Theoretical study of work function modification by organic molecule-derived linear nanostructure on H-silicon(100)-2 x 1

Tuning of the electronic properties of semiconductors can be achieved by surface modification with organic molecules. In this work, we study, by periodic density functional theory, the change in work function that occurs upon the modification of nominally hydrogen-terminated Si(100)-2 x I by chemisorption of substituted styrene molecules. Our results show that monolayers derived from 4-X-styrene molecules, with X being electron donating groups or hydrogen, decrease the work function of the system. Conversely, monolayers derived from 4-X-styrene molecules, with X being electron withdrawing groups, increase the work function of the system. For the molecules used in the modeling, the calculations indicate that the work function can be substantially modified from -1.4 eV (X=N(CH3)(2)) to +1.9 (X=NO2) eV relative to H-Si(100)-2 x 1. Because the direction and magnitude of charge transferred upon chemisorption is the same for all molecules, the work function changes are not the result of band bending. The work function modification comes exclusively from the inherent dipoles of the molecules chemisorbed on the surface. The computed dipoles for the monolayers range from -1.3 (X=N(CH3)(2)) to +1.4 (X=NO2) Debye. We conclude that substantial local control over some of the electronic properties of silicon can be achieved by the chemisorption of dipole-containing molecules.