Sensitive Cross-Linked SnO2:NiO Networks for MEMS Compatible Ethanol Gas Sensors.

Nowadays, it is still technologically challenging to prepare highly sensitive sensing films using microelectrical mechanical system (MEMS) compatible methods for miniaturized sensors with low power consumption and high yield. Here, sensitive cross-linked SnO2:NiO networks were successfully fabricated by sputtering SnO2:NiO target onto the etched self-assembled triangle polystyrene (PS) microsphere arrays and then ultrasonically removing the PS microsphere templates in acetone. The optimum line width (~ 600 nm) and film thickness (~ 50 nm) of SnO2:NiO networks were obtained by varying the plasma etching time and the sputtering time. Then, thermal annealing at 500 °C in H2 was implemented to activate and reorganize the as-deposited amorphous SnO2:NiO thin films. Compared with continuous SnO2:NiO thin film counterparts, these cross-linked films show the highest response of ~ 9 to 50 ppm ethanol, low detection limits (< 5 ppm) at 300 °C, and also high selectivity against NO2, SO2, NH3, C7H8, and acetone. The gas-sensing enhancement could be mainly attributed to the creating of more active adsorption sites by increased stepped surface in cross-linked SnO2:NiO network. Furthermore, this method is MEMS compatible and of generality to effectively fabricate other cross-linked sensing films, showing the promising potency in the production of low energy consumption and wafer-scale MEMS gas sensors.

Crystal-Defect-Dependent Gas-Sensing Mechanism of the Single ZnO Nanowire Sensors.

Though the chemical origin of a metal oxide gas sensor is widely accepted to be the surface reaction of detectants with ionsorbed oxygen, how the sensing material transduces the chemical reaction into an electrical signal (i.e., resistance change) is still not well-recognized. Herein, the single ZnO NW is used as a model to investigate the relationship between the microstructure and sensing performance. It is found that the acetone responses arrive at the maximum at the NW diameter ( D) of ∼110 nm at the D range of 80 to 400 nm, which is temperature independent in the temperature region of 200 °C-375 °C. The electrical properties of the single NW field effect transistors illustrate that the electron mobility decreases but electron concentration increases with the D ranging from ∼60 nm to ∼150 nm, inferring the good crystal quality of thinner ZnO NWs and the abundant crystal defects in thicker NWs. Subsequently, the surface charge layer ( L) is calculated to be a constant of 43.6 ± 3.7 nm at this D range, which cannot be explained by the conventional D- L model in which the gas-sensing maximum appears when D approximates 2 L. Furthermore, the crystal defects in the single ZnO NW are probed by employing the microphotoluminescence technique. The mechanism is proposed to be the compromise of the two kinds of crystal defects in ZnO (i.e., more donors and fewer acceptors favor the gas-sensing performance), which is again verified by the gas sensors based on the NW contacts.

Transilient Response to Acetone Gas Using the Interlocking p+n Field-Effect Transistor Circuit.

Low concentration acetone gas detection is significantly important for diabetes diagnosis as 1.8⁻10 ppm of acetone exists in exhaled breath from diabetes patients. A new interlocking p+n field-effect transistor (FET) circuit has been proposed for Mn-doped ZnO nanoparticles (MZO) to detect the acetone gas at low concentration, especially close to 1.8 ppm. It is noteworthy that MZO in this interlocking amplification circuit shows a low voltage signal of <0.3 V to the acetone <2 ppm while it displays a transilient response with voltage signal >4.0 V to >2 ppm acetone. In other words, the response to acetone from 1 ppm to 2 ppm increases by ~1233%, which is competent to separate diabetic patients from healthy people. Moreover, the response to 2 ppm acetone is hardly influenced by high relative humidity of 85%. In the meanwhile, MZO in this interlocking circuit possesses a high acetone selectivity compared to formaldehyde, acetaldehyde, toluene and ethanol, suggesting a promising technology for the widespread qualitative screening of diabetes. Importantly, this interlocking circuit is also applicable to other types of metal oxide semiconductor gas sensors. The resistance jump of p- and n-FETs induced by the change of their gate voltages is deemed to make this interlocking circuit produce the transilient response.

Coupling p+n Field-Effect Transistor Circuits for Low Concentration Methane Gas Detection.

Nowadays, the detection of low concentration combustible methane gas has attracted great concern. In this paper, a coupling p+n field effect transistor (FET) amplification circuit is designed to detect methane gas. By optimizing the load resistance (RL), the response to methane of the commercial MP-4 sensor can be magnified ~15 times using this coupling circuit. At the same time, it decreases the limit of detection (LOD) from several hundred ppm to ~10 ppm methane, with the apparent response of 7.0 ± 0.2 and voltage signal of 1.1 ± 0.1 V. This is promising for the detection of trace concentrations of methane gas to avoid an accidental explosion because its lower explosion limit (LEL) is ~5%. The mechanism of this coupling circuit is that the n-type FET firstly generates an output voltage (VOUT) amplification process caused by the gate voltage-induced resistance change of the FET. Then, the p-type FET continues to amplify the signal based on the previous VOUT amplification process.