QCM-D

« Quartz Crystal Microbalance with Dissipation » or QCM-D uses the piezoelectric effect to monitor the growth of a thin film on a surface and the resulting energy dissipation. The way it works is quite simple: a quartz crystal placed between two electrodes is excited by an oscillating electric field and begins to vibrate if the frequency of the field is close to its resonance frequency. This frequency can be measured by incorporating the crystal in an electric circuit given that it is equivalent to a RLC resonator. Indeed, the mechanical vibration generates in turn an oscillating electric field capable of interacting with the surrounding circuit.

The resonance frequency of the crystal-electrodes system depends on its thickness. For example, when material is deposited on an electrode, the system becomes thicker and the increase of mass causes a lowering of the resonance frequency. That’s why this device is called « microbalance » since it allows following mass variations at the surface of the sensing electrode. In addition to the fundamental resonance frequency, QCM-D also monitors several overtones which can be used to get more information about the layer(s) attached to the electrode.

The link between ∆frequency and ∆mass can be straight deduced if the layer is homogeneous, thin (a few µm maximum) and rigid (i.e. the layer has mechanical properties close to those of quartz). If these hypotheses are confirmed, the Sauerbrey equation is applicable:

where ∆m, ∆f, C and n, are variation of mass [ng.cm^-2], of resonance frequency [Hz], a constant for the crystal [ng.Hz-1.cm^-2] and the overtone number (n=1 for the fundamental frequency), respectively.

In addition to resonance frequencies, QCM-D measures the dissipation of energy by the system, which is the part of accumulated energy lost at each oscillation, after switching off the exciting electric field. This parameter is determined by the viscoelastic properties of the layer(s) deposited on the sensing electrode and can give an idea of their stiffness. For example, adding a soft layer on the electrode will enhance the damping of the vibration and will result in a higher dissipation.

The group “Surf” owns a E4 QCM-D system provided by Q-Sense. The complete specification set of this equipment can be consulted at that URL: http://www.qsense.com/q_sense_e4--5.asp