具有極高靈敏度之單根氧化鋅奈米線一氧化氮氣體感測器

Single ZnO Nanowire Sensors for Nitric Oxide Gas Sensing with Ultrahigh Sensitivities

陳信宏(Shin-Hong Chen)

指導教授 ： 林鶴南

國立清華大學/工學院/材料科學工程學系/碩士(2013年)

https://doi.org/10.6843/NTHU.2013.00496

摘要

本實驗以單根氧化鋅奈米線為基礎，利用黃光微影技術製作具有極高靈敏度的電阻式一氧化氮氣體感測器，其感測原理乃由於一氧化氮為氧化性氣體，吸附在氧化鋅表面時會造成奈米線導電性下降。將線徑約略300 nm的氧化鋅奈米線兩端連接100 nm、300 nm以及500 nm厚度的鈦電極，在室溫下進行濃度介於0.1 ppm至2 ppm的一氧化氮氣體感測。其中100 nm及300 nm較薄電極的元件，對2 ppm一氧化氮的靈敏度高達104以上，其電流電壓曲線在感測後由歐姆接觸轉變為蕭特基接觸；而500 nm較厚電極的元件，靈敏度則低了兩個數量級，並在感測前後維持歐姆接觸的特性。此結果說明奈米線與電極的接觸特性正是元件擁有高靈敏度的主要機制，且此一特性會受電極厚度所影響。

在不同氣體濃度的感測結果中，對同一元件而言，當濃度越高時由於氣體有較快的吸附速率，因此電流下降到初始值10 %所需的反應時間就越短。而當電極厚度100 nm、300 nm及500 nm的元件同樣感測0.1 ppm一氧化氮時，其反應時間則分別為331秒、183秒及26秒，顯示接觸電極越厚，反應越快的趨勢。此外，藉由歐姆接觸式元件在感測前後的電性變化，可根據吸附動力學得到氣體分子對奈米線的吸附及脫附速率常數，並推算出氧化鋅奈米線的表面缺陷密度及載子濃度。因此，我們藉由改變不同的厚度的電極，可根據需求製作極高靈敏度或反應快速的一氧化氮氣體感測器，並同時具有室溫操作性及可重複感測等優點。

關鍵字

氧化鋅奈米線 ； 一氧化氮 ； 氣體感測器

並列摘要

In this work, nitric oxide (NO) gas sensors with ultrahigh sensitivities based on single ZnO nanowires (NWs) are reported. The sensing principle is because NO is an oxidizing gas and the adsorption of gas molecules on the NW surface causes the NW conductance to decrease. An individual sensor is composed of a single ZnO NW with a diameter of around 300 nm in contact with Ti electrodes of 100, 300 or 500 nm thick created by photolithography. The fabricated devices have been utilized to sense NO gas in a concentration range between 0.1 and 2 ppm at room temperature. For the sensors with electrodes of 100 and 300 nm thick, ultrahigh sensitivities over 104 are achieved for 2 ppm NO sensing. From the I−V curves, it is found the contact is Ohmic type prior to exposure to NO and turns to Schottky type afterwards. For the sensor with electrodes of 500 nm thick, the sensitivity is around 102 and much lower than the previous values. In addition, the contact remains Ohmic type after NO sensing. The results indicate that the contact characteristic change at NW-electrodes interface is the dominant mechanism for the ultrahigh sensitivity of gas sensing and it can be modified by the electrode thickness.

The results show the response time decreases as the NO concentration increases due to faster adsorption of NO gas under higher concentration. The response time is defined as time needed for the current to become 10 percent of the initial current. In addition, under the same concentration of 0.1 ppm, the response time of the sensors with 100, 300 and 500 nm thick electrodes are 331, 183 and 26 s, which means the response is faster as the electrode thickness increases. Besides, the adsorption and desorption rate constants of Ohmic type device can be calculated to derive the carrier concentration of ZnO NW. Therefore, we can fabricate a NO gas sensor with high sensitivity or fast response by changing the thickness of electrodes, and it has the advantages of room-temperature functionality and repeatability.

並列關鍵字

ZnO nanowire ； nitric oxide ； gas sensor

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