The most high resolution AFM images are obtained in vacuum by Frequency-Modulation AFM (FM-AFM). Recently, Fukuma et al. (2005) succeeded in obtaining true atomic resolution by FM-AFM in spite of liquid environment [1]. Subsequently, not soluble crystals but insoluble crystals have been observed in liquid by FM-AFM. Dissolution and growth processes are inconvenience for AFM observation. However, such processes at atomic levelare very important for crystal growth. Therefore, we tried to observe several soluble crystals in liquid by FM-AFM [2].

FM-AFM images were obtained using a modified commercial AFM (Shimadzu, SPM-9600) with open fluid cell. We intentionally observed soluble crystals in undersaturated solution at first. The concentration of solution gradually increased because of evaporation of water. Therefore, dissolution, near equilibrium, and growth conditions could be observed with increasing elapsed time. By using this method, crystal growth speed of vertical direction was suppressed and atomic resolution images could be obtained.

Figure 1 shows a KCl(100) cleaved surface in solution. We could observe a periodic structure, in which the wavelength is 6.29 Å. Therefore, only one kind of atom was probablyimaged in liquid environment as well as ultrahigh vacuum environment.

Figure 2 shows amolecular resolution image of lysozyme (110) in solution[2]. The surface unit cell (a black rectangle in fig. 2b, 11.2 × 3.8 nm) involves four molecules with the four unique orientations, which make two kinds of zigzag structures (circles and triangles) along the [001] direction. Figure 2 shows the individual four molecules in the unit cell and the image has higher resolution than images obtained by conventional contact mode or amplitude-modulation (tapping) mode AFM [3]. In addition, we could observe admolecule and point defect on the lysozyme(110) face (fig. 3).

Acknowledgments

We thank Prof. S. Morita of Osaka University, M. Abe of Nagoya University, and Shimadzu Corporation for observation of AFM.

[1] T. Fukuma rt al., Appl. Phys. Lett. 87, 034101 (2005).

[2] K. Nagashima et al., J. Vac. Sci. Technol. B 28, C4C11 (2010).

[3] J. H. Konnert et al., Acta Crystallogr. D 50, 603 (1994).

Fig.1 An atomic resolution image of KCl (100) surface in saturated solution. Crystal growth speed in the vertical direction V < 0.005 nm/s.

Fig.2 (a) Molecular resolution image of lysozyme(110) face in saturated solution. The image was already corrected for XY drift. (b) theoretical molecular packings on (a).

Fig.3 Admolecule and point defect on the lysozyme(110) face in saturated solution.

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