PeakForce Tapping AFM Technology for High Resolution Imaging and Simultaneous Nanoscale Property Mapping

Bruker’s PeakForce Tapping is an exclusive operating mode in atomic force microscope (AFM) technology following the launch of TappingMode. This mode provides high resolution imaging, measuring samples that were not previously accessed and enabling simultaneous nanoscale property mapping.

Overview of PeakForce Tapping

PeakForce Tapping involves periodic tapping of the sample through probe and measure the pN-level interaction force by cantilever deflection. The peak force constant is maintained down to 10pN in both fluid and air by the feedback loop using superior force control. The peak force constant is comparatively lower than that used in other modes including TappingMode (∼1nN).

Besides material research, PeakForce Tapping is suitable for the measurement of biological samples owing to its exceptional low imaging forces. It overcomes several limitations of TappingMode operation in fluid including the need for constant tuning of the cantilever. PeakForce Tapping technology is provided with routine measurements of live-cell mechanical properties.

The sample integrity and tip shape can be maintained through superior force control, which can lead to constant accurate and high-resolution measurements of the tiny biological structures including double helix DNA.

The key aspect of the PeakForce Tapping technology is its ability to enhance and enable other correlative and quantitative mapping techniques at the same time, providing new opportunities in chemical, electrical, biological, mechanical and topographical at the nanoscale.

Benefits of PeakForce Tapping

Using PeakForce Tapping’s piconewton (pN) force sensitivity, it is possible to uniquely and simultaneously combine the highest resolution AFM imaging with quantitative, nanoscale electrical, mechanical, biological, and chemical property mapping thereby allowing the researchers of various expertise to make new inventions.

PeakForce Tapping allows the researchers to accurately control probe-to-sample interaction. Further, the superior force control facilitates the most consistent, highest resolution AFM imaging for a wide range of samples, right from the softest biological samples to very hard materials.

It also provides unmatched ease of use with ScanAsyst image optimization software, and the low forces maintain the probe shape for a long period of time while providing consistent imaging.

PeakForce QNM

PeakForce QNM uses PeakForce Tapping technology to map and differentiate the nanomechanical properties including deformation, dissipation, adhesion and modulus while imaging sample topography at atomic scale resolution. It is non-destructive to both the sample and tip as it reduces the lateral force on the probe and controls the peak normal force.

As the force distance data is directly analyzed, the uncertainty about the image contrast source is completely avoided. In addition, the variation and position of mechanical properties along the surface can be studied easily.

PeakForce QNM also enables the bio-researchers to correlate submolecular AFM imaging with quantitative mapping of biological, chemical and mechanical interactions including recognition imaging via the adhesion channel, with low force control and high resolution.

In addition, it provides simultaneous, high-resolution imaging and mechanical property mapping of tissues, viruses and live cells. This could potentially pave way for new researches related to the mechanics of biological processes and structures.

PeakForce QNM is free of limitations of other modes of cell imaging and acquires images at regular AFM imaging rates. Moreover, the force-ramp rates facilitate visco-elastic property measurements over a wide frequency range. Figures 1A, 1B and 1C show the PeakForce QNM images revealing a molecular defect on a polydiacetylene crystal, in air.

Figure 1. Individual molecules are resolved in height (A) as well as adhesion (B) and stiffness (C) maps, with a notable decrease in stiffness at the defect site.

The key features of PeakForce QNM include:

• High-speed, most quantitative nanomechanical mapping
• Highest resolution mapping of nanomechanical properties
• Wide operating range for samples, from extremely soft materials to hard metals
• High-speed, quantitative mechanical property and adhesion mapping of live cells
• Submolecular resolution mapping of mechanical, chemical and biological interactions
• Ease of use

PeakForce TUNA

PeakForce TUNA is suitable for studying the conductivity of fragile samples such as nanoparticles, conductive nanotubes, and organic photovoltaics. Unlike traditional contact-mode-based conductive AFM techniques, it avoids sample damage or probe tip contamination due to CAFM lateral forces. PeakForce TUNA features direct, precise force control, eliminates lateral forces and enables and high-resolution current imaging and high sensitivity.

The combination of directly correlated quantitative nanomechanical property imaging with PeakForce QNM and easy-to-use ScanAsyst ensures the full fA to µA current range. Regulation of water and oxygen levels is also possible through the integration of PeakForce TUNA with environmental control function, which allows accommodation of sensitive samples. Figure 2A, 2B and 2C shows the height and current maps of a carpet of vertical carbon nanotubes, obtained with PeakForce TUNA.

Figure 2. Height (A) and current (B) maps of a carpet of vertical carbon nanotubes, obtained with PeakForce TUNA

The key features of the PeakForce TUNA include:

• Correlated nanomechanical and nanoelectrical properties
• Exceptional repeatability and consistency in nanoelectrical measurements
• High resolution current mapping on the most fragile samples

PeakForce KPFM

PeakForce KPFM provides the most accurate measurements of surface potential at the highest spatial resolution. It has an improved measurement performance over conventional KPFM techniques. The combination of PeakForce Tapping mode, scan algorithms and Bruker’s unique in-house probe developments supports the improvements.

PeakForce KPFM ensures accurate probe-to-probe surface potential measurements with better consistency across different types of material. It can be combined with PeakForce Tapping QNM to simultaneously provide highly correlated nanoscale topography, electrical and mechanical property mapping for a wide range of samples. Using the ScanAsyst mode, users can acquire all kinds of data easily. Figures 3A, 3B and 3C show height, adhesion, and surface potential images of Sn-Pb obtained with PeakForce KPFM.

Figure 3. Height (A), adhesion (B), and surface potential (C) images of Sn-Pb obtained with PeakForce KPFM.

The key features of the PeakForce KPFM include:

• Correlated quantitative nanomechanical property mapping
• Leading-edge spatial resolution combined with artifact-free potential contrast
• Accurate, repeatable, and sensitive work function measurements

PeakForce IR

PeakForce IR performs nanoscale mapping that is far beyond the capabilities of photothermal, sSNOM or AFM techniques so as to probe previously inaccessible sample information. It provides a full, combined set of information as it interleaves sSNOM signal acquisition with PeakForce Tapping feedback. Nanoscale chemical maps having monolayer sensitivity and 10nm spatial resolution can be directly correlated with quantitative nanomechanical information including deformation, adhesion and modulus.

PeakForce IR avoids the need to mount the sample onto a hydrophobic support or microtome a thin slice. It can also determine variations in film thickness with molecular monolayer sensitivity and material types.

In addition, it facilitates scanning probe IR reflection and absorption imaging by exploiting the scattering SNOM for chemical, materials, and plasmon imaging at the highest spatial resolution. Figure 4 shows the PeakForce IR images providing instantly correlated chemical and nanomechanical information of a Polystyrene/LDPE blend.

Figure 4. PeakForce IR images providing instantly correlated chemical and nanomechanical information of a Polystyrene/LDPE blend.

The key features of the PeakForce IR include:

• Unique correlation of plasmonics and electronic structures of graphene and other 2D materials
• Powders and polymer brushes where neither contact nor TappingMode succeed
• Rubbery components, metals, semiconductors, ceramics, and other samples not conducive to photothermal approaches

PeakForce Capture

PeakForce Capture provides the actual force curves at every pixel apart from the calculated property channels. On enabling, the force curves from the PeakForce QNM image are saved along the standard image file in a proprietary 3D data cube file format.

This also provides direct calculations with the NanoScope analysis functions in addition to simple to other systems or programs for further analysis. It provides high resolution mechanical mapping capabilities and analysis when integrated with PeakForce QNM, which can be used with standard biomechanics model. Figures 5A and 5B show the height and stiffness maps of calcite.

Figure 5. Height (A) and stiffness (B) maps of calcite obtained with PeakForce Capture

The key features of the PeakForce Capture include:

• User-specific models through data export features
• High resolution force mapping
• Sensitivity to reveal unexpected events that not captured using other techniques

ScanAsyst

ScanAsyst is an image optimization technique based on PeakForce Tapping which allows the user to the highest resolution AFM images with the help of single-touch scanning.

ScanAsyst avoids the need to navigate complex AFM interfaces and parameter settings thereby automating PeakForce Tapping to produce exceptionally high-quality images by any user. The image quality can be automatically and continuously monitored through “intelligent” algorithms which also ensure parameter adjustments.

The user can simply select a scan size and a scan area for a sample, in air or fluid, enabling a turnkey solution for AFM imaging. ScanAsyst uniquely allows live cell imaging while simultaneously providing high-resolution cellular detail. Figure 6 shows the DNA imaged using ScanAsyst on a MultiMode 8.

Figure 6. DNA imaged using ScanAsyst on a MultiMode 8

The key features of the ScanAsyst include:

• Most stable high-resolution imaging of cells and molecules
• Single-button, repeatable roughness measurements for wafer applications
• Consistent measurement of a wide range of samples for material research

PeakForce Tapping Applications

The table shows the PeakForce Tapping applications.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.