彎曲式與扭曲式原子力顯微術之研究與應用

Abstract

原子力顯微術已廣泛地應用於物質表面結構與機械特性的研究。靜態模式與輕敲模式是AFM最常用的兩種傳統模式。靜態模式，通常易於造成軟性物質的破壞與影響；而輕敲模式，亦稱振幅調制(amplitude-modulation, AM)模式，其力靈敏度與品質因子(Q值)高度相關，所以，這兩種模式存在許多的問題與缺點，尤其是操作於液體環境下。本論文，使用不同於傳統AFM之模式，以得到較高的力靈敏度與較佳的空間解析度，尤其是針對液體環境成像能力提升。這些另類之AFM模式，利用所謂之〝頻率調制 (frequency-modulation, FM)〞偵測技術，此技術可以量測AFM探針之共振頻率的變化。此外，我們亦提供另一激振方式使探針作一新穎之扭曲運動；此種側向之扭曲式(Torsion mode)振動方式，對比於傳統之彎曲式(Flexure mode)振動。 本論文，以實驗的方式，客觀地討論彎曲式與扭曲式兩種動態模式AFM之特性。我們想要了解何種操作模式可以提供較高的力靈敏度與較佳的空間解析度，也同時針對軟性樣品於液體或原始生理環境進行解析能力的比較。實驗上，我們証實頻率調制偵測技術比傳統之振幅調制偵測技術具有較高之力靈敏度。因此，頻率調制模式，可施以軟性樣品表面較小的作用力，因而可以得到較為真實的表面形貌。此優異之表面形貌解析能力，在液體環境中更是明顯。此外，相較於傳統之輕敲式，扭曲激振模式可以提高表面訊息之對比度(contrast)，並可取得更細微之結構訊息。而且，我們更利用實驗室所發展之〝頻率調制扭曲模式AFM( FM-Torsion mode AFM)〞成功地獲得雲母(Mica)原子級表面形貌解析成像。總之，由於〝頻率調制扭曲模式〞之高力靈敏度的特性，可施以軟性樣品表面較小之作用力，以量測表面微小力場變化所造成之表面細微訊息，繼而開創動態模式原子力顯微術新的應用優勢。

Atomic force microscopy (AFM) has been widely used to investigate structures and mechanical properties of materials on surfaces. Static force mode and the so-called tapping mode are two most used operation modes. For operation in static force mode, the AFM tip tends to damage or dislodge the soft materials during scanning. The tapping mode is also named as the amplitude-modulation (AM) mode. Its force sensitivity is dependent on the quality factor (Q-factor) of the oscillating cantilever. Therefore, these two modes have some disadvantages, especially for imaging in aqueous environment. In our work, we test alternative methods to achieve a higher force sensitivity and a better spatial resolution as compared with the conventional AFM modes. In these methods, we use the frequency-modulation (FM) detection scheme, which can track the frequency shift of the vibrating cantilever during scanning. Moreover, a new driving mode, torsional-vibration mode, is excited. In this mode, the cantilever vibrates laterally as compared to conventional vertically vibrating in flexural-vibration mode. We try to make an objective study of the torsional and flexural modes atomic force microscopy. We want to compare which operation mode can provide a higher force sensitivity and a better spatial resolution in ambient environment, especially for soft materials in aqueous or native physical environment. Our results show that the frequency-modulation detection scheme is more sensitive to the conventional amplitude-modulation detection scheme. In the FM mode, the tip exerts a much more gentle force on soft materials and provides a height measurement closer to the true value. Also, the torsional-vibration excitation mode can provide better contrast and more detailed information of the surface region as compared with the tapping mode. The difference is even more prominent in liquid. Moreover, atomic resolution of Mica surface can be obtained with FM-Torsion mode. In summary, by taking advantage of this increased force sensitivity, the FM-Torsion mode allows the measurement of weaker force gradients and opens new applications for dynamic force microscopy.