Electrochemical scanning tunnelling microscopy: Concept, experiment, and application to organic layers on electrified surfaces.

In this work, we present the concept and experimental possibilities of electrochemical scanning tunnelling microscopy (EC-STM). We describe the underlying physical principles of electron tunnelling microscopy and cyclic voltammetry, our design and construction of an integrated experimental set-up of both methods, as well as the operation of this home-built instrumentation. Exemplary results for bare and iodide and/or porphyrin-covered Cu(100), Cu(111), Au(100), and Au(111) surfaces, obtained with the use of this device, demonstrate the power of real-space imaging of solid surfaces 'in situ', that is, in solution, and 'in operando' with atomic resolution. The images are recorded in potentiostatic, potentiodynamic, and quasi spectroscopic modes of microscope operation, and enable the morphological and structural characterisation of crystalline electrode surfaces before and after adsorption of ions from solution as a function of the electrode potential. Here we present results of (i) the reconstructed and unreconstructed bare electrode surfaces, (ii) their surface modification caused by adsorbed iodide anions, and (iii) the self-assembly of co-adsorbed porphyrin molecules with characteristic ligands and empty cores. Detailed analyses of the high-resolution data yield complete sets of lattice parameters and transformation matrices, which correlate the structure of the respective porphyrin overlayer with the preadsorbed iodide as well as the crystalline substrate underneath. The systematic combination of 'in situ' STM and cyclic voltammetry (CV) data enables the elucidation of potential driven processes at the electrode surface, with or without charge transfer. These processes include the adsorption and desorption of atomic and molecular ions, the structural self-assembly and phase transitions of the atomic/molecular adsorbates as well as with-surface and on-surface reactions. In the present context, we place emphasis on 2D phase transitions within the adsorbed iodide layers and the self-assembly of the porphyrin molecules on the bare or iodide-covered surfaces recorded potentiostatically and potentiodynamically across a wide potential range. The potentiodynamic data are presented herein in the form of a movie. These model studies demonstrate the importance of combined 'in situ' STM and CV investigations - in short 'electrochemical scanning tunnelling microscopy (EC-STM)' - in the context of modern two-dimensional materials science. This includes the formation of functionalised surfaces, as well as electrocatalysis and electrosynthesis in a realistic aqueous environment. Lay description: The work concerns the concept and experimental possibilities of electrochemical scanning tunnelling microscopy (EC-STM) designed and built at the University of Bonn. The physical principles of electron tunnelling, cyclic voltammetry, and experimental set-up are presented. Exemplary results for bare, iodide, and porphyrin-covered copper and gold monocrystals are shown with atomic resolution. The data enable the morphological and structural characterisation of crystalline electrode surfaces before and after adsorption of ions from solution as a function of the electrode potential. The images were recorded after the surface modification caused by adsorbed iodide anions and during the self-assembly of co-adsorbed porphyrins. The data enable to reveal the qualitative and quantitative information concerning lattice parameters, which correlate the structure of the respective porphyrin overlayer with the preadsorbed iodide, as well as the crystalline substrate underneath. The combination of EC-STM and cyclic voltammetry (CV) results enables the elucidation of adsorption and desorption processes at the electrode surface including the involved charge transfer. These experimental investigations demonstrate the importance of combined EC-STM and CV investigations in the context of modern two-dimensional materials science.