Surface dynamics studied by time-dependent tunneling current

Quinacridone on Ag(111): Hydrogen Bonding versus Chirality

Quinacridone (QA) has recently gained attention as an organic semiconductor with unexpectedly high performance in organic devices. The strong intermolecular connection via hydrogen bonds is expected to promote good structural order. When deposited on a substrate, another relevant factor comes into play, namely the 2D-chirality of the quinacridone molecules adsorbed on a surface. Scanning tunneling microscopy (STM) images of monolayer quinacridone on Ag(111) deposited at room temperature reveal the formation of quasi-one-dimensional rows of parallel quinacridone molecules. These rows are segmented into short stacks of a few molecules in which adjacent, flat-lying molecules of a single handedness are linked via hydrogen bonds. After annealing to a temperature of T = 550-570 K, which is close to the sublimation temperature of bulk quinacridone, the structure changes into a stacking of heterochiral quinacridone dimers with a markedly different intermolecular arrangement. Electron diffraction (LEED) and photoelectron emission microscopy (PEEM) data corroborate the STM findings. These results illustrate how the effects of hydrogen bonding and chirality can compete and give rise to very different (meta)stable structures of quinacridone on surfaces.

Identifying multiple configurations of complex molecules on metal surfaces

Experimental identification of molecular configurations in diffusion processes of large complex molecules has been a demanding topic in the field of molecular construction at solid surfaces. Such identification is needed in order to control the self-assembly process and the properties and configurations of the resulting structures. This paper provides an overview of state-of-the-art techniques for identification of molecular configurations in motion. First, a brief introduction to the conventional tools is presented, for example, low-energy electron diffraction and IR/Raman spectroscopy. Second, currently used techniques, scanning probe microscopy, and its application in molecular configuration identification are reviewed. In the last part, a methodology combining time-resolved tunneling spectroscopy and density functional theory calculation is reviewed in detail; this strategy has been successfully applied to two typical molecular systems, (t-Bu)₄ -ZnPc and FePc (where Pc is phthalocyanine), with molecular rotation and laterial diffusion on the Au(111) surface.