Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) is a real-time, nanoscale technique for analyzing surface phenomena including thin film formation, interactions and reactions.
A QCM sensor consists of a thin quartz disc sandwiched between a pair of electrodes. The sensor can be excited to oscillate at its resonance frequency by the application of an alternating voltage. The resonance frequency depends on the total oscillating mass of the sensor and sensor surface adhering layers, including coupled water. The frequency decreases when a thin film is attached to the sensor. If the film is thin and rigid the decrease in frequency is proportional to the mass of the film. In this way, the QCM operates as a very sensitive balance. Unlike all other QCMs, QCM-D monitors the frequency and energy dissipation response of the freely oscillating sensor, thus generating results more accurately and faster.
Common applications of QCM-D include measurements of proteins, polymers, surfactants and cells interacting with 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. The kinetics of structural changes and mass changes involved are obtained simultaneously with QCM-D.
You can view an animation here to learn more about the measurement principle and the type of data that can be obtained with QSense® QCM-D. The animation also highlights common application areas, shows the set-up and how the instrument is operated.
For more information, see:
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. You can watch a demonstration of the principles behind the QCM-D technology in the video below.
The quartz sensor
A QCM sensor 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, the QCM operates as a very sensitive balance.
Sauerbrey relation for rigid films
The mass of the adhering layer is calculated using the Sauerbrey relation:
|∆m = – (C · ∆f)/n||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 estimate the thickness (d) of the adhering layer:
|deff = ∆m/ρeff||where ρeff is the effective density of the adhering layer.|
Soft films and the importance of “D”
In most applications, 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 underestimate the 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 film’s softness (viscoelasticity).
D is defined as:
|D = Elost / 2πEstored||where Elost is the energy lost (dissipated) during one oscillation cycle and Estored|
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), which provides additional information of the system under study. The adhering film can hence be characterized in detail by measuring at multiple frequencies and applying a viscoelastic model (e.g. the so called Voigt model) incorporated in QSense software QTools; viscosity, elasticity and correct thickness may be extracted even for soft films when certain assumptions are made.
The basic QCM has been used for some time to monitor thin film deposition in vacuum or gas. The number of applications for the QCM increased dramatically after it was shown that the QCM may also be used in the liquid phase.