Magnetic graphene

Abstract

A patterned magnetic graphene made from the steps of transferring or growing a graphene film on a substrate, functionalizing the graphene film, hydrogenating the graphene film and forming fully hydrogenated graphene, manipulating the extent of the hydrogen content by using an electron beam from a scanning electron microscope to selectively remove hydrogen, wherein the step of selectively removing hydrogen occurs under a vacuum, and forming areas of magnetic graphene and non-magnetic graphene. A ferromagnetic graphene film comprising film that has a thickness of less than two atom layers thick.

Claims

1. A patterned magnetic graphene made from the steps of: transferring or growing a graphene film on a substrate; functionalizing the graphene film; hydrogenating the graphene film and forming fully hydrogenated graphene; manipulating the extent of the hydrogen content by using an electron beam from a scanning electron microscope to selectively remove hydrogen, wherein the step of selectively removing hydrogen occurs under a vacuum; and forming areas of magnetic graphene and non-magnetic graphene.

2. The patterned magnetic graphene of claim 1, wherein the step of forming areas of magnetic graphene and non-magnetic graphene comprise the steps of forming an area of fully hydrogenated graphene, forming an area of partially hydrogenated graphene, and forming an area of graphene.

3. The patterned magnetic graphene of claim 2, wherein the area of highly hydrogenated graphene is non-magnetic and the area of graphene is non-magnetic and the area of partially hydrogenated graphene is magnetic.

4. The patterned magnetic graphene of claim 1 wherein the step of manipulating the extent of the hydrogen content comprises using heat or pressure.

5. The patterned magnetic graphene of claim 1 wherein the step of hydrogenating the graphene film comprises reacting the graphene film with anhydrous liquid ammonia and lithium.

6. The patterned magnetic graphene of claim 5 wherein the graphene film reacts with the anhydrous liquid ammonia and lithium for about 5 to about 2 minutes.

7. A uniform ferromagnetic graphene film comprising film that has a thickness of less than two atom layers thick wherein the graphene film is patterned by locally controlling the extent of hydrogenation and wherein the pattern is achieved by an electron beam.

8. The uniform ferromagnetic graphene film of claim 7 wherein the strength of the ferromagnetism is controlled by the extent of the coverage of the hydrogen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates a schematic of Birch reduction and illustrates electron beam dehydrogenation of pHG.

(2) FIG. 2 illustrates MFM characterization of patterned pHG surface by electron beam dehydrogenation patterning.

(3) FIG. 3 illustrates magnetic gradient patterned with different e-beam doses.

DETAILED DESCRIPTION

(4) A method of making magnetic graphene comprising transferring or growing a graphene film on a substrate, functionalizing the graphene film, hydrogenating the graphene film and forming fully hydrogenated graphene, manipulating the extent of the hydrogen content, and forming areas of magnetic graphene and non-magnetic graphene. A ferromagnetic graphene film comprising film that has a thickness of less than two atom layers thick.

(5) Here it has been demonstrated that hydrogen atoms can be removed efficiently from graphene by heat, by pressure, or by electron beam to recover its original characteristics. This enables the use of pHG as a host material for patterning a surface with magnetic and non-magnetic regions. Electron beam (e-beam) lithography can break chemical bonds of CMGs.

(6) The use of pHG as a patterning host surface has not been realized until now. This invention concerns a method of preparing uniform and stable pHG, combined with an e-beam dehydrogenation (i.e. removal of hydrogen atoms) technique. By using the e-beam to selectively remove hydrogen atoms from certain areas of pHG, arrays of magnetic pHG and non-magnetic graphene patterns can be fabricated. Other methods for patterning the hydrogen content can be utilized. Such patterned surfaces can find use in particular in high density data storage application.

(7) The generic approach requires the formation of hydrogenated graphene on the desired substrate. This may be achieved either by transferring graphene onto the desired substrate and then functionalizing it or by transferring a previously functionalized film onto the desired substrate. If a patterned magnetic film is desired, it can be either patterned before transfer or patterned after transfer if, for instance, precise registry with existing features is required.

Example 1

(8) Graphene was transferred onto a technologically relevant substrate, SiO.sub.2, and functionalized in place.

(9) A CVD-grown single layer graphene film transferred onto SiO.sub.x/Si was placed in a N.sub.2-flushed vessel into which 10 mL of anhydrous liquid ammonia was distilled using a dry ice bath.

(10) Approximately 50 mL of lithium wire was added in pieces to the vessel, and the mixture was allowed to react for different durations depending on the desired degree of hydrogenation: 5 to 30 seconds for pHG, and 2 minutes for fully hydrogenated graphene.

(11) The reaction was then quenched with an alcoholic proton donor (methanol, ethanol, or isopropanol) and the devices were washed with additional alcohol and dried under a stream of N.sub.2. The sheet resistance of pHG was controlled at an average of 150 K/.

(12) Hydrogen was then selectively removed with the electron beam from a scanning electron microscope (SEM) under a mild vacuum (P 1.010.sup.6 Torr).

(13) The pHG was imaged with low magnification (up to 5000 at 5.0 kV), which did not affect the electronic and magnetic properties of HG. Using reduced raster scan in the SEM software, selected areas were exposed on pHG sheets to electrons.

Example 2

(14) The magnetic properties of the pHG and e-beam dehydrogenated area could be characterized by magnetic force microscopy (MFM), which uses a magnetized cantilever to detect long-range magnetic forces between the cantilever and surface. On the first pass, the topography of the surface was imaged in tapping mode; on the second pass, the cantilever was raised to a set height (40 nm) above the surface to detect the magnetic response. The magnetic cantilever can be oriented with either the north pole (+B) or the south pole (B) pointing toward the surface. FIG. 2 shows the topography image of patterned features obtained by tapping mode in the first pass. While the e-beam removes hydrogen atoms from pHG, carbonaceous contamination from SEM chamber that is neither magnetic nor conductive was also deposited on the e-beam irradiated features.

(15) FIG. 2 shows the magnetic responses of pHG for e-beam dehydrogenated features with (B) and (+B) orientations, respectively. The e-beam dehydrogenated features displayed negative (darker) phase shift against a background of pHG, while these features displayed a positive phase shift (brighter) with the opposite magnetization used in FIG. 2. The phase shifts indicate that pHG responded to the cantilever's magnetic field but the e-beam irradiated lines did not. The insets in FIG. 2 help the understanding of the magnetic response of the cantilever with tip polarization.

(16) In brief, the magnetized cantilever is bent (upward or downward) when it is above magnetic pHG due to the magnetic force between the tip and the surface, while the cantilever recovers the original state above the patterns of non-magnetic squares. Therefore, this observation indicates that the e-beam dehydrogenation of the pHG quenched its magnetic properties. Notably, the phase shifts were opposite with the south (FIG. 2) and north (FIG. 2) poled cantilevers, suggesting that pHG displays ferromagnetism at room temperature.

(17) In FIG. 3, the electron doses of the lines increase from 0.25 to 2.0 C/cm.sup.2 leading to more pronounced MFM signal shifts (darker contrast). Elimination of magnetism begins to plateau by 0.25 C/cm.sup.2. This shows that the ferromagnetism of the e-beam dehydrogenated lines was gradually quenched to fall into the plateau around the dose of 1.5 C/cm.sup.2. This technique could be used to generate alternating patterns with different magnetic strengths on single pHG sheet without additional materials or processes.

(18) This approach provides for a highly uniform ferromagnetic film that appears not to have grain boundaries. The film can be placed on a wide range of substrates. The film is ultra-thin, being on average less than two atom layers thick. The strength of the ferromagnetism can be controlled by the extent of the coverage of hydrogen.

(19) By locally controlling the extent of hydrogenation, the film can be readily patterned. Patterning can be achieved, at a minimum, by using heat, electron beam desorption, chemically, mechanical stress, and light, or other methods of disassociating the hydrogen.

(20) Many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any reference to claim elements in the singular, e.g., using the articles a, an, the, or said is not construed as limiting the element to the singular.