Researchers continue to be captivated by the views that scanning probe microscopy provides into some of the smallest structures of nature. Arguably, no technique reaches farther into the atomic world than scanning tunneling microscopy (STM), which makes those results especially mesmerizing.
Scanning Tunneling Microscopy (STM)
STM is the Nobel-prize-winning invention [i] in which an atomically sharp stylus is rastered back and forth at a distance of a few angstroms from a surface, and fractions of nanoamps of current are measured without causing damage to the tip. It’s a special case where a quantum mechanical process can be seen and used in the macroscle world; electron tunneling. It is incredible, but indeed STM can control the tip-sample distance to within an atomic radii and measure the current that depends exponentially on that distance, as shown in Figure 1.
Figure 1. Scanning Tunnelling Microscopy in the Cypher ES. (Left) A magnified view of a STM probe approaching the HOPG surface inside the Cypher ES. (Right) A schematic showing some of the principles of electron tunneling. Vapp is applied bias, e is the elementary charge, I is current, z is vertical distance, εF is Fermi level, Φ is work function, Ψ is the electron wave function, and k ~ Φ½ is the characteristic inverse decay length. Assuming Φ ≈ 5 eV, each 1 Å change in z causes an order of magnitude change in current, yielding extreme z sensitivity.
Despite skepticism, it has been proven that the STM does work in most cases, and much of that is due to improvements in instrument precision. Improvements that have been seen are fine position control with piezoelectric crystals [ii], noise reduction through the engineering of a smaller sample-tip mechanical loop such as that found in Asylum’s Cypher line of instruments, and most recently, ultra-low-noise current amplifiers enabling very small tunneling current setpoints.
Consequently, this means that STM is regularly achievable in a variety of conditions with detectable currents below 1 pA. Conventionally, high resolution STM imaging is a challenge in ambient and liquid conditions. Many of the most impressive images come from ultra-high vacuum measurements at ultra-low temperatures. As the Cypher instruments are intended for stable operation in ambient and liquid conditions, developers worked on extending the low-current range for STM on this platform. Asylum installed one of these new, ultra-low-current amplifiers on a Cypher ES.
Using STM to Image a Self-Assembled 2D Molecular Lattice
Researchers at Asylum were called upon separately by Professor Kerry W. Hipps from Washington State University and Professor Michael D. Hopkins from University of Chicago with the challenging task of obtaining low-current-STM images of self-assembled 2D metalloporphyrin crystals on HOPG under ambient conditions. The sample preparation is straightforward and involves ~1 μL of a solution of Cobalt or Nickel octaethylporphyrin (CoOEP or NiOEP, respectively) in phenyloctane that is deposited on freshly-cleaved HOPG. This allows metalloporphyrins to spontaneously form the desired conformal lattice through π-π interactions with the carbon substrate, meanwhile the phenyloctane remains on the surface.
Figure 2. Tip current images of the 2D lattice of NiOEP on HOPG. (A) A flattened survey scan showing the NiOEP grain boundary, as depicted by blue arrows, zoom regions, and moiré pattern. (B) Zoom region imaged at 300 fA setpoint. (C) Zoom region showing sub-nm molecular resolution. Inset: the CPK molecular model of NiOEP (adapted from ref. [iii]).
Professor Hopkins sent a solution of NiOEP, and Professor Hipps sent a solution of CoOEP. Researchers at Asylum followed the simple deposition procedure, as mentioned above, to form the 2D lattices. With Asylum’s house-made 80/20 Pt/Ir STM probes, and logarithmic current feedback, stunning resolution was achieved at a variety of imaging conditions for both samples. As shown in Figure 2, a grain boundary in the lattice of NiOEP molecules was identified.
Ambient STM Resolution that Begins to Rival UHV STM Resolution
Using an extremely low 300 fA setpoint (that’s a meager ~2 million electrons/second) allows one to obtain sub-nanometer molecular resolution of the porphyrin rings. Figure 3 depicts that the CoOEP lattice is also readily apparent, along with the porphyrin ring structures. For those images, current and applied voltage were adjusted and the integral gain was increased to resolve the lattice in the height channel. All of these data were acquired at ambient conditions in air, with no specific cleaning or extra sample preparation steps. Though not quite the same stunning atomic level view as UHV STM, this remarkable resolution under ambient conditions opens a wide range of opportunities to explore, especially when combined with other techniques.
Figure 3. Tip height images of the 2D lattice of CoOEP on HOPG. (A) Survey scan showing the CoOEP 2D lattice. (B) Zoomed view showing sub-molecular structure. (C) Further zoom showing sub-nm molecular topographic resolution.
[i] G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Applied Physics Letters 40 (1982), 178-180, https://aip.scitation.org/doi/pdf/10.1063/1.92999.
[ii] (a) G. Binnig et al., Piezo drive with coarse and fine adjustment, IBM Technical Disclosure Bulletin 22(7) (1979), 2897-2898, New York, US; (b) See also V. B. Elings, J. A. Gurley, Positioning device for a scanning tunneling microscope, US Patent 4,871,938 (1988) and its cited and citing patents: https://patents.google.com/patent/US4871938A/en.
[iii] L. Scudiero, D. E. Barlow, K. W. Hipps J. Phys. Chem. B, 2002, 106 (5), pp 996–1003. https://pubs.acs.org/doi/abs/10.1021/jp012436m
This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.
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