Molecular-scale shear response of the organic semiconductor β -DBDCS (100) surface

In this work we present friction-force microscopy (FFM) lattice-resolved images acquired on the (100) facet of the semiconductor organic oligomer ( 2 Z , 2 ′ Z ) − 3 , 3 ′ -(1,4-phenylene)bis(2-(4-butoxyphenyl)acrylonitrile) ( β -DBDCS) crystal in water at room temperature. Stick-slip contrast, lateral contact stiffness, and friction forces are found to depend strongly on the sliding direction due to the anisotropic packing of the molecular chains forming the crystal surface along the [010] and [001] directions. The anisotropy also causes the maximum value of the normal force applicable before wearing to increase by a factor of 3 when the scan is performed along the [001] direction on the (100) face. Altogether, our results contribute to achieving a better understanding of the molecular origin of friction anisotropy on soft crystalline surfaces, which has been often hypothesized but rarely investigated in the literature.This work was supported by the Spanish Ministry of Economy and Competitiveness (MINECO) (Projects No. MAT2012-38810 and No. CTQ2014-58801), by the Comunidad de Madrid (project Mad2D, Grant No. S2013/MIT- 3007), by the Campus of International Excellence (CEI) UAM+CSIC, and by theNational Research Foundation ofKorea (NRF) through a grant funded by the Korean government (MSIP; Grant No. 2009-0081571).Peer reviewe