Electrical Characterization of Semiconductor Oxide-Based Gas Sensors Using Impedance Spectroscopy: A Review

Abstract

Impedance spectroscopy is essential in understanding the physical and chemical processes that affect the electronic behavior of semiconducting metal oxide-based gas sensor materials, but is often overlooked or avoided by researchers because of the complexity of proper data interpretation. These metal oxide nanomaterials are promising as resistive-type gas sensors because they have the potential to meet the critical need that has developed for low-power, low-cost, portable gas sensors. However, the mechanisms that determine these materials' gas-sensing performance is still not understood well enough to advance the field toward bottom-up design for specific applications. Direct current (DC) measurements give information on the device performance such as sensitivity, selectivity and response time. Alternating current (AC) measurements are a lesser-used approach that can give the same information, but also allow the varying contributions from the bulk, surfaces and interfaces, grain boundaries, electrode contacts and even substrate to be quantified, whether we work with polycrystalline materials or single-crystal structures such as nanowires and nanorods. With this technique it is possible to extract fundamental information about intergranular potential energy barrier height, width and donor concentration changes in different atmospheres and temperatures. The equivalent circuit method is the most common method of relating the impedance data to the physical system. However, the main weakness of this method is that the ideal circuit elements used in the fitting can be connected in many different ways to fit the same signal. To properly apply this technique, the model development should consider both the impedance measurement and the physical and chemical characteristics of the system being studied. This review presents a detailed description of impedance spectroscopy techniques for fundamental analysis of materials behavior in gas sensing applications, including proper data interpretation and types of fitting models commonly used, to enable researchers to better apply these techniques toward understanding of their materials systems.

Room temperature selective sensing of aligned Ni nanowires using impedance spectroscopy

“…As shown in figure 4(a), the impedance modulus decreases with an increase in frequency which is an indication of mix capacitive and resistive behavior [17]. Also, by increasing frequency, the phase difference plot moves toward a lower angle.…”

“…Alternative current (AC) measurement which is rarely used by researchers can give the same information as DC measurement. Besides, it clarifies the contribution from the bulk, surfaces and interfaces, grain boundaries, electrode contacts and even substrate whether it is polycrystalline material or single-crystal structure such as nanowires and nanorods [17]. Bode plot includes the modulus (|Z|) and phase (Ө) and Nyquist plot includes the imaginary (Z′) and real components (Z′).…”

“…The relaxation frequency is defined as the value where the charge carriers have to overcome the impedances of two different regions of the bulk and the boundaries. Also, a circuit has a relaxation frequency at the value where the currents switch from one element of the circuit to the other [17]. The semicircle impedance spectroscopy (Nyquist plot) in figure 4(c) shows the imaginary part of the impedance (- Z ) versus the real part, ( ¢ Z ) with a center depressed below the real axis.…”

“…The equivalent electrical circuit includes a resistor (R) in parallel with a capacitor (C), each representing a physical part of the device that has resistance and an alternating-current capacitance. In an ideal case, the Nyquist plot can take the form of a series of semicircles each representing one separate electrical phenomena attributed to the bulk material, grain boundary effects and the electrode phenomena [17]. Also, each semicircular arc has a characteristic maximum occurring at a unique relaxation frequency.…”

“…Alternative current (AC) measurement which is rarely used by researchers can give the same information as DC measurement, but also allow understanding contributions from the bulk, surfaces and interfaces, grain boundaries, electrode contacts and even substrate whether we work with polycrystalline materials or single-crystal structures such as nanowires and nanorods [17]. Impedance spectroscopy (IS) is known as a powerful measurement technique in understanding the mechanism of interactions based on a physical and chemical process that affects the electronic behavior of the materials [17][18][19][20][21][22][23][24]. A small AC voltage with variable frequency will be applied to measure the impedance that is defined as the ratio of the complex voltage to the complex current.…”