Recent progress on the mechanical exfoliation of 2D transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted extensive attraction due to their unique properties in novel physical phenomena, such as superconductors, Moiré superlattices, ferromagnetics, Weyl semimetals, which all require the high quality of 2D TMDs. Mechanical exfoliation (ME) as a top-down strategy shows great potential to obtain 2D TMDs with high quality and large scale. This paper reviews the theoretical and experimental details of this method. Subsequently, diverse modified ME methods are introduced. Significantly, the recent progress of the Au-assisted ME method is the highlight. Finally, this review will have an insight into their advantages and limitations, and point out a rational direction for the exfoliation of TMDs with high quality and large size.


Introduction
Since graphene came into our view in 2004 [1], two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted extensive attention due to their exclusive electronic properties, optical properties, and abundant applications in novel physical phenomena, such as superconductor, Moiré superlattices, ferromagnetics, Weyl semimetals. 2D TMDs, which show a similar structure to graphene, have a sandwich structure that is composed of transition metal elements and one of the S, Se, and Te three elements. However, different from graphene, the diverse configurations of transition metals with chalcogenide make 2D TMDs materials form varied crystal phases with abundant properties. They can present insulator, semiconductor, metal and even superconducting characteristics in terms of electrical and optical properties as the filling situation of d electron orbital of transition metal atoms makes it no bandgap [2,3]. Besides, 2D TMDs show thickness-dependent electronic and optical properties. For example, MoS 2 features three-phase including 2H, 3R, and 1T/1T'. The 2H and 3R phases display semiconducting properties which can be used as a semiconductor with high carrier mobility, and show great potentials in electrocatalytic hydrogen evolution, energy storage like lithium battery, sodium battery, supercapacitor, and flexible transistors [4][5][6]. Intsead, the 1T/1T' phase shows a metallic property that can be applied in electrocatalytic hydrogen production because of its excellent performance of conductivity.
Different methods for preparing 2D TMDs from bulk have been developed during this time, such as mechanical exfoliation, ion intercalation exfoliation, and liquid-phase ultrasonic exfoliation, which are the typical top-to-down exfoliation methods [7][8][9][10]. However, alkali metal ion intercalation exfoliation may bring difficulty in handling alkali metals, the tendency for aggregation and impurity from the outside, the same as the liquid-phase exfoliation methods. Besides, using chemical exfoliation will introduce additional indeterminate factors that may change the physical and chemical properties. Meanwhile, it needs to take a long time to achieve atomically thinness. In contrast, CVD is another bottom-up method [11]. In the CVD technology, the 2D TMDs are grown on a substrate through thermal vaporization of the metal precursor that reacts with the thermally evaporated chalcogen elements such as sulfur or selenium [12,13]. The CVD method is a powerful method to grow monolayer or bilayer of 2D materials with large sizes. However, the quality of the monolayer obtained by the CVD method is a serious drawback, which hinders the applications of 2D materials in physical phenomena and nanotechnology. Besides, the CVD method requires strict conditions, such as high temperature in the synthesis process, and introduces abundant crystallographic defects and impurities under transfer from the substrate [14,15]. Overall, mechanical exfoliation is still the most popular method to make new monolayer 2D material from bulk crystal as it is straightforward, and the products have high crystal quality with excellent electronic and optical properties compared to 2D materials obtained by other methods. Notably, mechanical exfoliation offers infinite possibilities for constructing heterostructures or twistronics without limited substrates. For example, Sikandar et al fabricated p-MoTe 2 /n-MoTe 2 homojunction p-n diode using mechanical exfoliation achieved excellent electrical and photovoltaic characteristics when they used ultra-low resistive ohmic metal contacts for the homojunction p-n diode [16]. Matthew et al find a zero-density trion binding energy of about 62 meV in the MoSe 2 /WS 2 heterostructure, which opens up a new platform to study excitonic physics in low-dimensional systems and the Coulomb interaction between charge carriers [17]. Hou et al successfully detach monolayer graphene through the mechanical exfoliation method, then via the wetting transfer method to form the clean surface and interface tBLG (twisted bilayer graphene) with controllable twist angles [18]. In this review, we will focus on the technology of mechanical exfoliation over the past 20 years, from traditional mechanical exfoliation to assisted exfoliation, especially introducing the progress of the assisted exfoliated technology and discussing the prospect of its application.

Traditional mechanical exfoliation
Mechanical exfoliation is also called micro-mechanical exfoliation as the applied small mechanical forces during the operation. 2D TMDs can be exfoliated via breaking Van der Waals force between adjacent layers through some mechanical forces when the tape adhesion force is greater than the newly produced surface energy. The first example of mechanical exfoliation is monolayer graphene exfoliated from graphite [1]. Like graphene, many 2D TMDs like WS 2 , NbSe 2 , and MoS 2 also have low surface energy, so it is possible to exfoliate these 2D TMDs. Because of its abundance in nature and good stability under ambient conditions, MoS 2 , a representative of TMDs, has been studied widely. The standard processes of the mechanical exfoliation method for single-layer 2D TMDs crystals are illustrated in figure 1. The bulk crystal are pressed onto the scotch tape (3M) and folded the tape several times. A suitable pressure is applied to make the tape attached with MoS 2 samples effectively adhere to the substrate. Then, the scotch tape is removed, leaving many TMD flakes on the surface of SiO 2 /Si substrate. With this method, almost all high-quality monolayers of 2D TMDs crystals can be obtained. For example, Radisavljevic et al used this method exfoliated single-layer MoS 2 in 2011 [19], which was used as the channel material to fabricate an interband tunnel FET and shows a lower power consumption than classical transistors. This result takes a successful step on the low-standby-power integrated circuits based on two-dimensional materials. Li et al exfoliated single and few-layer nanosheets of WSe 2 , TaS 2 and TaSe 2 in 2013, and focused on the study of suitable laser excitation power for the Raman measurement of 1L to 5L WSe 2 nanosheets [20]. Khan et al prepared thin films of TiTe 2 in 2012, and demonstrated intriguing nonlinear I-Vs as the channel layers in FETtype devices with Ti/Al/Au-metal contacts [21]. Zhang et al improved the above method from heat temperature and time aspects [22]. Speaking it clearly, firstly, they prepared two Si/SiO 2 substrate groups named A and B. Group A contained 48 substrates and was divided into eight batches, the scotch tape with MoS 2 bulk was transferred onto the substrates. Then, heated the complex from 35°C (batch 1) to 140°C (batch 8) for 5h at intervals of 15°C. Then, choose three substrates to direct exfoliate and the others to pre-cooling exfoliate at one batch. The complex of group B (containing 30 substrates, ten batches) was heated at 110°C from 30 min (batch 1) to 300 min (batch 10) at intervals of 30 min, then exfoliated after cooling. It was found that the size and percentage of the exfoliated flakes were enhanced 152-fold larger size after thermal treatment for 2 h at a temperature of 110°C followed by pre-cooling for 10 min in ambient air. DiCamillo et al in 2018 put forward an automated method with good reproducibility instead of peeling off the tape by manual method [23]. It used controlled shear and normal forces imposed by a parallel plate rheometer to exfoliate single layer and few-layer MoS 2 and MoTe 2 mechanically. Scotch tape, Easy Draw tape, Lensguard tape, SWT 10 + tape, and SWT 20 + tape, these five different types of tape were tested. The scotch tape was abandoned immediately due to the heavy residue left behind on the substrate, while the SWT 10 + tape had the best performance on the least adhesive residue, and it could also obtain 3-5 few-layer flakes. But if transferred these flakes to the substrate successfully, all the above five tapes needed the help of thermal release tape (TRT).
However, the traditional ME method shows a severe drawback, which generates a small crystal size ranging from tens to hundreds of microns and a very low yield, and may also leave traces of adhesive, which hamper the application development of 2D TMD single crystals [7]. Then, several modification ME methods to obtain monolayer TMD crystals have been developed. Next, we will introduce several assistant exfoliation methods which have been reported more recently.

Gel film-assistant exfoliation
Gel film, which is transparent and easy to handle, also gives the flexibility to transfer the exfoliated target substance to a designated substrate. This method uses the correlation between the rate of separation and the adhesion which is between the substrate and the Gel film when external forces were applied to separate them. The separation rate is proportional to the adhesive force. So when the adhesive force is larger than the Van der Waals interactions between the substrate and the 2D TMD, it can be used to transfer 2D TMDs from one substrate to another.
Budania et al first proposed a gel-assisted exfoliation [24]. The PF-X4 gel film was put on a glass slide, and made sure that no air bubbles were between them. Then the Scotch tape with flakes was put onto the gel film as shown in figure 2, and the inversion was placed onto an oxidized substrate, then pressed hard with tweezers before being removed after 3 ∼ 4 min, illustrating this transfer technique did not leave the tape residue on the substrate. Y. Huang et al reported a method by adding an annealing step of the SiO 2 /Si substrate about 2 ∼ 5 min in contact with the layered crystal at the temperature of 100°C in 2015 [25]. It was demonstrated that the size of exfoliated flakes increased 20 to 60 times, and no additional defects were introduced by atomic force microscopic (AFM) imaging and Raman spectroscopy.
Notably, the transparent and flexible stamp which Polydimethylsiloxane (PDMS) can be used to get diverse Van der Waals heterostructures, such as monolayer MoSe 2 /MoS 2 heterostructure, as reported by F. Ceballos et al in 2014 [26]. Firstly, flakes of MoS 2 and MoSe 2 were respectively exfoliated from bulk crystals onto suitable polydimethylsiloxane (PDMS) substrates using adhesive tapes, then the flakes of MoSe 2 were transferred to SiO 2 /Si substrate, next annealed at 200°C for 2 h under an H 2 /Ar at a base pressure of 3 Torr. Flakes of MoS 2 were precisely transferred onto the MoSe 2 flakes, then got the heterostructure flake using the same thermally annealed as mentioned above. The test results showed a strong PL quenching effect in heterostructures and higher efficient charge transfer. Here many researchers used a similar method to fabricate bilayer heterostructures, N. Huo et al found MoS 2 /WS 2 heterojunctions novel and excellent field-effect, photosensitivity with new functionalities, superior electrical and optoelectronic properties that far exceed the one for their constituents in 2017 [27]. This method of free stack 2D TMDs which opens a new platform to investigate the exciting physical phenomena of 2D TMDs.

Au-assistant exfoliation
The principle of the Au-assistant exfoliation method is to use the differences in interfacial energy between Au with TMDs, interlayers of TMDs, and TMDs with a substrate. This idea utilizes the chemical affinity of the S atoms, which can bind more strongly to the Au surface than the interaction force between layers of the TMDs [28]. Meanwhile, H. Hakkinen et al discovered that the large areas of single-layer samples could be exfoliated efficiently at the same time without affecting the physical properties of the material. Furthermore, Magda et al proved that Au-assistant exfoliation could not perform successful exfoliation of graphene, which supports the mechanism behind this exfoliation process. Meanwhile, it proves this method is fully reversible without significant structural modification [29]. Figure 3 shows this team reported a modification ME method with Auassisted in 2015. Unlike traditional SiO 2 /Si substrates, Au substrates with large, atomically flat, and clean Au (111) surfaces are evaporated on Mica. Then, bulk MoS 2 crystals are exfoliated on the fresh gold substrates, generating monolayer MoS 2 with lateral sizes up to several hundreds of microns. This team also proved this method well for various layered chalcogenides, including selenides, tellurides. They successfully exfoliate bulk WSe 2 and Bi 2 Te 3 crystals, and get the single layers with several hundreds of microns lateral scale. Wu et al also used a similar method to prepare monolayer 2D materials [30]. Firstly, TRT attached with MoS 2 crystals was pressed on 200 nm Au (111) thick films coated on Mica substrate, after flame annealing treatment, then gently press the back to ensure the good contact. After that, transfer the sample onto a 90°C hot plate to release the tape. Meanwhile, utilize fine tweezers to peel off the MoS 2 flakes from the edge of the flakes. Finally, millimeter size single-layer MoS 2 is obtained on the Au surface. The AFM topography image shows there are no tape residues. Besides, Velicky et al also reported a similar Au-assisted ME method [31]. With this method, monolayer MoS 2 with a size up to a centimeter is obtained. Furthermore, they found that exfoliation highly depends on the smoothness and cleanliness of Au surface. There are negligible exfoliated nanoflakes while the surface of Au is rough or exposed to air >15 min Notably, a near-unity percentage of monolayer MoS 2 can be obtained on the fresh and flat Au substrates under the condition of Au exposed to air for less than 6 min Meanwhile, this method is expanded to exfoliate bulk MoSe 2 , WS 2 , WSe 2 , MoTe 2 , WTe 2 , and GaSe crystals. Desai et al at. 2016 reported another Au-assisted ME method as shown in figure 4 [32]. Bulk MoS 2 crystals adhered on a tape, then evaporated Au film on the surface of bulk MoS 2 . A TRT is used to peel off Au film and the topmost layer of bulk MoS 2 , which is transferred to a suitable substrate (SiO 2 /Si or SiO 2 ). The part of TRT and Au film are removed by heated and etched by KI/I 2 solution, respectively, leaving MoS 2 monolayer remaining on the surface of the substrate. With this method, MoS 2 monolayer with a lateral size of up to 500 μm is obtained. Similarly, Higashitarumizu et al also used Au-mediated to exfoliate the SnS [33], then Au residue was etched in the KI/I 2 solution for 5 min, following a rinse via deionized water. Large SnS flakes near to the size of 100 μm 2 and the thickness exhibited a wide distribution down to 1.1 nm (monolayer SnS about 0.6 nm) were discovered, and the chemical/thermal stability of monolayer SnS was demonstrated by Raman spectra after laser-annealed.
Recently, Liu et al demonstrated a new Au-assisted ME method [34], which can permit nondestructive, highthroughput, scalable, and large TMD monolayers. Figure 5 illustrates the process of modification Au-assisted ME method. Au film is evaporated on an ultra-flat surface of SiO 2 /Si substrate, and then the Au film is peeled off the substrate by a polyvinylpyrrolidone (PVP) film and TRT. Bulk TMD crystals are adhered on the ultra-flat Au tape due to the strong Van der Waals interactions between Au and the topmost of TMD crystals, resulting in a large monolayer of TMD crystals which is transferred onto a given substrate. The TRT is released under a given temperature, PVP film is washed off by deionized water, and Au film is etched in the Au etchant solution. Finally, the monolayer TMD crystal with macroscopic dimensions up to centimeters is obtained on the desired substrate. This method also can be used to exfoliate other 2D TMD monolayers such as WS 2 , MoS 2 , MoSe 2 , and ReS 2 on diverse substrates, including SiO 2 /Si substrate, quartz, and sapphire. Furthermore, the quality of these exfoliated 2D TMD monolayers is investigated by AFM and photoluminescence (PL) spectroscopy show better than the traditional ME method. It is worthy to note that this Au-assisted ME method can provide a larger lateral size and higher percentage of the monolayer with two or three orders of magnitude than that of the traditional scotch tape method. Panasci et al reported a similar method to exfoliate a very large area 1L MoS 2 in 2021 [35], which replaces the PVP to the PMMA, changes SiO 2 /Si substrate to the Al 2 O 3 /Si substrate, PMMA film is removed by acetone. In addition, there is another bright spot, a 10 nm thick Ni film is sputtered to improve the adhesion onto the substrate before sputtering 15 nm Au film. More recently, Huang et al expanded the Auassisted ME method to exfoliate 40 types of monolayer 2D materials [36], including 2D elements crystals, TMD crystals, magnets, and superconductors. They thoroughly calculated the Van der Waals forces of 2D crystals and Van der Waals interactions between 2D materials and Au film (111) via density functional theory (DFT) which demonstrates the presence of considerable interactions between Au and other elements, including S, Se, Te, Cl, Br and I. They suggested the great potential applications of Au-assisted ME method in exfoliation of 2D crystal monolayers with large size. According to this method, monolayers of PtSe 2 , PtTe 2 , PdTe 2 and so on, which are very hard to exfoliate by the traditional ME method, are successfully exfoliated to monolayer with the size up to millimeters. Zhao et al also used the same method to prepare the monolayer WSe 2 samples from bulk crystals in their paper in 2019 [37]. Figure 6 shows that Heyl M et al used Kapton ® tape adhered bulk MoS 2 , then the entirety was pressed onto a freshly-stripped Au substrate in 2020 [38]. Annealed at the condition of 200°C for 60 s, cooled down for 10-20 s before peeling the tape, then respectively etched Cu and Au, get single-layer MoS 2 whose size can reach 37 mm 2 . It was indicated that the monolayer area is continued over several square millimeters by optical micrograph.  In 2021, Tao L et al reported an Au-assist exfoliation and nondestructive transfer method to fabricate a largescale Bi 2 Se 3 thin nanosheet, as shown in figure 7 [39]. Furthermore, combing 2H-MoTe 2 and Bi 2 Se 3 by drytransfer method formed a vertically stacked Bi 2 Se 3 /MoTe 2 heterostructure. The manufactured device showed an ultralow dark current of ∼0.2 pA and superior optoelectronic performance of I light /I dark ratio ≈ 10 6 , a fast response time of 21 ms. This result demonstrates a new universal method to fabricate topological insulators and paves a new strategy for the construction of novel van der Waals tunneling structures.

Conclusions and outlook
In this review, we have discussed the primitive mechanical exfoliation method to the following assisted exfoliation and the corresponding mechanisms of the exfoliation process. The tape exfoliation method predicts a huge potential for preparing monolayer 2D TMDs materials when Novoselov and Geim used it to win the Nobel Prize for physics. Table 1 gives a detailed comparison of the methods described in this paper. It turns out typical mechanical exfoliation via Scotch tape is the universal method to obtain monolayer or few-layer 2D materials, due to its low cost, easy-manipulation, universality for all 2D materials. But the size, the thickness and the shape for exfoliated nanoflakes are also difficult to control. Besides, Scotch tapes always leave adhesive residue on the substrate. Sometimes Nitto S.W.T. 10 + Scotch tapes will replace Scotch tapes during the process of mechanical exfoliation due to its less residues. Gel film (such as PMDS)-assistant exfoliation as another method can produce large monolayer 2D materials. Besides, it provides the flexibility to transfer the exfoliated flakes onto a predefined    substrate. However, PDMS also leaves residues. This will expend more time to clean it. PDMS coated by polycarbonates (PC) maybe a good candidate to transfer few-layers or monolayer 2D materials with less or no residues. It is notably that Au-assistant exfoliation can provide monolayer materials with size up to cm and high quality [40]. Notably, the thickness and roughness of Au surface are crucial factors which will affect the exfoliation results. Besides, it needs many rigorous steps and uses KI/I2 etching solution, which will expand more time and introduce impurity to the surface of exfoliated 2D materials. In a word, to obtain clean surface of monolayer materials, mechanical exfoliation with assistance of Nitto S.W.T. 10 + tapes or PDMS coated with PC is one of best candidate. To obtain more monolayers with very large size, Au-assistant exfoliation method is a good selection. All these methods can provide high quality nanoflakes.
With the improvement of ME methods from simple tape to Au-assisted, various TMDs monolayers with high quality and large lateral dimensions are obtained. Such as bulk-like MoS 2 flakes are left on the substrate. Meanwhile, the low percentage of the traditional ME method is being overcome by these modified ME methods. In spite of the vital prospects of mechanical exfoliation, several issues are still required continuous attention, such as the obvious shortcomings that all the mechanical exfoliation techniques have fragmentation effects. The small flakes will limit the size of electronic devices and circuits, so we need to consider how to minimize the fragmentation effects and produce 2D TMDs monolayers with high percentages, uniform size and good quality.
The current popular smart wearables can have more powerful and complex functions even though their size is not significantly increased or even reduced, as the improvement of circuit integration brought by the continuous development of transistor preparation technology. With this technology, a position card can be carried on a safety helmet in some nuclear power plants which have more than 10000 people. In the trend of miniaturization and flexibility of semiconductor devices, the single layer or less more atomic layer of 2D TMDs (the typical representative is MoS 2 ) show unique advantages due to the excellent electrical, optical, and mechanical properties and more degrees of freedom regulation, making it promising in the future lighter, thinner, faster, more sensitive electronic devices.