Physicochemical Modulation of Electronic and Spatial Density of States in Shape-Limited Self-Doped Polyaniline

概要

Conducting polymers have been applied in a wide range of electronic devices. In general, the conduction mechanism is described by the variable range hopping (VRH) model, in which electrical conductivity is realized by the transport of conduction carriers between localized states. In the electrical properties of conducting polymers in the bulk state, it is difficult to clarify the correlation between the localized state and the electrical properties because the conduction carriers are transported in three dimensions and the electrical properties are dominated through the highly conductive region. Therefore, the activation energy (Ea) has been widely used to evaluate the properties. In order to realize highly integrated molecular devices in the future, which utilize the intrinsic electrical properties of molecules, it is necessary to make molecular thin films in nanoscale. The reduction in the size of molecular thin films may lead to the possibility that electrical conduction through a small number of localized states may affect the overall electrical properties. However, previous studies have not experimentally elucidated the activation energy in terms of the energy state of the localized state and its distribution. Therefore, in order to understand the mechanism and the effect on the electrical properties associated with the heterogeneity of the localized state, new research approaches are required.

In this work, in order to modulate and understand the energy and spatial distribution of localized states respectively, the original fountain-pen lithography (FPL) was utilized for the fabrication of well-defined line patterns of conductive polymers (shape-limited- material). The control of the energy and the spatial distribution of localized states were achieved by changing the dopant level of self-doped polyaniline sulfonate (SPAN) and by line thickness. The chemical analysis of SPAN lines was conducted by Raman spectroscopy and the electrical properties of them was conducted by two-probe I-V measurements under different temperature conditions.

As a result, the macroscale uniformity and reproducibility on chemical state and I-V properties of SPAN lines fabricated by FPL method was confirmed at first by Raman spectroscopy (J. Bao et. al, Jpn. J. Appl. Phys., 60, 015002, 2021). The change in the peak intensities of the polaron and the quinoid indicated the correspondence with the electrical conductivity, thus the control of energy distribution in SPAN line is confirmed. Next, by decreasing both the dopant level and sample thickness in nanoscale, I found the decreased conductivity resulted in the increase in Ea and the generation of nonlinear I-V characteristics. A parameter 𝑐(⟨𝑑⟩, 𝑎) which related to the spatial distribution of localized states was introduced to the VRH model. By inducing a parameter 𝑐(⟨𝑑⟩, 𝑎) which related to the spatial distribution of localized states, the presence of less conductive “bottle-neck” regions is proposed in samples shown nonlinear I-V properties.

At last, the on-site generation of current noise with SPAN lines is reported. The SPAN lines fabricated on SiO2/Si indicated the current fluctuation under light stimulation. The study of the effect of substrate, laser power and power spectral density support the presence of “bottle-neck” regions in the shape-limited SPAN lines. (J. Bao et. al, Nanotechnology, 31, 365203, 2020)

In this study, a new approach of physicochemical modulation of shape-limited molecular lines with nanoscale thickness is proposed to understand the mechanism of electrical conduction through the localized state of conducting polymers. The modulation of energy and spatial distribution of localized states in SPAN samples is achieved by the FPL with controlling the dopant level, sample thickness, and the external electric field stimulation. For the samples which showed nonlinear I-V properties, a parameter 𝑐(⟨𝑑⟩, 𝑎) was used to describe the effect of electric field, which the presence of “bottle-neck” region was used to explain the observed large 𝑐(⟨𝑑⟩, 𝑎) in nonlinear samples. From systematic experimental investigations and physical model-based analyses, it was shown that the transition from linear to nonlinear I-V properties was attributed to the emergence of a bottleneck region of electrical conduction due to the localized heterogeneity of the molecular lines, indicating the possibility of physicochemical modulation of the electrical conductivity of conducting polymers.

The results of this research are significant from the viewpoint of constructing a new concept for understanding and controlling the intrinsic conduction mechanism of conductive polymers at the nanoscale.

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