Produce reliable atomic-scale images of 2D materials with AFM


A new article has appeared in the journal Nanomaterials present research on improving the ability of atomic force microscopy (AFM) to image 2D materials at the atomic level. The research was carried out by scientists from Hanyang University in Korea and the University of Texas at Austin in the United States.

Study: Accurate atomic-scale imaging of two-dimensional networks using atomic force microscopy under ambient conditions. Image Credit: sanjaya viraj bandara/Shutterstock.com

Structures at the atomic level

Materials with atomic-level thin films and van der Waals heterostructures have been widely studied due to their unique properties and commercial applications for nanophotonics and quantum electronics. These materials are typically synthesized using chemical vapor deposition and mechanical exfoliation techniques, and the material properties can be tuned by techniques such as controlling interlayer twist angles and changing the number of layers. .

Raw lateral force microscopy (LFM) images obtained with or without engagement of XY closed-loop control in air (adjusted color gamut).  (a) Schematic representation of a scrambled LFM image due to the use of XY closed-loop control.  The orange dashed lines indicate locations where the piezo stage was abruptly repositioned due to XY closed-loop control;  (b) LFM image of a MoS2 monolayer obtained with closed-loop control (calibrated scale bar: 2 nm; scanning frequency: 21 Hz);  (c) schematic illustration of the LFM image of a two-dimensional (2D) network expected in the absence of the XY closed-loop control;  (d) LFM image of a MoS2 monolayer obtained without closed-loop control, which revealed a periodic pattern of the atomic lattice (calibrated scale bar: 2 nm; scan frequency: 21 Hz).  The gray dots are a guide for the eye and correspond to the MoS2 rate network.

Raw lateral force microscopy (LFM) images obtained with or without engagement of XY closed-loop control in air (adjusted color gamut). (a) Schematic representation of a scrambled LFM image due to the use of XY closed-loop control. The orange dashed lines indicate locations where the piezo stage was abruptly repositioned due to XY closed-loop control; (b) LFM image of a MoS2 monolayer obtained with closed-loop control (calibrated scale bar: 2 nm; scanning frequency: 21 Hz); (vs) schematic illustration of the LFM image of a two-dimensional (2D) network expected in the absence of the XY closed-loop control; (D) LFM image of a MoS2 monolayer obtained without closed-loop control, which revealed a periodic pattern from the atomic lattice (calibrated scale bar: 2 nm; scanning frequency: 21 Hz). The gray dots are a guide for the eye and correspond to the MoS rate lattice2. Image Credit: Kim, S et al., Nanoamaterials

Remarkable quantum properties and phases have been discovered by fitting these materials, including quantized exciton states, the Hofstadter butterfly effect, and unconventional superconductivity. However, some imperfections that arise during synthesis can limit the effectiveness of these materials, such as bubbles and wrinkles in unstacked van der Waals bilayers. This can cause unwanted spatial variations in disorder and distortion in superlattices.

Imaging structures at the atomic level

Improving techniques for imaging structures at the atomic level is currently a major concern for researchers. Non-destructive ultrafast laser imaging methods can image structural properties, stresses and strains at the scale of tens to hundreds of nanometers. Some studies have demonstrated that it is possible to image lattice strain distortion at the atomic level or use scanning probe techniques to control torsional angles in heterogeneous nanoscale structure of materials, but right now it’s hard.

Various atomic-scale imaging techniques have been developed in recent years, including scanning tunneling microscopy (STM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and lateral force microscopy (LFM), which is a type of AFM technique. . Each method has different capabilities and provides different complementary information.

The effect of slew rate on LFM grating images (adjusted color gamut).  (a–c) LFM raw data of MoSe2 at different scan rates (calibrated scale bar: 1 nm);  (d–f) LFM raw data of graphene at different scan speeds (scale bar: 1 nm).  All green arrows are aligned in the direction of the zigzag.

The effect of slew rate on LFM grating images (adjusted color gamut). (avs) LFM raw data of MoSe2 at different scanning speeds (calibrated scale bar: 1 nm); (DF) LFM raw data of graphene at different scan speeds (scale bar: 1 nm). All green arrows are aligned in the direction of the zigzag. Image Credit: Kim, S et al., Nanoamaterials

LFM has proven attractive for imaging structures at the atomic level due to its advantages over other techniques. Unlike STM and TEM, for example, it does not require elaborate sample preparation processes that would otherwise damage samples or the need for conductive substrates. The flexibility of this method makes it a versatile tool for the non-destructive characterization of materials at the atomic scale under ambient conditions. LFM works in contact mode.

Although LFM is a powerful, flexible, and versatile nondestructive imaging technique, several current challenges hinder its widespread adoption. Less than optimal settings and thermal drift can distort images, reducing the accuracy of routine imaging of structures such as 2D atomic lattices. Additionally, if the quality of the original image is poor or if the researchers do not follow careful filtering procedures, the authenticity of the filtered LFM images may be in question.

Specialized approaches using symmetrically aligned or custom-made AFMs, carbon nanotube tips, or specially functionalized tips have been used to produce contact mode images of 2D lattice structures, but adoption of these specialized processes is challenging.

The study

The authors presented a protocol based on an intimate understanding of feedback loops and parameters such as gain, scan size, and scan rate that influence unfiltered and unflattened LFM images in the presence of drifts. This new imaging protocol enhances the LFM’s ability to produce clear and accurate images of 2D atomic lattices.

The effect of load (i.e. set point) on LFM network images.  (a–c) LFM raw data of MoS2 at various loadings (calibrated scale bar: 5 nm; frequency: 17.1 Hz).  The insets in (a) represent the filtered FFT images of the area bounded by the white rectangle and its corresponding inverse FFT image;  (d–f) Corresponding unfiltered FFT images of MoS2.  The six FFT spots associated with the hexagonal lattice structure are highlighted by dashed circles.  The spots were less visible at a load of 33.38 nN, as seen in (f).

The effect of load (i.e. set point) on LFM network images. (avs) MoS LFM raw data2 at different loads (calibrated scale bar: 5 nm; frequency: 17.1 Hz). The inserts in (a) represent the filtered FFT images of the area bounded by the white rectangle and its corresponding inverse FFT image; (DF) corresponding unfiltered FFT images of MoS2. The six FFT spots associated with the hexagonal lattice structure are highlighted by dashed circles. The spots were less visible at a load of 33.38 nN, as seen in (F). Image Credit: Kim, S et al., Nanoamaterials

Examples of scanning targets used in the study include TMD monolayers and graphene. Selecting the optimal scan parameters decreases geometric distortions in raw LFM images and significantly improves the signal-to-noise ratio. The authors stated that the protocol enables the precise identification and interpretation of crystallographic structures in self-folded and torn edges in graphene, improving the determination of crystal axes present in folded graphene nanoribbons.

The authors also discussed the effect of substrate roughness and loading on the accuracy and sharpness of LFM images of 2D atomic lattice structures. A comprehensive analysis of ninety-nine studies and articles in the current literature was carried out by the researchers.

The new protocol developed by the researchers greatly improves the ability of LFM techniques to produce clear and precise images of structures at the atomic level. It was observed in the study that the optimal values ​​of scanning parameters vary between laboratories due to their specific drift rate. The Hertz model was used to estimate the stiffness between the AFM tip and the Si substrate contact area, which the authors say allows other research teams to adapt their protocol.

In summary, atomic-scale images produced using this new LFM protocol can be used to identify twist angle variations and local stresses to improve the quality of commercially important 2D materials such as MXenes, graphene, perovskites and van der Waals heterostructures.

Further reading

Kim, S et al. (2022) Accurate Atomic Scale Imaging of Two-Dimensional Lattices Using Atomic Force Microscopy Under Ambient Conditions Nanomaterials 12(9) 1542 [online] mdpi.com. Available at: https://www.mdpi.com/2079-4991/12/9/1542

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