During the last Multifrequency AFM Conference in Madrid [i], Kristen Burson from the Fritz Haber Institute in Berlin presented a stunning talk, “Resolving 2D Amorphous Materials with Atomic Force Microscopy”. She presented results where the Cypher AFM was used for high-resolution imaging of an amorphous silica bilayer. This work was also recent published in Applied Physics Letters [ii].
High Resolution Imaging of Amorphous Glass
For many reasons, this study sparked interest. Glass is a ubiquitous and fascinating material with a variety of mechanical and electrical properties, thus making it a perfect material for testing the resolution of AFM. Unlike a calcite or other crystal, it is basically just a collection of defects. It interested other researchers to see the amorphous surface imaged in liquid with the Cypher ES system.
Investigators at Asylum Research were inspired by this study and started simple tests on the glass to achieve amorphous lattice resolution. They acquired a microscope slide manufactured by Fischer Scientific that was made from common soda-lime glass. In contrast to calcite, glass is not self-cleaning and so care needed to be taken in the preparation and handling stages. A simple protocol was followed:
- The glass surface was vigorously scrubbed with a laboratory dishwashing detergent.
- It was rinsed copiously with distilled water.
- Then, blown dry with dry nitrogen.
- A droplet of the imaging solution, ultrapure distilled water, was put onto the glass and spread out evenly, demonstrating that the glass surface is clean and “hydrophilic.”
Imaging conditions were chosen using Nanoworld Arrow UHF cantilever in tapping mode using a free amplitude of ~4 nm and a setpoint of ~2 nm. Similar to Burson et al.’s investigations, blueDrive photothermal excitation for a stable, well-controlled amplitude was used and typical scan ranges were 5-10 nm.
Although the glass slides were not particularly smooth, it was mostly easy to find smooth hilltops or valleys where the amorphous structure could be resolved. Examples of this are shown below.
Figure 1. Topography images of disordered lattice imaged at 2nm amplitude setpoint. a) 10 nm scan and b) 5 nm scan. Both images clearly reveal sub-nm amorphous glass surface.
Figure 2. a) Surface topography and b) Tip-sample stiffness of a region of the glass sample imaged using AMFM stiffness mapping. 10 nm scan.
Mapping Topography and Tip-Sample Stiffness with AM-FM Mode
By using blueDrive and the Arrow UHF, researchers could simultaneously map the topography and tip-sample stiffness using AM-FM mode (Figure 2). Comparable to Burson et al., a disordered-appearing surface was revealed, with length scales similar to those reported in that paper. Remarkably, these structures were visible with slightly different resolutions in any attempt made. This is a testament to the low noise of the Cypher AFM and to the consistent sharpness of the Arrow UHF cantilevers.
[i] The next Multifrequency AFM meeting is this April in Madrid – see http://www.icmm.csic.es/multifrequency-afm/ for more info.
[ii] Kristen M. Burson, Leonard Gura, Burkhard Kell, Christin Büchner, Adrian L. Lewandowski, Markus Heyde, and Hans-Joachim Freund, Applied Physics Letters 108, 201602 (2016); http://aip.scitation.org/doi/abs/10.1063/1.4949556.
This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.
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