Nanoscale Chemical Mapping with Extreme Surface Sensitivity

Extreme surface sensitivity is expected to be provided by PiFM, as dipole-dipole interaction between the image tip dipole and the induced sample dipole will change with ˜ 1/z⁴ dependence, where z refers to the tip-sample spacing. The surface sensitivity is demonstrated using PS (polystyrene)-b-PTMSS [poly (4-trimethylsilylstyrene)] block copolymer (PS-PTMSS BCP) with horizontal lamellae. BCP is a great sample to demonstrate both depth and spatial resolution, as each component’s thickness can be precisely controlled by altering the molecular weights of the components. The pitch (L₀) of the BCP, in this case, is ~22 nm, as depicted in Figure 1a. One sample having island features with PTMSS at the top (Figure 1b shows Sample-I) and another sample having holes with PS at the bottom (Figure 1c shows Sample-H) are prepared on a silicon substrate.

Figure 1. Block Co-polymer (BCP) sample description. The island and hole features (shown in (b) and (c) respectively) have a half-full pitch height/depth of 11 nm; each chemical block is only 5.5 nm thick.

Figure 2. Topography, cross-section and phase images of Sample-I and Sample-H, showing the experimental results match the design of the BCP. The island and hole heights shown, at 10.83 nm and 10.71 nm respectively, agree very well with predicted model of 11 nm half-full pitch height/depth.

Topography and Phase Images of Samples I and H

Figure 2 indicates the type of phase images and characteristic topography formed on these two samples by a standard AFM. It should be noted that the depth of the hole feature and the height of the island feature measure about 11 nm, as shown in Figure 1b and 1c. Although the height or depth can be confirmed, AFM topography and phase cannot find out whether the top of the island structure is formed of PTMSS. Likewise, from AFM measurements it is not possible to determine whether PS is the bottom of the hole structure. Actually, it is hard to find an analytical technique that integrates the nanoscale spatial resolution with the surface chemical sensitivity that are needed to gain an insight into the chemical nature of the molecules related to these types of structures.

PiFM Spectra and PTMSS/PS Homopolymer Samples

The PiFM spectra related to PTMSS and PS Homopolymer films on silicon substrate are shown in Figures 3a and 3b, respectively. It is evident that PiFM must be able to highlight the two different polymer molecules by using 1493 cm^-1 for PS and 1599 cm^-1 for PTMSS as IR excitation light.

Figure 3. PiFM spectra for PTMSS and PS Homopolymer samples. The vibrational bands at 1599 cm^-1 (a) and 1493cm^-1 (b) will be used to identify the chemical species PTMSS and PS molecules respectively.

Identification of PS and PTMSS Molecules

PiFM images obtained at 1493 cm^-1 and 1599 cm^-1 along with topography for both Sample H (bottom row) and Sample I (top row) are shown in Figure 4. The color blue is used to indicate the PTMSS molecules (imaged at 1599 cm^-1) and the color red is used to indicate the PS molecules (imaged at 1493 cm^-1) in accordance with the colors used for the sketches in Figure 1. For Sample I, areas adjacent to the island features are covered by PS molecules (recognizable by taller aspects in topography) while the island is covered by PTMSS molecules. Also for Sample H, the hole features are covered by PS molecules (recognizable as depressions in topography) while areas adjacent to hole features are covered by PTMSS molecules. Thus, PiFM has the ability to recognize the molecules covering different regions, although the layer under measurement is only 5.5 nm thick. Even though the PiFM images of Sample I show that there are no PS molecules related to the island features, it is known that there are PS molecules below the top PTMSS molecules. From other experiments where PiFM’s thickness dependence was calculated, it known that about ~ 15 nm of the sample depth can be probed by PiFM. Although the images show the absence of PS molecules, it is only because of the contrast based on relative signal strength.

Figure 4.

Left figures show cross-sectional cartoon view of the BCP. The following images are surface topography and PiFM images at 1493 cm^-1 and 1599 cm^-1 to identify PS and PTMSS molecules. PS and PTMSS molecules are colored artificially red and blue respectively, to be consistent with the cartoon colors.

For example, for the island geometry (top row), where the topography is high (white), the PiFM chemical map shows the presence of PTMSS represented in blue. For the hole geometry (second row), where the topography is low (black), the PiFM chemical map shows the presence of PS represented in red.

PiFM Spectra of Bilayer Samples

Figure 5 shows the PiFM spectra related to two different bi-layer samples, one with PTMSS on top of PS and another with PS on top of PTMSS. It can be seen that even in the condition of PTMSS on top of PS, the two PS absorption bands are clearly observed in the spectrum albeit extensively reduced from that of PS on top of PTMSS. The quick reduction in the PiFM signal is because of the dipole-dipole nature of PiFM, resulting in its extreme surface sensitivity.

Overlaying Chemical Map on 3D Topography

Since topographically data are acquired simultaneously with PiFM images, the chemical map (with the color designation of molecules) can be overlaid on top of 3D topography of the sample. Such renderings are indicated in Figure 6 for Sample I and Sample H, clearly demonstrating the 3D structure of the samples.

Figure 5. . Sample A has PS on top, and Sample B has PS buried under the 5.5 nm thick PTMSS monolayer. The signal strength at wave numbers 1452 cm^-1 and 1492 cm^-1, which are the vibrational bands for PS, is very strong for Sample A versus Sample B. The precipitous reduction in signal strength of Sample B at those wave numbers demonstrates the surface sensitivity of the PiFM method; even though the PS is buried under only one monolayer, a significant signal drop is easily detected.

Figure 6. PiFM chemical map data overlaid on AFM 3D topography renderings of both sample types, match the model shown in the cross-sectional cartoon.

This information has been sourced, reviewed and adapted from materials provided by Molecular Vista.

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