Size Effect and Odd-Even Alternation in the Melting of Single and Stacked AgSG? Layers: Synthesis and Nanocalorimetry Measurements

摘要

We report a systematic study of melting of layered lamella of silver alkanethiolates (AgSCn). A new synthesis method allows us to independently change the thickness of the crystal in two ways-by modulating chain length (n = 7-18) and by stacking these crystals to a specific layer number (m = 1 -10). This method produces magic size lamella, having a well-spaced discrete melting point, T_m, distribution. Nanocalorimetry shows stepwise increases in T_m, as the lamella thickness increases by integer increments of chain length. The relationship between T_m and the inverse thickness follows the linear scaling law of Gibbs-Thomson effect. Layer stacking dramatically changes the degree and nature of size-effect melting. There is odd/even effect in stacked 2, 3, and 4 layers. T_m values of single-layer and multilayer samples do not show noticeable odd/even alternation. We develop a phenomenological model of size effect based on the cumulative excess free energy, G_(excess), contributions of four spatially separate regions of the crystal: surface, Ag-S central plane, substrate interface, and interlayer interface. The selective appearance of the odd/even effect is due to the significant stabilization (1.4 kj/mol) of interlamellae interfaces of odd-chain samples, possibly due to registration/packing. Stabilization occurs only for the mobile lamellae situated close to the free surface, and thus 2-layer samples show the highest degree of stabilization. X-ray diffraction shows that the chains are tilted 18° with respect to the basal plane normal but that the van der Waals gap is 0.3 A smaller for crystals with odd chains.