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

Abstract

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 C60 following [4 + 2] Diels–Alder cycloaddition. After cycloaddition of C60 on the anthracene layer, the surface roughness determined by atomic force microscopy increased from 3.0 ± 0.7 to 5.0 ± 1.0 Å 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°′ = −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) × 10–10 mol cm–2, which is consistent with a densely packed monolayer. In contrast, the voltammogram of the C60-modified monolayer did not show multiple reversible one-electron transfers characteristic of the anthracene:C60 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 C60 within a certain potential range. This hypothesis is consistent with the flat band potential Efb 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 C60. The SECM approach curves suggest that both the anthracene and anthracene:C60 layers displayed good conductivity, presumably by electron hopping between adjacent redox sites.