A review on transfer methods of two-dimensional materials

Over the years, two-dimensional (2D) materials have attracted increasing technological interest due to their unique physical, electronic, and photonic properties, making them excellent candidates for applications in electronics, nanoelectronics, optoelectronics, sensors, and modern telecommunications. Unfortunately, their development often requires special conditions and strict protocols, making it challenging to integrate them directly into devices. Some of the requirements include high temperatures, precursors, and special catalytic substrates with specific lattice parameters. Consequently, methods have been developed to transfer these materials from the growth substrates onto target substrates. These transfer techniques aim to minimize intermediate steps and minimize defects introduced into the 2D material during the process. This review focuses on the transfer techniques directly from the development substrates of 2D materials, which play a crucial role in their utilization.


Introduction
The first successful exfoliation of a graphene monolayer ignited a tremendous wave of interest in the exploration and research of two-dimensional (2D) materials [1,2].In addition to the pursuit of various 2D materials, a substantial amount of scientific investigation has been dedicated to uncovering their vast potential in electronic devices, microelectronics, nanoelectronics, optoelectronics, and sensors [3][4][5][6].Transition metal dichalcogenides (TMDs) and insulators, such as hexagonal boron nitride (hBN), has emerged as captivating subjects of study due to their exceptional electronic, physical, and mechanical properties [7][8][9][10][11].For instance, graphene, known for its remarkable hardness and mechanical stress resistance, has captivated researchers who are now delving into the prospect of replacing conventional silicon technology with these extraordinary materials [12,13].The unique properties of 2D materials offer considerable advantages in terms of scalability and energy loss reduction [14][15][16][17][18].However, the specific conditions required for the synthesis and development of these materials present challenges that hinder their immediate application in electronic and optical devices [19][20][21][22].Common manufacturing methods for 2D materials encompass techniques such as molecular beam epitaxy, chemical vapor deposition (CVD), and physical vapor deposition [23][24][25][26][27][28][29][30].Yet, one significant obstacle lies in the exceedingly high temperatures at which these materials are produced, often surpassing 800 • C, and the potential adverse consequences associated with the precursors and chemical compounds utilized in the manufacturing process [31][32][33].
Given the aforementioned challenges intricately linked with the manufacturing processes of 2D materials, continuous and diligent research efforts are indispensable to identify and implement viable solutions for transferring these materials from the growth substrate to the target wafers [34].This approach involves fabricating the 2D materials on growth substrates and subsequently employing various techniques to transfer them onto the desired target devices [35].By enabling the controlled production of highquality layered materials independently of the growth conditions of the devices.This approach not only offers greater flexibility in selecting target substrates, but also mitigates limitations and dependencies on the growth conditions of the 2D materials [36][37][38][39][40][41][42].Nevertheless, ongoing research endeavors are focused on refining and optimizing the transfer techniques employed for these materials, since certain stages of the process have the potential to introduce defects into the target wafer, thereby significantly altering its inherent properties [43,44].Moreover, researchers are actively investigating techniques that can effectively mitigate the challenge of high growth temperatures, which currently present a significant impediment in existing technology, preventing the direct growth of 2D materials on a chip [45,46].
Several studies have demonstrated the immense potential of devices based on 2D materials in terms of cost reduction and energy efficiency [47][48][49][50].Consequently, there is a growing demand to replace the conventional substrates used in manufacturing these devices with more cost-effective options, such as glass and flexible polymers [51,52].However, the elevated temperatures required for the synthesis of 2D materials present a formidable obstacle to achieving high-quality growth on most of these substrates [53][54][55].Therfore, there is an urgent need for transfer techniques that ensure minimal defects in the material's structure while preserving its inherent properties.Various defects can be introduced during the transfer of the 2D material from the growth substrate, including lattice defects, such as bond or atom deficiencies, as well as the inadvertent introduction of polymeric residues from supporting structures utilized in the transfer process [56][57][58][59].Additionally, the strong forces exerted on the materials during the transfer can alter bond lengths, which can be detected and analyzed using advanced techniques like Raman spectroscopy [56].Furthermore, new bonds may form between atoms of the surrounding environment and the 2D material, further influencing its properties and behavior [60][61][62].
Within the vast landscape of scientific literature, numerous review publications discuss various transfer techniques for 2D materials.However, the primary focus of this article is to provide an extensive and comprehensive review specifically tailored to techniques for transferring the material from its growth substrate to the target substrate, encompassing all the crucial steps and processes involved.It aims to offer a thorough exploration, starting from the immediate post-growth stage, continuing through the meticulous removal from the growth substrate, and concluding with the precise deposition onto the target substrate.Subsequently, the article delves into the potential introduction of defects during these intricate processes and investigates innovative approaches described in the existing literature, with the ultimate goal of further enhancing the quality of the transfer and advancing the field of 2D materials research.

Wet transfer
Since the discovery of 2D graphene, its continuous integration into an increasing number of applications has become possible due to improvements in its quality.The main contribution to this progress has been the development of a growth method through CVD, which requires the use of a special catalytic substrate and high temperature [63].However, in order to utilize graphene in electrical devices, it is necessary to remove the material from these substrates [64,65].Several methods have been extensively studied for the transfer of graphene from the growth substrate, and these are commonly referred to as wet transfer techniques, due to their utilization of a wet environment (figure 1) [66].In recent years, wet transfer has also been applied to other 2D materials, including molybdenum disulfide (MoS 2 ), WS 2 , hBN, etc [67][68][69][70][71].In these techniques, a coated polymeric supporting layer, often polymethylmethacrylate (PMMA), is commonly employed and subsequently removed after the transfer process [72][73][74].The detachment of the 2D-material from the substrate can be achieved either through a process involving chemicals that etch the growth substrate, or by employing an electrochemical method that relies on the creation of bubbles capable of detaching the polymer/2D stack from the substrate [75][76][77].Although these two methods fall within the same category of wet transfer techniques, their basic principles and intermediate steps differ significantly.A detailed analysis of each method is provided below.

Chemical etchant assisted methods
The most commonly used method for producing high-quality monolayer graphene in large quantities is through a process called CVD, which involves synthesizing graphene on copper or nickel substrates [79].However, in order to use graphene for various applications, it must first be transferred from the copper growth substrate onto a different surface, typically aiming to become the surface of a device [74].This is typically achieved through a process called wet transfer assisted by chemical etching, during wich a polymer layer, such as PMMA, is applied onto the graphene/copper sample, followed by the use of a chemical etchant, like FeCl 3 , to dissolve the copper foil and detach the graphene layer [78,80].The graphene layer can then be transferred to the desired substrate.The polymeric layer deposited on the 2D material acts as a supporting and adhesive layer during the transfer process [29,53,78].This chemical etching approach can also be used for other materials, like TMD and hBN, grown on different metal foils or other substrates [81,82].Graphene on copper is known for its high flexibility, making it ideal for rollto-roll production [83].Hot pressing techniques have been applied to this process to enable the patterned transfer of graphene [84].
In detail, the process consists of very specific steps, the key points of which are often found in the literature in different ways in order to optimize the process.After the development of the 2D material, for example, in the case of CVD graphene where copper foils are typically used as substrates and temperatures greater than 900 • C are employed, the sample must be prepared for the transfer of the layered material from its surface [25,63].In the case of CVD graphene, it is common to grow the material from both sides of the copper foil.Therefore, the backside graphene is first removed via O 2 plasma treatment [78,85].Then, as shown in figure 2(a), spin coating of PMMA is performed on the surface where the graphene is present [78].During this process, the sample will usually need to be heated on a hot plate at a temperature of approximately 60 • C [78].After the deposition of the polymeric layer, which can support the graphene, the process of etching the copper follows.FeCl 3 is typically used to dissolve the metal substrate during this step.Solutions such as (NH 4 ) 2 S 2 O 8 can often be found in the literature, and their use requires immersing the sample in the etchant liquid for about 4 h, assuming the growth substrate is a 35 µm Cu foil [78,85].Along the way, the floating graphene/PMMA stack is collected and placed in a container of deionized water to wash the sample and remove residues from the previous processes, such as the chemicals used for etching [78,85,86].The stack can now be transferred to the target substrate, which is usually a silicon dioxide (SiO 2 )/Si wafer, and allowed to dry, sometimes even under reduced pressure, at room temperature [78].In the work of Zhao et al [78], a water-assisted process was employed, where a droplet of water was placed at the interface of the graphene and the target substrate, while simultaneously heating the latter.The presence of water at the interface facilitates better contact between the graphene and the target substrate.This step appears to be useful because the graphene/polymer stack retains the roughness of the Cu foil, resulting in less-than-ideal contact between the 2D material and the wafer and weaker van der Waals (vdW) forces between the graphene and the target.When a small amount of water is present at the interface, it gradually evaporates during heating.As the water gradually recedes from the interface, the graphene comes into contact with the exposed wafer surface.Thus, the contact area between the graphene and the target wafer increases, and the vdW forces between them become stronger.The result is that the material binds so strongly to the substrate, that the polymer can now be removed even by simple peeling off.As for the last stage, which involves removing the polymeric supporting layer, besides peeling off, it can also be achieved by immersing the sample in acetone to dissolve the polymeric membrane [78,80].
Another critical point that can be modified during the transfer process to improve the quality of the 2D material on the target substrate is the choice of the polymer material serving as the supporting layer.In the case of graphene, Zhang et al [79] replaced PMMA with Paraffin, while keeping the remaining process steps identical and compared the new process with the previous one that utilized PMMA.The Paraffin/Graphene/Cu stack was etched using copper at 20 • C and then immersed in deionized water at 40 • C before being transferred to a SiO 2 substrate.As depicted in figure 2(b), the outcome of this study was the production of high-quality graphene with significantly higher carrier mobility values (hole mobility = 14 215 cm 2 V −1 s −1 and electron mobility = 7438 cm 2 V −1 s −1 ).The Dirac voltage (V Dirac ) of the device fabricated with paraffin-transferred graphene was much smaller and closer to zero compared to that of PMMA.The mean value of V Dirac was lower for the paraffin-transferred graphene, indicating that its electrical properties were more similar to intrinsic graphene when compared to PMMAtransferred graphene.
The process for transferring CVD-grown vdW materials, like MoS 2 , onto target substrates involves a different chemical etching process.For example, when transferring MoS 2 to an insulating layer for device applications, PMMA is first applied onto the sample and then floated in an etching solution of potassium hydroxide (KOH) at a concentration of 1 M [87,88].This selectively etches the SiO 2 growth substrate, causing the remaining Si substrate to fall apart and lift the PMMA/MoS 2 film onto the solution surface [88,89].The film is then washed in deionized water to remove any residual etchants while being held onto a polyethylene terephthalate (PET) sheet [88,89].Finally, the PMMA layer is removed through acetone [87,89].Apart from KOH, SiO 2 can also be chemically etched using other solutions, such as sodium hydroxide (NaOH), buffered oxide etch, or hydrofluoric acid [81,90].The choice of etchant will depend on the specific application and desired substrate.Overall, the process of transferring CVDgrown vdW materials to different substrates involves several critical steps, including coating with a polymer layer, etching the substrate, detaching the film, In detail, all the steps of the chemical etching method are as follows: i.Initially, the two-dimensional material (in this particular case, graphene on copper) is on its growth substrate.ii.A thin layer of polymer (usually PMMA) is then deposited.iii.The system is then immersed in a bath to etch the substrate.iv.After etching the substrate, the 2D material and the polymer are immersed in deionized water to remove the copper and solvent residues.v.The 2D material and the polymer are then placed on the target wafer.vi.Finally, the polymer is removed, leaving the 2D material alone on the target substrate.(b), (c) Optical microscopy images of the transfer of graphene onto a SiO2 substrate using the chemical etching-assisted method are shown.Reproduced from [78].© IOP Publishing Ltd.All rights reserved.(d) i.This specific image demonstrates the graphene transfer process using paraffin.ii-v.Optical microscopy images of a Graphene-FET and electrical measurements during the transfer process, enabling the derivation of mobility.Reproduced from [80].CC BY 4.0.(e) This is a representation of the roll-to-roll process.i.The thermal release tape is adhered onto the graphene/copper sample using two rollers with moderate pressure.ii.The copper foil is etched by passing it through a copper etching bath.iii.The detached tape/graphene is then placed on the target substrate and they are plugged together into two hot rollers.
transferring it to the target substrate, and removing the polymer layer [87,88].
In conclusion, the chemical etching process is an essential step in transferring CVD-grown materials onto different substrates for various applications.This process requires careful handling and precision to ensure the high-quality transfer of materials [88].The choice of etchant will depend on the specific application and desired substrate.With further improvements in the process, the transfer of CVDgrown materials onto different substrates will become even more efficient and accurate, enabling the widespread use of these materials in diverse applications.
The potential of this technique can be seen in its ability to be applied to large-scale transfer.An important example is the application of chemical etchant-assisted wet transfer in a roll-to-roll process [83].The work of Li et al [91] has opened new avenues in the production of graphene on non-rigid substrates, thus unlocking a new path for large-scale graphene production and transport.The most common process used is roll-to-roll, which involves using graphene grown on flexible Cu foils that can be rolled [83].The roll-to-roll process, as shown in figure 2(c), is characterized by the basic processes discussed in the previous paragraphs.The first step after the growth of graphene on the copper sheet is the deposition of a supporting layer of polymer material, which supports the 2D material during the transfer process.The adhesion of the supporting layer is achieved by passing the stack through two rollers, which press the materials together [83].This is followed by the etching stage, where a suitable solution is used for the selective destruction of the growth substrate.Finally, the removal of the graphene from the polymer is required, and at this stage, the 2D material is transferred to the target substrate by removing the adhesive force that holds it together [83].Thermal release tapes (TRTs) can be used to detach graphene films from the tapes and transfer them to target substrates through thermal treatment [83].However, the third step is unnecessary if the target substrate is directly adhered to the copper foil in the initial step using a permanent adhesion method.

Etchant free methods (electrochemical bubbling transfer)
The traditional method of separating graphene from a metal substrate involves chemical etching, which is both expensive and time-consuming due to the long treatment time and complete dissolution of the metal without recycling [92].An alternative approach is to directly peel off the CVD samples from the metal substrate, similar to mechanical exfoliation, or to use electrochemical bubbling transfer, which is non-destructive, cost-effective, and time-efficient [93].One of the most important studies that has been published on the technique that makes use of bubbles for the detachment of graphene from a Cu foil substrate is that of Wang et al [93] They demonstrate a nondestructive route to delaminate graphene by electrochemical means.Along the way, exfoliations of other 2D materials have been published using this method, such as hBN and MoS 2 [94,95].
This particular method first requires the use of an aqueous solution that acts as an electrolyte in the electrochemical process.In the literature, we can find several solutions suitable for this purpose.An example is K 2 S 2 O 8 , while another is NaOH or Na 2 SO 4 [75,93].To initiate the process, before immersing the sample of the 2D material with the growth substrate in the solution, a polymeric supporting layer must be deposited on the surface of the 2D material [93].The most commonly used polymer for this purpose is PMMA, which protects the 2D material from folding during the process [94].Therefore, in the case of graphene grown on Cu foils, PMMA is first deposited on the surface via spin-coating [93].The sample is then immersed in the aqueous solution, which can be K 2 S 2 O 8 at 0.05 mM, and a potential is applied as shown in figure 3(c) [93].An electrode needs to be immersed in the aqueous solution to create a potential difference between the electrode and the copper, which is chosen to be cathodically polarized [93].As a direct consequence of this polarization, hydrogen bubbles emerge at the interface of graphene with copper, originating from the following reaction: [75,93].The bubbles formed at the interface have the potential to remove the graphene from the surface of the growth substrate without damaging the 2D material [93].The generations of bubbles starts from the edges of the sample, and as the delamination progresses, the bubbles move inward until the material is completely detached [75,95].The delamination time can be reduced by increasing the DC voltage or using an electrolyte with a higher concentration [93].
Despite the fact that the main phenomenon observed during the above-mentioned process is the detachment of the material due to the hydrogen bubbles, there is also a slight dissolution of the copper due to the presence of K 2 S 2 O 8 , as indicated by the following reaction: [93].However, alongside this process, copper oxidation is also observed due to the presence of hydroxyl ions.The chemical reaction that occurs is as follows: 3Cu 2+ (aq) [93].This particular oxidation prevents further etching of the substrate, which, according to the work of Yu Wang et al., resulted in the removal of less than 40 nm from the surface of the Cu foil [93].
At this point, as shown in figure 3(e), the 2D material has been completely detached from the surface of the growth substrate and floats in the aqueous solution along with the polymeric supporting layer, which was deposited on its surface at the first stage Figure 3.This is the etchant-free wet transfer method, also known as the electrochemical bubbling technique.(a) The two-dimensional material, which in this case is graphene, sits on its growth substrate (Cu).(b) A layer of polymeric material (PMMA) is then deposited.(c) The system is placed in a NaOH bath, in which an electrode is also immersed, while a DC voltage of 5 V is applied between the electrode and the Cu, causing an electrochemical reaction.(d) During the decomposition of water, hydrogen bubbles appear and aggregate at the interfaces of the graphene/copper, leading to the detachment of the graphene from the growth substrate.Reprinted with permission from [93].Copyright (2011) American Chemical Society.(e) The graphene, together with the polymer, floats on the surface of the bath.(f) The graphene/polymer system is then transferred to a deionized water tank to remove any residues from the process, before being collected onto the target wafer.(g) The graphene/polymer is then left to dry on the surface of the target wafer.(h) PMMA is removed using acetone.(i) Finally, the graphene remains on the target substrate.
of the process [75,93,95,96].The next step is the transfer of the material to the target substrate, which can occur in two ways.According to the first method, the target wafer can be immersed in the aqueous solution to collect the 2D material from the liquid's surface, as depicted in figure 3(f) [97].Alternatively, the PMMA/graphene structure can be collected and then transferred by a stamping technique to the target substrate [75].Finally, the polymeric supporting layer needs to be removed, a process that is carried out by immersing the sample in acetone, similar to the case of PMMA [93,97].
The realization that the aforementioned process does not fully preserve the surface of the growth substrate, resulting in the destruction of approximately 40 nm thickness and causing oxidation, prompted further investigation into the process.Shortly afterwards, Gao et al [75] published their own work on graphene transfer using the bubbling transfer technique.The key difference they implemented, was the utilization of a Pt growth substrate for graphene.This substrate is incompatible with transfer methods involving etching [75].During the process, PMMA is employed as a supporting layer, and an aqueous solution of NaOH is used.It is important to note that graphene is prone to oxidation if the PMMA/Graphene/Pt structure is used as the anode during the electrolysis process.As mentioned earlier, when the growth substrate is polarized as the anode, hydrogen bubbles form at its interface with graphene, leading to the detachment of the PMMA/graphene structure.After complete delamination, the stack is cleaned with pure water and can be transferred to the target substrate via stamping, while the PMMA is removed as usual with acetone.A significant advantage of this technique is the Pt substrate's inactivity in the electrolyte, which prevents it from participating in any chemical reactions [75].Consequently, the substrate can be reused for new graphene deposition on its surface via CVD, as it remains intact.Additionally, this technique offers the advantage of minimal metallic residues in the transferred graphene, which are commonly observed when etching-based transfer methods are employed.Reported values in the literature indicate that graphene mobility achieved through the bubble technique can reach up to 7100 cm 2 V 1 s 1 [75].This value is comparable to the electron mobility achieved through chemical etchant wet transfer assisted by a paraffin supporting layer, as mentioned earlier [80].
This method is also applicable to other materials, such as TMD or hBN [94,95].An important example is the work of Van Ngoc et al [94], who applied the technique to hBN and MoS 2 in addition to graphene.They also introduced an important innovative step, which resulted in an improvement in the quality of the transfer.Specifically, it was observed that introducing a thin layer of 100 nm polyvinyl alcohol (PVA) between the PMMA and the 2D material, facilitated the transfer to the target substrate without any damages.This approach also prevented the etching of the PMMA layer, ensuring contamination-free, high-quality 2D materials and avoiding detrimental effects related to surface contaminants [94].This specific method does not necessitate the use of acetone to remove the PMMA layer.Instead, it is only necessary to use water at a temperature of 130 • C to dissolve the PVA and remove the PMMA from the surface, leaving the 2D material free on the surface [94].A key advantage of this technique is that PVA can be easily dissolved in hot water, leaving no residue on the surface of the 2D material.The process begins with the deposition of a thin layer of PVA, followed by the classical deposition of PMMA.By using electrolysis, it is possible to detach the 2D material from the growth substrate by creating bubbles within the aqueous environment.Finally, the stack of the 2D material and the supporting polymers is placed on the target substrate and placed in a container with hot water to dissolve the PVA and remove the PMMA, as described in the aforementioned process.The PMMA/PVA method results in a much cleaner sample, with minimal polymer residue compared to the PMMA method.One significant reason why PMMA residues remain on the surfaces of 2D materials is that, despite using acetone to dissolve the polymer, the strong vdW forces between the PMMA and the 2D material prevent complete removal [64,98].
The electrochemical bubbling transfer method provides several advantages over the conventional the chemical etching method.These advantages include cost-effectiveness, time efficiency, and nondestructiveness [93].This method presents an appealing alternative for the large-scale production of topnotch graphene and can be expanded to encompass other materials, like TMD or hBN grown on different metal substrates [94,95].Although caution is necessary when applying this method to alternative metal substrates, the electrochemical bubbling transfer method has the potential to revolutionize the manufacturing process of high-quality graphene and other 2D materials.This advancement could make them more accessible and affordable for a wide range of applications.

Wet capillary method
Wet transfer methods, utilizing capillary forces through the hydrophobic effect, enable the relocation of 2D materials from growth substrates to target wafers.These techniques are effective for water entry angles of 30 • -45 • [99,100].They also offer reversibility for re-deposition of the supporting polymer layer.Schneider et al [100] successfully extracted graphene flakes using this method, involving cellulose acetate butyrate in ethyl acetate.The capillary force at the hydrophilic/hydrophobic interface drives the dynamic separation [100].
The presented approach focuses on the efficient exfoliation of 2D materials from their growth substrates.Xu et al [101] successfully demonstrated a technique for transferring WS 2 from a sapphire growth substrate using a surface-energy-assisted method that involves an additional NaOH etching step.This method is crucial, as it enables the reuse of the sapphire substrate.The process begins with the deposition of a 100 nm thick hydrophobic polystyrene (PS) membrane onto the WS 2 surface using spin-coating.The PS film can then be easily removed from the sapphire by etching with NaOH for 5 min and immersion in ambient water, resulting in exfoliation at a rate of 0.3 cm 2 s −1 .Capillary forces between the hydrophobic PS and the hydrophilic sapphire play a crucial role in the detachment process [102].Strong adhesion between the carrier polymer (PS) and the 2D material is essential for successful exfoliation, and PS has been found to be superior to alternatives like PMMA due to its higher hydrophobicity [102].The exfoliated WS 2 flakes, when transferred from the sapphire to a SiO 2 /Si substrate and examined with photoluminescence (PL) emission, exhibit a noticeable 10 nm shift in the 620 nm peak, specifically a red shift.This shift is attributed to water intercalation in the atomic steps of the recycled sapphire surface, which enhances the PL by inducing p-type doping in the monolayer WS2 flakes.Furthermore, characterizations reveal that the mobility of the WS 2 flakes can reach up to 4.1 cm 2 V −1 s −1 , with recycled sapphirebased flakes achieving values of up to 3.8 cm 2 V −1 s −1 [102].In summary, the approach introduced by Xu and their team involves the use of PS membranes and etching techniques to efficiently exfoliate 2D materials, such as WS 2 , from their growth substrates, allowing for the reuse of the substrate.This method demonstrates the importance of achieving strong adhesion between the carrier polymer and the material, while also revealing interesting optical and electrical properties of the exfoliated flakes.
In their study, Gurarslan and their colleagues, as documented in [102], presented an innovative technique for the transfer of monolayer and few-layer MoS 2 onto a diverse range of substrates.This method leverages capillary forces to facilitate the transfer process, eliminating the need for etching.The process commences with the application of a PS layer onto MoS 2 , grown on sapphire wafers via spin coating, followed by a brief baking step at 80 • C for 15 min.The transfer operation is initiated by introducing a water droplet onto the polymer/MoS 2 /sapphire structure.However, the robust adhesion between the film and the substrate initially obstructs water penetration.To circumvent this, a delicate puncturing procedure is employed to create small cracks at the edges, facilitating water infiltration.Once initiated, the polymer/MoS 2 assembly can be lifted and floated on the water droplet, subsequently dried, and then meticulously transferred to the target wafer with the aid of precision tweezers.This transfer process is comprehensively documented with accompanying detailed illustrations.Following these primary steps, additional measures involve baking to remove residual water and ensure uniform polymer distribution, followed by polymer dissolution utilizing toluene.This method has proven to be highly successful in transferring centimeter-scale MoS 2 flakes.Raman spectroscopy analysis confirmed the presence of characteristic material peaks both before and after the transfer process.When subjected to testing in field-effect transistor (FET) devices, the transferred MoS 2 demonstrated a remarkable mobility of 0.03 cm 2 Vs −1 .
Researchers have devised an innovative technique for transferring large-scale multilayer MoS 2 films onto wafers, which is meticulously documented in a research paper bearing the [103].The methodology leverages polydimethylsiloxane (PDMS) as both a polymer support and a hydrophobic layer, while sapphire wafers serve as the foundational substrate for growing MoS 2 .This approach capitalizes on the hydrophilic/hydrophobic interaction between MoS 2 and sapphire, facilitating the seamless detachment of MoS 2 films from the substrate.However, it is important to note that these techniques have not yet gained widespread adoption for large-scale MoS 2 film transfers.The transfer process is orchestrated by immersing PDMS and MoS 2 in a water bath, where capillary forces deftly disengage the polymer and 2D material from the sapphire substrate.The next steps involve carefully positioning the MoS 2 and polymer onto the target wafer, followed by the methodical removal of PDMS via a peeling process within a glovebox to mitigate any contamination risks.This method boasts a remarkable 100% success rate in transferring MoS 2 films, yielding high-quality transfers with impressive mobility measurements of up to 40 cm 2 V −1 s −1 [106].Moreover, this pioneering technique extends its capabilities to transfer both continuous and prepatterned MoS 2 flakes onto a SiO 2 substrate, facilitating the assembly of multilayered MoS 2 structures.Furthermore, this methodology has been adapted to transfer WSe 2 from the sapphire growth substrate by employing PMMA as the supporting polymer and hydrophobic layer.This groundbreaking research has culminated in the successful transfer of both MoS 2 and WS 2 from the sapphire growth substrate to target wafers, paving the way for the fabrication of FET devices.Notably, FET devices with WS 2 channels have exhibited remarkable mobility values of up to 33 cm 2 V −1 s −1 , whereas those featuring MoS 2 channels recorded values of 23.9 cm 2 V −1 s −1 , only slightly lower than those reported in the original work [106].These developments hold great promise for the scalable production of 2D material-based electronic devices.

Chemical etchant assisted dry transfer
As mentioned in the previous paragraphs, one of the main characteristics of the wet-transfer process is that the floating 2D material/supporting polymer is collected from the target wafer through a process in which the latter is immersed in the water environment.On the contrary, the main distinction observed during the echemical etcahnt assisted dry transfer process, which is recommended for largescale applications requiring significant material transfer, is that after being detached from the growth substrate, it dries before being deposited on the final substrate [104].This process primarily relies on the difference in adhesion between the 2D material of its existing substrate and the target substrate [105].Therefore, the adhesion between the 2D material and the receiver substrate must be greater than the attraction force with the existing film.The choice of substrate on which the 2D material sits is critical, as it must have the necessary bonding action to protect the 2D material from external factors and other forces before attachment to the target substrate [105].
The specific process described in the literature is called dry transfer.However, it requires the use of an aqueous environment at its initial stages, as it involves the removal of the growth substrate, often achieved through etching, as shown in figure 4(a) [105,107].The preparation of the substrate bearing the 2D material requires the deposition of a thin layer of polymer, which serves as a supporting layer.For instance, in the case of graphene on Cu foil, PDMS is deposited on the surface of the 2D material [105].The literature also suggests the use of PDMS as a compliant layer, combined with a supporting PET substrate.
To ensure optimal adhesion between the materials, it is recommended to perform plasma surface treatment on the PET surface prior to depositing a thin layer of PDMS coating [107].Therefore, the specific structure can be placed on the 2D material, while it is still on the growth substrate.The specific preparation of the substrate is crucial, as mentioned before, .This is a schematic representation of the echemical etchant assisted dry transfer process: i.The 2D material is placed on its growth substrate.ii.Preparation of a supporting layer, on which a compliant layer is placed.iii.Placing the compliant layer on top of the 2D material.iv.The system is placed inside a bath where the etching of the growth substrate is carried out.v.After the growth substrate has been removed, the system is placed in deionized water to remove any residues from the process.vi.The stack is removed from the water and allowed to dry.vii.The system is placed on the target substrate so that the graphene rests on the receiver substrate.viii.Removal of the supporting and compliant layer.ix.The 2D material is now on top of the target wafer.(b) An illustration of the stamping technique using dry transfer is provided, along with optical microscopy images ii, iii) showcasing patterned graphene transfers using this method.Scale bars are set at 100 µm.Reprinted with permission from [106].Copyright (2015) American Chemical Society.
in the wet-transfer process, in order to provide support to the 2D material and prevent it from folding during the transfer.In addition to PDMS as a compliant supporting layer, Chen et al developed a transfer film consisting of a thin Si-based compatible layer and a support film [108], while Sebastian et al utilized a pressure-sensitive adhesive film (PSAF) [106].
In the next stage, a process is required to remove the growth substrate.Therefore, the structure is immersed in a solution for copper etching, such as 0.1 M ammonium persulfate [109], or in FeCl 3 as mentioned in the Wet Transfer method [108].Along the way, it needs to be placed in deionized water to ensure it is cleaned from any remnants of the previous process [105].
To complete the process of transferring the 2D material, after the drying stage of the 2D material/supporting layer structure, it must be placed onto the desired target substrate.However, achieving sufficient adhesion at the interface between the transfer film and the target wafer remains a challenge, and therefore, the combination of appropriate materials is crucial.For example, the stack mentioned above for the exfoliation of graphene from Cu foil is used.Similarly, for other materials like hBN developed on Fe foil, the literature proposes the use of PMMA as a supporting layer and PVA as a compliant layer between the 2D material and PMMA [107].However, this approach limits the partial transfer of the 2D material to the target substrate.To prevent mechanical damage during the transfer process, controlling the adhesion of the transfer film is important to ensure successful and damage-free transfer of the 2D material.After the material is adhered to the target wafer, the polymer layers are typically peeled off [105].This specific process has shown potential to leave less polymer residue on the surface of graphene compared to wet transfer, which can leave PMMA residues as thick as 1.5 nm [104].In the dry transfer method, polymer residues are not observed, but trapped bubbles between the graphene (Gr) and the target substrate are noticeable [105,108,109].
The dry-transfer technique has shown potential for use in the stamping process, aiming to eliminate the lithography stage typically used for patterning the 2D material on the target substrate's surface [106].As depicted in figure 4(b), PSAF can be utilized as a supporting layer for graphene transfer [106].By employing a mask with pre-designed structures in the form of holes on its surface, patterning can be achieved on the graphene surface.The selected mask should exhibit stronger adhesion to graphene compared to PSAF.The mask is placed on top of the graphene/PSAF structure, ensuring contact with the graphene.Upon mask removal, the graphene remains solely on the surface of PSAF in the areas where the hole structures were present.To complete the transfer, the graphene/PSAF structure needs to be placed onto the target substrate, with the graphene in direct contact.Subsequently, by peeling off the PSAF layer, the graphene pattern adheres to the target substrate, achieving the desired patterning [106].

Mechanical exfoliation
As the technology of 2D materials progresses, in terms of both manufacturing the materials themselves and their transfer, there is a demand for ever higher quality.Therefore, attempts are being made to reduce the intermediate steps required during the transfer of the 2D-material from its growth substrate to the target substrate.Special emphasis is placed on eliminating steps that involve the use of chemicals, such as those used to etch the growth substrate.One technique that aims to avoid this specific step is mechanical exfoliation (table 1) [110].This particular technique, as elaborated below, fundamentally falls within the broader spectrum of fully dry processes.In this case, the transfer of 2D material to the surface of the target wafer also transpires in a dry environment [110].The key distinction of the mechanical exfoliation method lies in the variance of adhesion that a 2D material exhibits when placed on different substrates [110].Consequently, by employing materials with superior adhesion characteristics, we can successfully transfer 2D materials from one substrate to another [111].In this technique, an adhesive layer is placed on the surface of the 2D material, while it is still on the growth substrate [110].Then, by peeling-off the 2D material, it lifts together with the deposited layer on its surface [110].This particular technique can be considered a form of dry transfer, as it does not require an aqueous environment for transferring the 2D materials to the target substrate [110][111][112].Additionally, the adhesive layer between the 2D material and the substrates that come into contact, plays a crucial role in this process, and no liquid solvents are used to destroy the growth substrate [111].In contrast to mechanical exfoliation, processes such as wet chemical etching, unavoidably introduce trapped moisture and organic contaminants at the interface between graphene and the substrate, which act as charge carrier trap sites [111].There are significant limitations in using the etching technique within the current mainstream CMOSbased manufacturing process [111].
In figure 5(a), all the necessary steps of the mechanical exfoliation process using peeling-off are clearly shown.This process is solvent-free and aims to remove the 2D material from the surface of the growth substrate.At the initial stage of the transfer process, an adhesive layer must be deposited on the surface of the developed 2D material, as it plays a crucial role in the subsequent peeling-off process.One example of a material that can act as an adhesive layer for graphene is MoO 3 [112].In the work by Moon et al [112], graphene grown via conventional lowpressure CVD was transferred onto Ge using a 30 nm amorphous MoO 3 thin film.On top of the adhesive layer, another substrate needs to be placed as a supporting layer to prevent folding of the 2D material during the transfer.A TRT can be utilized for this purpose, and for better contact with the surface of the adhesive layer, a thin layer of PMMA can be applied.The exfoliation of graphene from the growth substrate's surface can be carefully carried out through peeling-off, resulting in the 2D material being located on top of the adhesive layer, which is supported by the TRT and PMMA [112].The growth substrate can be reused for the growth of additional 2D material, as it remains undamaged throughout the process, which relies on the adhesive properties between the substrates [110][111][112][113].
The next step of the process involves removing the 2D material from both the adhesive layer and the supporting substrate.Initially, the TRT is heated to eliminate its adhesive properties, allowing the remaining materials to stand independently [77,114,115].As mentioned in the preceding paragraphs, materials like PMMA can be removed using acetone, while deionized water is employed for adhesive layers, such as MoO 3 [113].Thus, when the graphene, along with the adhesive layer, is on the target substrate, the entire structure can be immersed in deionized water to eliminate the adhesive layer.The utility of MoO 3 extends to its ability to address issues that arise when using polymers like PMMA as adhesive layers.In comparison to PMMA-assisted transferred graphene, which exhibits a high density of polymeric impurities due to ineffective dissolution of entangled polymer chains in solvents, MoO 3 -assisted transferred graphene proves advantageous [112].This technique has also been applied to transfer other 2D materials, including MoS 2 .For instance, a PVA film [111], a PDMS [104,[116][117][118] or a PMMA [119] can serve as  [78,80] ii.MoS2 [87,89] iii.hBN [81,82] i. CVD on metal [78,80] ii.CVD on SiO2 [87,88] i. PMMA [78,87,88] ii.Paraffin [80] i. Less mechanical damages [80] ii.Reduced strain effect on the flakes [80] iii.>8-inch graphene transfer [83] iv.
Transfer Technique

Suitable substrates
Carier substrate Advantages Limitations Laser-induced forward transfer i. Graphene [127][128][129] ii.MoS2 [130] Pre-transferred substrates [127][128][129][130] i. Ni [127,130] ii.Cu [128] iii.PMMA [129] i. Easy patterning [127][128][129][130]149] ii.Less polymer residue [127] iii.Controlling the size of the transferred material surface by adjusting the laser beam's size [127,128,130] iv.It is possible to manufacture larger surfaces by transferring small pixels of 2D materials through digital printing [127,130] v.It is possible to transfer the 2D material to the target wafer without the need for any adhesion or supporting layer [127,130] There is a need for a pre-transfer method before laser transfer [127][128][129][130] Metal Assisted Transfer i. MoS2 [135] ii.MoX2 [138] iii.Graphene [139] iv.hBN [134] v. WSe2 [134] vi.MoSe2 [134] i. SiO2 growth substrate [135] ii.CVD metal growth substrate [139] i. Cu [137] ii.Au [125] iii.Ni [126] i. Transfer without leaving polymer residues [135] ii.Transfer without leaving residue avoids wrinkling or creating surface imperfections on the 2D material [135] Use of chemicals for etching the metallic carrier adhesive layer [135,139]  an adhesive layer for detaching MoS 2 from its growth substrate through the peeling-off process.Particular importance should be given to the fact that the specific process is characterized by the possibility of reusing the growth substrate.As can be seen in figure 5(b), Moon et al [112] succeeded in reusing the same substrate 50 times without showing signs of degradation.In figure 5(b-ii.), the Raman spectrum for graphene grown on a Ge substrate is shown.From the spectra, it can be seen that for the 50 consecutive times graphene has been manufactured, it does not appear to change its properties, and its quality remains high [112].Additionally, electrical characterizations performed on the 1st and the 50th graphene, which were manufactured on the same growth substrate and transferred using a specific mechanical method, yield good values for sheet resistivity, charge neutral point, and mobility, with values >6500 cm 2 V 1 s 1 [112].
The mechanical exfoliation has been established as a precise method for transferring 2D materials to designated locations on a target wafer's surface.A notable approach involves employing a viscoelastic stamp.Following the mechanical exfoliation process, the 2D material is transferred onto the PDMS surface and, subsequently, positioned at a specific location on the target substrate using optical microscopy.Castellanos-Gomez et al [120] have demonstrated the exfoliation of 2D material flakes through mechanical means, placing them on the surface of a viscoelastic stamp adhered earlier to a glass support plate.Precise alignment of the flake with the target substrate is achieved under the microscope's guidance.The stamp and target wafer are then brought into contact, securing the wafer to the 2D material.A careful and gradual separation of the PDMS from the surface is necessary to detach it from the 2D material, owing to the viscoelastic properties of the material.This particular technique has been successfully applied for depositing graphene on the hBN surface, as well as MoS 2 on the hBN surface.Yang et al [104] have also demonstrated that the same method can be used to transfer MoS 2 onto a target wafer with fabricated source and drain electrodes.Through this method, they have achieved the fabrication of a FET without requiring additional lithography steps post-transfer, a notable advantage due to the precise positioning of the 2D material within the device.The reported mobility value for MoS 2 after this specific transfer is 76 cm 2 V −1 s −1 .
Building upon this process, subsequent methods have been developed where the adhesion layer used for transferring the 2D material is not a polymer but another 2D material, commonly known as the vdW pick-up method [121][122][123].Wang et al [124] have demonstrated how an hBN flake, which was mechanically exfoliated onto a PDMS layer, can serve as an adhesion layer for a graphene layer that has been exfoliated and placed on the surface of a SiO 2 wafer.The adhesion between graphene and hBN is notably stronger than that with SiO 2 , allowing for the adhesion of graphene to hBN and its removal from SiO 2 .This enhanced adhesion is attributed to the substantial contact area between hBN and the 2D material intended for transfer.In this work, the adhered hBN-graphene structure was then positioned onto another hBN flake, resulting in an hBN/graphene/hBN layered structure, while the stamp was removed by peeling it off.Mobility measurements of this graphene structure revealed impressive values of up to 140 000 cm 2 V −1 s −1 .
In view of the production and the transfer of large-scale graphene, it should be noted that the specific process is compatible with the roll-to-roll method, as shown in figure 5(c).Hong et al achieved the transfer of graphene grown on a Cu substrate via peeling-off [113].More specifically, this technique requires the use of ethylene vinyl acetate (EVA) as an adhesive layer, while the use of PET above the EVA as a supporting layer is mandatory.Since graphene can be grown on both sides of Cu, it is possible to place the adhesive layer and the supporting layer on both sides of the Cu.The structure needs to be passed through rolls and heated to enhance the adhesive.Then, through simple exfoliation, the 2D material can be removed from the surface of the growth substrate, and the copper foil can be reused for new graphene growth on its surface.
In their work, Shivayogimath et al [125], they employed the PVA lamination transfer technique, leveraging PVA as an adhesion layer.This technique facilitates the successful transfer of graphene from both copper [125] and sapphire growth substrate [125].Notably, for the exfoliated graphene from the copper substrate, it was reported that after characterization using graphene-FET devices, the observed mobility values reached up to 800 cm 2 V −1 s −1 .In the case of graphene grown on sapphire, post-transfer via the PVA lamination process, mobility values of 2000 cm 2 Vs −1 for electrons and 2300 cm 2 V −1 s −1 for holes were reported.In the case of graphene grown on copper, the initial step involves immersing it in DI water at room temperature to oxidize the copper's surface.This process aims to mitigate the binding of the 2D material to the growth substrate.Subsequent to surface drying, PVA is applied to the graphene.In their research, Shivayogimath et al [125] applied the PVA film by laminating it onto the graphene-copper foil at 110 • C and a speed of 20 cm min −1 using a commercial hot-roll office laminator.Following this, it was placed on a hot plate for 30 s at a temperature of 110 • C to enhance adhesion.The subsequent step in the process involves the mechanical exfoliation of the 2D material through peeling off the PVA/Graphene structure from the growth substrate and placing it onto a target wafer, such as SiO 2 .Furthermore, materials such as paper can be employed as a supporting layer atop the PVA, and it can be subsequently removed by placing the paper-/PVA/Graphene/Target Wafer structure on a hot plate and heating it to a temperature of 110 • C for 1 min.The paper can then be peeled off.During this process, the stack is placed in DI water at room temperature to dissolve the PVA.The aforementioned method has been employed for transferring wafer-scale graphene that was grown on sapphire substrates [126].
While the mechanical transfer process has been successfully used to create flexible electronics, there are still some problems associated with the process.For example, once the 2D material has been separated from the donor substrate and adhered to the adhesive, it cannot be transferred again to a desired substrate [111,112].Additionally, the process of mechanically separating the 2D material from the donor substrate can result in damage to the material, reducing its electrical properties [113].Despite these challenges, the mechanical transfer process has significant advantages, including the ability to transfer large areas of material continuously and reuse donor substrates for 2D material synthesis, making it a valuable technique in the field of materials science.

Laser-induced transfer (LIT)
The utilization of 2D-materials for controllable patterning on the surface of target substrates, typically devices, holds significant importance.With this objective in mind, extensive efforts have been devoted to the digital transfer of 2D materials through the LIT method.This method can be categorized into two major types: laser-induced forward transfer (LIFT) (figure 6(a-ii.))and laser-induced backward transfer (LIBT) (figure 6(a-iii)) [127,128].These categories employ pulsed lasers with nanosecond pulse durations and exhibit distinct differences that will be discussed below.A key requirement of this technique is that the 2D material must be present on a donor substrate, having been transferred from its growth substrate [129].To date, no published work has reported successful transfer of uniform and controlled portions of 2D materials from a growth substrate using the LIT technique.Consequently, the fabrication of the donor substrate necessitates growing the 2D material on the growth substrate, such as Cu foil for graphene, for instance.As described in the works of Smits et al [129] and Papazoglou et al [127], the graphene is detached from the growth substrate using a polymeric-assisted transfer wet transfer technique.This involves initially applying a PMMA layer onto the graphene/Cu followed by etching of the Cu foil.The resulting PMMA/graphene film is then cleansed in water to remove any excess Cu etching solution before being placed onto the donor substrate through submersion in water.Finally, the PMMA layer is eliminated using acetone.Hence, the manufacturing of the donor substrate requires incorporating specific stages from one of the aforementioned techniques.
The structure of the donor substrate holds particular importance for the successful transfer of 2Dmaterials through LIT techniques.Firstly, there is a requirement for an intermediate thin film between the 2D-material and the donor substrate, which will absorb the radiation of the laser pulse [129].Typically, a few nanometres thick metal layer is used in direct contact with the 2D-material, referred to as the dynamic release layer (DRL) [127][128][129].For the transfer of graphene monolayer and MoS 2 , a 50 nm thick Ni layer can be employed as the DRL [127,130].The 2D-material is placed on the surface of the DRL using the aforementioned process.Nevertheless, other materials, such as Triazene, are also used in the literature [131].The role of the DRL is to absorb the energy of the laser pulses and is selected based on its absorption coefficient.This coefficient ensures that the DRL absorbs the entire energy of the pulse at a depth smaller than its thickness [127,132,133].After energy absorption, two mechanisms are observed.The first mechanism involves the strong decomposition of the DRL due to irradiation, resulting in the release of gases that act as propellants for the transferred material [129].According to the second mechanism, a mechanical wave is generated within the metal DRL due to the oscillation of the metal ions [127].This wave propagates towards the contact interface with the 2D-material, providing it with the necessary kinetic energy to move away from the donor substrate and transfer to the receiver.Ab initio molecular dynamics simulations have been utilized to study the behavior of this layer, confirming the contribution of the mechanical wave during the transfer of 2D-materials.The DRL, as a thin metal layer, must be situated on a supporting substrate, which serves as the main substrate of the donor [127].In the case of LIFT, this substrate must be permeable to the wavelength of the laser pulses, with Quartz being commonly chosen.As the pulse passes through the back of the donor, it is absorbed by the back of the DRL, generating the mechanical wave that propagates towards the 2D-material located on the front side of the donor [127].In contrast to the LIBT technique, the pulse is incident on the front of the donor where the 2D-material is present and is absorbed by the DRL directly beneath it [128].Thus, in LIBT, the donor substrate does not need to be permeable to laser irradiation.However, in order to reach the donor, the pulse must pass through the receiver, necessitating the receiver to be permeable to the laser beam's wavelength.One receiver option is silicon, which exhibits a transmittance of 1 in infrared wavelength.
One of the earliest works demonstrating the successful transfer of digitally transferred graphene pixels was conducted by Smits et al [129], utilizing the LIFT technique to transfer monolayer graphene (figure 6(b)).As mentioned previously, the LIT was not performed directly from the growth substrate, but rather another substrate was used as the donor.Specifically, through chemical etching of the Cu foil growth substrate and employing a PMMA supporting layer, the graphene monolayer was transferred onto the donor.The donor substrate comprises quartz and carries a 200 nm thin film of Triazene on one side, which serves as the DRL with a slightly different function compared to the one discussed earlier.The graphene/PMMA stack is placed on top of the donor, with PMMA in contact with the Triazene.Therefore, the stack used as the donor is Graphene/PMMA/Triazene/Quartz.During the transfer process, the donor is positioned above the receiver, with the 2D material directly facing the receiver substrate, as depicted in figure 6(b).The distance between the receiver and the donor must be less than 100 µm, and the substrates should not come into contact for successful transfer to occur.At a specific point, the donor is irradiated with a laser pulse from the Quartz side, and the pulse reaches the DRL where it is absorbed.Triazene exhibits an absorption maximum in the region around 355 nm [134].Irradiation with the specific wavelength results in its strong decomposition and the release of N 2 , which acts as a propellant gas to remove the Graphene/PMMA stack from the donor surface.An optimal fluence for achieving graphene transfer through this process ranges from 42 mJ cm −2 to 54 mJ cm −2 , using laser pulses with a wavelength of 355 nm.The shape and size of the pulse can be controlled by adjusting the shape and size of the laser beam.In the aforementioned study [129], a square pixel with an edge dimension of approximately 100 µm was successfully transferred.Up to this point, the significant potential of LIFT in transferring patterned graphene pixels has been demonstrated simply by controlling shape of the laser beam, without the need for pre-patterned stages of the 2D-material on the donor substrate before transferring it to the target substrate, as required in Dry Transfer, which was discussed earlier.
The LIFT technique offers a significant advantage in digital transfer of graphene without the need for pre-patterning, however a polymeric supporting layer of PMMA is typically required for graphene transfer, which is also transferred to the receiver substrate [129].Papazoglou et al [127], published a work demonstrating the successful transfer of intact graphene monolayer pixels using the digital LIFT technique without any supporting layer.In their study, the 2D-material is grown on Cu foil and transferred to the donor surface with the aid of etching and a PMMA supporting layer, making direct contact with the DRL composed of a 50 nm thin film of Ni.Thus, the donor structure consists of Quartz/Ni(50 nm)/Graphene/PMMA stack.The PMMA is subsequently removed using acetone, resulting in the final donor structure: Quartz/Ni(50 nm)/Graphene.For successful transfer without the use of the polymeric supporting layer, contact between the donor and receiver substrates is necessary [127].Additionally, the work mentions the requirement of reduced pressure conditions in the range of a few tens of mbar.Controlling the size and shape of the pulse allows for precise transfer of graphene pixels with dimensions up to 30 µm × 30 µm.The investigated substrates include SiO 2 (figures 6(c-i., ii.)) as well as a flexible PDMS substrate.The graphene monolayer is transferred with significant repeatability, as evidenced by the Raman spectra in figure 6(c-iii.),showing no differences among 10 sequentially printed graphene pixels.The optimal fluence for effective graphene transport using 355 nm laser pulses is determined to be 50 mJ cm −2 .This energy is absorbed by the Ni DRL, as discussed earlier, generating a mechanical wave that delivers the necessary energy to the graphene, causing it to detach from the donor surface and break the bonds between neighboring carbon atoms, resulting in the formation of graphene pixels of specific dimensions determined by the shape of the laser pulse [127].Importantly, the feasibility of transferring graphene to the receiver substrate without damaging the DRL substrate and the concurrent transfer of Ni ions to the target wafer along with graphene have been investigated [127,130].Using the same technique, Logotheti et al [130], achieved the transfer of arrays of monolayer and bilayer MoS 2 flakes from a Quartz/Ni(50 nm)/MoS 2 donor substrate.This was accomplished using 355 nm laser pulses with a fluence of 80 ± 5 mJ cm −2 , further confirming the efficacy of the technique for digital transfer of 2D materials and its ability to achieve controlled patterning with ease.Finally, successful graphene transfer has also been achieved using the LIBT technique [128], where the laser beam initially passes through the receiver substrate, as illustrated in figure 5(a-iii).In this process, a laser beam with a wavelength that can penetrate the receiver substrate is utilized.Praeger et al [128], demonstrated the transfer of graphene via LIBT when the receiver substrate was made of silicon or glass, and the laser radiation wavelength was in the infrared region.The fabrication of the donor substrate follows the description mentioned earlier.The laser pulses that penetrate the receiver substrate reach the interface between the 2D-material and the donor substrate, where they are absorbed.For graphene, a DRL such as polycrystalline nickel or another metal can be used on the donor substrate [128].The laser pulse is absorbed by the metal layer a few nanometers below the interface with graphene, providing the necessary energy for successful transfer to the receiver substrate.Similar to the LIFT technique, the LIBT process requires the application of reduced pressure, typically a few mbar [128].

Metal assisted transfer
In this paragraph, the method of transferring 2D materials through the assistance of a metal layer is described.Transfers of materials such as TMDs, like MoS 2 , from their growth substrate to a target wafer, have been documented in literature.This particular method holds distinct value as it does not utilize any polymeric material as an adhesive layer or supporting layer that comes into direct contact with the 2D material intended for transfer [135,136].The specific materials often leave a residue on the surface of the transferred 2D materials, necessitating their replacement [102,136,137].This technique, however, enables the transfer without leaving polymer residues, and avoids wrinkling or creating surface imperfections on the 2D material [136].
Metal-assisted transfer, utilizing a very thin layer of Cu, has been employed for transferring 2D MoS 2 from its growth substrate (figure 7).Specifically, MoS 2 flakes grown on a SiO 2 /Si substrate through the CVD technique were transferred using this method [136].Materials like MoS 2 can only be grown on certain substrates, such as SiO 2 and Sapphire, due to the high temperatures required [22].Therefore, it becomes necessary to transfer it onto the desired substrate.To prevent the material from coming into direct contact with the polymeric adhesive and supporting layer, a thin copper layer of approximately 60 nm can be deposited on the MoS 2 when it is on the growth substrate [136].This copper layer provides significant adhesion for the exfoliation of the material from its substrate [136].Following the deposition of the thin metal layer, a supporting layer must be applied to support the entire structure during the transfer.One suitable material is TRT, which exhibits strong adhesion to copper at room temperature.Once the TRT/Cu/MoS 2 / SiO 2 stack is created, vertical forces can be applied to peel off the TRT/Cu/MoS 2 stack from the surface of the growth substrate [136].The structure can then be placed onto the target substrate.The TRT can be removed by heating the structure from room temperature to 120 • C, causing it to lose its adhesion forces with the Cu [136].Selective etching is then used to remove the thin metal layer.This specific technique allows for the transfer of the 2D material with fewer defects compared to direct contact transfer using a polymer on the 2D surface, resulting in fewer folds.Similar metal-assisted transfer processes can be achieved using other metals such as Au [138] for the transfer of MoX 2 and Ni for the transfer of graphene [139,140].Using a similar procedure, Lai et al achieved the detachment of MoS 2 from the SiO 2 growth substrate by employing a metallic Cu adhesive layer, along with a supporting layer of the PDMS/PMMA stack [141].However, in this specific case, the overall process involves water penetration assistance.After the delamination of the PDMS/PMMA/Cu/ MoS 2 stack from the growth substrate through peeling off, the structure needs to be transferred onto the target substrate.To accomplish this, the structure is immersed in a container of water, along with the target wafer.The buoyancy force of the water provides mechanical support, minimizing the formation of cracks and wrinkles [141].The PDMS can be removed by peeling off, the PMMA by immersing the structure in acetone, and the Cu by etching in a FeCl 3 solution.The metal-assisted transfer method can be utilized to transfer a monolayer from a multilayer material while it remains on its growth substrate.This method has been successfully employed for the exfoliation of various materials, including hBN, tungsten disulfide (WS 2 ), tungsten diselenide (WSe 2 ), MoS 2 , and molybdenum diselenide (MoSe 2 ) [134].It enables the high-throughput production of monolayer 2D materials with precise single-atom thickness, facilitating the fabrication of wafer-scale vdW heterostructures [135].The method involves using a 600 nm Ni layer as an adhesive layer, which is placed on top of the multilayered 2D material, while it remains on its sapphire growth substrate.The Ni layer binds some of the layers of the 2D material and removes them from the as-grown material.Subsequently, another Ni layer is deposited to create the Ni/2D/Ni stack, and peeling-off is repeated to separate the layers once again.This process results in a monolayer 2D material, which is then transferred onto a SiO 2 /Si substrate, while the Ni layer is etched away.Metal-assisted transfer process involves the following steps: i.The 2D material is placed on its growth substrate.ii.A thin metal layer is deposited on top of the 2D material.iii.The stack of metal with 2D material and the substrate is covered with thermal release tape (TRT).iv.The TRT is mechanically peeled off to separate the stack from the growth substrate.v.The stack is then pressed onto the target substrate.vi.The TRT is simply removed using heat.vii.The metal layer is selectively etched, leaving the 2D film in place.

Conclusion and future challenges
The field of 2D materials has witnessed an exponential growth in research, leading to remarkable advancements in the techniques used for transferring these materials from growth substrates to target substrates.This pursuit has been driven by the desire to fully exploit the physical and mechanical properties of 2D materials, while minimizing the introduction of defects during the transfer process.One of the key challenges encountered in the development of transfer techniques revolves around the detachment of these materials from their substrates.To overcome this challenge, researchers have explored various approaches, including selective etching of the substrate, electrochemical methods, and the utilization of adhesive materials that facilitate mechanical detachment from the growth substrate.By carefully addressing the detachment issue, significant progress has been made in reducing the occurrence of defects during the transfer process.Another hurdle that researchers have successfully tackled is the elimination of folds, which have a detrimental impact on the quality of the transferred material.To address this issue, supporting layers composed of polymer materials have been employed.However, a major concern arises when removing these polymer layers, as they often leave residues on the surface of the 2D material.Consequently, researchers have redirected their efforts towards the use of intermediate adhesive layers, such as PVA and thin metal layers, which aid in minimizing residue formation.
While transfer techniques have undeniably made substantial strides, the ultimate challenge lies in scaling up these methods to industrial levels.One potential solution that has been explored is the roll-to-roll technique, which is compatible with various transfer methods and holds promise for largescale transfers of 2D materials.However, reducing the number of defects introduced during large-scale transfers remains a critical goal for future research.Furthermore, the development of new transfer methods and the streamlining of existing ones are essential to minimize intermediate stages in the transfer process.By reducing the number of steps involved, researchers aim to curtail the introduction of defects while seeking alternatives to polymeric materials and etching chemicals.The goal is to find novel approaches that can achieve high-quality transfers without compromising the integrity of the 2D materials.Moreover, the compatibility of transfer methods with patterning techniques like stamping is of paramount importance in the utilization of 2D materials in electronic, optoelectronic, and photonic devices.The ability to incorporate 2D materials seamlessly without requiring additional lithography processes on the device would greatly enhance the efficiency and cost-effectiveness of device fabrication, like the LIT process.
In conclusion, the progress in the field of 2D materials transfer techniques has been remarkable, driven by the continuous efforts to harness the exceptional properties of these materials.By addressing challenges related to detachment, fold elimination, scalability, and the reduction of introduced defects, researchers are paving the way for the widespread utilization of 2D materials in various industries.The ongoing exploration of alternative methods, the streamlining of existing processes, and the compatibility with patterning techniques will shape the future of this field, enabling the integration of 2D materials into cutting-edge technologies.

Figure 1 .
Figure 1.The main transfer methods for two-dimensional materials include the wet transfer technique assisted by etching, the wet transfer technique in which no substrate etching is performed, the dry transfer technique, mechanical exfoliation, and metal-assisted transfer.

Figure 2 .
Figure2.This is a schematic representation of the chemical etching wet transfer method.(a) In detail, all the steps of the chemical etching method are as follows: i.Initially, the two-dimensional material (in this particular case, graphene on copper) is on its growth substrate.ii.A thin layer of polymer (usually PMMA) is then deposited.iii.The system is then immersed in a bath to etch the substrate.iv.After etching the substrate, the 2D material and the polymer are immersed in deionized water to remove the copper and solvent residues.v.The 2D material and the polymer are then placed on the target wafer.vi.Finally, the polymer is removed, leaving the 2D material alone on the target substrate.(b), (c) Optical microscopy images of the transfer of graphene onto a SiO2 substrate using the chemical etching-assisted method are shown.Reproduced from[78].© IOP Publishing Ltd.All rights reserved.(d) i.This specific image demonstrates the graphene transfer process using paraffin.ii-v.Optical microscopy images of a Graphene-FET and electrical measurements during the transfer process, enabling the derivation of mobility.Reproduced from[80].CC BY 4.0.(e) This is a representation of the roll-to-roll process.i.The thermal release tape is adhered onto the graphene/copper sample using two rollers with moderate pressure.ii.The copper foil is etched by passing it through a copper etching bath.iii.The detached tape/graphene is then placed on the target substrate and they are plugged together into two hot rollers.

Figure 4
Figure 4.This is a schematic representation of the echemical etchant assisted dry transfer process: i.The 2D material is placed on its growth substrate.ii.Preparation of a supporting layer, on which a compliant layer is placed.iii.Placing the compliant layer on top of the 2D material.iv.The system is placed inside a bath where the etching of the growth substrate is carried out.v.After the growth substrate has been removed, the system is placed in deionized water to remove any residues from the process.vi.The stack is removed from the water and allowed to dry.vii.The system is placed on the target substrate so that the graphene rests on the receiver substrate.viii.Removal of the supporting and compliant layer.ix.The 2D material is now on top of the target wafer.(b) An illustration of the stamping technique using dry transfer is provided, along with optical microscopy images ii, iii) showcasing patterned graphene transfers using this method.Scale bars are set at 100 µm.Reprinted with permission from[106].Copyright (2015) American Chemical Society.

Figure 5 .
Figure 5. Mechanical transfer of 2D materials can be achieved by two methods: (a) i.The 2D material is placed on its growth substrate.ii.An adhesive layer is coated on top of the 2D material, followed by the placement of the target substrate on top.iii.The 2D material is mechanically exfoliated from the growth substrate.iv.The 2D material is now on top of the target wafer.(b) i. Optical microscopy image of the transfer process.ii.Raman spectra after 50 repetitions of graphene growth and transfer using the same substrate.iii.A Graphene-FET fabricated through the aforementioned technique.iv.Measurement of the sheet resistance (Rsh) of the transferred graphene.[112] John Wiley & Sons.© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(c) A schematic illustration of the roll-to-roll process for mechanical transfer.[113] John Wiley & Sons.© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 6 .
Figure 6.(a) Schematic representation of the process of laser-induced transfer of 2D-materials.i.Schematic representation of the first step of the process, which requires the construction of the donor substrate of the 2D-material.ii.Schematic representation of the process of laser-induced forward transfer of 2D-materials.iii.Schematic representation of the process of laser-induced backward transfer of 2D-materials.(b) Schematic representation of the transfer of the graphene monolayer through the laser-induced forward transfer technique with the parallel use of a PMMA supporting layer.Reprinted from [129], with the permission of AIP Publishing.(c) i. Transfer of an array of graphene pixels through the laser-induced forward transfer technique without the use of a PMMA supporting layer.ii.SEM image of a transferred graphene monolayer pixel via LIFT.iii.Raman spectra from 10 pixels of graphene monolayer via laser-induced forward transfer.Reproduced from [127].© The Author(s).Published by IOP Publishing Ltd.CC BY 4.0.

Figure 7 .
Figure7.Metal-assisted transfer process involves the following steps: i.The 2D material is placed on its growth substrate.ii.A thin metal layer is deposited on top of the 2D material.iii.The stack of metal with 2D material and the substrate is covered with thermal release tape (TRT).iv.The TRT is mechanically peeled off to separate the stack from the growth substrate.v.The stack is then pressed onto the target substrate.vi.The TRT is simply removed using heat.vii.The metal layer is selectively etched, leaving the 2D film in place.

Table 1 .
A comparative analysis of various transfer methods for 2D materials.