Capillary Condensation in Peak Force AFM Imaging of Nanopores

Capillary condensation in nano-confinements between two surfaces is a common phenomenon in micro-nano systems [1]. Based on the Kelvin equation, the length scale for capillary condensation in cylindrical nanopores is less than 3 nm [2]. Here we fabricated conical nanopores of avg. dia. 30 nm on a 100 nm thick silicon nitride film and performed Peak Force Imaging in AFM in air using a sharp silicon nitride tip. As the AFM tip scans the nanopore, the small gap between the periphery of the tip and walls of the conical pore leads to capillary condensation. Increasing the peak force should reduce the gap and increase the area of condensation on the tip. Therefore, increasing the peak force from 420 pN to 8.4 nN, increases the adhesion of tip-nanopore from 5 nN to 10 nN and the adhesion force profile also broadens. In contrast, the adhesion of tip-flat SiN substrate remains constant at 1.7 nN. We also performed imaging of nanopores for relative humidity values varying from 57% to 97%, for which the measured adhesion profile resembled that corresponding to the increasing peak force setpoint. These AFM measurements allow us to study capillary condensation even in much larger nanostructures.

References:
[1] Charlaix, E. & Ciccotti, M. CRC Press (2010).
[2] Yang, Q. et al. (2020) ArXiv.