Special CPM Seminar

Revealing molecular dynamics by scanning noise microscopy and spectroscopy

Rolf Möller

Faculty of Physics, Center for Nanointegration Duisburg-Essen (CeNIDE)
University of Duisburg-Essen

Noise is generally considered as a nuisance. However, it may contain valuable information about atomic or molecular motion. Here, we show how signals can be extracted by a full real-time characterisation of the random telegraph noise in the current of a scanning tunneling microscope. The hopping rate, the noise amplitude and the relative occupation of the involved states are measured as a function of the tunneling parameters providing spatially resolved maps. In contrast to standard STM, our data give access to transiently populated states revealing an electron-driven hindered rotation between the equilibrium and two metastable positions of an individually adsorbed molecule. The results for individually adsorbed copper phthalocyanine molecules on Cu(111) are corroborated by density functional theory calculations. (J. Schaffert, M. C. Cottin, A. Sonntag, H. Karacuban, C. A. Bobisch, N. Lorente, J.-P. Gauyacq & R. Möller, Nature Materials 12, 223-227 (2013)).

1. Constant current image of several isolated CuPc molecules on Cu(111) observed at U_sample = -1 V, I = 40 pA.
2. Simultaneously measured spatial distribution of the switching rate. The rate ranges from 0 to 500 Hz.
3. Constant current image of an individual CuPc molecule at U_sample = -0.4 V and I = 40 pA
4. Corresponding switching rate. The rate ranges from 0 to 200 Hz.