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Odd-Even Effect in Self Assembly and Phase Transition of Alkanethiols Monolayers (SAMs) on Au (111) Surfaces

L. Ramin, A. Jabbarzadeh
The University of Sydney, AU

Keywords: Alkanethiols, self assembled monolayers, SAMs, gold substrate, phase transition, tilt angle


Molecular dynamic simulations were conducted to predict the structural properties of n-alkanethiols CH3(CH2)n-1SH (n=4-22) self assembled monolayers (SAMs) on Au (111) surfaces. We studied the effects of chain length on the structural properties, including orientation and tilt angles. We found clear dependence of the structural properties, on the number of carbon atoms, n, in the chain being odd or even. For n=odd, larger tilt angles, oriented in the direction of their next next nearest neighbour (NNNN); and for n=even, lower tilt angle, mostly tilted towards next nearest neighbour (NNN), were observed. For all alkanethiols a gradual decrease of the tilt angle occurred by increasing the temperature from 300 to 420 K. However, the decrease was smaller for the even alkanethiols than that for the odd ones. We also observed solid-liquid phase transitions at a certain temperature, signified by abrupt instabilities in the tilt orientation angle, and sometimes preceded by brief solid-solid transition events. This transition temperature, which increased with the length of the molecule, was ~65-105ÂșC above the melting point of the corresponding n-alkane bulk system. The critical tilt angle at transition temperature increased monotonically with n. Overall, we found odd alkanethiols to show smaller gauche defects and better structural and thermal stability.
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