QCM-D Technology
Quartz Crystal Microbalance with Dissipation monitoring, QCM-D, can be used in surface science to characterize the formation of thin films (nm).
Common applications include measurements on proteins, polymers, surfactants and cells onto surfaces in liquid.
Mass and structural changes detected
The amount of water in an adsorbed film can be as high as 95% depending on the kind of molecule and the type of surface you are studying.
If some elongated molecules adsorb flat on a surface, little water will be coupled to the molecules. However, if they adsorb standing up, lots of water will be coupled.
With QCM-D the kinetics of both structural changes and mass changes are obtained simultaneously.
Questions
You are welcome to contact our Application Specialists to discuss if QCM-D is applicable to your research.
Q-Sense QCM-D full animation including introduction, measurement principle and output data possible to obtain with QCM-D. The animation also highlights common application areas and shows the set-up and how the instrument is operated. You can also find more information in our Technology note on the QCM-D Principle.
The QCM-D Principle
Unlike all other QCMs, QCM-D monitors the frequency and energy dissipation response of the freely oscillating sensor. It is faster and more accurate than the usual frequency sweep principle. Here is a demonstration of the principles behind the QCM-D technology.
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The quartz sensor
A QCM consists of a thin quartz disc sandwiched between a pair of electrodes. Due to the piezoelectric properties of quartz, it is possible to excite the crystal to oscillation by applying an AC voltage across its electrodes. Normally the electrodes are made of gold, which can be coated with a wide range of different materials.
The resonance frequency (f) of the sensor depends on the total oscillating mass, including water coupled to the oscillation. When a thin film is attached to the sensor, the frequency decreases. If the film is thin and rigid the decrease in frequency is proportional to the mass of the film. In this way, QCMs operate as a very sensitive balance.
Sauerbrey for rigid films
The mass of the adhering layer is calculated by using the Sauerbrey relation:
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C = 17.7 ng Hz-1 cm-2 for a 5 MHz quartz crystal.
n = 1,3,5,7 is the overtone number. |
It is also possible to get an estimation of the thickness (d) of the adhering layer:
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where ρeff is the effective density of the adhering layer. |
Soft films and the importance of "D"
In most situations the adsorbed film is not rigid and the Sauerbrey relation becomes invalid. A film that is "soft" (viscoelastic) will not fully couple to the oscillation of the crystal, hence the Sauerbrey relation will underestimatethe mass at the surface.
A soft film dampens the sensor's oscillation. The damping or energy dissipation (D) of the sensor's oscillation reveals the the film's softness (viscoelasticity).
D is defined as
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where Elost is the energy lost (dissipated) during one oscillation cycle and Estored is the total energy stored in the oscillator. |
The energy dissipation of the sensor is measured by recording the response of a freely oscillating sensor that has been vibrated at its resonance frequency. This also gives the opportunity to jump between the fundamental frequency and overtones (e.g. 15, 25 and 35 MHz). By measuring at multiple frequencies and applying a viscoelastic model (e.g. the so called Voigt model) incorporated in Q-Sense software QTools, the adhering film can be characterized in detail; viscosity, elasticity and correct thickness may be extracted even for soft films when certain assumptions are made.
The basic quartz crystal microbalance (QCM) has been used for a long time to monitor thin film deposition in vacuum or gas. After it was shown that the QCM may be used in the liquid phase, the number of applications for the QCM has increased dramatically.
Read more about QCM-D in our Technology note and in over 900 peer-reviewed publications citing the use of the technology.
In our FAQ section you can also find more details on QCM-D.
