Optical imaging beyond the diffraction limit by SNEM: Effects of AFM tip modifications with thiol monolayers on imaging quality

摘要

Tip enhanced nanoscale optical imaging techniques such as apertureless scanning near field optical microscopy (a-SNOM) and scanning near held ellipsometric microscopy (SNEM) applications can suffer from a steady degradation in performance due to adhesion of atmospheric contaminants to the metal coated Lip. Here, we demonstrate that a self assembled monolayer (SAM) of ethanethiol (ELSH) is an effective means of protecting gold coated atomic force microscopy (AFM) probe Lips from accumulation of surface contaminants during prolonged exposure to ambient air. The period over which they yield consistent and reproducible results for scanning near field ellipsometric microscopy (SNEM) imaging is thus extended. SNEM optical images of a microphase separated polystyrene-block-poly (methylmethacrylate) (PS-b-PMMA) diblock copolymer film, which were captured with bare and SAM protected gold coated AFM probes, both immediately after coating and following five days of storage in ambient air, were compared. During this period the intensity of the optical signals born the untreated gold tip fell by 66%, while those horn the SAM protected tip fell by 14%. Additionally, gold coated AFM probe tips were modified with various lengths of alkanethiols to measure the change in intensity variation in the optical images with SAM layer thickness. The experimental results were compared to point dipole model calculations. While a SAM of 1-dodecanethiol (DoSH) was found to strongly suppress held enhancement we find that it can be locally removed from the tip apex by deforming the molecules under load, restoring SNEM image contrast. (C) 2014 Elsevier B.V. All rights reserved.