Relative reactivities of amino and ethenyl groups in allylamine on Si(100)2 x 1: Temperature-dependent X-ray photoemission and thermal desorption studies of a common linker molecule

The room-temperature adsorption and thermal evolution of allylamine on Si(100)2 x 1 have been investigated by using temperature-dependent X-ray photoelectron spectroscopy (XPS) and thermal desorption spectrometry (TDS). The presence of a broad N 1 s feature at 398.9 eV, attributed to a N-Si bond, indicates N-H dissociative adsorption. On the other hand, the presence of C 1 s features at 284.6 eV and 286.2 eV, corresponding to C=C and C-N, respectively, and the absence of the Si-C feature expected at 283.2 eV shows that [2 + 2] C=C cycloaddition does not occur at room temperature. These XPS data are consistent with the unidentate staggered and eclipsed allylamine conformer adstructures arising from N-H dissociation and not [2 + 2] C=C cycloaddition. The apparent conversion of the N 1 s feature for Si-N(H)-C <- at 398.9 eV to that for Si-N(H) at 397.7 eV and the total depletion of C 1 s feature for C-N at 286.2 eV near 740 K indicates cleavage of the C-N bond, leaving behind a Si-N(H)center dot radical. Furthermore, the C=C C 1 s feature at 284.6 eV undergoes steep intensity reduction between 740 K and 825 K, above which a new C 1 s feature at 283.2 eV corresponding to SiC is found to emerge. These spectral changes suggest total dissociation of the ethenyl fragment and the formation of SiC. Moreover, while the total N 1 s intensity undergoes a minor reduction (24%) upon annealing up to 1090 K, a considerable reduction (43%) is found in the overall C 1 s intensity. This observation is consistent with our TDS data, which shows the desorption of C-containing molecules including propene and ethylene at 580 K and of acetylene at 700 K. The lack of N-containing desorbates suggests that the dissociated N species are likely bonded to multiple surface Si atoms or diffused into the bulk. Interestingly, both the staggered and eclipsed N-H dissociative adstructures are found to have a less negative adsorption energy than the [N, C, C] tridentate or the [2 + 2] C=C cycloaddition adstructures by our DFT calculations, which suggests that the observed formation of N-H dissociative adstructures is kinetically favored on the Si(100)2 x 1 surface. (C) 2010 Elsevier B.V. All rights reserved.