A METHOD FOR PRODUCING GRAPHENE NANOSTRUCTURES

Abstract

A method for fabricating graphene nanoribbons by depositing molecules of a precursor directly on a surface of a substrate that is atomically pure, wherein the precursor is a polycyclic aromatic compound having halogen atoms; polymerizing the molecules of the precursor on the surface; and cyclodehydrogenating the polymerized structures under high vacuum conditions to obtain the graphene nanoribbons. A method for fabricating graphene nanoflakes by: depositing molecules of a precursor directly on a surface of a substrate, wherein the precursor is a polycyclic aromatic compound; and cyclodehydrogenating the precursor under high vacuum conditions to obtain the graphene nanoflakes.

Claims

1. A method for fabricating graphene nanoribbons, the method comprising: depositing molecules of a precursor directly on a surface of a substrate, wherein the surface is atomically pure, and the precursor is a polycyclic aromatic compound having halogen atoms; polymerizing the molecules of the precursor on the surface; and cyclodehydrogenating the polymerized structures under high vacuum conditions, with the pressure of molecular hydrogen in a cracker not exceeding 110.sup.7 mbar, at a temperature within the range of 200 to 220 C., while exposing the polymerized structures to atomic hydrogen, to obtain the graphene nanoribbons.

2. The method according to claim 1 wherein the depositing is accomplished using a thermal-deposition technique.

3. The method according to claim 1 wherein the depositing is performed on a substrate made of a non-metallic material.

4. The method according to claim 1 wherein the depositing is performed on a substrate having a surface made of a semiconductor selected from the group consisting of titanium dioxide (TiO.sub.2), silicon (Si), and germanium (Ge).

5. The method according to claim 1 wherein the depositing is performed on a substrate having a surface made of an insulator selected from the group consisting of sodium chloride (NaCl) and silicon dioxide (SiO.sub.2).

6. The method according to claim 1, comprising preparing the surface of the substrate by: a) bombarding the surface of the substrate with argon ions (Ar.sup.+); b) heating the surface of the substrate by means of alternating current to a temperature above a room temperature; c) cooling the surface of the substrate gradually to a room temperature; and d) repeating steps (a) to (c) until the surface is atomically pure.

7. The method according to claim 1, comprising cyclodehydrogenating for a time from 20 to 120 minutes.

8. A method for fabricating graphene nanoflakes, the method comprising: depositing molecules of a precursor directly on a surface of a substrate, wherein the precursor is a polycyclic aromatic compound; and cyclodehydrogenating the precursor under high vacuum conditions, with a pressure of atomic hydrogen not exceeding 110.sup.7 mbar, at a temperature within the range of 200 to 220 C., while exposing the precursors to atomic hydrogen, to obtain the graphene nanoflakes.

9. The method according to claim 8, wherein the precursor is a polycyclic aromatic compound containing hydrogen and carbon atoms.

10. The method according to claim 8, wherein the substrate is non-metallic.

11. The method according to claim 8, wherein the substrate is a semiconductor surface selected from the group consisting of titanium oxide (TiO.sub.2), silicon (Si) and germanium (Ge).

12. The method according to claim 8, wherein the substrate is an insulator surface selected from the group consisting of sodium chloride (NaCl) and silicon dioxide (SiO.sub.2).

13. The method according to claim 8, comprising comprising cyclodehydrogenating for a time from 20 to 120 minutes.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0059] Aspects and features of the present invention will become apparent by describing, in detail, exemplary embodiments of the present invention with reference to the attached drawings, in which:

[0060] FIG. 1 shows schematically structures related to production of nanoribbons;

[0061] FIG. 2 shows nanoribbons produced on a surface of the TiO.sub.2 crystal (011) according to a first example;

[0062] FIG. 3 shows schematically structures related to production of nanoflakes;

[0063] FIG. 4 shows nanoflakes produced on the TiO.sub.2(110) substrate according to a second example;

[0064] FIG. 5 shows nanoflakes produced on the TiO.sub.2(011) substrate according to a third example.

[0065] FIG. 6 shows spectra registered for nanoflakes produced on the SiO.sub.2/Si sample in a fourth example;

[0066] FIG. 7 shows spectra registered for nanoflakes produced on the bulk NaCl sample in a fifth example;

[0067] FIG. 8 shows STM imaging related to nanoflakes produced on the thin NaCl layer on Cu(111) in a sixth example.

DETAILED DESCRIPTION OF EMBODIMENTS

[0068] Reference will now be made to embodiments, examples of which are illustrated in the accompanying drawings. Aspects and features of the embodiments will be described with reference to the accompanying drawings. In the drawings, like reference numerals denote like elements. The present invention, however, may be embodied in various forms and should not be construed as being limited only to the illustrated embodiments. Rather, these embodiments are provided as examples so that this disclosure will be thorough and complete, and will fully convey the aspects and features of the present invention to those skilled in the art. It shall be understood that not all of the features shown in the embodiments are essential and the scope of the protection is defined not by means of literally shown embodiments, but by the features provided in the claims.

First ExampleProduction of Graphene Nanoribbons

[0069] The graphene nanoribbons were prepared using 10,10-dibromo-9,9-biantracene (DBBA) (Sigma Aldrich 808245) as the precursor having the structure shown in FIG. 1. (The same precursor is known from its use in fabrication of 7-AGNR graphene nanostructures by thermally induced cyclodehydrogenation on a metallic Au (111) surface, as described in Cai, Jinming, et al. Atomically precise bottom-up fabrication of graphene nanoribbons, cited above. Nature 466.7305 (2010): 470-473).

[0070] The following technical devices were used to carry out the method: an ultra-high vacuum chamber with a base vacuum of approximately 1.Math.10.sup.10 mbar together with a resistive heater and a hydrogen cracker of the type as described in Tschersich, K. G., J. P. Fleischhauer, and H. Schuler. Design and characterization of a thermal hydrogen atom source. Journal of applied physics 104.3 (2008): 034908. In this type of cracker, molecular hydrogen (H.sub.2) is thermally split into atomic hydrogen on a tungsten cathode whose temperature during operation is about 2500 C.

[0071] A substrate made of semiconductor i.e. pure TiO.sub.2 crystal (011) was prepared according to known annealing procedures. Preparation of the surface of TiO.sub.2 (011) substrate, under UHV conditions, was carried out as follows: [0072] at first step, the surface of TiO.sub.2 (011) substrate was subjected to bombardment with argon ions (Ar.sup.+) using an ion gun; the argon pressure in the ion gun was set at 510.sup.7 mbar, the bombardment time was 10 min; [0073] next, the TiO.sub.2 crystal (011) was heated with alternating current to a temperature of about 770 C. for 10 min; the temperature was measured with a pyrometer; [0074] and at the final step, the TiO.sub.2 crystal (011) was slowly cooled (for approx. 30 min.) to room temperature; a low-temperature STM microscope was used to check the surface quality of the substrate; the process as described above (bombarding, heating, cooling), was repeated until the surface of atomically pure crystal (TiO.sub.2(011)) was obtained.

[0075] Next, onto the atomically pure crystal surface of TiO.sub.2(011) substrate the precursor (DBBA) molecules were thermally deposited. This was followed by thermally induced polymerization process which was carried out as follows: the surface of the substrate having deposited thereon the molecules of the precursor (DBBA) was heated to 260 C. for 15 min. The polymerization process occurred in said conditions resulted in polymerized structures (shown in FIG. 1). The details of how the polymerization process can be carried out are known to specialists, e.g., from the publication by Sun, Kewei, Yuan Fang, and Lifeng Chi On-Surface Synthesis on Nonmetallic Substrates ACS Materials Letters 3.1 (2020): 56-63 and Kolmer, Marek, et al. Polymerization of Polyanthrylene on a Titanium Dioxide (011)-(21) Surface. Angewandte Chemie International Edition 52.39 (2013): 10300-10303.

[0076] Subsequently, the polymerized structures present on the surface of the substrate were subjected to dehydrogenation (and speaking more specifically, cyclodehydrogenation) resulting in flattening of the polymerized structures due to formation of new (additional) carbon-to-carbon bonds between the adjacent polycyclic aromatic structures. Therefore, as a result of the cyclodehydrogenation, the graphene nanoribbons were obtained.

[0077] The cyclodehydrogenation was induced with atomic hydrogen of the following parameters: [0078] atomic hydrogen was obtained by splitting molecular hydrogen (99.99% H.sub.2) in a hydrogen cracker; the cracker was positioned so that its output was facing a sample with the polymerized structures, wherein the distance between the cracker output and the polymerized structures was set at approximately 10 cm; [0079] sample temperature during the process: 200-220 C. (the temperature was chosen experimentally to achieve essentially complete conversion of the polymers into graphene nanoribbons); [0080] the molecular hydrogen stream was directed toward the sample with the polymerized structures for 30 minutes (optimum time of treatment with the molecular hydrogen stream, selected experimentally); [0081] partial pressure of the molecular hydrogen introduced into the cracker system: 1.Math.10.sup.7 mbar; [0082] experimentally determined efficiency of molecular-to-atomic-hydrogen splitting was approximately 10%.

[0083] The course of the cyclodehydrogenation reaction was verified in a low-temperature STM microscopeimages of the resulting 7 (7-AGNR) nanoribbons on the substrate having atomically pure crystal surface of TiO.sub.2 (011), are shown in FIG. 2. The obtained nanoribbons were the same as those described in the paper: Cai, Jinming, et al. Atomically precise bottom-up fabrication of graphene nanoribbons. Nature 466.7305 (2010): 470-473

Second ExampleProduction of Graphene Nanoflakes, TiO.SUB.2 .(110) Substrate

[0084] The graphene nanoflakes were prepared using a precursor shown in FIG. 3, wherein the precursor is shown in the top part and the nanoflake after the cyclodehydrogenation is shown in the bottom part. The same precursor is known from its use in fabrication of graphene nanoflakes by thermally induced cyclodehydrogenation on a metallic Au (111) surface, as described in R. Zuzak et al. Chemical Communication 2018.

[0085] The following technical devices were used to carry out the method: an ultra-high vacuum chamber with a base vacuum of approximately 1.Math.10.sup.10 mbar together with a resistive heater and a hydrogen cracker of the type as described in Tschersich, K. G., J. P. Fleischhauer, and H. Schuler. Design and characterization of a thermal hydrogen atom source. Journal of applied physics 104.3 (2008): 034908. In this type of cracker, ultrapure molecular hydrogen (99.99% H.sub.2) is thermally split into atomic hydrogen on a tungsten cathode whose temperature during operation is about 2500 C.

[0086] A substrate made of pure TiO.sub.2 (110) was prepared according to known surface cleaning procedures. Preparation of the surface of TiO.sub.2 (110) substrate, under UHV conditions, was carried out as follows: [0087] at first step, the surface of TiO.sub.2 (110) substrate was subjected to bombardment with argon ions (Ar.sup.+) using an ion gun; the argon pressure in the ion gun was set at 510.sup.7 mbar, the bombardment time was 10 min; [0088] next, the TiO.sub.2 crystal (110) was heated with alternating current to a temperature of about 770 C. for 10 min; the temperature was measured with a pyrometer; [0089] and at the final step, the TiO.sub.2 crystal (110) was slowly cooled (for approx. 30 min.) to room temperature; a low-temperature STM microscope was used to check the surface quality of the substrate; the process as described above (bombarding, heating, cooling), was repeated until the surface of atomically pure crystal (TiO.sub.2 (110)) was obtained.

[0090] Subsequently, the molecular precursors present on the surface of the substrate were subjected to dehydrogenation (and speaking more specifically, cyclodehydrogenation) with the following parameters: [0091] sample temperature during the process: 200-220 C. (the temperature was chosen experimentally to achieve essentially complete conversion of the precursor to nanographene); [0092] the time of carrying out the procedure was selected to 30 minutes (time of treatment necessary to achieve substantially complete conversion of the precursor to the nanographene, selected experimentally); [0093] partial pressure of the molecular hydrogen introduced into the cracker system: 1.Math.10.sup.7 mbar; [0094] experimentally estimated efficiency of molecular-to-atomic-hydrogen splitting was 10%.

[0095] The course of the cyclodehydrogenation reaction was verified in a low-temperature STM microscopeimages of the obtained nanoflakes on the substrate having atomically pure crystal surface of TiO.sub.2 (110), are shown in FIG. 4.

Third ExampleProduction of Graphene Nanoflakes, TiO.SUB.2 .(011) Substrate

[0096] The third example was carried out in a manner similar to the second example, except that a TiO.sub.2(011) substrate was used.

[0097] A substrate made of clean TiO.sub.2 (011) was prepared according to known surface cleaning procedures. Preparation of the surface of TiO.sub.2 (011) substrate, under UHV conditions, was carried out as follows: [0098] at first step, the surface of TiO.sub.2 (011) substrate was subjected to bombardment with argon ions (Ar.sup.+) using an ion gun; the argon pressure in the ion gun was set at 510.sup.7 mbar, the bombardment time was 10 min; [0099] next, the TiO.sub.2 crystal (011) was heated with alternating current to a temperature of about 770 C. for 10 min; the temperature was measured with a pyrometer; [0100] and at the final step, the TiO.sub.2 crystal (011) was slowly cooled (for approx. 30 min.) to room temperature; a low-temperature STM microscope was used to check the surface quality of the substrate; the process as described above (bombarding, heating, cooling), was repeated until the surface of atomically pure crystal (TiO.sub.2(011)) was obtained.

[0101] Subsequently, the molecular precursors present on the surface of the substrate were subjected to dehydrogenation (and speaking more specifically, cyclodehydrogenation) with the following parameters: [0102] sample temperature during the process: 200-220 C. (the temperature was chosen experimentally to achieve essentially complete conversion of the precursor to nanographene); [0103] the time of carrying out the procedure was selected to 30 minutes (time of treatment necessary to achieve substantially complete conversion of the precursor to the nanographene, selected experimentally); [0104] partial pressure of the molecular hydrogen introduced into the cracker system: 1.Math.10.sup.7 mbar; [0105] experimentally estimated efficiency of molecular-to-atomic-hydrogen splitting was 10%.

[0106] Images of the resulting nanoflakes on the TiO.sub.2(011) substrate are shown in FIG. 5. Since the obtained flakes are identical to those of the second example, this demonstrates that the method can be successfully applied to a variety of surfaces.

Fourth ExampleProduction of Graphene Nanoflakes, SiO.SUB.2./Si Interface

[0107] In this example, the substrate was made of a silicon bulk crystal covered by a layer of silicon oxide (300 nm thick) purchased from PI-KEM. This is a typical wafer that is used, for instance, in electronics industry. The oxide layer is an insulator, while the silicon bulk crystal is a semiconductor.

[0108] At first the molecular precursors were thermally evaporated onto the substrate. Subsequently, the molecular precursors present on the surface of the substrate were subjected to dehydrogenation (and speaking more specifically, cyclodehydrogenation) with the following parameters: [0109] sample temperature during the process: 200-220 C. (the temperature was chosen experimentally to achieve essentially complete conversion of the precursor to nanographene); [0110] the time of carrying out the procedure was selected to 30 minutes (time of treatment necessary to achieve substantially complete conversion of the precursor to the nanographene, selected experimentally); [0111] partial pressure of the molecular hydrogen introduced into the cracker system: 1.Math.10.sup.7 mbar; [0112] experimentally estimated efficiency of molecular-to-atomic-hydrogen splitting was 10%.

[0113] Since the used sample surface is non-conductive (insulating oxide layer), in this case measurements were carried out using TOF-SIMS (time of flightsecondary ion mass spectroscopy, as described for example at bups://sere.carleto ds/ToFSIMS.html). The spectra recorded for the sample are shown in FIG. 6. The nanographene flake obtained after successful dehydrogenation process are characterized by a mass of 816 m/z, while the molecular precursor by 828 m/z. TOF-SIMS measurements prove successful generation of C.sub.66H.sub.24 nanographenes by application of the atomic hydrogen at 200 C. on the SiO.sub.2/Si substrate.

[0114] The TOF-SIMS results for the generation of C.sub.66H.sub.24 nanographenes on the SiO.sub.2/Si interface by atomic hydrogen treatment are shown in FIG. 6. The lower panel shows the results after formation of nanographenes, i.e. both precursors and nanographenes are present on the surface; the upper panel shows data before treatment with atomic hydrogenonly precursors are present on the surface.

Fifth ExampleProduction of Graphene Nanoflakes, NaCl (001) Substrate

[0115] The crystal was cleaved and subsequently annealed at 300 C. to generate a fresh surface. Further the molecular precursors were thermally evaporated onto the substrate. Subsequently, the molecular precursors present on the surface of the substrate were subjected to dehydrogenation (and speaking more specifically, cyclodehydrogenation) with the following parameters: [0116] sample temperature during the process: 200 C. (the temperature was chosen experimentally to balance between the conversion rate from precursors into nanoflakes and the desorption from the substrate); [0117] the time of carrying out the procedure was selected to 30 minutes (time of treatment necessary to achieve substantially complete conversion of the precursor to the nanographene, selected experimentally); [0118] partial pressure of the molecular hydrogen introduced into the cracker system: 1.Math.10.sup.7 mbar; [0119] experimentally estimated efficiency of molecular-to-atomic-hydrogen splitting was 10%.

[0120] The cyclodehydrogenation was inspected using TOF-SIMS technique. The result is shown in FIG. 7. C.sub.66H.sub.24 nanographene synthesis was achieved here in a similar manner as for SiO.sub.2/Si. In general, the yield of the achieved transformation is lower compared to the SiO.sub.2/Si substrate because the cyclodehydrogenation proceeds in the same temperature range as the desorption of precursors from the substarte. This is because of the lower adsorption energy compared to SiO.sub.2/Si. Consequently, a large fraction of precursors desorbs from the surface limiting the transformation efficiency. Nevertheless, the TOF-SIMS measurements (FIG. 7, upper panel) doubtlessly prove formation of C.sub.66H.sub.24 nanoflakes on bulk NaCl.

Sixth ExampleProduction of Graphene Nanoflakes, NaCl/Cu (111) Substrate

[0121] Following the method described in the literature, a thin layer of salt was prepared on the top of copper (111) surface (Mishima, Ryota, Masaki Takada, and Hirokazu Tada. STM studies of NaCl thin films on Cu (111) surface at low temperature. Molecular Crystals and Liquid Crystals 472.1 (2007): 321-711). Further the molecular precursors were thermally evaporated onto the substrate. Subsequently, the molecular precursors present on the surface of the substrate were subjected to dehydrogenation (and speaking more specifically, cyclodehydrogenation) with the following parameters: [0122] sample temperature during the process: 200-220 C. (the temperature was chosen experimentally to achieve essentially complete conversion of the precursor to nanographene); [0123] the time of carrying out the procedure was selected to 30 minutes (time of treatment necessary to achieve substantially complete conversion of the precursor to the nanographene, selected experimentally); [0124] partial pressure of the molecular hydrogen introduced into the cracker system: 1.Math.10.sup.7 mbar; [0125] experimentally estimated efficiency of molecular-to-atomic-hydrogen splitting was 10%.

[0126] By the application of the atomic hydrogen at 200-220 C. the C.sub.66H.sub.24 nanographenes were generated from molecular precursors on NaCl/Cu (111). Successful synthesis has been doubtlessly verified by the high resolution STM imaging shown in FIG. 8. The ovals mark C.sub.66H.sub.24 graphene nanoflakes immobilized at the surface steps.

[0127] While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made. Therefore, the claimed invention as recited in the claims that follow is not limited to the embodiments described herein.