How to obtain atomic-scale structural information from technologically relevant nanomaterials using scanning probe microscopy?

抄録

Scanning probe microscopy (SPM) is one of the powerful techniques to investigate structures of material surfaces as well as adsorbed species at the atomic and submolecular scale. It also provides information on local electronic and mechanical properties using spectroscopic measurements of scanning tunneling microscopy (STM) and atomic force microscopy (AFM). However, one of the weak points of the high-resolution SPM is that the sample must be atomically flat and homogeneous. It is thus challenging to observe samples such as nanoparticles, porous materials, and nano-scale devices. Such a problem is known as the “materials gap.” To overcome this issue, our group is currently developing methodologies for the characterization of samples prepared in solution processes such as oxide nanoparticles, carbon black particles, and porous organic thin films. In this talk, I will discuss our recent study on porous organic thin film using atomic force microscopy (AFM) and molecular dynamics (MD) simulation.

Two-dimensional metal-organic frameworks (2D-MOFs), covalent-organic frameworks (2D-COFs), and hydrogen-bonded organic frameworks (2D-HOFs) are useful materials for various applications such as adsorbents, separation membranes, catalysts, and sensor devices. Structural characterization is one of the keys to the improvement of crystallinity and properties, but obtaining atomic-scale structural information on organic porous films, in both horizontal and vertical directions, is challenging. In this work, we prepared hydrogen-bonded organic thin films at the air/liquid interface using a Langmuir trough (Fig. 1 (a), (b)) and characterized them using ambient AFM. The evidence of A-A stacked honeycomb structures was obtained by resolving periodic pores over the films with a thickness variation of more than several nanometers (Fig. 1 (c), (d)) [1]. AFM images of samples prepared with different parameters provided a hint for understanding the film formation mechanism. Our expectation was supported by molecular dynamics simulations (Fig. 1(e)) [2], which revealed that the molecules are self-assembled without external forces in the quasi-two-dimensional system. In addition to hydrogen bonding and π-π interaction, long-range Coulomb interaction was found to play a vital role in the film growth.

This study was a good example of applications of ambient SPM measurements of solution-processed samples to extract atomic-level information.

References

[1] Yamanami et al., Langmuir 38, 1910 (2022).

[2] Matsui et al., AIP Advances 12, 105109 (2022).