1.School of Physics and Electronic Technology, Liaoning Normal University, Dalian 116029, China 2.School of Microelectronics, Dalian University of Technology, Dalian 116024, China
Abstract:Zinc oxide (ZnO) is a wide direct band gap (3.37 eV) II-VI semiconductor material with a wide range of applications in light emitting devices, solar cells, field emission devices, gas sensors, etc. Over the past decades, metal oxide semiconductors have been investigated extensively for sensing various types of vapors and toxic gases. Among the various metal oxides for gas sensing applications, ZnO is one of the potential materials for high response, stability and sensitivity to volatile organic gases. At present, the ZnO-based gas sensor has a relatively high operating temperature. However, the stability and lifetime of gas sensors operating under high temperature conditions with a long term will be greatly reduced. In addition, the power consumption of gas sensors is also significantly increased. Furthermore, there can exist the potential of explosion when gas sensors are used to detect the flammable gases at high temperature. Therefore, it is necessary to improve the sensing properties and reduce the operating temperature of gas sensors. In this paper, ultra-long, large-sized Sb doped ZnO microwires are successfully prepared by using chemical vapor deposition. The gas sensor is based on the principle of non-balance electric bridge, and a single Sb doped ZnO microwire is used as a bridge arm R4 of non-balance electric bridge to produce a gas sensor that can work at room temperature. The results show that the response-recovery curves of sensors at the acetone and ethanol concentrations of 20 ppm, 50 ppm, 100 ppm and 200 ppm (1 ppm = 10–6) are rectangular at room temperature, and have stable current values in air and measured gas, and the response value of the device gradually increases with gas concentration increasing. Furthermore, the detection of acetone and ethanol gas reveal that the device has better selectivity for acetone gas. The response of the gas sensor to 200 ppm acetone is about 243%, with response and recovery time of 0.2 s and 0.3 s, respectively. Compared with the traditional resistive gas sensor, this non-balanced electric bridge sensor has high response, fast response and recovery time. In addition, the sensing mechanism of the device is also studied. Keywords:chemical vapor deposition/ Sb doped ZnO microwires/ non-balance electric bridge/ gas sensor
3.结果与讨论图2(a)是刚生长出来的Sb掺杂ZnO微米线的实物照片. 由图2(a)可知, 具有大尺寸、高密度的Sb掺杂ZnO微米线阵列被生长出来, 且微米线的长度较长, 其长度约为1.5—2.5 cm. 图 2 (a) Sb掺杂ZnO微米线阵列实物照片; (b)单根Sb掺杂ZnO微米线的SEM图 Figure2. (a) Photograph of Sb doped ZnO microwire arrays; (b) SEM image of the single Sb doped ZnO microwire.
为了进一步观察单根Sb掺杂ZnO微米线的表面形貌, 利用扫描电子显微镜(SEM)对单根微米线的表面形貌进行了表征, 结果如图2(b)所示. 该测试选用的Sb掺杂ZnO微米线的长度为1.5 cm, 从图2(b)可以看出单根Sb掺杂ZnO微米线的表面非常光滑, 没有颗粒或者团簇附着在其表面, 而且微米线的直径比较均匀, 其直径约为35 μm. 通过多次对不同单根Sb掺杂ZnO微米线的表面形貌测试后发现, 微米线的表面都十分的光滑. 此外, 还对Sb掺杂ZnO微米线进行了元素成分的分析, 其能量色散X射线光谱(EDS)结果如图3所示. 从图3可以看出, 该微米线是由Zn, O以及Sb元素所组成, 且Sb掺杂ZnO微米线中Zn, O以及Sb元素的摩尔百分含量分别约为51%, 46%和3%. 图 3 单根Sb掺杂ZnO微米线的EDS Figure3. EDS for the single Sb doped ZnO microwire.
为了制作气体探测器件, 从Sb掺杂ZnO微米线阵列中挑选出长度为1.5 cm, 两个电极之间微米线的长度约为1 cm, 直径约为35 μm的微米线作为非平衡电桥的桥臂电阻R4. 图4给出了室温下单根ZnO微米线分别在空气和浓度为200 ppm (1 ppm = 10–6)丙酮及乙醇气体中的I-V特性曲线, 电压的测量范围从–10—10 V. 由图4可以看出这3条I-V特性曲线随着电压的增加, 电流也随之增加且基本呈线性变化, 这说明导电银胶与Sb掺杂ZnO微米线之间形成了良好的欧姆接触. 此外, 还发现当微米线分别放置于空气和不同的测试气体(丙酮和乙醇)中时, 3条曲线的斜率出现了明显的不同, 这表明当Sb掺杂ZnO微米线放在丙酮和乙醇气体中时, 电阻会比在空气中时明显增大, 且在丙酮中的电阻要大于在乙醇气体中的数值. 此外, 单根微米线在空气中的电阻率约为0.3 Ω·cm. 图 4 室温下单根Sb掺杂ZnO微米线分别在空气和浓度为200 ppm丙酮及乙醇气体中的I-V特性曲线 Figure4. The I-V characteristic curves of the single ZnO micronwire for air, acetone (200 ppm) and ethanol (200 ppm) at room temperature.
为了测试器件在室温下的气体探测性能, 对器件在20—200 ppm不同浓度丙酮和乙醇气体下的气敏特性进行了研究, 结果如图5所示. 图5(a)和图5(b)分别为室温下, 器件在不同浓度丙酮和乙醇气体中的响应-恢复曲线. 从图5(a)和图5(b)可以看出, 传感器在不同浓度丙酮和乙醇气体中的响应-恢复曲线的形状, 都接近于矩形, 这表明室温下该器件对两种气体都有着快速的响应和恢复时间. 另外, 还发现每次测量器件在空气中的初始电流值Ia基本不变, 但随着丙酮和乙醇气体浓度的变大, 传感器在待测气体中的稳态电流值Ig却不断地增大, 即电流的变化量(Ig–Ia)值在逐渐变大. 图5(c)和图5(d)为室温条件下器件分别对20, 50, 100和200 ppm的丙酮与乙醇气体浓度的响应对比. 从图5(c)和图5(d)可以清晰地看出, 随着被测丙酮与乙醇气体浓度的增加, 器件的响应值也在逐渐增加. 当乙醇和丙酮气体的浓度为200 ppm时, 室温下探测器对乙醇的响应度可以达到185%, 其响应时间为0.3 s, 恢复时间为0.6 s, 对丙酮的响应度为243%, 其响应时间和恢复时间分别为0.2 s和0.3 s. 在相同气体浓度下, 器件对丙酮气体显示出相对较大的响应度, 这表明该器件对丙酮气体具有更好的选择性. 图 5 室温条件下传感器在不同浓度丙酮和乙醇气体中的响应和恢复曲线 (a)丙酮; (b)乙醇; (c), (d)室温下传感器对不同浓度丙酮及乙醇气体的响应度曲线 Figure5. Response and recovery curves of the gas sensor to different concentrations of acetone (a) and (b) ethanol at room temperature; (c), (d) the response of the gas sensor to different concentrations of acetone and ethanol at room temperature.
对于气敏传感器的实际应用而言, 器件的可重复性和稳定性都是非常重要的参数. 图6为室温下气体传感器对200 ppm浓度丙酮和乙醇气体的响应和恢复曲线(4个循环). 由图6可知, 当器件放置在空气中时, 测得器件BD两端的电流值Ia基本稳定在10.9 μA; 而当器件分别插入丙酮和乙醇气体中时, 电流则迅速增加, 而后其电流值又逐渐趋于平稳, 达到另一个稳定值Ig, 当丙酮和乙醇气体浓度分别为200 ppm时, BD两端的电流值基本稳定在37.4和31.1 μA. 随后将器件再次暴露于空气中时, 器件产生的电流均恢复到初始电流值Ia, 即完成了一个循环操作. 由此可以得出该器件在丙酮和乙醇气体中的响应和恢复特性都能够重复再现, 这说明器件具有非常好的稳定性, 可以重复使用. 图 6 室温下气体传感器对200 ppm被测气体的响应恢复曲线 (a)丙酮; (b)乙醇 Figure6. Response and recovery curves of gas sensor to 200 ppm acetone (a) and ethanol (b) at room temperature.
为了对非平衡电桥结构气敏传感器和传统电导式气敏传感器, 在检测气体方面进行比较. 将微米线R4从电桥中拿出来, 两侧分别接电源的正负极(组成传统电导式气敏传感器), 在相同温度、相同直流电压(5 V)和相同测试气体浓度下(200 ppm的丙酮和乙醇气体), 测试两种类型传感器的响应-恢复曲线, 结果如图7所示. 由图7可以看出, 非平衡电桥结构传感器对200 ppm丙酮气体的响应时间为0.2 s, 恢复时间为0.3 s, 响应度为243%; 对相同浓度的乙醇气体的响应时间为0.3 s, 恢复时间为0.6 s, 响应度为185%; 而单根ZnO微米线传统电导式气敏探测器对200 ppm丙酮气体的响应和恢复时间分别为9.7 s和72.4 s, 响应度为9%; 对相同浓度的乙醇气体的响应时间为18.7 s, 恢复时间为12.8 s, 响应度为0.6%. 由此可见, 通过对两种不同结构器件的对比, 发现非平衡电桥结构的器件有着较好的气敏探测特性, 即对乙醇和丙酮气体有着更大的响应度和更短的响应和恢复时间. 图 7 两种气敏传感器室温下对200 ppm丙酮和乙醇气体的响应和恢复曲线 (a), (b)非平衡电桥式; (c), (d)传统电导式 Figure7. Response and recovery curves of two gas sensors to 200 ppm acetone and ethanol gases at room temperature: (a), (b) Non-balance electric bridge type; (c), (d) conventional conductance type.
图 1 气体传感器的结构示意图
图 2 (a) Sb掺杂ZnO微米线阵列实物照片; (b)单根Sb掺杂ZnO微米线的SEM图
图 3 单根Sb掺杂ZnO微米线的EDS
图 4 室温下单根Sb掺杂ZnO微米线分别在空气和浓度为200 ppm丙酮及乙醇气体中的I-V特性曲线
图 5 室温条件下传感器在不同浓度丙酮和乙醇气体中的响应和恢复曲线 (a)丙酮; (b)乙醇; (c), (d)室温下传感器对不同浓度丙酮及乙醇气体的响应度曲线
图 6 室温下气体传感器对200 ppm被测气体的响应恢复曲线 (a)丙酮; (b)乙醇
图 7 两种气敏传感器室温下对200 ppm丙酮和乙醇气体的响应和恢复曲线 (a), (b)非平衡电桥式; (c), (d)传统电导式
