Tip treatment for subnanoscale atomic force microscopy in liquid by atomic layer deposition Al2O3 coating

Atomic force microscopy (AFM) allows direct imaging of atomic- or molecular-scale surface structures in liquid. However, such subnanoscale measurements are often sensitive to the AFM tip properties. To overcome this problem, 30 nm Si-sputter coating was proposed, and its effectiveness in improving stability and reproducibility has been demonstrated in atomic-scale imaging of various materials. However, this method involves tip blunting, enhancing the tip-induced dilation effect. As an alternative method, here we investigate atomic layer deposition (ALD) Al2O3-coating, where the film thickness is atomically well-controlled. Our transmission electron microscopy, contact angle and force curve measurements consistently suggest that as-purchased tips are covered with organic contaminants, and the initial 20 cycles gradually remove them, reducing the tip radius (Rt) and hydrophobicity. Further deposition increases Rt and hydrophilicity and forms an intact Al2O3 film over 50 cycles. We compared 50-cycle ALD-coated tips with 30 nm Si-sputter-coated tips in imaging mica and chitin nanocrystals (NCs). On mica, ALD coating gives slightly less stability and reproducibility in hydration force measurements than the Si sputter coating, yet they are sufficient in atomic-scale imaging. In imaging chitin NCs, ALD-coated tips give a less tip-induced dilation effect while maintaining molecular-scale imaging capability. We also found that 10-cycle-ALD coated tips covered with carbon give a better resolution and reproducibility in observing subnanoscale features at chitin NC surfaces. This result and our experience empirically suggest carbon-coated tips' effectiveness in observing carbon-based materials.