Advance in twisted transition metal dichalcogenides: synthesis, characterization, and properties

The twist angle regulation strategy provides a feasible tool for studying the emerging properties of transition metal dichalcogenides (TMDCs). For the twisted TMDCs (t-TMDCs), there is the lattice mismatch and twist between layers, thus forming moiré superlattice. The formation of moiré superlattice brings about innovative properties to the t-TMDCs. These innovative properties have attracted more and more attention from researchers. This review firstly focuses on the synthesis methods of t-TMDCs, as well as the merits and shortcomings of each method. Secondly, the common spectral characterization and microscopic characterization methods are discussed. Thirdly, the prominent properties of t-TMDCs are briefly demonstrated, including ferroelectricity, flat band, and interlaminar excitons. Finally, we look forward to the potential application prospect and research direction of t-TMDCs.


Introduction
Since the graphene has been obtained by mechanical exfoliation in 2004 [1], other two-dimensional (2D) materials such as hexagonal boron nitride (hBN) [2,3], black phosphorus [4,5] and transition metal dichalcogenides (TMDCs) [6][7][8] have been gradually discovered and studied by the scientists.Among them, TMDCs materials have a broad application prospect in photoelectric devices because of their excellent electrical [9], optical [10] and catalytic [11] properties.It is well known that the properties of a material are determined by the structure.Previous studies on the property regulation of TMDCs has mainly focused on the size, morphology, vertical or horizontal heterojunction construction and other aspects.As these studies progressed, many properties have been discovered.Therefore, more efforts have been made to promote the structural design and enrich the properties of TMDCs.
One important strategy is to construct homogeneous or heterogeneous junctions stacked with special twist angles, instead of limiting to the simple 2H or 3R phase stacking.This twisting-based strategy has been proved to be an effective way to generate moiré superlattice, resulted from the lattice mismatches and rotations between two assembled TMDCs monolayers [12].When the twist angle between layers changes, a rigid-lattice moiré pattern for large angles and atomic reconstruction for small angles occurs, resulting in a change in the band structure and electronic structure [13].Therefore, the moiré superlattice can change the inherent properties of the material [14].
During the development of 2D materials, there are two pioneering studies.One is the study on constructing a moiré superlattice structure in graphene/hBN heterojunction in 2013, which has realized the transformation of a 2D superlattice to a single particle band [15].Another study is the study on preparing a magic-angle graphene in 2018, which has produced flat energy bands, unconventional superconductivity and other properties that have caused people to study magic-angle materials [16].The research began with graphene, but was not limited to graphene, and these findings have triggered a great enthusiasm in the field of twisted 2D materials, such as the twisted TMDCs (t-TMDCs).
TMDCs have been chosen as the main research object of this review, for the following reasons.First, there are abundant TMDCs materials, with unique band gap structure.The general formula of TMDCs is MX 2 (M represents the transition metal, X stands for chalcogenide elements such as S, Se, Te, etc) [17], which is rich in types of materials.Each specific TMDC has a unique band gap structure [18], which is easy to select one or more TMDCs with the suitable band gap, and then synthesize t-TMDCs with rich categories.Second, the flat band of the t-TMDCs moiré superlattice can be achieved under more relaxed conditions.Some research show that it was necessary to generate the flat band in twisted bilayer graphene (t-BLG) within a very small magic angle, such as 1.1 ± 0.1 • [16,[19][20][21], while the flat band in t-TMDC can be achieved at a relatively large twist angle of 4 • to 5 • [22].In addition, compared with t-BLG, few related reviews have been published on t-TMDCs, so it is urgent to summarize the preparation methods of t-TMDCs to guide the synthesis and property exploration of high-quality t-TMDCs.Based on this, the synthesis, characterization and properties of t-TMDCs have been reviewed in this paper.
In this review, we first highlight current physical and chemical methods for synthesizing t-TMDCs, as well as comparing their advantages and disadvantages.Secondly, we review the commonly used characterization methods of t-TMDCs, mainly including spectroscopy characterization and microscopy characterization, especially Raman spectroscopy, photoluminescence (PL), and second harmonic generation (SHG) in spectroscopy characterization.After introducing the synthesis and characterization methods, the key properties of t-TMDCs have been briefly analyzed, such as special ferroelectricity, flat band and interlayer excitons, and their potential applications have been also discussed.Finally, the potential prospects on the synthesis, characterization, and properties of t-TMDCs are summarized.

Synthesis method of t-TMDCs
In recent years, the research on twisted 2D materials has deepened, and the synthesis methods have become more diverse.Currently, the strategies for synthesizing t-TMDCs can be divided into physical and chemical methods, as shown in figure 1.Each synthesis method has its advantages and disadvantages, and the quality of the fabricated materials varies, which directly affects their performance.During the synthesis of t-TMDCs, factors such as surface cleanliness, twist angle, size, and number of layers need to be considered.This section will provide an overview of several common t-TMDCs synthesis methods, as well as comparing their advantages and disadvantages.

Physical methods
The physical synthesis method of t-TMDCs primarily relies on stacking and folding technique, which are strategies for material assembly.Since the physical properties of t-TMDCs largely depend on the quality and integrity of its structure, obtaining high-quality monolayer t-TMDCs from the original material is crucial.Any defects [23] or doping [24] can potentially impact the final performance of the t-TMDCs.During the stacking, the crystal orientation is a key factor.This is because the properties of t-TMDCs are closely related to the relative orientation between the two layers [25].Thus, it is essential to understand and control the crystal orientation of the involved monolayer before stacking.Once the layers are stacked together, readjusting the angle between them becomes very challenging.Therefore, initial control over the stacking angle is of utmost importance.Precise angle control can ensure the desired twist angle, thereby achieving the expected physical structure.

Stacking technique
Stacking technique refers to the method of combining two parts of TMDCs at a specific angle.Homojunctions can be formed by stacking two independently synthesized monolayers or by recombining a monolayer after it has been torn or cut.In contrast, heterojunctions are created by layer-by-layer stacking of two different monolayer TMDCs monolayers at a specific twist angle.
The most commonly used method for synthesizing homojunctions is the tear-stack technique [26], as shown in figure 2(a).The basic steps of tear-stack technique are as follows: the polymer is first dripped onto the glass, and half of the TMDC is picked up on the substrate.During the lifting, the TMDC is torn in half, then rotated to a certain angle, and the remaining TMDC is picked up again.Finally, the TMDC is transferred to a new substrate.After removing the polymer layer, t-TMDC is obtained.In the dry transfer method, polydimethylsiloxane (PDMS) is a commonly used polymer.Lin et al used this method to synthesize t-WSe 2 [27].They first mechanically exfoliated WSe 2 onto PDMS, then stamped part of it onto a silicon wafer, after the substrate was rotated, the remaining part was stacked.The adhesion between PDMS and TMDC is weak, often involving other polymers in the tear-stack process.Zeng et al also synthesized t-WSe 2 with a twist angle of about 1 • using dry transfer method [28].Since the viscosity of polycarbonate (PC) was temperature-sensitive, and its interaction force with WSe 2 was associated with the viscosity, a PDMS/PC/hBN Above method was simple, however, due to issues such as poor polymer flow, uneven stress, and lattice relaxation, the synthesized twist angle sometimes did not meet expectations.To reduce the strain during tearing, research show that the material can be pre-divided into two parts using an atomic force microscopy (AFM) tip [35], the laser [36] and other strategies.In Puretzky's study, the grown MoSe 2 substrate was first cut into two parts [31].One part was coated with PMMA, followed by removing the SiO 2 epilayer that was performed using etching in KOH solution.As a result of this procedure, we obtained a free-floating PMMA film with the MoSe 2 monolayer crystals on the solution surface.After the KOH was removed, that part was stacked with another part of monolayer MoSe 2 .And then acetone was used to remove PMMA, followed by annealing at 300 • C in flowing Ar (95%)/H 2 (5%) at atmospheric pressure for 2 h, to obtain t-MoSe 2 .The method of transfer involving the solution is called the wet transfer method.Using this method, W-doped t-MoSe 2 can also be prepared, as shown in figure 2(b).The low-frequency (LF) Raman vibration mode is derived from the weak interlayer vdW restoring force [37], which can be used as an indicator to directly detect the interlayer coupling effect in vdW-based layered structures [38].In the work of Puretzky et al, it was also found that the LF Raman peaks could not be detected without annealing the synthesized product.Therefore, annealing plays a key role in interlayer coupling.The dry transfer printing process is reported in the study of Huang et al, and the specific operation process is shown in figure 2(c) [32,39].During the fabrication, first, the PMMA is spin-coated on the top layer MoS 2 .After KOH is etched, PMMA-MoS 2 can be obtained, and then the film is inverted on the PDMS elastomer attached to the glass slide.At this point, the bottom MoS 2 sheet on the SiO 2 /Si chip is placed under the slide, and the two layers of MoS 2 can be stacked on top of each other at various twist angles.Lower the slide to touch the bottom layer and then carefully raise to remove the PDMS, running the annealing step to remove the PMMA.This technique does not introduce PMMA between the two layers of MoS 2 , and the interlayer coupling effect of obtained t-MoS 2 is strong.Similarly, Zheng et al also prepared t-WS 2 with twist angles from 0 • to 60 • by dry transfer method [40].Among them, one layer of WS 2 was first picked up by the poly propylene carbonate (PPC) membrane, and the twist angle was adjusted through the transfer platform.Then the WS 2 layer was stacked to another layer of WS 2 .The PPC can also be removed during heating and annealing.
It is worth noting that stacking technique is not limited to stacking t-TMDCs with micron-sized dimensions, it can also realize the synthesis of centimeter-level t-TMDCs.In 2020, Liao et al used a cutting machine to cut MoS 2 , and successfully synthesized centimeter-level t-MoS 2 with twist angles of 2    , and other angles using water-assisted transfer and stacking technique, as shown in figure 2(d) [33].Precise control of twist angles and synthesis of large-scale t-MoS 2 provide a pathway for industrial applications in twistable electronics and photonics.
Research has shown that the thickness of the thin film plays a crucial role in material synthesis.Thicker films tend to form uneven stress interfaces, while thinner ones are more conducive to obtaining conformal contact and even strain distribution [41].Based on these findings, Zhang et al introduced the thin-film assisted transfer technique [42].The technique uses a transparent, ultra-thin Formvar film on a TEM grid as the transfer medium, and the grid is precisely controlled by a robotic hand in the direction of movement.The monolayer TMDC was stripped from the underlying substrate by the ultra-thin support film, and then the saturated KOH solution was dropped into the TEM grid boundary as an etchant through a transfer method similar to that of PMMA, and the monolayer TMDC was obtained on the support film.t-TMDC can be obtained by repeating the stacking process.This method has three advantages.Firstly, the ultra-thin film as the transfer medium can ensure good contact between TMDC and the transfer medium, and obtain a structurally complete material.Secondly, the transferred product can be directly used for TEM analysis, which has great advantages in atomic-resolution imaging.Thirdly, the strategy can precisely control the twist angle with an accuracy of 0.37 • .
Heterojunctions are usually synthesized using stacking methods.Wang et al first grew monolayers of WS 2 and WSe 2 using the chemical vapor deposition (CVD) method, and then manually stacked them using a wet transfer method, resulting in a WSe 2 /WS 2 bilayer heterojunction with different twist angles.Structural schematic diagram and OM image of the twist WSe 2 /WS 2 are shown in figure 2(e) [34].Additionally, Lim et al obtained monolayers of WSe 2 and MoSe 2 through mechanical exfoliation method, and stacked the WSe 2 monolayer on MoSe 2 using a PDMS-assisted dry pressing method.The resulting WSe 2 /MoSe 2 has 2H-like and 3R-like twist angles [43].Similarly, Debnath et al described the preparation method for the MoS 2 /WS 2 heterojunction, and the micro-manipulation platform they used can precisely control the twist angle with an accuracy of 0.1 • [44].In addition, heterostructures with multiple twist interfaces can also be synthesized by this method.For example, Zheng et al prepared the heterostructure of WSe 2 /WS 2 /WSe 2 by PMMA-assisted transfer method, in which the top and bottom WSe 2 sheets were aligned, and the twist angle of the middle WS 2 sheet relative to the WSe 2 sheet was 3 • [45].At the interface of this heterostructure, two moire´period were generated, leading to new quantum phenomena.The stacking techniques basically involve transfer operations.Common polymers used in the transfer operations include PMMA [31,45,46], PDMS [27,43], and PPC [40].These polymers can enhance the adhesion to 2D thin films, thereby facilitating the transfer of TMDCs from the original substrate.In the wet transfer method, PMMA is one of the most used polymers.It is facile to operate, and the samples are easily accessible, but removing it requires immersion treatment in solution.The dry transfer method often uses PDMS polymer film as a carrier.This method does not require spin coating and avoids contact with the solution, making it suitable for 2D materials that are prone to hydrolysis or moisture absorption.However, in the dry transfer method, the quality of the samples is significantly influenced by the surface smoothness of the substrate and the contact pressure, thus it is necessary to precisely control the pressure during the transfer.

Folding technique
Mechanical stacking techniques cannot meet the construction needs of complex twisted structures, hence the need to develop new folding techniques.2D materials like graphene, MoS 2 , hBN, etc, due to their outstanding mechanical properties [47], these materials can be folded without damaging the structure under appropriate conditions.Initially, the folding strategy has applied water flow to gently flush the sample to make a folded layer [48], but an adsorption layer may be formed between the folded layers.During the preparation, water, oxygen, or nitrogen molecules may remain in the folding area, thus affecting the interlayer interactions.To achieve a cleaner product, Castellanos-Gomez et al employed a method in which they first moved mechanically exfoliated MoS 2 flakes to a pre-stretched elastic substrate [49].When the pre-stress applied by elastic substrate was released, the sheet on the substrate would be compressed, leading to wrinkled MoS 2 .The fold was induced by strain, as shown in figure 3(a) [50].However, the aforementioned folding method produce limited twist angles, making it difficult to control and form a specific twist angle for t-TMDCs.Zhao et al developed a fluid dynamics strategy for preparing bilayer t-MoS 2 with controllable twist angles [51].Polystyrene (PS) particles were applied as the fluid, thus forming different twist angles by controlling the flow direction.The results show that, the PL peak of folded bilayer structure prepared by this strategy showed a certain regularity, and its corresponding PL peak of direct bandgap transition was gradually weakened with the reappearance of the indirect bandgap transition peak.Thus, the fluid dynamics method is one of the effective strategies to regulate interlayer coupling.
Most research on twisted 2D materials focuses on bilayer structures.However, compared to bilayers, multilayer structures provide more interfaces, thus enhancing their tunability and complexity.Wang et al proposed an optimized method to prepare wrinkle arrays on rigid substrates, which involves epitaxially growing a single layer on a quartz substrate and then quickly cooling it to form a wrinkle array, as shown in figure 3(b) [52].Unlike the product obtained earlier, there are fractures at the folds due to the high stiffness of WS 2 .The top layer and the bottom layer maintain the same arrangement as the adjacent layer, respectively, and the middle layer shows a anticlockwise rotation of 15 • .In order to obtain a multilayer t-TMDC with a wider range of twist angles, Zhang et al introduced a paraffin-assisted folding technique in 2021 [53] and studied multilayer MoS 2 with a twisted interfaces prepared using this method in the following year [54].The specific operation diagram is shown in figure 3(c).Firstly, the paraffin layer is spin-coated on the MoS 2 /SiO 2 /Si substrate.After curing, the paraffin shrinks at low temperature, resulting in folding.Then, it is transferred to the target substrate, and the paraffin is dissolved with n-hexane.Finally, the twisted interface of t-MoS 2 can be obtained by this method, with a twist angle of 7 • , 10 • , 15 • , 18 • , 22 • and 27 • .Compared to previous folding techniques, it exhibits a wider range of twist angles.AFM and PL mapping results indicate that there are no significant cracks, wrinkles, or contamination in the product, confirming the high quality of the obtained materials.

Chemical methods
Chemical methods are often referred to as the bottom-up synthesis methods.Compared to physical methods, one of the biggest advantages of chemical methods is that the interfaces of obtained t-TMDCs can be cleaner.Currently, the chemical methods for synthesizing t-TMDCs mainly include CVD, epitaxial growth, and hydrothermal methods.The CVD method has been researched more extensively compared to the other two methods.Therefore, this section will focus on the applications and influencing factors of CVD, while briefly outlining the applications of epitaxial growth and hydrothermal methods.

CVD
The stacking and folding methods mentioned in section 2.1 usually require the preparation of single-layer TMDCs through the CVD method first.However, the application of CVD is not limited to the synthesis of single-layer TMDCs, it can also directly synthesize bilayer t-TMDCs.This method allows for the direct acquisition of high-quality and controllable t-TMDCs without the requirement for sample transfer.
MoO 3 and sulfur powder are the most commonly used precursors for the synthesis of MoS 2 , Liu et al successfully synthesized bilayer MoS 2 using these two precursors with a yield of 30%.However, the main twist angles in the bilayer were still 0 • and 60 • , and the proportion of special twist angles was only 5%.Notably, N 2 was chosen as the carrier gas in this study, rather than the typical Ar or a mixture of Ar and H 2 .They mentioned that the key factor for synthesizing t-MoS 2 via CVD is to slow down the growth rate in the initial reaction stage, thus promoting the vertical growth [55].Apart from MoO 3 , molybdenum foil is also a promising precursor for synthesizing t-MoS 2 .Han et al reported the synthesis of bilayer t-MoS 2 with twist angles from 0 • to 60 • by face-to-face CVD method using molybdenum foil and sulfur powder as precursors [56,57].Relevant OM images can be seen in figure 4(a).
WS 2 is also a key member of the TMDCs family.In 2013, Zhang et al synthesized WS 2 on a sapphire substrate using low-pressure CVD [63].They observed specific moire´patterns at the folded edges of 2L and 3L and determined its twist angle to be 19.7 • .The authors also found that adjusting the distance between the precursor and the substrate can effectively control the thickness and twist angle of WS 2 .Additionally, the edge morphology of WS 2 can be significantly improved by H 2 .In studies on WS 2 synthesis, Zheng et al particularly emphasized the significant influence of temperature on t-WS 2 synthesis [58].They observed that at a high temperature of 1100 • C, the growth of t-WS 2 is more favorable.However, at 850 • C, the formation of AA and AB bilayers was mainly observed.This might be attributed to the top layer tends to grow following the nucleus orientation and overcome the angle mismatch with the bottom layer at a high temperature.In this experiment, t-WS 2 with twist angles of 0 • , 13 • , 30 • , 41 • , 60 • , and 83 • was synthesized at 1100 • C (figure 4(b)).Similarly, Fang et al found that increasing the growth temperature promoted the formation of bilayer and multilayer WSe 2 when studying the growth mechanism of bilayer WSe 2 [64].
The one-step CVD method holds potential in the synthesis of twisted hetero-TMDCs.However, due to its single-step synthesis properties, it may face competitive reactions including alloying or doping [65][66][67].To overcome this challenge, Zhang et al designed a core-shell nanowire structure, where WO 3−x acts as the core and MoO 3−x as the shell [68].Since the sublimation temperature of WO 3−x was significantly higher than that of MoO 3−x , ensuring that Mo and W were supplied in sequence.During the continuous growth of MoS 2 and WS 2 , a WS 2 /MoS 2 structure with a twist angle of 20 • can be successfully synthesized.Through proper design of precursor structure, Mo and W can be sequentially fed, thereby ensuring the layered growth of MoS 2 and WS 2 .
Bilayer TMDCs with 0 • and 60 • are the most stable configurations because of the lowest energy.This is also why the proportion of t-TMDCs with specific angles obtained with conventional CVD methods is not high.The orientation of 2D materials is determined during the nucleation phase and influenced by the environment around the nucleation site.Therefore, adjusting the environment during the nucleation phase is crucial in the synthesis.To efficiently synthesize t-TMDCs, Shao et al proposed a heteroatom-assisted strategy [59].SnO 2 powder was placed in front of WO 3 powder, NaCl was used as a flux because of the high melting points of these two powders.Eventually, the product was deposited on a 1 × 3 cm SiO 2 /Si substrate.In this strategy, the introduction of the Sn as the heteroatom helped overcome stacking free energy, thereby increasing the proportion of WS 2 with a 60 • twist angle, and successfully synthesizing WS 2 with twist angles of 15 • , 30 • , and 45 • .The OM images can be seen in figure 4(c).Zheng et al used a similar heteroatom-assisted growth strategy and gas disturbance strategy to successfully synthesize WSe 2 with twist angles of 1.5 • , 24 • , and 30 • etc [69].In addition to the previously mentioned precursors, they also attempted to mix ammonium tungstate and KOH to prepare a tungsten source, and H 2 flow rate was adjusted to optimize the nucleation environment.An important finding of the study is that using this method successfully synthesized t-WSe 2 with an angle of about 1.5 • .The WSe 2 grown by the CVD exhibited strong interfacial coupling, with its moire´potential depth increasing by 155% compared to the manual stacking method.This implied that enhanced interface coupling increased the moire´flow potential.

Other methods
Epitaxial growth is a method to synthesize single crystals with specific requirements and the same crystal orientation.The t-TMDCs can be synthesized through the multi-step epitaxial growth method.During epitaxial growth, to obtain products with a smooth surface and high quality, the substrate must undergo rigorous polishing, control the right temperature and select the appropriate gas flow.In 2018, Ahn et al first synthesized t-BLG with a twist angle of 30 • via the epitaxial growth [70].Following this, this strategy has also been applied to synthesize other twisted 2D materials.Sutter et al utilized van der Waals epitaxy and solid-state transition methods to synthesize t-TMDCs [60].The transformation process is shown in figure 4(d).This reaction uses high-quality SnS 2 single crystals as a substrate, and then the precursor of SnS powder is evaporated onto SnS 2 , which in turn provides an S source to convert SnS to SnS 2 .The source of sulfur can be decomposition of the underlying SnS 2 or externally supplied.In the study of universality, it has been found that when the original substrate such as MoS 2 and WS 2 has a high bond dissociation energy, it is difficult to decompose, so sulfur is mainly dependent on external supply.Therefore, this strategy has a broad applicability.By adding an additional sulfur source in the vapor phase, it might also be suitable for other non-sulfur TMDCs, further promoting the preparation of twisted heterostructures related to SnS 2 .
The epitaxial growth technique discussed above mainly generates heterojunctions or homojunctions with a twist angle of 30 • , and its twist angle is relatively single.Zhao et al successfully prepared TiTe 2 /TiSe 2 heterostructures with twist angles of 0.5 • , 15 • , and 34 • by molecular beam epitaxy [61], the structure diagram of the product is shown in figure 4(e).Different from conventional SiC epitaxial growth, this method uses BLG/SiC as a substrate, first grow TiSe 2 bilayers on it, and then continue to grow monolayer TiTe 2 vertically.To reduce the competitive reaction of Se, TiTe 2 needs to grow in Te-rich environment.In particular, numerous heterostructures with small twist angles were synthesized in this work.Besides, Xie et al used the hydrothermal method, taking (NH 4 ) 10 W 12 O 41 •xH 2 O and CH 4 N 2 S as raw materials, to synthesize WS 2 superlattices in a single step [62].The formation of the moire´superlattice is due to the sliding of the S-W-S layers during bending and twisting, as show in figure 4(f).Studies have shown that its twist angle is between 13 • and 14 • .

Discussion
As research on t-TMDCs progresses, the synthesis methods are becoming increasingly diverse, each with its own unique characteristics.Overall, physical methods can more precisely control the twist angle.Stacking methods are suitable for the preparation of homojunctions and heterojunctions, while the folding method is mainly used for the synthesis of homojunctions.The stacking process also involves the corresponding transfer techniques.Importantly, the interlayer impurities play a significant role in coupling, making it crucial to enhance interlayer coupling after stacking.When using wet transfer stacking, it is common to use acetone to remove the polymer, followed by annealing.The purpose of these steps is primarily to remove any residues and adsorbates to ensure they do not impact interlayer coupling.For dry transfer methods, annealing is usually performed at the end to remove organic residues and air gaps, thereby enhancing interlaminar coupling.
Apart from the conventional methods, designing boundary is also a strategy for precisely controlling the twist angle of 2D materials.For instance, the desired twist angle can be achieved by adjusting the specific hydrophilic and hydrophobic boundaries.Its folding angle is altered by varying the angle between the hydrophilic boundary and the edge of the substrate used for folding (folding angle), resulting in the desired twist angle [71].Folding and stacking methods are relatively time-consuming, and utilizing a microscope for precision operations demands high instrument accuracy.A recent research report has indicated that immersing CVD-synthesized BLG in 1,2-dichloroethane can weaken its interlayer interactions, allowing the graphene to slide and rotate.A t-BLG ratio of up to 84.4% can be achieved, with twist angles predominantly near 0 • and 30 • [72].These synthesis strategies for t-BLG may also be applicable to the preparation of t-TMDCs.
Overall, physical methods involve transfer step are intricate, prone to interlayer contamination, and lack scalability and uniformity.Therefore, it is imperative to study the chemical methods for synthesizing high-quality t-TMDCs.CVD is a mainstream technique in chemical methods, and its synthesis outcomes can be affected by factors such as growth temperature, growth time, and gas flow rate.Epitaxial growth mainly depends on the orientation of different crystals and substrates, such as the mentioned t-SnS 2 synthesis which relies on a specific intermediate product substrate, limiting its range of applications.While current chemical methods can prevent interface contamination, the yield of synthesized t-TMDCs remains low, and angle control is challenging.Studies have found that the use of hetero-site nucleation strategy in the synthesis can increase the proportion of t-BLG to 88% [73].The gas-flow perturbation strategy could offer valuable insights into controlling the growth of t-TMDCs and hold promise for future t-TMDCs synthesis.

Characterization method of t-TMDCs
Characterization plays critical role in studying the internal structure, chemical composition, and properties of t-TMDCs.Due to the limitations of a single method, a combination of multiple characterization methods is generally employed.This section will focus on two common methods, spectroscopy characterization and microscopy characterization.Spectroscopy characterization mainly reveals interlayer properties and their angle-dependency.Determining the twist angle is challenging since the potential similarities in spectra across t-TMDCs with different angles.Complementary to the spectroscopy characterization, microscopy characterization holds significant advantages in determining twist angles, atomic arrangements, and moireṕ eriods.

Spectroscopy characterization
Spectroscopy characterization plays very important role in understanding the interaction between t-TMDCs layers.This section mainly introduces common spectroscopy characterization methods of t-TMDCs, including Raman spectroscopy, PL, and SHG.

Raman spectroscopy
Raman spectroscopy can obtain information about phonon vibrations through the mechanical vibration of molecules.When a laser source is incident on a molecule, the scattering light can be changed by its chemical structure.A small number of scattered light that differs from the wavelength of the incident light is called Raman scattering.Raman spectroscopy is non-destructive, which can quickly analyze the number of layers, twist angle, type of doping, and strain of 2D materials.Therefore, it is crucial in characterizing t-TMDCs.
The most common high-frequency (HF) Raman spectra can be used to identify the type of material and the state of interlayer coupling.In the research of Liao et al, using a water-assisted transfer method, t-MoS 2 with different twist angles can be obtained, such as 2 • , 4 • , 6 • , and etc [33].The HF Raman spectra are shown in figure 5(a).The peaks at 384 cm −1 and 407 cm −1 correspond to the E 2g in-plane mode and the A 1g out-of-plane mode, respectively.The E 2g peak remains stable at different twist angles, while the intensity and position of the A 1g peak vary with the twist angle.From the perspective of peak position, the increasing distance between the E 2g and A 1g peaks indicates enhanced interlayer coupling [55,74,75], which is most evident in the stacking mode of 0 • .From the perspective of peak intensity, the peak intensity of A 1g decreases significantly with the increase of twist angle, which reflects that the electron-phonon coupling of the A 1g mode is stronger than that of the E 2g mode [76].When the twist angle is greater than 8 • , a new peak appears at 411 cm −1 , which is related to the eccentric phonon associated with the lattice vectors in the moireŕ eciprocal space.In 2018, Lin et al also observed the new mode of t-MoS 2 at 411 cm −1 , showing dependence with twist angle [77].
In LF Raman spectroscopy, t-TMDCs exhibits two characteristic peaks: the in-plane shear mode and the out-of-plane breathing mode, both of which are originated from interlayer relative vibrations.Compared to HF Raman, LF Raman is more sensitive to interlayer coupling and changes in the layer number [78], thus providing more comprehensive information.The LF Raman test can be considered a fingerprint for measuring interlayer coupling.Huang et al conducted a LF Raman spectroscopy study on MoS 2 homojunctions with different twist angles [39].As shown in figure 5(b), the MoS 2 of the 2H stack (Exf.2L)exhibits a sharp shear pattern at 22.9 cm −1 and a wide breathing pattern at 38.1 cm −1 .This is due to the mixing of multiple highly symmetrical stacks around 0 • or 60 • , resulting in significant changes in stacking and interface coupling upon twisting.As a result, the frequency and intensity of the Exf.2L's shear and breathing mode vary greatly, while the t-MoS 2 exhibits a lower intensity shear mode and a higher intensity breathing mode, and even no shear mode at certain twist angles.In addition, t-MoS 2 with different twist angles also exhibits different shear and breathing mode characteristics.A similar phenomenon can be observed in other bilayer TMDCs materials [69,79,80], with shear mode and breathing mode having some angular dependence.
Raman spectroscopy can be used to characterize not only homojunctions but also heterojunctions.As shown in figure 5(c), the LF Raman spectra of individual bilayer MoSe 2 , individual bilayer WSe 2 , and MoSe 2 /WSe 2 with different twist angles are displayed [43].In the 2H-like and 3R-like structures, the layer shear and layer breathing modes are clearly visible, and the peak intensities of the two modes are opposite.In the range from 4.2 • to 28.9 • , only the layer breathing mode peak can be observed.Both experimental and theoretical results indicate that, the layer shear mode mainly appears at the twist angles close to the 2H and 3R phases, as show in figure 5(d).The layer shear mode occurs because of the domain caused by reconstruction [81].

PL spectroscopy
PL describes the phenomenon where a material emits photons after absorbing energy from a light source and gets excited.Here, how the PL intensity and shift change with the twist angle or number of layers of t-TMDCs has been primarily discussed.
The PL displays different characteristics with the change of twist angle of t-TMDCs.Moreover, PL is highly sensitive to the layer number of TMDCs.As it transfers from a monolayer to multilayers, the electronic structure changes from a direct bandgap to an indirect bandgap, leading to a decrease in PL intensity [63,82].Figure 6(a) shows the PL spectrum of monolayer MoS 2 and t-MoS 2 [55].Among them, peak I represents the direct band gap and peak II represents the indirect band gap.The indirect band gap formed by interlayer electron coupling leads to the formation of peak II, whose intensity reflects the strength of interlayer electron coupling.For example, the intensity of peak II at 15 • exceeds that at 0 • and 60 • , indicating that the interlayer coupling strength is higher at 0 • or 60 • .Compared to the monolayer, the peak I intensity of the bilayer is significantly reduced.Besides, Zhao et al also obtained a similar phenomenon by PL characterization of t-MoS 2 synthesized by liquid phase exfoliation, as shown in figure 6(b) [83].This phenomenon is not only observed in t-MoS 2 , but also in t-WSe 2 .Chen et al characterized the PL of t-WSe 2 , as shown in figure 6(c), the direct bandgap peak is highly correlated with the number of layers [84].As the number of layers increases, the peak intensity decreases (with the bilayer peak intensity amplified by 2 times) and undergoes a redshift, indicating a decrease in the bandgap.Additionally, the intensity of the indirect bandgap increases with angle within the range of 1 • to 32 • .However, the intensity of indirect bandgap peak significantly decreases at 60 • , indicating stronger interlayer coupling at 1 • and 60 • .It should be noted that since these samples are prepared using the dry transfer method, inevitable interlayer contamination may affect their interlayer coupling.
To sum up, combined with the spatial repulsion effect in figure 6(d), the standard for interlayer coupling can be summarized as: the smaller interlayer distance, the weaker the indirect bandgap, indicating stronger interlayer coupling.

SHG spectroscopy
SHG belongs to the category of nonlinear optics, where the symmetry and orientation of the crystal can be sensitively detected with the second harmonic response.It is a non-invasive and efficient characterization tool.The interlayer twist angle of t-TMDCs can be determined by analyzing the polar plots of the upper and lower layers.The SHG response of t-TMDCs strongly depends on its layer number and twist angle.The SHG of the twisted bilayer is formed by the coherent superposition of the SHG field of the monolayer, with its phase difference affected by the stacking angle.SHG signals are often displayed as mapping images and polar coordinate images.For instance, figure 7(a) shows the SHG mapping of t-MoS 2 at different twist angles [85].As the twist angle increases, the SHG intensity in the bilayer region changes significantly.The SHG intensity of the stacking area significantly weakens when approaching antiparallel, highlighting that SHG requires a medium without inversion symmetry.
Similar phenomena exist in twist heterostructures, as shown in the work reported by Yuan et al (figures 7(b)-(e)) [86], for twisted WSe 2 /WSe 2 and WSe 2 /WS 2 , when the stacking angle is less than 10 • , the SHG intensity is significantly suppressed, while for the twisted WSe 2 /MoS 2 , the SHG intensity is much lower than predicted by the model at all stacking angles.This bias is also attributed to the influence of interlayer coupling on the SHG of t-TMDCs bilayer.The angle-dependence of the SHG response cannot be explained by a simple interference model of the coherent superposition of two monolayers, it is also related to the composition of the material.Besides, the twist angle of the sample can be determined based on the polar plots.Figures 7(f) and (g) display the polar plots of SHG in two monolayer regions.In parallel polarization, the direction of maximum intensity aligns with the direction of the incident light polarization, and the polar coordinate map displays a six-petal pattern, with the petals aligning with the perpendicular bisector of the triangle.Figure 7(h) is the overlaid polar plots, where the six petals are located between the perpendicular bisectors of the two monolayers, with the corresponding twist angle of 25 • .
Since strain changes the symmetry of the crystal, strain also has a significant effect on the response of SHG [87].For example, Hung et al took heterojunction MoSSe/MoS 2 as the research object and optimized the nonlinear optical response in SHG of the MoSSe/MoS 2 as a function of strain and stacking order.The effect of strain on SHG was further evaluated [88].Figure 8 shows the absolute values of photon energies for AA and AB stacked with biaxial and tensile strains.Under biaxial strain, the lattice deformation does not change the point group symmetry, so the second-order nonlinear susceptibilities can be obtained by both AA and AB stacked heterojunctions.The symmetry is broken under tensile strain, and three independent in-plane susceptibilities appears.Compared with the second-order nonlinear susceptibilities without strain, both the biaxial strain and tensile strain can increase the second-order nonlinear polarizability by two times.After the application of biaxial strain and tensile strain, the second-order nonlinear susceptibilities changes, and the second-order nonlinear polarization rate of the AA stacked structure is not the same as that of the  (2) (χxxx (2), χxyy (2) , and χyxy (2) = χyyx (2) )of AA-MoSSe/MoS2 as a function of photon energy for biaxial strain sxy = 0.04, and tensile strains along x-(sxx = 0.04) and y-directions (syy = 0.04), respectively.(b) |χ (2) | of AB-MoSSe/MoS2 as a function of photon energy for sxy, sxx, and syy, respectively.χxxx (2) without strain (black line) is plotted as the reference value.Reprinted with permission from [88].Copyright (2023) American Chemical Society.
AB stacked structure.Therefore, the SHG intensity can effectively distinguish between AA and AB stacked heterojunctions.

Microscopy characterization
OM uses optical principles to magnify and image tiny substances, making details that are difficult for the human eye to discern visible.It is a commonly used instrument for obtaining microscopic structural information.Figures 2(a),(b), 3(a),(b) and (d) display typical OM images of t-TMDCs.From these images, the twist angle of the t-TMDCs can be directly estimated and the cleanliness of the product surface can be observed.However, due to the limited resolution of OM, it is not possible to accurately measure twist angles less than 5 • , so selected area electron diffraction (SAED) is typically used for conducting more precise measurements.Mandyam et al characterized a WSe 2 /WSe 2 homojunction with a twist angle of 15 • was characterized using SAED, as shown in figure 9(a) [89].According to the relative angles of the two sets of diffraction points, the twist angle of the structure can be determined.SAED allows the analysis of twist angle at the atomic level, which is more accurate in measuring twist angles.However, because it requires preliminary transfer operations, the operability of SAED is not so intuitive and convenient as OM.
The surface topography can be detected by AFM through the atomic force relationship between the probe and the sample.AFM plays a key role in characterizing the layer number and thickness of TMDCs.AFM can determine the boundary between monolayer and bilayer of t-TMDCs, assess interlayer coupling using spatial repulsion forces, and identify the quality of the product.On a microscopic level, conductive atomic force microscopy (CAFM), a derivative technique of AFM, can be used to investigate the conductivity of different regions of the material.Rosenberger et al used CAFM to measure t-TMDCs and found that under small twist angles of δ ⩽ 1 • , the heterostructure of MoSe 2 /WSe 2 underwent significant atomic restructuring [13].This R-type and H-type MoSe 2 /WSe 2 heterostructure at small angles differs from the traditional rigid lattice moiré theory.Figure 9(b) shows a discrete triangular lattice with alternating high and low conductivity at 0 • + δ. Figure 9(c) shows the CAFM measurement results at 60 • + δ, where the large hexagon with different conductivity domain boundaries is clearly visible.Similarly, Weston et al used CAFM to study the atomic structure and electronic properties of WS 2 and MoS 2 with twist angles less than 3 • [12].
The resolution of scanning transmission electron microscopy (STEM) far exceeds that of AFM and OM, providing structural details of materials at the atomic level.For t-TMDCs, STEM imaging enables direct imaging of moiré superlattices and surface reconstructions.Zhao et al prepared t-TaS 2 by liquid phase exfoliation, and its STEM characterization results are shown in figures 9(d)-(h).These results clearly show the moiré stripes at different twist angles, indicating that small twist angles lead to large moiré periods [83].In addition, STEM is often used in conjunction with other detectors, and characterization techniques such as aberration corrected annular dark-field STEM(ADF-STEM), four dimensional STEM(4D-STEM) have emerged.Under the same imaging conditions, STEM-ADF is less affected by aberration than ordinary STEM, so the image contrast is better.Zhao et al used STEM-ADF and an advanced STEM corrector to study the atomically fine moiré features and associated strain distributions of the t-TMDCs bilayer [90].The 4D-STEM has more powerful functions, it can not only reconstruct any conventional STEM images, but also obtain more information through reconstruction, such as the structure of materials, crystal orientation, stress or strain distribution, iDPC images [91], etc.The technique can even further improve spatial resolution through tandem diffraction imaging.Sung et al and Van Winkle et al obtained information on twist low-dimensional materials using 4D-STEM.The former observes molar lattice recombination in twist angle low-dimensional materials [92], and the latter identifies different reconstruction mechanisms in moiré homobilayers and heterobilayers [93].

Ferroelectricity
Ferroelectricity refers to materials that exhibit spontaneous polarization within a certain temperature range, and their direction of polarization can be altered by an external electric field.These materials can be widely applied in non-volatile memory, field-effect transistors, and microwave devices.The 2D van der Waals ferroelectric materials typically show a low-symmetry atomic structure and can mainly be divided into two types: intrinsic ferroelectrics and twisted stacked ferroelectrics.The former is mainly derived from layered polar materials fabricated through a top-down approach, creating 2D ferroelectrics like In 2 Se 3 [94][95][96] and CuInP 2 S 6 [97,98].The latter is prepared with a bottom-up strategy based on van der Waals assembly, through which the 2D ferroelectric materials can be designed from non-ferroelectric parent materials.When two layers of non-polar materials are stacked at a specific angle, both their band structure and crystal symmetry can be regulated by this twist angle, resulting in the formation of new synthetic ferroelectrics.This phenomenon is first discovered in t-BLG/BN (figures 10(a)-(c)) and later applied to other van der Waals layered systems [99].
In t-TMDCs materials, ferroelectric phenomena have also been observed.Fei et al have reported that bior tri-layer stacked WTe 2 exhibited switchable and spontaneous out-of-plane electric polarization, as shown in figure 10(d), arising from electron-hole correlation effects instead of lattice instability [100].Notably, bulk crystalline WTe 2 was a confirmed room-temperature ferroelectric semimetal [103].Thus, the application of such materials is constrained by the necessity of a polar space group in the original bulk crystal.Wang et al revealed interfacial ferroelectricity in rhombohedral stacked bilayer TMDCs.The rhombohedral-stacked (R-stacked) configuration comprises of two parallel layers and breaks the out-of-plane mirror symmetry, resulting in the interlayer charge transfer and out-of-plane electric dipole (figure 10(e)).The ferroelectricity can be achieved in these four twisted R-stacked MoS 2 , MoSe 2 (figure 10(f)), WS 2 and WSe 2 , whose intrinsic properties are non-ferroelectric at monolayer.The ferroelectricity found in t-TMDCs not only enriches the families of 2D ferroelectrics, but also provides the opportunity to explore the mechanism of ferroelectricity.The correlation between ferroelectricity with the electric and optical properties of TMDCs was also revealed [101].These 2D materials hold potential in the construction of ferroelectric semiconductor field-effect transistors.Besides, Weston et al observed robust room-temperature ferroelectricity in marginally t-MoS 2 .In t-MoS 2 bilayers, ferroelectric domain networks can be modulated by an external out-of-plane electric field (figure 10(g)), eliciting a significant response in the lateral electronic properties of the twisted layers [102].The observed ferroelectricity in t-MoS 2 offers promising applications in future atom-level electronic devices with memory functionalities.

Flat band
Band structures are essential to influence the electronic, optoelectronic, and magnetic properties of materials.Achieving flat bands in 2D materials has always been a focal point for scientists.When the flat band in materials is close to the Fermi surface, it often leads to a high electronic state density and the appearance of van Hove singularities.It is conducive to inducing special quantum states, such as superconductivity [19,20].Thus, flat bands are an ideal platform for realizing various special quantum states.Currently, scientists have adopted multiple methods to achieve flat bands, such as applying external magnetic fields [104], strain engineering [105,106], and tuning twist angles.Among these, preparing 2D material structures through Reproduced from [22], with permission from Springer Nature.twist angles and stacking sequence is the most straightforward.The electronic band structure varies with the change of angle, potentially leading to localized flat bands and enhanced electron correlation.The flat bands have been reported in t-BLG [107], bringing about properties such as superconductivity [108] or correlated insulators [16].The flat band was generated at a small twist angle of approximately 1.1 • .Theoretical predictions suggested that TMDCs can form moiré superlattices and flat bands over a broader range of angles [109].Experiments have confirmed the existence of flat bands in t-TMDCs.Zhang et al used a dry transfer technique to precisely control the stacking angle of two layers of WSe 2 on a graphene substrate, resulting in samples with twist angles of 3 • and 57.5 • .The two t-WSe 2 samples were characterized with scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS).As shown in figures 11(a)-(d), there are spectral features of flat band at two different angles and their localized properties are successfully imaged [110].The spatial positions of flat bands in 3 • and 57.5 • t-WSe 2 samples were different, which were consistent with first-principles density functional theory calculation [109].
The flat bands of t-TMDCs have been reported in several studies.When the stacking angle of bilayer t-WSe 2 is within the range of 60 ± 3 • , Li et al found that a flat band with a bandwidth of less than 10 meV appears at its valence band edge.Specifically, when the twist angle approaches 60 • , the number of flat bands increases, and the bandwidth decreases (figure 11(e)).The theoretical calculations further suggest that the emergence of the flat band is related to both the interlayer hybridization and atom-level reconstruction in the moiré superlattice.The observed number of flat bands and the spatial distribution of their wave functions also confirmed the specific impact of this reconstruction on the flat band [111].Wang et al prepared devices with high-quality bilayer t-WSe 2 (figures 11(f)-(h)).For the bilayer t-WSe 2 with angles ranging from 4 • to 5.1 • , by controlling the carrier concentration at hole side to one hole per moiré unit cell, a resistance peak appears, exactly corresponding to the half-filling of the flat band.Moreover, at the angles of 4 • and 4.2 • , resistance peaks are observed, corresponding to the full-filling of the moiré minibands.It confirms that the moiré superlattice induced by the twist angle will generate flat bands [112], and the resistance peaks at half-filling correspond to correlated electronic states (figure 11(i)).Subsequently, a series of behaviors associated with the insulation states were measured at half-filling, as a function of twist angle and electric field.Finally, at a twist angle of 5.1 • , by controlling the electric field to achieve the strongest correlated state, zero-resistance phenomena were observed at temperatures below 3 K on both sides of half-filling, indicating the possible existence of a superconducting state in the system [22].The flat bands can be designed in the corner-transitioning bilayer WSe 2 system, whose correlated insulating states is similar to those in moiré superlattices of graphene, but with different degeneracy and single-particle band structures.Reproduced from [119], with permission from Springer Nature.(e) Reflection contrast spectra of t-WSe2/WS2 heterostructures with a small twist angle (light blue, top) and a large twist angle (black, bottom), respectively.(f) Detailed reflection contrast spectra in the range of 1.6-1.9eV (the WSe2 A exciton) on the electron-doping side.Units of the electron concentration: cm −2 .Reproduced from [120], with permission from Springer Nature.(g), (h) Doping-dependent PL spectra of the R-stacked (g) and H-stacked (h) WS2/WSe2 device.Reproduced from [125], with permission from Springer Nature.(i) Gate-dependent PL spectra of the hBN encapsulated WS2/WSe2 heterobilayer device at 4.2 K, with CW excitation centered at 1.959 eV with an excitation power of 5 µW.(j) PL peak position (red) extracted from figure 11(i) as a function of the gate voltage, correlated with microwave impedance microscopy (MIM) measurements of the local conductivity (blue) taken at 14 K. Dashed lines are fillings corresponding to the insulating states determined from the MIM measurements.Reproduced from [123].CC BY 4.0.(k) Schematic of probing interlayer exciton transport at the K-K valleys using transient absorption microscopy (TAM).The valance and conductions bands are marked by solid and dashed lines, respectively.Reproduced from [126], with permission from Springer Nature.(l) TAM morphology imaging of the two WS2/WSe2 heterobilayers with 0 • and 60 • twist angles, respectively.Reprinted with permission from [126].Copyright (2020) Springer Nature.(m) Reflectance contrast spectra of bilayers of different twist angles θ i , for i = 1-6.Reproduced from [127].CC BY 4.0.

Moiré excitons
Similar to electronic bands, excitons form multiple moiré bands under moiré periodic potential, leading to the formation of moiré excitons.These interlayer excitons have various intriguing properties, such as valley-contrasting physics [113,114], long lifetimes [115,116], high tunability with electric fields [114,117], and so on.Tran et al observed multiple interlayer exciton resonances with positive or negative circularly polarized emission in MoSe 2 /WSe 2 bilayer heterostructures with small twist angles (figures 12(a) and (b)) [118].The interlayer moiré exciton was first experimentally verified by Seyler group, and the interlayer valley excitons can be captured by the moiré potential in MoSe 2 /WSe 2 heterostructures (figures 12(c) and (d)).The observed effects originate from the capture of interlayer excitons in a smooth moiré potential and inherit the valley-contrasting physics characteristics [119].Moreover, Jin et al explored the moiré superlattice exciton states in WSe 2 /WS 2 heterostructures.In the adsorption spectrum, these moiré exciton states display multiple peaks around the initial WSe 2 A exciton resonance (figure 12(e)).Compared to the A exciton in single-layer WSe 2 and in WSe 2 /WS 2 heterostructures with large twist angles, they show different gate dependencies (figure 12(f)) [120].Moiré exciton bands provide an enticing platform for exploring and controlling the excited states of materials, such as topological excitons and the associated exciton Hubbard model in TMDCs.Wu et al studied on the impact of moiré patterns in TMDCs like MoS 2 and WS 2 , revealing that optical absorption spectra in bilayers are significantly altered by extended-period moiré patterns.These patterns introduce excitonic peaks associated with the twist angle, paving the way for engineering topological collective excitations [121].In a related study, Arsenault et al investigated the stability of correlated states in WSe 2 /WS 2 moiré superlattices, uncovering crucial evidence of electron-phonon coupling.This was further supported by DFT calculations of the one-hole Mott state, confirming polaron formation and highlighting the complex interplay of electron-electron and electron-phonon interactions in stabilizing polaronic Mott insulators [122].Additionally, Miao et al explored how interlayer excitons interact with correlated states in angle-aligned WS 2 /WSe 2 moiré superlattices, significantly affecting the PL of interlayer excitons (figures 12(i) and (j)) [123].In parallel research, Xiong et al and Lian et al independently reported on a correlated insulator in WSe 2 /WS 2 stacked heterobilayers.Xiong et al identified a bosonic correlated insulator composed of excitons and developed a pump-probe spectroscopy method to observe an exciton incompressible state, proving the existence of a correlated insulator of excitons [124].Lian et al focused on determining the spatial extent of interlayer excitons and the band hierarchy in WS 2 /WSe 2 moiré superlattices (figures 12(g) and (h)), which demonstrated their potential for realizing excitonic Mott insulators, mainly due to strong electron-interlayer exciton repulsion.This study also noted a significant enhancement in valley polarization in these systems [125].
Numerous studies have reported on how moiré excitons are affected by the stacking angle of TMDCs [126][127][128][129]. Zhang et al have noted that in WS 2 /MoSe 2 heterostructures, a twist angle between 0 • and 60 • did not suppress moiré excitons, in addition, it provided a sensitive tuning knob.Moiré excitons remain stable even in bilayers with larger twist angles (figure 12(m)).Their characteristics depend on the moiré superlattice period, which can be controlled by adjusting the twist angle [127].Such findings pave the way for understanding and engineering the various properties induced by moiré lattices in angle-twisted semiconductor van der Waals heterostructures.Apart from the WS 2 /MoSe 2 heterostructure, twist angle-dependent interlayer exciton diffusion was also observed in WS 2 /WSe 2 bilayers [126,130].It was found that interlayer exciton transport heavily relied on exciton density and the depth of the moiré potential, which was also regulated by the twist angle.When the exciton motion is modulated by moiré potentials and changed with the twist angle, it deviates from normal diffusion due to the interactions between the moiré potentials and strong exciton-exciton interactions (figures 12(k) and (l)).Additionally, the K-Q interlayer exciton is proved to be the ground state, contrary to the commonly assumed K-K exciton.These insights lay the groundwork for research into exciton and spin transport in van der Waals heterostructures, shedding light on the design of quantum communication devices [126].

Conclusion
The synthesis of t-TMDCs provides more possibilities for constructing TMDCs with special properties.Physical methods can synthesize materials with precise twist angles, but it is difficult to maintain interface cleanliness.Chemical method can prepare clean t-TMDCs with relatively simple operation.To better understand the performance of t-TMDCs, spectroscopy and microscopy characterization need to work together.First, in terms of synthesis methods, it is generally necessary to make a compromise between obtaining a clean interface and controlling the twist angle accurately (especially the small twist angle), which also limits some applications of t-TMDCs, and further research is needed on synthetic methods that can both generate a clean interface and precisely control the twist angle.Second, most of current research stays in the laboratory, and it is still difficult to achieve large-scale fabrication.At the same time, for folding technology, the kinetic study is still insufficient.For CVD growth, there are challenges such as limited methods to break the minimum energy limits of 0 • and 60 • , and a low proportion of small twist angles during synthesis.Third, the research of t-TMDCs have still mainly focused on the twisted bilayer or single twisted interface, and the research on twisted multiple interfaces are still relatively limited.Finally, most of the current research stays on performance research and potential applications, and the performance evaluation in practical applications needs to be further studied.However, it is undeniable that the synthesis and property study of t-TMDCs is of great significance for exploring the application of TMDCs in the fields of optoelectronic device arrays, nanophotonics and quantum information, which deserves study in depth.

Figure 3 .
Figure 3. Folding method to synthesize t-TMDCs.(a) Schematic diagram of the process of forming folded MoS2 nanolayer by strain.Reprinted with permission from [50].Copyright (2013) American Chemical Society.(b) Schematic diagram of preparing a WS2 fold array on a rigid substrate.Reprinted with permission from [52].Copyright (2021) American Chemical Society.(c) Schematic diagram of the manufacturing process for preparing multilayer MoS2 by paraffin compression folding strategy.Reprinted with permission from [53].Copyright (2021) American Chemical Society.

Figure 4 .
Figure 4.Chemical method to synthesize t-TMDCs.(a) OM image of t-MoS2 synthesized by face-to-face CVD.Scale bar: 10 µm.Reproduced from [57] with permission from the Royal Society of Chemistry.(b) OM images of t-WS2 bilayers synthesized by CVD.The twist angle is defined by the rotation of top triangle with respect to the bottom one in counterclockwise direction.Scale bar: 10 µm.[58] John Wiley & Sons.[© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim].(c) OM image of t-WS2 synthesized by heteroatom-assisted method and corresponding schematic diagram of its atomic structure.Scale bar: 5 µm.Reprinted with permission from [59].Copyright (2020) American Chemical Society.(d) LEED image of epitaxial growth t-SnS2.Reproduced from [60].CC BY 4.0.(e) Schematic of the vertically grown TiTe2/TiSe2 heterostructure on a BLG/SiC (0001) substrate.Reproduced from [61], with permission from Springer Nature.(f) Schematic diagram of moire´superlattices formed by S-W-S layer slipping.Yellow and cyan balls represent S and W atoms, respectively.Reproduced from [62].CC BY 4.0.

Figure 5 .
Figure 5. Raman spectral characterization of t-TMDCs.(a) HF Raman spectra of t-MoS2.Reproduced from [33].CC BY 4.0.(b) LF Raman spectra of t-MoS2.The S and B represent the shear mode and the breathing mode, respectively.Reprinted with permission from [39].Copyright (2016) American Chemical Society.(c) LF Raman spectra of MoSe2/WSe2 at different twist angles.(d) Comparison of experimental and theoretical twist angle correlations of peak positions in layer shear (LS) and layer breathing (LB) modes.The red triangle and circle represent the LS and LB modes measured by the experiment, respectively, and the blue squares represent the theoretically calculated LB and LS modes.Reprinted with permission from [43].Copyright (2023) American Chemical Society.

Figure 6 .
Figure 6.PL characterization of t-TMDCs.(a) PL spectra of CVD synthesized t-MoS2.Reproduced from [55], with permission from Springer Nature.(b) PL spectra of t-MoS2 synthesized by liquid phase exfoliation.Reprinted with permission from [83].Copyright (2022) American Chemical Society.(c) PL spectra of t-WSe2.Reproduced from [84], with permission from Springer Nature.(d) Scheme of MoS2 bilayers with AA, AB and different twisted configurations.Mo atoms are shown as green spheres; two S atoms of the same horizontal position are presented by one yellow sphere.Reproduced from [55], with permission from Springer Nature.

Figure 10 .
Figure 10.Ferroelectricity in 2D twisted materials.(a) Schematic of BN-encapsulated bilayer graphene device with top (VTG) and bottom (VBG) gates.Left: schematic of the BLG/BN moiré superlattice pattern.(b) and (c) Four-probe resistance for the hysteretic devices H2 (b) and H4 (c).In devices H2 and H4, the top and bottom BN flakes have a relative angle of about 30 • and 0 • , respectively.Reproduced from [99], with permission from Springer Nature.(d) Conductance G of trilayer (left) and bilayer WTe2 device (right) as a function of an electric field E ⊥ for temperatures from 4 K to 300 K (as labeled).Inset to (d) optical image of a representative double-gated device.The WTe2 flake has been artificially colored red.Scale bar: 10 µm.Reproduced from [100], with permission from Springer Nature.(e) Crystal structure of H-stacked and R-stacked bilayer TMDCs.(f) Piezoelectric force microscopy (PFM) images of t-MoSe2.Reproduced from [101], with permission from Springer Nature.(g) Back-scattered electron channeling contrast images (BSECCIs) of domain switching under different in-situ transverse electric field.Reproduced from [102].CC BY 4.0.

Figure 11 .
Figure 11.The observation of flat band in t-TMDCs.(a), (b) STM topography image (a) and the local density of states (LDOS) map at the flat-band energy of 3 • t-WSe2 sample (b).(c) Topography image of t-WSe2 sample with the illustration of the line along those marked high-symmetry locations.(d) LDOS map at the flat band of 57.5 • t-WSe2 sample.Reproduced from [110], with permission from Springer Nature.(e) STS of bilayer t-WSe2 with different twist angles.Reproduced from [111].CC BY 4.0.(f) Schematic illustration of bilayer t-WSe2 device structure.(g) Left: real-space representation of the moiré pattern that results from a small angle twist between the two WSe2 layers.Right: Brillouin zones of the top (solid line) and bottom (dashed line) layers.(h) Schematic representation of the bilayer t-WSe2 hybridized band structure in the K valley, spin up (left), and K ′ valley, spin down (right).(i) Resistance as a function of carrier density n, measured at T = 1.8K for six WSe2 bilayer configurations.Reproduced from[22], with permission from Springer Nature.

Figure 12 .
Figure 12.Interlayer excitons in t-TMDCs.(a) Different local atomic alignments occur in a MoSe2/WSe2 vertical heterostructure with a small twist angle.(b) Representative PL spectra for heterobilayers with twist angles of 1 • (bottom) and 2 • (top).Each spectrum is fitted with four (1 • ) or five (2 • ) Gaussian functions.Reproduced from [118], with permission from Springer Nature.(c) AFM image of MoSe2/WSe2.The heterobilayers have different twist angles.(d) Corresponding PL spectrum of 2 • t-MoSe2/WSe2 heterostructure at excitation powers of 10 µW (dark red) and 20 nW (blue).Inset, a Lorentzian fit to a representative PL peak indicates a line width of approximately 100 µeV (excitation power of 20 nW).Reproduced from[119], with permission from Springer Nature.(e) Reflection contrast spectra of t-WSe2/WS2 heterostructures with a small twist angle (light blue, top) and a large twist angle (black, bottom), respectively.(f) Detailed reflection contrast spectra in the range of 1.6-1.9eV (the WSe2 A exciton) on the electron-doping side.Units of the electron concentration: cm −2 .Reproduced from[120], with permission from Springer Nature.(g), (h) Doping-dependent PL spectra of the R-stacked (g) and H-stacked (h) WS2/WSe2 device.Reproduced from[125], with permission from Springer Nature.(i) Gate-dependent PL spectra of the hBN encapsulated WS2/WSe2 heterobilayer device at 4.2 K, with CW excitation centered at 1.959 eV with an excitation power of 5 µW.(j) PL peak position (red) extracted from figure11(i) as a function of the gate voltage, correlated with microwave impedance microscopy (MIM) measurements of the local conductivity (blue) taken at 14 K. Dashed lines are fillings corresponding to the insulating states determined from the MIM measurements.Reproduced from[123].CC BY 4.0.(k) Schematic of probing interlayer exciton transport at the K-K valleys using transient absorption microscopy (TAM).The valance and conductions bands are marked by solid and dashed lines, respectively.Reproduced from[126], with permission from Springer Nature.(l) TAM morphology imaging of the two WS2/WSe2 heterobilayers with 0 • and 60 • twist angles, respectively.Reprinted with permission from[126].Copyright (2020) Springer Nature.(m) Reflectance contrast spectra of bilayers of different twist angles θ i , for i = 1-6.Reproduced from[127].CC BY 4.0.