Parallel Angle-resolved XPS Analysis of a Self Assembled Monolayer with the Theta Probe

Importance of the Topic

Self-assembled monolayers (SAMs) of alkanethiols on gold surfaces represent a class of ultra-thin organic coatings with applications in sensor technology, surface protection, lubrication, and as model biological interfaces. Their uniform thickness and well-defined molecular architecture make them ideal subjects for surface-sensitive analytical methods. Angle-resolved X-ray photoelectron spectroscopy (ARXPS) matches the sampling depth of these films, enabling precise, non-destructive investigation of layer composition, thickness, and molecular orientation.

Objectives and Study Overview

This study aims to evaluate the capabilities of the Thermo Scientific Theta Probe instrument for characterizing a self-assembled monolayer of dodecanethiol on a gold substrate using parallel ARXPS. The goals include determining the film thickness, estimating the molecular tilt angle, confirming elemental ordering within the layer, and constructing a non-destructive depth profile.

Methodology and Instrumentation

A gold film was evaporated onto a silicon wafer, followed by deposition of dodecanethiol (C12H25SH) from solution to form the SAM. Analysis was performed on the Theta Probe system with a microfocusing monochromator delivering Al Kα X-rays to a 400 µm spot. XPS spectra were acquired for C 1s, S 2p, and Au 4f regions across 16 angular channels, each spanning 3.75°. The Avantage data system provided relative depth plots for elemental ordering, while a multi-layer thickness calculator determined film thickness. Non-destructive depth profiling employed a Maximum Entropy method as detailed in application note 31014.

Main Results and Discussion

• Layer thickness measured at 1.6 nm, compared to the theoretical molecular length of 1.8 nm, indicating a tilt angle of approximately 27° from the surface normal, consistent with literature reports.
• Relative depth plotting confirmed the expected sequence of sulfur and carbon signals over the gold substrate.
• The non-destructive depth profile achieved high resolution, accurately reproducing the monolayer thickness and internal structure without sputter damage.

Benefits and Practical Applications

• Non-destructive determination of film thickness and molecular orientation in ultra-thin organic layers.
• Accurate depth profiling without sample alteration, suitable for delicate or reactive surfaces.
• Broad applicability to sensor development, surface coatings, and model biointerfaces.

Future Trends and Potential Applications

As ARXPS techniques and data analysis methods evolve, enhanced depth resolution and real-time monitoring of film formation are expected. Integration with in-situ environments and advanced computational modeling will expand applications to multilayer assemblies, responsive coatings, and dynamic interfacial processes.

Conclusion

Parallel ARXPS using the Theta Probe instrument provides detailed insights into self-assembled monolayer structure, delivering reliable measurements of thickness, molecular tilt, and depth distribution. The findings align closely with established literature and demonstrate the technique’s value for surface analysis in research and industrial quality control.

Reference

1. A. Ulman, Chem. Rev. 96 (1996) 1553.