Effect of substrate temperature on vapor-phase self-assembly of n-octylphosphonic acid monolayer for low-voltage organic thin-film transistors

N-octylphosphonic acid (C₈PA) monolayer was self-assembled on aluminum oxide (AlO_x) from vapor in vacuum, while the substrate temperature was varied between 25 and 150 °C. The capacitance, water contact angle measurement, Fourier transform infrared (FTIR) spectroscopy, and atomic force microscopy (AFM) confirmed the presence of C₈PA on AlO _x for all growth temperatures. However, the structural and electrical properties of such monolayers depend on their growth temperature. The minimum surface roughness of 0.36 nm, the maximum water contact angle of 113.5° ± 1.4°, the lowest leakage current density of ∼10^-7 A/cm² at 3 V, and the capacitance of 0.43 μF/cm² were obtained for AlO_x/ C₈PA bi-layers with C₈PA deposited at 25 °C. The elevated temperature led to increased surface roughness, decreased water contact angle, increased leakage current, inferior molecular ordering, and lower molecular coverage; while the effect on the chemisorption of the phosphonate was minimal. Methyl and methylene FTIR vibrations associated with C₈PA aliphatic tails exhibited similar centre-peak wavenumbers to those observed for C₈PA monolayers assembled from solutions, presenting a viable 'dry' alternative to the existing solution process. The substrate temperature applied during C₈PA self-assembly also affected the parameters of pentacene thin-film transistors with AlO_x/C₈PA gate dielectrics. The increase in the growth temperature from 25 to 150 °C decreased the field-effect mobility from 0.060 to 0.026 cm²/V.s and increased the threshold voltage from -1.19 to -1.38 V, while maintaining the off-current at or below 10^-12 A and the subthreshold slope near 90 mV/decade.