Negative ions detection in air using nano field-effect-transistor (nanoFET)
© Seo et al.; licensee Springer. 2014
Received: 9 June 2014
Accepted: 6 August 2014
Published: 15 August 2014
We firstly demonstrated the detection of anions in air using a nano field-effect transistor (nanoFET) device. Negative ions in air charged the top surface of the silicon nanoFET channel affecting the fieldeffect and making a conductance change of the channel proportional to anion concentration around the nano channel sensing surface. The real-time detection of anions in air with the nanoFET was performed for various anion concentrations which were differentiated by the distance of the anion generator to the nanoFET sensor. The air anions detection characteristics of the nanoFET device were evaluated with sensitivity and conductance change rates analysis.
KeywordsNegative ions in air Air anion detection NanoFET Quantitative detection of anions Semiconductor sensor Negative ion sensor
Negative ions in air are of great interest for the benefits for air purification and the positive effect on air conditions for healthy living -. Recently, a number of consumer products utilizing ion-generating technology have become available to eliminate airborne pollutants, such as dust or cigarette smoke, allergens and viruses from immediate breathing spaces ,. These devices work by generating a flow of negative ions that charge and bind together with airborne particulate matter. The charged matter gathers together and precipitates out of the air. In addition to eliminating harmful particulates from the air, negative ions also have a number of unique health benefits -. A growing number of people are using personal and home air filtration products that generate negative ions to charge and remove airborne particulate matter to create localized zones of improved air quality -.
Various devices exist to detect ions in the air. Some notable examples include the Geiger-Muller tube based radiation detector and parallel plate- or cylindrical plate-based electrostatic type detector -. These air ion counter devices are complicated systems that are much larger and expensive when compared to most ion generating devices. For instance, the electrostatic type detector has an additional fan to create an intake of air to the electrode plate detector. In this paper, we propose an airborne anion detecting sensor using nanoFETs which enables the realization of a very small and low cost air ion detecting device. Semiconductor based airborne ion sensors can be incorporated into consumer electronics allowing for applications such as the performance monitoring of anion generating device often found in air cleaning units today. We fabricated a nanoFET device and evaluated the air anion detection characteristics of the nanoFET sensor device. We measured the real-time conductance change and response properties of the nanoFET when exposed to different anion concentrations in air generated by an air ionizer. Our results demonstrate the possibility of an application of the nanoFET as a negative ion sensor.
where, Cbox is the capacitance of BOX layer and Vbg is the applied back-gate voltage. Under the experimental conditions when the conductance level, Ids, of the nanochannel by the applied back-gate voltage is equal or similar to that by the electric field formed by the amount of the adsorbed charges on the sensing area of the top capacitor, then the adsorbed charges, Q, equals to Qbg. Therefore, since Cbox = 6.1×10−15 Farad, Vbg = 15 V, then, Q = 9.15×10−14 C = 5.7×105 charges. The ionizer is known to generate the anions at a rate of approximately 2×106 ea./cc at 5 cm away from the ionizer. Since the amount of adsorbed charges is proportional to adsorption coefficient α and ion concentration in air D, then the adsorbed charges on the sensing channel Q = αD. The coefficient α is 1 at the maximum, confirming that the numerical calculation using the experimental result is reasonable.
Figure 4(b) shows the result of quantitative detection of negative ions using the nanoFET. In order to confirm the feasibility of quantitative detection of the air ion concentration, the response of the nanoFET to the different ion concentration in air was measured. The air anion concentration was differentiated by varying the distance of the sensor to the ionizer as shown in Figure 2(b). After the ionizer was turned on at t = 50 sec, the Ids responses increased, but showed different delay times depending on the ionizer position. This was due to the increase in time for the generated anions to reach the sensor. The distance between ion generator and nano FET sensor was set to 4 different positions from 25 cm to 10 cm in decreasing order. The results are represented by a dashed line and lines with circle, triangle, and rectangle markers, respectively, as shown in Figure 4(b) and (c). In Figure 4(b), the slope of Ids before saturation was larger as larger amounts of negative ions reached the sensor at closer distances. Figure 4(c) shows the plot of the change rate of the slope of Ids at each sampling time. The measurement of air anion detection was repeated twice at each ionizer position. When the initial level of Ids for the measurement was of similar value, near 22.5 μA then, the plot nearly overlapped, which indicated that the measurement results would be reproducible if the initial level of Ids is in the same condition. The slope of the Ids increasing rate produces the same peak value even with different initial levels of Ids as shown in Figure 4(c) and the inset. The inset of Figure 4(c) shows the maximum value of the slope versus the distance which is related to concentration of negative ions in the air. Note that the anion concentration near the nanoFET sensor reduces as the ionizer is placed at farther distances, which corresponds with the experimental measurement results. The initial Ids level cannot be controlled actively for the consecutive measurements because of the variable amount of adsorbed negative ions remaining on the channel surface which reduces gradually over time. However, even in this case, the quantitative detection of air anions using the nanoFET sensor may be possible because the increasing rates of Ids are related to the ion concentration. The measurement of Ids change rate for the quantification and calibration of the concentration of the immobilized negatively charged biomolecules on the channel, such as proteins in liquid solutions, was already demonstrated by the M. Reed group in Yale University ,. They showed the possibility of quantification of the concentration of the charged protein molecules in liquid solution, but did not report an application with repeatable consecutive measurements. After the ionizer was turned off at t = 200 sec, Ids decreased due to the reduction of adsorbed negative ions on the channel as shown in Figure 4(b). This corresponded to the result described above and in Figure 3. The results of the experiment showed that the nanoFET sensor could detect negative ions in air quantitatively and the concentration could be analyzed by the slope of the channel conductance change rate during a real-time measurement. However, the initial, so-called ‘ready,’ level of Ids for the air ion measurement was not actively controllable and was too slow to recover to the initial state after a measurement due to the residual charge caused by the adsorbed negative ions on the nanoFET channel surface. More experiments on sensor performance are required to verify the capability of reproducible quantitative measurements of air anions utilizing the nanoFET. These properties are essential for the commercialization of nanoFET device as an air anion detection sensor.
In this paper, we have characterized the response of the developed nanoFET sensor to the concentration of negative ions in air. We demonstrated the quantitative detection of negative ions in air during real-time measurement. The fabricated nanoFET showed the high performance characteristics of the p-type field-effect transistor. The nanoFET could detect and differentiate the concentration of negative ions in air generated by the ionizer indicated by the slope of the channel conductance change rate which was proportional to the ion concentration. From the experimental results, the conductance change of nanoFET channel was shown to be related with the anion concentration in the air. Furthermore, it was observed that the conductance of Ids decreased after reaching its saturation level in a resting state when the ion generator was turned off. Further research is underway to evaluate the limit of detection and dynamic range to differentiate the air anion concentration, and evaluate characteristics of reliability and reproducibility of the nanoFET anion sensor for commercialization.
This work was financially supported by the research fund of Ministry of Trade, Industry and Energy (MOTIE), Korea.
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