the crystal stops oscillating quickly and a high value of dissipation is
reported. Conversely, if the film is thin and rigid, it oscillates together with
the crystal and does not dissipate the crystal’s energy effectively. Hence, it
takes a longer amount of time for the crystal to stop oscillating and a low
dissipation value is reported. In this way, dissipation is a measure of the
“floppiness” of the adsorbed film (or better, its lack of rigidity).
By using a suitable mathematical model, the dissipation and frequency
shifts resulting from the adsorption of a nanofilm to a QCM-D crystal
surface can be used to calculate the viscoelastic properties of the film.
Viscoelasticity is the property of materials that exhibit both viscous and
elastic characteristics when undergoing some kind of deformation.
Viscous materials resist flow when a stress is applied. Elastic materials
when stretched very rapidly return to their original state once the stress is
removed. Viscoelastic materials have elements of both of these properties. While the amount of valuable information obtained using such a
mathematical model is impressive, caution should be observed in its use
as the model inevitably requires the input of one known parameter (such
as thickness, density, etc.) in order to calculate the other viscoelastic
properties. If this required parameter is unknown or not known precisely,
then the resulting calculated viscoelastic properties are unreliable. For
these reasons, it is often helpful to perform a QCM-D experiment in
conjunction with measurements taken from ellipsometry, SPR, or DPI.
8.2.4 Modern QCM-D setup
Traditional QCM was developed in the 1960s as a method to detect the
adsorption of gas molecules to surfaces, and has been used for decades
for monitoring the formation of thin films from the gas phase. With more
recent advances, the capabilities of QCM have been extended to detect
surface adsorption at the solid–liquid interface, and QCM-D has become
a valuable tool in the characterization of thin films under solution.
A typical QCM-D setup involves the quartz crystal being mounted in a
flow cell with electrodes mounted on either side of the crystal, as shown
in Figure 8.4. The QCM-D crystal is generally prepared with an active
sensor surface such as gold, hydroxyapatite, or SiO 2 . The resonant frequency of the QCM crystal is monitored as a function of time during the
exposure of the crystal to a given solution under continuous flow conditions, typically in the range of 0.100–0.300 mL/min. If the solution contains materials that have some sort of affinity for the QCM crystal surface,
they adsorb preferentially, increasing the mass of the quartz crystal and
CHAPTER 8: Surface Characterization and Imaging Methods
264
reported. Conversely, if the film is thin and rigid, it oscillates together with
the crystal and does not dissipate the crystal’s energy effectively. Hence, it
takes a longer amount of time for the crystal to stop oscillating and a low
dissipation value is reported. In this way, dissipation is a measure of the
“floppiness” of the adsorbed film (or better, its lack of rigidity).
By using a suitable mathematical model, the dissipation and frequency
shifts resulting from the adsorption of a nanofilm to a QCM-D crystal
surface can be used to calculate the viscoelastic properties of the film.
Viscoelasticity is the property of materials that exhibit both viscous and
elastic characteristics when undergoing some kind of deformation.
Viscous materials resist flow when a stress is applied. Elastic materials
when stretched very rapidly return to their original state once the stress is
removed. Viscoelastic materials have elements of both of these properties. While the amount of valuable information obtained using such a
mathematical model is impressive, caution should be observed in its use
as the model inevitably requires the input of one known parameter (such
as thickness, density, etc.) in order to calculate the other viscoelastic
properties. If this required parameter is unknown or not known precisely,
then the resulting calculated viscoelastic properties are unreliable. For
these reasons, it is often helpful to perform a QCM-D experiment in
conjunction with measurements taken from ellipsometry, SPR, or DPI.
8.2.4 Modern QCM-D setup
Traditional QCM was developed in the 1960s as a method to detect the
adsorption of gas molecules to surfaces, and has been used for decades
for monitoring the formation of thin films from the gas phase. With more
recent advances, the capabilities of QCM have been extended to detect
surface adsorption at the solid–liquid interface, and QCM-D has become
a valuable tool in the characterization of thin films under solution.
A typical QCM-D setup involves the quartz crystal being mounted in a
flow cell with electrodes mounted on either side of the crystal, as shown
in Figure 8.4. The QCM-D crystal is generally prepared with an active
sensor surface such as gold, hydroxyapatite, or SiO 2 . The resonant frequency of the QCM crystal is monitored as a function of time during the
exposure of the crystal to a given solution under continuous flow conditions, typically in the range of 0.100–0.300 mL/min. If the solution contains materials that have some sort of affinity for the QCM crystal surface,
they adsorb preferentially, increasing the mass of the quartz crystal and
CHAPTER 8: Surface Characterization and Imaging Methods
264