Systems and methods for metal-to-polymer exfoliation and transfer of large area van der waals monolayers

Abstract

High-quality single crystal monolayers (SCMs) are transferred to target substrates such as silicon and oxides thereof. The SCMs are exfoliated from material sources with van der Waals (vdW) crystal structures, e.g., transition metal di chalcogenides (TMDs) including WS2, WSe2, M0S2, MoSe2, etc., via tape sheets with metal exfoliant layers, e.g., elemental gold (Au). The metal exfoliant layers are pressed to surfaces of the material source to remove an SCM. A viscoelastic polymer layer, e.g., including polydimethyl siloxane (PDMS), is positioned so the SCM is between the polymer layer and the tape sheet. The exfoliant layer is etched away, and the etched construct is positioned on a final substrate with the SCM between the polymer layer and the final substrate. Applied heat separates the polymer layer from the SCM. SCMs having a width:thickness on the final substrate of at least about 500,000:1 are easily achievable without surface defects. These transfer methods are deterministic, unlike traditional methods that rely on stochastic tape exfoliation.

Description

SYSTEMS AND METHODS FOR METAL-TO-POLYMER EXFOLIATION AND TRANSFER OF LARGE AREA VAN DER WAALS MONOLAYERSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 529,846, filed July 31, 2023, which is incorporated by reference as if disclosed herein in its entirety.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0002] This invention was made with government support under N00014-18-1-2080 awarded by the Office of Naval Research and 2011738 awarded by the National Science Foundation. The government has certain rights in the invention.BACKGROUND

[0003] Two-dimensional (2D) materials exfoliated from van der Waals (vdW) crystals show great potential for applications in electronic, optoelectronic, and quantum devices, and are one of the most actively researched materials in science and engineering today. Transition metal dichalcogenides (TMDs), a class of two-dimensional (2D) semiconductors, are particularly promising candidates for integration into semiconductor technologies. While possessing electronic and optoelectronic properties on par with the best semiconductors in use today, e.g., Si, III-Vs, etc., 2D TMDs offer the possibility of downsizing devices to subnanometer thickness limits without introducing interfacial defects that are detrimental to conventional semiconductors at those limits. Despite the technological potential, commercializing 2D TMD materials has been largely hindered due to a lack of high-throughput techniques for exfoliating and transferring single-crystal monolayers with macroscopic size and sufficient quality. There are a number of desired characteristics for the commercial applications of 2D monolayer materials and 2D semiconductors, such as that the 2D monolayers be of sufficient crystalline-quality to maintain their intrinsic physical properties; the 2D monolayers be of macroscopic dimensions; the production of the 2D monolayers be highly reproducible; the macroscopic 2D monolayers be produced in large quantities, conveniently stored, and readily available for use, etc.

[0004] A “Scotch” tape exfoliation method is most widely used in research today, and can produce high quality monolayers of 2D single crystals. However, the “Scotch” tape methodyields 2D materials of microscopic dimensions, is stochastic in producing the monolayers, and is of low yield. Chemical vapor deposition can produce 2D materials of macroscopic dimensions (whole wafers), but the monolayers are mainly poly crystalline, of low quality, and may not retain their physical crystalline physical properties.

[0005] Metal assisted exfoliation, particularly the “gold-tape” exfoliation method, has emerged as a scalable method to deterministically exfoliate high quality, large area, single crystal monolayers, e.g., of TMD. However, metal-assisted exfoliation methods reported to date leave the TMD monolayers on the surfaces of the metal, an oxide, or a semiconductor. Removing the monolayers from these surfaces for subsequent use involves lengthy and cumbersome transfer steps and, thus, do not produce monolayers in sufficiently large quantities or in a state enabling convenient storage and / or use. What is desired, therefore, are methods enabling deterministic production and transfer of single crystal monolayers that are also scalable, e.g., for use in commercial applications.SUMMARY

[0006] Aspects of the present disclosure are directed to methods of exfoliating, storing, and transferring single crystal monolayers to target substrates from material sources with van der Waals (vdW) crystal structures. In some embodiments, a method for single crystal monolayer transfer is disclosed where a material source including one or more transition metal dichalcogenides (TMDs) including WS2, WSe2, M0S2, MoSe2, or combinations thereof is provided. In some embodiments, a tape sheet is prepared by positioning a layer of elemental gold on a template substrate, spin-coating a polymer protection layer (PPL) onto the elemental gold layer, adhering a thermal release tape layer to the PPL to form the tape sheet, and lifting the tape sheet from the template substrate. In some embodiments, the elemental gold layer is contacted with the material source and a single crystal monolayer is exfoliated from the material source to the elemental gold layer. In some embodiments, a polymer layer, e.g., a viscoelastic polymer layer composed of polydimethylsiloxane (PDMS), is then positioned on the tape sheet so that the exfoliated single crystal monolayer is positioned between the polymer layer and the tape sheet, forming a monolayer construct. In some embodiments, the elemental gold layer is etched away, and the etched monolayer construct is positioned on a final substrate with the single crystal monolayer positioned between the polymer layer and the final substrate. In some embodiments, heat is applied to the monolayer construct to separate the polymer layer from the single crystal monolayer. In some embodiments, the single crystal monolayer has a ratio of a width to a thickness of the single crystal monolayer on the final substrate of at leastabout 500,000: 1. In some embodiments, the monolayer construct can be stored for a predetermined time period.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The drawings show embodiments of the disclosed subject matter for the purpose of illustrating the invention. However, it should be understood that the present application is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein:

[0008] FIG. l is a schematic representation of a single crystal monolayer construct according to some embodiments of the present disclosure;

[0009] FIG. 2A is a chart of a method for single crystal monolayer transfer according to some embodiments of the present disclosure;

[0010] FIG. 2B is a chart of a method for single crystal monolayer transfer according to some embodiments of the present disclosure;

[0011] FIG. 2C is a chart of a method for single crystal monolayer transfer according to some embodiments of the present disclosure;

[0012] FIG. 3 is a chart of a method for single crystal monolayer transfer according to some embodiments of the present disclosure;

[0013] FIG. 4A is an image showing a single crystal monolayer positioned between an elemental gold layer and a viscoelastic polymer layer according to some embodiments of the present disclosure;

[0014] FIG. 4B is an image showing a single crystal monolayer on a final substrate according to some embodiments of the present disclosure;

[0015] FIG. 5 A is a microscope image of a millimeter scale twisted WSe2 homobilayer according to some embodiments of the present disclosure, with a relative twist angle of 21.9 degrees determined by polarization resolved second harmonic generation;

[0016] FIG. 5B portrays atomic force microscopy (AFM) topography images for the twisted WSe2 homobilayer imaged in FIG. 4A and according to some embodiments of the present disclosure, as compared to a MoSe2 single crystal monolayer exfoliated directly by a gold-tape method; and

[0017] FIG. 6 is an image of a comparison of AFM topography of three transition metal di chalcogenide (TMD) monolayers transferred using the methods according to embodiments of the present disclosure onto silicon with a 285nm thermal oxide layer under various conditions and surface treatments.DETAILED DESCRIPTION

[0018] Referring now to FIG. 1, some embodiments of the present disclosure are directed to a monolayer construct 100. In some embodiments, monolayer construct 100 includes a tape sheet 102. In some embodiments, tape sheet 102 includes at least one layer 102A including one or more metal exfoliants. In some embodiments, the metal exfoliants are any metals capable of exfoliating single crystal monolayers from a material source having a van der Waals (vdW) crystal structure. In some embodiments, the metal exfoliants include elemental gold. As will be discussed in greater detail below, in some embodiments, tape sheet 102 includes a polymer protection layer positioned on metal exfoliant layer 102 A, and further a thermal release tape layer adhered to the polymer protection layer (not pictured). In some embodiments, the polymer protection layer includes polyvinylpyrrolidone (PVP).

[0019] Still referring to FIG. 1, in some embodiments, monolayer construct 100 includes one or more polymer layers 104. In some embodiments, polymer layer 104 is viscoelastic. In some embodiments, polymer layer 104 includes poly dimethylsiloxane (PDMS). In some embodiments, monolayer construct 100 includes a single crystal monolayer 106. In some embodiments, single crystal monolayer 106 is positioned between tape sheet 102 and polymer layer 104. As discussed above, in some embodiments, single crystal monolayer 106 is obtained via exfoliation from a material source having a vdW crystal structure by metal exfoliant layer 102A. The monolayer constructs 100 consistent with the embodiments of the present disclosure, as well as the corresponding methods described below, expand the capabilities of crystal layer exfoliation, e.g., gold-tape exfoliation, at least in part by utilizing intermediate viscoelastic polymer substrates enabling storage and transfer of exfoliated single crystal monolayers, e.g., single crystal monolayer 106. In some embodiments, single crystal monolayer 106 is a single vdW layer of the material source. In some embodiments, single crystal monolayer 106 includes one or more transition metal di chalcogenides (TMD). In some embodiments, the TMD includes WS2, WSe2, M0S2, MoSe2, or combinations thereof. In some embodiments, single crystal monolayer 106 is obtained via exfoliation from one or more TMDs by metal exfoliant layer 102A, e.g., an elemental gold layer.

[0020] In some embodiments, single crystal monolayer 106 has a ratio of its width to its thickness that is suitable for its intended use. In some embodiments, the width of single crystal monolayer 106 is macroscopic. As used herein, the width to thickness “ratio” refers to the ratio of the mean “width” and mean “thickness” of the single crystal monolayer, as would be understood by those of ordinary skill in the art. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is at least about 500,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is between about 500,000: 1 and about 5,000,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is at least about 600,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is at least about 700,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is at least about 800,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is at least about 900,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is at least about 1,000,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is at least about 1,500,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is at least about 2,000,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is at least about 2,500,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is at least about 3,000,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is at least about 3,500,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is at least about 4,000,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is at least about 4,500,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is at least about 5,000,000: 1. In some embodiments, the ratio of the width to the thickness of single crystal monolayer 106 is greater than about 5,000,000: 1.

[0021] Referring now to FIG. 2A, some embodiments of the present disclosure are directed to a method 200 for monolayer transfer. In some embodiments, method 200 includes metal-to- polymer exfoliation and transfer of large area vdW monolayers to a final substrate. In some embodiments, at 202, a monolayer construct is provided. In some embodiments, the monolayer construct is configured consistent with the embodiments of monolayer construct 100 discussed above. In some embodiments, the monolayer construct includes one or more single crystal monolayers disposed on a metal exfoliant layer, and positioned between the metal exfoliant layer and a polymer layer. In some embodiments, the monolayer construct includes a tape sheetincluding the metal exfoliant layer. In some embodiments, the metal exfoliant layer includes elemental gold. In some embodiments, the polymer layer includes one or more viscoelastic polymers. In some embodiments, the polymer layer includes PDMS. In some embodiments, the single crystal monolayer includes one or more TMDs. In some embodiments, the TMD includes WS2, WSe2, M0S2, MoSe2, or combinations thereof.

[0022] Still referring to FIG. 2A, at 204, the metal exfoliant is removed from the single crystal monolayer. In some embodiments, the metal exfoliant is removed 204 to expose the single crystal monolayer to the surrounding environment without affecting the crystalline structure of the single crystal monolayer. In some embodiments, all of the metal exfoliant from the monolayer construct is removed at 204. In some embodiments, removal 204 of the single crystal monolayer is performed via etching. In some embodiments, any suitable etchant or combination of etchants can used to remove 204 the metal exfoliant. In some embodiments, an appropriate metal etchant is used to dissolve the metal exfoliant. In some embodiments, an I2 / F etchant solution is used to remove 204 a metal exfoliant including elemental gold. In some embodiments, an additional rinse with an appropriate solvent is used to remove residual metal etchant, e.g., in some embodiments, water is used to remove I2 / F etchant solution.

[0023] In some embodiments, at 206, the monolayer construct is positioned on a final substrate. In some embodiments, the monolayer construct, e.g., the exfoliated monolayer and viscoelastic polymer substrate, are inverted and placed onto the final substrate at 206. With the metal exfoliant being removed at 204, in some embodiments, the single crystal monolayer is positioned 206 directly on a surface of the final substrate, with the single crystal monolayer disposed between the polymer layer and the final substrate. In some embodiments, at 208, the polymer layer is separated from the single crystal monolayer and final substrate. In some embodiments, the polymer layer is separated 208 by any suitable process to leave the single crystal monolayer behind on the final substrate. In some embodiments, substantially all of the polymer layer is separated 208, leaving behind a single crystal monolayer disposed on a target substrate. In some embodiments, separating 208 includes applying heat to the monolayer construct, polymer layer, final substrate, etc., or combinations thereof, to separate the polymer layer from the single crystal monolayer.

[0024] In some embodiments, the final substrate includes any material or combination of materials upon which the deposition of one or more single crystal monolayers is desired. In some embodiments, the final substrate includes silicon, silicon oxide, or combinations thereof. As discussed above, in some embodiments, the ratio of a width to a thickness of the singlecrystal monolayer on the final substrate is at least about 500,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is between about 500,000: 1 and about 5,000,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 600,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 700,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 800,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 900,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 1,000,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 1,500,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 2,000,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 2,500,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 3,000,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 3,500,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 4,000,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 4,500,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 5,000,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is greater than about 5,000,000: 1. In some embodiments, method 100 is used to store one or more single crystal monolayers for a predetermined amount of time. In some embodiments, monolayer constructs are stored after etching 204 the metal exfoliant and before positioning 206 the monolayer construct e.g„ on the final substrate, on a single crystal monolayer stack (as will be discussed in greater detail below), etc.

[0025] Referring now to FIG. 2B, in some embodiments, providing 202 a monolayer construct includes providing 202A a material source having a van der Waals crystal structure. As discussed above, in some embodiments, the material source includes one or more TMDs. In some embodiments, the TMD includes WS2, WSe2, M0S2, MoSe2, or combinations thereof.

[0026] At 202B, the tape sheet is contacted with a surface of the material source. In some embodiments, contacting 202B includes depositing a layer of metal exfoliant, e.g., elementalgold, on a template substrate. A polymer protection layer (PPL) is then deposited on to the metal exfoliant layer, e.g., via spin-coating. In some embodiments, the PPL includes polyvinylpyrrolidone (PVP). A thermal release tape layer is then adhered to the PPL to form the tape sheet, which is then lifted from the template substrate. The metal exfoliant layer of the tape sheet can then be contacted with the material source.

[0027] Referring again to FIG. 2B, at 202C, a single crystal monolayer is exfoliated from the material source to the metal exfoliant layer. In some embodiments, the metal exfoliant layer is pressed onto the surface of the material source. In some embodiments, the tape sheet is removed from the bulk material source, deterministically exfoliating 202C a single crystal monolayer, e.g., the top monolayer of the bulk material source, onto the tape sheet. At 202D, a polymer layer is then positioned on the tape sheet, such that the single crystal monolayer is positioned between the polymer layer and the tape sheet. In some embodiments, heat, solvents, etc., or combinations thereof can be used to remove the PPL, thermal release tape layer, etc. from the monolayer construct and / or the tape sheet.

[0028] Referring againReferring now to FIG. 2A2C, in some embodiments of method 200, one or more additional monolayers are exfoliated and positioned on a previously transferred monolayer to form a multi-layer stack / structure / construct. In some embodiments, at 210, at least a second single crystal monolayer is exfoliated. In some embodiments, exfoliating 210 is consistent with exfoliating 202C discussed above. In some embodiments, the second single crystal monolayer is exfoliated 210 from a material source having a van der Waals crystal structure. In some embodiments, the second single crystal monolayer is exfoliated 210 from the same or different material source as the previous single crystal monolayer, e.g., from exfoliating 202C. In some embodiments, the second single crystal monolayer is exfoliated 210 to a second tape sheet including a layer composed of a metal exfoliant. As discussed above, in some embodiments, the metal exfoliant includes elemental gold. At 212, a second polymer layer is positioned on the second tape sheet such that the second single crystal monolayer is positioned between the second polymer layer and the second tape sheet, thus forming a second monolayer construct. At 214, the metal exfoliant is removed, e.g., as in 204 described above, to remove the metal exfoliant from the second single crystal monolayer. In some embodiments, the metal exfoliant is removed 214 via etching. At 216, the second monolayer construct is positioned on the final substrate. As above, with the metal exfoliant being removed at 214, in some embodiments, the second single crystal monolayer is positioned 216 directly on the first single crystal monolayer, with the second single crystal monolayer disposed between the second polymer layer and the first single crystal monolayer, and the first single crystal monolayer isitself positioned between the second single crystal monolayer and the final substrate. In some embodiments, at 218, the second polymer layer is separated from the second single crystal monolayer and final substrate. In some embodiments, the second polymer layer is separated 218 by any suitable process to leave the second single crystal monolayer behind on the first single crystal monolayer, forming a multilayer stack of single crystal monolayers. In some embodiments, substantially all of the second polymer layer is separated 218. In some embodiments, separating 218 includes applying heat to the second monolayer construct, second polymer layer, final substrate, multi-layer single crystal monolayer stack, etc., or combinations thereof, to separate the polymer layer from the second single crystal monolayer.

[0029] Referring now to FIG. 3, a method 300 for single crystal monolayer transfer consistent with some embodiments of the present disclosure is shown. At 302, a material source having a van der Waals crystal structure is provided. As discussed above, in some embodiments, the material source includes one or more transition metal di chalcogenides including WS2, WSe2, M0S2, MoSe2, or combinations thereof. At 304, a layer of elemental gold is deposited on a template substrate. At 306, a PPL is deposited, e.g., spin-coated, on to the elemental gold layer. At 308, a thermal release tape layer is adhered to the PPL to form the tape sheet. At 310, the tape sheet is lifted from the template substrate.

[0030] At 312, the elemental gold layer is contacted with the material source, e.g., a surface thereof. At 314, a single crystal monolayer is exfoliated from the material source to the elemental gold layer. At 316, a polymer layer is positioned on the tape sheet to form a monolayer construct. As discussed above, the polymer layer is positioned 316 such that the single crystal monolayer is positioned between the polymer layer and the tape sheet. In some embodiments, the polymer layer is a viscoelastic polymer layer. In some embodiments, the polymer layer includes PDMS.

[0031] At 318, the elemental gold layer is etched to remove it from the monolayer construct. In some embodiments, at 320, the monolayer construct is stored for a predetermined time period.

[0032] At 322, the monolayer construct is positioned on a final substrate with the single crystal monolayer positioned between the polymer layer and the final substrate. At 324, heat is applied to the monolayer construct to separate the polymer layer from the single crystal monolayer. As discussed above, in some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 500,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is between about 500,000: 1 and about 5,000,000: 1. In some embodiments, the ratio of the width tothe thickness of the single crystal monolayer on the final substrate is at least about 600,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 700,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 800,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 900,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 1,000,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 1,500,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 2,000,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 2,500,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 3,000,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 3,500,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 4,000,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 4,500,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 5,000,000: 1. In some embodiments, the ratio of the width to the thickness of the single crystal monolayer on the final substrate is greater than about 5,000,000: 1.

[0033] Referring now to FIGs. 4A-4B, methods according to some embodiments of the present disclosure transferred a single crystal monolayer onto a final substrate. In this embodiment, elemental gold on a tape sheet was used to exfoliate and transfer a single crystal monolayer of MoSe? onto a silicon substrate. FIG. 4A shows an exfoliated centimeter-scale MoSe? monolayer on the elemental gold layer of the tape sheet as imaged through the viscoelastic polymer substrate. After etching away the gold, the viscoelastic polymer was inverted and placed on the final substrate. The transparent viscoelastic polymer enabled careful control of placement and orientation, and in some embodiments, its flexibility allowed it to form conformal contact with the final substrate. In this exemplary embodiment, the alignment and transfer are done on a microscope transfer stage with a Teflon frame to suspend the viscoelastic polymer above the final substrate. In some embodiments, the viscoelastic polymer is slowly lowered until it contacts the final substrate. In some embodiments, it then delaminates from the Teflon support and covers the final substrate. In some embodiments, heat, time, or combinations thereof allowthe viscoelastic polymer to conform to the final substrate, allowing the TMD monolayer to make uniform contact. In some embodiments, the slow removal of the viscoelastic polymer leaves the TMD monolayer on the final substrate. FIG. 4B shows the MoSe? single crystal monolayer on the silicon substrate with a 285nm thermal oxide layer and a 40nm thin-film of Sb20s on half of the chip. Without wishing to be bound by theory, placing a monolayer onto this substrate would be impossible for gold-tape exfoliation due to the tendency for water to delaminate and damage the fragile Sb20s thin-film.

[0034] Referring now to FIGs. 5 A-5B, the quality of single crystal monolayers prepared consistent with methods according to embodiments of the present disclosure was validated with optical images and atomic force microscopy (AFM) topography. Optically, samples prepared via the methods of the present disclosure display large homogeneous regions on the order of millimeters. Microscopically, AFM measurements show comparable topography to monolayers exfoliated directly by the gold-tape-method. Referring specifically to FIG. 5 A, a twisted WSe2 homobilayer sample was prepared via methods consistent with the present disclosure. The control of the positioning and orientation is demonstrated by the ability to maximize the high- quality sample area at a chosen twist angle of 22 degrees. FIG. 5B compares the surface topography of this twisted WSe2 homobilayer prepared according to embodiments of the present disclosure with a MoSe2 single crystal monolayer exfoliated directly by the gold-tape method. The mean-squared roughness of each height map is about 0.3nmm, demonstrating that the methods of the present disclosure are able to transfer TMD single crystal monolayers without damaging those monolayers.

[0035] Referring now to FIG. 6, AFM topography was used to compare three TMD single crystal monolayers transferred onto silicon consistent with embodiments of the present disclosure, with a 285nm thermal oxide layer under various conditions and surface treatments. The first single crystal monolayers was transferred onto unpassivated silicon. The second single crystal monolayer was transferred onto silicon passivated with a hydrophobic self-assembled monolayer of 1 -dodecanol. The third single crystal monolayer was transferred onto unpassivated silicon in a dry environment. The monolayer transferred in air showed a high density of bubbles with areas on the order of lOOnm and heights on the order of lOnm. Neither the monolayer transferred onto the passivated silicon, nor the monolayer transferred in a dry nitrogen environment, showed bubbles.

[0036] Without wishing to be bound by theory, the transfer environment and substrate choice affect the topography monolayers prepared with the methods of the present disclosure. Asshown in FIG. 6, transfers in air can result in TMD monolayers with a high density of bubbles. Without wishing to be bound by theory, the formation of these bubbles was attributed to the deformation of the viscoelastic polymer during transfer. However, these bubbles did not form on substrates passivated by a hydrophobic self-assembled monolayer of 1-dodecanol. Additionally, unpassivated substrates did not form bubbles in a dry nitrogen environment. Without wishing to be bound by theory, bubble formation was attributed to water trapped between the substrate and the monolayer.

[0037] Systems and methods of the present disclosure are advantageous to provide metal-to- polymer exfoliation and transfer. The TMD monolayers it can produce have an unprecedented quality and size coupled with a scalable fabrication method. Although embodiments of the present disclosure specify gold as the exfoliation metal, the tape used in the exfoliation step can be covered with different types of metals. The exemplary embodiments have shown to be effective on TMDs, and can be applicable to 2D van der Waals materials in general.

[0038] The systems and methods of the present disclosure provide advantages and beneficial operational principles. Previously demonstrated gold-tape methods are incompatible with some substrates, provide poor control over monolayer orientation and position, and use separate exfoliation steps for each layer in a device. The metal-to-polymer methods according to embodiments of the present disclosure solve at least these problems. By way of example, in some embodiments, the final substrate in the metal-to-polymer methods according to the present disclosure are not exposed to the etchant or the solvents used in convention gold-tape methods. Further, the transparency of viscoelastic polymer substrates according to embodiments of the present disclosure allows monolayer transfer with excellent control of orientation and position. Also beneficial for the implementation in device fabrication, monolayers on viscoelastic polymer substrates can be prepared simultaneously and stored until the transfer step. Device fabrication time while using the methods of the present disclosure now takes only a few minutes, unlike the conventional gold-tape exfoliation which can take a few days.

[0039] The methods of the present disclosure can also produce monolayers with areas up to and beyond a million times larger than viscoelastic polymer transfer of traditional tape exfoliated monolayers. Additionally, the of the present disclosure are deterministic, unlike methods that rely on traditional stochastic tape exfoliation. The methods of the present disclosure also advantageously do not use the viscoelastic polymer for exfoliation. The viscoelastic polymer transfer of traditional tape exfoliated monolayers use viscoelastic polymers “sticky” enough to exfoliate monolayers from the “parent” tape. However, if the viscoelastic polymer is madestickier, the transfer onto the final substrate becomes more difficult. The methods of the present disclosure avoid this compromise and the viscoelastic polymer can be as un-sticky as possible to ensure easy transfer when the 2D monolayer “wafers” are ready for use.

[0040] Additional disclosure relevant to the instant disclosure can be found at U.S. Patent 11,866,847, which is incorporated by reference herein in its entirety.

[0041] Although the invention has been described and illustrated with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without parting from the spirit and scope of the present invention.

Claims

CLAIMSWhat is claimed is:

1. A method for monolayer transfer, comprising: providing a monolayer construct, the monolayer construct including: a tape sheet including a layer composed of a metal exfoliant; a polymer layer; and a single crystal monolayer is positioned between the polymer layer and the tape sheet, etching the metal exfoliant to remove the metal exfoliant from the single crystal monolayer; positioning the monolayer construct on a final substrate, wherein the single crystal monolayer is positioned between the polymer layer and the final substrate; and separating the polymer layer from the single crystal monolayer and final substrate.

2. The method according to claim 1, further comprising: exfoliating at least a second single crystal monolayer from a material source having a van der Waals crystal structure to a second tape sheet including a layer composed of a metal exfoliant; positioning a second polymer layer on the second tape sheet to form a second monolayer construct, wherein the second single crystal monolayer is positioned between the polymer layer and the second tape sheet; etching the metal exfoliant to remove the metal exfoliant from the second single crystal monolayer; positioning the second monolayer construct on the final substrate, wherein the second single crystal monolayer is positioned between the second polymer layer and the second single crystal monolayer; andseparating the second polymer layer from the second single crystal monolayer to form a multilayer stack of single crystal monolayers.

3. The method according to claim 1, wherein providing a monolayer construct further comprises: providing a material source having a van der Waals crystal structure; contacting the tape sheet with a surface of the material source; exfoliating a single crystal monolayer from the material source to the metal exfoliant layer; and positioning a polymer layer on the tape sheet, wherein the single crystal monolayer is positioned between the polymer layer and the tape sheet.

4. The method according to claim 3, wherein contacting a tape sheet with a surface of the material source includes: depositing a layer of metal exfoliant on a template substrate; spin-coating a polymer protection layer (PPL) on to the metal exfoliant layer; adhering a thermal release tape layer to the PPL to form the tape sheet; lifting the tape sheet from the template substrate; and contacting the metal exfoliant layer with the material source.

5. The method according to claim 4, wherein the PPL includes polyvinylpyrrolidone (PVP).

6. The method according to claim 1, wherein separating the polymer layer from the single crystal monolayer and final substrate includes: applying heat to the monolayer construct to separate the polymer layer from the single crystal monolayer.

7. The method according to claim 1, wherein the metal exfoliant includes elemental gold.

8. The method according to claim 1, wherein the single crystal monolayer includes one or more transition metal dichalcogenides (TMD).

9. The method according to claim 8, wherein the TMD includes WS2, WSe2, M0S2, MoSe2, or combinations thereof.

10. The method according to claim 1, wherein the polymer layer includes polydimethylsiloxane (PDMS).

11. The method according to claim 1, wherein the final substrate includes silicon, silicon oxide, or combinations thereof.

12. The method according to claim 1, wherein the ratio of a width to a thickness of the single crystal monolayer on the final substrate is at least about 500,000: 1.

13. The method according to claim 12, wherein the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 1,000,000: 1.

14. The method according to claim 13, wherein the ratio of the width to the thickness of the single crystal monolayer on the final substrate is at least about 5,000,000: 1.

15. The method according to claim 1, further comprising storing the monolayer construct after etching the metal exfoliant and before positioning the monolayer construct on a final substrate.

16. A monolayer construct, comprising: a tape sheet; a polymer layer; and a single crystal monolayer positioned between the tape sheet and the polymer layer, wherein the tape sheet includes a layer composed of elemental gold, and the single crystal monolayer is obtained via exfoliation from one or more transition metal dichalcogenides (TMD) by the elemental gold layer.

17. The monolayer construct according to claim 16, wherein the TMD includes WS2, WSe2, M0S2, MoSe2, or combinations thereof.

18. The monolayer construct according to claim 16, wherein the polymer layer includes polydimethylsiloxane (PDMS).

19. The monolayer construct according to claim 16, wherein the ratio of a width to a thickness of the single crystal monolayer is at least about 500,000: 1.

20. A method for single crystal monolayer transfer, comprising: providing a material source having a van der Waals crystal structure, wherein the material source includes one or more transition metal dichalcogenides including WS2, WSe2, M0S2, MoSe2, or combinations thereof; depositing a layer of elemental gold on a template substrate; spin-coating a polymer protection layer (PPL) on to the elemental gold layer; adhering a thermal release tape layer to the PPL to form the tape sheet; lifting the tape sheet from the template substrate; contacting the elemental gold layer with the material source; exfoliating a single crystal monolayer from the material source to the elemental gold layer; positioning a polymer layer on the tape sheet to form a monolayer construct, wherein the single crystal monolayer is positioned between the polymer layer and the tape sheet; etching the elemental gold layer to remove the elemental gold layer from the monolayer construct; storing the monolayer construct for a predetermined time period; positioning the monolayer construct on a final substrate, wherein the single crystal monolayer is positioned between the polymer layer and the final substrate and has a ratio of a width to a thickness of the single crystal monolayer on the final substrate of at least about 500,000: 1; and applying heat to the monolayer construct to separate the polymer layer from the single crystal monolayer, wherein the polymer layer includes polydimethylsiloxane (PDMS).

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