Parallel Angle-resolved XPS Analysis of SAMs with the Theta Probe

Self-assembled monolayers or SAMs of alkanethiols on gold create organic layers. Chemical characteristics of organic layers are ascertained by the functional groups on the thiol molecule. Possible applications comprise protective layers, lubricants, transducers, and sensor technology.

In addition, these materials exhibit a role as prototype systems for biological interfaces. They are capable of forming very thin, even layers. These films have layer thickness, which complements the sampling depth of XPS and angle resolved XPS (ARXPS). Hence, Thermo Scientific’s Theta Probe instrument is ideal for investigating these materials.

Specimen

First, a metal film was evaporated onto a silicon wafer to prepare a gold surface, and next dodecanethiol (C12H25SH) was deposited on the gold surface to prepare SAM. The structure is shown in Figure 1.

Self assembled monolayer on a gold substrate, (in this case R = C5H9).

Figure 1. Self assembled monolayer on a gold substrate, (in this case R = C5H9).

Analysis

The Theta Probe instrument was used for studying this material by means of parallel angle-resolved XPS method, and the entire data were obtained using a 400µm spot of Al Ka X-rays from a micro- focusing monochromator.

Subsequently, XPS spectra were obtained from the C 1s, S 2p and Au 4f regions in 16 angular channels, each measuring 3.75° wide. Using the relative depth plot, the arrangement of the elements in the layers was validated. The relative depth plot is a feature of the Avantage data system. This is illustrated in Figure 2. The layer’s thickness was measured by means of the multi-layer thickness calculator, which is also a part of the Avantage data solution.

Relative depth plot for the SAM on gold.

Figure 2. Relative depth plot for the SAM on gold.

It was observed that the thickness of the SAM layer was 1.6 nm. For the dodecanethiol molecule, the chain length is approximately 1.8nm. In case the quantified layer thickness is accurate, it indicates that the molecules are sloping by approximately 27° from the surface normal. This value thus obtained is close to the value specified in the literature. However, the C 1s signal may include certain carbonaceous impurity, if present in the layer.

Based on the ARXPS, a non-destructive depth profile (Figure 3) was built by utilizing a technique that involves Maximum Entropy methods. This profile displays excellent depth resolution and regenerates the thickness of the SAM layer precisely.

Non-destructive depth profile of the self-assembled monolayer.

Figure 3. Non-destructive depth profile of the self-assembled monolayer.

Conclusion

ARXPS serves as a powerful research tool for the characterization of SAMs. It is possible to determine molecular tilt angles and layer thickness. These results obtained are similar to the values reported in the literature. It was observed that the depth profile, which was constructed from the ARXPS data, precisely reflects the specimen’s structure.

This information has been sourced, reviewed and adapted from materials provided by Thermo Scientific – Surface Analysis and Microanalysis.

For more information on this source, please visit Thermo Scientific – Surface Analysis and Microanalysis.

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