Lateral Force Microscopy Studies of the Nanoscale Frictional Properties of Two-Dimensional SnSe and SnSe₂ as Solid Lubricants

Solid-state lubrication at the micro- and nanoscale poses challenges for high-performance microelectromechanical systems and space mechanisms, as conventional liquid lubricants are impractical. Two-dimensional (2D) layered materials offer promising solutions due to their weak van der Waals forces, enabling shear with minimal energy loss. This study presents the first experimental and theoretical analysis of the nanoscale friction behavior of mechanically exfoliated SnSe and SnSe₂, known for their thermoelectric and optoelectronic properties. Lateral-force microscopy conducted on Sn-based flakes demonstrates superior performance compared to graphene flakes of similar thickness. We measured the friction coefficients of 0.023 ± 0.004 and 0.027 ± 0.019 for SnSe and SnSe₂, respectively, while graphene exhibited a coefficient of 0.042 ± 0.060. Density functional theory calculations suggest that pristine monolayers of SnSe₂ should have a lower friction than SnSe, which contrasts with our experimental findings. X-ray photoelectron spectroscopy measurements indicate that both materials possess a thin oxide layer on their surfaces. Ab initio calculations show that the formation of the oxide layer increases the energy barrier by approximately 10× for SnSe and 100× for SnSe₂. Further potential energy surface analyses indicate that SnSe₂ has an intrinsically lower interlayer shear strength, implying that removing surface oxides could lead to significantly lower friction. This positions SnSe₂ as a promising candidate for applications in space exploration.