Quartz Crystal Microbalance (QCM) technology has been a cornerstone in surface science since the 1960s. It is a precise and sensitive method for characterizing surface interactions at the nanoscale and was originally used for monitoring thin-film deposition and characterizing thin films. In this post, we briefly introduce how QCM technology works.
The Quartz Crystal Microbalance (QCM) is a highly sensitive instrument used to measure minute mass changes on a quartz crystal sensor. It is particularly useful for applications requiring precise monitoring of thin films and surface interactions.
The origins of QCM technology date back to the discovery of the piezoelectric effect in the late 19th century. However, it wasn't until 1959 that Günter Sauerbrey formulated the Sauerbrey equation, establishing a linear relationship between the resonance frequency of a quartz crystal and its mass. This breakthrough enabled the development of the Quartz Crystal Microbalance, which became widely used in the 1960s for monitoring thin-film deposition and characterizing thin films. Over the decades, QCM technology has evolved, leading to advanced versions like QCM-D, which provide additional insights into the viscoelastic properties of materials.
The core of the QCM technology is the quartz crystal. The QCM sensor consists of a thin quartz crystal disk sandwiched between two electrodes, Fig. 1A. Quartz is a piezoelectric material, meaning it generates an electric charge in response to mechanical stress and vice versa. When a voltage is applied across the electrodes, the quartz crystal deforms. Practically speaking, if a voltage is applied over the QCM sensor electrodes it will change shape. The nature of this deformation depends on the crystal’s cut and the direction of the applied voltage.
Quartz crystals used in QCM applications are cut in such a way that the resulting deformation shears the disk in the thickness direction as indicated in Fig. 1B. If an alternating voltage is applied, the disk will oscillate back and forth in synch with the applied voltage, Fig. 1C, in what is referred to as thickness-shear mode oscillation.
Figure 1. A.) Schematic side and top views of a QCM sensor. The QCM sensor consists of a quartz crystal disk, sandwiched between two metal electrodes. B.) Applying a voltage across the disk causes it to deform. The direction of the deformation depends on the sign of the applied voltage and the crystal cut. C.) When an alternating voltage is applied to the quartz crystal it oscillates back and forth.
When an alternating voltage is applied to the QCM sensor, the quartz crystal oscillates back and forth in sync with the voltage. At a specific frequency, f, known as the resonance frequency, the oscillation is most efficient, and the oscillating disk will be in resonance. The resonance condition is described in Eq. 1, and is a function of the thickness, h, of the disk. The parameters λ, υq, and n are the wavelength, speed of sound in quartz, and overtone number respectively.
f = n·υq/λ = n·υq/(2h) (1)
Looking at eq. 1, we see that the thicker the disk, the lower the resonance frequency. Therefore, information about the resonance frequency can reveal the disk thickness. Based on this, a relation was formulated by Günter Sauerbrey in 1959, the so-called Sauerbrey equation. The relation says that if the sensor disk is loaded with a thin and rigid film of some other material, where the film is tightly coupled to the oscillating sensor, there is a proportional change in the sensor’s resonant frequency. I.e. as mass is added or removed from an oscillating sensor, there will be a corresponding frequency change, Fig. 2. Mass or thickness changes can hence be revealed via measurements of the resonance frequency, and the QCM technology was born.
Figure 2. Schematic illustration of a QCM measurement, where the change in resonance frequency, Δf, reveals molecular adsorption to the sensor surface. In the schematic adsorption scenario, the measurement starts with (I) a bare surface and a stable baseline of Δf. In (II), molecules adsorb to the surface, and as a result, the frequency decreases, indicating mass uptake. In (III), the surface uptake has been completed and the frequency response has stabilized.
The Quartz Crystal Microbalance (QCM) is a highly sensitive, real-time technique that monitors changes in mass or thickness of layers adhering to the surface of a quartz crystal. This is achieved by measuring the change in resonance frequency of the quartz crystal upon excitation by a driving voltage. QCM is particularly useful for applications requiring precise monitoring of thin films and surface interactions, making it an indispensable tool in various scientific and industrial fields.
Download the overview to read more about how QCM technology works
1 In certain cases, the mass change can be calculated with an equation called the Sauerbrey equation, which is derived from eq. 1.
Editor’s note: This post was originally published in April 2019 and has been updated
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