Surface potential imaging mode

Keywords Atomic force microscopy Charge dissipation Charge domains Electric charges on polymer surfaces Electric field imaging mode Surface potential imaging mode... 

Fig. 2. Surface potential image and horizontal relief of a vertical line written on the ferroelectric LB film in a contact mode with Udc= -15V applied to the bottom electrode with respect to the grounded cantilever. Reading in a non-contact mode.

In addition to the conventional lithographic techniques, surface patterning was performed by means of local polymerisation of the monomers under the SPM tip. These studies have been mainly focused towards electrically conductive polymers such as polypyrole, polythyophene and polyaniline. The easiest way to implement polymerisation is to set either the tip or sample potential sufficiently positive to cause the electrochemical oxidation of the monomer [438, 451 -455]. This technique enabled controlled removal and deposition of polymer dots as small as 1 nm to in a well defined pattern [453]. After deposition, the dots could be read using a conventional imaging mode (Fig. 49). 

We are able to distinguish between the charge carrier density, respectively, the density of 7i-electrons and the surface potential, on the other side. Thus the combination of p-PES mode and the imaging mode of PEEM offers a new analytical method for the characterisation of active layers in operating devices. 

Figure 9.22 Simultaneous images of (a, top) topography and (b, bottom) surface potential acquired with an as-grown PBT film in the KFM feedback mode. (Reprinted with permission from Journal of Physical Chemistry B., On the origin of Mesoscopic Inhomogenity of Conducting Polymers by K. D. O Neil et al., ill, 31. Copyright (2007) American Chemical Society)...

To circumnavigate the problem of dragging poorly immobilized specimens across the substrate and/or the potential damage of debcate samples, alternative imaging modes were subsequently developed. The first of these was termed non-contact mode. In this mode the cantilever is oscillated above the surface at its resonant frequency, with... 

In [12, 13] there was only used a non-contact regime of imaging [9] which probed a surface potential influenced by many factors. In the present paper we make a comparative study of ultrathin dielectric and ferroelectric LB films poled locally by a strong d.c. field using two modes, a contact and non-contact ones. The latter allows for the measurement of the piezoelectric distortion of the film due to the d.c. field aligned spontaneous polarization [6,14]. The piezoelectric response has, indeed, been observed on ferroelectric LB films. Therefore, their local ferroelectric switching by an AFM tip has been confirmed. 

Eqs. (3) and (9) allow one to explain the observed variations in the contrast of images obtained in a non-contact (surface potential) mode, in particular, the inverse contrast (small P, large AA and o) which, otherwise, would require an ill-defined concept of overscreening the polarization by free charges [8, 14]. We can illustrate this with two EFM images shown in Figs. 2 and 3. 

Langmuir-Blodgett film. The left pair of images is taken in the contact (piezo-electric) mode, the right pair in non-contact (surface potential) mode. 

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