阅读说明：本技术 在2d材料上的分子的组装以及电子设备 (Assembly of molecules on 2D materials and electronic devices ) 是由 萨穆埃尔·拉拉-阿维拉 汉斯·赫 谢尔盖·库巴特金 于 2018-12-06 设计创作，主要内容包括：本发明涉及用于在形成在基底(102)上的二维材料(104)的表面上组装分子(108)的方法,该方法包括：在二维材料的表面上形成包括电绝缘化合物或半导体化合物中的至少一种的间隔层(106),在间隔层上沉积分子,在升高的温度下对具有间隔层和分子的基底进行退火持续退火时间段,其中,温度和退火时间为使得分子的至少一部分被允许通过间隔层朝向二维材料的表面扩散以在二维材料的表面上组装。本发明还涉及电子设备。(The invention relates to a method for assembling molecules (108) on a surface of a two-dimensional material (104) formed on a substrate (102), the method comprising: forming a spacer layer (106) comprising at least one of an electrically insulating compound or a semiconducting compound on a surface of the two-dimensional material, depositing molecules on the spacer layer, annealing the substrate with the spacer layer and the molecules at an elevated temperature for an annealing time period, wherein the temperature and the annealing time are such that at least a portion of the molecules are allowed to diffuse through the spacer layer towards the surface of the two-dimensional material for assembly on the surface of the two-dimensional material. The invention also relates to an electronic device.)

1. A method for assembling molecules (108) on a surface of a two-dimensional material (104) formed on a substrate (102), the method comprising:

-forming a spacer layer (106) comprising at least one of an electrically insulating compound or a semiconducting compound on a surface of the two-dimensional material,

-depositing molecules on the spacer layer,

-annealing the substrate with the spacer layer and the molecules at an elevated temperature for an annealing time period, wherein the temperature and annealing time are such that at least a part of the molecules is allowed to diffuse through the spacer layer towards the surface of the two-dimensional material for assembly on the surface of the two-dimensional material.

2. The method of claim 1, comprising:

-encapsulating the spacer layer with at least one encapsulation layer (114) comprising an electrically insulating compound after the molecules have been deposited on the spacer layer.

3. The method of claim 2, comprising:

-depositing at least one metal layer (116) on the encapsulation layer.

4. A method according to any of the preceding claims, wherein the electrically insulating compound in the spacer layer is a polymer, wherein the annealing temperature is above the glass transition temperature of the electrically insulating polymer.

5. The method of any preceding claim, wherein the molecule is a molecular dopant, wherein the molecular dopant diffuses through the spacer layer towards the surface of the two-dimensional material to assemble on the surface of the two-dimensional material to dope the two-dimensional material.

6. The method of claim 5, wherein the annealing time and annealing temperature are based on a desired doping level of the two-dimensional layer.

7. The method of any preceding claim, wherein forming the spacer layer comprises:

-coating a layer of two-dimensional material with a liquid comprising an electrically insulating polymer, and

-annealing the coated substrate comprising the two-dimensional material at a temperature above the glass transition temperature of the electrically insulating polymer for a second period of time to form the spacer layer on the two-dimensional material.

8. The method of claim 7, wherein a liquid comprising an electrically insulating polymer is spin coated onto the two-dimensional material on the substrate.

9. The method of any of claims 1-6, wherein forming the spacer layer comprises depositing the electrically insulating polymer by at least one of physical vapor deposition or chemical vapor deposition.

10. The method of any preceding claim, wherein depositing molecules on the spacer layer comprises:

-coating the spacer layer with a liquid solution comprising an electrically insulating polymer and the molecules.

11. The method of claim 10, further comprising encapsulating the layer of annealing molecules with at least one encapsulation layer comprising an electrically insulating compound.

12. The method of claim 10, comprising depositing a metal layer on the encapsulation layer formed on the layer of annealing molecules.

13. The method of any one of claims 10 to 12, wherein the concentration of the molecular dopant in the liquid solution is at least 0.2% by weight.

14. The method of any one of claims 10 to 13, wherein the liquid solution comprising the electrically insulating polymer and the molecular dopant is spin coated onto the spacer layer.

15. The method of any of the preceding claims, wherein the spacer layer encapsulates the two-dimensional material on the substrate.

16. A method according to any of the preceding claims, wherein the spacer layer has a thickness of at least 5 nm.

17. The method of any of the preceding claims, wherein at least one of the electrically insulating polymers comprises PMMA or MMA, or a combination thereof.

18. The method of any preceding claim, wherein the two-dimensional material is epitaxial graphene.

19. A method according to any preceding claim, wherein the substrate is silicon carbide.

20. The method of any one of the preceding claims, wherein the molecular dopant is at least one of F4TCNQ and/or TCNQ.

21. An electronic device (700), comprising:

-a substrate (102);

-a two-dimensional material (104) formed on the substrate;

-a spacer layer (106) on a surface of the two-dimensional material, the spacer layer (106) comprising at least one of an electrically insulating compound or a semiconducting compound;

-a layer (112) of electrically insulating compounds and molecules (108) formed on the spacer layer;

-an encapsulation layer (114) formed on the layer comprising the molecules, the encapsulation layer (114) comprising at least one of an electrically insulating compound or a semiconducting compound;

-a metal layer (116) formed on the encapsulation layer,

wherein a layer having the same kind of molecules as the molecules in the layer on the spacer layer is assembled on the layer of the two-dimensional material.

22. The electronic device of claim 21, wherein the molecule is a molecular dopant, whereby the molecular dopant on the spacer layer thereby causes doping of the two-dimensional material.

23. The electronic device of any of claims 21 or 22, wherein the two-dimensional material is epitaxial graphene.

24. The electronic device of any of claims 21-23, wherein the substrate is silicon carbide.

25. The electronic device of any of claims 21-24, comprising four connection pads connected to the two-dimensional material, wherein two of the connection pads are arranged as input ports for providing current to the two-dimensional material, and wherein the other two connection pads are arranged as output ports for sensing voltage on the two-dimensional material in response to an input signal acting on the two-dimensional material.

26. The electronic device of claim 25, wherein the metal layer is configured to provide a gate to an electrostatic gate of the doped two-dimensional material.

27. The electronic device of any of claims 21-26, wherein the electronic device is a quantum resistance standard device.

Technical Field

The present invention relates to a method for assembling molecules on a surface of a two-dimensional material formed on a substrate. The invention also relates to an electronic device.

Background

Recently the possibility of assembling organic molecules on two-dimensional materials (2D materials) such as graphene to provide enhanced electronic properties of the 2D materials has been proposed. The assembly of molecules on the 2D material may also provide a means for creating new 2D materials with properties that are not available in bare 2D crystals.

It appears that the organization and conformation of molecules on the 2D crystal can influence the electronic structure of the 2D material through interactions between the 2D material and the deposited molecules. However, it is important that the molecules form a layer on the 2D material rather than forming closely packed islands.

Traditionally, molecules are deposited onto 2D materials under Ultra High Vacuum (UHV) conditions. However, the molecule-2D material composite prepared by sublimation of molecules onto the 2D material under UHV conditions is chemically unstable and the molecule-2D material composite degrades when exposed to ambient conditions, and this complicates the use of the molecule-2D material for some implementations, such as implementations employing doped 2D materials, where doping can be achieved by assembly of dopant molecules on the surface of the 2D material.

Therefore, there is room for improvement in a process for preparing a composite of a 2D material having a molecular layer on a surface of the 2D material. There also appears to be a need for such composites having improved chemical stability.

Disclosure of Invention

In view of the above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide a method allowing the preparation of a composite comprising a molecular layer on a 2D material, which composite is not only chemically stable under ultra-high vacuum and low temperature, but also under higher temperature and pressure conditions, such as ambient conditions.

According to a first aspect of the present invention, there is therefore provided a method for assembling molecules on a surface of a two-dimensional material formed on a substrate, the method comprising: forming a spacer layer comprising at least one of an electrically insulating compound or a semiconducting compound on a surface of the two-dimensional material, depositing molecules on the spacer layer, annealing the substrate with the spacer layer and the molecules at an elevated temperature for an annealing time period, wherein the temperature and the annealing time are such that at least a portion of the molecules are allowed to diffuse through the spacer layer towards the surface of the two-dimensional material for assembly on the surface of the two-dimensional material.

The invention is based on the following realization: the molecules are allowed to diffuse through the spacer layer to assemble on the surface of the two-dimensional material. Thus, the molecules are not deposited directly on the surface of the 2D material; but first a spacer layer is formed on the 2D material. Next, molecules are deposited on the spacer layer and diffused through the spacer layer towards the surface of the 2D material at a predetermined temperature for a predetermined period of time during the annealing process.

According to the inventive concept, no ultra-high vacuum is required in depositing molecules. Furthermore, the spacer layer provides for the embedding of molecules assembled on the surface of the 2D material, which is at least partly responsible for providing chemical stability of the molecular assembly on the surface of the 2D material.

The 2D material according to the inventive concept preferably comprises only a single atomic layer or only a few atomic layers of one or more atomic species.

The electrically insulating or semiconducting compound forming the spacer layer may be any such compound that allows diffusion of molecules through the compound during the annealing process. The spacer layer is preferably a solid spacer layer.

In some possible implementations, the semiconductor compound may be a wide band gap semiconductor. The wide bandgap semiconductor may have a bandgap greater than 2 eV.

The annealing temperature should be broadly construed as being at an elevated temperature, but preferably above room temperature. The anneal time and temperature may be based on several factors such as the characteristics of the compound of the spacer layer. In general, the interaction between the compound of the spacer layer and the annealing time and temperature should be such that molecules are allowed to diffuse through the spacer layer during annealing.

In some embodiments, the spacer layer is encapsulated with at least one encapsulation layer comprising an electrically insulating compound after the molecules have been deposited on the spacer layer. Thus, the molecules deposited on the spacer layer are provided with an encapsulation, which advantageously provides a further improved chemical stability to the molecular assembly on the 2D material. The encapsulation layer may comprise the same compound as the spacer layer.

According to further embodiments, at least one metal layer may be deposited on the encapsulation layer. The metal layer hinders the escape of molecules from the surface of the 2D material and also hinders the escape of molecules from the spacer layer, in particular when the spacer layer is a polymer matrix, thereby providing further improved stability.

In some possible embodiments, the electrically insulating compound in the spacer layer may comprise a polymer, wherein the annealing temperature is above the glass transition temperature of the electrically insulating polymer. Annealing above the glass transition temperature advantageously allows the molecules to diffuse more quickly through the spacer layer.

According to an embodiment of the present invention, the molecule may be a molecular dopant, wherein the molecular dopant diffuses toward the surface of the two-dimensional material through the spacer layer to assemble on the surface of the two-dimensional material, thereby doping the two-dimensional material. Thus, a method for doping a two-dimensional material in a stable manner under ambient conditions is provided. In this way, a high mobility and stable 2D material may be provided.

The anneal time and anneal temperature may be based on a desired doping level of the two-dimensional layer.

The formation of the spacer layer may be performed in various ways. In one implementation, forming the spacer layer includes: coating a layer of a two-dimensional material with a liquid comprising an electrically insulating polymer, and annealing the coated substrate comprising the two-dimensional material at a temperature above the glass transition temperature of the insulating polymer for a second period of time to form a spacer layer on the two-dimensional material. Thereby, a relatively simple method for forming the spacer layer is provided.

A liquid comprising an electrically insulating polymer may be, for example, spin coated onto the two-dimensional material on the substrate. However, the liquid comprising the electrically insulating polymer may also be applied by dipping the substrate in the liquid or spraying the liquid onto the 2D material. Spin coating provides a simple and reliable method for coating two-dimensional materials with liquids.

According to other possible implementations, the spacer layer may be formed by depositing an electrically insulating polymer by at least one of physical vapor deposition or chemical vapor deposition.

Depositing the molecules on the spacer layer may include coating the spacer layer with a liquid solution including an electrically insulating polymer and the molecules. This allows a relatively simple preparation for depositing molecules on the spacer layer. Furthermore, it allows spin coating of the spacer layer with a liquid solution comprising an electrically insulating polymer and a molecular dopant in a similar way as the spacer layer. In addition, chemical stability in air is further improved by embedding the molecules into a suitable polymer matrix to form a polymer blended dopant layer.

The concentration of molecules in the liquid solution is selected based on their molecular mass and density. For example, the concentration of the molecular dopant in the liquid solution may be at least 0.2%, such as 0.5%, 0.8%, 1% or 2% by weight.

The spacer layer may advantageously encapsulate the two-dimensional material on the substrate. Thereby, molecules assembled on the surface of the 2D material are more reliably retained on the surface.

The spacer layer has a thickness of at least 5 nm. For example, the spacer layer may be about 100nm, 200nm, or even 500nm, 700nm, or 1 micron.

At least one of the electrically insulating polymers comprises PMMA or a copolymer of PMMA.

The two-dimensional material may be any two-dimensional material that is peeled from its parent material.

In a possible implementation, the two-dimensional material is epitaxial graphene. Graphene may be produced by chemical vapor deposition.

The substrate is preferably silicon carbide, particularly where the two-dimensional material is epitaxial graphene.

Various types of molecular dopants may be used and are within the scope of the claims, however, in one possible implementation, the molecular dopant is at least one of tetrafluoro-tetracyanoquinodimethane (F4TCNQ) and Tetracyanoquinodimethane (TCNQ).

According to a second aspect of the present invention, there is provided an electronic apparatus comprising: a substrate; a two-dimensional material formed on a substrate; a spacer layer on a surface of the two-dimensional material, the spacer layer comprising at least one of an electrically insulating compound or a semiconducting compound; a layer of electrically insulating compound and molecules formed on the spacer layer; an encapsulation layer formed on the layer including the molecules, the encapsulation layer including at least one of an electrically insulating compound or a semiconductor compound; a metal layer formed on the encapsulation layer, wherein a layer having the same kind of molecules as the molecules in the layer on the spacer layer is assembled on the layer of the two-dimensional material.

The molecules may be molecular dopants, whereby the molecular dopants on the spacer layer thereby cause doping of the two-dimensional material. The molecular dopant advantageously causes an increase in mobility of the two-dimensional material. When the molecular dopants have diffused through the spacer layer, they are advantageously arranged on the surface of the two-dimensional material, which leads to a so-called modulation doping.

Furthermore, if the encapsulation layer is removed from the electronic device according to the embodiment of the second aspect of the inventive concept, a decrease in electron mobility of the two-dimensional material may be observed. This may be due to desorption of the molecular dopants from the surface of the two-dimensional layer or chemical degradation due to removal of the encapsulation.

According to further embodiments of the inventive concept, an electronic device may comprise at least four connection pads connected to the two-dimensional material, wherein two of the connection pads are arranged as input ports for providing current to the two-dimensional material, and wherein the other two connection pads are arranged as output ports for sensing a voltage over the two-dimensional material in response to an input signal acting on the two-dimensional material. Thus, the electronic device may act as a hall bar and the input signal may be a magnetic field applied perpendicular to the surface of the two-dimensional material.

Further, the electronic device may be a quantum resistance standard.

The metal layer may advantageously be configured as a gate for providing an electrostatic gate to the doped two-dimensional material. In this way, the mobility and carrier density of the two-dimensional material can be adjusted.

Further embodiments of the second aspect of the invention and the effects obtained by the second aspect of the invention are substantially similar to those described above for the first aspect of the invention.

According to a third aspect of the present invention there is provided use of the electronic device according to any one of the embodiments of the second aspect as a quantum resistance standard.

Other features and advantages of the invention will become apparent when studying the appended claims and the following description. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

Drawings

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing exemplary embodiments of the invention, wherein:

fig. 1a to 1d schematically show method steps for assembling molecules on a surface of a two-dimensional material;

FIG. 1e is a flow chart of method steps according to an embodiment of the invention;

fig. 2a to 2f schematically show method steps for assembling molecules on a surface of a two-dimensional material;

FIG. 3 is a flow chart of method steps according to an embodiment of the invention;

FIG. 4 is a flow chart of method steps according to an embodiment of the invention;

FIG. 5a is a cross-sectional view of a first test apparatus;

FIG. 5b is a cross-sectional view of a second testing apparatus;

FIG. 5c is a cross-sectional view of a third testing device according to an embodiment of the present invention;

FIG. 5d shows the carrier concentration as a function of temperature for the test device in FIGS. 5a to 5 c;

FIG. 5e shows the mobility of the test device in FIGS. 5a to 5c as a function of temperature;

FIGS. 6a to 6c show the chemical distribution of polymer heterostructures and underlying (doped) graphene using ToF-SIMS;

FIG. 7 schematically shows a cross-sectional view of an electronic device according to an embodiment of the invention;

FIG. 8 schematically illustrates an electronic device according to an embodiment of the invention; and

fig. 9 shows the longitudinal and transverse resistances of an electronic device versus an applied magnetic field for an electronic device as shown in fig. 8.

Detailed Description

In this detailed description, various embodiments of the inventive concept are described primarily with reference to a two-dimensional material in the form of graphene and a molecular dopant in the form of F4 TCNQ. It should be noted, however, that this by no means limits the scope of the invention, which is equally applicable to any two-dimensional material that delaminates from its parent material and any molecules that may diffuse through a suitable spacer layer.

Fig. 1a to 1d schematically show a method for assembling molecules on the surface of a two-dimensional material. Fig. 1a to 1d will be described in connection with a flow chart of the method steps shown in fig. 1 e.

Fig. 1a shows a substrate 102 having a layer of two-dimensional material 104 thereon. The two-dimensional material may be any two-dimensional material that is peeled from its parent material. In a preferred implementation, the two-dimensional material 104 is epitaxial graphene produced by chemical vapor deposition on the silicon carbide substrate 102. The epitaxial graphene may be wafer-level graphene grown on a silicon carbide substrate 102.

Fig. 1b shows a spacer layer 106 that has been formed on the two-dimensional material 104 (step S102, fig. 1 e). The spacer layer comprises an electrically insulating compound or a semiconducting compound. The spacer layer 106 may be created in various ways, for example by coating the two-dimensional material 104 with a liquid comprising an electrically insulating polymer and subsequently annealing the substrate with a liquid solution above the glass transition temperature of the electrically insulating polymer. Alternatively, the spacer layer 106 may be produced by Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD). The manufacturing process (e.g., with liquid coating, PVD, CVD) depends at least in part on the type of electrically insulating compound or the type of semiconductor compound. PVD and CVD are standard microfabrication processes known to those skilled in the art. The thickness of the spacer layer 106 depends on the type of spacer layer material, but is preferably at least 5nm thick, but the thickness of the spacer layer 106 may be as high as even 500nm thick.

Turning now to fig. 1c, it is shown that molecules 108 are deposited on the surface of the spacer layer 106 (step S104, fig. 1 e). Only one of the molecules 108 is numbered to avoid cluttering the drawing. There are a number of ways to deposit molecules on the spacer layer, one of which will be described with reference to the subsequent figures. Example methods include evaporation processes, deposition from solution, and spray coating.

The substrate with the spacer layer and the molecules deposited on the spacer layer is annealed at an elevated temperature for a predetermined period of time (step S106, fig. 1 e). The elevated temperature is above room temperature and is generally determined based on the material of the spacer layer 106 and the properties of the molecules 108. The annealing temperature and the annealing time are selected such that the molecules 108 are allowed to diffuse through the spacer layer 106 towards the two-dimensional material 104. As conceptually illustrated in fig. 1d, during annealing, the molecules 108 assemble on the surface of the two-dimensional material 104 after having diffused through the spacer layer 106.

Turning now to fig. 2a to 2d, a method for assembling molecules on a surface of a two-dimensional material is schematically illustrated. Fig. 2a to 2d will be described in connection with a flow chart of the method steps shown in fig. 3.

Similar to fig. 1a, fig. 2a shows a substrate 102 having a layer of two-dimensional material 104 thereon. The two-dimensional material 104 may be any two-dimensional material that is peeled away from its parent material. In one possible implementation, the two-dimensional material 104 is epitaxial graphene produced by chemical vapor deposition on the silicon carbide substrate 102. The epitaxial graphene may be wafer-level graphene grown on a silicon carbide substrate 102.

The spacer layer 106 as shown in fig. 2b may be formed by first coating a layer of the two-dimensional material 104 with a liquid comprising an electrically insulating polymer, also as indicated by step S202 in fig. 3. The substrate 102 having the two-dimensional material 104 coated with the liquid solution is annealed at an elevated temperature above the glass transition temperature of the electrically insulating polymer for a certain period of time (step S204, fig. 3). In this manner, the spacer layer 106 is formed in this presently described example embodiment. Coating the layer of two-dimensional material 104 with a liquid comprising an electrically insulating polymer may be performed by spin coating methods known per se to the person skilled in the art.

In one possible implementation, the electrically insulating polymer is poly (methyl methacrylate) (PMMA). In the case of PMMA being used in the spacer layer 106, the PMMA is typically dissolved in a suitable solvent and the annealing temperature should be high enough to exceed the glass transition temperature (which depends on the molecular weight of the PMMA). For example, the annealing time period may be about 5 minutes and the annealing temperature may be about 160 ℃, such that a solid spacer layer is formed.

Fig. 2c shows that the spacer layer 106 has been coated with a liquid solution comprising an electrically insulating polymer 110 and molecules 108 (step S206). The coating may be performed by a spin coating method.

The substrate 102 with the spacer layer 106 and the liquid solution comprising the molecules 102 and the electrically insulating polymer 110 is annealed (step S208, fig. 3) at an elevated temperature above the glass transition temperature of the electrically insulating polymer 110 for a certain period of time. Additionally, the electrically insulating polymer may be PMMA or MMA, or a copolymer of PMMA.

The annealing temperature and the annealing time are selected such that the molecules 108 are allowed to diffuse through the spacer layer 106 towards the surface of the two-dimensional material 104. As conceptually illustrated in fig. 2d, during annealing, the molecules 108 assemble on the surface of the two-dimensional material 104 after having diffused through the spacer layer 106. However, some molecules typically remain in the layer of annealing molecules 112 including the electrically insulating polymer 110 and the molecules 108.

Reference is now made to fig. 2e to 2f and to the flow chart in fig. 4. In a further embodiment, as schematically illustrated in fig. 2e, an encapsulation layer 114 is formed on the annealed molecular layer 112 (step S210, fig. 4). The encapsulation layer 114 may include an electrically insulating compound such as a polymer (e.g., PMMA or MMA, or a copolymer of PMMA). The generation of the encapsulation layer 114 may be performed in the same manner as the above-described layers comprising the electrically insulating compound (i.e. coating and annealing). The encapsulation layer improves the chemical stability of the molecules 108 assembled on the two-dimensional material 106. In particular, the encapsulation layer at least partially prevents drift in carrier concentration due to exposure to ambient dopants.

Furthermore, and as schematically shown in fig. 2f, a metal layer 116 may be deposited on the encapsulation layer 114 (step S212, fig. 4). Metal layer 116 may comprise, for example, gold or aluminum and may be deposited using known processes such as sputtering, physical vapor deposition, chemical vapor deposition, and the like. The metal layer 116 shields the molecules 108 assembled on the two-dimensional material 104 so that the chemical stability is further improved. In addition, in embodiments where the molecules are molecular dopants, the metal layer 116 may serve as a gate. The metal gate can then be used to provide an electrostatic gate to the doped two-dimensional material.

In some embodiments, the two-dimensional material is epitaxial graphene 104 grown on a silicon carbide substrate 102. Furthermore, the electrically insulating compound of the spacer layer 106 may be PMMA, and the electrically insulating compound in the annealing molecule layer 112 and the encapsulation layer 114 may also be PMMA. Molecule 108 can be tetrafluoro-tetracyanoquinodimethane (F4TCNQ), but other molecules are also suitable, such as Tetracyanoquinodimethane (TCNQ).

Spin coating and annealing methods are known per se to the person skilled in the art, as are chemical vapor deposition and physical vapor deposition.

Fig. 5a to 5c each show a cross-sectional view of a corresponding test apparatus for comparing the carrier density and electron mobility (fig. 6c) of an electronic device manufactured according to the inventive concept with other test apparatuses.

Fig. 5a shows a cross-sectional view of a first test apparatus 502 comprising a silicon carbide substrate 102 having thereon a layer 104 of graphene and a layer 106 of PMMA, which layer 106 of PMMA may correspond to the spacer layer 106.

Fig. 5b shows a cross-sectional view of a second test apparatus 504 comprising a silicon carbide substrate 102 having thereon an annealed layer 112 of PMMA110 and molecular dopants 108 (only one numbered), in this case F4 TCNQ.

Fig. 5c shows a cross-sectional view of a third test apparatus 506 comprising a layer of graphene 104 on a silicon carbide substrate 102, a PMMA spacer layer 106, a layer of annealed molecules 112 comprising PMMA110, and an encapsulation layer 114 comprising PMMA.

All of the devices shown in fig. 5a to 5c include gold contacts 120 electrically connected to the graphene layer 104 to enable the hall measurements to extract the carrier density and mobility of the graphene layer 104. Thus, the device is patterned as a hall bar, but only a part of the device is shown in cross-section in fig. 5a to 5 c.

Fig. 5d shows the carrier concentration as a function of temperature extracted from the hall measurements for the pristine epitaxial graphene ("as-grown") and the test devices (502, 504, 506) shown in fig. 5a to 5 c. Both PMMA (test device 502) and F4TCNQ (test device 504) independently act as p-dopants, which can result in lower carriers than graphene in the as-grown state from the curves for test devices 502 and 504Concentrations are seen where the former is a weaker p-dopant. In the case of direct deposition onto graphene, the carrier density of epitaxial graphene decreases by three orders of magnitude from 10 at T-4K only when PMMA spacer layer 106 is included between graphene and F4TCNQ layer 112 (test apparatus 506)¹³cm^-2To 10¹⁰cm^-2(a drop of almost 2 orders of magnitude at room temperature).

Fig. 5e shows the hall carrier mobility as a function of temperature from measurements for raw epitaxial graphene and the test apparatus (502, 504, 506) shown in fig. 5a to 5 c. Carrier mobility for the test device (502) in FIG. 5a and the test device (504) in FIG. 5b does not exceed 10,000cm²Vs. However, for the third test apparatus 506 shown schematically in FIG. 5c with a molecular dopant layer 112 on the spacer layer 106, the carrier mobility exceeded 50,000cm²/Vs。

Thus, as can be appreciated from the above, the molecules deposited on the spacer layer can be a molecular dopant such as F4TCNQ or TCNQ. Thus, air stable functionalization of graphene with molecular dopants is achieved, which enables high mobility epitaxial graphene.

The thickness of the spacer layer 106 does not appear to affect the improvement in carrier density and carrier mobility, at least in the range of 100nm to 500nm, indicating that the diffusion of the F4TCNQ molecules through the polymer is relatively fast. The spacer layer is preferably at least 5nm thick.

The chemical composition of the fabricated electronic devices has been studied using time-of-flight secondary ion mass spectrometry (ToF-SIMS) deep analysis. Fig. 6a schematically shows a cross-sectional view of a device 600 that has been investigated and produced using a method according to the inventive concept. The cross-sectional view in fig. 6a shows a graphene layer 104 on a silicon carbide substrate 102, a PMMA spacer layer 106 (about 100nm thick) in direct contact with the substrate 102 and graphene layer 104, an annealed molecular layer 112 comprising a PMMA-F4TCNQ blend (about 200nm thick, molecules not shown), an encapsulated PMMA layer 114 (about 100nm thick), and a gold pad 120 on the substrate 102 embedded through the spacer layer 106.

The results from the ToF-SIMS study are presented in fig. 6b to 6c and show that F4TCNQ species, which may form charge transfer complexes with graphene 104 and accumulate at the graphene/spacer interface, diffuse through the PMMA spacer layer 106 to reach the graphene 104 surface, as can be appreciated from fig. 6 b. Fig. 6b shows the chemical distribution of the three-layer polymer stack on the vertical axis relative to the substrate in the direction perpendicular to the surface of graphene 104 indicated by arrow 601a in fig. 6a, with ion intensity plotted as a function of sputtering time. The ionic strength for fluorine (F) and Cyano (CN), both representing the dopant F4TCNQ, is shown. The ionic strength for silicon (Si) is also included in fig. 6 b.

In fig. 6b, it can be seen that the intensity of the F4TCNQ counts indicated by lines 602(CN) and 603(F) has a small increase in the annealing molecular layer 112, indicating that some F4TCNQ molecules still remain in the annealing molecular layer 112. At the interface between the graphene layer 104 and the PMMA spacer layer 106, there is a large increase indicated by the peak 604 (the curve for CN, see also the peak in F-intensity in curve 603), which indicates the accumulation of F4TCNQ molecules at the surface of the graphene layer 104.

Fig. 6c shows at three different positions on the substrate in a direction perpendicular to the surface of the substrate: comparison of the chemical distributions on the vertical axis of the three-layer polymer stack of graphene (at arrow 601a in fig. 6 a), bare SiC (at arrow 601b in fig. 6 a), and thin Au film on graphene (at arrow 601c in fig. 6 a), with ionic strength plotted as a function of sputtering time. The legend in fig. 6c indicates where chemical distribution is obtained on the apparatus 600 (e.g., at arrows 601a to 601 c).

Fig. 6c shows the chemical signature resulting from the molecular dopant F4TCNQ (i.e., CN signal as described with reference to fig. 6 b) at the location of graphene (601a), on SiC (601b) and on Au (601 b). The features acquired at SiC or Au serve as an indicator of the polymer spacer layer substrate interface (visible at about 250 sputter seconds, also visible in fig. 6 b). Note that the thickness of each layer as estimated from SIMS is approximate because the etching rate differs depending on the material. Non-uniform sputtering, for example due to surface roughness, can also smear and widen interfaces, such as the F4TCNQ accumulation layer near graphene.

ToF-SIMS revealed that not only was F4TCNQ found at the annealed molecular layer 112 and the PMMA spacer layer 106, which indicated that F4TCNQ diffused rapidly from the intermediate dopant layer 112 including F4TCNQ and PMMA, but also that the dopant (F4TCNQ) reached the substrate 102 surface and accumulated at the conductive surfaces of the graphene 104 and gold 120 (fig. 6b and 6 c).

Thus, F4TCNQ is mobile in a polymer film, and the diffusion of F4TCNQ depends on a number of parameters of the host polymer matrix (e.g., PMMA, MMA, or copolymers of PMMA), in particular on the polarity and the glass transition temperature (T |)_g). Considering the polarity of PMMA and the thermal annealing step of the above described process is above the glass transition temperature (T) of the polymer_gAbout 105 ℃), a conservative estimate of the lower flux limit for F4TCNQ at the substrate surface is

This means that initially an amount of F4TCNQ equivalent to a solid layer of 10nm thickness per second reaches the spacer/substrate interface. Here, we used a D of about 10 as measured for diffusion of neutral F4TCNQ in nonpolar P3HT at about 50 deg.C^-10cm²s^-1(see, e.g., Quantitative Measurements of the Temperature-Dependent Micrococcus and macromolecular Dynamics of a Molecular dose in an aConjuncted Polymer, "Macromolecules, vol.50, No.14, pp.5476-5489, Jul.2017.), Δ c 5.10^-4mol cm^-3An initial concentration gradient of F4TCNQ between the molecular layer 112 and the spacer layer 106 (density ρ of F4TCNQ is about 1.4g cm)^-3(ii) a The molar mass M is about 276g mol^-1) And Δ x ═ 100nm is the thickness of spacer layer 106.

Observed p-doping effects on graphene (see fig. 5 a-5 e and discussion above) and accumulation of F4TCNQ at graphene 104 and gold surface 120-spike by CN species followed by Si signal or Au signal representation (fig. 6b and 6c) -are observedThis can be explained by the formation of charge transfer complexes that produce F4TCNQ anions that must be retained at the graphene interface to maintain overall charge neutrality. In addition, in the case of poly (3-hexylthiophene) (P3 HT): slower diffusion of F4TCNQ anions in the polymer matrix has been observed in F4TCNQ blends, where the diffusion coefficient for neutral F4TCNQ is 10 for F4 TCNQ-anions^-11cm²s^-1Two orders of magnitude lower (see, e.g., quantitative measurements of the Temperature-Dependent Micrococcus and macromolecular dynamics of a Molecular Doppler in a Conjugated Polymer, "Macromolecules, vol.50, No.14, pp.5476-5489, Jul.2017.). In the case of using PMMA as the host matrix for F4TCNQ, the F4TCNQ remains neutral both in the doped layer and as it diffuses through the PMMA spacer layer. Charge transfer may only occur when it comes into contact with an electron donor, such as graphene.

With further reference to fig. 6c, there appears to be little accumulation of F4TCNQ at the polymer spacer/SiC interface, as indicated by the relatively low peak 606 in the CN-signal (at SiC, 601c) along the vertical axis toward the SiC substrate 102, versus the signal measured at the doped layer (1.4 × 10)¹⁴ions cm^-2) In contrast, the intensity of the CN-signal at the graphene/PMMA spacer layer interface, indicated by peak 608, is roughly 50% higher (6 times higher at gold/PMMA, see peak 610). From SIMS measurements, an estimate of the fraction of molecules reaching graphene 104 can be calculated from the areas under the ionic strength curves 612(SiC, at arrow 601c), 614 (graphene, at arrow 601a of fig. 6 a), 616 (gold, at arrow 601b) in fig. 6 c. The total amount of available molecular dopants (F4TCNQ molecules) was calculated using a known density of PMMA, an anisole solvent in which PMMA was initially dissolved, F4TCNQ molecules, and the thickness of the F4TCNQ dopant layer after spin coating (assuming a plate of PMMA and F4TCNQ molecules only). Finally, this resulted in an estimated number of F4TCNQ on the graphene surface of roughly about 7x10¹⁴Molecule/cm²。

Fig. 7 schematically shows a cross-sectional view of an electronic device 700 according to an embodiment of the invention. An electronic device includes a substrate 102 and a two-dimensional material 104 formed on the substrate. The substrate 102 may be a silicon carbide substrate, and the two-dimensional material may be epitaxial graphene 104 grown on the substrate 102. There is also a spacer layer 106 comprising at least one of an electrically insulating compound or a semiconducting compound on the surface of the two-dimensional material 104. The spacer layer 106 may comprise an electrically insulating compound, for example in the form of PMMA or MMA or a combination thereof. On the spacer layer 106 there is a layer 112 of electrically insulating compound and molecules 108. The electrically insulating compound in layer 112 may also comprise PMMA or MMA, or a combination thereof.

An encapsulation layer 114 comprising at least one of an electrically insulating compound (e.g., PMMA or MMA, or a combination thereof) or a semiconducting compound is formed over the layer 112 comprising the molecules 108. A metal layer 116 is also formed on the encapsulation layer 114. A layer of molecules 108 of the same species as the molecules in the layer 112 on the spacer layer 106 is assembled on the layer of two-dimensional material 104.

In some embodiments, the molecule is a molecular dopant in the form of, for example, tetrafluoro-tetracyanoquinodimethane (F4TCNQ) and/or Tetracyanoquinodimethane (TCNQ). The molecular dopant allows doping of the two-dimensional material 104. The metal layer 116 may serve to further improve the chemical stability of the device under ambient conditions by preventing desorption of the molecular dopants from the polymer matrix into the surrounding environment.

In addition, the metal layer 116 may serve as a gate for adjusting the carrier concentration in the two-dimensional material 104.

Fig. 8 is a schematic top view of an exemplary conceptual electronic device 700. The cross-sectional view shown in fig. 7 is indicated by line a-a in fig. 8. The electronic device is shown here as a hall bar 701, which hall bar 701 may be used in a device as an implementation or realization of a quantum resistance standard by using the quantum hall effect.

The electronic device 700 may be manufactured using conventional lithography, such as e-beam lithography and/or photolithography, known per se to the person skilled in the art.

The electronic device 700 includes at least four connection pads (see fig. 7) connected to the two-dimensional material 104. The two connection pads 702, 704 are arranged such that a current (I) can pass through the two-dimensional material in the x-direction, the longitudinal direction of the hall bar 700. The two connection pads 706, 708 are arranged as output ports for measuring the transverse voltage (Vxy) when a current (I) is passed through the two-dimensional material in the device 700 in the longitudinal direction (x). The two connection pads 706, 708 are spatially separated in the lateral direction (y). Further, a longitudinal voltage (Vxx) may be measured between the connection pad 706 and an additional connection pad 710 spatially separated from the connection pad 706 in the longitudinal direction. The dimensions of the hall bar 700 may be, for example: w is 5mm × L3 mm, W is 30 μm × L100 μm, and W is 2 μm × L10 μm.

The doping uniformity of the two-dimensional material may be used to determine that the molecular dopant is uniformly dispersed over the surface of the two-dimensional material. The doping uniformity of the hall bar 701 is evaluated using magnetic transmission measurements at low temperatures (e.g., 2 kelvin), and the doping uniformity of the hall bar 701 indicates that the chemical doping of the graphene 104 in the device 700 is significantly conformal over the entire hall bar 701 only if the spacer layer 106 is included between the graphene 104 and the dopant layer 112. The doping uniformity evaluation will now be described with reference to fig. 9.

Fig. 9 is a graph showing the longitudinal resistance (Rxx) measured between the connection pads 706 and 710 and also the lateral resistance (Rxy1, Rxy2) measured between the connection pads 706 and 708 of the hall bar 701 in fig. 8 for a chemically doped device such as a hall bar 701, the magnetic transmission measurement at T2K in a hall bar (W30 μm × L100 μm) device 701 including the spacer layer 106 and the molecular doping layer 112 shows for a magnetic field | B | n |, for a chemically doped device such as a hall bar 701<Linear transverse resistance of 80mT (Rxy1, Rxy2), after which quantum Hall plateaus (quantum Hall plateaus)902, 904 begin to develop and exist at | B |>Obtaining their exact quantization value Rxy h/2e at 300mT²(h is Planck's constant). The magnetic field is applied perpendicular to the plane of the hall bar 700.

With further reference to fig. 9, it additionally shows the longitudinal resistance Rxx as a function of the applied magnetic field (B). Another test of charge carrier uniformity within the measurement region of the hall bar 700 is the observation of a fully developed quantum hall effect (represented by plateaus 902, 904) where Rxx-0 and the quantization stabilization in Rxy-h/2 e2 were simultaneously observed. Under quantitative conditions, observations of finite Rxx are actually determined by the amount of disorder in the sample, which can manifest as Oscillations in Rxx once Rxy steady state is reached (see, e.g., "Transport in two-dimensional distributed semi-metrics," Phys. Rev. Lett., vol.113, No.18, pp.1-5,2014, or "pure-Induced Resistance Oscillations in the Breakdown of the Graphene Quantum Hall Effect," Phys. Rev. Lett., vol.117, No.23, pp.1-5,2016).

Thus, the foregoing magnetic transfer characteristics indicate that a chemically doped sample with a PMMA spacer layer and an F4TCNQ dopant layer behaves as a system with a single electron band and spatially uniform carrier density over the two-dimensional material 104.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the word "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims shall not be construed as limiting the scope.