The role of the molecule-metal interface is a key issue in molecular electronics. Interface charge transfer processes for 4-fluorobenzenethiol monolayers with different molecular orientations on Au(111) were studied by resonant photoemission spectroscopy. The electrons excited into the LUMO or LUMO+1 are strongly localized for the molecules standing up on Au(111). In contrast, an ultrafast charge transfer process was observed for the molecules lying down on Au(111). This configuration-dependent ultrafast electron transfer is dominated by an adiabatic mechanism and directly reflects the delocalization of the molecular orbitals for molecules lying down on Au(111). Theoretical calculations confirm that the molecular orbitals indeed experience a localization-delocalization transition resulting from hybridization between the molecular orbitals and metal surface. Such an orientation-dependent transition could be harnessed in molecular devices that switch via charge transfer when the molecular orientation is made to change.
[Show abstract][Hide abstract] ABSTRACT: The molecular orientation and order in the initial growth of copper phthalocyanine on Si(111) was investigated in situ by angular-dependent X-ray absorption spectroscopy. A transition from lying-down to standing-up configuration occurs in less than one monolayer. After the Si surface is passivated by a monolayer of the molecules, only the standing-up structure persists in subsequent growth. The molecular film also exhibits an order−disorder−order transition in the distribution of molecular tilt angle during growth. These transitions are related to the changes of interactions in the system at different stages.
The Journal of Physical Chemistry C 02/2007; 111(8). DOI:10.1021/jp066600n · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: (Chemical Equation Presented) Intermolecular hydrogen bonding of N,N′-diphenyl oxalic amides deposited on Au(111) drives a rotation of the aromatic substituents (see picture). This low-dimensional supramolecular surface nanosystem, based on conformationally adaptive tectons, is identified by molecular-level scanning tunnelling imaging, X-ray absorption measurements and first-principles modeling.
[Show abstract][Hide abstract] ABSTRACT: The adsorption behavior of CuPc on Si(111) and the substrate effect on the valence state of the Cu atoms in CuPc is investigated by synchrotron-based photoemission and X-ray absorption spectroscopy. The monolayer of CuPc on Si(111) is disordered, in contrast to the ordered multilayer CuPc. The formation of Si–C and Si–N bonds at the interface is clearly observed in the Si 2p and C 1s core level spectra. For the CuPc monolayer, the valence state for a portion of Cu atoms is reduced from 2+ to 1+. Further post-annealing significantly enhances the formation of Si–C and Si–N bonds between the molecules and the substrate, but the Cu 2p core level spectra unambiguously reveal that the Cu valence state remains unchanged. This suggests that the Cu atoms are well shielded from the surrounding environment by the distortion of the CuPc molecules.
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