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挥发性有机化合物（VOCs）由于其潜在的毒性、致癌性和诱变性，是人类健康的隐形杀手，这使得大范围监控环境中VOCs的含量极为迫切。另外，某些VOCs可以作为某些疾病的生物标志物，但准确地测定呼吸中微量VOCs的含量富有挑战性，金属氧化物（MOX）传感器在这两个方面均有应用前景。本文探索ZnO的气敏传感机制并设计传感器信号放大技术，旨在提高MOX的灵敏度和选择性（考虑湿度），最后将开发的新材料和新技术应用到微量二甲苯和丙酮的检测。本论文的研究内容和结果如下：（1）以单根ZnO纳米梳（nanocomb）气敏器件为模型研究贵金属金（Au）对纳米梳的气敏性能的影响。采用单温区化学气相沉积（CVD）法，以ZnO粉末和石墨为原料制备ZnO纳米梳，进一步负载不同尺寸的Au颗粒以考察其对丙酮气体响应的影响。研究发现负载3.7±0.7 nm金的纳米梳对0.2 ppm（百万分之一）和5 ppm丙酮的响应值分别为3和21，远高于纯相纳米梳，而且丙酮的选择性得到增强。但进一步增加金颗粒尺寸到11.7±3.0 nm，气敏性能严重恶化。（2）以单根ZnO纳米线气敏器件为模型研究施主缺陷和受主缺陷在气敏机制中的作用。采用双温区CVD，以ZnO粉末和石墨为原料制备ZnO纳米线，构建单根纳米线场效应晶体管（FET），研究纳米线的电子迁移率和电子浓度随直径变化的规律，并由此计算出表面电荷层为43.6±3.7 nm。丙酮气敏测试结果表明暴露于5 ppm丙酮时，直径110 nm纳米线的响应为42，而直径80 nm纳米线的响应为5，这与颗粒模型相矛盾。重要的是，这种趋势在200 ℃–375 ℃范围内与温度无关，同时几乎不受纳米线之间接触的影响。利用微光致发光技术研究不同直径ZnO纳米线的光致发光特性获得单根纳米线的晶体缺陷。结果表明110 nm ZnO纳米线的施主缺陷含量最多和受主缺陷含量最少。因此，施主和受主缺陷在气敏性能中起着主导作用。最后提出一种气体传感机制：施主缺陷越多，受主缺陷越少，则气敏性能越好。（3）利用不同类型的FET对MOX传感器的信号进行调制（即放大和缩小）。一是利用p型和n型耗尽型FET来捕捉和放大MOX传感器的微弱信号，可以将响应值放大5–6倍，向更低浓度方向拓宽检测限，且对MOX传感器的响应-恢复时间均无影响。放大原理是晶体管微小的绝对栅压增加所导致的电阻指数增加。这种放大技术有别于锁相放大器和运算放大器，设计简便易行，适用于所有的电阻型MOX传感器。因此该信号放大技术增强了MOX气体传感器检测低浓度气体的能力，在空气质量监测和疾病呼吸分析方面具有巨大潜力。二是p型和n型增强型FET可以强烈缩小MOX传感器的信号，缩小原理是晶体管微小的绝对栅压增加所导致的电阻指数降低。（4）开发新材料和晶体管放大技术用于微量VOCs的检测。一方面，协同p+n晶体管可以让商用传感器（费加罗TGS2602）能够检测到10 ppb（十亿分之一）二甲苯（响应值为2.6）。在高湿度环境中，协同p+n晶体管使TGS2602传感器依然保持很好的二甲苯灵敏度和选择性，且受湿度的影响较小。因此，协同p+n晶体管放大电路可以方便地判断二甲苯浓度是否超过室内空气标准（~42 ppb）。协同p+n晶体管的放大机制被认为是两个晶体管放大效应的简单叠加；另一方面，利用共沉淀法制备Mn掺杂ZnO（MZO）气敏材料，该材料对丙酮有较好的灵敏度和选择性。为加强MZO传感器对微量丙酮的响应，我们设计互锁p+n晶体管。在高湿度条件下，它能够使MZO传感器对2 ppm丙酮的响应产生一个突变，而且MZO传感器有较好的抗干扰能力，这非常有利于糖尿病患者的定性筛查，极大地节省时间和成本。（5）设计缩放p+n晶体管电路解决MOX传感器在高浓度目标气体中信号饱和的问题，它能够抑制低浓度气体的灵敏度而增强很高浓度气体的灵敏度，实现MOX传感器的有效检测范围向高浓度拓展的目标。工作原理是n型增强型晶体管率先工作并缩小信号，而p型耗尽型晶体管的放大作用则完全依赖于目标气体的浓度，只有当气体浓度足够大的时候才可以驱动p型晶体管起到放大效应。至此，人们可以根据实际需要，选择合适类型的晶体管，灵活地改变MOX传感器的有效检测范围。;The volatile organic compounds (VOCs) are silent killers of human health due to their potential toxicity, carcinogenicity and mutagenicity, making it very urgent to monitor the content of VOCs in the environment on a large scale. In addition, some specific VOCs can be used as biomarkers of certain diseases. But it is challenging to determine the exact content of the trace VOCs in the breath. The metal oxide (MOX) sensors have promising applications in these two aspects. In this paper, in order to improve the sensitivity and selectivity (considering the humidity) of MOX sensors, the gas-sensing sensing mechanism of ZnO is explored and the sensor’s signal amplification technology is designed. Finally, new materials and new technologies are tried to apply to detect the trace concentration xylene and acetone. The content and results are as follows:(1) The single ZnO nanocomb gas-sensing device is used as a model to investigate the influence of the noble metal Au on the gas-sensing performance of nanocombs. The single-temperature-zone chemical vapor deposition (CVD) method is utilized to prepare the ZnO nanocombs by using ZnO powders and graphite as the raw materials. The different sizes of Au particles are further loaded to investigate their effects on the response to the acetone gas for ZnO nanocombs. It is found that the nanocombs loaded with the 3.7±0.7 nm Au have the responses of 3 and 21 to 0.2 ppm (part per million) and 5 ppm acetone respectively, much higher than those of the pure-phase nanocombs. At the same time, the selectivity to acetone is enhanced. However, if the size of the gold particles further increases to 11.7±3.0 nm, the gas-sensing performance is seriously deteriorated.(2) The single ZnO nanowire gas-sensing device is used as a model to investigate the role of donor- and acceptor-defects in gas-sensing mechanism. ZnO nanowires are synthesize by dual-temperature-zone CVD, also with the ZnO powders and graphite as raw materials. A single nanowire field-effect transistor (FET) is constructed to study the electron mobility and electron concentration versus the diameter of the nanowire, from which the surface charge layer is calculated to be 43.6±3.7 nm. The results of the acetone gas-sensing test show that when exposed to 5 ppm of acetone, the nanowires with the diameter of 110 nm exhibit a response of 42, while those with the diameter of 80 nm exhibit a response of 5, which is contradictory to the particle model. Importantly, this trend is independent of temperature in the range of 200 ℃–375 ℃ and is hardly affected by the contact between the nanowires. The photoluminescence properties of ZnO nanowires with different diameters are probed by using the micro-photoluminescence technology to obtain the crystal defects of the single nanowire. The results show that the donor content of the 110 nm ZnO nanowire is the maximum and the content of the acceptor is the minimum. As a result, donor- and acceptor-defects play a predominant role in gas-sensing performance. Finally, a gas-sensing mechanism is proposed: the more the donors are, the fewer the acceptors are, the better the gas-sensing performance is.(3) The signals of the MOX sensor are modulated (i.e., amplified and reduced) by using different types of FETs. Firstly, the p- and n-type depletion mode FETs are employed to capture and amplify the weak signal of the MOX sensors, with amplifying the responses by 5–6 times. The limit of detection can be extended to the lower concentration but the response-recovery time of the MOX sensors are not affected. The amplifying mechanism is that the exponential increase of its resistance is induced by the slight increase of the absolute gate voltage of the FET. The amplifying technology, which is different from the lock-in amplifier and operational amplifier, is simple, easy to design, suitable for all of resistive MOX sensors. Thus, the signal amplification technology enhances the capability of the MOX sensors to detect the low-concentration gas and has great potential in the air quality monitoring and breath analysis for diseases. Secondly, the p- and n-type enhancement mode FETs can strongly reduce the signals of the MOX sensors. The reducing mechanism is that the exponential decrease caused by the increase of the absolute gate voltage of the FET.(4) The new materials and new FET amplification technologies are developed to detect the trace VOCs. On one hand, the synergetic p+n FETs allow the commercial sensors (Figaro TGS2602) to detect 10 ppb (part per billion) of xylene (response is 2.6). In the high-humidity environment, the synergetic p+n FETs make TGS2602 sensors still remain better sensitivity and selectivity to the xylene, which can be affected by the humidity a little. Therefore, whether or not the xylene concentration is above the standard in indoor air (~42 ppb) can be easily assessed by using the synergetic p+n FETs circuit. On the other hand, Mn-doped ZnO (MZO) gas-sensing materials are fabricated by the coprecipitation method and they display a high sensitivity and selectivity to acetone. To enhance the response of the MZO sensors to trace concentration acetone, the interlocking p+n FETs are designed, causing a transilient response of the MZO sensors to 2 ppm acetone under the condition of high humidity, and the MZO sensors have a better ability of anti-interference, which is greatly beneficial to the qualitative screening of the diabetic patients, saving the time and the cost greatly.(5) Zooming p+n FETs are designed to handle the problem of signal saturation of the MOX sensors in the high-concentration target gas. It can reduce the sensitivity to the low-concentration gas but increase the sensitivity to the very high-concentration gas, realizing the goal that the effective detection range of the MOX sensors is tuned to the higher concentration. The principle of the zooming p+n FETs is that the n-type enhancement mode FET works firstly to suppress the signals and the amplifying role of the p-type depletion mode FET depends completely on the concentration of the target gas. This p-type FET will be driven to play an amplifying role only if the concentration is large enough. Now, the effective detection range of the MOX sensors can be flexibly changed by selecting a suitable type of FET according to the actual need.
|周新愿. 氧化锌气敏机制的研究及传感器信号放大的应用[D]. 中国科学院大学,2019.|
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