From Salt to Germanene: A Cookbook for Electrochemical Formation of 2D Materials (Inspired by R. Adžić)
Authors: Drnec Jakub , Stickney John Lewellen , Harrington David A. 
Published: February 2020

The re-discovery of graphene[1], until now the most studied 2D material, opened a large playground for scientists to study exotic properties of confined electrons. A few monolayers thick 2D sheets of insulating or semiconducting materials with various atomic compositions rapidly became a hot topic. Such sheets were shown to behave differently compared to their 3D counterparts, in large part due to the different electronic structure arising from the 2D nature of the material. They bear great promise for applications in electronics and optoelectronics, sensors, composite materials, photovoltaics, medicine, quantum dots, energy storage and cryptography. The most renowned 2D material is certainly graphene, but its analogs silicene and germanene have also drawn increased attention due to their potentially simpler incorporation in current semiconductor electronics.

There are several means of preparation for 2D materials: exfoliation or cleavage, chemical vapor deposition (CVD) and, as recently shown, also by underpotential deposition (UPD) [2]. Because standard UPD of transition metals also effectively results in formation of 2D layers, it might therefore be possible to explain the recently observed electrochemical formation of germanene by some of the principles discovered by R. Adžić et al. regarding UPD growth. They suggested that depending on the nature of the bond within the 2D layer (ionic vs. covalent) one can perhaps predict the general 2D structure of the layer (mixed monolayer vs. bilayer) [3].

We tested this idea in both UHV and electrochemical environments and found that this suggestion indeed holds in the case of truly ionic CsI layers [4]. The CsI layers, prepared in UHV by coadsorption of Cs and iodine, undergo several phase transitions from mixed monolayer with a honeycomb motif through to a bilayer. Simple electrostatic calculations showed that this evolution is expected from a theoretical perspective. To further test the hypothesis, we also prepared 2D CsO layers which are also strongly ionic due to the high oxygen electronegativity. Indeed, the observed 2D structures are the same as in the CsI case pointing to the universality of the hypothesis proposed by R. Adžić et al.

These results later sparked the idea that, by following the same principles, the honeycomb structure could be patterned onto the surface also for different elements because the presence of the anion in the electrolyte forces the layer to be deposited in the honeycomb arrangements. To show that this approach is feasible, we deposited germanium on Au(111) as the UPD has been studied before and it is still difficult to prepare germanene in UHV environment. We found that indeed the germanium deposition results in a honeycomb layer after following a certain protocol [2]. Furthermore the growth process was also characterized by STM, Raman spectroscopy and Surface X-ray diffraction (SXRD).

[1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science, 306 (2004), 666–669

[2] M. Ledina, N. Bui, X. Liang, Y. -G. Kim, J. Jung, B. Perdue, C. Tsang, J. Drnec, F. Carla, M. P. Soriaga, T. J. Reber, J. L. Stickney, J. Electrochem. Soc., 164 (2017), D469-D477

[3] J. X. Wang, I. K. Robinson, J. E. DeVilbiss and R. Adžić, J. Phys. Chem. B, 104 (2000), 7951–7959

[4] J. Drnec and D. A. Harrington, Surface Science, 604 (2010), 2106–2115; J. Drnec and D. A. Harrington, Surface Science, 630 (2014), 9-15