Abstract
AbstractConducting polymers are semiconductors whose applications cover a wide range of devices. Their versatility is due, in addition to other factors, to properties that can be easily modulated according to the intended application. It is therefore important to study and map the electronic structure of these materials for a better correlation between structure and properties. Electrochemical scanning tunneling spectroscopy (EC‐STS) can be a powerful tool to characterize the electronic structure of the semiconductor interface. In this work, we have used image‐based EC‐STS (IB‐EC‐STS) to describe quantitatively the band structure of an electrochemically deposited polypyrrole film. IB‐EC‐STS located the band edge of the polymer's valence band (VB) at 0.95 V vs. RHE (‐5.33 eV in the absolute potential scale) and the intragap polaron states formed when the polymer is doped, at 0.46 V vs. RHE (‐4.84 eV). The IB‐EC‐STS data were cross‐checked with electrochemical impedance spectroscopy (EIS) and Mott‐Schottky analysis of the interfacial capacitance. The DOS spectrum obtained from EIS data is consistent with the STS‐deduced location of the VB and the polarons.
Highlights
Understanding the electronic structure of conducting polymers (CPs) is necessary for a detailed understanding of the behavior of these materials, whose mechanism of electronic conduction is different from that characteristic of inorganic semiconductors
It is important to acquire the experimental information required to build a complete model of their band structure, which governs their properties.[1,2]
The tunneling current depends strongly on the density of states, and tunneling current versus bias plots obtained during a fast scan of the tip-sample bias have been used to elucidate the band structure of conducting polymers in ultra-high vacuum (UHV).[19]
Summary
Understanding the electronic structure of conducting polymers (CPs) is necessary for a detailed understanding of the behavior of these materials, whose mechanism of electronic conduction is different from that characteristic of inorganic semiconductors. Scanning Tunneling Microscopy (STM) is a powerful technique based on the quantum tunneling effect that allows obtaining images of surfaces with atomic resolution.[16] When a sharp metallic tip is placed very close to a sample surface and a small bias voltage is applied, a tunneling current flows through the gap between them This tunneling current can flow through a liquid electrolyte, and it is measurable if the tip is properly isolated to decrease the magnitude of any faradaic currents below the level of the target tunneling current, enabling atomic resolution images of electrode surfaces.[17,18] The tunneling current depends strongly on the density of states, and tunneling current versus bias plots obtained during a fast scan (typically in the V/s region) of the tip-sample bias (ie, scanning tunneling spectroscopy, STS) have been used to elucidate the band structure of conducting polymers in ultra-high vacuum (UHV).[19] A problem with this approach, is that the energy levels obtained by UHV-based STS cannot be related to any useful energy scale, like the reversible hydrogen electrode (RHE) or the absolute potential scale. IB-EC-TS and energy-resolved electrochemical impedance spectroscopy (EIS) were combined to map the electronic structure of a PPy film and to obtain a quantitative description of its electronic structure
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