What to consider when analyzing combined QCM-D and ellipsometry data

QCM-D and ellipsometry are two surface sensitive real-time technologies that can be used in combination to give synergistic effects. To make to most of the combined output data however, there are some aspects to consider regarding the differences and similarities of the two technologies. So what aspects should you keep in mind when setting up the combined experiment and then later analyze the captured data?

Optimizing the output of combined QCM-D and ellipsometry data analysis

The overall goal is to extract information about the system under study, not attainable with either of the two techniques alone. Therefore, to make the most of the combined output data, it must be recognized what information the respective method generates, and how to make the most of this information.

Understanding the QCM-D and spectroscopic ellipsometry (SE) information output

To recognize what synergistic effect that can result from the combination, a good start is to understand the output from the respective technology, and how these can be combined to generate new output not achievable with the respective technology alone.

QCM-D and SE are both surface sensitive real-time technologies that can monitor mass changes at the surface. QCM-D extracts info about wet mass and ellipsometry extracts the dry mass. Combined, it is therefore possible not only to get information about the mass, thickness, mechanical and optical properties during the formation and modification of thin layers at the solid-liquid interface, but it is also possible to monitor the organization and structure of layers in real time. Combining the wet and dry masses, info about the film porosity, conformation and swelling state can be deduced.

The analysis starts by extracting the independently measured SE and QCM-D thicknesses and masses; d^SE, d^QCM-D, m^SE and m^QCM-D, respectively. Then it is possible to estimate the degree of solvation in the film. We get the volume fraction as:

S⁰_v = d^SE / d^QCM-D                                (1)

and the mass fraction parameter as:

S⁰_m = m^SE / m^QCM-D                       (2)

Optimizing the quality of the combined QCM-D and ellipsometry data

Once the respective method and requirements for optimal data output is understood, the experiments should be planned and executed to optimize the quality of the data from the perspective of the combined data analysis.

• Both technologies are sensitive to contamination. Therefore, instrument and sensor surface cleanliness are crucial.
• For both methods, and for QCM-D measurements in particular, temperature stability is very important. Care should be taken to activate the instrument temperature control and to avoid large fluctuations of the ambient temperature.
• From the SE perspective, the modeling of the SE data requires SE reference measurements that provide information on the optical properties of the substrate, ambient medium, adsorbate, and other effects caused by the experimental setup at all stages of the measurement, Fig. 1. The following spectra hence must be captured:

• Another aspect to keep in mind is the measurement alignment with respect to time. The adsorbate fraction parameter is calculated as a function of time, and so the ellipsometry- and QCM-D-obtained thickness or mass parameters need to align closely with respect to time.
• To facilitate both the analysis of the respective data set as well as the alignment of data, it is good to capture a base line of at least 5 - 10 minutes.

Figure 1. A schematic cross-section of an ellipsometry module with a mounted sensor and the capture of an SE measurements.

Concluding remarks

QCM-D and ellipsometry are two complementary technologies which when used in combination can result in synergy effects and a deeper understanding of the system under study than can be achieved with either of the two technologies alone. Understanding the information output offered by the respective technology, additional info, such as adsorbate fraction parameters, can be obtained. To optimize the data analysis, it is important to prepare the combined experiment with the respective technology needs in mind.

Download our white paper to read more about the theory behind the data quantification and to get a step-by-step procedure on how to do the combined QCM-D and Ellipsometry analysis.