Nanoarchitectonics for Hierarchical Fullerene Nanomaterials

Nanoarchitectonics is a universal concept to fabricate functional materials from nanoscale building units. Based on this concept, fabrications of functional materials with hierarchical structural motifs from simple nano units of fullerenes (C60 and C70 molecules) are described in this review article. Because fullerenes can be regarded as simple and fundamental building blocks with mono-elemental and zero-dimensional natures, these demonstrations for hierarchical functional structures impress the high capability of the nanoarchitectonics approaches. In fact, various hierarchical structures such as cubes with nanorods, hole-in-cube assemblies, face-selectively etched assemblies, and microstructures with mesoporous frameworks are fabricated by easy fabrication protocols. The fabricated fullerene assemblies have been used for various applications including volatile organic compound sensing, microparticle catching, supercapacitors, and photoluminescence systems.

In addition to these nanomaterials and materials with internal nanostructures, materials prepared through assembly and aggregation of small units have also become important in the exploration of functional materials. Such materials have been prepared through self-assembly processes in supramolecular chemistry [70][71][72][73] as well as artificial fabrication processes such as the self-assembled monolayer (SAM) method [74][75][76], Langmuir-Blodgett (LB) technique [77][78][79][80], and layer-by-layer (LbL) assembly [81][82][83]. These approaches have been thus far examined as separated and independent methodologies. In order to achieve more effective and versatile developments of materials exploration with nanoscale structural features, a unified methodology must be established as a post-nanotechnology concept. This task can be assigned as an emerging concept, nanoarchitectonics [84,85], Figure 1. Nanoarchitectonics is a conceptual methodology to combine nanotechnology w research fields, in which functional material systems are prepared from nanoscale units.
Nanoarchitectonics approaches have two distinct features. One of them is harmonized nature between contributing processes [122,123]. Unlike actions between objects at microscopic scales, the interaction between nanoscopic objects often includes uncertainties such as thermal fluctuations, statistical distributions, and quantum effects. Total effects are not always the same as a summation of individual actions. Therefore, materials productions have to be considered with the harmonization of contributing interactions rather than their simple summation. Another feature of the nanoarchitectonics approaches is advantageous features to construct asymmetric and/or hierarchical material systems [124]. Self-assembling processes are mostly driven through an energy-consume-less equilibrium. Unlike conventional self-assembling processes, the nanoarchitectonics approaches can include multiple steps where energy-consuming non-equilibrium processes are often involved. Stepwise processing and sequential treatment for materials fabrications result in the formation of materials with hierarchical structural motifs. It can be said that the nanoarchitectonics approaches are advantageous for the constraction of hierarchical materials structures.
Based on these backgrounds, fabrications of functional materials with hierarchical structural motifs from simple nano units of fullerenes (C 60 and C 70 molecules) are described in this review article. Because fullerenes can be regarded as simple and fundamental building blocks with mono-elemental and zero-dimensional natures, these demonstrations for hierarchical functional structures impress the high capability of the nanoarchitectonics approaches [125,126]. Especially, this review article exemplifies three classes of structures and functions of hierarchical fullerene materials: (i) hierarchically structured fullerene assembly for vapor sensor usage; (ii) fullerene assembly with microscopic recognition capability; (iii) fullerene microstructure with a mesoporous framework for advanced functions.

Hierarchically Structured Fullerene Assembly for Vapor Sensor Usage
Recently, hierarchical nanostructures have gained immense attention due to their multimodal versatility, such as porous architecture with high surface area, diverse functionality, synergistic interactions, multiple functionalities, and easy bottom-up synthesis method [127,128], which are in general difficult to achieve in cases of conventional nanomaterials. Owing to their versatility, hierarchical nanostructures have been used in areas of advanced applications, such as optoelectronics, energy harvesting, sensing, and photonics [129,130]. The selection of buildings blocks for hierarchical nanostructures is a challenging task and extremely important to obtain the desire materials for the specific application. In this regard, fullerenes (C 60 or C 70 ) have recently received significant interest due to their extended π-conjugation and applications in many advanced research fields such as biomedical, semiconductors, optics, electronics, and spintronics [131][132][133]. Additionally, fullerene readily undergoes self-assembly to form dimensionally-controlled nano or microstructures via the simple liquid-liquid interfacial precipitation method ( Figure 2) [134,135]. Along with the liquid-liquid interfacial precipitation method, precise solvent engineering is also very important to control or transform the hierarchical fullerene structure. The fabricated hierarchical fullerene assemblies often exhibit high capabilities in vapor sensing.

Fullerene C70 Cube for Sensing Platform for Volatile Aromatic Solvent Vapor
Bairi et al. demonstrated the preparation of hierarchically structured fullere using C70 as building blocks through the liquid-liquid interfacial precipitation ( Figure 3) [136]. Structural analysis through scanning electron microscopy (SE transmission electron microscopy (TEM) suggested that these cubes are composed oporous fullerene C70 nanorods with crystalline pore walls, which make them an e candidate as a receptor layer for volatile solvent detection. Detailed study of the fo mechanism highlights the importance of precise solvent engineering to control su formation of C70 fullerene cubes to hierarchically structured fullerene cubes. In t isopropyl alcohol was used as an additional solvent to complete this transformat cess of C70 fullerene cubes to hierarchically structured fullerene cubes via hand followed by incubation at 25 °C for 1 h. SEM observations confirmed that isopro hol triggers the structural changes of the cubes, resulting in the formation of fulle nanorods, which subsequently formed the cube surface. The high surface area and architecture of hierarchically structured fullerene cubes make them potential rece terials for volatile organic compound sensing in combination with quartz crysta balance. These hierarchically structured fullerene cubes show excellent select wards aromatic vapors over other organic volatile organic compounds due to th π-π interactions between host and guest. Sensitivity towards toluene is the highes all the aromatic vapors. Additionally, sensitivity towards water vapor is very low Figure 2. Liquid-liquid interfacial precipitation method for self-assembly of fullerene to form dimensionally-controlled nano or microstructures. An example to form a one-dimensional assembly is displayed.

Fullerene C 70 Cube for Sensing Platform for Volatile Aromatic Solvent Vapor
Bairi et al. demonstrated the preparation of hierarchically structured fullerene cubes using C 70 as building blocks through the liquid-liquid interfacial precipitation method ( Figure 3) [136]. Structural analysis through scanning electron microscopy (SEM) and transmission electron microscopy (TEM) suggested that these cubes are composed of mesoporous fullerene C 70 nanorods with crystalline pore walls, which make them an excellent candidate as a receptor layer for volatile solvent detection. Detailed study of the formation mechanism highlights the importance of precise solvent engineering to control such transformation of C 70 fullerene cubes to hierarchically structured fullerene cubes. In this case, isopropyl alcohol was used as an additional solvent to complete this transformation process of C 70 fullerene cubes to hierarchically structured fullerene cubes via handshaking, followed by incubation at 25 • C for 1 h. SEM observations confirmed that isopropyl alcohol triggers the structural changes of the cubes, resulting in the formation of fullerene C 70 nanorods, which subsequently formed the cube surface. The high surface area and porous architecture of hierarchically structured fullerene cubes make them potential receptor materials for volatile organic compound sensing in combination with quartz crystal microbalance. These hierarchically structured fullerene cubes show excellent selectivity towards aromatic vapors over other organic volatile organic compounds due to the strong π-π interactions between host and guest. Sensitivity towards toluene is the highest among all the aromatic vapors. Additionally, sensitivity towards water vapor is very low, which is highly desirable for any kind of receptor material for gas sensing.
Nanomaterials 2021, 11, x FOR PEER REVIEW Figure 3. Preparation of hierarchically structured fullerene cubes and mesoporous fulleren norods with crystalline pore walls. Reprinted with permission from Reference [136]. Copyr American Chemical Society.

Dimension-Dependent Face-Selective Etching of Fullerene Assembly
Controlled structural modification and surface functionalization of such hier fullerene microstructure is highly desirable to widen their sensing ability towards matic vapors. Additionally, conversion of hydrophobic fullerene to their hyd counterpart through surface functionalization opens the possibility to use them in biological applications. Hsieh et al. demonstrated such modification through fa tive chemical etching of fullerene assemblies ( Figure 4) [137]. Their results showe ple and scalable strategy for the fabrication of hollow and hierarchical f nanostructures via face-selective etching of the self-assembled fullerene crystals.
The fabrication of fullerene C60 nanorods, fullerene C60 nanosheets, and fulle cubes was demonstrated by using the modified liquid-liquid interfacial prec method called ultrasound-assisted liquid-liquid interfacial precipitation. SEM im vealed the formation of fullerene C60 nanorods, fullerene C60 nanosheets, and fulle cubes with a uniform size distribution. X-ray diffraction (XRD) confirmed the cr

Dimension-Dependent Face-Selective Etching of Fullerene Assembly
Controlled structural modification and surface functionalization of such hierarchical fullerene microstructure is highly desirable to widen their sensing ability towards nonaromatic vapors. Additionally, conversion of hydrophobic fullerene to their hydrophilic counterpart through surface functionalization opens the possibility to use them in several biological applications. Hsieh et al. demonstrated such modification through face-selective chemical etching of fullerene assemblies ( Figure 4) [137]. Their results showed a simple and scalable strategy for the fabrication of hollow and hierarchical fullerene nanostructures via face-selective etching of the self-assembled fullerene crystals. diamine, and π-electron-rich fullerene. Additionally, the chemical etching bled fullerene crystals is highly dependent on the co-solvent as well as d forms. This effect can be attributed to the difference in solubility of fullere co-solvents. On the other hand, face-selectivity can be explained by the diffe of the different faces of the as-prepared fullerene assemblies. This kind of chemical etching leads to changes of fullerene assembly hydrophilic to hydrophobic. Additionally, porosity is also improved by etching, which leads to better sensing performance. Selectivity of these hy erene assemblies towards hydrophilic acidic volatile components is highe aromatic volatile organic compounds due to the favorable interaction w group, although, selectivity towards other hydrophilic solvents vapor like low. Additionally, such water-dispersible fullerene assemblies are highly their use in biological fields.

Bitter Melon Shaped Nanoporous Fullerene C60 Assembly
As a continuation of the previous discussion, not only the surface fun also the shape of the fullerene hierarchical nanostructures has great imp prove their potential application in a different field. In this regard, the c The fabrication of fullerene C 60 nanorods, fullerene C 60 nanosheets, and fullerene C 70 cubes was demonstrated by using the modified liquid-liquid interfacial precipitation method called ultrasound-assisted liquid-liquid interfacial precipitation. SEM images revealed the formation of fullerene C 60 nanorods, fullerene C 60 nanosheets, and fullerene C 70 cubes with a uniform size distribution. X-ray diffraction (XRD) confirmed the crystalline nature of the as synthesized fullerene assemblies. Chemical etching of these fullerene assemblies was performed in solution with ethylene diamine treatment for 10 min under ultra-sonication followed by incubation at 25 • C for 0 to 24 h depending on their shape. SEM observations revealed that ethylene diamine selectively etches at the ends of the one-dimensional nanorods, leading to the formation of hollow tubular structures, whereas for two-dimensional fullerene C 60 nanosheets, ethylenediamine etches largely at their upper and lower surfaces and partially their edges. Similarly, ethylenediamine etches the faces of the three-dimensional cube, not the edges. However, there were no changes in the overall dimensions after etching indicated the perfect example of faceselective etching of the fullerene assemblies. Chemical analysis suggested that the etching of self-assembled fullerene assemblies is due to the amination reaction between the primary amine, ethylenediamine, and π-electron-rich fullerene. Additionally, the chemical etching of post-assembled fullerene crystals is highly dependent on the co-solvent as well as different crystal forms. This effect can be attributed to the difference in solubility of fullerene in different co-solvents. On the other hand, face-selectivity can be explained by the different reactivity of the different faces of the as-prepared fullerene assemblies.
This kind of chemical etching leads to changes of fullerene assembly surfaces from hydrophilic to hydrophobic. Additionally, porosity is also improved by such chemical etching, which leads to better sensing performance. Selectivity of these hydrophilic fullerene assemblies towards hydrophilic acidic volatile components is higher than that of aromatic volatile organic compounds due to the favorable interaction with the amine group, although, selectivity towards other hydrophilic solvents vapor like alcohol is very low. Additionally, such water-dispersible fullerene assemblies are highly important for their use in biological fields.

Bitter Melon Shaped Nanoporous Fullerene C 60 Assembly
As a continuation of the previous discussion, not only the surface functionality but also the shape of the fullerene hierarchical nanostructures has great importance to improve their potential application in a different field. In this regard, the combination of good solvent and poor solvent during the self-assembly of pristine fullerene via the liquidliquid interfacial precipitation method is the determining factor to control the hierarchical structure. Furuuchi et al. demonstrated the assembly of C 60 into exceptional morphology called "bitter melon" shaped nanoporous C 60 assemblies by tuning the liquid-liquid interfacial precipitation method at room temperature (25 • C) ( Figure 5) [138]. In this case, isopropyl alcohol was used as a poor solvent, and C 60 solution in dodecylbenzene as a good solvent to form a clear liquid-liquid interface. Ultrasonication and vortex mixing were applied to modify the conventional liquid-liquid interfacial precipitation method before 24 h incubation at 25 • C. XRD and high-resolution transmission electron microscopy (HRTEM) analysis confirmed the crystalline nature of the self-assembled fullerene structure. Powder XRD patterns of the as synthesized "bitter melon" shaped C 60 assemblies showed mixed crystal phases, including fcc and hcp phases. TEM analysis confirmed the nanoporous nature of the fullerene structure. Surface textural properties and nanoporous architectures of bitter melon shaped fullerene C 60 assemblies make them excellent receptor materials of quartz crystal microbalance for sensing toxic volatile organic compounds. Here also, selectivity towards aromatic solvent vapors is excellent over nonaromatic compounds due to the favorable π-π interaction. Among aromatic vapor, selectivity towards aniline is the highest. Moreover, micro or mesoporous self-assembled hierarchical fullerene nanomaterials are a class of materials that display excellent efficiency as volatile organic compound sensors, with selectively for aromatic vapors due to their extended π-conjugated structure and favorable π-π interaction with the aromatic vapors. Additionally, functional modification from hydrophobic to hydrophilic while maintaining the morphology improves their selectivity from aromatic to nonaromatic solvent vapor. the nanoporous nature of the fullerene structure. Surface textural properties and rous architectures of bitter melon shaped fullerene C60 assemblies make them receptor materials of quartz crystal microbalance for sensing toxic volatile orga pounds. Here also, selectivity towards aromatic solvent vapors is excellent over matic compounds due to the favorable π-π interaction. Among aromatic vapor, ity towards aniline is the highest. Figure 5. "Bitter melon" shaped nanoporous C60 assemblies prepared through the liquidterfacial precipitation method at room temperature [138].

Fullerene Assembly with Microscopic Recognition Capability
The concept of the nanoporous hierarchical structure has been studied extensively and is now well established. However, the manipulation of micron-sized hollow objects with self-assembled hierarchical structures remains very challenging. Such kinds of microscale hollow objects and manipulation of such hollow structures are highly important for advanced applications, which include loading, transportation, and release of nano/micron-sized objects, especially cells, bacteria, biomolecules, and functional nanoparticles [139][140][141]. Therefore, researchers are giving extensive effort to fabricate such micron-sized hollow hierarchical objects, and there are only a few successful examples. In the following examples, fullerene C 70 offers such manipulable micron-sized hollow structures via the controlled liquid-liquid interfacial precipitation method. Additionally, it was established that the process to close and open the holes can be done purposefully. Fullerene C 70 cubes with open or closed holes were produced by the dynamic liquid-liquid interfacial precipitation method at 25 • C using mesitylene as a good solvent and tertiary butyl alcohol as a poor solvent followed by 24 h incubation under 25 • C. The formation mechanism of such an open hole cube is different from the previously discussed solvent or chemical etching mechanisms. Detailed structural analysis via cross-section SEM confirmed that the holes are not hollow through. Additionally, TEM images of fullerene assemblies formed just after the mixing of C 70 -mesitylene with tertiary butyl alcohol confirmed the formation of a smaller cube with no holes. It was suggested from this observation that the open hole formation is not driven by the solvent etching mechanism, but rather the growth of open hole cubes involves a two-step process including a solid core formation at the first step followed by slow growth to the final open hole cube formation. In the second step, concentration depletion and the different reactivity of the corner, as well as the edges, play important roles to form such a uniform open hole cube. This observation is in line with the formation mechanism of the closed hole cube. When the mesitylene/tertiary butyl alcohol ratio is fixed at 1:2, the C 70 concentration is below the critical concentration level to form the new core. Therefore, C 70 molecules tend to grow over each face of the open hole cube to form the closed hole cube. These open hole cubes show excellent microscopic recognition properties towards the micron size particles. Open hole cubes specifically recognize the graphitic carbon particles over resorcinol-formaldehyde polymeric resin particles with similar dimensions. SEM observations show that the open holes of the cube were mostly occupied by the gra- These open hole cubes show excellent microscopic recognition properties towards the micron size particles. Open hole cubes specifically recognize the graphitic carbon particles over resorcinol-formaldehyde polymeric resin particles with similar dimensions. SEM observations show that the open holes of the cube were mostly occupied by the graphitic particles, whereas polymeric resin particles were not recognized by these open hole cubes. This phenomenon can be attributed to the favorable π-π interaction of the graphitic carbon particles with the fullerene cube. Moreover, controlled synthesis of such functional materials with precise manipulation of open or closed hollow structures is highly important for many advanced applications, such as the controlled release of drugs, protection of biologically active species, and removal of pollutants from air or water.

Fullerene Microhorns with Microscopic Recognition Properties
We discussed in the previous section that the manipulation of the hollow structure with specific morphology is highly important for advanced nanotechnology. In addition to this, the structural transformation from one morphology to another is also extremely important. It is very important to find out an effective and versatile strategy to manipulate the morphology of such self-assembled nanostructures. In this regard, solvent engineering is showing extreme potential, which involves the use of different types of solvents or a mixture of solvents with different ratios to control the self-assembly conditions to achieve desired materials.
Recently, Tang et al. demonstrated the fabrication of fullerene microstructures with a hollow framework from a mixture of C 60 and C 70 based on the dynamic liquid-liquid interfacial precipitation method (Figure 7) [143]. Additionally, considering the different crystalline phases and solubility of C 60 and C 70 and precise solvent engineering, they can transform the microstructure into unique conical-shaped fullerene microhorns. A C 60 and C 70 fullerene mixture with a 4:1 volume ratio was used in mesitylene as a good solvent. A mixed fullerene microtube was fabricated by the dynamic liquid-liquid interfacial precipitation method using tertiary butyl alcohol as a poor solvent. The overall reaction was very fast and completed in a few seconds after the addition of the fullerene fixture into the poor solvent. Details analysis suggests that the mixing ratio of fullerene C 60 and C 70 plays an important role to control the formation of the microtube. Interestingly, the microtube formation of fullerene C 70 is not possible due to their crystal packing. However, with the help of fullerene C 60 , fullerene C 70 was forced to form the microtube through hexagonal closed packing. Fullerene microtube-to-microhorn transformation was accomplished by precise solvent engineering. Then, a mesitylene/tertiary butyl alcohol (volume ratio of 1:3) solvent mixture was used to wash the microtube to form the perfectly homogeneous microhorn. Interestingly, there was no change in crystallinity of the microhorn, which suggests that the process of such morphological transformation is fully dominated by the physical changes. Additionally, such solvent engineering helps us to generate a nanoporous architecture inside the microhorn.
Real-time optical microscope imaging suggests that the formation of microhorns directs through the solid core dissolution of the fullerene microtube. This can be explained by the difference in solubility in good and poor solvents. Solubility of fullerene C 70 is lower in alcohol than that of fullerene C 60 . Therefore, fullerene C 70 molecules tend to form tiny aggregates when the mixture of C 60 and C 70 starts to form the crystal, and that core forms the center solid part of the microtube. Therefore, there is a higher concentration of C 70 at the central part of the microtube. When the mesitylene-tertiary butyl alcohol solvent mixture is added to the microtube, C 70 molecules tend to dissolve faster in mesitylene due to their higher solubility over C 60 . In this way, these fullerene microtubes transfer to their secondary morphology microhorn.  Real-time optical microscope imaging suggests that the formation of microh rects through the solid core dissolution of the fullerene microtube. This can be e by the difference in solubility in good and poor solvents. Solubility of fullerene C7 in alcohol than that of fullerene C60. Therefore, fullerene C70 molecules tend to f aggregates when the mixture of C60 and C70 starts to form the crystal, and that co the center solid part of the microtube. Therefore, there is a higher concentration the central part of the microtube. When the mesitylene-tertiary butyl alcohol solv ture is added to the microtube, C70 molecules tend to dissolve faster in mesitylen their higher solubility over C60. In this way, these fullerene microtubes transfer These hollow microhorns also show excellent microscopic recognition properties to the silica particles over polystyrene particles. However, the recognition property towards fullerene C 70 nanoparticles is not so satisfactory, which suggests that this phenomenon is not directed by typical π-π interactions between the microhorn and the C 70 particles. Zeta potential charge analysis suggests that the microscopic recognition phenomenon is governed by the electrostatic surface charge of the microhorn. Moreover, such morphological transformation of fullerene assemblies through solvent engineering delivers important perceptions into the field of supramolecular self-assembly. Additionally, specific microscopic recognition by such hollow structures has immense potential for utilization in different fields, as stated earlier.

Fullerene Microstructure with Mesoporous Framework for Advanced Function
Not only the microscopic hollow structure but also the mesoporous structural materials with crystalline frameworks also have great importance in many advanced fields. Additionally, such post-synthesis modification of self-assembled materials to their mesoporous architecture critically influences their overall properties including optical, electrical, charge storage, etc. Thus, mesoporous materials with crystalline frameworks and tunable pore sizes might have substantial technological advantages. Therefore, a proper synthesis methodology and suitable building blocks for novel crystalline mesoporous materials remain a high point of interest due to their significant potential in many advanced fields. Here, fullerene also plays an important role to form such a crystalline framework via selfassembly, and transformation to the mesoporous framework can be achieved by a simple post-synthesis treatment, which leads to changes in its optical, charge storage mechanism.

Mesoporous Fullerene C 70 Cube with Enhanced Photoluminescence Property
Bairi et al. have recently demonstrated one such novel piece of work, which includes the formation of mesoporous fullerene C 70 cubes with highly crystalline frameworks with excellent photoluminescence properties [144]. Crystalline fullerene cubes with sharp edges were fabricated by the ultrasound liquid-liquid interfacial precipitation method from tertiary butyl alcohol as a poor solvent and a solution of C 70 in mesitylene as a good solvent at 25 • C. A good solvent to poor solvent ratio is one of the important parameters to control the desired morphology, and here in this case it was 1:5. After the preparation of the fullerene cube, the mother liquor was stirred at 300 rpm for 72 h at 75 • C and then drop-casted on a silicon wafer and dried at 80 • C to prepared the mesoporous crystalline fullerene cube. An adsorption isotherm confirmed the formation of mesopore inside the fullerene cube, as the surface area of the mesoporous structure was higher than that of the fullerene cube. Crystal structure analysis via XRD confirmed that the as-synthesized fullerene cube contained the simple cube packing, whereas mesoporous has a mixed crystal phase with a simple cube and hexagonal closed pack. This phenomenon can be attributed to the entrapped solvent molecules, which change the crystal packing of C 70 and reducing the crystal symmetry. Photoluminescence properties of mesoporous fullerene cubes are improved by such crystallographic modification, which can be confirmed from the comparison of photoluminescence spectra of pristine C 70 , fullerene cubes, and mesoporous fullerene cubes. It is worth noticing that for π-conjugated molecules, photoluminescence intensity is quenched in the solid-state. However, the photoluminescence intensity of mesoporous fullerene cubes is higher in the solid-state, which indicates the importance of such structural and crystalline framework modification.

Mesoporous Carbon Cubes for Supercapacitors
We have discussed earlier that the incorporation of mesopores creates substantial changes in material properties and makes them suitable for many advanced applications. One such application of the mesoporous materials is in electrochemical charge storage. During the last three decades, researchers have given enormous efforts to finding suitable mesoporous materials for charge storage applications [145][146][147]. Carbon-based materials with mesoporous architectures are supposed to be the best materials for such charge storage properties [148][149][150]. However, unwanted surface functionality and poor electrochemical conductivity of such carbon materials have to be addressed to improve their performance. Fullerene C 60 or C 70 is a π-conjugated molecule and readily self-assembles to form a homogeneous shape-controlled microstructure. Recently, researchers have focused on these dimensionally-controlled fullerene assemblies as an exceptional source of π-electronrich carbon materials. Such extended conjugated π-systems with high surface areas and mesoporous frameworks will be advantageous for energy storage applications such as supercapacitors, battery, etc. [151][152][153].
Bairi et al. demonstrated the direct transformation of porous crystalline fullerene C 70 cubes into mesoporous carbon cubes that possess a very high specific surface area [154]. Synthesis of a porous crystalline C 70 cube was done by the liquid-liquid interfacial precipitation method followed by mild heat treatment at 70 • C. XRD and TEM analysis confirmed the crystalline framework of the C 70 cube. Conversion of porous crystalline fullerene C 70 cubes to high surface area mesoporous carbon cubes was performed by high-temperature heat treatment (900 • C) under a continuous flow of N 2 gas in a tube furnace. A nitrogen adsorption isotherm confirmed the formation of a mesoporous architecture with a narrow pore size distribution. The Brunauer-Emmett-Teller surface area of the mesoporous carbon cube was very high at ca. 642.6 m 2 g −1 , which is almost 14 times higher than that of the porous crystalline fullerene cube (ca. 47.7 m 2 g −1 ). Therefore, high-temperature heat treatment is an essential step for such a large improvement of specific surface area. Pore size analysis by Barrett-Joyner-Halenda and the non-local density functional theory method confirmed the presence of both micropores and mesopores. The average pore size of the obtained carbon cube was 3.44 nm with a high pore volume of 0.367 cm 3 g −1 . XRD and Raman analysis of the obtained carbon cube confirmed the graphitic nature of carbon.
This newly synthesized mesoporous carbon cube with a high surface area showed excellent electrochemical charge storage properties. Electrochemical measurements via cyclic voltammetry and charge-discharge measurements revealed that this microstructure carbon cube showed excellent specific capacitance of ca. 286 F g −1 at a scan rate of 5 mV s −1 and 205 F g −1 at a current density of 1 A g −1 . These values are highly comparable with other nanocarbon materials, such as graphene and carbon nanotubes. The rate capability of this material is excellent, with a very high retention of specific capacitance of 56.0% at a very high current density of 20 A g −1 . Additionally, the cyclic stability of this material is also very high, with almost no loss of specific capacitance even after 10,000 chargedischarge cycles.

Quasi Two-Dimensional Mesoporous Carbon Microbelts for Supercapacitors
Electrochemical charge storage is highly dependent on the effective surface area accessible to the electrolyte ions. Therefore, it is very important to change the morphology of the fullerene-derived carbon to obtain maximum charge storage capacity. Here, fullerenes offer a great opportunity, as they can form almost any kind of morphology starting from one-dimensional rod to two-dimensional sheet to three-dimensional cube through the supramolecular assembly. Studies on charge storage mechanisms suggest that the sheet-like structure shows great performance.
Recently, Tang, et al. reported a novel method for fabrication of two-dimensional fullerene microbelts, which can be transferred to mesoporous carbon with the retention of their original sheet-like structure (Figure 8) [155]. Fullerene microbelts were made using the conventional liquid-liquid interfacial precipitation method taking CS 2 as a good solvent and isopropyl alcohol as a poor solvent followed by 24 h incubation at 25 • C to complete the full growth of the fullerene microbelts. Powder XRD analysis of the fullerene microbelts confirmed the mixed fcc and monoclinic crystal phase, which is different from the fcc crystal phase of the pristine C 60 powder. Mesoporous carbon microbelts were prepared by high-temperature heat treatment at the two different temperatures of 900 • C and 2000 • C. A nitrogen adsorption isotherm confirmed the formation of the porous architecture of the fullerene microbelt-derived carbon. Brunauer-Emmett-Teller surface areas of mesoporous carbon microbelts derived at 900 • C and 2000 • C were 980 m 2 g −1 and 297 m 2 g −1 , respectively. The surface area of the fullerene microbelt was much lower compared to that of the fullerene microbelt-derived carbon materials, which confirmed the importance of high-temperature heat treatment. Raman and XRD analysis confirmed the graphitic nature of the derived mesoporous carbon belt. TEM and HRTEM analysis of the derived mesoporous carbon confirmed the amorphous nature of the carbon. mesoporous carbon microbelts derived at 900 °C and 2000 °C were 980 m 2 g −1 and g −1 , respectively. The surface area of the fullerene microbelt was much lower comp that of the fullerene microbelt-derived carbon materials, which confirmed the imp of high-temperature heat treatment. Raman and XRD analysis confirmed the g nature of the derived mesoporous carbon belt. TEM and HRTEM analysis of the mesoporous carbon confirmed the amorphous nature of the carbon. These mesoporous microbelts were used as active electrode materials for s pacitive charge storage applications. Cyclic voltammetry curves of the mesopor bon microbelts showed quasi-rectangular shapes even at a very high scan rates indicated the typical feature of electrical double-layer capacitors. The specific capa of mesoporous carbon microbelts obtained from cyclic voltammetry curves was 3 at 5 mV s −1 , which was very high compared to the fullerene microbelts (38.4 F g −1 s −1 ). This improvement in specific capacitance can be attributed to the high surface the mesoporous carbon microbelts. Chronopotentiometry (charge-discharge) cu the mesoporous carbon microbelts showed a triangular nature, which is also the si of the electrical double-layer capacitors. The specific capacitance obtained fr charge-discharge curves for mesoporous carbon microbelts prepared at 900 °C w g −1 at a current density of 1 A g −1 . Additionally, these derived carbon materials excellent rate capability with retention of 48.8% specific capacitance even at a ve current density of 10 A g −1 . A cyclic stability test of the mesoporous carbon mi prepared at 900 °C was performed over 10,000 charge-discharge cycles at 10 A g − showed positive cyclic stability. This phenomenon can be recognized as addition vation of intercalation or deintercalation of electrolyte ions during the charging/d ing cycles. These mesoporous microbelts were used as active electrode materials for supercapacitive charge storage applications. Cyclic voltammetry curves of the mesoporous carbon microbelts showed quasi-rectangular shapes even at a very high scan rates, which indicated the typical feature of electrical double-layer capacitors. The specific capacitance of mesoporous carbon microbelts obtained from cyclic voltammetry curves was 360 F g −1 at 5 mV s −1 , which was very high compared to the fullerene microbelts (38.4 F g −1 at 5 mV s −1 ). This improvement in specific capacitance can be attributed to the high surface area of the mesoporous carbon microbelts. Chronopotentiometry (charge-discharge) curves of the mesoporous carbon microbelts showed a triangular nature, which is also the signature of the electrical double-layer capacitors. The specific capacitance obtained from the charge-discharge curves for mesoporous carbon microbelts prepared at 900 • C was 290 F g −1 at a current density of 1 A g −1 . Additionally, these derived carbon materials showed excellent rate capability with retention of 48.8% specific capacitance even at a very high current density of 10 A g −1 . A cyclic stability test of the mesoporous carbon microbelts prepared at 900 • C was performed over 10,000 charge-discharge cycles at 10 A g −1 , which showed positive cyclic stability. This phenomenon can be recognized as additional activation of intercalation or deintercalation of electrolyte ions during the charging/discharging cycles.

Summary and Perspectives
This review article summarized several examples focused on syntheses and function explorations of fullerene assemblies with hierarchical and asymmetric structural features. Basic structures of these fullerene assemblies are fabricated first through very simple methods, mostly the liquid-liquid interfacial precipitation method, and then post-treatment often results in conversion of rather simple symmetric structures to complicated hierarchical structural motifs. Process-integrated features in nanoarchitectonics approaches are advantageous for the preparation of hierarchical functional structures. Even though very simple fullerenes with mono-elemental (carbon) zero-dimensional structures are used as building units, various hierarchical structures such as cubes with nanorods, hole-incube assemblies, face-selectively etched assemblies, and microstructures with mesoporous frameworks are fabricated upon easy fabrication protocols. Accordingly, the fabricated fullerene assemblies are used in various applications including volatile organic compound sensing, microparticle catching, supercapacitors, and photoluminescence systems.
It must be noted that these rich varieties of structures and functions are obtained through nanoarchitectonics processes of simple building units. Expansion of these methodologies to a wide range of materials will create further huge possibilities in the fabrication of functional materials with hierarchical structural motifs. Various structural units with specific interactions such as host-guest systems [156][157][158][159] and biomolecules (and their mimics) [160][161][162] would have high potentials as active building blocks. In addition, the other material-based building units [163][164][165] and their hybrids/composites [166][167][168] would open further possibilities in nanoarchitectonics materials predictions. This wide range of possibilities could be logically handled by emerging approaches such as machine learning [169][170][171].
This review article mainly discusses recent progress on hierarchical fullerene nanoarchitectonics, mainly from our own results that would have some progress from the first step fullerene assemblies [172]. Not limited to our results, some examples on fullerene assemblies with certain complexities have been reported from some groups [173][174][175]. Various factors including molecular designs and assembling conditions would effectively work on the construction of desired structures, which is not fully clear yet. Accumulation of research facts would lead to total understanding of hierarchical fullerene nanoarchitectonics in the near future. From basic science to practical applications such as biomedical usages, various outputs can be highly expected.