06) 2278–2281www.elsevier.com/locate/matlet

Materials Letters 60 (20

Synthesis of three-dimensional C60 micro-flowers: A scanning electronmicroscopy study

Vinay Gupta a,b,c,⁎, Peter Scharff c, Norio Miura a

a Art, Science and Technology Center for Cooperative Research, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japanb Japan Science and technology Agency, Kawaguchi-shi, Saitama 332-0012, Japan

c Institute of Physics, Ilmenau Technical University, Ilmenau 98684, Germany

Received 24 April 2005; accepted 29 December 2005Available online 30 January 2006

Abstract

A new class of C60 microstructure materials, the C60 micro-flowers, has been synthesized from solution based growth process. Scanningelectron microscopy images showed that the C60 micro-flowers having sizes of a few micrometers were formed on Si surface. No such flowersformation was observed on glass or metal surfaces. The wide varieties of obtained C60 micro-flowers are the smallest real-looking flowers eversynthesized.© 2006 Elsevier B.V. All rights reserved.

Keywords: C60; Micro-flower; Toluene; Scanning electron microscopy

Nature blesses us with a wide range of exquisite-patternstructures. Materials with micrometer-scale regularity are ofgreat technological applications in microelectronics, informa-tion storage, optics, biomedical implants, catalysis andseparation technologies [1–3]. To control crystalline patterning,the important parameters are nucleation, morphology, size,stability and architecture. Attempts were made to synthesizenano and micro-size crystals of fullerenes in the recent years byvarious techniques [4–9]. The aerosol technique at temperaturesbetween 200 °C and 500 °C resulted in the C60 and C70 crystalsof sizes between 20 nm to a few hundred nanometers [5]. Themost remarkable observation was the pseudo-five-fold symme-try of the crystals. In the solution-growth of C60 crystals,fullerenes needles were synthesized on a silver plate electro-chemically [8]. The size of such needles was determined by thespeed of evaporation. It has been demonstrated that the growthof fullerene crystals has large anisotropy due to anisotropicactivation mechanism occurring in the thermal equilibriumcondition for assembled C60 from solution. In the photo-assistedgrowth of C60 nano-whiskers (C60 NWs) by the liquid–liquid

⁎ Corresponding author. KASTEC, Kyushu University, Kasuga-shi, Fukuoka,816-8580, Japan. Tel.: +81 9028501502; fax: +81 925837886.

E-mail address: drvinaygupta@netscape.net (V. Gupta).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2005.12.145

interfacial precipitation method [9], it was shown that thegrowth rate of C60 NWs depends strongly on the wavelength ofthe illuminated light. Solution-grown C60 crystals have variousstructures and shapes depend on organic solvent and growthtemperature. Here, we report the synthesis of micron-sizefullerene flowers on the silicon (Si) surface for the first time viaa solution-based growth.

C60 (99.9% pure) was procured from MER Corporation. Asolution of C60 in toluene containing 1.5 mg/ml was prepared. Adrop of this solution was applied onto smooth Si surface anddried overnight. The surface of the Si surface was analyzed inscanning electron microscope (SEM). C60 structures withflower like morphology were observed as shown in Fig. 1.These flowers have perfect leaves patterns having length of afew micrometers. The flowers were of various sizes and shapes,and are typically of several micrometer sizes. The flowerstructures looked like natural flowers. The peculiar feature ofsuch flowers is the bud like formation at the center of theflowers as shown in Fig. 1(d). Interestingly, such structures arealso a feature of the natural flowers. These perpendicularstructures are in various types and mostly in tubular form of C60

films as shown in Fig. 2. The most plausible mechanism of suchnanotube formation is due to the rolling of C60 films over theflowers surface as is clear in Fig. 3(a), where partially formed

Fig. 1. (a–h) Scanning electron microscopy images of C60 micro-flowers on Si substrate.

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flower clearly shows the half-rolled elongated C60 films. Fig. 3(b) indicates the three-dimensional evolution of flower-pattern.It can be seen in Fig. 3(b) that growth of new C60 leaves takesplace on the top of previously grown C60 leaves.

The formation mechanism of such flowers can be understoodin terms of i) activation energy for anisotropicity of C60 leavesand ii) the interaction of Si surface with C60. It has beenestimated that the relative anisotropic activation energy(ΔEact∼280 meV) mechanism can take place in the thermal-equilibrium for assembled C60 from solution by Ogawa et al.[8]. The process of growth of flowers is shown in Fig. 4. In Fig.4(a), a droplet of the C60/toluene is shown. In the drop, the

concentration of C60 molecules is more in the center and less onthe boundary. During the evaporation of the toluene, the dropcontract and the C60 molecules begin to coagulate in the form asshown in Fig. 4(b). Due to more concentration in the center andless on the boundary, the structure is in the form of flower. Thisprocess may also take place in the three dimensions. Anotherless plausible explanation is that such a growth begins with anucleation center and a uniform growth of C60 structure in alldirections in the leaf form, than C60 structure would look likeflower pattern.

Interestingly, such three-dimensional flowers could not beobtained on glass or metal surfaces. Therefore, the role of

Fig. 2. (a, b) Scanning electron microscopy images of C60 nanotubes at the top ofC60 micro-flowers on Si substrate.

Fig. 3. Scanning electron microscopy images of (a) partially formed C60 micro-flowers; (b) secondary growth of new C60 leaves on the top of C60 micro-flowerson Si substrate.

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surface can be of importance here. Fig. 5 shows themicrostructure of the C60 film obtained on a glass surface.Fig. 5(a) shows the large-area SEM image, whereas Fig. 5(b)shows the magnified SEM image where a fractal-like structureis visible. These are actually leaves attached to the surface andmerging into each other in two dimensions. It is clear from theseimages that C60 tends to adhere strongly to a glass surface andtherefore no three-dimensional evolution of such leaves ispossible. Such leaves merge quickly to form a uniform film asshown in Fig. 5(c). This is the main reason that such phenomenacould not be noticed before. On the other hand, adhesion of theC60 molecules to Si surface is very low so C60 can make three-dimensional structures and therefore the growth of flowerspatterns flourish. Whereas on the other surfaces, it mostly takesplace in two dimension and merged into each other quicklyenough to become a uniform film.

In summary, C60 micro-flowers have been synthesized by asimple solution based technique on the Si surface. The simpleand low cost process is suitable for large-scale production ofC60 crystals with specific shapes. Since the new applications ofC60 are emerging, such structures can both be fundamentally

and technologically important, i.e., in gears and wheels inMEMS devices, micro-antenna, etc.

Acknowledgements

Financial support from the AvH foundation is gratefullyacknowledged.

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Fig. 4. (a–c) Schematic diagrams of C60 micro-flower formation.

Fig. 5. (a–c) Scanning electron microscopy images of C60 film on a glasssurface.

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