Graphene nanoribbon precursor and method for producing graphene nanoribbon

A graphene nanoribbon precursor having a structural formula represented by a following chemical formula (1), wherein in the following chemical formula (1): n is an integer greater than or equal to 0; X is bromine, iodine or chlorine; and Y is hydrogen or fluorine.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2019-63818, filed on Mar. 28, 2019, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a graphene nanoribbon precursor and a method for producing a graphene nanoribbon.

BACKGROUND

As nano-sized graphene, a quasi-one-dimensional graphene of a ribbon shape with a width of several nanometers, which is called a graphene nanoribbon (GNR) is known. It is known that the GNR has a band gap whose size is roughly inversely proportional to the width.

As an application of the GNR, there is a semiconductor device having a PN junction. The GNR tends to operate as a P-type semiconductor due to doping derived from oxygen in the atmosphere. On the other hand, it is difficult to produce a GNR that operates as an N-type semiconductor. Theoretically, it is considered possible to make a GNR operate as an N-type semiconductor by replacing hydrogen (H) at an edge of a GNR whose edge structure is of armchair type with fluorine (F). However, it has been impossible to stably produce a GNR whose edge structure is of armchair type and whose edge hydrogen is replaced with fluorine. In view of the above, it is desirable to be able to provide a graphene nanoribbon precursor that is capable of stably producing a graphene nanoribbon whose edge structure is of armchair type and whose edge hydrogen is replaced with fluorine, and a method for producing the graphene nanoribbon.

SUMMARY

According to an aspect of the embodiments, a graphene nanoribbon precursor having a structural formula represented by a following chemical formula (1), wherein in the following chemical formula (1): n is an integer greater than or equal to 0; X is bromine, iodine or chlorine; and Y is hydrogen or fluorine.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present disclosure will be specifically described below with reference to the accompanying drawings. Note that, in this specification and the drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and explanation thereof will not be unnecessarily repeated.

First Embodiment

A first embodiment relates to a graphene nanoribbon (GNR) precursor that is suitable for producing a GNR.FIG. 1is a diagram illustrating a structural formula of the GNR precursor according to the first embodiment.

A GNR precursor100according to the first embodiment has the structural formula illustrated inFIG. 1. InFIG. 1, n is an integer greater than or equal to 0, X is bromine (Br), iodine (I) or chlorine (CI), and Y is hydrogen (H) or fluorine (F).

Here, a method for producing the GNR using the GNR precursors100according to the first embodiment will be described.FIGS. 2A and 2Bare diagrams illustrating a method for producing the GNR using the GNR precursors according to the first embodiment.

First, a surface cleaning process of a catalytic metal substrate on which the GNR is grown is performed in an ultrahigh vacuum. By the surface cleaning process, organic contaminants on the surface of the catalytic metal substrate may be removed and the surface flatness may be enhanced. As the catalytic metal substrate, for example, a gold (Au) substrate, a silver (Ag) substrate or a copper (Cu) substrate having a Miller index on the surface of (111), (110), (100) or (788) may be used.

Next, without exposing the catalytic metal substrate subjected to the surface cleaning process to the atmosphere, the catalytic metal substrate is heated such that the surface temperature of the catalytic metal substrate is a first temperature which is equal to or higher than the elimination temperature TXof X and lower than the elimination temperature TYof Y atoms, for example, 200° C. to 300° C., under an ultra-high vacuum, and the surface temperature of the catalytic metal substrate is held at the first temperature. Then, the GNR precursors100are vacuum-deposited on the surface of the catalytic metal substrate. It is desirable to control a deposition amount of the GNR precursor100so that about one-molecule layer will be formed. On the surface of the catalytic metal substrate'whose temperature is the first temperature, X atoms are eliminated from the GNR precursors100to cause an Ullmann reaction, and a C—C binding reaction is induced. As a result, as illustrated inFIG. 2A, a polymer110in which aromatic compounds are connected and in which a plurality of molecules of the GNR precursors100are arranged in one direction while reversing the direction of protrusion, is stably formed.

Next, the catalytic metal substrate is heated such that the surface temperature thereof is a second temperature, which is equal to or higher than the elimination temperature THof H, for example, 350° C. to 450° C., and the surface temperature of the catalytic metal substrate is held at the second temperature. As a result, as illustrated inFIG. 2B, H and Y are eliminated from the polymer110and a C—C binding reaction is induced to aromatize the polymer110, and a GNR150whose edge structure is of armchair type and whose edge H is replaced with F is formed.

In this way, upon heating the GNR precursors100, X's are eliminated and C's, to which X's have been bound, are bound with each other between the GNR precursors100, and thereafter, H's and Y's are eliminated and, then C's, to which H's have been bound, are bound with each other between the GNR precursors100and C's, to which Y's have been bound, are bound with each other between the GNR precursors100. The sequence of the GNR precursors100is determined by bonding of C's, to which X's have been bound, with each other, and then the structure of the GNR150is fixed by subsequent bonding of C's, to which H's have been bound, with each other and bonding of C's, to which Y's have been bound, with each other, Therefore, it is possible to stably produce the GNR150whose edge structure is of armchair type and whose edge H is replaced with F.

Next, a method for producing the GNR precursor100according to the first embodiment will be described.FIGS. 3A and 3Bare diagrams illustrating the method for producing the GNR precursor100according to the first embodiment.

First, a substance160whose structural formula is illustrated inFIG. 4Aand a substance130whose structural formula is illustrated inFIG. 4Bare prepared. The substance160illustrated inFIG. 4Ais 1,4-dibromo-2,3-diiodobenzene, and the substance130illustrated inFIG. 4Bis a boronic acid of benzene or an acene. As the acene, for example, naphthalene, anthracene, naphthacene, pentacene or hexacene may be used.

Next, these substances are dissolved in a solvent, a catalyst is added thereto and the mixture is stirred in the presence of a base to cause a Suzuki coupling reaction. By continuing stirring to evaporate the solvent, as illustrated inFIG. 3A, a substance140obtainable by monocoupling of one iodine (I) contained in the substance160with the substance130may be obtained.

Thereafter, the substance140illustrated inFIG. 3Aand the substance130are dissolved in a solvent, a catalyst is added thereto and the mixture is stirred in the presence of a base to cause a Suzuki coupling reaction. By continuing stirring to evaporate the solvent, as illustrated inFIG. 3B, the GNR precursor100obtainable by monocoupling of iodine (I) contained in the substance140with the substance130may be obtained.

Then purification of the GNR precursor100is carried out, for example, by column chromatography.

In this way, the GNR precursor100may be produced by a bottom-up method.

1,4-dibromo-2,3-diiodobenzene may be synthesized, for example, by the following method.FIG. 5is a diagram illustrating a method for synthesizing 1,4-dibromo-2,3-diiodobenzene.

First, 2,5-dibromoaniline (compound11), chloral hydrate (compound12) and hydroxylammonium chloride (compound13) are reacted in an aqueous solution of ethanol to give (2,5-dibromophenyl)-2-(hydroxyimino)acetamide (compound14). This reaction is carried out, for example, at 80° C. for 12 hours.

Next, (2,5-dibromophenyl)-2-(hydroxyimino)acetamide (compound14) is added to concentrated sulfuric acid and the mixture is heated to give 4,7-dibromoindoline-2,3-dione (compound15). This reaction is carried out, for example, at 100° C. for 30 minutes.

Thereafter, 4,7-dibromoindoline-2,3-dione (compound15) is dissolved in a sodium hydroxide aqueous solution, a hydrogen peroxide solution is added dropwise thereto and the mixture is stirred. Subsequently, filtration is performed, hydrogen is added to the carboxyl group using hydrochloric acid, and the pH is adjusted to 4. Then, filtration is carried out to give 2-amino-3,6-dibromobenzoic acid (compound16).

Next, 2-amino-3,6-dibromobenzoic acid (compound16) is added dropwise to a solution of 1,2-dichloromethane, iodine and isoamyl nitrite to give 1,4-dibromo-2,3-diiodobenzene (compound17). This reaction is carried out, for example, at 80° C. for 16 hours.

In this way, 1,4-dibromo-2,3-diiodobenzene may be synthesized.

Note that n is not particularly limited as long as it is an integer greater than or equal to 0, but in order to obtain a stable GNR precursor100, n is preferably an integer greater than or equal to 0 and less than or equal to 5. Furthermore, the length of a GNR is not particularly limited, and may be, for example, several tens of nanometers. When iodine is used as X, it is easy to produce a long GNR.

Second Embodiment

A second embodiment relates to a GNR precursor that is suitable for producing a GNR.FIG. 6is a diagram illustrating a structural formula of the GNR precursor according to the second embodiment.

A GNR precursor200according to the second embodiment has the structural formula illustrated inFIG. 6. That is, the GNR precursor200according to the second embodiment has the structural formula illustrated inFIG. 1where n is 0, X is Br, and Y is F. The GNR precursor200is, so to speak, 3′,6′-dibromo-3′,3″,4′,4″,5′,5″-hexafluoro-1,1′:2′,1″-terphenyl.

Here, a method for producing the GNR using the GNR precursors200according to the second embodiment will be describedFIGS. 7A and 7Bare diagrams illustrating a method for producing the GNR using the GNR precursors according to the second embodiment.

First, a surface cleaning process of a catalytic metal substrate on which the GNR is grown is performed in an ultrahigh vacuum. In this surface cleaning process, for example, Ar ion sputtering to the surface and annealing under an ultra-high vacuum are set as one cycle, and this cycle is performed for a plurality of cycles. For example, in each cycle, in the Ar ion sputtering, the ion acceleration voltage is set to 1.0 kV, the ion current is set to 10 μA, and the time is set to 1 minute, and in the annealing, while maintaining the degree of vacuum of 5×10−7Pa or less, the temperature is set to 400° C. to 500° C. and the time is set to 10 minutes. For example, the number of cycles is three (three cycles). By the surface cleaning process, organic contaminants on the surface of the catalytic metal substrate may be removed and the surface flatness may be enhanced. Here, a gold (Au) substrate having a Miller index on the surface of (111) is used as the catalytic metal substrate. Hereinafter, the (111) plane of the Au substrate is sometimes referred to as an “Au (111) plane”.

Next, without exposing the catalytic metal substrate subjected to the surface cleaning process to the atmosphere, the catalytic metal substrate is heated such that the surface temperature of the catalytic metal substrate is a first temperature which is equal to or higher than the elimination temperature of Br and lower than the elimination temperature of H, for example, 200° C., under an ultra-high vacuum, and the surface temperature of the catalytic metal substrate is held at the first temperature. Then, the GNR precursors200are vacuum-deposited on the surface of the catalytic metal substrate. It is desirable to control a deposition amount of the GNR precursors200so that about one-molecule layer will be formed. On the surface of the catalytic metal substrate whose temperature is the first temperature, Br is eliminated from the GNR precursors200to cause an Ullmann reaction, and a C—C binding reaction is induced. As a result, as illustrated inFIG. 7A, a polymer210in which aromatic compounds are connected and in which a plurality of molecules of the GNR precursors200are arranged in one direction while reversing the direction of protrusion, is stably formed.

Next, the catalytic metal substrate is heated such that the surface temperature thereof is a second temperature, which is equal to or higher than the elimination temperature THof H, for example, 400° C., and the surface temperature of the catalytic metal substrate is held at the second temperature. As a result, as illustrated inFIG. 7B, H and F are eliminated from the polymer210and a C—C binding reaction is induced to aromatize the polymer210, and a GNR250whose edge structure is of armchair type and whose edge H is replaced with F is formed.

In this way, upon heating the GNR precursors200, Br's are eliminated and C's, to which Br's have been bound, are bound with each other between the GNR precursors200, and thereafter, H's and F's are eliminated and then C's, to which H's have been bound, are bound with each other between the GNR precursors200and C's, to which F's have been bound, are bound with each other between the GNR precursors200. The sequence of the GNR precursors200is determined by bonding of C's, to which Br's have been bound, with each other, and then the structure of the GNR250is fixed by subsequent bonding of C's, to which H's have been bound, with each other and bonding of C's, to which F's have been bound, with each other, Therefore, it is possible to stably produce the GNR250whose edge structure is of armchair type and whose edge H is replaced with F.

Next, a method for producing the GNR precursor200according to the second embodiment will be described.

First, 1,4-dibromo-2,3-diiodobenzene and 3,4,5-trifluorophenylboronic acid are prepared, 3,4,5-Trifluorophenylboronic acid has a structural formula illustrated inFIG. 4Bin which n is 0.

Next, by a method similar to that of the method for producing the GNR precursor100according to the first embodiment, Suzuki coupling reactions are caused twice and the solvent is evaporated to give the GNR precursor200. Then, purification of the GNR precursor200is carried out, for example, by column chromatography.

In this way, the GNR precursor200may be produced by a bottom-up method.

FIG. 8Aillustrates a scanning tunneling microscope (STM) image of the GNR produced according to the second embodiment.FIG. 8Bis a diagram schematically illustrating the contents of the STM photograph illustrated inFIG. 8A.

As illustrated inFIGS. 8A and 8B, a one-dimensional structure having a width of slightly less than 1.5 nm, a length of about 10 nm to about 20 nm, and a thickness of about a monoatomic layer was obtained. That is, it was confirmed that a GNR was produced.

FIGS. 9A and 9Billustrate X-ray photoelectron spectroscopy (XPS) spectra of the produced GNR. As illustrated inFIG. 9A, a peak21of the 1s orbit of fluorine was confirmed in addition to a peak22of the 4p orbit of Au of the catalytic metal substrate. Furthermore, as illustrated inFIG. 9B, a peak23of C-F bond and a peak24of C of the 1s orbit located inside edges were confirmed. The intensity of the peak24is 3.4 times the intensity of the peak23. As illustrated inFIG. 9C, when focusing on a pair of armchairs, the number of C atoms26located inside edges is 14, and the number of C atoms25located at the edges and bound to F atoms is 4. Therefore, the number of the C atoms26is 3.5 times the number of the C atoms25. From this, it may be said that F atoms are bound to almost all the C atoms25located at the edges.

FIG. 10illustrates a microscopic Raman optical spectrum of the produced GNR. As illustrated inFIG. 10, a peak31of the G band indicating the presence of graphene was confirmed. Furthermore, a peak32of the D band indicating that the GNR was of armchair type was also confirmed.

From the results illustrated inFIGS. 9A, 9B and 10, it was confirmed that a GNR whose edge structure is of armchair type and whose edge H is replaced with F was produced.

FIG. 11illustrates electronic structures of 9AGNR and F-9AGNR side by side. 9AGNR is a GNR of armchair type in which four six-membered rings are arranged in the ribbon width direction and which has a width of nine carbon atoms. F-9AGNR is obtainable from 9AGNR by replacing the edge H of 9AGNR with F. When the electronic states of 9AGNR and F-9AGNR were calculated by density functional theory (DFT), the work function of F-9AGNR was 4.75 eV, and the work function of 9AGNR was 3.50 eV. This indicates that a PN junction may be realized by joining 9AGNR and F-9AGNR.