石墨結構奈米材料之元件特性與應用之研究

Abstract

本論文旨在研究具石墨結構的奈米材料，包含奈米碳管與石墨烯，其低溫、大面積成長製備暨其在場發射照明、生醫感測和電晶體之應用以及其相關物理、化學特性之研究。 本論文首次利用模擬軟體設計於不同陣列分佈之金字塔結構基板上，最佳化其奈米碳管薄膜結構之場發射特性。對奈米碳管薄膜在一金字塔陣列的結構上而言，當金字塔高度為30微米、相鄰金字塔頂端間距與其高度的比值R/H為2時，為奈米碳管薄膜在此結構之最佳場發射條件。接著，將奈米碳管薄膜沉積於聚二甲基矽氧烷(polydimethylsiloxane, PDMS)與電鍍銅兩種具有金字塔結構的軟性基板，以進行該奈米碳管薄膜之場發射特性的探討。對於奈米碳管沉積於具金字塔結構之軟性PDMS基板而言，其起始電場可有效下降至1.39 V/μm，且其在高電場4 V/μm 的操作下，經過6000秒的穩定度測試(stability test)後，場發射電流密度仍可保有200 uA/cm2的值。此外，若將奈米碳管轉移於具金字塔結構之軟性電鍍銅基板時，其起始電場更可進一步下降至1.12 V/μm，而場發射電流密度在高電場4 V/μm 操作下，經過6000秒的穩定度測試後，可維持更高的電流密度達690 uA/cm2。 接著，我們展示了一種經由連續波雷射處理後的奈米碳管薄膜，使其應用在延伸式閘極感測場效電晶體(Extended-Gate Field-Effect Transistor, EGFET)之酸鹼離子感測膜上能具有更佳的感測特性。經由雷射處理後的奈米碳管能夠被展開(unzipped)成為多層結構的石墨烯碎片，而這些多層結構的石墨烯碎片具有較好的導電特性以及更多的感測位置(sensing sites)。因此，相較於沒有雷射處理的奈米碳管薄膜，其電壓感測度僅37.8 mV/pH、感測線性度0.9823而言，經過5瓦雷射能量處理過後具石墨烯碎片的奈米碳管薄膜，在酸鹼值範圍從1到13的區間，展現出較高之電壓感測度達49.2 mV/pH以及更高的感測線性度0.9956。 為了更進一步提升奈米碳管薄膜的酸鹼值感測特性，我們提出利用氧電漿處理奈米碳管薄膜，使碳管表面能夠有效地提供更多的感測位置(sensing sites)。經由氧電漿處理過後的奈米碳管薄膜製作於玻璃基板時，其應用於延伸式閘極感測場效電晶體之酸鹼離子感測膜上，在酸鹼值範圍從1到13的區間，能夠具有優異的電壓感測度達56.8 mV/pH及感測線性度0.9995。此外，這種電漿處理的奈米碳管薄膜亦製作於聚醯亞胺(Polyimide, PI)的基板上形成可撓式感測膜，在延伸式閘極感測場效電晶體之酸鹼離子感測上，即使經過5次循環的撓曲測試後，依舊能夠保有良好的電壓感測度達53.6 mV/pH及感測線性度0.9943。 此外，一種利用連續波雷射在碳化矽薄膜的表面形成石墨烯的新穎方式首次被提出。藉由控制雷射不同的照射能量，可以調控所成長的石墨烯之層數。而石墨烯電晶體便可使用這些雷射致使成長的石墨烯直接製作於碳化矽薄膜的表面，因此，當我們利用10瓦雷射能量所製備的石墨烯，其上閘極(top-gate)結構的石墨烯電晶體能夠具有良好的載子遷移率(mobility)達5200 cm2/V-s，而電流開關具有3的值。 最後，亦提出論文結論與針對未來研究可著重的工作方向。

In this thesis, the fabrication processes and physical/chemical characteristics of the graphite-structured nanomaterials, including carbon nanotube (CNT) and graphene, are investigated with the low-temperature and large-area synthesis approaches to improve the performance of the field-emission lighting devices, biosensors, and transistors. At first, the pyramid structures for the field-emission properties of carbon nanotube thin films (CNTFs) are optimized with the simulated software for the first time. The optimal field-emission conditions for the CNTF on a 3 x 3 pyramid array are the pyramid height of 30 μm and the R/H ratio of 2, where the R/H ratio is the interspacing R between the tips of the neighbor pyramids against the pyramid height H. Then, the CNTFs are deposited on two kinds of flexible pyramid-structured substrates, polydimethylsiloxane (PDMS) and electroplating copper substrates, to investigate the field-emission properties of such CNTFs. The turn-on field of the CNTF on the pyramid-structured PDMS substrate after the oxygen-plasma treatment is achieved to be 1.39 V/μm and the field-emission current density is kept at 200 uA/cm2 after 6000-second stress at the field of 4 V/μm. In addition, the turn-on field of the transferred CNTF on the pyramid-structured electroplating Cu substrate with the bilayer seed of Ti/Cu can be further reduced to 1.12 V/μm. Moreover, the field-emission current density is still maintained at 690 uA/cm2 after 6000-second stress at the field of 4 V/μm. Secondly, the continuous-wave (CW) laser irradiation is applied on the CNTFs to achieve the better pH sensing characteristics of extended-gate field-effect transistors (EGFETs). The CNTs are unzipped into the multi-layer graphene sheets after the laser irradiation, and these multi-layer graphene sheets possess the higher conductance and more sensing sites. Therefore, the 5-Watt laser-irradiated CNTFs as the pH-EGFET sensing membranes can accomplish a higher voltage sensitivity of 49.2 mV/pH and a larger voltage linearity of 0.9956 in the wide sensing range of pH=1 to pH=13 as compared to the sensitivity of 37.8 mV/pH and the linearity of 0.9823 for the as-sprayed ones. To further improve the pH sensing characteristics of CNTFs, the oxygen-plasma treatment is utilized to effectively decorate a sufficient number of sensing sites on the CNTFs. The sensing characteristics of pH-EGFET sensors based on these oxygen-plasma-treated CNTFs with a higher voltage sensitivity of 56.8 mV/pH and a larger voltage linearity of 0.9995 in the wide sensing range of pH=1 to pH=13 are accomplished. Moreover, the flexible pH-EGFET sensors with these functionalized CNTFs are realized and exhibit the impressive voltage sensitivity of 53.6 mV/pH and linearity of 0.9943 even after a 5-cycles bending test. Next, a novel method for the graphene growth is proposed via the precisely-controlled CW laser irradiation on the SiC thin film. The thickness of the graphene layer can be adjusted by varying the applied laser energy. Then, the top-gate graphene field-effect transistors are directly fabricated with such laser-induced graphene layers on the relatively insulating SiC thin films. Consequently, the graphene field-effect transistors can achieve the electrical properties of the mobility up to 5200 cm2/V-s and the Ion /Ioff current ratio up to 3 for the 10-Watt laser irradiation. Finally, conclusions as well as prospects for the further research are also proposed.