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Force detection of IR response at sub-10nm resolution
Wickramasinghe, H., Park, S.
SPIE Newsroom
Abstract
FIGURE 1. Spectral photoinduced force microscopy (PiFM) images of a polystyrene-poly(methyl methacrylate) (PS-PMMA) block copolymer sample with a pitch of 40nm. Left: Image at 1492cm_-1_. Center: Image at 1732cm_-1_. Right: Topography of sample. PiFM at 1492 and 1732c_-1_ images the PS and PMMA polymer components selectively. Figure 2 [/news/6170-force-detection-of-ir-response-at-sub-10nm-resolution#fig2] shows topography, AFM phase, and PiFM images of collagen molecules at 1754 and 1666cm−1. In Figure 2 [/news/6170-force-detection-of-ir-response-at-sub-10nm-resolution#fig2](c)—the PiFM image taken at 1754cm−1—the collagen molecules appear dark, showing a stronger signal from the mica background. The dark patterns associated with the collagen molecules correlate well with the topography, shown in Figure 2 [/news/6170-force-detection-of-ir-response-at-sub-10nm-resolution#fig2](a), and with phase—see Figure 2 [/news/6170-force-detection-of-ir-response-at-sub-10nm-resolution#fig2](b)—acquired at the same time. When we tuned the excitation laser to the amide I band associated with the collagen molecules (1666cm−1), the bright features (the stronger dipole-dipole force as anticipated) associated with the collagen molecules appear broader in width than the topography or phase features. Note that the PiFM image at 1666cm−1—see Figure 2 [/news/6170-force-detection-of-ir-response-at-sub-10nm-resolution#fig2](d)—is slightly shifted from the images in Figure 2 [/news/6170-force-detection-of-ir-response-at-sub-10nm-resolution#fig2](a–c) because of thermal drift. This broadening is well observed for the single collagen molecule running vertically within the center of the highlighted region. The dark regions of the undulating signal running along the center of the collagen features in the PiFM image—see Figure 2 [/news/6170-force-detection-of-ir-response-at-sub-10nm-resolution#fig2](d)—correlate with tall features of topography and phase. We do not yet understand the significance of this result. However, this set of images demonstrates the potential strength of PiFM in studying the chemical composition and molecular structure of biological specimens with extreme sensitivity and ultra-high spatial resolution, without the need for labeling.