title: Optical imaging beyond the diffraction limit by SNEM: effects of AFM tip modifications with thiol monolayers on imaging quality.
abstract: Tip-enhanced nanoscale optical imaging techniques such as apertureless scanning near-field optical microscopy (a-SNOM) and scanning near-field ellipsometric microscopy (SNEM) applications can suffer from a steady degradation in performance due to adhesion of atmospheric contaminants to the metal coated tip. Here, we demonstrate that a self-assembled monolayer (SAM) of ethanethiol (EtSH) is an effective means of protecting gold-coated atomic force microscopy (AFM) probe tips 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 from the untreated gold tip fell by 66%, while those from 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 field enhancement we find that it can be locally removed from the tip apex by deforming the molecules under load, restoring SNEM image contrast.
authors.affiliation: Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, Enschede NL-7500, The Netherlands; Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX, Eindhoven, The Netherlands.
authors.forename: Aysegul
authors.initials: A
authors.lastname: Cumurcu
authors.affiliation: Scientific and Technological Centers of the University of Barcelona, C/ Lluís Solé i Sabaris, 1-3, 08028 Barcelona, Spain.
authors.forename: Jordi
authors.initials: J
authors.lastname: Diaz
authors.affiliation: Nanophysics and Soft Matter Group, H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, United Kingdom.
authors.forename: Ian D
authors.initials: ID
authors.lastname: Lindsay
authors.affiliation: Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, Enschede NL-7500, The Netherlands.
authors.forename: Sissi
authors.initials: S
authors.lastname: de Beer
authors.affiliation: Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, Enschede NL-7500, The Netherlands.
authors.forename: Joost
authors.initials: J
authors.lastname: Duvigneau
authors.affiliation: Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, Enschede NL-7500, The Netherlands; NanoBioInterface, Research Center Design and Technology, Saxion University of Applied Sciences, 7500 KB Enschede, The Netherlands.
authors.forename: Peter
authors.initials: P
authors.lastname: Schön
authors.affiliation: Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, Enschede NL-7500, The Netherlands. Electronic address: g.j.vancso@utwente.nl.
authors.forename: G
authors.initials: G
authors.lastname: Julius Vancso
keywords: AFM tip modification; Air contamination; Metal tip degradation; SAM; SNEM; Tip enhanced