High Resolution AFM Imaging of a Wide Range of Samples Using Peakforce Tapping

After the introduction of the TappingMode™, Bruker’s exclusive PeakForce Tapping^® is a very important scientific advancement in atomic force microscope (AFM) technology. It offers an excellent high resolution imaging, extends AFM measurements into a sample range that has not been accessed previously and allows simultaneous nanoscale property mapping.

Key Features of PeakForce Tapping^®

The key features of this AFM technology include:

• Highest Resolution Imaging PeakForce Tapping^® - PeakForce Tapping enables the researcher to accurately control probe-to-sample interaction allowing the lowest available imaging forces. Highly consistent, highest resolution AFM imaging for a broad range of sample types from very soft biological samples to very hard materials has been made possible due to this superior force control.
• Unique, quantitative results - PeakForce Tapping’s piconewton (pN) force sensitivity, innovatively and concurrently integrates the highest resolution AFM imaging with quantitative, nanoscale mechanical, electrical, chemical and biological property mapping, allowing researchers of different experience levels to make new discoveries.
• Easy to Use - PeakForce Tapping’s superior force control offers the user unrivaled ease in operating made possible with the ScanAsyst^® image optimization software, and the low forces help retain the shape of the probe so that it is long-lasting and offers reliable imaging.

Figure 1 shows multimodal graphene characterization with PeakForce Tapping, Peak Force KPFM, PeakForce IR and PeakForce QNM.

Figure 1. Multimodal graphene characterization with PeakForce Tapping, PeakForce KPFM, PeakForce QNM and PeakForce IR. Image size: 15µm top left and right; 5µm bottom left and right.

Overview of PeakForce Tapping

In PeakForce Tapping, periodic tapping of the sample is done using the probe and the pN-level interaction force is directly determined by cantilever deflection. Using superior force control, the feedback loop maintains the peak force at a constant level, down to 10pN, in both fluid and air, which is considerably lesser than is used typically with other modes like the TappingMode (~1 nN).

Further to material research, PeakForce Tapping is suited for measuring biological samples because of its extremely low imaging forces. The limitations of TappingMode operation in fluid has been overcome such as the requirement of constantly tuning the cantilever. Using PeakForce Tapping technology, it is now possible to conduct regular measurements of live-cell mechanical properties.

The superior force control maintains tip shape and sample integrity, leading to consistently accurate and high-resolution measurements of the tiniest biological structures like double helix DNA.

The greatest benefit of the technology is its capability to concurrently allow and improve other quantitative and correlative mapping techniques, offering a wide range of prospects in ever-expanding applications at the nanoscale in mechanical, topographical, biological, electrical and chemical fields.

Figure 2 shows submolecular resolution of the major and minor grooves of the DNA double helix.

Figure 2. Submolecular resolution of the major and minor grooves of the DNA double helix. Image obtained with BioScope Resolve on an inverted microscope. Image size 15 nm x 43 nm.

PeakForce QNM for Materials - Quantitative Nanoscale Mechanical Characterization

Using PeakForce Tapping technology, PeakForce QNM maps and differentiates between nanomechanical properties including adhesion, dissipation, modulus and deformation, while imaging sample topography at atomic scale resolution at the same time.

It is does not damage both the sample and the tips as it controls directly the peak normal force and reduces the probe’s lateral force. As force distance data is directly analyzed there is no ambiguity with regards to the image contrast source as normally occurs in other methods.

The produced quantitative data can enable researchers to answer the crucial question of what materials they are observing in their topographic images. Furthermore, it is also feasible to analyze the position and variation of mechanical properties conveniently across a surface easily and at a resolution that was not possible before.

The key features of the PeakForce QNM for materials applications include:

• Superior resolution mapping of nanomechanical properties
• High-speed, most quantitative nanomechanical mapping
• Widest operating range for samples, from extremely soft materials (~1 kPa) to hard metals (100 GPa)

Figure 3 shows the subtle transition between the ULDPE tie layer to the PS/LDPE sealant layer of cross sectioned packaging material.

Figure 3. This modulus image shows the subtle transition between the ULDPE tie layer to the PS/LDPE sealant layer of cross sectioned packaging material. Mapping the modulus at high spatial resolution reveals lamella from the tie layer act as nucleation sites penetrating into the sealant and increasing lamellar ordering within 1µm from the interface. Image size 3µm.

PeakForce QNM for Life Sciences - Submolecular, Quantitative Mapping of Biological Interactions

PeakForce QNM enables bio- researchers to correlate submolecular AFM imaging with quantitative mapping of chemical, mechanical, and biological interactions, which include recognition imaging via the adhesion channel with low force control and high resolution. Furthermore, it offers high-resolution, simultaneous imaging and mechanical property mapping of viruses, live cells and tissues.

This enables new studies into the role of mechanics in biological structures and processes. PeakForce QNM overcomes the large number of limitations of other modes of cell imaging, obtaining images at regular AFM imaging rates. Its force-ramp rates allow visco elastic property measurements over a wide frequency range.

PeakForce QNM for Life Sciences enables the following:

• Submolecular resolution mapping of mechanical, chemical and biological interactions
• High-speed, quantitative mechanical property and adhesion mapping of live cells
• Easiest to use, making every user an AFM expert

Figure 4 shows the topography and corresponding modulus images of living MDCK cells.

Figure 4. Topography (A) and corresponding modulus (B) images of living MDCK cells. Cell structures corresponding to actin fibers show higher modulus (lighter) while cell surface features, believed to be microvilli, appear softer (darker) than the cell membrane. Image size 32µm.

Figure 5 shows the PeakForce QNM images showing a molecular defect on a polydiacetylene crystal, in air.

Figure 5. PeakForce QNM images revealing a molecular defect on a polydiacetylene crystal, in air. 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. Image size 10nm.

PeakForce TUNA - Highest Resolution Current Mapping on the Most Fragile Samples

PeakForce TUNA™ is suitable for studying conductivity of delicate samples like conductive nanotubes, organic photovoltaics and nanoparticles. Conventional contact-mode –based conductive AFM techniques have limitations of probe tip contamination or sample damage, however the PeakForce TUNA™ overcomes these drawbacks.

Direct and accurate force control is possible and lateral forces are eliminated with the PeakForce TUNA™. Thus high-resolution current imaging and high-sensitivity is possible with the instrument. Now in a single module itself, the full fA to µA current range, correlated quantitative nanomechanical property imaging with PeakForce QNM, and the ease-of-use of ScanAsyst are available. It is also possible to combine PeakForce TUNA with an environmental control for monitoring water and oxygen levels down to ppm in order to accommodate the most fragile samples.

The benefits of PeakForce TUNA include:

• Highest resolution current mapping on the most delicate samples
• Superior consistency and repeatability in nanoelectrical measurements
• Correlated nanomechanical and nanoelectrical properties

Figure 6 shows the height and current maps of a vertical carbon nanotube carpet obtained with the PeakForce TUNA.

Figure 6. Height (A) and current (B) maps of a carpet of vertical carbon nanotubes, obtained with PeakForce TUNA, which are impossible to image with contact mode.Image size 1µm.

Figure 7 shows the PeakForce TUNA topography, current and adhesion maps of a semiconducting polymer composite, poly(3- hexylthiophene) (P3HT)with embedded carbon nanotubes.

Figure 7. PeakForce TUNA (A) topography, (B) current, and(C) adhesion maps of a semiconducting polymer composite, poly(3- hexylthiophene) (P3HT)with embedded carbon nanotubes. The high resolution in all channels reveals the influence of the embedded nanotube on P3HT lamellar ordering and current pathways. Image size 500nm. Image courtesy of Philippe Leclère et al, University of Mons (UMONS) Belgium.

PeakForce KPFM - Industry-Leading Spatial Resolution and Artifact-Free Potential Contrast

PeakForce KPFM™ enhances the measurement performance when compared to conventional KPFM methods by offering an excellent spatial resolution and highly precise measurements of surface potential.

By combining the new PeakForce tapping mode, Bruker’s innovative in-house probe developments and scan algorithms, these enhancements have been achieved. PeakForce KPFM offers highly precise probe-to-probe surface potential measurements enabling reliable measurements across a range of material types.

It can be deployed in tandem with PeakForce Tapping QNM for providing concurrent, highly correlated nanoscale topography, mechanical and electrical property mapping on a large number of samples.

PeakForce KPFM operates in the ScanAsyst mode, thus overcoming ease-of-use challenges encountered in the conventional versions. This mode helps acquire expert-quality data by users of all experience levels.

PeakForce KPFM delivers the following:

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

Figure 8 shows the height, adhesion, and surface potential images of Sn-Pb obtained with PeakForce KPFM

Figure 8. Height (A), adhesion (B), and surface potential (C) images of Sn-Pb obtained with PeakForce KPFM. The workfunction difference is accurately mapped while nanoscale phase structure in the adhesion map is simultaneously revealed. Image size 4µm.

PeakForce IR - Unprecedented QuantitativeNanochemical Characterization

PeakForce IR™ is an extension of nanoscale mapping that surpasses the range of conventional AFM, sSNOM, or photothermal methods to offer not only more comprehensive data but also to study previously inaccessible samples. In this mode, sSNOM signal acquisition and PeakForce Tapping feedback are interleaved to offer a complete integrated set of data.

A direct correlation is observed between nanoscale chemical maps with 10nm spatial resolution and monolayer sensitivity with quantitative nanomechanical information like adhesion, modulus, and deformation.

Using PeakForce IR there is no need for microtoming a thin slice or mounting a sample onto a hydrophobic support. Addressable samples include those which are not transparent or reflective, not Raman active or not conducive to contact or TappingMode feedback.

It is possible to detect the type of material and identify variations in film thickness with molecular monolayer sensitivity. It offers absorption imaging and scanning probe IR reflection with all the benefits of scattering SNOM for chemical, materials, and plasmon imaging at a very high spatial resolution.

Using PeakForce IR, nanoscale characterization has been extended to:

• Powders and polymer brushes where both TappingMode and contact has been unsuccessful
• Rubbery components, semiconductors, metals, ceramics, and other samples, which are suited to photothermal approaches
• Unique correlation of plasmonics and electronic structures of graphene and other 2D materials

Figure 9 shows the instantly correlated chemical and nanomechanical information of a Polystyrene/LDPE blend.

Figure 9. PeakForce IR images providing instantly correlated chemical and nanomechanical information of a Polystyrene/LDPE blend. Image size 1µm x 2µm.

PeakForce Capture - High-Sensitivity NanomechanicalData at Every Pixel

PeakForce QNM studies each force curve in PeakForce Tapping in real-time to obtain material property maps, however, PeakForce Capture™ offers the actual force curves at each pixel further to the computed property channels. When enabled, force curves from the PeakForce QNM image are saved along with the standard image file in a proprietary 3D data cube file format. This enables direct computations using the NanoScope^®analysis functions, and also simple export to other programs or systems for further analysis. When coupled with PeakForce QNM, it offers excellent resolution mechanical mapping capabilities and analysis, which can be used along with standard biomechanics models.

PeakForce Capture offers the following:

• Highest resolution force mapping
• Sensitivity to discover unpredicted events that are not captured with other techniques
• User-specific models through data export features

Figure 10 shows the height and stiffness maps of calcite.

Figure 10. Height (A) and stiffness (B) maps of calcite. This unique atomic scale nanomechanical information reveals increased contact stiffness for alternate rows of atoms.

ScanAsyst - Self-Optimizing AFM for High-Resolution Imaging

ScanAsyst is a PeakForce Tapping - based image optimization technique which helps every user to create AFM images of the highest resolution AFM images using single-touch scanning.

Using ScanAsyst, there is no need for navigating complex AFM interfaces and parameter settings, PeakForce Tapping is automated so that very high-quality images can be obtained by any user irrespective of the level of experience.

“Intelligent” algorithms are capable of constantly and automatically regulating image quality and making suitable parameter adjustments. The user chooses a scan size and scan area for a sample in fluid or air allowing a turnkey solution for AFM imaging. Convenient imaging of live cells while concurrently offering high-resolution cellular detail is possible with the ScanAsyst.

ScanAsyst enables:

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

Figure 11 shows DNA imaged using ScanAsyst on a MultiMode 8.

Figure 11. DNA imaged using ScanAsyst on a MultiMode 8. Image size 1 µm.

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.