Soft Intermittent Contact - An Important AFM Mode

Concept Scientific Instruments has successfully developed Soft Intermittent Contact mode (or Soft IC) - an alternative measurement mode that offers the key benefits of contact mode and force spectroscopy, while addressing the intrinsic slowness and friction forces commonly associated with these techniques.

Soft IC can be employed within a wide range of applications including electrical characterization, material characterization, polymer science, biology applications, investigating semiconductors and work with soft samples.

Soft IC mode - PDMS sample - 20 µm - Topography (left) & stiffness (right)

PDMS – Soft IC mode – 20µm. Topography (left) & Stiffness (right). Video Credit: CSInstruments

Principles of Soft IC

A Soft IC workflow is displayed in Figure 2. The process begins with the tip being moved to a safe distance (referred to as ‘lift height’) from the surface before being rapidly moved to the next measurement point. Once there, the tip performs a force spectroscopy curve.

Soft IC Video By CSInstruments

Video Credit: CSInstruments

Measurement points are determined by the number of pixels in the image, while the maximum force applied is user-defined via a setpoint value, similar to those used in contact mode.

Soft IC offers several advantages over traditional contact mode methods.Soft IC helps users avoid instabilities resulting from changes in adhesion force during scanning - a frequent problem in contact mode. It does this by setting a lift height that is higher than the adhesion force, therefore allowing the tip to be completely disengaged from the surface. Softer cantilevers can be used with this mode, even though they typically experience high adhesion forces.

Adhesion and stiffness can be acquired directly from each individual measured point, and stiffness (the ratio of applied force and sample deformation) can be used in conjunction with the Soft Mechanic software module to calculate the Young’s modulus.

Soft Intermittent Contact - An Important AFM Mode

Image Credit: CSInstruments

An example of Soft IC application is displayed below, in comparison with an application using standard contact mode. Here, a PS/PMMA blend sample has been imaged in contact mode using a stiff cantilever (k = 37 N/m, ACT from AppNano). The surface shows two distinct separated phases of PS islands (yellow domains) embedded in PMMA matrix (brown domains).

The applied force (45 nN) was not high enough to result in permanent deformation, but both friction and adhesion forces created during the scan do drag parts of the PMMA onto the PS islands and sides of the scanned area (white spots along blue-dashed square, Figure 2).

The same amount of force was applied using Soft IC mode (red square), increasing lift height up to 300 nm and completely releasing the tip from the surface. The image from Soft IC mode does not indicate any permanent damage on the sample, nor does the bottom section of the image which corresponds with measurement in resonant mode (green-dashed square).

Resonant mode does not provide an easy means of ascertaining the value of the applied force. It was estimated that the average applied force was 38 nN for these experimental conditions.

Using Soft IC With Polymer Materials

The figure below illustrates an application of Soft IC mode on a polymer sample. In this instance, a polydimehtylsiloxane (PDMS) sample was covered with a TEM grid before being irradiated with ultraviolet (UV) light. This irradiation created areas with varying mechanical properties.

Areas inside the TEM grid squares become more stiff, exhibiting UV light induced cross-linking. Stiffness and adhesion are displayed in Figures a and b, while Figure 3a shows that PDMS areas (green areas) exhibit lower stiffness than cross-linked PDMS (blue squares). A FORT cantilever (AppNano, k = 1.7 nN, 80 nN applied force) was used in this example. In contrast, adhesion was found to be similar in each case.

Both cross-sections offer notable histograms of values, displaying two well defined peaks for stiffness, but only one for adhesion.

The figure below explores the concept of stiffness, displaying two force spectroscopy curves on both regions. For a specified value of the applied force (the deflection setpoint utilized during the image), stiffness can be understood to be the force/deformation ratio – essentially the slope around the setpoint value. A higher stiffness indicates less deformation for an identical amount of applied force.

Soft IC mode on PDMS irradiated to UV. (a) stiffness and (b) adhesion maps with corresponding cross-sections and histograms.

Soft IC mode on PDMS irradiated to UV. (a) stiffness and (b) adhesion maps with corresponding cross-sections and histograms. Image Credit: CSInstruments

Soft IC is highly suited to identifying compounds present in polymer blends using information provided via the adhesion and stiffness. For example, a blend of PS/PMMA is displayed in Figure 5 with an ACT probe (k = 37 N/m, AppNano, Fsetpoint = 12 nN).

The topography image displayed in Figure 5a does not include a lot of detail. This is because the surface had been polished previously, revealing a roughness under 12 nm except in the cases of two 10 nm deep trenches located on either side the image (dark brown stripes) and a smaller trench located in the middle.

Both adhesion (Figure 5b) and stiffness (Figure 5c) display two well differentiated phases. These phases can be matched with PS and PMMA. In the stiffness image, red areas (higher stiffness values) correspond with PS, while blue areas correspond with PMMA. Meanwhile in the adhesion image, orange areas (higher adhesion values) correspond with PMMA while yellow areas correspond with PS.

Legend : Force vs deformation curves on the PDMS sample of Fig. 3. The stiffness magnitude is the ratio between the applied force and the deformation of the surface. A stiffer material (blue curve-inside square) has higher slope. Image Credit: CSInstruments

Interestingly, while the trenches visible in the topography (dark brown areas) have the same stiffness values, their adhesion values differ and these are also different to the PMMA and PS values on the flat areas.

One potential explanation for these differences lies in the fact that adhesion may be more sensitive to the contact area, and that this contact area is considerably higher in the deep trenches when compared to the flat parts. It should also be noted that a number of tilted lines of PMMA on the PS areas are present in the stiffness image - presumably small parts dragged as the surface was prepared.

As well as being suitable for characterizing polymer blends, Soft IC mode can be used to characterize several phase states within the same polymer.

The figure below displays the topography (Figure a), stiffness (Figure b) and adhesion (Figure c) of a PDES film spin that has been coated on a silicon substrate. This example employs a similar cantilever to the previous example, using an applied force of 17.8 nN. 

Soft IC on PS/PMMA mixture.(a) topography, (b) adhesion and (c) stiffness.

Soft IC on PS/PMMA mixture.(a) topography, (b) adhesion and (c) stiffness. Image Credit: CSInstruments

It is anticipated that the sample’s morphology will include stiffer lamellar domains surrounded by amorphous softer phases. Topography confirms the presence of two phases that are differentiated by two well defined heights (brown areas and yellow areas). These phases have an average height of 8.9 nm.

The layer covering the center of the image includes horizontal lines that may represent horizontal crystalline lamellae; a phenomenon in similar substrates explore in existing literature. Circular droplets with similar heights are present on the sides of the image, suggesting that in principle, two different phases may exist.

Information obtained via stiffness (Figure 6b) and adhesion maps, however, suggests that there are may three different types of phases in this case, each with an increasing stiffness value (blue, yellow and red respectively). The adhesion map sees the same three phases defined by decreasing adhesion values (yellow, dark blue and blue).

The central domain would appear to correspond with a well ordered crystalline lamella. In addition, the circular droplets appear to correspond to a semi-crystalline phase while the surrounding matrix (blue regions in the stiffness image) would appear to correspond to an amorphous phase.

Soft IC on PDES sample.(a) topography, (b) stiffness and (c) adhesion

Soft IC on PDES sample.(a) topography, (b) stiffness and (c) adhesion. Image Credit: CSInstruments

High Resolution Imaging with Soft IC

Soft IC effectively facilitates high resolution imaging because friction forces do not impact the imaging at all, and maximum force may be more precisely controlled than in resonant mode. Figures 7a and 7b feature examples of circular DNA chains of DNA and block copolymer PS/PMMA blends, used with an ACT cantilever (k = 28.9 N/m, AppNano).

Figure 7a shows a topography (image size 1.5 µm) acquired using an applied force of 2.9 nN. When investigated at this force, the DNA chains display the anticipated circular geometry: a diameter of approximately 50 nm with a lateral resolution of 8-10 nm; allowing the inner part of the chain to be resolved.

Figure 7b shows a topography (image size 5 µm) acquired with Soft IC, which displays the lamellar structure of the PS/PMMA block copolymer sample.

The bottom section of the image displays results whereby an applied force of 1 nN is increased to 5 nN, 10 nN and 20 nN respectively. Using 1 nN of applied force did not provide sufficient contrast to clearly see both phases, but employing 15 nN and 20 nN of force led to deformation being induced. These results confirm that the best conditions are achieved when using 5 nN of force. Where this amount of force is applied, the phases of the PS and PMMA blocks are resolved with 38.6 nm of spacing. This can be seen in the bottom section of the image.

High resolution imaging with Soft IC on (a) 50 nm dna rings and (b) PS/PMMA block copolymer with 38.6 nm spacing.

High resolution imaging with Soft IC on (a) 50 nm dna rings and (b) PS/PMMA block copolymer with 38.6 nm spacing. Image Credit: CSInstruments

Applications

  • Material characterization
  • Polymer science
  • Electrical characterization
  • Semiconductor
  • Soft sample
  • Biology

Soft IC mode – PS-PMMA – 50µm.

Soft IC mode – PS-PMMA – 50µm. Image Credit: CSInstruments

This information has been sourced, reviewed and adapted from materials provided by CSInstruments.

For more information on this source, please visit CSInstruments.

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