近年來由於人類科技高度發展以及人口大量增長，對於石油能源需求日益增加，為了減少對石油能源的依賴性，發展可再生能源以取代石油能源需求成為了科學研究中重要的領域之一。在人類生活中，機械能經常透過走路、跑步、點擊滑鼠等形式產生，這些無意間產生的機械能很容易被忽略且日常生活中可源源不斷產生，而利用壓電奈米發電機將機械能收集起來並轉化為電能做使用則是一種常被研究的方法。

氧化鋅作為典型的壓電材料以及具有半導體特性，經常被用來製備壓電奈米發電機，但因其低下的壓電係數(12.4 pm/V)，相較於其他壓電材料遜色不少，因此會透過材料改質的方式來提高壓電輸出，例如摻雜，形成PN接面，基板施加預應力等。此外，孔洞化也是其中一種改質方法，透過孔洞化可以降低壓電材料的楊式係數以及增強應力限制效應，這兩點都可以提升元件的壓電輸出，並且降低壓電材料的楊式係數則可使壓電奈米發電機的設計朝可撓式元件發展，更符合現代趨勢。再者，孔洞化也可以提高氧化鋅薄膜材料的比表面積，這對於氣體感測、光降解的應用是有益的。

本研究中透過醋酸蝕刻的方式製備出多孔氧化鋅薄膜，經由奈米壓痕量測後確認楊氏係數隨著孔洞率增加而下降，且透過AFM電性量測發現在蝕刻時間為5分鐘時薄膜具有最高的壓電係數，其也顯示了適當的孔洞率可以提升材料的壓電性質。在壓電輸出部分，蝕刻5分鐘的氧化鋅薄膜展現出了最大的電壓以及電流輸出，峰值分別可以達到約18mV以及40nA，相較於退火氧化鋅薄膜，分別提升了約4.5倍以及2.35倍。蝕刻時間超過5分鐘時，其輸出呈現大幅下降的趨勢，再次說明孔洞的改質依舊存在最佳孔洞率，並非多多益善。

In this research, we successfully fabricated porous ZnO thin films by acidic etching. By controlling etching time, ZnO films with different degrees of porosity were obtained. From nano-indenter test, the Young’s modulus decreases as the increase of porosity if ignoring substrate effect. The FWHM of ZnO (002) peak increases results from the micro-strain due to etching. From IV curves, the change of Schottky barrier height is largest in etched ZnO films for 5 mins, which implies it has the largest piezoelectric coefficient. Besides, the trend of the force-dependent iv curves for as-grown and etched ZnO films is opposite due to the reverse of polarity. The as-grown ZnO films demonstrate oxygen-terminated polarity while etched ZnO films exhibit zinc-terminated polarity. The reverse of polarity results from the difference of tolerance of etching between Zn-polar and O-polar ZnO grains. The polarity of annealed and etched porous ZnO films is confirmed by XPS. The lattice constant of ZnO decreases due to the pore effect. Finally, we also fabricated the ZnO piezoelectric nanogenerators to check the output. The output of PENG obtained great improvement when etching time is 5 mins. the voltage and current output for etching 5 mins are about 4.5 and 2.35 times of that of as-grown ZnO. However, too much pores will lead to the decrease of piezoelectric output voltages, which implies that the porosity needs to be controlled carefully.

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