Revealing the nature of size and odd/even effect observed by NanoDSC in 2D organic metal chalcogenides: atomic resolution crystallography via diffraction and ab initio calculations*

Layered silver alkanethiolate (AgSCn) is currently of growing scientific interest as 2D organic metal chalcogenides. Exploring the structure of AgSCn with atomic resolution is critical to understand its size and odd/even effect observed in properties such as melting point/enthalpy (NanoDSC). Crystallography determination is often challenging for beam-sensitive hybrid materials with small crystal sizes. In this work, we innovatively overcome this obstacle by combining synchrotron powder X-ray diffraction, nanobeam electron diffraction, and ab initio calculations. Preliminary findings show AgSCn structure consisting of three segments: (i) Ag-S inorganic backbone, which undergoes deformation modulated by organic chain length due to the lattice mismatch between Ag-S framework and the epitaxial hydrocarbon chains; (ii) Interlayer interface: the structural origin of odd/even effect lies in the parity-dependent H-H interactions exhibited among the terminal methyl groups; (iii) Alkyl chains: the epitaxial mismatch between the backbone and the alkyl chains triggers the progression of distortion in the local alkyl configurations towards the optimal chain packing.