氧化鋅和二氧化錫奈米線應用於高效能光電元件暨生醫感測器之研究

Abstract

本論文旨在探討氧化鋅(ZnO)和二氧化錫(SnO2)奈米線應用於高效能光電元件暨生醫感測器之研究，內容涵蓋其在太陽能電池、場發射元件、酸鹼值感測器以及葡萄醣生醫感測器之應用。 針對太陽能電池的開發，我們首次提出了一個新穎的太陽能電池結構，結合p-i-n非晶矽薄膜及鋁摻雜氧化鋅奈米線形成一同軸結構的奈米線太陽能電池，首先，我們探討本質非晶矽薄膜厚度對電性的影響，當本質非晶矽薄膜的厚度從25 nm增加到150 nm，太陽能電池轉換效率從3.92 %提升到4.27 %，不過，當我們繼續增加本質非晶矽薄膜的厚度到250 nm，轉換效率反而下降到3.66 %，這是由於過厚的本質非晶矽薄膜會造成p-n介面的內建電場下降，進而影響到因為照光所產生電子電洞對的分離使得太陽能電池效率反而下降。接著，我們探討鋁摻雜氧化鋅奈米線長度對電性的影響，當沈積150 nm最佳化的本質非晶矽薄膜厚度，改變鋁摻雜氧化鋅奈米線長度從1 μm到2 μm，電池轉換效率從4.27 % 提升到4.73 %，此外，我們提出的同軸結構奈米線太陽能電池相較於平面結構的非晶矽薄膜太陽能電池，太陽能電池效率有將近46 %的提升。 針對場射元件的開發，我們提出利用低溫水熱法合成鋁摻雜氧化鋅奈米柱，搭配稀釋過的醋酸進行蝕刻得到鋁摻雜氧化鋅奈米尖錐的陣列並探討其場發射特性，相較於沒有經過摻雜、沒有經過削尖的氧化鋅奈米柱，經過削尖的鋁摻雜氧化鋅奈米尖錐擁有較好的場發射特性，包括較高的電流密度，較大的電場增強因子(β = 3102)，較低的開啟電場(2.25 V/µm)，和較低臨界值的電場(3 V/µm)，這是由於摻雜和削尖效應造成場發射特性的改善，此外，我們透過施加3.08 V/µm的電場進行1800 s可靠度測試，沒有明顯的劣化現象顯示鋁摻雜氧化鋅奈米尖錐擁有良好的場發射電流穩定性。 針對酸鹼值感測器的開發，我們首次提出利用低溫水熱法合成二氧化錫奈米柱作為延伸式閘極場效電晶體之酸鹼值感測器，二氧化錫奈米柱感測器相較於薄膜結構感測器擁有較高的感測度55.18 mV/pH以及較大的線性度0.9952在廣泛的感測區間(pH 1–13)，與二氧化錫薄膜感測器相比，奈米柱結構對於酸鹼感測度有將近15 % 的提升，這是由於奈米線結構的高比表面積能提供更多有效感測的區域，時漂特性的量測也指出二氧化錫奈米柱感測器有很好的可靠度及耐久性，除此之外，磁滯特性的量測也顯示二氧化錫奈米柱感測器在經過一連串中性→酸性→中性→鹼性再回到中性溶液的測試只有很小的磁滯電壓3.69 mV。 為了改善氧化物感測膜在鹼性溶液的感測度，我們提出一個簡單和低溫的無電極電鍍法和原子層沈積法來製作同軸結構氧化鋅與矽奈米線延伸式閘極場效電晶體，氧化鋅與矽奈米線酸鹼值感測器經過不同的感測區間(pH 1–pH 13)的量測，可以得到感測度為46.25 mV/pH以及線性度為0.9902，與氧化鋅薄膜結構的感測器相比，氧化鋅與矽奈米線結構擁有較高的感測度和較大的性性度，這是由於奈米線結構的高比表面積能提供更多有效感測的區域，時漂特性的量測也指出二氧化錫奈米柱感測器有很好的可靠度及耐久性，除此之外，磁滯特性的量測也顯示氧化鋅與矽奈米線感測器在經過一連串中性→酸性→中性→鹼性再回到中性溶液的測試只有很小的磁滯電壓9.74 mV。 針對葡萄糖生醫感測器的應用，我們首次提出利用低溫水熱法合成二氧化錫奈米柱作為酵素葡萄糖生醫感測器，二氧化錫奈米柱生醫感測器相較於薄膜結構的生醫感測器在葡萄糖濃度60~360 mg/dl的感測區間有較高的感測度0.661 mV(mg/dl)−1，與二氧化錫薄膜感測器相比，奈米柱結構對於葡萄糖感測度有近六倍的提升，這是由於奈米線結構的高比表面積能提供更多有效感測的區域，此外，二氧化錫奈米柱相較於薄膜結構擁有較好的結晶性，可以使感測膜表面電位變化造成的自由載子更容易流向電極被我們所收集，因此，與薄膜結構感測器相比，二氧化錫奈米柱對於葡萄糖感測有較快的響應時間，如此優越的感測特性揭示了二氧化錫奈米柱是非常有潛力被應用在葡萄糖感測。 最後，亦提出論文結論與針對未來可著重的研究方向。

In this thesis, ZnO and SnO2 nanowires had been demonstrated for the applications in the high performance photoelectric devices and biosensors, including solar cells, field-emission devices, pH sensors, and glucose biosensors. A novel coaxial-structured amorphous-silicon (a-Si) p-i-n solar cell with the low-temperature hydrothermally synthesized AZO nanowires was demonstrated for the first time. The conversion efficiency increased from 3.92 % to 4.27 % when the intrinsic a-Si thickness was increased from 25 to 150 nm and then decreased to 3.66 % when the intrinsic layer thickness was further increased to 250 nm. It was attributed to an excessively thick intrinsic a-Si layer that would decrease the internal electrical field and interfere with charge separation. With the optimum intrinsic a-Si thickness of 150 nm, the conversion efficiency increased from 4.27 % to 4.73 % when the AZO wire length was increased from 1 to 2 μm. Moreover, the proposed coaxial-structured solar cell exhibited a nearly 46 % efficiency enhancement over a conventional a-Si thin-film solar cell. The field emission (FE) properties of low-temperature hydrothermally synthesized AZO nanotip arrays sharpened using a dilute CH3COOH solution were investigated. As compared to the undoped and unsharpened ZnO nanorod arrays, the sharpened AZO nanotip arrays exhibited superior FE properties, including a higher current density, a larger field enhancement factor (β = 3102), a lower turn-on field (2.25 V/µm), and a lower threshold field (3 V/µm). The improvement was attributed to the tip-shaped emitter and Al doping effect of the AZO nanotips. No obvious degradation in the FE current density was observed under a constant applied electric field of 3.08 V/µm for 1800 s, indicating that the AZO nanotip arrays had good emission current stability. An extended-gate field-effect transistor (EGFET) with low-temperature hydrothermally synthesized SnO2 nanorods as the pH sensor was demonstrated for the first time. The SnO2 nanorod sensor exhibited the higher sensitivity of 55.18 mV/pH and larger linearity of 0.9952 in the wide sensing range of pH 1–13 with respect to the thin-film one. The nearly 15 % sensitivity enhancement for such a sensor was attributed to the high surface-to-volume ratio of the nanorod structure, reflecting larger effective sensing areas. The characteristics of the output voltage versus sensing time also indicated good reliability and durability for the SnO2 nanorod sensor. Furthermore, the hysteresis was only 3.69 mV after the solution was changed as pH 7→pH 3→pH 7→pH 11→pH 7. A simple and low temperature method was proposed to fabricate such a coaxial-structured ZnO/silicon nanowire EGFET sensor, based on the electroless metal deposition and atomic layer deposition methods. The transfer characteristics ( – ) of such ZnO/silicon nanowire sensor exhibited the sensitivity and linearity of 46.25 mV/pH and 0.9902, respectively, for the different pH solutions (pH 1–pH 13). In contrast to the ZnO thin-film ones, the ZnO/silicon nanowire sensor achieved much better sensitivity and superior linearity. It was attributed to a high surface-to-volume ratio of the nanowire structures, reflecting a larger effective sensing area. The output voltage and time characteristics were also measured to indicate good reliability and durability for the ZnO/silicon nanowires sensor. Furthermore, the hysteresis was only 9.74 mV after the solution was changed as pH 7→pH 3→pH 7→pH 11→pH 7. Low-temperature hydrothermally synthesized SnO2 nanorods as the enzyme glucose biosensors were demonstrated for the first time. The SnO2 nanorod biosensor exhibited a higher sensitivity of 0.661 mV(mg/dl)−1 in the 60~360 mg/dl glucose concentration as compared with the thin-film one. The nearly sextuple sensitivity enhancement was attributed to a high surface-to-volume ratio of the nanorod structure, reflecting a larger effective sensing area. Moreover, the better crystallinity of the SnO2 nanorods facilitated free carriers flowing to the electrode, resulting in a faster sensing response. Consequently, the superior sensing properties revealed the potential application in glucose sensing using such the SnO2 nanorods. Finally, the summary and conclusions as well as the prospects for the further research were also proposed.