AFM Metrology and Analysis of Membranes
Introduction
One clean membrane sample (NF-ESNAI-4040) and three used membrane samples (NF-CRW, UF-PVWC, NF-PVWC) were submitted for
Nano-R2™ atomic force microscope (AFM) measurement and analysis. Samples were scanned in air and liquid using the environmental
cell. This work is to show the capability of Pacific Nanotechnology’s Nano-R2™ AFM in topographic imaging and quantitative analysis.
Examples of measurement results are shown and discussed below.
Experiment
All sample measurement was carried out using the Close-Contact Mode of Nano-R2™ AFM equipped with a light lever scanner. The
Close-Contact Mode was found the most suitable for present samples. The regular close-contact probes were used to scan in air. Softer
silicon nitride probes with a spring constant of 0.32 N/m were used to scan samples in liquid phase.
The Nano-R2™ system was calibrated in the X, Y, and Z axes before measurement. We utilized the standard samples to calibrate
Z-height channel, and resulting system accuracy was ± 1 %. The resonant frequency of used probes was tested to comply with the
manufacturers’ datasheet. We minimized the drive amplitudes to enhance the measurement stability in the Close-Contact Mode. Raw
scanning data were processed using the NanoRule+ (V 2.5.04) software. Different scanning rates, resolutions and angles were used
to achieve optimal results.
Results and Discussion
Clean Membrane
We scanned many different localities of the sample, and found that the sample was clean, uniform and porous. Figure 1 is a typical
topographic image (5.35 × 5.35 μm) acquired by the Close-Contact Mode. The top view image (Figure 1A) is comparable to the SEM
image reference (5.2 μm), but this AFM image gives much more information about the detailed structure of the membrane (Figure 1B),
especially for information on the Z-direction. Data analysis about pores will be shown below.
Figure 1: Close-contact micrographs of the clean membrane (NF-ESNAI-4040) in air. (A) Top view; (B) 3-D view;
the peak to peak height is 333.43 nm. The images show porous microstructures of the sample.
Figure 2: Topographic change of the clean membrane (NF-ESNAI-4040) after submerged in DI water.
(A) Before submerging; (B) After submerging; the Close-Contact image was in-situ acquired in water
using the environmental cell.
The Environmental Cell (EC) option for the Nano-RTM AFM is designed to allow users to study samples in liquid and gas phases. In the
Close-Contact mode, because the damping of the cantilever induced by surrounding media would decrease the mechanic Q-factor of
vibrating probes, the soft triangular silicon nitride probe was used to obtain optimal results. Figure 2 shows the typical topographic
change after the membrane was submerged into water, presumably because of water adsorption in filter pores. After water interaction
with the membrane, the typical pores disappeared. Instead, submicron-sized particles appeared on all over the surface of the membrane
submerged in DI water.
Figure 3: The typical topography of the dirty membrane
(taken
on UF-PVWC). Three dirty membranes have similar
surface
structures, characterized by ditch, particles or
aggregates, and
exposed supporting layer (arrow indicated).
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Figure 4: A close-look image of the typical topography of the dirty
membrane (taken on NF-PVWC).
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Dirty Membrane
The three dirty membrane samples have similar topography (Figure 3). The peak-to-peak height and roughness varied patch by patch.
We found that there were three common features on the surface: ditch, particles and their aggregates, and exposed substrates, which
are indicated by arrows in Figure 3. It is clear that the average roughness of dirty membrane is much greater than clean membrane
(Figure 2A) and fresh water-submerged membrane (Figure 2B). Figure 4 shows a 5.16 ? 5.16 ?m scan of the typical dirty membrane
sample.
Measurement and Analysis
NanoRule+™ is a powerful tool to quantitatively analyze acquired images. We used NanoRule+™ to measure 3D data of the samples.
Example results are shown in Figure 5.
Figure 5A shows a dimension analysis for a pore found on surface of the clean membrane. Pair 1 and 2 represents the pore diameter
of 308 nm and the depth of 148 nm respectively. The roughness analysis could be useful for indication of surface changes during
filtering. Figure 5B shows the surface roughness of ~38 nm (RMS) for a clean sample. It is obvious that the dirty membrane became
much rougher than the clean sample and fresh water-submerged sample, presumably because of precipitates of filtered materials. The
tested roughness of the dirty membrane was ~61 nm (not shown).
The particle analysis function of the NanoRule+™ could be useful in characterization of particles for freshly water-submerged
membranes and the dirty membranes. Figure 6 shows the analysis of the particle formation during interaction between water and the
clean membrane (a part of Figure 2B). The total of 141 particles larger than 100 nm was counted to this measurement. The average
radius, for example, is 230 nm, and the average area is 0.3 μm2. The other common statistical data, such as max., min., range, and
standard deviation for various parameters of particles were also shown in the Figure 6.
Figure 5: (A) Line analysis for a pore (crossed by the red line) on the clean membrane. Pair 1 and 2 represents the pore diameter of 308 nm and
the depth of 148 nm respectively. (B) shows the surface roughness of 38 nm (RMS).
Figure 6: Analysis of particles formed after the clean membrane was submerged in DI water. The green color and
label indicate the 141 counted particles.
Conclusion
This measurement shows example results of the topography of clean and dirty membranes. The Close-Contact Mode was used for most
cases. Scanning of membranes in liquid was obtained by using the environmental cell and silicon nitride probes. Quantitative analysis of
pores and particles was also performed by using NanoRule+™. We did not take the average of data.
Pacific Nanotechnology’s Nano-R2™ AFM is shown fully capable for analysis and study of various membranes both in air and liquid.
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