p-Benzoquinone on Si(111)-7x7: [6+2]-like Cycloaddition

The Covalent binding of p-benzoquinone (O=C6H4=O) and the formation of an aromatic ring (-OC6H4-O-) on Si(111)-7 x 7 have been investigated by using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. For chemisorbed p-benzoquinone, the absence of v(C=O) at 1659 cm(-1), the retention of sp(2-)u(C-H) at 3050 cm(-1), the appearance of u(Si-O) at 824 cm(-1), and aromatic v(C=C)/delta(ip) (C-H) at 1600/1505 cm(-1) demonstrate that the molecule reacts with the surface in a [6+2]-like cycloaddition mode, which is further confirmed by XPS and density functional theory (DFT) vibrational calculations. DFT calculations indicate that the [6+2]-like cycloadduct (-O-C6H4-O-) bridging two nearest adatoms in neighboring half-unit cells is the most stable. This binding scheme may prove useful for chemical and electronic modification of the semiconductor surfaces.