Nanotechnology-Enabled Sensors

5.9 Scanning Probe Microscopy (SPM) 269
nated. It is also advantageous for materials that are poorly adsorbed on a
substrate. In general, for delicate samples, constant force or tapping modes
are utilized.
The sensitivity of the force measurements is directly related to the size
and stiffness of the cantilever, which can range from 1 pN to 1 nN per nm,
and the deflection sensor measures motions ranging from several microns
to even 0.01 nm. The minimum force that can be measured with the AFM
is typically 5 pN.
Fig. 5.46 illustrates how the AFM can be utilized to characterize the surface
of a sensor. These AFM images are of WO3 crystals, which were deposited
onto a multilayered ZnO/LiTaO3 SAW transducer (described in
Chap. 3). WO3 is known for its sensitivity towards different gas species.
On this device, some of the deposited WO3 covers the regions where the
metallic interdigital electrodes are located beneath. From Fig. 5.46 it is
clear that the presence of the underlying metal results in the deposited
WO3 having vastly different surface morphology to that on the areas without
underlying metal. Such observations are important because in gas sensing
applications the surface morphology and surface-to-volume ratio can
have a strong influence on its sensitivity. From the figure the average grain
size for WO3 over the non-metalized to metalized regions are 132 nm and
156 nm, respectively. Furthermore rms surface roughness also changes for
both of the regions, being ~12 nm on the non-metalized and ~16 nm on the
metalized regions.
Fig. 5.46 AFM surface images of 150 nm thick WO3 sensing layer deposited on a
ZnO/36° YX LiTaO3 SAW device: (a) WO3 deposited on the non-metalized region;
(b) WO3 deposited on the metalized region. Reprinted with permission from
the Elsevier publications. 107