On-surface Chemistry Explored by STM and XPS

On surface chemical reactions are of profound importance for the modification of surfaces by adding specific covalent/ionic molecular assemblies. Fast, highly directive reactions with small functional units serving as reactants, so called “click” reactions, are well established in nature’s biochemistry and are particularly useful also to design specific ad-surface molecular layers in subsequent reactions. Current understanding, however, of chemistry in the solvent-free on-surface environment is just at its start: experience gathered and reaction mechanisms derived for “in-solution” chemistry are not necessarily transferrable to solution-free processes occurring at surfaces.


In order to control on-surface reactions, it is possible to modify either the reactants or the substrate itself. The former can be changed by, for example, replacing substituents, while the latter can be modified by several methods including chemical modifications as well as varying the number of active sites by changing roughness. The approach we chose in our study [1] involves modifying the reactive surface with an atomically thin layer of adsorbates and investigating its influence on the on-surface metalation reaction of a 2HTPP porphyrin.

Metalation fig.gif
Currently we are investigating the influence of other adsorbate-induced (e.g. N and Cl) surface superstructures on the availability of reactive metal atoms by studying the porphyrin metalation. We are able not only to promote a reaction, but also inhibit it completely.

One of the surface science’s quests is to create 2D nanomaterials with new and more advanced functionalities. In order to achieve that goal, extended, defect-free 2D structures need to be created. Scanning-probe repositioning techniques allow for creation of well-defined, functional structures. Such structures however, do not meet other requirement valuable for future devices – scalability, since creation of large (few hundred nanometers) structures would be very time-consuming. The approach presented here allows for fabrication of extended ionic 2D layers with potentially interesting electronic properties by means of self-assembly.



In our work [2], a charge-transfer (CT) reaction between 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and NaCl, both sublimed onto a Au(111) surface in a sub-monolayer regime, has been studied. STM has been used to probe self-assembly of TCNQ/Au(111) and NaCl+TCNQ/Au(111) (Fig. 1a). In the former case, self assembly is mainly ascribed to H-bonding of the cyano groups [3]. In the latter, arrangement of TCNQ is significantly influenced by the addition of NaCl.


XPS data (Fig. 2) indicate the reduction of TCNQ0 to TCNQ- due to an on-surface CT reaction with NaCl. Further photoelectron spectroscopy studies prove that the CT occurs between TCNQ and NaCl without involving the Au(111) substrate. As a result, Cl2 gas is released. Thanks to this reaction, extended 2D molecular structure could be obtained.


molecules.png


Another on-surface chemical reaction [4] was observed for a zinc porphyrin (ZnTPP, see Fig. 1b) on Au(111) substrate. When exposed to TCNQ molecule at room temperature, a covalent bond between the two molecules is formed and a new molecule (Fig. 1b) arises. The XPS (Fig. 3) data unequivocally prove the covalent bond formation and creation of the ZnTPP-TCNQ complex.

References

[1] Porphyrin metalation providing an example of a redox reaction facilitated by a surface reconstruction
J. Nowakowski, C. Wäckerlin, J. Girovsky, D. Siewert, T. A. Jung, N. Ballav
Chem. Commun. 49, 2347 (2013)

[2] Assembly of 2D ionic layers by reaction of alkali halides with the organic electrophile 7,7,8,8-tetracyano-p-quinodimethane (TCNQ)
C. Wäckerlin, C. Iacovita, D. Chylarecka, P. Fesser, T. A. Jung, N. Ballav
Chem. Commun. 47, 9146 (2011)

[3] Structure and electronic configuration of tetracyanoquinodimethane layers on a Au(111) surface
I. Fernandez-Torrente, K. J. Franke, J. I. Pascual
Int. J. Mass Spectrom. 277, 269 (2008)

[4] Visualizing the Product of a Formal Cycloaddition of 7,7,8,8-Tetracyano-p-quinodimethane (TCNQ) to an Acetylene-Appended Porphyrin by Scanning Tunneling Microscopy on Au(111)
P. Fesser, C. Iacovita, C. Wäckerlin, S. Vijayaraghavan, N. Ballav, et al.
Chem. Eur. J. 17 : 19, 5246 (2011)