Configuration-dependent interface charge transfer at a molecule-metal junction.
ABSTRACT 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.
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ABSTRACT: The improvement of molecular electronic devices such as organic light-emitting diodes requires fundamental knowledge about the structural and electronic properties of the employed molecules as well as their interactions with neighboring molecules or interfaces. We show that highly resolved scanning tunneling microscopy (STM) and spectroscopy (STS) are powerful tools to correlate the electronic properties of phosphorescent complexes (i.e., triplet emitters) with their molecular structure as well as the local environment around a single molecule. We used spectroscopic mapping to visualize several occupied and unoccupied molecular frontier orbitals of Pt(II) complexes adsorbed on Au(111). The analysis showed that the molecules exhibit a peculiar localized strong hybridization that leads to partial depopulation of a dz² orbital, while the ligand orbitals are almost unchanged. We further found that substitution of functional groups at well-defined positions can alter specific molecular orbitals without influencing the others. The results open a path toward the tailored design of electronic and optical properties of triplet emitters by smart ligand substitution, which may improve the performance of future OLED devices.Beilstein Journal of Nanotechnology 01/2014; 5:2248-58. · 2.33 Impact Factor
- The Journal of Physical Chemistry C 09/2014; · 4.84 Impact Factor
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ABSTRACT: Charge transfer dynamics across the interface of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) organic molecules and the reduced rutile TiO2 (110) 1 × 1 surface has been investigated using core-hole clock implementation of resonant photoemission spectroscopy (RPES). It is found that ultrafast charge transfer from PTCDA molecules to TiO2 substrate takes place on the time scale of 8–20 fs due to their strong electronic coupling. Moreover, the charge transfer time scale at the PTCDA/TiO2 (110) interface shows evident orientational dependence which varies with the molecular site owing to different electronic coupling strengths.The Journal of Physical Chemistry C 02/2014; 118(8):4160–4166. · 4.84 Impact Factor