Zero-to-Two Nanoarchitectonics: Fabrication of Two-Dimensional Materials from Zero-Dimensional Fullerene

Nanoarchitectonics of two-dimensional materials from zero-dimensional fullerenes is mainly introduced in this short review. Fullerenes are simple objects with mono-elemental (carbon) composition and zero-dimensional structure. However, fullerenes and their derivatives can create various types of two-dimensional materials. The exemplified approaches demonstrated fabrications of various two-dimensional materials including size-tunable hexagonal fullerene nanosheet, two-dimensional fullerene nano-mesh, van der Waals two-dimensional fullerene solid, fullerene/ferrocene hybrid hexagonal nanosheet, fullerene/cobalt porphyrin hybrid nanosheet, two-dimensional fullerene array in the supramolecular template, two-dimensional van der Waals supramolecular framework, supramolecular fullerene liquid crystal, frustrated layered self-assembly from two-dimensional nanosheet, and hierarchical zero-to-one-to-two dimensional fullerene assembly for cell culture.

On the basis of these developments of science and technology, various functional materials and systems have been continuously produced upon important social demands in energy [37][38][39][40], environmental [41][42][43][44], and biomedical fields [45][46][47][48]. It becomes realized that regulation of nanostructures within materials systems is crucially important for getting better performances with higher efficiencies. Interactions in well-coordinated components leads to efficient processes in many events including conductivity, reactivity, adhesion, and optical response. Both materials' intrinsic qualities and their precise internal structures are indispensable factors to produce more desirable materials for target functions. Therefore, features can be commonly applied to a wide range of materials. There tectonics concept has been applied in various research fields includin tion [58,59], structure regulations [60,61], catalysis [62,63], sensor [64 bio-related science [68][69][70], biomedical applications [71,72], energy- [73,74], and environment-oriented applications [75,76].  Two-dimensional materials can be fabricated with some physical methods such as exfoliation [95,96] and chemical vapor deposition [97,98] as standard procedures. Some bottom-up-type fabrications of two-dimensional materials have been also explored as illustrated in Figure 2. For example, syntheses of graphene-like two-dimensional nanocarbon pieces such as two-dimensional graphene nanoribbon upon well-designed organic synthesis have been also demonstrated ( Figure 2A) [99,100]. Preparation of two-dimensional nanocarbon films from molecular carbon rings was realized using a novel technique, vortex Langmuir-Blodgett method, and subsequent calcination procedure ( Figure 2B) [101]. Among bottom-up-type fabrications, fabrications of two-dimensional structures and assemblies from zero-dimensional fullerenes are mainly introduced in this short review article ( Figure 2C). Fullerenes are very simple objects with mono-elemental (carbon) composition and zero-dimensional structure. As demonstrated in the following sections, simple unit components of fullerenes and their derivatives can create various types of two-dimensional materials. In this review article, essences of zero-to-two nanoarchitectonics for the fabrication of two-dimensional materials from zero-dimensional fullerene molecules are discussed.

Two-Dimensional Fullerene Nanoarchitecture
For the preparation of self-assembled fullerene nanocrystals, a simple liquid-liquid interface precipitation method is often used [102,103]. In the liquid-liquid interface precipitation method, the crystal formation mechanism is supposed to be driven by supersaturation of fullerene molecules at interfaces between good solvent (with higher fullerene solubility) and bad solvent (with lower fullerene solubility). Although the preparation procedure of this technique is very simple, fullerene assemblies in various dimensionsincluding one-dimensional rods/tubes [104,105], three-dimensional cubes [106,107], and their hierarchical nanoarchitectures [108,109]-can be obtained. For example, Sathish and Miyazawa synthesized size-tunable C60 hexagonal thin crystalline nanosheets through the liquid-liquid interface precipitation method at the carbon tetrachloride (CCl4)/alcohol interface for the first time [110]. The size of the hexagonal nanosheets could be tailored

Two-Dimensional Fullerene Nanoarchitecture
For the preparation of self-assembled fullerene nanocrystals, a simple liquid-liquid interface precipitation method is often used [102,103]. In the liquid-liquid interface precipitation method, the crystal formation mechanism is supposed to be driven by supersaturation of fullerene molecules at interfaces between good solvent (with higher fullerene solubility) and bad solvent (with lower fullerene solubility). Although the preparation procedure of this technique is very simple, fullerene assemblies in various dimensionsincluding one-dimensional rods/tubes [104,105], three-dimensional cubes [106,107], and their hierarchical nanoarchitectures [108,109]-can be obtained. For example, Sathish and Miyazawa synthesized size-tunable C 60 hexagonal thin crystalline nanosheets through the liquid-liquid interface precipitation method at the carbon tetrachloride (CCl 4 )/alcohol interface for the first time [110]. The size of the hexagonal nanosheets could be tailored merely by selecting proper alcohol for the interfacial precipitation. Hexagonal nanosheet with~7.5 µm diameter was observed for the CCl 4 /isopropyl alcohol interface, whereas the hexagonal nanosheets with~2.5 µm diameter and 500 nm diameter at the CCl 4 /ethanol and CCl 4 /methanol interface, respectively. This work opens a new door for synthesizing fullerene nanosheets by simple self-assembly.
It was demonstrated in further studies that variations in the shape and size of twodimensional fullerene assemblies can be fabricated with the appropriate choice of solvent combinations for the liquid-liquid interfacial precipitation method ( Figure 3A) [111]. Formation of hexagonal C 60 nanosheets was achieved at the interfaces between toluene and isopropyl alcohol, while two-dimensional nanosheets with uniform rhombi and hexagonal shapes were obtained through the self-assembly of C 60 molecules at the interfaces of toluene/tert-butyl alcohol and CCl 4 /isopropyl alcohol, respectively. The lattice formation favoring these two-dimensional crystalline assemblies may be the most influential factor in determining the shape of the two-dimensional nanosheets. Under certain stimuli, such as contact with water, two-dimensional rhombi-shaped nanosheets were transformed into one-dimensional nanorods. The latter transformation would be occurred accompanied by the release of high surface energy.
Pore formation within a two-dimensional C 60 nanosheet was achieved by faint adaptations of solvent systems to the liquid-liquid interfacial precipitation system [112]. Twodimensional nanosheets possessing dimorphic macroporous/interporous structures were fabricated via C 60 assembly at interfaces between isopropyl alcohol and CCl 4 /benzene mixtures ( Figure 3B). At high CCl 4 contents, macropores were formed in the two-dimensional hexagonal nanosheets. In addition, smaller mesopores were also formed within the macropores under suitable mixing conditions between CCl 4 and benzene. The average mesopore sizes came to be 15 to 25 nm and 20 to 40 nm at CCl 4 /benzene mixing ratios of 80:20 and 90:10, respectively. The latter two-dimensional structures with bimodal features were accompanied by mixed fcc and hcp crystal structures. It is expected that the evaporation of solvents trapped in them would be the cause of mesopore formation. The formed bimodal nanoarchitectures have high surface areas with enhanced electrochemical activities, which would be useful for energy-related applications.
Chen and co-workers developed a simple but effective co-solvent induced crystal transformation strategy to transform two-dimensional C 60 microplates to nano-meshes made of ordered nanorods ( Figure 4) [113]. The transformation was triggered by removing the solvent molecule and then re-incorporating another solvent molecule. Specifically, C 60 plates were obtained first by adding isopropyl alcohol to the CCl 4 /m-xylene mixture with gentle swirling. The C 60 plates were concentrated by centrifugation and then diluted with isopropyl alcohol to transform to two-dimensional mesh networks. After diluting with isopropyl alcohol, the incorporated CCl 4 /m-xylene would then leave the C 60 plate, leading to the formation of pores in the C 60 microplates. Under appropriate conditions, m-xylene can be re-incorporated during the C 60 recrystallization and eventually gives rise to the mesh network. This work opens a new field for designing hierarchical twodimensional superstructures. The fabricated complicated two-dimensional mesh-like fullerene superstructures would be useful separation and filtration such as oil/water separation and biomolecular segregations.  Chen and co-workers developed a simple but effective co-solvent induced crystal transformation strategy to transform two-dimensional C60 microplates to nano-meshes with isopropyl alcohol, the incorporated CCl4/m-xylene would then leave the C60 plate, leading to the formation of pores in the C60 microplates. Under appropriate conditions, mxylene can be re-incorporated during the C60 recrystallization and eventually gives rise to the mesh network. This work opens a new field for designing hierarchical two-dimensional superstructures. The fabricated complicated two-dimensional mesh-like fullerene superstructures would be useful separation and filtration such as oil/water separation and biomolecular segregations.

Two-Dimensional Nanoarchitecture from Fullerene with Other Component
Steigerwald, Nuckolls, Roy, and coworkers proposed a method that utilizes dimensionally confined fullerene as a strippable layer to obtain two-dimensional fullerene (Figure 5) [114]. They firstly synthesized a layered van der Waals solid from a structure-directing building block and C60 fullerene by self-assembly method. Transition metal chalcogenide cluster (Co6Se8(PEt2phen)6) was employed as the structure-directing building block that associated with C60 and thereby directed the spontaneous assembly of C60 into crystalline solid of [Co6Se8(PEt2phen)6][C60]5. The structure of [Co6Se8(PEt2phen)6][C60]5 is composed of two layers stacked along the hexagonal c-axis in which a slightly corrugated trigonal array of C60 was separated by a layer comprised of a molecular cluster with stoichiometry [Co6Se8(PEt2phen)6][C60]2. The intralayer bonding of [Co6Se8(PEt2phen)6][C60]5 is strong enough while interlayer bonding is weak enough that the layered solid may be mechanically exfoliated. Therefore, they exfoliated the layered crystals to produce twodimensional sheets as thin as ~130 nm with smooth surfaces and transferred them onto Si wafers. In this example, flexible molecular recognition between fullerene and ligand units enables us to nanoarchitect van der Waals materials from multiple components in desired layered structures.

Two-Dimensional Nanoarchitecture from Fullerene with Other Component
Steigerwald, Nuckolls, Roy, and coworkers proposed a method that utilizes dimensionally confined fullerene as a strippable layer to obtain two-dimensional fullerene ( Figure 5) [114]. They firstly synthesized a layered van der Waals solid from a structuredirecting building block and C 60 fullerene by self-assembly method. Transition metal chalcogenide cluster (Co 6 Se 8 (PEt 2 phen) 6 ) was employed as the structure-directing building block that associated with C 60 and thereby directed the spontaneous assembly of C 60 into crystalline solid of [Co 6 Se 8 (PEt 2 phen) 6 ][C 60 ] 5 . The structure of [Co 6 Se 8 (PEt 2 phen) 6 ][C 60 ] 5 is composed of two layers stacked along the hexagonal c-axis in which a slightly corrugated trigonal array of C 60 was separated by a layer comprised of a molecular cluster with stoichiometry [Co 6 Se 8 (PEt 2 phen) 6 ][C 60 ] 2 . The intralayer bonding of [Co 6 Se 8 (PEt 2 phen) 6 ][C 60 ] 5 is strong enough while interlayer bonding is weak enough that the layered solid may be mechanically exfoliated. Therefore, they exfoliated the layered crystals to produce twodimensional sheets as thin as~130 nm with smooth surfaces and transferred them onto Si wafers. In this example, flexible molecular recognition between fullerene and ligand units enables us to nanoarchitect van der Waals materials from multiple components in desired layered structures.  Figure 5. Two-dimensionally confined fullerene as strippable layer of van der Waals solid from a structure-directing building block (Co6Se8-based cluster) and C60 fullerene by self-assembly method.
Reprinted with permission from [114]. Copyright 2016 American Chemical Society.
Based on the above-mentioned work, Zhu, Roy, and coworkers made an effort to enhance the intralayer interactions of the layered van der Waals solid [Co6Se8(PEt2phen)6][C60]5 by photo converting method and isolated free-standing two-dimensional crystalline C60 structure by solvent-induced exfoliation ( Figure 6) [115]. Specifically, they first used a 532 nm laser to polymerize [Co6Se8(PEt2phen)6][C60]5 for 5-10 days to convert the layer into a covalently linked sheet of polymerized C60. The irradiated crystals of [Co6Se8(PEt2phen)6][C60]5 readily dissolve in toluene to produce a brown solution with a dark brown suspension of polymerized C60 sheets. The two-dimensional polymerized C60 sheets were composed of a layer of the inorganic cluster, sandwiched between two monolayers of polymerized C60. The nanosheets with 5 nm thick can be transferred onto solid substrates and depolymerized by heating. Figure 5. Two-dimensionally confined fullerene as strippable layer of van der Waals solid from a structure-directing building block (Co 6 Se 8 -based cluster) and C 60 fullerene by self-assembly method. Reprinted with permission from [114]. Copyright 2016 American Chemical Society.
Based on the above-mentioned work, Zhu, Roy, and coworkers made an effort to enhance the intralayer interactions of the layered van der Waals solid [Co 6 Se 8 (PEt 2 phen) 6 ][C 60 ] 5 by photo converting method and isolated free-standing two-dimensional crystalline C 60 structure by solvent-induced exfoliation ( Figure 6) [115]. Specifically, they first used a 532 nm laser to polymerize [Co 6 Se 8 (PEt 2 phen) 6 ][C 60 ] 5 for 5-10 days to convert the layer into a covalently linked sheet of polymerized C 60 . The irradiated crystals of [Co 6 Se 8 (PEt 2 phen) 6 ][C 60 ] 5 readily dissolve in toluene to produce a brown solution with a dark brown suspension of polymerized C 60 sheets. The two-dimensional polymerized C 60 sheets were composed of a layer of the inorganic cluster, sandwiched between two monolayers of polymerized C 60 . The nanosheets with 5 nm thick can be transferred onto solid substrates and depolymerized by heating. Molecules 2021, 26, x FOR PEER REVIEW 9 of 19 Figure 6. Isolated free-standing two-dimensional crystalline C60 structure fabricated by photopolymerization and solvent-induced exfoliation.
Wakahara, Tateyama, and coworkers prepared C60/ferrocene nanosheets by a simple liquid-liquid interfacial precipitation method through the formation of an interface between isopropyl alcohol and toluene solution containing C60 and ferrocene ( Figure 7) [116]. A strong charge-transfer band of diffuse reflectance spectra between ferrocene and C60 was observed at 782 nm, indicating the presence of donor−acceptor interaction in the nanosheets. The average size of the C60/ ferrocene hexagonal nanosheets was 9.1 ± 6.2 μm and the thickness is about 250−550 nm. Size would be controlled by conditions at the liquid-liquid interfacial precipitation method. In the case of C60/ferrocene nanosheets, each C60 is surrounded by two ferrocene molecules, showing a triclinic C60(ferrocene)2 structure. Upon heating the nanosheets to 150 °C to sublimate ferrocene, the C60/ferrocene hexagonal nanosheets can be converted to fcc C60 hexagonal nanosheets with the same shape and size as the C60/ferrocene nanosheets. The successful preparation of C60/ferrocene nanosheets based on the co-crystallization in the liquid-liquid interface is expected to be an important steppingstone to the fabrication of novel fullerene nanosheets and to lead to a new field relative to fullerene-derived co-crystals engineering.  Wakahara, Tateyama, and coworkers prepared C 60 /ferrocene nanosheets by a simple liquid-liquid interfacial precipitation method through the formation of an interface between isopropyl alcohol and toluene solution containing C 60 and ferrocene (Figure 7) [116]. A strong charge-transfer band of diffuse reflectance spectra between ferrocene and C 60 was observed at 782 nm, indicating the presence of donor−acceptor interaction in the nanosheets. The average size of the C 60 / ferrocene hexagonal nanosheets was 9.1 ± 6.2 µm and the thickness is about 250−550 nm. Size would be controlled by conditions at the liquid-liquid interfacial precipitation method. In the case of C 60 /ferrocene nanosheets, each C 60 is surrounded by two ferrocene molecules, showing a triclinic C 60 (ferrocene) 2 structure. Upon heating the nanosheets to 150 • C to sublimate ferrocene, the C 60 /ferrocene hexagonal nanosheets can be converted to fcc C 60 hexagonal nanosheets with the same shape and size as the C 60 /ferrocene nanosheets. The successful preparation of C 60 /ferrocene nanosheets based on the co-crystallization in the liquid-liquid interface is expected to be an important steppingstone to the fabrication of novel fullerene nanosheets and to lead to a new field relative to fullerene-derived co-crystals engineering. Wakahara, Tateyama, and coworkers prepared C60/ferrocene nanosheets by a simple liquid-liquid interfacial precipitation method through the formation of an interface between isopropyl alcohol and toluene solution containing C60 and ferrocene (Figure 7) [116]. A strong charge-transfer band of diffuse reflectance spectra between ferrocene and C60 was observed at 782 nm, indicating the presence of donor−acceptor interaction in the nanosheets. The average size of the C60/ ferrocene hexagonal nanosheets was 9.1 ± 6.2 μm and the thickness is about 250−550 nm. Size would be controlled by conditions at the liquid-liquid interfacial precipitation method. In the case of C60/ferrocene nanosheets, each C60 is surrounded by two ferrocene molecules, showing a triclinic C60(ferrocene)2 structure. Upon heating the nanosheets to 150 °C to sublimate ferrocene, the C60/ferrocene hexagonal nanosheets can be converted to fcc C60 hexagonal nanosheets with the same shape and size as the C60/ferrocene nanosheets. The successful preparation of C60/ferrocene nanosheets based on the co-crystallization in the liquid-liquid interface is expected to be an important steppingstone to the fabrication of novel fullerene nanosheets and to lead to a new field relative to fullerene-derived co-crystals engineering. Figure 7. Two-dimensional C60/ferrocene nanosheets prepared by liquid-liquid interfacial precipitation method. Reprinted with permission from [116]. Copyright 2009 American Chemical Society. Figure 7. Two-dimensional C 60 /ferrocene nanosheets prepared by liquid-liquid interfacial precipitation method. Reprinted with permission from [116]. Copyright 2009 American Chemical Society.
In addition to synthesizing C 60 /ferrocene hexagonal nanosheets co-crystals nanosheets, Wakahara, Bradley, Anthopoulos, and coworkers also reported the preparation of novel co-crystals nanosheets comprising C 60 and 5,10,15,20-tetrakis(4-methoxyphenyl) porphyrinato cobalt(II) (CoTMPP) using liquid-liquid interfacial precipitation method [117]. The hybrid nanosheets were formed at an interface between isopropyl alcohol and toluene solution containing C 60 /CoTMPP. The thickness of the nanosheets is~50-200 nm. The C 60 /CoTMPP co-crystals nanosheets exhibited ambipolar transport characteristics with nearly balanced hole/electron mobilities. The successful preparation of fullerene-derived co-crystals nanosheets by using the liquid-liquid interfacial precipitation method to cocrystallization would enrich the fabrication of two-dimensional fullerene.
Zeng, Wang, and coworkers reported a two-dimensional molecular template with two types of cavities that have different sizes and symmetry at the liquid-solid interface formed by an azobenzene derivative (NN4A) [118] (Figure 8). Both the A-(diameter: 12.0 Å) and B-type (diameter: 8.6 Å) cavities were large enough in diameter to accommodate guest molecules such as fullerene molecules. Therefore, highly ordered fullerene arrays were constructed at the liquid-solid interface of graphite by coadsorption of fullerenes and NN4A. The experimental and theoretical results indicate that C 60 molecules (diameter:6.80 Å) occupy both types of the cavity, while the larger C 80 (diameter: 8.22 Å) and Sc 3 N@C 80 molecules were trapped exclusively in the A-type cavities, which implies appreciable site selectivity. These template-directed fabrications of fullerene arrays provide a new method for the formation of two-dimensional fullerene. In addition to synthesizing C60/ferrocene hexagonal nanosheets co-crystals nanosheets, Wakahara, Bradley, Anthopoulos, and coworkers also reported the preparation of novel co-crystals nanosheets comprising C60 and 5,10,15,20-tetrakis(4-methoxyphenyl) porphyrinato cobalt(II) (CoTMPP) using liquid-liquid interfacial precipitation method [117]. The hybrid nanosheets were formed at an interface between isopropyl alcohol and toluene solution containing C60/CoTMPP. The thickness of the nanosheets is ~50-200 nm. The C60/CoTMPP co-crystals nanosheets exhibited ambipolar transport characteristics with nearly balanced hole/electron mobilities. The successful preparation of fullerene-derived co-crystals nanosheets by using the liquid-liquid interfacial precipitation method to co-crystallization would enrich the fabrication of two-dimensional fullerene.
Zeng, Wang, and coworkers reported a two-dimensional molecular template with two types of cavities that have different sizes and symmetry at the liquid-solid interface formed by an azobenzene derivative (NN4A) [118] (Figure 8). Both the A-(diameter: 12.0 Å) and B-type (diameter: 8.6 Å) cavities were large enough in diameter to accommodate guest molecules such as fullerene molecules. Therefore, highly ordered fullerene arrays were constructed at the liquid-solid interface of graphite by coadsorption of fullerenes and NN4A. The experimental and theoretical results indicate that C60 molecules (diameter:6.80 Å) occupy both types of the cavity, while the larger C80 (diameter: 8.22 Å) and Sc3N@C80 molecules were trapped exclusively in the A-type cavities, which implies appreciable site selectivity. These template-directed fabrications of fullerene arrays provide a new method for the formation of two-dimensional fullerene. Gao, Pan, Guo, and coworkers investigated a two-dimensional van der Waals supramolecular framework consisting of C60 molecules and octane thiol molecules on the Au(111) substrate by self-assembled co-crystallization method [119]. The porous framework was a result of cooperative self-organization with the C60 molecule and octane thiol as cohosts of the framework, in which the pores were filled by RS-Au-SR (R = CH3(CH2)7S) staples. The pairing of two adjacent C60 molecules and the formation of the C60 dimer were visualized, showing a charge transfer in each C60 dimer. The presence of octane thiol molecules prevents C60 from forming the preferred close-packed structure. The shapes of grown frameworks should be mainly influenced by the van der Waals interactions between the alkane chain and the C60 molecules. Gao, Pan, Guo, and coworkers investigated a two-dimensional van der Waals supramolecular framework consisting of C 60 molecules and octane thiol molecules on the Au(111) substrate by self-assembled co-crystallization method [119]. The porous framework was a result of cooperative self-organization with the C 60 molecule and octane thiol as cohosts of the framework, in which the pores were filled by RS-Au-SR (R = CH 3 (CH 2 ) 7 S) staples. The pairing of two adjacent C 60 molecules and the formation of the C 60 dimer were visualized, showing a charge transfer in each C 60 dimer. The presence of octane thiol molecules prevents C 60 from forming the preferred close-packed structure. The shapes of grown frameworks should be mainly influenced by the van der Waals interactions between the alkane chain and the C 60 molecules.

Two-Dimensional Nanoarchitecture from Alkylated Fullerene Derivative
Nakanishi et al. used alkylated C 60 derivatives for the fabrication of variously shaped fullerene assemblies (Figure 9) [120]. The used C 60 derivative possesses sp 3 -carbon-based three C 16 alkyl chains and sp 2 -carbon-based C 60 moiety in one molecule. Because these molecular parts have different affinities to aliphatic and aromatic solvents, this molecule behaves as a solvent-dependent amphiphile. Accordingly, cast films of this C 60 derivative from various solvents included variously shaped objects-such as spherical vesicles, onedimensional and two-dimensional fibers, and tapes, two-dimensional discs, and threedimensional cones. Self-assembled single bilayer discs were obtained from the browncolored supernatant of its 1,4-dioxane solution. The discs have uniform two-dimensional motifs with a thickness of 4.4 nm, which corresponds to the thickness of interdigitated bilayer. The two-dimensional bilayer is probably formed through π-stacking of C 60 parts and interdigitation of long alkyl chains.

Two-Dimensional Nanoarchitecture from Alkylated Fullerene Derivative
Nakanishi et al. used alkylated C60 derivatives for the fabrication of variously shaped fullerene assemblies (Figure 9) [120]. The used C60 derivative possesses sp 3 -carbon-based three C16 alkyl chains and sp 2 -carbon-based C60 moiety in one molecule. Because these molecular parts have different affinities to aliphatic and aromatic solvents, this molecule behaves as a solvent-dependent amphiphile. Accordingly, cast films of this C60 derivative from various solvents included variously shaped objects-such as spherical vesicles, onedimensional and two-dimensional fibers, and tapes, two-dimensional discs, and three-dimensional cones. Self-assembled single bilayer discs were obtained from the brown-colored supernatant of its 1,4-dioxane solution. The discs have uniform two-dimensional motifs with a thickness of 4.4 nm, which corresponds to the thickness of interdigitated bilayer. The two-dimensional bilayer is probably formed through π-stacking of C60 parts and interdigitation of long alkyl chains. Tu, Zhu, and coworkers demonstrated a new strategy to introduce a soft group onto fullerene so that the molecules will self-organize to form supramolecular assembly ( Figure  10) [121]. They succeeded in the synthesis of a thermotropic C60 supramolecular nanosheet with hierarchical structures. A typical molecule consists of a rigid C60, a gallic ester segment substituted with three long alkyl chains as the soft part, and a multi-methylene unit as a flexible spacer connecting the two segments. The C60 dyads underwent self-organization driven by π-π interactions to form triple-layer two-dimensional fullerene crystals sandwiched between layers of alkyl chains. Tu, Zhu, and coworkers demonstrated a new strategy to introduce a soft group onto fullerene so that the molecules will self-organize to form supramolecular assembly ( Figure 10) [121]. They succeeded in the synthesis of a thermotropic C 60 supramolecular nanosheet with hierarchical structures. A typical molecule consists of a rigid C 60 , a gallic ester segment substituted with three long alkyl chains as the soft part, and a multimethylene unit as a flexible spacer connecting the two segments. The C 60 dyads underwent self-organization driven by π-π interactions to form triple-layer two-dimensional fullerene crystals sandwiched between layers of alkyl chains. Molecules 2021, 26, x FOR PEER REVIEW 12 of 19 Figure 10. Formation of a thermotropic C60 supramolecular nanosheet from molecules consists of a rigid C60, a gallic ester segment substituted with three long alkyl chains as the soft part, and a multimethylene unit as a flexible spacer connecting the two segments. Reprinted with permission from [121]. Copyright 2015 Wiley-VCH.
According to the work of the above-mentioned work, Tu, Ungar, and coworkers further synthesized the fullerene block molecule C7-C8-C60 with a rigid fullerene sphere and a soft cone connected by a flexible spacer. They proposed an unprecedented type of selfassembly, where the block molecules organize into a semiconducting superlattice of isolated two-dimensional-ordered C60 layers ( Figure 11) [122]. The frustration caused by cross-sectional area mismatch between the spheres can be resolved by direction-flipping steric dipoles. The obtained flat nanosheets possess both sublayer (~2 nm) and superlayer (~10 nm) lamellar thickness. Moreover, the superlattice is found to have unexpectedly high electrical conductivity. Figure 11. Frustrated layered superlattice as flat nanosheets with sublayer (~2 nm) and superlayer (~10 nm) lamellar thickness. Reprinted with permission from [122]. Copyright 2020 American Chemical Society. According to the work of the above-mentioned work, Tu, Ungar, and coworkers further synthesized the fullerene block molecule C 7 -C 8 -C 60 with a rigid fullerene sphere and a soft cone connected by a flexible spacer. They proposed an unprecedented type of self-assembly, where the block molecules organize into a semiconducting superlattice of isolated two-dimensional-ordered C 60 layers ( Figure 11) [122]. The frustration caused by cross-sectional area mismatch between the spheres can be resolved by direction-flipping steric dipoles. The obtained flat nanosheets possess both sublayer (~2 nm) and superlayer (~10 nm) lamellar thickness. Moreover, the superlattice is found to have unexpectedly high electrical conductivity. According to the work of the above-mentioned work, Tu, Ungar, and coworkers further synthesized the fullerene block molecule C7-C8-C60 with a rigid fullerene sphere and a soft cone connected by a flexible spacer. They proposed an unprecedented type of selfassembly, where the block molecules organize into a semiconducting superlattice of isolated two-dimensional-ordered C60 layers ( Figure 11) [122]. The frustration caused by cross-sectional area mismatch between the spheres can be resolved by direction-flipping steric dipoles. The obtained flat nanosheets possess both sublayer (~2 nm) and superlayer (~10 nm) lamellar thickness. Moreover, the superlattice is found to have unexpectedly high electrical conductivity. Figure 11. Frustrated layered superlattice as flat nanosheets with sublayer (~2 nm) and superlayer (~10 nm) lamellar thickness. Reprinted with permission from [122]. Copyright 2020 American Chemical Society. Figure 11. Frustrated layered superlattice as flat nanosheets with sublayer (~2 nm) and superlayer (~10 nm) lamellar thickness. Reprinted with permission from [122]. Copyright 2020 American Chemical Society.

Two-Dimensional Hierarchical Fullerene Nanoarchitecture from Fullerene
Fabrication methods of two-dimensional structures from zero-dimensional fullerene molecules are not limited to direct zero-to-two dimensional nanoarchitectonics. A multiphase strategy such as zero-to-one to two nanoarchitectonics is also possible. In these cases, hierarchical two-dimensional structures are often obtainable. Minami et al. proposed the fabrication of two-dimensional films of one-dimensional fullerene (C 60 ) nanowhiskers that were used as a scaffold for living cells [123]. In the initial step, zero-dimensional C 60 molecules were converted into one-dimensional fullerene nanowhiskers through the liquid-liquid interfacial precipitation method using toluene as a good solvent and isopropyl alcohol as a poor solvent. The obtained one-dimensional fullerene nanowhiskers were then assembled and aligned at the air-water interface and transferred as a two-dimensional film onto a solid substrate through the conventional LB method. This nanoarchitectonics method provides hierarchical structures of two-dimensional structures in a centimeter area with highly aligned one-dimensional fullerene nanowhiskers. The resulting structures were subjected to cell cultures to control cellular orientation and differentiation to muscle cells. Because of the simplicity and low-cost nature of the proposed approach, this hierarchical nanoarchitectonics strategy would be a promising cell scaffold for tissue engineering.
Krishnan et al. demonstrated a modified method to modulate aligning curvature of one-dimensional fullerene (C 60 ) nanowhiskers within the two-dimensional film. Instead of using a conventional LB technique, a newly developed vortex LB method was employed for this purpose [124]. The C 60 nanowhiskers floated on a water surface were spontaneously assembled according to rotational flows of water subphase. Their alignment curvatures can be controlled at the position from a center of rotating motions. The aligned fullerene nanowhiskers with controlled alignment curvatures were then transferred onto glass substrates used for the culture of bone-forming human osteoblast MG63 cells. It was observed that the growth of the MG63 cells was highly oriented with an axis of aligned one-dimensional C 60 nanowhiskers. Cell proliferation experiments confirmed the low toxicity nature of the used hierarchical two-dimensional fullerene scaffold, suggesting the potential availability of the proposed method in various biomedical applications.
Very recently, Song et al. investigated the regulation of adhesion and function of human mesenchymal stem cells on two-dimensional fullerene (C 60 ) nanowhisker nanopatterns with controlled alignments that were prepared with conventional LB technique ( Figure 12) [125]. It was found that regenerative capacity and long-term multipotency were enhanced on two-dimensional nanopatterns with highly aligned fullerene nanowhisker arrays. Appropriately architected nanotopographic surface with certain hydrophobicity gave limited contact area between material surface and cells. This situation induced decreased cell spreading and focal adhesion areas of human mesenchymal stem cells. Enhancement of stemness retention of human mesenchymal stem cells would be related to the diminished cytoskeletal tensions. Regulated contact with surfaces resulted in the appropriate cell contractility with localization of Yes-associated proteins in the nucleus. The proposed strategy would be useful in biomedical applications, such as tissue engineering as a platform for in vitro stem cell expansion. This example demonstrates important roles of two-dimensional assembly to bridge single molecules and life control.

Future Perspective
In this review, various types of fabrication methods of two-dimensional structures from zero-dimensional fullerene molecules are briefly introduced, which is called 'zeroto-two nanoarchitectonics'. Although two-dimensional nanomaterials have been paid much attention, most of them were fabricated through material-based physical methods such as exfoliation and chemical vapor depositions. Unlike these common methods, twodimensional materials described in this review are prepared by supramolecular assembling processes under wet conditions. The presented nanoarchitectonics approaches can be applied to various kinds of solvent-soluble chemical species. In fact, in addition to pristine fullerene molecules, their derivatives and composites are used in the fabrication of two-dimensional materials at interfaces. The presented approaches demonstrated fabrications of various two-dimensional structures including size-tunable hexagonal fullerene nanosheets, two-dimensional fullerene nano-meshes, van der Waals two-dimensional fullerene solids, fullerene/ferrocene hybrid hexagonal nanosheets, fullerene/cobalt porphyrin hybrid nanosheets, two-dimensional fullerene arrays in the supramolecular template, two-dimensional van der Waals supramolecular frameworks, supramolecular fullerene liquid crystals, frustrated layered self-assembly from two-dimensional nanosheets, and hierarchical zero-to-one-to-two fullerene assemblies for cell culture.
Examples shown in this short review article strikingly demonstrated wide varieties in preparation of functional two-dimensional materials even from zero-dimensional nanounits, fullerenes. Therefore, the use of various nano units and their mixtures would create huge possibilities to produce variously functionalized two-dimensional materials, which cannot be obtained from conventional processes such as exfoliation. Coupling with the other interfacial techniques, such as interfacial fabrications of metal-organic frameworks (MOFs) [126] and covalent organic frameworks (COFs) [127], lead to further varieties in structures and functions. Introduction of forefront concepts such as interfacial controls of molecular machines and molecular receptors [128] results in innovative functions

Future Perspective
In this review, various types of fabrication methods of two-dimensional structures from zero-dimensional fullerene molecules are briefly introduced, which is called 'zeroto-two nanoarchitectonics'. Although two-dimensional nanomaterials have been paid much attention, most of them were fabricated through material-based physical methods such as exfoliation and chemical vapor depositions. Unlike these common methods, twodimensional materials described in this review are prepared by supramolecular assembling processes under wet conditions. The presented nanoarchitectonics approaches can be applied to various kinds of solvent-soluble chemical species. In fact, in addition to pristine fullerene molecules, their derivatives and composites are used in the fabrication of two-dimensional materials at interfaces. The presented approaches demonstrated fabrications of various two-dimensional structures including size-tunable hexagonal fullerene nanosheets, two-dimensional fullerene nano-meshes, van der Waals two-dimensional fullerene solids, fullerene/ferrocene hybrid hexagonal nanosheets, fullerene/cobalt porphyrin hybrid nanosheets, two-dimensional fullerene arrays in the supramolecular template, two-dimensional van der Waals supramolecular frameworks, supramolecular fullerene liquid crystals, frustrated layered self-assembly from two-dimensional nanosheets, and hierarchical zero-to-one-to-two fullerene assemblies for cell culture.
Examples shown in this short review article strikingly demonstrated wide varieties in preparation of functional two-dimensional materials even from zero-dimensional nanounits, fullerenes. Therefore, the use of various nano units and their mixtures would create huge possibilities to produce variously functionalized two-dimensional materials, which cannot be obtained from conventional processes such as exfoliation. Coupling with the other interfacial techniques, such as interfacial fabrications of metal-organic frameworks (MOFs) [126] and covalent organic frameworks (COFs) [127], lead to further varieties in structures and functions. Introduction of forefront concepts such as interfacial controls of molecular machines and molecular receptors [128] results in innovative functions in two-dimensional materials. Preparation of two-dimensional materials by bottom-up nanoarchitectonics at interfacial media with novel materials and methodologies would also open new ways in the development of functional materials in various necessary fields such as energy, environmental, and biomedical applications.
Author Contributions: G.C. made the first draft. L.K.S. and K.A. revised it together. All authors have read and agreed to the published version of the manuscript.
Funding: This study was partially supported by JSPS KAKENHI grant no. JP20H00392, JP20H00316 and JP20K05590.