Dynamic Temperature Modulation Sensing Technique of Electronic Nose: A Review

Electronic nose (E-nose) is a simulation of human nose, which consists of a gas sensor array and an artificial intelligent algorithm. The gas sensing properties of semiconductor sensors are affected by the heating temperature. For most gases, there exists the optimum oxidation temperature. If sensor response is recorded, we can obtain the data with abundant information at different working temperatures. The selectivity and sensitivity of a gas sensor array are the bottleneck of its development. Dynamic temperature modulation sensing technique is a use of semiconductor sensor temperature modulation characteristics by modulating its heating voltage to realize the heating temperature in a range of changes, people can record the corresponding response. The temperature modulation sensing technique can effectively improve the sensitivity of E-nose and realize the detection of low concentration gas, so it is of great practical significance to development technique of E-nose, which is based on temperature modulated sensing system for promoting the detection speed. But so far, the technique is only used for the detection of several common gases (such as methanol, ethanol, carbon monoxide, et al). The aim of this review is to supply a summary of the development and significant achievements of dynamic temperature modulation sensing technique used in E-nose in recent years. We are also looking forward to seeing dynamic temperature modulation sensing technique to accomplish more breakthroughs and get more achievements

Enhanced Sensing Performance of Novel Nanostructured ZnO Gas Sensors in Ethanol Vapor Concentration Detection Applications

Sensors are devices which have been commonly used to measure the functional dependence and the variability of physical parameters like temperature, pressure, pH, voltage, current, concentration, and others. Among the numerous kinds of sensors in different areas, gas sensors have been widely used and investigated for gas monitoring. Gas sensors are of crucial importance for the detection of hazardous atmospheres, because toxic gases are frequently odorless, colorless, invisible, rapidly evaporating, and flammable, and would otherwise go unnoticed. Gas sensors have been used in a variety of applications among others for the detection of specific gas species and the detection of gas concentrations. Compared to other materials systems used in gas sensor applications, Metal Oxide Semiconductor (MOS) sensors have attracted much attention for gas detection due to their low cost, simple design and ease of production, short response time, wide detection range, and resistance to harsh working environments. Among various semiconductor materials used in MOS gas sensors, ZnO is a well-known semiconducting metal oxide material used in gas sensor applications due to its good electrical property, wide band gap of 3.37 eV, ~60 meV exciton binding energy, low cost, and high mechanical stability. ZnO has been applied for MOS gas sensor applications due to its high electrochemical stability, non-toxicity, ease of doping, and low cost. In general, gas sensors based on ZnO tend to exhibit exceptional performance for ethanol detection with respect to high sensitivity, short response time, and fast recovery time. In this dissertation, the sensing performance of novel innovative ZnO nanostructure gas sensor designs to ethanol vapor concentration detection were investigated and analyzed in terms of their sensing response, their response time, and recovery time. Currently, the shortcomings of state-of-the-art thin film ZnO gas sensors are lack of sufficient sensitivity, long response times, and long recovery times relying only on one reactive surface. My research is addressing these shortcomings by designing, fabricating and testing novel innovative 3-dimensional nanoscale ZnO sensor device architectures with increased surface-to-volume ratio using an integrated process approach combining hydrothermal growth of nanorods with Atomic Layer Deposition (ALD) wrap-around coatings. First and foremost, Aluminum doped ZnO (AZO) thin films were introduced to enhance the sensing performance of ZnO nanorod gas sensors by providing additional oxygen vacancies and extra electrons for the redox reactions using ALD technology. A roughly 100% improvement was achieved on the sensing response of ZnO nanorod gas sensors equipped with ALD AZO 3-D wrap-around coatings compared to conventional ZnO nanorod gas sensors. Secondly, the other key approach in this dissertation was to conceive a unique novel sensor architecture design to further improve the sensing performance of ZnO nanostructure gas sensors with an innovative increase of the surface-to-volume ratio. These novel nested ZnO nanorod/nanotube gas sensors exhibited a large improvement in their sensing response due to the increased surface-to-volume ratio with two additional reaction surfaces and extra reaction sites. The sensing response of ZnO gas sensors was improved up to a maximum of 150% with the novel nested coaxial nanorod/nanotube architecture compared to the sensing response of conventional ZnO nanorod gas sensors. The third approach was to investigate the sensing performance of ZnO nanotube sensors synthesized within porous templates by utilizing ALD and Al2O3 sacrificial layers. The sensing performance of these ZnO nanotube gas sensors was enhanced with increased surface-to-volume ratio by adding additional coaxial ZnO nanotubes. The enhancement can be further improved by adding additional coaxial ZnO nanotubes layers which provide each 2 additional reaction surfaces. Furthermore, ALD AZO coatings were introduced to further enhance the sensing performance of ZnO nanotube gas sensors synthesized in porous templates. With the combined benefits from approaches 1 and 2, the maximum gained enhancement reached up to 136% for the template replication case. The first two approaches established a bottom-up technology, which is subject to high variability from batch to batch hydrothermal growth. In contrast, nested ZnO nanotubes synthesized within porous templates enables a true top-down technology by using mask and photolithography patterning techniques from microelectronics to guarantee the reproducibility, which would render it ready for commercialization and to be transferred for industrial applications

Cellulose Nanocrystal-Templated Tin Dioxide Thin Films for Gas Sensing.

Porous tin dioxide is an important low-cost semiconductor applied in electronics, gas sensors, and biosensors. Here, we present a versatile template-assisted synthesis of nanostructured tin dioxide thin films using cellulose nanocrystals (CNCs). We demonstrate that the structural features of CNC-templated tin dioxide films strongly depend on the precursor composition. The precursor properties were studied by using low-temperature nuclear magnetic resonance spectroscopy of tin tetrachloride in solution. We demonstrate that it is possible to optimize the precursor conditions to obtain homogeneous precursor mixtures and therefore highly porous thin films with pore dimensions in the range of 10-20 nm (ABET = 46-64 m2 g-1, measured on powder). Finally, by exploiting the high surface area of the material, we developed a resistive gas sensor based on CNC-templated tin dioxide. The sensor shows high sensitivity to carbon monoxide (CO) in ppm concentrations and low cross-sensitivity to humidity. Most importantly, the sensing kinetics are remarkably fast; both the response to the analyte gas and the signal decay after gas exposure occur within a few seconds, faster than in standard SnO2-based CO sensors. This is attributed to the high gas accessibility of the very thin porous film

Detection mechanism in highly sensitive ZnO nanowires network gas sensors

Metal-oxide nanowires are showing a great interest in the domain of gas sensing due to their large response even at a low temperature, enabling low-power gas sensors. However their response is still not fully understood, and mainly restricted to the linear response regime, which limits the design of appropriate sensors for specific applications. Here we analyse the non-linear response of a sensor based on ZnO nanowires network, both as a function of the device geometry and as a response to oxygen exposure. Using an appropriate model, we disentangle the contribution of the nanowire resistance and of the junctions between nanowires in the network. The applied model shows a very good consistency with the experimental data, allowing us to demonstrate that the response to oxygen at room temperature is dominated by the barrier potential at low bias voltage, and that the nanowire resistance starts to play a role at higher bias voltage. This analysis allows us to find the appropriate device geometry and working point in order to optimize the sensitivity. Such analysis is important for providing design rules, not only for sensing devices, but also for applications in electronics and opto-electronics using nanostructures networks with different materials and geometries

Localized Charge Transfer Process and Surface Band Bending in Methane Sensing by GaN Nanowires

The physicochemical processes at the surfaces of semiconductor nanostructures involved in electrochemical and sensing devices are strongly influenced by the presence of intrinsic or extrinsic defects. To reveal the surface controlled sensing mechanism, intentional lattice oxygen defects are created on the surfaces of GaN nanowires for the elucidation of charge transfer process in methane (CH4) sensing. Experimental and simulation results of electron energy loss spectroscopy (EELS) studies on oxygen rich GaN nanowires confirmed the possible presence of 2(ON) and VGa-3ON defect complexes. A global resistive response for sensor devices of ensemble nanowires and a localized charge transfer process in single GaN nanowires are studied in situ scanning by Kelvin probe microscopy (SKPM). A localized charge transfer process, involving the VGa-3ON defect complex on nanowire surface is attributed in controlling the global gas sensing behavior of the oxygen rich ensemble GaN nanowires.Comment: 42 pages, 6 figures, Journa