Leu-M

Metal-oxide-semiconductor (MOS) based gas sensors have been considered a promising candidate for gas detection over the past few years. However, the sensing properties of MOS-based gas sensors also need to be further enhanced to satisfy the higher requirements for specific applications, such as medical diagnosis based on human breath, gas detection in harsh environments, etc. In these fields, excellent selectivity, low power consumption, a fast response/recovery rate, low humidity dependence and a low limit of detection concentration should be fulfilled simultaneously, which pose great challenges to the MOS-based gas sensors. Recently, in order to improve the sensing performances of MOS-based gas sensors, more and more researchers have carried out extensive research from theory to practice. For a similar purpose, on the basis of the whole fabrication process of gas sensors, this review gives a presentation of the important role of screening and the recent developments in high throughput screening. Subsequently, together with the sensing mechanism, the factors influencing the sensing properties of MOSs involved in material preparation processes were also discussed in detail. It was concluded that the sensing properties of MOSs not only depend on the morphological structure (particle size, morphology, pore size, etc.), but also rely on the defect structure and heterointerface structure (grain boundaries, heterointerfaces, defect concentrations, etc.). Therefore, the material-sensor integration was also introduced to maintain the structural stability in the sensor fabrication process, ensuring the sensing stability of MOS-based gas sensors. Finally, the perspectives of the MOS-based gas sensors in the aspects of fundamental research and the improvements in the sensing properties are pointed out.

Metal-oxide-semiconductor based gas sensors: screening, preparation, and integration

Investigations into the mechanisms governing the behavior of metal oxide gas sensors continue to be of great interest. Oxygen vacancies are a ubiquitous defect in this class of materials and their characteristics can be affected by synthesis, processing and operating parameters. The primary role of oxygen vacancies in modifying sensing performance cited in the gas sensing literature is based on a modulation of the amount of surface adsorbed oxygen or alternatively, the baseline resistance. Unfortunately, this generalized description does not provide a complete representation of the role of oxygen vacancies that would aid in more a fundamental understanding of their role in gas sensing. To this end, an attempt is made to distinguish between the role of surface and bulk oxygen vacancies where emphasis on proper characterization is first highlighted. The influence of surface oxygen vacancies on factors affecting adsorption, such as surface structure, are examined to gain understanding on improved sensing performance. The effect of bulk oxygen vacancy concentration and distribution on sensing are also discussed. Finally, the importance of these concepts within the context of doped and heterostructured gas sensors are then briefly discussed.

Role of Oxygen Vacancies in Nanostructured Metal-Oxide Gas Sensors: A Review

In this study we report the enhancement of UV photodetection and wavelength tunable light induced NO gas sensing at room temperature using Au-ZnO nanocomposites synthesized by a simple photochemical process Plasmonic Au-ZnO nanostructures with a size less than the incident wavelength have been found to exhibit a localized surface plasmon resonance (LSPR) that leads to a strong absorption, scattering and local field enhancement The photoresponse of Au-ZnO nanocomposite can be effectively enhanced by 80 times at 335 nm over control ZnO We also demonstrated Au-ZnO nanocomposite's application to wavelength tunable gas sensor operating at room temperature The sensing response of Au-ZnO nancomposite is enhanced both in UV and visible region, as compared to control ZnO The sensitivity is observed to be higher in the visible region due to the LSPR effect of Au NPs The selectivity is found to be higher for NO gas over CO and some other volatile organic compounds (VOCs), with a minimum detection limit of 01 ppb for Au-ZnO sensor at 335 nm

/pdf/multifunctional-au-zno-plasmonic-nanostructures-for-enhanced-275hyxzwnv.pdf

Multifunctional Au-ZnO Plasmonic Nanostructures for Enhanced UV Photodetector and Room Temperature NO Sensing Devices

https://www.econstor.eu/bitstream/10419/244006/1/1693663112.pdf

Enhanced sensing performance of ZnO nanostructures-based gas sensors: A review

Zinc oxide (ZnO) is one of the most promising semiconducting metal oxides, particularly for gas sensing applications. Impurity doped ZnO has offered much improved sensing performance, as compared to its undoped counterpart. In this work, undoped as well as indium doped nanocrystalline ZnO thin films are synthesized by a low cost chemical solution deposition route. X-ray diffraction patterns of the synthesized films reveal preferential orientation along the (002) plane. The surface and cross section morphology has clearly changed with the variation of indium content. Optical transmittance values increase with the increase in indium concentration and the band gap energy is found in the range 3.210 eV to 3.221 eV. PL spectra reveal three characteristic peaks, owing to band to band transition and defect level emissions. The effect of indium doping on the electrical parameters of ZnO is analyzed through Hall effect measurements at room temperature. The gas sensing characteristics of these sensors offer good reproducibility and stability towards various reducing gases, with an enhancement of response% and lower detection limit as compared to undoped ZnO. A doping level of 3 wt% of indium in ZnO is found to give optimum response and the lowest detection limit of hydrogen of 1 ppm or even lower. However, further increase in the doping concentration resulted in reduced sensing performance. This is attributed to the gas sensing mechanism related to the substitution of In3+ ions at Zn2+ ion sites enhancing the number of free charge carriers at the optimum level of indium. Through exploring this gas sensing mechanism, it is argued that the sensor performance can be dramatically improved by tailoring the indium concentration in ZnO for its practical application in various sectors as an effective gas sensor.

Properties of indium doped nanocrystalline ZnO thin films and their enhanced gas sensing performance

Role of oxygen vacancy in optical and gas sensing characteristics of ZnO thin films

n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors

The present work is aimed to investigate the influence of microstructure on the gas sensing characteristics of ZnO thin films. By controlling the deposition parameters we have successfully grown ZnO thin films of different microstructures on quartz substrates by MOCVD growth technique using diethyl zinc and tert-butanol as zinc and oxygen precursors, respectively. X-ray diffraction pattern shows that the films are textured along (0 0 2) plane. FESEM images reveal the uniform deposition of fine grains with varying microstructures. Growth mode dependent microstructure formation is explained on the basis of a simple model. The optical characterization reveals that the films are transparent in the visible range with band gap energy in the range 3.234–3.217 eV. The effect of microstructure on the electrical parameters of the ZnO such as carrier concentration, resistivity and Hall mobility is understood through Hall effect measurement using standard Van der Pauw geometry. The gas sensing characteristics of the grown films is investigated as a function of temperature and gas concentration in CO test gas environment. The observed results are analyzed on the basis of the variation of microstructures of the grown films.

MOCVD grown ZnO thin film gas sensors: Influence of microstructure

In the present work we have grown highly textured, ultra-thin, nano-crystalline zinc oxide thin films using a metal organic chemical vapor deposition technique and addressed their selectivity towards hydrogen, carbon dioxide and methane gas sensing. Structural and microstructural characteristics of the synthesized films were investigated utilizing X-ray diffraction and electron microscopy techniques respectively. Using a dynamic flow gas sensing measurement set up, the sensing characteristics of these films were investigated as a function of gas concentration (10–1660 ppm) and operating temperature (250–380 °C). ZnO thin film sensing elements were found to be sensitive to all of these gases. Thus at a sensor operating temperature of ∼300 °C, the response% of the ZnO thin films were ∼68, 59, and 52% for hydrogen, carbon monoxide and methane gases respectively. The data matrices extracted from first Fourier transform analyses (FFT) of the conductance transients were used as input parameters in a linear unsupervised principal component analysis (PCA) pattern recognition technique. We have demonstrated that FFT combined with PCA is an excellent tool for the differentiation of these reducing gases.

Sumati Pati

Papers

Properties of indium doped nanocrystalline ZnO thin films and their enhanced gas sensing performance

Role of oxygen vacancy in optical and gas sensing characteristics of ZnO thin films

n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors

MOCVD grown ZnO thin film gas sensors: Influence of microstructure

Qualitative and quantitative differentiation of gases using ZnO thin film gas sensors and pattern recognition analysis