氮改質多層石墨烯於氣體感測之應用

Abstract

氣體感測器在我們的生活中扮演非常重要的角色，在無色無味的氣體環境下，人們無法依靠本身的視覺以及嗅覺去了解環境的狀況，透過氣體感測器，人們能了解環境中的危害，並進一步逃離，避免悲劇的產生。根據氣體濃度資料所示，室內二氧化碳濃度在達到1000 ppm以上時，會影響人類的呼吸；一氧化碳則是在數百ppm時，會使人體昏迷，2000 ppm以上甚至可能致死；氨氣則是在20至25 ppm時，就會刺激我們的皮膚、眼睛、呼吸系統等等；而在醫學方面，有些研究指出有肝疾病的患者呼出的氨氣濃度會超過0.7 ppm，而普通人呼出的氨氣濃度在 0.3 ppm以下，而在食品保存方面，食物腐壞也會產生二氧化硫、氨氣等氣體；因此若是能研發及改良氣體感測器，並在實際氣體感測應用得到驗證的話，將有助於提升感測器對於氣體的靈敏度與選擇性；其中，以石墨稀優越的物理特性、良好的電子傳輸速度、高質量晶體以及低電阻的特性等，已經被廣泛應用在各個領域上，其中也包含氣體感測器。 本研究主要是以碳材料為主的不同氣體感測器，透過不同時間的氮改質，分析及比較改質前後對於二氧化碳、氨氣以及一氧化碳的反應程度，進一步整理並歸納出感測器元件適用的氣體種類以及濃度，以及進一步改良的展望。 碳材料為主的氣體感測器，分為單層石墨烯、雙層石墨烯以及雷射誘導石墨烯(LIG)；其中單層石墨烯，是以化學氣相沉積法(CVD)來製備，首先將銅箔拋光並放入高溫爐管內，通入氬氣、氫氣及甲烷，將碳原子沉積在銅箔上來成長出單層石墨烯，之後轉印在基板上；而雙層石墨烯則是轉印至單層石墨烯的銅箔上後，再轉印至基板上；將製備好的感測器元件進行低損傷電漿(LD-plasma)系統進行氮改質，其中低損傷電漿系統中利用互補式遮板的架設，阻擋了大部份的離子撞擊與紫外光幅射，因此大幅地減少對於石墨烯的表面傷害，且透過調整加熱溫度並配合拉曼光譜儀的分析，選擇最合適的溫度去穩定控制氮改質石墨稀的程度；最後再經由改變氮改質的時間去觀察拉曼波形圖的G peak和 D peak的峰值比例，初步判斷其改質的狀況與成功與否，並找出哪一個改質時間下的氣體感測元件有最好的感測能力。 接下來再以雷射誘導石墨烯(LIG)元件進行電阻對溫度的變化測試，討論在定區間的溫度下，溫度與電阻的線性關係，並使用與單層石墨烯、雙層石墨烯相同的改質條件進行低損傷電漿系統氮改質，判斷在不同的氮改質時間下，所得到的拉曼光譜圖作分析及討論。 最後，將改質後的單層、雙層石墨烯以及LIG分別進行不同濃度的二氧化碳、氨氣和一氧化碳的量測，其中二氧化碳的濃度為1000 ppm至5000 ppm的區間；氨氣以及一氧化碳濃度則是0 ppm 至 10 ppm以及100 ppm去作電性量測，比較三種感測器對於不同氣體和濃度的反應。

Gas sensors play a crucial role in our lives. In environments where gases are colorless and odorless, relying on our own vision and sense of smell is insufficient to understand the conditions. Through gas sensors, people can assess the hazards in the environment and take necessary actions to avoid tragedies. Based on the data on gas concentrations, when indoor carbon dioxide (CO2) levels exceed 1000 ppm, it affects human respiration. Carbon monoxide (CO), even at a few hundred ppm, can cause unconsciousness, and concentrations above 2000 ppm may even be lethal. Ammonia (NH3) at 20 to 25 ppm can irritate our skin, eyes, and respiratory system. In the field of medicine, some studies indicate that patients with liver diseases exhale ammonia concentrations exceeding 0.7 ppm, while the exhaled ammonia concentration in healthy individuals is below 0.3 ppm. In terms of food preservation, the spoilage of food can produce gases such as sulfur dioxide (SO2) and ammonia. Therefore, the development and improvement of gas sensors, validated through practical gas sensing applications, would enhance the sensitivity and selectivity of the sensors towards gases. Graphene, with its superior physical properties, excellent electron transport speed, high-quality crystals, and low resistance, has been widely applied in various fields, including gas sensors. This study primarily focuses on carbon-based gas sensors, including monolayer graphene, bilayer graphene, and laser-induced graphene (LIG). Monolayer graphene is prepared through chemical vapor deposition (CVD) by polishing copper foil, placing it in a high-temperature furnace, and introducing argon gas, hydrogen gas, and methane. Carbon atoms are deposited on the copper foil to grow monolayer graphene, which is then transferred onto a substrate. Bilayer graphene is transferred onto the copper foil of monolayer graphene and subsequently onto the substrate. The prepared sensor devices undergo nitrogen modification using a low-damage plasma (LD-plasma) system. In the LD-plasma system, complementary shielding plates are used to block most of the ion collisions and ultraviolet radiation, significantly reducing surface damage to graphene. By adjusting the heating temperature and analyzing the Raman spectra with a Raman spectrometer, the most suitable temperature for stabilizing and controlling the nitrogen-modified graphene is selected. Subsequently, by varying the nitrogen modification time and observing the peak ratio of the G peak and D peak in the Raman spectra, the modification status and success of the gas sensing elements are preliminarily determined, identifying the gas sensing devices with the best sensing ability at different modification times. Next, the LIG devices are tested for the resistance-temperature variation to discuss the linear relationship between temperature and resistance within a specific temperature range. The same modification conditions as monolayer graphene and bilayer graphene are applied using the low-damage plasma system to nitrogen-modify the LIG. Raman spectroscopy is used to analyze and discuss the obtained Raman spectra under different nitrogen modification times. Finally, the modified monolayer graphene, bilayer graphene, and LIG are subjected to measurements of different concentrations of carbon dioxide, ammonia, and carbon monoxide. The carbon dioxide concentration ranges from 1000 ppm to 5000 ppm, while ammonia and carbon monoxide concentrations range from 0 ppm to 10 ppm and 100 ppm, respectively. A comparison is made among the three types of sensors regarding their responses to different gases and concentrations.