Adsorption and self-assembly of fullerenes C60 and C70 at the Au(111)/n-tetradecane interface

N. Katsonis, A. Marchenko, D. Fichou

A number of studies have been devoted to the organization and local properties of C₆₀ films deposited on metallic or semi-conducting surfaces. Much less attention has been paid to C₇₀ overlayers, which present a greater variety of morphologies and properties than C₆₀ because the molecule symmetry is lowered from I_h to D_h and has an ellipsoidal shape.

A few STM studies of C₇₀ overlayers on Au, Ag, Cu, Si, and GaAs evidence the existence of a local order. In most cases, the C₇₀ overlayer is produced and observed in UHV. STM observation of C₇₀ in air has also been reported but leads to a poor resolution. Surprisingly no attempt has been made to observe C₇₀ by STM at a liquid-solid interface, a convenient technique which allows to prepare molecular self-assemblies and observe them in situ, in ambient conditions and without contamination.

We report here on the STM study of C₇₀ (sub)-monolayers deposited at the tetradecane/Au(111) interface. We compare adsorption of C₇₀ with previous STM results that we obtained with C₆₀ in the light of their geometric and electronic differences.

Similarly to C₆₀, at room temperature C₇₀ molecules keep rotate at the liquid/gold interface thus preventing STM intramolecular resolution. Our study shows that the C₇₀ molecules adopt two types of arrangements: 1/ a 2Ö3´2Ö3 R30° structure with long molecular axis perpendicular to the Au (111) surface (“a-domains”), and 2/ a quasi-hexagonal in-phase packing with long axis parallel to the surface (“b-domains”).

In the a-domains, C₇₀ molecules form a commensurate structure against the <112> direction. C₇₀ adopt an upright position, with the short axis parallel to the surface. In the b-domains, the molecular rows are parallel to the <110> direction of gold. Their structure is quasi-hexagonal with a unit cell elongated in the <110> direction. This type of packing is reminiscent of the in-phase structure observed for C₆₀ monolayers on gold in UHV.

Adsorption and self-assembly of fullerenes C60 and C70 at the Au(111)/n-tetradecane interface

Top: STM image (24×17 nm2; Vt=0.3 V, It =100 pA) of the co-adsorption of C60 and C70 molecules (80:20) deposited on Au(111) from n-tetradecane. Bottom: Surface profile recorded along the black line. C70 molecules are recognizable due to a larger apparent height.

Finally, we checked the possibility to distinguish C₇₀ from C₆₀ in a binary C₆₀/C₇₀ mixture (80:20). The deposition of this mixture on Au(111) generates an overlayer presenting close-packed areas among which a number of brighter spots are observed (see Figure). Based on their larger apparent height (by ~1-2 Å) we attribute these bright spots to C₇₀.

Further work is presently underway to investigate the conduction properties of both structures of C₇₀ on Au(111) by means of STS.

REFERENCES:

1. N. Katsonis, A. Marchenko, D. Fichou, Adv. Mater. 16, 309-312 (2004).

2. N. Katsonis, A. Marchenko and D. Fichou, J. Photochem. Photobiol. A: Chem., 158, 101-104 (2003).

3. N. Katsonis, A. Marchenko and D. Fichou, Synth. Met., 137, 1453-1455 (2003).

#862 - Màj : 09/07/2007