Theoretical study of functionalization of the Ge(100) surface by organic molecules

Theoretical study of functionalization of the Ge(100) surface by organic molecules^*

Suklyun Hong^†

Department of Physics, Sejong University, Seoul 143-747, Korea

Functionalization of semiconductor surfaces through chemisorption of organic molecules has generated much recent interest on account of its potential applications in molecule-based devices, bio-sensors, nanolithography and organic dielectrics.

For multifunctional organic molecules, it is important to understand how to produce well-ordered monolayers of the organic molecules that bind to the surface by only one of their functional groups, leaving the other group available for subsequent layer attachment. Among the functional organic molecules studied to date, N-containing aromatic compounds such as pyridine, pyrrole, pyrimidine, purine and aniline have attracted particular interest due to their useful physical and chemical properties, deriving from the lone-pair electrons.

Pyridine is a heterocyclic amine with an aromatic ring. A pyridine molecule with two different functional groups can be attached to the Ge(100) surface via two different reactions; one is [2+2] or [4+2] cycloaddition between the π-conjugated aromatic ring and the Ge dimers, and the other is the Lewis acid-base reaction in which the lone-pair electrons of pyridine are donated into the electron-deficient down-Ge atom in a Ge dimer. To investigate the detailed adsorption geometry and the electronic structure of pyridine adsorbed on Ge(100), we performed ab initio pseudopotential total energy calculations. The simulated filled-state STM image generated from the optimized adsorption configuration is in good agreement with the experimental STM image. Our results show that pyridine molecules adsorb on the electron-deficient down-Ge atoms of Ge(100) via Ge-N dative bonding to form highly ordered and oriented organic monolayers without loss of aromaticity. The plane of the aromatic ring is tilted to the surface for 0.25 ML pyridine on Ge(100), while it is perpendicular to the surface for 0.5 ML pyridine. The phase transition from the c(4x2) structure for 0.25 ML pyridine to the p(2x2) structure for 0.5 ML pyridine occurs due to steric repulsion between adsorbed pyridine molecules.

Similarly, we investigate possible configurations for the pyrrole/Ge(100) system. In addition to cycloadditions and dative bonding configurations, dissociative adsorption models are considered. N-endon tilted structures obtained after N-H bond dissociation are found to be the most favorable. Theoretical STM images are compared with the experimental ones. Finally, the difference between the Ge(100) and Si(100) surfaces is discussed.

^*Supported by the Korea Research Foundation Grant (KRF-2003-041-C00096).

^†In collaboration with Y.E. Cho, J.Y. Maeng, D.H. Kim, D.S. Choi, J.Y. Lee, S.J. Jung, and S. Kim at KAIST.