Structural characterization of metal-organic framework thin films using infrared scattering-type scanning near-field optical microscopy

抄録

Thin films of metal-organic frameworks (MOFs) have potential applications as gas sensors, filters, and electrocatalysts due to its efficient and specific gas capturing properties. Various types of monolayer and thicker MOF thin films have been fabricated, but the structural details remain unclear. In this study, we fabricated one of the MOF thin film called NAFS-1 using a Langmuir-trough, following the recipe in previous study[1], and carried out the structural characterization of monolayer and thicker films using infrared scattering-type scanning near-field optical microscopy (IR s-SNOM).

For sample preparation, 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrinato-cobalt(II) (CoTCPP) and pyridine were first dissolved in mixed chloroform/methanol solvent. This CoTCPP/pyridine solution was spread onto CuCl₂ subphase, which spontaneously formed the NAFS-1 at the air-liquid interface. The NAFS-1 film was then transferred onto a Si substrate by the horizontal dipping method.

We observed these samples using the amplitude-modulation atomic force microscopy (AFM) and confirmed the fabrication of the gathered islands of NAFS-1 films as shown in Figure 1(a). The thickness of the film is approximately 1.5 nm, which is consistent with the expected film thickness based on the structural model[1].

The same sample was further characterized by IR s-SNOM with a broad-band infrared laser ranging from 1000 to 1700 cm^-1 and a PtIr5-coated tip. Figure 1(b) shows the IR s-SNOM spectra obtained on the NAFS-1 film. Each spectrum was recorded on the spot indicated in the AFM image in Figure 1(a). As indicated by dashed black line in Figure 1(b), these spectra show almost the same characteristics. Three IR s-SNOM spectra show characteristic IR peaks of the NAFS-1 around 1400 cm^-1 (corresponding to C=O symmetric stretching mode of carboxylate ion in paddlewheel structure), 1620 cm^-1 (corresponding to C=O asymmetric stretching mode of carboxylate ion in paddlewheel structure), and 1680 cm^-1 (corresponding to C=O stretching mode of carboxy group). It indicates the nanoscale structural homogeneity of the fabricated NAFS-1 films. These results demonstrate the capability of the IR s-SNOM in characterizing MOF thin films. This technique is expected to be readily applicable to various types of other organic porous thin films. Attribution of the other peaks shown in Figure 1(b) and further experiment to reveal the dependence on the number of layers will also be discussed.

References

[1] R. Makiura et al., Nature Mater. 9, 565 (2010).