Ethylenediamine Grafting on Oxide-Free H-, 1/3 ML F-, and Cl-Terminated Si(111) Surfaces

Amine termination of surfaces constitutes a core platform for fields as diverse as microelectronics and bioengineering, and for nanotechnology in general. Diamines are particularly attractive for surface amination because unlike ammonia or simple amine molecules, they have a metal chelating capability useful in fabricating heterostructures. They can act as a linker molecule between inorganic electronic materials and biomolecules or photoactive quantum dots for applications in microelectronic, photonics, and biosensing. In contrast to ammonia modification of silicon surfaces, the direct grafting of diamine on silicon surfaces has been less explored. In this work, the attachment of liquid and vapor-phase ethylenediamine (EDA) on three types of oxide-free (H-, 1/3 ML F-, and Cl-terminated) Si(111) surfaces is therefore examined by infrared absorption spectroscopy and X-ray photoelectron spectroscopy in conjunction with first-principle calculations. We find that EDA chemisorption is only possible on 1/3 ML F- and Cl-terminated Si(111) surfaces: EDA only physisorbs on H-terminated Si(111) surfaces. On Cl-terminated Si(111) surfaces, EDA molecules adsorb in a mixture of monodentate and bridging configurations (chemical reaction of both EDA end groups), while on 1/3 ML F-terminated Si(111) surfaces the adsorption occurs primarily at one end of the molecule. EDA reaction with Cl-terminated Si(111) surfaces is also characterized by complete removal of Cl and partial Si–H (∼25% ML) formation on the surface. This unexpected Si–H product suggests that a proton–chlorine exchange may take place, with the endothermic barrier possibly reduced via concerted chemical reactions after an initial attachment of EDA to the surface.