# 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

- Zheng, H.Y.; Li, Y.J.; Liu, H.B.; Yin, X.D.; Li, Y.L. Construction of heterostructure materials toward functionality. Chem. Soc. Rev.
**2011**, 40, 4506–4524. [Google Scholar] [CrossRef] [PubMed] - Carbone, L.; Cozzoli, P.D. Colloidal heterostructured nanocrystals: Synthesis and growth mechanisms. Nano Today
**2011**, 5, 449–493. [Google Scholar] [CrossRef] - Androulidakis, C. Zhang, K.H.; Robertson, M.; Tawfick, S. Tailoring the mechanical properties of 2D materials and heterostructures. 2D Mater.
**2018**, 5, 032005. [Google Scholar] [CrossRef] - Frisenda, R.; Molina-Mendoza, A.J.; Mueller, T.; Castellanos-Gomez, A.; Van Der Zant, H.S.J. Atomically thin p–n junctions based on two-dimensional materials. Chem. Soc. Rev.
**2018**, 47, 3339–3358. [Google Scholar] [CrossRef][Green Version] - Allen, M.J.; Tung, V.C.; Kaner, R.B. Honeycomb carbon: A review of graphene. Chem. Rev.
**2010**, 110, 132–145. [Google Scholar] [CrossRef] [PubMed] - Novoselov, K.S.; Mishchenko, A.; Carvalho, A.; Neto, A.H.C. 2D materials and van der Waals heterostructures. Science
**2016**, 353, aac9439. [Google Scholar] [CrossRef] [PubMed][Green Version] - Zhou, C.C.; Li, X.D.; Hu, T.T. Structural and Electronic Properties of Heterostructures Composed of Antimonene and Monolayer MoS
_{2}. Nanomaterials**2020**, 10, 2358. [Google Scholar] [CrossRef] - Yang, W.; Chen, G.R.; Shi, Z.W.; Liu, C.C.; Zhang, L.C.; Xie, G.B.; Cheng, M.; Wang, D.; Yang, R.; Shi, D.; et al. Epitaxial growth of single-domain graphene on hexagonal boron nitride. Nat. Mater.
**2013**, 12, 792–797. [Google Scholar] [CrossRef] - Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-layer MoS
_{2}transistors. Nat. Nanotechnol.**2011**, 6, 147–150. [Google Scholar] [CrossRef] - Li, Y.P.; Li, D.Y. Experimental studies on relationships between the electron work function, adhesion, and friction for 3d transition metals. J. Appl. Phys.
**2004**, 95, 7961–7965. [Google Scholar] [CrossRef] - Liu, S.Y.; Lu, H.; Li, D.Y. The relationship between the electron work function and friction behavior of passive alloys under different conditions. Appl. Surf. Sci.
**2015**, 351, 7961–7965. [Google Scholar] [CrossRef] - Filleter, T.; Emtsev, K.V.; Seyller, T.; Bennewitz, R. Local work function measurements of epitaxial graphene. Appl. Phys. Lett.
**2008**, 93, 133117. [Google Scholar] [CrossRef] - Filleter, T.; McChesney, J.L.; Bostwick, A.; Rotenberg, E.; Emtsev, K.V.; Seyller, T.; Horn, K.; Bennewitz, R. Friction and dissipation in epitaxial graphene films. Phys. Rev. Lett.
**2009**, 102, 086102. [Google Scholar] [CrossRef] [PubMed][Green Version] - Lavini, F.; Caló, A.; Gao, Y.; Albisetti, E.; Cao, T.F.; Li, G.Q.; Cao, L.Y.; Aruta, C.; Riedo, E. Friction and work function oscillatory behavior for an even and odd number of layers in polycrystalline MoS
_{2}. Phys. Rev. Lett.**2018**, 10, 8304–8312. [Google Scholar] [CrossRef] [PubMed] - Sadewasser, S.; Glatzel, T. Kelvin Probe Force Microscopy Measuring and Compensating Electrostatic Forces; Springer Series in Surface Sciences; Springer: Berlin/Heidelberg, Germany, 2011. [Google Scholar]
- Sadewasser, S.; Glatzel, T. Kelvin Probe Force Microscopy—From Single Charge Detection to Device Characterization; Springer International Publishing: Berlin/Heidelberg, Germany, 2018. [Google Scholar]
- Gnecco, E.; Meyer, E. Fundamentals of Friction and Wear on the Nanoscale; Springer International Publishing: Berlin/Heidelberg, Germany, 2015. [Google Scholar]
- Li, B.; Geng, Y.Q.; Yan, Y.D. Nano/microscale thermal field distribution: Conducting thermal decomposition of pyrolytic-type polymer by heated AFM probes. Nanomaterials
**2020**, 10, 483. [Google Scholar] [CrossRef][Green Version] - Liu, Z.; Wang, Y.F.; Glatzel, T.; Hinaut, A.; Zhang, J.Y.; Meyer, E. Low Friction at the nanoscale of hydrogenated fullerene-like carbon films. Coatings
**2020**, 10, 643. [Google Scholar] [CrossRef] - Kawai, S.; Glatzel, T.; Koch, S.; Such, B.; Baratoff, A.; Meyer, E. Systematic achievement of improved atomic-scale contrast via bimodal dynamic force microscopy. Phys. Rev. Lett.
**2009**, 103, 220801. [Google Scholar] [CrossRef] [PubMed] - Kawai, S.; Glatzel, T.; Koch, S.; Such, B.; Baratoff, A.; Meyer, E. Ultrasensitive detection of lateral atomic-scale interactions on graphite(0001) via bimodal dynamic force measurements. Phys. Rev. B
**2010**, 81, 085420. [Google Scholar] [CrossRef] - Bhushan, B. Nanotribology and nanomechanics. Wear
**2005**, 259, 1507–1531. [Google Scholar] [CrossRef] - Liu, Z.; Hinaut, A.; Peeters, S.; Scherb, S.; Meyer, E.; Righi, M.C.; Glatzel, T. Reconstruction of a 2D layer of KBr on Ir(111) and electromechanical alteration by graphene. Beilstein J. Nanotechnol.
**2021**, 12, 432–439. [Google Scholar] [CrossRef] [PubMed] - Perdew, J.P.; Zunger, A. Self-interaction correction to density-functional approximations for many-electron systems. Phys. Rev. B
**1981**, 23, 5048–5079. [Google Scholar] [CrossRef][Green Version] - Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G.L.; Cococcioni, M.; Dabo, I.; et al. QUANTUM ESPRESSO: A modular and open-source software project for quantum simulations of materials. J. Phys. Condens. Matter
**2009**, 21, 395502. [Google Scholar] [CrossRef] [PubMed] - Giannozzi, P.; Andreussi, O.; Brumme, T.; Bunau, O.; Nardelli, M.B.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Cococcioni, M.; et al. Advanced capabilities for materials modelling with quantum ESPRESSO. J. Phys. Condens. Matter
**2017**, 29, 465901. [Google Scholar] [CrossRef] [PubMed][Green Version] - Peeters, S.; Restuccia, P.; Loehlé, S.; Thiebaut, B.; Righi, M.C. Tribochemical reactions of MoDTC lubricant additives with iron by quantum mechanics/molecular mechanics simulations. J. Phys. Chem. C
**2020**, 124, 13688–13694. [Google Scholar] [CrossRef] - Corsini, C.; Peeters, S.; Righi, M.C. Adsorption and dissociation of Ni(acac)
_{2}on iron by ab initio calculations. J. Phys. Chem. A**2020**, 124, 8005–8010. [Google Scholar] [CrossRef] [PubMed] - Peeters, S.; Charrin, C.; Duron, I.; Loehlé, S.; Thiebaut, B.; Righi, M.C. Importance of the catalytic effect of the substrate in the functionality of lubricant additives: The case of molybdenum dithiocarbamates. J. Mater. Today Chem.
**2021**, 21, 100487. [Google Scholar] [CrossRef] - Losi, G.; Peeters, S.; Delayens, F.; Vezin, H.; Loehlé, S.; Thiebaut, B.; Righi, M.C. Experimental and ab initio characterization of mononuclear molybdenum dithiocarbamates in lubricant mixtures. Langmuir
**2021**, 37, 4836–4846. [Google Scholar] [CrossRef] - Janthon, P.; Viñes, F.; Sirijaraensre, J.; Limtrakul, J.; Illas, F. Adding pieces to the CO/Pt(111) puzzle: The role of dispersion. J. Phys. Chem. C
**2017**, 121, 3970–3977. [Google Scholar] [CrossRef][Green Version] - Cutini, M.; Maschio, L.; Ugliengo, P. Exfoliation energy of layered materials by DFT-D: Beware of dispersion! J. Chem. Theory Comput.
**2020**, 16, 5244–5252. [Google Scholar] [CrossRef] - Tang, W.; Sanville, E.; Henkelman, G. A Grid-based Bader analysis algorithm without lattice bias. J. Phys. Condens. Matter
**2009**, 21, 084204. [Google Scholar] [CrossRef] [PubMed] - Sanville, E.; Kenny, S.D.; Smith, R.; Henkelman, G. Improved grid-based algorithm for Bader charge allocation. J. Comput. Chem.
**2007**, 28, 899–908. [Google Scholar] [CrossRef] [PubMed] - Henkelman, G.; Arnaldsson, A.; Jónsson, H. A fast and robust algorithm for Bader decomposition of charge density. Comp. Mater. Sci.
**2006**, 36, 354–360. [Google Scholar] [CrossRef] - Yu, M.; Trinkle, D.R. Accurate and efficient algorithm for Bader charge integration. J. Chem. Phys.
**2011**, 134, 064111. [Google Scholar] [CrossRef][Green Version] - Wolloch, M.; Levita, G.; Restuccia, P.; Righi, M.C. Interfacial charge density and its connection to adhesion and frictional forces. Phys. Rev. Lett.
**2018**, 121, 026804. [Google Scholar] [CrossRef] [PubMed][Green Version] - Kokalj, A. Computer graphics and graphical user interfaces as tools in simulations of matter at the atomic scale. Comp. Mater. Sci.
**2003**, 28, 155–168. [Google Scholar] [CrossRef] - Glatzel, T.; Zimmerli, L.; Koch, S.; Such, B.; Kawai, S.; Meyer, E. Determination of effective tip geometries in Kelvin probe force microscopy on thin insulating films on metals. Nanotechnology
**2009**, 20, 264016. [Google Scholar] [CrossRef] [PubMed] - Filleter, T.; Paul, W.; Bennewitz, R. Atomic structure and friction of ultrathin films of KBr on Cu(100). Phys. Rev. B
**2008**, 77, 035430. [Google Scholar] [CrossRef] - Holmberg, N.; Laasonen, K.; Peljo, P. Charge distribution and Fermi level in bimetallic nanoparticles. Phys. Chem. Chem. Phys.
**2016**, 18, 2924–2931. [Google Scholar] [CrossRef][Green Version] - Lüthi, R.; Meyer, E.; Howald, L.; Bammerlin, M.; Güntherodt, H.J.; Gyalog, T.; Thomas, H. Friction force microscopy in ultrahigh vacuum: An atomic-scale study on KBr(001). Phys. Chem. Chem. Phys.
**1995**, 1, 129–138. [Google Scholar] - Wieferink, C.; Krüger, P.; Pollmann, J. Simulations of friction force microscopy on the KBr(001) surface based on ab initio calculated tip-sample forces. Phys. Rev. B
**2011**, 83, 235328. [Google Scholar] [CrossRef] - Coraux, J.; Plasa, T.N.; Busse, C.; Michely, T. Structure of epitaxial graphene on Ir(111). New J. Phys.
**2008**, 10, 043033. [Google Scholar] - Busse, C.; Lazić, P.; Djemour, R.; Coraux, J.; Gerber, T.; Atodiresei, N.; Caciuc, V.; Brako, R.; Blügel, S.; Zegenhagen, J.; et al. Graphene on Ir(111): Physisorption with chemical modulation. Phys. Rev. Lett.
**2011**, 107, 036101. [Google Scholar] [CrossRef] [PubMed][Green Version] - Torrel, S. Chemical Vapor Deposition Growth of Molybdenum Disulfide and Its Nanoscale Tribological Correlation with Raman Spectroscopy; Rutgers The State University of New Jersey-New Brunswick: New Brunswick, NJ, USA, 2017. [Google Scholar]

**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