Thermodynamics of decanethiol adsorption on Au(111):: Extension to 0 °C

The coverage-dependent phase behavior of the thiolate formed from docanethiol, CH3(CH2)(9)SH, on Au(111) was studied at 0 degreesC using variable-temperature scanning tunneling microscopy and compared to analogous results for temperatures between 25 and 65 degreesC. At 0 degreesC, the lowest density striped phase, beta, converts to higher density striped phases, delta and chi, at exposures that are significantly less than those required at room temperature. The upright saturation phase, phi, is also obtained with a lower relative exposure. We discuss these results using an extrapolation of the schematic two-dimensional pressure (pi) versus temperature phase diagram developed in previous work. The observed low-temporature phase behavior is rationalized on the basis of thermodynamic considerations. By use of a schematic plot of phase chemical potential versus lateral pressure, the range of exposures over which various phases are thermodynamically stable is assessed as a function of temperature between 0 and 65 degreesC.