The unique electronic properties of semiconductor nanowires, in particular silicon nanowires (SiNWs), are attractive for the label-free, real-time, and sensitive detection of various gases. Therefore, over the past two decades, extensive efforts have been made to study the gas sensing function of NWs. This review article presents the recent developments related to the applications of SiNWs for gas sensing. The content begins with the two basic synthesis approaches (top-down and bottom-up) whereby the advantages and disadvantages of each approach have been discussed. Afterwards, the basic sensing mechanism of SiNWs for both resistor and field effect transistor designs have been briefly described whereby the sensitivity and selectivity to gases after different functionalization methods have been further presented. In the final words, the challenges and future opportunities of SiNWs for gas sensing have been discussed.

Silicon Nanowires for Gas Sensing: A Review

A green approach for the preparation of nanostructured zinc oxide: Characterization and promising antibacterial behaviour

Predicting gases sensing performance of α-MoO3 from nano-structural and electronic properties

Accuracy of CO2 measurement is affected by ambient air fluctuations, making the compensation of such variations in drift-like sensor response essential for concentration level assessment. Here, a series of experiments were carried out with a chamberless approach in a nondispersive infrared (NDIR) gas sensor to examine the combined effect of environmental temperature and relative humidity fluctuations on sensor responses at different concentrations of CO2. To eliminate the drift-like terms caused by environmental fluctuations, the behavior of the sensor was modeled to include ambient temperature, relative humidity, the measured responses as the inputs, and the concentration level as the output. The sensor was fabricated by a light source with an embedded parabolic reflector, a thermopile detector, and two reflective walls that are exposed to the applicable range of CO2 gas. The predicted concentration level was determined by analyzing the system and acquiring a heuristic function based on an ensemble regression model. The created model's reliability and sensor's performance were evaluated by the test and validation data, and the respective accuracies of 99.83 and 98.90% demonstrated the model effectiveness. The chamberless structure of the sensor provides reduction in diffusion time, improves the linearity of responses accompanied by eliminating drift-like variation of responses in varying ambient conditions, and prepares the sensor for industrial applications.

Chamberless NDIR CO2 Sensor Robust against Environmental Fluctuations.

In this paper, we show that the ionic conduction through surface chemisorbed ambient moisture leads to a remarkably high and selective response towards hydrogen gas at room temperature. The surface adsorbed water molecules acts as surface states, due to porous and granular nature of ZnO nanoparticles of 20 ± 5 nm size. This is depicted as deviation from Arrhenius behavior near room temperatures. The response to hydrogen gas is further enhanced remarkably from 5% to 71% for 1200 ppm when p-type NiO quasi-nanowires (width 15–20 nm) are mixed with these n-type ZnO nanoparticles to form a homogenous NiO/ZnO nano-bulk p-n heterostructures. The maximum response is obtained for about 50–50% composition of NiO/ZnO although it is of still n-type character which signifies the dominance of ZnO in the sensing mechanism. The carrier type reversal from n-type to p-type takes place at a rather high NiO content of about 60–80% NiO in ZnO. The parallel surface ionic current through chemisorbed moisture (surface states) has been identified as a primary factor for high sensitivity to hydrogen gas at room temperature. Further, the presence of heterojunction barriers at the NiO-ZnO interface along with surface ionic conduction synergistically enhanced the selective response to hydrogen at room temperature.

Protonic conduction induced selective room temperature hydrogen response in ZnO/NiO heterojunction surfaces

Using thermoelectricity as a platform for sensor design and fabrication commercially and historically precedes its power generation aspect, but recent advancements in science and engineering of materials have failed to elevate this platform as the researchers mainly pursue higher figure of merit (zT) materials. It has been shown that thermoelectricity in polycrystalline semiconductors is partly related to the grain boundary effects, and the profound contribution of grain boundaries to the Seebeck voltage (SV) generation has been demonstrated. It immediately follows that all grain boundary-related effects should be detectable via SV measurement, as well. Here, we report the variations of the SV generated at the presence of a constant temperature gradient along a ZnO pellet with respect to the concentration of the various volatile organic compounds (VOCs) in the surrounding atmosphere. The presence of VOCs reversibly reduces the SV generated along the sample. Similar to the case of chemoresistivity, the recorded chemo-thermoelectric responses obey the power law; providing another reason for the common origin of these two apparently distinct phenomena.

Zinc oxide-based direct thermoelectric gas sensor for the detection of volatile organic compounds in air

A common way of determining the majority charge carriers of pristine and doped semiconducting polymers is to measure the sign of the Seebeck coefficient. However, a polarity change of the Seebeck coefficient has recently been observed to occur in highly doped polymers. Here, it is shown that the Seebeck coefficient inversion is the result of the density of states filling and opening of a hard Coulomb gap around the Fermi energy at high doping levels. Electrochemical n‐doping is used to induce high carrier density (>1 charge/monomer) in the model system poly(benzimidazobenzophenanthroline) (BBL). By combining conductivity and Seebeck coefficient measurements with in situ electron paramagnetic resonance, UV–vis–NIR, Raman spectroelectrochemistry, density functional theory calculations, and kinetic Monte Carlo simulations, the formation of multiply charged species and the opening of a hard Coulomb gap in the density of states, which is responsible for the Seebeck coefficient inversion and drop in electrical conductivity, are uncovered. The findings provide a simple picture that clarifies the roles of energetic disorder and Coulomb interactions in highly doped polymers and have implications for the molecular design of next‐generation conjugated polymers.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/adfm.202112276

On the Origin of Seebeck Coefficient Inversion in Highly Doped Conducting Polymers

The main challenge in thermoelectric metrology is an accurate Seebeck coefficient measurement on high electrical resistance bulk and thin-film samples, as the loading by the measuring device causes a considerable voltage drop on the internal resistance of the sample. It has been shown that this technical problem can be solved by utilizing the zero current technique of measurement. Here, an apparatus for the simultaneous measurement of the Seebeck coefficient and electrical resistance of bulk and thin-film samples using zero current measurement technique is presented, which automatically measures these quantities at different temperature profiles at steady-state, quasi-steady-state, and transient conditions in different atmospheres. Thick-film tin oxide microheaters are utilized to provide the desired temperature gradient along with the sample from room temperature up to 1200 K, and zero current method is used to eliminate errors originating from the high internal resistance of the sample. As examples, Seebeck voltages and electrical resistances are measured on the bulk and thin-film zinc oxide samples with internal resistances in the megohm ( $\text{M}\Omega$ ) range. Operating the device for measurements involving time-varying temperature gradients is demonstrated.

Apparatus for Seebeck Coefficient Measurements on High-Resistance Bulk and Thin-film Samples

The profound contribution of grain boundaries to the Seebeck voltage generation in the polycrystalline semiconductors has recently been stated. Considering the direct link between the chemiresistiv...

Atmospheric Dependence of Thermoelectric Generation in SnO2 Thin Films with Different Intergranular Potential Barriers Utilized for Self-Powered H2S Sensor Fabrication

Morteza Shokrani

Papers

Zinc oxide-based direct thermoelectric gas sensor for the detection of volatile organic compounds in air

On the Origin of Seebeck Coefficient Inversion in Highly Doped Conducting Polymers

Apparatus for Seebeck Coefficient Measurements on High-Resistance Bulk and Thin-film Samples

Atmospheric Dependence of Thermoelectric Generation in SnO2 Thin Films with Different Intergranular Potential Barriers Utilized for Self-Powered H2S Sensor Fabrication