Single-harmonic vs multi-harmonic QCM-D – what is the difference?

There is a wide variety of QCM:s available in the market. These range from standard QCM models to extended versions like QCM-D. Additionally, within these categories, there are single-harmonic and multi-harmonic instruments. Here we explain what that means and what the difference is between them.

Single-harmonic and multi-harmonic QCM - what does it mean?

QCM technology can monitor changes in mass, or thickness, of layers at the sensor surface by measuring changes in the resonance frequency of the quartz crystal upon excitation by a driving voltage.

A single harmonic QCM excites the crystal at one single frequency, typically the fundamental. A multi-harmonic QCM, on the other hand, excites the crystal at multiple frequencies. Typically, the fundamental frequency and one or several overtones are used. For a QCM to qualify as multi-harmonic, the number of frequencies measured needs to be more than one, but the number can vary from two and up.

What do the different QCM measurements look like?

Let’s take an imaginary scenario of molecules adsorbing to the sensor surface. In terms of information output, it is interesting to compare measurements captured by the three different QCM versions single-harmonic QCM, single-harmonic QCM-D, and multi-harmonic QCM-D. Figure 1 shows schematic illustrations of what measurements of molecule adsorption could look like when measured by these three different instruments.

1. In step (I), there is a bare sensor surface and a stable baseline of Δf.
2. In (II), molecules adsorb to the surface, and as a result, the frequency decreases, indicating mass uptake.
3. In (III), the surface uptake has been completed and the frequency response has stabilized.

B. Single harmonic QCM-D provides time-resolved information on f and D from one frequenct, typically the fundamental mode. The schematic measurement of an adsorption scenario in Fig 1 (middle) shows

1. In step (I), there is a bare sensor surface and stable baselines of Δf and ΔD.
2. In (II), molecules adsorb to the surface, and as a result, the frequency decreases and the dissipation increases, indicating mass uptake and increasing energy loss.
3. In (III), the surface uptake has been completed and the frequency and dissipation responses have stabilized.

C. Multi-harmonic QCM-D provides time-resolved information on f and D at multiple harmonics. The schematic measurement of an adsorption scenario in Fig 1 (bottom) shows

1. In step (I), there is a bare sensor surface and the measurement shows stable baselines of multiple Δf:s and ΔD:s.
2. In (II), molecules bind to the surface, and as a result, the frequencies decrease and the dissipations increase, indicating mass uptake and increasing energy loss.
3. In (III), the surface uptake has been completed and the frequency and dissipation responses have stabilized.

Figure 1.  Schematic illustration of molecules adsorbing to the sensor surface, characterized by single-harmonic QCM (top), single-harmonic QCM-D (middle), and multi-harmonic QCM-D (bottom).

Comparison of information output

As the information collection between single-harmonic and multi-harmonic instruments differs, so does the information output. Single-harmonic QCM can be used to quantify mass, or thickness, in situations where the Sauerbrey equation is valid. Single-harmonic QCM-D, which also collects information on the energy losses of the system, can reveal whether the film is rigid or not, and if the Sauerbrey equation can be used for the quantification of mass. If f and D are collected at multiple harmonics, it is also possible to quantify mass, thickness, and viscoelastic properties of soft layers at the sensor surface.

Concluding remarks

To conclude, the wide array of QCM instruments available in the market, ranging from standard QCM:s to extended versions like QCM-D, offer varying capabilities for monitoring changes in mass or thickness of layers at the sensor surface. Single-harmonic QCMs excite the crystal at a single frequency, while multi-harmonic QCMs utilize multiple frequencies, providing different types of information output. Understanding the differences between single-harmonic and multi-harmonic QCM:s can help in selecting the most suitable instrument based on your specific measurement needs, whether it involves obtaining a qualitative indication of molecule-surface interaction or quantifying mass or analyzing viscoelastic properties of layers.

Download the overview to learn more about the working principles of QCM and QCM-D  and the difference between them.