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IMM colloquium March 26, 2019: "The qPlus sensor, a powerful core for the atomic force microscope" (Lecture)

Tuesday 26 March 2019Add to my calendar
from 16:00
prof. Franz Giessibl (University of Regensburg, Germany)
Preceding lecture
Fleur van Zelst (Solid State NMR): "Analysing complex mixtures by hyphenation of supercitical fluid chromatography and NMR"

Franz GiessiblThe scanning tunneling microscope (STM) has opened a new era of small things. STM relies on vacuum tunneling with an exponential increase of a tunneling current between two biased conductive electrodes at a factor of ten per Å (100 pm). If a tip has one atom that sticks out one Å more than all the others, this front atom carries ten times more current than the other atoms. The monotonic decrease of current with distance facilitates distance feedback and allows to scan the tip across a sample with atomic precision. In 1986, Binnig, Gerber and Quate introduced atomic force microscopy (AFM), a method that also images insulators by relying on forces. Unlike the current, the force between tip and sample is non-monotonic and includes long- and short range components. AFM has been inferior in resolution to STM for a long time. Today, AFM exceeds STM in spatial resolution by utilizing Pauli repulsion forces that change even stronger with distance than the tunneling current. That progress was enabled by advances in measuring small forces and by the isolation of chemical bonding forces from strong background forces. The special challenges of AFM are met by the qPlus sensor,1 a quartz force sensor that measures force gradients by frequency changes and was initially based on tuning forks used in Swatch wristwatches. Using the outstanding precision of frequency measurements, we can today measure the forces that act in atomic manipulation, measure exchange interactions with sub-pN sensitivity, image clusters and molecules with atomic resolution and single adatoms with subatomic resolution. Highest precision measurements require vacuum and low temperatures, and measuring the deflection of a force sensor usually introduces heat. Nevertheless, we could show that the tip of a qPlus sensor remains superconducting during its operation.

Left: AFM image of a Fe trimer next to a Fe dimer on Cu(111). Inset: AFM image of a single Fe atom on Cu(111)2. Right: qPlus sensor.

  1. F. J. Giessibl, Rev. Sci. Instrum. 90, 011101 (2019).
  2. M. Emmrich et al., Science348 308 (2015).

prof. Alex Khajetoorians, dr. Nadine Hauptmann