Surface Sensitivity of PiFM

Since photo-induced force (PiF) is generated from the tip-enhanced field, which extends only about 20 nm from the apex of the AFM tip, PiFM enjoys excellent surface sensitivity along with ~5 nm spatial resolution. PS(polystyrene)-b-PTMSS[poly(4-trimethylsilylstyrene)] block copolymer (PS-PTMSS BCP) with horizontal lamellae is used to demonstrate the surface sensitivity. BCP is an excellent sample to demonstrate both spatial and depth resolution since the thickness of each block component can be controlled precisely by adjusting the molecular weights of the components. In this case, the full pitch L₀ of the BCP is ~22 nm as shown in figure 1. One sample with island features with PTMSS at the top (shown in the top row of figure 1) and another sample with holes with PS at the bottom (shown in the bottom row of figure 1) were prepared on silicon substrate.

Figure 2 shows what a conventional AFM would produce as topography and phase images. Note that the height of the island features and the depth of the hole features measure about 11 nm, as expected since they should measure half of the full pitch L₀. While the height (or depth) can be confirmed, AFM topography and phase cannot determine whether the top of the island (or bottom of the hole) structure is composed of PTMSS (or PS). In fact, it will be difficult to find an analytical technique that combines the surface chemical sensitivity and the nanoscale spatial resolution that are required to shed light on the chemical nature of the molecules associated with this type of structures.

Figure 2 shows PiFM images acquired at 1493 cm⁻¹ for PS molecules (colored red) and 1599 cm⁻¹ for PTMSS molecules (colored green) along with the combined PiFM images overlaid onto the 3D topography for both sample types. One can see that for island features (top row), PS (red) molecules cover regions surrounding the features while PTMSS (green) molecules cover the features. One can also see that for the hole features (bottom row), PS (red) molecules cover the bottom surface of the features while PTMSS (green) molecules cover regions surrounding the features. Thus, PiFM can identify which molecules cover the different regions even though the top layer that we are measuring is only 5.5 nm thick and there are more of the same molecules in the adjoining regions, buried by only 5.5 nm; this goes to show that PiFM derives most of its signal from the top few nanometers even though PiF extends down to ~20 nm depth.

Looking at the PS (red) image at the top row of figure 2, the contrast seems to indicate that there is no PS molecules associated with the island structure. In fact, we know that there are PS molecules 5.5 nm below the surface. However, if we instead locate the AFM tip on top of the island structure and acquire a PiFM spectrum, we should see infrared (IR) signatures of both PTMSS and PS since PiF extends down to ~20 nm.

Figure 3 shows PiFM spectra associated with two different bi-layer samples, one with PS on top of PTMSS and another with PTMSS on top of PS. One can see that when there is a layer of PTMSS on top of PS, the two PS absorption bands (at 1452 and 1492 cm⁻¹) are still seen in the PiFM spectrum (blue spectrum) albeit greatly reduced when compared to the spectrum acquired with PS on top of PTMSS (orange spectrum). The PTMSS peak at 1599 cm⁻¹ shows a similar behavior with a sharp reduction in its peak strength when PS is on top of the PTMSS layer. For both PS and PTMSS, the peak strength drops by about ~80% when there is 5.5 nm of an intervening polymer layer, demonstrating the excellent surface sensitivity of PiFM.

Two-dimensional crystals of organic semiconductors (2DCOS) have attracted attention for large-area and low-cost flexible optoelectronics. Figure 4 shows topography, PiFM images at 1730 cm⁻¹ and 1519 cm⁻¹, and cross-section profile of the topography of 2DCOS, 2,6-bis(4-hexylphenyl) anthracene (C6-DPA) formed on top of a PMMA film. The measured C6-DPA thickness is around 2.5 nm, indicating it could be a single layer.  The image at 1730 cm⁻¹ highlights the PMMA substrate nicely while the image at 1519 cm⁻¹ highlights the C6-DPA well.  Again, the contrast obtained for both materials are excellent due to the surface sensitivity of PiFM even for such thin layers. When we zoom into a single flake of C6-DPA (figure 5), nanoscale variations are observed in the signal strength of 1519 cm⁻¹ band, suggestive of chemical variations on the scale of ~10 nm.

We would like to thank Michael Maher and Prof. Grant Wilson of Univ. of Texas at Austin for the PS-PTMSS samples. We would like to thank Prof. Lang Jiang and Dr. Mingchao Xiao of ICCAS for the 2DCOS sample.