AFM Reveals Detail of a Molecular Resolution 2D Lattice Previously Observed Only by STM

Previously, Dr. Nate Kirchhofer from Asylum Research made a note about low-current STM imaging of self-assembled 2D lattices of cobalt and nickel octaethylporphyrin (CoOEP and NiOEP, respectively) on HOPG. After viewing the resolution of the images produced from this, a colleague suggested that it might be possible to see the CoOEP lattice by implementing using tapping mode on a Cypher AFM, owing to its exceptional stability [ii]. Despite considerable skepticism about the idea, he tried it and was amazed by the results.

AFM vs. STM

Asylum Research used a FS-1500AuD probe, known for its sharp, quick, moderately stiff, gold-coated silicon lever (nominally, Rtip = 10 ± 2 nm, fair ≈ 1.5 MHz, k ≈ 6 N/m). After engaging the tip to the surface, the thin layer of phenyloctane wicked up onto the probe cantilever as shown in Figure 1. The resonant frequency instantly dropped to 0.66 MHz in the now hybrid air-liquid oscillatory medium. In this case, it took about 10 minutes for the wicked solution to equilibrate, and was stable even as the surface was moved.

Time series of images: interference fringes at the phenyloctane/HOPG interface. These images were taken through the Cypher ES topview optics. The fringes arise from a phenyloctane meniscus acting as a diffractor as the solution wicks up onto the AFM cantilever.

Figure 1. Time series of images: interference fringes at the phenyloctane/HOPG interface. These images were taken through the Cypher ES topview optics. The fringes arise from a phenyloctane meniscus acting as a diffractor as the solution wicks up onto the AFM cantilever.

Afterwards, the cantilever was tuned in the equilibrated solution, and investigators were able to maintain a stable free drive amplitude of ~1.44 nm (90 mV) and a setpoint of ~0.34 nm (21 mV). The solidity is due to the excellent stability of blueDrive photothermal excitation [iii].

Consequently, the CoOEP lattice was seen, with the expected molecular spacing of ~1.4 nm. The image in Figure 2 showed the low-amplitude tapping mode molecular lattice resolution of CoOEP

Low-amplitude tapping mode molecular lattice resolution of CoOEP. (A) 100 nm survey scan. (B) Line trace along the white line in (A) presenting the regular spacing of CoOEP molecules. (C) 3D rendering of the 100 nm image for perspective.

Figure 2. Low-amplitude tapping mode molecular lattice resolution of CoOEP. (A) 100 nm survey scan. (B) Line trace along the white line in (A) presenting the regular spacing of CoOEP molecules. (C) 3D rendering of the 100 nm image for perspective.

Figure 3 shows a zoomed in version in tapping mode AFM images of the CoOEP lattice. Researchers were convinced they saw a porphyrin ring structure in part of the phase image. Measurements were observed to be much more sensitive to contamination compared to the STM images seen prior.

Amorphous globs were seen on the surface, in addition to changes in the image as the scan progressed. This was because of the tip-glob interactions. This simply means that prior to AFM measurements, users need to be extra meticulous about cleaning the sample, the probe, and the probe holder.

Zoomed in tapping mode AFM images of the CoOEP lattice. (A) Height topography; 20 nm scan. (B) Phase; 20 nm scan. Note: the porphyrin ring structure apparent in the upper section of the image.

Figure 3. Zoomed in tapping mode AFM images of the CoOEP lattice. (A) Height topography; 20 nm scan. (B) Phase; 20 nm scan. Note: the porphyrin ring structure apparent in the upper section of the image.

Results

These data are comparable to previous data from AFM studies [iv-vi] using Cypher in ambient conditions. These were published by Prof. Rob Atkin from The University of Newcastle, Prof. Peter Beton from University of Nottingham, and Prof. Xinran Wang  from Nanjing University. The first study [iv] clarified the nanostructure of an ionic liquid (EMIm+ TFSI) under potentiostatically-controlled bias at a graphite (HOPG) surface, as seen in Figure 4A.

Furthermore, the applied bias was varied methodically around the open circuit potential, and the molecular Stern layer reorganized as a function of bias (as well as ionic components, such as Li+ and Cl). The second study [v] explored the supramolecular organization of 5,10,15,20-tetrakis (4-carboxylphenyl) porphyrin (TCPP) which was adsorbed on hexagonal boron nitride (hBN) and other surfaces, and the consequence that resulted from that adsorption on the optoelectronic properties of the TCPP molecules.

Figure 4B demonstrates the square lattice structure of TCPP on hBN. The third study [vi] investigated van der Waals epitaxy of few-layer 2D molecular crystals of high-mobility dioctylbenzothienobenzothiophene (C8-BTBT) on HOPG and hBN, particularly for applications in organic field-effect transistors. High-resolution topography of the lattice of C8-BTBT grown on hBN can be seen in Figure 4C.

AFM imaging of 2D molecular lattices from previous studies. (A) Phase image of a pure EMIm+ TFSI– Stern layer adsorbed to a HOPG substrate; 30 nm scan, imaged in the bulk EMIm+ TFSI– ionic liquid (as seen in Ref. [iv]). (B) Height image of the square lattice of TCPP assembled on hBN substrate; 50 nm scan, imaged in air (as seen in Ref. [v]). (C) Height image of the lattice of C8-BTBT grown on hBN substrate; 10 nm scan, imaged in air (as seen in Ref. [vi]).

Figure 4. AFM imaging of 2D molecular lattices from previous studies. (A) Phase image of a pure EMIm+ TFSI Stern layer adsorbed to a HOPG substrate; 30 nm scan, imaged in the bulk EMIm+ TFSI ionic liquid (as seen in Ref. [iv]). (B) Height image of the square lattice of TCPP assembled on hBN substrate; 50 nm scan, imaged in air (as seen in Ref. [v]). (C) Height image of the lattice of C8-BTBT grown on hBN substrate; 10 nm scan, imaged in air (as seen in Ref. [vi]).

Imaging ionic liquids has resulted in STM being the standard analytical technique. AFM has discovered and published fresh detail about their interfacial structures. This is mainly because, as an ambient technique, it evades the conventional artifacts that are induced by freezing monolayers of an ionic liquid under ultra-high vacuum.

Conclusion

Additionally, STM is only possible with conductive samples, which many materials are not, as a new avenue of characterization for interesting semiconductor and insulator nanomaterials is allowed by high-resolution AFM. Sub-nanometer molecular resolution has now been forgotten as the province of only STM. It is obvious that next-generation AFMs, like Cypher, are steadily leading into that resolution realm, presenting new nanoscale features that might not have been observable with STM previously.

References

[i] April Current Amplifiers Bring May Ultra-Low-Current STM
[ii] Learn more about Cypher here: https://www.oxford-instruments.com/products/atomic-force-microscopy-systems-afm/asylum-research/high-resolution-fast-scanning-afm.
[iii] (a) Learn more about blueDrive at https://afm.oxinst.com/bluedrive and at https://pdfs.semanticscholar.org/e807/9171fb282e6340f6813a0f6b8cee8b4bae74.pdf. (b) A. Labuda, K. Kobayashi, Y. Miyahara, and P. Grütter, Retrofitting an atomic force microscope with photothermal excitation for a clean cantilever response in low Q environments, Review of Scientific Instruments, 2012 83, 053703. https://aip.scitation.org/doi/abs/10.1063/1.4712286.
[iv] A. Elbourne, S. McDonald, K. Voïchovsky, F. Endres, G. G. Warr, and R. Atkin, Nanostructure of the Ionic Liquid–Graphite Stern Layer, ACS Nano, 2015, 9(7), 7608–7620. https://pubs.acs.org/doi/abs/10.1021/acsnano.5b02921.
[v] V. V. Korolkov, S. A. Svatek, A. Summerfield, J. Kerfoot, L. Yang, T. Taniguchi, K. Watanabe, N. R. Champness, N. A. Besley, and P. H. Beton, van der Waals-Induced Chromatic Shifts in Hydrogen-Bonded Two-Dimensional Porphyrin Arrays on Boron Nitride, ACS Nano, 2015, 9(10), 10347–10355. https://pubs.acs.org/doi/10.1021/acsnano.5b04443.
[vi] D. He, Y. Zhang, Q. Wu, R. Xu, H. Nan, J. Liu, J. Yao, Z. Wang, S. Yuan, Y. Li, Y. Shi, J. Wang, Z. Ni, L. He, F. Miao, F. Song, H. Xu, K. Watanabe, T. Taniguchi, J.-B. Xu & X. Wang, Two-dimensional quasi-freestanding molecular crystals for high-performance organic field-effect transistors, Nature Communications, 2014, 5:5162, 1–7. https://www.nature.com/articles/ncomms6162.

This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.

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