Conductivity of Styrene Lines on Si(100)-(2×1):H: Effects of Molecular Orientation and Surface Charge

First-principles electronic structure calculations of styrene lines on H-covered Si(100) show that phenyl ring orientations at chain ends are fluxional, favoring structures with the terminal ring arranged perpendicular to the molecular line. Simulated scanning tunneling microscopy images show that the tunneling current depends strongly on the phenyl ring orientation, because it controls the coupling between the charged dangling bond and the styrene π system. The perpendicular-ring orientation shows higher conductivity than when all rings are parallel, increasing the apparent height of the molecule at the end of the row. This is consistent with experimental observations, suggesting that STM images provide evidence for the perpendicular-ring structure. Experiments have also shown that on highly n-doped surfaces, where the dangling bond is negatively charged, an electrostatic field modulates the conductivity of the styrene molecules as a function of distance from the dangling bond. This effect is also reproduced in the simulated images. This “slope effect” can be quantitatively modeled by a simple surface screening model, which provides some new insight into the structure of the system.