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

• Studied n-octylphosphonic acid (C8PA) SAM using vapor-phase growth at various T.
• 25 °C-growth temperature leads to more ordered SAMs with standing up aliphatic tails.
• Higher growth temperature leads to less ordered SAMs; 150 °C – low surface coverage.
• Structural properties of optimized SAM are similar to those prepared from solutions.
• Al2O3/C8PA exhibits low roughness and leakage current and high capacitance in OFETs.

N-octylphosphonic acid (C8PA) monolayer was self-assembled on aluminum oxide (AlOx) 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 AlOx 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 AlOx/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 AlOx/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.

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