Abstract
A quartz tuning fork (QTF) has been widely used as a force sensor of the frequency modulation atomic force microscope due to its ultrahigh stiffness, high quality factor and self-sensing nature. However, due to the bulky structure and exposed surface electrode arrangement, its application is limited, especially in liquid imaging of in situ biological samples, ionic liquids, electrochemical reaction, etc. Although the complication can be resolved by coating insulating materials on the QTF surface and then immersing the whole QTF into the liquid, it would result in a sharp drop of the quality factor, which will reduce the sensitivity of the QTF. To solve the problem, a novel method, called the balanced trolling quartz tuning fork (BT-QTF), is introduced here. In this method, two same probes are glued on both prongs of the QTF separately while only one probe immersed in the liquid. With the method, the hydrodynamic interaction can be reduced, thus the BT-QTF can retain a high quality factor and constant resonance frequency. The stable small vibration of the BT-QTF can be achieved in the liquid. Initially, a theoretical model is presented to analyze the sensing performance of the BT-QTF in the liquid. Then, the sensing performance analysis experiments of the BT-QTF have been performed. At last, the proposed method is applied to atomic force microscope imaging different samples in the liquid, which proves its feasibility.
Highlights
Since the atomic force microscope (AFM) was introduced in 1986 [1], it has been an important tool in nanotechnology to simultaneously achieve high-resolution topography imaging [2,3] and quantify the physical properties of samples [4,5] in various environments
During the frequency modulation atomic force microscope (FM-AFM) imaging, the BT-quartz tuning fork (QTF) works at its real-time resonance frequency and scans over the sample
The vibration amplitude of the QTF in the FM-AFM is about 100 pm [27], which is far less than the capillary length κ −1 (The capillary length is in millimeters for a tungsten probe with a diameter of 100 μm in pure water)
Summary
Since the atomic force microscope (AFM) was introduced in 1986 [1], it has been an important tool in nanotechnology to simultaneously achieve high-resolution topography imaging [2,3] and quantify the physical properties of samples [4,5] in various environments. Frequency modulation atomic force microscope (FM-AFM) is becoming a better choice to image samples in the liquid because of its super high resolution and force sensitivity [9]. The Q-factor of the cantilever will be reduced by more than two orders of magnitude, which means a severe decrease in the sensitivity and the measurement accuracy This is mainly caused by the hydrodynamic interaction between the cantilever and the liquid [11]. When a QTF is used as a force sensor of the AFM, a probe is glued on one of its prongs and the probe-sample interaction results in the changes of QTF’s resonance frequency f 0 and vibration amplitude A. The images of actual samples have been obtained in the liquid with the proposed method to prove its feasibility
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