Anthracene and Anthracene:C60 Adduct-Terminated Monolayers Covalently Bound to Hydrogen-Terminated Silicon Surfaces

Anthracene monolayers covalently bound to hydrogen-terminated n-type Si(100) surfaces have been prepared from the attachment of an amine-substituted anthracene derivative to a preassembled acid-terminated alkyl monolayer using carbodiimide coupling. The anthracene headgroups were then used as anchoring sites for C-60 following [4 + 2] Diels-Alder cycloaddition. After cycloaddition of C-60 on the anthracene layer, the surface roughness determined by atomic force microscopy increased from 3.0 +/- 0.7 to 5.0 +/- 1.0 angstrom and the morphology showed uniformly distributed globular features a few nanometers high. Cyclic voltammograms of the anthracene-modified monolayer in the dark were characterized by an ill-defined reversible system at E-o' = -2.05 V vs saturated calomel electrode (SCE), which compares well with the value determined for the anthracene derivative in solution on a platinum electrode. Furthermore, the surface coverage of attached anthracene units was estimated to be (4.6 +/- 0.3) x 10(-10) mol cm(-2), which is consistent with a densely packed monolayer. In contrast, the voltammogram of the C-60-modified monolayer did not show multiple reversible one-electron transfers characteristic of the anthracene:C-60 adduct. Instead, one irreversible cathodic peak at -1.50 V followed by a reversible system at -2.15 V was observed. These electrochemical differences between surface-confined and dissolved species are assigned to reduced charge transfer kinetics between the underlying semiconductor and bound C-60 within a certain potential range. This hypothesis is consistent with the flat band potential E-fb, value of -0.80 +/- 0.05 V vs SCE, determined from capacitance measurements. Moreover, scanning electrochemical microscopy (SECM) measurements in feedback mode provided clear evidence for the electroactive properties of bound C-60. The SECM approach curves suggest that both the anthracene and anthracene:C-60 layers displayed good conductivity, presumably by electron hopping between adjacent redox sites.