利用雷射處理奈米碳管薄膜應用於光電與生醫元件特性之研究

Abstract

本論文旨在研究利用雷射處理之奈米碳管或其表面具鉑金屬奈米粒子鑲嵌之奈米碳管水平薄膜製備及其奈米電子、光電及生醫元件之應用及相關特性之研究。 首先，利用一強酸氧化之表面改質使奈米碳管表面鍵結上含氫氧之官能基，此官能基能改善原奈米碳管之斥水性，使奈米碳管能均勻分散於有機溶液中。接著使用一低溫大面積超音波噴塗設備將此含奈米碳管溶液噴塗於銦錫氧化物(CNT/ITO)之玻璃基板上，形成延伸式閘極感測場效電晶體(Extended-gate field-effect transistor, EGFET)之酸鹼離子感測膜。同時與金屬-氧化物-半導體場效電晶體(Metal-oxide-semiconductor field-effect transistor, MOSFET)元件結合後，將製備的酸鹼離子感測膜置於不同pH緩衝液，探討此感測膜材料之元件特性與酸鹼離子感測度的變化情形。首先，由實驗結果顯示，經過強酸氧化處理的奈米碳管表面會含有氫氧的官能基，在拉曼光譜的量測也發現結構缺陷的出現。在酸鹼值量測的表現上，可發現隨著沉積次數的提升，此奈米碳管薄膜可達到一優異的電壓感測度(51.27 mV/pH)、較高之電流感測度(0.831 μA1/2/pH)、較佳之線性度(0.9942)、較小之遲滯寬度(1.73 mV)、較低之時漂速率(5.83 mV/hour)與較廣之感測範圍(pH=1~pH=13)，相較於純奈米碳管做為感測膜之延伸式閘極感測場效電晶體，此CNT/ITO因具三維的感測結構且與ITO基板較佳的附著性，所以可得到較好的特性。此外，本論文亦提出利用連續波雷射(Continuous-wave laser)作用沉積於石英機板之奈米碳管薄膜上。利用此結構可將奈米碳管感測膜同時作為電極，並藉由雷射的高能處理，展開奈米碳管的同心圓結構形成石墨片狀結構，造成一奈米碳管與石墨的混成結構，也可改善奈米碳管與基板之附著性，進而降低阻抗。由於展開的石墨結構可提升整體的感測鍵結位置，進而改善感測膜之感測特性。在酸鹼值量測的表現上，可發現隨著沉積次數的提升，此奈米碳管薄膜也可達到一優異的電壓感測度(46.7 mV/pH)、較高之電流感測度(0.767 μA1/2/pH)、較佳之線性度(0.9843)、較小之遲滯寬度(2.924 mV)、較低之時漂速率(4.33 mV/hour)與較廣之感測範圍(pH=1~pH=13)。由於此感測器結構易於製造與封裝，故可應用於軟性與拋棄式生物感測器。 接著，本論文提出一利用KrF準分子雷射作用於奈米碳管薄膜。在雷射單次作用(Single-scanned mode)無重疊的情況下，可發現在奈米碳管薄膜表面對應至高能量區間會發現濺蝕(Ablation)區的表面形貌，另外在低能量區間會觀察到退火(Annealing)區的表面形貌。此外，由於雷射瞬間的熱應力濺蝕會在濺蝕以及退火區的界面產生表面突出。另外，在重疊作用(Multiple-scanned mode)的情況下，發現隨著雷射的重疊百分比的提升，奈米碳管薄膜的結晶性也能同時提升，進而增強奈米碳管薄膜整體抵抗雷射濺蝕的能力。因此，在重疊照射下，奈米碳管薄膜的所需的濺蝕能量(Ablation energy)也會增加。在本論文中發現，隨著重疊照射的次數以及退火能量的提升，均能大幅提升濺蝕能量。此外，根據前述的結果，提出一緩步提升照射能量(Irradiation with the step elevated laser energy density, ISLED)的方法，利用ISLED的方式可確保奈米碳管薄膜在逐漸提升的雷射能量下仍能保持在退火的區間，避免對表面造成濺蝕的破壞。發現在ISLED高能量的作用下，多數的碳管會因受到高能量退火由管狀結構剝開形成石墨烯的片狀結構。藉由此章節所提出的雷射機制，可得到在不同雷射能量照射及不同的重疊比率下奈米碳管表面形貌的變化。 針對場發射元件之開發，我們利用KrF準分子雷射作用於奈米碳管薄膜，並操作在突起區間，利用雷射造成表面週期性突起，並能達到提升碳管結晶性已提高電子的遷移率，可大幅改善奈米碳管薄膜之場發射特性。經實驗發現，在90%重疊率下，奈米碳管可在雷射能量180mJ/cm2下，可達到最佳的結晶性(ID/IG=1.89)、最低的開啟電場(1.56 V/µm)及較高之場增強因子(4446)。進一步發現，在95%重疊率下，由於重疊率的提升，可改善奈米碳管薄膜之結晶性，因此在雷射能量240mJ/cm2作用下，可達到優異的結晶 性(ID/IG=1.203)、最低的開啟電場(1.44 V/µm)、最佳的臨界電場(2.16 V/µm)及較高之場增強因子5085)。利用本實驗結果，對於未來低溫場發射顯示器製作於可撓性基板將有非常大的助益。 針對染料敏化太陽能電池之開發，首先我們利用純奈米碳管薄膜作為染敏電池之對電極，碳管薄膜因具有三維結構，能使電解液能均勻分布於碳管間隙中，大幅增加接觸面積，改善電子由對電極導入電解液的能力，但實驗結果發現，利用純碳管薄膜的光電轉換效率只能到達5.168 %，相較於傳統鉑電極元件之效率7.12 %，仍具有改善的空間。因此，為了進一步增加碳管與電解液的反應面積，本論文使用KrF準分子雷射對碳管薄膜做熱退火處理，發現隨著雷射能量增加，多數的碳管會因受到高能量退火由管狀結構剝開形成石墨烯的片狀結構，此奈米碳管與石墨烯的混成薄膜可大幅增加與電解液的反應面積。此外，雷射退火可使碳管再結晶化，增加其導電性，並改善碳管與基板之間的附著力，進而增加載子的傳遞能力。在雷射能量為600 mJ/cm2時，其元件光電轉換效率由原本的5.168 %提升至6.352 %，增加了23 %，足見雷射熱退火對碳管薄膜特性有顯著的提升。由於鉑金屬對電解液催化的效果較碳管優異，為了進一步增加對電極的載子交換能力，本論文利用雷射退火製備鑲嵌有鉑奈米粒子之奈米碳管/石墨烯片混成薄膜作為染敏電池對電極，也因使用鉑奈米粒子做沉積，相較於傳統鉑電極，其鉑金屬用量大幅減少，降低製程成本。實驗結果發現，由於雷射處理過的碳管薄膜提供較大的比表面積，沉積在碳管上的鉑奈米粒子與電解液接觸面積也因此增加，載子傳遞更加容易，因此光電轉換效率由6.352 %提高到8.791 %，增加了約40 %。相較傳統鉑電極之效率7.12 %，元件效率提高約 23%。本研究利用雷射退火製備鑲嵌有鉑奈米粒子之奈米碳管/石墨烯片混成薄膜可提供高比表面積與電解液進行載子交換外，特性穩定且導電性良好也為其優勢，未來在可撓性及半穿透性的光電元件中深具潛力。 最後，亦提出本論文之結論，並針對未來研究可著重的工作方向加以闡述。

In this thesis, the physical/chemical properties of laser-irradiated carbon nanotubes (CNTs), including CNT thin films and Pt nanoparticles decorated on CNTs were investigated. This study demonstrates the potential of carbon-based nanostructures in the applications of the nano electronic, optoelectronic devices, and biosensors. First, the CNT thin films were deposited onto the indium tin oxide (ITO) substrate by the low-temperature scanning ultrasonic spraying method to fabricate the CNT-based extended-gate field-effect transistors (EGFETs) as the pH sensor. With an acid-treated process, the chemically functionalized two-dimensional CNT thin films could provide plenty of functional groups which exhibit the hydrophilic property and serve as hydrogen sensing sites. The EGFET using a CNT/ITO structure could achieve a wide sensing rage from pH=1 to pH=13. Futhurmore, the pH sensitivity and linearity values of the CNT pH-EGFET devices were enhanced to 51.74 mV/pH and 0.9948 from pH=1 to pH=13 while the sprayed deposition reached 15 times. The hydrogen ions sensing show significantly dependent on the sprayed deposition times, morphologies, crystallinity and chemical bonding of acid-treated CNTs. In addition, the EGFETs with only the CNT thin films as both the sensing membrane and the contact electrode have been demonstrated for the first time to exhibit superior pH sensing characteristics. The continuous-wave (CW) laser was necessary to improve the pH sensitivity to be 46.7 mV/pH and linearity values to be 0.9843 for pH=3 to pH=13 wide sensing range, respectively. It implied that the laser energy would unzip the chemically modified MWCNTs into numerous graphite slices, resulting in the elevated sensing sites, the improved electrical and sensing properties. Therefore, the laser-irradiated MWCNT thin film is promising for the applications in the flexible and transparent pH-EGFETs. These results demonstrate that the transparent CNT-EGFETs are very promising for the applications in the pH and biomedical sensors. Secondly, the surface morphology transformation of the sprayed CNT thin films irradiated with the excimer laser has been systematically investigated. Under the excimer-laser irradiation, two phenomena, including the annealing and ablation effects, were found to be dependent on the incident laser energies and overlapping ratios. Moreover, the extremely high protrusions would be produced in the interface between the annealing and ablation regions. The mechanism of the CNT thin films under the excimer laser irradiation was, therefore, proposed to derive the surface morphology modifications and the further reinforced crystallinity with proper laser energy densities and overlapping ratios. In addition, the irradiation with step laser energy densities (ISLED) was developed to irradiate on the CNT thin films to guarantee the whole device be operated under the annealing condition. Under the high laser energy of ISLED, the structures of CNTs would begin to be modified owing to the laser energy. With the laser irradiation mechanism, the surface morphology and crystallinity could be precisely predicted under different laser energy densities and overlapping ratios. For the field emission applications, the field emission characteristics of the CNT thin films irradiated with the excimer laser with the 90% and 95% overlapping ratios under different laser energy densities were systematically addressed. According to the previous laser mechanism, the field emission devices were operated under the protrusion conditions. Such a CNT thin film was protruded as the field-emitted cathode to achieve a low turn-on voltage of 1.44 V/µm, a low threshold voltage of 2.16 V/µm, and a high field enhancement factor β with 5085. It was attributed to the aspect ratio effect from the surface protrusions for the electron emitting. The excellent field emission characteristics and low temperature fabrication process suggest that the laser-irradiated CNT thin films show potential in the future field-emission -based applications. For the solar cell applications, the CNT thin films decorated with Pt nanoparticles (PtCNT) irradiated by excimer laser as the counter electrode (CE) of dye-sensitized solar cells (DSSCs) has been demonstrated for the first time. First, the pure CNT was used to the CE of DSSC. The CNT thin films kept three-dimensional structure to increase the contact area with electrolyte. It would increase the charge-transfer ability between CE and electrolyte. However, to compare with the performance of traditional Pt CE which was 7.12 %, the performance of DSSC with pure CNT was just 5.168 % which could not reach the expectation for commercializing. Therefore, the CNT thin films were irradiated by excimer laser with ISLED for instantaneous annealing in order to elevate the contact surface area. The surface morphology variations of CNT thin films were systematically compared under different excimer laser energy irradiation. Part of the CNTs would be unzipped into GSs resulting in the CNT and GS hybrid structures under high energy irradiation. Moreover, the laser energy also assisted the CNT recrystallization for improving the charge transfer ability. Finally, the adhesion between FTO and CNT film would also be enhanced. The efficiency of CNT CE under 600 mJ/cm2 laser irradiation was promoted to 6.352 %, which shown a considerable 23 % improvement in performance. After decorating the circular Pt nanoparticles on CNT and GS structures, the conversion efficiency was improved from 6.352 % to 8.791 % with 40 % enhancement with ISLED of 600 mJ/cm2. It was attributed to the enhanced catalytic surface from Pt nanoparticles on the three-dimensional structures and the improved conductivity of PtCNT films due to the adjoining phenomenon of CNTs irradiated by laser. Moreover, the laser annealing could also promote the interface contact between PtCNT CE and conductive glass. Therefore, such a simple laser-irradiated PtCNT network is promising for the future applications in the flexible DSSCs. Finally, the conclusions as well as prospects for the further research were also proposed.