Quartz Crystal Microbalance - Gravimetric and Non-gravimetric QCM

Gravimetric and Non-gravimetric QCM

The classical sensing application of quartz crystal resonators is microgravimetry. Many commercial instruments, some of which are called thickness monitors, are available. These devices exploit the Sauerbrey relation. For thin films, the resonance frequency is – by-and-large – inversely proportional to the total thickness of the plate. The latter increases when a film is deposited onto the crystal surface. Monolayer sensitivity is easily reached. However, when the film thickness increases, viscoelastic effects come into play. In the late 80’s, it was recognized that the QCM can also be operated in liquids, if proper measures are taken to overcome the consequences of the large damping. Again, viscoelastic effects contribute strongly to the resonance properties.

Today, microweighing is one of several uses of the QCM. Measurements of viscosity and more general, viscoelastic properties, are of much importance as well. The “non-gravimetric” QCM is by no means an alternative to the conventional QCM. Many researchers, who use quartz resonators for purposes other than gravimetry, have continued to call the quartz crystal resonator “QCM”. Actually, the term "balance" makes sense even for non-gravimetric applications if it is understood in the sense of a force balance. At resonance, the force exerted upon the crystal by the sample is balanced by a force originating from the shear gradient inside the crystal. This is the essence of the small-load approximation.

Crystalline α–quartz is by far the most important material for thickness-shear resonators. Langasite (La₃Ga₅SiO₁₄, “LGS”) and gallium-orthophosphate (GaPO₄) are investigated as alternatives to quartz, mainly (but not only) for use at high temperatures. Such devices are also called “QCM”, even though they are not made out of quartz (and may or may not be used for gravimetry).