2D Layered Material‐Based van der Waals Heterostructures...

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© 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

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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.



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

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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.



(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

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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.



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

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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.



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.



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

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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]



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

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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.



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

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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]



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).

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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.



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

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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.



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.



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 (



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

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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.



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.

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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.



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.



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

Adv. Funct. Mater. 2018, 28, 1706587



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

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