Formation and reinforcement of clusters composed of C60 molecules

We carry out two experiments: (1) the formation of clusters composed of C60 molecules via self-assembly and (2) the reinforcement of the clusters. Firstly, clusters such as fibres and helices composed of C60 molecules are produced via self-assembly in supercritical carbon dioxide. However, C60 molecules are so weakly bonded to each other in the clusters that the clusters are broken by the irradiation of electron beams during scanning electron microscope observation. Secondly, UV photons are irradiated inside a chamber in which air is filled at 1 atm and the above clusters are placed, and it was found that the clusters are reinforced; that is, they are not broken by electron beams any more. C60 molecules located at the surface of the clusters are oxidised, i.e. C60On molecules, where n = 1, 2, 3 and 4, are produced according to time-of-flight mass spectroscopy. It is supposed that oxidised C60 molecules at the surface of the clusters may have an important role for the reinforcement, but the actual mechanism of the reinforcement of the clusters has not yet been clearly understood and therefore is an open question.

The gas-liquid coexistence curves terminate at the critical points [31]. Incident light cannot penetrate fluids as they approach their critical points, known as critical opalescence, due to the formation of large molecular clusters [31]. It was recently shown that fibres, fibre networks, sheets and helices composed of C 60 molecules were selfassembled by leaving C 60 crystals in ethane, xenon or carbon dioxide under supercritical conditions for 24 h [32]. Those structures were formed via van der Waals interactions between C 60 and the fluids' molecules.
In this letter, we create clusters composed of C 60 molecules via self-assembly in supercritical carbon dioxide and reinforce the clusters by attaching oxygen atoms to the surface of C 60 molecules. Figure 1 shows an outline of the experimental apparatuses. We carried out two experiments. First, clusters composed of C 60 molecules are produced by leaving C 60 crystals in carbon dioxide under supercritical conditions for 24 h [32] (see Figure 1a). The inner volume of the supercritical fluid chamber made of aluminium was 11.7 ml. Of the crystals composed of C 60 molecules, 0.2 mg was placed on the surface of a silicon plate set at the bottom of the supercritical fluid chamber and carbon dioxide of critical density was introduced into the chamber. The temperature of the fluid was set at 36.0°C by a heater installed around the chamber, which was regulated by a PID controller (C541, Technol Seven Co. Ltd., Tokyo, Japan). The temperature was monitored by a thermistor (SZL-64, Takara Thermistor Co. Ltd., Tokyo, Japan) embedded inside the chamber wall. Note that the critical temperature, pressure and density of carbon dioxide are respectively 31.0°C, 7.38 MPa and 468.0 kg m -3 [33]. After the experiment, the fluid in the chamber was gradually released by controlling a valve switch. Clusters formed by C 60 molecules were observed by a scanning electron microscope [SEM] (JSM-7400F, JEOL, Tokyo, Japan). Secondly, the clusters formed on the silicon plate were moved to another chamber made of stainless steel for irradiation of UV light (Figure 1b). The inner height and diameter of the chamber were 500 and 254 mm. The clusters were placed 150 mm under an Hg lamp (200 W, low-pressure Hg lamp, SEN Light Co. Ltd., Osaka, Japan). The chamber was filled with air at 1 atm and the air irradiated with UV light, the primary wavelengths of which were 184.9 and 253.7 nm, for 3 h. After the experiment, the chamber was vacuumed via the ejection port and fresh air was injected. The structures of the clusters were observed by an SEM. Mass spectroscopic analysis of the clusters was also carried out by matrixassisted laser desorption ionisation time-of-flight mass spectroscopy (Brücker Daltonics, Autoflex, Bremen, Germany).

Results
First of all, clusters such as fibres and helices were formed by C 60 molecules after having left the crystals composed of C 60 molecules in carbon dioxide under supercritical conditions (36.0°C) for 24 h [32] (see Figure 2a). However, C 60 molecules were so weakly bonded to each other that the clusters were broken by electron beams during the SEM observation ( Figure 2b; see also Additional file 1 for the movie). Note that the accelerating voltage, current and diameter of the electron beams were 1 kV, 4.7 × 10 -2 nA and 2.0 nm, respectively, and therefore the energy flux of the electron beams was 1.5 nW nm -2 .
As mentioned, the clusters were placed in another chamber filled with air at 1 atm and irradiated with UV light in the chamber. Figure 3 shows the fibres and helices composed of C 60 molecules after irradiation of UV light for 3 h. Those structures were not broken by electron beams anymore even when the accelerating  voltage was raised up to 10 kV (see Additional file 1 for the movie). Note that those structures were broken by electron beams when the clusters were placed in a vacuumed chamber irradiated with UV light. Mass spectroscopic analysis of those clusters was carried out to investigate the component of the structures. Figure 4 shows the result of the mass spectroscopic analysis. Interestingly, C 60 O n molecules (where n = 1, 2, 3 and 4), that is, C 60 molecules to which oxygen atoms are bonded, were detected, but neither C 120 nor C 60 -O-C 60 molecules were found. In other words, the reinforced clusters were not polymerised via a chemical bond. Note that when the chamber was vacuumed and UV light was irradiated, the clusters were broken as mentioned, but dimers such as C 108 , C 110 , C 112 , C 114 , C 116 and C 118 were created (see Figure 5). It is therefore supposed that air and irradiation of UV photons are essential for the reinforcement of clusters composed of C 60 molecules. The dissociation energy of an oxygen molecule, O 2 O + O, is 5.1 eV [34]; therefore, it is supposed that oxygen molecules in the chamber were dissociated by photons of 184.9-nm wavelength, the energy of which is 6.48 eV, and oxygen atoms were bonded to C 60 molecules. The order of the diameter of the clusters being 10 nm, it is supposed that C 60 molecules located at the surface of the clusters were oxidised (see Figure  6) and the clusters somehow reinforced. It is inferred that oxidised C 60 molecules located at the surface of the clusters may have an important role for the reinforcement of the clusters.
We will be investigating the mechanism of the reinforcement of the clusters, that is, the role of oxidised C 60 molecules (C 60 O n ) located at the surface of the clusters, in the reinforcement process in detail, carrying out quantum mechanical calculations. We will also be measuring the electric, electronic, mechanical and thermal   1, 2, 3, 4). It is supposed that oxygen atoms are bonded to C 60 molecules at the surface of the clusters. properties of the fibres and helices so that the clusters may be utilised for the development of nano electron devices, nano/microelectromechanical systems and micro-total analysis systems.

Summary
We carried out two experiments: (1) Crystals composed of C 60 molecules were placed in supercritical carbon dioxide (36.0°C), and it was found that fibres, fibre networks and helices composed of C 60 molecules were selfassembled. Since C 60 molecules in the clusters were bonded to each other via van der Waals interactions [32], the clusters were easily broken by the irradiation of electron beams during the SEM observation. (2) The clusters were placed in another chamber filled with air at 1 atm and irradiated with UV photons. Oxygen molecules were dissociated by UV photons, C 60 molecules at the surface of the clusters were oxidised, and C 60 O n molecules were created. The clusters were not broken by the electron beams any more. It is supposed that C 60 O n molecules located at the surface of the clusters may have an important role in the reinforcement process, but the actual mechanism of the reinforcement of the clusters has not yet been clearly understood and therefore is an open question.

Additional material
Additional file 1: Supplementary materials. Supplementary Material 1 -SEM observation of clusters composed of C 60 molecules which were self-assembled in supercritical carbon dioxide. The accelerating voltage of electron beams is 1.0 kV. The clusters are broken during the SEM observation. Supplementary Material 2 -SEM observation of clusters. C 60 molecules located at the surface of the clusters were oxidised. The accelerating voltage of electron beams is 1.0 kV. The clusters are not broken any more during the SEM observation. Figure 6 Outline of a reinforced fibre composed of C 60 molecules. Originally, the fibre composed of C 60 molecules was self-assembled in supercritical carbon dioxide (36.0°C). C 60 molecules were so weakly bonded to each other that the fibre was broken by electron beams. The fibre was then placed in another chamber filled with air at 1 atm, which was irradiated with UV photons. C 60 molecules located at the surface of the fibre were oxidised. The fibre was not broken any more by electron beams.