本研究利用原子力顯微鏡的技術，來探討奈米尺度下，脈衝雷射沉積法所成長的氧化鋅薄膜其表面摩擦力對探針掃描速度的關係。我們發現氧缺陷的存在對於氧化鋅表面的摩擦性質扮演著重要角色。在低相對溼度的環境下，由於熱擾動造成的黏滑效應之影響，摩擦力與探針掃描速度呈正相關。然而，在高相對溼度的環境下，探針與氧化鋅表面的摩擦力在掃描速度小於2.7μm/s時與掃描速度呈現負相關，而在大於2.7μm/s時則呈正相關。這是由於氧缺陷可以吸附大氣中的水分子，因此在低速時，水分子有足夠的時間可以在探針與樣品粗糙表面間的空隙形成毛細水橋，使得摩擦力增大。但隨著速度增加，毛細水橋對摩擦力造成的效應將漸漸降低，使得摩擦力隨速度增加而減小。當毛細水橋不再有足夠時間形成時，摩擦力則再度與速度呈正相關。接著我們又利用光催化效應來操控氧化鋅表面的親疏水性。氧化鋅表面在被波長為365 nm的紫外光照射後將由疏水性轉變為親水性。此時所量得的摩擦力不僅大幅加，並且在不同濕度下，摩擦力對掃描速度皆呈現負相關。這是因為光催化反應可以大幅增加氧化鋅表面的氧缺陷，促進更多水分子的吸附，讓毛細水橋更容易在探針與氧化鋅間形成，進而影響了摩擦力對速度的關係。最後，利用光催化反應，氧化表面的溼潤性的轉變具有可逆性，因此我們可以利用此性質調控氧化表面摩擦力對速度的變化關係，這對未來利用氧化鋅為奈米元件材料的奈米機電系統將有應用的潛力。

In this work, we investigate the velocity dependent friction on pulsed laser deposited (PLD) Zinc Oxide (ZnO) thin films by using atomic force microscopy (AFM). We found that the oxygen vacancies present in ZnO surface play an important role in the frictional properties of ZnO. At low relative humidity (R.H.), the measured friction shows a positive dependence on the sliding velocity due to the thermal activated stick-slip effect. At high R.H., when the sliding velocity of the AFM probe is less than 2.7 μm/s, the measured friction has a negative dependence on the sliding velocity, but becomes positively dependent when the velocity is larger than 2.7 μm/s. This is because oxygen vacancies present in ZnO surface tend to absorb water molecules from the ambient. Therefore, at low sliding velocity, water meniscus has sufficient time to form at the tip-ZnO contact, leading to the enhancement of friction force. With the increasing velocity, there will no sufficient time for water bridges to form, thus the friction will show positive dependence on the sliding velocity. In addition, we convert the wettability of ZnO surface from being hydrophobic to hydrophilic via the photo-catalyst effect. UV light with 365 nm wavelength is used to illuminate the ZnO surface. After UV exposure, the measured friction forces not only increase significantly but also show a negative dependence on the sliding velocity for all investigated R.H.. The photo-catalyst effect can generate excess oxygen vacancies on the surface, giving rise to the enhancement of water absorption that further promotes the formation of water bridge at the tip-ZnO contact. Furthermore, since the wettability conversion using photo-catalyst effect on the ZnO surface is repeatable, we might be able to use this method to control the velocity dependent friction of ZnO based nano-devices.

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