Advances in Nanoscale Infrared Spectroscopy to Explore Multiphase Polymeric Systems

Our group focuses on advancing the field of nanoscale functional imaging and spectroscopy to study complex systems with an emphasis on nanoscale infrared spectroscopy. Our research aims to develop methods that allows us to make sense of the behavior of heterogeneous systems by studying their nanoscale properties. Understanding the role of actuations at different frequencies to extract information about the sample is essential as this is complicated developing samples that can be modeled with physics simulation packages is also important.

A nanoscale infrared spectroscopy determining the penetration depth of the measurement and understanding how subsurface features affect the signal detected by the AFM tip is very important. This protocol proposes a simple approach to create a model system that enables quantifying the penetration depth of nanoscale infrared spectroscopy, and the effect of JUUL heating on the sample response involved in the measurement of multiphase polymer materials. Our team will continue to focus on developing new ideas to improve nanoscale infrared spectroscopy and other nanoscale functional tools to explore real-life systems at those small scale.

To begin, clean a silicon substrate using isopropyl alcohol and let it air dry. Using a pipette, place one microliter of polystyrene beads suspended in water onto the center of the substrate. Place the polystyrene beads added substrate in a storage compartment containing bentonite clay desiccant to allow the water to evaporate.

Examine the dried polystyrene beads substrate and a previously prepared polyvinyl alcohol or PVA-coated substrate under an optical microscope. Using ultra fine tweezers, gently loosen the beads, then collect a few beads using a fine hair paint brush, and gently tap the hairs of the paint brush over the freshly PVA-coated wafer. Repeat collecting the beads until individual polystyrene beads adhere to the PVA surface as confirmed by optical microscopy.

Deposition of the polystyrene beads was confirmed by scanning electron microscopy imaging of the five-micrometer bead on top of a pristine silicon substrate, on top of a PVA film, and covered with PVA. For nano IR measurements, position the atomic force microscope or AFM tip on the feature of interest identified from the topography image. Select the tuning fork icon in the nano IR control panel to determine the contact resonance frequencies of the cantilever.

Then set an illumination wave number to excite photothermal expansion in the material. Next, set a range of laser pulse frequency to sweep and set the duty cycle of the nano IR laser. Select acquire within the laser pulse tune window.

Position the marker bar at the peak to select the second contact resonance of the tip sample system for nano IR measurements. Click on the optimize button to align the center of the IR laser focal region with the position of the cantilever tip. Acquire IR laser illumination background.

Select the wave number range, step size, and number of averages for the nano IR spectrum. Then perform a background correction of the spectra by dividing the photothermal amplitude by the attenuated background. Enable phase locked loop or PLL autotune in the laser pulse tune window.

Then adjust the maximum and minimum frequency to create a sweep range centered at the second resonance mode in the general control panel. Click on zero in the PLL control panel and then click on OK in the laser pulse tune window. Enable IR imaging by selecting the checkbox of IR imaging enabled in the nano IR control panel.

In the imaging view control panel, choose height, amplitude 2, and phase 2 to acquire the topographical and chemical images of the sample. Then set the acquisition direction to trace or retrace. Select the scan icon in the AFM scan control panel.

Then select the now or end of frame icon in the capture control panel to save the image. To export the data, right click on the image or spectrum file names within the data lists. Select export, and then choose the file format to export.

Finally, save the file in the desired computer folder. The nano IR spectrum of polystyrene yielded two IR bands corresponding to the stretch mode of the phenyl moiety at 1600 wave number and a subset of the ring stretching at 1730 wave number. The nano IR spectrum of PVA exhibited better agreement with the FDIR spectrum with a predominant absorption band centered at 1730 wave number.

The nano IR spectra were used to select the illumination wave numbers for chemical imaging of the polystyrene bead deposited at the surface of PVA and of the polystyrene bead coated with PVA. Next, spectra were collected on top of a polystyrene bead covered with PVA at different laser powers. The polystyrene signal at 1600 wave number was significantly lower than that of PVA at 1750 wave number.

However, it was noted that increasing the laser power led to a higher ratio.