During the last years, surface-bound coordination network compounds (CNCs) have found application for sensing purposes (electrical, optical, and magnetic read-out), charge storage, electrochromic and optoelectronic layers, transparent coatings, bipolar switches, thermoelectric materials, catalytic, photocatalytic and light harvesting applications [1]. However, most of these applications require electrically conductive coordination network compounds (CCNCs). This requirement severely limits the choice of CNCs for the above-mentioned applications. Hexacyanometallates are coordinative networks with high electrical and ion conductivity that possess interesting redox properties and tuneable electrochemical redox transitions associated with clear colour changes [2].

We developed and characterized CCNCs such as copper hexacyanoferrate as thin crystalline films and characterized their optical, magnetic, electrical and dielectric properties. Thin films of these compounds were fabricated on gold and silicon oxide coated by self-assembled, cyanide-terminated monolayers using alternating immersion in their precursor solutions. Atomic force microscopy (AFM) confirms the formation of copper hexacyanoferrate with cubic structure on the gold surface (Fig. 1).

Fig. 1: AFM image of copper hexacyanoferrate film on a cyanide-terminated self-assembled monolayer on gold surface via layer-by-layer method.

X-ray diffraction (XRD) analysis confirmed the crystalline nature of the films. The thin films can be switched by exploiting the redox chemistry of the metal centres. The modulation of the electrical transport properties provides great potential benefits for numerous future applications in microelectronic device technologies. Structural and electronic changes of the films were probed by X-ray photoelectron spectroscopy (XPS) and polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) before and after electrochemical oxidation and reduction [3]. Results demonstrate the possibility for electrochemical switching of the material properties for each compound but also indicates the complications in those transitions originating in the existence of different structures and defects in which (mobile) alkali metal cations may or may not reside in the pores and provide charge compensation.

Acknowledgement(s) Financial supports from DFG priority programme (SPP1928, Wi1617/24) is gratefully acknowledged.

References:

[1] J. Liu, W. Zhou, J. Liu, I. Howard, G. Kilibarda, S. Schlabach, D. Coupry, M. Addicoat, S. Yoneda and Y. Tsutsui, Angew. Chem. Int. Ed., 7441, 54 (2015).

[2] J. J. Alexander and H. B. Gray, J. Am. Chem. Soc., 4260, 90 (1968).

[3] P. Hosseini, G. Wittstock and I. Brand, J. Electroanal. Chem., 199, 812 (2017).