The spatial resolution of AFM IR instrument is determined by several criteria: (1) effective volumes of the tip and sample that are interacting; (2) sensitivity of the detection technique; and (3) background signal that will determine the signal-to-noise. The table below shows how PiFM compares with the competing techniques in these areas:
As can be seen, compared to alternative AFM IR techniques, PiFM enjoys favorable operating conditions on all of the above criteria, which provides the basis for its superior performance, both in spatial resolution and surface sensitivity.
Since our Science Advances paper, we have enhanced the spatial resolution of PiFM to the point where we are now able to resolve the different chemical blocks of a PS-b-PMMA sample with a pitch of about 22 nm. Figure 1 shows the PS and PMMA molecules in red and green colors respectively. Cross-sections of the PiFM images for PS and PMMA anti-correlate with each other as they should and show the measured pitch to be about 21 nm (in the cross-section, two pitches are measured). Each polymer molecular block with width of about 11 nm is imaged clearly, and the rise of the signal measures less than 6 nm, which is used by many as the instrument’s spatial resolution. Given that there are straightforward ways to increase the quality factor Q of the cantilever, we are hopeful that the spatial resolution of PiFM will continue to improve.