As you prepare to purchase a QCM instrument, you may find yourself overwhelmed by the variety of options available. There are several versions to consider, such as QCM, QCM-I, QCM-D, and QCM-R. Additionally, there are also single-harmonic QCM:s and multi-harmonic ones. So which version is the right choice for your needs? Here we present guidelines to help you decide which aspects are important for your specific experiments and how to select the most suitable QCM instrument.
Which instrument to choose depends on your application. Therefore, to identify which QCM version will be most suitable, you have to define the intended use of the instrument. Aspects to consider are
Below, we explain how your answers to these questions will determine what qualities to look for in a QCM.
Essentially, if you plan to study anything other than rigid films in vacuum, then you need a QCM that also monitors the energy losses in addition to the frequency. Even though there is seemingly a large range of different QCM types available out there, which all measure the energy loss, there are only three principal ways to measure it - via i) impedance spectroscopy, via ii) the oscillation decay time, or via the iii) resistance of the equivalent circuit. It is important to note that these three method approaches are not equivalent in terms of information content, and which method to go for depends on the intended application and information needs.
Although perhaps obvious, it is worth mentioning that, in order to characterize an unknown system, the information quality has to be sufficient to allow for the film properties to be resolved. I.e. if you are not 100% sure that the film on your QCM sensor is rigid the measurement result will be in doubt if only the resonant frequency is used to estimate the mass of the film. If only frequency and dissipation of one harmonic are measured and it is detected that the film causes a change in dissipation, then you are at loss since you have no way of estimating what type of film you have on your sensor (more than that it is not solid)
If you are only interested in a qualitative measure of the layers, then information from one harmonic might be sufficient. If, however, you would like to quantify viscoelastic layers, then information about f and D at multiple harmonics should be collected. From a theoretical perspective, this information extraction is only possible with the impedance method and the decay time method. These two methods are therefore the only ones that enable viscoelastic modeling. The third method, the resistance approach, can only offer a qualitative estimate of the energy losses in the system. Note, however, that even though an instrument may be based on the impedance or decay time method, this is not a guarantee for overtone capabilities, as not all instruments may have this functionality implemented. If an evaluation of viscoelastic films is of interest, overtone capabilities are thus something to look for.
Another feature to consider is the rate with which measurements are possible, i.e., the measurement time resolution. If you plan to study slow processes, for example, films that take a long time to form or slow layer restructuring events, or if the layer buildup is not of interest but only the final equilibration values, then the time resolution may not be so important. If, however, you are interested in following fast processes and surface changes, then the time resolution is a specification to keep an eye on.
Again, in order to characterize an unknown system, the information content must be sufficient to allow for the process characteristics to be resolved. I.e. if it is not known beforehand whether any fast processes are to be dealt with, then an instrument with high time resolution will be needed in order to reveal whether this is the case or not. The time resolution will, to some extent, be limited by the technical principle on which the QCM is based. For example, the decay time method which is based on pinging is a lot faster than the impedance method, which captures slow spectrum sweeps.
The technical principle on which the QCM is based says a lot, but not all. Indeed, the technical principle largely determines the capabilities in terms of data quality and time resolution. However, there is a lot more to a well-functioning and useful piece of equipment than just the method that it is based on. The method is a good start, but then the supplier must make the most of the inherent capabilities and design a robust and well-functioning instrument suited for the purpose. For example, not all impedance-based QCMs will have overtone capabilities. Other parameters that are important to keep an eye on from a QCM perspective are, for example, the temperature stability and the mechanical design. Two aspects of utmost importance for stable and reproducible QCM measurements.
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Malin graduated in engineering physics in 2006, where her research focused on the QCM-D technology. Since then, she has been scrutinizing the how’s and why’s of the world in general, and the world of QCM-D in particular.
