Influences on the Dynamics and Stability of Self-Assembly: Solvent, Substrate, and Concentration

The formation dynamics and stability of CoOEP at the solution/Au(111) interface are captured in situ using scanning tunneling microscopy (STM) in a dynamic solution flow cell at room temperature. The intermediate steps of self-assembly of CoOEP into an ordered monolayer were captured, and fractional coverage as a function of time was measured to extract characteristic parameters of the self-assembly process. Adlayer structure and formation under various solvents are compared to previous studies conducted on HOPG. The choice of substrate is found to have a dramatic influence on adlayer structure and stability. It was found that the CoOEP adlayer assembly on HOPG is an equilibrium process, and the monolayer can be readily formed within minutes of contact with solution (above a solvent dependent threshold solution concentration); the dissolution of the formed adlayer is feasible, though the rate of dissolution is solvent-dependent. The assembly of an adlayer on Au(111) is kinetically driven, monolayer formation occurs within minutes, and dissolution is very slow only minimal island dissolution was achieved after hours of pure solvent flow. Solvent incorporation of 1,2,4-trichlorobenzene (TCB) was observed to form a CoOEP pseudorectangular adlayer structure (REC) on both HOPG and Au(111), though a solvent-free pseudohexagonal structure (HEX) occurred at much higher concentrations of CoOEP on HOPG than on Au(111). This is likely due to the fact that CoOEP binds more strongly to Au(111) than HOPG, which promotes the REC to HEX transition on Au(111) at lower concentrations. Solvent incorporation of toluene (Tol) into a CoOEP adlayer on Au(111) was observed, but it did not incorporate into the adlayer on HOPG. There is a significant increase in the Arrhenius desorption rate factor (similar to 350) of toluene on HOPG relative to Au that is likely a driving factor for Tol coadsorption on Au. A very short-lived decane incorporated adlayer was also observed. The transformation on Au(111) from REC to HEX structure under 1 mu M CoOEP in Tol occurred within similar to 10 min, while under a solution of 470 mu M CoOEP in TCB the transformation required similar to 102 min. This variance is primarily due to the relative residence times of the solvent molecule on the Au(111) surface, where Tol has an estimated desorption rate 500 times greater than TCB. The unit cells of the CoOEP adlayer are also substrate-dependent. The commensurate TCBincorporated REC structure on Au(111) contains two CoOEP but only one CoOEP on HOPG. Thus, the adlayer formation of CoOEP on Au(111) was more significantly affected by solvent than for adsorption on HOPG.