Self assembled monolayers (SAMS) serve as an important means of functionalizing surfaces to change reactivity or surface properties. The length of different molecules present in the monolayers and the layer thickness is less than the sampling depth of the XPS process.
This shows that the XPS technique is well-suited for the layer characterization. The additional advantages of Angle Resolved XPS (ARXPS) also contribute to the determination of thickness and layer orientation.
In this study, a set of alkane thiols on gold and 1-mercapto-11-undecyl-tri (ethylene glycol) on gold were used as the monolayers. The structure of these molecules are shown in Figure 1. The expected structures of the molecules during the interaction with a gold surface is shown in Figure 2.
The self assembled monolayers on a gold surface were characterized using the Thermo Scientific Theta Probe with ARXPS. This also enables the determination of the attachment method of the layers, the layer thickness and the way in which the chemical states or material concentration vary across the few nanometers on top of the material.
Figure 1. The structure of the molecules used in this study. a) Hexadecanethiol (an example of an alkane thiol) and b) 1-Mercapto-11-undecyl-tri(ethylene glycol).
Figure 2. The structure of an alkane thiol SAM on gold, sulfur atoms are shown in red, carbon in blue and hydrogen in white
The Thermo Scientific Theta Probe is a unique tool which was used to collect the data. It can also carry out ARXPS measurements without the need to tilt the sample. Some of the benefits of the tool include:
- Generation of thickness maps using ARXPS measurements
- Analysis of small features
- Analysis of large samples
XPS spectral regions including Au4f, C1s, O1s and S2p are measured with respect to over 60 ° angle ranges. The obtained data were then used to collect information of different complexities involved in the spectra.
Results and Discussions
Relative Depth Plots
Taking the logarithm of the ratio between the signals emitted at large angles and signals emitted at small angles followed by arranging the values in order is the easiest method of using the ARXPS data. In this way, the information on ordering of the layer can be obtained.
For instance, the results of this type of measurement for the SAMS are shown in Figure 3. This validates the expected molecular orientation of gold as substrate bonded via sulfur, above which is the the hydrocarbon chain and atop it is the the functional group. Although this technique helps in determining the order of sample layers, it does not provide any quantitative information.
Figure 3. Relative depth plots from the two SAMS formed from the molecules in Figure 1
The Thermo Scientific data system, Avantage, comes in all XPS instruments and includes software for calculating layer thickness from ARXPS data set. The samples with three ultra-thin layers on a substrate can be used in this system.
When applied to the alkane thiol SAMs, the system can measure hydrocarbon layer thickness with respect to the number of carbon atoms present in the chain. Figure 4 shows the expected linear relationship.
Figure 4. Layer thickness, determined by ARXPS, as a function of the number of carbon atoms in a series of alkane thiol SAMS
Depth Profile Generation
Non-destructive depth profiles can be generated with the application of mathematical methods to ARXPS data. Non-destructive profiles refer to the fact that no material is eliminated during the measurement. Such profiles can be produced using the algorithms present in Avantage.
The techniques used for creating these profiles have maximum entropy. Figure 5 shows a profile generated from the SAM based on the molecule illustrated in Figure 1b. The chemical species in this appear to be in appropriate order.
Figure 5. A non-destructive depth profile from the SAM derived from the molecule shown in Figure 1b
ARXPS is a versatile tool used for characterizing ultra-thin layers, using the software containing the advanced algorithms for data processing. With the parallel angle resolved technique in Theta Probe, the method is easy to access as the analysis times are minimized.
In addition, small areas and large samples can also be analyzed. These instruments also feature ARXPS mapping which enables the analyst to produce, for instance, thickness maps.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.
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