Reconstruction of a 2D layer of KBr on Ir(111) and electromechanical alteration by graphene

Abstract

A novel reconstruction of a two-dimensional layer of KBr on an Ir(111) surface is observed by high-resolution noncontact atomic force microscopy and verified by density functional theory (DFT). The observed KBr structure is oriented along the main directions of the Ir(111) surface, but forms a characteristic double-line pattern. Comprehensive calculations by DFT, taking into account the observed periodicities, resulted in a new low-energy reconstruction. However, it is fully relaxed into a common cubic structure when a monolayer of graphene is located between substrate and KBr. By using Kelvin probe force microscopy, the work functions of the reconstructed and the cubic configuration of KBr were measured and indicate, in accordance with the DFT calculations, a difference of nearly 900 meV. The difference is due to the strong interaction and local charge displacement of the K+/Br− ions and the Ir(111) surface, which are reduced by the decoupling effect of graphene, thus yielding different electrical and mechanical properties of the top KBr layer.

2D KBr/Graphene Heterostructures—Influence on Work Function and Friction

“…In addition, several defects can also be observed within or between the lines, in particular pairs of vacancies on top of and free ions between the stripes, represented with white and black squares, respectively. The high density of defects within the KBr islands compared to typical observations of alkali halides on metal surfaces is due to a large lattice strain in this reconstruction as we have shown before [ 23 ]. This reconstruction is quite different in comparison to the known cubic KBr reconstruction on most metal surfaces [ 39 , 40 ].…”

“…Table 1 shows the experimentally measured and calculated work function of the different systems measured by KPFM [ 12 ] and determined by Density Functional Theory (DFT), respectively [ 23 ]. If we take the work function measured on Gr/Ir(111) as a reference, the reconstructed KBr directly on Ir(111) shows a difference.…”

“…To illustrate the change in work function, the charge distribution of the two different KBr structures was calculated with DFT, as can be seen in Figure 2 showing the side view of the simulated models [ 23 ]. For the reconstructed KBr on Ir(111), the anions (Br ) and cations (K ) are shifted into different sublayers, introducing a charge separation and creating a dipole moment perpendicular to the iridium surface ( e per KBr unit in total).…”

“…All of the Density Functional Theory (DFT) calculations were performed following the computational approach described in our previous publication [ 23 ]. The local density approximation, using the Perdew-Zunger (PZ) [ 24 ] parameterization, was chosen to describe the exchange correlation functional.…”

“…For the Bader charge analysis, the code by Henkelman’s group [ 33 , 34 , 35 , 36 ] was used, and the details concerning this approach are reported in our recent work [ 23 ]. For the calculation of the work functions, symmetric systems were generated to avoid the formation of any electric dipoles in the vacuum region due to the periodic boundary conditions.…”