
2DLMs such as slow-light wave guides, microcavities, coherent
con-trol, interference effects, doping, evanescent mode
integration, stacking multiple monolayers.[277,290–294] 2DLM-based
hetero-structures such as MoS2–WSe2[295] and MoS2–WS2[196] have
also been demonstrated for new-type linear and nonlinear optical
device constructions with tunable optical properties (carrier
dynamics and reflectance). However, 2DLM heterostructure-based
nonlinear optics is in its initial stage and deserves further
attention. Till now, most reported all-optical photonic devices
based on 2DLMs depend on third-order nonlinear processes.
Graphene shows weak second-harmonic generation due to
centrosymmetry.[296] However, other 2DLMs (such as WSe2,[297]
WS2,[298] hBN,[299] MoS2[300]) have exhibited strong second-order
nonlinearity with an odd number of layers due to the broken
symmetry. High optical nonlinearity in 2DLMs shows highly promising
applications in quantum optical switches[301] and high-purity
quantum emitters for integrated quantum circuits.
4.3.2. Electro-Optic Modulators
The electro-optic effect modulators are highly promising for
data communication link applications. Graphene is a prom-ising
material for optical modulator due to its tunable dielec-tric
constant. Thus, many reported 2DLM-based electro-optic modulators
are based on graphene.[13,270,302–307] Typical modula-tion speeds
of electroabsorption modulators based on graphene at the visible
and near-infrared range are on the order of giga-hertz (≈1
GHz[13,308] and 30 GHz[309]). Though 2DLMs show strong light–matter
interaction, the absolute value is very small for atomic-scale
materials.[270,310] For example, monolayer gra-phene can only
absorb ≈2.3% of white light,[310] suggesting that the intrinsic
modulation of monolayer graphene can only be up to ≈0.1 dB.
However, this value is far lower than ≈50% required signal
modulations for practical applications. Therefore, diverse methods
have been proposed to improve the modulation depth such as
employing multilayer devices (few-layer graphene[311]),
cavities,[312] evanescent-mode coupling,[313] interference
enhancement,[314] patterned structure.[315] 2D
heterostructure-based electro-optic modulators have attracted
attention recently.
Adv. Funct. Mater. 2018, 28, 1706587
Figure 11. a) Schematic of the WSe2/graphene/MoS2
heterostructure. b) Photoresponse and detectivity of the
heterostructure at a wavelength range from 400 to 2400 nm. (a, b)
Reproduced with permission.[267] Copyright 2016, American Chemical
Society. c) Schematic of the MoTe2/MoS2 heterostruc-ture under
illumination. d) Band diagram of the heterostructure with interband
excitation process. (c, d) Reproduced with permission.[268]
Copyright 2016, American Chemical Society.
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Zhang and co-workers[13] illustrated a high-speed, broadband,
waveguide-integrated electroabsorption modulator based on
integrated monolayer graphene and silicon (Figure 12a).
Fre-quencies of the incident light can be modulated over 1 GHz,
with a broad operation spectrum ranging from 1.35 to 1.6 µm. This
integration of graphene and silicon for optical modu-lator may pave
a new way for on-chip optical communications. Englund and
co-workers[316] demonstrated a graphene–BN heterostructure-based
electro-optic modulator integrated with a silicon photonic crystal
cavity (Figure 12b), showing operation frequency up to 1.2 GHz with
a modulation depth of 3.2 dB. Similarly, Lipson and co-workers[309]
demonstrated a silicon-based microring resonator (Figure 12c),
providing efficient light modulation with diverse advantages such
as small energy consumption, large modulation depth, and small
footprint. Interestingly, Li and co-workers[317] illustrated a
single gra-phene-based device (Figure 12d) that simultaneously
provides both efficient optical modulation (modulation depth of
64%) and photodetection (near-infrared photodetection responsivity
of 57 mA W−1). This novel multifunctional device may provide a new
platform for the applications in optoelectronics. Tera-hertz
research has been one of the most investigated research fields
recently, which is particularly desirable for the appli-cations
extending from health and environment to security. Graphene
modulators have been proposed to be suitable for working at the
terahertz region[318–322] with excellent modula-tion performance
(>94% modulation depth[323]). Therefore, gra-phene plasmonic
electro-optic modulators have been proposed to be promising in the
infrared and terahertz range,[324–326] due to the pristine
frequency response of graphene. Particu-larly, 2DLMs with
metamaterial structures are attracting intensive attention for
light modulation such as polarization,[324,327] phase,[328]
amplitude,[329,330] and wavelength modulation,[327] exhibiting
exciting modu-lation performance with broad operation bandwidth,
high modulation speed and depth. Besides, 2D polar materials
(hBN[331]) and their heterostructures (graphene–hBN
heterostructures[332]) have been proposed to improve the
light–matter interaction in 2DLMs either with surface–phonon
polari-tons, or plasmon–phonon polaritons for light modulation.
4.3.3. Thermo-Optic Modulators
The most common type of thermo-optic effects is based on the
change in the mate-rial refractive index with variations in
tem-perature, resulting in slow modulation speed (approximately
megahertz) due to the primi-tively slow thermal diffusivity. Thus,
thermo-optic modulators are usually employed for applications in
optical routing and switching, where high speed is not
indispensable. Because of the highly intrinsic thermal
con-ductivity, graphene-based electric heaters have
been successfully integrated into graphene-based silicon ring
resonators[333] and long-range surface plasmon waveguides[334] for
light modulation through change in thermoinduced refractive index.
Transparent flexible heat conductors based on gra-phene also have
been demonstrated to transfer localized heat in a microdisk
resonator and a silicon-based Mach–Zehnder interferometer.[335]
4.3.4. Other Modulation Approaches
Magneto-optic modulators employing magneto-optic effects
(Faraday effect or magneto-optic Kerr effect) for light modulation
obtain little attention than all-optical or electro-optic
modulators due to the operation simplicity of all-optical and
electrical strat-egies. Magneto-optic Faraday[336,337] and Kerr
rotation[338] have been realized in graphene at the
far-infrared,[336] terahertz,[338] and microwave range,[337]
suggesting the possibility of graphene-based magneto-optic
modulators for diverse nonreciprocal applications.
Magnetoplasmons[339,340] and metastructures[341] can also improve
the magneto-optic response (Faraday rota-tion and cyclotron
resonance). There exists the other type of modulators changing the
refractive index of the material for light diffraction and
frequency varying by acoustic waves. These acousto-optic modulators
have been demonstrated in signal modulation and pulse generation in
optical telecommunica-tions and displays. Graphene and other 2DLMs
are attracting intensive attention for the generation, propagation,
amplifica-tion, and detection of surface acoustic waves.[342,343]
2DLMs also have unique mechanical properties such as high
Young’s
Adv. Funct. Mater. 2018, 28, 1706587
Figure 12. a) Schematic of the graphene-based silicon waveguide
modulator. Reproduced with permission.[13] Copyright 2011, Nature
Publishing Group. b) Schematic of a graphene–hBN
heterostructure-based planar photonic crystal (PPC) cavity
modulator. Reproduced with permission.[316] Copyright 2015,
American Chemical Society. c) Schematic of a graphene-based silicon
nitride ring resonator modulator. Reproduced with permission.[309]
Copyright 2015, Nature Publishing Group. d) Schematic illustration
of the dual layer graphene modulator/detector integrated on a
planarized waveguide. Reproduced with permission.[317] Copyright
2014, American Chemical Society.
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modulus combined with a low loss, indicating promising
appli-cations for mechano-optic modulators. For instance, graphene
can be actuated up to high mechanical vibration frequencies (more
than few hundred megahertz), which may be desirable for modulation
of microwave photons.[344] These novel types of optical modulators
provide more possibilities for applications in various fields and
still need more attention.
5. Conclusions and Outlook
2DLM-based vdWHs have a booming development in recent years. In
this feature article, we have comprehensively pre-sented the
preparation methods, energy band alignments, and applications in
optoelectronics in three primary aspects: light-emitting diodes
(such as single defect light-emitting diodes, circularly polarized
light emission arising from valley polari-zation), photodetectors
(such as photo-thermionic, tunneling, electrolyte-gated, and
broadband photodetectors), and optical modulators (such as graphene
integrated with silicon tech-nology, and graphene/hBN
heterostructure). These reports on 2D vdWHs suggest significant
applications in the next-genera-tion optoelectronics. However, the
family of 2D vdWHs is still developing, both in terms of variety of
materials and construc-tion of devices, and it seems like just
beginning. There are still so many kinds of 2D materials unknown,
which may impede the diversity and multifunctionality of 2D vdWHs.
Although mechanical transfer is beneficial for investigating the
physical properties of 2D vdWHs, it is difficult for integration
with the industrial semiconductor technology due to the
uncontrollable size and thickness. Although some reports on the
synthesizing 2D vdWHs by CVD methods,[63,65,73,85] it is still a
great challenge to controlled-synthesize more novel 2D vdWHs based
on a variety of 2D materials by CVD methods, which is a promising
route for large-area and high-quality 2D vdWHs and is prospec-tive
for compatibility with industrial optoelectronics. Recently, 2D
nonlayered materials have attracted significant attention due to
possessing both novel properties of their bulk counterparts and
unique characteristics induced by the 2D morphology.[345] For
example, the surfaces of 2D nonlayered materials are filled with
dangling bonds, which do not exist in layered mate-rials,
modulating the charge transfer which may induce novel transfer
characteristic in electronics and optoelectronics. How-ever, it is
much difficult to realize the controllable synthesis of 2D
nonlayered materials due to the intrinsic isotropic chemi-cally
bonded nature, not to mention the 2D nonlayered mate-rial-based
heterostructures. CVD growth is still an efficient way to realize
the controllable synthesis of 2D nonlayered material-based
heterostructures if the controllability of kinetics can be
introduced to stimulate the 2D anisotropic growth. Besides, the
lateral vdWHs have been reported rarely due to the diffi-culty to
obtain sharp and clean interfaces.[63] Unknown physics from
different vdWHs such as the band alignments, built-in electric
field, charge transfer, and surface reconstruction need to be
further probed.[59] Although most physics of 2D mate-rials show
strong relations with the layers such as the energy band
structures, the layer-dependent physical properties of 2D vdWHs
still need significant attention, which may generate new
functionalities.[346] Notably, the angle-dependent physics
of 2D vdWHs have also exhibited much significances in the recent
years such as the interaction at the interface, which may provide a
new degree of freedom for modulating the interaction in 2D
vdWHs.[187,347] The controllable preparation of large-scale 2D
vdWHs is also indispensable for the industrial integra-tion. The
main obstacle to obtain large-scale 2D vdWHs is the well-controlled
crystallinity, uniformity, and thickness. Gener-ally, direct growth
via vapor deposition is one of the most used methods.[348] The
other way is chalcogenization of the metal precursors predeposited
on the substrate.[349] In order to obtain more uniform large-scale
2DLMs, there are some reports on chalcogenization of the metal
precursors with the substrates pretreated by some organics, which
can adsorb metal precur-sors and disperse them uniformly.[350]
5.1. Outlook for LEDs
Monolayer TMD semiconductors with direct bandgaps of vis-ible to
near-infrared ranges have demonstrated promising applications in
LEDs. Compared with lateral p–n junction with the active area
localized by the depletion region, vertical hetero-structure based
on monolayer TMDs may be more efficient for carrier injection due
to the large active region at the whole over-lapping region.
However, the EL performance is drastically lim-ited by the leakage
current at the interface of vertically stacked p–n junctions. Thus,
there are some directions to improve the performance of LEDs:
(a) The leakage current can be efficiently weakened by inserting
tunneling layers such as hBN into the p–n junction, resulting in
long lifetime of excitons in the vertically p–n junction. Thus, the
EL quantum efficiency can be improved by this strategy for
verti-cally p–n junction.[74] The tunneling layer can also be
extended to other insulators such as Al2O3 and Ta2O5, resulting in
different tunneling barriers, which may induce more light emission
properties;
(b) Using graphene as contacting electrodes can also help to
promote efficient injection of both holes and elec-trons in the
whole junction region;
(c) Monolayer TMD-based LEDs usually show low efficient carrier
injection. An alternative way to possess both the direct bandgap
and high efficient carrier injection is to employ the multilayer
direct bandgap 2DLMs such as GaTe and In2Se3;[351,352]
(d) The white LED also needs more attention due to the
significant potential in lighting and display applica-tions, which
can be realized by multilayer-stacked TMDs with colorful emission
spectra;
(e) Thermal-induced light emission can improve the brightness of
the 2DLM-based LEDs. Thus, it may have exciting effects in
vdWH-based LEDs.
Apart from the common LEDs based on p–n junctions emitted by
recombination of injected electrons and holes, LEDs with new
mechanisms and applications are also very impor-tant. For example,
the electrically tunable and circularly valley-LED was realized in
p+-Si/WS2/n-Indium tin oxide (ITO)-based
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heterojunction, which opens up new opportunities for the
emerging valley-based optoelectronics. Single photons can be
generated by quantum emitters (from quantum confined struc-tures
such as defects), which is much significant for the applica-tions
in high-resolution metrology and quantum information. For example,
single photon emitter based on graphene/hBN/WSe2/hBN/graphene has
been fabricated with a very sharp photon emission spectrum, which
provides a new platform for applications of LEDs. Furthermore,
laser (coherent light source) has been realized in the 2D
system.[225] However, the reports on laser generated by 2D vdWHs
are rare, which still needs sig-nificant attempts. Besides,
scalable fabrication of 2D vdWHs will be the first obstacle for the
industrial integration. As dis-cussed above, CVD growth is more
promising than mechanical transfer. However, it is very difficult
to control the physical prop-erties at the interface especially for
the increasing layers such as the atomic dispersing at the
interfaces. Thus, the controllable preparation of 2D vdWHs still
remains a great challenge. Fur-thermore, flexible LEDs are also
attractive for many applications in wearable optoelectronics, while
the flexible LEDs based on 2D vdWHs are rarely reported.
5.2. Outlook for Photodetectors
Various kinds of photodetectors based on 2D vdWHs, including
photovoltaic, photo-thermionic, tunneling, electrolyte-gated, and
broadband photodetectors, have been reported in recent years and
have demonstrated great promising applications in optoelectronics.
However, there are many challenges left in this field. In order to
fabricate high-performance photodetectors, reasonable device
designs and suitable materials for different parts of
photodetectors are required. The most common photo-detectors based
on 2D vdWHs are p–n diodes, which usually possess fast response.
However, those p–n diodes show low responsivities due to the low
carrier concentration. Photode-tectors based on 2D vdWHs dominated
by tunnelin