Functionalizing the GaN(0 0 0 1)-(1×1) surface II. Chemisorption of 3-pyrroline

Chemisorption of 3-pyrroline (C4H6NH) on the GaN(0 0 0 1)-(1×1) surface has been studied using mainly X-ray-excited Auger electron, electron energy loss and ultraviolet photoemission spectroscopies (XAES, ELS and UPS, respectively). The XAES data show adsorption near room temperature with a total sticking probability of ∼0.05 and a saturation coverage of ∼0.34 molecules per surface lattice site. The ELS data show removal of the characteristic surface-state band centered at ∼3.4 eV and the appearance of a π→π* loss at 7.2 eV due to CC bonds. The UPS data, which have been analyzed with the aid of density functional theory molecular orbital calculations, indicate adsorption with the 3-pyrroline ring remaining intact. The results are consistent with bonding via the molecular N atom in a fourfold coordinated configuration bridging two surface Ga atoms, as proposed earlier for ammonia and aniline. However, in this case adsorption by charge donation from the molecular N non-bonding lone-pair orbital to a single surface Ga dangling orbital cannot be excluded on the basis of UPS alone. Adsorption causes a decrease in electron affinity (δχ≈−0.92 eV) due to a surface dipole layer, and measurement of δχ and of changes in band bending have been used to construct a partial energy-level diagram for the adsorbate-covered surface. Some results (ELS and UPS) are also given, for comparison, for the chemisorption of pyridine.