Method for preparing nanodiamonds

Abstract

The present invention relates to a process for preparing diamond particles, comprising subjecting a nanodiamond precursor material to a pressure of at least 8 GPa and a temperature of at least 900°C, wherein the diamond precursor material comprises a polycyclic aromatic compound which contains from 10 to 200 fused six-membered aromatic rings.

Description

Method for preparing nanodiamondsThe present invention relates to a method for preparing nanodiamonds.Nanodiamonds (NDs) have excellent mechanical and optical properties, large surface area, easy bioconjugation and high biocompatibility, which makes them appealing for various biomedical applications from drug delivery to diagnostics, but also for spintronic and photonic applications.By introducing non-carbon defect atoms (such as N, Si, Ge, B, Sn, etc.) and vacancy centers into the nanodiamond lattice, color centers (e.g. nitrogen -vacancy (NV) center, silicon-vacancy (SiV) center) are formed that provide several unique properties such as emission and magneto-optical features. The heteroatom (e.g. nitrogen) vacancy center is a point defect consisting of a substitutional heteroatom such as nitrogen and a nearby vacancy revealing stable fluorescence without photobleaching or photoblinking, far-red emission, long lifetime, and high quantum efficiency. Consequently, nanodiamonds with these defects have attracted much attention for bioimaging applications. Boron-doped nanodiamonds might be used as electrodes for various applications in electrochemistry (see e.g. Y. Einaga, Acc. Chem. Res., 2022, 55, 24, pp. 3605-3615).Commercially available nanodiamonds are typically prepared by a so-called detonation method (“detonation nanodiamonds”) or by a high pressure high temperature (HPHT) method (“HPHT nanodiamonds”).According to the detonation method, explosives like TNT detonate in a closed chamber, resulting in very high temperatures and pressures inside the reaction vessel. As a result, nanodiamonds are formed in larger clusters with sizes down to 5 nm of a single nanodiamond. The main disadvantage of this technique is the formation of graphitic shells and soot-like structures (= 25 wt%) which surround the nanodiamond particlesand the less crystalline structure of the nanodiamonds, which may result in less stable color centers.EP 3 950 586 A1 discloses heteroatom-doped nanodiamonds having a BET specific surface area of from 20 to 900 m2 / g, and an average size of primary particles of from 2 to 70 nm.EP 3 950 109 A1 discloses a method for producing a nanodiamond doped with a Group 14 element, the method comprising (i) detonating by exploding an explosive composition containing at least one explosive and at least one Group 14 element compound in a sealed container to obtain a nanodiamond doped with at least one Group 14 element selected from the group consisting of Si, Ge, Sn, and Pb; and (ii) subjecting the nanodiamonds doped with the Group 14 element to an alkali treatment to remove the Group 14 element and / or oxide thereof.According to the high-pressure high-temperature (HPHT) method, precursor materials are exposed to pressures of at least 7 GPa and temperatures up to 2200 °C, optionally in the presence of metal catalysts (e.g. Fe / Ni). Typically, this approach yields microdiamonds, which are further processed by high energy ball milling to break the microdiamonds to nanodiamonds. As a result of the ball milling post-treatment for reducing diamond particle size, the ground carbon material may have a high content of metal impurities which in turn may adversely affect its performance in the intended applications. The high energy impact during the ball milling treatment may promote the formation of a non-diamond layer on the particle surface. Furthermore, ball-milled nanodiamonds exhibit non-uniform shapes with sharp edges which make them less suitable for biomedical applications or quantum sensing in cells. Additionally, ball-milling of micron-sized fluorescence nanodiamonds results in nanodiamonds with non-uniform photoluminescence properties and non-uniform distribution of color centers.Reviews on properties, applications and preparation methods of nanodiamonds (both doped and non-doped) are provided by the following publications:Xi-Gui Yang et at., “Nanodiamonds: Synthesis, properties and applications in nanomedicine’’ , Materials & Design, 2021 , 210, 110091 ;T. Weil, Y. Wu, “Recent Developments of Nanodiamond Quantum Sensors for Biological Applications’’ , Adv. Sci., 2022, 9, 2200059;E.K.-H. Chow et al., “Nanodiamonds: The intersection of nanotechnology, drug development, and personalized medicine" , Sci. Adv. 2015; 1 :e1500439;E.K.-H. Chow et al, “Biomedical applications of nanodiamonds: From drugdelivery to diagnostics" , SLAS Technology 28 (2023) 214-222;A. Miotello et al., “Nanodiamonds: Synthesis and Application in Sensing, Catalysis, and the Possible Connection with Some Processes Occurring in Space"-, Appl. Sci., 2020, 10, 4094.As mentioned above, high pressure high temperature (HPHT) methods for preparing diamonds are typically carried out in the presence of metal catalyst, thereby resulting in diamonds having a high level of metal contaminants which need to be removed by a laborious post-treatment.However, it is also known that diamond-containing materials might be prepared by HPHT methods in the absence of metal catalysts.V.A. Davydov et al., “Conversion of polycyclic aromatic hydrocarbons to graphite and diamond at high pressures", Carbon, 2005, 42, pp.261 -269, describe a preparation method wherein the polycyclic aromatic hydrocarbons naphthalene, anthracene, pentacene, perylene, and coronene were submitted to temperatures up to 1500 °C at 8 GPa. Diamonds in the form of single crystals having a crystal size of 5 to 40 pm are obtained.Xi-Gui Yang et al., “Two-step high-pressure high-temperature synthesis of nanodiamonds from naphthalene", Chin. Phys. B Vol. 29, No. 10 (2020) 108102, describe a two-step HPHT method for synthesizing nanodiamonds from a naphthalene precursor. This method includes a carbonization step at 11 .5 GPa and 700 °C and a subsequent diamondation step at 11 .5 GPa and 1700 °C, and results in diamond particles having an average size of 62 nm to 457 nm.WO 2018 / 122321 A1 describes a method of forming polycrystalline diamond, comprising (i) placing a plurality of graphene nano-platelets into a capsule; and (ii) subjecting the platelets to a pressure of around 10 GPa to around 20 GPa and a temperature of around 1600 °C to around 3000 °C to convert the graphene platelets to nano-polycrystalline diamond. The X-Y (i.e. in-plane) dimension of the graphene nano-platelets used as diamond precursors ranges from 20 nm to 25000 nm. The polycrystalline diamondobtained by said process is a mass of sintered diamond grains being directly interbonded with each other.For some applications, in particular biological applications, it is preferred or even necessary to use nanodiamond particles having an average particle size of less than 100 nm, sometimes less than 20 nm or even less than 10 nm sub-10 nm NDs"). Small sized (less than 100 nm, sometimes even less than 10 nm) and round shaped NDs are in particular critical for those biological applications which require the NDs to pass through the cell membrane.As mentioned above, nanodiamonds prepared via HPHT methods typically have an average particle size well above 10 nm. For providing nanodiamonds having an average particle size of less than 10 nm, the HPHT micro- and nanodiamonds might be subjected to a ball milling post-treatment which, however, is laborious and may result in nanodiamonds having a high level of metal contaminants or promote the formation of a non-diamond structure on the particle surface.S. Stehlik et al., “Size and Purity Control of HPHT Nanodiamonds down to 1 nm", J. Phys. Chem. C, 2015, 119, 27708-27720, describe a process wherein HPHT nanodiamonds are subjected to a thermal treatment at 450°C and subsequently centrifuged so as to separate a nanodiamond fraction having an average particle size of less than 10 nm.A non-HPHT process for preparing nanodiamonds is described by R.S. Ruoff et al., “Sub-4 nm Nanodiamonds from Graphene Oxide and Nitrated Polycyclic Aromatic Hydrocarbons at 423 K’, ACS Nano, 2021 , 15, 11 , 17392-17400. Graphene oxide and nitrated naphthalene, anthracene, phenanthrene, and pyrene are subjected to a hydrothermal treatment at 423 K and transformed into nanodiamonds having diameters of less than 4 nm. The nanoparticles have a structure in which the outer layer is made of amorphous carbon. In addition, the nanoparticles are themselves embedded in a significant amount of amorphous carbon, originating mainly from residues. The residues include amorphous carbon, nitric salts, and precursors that have been modified during the hydrothermal process.An object of the present invention is to provide diamonds of defined particle shape and narrow particle size distribution. The diamonds should be obtainable at high yield andhigh purity (e.g. low amount of amorphous or graphitic carbon) and have a small particle size (e.g. an average particle size of less than 100 nm or even less than 20 nm).The object is solved by a process for preparing diamond particles, comprising subjecting a diamond precursor material to a pressure of at least 8 GPa and a temperature of at least 900°C, wherein the diamond precursor material comprises a polycyclic aromatic compound which contains from 10 to 200 fused six-membered aromatic rings.As known to the skilled person, a fused aromatic ring is an aromatic ring that shares with a neighbouring aromatic ring two of its ring atoms and one bond (wherein the shared ring atoms are adjacent ring atoms and the shared bond connects the shared ring atoms). In principle, a six-membered aromatic ring can be fused to up to six other aromatic rings which, in turn, can be fused to further aromatic rings.The polycyclic aromatic compound used in the HPHT process of the present invention as a diamond precursor material can be transformed to diamond particles having an average particle size of less than 100 nm, more preferably less 50 nm, even more preferably less than 20 nm or even less than 10 nm and optionally having a narrow particle size distribution without using a metal catalyst. Furthermore, as the diamond particles obtained by the HPHT treatment already have an average particle size of less than 100 nm or even less than 20 nm, no ball-milling post treatment is necessary.Accordingly, apart from having an average particle size of less than 100 nm, more preferably less than 50 nm, even more preferably less than 20 nm or even less than 10 nm (which is needed for some applications of nanodiamonds) and optionally having a narrow particle size distribution, the diamond particles obtainable by the process of the present invention may have a very low amount of or even no metal contaminants.Furthermore, although no metal catalyst is used in the process of the present invention, the diamonds are obtained at high yield whereas the amount of non-diamond carbon (such as graphitic carbon) is very low.As mentioned above, the polycyclic aromatic compound used as a diamond precursor material contains from 10 to 200 fused six-membered aromatic rings. Accordingly, each of these fused six-membered aromatic rings shares with a neighbouring aromatic ring towhich it is fused two ring atoms and one bond (wherein the shared ring atoms are adjacent ring atoms and the shared bond connects the shared ring atoms).Such polycyclic aromatic compounds are known to the skilled person and are commercially available or can be prepared by commonly known synthesis methods which are described e.g. in the following publications:K. Mullen et al., “From Nanographene and Graphene Nanoribbons to Graphene Sheets: Chemical Synthesis, Angew. Chem. Int. Ed., 2012, 51 , 7640-7654;K. Mullen et al., “Graphene as Potential Material for Electronics", Chem. Rev., 2007, 107, 718-747;K. Mullen et al., “New advances in nanographene chemistry", Chem. Soc. Rev., 2015, 44, 6616-6643;K. Mullen et al., “Large polycyclic aromatic hydrocarbons: Synthesis and discotic organization” , Pure Appl. Chem., Vol. 81 , No. 12, pp. 2203-2224, 2009;A. Tran-Van, H.A. Wegner, “Strategies in Organic Synthesis for Condensed Arenes, Coronene, and Graphene", Top. Curr. Chem., 2014, 349, 121 -158.In the following, a six-membered aromatic ring might also be referred to as a “sixfold fused” or “fully fused” six-membered aromatic ring if being fused to six other aromatic rings; and a “partially fused” six-membered aromatic ring if being fused to less than six other aromatic rings.A ring atom of one of the fused six-membered aromatic rings might be referred to as a “non-shared” ring atom RA-1 if not being shared with another fused aromatic ring; a “twofold-shared” ring atom RA-2 if being shared with exactly one other fused aromatic ring; a “threefold-shared” ring atom RA-3 if being shared with exactly two other fused aromatic rings.A non-shared ring atom and a twofold-shared ring atom might also be referred to as edge-located ring atoms as they are located on an edge of the polycyclic aromatic compound. A threefold-shared ring atom might also be referred to as an internal ring atom as it is located in the internal part of the polycyclic aromatic compound.Non-shared, twofold-shared and threefold-shared ring atoms are illustrated by the following Formula (i) which is a cutout from a larger polycyclic aromatic compound:According to an exemplary embodiment, the polycyclic aromatic compound may contain from 10 to 100, more preferably 12 to 55 fused six-membered aromatic rings.Optionally, the polycyclic aromatic compound may contain, in addition to the fused sixmembered aromatic rings, fused five-membered aromatic rings.In an exemplary embodiment, the polycyclic aromatic compound does not contain any fused aromatic rings other than the six-membered aromatic rings.The polycyclic aromatic compound may consist of carbon atoms and hydrogen atoms. Alternatively, the polycyclic aromatic compound may additionally contain one or more heteroatoms (such as nitrogen, boron, silicon, germanium or tin or a combination of at least two of these heteroatoms).Each of the fused six-membered aromatic rings of the polycyclic aromatic compound can be a ring containing no ring atoms other than carbon (aromatic all-carbon ring) or a heteroaromatic ring comprising at least one, preferably one or two heteroatom (i.e. noncarbon) ring atoms e.g. nitrogen or boron atoms.According to an exemplary embodiment, at least one of the fused six-membered aromatic rings of the polycyclic aromatic compound might be a six-membered heteroaromatic ring. The one or more six-membered heteroaromatic rings may comprise at least one, preferably one or two heteroatom (i.e. non-carbon) ring atoms. Preferably, the heteroatom is nitrogen or boron, more preferably nitrogen.If one or more heteroaromatic rings are present in the polycyclic aromatic compound, non-carbon defect atoms (such as N or B) can be introduced into the diamond lattice, thereby obtaining diamond particles which comprise color centers.Alternatively, none of the fused six-membered aromatic rings may comprise a ring atom other than carbon.Preferably, at least one of the fused six-membered aromatic rings is a fully fused aromatic ring (i.e. an aromatic ring being fused to six other aromatic rings). More preferably, two or more of the fused six-membered aromatic rings are fully fused aromatic rings.According to an exemplary embodiment, the polycyclic aromatic compound complies with the following requirement:N(RA-2) + N(RA-3) > N(RA-1 ) whereinN(RA-2) is the total number of the twofold-shared ring atoms of the fused six-membered aromatic rings,N(RA-3) is the total number of the threefold-shared ring atoms of the fused six-membered aromatic rings,N(RA-1 ) is the total number of the non-shared ring atoms of the fused six-membered aromatic rings.According to an exemplary embodiment, the polycyclic aromatic compound complies with the following requirement:N(RA-3) > N(RA-2) wherein N(RA-3) and N(RA-2) have the same meaning as outlined above.Preferably, the following requirement is met: N(RA-3) > N(RA-2)In an exemplary embodiment, the polycyclic aromatic compound complies with the following requirements:N(RA-2) + N(RA-3) > N(RA-1 ); andN(RA-3) > N(RA-2)Preferably, the following requirements are met:N(RA-2) + N(RA-3) > N(RA-1 ); andN(RA-3) > N(RA-2); andAccording to an exemplary embodiment, at least one of the fused six-membered aromatic rings is a substituted aromatic ring to which at least one substituent Rsis attached, wherein the substituent Rsis selected from a C1-20 alkyl group (e.g. C1-4 alkyl); an aryl (e.g. phenyl) group; an amine group; a carboxylic acid group (-COOH) or a salt or an ester (e.g. a C1-4 alkyl ester) thereof; a silyl group; a germanyl group; and a boron- containing group.The amine group might have the following formula:-N(R1)(R2) wherein R1and R2are, independently from each other, hydrogen, C1-4 alkyl or aryl (e.g. phenyl).The silyl group might have the following formula: -Si(R1)x(OR2)ywherein x is 0, 1 , 2 or 3; y is 0, 1 , 2, or 3, under the provision that x + y = 3;R1and R2are, independently from each other, C1-4 alkyl or aryl (e.g. phenyl).According to an exemplary embodiment, the silyl group might be -Si(R1)s (i.e. x=3; y=0).The germanyl group might have the following formula: -Ge(R1)x(OR2)ywherein x is 0, 1 , 2 or 3; y is 0, 1 , 2, or 3, under the provision that x + y = 3;R1and R2are, independently from each other, C1-4 alkyl or aryl (e.g. phenyl).According to an exemplary embodiment, the germanyl group might be -Ge(R1)s (i.e. x=3; y=0).The boron-containing group might be a boronic acid group or a boronic acid ester group.If the one or more substituents Rsare heteroatom-containing substituents, non-carbon atoms can be introduced into the diamond lattice, thereby obtaining heteroatom-doped diamond particles.Furthermore, the presence of substituents Rsmay improve solubility of the polycyclic aromatic compound in solvents and thereby assist in preparing a homogeneous mixture of the polycyclic aromatic compound with a second compound. Accordingly, if the diamond precursor material comprises a mixture of the polycyclic aromatic compound and a second compound (e.g. a mixture of a heteroatom-free polycyclic aromatic compound and a heteroatom-containing compound), both compounds can be dissolved in an appropriate solvent, followed by evaporating the solvent, thereby obtaining a very homogeneous powder mixture of these compounds which is subsequently subjected to the high-pressure high-temperature treatment.Alternatively, according to an exemplary embodiment, none of the fused six-membered aromatic rings of the polycyclic aromatic compound is a substituted aromatic ring.The polycyclic aromatic compound may have one of the following Formulas (I) to (XVI):- £1 -wherein in each of the Formulas (I) to (XVI) the residues R are, independently from each other, hydrogen or a substituent Rs, wherein in Formula (II) the ring atoms X are, independently from each other, are C or N.As mentioned above, the subsituent Rsmight be selected from a C1-20 alkyl group (e.g. C1-4 alkyl); an aryl (e.g. phenyl) group; an amine group; a carboxylic acid group (-COOH) or a salt or an ester (e.g. a C1-4 alkyl ester) thereof; a silyl group; a germanyl group; and a boron-containing group.Exemplary amine, silyl, germanyl and boron-containing groups include those mentioned above.According to an exemplary embodiment, the residues R are, independently from each other, hydrogen or a C1-20 alkyl group (e.g. C1-4 alkyl).If the polycyclic aromatic compound comprises an edge part having one of the following Formulas (a) and (b)it might be preferred that the adjacent groups R in Formulas (a) and (b) are hydrogen. The dotted lines in Formulas (a) and (b) represent bonds to other ring atoms of the fused six-membered aromatic rings.An exemplary polycyclic aromatic compound of Formula (II) may have the following Formula (Ila):wherein the residues R are, independently from each other, hydrogen or the above-mentioned substituent Rs, the ring atoms X are, independently from each other, C or N.An exemplary polycyclic aromatic compound of Formula (III) may have the following Formula (Illa):wherein the residues R are, independently from each other, hydrogen or the above-mentioned substituent Rs.An exemplary polycyclic aromatic compound of Formula (VII) may have the followingFormula (Vila):wherein the residues R are, independently from each other, hydrogen or the above-mentioned substituent Rs.An exemplary polycyclic aromatic compound of Formula (XI) may have the following Formula (Xia):wherein the residues R are, independently from each other, hydrogen or the above-mentioned substituent Rs.An exemplary polycyclic aromatic compound of Formula (XII) may have the followingFormula (Xlla):wherein the residues R are, independently from each other, hydrogen or the above-mentioned substituent Rs.According to an exemplary embodiment, the residues R are, independently from each other, hydrogen or a C1-20 alkyl group (e.g. C1-4 alkyl) in each of the Formulas (Ila), (Illa), (Vila), (Xia) and (Xlla).The diamond precursor material may comprise just one of the polycyclic aromatic compounds described above or may comprise two or more of the polycyclic aromatic compounds described above. Just as an example, if the diamond precursor material comprises at least two of the polycyclic aromatic compounds described above, one of these might be a polycyclic aromatic compound consisting of carbon and hydrogen atoms, whereas the other might be a polycyclic aromatic compound wherein at least one of the fused six-membered aromatic rings is a six-membered heteroaromatic ring or at least one of the fused six-membered aromatic rings is a substituted aromatic ring to which at least one silyl group, germanyl group or boron-containing group is attached.The polycyclic aromatic compound used in the process of the present invention is preferably not a carbon nanotube or a fullerene.In addition to the polycyclic aromatic compound, the diamond precursor material may comprise one or more heteroatom-containing dopant compounds.The heteroatom-containing dopant compound may assist in introducing heteroatoms into the diamond particles.The heteroatom might be an element of Group 13 (e.g. boron), Group 14 (e.g. Si, Ge, Sn or Pb) or Group 15 (e.g. N or P) of the Periodic Table or a transition metal atom (e.g. Ni, Ti or Co).The heteroatom might be selected from nitrogen, silicon, germanium, tin, lead, boron, phosphorus, and nickel. The heteroatom is preferably selected from nitrogen, silicon, germanium, boron, and tin.The at least one heteroatom-containing dopant compound, if present, differs from the polycyclic aromatic compound. As an example, the polycyclic aromatic compound may comprise no atoms other than carbon and hydrogen. Alternatively, the polycyclic aromatic compound may comprise one or more heteroatoms (preferably selected fromnitrogen, silicon, germanium, tin, lead, boron, phosphorus, and nickel; more preferably nitrogen, silicon, germanium, boron, and tin) which are different from the one or more heteroatoms being present in the heteroatom-containing dopant compound.Preferably, the heteroatom-containing dopant compound is a solid at a temperature of 25°C and atmospheric pressure (101 .325 kPa).If the dopant compound is a nitrogen-containing dopant compound, it might be an amine or a nitrogen-containing polycyclic aromatic compound comprising 2 to 4 (e.g. 3) fused aromatic rings, wherein at least one of the fused aromatic rings comprises nitrogen as a ring atom and / or is substituted with a cyano (i.e. -CN) group. The amine might be of the following Formula (1 )N(R1)(R2)(R3) (1 ) wherein R1, R2and R3are, independently from each other, selected from aryl (e.g. phenyl) and C1-20 alkyl. An exemplary amine is triphenylamine. Exemplary nitrogencontaining polycyclic aromatic compounds are acridine and a cyanoanthracene.If the dopant compound is a Si-containing dopant compound, it might be an organosilane compound, such as a compound of the following Formula (2) Si(R1)(R2)(R3)(R4) (2) wherein R1, R2, R3and R4are, independently from each other, selected from aryl (e.g. phenyl) and C1-20 alkyl. A preferred Si-containing dopant compound is tetraphenyl silane (SiPhu). Another preferred Si-containing dopant compound is tetrakis(trimethylsilyl)silane. Other exemplary Si-containing dopant compounds, in particular silanes, that might be used in the process of the present invention are those mentioned in paragraph

[0057] of EP 3 950 586 A1 .If the dopant compound is a Ge-containing dopant compound, it might be an organogermanium compound, such as a compound of the following Formula (3) Ge(R1)(R2)(R3)(R4) (3) wherein R1, R2, R3and R4are, independently from each other, selected from aryl (e.g. phenyl) and C1-20 alkyl. A preferred Ge-containing dopant compound is germaniumtetraphenyl (GePhu). Other exemplary Ge-containing dopant compounds that might be used in the process of the present invention are those mentioned in paragraph

[0064] of EP 3 950 586 A1 .If the dopant compound is a Sn-containing dopant compound, it might be an organotin compound, such as a compound of the following Formula (2)Sn(R1)(R2)(R3)(R4) (4) wherein R1, R2, R3and R4are, independently from each other, selected from aryl (e.g. phenyl) and C1-20 alkyl. A preferred Sn-containing dopant compound is tetraphenyltin (SnPhu). Other exemplary Sn-containing dopant compounds, in particular organotin compounds that might be used in the process of the present invention are those mentioned in paragraph

[0065] of EP 3 950 586 A1 .If the dopant compound is a Pb-containing dopant compound, it might be an organolead compound, such as a compound of the following Formula (5)Pb(R1)(R2)(R3)(R4) (5) wherein R1, R2, R3and R4are, independently from each other, selected from aryl (e.g. phenyl) and C1-20 alkyl. A preferred Pb-containing dopant compound is tetraphenyl lead (PbPhu). Other exemplary Sn-containing dopant compounds, in particular organolead compounds that might be used in the process of the present invention are those mentioned in paragraph

[0076] of EP 3 950 586 A1 .Exemplary boron-containing dopant compounds, in particular organic boron-containing compounds, that might be used in the process of the present invention are those mentioned in paragraphs

[0060] and

[0061] of EP 3 950 586 A1 .Exemplary phosphorus-containing dopant compounds, in particular organic phosphorus- containing compounds, that might be used in the process of the present invention are those mentioned in paragraphs

[0062] and

[0063] of EP 3 950 586 A1 .The diamond precursor material may comprise just one of the heteroatom-containing dopant compounds described above or may comprise two or more of the heteroatom - containing dopant compounds described above. Just as an example, if the diamond precursor material comprises at least two of the heteroatom-containing dopant compounds described above, they may contain different heteroatoms, thereby introducing at least two different heteroatoms into the diamond particles.If the diamond precursor material comprises both the polycyclic aromatic compound and the heteroatom-containing dopant compound, the molar ratio of the polycyclic aromatic compound to the heteroatom-containing dopant compound might be in the range of500:1 to 1 :10. However, depending on the intended application of the heteroatom-doped nanodiamonds, a ratio of more than 500:1 or less than 1 :10 might be used as well.If the diamond precursor material comprises both the polycyclic aromatic compound and the heteroatom-containing dopant compound, it is preferred to provide an intimate mixture of these compounds.The diamond precursor material might be prepared by dissolving or dispersing the polycyclic aromatic compound and the heteroatom-containing dopant compound in a liquid medium, followed by evaporating the liquid medium, thereby obtaining a mixture of the polycyclic aromatic compound and the heteroatom-containing dopant compound.According to an exemplary embodiment, the diamond precursor material comprises compounds other than the polycyclic aromatic compound and the optional heteroatomcontaining dopant described above in a total amount of less than 3 wt%, more preferably less than 1 wt%. In a preferred embodiment, the diamond precursor material does not comprise compounds other than the polycyclic aromatic compound and optionally the heteroatom-containing dopant described above. Accordingly, the diamond precursor material may comprise the one or more polycyclic aromatic compounds and, if present, the one or more heteroatom-containing dopants described above in a total amount of at least 97 wt%, more preferably at least 99 wt%. Preferably, the diamond precursor material consists of the one or more polycyclic aromatic compounds and, if present, the one or more heteroatom-containing dopants.As indicated above, the diamond precursor material is subjected to a pressure of at least 8 GPa and a temperature of at least 900°C.Typically, said treatment of the diamond precursor material is carried out in a press cell.Press cells for HPHT processes at a pressure of at least 8 GPa and a temperature of at least 900°C are known to the skilled person.The process might be carried out in a multi-anvil press. The multi-anvil press may comprise four or more anvils. Multi-anvil presses are well known to the skilled person, see e.g. R.C. Liebermann, “Multi-anvil, high pressure apparatus: a half-century of development and progress" , High Pressure Research, Vol. 31 , No. 4, 2011 , pp. 493-532.The diamond precursor material might be introduced into the press cell such as a multianvil press (e.g. at atmospheric or subatmospheric pressure), followed by raising temperature and pressure inside the press cell until the diamond precursor material is subjected to a pressure of at least 8 GPa and a temperature of at least 900°C. Just as an example, after having introduced the diamond precursor material into the press cell, the pressure inside the press cell might be raised to at least 8 GPa, followed by raising the temperature inside the press cell to at least 900°C.Preferably, any non-gaseous material being introduced into the press cell comprises the diamond precursor material in an amount of at least 97 wt%, more preferably at least 99 wt%. In a preferred embodiment, any non-gaseous material being introduced into the press cell consists of the diamond precursor material. The term “non-gaseous material” relates to a material which is in a solid or liquid state at 25°C and 101 .325 kPa.In a preferred embodiment, any non-gaseous material being introduced into the press cell comprises the diamond precursor material described above in an amount of at least 97 wt%, and the diamond precursor material comprises compounds other than the polycyclic aromatic compound and the optional heteroatom-containing dopant described above in a total amount of less than 3 wt%. More preferably, any non-gaseous material being introduced into the press cell comprises the diamond precursor material described above in an amount of at least 99 wt%, and the diamond precursor material comprises compounds other than the polycyclic aromatic compound and the optional heteroatomcontaining dopant described above in a total amount of less than 1 wt%. Even more preferably, any non-gaseous material being introduced into the press cell consists of the diamond precursor material described above, and the diamond precursor material does not comprise compounds other than the polycyclic aromatic compound and optionally the heteroatom-containing dopant described above.According to an exemplary embodiment, the pressure is at least 10 GPa and the temperature is at least 1100°C.The diamond precursor material might be subjected to a pressure of at least 8 GPa, more preferably at least 10 GPa and subsequently heated at said pressure to a temperature of at least 900°C, more preferably at least 1100°C.The time needed for transforming the diamond precursor material into diamond particles can be determined by routine experiments.Optionally, the diamond particles might be post-treated by a liquid or gaseous posttreatment medium containing one or more oxidants. Such post-treatment media are known to the skilled person and assist in removing non-diamond residues (e.g. graphitic or amorphous carbon residues).Typically, for post-treatment, the diamond particles are transferred from the press cell to a post-treatment container.An exemplary liquid post-treatment medium can be an aqueous composition comprising HNO3, H2SO4 and optionally HCIO4. The post-treatment might be carried out at a temperature of 70-100°C. However, higher or lower post-treatment temperatures might be used as well.The gaseous post-treatment medium might be an oxygen-containing gas (e.g. air). The post-treatment might be carried out at a temperature of 400-600°C. However, higher or lower post-treatment temperatures might be used as well.Optionally, the process may include a post-treatment of the diamond particles by sonication.The diamond particles obtained by the process of the present invention are preferably nanodiamond particles and may have an average particle size, based on a particle size number distribution, of less than 100 nm, more preferably less than 50 nm, even more preferably less than 20 nm or even less than 10 nm. The average particle size might be determined by transmission electron microscopy according to ISO 21363:2020.According to an exemplary embodiment, the diamond particles obtained by the process of the present invention are non-doped (i.e. heteroatom-free) diamond particles havingan average particle size, based on a particle size number distribution, of less than 20 nm, more preferably less than 10 nm.According to another exemplary embodiment, the diamond particles obtained by the process of the present invention are doped (i.e. heteroatom-containing) diamond particles having an average particle size, based on a particle size number distribution, of less than 100 nm, more preferably less than 50 nm, even more preferably less than 20 nm or even less than 10 nm.As already mentioned above, the polycyclic aromatic compounds used in the HPHT process of the present invention as a diamond precursor material can be transformed to diamond particles having an average particle size of less than 100 nm, more preferably less than 50 nm or even less than 20 nm and optionally having a narrow particle size distribution without using a metal catalyst. Furthermore, as the diamond particles obtained by the process of the present invention already have a small average particle size, no ball-milling post treatment is necessary. Accordingly, apart from having a small average particle size (which is needed for some applications of nanodiamonds) and optionally a narrow particle size distribution, the diamond particles obtainable by the process of the present invention have a very low amount of or even no metal contaminants. Furthermore, although no metal catalyst is used in the process of the present invention, the diamonds are obtained at high yield whereas the amount of nondiamond carbon (such as graphitic) is very low.The diamond particles might be present in the form of a powder or might be dispersed in a liquid (e.g. aqueous) dispersion medium.The present invention also relates to the use of the polycyclic aromatic compound described above as a precursor in diamond particle synthesis, in particular HPHT (high pressure high temperature) diamond particle synthesis.With regard to preferred properties of the polycyclic aromatic compound, reference can be made to the statements made above.ExamplesInventive Example IE1A multi anvil cell was charged with 8 mg of a polycyclic aromatic compound containing34 fused aromatic rings and having the following Formula:The polycyclic aromatic compound was prepared as described by K. Mullen et al., Angew. Chem. Int. Ed. EngL, 1997, 36, No. 15, pp. 1604-1607.Pressure was applied up to the set-point of 12 GPa. At the set pressure, the sample was heated with a heating rate of 100 K / min and held at 1200 °C for 2 h. A powder was obtained which was subjected to a post-treatment with a HCIO4 / H2SO4 / HNO3 mixture (vol. ratio 1 :1 :1 ) at 90 °C for about 4 h.The obtained powder was analyzed by Raman spectroscopy, X-ray diffraction (XRD), selected area electron diffraction (SAED) and transmission electron microscopy (TEM).Raman spectroscopy was conducted using a customized confocal microscope with 532 nm excitation laser (LaserQuantum Tau532) typically operating at 30 mW output power focused onto the sample using a 10x Mitutoyo air objective with long working distance. The high resolution transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) were obtained with a field-emission transmission electron microscopy (FE-TEM, Tecnai G2 F20 U-TWIN). Powder X-ray diffraction (PXRD) measurements were performed on a Rigaku SmartLab spectrometer using a Cu K-a anode (A = 1 .5406 A). The powders were placed on a zero reflection Si substrate.Raman analysis confirmed the diamond lattice with a peak at 1324 cm-1. No graphitic material was detected, as the G peak (~ 1590 - 1620 cm-1) was missing in the Raman spectrum.X-ray powder diffraction (XRPD) analysis also confirmed the diamond lattice, showing the relevant peaks of the D111 (2.07 A), D220 (1 .26 A) and D311 (1 .08 A) planes of the crystals.Selected area electron diffraction (SAED) proved again the presence of the diamond lattice by showing the according D111 , D220 and D311 reflections.TEM analysis showed nanodiamond particles of narrow size distribution with diameters of = 5 nm and uniform, almost spherical shapes.Inventive Example IE2The same polycyclic aromatic compound as in Inventive Example IE1 was used.The same multi anvil cell as used in Inventive Example IE1 was charged with 8 mg of the polycyclic aromatic compound. Pressure was applied up to the set-point of 19 GPa. At the set pressure, the sample was heated with a heating rate of 100 K / min and held at 1600 °C for 2 h. A powder was obtained which was subjected to a post-treatment with a HCIO4 / H2SO4 / HNO3 mixture (vol. ratio 1 :1 :1 ) at 90 °C for about 4 h.Again, the obtained powder was analyzed by Raman spectroscopy, X-ray diffraction (XRD), selected area electron diffraction (SAED) and transmission electron microscopy (TEM) under the same measuring conditions as in Inventive Example IE1 .Raman analysis confirmed the diamond lattice with a peak at 1333 cm-1. No graphitic material was detected, as the G peak (~ 1590 - 1620 cm-1) was missing in the Raman spectrum.XRPD analysis also confirmed the diamond lattice, showing the relevant peaks of the D111 (2.06 A), D220 (1.26 A) and D311 (1.08 A) plane of the crystals. Selected area electron diffraction (SAED) proved again the presence of the diamond lattice by showing the D111 , D220 and D311 reflections.TEM analysis showed nanodiamond particles of narrow size distribution with diameters of = 15 nm and uniform, almost spherical shapes.Inventive Example IE 3A multi anvil cell was charged with 8 mg of a polycyclic aromatic compound containing12 fused aromatic rings and having the following Formula:The polycyclic aromatic compound was prepared as described by K. Mullen et al., J.Am. Chem. Soc., 2021 , 143, pp. 10403-10412 and Supporting Information.Pressure was applied up to the set-point of 12 GPa. At the set pressure, the sample was heated with a heating rate of 100 K / min and held at 1200 °C for 2 h. A powder was obtained which was subjected to a post-treatment with a HCIO4 / H2SO4 / HNO3 mixture (vol. ratio 1 :1 :1 ) at 90 °C for about 4 h.The obtained powder was analyzed by Raman spectroscopy under the same measuring conditions as in Inventive Example IE1 .Raman analysis confirmed the diamond lattice with a peak at 1333 cm-1. No graphitic material was detected, as the G peak (~ 1590 - 1620 cm-1) was missing in the Raman spectrum.Inventive Example IE4A multi anvil cell was charged with 8 mg of a polycyclic aromatic compound containing 13 fused aromatic rings and having the following Formula:The polycyclic aromatic compound was prepared as described by K. Mullen et al., Angew. Chem. Int. Ed. EngL, 1997, 36, No. 15, pp. 1604-1607.Pressure was applied up to the set-point of 12 GPa. At the set pressure, the sample was heated with a heating rate of 100 K / min and held at 1200 °C for 2 h. A powder was obtained which was subjected to a post-treatment with a HCIO4 / H2SO4 / HNO3 mixture (vol. ratio 1 :1 :1 ) at 90 °C for about 4 h.The obtained powder was analyzed by Raman spectroscopy under the same measuring conditions as in Inventive Example IE1 .Raman analysis confirmed the diamond lattice with a peak at 1333 cm-1. No graphitic material was detected, as the G peak (~ 1590 - 1620 cm-1) was missing in the Raman spectrum.Comparative Example CE1A multi anvil cell was charged with 8 mg of coronene which is a polycyclic aromatic compound containing 7 fused aromatic rings and has the following Formula:Pressure was applied up to the set-point of 12 GPa. At the set pressure, the sample was heated with a heating rate of 100 K / min and held at 1200 °C for 2 h. A powder was obtained which was subjected to a post-treatment with a HCIO4 / H2SO4 / HNO3 mixture (vol. ratio 1 :1 :1 ) at 90 °C.The obtained powder was analyzed by Raman spectroscopy and X-ray diffraction (XRD) under the same measuring conditions as in Inventive Example IE1 .Raman analysis showed two peaks at 1372 cm-1and 1613 cm-1corresponding to the D and G peaks of graphitic materials. No diamond peak (~ 1333 cm-1) was detected.X-ray powder diffraction (XRPD) analysis confirmed the presence of graphitic material, showing the relevant peak of the G002 plane at 26 °. Additionally, a peak of rather low intensity at 44 ° was detected, said peak originating from the D111 plane of the diamond structure. Accordingly, the powder obtained in Comparative Example 1 was mainly made of a graphitic material, whereas diamond particles were only present in minor amounts.In Inventive Examples IE5 to IE7 described below, heteroatom-doped nanodiamonds were prepared. Inventive Examples IE5 to IE6 relate to Si-doped nanodiamonds, whereas Inventive Example IE7 relates to Ge-doped nanodiamonds.Inventive Example IE5The polycyclic aromatic compound used in Inventive Example IE5 corresponds to the one of Inventive Example IE1 . As a dopant, Si(Si(CHs)3)4was used.The dopant Si(Si(CHs)3)4was added to the polycyclic aromatic compound in a molar ratio of 1 :1 and mixed in a mortar. The mixture (8 mg) was introduced into a multi anvil cell.Pressure was applied up to the set-point of 12 GPa. At the set pressure, the sample was heated with a heating rate of 100 K / min and held at 1200 °C for 2 h. A powder was obtained which was subjected to a post-treatment with a HCIO4 / H2SO4 / HNO3 mixture (vol. ratio 1 :1 :1 ) at 90 °C for about 4 h.The obtained powder was analyzed by Raman spectroscopy, X-ray diffraction (XRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM), and photoluminescence spectroscopy.Raman spectroscopy was conducted using a customized confocal microscope with 532 nm excitation laser (LaserQuantum Tau532) typically operating at 30 mW output power focused onto the sample using a 10x Mitutoyo air objective with long working distance. The high resolution transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) were obtained with a field-emission transmission electron microscopy (FE-TEM, Tecnai G2 F20 U-TWIN). Powder X-ray diffraction (PXRD) measurements were performed on a Rigaku SmartLab spectrometer using a Cu K-a anode (A = 1 .5406 A).Raman analysis confirmed the diamond lattice with a peak at 1324 cm-1. No graphitic material was detected, as the G peak (~ 1590 - 1620 cm-1) was missing in the Raman spectrum.X-ray powder diffraction (XRPD) analysis also confirmed the diamond lattice, showing the relevant peaks of the D111 (2.07 A), D220 (1 .26 A) and D311 (1 .08 A) planes of the crystals.Selected area electron diffraction (SAED) proved again the presence of the diamond lattice by showing the according D111 , D220 and D311 reflections.TEM analysis showed nanodiamond particles of narrow size distribution with diameters of = 30 nm.The photoluminescence spectrum showed a peak at 739 nm which indicates the presence of SiV- vacancy centers in the nanodiamonds.Inventive Example IE6The polycyclic aromatic compound used in Inventive Example IE6 corresponds to the one of Inventive Example IE1 . As a dopant, Si(Si(CHs)3)4was used.The dopant Si(Si(CHs)3)4was added to the polycyclic aromatic compound. Molar ratio of polycyclic aromatic compound to Si(Si(CH3)3)4: 5.2 : 1The mixture (8 mg) was introduced into a multi anvil cell.Pressure was applied up to the set-point of 12 GPa. At the set pressure, the sample was heated with a heating rate of 100 K / min and held at 1200 °C for 2 h. A powder was obtained which was subjected to a post-treatment with a HCIO4 / H2SO4 / HNO3 mixture (vol. ratio 1 :1 :1 ) at 90 °C for about 4 h.The obtained powder was analyzed by Raman spectroscopy, X-ray diffraction (XRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM) and photoluminescence spectroscopy.Raman spectroscopy was conducted using a customized confocal microscope with 532 nm excitation laser (LaserQuantum Tau532) typically operating at 30 mW output power focused onto the sample using a 10x Mitutoyo air objective with long working distance. The high resolution Transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) were obtained with a field-emission transmission electron microscopy (FE-TEM, Tecnai G2 F20 U-TWIN). Powder X-ray diffraction (PXRD) measurements were performed on a Rigaku SmartLab spectrometer using a Cu K-a anode (A = 1 .5406 A).TEM analysis showed nanodiamond particles of narrow size distribution with diameters of = 30 nm.The photoluminescence spectrum showed a peak at 739 nm which indicates the presence of SiV- vacancy centers in the nanodiamonds.Inventive Example IE7The polycyclic aromatic compound used in Inventive Example IE7 corresponds to the one of Inventive Example IE1 . As a dopant, tetraphenylgermanium (GePh4) was used.The dopant tetraphenylgermanium was added to the polycyclic aromatic compound in a molar ratio of 1 :1 and mixed in a mortar. The mixture (8 mg) was introduced into a multi anvil cell.Pressure was applied up to the set-point of 12 GPa. At the set pressure, the sample was heated with a heating rate of 100 K / min and held at 1200 °C for 2 h. A powder wasobtained which was subjected to a post-treatment with a HCIO4 / H2SO4 / HNO3 mixture (vol. ratio 1 :1 :1 ) at 90 °C for about 4 h.The obtained powder was analyzed by Raman spectroscopy, X-ray diffraction (XRD), selected area electron diffraction (SAED) and photoluminescence spectroscopy.Raman spectroscopy was conducted using a customized confocal microscope with 532 nm excitation laser (LaserQuantum Tau532) typically operating at 30 mW output power focused onto the sample using a 10x Mitutoyo air objective with long working distance. The high resolution transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) were obtained with a field-emission transmission electron microscopy (FE-TEM, Tecnai G2 F20 U-TWIN). Powder X-ray diffraction (PXRD) measurements were performed on a Rigaku SmartLab spectrometer using a Cu K-a anode (A = 1 .5406 A).Raman analysis confirmed the diamond lattice with a peak at 1324 cm-1. No graphitic material was detected, as the G peak (~ 1590 - 1620 cm-1) was missing in the Raman spectrum.X-ray powder diffraction (XRPD) analysis also confirmed the diamond lattice, showing the relevant peaks of the D111 (2.07 A), D220 (1 .26 A) and D311 (1 .08 A) planes of the crystals.Selected area electron diffraction (SAED) proved again the presence of the diamond lattice by showing the according D111 , D220 and D311 reflections.The photoluminescence spectrum showed a peak at 603 nm which indicates the presence of GeV- vacancy centers in the nanodiamonds.The precursor materials, pressure and temperature conditions and properties of the products of Inventive Examples IE1 to IE7 and Comparative Example CE1 are summarized in Table 1 .Table 1 : Precursor materials, pressure and temperature conditions and product propertiesThe polycyclic aromatic compound used in the HPHT process of the present invention as a diamond precursor material can be transformed to diamond particles having a narrow particle size distribution and an average particle size of less than 100 nm (in particular less than 50 nm, in some cases less than 20 nm or even less than 10 nm) without using a metal catalyst. Furthermore, as the diamond particles obtained by the HPHT treatment already have a small average particle size, no ball-milling post treatment is necessary. Furthermore, although no metal catalyst is used in the process of the present invention, the diamonds are obtained at high yield whereas the amount of non-diamond carbon (such as graphitic carbon) is very low.As shown by Inventive Examples IE5 to IE7, heteroatom-doped nanodiamonds of high quality are obtainable by the process of the present invention.

Claims

Claims1 . A process for preparing diamond particles, comprising subjecting a diamond precursor material to a pressure of at least 8 GPa and a temperature of at least 900°C, wherein the diamond precursor material comprises a polycyclic aromatic compound which contains from 10 to 200 fused six-membered aromatic rings.

2. The process according to claim 1 , wherein the polycyclic aromatic compound does not contain any fused aromatic rings other than the six-membered aromatic rings.

3. The process according to claim 1 or 2, wherein at least one of the fused sixmembered aromatic rings is a six-membered heteroaromatic ring.

4. The process according to claim 3, wherein the one or more six-membered heteroaromatic rings comprise at least one, preferably one or two non-carbon ring atoms, wherein the non-carbon ring atoms are selected from nitrogen and boron.

5. The process according to claim 1 or 2, wherein none of the fused six-membered aromatic rings comprises a ring atom other than carbon.

6. The process according to one of the preceding claims, wherein at least one of the fused six-membered aromatic rings is a substituted aromatic ring to which at least one substituent Rsis attached, wherein the substituent Rsis selected from a C1-20 alkyl group; an aryl group; an amine group; a carboxylic acid group or a salt or an ester thereof; a silyl group; a germanyl group; and a boron-containing group.

7. The process according to one of the claims 1 to 5, wherein none of the fused six- membered aromatic rings is a substituted aromatic ring.

8. The process according to one of the preceding claims, wherein the polycyclic aromatic compound complies with the following requirement:N(RA-2) + N(RA-3) > N(RA-1 ) whereinN(RA-2) is the total number of twofold-shared ring atoms of the fused six-membered aromatic rings, wherein a twofold-shared ring atom of one of thesix-membered aromatic rings is a ring atom being shared with exactly one other fused aromatic ring;N(RA-3) is the total number of threefold-shared ring atoms of the fused six-membered aromatic rings, wherein a threefold-shared ring atom of one of the six-membered aromatic rings is a ring atom being shared with two other fused aromatic rings;N(RA-1 ) is the total number of non-shared ring atoms of the fused sixmembered aromatic rings, wherein a non-shared ring atom of one of the sixmembered aromatic rings is a ring atom not being shared with another fused aromatic ring.

9. The process according to one of the preceding claims, wherein the polycyclic aromatic compound has one of the following Formulas (I) to (XVI):wherein in each of the Formulas (I) to (XVI) the residues R are, independently from each other, hydrogen or a substituent Rs, wherein the subsituent Rsis selected from a C1-20 alkyl group; an aryl group; an amine group; a carboxylic acid group or a salt or an ester thereof; a silyl group; a germanyl group; and a boron-containing group, wherein in Formula (II) the ring atoms X are, independently from each other, C or N.

10. The process according to one of the preceding claims, wherein the diamond precursor material additionally comprises a heteroatom-containing dopant compound.11 . The process according to claim 10, wherein the heteroatom is selected from an element of Group 13 (e.g. boron), Group 14 (e.g. Si, Ge, Sn or Pb) or Group 15 (e.g. N or P) of the Periodic Table or a transition metal atom (e.g. Ni, Ti or Co).

12. The process according to one of the preceding claims, wherein the process is carried out in a multi-anvil press.

13. Use of the polycyclic aromatic compound according to one of the claims 1 to 9 as a precursor in diamond particle synthesis.

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