The excellent electronic and mechanical properties of fullerene (C₆₀) make it a promising carbon nanomaterial for advanced energy conversion and storage devices ^{[1, 2, 3, 4]}. To improve the performance of devices utilizing them,morphology control of fullerene nanostructures has been considered a crucial factor in tailoring its properties ^{[5, 6, 7]}. In this regard,much attention has been paid to controlled construction of diverse nano/microstructures of C₆₀ ^{[8, 9]}. For example,rods ^[10],nanowires ^[11], nanowhiskers ^[12],sheets ^[13],and spheres ^[14] have been fabricated from pristine C₆₀solution,by direct evaporation ^{[11, 14]}, liquid-liquid interfacial precipitation ^{[12, 13]},and re-precipitation ^[10]. These approaches,however,are hampered by a very limited numbers of solvents due to the poor solubility of pristine C₆₀in common organic solvents. Alternatively,well-defined supramolecular hierarchical architectures have been fabricated from amphiphilic C₆₀ derivatives with various appends ^{[15, 16, 17]}. However,it is necessary to covalently attach insulated appends onto the periphery of the C₆₀cage,which may weaken its intact properties. The controlled fabrication of various architectures of unmodified C₆₀viasimple construction processes is still highly demanded.

Here we report a facile approach for constructing diverse architectures of unmodified C₆₀viasimple evaporation of pure C₆₀ solution in CS₂ under various atmospheres. In our approach,CS₂ was selected as a good solvent that has a solubility of 8 mg/mL for C₆₀,whereas 19 poor solvents,such as alcohols,tetrahydrofuran, andN,N-dimethylformamide were used as atmosphere to control the self-assembly of C₆₀^[18]. Diverse nanostructures such as belts, sheets,and starfishes,were successfully constructed under different experimental conditions. C₆₀belts obtained under EtOH atmosphere were also characterized by TEM and XRD,and a facecentered cubic (fcc) structure was confirmed. We believe that the concept of self-assembly under poor solvent atmosphere represents a novel method for preparation of nanostructures of C₆₀and it could also be applied for other functional organic molecules. 2. Experimental

C₆₀(BuckyUSA,purity 99.5%) and all solvents (anhydrous) were purchased from XFNANO and J&K and used as received. The objects obtained were characterized with SEM (TM-1000,Hitachi),TEM (JEM-2100,JEOL),and XRD (RIGAKU D/max-2550 PC).

Briefly,the typical procedure for fabrication of C₆₀microbelt was as follows: 20 μL C₆₀stock solution (1 mg/mL in CS₂) was transferred onto a Si wafer (5 mm×5 mm) and evaporated in a glass petri dish (diameter 70×height 20 mm) under 1 mL EtOH atmosphere at room temperature (ca.20°C),where the Si wafer was simply washed by acetone and dried in air before use; the C₆₀ solution was completely dried within 2 h,and various objects were then obtained for SEM observation. Others architectures were similarly constructed by only changing the solvent atmosphere to the corresponding one. 3. Results and discussion

Self-organized objects of C₆₀were prepared by evaporation of carbon disulfide solution of pure C₆₀on Si wafer under various solvent atmospheres,and their SEM images are shown in Fig. 1. The diverse micron-sized morphologies,such as belts,sheets,porous belts,and starfishes were obtained under different experimental conditions (Fig. 1). Evaporation of carbon disulfide solution of pure C₆₀without additional solvent atmosphere leads to formation of flat belts with a width ofca.5-30 μm,where CS₂molecules may assist the self-assembly of C₆₀^[19]. Some solvent atmospheres did not affect the belt morphology (Fig. 1b-f,n-p,r,and s),but the width distribution was 1-5mm under EtOH atmosphere. The other solvents completely changed the final morphology (Fig. 1g-m,q, and t). These clearly demonstrate that it is possible to control the morphology of pure C₆₀by direct evaporation of its solution in good solvent under poor solvent atmospheres. Although the exact details of how these morphologies are controlled are not well understood now,it has been recognized that a delicate balance among supramolecular interactions ^[15],the surface free energy of substrate ^[20],and the solvent geometry structure ^[21] have critical effects on fullerene self-assembly into various architectures. Therefore,the fabrication of fine shape- and size-controlled architecture could be achieved by comprehensive influences of these factors. The effect of solvent atmosphere on morphology can be classified into three categories. The first one is alcohols,ethyl acetate,acetonitrile,hexane,acetone,and acetic acid,where the belt morphology did not change so much,but the length and width could be tuned. The second one is chloroform,tetrachloromethane, tetrahydrofuran,1,4-dioxane,N,N-dimethylformamide,cyclohex anone,triethylamine,where the morphology changed into sheets, starfishes,and rods with particles,respectively. The third one is toluene and 1,2-dichloroethane,where the porous morphology was obtained. It is noted that these categories seems to be related to the solubility of C₆₀in these solvents,namely,from alcohol, chloroform to toluene series,the solubility of C₆₀ is highly enhanced ^[18]. Therefore,the solvent atmospheres not only slowed down the evaporation speed,but also reorganized the self-assembly of C₆₀ by partially re-dissolving the formed nanostructure. This approach could be extended to other goodpoor solvents pairs for constructing various nanostructures of pristine C₆₀.

Fig. 1

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Fig. 1. SEM images of diverse architectures obtained from unmodified C60in CS2under (a) air, (b) MeOH, (c) EtOH, (d) PrOH, (e)i-PrOH, (f) BuOH, (g) chloroform, (h) tetrachloromethane, (i) 1,2-dichloroethane, (j) toluene, (k) tetrahydrofuran, (l) 1,4-dioxane, (m)N,N-dimethylformamide, (n) ethyl acetate, (o) acetonitrile, (p) hexane, (q) cyclohexanone, (r) acetone, (s) acetic acid, and (t) triethylamine atmospheres. The inset bar represents 10μm.

The belt architectures obtained under EtOH atmosphere were also characterized by TEM and XRD. As shown in Fig. 2a,these microbelts are almost transparent under the electron beam, indicating that they are ultrathin in nature. X-ray diffraction (XRD) patterns for belts recorded on a Si substrate are shown in Fig. 2b. The well-defined XRD patterns of microbelts match well with those of pristine C₆₀. Thus,these patterns were readily assigned having a fcc structure,where four strong peaks are indexed as (1 1 1),(2 2 0),(3 1 1),and (2 2 2),indicating that the belt structures are mainly packed in a fcc way.

Fig. 2

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Fig. 2. TEM image (a) and XRD pattern (b) of belts obtained from C60in CS2 under EtOH atmosphere. The XRD pattern of pristine C60powder was also provided for comparison.

A facile approach for constructing diverse architectures of unmodified C₆₀was developedviasimple evaporation of pure C₆₀ solution in CS₂ under various solvent atmospheres. In our approach,CS₂ was selected as a good solvent for C₆₀,whereas 19 poor solvents such as alcohols,tetrahydrofuran,N,N-dimethylformamide,toluene,and chloroform were used as atmosphere to control the self-assembly. Various architectures such as belts, sheets,and starfishes were successfully constructed under different experimental conditions. The C₆₀belts obtained under EtOH atmosphere were also characterized by TEM and XRD,and a fcc structure was confirmed. The solvent atmospheres not only slowed down the evaporation speed,but also could reorganize the self-assembly of C₆₀by partially re-dissolving the initially formed architectures. This simple but effective strategy could be applied for controlling of the self-assembly of other functional organic molecules. Acknowledgments

This work was financially supported by the Innovation Program of Shanghai Municipal Education Commission (No. 12ZZ067), Shanghai Pujiang Program (No. 11PJ1400200),the Research Fund for the Doctoral Program of Higher Education of China (No. 20120075120018),and the Fundamental Research Funds for the Central Universities