Differences in Charge Transfer between Surface and Bulk Species in Tetrathiafulvalene- Tetracyanoquinodimethane

The charge-transfer complex tetrathiafulvalene-tetracyanoquinodimethane has been studied by x-ray-photoelectron spectroscopy at both normal and grazing electron takeoff angles. Changes in the ${\mathrm{N}}_{1s}$ and ${\mathrm{S}}_{2p}$ spectra with angle show that a discrete layer exists at the surface of the $a-b$ plane. Essentially zero electron transfer takes place in this layer versus ∼ 0.8±0.2 in the bulk. The surface layer is ca. 1 to 5 layers of complex thick and appears to be intrinsic to "good" organic conductors.