Factors influencing surface coverage and structural organization of phosphonic acid self-assembled monolayers on zinc oxide

This study investigates the effects of solvent type, ODPA concentration, and solution temperature on the growth, surface coverage, and structural order of n-octadecylphosphonic acid (ODPA) self-assembled monolayers (SAMs) on zinc oxide (ZnO) surfaces. We explore the impacts of isopropanol and ethanol as solvents, considering their differing dielectric constants and lower toxicity compared to non-polar alternatives like toluene. The results indicate that ODPA SAMs formed more rapidly in isopropanol, with a growth rate of 0.7% per minute, compared to 0.2% per minute in ethanol. However, ethanol resulted in slightly higher surface coverage (97%) than isopropanol (96%). Moreover, increasing the ODPA concentration from 0.03 to 0.14 mM enhanced surface coverage; however, further increases in concentration did not lead to a corresponding increase in SAM coverage due to molecular interactions. The solution temperature also significantly influenced the growth rate of the SAM, with temperatures rising from 30 to 60 °C enhancing growth. However, higher temperatures (above 60 °C) affected the SAM structure, leading to defects in the monolayer. This study provides a comprehensive understanding of how solvent type, concentration, and solution temperature influence SAM formation, offering valuable insights for optimizing deposition conditions to enhance device performance in optoelectronic applications.