Recent Advances of Preparation and Application of Two-Dimension van der Waals Heterostructure

Abstract

With paramount electrical, optical, catalytic, and other physical and chemical properties, van der Waals heterostructures (vdWHs) have captured increasing attention. vdWHs are two-dimension (2D) heterostructures formed via van der Waals (vdW) force, paving the way for fabricating, understanding, and applications of 2D materials. vdWHs materials of large lattice constant difference can be fabricated together, forming a series of unique 2D materials that cannot form heterostructures earlier. Additionally, vdWHs provide a new platform to study the interlayer interactions between materials, unraveling new physics in the system. Notably, vdWHs embody short-range bonds weaker than covalent and ionic bonds, almost only interactions between nearest particles are considered. Owing to a clear interface, vdW interaction between two different components, devices made by vdWHs can bring amazing physicochemical properties, such as unconventional superconductivity, super capacitance in intercalation 2D structure, etc. Recently, impressive progress has been achieved in the controlled preparation of vdWHs and various applications, which will be summarized in this review. The preparation methods comprise mechanical exfoliation, liquid phase stripping, physical vapor deposition, chemical vapor deposition, and metalorganic chemical vapor deposition. The applications sections will focus on photoelectric devices, logic devices, flexible devices, and piezotronics. Finally, some perspectives in the future on the controlled preparation of vdWHs with desired properties for advanced applications will be discussed.

1. Introduction

After the epochal exfoliation of graphene [1], many two-dimensional (2D) materials, such as transitional metal dichalcogenides (TMDs, MX₂), black phosphorous (BP), hexagonal boron nitride (h-BN), etc., have been extensively developed and investigated. Two-dimensional (2D) van der Waals heterostructure (vdWH) functional films can be formed without the requirements for lattice matching and processing compatibility, giving rise to the flexible integration of radically different materials with distinct crystal structures. Integrating dissimilar materials with pristine devices by design [2] is essential for creating novel functional devices for which vdWH is necessary. With precise-tuning stacks being gradually manifested to affect the intrinsic physicochemical properties of vdWH functional thin films, researchers have tried stacking different 2D materials, including graphene [3,4,5,6,7,8,9,10], TMDs [11], h-BN [12], BP [13], graphitic carbon nitrides (g-C₃N₄) [14], and so on. 2D vdWH thin films have captured tremendous attention due to their excellent physical, chemical, and mechanical properties that are beneficial in electronics, optoelectronics, quantum, exciton, phonon, photocatalysis, etc. Various methods of preparation of 2D vdWHs such as mechanical exfoliation, liquid-phase stripping, physical vapor deposition (PVD), chemical vapor deposition (CVD), and metalorganic chemical vapor deposition (MOCVD), have been proposed to facilitate related research.

The electronic properties can be modulated by mechanical strain [15], interfacial twist [16], external electric field [17], stacking pattern [18], laser illumination [19], pressure control [20], interfacial distance control [21], etc. Significantly, researchers have disclosed their potential applications in rectifiers [22], field effect transistors (FETs) [23], tunneling transistors [24] and digital logic devices [25], rechargeable batteries [26], electrocatalysis [27], and so on. Furthermore, 2D vdWH functional films are promising candidates for photovoltaics [28], photodetectors [19], quantum emitters [29], and phototransistors [30] due to their ultrafast transfer processes, high responsivity, gate-tunable sensitivity, and wavelength selectivity. In addition, thanks to the unique properties of vdWHs, such as their tunable electronic bandgap, band alignment, optical absorption, coupling effect, and efficient charge transfer and separation [31], vdWH functional films have potential novel applications in photocatalysis [32].

The required structure can avoid hurdles stemming from precise control of the magic angle, doping profile, layer number, etc., which gives rise to a plethora of opportunities for modulating physical and chemical properties such as energy bandgap, optical absorption, and photocatalysis. Consequently, to realize the scalable application of 2D vdWHs needs perennial effort for exploring advanced preparation methods to efficiently leverage their laudable properties.

Some of the milestones of the research on 2D vdWHs are summarized in Figure 1 [2,33,34]. In 2004, Novoselov et al. [1], firstly exfoliated graphene, and since then the study of 2D materials have sprung up, and they demonstrated the exfoliation other 2D materials in 2005 [35]. The first 2D vdWH of graphene/h-BN was reported in 2010 [36], and the vdWH with a substrate beyond was implanted in 2011 [37]. After that, the structure of vdWHs became more complex, some structures such as the first tunneling transistor with 2D vdWH and complex superlattice were realized in 2012 [38,39]. Electrical edge-contact in h-BN/graphene/h-BN and first vertical logic integration with graphene/h-BN/graphene are reported in 2013 [40,41]. Kang et al., realized wafer-scale 2D vdWH through layer-by-layer assembly in 2017 [42]. Since 2018, consecutive works of literature have unleashed the potential for the investigation of the exotic electronic properties in 2D vdWHs [7,43]. Three works in 2019 demonstrated the moiré excitons in vdWHs [44,45,46]. After that, Wu et al. [47], observed the Néel-type skyrmion in 2D vdWH in 2020.

Herein, firstly, fabrication methods including mechanical exfoliation, liquid phase stripping, physical vapor deposition, chemical vapor deposition, metal-organic chemic vapor deposition, and atomic layer deposition will be reviewed, with the facile introduction of their advantages and disadvantages, respectively. The applications focused on photoelectric devices, logic devices, flexible devices, and piezotronics are reviewed on the heels of the fabrication methods. Finally, some perspectives in the future on the controlled preparation of vdWHs with desired properties for advanced applications will be discussed.

2. Fabrication Techniques

2.1. Mechanical Transfer Techniques

The mechanical exfoliation method was first used to exfoliate graphene [1]. Recently, with the rise of 2D materials, this method has also been used in the preparation of other 2D materials such as TMD and h-BN. The operation method is relatively simple. As can be seen in Figure 2, the sample fluctuation can induce the 2D materials to detach from PDMS, which is utilized to transfer 2D materials. The conventional process can be seen in Figure 2a, the required 2D materials are prepared on PDMS and then directly transferred to the silicon substrate; this individual process is repeated and different 2D layers can be stacked on the substrate to form 2D a vdWH. Nevertheless, it is hard for PDMS to reach a uniform contact with the substrate, rendering a gap between 2D materials and the substrate or the monolayer below, which hamstrings the effective coupling.

The ameliorated section proposed by Lei [48] can be seen in Figure 2b. After transferring a monolayer, acetone, isopropyl alcohol, and deionized water are phased in cleaning the sample accompanied by blow-drying. The hot plate is heated to evaporate the solution between the sample and substrate on the heels of the cleaning process. And the interlayer coupling of the vdWH fabricated by this method is corroborated by the emerging interlaminar exciton emission peak in conjunction with a newly measured interlaminar Raman model. This method is relatively simple and can avert redundant time-costing.

Furthermore, Tian et al. [49], fabricate vdWH by dint of mechanical transferring to make a PN vdWH photodetector, which shows excellent responsivity up to 709 mA/W when electrostatically dopped. Tan et al. [50], utilize mechanical exfoliation to fabricate GaTe/MoS₂ vdWH, and the photocurrent polarization ratio of the photodetector is shown to be 2.9. Nevertheless, since mechanical exfoliation is implemented in an open environment, contaminants are usually trapped between the layers, aggregating into randomly located blisters, incompatible with scalable fabrication processes [51]. Besides, lack of precise alignment between different material layers also impedes the usage of mechanical exfoliation.

2.2. Liquid Phase Stripping

Liquid-phase stripping overcomes the problem of low yield and environmental contamination of the mechanical transferring method and can produce single-layer and multi-layered 2D materials in batches. Among the liquid-phase stripping methods, there are two most widely used methods, the lithium-ion intercalation method, and the liquid-phase ultrasonic method. Initially, Morrison et al. [52] stripped a single-layer sample of MoS₂ by the lithium-ion intercalation method.

Briefly, the sample was immersed in a liquid containing a lithium-ion intercalating agent and lithium ions were inserted into the bulk sample to form a combined body. Then, this combination was submerged in water, dilute acid, or alcohol solution with a lower boiling point. The violent reaction of lithium ions with the solvent generated a large amount of hydrogen, thereby increasing the distance between layers and realizing the layer separation in the samples [53]. The lithium-ion intercalation method has a relatively high peeling input and output, and a high peeling repeatability rate. However, high preparation effort and temperature are required. Additionally, the reaction time is too long, and impurities are easily introduced, resulting in a change in the crystal configuration and optical properties. The inability to control the parameters well is also one of the bottlenecks of the lithium-ion intercalation method. Fundamentally, the liquid-phase exfoliation method uses the model [54] shown in Figure 3 below to obtain a 2D vdWH.

As can be seen in Figure 3, Sui et al. [54], show the process of stripping heavy rare earths (HREs) from the organic top phase and then from the poly(ethylene glycol)-rich mesophase to an aqueous solution. The water-soluble complexing agent in the aqueous phase contends with the extraction agent in the organic phase, complexing with HRE ions. The method is simple and convenient to operate and caters to large-scale preparation. However, the crystal quality is not so high when applied by ultrasound, and there are defects, not conducive to the application of optoelectronic devices. The materials prepared by the solution method have great potential in photocatalysis [55] and electrocatalysis [56] since the 2D materials can be conveniently modified by controlling the solution content during stripping.

2.3. Physical Vapor Deposition

Physical vapor deposition (PVD) is the preparation of samples by physical methods involving vaporizing substances into molecules or ionizing them into ions, and then depositing them into thin films [57,58,59,60,61,62]. PVD methods primarily include electron beam evaporation, magnetron sputtering, molecular beam epitaxy(MBE), pulsed laser deposition(PLD), etc. [63] As can be seen in Figure 4, Zatko et al. [64], fabricate a WS₂/WSe₂/WS₂ 2D vdWH with PLD. The PLD chamber is filled with Ar atmosphere and equipped with WS₂ and WSe₂ targets. A Nd:YAG laser of 355 nm, 80 mJ power, and the Ar pressure is set at 10⁻¹ mbar to mitigate the plume energy and facilitate the lateral crystallization. The in situ target allows the 2D vdWH to refrain from exposure to air, which is of significance to the quality of vdWH.

Compared with other methods, PVD has its inherent advantages, such as better flatness of the sample surface, fewer defects, excellent photoelectric properties, and convenient ion transfer, making it suitable for basic research. However, PVD also has inherent shortcomings that cannot be ignored, such as low preparation efficiency, small sample size, high preparation cost of single-layer materials, and low repetition rate, which renders PVD prepared samples unfit for large-area device preparation. For these reasons, this method is currently only used in the front-end research, and the small area of high-quality samples also determines that the hand-tear method is only suitable for the exploration of the original properties of single-layer material [65,66,67,68].

Among all kinds of PVD methods, molecular beam epitaxy (MBE) is an impresive one, which develops a reputation for ultra-high purity of the grown films. MBE takes place in an ultra-high vacuum chamber, accompanied by a low-temperature substrate and low-speed deposition rate. The beam intensity can be meticulously controlled, and the composition and doping concentration can be adjusted in deference to the source. Apparently, one of its constitutional deficiencies is its low deposition rate. Ribeiro et al. [69], demonstrate the large-area growth of single-crystal ultrathin films of stoichiometric Fe₅GeTe₂ using MBE in ultrahigh vacuum deposition system with a base pressure in the low 10⁻¹⁰ mbar.

As can be seen in Figure 5a, the top left, bottom left, top right and bottom right side panels show RHEED patterns of the Al₂O₃ (001) surface before and after the growth of Fe₅GeTe₂ layers, with the electron-beam aligned along [110] and [210], respectively. In Figure 5b, the sharp streaks and anisotropic RHEED patterns corroborate the single-crystalline character of Fe₅GeTe₂. Besides, the root mean square roughness in Figure 5c substantiates the smoothness of the surface [69].

2.4. Chemical Vapor Deposition

Chemical vapor deposition (CVD) can synthesize 2D materials with a large area, uniform thickness, and controlled size of atomic layer thickness, and can be directly grown on Si/SiO₂, sapphire, and other substrates. It is a method of obtaining 2D materials by passing one or several reactants or gases containing the precursor materials into a tube furnace and reacting at a high temperature [70,71]. It is currently the most common method for preparing TMD materials [72,73,74,75,76,77,78,79,80].

Generally, there are two main CVD approaches for fabricating vdWHs, (1) direct growth of atomic heterostructures and (2) the sequential CVD growth of 2D layers on top of others. Some vdWHs, especially TMD/TMD vdWHs, can be fabricated via the one-step CVD approach [70,71].

The advantages include the adjustability of a variety of test parameters, which leads to the control of sample morphology, thickness, etc., good reproducibility, and the huge size of the prepared 2D nanosheet. In addition, CVD shows general characteristics for other types of chalcogenides, alloy products, and even heterojunctions. However, because the preparation process is still immature, the samples prepared by CVD still have many shortcomings, such as poor control of the degree of sulfidation. Various experimental parameters and methods are being studied by the researchers for optimization and improvement of the process for suitable industrial applications.

Tian et al. [81], propose an in situ CVD method to fabricate the graphene/h-BN/graphene 2D vdWH, which can be seen in Figure 6. Primary steps are summarized as follows: (1) The CuNi(111) film is deposited on the sapphire substrate, in which the carbon atoms are dissolved to form C-CuNi(111); (2) Hydrogen-argon plasma is used to eliminate the graphene islands on the CuNi(111) surface; (3) The monolayer h-BN and graphene are synthesized sequentially on the cleaned C-CuNi(111) substrate; (4) The pre-dissolved carbon atoms in the C-CuNi(111) diffuse into the interface between C-CuNi(111) and h-BN film to form the graphene layer. Ultimately, the raphene/h-BN/graphene is fabricated.

What’s more, various technologies endow CVD with stirring developments. Plasma-enhanced CVD (PECVD) is also comprehensively implemented in the 2D vdWH fabrication process. Plasma is leveraged to decompose precursors into highly reactive species, and the 2D materials can be directly grown on noncatalytic substrates such as SiO₂/Si or sapphire at a comparatively lower temperature than conventional CVD. Plasma is usually generated by the radiofrequency between two electrodes. PECVD has developed a reputation for its low-temperature ambient conditions, transfer-free process, and industrial compatibility, which enables the scalable and low-cost preparation of 2D materials [82,83,84,85,86,87]. Moreover, artificial neuron networks can be used in a CVD process to identify and characterize 2D vdWH, ameliorating the efficiency of identifying sample quality and optimizing synthesis parameters [88].

2.5. Metalorganic Chemical Vapour Deposition

MOCVD belongs to CVD, which uses metalorganics as precursors. Generally, MOCVD can be performed at a lower deposition temperature than conventional CVD, due to the lower pyrolysis temperature of metalorganics precursors than CVD use [89].

MOCVD was recently extensively used in the growth of vdW materials [90]. MOCVD is an attractive process as it is readily transportable for the high purity organometallic compounds that are used [91]. The larger free energy change of the source chemicals facilitates the pyrolysis and reduced grain boundaries on substrates that allow MOCVD to be the mere technique to grow various 2D TMD [92].

Figure 7 presents cross-sectional TEM images, with the protected region in the top row and the unprotected ones in the bottom row. It is apparent that MOCVD h-BN has a layered structure. Nevertheless, the MOCVD samples contain multiple twin boundaries, and their interfaces are more defective and rougher than that of the samples prepared by mechanical exfoliation [93]. Moreover, metal-organic precursors are very expensive compared to their counterparts, and most metalorganics are volatile, requiring accurate pressure control [89].

Advantages and disadvantages of different fabrication techniques for 2D vdWHs are summarized in

Table 1. Advantages and disadvantages of different fabrication techniques.

Techniques	Advantages	Disadvantages	References
Mechanical transfer	Relatively simple	Cumbersome, easily contaminated, low yield, unscalable,	[48]
Liquid phase stripping	High yield and low contamination	Defective, high efficiency	[54,55,56]
PVD	Better flatness of surface, fewer defects	Unscalable, low efficiency	[64,65,66,67,68,69]
CVD	Scalable, uniform	Defective	[81,82,83,84,85,86,87,88]
MOCVD	Lower temperature	Defective, precursors are expensive, metalorganics are volatile and require precise pressure control	[90,93,94]

.

3. Progress of Applications

3.1. Photoelectric Devices

Versatile photoelectric devices based on vdWHs, i.e., photovoltaics, photocatalytics, photoelectrochemistry devices (PEC), photodetectors, etc., have witnessed great developments, which can be attributed to the ultrafast charge transfer processes, high responsivity, gate-tunable sensitivity, and wavelength selectivity.

Jiang et al. [94] design a WSe₂/MoS₂/Wse₂-based photovoltaic field-effect photodiode (PVFED), featuring in one vdWH modulate some optoelectronic characteristics of another vdWH. This double vdWH exhibit a high responsivity(R) of 715 mA·W⁻¹ and a fast response time of 45 μs.

As can be seen in Figure 8, the photogenerated electrons can be swept into the middle MoS₂ from both sides of WSe₂, while the residual holes have no access to recirculating, offering a photogate effect. Consequently, this double vdWH is endowed with high sensitivity by dint of suppressing the dark current and enhancing the photocurrent.

Gao et al. [95] construct vdWHs with 2D CrXh (X = S/Se, h = Cl/Br/I) for magnetic-field-modulated photoelectric devices. The robust magnetic ordering and distinctive spin-polarized band alignment of this kind of vdWH facilitate the laudable ultrarapid and reversible “write-read” processes.

In the field of photodetection, Ahn et al. [96] suggest a MoTe₂/ReS₂ vdWH for a high-performance photodiode with a fairly high R of 0.54 A/W and excellent linearity. Various studies have also focus on graphene-based vdWH; Feng et al. [97], construct a WSe₂/graphene/MoTe₂ vdWH with a responsivity of 40.84 mA/W and a detectivity of 1.21 × 10¹¹ Jones for 550 nm light. Peng et al. [98], fabricate photodetectors based on 2D Te/graphene vdWH, which shows a detectivity of 1.04 × 10⁹ Jones and responsivity of 96.4 mA/W under 2 μm laser irradiation. Zhong et al. [99], fabricate a graphene/PdSe₂/MoSe₂/graphene vdWH based photodetector, and obtain detectivity of 5.29 × 10¹¹ Jones, a responsivity of 651 mA/W, and a response time of 41.7 μs. Some of the principle characteristics of 2D vdWH-based photodetectors are summarized in

Table 2. Performance of photodetectors of 2D vdWHs.

Material	Detectivity/Jones	Responsivity mA/W	Response Time/μs	Reference
WSe2/MoS2/WSe2	1.59 × 1013	715	45	[94]
MoTe2/ReS2	109–1010	540	-	[96]
WSe2/graphene/MoTe2	1.21 × 1011	40.84	468	[97]
Te/graphene	1.04 × 109	96.4	28	[98]
graphene/MoSe2/PdSe2/graphene	5.29 × 1011	651	41.7	[99]
Cs2AgBiBr6/WS2	1.5 × 1013	520	52.3	[100]
graphene/WSe2/Fe3GeTe2	3.4 × 1010	116.38	370	[101]
WSe2/Bi2O2Se	-	284	2.6	[102]
PdS decorated-MoS2/WSe2	5.15 × 1011	760	43	[103]
NiTe2/graphene	-	1310	8.5	[104]

.

3.2. Logic Devices

The memory devices built with 2D vdWHs, which have excellent tunability under external signals, exhibit low power consumption, superior scalability, conspicuously fast operation speed, and high endurance. Therefore, memories implemented by 2D vdWHs pave the way for ameliorating energy efficiency, computing accuracy, operation speed, and learning capability in in-memory computing. [25,105]

In memory devices based on 2D vdWHs for in-memory computing logic operations are executed in situ within an individual memory unit which basically includes charge-based memory devices, i.e., static random-access memory(SRAM), dynamic random access memory(DRAM), flash, and resistive switching memories, i.e., resistive random access memories (RRAM), phase change memories (PCM), ferroelectric random-access memories (FeRAM), and ionic transistors [25].

As can be seen in Figure 9, the memories primarily comprise two categories, charge-based memory and resistive switching (RS) memory. Charge-based memory mainly refers to the conventional SRAMs, DRAMs, and flash memories, i.e., floating gate transistors, charge-trapping devices, and ferroelectric field-effect transistors (FeFETs). In addition, resistive switching memory consists of vertical RRAMs, PCMs, and FeRAMs, lateral memtransistors, and ionic transistors.

Xu et al. [105], find that the polarization of In₂Se₃ can be tuned by optical stimuli, which is attributed to the mechanism that photogenerated carriers in In₂Se₃ can alter the screening field, thus leading to polarization reversal. They fabricate an In₂Se₃-based logic gate and photonic memory. This dual electrical and optical operation can simplify the device architecture and furnish extra functionalities, such as ultrafast optical erasure of large memory arrays. Furthermore, a photoinduced electrostatic modulation of WSe₂ FET with charge-trapping h-BN/SiO₂ was realized [106] for n- and p-type polarities, demonstrating an arresting on/off ratio exceeding 10⁶, which facilitates the AC–DC conversion and cascade logic NOT. As graphene is endowed with high mobility, graphene-based 2D vdWH is extensively investigated to ameliorate the logic performance of devices. Bai et al. [107], fabricate field-effect tunneling transistors with graphene/Ws₂/graphene and observe off-state current below 1 pA and an ON/OFF ratio surpassing 10⁶. Wu et al. [108], report a MOSFET based on MoS₂/hBN/graphene vdWH, which possesses logic, nonvolatile, and rectification functions. The device has ON/OFF ratios of 10⁵, rectification ratios of 10³, and 10-year retention as a floating-gate MOSFET.

3.3. Solar Cells

As a competitive candidate for solar cells, 2D vdWHs possess several advantages. By virtue of strong Coulomb interaction and carrier–carrier scattering, weak carrier–phonon coupling, and high exciton binding energy, the slow HC cooling and restricted loss channels remain prominent. Contingent on their laudable properties, the application of 2D vdWHs on solar cells garners enormous attention [12,109,110,111,112,113,114]. Zhao et al. [115], explore the optical properties of the type-II AsP/MX₂ heterostructures (M = Mo, W; X = S, Se) by first-principle calculations. The results demonstrate that the power conversion efficiency (PCE) exceeds 15%, showing the promise for this kind of structure’s applications in solar cells. Similarly, by dint of first-principle calculations, it revealed a PCE of 21.2% in GaSe/C2N. [116], 22.2% in BP/MoSi₂P₄ [117], 21.56% in K₂O/Cs₂O [118], and 23.20% in Sc₂CCl₂/SiS₂ [119], respectively.

Apart from the theoretical research mentioned, various experimental implementations have also been investigated recently. Zeng et al. [103], constructed a 2D vdWH MoS₂/WSe₂ in conjunction with a PbS quantum dot layer which facilities the collection of photogenerated electrons. This structure can drastically enhance the photovoltaic response of the device, with a PCE of 7.65%, broadband from 405 to 1064 nm, the maximal responsivity of 0.76 A/W, and the specific detectivity of 5.15 × 10¹¹ Jones. Vikraman et al. [120] demonstrate that the MoSe₂/WS₂ vdWH is utilized to act as a counter electrode for dye-sensitized solar cells. The electrode endows the solar cells with a high PCE of 9.92% and a photocurrent density of 23.10 mA⋅cm⁻². It can be noticed that PCE in experiments is manifested to be a large discrepancy compared to the results in theoretical research, which is ascribed to the large traps-induced recombination of electrons and holes. PCE can be written as

$$\mathrm{PCE}=\frac{\mathrm{FF}\times {\mathrm{V}}_{\mathrm{OC}}\times {\mathrm{I}}_{\mathrm{SC}}}{{\mathrm{P}}_{\mathrm{in}}}$$

(1)

where P_in is the incident optical power, FF is the fill factor, V_OC is the open-circuit voltage, and I_SC is the short circuit current. On account of the enormous charge traps in 2D materials, recombination roots in the electronic tail states can palpably affect FF, I_SC and V_OC, thus exacerbating the PCE obtained in experiments [121]. Consequently, in order to unleash the potential of the 2D vdWH solar cells, it is imperative to refrain from the contamination and defects during the growth of 2D materials, facilitating the dissociation of the excitons formed by excited electrons and holes.

3.4. Flexible Devices

Flexible and wearable devices have seen flourishing development due to their promising applications. However, conventional chemical-bonded heterostructures with dissimilar materials suffer from interfacial strain. On the contrary, components of vdWHs can slide when compressed or stretched due to the exhilarating bond-free characteristic, which effectively releases tension [2]. Consequently, vdWHs have broad application prospects in the field of flexible electronics due to the flexibility of 2D materials, their excellent ductility, and the bendability of the products processed via vdWH. For flexible devices, the structures are either flexible or embody extremely thin constituents with excellent bendability. There are two noteworthy characteristics: (1) the bending strain decreases with thickness in adequately thin films; and (2) the bending strain of 2D materials can be minimized if they are placed into a neutral mechanical plane. Conversely, the comparatively weak vdW force between layers can afford the needed flexible structure. 2D vdWHs remain comparatively immaculate for fabricating flexible devices and unleash the potential of 2D vdWH-based flexible devices.

As seen in Figure 10, Zhang et al. [122], construct a Ti₃C₂T_x/g-C₃N₄ 2D vdWH by the self-assembly method to form a flexible solid-state supercapacitor. This device is endowed with a high capacitance of 414F/g due to the combination of the stable pseudo-capacitance of Ti₃C₂T_x and the amelioration from the nitrogen-containing functional groups in g-C₃N₄. Intriguingly, the capacitance of the device changes inconspicuously under bending angles of 0°, 90°, and 180°. Furthermore, the capacitance remains at 94.93% after 2500 cycles of charge/discharge curves, which means that the Ti₃C₂T_x/g-C3N₄-based supercapacitor has excellent stability as well as flexibility.

2D vdWH-based flexible devices have promising applications in intelligent products, including healthcare, wearable sensors, electronic skins, the automotive industry, and foldable displays by dint of their retainability of commendable electronic and optoelectronic properties after a plethora of bending cycles [123,124,125,126,127,128,129,130,131,132]. Ko et al. [126], report 2D bis (trifluoromethanesulfonyl)-amide (TFSA)-doped graphene/MoS₂/triethylene tetramine (TETA)-doped graphene vdWH semitransparent photodetectors (PDs) on rigid/flexible substrates. The flexible PD maintains 32% of its initial R even after 2000 bending cycles at a radius of curvature of 2 mm. Jang et al. [133], find that the PDs fabricated by doped-graphene/WS2 vdWH maintain 88% of the initial R even after 3000 bending cycles with the radius of curvature of 4 mm. Both of them manifest excellent flexibility.

Furthermore, vdWH-based flexible devices exhibit excellent prospects in applications for the field of solar cells and thermoelectric devices [134,135,136,137]. An HfN₂/MoTe₂ vdWH-based excitonic solar cell has been designed [138] with a very high PCE and a high figure of merit (Z = 2.28). By virtue of laudable photovoltaic, thermoelectric properties and the intrinsic flexibility of 2D vdWH, a flexible multifunctional device can be designed to harness both solar energy and the waste heat dissipated by the photovoltaic devices. Li et al. [139] theoretically investigated the mechanical and thermoelectrical anisotropy of S₃N₂/BP vdWH. Apart from the thermal conductivity of AB₁ stacking of this heterostructure being half that of the pristine monolayer BP, Young’s modulus and Poisson’s ratio of AB₁ stacking manifest as fairly small and anisotropic, indicating that S₃N₂/BP is relatively appropriate for flexible and thermoelectric devices.

3.5. Piezotronics

The piezoelectric effect arises when non-centrosymmetric materials are applied by external stress. Among the 21 non-centrosymmetric point groups, polar ones demonstrate spontaneous polarization, and the reversing of the electric dipole moments aroused from the external electric field stipulates ferroelectricity. Linear piezoelectricity combines the linear electrical and elastic behaviors of materials, which can be written as [140]

$$\left\{\begin{array}{l}{\mathrm{S}}_{\mathrm{ij}}={\displaystyle \sum _{\mathrm{k},\mathrm{l}}{\mathrm{s}}_{\mathrm{ijkl}}{\mathrm{T}}_{\mathrm{kl}}+{\displaystyle \sum _{\mathrm{k}}{\mathrm{d}}_{\mathrm{ijk}}^{\mathrm{t}}{\mathrm{E}}_{\mathrm{k}}}}\\ {\mathrm{D}}_{\mathrm{i}}={\displaystyle \sum _{\mathrm{j},\mathrm{k}}{\mathrm{d}}_{\mathrm{ijk}}{\mathrm{T}}_{\mathrm{jk}}+{\displaystyle \sum _{\mathrm{j}}{\mathsf{\epsilon }}_{\mathrm{ij}}{\mathrm{E}}_{\mathrm{j}}}}\end{array}\right.$$

(2)

where S is linearized strain, T is stress, E is the electric field, D is electric displacement, s is Young’s modulus, ε is dielectric constant, and d and d^t represent the matrices for direct and converse piezoelectricity, respectively. Furthermore, the piezoelectric coefficient d_ijk can be rewritten as d_ij in Voigt notation on account of the symmetric essence of the stress and strain tensor, where i = 1, 2, 3 represent x, y, z directions of the materials, j = 1, 2, 3 represent the diagonal components of the tensor, j = 4, 5, 6 represent the off-diagonal components of the tensor, respectively.

Apart from the in-plane piezoelectricity in 2D materials, out-of-plane(OOP) piezoelectricity arose in 2D vdWHs [141] as well as multilayers [142] has been corroborated both theoretically [143,144] and experimentally [145]. Initially, OOP piezoelectricity in the BP/GaN [141] was theoretically predicted, and the tremendous OOP piezoelectric coefficient |d₃₃|_max ≈ 40.33 pm V⁻¹, which is about four times larger than that of the MoSTe multilayers [142]. Specifically, d₃₃ is the largest when the two monolayers are in exact alignment and have charges of opposite signs, with the largest electric dipole moment. Subsequently, various theoretical predictions in conjunction with experiments have been demonstrated. A plethora of 2D vdWHs exhibits OOP piezoelectricity or spontaneous OOP polarization that is impalpable in individual 2D materials or perceptibly enhanced compared with them [146]. OOP piezoelectricity and ferroelectricity arise in twisted graphene [147], hBN [148] and TMD [149] by dint of moiré pattern and interlayer sliding. Rogée et al. [150], observe d₃₃ of from 1.95–2.09 pm/V in untwisted MoS₂/WS₂ vdWH, which vanishes in individual flakes, and intriguingly, this value is six times larger than that of In₂Se₃, in unison with the DFT theory. The commensurate stacking of the untwisted structure nullifies the impact of the moiré pattern. Besides, the symmetry breaking from D_3h to C_3v can be considered to introduce the OOP piezoelectricity, in conjunction with interlayer sliding to generate ferroelectricity.

Various piezoelectric devices based on 2D vdWH have been investigated. Weston et al. [139], demonstrate a proof-of-principle field-effect-transistor based on twisted MoS₂ vdWH. Besides, a 10-year memory retention is realized in MoS₂/h-BN/graphene/CuInP₂S₆ [151] vdWH-based FeFET memory cell. Guo et al. [152], propose a synaptic device based on 2D α-In₂Se₃/GaSe vdWH, which can perform logic and memory functions to mimic the visual cortex of the brain by virtue of ferroelectricity.

The optical processes can be effectively modulated by applied strain by dint of piezotronics, which is called piezo-phototronics [153]. A 2D In₂Se₃/WSe₂ vdWH [128] has been fabricated to investigate the piezo-phototronic effect on the photodetectors. The strain-induced piezo-potential can enhance the separation efficiency of photogenerated carriers. Applied by the tensile strain of 0.433%, the photocurrent can be improved by 18 times, and the ultrahigh photoresponsivity and detectivity are 4.61 × 10⁵ A/W and 4.34 × 10¹⁴ Jones, respectively. Similarly, the photoresponsivity of the SnS/MoS₂ [154] vdWH-based photodetector is improved by ~97% at ~2% tensile strain.

The primary applications of 2D vdWHs are summarized above in Figure 11, which is divided into two categories, optoelectronics and electronics. Among the subsections, photodetectors and solar cells belong to optoelectronics, logic devices belong to electronics, and flexible devices and piezotronics fall into both categories.

4. Summary and Prospects

In the past few years, tremendous achievements have been made in the field of vdWHs due to their tolerance for lattice mismatch, orientation free layering, and other material incompatibilities, which enable researchers to assemble dissimilar 2D materials almost freely. Subsequently, their remarkable physical, chemical, and mechanical properties have been found at an amazing speed. The clear interface among materials makes vdWHs a great platform for researching interlayer interactions, interlayer transportation, and other novel physical effects related to the surface/interface effects. Furthermore, these unique properties also enable vdWHs to support a series of applications with enhanced tunable physicochemical properties.

In this article, we summarized five ways to fabricate vdWHs: the mechanical transfer technique, liquid-phase sipping, PVD, CVD, and MOCVD. The mechanical transfer technique is simple yet cannot be used in large-scale manufacturing, and is hard to control, For the liquid-phase sipping, it can realize large-scale preparation. However, it can easily lead to defects, so the quality is not great. PVD can be applied to make a wide range of coating materials, and there is no pollution, so a high-quality sample can be produced. However, it needs a relatively high requirement to the environment with high vacuum and high temperature, so it is impossible to make large-scale manufacture in this way. CVD can synthesize two-dimensional materials with a large area, and high accuracy thickness on the atomic scale; however, it has poor control of the degree of sulfidation and the preparation of CVD still needs to be optimized. MOCVD, compared with CVD, can perform at a lower deposition temperature; compared with PVD, it can also support a wide range of material growth. However, it is expensive and needs a strict pressure environment.

Although the relevant research has achieved brilliant progress that is quite exciting, there is still a long way to go before its mature industrial application is realized, especially in the following aspects: (1) The scalable fabrication of vdWHs remains a primary barrier to future advancements, which encounters problems of high-quality Ohmic contacts, homogeneity problems for the deposition of high-k dielectrics and interaction between devices and the environment, etc. (2) Sequential chemical synthesis is intractable to grow each successive layer which remains detrimental to physical and chemical properties of the underlying layers [155]. (3) How to improve the precise tunability of the twist angle, surface, interface width, doping profile, layer number, etc. in the synthesis process.

Clearly, more advanced fabrication strategies with controlled surface and interface microstructures of 2D material thin films or even heterostructures at a large scale are keys to their commercial applications. With the development of new fabrication methods, in the near future, their applications may not be limited to the above list of applications on photoelectric, logic, solar cells, flexible devices, and piezotronics, which can be applied to various electric sensors, but can be expanded to more amazing areas, such as the construction of long-range ordered single atomic p–n junctions for flexible transistors, potable rolling solar cells and computers, flexible invisibility cloaks, etc. As piezotronics is in full swing, considering the large amendment piezo-photoelectric coupling brings, how to effectively utilize piezoelectricity in 2D vdWH to improve photovoltaic performance is a promising direction. Furthermore, combining the piezoelectric effect with artificial synapses to emulate the human brain’s cortex is another promising field.