The effect of orientation of terminal groups in SAMs has been observed with a large head group, ferrocenes (larger than –CH3),75 and this manifests in the rectification ratios of molecular diodes (Figure 12a). This phenomena has also been reported in capacitance and
contact resistance.38 All these studies have, however, been on SAMs formed on Au or Ag (hard) substrates. Recently, it has been shown that even for flexible substrates the odd-even effect persists. The alkylamine SAMs formed on graphene demonstrates such phase evolution with chain length, as reported by Song et al.74 The calculated coverage of molecules on surface was estimated through molecular dynamics, showing three regions (Figure 12c) transitioning from comparatively low coverage (≤C3) to a higher coverage (≥C16). In addition, this transition was also observed in the calculated packing energy (Epack) and the physisorption energy (Eads) as shown (Figure 12d).
Besides wetting and charge transport properties, the odd-even effect – which as discussed above is dependent on the symmetry and conformation of the molecules in question, has been observe in other molecular properties. The odd-even effect in crystallization rate was observed in α,ω-bis-(pentane-2,4-dione-3-ylmethylsulfanyl)alkanes using polarized optical technique.76 In biological applications, the cell parameters (cell volume, angle, space group etc.) is dependent on the number of carbons in the ester groups in chiral crystal solids,77 which shows odd-even oscillation. Even in amorphous materials, an odd-even effect was observed in the glass transition temperature of network-forming ionic glasses.78
One can thus conclude that the chain length dependence is a universal properties for molecular assemblies, especially the alkyl chain based molecules, which is irrelevant to substrates nature. The chain length affects the intermolecular interaction and the inherent structure of the assemblies and the corresponding properties.
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CHAPTER 3. REVEALING THE QUALITY OF LARGE AREA TUNNELING