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

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.

Sub-nanometer expansions of redox responsive polymer films monitored by imaging ellipsometry.

We describe a novel approach to quantitatively visualize sub nm height changes occurring in thin films of redox active polymers upon reversible electrochemical oxidation/reduction in situ and in real-time with electrochemical imaging ellipsometry (EC-IE). Our approach is based on the utilization of a micro-patterned substrate containing circular patterns of passive (non-redox active) 11-mercapto-1-undecanol (MCU) within a redox-responsive oligoethylene sulfide end-functionalized poly(ferrocenyldimethylsilane) (ES-PFS) film on a gold substrate. The non-redox responsive MCU layer was used as a molecular reference layer for the direct visualization of the minute thickness variations of the ES-PFS film. The ellipsometric microscopy images were recorded in aqueous electrolyte solutions at potentials of -0.1 V and 0.6 V vs. Ag/AgCl corresponding to the reduced and oxidized redox states of ES-PFS, respectively. The ellipsometric contrast images showed a 37 (±2)% intensity increase in the ES-PFS layer upon oxidation. The thickness of the ES-PFS layer reversibly changed between 4.0 (±0.1) nm and 3.4 (±0.1) nm upon oxidation and reduction, respectively, as determined by IE. Additionally, electrochemical atomic force microscopy (EC-AFM) was used to verify the redox controlled thickness variations. The proposed method opens novel avenues to optically visualize minute and rapid height changes occurring e.g. in redox active (and other stimulus responsive) polymer films in a fast and non-invasive manner.

Covalent layer-by-layer assembly of redox-active polymer multilayers.

Poly(ferrocenyl(3-bromopropyl)methylsilane) and poly(ethylene imine) are employed in a layer-by-layer deposition process to form covalently connected, redox-active multilayer thin films by means of an amine alkylation reaction. The stepwise buildup of these multilayers on silicon, ITO, and quartz substrates was monitored by UV-vis absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR), static contact angle measurements, surface plasmon resonance (SPR), atomic force microscopy, ellipsometry, and cyclic voltammetry, which provide evidence for a linear increase in multilayer thickness with the number of deposited bilayers. Upon oxidation and reduction, these covalently interconnected layers do not disassemble, in contrast to poly(ferrocenylsilane) (PFS) layers featuring similar backbone structures that are held together by electrostatic forces. The PFS/PEI multilayers are effective for the electrochemical sensing of ascorbic acid and hydrogen peroxide and show improved sensing performance at higher bilayer numbers. These covalently linked layers are readily derivatized further and can therefore be regarded as a versatile platform for creating robust, tailorable, redox-active interfaces with applications in sensing and biofuel cells.