JP2012096968A - Method of producing graphene substrate and graphene substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of processing graphene, the method forming and controlling an end structure on an atomic scale.SOLUTION: A method of producing the graphene includes: adsorbing oxygen atoms on a graphene layer; arranging the oxygen atoms in a row between carbon bonds by using adsorption/diffusion anisotropy and then diffusing them to form an ether bond; and cleaving the ether bond to form an end.

Description

  The present invention relates to a method for producing a graphene substrate and a graphene substrate produced by the production method, and in particular, next-generation electronics derived from exceptional electronic properties and optical properties, excellent mechanical properties and chemical properties, The present invention relates to a graphene substrate manufacturing method applicable to optoelectronics and spintronics, and a graphene substrate.

  Supporting the current information society is a semiconductor device represented by silicon-based CMOS (complementary metal oxide semiconductor). Until now, the silicon semiconductor industry has achieved miniaturization by continuously reducing the application range of microfabrication technologies such as lithography technology, etching technology, and film formation technology from micrometers to several tens of nanometers. Performance has been realized at the same time. In recent years, the thinness of a silicon layer serving as a semiconductor channel has reached the atomic layer level, and its material and physical limits have been pointed out.

  Although it is the ultimate thin two-dimensional atomic layer thin film, chemically and thermodynamically stable graphene is a new semiconductor material with great potential to meet this demand, and utilizes its excellent physical properties By doing so, there is a possibility that a new element that surpasses the performance of the existing element can be realized.

Graphene is a single layer of graphite, a layered material composed only of sp ² hybridized carbon. As mentioned above, it is extremely robust against chemical reactions and is also thermodynamically stable. It is a simple monoatomic layer planar material.

  The graphene structure is a honeycomb-like quasi-two-dimensional sheet in which hexagonal hexacarbon rings with carbon atoms at the top are laid without gaps, and the carbon-carbon distance is about 1.42 angstroms. The thickness of the substrate is about 3.3 to 3.4 angstroms if the base is graphite, and about 10 angstroms on other substrates.

  The size of the graphene plane can be assumed to vary from a molecular size with a piece length of nanometer order to a theoretically infinite size. Normally, graphene refers to a single graphite layer, but often includes two or more layers. In that case, the one-layer, two-layer, and three-layer ones are referred to as single-layer (monolayer) graphene, two-layer (bilayer) graphene, and three-layer (trilayer) graphene, respectively. Up to about 10 layers are collectively referred to as several layers (few-layer) graphene. In addition, layers other than single-layer graphene are referred to as multilayer graphene.

  The electronic state of graphene can be described by the Dirac equation in the low energy region (Non-Patent Document 1). This is in contrast to the fact that the electronic states of substances other than graphene are well described by the Schrodinger equation.

The electron energy of graphene has a linear dispersion relationship with respect to the wave number in the vicinity of the K point, and more specifically can be expressed by two straight lines having positive and negative slopes corresponding to the conduction band and the valence band. The point at which they intersect is called the Dirac point, where graphene electrons have a unique electronic property that they behave as zero-mass fermions. As a result, the mobility of graphene shows the highest value among the existing substances of 10 ⁷ cm ² V ⁻¹ s ⁻¹ or more, and has a feature that temperature dependence is small.

  Single-layer graphene is basically a metal or semimetal with a zero band gap. However, when the size is on the order of nanometers, the band gap opens, and the semiconductor has a finite band gap depending on the width and edge structure of graphene. The bilayer graphene has zero band gap without perturbation, but perturbation that breaks the mirror symmetry between the two graphenes, for example, when an electric field is applied, a finite gap is formed according to the magnitude of the electric field. To have.

For example, the gap opens about 0.25 eV with an electric field of 3 Vnm- ¹ . In the case of three-layer graphene, it exhibits semi-metallic electronic properties in which a conduction band and a valence band are overlapped with a width of about 30 meV. The point that the conduction band and the valence band overlap is close to that of bulk graphite. Graphene with four or more layers also exhibits semi-metallic properties, and as the number of layers increases, it gradually approaches the electronic properties of bulk graphite.

  Graphene is also excellent in mechanical properties, and the Young's modulus of one layer of graphene jumps to 2 TPa (terrapascal) and is large. Tensile strength is the highest level among existing materials.

In addition, graphene has unique optical properties. For example, in a wide electromagnetic wave region from an ultraviolet light region (wavelength: up to 200 nm) to a near terahertz light region (wavelength: up to 300 μm), the transmittance of graphene is 1−nα (n: the number of graphene layers, n = 1 to 10: α: fine structure constant, α = e ² / 2hcε ₀ = 0.0229253012, e: elementary electric charge, h: Planck's constant, ε ₀ : dielectric constant of vacuum) Represented only by physical constants. This is a characteristic characteristic of graphene that is not found in other material materials.

  Furthermore, the transmittance and reflectance of graphene show carrier density dependence in the terahertz light region. This means that the optical properties of graphene can be controlled based on the field effect. It is known that other two-dimensional atomic layer thin films also have unique physical properties based on dimensionality.

  As described above, graphene is expected to be used in a wide range of industrial fields from chemistry to electronics because of its exceptional electronic properties and optical properties, as well as excellent mechanical and chemical properties. In particular, the development of semiconductor devices and micromechanical devices in the fields of next-generation electronics, spintronics, optoelectronics, micro / nanomechanics, and bioelectronics is being promoted around the world. As with graphene, other two-dimensional atomic layer thin films are actively being researched and developed for industrial use.

  Here, the size of the graphene plane can be assumed to have various shapes from the molecular size of nanometer order to the theoretically infinite, but when the molecular size becomes nanometer order, the macroscopic bulk state It exhibits completely different electronic properties.

  This is the so-called quantum size effect, and graphene has the properties of a normal graphite-like metal in macro size, but when it becomes nanometer size, depending on the edge structure, the property as metal, Alternatively, it exhibits properties as a semiconductor with a band gap.

  Specifically, the shape of the end of graphene has two shapes, “zigzag end” and “armchair end”. For example, when the width is 20 nm or less and the end is “zigzag end” It behaves as metal, and in the case of “armchair end” it behaves as a semiconductor.

  Therefore, in manufacturing a device using graphene, it is necessary to process the edge according to the electronic properties required for graphene.

  As a conventional method for processing the shape of graphene, there is a method using plasma etching (Non-patent Document 2).

In addition, the carbon nanotubes are linearly oxidized in the direction along the tube axis under heating in an O ₂ atmosphere to expand the cylindrical carbon nanotubes and to form strip-shaped graphene, thereby flattening at the atomic level. There is a method of obtaining a “zigzag end” and an “armchair end” (Patent Document 1).

Japanese Unexamined Patent Publication No. 2009-184873

T.Ando, “The electronic properties of graphene and carbon nanotubus”, NPG asia materials 1 (1), 2009, p17-21 M.Y.Han, B. Ozyilmaz, Y. Zhang, and P. Kim, "Energy Band-Gap Engineering of Graphene Nanoribbons" Physical Review Letters, 2007, 98, p206805

  However, the prior art method for treating the edge of graphene has the following problems.

  First, in the plasma etching method described in Non-Patent Document 2, etc., the accuracy of the mask for determining the structure is about 0.1 μm, and the processing accuracy on the atomic scale necessary for nanodevice application is obtained. Was very difficult.

Further, in the method of forming graphene having a specific end structure by disposing and cleaving carbon nanotubes under heating in an O ₂ atmosphere of Patent Document 1, carbon having a known atomic scale winding structure called chirality is known. There was a problem that the nanotubes had to be pre-arranged. In addition, with this method, it is possible to construct a graphene ribbon structure of the same width, but it is very difficult to form a complicated structure such as changing the width or connecting to another structure.

  The present invention has been made in view of the above problems, and an object thereof is to provide a graphene processing method capable of forming and controlling an end structure on an atomic scale.

  In order to solve the above problems, the first aspect of the present invention is as follows: (a) an oxygen atom is adsorbed on the graphene layer; and (b) the oxygen atom is bonded to the carbon by utilizing the adsorption / diffusion anisotropy of graphene. It is a method for producing a graphene substrate characterized in that an ether bond is formed by arranging and diffusing them in between, and (c) cleaving the ether bond to form an end.

  A second aspect of the present invention is a graphene substrate manufactured using the method for manufacturing a graphene substrate according to the first aspect.

  ADVANTAGE OF THE INVENTION According to this invention, the processing method of the graphene which can form and control an end structure on an atomic scale can be provided.

It is a flowchart which shows the outline of the formation method of the edge structure which is the most characteristic part among the manufacturing methods of the graphene substrate which concerns on this embodiment. It is detail drawing which shows the manufacturing method of the graphene board | substrate which concerns on this embodiment. It is a schematic diagram showing the zigzag direction on the graphene substrate and the direction of the extension force applied to stabilize the adsorption structure of oxygen atoms. It is the figure which shows the result of the first principle calculation based on the density functional method of the potential energy depending on the adsorption position of the oxygen atom adjacent to the oxygen atom (black circle) adsorbed on the graphene substrate and the energy barrier for diffusion, The smaller the energy value, the more stable it is.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail based on the drawings.

  First, the outline of the formation of the end structure, which is the most characteristic part of the present invention, of the method for manufacturing a graphene substrate according to the first embodiment will be described with reference to FIG.

First, oxygen atoms are adsorbed on the graphene layer (S1 in FIG. 1).
Next, utilizing the adsorption / diffusion anisotropy of graphene, the oxygen atoms are arranged in rows between carbon bonds and diffused to form ether bonds (S2 in FIG. 1).
Finally, the ether bond is cleaved to form an end (S3 in FIG. 1).

  In this way, after forming an ether bond by arranging oxygen atoms by utilizing the adsorption / diffusion anisotropy of graphene, the edge of graphene is formed with a processing accuracy of 1 nm or less by cleaving it. Can do.

  Next, the method for manufacturing a graphene substrate according to the present embodiment will be described more specifically with reference to FIGS.

First, graphene is formed on a substrate (S11 in FIG. 2).
Examples of the substrate include SiO _2, but graphite, SiC, or the like may be used.
In addition, as a method for forming graphene, for example, a transfer method may be mentioned, but it may be formed by chemical vapor deposition on a Cu substrate, or by using other methods such as a method of forming by thermal oxidation of SiC. Also good.

  Next, the crystal orientation of graphene is examined (S12 in FIG. 2). Specifically, the crystal orientation of graphene is determined using X-ray diffraction or the like.

  Next, a resist is applied to the graphene substrate and patterned by an electron beam drawing method (S13 in FIG. 2).

  During patterning, patterning is performed so that the ends of the pattern are parallel to the armchair direction and the zigzag direction as necessary, based on the previously determined crystallographic orientation of graphene.

In order to eliminate the influence of contaminants, this substrate may be loaded into a vacuum chamber after patterning and evacuated at a degree of vacuum of about 10 ⁻⁶ Pa.

  Next, after raising the substrate temperature to about 120 ° C., a very small amount of oxygen gas is introduced into the chamber and left for about 1 hour to adsorb oxygen atoms on the substrate (S14 in FIG. 2).

  Note that the adsorption of oxygen gas can be promoted by further increasing the substrate temperature, but the resolution by the resist is lowered, so that the range of 50 to 150 ° C. is desirable. However, if the resist has both high heat resistance and high resolution, the substrate temperature is not limited to this, and the heating temperature can be arbitrarily set in the range of 100 to 900 ° C. according to the type of resist.

  The standing time may be shorter when the substrate temperature is high. Conversely, when the substrate temperature is set low, exposure for a longer time is required. Whether or not the adsorption and diffusion have been sufficiently performed can be confirmed by scanning the vicinity of the resist mask using a scanning tunneling microscope (STM), an atomic force microscope (AFM), or the like as necessary.

  Oxygen atoms adsorbed on the substrate during the exposure time spontaneously diffuse so as to be arranged in a line along the armchair direction due to diffusion and alignment anisotropy on the graphene.

  As a result, the oxygen atoms adsorbed in a line form further cut the carbon-carbon bond immediately below the adsorption site and finally inserted between the carbon bonds to form an ether bond (S15 in FIG. 2).

  The resolution of the resist mask is much coarser than the atomic scale as described above, but oxygen atoms are adsorbed linearly on the atomic scale along the edges of the resist due to the diffusion and adsorption anisotropy of oxygen atoms.

Oxygen atoms are diffused and adsorbed extensively in areas where no resist is applied, but the vicinity of the resist edge tends to adsorb because the potential is particularly low, and the one-dimensional array structure near the resist edge is dense. There are few defects.
In addition, said method is a method in the case of forming an armchair end.

  When it is desired to form the zigzag end, as shown in FIG. 3, oxygen atoms are arranged in the zigzag direction by applying a tensile pressure in the armchair direction.

  Next, after a sufficient diffusion time, the formed ether bond is cleaved by further oxidation treatment with hydrogen halide such as hydrogen chloride or Lewis acid to form ends (S16 in FIG. 2). .

  Through the above procedure, a graphene substrate having a desired atomic scale edge structure is manufactured.

As described above, according to the first embodiment, oxygen atoms are adsorbed on the graphene layer, and oxygen atoms are arranged and diffused in rows between carbon bonds using the adsorption / diffusion anisotropy of graphene. Thus, an ether bond is formed, and the ether bond is cleaved to form an end.
Therefore, the end portion of graphene can be formed with a processing accuracy of 1 nm or less.

Next, a second embodiment will be described.
In the second embodiment, the substrate is not heated in the first embodiment, but a voltage is applied to the probe of the scanning electron microscope to change the potential of the surface of the graphene layer. The oxygen atoms are arranged in a row along the direction of the armchair.

Hereinafter, a method for manufacturing a graphene substrate according to the second embodiment will be described.
First, in the graphene substrate manufacturing method according to the second embodiment, an outline of forming an end structure, which is the most characteristic part of the present invention, will be described with reference to FIG.

First, oxygen atoms are adsorbed on the graphene layer (S1 in FIG. 1).
Next, utilizing the adsorption / diffusion anisotropy of graphene, the oxygen atoms are arranged in rows between carbon bonds and diffused to form ether bonds (S2 in FIG. 1). Here, the potential of the applied position is changed (lower than the surroundings) by bringing the probe of the scanning electron microscope into contact with graphene and applying the potential. As a result, oxygen atoms are adsorbed at the position where the potential is applied, and an ether bond is formed.
Finally, the ether bond is cleaved to form an end (S3 in FIG. 1).

  Next, a method for manufacturing a graphene substrate according to the second embodiment will be described more specifically with reference to FIG.

First, graphene is formed on a substrate in the same procedure as in the first embodiment (S11 in FIG. 2).
Next, the crystal orientation of graphene is examined by the same procedure as in the first embodiment (S12 in FIG. 2).
Next, this substrate is sealed in a vacuum chamber, and a pattern corresponding to a device structure to be formed is patterned using a resist in the same procedure as in the first embodiment (S13 in FIG. 2).

  At this time, patterning is performed based on the previously determined crystallographic orientation of graphene so that the end of the pattern is parallel to the armchair direction or the zigzag direction as necessary.

  Next, oxygen gas is introduced into the chamber to adsorb oxygen atoms on the substrate (S14 in FIG. 2).

  At this time, the scanning electron microscope probe is brought into contact with the graphene, and an applied potential is applied by applying a potential of +1 V / nm to +20 V / nm, preferably +1 V / nm to +10 V / nm. Change the potential of (lower than the surroundings). As a result, oxygen atoms are adsorbed at the position where the potential is applied, and an ether bond is formed (S15 in FIG. 2).

  Further, by scanning the probe in this state, oxygen atoms can be arranged and adsorbed in a line along the scanning locus.

  In the case of the second embodiment, since oxygen atoms are arranged along the scanning trajectory of the probe, the structure is formed without greatly depending on the ease of arrangement in the armchair direction and the zigzag direction. be able to. Therefore, it is not essential to apply local strain due to tensile stress when forming the zigzag end.

Next, as in the first embodiment, the formed ether bond is cleaved by further oxidation treatment with hydrogen halide such as hydrogen chloride or Lewis acid to form an end (S16 in FIG. 2). ).
The graphene substrate is manufactured by the above procedure.

As described above, according to the second embodiment, oxygen atoms are adsorbed on the graphene layer, and oxygen atoms are arranged in rows between carbon bonds and diffused using the adsorption / diffusion anisotropy of graphene. Thus, an ether bond is formed, and the ether bond is cleaved to form an end.
Therefore, the same effect as that of the first embodiment is obtained.

Further, according to the second embodiment, oxygen atoms are arranged by changing the surface potential by applying a voltage to the probe of the scanning electron microscope.
Therefore, compared with the first embodiment, there is an effect that patterning on an atomic scale becomes possible without largely depending on the resolution of the resist.

  Hereinafter, based on an Example, this invention is demonstrated in detail.

Example 1
The stable structure of adsorbed oxygen atoms and the diffusion barrier for diffusion were obtained by a first-principles electronic state calculation program using the density functional method developed originally. The pseudopotentials by Troullier-Martins were used as potentials for carbon and oxygen nuclei. The wave function was developed by plane waves, and the energy cutoff was 60 Ry (60 × 13.6 eV). A 6 × 6 hexagonal supercell will contain 72 carbon atoms and optionally one or two oxygen atoms, imposing periodic boundary conditions. The specific procedure is as follows.

The stable adsorption position of oxygen atoms on graphene is directly above the carbon-carbon bond and is called an epoxy structure.
The diffusion barrier between the most stable structures was 0.9 eV.

  Further, as shown in FIG. 4, after the first oxygen atom is adsorbed, the second oxygen atom is adsorbed at various metastable adsorption positions and the total energy is calculated. It was found that it was the adsorption position in the chair direction.

  In other words, oxygen atoms are more stable when lined up in the armchair direction on graphene.

  Furthermore, when the activation barrier for oxygen atoms to be diffused was found, it was found that the barrier was lower than that arranged in the zigzag direction because the diffusion was similarly arranged in the armchair direction.

  Here, on the graphene substrate, the armchair direction has three-fold symmetry, but the stable adsorption position of oxygen is between two adjacent carbon atoms, so that it is further limited to two directions.

  In addition, when oxygen atoms are adsorbed, the oxygen atoms further form a one-dimensional array in order to alleviate distortion of the crystal structure due to the increased bond distance between carbon atoms.

  Note that the direction of the one-dimensional array in which oxygen atoms are adsorbed can be further limited by applying a potential due to an external field using a resist mask or an STM probe.

  Further, since the activation energy for diffusing the adsorbed oxygen atoms is about 1 eV, it is possible to dramatically promote the alignment of the adsorbed oxygen atoms in the armchair direction by raising the substrate temperature to about 700 ° C. it can.

  On the other hand, when the heat resistance of the resist material is low, it is necessary to set the substrate temperature to about 100 ° C. and to set the alignment time as long as 1 hour or more.

(Example 2)
An attempt was made to produce a graphene substrate using the method for producing a graphene substrate described in the first embodiment. The specific procedure is as follows.

First, a graphene sheet was transferred and formed from highly oriented pyrolytic graphite by a transfer method onto a silicon substrate (size 100 mn × 100 mm, oxide film thickness 4 nm) on which a surface oxide film was formed by thermal oxidation.
Next, the crystal orientation of this substrate was previously determined using an X-ray diffractometer.
Next, on this substrate, a wiring structure parallel to the armchair direction and the zigzag direction (pattern on a straight line crossing the graphene substrate, length of about 100 nm) was applied with a photoresist, and then patterned by electron beam lithography.
Next, the substrate was sealed in a vacuum chamber.

When the substrate is sealed, oxygen gas is introduced into the vacuum chamber, the pressure in the chamber is set to 10 ⁻⁵ Pa, oxygen is adsorbed on the graphene substrate, and then the substrate temperature is increased to 100 ° C. at a rate of temperature increase of 10 ° C./min. And kept constant for 180 minutes.

  At this time, the wiring structure portion parallel to the zigzag direction was bent by applying a stress to the substrate to apply a tensile force in the direction perpendicular to the pattern. The substrate was cooled to room temperature, washed with hydrochloric acid, and observed with a transmission electron microscope.

  The formed end structure has a flat structure on an almost atomic scale, and it was found from detailed observation that end structures in the armchair direction and the zigzag direction were formed.

  Although the embodiments and examples of the present invention have been specifically described above, the present invention is not limited to the above-described embodiments and examples, and various modifications based on the technical idea of the present invention are possible. is there.

Claims (10)

(A) adsorbing oxygen atoms to the graphene layer;
(B) Utilizing the adsorption / diffusion anisotropy of graphene, the oxygen atoms are arranged in rows between carbon bonds and diffused to form ether bonds,
(C) A method for producing a graphene substrate, wherein the ether bond is cleaved to form an end. (B)
2. The method for producing a graphene substrate according to claim 1, wherein by heating the graphene layer, oxygen atoms are arranged in a line along the most stable armchair direction.   2. The graphene substrate according to claim 1, wherein the oxygen atom is arranged in a zigzag direction of the graphene layer by applying a tensile stress in the armchair direction while heating the graphene layer. Production method. (B)
The said heating temperature is 100 to 900 degreeC, The manufacturing method of the graphene substrate as described in any one of Claim 2 or 3 characterized by the above-mentioned. (B)
By applying a potential to the probe of a scanning electron microscope and changing the potential of the surface of the graphene layer by the electric field, oxygen atoms are arranged in a line along the most stable armchair direction. The manufacturing method of the graphene board | substrate of Claim 1.   The graphene substrate manufacturing method according to claim 5, wherein the electric field has an intensity of 1 to 20 V / nm. (C)
The method for producing a graphene substrate according to any one of claims 1 to 6, wherein the ether bond is cleaved using hydrogen halide or a Lewis acid. Before (a)
(D) forming a graphene layer on the substrate;
(E) determining the crystal orientation of the graphene layer;
(F) A method for producing a graphene substrate, wherein the graphene layer is patterned to have a shape parallel to an armchair direction and / or a zigzag direction.   9. The method of manufacturing a graphene substrate according to claim 8, wherein (e) determines a crystal orientation of the graphene layer by X-ray diffraction.   A graphene substrate manufactured by the method for manufacturing a graphene substrate according to claim 1.

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