The main purpose of microscopy is to make possible the observation of objects and their details. Both objects and details are sometimes extremely small, and thus, cannot be observed without any supplementary help. The scientific method naturally requires testing the boundaries of available metrology techniques, and, in this regard, Atomic Force Microscopy (AFM) is no exception.
Using AFM for Observation of Objects
The AFM has demonstrated its proficiency in creating high resolution images, at which it becomes possible to visualize sample features measured in fractions of nanometers. Images at such a high resolution are usually referred to as having achieved “atomic resolution.” Nevertheless, this moniker should not be taken literally, as the resolution of the images does not show individual atoms. It does, however, reveal the resonance of the spaces between the atoms that make up materials with atomically flat surfaces, such as graphite or mica [1, 2, 3].
Uniform lattices with constants form the surfaces of these materials. The constants are several tenths of a nanometer wide. They are also scanned with probes which feature tip radius curvatures that are greater in size by an order of magnitude (2-5 nm). The way the tip detects the resonance of features smaller than its radius curvature resembles a palm running over the keys of a keyboard. Using this tactile feedback alone, a user would be able to render a mental image of the approximate layout of the keyboard, but it would be hard to distinguish single keys.
Benefits of AFM systems from Park Systems
Contemporary AFM systems, including those produced by Park Systems, have the ability to produce “atomic resolution” images with features as small as few tenths of an angstrom (Å). Imaging via resonance, versus imaging solely from the cantilever feedback response, has to be distinguished. Probe manufacturing must advance far enough to allow tip radius curvatures to be on the order of widths of single atoms. Until then, innovations in ultra-high resolution AFM must come from other avenues.
The resolution limit of AFM imaging depends on the geometry of the probe tip. To keep the highest possible image quality, for the longest amount of time, one must consider preserving tips across multiple scans.
The most elementary AFM operation technique is the contact mode, in which the probe needs to be dragged across the surface of the sample to obtain topography data. As a consequence, probe blunting becomes a concern as higher tip radius curvatures lead to decreased spatial resolution, and hence, less accurate imaging. Another concern a lot of laboratories face is how they can maintain stocks of contact mode probes, which typically increase supply expenses in the long run.
There is, however, a way to sustain the highest ultra-high resolution for the longest period of time and in the most cost-effective manner, and that is to take images in non-contact mode. In this mode, the distance of the tip of the probe that oscillates the sample surface during the scan is maintained by a precise, high-speed feedback loop. Such advanced electronics can be found in the NX-series system architecture from Park Systems . They sustain the non-contact regime of the van der Waals forces between the atoms of its tip and those of the sample.
Topography images are created using the records of the deviations in the amplitude of the probe’s oscillation which the sample has traced. Due to the fact that the tip and probe are not in direct contact, the longevity of the tip is increased significantly over dozens of scans with no observable loss in resolution .
Challenges Presented During AFM
To demonstrate how efficient the non-contact mode for ultra-high resolution AFM imaging is, a sample with features that rival the smallest tip radius curvatures of commercially available AFM probes is required. To support the argument for the non-contact mode’s increased tip longevity across multiple scans, the sample used must be regarded by the research community in terms of its difficulty (as a subject) for consistent, reproducible nanoscale imaging.
Nanomaterials that feature moiré patterns meet both of the aforementioned criteria. The moiré patterns are secondary, visually evident and result from periodic patterns (such as atomic lattices). The periodic patterns are placed one on top of the other and are then rotated to create the new distinct and offset design.
When repeated high-resolution imaging of the moiré patterns (and the superlattice constants that comprise them) are not carried out using non-contact mode AFM, significant challenges are presented. One of which is that the resolution that has to be achieved is about the size of the tip radius curvature. Therefore, any loss of the sharpness of the tip, for example, in one resulting from the blunting effect that is typical for contact mode AFM, would impede the repeatability of data acquisition.
This obstacle is not easy to overcome even by experienced AFM operators, as hardware limitations and constant scan parameter optimization via trial-and-error complicate the task even more. Overcoming this issue will be important to achieve serviceable AFM image and data at ultra-high resolutions. Removing increased tip deterioration would be the welcome reprieve for this application.
Park Systems conducted an evaluation of a graphene/hexagonal boron nitride (hBN) sample using Park NX10 AFM system powered by Park SmartScan operation software. The evaluation showed the ultra-high resolution of AFM, demonstrating the advantages of using contemporary system architecture and robust software automation.
The sample consisted of an hBN substrate overlaid with a graphene layer and was scanned under ambient air. The purpose of the evaluation was to assess the Park NX10’s ability to characterize the topography of the moiré pattern that was created when one layer was set on top of the other and offset by rotation. The Park NX10 succeeded in imaging the moiré pattern superlattice constant of the sample  in scans as large as 500 x 500 nm (see Figure 1a). The imaging was done using non-contact AFM mode and a standard AFM probe tip .
A second scan (see Figure 1b), this time at 250 x 250 nm, was collected referencing a single isolated sample defect in the upper-left quadrant of the initial 500 x 500 nm scan as a landmark. The superlattice pattern around the defect in the center is even more visually evident in the second scan than in the first.
Figure 1. A series of four non-contact AFM topographical images of a graphene sample exhibiting moiré patterns: (a) at 500 x 500 nm, (b) at 250 x 250 nm, (c) at 125 x 125 nm, and (d) at 60 x 60 nm. All images were taken with a Park NX10 AFM system using the Park SmartScan operating software’s Auto mode.
The most convincing evidence for the Park NX10’s capability to maintain its ultra-high resolution, even after multiple scans with the same AFM tip, comes from the final two scans. Figure 1 shows the diagonal striations superimposed onto the moiré pattern that are evident as they repeat across the surface of the sample.
The last image in the series (Figure 1d) was taken at a scan size of 60 x 60 nm, and provides the clearest evidence that not only are the superlattice constants of the moiré pattern about 15 nm  in width, but also that the spacing between each striation on the moiré pattern is roughly 4-5 nm in length.
Similar observations as those in graphene/hBN systems have been previously reported . The latter distance matches the expected tip radius curvature values for the AFM tip used to acquire all four sets of data. This achievement is truly exceptional for two main reasons: the consistency and clarity of the data. The acquired images increase in magnification and this is generally difficult to characterize with AFM.
The data can be now collected even by inexperienced researchers with the guidance of automation software. Hence, the accomplishment of resolving five nanometer periodic features is even greater. These demonstrated advantages of non-contact mode for ultra-high resolution AFM imaging can only become more valuable as tip radius curvatures decrease and the effect of tip blunting becomes more pronounced as we characterize smaller and smaller sample features.
We are thankful to Patrick Gallagher of Stanford University for providing the graphene/hBN sample used to acquire the images presented in this report.
References and Further Reading
- Park, S. Ultimate Resolution of AFM in Air.
- Mizes, H., Park, S., & Harrison, W. Phys. Rev. B 36 4491 (1987).
- Albrecht, T.A., Mizes, H.A., Nogami, J., et al. App. Phys. Lett. 52, 362 (1988)
- Park NX10 – Technical Info. Retrieved from http://www.parkafm.com/index.php/products/research-afm/park-nx10/technical-info (2016)
- True Non-Contact™ Mode. Retrieved from http://www.parkafm.com/index.php/park-spm-modes/91-standard-imaging-mode/217-true-non-contact-mode (2016)
- AFM tips PPP-NCHR. Retrieved from http://www.nanosensors.com/PointProbe-Plus-Non-Contact-Tapping-Mode-High-Resonance-Frequency-Reflex-Coating-afm-tip-PPP-NCHR (2016)
- Zandiatashbar, A.Automated Non-Destructive Imaging and Characterization of Graphene/hBN Moiré Pattern with Non-Contact Mode AFM.NanoScientific, Fall 2015 14-17 (2015)
- Gallagher, P., Lee, M., Amet, F., et.al. Nature Comm. 7 10745 (2016)
This information has been sourced, reviewed and adapted from materials provided by Park Systems Inc.
For more information on this source, please visit Park Systems Inc.