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Transfer of Electron Density and Formation of Dative Bonds in Chemisorption of Pyrrolidine on Si(111)-7 × 7

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journal contribution
posted on 02.10.2008, 00:00 by Feng Tao, Yinghui Cai, Yuesheng Ning, Guo-Qin Xu, Steven L. Bernasek
The chemical binding of pyrrolidine on Si(111)-7 × 7 was studied using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and DFT calculation, to obtain a thorough mechanistic understanding of the formation of dative bonds of nitrogen-containing organic molecules on semiconductor surfaces. This study focuses on the electronic and geometric structures of the surface reactive sites. XPS of a chemisorbed monolayer suggests two adsorbates (β1 and β2) associated with the formation of a N···Siad dative bond. At low exposure, the adsorbate β1 forms a dative bond as indicated by a higher N1s binding energy compared to the Si−N σ bond in the dissociated product formed at 250 K. At higher exposure, the formation of the second N···Siad dative bond in adsorbate β2 was evidenced by an obvious upshift of the N1s core-level in contrast to that observed for physisorbed molecules or Si−N σ-bonded molecules. Vibrational studies show a down-shift of the N−H stretching frequency by ∼95 cm−1 upon the formation of the N···Siad dative bond in contrast to the free pyrrolidine molecule, providing new evidence for the dative bonding between amines and silicon surfaces. These studies suggest that the β2 state is an adsorbate directly bonded to the silicon surface adatom via the formation of a N···Siad dative bond, and the β1 state is an adsorbate which forms a N···Siad dative bond with an adatom and a weak N−H···Sire hydrogen bond with an adjacent electron-rich rest atom. In the β1 adsorbate, both electron acceptance by the adatom and electron donation by the rest atom occur simultaneously with one pyrrolidine molecule through dative bonding and hydrogen bonding, respectively. The extra electron-deficient adatom sites on Si(111)-7 × 7 provide an opportunity for forming a second dative bond after the formation of the first in contrast to Ge(100) and Si(100) where the number of electron-deficient and electron-rich sites are equivalent. The difference in electronic and geometric structures of the reactive sites on different semiconductor surfaces offers a useful channel to shape reactivity and selectivity of organic functional groups on silicon surfaces.