Cross-sectional IR PiFM Analysis

Multi-layers of different materials find usage across various applications. The thicknesses of the layers range from nanometers to millimeters. Various analytical and microscopy techniques are utilized to study the interfaces of the layers by preparing cross-sections of the samples. To identify the layers or to study the chemical nature of the interfaces, Raman spectroscopy or infrared (absorption) spectroscopy can be used if the layers are thick enough (millimeters for practical purposes). For layers that are sub-micron in thickness, there currently is no viable technique to chemically identify the layers and analyze the interface between the different layers.  IR PiFM with ~5 nm spatial resolution in IR spectral analysis is well suited for such applications. Figure 1 shows a sulfur containing polymer Poly(Sulfur-random-1, 3-diisopropenylbenzene) copolymer (SDIB) capped with a thin layer of Poly(DiethyleneGlycol DiAcrylate) (pDEGDA)  to prevent degradation of the SDIB layer from exposure to ambient condition. Given its high index of refraction, it is formed into a curved shape for optical applications. While low angle neutron scattering can be used to study the interfacial mixing, the presence of the curved surface introduces difficulty. The sample is cast into resin and cyro-sectioned for PiFM analysis. Figure 1 shows the optical micrograph of the region analyzed by PiFM along with topography and PiFM images that highlight the SDIB at 1679 cm⁻¹ and pDEGDA at 1721 cm⁻¹. In order to analyze the interface in detail, a hyPIR image of 128 x 128 pixels at 1 x 1 mm2 scan    size    was    acquired    across    the interface. 30 spectra across the interface with approximately 10 nm spacing are displayed along with topography and a combined PiFM image in figure 2. We can see that there are three regions with distinct strength of the 1734 cm⁻¹ peak; spectra #1 – 6, #7 – 11, #12 and beyond with a sharp jump between 6 and 7 and a gradual reduction from 12 to about 19. The peak at 1679 cm⁻¹ starts to show up by 12 or 13 and reaches full strength by 19 or 20 while the peak at 1512 cm⁻¹ starts to drop from 5 and becomes negligible after 7. One can see that gradual changes in peaks are observable with 10 nm steps, demonstrating the capability of PiFM to elucidate local chemical information with ~10 nm spatial resolution. In figure 3, we plot peak strengths of 1670 cm⁻¹ (B) and 1512 cm⁻¹ (C) divided by the common peak at 1734 cm⁻¹ (A) as a function of distance and see that of the ~140 nm of pDEGDA, there is about 70 nm and 25 nm of mixing with SDIB and the resin, respectively.

Figure 4 shows a topography of a fiber/resin interface across which 30 PiFM spectra, with 10 nm spacing between each spectrum, were acquired. We can see that spectra 1 – 8 and 16 – 30 are distinct, demonstrating the repeatability of PiFM while 9 – 15 display a gradual development of spectral features with each 10 nm step.

Figure 5 shows that PiFM works equally well with inorganics, where 25 spectra with 10 nm spacing are acquired across a cleaved silicon with a trench that is filled with another material. PiFM images along with the 25 spectra show how different materials are distributed.

In summary, we have demonstrated that PiFM can acquire IR spectra that characterize local regions of ~10 nm in size laterally.