Size Effect and Odd–Even Alternation in the Melting of Single and Stacked AgSC<i>n</i> Layers: Synthesis and Nanocalorimetry Measurements

We report a systematic study of melting of layered lamella of silver alkanethiolates (AgSC<i>n</i>). A new synthesis method allows us to independently change the thickness of the crystal in two waysby modulating chain length (<i>n</i> = 7–18) and by stacking these crystals to a specific layer number (<i>m</i> = 1–10). This method produces magic size lamella, having a well-spaced discrete melting point, <i>T</i>_m, distribution. Nanocalorimetry shows stepwise increases in <i>T</i>_m, as the lamella thickness increases by integer increments of chain length. The relationship between <i>T</i>_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. <i>T</i>_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, <i>G</i>_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 Å smaller for crystals with odd chains

Critical Size for Bulk-to-Discrete Transition in 2D Aliphatic Layers: Abrupt Size Effect Observed via Calorimetry and Solid-State ¹³C NMR

Anomalous changes of physical properties are observed in an abrupt bulk-to-discrete transition in layered silver alkanethiolate (AgSC<i>n</i>, <i>n</i> = 1–16). A critical chain length of <i>n</i>_cr = 7 marks the sharp boundary between the bulk (uniform, <i>n</i> ≥ 7) and discrete (individualistic, <i>n</i> ≤ 6) forms of AgSC<i>n</i>. Solid-state ¹³C NMR analysis reveals that none of the carbons share identical chemical environment in the discrete range, making each AgSC<i>n</i> with <i>n</i> = 2–6 uniquely different material, even though the crystal structure is preserved throughout. Extraordinary changes of thermodynamic properties appearing at this bulk-to-discrete transition include ∼500% increases of melting enthalpy (Δ<i>H</i>_m), ∼50 °C increases of melting point (<i>T</i>_m), and an atypical transition between size-dependent <i>T</i>_m depression and <i>T</i>_m enhancement. We develop a new comprehensive Gibbs–Thomson model with piecewise excess free energy (Δ<i>G</i>_excess) to predict the nature of the abrupt size effect melting. A new 3D spatial model is constructed to divide the aliphatic chains of AgSC<i>n</i> into three bulk or discrete segments: (a) tail segment containing three carbons, (b) head segment containing two carbons, and (c) bulk mid-chain segment containing (<i>n</i> – 5) carbons. Odd/even effect of <i>T</i>_m and Δ<i>H</i>_m is described by a constant Δ<i>G</i>_excess over the entire chain length range of AgSC<i>n</i> and is exclusively attributed to the localized tail segment. Bulk-to-discrete transition occurs when material properties are dominated by the discrete head and tail segments at <i>n</i> < <i>n</i>_cr. Values of <i>n</i>_cr are independently measured by both calorimetry and ¹³C NMR. This analysis is generalized to other aliphatic layers including <i>n</i>-alkanes with <i>n</i>_cr ≈ 11. This work is seminal to the design of novel aliphatic layers with tailorable properties (e.g., <i>T</i>_m) and has applications in molecular electronics and biophysics