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

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## Abstract

**:**

## 1. Introduction

_{2}) and insulators (e.g., h-BN) [6,7,8]. They differ significantly in their behaviors depending on their bulk composition and therefore offer several additional possibilities for observing interesting phenomena [9].

## 2. Materials and Methods

#### 2.1. Sample Preparation

#### 2.2. Atomic Force Microscopy

#### 2.3. Computational Method

## 3. Results

#### 3.1. Structural Characterization

#### 3.2. Work Function & Formation of Dipoles

#### 3.3. Friction and Adhesion Forces

## 4. Conclusions

## Supplementary Materials

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 1.**nc-AFM images of KBr accompanied with graphene on an Ir(111) surface: (

**a**) Topography with both irregular KBr (top in red) and cubic KBr (bottom in blue). (

**b**) Topography of KBr islands rotated in two directions (red lines) and graphene moiré (black spots) on Ir(111). (

**c**) Atomic resolution of double parallel lines of reconstructed KBr on Ir(111), with lattice vacancies and adatoms marked with white and black squares, as well as pairs of protrusions marked with white spots, respectively, (the inset shows the atomic configurations with Br in red and K in blue). (

**d**) Topography of cubic KBr on Gr/Ir(111). (

**e**) Atomic resolution of cubic KBr on graphene on Ir(111), with the left topography image showing the graphene moiré and the right corresponding to the torsional frequency shift illustrating the cubic KBr lattice taken simultaneously at the same region. Scale bars for (

**a**): 100 $\mathrm{n}$$\mathrm{m}$, (

**b**,

**d**): 20 $\mathrm{n}$$\mathrm{m}$ and (

**c**,

**e**): 2 $\mathrm{n}$$\mathrm{m}$.

**Figure 2.**Simulated model (lateral view) of both KBr structures with the partial charges in the dotted box of the model calculated by the Bader method (K in blue, Br in red, C in black and Ir in gray): (

**a**) KBr/Ir(111), (

**b**) KBr/Gr/Ir(111).

**Figure 3.**Frictional images (forward direction of the two KBr structures at a normal load of ${\mathrm{F}}_{\mathrm{n}}=5$ $\mathrm{n}$$\mathrm{N}$: (

**a**) KBr/Ir(111), (

**b**) KBr/Gr/Ir(111). Scale bar: 1 $\mathrm{n}$$\mathrm{m}$. The corresponding friction loops of a single line marked in both images are presented in (

**c**).

**Figure 4.**Adhesion force and energy of KBr on Ir(111) and Gr/Ir(111) evaluated by: (

**a**) force spectroscopy in FFM and (

**b**) DFT simulations.

**Figure 5.**Top: charge density differences $\rho $${}_{\mathrm{diff}}$ of (

**a**) KBr on Gr, (

**b**) Gr on Ir(111) and (

**c**) KBr on Ir(111). Red and blue regions represent charge accumulation and depletion, respectively. Bottom: $\overline{\rho}$${}_{\mathrm{diff}}\left(\mathrm{z}\right)$, i.e., planar averages of $\rho $${}_{\mathrm{diff}}$. The horizontal axis indicates the height in angstrom and is centered on the middle of the interface. The vertical axis indicates the values $\overline{\rho}$${}_{\mathrm{diff}}\left(\mathrm{z}\right)$ multiplied by ${10}^{3}$. The amount of redistributed charge, $\rho $${}_{\mathrm{redist}}$, is indicated by the gray shaded areas on the graph below.

**Table 1.**Work function values experimentally observed by KPFM measurements and theoretically by DFT simulation results [23].

System | ${\mathbf{\Phi}}_{\mathit{EXP}}$ (eV) | ${\mathbf{\Phi}}_{\mathit{DFT}}$ (eV) |
---|---|---|

Ir(111) | 5.76 | 5.77 |

KBr /Gr/Ir(111) | 4.71 | 4.96 |

Gr/Ir(111) | 4.56 | 4.74 |

KBr/Ir(111) | 4.06 | 4.16 |

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**MDPI and ACS Style**

Liu, Z.; Hinaut, A.; Peeters, S.; Scherb, S.; Meyer, E.; Righi, M.C.; Glatzel, T.
2D KBr/Graphene Heterostructures—Influence on Work Function and Friction. *Nanomaterials* **2022**, *12*, 968.
https://doi.org/10.3390/nano12060968

**AMA Style**

Liu Z, Hinaut A, Peeters S, Scherb S, Meyer E, Righi MC, Glatzel T.
2D KBr/Graphene Heterostructures—Influence on Work Function and Friction. *Nanomaterials*. 2022; 12(6):968.
https://doi.org/10.3390/nano12060968

**Chicago/Turabian Style**

Liu, Zhao, Antoine Hinaut, Stefan Peeters, Sebastian Scherb, Ernst Meyer, Maria Clelia Righi, and Thilo Glatzel.
2022. "2D KBr/Graphene Heterostructures—Influence on Work Function and Friction" *Nanomaterials* 12, no. 6: 968.
https://doi.org/10.3390/nano12060968