FTIR spectroscopy reveals molecular orientation

Researchers from Osaka Prefecture University (OPU), led by Professor Masahide Takahashi, have used laboratory-grade equipment with 3D-printable optical set-ups to establish an easy, versatile and highly sensitive approach to identify the orientation of molecules and chemical bonds in crystalline organic–inorganic hybrid thin film deposited on a substrate as thin as 10 nm — even film with three molecular layers. Their method has been published in the journal Chemical Science.

The equipment used was a spectrometer that employs a technique called Fourier-transform infrared spectroscopy (FTIR) and polarised infrared light with an originally designed 3D-printed attenuated total reflectance (ATR) unit. FTIR spectrometers are found in most laboratories, in part because they show what molecules are found in a sample, but they have not been able to reveal the three-dimensional orientation of these molecules relative to the substrates. This is important to the manufacturing of thin-film devices that can be nanometres in size, as an unpredicted shift in molecular orientation at that level can cause the entire structure of the device to break down.

Conventionally, in FTIR spectroscopy in transmission configuration, infrared light penetrates from the top of the sample like a skewer. This narrow point of entry and exit does not allow the sample enough interaction with the light to excite the molecules in their chemically bonded states.

“We realised that by reorienting the sample, we could introduce polarised light directly into the substrate of the thin film, generating an evanescent wave that heats up the sample, exciting certain molecules and betraying their orientation,” said Bettina Baumgartner, a visiting researcher on the OPU team.

“We just needed a new kind of sample interface,” added OPU Associate Professor Kenji Okada.

This is where the team designed a brand-new ATR optical set-up that bounces polarised infrared light through the entirety of the sample substrate, allowing them to observe the vibration of all molecules aligned with the electric field component of the infrared light, revealing their orientation. Any lab with a 3D printer can make these ATR optical set-ups.

This method, which the team used to obtain the structural information of metal–organic framework thin film with a degree of crystal orientation comparable to X-ray structural analysis, is expected to be useful in many situations in materials science, such as where orientation control is linked to controlling physical properties, the functional improvement of porous materials used for CO₂ capture and the development of new heterogeneous catalysts.

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