On-surface chemistry using local high electric fields

On-surface chemistry is a promising topic in nanoscience and is widely applied for the generation of nanostructures and to the chemical modification of adsorbed molecules in order to tune their properties. In this work[1] we developed an approach based on the application of a high electric field between the tip of a scanning tunnelling microscope(STM) and a metallic substrate across well-organized layers of organic molecules to induce chemical reactions. Using an electric field to control chemical reactions is a concept that has been discussed in the early works of Shaik et al.[2] In fact, electro-chemistry is based on the modification of the chemical potential by applying a voltage between the two electrodes of an electro-chemical cell in order to initiate redox reactions. This implies, however, a charge transport using an electrolyte and this scheme is not applicable to ultra-high vacuum. In this work we address the following question: can a dehydrogenation reaction in vacuum be triggered by an electric field gradient in the absence of electron tunneling and thus in the zero-current limit? This mechanism has been successfully demonstrated for some rare cases[3]. Here we extended this idea from the chemical modification of a single molecule to an extended molecular layer. As a prototype molecule, we use dihydrotetraazapentacene (DHTAP) molecule, which was previously reported to form layers on Au(111). By locally applying a high electric field synthesized TAP and MHTAP molecules derived from DHTAP by the double or single dehydrogenation, respectively, in a self-assembled layer. We have also shown that it is possible to dehydrogenate either individual molecules by voltage pulses or a group of molecules by scanning a larger area at high voltages. Our observations suggest that it could be possible to prepare large-scale TAP layers via the non-local application of a high electric fields.

[1] T. Leoni, T. Lelaidier, A. Thomas, A. Ranguis, O. Siri, C. Attaccalite and C. Becker, Nanoscale Adv., 2021, 3, 5565
[2] S. Shaik, S. de Visser and D. Kumar, J. Am. Chem. Soc., 2004, 126, 11746
[3] M. Alemani, et al., J. Am. Chem. Soc., 2006, 128, 14446–14447