Towards atomic resolution non-contact dynamic force microscopy in a liquid

Abstract

Initially, calcite surfaces were imaged with regular tapping mode atomic force microscopy (AFM) at nearly true atomic resolution with a soft cantilever (k = 0.2 N m -1 ) resonantly oscillating in a liquid. Now, at least the same imaging quality in water could be achieved by driving a rather stiff (k = 20-25 N m -1 ) lever at ultrasonic frequencies of ∼ 150 kHz and selecting the working point in quite a small region on the amplitude vs. distance curve where the amplitude steeply increases with decreasing tip-sample distance. This amplitude increase by ∼10% over the lever's free amplitude in the liquid occurs just before the onset of the regular tapping (intermittent repulsive contact) regime, where the amplitude rapidly falls off upon further tip-sample approach. Thus, very gentle forces were utilized here for imaging, probably even non-contact forces, where the use of relatively stiff levers eliminates uncontrollable jump-into-contact effects and demonstrates the high sensitivity of this particular dynamic force microscopy (DFM) mode. Constant interaction images of the amplitude signal at this working point, where the feedback response is inverted compared to regular tapping, revealed defects on the atomic scale but as yet not with clear-cut true atomic resolution. These results could represent an important step towards reliable atomic resolution non-contact imaging in a liquid with feedback-control, which is essential for the goal of atomic scale imaging on corrugated and delicate samples such as biomolecules.