Film Formation Behavior of the Endohedral Metallofullerene DY@C 82
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63 Mat. Res. Soc. Symp. Proc. Vol. 593 ©2000 Materials Research Society
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_
-_ E
in air
.......... in A r
30'
CL
ca 20:3
0.
10
'
20
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Area/molecule (2)
Fig. 1 it-A isotherm curves of Dy@C 8 2 compressed in air and Ar. For LB film preparation, freshly distilled ultra pure water (through an ultra pure water system) was used as the sub-phase. Before spreading, the water surface was cleaned using a 100 mL syringe to ensure that the maximum surface pressure difference was less than 0.2 mN/m upon 5 compression. The initial concentration of the metallofullerene toluene solution (2.05 x 10- M) was determined by weighing a vacuum dried solid in a known volume of solution using a precision balance (Autobalance Model AD-6). In each run of experiments, a known volume (1-10 ml) of the 2 metallofullerene solution was carefully dropped onto the water surface (554 cm ) using a 1 ml was compressed syringe. After the solvent (toluene) evaporated completely (-30 min.), the barrier slowly (-1 cm/min.). The metallofullerene films formed at the air (or Ar)/water interface were transferred onto fused quartz plates or thin gold films using the vertical transfer method for UV-visible absorption spectroscopy, XPS or SEM, respectively. The surface pressure was kept at 20 mN/m during the transfer. UV-visible absorption spectra were recorded with a Milton Roy spectrometer (Spectronic 3000). High-resolution transmission electron microscopy was carried out using JEOL2010 TEM operated at 200 KV. SEM for the deposited LB films was performed in an instrument JEOL 6300F. RESULTS AND DISCUSSION Shown in Fig. 1 are the nt-A isotherm curves of the endohedral metallofullerene Dy@C 82 at air/water and Ar/water interfaces after two pre-compression cycles reversed at a lower surface pressure (< 20 mN/m). The most striking feature of the it-A isotherms is that the 'area/molecule' 2 compressed in Ar (-63 A2 ) is obviously larger than the figure obtained in air (-36 A), suggesting that the LB film formed at the Ar/water interface is more prone to form a mono-layer. This was confirmed by the observation that the surface pressure decreased rapidly when air was leaked into the system. Another noteworthy phenomenon is that the surface pressure remained constant (e.g. 20 mN/m) in the Ar atmosphere for hours when the barrier was fixed. However, the surface
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Fig. 2 (a) Optical microscope image of a Dy@C. 2 LB film deposited on a quartz plate in air at a surface pressure of 20 mN/m. (b) Optical microscope image of a Dy@C8 2film prepared by dropping a metallofullerene solution on a quartz plate and evaporating the solvent. pressure kept on decreasing for the LB film formed at the air/water interface even though the barrier did not move at all. These findings indicate that the metallofullerene cages are sensitive to oxygen. However, from high resolution TEM results, the metallofullerene cages could be clearly identified and formed short range ordered structures [17], suggesting that the metallofullerene cage structure could
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