Development of High-Resolution Imaging of Solid–Liquid Interface by Frequency Modulation Atomic Force Microscopy

Abstract

The high-resolution imaging technique used in liquid environments involving dynamic mode atomic force microscopy (AFM) with a frequency modulation (FM) detection technique has been newly developed on the basis of a commercial atomic force microscopy (AFM) apparatus, which is, generally, extremely difficult because of the large decrease in Q-factor caused by hydrodynamic damping in liquids. Through various improvements and optimization, the noise density of the improved deflection sensor was 29 fm/ ffiffiffiffiffiffi Hz p (f 0 ¼ 141 kHz) in liquid. In addition, the noise level and bandwidth of the FM detector were improved to 6 mHz/ ffiffiffiffiffiffi Hz p and 6 kHz, respectively. Thermal drift was successfully regulated at less than 1 nm/min in air for a sufficiently long time without any special air conditioning treatments. As a result, we succeeded in obtaining highresolution images of polypropylene sheet, Au thin film, and DNA structures in a buffer solution. Therefore, liquid environments are proved to be suitable for high-resolution imaging by FM-AFM under ultrahigh vacuum (UHV) condition. #

Measurement and modelling of non-contact atomic force microscope cantilever properties from ultra-high vacuum to normal pressure conditions

“…Figure 2 shows the normalized shift f/f 0 of the resonance frequency as a function of pressure p compared to theory. As evident from equation (6), the pressure-dependent shift of the resonance frequency depends on the ratio of length and thickness of the cantilever. For the dimensions of cantilever 1, this ratio is about three times larger than for the other cantilevers explaining the higher sensitivity of the resonance frequency to air damping.…”

“…Cantilevers 1, 3 and 4 are of similar length, 3 and 4 have similar resonance frequencies but all cantilevers differ from each other significantly in thickness. In table 2, the resonance frequency f 0 measured in UHV is compared to f air,exp 0 measured under ambient pressure conditions and a theoretical value f air,theo 0 calculated using equation (6). The theoretical resonance frequency in UHV is not mentioned here because an accurate calculation requires precise knowledge of the tip mass.…”

“…Due to the reduction in the Q-factor, the force gradient detection sensitivity is strongly diminished [3]. However, it has been shown that the detection sensitivity for NC-AFM imaging in air and liquids is high enough to allow for atomic resolution imaging provided that the opto-electronic detection system is optimized to achieve its technical limits [4][5][6]. The quality of NC-AFM images obtained in air can further be enhanced by applying advanced experimental techniques such as robust feedback loops [7], Q-control [8] or bimodal detection [9].…”

Unraveling DNA dynamics using atomic force microscopy

“…The development of ‘high‐speed AFM’ enabled researchers to image dynamic process of single molecules with close to millisecond temporal resolution 28–30. It is noteworthy that recent advances in frequency modulation AFM (FM‐AFM) achieved further high resolution imaging of DNA strands in liquid 31. The periodic structure with a 3.5 nm pitch, which consistent with the periodicity of the double‐helical DNA, can be recognized (Figure 1(d)).…”

The structure of uniaxially stretched isotactic polypropylene sheets: Imaging with frequency-modulation atomic force microscopy

“…A number of soft materials have been imaged with liquid-compatible FM-AFM to date, including crystalline p-nitroaniline [21], rubrene [22], lysozyme [23], dodecanol adsorbed on graphite [24], hydrophilic thiolate monolayers on gold [25,26], lipid bilayers [27], purple membranes [28], peptide nanotubes [29], antibodies [30], and DNA [31]. As synthetic polymers, commercial sheets of polyethylene [32] and polypropylene [33] have also been imaged in liquids. In these previous works, the molecular-resolution images were obtained without using a Fourier filter.…”