RU2772338C1 - Diamond-like films based on modified graphene - Google Patents

Abstract

FIELD: nanotechnology.

SUBSTANCE: invention relates to nanotechnology and can be used to produce wide-band films of nanometer thickness for optical devices, dielectric substrates, interlayers in supercapacitors and layered heterostructures. In diamond-like films based on modified graphene, graphene layers are rotated relative to each other and are connected by interlayer covalent bonds formed during hydrogenation or fluorination of graphene. The rotation angles of graphene layers relative to each other are in the range from 20 to 40°.

EFFECT: such films have a high hardness and an ultra-wide forbidden bandwidth, the width of which exceeds the width of the forbidden bandwidth for known non-twisted and twisted graphene-based structures.

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Description

The invention relates to the field of nanomaterials and can be used to obtain wide-gap films of nanometer thickness for use as elements of optical devices, dielectric substrates, interlayers in supercapacitors and layered heterostructures as mechanically strong ultrathin carbon films and in other fields of science and technology.

Diamond films, which have a wide range of applications in various fields of science and technology, are obtained only in the form of micron polycrystals or nanodiamonds [S. Mandal, Nucleation of diamond films on heterogeneous substrates: a review RSC Adv., 2021, 11, p. 10159-10182]. The need to reduce the film thickness is dictated by the further miniaturization of elements based on them using the unique properties of crystalline nanofilms. In recent years, research has been actively developing in the development of new nanofilm materials based on graphene - diamond-like films of nanometer thickness, the unique properties of which open up the possibility of their wide application in the field of nanophotonics [F. Piazza, et al. Progress on Diamane and Diamanoid Thin Film Pressureless Synthesis - C, 2021, 7, 9].

In [LA Chernozatonskii, et al. Diamond-like C ₂ H nanolayer, diamane: simulation of the structure and properties, JETP Lett. 90 (2009) p. 134-138] modeled and substantiated by calculations using modern quantum-chemical methods, the structure of C ₂ H based on bigraphene, called "diaman", formed during the adsorption of hydrogen atoms on two-layer graphene, in which, as in graphane, each carbon atom is sp ³ -hybridized in a tetragonal diamond configuration. In recent years, studies have been carried out on the creation of diamanes from two or more graphene layers not rotated relative to each other and subjected to complete hydrogenation [Piazza, F. et al. Low temperature, pressureless sp ² to sp ³ transformation of ultrathin, crystalline carbon films. Carbon 2019, 145, 10-22] or fluorination [Bakharev PV et al. Chemically induced transformation of chemical vapor deposition grown bilayer graphene into fluorinated single-layer diamond. Nat. Nanotechnol. 15 (2020) p. 59-66].

Increasing interest is attracted by the so-called. moiré diamonds, which are formed during the functionalization of several graphene layers twisted relative to each other. In the US patent [US 10562278 B2, publ. February 18, 2020], taken as a prototype, structures are described that include graphene layers twisted relative to each other at a fixed twist angle 9, ranging from 0° to 16° or from 44° to 60°, and connected to each other by interlayer covalent bonds, formed due to the chemical functionalization of graphene by hydrogenation or fluorination. Although the mechanical properties of such structures, called "moiré" or twisted structures, are superior to the initially non-functionalized non-twisted structures, they, nevertheless, are significantly inferior to diamond in these indicators. According to the data given in the work of the authors of the invention on the prototype [Machado et al. Tunable mechanical properties of diamond superlattices generated by interlayer bonding in twisted bilayer graphene Appl. Phys. Lett. 103, 013113 (2013)], the shear modulus for them does not exceed 250 GPa, while for diamond it is more than 400 GPa. In addition, the band gap (dielectric gap) for structures according to the prototype, as follows from the work of the authors [A.R. Muniz, Maroudas D. Opening and tuning of band gap by the formation of diamond superlattices in twisted bilayer graphene. Phys Rev B. 2012; 86(7): 75404], does not exceed 1.2 eV, which limits the possibility of their use in laser technology in the visible and UV ranges, in lighting engineering in computer technology for the manufacture of optical discs and full-color solid-state displays, as well as in other areas where wide-gap semiconductor materials.

Further in the text, the terms "twisted graphene", "twisting angles", "rotation angles" will be used as Russian-language equivalents to characterize the structures denoted by the English term "twisted".

The technical problem solved by the present invention is to provide diamond-like film structures based on twisted graphene, characterized by an ultra-wide band gap and high mechanical properties comparable to diamond.

The technical problem is solved by the proposed wide-gap nanoscale film material, which is a structure in which the graphene layers twisted relative to each other are connected by interlayer covalent carbon-carbon bonds formed during the hydrogenation or fluorination of graphene, and the twisting angles of the graphene layers relative to each other lie in the range from 20° up to 40°.

The technical result of the invention is that the claimed material is characterized by an ultra-wide dielectric gap (gap) exceeding the band gap for twisted prototype structures and known diamans based on non-twisted graphenes. Also, the material is characterized by high hardness, exceeding the hardness of diamonds based on non-twisted graphenes and prototype structures.

The essence of the invention is illustrated by the following graphic illustrations, in which the designation Dnθ corresponds to the twisted structure proposed in the invention, in which the angle of twisting of the graphene layers relative to each other is equal to θ:

On FIG. 1 schematically shows the atomic structure of structures according to the prototype and according to the invention (hydrogen atoms are indicated by white circles, carbon atoms of the upper layer are black, carbon atoms of the lower layer are gray).

a - Structure of hydrogenated bigraphene (prototype), twist angle θ=13.17°; top view (top), side view (bottom); the structure of its unit cell C ₇₆ H ₆ is highlighted in the rhombus.

b - The structure of the hydrogenated bigraphene according to the invention Dn21.8; top view (top), side view (bottom); the structure of the unit cell C ₂₈ H ₁₈ is highlighted in the rhombus. The inset schematically shows the mutual "cruciform" arrangement of C-C and C'-C' bonds belonging to the upper and lower layers, respectively. C _d -C' _d - interlayer bonds.

On FIG. 2 schematically shows the atomic structure of the films according to the invention:

a - Structure of hydrogenated bigraphene according to the invention Dn27.8 (θ=27.8°); top view (top), side view (bottom); in the rhombus, the structure of its unit cell of the C ₅₂ H ₃₀ superlattice is highlighted.

b - The structure of the hydrogenated bigraphene according to the invention Dn29.4 (θ=29.4°); top view (top), side view (bottom); the structure of its unit cell, the C ₃₈₈ H ₁₇₄ superlattice, is highlighted in the rhombus.

On FIG. 3 shows the electronic density of states (DOS spectra) of structures according to the invention. Forbidden zones E _g are highlighted in gray; (a, b, and c) hydrogenated structures Dn21.8 (E _g = 3.2 eV), Dn27.8 (E _g = 3.3 eV), and Dn29.4 (E _g = 3.6 eV), respectively; (d, e, and f) fluorinated structures F-Dn21.8 (E _g =4.2 eV), F-Dn27.8 (E _g =4.5 eV), and F-Dn29.4 (E _g =4.1 eV), respectively.

On FIG. Figure 4 shows the scheme of molecular dynamics simulation of the mechanical action of the probe on the disks of diamond-like films (the diameter of the disk fixed at the edges is 3 nm). a - structure according to the invention Dn29.4; b - "untwisted" diaman described in [LA Chernozatonskii, et. al. Diamond-like C ₂ H nanolayer, diamane: simulation of the structure and properties, JETP Lett. 90 (2009) p. 134-138] Above - the initial view of the disks in the absence of impact (impact force F=0); below - view of the disks after exposure to the force F=157 nN.

On FIG. Figure 5 shows the spectra of the Dn21.8 structure: a - Raman spectrum; b - IR spectrum. The dashed lines correspond to rare lines in the spectra of the "untwisted" diaman.

The essential difference between the proposed structures and the prototype, which ensures the achievement of a technical result, is the choice of twisting angles of the graphene layers relative to each other. Analysis of the materials given in [US 10562278 B2, publ. 02/18/2020], allows us to conclude that the spatial arrangement of carbon atoms belonging to neighboring layers of a multilayer graphene structure according to the prototype, in which the twisting angles θ correspond to the ranges of 0°-16° and 44°-60°, does not allow achieving full functionalization of carbon atoms during hydrogenation or fluorination. In the structures of the prototype (see Fig. 1a) provides a steric possibility of only partial formation of diamond-like areas in the film, which are surrounded by a graphene matrix, in which there is no complete functionalization of carbon atoms by hydrogen or fluorine. The structure of the C ₇₆ H ₆ unit cell for the prototype structure, for which the twisting angle θ=13.17° corresponds to the interval 0°-16°, is highlighted in the rhombus in FIG. 1a (top). Thus, the structures of the prototype include both areas with sp ³ hybridized carbon atoms - diamane domains, both in non-twisted diamans, and areas in which sp ² hybridized carbon atoms remain. As can be seen from the figure in Fig. 1a (bottom), the surfaces of such a bigraphene structure are only partially functionalized with light atoms. A similar picture is observed for structures with twist angles related to the symmetrical interval of 44°-60°. It is these features of the structures according to the prototype, due to the magnitude of the angle of twist from the declared in the prototype interval, that negatively affect the properties of the material: the resulting structures are characterized by relatively low rigidity in comparison with the untwisted diaman. [Machado et al. Tunable mechanical properties of diamond superlattices generated by interlayer bonding in twisted bilayer graphene Appl. Phys. Lett. 103, 013113 (2013)], and due to the presence of the graphene environment of nanodiamond regions, the size of the dielectric gap does not exceed 1.2 eV [AR Muniz, Maroudas D. Opening and tuning of band gap by the formation of diamond superlattices in twisted bilayer graphene . Phys Rev B. 2012; 86(7): 75404].

Unlike the prototype, in our proposed solution at twisting angles θ in the range from 20° to 40°, functionalization with light atoms gives rise to special moiré structures, in which the possibility of forming a film with a continuous diamond-like crystal structure opens up. They are characterized by such a mutual arrangement of carbon atoms, which ensures the possibility of their complete functionalization during hydrogenation or fluorination, as a result of which the carbon atoms located in all layers, initially covalently bonded into a hexagonal sp ² -hybridized structure, become sp ³ -hybridized, as in diamond . As an illustration, in FIG. 1b shows the structure of the hydrogenated bigraphene according to the invention Dn21.8 (θ=21.8°). As can be seen from the figure in Fig. 1b (top), at a given angle of twisting of the layers relative to each other, a moiré structure is formed containing complexes in which the C-C and C'-C' carbon bonds located in adjacent graphene layers are arranged crosswise one above the other (see inset on Fig. 1b). In this case, a superlattice arises, the unit cell of which C ₂₈ H ₁₈ (highlighted in a rhombus in Fig. 1b at the top) differs sharply from non-twisted or twisted, as in the prototype, structures in that it includes carbon-carbon bonds arranged crosswise almost at right angles, related to neighboring layers. In each such crosswise pair of carbon bonds located in adjacent layers, upon chemical modification with hydrogen or fluorine, an energetically favorable addition of a pair of hydrogen or fluorine atoms occurs, as occurs, for example, in graphane with a “boat” configuration [VI Artyukhov, LA Chernozatonskii, Structure and layer interaction in carbon monofluoride and graphane: a comparative computational study, J. Phys. Chem. A 114 (2010) 5389]. This leads to the movement of carbon atoms adjacent to the "cross", closely located one above the other in adjacent layers, and their connection - the formation of an interlayer C _d -C' _d bond, resulting in a diamond-like configuration. In this case, as shown in the figure (Fig. 1b below), the possibility of complete functionalization by light atoms of both surfaces of the rolled graphene structure is provided, in which a two-dimensional diamond-like film of nanometer thickness is formed, which does not contain graphene regions. The presence throughout the film of strained C _d -C' _d bonds between sp ³ hybridized carbon atoms located in adjacent layers provides, as will be shown below, high mechanical properties of the material, in which it surpasses the usual untwisted diamond, characterized by a rigidity comparable to diamond.

A similar picture is observed for other twisted structures according to the invention, in which the angle of twisting θ is in the ranges of 20°-30° and symmetrical to it in the range of 30°-40°. As examples in FIG. Figure 2 schematically shows the structure of the proposed structures Dn27.8 (Fig. 2a) Dn29.4 (Fig. 2b), in the unit cells of which there are also C-C and C'-C' carbon oriented crosswise at angles close to right -carbon bonds located in adjacent graphene layers. Similarly to the example described above for the structure Dn21.8, in these structures, upon functionalization with hydrogen or fluorine, strained interlayer C _d -C' _d bonds are also formed between sp ³ hybridized, located in different layers, neighboring carbon atoms, causing the formation of nanofilm materials with completely functionalized top and bottom surfaces (see Figs. 2a and 2b below).

Depending on the specific value of the twisting angle θ, the mechanical characteristics of the obtained materials may differ from each other, but in any case, due to the diamond-like structure, they are superior in terms of the corresponding indicators of the prototype structure.

Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Computational Materials Science. 6 (1996) 15-50. G. Kresse, J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B. 47 (1993) 558-561. G. Kresse, J. Hafner, Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium, Phys. Rev. B. 49 (1994) p. 14251-14269]. Результаты обобщены в таблице.The described features of the structure of the proposed diamond-like structures have a significant impact on the electronic and optical properties of materials. On FIG. 3 compares the electronic density of states spectra (DOS spectra) of structures according to the invention. Calculations of atomic structures and electronic spectra (dielectric gap) are carried out using quantum chemical methods within the framework of the VASP program [G. Kresse, J.

Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Computational Materials Science. 6 (1996) 15-50. G. Kresse, J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B. 47 (1993) 558-561. G. Kresse, J. Hafner, Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium, Phys. Rev. B. 49 (1994) p. 14251-14269]. The results are summarized in the table.

Thus, the structures according to the invention are characterized by an ultra-wide band gap, close to the band gap of diamond, and significantly exceed the prototype in this indicator.

The mechanical properties of the proposed materials are evaluated using molecular dynamics simulation of the mechanical impact on the material as described in [C. Lee, X. Wei, JW Kysar, J. Hone, Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene, Science. 321 (2008) 385-388]. On FIG. Figure 4 shows, as an example, the results of the mechanical action of the probe on the disks of diamond-like films (the diameter of the disk fixed along the edges is 3 nm): a - Dn29.4 structure according to the invention; b - "untwisted" diaman described in [LA Chernozatonskii, et al. Diamond-like C ₂ H nanolayer, diamane: simulation of the structure and properties, JETP Lett. 90 (2009) 134-138]. Above - the initial view of the disks in the absence of impact (impact force F=0); below - view of the disks after exposure to the force F=157 nN. As can be seen from the figure, while the “untwisted” diamond begins to collapse under the probe at a critical penetration depth δ _crit = 0.43 nm, the moiré diamond according to the invention Dn29.4 only bends under the same impact to a depth of δ = 0.34 nm without violating integrity films. Moiré diamonds lose their integrity with more applied force than an untwisted diamond. The prototype structures are inferior to the non-twisted diamond in terms of rigidity [US 10562278 B2, publ. 02/18/2020], directly related to the strength of the material. Thus, the mechanical properties of the proposed structures are superior to the prototype and untwisted diaman.

The proposed twisted structures can be obtained by known methods, for example, in the same way as described in the patent [US 10821709 B2, publ. November 03, 2020], with the difference that lies in the provision of relative angles of rotation (twisting) of graphene monolayers in the range from 20 to 40°.

The most accurately proposed structures can be identified using IR and Raman spectra. As an example, in FIG. 5 shows the Raman spectrum (Fig. 5a) and IR spectrum (Fig. 5b) of the Dn21.8 structure according to the invention, which clearly differ from similar spectra of the "untwisted" diaman, shown in the same figures as rare discontinuous bands. The most active Raman frequencies of these moiré diamond-like structures have a blue shift relative to untwisted diamonds.

Due to the combination of high mechanical strength and wide-gap electronic structure, the proposed diamond-like nanofilm materials have a high potential for use as elements of optical and optoelectronic devices, ultrasensitive sensors, dielectric substrates in biomedical applications, interlayers in layered heterostructures, as mechanically strong transparent ultrathin carbon films. and in other areas of science and technology.

Claims (1)

Diamond-like films based on modified graphene, in which the graphene layers are rotated relative to each other and are connected to each other by interlayer covalent bonds formed during the hydrogenation or fluorination of graphene, characterized in that the angles of rotation of the graphene layers relative to each other are in the range from 20 to 40° .

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