On the Origin of Mesoscopic Inhomogeneity of Conducting Polymers

Abstract

The mesoscopic inhomogeneity of conducting polymer films obtained by electropolymerization and spin-coating was studied using Kelvin probe force microscopy (KFM) and current-sensing atomic-force microscopy (CS-AFM). A well-pronounced correlation was established between the polymer morphology, on the one hand, and its local work function (which is related to the polymer oxidation degree) as well as polymer conductivity, on the other. The most conducting regions were associated with the tops of the polymer grains and showed Ohmic behavior. They were surrounded first by semiconducting and then by insulating polymer. The conductivity of the grain periphery could be lower by as much as 2 orders of magnitude. The grain cores also showed consistently higher values of the local work function as compared to the grain periphery. This fact suggested that the grain cores were more oxidized and/or more ordered as compared to the grain periphery, which is in good agreement with the local conductivity data. More uniform morphology corresponded to less variability in the other properties of the polymer. A model is proposed that relates the observed inhomogeneity to preferential deposition of polymer molecules with higher molecular weight at the early stages of the polymer phase formation. The polymer deposition in either electropolymerization or various solution-casting techniques involves the nucleation of a new phase from a solution containing polymer fractions of different molecular weights. The driving force of the nucleation process depends on the solubility of the polymer fractions, which decreases with an increase in the molecular weight. This gives rise to preferential deposition of more crystalline, higher molecular weight polymer at the early stages of the polymer deposition to form the cores of the polymer grains. The fractions with lower molecular weights are deposited later and form less ordered/less conducting grain periphery. On the basis of this model, we conclude that, to ensure the formation of materials with low inhomogeneity and high quality, one should use the starting polymer with as narrow molecular weight distribution as possible. Yet another possibility is to use solvents which would reduce the differences in the solubilities of polymer fractions with different molecular weight.

Quantifying fibronectin adhesion with nanoscale spatial resolution on glycosaminoglycan doped polypyrrole using Atomic Force Microscopy

“…The extent of the homogenous doping was reflected in the calculated surface area o f co n d u ct i ve regions (threshold > 5 pA) that was 97.7% for PPy/CS compared to 54% for PPy/HA. Similarly, conductive AFM imaging of polybithiophene films has revealed higher conductivity in the nodules compared to the nodular periphery 29 . These observations are further supported by Kelvin Probe Force Microscopy (KPFM) of PPy/pTS films showing a more negative potential at the nodules and explained by these regions as having a higher work function, or increased doping, than their peripheries 27 .…”

“…conductivity, surface potential) associated with the morphology of conducting polymers has also been revealed using AFM phase imaging. For example, both the modulus 29 and surface charge/energy 30 was shown to undergo phase separation.…”

“…Furthermore, the complex surface properties of conducting polymers, due to different oxidation states 27 , inhomogenous 44 doping 28 and phase separation of properties (e.g. surface potential and conductivity) 29,30 , that have been shown to display nanoscale lateral variation are expected to amplify variations in the conformation and adhesion of individual proteins across the polymer surface. Whilst the above studies have focused on conformation 17,20,21 , the FN-polymer interfacial forces are also interest as they play an important role in force-dependent signal transduction processes such as cellular forces exerted on FN through cell receptors and intracellular proteins to regulate cell function.…”

Vapor Phase Polymerization of EDOT from Submicrometer Scale Oxidant Patterned by Dip-Pen Nanolithography

“…C-AFM involves placing an electrical bias on the sample and measuring local conductivity via the current passing through a conductive AFM tip. 31,32 As direct electrical contact between the sample and tip must be maintained, contact mode imaging is used while simultaneously recording topography and current channels. Parts A and B of Figure 5 show a C-AFM image of a PEDOT microwire patterned across a gold-glass substrate interface.…”

Electrical Scanning Probe Microscopy on Active Organic Electronic Devices

“…[18] In addition to the anode layers, c-AFM has also been used to examine local transport through the semiconducting-polymer layer. [19][20][21] The observed variations in transport are often correlated with specific topographic features, but the overall theme among almost all of the c-AFM and STM studies performed so far is the extremely heterogeneous nature of these materials on fine length scales.…”