Passive probe model

We examine here the passive probe model [71], which ignores the effect of the probe on the SNOM image and assumes that the signal detected is proportional to the near-field intensity at the nanostructure surface in the absence of the probe. This hypothesis may be valid either if the field scattered by the tip is very small or if it is not reflected back by the sample. Thus, from this qualitative analysis, we may expect the probe to be passive either if the tip is very small or if the sample has a low reflectivity. Therefore, a metallic tip close to a metallic sample may not satisfy the assumption of a passive probe, whereas a tiny metallic tip above a dielectric (or magnetic) might be considered as a passive probe. 

A passive probe model simplifies calculation substantially. Indeed, such an approach enables us to work in the first Born approximation, while for calculation of the near field we only need to calculate the scattered field [i (z,to, n) by equation (19) with the Fourier transform chosen to be compati-... 

At smaller values of the tip-substrate separation distance, d, feedback and hindering effects, as those observed in the feedback modes at amperometric tips, perturb the transport processes. The tip then starts to interfere with the source, which complicates the quantitative data analysis without numerical modeling.Such perturbing effects are not observed when passive probes such as potentiometric " or biosensor microelectrodes are used, but then the positioning of these substance-selective sensors is difficult. An alternative would be to consider the use of the scanning ion conductance microscopy (SICM). Indeed, recently, the SICM afforded the opportunity to image and quantify precisely local K+ and Cl ionic fluxes. ... 

One may now extend such detailed study of the molecular/water interfaces to shed light on processes more directly relevant to corrosion. For instance, specific details regarding the reactivity of water molecules at the metal-water interface have been modeled with varying degrees of sophistication [27, 28, 112-116]. The dissociation of H2O at the surface into products presents an important first probe reaction and bas been related to the initiation of passivity of fresh metal surfaces when first exposed to aqueous solution ... 

The permeation and clearance of model ionic permeants after subconjunctival injection was assessed with NMR imaging. New Zealand white rabbit was the animal model and manganese ion (Mn ) and manganese EDTA complex (MnEDTA ) were the model permeants. The current study was divided into three parts in vitro, postmortem, and in vivo. Transscleral passive permeation experiments were conducted with excised sclera in side-by-side diffusion cells in vitro. Sub-conjunctival delivery experiments were conducted with rabbits postmortem and in vivo. The distribution and elimination of the probe permeants from the subconjunctival space after subconjunctival injections were determined by MRI. The permeability coefficients of Mn and MnEDTA across the sclera in vitro were 3.6 x 10 cm/s and 2.4 x 10 cm/s, respectively. 

Extensive work has also been performed on characterizing the electronic structure of passive films on various metals in terms of semiconductor models (Gerischer, 1990 Di Paola, 1989 Schultze, 1978). It is well recognized that passive Elms exhibit semiconductor properties, and numerous attempts have been made to interpret these in terms of classical theory for n- or p-doped semiconductors. The application of EIS has proven very successful (Quarto et al., 1981 Schmuki and Bohni, 1992 Stimming 1986) provided that the frequency is sufficiently high that only the electronic properties are being probed (f> 1 kHz), and ion and vacancy relaxation processes are unimportant. 

The passive permeability of the NPCs to various probe ions thus determined by SECM was quantitatively analyzed to assess the dimensions of the NPCs and the strength of NPC-probe interactions. Notably, the permeability values were proportional to ion diffusion coefficients in the aqueous phase (Figure 18.3). This proportionality is expected theoretically when the probe ions freely diffuse through nanopores as established by the SECM study of a pnc-Si membrane as a model (see Section 18.2.2). Theoretically, the permeability of a nanopore membrane to a probe ion, k, is based on three diffusion steps the probe ion accesses from the aqueous solution to the pore, moves through the pore, and escapes from the pore to the aqueous solution. This theoretical analysis gives ... 

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