Special CPM Seminar

Theoretical Insight into Multi-Channel Scanning Probe Imaging of Metal Oxide Surfaces

Milica Todorović

Department of Condensed Matter Physics Theory
Universidad Autónoma de Madrid

Scanning probe microscopy (SPM) methods have matured into sophisticated tools for surface studies with the capacity to uncover much more than surface morphology: the site-specific quantification of forces and currents at the surface of industrially-relevant materials is key to unlocking their potential for future applications. Multi-channel SPM techniques that combine force and tunnelling microscopy (AFM and STM) were developed in a bid towards functional and species-specific imaging. This method has generated much new information, but image interpretation remains challenging. Imaging modes in individual channels bear a complex dependence on experimental conditions, and moreover, the contrast formation mechanisms in simultaneously acquired images arise from distinct quantum mechanical processes so the imaging signal may be related to different underlying surface features.

With the aim of characterising materials for industrial applications, we employ density functional theory (DFT) calculations combined with non equilibrium Green�~@~Ys functions (NEGF) conductance methods to explore simultaneous AFM and STM imaging modes on metal oxide surfaces in close collaboration with experimental partners. In the study of a model Cu(100)-O surface imaged with 3D NC-AFM/STM, we clarified the dominant contrast modes and thus identified the common defect on this surface [1]. By systematically probing contrast dependence on imaging parameters such as tip height, chemical termination and bias voltage, we determined the conditions for chemically-specific imaging [2]. We drew on these findings to gain insight into the AFM/STM characterisation of the optically active (101) surface of TiO2 anatase and its defects [3], before proceeding to analyze the attachment and imaging of pentacene monolayers on anatase with a view to explore their potential for organic photovoltaics. We demonstrate that high-accuracy simulations can help make conceptual leaps in understanding the fundamental mechanisms that underpin multi-channel SPM techniques, and thus aid image interpretation to extract both qualitative surface features and quantitative information on material functionality.

[1] M. Z. Baykara, M. Todorović, H. Mönig, et al. Phys. Rev. B 87, 155414 (2013).
[2] H. Mönig, M. Todorović, M. Z. Baykara, et al. ACS Nano 7, 10233 (2013).
[3] O. Stetsovych, M. Todorović, T. K. Shimizu, et al. (in preparation).