Nanoscale physics
Effect of the crystallographic c-axis orientation on the tribological properties of the selected few-layer transition metal dichalcogenides
A. Kozak1, M. Sojkova2, M. Bodik3, K. Vegso3, M. Hulman2, P. Siffalovic1,3, and M. Tapajna1,2
1 Centre for Advanced Materials Application SAS, Dubravska cesta 9, Bratislava 84511, Slovakia,
E-mail: andrii.kozak@savba.sk
2 Institute of Electrical Engineering SAS, Dubravska cesta 9, 84104 Bratislava, Slovakia,
3 Institute of Physics SAS, Dubravska cesta 9, 84511 Bratislava, Slovakia

Transition metal dichalcogenides, a member of two-dimensional materials group, have received increased scientific attention thanks  to their unique electronic, physical, and chemical properties. In this work, a comparative investigation of nanoscale friction behavior of MoS2 and PtSe2 layers with the in-plane (||) and out-of-plane (⊥) crystallographic orientation of the c-axis have been performed based on the structural, morphological, and wettability (contact angle, Θ) measurements. MoS2 and PtSe2 ultra-thin films were prepared by sulfurization and selenization of 3 nm magnetron pre-deposited Mo and Pt layers, respectively. The growth of the layers with a different c-axis orientation was controlled by changing the synthesis parameters during the thermal sulfurization/selenization process.
The rotation of c-axis from ⊥ to || shows a different impact on the nanotribological and morphological properties of MoS2 and PtSe2. In particular, ⊥ oriented MoS2 shows more than six times lower friction and higher Θ in comparison with || oriented films. However, the identical change in the c-axis orientation for PtSe2 caused increased friction (approx. by half) and almost unchanged Θ. Moreover, switching the c-axis from ⊥ to || is accompanied by an increased RMS roughness of MoS2 and PtSe2 surfaces (from 1.3 to 2.3 nm and 0.69 to 0.81 nm, respectively). Thus, the observed nanoscale friction behavior can be a result of a different configuration of S and Se at the edges due to the different coordination of MoS2 and PtSe2 (2H and 1T, respectively). 
This work was performed during the implementation of the project Building-up Centre for advanced materials application SAS, ITMS project code 313021T081 supported by Research & Innovation Operational Programme funded by the ERDF (50%) and Research and Development Agency under the contract no. APVV-17-0560.